Method for manufacturing anode electrodes for water electrolysis

Abstract

To manufacture an electrode material having a high oxygen generation catalytic activity using an electrocrystallization method in an organic solvent, the electrode material containing a non-metallic element such as oxygen besides a multi-component transition metal and having a nanocrystal or amorphous structure.SOLUTION: An organic solvent is made up by salt of a metal. DMF (N,N-dimethylformamide), CH3CN (acetonitrile), or DMSO (dimethyl sulfoxide) is selected but other organic solvents may be selected. Salt of transition metal is mainly used as salt of metal but other metal salts may also be used. Constant voltage (electric potential), constant current, pulse voltage (electric potential) / current, or voltage of an arbitrary waveform (electric potential) / current or a combination of these is applied to a conductive electrode substrate, and a single kind or multi-component transition metal material containing a non-metallic element such as oxygen is formed. The formed material includes powders collected from a substrate and a bath liquid other than films staying in the substrate. Such film electrodes or powders are applied as an oxygen generation electrode material for water electrolysis.SELECTED DRAWING: Figure 13

Description

Technical Field

[0001] The present invention manufactures electrode materials using electrodeposition in an organic solvent and further applies these electrode materials as oxygen evolution electrodes. In particular, it is applied as an anode electrode for oxygen evolution in the electrolysis of water.

Background Art

[0002] To achieve the goal of carbon neutrality, the use of hydrogen as clean energy has attracted attention. When hydrogen is produced by electrolysis of water, it is necessary to improve the efficiency of the hydrogen generation reaction and the oxygen generation reaction. Among them, the oxygen generation reaction affects the efficiency of water splitting. At present, electrodes for the oxygen generation reaction mainly use ruthenium oxide (RuO2) and iridium oxide (IrO2) with high catalytic activity. Since RuO2 and IrO2 are expensive and rare, research and development of catalysts using transition metal oxides have been actively carried out. There are the following prior art documents regarding this. All of these documents are crystalline electrodes made using methods such as high temperature or high pressure, and the process is complex and cannot be said to be resource-saving or material-saving. Also, when used as an electrode, it becomes amorphous over time, and the catalytic function deteriorates

[0003] On the other hand, in the electrolysis of water, oxygen evolution utilizes the surface function of the electrode. Therefore, from the perspective of resource saving, etc., a thin film type electrode attached to an inexpensive conductive substrate is more necessary than a bulk electrode. To obtain a thin film, electrodeposition, which is cheaper and can be carried out at room temperature compared to thermal spraying, magnetron sputtering, plasma ion implantation, etc., can coat a structurally complex substrate and is suitable for creating nanomaterials and amorphous structures.

[0004] When preparing electrodes containing transition metals by electrodeposition, a potential lower than the reduction potential of the transition metal ions must be applied. However, many transition metal ions have a lower reduction potential than hydrogen ions, leading to hydrogen generation during electrodeposition, which not only reduces current efficiency but can also prevent the reduction of the target element. This problem can be avoided by using an organic solvent instead of an aqueous solvent. Furthermore, simultaneously depositing multi-component transition metals onto a substrate is not easily achieved. A literature search reveals the following prior art documents on the creation of multi-component elemental thin films by electrodeposition. However, these electrodeposited films do not contain many non-metallic elements such as oxygen, and no research or applications applying them to oxygen-generating electrodes with effective catalytic activity have been found. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 6745733: Oxygen evolution reaction catalyst, oxygen evolution reaction electrode, and oxygen evolution reaction method. Authors: Yoshito Hirai, Tomoya Ohno, Tsuyoshi Matsuda, and Shunsuke Yagi. [Non-patent literature]

[0006] [Non-Patent Document 1] Shunsuke Yagi, Goichi Ikeno, and Ikuya Yamada, "New Developments in Oxygen Evolution Catalyst Development," Journal of MMIJ, Vol.133, No.11, 2017, pp.264-269.

[0007] [Non-Patent Document 2] Thi Xuyen Nguyen, Yi-Cheng Liao, Chia-Chun Lin, Yen-Hsun Su, Jyh-Ming Ting, Advanced High Entropy Perovskite Oxide Electrocatalyst for Oxygen Evolution Reaction, Advanced Functional Materials, 2021, 2101632.

[0008]

Table 3

[0009]

Fashion 4

[0010] In this invention, to produce an electrode material for oxygen generation with effective catalytic activity, a bath solution is used in which salts of the target metal to be included in the film are added to an organic solvent. The organic solvent is selected to have low hydrogen generation during electrodeposition and a wide potential window, such as DMF (N,N-dimethylformamide), CH3CN (acetonitrile), or DMSO (dimethyl sulfoxide). The bath solution contains (i) metal cations and anions derived from the metal salt, and (ii) nonmetallic elements such as oxygen, nitrogen, carbon, or sulfur derived from the organic solvent. By utilizing the complex deposition behavior when a predetermined potential or current is applied to a conductive substrate in the bath solution, a single-metal or multi-metallic thin film containing various metals and elements such as oxygen, carbon, nitrogen, and sulfur is formed on the substrate. A wide range of potentials can be freely applied to the conductive substrate, and the chemical composition, thickness, and structure of the film can be controlled by adjusting conditions such as the concentration and temperature of the bath solution, the applied potential and current, and the application time. In addition to materials that remain as a film on the substrate, conventional electrodes can be fabricated using powder materials collected from the substrate or from the bath solution. Furthermore, these electrode materials have nanocrystalline or amorphous structures, resulting in less degradation due to catalytic reactions and a longer lifespan. [Overview of the Initiative] [Problems that the invention aims to solve]

[0011] This invention provides a method for producing an electrode material with high oxygen-evolving catalytic activity for the electrolysis of water using an electrodeposition method in an organic solvent. This material exists in film or powder form, contains multi-component transition metals as well as non-metallic elements such as oxygen, and has a nanocrystalline or amorphous structure. The material may also contain a single transition metal or other metals. Furthermore, the produced material is applied to an oxygen-evolving electrode for the electrolysis of water. [Means for solving the problem]

[0012] This invention involves preparing a bath by adding salts of the target metal to be included in the film to an organic solvent. The organic solvent is selected to have low hydrogen generation during electrodeposition and a wide potential window, such as DMF (N,N-dimethylformamide), CH3CN (acetonitrile), or DMSO (dimethyl sulfoxide), but other organic solvents may also be used. The metal salts are mainly transition metal salts, but other metal salts may also be used. A film of a single metal element or a multi-component metal element containing nonmetallic elements such as oxygen is formed on the surface of the conductive substrate by applying a constant voltage (potential), constant current, pulsed voltage (potential) / current, voltage (potential) / current of any waveform, or a combination thereof, to the electrode substrate in the bath solution. Furthermore, the chemical composition, thickness, and structure of the film are controlled by adjusting various electrodeposition conditions (solution concentration and temperature, applied potential and current, application time, etc.). The material formed in this way includes not only the film form that remains on the substrate, but also powder collected from the substrate or powder collected from the bath solution. The substrate used for electrodeposition only needs to be conductive. The thin film electrodes on the substrate prepared in this way, or the electrodes made from the collected powder, are used as oxygen-evolving electrodes in the electrolysis of water. [Effects of the Invention]

[0013] The oxygen-evolving electrode material of the present invention uniformly and easily forms a film of a single or multi-component transition metal element containing nonmetallic elements such as oxygen on a conductive substrate of any shape in a low-temperature organic bath. The thin film formed on the substrate can be used as an electrode together with the substrate, or it can be powdered during or after electrodeposition and used as a suitable electrode material. This material has a low oxygen-evolving overpotential and a long lifespan as an oxygen-evolving catalyst. Such oxygen-evolving electrodes can be applied to the electrolysis of water, replacing expensive ruthenium oxide (RuO2) and iridium oxide (IrO2), avoiding wasted electrical energy and unnecessary damage to electrodes. Furthermore, it can greatly contribute to the accumulation of surplus power in power generation, the smoothing of power plant output, and the promotion of solar, wind, and hydroelectric power generation. Of course, it can also contribute to achieving carbon neutrality as a measure against global warming, while bringing about significant social and economic benefits. [Brief explanation of the drawing]

[0014] [Figure 1] Figure 1 is a schematic diagram of electrolysis. (Example 1)(Example 2) [Figure 2] Figure 2 is a schematic diagram of the film-type electrode material (a) and the powdery electrode material (b) on the conductive substrate. (Example 1)(Example 2) [Figure 3] Figure 3 is the surface morphology of the electrolytic film. (Example 1) [Figure 4] Figure 4 is the cross-sectional morphology of the electrolytic film. (Example 1) [Figure 5] Figure 5 is the X-ray diffraction pattern of the electrolytic film. (Example 1) [Figure 6] Figure 6 is a high-magnification cross-sectional photograph of the electrolytic film observed using a transmission electron microscope. (Example 1) [Figure 7] Figure 7 is the composition (atomic concentration) of the electrolytic film obtained using SEM-EDS. (Example 1) [Figure 8] Figure 8 is the composition (atomic concentration) of various chemical bonding states of the film obtained based on the XPS spectrum. (Example 1) [Figure 9] Figure 9 is the anodic polarization curves of the copper plate, RuO2 and the electrolytic film in 1.0 M KOH aqueous solution. (Example 1) [Figure 10] Figure 10 is the oxygen evolution overpotential of various electrode materials. (Example 1) [Figure 11] Figure 11 is the change in overpotential when a 50-hour electrolysis reaction (current density 10 mA / cm2) is carried out in 1.0 M KOH aqueous solution. (Example 1) [Figure 12] Figure 12 is the anodic polarization curves of the electrolytic films of (Example 1) and (Example 2) compared in 1.0 M KOH aqueous solution. (Example 1)(Example 2) [Figure 13] Figure 13 is the oxygen evolution overpotential of various electrode materials. (Example 2)

Modes for Carrying Out the Invention

[0015] Polished and degreased copper plates were used as substrates for electrodeposition. Metal salts such as CrCl3, MnCl2, FeCl2, CoCl2, NiCl2, MoCl5, and WCl6 were dissolved in an organic solvent (DMF-CH3CN (4:1 volume)) and electrodeposited onto the copper plates by applying a predetermined potential. Before electrodeposition, the bath solution was dehydrated using a molecular sieve and degassed with nitrogen gas. Degassing with nitrogen gas was continued during electrodeposition. Dehydration and degassing of the bath solution are not essential conditions. A schematic diagram of the electrodeposition method is shown in Figure 1. A schematic diagram of the electrode material on the conductive substrate after electrodeposition is shown in Figure 2(a). The powder form collected from the substrate or from the bath solution is shown in Figure 2(b). [Explanation of symbols]

[0016] 1. Potential / Current Control Device 2. Control input and result recording equipment 3 Reference electrode 4. Chloride solution 5 Glass containers 6 glass tubes 7. Conductive substrate 8 pairs of electrodes 9 Bath liquid 10 water 11 Temperature control device 12. Glass tube for gas introduction 13 Shiobashi 14 Electrode materials 15 Conductive substrate 16 Powdered electrode materials

[0017] The surface and cross-section of the electrodeposited film were observed using a scanning electron microscope and a transmission electron microscope. The crystal structure was analyzed using X-ray diffraction and a transmission electron microscope. The component composition of the film was analyzed using energy-dispersive X-ray spectroscopy (SEM-EDS) on a scanning electron microscope. In addition, the chemical bonding state of various elements in the film was analyzed using X-ray photoelectron spectroscopy.

[0018] As for mechanical properties, the hardness of the coating was measured using nanoindentation up to a maximum indentation load of 40 nN.

[0019] The oxygen-evolving catalytic properties of the film were evaluated in a 1.0 M KOH aqueous solution. [Example 1]

[0020] CrCl3, MnCl2, FeCl2, CoCl2, and NiCl2 were dissolved at predetermined concentrations in an organic solvent (DMF-CH3CN (4:1 volume)) to create a bath solution. A predetermined potential (-2.0V, -2.5V, -3.0V vs. SSE) was applied to a copper substrate for 0.6 ks (1 ks = 1,000 seconds) or 1.8 ks to obtain a film (represented as CrMnFeCoNi in the figure). Here, SSE refers to the silver / silver chloride electrode used as the reference electrode.

[0021] Figure 3 shows the surface morphology of the film electrodeposited over 0.6 ks at -2.0 V, -2.5 V, and -3.0 V (vs. SSE). Uniform spherical particles were observed on the film surface.

[0022] Figure 4 shows cross-sections of films electrodeposited at -2.0V and -3.0V (vs. SSE). The film thickness ranged from 0.5 to 2.0 μm depending on the applied potential and application time. The film thickness increased as the applied potential became more negatively base.

[0023] Figure 5 shows the X-ray diffraction patterns of the specimens before and after electrodeposition. Diffraction peaks of Cu(111), (200), and (220) were detected from the copper plate. The longer the electrodeposition time, the weaker the Cu diffraction peak. The Cu(200) and (220) peaks were hardly detectable from the specimens electrodeposited for 1.8 ks. Since no peaks other than Cu were detected from any of the specimens, it was confirmed that the electrodeposited film was mainly amorphous (non-crystalline).

[0024] Figure 6 is a high-magnification image of a cross-section of a film electrodeposited at -2.5V (vs. SSE) for 0.6 ks, observed with a transmission electron microscope. This image confirms that the film has a fine amorphous (non-crystalline) structure containing several nanocrystals.

[0025] Figure 7 shows the relative mole fractions (atomic concentrations) of Cr, Mn, Fe, Co, and Ni, analyzed by scanning electron microscope energy-dispersive X-ray spectrometer (SEM-EDS) on films electrodeposited for 0.6 ks at -2.0 V, -2.5 V, and -3.0 V (vs. SSE). The film obtained at -3.0 V (vs. SSE) had nearly identical content of the five metals. Furthermore, calculations based on the content of all elements including O and C revealed that each film contained more than 50% (atomic ratio) of oxygen atoms and about 10% of carbon atoms.

[0026] After sputter etching of approximately 20 nm (vs. SiO2) with argon ions, the XPS spectrum of the film was measured. 2p spectra of Cr, Mn, Fe, Co, and Ni were detected in all films. Cr3+ and Mn2+ were detected at 576.6 eV and 641.3 eV, respectively, but Cr0 and Mn0 were not observed. On the other hand, Fe 2p could be separated into Fe3+, Fe2+, and Fe0. Co2+, Co0, Ni2+, and Ni0 were identified from Co 2p and Ni 2p, respectively. Furthermore, components were identified in the O1s spectra as O(-M) and HO(-M). However, the Mn peak was not detected at -2.0 V (vs. SSE).

[0027] Figure 8 shows the atomic concentrations of each element under various chemical states, obtained based on X-ray photoelectron spectroscopy (XPS) analysis. Cr, Mn, Fe, and Co increased with decreasing applied potential. In all cases, the amount of Ni2+ detected was lower than that of NiO. Fe2+ and Fe3+ accounted for approximately 87% of the total iron, and about half of the total cobalt was Co2+. For Cr and Mn, only Cr3+ and Mn2+ were detected; CrO and MnO were not. The oxygen content in the film was approximately 22-39%, about half of that detected by SEM-EDS analysis.

[0028] The plastic hardness values ​​(load 40nN) of the coating (electrodeposition time: 1.8ks) were 2670 MPa (-2.0 V vs. SSE), 2660 MPa (-2.5 V), and 2250 MPa (-3.0 V), exceeding those of a 1600 MPa copper plate. This level of hardness indicates that the coating possesses sufficient mechanical properties. This demonstrates that, when applied as an electrode, it can adequately withstand water flow and impact from fine particles.

[0029] Figure 9 shows the anodic polarization curves of a Cu substrate, a commercially available RuO2 electrode, and various coatings obtained in a 1.0 M KOH aqueous solution. For the electrolysis of water, the overpotential (η10: Figure 10) obtained from the potential corresponding to a benchmark current density of 10 mA / cm² relative to the standard potential (1.23 V vs. RHE (hydrogen standard electrode)) of the oxygen evolution reaction 2H₂O = O₂ + 4H⁺ + 4e was used to evaluate the oxygen evolution catalytic activity of the electrode. The smaller the overpotential, the easier it is for oxygen to be generated. Compared to the Cu plate at 628 mV, the overpotential of the coating at 325-333 mV shows high catalytic activity. This value is clearly smaller than the 342 mV for commercially available RuO2. Furthermore, the taffel gradient of the coating (39.1 - 44.3 mV / decade) is also significantly lower than that of copper (125.4 mV / decade) and RuO2 (66 mV / decade). These results suggest that it can be used as an oxygen-evolving electrode instead of the expensive RuO2. The film has an amorphous structure containing a small amount of nanocrystals, and it can be assumed that there are many defects in the electrode that are not oxygen-saturated. This is presumed to be the cause of the low oxygen-evolving overpotential. Of course, it could also be due to the unique electronic structure of the transition metal.

[0030] Figure 11 shows the change in overpotential when an electrodeposited film (electrodeposited at -2.0V for 0.6 ks) was used as an oxygen evolution electrode and oxygen was generated in a 1.0 M KOH aqueous solution at a benchmark current density of 10 mA / cm2 for 50 hours. From this, it was found that there was no significant increase in overpotential even after 50 hours, indicating excellent durability as an electrode catalyst for the oxygen evolution reaction. This excellent durability is thought to be related to the amorphous structure of the electrode material, which contains a unique small amount of nanocrystals. [Example 2]

[0031] MoCl5, WCl6, or MoCl5 and WCl6 were further added to the bath solution of (Example 1), and a potential was applied to the copper substrate in each bath solution to obtain a film (represented as CrMnFeCoNiMo and CrMnFeCoNiW in the figure).

[0032] SEM-EDS analysis revealed that the film contained Cr, Mn, Fe, Co, Ni, as well as Mo, W, or Mo / W, respectively. The content of Cr, Mn, Fe, Co, Ni, O, C, etc., in the film of (Example 2) differed slightly from that of the film of (Example 1). For example, when Mo and W were simultaneously contained in the film, the amount of Mn detected was low. Figure 12 shows the polarization curves of the oxygen evolution potential of the various films obtained in (Example 2). Some results from (Example 1) are also shown in this figure for comparison. As shown in Figure 13, the oxygen evolution overpotential η10 of the film of (Example 2) (containing Mo or W) is about the same as that of the RuO2 film of (Example 1), but the oxygen evolution overpotential η10 corresponding to a current density of 10 mA / cm2 is lower than that of the film of (Example 1) (not containing Mo or W), indicating a higher oxygen evolution current efficiency. From this, it is considered that the inclusion of Mo or W in the film is effective for oxygen evolution catalytic activity.

Claims

1. A method for producing an anode electrode for water electrolysis, comprising dissolving chlorides of metal elements Cr, Mn, Fe, Co, and Ni as solutes in a mixed bath of the organic solvents DMF (N,N-dimethylformamide) and CH3CN (acetonitrile), and then using a cathode electrode deposition method to produce a thin film material containing oxygen, carbon, and the aforementioned metal elements on a conductive substrate.

2. The method for producing an anode electrode for water electrolysis according to claim 1, wherein the solute further comprises one of the following: a chloride of the metal element Mo, a chloride of the metal element W, or a chloride of the metal element Mo and the metal element W.

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