γ-FeOOH Nanosheet with Enormous Cationic Defect: Efficient and Durable Bifunctional Electrocatalyst Suitable for an Industrial-Scale AEM Electrolyzer

Abstract

Anion exchange membrane (AEM)-based electrolysis of alkaline water using a transition metal electrocatalyst is supposed to be the effective route for next-generation pure green hydrogen production, but development of a suitable electrocatalyst is challenging. Herein, we report the development of a simple and scalable protocol to grow a highly aligned ultrathin iron(III) oxyhydroxide (lepidocrocite, γ-FeOOH) nanosheet on nickel foam (γ-FeOOH-NS-NF) at room temperature (RT) through controlled simultaneous oxidation and hydrolysis in the presence of hydrazine. During synthesis, hydrazine plays crucial multiple roles, one of which is the generation of enormous Fe vacancies (VFe). The synthesized γ-FeOOH-NS-NF showed superior bifunctional water splitting activity because of its thin sheet microstructure and enormous cationic defect. Particularly at high current density, it showed an exceptionally low overpotential of 320 at η1000 and a Tafel slope of 29 for the OER, and 309 mV at η1000 and 65 mV dec–1 for the HER in aqueous 1 M KOH solution. For overall water splitting, a 10 mA cm–2 current density was observed at a low potential of 1.6 V. It showed 98% Faradaic efficiency and excellent stability for continuous operation over 100 h at 500 mA cm–2 current density. More importantly, a membrane electrode assembly (MEA) having γ-FeOOH-NS-NF in both the anode and cathode in a prototype anion exchange membrane (AEM) electrolyzer (4 cm2) showed outstanding water splitting performance and stability. The experimental results evidenced that the ultrathin sheet microstructure grown on NF and the generated VFe are primarily responsible for the efficient water splitting. Thus, the scalable and robust synthetic technique, direct usability in an AEM electrolyzer, and the correspondingly high AEM activity and excellent electrode stability make it suitable as an industrial-scale AEM electrolyzer for green hydrogen production.