As 1 H NMR could not accurately quantify the environments of the cations due to poor resolution of the proton spectra (SI Figure ), EQCM experiments were performed with Ni3(HITP)2 with 1 M NEt4BF4 in acetonitrile electrolyte to further investigate the charging mechanism (Figure ). In EQCM, the frequency change, Δf, of a crystal is measured and used to determine the change in mass, Δm, of an electrode during charging via Sauerbrey's equation, as long as the gravimetric approach is valid (see SI for details, SI Figure ). This change in mass can be attributed to movement of cations, anions and solvent into and out of the electrode. To prevent possible decomposition reactions that occur on the MOF/Au electrode of the EQCM cell, the potential window was restricted to +200 mV to +500 mV vs. Ag during the EQCM experiment (Figure ; CVs shown in outside lines with frequency change (Δf) shown inside). Given that the open circuit potential of the electrode, prior to any polarisation, was +284 mV relative to Ag, the selected potential window covers in part both positive and negative charging regimes of the ex-situ NMR experiments above. Furthermore, by using a slow voltage sweep of 1 mV s -1 , we assume that the mechanistic charging processes observed via EQCM can be directly compared to the results of the earlier ex-situ NMR experiments, which used constant voltage holds. A reversible and negative correlation between Δm and ΔQ was found during cathodic polarisation (Figure ), which by comparison with the (i) all cation and (ii) all anion scenarios described above indicates that cations are the dominant charge carriers in this system. Therefore, the mass increase with negative electronic charge accumulation indicates net cation adsorption in the cathodic charging process, while a reversible net cation desorption process is observed in the anodic regime. Importantly, this agrees qualitatively with the ex-situ NMR results above and confirms the dominant contribution of the cations to the charging mechanism. The slope of the EQCM data on both cathodic and anodic charging corresponds to a relatively small change in mass on charging relative to the range expected from the experimental ex-situ NMR data (Figure ). This result suggests that, in addition to the dominant cation adsorption during cathodic charging, there must be a net loss of solvent and or/anion desorption, and vice versa on anodic charging. As only the net mass change is recorded in EQCM experiments, it is difficult to accurately quantify the absolute solvent and/or anion movement as molecules may simultaneously be moving into and out of the pores during both charging and discharging. To investigate the solvent contribution to the charging mechanism further, 2 D NMR of the deuterated acetonitrile solvent in 1 M NEt4BF4/d3ACN was attempted, with the aim to quantify the 'inpore' solvent environment (SI Figure ). However, due to poor resolution, fitting these spectra is associated with high uncertainty, and coupled with partial evaporation of acetonitrile in ex-situ measurements, makes the application of NMR limited in studying the solvent in this system. Nevertheless, this works highlights the complementary nature of EQCM together with NMR to study anion, cation and solvent contributions to the charging mechanism of layered MOFs in supercapacitor devices. Qualitatively, a cation-dominated charging mechanism is observed in both NMR and EQCM charging experiments for this MOF system, although measuring the quantitative agreement in the percentage contribution of anion movement to charge storage is not possible.