Influence of Carbon Surface Defects on Double Layer Charge Storage and Ion Dynamics in Electrochemical Capacitors
Authors: Dyatkin Boris , Mamontov Eugene , Gogotsi Yury 
Published: February 2020

Supercapacitors, which rely on electrosorption of ions in a porous carbon electrode, can supplement or even replace traditional batteries in energy harvesting and storage applications. While supercapacitors offer > 10 kW/kg power densities, their < 5 Wh/kg energy density levels are insufficient for many automotive and grid storage applications. Most prior efforts have focused on novel high-performing ionic liquid electrolytes and porous carbons with tunable pore diameters and high accessible surface areas. However, we still lack fundamental understanding of the influence of electrode surface properties, such as graphitic defects and functional groups, on capacitance limitations, charge/discharge dynamics, and electrochemical stability of the electrolyte under high potentials. These surface properties significantly impact quantum capacitance contributions and must be investigated to optimize ion screening, charge mobilities, and operating voltage windows.
Our efforts focused on the influence of surface functional groups and structural ordering on electrochemical behavior of room-temperature ionic liquid electrolytes [1]. 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.
Reference:
1. Dyatkin, B.; Gogotsi, Y., Effects of Structural Disorder and Surface Chemistry on Electric Conductivity and Capacitance of Porous Carbon Electrodes. Faraday Discussions 2014, 172
(1), 139 - 162.

Figure 1