Selective Anion Anchoring in MOF-Based Supercapacitors Revealed with Operando Small-Angle X-Ray Scattering
Malina Seyffertitz, Chloe J. Balhatchet, Max Valentin Rauscher, Sebastian Stock, Gerhard Fritz‐Popovski, Thomas Leiner, David Holec, Heinz Amenitsch, Alexander C. Forse, Oskar Paris
Abstract
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. Understanding charge storage in supercapacitors remains a challenge. Here, authors use operando X-ray scattering to show that selective anion immobilization in MOF-based electrodes leads to a cation-driven charge storage mechanism.