An efficient bifunctional oxygen catalyst of CNT@FeCo–C3N4 & Ni7Fe-LDH composite for rechargeable ammonium ion-air battery

Abstract

Metal and nitrogen co-doped carbon-based materials have garnered significant attention due to their high oxygen reduction reaction (ORR) activity and earth-abundance, but their practical applications are limited by susceptibility to oxidation and insufficient oxygen evolution reaction (OER) performance, necessitating strategies to enhance both stability and bifunctionality. To address these challenges, this work proposes a hybrid composite catalyst, CNT@FeCo–C₃N₄ & Ni₇Fe-LDH, which synergistically integrates the advantages of multiple components. The core–shell structured carbon nanotubes (CNTs) coated with FeCo-doped g-C₃N₄ not only enhance the ORR activity and stability through the synergistic effect of Fe/Co active sites and the high surface area of C₃N₄, but also provide a conductive framework to facilitate electron transfer. Furthermore, the integration of NiFe-layered double hydroxide (NiFe-LDH) introduces robust OER-active sites, addressing the OER bottleneck. The composite exhibits exceptional ORR activity with a half-wave potential of 0.753 V vs. RHE, decaying 11 mV after 1000 accelerated durability test (ADT) cycles. Simultaneously, it achieves an OER overpotential of 445 mV at 10 mA cm⁻² with merely 152 mV degradation post-cycling, The optimized bifunctional index (ΔE = 0.921 V) outperforms commercial benchmarks Pt/C & RuO₂ (ΔE = 0.944 V). When implemented in an ammonium ion-air battery, the system delivers high specific capacity (70.084 mA h g⁻¹ at 1 A g⁻¹) with 77.7% capacity retention after 2000 cycles.