Densely colonized isolated Cu-N single sites for efficient bifunctional electrocatalysts and rechargeable advanced Zn-air batteries

The rational design of earth-abundant, highly efficient, and robust bifunctional oxygen electrocatalysts remains a contemporary challenge toward the widespread implementation of reversible metal-air batteries and fuel cells. Here, we report a universal strategy for the fabrication of single-atom (Cu, Co, and Fe) incorporated hollow nano-spheroids of nitrogen-deficient carbon nitride frameworks (CuSA@HNCNx. The interconnected three-dimensional 3D porous and hollow robust single-atom spheroid frameworks display a high surface area of 1286 m² g⁻¹, favorable electronic structure, local chemical coordination, effective density of active sites (Cu-N_x, pyridinic, graphitic C–N etc.), and mass transport pathways. The obtained CuSA@HNCNx exhibited outstanding bifunctional reversible electrocatalytic activity and robustness for oxygen reduction and evolution reactions (ORR half-wave potential of 0.91 V, OER overpotential of 1.55 V at 10 mA cm^-2, ΔE = 0.64 V, 5000 cycles), outperforming benchmarked Pt/C and RuO₂. Electrocatalytic activity towards ORR/OER was analyzed by D-band center correlation using density functional theory (DFT) calculations. Moreover, reversible alkaline Zn-air batteries with the designed CuSA@HNCNx cathode illustrated a high power 212 mW cm^-2, high energy density 1031 Wh kg_Zn⁻¹, and excellent discharge-charge cycle life of 1800 cycles for 300 h @10 mA cm^-2 with voltaic efficiency of 64.51 %. Notably, all-solid-state flexible ZABs showed long cycle life of 250 h with 1500 cycles at 25 mA cm^-2 with efficiency of 66.31 %. This unique strategy offers controlled design of entangled single-atom frameworks as advanced cathodes for next-generation energy storage technology.