Anchored Anions, Mobile Cations: Charge Storage in MOF-based Supercapacitors Studied 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 a Ni ₃ (HITP) ₂ metal–organic framework (MOF), with well-defined pores as an electrode model system. Using 1 M NaTFSI aqueous electrolyte, we show that TFSI ⁻ anions are immobilized near MOF pore walls via fluorine–hydrogen interactions with N-H functional groups. 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 outer-pore environments. Additionally, we track reversible adsorption induced pore swelling, rule out ion intercalation, and observe minor irreversible structural expansion after prolonged negative bias.