Water electrolyzers employing anion exchange membranes

Abstract

A water electrolyzer comprises an electrolyzer stack comprising at least two electrochemical cells. Each cell comprises an anion exchange membrane, a base metal anode electrocatalyst, a base metal cathode electrocatalyst, and a sufficiently long ion conduction path between adjacent cells such that shunt currents are less than 1% of the total current supplied to the stack.

Claims

1. A water electrolyzer for producing hydrogen gas comprising an electrolyzer stack, the stack comprising at least two electrochemical cells, each of said cells comprising: (a) an anode comprising an anode gas diffusion layer (GDL) and a quantity of anode electrocatalyst; (b) a cathode comprising a cathode GDL and a quantity of cathode electrocatalyst; (c) a source of a solution comprising water directed to at least one of said anode and said cathode; (d) an anion exchange membrane interposed between said anode and said cathode, wherein each of said anion exchange membranes has an average thickness (t) and, wherein a shortest ion conduction path (L.sub.GDLA) between the anode GDL of any two of the cells in the stack is: L.sub.GDLA>10*t*σ.sub.sol/σ.sub.mem where σ.sub.sol is the conductivity (S/cm) of the solution at 60° C. and σ.sub.mem is the conductivity of each of said membranes measured in 1 M KOH at 60° C., wherein L.sub.GDLA>3 cm.

2. The water electrolyzer of claim 1, wherein:
L.sub.GDLA>50*t*σ.sub.sol/σ.sub.mem.

3. The water electrolyzer of claim 1, wherein the shortest ion conduction path (L.sub.GDLC) between the cathode GDL of any two of the cells in the stack is:
L.sub.GDLC>10*t*σ.sub.sol/σ.sub.mem.

4. The water electrolyzer of claim 1, wherein said solution has a pH greater than 8 at 25° C.

5. The water electrolyzer of claim 4, wherein said solution has a pH greater than 12 at 25° C.

6. The water electrolyzer of claim 1, wherein said solution has a conductivity greater than 0.05 S/cm at 25° C.

7. The water electrolyzer of claim 1, wherein said cathode electrocatalyst in each of said cells is in the form of unsupported particles or particles supported on an electrically conductive support, said particles having an average size between 0.6 nm and 100 nm.

8. The water electrolyzer of claim 1, wherein said anode electrocatalyst in each of said cells comprises a base metal catalyst.

9. The water electrolyzer of claim 8, wherein said anode electrocatalyst in each of said cells consists essentially of one or more base metal catalysts.

10. The water electrolyzer of claim 1, wherein said cathode electrocatalyst in each of said cells comprises a base metal catalyst.

11. The water electrolyzer of claim 10, wherein said cathode electrocatalyst in each of said cells consists essentially of one or more base metal catalysts.

12. The water electrolyzer of claim 1, wherein said membrane in each of said cells comprises positively charged cyclic amines.

13. The water electrolyzer of claim 12, wherein said positively charged amines are imidazoliums, piperidiniums or pyridiniums.

14. The water electrolyzer of claim 1, wherein said cathode in each of said cells comprises an ionomer.

15. An AEM water electrolyzer for converting water into substantially O.sub.2 and H.sub.2 comprising: an electrolyzer stack, said electrolyzer stack comprising at least two electrochemical cells, each of said cells comprising: (a) an anode comprising an anode gas diffusion layer (GDL) and a quantity of anode electrocatalyst; (b) a cathode comprising a cathode GDL and a quantity of cathode electrocatalyst; (c) a source of a solution comprising water directed to at least one of said anode and said cathode; (d) an anion exchange membrane interposed between said anode and said cathode, wherein a shortest ion conduction path (L.sub.GDLA) between the anode GDL of any of the two cells of said at least two electrochemical cells in the stack is greater than 3 cm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a perspective view of an electrolyzer stack comprising three electrolyzer cells 101, 102 and 103.

(2) FIG. 2 is an exploded side view of an electrolyzer cell assembly including a membrane electrode assembly interposed between two fluid flow field plates having reactant flow channels formed in the major surfaces of the plates facing the electrodes.

(3) FIG. 3. is a perspective view, partially transparent, of a section of the electrolyzer stack of FIG. 1, with the minimum ion conduction path between the anode 101a and the anode 102b indicated as path 200.

(4) FIG. 4. is a perspective view of the water electrolyzer built according to the description in Specific Example 1.

(5) FIG. 5. is an exploded perspective view of a 5-cell electrolyzer stack with internal fluid connections. The top cell of the stack has been further expanded to reveal the internal components.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT(S)

(6) It is understood that the process is not limited to the particular methodology, protocols and reagents described herein, as these can vary as persons familiar with the technology involved here will recognize. It is also to be understood that the terminology used herein is used for the purpose of describing particular embodiments only, and is not intended to limit the scope of the process. It also is to be noted that as used herein and in the appended claims, the singular forms “a,” “an,” and “the” include the plural reference unless the context clearly dictates otherwise. Thus, for example, a reference to “a linker” is a reference to one or more linkers and equivalents thereof known to those familiar with the technology involved here. Similarly, the phrase “and/or” is used to indicate one or both stated cases can occur, for example, A and/or B includes (A and B) and (A or B).

(7) Unless defined otherwise, technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which the process pertains. The embodiments of the process and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments and/or illustrated in the accompanying drawings and detailed in the following description. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale, and features of one embodiment can be employed with other embodiments as a person familiar with the technology here would recognize, even if not explicitly stated herein.

(8) Any numerical value ranges recited herein include all values from the lower value to the upper value in increments of one unit, provided that there is a separation of at least two units between any lower value and any higher value. As an example, if it is stated that the concentration of a component or value of a process variable such as, for example, size, angle size, pressure, time and the like, is, for example, from 1 to 98, specifically from 20 to 80, more specifically from 30 to 70, it is intended that values such as 15 to 85, 22 to 68, 43 to 51, 30 to 32, and the like, are expressly enumerated in this specification. For values which are less than one, one unit is considered to be 0.0001, 0.001, 0.01 or 0.1 as appropriate. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value are to be treated in a similar manner.

(9) Moreover, provided immediately below is a “Definitions” section, where certain terms related to the process are defined specifically. Particular methods, devices, and materials are described, although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the process.

Definitions

(10) The term “polymer electrolyte membrane” as used herein refers to both cation exchange membranes, which generally comprise polymers having multiple covalently attached negatively charged groups, and anion exchange membranes, which generally comprise polymers having multiple covalently attached positively charged groups. Typical cation exchange membranes include proton conducting membranes, such as the perfluorosulfonic acid polymer available under the trade designation NAFION from E. I. du Pont de Nemours and Company (DuPont) of Wilmington, Del.

(11) The term “anion exchange membrane electrolyzer” as used herein refers to an electrolyzer with an anion-conducting polymer electrolyte membrane separating the anode from the cathode.

(12) The term “Hydrogen Evolution Reaction,” abbreviated “HER,” as used herein refers to the electrochemical reaction 2H.sup.++2e.sup.−.fwdarw.H.sub.2.

(13) The term “MEA” as used herein refers to a membrane electrode assembly.

(14) The term “UEA” as used herein refers to a unitized electrode assembly.

(15) The term “Millipore water” as used herein refers to water that is produced by a Millipore filtration system with a resistivity of at least 18.2 megaohm-cm.

(16) The term “imidazolium” as used herein refers to a positively charged ligand containing an imidazole group. This includes a bare imidazole or a substituted imidazole. Ligands of the form:

(17) ##STR00001##
where R.sub.1-R.sub.5 are each independently selected from hydrogen, halides, linear alkyls, branched alkyls, cyclic alkyls, heteroalkyls, aryls, heteroaryls, alkylaryls, heteroalkylaryls, and polymers thereof, such as the vinyl benzyl copolymers described herein, are specifically included.

(18) The term “pyridinium” as used herein refers to a positively charged ligand containing a pyridine group. This includes a bare pyridine or a substituted pyridine. Ligands of the form:

(19) ##STR00002##
where R.sub.6-R.sub.11 are each independently selected from hydrogen, halides, linear alkyls, branched alkyls, cyclic alkyls, heteroalkyls, aryls, heteroaryls, alkylaryls, heteroalkylaryls, and polymers thereof, such as the vinyl benzyl copolymers described herein, are specifically included.

(20) The term “phosphonium” as used herein refers to a positively charged ligand containing phosphorous. This includes substituted phosphorous. Ligands of the form:
P.sup.+(R.sub.12R.sub.13R.sub.14R.sub.15)
where R.sub.12-R.sub.15 are each independently selected from hydrogen, halides, linear alkyls, branched alkyls, cyclic alkyls, heteroalkyls, aryls, heteroaryls, alkylaryls, heteroalkylaryls, and polymers thereof, such as the vinyl benzyl copolymers described herein, are specifically included.

(21) The term “piperidinium” as used herein refers to a positively charged ligand of the form:

(22) ##STR00003##
where R.sub.19-R.sub.25 are each independently selected from hydrogen, halides, linear alkyls, branched alkyls, cyclic alkyls, heteroalkyls, aryls, heteroaryls, alkylaryls, heteroalkylaryls, and polymers thereof, such as the vinyl benzyl copolymers described herein, are specifically included.

(23) The term “positively charged cyclic amine” as used herein refers to a positively charged ligand containing a cyclic amine. This specifically includes imidazoliums, pyridiniums, pyrazoliums, pyrrolidiniums, pyrroliums, pyrimidiums, piperidiniums, indoliums, triaziniums, and polymers thereof, such as the vinyl benzyl copolymers described herein.

(24) The term “simple amine” as used herein refers to a species of the form:
N(R.sub.16R.sub.17R.sub.18)
where R.sub.16-R.sub.18 are each independently selected from hydrogen, linear alkyls, branched alkyls, cyclic alkyls, heteroalkyls, aryls, heteroaryls, alkylaryls, and heteroalkylaryls, but not polymers.

(25) The term “precious metal” as used herein refers to one or more of Ru, Rh, Pd, Ag, Re, Os, Ir, Pt, and Au.

(26) The term “base metal catalyst” as used herein refers to a catalyst that is substantially free of precious metals.

(27) The term “shunt current” as used herein refers to a portion of the electrical current applied to the stack that follow a path through the fluid connections between adjacent cells rather than through the membrane. The current can be transferred through the fluid in the connections or through the conduits and fittings.

(28) The term “bipolar plate” as used herein is a single electrically conductive plate that serves as the cathode or cathode flow field for one cell, and an anode or anode flow field for an adjacent cell in a stack.

(29) The term “shortest ion conduction path between anodes of adjacent cells” as used herein refers to the shortest ion conduction path from the anode of a given cell and to the anode of an adjacent cell.

Specific Example 1

(30) The objective of this example is to provide an electrolyzer design that has low shunt current when fed solutions with a conductivity above 0.1 S/cm.

(31) FIG. 1 shows a diagram of a three cell electrolyzer stack. The stack consists of: (a) electrochemical cell 101 comprising anode 101a and cathode 101c, (b) electrochemical cell 102 comprising anode 102a and cathode 102c, (c) electrochemical cell 103 comprising anode 103a and cathode 103c, (d) anode inlet manifold 110, (e) pipes 111, 112 and 113 connecting the anode inlet manifold 110 to respective anodes 101a, 102a and 103a, (f) cathode inlet manifold 120, (g) pipe 121 connecting cathode inlet manifold 120 to cathode 101c. There are also pipes (hidden in FIG. 1), numbered 122 and 123, that connect cathode inlet manifold 120 to respective cathodes 102c and 103c, (h) anode outlet manifold 130, (i) pipes 131, 132 and 133 connecting anode outlet manifold 130 to respective anodes 101a, 102a and 103a, (j) cathode outlet manifold 140, and (k) pipes 141, 142 and 143 connecting cathode outlet manifold 140 to respective cathodes 101c, 102c and 103c.

(32) Cathode 101c is in electrical contact with anode 102a. Cathode 102c is in electrical contact with anode 103a. FIG. 1 shows adjacent anodes and cathodes (for example, cathode 101c and anode 102a), as separate components, but the adjacent components could be combined into a single bipolar plate.

(33) FIG. 2 illustrates an exploded version of one of the cells in the stack illustrated in FIG. 1. Cell 30 includes a membrane electrode assembly 32 interposed between rigid flow field plates 34 and 36. The membrane electrode assembly 32 comprises an anion exchange membrane 42 interposed between two gas diffusion layers (GDLs), namely, anode GDL 44 and cathode GDL 46. Anode GDL 44 and cathode GDL 46 are typically formed of porous electrically conductive sheet material such as carbon fiber paper or nickel fiber paper, and have planar major surfaces. Electrodes 44 and 46 have a thin layer of catalyst material disposed on their major surfaces at the interface with membrane 42 to render them electrochemically active. Anode flow field plate 34 has at least one open faced channel 34a formed in its major surface facing membrane electrode assembly 32. Similarly, cathode flow field plate 36 has at least one open faced channel 36a formed in its major surface facing membrane electrode assembly 32. When assembled against the cooperating surfaces of electrodes 44 and 46, channels 34a and 36a form the reactant flow field passages for the anode reactant (fuel) stream and cathode reactant (oxidant) stream, respectively.

(34) The cells can also contain a metal mesh, a perforated metal sheet, or metal foam between the GDL and the flow plates. Optimal openings are between 1 and 20 mm. The flow plates can have a flow channel machined into their body or flat plates can be used, and the metal mesh or metal plate serves as a flow channel.

(35) Alternatively, the catalyst material can be deposited directly on the membrane.

(36) In the case of an AEM water electrolyzer, anion exchange membrane 42 (see FIG. 2) comprises a hydrocarbon backbone such as a copolymer of styrene and chloromethyl styrene, and a positively charged amine. Positively charged cyclic amines, such as imidazoliums, pyridiniums and piperidiniums are typically employed.

(37) U.S. Pat. No. 9,340,882 teaches that the catalyst layer on the cathode should not contain any ionomer, but superior performance has been found in the present electrolyzer when an ionomer is used. Superior performance has also been found when membrane 42 and electrodes 44 and 46 are bound together to form a unified electrode assembly (UEA).

(38) During operation, a mixture comprising water and a base is fed into the anode and/or cathode inlet manifolds. The solution flows through the cell and out through the outlet manifolds. The solution typically has a pH above 8 and may contain sea water, salt and other components.

(39) If high currents of 1 A/cm.sup.2 or more are desired, the solution should have a pH of at least 12.

(40) Experimentally, it has been found that suitable performance occurs when the conductivity of the solution is above 0.05 S/cm, with the best performance occurring when the solution conductivity is between 0.1 and 0.4 S/cm

(41) Once the solution is flowing, a voltage is applied between the anode of one of the cells (for example, anode 101a) and the cathode of a different cell (for example, cathode 103c) to promote a desired electrochemical reaction. In an AEM water electrolyzer, the following reaction occurs at the cathode:
2H.sub.2O+2e.sup.−.fwdarw.H.sub.2+2OH.sup.−  (1)
The hydroxyls then travel through the membrane where they react at the anode to produce oxygen:
2OH.sup.−.fwdarw.½O.sub.2+H.sub.2O+2e.sup.−  (2)
The net reaction becomes:
H.sub.2O.fwdarw.½O.sub.2+H.sub.2  (3)

(42) It would be particularly desirable for all of the current supplied to the cell to go toward producing hydrogen and oxygen, but in practice a portion of the current, called the “shunt current”, bypasses the MEA. For example, if the solution flowing into the inlet manifold 110 in FIG. 1 is a good electrical conductor, then current can flow from anode 101a, out the pipe 111, into the manifold 110, and back into pipe 112, and then to anode 102a, bypassing the MEA in cell 101. Current that bypasses the MEA is wasted since it does not produce any reaction product(s), so eliminating or mitigating the shunt current is desirable.

(43) The previous patent literature describes several ways to avoid shunt current in electrolyzers. Generally, the approach has been to include a polytetrafluoroethylene (PTFE) or other hydrophobic material in the flow path between adjacent cells. Gas bubbles generated during the electrochemical process build up in the flow path, thereby blocking the shunt current. Such an approach works when sufficient gas builds up near the inlet and outlet of the cell but that approach does not work for commercial scale AEM water electrolyzers because there are few or no bubbles near the inlet to the cathode and anode of the cell and the solution flowrate is high, so bubbles do not accumulate. So a different approach to reduce the shunt current is needed.

(44) The AEM electrolyzer design of FIG. 1 employs two different approaches to reduce the shunt current: (a) The connecting tubes (pipes) 111, 112, 113, 121, 122, 123, 131, 132, 133, 141, 142 and 143 are all formed of non-conducting materials, thereby eliminating a path for shunt current via electron conduction through the metal. (b) The ionic path back through the manifold is long, so that very few ions flow from anode 101a to anode 102a.

(45) FIG. 3 illustrates the shortest ion conduction path between the anodes of cells 101 and 102. In this case, the shortest conduction path 200 extends from the point on the anode next to the fitting, down through pipe 111, through the manifold 110 and back up through pipe 112.

(46) It has been found that the shunt current can be lowered to below 5% of the cell current if, for each of the cells in the stack, the length of the shortest ion conduction path (L.sub.ano) between anodes of different cells in the stack is:
L.sub.ano>10*t*σ.sub.sol/σ.sub.mem  (4)
where t is the membrane thickness, σ.sub.sol is the conductivity of the solution being fed into the anode in S/cm, and σ.sub.mem is the ion conductivity of the membrane measured in the solution at the operating temperature. Typically, σ.sub.mem is measured in 1 M KOH at 60° C.

(47) L.sub.ano is calculated by the following method: (a) Number the cells: 1, 2, 3, . . . ; (b) Make a list of all of the pairs of cells; (c) Calculate the minimum ion conduction path (L.sub.ano(i,j),) between the anode of cell i and the anode of cell j, for all of the pairs of cells in the stack; (d) L.sub.ano equals the minimum value of all of the L.sub.ano(i,j) values calculated.

(48) For example, if a stack has 4 cells numbered 1, 2, 3, and 4, then there are six pairs of cells (1,2), (1,3), (1,4), (2,3), (2,4), (3,4). L.sub.ano(1,2), L.sub.ano(1,3), L.sub.ano(1,4), L.sub.ano(2,3), L.sub.ano(2,4), and L.sub.ano(3,4) would then be calculated or measured. L.sub.ano=minimum(L.sub.ano(1,2), L.sub.ano(1,3), L.sub.ano(1,4), L.sub.ano(2,3), L.sub.ano(2,4), L.sub.ano(3,4)).

(49) Similarly, the shunt current between adjacent anodes will typically be less than 1% of the cell current if:
L.sub.ano>50*t*σ.sub.sol/σ.sub.mem  (5)

(50) Shunt current between cathodes must also be considered. The shunt current between adjacent cathodes will be less than 5% of the cell current if:
L.sub.cat>10*t*σ.sub.sol/σ.sub.mem  (6)
where L.sub.cat is the length of the shortest ion conduction path between the cathodes of two different cells in the stack calculated by the following method: (a) Number the cells: 1, 2, 3, . . . ; (b) Make a list of all of the pairs of cells; (c) Calculate the minimum ion conduction path (L.sub.cat(i,j)) between the cathode of cell i and the cathode of cell j for all of the pairs of cells in the stack; (d) L.sub.cat equals the minimum value of all of the L.sub.cat(i,j) values calculated.

(51) Similarly, the shunt current between adjacent cathodes will be less than 1% of the cell current if:
L.sub.cat>50*t*σ.sub.sol/σ.sub.mem  (7)

(52) FIG. 4 is an illustration of the actual AEM water electrolyzer. Note that the electrolyzer has long tubes 221, 222 and 223 to connect cells 201, 202 and 203 to cathode header 220. The long tubes minimize, or at least mitigate, the shunt current.

Specific Example 2

(53) Anode inlet manifold 110 in FIG. 1 is external to the stack. Specific Example 2 describes an alternate design for an AEM water electrolyzer in which the inlet manifold is internal to the stack and insulated from the metal plates.

(54) FIG. 5 illustrates such a design with an internal inlet manifold. The stack shown includes cells 301, 302, 303, 304 and 305. Cell 305 includes: (a) a bipolar plate 355 that serves as the anode of a given cell and the cathode of an adjacent cell; (b) a plastic frame 365; (c) an opening 310 through the plastic frame that serves as an anode header; (d) a notch 315 that serves as a connection between anode header 310 and anode GDL 375; (e) an insulating gasket 385 that isolates bipolar plate 355 from anode header 310 and the notch 315; and (f) a second hole 320 that serves as a cathode header.

(55) In such a case, the shortest ion conduction path may not be between adjacent anodes/bipolar plates. Instead, the shortest ion conduction path may be between the anode GDL in one cell, and the anode GDL in a second cell. In that case, the equations 4, 5, 6 and 7 need to be replaced by
L.sub.GDLA>10*t*σ.sub.sol/σ.sub.mem  (8)
L.sub.GDLA>50*t*σ.sub.sol/σ.sub.mem  (9)
L.sub.GDLC>10*t*σ.sub.sol/σ.sub.mem  (10)
L.sub.GDLC>50*t*σ.sub.sol/σ.sub.mem  (11)
where L.sub.GDLA is the length of the shortest ion conduction path between the anode GDL of one cell in the stack and the anode GDL in another cell in the stack. L.sub.GDLC is the length of the shortest ion conduction path between the cathode GDL of one cell in the stack and the cathode GDL in another cell in the stack.

(56) L.sub.GDLA is calculated by the following method: (a) Number the cells: 1, 2, 3, . . . ; (b) Make a list of all of the pairs of cells; (c) Calculate the minimum ion conduction path (L.sub.GDLA(i,j)) between the anode GDL of cell i and the anode GDL of cell j for all of the pairs of cells in the stack; (d) L.sub.GDLA equals the minimum value of all of the L.sub.GDLA(i,j) values calculated.

(57) L.sub.GDLC is calculated by the following method: (a) Number the cells: 1, 2, 3, . . . ; (b) Make a list of all of the pairs of cells; (c) Calculate the minimum ion conduction path (L.sub.GDLC(i,j)) between the cathode GDL of cell i and the cathode GDL of cell j for all of the pairs of cells in the stack; (d) L.sub.GDLC equals the minimum value of all of the L.sub.GDLDC(i,j) values calculated.

(58) If equations 9 and 11 are satisfied, the shunt current will be less than 1% of the total cell current.

(59) While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood that the invention is not limited thereto since modifications can be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings.

(60) The examples given above are merely illustrative and are not meant to be an exhaustive list of all possible embodiments, applications or modifications of the present electrochemical device. Thus, various modifications and variations of the described methods and systems of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention which are obvious to those skilled in the chemical arts or in the relevant fields are intended to be within the scope of the appended claims.