The feasibility of energy extraction by carbon xerogel electrodes – A question of ionizable or redox active surface groups?

Carbon Xerogel electrodes' materials were prepared by polycondensation of resorcinol and formaldehyde. They show accommodation of anions in preference to cations by their porous structure. In addition, their selectivity toward anion electroadsorption was found to be pH dependent, with an increase in electrodes' capacitance in acidic solutions. This dependence of the capacitance on the pH of the solution is explained by the presence of functional carboxylic surface groups in the Xerogel carbon micropores, determined by Boehm titration to be 3.26 meq/g. These functional surface groups give rise to the Donnan exclusion effect, whereby they cause partial repulsion of anions from the microporous structure. When the pH of the electrolyte solution is acidic (<2), most of these acidic functional surface groups are in their non-ionized state, and hence anions can freely penetrate the Xerogel carbon microporous structure. The pH dependent capacitance of these carbons was utilized to examine the feasibility of energy extraction from mixing of saline and acidic solutions following the four basic steps of capacitive mixing processes – charge, solution exchange, discharge, and another solution exchange. As opposed to what we expected, a potential rise was observed when the solution was exchanged from acidic to saline, rather than during the other exchange, from saline to acidic solution. It appears that upon exchange of the solutions from saline to acidic, the expected potential rise resulting from the change in the electrodes capacitance, is cancelled by the new quinone-hydroquinone equilibrium that is established on the electrodes by their surface groups. The ability of these surface redox functional groups to store charge provides a basis for energy extraction from the neutralization of acidic solutions, as described herein.