Multilayer electrodes

Abstract

The present invention relates to a multilayer electrode for an electrocatalytic reaction. The multilayer electrode comprises a first layer comprising a substrate, a second layer comprising a metallic magnetic material and a third layer comprising an electrocatalyst, wherein the second layer is located between the first and the third layer. The present invention also relates to an electrochemical cell, a method of making the multilayer electrode, a method of modifying the activity of an electrocatalyst, a method of achieving a desired electrocatalytic activity in an electrocatalytic reaction, the use of a metallic magnetic material to modify the activity of an electrocatalyst and the use of the multilayer electrode to control the rate of an electrocatalytic reaction.

Description

[0001]P258212GB00 - 1 - MULTILAYER ELECTRODES FIELD OF THE INVENTION The present invention relates to a multilayer electrode for an electrocatalytic reaction, an electrochemical cell, a method of making the multilayer electrode, a method of modifying the activity of an electrocatalyst, a method of achieving a desired electrocatalytic activity in an electrocatalytic reaction, the use of a metallic magnetic material to modify the activity of an electrocatalyst and the use of the multilayer electrode to control the rate of an electrocatalytic reaction. BACKGROUND An electrochemical process involves the transfer of electrons between an electrode surface and an electrolyte. The simplest electrocatalytic reaction is the well-known Hydrogen Evolution Reaction (HER). Under neutral / basic conditions this reaction is represented by the general formula: !"#$ % &!'()& "# % !$"( Under acidic conditions this reaction is represented by the general formula: !"*% &!'()& "#The generally accepted HER mechanisms in acid solutions include the Volmer Reaction (1), which involves Hydrogen adsorption to the electrode surface (S), the Heyrovsky Reaction (2) which involves electrochemical desorption from the electrode surface, and the Tafel Reaction (3) which involves chemical desorption from the electrode surface. The formation of a dihydrogen molecule can be realised by both the Tafel and Heyrovsky steps; however, the proportion between the two pathways or the predominance of one pathway over the other can change with the electrolyte acidity and electrode material. "*% + %&'(, + - ".(1) +- ". %&"*% '(, + % "# (2)15713001.CXB.CXB P258212GB00 - 2 - !+ - "., !+ % "# (3)Equivalent HER mechanisms in neutral / basic pH solutions for the Volmer Reaction (4), the Heyrovsky Reaction (5), and the Tafel Reaction (6) are provided below. "#$ % + %&'(, + - ". %&$"((4) +- ".%&"#$ % '(, + % "# %&$"((5) !+ - "., !+ % "# (6)Due to its simplicity this reaction is often selected as a model reaction for electrochemical studies. The mechanism and kinetics of the HER have been extensively studied to improve understanding of the rate limiting steps of electrocatalytic reactions and the interaction between electrolytes and the electrocatalytic surfaces. Electrodes containing or consisting of non-ferromagnetic electrocatalyst material are routinely used in electrochemical processes to control the rate of the electrocatalytic reaction through modification of the free energy of adsorbed intermediates and transition states (TS). However, the extent to which the catalyst can control the rate of the reaction is limited by the requirement of spin conservation during the reaction which can lead to large kinetic barriers. For example, in the HER the Tafel reaction can often be the rate limiting step as it requires that the surface adsorbed hydrogen atoms have opposite spins to couple. Spin polarisation has emerged as a route to address spin-related reaction rate limitations in electrocatalytic reactions which involve intermediates and transition states of different spin- multiplicities. Particularly, attempts to enhance the rate of electrocatalytic reactions include the use of chiral molecules / structures for spin filtering or the use of an external magnetic field in tandem with a ferromagnetic electrocatalyst. However, these approaches are not compatible with conventionally used non-ferromagnetic electrocatalyst materials. Furthermore, the extent of spin polarisation achieved by applying an external magnetic field is necessarily limited by the strength and distance of the applied field, resulting in scalability and performance drawbacks. 15713001.CXB.CXB P258212GB00 - 3 - Additionally, these spin polarisation routes fail to cater for in-situ tuneability of the reactivity at the electrode surface. During an electrochemical process, the desired electrocatalytic reaction rate may require a reduction rather than an enhancement of the catalytic activity without significantly modifying the electrocatalyst. It is one object of the present invention to overcome at least some of the disadvantages of the prior art or to provide a commercially useful alternative thereto. It is a further object of the present invention to provide a multilayer electrode that generates in-situ magnetic field at the electrode surface in electrocatalytic reactions so that no external magnetic field is required. It is a further object of the present invention to provide a multilayer electrode with tuneable reactivity at the electrode surface. It is a further object of the present invention to provide a method of modifying the activity of an electrocatalyst in a multilayer electrode without having to substantially modify the electrocatalyst. It is a further object of the present invention to provide a method of achieving a desired electrocatalytic activity in an electrocatalytic reaction that is compatible with a range of electrocatalytic materials, including conventional electrocatalytic materials. It is a further object of the present invention to provide a multilayer electrode for use in an electrocatalytic reaction with improved scalability and enhanced magnetic gradient at the electrolyte surface. SUMMARY OF THE INVENTION In a first aspect the invention provides a multilayer electrode for an electrocatalytic reaction comprising: a first layer comprising a substrate; a second layer comprising a metallic magnetic material; and a third layer comprising an electrocatalyst; wherein the second layer is located between the first layer and the third layer. 15713001.CXB.CXB P258212GB00 - 4 - In a second aspect the invention provides an electrochemical cell comprising the multilayer electrode of the first aspect, the electrochemical cell further comprising: a counter electrode; and an electrolyte. In a third aspect the invention provides a method of making the multilayer electrode of the first aspect, the method comprising: i) providing a first layer comprising a substrate; ii) applying a second layer on a surface of the first layer, the second layer comprising a metallic magnetic material; iii) applying a third layer on a surface of the second layer, the third layer comprising an electrocatalyst. In a fourth aspect the invention provides a method of modifying the activity of an electrocatalyst, the method comprising: i) providing a multilayer electrode comprising: a first layer comprising a substrate; a second layer comprising a metallic magnetic material; and a third layer comprising the electrocatalyst; wherein the second layer is located between the first layer and the third layer; ii) determining a spin diffusion length lsd of the electrocatalyst; iii) determining the thickness lc of the third layer; and iv) modifying the lc:lsdratio. In a fifth aspect the invention provides a method of achieving a desired electrocatalytic activity in an electrocatalytic reaction, the method comprising: i) determining the desired electrocatalytic activity; ii) preparing a multilayer electrode comprising: a first layer comprising a substrate; a second layer comprising a metallic magnetic material; and a third layer comprising an electrocatalyst; 15713001.CXB.CXB P258212GB00 - 5 - wherein the second layer is located between the first layer and the third layer; and selecting a lc:lsdratio; wherein lc is the thickness of the third layer and wherein lsd is the spin diffusion length of the electrocatalyst; and iii) using the multilayer electrode in the electrocatalytic reaction. In a sixth aspect the invention provides a use of a metallic magnetic material to modify the activity of an electrocatalyst, the use comprising: i) inserting the metallic magnetic material as a second layer between a first layer comprising a substrate and a third layer comprising the electrocatalyst. In a seventh aspect the invention provides a use of the multilayer electrode of the first aspect to control the rate of an electrocatalytic reaction. It has been found that using a multilayer electrode comprising a metallic magnetic material beneath a layer of an electrocatalyst can achieve spin selective electrolysis at non- ferromagnetic surfaces. Without wishing to be bound by theory, it is believed that this is due to the generation of an in-situ magnetic field, intrinsic to the electrode. As a result of the in-situ magnetic field no modification of the electrocatalyst used is necessary and a wider range of electrocatalytic materials can be used. It has also been found that the in-situ generated magnetic field in the multilayer electrodes of the present invention provides enhanced magnetic gradient at the electrolyte surface in comparison to externally applied magnetic fields. This in turn results in improved scalability of the electrochemical process. It has been found that the use of a ferromagnetic material beneath an electrocatalyst layer can result in, for example, a two-fold change in rate in the Hydrogen Evolution Reaction (HER). Without wishing to be bound by theory, it is believed that there is a transfer of ferromagnetism from the ferromagnet to the originally non-magnetic transition metal through proximity induced magnetism (PIM). It is believed that the induced magnetism results in spin polarisation effects at the electrode surface which break the scaling relationships for the electrochemical protons to H2(hydrogen) step in the Hydrogen Evolution Reaction (HER). Scaling relationships are understood by the skilled person to govern the energy profile of a reaction. It is believed that spin polarisation effects may 15713001.CXB.CXB P258212GB00 - 6 - break scaling relationships between reactants and products in the presence of a catalyst by stabilising or destabilising the transition state. It has also been found that the activity of an electrocatalyst can be modified by changing the ratio of the spin diffusion length lsdof the electrocatalyst and the thickness of the outermost electrode layer comprising the electrocatalyst lc. It has also been found that the tuneability of the electrocatalyst activity can be used to control and achieve a desired electrocatalytic activity without substantial structural modification of the electrocatalyst. Other preferred embodiments of the compositions, formulations, methods, and uses provided herein appear throughout the specification and in particular in the examples. Each aspect or embodiment as defined herein may be combined with any other aspect(s) or embodiment(s) unless clearly indicated to the contrary. In particular any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. DETAILED DESCRIPTION OF THE INVENTION Unless otherwise defined herein, scientific and technical terms used in connection with the present invention shall have the meanings that are commonly understood by those of ordinary skill in the art. The meaning and scope of the terms should be clear, however, in the event of any latent ambiguity, definitions provided herein take precedence over any dictionary or extrinsic definition. As used in the specification and the appended claims, unless specified to the contrary, the following terms have the meaning indicated: As used herein, the term “perpendicular magnetic anisotropy ferromagnetic material” refers to a material which exhibits perpendicular magnetic anisotropy (PMA). Materials with perpendicular magnetic anisotropy have a strong stray field in remanence when an out-of- plane field is applied, i.e., these materials allow magnetization to remain out-of-plane once the external out-of-plane magnetic field is removed. This is illustrated in the magnetogram 15713001.CXB.CXB P258212GB00 - 7 - of Fig 2. Perpendicular magnetic anisotropy ferromagnetic materials are known in the art. Perpendicular magnetic anisotropy ferromagnetic materials may include superlattice structures comprising at least one magnetic material layer with a thickness less than about 1.5 nm. An example of such a superlattice structure is [Pt / Co68B32 / Ir]n, where the magnetic material is the Cobalt Boride, the Cobalt Boride layer having a thickness of less than about 1.5 nm, and n is the number of repetitions of the three sublayers. As used herein, the term “non-perpendicular magnetic anisotropy ferromagnetic material” refers to a material which does not exhibit perpendicular magnetic anisotropy (non-PMA). These materials are reticent to hold out-of-plane magnetisation and exhibit small or negligible remanence of the out-of-plane magnetic field once the out-of-plane magnetic field is removed. This is illustrated in the magnetogram of Fig 3. Non-Perpendicular magnetic anisotropy ferromagnetic materials are known in the art. Non-Perpendicular magnetic anisotropy ferromagnetic materials may include superlattice structures comprising a magnetic material layer with a thickness greater than about 1.5 nm. An example of such a superlattice structure is [Pt / Co68B32 / Ir]n, where the magnetic material is the Cobalt Boride, the Cobalt Boride layer having a thickness of greater than about 1.5 nm, and n is the number of repetitions of the three sublayers. As used herein, the term “electrocatalyst” refers to a catalyst that participates in electrochemical reactions at the electrode surface or as the electrode surface itself. Electrocatalysts may assist in the formation and transformation of reactant intermediates and in the transfer of electrons between the electrode and reactants. Suitable electrocatalyst materials include Transition Metals such as Iron, Rhodium, Palladium, Iridium, Platinum, Gold, Copper and Silver and their alloys, as well as oxides, nitrides and sulfides thereof. As used herein, the term “spin diffusion length of the electrocatalyst, lsd” is the distance at which the spin polarisation falls by 1 / e. As understood by the skilled person in the art, spin diffusion lengths are dependent on multiple variables including temperature and the specific electrocatalyst metal present in the third layer. For example, for Au, !sdis about 32 nm at 300 K. Meanwhile, for Pt, !sdis about 2.0 nm at 300 K. For Cu, !sdis about 100 nm at 300 K. For Ag, !sd is about 180 nm at 300 K. 15713001.CXB.CXB P258212GB00 - 8 - In a first aspect there is provided a multilayer electrode for an electrocatalytic reaction comprising: a first layer comprising a substrate; a second layer comprising a metallic magnetic material; and a third layer comprising an electrocatalyst; wherein the second layer is located between the first layer and the third layer. Preferably, the metallic magnetic material is a ferromagnetic material. More preferably, the second layer comprises a perpendicular magnetic anisotropy ferromagnetic material or a non–perpendicular magnetic anisotropy ferromagnetic material. In one preferred embodiment, the second layer comprises a perpendicular magnetic anisotropy ferromagnetic material. In an alternative preferred embodiment, the second layer comprises a non–perpendicular magnetic anisotropy ferromagnetic material. Without wishing to be bound by theory, it is believed that a second layer comprising a perpendicular magnetic anisotropy ferromagnetic material (PMA) or a non–perpendicular magnetic anisotropy ferromagnetic material (non-PMA) results in spin polarising effects at the electrode surface which may influence the rate of electrocatalytic reactions. The second layer may comprise two or more sublayers. Preferably, the second layer comprises at least one sublayer of the metallic magnetic material. Preferably, the second layer further comprises at least one sublayer comprising a heavy metal selected from the group consisting of Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg and Bi. More preferably, the second layer comprises at least one sublayer comprising platinum and / or at least one sublayer comprising iridium. Even more preferably, the second layer comprises at least one sublayer comprising platinum and at least one sublayer comprising iridium. Without wishing to be bound by theory, it is believed that the presence of at least one sublayer comprising platinum and / or at least one sublayer comprising iridium results in spin polarisation effects at the electrode / electrolyte interface. Alternatively, in some embodiments, the second layer consists of one layer of the metallic magnetic material. 15713001.CXB.CXB P258212GB00 - 9 - Preferably, the metallic magnetic material is selected from one or more of chromium, manganese, iron, nickel, cobalt, and borides, oxides, or alloys thereof. In one preferred embodiment, the metallic magnetic material is selected from one or more of chromium, manganese, iron, nickel, and borides, oxides, or alloys thereof, or a boride or oxide of cobalt. In some embodiments, the metallic magnetic material comprises iron cobalt boride (FeaCobBc), wherein a = 1 to 59, b = 1 to 59, and c = 1 to 40, and / or cobalt boride (CoxBy), wherein x = 60 to 99 and y = 1 to 40, preferably wherein x = 65 to 99 and y = 1 to 35. In one preferred embodiment, the metallic magnetic material comprises or consists of a cobalt boride (CoxBy). In a further preferred embodiment, the metallic magnetic material comprises CoxBy, wherein x = 60 to 99 and y = 1 to 40, preferably wherein x = 65 to 99 and y = 1 to 35. Suitable Cobalt Borides include Co68B32, for example. The use of borides is believed to improve the electrode properties. Particularly, borides are thought to lead to smoother surfaces and reduced grain boundaries. In alternative embodiments, the metallic magnetic material comprises or consists of a cobalt and iron alloy, preferably CodFee, wherein d = 1 to 99 and e = 1 to 99, more preferably wherein d = 1 to 75 and e = 25 to 99 or wherein d = 1 to 50 and e = 50 to 99. Suitable cobalt and iron alloys are known in the art, including Co25Fe75, for example.The combined presence of a metallic magnetic material, e.g., cobalt boride, and platinum and / or iridium sublayers, is thought to result in a superlattice structure. For example, the superlattice structure may be [Pt / Co68B32 / Ir]n where the magnetic material is the Cobalt Boride and n is the number of repetitions of the three sublayers. In some preferable embodiments, the sublayer comprising a metallic material, preferably cobalt boride, has a thickness of less than about 1.5 nm, which is believed to result in a material with intrinsic perpendicular magnetic anisotropy. This is illustrated in Fig 2. In other preferable embodiments, the sublayer comprising a metallic material, preferably cobalt boride, has a thickness greater than about 1.5 nm, which is believed to result in a non-perpendicular magnetic anisotropy material. This is illustrated in Fig 3. 15713001.CXB.CXB P258212GB00 - 10 - Without wishing to be bound by theory, it is believed that these perpendicular magnetic anisotropy and non-perpendicular magnetic anisotropy superlattice structures induce spin polarisation at the electrode / electrolyte interface, thereby affecting the activity of the electrocatalyst. In a preferred embodiment, the second layer consists of a multilayer repeat structure represented by the following formula: (X / M / X)n wherein X is a heavy metal individually selected from Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, and Bi; wherein M is a magnetic material sublayer; wherein n is the number of repeat units of the multilayer structure present in the second layer and is in the range of 1 to 10. Preferably, each X is individually selected from Pt and Ir. Preferably, M is selected from one or more of chromium, manganese, iron, nickel, cobalt, and borides, oxides, or alloys thereof. In one preferred embodiment, M is selected from one or more of chromium, manganese, iron, nickel, and borides, oxides, or alloys thereof, or a boride or oxide of cobalt. In a preferred embodiment, M is iron cobalt boride (FeaCobBc, wherein a = 1 to 59, b = 1 to 59, and c = 1 to 40) and / or cobalt boride (CoxBy wherein x = 60 to 99 and y = 1 to 40, preferably wherein x = 65 to 99 and y = 1 to 35). Preferably, n is in the range of 1 to 9, or 1 to 8, or 2 to 8, or 2 to 7, or 2 to 6, or 3 to 6, or 3 to 5. In a preferred embodiment, the second layer consists of a multilayer repeat structure selected from (Pt / CoxBy / Ir)n, (Pt / CoxBy / Pt)n, (Ir / CoxBy / Ir)n, (Pt / FeaCobBc / Ir)n, (Pt / FeaCobBc / Pt)n, or (Ir / FeaCobBc / Ir)n, wherein n is 3 to 5, and wherein x = 60 to 99 and y = 1 to 40, preferably wherein x = 65 to 99 and y = 1 to 35, and wherein a = 1 to 59, b = 1 to 59, and c = 1 to 40. 15713001.CXB.CXB P258212GB00 - 11 - The third layer may comprise two or more sublayers, with the proviso that the outermost sublayer of the third layer comprises or consists of the electrocatalyst. It is understood that the outermost sublayer of the third layer is the sublayer that is furthest away from the first and second layers in the multilayer electrode, and, in use, the outermost sublayer of the third layer is the sublayer that will be in contact with the electrolyte. In some embodiments, the third layer may comprise one or more protective sublayers beneath the electrocatalyst sublayer. When one or more protective sublayers are present, one of the protective sublayers will be in direct contact with the second layer. A protective sublayer can be made of any non-magnetic material and are substantially free of the magnetic material present in the second layer of the electrode. Furthermore, the protective sublayer is void of a heavy metal individually selected from Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, and Bi. In one preferred embodiment, the third layer consists of one protective sublayer and one (outermost) sublayer comprising or consisting of the electrocatalyst. Alternatively, in more preferred embodiments, the third layer consists of the electrocatalyst, preferably one layer of the electrocatalyst. Preferably, the electrocatalyst comprises at least one metal selected from gold, platinum, copper, and silver. In one preferred embodiment, the electrocatalyst comprises gold or platinum. Preferably, the electrocatalyst comprises or consists of gold. Alternatively, preferably, the electrocatalyst comprises or consists of platinum. Alternatively, in another preferred embodiment, the electrocatalyst comprises copper or silver. Preferably, the electrocatalyst comprises or consists of copper. Alternatively, preferably, the electrocatalyst comprises or consists of silver. Preferably, the multilayer electrode is further characterised by a lc:lsd ratio in the range of 100:1 to 1:100, where lc is the thickness of the third layer, and lsd is the spin diffusion length of the electrocatalyst. More preferably the lc:lsd ratio is in the range of 50:1 to 1:50, more preferably 30:1 to 1:30. 15713001.CXB.CXB P258212GB00 - 12 - In a preferred embodiment, the electrocatalyst comprises or consists of gold and the lc:lsdratio is in the range of 30:1 to 1:30, more preferably in the range of 30:1 to 1:20, or 30:1 to 1:10, or 30:1 to 1:6, or 20:1 to 1:20, or 10:1 to 1:20, or 1:1 to 1:20, or 1:5 to 1:20, or 1:10 to 1:20. Alternatively, preferably, the electrocatalyst comprises or consists of gold and the lc:lsd ratio is in the range of 20:1 to 1:30, or 10:1 to 1:30, or 5:1 to 1:30, or 1:1 to 1:20, or 1:2 to 1:10, or 1:2 to 1:6. In another preferred embodiment, the electrocatalyst comprises or consists of platinum and the lc:lsd ratio is in the range of 100:1 to 1:10, or 100:1 to 1:5, or 10:1 to 1:3, or 10:1 to 1:1. Alternatively, preferably, the electrocatalyst comprises or consists of platinum and the lc:lsd ratio is in the range of 50:1 to 1:10, or 10:1 to 1:10, or 1:1 to 1:10, or 1:2 to 1:10. The first layer comprises a substrate. Suitable substrates are known in the art. For example, the substrate may comprise or consist of non-conductive or conductive supports. Examples of non-conductive supports include silicon dioxide. Examples of conductive supports include carbon-based materials, conductive glasses such as Fluorine-doped Tin Oxide (FTO), and metallic supports such as Ni, stainless steel or Ti based supports. In one preferable embodiment, the substrate comprises or consists of a non-conductive support, for example the substrate may comprise or consist of silicon dioxide. In an alternative embodiment, the substrate comprises or consists of conductive material. The first layer may comprise two or more sublayers, with the proviso that the first layer comprises at least one substrate sublayer. Preferably, the first layer further comprises a seed sublayer deposited on the substrate, and the seed sublayer is in contact with the second layer. In a preferred embodiment, the seed sublayer comprises tantalum and / or titanium. In a further preferred embodiment, the seed sublayer comprises or consists of tantalum. Without wishing to be bound by theory, it is believed that when the second layer comprises at least one sublayer of a heavy metal selected from Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg and Bi, preferably platinum and / or iridium, the presence of tantalum and / or titanium in the seed sublayer promotes crystallographic growth of the at least one sublayer of the heavy metal. Crystallographic growth of the at least one sublayer of the heavy metal is thought to promote perpendicular magnetic anisotropy in the second layer. 15713001.CXB.CXB P258212GB00 - 13 - In preferred embodiments, the first layer consists of one layer of the substrate and one seed sublayer deposited on the substrate. Alternatively, in some embodiments, the first layer consists of one layer of the substrate. Preferably, the sum of the thickness of the second layer and the thickness of the third layer is from 10 nm to 30 nm, or from 10 nm to 20 nm, or from 15 nm to 18 nm. Without wishing to be bound by theory, it is believed that a combined thickness of the second and third layers of from 10 nm to 30 nm, particularly from 15 nm to 18 nm, may ensure that the spin polarising effect of the second layer extends to the electrode surface, thereby affecting the activity of the electrocatalyst. When the second layer comprises a perpendicular magnetic anisotropy ferromagnetic material, it is believed that a combined thickness of the second and third layers of from 10 nm to 30 nm, particularly from 15 nm to 18 nm, may ensure that the magnetising effect of the in situ magnetic field extends to the electrode surface. Preferably, the thickness of the third layer is from 5 nm to 15 nm, or from 10 nm to 15 nm, or from 5 nm to 10 nm, or from 6 nm to 14 nm, or from 7 nm to 13 nm, or from 8 nm to 12 nm, or from 9 nm to 11 nm, preferably about 10 nm. Alternatively, in some embodiments, the thickness of the third layer is from 0.5 nm to 10 nm, or from 1.0 nm to 9 nm, or from 1.5 nm to 8 nm, or from 2 nm to 7 nm or from 2.5 nm to 6 nm, or from 3 nm to 5 nm. Without wishing to be bound by theory, it is believed that a third layer thickness of 5 nm to 15 nm allows for improved control of the electrocatalyst activity at the electrode / electrolyte interface, when the electrocatalyst layer comprises gold, for example. Particularly, it is believed that a thickness of about 10 nm may be an optimum thickness to ensure that spin polarisation effects are present at the electrode surface. When the second layer comprises a perpendicular magnetic anisotropy ferromagnetic material, it is also believed that a thickness of about 10 nm may be an optimum thickness to ensure that magnetic effects are present at the electrode surface. 15713001.CXB.CXB P258212GB00 - 14 - Without wishing to be bound by theory it is believed that a third layer thickness of 2 to 5 nm allows for improved control of the electrocatalyst activity at the electrode / electrolyte interface, when the electrocatalyst comprises platinum, for example. Particularly, it is believed that a thickness of about 3 to 5 nm may be an optimum thickness to ensure that spin polarisation effects are present at the electrode surface. When the second layer comprises a perpendicular magnetic anisotropy ferromagnetic material, it is also believed that a thickness of about 3 to 5 nm may be an optimum thickness to ensure that magnetic effects are present at the electrode surface. In a second aspect, the invention provides an electrochemical cell comprising the multilayer electrode of the first aspect, the electrochemical cell further comprising: a counter electrode; and an electrolyte. Preferably, the electrolyte is a solid, a liquid, or a gel. Solid electrolytes may include ionically conductive polymers such as ionomers, cation exchange membranes, anion exchange membranes or porous diaphragms, and ionically conductive ceramics such as ceramic oxide electrolytes. Liquid electrolytes may include acidic solutions, neutral solutions, alkaline solutions, and ionic liquids. For example, a liquid electrolyte may be an aqueous or non-aqueous solvent containing a dissolved salt as a supporting electrolyte. Suitable salts include KCl, HCl, H2SO4, NaOH, KOH or KHCO3. Gel electrolytes may include any gel polymer electrolyte known in the art. More preferably, the electrolyte is a solid and comprises a polymeric and / or ceramic material. More preferably, the electrolyte is an ionomer or a ceramic electrolyte. Ionomeric electrolytes include commercially available ionomers such as Nafion®, Sustainion® and Fumapem®. Ceramic materials may include oxide-based inorganic solids. Examples of oxide-based inorganic solids include materials with a YSZ-based structure, a ScSZ-based structure, a ceria-based structure, and a lanthanum gallate structure. Preferably, the electrolyte comprises one or more ionomers. In one preferred embodiment, the electrolyte comprises at least one ionomer selected from a perfluorinated polymer with sulfonic or carboxylic ionic functional groups (such as commercially available Nafion® or Fumapem®) and an imidazolium functionalized styrene polymer (such as commercially available Sustainion®). 15713001.CXB.CXB P258212GB00 - 15 - Preferably, the electrolyte generates an acidic environment at the electrocatalyst with a local pH of from 1 to 5, or from 2 to 4, or from 1 to 3, or from 1 to 2. Preferably, the electrolyte is an acidic solution with a pH of from 1 to 5, or from 1 to 4, or from 1 to 3, or from 1 to 2, or from 2 to 4, or from 2 to 3. Alternatively, preferably, the electrolyte is a basic solution with a pH of from 8 to 14, or from 8 to 13, or from 8 to 12, or from 8 to 11, or from 8 to 10, or from 8 to 9, or from 8.5 to 9, preferably about 8.8. Alternatively, in some embodiments, the electrolyte is a basic solution with a pH of from 8 to 14, or from 9 to 14, or from 9 to 13.5, or from 9 to 13, or from 10 to 14, or from 10 to 13.5, or from 10 to 13, or from 11 to 14, or from 11 to 13.5, or from 11 to 13, or from 12 to 14, or from 12 to 13.5, or from 12 to 13, or from 13 to 14, or from 13.5 to 14. The counter electrode may comprise or consist of any material suitable for the counterpart reaction. As understood by the skilled person in the art, the counterpart reaction is the other half of the reaction not occurring at the working electrode, for example, an oxidation reaction such as an oxygen evolution reaction, chlorine evolution reaction or carbon oxidation reaction. Examples of counter electrodes include supported oxides such as IrOx, RuOx and mixtures thereof, Ni, Ni(Fe) oxides, carbon, Pt, Au, and Ag. Preferably, the counter electrode comprises or consists of platinum or iridium. In some embodiments, the electrochemical cell is a battery. In some embodiments, the electrochemical cell further comprises a working chamber and a counter chamber, wherein the counter electrode is located inside the counter chamber. The working chamber may comprise a reference electrode. The reference electrode may comprise or consist of Pt, Hg / Hg2Cl2, Hg / Hg2SO4, Ag / AgSO4, Pd / H2, Hg / HgO or Ag / AgCl. Preferably, the reference electrode comprises or consists of Hg / HgO or Ag / AgCl. In some embodiments, the electrochemical cell contains three electrodes: a counter electrode, a reference electrode, and the multilayer electrode of the first aspect of the invention. The counter electrode may be located in a counter chamber and the multilayer 15713001.CXB.CXB P258212GB00 - 16 - electrode of the first aspect of the invention and the reference electrode may be located in a working chamber. In a third aspect, the invention provides a method of making the multilayer electrode of the first aspect, the method comprising: i) providing a first layer comprising a substrate; ii) applying a second layer on a surface of the first layer, the second layer comprising a metallic magnetic material; iii) applying a third layer on a surface of the second layer, the third layer comprising an electrocatalyst. Methods of applying layers to multilayer electrodes are known in the art. For example, layers / sub-layers may be applied to a substrate / other layers / sub-layers using sputtering techniques known in the art, for example by using DC magnetron sputtering in an inert atmosphere at low pressures or in a vacuum chamber. Preferred embodiments for the multilayer electrode, first layer, substrate, second layer, metallic magnetic material, third layer, and electrocatalyst are as described above in relation to the multilayer electrode of the first aspect. In a fourth aspect the invention provides a method of modifying the activity of an electrocatalyst, the method comprising: i) providing a multilayer electrode comprising: a first layer comprising a substrate; a second layer comprising a metallic magnetic material; and a third layer comprising the electrocatalyst; wherein the second layer is located between the first layer and the third layer; ii) determining a spin diffusion length lsd of the electrocatalyst; iii) determining the thickness lc of the third layer; and iv) modifying the lc:lsd ratio. The spin diffusion length lsdof the electrocatalyst may be determined in step (ii) by spin absorption, as described in Miren Isasa, Estitxu Villamor, Luis E. Hueso, Martin Gradhand, 15713001.CXB.CXB P258212GB00 - 17 - and Fèlix Casanova, Temperature dependence of spin diffusion length and spin Hall angle in Au and Pt. Physics Review B 91, 024402 (2015), which is incorporated herein by reference insofar as it relates to the determination of spin diffusion length by spin absorption (see e.g., page 024402-1 column 2 lines 17 to 22, and equation (1) on page 024402-2, as well as the spin diffusion lengths reported for Pt and Au at 300 K in Table I on page 024402-3). In preferred embodiments, the step of ii) determining a spin diffusion length lsdof the electrocatalyst may be carried out by checking the value reported in published literature for the relevant electrocatalyst, as determined by spin absorption at a specific temperature (e.g., 300 K). Without wishing to be bound by theory, it is believed that the activity of the electrocatalyst at the electrode surface can be tuned by modifying the thickness of the third layer (electrocatalyst layer) and / or the spin diffusion length of the electrocatalyst. Preferably, the lc:lsdratio is modified by altering the thickness lcof the third layer. Alternatively, the lc:lsd ratio is modified by altering the spin diffusion length lsd of the electrocatalyst. Alternatively, the lc:lsd ratio is modified by altering the spin diffusion length lsdof the electrocatalyst and the thickness lcof the third layer. The multilayer electrode, first layer, substrate, second layer, metallic magnetic material, third layer, electrocatalyst, spin diffusion length lsdof the electrocatalyst, thickness lcof the third layer, and lc:lsdratio, and preferred embodiments thereof, are as described above in relation to the multilayer electrode of the first aspect. Preferably, the lc:lsdratio is modified to the lc:lsdratio of the first aspect, that is, preferably, the modified lc:lsd ratio is in the range of 100:1 to 1:100, more preferably 50:1 to 1:50, more preferably still 30:1 to 1:30. In a preferred embodiment, the electrocatalyst comprises or consists of gold and the lc:lsd ratio is modified to be in the range of 30:1 to 1:30, more preferably 30:1 to 1:20, or 30:1 to 1:10, or 30:1 to 1:6, or 20:1 to 1:20, or 10:1 to 1:20, or 1:1 to 1:20, or 1:5 to 1:20, or 1:10 to 1:20. Alternatively, preferably, the electrocatalyst comprises or consists of gold and the lc:lsd ratio is modified to be in the range of 20:1 to 1:30, or 10:1 to 1:30, or 5:1 to 1:30, or 1:1 to 1:20, or 1:2 to 1:10, or 1:2 to 1:6. 15713001.CXB.CXB P258212GB00 - 18 - In another preferred embodiment, the electrocatalyst comprises or consists of platinum and the lc:lsdratio is modified to be in the range of 100:1 to 1:10, or 100:1 to 1:5, or 10:1 to 1:3, or 10:1 to 1:1. Alternatively, preferably, the electrocatalyst comprises or consists of platinum and the lc:lsd ratio is modified to be in the range of 50:1 to 1:10, or 10:1 to 1:10, or 1:1 to 1:10, or 1:2 to 1:10. In a fifth aspect, the invention provides a method of achieving a desired electrocatalytic activity in an electrocatalytic reaction, the method comprising: i) determining the desired electrocatalytic activity; ii) preparing a multilayer electrode comprising: a first layer comprising a substrate; a second layer comprising a metallic magnetic material; and a third layer comprising an electrocatalyst; wherein the second layer is located between the first layer and the third layer; and selecting a lc:lsd ratio; wherein lc is the thickness of the third layer and wherein lsd is the spin diffusion length of the electrocatalyst; and iii) using the multilayer electrode in the electrocatalytic reaction. The multilayer electrode, first layer, substrate, second layer, metallic magnetic material, third layer, electrocatalyst, spin diffusion length lsd of the electrocatalyst, thickness lc of the third layer, and lc:lsd ratio, and preferred embodiments thereof, are as described above in relation to the multilayer electrode of the first aspect. Preferably, the method further comprises contacting the multilayer electrode with an electrolyte. The electrolyte, and preferred embodiments thereof, are as described above in relation to the electrochemical cell of the second aspect. Preferably, the electrocatalytic reaction is selected from the group consisting of a hydrogen evolution reaction, a water oxidation reaction, an oxygen reduction reaction, a carbon dioxide reduction reaction, and a nitrogen reduction reaction. More preferably, the electrocatalytic reaction is a hydrogen evolution reaction. 15713001.CXB.CXB P258212GB00 - 19 - An electrocatalytic reaction may require a higher / lower reaction rate based on reaction conditions or desired outcome. Without wishing to be bound by theory, it is believed that the activity of the electrocatalyst at the electrode surface can be tuned by modifying the thickness of the third layer (electrocatalyst layer) and / or the spin diffusion length of the electrocatalyst. Therefore, the claimed method allows a desired electrocatalytic activity to be achieved to meet these needs. In particular, when a faster reaction rate is required, an enhanced electrocatalytic activity may be achieved by selecting an appropriate ratio. Similarly, by selecting a different lc:lsdratio, the activity of the electrocatalyst may be reduced, thereby allowing a lower reaction rate to be achieved, if desired. The temperature of the electrocatalytic reaction may be in the range of less than or equal to 1,300 Kelvin (K), preferably less than or equal to 600 K, more preferably less than or equal to 500 K, more preferably less than or equal to 400 K, more preferably less than or equal to 373 K, most preferably less than or equal to 353 K. In some preferred embodiments, the temperature of the electrocatalytic reaction is in the range of from 0 K to 1,300 K, preferably 100 K to 600 K, more preferably 150 K to 500 K, more preferably 200 K to 400 K, more preferably 273 K to 373 K, most preferably 273 K to 353 K, or 293 K to 353 K. In a sixth aspect the invention provides a use of a metallic magnetic material to modify the activity of an electrocatalyst, the use comprising: i) inserting the metallic magnetic material as a second layer between a first layer comprising a substrate and a third layer comprising the electrocatalyst. The first layer, substrate, second layer, metallic magnetic material, third layer, electrocatalyst, spin diffusion length lsdof the electrocatalyst, thickness lcof the third layer, and lc:lsdratio, and preferred embodiments thereof, are as described above in relation to the multilayer electrode of the first aspect. In a seventh aspect the invention provides a use of the multilayer electrode of the first aspect to control the rate of an electrocatalytic reaction. FIGURES Figure 1 illustrates the structure of a multilayer electrode according to the present invention. 15713001.CXB.CXB P258212GB00 - 20 - Figure 2 illustrates the structure and composition of a multilayer electrode with Perpendicular Magnetic Anisotropy (PMA). The hysteresis loops represent the sample’s magnetic behaviour when an out-of-plane and in-plane magnetic field is applied. As can be observed, the out-of-plane magnetic field results in large remanence which is indicative of PMA. Figure 3 illustrates the structure and composition of a multilayer electrode without Perpendicular Magnetic Anisotropy (PMA). The hysteresis loops represent the sample’s magnetic behaviour when an out-of-plane and in-plane magnetic field is applied. As can be observed, the out-of-plane magnetic field results in small or negligible remanence which is indicative of a lack of PMA.Figure 4 (b) refers to a geometric current density measured on a series of Pt capped multi-layer magnetic electrodes at -0.15 V versus the RHE reference electrode during a series of linear sweep voltammograms (0 to -0.15 V, 20mV s-1) in Ar-saturated 0.5 M KHCO3 solution. The current is due to the hydrogen evolution reaction. Figure 4 (a) refers to electrochemical data recorded during the 50thlinear sweep voltammogram of each sample. In this Figure, “IP” stands for in-plane magnetisation (i.e. non-PMA materials), “non-mag” refers to a non-magnetic sample, “big-dom” and “mazy” represent samples with out-of- plane magnetisation (PMA). Figure 5(a) is a schematic illustration of different HER reaction mechanisms under acidic and basic conditions. Figure 5(b) refers to a cathodic polarisation test on Au surfaces in ultrapure 0.1 M HClO4 solution (acidic conditions). Figure 5(c) represents the same test inpre-electrolysed 0.5 M KHCO3 solution (near-alkaline conditions). Figure 5(D) representsthe same test in pre-electrolysed 1 M KOH solution (alkaline conditions). In these graphs, “PCI” is a superlattice with structure Pt / Co68B32 / Ir and NMS stands for non-magnetic substrate. Figure 6 refers to a Linear sweep voltammograms (0 to -0.15 V, 20 mV s-1) in Ar-saturated 0.5 M KHCO3 solution for a series of Au capped multi-layer magnetic electrodes. In this figure, “non-mag” refers to nonmagnetic samples, “no OOP” refers to samples with no out- of-plane magnetisation (i.e. non-PMA materials), “big dom” and “mazey” refer to samples with out-of-plane magnetisation (i.e. PMA materials). 15713001.CXB.CXB P258212GB00 - 21 - Figure 7(a) refers to a cathodic polarisation test on Au surfaces in ultrapure 0.5 M KHCO3 solution (near-alkaline conditions) for different thickness layers of Au electrocatalyst. Figure 7(b) shows the current recorded during repeated experiments. Figure 7(c) is a schematic illustration of different HER reaction mechanisms under acidic and basic conditions. Figure 7(d) shows morphology of an Au capped electrode with a magnetic underlayer as measured by AFM prior to electrochemical studies and after the experiments shown in Figure 7. There is no significant roughening of the Au surface measured. EXPERIMENTAL SECTION Test Methods As is known in the art, spin diffusion lengths lsdof electrocatalysts can be measured by a range of techniques, including spin absorption, spin-torque ferromagnetic resonance, Hall Cross, spin pumping, and lateral spin valve. For the purposes of this application, the spin diffusion length lsdof the electrocatalyst is measured by spin absorption as described in Miren Isasa, Estitxu Villamor, Luis E. Hueso, Martin Gradhand, and Fèlix Casanova, Temperature dependence of spin diffusion length and spin Hall angle in Au and Pt. Physics Review B 91, 024402 (2015), which is incorporated herein by reference insofar as it relates to the determination of spin diffusion length by spin absorption (see e.g., page 024402-1 column 2 lines 17 to 22, and equation (1) on page 024402-2, as well as the spin diffusion lengths reported for Pt and Au at 300 K in Table I on page 024402-3). Unless otherwise indicated, spin diffusion lengths lsddisclosed herein are measured at 300 Kelvin (K). The thickness lcof the third layer, or indeed any layer or sublayer, can be measured by X- ray reflectivity. The pH of the electrolyte can be measured by using a pH probe, for example a HI 83141 pH probe (Hanna Instruments). Perpendicular Magnetic anisotropy can be determined by measuring magnetisation using a magnetometer. A hysteresis loop can be generated by measuring the magnetic flux of the sample as the strength of an applied magnetic field is changed. The shape of the magnetic hysteresis loop informs of the presence or absence of perpendicular magnetic anisotropy 15713001.CXB.CXB P258212GB00 - 22 - and can be determined by the Optical Kerr effect or SQUID. A PMA material will show a small saturation field and large remanence when an out of plane magnetic field is applied. A non-PMA material will exhibit a high saturation field and small remanence when an out- of-plane magnetic field is applied. Electrode Preparation Working electrodes were prepared on a thermally oxidised silicon substrate with a crystallographic orientation <100> (Si-Mat, Germany) cut to a square (10 mm × 10 mm). Substrates were cleaned in acetone and then isopropanol before loading them into the vacuum chamber for growth. Samples were deposited using DC magnetron sputtering in an Ar pressure of about 3 x 10-3mbar within a vacuum chamber with a base pressure of the order of 10-8mbar. An alloy target of composition Co68B32was used to grow the cobalt boride layers, and all other layers were deposited from 99.99% commercially available products obtained from PI-KEM. Cell assembly A sample was mounted onto a bottom-mount front-contact cell (BM FC H-CELL 50 mL, Redoxme, Sweden). The sample surface was oriented upward during the electrocatalysis such that the gaseous products can be released by buoyance. A Pt counter electrode was located inside the counter chamber and separated from the working chamber using a membrane (Fumasep FS-990-PK, Fumatech, Germany). The reference electrode was chosen according to the type of the electrolyte used. A Hg / HgO electrode (1 M KOH, +119 mV versus standard hydrogen electrode, SHE; BASi EF-1369, ALS Co., Japan) was inserted into the working chamber which was filled with an acidic, near-alkaline or alkaline electrolyte as set out below. Otherwise, a Ag / AgCl electrode (3 M KCl, +210 mV versus SHE; Redoxme, Sweden) was plugged into the working chamber. Electrolyte preparation Acidic Electrolyte (pH 1-2) A 0.1 M HClO4electrolyte was formulated from deionised water (18.2 M" cm; Direct-Q®3 UV water purification system, Merck) and an ultrapure quality chemical (70% HClO4-basis, 15713001.CXB.CXB P258212GB00 - 23 - 99.999% trace metals-basis; Sigma-Aldrich, United States), after which the acidity was measured as pH 1.0 using a pH probe (HI 83141, Hanna Instruments). Near-alkaline Electrolyte (pH 8-9) A 0.5 M KHCO3 electrolyte was prepared by dissolving the chemical (#99.5%; Sigma- Aldrich, Spain) in deionised water, after which 1 L of the solution was pre-electrolysed to remove trace-level impurities at a constant current of 0.1 mA for 16 hours under agitation by magnetic stirring without purging. This process employed a two-electrode system where the working and counter electrodes are a titanium plate and a platinum coil, respectively. The pre-electrolysed electrolyte was purged in Ar (99.998%; BOC, United Kingdom) at a flow rate of 20 cm3min1(sccm) for 30 minutes and then the basicity was measured as pH 8.8 using a pH probe. Alkaline Electrolyte (pH 13-14) A 1 M KOH electrolyte was prepared by dissolving the chemical (85% KOH-basis, 99.98% metals-basis; Alfa Aesar, United States) in deionised water, after which the 1 L of the solution was pre-electrolysed at a constant current of 0.25 mA for 20 hours with Ar purging at a flow rate of 30 sccm under agitation by magnetic stirring. This process employed a two-electrode system where both working and counter electrodes are platinum coils. The pre-electrolysed electrolyte was purged in Ar at a flow rate of 20 sccm for 30 minutes and the basicity was measured as pH 13.9 using a pH probe. Example 1 A study to determine the effects of in-plane vs out-of-plane magnetic fields on electrocatalytic activity was performed under acidic conditions. Pt capped multilayer electrodes were prepared with varying compositions as set out in Table 1 below. 15713001.CXB.CXB P258212GB00 - 24 - Table 1- Pt Capped Multilayer Electrode Sample Structure and Composition *CEx= Comparative Example.Geometric current density was measured on the Pt capped multi-layer magnetic electrodes of Table 1 at -0.15 V versus the RHE reference electrode during a series of linear sweep voltammograms (0 to -0.15 V, 20mV s-1) in Ar-saturated 0.5 M KHCO3 solution. The current was due to the hydrogen evolution reaction. The results are illustrated in Figure 4(b). Figure 4(a) shows the electrochemical data recorded during the 50th linear sweep voltammogram of each sample. Comparison of Pt non-mag (CEx1.5) with the magnetic samples (Ex1.1-1.3 and 1.6) in Figure 4, shows that the presence of the magnetic layer (Co68B32) with a Pt capped electrode results in an increase in the current density which is due to an acceleration in the rate of the hydrogen evolution reaction. 15713001.CXB.CXB P258212GB00 - 25 - It was also found that when a magnetic layer is present, a thicker (!c = 10 nm) Pt cap (see Ex 1.1, 1.3, and 1.6) leads to a smaller increase in hydrogen evolution activity when compared to a corresponding electrode sample with a thinner Pt cap (!s = 5 nm) (see Ex 1.2 and 1.4). A sample with in-plane magnetisation, i.e. without perpendicular magnetic anisotropy, (see Ex 1.6 (Pt 10 nm IP)) was found to have the same activity as those where the magnetism is out of plane (Pt 10 nm big), i.e. samples with perpendicular magnetic anisotropy. This suggests that the stray magnetic field present in perpendicular magnetic anisotropy materials is not the cause of the increased level of electrocatalytic activity. Furthermore, this data suggests that when the magnetic layer thickness (Co68B32) exceeds a particular threshold, the sample exhibits in-plane magnetisation (no PMA) as opposed to out-of-plane magnetisation (PMA). It was found that this threshold was about 1.5 nm. This effect is also illustrated in Figures 2 and 3. Example 2 Four multilayer electrodes labelled Examples 2.1-2.4 were prepared. The composition and structure of each is set out in Table 2 below. Pt / Co68B32 / Ir (PCI) superlattices were grown onto thermally oxidised Si substrate deposited with a Ta seed layer in line with the generic structure of Fig. 1 and capped with a conventionally non-ferromagnetic electrocatalyst. The full sample structure is: SiO2 / / Ta / [PCI]x / NM. The electrocatalyst layer thickness was kept at 10 nm and the magnetic layer thickness was kept below the 1.5 nm threshold so that all magnetic samples exhibited an out-of-plane magnetic domain (with Perpendicular Magnetic Anisotropy). The domain state was set on all samples before catalysis measurements by applying an AC magnetic field of 20 mT which was gradually reduced to 0 mT. 15713001.CXB.CXB P258212GB00 - 26 - Table 2 – Au and Pt capped Multilayer Electrodes Structure and Composition *CEx= Comparative Example.Au Electrodes The presence of spin polarisation at an Au electrode surface was examined through the Hydrogen Evolution Reaction. The reaction was performed under acidic (Fig 5(b)), near- alkaline (Fig 5(c)), and alkaline conditions (Fig 5(d)). Two separate HERs were performed at each of the pH conditions using different Au electrodes. A first reaction was performed with an Au electrode comprising a Pt / Co68B32 / Ir (PCI) superlattice beneath the Au outer surface (Ex 2.2) and a second reaction was performed with an Au electrode void of Pt / Co68B32 / Ir (PCI) i.e. void of magnetic material (Ex 2.1). The detailed structure of each electrode is set out in Table 2. Direct comparison of the two allowed to assess the effect of using a layer of magnetic material beneath the Au surface on the activity of the electrocatalyst. The HER can be broken down into two steps: hydrogen adsorption and molecular dihydrogen desorption steps (Fig. 5a). Acidic Conditions (pH 1) As shown in Fig 5(b), the electroanalytical signals obtained from Au / PCI / NMS exhibit a higher overpotential than that from Au / NMS. Without wishing to be bound by theory, it is believed that the underperformance can be attributed to the involvement of spin-polarised *H intermediates. 15713001.CXB.CXB P258212GB00 - 27 - Near-alkaline conditions (pH 8-9) The presence of spin polarisation at the Au electrode surface was examined through the Hydrogen Evolution Reaction in mildly basic electrolyte (pH 8.8). As can be observed in Fig 5(c), a decrease in the electrocatalytic activity is observed for the Au electrode comprising PCI compared to the Au electrode without PCI. The underperformance is more obvious than in acid (Fig 5 (b)). These observations may suggest that spin-related inhibition of the HER is more evident when the coupling occurs preferentially via the Tafel step. Alkaline Conditions (pH 13.9) As shown in Fig 5(d) the outperformance is observed in the magnetic sample (with PCI), indicating that the magnetic enhancement outweighs the inhibition caused by the spin- incoherent coupling via the Tafel step under basic conditions. Therefore, it was found that the in-situ magnetic field of the multilayer electrode exerted a pH dependent influence on the HER activities. It has been found that the use of a ferromagnetic material beneath an electrocatalyst layer results in a 2-fold change in rate in the Hydrogen Evolution Reaction (HER) on an Au electrode. Without wishing to be bound by theory, it is believed that there is a transfer of ferromagnetism from the ferromagnet to the originally non-magnetic transition metal through proximity induced magnetism (PIM). It is believed that the induced magnetism results in spin polarisation effects at the electrode surface which break the scaling relationships for the electrochemical protons to H2 (hydrogen) in the Hydrogen Evolution Reaction (HER). This approach can provide a way of circumventing scaling relationships on existing electrocatalyst surfaces which could be applied to a range of sustainable chemistry and energy applications. 15713001.CXB.CXB P258212GB00 - 28 - Example 3 The HER can be broken down into two steps: hydrogen adsorption and molecular dihydrogen desorption steps as shown in Figure 5(a). Adsorbed hydrogens result from the Volmer step which can be realised by the electrosorption of a solvated proton in acidic media and by water dissociation-coupled proton adsorption in basic media. The as-formed *H couples with an adjacent *H via the Tafel step to produce H2 (i.e. an electrochemical analogue of the Langmuir–Hinshelwood mechanism) or with a proton donor present in the electrolyte via the Heyrovsky step (i.e. an electrochemical analogue of the Eley–Rideal mechanism). The formation of a dihydrogen molecule can be realised by both the Tafel and Heyrovsky steps; however, the proportion between the two pathways or the predominance of one pathway over the other can change with the electrolyte acidity. It has been reported that an increase in H+concentration at Au surfaces can initiate the production of H2via the Heyrovsky step, in addition to the Tafel step. In other words, when the HER proceeds on Au in basic electrolyte, the contribution of the Tafel step will predominate over that of the Heyrovsky step. Au capped multilayer electrodes were prepared with a capping thickness layer of 5nm or 10nm as shown in Figure 6. The samples labelled (ii), (iii) and (v) showed out of plane magnetism. Sample (i) did not have a magnetic layer. It was found that the presence of the magnetic underlayer retards hydrogen evolution on Au under near-alkaline conditions (0.5 M KHCO3, pH 8.8). Example 4 A study to evaluate the effect of varying the thickness of the electrocatalyst layer was carried out with Au capped multilayer electrodes as shown in Figure 7. All samples except (Au(10nm) / NM had a multilayer magnetic layer underneath the Au capping layer. Figure 7 shows samples with PMA structure under the Au electrocatalyst layer, compared to an Au control electrode (non-magnetic sample) using 0.5 M KHCO3 electrolyte (near-alkaline conditions). It has been found that the magnetic underlayers decrease the hydrogen evolution reaction rate. The thinner the Au layer the more pronounced the effect. 15713001.CXB.CXB P258212GB00 - 29 - SPECIFIC EMBODIMENTS (CLAUSES) The invention will now be further described with reference to a list of specifically preferred embodiments, as set out in the following clauses. 1. A multilayer electrode for an electrocatalytic reaction comprising: a first layer comprising a substrate; a second layer comprising a metallic magnetic material; and a third layer comprising an electrocatalyst; wherein the second layer is located between the first layer and the third layer. 2. The multilayer electrode of clause 1, wherein the metallic magnetic material is a ferromagnetic material. 3. The multilayer electrode of clauses 1 or 2, wherein the second layer comprises a perpendicular magnetic anisotropy ferromagnetic material. 3a. The multilayer electrode of clauses 1 or 2, wherein the second layer comprises a non-perpendicular magnetic anisotropy ferromagnetic material. 4. The multilayer electrode of any of the preceding clauses, wherein the second layer comprises at least one sublayer of the metallic magnetic material. 5. The multilayer electrode of any of the preceding clauses, wherein the metallic magnetic material is selected from one or more of chromium, manganese, iron, nickel, cobalt, and borides, oxides, or alloys thereof. 6. The multilayer electrode of any of the preceding clauses wherein the metallic magnetic material is selected from one or more of chromium, manganese, iron, nickel, and borides, oxides, or alloys thereof, or a boride or oxide of cobalt. 7. The multilayer electrode of any of the preceding clauses wherein the metallic magnetic material comprises iron cobalt boride (FeaCobBc), wherein a = 1 to 59, b = 1 to 59, and c = 1 to 40, and / or cobalt boride (CoxBy), wherein x = 60 to 99 and y = 1 to 40, preferably wherein x = 65 to 99 and y = 1 to 35. 15713001.CXB.CXB P258212GB00 - 30 - 7a. The multilayer electrode of any of the preceding clauses wherein the metallic magnetic material comprises or consists of a cobalt boride (CoxBy). 7b. The multilayer electrode of any of the preceding clauses, wherein the metallic magnetic material comprises CoxBy, wherein x = 60 to 99 and y = 1 to 40, preferably wherein x = 65 to 99 and y = 1 to 35. 7c. The multilayer electrode of any of clauses 1 to 6, wherein the metallic magnetic material comprises or consists of a cobalt and iron alloy, preferably CodFee, wherein d = 1 to 99 and e = 1 to 99, more preferably wherein d = 1 to 75 and e = 25 to 99 or wherein d = 1 to 50 and e = 50 to 99. 8. The multilayer electrode of any of the preceding clauses wherein the metallic magnetic material comprises Co68B32.9. The multilayer electrode of any of clauses 4 to 8, wherein the second layer further comprises at least one sublayer comprising a heavy metal selected from the group consisting of Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg and Bi. 9a. The multilayer electrode of any of clauses 4 to 9, wherein the second layer comprises at least one sublayer comprising platinum and / or at least one sublayer comprising iridium. 10. The multilayer electrode of any of the preceding clauses wherein the electrocatalyst comprises at least one metal selected from gold, platinum, copper, and silver. 11. The multilayer electrode of any of the preceding clauses wherein the electrocatalyst comprises gold or platinum. 12. The multilayer electrode of any of clauses 1 to 10 wherein the electrocatalyst comprises copper or silver. 12a. The multilayer of any one of clauses 1 to 12, wherein the second layer consists of a multilayer repeat structure represented by the following formula: (X / M / X)n 15713001.CXB.CXB P258212GB00 - 31 - wherein X is a heavy metal individually selected from Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, and Bi; wherein M is a magnetic material sublayer; wherein n is the number of repeat units of the multilayer structure present in the second layer and is in the range of 1 to 10. 12b. The multilayer electrode of clause 12a, wherein each X is individually selected from Pt and Ir. 12.c The multilayer electrode of clause 12a or 12b, wherein M is selected from one or more of chromium, manganese, iron, nickel, cobalt, and borides, oxides, or alloys thereof. 12d. The multilayer electrode of clause 12a or 12b, wherein M is selected from one or more of chromium, manganese, iron, nickel, and borides, oxides, or alloys thereof, or a boride or oxide of cobalt. 12e. The multilayer electrode of clause 12a or 12b, wherein M is iron cobalt boride (FeaCobBc, wherein a = 1 to 59, b = 1 to 59, and c = 1 to 40) and / or cobalt boride (CoxBywherein x = 60 to 99 and y = 1 to 40, preferably wherein x = 65 to 99 and y = 1 to 35).12f. The multilayer electrode of any of clauses 12 to 12e, wherein n is in the range of 1 to 9, or 1 to 8, or 2 to 8, or 2 to 7, or 2 to 6, or 3 to 6, or 3 to 5. 12g. The multilayer electrode of any of clauses 12 to 12f, wherein the second layer consists of a multilayer repeat structure selected from (Pt / CoxBy / Ir)n, (Pt / CoxBy / Pt)n, (Ir / CoxBy / Ir)n, (Pt / FeaCobBc / Ir)n, (Pt / FeaCobBc / Pt)n, or (Ir / FeaCobBc / Ir)n, wherein n is 3 to 5, and wherein x = 60 to 99 and y = 1 to 40, preferably wherein x = 65 to 99 and y = 1 to 35, and wherein a = 1 to 59, b = 1 to 59, and c = 1 to 40. 13. The multilayer electrode of any of the preceding clauses, further characterised by a lc:lsd ratio in the range of 100:1 to 1:100; wherein lc is the thickness of the third layer; and wherein lsd is the spin diffusion length of the electrocatalyst. 15713001.CXB.CXB P258212GB00 - 32 - 14. The multilayer electrode of clause 13 wherein the electrocatalyst comprises gold and wherein the ratio is in the range of 30:1 to 1:30. 15. The multilayer electrode of clause 13 wherein the electrocatalyst comprises platinum and the lc:lsd ratio is in the range of 100:1 to 1:10. 16. The multilayer electrode of clause 13 or 15, wherein the electrocatalyst comprises platinum and the lc:lsd ratio is in the range of 100:1 to 1:1. 17. The multilayer electrode of clause 13 wherein the electrocatalyst comprises copper or silver and the ratio is in the range of 30:1 to 1:100. 18. The multilayer electrode of any of the preceding clauses wherein the first layer further comprises a seed sublayer deposited on the substrate, and wherein the seed sublayer is in contact with the second layer. 19. The multilayer electrode of clause 18 wherein the seed sublayer comprises tantalum. 20. The multilayer electrode of any one of the preceding clauses wherein the sum of the thickness of the second layer and the thickness of the third layer is from 10 nm to 30 nm. 21. The multilayer electrode of any one of the preceding clauses wherein the thickness of the third layer is from 5 nm to 15 nm. 21a. The multilayer electrode of any one of clauses 1-20 wherein the thickness of the third layer is from 0.5 nm to 10 nm. 22. An electrochemical cell comprising the multilayer electrode of any of one of the preceding clauses, the electrochemical cell further comprising: a counter electrode; and an electrolyte. 23. The electrochemical cell of clause 22 wherein the electrolyte is a solid, a liquid, or a gel. 15713001.CXB.CXB P258212GB00 - 33 - 24. The electrochemical cell of clause 22 or clause 23, wherein the electrolyte comprises one or more ionomers. 25. The electrochemical cell of any of clauses 22 to 24, wherein the electrolyte generates an acidic environment at the electrocatalyst with a local pH of from 1 to 5. 26. The electrochemical cell of clause 22 wherein the electrolyte is an acidic solution with a pH of from 1 to 5. 27. The electrochemical cell of clause 22 wherein the electrolyte is a basic solution with a pH of from 8 to 14. 28. A method of making the multilayer electrode of any one of clauses 1 to 21a, the method comprising: i) providing a first layer comprising a substrate; ii) applying a second layer on a surface of the first layer, the second layer comprising a metallic magnetic material; iii) applying a third layer on a surface of the second layer, the third layer comprising an electrocatalyst. 29. A method of modifying the activity of an electrocatalyst, the method comprising: i) providing a multilayer electrode comprising: a first layer comprising a substrate; a second layer comprising a metallic magnetic material; and a third layer comprising the electrocatalyst; wherein the second layer is located between the first layer and the third layer; ii) determining a spin diffusion length lsd of the electrocatalyst; iii) determining the thickness lc of the third layer; and iv) modifying the lc:lsd ratio. 30. A method of achieving a desired electrocatalytic activity in an electrocatalytic reaction, the method comprising: 15713001.CXB.CXB P258212GB00 - 34 - i) determining the desired electrocatalytic activity; ii) preparing a multilayer electrode comprising: a first layer comprising a substrate; a second layer comprising a metallic magnetic material; and a third layer comprising an electrocatalyst; wherein the second layer is located between the first layer and the third layer; and selecting ratio; wherein lcis the thickness of the third layer and wherein lsdis the spin diffusion length of the electrocatalyst; and iii) using the multilayer electrode in the electrocatalytic reaction. 31. The method of clause 30 further comprising contacting the multilayer electrode with an electrolyte. 32. The method of clause 31 wherein the electrolyte is an acidic solution with a pH of from 1 to 5. 33. The method of clause 31 wherein the electrolyte is a basic solution with a pH of from 8 to 14. 34. The method of any one of clauses 30 to 33 wherein the electrocatalytic reaction is selected from the group consisting of a hydrogen evolution reaction, a water oxidation reaction, an oxygen reduction reaction, a carbon dioxide reduction reaction, a nitrogen reduction reaction, a nitrogen oxide redox reaction, and a hydrogen sulfide redox reaction. 34a. The method of any one of clauses 30 to 34 wherein the electrocatalytic reaction is selected from the group consisting of a hydrogen evolution reaction, a water oxidation reaction, an oxygen reduction reaction, a carbon dioxide reduction reaction, and a nitrogen reduction reaction. 35. The method of any one of clauses 30 to 34 wherein the electrocatalytic reaction is a hydrogen evolution reaction. 36. The method of any one of clauses 30 to 35 wherein the second layer comprises a perpendicular magnetic anisotropy ferromagnetic material. 15713001.CXB.CXB P258212GB00 - 35 - 37. The method of any one of clauses 30 to 36 wherein the second layer comprises at least one sublayer of the metallic magnetic material. 38. The method of any one of clauses 30 to 37 wherein the metallic magnetic material is selected from one or more of chromium, manganese, iron, nickel, cobalt, and borides, oxides, or alloys thereof. 39. The method of any one of clauses 30 to 38 wherein the metallic magnetic material comprises cobalt boride. 40. The method of any one of clauses 30 to 39 wherein the second layer further comprises at least one sublayer comprising platinum and / or at least one sublayer comprising iridium. 41. The method of any one of clauses 30 to 40 wherein the electrocatalyst comprises a metal selected from gold, platinum, copper, and silver. 42. The method of clause 41 wherein the electrocatalyst comprises gold or platinum. 43. The method of clause 41 wherein the electrocatalyst comprises copper or silver. 44. The method of any one of clauses 30 to 43 wherein the lc:lsd ratio is in the range of 100:1 to 1:100. 45. The method of clause 42 wherein the electrocatalyst comprises gold and wherein the lc:lsd ratio is in the range of 30:1 to 1:30. 46. The method of clause 42 wherein the electrocatalyst comprises platinum and the lc:lsdratio is in the range of 100:1 to 1:10. 47. The method of clause 43 wherein the lc:lsdratio is in the range of 30:1 to 1:100. 48. The method of any of clauses 30-47 wherein the sum of the thickness of the second layer and the thickness of the third layer is from 10 nm to 30 nm. 15713001.CXB.CXB P258212GB00 - 36 - 49. The method of clause 30-48 wherein the thickness of the third layer is from 5 nm to 15 nm. 50. The method of any one of clauses 30-49 wherein the first layer further comprises a seed sublayer deposited on the substrate, and wherein the seed layer is in contact with the second layer. 51. The method of clause 50 wherein the seed sublayer comprises tantalum. 52. Use of a metallic magnetic material to modify the activity of an electrocatalyst, the use comprising: i) inserting the metallic magnetic material as a second layer between a first layer comprising a substrate and a third layer comprising the electrocatalyst. 53. Use of the multilayer electrode of any one of clauses 1 to 21 to control the rate of an electrocatalytic reaction. 15713001.CXB.CXB

Claims

P258212GB00 - 37 - CLAIMS:

1. A multilayer electrode for an electrocatalytic reaction comprising: a first layer comprising a substrate; a second layer comprising a metallic magnetic material; and a third layer comprising an electrocatalyst; wherein the second layer is located between the first layer and the third layer.

2. The multilayer electrode of claim 1, wherein the second layer comprises a ferromagnetic material.

3. The multilayer electrode of any of the preceding claims, wherein the second layer comprises at least one sublayer of the metallic magnetic material.

4. The multilayer electrode of any of the preceding claims wherein the metallic magnetic material is selected from one or more of chromium, manganese, iron, nickel, cobalt, and borides, oxides, or alloys thereof.

5. The multilayer electrode of any of the preceding claims wherein the metallic magnetic material is selected from one or more of chromium, manganese, iron, nickel, and borides, oxides, or alloys thereof, or a boride or oxide of cobalt.

6. The multilayer electrode of any of the preceding claims wherein the metallic magnetic material comprises cobalt boride.

7. The multilayer electrode of any of claims 4 to 6, wherein the second layer further comprises at least one sublayer comprising platinum and / or at least one sublayer comprising iridium.

8. The multilayer electrode of any of the preceding claims wherein the electrocatalyst comprises at least one metal selected from gold, platinum, copper, and silver.

9. The multilayer electrode of any of the preceding claims wherein the electrocatalyst comprises gold or platinum. 15713001.CXB.CXBP258212GB00 - 38 - 10. The multilayer electrode of any of claims 1 to 7 wherein the electrocatalyst comprises copper or silver.

11. The multilayer electrode of any of the preceding claims, further characterised by a lc:lsdratio in the range of 100:1 to 1:100; wherein lc is the thickness of the third layer; and wherein lsd is the spin diffusion length of the electrocatalyst.

12. The multilayer electrode of claim 11 wherein the electrocatalyst comprises gold and wherein the lc:lsd ratio is in the range of 30:1 to 1:

30.

13. The multilayer electrode of claim 11 wherein the electrocatalyst comprises platinum and the ratio is in the range of 100:1 to 1:

10.

14. The multilayer electrode of any of the preceding claims wherein the first layer further comprises a seed sublayer deposited on the substrate, and wherein the seed sublayer is in contact with the second layer.

15. The multilayer electrode of claim 14 wherein the seed sublayer comprises tantalum.

16. The multilayer electrode of any one of the preceding claims wherein the sum of the thickness of the second layer and the thickness of the third layer is from 10 nm to 30 nm.

17. The multilayer electrode of any one of the preceding claims wherein the thickness of the third layer is from 5 nm to 15 nm.

18. An electrochemical cell comprising the multilayer electrode of any of one of claims 1 to 17, the electrochemical cell further comprising: a counter electrode; and an electrolyte.

19. The electrochemical cell of claim 18, wherein the electrolyte is a solid, a liquid, or a gel. 15713001.CXB.CXBP258212GB00 - 39 - 20. The electrochemical cell of claim 18 or claim 19, wherein the electrolyte comprises one or more ionomers.

21. The electrochemical cell of any of claims 18 to 20, wherein the electrolyte generates an acidic environment at the electrocatalyst with a local pH of from 1 to 5.

22. The electrochemical cell of claim 18 wherein the electrolyte is an acidic solution with a pH of from 1 to 5.

23. The electrochemical cell of claim 18 wherein the electrolyte is a basic solution with a pH of from 8 to 14.

24. A method of making the multilayer electrode of any one of claims 1 to 17, the method comprising: i) providing a first layer comprising a substrate; ii) applying a second layer on a surface of the first layer, the second layer comprising a metallic magnetic material; iii) applying a third layer on a surface of the second layer, the third layer comprising an electrocatalyst.

25. A method of modifying the activity of an electrocatalyst, the method comprising: i) providing a multilayer electrode comprising: a first layer comprising a substrate; a second layer comprising a metallic magnetic material; and a third layer comprising the electrocatalyst; wherein the second layer is located between the first layer and the third layer; ii) determining a spin diffusion length lsd of the electrocatalyst; iii) determining the thickness lc of the third layer; and iv) modifying the lc:lsd ratio.

26. A method of achieving a desired electrocatalytic activity in an electrocatalytic reaction, the method comprising: 15713001.CXB.CXBP258212GB00 - 40 - i) determining the desired electrocatalytic activity; ii) preparing a multilayer electrode comprising: a first layer comprising a substrate; a second layer comprising a metallic magnetic material; and a third layer comprising an electrocatalyst; wherein the second layer is located between the first layer and the third layer; and selecting alc :lsd ratio; whereinlc is the thickness of the third layer and whereinlsd is the spin diffusion length of the electrocatalyst; and iii) using the multilayer electrode in the electrocatalytic reaction.

27. The method of claim 26 further comprising contacting the multilayer electrode with an electrolyte.

28. The method of claim 27 wherein the electrolyte is an acidic solution with a pH of from 1 to 5.

29. The method of claim 27 wherein the electrolyte is a basic solution with a pH of from 8 to 14.

30. The method of any one of claims 26 to 29 wherein the electrocatalytic reaction is selected from the group consisting of a hydrogen evolution reaction, a water oxidation reaction, an oxygen reduction reaction, a carbon dioxide reduction reaction, and a nitrogen reduction reaction.

31. The method of any one of claims 26 to 30 wherein the electrocatalytic reaction is a hydrogen evolution reaction.

32. The method of any one of claims 26 to 31 wherein the second layer comprises a perpendicular magnetic anisotropy ferromagnetic material.

33. The method of any one of claims 26 to 32 wherein the second layer comprises at least one sublayer of the metallic magnetic material. 15713001.CXB.CXBP258212GB00 - 41 - 34. The method of any one of claims 26 to 33 wherein the metallic magnetic material is selected from one or more of chromium, manganese, iron, nickel, cobalt, and borides, oxides, or alloys thereof.

35. The method of any one of claims 26 to 34 wherein the metallic magnetic material comprises cobalt boride.

36. The method of any one of claims 26 to 35 wherein the second layer further comprises at least one sublayer comprising platinum and / or at least one sublayer comprising iridium.

37. The method of any one of claims 26 to 36 wherein the electrocatalyst comprises a metal selected from gold, platinum, copper, and silver.

38. The method of claim 37 wherein the electrocatalyst comprises gold or platinum.

39. The method of claim 37 wherein the electrocatalyst comprises copper or silver.

40. The method of any one of claims 26 to 39 wherein the lc:lsd ratio is in the range of 100:1 to 1:

100.

41. The method of claim 40 wherein the electrocatalyst comprises gold and wherein the lc:lsdratio is in the range of 30:1 to 1:

30.

42. The method of claim 40 wherein the electrocatalyst comprises platinum and the lc:lsdratio is in the range of 100:1 to 1:

10.

43. The method of any one of claims 26 to 42 wherein the first layer further comprises a seed sublayer deposited on the substrate, and wherein the seed layer is in contact with the second layer.

44. The method of claim 43 wherein the seed sublayer comprises tantalum.

45. Use of a metallic magnetic material to modify the activity of an electrocatalyst, the use comprising: 15713001.CXB.CXBP258212GB00 - 42 - ii) inserting the metallic magnetic material as a second layer between a first layer comprising a substrate and a third layer comprising the electrocatalyst.

46. Use of the multilayer electrode of any one of claims 1 to 17 to control the rate of an electrocatalytic reaction. 15713001.CXB.CXB

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