Thin-Film Photoelectrode of p-Type Ni-Doped Co3O4 Nanosheets for a Single Hybrid Energy Conversion and Storage Cell

Abstract

A thin-film Ni0.07Co2.93O4 p-type semiconductor finely tuned with better electronic conductivity and electrochemical activity than NiO and Co3O4 is proposed as a new photoelectrode of single asymmetric hybrid energy conversion and storage cell (HECS). Photoactive Ni0.07Co2.93O4 and N-doped reduced graphene oxide aerogel (N-rGOAE) were used as positive and negative electrodes of the HECS. Under light illumination, the Ni0.07Co2.93O4 can generate a hole (photogenerated carrier, Ni0.07Co2.93O4+) at the valence band (VB) and a photoelectron at the conduction band (CB) via the photoelectric effect. During charging, the active hole (Ni0.07Co2.93O4+) of the positive electrode can chemically react with OH− and H2O forming CoO(OH) and NiO(OH). This process can enhance the redox mechanism leading to the charge storage performance of the Ni0.07Co2.93O4 electrode since OH− can easily react with the active Ni0.07Co2.93O4+ hole having the lower energy level of the VB when compared with the electrode under the dark condition. The changes in the oxidation state of Ni and Co in Ni0.07Co2.93O4 were fundamentally investigated by an in situ electrochemical X-ray absorption spectroscopy. The electrochemical performance of Ni0.07Co2.93O4//N-rGOAE HECS can be enhanced under light illumination about 1.9-fold higher than that under dark condition. The HECS device may be useful for renewable energy applications.