A Fully Reversible Water Electrolyzer Cell Made Up from FeCoNi (Oxy)hydroxide Atomic Layers

Abstract Renewable electricity powered water electrolysis is a promising solution for the conversion and storage of the intermittent renewable energy resources in the form of hydrogen. Herein, atomically thin FeCoNi ternary (oxy)hydroxide nanosheets (FeCoNi‐ATNs) are developed as efficient and robust bifunctional electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in 1 m KOH solution (pH = 14). The electrocatalyst shows remarkably high apparent catalytic performance (400 mA cm −2 at 350 mV for OER and 240 mV for HER) and mass activities at modest overpotentials (1931 A g −1 at 330 mV for OER; 1819 A g −1 at 200 mV for HER). Moreover, the OER and HER performance of FeCoNi–ATNs are fully reversible and electrochemically switchable, due to the interconversion attribute of two catalytic states for OER and HER. Using the dual functional properties of this catalyst, a fully reversible water electrolyzer cell is fabricated, exhibiting a robust reversibility between two half reactions in water electrolysis under a high current density (100 mA cm −1 ), which can effectively overcome the stability issues caused by electrode depolarization during frequent power interruptions, an inevitable phenomenon commonly brought about by the usage of intermittent renewable energy supplies.