Speaker
Description
Understanding ion behaviour at electrified interfaces is central to improving electric double-layer capacitors (EDLCs), yet many molecular-scale charge storage mechanisms and specific electrode-electrolyte interactions remain incompletely resolved. Operando synchrotron techniques such as Small Angle X-Ray Scattering (SAXS) and X-Ray Diffraction (XRD) are powerful tools to probe ion behaviour in nanoporous electrodes under working conditions. However, in conventional activated carbon electrodes, the disordered pore structure gives rise to broad and complex scattering features, hindering a more direct interpretation and quantitative analysis. To enable more direct insight, we use an ordered metal-organic framework (MOF) as a model material, in which cylindrical nanopores arranged on a hexagonal lattice give rise to well-defined Bragg peaks, facilitating data interpretation.
Here, we investigate charge storage in the electrically conductive Ni3(HITP)2 MOF using a 1 M NaTFSI aqueous electrolyte. Operando SAXS measurements were performed at the Austrian SAXS beamline at Elettra, complemented by XRD measurements at the ID22 beamline at the European Synchrotron Radiation Facility (ESRF). Using this combined synchrotron approach, we show that TFSI- anions are immobilized near MOF pore walls via fluorine–hydrogen interactions with N-H functional groups at the electrode. We quantify the concentration of pinned anions and demonstrate that their immobilisation persists across applied cell voltages, resulting in a cation-dominated charge storage mechanism governed solely by Na+ adsorption and desorption.
These findings reveal how combining operando synchrotron techniques with a well-defined model system enables direct insight into interfacial ion behaviour, enabling a mechanistic explanation for the cation-dominated charge storage observed in many MOF-based systems. More broadly, this work establishes a framework for understanding ion electrosorption during electric double-layer formation in aqueous supercapacitors, enabling the targeted design of charge storage mechanisms through controlled interfacial interactions.