https://www.aist.go.jp/aist_j/press_release/pr2020/pr20201009/pr20201009.html
https://www.aist.go.jp/aist_j/press_release/pr2020/pr20201009/pr20201009.html
Contributing to the promotion of the practical application of solar hydrogen production technology using seawater.
Key Points
Achieving selective oxygen production by modifying part of the photo-responsive electrode with manganese
Elucidating the mechanism by which the surface-modified manganese promotes oxygen production as a catalyst and suppresses side reactions
Expectations for the practical application of artificial photosynthesis technology for low-cost hydrogen production using inexhaustible sunlight and seawater
Summary
Kazuhiro Sayama, Research Team Leader, Yugo Mitsuishi, Senior Researcher, and Sayuri Okunaka, Researcher, of the Artificial Photosynthesis Research Team, Zero-Emission International Research Center, National Institute of Advanced Industrial Science and Technology (AIST) [President: Kazuhiko Ishimura] (hereinafter referred to as “AIST”) have developed a technology that selectively produces hydrogen and oxygen from aqueous solutions containing chloride ions (Cl-) such as salt water and seawater at low electrolysis voltage by irradiating sunlight on a photoelectrode of a visible light-responsive oxide semiconductor. have developed an artificial photosynthesis technology that selectively produces hydrogen and oxygen at low electrolysis voltage from aqueous solutions containing chloride ions (Cl-) such as salt water and seawater by irradiating sunlight onto a photoelectrode made of an oxide semiconductor that responds to visible light. They found that simply loading a small amount of manganese (Mn) oxide on the surface of this photoelectrode suppresses the generation of hypochlorous acid (HClO) by oxidizing chloride ions, which is a side reaction. The results of this study not only show the potential for the realization of a hydrogen production system using artificial photosynthesis technology with photoelectrodes, but also suggest that they may be a key to solving the question of 'why manganese was chosen' in the evolutionary process of the oxygen-evolving center of natural photosynthesis systems, and are expected to make a significant contribution to both practical and basic research.
Further details of this technology will be published online in the American academic journal iScience on October 9, 2020 (Japan time).
Overview
Schematic of the reaction of the Mn-modified photoelectrode in artificial photosynthesis and the production of oxygen at the oxygen-evolving center in natural photosynthesis
Social Background of the Development
The technology of producing hydrogen and oxygen by using sunlight to split water with photoelectrodes and photocatalysts is a low-cost, clean production method that is the subject of much research as a fundamental technology for the realization of a hydrogen society in the future. In order to further reduce the cost of this system, it is desirable to use seawater, which is abundant, as the reaction solution for the system. However, when water containing chloride ions (Cl-) such as seawater or brine is used as the reaction solution, chloride ions are oxidized to produce hypochlorous acid (HClO) at the same time as oxygen is produced by the oxidation of water. While HClO is expected to be a chemical product with a higher added value than oxygen for sterilization and disinfection, in large-scale water electrolysis hydrogen production systems, it is a problematic substance that accelerates the corrosion and deterioration of the system. For this reason, there is a need to clarify the mechanism of HClO generation and to develop a photoelectrode that can generate only oxygen.
Research Background
AIST has independently developed an oxide semiconductor photoelectrode (BiVO4/WO3/FTO) that can efficiently decompose water into hydrogen and oxygen using low voltage and sunlight. We have also developed a method for synthesizing inorganic oxidants such as hydrogen peroxide, persulfuric acid, and hypochlorous acid, and a method for efficiently synthesizing organic substances such as KA oil from cyclohexane, using oxide semiconductor photoelectrodes that absorb visible light, such as tungsten oxide (WO3) and bismuth vanadate (BiVO4). we have developed methods for synthesizing inorganic oxidants such as hydrogen peroxide, persulfuric acid, and hypochlorous acid, as well as methods for efficiently synthesizing organic substances such as KA oil from cyclohexane (AIST press releases, March 12, 2012, March 6, 2015, August 2, 2018).
In this study, we fabricated photoelectrodes with partially modified surfaces with metal oxides, and investigated the effect of various reaction conditions, such as a wide range of chloride ion (Cl-) concentrations and low pH ranges, on reaction selectivity, in order to elucidate the catalytic mechanism and improve the water electrolysis hydrogen production system.
Part of this research and development was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) through the “Creation of Innovative Photochemical-Material Conversion Systems by Elucidating the Molecular Mechanism of Photosynthesis and Controlling Space-Time” (FY2017-2021).
Research content
Visible-light-absorbing BiVO4/WO3/FTO photoelectrodes can be easily fabricated by spin-coating precursor solutions containing tungsten or bismuth ions and vanadium ions onto conductive glass (fluorine-doped tin oxide, FTO) and firing in air. In this study, a photoelectrode was prepared by spin-coating a precursor solution containing various metal ions such as manganese onto such a photoelectrode and firing it in air.
The photoelectrode and counter electrode were placed in a two-chamber reaction vessel with an ion exchange membrane, and the ability to produce hydrogen and oxygen by reducing and oxidizing water, and the ability to produce HClO by oxidizing Cl- ions, were evaluated in an electrochemical reaction system using a reaction solution containing sodium chloride (NaCl) (Figure 1, left). Hydrogen generation occurs at the counter electrode. When the photoelectrode was irradiated with simulated sunlight, oxygen and HClO were produced simultaneously when using a photoelectrode with no surface modification. On the other hand, when using a photoelectrode with a manganese-modified surface, almost no HClO was produced, and only oxygen was produced with high selectivity (over 90%) (Figure 1, right). When using a photoelectrode with a surface modification of a metal element other than manganese, HClO and oxygen were produced. The behavior of the significant suppression of HClO production on the manganese-modified photoelectrode was almost unchanged by the pH and Cl- concentration of the NaCl aqueous solution used, the difference in the crystal structure of the manganese precursor or manganese oxide, or the combination with a different element (such as the calcium combination of Mn/Ca = 4), and it was clarified that manganese is a very unique element that can selectively suppress HClO production while generating oxygen under a very wide range of conditions (Figure 2). We confirmed that this behavior is also reproduced in artificial seawater containing a wide variety of coexisting ions. This unique property of manganese is thought to be due to the catalytic action inherent in the manganese element, which causes the overpotential for HClO production to be significantly higher than that for oxygen production.
In addition to contributing to the practical application of solar hydrogen production technology using seawater, the results of this study will also contribute to a deeper understanding of natural photosynthesis. The oxygen-evolving center of natural photosynthesis is composed of manganese oxide aggregates, but the reason why the element manganese is used was unclear. The results of the experiment using the photoelectrode have allowed us to propose a new hypothesis: “The specific properties of manganese, which can suppress the generation of HClO, which is harmful to living organisms, under a wide range of conditions, are involved in the evolution of the oxygen-evolving center”. We believe that the mutual understanding and fusion of natural photosynthesis and artificial photosynthesis research will contribute to the construction of innovative systems.
Figure 1
Figure 1: Left: Apparatus for evaluating the reaction of oxygen and HClO production from a NaCl aqueous solution using a photoelectrode
Right: Effect of surface-modified oxide on the selectivity of this reaction system
Figure 2
Figure 2: Effect of reaction solution conditions on the reaction of oxygen and HClO formation from a NaCl aqueous solution using a photoelectrode modified with manganese
Future Plans
In the future, we will conduct research and development aimed at the practical application of hydrogen production using sunlight, including improving the long-term stability of the photoelectrode developed this time. We will also work to clearly prove the hypothesis we have proposed regarding the evolution of natural photosynthesis.
Glossary
◆Photoelectrode
A photocatalyst with semiconductor properties is deposited on a conductive substrate such as fluorine-doped tin oxide (FTO) or tin-doped indium oxide (ITO), and then connected to a conductor to form an electrode. When the semiconductor absorbs light, the electrons in the valence band (e-) move up to the conduction band, and these electrons flow into the counter electrode to produce a photocurrent. At the counter electrode, the electrons promote a reduction reaction. In this research, water is reduced to produce hydrogen. On the photoelectrode, however, the holes (h+) that are produced in the valence band as a result of the electrons leaving the valence band promote an oxidation reaction. By using light energy, the reaction can proceed at a lower voltage than normal electrode reactions. [Back to reference source]
◆Hypochlorous acid (HClO)
One of the oxoacids of chlorine, hypochlorous acid ions (ClO-) and salts of hypochlorous acid are used as oxidants, bleaching agents, disinfectants and disinfectants. Hypochlorous acid is generally produced by dissolving chlorine in aqueous solution or electrolyzing brine. [Back to reference source]
◆Oxygen-evolving center
In the protein complex called photosystem II, which is found in the chloroplasts of algae and plants, the oxygen-evolving reaction, which produces oxygen from water, occurs. The oxygen-evolving center is the part that efficiently promotes the oxygen-evolving reaction. The oxygen-evolving center of natural photosynthesis is composed of a Mn4CaO5 cluster consisting of four manganese and one calcium atoms, and it plays the role of a catalyst for oxygen evolution. [Back to the reference source]
◆Selectivity
This refers to the ratio of the desired product obtained from the oxidation products. On electrodes and photoelectrodes, oxygen and oxidants are simultaneously produced as oxidation products, so this refers to the ratio of the desired product to the current that flows. In this study, it refers to the ratio of the production of oxygen and HClO. [Back to reference source]
◆Spin coating
This is a simple method that has been used for many years to form a uniform thin film on a substrate. In general, a thin film is formed by dropping a solution onto the substrate surface and spinning it at high speed for a certain period of time. [Back to the reference source]
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