Influence of Carbon Surface Defects on Double Layer Charge Storage and Ion Dynamics in Electrochemical Capacitors
Our efforts focused on the influence of surface functional groups and structural ordering on electrochemical behavior of room-temperature ionic liquid electrolytes . We synthesized carbide-derived carbon via halogen etching of TiC precursor at 800 °C, yielding powder with 0.67 nm pores. Using 700-1800 °C vacuum annealing, 800 °C H2, 600 °C NH3, or 400 °C air treatments, we, respectively, defunctionalized, aminated, hydrogenated, and oxidized pore surfaces. Room-temperature 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonimide) was subsequently confined in the pores of functionalized CDCs. By maintaining a consistent pore structure, we decoupled the key properties that affect double layer charge storage and ion dynamics. We demonstrated the structural changes and resulting changes in surface properties, such as hydrophobicity and electrical conductivity, and correlated them with capacitance and ionic resistance. In particular, we confirmed the detrimental influence of defect removal on capacitance but found its positive influence on rate handling and electrochemical stability. Quasi-elastic neutron scattering showed a logarithmic decay relationship between relaxation time and scattering intensity, as well as surface chemistry-dependent coefficients of elastic scattering. Electrochemical results correlated neutron scattering behavior with ionic resistance and rate handling and underscored the significance of intermolecular interactions between electrolyte ions and functional groups on pore surface. Capacitance and cyclability differences between functionalized CDCs underscored the selective effects of surface porous carbons with similarly modified non-porous ssamples including graphene, carbon onions, and carbon black. We demonstrate divergent electrochemical behaviors that depend on ion confinement in similarly sized pores, shedding fundamental insights into key surface properties that govern electrochemical capacitor performance.