Deriving paired Fe sites on N-doped carbon from Fe(iii)-tetrapyridinylporphyrin-modified ZIF-8: robust electrocatalysts for air electrodes in zinc–air batteries

Abstract

Sustainable electrochemical energy conversion and storage systems are expected to alleviate the energy shortages and environmental crisis that are currently facing the world. To bring the large-scale deployment of these energy systems to reality, the development of highly efficient, sustainable, and stable electrocatalysts is key. In this work, an electrocatalyst composed of homonuclear paired Fe sites coordinated onto N-doped carbon, labelled as Fe₂–N–C-900, is reported. The material is synthesized by preparing, and then pyrolyzing at 900 °C, Fe(III)-tetrapyridinylporphyrin-modified zeolitic imidazolate framework-8 (ZIF-8). The electrocatalyst displays excellent activity for the oxygen reduction reaction (ORR), with an onset potential of 0.990 V vs. RHE, a positive half-wave potential of 0.869 V vs. RHE, and a limiting current density of 6.7 mA cm⁻², which are all superior to those of the benchmark noble metal-based ORR catalyst Pt/C. This is attributed to its high-density of catalytically active Fe₂–N₆ sites, large electrochemically active surface area, and small charge transfer resistance. The electrocatalyst also functions well for the ORR on an air electrode in an aqueous zinc–air battery while showing high catalytic activity, remaining stable, and giving a peak power density of 166.3 mW cm⁻². Its performance here, too, exceeds that of a Pt/C-based zinc–air battery.