A design of electrocatalysts with controlled oxidation state and pore structure for oxygen evolution reaction in acidic media

Abstract

To maximize the efficiency of renewable energy utilization, it has been suggested that the water electrolysis using surplus power to produce hydrogen, which is a promising energy carrier. In particular, a proton exchange membrane water electrolyzer (PEMWE) has many advantages of high purity H2 production, rapid response rate against load, and supports high current density operation, etc. However, there are still challenges to the commercialization of PEMWE technology as follows: (1) performance (2) durability, and (3) cost. Since the oxygen evolution reaction (OER), which occurs at the anode of the WE, is sluggish and proceeds under harsh conditions (e.g., low pH, high voltage), noble metals such as Ir and Ru have been used as OER active materials. This degrades the price competitiveness of the PEMWE system. To resolve this issue, many researchers had made an effort to realize highly active and stable OER catalysts. In this dissertation, we aimed to design high efficient OER catalysts for PEMWE, by controlling oxidation state properties and the pore structure of nano-materials. As a first strategy, we designed the series of Ir/IrOX catalysts with the different ratios of highly active Ir(Ⅲ) to stable Ir(Ⅳ) at the catalyst surface via a novel solution-reduction process, which is suitable for mass production. These catalysts have also the unique feature that a hierarchical distribution of oxidation states of Ir in nanostructures. By evaluating electrochemical properties at the level of practical device (i.e., single-cell) as well as the half-cell, the enhanced catalytic performance for OER could be confirmed in acidic media. In addition, it was investigated that the effect of thermally treated electrodes on cell performance. As the second approach, by preparing a well-defined mesoporous structure via the sequential combination of hard template and the Adams method, we tried to understand the effects of the pore structures on the catalytic activity and stability for OER.