Synthesis of NiCo ₂ O ₄ Nanocrystal and Its Use as a Bifunctional Electrocatalyst for the Overall Water Electrolysis

Development of Earth‐abundant bifunctional electrocatalysts like NiCo ₂ O ₄ for water electrolysis is a potential key enabler of hydrogen economy. Nevertheless, the influence of synthesis parameters on the morphology, and thus, the catalytic activity of NiCo ₂ O ₄ remained insufficiently investigated. Herein, porous NiCo ₂ O ₄ spinel nanowires synthesized via simple hydrothermal method coupled with thermal annealing in air were employed as bifunctional electrocatalyst for overall water electrolysis in alkaline condition. Specifically, the effect of hydrothermal temperature on the microstructure of NiCo ₂ O ₄ and the eventual catalytic activity is emphasized. Employing ECSA‐corrected evaluation of electrochemical performance and standardized characterization tools, an optimum hydrothermal temperature of 120 °C was determined. The corresponding NiCo ₂ O ₄ exhibited respectable intrinsic activity on graphite substrate without additional co‐catalyst. Catalytic current density of 30 and 10 mA cm ^{− 2} was achieved with overpotentials of 343 and 360 mV for hydrogen and oxygen evolution reactions, respectively. For overall water electrolysis, a cell voltage of 2.2–2.0 V was enough to sustain 10 mA cm ⁻² for 35 h, reaching near 100% Faradaic efficiency. The evolution of the catalyst layers was also monitored using SEM, EDX, XPS, and Raman spectroscopy. Insights into the formation and evolution of the catalyst layer and of the working mechanism of the resultant electrolyzer were provided.