Recent Progress of Bifunctional Electrocatalysts for Oxygen Electrodes in Unitized Regenerative Fuel Cells

Abstract

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 catalysts combine Pt and IrO₂ with excellent ORR and OER activities 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 on developing efficient, durable and low-cost bifunctional oxygen catalysts. In this review, we mainly introduce the recent advances in bifunctional oxygen catalysts for URFCs focusing 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 relationship 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 relationship. 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 literature show promising ORR and OER activities superior to Pt+IrO₂ in an alkaline environment. In general, although great progress has been made on PGM-free bifunctional electrocatalysts, their cycling durability is 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 challenge and future development of designing optimal bifunctional oxygen catalysts are discussed.

Keywords

Unitized regenerative fuel cell / Bifunctional electrocatalyst / Oxygen reduction reaction / Oxygen evolution reaction / Pt-based electrocatalyst / Non-Pt group electrocatalyst

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