context: EIS experiments. The electrolyte 65 mM Na[FeIII-racEDDHA], 0.7 M Na2SO4, 155 mM borate pH 9, was reduced at the negative electrode to reach 50% SoC (determined by photometry), using the same setup outlined in the charge–discharge cycling experiments. Potassium ferro/ferricyanide at 50% SoC was directly prepared with 32.5 mM K3[FeIII(CN)6], 32.5 mM K4[FeII(CN)6], 0.7 M Na2SO4, 1 mM borate, pH 9. Three-electrode configuration with two CRLEs used as working and counter electrode and a reference electrode (Ag/AgCl 3 M KCl) were connected to a BioLogic VSP potentiostat. The impedance spectra were recorded from 3 × 104 Hz to 5 × 10−2 Hz. An AC amplitude of 100 mV was applied on top of a DC potential matching the open cell potential at 50% SoC. All electrodes used had a similar roving length (72 mm) and weight (0.23 g).
answer: 65 mM Na[FeIII-racEDDHA], 0.7 M Na2SO4, 155 mM borate pH 9