Supercritical CO2-assisted synthesis of high-density Co clusters/N-doped porous carbon as bifunctional oxygen electrocatalyst for rechargeable Zn-air batteries

Rechargeable zinc-air batteries (ZABs) hold great promise for next-generation energy storage, but their practical application depends on the development of efficient bifunctional catalysts capable of catalyzing both the oxygen reduction reaction (ORR) during discharge and the oxygen evolution reaction (OER) during recharge. Herein, we propose a high-performance bifunctional oxygen electrocatalyst for ZABs, fabricated by encapsulating Co clusters into zeolite imidazole framework-8 (ZIF-8)-derived carbon via a supercritical CO₂ (scCO₂) fluid-assisted method. The dual-protection combining spatial confinement from porous carbon frameworks and strong Co-N coordination anchoring enables the stabilization of high-density Co clusters and preventing its aggregation. Furthermore, the scCO₂ treatment reconstructs the mesoporous structure, significantly improving mass transport and exposing more accessible active sites. Density functional theory (DFT) calculations demonstrate that the surface Co-N moieties serve as highly active centers for the ORR, whereas the metallic Co sites within the clusters predominantly drive the OER. As a result, Co@N–C exhibits a remarkably low potential gap (ΔE = 0.68 V) between ORR and OER. When applied in aqueous ZABs, the catalyst delivers an impressive specific capacity of 780 mAh g_Zn⁻¹ and a peak power density of 170 mW cm⁻², surpassing Pt/C + RuO₂. Moreover, the solid-state ZABs assembled with this catalyst achieve a high peak power density of 87.0 mW cm⁻² along with long-time cycling stability of 200 h, demonstrating great potential for flexible and portable energy storage devices.