Field

[0001] The invention relates to electrocatalysts for use in electrochemical processes, such as electrolysis, electrochemical hydrogen compression, electrochemical purification and battery processes.

Background

[0002] Electrochemical processes are critical to many aspects of modern life, notably they are integral to a variety of energy sources, such as fuel cells, photovoltaic cells and batteries; they form the basis for many sensor technologies; and electrochemistry forms the basis of electrolysis processes, including the production of hydrogen and oxygen from water, the chlor-alkali process, and electrosynthesis, for instance of ethylene from carbon dioxide. There is therefore a continual search for materials and methods which can improve these every day processes and one element of this is the provision of improved electrocatalysts.

[0003] In an electrochemical reaction, an electrical bias exists between the cathode and the anode to provide the energy for any electrochemical process, allowing the reaction to occur. However, such processes are often slow, and can benefit from catalytic enhancement of the reaction rate for a given energy input. For example, water electrolysers are seen as an attractive and efficient method of large-scale hydrogen production, but splitting water without catalysts requires too much energy to produce a given amount of hydrogen and oxygen to make the process commercially relevant. Catalysts that can reduce the energy required for an electrochemical process are therefore advantageous.

[0004] A problem with known electrocatalysts is cost; they are often expensive by virtue of the materials included. Specifically, noble metals, often platinum, are generally used. Where the catalytic effect of the electrocatalyst is good, the cost savings in the process can warrant their use, but in some cases, the cost of the electrocatalyst can be prohibitive. A reduction in cost without loss of efficacy in terms of reaction rate and/or reduction in energy required would therefore also be desired. [0005] Steps have been taken to reduce catalyst costs, for instance, the applicants have developed palladium-iridium catalysts (WO 2013/021145). However, it would be useful to further develop this work by providing alternative electrocatalysts, suitable for a wider utility by combining favourable attributes of other noble metals and non-noble metal catalysts into a single catalyst system.

[0006] Core-shell electrocatalysts have been described, for instance in US 7,855,021 (Brookhaven Science Associates LLC), which describes platinum coated particles where the core comprises a noble metal, such as palladium, for use in fuel cell technology, specifically for fuel cell cathodes.

[0007] The invention is intended to overcome or ameliorate at least some aspects of these problems.

Summary

[0008] Accordingly, in a first aspect of the invention there is provided an electrolyser electrocatalyst, hydrogen purification electrocatalyst, hydrogen compression electrocatalyst or battery electrocatalyst comprising particles of core-shell structure, the core comprising palladium and iridium, the shell comprising one or more noble metals. It has been found that the use of a palladium iridium core with a noble metal shell reduces the costs of the electrocatalyst, relative to traditional noble metal electrocatalysts, such as platinum, and gives commercially relevant performance despite using less of the more expensive noble metal. This configuration has been found to be robust to cycling at high positive potentials, and to shut down/power up cycles of the electrochemical systems. For instance, cycling between potentials in the range -0.3V to +1.2V can be tolerated by the electrocatalyst of the current invention, without loss of commercially relevant activity, for over 1000 cycles.

[0009] Two or more different noble metals may be used in the shell of the various aspects of the invention.

[0010] The electrocatalyst may be an anode electrocatalyst, or a cathode electrocatalyst. Further, the electrocatalyst may be present at both the anode and the cathode as appropriate for the application. [0011] The core may comprise Pd _x Ir _y wherein x and y are both independently in the range 1- 99. X and y may be independently in the range 1 - 10, or 1 - 5. Sometimes x and y may both be 1. Both palladium and iridium will be present in the core, as this offers beneficial catalytic activity relative to the use of a core of either component alone. The core may be palladium or iridium rich, such that x may be greater than y, or y greater than x, such that x may be in the range 6 - 10 while y is in the range 1 - 5, or x may be in the range 1 - 5 while y may be in the range 6 - 10.

[0012] The core is typically of diameter in the range 2 - 10 nm, often in the range 2 - 6nm. Fine particulates offer more commercially relevant activity due to the increase in surface area to volume ratio providing more surface area upon which the reaction or reactions occur. As used herein the term "diameter" means the diameter across the widest axis of the particle although the particles will typically be substantially spherical.

[0013] Often the noble metal described in this and subsequent aspects of the invention will comprise a noble metal other than palladium or iridium. Typically the noble metal shell will comprise a noble metal or noble metals at least one of which will have a higher reduction potential than palladium or iridium. Without being bound by theory, it is believed that this is advantageous as the palladium-iridium core acts as a sacrificial component protecting the noble metal shell and also conveys certain advantages the palladium-iridium alloy alone exhibits above certain noble metals (e.g. platinum) in several electrochemical applications. Often the shell will not comprise palladium or iridium. Often the noble metal will comprise platinum or alloys of platinum. It is believed that a palladium-iridium alloy is more durable than a palladium or iridium only core. Often the noble metal will be platinum, such that the shell consists of platinum, used alone as the shell material. When platinum is used as a component of the shell, the combination of catalytic activity and durability has been found to be maximised. In many examples, the shell comprises a monolayer of the noble metal, although bilayers and layers of a few atomic thicknesses may also be used. As defined herein the term shell is intended to refer to a complete, or substantially complete, coating of the core, such that 80 - 100%, or 90 - 100% of the core is coated by the shell in more than 50% of the catalyst particles in the electrocatalyst used.

[0014] In a second aspect of the invention there is provided an electrochemical system selected from an electrolysis system, an electrochemical hydrogen compressor, an electrochemical purifier and a battery; the electrochemical system comprising an electrocatalyst, wherein the electrocatalyst comprises particles of core-shell structure, the core comprising palladium and iridium, the shell comprising a noble metal. The electrocatalyst described can surprisingly be used in these applications with good activity and stability. Typically core-shell electrocatalysts are used for oxygen reduction in hydrogen fuel cells, a specific reaction with slow kinetics which is performed at low positive voltage. It is surprising that catalysts of this structure can be used in other applications across a wide range of different voltages without loss of catalytic activity. The electrocatalyst described, surprisingly, has beneficial features of both the core constituent and noble metal(s) shell constituent in these applications. For instance, a palladium- iridium core with a platinum shell is, surpri singly, a more effective oxygen evolution catalyst than a pure platinum nanoparticle. It is believed that enhanced oxygen evolution kinetics lends additional durability to the core-shell structure than a platinum nanoparticle during cell reversal or other transient events common in these electrochemical applications. It is also surprising a core-shell structure, even with cores composed of components with much lower reduction potentials, can exhibit the same durability as a pure nanoparticle with a higher reduction potential.

[0015] Typically, the electrochemical system will comprise a cathode, an anode, and an electrolyte or electrolytes, wherein the cathode, the anode or both the cathode and the anode comprise an electrocatalyst, the electrocatalyst comprising particles of core-shell structure, the core comprising palladium and iridium, the shell comprising one or more noble metals.

[0016] Often, the electrochemical system comprises an electrolysis system wherein the electrolysis system is a water electrolyser. Water electrolysers produce hydrogen and oxygen at the cathode and anode respectively and the electrocatalyst promotes the reaction rate for a given energy input. In such cases, it may be the cathode of the electrolysis system that comprises the electrocatalyst.

[0017] Often the electrochemical system comprises a hydrogen compressor. Such systems compress hydrogen by splitting the hydrogen into protons and electrons which are reformed in a confined storage space after passage of the protons across the electrolyte. Often in these systems the electrolyte will be a proton exchange membrane. [0018] Often the electrochemical system comprises a hydrogen purifier. Such systems purify hydrogen by splitting the hydrogen in the presence of other components into protons and electrons which are reformed away from the other components after the passage of the protons across the electrolyte. Often in these systems the electrolyte will be a proton exchange membrane. Often these purification systems will be combined with hydrogen compression.

[0019] Often the electrochemical system comprises a battery. Often, the battery is a flow battery, although a wide range of chargeable and rechargeable battery technologies are compatible with the electrocatalyst described herein. When used in a battery, the electrocatalyst promotes the rate of charging or discharge of the battery by reducing voltage losses associated with kinetics. The use of both solution based (redox-flow batteries) and hybrid flow batteries is envisaged. Often, the flow battery is selected from a metal-hydrogen battery and / or a hydrogen bromide battery. Often the metal hydrogen- battery is selected from a vanadium hydrogen battery.

[0020] In a third aspect of the invention there is provided the use of an electrocatalyst for catalysing an electrochemical process selected from an electrolysis process, electrochemical compression of hydrogen, electrochemical purification of hydrogen and the release or storage of chemical energy in a battery; wherein the electrocatalyst comprises particles of core-shell structure, the core comprising palladium and iridium, the shell comprising one more noble metals.

[0021] The electrolysis process may be selected from an electrosynthesis process, the electrolysis of water, the release of stored chemical energy from a battery, and the storage of chemical energy in a battery. As described above, often the battery will be a metal hydrogen or a hydrogen bromide battery.

[0022] In the uses described, the electrocatalyst may be part of a cathode and / or an anode. For instance, the electrocatalyst may be used to catalyse the production of hydrogen at the cathode, and / or to catalyse the production of oxygen, or protons at the anode. For instance, in water electrolysis, hydrogen will be produced at the cathode and oxygen at the anode. For hydrogen compression protons will be produced at the anode, such that hydrogen is reformed. [0023] In a fourth aspect of the invention there is provided 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 particles of core-shell structure, the core comprising palladium and iridium, the shell comprising a noble metal;

(ii) contacting the water electrolyser with water;

(iii) creating an electrical bias between the cathode and the anode; and

(iv) generating hydrogen and/or oxygen.

[0024] A fifth aspect provides a method for compressing hydrogen comprising the steps of:

(i) providing an electrochemical hydrogen compressor comprising an anode, a cathode, and an electrolyte or electrolytes, wherein at least one of the anode and the cathode comprises an electrocatalyst comprising particles of core-shell structure, the core comprising palladium and iridium, the shell comprising one or more noble metals;

(ii) creating an electrical bias between the cathode and the anode;

(iii) forming protons and electrons by contacting the compressor with hydrogen;

(iv) recombining the protons with the electrons to generate hydrogen; and optionally

(v) storing the hydrogen.

The hydrogen may be stored under pressure.

[0025] Often the method of the fifth aspect of the invention will comprise, as the electrolyte or electrolytes, a proton exchange membrane.

[0026] A sixth aspect provides a method for purifying hydrogen comprising the steps of:

(i) providing an electrochemical hydrogen purifier comprising an anode, a cathode, and an electrolyte or electrolytes, wherein at least one of the anode and the cathode comprises an electrocatalyst comprising particles of core-shell structure, the core comprising palladium and iridium, the shell comprising a noble metal or noble metals;

(ii) creating an electrical bias between the cathode and the anode;

(iii) forming protons and electrons by contacting the compressor with hydrogen in the presence of additional impurities;

(iv) recombining the protons with the electrons away from the impurities to generate hydrogen with less impurities

[0027] Often the method of the sixth aspect of the invention will comprise, as the electrolyte or electrolytes, a proton exchange membrane

[0028] Often the method of the sixth aspect of the invention will also comprise an electrochemical pressurisation as per the fourth aspect of the invention and storing the purified hydrogen under pressure

[0029] A seventh aspect provides a method for releasing stored chemical energy from a battery comprising:

(i) providing a battery comprising an anode, a cathode, and an electrolyte or electrolytes, wherein at least one of the anode and the cathode comprises an electrocatalyst comprising particles of core-shell structure, the core comprising palladium and iridium, the shell comprising a noble metal;

(ii) creating an electrical bias between the cathode and the anode;

(iii) connecting the battery to an external circuit; and

(iv) allowing electrons to flow from the cathode into the external circuit.

[0030] And an eighth aspect provides for a method for recharging a battery comprising:

(i) providing a battery comprising an anode, a cathode, and an electrolyte or electrolytes, wherein at least one of the anode and the cathode comprises an electrocatalyst comprising particles of core-shell structure, the core comprising palladium and iridium, the shell comprising one or more noble metals;

(ii) creating an electrical bias between the cathode and the anode;

(iii) connecting the battery to an external circuit; and (iv) allowing electrons to flow from the external circuit into the battery reversing the catalysed reaction.

[0031] Often the battery is a flow battery or a hybrid flow battery.

[0032] A ninth aspect of the invention provides a method for preparing an electrochemical system selected from an electrolysis system, a battery and an electrochemical hydrogen compressor, as defined in the second aspect of the invention 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 particles of core-shell structure, the core comprising palladium and iridium, the shell comprising one or more noble metals.

[0033] A tenth aspect of the invention provides for the use of a cathode electrocatalyst comprising particles of core-shell structure, the core comprising palladium and iridium, the shell comprising one or more noble metals for producing hydrogen via an electrolysis process.

[0034] An eleventh aspect of the invention provides for the use of an anode electrocatalyst comprising particles of core-shell structure, the core comprising palladium and iridium, the shell comprising a noble metal or noble metals for producing oxygen via the electrolysis of water.

[0035] A twelfth aspect of the invention provides for the use of an anode electrocatalyst comprising particles of core-shell structure, the core comprising palladium and iridium, the shell comprising a noble metal or noble metals for producing compressed hydrogen, via the electrochemical splitting and reforming of hydrogen.

[0036] A thirteenth aspect of the invention provides for the use of an electrocatalyst comprising particles of core-shell structure, the core comprising palladium and iridium, the shell comprising one or more noble metals for producing purified hydrogen, via the electrochemical splitting of hydrogen in the presence of other impurities and reforming the hydrogen away from the impurities.

[0037] A fourteenth aspect of the invention provides for the use of an electrocatalyst comprising particles of core-shell structure, the core comprising palladium and iridium, the shell comprising one or more noble metals for the release or storage of chemical energy from a battery. [0038] Unless otherwise stated each of the integers described may be used in combination with any other integer as would be understood by the person skilled in the art. Further, although all aspects of the invention preferably "comprise" the features described in relation to that aspect, it is specifically envisaged that they may "consist" or "consist essentially" of those features outlined in the claims. In addition, all terms, unless specifically defined herein, are intended to be given their commonly understood meaning in the art.

[0039] Further, in the discussion of the invention, unless stated to the contrary, the disclosure of alternative values for the upper or lower limit of the permitted range of a parameter, is to be construed as an implied statement that each intermediate value of said parameter, lying between the smaller and greater of the alternatives, is itself also disclosed as a possible value for the parameter.

[0040] In addition, unless otherwise stated, all numerical values appearing in this application are to be understood as being modified by the term "about".

[0041] Often the metal hydrogen-battery is selected from a vanadium hydrogen battery. These systems have been found to have good stability in the presence of the electrocatalyst.

Brief Description of the Drawings

[0041] In order that the invention may be more readily understood, it will be described further with reference to the figures and to the specific examples hereinafter.

[0042] Figures la and lb show schematic drawings of a core shell structure.

[0043] Figure 2 shows water electrolyser performance of the present invention having a platinum- shell compared with an equivalent commercial platinum cathode. Figure 2 demonstrates the core-shell catalyst membrane electrode assembly of the present invention has equivalent performance to the membrane electrode assembly using a pure platinum cathode despite using significantly less platinum overall.

[0044] Figure 3 shows the present core-shell invention with a platinum shell shows fundamentally better performance for oxygen evolution (higher current densities at equivalent potentials) than a pure platinum nanoparticle. This feature offers a superior protection mechanism for both the catalyst and the catalyst support in a number electrochemical applications when they undergo transient events.

[0045] Figure 4 shows the present core-shell invention with a platinum shell shows fundamental superior durability under cell reversal conditions than an equivalent amount of a commercially available platinum.

[0046] It should be appreciated that the processes and apparatus of the invention are capable of being implemented in a variety of ways, only a few of which have been illustrated and described above.

Synthetic Method

[0047] The synthesis of the core-shell catalyst of the present invention is a two-stage process: first the core is synthesised and then the shell material deposited upon the core particle. The core is generally dispersed on a support although can be an unsupported metal nanoparticle as well. The support is often carbonaceous with examples being carbon blacks such as Vulcan XC-72R (Cabot) or Ketjen EC-300J (Akzo Nobel). The support could also be an inorganic support, such as tungsten, tungsten carbide, titanium, titanium oxides. Inorganic support with electrically conducting components could also be acceptable support materials. The nature of the support is not meant to limit the invention.

[0048] Furthermore, the support may or may not be pre-treated or functionalised in some manner to improve the catalyst surface area, dispersion, or otherwise improve the catalyst properties. The nature of the pre-treatment or functionalisation is not meant to limit the invention.

Example 1

[0049] One method of fun ctionali sing l .Og of Vulcan XC-72R carbon would be to disperse it in 50ml of deionised water and heating it to 80°C. When the slurry reaches 80°C, 3.3g of potassium hydroxide is added to the slurry and this is stirred at temperature for 5 hours. The slurry is allowed to cool overnight. The carbon is then filtered, washed thoroughly, dried and ground fine. Other methods of functionalising the carbon could be to disperse the carbon in dilute acetic acid at room temperature and dry the slurry at 300°C. The method of functionalisation is not meant to limit the invention. [0050] To apply the palladium-iridium core to the support, 0.6g of a support is dispersed in 30ml deionised water where it is well-mixed. The water is heated above 80°C, with 90°C being an example temperature, and held there while continuously mixing. Functionalised Vulcan XC-72R is an example of such a support.

[0051] The present example is for an alloy with a 1 : 1 atomic ratio of palladium to iridium. Other alloy ratios are effective and the ratio is not meant to limit the invention. 100 mg of palladium metal is needed in the present example. The palladium will generally be in salt for with palladium (II) nitrate being one example and sodium tetrachloropalladate (II) being an exemplary example. The palladium salt can be in solid form - where the appropriate amount of salt is dissolved in deionised water, nitric acid, or some other suitable solute. The dissolved palladium salt is added to the support mixture described above.

[0052] 180 mg of iridium metal is needed in the present example. The iridium will generally be in salt form with iridium (III) chloride being an exemplary example. The iridium salt is weighed out to the appropriate amount and added to the support and palladium mixture.

[0053] Sufficient time is given to allow the iridium salt to fully dissolve and for the support, palladium, and iridium to be well-mixed. The pH of the resulting solution will be acidic. The solution is brought up to a neutral pH with the addition of an alkaline agent or buffering agent. Potassium hydroxide is one example; sodium bicarbonate is an exemplary example. The pH adjusting agent is generally dissolved into deionised water with a concentration sufficient to adjust the pH towards neutral without the addition of a great excess of deionised water.

[0054] The pH of the slurry is monitored continuously at this stage and will be acidic. The solution of the pH will be added until the slurry reaches a pH of at least 6.5 but never goes above pH 7. In this example, 10ml of 1M sodium bicarbonate is generally sufficient.

[0055] When the pH is in the desired range, the metals are reduced onto the support via a chemical reduction process. This reduction can be accomplished through a number of routes: simple hydrogenation, solution of sodium hypophosphite, ethylene glycol or other polyols, or, in an exemplary example, sodium borohydride stabilised in a solution of potassium hydroxide. The concentration of the reducing agent used is a molar excess relative to the metals being reduced from their oxidation state within the slurry to the metal. This molar excess could be between 2 - 20 times excess relative to the oxidation state of the metals with 6 - 10 times excess being an exemplary example.

[0056] The reaction is given sufficient time to proceed to completion although the reaction is very fast. The catalyst slurry is then cooled, filtered, washed thoroughly and the catalyst powder dried and ground fine.

[0057] Often the catalyst powder then undergoes a heat activation step. The finely divided catalyst is heated in a suitable oven or furnace. The temperature of the heat activation step can be 80°C - 600°C with 150°C being a typical example. The catalyst is typically held at this temperature for 1 - 12 hours with 1 - 2 hours being typical of this example. The atmosphere of the heat activation step is an inert gas such as nitrogen or argon. Often the inert gas is mixed with hydrogen with the inert carrier being over 80% of the total gas content. This completes the synthesis of the catalyst core.

[0058] A shell of a noble metal or noble metals is then applied to the catalyst core. Several synthesis routes to apply the shell can be employed. One such route would involve the reduction platinum via a sodium borohydride method in an aqueous solvent vehicle which may or may not contain components to promote the formation of the platinum shell on the core particle. The method of applying the noble metal(s) shell to the core is not intended to limit the present invention.

[0059] An exemplary example to create a noble metal(s) shell on the core is a reduction route using ethanol. The catalyst core is described above is dispersed and stirred in ethanol, anhydrous and denatured ethanol being a typical example. A noble metal or noble metals are dissolved in the ethanol. An exemplary example of the noble metal constituting the shell is platinum and an example of the platinum salt is dihydrogen hexachloroplatinate (IV) sometimes known as chloroplatinic acid.

[0060] In the current example 120 mg of platinum is needed to form a shell of platinum on the core particle. A contiguous monolayer of platinum on the core particle is desired but the current invention should not be limited by pinholes within the shell, incomplete monolayer coverage, coverage greater than a monolayer, or even some discrete platinum nanoparticle formation. The core particles of the present description generally have an average diameter of - 4nm. Core particles with different average diameters would require different amounts of platinum to achieve contiguous monolayer shells.

[0061] Sufficient chloroplatinic acid is weighed out for the amount of platinum required. The chloroplatinic acid is introduced to the ethanol and core catalyst mixture. The mixture continues to stir and sufficient time is allowed for the chloroplatinic acid to fully dissociate.

[0062] Temperature is a critical parameter for shell formation. The ethanol / core / dissociated noble metal(s) is heated. The temperature can be between 30°C - 80°C with an exemplary range being 60°C - 70°C for platinum. Different cores and different noble metal shells may have a different optimum temperature range as different materials may be reduced by ethanol at different temperatures. The key is the temperature must be high enough for the reduction of the noble metal(s) to take place but not too fast as to promote the formation of discrete platinum particles alongside the shell formation.

[0063] The ethanol / core / noble metal(s) mixture is kept at the desired temperature for a few hours. A final reduction step is performed at the end of this time to ensure complete reduction of the noble metal(s) for the shell. One example is simply increasing the temperature of the mixture. An exemplary method is the addition of a small excess of potassium hydroxide to the mixture.

[0064] After the completion of the final reduction step the slurry is filtered, washed thoroughly, dried, and ground into fine powder.

[0065] Often the final core-shell catalyst powder undergoes a heat activation step. The finely divided catalyst is heated in a suitable oven or furnace. The temperature of the heat activation step can be 80°C - 600°C with 150°C being a typical example. The catalyst is typically held at this temperature for 1 - 12 hours with 1 - 2 hours being typical of this example. The atmosphere of the heat activation step is an inert gas such as nitrogen or argon. Often the inert gas is mixed with hydrogen with the inert carrier being over 80% of the total gas content. This completes the synthesis of the present invention.

[0066] Figures la and lb show general schematics of the core-shell structure wherein the platinum 2 acts as a shell encapsulating an iridium 3 / palladium 1 core. This is shown more general in figure lb wherein the core 4 of the particle is formed from a palladium iridium alloy and the shell 5 of the particle is comprises platinum. [0067] Figure 2 shows the performance in water electrolyser configuration of the electrocatalyst of the current invention and a commercial platinum offering in two otherwise similar membrane electrode assemblies. In this configuration the current invention and the platinum form the cathode of the membrane electrode assembly while the anode is composed of an iridium-ruthenium oxide. The figure shows two polarisation curves: initial performance curves for both the platinum cathode MEA and the MEA of the current invention. This data demonstrates the current invention has the same performance in a water electrolyser as a full platinum cathode despite the current invention having significantly less platinum content.

[0068] Figure 3 shows the performance of a commercial platinum electrode and an electrode formed from the electrocatalyst of the current invention for the oxygen evolution reaction (OER). Surprisingly, the core-shell catalyst performs much better for OER than the pure platinum nanoparticles. The onset potential for OER is much lower with the current invention and the electrocatalyst of the current invention produces more current (i.e. more oxygen) at equivalent potentials relative to the commercial platinum. This feature will help protect the nanoparticle or the nanoparticle support during transient events in electrochemical systems.

[0069] Figure 4 shows the performance of a commercial platinum electrode and an electrode formed from the electrocatalyst of the current invention during a 2mA/cm ² current hold to simulate cell reversal conditions. The electrocatalyst of the current invention demonstrates a lower potential to maintain the current than the platinum example and keeps the potential of the electrode well below the ~ 1.7V potential where runaway carbon corrosion of the nanoparticle support begins.

[0070] Figure 4 shows that the electrocatalysts of the invention maintain current at much lower potential before significant carbon corrosion destroys the electrode as in the platinum example shown.