The CV data also shows that Cu 3 (HHTP) 2 displays a clear asymmetric charging response with all TAA + electrolytes, with the capacity upon positive charging greater than the capacity upon negative charging (Fig. ). It is worth noting that there is an increase in the degree of asymmetry as the cation size increases, with a significant increase from TPA + to TBA + , and THA + having the greatest difference in capacity between positive (41.6 ± 1.4 C g -1 ) and negative (19.5 ± 1.4 C g -1 ) charging. Interestingly, this charging behaviour is reminiscent of porosity saturation that has been observed previously in low porosity carbon electrodes with electrolytes with large cations. These systems also exhibit asymmetric charging, with a significant decrease in charge storage at high cell voltages as the carbon pores become saturated by large TAA + cations during charging. Porosity saturation in Cu 3 (HHTP) 2 is plausible due to the relatively low total porosities of the MOF electrode films compared to other traditional supercapacitor electrode materials, such as porous carbons. While there is only a small decrease in the negative charging capacity when increasing the cation size from TEA + (42.1 ± 1.9 C g -1 ) to TPA + (40.4 ± 2.0 C g -1 ), there is a much larger drop in capacity between TPA + and TBA + (31.2 ± 2.0 C g -1 ). This strongly suggest that porosity saturation only becomes significant for cation sizes of TBA + and above, and provides another explanation for the higher charge storage performances of smaller cations noted above. Three-electrode rate capability and electrochemical impedance spectroscopy (EIS) measurements support this hypothesis, as both measurements indicate large decreases in ion mobility during negative charging for TBA + and THA + compared to TEA + and TPA + , consistent with the larger cations struggling to fit into the near-saturated pore network of the electrodes (SI Fig. -S11; SI Table ). Ion sieving, a related phenomenon which has been seen in other layered materials, also gives rise to similar asymmetric charging behaviour, but is unlikely in these systems as the naked (desolvated) cation sizes are all significantly smaller than the electrode pore size.