Controlling Electrode-Electrolyte Interactions to Enhance Capacitance.

Abstract

Understanding how ions interact with electrode surfaces at the molecular level is essential for improving the performance of energy storage devices and electrocatalysts. However, progress has been limited by the structural disorder and poorly defined surface chemistries of conventional carbon-based electrodes. In this work, we use layered metal-organic frameworks (MOFs) as model systems to investigate how different functional groups influence electric double-layer capacitance. We find that electrodes with deprotonated M-O and M-S groups exhibit significantly enhanced capacities with alkali metal cations, most notably Li+, compared to tetraethylammonium (TEA+), while no enhancement is observed for MOFs with protonated M-NH groups. The largest capacity increase is seen for MOF electrodes with metal-hydroxy linkages paired with Li+ electrolytes, which we attribute to strong Li-O interactions and improved charge screening. This mechanism is supported by solid-state nuclear magnetic resonance spectroscopy experiments and molecular simulations, which reveal specific Li+ binding at oxygen-rich sites, while operando X-ray techniques rule out cation intercalation as a contributing factor. Overall, these results highlight a chemically tunable strategy for enhancing charge storage in porous electrodes and offer new insights into how surface functionality impacts electric double-layer behavior.