Origin of High Areal Capacitances of Low Apparent Surface Area Carbons

The energy storage mechanism of low apparent specific surface carbons derived from direct pyrolysis of biomass or N-rich polymers has puzzled the researchers in the supercapacitor field in recently years. In order to explore this scientific problem, such carbon materials were prepared through pyrolysis of seaweed and nitrogen-rich polymers such as melamine formaldehyde resin and polyaniline. The chemical and physical properties of these carbons were investigated intensively by nitrogen sorption analysis, CO₂ sorption analysis, X-ray photoelectron spectroscopy and X-ray diffraction. Besides, the capacitive properties of these carbon materials were investigated by galvanostatic charge-discharge, cyclic voltammetry and electrochemical impedance spectroscopy. The results show that all the carbons derived from direct pyrolysis of seaweed or polymers possess low apparent specific surface areas of no more than 60 m² g^-1, and their areal capacitance could reach up to an abnormally high value of 252 μF cm^-2. Combining the results of systematical pore structure analyses and electrochemical measurements, it was found that these carbons contain numerous ultramicpores which could not be detected by the adsorbate of N₂ but are accessible to CO₂ and electrolyte ions. These ultramicropores play dominant roles in the charge storage process for these low apparent surface area carbons. We concluded that the energy storage mechanism of these carbons is mainly electric double layer capacitance and that the contribution of pseudocapacitance to the total capacitance is less than 15 %. This finding challenges the widely accepted viewpoint that the high capacitance of O-rich or N-doped carbon is mainly attributed to the pseudocapacitance generated from the faradic reactions between oxygen or nitrogen functionalities and electrolyte.