Recent advances in design of hierarchically porous Fe1-Nx-C based electrocatalysts for zinc-air batteries

Zinc-air batteries with high theoretical energy density, earth-abundant raw materials, eco-friendliness and safety are considered as promising next generation energy devices. Their commercial advancement can be boosted with the development of inexpensive and high-performing oxygen reduction reaction (ORR) catalysts. The precious platinum-group metal-based nanoparticles dispersed in conducting carbon black (e.g., Pt/C) are the typical ORR catalysts. The iron-nitrogen-carbon-based materials, specifically comprising atomic-level iron-nitrogen coordination in hierarchical porous carbon support (usually denoted as Fe1-Nx-C), have shown promising electrocatalytic activities by delivering important half-wave and on-set potentials and reduction current densities along with high durability. This has been attributed to the favorable adsorptive and reduction ability of Fe1-Nx centers for molecular oxygen in alkaline electrolyte. Numerous studies have been focused on rational design of the hierarchically porous structures to enhance the accessibility of active Fe1-Nx sites and mass-transfer characteristics for efficient oxygen reduction and intermediate species. Therefore, in this review, several design strategies relevant to the template and self-template synthesis routes for hierarchically porous Fe1-Nx-C catalysts are insightfully presented. A detailed discussion is offered on the ORR activity and performance of Fe1-Nx-C catalysts in zinc-air batteries. Further opportunities and challenges for the rational design and application of Fe1-Nx-C catalysts are also discussed.