METHOD OF PRODUCING ELECTROCATALYST COATED ELECTRODE BY ELECTROCHEMICAL OXIDATION
20250297386 ยท 2025-09-25
Inventors
Cpc classification
C25B11/052
CHEMISTRY; METALLURGY
C25B11/054
CHEMISTRY; METALLURGY
International classification
Abstract
A method comprises applying a slurry to one or more surfaces of a conductive substrate to form a precursor coating and provide a precursor coated substrate, wherein the slurry includes precursor particles comprising one or more precursor compounds in a slurry medium, and electrochemically oxidizing the one or more precursor compounds to chemically convert the one or more precursor compounds to one or more catalyst compounds and form catalyst particles adhered to the one or more surfaces of the conductive substrate to provide a catalyst coated substrate.
Claims
1. A method comprising: applying a slurry to one or more surfaces of a conductive substrate to form a precursor coating and provide a precursor coated substrate, wherein the slurry includes precursor particles comprising one or more precursor compounds in a slurry medium; and electrochemically oxidizing the one or more precursor compounds to chemically convert the one or more precursor compounds to one or more catalyst compounds and form catalyst particles adhered to the one or more surfaces of the conductive substrate to provide a catalyst coated substrate.
2. The method of claim 1, wherein electrochemically oxidizing the one or more precursor compounds comprises exposing the precursor coated substrate to a caustic solution while performing a specified electrochemical processing method to the precursor coated substrate that chemically converts the one or more precursor compounds to the one or more catalyst compounds.
3. The method of claim 2, wherein the specified electrochemical processing method comprises chronoamperometry, chronopotentiometry, cyclic voltammetry, or a combination thereof.
4. The method of claim 1, wherein the one or more catalyst compounds comprise a spinel with the general chemical formula AB.sub.2O.sub.4, wherein both A and B are elements comprising at least one of: a Group 1 metal, a Group 2 metal), a transition metal, a lanthanide series metal, a post-transition metal, a metalloid, sulfur (S), or phosphorus (P).
5. The method of claim 4, wherein A comprises one or more of: nickel (Ni), zinc (Zn), copper (Cu), cobalt (Co), iron (Fe), lithium (Li), magnesium (Mg), calcium (Ca), barium (Ba), manganese (Mn), germanium (Ge), cadmium (Cd), lanthanide (La), Yttrium (Y), chromium (Cr), molybdenum (Mo), ruthenium (Ru), and iridium (Jr).
6. The method of claim 4, wherein B comprises one or more of: cobalt (Co), iron (Fe), chromium (Cr), aluminum (Al), manganese (Mn), nickel (Ni), gallium (Ga), selenium (Se), molybdenum (Mo), ruthenium (Ru), sulfur (S), and tellurium (Te).
7. The method of claim 1, further comprising drying the precursor coating to provide a dried precursor coating before electrochemically oxidizing the one or more precursor compounds to chemically convert the one or more precursor compounds to one or more catalyst compounds.
8. The method of claim 1, further comprising electrically activating active sites on the catalyst particles.
9. The method of claim 8, wherein electrically activating the active sites on the catalyst particles comprises synthesizing a reaction-enhancing compound at the active sites.
10. The method of claim 9, wherein the reaction-enhancing compound comprises a metal oxyhydroxide.
11. The method of claim 8, wherein electrochemically oxidizing the one or more precursor compounds to chemically convert the one or more precursor compounds to one or more catalyst compounds and electrically activating the active sites on the catalyst particles occur simultaneously or substantially simultaneously.
12. The method of claim 1, wherein the one or more precursor compounds comprise one or more metal hydroxides.
13. The method of claim 1, wherein the slurry comprises one or both of a thickener and a binder.
14. The method of claim 1, wherein apply the slurry to the one or more surfaces of the conductive substrate comprises at least one of: dip coating the conductive substrate with the slurry, painting the one or more surfaces of the conductive substrate with the slurry, spraying the slurry onto the one or more surfaces of the conductive substrate, applying the slurry onto one or more surfaces of the conducive substrate with a doctor blade, applying the slurry onto one or more surfaces of the conducive substrate with a slot die, and applying the slurry onto one or more surfaces of the conducive substrate with a comma bar.
15. The method of claim 1, wherein the conductive substrate comprises a metal mesh.
16. The method of claim 1, wherein the conductive substrate comprises one or more of: nickel metal, titanium metal, steel, gold metal, copper metal, and a conductive carbon-based material.
17. The method of claim 1, further comprising: coprecipitating one or more precursor compounds from a precursor solution comprising one or more first metal salts and one or more second metal salts to produce precipitated precursor particles comprising the one or more precursor compounds; and forming the slurry from the precipitated precursor particles and the slurry medium.
18. The method of claim 17, further comprising washing the precipitated precursor particles before forming the slurry.
19. The method of claim 17, wherein the one or more catalyst compounds comprise a spinel oxide having the general chemical formula AB.sub.2O.sub.4, wherein the one or more first metal salts comprise one or more first salts of A, and wherein the one or more second metal salts comprises one or more second salts of B.
20. The method of claim 19, wherein the one or more first salts and the one or more second salts each comprise at least one of: a metal nitrate, a metal acetate, a metal sulfate, or a metal halide.
21. The method of claim 20, wherein the one or more first salts are different from each of the one or more second salts.
Description
BRIEF DESCRIPTION OF THE FIGURES
[0006] The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document.
[0007]
[0008]
[0009]
[0010]
[0011]
[0012]
[0013]
[0014]
[0015]
[0016]
[0017]
[0018]
[0019]
DETAILED DESCRIPTION
[0020] The following detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments, which are also referred to herein as examples, are described in enough detail to enable those skilled in the art to practice the invention. The example embodiments may be combined, other embodiments may be utilized, or structural, and logical changes may be made without departing from the scope of the present invention. While the disclosed subject matter will be described in conjunction with the enumerated claims, it will be understood that the exemplified subject matter is not intended to limit the claims to the disclosed subject matter. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims and their equivalents.
[0021] References in the specification to one embodiment, an embodiment, an example embodiment, etc., indicate that the embodiment described can include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.
[0022] Values expressed in a range format should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a recited range of values of about 0.1 to about 5 should be interpreted to include not only the explicitly recited values of about 0.1 and about 5, but also all individual concentrations within the indicated range of values (e.g., 1, 1.23, 2, 2.85, 3, 3.529, and 4, to name just a few) as well as sub-ranges that fall within the recited range (e.g., 0.1 to 0.5, 1.21 to 2.36, 3.3 to 4.9, or 1.2 to 4.7, to name just a few). The statement about X to Y has the same meaning as about X to about Y, unless indicated otherwise. Likewise, the statement about X, Y, or about Z has the same meaning as about X, about Y, or about Z, unless indicated otherwise.
[0023] In this document, the terms a, an, or the are used to include one or more than one unless the context clearly dictates otherwise. The term or is used to refer to a nonexclusive or unless otherwise indicated. Unless indicated otherwise, the statement at least one of when referring to a listed group is used to mean one or any combination of two or more of the members of the group. For example, the statement at least one of A, B, and C can have the same meaning as A; B; C; A and B; A and C; B and C; or A, B, and C, or the statement at least one of D, E, F, and G can have the same meaning as D; E; F; G; D and E; D and F; D and G; E and F; E and G: F and G; D, E, and F; D, E, and G; D, F, and G; E, F, and G; or D, E, F, and G. A comma can be used as a delimiter or digit group separator to the left or right of a decimal mark; for example, 0.000,1 is equivalent to 0.0001.
[0024] In the methods described herein, the steps can be carried out in any order without departing from the principles of the invention, except when a temporal or operational sequence is explicitly recited. Furthermore, specified steps can be carried out concurrently unless explicit language recites that they be carried out separately. For example, a recited act of doing X and a recited act of doing Y can be conducted simultaneously within a single operation, and the resulting process will fall within the literal scope of the process. Recitation in a claim to the effect that first a step is performed, and then several other steps are subsequently performed, shall be taken to mean that the first step is performed before any of the other steps, but the other steps can be performed in any suitable sequence, unless a sequence is further recited within the other steps. For example, claim elements that recite Step A, Step B, Step C, Step D, and Step E shall be construed to mean step A is carried out first, step E is carried out last, and steps B, C, and D can be carried out in any sequence between steps A and E (including with one or more steps being performed concurrent with step A or Step E), and that the sequence still falls within the literal scope of the claimed process. A given step or sub-set of steps can also be repeated.
[0025] Furthermore, specified steps can be carried out concurrently unless explicit claim language recites that they be carried out separately. For example, a claimed step of doing X and a claimed step of doing Y can be conducted simultaneously within a single operation, and the resulting process will fall within the literal scope of the claimed process.
[0026] The term about as used herein can allow for a degree of variability in a value or range, for example, within 10%, within 5%, within 1%, within 0.5%, within 0.1%, within 0.05%, within 0.01%, within 0.005%, or within 0.001% of a stated value or of a stated limit of a range, and includes the exact stated value or range.
[0027] The term substantially as used herein refers to a majority of, or mostly, such as at least about 50%, 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5%, 99.9%, 99.99%, or at least about 99.999% or more, or 100%.
[0028] In addition, it is to be understood that the phraseology or terminology employed herein, and not otherwise defined, is for the purpose of description only and not of limitation. Furthermore, all publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls.
Water Electrolysis for Hydrogen Gas Production
[0029] Hydrogen gas (H.sub.2) can be formed electrochemically by a water-splitting reaction where water is split into H.sub.2 gas and (optionally) oxygen gas (O.sub.2) at a cathode and an anode of an electrochemical cell, respectively. Examples of such electrochemical processes include, without limitation, proton electrolyte membrane (PEM) electrolysis and alkaline water electrolysis (AWE). In such electrochemical reactions, the operating energy necessary to drive the water-splitting electrolysis reaction is high due to additional energy costs as a result of various energy inefficiencies. For example, to reduce unwanted migration of ionic species between the electrodes, the cathode and the anode may be separated by a separator, such as a membrane, which can reduce migration of the ionic species. Although the separator can improve the overall efficiency of the cell, it can come at a cost of additional resistive losses in the cell, which in turn increases the operating voltage. Other inefficiencies in water electrolysis can include solution resistance losses, electric conduction inefficiencies, and/or electrode over-potentials, among others.
[0030]
[0031] In examples where the separator 16 is a cation exchange membrane, the cation exchange membrane can be a conventional membrane such as those available from, for example, Asahi Kasei Corp. of Tokyo, Japan, or from Membrane International Inc. of Glen Rock, NJ, USA, or from The Chemours Company of Wilmington, DE, USA. Examples of cation exchange membranes include, but are not limited to, the membrane sold under the N2030WX trade name by The Chemours Company and the membrane sold under the F8020/F8080 or F6801 trade names by the Asahi Kasei Corp. Examples of materials that can be used to form a cationic exchange membrane include, but are not limited to, a perfluorinated polymer containing anionic groups, for example sulphonic and/or carboxylic groups. It may be appreciated, however, that in some examples, depending on the need to restrict or allow migration of a specific cation or an anion species between the electrolytes, a cation exchange membrane that is more restrictive and thus allows migration of one species of cations while restricting the migration of another species of cations may be used. Similarly, in some embodiments, depending on the need to restrict or allow migration of a specific anion species between the electrolytes, an anion exchange membrane that is more restrictive and thus allows migration of one species of anions while restricting the migration of another species of anions may be used. Such restrictive cation exchange membranes and anion exchange membranes are commercially available and can be selected by one ordinarily skilled in the art.
[0032] In some examples, the separator 16 can be selected so that it can function in an acidic and/or an alkaline electrolytic solution, as appropriate. Other properties for the separator 16 that may be desirable include, but are not limited to, high ion selectivity, low ionic resistance, high burst strength, and high stability in electrolytic solution in a temperature range of room temperature to 150 C. or higher.
[0033] In an example, the separator 16 is stable in a temperature range of from about 0 C. to about 150 C., for example from about 0 C. to about 100 C., such as from about 0 C. to about 90 C., for example from about 0 C. to about 80 C., such as from about 0 C. to about 70 C., for example from about 0 C. to about 60 C., such as from about 0 C., to about 50 C., for example from about 0 C. to about 40 C., or such as from about 0 C. to about 30 C.
[0034] It may be useful to use an ion-specific ion exchange membrane that allows migration of one type of ion (e.g., cation for a CEM and anion for an AEM) but not another, or migration of one type of ion and not another, to achieve a desired product or products in the electrolyte solution.
[0035] In an example, the first half cell 12 defines a first chamber 18 that at least partially houses a first electrode 20 and a first electrolyte solution 22 and the second half cell 14 defines a second chamber 24 that at least partially houses a second electrode 26 and a second electrolyte solution 28. In an example, described in more detail below, each electrode 20, 26 can comprise a high surface area metal, such as a fine metal mesh. In an example, each electrode 20, 26 comprises a nickel mesh. Examples of solutions that can comprise the first electrolyte 22 and the second electrolyte 28 include, but are not limited to, one or more of: a solution of potassium hydroxide (KOH) in water, a solution of sodium hydroxide (NaOH) in water, and a solution of lithium hydroxide (LiOH) in water.
[0036] In an example, the first and second electrodes 20, 26 are positioned proximate to the separator 16, such as by being abutted against a corresponding face of the separator 16. In an example, the first electrode 20 is positioned proximate to (e.g., abutted against) a first separator face on a first side of the separator 16 and the second electrode 26 is positioned proximate to (e.g., abutted against) a second separator face on a second side of the separator 16 that opposes the first separator face.
[0037] In an example, the first electrode 20 is the anode for the electrolyzer cell 10 and the second electrode 26 is the cathode for the electrolyzer cell 10. Therefore, for the sake of simplicity of identification, the first half cell 12 may also be referred to as the anode half cell 12, the first electrode 20 may also be referred to as the anode 20, the first electrolyte solution 22 may also be referred to as the anode electrolyte solution 22 or as the anolyte 22, the second half cell 14 may also be referred to as the cathode half cell 14, the second electrode 20 may also be referred to as the cathode 20, and the second electrolyte solution 28 may also be referred to as the cathode electrolyte solution 28 or as the catholyte 28.
[0038] The electrodes 20, 26 are the locations of the cell 10 where electron transfer half reactions occur, e.g., by reacting with one or more components of the electrolyte solutions 22, 28 in the chambers 18, 24 to generate O.sub.2 gas and/or H.sub.2 gas, respectively. Each of the electrodes 20, 26 can be coated with one or more electrocatalysts to speed reaction toward H.sub.2 gas and/or toward O.sub.2 gas. In a typical example, one of both of the electrodes 20, 26 comprises a conductive substrate, such as a nickel substrate body, with or without an electrocatalyst material coated onto one or more surfaces of the conductive substrate. One or more binders can be used to adhere an electrocatalyst material onto the conductive substrate of one or both of the electrodes 20, 26. The electrocatalyst material can lower the activation energy for the electrochemical reaction so that the reaction can proceed without the electrocatalyst being consumed by the reaction. By lowering the activation energy, an electrocatalyst is able to facilitate specific reactions at the electrode so that the electrochemical device has a reduced energy demand and lower operating costs. Examples of electrocatalyst materials include, but are not limited to, metals, metal alloys, metal-metalloid alloys, metal oxides, metal phosphides, and metal sulfides. Further details of specific examples of electrocatalyst materials that can be applied to one or both electrodes 20, 26 are described in more detail below.
[0039] Each of the electrodes 20, 26 can be configured for a particular electrochemical half reaction, such as the half reactions for the overall water electrolysis process described below. For example, the first electrode 20 can be configured to perform a first electrochemical half reaction and the second electrode 26 can be configured to perform a second electrochemical half reaction. The actual half reactions that take place at each electrode 20, 26 can depend on the type of local environment that is present at each electrode 20, 26 during operation of the electrolyzer cell 10, and in particular on the alkalinity (e.g., pH) of the anolyte 22 at the anode 20 and of the catholyte 28 at the cathode 26. Half Reaction [1], below, is an example of a reaction that can take place at the anode 20 when the anolyte 22 is alkaline (e.g., with a pH>7):
4OH.sup..fwdarw.O.sub.2+2H.sub.2O+4e.sup.[1]
Half Reaction [1] is also referred to as the Oxygen Evolution Reaction [1] or the OER [1]. The O.sub.2 gas that is generated by the OER [1] can form oxygen bubbles 30 in the anolyte 22 within the anode chamber 18, as shown in
[0040] Half Reaction [2], below, is an example of a reaction that can take place at the cathode 26 when the catholyte 28 is alkaline (e.g., with a pH>7):
2e.sup.+2H.sub.2O.fwdarw.H.sub.2+2OH.sup.[2]
Half Reaction [2] is also referred to as the Hydrogen Evolution Reaction [2] or the HER [2]. The H.sub.2 gas that is generated by the HER [2] can form hydrogen bubbles 32 in the catholyte 28 within the cathode chamber 24, as shown in
[0041] In an example, the anode 20 is electrically connected to an external positive conductive lead 34 (also referred to as the anode lead 34) and the cathode 26 is electrically connected to an external negative conductive lead 36 (also referred to as the cathode lead 36). In an example, when the separator 16 is wet and is in electrolytic contact with the electrodes 20, 26, and an appropriate voltage is applied across the leads 34 and 36, Half Reactions [1] and [2] are activated. As noted above, in OER [1], OH.sup. ions are oxidized at the anode 20, which generates O.sub.2 gas (e.g., as the O.sub.2 bubbles 30 in the anolyte 22) and forms additional H.sub.2O molecules in the anolyte 22. In HER [2], H.sub.2O is reduced at the cathode 26, which generates H.sub.2 gas (e.g., as the H.sub.2 bubbles 32 in the catholyte 28) and forms additional OH.sup. ions in the catholyte 28. In some examples, at least a portion of the OH.sup. ions pass through the separator 16 (e.g., if the separator 16 is an anion exchange membrane) so that they are available to be oxidized via OER [1] at the anode 20.
[0042] The electrolyzer cell 10 can be configured so that the electrolyte solutions 22, 28 flow through the chambers 18, 24 so that each electrolyte solution 22, 28 can pick up the gas generated by its corresponding electrode 20, 26 and carry the produced gas out of the electrolyzer cell 10. For example, the anolyte 22 can flow into the anode half cell 12 through an anolyte inlet 38 and can exit the anode half cell 12 through an anolyte outlet 40. Similarly, the catholyte 28 can flow into the cathode half cell 14 through a catholyte inlet 42 and can exit the cathode half cell 14 through a catholyte outlet 44. In an example, the flow of the anolyte 22 through the anode chamber 18 picks up the produced O.sub.2 gas as the O.sub.2 bubbles 30 and exits the anode chamber 18 through the anolyte outlet 40 and the flow of the catholyte 28 through the cathode chamber 24 picks up the produced H.sub.2 gas as the H.sub.2 bubbles 32 and exits the cathode chamber 24 through the catholyte outlet 44. One or both of the gases can be separated from their respective electrolyte solution 22, 28 downstream of the electrolyzer cell 10 with one or more appropriate separators. In an example, the produced H.sub.2 gas is separated from the catholyte 28, dried, and harvested into high pressure canisters or fed into further process elements. The produced O.sub.2 gas can be allowed to simply vent into the atmosphere or can be stored for other uses. In an example, the electrolyte solutions 22, 28 are recycled from the electrolyte outlets 40, 44 back to the electrolyte inlets 38, 42, as needed.
[0043] In an example, a typical voltage across the electrolyzer cell 10 (e.g., the voltage difference between the anode lead 34 and the cathode lead 36) is from 1.5 volts (V) to 3.0 V. In an example, an operating current density for the electrolyzer cell 10 is from 0.1 A/cm.sup.2 to 3 A/cm.sup.2. Each cell 10 has a size that is sufficiently large to produce a sizeable amount of H.sub.2 gas when operating at these current densities. In an example, an active area of each cell 10 (e.g., a width multiplied by a height for a rectangular cell) is from 0.25 square meters (m.sup.2) to 15 m.sup.2, such as from 1 m.sup.2 to 5 m.sup.2, for example from 2 m.sup.2 to 4 m.sup.2, such as from 2.25 m.sup.2 to 3 m.sup.2, such as from 2.5 m.sup.2 to 2.9 m.sup.2. In an example, the total volume of each cell (e.g., a width multiplied by a height multiplied by a depth) is from 0.1 cubic meter (m.sup.3) to 2 m.sup.3, such as from 0.15 m.sup.3 to 1.5 m.sup.3, for example from 0.2 m.sup.3 to 1 m.sup.3, such as from 0.25 m.sup.3 to 0.5 m.sup.3, for example from 0.275 m.sup.3 to 0.3 m.sup.3. In a non-limiting example, the total volume of the entire electrolyzer system (e.g., the combined volume of all the cells in all the stacks in the plant) is from 1 m.sup.3 to 25,000 m.sup.3, such as from 5 m.sup.3 to 2,500 m.sup.3, for example from 10 m.sup.3 to 100 m.sup.3, such as from 25 m.sup.3 to 75 m.sup.3, for example from 30 m.sup.3 to 50 m.sup.3.
[0044] The efficiency of an electrolyzer cell can depend on resistive losses between the anode and cathode, also referred to as ohmic resistance. The ohmic resistance of the separator 16 can affect the voltage drop across the anode 20 and the cathode 26 (and thus, the overall efficiency of the cell 10). For example, as the ohmic resistance of the separator 16 increases, the voltage across the anode 20 and the cathode 26 that is required may also increase, and vice versa. In an example, the separator 16 has a relatively low ohmic resistance and a relatively high ionic mobility. In an example, the separator 16 has a relatively high hydration characteristics that increase with temperature, and thus decreases the ohmic resistance. By selecting a separator 16 with lower ohmic resistance known in the art, the voltage drop across the anode 20 and the cathode 26 at a specified temperature can be lowered.
[0045] One parameter that can affect the ohmic resistance between electrodes in an electrolyzer cell is the distance between the anode and the cathode, with a larger gap between the electrodes resulting in a correspondingly larger resistance compared to a smaller gap. Therefore, in an example, an electrolyzer cell can be configured so that the space or gap between the anode and the cathode is as small as possible. In one example configuration, one or both of the anode and the cathode are positioned so that the electrode is in contact with the separator, which is also referred to as a zero-gap configuration. In an example of a zero gap configuration, one face or surface of the anode is in contact with a first surface of the separator and one face or surface of the cathode is in contact with an opposing second surface of the separator.
[0046]
[0047] In an example, the cell assembly 50 includes a housing that at least partially encloses a cell interior, wherein a first electrode 52, a second electrode 54, and a separator 56 are enclosed within the cell interior. In an example, each electrode 52, 54 can be part of a corresponding half cell. For example, the first electrode 52 can be included as part of a first half cell and the second electrode 54 can be included as part of a second half cell. In a non-limiting example, the first electrode 52 is the anode of the cell assembly 50 and the second electrode 54 is the cathode of the cell assembly 50, such that the first and second electrodes 52, 54 may also be referred to as the anode 52 and the cathode 54, respectively, and the corresponding half cells will also be referred to as the anode half cell (i.e., the half-cell associated with the anode 52) and the cathode half cell (i.e., the half-cell associated with the cathode 54). There are also instances when the anode 52 and the cathode 54 are referred to more generically as the electrode 52, 54 or the electrodes 52, 54.
[0048] The separator 56 can be situated between the anode half cell and the cathode half cell, for example by being positioned between the anode 52 and the cathode 54. As discussed above, the separator 56 can be configured to reduce migration of certain species between the electrodes 52, 54 while allowing one or more other species to pass from the anode half cell to the cathode half cell and/or from the cathode half cell to the anode half cell. In an example, the separator 56 comprises a diaphragm, a membrane electrode assembly (MEA), or a membrane, such as an ion exchange membrane (IEM) (e.g., an anion exchange membrane (AEM), a cation exchange membrane (CEM), or a proton exchange membrane (PEM)), a bipolar ion exchange membrane (BEM), an ion solvating membrane (ISM), or a microporous or nanoporous membrane. In some examples, the separator 56 can comprise more than one type of separator, e.g., more than one type of membrane (as is the case with a bipolar ion exchange membrane), and/or can be part of a composite structure (such as a membrane electrode assembly (MEA)), which can also include one or more separator components (e.g., to separate an anion exchange membrane (AEM) from a cation exchange membrane (CEM)), or one or more support structures to provide mechanical integrity to the one or more separators. In addition to these components, individual gaskets or gasket tape may be provided in between and along the outer perimeter of the components to seal the compartments from fluid leakage.
[0049] As discussed above, in an example, each of the electrodes 52, 54 is situated in a zero-gap configuration relative to the separator 56. Although the term zero-gap would typically imply that one or both electrodes 52, 54 are in actual physical contact with the separator 56, in the present disclosure, the term zero-gap is expanded to mean that all structures between the two current collectors 70, 74 (described below) are in mechanical contact with no space for the liquid electrolyte to congregate. In other words, there could be one or more spacer materials inserted between one or both of the current collectors 70, 74 and the separator 56, and the overall structure would still be considered a zero-gap architecture, as that term is being used herein, so long as there is not a liquid electrolyte gap between the two current collectors 70, 74.
[0050] The housing of the cell assembly 50 can comprise a pan assembly 58, 60 for one or both of the half cells, such as an anode-side pan assembly 58 for the anode half cell and a cathode-side pan assembly 60 for the cathode half cell (also referred to as the anode pan assembly 58 and the cathode pan assembly 60, respectively). In an example, each pan assembly 58, 60 includes a pan 62, 64 with an interior for receiving an electrolyte solution. For example, the anode pan assembly 58 can comprise an anode-side pan 62 (also referred to as the anode pan 62) for receiving an anolyte and the cathode pan assembly 60 can comprise a cathode-side pan 64 (also referred to as the cathode pan 64) for receiving a catholyte. The pan assemblies 58, 60 can be configured so that the electrolyte flowing through each pan 62, 64 will come into contact with its corresponding electrode 52, 54, e.g., so that H.sub.2 gas can be evolved from the cathode 54 and so that O.sub.2 gas can be evolved from the anode 52. Each pan assembly 58, 60 can also include an inlet for receiving electrolyte into the interior of the pan 62, 64, and one or more outlets so that electrolyte and evolved gas can exit the pan 62, 64 (not shown in
[0051] In an example, each electrode 52, 54 is electrically connected to its corresponding pan 62, 64 so that electrical current can flow from the pan 62, 64 to the electrode 52, 54 (as is the case for current flowing from an anode pan 62 to an anode 52) or from the electrode 52, 54 to the pan 62, 64 (as is the case for current flowing from a cathode 54 to a cathode pan 64). Each half cell can include one or more additional structures to provide for the electrical connection between the electrodes 52, 54 and the pans 62, 64. For example, one or both of the electrodes 52, 54 can be part of a corresponding electrode assembly comprising the electrode 52, 54 and one or more additional structures. For example, the first electrode 52 (e.g., the anode 52) can be part of a first electrode assembly 66 (which will also be referred to herein as the anode assembly 66) and the second electrode 54 (e.g., the cathode 54) can be part of a second electrode assembly 68 (which will also be referred to herein as the cathode assembly 68).
[0052] One or both of the electrode assemblies 66, 68 can include its corresponding electrode 52, 54, a current collector, and an optional elastic element (also sometimes referred to as a mattress). For example, the anode assembly 66 can include the anode 52, an anode-side current collector 70 (also referred to simply as the anode current collector 70), and an optional anode-side elastic element 72 (also referred to simply as the anode elastic element 72). Similarly, the cathode assembly 68 can include the cathode 54, a cathode-side current collector 74 (also referred to simply as the cathode current collector 74), and an optional cathode-side elastic element 76 (also referred to simply as the cathode elastic element 76).
[0053] Each electrode assembly 66, 68 is coupled to its respective pan 62, 64, i.e., so that there is an electrical connection between the anode 52 and the anode pan 62 and between the cathode 54 and the cathode pan 64. In an example, one or both of the electrodes 52, 54 comprise a fine mesh structure, such as a fine woven mesh, for example the woven mesh electrode shown in
[0054] In an example, a differential fluid pressure can be applied across the separator 56 (e.g., with a pressure on the cathode side of the separator 56 being larger than on the anode side, or vice versa). The differential pressure, in addition to or in place of one or both of the elastic elements 72, 76, can act to load the electrodes 52, 54 and create effective electrical contact between the electrode 52, 54 and the separator 56 across the active area of the electrodes 52, 54 without requiring welding to couple the electrodes 52, 54 to other structures in the cell assembly 50, particularly with fine mesh electrodes.
[0055] As described in more detail below, in an example, the woven mesh of one or both of the electrodes 52, 54 can comprise a network of sets of crossing wires, which can be perpendicular or angled relative to one another, that alternative cross and bend over one another. For example, any particular wire alternates between passing under an adjacent cross wire and then over the next cross wire. In an example, one or both of the electrodes 52, 54 can comprise a woven wire mesh electrode formed from wires having a wire diameter of 0.18 mm diameter with openings in the mesh of 0.44 mm and with an open area of from about 50% to about 60%, such as from about 50% to about 55%. In an example, one or both of the electrodes 52, 54 is formed from an expanded mesh wherein one or both of the electrodes 52, 54 are fabricated from a sheet of material that is 0.13 mm thick with a long way of the diamond shape (LWD) of about 2 mm and a short way of the diamond (SWD) of about 1 mm.
[0056] In an example, one or both of the anode 52 and the cathode 54 is made primarily or entirely from nickel. Fabricating both the anode 52 and the cathode 54 out of nickel enables the use of non-welded electrodes fabricated from fine woven meshes for both electrodes 52, 54, for example because nickel has a very low contact resistance. In an example, one or both of the anode 52 and the cathode 54 is coated with one or more catalyst materials, e.g., in the form of one or more catalyst coating layers on the electrode 52, 54. In an example, the one or more catalyst materials can be electrically conducting.
[0057] The current collector 70, 74 of each electrode assembly 66, 68 acts to distribute current flowing into or out of its respective electrode 52, 54. In an example, the current collector 70, 74 of each electrode assembly 66, 68 comprises a rigid structure, such as a rigid metal plate or mesh, which is electrically connected to its corresponding electrode 52, 54, either directly or indirectly. In an example, each current collector 70, 74 can comprises an expanded metal sheet, such as an expanded nickel sheet.
[0058] In an example, each elastic element 72, 76, if present, comprises a compressible and expandable structure that provides a controlled load when compressed. For example, the elastic element 72, 76 can be compressed between the separator 56 and the current collector 70, 74, and the resulting load that results as the elastic element 72, 76 tries to expand back to its fully expanded state biases its corresponding electrode 52, 54 toward the separator 56 to provide a zero-gap configuration between the electrode 52, 54 and the separator 56. In an example, the elastic element 72, 76 is also electrically conductive (e.g., the elastic element 72, 76 is made from or is coated with an electrically conductive material, such as nickel) so that the elastic element 72, 76 will conduct electricity from the current collector 70, 74 to the electrode 52, 54 or vice versa. In an example, each of the one or more elastic elements 72, 76 comprise one or more electrically conductive filaments that are woven together into an elastic layer that can expand and collapse to apply the controlled load when the elastic layer is compressed. In some examples, one or both of the elastic elements 72, 76 can be a corrugated knitted mesh having a pre-load of 2 pounds per square inch at about 3 mm of compression. In an example, an uncompressed thickness of one or both of the elastic elements 72, 76 can be from about 5 mm to about 7 mm. One or both of the elastic elements 72, 76 can have a corrugation pitch of about 10 mm. In an example, one or both of the elastic elements 72, 76 are formed from wire having a wire diameter of 0.15 mm.
[0059] In the example shown in
[0060] In an example, the current collectors 70, 74 can be coupled to their respective pans 62, 64, e.g., so that the current collector 70, 74 is electrically connected to its corresponding pan 62, 64, which provides part of the electrical path between an electrode 52, 54 and its corresponding pan 62, 64. In order to accommodate this electrical connection between the current collector 70, 74 and its corresponding pan 62, 64, in an example, each pan assembly 58, 60 includes one or more ribs that extend between the electrode assembly 66, 68 and a back wall of the pan. For example, the anode pan assembly 58 can include one or more ribs 78 that extend between a back wall 80 of the anode pan 62 and the anode assembly 66, while the cathode pan assembly 60 can include one or more ribs 82 that extend between a back wall 84 of the cathode pan 64 and the cathode assembly 68. The one or more ribs 78 can be welded to the back wall 80 of the anode pan 62 while the one or more ribs 82 can be welded to the back wall 84 of the cathode pan 64.
[0061] The one or more ribs 78, 82 of each pan assembly 58, 60 can be electrically coupled to its corresponding electrode assembly 66, 68 by one or more welds, e.g., one or more welds 86 that electrically couple the anode assembly 66 to the one or more ribs 78 of the anode pan assembly 58 and one or more welds 88 that electrically couple the cathode assembly 68 to the one or more ribs 82 of the cathode pan assembly 60. As shown in
[0062] In an example, the electrodes 52, 54 can be electrically connected to the one or more ribs 78, 82 and the one or more welds 86, 88. In examples where the electrode assembly 66, 68 includes the current collector 70, 74 that is welded to the one or more ribs 78, 82, then the electrode 52, 54 of the electrode assembly 66, 68 can be electrically coupled to the current collector 70, 74 via physical contact between the electrode 52, 54 and the current collector 70, 74, e.g., such as by wrapping the flexible electrode 52, 54 around a back side of the current collector 70, 74 so that there is physical contact between the mesh electrode 52, 54 and a side or back surface of the current collector 70, 74 (not shown in
[0063] The electrodes 52, 54 can be electrically coupled to the supplied electrical current via the one or more ribs 78, 82 and the one or more welds 86, 88. During operation of the cell assembly 50, current flows from a conductor contacting the anode pan 62 (similar to the anode lead 34 in the electrolyzer cell 10 of
[0064] The geometry and spacing of the one or more ribs 78, 82 can dictate current flow through the pan assemblies 58, 60. The geometry of the ribs 78, 82 can include, but is not limited to, the number of the ribs 78, 82, the height of the ribs 78, 82 (e.g., the distance between the back wall 80, 84 and the electrode assembly 66, 68 to which the ribs 78, 82 are connected), the physical design of the ribs 78, 82, the pitch between adjacent ribs 78, 82, and/or the thickness of the ribs 78, 82. As the current flows in through the ribs 78, 82 and the welds 86, 88, the geometry, spacing or density, and/or cross-sectional area of the welds 86, 88 can also impact current flow through the pan assemblies 58, 60. For example, as increasingly high currents flow through the cell, the density and the cross sectional area of the welds 86, 88 can impact local Joule heating and the formation of local hot spots, which can cause damage to the separator 56. In an example, the geometry, spacing, and cross-sectional area of the ribs 78, 82 and/or the welds 86, 88 can facilitate efficient operation of the cell assembly 50 at high current densities.
[0065] Further details regarding geometries and other configurations of the one or more ribs 78, 82 and the one or more welds 86, 88 for coupling the one or more ribs 78, 82 to the electrode assemblies 66, 68 (e.g., to the current collectors 70, 74) are described in U.S. Pat. No. 11,390,956, issued on Jul. 19, 2022, entitled ANODE AND/OR CATHODE PAN ASSEMBLIES IN AN ELECTROCHEMICAL CELL, AND METHODS OF USE AND MANUFACTURE THEREOF, the disclosure of which is incorporated herein by reference in its entirety.
[0066] In an example, one or both of the pan assemblies 58, 60 includes a baffle plate that is fitted within its corresponding pan 62, 64 that is generally aligned with the orientation of the pan 62, 64 and the electrode assembly 66, 68 of that particular pan assembly 58, 60. For example, the anode pan assembly 58 can include an anode-side baffle plate 90 located within the interior of the anode pan 62 and the cathode pan assembly 60 can include a cathode-side baffle plate 92 located within the interior of the cathode pan 64. Each baffle plate 90, 92 is coupled to its corresponding set of one or more ribs 78, 82 to position the baffle plate 90, 92 within its corresponding pan 62, 64, e.g., at a specified position relative to its corresponding electrode assembly 66, 68 and/or its corresponding back wall 80, 84.
[0067] In an example, one or both of the baffle plates 90, 92 comprise a solid plate that is configured to fit over or within the one or more ribs 78, 82 of its corresponding pan assembly 58, 60. In other examples, one or both of the baffle plates 90, 92 can comprise an expanded metal plate or a mesh. In an example, one or both of the baffle plates 90, 92 are made from a conductive metal, such as, but not limited to, nickel, stainless steel, and the like. In another example, one or both of the baffle plates 90, 92 are made from a polymeric material.
[0068] As will be appreciated by those having skill in the art, the contribution of internal power dissipation to the internal temperature distribution within the cell assembly 50 can be reduced or minimized through operating conditions such as the temperature and flow rate of the electrolyte flowing through the cell assembly 50 (e.g., through the pan assemblies 58, 60). High electrolyte flow rates can increase and in some examples maximize convective heat transfer within the cell assembly 50, thereby helping to reduce or minimize heat buildup and the corresponding concomitant temperature rise within the cell 200 that could otherwise result from high current densities. The baffle plates 90, 92 can provide for mixing of electrolyte as it flows through the pan assemblies 58, 60 to enhance convective heat transfer within the electrolyte during electrolysis.
[0069] In some examples, the baffle plate 90, 92 is designed and positioned in its corresponding pan 62, 64 in such a way that the gas produced at the electrode assembly 66, 68 can mix with the electrolyte on the side of the baffle plate 90, 92 closest to the electrode assembly 66, 68, resulting in a relatively low density column and defining a riser section. The low density mixture can rise relatively quickly through the riser section. Once above the top of the baffle plate 90, 92, the gas can disengage and flow into an outlet (such as a manifold, not shown in
[0070] The gas evolved at the electrode 52, 54 impacts the flow of the electrolyte, dragging some of the electrolyte up, and buffeting some of the electrolyte laterally. Gas lift occurs along the region adjacent to the electrode assembly 66, 68. The presence of the baffle plate 90, 92 can create a strong circulation within the pan assembly 58, 60. The flow of electrolyte in the riser section on the side of the baffle plate 90, 92 closest to the electrode assembly 66, 68 can be strongly oriented upward due to gas lift, and the flow in the down-comer section on the side of the baffle plate 90, 92 closest to the back wall 80, 84 can be strongly oriented downward. The relatively high velocities and shear rates in the riser section can help sweep gas from the electrode assembly 66, 68, provide efficient top to bottom mixing within the pan 62, 64, and drive increased convective cooling.
[0071] The baffle plate 90, 92 can be used to create a rapidly flowing circulation loop so that the electrolyte remains substantially isothermal as it flows through the pan assemblies 58, 60. Due to the high degree of top-bottom mixing and circulation, rapid thermal equilibration of the electrolyte can be achieved as it flows into and through the pan assemblies 58, 60. Another advantage is that relatively cold electrolyte can be introduced into the pan assembly 58, 60 which can equilibrate with warm circulating electrolyte fluid relatively quickly. The circulation rate (or laps of the recirculation loop during electrolyte transit through the pan 62, 64) can be anywhere from 1 to 200. The high circulation rate can also drive larger shear rates adjacent to the separator 56, helping to sweep gas away from the separator 56 and/or enhance or maximize heat transfer from the separator 56 to the electrode 52, 54.
[0072] Further details regarding a baffle plate in the pan assemblies 58, 60 are described in U.S. Pat. No. 11,444,304, issued on Sep. 13, 2022, entitled ANODE AND/OR CATHODE PAN ASSEMBLIES IN AN ELECTROCHEMICAL CELL, AND METHODS TO USE AND MANUFACTURE THEREOF, the disclosure of which is incorporated herein by reference in its entirety.
[0073] The pan assemblies 58, 60 can be coupled together to enclose the interior of the cell assembly 50. For example, each of one or more flanges 94 extending from the main housing of the anode pan 62 can be coupled to a corresponding flange 96 extending from the main housing of the cathode pan 64, such as with one or more fasteners 98. In the example shown in
Catalyst Coated Electrode
[0074] As mentioned above, an electrode of an electrochemical cell (such as one or both of the electrodes 20, 26 of the cell 100 of
[0075] The example woven mesh electrode 100 can comprise a network of wires 104, 106 that form a conductive substrate for carrying current for the purpose of activating the half reactions within the electrolyzer cell (e.g., OER [1] and HER [2] described above). Examples of materials that can be used to form the wires 104, 106 or other conductive substrate of the electrode 100 include, but are not limited to, a metal (such as nickel metal (Ni), titanium metal (Ti), a steel alloy (such as a low-alloy steel, stainless steel, or carbon steel), gold metal (Au), or copper metal (Cu)) and a conductive carbon-based material (such as a carbon cloth, a carbon fiber, a plurality of carbon rods, or graphite).
[0076] In the example, the woven mesh electrode 100 comprises sets of crossing wires 104, 106 that alternatively cross and bend over one another. One portion of the wires 104 can extend in a first direction and another portion of the wires 106 can extend in a second direction that is angled relative to the first direction. For example, the configuration of woven mesh electrode 100 shown in
[0077] In an example, the catalyst coating 102 comprises a plurality of particles of one or more catalyst compounds that are adhered or otherwise coupled to the conductive material of the electrode, such as the metal wires 104, 106.
[0078] In some examples, the catalyst particles 114 are nanosized particles, e.g., having a largest particle size that is less than or equal to 900 nanometers (nm), for example less than or equal to 750 nm, such as less than or equal to 500 nm, for example less than or equal to 450 nm, such as less than or equal to 400 nm, for example less than or equal to 350 nm, such as less than or equal to 325 nm, for example less than or equal to 300 nm, such as less than or equal to 275 nm, for example less than or equal to 250 nm, such as less than or equal to 250 nm, for example less than or equal to 225 nm, such as less than or equal to 200 nm. The method 150 described below for the making of a catalyst coated substrate is able to achieve nanosized particles 114, which in some examples are adhered directly to the surfaces of the conductive substrate 110 during the method 150 (e.g., without requiring a binder to adhere the particles 114 to the conductive substrate 110), without requiring very high temperatures (e.g., higher than 400 C.) or a pressure above atmospheric pressure and without requiring an inert atmosphere (e.g., with a high concentration of nitrogen gas or another inert gas), and without requiring templating or other specialized methods of patterning the coating onto the conductive substrate 110.
[0079] In some examples, the method 150 described below for the making of a catalyst coated substrate is able to achieve nanosized spinel particles 114 that are adhered directly to the surfaces of the conductive substrate, e.g., without requiring a binder to adhere the particles 114 to the conductive substrate 110 or in examples wherein a binder is present (e.g., to provide for better inter-particle cohesion of the catalyst coating 102) it is not necessary for the particles 114 to adhere or be otherwise bound to the surface of the conductive substrate 110.
[0080] In some examples, the method 150 described below is able to produce catalyst particles 114 wherein the catalyst material comprises a spinel crystal structure (described below) or a spinel-like mixed metal oxide structure, although the present disclosure is not limited to catalyst compounds having a spinel or spinel-like mixed metal oxide structure. The method 150 described below can be implemented to form other metal oxide catalyst compounds, such as mixed metal oxides other than spinel catalyst compounds.
[0081] Spinel materials have been shown to provide for enhanced performance over other crystal structures for various electro-reactions, and in particular for oxidation reactions (such as the OER [1] to produce oxygen gas at the anode of a water electrolysis cell). Because oxidation reactions are often a rate-limiting bottleneck for green technologies, the scalable synthesis of electrocatalysts comprising spinel crystal structures can provide for more economical and efficient green technologies, including water electrolysis.
[0082] A spinel crystal structure has the general chemical formula [3], below:
AB.sub.2O.sub.4[3]
wherein both A and B in spinel formula [3] comprise one or more metal element ions at particular positions within the crystal structure. Specifically, the one or more A metal ions occupy octahedral sites and the one or more B metal ions are distributed over both octahedral sites and tetrahedral sites, as would be understood by a person having ordinary skill in the art.
[0083] Both the A and B metal elements can be any metal that is suitable for forming a catalyst to the electrochemical reaction of interest (such as the OER [1] or the HER [2] described above), including, but not limited to: an alkali metal (i.e., Group 1 metal), an alkaline earth metal (i.e., Group 2 metal), a transition metal (i.e., Groups 3 through 12), a lanthanide series metal, a post-transition metal, a metalloid, some select non-metals (such as sulfur (S) or phosphorus (P)), and combinations thereof.
[0084] In an example, the A metal ion of the spinel formula can include, but is not limited to: nickel (Ni), zinc (Zn), copper (Cu), cobalt (Co), iron (Fe), lithium (Li), magnesium (Mg), calcium (Ca), barium (Ba), manganese (Mn), germanium (Ge), cadmium (Cd), lanthanide (La), Yttrium (Y), chromium (Cr), molybdenum (Mo), ruthenium (Ru), iridium (Jr), and combinations thereof.
[0085] In an example, the B metal ion of the spinel formula can include, but is not limited to: cobalt (Co), iron (Fe), chromium (Cr), aluminum (Al), manganese (Mn), nickel (Ni), gallium (Ga), selenium (Se), molybdenum (Mo), ruthenium (Ru), sulfur (S), tellurium (Te), and combinations thereof.
[0086] Specific examples of spinel electrocatalyst materials that have been found to be useful for water splitting electrolysis, such as the catalyst for the OER [1], include, but are not limited to, a cobalt-based spinel (sometimes also referred to as a cobaltite) having the general formula MCo.sub.2O.sub.4, an iron-based spinel (sometimes also referred to as a ferrite) having the general formula MFe.sub.2O.sub.4, a manganese-based spinel (sometimes also referred to as a manganite) having the general formula MMn.sub.2O.sub.4. For each of these spinel compound formulas, M is a metal ion such as, but not limited to one or more of: nickel (N), zinc (Zn), copper (Cu), cobalt (Co), iron (Fe), lithium (Li), magnesium (Mg), calcium (Ca), barium (Ba), manganese (Mn), germanium (Ge), cadmium (Cd), and lanthanide (La). Specific examples of cobalt-based spinels that can be used as an electrolysis catalyst include, but are not limited to, nickel cobaltite (NiCo.sub.2O.sub.4), cobalt (II,III) oxide (Co.sub.3O.sub.4), manganese cobaltite (MnCo.sub.2O.sub.4), copper cobaltite (CuxCo.sub.3-xO.sub.4, such as CuCo.sub.2O.sub.4), zinc cobaltite (ZnCo.sub.2O.sub.4), lithium cobaltite (LiCoO.sub.2 or LiCo.sub.2O.sub.4). Specific examples of iron-based spinels that can be used as an electrolysis catalyst include, but are not limited to, iron (II,III) oxide (Fe.sub.3O.sub.4), cobalt ferrite (CoFe.sub.2O.sub.4), nickel ferrite (NiFe.sub.2O.sub.4), manganese ferrite (MnFe.sub.2O.sub.4), and zinc ferrite (ZnFe.sub.2O.sub.4). Specific examples of manganese-based spinels that can be used as an electrolysis catalyst include, but are not limited to, manganese (II,III) oxide (Mn.sub.3O.sub.4), cobalt manganite (CoMn.sub.2O.sub.4), and lithium manganite (LiMn.sub.2O.sub.4).
[0087] As will be appreciated by those of skill in the art, in some examples, both A or B in the chemical formula [3] can be a single element (such as nickel cobaltite (NiCo.sub.2O.sub.4), where the only A element is nickel (Ni) and the only B element is cobalt (Co)). Alternatively, in some examples, there can be more than one A element and/or more than one B element in a spinel of formula [3]. For example, a spinel can be a ferrite (i.e., wherein the B metal ion is iron (Fe)) with A metal ions comprising both nickel and zinc, which would have a relative chemical formula of Ni.sub.aZn.sub.1-aFe.sub.2O.sub.4. In another example, the only A metal ion of a particular spinel can be copper with B metal ions comprising both cobalt and manganese, which would have a relative chemical formula of CuCobMn.sub.2-bO.sub.4. Those having skill in the art can also contemplate examples of spinels having three or more A metal ions and/or three or more B metal ions.
[0088] The catalyst particles 114 can comprise other types and configurations of catalyst materials in place of or in addition to spinel materials. Other catalyst materials that can form some or all of the catalyst particles 114 include, but are not limited to, metals, metal alloys, metal-metalloid alloys, metal oxides (including mixed metal oxides, metal alloy oxides, mixed metal alloy oxides, metal hydroxides, and metal oxyhydroxides), metal phosphides, and metal sulfides.
[0089] The present disclosure describes methods of producing an electrode for use in an electrochemical reaction, such as water electrolysis, comprising an electrocatalyst materialfor example catalyst particles comprising one or more metal oxide catalyst compounds, such as a mixed metal oxide catalyst compound, for example one having a spinel crystal structure, and in particular nanostructured spinel-based material that can be a catalyst for electrochemical oxidation reactions such as the OER [1]. The present disclosure also describes the electrodes that result from these methods.
[0090]
[0091] The method 150 of
[0092] The precursor particles comprise one or more precursor compounds that can be chemically converted via oxidation to one or more metal oxide catalyst compounds, and in particular one or more precursor compounds that can be converted by the electrochemical oxidation step 158 described below. An example of a type of precursor compound for the precursor particles that can be used to form the precursor slurry in step 152 include, but are not limited to: a metal hydroxide, e.g., having the general formula M.sub.x(OH).sub.y; a metal carbonate, e.g., having the general formula M.sub.x(CO.sub.3).sub.y; or a metal oxyhydroxide, e.g., having the general formula MO(OH). In each of the example precursor compounds described above, M is a metal element including, but not limited to one or more of: an alkali metal, an alkaline earth metal, a transition metal, a lanthanide series metal, a post-transition metal, a metalloid, and combinations thereof, x is the relative molecular amount of the metal element, and y is the relative molecular amount of the counterion or counterions (e.g., OH.sup., CO.sub.3.sup.2).
[0093] The values of x and y can depend on the oxidation state of the metal element M and on the charge of the counterion (i.e., a hydroxide ion (OH.sup.) has a 1 charge, while a carbonate ion (CO.sub.3.sup.2) has a 2 charge). For x (the relative molecular amount of the metal atom), the most common value is 1 (e.g., as in nickel (II) hydroxide (Ni(OH).sub.2), copper (II) hydroxide (Cu(OH).sub.2), iron (II) hydroxide (Fe(OH).sub.2), manganese (II) hydroxide (Mn(OH).sub.2), lithium hydroxide (LiOH), nickel (II) carbonate (NiCO.sub.3), copper (II) carbonate (CuCO.sub.3), iron (II) carbonate (FeCO.sub.3), cobalt (II) carbonate (CoCO.sub.3), etc.). But x can also be 2, either because a metal ion with an oxidation state of +1 (e.g., Li.sup.+, La.sup.+, etc.) is paired with a counterion having a 1 charge (e.g., OH.sup., etc.) (e.g., as in lithium carbonate (Li.sub.2CO.sub.3), lanthanum carbonate (La.sub.2(CO.sub.3).sub.3), etc.) or because a metal ion with an oxidation state of +3 (e.g., Al.sup.3+, Y.sup.3+, Ru.sup.3+, etc.) is paired with a counterion having a 2 charge (e.g., CO.sub.3.sup.2, etc.) (e.g., as in aluminum carbonate (Al.sub.2(CO.sub.3).sub.3), yttrium carbonate (Y.sub.2(CO.sub.3).sub.3), ruthenium carbonate (Ru.sub.2(CO.sub.3).sub.3), etc.). For y (the relative molecular amount of the counterion(s)), the most common value is 2, because many of the metal ions have an oxidation state of +2 (e.g., Ni.sup.2+, Zn.sup.2+, Cu.sup.2+, Co.sup.2+, Fe.sup.2+, etc.) (e.g., as in nickel (II) hydroxide (Ni(OH).sub.2), copper (II) hydroxide (Cu(OH).sub.2), iron (II) hydroxide (Fe(OH).sub.2), etc.). But y can also be 1, either because the oxidation state of the metal ion is +1 (e.g., as in lithium hydroxide (LiOH), lanthanum hydroxide (LaOH), lithium carbonate (Li.sub.2CO.sub.3), etc.) or because a metal ion with oxidation state +2 is paired with a counterion having a 2 charge (i.e., CO.sub.3.sup.2) (e.g., as in nickel (II) carbonate (NiCO.sub.3), copper (II) carbonate (CuCO.sub.3), iron (II) carbonate (FeCO.sub.3), cobalt (II) carbonate (CoCO.sub.3), etc.). The value of y can also be 3, such as when the oxidation state of the metal ion is +3 (as with Al.sup.3+, Y.sup.3+, Ru.sup.3+, etc.) (e.g., as in aluminum hydroxide (Al(OH).sub.3), yttrium (III) hydroxide (Y(OH).sub.3), ruthenium (III) hydroxide (Ru(OH).sub.3), lanthanum carbonate (La.sub.2(CO.sub.3).sub.3), aluminum carbonate (Al.sub.2(CO.sub.3).sub.3), yttrium carbonate (Y.sub.2(CO.sub.3).sub.3), ruthenium carbonate (Ru.sub.2(CO.sub.3).sub.3), etc.).
[0094] In some examples, M for the precursor metal hydroxide can comprises any of the A elements or B elements described above with respect to the spinel formula [3]. In some examples, the precursor particles can comprise only a single hydroxide precursor, even if the final catalyst compound to be formed is a mixed metal oxide having different A and B elements, or having more than one A element, more than one B element, or more than one A element and more than one B element. For example, if the desired final catalyst compound is nickel cobaltite (NiCo.sub.2O.sub.4), then the precursor particles can comprise a nickel-doped cobalt hydroxide (Ni-doped Co(OH).sub.2). In other examples, the precursor compounds that form the precursor particles can include one or more first precursor compounds (e.g., one or more first metal hydroxides) and one or more second precursor compounds (e.g., one or more second metal hydroxides). For example, if the desired final catalyst compound is nickel ferrite (NiFe.sub.2O.sub.4), the precursor particles could comprise particles of nickel hydroxide (Ni(OH).sub.2) and particles of iron (II) hydroxide (Fe(OH).sub.2) and/or composite particles that include both nickel hydroxide and cobalt hydroxide. As described in more detail below, in an example, the precursor particles can be formed by coprecipitation of the one or more precursor compounds from a precursor salt solution to provide the precursor particles.
[0095] After forming the precursor slurry, the method 150 can include, at step 154, applying the precursor slurry to one or more surfaces of a conductive substrate (such as the conductive substrate 110) to form a slurry coating on the conductive substrate, which produces a precursor coated substrate. The slurry coating comprises the precursor particles such that the slurry coating may also referred to as the precursor coating.
[0096] The precursor slurry formed in step 152 and coated onto the conductive substrate in step 154 can optionally include one or more additives in addition to the precursor particles and the slurry medium in order to impart one or more additional properties to the precursor slurry, the precursor coating, or to the final catalyst coating. Examples of additives that can be included in the precursor slurry include, but are not limited to one or more of: a thickener and a binder.
[0097] If included, a thickener can act to thicken the precursor slurry to a specified thickened viscosity, such as to a thickened viscosity of from about 1 centipoise (cP) to about 200 cP. A non-limiting example of a thickener that can be included in the precursor slurry is a modified cellulose.
[0098] A binder can be included in the precursor slurry to improve adhesion between two or more structures in the precursor coating and/or in the catalyst coating that is formed after converting the one or more precursor compounds to one or more catalyst compounds (described below). For example, a binder, if included, can improve adhesion between the conductive substrate and the precursor particles, and/or can improve inter-particle adhesion between the precursor particles in the precursor coating, and/or can improve adhesion between the catalyst particles and the conductive substrate, and/or can improve inter-particle adhesion between the catalyst particles within the catalyst coating. Examples of binders that can optionally be included in the precursor slurry include, but are not limited to: ionomer binders (such as ionomeric binders sold under the NAFION trade name by The Chemours Co., Wilmington, DE, USA, or ionomeric binders sold under the SUSTAINION trade name by Dioxide Materials, Inc., Boca Raton, FL, USA), or polymeric binders (such as a binder comprising polytetrafluoroethylene (PTFE)). In an example, if included, a binder can be from 0 wt. % to 1 wt. % of the precursor slurry.
[0099] Applying the precursor slurry to the conductive substrate (step 154) can be performed with any method capable of applying a precursor slurry onto the conductive substrate, which can include, but is not limited to, one or more of: dip coating the conductive substrate with the precursor slurry, painting the one or more surfaces of the conductive substrate with the precursor slurry, spraying the precursor slurry onto the one or more surfaces of the conductive substrate, applying the precursor slurry onto one or more surfaces of the conducive substrate with a doctor blade, applying the precursor slurry onto one or more surfaces of the conducive substrate with a slot die (with or without a tensioned web), or applying the precursor slurry onto one or more surfaces of the conducive substrate with a comma bar. In some examples, the method 150 of the present disclosure does not require any specialized methods to pattern or template the precursor slurry when applying it to the conductive substrate, e.g., so that the final particles formed have a desired size and/or shape. In other words, the method 150 of the present disclosure is capable of forming nanosized particles (e.g., particles that are 500 nm or less, for example 200 nm or less) without having to use templating or other specialty methods of patterning the precursor slurry onto the conductive substrate.
[0100] In an example, the thickness of the precursor slurry layer that has been coated onto the conductive substrate in step 154 can be from 0.5 micrometers (m) to 25 m, such as from 1 m to 20 m, for example from 2 m to 17.5 m, such as from 5 m to about 15 m. In an example, the coating layer of the precursor slurry on the conductive substrate is no more than 10 wt. % water before performing the baking step (step 156, described below), for example no more than 9 wt. % of the precursor coating being water, no more than 8 wt. % being water, no more than 7.5 wt. % being water, no more than 7 wt. % being water, no more than 6.5 wt. % being water, no more than 6 wt. % being water, no more than 5.5 wt. % being water, no more than 5 wt. % being water, no more than 4.5 wt. % being water, no more than 4 wt. % being water, no more than 3.5 wt. % being water, no more than 3 wt. % being water, or no more than 2.5 wt. % being water.
[0101] In an example, the conductive substrate onto which the precursor slurry is coated can comprise any conductive material that is useful as an electrode for an electrolyzer, such as the example hydrogen gas electrolyzer 10 described above with respect to
[0102] After coating the one or more surface of the conductive substrate with the precursor slurry to provide the precursor coated substrate (step 154), the method 150 can optionally include, at step 156, drying the precursor coating to provide a dried precursor coating on the conductive substrate. As discussed in more detail below, conversion of the precursor compounds of the precursor particles to the final metal oxide catalyst material (e.g., spinel structured catalyst particles) is accomplished by placing the precursor coated substrate in a bath of an electrochemical oxidation solution (also referred to simply as an ECO solution) while then subjecting the precursor coated substrate to electrochemical oxidation. In an example, the purpose of drying the precursor coating (step 156) is so that the dried precursor coating is less likely to be washed off by the ECO solution before the electrochemical oxidation converts the precursor material (e.g., the metal hydroxides) to the final metal oxide catalyst material (e.g., the spinel or other mixed metal oxide). However, it may be possible to configure the ECO solution bath and/or the electrochemical oxidation process in such a way that conversion to the desired final metal oxide catalyst material can be achieved before the ECO solution can wash away a substantial portion of the precursor particles from the conductive substrate.
[0103] In an example, drying the precursor coating (step 156) comprises removing a specified portion of the slurry medium from the coating, such as by evaporation of the water and/or alcohol, so that the resulting dried precursor coating has a specified solids content. In an example, drying of the precursor coating (step 156) comprises heating the precursor coated substrate to a drying temperature above the evaporation point of one or more components of the slurry medium and/or placing the precursor coated substrate in an atmosphere wherein the partial pressure of one or more components of the slurry medium is sufficiently below the equilibrium vapor pressure of the same component of the slurry medium so that the slurry medium will evaporate away. For example, if the slurry medium is water only, then drying of the precursor coating can include heating the precursor coated substrate to a temperature above the boiling point of water (e.g., above 100 C. under normal atmospheric conditions) or exposing the precursor coated substrate to air with a low relatively humidity, or both, so that a substantial portion of the water will evaporate. The remaining dried precursor coating can comprise precursor particles that are loosely bound to the conductive substrate, e.g., by tackiness or because of a binder additive (if used).
[0104] In an example, the dried precursor coating that remains after drying the precursor coating (step 156) has a specified solids content of at least 50 wt. % total solids (e.g., the remaining precursor particles make up at least 50 wt. % of the dried precursor coating), for example at least 55 wt. % total solids, at least 60 wt. % total solids, at least 65 wt. % total solids, at least 66 wt. % total solids, at least 67 wt. % total solids, at least 68 wt. % total solids, at least 69 wt. % total solids, at least 70 wt. % total solids, at least 71 wt. % total solids, at least 72 wt. % total solids, at least 73 wt. % total solids, at least 74 wt. % total solids, at least 75 wt. % total solids, at least 76 wt. % total solids, at least 77 wt. % total solids, at least 78 wt. % total solids, at least 79 wt. % total solids, at least 80 wt. % total solids, at least 81 wt. % total solids, at least 82 wt. % total solids, at least 83 wt. % total solids, at least 84 wt. % total solids, at least 85 wt. % total solids, at least 86 wt. % total solids, at least 87 wt. % total solids, at least 88 wt. % total solids, at least 89 wt. % total solids, at least 90 wt. % total solids, at least 91 wt. % total solids, at least 92 wt. % total solids, at least 93 wt. % total solids, at least 94 wt. % total solids, at least 95 wt. % total solids, at least 96 wt. % total solids, at least 97 wt. % total solids, at least 98 wt. % total solids, at least 99 wt. % total solids or more. In examples where the slurry medium is water only or primarily water, then the dried precursor coating after drying the precursor coating (step 156) can have a moisture content (i.e., percentage of the total weight of the dried precursor coating that is water, or the mass of water present in the dried precursor coating divided by the total weight of the dried precursor coating (which includes the precursor particles and any remaining water)) of 25 wt. % or less, for example 20 wt. % or less, such as 19 wt. % or less, 18 wt. % or less, 17 wt. % or less, 16 wt. % or less, 15 wt. % or less, 14 wt. % or less, 13 wt. % or less, 12 wt. % or less, 11 wt. % or less, 10 wt. % or less, 9 wt. % or less, 8 wt. % or less, 7 wt. % or less, 6 wt. % or less, or 5 wt. % or less.
[0105] After applying the precursor slurry to the conductive substrate to form the precursor slurry coating and provide the precursor coated substrate (step 154) and optionally drying the precursor slurry coating (step 156), the method 150 includes, at step 158, chemically converting the one or more precursor compounds of the precursor particles (which are part of the precursor coating or the dried precursor coating) to one or more catalyst compounds, such as one or more metal oxide catalyst compounds (such as the spinel or spinel-like mixed metal oxides described above), which produces a catalyst coating on the conductive substrate and provides a catalyst coated substrate. In an example, the one or more catalyst compounds can be in the form of catalyst particles, such as the catalyst particles 114 in
[0106] Chemical conversion of a precursor compound (such as a metal hydroxide precursor) to an oxide catalyst compound for an electrochemical electrode (such as a spinel oxide) has typically comprised thermal oxidation of the precursor compound. Thermal oxidation involves supplying heat energy to the precursor compound to drive the reaction or reactions that convert the precursor compound to the oxide catalyst compound. However, thermal oxidation of catalyst compounds that are useful for electrolysis, such as water electrolysis to form H.sub.2 gas, has tended to require high temperatures of at least 400 C., and often much higher, such as 500-700 C. or even higher, in particular if the goal was to achieve nanoscale sized catalyst particles. Even if satisfactory conversion could be achieved at lower temperatures, thermal oxidation typically requires keeping the precursor coated substrate at the elevated temperature for a long period of time, typically for at least about 1 hour and sometimes for as much as 2 to 5 hours, for full conversion to the desired spinel or spinel-like metal oxide compound. Thermal oxidation also often requires high pressure (e.g., high enough for solvothermal synthesis of spinel materials, such as 10 atm or more) and/or exposure to a non-air environment (such as an all nitrogen or highly nitrogen rich environment) during conversion. These more onerous conditions often require specialized equipment, such as a large furnace or specialized deposition equipment, which increases both initial capital expenses and continuing operating costs.
[0107] In addition, once thermal oxidation conversion is complete, it is also often necessary to activate the catalyst material after it has been formed on the electrode substrate. As used herein, the term activate, activated, activation, and the like refers to one or more modifications to locations on the catalyst particles where the electrochemical reaction of interest can occur (e.g., the OER [1]), which are also referred to as active sites for the electrochemical reaction. Activation can involve one or more processes that make active sites on the catalyst material more accessible and/or more reactive for the electrochemical reaction of interest. In some examples, activation of a catalyst compound can include synthesis of a reaction-enhancing compound on exposed surfaces of the catalyst particles to form and/or enhance active sites. Activation of catalyst compounds for electrochemical reactions can require even more specialized equipment to successfully perform thermal oxidation to convert a precursor compound to a catalyst compound and then to activate the catalyst compound so that the catalyst coated electrode will have sufficiently high activity for economically efficient operation of the electrochemical cell in which the electrode is being used.
[0108] In order to provide for lower energy fabrication and to avoid the high temperature, high pressure, and/or inert atmosphere required for thermal oxidation, the chemical conversion to the one or more catalyst compounds of the present disclosure (step 158) comprises electrochemically oxidizing the one or more precursor compounds of the precursor particles (which are part of the precursor slurry coating or the dried precursor coating). Electrochemical oxidation converts the one or more precursor compounds to one or more catalyst compounds (such as the spinel or spinel-like mixed metal oxides described above) to form catalyst particles (e.g., the catalyst particles 114 in
[0109] In an example, the electrochemical oxidation of step 158 comprises exposing the precursor coated substrate to a caustic solution while performing an electrochemical processing method to supply a specified amount of energy that is required to oxidize the one or more precursor compounds to form the desired one or more catalyst compounds (also referred to herein as the specified conversion energy to distinguish it from the specified activation energy described below). In an example, the caustic solution stabilizes the precursor compound and any catalyst intermediate compounds during the electrochemical oxidation. Examples of specific caustic solutions that can be used include, but are not limited to, solutions of sodium hydroxide (NaOH), potassium hydroxide (KOH), sodium carbonate (Na.sub.2CO.sub.3), or potassium carbonate (K.sub.2CO.sub.3) having a specified pH or a specified concentration of the solute (e.g., the NaOH, KOH, Na.sub.2CO.sub.3, or K.sub.2CO.sub.3). In an example, the caustic solution that the precursor coated substrate is exposed to has a concentration of at least 0.1 M, such as at least 0.11 M, at least 0.12 M, at least 0.125 M, at least 0.13 M, at least 0.14 M, at least 0.15 M, at least 0.16 M, at least 0.17 M, at least 0.175 M, at least 0.18 M, at least 0.19 M, at least 0.2 M, at least 0.225 M, at least 0.25 M, at least 0.275 M, at least 0.3 M, at least 0.325 M, at least 0.35 M, at least 0.375 M, at least 0.4 M, at least 0.45 M, at least 0.5 M, at least 0.55 M, at least 0.6 M, at least 0.65 M, at least 0.7 M, at least 0.75 M, at least 0.8 M, at least 0.85 M, at least 0.9 M, at least 0.95 M, at least 1 M, at least 1.1 M, at least 1.2 M, at least 1.25 M, at least 1.3 M, at least 1.4 M, at least 1.5 M, at least 1.6 M, at least 1.75 M, at least 1.8 M, at least 1.9 M, at least 2 M, at least 2.1 M, at least 2.2 M, at least 2.3 M, at least 2.4 M, at least 2.5 M, at least 2.6 M, at least 2.7 M, at least 2.8 M, at least 2.9 M, at least 3 M, at least 3.25 M, at least 3.5 M, at least 3.75 M, at least 4 M, at least 4.25 M, at least 4.5 M, at least 4.75 M, at least 5 M, at least 5.5 M, at least 6 M, at least 6.5 M, at least 7 M, at least 7.5 M, at least 8 M, at least 8.5 M, at least 9 M, at least 9.5, or at least 10 M.
[0110] In an example, the caustic solution that the precursor coated substrate is exposed to has a pH of at least about 12, for example at least about 12.5, at least about 13, at least about 13.5, at least about 14, at least about 14.5, or at least about 15. Examples of specific pH values for the solution used in step 158 can include, but are not limited to: a pH of 12 or more, such as 12 or more, 12.1 or more, 12.2 or more, 12.25 or more, 12.3 or more, 12.4 or more, 12.5 or more, 12.6 or more, 12.7 or more, 12.75 or more, 12.8 or more, 12.9 or more, 13 or more, 13.05 or more, 13.1 or more, 13.15 or more, 13.2 or more, 13.25 or more, 13.3 or more, 13.35 or more, 13.4 or more, 13.45 or more, 13.5 or more, 13.55 or more, 13.6 or more, 13.65 or more, 13.7 or more, 13.75 or more, 13.8 or more, 13.85 or more, 13.9 or more, 13.95 or more, 14 or more, 14.05 or more, 14.1 or more, 14.15 or more, 14.2 or more, 14.25 or more, 14.3 or more, 14.35 or more, 14.4 or more, 14.45 or more, 14.5 or more, 14.55 or more, 14.6 or more, 14.65 or more, 14.7 or more, 14.75 or more, 14.8 or more, 14.85 or more, 14.9 or more, 14.95 or more, 15 or more, 15.05 or more, 15.1 or more, 15.15 or more, 15.2 or more, 15.25 or more, 15.3 or more, 15.35 or more, 15.4 or more, 15.45 or more, 15.5 or more, 15.55 or more, 15.6 or more, 15.65 or more, 15.7 or more, 15.75 or more, 15.8 or more, 15.85 or more, 15.9 or more, or 15.95 or more.
[0111] Examples of electrochemical processing methods that can be used to provide the specified conversion energy for the electrochemical oxidation of the one or more precursor compounds to the desired one or more catalyst compounds (step 158) include, but are not limited to: chronoamperometry, chronopotentiometry, cyclic voltammetry, or a combination of two or all three of these techniques (either under dynamic conditions such as a linear sweep, or discrete step conditions).
[0112] The term chronoamperometry (also referred to hereinafter as CA), as used in reference to the electrochemical oxidation of step 158, refers to applying a specified steady-state conversion potential to the precursor coated substrate for a specified period of time (also referred to as the specified conversion period) while it is exposed to the caustic solution. The term specified conversion potential, as used herein for electrochemical oxidation by CA, refers to the potential that is required to supply the specified conversion energy to electrochemically oxidize the one or more precursor compounds to the desired one or more catalyst compounds (e.g., to one or more spinel or spinel-like mixed metal oxide compounds), which is used to distinguish the conversion potential from the activation potential that is required to be applied to activate active sites on the catalyst particles, as described below.
[0113] The term steady-state, as used herein when referring to the specified conversion potential that is applied to the precursor coated substrate during CA, means that the applied conversion potential remains constant or substantially constant for an appreciable period of time. However, the applied conversion potential can change during the CA process and still be considered steady-state for the purposes of the present disclosure so long as the applied conversion potential is held constant for a specified portion of the specified conversion period each time it is changed. For example, the CA process can include a stepped applied conversion potential wherein the applied conversion potential is held constant at a first specified conversion potential for a first specified period of time, and then raised or lowered to a second specified conversion potential and held constant for a second specified period of time, and then (optionally) is raised or lowered to a third specified conversion potential for a third period of time, and so on. The periods of time in the stepped applied conversion potential (e.g., the first period of time for the first specified conversion potential, the second period of time for the second specified conversion potential, and so on) can be the same or different.
[0114] In a specific non-limiting example, the applied conversion potential may be stepped up or down for a specified interval during the CA process. For example, if the applied conversion potential is being stepped up at an interval of 50 millivolts (mV), then the stepped CA process can include applying 50 mV (referenced to a standard Hg/HgO reference electrode) to the precursor coated substrate for a first period of time, followed by applying 100 mV (i.e., stepping up 50 mV from the previous value of 50 mV) for a second period of time, followed by applying 150 mV (i.e., stepping up 50 mV from the previous value of 100 mV) for a third period of time, followed by applying 200 mV (i.e., stepping up 50 mV from the previous value of 150 mV) for a fourth period of time, and so on until a specified maximum applied conversion potential is reached at which the precursor coated substrate can be held for a specified holding time period, after which the applied conversion voltage can be stepped back down by the same potential interval of 50 mV or by a different potential interval (including immediately dropping the applied conversion potential down to 0).
[0115] All potential values provided in the present disclosure are versus a standard mercury/mercury oxide (Hg/HgO) reference electrode, as will be understood by those of skill in the art.
[0116] The specified conversion potential that can be applied to the precursor coated substrate during CA can be anywhere from a specified minimum conversion potential to a specified maximum conversion potential. The specified minimum conversion potential is the potential that supplies sufficient energy to electrochemically convert the one or more precursor compounds that are coated on the substrate to the desired one or more catalyst compounds (e.g., one or more spinel or spinel-like mixed metal oxide compounds). However, the inventors have found that if the applied potential is too high, then the catalyst compound will not have the spinel or spinel-like mixed metal oxide structure that are preferred for electrolysis (and in particular for water electrolysis to produce H.sub.2 gas and O.sub.2 gas). Rather, when the applied potential is above a specified maximum conversion potential, the electrochemical oxidation will tend to bypass the spinel or spinel-like mixed metal oxide structures.
[0117] Examples of a specified minimum conversion potential for CA as part of step 158 (referenced to a standard Hg/HgO reference electrode) include, but are not limited to: 1 mV or more, 5 mV or more, 10 mV or more, 15 mV or more, 20 mV or more, 25 mV or more, 30 mV or more, 35 mV or more, 40 mV or more, 45 mV or more, 50 mV or more, 55 mV or more, 60 mV or more, 70 mV or more, 75 mV or more, 80 mV or more, 85 mV or more, 90 mV or more, 100 mV or more, 110 mV or more, 120 mV or more, 125 mV or more, 130 mV or more, 140 mV or more, 150 mV or more, 160 mV or more, 170 mV or more, 175 mV or more, 180 mV or more, 190 mV or more, or 200 mV or more. Examples of a specified maximum conversion potential for CA as part of step 158 (referenced to a standard Hg/HgO reference electrode) include, but are not limited to, 500 mV or less, 510 mV or less, 520 mV or less, 525 mV or less, 530 mV or less, 540 mV or less, 550 mV or less, 560 mV or less, 570 mV or less, 575 mV or less, 580 mV or less, 590 mV or less, 600 mV or less, 610 mV or less, 620 mV or less, 625 mV or less, 630 mV or less, 640 mV or less, 650 mV or less, 660 mV or less, 670 mV or less, 675 mV or less, 680 mV or less, 690 mV or less, 700 mV or less, 710 mV or less, 720 mV or less, 725 mV or less, 730 mV or less, 740 mV or less, 750 mV or less, 760 mV or less, 770 mV or less, 775 mV or less, 780 mV or less, 790 mV or less, 800 mV or less, 810 mV or less, 820 mV or less, 825 mV or less, 830 mV or less, 840 mV or less, 850 mV or less, 860 mV or less, 870 mV or less, 875 mV or less, 880 mV or less, 890 mV or less, 900 mV or less, 910 mV or less, 920 mV or less, 925 mV or less, 930 mV or less, 940 mV or less, 950 mV or less, 960 mV or less, 970 mV or less, 975 mV or less, 980 mV or less, 990 mV or less, or 1000 mV or less.
[0118] In an example, the specified conversion period that the specified conversion potential (either fixed steady-state or stepped steady-state) is applied to the precursor coated substrate during CA is from about 5 seconds to about 10 minutes. The specified conversion period that the specified conversion potential is applied can depend on the concentration of the caustic solution and the solids content of the precursor coating, and is selected to provide sufficient energy to overcome the energetic barrier to convert the one or more precursor compounds (e.g., the metal hydroxide) to the one or more catalyst compounds (e.g., spinel or spinel-like mixed metal oxide compound).
[0119] The term chronopotentiometry (also referred to herein as CP), as used in reference to the electrochemical oxidation of step 158, refers to applying a specified steady-state conversion current density to the precursor coated substrate for a specified conversion period while the precursor coated substrate is exposed to the caustic solution. The term specified conversion current density, as used herein for electrochemical oxidation by CP, refers to the current density that is required to supply the specified conversion energy to electrochemically oxidize the one or more precursor compounds to the desired one or more catalyst compounds (e.g., to one or more spinel or spinel-like mixed metal oxide compounds), which is used to distinguish the conversion current density from the activation current density that is required to be applied to activate active sites on the catalyst particles, as described below. The term steady-state, as used herein when referring to the specified conversion current density that is applied to the precursor coated substrate during CP, means that the applied conversion current density remains constant or substantially constant for an appreciable period of time. Similar to the description of CA above, the applied conversion current density can change during the CP process and still be considered steady-state for the purposes of the present disclosure so long as the applied conversion current density is held constant for a specified period of time each time it is changed.
[0120] Similar to that which is described for CA processes, the specified steady-state conversion current density that is applied to the precursor coated substrate during CP is at least at a specified minimum conversion current density that is high enough to convert the one or more precursor compounds of the precursor particles that are coated on the substrate to the desired one or more catalyst compounds (e.g., one or more spinel or spinel-like mixed metal oxide compounds). However, the inventors have found that if the current density is too high, then the catalyst compound will not have the spinel or spinel-like mixed metal oxide structure that are preferred for electrolysis (and in particular for water electrolysis to produce H.sub.2 gas and O.sub.2 gas). Rather, when the applied current density is above a specified maximum conversion current density, the electrochemical oxidation will tend to bypass the spinel or spinel-like mixed metal oxide structures.
[0121] Examples of a specified conversion minimum current density for CP as part of step 158 include, but are not limited to: about 3 milliamps per square centimeter (mA/cm.sup.2), for example about 3.1 mA/cm.sup.2, about 3.2 mA/cm.sup.2, about 3.25 mA/cm.sup.2, about 3.3 mA/cm.sup.2, about 3.4 mA/cm.sup.2, about 3.5 mA/cm.sup.2, about 3.6 mA/cm.sup.2, about 3.7 mA/cm.sup.2, about 3.75 mA/cm.sup.2, about 3.8 mA/cm.sup.2, about 3.9 mA/cm.sup.2, about 4 mA/cm.sup.2, about 4.1 mA/cm.sup.2, about 4.2 mA/cm.sup.2, about 4.25 mA/cm.sup.2, about 4.3 mA/cm.sup.2, about 4.4 mA/cm.sup.2, about 4.5 mA/cm.sup.2, about 4.6 mA/cm.sup.2, about 4.7 mA/cm.sup.2, about 4.75 mA/cm.sup.2, about 4.8 mA/cm.sup.2, about 4.9 mA/cm.sup.2, about 5 mA/cm.sup.2, about 5.25 mA/cm.sup.2, about 5.5 mA/cm.sup.2, about 5.75 mA/cm.sup.2, about 6 mA/cm.sup.2, about 6.25 mA/cm.sup.2, about 6.5 mA/cm.sup.2, about 6.75 mA/cm.sup.2, about 7 mA/cm.sup.2, about 7.25 mA/cm.sup.2, about 7.5 mA/cm.sup.2, about 7.75 mA/cm.sup.2, about 8 mA/cm.sup.2, about 8.25 mA/cm.sup.2, about 8.5 mA/cm.sup.2, about 8.75 mA/cm.sup.2, about 9 mA/cm.sup.2, about 9.25 mA/cm.sup.2, about 9.5 mA/cm.sup.2, about 9.75 mA/cm.sup.2, or about 10 mA/cm.sup.2. Examples of a specified conversion maximum current density for CP as part of step 158 include, but are not limited to, 20 mA/cm.sup.2, 20.5 mA/cm.sup.2, 21 mA/cm.sup.2, 21.5 mA/cm.sup.2, 22 mA/cm.sup.2, 22.5 mA/cm.sup.2, 23 mA/cm.sup.2, 23.5 mA/cm.sup.2, 24 mA/cm.sup.2, 24.5 mA/cm.sup.2, 25 mA/cm.sup.2, 25.5 mA/cm.sup.2, 26 mA/cm.sup.2, 27 mA/cm.sup.2, 27.5 mA/cm.sup.2, 28 mA/cm.sup.2, 28.5 mA/cm.sup.2, 29 mA/cm.sup.2, 29.5 mA/cm.sup.2, or about 30 mA/cm.sup.2.
[0122] In an example, the specified conversion period that the specified conversion current density is applied to the precursor coated substrate during CP is essentially instantaneously (e.g., within a millisecond of the conversion current density being applied) to about 10 minutes. The specified conversion period that the specified conversion current density is applied can depend on the concentration of the caustic solution and the solids content of the precursor coating, and is selected to provide sufficient energy to overcome the energetic barrier to convert the one or more precursor compounds (e.g., the metal hydroxide) to the one or more catalyst compounds (e.g., spinel or spinel-like mixed metal oxide compound).
[0123] The term cyclic voltammetry (also referred to herein as CV), as used in reference to the electrochemical oxidation of step 158, refers to cycling between applying a first specified conversion voltage to the precursor coated substrate and applying a second specified conversion voltage to the precursor coated substrate for a specified conversion period. The term specified conversion voltage, as used herein for electrochemical oxidation by CV refers to a specified voltage (e.g., the first specified conversion voltage or the second specified conversion voltage) that is required to supply the specified conversion energy to electrochemically oxidize the one or more precursor compounds to the desired one or more catalyst compounds (e.g., to one or more spinel or spinel-like mixed metal oxide compounds). Because the conversion voltage applied during electrochemical oxidation via CV is also an electrical potential, the concept could have been referred to as the conversion potential for CV, like it was for CA. However, to avoid confusion between the two different types of electrochemical oxidation, the concept is referred to as the conversion voltage for CV to distinguish it from the conversion potential for CA.
[0124] In an example, the cycling between the first and second specified conversion voltages can be via a regular or substantially regular waveform for each cycle. In an example, the regular or substantially regular waveform comprises a triangular (e.g., sawtooth) waveform wherein each cycle comprises the voltage ramps up (or down) linearly or substantially linearly at a specified rate from the first specified conversion voltage up (or down) to the second specified conversion voltage and then ramps back down (or up) linearly at the same specified rate from the second specified conversion voltage down (or up) to the first specified conversion voltage. The regular or substantially regular waveform can have a specified period or a specified frequency. In an example, the first specified conversion voltage is negative relative to the standard Hg/HgO reference electrode and the second specified conversion voltage is positive relative to the standard Hg/HgO reference electrode, e.g., so that each cycle of the CV waveform comprises cycling between the negative first specified conversion voltage and the positive second specified conversion voltage.
[0125] Examples of a first specified conversion voltage (e.g., the negative voltage referenced to a standard Hg/HgO reference electrode)) include, but are not limited to: about 2 V, about 1.9 V, about 1.8 V, about 1.75 V, about 1.7 V, about 1.6 V, about 1.5 V, about 1.4 V, about 1.3 V, about 1.25 V, about 1.2 V, about 1.1 V, about 1 V, about 0.9 V, about 0.8 V, about 0.75 V, about 0.7 V, about 0.6 V, about 0.5 V, about 0.4 V, about 0.3 V, about 0.25 V, about 0.2 V, about 0.1 V, or about 0.05 V. Examples of a second specified conversion voltage (e.g., the positive voltage referenced to a standard Hg/HgO reference electrode) include, but are not limited to: 0.05 V, about 0.1 V, about 0.15 V, about 0.2 V, about 0.25 V, about 0.3 V, about 0.35 V, about 0.4 V, about 0.45 V, about 0.5 V, about 0.55 V, about 0.6 V, about 0.65 V, about 0.7 V, about 0.75 V, about 0.8 V, about 0.85 V, about 0.9 V, about 1 V, about 1.05 V, about 1.1 V, about 1.15 V, about 1.2 V, about 1.25 V, about 1.3 V, about 1.35 V, about 1.4 V, about 1.45 V, about 1.5 V, about 1.55 V, about 1.6 V, about 1.65 V, about 1.7 V, about 1.75 V, about 1.8 V, about 1.85 V, about 1.9 V, or about 2 V.
[0126] In an example, the specified period of the CV waveform was about 75 seconds, performed from 5 to 10 times. In an example, the specified conversion period that the first and second specified conversion voltages are applied in total to the precursor coated substrate during CV is from about 2 minutes to about 15 minutes. The specified conversion period that the specified first and second conversion voltages are applied during CV can depend on the concentration of the caustic solution and the solids content of the precursor coating, and can be selected to provide sufficient energy to overcome the energetic barrier to convert the one or more precursor compounds (e.g., the metal hydroxide) to the one or more catalyst compounds (e.g., spinel or spinel-like mixed metal oxide compound).
[0127] The method 150 can also include, at step 160, activating active sites on the catalyst particles to provide an activated catalyst coated substrate. As described above, activating the catalyst particles can comprise modifying the active sites on the catalyst particles, such as by synthesizing a reaction-enhancing compound at the active sites or otherwise making the active sites more available and reactive toward the electrochemical reaction of interest (e.g., the OER [1]).
[0128] Typically, catalyst compounds for electrolysis that have been synthesized by thermal oxidation are activated by subjecting the catalyst compound to an electrical process, wherein the applied electrical energy produces one or more physical or chemical changes at active sites on the catalyst particles, wherein the physical or chemical changes result in higher activity for the electrochemical reaction of interest. In the case of spinel or spinel-like mixed metal oxide catalyst compounds, in an example, activation (and in particular electrical activation) comprises formation of a metal oxyhydroxide compound on at least a portion of the surfaces of the one or more metal oxide compounds that form the catalyst particles. In an example, a metal oxyhydroxide can have the general chemical formula MO(OH), wherein M is a metal element including, but not limited to one or more of: an alkali metal, an alkaline earth metal, a transition metal, a lanthanide series metal, a post-transition metal, a metalloid, and combinations thereof. In a specific example, M in the metal oxyhydroxide formula MO(OH) is one of the A or B metal element ions discussed above for the spinel chemical formula [3], i.e., one of the metal elements that are present in the one or more catalyst compounds of the catalyst particles 114.
[0129] In an example, activating active sites on the one or more catalyst compounds (step 160) comprises performing an electrochemical processing method to supply a specified amount of energy to the catalyst coated substrate that is required to activate active sites on the one or more catalyst compounds (also referred to herein as the specified activation energy to distinguish it from the specified conversion energy described above).
[0130] In an example, supplying the specified activation energy comprises applying a specified activation potential and/or a specified activation current density to the catalyst coated substrate provided by step 158 for a specified period of time (also referred to as the specified activation period to distinguish it from the specified conversion period described above for the electrochemical oxidation processes of step 158 described above. The term specified activation potential, as used herein, refers to the potential that is required to supply the specified activation energy to the catalyst particles to activate active sites on the catalyst particles, which is used to distinguish the activation potential from the specified conversion potential that is required to supply the specified conversion energy to the one or more precursor compounds to convert them to the desired one or more catalyst compounds (e.g., to one or more spinel or spinel-like mixed metal oxide compounds), as described above. Similarly, the term specified activation current density, as used herein, refers to the current density that is required to supply the specified activation energy to the catalyst particles, which is used to distinguish the activation current density from the specified conversion current density to supply the specified conversion energy to the one or more precursor compounds, as described above.
[0131] In some examples, the specific parameters of the electrochemical processing method (e.g., one or more of CA, CP, and CV) used for electrochemical oxidation of the one or more precursor compounds to the one or more catalyst compounds (step 158) is also sufficient to activates active sites on the catalyst particles such that the step of electrochemical oxidation (step 158) and activation (step 160) can be performed simultaneously or substantially simultaneously via the same electrochemical processing method. In other words, the electrochemical processing method that provides the energy for electrochemical oxidation of the one or more precursor compounds to the one or more catalyst compounds (step 158), e.g., by providing one or both of the conversion current density or the current voltage for a sufficient conversion period) also provides sufficient energy to supply the specified activation potential and/or the specified activation current density to the catalyst coated substrate provided by step 158 during the same electrical processing step.
[0132] In other examples, the electrochemical oxidation of the one or more precursor compounds to the one or more catalyst compounds (step 158) and electrical activation of active sites on the one or more catalyst compounds (step 160) can be performed as separated electrical processing steps, for example if the amount of energy provided by the specified conversion potential, the specified conversion current density, or the specified conversion voltage is less than is required for the activation potential or the activation current density. As described in more detail below, applying the specified conversion energy (e.g., what is provided by one or more of the specified conversion potential, the specified conversion current density, or the specified conversion voltage) separate from applying the activation energy (e.g., the energy provided by one or both of the specified activation potential and the specified activation current density) may be desirable if the energy provided by the specified activation potential or the specified activation current density is so high that it tends to cause the one or more precursor compounds to bypass a desired form of catalyst compound, e.g., by bypassing a spinel or spine-like metal oxide.
[0133] As a non-limiting example to show the differences between the specified conversion energy and the specified activation energy, the specified conversion current density for converting a precursor compound (such as a metal hydroxide, e.g., cobalt (II) hydroxide (Co(OH).sub.2) to a spinal oxide catalyst compound (e.g., cobalt (II, III) oxide (Co.sub.3O.sub.4)) was found to typically on the order of a microamp per square centimeter (A/cm.sup.2) or about 0.001 milliamps per square centimeter (mA/cm.sup.2), while the specified activation current density for activating the same spinal oxide catalyst compound (e.g., cobalt (II, III) oxide (Co.sub.3O.sub.4)) was found to typically be on the order of around 0.01 mA/cm.sup.2 up to about 1 mA/cm.sup.2, such as about 0.1 mA/cm.sup.2. In other words, the specified activation current density was anywhere from about 10 times to about 1000 times greater than the specified conversion current density. Therefore, if a current density that is as high as these specified activation current densities was used during the electrochemical processing method or methods used for step 158for example, if the specified steady-state current density for a CP process was as high as these specified activation current densities or if a CA or CV process were employed that resulted in a current density that was as high as these typical activation current densitiesthen it would tend to bypass the formation of a spinel or spinel-like mixed metal oxide compound that is a preferred structure of catalyst compound for electrolysis (and in particular for water electrolysis to produce H.sub.2 gas and O.sub.2 gas). Therefore, as described above, the specified conversion current density that is applied for the electrochemical oxidation step 158 to produce the desired catalyst compound or compounds is generally substantially lower than the typical activation current density and/or than the specified activation potential.
[0134] The inventors have found that, surprisingly, even when performing the electrochemical oxidation of step 158 and the activation of step 160 at a specified maximum current density and/or a specified maximum applied potential that is substantially lower than the typical activation current density and/or the typical activation potential, the resulting catalyst particles are both (a) formed from one or more catalyst compounds having a spinel or spinel-like mixed metal oxide structure; and (b) activated (e.g., comprise a substantial number of active sites, such as by formation of one or more metal oxyhydroxides on the catalyst particles). This is particularly beneficial because it means that the method 150 of the present disclosure allows for conversion to the desired spinel or spinel-like mixed metal oxide compounds and activation of the catalyst particles with a lower energy requirement, and thus lower operating costs, than typical activation methods.
[0135] Although activation of the one or more catalyst compounds (step 160) is described as applying electrical energy to the catalyst coated substrate (e.g., by applying one or both of a specified activation potential and a specified activation current density for a specified activation period), other methods of activating active sites of the catalyst particles can be performed as part of step 160 in addition to or in place of electrical activation. Examples of other methods of activation that can be used include, but are not limited to: one or both of reducing or oxidizing the surface of the catalyst particles with gases (e.g., nitriding); applying one or more adsorbates to the catalyst particles that can activate active sites; and intercalation of one or more compounds into the catalyst particles.
[0136] As described above, the method 150 of
[0137] The method 170 of
[0138] In an example, each of the one or more first salts of the one or more first metal elements are a different type of salt from each of the one or more second salts of the one or more second metal elements. For example, if the first salts comprise nitrates of the one or more first metal elements, then in an example the one or more second salts are not nitrates of the one or more second metal elements (i.e., the one or more second salts could be halides of the one or more second metal elements, and/or acetates of the one or more second metal elements, and/or sulfates of the one or more second metal elements, but not nitrates of the one or more second metal elements). In another example, if the first salts comprise nitrates and acetates of the one or more first metal elements, than the second salts can include halides and/or sulfates of the one or more second metal elements, but not nitrates or acetates of the one or more second metal elements. However, those having skill in the art will appreciate that the method 170 is not limited to the one or more first salts being of a different type from the one or more second salts. For example, each first salt can be a nitrate of one or more of the first metal elements and each second salt can be a nitrate of the one or more second metal elements; or each first salt can be a halide of one or more of the first metal elements and each second salt can be a halide of one or more of the second metal elements, and so on.
[0139] The one or more first metal elements and the one or more second metal elements, e.g., that form the one or more first salts and the one or more second salts, can be any metal element that will be able to form either the one or more A metal ions or the one or more B metal ion in the spinel formula [3], described above. In an example, the one or more first salts of the one or more first metal elements that are included in the salt solution comprise all of the one or more A metal elements of the final spinel formula [3] and the one or more second salts of the one or more second metal elements that are included in the salt solution comprise all of the one or more B metal elements of the final spinel formula [3]. In another example, the first and second salts are reversed, i.e., with the one or more first salts of the one or more first metal elements including all of the one or more B metal elements of the final spinel formula [3], and the one or more second salts of the one or more second metal elements including all of the one or more A metal elements of the final spinel formula [3].
[0140] As a non-limiting example, if the desired catalyst compound is nickel cobaltite (NiCo.sub.2O.sub.4), then the first metal salt that is used to form the salt solution in step 174 can comprise a nickel-containing salt such as nickel nitrate (Ni(NO.sub.3).sub.2) and the second metal salt can comprise a cobalt-containing salt such as cobalt chloride (CoCl.sub.2)). In another example, wherein the desired catalyst compound comprises more than one A metal ion and/or more than one B metal ion, such as a ferrite (B metal ion is Fe) with A metal ions comprising nickel and zinc (with the relative chemical formula of Ni.sub.aZn.sub.1-aFe.sub.2O.sub.4), the salt solution can be formed from first metal salts comprising both a nickel-containing salt (such as nickel nitrate (Ni(NO.sub.3).sub.2) and a zinc-containing salt (such as zinc nitrate (Zn(NO.sub.3).sub.2) and a second metal salt comprising an iron-containing salt (such as iron (II) chloride (FeCl.sub.2)).
[0141] The surfactant that can optionally be included in the salt solution that is formed in step 174 can be any surfactant that can act as a template for the formation of precursor particles during coprecipitation (step 154, discussed below) such that the co-precipitated precursor particles will have a specified size. In an example, the specified size provided for by the surfactant (if present) is 1 micrometer (m) or less, for example 975 nanometers (nm) or less, such 950 nm or less, 925 nm or less, 900 nm or less, 875 nm or less, 850 nm or less, 825 nm or less, 800 nm or less, 750 nm or less, 725 nm or less, 700 nm or less, 675 nm or less, 650 nm or less, 625 nm or less, 600 nm or less, 575 nm or less, 550 nm or less, 525 nm or less, 500 nm or less, 475 nm or less, 450 nm or less, 425 nm or less, 400 nm or less, 375 nm or less, 350 nm or less, 325 nm or less, or 300 nm or less. Examples of surfactant that can be used for the method of the present disclosure include, but are not limited to, polyethylene glycol (PEG), polyethylene glycol dodecyl ether (such as the surfactant sold under the trade name BRIJ L4 by Sigma-Aldrich, Inc., St. Louis, MO USA), and oleic acid.
[0142] In an example, forming the salt solution (step 174) comprises dissolving the one or more first metal salts and the one or more second metal salts in a solvent, such as water. In examples that include a surfactant, the one or more first metal salts and the one or more second metal salts can be dissolved in the solvent, and then the surfactant can be added to the resulting salt solution, or the surfactant can first be added to the solvent and then the one or more first metal salts and the one or more second metal salts can be dissolved in the solvent and surfactant mixture, or the surfactant and the one or more first and second metal salts can be added to the solvent at the same time or substantially the same time.
[0143] In an example, the relative amount of each of the one or more first metal salts and the one or more second metal salts that are dissolved in the salt solution correspond to the stoichiometric amount of each metal ion in the final spinel material that is to be formed. Using the same non-limiting examples as above, if the desired final spinel is nickel cobaltite (NiCo.sub.2O.sub.4), then the concentration of the nickel-containing salt (e.g., nickel nitrate (Ni(NO.sub.3).sub.2) in the salt solution can be about one half the concentration of the cobalt-containing salt (e.g., cobalt chloride (CoCl.sub.2)). When the final spinel has more than one A metal ion or more than one B metal ion, the corresponding concentrations of each salt will be selected accordingly. For example, if the final desired spinel is a ferrite (e.g., with the B metal ion being Fe) with A metal ions comprising nickel and zinc with a relative chemical formula of Ni.sub.0.4Zn.sub.0.6Fe.sub.2O.sub.4, the ratio of the concentrations of the nickel containing salt (e.g., nickel nitrate (Ni(NO.sub.3).sub.2)), the zinc-containing salt (e.g., zinc nitrate (Zn(NO.sub.3).sub.2)), and the iron-containing salt (e.g., iron (II) chloride (FeCl.sub.2)) can be about 0.4:0.6:2=about 1:1.5:5 (i.e., about 1.5 parts Zn-containing salt and about 5 parts Fe-containing salt for every part of the Ni-containing salt, on a molar basis).
[0144] Next, the method 170 can include, at step 176, coprecipitating one or more precursor compounds from the salt solution formed in step 174 to provide precipitated precursor particles, wherein the precipitated precursor particles comprise the one or more precursor compounds. After the precipitated precursor particles have been formed, the liquid component of the salt solution (e.g., the solvent or solvents that form the salt solution, the one or more surfactants, and any unprecipitated portion of the first and second method salts) and the precipitated precursor particles form a reaction mixture. The precursor compound or compounds of the precipitated precursor particles can comprise one or more metal salts of the one or more first metal ions and the one or more second metal ions that are insoluble or substantially less soluble in the solvent than the one or more first metal salts and the one or more second metal salts that are dissolved to form the salt solution in step 174. An examples of a type of precursor compounds that can be formed by the coprecipitation of step 176 include, but are not limited to a metal hydroxide.
[0145] In one example, the one or more precursor compounds of the precipitated precursor particles comprises a single compound that includes both the one or more A elements and the one or more B elements of the spinel formula [3] that was precipitated from the one or more first salts of the one or more first metal elements and the one or more second salts of the one or more second metal elements in step 174. For example, if the first metal salt comprises nickel nitrate (Ni(NO.sub.3).sub.2) and the second metal salt comprises cobalt chloride (CoCl.sub.2), the resulting precursor particles can comprise a nickel-doped cobalt hydroxide (Ni-doped Co(OH).sub.2).
[0146] In another example, the one or more precursor compounds that form the precipitated precursor particles can include one or more first precursor compounds corresponding to the one or more first metal salts and one or more second precursor compounds corresponding to the one or more second metal salts. For example, if the first metal salt comprises nickel nitrate (Ni(NO.sub.3).sub.2) and the second metal salt comprises cobalt chloride (CoCl.sub.2), the one or more precursor compounds could comprise particles of nickel hydroxide (Ni(OH).sub.2) and particles of cobalt hydroxide (Co(OH).sub.2) and/or composite particles that include both nickel hydroxide and cobalt hydroxide).
[0147] In an example, coprecipitating the precursor particles comprising the one or more precursor compounds from the salt solution (step 176) comprises adding one or more precipitating agents to the salt solution that react with the one or more first metal salts and the one or more second metal salts in the salt solution to form the one or more precursor compounds, wherein the one or more precursor compounds are insoluble or substantially insoluble in the solvent so that they precipitate out of the salt solution.
[0148] In an example, the precipitating agent comprises a hydroxide compound (such as sodium hydroxide (NaOH), potassium hydroxide (KOH), or tetramethylammonium hydroxide (N(CH.sub.3).sub.4(OH)), wherein the hydroxide of the precipitating agent reacts with the one or more first metal salts and the one or more second metal salts to form hydroxide compounds of the one or more first metals that form the one or more first metal salts and second hydroxide compounds of the one or more second metals that form the one or more second metal salts. For example, if the first metal salt comprises a nickel-containing salt (e.g., nickel nitrate (Ni(NO.sub.3).sub.2) and the second metal salt comprises an iron-containing salt (e.g., iron chloride (FeCl.sub.2)) to eventually form nickel ferrite (NiFe.sub.2O.sub.4), then a first precursor compound that is precipitated can be nickel (II) hydroxide (Ni(OH).sub.2) and a second precursor compound that is precipitated can be iron (II) hydroxide (Fe(OH).sub.2).
[0149] Precipitating agents other than hydroxides that can be used for the coprecipitation step (step 176) include, but are not limited to, a carbonate compound (such as sodium carbonate (Na.sub.2CO.sub.3) or ammonium carbonate ((NH.sub.4).sub.2CO.sub.3)), a bicarbonate compound (such as sodium bicarbonate (NaHCO.sub.3) or ammonium bicarbonate (NH.sub.4)HCO.sub.3), a dicarboxylic acid (such as oxalic acid (C.sub.2H.sub.2O.sub.4)), ammonia (NH.sub.3), sodium dodecyl sulfate (CH.sub.3(CH.sub.2).sub.11OSO.sub.3Na), urea (CO(NH.sub.2).sub.2), and thiourea (CS(NH.sub.2).sub.2).
[0150] In one example, coprecipitating the precursor particles (step 176) can comprise titrating the one or more precipitating agents into the salt solution comprising the first and second metal salts and the surfactant (e.g., slowly increasing the amount of the precipitating agent added to the salt solution, such as in a dropwise manner). In an example, the salt solution can be combined with the one or more precipitating agents until the resulting mixture reaches a specified concentration of the precipitating agent, wherein the specified concentration of the one or more precipitating agents corresponds to reaction and formation of the one or more precursor compounds. In the case where the precipitating agent comprises a hydroxide such as NaOH, the hydroxide can be titrated into the salt solution until the resulting mixture has a specified pH that is known to result in precipitation of the first and second precursor particles (e.g., a pH of 10 or higher).
[0151] In another example, forming the salt solution (step 174) can comprise forming a first metal salt solution comprising the one or more first metal salts dissolved in a first solvent, such as water, and forming a second metal salt solution comprising the one or more second metal salts dissolved in a second solvent (which can be the same or different from the first solvent). One of the first and second metal salt solutions can also include a precipitating agent, such as a hydroxide solution, for example sodium hydroxide (NaOH). Then, during the coprecipitation step (step 176), the first metal salt solution and the second metal salt solution can be combined, such as by titrating the first metal salt solution into the second metal salt solution or vice versa, to initiate coprecipitation by reaction of the precipitating agent and the one or more first metal salts and/or the one or more second metal salts.
[0152] Other parameters for the coprecipitation step 176 can include, but are not limited to: a specified pH for the salt solution during precipitation step 176, a specified temperature of the salt solution during precipitation step 176, or a dosing rate for the introduction of the one or more precipitation agents into the salt solution (or vice versa for the introduction of the salt solution into a second solution comprising the one or more precipitation agents), e.g., a titration rate.
[0153] In an example, a specified pH for the salt solution during the coprecipitation step 176 is from about 10 to about 14, such as from about 11 to about 13, for example from about 11.5 to about 12, such as a pH of 10, 10.1, 10.2, 10.3, 10.4, 10.5, 10.6, 10.7, 10.8, 10.9, 11, 11.1, 11.2, 11.3, 11.4, 11.5, 11.6, 11.7, 11.8, 11.9, 12, 12.1, 12.2, 12.3, 12.14, 12.15, 12.6, 12.7, 12.8, 12.9, 13, 13.1, 13.2, 13.3, 13.4, 13.5, 13.6, 13.7, 13.8, 13.9, or 14.
[0154] In an example, a specified temperature for the salt solution during the coprecipitation step 176 is from about 10 C. to about 100 C., such as from about 10 C. to about 90 C., from about 10 C. to about 80 C., from about 10 C. to about 75 C., from about 10 C. to about 70 C., from about 10 C. to about 60 C., or from about 10 C. to about 50 C., for example about 10 C., about 11 C., about 12 C., about 13 C., about 14 C., about 15 C., about 16 C., about 17 C., about 18 C., about 19 C., about 20 C., about 21 C., about 22 C., about 23 C., about 24 C., about 25 C., about 26 C., about 27 C., about 28 C., about 29 C., about 30 C., about 31 C., about 32 C., about 33 C., about 34 C., about 35 C., about 36 C., about 37 C., about 38 C., about 39 C., about 40 C., about 41 C., about 42 C., about 43 C., about 44 C., about 45 C., about 46 C., about 47 C., about 48 C., about 49 C., about 50 C., about 51 C., about 52 C., about 53 C., about 54 C., about 55 C., about 56 C., about 57 C., about 58 C., about 59 C., about 60 C., about 61 C., about 62 C., about 63 C., about 64 C., about 65 C., about 66 C., about 67 C., about 68 C., about 69 C., about 70 C., about 71 C., about 72 C., about 73 C., about 74 C., or about 75 C.
[0155] In an example, a dosing rate of the one or more precipitation agents into the salt solution (or of the salt solution into the one or more precipitation agents) is from about 0.01 milliliters per minute (mL/min) to 25 mL/min, such as from 0.05 mL/min to 20 mL/min, for example from 0.1 mL/min to 15 mL/min, such as from 0.2 mL/min to 10 mL/min. Specific examples of dosing rates include, but are not limited to, 0.01 mL/min or more, 0.025 mL/min or more, 0.05 mL/min or more, 0.075 mL/min or more, 0.1 mL/min or more, 0.125 mL/min or more, 0.15 mL/min or more, 0.175 mL/min or more, 0.2 mL/min or more, 0.225 mL/min or more, 0.25 mL/min or more, 0.275 mL/min or more, 0.3 mL/min or more, 0.325 mL/min or more, 0.35 mL/min or more, 0.375 mL/min or more, 0.4 mL/min or more, 0.425 mL/min or more, 0.45 mL/min or more, 0.475 mL/min or more, 0.5 mL/min or more, 0.525 mL/min or more, 0.55 mL/min or more, 0.575 mL/min or more, 0.6 mL/min or more, 0.625 mL/min or more, 0.65 mL/min or more, 0.675 mL/min or more, 0.7 mL/min or more, 0.725 mL/min or more, 0.75 mL/min or more, 0.775 mL/min or more, 0.8 mL/min or more, 0.825 mL/min or more, 0.85 mL/min or more, 0.875 mL/min or more, 0.9 mL/min or more, 0.925 mL/min or more, 0.95 mL/min or more, 0.975 mL/min or more, 1 mL/min or more, 1.1 mL/min or more, 1.2 mL/min or more, 1.3 mL/min or more, 1.4 mL/min or more, 1.5 mL/min or more, 1.6 mL/min or more, 1.7 mL/min or more, 1.8 mL/min or more, 1.9 mL/min or more, 2 mL/min or more, 2.1 mL/min or more, 2.2 mL/min or more, 2.3 mL/min or more, 2.4 mL/min or more, 2.5 mL/min or more, 2.6 mL/min or more, 2.7 mL/min or more, 2.8 mL/min or more, 2.9 mL/min or more, 3 mL/min or more, 3.1 mL/min or more, 3.2 mL/min or more, 3.3 mL/min or more, 3.4 mL/min or more, 3.5 mL/min or more, 3.6 mL/min or more, 3.7 mL/min or more, 3.8 mL/min or more, 3.9 mL/min or more, 4 mL/min or more, 4.1 mL/min or more, 4.2 mL/min or more, 4.3 mL/min or more, 4.4 mL/min or more, 4.5 mL/min or more, 4.6 mL/min or more, 4.7 mL/min or more, 4.8 mL/min or more, 4.9 mL/min or more, 5 mL/min or more, 5.1 mL/min or more, 5.2 mL/min or more, 5.3 mL/min or more, 5.4 mL/min or more, 5.5 mL/min or more, 5.6 mL/min or more, 5.7 mL/min or more, 5.8 mL/min or more, 5.9 mL/min or more, 6 mL/min or more, 6.1 mL/min or more, 6.2 mL/min or more, 6.3 mL/min or more, 6.4 mL/min or more, 6.5 mL/min or more, 6.6 mL/min or more, 6.7 mL/min or more, 6.8 mL/min or more, 6.9 mL/min or more, 7 mL/min or more, 7.1 mL/min or more, 7.2 mL/min or more, 7.3 mL/min or more, 7.4 mL/min or more, 7.5 mL/min or more, 7.6 mL/min or more, 7.7 mL/min or more, 7.8 mL/min or more, 7.9 mL/min or more, 8 mL/min or more, 8.1 mL/min or more, 8.2 mL/min or more, 8.3 mL/min or more, 8.4 mL/min or more, 8.5 mL/min or more, 8.6 mL/min or more, 8.7 mL/min or more, 8.8 mL/min or more, 8.9 mL/min or more, 9 mL/min or more, 9.1 mL/min or more, 9.2 mL/min or more, 9.3 mL/min or more, 9.4 mL/min or more, 9.5 mL/min or more, 9.6 mL/min or more, 9.7 mL/min or more, 9.8 mL/min or more, 9.9 mL/min or more, or 10 mL/min or more.
[0156] After coprecipitating the one or more precursor compounds as the precipitated precursor particles (step 176), the method 170 can include, at step 178, separating at least a portion of the precipitated precursor particles from the liquid component of the reaction mixture. Separating the precipitated precursor particles from the liquid component (step 178) can comprise any method of solid-liquid separation including, but not limited to, filtering the reaction mixture with a filter or screen having a filtration size that is smaller than the expected size of the precipitated precursor particles, centrifugation, and the like. After separating at least the portion of the precipitated precursor particles from the liquid component (step 178), the method 170 can optionally include, at step 180, washing the separated precipitated precursor particles to provide washed precursor particles. In an example, the washing of step 180 can be to remove surfactant from the surfaces of the precipitated precursor particles and/or so that a pH of the washed precursor particles is neutral (e.g., from about 6.5 to about 7.5, e.g., about 7). In an example, the precipitated precursor particles can be washed (step 180) with a washing medium such as, but not limited to, one or any mixture of: water, an alcohol (such as ethanol or isopropanol), or a ketone (such as acetone or methyl ethyl ketone).
[0157] The particles that are separated from the liquid component of the reaction mixture in step 178 and/or the washed particles after the washing of step 180 can comprise the precursor particles 172 that are then converted to catalyst particles, such as via the method 150. In other words, the precursor particles 172 that are the result of the method 170 can be the feedstock precursor particles that are combined with a slurry medium to form a precursor slurry in step 152 of the method 150, followed by the remaining steps of the method 150. Those having skill in the art will appreciate that the precursor particles that are used in step 152 of the method 150 can be formed by synthesis methods other than the coprecipitation-based method 170 described above with respect to
[0158] The method 150 of the present disclosure including the electrochemical oxidation step 158 allows for several advantages over conventional thermal oxidation processes. These advantages can include one or more of the following: (1) the energy requirement that is necessary for the electrochemical oxidation-based method 150 is substantially lower than what is required for conventional thermal oxidation methods; (2) the one or more catalyst compounds that make up the catalyst coating on the electrodes can be both chemically synthesized and activated in a single method step; (3) the method 150 does not require external heating, e.g., to bring the temperature of the one or more precursor compounds up to a calcination temperature; (4) the method 150 is sufficiently adaptable that catalyst coated electrodes can be manufactured by a batch pre-fabrication process, a continuous pre-fabrication process, or an in situ process (described in more detail below) without requiring relatively large or relatively expensive equipment.
[0159] Regarding the energy requirement for the electrochemical oxidation based method 150 compared to conventional thermal oxidation based methods, the inventors have found that the application of an electrochemical processing method for the purposes of electrochemical oxidation of the one or more precursor compounds to form one or more catalyst compounds (step 158 of the method 150) is more energy efficient in that a higher percentage of the electrical energy input ultimately contributes to chemical conversion of the one or more precursor compounds to the one or more catalyst compounds by electrochemical oxidation compared to the percentage of thermal energy that ultimately contributes to the same conversion by thermal oxidation. This higher energy conversion efficiency allows for lower operating costs over time as well as lower capital expenditure required for the equipment necessary to perform the electrochemical oxidation compared to the equipment needed for thermal oxidation. Also, the higher energy conversion efficiency means that the method 150 can result in a lower fuel requirement and, therefore, a smaller greenhouse gas emission footprint.
[0160] In addition to the energy conversion efficiency being better for the electrochemical oxidation based method 150 compared to conventional thermal oxidation processes, in some examples, the method 150 can also allow for electrochemical oxidation to form the one or more catalyst compounds (step 158) and activation of the catalyst particles (step 160) as part of the same unit operation, as described above. Specifically, the electrochemical processing method or methods that are used for the electrochemical oxidation step 158 can also be configured to activate the formed catalyst compound or compounds at substantially the same time that the one or more catalyst compounds or formed or very shortly thereafter. This simultaneous or substantially simultaneous occurrence of the electrochemical oxidation and the catalyst activation still further reduces the energy requirement of the electrochemical oxidation based method 150 compared to conventional thermal oxidation.
[0161] The electrochemical oxidation based method 150 also offers flexibility in how the resulting catalyst coated electrode is manufactured depending on the capacity requirements for the method 150 (e.g., the size and number of catalyst coated electrodes that are to be made by the method 150 over a certain period of time) as well as other considerations.
[0162] In an example, the anode substrate 186 is coated with a precursor coating 192 of precursor particles comprising one or more precursor compounds (such as the precursor coating described above with respect to steps 152-156 of the method 150). The voltage source 190 is configured so that it can apply one or more of the electrochemical processing methods described above for step 158 of the method 150 (e.g., one or more of CA, CP, and CV) to the precursor coated anode substrate 186 and the cathode substrate 188 so as to covert the one or more precursor compounds of the precursor coating 192 to one or more catalyst compounds (e.g., one or more spinel or spinel-like mixed metal oxides, as described above) to form a catalyst-coated electrode comprising the anode substrate 186 coated with a coating of catalyst particles of the one or more catalyst compounds.
[0163] The batch reactor 180 can allow catalyst-coated electrodes to be produced by the method 150 in a batch manner (e.g., one electrode at a time). Batch production of catalyst-coated electrodes can be beneficial if only a small volume of electrodes are required (e.g., if only a single electrode or a small number or size of electrode), and/or if it is desired to have on-demand production of a particular configuration of electrode (e.g., a specified size of electrode, specified material of the electrode substrate, specified catalyst compound or compounds, etc.), and/or if a simplified system and process is desired.
[0164]
[0165] A voltage source 220 is electrically connected to first and second electrodes that are located within the caustic bath 204. In the example shown in
[0166] As the precursor coated substrate 218 is pulled through the caustic bath 204, the substrate 218 comes into contact with the roller anode 212. The voltage source 220 is configured so that as the precursor coated substrate 218 is passed over the roller anode 212, one or more of the electrochemical processing methods described above for step 158 of the method 150 (e.g., one or more of CA, CP, and CV) is applied between the roller anode 212 and the cathode 222 so as to covert the one or more precursor compounds of the precursor coating on the substrate 218 to one or more catalyst compounds (e.g., one or more spinel or spinel-like mixed metal oxides, as described above), which produces a catalyst catalyst-coated substrate 224 comprising the elongated substrate 206 coated with a coating comprising catalyst particles of the one or more catalyst compounds. The catalyst-coated substrate 224 can then be rolled into a product roll 226 for storage. When it is desired to form a catalyst-coated electrode of a specified size, a corresponding amount of the catalyst-coated substrate 224 can be unrolled from the product roll 226 and cut to the specified size before being installed into an electrolyzer cell, such as the example cell 10 or 50 described above.
[0167] In yet another example, electrochemical oxidation (step 158 of the method 150) can be performed in situ simultaneously, substantially simultaneously, or immediately before performing water electrolysis to generate H.sub.2 gas and O.sub.2 gas. As used herein, the term in situ, in relation to the formation of the catalyst coating on an electrode substrate, refers to performing the electrochemical oxidation (step 158) at the same time as, i.e., concurrent with, substantially concurrent with, or very shortly before the electrolysis operation of an electrolyzer cell and while the electrode substrate is installed as one of the electrodes of the electrolyzer cell.
[0168] In an example, in situ electrochemical oxidation can include providing a precursor coated substrate (e.g., by performing steps 152, 154, and optionally 156 of the method 150) and placing the precursor coated substrate into an electrolyzer cell as either the anode or as the cathode. For example, the precursor coated substrate can be placed in the electrolyzer cell 10 of
[0169] During an initial period of operation, the power source can be operated according to one of the electrochemical processing methods that provide for the electrochemical oxidation of step 158 (e.g., CA, CP, CV, or a combination thereof) to convert the one or more precursor compounds of the precursor coating to the one or more catalyst compounds. After the electrochemical oxidation has been completed, the power source can be operated according to a specified procedure for generating H.sub.2 gas at the cathode 26, 54 and O.sub.2 gas at the anode 20, 52 (e.g., at a specified current density, a specified electrolyte flow rate, etc.). In some examples, even during the initial period when the electrochemical oxidation process (e.g., one or more of CA, CP, and CV) is being performed, H.sub.2 gas may be generated at the cathode 26, 54 and O.sub.2 gas may be generated at the anode 20, 52.
EXAMPLES
[0170] Various embodiments of the present invention can be better understood by reference to the following Examples which are offered by way of illustration. The present invention is not limited to the Examples given herein.
Example 1
[0171] A nickel mesh electrode substrate (similar to the mesh substrate of the mesh electrode 100 shown in
Example 2
[0172] The precursor coated substrate from EXAMPLE 1 was placed in a bath of 0.1 M KOH and subjected to cyclic voltammetry (CV) cycling from 1.2 V to 0.65 V (vs. Hg/HgO reference electrode) with a triangular sawtooth waveform with a scan rate of 50 millivolts per second (mV/s) for a total of 10 cycles, resulting in a catalyst coated substrate.
[0173]
Example 3
[0174] A precursor coated substrate identical to that of EXAMPLE 1 was placed in a bath of 0.1 M KOH and subjected to cyclic voltammetry (CV) cycling from 1.2 V to 0.65 V (vs. Hg/HgO reference electrode) with a triangular sawtooth waveform with a scan rate of 50 mV/second for a total of 10 cycles, followed by chronoamperometry (CA) at a steady-state potential of 800 mV (vs. Hg/HgO reference electrode) for 2 minutes.
Comparative Example 4
[0175] A precursor coated substrate identical to that of EXAMPLE 1 was subjected to thermal oxidation by ramping up the temperature of the precursor coated substrate from room temperature to 325 C. at a heating rate of 4 C. per minute. The coated substrate was then held at 325 C. for four (4) hours to produce a thermally oxidized catalyst coated substrate. The catalyst coated substrate was then allowed to cool by air cooling back down to room temperature.
[0176]
[0177]
[0178]
[0179] The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as examples. Such examples can include elements in addition to those shown or described. However, the present inventors also contemplate examples in which only those elements shown or described are provided. Moreover, the present inventors also contemplate examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein.
[0180] In the event of inconsistent usages between this document and any documents so incorporated by reference, the usage in this document controls.
[0181] In this document, the terms a or an are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of at least one or one or more. In this document, the term or is used to refer to a nonexclusive or, such that A or B includes A but not B, B but not A, and A and B, unless otherwise indicated. In this document, the terms including and in which are used as the plain-English equivalents of the respective terms comprising and wherein. Also, in the following claims, the terms including and comprising are open-ended, that is, a system, device, article, composition, formulation, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms first, second, and third, etc. are used merely as labels, and are not intended to impose numerical requirements on their objects.
[0182] Method examples described herein can be machine or computer-implemented at least in part. Some examples can include a computer-readable medium or machine-readable medium encoded with instructions operable to configure an electronic device to perform methods as described in the above examples. An implementation of such methods can include code, such as microcode, assembly language code, a higher-level language code, or the like. Such code can include computer readable instructions for performing various methods. The code may form portions of computer program products. Further, in an example, the code can be tangibly stored on one or more volatile, non-transitory, or non-volatile tangible computer-readable media, such as during execution or at other times. Examples of these tangible computer-readable media can include, but are not limited to, hard disks, removable magnetic disks, removable optical disks (e.g., compact disks and digital video disks), magnetic cassettes, memory cards or sticks, random access memories (RAMs), read only memories (ROMs), and the like.
[0183] The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. 1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description as examples or embodiments, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various combinations or permutations. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.