Influence of electrolyte on the photo-charging capability of a ZnO–FTO supercapacitor

Abstract

Photo-rechargeable supercapacitors serve as a bridge between batteries and solar cells for developing efficient energy storage devices. Unlike previous studies where there was either (a) a heterostructure of zinc oxide (ZnO) with other electroactive materials or (b) a coating of ZnO powder on the current collector, here, ZnO nanorods grown on fluorine-doped tin oxide (FTO) were used as photoelectrodes. Our devices tested with (i) an ionic liquid electrolyte and (ii) a gel electrolyte exhibited a capacitance rise of ∼3006% (>10× the previous record) and ∼500% (2× the previous record) compared to that under dark conditions, which is attributed to the electrode synthesis technique adopted in this study. A higher ionic diffusivity in the ionic liquid electrolyte leads to a higher capacitance rise. Notably, a novel necking behavior (a possibly higher incremental capacitance at higher voltages) was observed during galvanostatic charge/discharge tests under UV illumination for both electrolytes, which is attributed to the high electrode porosity. Contrary to conventional wisdom, higher capacitance was recorded at higher current densities during galvanostatic charge/discharge tests under UV illumination for ionic liquid-based supercapacitor cells. A first-ever physics-based continuum scale model for photo-rechargeable supercapacitors is proposed to explain the above intriguing experimental observations, paving a path for the design of fast-charging high-capacitance supercapacitors.