Approaches to active-site formation of transition metal oxide for electrocatalytic oxygen evolution in alkaline media

Abstract

Hydrogen is one of the rising energy carriers due to its abundance and high energy density. Among several hydrogen production technologies, water electrolysis has been getting attention as a sustainable process that is easy to be integrated with renewable energy sources. To maximize the hydrogen production efficiency, it is crucial to develop both electrocatalyst for kinetically sluggish oxygen evolution reaction (OER) and ion exchange membrane. Anion exchange membrane water electrolysis (AEMWE) is based on a zero-gap cell using a non-porous membrane for hydroxyl ion transport, which is able to directly produce high purity of hydrogen gas at high current density. The alkaline environment can also extend OER catalyst candidate from expensive precious metal toward 4d transition metal such as Co and Mn. OER catalyst undergoes successive oxidation from metallic state or metal hydroxide to highly oxidized species, hence the catalyst oxidation state and structure are different from the initial state. Therefore, it is difficult to observe “real” active site and reaction mechanism by classical spectroscopy analysis, which also makes rational catalyst design difficult. With the series of researches in this dissertation, we aimed to maximize the active site in 4d transition metal for enhanced electrocatalytic oxygen evolution and understand the activity origins based on mechanistic studies using in situ technique. By considering the catalyst at the level of from atomic scale to crystal and mechanism, we extended the OER active site through three strategies: (i) increasing of metal oxidation state, (ii) complete phase transition, and (iii) shift of reaction mechanism, and confirmed the improvement of OER activities. Furthermore, we evaluated the AEMWE performance and hydrogen gas production rate of the developed OER catalysts.