Transient and steady state electrochemical studies of transport dynamics and equilibria in thin polymer films containing redox centers

• Main Author: Bruns, Mark William
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-1990
• Subjects: Analytical chemistry
• ISBN: 9798207083988

Electron transport dynamics and equilibrium effects were studied for the ferricyanide/ferrocyanide (Fe(CN)$\sb6\sp{3-/4-}$) redox couple incorporated in partially-quaternized poly(4-vinylpyridine) (QPVP) modified electrodes in contact with potassium nitrate (KNO$\sb3$) solution. The goal was to intercompare the electron diffusion coefficient (D$\sb{\rm E}$) of the system obtained by transient and steady state electrochemical methods and to evaluate the effect of equilibrium changes in the film on the magnitude of these values. Chronocoulometry (CC), ac impedance spectrometry (AC), and a form of steady state voltammetry (SS) were the methods chosen for the intercomparative study. Time-dependent equilibrium effects were determined to profoundly alter the measured D$\sb{\rm E}$ values for this system as it soaked in electrolyte solution for extended periods of time; the film thickness ($\phi$) was unaffected by soaking. Final D$\sb{\rm E}$/$\phi\sp2$ values (after soaking times $>$ 100 h) were 0.5 to 1 s$\sp{-1}$, 3 to 8 times smaller than initial values obtained immediately after the film was lightly loaded with redox centers (initially, D$\sb{\rm E}$/$\phi\sp2$ $\approx$ 3 to 4 s$\sp{-1}$; (Fe(CN)$\sb6\sp{3-}$) $\approx$ 0.25 to 0.8 M). D$\sb{\rm E}$/$\phi\sp2$ values fell rapidly during the first 10 h of equilibration, but become constant at long soaking times. The redox centers partitioned between the film and the surrounding electrolyte solution to effectively lower the initial concentration in the film by 30-50%. Activation energies (E$\sb{\rm a}$) for electron transport were shown to decrease slightly as the film aged (from 6.5 kcal/mole to 5 kcal/mole), and were found to be significantly greater for more heavily loaded, extensively crosslinked films ((Fe(CN)$\sb6\sp{3-}$) $\approx$ 2 M, E$\sb{\rm a}$ = 13.5 kcal/mole; whereas, at $\sim$0.25 M, E$\sb{\rm a}$ = 5 kcal/mole). Changes in the permeability of the membrane with increasing soaking time revealed that it assumed a more fluid, open structure during equilibration. Cyclic voltammograms demonstrated a conversion to a more reversible and more energetically equivalent system as the redox centers redistributed themselves uniformly within the film structure. It was determined that valid, comparable D$\sb{\rm E}$ values from all methods could only be achieved with fully equilibrated films ($>$100 h old). The intercomparability study revealed that CC and AC yield essentially equal D$\sb{\rm E}$ values for thin ($\phi\ \approx$ 4500A), lightly loaded QPVP films, especially at long soaking times (when D$\sb{\rm E}$/$\phi\sp2$ $\approx$ 0.7 to 1 s$\sp{-1}$); D$\sb{\rm E}$'s from CC, AC, and SS agreed fairly well for thick, more heavily loaded films. CC and AC results suggested migration was prominent at short timescale on freshly loaded films, but diminished as the film aged.