Study on Ion and solvent transport mechanism for polypyrrole films using the impedance techniques

Abstract

Water transport during cation and anion transport in polypyrrole(PPy) and poly(N-methylpyrrole) (PMPy) films has been investigated by employing the electrochemical quartz crystal microbalance (EQCM) technique, the electrochemical impedance technique, and the electrogravimetric impedance technique. The number of accompanying waters per cation in poly(pyrrole/ copper phthalocyaninetetrasulfonate) (PPy/CuPTS) films, where cation transport prevails, increases with the hydration number of cation in an electrolyte solution ; it increases with the sequence of $Cs^+＜K^+＜Na^+＜Li^+＜Mg^{2+}$. The number of accompanying waters per cation exhibits hysteresis behavior during the redox cycle; it increases during the insertion of a cation whereas it is uniform during the exclusion of a cation. The number of accompanying waters per $Cl^-$ or $SO_4^{2-}$ in poly(N-methylpyrrole/chloride) (PMPy/Cl) and poly(N-methylpyrrole/sulfate) (PMPy/$SO_4$) films, where ion transport is anion-specific, is considerable, whereas the number per $NO_3^-$ in poly(N-methylpyrrole/nitrate) (PMPy/$NO_3$) films is not large. The ionic conductivity of a divalent cation and anion is much smaller than that of a monovalent cation and anion, respectively, because of the strong ion-ion interaction in a film. The contribution of cation and anion to ion transport in polypyrrole/poly(styrenesulfonate) (PPy/PSS) and polypyrrole/nitrate (PPy/$NO_3$) films has also been obtained to investigate frequency- dependent ion transport mode. In the PPy/PSS film, cation transport is dominant during the fast faradaic process whereas anion transport is dominant during the slow one. On the contrary, in the PPy/$NO_3$ film, anion transport is dominant during the fast faradaic process whereas cation transport is considerable during the slow one. Finally, the electromechanical impedance technique has been employed to verify the relation between mass and resonant frequency. The morphology change of a PPy film does not affect ...