Bifunctional catalysts for overall water splitting: CoNi oxyhydroxide nanosheets electrodeposited on titanium sheets

In this work, CoNi oxyhydroxide nanosheets [CoNi-OOH-30(40)] are synthesized, through two simple steps, as efficient bifunctional catalysts in alkaline media. First, a shape-controlled CoNi-X alloy (X denotes electrodeposition time) with a ridge-like morphology that in-situ deposited on titanium sheets is obtained by electrodeposition. Then, CoNi-30 with ridge-like structure is further electrooxidized to improve the electrochemical activity. After 40 h oxidation, the CoNi-OOH-30(40) oxyhydroxide with nanosheets structure is obtained, which exhibits good catalytic performance for overall water splitting than the other as-prepared catalysts. It is found that CoNi-OOH-30(40) to reach +/- 10 mA cm(-2) requires small overpotentials of -210 and 279 mV, Tafel slopes of 67 and 62 mV dec(->)1 and exhibit good stabilities for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline solution, respectively. A water electrolyzer, using CoNi-OOH-30(40) as anode and cathode catalysts, reaches 10 mA cm(-2) at a voltage of 1.76 V. Furthermore, due to the Co-Ni alloy and CoNi oxyhydroxide, CoNi-OOH-30(40) maintains 10 mA cm(-2) for 60 h in alkaline media without activity losses. It is believed that the present work provides a facile, fast and feasible strategy to fabricate cost-efficient CoNi oxyhydroxide nanosheets catalysts with high water splitting efficiency and stability in alkaline conditions. (C) 2019 Elsevier Ltd. All rights reserved.