Understanding the electrolyte/electrode interfacial interactions for the development of high-performance aqueous redox-enhanced supercapacitors

Abstract

Aqueous redox-enhanced supercapacitors (redox SCs) have versatile features such as high energy, high power, non-flammability and eco-friendliness, thus have received increasing research interest. But there are obstacles to overcome such as lower energy density compared to secondary metal ion-based batteries and self-discharge issues due to cross-diffusion of dissolved redox species. In this dissertation, by quantifying the concentration of redox-active electrolytes adsorbed on the electrode surface, defined as effective concentration (i.e., maximum possible number of ions available for faradaic charging) and studying the correlation between the effective concentration and the cell level specific energy of , we optimize the performance of redox SCs. Based on the electrical energy storage mechanisms of redox SCs, the hypotheses were established and confirmed. The established hypotheses were that (1) the adsorption concentration of redox-active electrolytes is different per substituents attached to the redox core; (2) amounts of adsorbed redox-active electrolyte on the electrode affect the cell level energy performance. To confirm that, several redox-active viologen candidates (1,1′-dimethyl-4,4′-bipyridinium dibromide, EV; 1,1′-dipentyl-4,4′-bipyridinium dibromide, PV; (1,1’)-di(2-ethanol)-4,4’-bipyridinium dibromide, 2OH; (1,1’)-di(6-hexanol)-4,4’-bipyridinium dibromide, 6OH; 1,1’ -bis[3-(trimethylammonio)propyl]- 4,4’ -bipyridinium tetrabromide, NV) were synthesized and their physicochemical and electrochemical properties were tested. By using UV-Visible spectroscopy, the adsorption concentration of viologen candidates on the electrode were measured and performance of candidates was measured. Among candidates, 6OH showing excellent performance was selected as the best candidate, and through enhancing adsorption concentration step, optimized performance was measured (61.1 Wh/kgdry at 1.4 V, 0.5 A/gdry). Further the performance of 6OH with differently structured carbon electrode material (p-NOMC) was compared