Photoinduced Charge Storage with p-Type NiO Nanoplates via Surface Trapped Holes

Existing photoelectrochemical cells for solar energy conversion are hindered by their inability to counteract the ever-changing sunlight conditions, which results in unstable electrical energy output for production of chemicals. Herein, a p-type NiO nanoplate thin film photocathode with rich Ni vacancies is reported, enabling the dual functions of sunlight harvesting and electrical energy storage in a single material system. Band gap excitation of NiO induces surface adsorptions of hydroxide anions and a concomitant storage of in situ-generated photogenerated holes to form NiOOH. In the absence of light, the stored energy in NiOOH is discharged to provide a sustainable current by desorbing the hydroxide anions. The energy storage capacities of the NiO depend on the electrolyte pH and the magnitude of the applied bias. The energy storage capacities of the NiO are greater with alkaline electrolytes due to the availability of hydroxide anions for maintaining the overall charge neutrality in the electrode system. In addition, a sufficiently large negative bias is also required during periodic irradiation to enhance the charge separation efficiency and enable photogenerated holes to be available for charge storage. The feasibility for light-to-electrical energy storage with a single material photocathode is demonstrated, which provides a versatile solution to mitigate the instability of solar irradiation.