Electrochemical behavior of porous carbons

Abstract

The electrochemical behavior of porous carbons based on carbon fibers and those based on phenolic was evaluated by cyclic voltammetry via the Fe 2+ Fe 3+ redox couple. An irreversible electrochemical response obtained from the fiber-based porous carbons was made reversible by coating of the porous carbons with graphite flakes or carbon black; the electron-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 electrochemical behavior, due to contamination by long-chain aliphatic hydrocarbons (comprising ≤ 10% of the porous carbon weight). Their electrochemical performance was rendered consistent from batch to batch and also enhanced (with increased k s ) by cleansing in methylene chloride. The electrochemical areas obtained for phenolic-based porous 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 specific geometric surface area (per unit volume) for phenolic-based porous carbons, while the specific electrochemical surface area did not vary much; at a pore density of 30 ppi, the electrochemical area was even higher than that of carbon black, due to the large pore size and the 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 (increasing fiber length). The 500 ppi phenolic-based porous carbon, after methylene chloride cleansing, exhibited higher k s , than conventional glassy carbon. The fiber-based porous carbons exhibited higher apparent density, lower resistivity and higher compressive strength than those based on phenolic.