Supercapacitors with Aqueous Electrolytes: What Can We Learn from In-Situ and Operando Synchrotron and Neutron Techniques?

This thesis investigates the fundamental processes governing supercapacitors with aqueous electrolytes through the development and application of in-situ and operando synchrotron and neutron techniques. Using a suite of complementary methods, including X-ray Transmission (XRT), X-ray Fluorescence (XRF), (Anomalous) Small-Angle X-ray Scattering ((A)SAXS), Small-Angle Neutron Scattering (SANS), X-ray Diffraction (XRD), and Neutron Diffraction (ND), a detailed experimental framework is established to probe supercapacitor behaviour under working conditions. The work focuses on experimentally accessing and interpreting the following aspects of supercapacitor operation: (1) ion and solvent concentration changes and charge-balancing mechanisms, (2) structural changes in the electrode during operation, (3) wetting effects, (4) local ion and solvent (re-)arrangement, (5) charging dynamics, (6) atomic-scale interactions via pair distribution functions, and (7) the influence of electrode geometry and cell design. These phenomena are explored in detail for two electrochemical systems: MSP-20X activated carbon electrodes with 1 M RbBr (aq.) electrolyte, and Ni3(HITP)2 conductive metal-organic framework electrodes with 1 M NaTFSI (aq.) electrolyte. For both, tailored experimental approaches are developed and critically assessed. While this thesis provides a practical framework for applying in-situ and operando synchrotron and neutron techniques to electrochemical systems, its central contribution lies in the insights it offers into electric double-layer formation and ion and solvent behavior in supercapacitors under operating conditions. By revealing how a range of interconnected processes unfold during charging and discharging, these findings offer a comprehensive and nuanced understanding of how supercapacitors function and how their performance might be more effectively tailored in the future.