Onion-like carbon (OLC), also known as carbon onions, is an attractive material for electrical energy storage in regards to high rate, high power applications. We report the most up to date, systematic, and extensive study of the electrochemical behavior of carbon onions in aqueous (1 M sulfuric acid, H 2SO 4) and organic (1 M tetraethylammonium tetrafluoroborate, TEA-BF 4, and 1 M tetrabutylammonium tetrafluoroborate, TBA-BF 4, in acetonitrile) electrolytes. The physical and electrical properties of OLC are studied as a function of the synthesis temperature and compared with diamond soot, carbon black, and activated carbon. To obtain a molecular scale picture of the processes at the OLC-electrolyte interface, we supplement the experimental work with molecular dynamics (MD) simulations of carbon onions in organic electrolytes. The capacitive performance of OLC exceeds other carbon materials at high charge/discharge rates (up to 50 V s -1; time constant τ ∼ 10 ms). OLC produced from detonation soot has a performance similar to that of OLC from highly purified nanodiamond. While OLC produced at 1500 °C has the largest specific surface area, OLC produced at 1800 °C has the highest conductivity and shows the best capacitive performance at high rates.