The influence of electrode and separator thickness on the cell resistance of symmetric cellulose-polypyrrole-based electric energy storage devices

The influence of the cell design of symmetric polypyrrole and cellulose-based electric energy storage devices on the cell resistance was investigated using chronopotentiometric and ac impedance measurements with different separator and electrode thicknesses. The cell resistance was found to be dominated by the electrolyte and current collector resistances while the contribution from the composite electrode material was negligible. Due to the electrolyte within the porous electrodes thin separators could be used in combination with thick composite electrodes without loss of performance. The paper separator contributed with a resistance of similar to 1.5 Omega mm(-1) in a 1.0 M NaNO3 electrolyte and the tortuosity value for the separator was about 2.5. The contribution from the graphite foil current collectors was about similar to 0.4-1.1 Omega and this contribution could not be reduced by using platinum foil current collectors due to larger contact resistances. The introduction of chopped carbon fibres into the electrode material or the application of pressure across the cells, however, decreased the charge transfer resistance significantly. As the present results demonstrate that cells with higher charge storage capacities but with the same cell resistance can be obtained by increasing the electrode thickness, the development of paper based energy storage devices is facilitated. (C) 2014 Elsevier B.V. All rights reserved.