A Simplified Equivalent Circuit Model for the Photo-Charging Process of Carbon-Based Quasi-Solid Photosupercapacitors

Abstract

Solar irradiation can be converted into electrical current by a solar cell, which in turn can be stored in a supercapacitor. The coupling of a solar cell and a supercapacitor, called photosupercapacitor, shows promising applications that demand multidisciplinary studies to understand its functionality and potential. Normally, supercapacitors are characterized with potentiostats or power sources that provide a constant current or voltage, however, we find that the photocurrent provided by a solar cell in a photosupercapacitor configuration largely depends on the voltage stored in the supercapacitor connected in parallel to the solar cell. Therefore, we use a simplified equivalent circuit model to demonstrate that the charging time of a photosupercapacitor depends mainly on its capacitance, and to a lesser extent, on its resistance. At the same time, the maximum output voltage of the photosupercapacitor depends on the saturation and short circuit currents of the solar cell. The numerical results confirm qualitatively the experimental behavior of the photo-charging curves of quasi-solid supercapacitors, which consist of polyvinyl alcohol (PVA)-H2SO4 electrolyte for both activated carbon or reduced graphene oxide-based electrodes. The latter presents better electrochemical characteristics that optimize the operation of the photosupercapacitor. The electrical circuit analysis is a useful tool to guide further improvements in the photosupercapacitor design and fabrication.