Through using calibration experiments, quantitative in-pore BF4 -populations were obtained at different charging voltages (SI Figure ). For both the propylene carbonate (Figure ), and the acetonitrile electrolytes (Figure ), the in-pore anion population was plotted against the specific electronic charge of the electrode, alongside different theoretical scenarios. First, (i) an 'all cation' scenario in which the cations account for all of the electrode's charge storage, and there is no net change in the in-pore anion population, i.e. charge is stored through cation adsorption (counter ion adsorption) for the negative electrode, and through cation desorption (co-ion desorption) for the positive electrode. And in the second case (ii) an 'all anion' scenario in which the anions account for all the electrode's charge storage. Both cases are for illustration of the gradient only, and the intercept is fixed to match that of the experimental regression slope. By comparison of the experimental slope from regression with the theoretical scenarios, the anions accounted for 9±13 % of the total charge storage in a propylene carbonate electrolyte, and 18±20 % in an acetonitrile electrolyte (95% confidence interval, see SI for details). Therefore, the experimental result for both electrolytes is closer to the "all cation" scenario. This gives confidence that regardless of the organic solvent, cations are the major contributor to charge storage in Ni3(HITP)2. This finding supports the cation-dominated charging mechanism proposed from theoretical and experimental work for the related MOF Cu3(HHTP)2. Therefore, whilst in-pore anion population was earlier shown to serve as a useful indicator of electrolyte-accessible pore volume and hence electrochemical performance (Figure ), the anions themselves are not primarily responsible for the charge storage in these systems.