Electrochemical fabrication of 3D quasi-amorphous pompon-like Co-O and Co-Se hybrid films from choline chloride/urea deep eutectic solvent for efficient overall water splitting

The development of earth-abundant, low-cost, highly active, and durable bifunctional catalysts competent for efficient electrochemical water-splitting is intensively demanding and of vital importance to realize high-purity hydrogen production . Herein, we report that 3D quasi-amorphous pompon-like Co-O and Co-Se hybrid films grown on copper foil (Co-O@Co-Se/Cu) can be electrochemically synthesized from a choline chloride and urea mixed deep eutectic solvent , denoted as Reline, via a facile one-step electrochemical deposition approach. Benefiting from the induction effect of Se, the obtained Co-O@Co-Se/Cu hybrid electrode exhibits strongly enhanced catalytic activity. The optimal Co-O@Co-Se/Cu sample shows good activity and excellent durability for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in an alkaline solution with low overpotentials of 85 mV for HER and 340 mV for OER, respectively to reach a catalytic current of 10 mA cm −2 . The Co-Se component within the hybrid films is gradually converted into Co-O species during the course of the OER, which are responsible for the good OER performance. In addition, when performed as bifunctional catalysts in a two-electrode configured electrolyzer, the Co-O@Co-Se/Cu couple displays a water-splitting current of 10 mA cm -2  at 1.65 V with reasonable stability for long-term electrolysis over 100 h. This simple one-step electrochemical synthesis route operated in the Reline-based deep eutectic solvent is demonstrated as an efficient strategy to fabricate active non-noble-free electrocatalysts toward overall water splitting applications.