Abstract
Recently, zinc–air batteries (ZABs) have been receiving attention due to their theoretically high energy density, excellent safety, and the abundance of zinc resources. Typically, the performance of the zinc air batteries is determined by two catalytic reactions on the cathode—the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Therefore, intensive effort has been devoted to explore high performance electrocatalysts with desired morphology, size, and composition. Among them, single-atom catalysts (SACs) have emerged as attractive and unique systems because of their high electrocatalytic activity, good durability, and 100% active atom utilization. In this review, we mainly focus on the advance application of SACs in zinc air batteries in recent years. Firstly, SACs are briefly compared with catalysts in other scales (i.e., micro- and nano-materials). A main emphasis is then focused on synthesis and electrocatalytic activity as well as the underlying mechanisms for mono- and dual-metal-based SACs in zinc air batteries catalysis. Finally, a prospect is provided that is expected to guide the rational design and synthesis of SACs for zinc air batteries.
Highlights
single-atom catalysts (SACs) have been introduced in the zinc–air batteries (ZABs) field as unique and new frontier catalysts
Among the diverse SACs, Co- and Fe-SACs perform best for the oxygen reduction reaction (ORR), while more kinds of metal that used to be considered less active catalysts have been exploited as active SAC catalysts for ZABs
To satisfactorily catalyze the reactions in ZABs, a sufficient number of active sites and a high intrinsic activity of the active sites are extremely critical for the catalysts
Summary
Nowadays, based on the current advanced technology, the energy density of lithium ion batteries can reach only approximately 100~250 Wh kg−1. The low cost M–Nx/C catalysts that use diverse transition metals as their centers exhibit outstanding catalytic activity for the ORR/OER in ZABs and have been regarded as alternatives to Pt-based catalysts [50–52,56,57]. It has been suggested that these catalysts’ highly dispersed as well as coordinatively unsaturated environment and enhanced charge-transfer effect by the interaction of the metal-support remarkably increase catalytic active sites. Their unique electronic structures result in the appropriate adsorption energies for oxygen and decrease the free energies, all of which eventually leads to improved intrinsic catalytic activity and the four-electron pathway for the ORR [58,59]. Among the diverse non-noble metals, Fe and Co are the most commonly employed metals used to construct M–Nx moiety sites and have been proven to perform best for various electrochemical catalytic reactions [37,60–65]
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