Photoactive Electrocatalyst with Staggered p-n Heterojunction for Rechargeable Zn-Air Batteries

Abstract

Rechargeable zinc-air batteries (RZBs) demonstrate significant potential as next-generation batteries as they are non-flammable, resource-abundant, cost-efficient, feasible for ambient air operation, and have remarkable theoretical energy density (1086 Wh kg-1). However, the development of RZBs needs to address many challenges to speed up their commercialization. For example, low round-trip efficiency during cycling is caused by retarded reactivity of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the air cathode. An air cathode with deficient electrocatalytic kinetics creates a large voltage gap, especially when the practical charge voltage is > 2.0 V, during the charging and discharging process. Moreover, the cycle life of the RZBs is limited owing to the structural degradation of air cathodes working under high charge voltage. Therefore, it is essential to improve both the round-trip efficiency and structural durability of air cathodes to ensure further development and commercialization of RZBs. For efficient energy generation and storage, photo-enhanced RZBs that combine photochemical and electrochemical energy conversion and storage with the help of solar energy, are a promising research focus. Photo-enhanced RZBs require a photoactive OER/ORR bifunctional cathode that is not merely an electrochemical catalyst but also a photo-active catalyst.In the present study, we developed a photoactive bifunctional air-electrocatalyst with a staggered p-n heterojunction band gap structure, comprising n-type g-C3N4 and p-type copper-doped ZIF-67 (CuZIF-67) composite, to accelerate sluggish OER/ORR processes via a durable structure and stable long-cycle-life performance for photo-enhanced RZBs. g-C3N4/CuZIF-67 has a suitable band gap, which ensures wide-range light absorption in the UV–Vis–NIR region, and stimulates more electrons to participate in ORR/OER electrocatalytic activity. Besides, it has a staggered bandgap structure and a BEF at the p-n junction of the catalyst, which improves electron–hole pairs separation efficiency, lowers overpotential, promotes ORR/OER electrocatalytic activity, and eventually enhances the efficiency of RZBs (the voltage gap was significantly decreased by 14% with simulated solar light irradiation). Moreover, a high concentration of pyrrolic-N groups in the cathode material, resulting in the excellent electrochemical durability of RZBs during charge/discharge cycles. The photo-enhanced RZB based on the g-C3N4/CuZIF-67 bifunctional catalyst exhibits good round-trip efficiency of 60% (2 mA cm-2) with a significantly decreased voltage gap (from 0.92 to 0.81 V) after 1000 cycles, under simulated solar light irradiation. It can be concluded that staggered p-n heterojunction and pyrrolic nitrogen-rich groups play an important role in efficient and durable photo-enhanced Zn-air batteries.