Thermodynamics and Kinetics of Redox Switching of Polyvinylferrocene Films in Perchlorate Solutions

Abstract

Cyclic voltammetry was used to create nonequilibrium populations of different solvation and configurational states of partially oxidized polyvinylferrocene (PVF). Oxidation levels were established by scanning either from a fully reduced state to the desired oxidation level or from a fully oxidized state to the desired level. Coulostatic conditions were then established by opening the external circuit, and subsequent mass and potential changes were followed. The film's approach to equilibrium involves configurational changes within the polymer and simultaneous and subsequent solvent transfer. At very short times (t e 0.2 s) the approach to equilibrium is limited by both solvation and reconfiguration processes. For a short time afterward (0.2 < t/s < 1.0) reconfiguration alone is rate limiting. At intermediate times (1 < t/s < 5) both processes play comparable roles. At long times (t > 5 s) solvation is the controlling step. The electroactive polymer film does not completely reach equilibrium even after 1 h at open circuit as evidenced by continuous small mass changes. These mass changes are the result of water transfer between the polymer film and the bathing electrolyte. A scheme of cubes model rationalizes mechanistic pathways leading to equilibrium. In particular, the observed extrema in mass (solvent population) are predicted. The electrode potential, after 1 h at open circuit, shows nearly Nernstian dependence on the redox composition for film states produced by either anodic or cathodic cyclic voltammetric scans. These Nernst plots are displaced by only a few millivolts because only a weak Nernstian dependence on film water content exists. Films that are 50% oxidized exhibit a sub-Nernstian response with respect to the perchlorate concentration in the bathing solution under permselective conditions.

Vinylferrocene Homopolymer and Copolymers: An Electrochemical Comparison

“…[18][19][20] Studies into the electrochemical behaviour of pVFc films have shown how the voltammetric response is dependant on the anion present within the solution. 8,10,[21][22][23][24][25][26][27][28] The stability of the oxidation wave upon repetitive scanning was found to be dependant on the size of the anion, with larger anions producing less stable signals. This anion sensitivity was minimized by controlling both the scan rate and potential range.…”

Two-dimensional correlation in cyclic voltammetry and electrochemical quartz crystal microbalance: A complementary tool to conventional techniques

“…With knowledge from earlier literature, this effect was heterogeneous electron transfer where electrons move across the interface and enter the electrode. Both ferrocene oxidation and FeðCNÞ 4À 6 oxidation were diffusion controlled or kinetically favored [9,12]. As cross valleys 5-8 suggested, both FeðCNÞ 4À 6 and Fc oxidations were negatively correlated with a process taking place at a lower potential of 0.02 V. This process was very likely solvation and reconfiguration of the polymer network [9].…”

“…Both ferrocene oxidation and FeðCNÞ 4À 6 oxidation were diffusion controlled or kinetically favored [9,12]. As cross valleys 5-8 suggested, both FeðCNÞ 4À 6 and Fc oxidations were negatively correlated with a process taking place at a lower potential of 0.02 V. This process was very likely solvation and reconfiguration of the polymer network [9]. It could also be a non-faradic process such as charging of a double layer on the polymer/solution interface [6].…”

“…The charge (Q) and the mass variation (M) can be measured by a potentiostat and a QCM. The ratio of Dm/DQ reflects the formula mass of the counter ions or ion complexes under permselective conditions [9].…”

“…Detailed surveys related to the break-in effects have revealed that the electrochemical behavior of PVF in aqueous solutions includes several aspects. Bruckenstein et al used a cubic model with three processes: salvation, reconfiguration and electron exchange [9]; while Tang and Zeng used a square model containing two processes, faradic and nonfaradic [6]. Pre-soaking of PVF-coated electrode in electrolyte media may reduce break-in effects [11], but not always.…”