Carbon xerogels as electrodes for supercapacitors. The influence of the catalyst concentration on the microstructure and on the electrochemical properties

Carbon xerogels, synthesized through the resorcinol–formaldehyde polycondensation and subsequently dried under subcritical condition, have been studied as electrodes for supercapacitors. In particular, the influence of the catalyst concentration has been investigated by systematically changing the amount of catalyst (Na₂CO₃) utilized to synthesize the xerogels. To clarify the effect of such variable, both the surface properties and the electrochemical behavior of xerogel-based supercapacitors have been examined. From the xerogels characterization, it can be inferred that the amount of catalyst used has a strong influence on the properties of the material. Contrary to what happens with carbon aerogels, the best properties are obtained with the xerogels synthesized with the least amount of catalyst: in this case the highest measured specific capacitance of the supercapacitor cells, which is assembled coupling two symmetric electrodes in series, is 25 F/g, value that corresponds to a single-electrode specific capacitance of 100 F/g. The maximum energy storage capacity in an aqueous electrolyte is 3.1 Wh/kg. Using more concentrated catalyst solutions, the gel microstructure becomes finer, composed of smaller particles and pores, which in turns leads to an increase of the capillary drying stresses and to the collapse of the organic structure. Consequently, the shrinkage of the gels is high and the final carbon xerogels do not posses sufficient surface area and porosity to store a significative amount of energy.