Rational design of ZIF-derived nanocarbon with dual metal active sites via molten salt strategy for advancing oxygen electrocatalysis

Abstract

Designing competent bifunctional oxygen electrocatalysts is imperative for developing sustainable metal–air batteries and reversible fuel cells. The issue is that efficient oxygen reduction reaction (ORR) catalysts are not necessarily good in oxygen evolution reaction (OER) and vice versa due to the difference in the binding energy of the intermediates of the oxygen reduction and evolution reactions. Thus, materials that can be employed in both are in great demand. Nevertheless, their synthesis is quite challenging due to the agglomeration of metal atoms to form nanoparticles. Therefore, here, we have synthesized Fe in Co-based zeolitic imidazolate frameworks (Fe-ZIF-67) for further formation of a bimetal catalyst comprising Fe–N₄ and Co–N₄ sites on 2D N-doped carbon (NC) nanosheets with a hierarchically porous structure (Fe,Co-HPNC). It was constructed as the cathode for a flexible zinc–air battery (ZAB) via a one-pot synthesis approach followed by an unusual molten-salt-assisted pyrolysis strategy using eutectic salts, which is responsible for the formation of the bimetal in the nanocarbon sheets with exceptionally high surface area. The as-prepared catalyst exhibits remarkable bifunctional activities with a high half-wave potential of 0.84 V for the ORR, an overpotential of 310 mV at 10 mA cm⁻² current density for the OER, and a narrow potential gap (ΔE) between the ORR and OER of 0.70 V in an alkaline medium. The augmented catalytic activity arises from the synergistic effect of the Fe–Co bimetal-sites facilitating intermediate dissociation and the hierarchically porous 2D nanosheet structure promoting the exposure of more active sites and mass transfer during the reaction process. An in situ FTIR experiment was conducted to investigate the intermediates formed within the reactions. The excellent bifunctional activity of Fe,Co-HPNC opens a new route for developing catalysts for ZABs.