CoFe-CoxN heterojunction encapsulated by lignin-derived nitrogen-doped biochar as bifunctional oxygen electrocatalysts for zinc-air batteries

The development of low-cost and efficient bi-functional electrocatalysts are still the main challenging for oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) in zinc-air batteries. In this work, CoFe-CoxN heterojunction nanocatalyst encapsulated by N-doped biochar was synthesized by combining carboxylated lignin with metal ions via self-assembly and in situ pyrolysis reduction. The heterojunction endowed the superior oxygen electrocatalytic performance, and the carbon-encapsulated structure improved the stability and corrosion resistance in electrolyte. It exhibited a positive half-wave potential of 0.79 V for ORR and a low overpotential of 404 mV at 10 mA cm−2 for OER. The corresponding zinc-air battery displayed a high-power density of 178 mW·cm−2 and excellent stability. DFT calculation indicated that the existence of CoFe-CoxN and the N-doped carbon layer could improve the electron transfer efficiency. This study demonstrated a viable and eco-friendly design for alloy and nitride heterojunction electrocatalysts for energy conversion by using lignin-metals complex.