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

As one of the most promising alternates for future energy system, rechargeable Zn-air battery (ZAB) has attracted extensive attentions due to their extraordinary 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 electrode, like noble metal catalysts (Pt/C or Ru/IrO2), it has posed 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 conclude the latest development of non-noble metal bifunctional ORR/OER electrocatalysts for ZABs. Firstly, the related reaction mechanisms of ORR and OER are introduced. Then, the latest development of bifunctional ORR/OER materials for ZABs is detailed discussed following three aspects: (i) MOFs-based catalysts, including pristine MOFs and their derivatives; (ii) metal-free based carbon catalyst, 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 electrode for rechargeable ZABs with high activity and ultra-long lifetime are put forward.