FIELD OF THE INVENTION The present invention relates to a novel electrocatalyst for use in the electrolysis of water. The invention further extends to an electrolytic reactor provided with said novel electrocatalyst and a method for the electrolysis of water using the aforementioned electrolytic reactor. BACKGROUND TO THE INVENTION

Electrolysis of water involves the decomposition (i.e., "splitting") of water into oxygen and hydrogen gas by the action of an electric voltage (i.e., current) being applied to the water across electrodes of opposite polarity in the presence of an electrolyte, which could be acidic, alkaline or include a salt.

Hydrogen is produced at the negative electrode (cathode) and oxygen is produced at the positive electrode (anode), as shown by the following well- known chemical equations (in the case of an acidic or proton exchange membrane electrolyser):

Cathode (reduction): 2H ⁺ (aq) + 2e ^" -> H ₂ (g)

Anode (oxidation): 2H ₂ 0(I) -> 0 ₂ (g) + 4H ⁺ (aq) + 4e ^" An electrocatalyst is a catalyst that participates in electrochemical reactions. Catalyst materials modify and increase the rate of chemical reactions without being consumed in the process. Electrocatalysts provide a specific form of catalysts that function at electrode surfaces or may be the electrode surface itself.

An electrocatalyst can be heterogeneous such as a platinum surface or nanoparticles, or homogeneous like a coordination complex or enzyme. The electrocatalyst typically assists in transferring electrons between the electrode and reactants, and/or facilitates an intermediate chemical transformation described by an overall half-reaction.

Various electrocatalysts are known and used in the electrolysis of water. For example, US patent 5,645,930 describes an electrode for use in electrochemical reactions comprising an electrically conducting, electrocatalytically inert metal substrate or a non-metallic substrate having an electrically conducting, electrocatalytically inert metallic surface thereon and an electrocatalytically active coating consisting of:

A) a porous, dendritic, heterogeneous, electrocatalytically active primary phase coating on said substrate having a substantial internal surface area comprising a platinum group metal matrix in admixture with a particulate material; B) a secondary phase intermediate coating comprising a water insoluble, adhesion promoting polymer having a nitrogen-containing functional group and an electroless metal plating catalyst; and

C) an outer phase metal reinforcement coating comprising a transition metal or alloy thereof.

The electrically conducting, electrocatalytically inert, metallic substrate or said metallic coating on said non-metallic substrate comprises a metal selected from the group consisting of iron, steel, nickel, stainless steel, copper, cobalt, silver, and alloys thereof.

The said porous, primary phase coating is formed on a nickel substrate by a non-electrolytic reductive deposition method, an electrodeposition method, a thermal spraying method, or a sintering method, and said primary phase coating comprises a platinum group metal in admixture with a metal oxide particulate material.

The said reinforcement coating comprises a transition metal or alloy selected from the group consisting of nickel, cobalt, copper, and alloys thereof with phosphorus, boron, or sulphur. A common disadvantage of known electrocatalysts is that they have a relatively inefficient electrolysis onset energy level and current density, with pure nickel electrocatalysts for alkaline electrolysis providing a benchmark level and density, which has hardly ever been improved on in the past.

OBJECT OF THE INVENTION

There is thus a need in the art for an electrocatalyst that performs relatively better than a pure nickel electrocatalyst in the alkaline electrolysis of water and it is an object of the present invention to provide such an electrocatalyst.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided an electrocatalyst for use in the electrolysis of water, the electrocatalyst including a combination of two or more metals described by the following formula:

R Ni _y Al _z wherein:

R is one or more metals selected from the platinum-group metals; x has a value of from 1 to 40;

y has a value of from 25 to 70; and z has a value of from 1 to 20.

According to a second aspect of the invention, there is provided an electrocatalyst for use in the electrolysis of water, the electrocatalyst including a combination of two or more metals described by the following formula:

R Ni _y Al _z wherein:

R is one or more metals selected from the platinum-group metals; x has a value of from 1 to 30;

y has a value of from 25 to 70; and

z has a value of from 25 to 70.

Preferably, the platinum group metals are selected from the group consisting of platinum, ruthenium and iridium and combinations thereof.

There is provided for the use of the electorcatalyst according to the first and second aspect of the invention in a hydrogen evolution reaction, wherein the platinum group metal is ruthenium.

In an embodiment of the invention, the combination of metals forming the electrocatalyst may be deposited on an electrode (cathode and/or anode) by means of the sputtering technique, as known in the art. Alternatively, the combination of metals forming the electrocatalyst may be deposited on an electrode by means of nanoparticle deposition, as known in the art. It may further readily be appreciated that a number of other known techniques for the production of an electrocatalyst may be used, such as electrochemical deposition, hot dipping or alloying of the electrocatalytic metals. According to a preferred embodiment of the invention; R is platinum with x having a value of from 20 to 40, y of from 50 to 70 and z of from 5 to 15.

According to a third aspect of the invention, there is provided an electrolytic reactor for use in the electrolysis of water provided with an electrocatalyst according to the first or second aspects of the invention. The electrolysis of water will be readily understood by persons knowledgeable in the art as including both the Oxygen Evolution Reaction (OER) on the anode, and the Hydrogen Evolution Reaction (HER), on the cathode. According to a fourth aspect of the invention, there is provided a method for the electrolysis of water using an electrolytic reactor according to the second embodiment of the invention. According to a fifth aspect of the invention, there is provided for the use of the electrolytic reactor according to the second embodiment of the invention in the electrolysis of water.

BRIEF DESCRIPTION OF THE FIGURES AND DRAWINGS

The invention will now be described further, by way of example only, with reference to accompanying figures and photographs wherein:

Figure 1 is a view of an electrolytic reactor according to a preferred embodiment of the invention (adapted from J.S. Cooper, P.J. McGinn, J. Power Sources 163, 330 (2006)); Figure 2 is a representation of current density versus potential difference of a number of platinum, nickel and aluminium-containing electrocatalysts in comparison to catalysts according to a preferred embodiment of the invention; Figure 3 is a representation of current density versus potential difference of a number of platinum, nickel and aluminium-containing electrocatalysts in comparison to catalysts according to a second embodiment of the invention; Figure 4 is a representation of current density of platinum, nickel and aluminium electrocatalysts in comparison to catalysts according to an embodiment of the invention, where the potential difference is 1 .39V; Figure 5 is a representation of current density at a specific voltage for a number of electrocatalysts deposited onto a glassy carbon (GC) electrode according to an embodiment of the invention. The current density values for non-annealed and annealed electrocatalysts are represented; and

Figure 6 is a representation of current density of ruthenium, platinum and nickel electrocatalysts in comparision to catalysts according to a further embodiment of the invention, where the potential difference is -0.2V.

DETAILED DESCRIPTION OF THE INVENTION

An electrocatalyst according to a preferred embodiment of the invention now be described by way of example only.

The electrocatalyst according to the invention is particularly suitable for use in the electrolysis of water using an electrolytic reactor (not shown) having an anode and a cathode located in a container for an electrolyte. The electrolyte is typically an alkaline solution such as potassium chloride.

The electrocatalyst is provided as a combination of metals described by the following formula: R NiyAlz wherein:

R is one or more metals selected from the platinum-group metals; x has a value of from 1 to 40;

y has a value of from 25 to 70; and

z has a value of from 1 to 20.

Preferably, the platinum group metals are selected from the group consisting of platinum, ruthenium and iridium and combinations thereof. More preferably, R is platinum with x having a value of from 20 to 40, y of from 50 to 70 and z of from 5 to 15.

The combination of metals forming the electrocatalyst is preferably deposited on an electrode (cathode and/or anode) by means of the spluttering technique, as known in the art. Alternatively, the combination of metals forming the electrocatalyst is deposited on an electrode by means of nanoparticle deposition, as known in the art. It will be appreciated that a number of other known techniques for the production of an electrocatalyst may be used, such as electrochemical deposition, hot dipping or alloying of the electrocatalytic metals. In an embodiment of the invention, the catalyst may be deposited on the electrode by means of the sputtering technique, as known in the art. In this instance evaporative sputtering is used, with a combinatorial approach being adopted for rapid development and testing of catalyst compositions.

In use, sputtering is proceeded by calibration of the device in order to reduce deposition rate/power curves for all three metals being deposited. Sputtering is carried out under a low-pressure argon atmosphere. Following calibration, deposition is carried out individually for each electrode in a multiple electrode array.

Following deposition, the electrodes are tested in an electrolytic reactor (10) as shown in Figure 1 . The reactor includes a reference electrode (12), counter electrode (14), and a silica wafer (16) for electrode-containing pads (18) and plurality of potentiostat connections (20) for screening of individual electrodes. Here, the known potential difference across the electrodes allows measurement of the relative current densities in comparison to the counter electrode. In use, the electrolytic reactor is filled with the potassium chloride electrolyte and the electrode formed from or coated with the electrocatalyst placed in the electrolyte. A potential difference is applied across the electrodes to effect electrolysis. In comparing various combinations in accordance with the formula according to the invention, optimal combinations of electrocatalytic metals were identified. The electrolysis onset energy and current density of different combinations of electrocatalytic metals were compared and these are set out in Figures 2 to 6.

A well-accepted measure of catalytic efficiency for this process lies in comparing the onset potential and current density obtained when testing a putative catalyst against both the existing state of the art and maximum values expected under ideal conditions.

During operation, the Applicant surprisingly found that the onset potential of the electrode was significantly lower when compared to existing platinum, nickel and nickel-alloy catalysts used currently in the art (as depicted in Figures 2 and 3). Similarly, the current density of the catalyst material was surprisingly found to be raised when compared to existing platinum, nickel and nickel-alloy catalysts at the same level of potential (as presented in Figure 4 to 6). When testing the preferred embodiment of the invention, the applicant surprising found that the amount of platinum-group metals required to achieve high catalytic activity to be significantly lower than in the prior art.

It will be appreciated that variations in detail are possible with an electrocatalyst according to the invention, without departing from the scope and/or spirit of this disclosure.