In Situ Electrochemical Dilatometry of Onion-Like Carbon and Carbon Black

High power electrochemical double layer capacitors (also called supercapacitors) rely on highly conductive electrode materials with a large specific surface area, which is easily accessible for ions. Exohedral materials with a large ion-accessible outer surface and little or no porosity within the particles are particularly attractive for supercapacitor electrodes because they decrease mass transport limitations and enable very high charge/discharge rates. This study focuses on the investigation of charge induced expansion effects of spherical exohedral carbons, that is, onion-like carbons (OLC, diameter: 5-7 nm) and carbon black (diameter: ≈40 nm). Employing electrochemical in-situ dilatometry we studied the expansion behavior within ±1 V potential window versus carbon in an organic electrolyte (tetraethylammonium-tetrafluoroborate in acetonitrile). The expansion had a very small amplitude (<0.2% at ±0.08 C·m^-2 accumulated charge; i.e., approximately ±1 V versus carbon) and was fully reversible. This was explained by ion adsorption on the exohedral carbons. Molecular dynamics (MD) simulations were employed to calculate the solvation shell of the respective cation and anion and the results were used to further evaluate the measured expansion. In summary, the experiments and simulations revealed that ion intercalation or ion sieving, which are commonly found in microporous (endohedral) carbons, were absent. Finally, low sweep rates resulted in a slight electrode compaction on a cycle-by-cycle basis, which can be explained by charge-induced restructuring of the nanoparticle network.