Architecting hydrangea-inspired nitrogen-doped hollow carbon with isolated Co atoms for superior oxygen reduction catalysis†

The oxygen reduction reaction (ORR) underpins energy technologies like metal–air batteries and fuel cells, yet it faces issues with slow kinetics and relies on costly, scarce noble metal catalysts. This work develops a facile approach by anchoring isolated Co atoms on defective nitrogen-doped carbon using low-temperature NH₄Cl pyrolysis along with metal ion adsorption. The as-prepared catalyst exhibits excellent ORR performance, achieving a high half-wave potential of 0.91 V in alkaline electrolytes and remarkable durability for 253 h with a retention of 97.47% at half-wave potential. Density functional theory (DFT) simulations confirm that the Co–N₄ moieties act as the active sites and elucidate substantial electronic structure modulations of Co centers during the ORR. These electronic structure modulations manifest as shifts in the Co atoms projected density of states and fluctuations in their local magnetic moments across various stages of the catalytic process. The findings presented herein advance both the practical synthesis of robust single-atom electrocatalysts and the theoretical understanding of their electronic structural properties.