Datasets of EQCM-controlled deposition and cycling of thin polypyrrole films in acetonitrile electrolyte solution

The paper presents three datasets obtained by electrochemical quartz microbalance technique which was applied to studies of conducting polymer film in contact with non-aqueous electrolyte solution. The first dataset describes the calibration procedure of gold-coated quartz crystal, immersed in acetonitrile silver ion-containing electrolyte, by means of silver layer electrodeposition. On the basis of experimentally measured dependence of the resonance frequency on the varying electrode mass in the course of electrochemical silver deposition/dissolution, the calibration coefficient was found to be equal to 13.6 ng/Hz. The second dataset has been collected when thus calibrated EQCM cell was used for determination of the mass change due to the polypyrrole film growth during anodic oxidation of pyrrole monomer from its acetonitrile solution. Its treatment reveals the proportionality between the mass change and the charge spent for pyrrole electrooxidation, the proportionality coefficient being 53.5 g per mole of electrons. The third dataset contains EQCM measurement data during repetitive charge-discharge treatment of the deposited polypyrrole film (cyclic voltammetry, CV) in monomer-free electrolyte. Collected data shows that continuous cycling of the polymer film leads to progressive increase of the cation-exchange contribution to the total ion flux which maintains the film's electroneutrality during variation of its redox state. These findings might be useful both for a qualitative consideration of the cycling stability of polypyrrole in non-aqueous medium and for a quantitative mathematical modelling of polypyrrole electropolymerization and its redox transformations.


a b s t r a c t
The paper presents three datasets obtained by electrochemical quartz microbalance technique which was applied to studies of conducting polymer film in contact with non-aqueous electrolyte solution. The first dataset describes the calibration procedure of gold-coated quartz crystal, immersed in acetonitrile silver ioncontaining electrolyte, by means of silver layer electrodeposition. On the basis of experimentally measured dependence of the resonance frequency on the varying electrode mass in the course of electrochemical silver deposition/dissolution, the calibration coefficient was found to be equal to 13.6 ng/Hz. The second dataset has been collected when thus calibrated EQCM cell was used for determination of the mass change due to the polypyrrole film growth during anodic oxidation of pyrrole monomer from its acetonitrile solution. Its treatment reveals the proportionality between the mass change and the charge spent for pyrrole electrooxidation, the proportionality coefficient being 53.5 g per mole of electrons. The third dataset contains EQCM measurement data during repetitive charge-discharge treatment of the deposited polypyrrole film (cyclic voltammetry, CV) in monomer-free electrolyte. Collected data shows that continuous cycling of the polymer film leads to progressive increase of the cation-exchange contribution to the total ion flux which maintains the film's electroneutrality during variation of its redox state. These findings might be useful both for a qualitative consideration of the cycling stability of polypyrrole in non-aqueous medium and for a quantitative mathematical modelling of polypyrrole electropolymerization and its redox transformations. © 2020 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons. org/licenses/by/4.0/).

Calibration of gold-coated quartz crystal
The dataset describes the calibration procedure of gold-coated quartz crystal in acetonitrile silvercontaining electrolyte by means of silver layer electrodeposition. Fig. 1 presents data on the determination of the proportionality coefficient between the shift of the resonant frequency and the electrode mass change during the electroreduction of silver in the CV mode (mass change was assumed to be only due to the one-electron silver deposition, the corresponding charge being determined via integration of the CV data in Fig. 1a). This coefficient was found to be equal to 13.6 ng/Hz. This value was subsequently used for determination of the polymer film mass variation.
Specifications Table   Subject Electrochemistry Specific subject area EQCM-analysis of transport processes of background electrolyte ions in conductive polymers Type of data

Value of the Data
The data provide a quantitative information on the participation of background electrolyte ions in the transport processes inside thin polypyrrole films obtained at low potentials in low-concentration monomer solutions. The data on the in situ EQCM characterization of the processes of electrodeposition and of redox cycling of thin polypyrrole films can be useful for further understanding of the properties and patterns of ion exchange in such films both for researchers dealing with polypyrrole-coating systems and for specialists dealing with polypyrrole-based sensors. The findings may be useful both for qualitative consideration of the cycling stability of polypyrrole in non-aqueous media and for quantitative mathematical modelling of the polypyrrole electropolymerization and its redox transformations.

Electrodeposition of polypyrrole films
The dataset in Fig. 2a (chronoamperogram and shift of the resonant frequency of the quartz crystal) has been collected when the calibrated EQCM cell was used for determination of the electrode mass changes due to the polypyrrole film growth on its surface as a consequence of the anodic oxidation of the pyrrole monomer in its acetonitrile solution. Similar measurements have earlier been described by J. Heinze and coworkers [1e4] and many scientific groups [5e7] for a different solution composition.  The instantaneous values of the EQCM frequency change, DF, in Fig. 2a were recalculated via the corresponding values of the deposited mass of the polymer film, Dm, with the use of the above proportionality coefficient. The corresponding values of the oxidation charge (derived from the current in Fig. 2a by integration) were divided by the Faraday constant, 96500 C/mol, to determine the number of moles of electrons, n, passed via the circuit till this moment. Fig. 2b shows that these two quantities, Dm and n, are proportional to one another within the whole time interval.
The slope of this dependence in Fig. 2b is 53.5 g/mol, which corresponds to the mass of the polymer film in its charged state (which includes those of the monomer units and of the charge-compensating ions) per mole of electrons. Earlier [8] in the solution of a similar composition, the redox equivalent of the pyrrole electrooxidation process was determined via the same procedure.

Characterization of polypyrrole film (CV test)
The dataset given in Fig. 3 and Table 1 contains data of EQCM measurements during repetitive charge-discharge (CV test) of the polypyrrole film (its deposition procedure has been described in section 2) in monomer-free acetonitrile electrolyte solution, with synchronous registration of the resonance frequency variation.
Collected data in Fig. 3a, b and c show that the CV treatment (repetitive cycling of potential) of the polymer film leads to progressive increase of the cation contribution to the total ionic exchange between the film and the electrolyte solution flux which maintains the film's electroneutrality in the course of variation of its redox state. These findings might be useful both for qualitative consideration of the cycling stability of polypyrrole in non-aqueous media and for quantitative mathematical modelling of polypyrrole redox transformations.
Optical microscopy (Fig. 4) confirms formation of a solid colored film uniformly covering the electrode surface in the range of its contact with electrolyte solution.

Experimental design, materials, and methods
Electrochemical quartz crystal microbalance (EQCM) measurements were carried out in electrochemical three-electrode cells with separated compartments, in which one of the two surfaces (covered with thin sprayed Au layers) of non-polished quartz crystal disk of 5 MHz resonant frequency was in contact with electrolyte solution and used as working electrode. The geometrical vibrationally active surface area of the working gold electrode was 0.79 cm 2 . Both electrodes on the crystal surface were connected to the SRS QCM 200 generator/analyzer of high-frequency oscillations. Potential of the working electrode immersed in the solution was simultaneously controlled by potentiostat. The two devices were synchronized in time, which made it possible to record simultaneously a chronoamperogram of the potentiostatic film electrodeposition on the working electrode (or a cyclic voltammogram) as well as the temporal variation of the resonant frequency shift of the crystal, which was proportional to the film's mass change.
All potentials are given relative to the reference electrode composed of Ag wire immersed into 10 mM AgNO 3 þ 0.1 M TBAPF 6 acetonitrile solution. Its equilibrium potential is 100 mV more negative than that of the ferrocene (Fc/Fc þ ) redox couple in the same solution. The counter electrode was platinum wire in background acetonitrile solution separated from the working electrode compartment by porous sintered glass frit.
To calibrate the quartz crystal, AgNO 3 (Fluka, > 99.5%) was used. Untreated acetonitrile (Panreac, HPLC) was used as solvent for preparation of calibration (AgNO 3 ) and monomeric (pyrrole) solutions (sections 1 and 2); it was dried with 4 Å molecular sieves prior to use in the solution for characterization of the polymer-modified electrode (section 3). The background electrolyte, TBAPF 6 (Fluka, > 99%), was kept in oven at 80 C for a day before the experiment. Pyrrole (Alfa Aesar, 98%) was preliminarily purified by fractional distillation under inert (Ar) atmosphere with the use of the Schlenk line.
Quartz crystal was calibrated in contact with 0.6 mM AgNO 3 þ 0.1 M TBAPF 6 acetonitrile solution in the potentiodynamic (CV) mode within the potential range from -0.5 V to 0.5 V with scan rate of 0.1 V/s. Potentiostatic deposition of the polypyrrole film from the 1 mM pyrrole þ 0.1 M TBAPF 6 acetonitrile solution was carried out at potential of 0.58 V. The electrolyte composition and deposition potential were selected based on papers [9,10]. The deposition charge density on the vibrationally active surface of the non-polished quartz crystal was 83.7 mC/cm 2 .
The redox response of the deposited polypyrrole films was studied by cyclic voltammetry (CV) in 0.1 M TBAPF 6 acetonitrile solution after their thorough rinsing with pure acetonitrile to remove unreacted monomer. The redox response was recorded in the potential range from -1.0 V to 0.1 V (Fig. 3a) and from -1.3 V to 0.3 V (Fig. 3b) with scan rate of 0.1 V/s. The total number of cycles was 25. The first five cycles (Fig. 3a) and the last five cycles (Fig. 3b) are presented above. Deaeration of the solutions before the experiment was carried out with the use of the Schlenk system, by imposing vacuum briefly to the cell, followed by filling it with high purity argon.  (c) its peripherical region which was partially (left side) in contact with the solution so that it was covered by the polypyrrole film while its right side was in contact with the insulating O-ring so that no polypyrrole film was deposited there.