Supercapacitor electrode energetics and mechanism of operation: Uncovering the voltage window

In recent years, supercapacitors have received enormous popularity as energy storage devices due to their high power density and long-lasting cycle life compared to Lithium-Ion batteries and other similar energy storage devices. To attain high energy density, pseudocapacitive materials are being primarily investigated for asymmetric configuration-based supercapacitors. While most of the research is focused on finding new pseudocapacitive materials with higher specific capacitance, only very limited knowledge is available about how high voltage can be achieved in supercapacitors. Here, we present a comprehensive review of the mechanism of operation of the asymmetric supercapacitors. We discuss in detail the factors affecting the voltage window of the asymmetric supercapacitors like 1) the work function of the electrodes, 2) the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the electrolyte and 3) hydrogen and oxygen evolution overpotentials. We included an in-depth review of how work function originates in pseudocapacitive electrodes and its relation to Fermi level and electronegativity. Though transition metal oxides and carbon-based materials are used as base materials for this study, their application can be extended to any electrode-active material as long as it operates on the pseudocapacitive principle.