Photocapacitive CdS/WOx nanostructures for solar energy storage

Abstract

Through a facile solvothermal procedure, a CdS/WO x nanocomposite has been synthesised which exhibits photocapacitive behaviour under white light illumination at a radiant flux density of 99.3 mW cm −2 . Photoelectrochemical experiments were undertaken to examine the self-charging properties of the material and to develop an understanding of the underlying electronic band structure responsible for the phenomenon. By employing XPS, UPS and UV-Vis diffuse reflectance spectroscopy for further characterisation, the ability of the composite to generate current following the removal of incident light was related to the trapping of photoexcited electrons by the WO x component. The presence of WO x yielded an order of magnitude increase in the transient photocurrent response relative to CdS alone, an effect attributed to the suppression of electron-hole recombination in CdS due to hole transfer across the CdS/WO x interface. Moreover, current discharge from the material persisted for more than twenty minutes after final illumination, an order of magnitude improvement over many existing binary composites. As a seminal investigation into the photocapacitive characteristics of CdS/WO x composites, the work offers insight into how the constituent materials might be utilised as part of a future self-charging solar device.

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

“…In particular, the outlined comparison of EIS behaviours suggests that whilst a selection of equivalent circuit elements are required to describe the measured impedance behaviours, these components may be regarded as approximately independent of irradiance levels or bias potential, with the exception of the series resistance Rin. In a previous publication by the present authors [27] it was demonstrated that the open-circuit potential of CdS/WOx becomes increasingly negative when exposed to white light, and hence it follows from the EIS analysis that this incident irradiation can be approximately described as a conventional source of voltage and current in series with the equivalent circuit shown in Fig. 7.…”

“…By investigating the transient photocurrent response (TPR) of the CdS/WOx nanocomposite in the presence of aqueous sodium sulphate (0.5 M), it was demonstrated in a previous publication by the present authors [27] that the addition of WOx could be exploited to increase photocurrent under one Sun irradiation by an order of magnitude relative to CdS alone. However, significant electrode instability was evident during the course of these experiments, with the photocurrent halving over a period of approximately twenty minutes from the onset of illumination.…”

“…The CdS/WOx nanocomposite was synthesised in accordance with a two-step solvothermal protocol reported previously by the present authors [27]. In the first phase of this process, CdS nanoparticles were produced by heating a 60 mL aqueous solution of cadmium(II) acetate (167 mM) and thiourea (333 mM) at 180 ˚C for ten hours inside a 125 mL stainless steel Parr acid digestion bomb.…”

“…Indeed, whilst the introduction of a common counter electrode serves to improve storage efficiency by minimising the electrical resistance encountered by charge travelling between the photovoltaic and supercapacitor modules, one cannot significantly reduce the size, complexity or cost of a solar cell/supercapacitor pairing without integrating the two devices in a more meaningful manner. To this end, the present authors recently explored the concept of simultaneously converting and storing energy from solar irradiation at a single photocapacitive working electrode [27], which was achieved through the low-temperature solvothermal synthesis of a cadmium(II) sulphide (CdS)/tungsten(VI) suboxide (WOx) nanocomposite. By combining X-ray and UV photoelectron spectroscopy (XPS and UPS, respectively) measurements with Tauc plots constructed from diffuse reflectance spectroscopy (DRS) data, it was possible to relate the three-electrode transient photocurrent response (TPR) of the material to the relative valence and conduction band energies of the CdS and WOx components.…”

Novel Aggregation-Induced Emission Materials/Cadmium Sulfide Composite Photocatalyst for Efficient Hydrogen Evolution in Absence of Sacrificial Reagent

“…−1.0 V vs. Ag/AgCl), due to the higher conduction band of TPE-Ca. The photocurrent density of CdS/TPE-Ca (2.55 mA cm −2 ) can be compared to the recent developed CdS/MoS 2 (1.9 mA cm −2 ) [41], CdS/WO x (10 µA cm −2 ) [42], and CdS nanowires (2.0 mA cm −2 ) [43]. Subsequently, the electrochemical impedance spectroscopy (EIS) Nyquist plots was conducted to demonstrate the electrochemical kinetics toward the interfaces between samples and electrolyte.…”