Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are a core pair in many energy storage and conversion systems, such as regenerative fuel cells and rechargeable metal-air batteries. Unfortunately, both reactions possess sluggish kinetics, involving multiple charge-/proton-transfers and oxygen bond breaking (in ORR)/recombining (in OER) processes at the “solid-liquid-gas” triple phase interfaces. Therefore efficient bifunctional catalysts are urgently needed. The most efficient oxygen-relative electrocatalysts are made from precious metal, but there is no a single catalyst based on precious metals that can serve the need for both ORR and OER.

Metal-organic frameworks (MOFs) are porous crystalline materials self-assembled with metal nodes and organic linkers with various compositions, high specific surface areas, tunable pore structures, and easily functionalized. Therefore, MOFs have provided opportunities for advancements in oxygen electrocatalysis. MOFs-based ORR/OER electrocatalysts are generally used in two ways: (1) by self-sacrificing through pyrolysis, and (2) in pristine phases. The former is at the expense of MOFs’ properties, while the latter falls short of meeting the practical target due to the poor conductivity of pristine MOFs, requiring carbon support. Conventional carbon supports are synthesized from fossil fuel precursors with harsh synthetic conditions and high energy consumption. Sustainable carbon materials should be derived from renewable resources (especially agriculture/food waste), and the preparation process should be green, facile and easy to scale up.

Herein, a Co(mIm)₂ (mIm=methyl-imidazole) MOF was incorporated with N-doped pomelo-peel-derived carbon (NPC) as a synergistic ORR/OER bifunctional catalyst in an alkaline electrolyte (Fig. 1A). Through rotating disk electrode voltammetry, an optimized composite exhibits an ORR/OER overpotential gap, ΔE (E_{OER@10 mAcm-2}-E_{ORR@-1 mAcm-2}), as small as 0.79V (Fig. 1B), putting the catalyst among the best of the reported bifunctional electrocatalysts so far. The catalyst also demonstrated a 4-electron transfer pathway and superior catalytic stability. The Co-N₄ ligation in MOF, electrochemical active surface area, and the strong interactions between MOF and support are attributed as the main contributors to the bifunctional catalytic activity. These factors act synergistically, resulting in substantially enhanced bifunctional catalytic activity and stability.