Recent Progress on the Hierarchically Porous Carbon for High-Performance Bifunctional Oxygen Electrocatalyst in Rechargeable Zn-Air Batteries

Zinc-Air batteries (ZABs) are in the spotlight as next-generation energy storage devices due to their high theoretical energy density and the inherent safety of aqueous electrolytes. Unfortunately, the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the air cathode limit the practical performance and lifespan of rechargeable ZABs. To overcome these issues, extensive efforts have been devoted to developing high-performance bifunctional OER/ORR electrocatalysts by enhancing the intrinsic activity of non-noble metal catalysts and by reducing the reliance on noble metals. Although substantial progress has been achieved at the catalyst level, performance often degrades due to unstable active sites and the complex three-phase reactions occurring at the catalyst/electrolyte/oxygen interfaces. In this context, hierarchically porous carbon (HPC) structures were widely adopted not only for enhancing the density of exposed catalytic active sites but also for promoting efficient mass and electron transport between the interfaces. In particular, the different sizes of pores in HPC enable distinct functions: micropores (<2 nm) provide abundant reaction sites, mesopores (2-50 nm) improve reactant accessibility to active sites, and macropores (>50 nm) enhance overall mass-transfer efficiency by facilitating gas/electrolyte transport pathways. This multi-scale pore structure is a key component for implementing ZAB with high performance and long-term durability. In this review, we summarize recent advances in synthesis strategies for HPC-based bifunctional electrocatalysts and highlight design principles for next-generation rechargeable ZABs.