CoFeO inverse opals with tunable catalytic activity for high-performance overall water splitting.

The development of earth-abundant and high-performance bifunctional catalysts for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in alkaline electrolytes is required to efficiently produce hydrogen by electrochemical water splitting, but remains a challenge. We have fabricated mesoporous cobalt iron oxide inverse opals (m-CFO IO) with different mole ratios of cobalt and iron by a wet chemical method using polystyrene beads as a hard template, followed by calcination in air. The performance of the m-CFO IO as OER and HER electrocatalysts was investigated. The as-prepared catalyst with equal concentrations of Fe and Co exhibits remarkable OER and HER performances with low overpotentials of 261 and 157 mV to attain 10 mA cm and small Tafel slopes of 63 and 56 mV dec, respectively. An alkaline water electrolyzer with a two-electrode configuration achieves 10 mA cm at 1.55 V with excellent long-term stability, outperforming the combination of noble metal IrO and Pt/C benchmark catalysts. The superior catalytic performance is ascribed to the synergistic effects of particle size, crystallinity, oxygen efficiency, a large number of active sites, and the large specific surface area of the porous inverse opal structure.