EQCM studies were initially performed on Cu 3 (HHTP) 2 with 1 M TEABF 4 in acetonitrile electrolyte (Fig. ). The CV was sub-divided into two sections based on the polarisation of the electrode: a cathodic regime with a negative current response (I < 0), and an anodic regime with a positive current response (I > 0) (Fig. ; outside lines). EQCM cells with TEA + were cycled between the potential limits of +0.29 V vs. OCV to -0.21 V vs. OCV. Although the potential window is narrower than that used in the three-electrode experiments above, it covers both positive and negative charging as defined for Figure , allowing the charging mechanism to be studied for both charging directions. The lower stable potential window of the EQCM cell was due to instabilities of the gold coating and its related titanium adhesion layer on the quartz crystal outside the chosen potential limits. Also shown in Figure (inside lines) is the change in resonance frequency response, Δf, of the Cu 3 (HHTP) 2 -coated quartz crystal during the CV experiment. In this work, Sauerbrey's equation applies as there was negligible change in motional resistance (ΔR) during CV cycling for the EQCM cells (SI Fig. ). As a result, Δf of the Cu 3 (HHTP) 2 -coated quartz crystal was translated into a mass change (Δm) of the working electrode during the cycling, where an increase in Δm corresponds to a decrease in Δf (see SI for details). Δm was then plotted against the accumulated electronic charge, ΔQ (Fig. ) to allow analysis of the charging mechanism in greater detail, including assigning the different species entering and leaving the electrode pores during charging and discharging. If Δm is negatively correlated with ∆Q, cations ("counterions") are the main charge carriers, while the opposite stands for a positive correlation. TEABF 4 showed an approximately rectangular EQCM CV response, consistent with the above threeelectrode experiments (Fig. ). The Δm-ΔQ plot for this system demonstrates a reversible and linear mass change during cycling, with the mass change upon both cathodic (negative) and anodic (positive) charging aligning closely to the average value (dashed orange line, Fig. ). This suggests a kinetically reversible process with the same dominant charge carrier for both charging directions. As the slope of the ∆m-∆Q plot is negative, this indicates that cation (counterion) adsorption and desorption are the dominant charge storage mechanisms. If anions (co-ions) were involved in the charging mechanism, as would be the case for an ion exchange mechanism, there would be an obvious decrease in the slope of the ∆m-∆Q with charge accumulation, resulting in a weaker ∆m-∆Q correlation. Both the cathodic and anodic charging regimes show an average mass change close to 1400 ng mC -1 , corresponding to an equivalent molecular weight of 139 ± 4 g mol -1 (SI Fig. ). This aligns closely with the molecular weight of the naked TEA + ion (130 g mol -1 ), and further confirms that the cations are the dominant charge carriers across the full potential range considered here. As the experimental mass change is consistent with adsorption and desorption of naked TEA + cations, this charging mechanism would require no net movement of anions or solvent molecules in or out of the electrode pores. However, the solvated ion size of TEA + (13.0 Å) in the bulk electrolyte is less than the average pore size of Cu 3 (HHTP) 2 in the electrode film (~14 Å), indicating that solvated TEA + cations are still able to enter and leave the pores of the MOF. If this were to occur during the charging mechanism, an equivalent amount of free solvent molecules would need to leave or enter the pores. This demonstrates a key limitation of EQCM experiments as the contributions of different electrolyte species to the net mass change cannot be distinguished. Overall however, our EQCM results suggest that cation desorption (positive charging) and cation adsorption (negative charging) are the primary charging mechanisms for Cu 3 (HHTP) 2 in 1 M TEABF 4 in the potential range from -0.05 V to +0.45 V vs. Ag (Fig. ). This is in contrast to the double-layer charge storage mechanism of traditional porous carbons, where both the anions and cations have been shown to play a role in the charging mechanisms. This finding also begins to explain our capacity data in Figure , where variation in the size of the cation (i.e. the dominant charge carrier) impacted the capacitance for both positive and negative charging in the present potential window. Excitingly, this finding is in line with recent computational work on this system, which suggested a cation-dominated charging mechanism for this system. Although the EQCM results suggest that the anions are not involved in the charging mechanism, their involvement cannot be ruled out. Simulations from Walsh et al. found that, while TEA + cations only have one possible adsorption site in the pores of Cu 3 (HHTP) 2 , BF 4 -anions have two environments: a weaker binding site at the centre of the pore, and a stronger binding site at the pore edge where the BF 4 -anions interact with the C-H moieties of the HHTP linker molecules (Fig. ). At the OCV, the simulations showed that anions exist in both environments. However, upon positive charging (cation desorption), simulations found that the anions shift and predominantly occupy the more strongly bound edge site. Upon negative charging (cation adsorption), the opposite occurs with the anions shifting to occupy the pore centre site. However, as there is no net change in the in-pore population of BF 4 -from this process, and thus no net anion flux, EQCM is not able to detect this potential contribution of the anions to the charging mechanism, although it is likely to be a more minor contribution to the overall charge storage compared to the adsorption/desorption of cations. This illustrates that a multi-technique approach is required to get a complete picture of charging. Other experimental techniques, such as NMR spectroscopy, should be used to better understand the role of both the anions and solvent in the charging mechanism of this system. Figure : Cartoon of the cation-dominated charging mechanism of CuHHTP with 1 M TEABF4 in acetonitrile, as deduced from EQCM and computational studies. (a) Upon positive charging, there is cation desorption from the MOF pores, with the anions shifting to a more strongly interacting edge adsorption site. (b) Upon negative charging, the reverse occurs with cation adsorption and corresponding movement of the anions to the more weakly bound centre adsorption site. Note that the ion and solvent sizes are not to scale, and the solvation of electrolyte ions is not shown.