Electrode catalyst for water electrolysis, electrochemical reaction device for water electrolysis, membrane electrode assembly for water electrolysis, alcohol synthesis device, method for manufacturing a structure, method for manufacturing an electrode catalyst for water electrolysis, and method for activating an electrode catalyst for water electrolysis

Abstract

To provide a catalyst capable of electrolyzing water in high activity.SOLUTION: A complex comprises a material having electric conductivity and an oxide of a transition metal supported on the material. The oxide of the transition metal includes an amorphous structure.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention provides Electrode for water electrolysis, electrochemical reactor for water electrolysis, membrane electrode assembly for water electrolysis, alcohol synthesis device, method for manufacturing a structure, method for manufacturing an electrode catalyst for water electrolysis, and method for activating an electrode catalyst for water electrolysis Regarding. [Background technology]

[0002] In recent years, the use of renewable energy sources has been considered as a means of reducing carbon dioxide emissions. Because it is difficult to store renewable energy generated as electricity from sources such as solar and wind power, methods using hydrogen, ammonia, and methylcyclohexane as energy carriers have been proposed as a means of storing surplus energy from renewable energy sources. In either case, it is necessary to obtain hydrogen from the surplus energy. One efficient method for obtaining hydrogen is known to be the steam reforming method from fossil fuels such as hydrocarbons. However, considering global environmental issues, it is important to obtain hydrogen through methods that do not rely on fossil resources.

[0003] Water electrolysis is known as such a method. Water electrolysis is an industrially established method for producing hydrogen, similar to steam reforming methods using fossil fuels. However, with the rise of petrochemicals, steam reforming of hydrocarbons has become more economically advantageous. Under these circumstances, technological developments are underway to utilize electrolysis industrially as a hydrogen production method, including high-temperature, high-pressure water electrolysis, solid polymer electrolyte (SPE) electrolysis, and high-temperature steam electrolysis.

[0004] The voltage of a water electrolysis cell is the sum of the theoretical electrolysis voltage, overvoltage due to the resistance of the reaction at the electrodes, and ohmic losses due to the electrical resistance of the electrolyte and diaphragm. The theoretical electrolysis voltage is the voltage (1.23 V) calculated from the amount of electricity required for electrolysis, the amount of electricity required, and the enthalpy change. In order to lower the voltage of a water electrolysis cell, it is possible to reduce any of the theoretical electrolysis voltage, overvoltage, or ohmic losses. Of these, the theoretical electrolysis voltage can be calculated, but the optimal value for ohmic losses can be found by modifying the cell design, and the overvoltage can be reduced by using electrodes with high catalytic activity that have a strong ability to promote electrochemical reactions.

[0005] Patent Document 1 discloses a technique in which water is electrolyzed at an anode using an iridium oxide catalyst, the generated protons are supplied to a cathode, and carboxylic acids are hydrogenated at the cathode to generate alcohol.

[0006] Non-Patent Document 1 investigated the electronic structure of electrodeposited iridium oxide films and revealed that both Ir(III) and Ir(V) oxidation states exist during the water electrolysis reaction.

[0007] Non-Patent Document 2 describes the evaluation of the anode performance of iridium oxide (T-IrO2) produced by calcination at 500°C, metallic iridium (A-Ir) obtained by further reducing this at 500°C, and E-Ir obtained by electrochemically oxidizing this. The study describes that Ir(III) is present in the region where the oxygen evolution reaction occurs, that there is more Ir(III) in E-Ir than in T-IrO2, and that E-Ir is amorphous, with many Ir atoms exposed on the porous surface. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2017 / 154743 [Non-patent literature]

[0009] [Non-Patent Document 1] Alessandro Miguzzi,Ottavio Lugarsei,Elisabetta Achilli,Cristina Locatelli,Alberto Vetova,Paolo Ghigna and Sandra Rondinini,Chem.Scil,2014,5,3591-3597 [Non-patent document 2] Primoz Jovanovic, Nejc Hodnik, Francisco Ruiz-Zepeda, Iztok Arcon, Barbara Jozinovic, Milena Zorko, Marjan Bele, Martin Sala, Vid Simon Selih, Samo Hocevar and Miran Gaberseck, J.Am,Chem.Soc., 2017 139,12837-12846 Summary of the Invention [Problem to be solved by the invention]

[0010] In the method disclosed in Patent Document 1, hydrogen can be produced by electrolyzing water using an iridium oxide catalyst, but the water electrolysis activity of this iridium oxide catalyst is not necessarily sufficiently high, and there has been a demand for the development of a catalyst that can electrolyze water with higher activity.

[0011] In general, some transition metal oxides have water electrolysis activity. For example, in the case of iridium oxide, it is clear that the valence of some Ir atoms changes during the oxygen evolution reaction in electrolysis. However, no practical electrode capable of stable and efficient oxygen evolution has been developed by focusing on the relationship between the operating environment (applied voltage, temperature, electrolyte concentration, etc.) and the electronic and morphological structure of Ir.

[0012] The present invention has been made in view of the above circumstances, and has as its object to provide a catalyst capable of electrolyzing water with high activity. [Means for solving the problem]

[0013] The inventors discovered that when a voltage is applied to an electrode containing an electrically conductive material and a transition metal oxide, the oxidation number of the metal changes, and that when the voltage application is released, the oxidation number changes again, and that at this time the transition metal oxide becomes amorphous. Furthermore, when this electrode is used as an anode for water electrolysis, overvoltage is significantly reduced, and thus completed the present invention. [1] A composite comprising an electrically conductive material and a transition metal oxide supported on the material, wherein the transition metal oxide has an amorphous structure. [2] The composite according to [1], wherein the oxidation number of the transition metal oxide changes flexibly and reversibly in response to an applied voltage. [3] The composite according to [1] or [2], wherein the electrically conductive material and the transition metal oxide are bridged via oxygen. [4] The composite according to any one of [1] to [3] above, wherein oxygen atoms are observed between the electrically conductive material and the transition metal oxide. [5] The composite according to any one of [1] to [4], wherein a hydroxyl group is present on the surface of the electrically conductive material. [6] The composite according to any one of [1] to [5], wherein a hydroxyl group is present on the oxide of the transition metal. [7] The composite according to any one of [1] to [5], wherein a double bond structure between a metal atom and an oxygen atom (metal=O) is present on the oxide of the transition metal. [8] The composite according to any one of [1] to [7], wherein the transition metal is at least one transition metal of Groups 8 to 10 of the periodic table. [9] The composite according to any one of [1] to [8], wherein the transition metal oxide is in the form of particles having an average particle size of 100 nm or less as determined from a transmission electron microscope (TEM) image.

[10] The composite according to any one of [1] to [9], wherein the transition metal oxide contains lattice defects.

[11] The composite according to any one of [1] to

[10] , wherein the electrically conductive material is at least one selected from the group consisting of carbonaceous materials and metal compounds.

[12] The electrical conductivity of the electrically conductive material is 1×10 -14 Scm -2 The complex according to any one of [1] to

[11] above.

[13] The composite according to

[11] or

[12] , wherein the metal compound used in the electrically conductive material is titanium oxide.

[14] The composite according to

[13] , wherein the titanium oxide has an anatase crystal structure.

[15] A catalyst comprising an electrically conductive material and a transition metal oxide supported on the material, wherein the transition metal oxide includes a complex having an amorphous structure.

[16] The catalyst according to

[15] , wherein the transition metal oxide has an oxidation number that changes flexibly and reversibly in response to an applied voltage.

[17] The catalyst according to

[15] or

[16] , wherein the electrically conductive material and the transition metal oxide are bridged via oxygen.

[18] The catalyst according to any one of

[15] to

[17] , wherein oxygen atoms are observed between the electrically conductive material and the transition metal oxide.

[19] The catalyst according to any one of

[15] to

[18] , wherein a hydroxyl group is present on the surface of the electrically conductive material.

[20] The catalyst according to any one of

[15] to

[19] , wherein a hydroxyl group is present on the oxide of the transition metal.

[21] The catalyst according to any one of

[15] to

[20] , wherein a double bond structure between a metal atom and an oxygen atom (metal=O) is present on the oxide of the transition metal.

[22] The catalyst according to any one of

[15] to

[21] , wherein the transition metal is at least one transition metal of Groups 8 to 10 of the periodic table.

[23] The catalyst according to any one of

[15] to

[22] , wherein the transition metal oxide is in the form of particles having an average particle size of 100 nm or less as determined from a transmission electron microscope (TEM) image.

[24] The catalyst according to any one of

[15] to

[23] , wherein the transition metal oxide contains lattice defects.

[25] The catalyst according to any one of

[15] to

[24] , wherein the electrically conductive material is at least one selected from the group consisting of carbonaceous materials and metal compounds.

[26] The electrical conductivity of the electrically conductive material is 1×10 -14 Scm -2 The catalyst according to any one of

[15] to

[25] above.

[27] The catalyst according to

[25] or

[26] , wherein the metal compound used in the electrically conductive material is titanium oxide.

[28] The catalyst according to

[27] , wherein the titanium oxide has an anatase crystal structure.

[29] A structure in which a composite comprising an electrically conductive material and a transition metal oxide supported on the material is supported on an electrically conductive substrate, wherein the transition metal is at least one transition metal from Groups 8 to 10 of the periodic table, the transition metal oxide has an amorphous structure, and the substrate is a porous material.

[30] The structure according to

[29] , wherein the transition metal oxide changes flexibly and reversibly in response to an applied voltage.

[31] The structure according to

[29] or

[30] , wherein the electrically conductive material and the transition metal oxide are bridged via oxygen.

[32] The structure according to any one of

[29] to

[31] , wherein oxygen atoms are observed between the electrically conductive material and the transition metal oxide.

[33] The structure according to any one of

[29] to

[32] , wherein a hydroxyl group exists on the surface of the electrically conductive material.

[34] The structure according to any one of

[29] to

[33] , wherein a hydroxyl group is present on the oxide of the transition metal.

[35] The structure according to any one of

[29] to

[34] , wherein a double bond structure between a metal atom and an oxygen atom (metal=O) is present on the oxide of the transition metal.

[36] The structure according to any one of

[29] to

[35] , wherein the transition metal oxide is in the form of particles having an average particle size of 100 nm or less as determined from a transmission electron microscope (TEM) image.

[37] The structure according to any one of

[29] to

[36] , wherein the oxide of the transition metal contains lattice defects.

[38] The structure according to any one of

[29] to

[37] , wherein the electrically conductive material is at least one selected from the group consisting of carbon-based materials and metal compounds.

[39] The electrical conductivity of the electrically conductive material is 1×10 -14 Scm -2 The structure according to any one of

[29] to

[38] above.

[40] The structure according to

[38] or

[39] , wherein the metal compound used in the electrically conductive material is titanium oxide.

[41] The structure according to

[40] , wherein the titanium oxide has an anatase crystal structure.

[42] A catalyst comprising a composite comprising an electrically conductive material and an oxide of a transition metal supported on the material, the composite being supported on an electrically conductive substrate, wherein the transition metal is at least one of transition metals of Groups 8 to 10 of the periodic table, the oxide of the transition metal has an amorphous structure, and the substrate is a porous material.

[43] The catalyst according to

[42] , wherein the transition metal oxide changes flexibly and reversibly in response to an applied voltage.

[44] The catalyst according to

[42] or

[43] , wherein the electrically conductive material and the transition metal oxide are bridged via oxygen.

[45] The catalyst according to any one of

[42] to

[44] above, wherein oxygen atoms are observed between the electrically conductive material and the transition metal oxide.

[46] The catalyst according to any one of

[42] to

[45] , wherein a hydroxyl group is present on the surface of the electrically conductive material.

[47] The catalyst according to any one of

[42] to

[46] , wherein a hydroxyl group is present on the oxide of the transition metal.

[48] ​​The catalyst according to any one of

[42] to

[47] , wherein a double bond metal=O (metal carbonyl) structure of a metal atom and an oxygen atom is present on the oxide of the transition metal.

[49] The catalyst according to any one of

[42] to

[48] , wherein the transition metal oxide is in the form of particles having an average particle size of 100 nm or less as determined from a transmission electron microscope (TEM) image.

[50] The catalyst according to any one of

[42] to

[49] , wherein the transition metal oxide contains lattice defects.

[51] The catalyst according to any one of

[42] to

[50] , wherein the electrically conductive material is at least one selected from the group consisting of carbonaceous materials and metal compounds.

[52] The electrical conductivity of the electrically conductive material is 1×10 -14 Scm -2 The catalyst according to any one of

[42] to

[51] above.

[53] The catalyst according to

[51] or

[52] , wherein the metal compound used in the electrically conductive material is titanium oxide.

[54] The catalyst according to

[53] , wherein the titanium oxide has an anatase crystal structure.

[55] The catalyst according to any one of

[42] to

[54] , wherein the catalyst is a catalyst for electrolysis of water.

[56] An electrode catalyst comprising a composite comprising an electrically conductive material and an oxide of a transition metal supported on the material, the composite being supported on an electrically conductive substrate, wherein the transition metal is at least one of transition metals of Groups 8 to 10 of the periodic table, the oxide of the transition metal has an amorphous structure, and the substrate is a porous material.

[57] The electrode catalyst according to

[56] , wherein the transition metal oxide changes flexibly and reversibly in response to an applied voltage.

[58] The electrode catalyst according to

[56] or

[57] , wherein the electrically conductive material and the transition metal oxide are bridged via oxygen.

[59] The electrode catalyst according to any one of

[56] to

[58] , wherein oxygen atoms are observed between the electrically conductive material and the transition metal oxide.

[60] The electrode catalyst according to any one of

[56] to

[59] , wherein a hydroxyl group is present on the surface of the electrically conductive material.

[61] The electrode catalyst according to any one of

[56] to

[60] , wherein a hydroxyl group is present on the oxide of the transition metal.

[62] The electrode catalyst according to any one of

[56] to

[61] , wherein a double bond structure between a metal atom and an oxygen atom (metal=O) is present on the oxide of the transition metal.

[63] The electrode catalyst according to any one of

[56] to

[62] , wherein the transition metal oxide is in the form of particles having an average particle size of 100 nm or less as determined from a transmission electron microscope (TEM) image.

[64] The electrode catalyst according to any one of

[56] to

[63] , wherein the transition metal oxide contains lattice defects.

[65] The electrode catalyst according to any one of

[56] to

[64] , wherein the electrically conductive material is at least one selected from the group consisting of carbonaceous materials and metal compounds.

[66] The electrical conductivity of the electrically conductive material is 1×10 -14 Scm -2 The electrode catalyst according to any one of

[56] to

[65] above.

[67] The electrode catalyst according to

[65] or

[66] , wherein the metal compound used in the electrically conductive material is titanium oxide.

[68] The electrode catalyst according to

[67] , wherein the titanium oxide has an anatase crystal structure.

[69] The electrode catalyst according to any one of

[56] to

[68] , which is an anode.

[70] An electrochemical reaction device using the electrode according to any one of

[56] to

[68] as an anode.

[71] The electrochemical reaction device according to

[70] , comprising: a standard electrode; a reaction vessel connected between the electrode and including a water-containing electrolyte, the electrode that oxidizes water to generate oxygen, and the standard electrode; and a potential varying device that can sweep an applied voltage in a range of −3.0 V to 1.0 V with respect to an onset potential between the electrode and the standard electrode, one or more times.

[72] A membrane electrode assembly comprising the anode according to

[69] , which comprises a structure in which a composite comprising an electrically conductive material and an oxide of a transition metal supported on the material is supported on an electrically conductive substrate; a cathode; and an electrolyte membrane provided between the anode and the cathode.

[73] The membrane electrode assembly according to

[72] , which is used for alcohol synthesis.

[74] An alcohol synthesis apparatus comprising the membrane electrode assembly according to

[72] or

[73] , a first supply means for supplying water or water vapor to the anode, a second supply means for supplying a carboxylic acid to the cathode, and a means for recovering alcohol produced at the cathode.

[75] The alcohol synthesis apparatus according to

[74] , comprising: a standard electrode; a measurement system for measuring an onset voltage of the anode with respect to the standard electrode; and a potential varying device capable of sweeping an applied voltage in a range of −3.0 V to 1.0 V with respect to the onset potential between the anode and the standard electrode one or more times.

[75] A method for producing a structure according to any one of

[29] to

[41] , comprising the steps of immersing an electrically conductive material in a solution of a precursor of a transition metal oxide, and heating the solution in which the material is immersed.

[76] A method for producing an electrode catalyst according to any one of

[56] to

[69] , comprising the steps of: treating a precursor of an oxide of a transition metal with an aqueous solution of an alkali metal or an aqueous solution of a polyhydric alcohol to obtain a transition metal; applying the resulting transition metal to a solid electrolyte membrane; and bonding the resulting membrane to a substrate carrying an electrically conductive material.

[77] The method for producing an electrode catalyst according to

[76] , wherein the electrically conductive material is titanium oxide, the transition metal oxide is iridium oxide, and the porous electrically conductive substrate is titanium.

[78] A method for activating an electrocatalyst, comprising sweeping an applied voltage in a range of -3.0 V to 1.5 V with respect to an onset potential at least once in a system of an electrocatalyst according to any one of

[56] to

[69] and a standard electrode placed in an electrolytic solution. [Effects of the Invention]

[0014] The catalyst of the present invention has high activity in the electrolysis reaction of water. The catalyst of the present invention can be suitably used as an electrode. The catalyst of the present invention is characterized in that the oxidation number of the metal contained in the transition metal oxide changes flexibly depending on the applied voltage. When the transition metal oxide is iridium oxide, the oxidation number of iridium changes flexibly depending on the applied voltage, and an oxidation number of +4 or higher appears. A decrease in overvoltage in the electrolysis reaction of water has been observed due to iridium oxide being in a high valence state, confirming that the high valence state of iridium oxide contributes to improved activity. [Brief explanation of the drawings]

[0015] [Figure 1] Figure 1(a) is a schematic diagram of the structure of the IrOx / TiO2-Ti mesh catalyst, Figure 1(b) is a schematic diagram of a localized enlargement of the TiO2 needle-shaped bodies of the IrOx / TiO2-Ti mesh catalyst, and Figure 1(c) is a schematic diagram of an enlargement of IrOx nanoparticles on the surface of the TiO2 needle-shaped bodies. [Figure 2] FIG. 2 is a schematic diagram showing the structure of an alcohol electrolytic synthesis cell. [Figure 3] Figure 3(a) is an SEM image of the TiO2-Ti mesh, Figure 3(b) is an SEM image of the IrOx / TiO2-Ti mesh catalyst, and Figure 3(c) is a high-resolution TEM image of the IrOx-TiO2-Ti mesh catalyst. [Figure 4] Figure 4(a) shows the cyclic voltammogram (CV) curve of the IrOx / TiO2-Ti mesh catalyst, and Figure 4(b) shows the Tafel plot of the IrOx / TiO2-Ti mesh catalyst (IrOx loading: 0.58 mg / cm2) at a sweep rate of 10 mV / s. [Figure 5]Figure 5(a) shows the cyclic voltammogram (CV) curves of the IrOx / TiO2-Ti mesh catalyst after different calcination treatments. Figure 5(b) shows the correlation between the OER performance and the OH concentration. Figure 5(c) shows the high-resolution XPS spectra in the O 1s orbital of the IrOx / TiO2-Ti mesh catalyst after different calcination treatments. [Figure 6] Figure 6 shows the setup for the in situ measurements. [Figure 7] Figure 7 shows the XANES spectra of the electrode sample and the standard sample at the Ir L3 absorption edge. [Figure 8] Figure 8 shows the EXAFS spectra after Fourier transformation of the electrode sample and the standard sample at the Ir L3 absorption edge. [Figure 9] Figure 9 shows the XANES spectra after Fourier transformation of the electrode sample and the standard sample at the Ti-K absorption edge. [Figure 10] Figure 10 shows the EXAFS spectra of the electrode sample and the standard sample at the Ti-K absorption edge. [Figure 11] Figure 11 shows the XANES spectra of the Ir-L3 absorption edge of the IrOx / TiO2-Ti mesh (unfired and after activation treatment) with and without applied voltage. [Figure 12] FIG. 12 shows the applied voltage dependence of the inflection point in the XANES spectrum of the Ir-L3 absorption edge X of the IrOx / TiO2-Ti mesh (unfired / after activation treatment). [Figure 13] Figure 13 shows the EXAFS spectra after Fourier transformation of the Ir-L3 absorption edge of the IrOx / TiO2-Ti mesh (unfired and after activation treatment) with and without applied voltage. [Figure 14] FIG. 14 shows the applied voltage dependence of the Ir-O interatomic distance in the EXAFS spectrum after Fourier transformation of the Ir-L3 absorption edge of the IrOx / TiO2-Ti mesh (unfired / after activation treatment). [Figure 15]Figure 15 shows the XANES spectra of the Ir-L3 absorption edge of the IrOx / TiO2-Ti mesh (after firing and activation treatment) with and without applied voltage. [Figure 16] FIG. 16 shows the applied voltage dependence of the inflection point in the XANES spectrum at the Ir-L3 absorption edge of IrOx / TiO2 (after firing and activation treatment). [Figure 17] Figure 17 shows the Fourier-transformed EXAFS spectra of the Ir-L3 absorption edge of the IrOx / TiO2-Ti mesh (after firing and activation treatment) with and without applied voltage. [Figure 18] FIG. 18 shows the applied voltage dependence of the Ir-O interatomic distance in the EXAFS spectrum after Fourier transformation of the Ir-L3 absorption edge of the IrOx / TiO2-Ti mesh (after firing and activation treatment). [Figure 19] FIG. 19 is a diagram showing the applied potential dependence of the inflection point in the XANES spectrum of the Ir-L3 absorption edge of each electrode sample. [Figure 20] FIG. 20 is a diagram showing the applied voltage dependence of the Ir-O interatomic distance in the EXAFS spectrum after Fourier transformation of the Ir L3 absorption edge of each electrode sample. [Figure 21] Figure 21(a) is a TEM image of TiO2 (commercial product), and Figure 21(b) is a TEM image of TiO2-lab. [Figure 22] Figure 22 shows photographs of IrOx / Ti and a commercially available DSE. [Figure 23] Figure 23 compares the OER performance of IrOx / C, IrOx / Ti, and a carbon-supported electrode (IrO2-wako / C) prepared using commercially available IrO2 (0.2 M Na2SO4 aq). [Figure 24] FIG. 24 is a graph comparing the OER characteristics of IrOx-Ti and a commercially available product. [Figure 25] FIG. 25 shows a comparison of the performance of IrOx-Ti (shown as IrO2 (Yamauchi)) and a commercially available DSE when used in 25PEAEC. [Figure 26]FIG. 26 is a diagram showing the OER characteristics of IrOx—Ti. [Figure 27] FIG. 27(a) is a diagram showing a method for synthesizing IrO2 particles via hydroxide, and FIG. 27(b) is a diagram showing a method for synthesizing IrO2 particles via metallic Ir particles. [Figure 28] FIG. 28 shows the XRD patterns of IrO2-large, IrO2-small, and IrO2-metal. [Figure 29] FIG. 29 shows TEM images of IrO2-large, IrO2-small, and IrO2-metal. [Figure 30] FIG. 30 shows the XRD patterns of Ir-RuO2-small and Ir-RuO2-metal. [Figure 31] FIG. 31 is a schematic diagram showing the structure of PEAEC used in the evaluation. [Figure 32] FIG. 32 is a diagram showing the change in current density over time. [Figure 33] FIG. 33 is a graph showing the conversion rate of oxalic acid, the faradaic efficiency of glycolic acid production, and the faradaic efficiency of glyoxylic acid production at each potential. [Figure 34] FIG. 34 is a diagram showing the change in current density over time. [Figure 35] FIG. 35 is a graph showing the conversion rate of oxalic acid, the faradaic efficiency of glycolic acid production, and the faradaic efficiency of glyoxylic acid production at each potential. [Figure 36] FIG. 36 is a diagram showing the change in current density over time. [Figure 37] FIG. 37 is a graph showing the conversion rate of oxalic acid, the faradaic efficiency of glycolic acid production, and the faradaic efficiency of glyoxylic acid production at each potential. [Figure 38] FIG. 38 is a diagram showing the change in current density over time. [Figure 39] FIG. 39 is a graph showing the conversion rate of oxalic acid, the faradaic efficiency of glycolic acid production, and the faradaic efficiency of glyoxylic acid production at each potential. [Figure 40]FIG. 40 is a diagram showing the change in current density over time. [Figure 41] FIG. 41 is a graph showing the conversion rate of oxalic acid, the faradaic efficiency of glycolic acid production, and the faradaic efficiency of glyoxylic acid production at each potential. [Figure 42] FIG. 42 is a diagram showing the change in current density over time. [Figure 43] FIG. 43 is a graph showing the conversion rate of oxalic acid, the faradaic efficiency of glycolic acid production, and the faradaic efficiency of glyoxylic acid production at each potential. [Figure 44] FIG. 44 is a diagram showing the change in current density over time. [Figure 45] FIG. 45 is a graph showing the conversion rate of oxalic acid, the faradaic efficiency of glycolic acid production, and the faradaic efficiency of glyoxylic acid production at each potential. [Figure 46] FIG. 46 is a diagram showing the change in current density over time. [Figure 47] FIG. 47 is a graph showing the conversion rate of oxalic acid, the faradaic efficiency of glycolic acid production, and the faradaic efficiency of glyoxylic acid production at each potential. [Figure 48] FIG. 48 is a schematic diagram showing the structure of PEAEC. DETAILED DESCRIPTION OF THE INVENTION

[0016] In the present invention, "flexibly changing oxidation state" means that the oxidation number of the transition metal changes when a voltage is applied to the composite of the present invention, and the oxidation number changes again when the voltage application is released. In many cases, the oxidation number changes in the opposite direction between when a voltage is applied and when the voltage is released. In the present invention, this change exhibits reversible potential responsiveness. In this specification, the structure of a catalyst supported on a carrier is referred to as "catalyst / carrier," and the structure in which this is attached to a substrate such as an electrode is referred to as "catalyst / carrier-substrate." Furthermore, catalyst / carrier may be referred to as "catalyst," and catalyst / carrier-substrate may be referred to as catalyst. Furthermore, when the substrate is an electrode, it may be referred to as an electrode catalyst.

[0017] [Complex] The composite of the present invention comprises an electrically conductive material and a transition metal oxide supported on the material, the transition metal oxide having an amorphous structure.

[0018] The composite of the present invention comprises an electrically conductive material carrying a component containing an oxide of a transition metal of Groups 8 to 10 of the periodic table, and the oxidation number of the metal contained in the oxide of the transition metal changes flexibly and reversibly in response to an applied voltage.

[0019] The composite may be formed by simply contacting an electrically conductive material with an oxide of a transition metal, or by contacting them and causing a chemical reaction by heating or the like. In the composite of the present invention, oxygen atoms are observed between the transition metal oxide and the material. These oxygen atoms may originate from either the transition metal oxide or the material. The inventors believe that these oxygen atoms contribute to maintaining the overall charge balance when the oxidation number of the transition metal changes under potential. Although it is difficult to clearly determine the structure of the oxygen atoms present between a conductive material and a transition metal oxide, analyses such as XPS have revealed the presence of hydroxyl groups on the surface of the transition metal oxide and double bonds between metal atoms and oxygen atoms (metal = O) on the transition metal oxide. For example, computational chemistry techniques have been able to show the presence of a bridge structure such as a Ti-OHO-Ir bond between titanium oxide and iridium oxide, suggesting that the oxygen present between the transition metal oxide and the material may be bonding them together. The composite of the present invention functions as a catalyst and also as an electrode.

[0020] (electrically conductive materials) The electrically conductive material is not particularly limited, and examples thereof include carbon-based materials such as acetylene black, Ketzen Black (registered trademark), carbon nanotubes, graphite, and graphene, transition metals such as Ni, V, Ti, Co, Mo, Fe, Cu, Zn, Sn, W, and Zr, and oxides, carbides, nitrides, and nitride-supported metals thereof. These materials may be used alone or in combination of two or more.

[0021] The electrical conductivity of the electrically conductive material is preferably 1×10 -14 Scm -2 or more, more preferably 1×10 -12 Scm -2 More preferably, 5 × 10 -12 Scm -2 When the electrical conductivity of the material satisfies the above conditions, electrons are transferred favorably in the catalytic reaction, improving activity.

[0022] The electrically conductive material is preferably a metal oxide, and in the case of a metal oxide, it is more preferable that the metal oxide has hydroxyl groups on the surface.

[0023] When the electrically conductive material is a metal oxide having hydroxyl groups on its surface, the electrically conductive material and the transition metal oxide are more likely to form a structure via oxygen, and when the oxidation number of the transition metal contained in the transition metal oxide changes, the hydroxyl groups of the material maintain the electrical neutrality of the transition metal and the high oxidation state of the transition metal, thereby making it possible to more flexibly change the oxidation number of the transition metal.

[0024] An example of such a metal oxide material is a material made of titanium oxide, which preferably has an anatase crystal structure, and more preferably has hydroxyl groups on the surface.

[0025] When the electrically conductive material is used as a carrier, the shape is not particularly limited, and examples thereof include granular, needle-like, tubular, and sheet-like shapes. Furthermore, when a metal is used as the substrate of the present invention, the metal often forms a metal oxide film in the air, and such metal oxide can be used as a carrier if it has electrical conductivity. Examples of such metals include titanium, and examples of metal oxides include titanium oxide. In such cases, a thinner titanium oxide is preferable.

[0026] (Oxides of transition metals) In the composite of the present invention, the material supported on the catalyst is an oxide of a transition metal of Groups 8 to 10 of the periodic table. When the material supports an oxide of a transition metal of Groups 8 to 10 of the periodic table, particularly high catalytic activity can be obtained.

[0027] The transition metal oxide of the present invention has the characteristic that the oxidation number of the transition metal in the transition metal oxide changes flexibly depending on the applied voltage. This characteristic will be described in detail in the Examples. This characteristic is involved in the high catalytic activity of the composite of the present invention. In this case, the transition metal oxide has an amorphous structure.

[0028] Suitable examples of the oxide of a transition metal of Groups 8 to 10 of the periodic table include oxides of one or more metals selected from the group consisting of ruthenium, iridium, platinum, and palladium.

[0029] The component supported on the material may contain a transition metal of Groups 8 to 10 of the periodic table in addition to an oxide of the transition metal. Suitable examples of the transition metal of Groups 8 to 10 of the periodic table include one or more metals selected from the group consisting of ruthenium, iridium, rhodium, platinum, and palladium.

[0030] The transition metal oxide supported on the material preferably has an amorphous structure. As described above, the composite has a characteristic in that the oxidation number of the transition metal contained in the transition metal oxide changes flexibly depending on the applied voltage. If the supported transition metal oxide has a crystalline structure, the structure of the transition metal oxide is less likely to be maintained when the oxidation number of the transition metal contained in the transition metal oxide changes, making it difficult for the oxidation number of the transition metal to change flexibly. On the other hand, if the supported transition metal oxide has an amorphous structure, the structure of the transition metal oxide is maintained even when the oxidation number of the transition metal contained in the transition metal oxide changes, making it easier for the oxidation number of the transition metal to change flexibly.

[0031] Such a transition metal oxide having an amorphous structure is produced by an electrochemical reaction, which will be described later. On the other hand, when the obtained catalyst is calcined, the transition metal oxide contained in the catalyst is thought to have a crystalline structure.

[0032] The component carried on the material preferably contains lattice defects. The component supported on the material is preferably, for example, ruthenium oxide (RuO), rhodium oxide (RhO), platinum oxide (PtO), or palladium oxide (PdO), with metallic iridium, iridium oxide (IV: iridium dioxide (IrO)), iridium oxide (III: iridium trioxide (IrO)), and / or mixtures thereof being more preferred. Furthermore, the material may include oxides having a highly disordered, irregular crystal structure, distinct from bulk iridium oxide. When the component supported on the material satisfies the above conditions, the oxidation state of the transition metal can be easily changed flexibly.

[0033] The average particle size obtained from a transmission electron microscope (TEM) image is usually 1000 nm or less, preferably 100 nm or less, more preferably 10 nm or less, and even more preferably 5 nm or less. The smaller the average particle size, the higher the catalytic activity. There is no particular lower limit for the average particle size of the catalyst component, and it is, for example, 2 nm. When the average particle size is in the above-mentioned preferred range, particularly high catalytic activity can be obtained.

[0034] The amount of the component supported on the material is preferably 0.01% by mass to 50% by mass, more preferably 0.1% by mass to 30% by mass, and even more preferably 0.2% by mass to 20% by mass, relative to the mass of the material. When the amount of the component supported is within the above range, particularly high catalytic activity can be obtained.

[0035] The transition metal oxide contained in the component is preferably capable of assuming a higher oxidation state than that of a normal oxide when a voltage of 1.23 V or higher is applied to a reversible hydrogen electrode (RHE) under acidic conditions. For example, when iridium oxide is used as the transition metal oxide, iridium oxide can assume an oxidation number of +4 or higher, i.e., a pentavalent or hexavalent oxidation number. The oxidation number can be determined, for example, using X-ray absorption fine structure (XAFS) analysis.

[0036] (Electrocatalysis and its reactions) The composite can be used as an electrode catalyst by being supported on a substrate that serves as an electrode. When the composite is used as an electrode catalyst, the electrically conductive material functions as a support, and the transition metal oxide or the like supported on the material functions as a catalytic component. When the composite is supported on a substrate, it becomes an electrode catalyst.

[0037] The electrode catalyst used in the present invention can be a structure having a porous structure as an electrode, an electrically conductive substrate, the substrate holding the electrically conductive material as a carrier, and the carrier supporting the transition metal oxide as a catalytic component. Furthermore, a porous structure generally refers to a material with a large number of open pores, and is classified into microporous materials, mesoporous materials, macroporous materials, etc. depending on the size of the pores. However, the substrate having a porous structure of the present invention also includes sheet-like or mesh-like carbon materials and metal materials. When the catalyst has a substrate, the substrate, carrier, and catalytic component are layered in this order in the thickness direction of the catalyst.

[0038] Furthermore, in a preferred embodiment of the catalyst, the substrate having a porous structure is a Ti mesh, the support is anatase-type TiO2, and iridium oxide particles are supported on the support as a catalytic component by IrO x An example of such a catalyst is a TiO2-Ti mesh catalyst. Here, x is unknown and is therefore set to x. Here, 0≦x≦3. This catalyst is produced, for example, by immersing a TiO2-Ti mesh composite prepared by a two-stage hydrothermal synthesis method in an IrCl3 solution under a N2 atmosphere for typically 1 to 24 hours, preferably 3 to 6 hours, and then heating the mixture at typically 55°C to 200°C, preferably 80°C to 200°C, more preferably 100°C to 200°C, and even more preferably 120°C to 150°C. By adjusting the concentration of the IrCl3 solution, the catalyst component IrO x The method for producing the catalyst of the present invention will be specifically described in detail in the Examples.

[0039] IrO obtained by this method x The TiO2-Ti mesh catalyst has needle-shaped TiO2 held on the Ti mesh, and IrO x Nanoparticles are supported. IrO x An example of the structure of the IrO / TiO2-Ti mesh catalyst is shown in Figure 1. Figure 1(a) shows the structure of the IrO x Schematic diagram of the structure of the TiO2-Ti mesh catalyst.x Fig. 1(c) shows the IrO on the surface of the TiO needles. x FIG. 1 is an enlarged schematic diagram of a nanoparticle.

[0040] Such IrO x In the TiO2-Ti mesh catalyst, the catalytic component IrO x The amount of supported titanium per unit area of ​​the Ti mesh is 0.1 mg / cm 2 ~1mg / cm 2 is preferably 0.4 mg / cm 2 ~0.6mg / cm 2 It is more preferable that:

[0041] Catalysts containing catalytic components such as iridium oxide are generally calcined before use, but as described above, the catalyst of the present invention is preferably a non-calcined product from the viewpoint of making the transition metal oxide contained in the catalytic component have an amorphous structure. However, in the present invention, the catalyst as an electrode catalyst can be made to have an amorphous structure by changing the applied voltage.

[0042] The catalyst of the present invention has particularly high activity as a catalyst for water electrolysis. The structure of the present invention is formed by supporting a transition metal oxide on an electrically conductive material and holding the supported oxide on a porous, electrically conductive substrate. Because the substrate is electrically conductive, it can be used as an electrode catalyst. Hydrogen and oxygen can be efficiently produced by performing a water electrolysis reaction using this electrode.

[0043] As described above, the catalyst of the present invention is characterized by the fact that the oxidation state of the transition metal in the transition metal oxide contained in the catalyst component changes flexibly in response to the applied voltage. The property of the transition metal in the transition metal oxide changing flexibly is due to the amorphous structure of the transition metal oxide. For example, in a system in which an electrode obtained using the catalyst of the present invention, in which iridium oxide is supported on the support, and a standard electrode are placed in an electrolyte (0.05 M H2SO4 aqueous solution), when the applied voltage is swept from 1.0 V to 2.2 V relative to the standard electrode, the valence of iridium in the catalyst changes within a range of 0 to +4 or more. More specifically, when the applied voltage is swept from 1.0 V to 2.2 V relative to the standard electrode in the system, the valence of iridium measured by XAFS (X-ray absorption fine structure) analysis changes within a range of 0 to +4 or more. That is, it can assume a pentavalent or hexavalent oxidation state. It is presumed that the catalyst of the present invention can exhibit high activity because the oxidation state of the transition metal can be increased without applying such a high voltage. A catalyst having such properties can be produced by the production method shown in the examples.

[0044] In the present invention, in order to use a transition metal oxide as a catalyst, the transition metal oxide must have an amorphous structure. To achieve an amorphous structure, the following procedure is performed; in the present invention, this procedure is referred to as "activation." To activate the catalyst, the applied voltage is swept back and forth between -3.0 V and 1.5 V relative to the onset potential in a system of an electrocatalyst and a standard electrode placed in an electrolyte (e.g., a 0.05 mmol / L H2SO4 aqueous solution). The voltage sweep conditions for activation are preferably swept back and forth between -3.0 V and 1.5 V relative to the onset potential, more preferably swept back and forth between -3.0 V and 0.5 V, and even more preferably swept back and forth between -0.4 V and 0.5 V, and even more preferably swept back and forth between -0.4 V and 0.5 V. The onset potential is defined as the potential at which the current density becomes 0 when the increase in current density resulting from the oxygen evolution reaction is extrapolated onto the X-axis in the current-voltage curve obtained when the electrode catalyst is used.

[0045] An electrode using the catalyst of the present invention can be suitably used as an anode. As a result, water electrolysis can be carried out efficiently. This point will be described in detail in Example 1. When an electrolyte membrane is used between the anode catalyst and the cathode catalyst, the electrode using the catalyst of the present invention transports protons generated by electrolysis to the cathode catalyst layer through the electrolyte membrane. With an appropriate cathode catalyst, the protons can be used to carry out a reduction reaction. For example, by combining the catalyst with a carboxylic acid hydrogenation catalyst, an alcohol electrosynthesis cell such as that shown in Figure 2 can be constructed. While there are no particular limitations on the electrolyte membrane as long as it is a known electrolyte membrane, as shown in Figure 2, a Nafion (NAFION®, a perfluoroalkylsulfonic acid-based polymer) membrane is preferably used. This electrolyte membrane is sandwiched between a cathode and an anode, and the cathode catalyst layer and anode catalyst layer are thermocompression-bonded to the electrolyte membrane to form a membrane electrode assembly (MEA). A catalyst layer made of the catalyst of the present invention is used as the anode catalyst layer. In Figure 2, a catalyst made of iridium oxide supported on a carbon support is used as the catalyst of the present invention. The cathode catalyst layer can be, for example, a TiO layer. Silicone rubber is sandwiched between both sides of the membrane electrode assembly, with an electrolysis reactor attached to the anode side and a carboxylic acid hydrogenation reactor attached to the cathode side. Water is supplied to the electrolysis reactor and carboxylic acid is supplied to the hydrogenation reactor, and a voltage is applied between the two electrodes. Water is then electrolyzed on the anode side, and the generated protons are supplied to the cathode through the electrolyte membrane, where the carboxylic acid is hydrogenated to produce alcohol. The catalyst of the present invention has particularly high activity as a water electrolysis catalyst, enabling efficient alcohol synthesis using this alcohol electrosynthesis cell. Such a device may be referred to herein as a PEAEC. As the PEAEC, those described in International Publication No. 2017 / 154743 can be used.

[0046] When PEAEC is used to activate the catalyst, the procedure is as follows. The device has a liquid reservoir in the anode flow path, and a reference electrode (Ag / AgCl electrode) is inserted into the reservoir (see Figure 48). Water containing a supporting electrolyte (e.g., 0.2 mol / L Na2SO4) is circulated through the anode side of the device, while water is circulated through the cathode side. The anode catalyst can be activated by sweeping the potential across the desired potential range multiple times. Then, water is circulated through the anode side and an aqueous carboxylic acid solution is circulated through the cathode side, allowing for conventional alcohol electrochemical synthesis.

[0047] (Method of manufacturing the structure) The structure of the present invention can be produced by first supporting a carrier, which is an electrically conductive material, on a substrate acting as an electrode, and then supporting a transition metal oxide catalyst thereon. Alternatively, it can be produced by a process in which a transition metal oxide is supported on an electrically conductive material and then supported on the substrate. In the former method, the surface of the substrate may be actively chemically treated to produce an electrically conductive material as a carrier, or the substrate surface may be naturally oxidized to form an oxide coating. When using an oxide coating, the thinner the coating, the better the performance as an electrode catalyst, since oxides have low electrical conductivity. When using a titanium mesh as the substrate, a method can be used in which needle-shaped titanium oxide grows on the surface by high-temperature treatment in an alkaline aqueous solution, and then a transition metal oxide precursor is reacted with this to support the titanium oxide. When this structure is used as the anode of a membrane electrode assembly, a transition metal oxide can be produced from a transition metal oxide precursor, applied to a solid electrolyte membrane, and then combined with titanium paper whose surface has been naturally oxidized in air to form the structure. The transition metal oxide may be calcined as a hydroxide by alkali treatment, or may be formed into nanoparticles and then calcined.

[0048] (Activation of electrode catalyst) In the electrode catalyst of the present invention, the transition metal must be made amorphous. To activate the catalyst, the applied voltage is swept back and forth at least once in the range of -3.0 V to 1.5 V relative to the onset potential in a system of an electrocatalyst and a standard electrode placed in an electrolyte (for example, a 0.05 mmol / L H2SO4 aqueous solution). The voltage sweep conditions for activation are preferably swept back and forth at least once in the range of -3.0 V to 1.5 V relative to the onset potential, more preferably swept back and forth at least five times in the range of -3.0 V to 0.5 V, and even more preferably swept back and forth at least 10 times in the range of -0.4 V to 0.5 V. The onset potential is defined as the potential at which the current density becomes 0 when the increase in current density resulting from the oxygen evolution reaction is extrapolated onto the X-axis in the current-voltage curve obtained when the electrode catalyst is used. [Example]

[0049] [Example 1] IrO x Synthesis of TiO2-Ti mesh catalyst The TiO2-Ti mesh was prepared by a two-step hydrothermal synthesis method. In the first step, a Ti mesh (area: 2 cm × 2 cm) was placed in an autoclave containing 30 mL of 1 M NaOH aqueous solution. The autoclave was then heated to 220 °C for 12 h to grow H2Ti2O5·H2O on the Ti mesh. Afterwards, the Ti mesh was washed with water, immersed in a 0.1 M HCl aqueous solution for 10 min, washed with water and ethanol, and air-dried.

[0050] In the second step, the treated Ti mesh was placed in an autoclave with 40 mL of water and kept at 200 °C for 24 h to convert H2Ti2O5·H2O into anatase TiO2. After that, the Ti mesh with TiO2 retained was washed with water and ethanol and then dried in air.

[0051] The obtained TiO2-Ti(1) mesh was immersed in a solution (IrCl3 solution) prepared by dissolving IrCl3 in a 6:4 mixture of ethylene glycol and deionized water under a N2 atmosphere for 6 hours, and then heated at 140 °C to form an IrOx The IrO / TiO2-Ti mesh catalyst was synthesized by adjusting the concentration of the IrCl3 solution. x The loading amount is 0.58 mg / cm 2 (1A), 0.437 mg / cm 2 (1B), 0.379 mg / cm 2 , (1C), 0.33 mg / cm 2 , (1D), 0.08 mg / cm 2 Five types of IrO x / TiO2-Ti mesh catalyst was prepared.

[0052] (Sample Identification) The obtained TiO2-Ti(1) and IrO x An example of an electron microscope image of the IrO / TiO2-Ti mesh catalyst (1A) is shown in Figure 3. Figure 3(a) is an SEM image of the TiO2-Ti mesh, and Figure 3(b) is an SEM image of the IrO x Figure 3(c) shows the SEM image of the IrO x This is a high-resolution TEM image of the TiO2-Ti mesh catalyst (1A).

[0053] As can be seen from Figure 3, the obtained IrO x The TiO2-Ti mesh catalyst (1A) has needle-shaped TiO2 held on a Ti mesh, and IrO x It was found that the nanoparticles were supported on the surface. x All nanoparticles were found to have a particle size of less than 10 nm and to be uniformly deposited on the surface of the porous TiO2 needles.

[0054] (catalytic properties) IrO x Loading amount (unit: mg / cm 2 ) several types of IrO x Cyclic voltammograms (CV) were obtained for the IrO / TiO2-Ti mesh catalyst. The results are shown in Figure 4. Figure 4(a) shows the IrO x Figure 4(b) shows the cyclic voltammogram (CV) curves of the IrO / TiO2-Ti mesh catalyst at a sweep rate of 10 mV / s.x / TiO2-Ti mesh catalyst (IrO x Loading amount: 0.58 mg / cm 2 , (1A)). The Tafel plot describes the relationship between the rate of an electrochemical reaction and overpotential by plotting the logarithm of the absolute value of the measured current versus the applied potential. This allows for a quantitative evaluation of how much voltage needs to be applied to obtain a certain current value, and a small slope indicates a high catalytic efficiency.

[0055] As can be seen from Figure 4, these IrO x The IrO / TiO2-Ti mesh catalyst was found to exhibit excellent oxygen evolution reaction (OER) performance in acidic media. x The OER performance improved with increasing loading of IrO. x The loading amount is 0.58 mg / cm 2 IrO x / TiO2-Ti mesh catalyst (1A) and known analogues 10 mAcm -2 Catalyst overpotential (mV) at 10 mAcm -2 The potential (EOER, V vs. RHE) and Tafel slope of the oxygen evolution reaction in the IrO x The loading of IrO is 0.58 mg / cm2. x The / TiO2-Ti mesh catalyst was found to exhibit significantly higher OER performance compared to Ir- or Ru-based catalysts reported in previously published papers, as shown in Table 1.

[0056] [Table 1]

[0057] (Reaction mechanism of IrOx / TiO2-Ti mesh catalyst for OER in acidic medium) (1) Presence of surface hydroxyl (OH) groups The oxygen evolution reaction (OER) is important in several energy conversion devices, especially in acidic media. Although various related catalysts have been reported, the catalytic mechanism remains largely unknown. Here, we performed several X-ray photoelectron spectroscopy (XPS) and XAFS (X-ray absorption fine structure) analyses to clarify the catalytic mechanism of the catalysts.

[0058] Figure 5 shows the IrO before and after firing at 350°C. x Figure 5(a) shows the CV and XPS measurements of the IrO / TiO-Ti mesh catalyst after different calcination treatments. x Figure 5(b) shows the cyclic voltammogram (CV) curves of the IrO / TiO2-Ti mesh catalyst. Figure 5(b) shows the correlation between the OER performance and OH concentration. Figure 5(c) shows the cyclic voltammogram (CV) curves of the IrO / TiO2-Ti mesh catalyst after different calcination treatments. x This is a high-resolution XPS spectrum of the TiO2-Ti mesh catalyst with respect to the 0 signal.

[0059] As can be seen from the CV curve shown in Figure 5(a), the highest activity was observed for the loading of 0.58 mg / cm 2 (1A) IrO x For the TiO2-Ti mesh catalyst, a large voltage gap was observed when the voltage was swept in the opposite directions, positive and negative. This is thought to indicate the accumulation of charge on the electrode. On the other hand, it was also found that as the calcination time increased, the gap decreased and the onset potential increased. Therefore, it is thought that a mechanism for charge accumulation is at work on a highly active electrode.

[0060] The firing time dependence of the XPS spectrum is shown in Figure 5(c). Looking at the O1s region, IrO x On the TiO2-Ti mesh catalyst, the oxygen (O 2- In addition to hydroxyl (OH - ) group was observed. It was also found that the spectral components of the hydroxyl group decreased as the calcination proceeded. Figure 5(b) shows the IrO calculated from the XPS spectrum. x / The ratio of hydroxyl groups to the total amount of oxygen on the TiO2-Ti mesh catalyst surface and 10 mA cm -2 The potential at 0.05V was plotted against the potential at 0.05V, and it was found that the potential decreased with increasing hydroxyl group abundance, i.e., the overpotential decreased. From these results, it is considered that the presence of hydroxyl groups on the catalyst surface is important for the activity.

[0061] (2) The existence of a highly oxidized state under applied voltage To investigate the state of the catalyst in detail, XAFS (X-ray absorption fine structure) analysis experiments were carried out on the catalyst.

[0062] Experimental Method: The sample measurements were performed using transmission, fluorescence, and conversion electron yield methods, each of which was appropriate for the sample form and the element species being measured.

[0063] To prepare the standard sample pellets, approximately 160 mg of boron nitride powder was added to approximately 5 mg of the standard sample powder and mixed, and then pelletized using a 10 mm diameter pelletizer under a pressure of approximately 7 t for 1 minute.

[0064] Kapton® film was fixed to a square-shaped plastic frame, and an electrode sample was fixed on the Kapton® film using cellophane tape. The plastic frame with the sample attached was fixed to a stand and adjusted to a position where the sample was irradiated with synchrotron light. Measurements were then performed at 293 K in air. For the Ir L3 absorption edge measurements, the standard sample pellet (metallic Ir, IrO2) was measured using the transmission method, and the electrode sample was measured using the fluorescence method. For the Ti K absorption edge measurements, the standard sample pellet (anatase-type TiO2) was measured using the transmission method, and the electrode sample was measured using the conversion electron yield method. The measurement times were 15 minutes for the transmission method, 90 minutes for the fluorescence method, and 10 minutes for the conversion electron yield method.

[0065] The electrode sample exhibited IrO x / TiO2-Ti mesh (unfired, before activation treatment), IrOx / TiO2-Ti mesh (unfired, after activation treatment), IrO x / TiO2-Ti mesh (after firing and activation treatment), IrO x TiO2 mesh before loading, IrO x / TiO2-Ti mesh (unfired, before activation treatment), IrO x / TiO2-Ti mesh (unfired, after activation treatment), IrO x Measurements were performed on the IrO / TiO2-Ti mesh (after firing and activation treatment). The size was 2 cm x 0.5 cm. Here, performing several cycles of CV measurement is called activation treatment. The firing was performed at 350°C for a specified time. x / TiO2-Ti mesh has a loading of 0.58 mg / cm 2 IrO x / TiO2-Ti mesh catalyst was used.

[0066] Supplement: Electrode adjustment scheme TiO2-Ti mesh (IrO x (Before loading) → IrO x / TiO2-Ti mesh (unfired, before activation treatment) → IrO x / TiO2-Ti mesh (unfired, after activation treatment) TiO2-Ti mesh (before IrOx support) → IrO x / TiO2-Ti mesh (unfired, before activation treatment) → IrO x / TiO2-Ti mesh (before firing and activation treatment), IrO x / TiO2-Ti mesh (after firing and activation treatment)

[0067] XAFS measurement (in situ) All procedures for the standard samples were carried out under the same conditions as above. All in situ measurements were carried out using the fluorescence method. The electrode sample is IrO x / TiO2-Ti mesh (unfired, after activation treatment), IrO x / TiO2-Ti mesh (after firing and activation treatment) was used, and the size was 2cm x 1cm.

[0068] Figure 6 shows a photograph of the setup for in situ measurements. The reaction in situ measurements was carried out in a polypropylene rectangular cell (120 mm × 80 mm × 70 mm, 460 cm) with a polyethylene lid. 3 A polypropylene rectangular parallelepiped cell (SHINKIGOSEI Co., LTD., Japan) was used. A 16 mm x 16 mm measurement window was cut out from one side (80 mm x 70 mm) of the polypropylene rectangular cell. A Kapton (registered trademark) sticker was attached to the inner wall of the cell so as to completely cover the measurement window.

[0069] The electrode sample was fixed to a stainless steel rod and used as the working electrode. The working electrode, platinum coil counter electrode, silver-silver chloride reference electrode, and stirrer tip were placed in a cell. 0.05 M H2SO4 aqueous solution (FUJIFILM Wako Pure Chemical Corp., Japan) was added to the cell at a volume of 340 cm 3 The electrode sample was immersed in the solution by 1 cm x 1 cm, and was positioned so that the entire immersed area was within the measurement window when observed from the measurement window side. The distance between the electrode sample and the Kapton® seal on the measurement window was 2 mm. The cell was positioned so that synchrotron light passed through the measurement window and was irradiated onto the electrode sample at a 45° angle, and the detector was positioned at 90° to the irradiation direction (45° to the electrode sample). Each electrode was connected to a potentiogalvanostat (VersaSta4, Hokuto Denko). While stirring the solution at 1000 rpm, synchrotron light was irradiated to the working electrode with or without a voltage of 1.0 V–2.9 V vs. RHE applied to the working electrode. Measurements were performed for approximately 60–120 minutes per condition.

[0070] (result) XAFS measurement (electrodes only) Figure 7 shows the results of XANES measurements at the Ir-L3 absorption edge. The peak top positions of the absorption edges of all electrode samples were intermediate between metallic Ir and IrO2, indicating that the Ir species in the prepared electrode samples had a valence intermediate between zero and four. Unsintered IrO before and after activation treatment xFocusing on the spectrum of the IrO2 / TiO2-Ti mesh, the position of the peak top of the absorption edge shifted to the lower energy side due to the activation treatment, suggesting that the Ir species were slightly reduced by the activation treatment. x When we look at the spectrum of the TiO2-Ti mesh, we see that the unsintered IrO x The spectrum of TiO2-Ti mesh is close to that of IrO2, and the calcined IrO x The spectrum of the TiO2-Ti mesh is close to that of metallic Ir, so it is x It was suggested that the Ir species in the / TiO2-Ti mesh were in a higher oxidation state.

[0071] Figure 8 shows the results of EXAFS measurements after Fourier transformation at the Ir L3 absorption edge. x In the spectrum of the IrO / TiO2-Ti mesh (unsintered, before activation treatment), peaks similar to those in the IrO2 spectrum were observed at 1.7 (Ir-O), 2.9 (Ir-Ir), and 3.5 (Ir-Ir) Å. In addition, the peak intensities at 2.9 and 3.5 Å in the second and third coordination spheres were smaller than those of IrO2, suggesting that IrO x / TiO2-Ti mesh (unfired, before activation treatment) IrO x The nanoparticles have a structure similar to that of IrO2, but their crystallinity is lower than that of IrO2. x Focusing on the spectrum of the IrO / TiO2-Ti mesh, the peaks at 2.9 and 3.5 disappeared in the electrode sample after activation, suggesting that the activation treatment reduced the crystallinity of the Ir nanoparticles and caused them to become amorphous. x Since the Ir nanoparticles in the TiO2-Ti mesh are amorphous with low crystallinity, the molar ratio of Ir to oxygen is non-stoichiometric, resulting in the formation of Ir. 0 and Ir 4+It is thought to have an intermediate valence between these two. In the spectrum of the IrOx / TiO2-Ti mesh (after calcination and activation treatment), a peak at 1.7 Å (Ir-O) similar to the spectrum of IrO2 and a peak at 2.9 Å (Ir-Ir) similar to the spectrum of metallic Ir were observed. This suggests that the calcination treatment caused the Ir nanoparticles to aggregate, and some of them transformed into metallic Ir species, resulting in the generation of iridium oxide species and metallic iridium species. These results suggest that amorphous iridium oxide species (IrO x ) was suggested to be the cause of the high activity of this electrode sample.

[0072] Figures 9 and 10 show the results of XANES and EXAFS measurements at the Ti-K absorption edge. The spectra of all electrode samples showed the same results as the anatase TiO2 standard sample. x It was revealed that the TiO2 mesh did not change before and after the nanoparticle loading.

[0073] XAFS measurement (during electrode reaction: in situ) IrO x Figure 11 shows the results of XANES measurements of the Ir-L3 absorption edge with and without applied voltage when using a TiO2-Ti mesh (unfired and after activation treatment). The spectrum shifted to higher energy as more positive voltages were applied, which revealed that the valence of the Ir species changed to a higher oxidation state as more positive voltages were applied. Figure 12 shows a graph plotting the inflection points at the Ir-L3 XANES absorption edge against applied voltage. The inflection points at no voltage and at 1.05 V vs. RHE were on the lower energy side than the inflection point of IrO4, whereas the inflection points shifted to higher energy than the inflection point of IrO2 when a voltage of 1.2 V vs. RHE or higher was applied. In other words, the inflection points of IrO x The valence of Ir species in the TiO2-Ti mesh (unfired and after activation treatment) changes with the application of voltage. When a voltage of 1.2 V vs. RHE or higher is applied, Ir 4+Furthermore, although the order in which the voltages were applied in the measurements was different from the order in which the voltages were applied, the valence of the Ir species depended proportionally on the applied voltage. x It was revealed that the redox ability of the Ir species in the / TiO2-Ti mesh (unsintered and activated) is very flexible and has a reversible voltage response.

[0074] IrO x Figure 13 shows the results of EXAFS measurements after Fourier transformation of the Ir L3 absorption edge when using the / TiO2-Ti mesh (unfired, after activation treatment) with and without voltage application. Under all conditions, only peaks derived from Ir-O were observed, and no other peaks were observed. This indicates that the Ir species remained in the amorphous state of IrO even when voltage was applied. x It was found that when a voltage of 1.2 V vs. RHE or less was applied, the Ir-O peak was observed at a position similar to that observed when no voltage was applied. However, when a voltage of 1.5 V vs. RHE or more was applied, the Ir-O peak shifted toward a shorter distance as a more positive voltage was applied. It is thought that the valence of Ir changed to a higher oxidation state with the application of voltage, and oxygen approached Ir to compensate for the charge. Figure 14 shows a graph plotting the position of the Ir-O peak top in the EXAFS spectrum against the applied voltage. The potential at which the Ir-O peak position changed was close to the onset potential of the OER, indicating that the Ir-O interatomic distance depends on the progress of the reaction. Furthermore, as with the results of XANES measurements, the Ir-O peak showed a systematic change, even though the order of voltage application in the measurements was different from the order of potentials, demonstrating that the Ir-O interatomic distance also exhibits a reversible potential response. In summary, IrO x The valence of Ir species in the / TiO2-Ti mesh (unfired and activated) depended uniformly on the applied voltage, and it was revealed that the Ir-O interatomic distance was due to the progress of the reaction.

[0075] IrO xFigure 15 shows the results of XANES measurements of the Ir-L3 absorption edge when using a / TiO2-Ti mesh (after calcination and activation treatment) with and without applied voltage. Regardless of the applied voltage, nearly identical spectra were observed. Figure 16 shows a graph plotting the inflection point at the Ir-L3 XANES absorption edge against applied voltage. Regardless of the applied voltage, the energy position of the inflection point was near 11,221.7 keV, suggesting that the valence of the Ir species is independent of the potential and is nearly constant.

[0076] IrO x Figure 17 shows the results of EXAFS measurements after Fourier transformation of the Ir L3 absorption edge when using a / TiO2-Ti mesh (after calcination and activation treatment) with and without applied voltage. Under all conditions, only peaks due to Ir-O and Ir-Ir were observed, demonstrating that the state of the Ir species does not change even when a voltage is applied. Figure 18 shows a graph plotting the position of the Ir-O peak top in the EXAFS spectrum against the applied voltage. The Ir-O interatomic distance becomes longer with applied voltage than when no voltage is applied, reaching a similar value of 1.8 Å at voltages below 2.2 V vs. RHE. Under voltages of 2.9 V vs. RHE, the Ir-O interatomic distance becomes shorter than when a voltage below 2.2 V vs. RHE is applied. x Compared with the results for the Ir-O / TiO2-Ti mesh (unsintered and after activation treatment), the interatomic distance of the sintered electrode does not appear to show a clear voltage dependence. x The valence and interatomic distance of Ir species in the / TiO2-Ti mesh (after calcination and activation treatment) are considered to be independent of the potential.

[0077] The graphs summarizing the potential dependence for each electrode are shown in Figures 19 and 20. From the results of the XAFS measurements, it is clear that the IrO x The nanoparticles are made of amorphous oxide IrO xThe flexible voltage response of the valence and the Ir-O interatomic distance, which is thought to be made possible by the small particle size and amorphous structure, and the Ir 4+ It is believed that the Ir active species exhibiting the above valence levels achieved high activity and durability in this electrode sample.

[0078] [Example 2] IrO x Supported carbon (IrO x / C):Aqueous solution method Based on a previous report (J Nanopart Res (2011) 13:1639-1646), H2IrCl6·nH2O (10 g), 19.4 mM) powder was first dissolved in 200 mL of ion-exchanged water.

[0079] The aqueous solution was then heated to 100° C. and stirred for 45 minutes. To promote the formation of Ir hydroxide, 1M sodium hydroxide solution (1M, 200 mL) was added, and the mixture was further stirred at 100° C. for 45 minutes.

[0080] The solution was then centrifuged for 15 minutes and filtered. The precipitate was washed with ion-exchanged water to remove chlorides. The resulting IrO x The particles were dried at 80°C for 5 hours, and then calcined in air at 400°C for 1 hour.

[0081] Figure 21 shows TEM images of commercial IrO2 (a) and IrO2 nanoparticles prepared by the Baglio procedure (IrOx-lab) (b). x The lab preparation showed the presence of agglomerates. However, the agglomerates in the lab-prepared samples were found to be much smaller than those in the commercially available samples. Furthermore, IrO x In the TEM image of the lab (Fig. 21(b)), numerous IrOx particles smaller than 10 nm were observed in addition to the aggregates. x Although the presence of small particles of IrO has not been confirmed, in our experiments, x-lab's majority of IrO2 has a diameter of less than 10 nm x It was revealed that it was composed of nanoparticles. That is, by the above method, IrO x Nanoparticles were obtained.

[0082] 4 mg of IrO x -lab, a mixture containing 24 μL of Nafion® solution (5 mass %), 240 μL of 2-propanol, and 240 μL of water was sonicated for several tens of minutes.

[0083] Next, 4 cm 2 After painting the ink onto a gas diffusion carbon paper with an area of ​​1000 μm (IrO x :1mg / cm 2 ), air-dried, and IrO x I got a -C.

[0084] [Example 3] IrO x Supported Ti paper (IrO x / TiO2-Ti):Aqueous solution method A mixture containing 4 mg of the IrOx-lab, 24 μL of Nafion (registered trademark) solution (5 mass %), 240 μL of 2-propanol, and 240 μL of water was sonicated for several tens of minutes.

[0085] Next, 4 cm 2 The ink was applied to a Ti paper (WEBTi-K (0.025 mm thickness, Toho Technical Service, Co.) with an area of ​​100 mm, and then air-dried. x The surface of commercially available Ti paper is oxidized, so TiO2 is present on the surface. Figure 22 shows the IrO x Photographs of the commercially available structurally stabilized electrode (DSE) used in the OER are shown.

[0086] [Example 4] IrO x / C and IrO x Comparison of OER performance of TiO2 / TiO2 IrO obtained in Example 2 in 0.2M NaSO aqueous solution x / C and IrO obtained in Example 3 x The LSV measurement results for IrO / TiO2-Ti are shown in Figure 23. x -C showed higher performance than the electrode synthesized using commercially available IrO2 particles (IrO2-wako-C). x / TiO2-Ti is IrO x It showed even higher catalytic activity than IrO / C. x The higher catalytic activity of / TiO2-Ti can be mainly attributed to the lower electrical resistivity of Ti paper than that of carbon paper.

[0087] [Example 5] IrO x Comparison of OER characteristics between TiO2-Ti and commercially available OER electrodes FIG. 24 shows the IrO obtained in Example 3. x The OER characteristics of IrO / TiO2-Ti (IrO2 (Yamauchi) in the figure) and a commercially available DSE are shown. x The overpotential on the IrO / TiO2-Ti anode was found to be lower than that on the commercially available anode, resulting in a higher current density. x It was revealed that / TiO2-Ti exhibited excellent OER ability.

[0088] [Example 6] IrO x Comparison of catalytic performance of commercial OER electrodes of TiO2 / TiO2-Ti in PEAEC FIG. 25 shows the IrO obtained in Example 3. x The PEAEC performance of a commercially available OER electrode made of IrO / TiO2-Ti was shown. x It was found that / TiO2-Ti showed the best catalytic activity.

[0089] [Example 7] Durability We also conducted a durability test of this catalyst (Figure 26). While many water electrolysis catalysts have a low durability of less than 20 hours (see reference paper and supporting data: T. Fujigaya, Y. Shi, J. Yang, H. Li, K. Ito, N. Nakashima, "A highly efficient and durable carbon nanotube-based anode electrocatalyst for water electrolyzers," J. Mater. Chem. A, 2017, 5, 10584-10590. DOI: 10.1039 / c7ta01318c), the water electrolysis potential of this catalyst remained almost unchanged even after 100 hours, demonstrating its extremely high OER durability.

[0090] [Example 8] IrO2 microparticles were obtained by the same method as in Example 2, except that the raw material was H2IrCl6·nH2O 1.0 g (1.9 mmol) and the amount of solvent was 1 / 10 of that in Example 2. In Example 8, IrO2 microparticles were synthesized by the method (a) shown in Figure 27.

[0091] [Example 9] 1.0 g (1.9 mmol) of H2IrCl6·nH2O was dissolved in a mixed solution of 15 mL of ethylene glycol and 10 mL of water, and the solution was dry-distilled in a nitrogen stream at 140°C for 6 hours to obtain a black suspension. This suspension was filtered and washed with water to obtain iridium nanoparticles. The obtained iridium nanoparticles were calcined in air at 400°C for 1 hour to obtain IrO2 microparticles. In Example 9, IrO2 microparticles were synthesized using method (b) shown in Figure 27.

[0092] [Example 10] The IrO2 obtained in Example 2 is designated IrO2-large (a), the IrO2 obtained in Example 8 is designated IrO2-small (b), and the IrO2 obtained in Example 9 is designated IrO2-metal (c), and TEM and XRD were compared. The XRD patterns of IrO2-large (a), IrO2-small (b), and IrO2-metal (c) are shown in Figure 28, and TEM images are shown in Figure 29.

[0093] IrO2-small (b) showed an XRD pattern very similar to IrO2-large (a), indicating that there was no significant change in crystallite size. However, TEM images revealed that the aggregate size of IrO2-small (b) was smaller than that of IrO2-large (a). (IrO2-large (a): 2 μm to 10 μm, IrO2-small (b): 0.2 μm to 1 μm). On the other hand, the peaks in the XRD pattern of IrO2-metal (c), prepared via metallic Ir particles, were broadened, indicating that the crystallite size was smaller. Furthermore, TEM images revealed that IrO2-metal (c) consisted of aggregates approximately 0.2 μm to 1 μm in size, but these were composed of aggregates of nanocrystals approximately 2 nm to 3 nm in diameter.

[0094] [Example 11] Ir-RuO2-small Ir-RuO2 was obtained in the same manner as in Example 2, except that 1.0 g (1.9 mmol) of H2IrCl6·nH2O and 55 mg (0.22 mmol) of RuCl3·nH2O were used as raw materials. The obtained Ir-RuO2 is referred to as Ir-RuO2-small.

[0095] [Example 12] Ir-RuO2-metal Ir-RuO2 was obtained by the same method as in Example 9, except that 1.0 g (1.9 mmol) of H2IrCl6·nH2O and 55 mg (0.22 mmol) of RuCl3·nH2O were used as raw materials. The obtained Ir-RuO2 is referred to as Ir-RuO2-metal.

[0096] [Example 13] The Ir-RuO2 obtained in Example 11 was designated Ir-RuO2-small (a), and the Ir-RuO2 obtained in Example 12 was designated Ir-RuO2-metal (b), and their XRD patterns were compared. The XRD patterns of Ir-RuO2-small (a) and Ir-RuO2-metal (b) are shown in Figure 30.

[0097] Since Ir-RuO2-small (a) and Ir-RuO2-metal (b) both have a single peak in the XRD patterns, it is believed that Ru is dissolved in IrO2. Furthermore, the half-width of the peak derived from the reflection from the 110 plane of each sample, observed around 28°, was 0.31° for IrO2-small, 0.31° for Ir-RuO2-small (a), 1.0° for IrO2-metal, and 0.74° for Ir-RuO2-metal (b). This suggests that IrO2 and Ir-RuO2, prepared using the same synthesis method, have similar crystallite sizes.

[0098] [Example 14] Ir-RhO x / TiO2-Ti mesh The TiO2-Ti mesh obtained in Example 1 was dissolved in IrCl3·nH2O (24 mg, 68 μmol) and Rh(C2H4O2) x The sample was immersed in a mixture of 45 mL of ethylene glycol and 30 mL of water containing 2.2 mg of Ir-H2O (7.6 μmol), and refluxed at 140 °C for 6 hours in a nitrogen stream to form Ir-RhO x / TiO2-Ti mesh was obtained.

[0099] [Example 15] Ir-PdO x / TiO2-Ti mesh Ir-PdO was prepared in the same manner as in Example 14, except that PdCl (1.3 mg, 7.6 μmol) was used instead of the Rh compound used in Example 14. x / TiO2-Ti mesh was obtained.

[0100] [Example 16] Ir-PtO x / TiO2-Ti mesh Ir-PtO was prepared in the same manner as in Example 14, except that 19.4 mmol / L HPtCl aq. (0.40 mL, 7.6 μmol) was used instead of the Rh compound used in Example 14. x / TiO2-Ti mesh was obtained.

[0101] [Example 17] Evaluation using PEAEC (IrO2-large) The performance of PEAEC equipped with IrO2-large was evaluated. FIG. 31 shows the structure of the PEAEC used in the evaluation.

[0102] A membrane electrode assembly (MEA) was fabricated by applying 3-5 mg of IrO2-large anode catalyst to a 2 cm x 2 cm area on one side of a Nafion® membrane, placing porous Ti paper (2 cm x 2 cm) on top of it, and then placing a TiO2-Ti mesh (2 cm x 2 cm) cathode catalyst on the other side of the Nafion® membrane and hot pressing it together.

[0103] The fabricated MEA was sandwiched between Ti current collecting blocks in which a flow path for the sample solution (serpentine flow path: 2 cm x 2 cm) was excavated from both the anode and cathode sides to assemble a PEAEC.

[0104] To maintain airtightness of the solution, a silicone gasket was placed between the MEA and each current collecting block. A 1 mol / L oxalic acid solution was flowed through the cathode channel at a flow rate of 0.50 mL / min, and pure water was flowed through the anode channel at a flow rate of 1.0 mL / min. A potential difference of 1.8 V to 3.0 V was applied between the anode and cathode current collecting blocks using a potentiostat. The PEAEC was operated at 60°C. Figure 32 shows the time course of the current density.

[0105] The solution that passed through the cathode channel was collected and analyzed by high-performance liquid chromatography (HPLC) to quantify the concentrations of the reactant oxalic acid and the products glycolic acid and glyoxylic acid. Figure 33 shows the conversion rate of oxalic acid, the faradaic efficiency of glycolic acid production, and the faradaic efficiency of glyoxylic acid production at each potential.

[0106] [Example 18] Evaluation using PEAEC (IrO2-small) The performance of the PEAEC equipped with IrO2-small was evaluated in the same manner as in Example 17, except that IrO2-small was used. The change in current density over time is shown in Figure 34. Figure 35 shows the conversion rate of oxalic acid, the faradaic efficiency of glycolic acid production, and the faradaic efficiency of glyoxylic acid production at each potential.

[0107] [Example 19] Evaluation using PEAEC (IrO2-metal) The performance of the PEAEC equipped with IrO2-metal was evaluated in the same manner as in Example 17, except that IrO2-metal was used. The change in current density over time is shown in Figure 36. Figure 37 shows the conversion rate of oxalic acid, the faradaic efficiency of glycolic acid production, and the faradaic efficiency of glyoxylic acid production at each potential.

[0108] When comparing IrO2 synthesized via hydroxide, the current density of IrO2-small (Figs. 34 and 35) synthesized by small-scale synthesis was 0.43 A cm when 3.0 V was applied, compared to IrO2-large (Figs. 32 and 33) synthesized by large-scale synthesis. -2 to 0.66Acm -2 The conversion rate of oxalic acid increased from 54% to 70%. Furthermore, in the case of IrO2-metal (Fig. 36, Fig. 37) synthesized via metallic Ir particles, the current density at 3.0 V was 1.0 Acm -2 The oxalic acid conversion rate improved to 81%. This improvement in PEAEC performance due to the use of IrO2-small and IrO2-metal is thought to be due to the high hydroxylation catalytic activity of IrO2. Meanwhile, the faradaic efficiency for glycolic acid production was 38%-92% for IrO2-large (Figure 33), but was 31%-91% for IrO2-small (Figure 35), and 33%-92% for IrO2-metal (Figure 37), showing no significant change. This is thought to be due to the absence of any changes to the cathode catalyst.

[0109] [Example 20] Evaluation using PEAEC (Ir-RuO2-small) The performance of the PEAEC equipped with Ir-RuO2-small was evaluated in the same manner as in Example 17, except that Ir-RuO2-small was used. The change in current density over time is shown in Figure 38. Figure 39 shows the conversion rate of oxalic acid, the faradaic efficiency of glycolic acid production, and the faradaic efficiency of glyoxylic acid production at each potential.

[0110] [Example 21] Evaluation using PEAEC (Ir-RuO2-metal) The performance of the PEAEC equipped with Ir-RuO2-metal was evaluated in the same manner as in Example 17, except that Ir-RuO2-metal was used. The change in current density over time is shown in Figure 40. Figure 41 shows the conversion rate of oxalic acid, the faradaic efficiency of glycolic acid production, and the faradaic efficiency of glyoxylic acid production at each potential.

[0111] Comparing IrO2-small (Fig. 34, Fig. 35) synthesized via hydroxide and Ir-RuO2-small (Fig. 38, Fig. 39), the current density was 0.66 cm when 3.0 V was applied. -2 from 0.62cm -2 The conversion rate of oxalic acid decreased from 70% to 49%. Furthermore, when comparing IrO2-metal (Fig. 36, Fig. 37) and Ir-RuO2-metal (Fig. 40, Fig. 41), which were synthesized via metallic Ir particles, the current density at 3.0 V was 1.0 A cm -2 from 0.84Acm -2 The oxalic acid conversion rate decreased from 81% to 72%, respectively. Therefore, it was found that the catalytic activity of the Ir-RuO2 composite oxide was inferior to that of IrO2.

[0112] [Example 22] Evaluation using PEAEC (Ir-RhO x / TiO2-Ti mesh) Ir-RhO xAn MEA was prepared by the same method as in Example 17, except that an Ir-RhO / TiO-Ti mesh (2 cm x 2 cm), Nafion (registered trademark), and another TiO-Ti mesh (2 cm x 2 cm) were stacked in this order and heat-pressed. x The performance of PEAEC equipped with TiO2-Ti mesh was evaluated.

[0113] The change in current density over time is shown in Figure 42. Figure 43 shows the conversion rate of oxalic acid, the faradaic efficiency of glycolic acid production, and the faradaic efficiency of glyoxylic acid production at each potential.

[0114] [Example 23] Evaluation using PEAEC (Ir-PdO x / TiO2-Ti mesh) Ir-PdO x An MEA was prepared by the same method as in Example 17, except that an Ir-PdO / TiO-Ti mesh (2 cm x 2 cm), Nafion (registered trademark), and another TiO-Ti mesh (2 cm x 2 cm) were stacked in this order and heat-pressed. x The performance of PEAEC equipped with TiO2-Ti mesh was evaluated.

[0115] The change in current density over time is shown in Figure 44. Figure 45 shows the conversion rate of oxalic acid, the faradaic efficiency of glycolic acid production, and the faradaic efficiency of glyoxylic acid production at each potential.

[0116] [Example 24] Evaluation using PEAEC (Ir-PtO x / TiO2-Ti mesh) Ir-PtO x An MEA was fabricated by the same method as in Example 17, except that an Ir-PtO / TiO-Ti mesh (2 cm x 2 cm), Nafion (registered trademark), and another TiO-Ti mesh (2 cm x 2 cm) were stacked in this order and heat-pressed. x The performance of PEAEC equipped with TiO2-Ti mesh was evaluated.

[0117] The change in current density over time is shown in Figure 46. Figure 47 shows the conversion rate of oxalic acid, the faradaic efficiency of glycolic acid production, and the faradaic efficiency of glyoxylic acid production at each potential.

[0118] When the current density at 3.0 V is compared with that of IrO2-large (Fig. 32, Fig. 33), it is 0.43 Acm -2 It was changed to Ir-RhO x / TiO2-Ti mesh (Figure 42, Figure 43) 0.19Acm -2 , Ir-PdO x / TiO2-Ti mesh (Figure 44, Figure 45) 0.19Acm -2 , Ir-PtO x / TiO2-Ti mesh (Figure 46, Figure 47) 0.21Acm -2 Similarly, the oxalic acid conversion rate at 3.0 V was x / TiO2-Ti mesh (Fig. 42, Fig. 43) 9.6%, Ir-PdO x / TiO2-Ti mesh (Fig. 44, Fig. 45) 21%, Ir-PtO x The results were 15% for the Ir-MO / TiO2-Ti mesh (Fig. 46, Fig. 47). x Although the performance of TiO2-Ti mesh (M = Rh, Pd, Pt) anode catalysts is lower than that of conventional IrO2, it was revealed that PEAEC performance can be achieved. x This is thought to be due to the small amount used. It is expected that the activity can be improved by optimizing the conditions in the future.

Claims

1. An electrode catalyst for water electrolysis, comprising a composite supported on an electrically conductive substrate, the composite comprising an electrically conductive material having hydroxyl groups on its surface and at least one transition metal oxide supported on the material, wherein the transition metal is at least one selected from the group consisting of ruthenium, iridium, rhodium, platinum, and palladium, the transition metal oxide has an amorphous structure, the oxidation number of the metal contained in the transition metal oxide changes flexibly and reversibly in response to an applied voltage, the particle size of the transition metal oxide is 100 nm or less, the substrate is a porous material, and the electrically conductive material and the transition metal oxide are bridged via oxygen.

2. 2. The electrode catalyst for water electrolysis according to claim 1, wherein the electrically conductive material is titanium oxide, and the titanium oxide has an anatase crystal structure.

3. 3. The electrode catalyst for water electrolysis according to claim 2, wherein the transition metal oxide is iridium oxide.

4. an electrochemical reaction device for water electrolysis using an electrode for water electrolysis as an anode, the electrode comprising: a structure supported on an electrically conductive substrate; a composite comprising an electrically conductive material having hydroxyl groups on its surface and at least one transition metal oxide supported on the material; the transition metal being at least one selected from the group consisting of ruthenium, iridium, rhodium, platinum, and palladium; the transition metal oxide having an amorphous structure; the oxidation number of a metal contained in the transition metal oxide changing flexibly and reversibly in response to an applied voltage; the particle size of the transition metal oxide being 100 nm or less; and the electrically conductive material and the transition metal oxide being bridged via oxygen.

5. 5. The electrochemical reaction device for electrolyzing water according to claim 4, wherein the electrically conductive material is titanium oxide, and the titanium oxide has an anatase crystal structure.

6. 6. The electrochemical reactor for water electrolysis according to claim 5, wherein the transition metal oxide is iridium oxide.

7. a cathode; and an electrolyte membrane provided between the anode and the cathode. The composite includes an electrically conductive material having hydroxyl groups on its surface and at least one transition metal oxide supported on the material. The composite includes an anode including a structure supported on an electrically conductive substrate, the anode including the structure being supported on an electrically conductive substrate, the cathode being supported on the anode, the cathode being supported on the cathode, the transition metal being at least one selected from the group consisting of ruthenium, iridium, rhodium, platinum, and palladium, the transition metal oxide having an amorphous structure, the oxidation number of a metal contained in the transition metal oxide changing flexibly and reversibly in response to an applied voltage, the transition metal oxide having a particle size of 100 nm or less, and the electrically conductive material and the transition metal oxide being bridged via oxygen.

8. 8. The membrane electrode assembly for water electrolysis according to claim 7, wherein the electrically conductive material is titanium oxide, and the titanium oxide has an anatase crystal structure.

9. 9. The membrane electrode assembly for water electrolysis according to claim 8, wherein the transition metal oxide is iridium oxide.

10. 10. An alcohol synthesis apparatus comprising the membrane electrode assembly for water electrolysis according to claim 7, a first supply means for supplying water or water vapor to the anode, a second supply means for supplying a carboxylic acid to the cathode, and a means for recovering alcohol produced at the cathode.

11. A method for producing a structure to be used for water electrolysis, the structure comprising a composite having an electrically conductive material with hydroxyl groups on the surface thereof and at least one transition metal oxide supported on the material, the composite being held on an electrically conductive substrate with a porous structure, the method comprising: The method includes a step of immersing an electrically conductive material in a solution of a precursor of a transition metal oxide, and heating the solution in which the material is immersed; The transition metal is at least one selected from the group consisting of ruthenium, iridium, rhodium, platinum, and palladium, the transition metal oxide has an amorphous structure, the oxidation number of the metal contained in the transition metal oxide changes flexibly and reversibly in response to an applied voltage, the particle size of the transition metal oxide is 100 nm or less, and the electrically conductive material and the transition metal oxide are bridged via oxygen.

12. The method for producing a structure according to claim 11, wherein the electrically conductive material is titanium oxide, and the titanium oxide has an anatase crystal structure.

13. The method for producing a structure according to claim 12, wherein the oxide of the transition metal is iridium oxide.

14. A method for producing an electrode catalyst for water electrolysis, comprising a composite supported on an electrically conductive substrate having a porous structure, the composite comprising an electrically conductive material having hydroxyl groups on the surface thereof and at least one transition metal oxide supported on the material, the method comprising: a step of applying a transition metal obtained by treating a precursor of an oxide of a transition metal with an aqueous solution of an alkali metal or an aqueous solution of a polyhydric alcohol to a solid electrolyte membrane, and bonding the applied transition metal to a substrate supporting an electrically conductive material; The method for producing an electrode catalyst for water electrolysis, wherein the transition metal is at least one selected from the group consisting of ruthenium, iridium, rhodium, platinum, and palladium, the transition metal oxide has an amorphous structure, the oxidation number of the metal contained in the transition metal oxide changes flexibly and reversibly in response to an applied voltage, the particle size of the transition metal oxide is 100 nm or less, and the electrically conductive material and the transition metal oxide are bridged via oxygen.

15. 15. The method for producing an electrode catalyst for water electrolysis according to claim 14, wherein the electrically conductive material is titanium oxide, the transition metal oxide is iridium oxide, and the porous electrically conductive substrate is titanium.

16. 16. The method for producing an electrode catalyst for water electrolysis according to claim 15, wherein the titanium oxide has an anatase crystal structure.

17. A method for activating an electrode catalyst for water electrolysis, comprising: In a system of an electrode catalyst for electrolysis of water provided in an electrolyte and a standard electrode, the applied voltage is swept back and forth one or more times in the range of −3.0 V to 1.5 V relative to the onset potential, The electrode catalyst for water electrolysis is an electrode for water electrolysis in which a composite comprising an electrically conductive material having hydroxyl groups on its surface and at least one oxide of a transition metal supported on the material is supported on an electrically conductive substrate, the transition metal being at least one selected from the group consisting of ruthenium, iridium, rhodium, platinum, and palladium, the transition metal oxide having an amorphous structure and a particle size of 100 nm or less, the substrate being a porous material, and the electrically conductive material and the transition metal oxide being bridged via oxygen, This is a method for activating an electrode catalyst for water electrolysis, in which the oxidation number of the metal contained in the transition metal oxide is made flexibly and reversibly changeable in response to the applied voltage by the sweep.

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