Direct storage of holes in ultrathin Ni(OH)2 on Fe2O3 photoelectrodes for integrated solar charging battery-type supercapacitors

Abstract

Energy storage is a very significant issue for utilization of solar energy due to its discontinuous and unstable energy flux. Herein, for the first time, we propose a Fe₂O₃@Ni(OH)₂ core–shell nanorod array as a photoelectrochemical battery-type supercapacitor for the direct storage of solar energy. Under light illumination, Fe₂O₃ absorbs solar energy and produces electron–hole pairs, while Ni(OH)₂ stores the photo-generated holes, which can be released as electricity when the light is switched off. In addition, by controlling the valence band position of the semiconductor and the thickness of Ni(OH)₂, the side reaction of water oxidation (electrolyte decomposition), which is harmful for the applications of a photoelectrochemical supercapacitor, can be completely suppressed. As a consequence, the specific capacitance of the Fe₂O₃@Ni(OH)₂ photoelectrochemical supercapacitor is enhanced up to 20.6 mF cm⁻² at a discharge current density of 0.1 mA cm⁻², which is about 4.5 times that of BiVO₄/PbO_x reported in a previous study. This study offers a very promising device for the direct storage of solar energy and deepens our understanding on the interface charge transfer between a photoelectrode and a battery-type capacitive material.