Recent Progress of Bifunctional Electrocatalysts for Oxygen Electrodes in Unitized Regenerative Fuel Cells
Unitized regenerative fuel cells (URFCs), which oxidize hydrogen to water to generate electrical power under the fuel cells (FCs) mode and electrolyze water to hydrogen under the water electrolysis (WE) mode for recycling, are known as clean and sustainable energy conversion devices. In contrast to the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) on the hydrogen electrode side, the sluggish kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on the oxygen electrode side requires highly efficient bifunctional oxygen catalysts. Conventional precious metal oxygen catalysts combine Pt and IrO2 with excellent ORR and OER activities, respectively, to achieve bifunctional electrocatalysis performance, but the scarcity and high cost of precious metals have restricted their applications. Although platinum group metal (PGM)-free bifunctional catalysts circumvent the problems of high price and scarce resources, they suffer from insufficient activity and poor stability. Therefore, much attention has been paid by researchers to developing efficient, durable and low-cost bifunctional oxygen catalysts. In this review, we mainly introduce the recent advances in bifunctional oxygen catalysts for URFCs with a focus on the catalyst design, activity, and durability. First of all, the fundamental understanding of the ORR and OER mechanisms is essential prior to discussing the development of bifunctional oxygen catalysts. Starting from activity descriptor-based approaches in the identification of catalyst activity, this review summarizes the alternative catalyst design strategies confronted with the unfavorable scaling relationships existing among the binding energies of different oxygen-containing reaction intermediates during ORR and OER. Subsequently, in addition to introducing the design strategies of conventional PGM-based bifunctional catalysts, the recent progress of PGM-free bifunctional catalysts, including perovskite oxides, spinel oxides, other transition metal compounds, and carbon-based (non-metal) catalysts is presented in terms of their structure-property relationships. Various strategies have been developed by researchers to optimize the performance of PGM-free bifunctional catalysts, such as nanostructuring, defects engineering, heteroatom doping, phase and composition modulation, support coupling and morphology engineering, etc. Some PGM-free bifunctional catalysts reported in the literatures show promising results with ORR and OER activities superior to Pt+IrO2 in alkaline environment. In general, although great progress has been made on PGM-free bifunctional electrocatalysts, their cycling durability are still far from that of precious metal catalysts, and few of them have been applied in acidic environments. Therefore, much more efforts are needed to improve the stability of PGM- free bifunctional catalysts. Lastly, the challenges and future development of designing optimal bifunctional oxygen catalysts are discussed.