Corresponding Author

Xin-Biao Mao (xbmao@zjut.edu.cn);
Qing-Gang He(qghe@zju.edu.cn)


Unitized regenerative fuel cells (URFCs), which oxidize hydrogen to water to generate electrical power under thefuel cells (FCs) mode and electrolyze water to hydrogen under the water electrolysis (WE) mode for recycling, areknown as clean and sustainable energy conversion devices. In contrast to the hydrogen oxidation reaction (HOR) andhydrogen 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 oxygencatalysts. Conventional precious metal oxygen catalysts combine Pt and IrO2 with excellent ORR and OER activities toachieve bifunctional electrocatalysis performance, but the scarcity and high cost of precious metals have restricted theirapplications. Although platinum group metal (PGM)-free bifunctional catalysts circumvent the problems of high priceand scarce resources, they suffer from insufficient activity and poor stability. Therefore, much attention has been paidby researchers on developing efficient, durable and low-cost bifunctional oxygen catalysts. In this review, we mainlyintroduce the recent advances in bifunctional oxygen catalysts for URFCs focusing on the catalyst design, activity, anddurability. First of all, the fundamental understanding of the ORR and OER mechanisms is essential prior to discussingthe development of bifunctional oxygen catalysts. Starting from activity descriptor-based approaches in the identificationof catalyst activity, this review summarizes the alternative catalyst design strategies confronted with the unfavorablescaling relationship existing among the binding energies of different oxygen-containing reaction intermediatesduring ORR and OER. Subsequently, in addition to introducing the design strategies of conventional PGM-basedbifunctional 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 compositionmodulation, support coupling and morphology engineering, etc. Some PGM-free bifunctional catalysts reported in theliterature show promising ORR and OER activities superior to Pt+IrO2 in an alkaline environment. In general, althoughgreat progress has been made on PGM-free bifunctional electrocatalysts, their cycling durability is still far from thatof precious metal catalysts, and few of them have been applied in acidic environments. Therefore, much more effortsare needed to improve the stability of PGM-free bifunctional catalysts. Lastly, the challenge and future development ofdesigning optimal bifunctional oxygen catalysts are discussed.

Graphical Abstract


Unitized regenerative fuel cell, Bifunctional electrocatalyst, Oxygen reduction reaction, Oxygen evolutionreaction, Pt-based electrocatalyst, Non-Pt group electrocatalyst

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Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

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