Influence of porosity on charging speed of polypyrrole

The rate of charging of supercapacitor and battery electrodes is often limited by the transport of ions through the active electrode material. One way to accelerate this transport is to increase the porosity of the electrode, at the cost of electrode capacity. This tradeoff is studied through the fabrication and characterization of porous carbon nanofibre/polypyrrole films used as storage electrodes. Electrospun poly (acrylonitrile-co-acrylamide) fibres were carbonized and then electrochemically coated with variable amounts of polypyrrole. The resulting hybrid materials were characterized using both conventional methods such as cyclic voltammetry, electron microscopy and conductivity, as well as ionic conductivity and pulsed-field-gradient nuclear magnetic resonance which directly probe ion transport. It is found that with modest porosity, these materials retain much of the capacitance (∼50%) of bulk polypyrrole with dramatically increased (∼300 times) charge and discharge rates, suggesting the potential of the approach for increasing the useful frequency range of polypyrrole-based supercapacitors, as well as other storage materials whose porosity can be varied. Simple transport models based on series or parallel arrangements of electrolyte and active electrode are developed to explain the results and identify the factors limiting charge/discharge rates.