In summary, this study has probed both the ion-size performance relationship and the charging mechanism of the layered MOF Cu 3 (HHTP) 2 to provide a detailed description of the electrochemical behaviour of this material. Three-electrode measurements revealed that smaller cations result in higher charge storage performances upon both positive and negative charging, showing that electrolytes with small ion sizes should be employed in layered MOF supercapacitors to maximise their performances. Furthermore, our results indicate that porosity saturation of the Cu 3 (HHTP) 2 electrode occurs when using electrolytes with larger TBA + and THA + cations. Charging mechanism studies with EQCM support these findings. These measurements revealed that Cu 3 (HHTP) 2 exhibits a cation-dominated charging mechanism with TEABF 4 , the electrolyte with the smallest cation used in this study, with coion desorption upon positive charging and counterion adsorption upon negative charging. No net movement of the solvent or anions in or out of the electrode pores was observed for this system. However, with THABF 4 , EQCM indicated that solvent molecules also likely participate in the charging mechanism, illustrating that increasing the cation size changes the charging mechanism of MOF-based supercapacitors. This result further supports saturation of the MOF pores with larger electrolyte ions, and confirms that electrolytes with smaller ion sizes should be targeted to optimise the performances of MOF-based devices going forwards. Our findings on this crystalline electrode system with unimodal porosity also validate previous results that demonstrated porosity saturation can occur in electrode materials with low overall total porosities relative to traditional porous carbons. We envisage that our study will guide the design of improved supercapacitors in the future.