Nickel cobalt phosphide-based nanoparticles and methods of making, and electrochemical systems and methods

Abstract

The present disclosure is directed to cobalt and nickel cobalt phosphide/phosphate electrocatalyst nanoparticles for catalyzing electrochemical reactions, such as water splitting. The nanoparticles are formed into electrodes that have bi-functional oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) capabilities.

Claims

1. A electrode, comprising: agglomerated nanoparticles; and a conductor; wherein the nanoparticles consist essentially of a metal, phosphorus and oxygen; and wherein the metal is a mixture of nickel and cobalt having a cobalt to nickel ratio in the range of 2:1 to 100:1; and wherein the cobalt to phosphorus ratio is between 0.8:1 and 1.2:1; and wherein the nanoparticles have a phosphide core surrounded by an oxide; and wherein the nanoparticles also have a phosphate phase.

2. An electrochemical device, comprising: an electrode, comprising: agglomerated nanoparticles; and a conductor; wherein the nanoparticles consist essentially of a metal, phosphorus and oxygen; and wherein the metal is a mixture of nickel and cobalt having a cobalt to nickel ratio in the range of 2:1 to 100:1; and wherein the cobalt to phosphorus ratio is between 0.8:1 and 1.2:1; and wherein the nanoparticles have a phosphide core surrounded by an oxide; and wherein the nanoparticles also have a phosphate phase.

3. The device of claim 2, further comprising: an aqueous electrolyte.

4. A method, comprising: energizing an electrode in an aqueous electrolyte; wherein the electrode comprises: agglomerated nanoparticles; and a conductor; wherein the nanoparticles consist essentially of a metal, phosphorus and oxygen; and wherein the metal is a mixture of nickel and cobalt wherein the metal is a mixture of nickel and cobalt having a cobalt to nickel ratio in the range of 2:1 to 100:1; and wherein the cobalt to phosphorus ratio is between 0.8:1 and 1.2:1; and wherein the nanoparticles have a phosphide core surrounded by an oxide; and wherein the nanoparticles also have a phosphate phase.

5. A water splitting method, comprising: energizing an electrode in an aqueous electrolyte; wherein the electrode comprises: agglomerated nanoparticles; and a conductor; wherein the nanoparticles consist essentially of a metal, phosphorus and oxygen; and wherein the metal is a mixture of nickel and cobalt wherein the metal is a mixture of nickel and cobalt having a cobalt to nickel ratio in the range of 2:1 to 100:1; and wherein the cobalt to phosphorus ratio is between 0.8:1 and 1.2:1; and wherein the nanoparticles have a phosphide core surrounded by an oxide; and wherein the nanoparticles also have a phosphate phase.

6. The method of claim 5, further comprising: generating hydrogen gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The figures depict embodiments of the present invention for purposes of illustration only, and are not necessarily drawn to scale. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein.

(2) FIG. 1 discloses a synthesis scheme for producing CPP and NCPP nanoparticle electrocatalysts according to an embodiment of the disclosure.

(3) FIG. 2 discloses an electrolyzer according to an embodiment of the disclosure.

DETAILED DESCRIPTION OF THE INVENTION

(4) The present disclosure is directed to compositions, methods of making, and systems and method for electrolysis that use high surface area, high bifunctional activity, nanoparticle electrocatalysts based on earth abundant, low-cost materials. The electrolysis may be a water splitting reaction.

(5) As used herein, the term nanoparticle includes particles having at least one cross-sectional dimension between 1 and 100 nanometers (nm) in size. In an embodiment, the particles are substantially spherical and have a particle size of between 1 nm and 100 nm. In an embodiment, the particles have a rod geometry and have an axial cross-section of between 1 nm and 100 nm. In an embodiment, the nanoparticles may have a particle size between 5 nm and 25 nm.

(6) The nanoparticle electrocatalysts include cobalt (Co), phosphorus (P) and oxygen (O) in one or more material phases. In an embodiment, the nanoparticle electrocatalysts may include Ni. The material phases may include one or more of phosphide, phosphate and oxygen containing phases. The nanoparticle electrocatalysts as initially synthesized are a Co or NiCo phosphide, however, upon oxidation with air and/or oxidation during an oxygen evolution reaction (OER), or by reaction with electrolyte, oxides and/or attachments to hydroxyl (OH) groups are formed that introduce or attach oxygen into the nanoparticle electrocatalysts.

(7) The nanoparticle electrocatalysts have a Co to P stoichiometry ratio (all ratios in this disclosure are stoichiometric and will be referred to hereinafter as ratio) of between 1:1 to 2:1. In an embodiment, the Co to P ratio may be between 0.8:1 and 1.2:1.

(8) The nanoparticle electrocatalysts have a nickel to cobalt range of between >0:1 to 1:1. In an embodiment, the ratio may be between Ni.sub.0.05Co.sub.2.95 and Ni.sub.1Co.sub.2. In an embodiment, the composition has a nickel to cobalt range of between Ni.sub.0.05Co.sub.2.95 and Ni.sub.1Co.sub.2 in a phosphide particle. In an embodiment, the nanoparticle electrocatalysts may be phosphide nanoparticles. In an embodiment, the nanoparticles may include an oxide phase. In an embodiment, the phosphide particle may have an oxide or hydroxide coating. In another embodiment, the nanoparticles may be oxide particles having phosphide phases having nickel to cobalt stoichiometry ranging from Ni.sub.0.05Co.sub.2.95O.sub.4 to Ni.sub.1Co.sub.2O.sub.4 or Ni.sub.0.02Co.sub.0.98O to Ni.sub.0.33Co.sub.0.66O.

(9) In an embodiment, Ni is added in the range of Co:Ni of 2:1 to 100:1 to provide for a resulting NixCoyP catalysts with higher surface areas and catalytic active site densities. In another embodiment, Ni is added in the ratio of Co:Ni of 19:1 to provide for a resulting NixCoyP catalyst with higher surface areas and catalytic active site densities.

(10) In another embodiment, Ni is added in the range Co:Ni of 2:1 to 100:1 to provide for an electrocatalyst material with lower overpotentials for both the OER and HER reactions and provides for a better bifunctional electrocatalyst material in that it can perform both the HER and the OER reaction. In another embodiment, Ni is added in the ratio of Co:Ni of 19:1 to provide for an electrocatalyst material with lower overpotentials for both the OER and HER reactions and provides for a better bifunctional electrocatalyst material in that it can perform both the HER and the OER reaction.

(11) In another embodiment, Ni is added in the range of Co:Ni of 2:1 to 100:1 to form nickel cobalt phosphide/phosphate nanoparticles (NCPP) that provide for a more active electrocatalyst such that less electrocatalyst material can be used to achieve the same activity as without nickel, cobalt phosphide/phosphate (CPP). In another embodiment, Ni is added in the ratio of Co:Ni of 19:1 provides for a more active electrocatalyst such that less electrocatalyst material can be used to achieve the same activity as without nickel (CPP).

(12) In an embodiment, the CPP and/or NCPP particles are oxidized by exposure to air to form an oxide. In an embodiment, the particle may be or may include an oxide phase containing phosphorus. In an embodiment, the oxide may be formed on the particle outer surface. In an embodiment, the particles may include a hydroxide from exposure to water splitting reaction kinetics.

(13) The present disclosure is further directed to methods for forming cobalt phosphide/phosphate (CPP) particles and cobalt oxide particles. In an embodiment the methods are further directed to forming nickel cobalt phosphide/phosphate (NCPP) particles and nickel cobalt oxide particles. According to an embodiment of the disclosure, a method for forming nickel cobalt phosphide nanoparticles is disclosed that includes providing or preparing precursor Co.sub.3O.sub.4 nanoparticles hydrothermally. In an embodiment, the CPP and/or NCPP particles are oxidized by exposure to air to form an oxide. In an embodiment, the particles may include an oxide phase. In an embodiment the oxide may be formed on the particle outer surface by exposure to air. In an embodiment, the particles may include a hydroxide from exposure to water splitting reaction and/or electrolytes.

(14) FIG. 1 discloses a synthesis scheme for producing CPP and NCPP nanoparticle electrocatalysts according to an embodiment of the disclosure. As can be seen in FIG. 1, the CPP nanoparticles have a rod structure, while the NCPP nanoparticles has a round or spherical structure. CPP and NCPP are prepared from cobalt oxide or nickel cobalt oxide, respectively, by a phosphidation reaction with phosphine gas.

(15) Referring to FIG. 1, Step (1) shows the formation of spinel Co.sub.3O.sub.4 prepared using an aqueous solvothermal reaction according to an embodiment of the disclosure. In an embodiment cobalt acetate and ammonium hydroxide are mixed together and then heated at 175? C. for 24 h in an autoclave. After reaction the autoclave is cooled and the Co.sub.3O.sub.4 is washed with a mixture of water and ethanol and then then dried using a rotary evaporator to isolate the Co.sub.3O.sub.4 nanoparticles and then dried in vacuo overnight. Less than 5% CoO phase may be present along with the spinel Co.sub.3O.sub.4, although this is not required.

(16) In other embodiments, other cobalt oxide synthesis methods may be used, such as, but not limited to solution precipitation reactions at ambient pressure, mechanochemical, thermal, pyrolysis and sonochemical methods. In other embodiments, the cobalt oxide may be other than spinel.

(17) Referring to FIG. 1, Step (2) shows an ion exchange procedure to partially replace some of the cobalt ions with nickel ions according to an embodiment of the disclosure. In an embodiment, Ni.sup.2+ incorporation (Ni.sub.xCo.sub.3-xO.sub.4) (where x is between 0.05 and 1) may be achieved by using an ion exchange process. In an embodiment, x may be between 0.05 and 0.25 Less than 5% CoO phase may be present along with the spinel, although this is not required. Transmission electron microscopy (TEM) images of the precursor Co.sub.3O.sub.4 nanoparticles demonstrated roughly spherical or cubic shaped particles with sizes of approximately 20 nm or less. Dispersion of the Co.sub.3O.sub.4 nanoparticles in an aqueous NiCl.sub.2 solution, followed by a simple workup, provided Ni-doped Co.sub.3O.sub.4 with a Co:Ni ratio of 20.5:1 (i.e. Ni.sub.0.15Co.sub.2.85O.sub.4), as determined from acid digestion/inductively coupled plasma-mass spectrometry (ICP-MS) analysis. Ni.sub.0.15Co.sub.2.85O.sub.4 particles were obtained with retention of size, phase and morphology. In an embodiment, nickel cobalt oxide may be prepared directly using the methods outlined above or by including Ni salts along with cobalt salts in the initial synthesis. In other embodiments, other nickel cobalt oxide synthesis methods may be used, such as, but not limited to solution precipitation reactions at ambient pressure, mechanochemical, thermal, pyrolysis and sonochemical methods.

(18) FIG. 1, Step (3) shows the conversion to the CPP or NCPP material: In an embodiment the cobalt oxide or cobalt nickel oxide precursor is heated rapidly in a furnace to 300? C. under flowing argon or nitrogen containing phosphine gas and held at 300? C. for 2 hours. In an embodiment, the phosphine gas may be generated in situ from the thermal decomposition of sodium hypophosphite. In an embodiment the sodium hypophosphite can be added directly to the oxide powder and the mixture is heated. In an embodiment the sodium hypophosphite can be placed upstream in the reaction chamber so that it evolves as a gas and the carrier gas of argon or nitrogen carries it to the oxide material where it reacts to form the CPP or NCPP.

(19) In other embodiments, other CPP and NCPP synthesis methods may be used, such as, but not limited to solution precipitation reactions at ambient pressure, mechanochemical, thermal, pyrolysis and sonochemical methods.

(20) The present disclosure is further directed to an electrode that incorporates the nanoparticle electrocatalysts. In an embodiment, an electrode is disclosed that includes agglomerated nanoparticle electrocatalysts. The electrodes are porous to maximize surface area exposure of the nanoparticle electrocatalysts. The nanoparticle electrocatalysts may be agglomerated by pressing, rolling, pressing and heating, calendaring or other agglomeration methods using the disclosed nanoparticle electrocatalysts. The electrodes include a support current collector, such a conductive mesh, wire or other support structure as known in the art. In an embodiment, the nanoparticles may be agglomerated with or without the use of a binder, such as, but not limited to hydrophobic polymers such as Teflon or may use charged polymers such as anionic (negatively charged) polymers such as Nafion or cationic polymers such as positively charged polyphenylenes, polysulfones, polyacrylamides, polyphenyleneoxides, which may be later removed. In an embodiment, the electrode may include conductive carbon such as graphite, amorphous carbon, carbon nanotubes, graphene, other high surface area carbons or other conductive materials, such as but not limited to metals, that give an overall composite with higher conductivity and lower the overpotential for the electrocatalytic OER and HER reactions.

(21) The choice of binder and carbon can be used to tune the electrode hydrophilicity, that is the electrode can be tuned from one that is hydrophilic in nature to one that is hydrophobic.

(22) The present disclosure is also directed to an electrolyzer including the disclosed agglomerated nanoparticle electrocatalyst electrode. FIG. 2 illustrates an electrochemical cell 200 according to an embodiment of the disclosure. In this exemplary embodiment, the electrochemical cell 200 is an electrolyzer includes a cell housing 205, electrodes 210 including a first or anode electrode 210 and a second or cathode electrode 210B, a separation membrane 230, an electrolyte 240 and a power supply 250. The anode and/or cathode electrodes 210A, 210B, which may be referred to as anode 210A and cathode 210B, respectively may be formed of the nanoparticle electrocatalysts of the present disclosure, formed into an electrode as discussed above. In this exemplary embodiment, the electrolyte 240 is acidic, water based. In other embodiments, the water based electrolyte can either be alkaline or acidic. When alkaline, the electrolyte consists of KOH, NaOH, LiOH or mixtures thereof, where the concentration ranges from 5-30 weight percent. When acidic, the electrolyte consists of H.sub.2SO.sub.4, HCl, HNO.sub.3, CH.sub.3CO.sub.2H, H.sub.3PO.sub.4 or other acidifying compound. In an embodiment, the acid concentration ranges from 5-30 weight percent, and the resultant reaction is water splitting for the formation of hydrogen and oxygen gasses. When connected to a power supply 250, a current is passed through the anode and cathode 210A, 210B, and the splitting of water occurs. For water splitting, the HER reaction occurs at the cathode 210B, and the OER reaction occurs at the anode 210A.

(23) The separation membrane 230 is permeable to ions but retards mechanical mixing and diffusion and can be made from asbestos, polymers or ceramics. The use of membranes in electrochemical devices is well understood in the art.

(24) The power supply 250 is selected to drive the electrochemical reaction. The use of power supplies in electrochemical processes is well understood in the art and includes being tied to the electric grid, the use of a battery or the power coming from a renewable energy source such as but not limited to a photovoltaic device.

(25) In the embodiment where an alkaline electrolyte is used the reaction at the cathode 210B the electrocatalyst promotes the combination of water and electrons (e?) to form H.sub.2 gas and hydroxide ions according to the equation (4H.sub.2O+4e?.fwdarw.2H.sub.2+4OH.sup.?) while at the anode 210A hydroxide ions are converted to O.sub.2 gas and water according to the equation (4OH.sup.?.fwdarw.O.sub.2+2H.sub.2O+4e?). The overall cell reaction is 2H.sub.2O.fwdarw.2H.sub.2+O.sub.2.

(26) In an embodiment where an acidic electrolyte is used the reaction at the cathode 210B the electrocatalyst promotes the combination of protons (H.sup.+) and electrons (e.sup.?) to form H.sub.2 gas according to the equation (4H.sup.++4e?.fwdarw.2H.sub.2) while at the anode 210A water molecules are converted to O.sub.2 gas and protons according to the equation (2H.sub.2O.fwdarw.O.sub.2+4H.sup.++4e?) the overall cell reaction is 2H.sub.2O.fwdarw.2H.sub.2+O.sub.2.

(27) In this exemplary embodiment, the electrochemical cell is an electrolyzer. In other embodiments, the disclosed nanoparticle electrocatalysts and electrodes formed therefrom may be used in other electrochemical processes and devices, such as, but not limited to use in photoelectrochemical cells, photoelectrochemical water splitting cells, thermoelectrochemical cells, thermoelectrochemical water splitting cells, metal or ceramic air batteries, lithium or other ion intercalation batteries and sensors. The general performance and structure of these other electrochemical processes and devices are well understood in the art.

(28) While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.