Preparation of photo-rechargeable asymmetric supercapacitors using S,W-codoped titania: Experimental and theoretical insights

Abstract

Photo-assisted charging has been shown to be a sustainable, highly efficient, cheap, and environmentally benign method for the successful enhancement of the capacitance of supercapacitors with low cost and good efficiency. Despite being promising supercapacitor electrode compounds, TiO2 nanotubes (TN) have poor electronic conductivity, which restricts their practical applications. Sulfur doped, tungsten doped and different S,W-codoped TN were prepared and applied as electrodes for supercapacitors with photo-assisted charging. A high average specific capacitance of 179 mF/cm2 at a 0.4 mA/cm2 current density, which is about 5 times the corresponding value for the pristine TN electrodes, is shown by codoped TN electrodes. To explain, the improvement of the capacitance of the fabricated doped and co-doped TN compared to the un-doped TNs, the density functional theory (DFT) calculations were employed. The higher capacitance of the doped and co-doped TNs compared to un-doped TiO2, was attributed to the presence of the occupied and unoccupied molecular orbitals which are related only to the doped elements (S and W) or have the contribution from both doped elements and TiO2 part of the nanotubes. The contribution of these molecular orbitals in the electron excitation due to the light illumination decreases the rate of the electron-hole recombination in the doped TN compared to un-doped TN. A photo-rechargeable SW(75:25)TN//polypyrrole/FTO structure asymmetric supercapacitor (ASC) device has also been fabricated, in which TN are used as the active material to provide direct charging by light in the absence of any external photovoltaic devices. This yields a high specific capacitance of 31 mF/cm2 at a 0.23 mA/cm2 current density.