Molecular Simulation Study on Interfacial Humid Ionic Liquids in Supercapacitors

Electrical double layer capacitors (EDLCs), also called supercapacitors, have attracted a great deal of attention in present electrical energy community, owing to their advantageous properties, such as high-power density and exceptional cycle life. Room-temperature ionic liquids are potentially ideal materials for electrochemical applications such as supercapacitors and batteries, and impressive progress has recently been demonstrated in well-controlled laboratory systems. However, complete removal of water from ionic liquids is nearly impossible, and such water content may bring significant effects on electrical double layers in IL-based supercapacitors since the electrolysis of water possibly arises due to the potential applied on IL supercapacitors larger than water decomposition voltage.

In this talk, using molecular dynamics (MD) simulation, we show the first work on the adsorption of water on carbon electrode surfaces in contact with humid ILs. We found that water is generally enriched within sub-nanometer distance from charged electrodes. The level of enrichment depends on the type of ions and typically increases as the surface charge density of electrodes increases. We clarified the mechanisms of these electrosorption behaviors. We highlighted the key role of ion-water association and interactions between water dipole and inhomogeneous electrical fields in double layers in determining water electrosorption, and rationalized the ion specificity and the asymmetrical dependence of electrosorption on the polarity of electrode charge. We also investigate influence of electrode materials (carbon vs. gold) on the interfacial humid ionic liquids. The comparison of MD between carbon and gold electrodes with the humid ILs and observed different water and ion accumulation, which may be attributed to the interaction between gold and ILs as well as water content.