Supercapacitors or ECs represent a class of energy storage devices that offer fast energy uptake and delivery2. EDLCs utilize reversible ion adsorption at the surface or inside pores to store charge. Thus, the best performing EDLC materials should have a high specific surface area (SSA), making nanostructured carbons the materials of choice, since they can easily deliver high SSA at low price and possess good electrical conductivity. Generally, EDLCs offer great cyclability and power densities and are characterized by nearly rectangular cyclic voltammograms (CVs) and linear galvanostatic charge–discharge profiles (Fig. 1a). It is important to note that the rectangular CV for EDLCs stems from the fundamental nature of the double-layer storage mechanism: in the absence of diffusion limitations, there is instantaneous charge separation (that is, polarization) in a capacitor under an external electrical field so that dV/dt is a constant. While ideal for power applications, EDLCs suffer from limited energy density (the amount of charge stored).