A hydrogen evolution reaction electrocatalyst and methods for preparation and use thereof

Abstract

The invention relates to a catalyst for the hydrogen evolution reaction (HER) and methods for using the catalyst in a water-splitting process. A hydrogen evolution reaction (HER) electrocatalyst, and a method and use for preparing a HER electrocatalyst are disclosed. The method comprises the step of forming a two-dimensional electron gas (2DEG) interface at a heterojunction of at least one of two or more alternating layers of a first complex oxide and a second complex oxide, wherein the or each 2DEG interface exhibits a current density, corresponding to an intrinsic hydrogen evolution reaction (HER) activity that renders the 2DEG interface(s) suitable as an HER electrocatalyst for producing hydrogen (H2) from a water-based electrolyte by water electrolysis.

Description

A HYDROGEN EVOLUTION REACTION ELECTROCATALYST AND METHODS FOR PREPARATION AND USE THEREOFField of the Invention

[0001] The present invention relates to a hydrogen evolution reaction electrocatalyst, and methods for the preparation and use thereof. Whilst the invention has been developed for use in electrocatalytic water splitting to produce hydrogen (H2), it will be appreciated that the invention is not limited to this particular field of use.Background of the Invention

[0002] The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It should be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0003] Green hydrogen (H2) production stands as a pivotal step towards the realization of a future hydrogen (H2) economy. However, despite it being a promising H2 production route, the electrolysis of water suffers from high cost due to the noble metal(s) used in the electrocatalysts and the high energy consumption.

[0004] Electrocatalytic water splitting is a promising means to produce green hydrogen (H2), a fuel which is regarded as the ideal solution for the future due to its high mass specific energy density and environmental friendliness. However, water electrolysis is an energy-intensive process which is prone to ohmic voltage drops, decomposition voltage, cathode overpotential, and anode overpotential.

[0005] Electrochemical catalysts (or electrocatalysts) are employed in water electrolysers to reduce the reaction overpotential, and thus reduce the total energy consumption (cell voltage). The hydrogen evolution reaction (HER) occurs on the cathode of the water electrolyser via the following reaction:2H++ 2e~ -► H2

[0006] Platinum-based catalysts are currently considered the most efficient catalysts for use in the hydrogen evolution reaction (HER), owing to their high electrical conductivity and intrinsic activity, which enables them to reduce the overpotential andminimize the ohmic voltage drop simultaneously. Nevertheless, the benefits of Ptbased catalysts have yet to warrant the substantial cost associated with these materials. Alternatives are currently under development to replace the expensive Ptbased catalysts for use in large-scale applications.

[0007] The discovery of earth-abundant electrocatalysts to replace noble metals for use in the Hydrogen Evolution Reaction (HER) represents a crucial endeavour to reduce the cost for green hydrogen (H2) production. For instance, transition metal oxides (TMOs) are promising electrocatalysts for use in water electrolysis due to their low-cost, rich variants, and environmental stability. However, their poor intrinsic activity and poor conductivity impede efficient charge transfer between the catalysts and the reactants required to be catalyzed, as well as hinder charge transport within the catalysts themselves. The energy barriers for such charge movements result in energy loss, which can be summarized as resistance in electrocatalysis.

[0008] Three groups of resistance are involved during the water electrolysis on the electrode, (i) the interface resistance between the reaction substances and catalyst (Rsa), (ii) the resistance due to the introduction of other multi-interfaces (Rss), and (iii) the material resistance (Rs) of the catalyst and support material.

[0009] The intrinsic activity of the catalyst determines the Rsa which represents the charge transfer resistance, meanwhile, the catalyst intrinsic conductivity significantly affects the Rs which reveals the charge transport performance. The intrinsic activity of the catalyst is strongly influenced by its electronic structure, which can be tuned through various mechanisms, including the doping effect, strain engineering, and defect engineering.

[0010] For instance, the addition of tungsten (W) favorably modulates the electronic structure of iron-cobalt oxides, with the obtained turnover frequency (TOF) showing a ten-fold increase. Also, CoO nano-rods with strained surfaces present a higher current density (normalized to the BET surface area) than non-strained CoO, which indicates an improved intrinsic HER activity.

[0011] Notably, the intrinsic electrical conductivities of catalysts are often overlooked because the charge transport in conventional catalysis strongly relies on the conductive materials (such as carbon cloth, carbon fibre paper, and foams and foils of metals) upon which the catalysts are supported. However, conventional catalysts stillsuffer from energy loss resulting from the multi-interface between the catalyst pellets and the interface between the support material and the catalyst.

[0012] A need exists, therefore, for a more effective non-noble metal HER catalyst for use in electrocatalytic water splitting to produce hydrogen (H2).

[0013] Accordingly, it is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

[0014] The present invention seeks to provide a hydrogen evolution reaction electrocatalyst, and methods for the preparation and use thereof, which will overcome or substantially ameliorate at least some of the deficiencies of the prior art, or to at least provide an alternative.Summary of the Invention

[0015] According to a first aspect, the present invention provides a method of preparing a hydrogen evolution reaction (HER) electrocatalyst, the method comprising the step of: forming a two-dimensional electron gas (2DEG) interface at a heterojunction of at least one of two or more alternating layers of a first complex oxide and a second complex oxide, wherein the or each 2DEG interface exhibits a current density corresponding to an intrinsic hydrogen evolution reaction (HER) activity that renders the 2DEG interface(s) suitable as an HER electrocatalyst for producing hydrogen (H2) from a water-based electrolyte by water electrolysis.

[0016] Surprisingly, the inventors have observed that the heterostructure that is formed by combining two such wide bandgap insulators to generate a two-dimensional electron gas (2DEG) at the heterointerface thereof, results in an electrocatalyst that exhibits a high intrinsic hydrogen evolution reaction (HER) activity of 300 mA / cm2at - 0.1 V (E vs RHE), which enables these HER electrocatalysts to outperform most electrocatalysts, including the conventional platinum-based catalysts. Moreover, this high intrinsic HER activity also means that it is possible to dispense with the use of a support material and co-catalyst, thereby realising a more cost effective solution that can be applied to electrocatalytic water splitting to produce hydrogen (H2). Crucially, this unexpected outcome addresses the inherent instability of platinum in alkaline solutions, showcasing its distinct competitive advantage.

[0017] In preferred embodiments, the first and second complex oxide layers are each formed from a perovskite oxide or a transition metal oxide.

[0018] Preferably, the first complex oxide layer is formed from a perovskite oxide selected from the group consisting of LaAlOa, NdGaOa, LaTiOa, LaVOa, GdTiOa, SrlrOa, LaNiOa, NdAlOa, GdTiOa, BiFeOa, PrAlOa, Laa / aSr aMnOa, LaAIOa / LaVOa / LaAlOa, Y-AI2O3, CaZrOa, LaAIOa / La / sSri / sMnOa, SrTiO3ZSrTio.8Nbo.2O3, SrTiOa, LaAlOa, LaNiOa, SrVOa, BaSnOa, LaTiOa, LaAlOa, SrTiOa, Nb-SrTiOa and LaFeOa.

[0019] Preferably, the second complex oxide layer is formed from a perovskite oxide selected from the group consisting of SrTiOa, BaTiOa, SrMnOa, SrVOa, SrCrOa, LaNiOa.

[0020] In one embodiment the first complex oxide layer is LaAlOa (LAO) and the second complex oxide layer is SrTiOa (STO).

[0021] In preferred embodiments, the alternating LAO and STO layers are deposited in sequence onto a surface of a substrate, wherein the substrate is a solid state material.

[0022] In some embodiments, the solid state material is selected from the group consisting of a perovskite oxide, a transition metal oxide, silicon (Si), silicon dioxide (SiOa), indium tin oxide (ITO), zinc oxide (ZnO), graphite, diamond, gallium nitride (GaN) and silicon carbide (SiC).

[0023] In some embodiments, the LAO layer has a thickness that falls within a range of about 4 unit cells (u.c.) to about 120 unit cells (u.c.).

[0024] In some embodiments, the LAO layer has a thickness that falls within a range of about 4 unit cells (u.c.) to about 80 unit cells (u.c.).

[0025] In a preferred embodiment, the LAO layer has a thickness of at least 4 unit cells (u.c.).

[0026] In a preferred embodiment, the surface of the or each STO layer is terminated with TiOa.

[0027] Preferably, the alternating layers of the first and second complex oxides are deposited on to the substrate surface using a deposition technique selected from the group consisting of atomic layer deposition (ALD), molecular beam epitaxy (MBE), pulsed laser deposition (PLD), RF magneton sputtering, physical vapour deposition (PVD), chemical vapour deposition (CVD) or any combination thereof.

[0028] In some embodiments, the deposition technique is PLD conducted at a temperature of between about 500 °C and about 900 °C at an oxygen pressure of about 10’9Torr to about 10’2Torr. For instance, PLD is conducted at a temperature of between about 500 °C to about 525 °C, about 525 °C to about 550 °C, about 550 °C to about 575 °C, about 575 °C to about 600 °C, about 600 °C to about 625 °C, about 625 °C to about 650 °C, about 650 °C to about 675 °C, about 675 °C to about 700 °C, about 700 °C to about 725 °C, about 725 °C to about 750 °C, about 750 °C to about 775 °C, about 775 °C to about 800 °C, about 800 °C to about 825 °C, about 825 °C to about 850 °C, and about 850 °C to about 900 °C, at an oxygen pressure of about 1 O’9Torr to about 10’2Torr.

[0029] In a preferred embodiment, the deposition technique is PLD conducted at a temperature of around 650 °C at an oxygen pressure of around 10’4Torr.

[0030] In some embodiments, the 2DEG interface is formed at the heterojunction of two alternating layers of LAO and STO, and wherein the 2DEG interface exhibits a carrier density that falls within a range of from about 3.0 x 1013cm2to about 2.4 x 1016cm2, with increasing LAO thickness, from 4 u.c. to 80 u.c. For instance, the 2DEG interface exhibits a carrier density that falls within a range of from about 3.0 x 1013cm2to about 3.5 x 1013cm2, about 3.5 x 1013cm2to about 4.0 x 1013cm2, about 4.0 x 1013cm2to about 4.5 x 1013cm2, about 4.5 x 1013cm2to about 5.0 x 1013cm2, about 5.0 x 1013cm2to about 5.5 x 1013cm2, about 5.5 x 1013cm2to about 6.0 x 1013cm2, about 6.0 x 1013cm2to about 6.5 x 1013cm2, about 6.5 x 1013cm2to about 7.0 x 1013cm2, about 7.0 x 1013cm2to about 7.5 x 1013cm2, about 7.5 x 1013cm2to about 8.0 x 1013cm2, about 8.0 x 1013cm2to about 8.5 x 1013cm2, about 8.5 x 1013cm2to about 9.0 x1013cm2, about 9.0 x 1013cm2to about 9.5 x 1013cm2, about 9.5 x 1013cm2to about 1.0 x 1014cm2, about 1.0 x 1014cm2to about 1 .5 x 1014cm2, about 1 .5 x 1014cm2to about 2.0 x 1014cm2, about 2.0 x 1014cm2to about 2.5 x 1014cm2, about 2.5 x 1014cm2to about 3.0 x 1014cm2, about 3.0 x 1014cm2to about 3.5 x 1014cm2, about 3.5 x1014cm2to about 4.0 x 1014cm2, about 4.0 x 1014cm2to about 4.5 x 1014cm2, about4.5 x 1014cm2to about 5.0 x 1014cm2, about 5.0 x 1014cm2to about 5.5 x 1014cm2, about 5.5 x 1014cm2to about 6.0 x 1014cm2, about 6.0 x 1014cm2to about 6.5 x 1014cm2, about 6.5 x 1014cm2to about 7.0 x 1014cm2, about 7.0 x 1014cm2to about 7.5 x 1014cm2, about 7.5 x 1014cm2to about 8.0 x 1014cm2, about 8.0 x 1014cm2to about8.5 x 1014cm2, about 8.5 x 1014cm2to about 9.0 x 1014cm2, about 9.0 x 1014cm2toabout 9.5 x 1014cm2, about 9.5 x 1014cm2to about 1 .0 x 1015cm2, about 1.0 x 1015cm2to about 1 .5 x 1015cm2, about 1.5 x 1015cm2to about 2.0 x 1015cm2, about 2.0 x 1015cm2to about 2.5 x 1015cm2, about 2.5 x 1015cm2to about 3.0 x 1015cm2, about 3.0 x 1015cm2to about 3.5 x 1015cm2, about 3.5 x 1015cm2to about 4.0 x 1015cm2, about 4.0 x 1015cm2to about 4.5 x 1015cm2, about 4.5 x 1015cm2to about 5.0 x 1015cm2, about 5.0 x 1015cm2to about 5.5 x 1015cm2, about 5.5 x 1015cm2to about 6.0 x 1015cm2, about 6.0 x 1015cm2to about 6.5 x 1015cm2, about 6.5 x 1015cm2to about 7.0 x 1015cm2, about 7.0 x 1015cm2to about 7.5 x 1015cm2, about 7.5 x 1015cm2to about 8.0 x 1015cm2, about 8.0 x 1015cm2to about 8.5 x 1015cm2, about 8.5 x 1015cm2to about 9.0 x 1015cm2, about 9.0 x 1015cm2to about 9.5 x 1015cm2, about 9.5 x 1015cm2to about 1.0 x 1016cm2, about 1.0 x 1016cm2to about 1 .5 x 1016cm2, about 1.5 x 1016cm2to about 2.0 x 1016cm2, about 2.0 x 1016cm2to about 2.4 x 1016cm2, with increasing LAO thickness, from 4 u.c. to 80 u.c.

[0031] In some embodiments, the method further comprises the step of: patterning a surface of the HER electrocatalyst to expose a portion of the or each 2DEG interface, wherein the exposed portion of the or each 2DEG interface contributes to the overall intrinsic HER activity of the HER electrocatalyst.

[0032] Preferably, the patterning is conducted using a technique selected from the group consisting of lithography, wet etching, and a combination thereof.

[0033] Suitably, the wet etching is performed with buffered hydrofluoric acid (HF) for about 5 minutes to about 7 minutes. For instance, the wet etching is performed with buffered HF for about 5 minutes to about 5.2 minutes, about 5.2 minutes to about 5.4 minutes, about 5.4 minutes to about 5.6 minutes, about 5.6 minutes to about5.8 minutes, about 5.8 minutes to about 6 minutes, about 6 minutes to about6.2 minutes, about 6.2 minutes to about 6.4 minutes, about 6.4 minutes to about6.6 minutes, about 6.6 minutes to about 6.8 minutes, about 6.8 minutes to about7 minutes.

[0034] According to a second aspect, the present invention provides a hydrogen evolution reaction (HER) electrocatalyst prepared according to the method of the first aspect.

[0035] In some embodiments, the current density exhibited by the HER electrocatalyst is defined by the equation jSum=N*jUnit, wherein N represents the number(N) of 2DEG interfaces formed in the HER electrocatalyst and junit represents the current density.

[0036] In some embodiments, the HER electrocatalyst exhibits a turnover frequency (TOF) that falls within the range of about 1 s’1to about 3000 s’1. For instance, the HER electrocatalyst exhibits a TOF that falls within the range of about 1 s’1to about 10 s’1, about 10 s’1to about 20 s’1, about 20 s’1to about 30 s’1, about 30 s’1to about 40 s’1, about 40 s’1to about 50 s’1, about 50 s’1to about 60 s’1, about 60 s’1to about 70 s’1, about 70 s’1to about 80 s’1, about 80 s’1to about 90 s’1, about 90 s’1to about 100 s’1, about 100 s’1to about 120 s’1, about 120 s’1to about 140 s’1, about 140 s’1to about 160 s’1, about 160 s’1to about 180 s’1, about 180 s’1to about 200 s’1, about 200 s’1to about 250 s’1, about 250 s’1to about 300 s’1, about 300 s’1to about 350 s’1, about 350 s’1to about 400 s’1, about 400 s’1to about 450 s’1, about 450 s’1to about 500 s’1, about 500 s’1to about 550 s’1, about 550 s’1to about 600 s’1, about 600 s’1to about 650 s’1, about 650 s’1to about 700 s’1, about 700 s’1to about 750 s’1, about 750 s’1to about 800 s’1, about 800 s’1to about 850 s’1, about 850 s’1to about 900 s’1, about 900 s’1to about 950 s’1, about 950 s’1to about 1000 s’1, about 1000 s’1to 1 100 s’1, about 1100 s’1to about 1200 s’1, about 1200 s’1to about 1300 s’1, about 1300 s’1to about 1400 s’1, about 1400 s’1to about 1500 s’1, about 1500 s’1to about 1600 s’1, about 1600 s’1to about 1700 s’1, about 1700 s’1to about 1800 s’1, about 1800 s’1to about 1900 s’1, about 1900 s’1to about 2000 s’1, about 2000 s’1to about 2100 s’1, about 2100 s’1to about 2200 s’1, about 2200 s’1to about 2300 s’1, about 2300 s’1to about 2400 s’1, about 2400 s’1to about 2500 s’1, about 2500 s’1to about 2600 s’1, about 2600 s’1to about 2700 s’1, about 2700 s’1to about 2800 s’1, about 2800 s’1to about 2900 s’1, and about 2900 s’1to about 3000 s’1.

[0037] In some embodiments, the HER electrocatalyst exhibits a current density of up to 300 mA / cm2at -0.1 V (E vs RHE). For instance, the HER electrocatalyst exhibits a current density of up to 30 mA / cm2, up to 60 mA / cm2, up to 90 mA / cm2, up to 120 mA / cm2, up to 150 mA / cm2, up to 180 mA / cm2, up to 210 mA / cm2, up to 240 mA / cm2, up to 270 mA / cm2, up to 300 mA / cm2, at -0.1 V (E vs RHE).

[0038] In one embodiment, the HER electrocatalyst exhibits a current density of up to 300 mA / cm2at -0.1 V (E vs Reversible Hydrogen Electrode) and a turnover frequency (TOF) of about 1600 s’1when N=1 .

[0039] According to a third aspect, the present invention provides a hydrogen evolution reaction (HER) electrode for producing hydrogen (H2) from a water-based electrolyte by water electrolysis, comprising the HER electrocatalyst according to the second aspect.

[0040] According to a fourth aspect, the present invention provides an electrochemical device for producing hydrogen (H2) from a water-based electrolyte by water electrolysis, comprising at least two electrodes and a power supply, wherein at least one of the at least two electrodes comprises the HER electrocatalyst prepared according to the method of the first aspect, or the HER electrode according to the fourth aspect.

[0041] Preferably, the at least other electrode comprises a non-noble metal with a work function lower than the work function of the substrate of the HER electrocatalyst.

[0042] Suitably, the non-noble metal is selected from the group consisting of Nickel (Ni), Titanium (Ti) and Aluminium (Al).

[0043] In some embodiments, the at least two electrodes are at least partially immersed in a water-based electrolyte solution with a pH of between 7 and 14. For instance, the at least two electrodes are at least partially immersed in a water-based electrolyte solution with a pH of between pH 7 to pH 7.2, pH 7.2 to pH 7.4, pH 7.4 to pH 7.6, pH 7.6 to pH 7.8, pH 7.8 to pH 8.0, pH 8 to pH 8.2, pH 8.2 to pH 8.4, pH 8.4 to pH 8.6, pH 8.6 to pH 8.8, pH 8.8 to pH 9.0, pH 9 to pH 9.2, pH 9.2 to pH 9.4, pH 9.4 to pH 9.6, pH 9.6 to pH 9.8, pH 9.8 to pH 10.0, pH 10.0 to pH 10.2, pH 10.2 to pH 10.4, pH 10.4 to pH 10.6, pH 10.6 to pH 10.8, pH 10.8 to pH 1 1.0, pH 1 1.0 to pH 11.2, pH 1 1 .2 to pH 11 .4, pH 1 1 .4 to pH 1 1 .6, pH 1 1 .6 to pH 1 1 .8, pH 1 1 .8 to pH 12.0, pH 12.0 to pH 12.2, pH 12.2 to pH 12.4, pH 12.4 to pH 12.6, pH 12.6 to pH 12.8, pH 12.8 to pH 13.0, pH 13.0 to pH 13.2, pH 13.2 to pH 13.4, pH 13.4 to pH 13.6, pH 13.6 to pH 13.8, pH 13.8 to pH 14.0,

[0044] In some embodiment, the water-based electrolyte solution comprises sodium hydroxide (NaOH) or potassium hydroxide (KOH) or a mixture thereof.

[0045] In some embodiments, the water-based electrolyte solution comprises potassium hydroxide (KOH) in a molar concentration of 1.0 to 10.0 Mol.

[0046] In some embodiments, the water-based electrolyte solution comprises potassium hydroxide (KOH) in a molar concentration of 1 .0 Mol to 1 .5 Mol, 1 .5 Mol to2.0 Mol, 2.0 Mol to 2.5 Mol, 2.5 Mol to 3.0 Mol, 3.0 Mol to 3.5 Mol, 3.5 Mol to 4.0 Mol, 4.0 Mol to 4.5 Mol, 4.5 Mol to 5.0 Mol, 5.0 Mol to 5.5 Mol, 5.5 Mol to 6.0 Mol, 6.0 Mol to 6.5 Mol, 6.5 Mol to 7.0 Mol, 7.0 Mol to 7.5 Mol, 7.5 Mol to 8.0 Mol, 8.0 Mol to 8.5 Mol, 8.5 Mol to 9.0 Mol, 9.0 Mol to 9.5 Mol, 9.5 Mol to 10.0 Mol.

[0047] According to a fifth aspect, the present invention provides a method of producing hydrogen (H2) from a water-based electrolyte by water electrolysis, the method comprising: providing an electrochemical cell comprising at least two electrodes and an electrolyte solution; contacting water with the at least two electrodes; and applying a voltage across the at least two electrodes, wherein one of the at least two electrodes comprises the HER electrocatalyst prepared according to the method of the first aspect, or the HER electrode according to the third aspect.

[0048] In one embodiment, the at least two electrodes are at least partially immersed in the water-based electrolyte solution.

[0049] In some embodiments, the water-based electrolyte solution has a pH of between 7 and 14.

[0050] For instance, the at least two electrodes are at least partially immersed in a water-based electrolyte solution with a pH of between pH 7 to pH 7.2, pH 7.2 to pH7.4, pH 7.4 to pH 7.6, pH 7.6 to pH 7.8, pH 7.8 to pH 8.0, pH 8 to pH 8.2, pH 8.2 to pH8.4, pH 8.4 to pH 8.6, pH 8.6 to pH 8.8, pH 8.8 to pH 9.0, pH 9 to pH 9.2, pH 9.2 to pH9.4, pH 9.4 to pH 9.6, pH 9.6 to pH 9.8, pH 9.8 to pH 10.0, pH 10.0 to pH 10.2, pH 10.2 to pH 10.4, pH 10.4 to pH 10.6, pH 10.6 to pH 10.8, pH 10.8 to pH 1 1.0, pH 1 1.0 to pH 1 1 .2, pH 11 .2 to pH 1 1 .4, pH 1 1 .4 to pH 1 1 .6, pH 11 .6 to pH 1 1 .8, pH 11 .8 to pH 12.0, pH 12.0 to pH 12.2, pH 12.2 to pH 12.4, pH 12.4 to pH 12.6, pH 12.6 to pH 12.8, pH 12.8 to pH 13.0, pH 13.0 to pH 13.2, pH 13.2 to pH 13.4, pH 13.4 to pH 13.6, pH 13.6 to pH 13.8, pH 13.8 to pH 14.0.

[0051] In some embodiment, the water-based electrolyte solution comprises sodium hydroxide (NaOH) or potassium hydroxide (KOH) or a mixture thereof.

[0052] In a preferred embodiment, the water-based electrolyte solution comprises potassium hydroxide (KOH)

[0053] In a preferred embodiment, the water-based electrolyte solution comprises potassium hydroxide (KOH) in a molar concentration of 1.0 to 10.0 Mol.

[0054] In some embodiments, the water-based electrolyte solution comprises potassium hydroxide (KOH) in a molar concentration of 1 .0 Mol to 1 .5 Mol, 1 .5 Mol to 2.0 Mol, 2.0 Mol to 2.5 Mol, 2.5 Mol to 3.0 Mol, 3.0 Mol to 3.5 Mol, 3.5 Mol to 4.0 Mol, 4.0 Mol to 4.5 Mol, 4.5 Mol to 5.0 Mol, 5.0 Mol to 5.5 Mol, 5.5 Mol to 6.0 Mol, 6.0 Mol to 6.5 Mol, 6.5 Mol to 7.0 Mol, 7.0 Mol to 7.5 Mol, 7.5 Mol to 8.0 Mol, 8.0 Mol to 8.5 Mol, 8.5 Mol to 9.0 Mol, 9.0 Mol to 9.5 Mol, 9.5 Mol to 10.0 Mol.

[0055] According to a sixth aspect, the present invention provides the use of a hydrogen evolution reaction (HER) electrocatalyst for producing hydrogen (H2) from a water-based electrolyte by water electrolysis, the HER electrocatalyst comprising: two or more alternating layers of a first complex oxide and a second complex oxide; and a two-dimensional electron gas (2DEG) interface formed at a heterojunction of at least one of the two or more alternating layers of a first complex oxide and a second complex oxide, wherein the or each 2DEG interface exhibits a current density corresponding to an intrinsic hydrogen evolution reaction (HER) activity that renders the 2DEG interface(s) suitable as an HER electrocatalyst for producing hydrogen (H2) from a water-based electrolyte by water electrolysis.

[0056] In preferred embodiments, the first and second complex oxide layers are each formed from a perovskite oxide or a transition metal oxide.

[0057] Preferably, the first complex oxide layer is formed from a perovskite oxide selected from the group consisting of LaAlOa, NdGaOa, LaTiOa, LaVOa, GdTiOa, SrlrOa, LaNiOa, NdAlOa, GdTiOa, BiFeOa, PrAlOa, Laa / aSr aMnOa, LaAIOa / LaVOa / LaAIOa / SrTiOa, Y-AI2O3, CaZrOa, LaAIOa / La / sSri / sMnOa, SrTiO3ZSrTio.8Nbo.2O3, SrTiOa, LaAlOa, LaNiOa, SrVOa, BaSnOa, LaTiOa, LaAlOa, SrTiOa, Nb-SrTiOa and LaFeOa.

[0058] Preferably, the second complex oxide layer is formed from a perovskite oxide selected from the group consisting of SrTiOa, BaTiOa, SrMnOa, SrVOa, SrCrOa and LaNiOa.

[0059] In a preferred embodiment, the first complex oxide layer is LaAlOa (LAO) and the second complex oxide layer is SrTiOa (STO).

[0060] Preferably, the alternating LAO and STO layers are deposited in sequence onto a surface of a substrate, wherein the substrate is a solid state material.

[0061] In some embodiments, the solid state material is selected from the group consisting of SrTiOs (STO), Niobium (Nb) doped STO, silicon (Si), silicon dioxide (SiO2), indium tin oxide (ITO), zinc oxide (ZnO) and graphite.

[0062] In some embodiments, the LAO layer has a thickness that falls within a range of about 4 unit cells (u.c.) to about 120 unit cells (u.c.).

[0063] In some embodiments, the LAO layer has a thickness that falls within a range of about 4 unit cells (u.c.) to about 80 unit cells (u.c.).

[0064] In a preferred embodiment, the LAO layer has a thickness of at least 4 unit cells (uc).

[0065] In a preferred embodiment, the surface of the or each STO layer is terminated with TiO2.

[0066] Other aspects of the invention are also disclosed in the following.Brief Description of the Drawings

[0067] Notwithstanding any other forms which may fall within the scope of the present invention, preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0068] Figure 1 shows a schematic representation that provides a comparison of the energy loss between conventional catalysis and ideal catalysis;

[0069] Figure 2 (a-b) Reflection High-Energy Electron Diffraction (RHEED) oscillations during LaAIOs (LAO) deposition on an SrTiOs (STO) substrate to produce an LAO / STO heterostructure; the inset image shows the RHEED pattern of the as- grown LAO / STO surface, where the clear streaky (1 x1 ) pattern indicates a flat surface after LAO deposition, (c). The 2theta Omega X-ray diffraction (XRD) scan for the LAO / STO heterostructure produced with different LAO thicknesses (u.c. is unit cell);

[0070] Figure 3 shows X-ray reciprocal space mapping (RSM) of the LAO / STO heterostructure with 20, 30, 50, 80 u.c LAO in the (013) and (103) orientations;

[0071] Figure 4 shows 30-unit cells (u.c.) of LaAIOs grown on bulk SrTiOs (001 ). (a) The ABOs perovskite crystal structure of SrTiOs (STO), and the heterostructure of LaAIOs (LAO) on STO. (b) High-resolution Scanning Transmission Electron Microscopy (STEM) image of the 30LAO / STO sample, (c) The left most High-Angle Annular Dark-Field (HAADF) image and the atomic-column-resolved ScanningTransmission Electron Microscopy-Electron energy-loss spectroscopy (STEM-EELS) maps constructed by selecting La-M, Al-K, Sr-L, Ti-L, O-K along the

[0001] direction of SrTiOs. (d) EELS spectra of Ti-L and O-K edge extracted from different regions cross the LAO / STO interface, two bold spectra are corresponding to two rectangle marks in (c). (e) The fine structure of Ti-L and O-K edge for the two bold spectra from the LAO / STO interface and the STO bulk region, respectively. Gaussian fits were performed to extract the position and intensity of the peaks, (f) l(L2):l(L3) and AE (distance between peaks on O-K edge) as a function of distance from the LAO / STO interface. The line shown in the graphs are guides to the eye;

[0072] Figure 5 (a) shows the temperature-dependent sheet resistance (Rs-T) of the 30LAO / STO heterostructure. As can be seen, the heterostructures exhibit metallic behaviour in the whole temperature range, (b) According to Hall measurements, the electron concentration of the 2DEG ranges from 3 x 1013cm’2to 2.4 x 1016cm’2with increasing LAO thickness from 4u.c to 80u.c. Such a high concentration of electrons is expected to contribute to a better catalytic performance;

[0073] Figure 6 (a) shows polarization curves of the 30LAO / STO sample, pristine STO and LAO. Data collected in 1 M KOH aqueous electrolyte at a 20 mV / s scan rate, (b) Schematic diagram of three electrochemical electrode devices: (i) LAO / STO [LAO Surface], (ii) LAO / STO [LAO Surface+lnterface] and (iii) LAO / STO [STO bulk]). Each electrode device is electrically connected via a conducting wire and is selectively sealed using a chemically inert epoxy, (c) Polarization curves of 30LAO / STO samples on the electrochemical electrode devices (i)-(iii), together with an additional electrochemical electrode device (iv) LAO / STO [LAO Surface 2+lnterface], Data collected in 1 M KOH aqueous electrolyte at a 20 mV / s scan rate, (d) A plot of current (A) measured at -0.8 V vs. reversible hydrogen electrode (RHE) as a function of top exposed area (Area (Top)) for the electrochemical electrode devices (i), (ii) and (iv);

[0074] Figure 7 (a) shows Electrochemical Impedance Spectroscopy (EIS) patterns recorded on the (i) 30LAO / STO [LAO Surface] and (ii) 30LAO / STO [Surface+lnterface] electrochemical electrode devices at cathodic overpotentials (- 0.17 V to -0.77 V vs. RHE), (b) EIS fitting parameters for Eapp= 2.23 vs RHE. [Note: Rsis a series resistor that accounts for resistance due to wires, clips, and the electrolyte solution. Ri represents the charge transfer resistance at the electrolyte and catalyst interface. R2 can be assigned to the charge transfer resistance at referenceelectrode / electrolyte interface], (c) Deconvolution of the linear sweep voltammetry (LSV) curves obtained for the 30LAO / STO electrode device, when measured in argon- saturated 1 M KOH at a scan rate of 10 mVs’1, separating the total current (A) contribution for the 30LAO / STO electrode device into two components: the LAO surface current (A) and the interface current (A) components, (d) The current density (mA / cm2) contributed (normalized to the 2DEG area) by the 2DEG interface, which is deducted from the interface current (A) in (c);

[0075] Figure 8 Comparing the HER performance of the LAO / STO 2DEG interface described herein with other reported electrocatalysts on the basis of current density (mA / cm2) at 100mV vs RHE;

[0076] Figure 9 shows (a) The corresponding surface topography change (nm) of the LAO / STO surface, before and after the HER reaction, (b) RHEED pattern of the LAO / STO surface, before and after the HER reaction;

[0077] Figure 10 shows (a) Polarization curves obtained for LAO / STO [LAO Surface] samples of varying LAO thickness (measured in an argon saturated 1 M KOH solution) after the HER reaction. Scan rate, 10 mv s’1, (b). The corresponding Tafel slopes (mV) obtained for each of the polarization curves of (a), (c) A plot of specific current density (mA / cm2) vs. LAO thickness (u.c.) for LAO / STO samples of varying LAO thickness (n=0.8 v). (d) A plot showing the Tafel slopes (mV) of (b) vs. LAO thickness (u.c.) for LAO / STO samples of varying LAO thickness;

[0078] Figure 11 shows electrochemical impedance spectroscopy (EIS) patterns recorded on (a) the 5LAO / STO[LAO Surface] sample and (c) the 30LAO / STO[LAO Surface] sample across a series of cathodic overpotentials (-0.17 V to -0.77 V vs. RHE). The corresponding Bode phase plots obtained for these two samples are shown in (b) and (d) respectively. Inset: Equivalent circuit used to fit experimental data and generate overlaid line;

[0079] Figure 12 shows (a) a series of cyclic voltammetry (CV) measurements for a range of LAO / STO samples of varying LAO thickness (measured in 1 M KOH) conducted in a non-faradic current region (0.0-0.1 V vs. RHE, no iR corrected) at scan rates of 20, 40, 50, 60, 80 and 100 mV s’1, (b) An electrochemical active surface area (ECSA) estimation of LAO / STO samples of varying LAO thickness (measured in 1 MKOH), as determined from double-layer capacitance (Cdi). (c) A plot of Cdl vs. LAO thickness (u.c.) for the LAO / STO samples of varying LAO thickness;

[0080] Figure 13 shows (a) an atomic force microscopy (AFM) image revealing the surface morphology of the 30LAO / STO sample, and (b) a Kelvin probe force microscopy (KPFM) image revealing the corresponding surface potential (mV) of the 30LAO / STO sample, (c-d) Plots of step height (nm) and surface roughness (mV) of the 30LAO / STO sample, measured along the dotted box in the images of (a) and (b), respectively, (e) A plot showing the dependence of LAO thickness (u.c.) on the interfacial sheet resistance (Rs) of the LAO layer(s) for a range of LAO / STO (dark dots) and LAO / Nb:STO (red dots) samples, measured using a four-point probe in the vender Pauw configuration, (f) The contact potential difference (VCPD) obtained for the surface of the LAO / STO (dark dots) and LAO / Nb:STO (red dots) samples of varying LAO thickness, where the STO and Nb:STO substrates represent a zero thickness. The inset in (f) shows a schematic representation of the measurement setup, (g) An energy band diagram of the contact between the metal tip of the KPFM probe and the surface of the LAO / STO heterostructure;

[0081] Figure 14 shows a plot of work function (0) vs. LAO thickness (u.c.) for a range of LAO / STO (dark dots) and LAO / Nb:STO (red dots) samples of varying LAO thickness, providing the surface effective work function (0) derived from the contact potential difference (VCPD) values for the surfaces of these samples, where the STO and Nb:STO substrates represent a zero thickness;

[0082] Figure 15 shows (a) ultraviolet photoelectron spectroscopy (UPS) spectra of STO, LAO, and the 4LAO / STO sample, where the cut-off energy (eV) of the valence electrons yields the work function (0) of the corresponding samples, (b) Fermi edge spectra of STO, LAO, and the 4LAO / STO sample, where the dashed lines indicate the linear extrapolation of the spectral onset used to determine the valence band minimum (VBM) energy for the corresponding samples;

[0083] Figure 16 shows (a) X-ray photoelectron spectroscopy (XPS) Ti2p spectra for bulk STO, Nb-doped STO, and the 4LAO / STO sample, (b) XPS AI2p spectra for the 30LAO / STO and 4LAO / STO samples, respectively, (c-d) Ultraviolet photoelectron spectroscopy (UPS) spectra for a range of LAO / STO samples of varying LAO thickness to define the work function ( ) and valence band maximum (VBM),respectively, for the corresponding samples, (e) The valence band offset (VBO) and conduction band offset (CBO) values (eV) obtained for the same range of LAO / STO samples correspond with the results obtained by hard X-ray photoelectron spectroscopy (HX-PES)[1]. (f) A schematic energy band diagram showing the relationship between the contact potential difference (VCPD) value for the LAO / STO sample and the corresponding work function (0);

[0084] Figure 17 shows schematic representations of the unit cells of SrTiOa (STO), LaAIOs (LAO) and SrTiOa / LaAIOs (STO / LAO) interface;

[0085] Figure 18 shows the projected density of states (PDOS) of (a) LAO, (b) STO, and (c) LAO / STO interface cells;

[0086] Figure 19 show the distribution of conductive electrons in the TiO2 layers near the interface of the LAO / STO heterostructure;

[0087] Figure 20 shows schematic representations of the top and side views of the unit cells of SrTiOa (100), LaAIOs (100) and the SrTiOa / LaAIOs (100) interface with BO2 termination;

[0088] Figure 21 shows schematic representations of the top and side views of the unit cells of SrTiOa (100), LaAIOs (100) and the SrTiOa / LaAIOs (100) interface with AO termination;

[0089] Figure 22 shows (a) the spatial variation of hydrogen adsorption free energy (AGH) on oxygen atom sites across the heterointerface (inset), and the reaction profile of HER on the oxygen sites (b) and metal sites (c) of BO2-terminated STO, LAO, and LAO / STO surfaces;

[0090] Figure 23 shows the reaction profile of HER on the (a) oxygen sites and (b) metal sites of AO-terminated STO, LAO, and LAO / STO surfaces;

[0091] Figure 24 shows the projected density of states (PDOS) of oxygen active sites and their nearest neighbour Ti atoms on the clean surface (left column) of the LAO / STO heterostructure, and on the surface of the LAO / STO heterostructure with H adsorption (right column);

[0092] Figure 25 shows the top (a) and side (b) views of a reconstruction of the oxygen atom (Site OE) of the LAO / STO heterostructure under the adsorption of hydrogen;

[0093] Figure 26 shows the crystal orbital Hamilton population (COHP) of the oxygen atom at site OE with its nearest neighbour Ti atoms before (a) and after the H adsorption (b) on the LAO / STO surface, and the COHP of the oxygen atom at site OG with its nearest neighbour Ti atoms before (c) and after the H adsorption (d) on the LAO / STO surface.

[0094] Figure 27 shows the 2DEG patterning process. 1. Lithography process, 2. Deposition of hard mask material, 3. Lift-off process, 4. 2DEG heterostructure deposition, 5. Coating resist, 6. Lithography process, 7. Wet etching, cleaning the protective resist;

[0095] Figure 28 shows (a) an optical image of an LAO / STO electrode device fabricated with square-shaped openings, (b) a close up view of the optical image of (a), and (c) a schematic representation of the LAO / STO electrode device. This device features a square 2DEG edge with a side length (W) of 20 pm;

[0096] Figure 29 shows (a) an optical image of an LAO / STO electrode device with a 2DEG edge length (w) of 5 pm, (b) a close up view of the optical image of (a), and (c, d) close up views of two additional LAO / STO electrode devices with 2DEG edge lengths (w) of 50 pm and 100 pm, respectively;

[0097] Figure 30 shows two linear sweep voltammetry (LSV) curves conducted over different ranges obtained for a series of LAO / STO electrode devices fabricated with openings of varying size and shape, an LAO surface electrode device, and two platinum (Pt) thin film electrodes with thicknesses of 5 nm and 10 nm, respectively. For brevity, the notation of 60LAO / STO[LAO Surface], 60LAO / STO[edge]-WxW pm2, 60LAO / STO[edge]-Lpm, are simplified to LAO surface, l / Vxl / V pm2, L pm, respectively;

[0098] Figure 31 shows plots of the current density (normalized to the geometry area) versus (a) the opening window size (cm2) and (b) the 2DEG length / geometry area (cm2deg / cmgeo2), whereas it shows a linear dependence on the 2DEG edge length, (c) LSV curves with current density normalized to the electrochemical active surface area (ECSA). As shown inset, ECSA increases with increasing 2DEG length (pm), (d) Benchmarking the intrinsic activity of the LAO / STO electrode devices in Figure 30 against a range of state-of-the-art HER electrocatalysts when measured in an alkaline solution. Current density (normalized to ECSA) at 100mV (vs. RHE) in 1 mol KOH provides a direct performance comparison;

[0099] Figure 32 shows exchange current density (normalized to the geometry area mA / cmgeo2) as a function of 2DEG edge length per geometric area (cm2deg / cm2geo);

[0100] Figure 33 shows a volcano plot of the exchange current density (mA / cm2) as a function of the DFT-calculated Gibbs free energy (eV) of adsorbed atomic hydrogen for 2DEG edge (red dot) and a series of representative pure metals;

[0101] Figure 34 shows (a) a schematic representational comparison of a nonpatterned (unpatterned) and lithography-patterned 2DEG heterostructures showing that a greater surface area of the 2DEG interface can be exposed via lithography engineering, (b) Optical images of the unpatterned electrocatalyst device and patterned electrocatalyst device obtained during a water splitting HER reaction in the presence of a suitable electrolyte;

[0102] Figure 35 shows linear sweep voltammetry (LSV) curves for non-patterned (unpatterned) and lithography-patterned 2DEG heterostructures (patterned); and

[0103] Figure 36 shows a schematic representation of an SrTiOs / LaAIOs / SrTiOs superlattice deposited as an HER electrocatalyst on an electrode device for application in the water splitting reaction, where the substrate of the device is TiO2-terminated SrTiOs (STO), and the alternating A and B layers stacked in vertical arrangement, represent epitaxially grown LaAIOs (LAO) and SrTiOs (STO), respectively, with corresponding 2DEGs formed at the heterointerfaces therebetween, with all layers of the electrode device being interconnected via a conducting wire.Description of Preferred Embodiments of the Invention

[0104] It should be noted in the following description that like or the same reference numerals in different embodiments denote the same or similar features.

[0105] Specifically, the inventors have surprisingly found that to eliminate the energy loss in conventional catalysts, a possible strategy is to induce high conductivity in catalysts with high intrinsic activity. The development of techniques for high-quality growth of epitaxial complex oxide thin films and heterostructures provides an intriguing opportunity to achieve excellent hydrogen evolution reaction (HER) activity with the contribution of high charge transport on the interface with the two-dimensional electron gas (2DEG) effect.

[0106] The 2DEG effect in transition metal oxides (TMOs) was originally observed at the heterointerface between two wide bandgap insulators, such as LaAIOs (LAO)and SrTiOa (STO). The leading consensus for the formation of the 2DEG at this interface is the electronic reconstruction, resulting in a metallic character with charge densities as high as 3x1014crrr2resided in Ti3d orbitals. The d-orbital electrons of transition metal atoms participate directly in electron transport and the O p orbital electron participate directly in the electron transfer during the catalytic redox reactions and have close relationship with the adsorption strength of the catalytic reaction intermediates, hence, to be a crucial factor in determining catalytic performance. Moreover, the electron-rich and electrically conducting region enables fast electron transfer at the catalytically active site, thereby improving electrocatalytic kinetics. Therefore, the ultra-high charge density and conductivity are expected to minimize the energy barriers in catalysts.

[0107] In this study, the inventors have innovatively applied the heterointerface that is formed by combining two insulators to generate a two-dimensional electron gas (2DEG) as the charge carrier pool and the current collector, realizing high-efficient charge exchange in water splitting process to significantly enhance the electrocatalytic HER. Such a 2DEG interface exhibits an interface current density of 300 mA / cm2at - 0.1 V (E vs RHE), which reflects the high intrinsic hydrogen evolution reaction (HER) activity associated with the 2DEG interface, outperforming most electrocatalysts, including heterostructure-based and Pt-based catalysts.

[0108] A non-noble-metal 2DEG based electrocatalyst provides distinct competitive advantages over the conventional Pt-based catalysts in terms of their good electrical conductivity and high intrinsic activity, which translates to a low intrinsic conductivity (Rs)and low solid-aqueous electrolyte charge transfer resistance (Rsa), thereby eliminating the need for a supporting material and co-catalyst; components which are typically required in the conventional noble metal-based catalysts.

[0109] For instance, Figure 1 shows a comparison of the energy loss between conventional catalysis and ideal catalysis. The left image shows that the ohmic voltage drop in the conventional catalytic system comes from intrinsic conductivity (Rs), solidsolid interface resistance (Rss), solid-aqueous electrolyte charge transfer resistance (Rsa).

[0110] Additionally, the inventors have also demonstrated fine-controlled 2DEG- based electrocatalysis by exposing 2DEG areas as designed, revealing the great potential to achieve cost-reduced green hydrogen (H2) in mass production.

[0111] It is also demonstrated that the 2DEG interface between the well-defined LAO / STO heterostructure sustains outstanding intrinsic activity in HER due to the ultra-low energy loss for charge transport from the electrode to the reaction interface. These 2DEGs are confined to within a few nanometers of the LAO / STO interface with the carrier density ranging from about 3x1013cm-2to about 2.4x1016cm-2, leading to enhanced conductivity and electron mobility.

[0112] As a result, the 2DEG interface demonstrates a remarkably high intrinsic HER activity, as evidenced by an exceptionally high current density, along with a large turnover frequency (TOF) value that falls within the range of about 1 s’1to about 3000 S’1.

[0113] As will be described in more detail below, the 2DEG interface generated at the heterojunction formed between alternating layers of LAO / STO yields a current density of up to 300 mA / cm2at -0.1 V (E vs RHE), corresponding to a TOF value of around 1660 s-1.

[0114] Moreover, a further ten-fold increase of the catalytic current can be achieved through the controlled exposure of more 2DEG interface regions by microstructural engineering.

[0115] Combining experimental results with density functional theory (DFT) calculations, the 2DEG heterointerface provides a catalysis pathway with ultra-low energy loss, owing to the following three factors: (i) the 2DEG intrinsic electrical conductivity ensuring the rapid transport of electrons injected from the electrode to the active sites, (ii) high intrinsic activity attributed to the proper band alignment, and (iii) the synergistic effects of the multiple catalytic sites at the interface.

[0116] Finally, the inventors have demonstrated that, through the construction of the 2DEG, the intrinsic activity and electrical conductivity can ideally be addressed simultaneously, leading to efficient electrocatalysis.

[0117] This promising approach offers an opportunity in advanced heterostructured electrocatalyst design by the combination of microstructural engineering techniques and non-noble metal catalysts.

[0118] Various embodiments of the present invention are described below. It will be appreciated by persons of ordinary skill in the art that numerous variations and / or modifications can be made to the present invention as disclosed in the specific embodiments without departing from the spirit or scope of the present invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.Methods of Use

[0119] The invention provides a method of evolving hydrogen through a water splitting process. The method is carried out in an electrochemical cell, which comprises at least two electrodes at least partially immersed in a suitable electrolyte solution, wherein at least one of the at least two electrodes comprises the HER electrocatalyst of the invention. In some embodiments, the electrochemical cell is a 2- electrode system comprising an anode and a cathode comprising the HER electrocatalyst of the invention.

[0120] In other embodiments, the electrochemical cell is a 3-electrode system comprising a working electrode comprising the HER electrocatalyst of the invention, a counter electrode and a reference electrode. The method comprises contacting water with the at least two electrodes of the electrochemical cell and applying a voltage across the at least two electrodes through the electrolyte solution.

[0121] The counter electrode may typically comprise a non-noble metal with a work function lower than the work function of the working electrode including but not limited to a non-noble metal selected from the group consisting of Nickel (Ni), Titanium (Ti), Aluminium (Al) or other non-noble metal(s) that have a work function lower than that of the substrate material.

[0122] As the HER half reaction typically occurs at the cathode of a 2-electrode system or the working electrode of a 3-electrode system, the cathode or the working electrode comprises the HER electrocatalyst of the invention. In some embodiments, the electrolyte solution is an aqueous electrolyte solution. The aqueous electrolyte solution may also be the source of the water. The electrolyte solution may have a pH of about 7 or greater, for example, a pH from about 7 to about 14. In some embodiments, when the pH is alkaline, the electrolyte solution comprises a strongbase, for example, a hydroxide base such as sodium hydroxide (NaOH) or potassium hydroxide (KOH).

[0123] In a preferred embodiment, the electrolyte solution comprises potassium hydroxide (KOH) in a molar concentration of 1 .0 Mol to 10.0 Mol.

[0124] Typically, the anode used in these methods will comprise an oxygen evolution reaction (OER) catalyst, such as Ir / C or RuCVC.

[0125] When used in these methods, the HER electrocatalyst of the invention provides improved catalytic activity to the present leading HER catalysts, and require a lower overpotential. Further, the HER electrocatalyst of the invention is surprisingly stable and active under neutral and alkaline electrolysis conditions.

[0126] The voltage applied across the anode should be selected to match or exceed the overpotential for the HER catalyst in order to drive HER.

[0127] Also provided is an electrolyser comprising an anode and a cathode, a power supply and optionally a reference electrode. Typically, the cathode comprises the HER electrocatalyst of the invention. In some embodiments, the power supply provides electricity generated from a low carbon intensive power source. The power source may be a renewable power source, for example, one or more solar panels or wind turbines, or a non-renewable power source.Results and Discussion

[0128] Structural characterization and electronic state investigation

[0129] It will be appreciated by persons of skill in the relevant art that the heterostructure formed between alternating layers of first and second complex oxides deposited in sequence on the surface of a suitable substrate may be produced by any one of a number of deposition techniques selected from the group consisting of atomic layer deposition (ALD), molecular beam epitaxy (MBE), pulsed laser deposition (PLD), RF magneton sputtering, physical vapour deposition (PVD), chemical vapour deposition (CVD) or any combination thereof.

[0130] The substrate may be any conductive substrate selected from the group consisting of SrTiOa (STO), Niobium (Nb) doped STO, silicon (Si), silicon dioxide (SiO2), indium tin oxide (ITO), zinc oxide (ZnO) and graphite.

[0131] As will be described in more detail below, HER electrocatalysts produced from alternating layers of the first and second complex oxides of LAO and STO, respectively, yield good HER activity when the LAO layer has a thickness of at least 4 unit cells (uc).

[0132] In this work, pulsed-laser-deposition (PLD) was used to prepare the heterostructure formed from alternating layers of LAO / STO formed on either an STO or Nb:STO substrate (see Materials and Methods section for details).

[0133] As is described in the Materials and Methods section, PLD is ideally conducted at a temperature of between about 500 °C and about 900 °C at an oxygen pressure of about 10’9Torr to about 10’2Torr, and more preferably at a temperature of around 650 °C at an oxygen pressure of around 10’4Torr.

[0134] During growth, reflection high-energy electron diffraction (RHEED) was used to realize the atomic control of the LAO thin film thickness. The growth parameter of the LAO / STO heterostructure was carefully optimized to show clear RHEED oscillation along the

[0100] azimuthal direction, corresponding to two-dimensional layer-by-layer growth, with an atomically flat surface with a step-terraced morphology (Figure 2(a- b))

[0135] The LAO / STO samples were also characterized by X-ray diffraction (XRD) in which the out-of-plane 2theta omega scan (Figure 2c) and X-ray reflectivity (XRR) confirms the single crystalline phases of the LAO film and the atomically sharp interface.

[0136] Moreover, the (013) and (103) reciprocal space mapping (RSM) shown in Figure 3 indicates the in-plane lattice constants of LAO are identical to the substrate, which means the LAO film is fully strained to the substrate. For thicker LAO / STO samples, no apparent relaxation occurs.

[0137] The high-resolution scanning transmission electron microscopy (STEM) image of the 30LAO / STO sample in Figure 4b demonstrates the epitaxial growth of LAO on the STO substrate with very limited defects in the probed area. The crystal structure of the resulting LAO / STO heterostructure is shown in Figure 4a.

[0138] An electron energy loss spectroscopy (EELS) scan was performed across the interface from the LAO side of the 30LAO / STO sample to the STO substrate to further investigate the electronic states in atomic resolution (Figure 4b).

[0139] Figure 4c presents the annular dark field (ADF) image (leftmost) and high spatial resolution EELS mapping image, which confirmed the (LaO)+ / (TiO2)° interface. The individual spectra of Ti-L2,3 and O-K edge are stacked in Figure 4d respectively, the spectra at the interface and the bulk STO are highlighted. The Ti L-2,3 spectra of the interface and bulk STO were carefully analysed and the peak decomposition was performed by Gaussian fit (Figure 4e), the Ti-Ls edge existed on the low-energy side (ev) and the Ti-L2 (ev) edge located on the high-energy side.

[0140] The first and second peak of the Ti L3 and L2 edge belong to the t2g and eg orbitals of Ti3d, respectively. The Ti spectra obtained from interface are significantly suppressed between the t2g and eg peaks for both the L2 and L3 edges, and the spectrum from the substrate has a sharper drop in the minimum between these peaks, which suggests a valence change and indicates the existence of Ti3+, while Ti2+is not likely to exist due to no apparent peak shift observed in the Ti-L2,3edge. The l(L2) / l(L3> intensity ratio of peak d and a in TLL2 reflects the occurrence of electron filling of the Ti 3d(t2g) orbital.

[0141] The upper panel of Figure 4f shows the ratio l(L2) / l(L3> as a function of distance (nm) from the LAO / STO interface at which the position is defined as zero. Region I (with a lower ratio value) can be assigned to the Ti4+ / Ti3+zone, while Region II (which represents bulk STO) can be assigned to the Ti4+only zone.

[0142] As complementary measurements of Ti-Ls,2 edges, O-K edges provide information relative to the Ti-0 chemical bonding and the Ti valence and spin state.

[0143] The lower panel of Figure 4d shows a comparison between the O-K edge spectrum far away from the LAO / STO interface and the spectrum at the LAO / STO interface. Peak A is assigned to 0 2p — ► Ti 3d(t2g) and Peak B is assigned to O 2p — ► Ti 3d(eg).

[0144] The energy separation (AE) between peak A and peak C has been recognized as a strong indicator of oxidation state change for cations in ABO3 type perovskites, with negligible contribution from atomic distortions.

[0145] With peak determination by Gaussian fits, the evolution of the AE across the interface reveals a clear decreasing region near the interface, which indicates the occurrence of charge transfer, where the resulting Ti4+ / Ti3+mixed region is localized within a spatial depth(s) of about 3 nm.

[0146] From experimental results or calculation, the reduced Ti3+suggests the occupation of extra electrons at the Ti 3d(t2g) dxy sub-bands, which lobes lie in the (001 ) plane and have the lowest energy compared, whereas the dxz and dyzorbitals are extended across the interface plane. These observations furnish compelling evidence that, in our sample, the two-dimensional electron gas (2DEG) is confined within 3 nm from the interface, and the chemical and electronic information on buried atoms located beyond 3 nm from the interface are in a bulk-like environment. This measurement aligns well with prior findings, which fall into distinct categories such as theoretical calculations addressing Poisson’s equation, first-principles electronic structure calculations for obtaining metal-induced gap states, direct probing of interface electronic properties through conductive AFM, spectroscopy methods [6] [8], and inline electron holography on TEM.

[0147] Based on the EELS spectra of the Ti-La,2 edges and O-K edges, the inventors distinguished the nanoscale evolution of oxidation states at the LAO / STO interface as two regions, Ti4+ / Ti3+mixed valence region (3 nm in depth) and Ti4+bulk STO region.

[0148] Additionally, one may also need to consider the existence of other conducting regions, the bulk STO contributed by the small amount of oxygen vacancies which cannot be detected as the resolution of the EELS. However, the bulk STO conductivity region is lower than that of the 2DEG channel by about four orders of magnitude, which has been directly determined from resistance profile mappings using a conducting-tip atomic force microscope (AFM). Meanwhile, the diffusion of oxygen vacancies of the perovskite oxide in the RT temperature range is highly limited, which is beneficial for their confinement near the interface.

[0149] The temperature-dependent sheet resistance (Rs-T) of the 30LAO / STO heterostructure presents a metallic behaviour in the whole temperature range (Figure 5a). According to Hall measurements, the electron concentration of the 2DEG ranges from 3 x 1013cm-2to 2.4 x 1016cm-2with increasing LAO thickness from 4u.c to 80u.c (Figure 5b). Such a high concentration of electrons is expected to contribute to a better catalytic performance.

[0150] Based on the polar catastrophe mechanism, full compensation of the internal electric field of the LAO layer requires 0.5 electrons per two-dimensional interface unit cell (- —) at the LAO / STO heterointerface, which is equivalent to 3.3 x 1014cm-2. aSTO

[0151] In the samples described herein, the 30LAO / STO sample has an electron concentration of 2.0 x 1014cm-2.

[0152] However, the thicker samples with a higher electron concentration beyond the “intrinsic” limit of -^-, have an electron distribution that affords the 2DEG with a aSTO3D character.

[0153] Therefore, the 30LAO / STO sample was selected to study the 2DEG catalytic performance.

[0154] Electrode Devices

[0155] A 2DEG based electrocatalyst electrode device was then fabricated to ascertain the alkaline HER electrocatalytic performance of the heterostructure. The 2DEG interface was connected via conducting wire such as an alumina (Al wire) to ensure good ohmic contact. The wire connection on the electrode device was then carefully sealed using a chemically inert epoxy (Loctite EA D609) to keep the heterostructure from being exposed to the electrolyte. The pristine STO and LAO, which were subjected to the same treatment in the vacuum chamber but without thin film deposition, were prepared as electrodes for comparison.

[0156] The size of the material system for the three electrodes is 5mm*5mm*0.5mm. The details of the sample electrode assembly are shown in the Materials and Methods section detailed further below.

[0157] Figure 6a presents the polarization curves obtained from linear sweep voltammetry (LSV) at a scan rate of 10 mV s-1and without insulation resistance compensation. The 30LAO / STO catalyst was observed to outperform the pristine STO and LAO samples in terms of the current by three orders of magnitude, suggesting the heterointerface constructed by LAO / STO plays a pivotal role in promoting the HER activity significantly. The negligible catalytic current for pristine STO and LAO imply their intrinsic inertness for HER due to their poor conductivity.

[0158] Notably, when observing the sample under an optical microscope with a two- electrode setup (with a gold (Au) wire as the counter electrode), H2 bubble generation was primarily observed at the top edge of the LAO / STO sample, without any bubbles seen on the top surface or bulk STO side. This implies that the catalytic activity ismainly concentrated at the edge with abundant active sites, while the base plane remains chemically inert.

[0159] The inventors hypothesize that this observation is due to the formation of 2DEG induced by the Ti4+ / Ti3+mixed valence region (Figure 4f) within 3 nm of the heterointerface, which assists the charge transfer and charge transport.

[0160] To pinpoint the catalytically active area, the inventors conducted a selective study on the catalytic activity across various regions [i.e., LAO surface, LAO / STO interface, STO bulk (substrate)], the samples 30LAO / STO [LAO surface], 30LAO / STO [LAO surface + interface], 30LAO / STO [STO bulk] were prepared accordingly, as shown in Figure 6b. A chemically inert epoxy was used to cover the edge and sides of the 30LAO / STO[LAO surface] electrode device, leaving only the top LAO surface (0.028 cm2) exposed.

[0161] For the 30LAO / STO [LAO surface+ interface] electrode device, the surface area was partially covered by epoxy while the interface was fully exposed to electrolyte. The uncovered surface area of the 30LAO / STO[LAO surface 1 + interface] electrode device is 0.018 cm2and that of the 30LAO / STO[LAO surface 2 + interface] electrode device is 0.012 cm2.

[0162] Figure 6c presents the linear sweep voltammetry (LSV) curves at a scan rate of 10 mV s-1and without insulation resistance compensation. It was observed that the samples with an edge of the 2DEG interface exposed to the electrolyte show a better catalytic performance. The current (-0.8V) of the 30LAO / STO [LAO surface 1 + interface] electrode device is four times higher than that of the 30LAO / STO [LAO surface] electrode device.

[0163] It is noteworthy that two 30LAO / STO [LAO surface + interface] electrode devices show not much difference on the current (-0.8V) although the 30LAO / STO [LAO surface 2 + interface] electrode device has a smaller top exposed area (Figure 6d).

[0164] In addition, to exclude the contribution from the STO bulk side, the inventors inverted the 30LAO / STO sample to make an electrode device, ensuring that the epoxy shielded the heterointerface, thereby exposing the majority of the STO side to the electrolyte, as illustrated in Figure 6b(iii). As expected, there is no apparent currentobserved as shown in Figure 6c. This outcome is mainly due to the bulk side STO being poorly conductive.

[0165] Based on the above observation, the inventors’ hypothesis that the 2DEG interface possesses a much higher HER catalytic activity than the surface of the 30LAO / STO sample has been confirmed.

[0166] The significant impact of the 2DEG interface on charge transfer kinetics between the LAO / STO sample and the electrolyte is further investigated by electrochemical impedance spectroscopy (EIS).

[0167] Figure 7a displays the Nyquist plots for the 30LAO / STO [LAO surface] and 30LAO / STO [LAO surface + interface] electrode devices at -0.17V, where accurate modelling of the data was obtained with the equivalent circuit model shown inset. It is evident that the 30LAO / STO [LAO surface + interface] electrode device exhibits a significantly lower charge transfer resistance compared to the 30LAO / STO [LAO surface] electrode device.

[0168] As Figure 7b shows, at -0.77 V (vs RHE), the 30LAO / STO [LAO surface + interface] electrode device has a charge transfer resistance of 247.8 Q, which is notably smaller than that of the 30LAO / STO [LAO surface] electrode device (4516.0 Q). The inventors therefore deduce that the 2DEG at the LAO / STO heterostructure promotes the HER catalytic performance, which not only demonstrates its high catalyst activity, but also provides an unimpeded charge transfer pathway between the catalyst and the electrolyte.

[0169] The specific activity (normalized to the catalyst surface area) is well known to reflect the intrinsic activity of a catalyst. To evaluate the HER performance of the 2DEG interface, the inventors extracted the current contributed by the 2DEG interface with a subtraction of the top current from the total current of the 30LAO / STO[LAO surface 1 + interface] electrode device (Figure 7c).

[0170] Then, the interface current was normalized to the effective area of the interface, which is the 2DEG channel area (3 nm in depth). The obtained polarization curve of the 2DEG interface is shown in Figure 7d, in which the interface current density is extremely high, being up to 300 mA / cm2at -0.1 V (E vs RHE).

[0171] Figure 8 summarizes the HER performance of other various representative state-of-the-art catalysts on the basis of current density (normalized to the catalystsurface area) at an -0.1 V overpotential. Under alkaline conditions (1 mol KOH), the LAO / STO 2DEG interface of the present invention shows excellent intrinsic HER activity, outperforming most representative heterostructure-based catalysts reported to date.

[0172] Indeed, it is noteworthy that the specific current density of the LAO / STO 2DEG interface of the present invention is almost two orders of magnitude higher than that of Pt / C at -0.1 V (E vs RHE).

[0173] AFM, RHEED and XPS analyses were also conducted on the 30LAO / STO electrode devices to further evaluate any possible morphological or compositional changes between samples, before and after the HER reaction.

[0174] As shown in Figure 9a, the surface terrace structure of the 30LAO / STO electrode device, post HER reaction, was well preserved, as confirmed by two observations: first, the retention of the 3.9 A step height, and second, the reflection high-energy electron diffraction (RHEED) pattern in Figure 9b showed the same three diffraction spots (labelled 0 1 , 0 0, 0 -1 ).

[0175] Evaluation of LAO / STO Surface HER performance

[0176] The catalytic performance of a series of LAO / STO [LAO surface] samples was investigated to determine the impact of the thickness of the LAO layers (Figure 10a). Tafel slopes were obtained by fitting the linear region (0.5-0.8 V) of the Tafel curves (Figure 10b) with the Tafel equation: q=bxlog ij i+a,

[0177] where q is the overpotential, b is the Tafel slope, and a represents the current density (A / cm2).

[0178] As shown in Figure 10c, the current density (A / cm2) of the series of LAO / STO [LAO surface] samples observed at the overpotential of 0.8 V (vs. RHE) shows an increasing trend across the thickness range from 4 u.c. to 30 u.c. Beyond 30 u.c, the current density of the LAO / STO [LAO surface] samples starts to fluctuate, which suggests that saturation has been reached. A Tafel slope is viewed as an intrinsic property of reactions at an electrode, and the value of the Tafel slope is determined by the rate limiting step of the reaction.

[0179] The high Tafel slope at 405 mV / dec for the 4LAO / STO surface sample in Figure 10d indicates the sluggish catalytic kinetics at the surface of LAO. However, when the LAO thickness was increased from 4 to 80 unit cells (u.c.), the Tafel slope decreased to approximately 150 mV / dec, indicating a faster reaction rate.

[0180] To reveal the relationship between the LAO thickness and the catalytic activity of the LAO / STO [LAO surface] electrode device, electrochemical impedance spectroscopy (EIS) was performed on the 5LAO / STO [LAO surface] sample and the 30LAO / STO [LAO surface] electrode device, with applied potentials ranging from -0.17 V to -0.77 V (see Figure 11).

[0181] A summary of the EIS fitting parameters including the resistance values are detailed in Table 1.

[0182] Table 1

[0183] Summary of EIS Fitting Parameters for Eapp = -0.17 V vs RHE and -0.77V vs RHE

[0184] The sum of series resistance (Rs) and LAO / STO interfacial charge transfer resistance (Ri) for the 5LAO / STO [LAO surface] electrode device is reported to be around 13-14 KQ, which is much greater than the 800 0 recorded for the 30LAO / STO[LAO surface] electrode device.

[0185] The series resistance (Rs) and interfacial charge transfer resistance (Ri) are correlated to each other, but represent the contribution of 2DEG transport at the heterointerfaces. Here, the R2 values for the 5LAO / STO [LAO surface] and 30LAO / STO [LAO surface] electrode devices are about 9.1 x 105Q and 4.3 x 105Q at -0.17 V (E vs RHE), respectively; values which continue to decrease to 4.4 x 103Q and 1 .4 x 103Q at -0.77 V (E vs RHE) over time. Such high resistance can be attributed to the LAO layer, which impedes the charge transfer, and subsequently impacts on the overall current density.

[0186] For the LAO dielectric layer, the electron conduction has two possible pathways, the first pathway is to overcome the energy barrier due to the Valence band maximum offset between LAO and STO, while the second pathway is through defect- assisted electron hopping through the LAO layer.

[0187] The thicker LAO layers have a significantly lower resistance in the 2DEG channel (Rs+Ri) compared to the thinner LAO layers. Consequently, the total current density at 0.8 V (E vs RHE) of the 30LAO / STO [LAO surface] electrode device is about double that of the 5LAO / STO [LAO surface] electrode device. This confirms that the 2DEG channel plays a vital role during the electrocatalytic process in the context of charge transfer.

[0188] Additionally, the true capacitance value converted from the constant phase element (CPE) is summarized below in Table 2 in which the 2DEG interface on the 30LAO / STO[LA O surface] electrode device becomes more metal-like, thereby rendering the capacitance as negligible.

[0189] Table 2

[0190] Summary of the calculated true capacitance value

[0191] It is worth noting that the LAO / electrolyte double layer capacitance (C2) for the 5LAO / STO [LAO surface] electrode device is 16.1 pF-crrr2at -0.77 V (E vs RHE) while the corresponding value for the 30LAO / STO [LAO surface] electrode device is about 1 .6 pF-cm-2.

[0192] As shown in Figure 12a, the inventors performed a series of cyclic voltammetry (CV) scans to determine the double layer capacitance (Cdi) of a range of LAO / STO electrode devices with increasing LAO thickness (measured in 1 M KOH) conducted in a non-faradic current region (0.0-0.1 V vs. RHE, no iR corrected) at scan rates of 20, 40, 50, 60, 80 and 100 mV s’1. As shown in Figure 12b, electrochemical active surface area (ECSA) estimation of these LAO / STO samples of varying LAO thickness (measured in 1 M KOH), was then determined from double-layer capacitance (Cdi). According to these measurements (see Figure 12c), the Cdi value of the 5LAO / STO [LAO surface] electrode device (15.9 pF-cm-2) is about 6 times larger than that of the 30LAO / STO [LAO surface] electrode device (2.4 pF-crrr2), which is consistent with the EIS measurements (See Table 2).

[0193] In brief, an LAO / STO [LAO surface] electrode device with an LAO layer thickness exceeding 30 unit cells (u.c.) may impede the electrocatalytic performance,mainly because of sluggish surface HER kinetics, which is attributed to an increased charge transport resistance in the LAO layer and a reduction in surface -active sites.

[0194] Surface potential and topography characterization

[0195] The surface potential is a crucial physical parameter that reflects the dynamic behaviour of electrons. Typically, for metals, the surface potential is referred to as work function (0), which indicates the capacity of a solid surface to donate free electrons.

[0196] Here, the inventors applied amplitude modulated Kelvin probe force microscopy (AM-KPFM) to spontaneously investigate the surface topography and surface potential of the as-grown LAO / STO surface under dark and ambient conditions, respectively. The resulting AFM topography and surface potential (mV) images (Figures 13a-b) of the surface of the 30uc LAO / STO electrode device show a high quality LAO thin film, and a clear terrace structure with a step height of 3.80 A (Figure 13c).

[0197] Kelvin probe calibration was performed using highly oriented pyrolytic graphite (HOPG), a material with a work function in the range of 4.4 eV to 4.8 eV.

[0198] Figure 13d shows a plot of surface potential (mV) measurements taken along the portion of surface of the 30uc LAO / STO electrode device, as indicated by the dotted box in the AFM image, and demonstrates that there is no clear difference in surface potential along the length of this terraced structure.

[0199] Figure 13e shows the dependence of LAO thickness (u.c.) on the interfacial sheet resistance (Rs) of the LAO layer(s) for a range of LAO / STO (dark dots), measured using a four-point probe in the van-der Pauw configuration. Additional samples of LAO / Nb:STO (red dots) were prepared on the highly conductive Nb:STO substrate (~1 KQ) with the same deposition parameters as those employed for the LAO / STO samples. The Rs value for LAO / STO gradually decreases with the increase in LAO thickness until the thickness of the LAO layer exceeds 30 u.c. However, the Rs value for LAO / Nb:STO remains unchanged due to the inherently low background resistance of Nb:STO.

[0200] The inset in Figure 13f shows a schematic representation of the setup for measuring VCPD using KPFM for a series of LAO / STO (dark dots) samples with increasing LAO thickness (from 4 u.c. to 80 u.c.).

[0201] KPFM measures the contact potential difference (VCPD) between the tip of the KPFM probe and a sample surface, without external bias voltage.

[0202] VCPD can be defined as:where cptiPand cpsare the work functions of the tip and sample surface, respectively, and e is the elemental charge.

[0203] The plot of VCPD) versus LAO thickness (u.c.) of these LAO / STO samples in Figure 13f shows that VCPD increases with increasing LAO thickness until it reaches a saturation state after 30 u.c.

[0204] Figure 13g shows an energy band diagram of the contact between the metal (stainless steel) tip of the KPFM probe and the surface of an LAO / STO heterostructure. The CPD value is determined from the surface states of the LAO layer, which reveals the surface effective work function (SEWF) of the LAO / STO surface for electron emission or transfer.

[0205] Figure 14 presents the work function (<t>) as a function of LAO thickness for both LAO / STO (dark dots) and LAO / Nb:STO (red dots) samples, with LAO thickness ranging from 4 to 80 unit cells (u.c.). This plot illustrates the surface electron work function (SEWF, <t>s) for each sample, calculated from contact potential difference (VCPD) values. The SEWF was found to increase with LAO thickness and plateau after 30 u.c. For instance, the SEWF for the 30 u.c. LAO / STO sample was between 5.2 eV and 5.6 eV. LAO / Nb:STO samples (red dots) exhibited a similar trend.

[0206] Given these results, it can be concluded that samples with a greater LAO thickness have a high threshold for releasing electrons, and thus a low ability to donate surface free electrons.

[0207] Figure 15 shows ultraviolet photoelectron spectroscopy (UPS) spectra of bulk STO, LAO, and the 4LAO / STO sample, where the cut-off energy (eV) of the valence electrons on the x-axis yields the work function (<t>) of the corresponding samples.

[0208] As shown in Figure 15a, the 4LAO / STO sample has a cut-off energy of 5.25 eV (as compared to 4.6 eV for STO), which is comparable to the results obtained inFigure 14. This cut-off energy of the valence electrons of the 4LA0 / ST0 sample being representative of the work function (0).

[0209] Figure 15b shows Fermi edge spectra of bulk STO, LAO, and the 4LAO / STO sample, which respectively indicate the Fermi level and valence band maximum (VBM) values for the LAO / STO interface, pristine LAO, and pristine STO. The dashed lines in this figure indicate the linear extrapolation of the spectral onset used to determine the valence band minimum (VBM) energy for the corresponding samples. As shown in Figure 15b, the VBM energy for the 4LAO / STO sample (3.1 ev) falls between the VBM energies for bulk STO (2.8 ev) and LAO (3.5 ev).

[0210] Figure 16a shows X-ray photoelectron spectroscopy (XPS) Ti2p spectra for bulk STO, Nb-doped STO, and the 4LAO / STO sample, while Figure 16b shows XPS AI2p spectra for the 30LAO / STO and 4LAO / STO samples, respectively. The absence of any notable asymmetrical broadening or shift of the peak for the AI2p core levels in LAO suggests a flat LAO band.

[0211] The appearance of Ti3+on the Ti 2p core-level spectrum (Figure 16a) reflects the extra electrons located at the Ti sites, which is consistent with the electron energy loss spectroscopy (EELS) results observed for the Ti3+ / Ti4+mix region confined within a few unit cells (u.c.) on the STO side. A comparison of UPS and Fermi edge spectra for a range of LAO / STO samples of varying LAO thickness (4 u.c. to 30 u.c.) is shown in Figure 16c and Figure 16d, respectively, revealing comparable cut-off and VBM energies, irrespective of LAO thickness.

[0212] Figure 16e shows plots of valence band offset (VBO) and conduction band offset (CBO) values (eV) obtained via the UPS study for a range of LAO / STO samples with increasing LAO thickness. These results were compared with those obtained by hard X-ray photoelectron spectroscopy (HX-PES), as evidenced by the shared region111.

[0213] The VBO and CBO values show an increasing trend with varying LAO thickness, while the corresponding results obtained by hard x-ray photoelectron spectroscopy (HX-PES) are slightly scattered. This is because the penetration by x- rays in HX-PES is much deeper than the penetration by UV rays in UPS and its data is mainly contributed to the bulk STO substrates. For UPS on the other hand, the SX- PES probes the region closer to the surface and interface.

[0214] A negative VBO value indicates that the VBM of STO exhibits a higher energy than that of LAO. Therefore, for LAO layers with thicknesses of 4 and 5 u.c., the VBM of LAO lies below that of STO, in agreement with the Type I band alignment. It is worth noting that the band alignment gradually switches from type I to type II with increasing LAO thickness as the upper shifting of the CBO and VBO. This result reveals the sluggish charge transport kinetics within the LAO layer, which is particularly evident in the case of thicker LAO samples.

[0215] Figure 16f shows a schematic energy band diagram for the 4LAO / STO sample showing the relationship between the observed VBM energy (3.1 ev) and corresponding SEWF (5.2 ev) for this sample. The energy-band diagram shows a type I alignment.

[0216] Determination of the electronic band structure

[0217] Table 3

[0218] Core-level peaks position and Valence band maximum position obtained from XPS and UPS respectively

[0219] The valence band offset (VBO), which is defined as AEV= EyT0- EyA0, is determined by the measured elements’ core-level binding energy differences between LAO film and STO substrate.

[0220] Four pairs of element’s core-level peaks were chosen to obtain an average value of the VBO, the expressions are shown below:

[0221] The first two terms in each equation represent for the core-level binding energy measured from 30LAO / STO and bulk STO, respectively. The last term is for the core-level binding energy measured for the specific LAO / STO whose VBO is to be calculated. The positive signs for the VBO and conduction band offset (CBO) are indicative of a type II band alignment, where the valence band maximum (VBM) and conduction band maximum (CBM) of the STO are at higher binding energies than those of LAO. The negative signs are indicative of a type I band situation.

[0222] The obtained results are summarized in Table 4, where the large valence band offset (-0.4 ± 0.1 eV) confirms the large transfer resistance of the LAO layer, resulting in sluggish catalytic kinetics at the LAO surface.

[0223] Table 4

[0224] The valence band offsets (VBO) and conduction band offsets (CBO) derived from two methods, the linear combination of VB spectra and core levels method, respectively

[0225] Theoretical insights on 2DEG HER activity

[0226] Electronic Structures:

[0227] Figure 17 shows schematic representations of the unit cells of SrTiOa (STO), and LaAIOs (LAO), together with a schematic representation of the unit cell for the interface formed between STO and LAO unit cells, when viewed from different angles in the (100) plane.

[0228] Figures 18(a-c) shows projected density of states (PDOS) analyses of the unit cells of (a) LaAIOs (LAO), (b) SrTiOa (STO) and (c) of the corresponding SrTiOa / LaAIOs (STO / LAO) interface. According to the PDOS analyses, electronic states of Ti were found just below the Fermi level, suggesting the presence of free electrons at the STO / LAO interface, which forms the 2DEG as discussed above.

[0229] These free electrons primarily contributed by the Ti 3d orbital of the LAO / STO heterointerface. The electron density gradually decreases in TiO2 layers further from the interface (Figure 19), consistent with STEM-EELS mapping.

[0230] Calculated HER performance:

[0231] To examine the HER performance, the adsorption free energy of hydrogen (AG(H)) was calculated for the three systems (STO, LAO, STO / LAO). A series of active sites [Oxygen sites (A,B,C,D,E,F,G) in the case of Figure 20 and (A,B,C,D,E) in the case of Figure 21 ; Metal sites (La, Al, Ti, Sr)] distributed from the interface to the further layers of STO were tested for both AX and BX terminated surfaces (Figures 20-21).

[0232] The calculated AG(H) for all considered systems is shown in Figure 22 and Figure 23. It is noteworthy that while the oxygen at the Site Oc of the BO2 terminatedheterointerface exhibits a |AG(H)| of 0.08 eV, the oxygen at the Site OE within the TiO2layer beneath the heterointerface demonstrates the lowest |AG(H)| of 0.02 eV among the investigated metal and oxygen sites in the three systems (pristine STO, pristine LAO, LAO / STO heterostructure with two considered terminations).

[0233] In addition to Site OE, the Sites OF and OG also show a low |AG(H)| (0.04 eV). These results indicate the superior intrinsic catalytic activity at the 2DEG heterointerface. Notably, the TiO2-terminated surface of the pure STO single crystal also exhibits a very low |AG(H)| value of 0.06 eV. However, its electrocatalytic performance is limited by poor electrical conductivity.

[0234] As shown in Figure 24, projected density of states (PDOS) calculations for the O active sites and their nearest neighbour Ti atoms on the surface of the LAO / STO heterostructure with H adsorption (right column) and without H adsorption (clean surface) were performed. Upon the H-0 bond formation, the PDOS in the conduction band shifts to the lower energy region, which helps to increase the number of occupied states around the Fermi level, thereby enhancing the surface conductivity.

[0235] For sites OE and OG, there is an increased amount of occupied O 2p states near the Fermi level compared to the case of the clean surface (left column). These occupied O 2p states appear at the same energy region with the Ti 3d states, which suggests a possible overlap between the two orbitals.

[0236] Under the hydrogen adsorption, the surface structure of the LAO / STO heterostructure is reconstructed, with O atoms being pulled off the surface, as indicated in Figure 25.

[0237] To further validate the orbital overlap, crystal orbital Hamilton population (COHP) analysis was performed on the oxygen atom at site OE and its nearest neighbour Ti atoms. A COHP analysis is a quantitative tool that effectively measures the extent of orbital interaction and overlap between two atoms. The calculated COHP, presented in Figure 26, provides confirmation that the O 2p state at the Fermi level overlaps with Ti 3d orbitals.

[0238] Figure 26a shows the COHP of the oxygen atom at site OE with its nearest neighbour Ti atoms before (a) and after the H adsorption (b) on the LAO / STO surface, and the COHP of the oxygen atom at site OG with its nearest neighbour Ti atoms before (c) and after the H adsorption (d) on the LAO / STO surface.

[0239] Based on above analysis, this electronic reconstruction of the underlying TiO2 layers and the charge exchange between the interface Ti atoms and O atoms renders the oxygen atoms at Site OE not only electrochemically active with the lowest hydrogen adsorption-free energy but also electrically conductive, endowed with a large reservoir of high-mobility electrons. This facilitates efficient charge transfer from oxygen to hydrogen, satisfying the three essential criteria for electrocatalysis and showcasing exceptional performance in the HER.

[0240] Lithography Patterning and Determining the intrinsic activity of LAO / STO edge site

[0241] A persistent challenge in electrocatalysis research lies in accurately isolating the intrinsic activity of nanoscale active sites. Techniques like linear sweep voltammetry (LSV) often reflect the overall electrode performance rather than the inherent activity of the catalyst itself. To address this challenge and investigate the role of 2DEG edge sites in LAO / STO heterostructures, the inventors developed a 2DEG patterning method with lithography process, which is commonly used for the fabrication of semiconductor microelectronic devices, to create a LAO / STO heterointerface positioned at the junction of a LAO-STO step to form the quasi-one- dimensional LAO / STO edge.

[0242] As Figure 27 shows, the 2DEG patterning method is primarily composed of the following steps: 1 . lithography process, 2. deposition of hard mask material, 3. liftoff process, 4. 2DEG heterostructure deposition, 5. Coating resist, 6. lithography process, 7. Wet etching, cleaning the protective resist.

[0243] Wet etching was performed with buffered hydrofluoric acid (HF) for about 5 to about 7 minutes in order to etch away any hard mask / LAO not covered by the photoresist mask. After etching, the photoresist was removed with acetone, revealing a clean LAO / STO patterned surface.

[0244] For instance, in step 1 , a first photoresist (Resist 1 ) is deposited on to the surface of an SrTiOa (STO substate, and then a pattern is produced in the photoresist by exposing the photoresist with a light source with a suitable UV wavelength through a photomask, in step 2, a layer of AIOx is sputter coated on to the photopatterned surface. In step 3, the photoresist is removed via a suitable lift-off process to reveal the patterned AIOx features on the surface of the STO substrate. In step 4, a layer ofLaAIOs (LAO) is deposited on to the surface of the patterned AIOx features and STO substrate via pulsed laser deposition (PLD) as part of the 2DEG heterostructure formation. A second photoresist (Resist 2) is then coated on to the newly patterned surface in step 5 to protect the LAO-coated parts of the STO substrate. In step 6, the coated substrate is then subjected to photopatterning using a suitable light source through a second photomask to form a protective coating over the LAO-coated parts of the STO substrate. The newly patterned surface is then wet etched and cleaned to remove the protective resist and the LAO-coated AIOx features to leave a patterned 2DEG STO / LAO heterostructure.

[0245] As shown in Figure 28 and Figure 29, the inventors fabricated a series of samples (LAO / STO edge) based on 60LAO / STO with varying LAO opening sizes using the precise 2DEG patterning technique of the present invention. These samples were designated as 60LAO / STO[edge]-WxWpm2, where W represents the side length of the LAO opening (e.g., a sample with a 5 pm by 5 pm LAO opening, and thus a 20 pm total 2DEG edge length, is denoted as 60LAO / STO[edge]-5x5pm2). This design ensures that the ratio of edge length to surface area increases as the LAO opening size decreases, allowing for a systematic study of edge site effects.

[0246] As shown in the linear sweep voltammetry (LSV) curves in Figure 30, the LAO / STO 2DEG samples fabricated in this manner were found to exhibit current densities (jgeo, normalized to geometric area) comparable to platinum thin films (5 nm and 10 nm thick) at low overpotentials, and notably higher current densities at high overpotentials.

[0247] Figure 31a shows a plot of the current density (normalized to the geometry area; jgeo) versus the opening window size (cm2) for the same LAO / STO 2DEG samples above. It was found that jgeo increased with decreasing geometric size, a trend indicating that the active sites are not uniformly distributed in the planar geometric areas.

[0248] To further analyze this relationship, the inventors also plotted jgeo at an overpotential of -0.3 V against the 2DEG edge length, revealing a clear linear correlation (see Figure 31 b). While some data scatter is present, likely due to measurement errors and experimental system variations, they align closely with abest-fit linear trend, characterized by a slope of 2.35 x 103mA / cm2deg (mA per cm degree-edge).

[0249] The observed linear relationship between the reaction rate (directly proportional to current density) and the 2DEG edge length provides strong evidence that these edge sites are the dominant active sites for HER catalysis in our LAO / STO system.

[0250] Figure 31c shows linear sweep voltammetry (LSV) curves with current density normalized to the electrochemical active surface area (ECSA). As shown inset, ECSA was found to increase with increasing 2DEG length (pm), demonstrating a linear relationship between the ECSA and the controlled 2DEG edge length, yielding an ECSA2DEG value of 8.09 x 10’5 cm2 / cm2DEG. This value confirms that the active area for the HER is confined to a narrow region along the 2DEG edge.

[0251] To evaluate the intrinsic catalytic activity, the inventors calculated the specific activity by normalizing the current to the electrochemical surface area (ECSA). The 60LAG / STO[edge]-20x20 m2sample demonstrates a remarkably high specific activity, reaching a current density (JECSA) of 14.40 mA / cm2ECSA at an overpotential (q) of 0.1 V, significantly exceeding that of both STO and SrRuOa, and outperforming numerous platinum-based and other state-of-the-art catalysts (see Figure 31 d).

[0252] Exchange current is another accurate descriptor of the intrinsic activity, which is heavily influenced by the reaction activation energy at the electrocatalyst surface since no overpotential is applied.

[0253] As Figure 32 shows, the exchange current density (iO) exhibits a linear relationship with the increased proportion of 2DEG edge length, which is consistent with trend observed for jgeo. This linear relationship, with a slope of 2.82 x 10’4mA / cm2DEG, underscores the ability to quantify the intrinsic activity of the quasi-one- dimensional active sites created by photolithographically patterning the LAO / STO heterointerface.

[0254] Based on EELS mapping (see Figure 4f), the most conductive 2DEG layer resides within a 3 nm depth from the interface. Assuming this region constitutes the primary reaction area, the estimated exchange current density (ist) is approximately 1 .27 x 103 mA / cm2, surpassing most state-of-the-art catalysts.

[0255] On the other side, the experimentally determined ECSA2DEG (8.09 x 10’5 cm2 / cm2DEG) suggests an active area two orders of magnitude larger than the electronically reconstructed region observed by EELS. This discrepancy can be rationalized by considering the presence of localized or itinerant electrons in the Ti layer beyond the immediate interface region. These electrons, potentially participating in HER via strong orbital hybridization with neighbouring oxygen atoms, could extend the active area beyond the confines of the most conductive 2DEG layer.

[0256] Normalizing the measured exchange current per unit length (2.82 x 10’4mA / cm2DEG) by the ECSA2DEG, we obtain an experimental exchange current density igXp) of 3.48 mA / cm2. This value surpasses that of most metals, including platinum, and is comparable to that of topological materials like MnSi, TiSi, and RhSi, which exhibit values above 10 mA / cm2.

[0257] The turnover frequency (TOF), relevant to the amount of gaseous hydrogen molecules evolving per active oxygen site per second, is another crucial parameter reflecting the intrinsic activity of an electrocatalyst. Using a well-established method shown in the Materials and Method section, the inventors calculate the averaged TOF values by normalizing it to a per surface site basis. Based on the experimental averaged exchange current density of 3.48 mA / cm2and estimated exchange current density of 1 .27 x 103 mA / cm2, the TOF of the 2DEG edge is determined to range from 7.94 s-1to 2897.64 s-1, substantially exceeding the reported TOF of 0.94 s1for Pt (11 1 ).

[0258] Volcano plot

[0259] For further insight into the catalytic nature of the 2DEG edge, the inventors have added their data (ippand ioST, DFT calculated Gibbs free energy change for atomic hydrogen adsorption (AGH) ) to a recent version of the volcano-type relations observed for HER catalysts, as shown in Figure 33.

[0260] The inventors additionally prepared several other metal electrodes by using physical vapour deposition (e-beam) for reference purposes; the calculated exchange current densities (A / cm2) for each of these reference electrodes has been summarized in Table 6.

[0261] Table 6

[0262] Calculated exchange current density and exchange current per site

[0263] The intrinsic activity of the 2DEG edge ( igxp= 3.48 mA / cm2, if1.27 x io3mA / cm2; DFT calculated AGH* = 0.02-0.04 eV) surpasses that of most metals, including platinum, and is comparable to that of topological materials like MnSi, TiSi and RhSi , yet it still follows the volcano trend. This agreement validates the predictive capacity of this DFT model, indicating its applicability beyond metal catalysts.

[0264] Figure 34a shows a schematic representational comparison of electrode devices comprising an unpatterned and lithography-patterned 2DEG STO / LAO heterostructure, each configured a with two-electrode connection configuration and a gold (Au) counter electrode, for application in a water splitting HER reaction when conducted in the presence of a suitable electrolyte.

[0265] By patterning the surface of the LAO film of the LAO / STO heterostructure in this manner, it becomes possible to expose a greater surface area of the 2DEG heterointerface to enhance the interaction between the 2DEG heterointerface and an electrolyte (argon saturated 1 M KOH (pH=14)).

[0266] Figure 34b shows that in the case of the unpatterned device, bubbles of hydrogen (H2) gas were generated during the water splitting HER reaction from the exposed edges of the 2DEG heterointerface around the external periphery of the device on interaction with the electrolyte.

[0267] The patterned device on the other hand shows that bubbles of hydrogen (H2) gas were not only generated from around the exposed edges around the periphery of the device, but also, and predominantly, from the exposed edges around the internal periphery of each of the individually patterned features of the 2DEG heterostructure, resulting in a tenfold increase in current (see Figure 35) compared to unpatterned samples.

[0268] Drawing from the experimental results and reasonable analysis, the inventors propose that the construction of a 2DEG interface in perovskites can be a universal way to boost alkaline HER in wide-bandgap materials.

[0269] Superlattice Electrocatalyst Electrode Device

[0270] According to another preferred embodiment of the present invention, the inventors have developed a superlattice-type structure comprising an STO substrate and alternating layers of A (LaAIOs) and B (SrTiOa) deposited in a vertically stacked arrangement on the surface of the STO substrate to generate a multi-layered structure with a corresponding 2DEG formed at the heterointerface between the alternating LAO / STO layers.

[0271] Figure 36 shows a schematic representation of one embodiment of a superlattice electrocatalyst electrode device comprising alternating pairs of LAO / STO layers deposited in a vertically stacked arrangement on the surface of an STO substrate (LAO is denoted “A” and STO is denoted “B”), operably connected to a power source (not shown) via a conducting wire, such as a gold (Au) or copper (Cu) wire.

[0272] According to this embodiment, a 2DEG layer (denoted “2DEG”) is generated at the heterointerface formed between each pair of alternating LAO / STO (A / B) layers, resulting in a multilayered heterostructure comprising four distinct 2DEG interfaces.

[0273] Because of the charge balance in the multilayered heterostructure shown in Figure 36, the interface formed between “adjacent’ pairs of LAO / STO layers, that interface being an AIO2 / SrO interface (best represented by “B / A”), theoretically actsas an insulator. As such, the AlC / SrO interface does not generate a 2DEG interface. In other words, only the interfaces formed between the LAO / STO (A / B) alternating pairs exhibit the conductive behaviour associated with 2DEGs.

[0274] Suffice to say, by virtue of there being a plurality of 2DEGs within the multilayered heterostructure in Figure 36, it is expected that the catalytic activity of the resulting HER electrocatalyst will occur at the four 2DEG edges around the external periphery of the electrode device, thereby enhancing the overall efficiency of the HER electrocatalyst.

[0275] Indeed, without wishing to be bound by any one particular theory, the inventors believe that if it is considered that if there is a parallel connection of 2DEGs in the multilayered heterostructure of the superlattice electrocatalyst device, then the current density will be expected to increase with the number (N) of 2DEGs in the multilayered heterostructure according to the equation: jsum=N*junit where junit represents the current density for a single 2DEG interface.

[0276] It will be appreciated by persons skilled in the art that a greater or fewer number of 2DEG interfaces can be formed by simply fabricating a multilayered heterostructure with greater or fewer alternating LAO / STO (A / B) layers, respectively.

[0277] In other embodiments, it is envisaged that the superlattice structure of the electrocatalyst electrode device could also be patterned using lithography to expose edges around the internal periphery of each individually patterned feature of one or more of the 2DEGs in the multi-layered structure to enhance the efficiency of the HER electrocatalyst still further.

[0278] Alternative Heterostructures

[0279] It will be appreciated by persons of skill in the relevant art that the present invention is not simply limited to producing a two-dimensional electron gas (2DEG) interface at the heterojunction between the two perovskite oxides (LaAIOs and SrTiOa) described in the embodiments above.

[0280] For instance, the inventors believe that 2DEG interfaces suitable for the purpose of producing a hydrogen evolution reaction (HER) electrocatalyst may also be generated from the following combination of first and second complex oxide layers,including but not limited to a first complex oxide layer selected from the group consisting of LaAIO3, NdGaO3, LaTiO3, LaVO3, GdTiO3, SrlrO3, LaNiOa, NdAlOa, GdTiOa, BiFeOa, PrAlOa, La2 / 3Sri / 3MnO3, LaAIOa / LaVOa / LaAIOa / SrTiOa, Y-AI2O3, CaZrO3, LaAIOa / La / sSr sMnOa, SrTiO3ZSrTio.8Nbo.2O3, SrTiO3, LaAIO3, LaNiOa, SrVOa, BaSnO3, LaTiO3, LaAIO3, SrTiO3, Nb-SrTiO3and LaFeO3, and a second complex oxide layer selected from the group consisting of SrTiO3, BaTiO3, SrMnO3, SrVOa, SrCrO3, LaNiOa.

[0281] In other words, the resulting 2DEG interfaces may include: NdGaO3 / SrTiO3, LaTiO3 / SrTiO3, LaVO3 / SrTiO3, GdTiO3 / SrTiO3, SrlrO3 / SrTiO3, LaNiO3 / SrTiO3, NdAIO3 / SrTiO3, GdTiO3 / SrTiO3, BiFeO3 / SrTiO3, PrAIO3 / SrTiO3, La2 / 3Sri / 3MnO3 / SrTiO3, LaAIO3 / LaVO3 / LaAIO3 / SrTiO3, y-Al2O3 / SrTiO3, CaZrO3 / SrTiO3, LaAIO3 / La7 / 8Sri / 8MnO3 / SrTiO3, SrTiO3ZSrTio.8Nbo.2O3 / SrTiO3, SrTiO3 / BaTiO3, LaAIO3 / SrMnO3, LaNiO3 / SrMnO3, SrVO3 / SrMnO3, BaSnO3 / BaTiO3, LaTiO3 / SrVO3, LaAIO3 / SrCrO3, SrTiO3 / SrTiO3, Nb-SrTiO3 / SrTiO3, LaFeO3 / LaNiO3.

[0282] Additionally, the inventors also believe that 2DEG interfaces suitable for the purpose of producing a hydrogen evolution reaction (HER) electrocatalyst may also be generated from the following combination of semiconductor materials, including but not limited to: GaAs / AIGaAs, AIGaAs / GaAs, InAs / GaSb, AlAs / GaSb, InAs / lnP, InGaAs / lnP, Si / SiGe, GaN / AIGaN, InAs / lnGaAs,, InGaAs / lnAIAs, InP / lnGaAs, GaAs / lnGaAs, Si / SiC, HgCdTe, GaSb / AISb, GalnNAs / GaAs„ ZnO / MgZnO, AIN / GaN, Bi2Se3 / Topological Insulator, Graphene / Boron Nitride, M0S2 / WS2.Conclusion

[0283] In summary, the inventors have successfully demonstrated that the 2DEG based electrocatalysts described above exhibit a remarkably high intrinsic HER activity, as evidenced by an exceptionally high current density of up to 300 mA / cm2at -0.1 V (E vs RHE) when normalized to the 2DEG area.

[0284] These values represent a top-tier catalytic performance, comparable even to the benchmark Pt / C catalyst, and surpassing that of all previously reported state-of- the-art catalysts.

[0285] Moreover, the combination of the 2DEG patterning process with the electrode preparation method described above can achieve good control of the exposing active site (2DEG interface) and compensate for the size induced mass transfer limitation,thereby enlarging the overall catalytic current, which makes the 2DEG based electrocatalyst one step closer to useful industrial application.

[0286] The outstanding HER performance of the 2DEG based HER electrocatalyst is strongly attributed to the following points:• good intrinsic electrical conductivity ensuring the rapid transport of electrons injected from electrode to the active sites; and• high intrinsic activity attributed to the electron rich states of Interfacial Oxygen.

[0287] To the best of the inventors’ knowledge, this is the first report of an insulator perovskite oxide-based heterostructure that exhibits remarkable efficiency in electrocatalytic hydrogen evolution under alkaline conditions.

[0288] The development of a perovskite oxide-based heterostructure with high- packing density and superior electrocatalytic performances based on 2DEG engineering is highly feasible and desirable.

[0289] This study sheds light on the intrinsic mechanism behind interface catalysis and elucidates that the intrinsic activity and electrical conductivity associated with nonnoble earth abundant metals are not mutually exclusive and can ideally be addressed simultaneously, leading to an ideal electrocatalysis. Importantly, this study offers a promising direction in the design of advanced heterostructured electrocatalysts and for optimizing the performance of non-noble metal electrocatalysts.

[0290] Materials and Methods

[0291] Electrode preparation

[0292] Before the growth, the bare STO substrate was treated by acid etching and then post-annealed in oxygen to achieve TiO2 termination.

[0293] Specifically, the acid etching process includes: 1. Clean the STO substrate with either acetone and methanol (1 :1 ) or acetone and isopropanol (1 :1 ) in an ultrasonic bath for 5 minutes. 2. Then rinse the cleaned STO substrate with Dl-water in an ultrasonic bath for 5 minutes. 3. Soak the cleaned STO substrate in Buffered HF (BHF) solution for 12 seconds to adjust the pH to around 3.7 to 4.3, and then immediately rinse it in Dl-water for 1 minute to remove any residual acid solution. 4. Anneal the cleaned STO substrate at around 1000 °C for 3 hours with an oxygen (O2) gas flow rate of 1.5L / min. (7 minutes holding at mouth zone and 3 hours on centralzone). 5. Then, clean the annealed substrates with either acetone and methanol (1 :1 ) or acetone and isopropanol (1 :1 ) in an ultrasonic bath for 5 minutes.

[0294] All the film reported here were grown by pulsed laser deposition (PLD) at 650°C at an oxygen pressure of 10’4Torr with a laser fluence of 3.6 J em-2and a laser repetition rate of 1 Hz. Then, the samples were slowly cooled at 30°C / min down to room temperature at an oxygen pressure ranging of 10’4Torr. During the growth, reflection high-energy electron diffraction (RHEED) analysis was used to realize the atomic control of the LAO thin film thickness. To precisely analyse the electrochemical effects from the surface and interface of the LAO / STO heterostructures, all the samples were prepared using a chemically inert epoxy (Loctite EA D609) with Al wire bonded at the interface.

[0295] Material characterization

[0296] HRTEM and EELS mapping were collected using a Hitachi HF-3300 microscope, with an acceleration voltage of 300 kV, equipped with a Bruker detector. Electrical transport properties for the LAO / STO samples were performed via the van der Pauw method using PPMS (model 6000, Quantum Design). KPFM measurements were performed with atomic probe microscopy (Bruker Dimension ICON SPM) using Pt coated Si tips (SCM-PTSI, Bruker). The work function of the tip was calibrated using Highly oriented pyrolytic graphite (HOPG, ZYB Grade). The crystal quality of the LAO / STO heterostructures was carried out by X-ray diffraction (XRD) were measured using a Rigaku Smartlab diffractometer operating at 40 kV with filtered Cu Ka radiation. X-ray photoelectron spectroscopy (XPS) measurements were conducted using a ThermoFisher Esca Lab 250XL Ultraviolet photoelectron spectroscopy (UPS) measurements were conducted using ThermoFisher Esca Lab 250Xi with a He(l) photo line of 21 .22 eV.

[0297] Electrochemical measurements

[0298] Linear sweep voltammetry (LSV) values for the LAO / STO samples in argon saturated 1 M KOH (pH=14). was measured using a CHI 660E electrochemistry workstation with a scan speed of 10 mV s-1. The RE-2BP Calomel Reference Electrode (saturated KCL) and Pt wire were used as the reference electrode, and counter electrode, respectively. All experiments were repeated more than three times. The measured results were converted from a saturated calomel reference electrodeto a Reversible Hydrogen Electrode (RHE) using the Nernst equation: ERHE=Eapp+0.241 +0.059xpH. The electrochemical double-layer capacitance (Cdi) was measured through the cyclic voltammetry (CV) technique within the potential window from 0 to 0.1 V vs. RHE of the non-faradic current region at the changed sweeping rate from 20 to lOOmV s-1. The electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range from 105to 0.1 Hz with an AC amplitude of 10 mV at several applied potentials ranging from -0.17 V to -0.77 V.

[0299] Extraction of catalytic current for 2DEG Interface

[0300] To calculate the pure current density (mA / cm2) contributed by the 2DEG heterointerface, the top current density (Jtop) was firstly measured on the 30LAO / STO [LAO surface] sample, then, the total current (Itotai) was measured for the 30LAO / STO [LAO surface-i- Interface] sample. The interface current density (Ji) can be calculated using the following equation:where ATopis the exposed top area covered by the LAO layer in the 30LAO / STO [LAO surface] sample (measured using optical microscopy, the surface roughness factor is close to 1 , due to the smooth surface of the sample), and Atis the effective area of the 2DEG ( At= 3 x 10’7x 0.5 cm2).

[0301] Turnover Frequency (TOF) Calculations

[0302] The following equation 1 was used to calculate the per-site turnover frequency (TOF):T003031 TOT —#number total hydr°3en turnovers / cmgeo# / -| \1 J#number of active sites / cm.geo# ' '

[0304] The total number of hydrogen turnovers was calculated from the current density according to the Supplementary Equation 2:

[0306] The number of active sites is estimated as the number of oxygen sites from the unit cell (u.c) of the STO based on our DFT calculation. The number of active sites per real surface for the STO / LAO interface is calculated from equations (3): 5 sites cmrgai(3)

[0308] The real surface area for HER is calculated from the electrochemical active surface area (ECSA), which can be converted from the specific capacitance (See details in the Materials and Method). The real active surface area per 2DEG length (ECSA2DEG =8.09 x 10’5cm2 / cm2DEG) is determined by the slope of the linear fit of ECSA versus 2DEG edge length. The exchange current per length II (2.82 x 10’4mA / cm2DEG) was determined by the slope of the linear fit of exchange current versus 2DEG edge length.

[0309] Then, the exchange current density io (mA / cm2) is calculated according to equation (4):

[0310] i0= / L / ECSA2D£G(4)

[0311] Finally, the exchange current density can be converted into corresponding TOF (Table S*) according to equation: (5)

[0312] TOF(S~') =(i0, A / cm2) / [(1.368 x 1015Reference:1 . Berner, G., Muller, A., Pfaff, F., Walde, J., Richter, C., Mannhart, J., Thiess, S., Gloskovskii, A., Drube, W, Sing, M. & Claessen, R. (2013). Band alignment in LaAIOa / SrTiOa oxide heterostructures inferred from hard x-ray photoelectron spectroscopy. Physical Review B, 88(11 ), 1 151 1 1.Embodiments:

[0313] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described inconnection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[0314] Similarly, it should be appreciated that in the above description of example embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, Figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description of Specific Embodiments are hereby expressly incorporated into this Detailed Description of Specific Embodiments, with each claim standing on its own as a separate embodiment of this invention.

[0315] Furthermore, while some embodiments described herein include some, but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.Different Instances of Objects

[0316] As used herein, unless otherwise specified the use of the ordinal adjectives "first", "second", "third", etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.Specific Details

[0317] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.Terminology

[0318] In describing the preferred embodiment of the invention illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar technical purpose. Terms such as "forward", "rearward", "radially", "peripherally", "upwardly", "downwardly", and the like are used as words of convenience to provide reference points and are not to be construed as limiting terms.Definitions

[0319] As used throughout, the singular forms “a,” “an,” and the include plural referents unless the context clearly dictates otherwise.

[0320] Ranges can be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another aspect includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

[0321] It will be understood that any use of the term “about” herein in reference to a recited numerical value (e.g., a temperature or pressure) includes the recited numerical value and numerical values within plus or minus 10% of the recited value.

[0322] It will be understood that any use of the term “between” herein in reference to a range of numerical values encompasses the numerical values at each endpoint of the range. For example, a temperature range of between 10SC and 15SC is inclusive of the temperatures 10SC and 15SC.

[0323] As used throughout, the singular forms “a,” “an,” and the include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “an electrocatalyst” includes a plurality of such electrocatalysts.

[0324] As used herein, the term “hydrogen evolution reaction (HER),” refers to the cathodic reaction 2H++2e“— ► H2 that accompanies, in aqueous electrolytes, anodic processes such as oxygen production via water electrolysis.

[0325] As used herein, the term “turnover frequency (TOF)”, refers to the number of molecules (e.g., H2) produced per second per site.

[0326] As used herein, the term “Tafel slope” refers to a measure used in electrochemistry to quantify the relationship between the overpotential (the difference between the actual electrode potential and the thermodynamic equilibrium potential) and the logarithm of the current density in an electrochemical reaction.

[0327] As used herein, the term “contact potential difference (VCPD)” refers to the difference in work function (<t>) between a sample and tip.

[0328] As used herein, the term “work function (<t>)” is generally defined as the minimum energy required to excite one electron from a material surface containing N electrons and move to an infinite distance.

[0329] For the purposes of description all documents referred to herein are hereby incorporated by reference in their entirety unless otherwise stated.Comprising and Including

[0330] In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" are used in an inclusive sense, i.e., to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

[0331] Any one of the terms: including or which includes or that includes as used herein is also an open term that also means including at least the elements / features that follow the term, but not excluding others. Thus, including is synonymous with and means comprising.Scope of Invention

[0332] Thus, while there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as fall within the scope of the invention. For example, any formulas given above are merely representative of procedures that may be used. Steps may be added or deleted to methods described within the scope of the present invention.

[0333] Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.Industrial Applicability

[0334] It is apparent from the above, that the arrangements described are applicable to the production of green hydrogen (H2) for fuel based applications.

Claims

AMENDED CLAIMS received by the International Bureau on 18 August 2025 (18.08.2025)1 . A hydrogen evolution reaction (HER) catalyst, comprising: a two-dimensional electron gas (2DEG) formed at a surface or heterointerface of at least one of two or more alternating layers of a first complex oxide and a second complex oxide, wherein the first and second complex oxide layers are each formed from a perovskite oxide or a combination of a perovskite oxide and a transition metal oxide, and wherein the or each 2DEG exhibits an exchange current density corresponding to an intrinsic hydrogen evolution reaction (HER) activity that renders the 2DEG suitable as an HER catalyst for producing hydrogen (H2) from a water-based electrolyte by water electrolysis.

2. The HER catalyst according to claim 1 , wherein the first complex oxide layer is formed from a perovskite oxide selected from the group consisting of LaAIO3, NdGaO3, LaTiO3, LaVO3, GdTiO3, SrlrO3, SrRuO3, LaNiO3, NdAIO3, GdTiO3, BiFeO3, PrAIO3, La2 / 3Sr1 / 3MnO3, LaAIO3 / LaVO3 / LaAIO3, γ-Al2O3, CaZrO3, LaAIO3 / La7 / 8Sr1 / 8MnO3, SrTiO3 / SrTi0.8Nb0.2O3, SrTiO3, LaAIO3, LaNiO3, SrVO3, BaSnO3, LaTiO3, LaAIO3, SrTiO3, Nb-SrTiO3and LaFeO3.

3. The HER catalyst according to claim 1 or claim 2, wherein the second complex oxide layer is formed from a perovskite oxide selected from the group consisting of SrTiO3, BaTiO3, SrlrO3, SrRuO3, SrMnO3, SrVO3, SrCrO3and LaNiO3.

4. The HER catalyst according to claim 1 , wherein the first complex oxide layer is LaAIO3(LAO) and the second complex oxide layer is SrTiO3(STO).

5. The HER catalyst according to claim 4, wherein the alternating LAO and STO layers are deposited in sequence onto a surface of a substrate, wherein the substrate is a solid state material.

6. The HER catalyst according to claim 5, wherein the solid state material is selected from the group consisting of silicon (Si), silicon dioxide (SiO2), indium tin oxide (ITO), zinc oxide (ZnO) and graphite.

7. The HER catalyst according to any one of claims 4 to 6, wherein the LAO layer has a thickness of at least 4 unit cells (uc).

8. The HER catalyst according to any one of claims 4 to 7, wherein the surface of the or each STO layer is terminated with TiO2.

9. The HER catalyst according to any one of claims 1 to 8, wherein the HER catalyst exhibits an exchange current density of up to 1270 mA / cm2, when N=1 .

10. The HER catalyst according to any one of claims 1 to 9, wherein the HER catalyst exhibits a turnover frequency (TOF) that falls within the range of about 1 S-1to about 3000 s-1.

11. The HER catalyst according to any one of claims 1 to 10, wherein the HER catalyst exhibits a current density of up to 300 mA / cm2at -0.1 V (E vs Reversible Hydrogen Electrode) and a turnover frequency (TOF) of about 1600 s-1, when N=1.

12. A method of preparing a hydrogen evolution reaction (HER) catalyst, the method comprising the step of: forming a two-dimensional electron gas (2DEG) at a surface or heterointerface of at least one of two or more alternating layers of a first complex oxide and a second complex oxide, wherein the first and second complex oxide layers are each formed from a perovskite oxide, and wherein the or each 2DEG exhibits an exchange current density corresponding to an intrinsic hydrogen evolution reaction (HER) activity that renders the 2DEG suitable as an HER catalyst for producing hydrogen (H2) from a water-based electrolyte by water electrolysis.

13. The method according to claim 12, wherein the alternating layers of the first and second complex oxides are deposited in sequence on to a surface of a substrate using a deposition technique selected from the group consisting of atomic layer deposition (ALD), molecular beam epitaxy (MBE), pulsed laser deposition (PLD), RF magneton sputtering, physical vapour deposition (PVD), chemical vapour deposition (CVD) or any combination thereof.

14. The method according to claim 13, wherein the deposition technique is PLD conducted at a temperature of between about 500 °C and about 900 °C at an oxygen pressure of about 10-9Torr to about 10-2Torr.

15. The method according to any one of claims 12 to 14, wherein the 2DEG is formed at the surface or heterointerface of two alternating layers of LAO and STO, and wherein the 2DEG exhibits a carrier density that falls within a range of from about 3 x 1013cm2to about 2.4 x 1016cm2, with increasing LAO thickness, from 4 uc to 120 uc.

16. A hydrogen evolution reaction (HER) catalyst prepared according to the method of any one of claims 12 to 15.

17. A method of preparing a hydrogen evolution reaction (HER) electrode for producing hydrogen (H2) from a water-based electrolyte by water electrolysis, the method comprising the steps of: patterning a surface of the HER catalyst according to any one of claims 1 to 11 to expose a portion of the or each 2DEG, wherein the exposed portion of the or each 2DEG contributes to the overall intrinsic HER activity of the HER catalyst; and electrically interconnecting the or each 2DEG using at least one conducting wire to ensure an ohmic contact.

18. The method according to claim 17, wherein the patterning is conducted using a technique selected from the group consisting of lithography, wet etching, and a combination thereof.

19. The method according to claim 18, wherein the wet etching is performed with buffered hydrofluoric acid (HF) for about 5 to about 7 minutes.

20. The method according to any one of claims 17 to 19, wherein the conducting wire(s) is deposited via metal deposition or wire bonding.

21. The method according to claims 17 to 20, wherein the conducting wire(s) is a metal selected from the group consisting of Nickel (Ni), Titanium (Ti), Aluminium (Al), Copper (Cu), Gold (Au), Silver (Ag) and Platinum (Pt) or an alloy thereof.

22. A hydrogen evolution reaction (HER) electrode for producing hydrogen (H2) from a water-based electrolyte by water electrolysis, comprising the HER catalyst according to any one of claims 1 to 11 or 16.

23. An electrochemical device for producing hydrogen (H2) from a water-based electrolyte by water electrolysis, comprising at least two electrodes and a power supply, wherein at least one of the at least two electrodes comprises the HER catalyst according to any one of claims 1 to 11 or 16, or the HER electrode according to claim 22.

24. The electrochemical device according to claim 23, wherein the at least other electrode comprises a non-noble metal with a work function closely matching a work function of the substrate of the HER catalyst, differing by less than 1 eV.

25. The electrochemical device according to claim 24, wherein the non-noble metal is selected from the group consisting of Nickel (Ni), Titanium (Ti) and Aluminium (Al).

26. The electrochemical device according to any one of claims 23 to 25, wherein the at least two electrodes are at least partially immersed in a water-based electrolyte solution with a pH of between 7 and 14.

27. The electrochemical device according to claim 26, wherein the water-based electrolyte solution comprises potassium hydroxide (KOH) in a molar concentration of 1.0 Mol to 10.0 Mol.

28. A method of producing hydrogen (H2) from a water-based electrolyte by water electrolysis, the method comprising: providing an electrochemical cell comprising at least two electrodes and an electrolyte solution; contacting water with the at least two electrodes; and applying a voltage across the at least two electrodes, wherein one of the at least two electrodes comprises the HER catalyst prepared according to the method of any one of claims 1 to 11 or 16, or the HER electrode according to claim 22.

29. The method according to claim 28, wherein the at least two electrodes are at least partially immersed in the water-based electrolyte solution.

30. The method according to claim 28 or 29, wherein the water-based electrolyte solution has a pH of between 7 and 14.31 .The method according to any one of claims 28 to 30, wherein the water-based electrolyte solution comprises potassium hydroxide (KOH) in a molar concentration of 1.0 Mol to 10.0 Mol.

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