Selective Anion Anchoring in MOF-Based Supercapacitors Revealed with Operando Small-Angle X-Ray Scattering

Understanding how ions interact with electrodes in electric double-layer capacitors (EDLCs) is key to advancing energy storage, yet many fundamental aspects remain unclear. Here, we employ operando small-angle X-ray scattering (SAXS) to investigate charge storage in metal-organic framework (MOF)-based supercapacitor electrodes as a model system. Using Ni₃(2,3,6,7,10,11-hexaiminotriphenylene)₂ (Ni₃(HITP)₂) MOF electrodes and 1 M aqueous sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) as the electrolyte, we show that TFSI^- anions are immobilised near MOF pore walls via fluorine-hydrogen interactions with N-H functional groups of the MOF. We quantify the concentration of pinned anions and demonstrate that their immobilization persists across different applied cell voltages, resulting in a cation-dominated charge storage mechanism governed solely by Na⁺ adsorption and desorption. Charge balancing is unaffected by whether voltage is applied stepwise or gradually, with no dynamic differences between in-pore and out-of-pore environments and no ion intercalation taking place.