Water electrolysis electrode and method using same for controlling overvoltage during water electrolysis

Abstract

A water electrolysis electrode for use in a water electrolysis device in which a hydrogen-producing reaction electrode and an oxygen-producing reaction electrode are brought into contact with water and a voltage is applied between both electrodes to produce hydrogen and oxygen, said water electrolysis electrode having, on at least a portion of a surface thereof, one or more structures selected from the group consisting of a plurality of nanostructures and a plurality of microstructures which, during water electrolysis, interact with at least one type of polarization selected from the group consisting of electronic polarization of a chemical reaction intermediate during water electrolysis and vibrational polarization of water molecules.

Description

Electrode for water electrolysis and method for controlling overvoltage during water electrolysis using the same

[0001] The present disclosure relates to an electrode for water electrolysis and a method for controlling overvoltage during water electrolysis using the electrode.

[0002] Water electrolysis is a fundamental technology supporting the next-generation hydrogen society, enabling the efficient conversion of electrical energy into chemical energy. For example, as described in Non-Patent Document 1, an overvoltage, which is an excess potential relative to the thermodynamic potential, is required to proceed with water electrolysis. Reducing the overvoltage makes it possible to reduce excess energy.

[0003] Shigenori Mitsushima and Koichi Matsuzawa, "Current Status and Issues of Water Electrolysis Technology," Journal of the Hydrogen Energy Association (Hydrogen Energy Systems), 2011, Vol. 36, No. 1, pp. 11-16

[0004] As partially described in Non-Patent Document 1, studies have been conducted to reduce the overvoltage by optimizing the component composition of the electrode to improve catalytic activity, but the results are still not sufficient.

[0005] The present disclosure has been made in light of the above circumstances, and one of its objectives is to provide an electrode for water electrolysis that can reduce overvoltage using a means different from conventional means, and a method for controlling overvoltage during water electrolysis using the electrode.

[0006] A first aspect of the present invention is an electrode for use in a water electrolysis device that generates hydrogen and oxygen by bringing a hydrogen generation reaction electrode and an oxygen generation reaction electrode into contact with water and applying a voltage between the two electrodes, the electrode for water electrolysis having, on at least a part of its surface, one or more selected from the group consisting of a plurality of nanostructures and a plurality of microstructures that interact with one or more polarizations selected from the group consisting of electronic polarization of chemical reaction intermediates in water electrolysis and vibrational polarization of water molecules during water electrolysis.

[0007] A second aspect of the present invention is the water electrolysis electrode according to the first aspect, which satisfies any one or more of the following (A) to (D): (A) The hydrogen generation reaction electrode has a plurality of nanostructures on at least a part of its surface so as to have an absorption peak in the energy range of 0.50 to 4.00 eV. (B) The oxygen generation reaction electrode has a plurality of nanostructures on at least a part of its surface so as to have an absorption peak in the energy range of 0.50 to 2.00 eV. (C) The hydrogen generation reaction electrode has a surface roughness of 600 to 4400 cm. －１ (D) The oxygen evolution reaction electrode has a plurality of microstructures on at least a part of its surface so as to have a resonance wave number in the range of 600 to 4400 cm －１ The optical waveguide has a plurality of microstructures on at least a portion of its surface so as to have a resonant wavenumber in the range of .

[0008] Aspect 3 of the present invention is characterized in that (A) has an absorption peak in the energy range of 0.55 to 2.39 eV, (B) has an absorption peak in the energy range of 1.00 to 1.44 eV, and (C) and (D) have an absorption peak of 1600±400 cm －１ and 3400 ± 500 cm －１ The electrode for water electrolysis according to Aspect 2, wherein the electrode has a resonance wavenumber within one or more ranges selected from the group consisting of:

[0009] A fourth aspect of the present invention is the water electrolysis electrode according to any one of the first to third aspects, wherein the nanostructure contains a metal.

[0010] A fifth aspect of the present invention is the water electrolysis electrode according to any one of the first to fourth aspects, wherein the microstructure includes a metal.

[0011] A sixth aspect of the present invention relates to the water electrolysis electrode according to any one of the first to fifth aspects, wherein a difference in gas generation onset potential between a portion of the water electrolysis electrode having any one or more selected from the group consisting of a plurality of nanostructures and a plurality of microstructures and a smooth electrode having the same catalytic layer configuration is 0.01 V or more.

[0012] A seventh aspect of the present invention relates to the water electrolysis electrode according to any one of Aspects 1 to 6, wherein a difference in Tafel slope between a portion of the water electrolysis electrode having any one or more selected from the group consisting of a plurality of nanostructures and a plurality of microstructures and a smooth electrode having the same catalyst layer configuration is 20 mV / decade or more.

[0013] Aspect 8 of the present invention is a method for controlling overvoltage, comprising performing water electrolysis using the electrode for water electrolysis according to any one of Aspects 1 to 7.

[0014] According to an embodiment of the present invention, it is possible to provide an electrode for water electrolysis that can reduce overvoltage using a means different from conventional means, and a method for controlling overvoltage during water electrolysis using the electrode.

[0015] 1 is a schematic top view of a plurality of nanostructures of Examples 1 and 2, as viewed from the height direction (Z-axis direction); 2 is a surface SEM image of Test Nos. 1-8; 3 is a surface SEM image of Test Nos. 2-6; 4 is a schematic top view of a plurality of microstructures of Example 3, as viewed from the height direction (Z-axis direction); and 5 is a surface SEM image of Test Nos. 3-10.

[0016] The present inventors conducted extensive research to develop a water electrolysis electrode capable of reducing overpotential using a non-conventional method. As a result, they discovered that providing a plurality of nanostructures and / or a plurality of microstructures, which interact with the electronic polarization of chemical reaction intermediates and / or the vibrational polarization of water molecules during water electrolysis, on at least a portion of the surface of a hydrogen evolution reaction (HER) electrode and / or an oxygen evolution reaction (OER) electrode. As an example, they discovered that providing a plurality of nanostructures so as to have an absorption peak in a predetermined energy range (corresponding to an energy range in which electronic polarization of reaction intermediates in the water electrolysis reaction can occur) and / or providing a plurality of microstructures so as to have a predetermined resonance wavenumber (corresponding to a wavenumber in which vibrational polarization of water molecules can occur) can reduce overpotential (i.e., lower the onset potential for gas generation). They also discovered that providing a plurality of nanostructures and / or a plurality of microstructures can reduce the Tafel slope. Details of each requirement specified by the embodiments of the present invention are described below.

[0017] The water electrolysis electrode according to an embodiment of the present invention is an electrode used in a water electrolysis (water electrolysis) device that generates hydrogen or oxygen by contacting a hydrogen generation reaction electrode and an oxygen generation reaction electrode with water (including, for example, water in an electrolyte aqueous solution) and applying a voltage between the two electrodes. This device uses electrical energy to decompose water and does not necessarily require the use of other energy sources such as heating and light irradiation. Furthermore, the water electrolysis electrode according to an embodiment of the present invention has, on its surface, a plurality of nanostructures and / or a plurality of microstructures that interact with the electronic polarization of chemical reaction intermediates (including those in a bonded state with metals) and / or the vibrational polarization of water molecules during water electrolysis. Water electrolysis (water electrolysis) is an electrochemical reaction that generates hydrogen or oxygen by contacting a hydrogen generation reaction electrode and an oxygen generation reaction electrode with water and applying a voltage between the two electrodes. Efficient conversion of provided electrical energy into chemical energy at the electrode reduces the overvoltage (excess energy) that exceeds the electrolysis voltage derived from thermodynamic theory. The inventors have found that forming specific nanostructures and / or microstructures on an electrode surface and allowing these structures to interact with the electronic polarization of reaction intermediates and / or the vibrational polarization of water molecules involved in a chemical reaction results in a difference in the onset potential for gas (hydrogen or oxygen) generation and a difference in the Tafel slope compared to a smooth electrode. They have also found that this interaction can suppress overpotential in the reaction of generating hydrogen or oxygen on the electrode surface, lower the onset potential for gas generation, and further reduce the Tafel slope. Here, a "smooth electrode" refers to an electrode that has the same catalyst layer configuration as a portion having multiple nanostructures and / or multiple microstructures, but does not have unevenness due to the multiple nanostructures and / or multiple microstructures, and does not interact with the electronic polarization of chemical reaction intermediates and / or the vibrational polarization of water molecules during water electrolysis. An example of a smooth electrode is an electrode with a flat surface.The onset potential difference for gas generation is thought to depend on the nanostructures and / or microstructures on the electrode surface, and it is possible to design a water electrolysis electrode that enables an onset potential difference of, for example, 0.01 V or more, and from the viewpoint of industrial use, preferably 0.05 V or more, more preferably 0.15 V or more, even more preferably 0.30 V or more, and still more preferably 0.50 V or more. The difference in Tafel slope during gas generation can be, for example, 20 mV / decade or more, preferably 30 mV / decade or more, more preferably 40 mV / decade or more, even more preferably 50 mV / decade or more, and even more preferably 60 mV / decade or more.

[0018] An electrode for water electrolysis according to one embodiment of the present invention may satisfy one or more of the following conditions (A) to (D): (A) The hydrogen generation reaction electrode has a plurality of nanostructures on at least a part of its surface so as to have an absorption peak in the energy range of 0.50 to 4.00 eV. (B) The oxygen generation reaction electrode has a plurality of nanostructures on at least a part of its surface so as to have an absorption peak in the energy range of 0.50 to 2.00 eV. (C) The hydrogen generation reaction electrode has a plurality of nanostructures on at least a part of its surface so as to have an absorption peak in the energy range of 600 to 4400 cm. －１ (D) The oxygen evolution reaction electrode has a plurality of microstructures on at least a part of its surface so as to have a resonance wave number in the range of 600 to 4400 cm －１ The electrode has a plurality of microstructures on at least a portion of its surface so as to have a resonance wavenumber in the range of 0.01 to 0.01. This strengthens the above-mentioned interaction and reduces the overvoltage during water electrolysis (reducing the onset potential for gas generation). The requirements (A) to (D) are described in detail below.

[0019] <(A) Hydrogen Evolution Reaction Electrode Having Multiple Nanostructures> In the above (A), the interaction with the electronic polarization of a reaction intermediate in the hydrogen evolution reaction (HER) can be utilized. Examples of the reaction intermediate include the nanostructure surface (the metal surface if the nanostructure is metal), a nanostructure surface with hydrogen adsorbed, and a nanostructure surface with water molecules bound thereto, and the reaction intermediate can vary depending on the binding state. The present inventors have found that by forming multiple nanostructures on at least a portion of the surface of a HER electrode so that the electronic polarization of the reaction intermediate occurs in an energy range of 0.50 eV to 4.00 eV and has an absorption peak in this energy range, the overpotential can be reduced (the onset potential of gas evolution can be lowered) and the Tafel slope can be reduced compared to when the multiple nanostructures are not formed. To further reduce the overvoltage (lower the onset potential of gas generation) and further reduce the Tafel slope, the energy range of the absorption peak is preferably 0.50 eV to 2.39 eV, more preferably 0.50 eV to 2.00 eV, and even more preferably 0.50 eV to 0.80 eV. Furthermore, the absorbance obtained by subtracting the absorbance of a smooth electrode having the same catalyst layer configuration as the portion containing the nanostructure from the absorbance of the absorption peak is preferably 0.1 or greater. Furthermore, the present inventors have found that by performing water electrolysis in a field capable of interacting with the electronic polarization, the electronic polarization of the reaction intermediate interacts with the field, thereby reducing the overvoltage (lowering the onset potential of gas generation) and further reducing the Tafel slope. Therefore, by using a HER electrode that satisfies the above (A), for example, 0.01 mW cm －２ Even when water electrolysis is performed under weak light (including in a dark room) below 0.01 mW cm, the overvoltage can be reduced (the onset potential of gas generation can be lowered) and the Tafel slope can be reduced. On the other hand, since the overvoltage can be reduced regardless of light irradiation, －２ Even when water electrolysis is carried out under relatively strong light, the overpotential can be reduced (i.e., the onset potential of gas generation can be lowered), and further the Tafel slope can be reduced.

[0020] Whether or not an absorption peak exists in the above energy range can be determined, for example, by obtaining an optical absorption spectrum of a hydrogen evolution reaction electrode containing a predetermined number of nanostructures and checking the energy (eV) of the peak position, or by obtaining an optical reflection spectrum and checking the energy (eV) of the position showing the minimum value. Furthermore, if the configuration of the nanostructures (material, shape, size, spacing between the nanostructures, etc.) is known, for example, by analysis using the fabrication conditions or SEM, the optical absorption spectrum (or optical reflection spectrum) may be determined by performing an FDTD simulation using that configuration.

[0021] The material, shape, size, and spacing of the nanostructures are not particularly limited as long as they are configured to have an absorption peak in the above energy range due to electronic resonance. By adjusting the material, shape, size, or spacing of the nanostructures, those skilled in the art can control the resonance energy range of electronic resonance (i.e., the absorption peak position) from the visible range to the near-infrared range. Therefore, the material, shape, size, and spacing of the nanostructures can be appropriately selected so that electronic resonance occurs in a predetermined energy range. The material of the nanostructures may be a conductive material, such as a metal selected from the group consisting of noble metals (gold, silver, platinum, palladium, etc.), iron, iron-rare earth alloys, iron-nickel alloys, and nickel, a conductive oxide, or carbon. The shape of the nanostructures may be, for example, one or more of stripes, spheres, hemispheres, prisms, cylinders (discs), and cones. The maximum length and maximum height of the nanostructures may be 1 nm or more but less than 1,000 nm. The distance between the centers of the nanostructures may be 1 nm or more and less than 1000 nm. A hydrogen evolution reaction electrode according to one embodiment of the present invention may include a substrate made of a non-metallic conductive material such as carbon or a conductive oxide, and a plurality of metallic nanostructures disposed on the substrate.

[0022] The number of the plurality of nanostructures may be 2 or more, preferably 10 or more, and more preferably 50 or more. The nanostructures may be present on at least a portion of the surface of the hydrogen generation reaction electrode, and may be formed over the entire surface of the hydrogen generation reaction electrode. When the plurality of nanostructures are viewed from above in the height direction, the occupancy (coverage) of the surface of the HER electrode by the plurality of nanostructures is not particularly limited, and may be, for example, 1 area % to 99 area %, or 5 area % to 90 area %, etc.

[0023] <(B) Oxygen Evolution Reaction Electrode Having Multiple Nanostructures> In (B) above, the interaction with the electronic polarization of a reaction intermediate in the oxygen evolution reaction (OER) can be utilized. Examples of the reaction intermediate include the nanostructure surface (if the nanostructure is metallic, the metal surface), a hydroxide adsorbed on the nanostructure surface, an oxygen atom adsorbed on the nanostructure surface, a hydrogen peroxide molecule adsorbed on the nanostructure surface, and a water molecule bonded to the nanostructure surface. The present inventors have found that by forming multiple nanostructures on at least a portion of the surface of an OER electrode so that the electronic polarization of the reaction intermediate occurs in an energy range of 0.50 eV to 2.00 eV and has an absorption peak in this energy range, the overpotential can be reduced (the onset potential for gas evolution can be lowered) and the Tafel slope can be reduced compared to when the multiple nanostructures are not formed. In order to further reduce the overvoltage (lower the onset potential of gas generation) and further reduce the Tafel slope, the energy range of the absorption peak is preferably 1.00 eV or more and 1.44 eV or less, more preferably 1.10 eV or more and 1.33 eV or less. The energy range of the absorption peak may also be 0.50 eV or more and 0.80 eV or less. The absorbance obtained by subtracting the absorbance of a smooth electrode having the same catalyst layer configuration as the portion containing the nanostructure from the absorbance of the absorption peak is preferably 0.1 or more. Similarly to the case of (A) above, an OER electrode satisfying (B) above can be used to obtain, for example, 0.01 mW cm －２Even when water electrolysis is performed under weak light (including in a dark room) below 0.01 mW cm, the overvoltage can be reduced (the onset potential of gas generation can be lowered) and the Tafel slope can be reduced. On the other hand, since the overvoltage can be reduced regardless of light irradiation, －２ Even when water electrolysis is performed under relatively strong light of 1000 kJ / cm or more, the overpotential can be reduced (the onset potential of gas generation can be lowered), and further, the Tafel slope can be reduced. In addition, whether or not there is an absorption peak in the above energy range can be determined in the same manner as in the case of (A) above.

[0024] The shape, size, and spacing of the nanostructures, as well as the coverage (coverage) of the surface of the OER electrode by the nanostructures, are the same as in the case of (A) above. Materials for the nanostructures include conductive materials such as noble metals (gold, silver, platinum, palladium, etc.), nickel, nickel-based alloys, iron, nickel-coated iron, and ion-plated nickel (Ag ＋ , Li ＋ , He ＋ , Kr ＋ Examples of the metal nanostructure include one or more metals selected from the group consisting of metals such as FTO, nickel-cobalt oxide, cobalt oxide, lanthanum-doped cobalt oxide, lanthanum strontium-cobalt oxide, zinc-cobalt oxide, and noble metal oxides, and carbon. The nanostructure may be present on at least a portion of the surface of the oxygen evolution reaction electrode, and may be formed over the entire surface of the oxygen evolution reaction electrode. An oxygen evolution reaction electrode according to one embodiment of the present invention may include a substrate made of a non-metallic conductive material such as carbon or a conductive oxide, and a plurality of metal nanostructures arranged on the substrate.

[0025] <(C) and (D): Hydrogen Evolution Reaction Electrode and Oxygen Evolution Reaction Electrode Having Multiple Microstructures> In the above (C) and (D), the interaction with the vibrational polarization of water molecules can be utilized. Water molecules have a wave number of 1600 cm －１ It has an HOH bending mode near 3400 cm －１ The inventors have investigated the OH stretching mode in the range of 600 to 4400 cm, including the vicinity of these wavenumbers. －１It has been found that by forming a plurality of microstructures on at least a portion of the surface of the HER electrode and / or OER electrode so that the resonant wavenumber is 1600±400 cm, the overvoltage can be reduced (the onset potential of gas generation can be lowered) and the Tafel slope can be reduced compared to when the plurality of microstructures are not formed. In order to further reduce the overvoltage (to further lower the onset potential of gas generation) and further reduce the Tafel slope, it is preferable to form a resonant wavenumber of 1600±400 cm. －１ and 3400 ± 500 cm －１ More preferably, 1600±170 cm －１ and 3400 ± 200 cm －１ The resonant wave number is 1600±400 cm. －１ and twice the resonant wave number is 3400±500 cm －１ It is more preferable that (C) and (D) are satisfied, whereby the overpotential can be further reduced by interaction with the OH stretching mode in addition to the HOH bending mode (the onset potential of gas generation can be further lowered), and the Tafel slope can be further reduced. Furthermore, the present inventors have found that by performing water electrolysis in a field capable of interacting with the vibrational polarization of water molecules, the field and the vibrational polarization of water molecules interact to reduce the overpotential and further reduce the Tafel slope. Therefore, even when water electrolysis is performed using a HER electrode and / or OER electrode that satisfies (C) and / or (D) without intentionally irradiating it with near-infrared to far-infrared rays, the overpotential can be reduced (the onset potential of gas generation can be lowered), and the Tafel slope can be further reduced.

[0026] The resonant wavenumber can be measured by a general method for determining a photonic band, for example, by measuring reflectance using microscopic infrared reflection spectroscopy and observing the extinction maximum, thereby measuring the resonant wavenumber. Furthermore, if the configuration of the multiple microstructures (material, shape, size, spacing between the multiple microstructures, etc.) is known, for example, by analysis using the fabrication conditions or an SEM, the resonant wavenumber may be determined by performing an FDTD simulation using that configuration. Furthermore, according to one embodiment of the present invention, when the multiple microstructures are arranged in the form of multiple stripes that are equally spaced, the resonant wavenumber (ω ｃａｖ ) (cm －１ ) can be calculated using the stripe spacing d (μm) and the refractive index n (=1.33) of water as in the following formula (1): ｃａｖ =10 ４ / 2nd...(1)

[0027] The material, shape, size, and spacing of the microstructures are not particularly limited as long as they are configured to interact with the vibrational polarization of water molecules in the above-mentioned wavenumber range. For example, the material of the microstructures may be a conductive material such as metal, conductive oxide, or carbon, and the shape of the microstructures may be one or more of stripe, sphere, hemisphere, prism, cylinder (disc), and cone. The maximum length and maximum height of the microstructures may be 1 μm or more (preferably 5 μm or more) and 1000 μm or less. The closest distance between the centers of multiple microstructures may be 1 μm or more (preferably 5 μm or more) and 1000 μm or less. A hydrogen evolution reaction electrode (and / or oxygen evolution reaction electrode) according to one embodiment of the present invention may include a non-conductive material substrate such as a quartz substrate and multiple metal microstructures arranged on the substrate.

[0028] The number of the multiple microstructures may be 2 or more, preferably 10 or more, and more preferably 50 or more. When the multiple microstructures are viewed from above in the height direction, the occupancy (coverage) of the multiple microstructures on the surface of the HER electrode or OER electrode is not particularly limited, and may be, for example, 1 area % or more and 99 area % or less, or 5 area % or more and 90 area % or less. The microstructures may be present on at least a part of the surface of the hydrogen generation reaction electrode and / or the oxygen generation reaction electrode, and may be formed over the entire surface of the hydrogen generation reaction electrode and / or the oxygen generation reaction electrode.

[0029] An electrode for water electrolysis according to one embodiment of the present invention preferably satisfies any two or more of the above (A) to (D), more preferably satisfies three or more, and most preferably satisfies all four. Here, microstructures and nanostructures may coexist on the surface of the HER electrode and / or OER electrode. For example, a microstructure may be provided on one part of the surface and a nanostructure may be provided on another part of the surface, or a microstructure may be provided on one part of the surface and a nanostructure may be further provided thereon.

[0030] The nanostructures and microstructures can be formed by using known methods such as lithography.

[0031] The configuration of the water electrolysis electrode according to the embodiment of the present invention is not particularly limited except for the nanostructure and microstructure, and known configurations (materials, shapes, etc.) of water electrolysis electrodes can be applied.

[0032] The water electrolysis electrode according to the embodiment of the present invention has a capacitance of, for example, 0.01 mWcm －２Even when water electrolysis is performed under weak light, the overpotential can be reduced (the onset potential for gas generation can be further lowered) and the Tafel slope can be reduced. Furthermore, the water electrolysis electrode having a plurality of nanostructures and / or microstructures according to the embodiment of the present invention interacts with the electronic polarization of chemical reaction intermediates and / or the vibrational polarization of water molecules during water electrolysis, resulting in a difference in the onset potential for gas (hydrogen or oxygen) generation and a difference in the Tafel slope compared to a smooth electrode. In other words, the presence or absence of this interaction can be determined by whether or not a difference in the onset potential for gas generation and / or a difference in the Tafel slope occurs compared to the smooth electrode. The smooth electrode may be, for example, a water electrolysis electrode having a plurality of target nanostructures (and / or a plurality of microstructures), in which only one nanostructure (and / or a plurality of microstructures) is selected from the plurality of nanostructures (and / or a plurality of microstructures) and the portion containing the other nanostructures (and / or the other microstructures) is removed (or an electrode that has been subjected to a process equivalent to the removal), or a smooth electrode (plain electrode) that is not patterned with a plurality of nanostructures (and / or a plurality of microstructures) may be prepared. The onset potential difference for gas (hydrogen or oxygen) generation (i.e., the overvoltage that can be reduced by embodiments of the present invention) may be, for example, 0.01 V or more, preferably 0.05 V or more, more preferably 0.15 V or more, even more preferably 0.30 V or more, and even more preferably 0.50 V or more. The difference in Tafel slope in the gas (hydrogen or oxygen) generation (i.e., the Tafel slope that can be reduced by the embodiments of the present invention) can be, for example, 20 mV / decade or more, preferably 30 mV / decade or more, more preferably 40 mV / decade or more, even more preferably 50 mV / decade or more, and still more preferably 60 mV / decade or more.

[0033] While water electrolysis becomes difficult as the pH of the electrolytic solution approaches neutrality, the present inventors have found that the water electrolysis electrode according to an embodiment of the present invention can reduce overvoltage (reduce the onset potential of gas generation) even in conditions other than, for example, strongly alkaline or strongly acidic conditions in which water electrolysis is usually performed. That is, the water electrolysis electrode according to an embodiment of the present invention can reduce overvoltage even in an electrolytic solution having a pH of 13 or higher (or 1 or lower), and can also reduce overvoltage even when water electrolysis is performed at a pH of more than 1 and less than 13, 2 or higher and 12 or lower, or 3 or higher and 11 or lower.

[0034] Water electrolysis is difficult if the electrolyte does not contain a buffer solution such as a phosphate buffer solution, but the inventors have found that the water electrolysis electrode according to an embodiment of the present invention can reduce the overvoltage (reduce the onset potential of gas generation) even if the electrolyte does not contain a buffer solution. That is, the water electrolysis electrode according to an embodiment of the present invention can reduce the overvoltage even if the electrolyte contains a buffer solution, and can also reduce the overvoltage even if the electrolyte does not contain a buffer solution.

[0035] The method for controlling overvoltage during water electrolysis according to the embodiment of the present invention can be carried out by using the water electrolysis electrodes according to the embodiment of the present invention. －２ This is a method for performing water electrolysis under weak light as described below. Furthermore, in the method for controlling overvoltage during water electrolysis according to the embodiment of the present invention, the pH of the electrolytic solution may be 13 or more (or 1 or less), or may be more than 1 and less than 13, or 2 or more and 12 or less, or 3 or more and 11 or less. Furthermore, in the method for controlling overvoltage during water electrolysis according to the embodiment of the present invention, the electrolytic solution may or may not contain a buffer solution such as a phosphate buffer solution.

[0036] The following examples are provided to more specifically describe the embodiments of the present invention. The embodiments of the present invention are not limited to the following examples, and may be modified as appropriate within the scope of the above-described and below-described aims, and all such modifications are within the technical scope of the embodiments of the present invention.

[0037] 1. Hydrogen Evolution Reaction Electrode Having Multiple Nanostructures First, a substrate (glassy carbon substrate) for forming nanostructures was prepared as follows: A commercially available glassy carbon substrate was immersed in aqua regia for 30 minutes for cleaning, then polished with diamond paste for 5 minutes, alumina paste for 5 minutes, and alumina polishing paper for 1 minute, and then ultrasonically cleaned for 10 minutes.

[0038] Next, the substrate was heated at 180°C for 3 minutes, and then a resist solution (ZEP520A (manufactured by Nippon Zeon Co., Ltd.): ZEPA (manufactured by Nippon Zeon Co., Ltd.) = 2:1) was spin-coated onto the substrate (300 rpm for 3 seconds and 4000 rpm for 60 seconds), and again heated at 180°C for 3 minutes. Subsequently, electron beam lithography (EBL) was performed using a scanning electron microscope (SEM, Elionix ELS-F125) at an acceleration voltage of 125 kV and a current of 50 pA or 100 pA (depending on the structure size) to form multiple nanostructures as shown in Figures 1 and 2 and Table 1, which will be described later. After electron beam writing, the substrate was immersed in a developer (ZED-N50, manufactured by Nippon Zeon Co., Ltd.) for 60 seconds and in a rinse solution (ZMD-B, manufactured by Nippon Zeon Co., Ltd.) for 10 seconds.

[0039] After development, 100 nm of Ag was deposited on the resist-coated substrate using an electron beam deposition device. The resist layer was removed by ultrasonic cleaning in N,N-dimethylacetamide (a stripping solution). The resist and stripping solution were then removed by rinsing with acetone. If any resist remained, it was similarly ultrasonically cleaned in trichloroethylene, and then rinsed five or more times with acetone.

[0040] In this manner, hydrogen evolution reaction electrodes having multiple nanostructures were prepared as Test Nos. 1-1 to 1-12. Figure 1 shows a top view of the multiple nanostructures of Test Nos. 1-1 to 1-12, viewed from the height direction (Z-axis direction). As shown in Figure 1, each of the multiple nanostructures was cylindrical and arranged at equal intervals in the X-axis and Y-axis directions. Test Nos. 1-1 to 1-12 were prepared by varying the closest distance a between the centers of the circles and the diameter D of the circles, as shown in Table 1 below. Note that L (the distance between the centers of the circles at both ends on the X-axis (or Y-axis)) shown in Figure 1 was set to 200 × a when D = 200 nm, 100 × a when D = 400 nm, and 50 × a when D = 800 nm. Figure 2 shows an example surface SEM image of Test No. 1-8 (scale bar in the figure: 1 μm). For comparison, a smooth electrode (glassy carbon substrate / Ag (100 nm)) not having a plurality of nanostructures was used as Test No. 1-13.

[0041]

[0042] Next, based on the pattern in Table 1, an FDTD simulation was performed under the following conditions to obtain a reflection spectrum of the hydrogen evolution reaction electrode, and the energy (eV) at the absorption peak of Test Nos. 1-1 to 1-12 was determined from the spectrum. Note that the absorbance at the absorption peak minus the absorbance of a smooth electrode (electrode corresponding to Test No. 1-13) having the same catalyst layer configuration as the portion containing the nanostructure was all 0.1 or more. [FDTD Simulation Conditions] FDTD software: Ansys Lumerical 2022R1.4 Structural model (mesh order): H ２ O (2), Ag disk (1), GC (1) Mesh setting (overall): uniform, x: 10 nm, y: 10 nm, Z: 10 nm Mesh step (near the structure): None Boundary condition: x: Bloch, y: Bloch, Z: PML Source: Plane wave, Bloch / periodic. Polarization angle: x axis (0 degree) Analysis area: 300-3500 nm, 1600 points Physical properties: H ２O: “H2O (Water)-200-3500nm”, Ag: “Ag (silver)-CRC100-3000”, GC: “Amorphous Carbon_Laruquert”

[0043] The hydrogen evolution reaction electrode prepared as described above was used as the working electrode, platinum was used as the counter electrode, and gold was used as the reference electrode. The potential was calibrated based on the reversible hydrogen electrode (RHE). The electrolyte was 0.5 M NaClO ４ An aqueous solution (pH 7) was used. The working electrode was set as the cathode, and the applied voltage was changed by -5 mV / sec. －２ Water electrolysis was performed under the following weak light conditions, and the applied voltage at which bubbles (i.e., hydrogen) were generated was investigated. The water electrolysis was performed under high pressure (75 MPa) by applying pressurizing oil around the sample cell to facilitate observation of bubble generation. The investigation was also performed by monitoring the state of the hydrogen generation reaction electrode, and the applied voltage at which gas generation was confirmed with the naked eye was defined as the applied voltage at which bubbles were generated. The results are summarized in Table 2, along with the energy (eV) at the absorption peak obtained as described above.

[0044]

[0045] As shown in Table 2, Test Nos. 1-1 to 1-12 are examples that satisfy the requirements of the embodiment of the present invention, and compared to the case where a smooth electrode without multiple nanostructures was used (Test No. 1-13), the onset potential of hydrogen generation was reduced (i.e., the overvoltage was reduced by, for example, 0.05 V or more). Furthermore, Test Nos. 1-3 to 1-12 satisfied the preferred requirements of the embodiment of the present invention (having multiple nanostructures on at least a portion of the surface so as to have an absorption peak in the energy range of 0.50 to 2.39 eV), and the onset potential of hydrogen generation was further reduced (the overvoltage was reduced by, for example, 0.15 V or more). Furthermore, Test Nos. 1-3 to 1-12 satisfied the more preferred requirements of the embodiment of the present invention (having multiple nanostructures on at least a portion of the surface so as to have an absorption peak in the energy range of 0.50 to 2.00 eV), and the onset potential of hydrogen generation was further reduced (the overvoltage was reduced by, for example, 0.30 V or more).

[0046] Furthermore, the Tafel slope was determined for Test Nos. 1-5, 1-7, 1-11, and 1-13 as follows. First, water electrolysis was performed in the same manner as above. During water electrolysis, each bubble generated at the working electrode was monitored (or videotaped) from a top view. This data was input, and the area of ​​each bubble generated at the working electrode and the position of the center of gravity of the circumscribed rectangle were determined. Assuming each bubble was a sphere, the radius (diameter) and volume of each bubble were determined from the area of ​​each bubble. Then, the number of moles for each generated bubble was calculated from the time, potential, diameter, volume, and surface volume of the position of the center of gravity of the circumscribed rectangle, and the total number of moles y was determined. The total number of moles y was plotted against the potential V and fitted to the following equation (2), and the current I was determined from the following equation (3): y = y ０ +Aexp(-(V-V ０ ) / t) (2) I=nF(δy / δt) (3) where y ０ , A., V. ０ and t are fitting parameters, n is the number of reaction electrons, where n = -2 for the hydrogen evolution reaction and n = +4 for the oxygen evolution reaction, F is the Faraday constant, and (δy / δt) is the amount of change in the total number of moles per minute period of time. The current density I / S was calculated from the current I and electrode area S calculated as described above, and the slope of the potential V relative to the logarithm log(I / S) was calculated as the Tafel slope. The results are shown in Table 3 below.

[0047]

[0048] As shown in Table 3, Test Nos. 1-5, 1-7, and 1-11 are examples that satisfy the requirements of the embodiments of the present invention, and the Tafel slope during hydrogen generation was reduced by 20 mV / decade or more compared to the case where a smooth electrode without multiple nanostructures was used (Test No. 1-13).

[0049] 2. Oxygen Evolution Reaction Electrode Having Multiple Nanostructures First, a substrate (FTO substrate) for forming the nanostructures was prepared as follows: The FTO substrate was ultrasonically cleaned in trichloroethylene for 15 minutes, rinsed five times with acetone, ultrasonically cleaned in acetone for 15 minutes, and then immersed in fresh acetone heated to 60°C for approximately 10 minutes to remove the trichloroethylene.

[0050] Next, the substrate was heated on a hot plate at 180°C for 3 minutes, and then a resist solution (ZEP520A (Zeon Corporation): ZEPA (Zeon Corporation) = 2:1) was spin-coated onto the substrate (300 rpm for 3 seconds and 4000 rpm for 60 seconds) and heated again at 180°C for 3 minutes. Subsequently, electron beam lithography (EBL) was performed using a SEM (Elionix ELS-F125) at an acceleration voltage of 125 kV and a current of 50 pA or 100 pA (depending on the structure size) to form multiple nanostructures as shown in Figure 1 and in Figure 3 and Table 4 (described below). After electron beam lithography, the substrate was immersed in a developer (ZED-N50, Zeon Corporation) for 60 seconds and a rinse (ZMD-B, Zeon Corporation) for 10 seconds.

[0051] After development, Au was vapor-deposited to a thickness of 100 nm on the substrate with the resist using a helicon sputtering device (MPS-4000C1 / HCl, manufactured by ULVAC). The resist layer was removed by ultrasonic cleaning in N,N-dimethylacetamide (stripping solution). After that, the resist and stripping solution were removed by rinsing with acetone. If the resist remained, it was similarly ultrasonically cleaned in trichloroethylene, and then rinsed five or more times with acetone. Furthermore, a plating solution containing 10 mM Ni(NO ３ ) ２ Using the solution, 31.8 μA / cm ２ The Ni plating treatment was carried out under the conditions of 1 minute, 2 minutes, and 45 seconds.

[0052] In this manner, oxygen evolution reaction electrodes having multiple nanostructures were prepared as Test Nos. 2-1 to 2-12. Figure 1 shows a top view of the multiple nanostructures of Test Nos. 2-1 to 2-12, viewed from the height direction (Z-axis direction). As shown in Figure 1, each of the multiple nanostructures was cylindrical and arranged at equal intervals in the X-axis and Y-axis directions. Test Nos. 2-1 to 2-12 were prepared by varying the closest distance a between the centers of the circles and the diameter D of the circles, as shown in Table 4 below. Note that L (the distance between the centers of the circles at both ends on the X-axis (or Y-axis)) shown in Figure 1 was approximately 60 μm. Figure 3 shows an example SEM image of the surface of Test No. 2-6 (scale bar in the figure: 1 μm). For comparison, a smooth electrode (FTO substrate / Au (100 nm) / Ni) without multiple nanostructures was prepared as Test No. 2-13.

[0053]

[0054] Next, based on the structure in Table 4, an FDTD simulation was performed under the following conditions to obtain the reflection spectrum of the oxygen evolution reaction electrode, and the energy (eV) at the absorption peak was determined. Note that the absorbance at the absorption peak minus the absorbance of a smooth electrode (electrode corresponding to Test No. 2-13) having the same catalyst layer configuration as the portion containing the nanostructure was 0.1 or more in all cases. [FDTD Simulation Conditions] FDTD software: Ansys Lumerical 2022R1.4 Structural model (mesh order): H ２ O (2), Au disk (1), GC (1) Mesh setting (overall): uniform, x: 10 nm, y: 10 nm, Z: 20 nm Mesh step (near the structure): None Boundary condition: x: Bloch, y: Bloch, Z: PML Source: Plane wave, Bloch / periodic. Polarization angle: x axis (0 degree) Analysis area: 200-1200 nm, 1258 points Physical properties: H ２ O: “H2O (Water) -100-2000nm”, Au: “Au Babar and Weaver 2015”, FTO: “FTO_Ameur (2017)”

[0055] The oxygen generating reaction electrode prepared as described above was used as the working electrode, platinum was used as the counter electrode, and Ag / AgCl was used as the reference electrode. The potential was calibrated based on the reversible hydrogen electrode (RHE). The electrolyte was a mixed solution (pH 11) of 1 M phosphate buffer and 3 M NaOH aqueous solution. The working electrode was used as the anode, and the applied voltage was changed by 5 mV / sec to obtain a voltage of 0.01 mW cm. －２ Electrolysis was performed under the following weak light at atmospheric pressure, and the applied voltage at which bubbles (i.e., oxygen) were generated was investigated. The investigation was carried out by monitoring the state of the oxygen evolution reaction electrode, and the applied voltage at which gas generation was confirmed with the naked eye was defined as the applied voltage at which bubbles were generated. The results are summarized in Table 5, along with the energy (eV) at the absorption peak obtained as described above.

[0056]

[0057] The following can be seen from Table 5. Test Nos. 2-1 to 2-12 are examples that satisfy the requirements of the embodiment of the present invention, and compared to the case where a smooth electrode without multiple nanostructures was used (Test No. 2-13), the onset potential of oxygen generation was reduced (i.e., the overvoltage was reduced by, for example, 0.05 V or more). Furthermore, Test Nos. 2-6 to 2-12 satisfied the preferred requirements of the embodiment of the present invention (having multiple nanostructures on at least a portion of the surface so as to have an absorption peak in the energy range of 1.00 to 1.44 eV), and the onset potential of oxygen generation was further reduced (the overvoltage was reduced by, for example, 0.15 V or more) without exception. Furthermore, Test Nos. 2-9 to 2-12 satisfied the more preferred requirements of the embodiment of the present invention (having multiple nanostructures on at least a portion of the surface so as to have an absorption peak in the energy range of 1.10 to 1.33 eV), and the onset potential of oxygen generation was further reduced (the overvoltage was reduced by, for example, 0.30 V or more) without exception.

[0058] Furthermore, for Test Nos. 2-2, 2-3, 2-6, 2-7, and 2-11 to 2-13, the Tafel slopes were determined in the same manner as in Example 1. The results are shown in Table 6 below.

[0059]

[0060] As shown in Table 6, Test Nos. 2-2, 2-3, 2-6, 2-7, 2-11, and 2-12 are examples that satisfy the requirements of the embodiments of the present invention, and the Tafel slope during oxygen generation was reduced by 20 mV / decade or more compared to the case where a smooth electrode without multiple nanostructures was used (Test No. 2-13).

[0061] 3. Oxygen Evolution Reaction Electrode Having Multiple Microstructures First, a substrate (quartz glass substrate) for forming the microstructure was prepared. The substrate was ultrasonically cleaned with acetone for 5 minutes, and then ultrasonically cleaned with methanol, ethanol, and / or ultrapure water for another 5 minutes, as necessary.

[0062] Next, a resist solution (AZP-1350, manufactured by AZ Electronic Materials) was spin-coated onto the substrate (300 rpm for 3 seconds and 3000 rpm for 60 seconds) and heated at 95°C for 90 seconds. To improve wettability, OAP (manufactured by Tokyo Ohka Kogyo Co., Ltd.) and LOR-3A (manufactured by Nippon Kayaku Co., Ltd.) were spin-coated before coating with the resist solution, as necessary. Laser writing was then performed using a laser writing device (DWL66+ manufactured by Heidelberg) under the following conditions to form multiple microstructures as shown in Figures 4 and 5 and Table 7 below. After laser writing, the substrate was immersed in a developer (NMD-3, manufactured by Tokyo Ohka Kogyo Co., Ltd.) for 60 seconds and then in ultrapure water for 30 seconds. [Laser writing conditions] Focus: -1% Intensity: 100% Laser power: 65 mW Filter: 5%

[0063] After development, Ni was vapor-deposited to a thickness of 200 nm on the substrate with the resist using a helicon sputtering device (MPS-4000C1 / HCl, manufactured by ULVAC). The resist layer was then removed by ultrasonic cleaning in acetone and ethanol, or by immersion in N,N-dimethylacetamide (stripping solution) and heating to 65°C. The resist and stripping solution were then removed by rinsing with acetone or the like as necessary. Furthermore, CHF was used as an etching gas. ３Etching was performed using argon (and Ar, if necessary) to completely remove the resist residue. After etching, the substrate was washed with Pure Etch CR101 (manufactured by Hayashi Pure Chemical Industries, Ltd.), acetone, etc. Furthermore, a plating solution containing 10 mM Fe(NO ３ ) ３ The solution was used to perform Fe plating treatment under conditions of 100 mV and 90 seconds.

[0064] In this manner, oxygen evolution reaction electrodes having multiple microstructures were prepared, Test Nos. 3-1 to 3-10. Figure 4 shows a top view schematic of the multiple microstructures of Test Nos. 3-1 to 3-10, viewed from the height direction (Z-axis direction). As shown in Figure 4, multiple stripes extending in the Y-axis direction were arranged at equal intervals in the X-axis direction. Test Nos. 3-1 to 3-10 were prepared by setting the stripe width s to 1.2 μm or 2.0 μm, the stripe length L to 60 μm, and varying the spacing d between the stripes as shown in Table 7 below. Figure 5 shows an example surface SEM image of Test No. 3-10 (scale bar in the figure: 1 μm).

[0065] The oxygen generating reaction electrode prepared as described above was used as the working electrode, platinum was used as the counter electrode, and Ag / AgCl was used as the reference electrode. The potential was calibrated based on the reversible hydrogen electrode (RHE). The electrolyte was a mixed solution (pH 11) of 1 M phosphate buffer and 3 M NaOH aqueous solution. The working electrode was used as the anode, and the applied voltage was changed by 5 mV / sec to obtain a voltage of 0.01 mW cm. －２ Electrolysis was carried out under the following weak light at atmospheric pressure, and the applied voltage when bubbles (i.e., oxygen) were generated was investigated. The investigation was carried out by monitoring the state of the oxygen evolution reaction electrode, and the applied voltage at which gas generation was confirmed with the naked eye was taken as the applied voltage when bubbles were generated. The results are shown in Table 7. The stripe spacing d (μm) of an actually produced electrode was measured at nine points on an SEM image, and the average value was used. The resonance wave number (ω ｃａｖ ) (cm －１ ) was calculated using the stripe spacing d (μm) and the refractive index n (=1.33) of water according to the above formula (1).

[0066]

[0067] The following can be seen from Table 7. Test Nos. 3-1 to 3-10 are examples that satisfy the requirements of the embodiment of the present invention, and compared to the case where a smooth electrode (not having a predetermined plurality of microstructures) was used (Test No. 3-11), the onset potential of oxygen evolution was reduced (i.e., the overvoltage was reduced by, for example, 0.05 V or more). Furthermore, Test Nos. 3-1 to 3-3 and 3-7 to 3-10 satisfy the preferred requirements of the embodiment of the present invention (1600±400 cm －１ and 3400 ± 500 cm －１ The test samples Nos. 3-2 and 3-8 to 3-9 satisfied the more preferable requirements of the embodiment of the present invention (having a resonance wave number within any one or more ranges of the group consisting of: 1600±170 cm －１ and 3400 ± 200 cm －１ The test samples Nos. 3-2 to 3-3 satisfied the more preferable requirements of the embodiment of the present invention (the resonance wave number is within one or more ranges of the group consisting of: －１ and twice the resonant wave number is 3400±500 cm －１ The onset potential of oxygen evolution was further reduced (overvoltage was reduced by, for example, 0.30 V or more).

[0068] Furthermore, samples having the same configuration as Test Nos. 3-1, 3-2, 3-4, and 3-11 were reproduced (referred to as Test Nos. 3-1′, 3-2′, 3-4′, and 3-11′, respectively), and the above electrolyte was replaced with a solution containing no buffer solution (1M NaClO ４ Water electrolysis was carried out in the same manner except that the electrolytic solution was changed to a mixed solution (pH 11) of a 0.003 M aqueous solution of 0.003 M NaOH and a 0.003 M aqueous solution of 0.003 M NaOH, and the applied voltage during oxygen generation was investigated. The results are shown in Table 8. The stripe spacing d (μm) of the actually fabricated electrode was measured at nine points on an SEM image, and the average value was used. The resonant wave number (ω ｃａｖ ) (cm －１) was calculated using the stripe spacing d (μm) and the refractive index n (=1.33) of water according to the above formula (1).

[0069] The following can be seen from Table 8. Test Nos. 3-1', 3-2', and 3-4' are examples that satisfy the requirements of the embodiment of the present invention, and compared to the case where a smooth electrode (not having a predetermined plurality of microstructures) was used (Test No. 3-11'), the onset potential of oxygen evolution was reduced (i.e., the overvoltage was reduced, for example, by 0.05 V or more). Furthermore, in Test Nos. 3-1', 3-2', and 3-4', the overvoltage was reduced even when the electrolyte did not contain a buffer solution. Furthermore, Test Nos. 3-1' and 3-2' satisfied the preferred requirements of the embodiment of the present invention (1600±400 cm －１ and 3400 ± 500 cm －１ The test samples 3-1' and 3-2' satisfied the more preferable requirements of the embodiment of the present invention (having a resonant wave number within any one or more ranges of the group consisting of: 1600±170 cm －１ and 3400 ± 200 cm －１ The resonance wave number was within one or more ranges of the group consisting of (a) and (b), and the onset potential of oxygen evolution was further reduced (the overvoltage was reduced by, for example, 0.30 V or more).

[0070] In Example 3, the wavelength range of 600 to 4400 cm －１ This is an example of an oxygen evolution reaction electrode having a plurality of microstructures on at least a part of its surface so as to have a resonance wavenumber in the range of . However, it is considered that a hydrogen evolution reaction electrode having a similar microstructure will also have a similar overvoltage reduction effect by interacting with the vibrational polarization of water molecules.

[0071] Furthermore, for the above Test Nos. 3-9 and 3-11, as well as for Test Nos. 3-12 to 3-14 in which the stripe spacing was changed relative to the above Test No. 3-1, etc., as shown in Table 9, the Tafel slopes were determined in the same manner as in Example 1. The results are shown in Table 9 below.

[0072]

[0073] As shown in Table 9, Test Nos. 3-9 and 3-12 to 3-14 are examples that satisfy the requirements of the embodiments of the present invention, and the Tafel slope during oxygen generation was reduced by 20 (mV / decade) or more compared to the case where a smooth electrode without multiple microstructures was used (Test No. 3-11).

[0074] 4. Hydrogen Evolution Reaction Electrode Having Multiple Microstructures First, a substrate (quartz glass substrate) for forming the microstructure was prepared. The substrate was ultrasonically cleaned with acetone for 5 minutes, and then ultrasonically cleaned with methanol, ethanol, and / or ultrapure water for another 5 minutes, as necessary.

[0075] Next, a resist solution (AZP-1350, manufactured by AZ Electronic Materials) was spin-coated onto the substrate (300 rpm for 3 seconds and 3000 rpm for 60 seconds) and heated at 95°C for 90 seconds. To improve wettability, OAP (manufactured by Tokyo Ohka Kogyo Co., Ltd.) or LOR-3A (manufactured by Nippon Kayaku Co., Ltd.) was spin-coated before coating with the resist solution, as necessary. Laser writing was then performed using a laser writing apparatus (Heidelberg DWL66+) in the same manner as in Example 3, so as to form the multiple microstructures shown in Table 10. After laser writing, the substrate was immersed in a developer (NMD-3, manufactured by Tokyo Ohka Kogyo Co., Ltd.) for 60 seconds and then in ultrapure water for 30 seconds. [Laser writing conditions] Focus: -1% Intensity: 100% Laser power: 65 mW Filter: 5%

[0076] After development, Ag was vapor-deposited to a thickness of 200 nm on the substrate with the resist using a helicon sputtering device (MPS-4000C1 / HCl, manufactured by ULVAC). The resist layer was then removed by ultrasonic cleaning in acetone and ethanol, or by immersion in N,N-dimethylacetamide (stripping solution) and heating to 65°C. The resist and stripping solution were then removed by rinsing with acetone or the like as necessary. Furthermore, CHF was used as an etching gas. ３ Etching was carried out using argon (and Ar, if necessary) to completely remove resist residues. After etching, the substrate was washed with Pure Etch CR101 (manufactured by Hayashi Pure Chemical Industries, Ltd.), acetone, etc.

[0077] In this manner, hydrogen evolution reaction electrodes having multiple microstructures were prepared as Test Nos. 4-1 and 4-2. The top view of these electrodes is the same as that of Example 3, shown in FIG. 4. As shown in FIG. 4, multiple stripes extending in the Y-axis direction were arranged at equal intervals in the X-axis direction. Test Nos. 4-1 and 4-2 were prepared by setting the stripe width s to 1.2 μm or 2.0 μm, the stripe length L to 60 μm, and varying the spacing d between the stripes as shown in Table 10 below. For comparison, a smooth electrode (quartz glass substrate / Ag (200 nm)) without multiple nanostructures was prepared as Test No. 4-3.

[0078] The hydrogen evolution reaction electrode prepared as described above was used as the working electrode, platinum was used as the counter electrode, and gold was used as the reference electrode. The potential was calibrated based on the reversible hydrogen electrode (RHE). The electrolyte was 0.5 M NaClO ４ An aqueous solution (pH 7) was used. The working electrode was set as the cathode, and the applied voltage was changed by -5 mV / sec. －２ Electrolysis was carried out under the following weak light at atmospheric pressure, and the applied voltage when bubbles (i.e., hydrogen) were generated was investigated. The investigation was carried out by monitoring the state of the hydrogen generation reaction electrode, and the applied voltage at which gas generation was confirmed with the naked eye was taken as the applied voltage when bubbles were generated. The results are shown in Table 10. The stripe spacing d (μm) of an actually produced electrode was measured at nine points on an SEM image, and the average value was used. The resonance wave number (ω ｃａｖ ) (cm －１ ) was calculated using the stripe spacing d (μm) and the refractive index n (=1.33) of water according to the above formula (1).

[0079]

[0080] The following can be seen from Table 10: Test Nos. 4-1 and 4-2 are examples that satisfy the requirements of the embodiment of the present invention, and compared to the case where a smooth electrode (not having the predetermined plurality of microstructures) was used (Test No. 4-3), the onset potential for hydrogen generation was reduced (i.e., the overvoltage was reduced by, for example, 0.05 V or more).

[0081] Furthermore, the Tafel slopes of the above Test Nos. 4-1 to 4-3 and Test No. 4-4, in which the stripe spacing d was changed to 5.8 μm, were determined in the same manner as in Example 1. The results are shown in Table 11 below.

[0082] As shown in Table 11, Test Nos. 4-1 to 4-2 and 4-4 are examples that satisfy the requirements of the embodiments of the present invention, and the Tafel slope during hydrogen generation was reduced by 20 (mV / decade) or more compared to the case where a smooth electrode without multiple microstructures was used (Test No. 4-3).

[0083] This application claims priority from Japanese Patent Application No. 2023-032065, filed March 2, 2023. Japanese Patent Application No. 2023-032065 is incorporated herein by reference.

Claims

1. An electrode used in a water electrolysis device that generates hydrogen and oxygen by bringing a hydrogen generation reaction electrode and an oxygen generation reaction electrode into contact with water and applying a voltage between the two electrodes (however, this does not include an electrode used to generate H 2 -containing gas from hydrogen isotope-mixed water and obtain water with an increased concentration of D 2 O and / or HDO in the hydrogen isotope-mixed water), The electrochemical cell has, on at least a part of its surface, one or more selected from the group consisting of a plurality of nanostructures and a plurality of microstructures that interact with one or more polarizations selected from the group consisting of electronic polarization of chemical reaction intermediates in water electrolysis and vibrational polarization of water molecules under weak light of 0.01 mWcm −2 or less; An electrode for water electrolysis for performing water electrolysis under weak light of 0.01 mWcm −2 or less, which satisfies at least one of the following (A) to (D): (A) The hydrogen evolution reaction electrode has a plurality of nanostructures on at least a part of its surface so as to have an absorption peak in the energy range of 0.50 to 2.00 eV. (B) The oxygen evolution reaction electrode has a plurality of nanostructures on at least a part of its surface so as to have an absorption peak in the energy range of 0.50 to 2.00 eV. (C) The hydrogen evolution reaction electrode has a plurality of microstructures on at least a part of its surface so as to have a resonance wave number in the range of 600 to 4400 cm −1 . (D) The oxygen evolution reaction electrode has a plurality of microstructures on at least a part of its surface so as to have a resonance wave number in the range of 600 to 4400 cm −1 .

2. An electrode used in a water electrolysis device that generates hydrogen and oxygen by contacting a hydrogen generating reaction electrode and an oxygen generating reaction electrode with water and applying a voltage between the two electrodes (excluding electrodes used to generate H2-containing gas from hydrogen isotope-mixed water and obtain water with an increased concentration of D2O and / or HDO in the hydrogen isotope-mixed water), An electrode for water electrolysis has, on at least a part of its surface, one or more selected from the group consisting of a plurality of nanostructures and a plurality of microstructures that interact with one or more polarizations selected from the group consisting of electronic polarization of chemical reaction intermediates in water electrolysis and vibrational polarization of water molecules under weak light of 0.01 mWcm −2 or less during water electrolysis, and satisfies one or more of the group consisting of the following (A'), (B'), (C) and (D): (A') The hydrogen evolution reaction electrode has a plurality of nanostructures on at least a part of its surface so as to have an absorption peak in the energy range of 0.50 to 1.34 eV. (B') The oxygen evolution reaction electrode has a plurality of nanostructures on at least a part of its surface so as to have an absorption peak in the energy range of 0.50 to 1.44 eV. (C) The hydrogen evolution reaction electrode has a plurality of microstructures on at least a part of its surface so as to have a resonance wave number in the range of 600 to 4400 cm −1 . (D) The oxygen evolution reaction electrode has a plurality of microstructures on at least a part of its surface so as to have a resonance wave number in the range of 600 to 4400 cm −1 .

3. (A) or (A') has an absorption peak in the energy range of 0.50 to 0.80 eV, (B) or (B') has an absorption peak in the energy range of 1.00 to 1.44 eV, The electrode for water electrolysis according to claim 1 or 2, wherein (C) and (D) have resonance wavenumbers within one or more ranges selected from the group consisting of 1600±400 cm−1 and 3400±500 cm−1.

4. The water electrolysis electrode according to claim 1 or 2, wherein the nanostructure comprises a metal.

5. The water electrolysis electrode according to claim 1 or 2, wherein the microstructure comprises a metal.

6. 3. The water electrolysis electrode according to claim 1, wherein a difference in gas generation onset potential between a portion of the water electrolysis electrode having any one or more selected from the group consisting of a plurality of nanostructures and a plurality of microstructures and a smooth electrode having the same catalytic layer configuration is 0.01 V or more.

7. 3. The water electrolysis electrode according to claim 1, wherein a difference in Tafel slope between a portion of the water electrolysis electrode having any one or more selected from the group consisting of a plurality of nanostructures and a plurality of microstructures and a smooth electrode having the same catalytic layer configuration is 20 mV / decade or more.

8. A method for controlling overvoltage, comprising performing water electrolysis under weak light of 0.01 mWcm −2 or less using the electrode for water electrolysis according to claim 1 .

9. A method for controlling overvoltage in which water electrolysis is performed using the water electrolysis electrode described in claim 2.