Clean energy storage technologies are critical to transform the fossil fuel-based transportation sector and to store energy from intermittent renewable sources. Supercapacitors have gained much attention due to their fast charging and discharging times and their long cycle lifetimes. These electrochemical devices usually consist of two solid electrodes, an ionically conductive separator, and a liquid electrolyte. Electrode materials for supercapacitors should have high electrical conductivity, high surface areas for charge storage, and high electrochemical stability.