J. Bácskai

Polynuclear nickel hexacyanoferrates: monitoring of film growth and hydrated counter-cation flux/storage during redox reactions

Abstract An electrochemical quartz crystal microbalance (EQCM) was employed to monitor directly the growth of nickel(II) hexacyanoferrate(III) (NiHCNFe) films on gold substrates during electrodeposition as well as a result of sol-gel aggregation in colloidal nickel ferricyanide solutions used for modification. Frequency changes due to mass changes of the gold/crystal working electrode were correlated with cyclic voltammetric (CV) data. Evidence is also provided for the sorption of counter-cations (Li+, Na+ and K+), and associated water molecules, during redox reactions of the film. There is a strict relationship between the amount of alkali metal ions incorporated into the film during reduction, or excluded from the film during oxidation, and the frequency changes during EQCM measurements. The amount of solvent (H2O) transferred and sorbed in the NiHCNFe film reflects the degree of hydration of the investigated counter-ions. Anions also seem to participate in NiHCNFe electrochemistry, but their role is much less pronounced.

Electrochemical quartz crystal microbalance study of the swelling of poly(vinylferrocene) films

Abstract The sorption of solvent molecules and ions in poly(vinylferrocene) films was studied under potential cycling conditions as a function of the electrolyte composition. The results indicate that only the redox behaviour of PVF films but also the film swelling is dependent on the nature and concentration of supporting electrolyte. The study of the break-in process revealed that significant accumulation of the solution species occurs in the neutral films. It was found that the degree of swelling depends also on the conditions of film preparation. Due to the extensive swelling an interplay between the elastic and viscous behaviour may complicate the evaluation of the results obtained by electrochemical quartz crystal microbalance method.

Monitoring of formation and redox transformations of poly(methylene blue) films using an electrochemical quartz crystal microbalance

Abstract An electrochemical quartz crystal microbalance and cyclic voltammetry have been used to monitor the growth of poly(methylene blue) films on gold during potential cycling between −0.6V and 1.0V vs. sce in sodium phosphate buffer (pH 8.2) and Na 2 SO 4 solutions. Both the adsorption/desorption of monomer methylene blue and the formation of poly(methylene blue) film can be followed by microgravimetry. The response of the polymer films is significantly separated from that of the monomer species in solution. It has been proved that anion sorption/desorption occur during the redox transformation of the polymer, while cations play no role. Preliminary results on the complex protonation/deprotonation equilibria are also discussed.

Impedance analysis of polymer film electrodes

Abstract A new model is elaborated which can explain some anomalies of the impedance spectra of polymer films on electrodes. The model assumes two capacitances associated with the double layers at the substrate/film and at the film/solution interfaces, respectively. The latter capacitance is considered as a constant phase element due to the inhomogeneity of the polymer network and to energetically non-equivalent bonding sites for counterions. The effect of a low dc current, ie a slow reaction of the redox sites in the film with solution species, is also investigated. The electrochemical impedances characteristic of the model have been derived. The simulated impedance spectra were compared with those obtained experimentally.

Effect of the temperature on the conductivity and capacitance of poly(aniline) film electrodes

Abstract The temperature and potential dependences of the behaviour of poly(aniline) modified electrodes were studied by impedance spectroscopy. It was rendered probable that the ohmic resistance which can be determined by impedance measurements is in connection with ionic conductivity and practically no information of the electronic conductivity of the polymer can be obtained. No temperature dependence of the low-frequency limiting capacitance was found. The high-frequency capacitance which can be assigned to double-layer at the polymer/electrolyte interface depends strongly on temperature. Both capacitances show cpe behaviour.

Quartz crystal microbalance study of the growth of indium hexacyanoferrate films during electrodeposition and coagulation

Abstract An electrochemical quartz crystal microbalance and cyclic voltammetry have been used to monitor fabrication of indium(III) hexacyanoferrate(III,II) films by electrodeposition through potential cycling and by interfacial coagulation during ageing in the modification solution containing potassium electrolyte, indium(III) and hexacyanoferrate(III). We have chosen indium hexacyanoferrate as a model system since its cyclic voltammetry shows a single set of reversible, well-defined peaks significantly separated from the solution responses. Simultaneous estimations of charge and mass have been used to characterize the efficiency of the deposition processes. Microgravimetry is particularly attractive for monitoring the film growth due to interfacial coagulation (at open circuit). Electrodeposition by potential cycling permits fast preparation of thin films. The coagulation method is useful for fabrication of thick films. We also discuss the response of the resulting indium hexacyanoferrate film in the supporting electrolyte. The data are consistent with the insertion and exclusion of K+ countercations during the systems reduction and oxidation respectively.

An electrochemical quartz crystal microbalance study of the influence of pH and solution composition on the electrochemical behaviour of poly(aniline) films

There is a significant accumulation of solution species in the film in all solution compositions and in the whole potential range studied. In 10 mol dm−3 HClO4 or H2SO4 solutions the films become highly swollen, which eventually leads to the dissolution of the films. Significant mass changes during the oxidation of the leucoemeraldine to emeraldine form can be observed only in the pH interval between pH 1 and 3, where the reduced poly(aniline) is practically uprotonated, while the half-oxidized material is protonated.

The effect of the counter ion concentration on the electrochemistry of tetracyanoquinodimethane polymer film electrodes

Abstract The effect of the counter in concentration on the cyclic voltammetric behavior of TCNQ polymer film electrodes has been studied in different aqueous electrolytes. The variation of the peak currents and peak potentials as a function of the counter ion concentration is explained in terms of salt or ion pair formation between the reduced redox sites in the film and the counter ions, as well as the polyelectrolyte behavior of the surface layer. The voltammetric behavior is simulated by an empirical model that takes into account ionic shielding of fixed-site charges by electrolyte ions. With increasing concentration of the supporting electrolyte, the reduced polymer film adopts a more compact structure and the diffusion of the counter ions and/or the segmental motion of the polymer chain becomes hindered. No dissolution of the film occurs at negative potentials in simple salt electrolytes or in buffers up to pH 10 at high enough electrolyte concentration.

Study of the sorption phenomenon accompanying the break-in process of polymer film electrodes using a quartz crystal microbalance

Abstract The sorption of solvent molecules and ions into poly(tetracyanoquinodimethane) surface films during the break-in process has been studied under potentiostatic steady-state and potential cycling conditions using the electrochemical quartz crystal microbalance technique. No sorption of solvent molecules and ions was found in the case of unelectrolysed virgin films. It has been shown that the mass change of the film follows the charge and discharge processes occurring in the films; however, a gradual decrease of the frequency values corresponding to the starting potential can be observed which can be considered as a measure of the electrolyte content in the discharged film layer. No equilibrium with regard to the electrolyte was attained during the potential cycling even at slow sweep rates. The time necessary for break-in, i.e. for the swelling and wetting of the polymer structure to be completed, increases with the film thickness and electrolyte concentration.

Combined electrochemical and quartz crystal microbalance study on the temperature dependence of ion and solvent sorption in electroactive polymer films on electrodes

Abstract The electrochemical quartz crystal microbalance (EQCM) technique has been applied to monitor the changes of ion and solvent content within poly(tetracyanoquinodimethane) (PTCNQ) films as a function of the redox state at different temperatures. The effect of concentration of the LiCI supporting electrolyte was also studied. It was observed that the mass of the neutral film increased as the temperature was increased corresponding to a greater swelling of the surface film. The electroactivity of the film (the total charge consumed during the electrochemical transformation of the redox groups of the film) and the mass change increased simultaneously with the temperature. This is consistent with the model that with increasing temperature, the polymer film adopts a less compact structure and the diffusion of the counter-ions as well as segmental motion of the polymer chain becomes less hindered. It was established that in the mass changes accompanying the redox processes occurring in PTCNQ films, the solvent sorption is predominant and the incorporation of ions plays a lesser role. The extent of the swelling depends on the concentration of the bulk electrolyte solution due to the decrease in water activity and the diminishing number of available water molecules for the hydration of Li+ ions. In contact with electrolyte solutions of low or medium concentration, the films are highly swollen at elevated temperatures and consequently an interplay between a rigid layer and viscous behaviour can be observed.