Three-electrode measurements were done using Ag/AgCl electrode and platinum coil as reference and counter electrode, respectively. The working electrode was obtained by dropcasting slurry composed of 60 wt. % of OTQC, 30 wt. % of Super C65 carbon black and 10 wt. % of polyvinylidene fluoride (PVDF) solution (10 mg/mL) in N-methyl-2-pyrrolidone (NMP) on a glassy carbon electrode and afterward drying at 50 °C for 2h. A continuous flow of nitrogen ensured an oxygen-free environment for the measurements. Different concentrations of ZnSO4 electrolytes were prepared by dissolving an appropriate amount of zinc sulfate heptahydrate (ZnSO4•7H2O) in water. 3 M ZnSO4 + H2SO4 (pH = 1) electrolyte was prepared by the addition of H2SO4 into 3 M ZnSO4 until the pH meter (827 pH Lab, Metrohm) measured pH = 1. 0.1 M ZnSO4 + H2SO4 (pH = 1) electrolyte was prepared by the addition of H2SO4 into 0.1 M ZnSO4 until the pH meter (827 pH Lab, Metrohm) measured pH = 1. H2SO4 (pH = 1) and H2SO4 (pH = 3.3) electrolyte solutions were obtained by the addition of H2SO4 into water until the appropriate pH meter (827 pH Lab, Metrohm) measured value.