Electrochemical Characterisations to Elucidate the Pseudocapacitance Mechanisms of a CdS/WOx Nanocomposite Photoanode in Acidic Aqueous Electrolytes

The electrochemical properties of a cadmium sulphide/tungsten(VI) sub-oxide (CdS/WO_x) nanocomposite have been explored using aqueous solutions of acetic acid (pH 2.2) and acidified sodium acetate (pH 5.0), for the purpose of evaluating the origin of pseudocapacitance within the material. Through transient photocurrent response, galvanostatic charge/discharge and electrochemical impedance measurements, it was established that cation-intercalation phenomena were principally responsible for charge-accumulation in the composite and that the incorporation of ionic species into interstitial surface sites was more energetically favourable for protons than for sodium ions. The composite displayed promising capacitive performance in the tested electrolytes, exhibiting Coulombic efficiencies of up to 88% under galvanostatic cycling at 1.0 mA cm^-2 alongside a peak differential capacitance value of 560 mF cm^-2 during the discharge phase. From electrochemical impedance spectroscopy data it was further determined that whilst illumination by white light acted to decrease the series resistance of the photoanode, all other resistive and capacitive components of the impedance characteristics were affected negligibly by the irradiation. In combination, the investigations detailed herein provide an instructive resource for the development of CdS/WO_x composites and the optimisation of electrolytes to improve the performance and chemical stability of such materials. Furthermore, the study serves as a potential foundation from which to advance the concept of integrating the conversion and storage of solar energy into a single dual-functional electrode, in turn facilitating a new generation of photo-supercapacitor devices.