Structure and electrochemical capacitance of carbon cryogels derived from phenol-formaldehyde resins

Highly porous carbon cryogels derived from phenol-formaldehyde (PF) resin formulated in ethanol have been prepared and investigated. Different P/F molar ratios have been used for obtaining a series of alcogels that have been freeze-dried and next pyrolysed at 900 °C. Carbonaceous materials having different bulk densities and textural properties have thus been obtained. These materials were fully characterised by helium and mercury pycnometry, mercury porosimetry, nitrogen adsorption, and tested as porous electrodes of electrochemical double-layer capacitors working in 4 M H₂SO₄ aqueous electrolyte. For that purpose, cyclic voltammetry and galvanostatic charge/discharge experiments have been carried out. The derived specific capacitances, around 100 F/g, were shown to depend strongly both on pore availability to the electrolyte and on average micropore width. The cryogel having the highest surface area and micropore volume is thus the one presenting the lowest capacitance because of its poorly accessible and too narrow microporosity.