The effect of electrochemical aging on self-discharge characteristics of carbon/iodide electrochemical capacitors operating in liquid and hydrogel electrolytes

Introducing redox additives into electrolytes is a widely considered strategy aiming at increasing the charge storage abilities of electrochemical capacitors. Such systems might, however, suffer from a high dynamic of self-discharge related to the shuttle effect, i.e. movement of charged species toward the opposite electrode and their back to the initial oxidation state. This research is dedicated to one of the most recognized redox electrolytes based on alkali metal iodides and shows the effect of electrochemical aging on the self-discharge related to the formation of polyiodide intermediates. Systems employing iodide-based liquid and gel polymer electrolytes are here investigated are analyzed by taking into account the electrolyte composition, i.e. the presence of follow-up anions and solution pH. In this respect, nitrates as co-anions are shown to have the highest accelerating effect on the shuttle processes. Their decomposition to NO/NO2, NO2−, and NH4+, confirmed using online mass spectrometry and colorimetric analysis, is proven to intensify the redox shuttling in liquid cells. Self-discharge increase in time in both buffer (pH = 7), and highly alkaline solution (pH = 14) is demonstrated as well. At elevated pH, however, the mitigation of self-discharge is shown upon the formation of a passive layer on titanium current collectors as confirmed with XPS analysis. In this respect, the alkalization effects, typically observed in neutral aqueous electrolytes without buffering properties, were examined using operando monitoring diagnostic tools, and their effect on self-discharge was discussed. Our research also shows the mitigation of the shuttle effect by trapping iodides in cellulose-based gel polymer electrolytes (GPE). In contrast to liquid electrolyte systems, both, cell potential and energy decay with time under open circuit conditions are significantly lower and these effects are maintained for the cells aged under harsh floating conditions, regardless of the co-anion. The impeded formation and mobility of undesirable polyiodides in a gel are confirmed using independent post-mortem UV–Vis, Raman, and EDX analyses.