High-Loading Single Atoms via Hierarchically Porous Nanospheres for Oxygen Reduction Reaction with Superior Activity and Durability

Rational design and facile synthesis of single-atom catalysts featuring high-density active sites and favorable mass transport are crucial for electrocatalysis. Herein, a facile route is reported to craft a battery of high-loading (up to 9.36 wt.%) and readily accessible single transition-metal atoms anchored on hierarchically porous hollow carbon nanospheres (denoted TM-SAC-HC; TM═Fe, Co, Ni, and Cu) as robust electrocatalysts for oxygen reduction reaction (ORR). Intriguingly, the TM-SAC-HC possesses a hollow interior with well-structured porosities on the carbon shell. Such hierarchically porous hollow carbon nanospheres adequately expose the dense metal-atom active sites, boosting the mass transport. Remarkably, Fe-SAC-HC in an alkaline electrolyte manifests a superior ORR activity (E_1/2 = 0.92 V) and an excellent durability (ΔE_1/2 = −15 mV after 30 000 potential cycles and 90% current retention after 48 h continuous operation), outperforming most state-of-the-art TM-based catalysts and commercial Pt/C. Zinc–air batteries assembles using Fe-SAC-HC as the air electrode deliver a peak power density of 186.6 mW cm⁻² and a special capacity of 805.7 mAh g⁻¹. Moreover, theoretical calculations reveal that Fe─N₄ moieties situated within micropores significantly lower energy barriers, leading to superior ORR activity. This work provides a foundation for the rational design of high-efficiency catalysts for energy conversion and storage.