Bifunctional electrocatalysts for Zn-air batteries: recent developments and future perspectives

As one of the most promising alternatives for future energy systems, the rechargeable Zn-air battery (ZAB) has attracted extensive attention due to its extraordinarily high theoretical specific energy density. However, several obstacles restrict its practical application. One challenge is the sluggish kinetics of oxygen-reduction reaction (ORR) and oxygen-evolution reaction (OER) in the discharging and charging processes of ZABs. In addition, when using unifunctional ORR or OER electrocatalysts as air electrodes, like noble metal catalysts (Pt/C or Ru/IrO2), there are the disadvantages of high cost and poor stability. Therefore, rational design of non-noble metal bifunctional ORR/OER electrocatalysts with high activity and stability is essential for the development of ZABs. In this review, we discuss the latest developments of non-noble metal bifunctional ORR/OER electrocatalysts for ZABs. Firstly, the related reaction mechanisms of ORR and OER are introduced. Then, the latest developments of bifunctional ORR/OER materials for ZABs are discussed in detail from three aspects: (i) MOF-based catalysts, including pristine MOFs and their derivatives; (ii) metal-free-based carbon catalysts, including heteroatom-doped carbon and defective carbon; (iii) metal-based catalysts, including metal-nitrogen-carbon materials (such as metals/alloys, single-atom) and metal compound materials. Finally, some challenges and outlooks for the optimal design of bifunctional air electrodes for rechargeable ZABs with high activity and ultra-long lifetime are put forward.