Rapid potential step charging of paper-based polypyrrole energy storage devices

Symmetric paper-based supercapacitor devices containing polypyrrole (PPy)-cellulose composite electrodes and aqueous electrolytes can be charged using either potential step or constant current charging. Potential step charging provides better control of the charging and can result in significantly shorter charging times, enabling charging in 22 s for devices with cell capacitances of 12.2 F when charged to 0.8 V. The paper-based electrode material was compatible with charging currents as large as 5.9 A g ^-1 due to the rapid counter ion mass transport resulting from the porous composite structure and the thin PPy coatings. The charging times were controlled by the RC time constants of the devices and the cell resistance was found to decrease with increasing electrode area. For small cells, the cell resistance was determined to a large extent by the electrolyte resistance and contact resistances, whereas the resistance of the current collectors dominated for larger cells. The specific cell capacitance was 38.3 F g ^-1 or 2.1 F cm ^-2, normalized with respect to the total electrode weight and electrode cross section area respectively, and the devices showed 80-90% capacitance retention after 10 000 potential step charge and discharge cycles. These results, which demonstrate that potential step charging can be advantageous for conducting polymer based energy storage devices, are very encouraging for the development of new up-scalable paper-based energy storage devices.