An Investigation of Morphology Dependent Charge Storage in Polypyrrole Membranes

PPy-based membranes exchange ions with electrolyte through reversible redox processes and hence are best suited as electrodes for batteries and super capacitors. The energy density of batteries and super capacitors are dependent on the specific capacitance of the conducting polymer and can be represented through a mechanistic model for ion transport. Through this model, the specific capacitance of polypyrrole-based membranes is shown to be dependent on the number of accessible redox sites at the electrolyte-polymer interface. The accessibility of redox sites at the electrolyte-polymer interface can be increased by controlling the morphological properties and distribution of dopant in the polymer backbone. Thus, by nanostructuring and by controlling the distribution of the dopant in the polymer, we have shown that the capacitance of PPy-based membranes can be increased to 490 F.g⁻¹ for a 50 mV.sec⁻¹ scan rate and 0.6 g.cm⁻² specific mass. Despite this value of specific capacitance being the highest reported for PPy-based membranes to date, it is estimated that only 69% of active redox sites are used for ion storage and hence can be increased further. Maximizing specific capacitance requires an understanding of spatial distribution of redox sites in the polymer backbone and its corresponding chemoelectrical activity. In order to generate a spatial map of ion storage in PPy-based membranes, this article presents for the first time a shear-force (SF) based topography imaging and scanning electrochemical microscopy (SECM) imaging of the PPy(DBS) under reduced and oxidized conditions. From a correlated topography and chemoelectrical activity of PPy-based membrane, the data shows the availability of redox sites in the polymer and it is projected that this result will enhance the design and nanostructuring of PPy-based membranes and distribution of dopant in the backbone.