The role of the electric conductivity of carbons in the electrochemical capacitor performance

The interpretation of the performance of electrochemical capacitors based exclusively on the textural features and surface chemistry of carbons can be insufficient, or even misleading, in the case of materials prepared at low temperatures (typically below 800 degrees C). It is suggested that the gradual improvement of the electrochemical performances of carbon-based capacitors at high current densities, following heat treatments up to 900 degrees C, is mainly a consequence of the simultaneous increase in conductivity. This is illustrated by a study of carbons based on a mesoporous carbon prepared at 550 degrees C, which displays poor electrochemical performances and a low conductivity (4.6 x 10(-6) S m(-1)). A first heat treatment at 700 degrees C leads to major structural, chemical and electrochemical changes, due to the collapse of the smaller mesopores and the formation of a microporous structure with average pore widths around 1.3 nm. One also observes a reduction in the surface oxygen density from 13 to approximately 5 mu mol m(-2). Further heat treatments at 800 and 900 degrees C do not modify significantly these characteristics, nor the surface-related capacitances at low current densities (1 mA cm(-2)) in the aqueous (2 M H(2)SO(4)) and organic (1 M (C(2)H(5))(4)NBF(4)/CH(3)CN) electrolytes. On the other hand, one observes increasingly high rate capabilities which may be ascribed to the simultaneous increase in conductivity from 7.3 to 147.8 S m(-1) between 700 and 900 degrees C. (C) 2011 Elsevier B.V. All rights reserved.