Bias-free, solar-charged electric double-layer capacitors

Abstract

The conversion of solar energy with simultaneous electric energy storage provides a promising means for optimizing energy utilization efficiency and reducing device volume. In this paper, a 3-dimensional mesoporous carbon coated branched TiO₂ nanowire composite is rationally designed for direct conversion and storage of solar energy as electric double-layer capacitive energy. The 1-dimensional, crystalline TiO₂ trunks serve as long light absorption and continuous charge transport pathways, and the high-density TiO₂ branches can efficiently increase the contact area with the surface coated mesoporous carbon layers. In addition, the ordered and uniformed mesopores provide large pore sizes for electrolyte penetration, and a high surface area for charge absorption and storage. Under a 1-sun illumination and no external electric bias, this branched TiO₂/mesoporous carbon composite exhibits specific capacitances of over 30 and 23.4 F g⁻¹, at current densities of 0.1 and 0.5 A g⁻¹, respectively. An excellent stability of >50 photocharging–electrical discharging cycles has also been demonstrated, suggesting the potential of further developing this hybrid material structure for simultaneous solar conversion and electric energy storage.