Amplifying Interfacial Capacitance Through Underpotential Deposition in Salt-in-Ionic Liquid Electrolytes

Rapid growth of intermittent energy sources, such as wind and solar, is causing a resurgence in research on materials and devices that store electrochemical energy. Electrochemical capacitors exhibit promising device characteristics to help level grid scale power fluctuations, including fast charge-discharge kinetics and long device lifetimes. This is especially true for ionic liquids, which promise increased safety and performance, as compared to volatile organic electrolytes commonly used in batteries. However, large scale implementation of ionic liquid-based capacitors remains limited by low device energy densities, as the interfacial capacitance of ionic liquid-electrode interfaces decreases significantly under large polarization. Here, we investigate how incorporating metal cations of varying size and valence into ionic liquids modifies electric double layer formation. We find that alkali cations substantially amplify interfacial capacitance in salt-in-ionic liquid electrolytes, overcoming capacitive limitations caused by ion crowding. Remarkably, we observe capacitive enhancement exceeding 350% in lithium- and sodium-containing electrolytes at Au and Cu electrodes under large polarization, where ion crowding diminishes interfacial capacitance in neat ionic liquids. Our data indicates that metal cation underpotential deposition plays a key role in capacitive enhancement, and we observe that this process can be highly reversible under cycling. Our findings suggest that tuning metal-electrolyte interactions to enable underpotential deposition provides avenues for increasing capacitor performance. This opens the door to additional opportunities for developing devices that could play an essential role in leveling power fluctuations that are inherent to renewable energy grids.