THE DOUBLE-LAYER OF ACTIVATED CARBON ELECTRODES .2. CHARGE-CARRIERS IN THE SOLID MATERIAL

Considerable differences in the electronic conductivity of activated carbon when conditions are varied can be accounted for formally by treating the material as an intrinsic semiconductor with a narrow bandgap. According to the temperature dependence of the resistance, the bandgap is 0.034 eV. The minimum conductivity of the material investigated was 10(3) S m-1. Upon exposing the formerly dry material to a humid atmosphere or dipping it in water or an electrolyte solution, the conductivity increases by 15%-20%. This effect can be attributed to a H2O monolayer at the pore surface, with the system H2O + CO surface group acting as an acceptor and creating holes. Upon shifting the electrode potential from the immersion potential (potential of zero charge), the conductivity changes considerably because, owing to the thinness of the solid walls between the pores, the space-charge layer extends throughout the solid material. Unusual effects are observed in the presence of H+ or OH- ions; they are attributed to interactions of the adsorbed ions with the CO surface groups. The large capacity of the material is due to the mobile charge carriers but not to the surface groups.