Here, we present a detailed study examining the impact of electrolyte cation size on the capacitive performance and charge storage mechanism of the layered MOF Cu 3 (HHTP) 2 , which has a well-defined unimodal pore size distribution. We find that smaller cations result in higher capacitive performances upon both positive and negative charging, with highly asymmetric charging seen with larger cations. EQCM measurements suggest a cation-dominated charging mechanism for this series of tetraalkylammonium (TAA + ) electrolytes, with potential involvement of the solvent for electrolytes with larger cation sizes. Overall, our findings indicate that larger electrolyte cations lead to porosity saturation in the MOF electrodes, resulting in more asymmetric charging and lower overall energy storage performance. This is the first work to examine the charging mechanisms of a layered MOF supercapacitor using operando EQCM, and it demonstrates that smaller electrolyte ions lead to higher capacitive performances in MOF-based supercapacitors.