ELECTROCHEMICAL-BEHAVIOR OF POROUS CARBONS

The electrochemical behavior of porous carbons based on carbon fibers and those based on phenolic was evaluated by cyclic voltammetry via th e Fe2+/Fe3+ redox couple. An irreversible electrochemical response obt ained from the fiber-based porous carbons was made reversible by coati ng of the porous carbons with graphite flakes or carbon black; the ele ctron-transfer rate constant k(s) and the capacitance were increased, while the electrochemical area was decreased. Porous carbons based on phenolic exhibited variation from batch to batch in their electrochemi cal behavior, due to contamination by long-chain aliphatic hydrocarbon s (comprising less than or equal to 10% of the porous carbon weight). Their electrochemical perfor mance was rendered consistent from batch to batch and also enhanced (with increased k(s)) by cleansing in methy lene chloride. The electrochemical areas obtained for phenolic-based p orous carbons were larger than those of the fiber-based porous carbons , even though their specific geometric surface areas (per unit volume) were much lower. The electrochemical area decreased with increasing s pecific geometric surface area (per unit volume) for phenolic-based po rous carbons, while the specific electrochemical surface area did not vary much; at a pore density of 30 ppi, the electrochemical area was e ven higher than that of carbon black, due to the large pore size and t he consequent penetrability by the electrolyte. Among the fiber-based porous carbons, both electrochemical area and specific electrochemical area decreased with decreasing specific geometric surface area (incre asing fiber length). The 500 ppi phenolic-based porous carbon, after m ethylene chloride cleansing, exhibited higher k(s) than conventional g lassy carbon. The fiber-based porous carbons exhibited higher apparent density, lower resistivity and higher compressive strength than those based on phenolic. (C) 1997 Elsevier Science Ltd.