Conducting polymers can be doped and dedoped rapidly to high charge density and as a result are potential active materials for use in electrochemical capacitors. We discuss three schemes by which conducting polymers can be utilized in electrochemical capacitors and in the third of these, which employs a conducting polymer that can be both n- and p-doped, high energy and power densities are demonstrated. Polythiophenes can be both n- and p-doped reversibly and we have tested a number of poly-3-arylthiophenes in order to achieve the best n-doping at thick polymer films. We have found that poly-3-(4-fluorophenyl)-thiophene, in a solution of 1moldm(-3) tetramethylammonium trifluoromethanesulfonate in acetonitrile, can be n- and p-doped, both reversibly and to high charge density. Such key properties for high power, energy storage devices have not been demonstrated before with conducting polymer active materials. We propose that the improvement in polymer n-dopability that results from derivitization of thiophene with aryl substituents in the 3-position, is probably achieved thanks to electron transfer from the negatively charged polythiophene backbone to the aryl substituent. This proposal is discussed in the light of modeling information and of voltammetric data. Further supporting information is provided by electron microscopy, impedance spectroscopy and electrochemical quartz crystal microbalance results.