Coupling isolated Ni single atoms with sub-10 nm Pd nanocrystals embedded in porous carbon frameworks to boost oxygen electrocatalysis for Zn–air batteries

Abstract

Developing effective bifunctional catalysts for the oxygen reduction and evolution reaction (ORR/OER) is essential for accelerating the cathode efficiency of Zn–air batteries. Herein, atomically dispersed Ni single atoms are supported by sub-10 nm Pd nanocrystals embedded in N-doped carbon frameworks (Ni SAs-Pd@NC), in an effort to achieve superior bifunctional activity in both the ORR and OER. The key synthetic point depends on the protection mechanism of 1-naphthylamine, which could provide a carbon source for Ni SAs and restrict the Pd size under sub-10 nm during 600 °C pyrolysis, simultaneously. The synergistic effect of sub-10 nm Pd with superior ORR activity and Ni–N₄ SAs with favourable OER activity leads to bifunctional catalytic performance, meanwhile the rod-like carbon frameworks with ultrathin, porous and N-doped features contribute to accelerated electron transfer and structural robustness. As a proof-of-concept application, Ni SAs-Pd@NC demonstrates ultrahigh ORR activity with a positive half-wave potential of 0.84 V and a low OER overpotential of 380 mV at 10 mA cm⁻², in an alkaline medium. For a rechargeable Zn–air battery, the Ni SAs-Pd@NC cathode delivers a low charge–discharge voltage gap of 0.87 V, a high energy density of 884.6 W h kg_Zn⁻¹, a high power density of 134.2 mW cm⁻² and remarkable long-term cyclability for operation over 700 cycles, outperforming commercial Pt/C + RuO₂ benchmarks. This work successfully integrates single atom sites with small-sized noble metals to break out their incompatibility in synthesis and to improve their thermostability, which offers a versatile approach to develop single atom-based bifunctional catalysts for energy devices.