Unravelling the role of iron carbide in oxygen reduction catalysts for rechargeable zinc–air batteries: a comprehensive kinetics & mechanistic study

Abstract

Rechargeable metal–air batteries are an emerging electrochemical energy storage technology wherein the invention of bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) plays a critical role in the device's performance. Recently, Fe₃C-based carbon materials have been reported for their excellent bifunctional activity. The lack of understanding in defining the role of Fe₃C impedes the design of high-performance catalysts. Fe₃C on carbon (Fe₃C/C) and Fe₃C on nitrogen-doped carbon (Fe₃C/NC) are synthesised using various carbon substrates. The mechanistic analysis from the rotating ring-disk electrode voltammogram demonstrates the formation of 4-electron active sites while introducing Fe₃C and nitrogen in the graphitic carbon substrate, and the reason for the superior ORR activity is discussed. The activity and stability are evaluated using a zinc–air battery experiment. Fe₃C & N-doped Vulcan carbon exhibits higher power density (167 mW cm⁻²) and specific capacity (756 mA h g_Zn⁻¹ @ 20 mA cm⁻²). Besides, Fe₃C & N-doped Vulcan carbon shows excellent stability with 88% efficiency after 300 cycles (20 minutes per cycle) of continuous charge–discharge operation at a current density of 5 mA cm⁻². All the Fe₃C/NC catalysts show excellent stability in the OER region, and this study guides the futuristic design of Fe₃C-based catalysts for zinc–air battery electrodes.