Strain-based in situ study of anion and cation insertion into porous carbon electrodes with different pore sizes

The expansion of porous carbon electrodes in a room temperature ionic liquid (RTIL) is studied using in situ atomic force microscopy (AFM). The effect of carbon surface area and pore size/pore size distribution on the observed strain profile and ion kinetics is examined. Additionally, the influence of the potential scan rate on the strain response is investigated. By analyzing the strain data at various potential scan rates, information on ion kinetics in the different carbon materials is obtained. Molecular dynamics (MD) simulations are performed to compare with and provide molecular insights into the experimental results; this is the first MD work investigating the pressure exerted on porous electrodes under applied potential in a RTIL electrolyte. Using MD, the pressure exerted on the pore wall is calculated as a function of potential/charge for both a micropore (1.2 nm) and a mesopore (7.0 nm). The shape of the calculated pressure profile matches closely with the strain profiles observed experimentally. Atomic force microscopy is used to monitor the expansion of porous carbon electrodes, which results from insertion/adsorption of ions in carbon pores during charging. The strain data collected at various potential scan rates are used to obtain information on anion and cation kinetics. Molecular dynamics simulations are performed to determine the molecular origins of charge-induced expansion in porous carbons.