Influence of counterion charge on the electrochemistry and impedance of polypyrrole

Polypyrroles doped with multiply charged anions are becoming increasingly important in a wide range of energy, environmental and biomedical applications. The increased counterion charge promotes anion binding and retention and can significantly increase stability and performance. The electrochemical properties of polypyrrole films prepared galvanostatically in Na₂SO₄ (PPySO₄) where found to be similar those of PPyClO₄ prepared in NaClO₄, although there was significantly more anion retention during potential cycling. In contrast, PPyPO₄ films prepared in Na₃PO₄ under the same conditions were over-oxidised and more dense, which is beneficial for corrosion protection and electroanalysis. Paradoxically, the low mobility of SO₄²⁻ counterions results in more facile charging and discharging of the film, as observed with large and polymeric counterions. This can create significant benefits in applications that require fast cycling, such as supercapacitors and high-rate batteries. These conclusions are corroborated by electrochemical impedance measurements in various aqueous electrolytes. Ionic conductivity was dominated by anion transport for both PPyClO₄ and PPySO₄, whilst PPyPO₄ was predominantly a cation conductor.