Towards an optimized hybrid electrochemical capacitor in iodide based aqueous redox-electrolyte: Shift of equilibrium potential by electrodes mass-balancing

Considering the cost-effectiveness, safety, and environmental friendliness for energy storage and delivery at high rates, hybrid electrochemical capacitors in aqueous electrolytes containing redox-active species are attractive alternatives to expensive organic electrolyte based electric double-layer capacitors (EDLCs). Here, the influence of electrode mass-balancing on the equilibrium potential of hybrid cells in aqueous sodium nitrate + sodium iodide (5 mol L−1 NaNO3 + 0.5 mol L−1 NaI) has been investigated. The shift of equilibrium potential determines, whether the positive electrode behaves fully battery-like (charge/discharge strictly in the iodide redox potential range) or shows a mixed battery-like and EDL capacitive behavior. With an appropriate mass-balancing of the positive and negative electrodes (mass ratio = 1:2), the equilibrium potential shows a negligible shift during galvanostatic charge/discharge cycles at 0.5 A g−1, which results in full battery-like behavior of the positive electrode. Consequently, the hybrid cell exhibits stable electrochemical performance. By contrast, an equal or higher mass of the positive compared to the negative electrode, leads to a shift of the equilibrium potential resulting in two different charge storage mechanisms at the positive electrode. As a result, the overall performance of the hybrid cell deteriorates. We show by thermogravimetric analysis and Raman spectroscopy that the formation of polyiodides (I3− and I5−) is controlled by the oxidation of iodide (I−) anions to molecular iodine in nanoporous carbon based positive electrode, and that more polyiodides are produced, if the positive electrode operates strictly within the iodide/iodine redox potential range.