Ion structures and populations inside microporous carbon electrodes of electrical double-layer capacitors

• Main Author: Wei, Jiaqi
• Other Authors: Li Shuzhou
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Science::Chemistry::Physical chemistry::Electrochemistry; Engineering::Materials::Energy materials
• Online Access: https://hdl.handle.net/10356/152010
• Institution: Nanyang Technological University

Electrical double-layer capacitor (EDLC) is one of the most important energy storage devices famous for its high power density and long cycle life but suffers from low energy density. To expand the energy density, microporous carbons with high specific surface area and tailored porosity below 2 nm have been developed. In addition to the high specific capacitance, carbon micropores also bring distinct charging mechanisms from electrical double-layer model due to their small pore sizes. To study the working principles of microporous carbon EDLC, various experimental and theoretical approaches based on nuclear magnetic resonance, molecular dynamics, etc. have been developed in the past decade. Molecular insights such as ion desolvation and breaking of the Coulombic ordering have been revealed inside carbon micropores. Nevertheless, the ion–ion and ion–pore interactions are still poorly understood due to the lack of knowledge on ion structures and packings. New approaches are needed to address these challenges. This thesis presents some efforts in exploring the ion structure and population inside microporous carbon EDLC with spectroscopical approaches. An in situ platform coupling an X-ray spectroscopy, an electrochemical workstation and a customized coin cell is developed for real-time analysis. Aqueous electrolytes of metal cations are prepared for studies for their wide applications in EDLC and other electrochemical systems. Using the set-up, the structure and population of metal cations in 1.0 M aqueous electrolyte inside microporous carbon EDLC are analyzed. Upon charging, the cations are electrostatically adsorbed into negatively charged micropores. Partial desolvation of the octahedrally solvated cations to tetrahedrally solvated ones is observed depending on the cation physicochemical properties as well as the carbon pore size. Moreover, an anion-specific effect on the extent of cation adsorption is discovered, which is due to different ion–ion and/or ion–pore interactions. The cations associate inside micropores due to the synergic effect of spatial confinement and Coulombic screening, which show sluggish dissociation kinetics and block the pores from further charge storage. Positive charging effectively repulses the cations and restores the capacitance. Therefore, an intermittent reverse cycling strategy that periodically evacuates micropores and revivifies the capacitance is designed, which leads to enhanced cycling performance of EDLCs. The results demonstrate new insights into ion structural evolutions and their effects on capacitive charge storage, providing guidelines for designing advanced EDLCs. Further investigations on new applications based on the abovementioned findings as well as ion structures in organic electrolytes would lead to bigger impacts in related fields such as energy storage and electrocatalysis.