Metal oxide derivatives as a co-catalyst for efficient and selective (photo)electrochemical reaction

Abstract

There are worldwide efforts to develop alternative energy to overcome climate change problems caused by an increase in atmospheric carbon dioxide concentration and environmental problems caused by industrial development. Although there are many types of alternative energy, the generation of chemical energy using solar energy or electricity has the advantage of being environmentally friendly, extensive, and can be made with relatively simple equipment. Chemical energy production reactions mentioned above include hydrogen evolution reaction (HER), oxygen evolution reaction (OER), carbon dioxide reduction reaction (CO2RR), and ammonia production reaction. Such chemical reactions have been conducted by noble metals, metal oxides or organic catalysts. There are many factors to determine the catalytic characteristics, including a reaction surface area, binding energy and activation energy, selectivity to a specific product, catalytic stability. In this study, firstly, the photoelectrochemical water oxidation reaction on hematite photoanode has been improved by activating the reaction site. The oxidation state of cobalt ions of the cobalt oxo/hydroxide cluster was reduced through nitrogen plasma treatment. As a result, electron/hole pairs were efficiently separated through nitrogen atoms doped in cobalt oxo/hydroxide. As the portion of $Co^{2+}$ ions increases, photoelectrochemical (PEC) water oxidation reaction has been enhanced by increasing the number of reaction sites while preventing charge recombination. In the second topic, the electrochemical ammonia generation reaction has been improved through introducing the co-catalyst at reaction active site. In the process of electrochemically converting nitrate ions into ammonia, various intermediate products are generated. Therefore, Fe-polyoxometalate (Fe-POM), which has the potential to generate hydrogen radicals, was attached to copper metal, which has good binding affinity to nitrate ions. As a result, not only the kinetics of the electrochemical ammonia synthesis but also the selectivity to ammonia were improved through providing hydrogen radicals to the reaction active site. Both studies use metal oxides as catalytic materials for facilitating the electrochemical reaction. In the case of cobalt or iron oxide, since it is relatively abundant on earth and can be used as a low-cost catalyst material, it has the advantage of being able to replace the noble metal-based catalyst material that has been widely used in chemical energy production. In addition, the strategy to activate reaction sites by plasma treatment or to enhance the selectivity to target product by introducing co-catalyst to reaction sites is expected to contribute to various catalytic reactions as an electrochemical catalyst surface treatment or modification method.