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 via solar cell, which can be stored in a supercapacitor. The coupling of a solar cell and a supercapacitor, called photosupercapacitor, shows promising applications demanding 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. Meanwhile, 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.