Metal–organic framework-derived Co single atoms anchored on N-doped hierarchically porous carbon as a pH-universal ORR electrocatalyst for Zn–air batteries

Abstract

Developing non-Pt-group-metal (non-PGM) electrocatalysts with excellent pH-universal catalytic activity offers bright prospects in the context of the electrochemical oxygen reduction reaction (ORR). Herein, we report a simple spatial-isolation strategy for the fabrication of atomic Co catalytic sites anchored on hierarchical porous N-doped carbon (Co-SAs/N–C/rGO) as a competitive candidate for ORR across a wide pH range. The resultant Co-SAs/N–C/rGO catalyst exhibits pH-universal ORR activity with half-wave potentials (E_1/2) of 0.84, 0.77, and 0.65 V vs. RHE in alkaline, acidic, and neutral media, respectively, which are comparable to the commercial Pt/C (0.85, 0.79, and 0.65 V vs. RHE, respectively). These superior ORR catalytic activities across a wide pH range are attributed to the combined effect of atomic active sites, a large specific surface area, and a hierarchical porous structure. As a demonstration, the homemade liquid Zn–air battery (ZAB) assembled with Co-SAs/N–C/rGO as the cathode catalyst displays an open-circuit voltage (OCV) of ∼1.52 V and a discharging specific capacity of 671.94 mA h g⁻¹, outperforming those of the device with commercial Pt/C + RuO₂ (1.49 V and 657.32 mA h g⁻¹, respectively). Notably, the corresponding flexible solid-state ZAB manifests state-of-the-art OCV, peak power density, foldability, and cycling stability. Based on density functional theory (DFT) calculations, the CoN₄ configuration is the real catalytically active center for ORR. Besides, the introduction of the Co active center can lower the energy barrier of the rate-determining step (RDS). This work opens an avenue for the rational design of non-PGM electrocatalysts with pH-universal adaptability for energy storage and conversion.