Bimetallic melamine doped ZnCo-ZIF-8 derived carbons as bifunctional ORR/OER electrocatalysts for zinc–air batteries

Abstract

Zinc–air batteries are promising for energy storage due to their high energy density, low-cost and safety, yet their performance is limited by sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics. Developing efficient bifunctional catalysts is thus crucial for advancing metal–air batteries. In this study, a porous carbon nanomaterial embedded with cobalt nanoparticles is synthesized through the pyrolysis of a Co–Zn bimetallic zeolitic imidazolate framework (ZIF) doped with melamine, demonstrating exceptional activity as a bifunctional ORR & OER catalyst. The catalyst yields a nitrogen-doped porous carbon with carbon nanotube structure. The further optimisation of the synthesis conditions revealed a catalyst (mCo₃Zn₁@CN(800)) showing significant bifunctional electrocatalytic activity for ORR and OER, giving a low potential gap (ΔE) of 0.68 V, which matches the precious metals based benchmark materials. The catalyst shows excellent activity in ORR with onset potential of 0.98 V and half-wave potential of 0.89 V with a Tafel slope of 44.6 mV dec⁻¹. Similarly, it shows remarkable OER activity with overpotential (η_OER) of 0.34 V at 10 mA cm⁻². Furthermore, a series of these catalysts were employed as air electrode in a Zn–air battery, achieving a high open-circuit voltage of 1.430 V, and exhibiting excellent cycling stability over 250 hours (1250 cycles) at 2 mA cm⁻². The enhanced electrochemical performance could be attributed to the characteristics of the catalysts, including high surface area with well dispersed, exposed, and stable Co–N_x active sites, as well as graphitic carbon and carbon nanotubes. These features collectively promote the oxygen reactions of both OER and ORR.