Iridium-containing water-oxidation catalysts in acidic electrolyte

With the goal of constructing a carbon-free energy cycle, proton-exchange membrane (PEM) water electrolysis is a promising technology that can be integrated effectively with renewable energy resources to produce high-purity hydrogen. IrO2, as a commercial electrocatalyst for the anode side of a PEM water electrolyzer, can both overcome the high corrosion conditions and exhibit efficient catalytic performance. However, the high consumption of Ir species cannot meet the sustainable development and economic requirements of this technology. Accordingly, it is necessary to understand the OER catalytic mechanisms for Ir species, further designing new types of low-iridium catalysts with high activity and stability to replace IrO2. In this review, we first summarize the related catalytic mechanisms of the acidic oxygen evolution reaction (OER), and then provide general methods for measuring the catalytic performance of materials. Second, we present the structural evolution results of crystalline IrO2 and amorphous IrOx using in situ characterization techniques under catalytic conditions to understand the common catalytic characteristics of the materials and the possible factors affecting the structural evolution characteristics. Furthermore, we focus on three types of common low-iridium catalysts, including heteroatom-doped IrO2 (IrOx)-based catalysts, perovskite-type iridium-based catalysts, and pyrochlore-type iridium-based catalysts, and try to correlate the structural features with the intrinsic catalytic performance of materials. Finally, at the end of the review, we present the unresolved problems and challenges in this field in an attempt to develop effective strategies to further balance the catalytic activity and stability of materials under acidic OER catalytic conditions.