Ion Bridging by Carbon Dioxide Facilitates Electrochemical Energy Storage at Charged Carbon-Ionic-Liquid Interfaces

Solvent free ionic liquid (IL) electrolytes facilitate high-voltage supercapacitors with enhanced energy density, but their complex ion arrangement and through that the electrochemical properties, are limited by strong Coulombic ordering in the bulk state and like-charged ion repulsion at electrified interfaces. Herein, a unique interfacial phenomenon resulting from the presence of carbon dioxide loaded in 1-Ethyl-3-methylimidazoliumtetrafluorborate electrolyte that simultaneously couples to IL ions and nitrogen-doped carbonaceous electrode is reported. The adsorbed CO2 molecule polarizes and mitigates the electrostatic repulsion among like-charged ions near the electrified interface, leading to an ion "bridge effect" with increased interfacial ionic density and significantly enhanced charge storage capability. The unpolarized CO2 possessing a large quadrupole moment further reduces ion coupling, resulting in higher conductivity of the bulk IL and improved rate capability of the supercapacitor. This work demonstrates polarization-controlled like-charge attraction at IL-electrode-gas three-phase boundaries, providing insights into manipulating complex interfacial ion ordering with small polar molecule mediators.