Porous carbon thin films for electrochemical capacitors

Activation effects on carbon films, derived from commercial aromatic polyimide films (Kapton, DuPont), in CO2 atmosphere at 1203 K on capacitance properties were studied. Two thicknesses of polyimide films were used: 7 and 25 μm. Pore formation during the activation process progresses in two steps due to the existence of a denser surface layer and a more porous core material. In the first step micropores are opening in the dense surface region of the material with average pore diameter smaller than 1 nm. During the second step, mesopores start opening, while micropore volume remains constant with the average micropore diameter of over 1 nm, producing bimodal texture. The first step finishes after 30 min for the thinner samples while for the thicker samples it finishes after 60 min of activation. As a consequence of such textural changes during activation, the thicker sample has a maximum areal capacitance of 0.35 F/cm2. The thinner sample activated for 30 min has a maximum volumetric capacitance of 220 F/cm3 and achieves a maximum gravimetric capacitance of 240 F/g when the texture becomes bimodal after 240 min of activation. These results confirm that activation of carbonized Kapton films gives promising electrode materials for supercapacitors.