Building on the results with TEABF 4 , EQCM studies were also performed with 1 M THABF 4 in acetonitrile electrolyte to assess how the charging mechanism changes upon increasing the cation size, and to provide insights into why larger cations lead to lower energy storage performances with Cu 3 (HHTP) 2 (Fig. ). Negligible change in motional resistance (ΔR) was also observed with this electrolyte. EQCM cells with THA + were cycled between the potential limits of +0.23 V vs. OCV to -0.27 V vs. OCV, allowing for investigation of the charge storage mechanism for both positive and negative charging as defined for Figure . As observed in the 3-electrode measurements, THA + exhibited an asymmetric CV (Fig. ), resulting in asymmetric Δm-ΔQ plot that displays a clear mass hysteresis about the average mass change of 1250 ng mC -1 (dashed orange line; Fig. ). This indicates clear differences in the charging mechanism between the cathodic and anodic charging regimes (Fig. ), in contrast with the results for TEA + (Fig. ). During the cathodic scan from ΔQ = 0 to ΔQ = -0.38 mC, there is an approximately linear increase in Δm with an average slope of 1660 ng mC -1 (Fig. ). However, the equivalent molecular weight corresponding to this mass increase was calculated to be only 163 ± 3 g mol -1 (SI Fig. ), less than the molecular weight of the naked THA + cation (298 g mol ). If this was due to involvement of anions in the charging process, a parabolic-like Δm-ΔQ plot would be expected, with an increasing slope as the total charge increases. However, as the mass change is approximately linear, and there's a negative correlation with ΔQ, it is likely that the anions do not play a significant role in the charge storage mechanism. Instead, involvement of solvent in the charging process is a more likely explanation for the lower observed mass change, and is consistent with a linear Δm-ΔQ plot. Therefore, the results suggest that cation insertion is accompanied by a net loss of solvent molecules from the electrode pores during cathodic charging, with adsorption of one THA + cation accompanied by the loss of 3.3 acetonitrile molecules on average (Fig. ; process from stage 1 to 2). This observation supports the hypothesis of porosity saturation in Cu 3 (HHTP) 2 when charging with larger TAA + cations, with a net expulsion of solvent molecules from the pores required following cation adsorption upon cathodic charging, resulting in lower observed equivalent molecular weight changes.