Surface Modulation Insights of High-Performing Ni–Fe Nitride Fuel Cell and Electrolyzer Electrocatalysts

Abstract

Catalysis has been crucial in advancing and commercializing energy conversion technologies. It is essential to identify abundant, active, and stable materials to enable the reliable and cost-efficient use of catalysts in renewable technologies, such as fuel cells (FCs) and electrolyzers. Suitable candidates, such as nonprecious metals, can be found in first-row transition metals, where materials such as bimetallics, metal oxides, and metal nitrides can be readily synthesized. Recently, these materials have exhibited high activity toward the oxygen reduction (ORR) and oxygen evolution (OER) reactions in alkaline media, which, in turn, were related to promising performance in FCs and electrolyzers. However, most of these studies have not gone beyond half-cell reactions. In this study, we explored the synthesis of a metal nitride, Ni₃FeN, and its application as an electrocatalyst for ORR and OER. We developed procedures for the synthesis of Ni₃FeN nanocrystals with different carbon loadings using a one-step ammonolysis route. We show that the pristine structure of the material encompasses a nitride core and an oxide shell with a thickness of a few nanometers. However, the bulk electronic structure is mainly dominated by the Ni₃FeN phase. The nitride exhibited an impressive and stable ORR performance in 1 M KOH favoring the 4 e^– pathway. The material exhibited a slight decrease in E_1/2 of 10 mV (from 0.85 to 0.84 V vs RHE) during a prolonged (100 K) accelerated stress test (AST). The AST degradation at ORR potentials indicates that the catalyst aggregates into larger nanoparticles, forming a Ni@NiFeOx structure. After tests at OER potentials, the catalyst breaks into smaller nanoparticles and mainly favors the NiFeOx structure. MEA testing of the Ni₃FeN ORR catalyst in a hydrogen-fueled alkaline exchange membrane fuel cell (AEMFC) yielded a peak power density of ca. 700 mW/cm²; among the highest reported for nitride and NiFe-based materials. We believe that this work could enable the use of NiFe-based materials as viable, inexpensive alternatives for fuel cell applications.