Cobalt Oxide Nanowires with Controllable Diameters and Crystal Structures for the Oxygen Evolution Reaction

Abstract

Herein, mesoporous cobalt oxides with nanowire morphology were size-selectively synthesized via nanocasting and used as a model system to reveal the impact of diameter, pore, and crystal structures toward the alkaline water electrolysis. A range of Co3O4 nanowires with variable diameters was prepared by replication of SBA-15 silica with different degree of interconnectivity and pore sizes. These nanowires could be further transformed to CoO rock-salt structure through a selective reduction process by keeping the initial morphology and textural parameters. The electrocatalytic screening showed that CoO with the smallest nanowire diameter and open pore structure showed superior activity in the electrochemical oxygen evolution reaction (OER) due to the higher amount of available active centers and defect sites. The overpotential to reach 10 mA/cm2 drops from 392 to 337 mV after reduction of Co3O4 to CoO. An in situ Raman spectroscopy investigation showed that Co3O4 retained its bulk crystalline spinel phase while CoO faced an irreversible phase transformation into a distorted spinel structure after OER.