The impact of ion size on the energy storage performance of Cu 3 (HHTP) 2 was studied by using a series of 1 M tetraalkylammonium tetrafluoroborate (TAABF 4 ) in acetonitrile electrolytes (Fig. ). While all electrolytes had the same anion (BF 4 -), and thus a constant anion size (4.8 Å naked; 11.6 Å solvated) , the TAA + cation was varied to systematically change the cation size. In total, four different cations were used: tetraethylammonium (TEA + ), tetrapropylammonium (TPA + ), tetrabutylammonium (TBA + ) and tetrahexylammonium (THA + ), with their naked and solvated sizes summarised in Figure . This allowed for a variation in unsolvated cation size between 6.8 -9.5 Å (solvated: 13.0 -15.7 Å). A three-electrode arrangement enabled the response of Cu 3 (HHTP) 2 upon both negative and positive charging relative to the open circuit voltage (OCV) to be studied separately (Fig. ; SI Fig. ). To evaluate the charge storage, specific capacity and specific capacitance, C g , were calculated (Fig. ; SI Table and; and SI Fig. ). All experimental capacity and capacitance values for Cu 3 (HHTP) 2 were calculated after removing the contributions of acetylene black and PTFE that are also present in the electrodes. Capacity is reported as the primary energy storage performance metric in this work rather than capacitance due to the non-linearity of the GCD discharge curves produced from these systems. This is in accordance with previous recommendations on data analysis. Low current densities and scan rates were used to attempt to ensure complete permeation of the ions into the MOF pores and to minimise the impact of differences in ion kinetics on the results.