Relationship between pore surface areas and electric double layer capacitance in non-aqueous electrolytes for air-oxidized carbon spheres

To investigate the relationship between pore structure and specific capacitance of electric double layer capacitors with non-aqueous electrolytes, phenol resin based carbon spheres were modified by oxidation in dry air, giving a wide range of pore structure. Surface area due to the presence of different pores was characterized through α_s plots of adsorption isotherms of N₂ gas at 77 K as microporous surface area (S_micro.) due to micropores and external surface area (S_ext.) mainly due to the mesopores. A simple model can be presented by recognizing the different contributions of S_micro. and S_ext. in forming electric-double layers. The capacitance per unit external surface area was larger by one order of magnitude than that per unit surface area of micropores (0.187 F/m² versus 0.017 F/m² at charge-discharge current of 4 mA/g), revealing greater contribution of external surface to specific capacitance for electric-double layers than surface of micropores. With the increase in charge-discharge current to 40 mA/g, the relative contribution of external surface became more significant, which was supposed to be due to large sizes of electrolyte ions, Et₃MeN⁺ and BF₄⁻.