The contribution of sulfate ions and protons to the specific capacitance of microporous carbon monoliths

The monoliths studied in this work show large specific surface areas (up to 1600 m2 g-1), high densities (up to 1.17 g cm-3) and high electrical conductivities (up to 9.5 S cm-1). They are microporous carbons with pore sizes up to 1.3 nm but most of them below 0.75 nm. They also show oxygen functionalities. The electrochemical behavior of the monoliths is studied in three-electrode cells with aqueous H2SO4 solution as electrolyte. This work deals with the contribution of the sulfate ions and protons to the specific capacitance of carbon monoliths having different surface areas and different contents of oxygen groups. Protons contribute with a pseudocapacitance (up to 152 F g-1) in addition to the double layer capacitance. Sulfate ions contribute with a double layer capacitance only. At the double layer, the capacitance of the sulfate ions (up to 291 F g-1) is slightly higher than that of protons (up to 251 F g-1); both capacitances increase as the surface area increases. The preference of protons to be electroadsorbed at the double layer and the broader voltage window of these ions account for their higher contribution (70 %) to the double layer capacitance.

Financial support through the projects MAT2011-25198, MP 1004 and PROMETEO/2009/047 is gratefully acknowledged. V.B. thanks MINECO for R&C contract.