US20150240369A1

US20150240369A1 - Electrolysis Electrocatalyst

Info

Publication number: US20150240369A1
Application number: US14/429,174
Authority: US
Inventors: Rhodri Jervis; Noramalina Mansor; Daniel Brett; Christopher Gibbs
Original assignee: UCL Business Ltd
Current assignee: UCL Business Ltd
Priority date: 2012-09-21
Filing date: 2013-09-20
Publication date: 2015-08-27
Also published as: GB201216920D0; WO2014045049A1; GB2508795A; ZA201501887B; CN104797742A; EP2898119A1

Abstract

Electrolysis electrocatalysts and their use in an electrolysis process, for example, in the electrolysis of water, more particularly for producing hydrogen at the cathode of a water electrolyser or producing oxygen at the anode of a water electrolyser, as well as a water electrolyser including such an electrocatalyst. The electrocatalyst includes a combination of palladium and iridium.

Description

FIELD OF THE INVENTION

The present invention relates to electrocatalysts for use in electrolysis processes, such as the electrolysis of water. In the context of the electrolysis of water, the invention relates to the use of such an electrocatalyst for producing hydrogen at the cathode of an electrolyser or producing oxygen at the anode of an electrolyser as well as a water electrolyser comprising such an electrocatalyst.

BACKGROUND OF THE INVENTION

A number of electrolysis processes have industrially useful applications. For example, the electrolysis of water provides a source of hydrogen and oxygen. Alternatively, the electrolysis of water containing sodium chloride provides a source of hydrogen, chlorine, and sodium hydroxide.
Water electrolysis processes fundamentally involve the splitting of water into its constituents although other chemistries also utilising the electrolysis of water are certainly possible. In a water electrolyser water is split into its components: hydrogen (gas) and oxygen (gas). An electrolyser essentially comprises a cathode, an anode, and an electrolyte. Hydrogen is produced at the cathode via the Hydrogen Evolution Reaction (HER). Oxygen is produced at the anode via the Oxygen Evolution Reaction (OER).
In an acidic medium, the half-cell and overall reactions are:
Anode: 2H₂O(l)→O₂(g)+4H⁺+4e⁻
Cathode: 4H⁺+4e⁻→2H₂
Overall: H₂O(l)→H₂(g)+½O₂(g)
In an alkaline medium, the half-cell and overall reactions are:
Anode: 2OH⁻ →½ O ₂(g)+H₂O+2e⁻
Cathode: 2H⁺+2e⁻→H₂(g)
Overall: H₂O(l)→H₂(g)+½O₂(g)
Other electrolysis chemistries include the chlor-alkali process where the half-cell and overall reactions are:
Anode: 2Cl⁻→Cl₂(g)+2e⁻
Cathode: 2H₂O+2e⁻→H₂+2OH⁻
Overall: 2H₂O(l)+2NaCl(aq)→2NaOH(aq)+H₂(g)+Cl₂(g)
Anodes in chlor-alkali processes are typically titanium based usually coated with rutile oxide catalysts, including platinum group metal oxides. Chlor-alkali cathodes are often nickel based and usually have some platinum group metal catalysts to significantly reduce overpotentials. Newer methods include oxygen gas diffusion electrodes which significantly reduce power consumption for producing sodium hydroxide which also employ catalysts to significantly reduce the overpotentials.
Other electrolysis chemistries exist such as anodic oxidations such as electrochlorination, chlorine evolution, Kolbe reaction and other organic electrosyntheses or cathodic reduction in electroplating, electrochemical nitro-group reductions and other organic electro-syntheses. The Kolbe reaction, for example, involves electrolytic decarboxylation of carboxylic acid in the presence of platinum and/or platinum alloys to produce free radicals which predominately leads to dimerisation of similar radicals. A review of several electro-syntheses is Rautenbach, Daniel, “The Development of an Electrochemical Process for the Production of Para-Substituted Di-Hydroxy Benzenes”, PhD Thesis, Nelson Mandela Metropolitan University, January 2005.
An electrical bias exists between the cathode and the anode to provide the energy for the reactions, but the reactions are often facilitated by catalysis on both electrodes in order to proceed at lower energy consumption per unit of electrolysis products. For example, water electrolysers are seen as an attractive and efficient method of large-scale hydrogen production, but splitting water without catalysts requires far more energy to produce the same amount of hydrogen and oxygen. In addition, the catalysts employed in many electrolysis processes are expensive. Electrocatalysts which reduce the energy consumption of an electrolysis process are desirable, especially relative to current-use materials. Electrocatalysts which are cheaper than current materials while maintaining similar efficiency relative to the current-use materials are desirable. Electrocatalysts which both reduce energy consumption and material costs are very desirable.
A basic water electrolyser comprises an anode, a cathode, an electrolyte, and a power supply providing electric current. The power supply keeps electrons flowing to the cathode where hydrogen ions consume them to form hydrogen gas. A suitable diaphragm is usually employed to keep the gas products separate so they may be collected for other applications. Multiple cells of this basic electrolyser unit may be put in series, in either monopolar or bipolar arrangements, in order to produce larger amounts of the chemical products, especially product gases. In alkaline applications the electrolyte is most often a solution of potassium hydroxide; for acid applications, the electrolyte is often a solid polymeric membrane which also acts as the diaphragm. A good review of alkaline water electrolysis is K. Zeng, D. Zhang, Progress in Energy and Combustion Science 36 (2010) 307-326; a good example of a typical water electrolyser is described in P. Millet, et al, International Journal of Hydrogen Energy 34 (2009) 4974-4982.
The chlor-alkali industry has, in the past, employed three different types of cells: the mercury, the membrane, and the diaphragm cell. The nature of the reactions involved in the production of chlorine limit the choice of electrode materials on the anode. A non-reactive metal such as titanium is often employed as an electrode substrate, with an electrocatalyst deposited on the substrate. On the cathode side, however, a wider range of choices is possible. Nickel is often used, and it is often coated or otherwise impregnated with platinum or other platinum group metal catalysts to lower the overpotential, and thus the required energy inputs, of the water electrolysis.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides the use of an electrocatalyst comprising palladium and iridium for catalysing an electrolysis process.
In one embodiment, the electrolysis process is an electrosynthesis process. The electrocatalyst may be part of the anode, part of the cathode, or part of both electrodes.
In one embodiment, the electrolysis process is the electrolysis of water.
In one embodiment, the electrolysis process is a chlor-alkali process; also known as the electrolysis of an aqueous sodium chloride solution (brine). The electrocatalyst may be part of the anode, part of the cathode, or part of both electrodes. In a specific embodiment, the electrocatalyst is used at the cathode in a chlor-alkali process.
In one embodiment, the electrolysis process is a Kolbe reaction electrolysis process.
In one embodiment, the electrolysis process is an electro-plating process.
In one embodiment, the electrolysis process is an electro-galvanizing process.
In one embodiment, the electrolysis process is one in which the electrocatalyst would increase the efficiency of the process.
In one embodiment, the electrocatalyst is part of an electrode in an electrolysis system such as a water electrolyser.
In one embodiment, the electrocatalyst comprising palladium and iridium is used for catalysing the production of hydrogen at the cathode of a water electrolyser. At the cathode, the electrocatalyst of the present invention exhibits equivalent or greater kinetic performance relative to platinum for the hydrogen evolution reaction and provides significant cost savings. Hydrogen is produced at the cathode of the water electrolyser and may be collected.
In one embodiment, the electrocatalyst comprising palladium and iridium is used for catalysing the production of oxygen at the anode of a water electrolyser. At the anode, the electrocatalyst of the present invention provides significant cost savings without compromising kinetic performance relative to iridium oxide for the oxygen evolution reaction. Oxygen is produced at the anode of the water electrolyser and may be collected.
In a further embodiment, the electrocatalyst comprising palladium and iridium is used for catalysing the production of hydrogen at the cathode of a water electrolyser and for catalysing the production of oxygen at the anode of the water electrolyser. Hydrogen produced at the cathode and oxygen produced at the anode may either or both be collected.
In one embodiment, the electrocatalyst comprising palladium and iridium is used for catalysing the production of hydrogen at the cathode of an electrolysis system wherein the production of hydrogen is generated by the electrolysis of an aqueous sodium chloride solution. Hydrogen is produced at the cathode of the electrolysis system and may be collected.
In a second aspect, the invention provides an electrolysis electrocatalyst comprising palladium and iridium. For example, the electrolyser may be a water electrolyser.
In a third aspect, the invention provides an electrolysis system comprising an electrocatalyst, wherein the electrocatalyst comprises palladium and iridium. For example, the electrolysis system may be a water electrolyser.
In one embodiment, the electrolysis system comprises a cathode, an anode and an electrolyte or electrolytes, wherein the cathode or the anode or both the cathode and the anode comprise an electrocatalyst comprising palladium and iridium.
In a fourth aspect, the invention provides a method for electrolysing water comprising the steps of:

- (i) providing a water electrolyser comprising an anode, a cathode, and an electrolyte or electrolytes, wherein at least one of the anode and the cathode comprises an electrocatalyst comprising palladium and iridium;
- (ii) contacting the water electrolyser with water; and
- (iii) generating hydrogen and/or oxygen.

Alternatively, the invention provides a method for electrolysing an aqueous sodium chloride solution comprising the steps of:

- (i) providing an electrolysis system comprising an anode, a cathode, and an electrolyte or electrolytes, wherein at least one of the anode and the cathode comprises an electrocatalyst comprising palladium and iridium;
- (ii) contacting the cathode and the anode with the electrolyte or electrolytes wherein at least one of the electrolytes comprises an aqueous sodium chloride solution; and
- (iii) generating hydrogen and/or chlorine and/or sodium hydroxide.

For example, the anode may be contacted with a sodium chloride solution and the cathode may be contacted with concentrated sodium hydroxide.
In any embodiment of the fourth aspect, generating hydrogen and/or chlorine and/or sodium hydroxide is achieved by an electrolysing step that is carried out by creating an electrical bias between the cathode and the anode.
In a further embodiment, the invention provides a method for electrolysing any suitable inputs comprising the steps:

- (i) providing an electrolysis system comprising an anode, a cathode, and an electrolyte or electrolytes, wherein at least one of the anode and the cathode comprises an electrocatalyst comprising palladium and iridium;
- (ii) contacting the cathode and the anode with the electrolyte or electrolytes;
- (iii) flowing current to the electrolysis system; and
- (iv) generating electrolysis products and/or new states of matter.

In a fifth aspect, the invention provides a method for producing hydrogen and/or oxygen comprising the steps of:

- (i) providing a water electrolyser comprising an anode, a cathode, and an electrolyte or electrolytes, wherein at least one of the anode and the cathode comprises an electrocatalyst comprising palladium and iridium;
- (ii) contacting the water electrolyser with water; and
- (iii) electrolysing the water to produce hydrogen and/or oxygen.

Alternatively, the invention provides a method for producing hydrogen and/or chlorine and/or sodium hydroxide comprising the steps of:

- (i) providing an electrolysis system comprising an anode, a cathode and an electrolyte or electrolytes, wherein at least one of the anode and the cathode comprises an electrocatalyst comprising palladium and iridium;
- (ii) contacting the cathode and the anode with the electrolyte or electrolytes wherein at least one of the electrolytes comprises an aqueous sodium chloride solution; and
- (iii) electrolysing the system to produce hydrogen and/or chlorine and/or sodium hydroxide.

For example, the above method may involve contacting the anode with an aqueous sodium chloride solution and contacting the cathode with concentrated sodium hydroxide.
In one embodiment, the electrolysing step is carried out by creating an electrical bias between the cathode and the anode.
In a sixth aspect, the invention provides a process for preparing an electrolysis system comprising assembling a cathode, an anode and an electrolyte or electrolytes, wherein the cathode, the anode or both the cathode and the anode comprise an electrocatalyst comprising palladium and iridium.
In a further aspect, the invention provides the use of a cathode electrocatalyst comprising palladium and iridium for producing hydrogen via an electrolysis process. Such a use may involve the electrolysis of water or the electrolysis of an aqueous sodium chloride solution.
In a further aspect, the invention provides the use of an anode electrocatalyst comprising palladium and iridium for producing oxygen via the electrolysis of water.
In a further aspect, the invention provides an electrolysis cathode electrocatalyst comprising palladium and iridium.
In a further aspect, the invention provides an electrolysis anode electrocatalyst comprising palladium and iridium.
In all aspects of the invention, the electrolysis system may be any electrolysis system such as a water electrolyser.
The electrolysis of the present invention may be employed in a number of applications, such as feeding into the natural gas infrastructure, refueling stations, energy storage, etc.

DETAILED DESCRIPTION OF THE INVENTION

The electrocatalyst employed in the present invention comprises palladium and iridium. The concentration of iridium in the electrocatalyst may be of any molar concentration, however in one embodiment, the atomic ratio of iridium to palladium can be anywhere from about 1:99 to about 99:1. For example, the palladium and iridium may be present in the electrocatalyst in a Pd:Ir atomic ratio of from about 9:1 to about 1:1, from about 5:1 to about 1:1, from about 3:1 to about 1:1, from about 5:1 to about 3:1, about 9:1, about 5:1, about 3:1, or about 1:1.
The iridium may be present as an alloy with the palladium, as a surface modification of the palladium, as an amorphous state material, as a structure similar to a core shell with surface layers rich in palladium, or any combination thereof. The palladium may be present as an alloy with the iridium, as a surface modification of the iridium, as an amorphous state material, as a structure similar to a core shell with surface layers rich in iridium, or any combination thereof. The metal constituents of the catalyst may be neat metal surfaces, may be in the oxide form, or may be both mixed oxides and neat metal surfaces.
Those skilled in the art will appreciate that the electrocatalyst will comprise functionally significant amounts of palladium and iridium, palladium and/or iridium alloys, palladium or iridium mixed amorphous state material and/or surface modified palladium/iridium, not merely tiny amounts present as impurities in other catalyst components. For present purposes, “functionally significant” amounts of palladium and iridium means sufficient to cause a detectable increase in electrolysis efficiency as measured by the production of electrolysis products or by the decrease in energy required to produce equivalent amounts of electrolysis products.
In one embodiment, the electrocatalyst may further comprise other catalyst components such as other metals. The catalyst may comprise two metals, three metals or even four or more different metals, in any desired or convenient ratio.
In one embodiment, the electrocatalyst may comprise palladium oxide and iridium oxide, or a mixture of palladium oxide and iridium oxide with the pure metals.
Also, the palladium and iridium may be present in substantially pure form (at least 99.1% pure), or may be present in a mixture with one or more additional elements.
The palladium and iridium in the electrocatalyst are believed to be intimately involved in the catalysis of the electrochemical reaction. However, other elements which may advantageously be included in the catalyst need not, necessarily, be actively involved in the catalysis. For example, they may exert a beneficial effect by improving or enhancing the stability of the palladium and/or iridium in either electrolytic medium, by promoting useful side reactions for the long term durability of the system, or in some other way. Reference to these materials forming part of, or being comprised within, the electrocatalyst does not therefore necessarily imply that the materials in question have themselves catalytic activity for the electrochemical reactions catalysed by the electrocatalyst, though this may in fact be the case.
Those skilled in the art will appreciate that the palladium, iridium, and other catalyst components if present, will preferably be in a form which has a high surface area e.g. very finely divided or nanoparticulate or the like.
Conventionally, platinum catalysts are the catalysts of choice for the electrolysis system cathode exhibiting far superior kinetic and stability properties when compared to other materials, but platinum is expensive. The surprising discovery of the present invention however allows the use of electrocatalysts containing low levels of platinum, and even allows the use of electrocatalysts containing no platinum. Platinum may be present in the electrocatalysts employed in the invention, although it will normally be preferred that platinum, if present, exists only in trace amounts (below 0.05 atomic %, preferably below 0.1 atomic %). More preferably, the electrocatalyst employed in the present invention contains no platinum. That is to say, the electrocatalyst employed in the present invention comprises palladium and iridium in the absence of platinum.
An electrolysis system such as an electrolyser typically comprises an electrolyte and two electrodes (a cathode and an anode). The cathode and anode may be connected to an electrical power source for the purpose of creating an electrical bias between the two.
By way of an example, a water electrolyser is operational when the anode and the cathode electrodes are brought into a suitable functional relationship with the electrolyte in such a way that useful amounts of hydrogen and/or oxygen are generated from the electrolysis of water. For example, this occurs when an electrical power source is connected to the two electrodes.
Water electrolysers represent a typical example of the use of the electrocatalyst in an electrolysis application, and is a preferred application. The scope of the invention is the use of the electrocatalyst in any electrolysis application (e.g. the electrolysis of water, the chlor-alkali process, the production of hydrogen, the production of chlorine gas, the production of sodium hydroxide, electrogalvanizing, electro-plating, electrosynthesis, Kolbe reaction, etc) and especially any electrolysis application employing platinum, platinum group metal catalysts, platinum group metal alloys, or alloys or other combinations employing platinum group metals and other elements that are not platinum group metals.
The invention intends to cover the use of the palladium-iridium electrocatalyst in electrolysis processes wherever this catalyst system increases efficiency in the electrolysis chemistry and/or reduces the overall costs of the processing. This is especially true in electrolysis processes using platinum, iridium, or other platinum group metal catalysts and their alloys, surface modified platinum group metal systems, and the like.
In the present invention, the electrocatalyst of the present invention is included in one or both of the cathode and the anode. In one embodiment, the anode comprises an electrocatalyst comprising palladium and iridium. In one embodiment, the cathode comprises an electrocatalyst comprising palladium and iridium. In one embodiment, both the cathode and the anode comprise an electrocatalyst comprising palladium and iridium.
The electrolyte employed in the present invention may comprise one electrolyte or may comprise more than one electrolyte. For example, the electrolyte employed in the present invention may be a combination of electrolytes.
An electrolyte employed in the context of the present invention is an ion conducting media within the water electrolyser, electrolysis system or other electrolysis process. The electrocatalyst of the present invention exhibits high catalytic efficiency over a wide range of pH. In one embodiment of the present invention, the electrolyte may be either acidic, alkaline or any suitable electrolytic system of neutral pH.
The acidic electrolyte may be any conventional acidic electrolyte. For example, the acidic electrolyte may be a polymeric electrolyte or a liquid electrolyte. More specifically, the acidic electrolyte may be a cation exchange membrane, a free-flowing liquid electrolyte or a liquid electrolyte contained within a porous matrix.
The alkaline electrolyte may be any conventional alkaline electrolyte. For example, the alkaline electrolyte may be a liquid electrolyte or even an anion conducting membrane. More specifically the alkaline electrolyte may be a free-flowing liquid electrolyte, a liquid electrolyte contained within a porous matrix, a cation exchange membrane, an anion exchange membrane or an anion exchange membrane in functional contact with an alkaline electrolyte.
The electrolyte or electrolytes of the electrolysis system may also be combinations of both acidic and alkaline electrolytes. These hybrid electrolysis systems can be organised in any way where the electrolytes have the appropriate functional relationship with the electrodes (cathode and anode). For instance, some alkaline water electrolysers may employ a cation exchange membrane where the cation is the charge carrier, alternatively they may employ an anion exchange membrane where the anion is the charge carrier. Use of combinations of electrolytes should not limit the scope of the invention.
Suitable electrolytes for use in the invention are further described below.
The electrolyte may be liquid, solid, or a combination of liquid and solid.
For liquid electrolyte applications, the electrolyte may be either an acidic solution, an alkaline solution, or even a suitably ionically conducting solution of neutral pH. Depending on the system and application, this solution can be either concentrated or dilute. Examples of acidic liquid electrolytes include solutions of sulphuric acid or phosphoric acid; examples of alkaline liquid electrolytes include solutions of potassium hydroxide or sodium hydroxide. Several other potential liquid acidic solutions, liquid alkaline solutions and ionic solutions of neutral pH are possible, and the examples given are not intended to limit the invention.
Suitable solid electrolytes include ionomers formed into ion conducting membranes. For example, a solid electrolyte may be either a cation conducting ionomer or an anion conducting ionomer. In one embodiment, the electrolyte is a cation exchange polymer membrane.
Alternatively, the electrolyte may be a liquid electrolyte impregnated matrix, membrane or gel, i.e. a combination of liquid and solid. Examples include acid impregnated porous membranes or similarly impregnated gels or other such matrices.
Nafion® (E. I. Du Pont De Nemours) is a suitable commercially available cation conducting exchange membrane and used in several electrolysis applications including water electrolysers, and the chlor-alkali industry. Several other cation exchange ionomers exist as well such as Aquivion™ (Solvay, Belgium) and Fumion® (Fumatech, Germany). The backbone of the acidic polymer need not be a polyfluorocarbon and several hydrocarbon based cation exchange membranes are also available. A review of nonperfluorosulfonic acid ionomer is J. Perron et al, Energy Environ. Sci., 2011, 4, 1575-1591. A-006 (Tokuyama Corporation, Japan) is a suitable commercially available anion conducting (alkaline) polymer electrolyte. Varcoe et al, Solid State Ionics 176 (2005) 585-597 exemplifies several anion exchange membrane technologies and chemistries which could be used for alkaline based electrolysis. However, the overwhelming majority of alkaline based electrolysis is currently performed with liquid electrolytes using matrices or diaphragms to prevent product gas or product chemical crossover. The above examples of solid polymeric electrolytes are given as expounding examples and not meant to limit the scope of the present invention.
To form electrodes for electrolysis applications the electrocatalysts may be coated onto a substrate and then brought into functional contact with any electrically conducting substrate appropriate for the application, may be coated directly onto an electrically conducting substrate or may be coated directly onto the electrically conducting substrate directly which is then contacted to another conductor for whatever reason. In the specific example of a water electrolyser anode, any electrically conducting support or electrically conducting substrate must tolerate high voltage potentials. For solid polymeric electrolytes, the electrocatalyst layers may be deposited directly onto the membrane by any appropriate means and then brought into functional contact with an electrically conducting substrate and/or electrical conductor appropriate for the application. Titanium mesh is an example of an electrically conducting substrate suitable for use in water electrolysers. Nickel or nickel mesh is another example of an appropriate conductor for the application. Other suitable electrically conducting substrates or appropriate electrical conductors are available, and the choice of electrically conducting substrate or electrical conductor or how the catalyst is brought into functional contact with the electrically conducting substrate, the electrical conductor and/or the electrolyte should not limit the invention.
The electrocatalysts in the form of an ink may be coated on the electrical conductor and/or electrically conducting substrate. The ink can be coated onto the electrical conductor or electrically conducting substrate in a variety of ways: brush coating, doctor blading, gravure, screen printing, roll-to-roll, or spray coating. Alternative methods of applying the electrocatalyst are perfectly acceptable: solution coating, dip coating, sputtering, electrospinning, vacuum deposition, etc. Methods of applying the electrocatalyst to form an electrode should not limit the invention including any methods of intermediate or post-deposition treatment, such as heating or treatment with a reactive gas. Ideally the electrodes will exhibit excellent mass transport properties, allowing water to ingress to the catalyst surface while simultaneously releasing any product gases at a rate commensurate with the turnover rate of the catalyst. The present invention does not intend to describe a proprietary method for making electrolyser electrodes, but seeks to protect any use of the electrocatalyst of the present invention in a water electrolyser or in any electrolysis application.
The construction of the anodes and cathodes will conveniently be generally very similar, but they may be different. Typically both the anode and cathode may be of essentially conventional construction and may comprise a conducting support including, but not limited to, one of the following: solid carbon, graphitic carbon, solid metal, metalized fabrics, metalized polymer fibres, metallic meshes, carbon cloth, carbon paper and carbon felt. The conducting support may be a material such as stainless steel, nickel, mild steel, titanium, tungsten carbide, but not necessarily limited to just these materials. The conducting support may also be in the form of a sintered powder, foam, powder compacts, mesh (e.g. titanium, nickel), woven or non-woven materials, perforated sheets, assemblies of tubes or the like, on which may be deposited, or otherwise associated therewith, electrocatalysts.
The electrocatalyst may be unsupported as a catalyst-black or may be supported on any appropriate support for either electrode. It is possible in some applications for one of the electrodes to be composed of the unsupported electrocatalyst while the other electrocatalyst on the other electrode is supported. For a typical electrolysis example, water electrolysers, the cathode support could be a high surface area carbon such as Denka Black, Vulcan XC-72R, and Ketjen Black® EC-300JD, nanotubes such as Nanocyl™, acetylene blacks or furnace blacks. The cathode could also be supported on metals or metal oxides such as nickel spheres, titanium oxides (e.g. Ti₄O₇Ebonex®), tungsten carbides, etc. Polymer supports could also be employed, for instance, polyaniline, polypyrrole and polythiophene assuming they are modified appropriately for electrical conduction. Metal, metal oxide, and polymer supports are more appropriate. The exact nature of the support (or lack of support) should not limit the invention. In other electrolysis applications where the anodes have more flexibility regarding possible support materials, these may be used. The examples above are simply given as potential examples and ways in which the electrocatalyst may be used in electrolysis applications; the nature of the catalyst support or lack of catalyst support should not limit the invention.
The electrocatalyst, either supported or unsupported, once formed into an appropriate electrode for the application, must be brought into a functional relationship with an electrolyte. This electrolyte may either be acidic, alkaline or of more neutral pH, may be either liquid or solid or a combination of liquid and solid (such as a porous polymeric substrate infused with liquid acid or liquid alkaline solutions). Examples of potential solid polymeric electrolytes have been given previously with Nafion® being the most widely used cation exchange membrane for many electrolysis processes.
In one embodiment, the electrocatalyst employed in the present invention is prepared into a suitable electrode using a method comprising of steps of:
contacting the palladium and iridium electrocatalyst on an electrically conducting support or substrate, optionally with a suitable binder, enabling the electrocatalyst to be electrically connected to the conducting substrate while also permitting any chemical reagents (i.e. water, brine, electrolytes) access to the catalyst surface while permitting the egress of any chemical products formed. The nature of the binder should not limit the invention. These binders are typically solvent dispersed variations of the solid polymeric membranes, such as Nafion® and Tokuyama A-006®, but they may also be inert polymers like PVDF, or other appropriate resins, epoxies, thermosets or the like which still maintain the required mass transport properties of the electrode. Application of the electrocatalyst can vary; the application and the use of rheological modifiers to apply the electrocatalyst should not limit the scope of the invention. Some examples of suitable methods of applying electrocatalysts to electrodes are disclosed in U.S. Pat No. 5,865,968, EP0942482, WO2003103077, U.S. 4,150,076, U.S. 6,864,204, and WO2001094668. These references are intended to elucidate typical procedures for making inks with electrocatalysts and applying to (carbon-based) substrates; methods utilising different solvents, different binders, different application methods or different substrate materials or any combination of these, or, in fact, omitting any of these should not limit the scope of the invention.
The general method for fabricating the electrocatalyst described in this invention includes (i) dispersing a palladium salt and/or iridium salt in an aqueous solution usually, but not necessarily, in the presence of an electrically conducting support. The pH of the aqueous solution is typically kept at neutral pH in this stage via the addition of suitable materials, sodium bicarbonate being one example, (ii) causing the precipitation of the palladium and iridium as either the metal or metal oxide onto the electrically conducting support via a chemical reducing agent, or thermally activated reduction (such as in the presence of ethylene glycol), (iii) filtering, washing, and drying the resulting precipitate, (iv) a final thermal reduction in the presence of hydrogen to clean the metal surfaces while preventing agglomeration.
Suitable palladium and iridium salts include palladium nitrate, palladium chloride, and iridium chloride. Suitable reducing agents include, but are not limited to, sodium hypophosphite (NaH₂PO₂) and sodium borohydride (NaBH₄). Thermally activated reductions in the presence of suitable agents can also be employed, an example being reduction in the presence of ethylene glycol at temperatures under 100° C. A suitable reducing atmosphere for step (iv) is 5%-20% hydrogen in nitrogen or argon. Example thermal reduction conditions are 150° C. for one hour. This heating process is advantageous to the present invention as it promotes the removal of hydroxides/oxides from the surface of the catalyst without promoting the sintering and the loss of surface area of the catalyst.
A general summation of these techniques follow: a mixture of one or more active catalytic materials and optionally a suitable binder is formed in the presence of a suitable solvent, and the mixture dried to cause the deposition and adhesion of the catalytic material to a suitable substrate in the construction of the electrode. The electrode is then brought into a functional relationship with the electrolyte of the cell. The opposing electrode may or may not also contain the electrocatalyst of the present invention. It too is brought into a functional relationship with the electrolyte of the cell. In liquid electrolytes, the electrode catalyst layer need only contact the electrolyte (which can be either flowing or stagnant). Usually in the liquid electrolyte the two electrodes are separated by a diaphragm or semi-permeable membrane. With solid polymer electrolytes, the electrode containing the electrocatalyst layer(s) may be bound to the membrane with heat and pressure in any appropriate lamination process suitable for membrane material, or the layer may simply be mechanically compressed against the membrane at any appropriate force for the cell or system. Direct current provides the energy (electrons) for the chemistry to take place.
Methods of forming and depositing catalysts are in general well-known to those skilled in the art and do not require detailed elaboration. The exact qualities an electrode layer—such as metal loading, support type, substrate, etc are dependent upon several factors such as chemical inputs, electrolyte, system operating conditions, chemical products, etc with general rules of thumb for fabrication well-known throughout the art. The electrocatalyst layer may be deposited upon any suitable electrically conducting material (e.g. titanium, nickel or carbon based) or may be deposited directly on the electrolytic membrane for cells employing a polymeric membrane or impregnated scaffold.
The electrolyser (cell) of the present invention contains at least one electrode which comprises palladium and iridium as described herein. The other electrode may comprise the same or different electrocatalyst. Both electrodes may contain the electrocatalyst as well.

DESCRIPTION OF THE FIGURES

FIG. 1 compares the present invention with platinum in an acidic half-cell arrangement for the HER. The data has been captured using Thin-Film Rotating Disk Electrode (TF-RDE) techniques described in Gasteiger et al., Applied Catalysis B, 2005. The experiment was performed using thin-film rotating disk electrode techniques where approximately 35 ug/cm²of each metal were deposited on a glassy carbon electrode. The electrolyte was 0.1M perchloric acid. Perchloric acid is used in lieu of sulphuric acid to prevent ion absorption on the platinum. In FIG. 1, the electrocatalyst employed in the present invention as a cathode electrocatalyst demonstrates improved efficiency at generating hydrogen in a water electrolyser (compared to platinum). At a given current density, the electrode with the electrocatalyst of the present invention requires less energy to generate the same amount of hydrogen as the platinum.

FIG. 2 demonstrates the efficacy of the present invention for the oxygen evolution reaction in an acidic environment. The electrocatalyst employed in the present invention maintains efficacy comparable to the state-of-the-art iridium electrocatalyst especially at low current densities. The lower cost of palladium however provides a superior cost to efficiency ratio to iridium. Clearly the electrocatalyst employed in the present invention exhibits greatly improved efficiency compared to platinum. The data in FIG. 2 was generated with a carbon supported palladium-iridium subject to corrosion potentials on the electrolyser anode half-cell.

FIG. 3 demonstrates the efficacy of using palladium-rich species of the catalyst for a water electrolyser cathode in an acidic environment. In FIG. 3, an additional palladium-rich embodiment of the present invention has been compared to a platinum catalyst. In this example, all of the catalysts have been coated onto a suitable electrode substrate—in this instance a bar of graphitic carbon—and immersed in a three-electrode cell filled with 1M sulphuric acid. FIG. 3 shows a palladium and iridium catalyst in an atomic ratio of 3:1 (palladium to iridium) is superior to a similar platinum electrode for the hydrogen evolution reaction.

FIG. 4 demonstrates that the platinum-iridium electrocatalyst can be used to generate hydrogen, not only in an acidic environment, but also under alkaline conditions. The electrolyte was 5 m potassium hydroxide, the wording electrode a graphite plate coated in the electrocatalyst. FIG. 5 demonstrates the efficacy of the invention for the oxygen evolution reaction in alkaline traditions. This illustration of the activity of the electrocatalyst across a broad pH range shows the flexibility of application available with this invention.

All of the data generated in FIGS. 1, 2, 4 and 5 has been performed with a 1:1 atomic ratio of palladium and iridium. The catalyst system has been supported on Ketjen carbon such that the metals constitutes approximately 40% of the mass of the resulting catalyst. FIG. 3 has been performed with an atomic ratio of 3:1 of palladium to iridium. These catalyst systems have also been supported on Ketjen carbon such that the metals constitute approximately 40% of the mass of the resulting catalyst.
The platinum examples in FIGS. 1-3 is a commercially available catalyst (Alfa Aesar). The platinum is supported on Vulcan carbon (XC72R) such that the metal constitutes approximately 40% of the mass of the resulting catalyst.
Methods of fabricating the catalyst have been disclosed in United Kingdom patent application GB1110045.0 (published as GB2481309A), the contents of which are hereby incorporated by reference.

EXAMPLES

Example 1

Palladium/Iridium Catalyst on Carbon [PdIr (1:1 atomic %)], 150° C.

Carbon black (Ketjen Black EC300JD, 0.8 g) was added to 1 litre of water and heated to 80° C. in round-bottom flask. The carbon was dispersed using an overhead stirrer and a paddle for 12 hours.
Into a second vessel, palladium nitrate (0.475 g, assay 42.0% Pd by weight) was carefully weighed and dissolved in 50 ml of deionised (DI) water. Into a third vessel iridium chloride (0.660 g, assay 54.4% Ir by weight) was carefully weighed and dissolved into 50 ml of DI water. The salts were then carefully introduced into the vessel containing the stirring carbon slurry at 80° C.
Once the metal salts had been transferred to the larger vessel, the remaining contents of the dropping funnel were washed into the larger vessel. Then the pH of the stirring slurry was carefully increased to 7.0 by the addition of a saturated solution of sodium bicarbonate (NaHCO₃). The pH of the slurry was maintained at 7.0-7.5 for 1 hour by further controlled addition of sodium bicarbonate.
A sodium hypophosphite (NaH₂PO₂, 0.495 g diluted in 50 ml of DI water) solution was prepared. Two and half times the molar amount of palladium in the catalyst is a suitable amount of sodium hypophosphite to use. Half of this solution was introduced to the reaction vessel containing the carbon-salt slurry. The slurry was maintained at 80° C. for an additional hour with continuous stirring.
After cooling the slurry down to room temperature, the filtrate was recovered and washed on a microporous filter until the filtrate conductivity was 2.42 mS. The catalyst was dried in an oven at 80° C. for 10 hours. The dried catalyst was then broken up in a pestle and mortar to give a fine powder, which was carefully placed into a ceramic boat to a maximum depth of 5 mm. The boat was placed in a tube-furnace and heated under a 20% H₂/80% N₂atmosphere for 1 hour at 150° C. The yield for 1.4 g for a 40 metal wt % was 1.23 g.

Claims

1-38. (canceled)

39. A method for catalysing an electrolysis process comprising providing an electrocatalyst comprising palladium and iridium.

40. The method of claim 39, wherein the electrolysis process is selected from the group consisting of an electrosynthesis process, the electrolysis of an aqueous sodium chloride solution.

41. The method of claim 39, wherein the electrocatalyst comprises palladium and iridium in an atomic ratio of about 9:1, about 5:1, about 3:1 or about 1:1.

42. The method of claim 39, wherein the electrocatalyst is part of a cathode and/or an anode in an electrolysis system, wherein the electrolysis system comprises a cathode, an anode and an electrolyte or electrolytes.

43. The method of claim 42, wherein the electrolysis system is selected from the group consisting of a water electrolyser and an aqueous sodium chloride electrolysis system.

44. The method of claim 42, wherein the electrode is a cathode and wherein the electrocatalyst process is the production of hydrogen at the cathode.

45. The method of claim 42, wherein the electrode is an anode and wherein the electrocatalyst is used to catalyse the production of oxygen, chlorine, or sodium hydroxide at the anode.

46. An electrolysis system comprising an electrocatalyst, wherein the electrocatalyst comprises palladium and iridium.

47. The electrolysis system of claim 46, comprising a cathode, an anode, and an electrolyte or electrolytes, wherein the cathode, the anode or both the cathode and the anode comprise an electrocatalyst comprising palladium and iridium.

48. The electrolysis system of claim 47, wherein the electrolysis system is a water electrolyser or an aqueous sodium chloride solution electrolysis system.

49. The electrolysis system of claim 47, wherein the electrolyte is a cation conducting polymer membrane.

50. A method for electrolysing water to produce hydrogen and/or oxygen comprising the steps of:

(i) providing a water electrolyser comprising an anode, a cathode, and an electrolyte or electrolytes, wherein at least one of the anode and the cathode comprises an electrocatalyst comprising palladium and iridium;

(ii) contacting the water electrolyser with water; and

(iii) generating hydrogen and/or oxygen.

51. The method of claim 50, wherein the hydrogen and oxygen are generated by creating an electrical bias between the cathode and the anode.

52. A method for electrolysing an aqueous sodium chloride solution to produce hydrogen and/or chlorine and/or sodium hydroxide comprising the steps of:

(i) providing an electrolysis system comprising an anode, a cathode, and an electrolyte or electrolytes, wherein at least one of the anode and the cathode comprises an electrocatalyst comprising palladium and iridium;

(ii) contacting the cathode and the anode of the electrolysis system with the electrolyte or electrolytes wherein one of the electrolytes is an aqueous sodium chloride solution; and

(iii) generating hydrogen and/or chlorine and/or sodium hydroxide.

53. The method of claim 52, wherein the hydrogen and chlorine are generated by creating an electrical bias between the cathode and the anode.

54. A process for preparing an electrolysis system as defined in claim 9 comprising assembling a cathode, an anode and an electrolyte or electrolytes, wherein the cathode, the anode or both the cathode and the anode comprise an electrocatalyst comprising palladium and iridium.

55. An electrolyser electrocatalyst comprising palladium and iridium.

56. The electrolyser electrocatalyst of claim 55, as a cathode.

57. The electrolyser electrocatalyst of claim 55, as an anode.