A Comparative Study on Thin Poly-N-methylpyrrole and Polypyrrole Films

Growth, cycling, and overoxidation of thin poly-N-methylpyrrole (PMePy) films was studied in 0.5 M LiClO4-propylene carbonate electrolyte. N-methylpyrrole starts to polymerize at about 3.53 V vs Li/Li+. The redox potential of PMePy is about 0.6 V more positive than that of polypyrrole (PPy). The overoxidation thresholds of both PMePy and PPy are, however, almost identical (3.8-3.9 V vs Li/Li+). Low doping levels, of the order of y ≈ 25%, are therefore reached during the nearly reversible potentiodynamic cycling of PMePy in the 2.0-4.0 V range, in contrast to y ≈ 42% attainable on polypyrrole (PPy) under the same conditions. Continuous slight decrease of the coulombic capacity of the polymer with the cycle number, attributed to an irreversible overoxidation, is observed. Addition of 1 vol% of water into the growth electrolyte does not improve the doping level of the PMePy film. Due to the poor electrochemical performance of thin PMePy films with respect to battery application, higher energy densities as offered from the Li/PPy system are not to be expected.