Rationalizing the Role of Polyoxometalate-Based Gel–Polymer Electrolytes to Achieve Five-Fold Increase in the Specific Capacitance of Hard-Carbon-Based Supercapacitors

Abstract

Boosting the charge-transport pathways in gel–polymer electrolytes (GPEs) is important to extract their full potential for non-Faradaic, supercapacitive energy-storage devices. Herein, a series of polyoxometalates (POMs) ([P6W18O79]20−, [PW9O34]9− and [PW12O40]3−) as electrochemical polarization promoters is demonstrated to achieve 15-fold enhancement in ionic conductivity of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide-based GPEs. Fundamentally, the three POMs offer systematically differing charge densities (200, 45, and 6.5 e− nm−3) that are linearly correlated to the observed enhancements in ionic conductivities. The charge density on the POMs contributes to the formation of additional charge-migrative pathways across the GPE, as evidenced from facile polarization characteristics. Importantly, the presence of POM beyond a critical concentration acts as charge-trapping centers to impede the ionic conductivity of the GPEs. The insights obtained through such detailed spectroscopic and electrochemical techniques are integrated into full-scale supercapacitive devices with a rationally designed porous hard carbon as the electrode material. The resulting electrode–electrolyte interface synergies achieve a best-in-class supercapacitor exhibiting energy density of 58 Wh kg−1, power density of 14 kW kg−1, and a relaxation time constant of 0.66 s, without compromising on cycling stability for direct integration with intermittent energy-harvesting devices. Thus, the fundamental insights presented here outline the design principles for extracting high-performance from GPE-based energy-storage devices.