Hyper-Cross-Linked Polymer-Derived Carbon-Coated Fe–Ni Alloy/CNT as a Bifunctional Electrocatalyst for Rechargeable Zinc–Air Batteries

Abstract

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are considered to be the most important processes in metal–air batteries and regenerative fuel cell devices. Metal–organic polymers are attracting interest as promising precursors of advanced metal/carbon electrocatalysts because of their hierarchical porous structure along with the integrated metal–carbon framework. We developed carbon-coated CNTs with Ni/Fe and Cu/Fe as active sites. Experimental observations from X-ray photoelectron spectroscopy and X-ray absorption analysis suggest that C@CNT[Ni] outperforms C@CNT[Cu] in the ORR and OER, which is further supported by density functional theory calculations. C@CNT[Ni] exhibits a higher onset potential (0.99 V vs RHE) and a smaller Tafel slope (40.2 mV decade^–1) compared to those of C@CNT/[Cu] in an alkaline electrolyte (0.94 V vs RHE and 46.5 mV decade^–1, respectively). Such circumstances are attributed to the alloying effect between Ni and Fe in C@CNT[Ni], in contrast to the existing copper iron oxide phase in C@CNT/[Cu]. It is noteworthy that C@CNT[Ni] also displayed an improved OER, demanding its bifunctional property. As a proof of concept, C@CNT[Ni] was utilized in zinc–air batteries, which shows a high energy efficiency of ∼60%, a small charge–discharge voltage gap of 0.78 V, and excellent cycling performance (∼120 h) at 5 mA cm^–2 and 25 °C. This protocol expands the utility of novel metal–organic hyper-cross-linked polymer-derived bimetallic electrocatalysts for clean energy research.