Impact of the Electrochemical Porosity and Chemical Composition on

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Impact of the Electrochemical Porosity and Chemical Composition on the Lithium Ion Exchange Behavior of Polypyrroles (ClO4-, TOS-, TFSI-) Prepared Electrochemically in Propylene Carbonate. Comparative EQCM, EIS and CV Studies Paweł Marek Dziewon´ski† and Maria Grzeszczuk* Faculty of Chemistry, UniVersity of Wrocław, 14 F. Joliot-Curie Street, 50-383 Wrocław, Poland ReceiVed: January 27, 2010; ReVised Manuscript ReceiVed: March 17, 2010

Conditions of electrodeposition, i.e. a potential window of the process, addition of water, the current density, and morphology of substrate electrodes (Pt, Pt/TiO2, Au), were shown to influence strongly ion-exchange properties of polypyrrole (PPy) synthesized in propylene carbonate (PC), doped with ClO4- or ptoluenesulfonate (TOS-). “Electrochemical porosity” and redox activity of PPy films were compared to the characteristics of poly(3,4-ethylenedioxythiophene) (PEDOT). A molecular indicator of the PPy film structure packing was bis(trifluoromethylsulfonyl)imide anion (TFSI-). Ion-exchange properties of PPy were found to be almost independent of chemical composition of the polymer, described in the literature as PPy(I), PPy(II), PPy(III). Instead, micro- and nanoscopic morphology of the polymer film and a molecular level packing of the polymer chains as well as the counterion nature are of the foremost importance. The polymer film structure/ properties are shown to change upon prolonged redox/ionic stimulations. Lithium exchange between PPy films and contacting phases (PC electrolyte, TiO2) proceeds in addition to the anion exchange, the latter being a dominant process under conditions of the reversible electrochemical p-doping of PPy, although diffusion coefficients of PC solvated lithium ions in PPy are higher than diffusion coefficients of perchlorate, p-toluenesulfonate or bis(trifluoromethylsulfonyl)imide anions. The highest flux of Li+ ions into/out of the PPy phase takes place about -1.0 V vs Ag/Ag+ which is clearly evidenced by the cathodic/anodic CV peaks. Cation transport phenomena can be analyzed independently from anion transport when observed at a longer time scale (low values of potential scan rate) as each prevails at different redox states of the polymer. However, in a shorter time scale (V g 10 mV s-1), the opposite fluxes of cations and anions were observed to interfere. Furthermore, a net uptake of propylene carbonate by the as grown PPy film occurs at initial cycles of the cation uptake causing irreversible swelling of the polymer phase. Mechanisms of the redox process and accompanying mass transport involving PPy films were investigated using comparatively three techniques: cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and electrochemical quartz crystal microbalance (EQCM). 1. Introduction Polypyrrole (PPy) is among the main representatives of conducting polymers which are a broad class of chemical substances possessing high electrical conductivity of the mixed electronic-ionic origin. The electrical conductivity of these materials can be achieved by chemical, electrochemical, photochemical, or interphasial doping, and numerous applications were found for them.1 Electrochemical and chemical doping of conducting polymers can be considered in terms of solid-state redox process, coupled with the charge-balancing influx and/or outflow of counterions (derived from an electrolyte), in which charge carriers are formed as a result of removing electrons from (partial oxidation, p-doping) or adding electrons to (partial reduction, n-doping) the system of π-delocalized bonds extended along the whole polymeric backbone. There are detailed reviews on using conducting polymers in solar cells,2 sensors and biosensors,3 electrocatalysis4 and polymer batteries.5 Ion exchange properties of conducting polymers are of foremost importance in electrochemical and chemical processes. * Corresponding author. E-mail: [email protected]. Tel: +48 71 3757336. † Ph.D. student at University of Wrocław (2005-2009). E-mail: [email protected].

The p-doping of polypyrrole entails anion, cation, or mixed ion transport properties of the polymer. It impacts the ion exchange between the polymer and contacting phases. Stoichiometry of the p-doping process can be described by the chemical equations as follows:

[(P)nx+xA-] + xe- T [(P)n] + xA-

(1)

[(P)nx+xA-] + xe- + yC+ T [(P)nyA-yC+] + (x-y)A(2) [(P)nx+xA-(Pd)d+dA- + (x + d)e- + dC+ T [(P)n(Pd)dA-dC+] + xA- (3) [(P)nx+xA-] + xe- + xC+ T [(P)nxA-xC+]

(4)

[(Pd)dA-dC+] - ye- + yA- T [(Pd)dA-dC+]y+yA-

(5) Above, (Pd) and (P) designate fragments of a polymer chain with or without structural (chemical) defects, respectively.

10.1021/jp100796a  2010 American Chemical Society Published on Web 05/11/2010

Lithium Ion Exchange Behavior of Polypyrroles Moreover, one shall differentiate the primary anions (A-), incorporated into the polymer phase during electropolymerization, from the secondary anions, that can be involved in the electrochemical redox switching of the polymer instead of or together with the primary anions.6 Cations are designated as (C+). An origin of the cation-exchange properties of the conducting polymer can be insufficient mobility of the dopant anion (counterion) (eq 2), which is related to nucleophilic properties, size and charge valency of the anion, solvent taking part in the ion exchange, hydrophobic-hydrophilic properties of the polymer and morphology of the polymer phase. The ionexchange properties of polypyrrole can be a result of a synergic effect due to many factors controlling the ion transport. Polypyrrole allowing for a partial cation exchange was obtained even in the presence of small anions,7-9 as for example Cland ClO4-. On the other hand, polypyrrole prepared with quite large primary anions, as for example dodecylsulfate DSentering the polymer phase from water solution, is performing as the ClO4- anion exchanger (eq 1) when undergoing redox cycles in an ethanolic solution of NaClO4.10 Immobilization of anions depends also on the thickness of the polymer layer, and, as a result, the anion-exchanging thin film of polypyrrole has changed to the mixed anion-, cation-exchanging phase when thickness of the polymer layer has increased.11 Moreover, hindering of the anion exchange was reported as a result of the potentiodynamic electrodeposition of the polymer when compared with the potentiostatic or galvanostatic synthesis cases12 and as a result of the multistage deposition methods.13 Polypyrrole doped with various sulfonic anions, when cycled in water, reveals the mixed ion-exchange properties but in favor of cation-exchange ability (dodecylbenzenesulfate DBS-,14,15 tosylate TOS-, benzenesulfonate BS-, and naphthalenedisulfonate NDS2- 16-18). Unfortunately, there is not enough consistence on the ion-exchange properties of polypyrrole in aprotic solvent systems, for example PPy(DBS) is exchanging anions in propylene carbonate PC14,15 but it is a cation exchanger in acetonitrile ACN.15 Moreover, according to many authors, the primary anions are not exchangeable in a secondary electrolyte and the redox properties of PPy have to be described using eqs 2-5. The secondary effects caused by different electrolytes used in the electropolymerization and electrochemical performance of the polymer were studied thoroughly using electrochemical quartz crystal microbalance, EQCM.19 The existence of the structural cationic defects (Pd)d+ in polymer chains was postulated to explain specific ion-exchange properties of polypyrrole (eqs 3 and 5). These defects might be responsible for a strong interaction with counterions (anions) causing, very often, a permanent immobilization of the anions in the polymer lattice. Different origins of the structural defects were considered: (i) strong hydrogen bonds between counterions and the polymer sites (ClO4-, F- CN- 9,20); (ii) protonation of polypyrrole in β-positions21 or R-positions.22-24 Protonation of polypyrrole at the both R- and β-positions is possible. Pyrrole can be protonated at the β-position during electropolymerization, and C-3 protonated pyrrole cations are linked with formation of 2,5-bis(2-pyrrole)pyrrolidinic units25 present in so-called PPy(III) having pure anionic-exchange properties.22-24 Then, cation-exchanging PPy(II), composed of the short chains, is due to hindered deprotonation steps in a course of the coupling process of the radical cations of pyrrole oligomers. On the other hand, anion-exchanging PPy(I) is free from the chemical defects characteristic for PPy(II) and PPy(III).22-24 Contributions of PPy(I), PPy(II) and PPy(III) to the polymer material depend on

J. Phys. Chem. B, Vol. 114, No. 21, 2010 7159 details of electropolymerization procedures, especially on pH of the reaction media, amounts of H2O or other protophilic substances,26 and electrical regimes of the electrolysis. In summary, to produce cation-exchanging PPy(II) in an aprotic solvent like acetonitrile, one should apply low polymerization currents (low polymerization potentials) and low concentrations of hydrogen ions of the order of 10-5 mol dm-3.24 The latter condition is difficult to fulfill because H+ ions are being generated in the course of the polymerization process. The excess of H+ in the reaction zone might be even more persistent in a viscous solvent like PC. Therefore, electropolymerization of polypyrrole in propylene carbonate was performed by means of cyclic voltammetry to ensure formation of a mixture of the polypyrrole variants and films having mixed-ionic conductivity. A goal of the presented work was to investigate the influence of various conditions of electropolymerization on the ionexchange properties of the thin PPy layers produced in solutions of lithium perchlorate and tetrabutylammonium tosylate in PC. Effects of the contributing PPy(I), PPy(II) and PPy(III) variants in the deposited polymers were analyzed using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and electrochemical quartz crystal microbalance (EQCM) measurements. The concept of the effective molar mass (Mef) of the chemical moieties that influx/efflux the polymer layer was used to monitor the ion-exchange properties of the synthesized polypyrrole samples during combined EQCM-CV measurements. Mef values can be determined using eq 6 combining Sauerbrey’s equation (eq 7)27 and Faraday’s equation (eq 8).

( )

Mef ) -1.106nF

( )

∆ft ∂ft ) -1.106nF |∆Qt | ∂ |Qt |

(6)

∆mt ) -Ceqcm ∆ft ) -1.106 ∆ft

(7)

∆mt)Mef(∆Qt /nF)

(8)

Sauerbrey’s equation relates changes of the resonant frequency of a quartz crystal with changes of the electrode mass. Experimental values of the Ceqcm-coefficient were found close to the theoretical one that is 1.106 ng Hz-1 for the 10 MHz AT quartz crystal furnished with the 0.25 cm2 electrodes.28 Values of the passing electric charge ∆Qt were determined from CV curves of the observed process. Various materials of the substrate electrodes (a smooth platinum disk, platinum disk covered with nanostructured TiO2, and a rough layer of gold on quartz) were employed to provide insight into the effect of the polymer layer morphology on properties of polypyrrole. Comparative studies of the redox switching processes of PPy and poly(3,4,-ethylenedioxythiophene) in the LiClO4-PC and LiTFSI-PC electrolytes were done for this purpose. In general, the undertaken studies were intended to reveal the most suitable conditions for the synthesis of the TiO2(anatase)/ conducting polymer composites effective for intercalation of lithium ions.29 2. Experimental Section 2.1. Equipment and Chemicals. All electrochemical testes were performed in a standard three-electrode cell with a platinum ribbon (geometric area: 8-10 cm2) as the counter electrode at 25 °C by the use of Solartron’s 1287 potentiostat/galvanostat coupled with Solartron’s 1260 frequency response analyzer.

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TABLE 1: Conditions for the Electrochemical Deposition Process of PPy and PEDOT Thin Filmsa sample nameb

composition of the electrolytes in propylene carbonate (molar concentrations)

potential range/V:c Es/Ev1/Ev2/Es

no. of cycles

ncpy1 ncpy1Ti, ncpy1eq ncpy2, ncpy3 ncpy4 tpy1 tpy1eq, tpy1eqtf tpy2Ti tfped4

0.1 M Py + 0.1 M LiClO4 0.1 M Py + 0.1 M LiClO4 0.1 M Py + 0.1 M LiClO4 0.1 M Py + 0.1 M LiClO4 + 1% (v/v) H2O 0.1 M Py + 0.1 M TBAT 0.1 M Py + 0.05 M TBAT 0.1 M Py + 0.05 M TBAT 0.042 M EDOT + 0.1 M LiTFSI

-0.36/+0.3/-0.6/-0.36 -0.4/+0.3/-0.6/-0.4 -0.4/+0.2/-0.6/-0.4 -0.4/+0.3/-0.6/-0.4 -0.36/+0.3/-0.6/-0.36 -0.4/+0.3/-0.6/-0.4 -0.36/+0.3/-0.6/-0.36 -0.4/+0.6/-0.45/-0.4

22 14, 16 26 16 15 16 3 11

Potential sweeping rate (V): 25 mV s-1 for PPy or 50 mV s-1 for PEDOT. b Meaning of the sample names: in the central part, -py- or -pedstands for polypyrrole or PEDOT, respectively; as suffixes, -eq stands for deposited on a gold EQCM electrode, -Ti for deposited on a Pt/TiO2 electrode, -tf for studied in a LiTFSI-PC solution; as prefixes, t- stands for primarily doped with tosylate anions, nc- for primarily doped with ClO4- anions, tf- for primarily doped with TFSI- anions. c Es, starting potential; Ev1 and Ev2, first and second switching potential. a

TABLE 2: The Electropolymerization Charge (Qp), Mass (m) and Thickness (h) Determined for Different Samples of Conducting Polymers sample name

Qp/mC

m/µg

h/nm

kind of substratea

sample name

Qp/mC

m/µg

h/nm

kind of substratea

ncpy1 ncpy1Ti ncpy1eq ncpy2 ncpy3 ncpy4

6.7 7.0 34.6 4.0 4.7 11.0

2.0 2.1 10.4 1.2 1.4 3.3

1592 1462 581 949 983 2608

dPt dPt/Plur1 eq dPt dAu dPt

tpy1 tpy1eq tpy1eqtf tpy2Ti tfped4

11.7 33.3 27.4 1.60 6.0

3.5 10.0 8.2 0.48 3.9

2776 559 460

dPt eq eq dPt/an4 dPt

2610

a dPt, a conventional platinum disk electrode (0.018 cm2); dAu, a conventional gold disk electrode (0.020 cm2); eq, gold-covered EQCM electrode (0.25 cm2); Pt/Plur1 or Pt/an4, platinum disk electrode modified with TiO2 thin film (plur1 or an4).

However, a GPES system (Autolab Electro-chemical Instruments, Ecochemie) and quartz crystal microbalance unit (type M106, UELKO, Warsaw, Poland) equipped with AT-cut and gold-coated quartz crystals (active area 0.25 cm2, fundamental frequency 10 MHz, International Crystal Mfg. Co., Oklahoma City, OK) were utilized to acquire EQCM data at room temperature. Platinum (0.018 or 0.020 cm2) and gold (0.020 cm2) disk electrodes served as the working electrode in the other electrochemical measurements. These electrodes were polished on special pads made from a commercial suede rayon loaded with alumina powder (average particle diameter: 10, 3, 1, and 0.3 µm) to obtain a mirrorlike surface and were subsequently chemically (Piranha solution, few seconds) and electrochemically (0.5 M H2SO4 (96%, PPH Standard)) cleaned before use. A silver wire immersed in a 0.01 M AgClO4 (Alfa Aesar 99.9%) + 1 M LiClO4 (Aldrich, reagent Plus)-propylene carbonate (PC (Huntsman, ultrapure)) solution acted as the reference electrode (Ag/Ag+). On the basis of voltammetric measurements of the formal potential of the 1,1′-bis(hydroxymethyl)ferrocene/1,1′bis(hydroxymethyl)ferrocenium ion internal standard couple (Aldrich, 98%) in a 1 M LiClO4-PC (E0′ ) -0.308 V vs Ag/ Ag+) and 1 M LiClO4-water solution (E0′ ) +0.188 V vs SCE), the equations E(Ag/AgCl/3 M KClaq) ) E(Ag/Ag+) + 0.53/V and E(Li/1 M Li+) ) E(Ag/Ag+) + 3.79/V were found and used to express the measured potential of working electrodes in different potential scales. In aqueous systems (the case of the electrochemical deposition process of thin layers of TiO2), measured potential values were quoted against SCE electrode, but potential values were referred to the aforementioned Ag/ Ag+ scale in all figures and in all other cases when the potential scale was not exactly indicated in the manuscript. Used electrolytes, except the experimental protocol concerning the deposition process of TiO2 layers, before electrochemical studies, were deoxygenated by nitrogen (Messer 5.0, 99.999%) bubbling for 15-20 min. All nonaqueous electrolytes were kept in an inert atmosphere under a nitrogen tent to minimize the influence of water on the accuracy and quality of obtained

electrochemical data. Additionally, LiClO4 was dried under vacuum conditions at 140 °C for two days. Tetrabutylammonium tosylate (TBAT) (Fluka, puriss) and lithium bis(trifluoromethylsulfonyl)imide (TFSI) (Aldrich, 99.95%) were used without any pretreatment. 2.2. Deposition of TiO2 and Conducting Polymer Films. Protocols of Electrochemical Studies. The conditions of the voltammetric deposition process of PPy or PEDOT samples and further characterization of their mass and thickness were put together in Table 1 and Table 2, respectively. Pyrrole (Py) (Merck, 98%) was distilled in an atmosphere of nitrogen under reduced pressure before use and was stored between experiments at about -20 °C in a freezing compartment and protected against the action of light and oxygen. The storage of 3,4-ethylenedioxythiophene (EDOT) (Aldich, g 96.5%) was identical with that for PPy monomers, but this compound was not subjected to any procedure of purification. Reported values of the mass of the obtained films were estimated on the assumption that 2.25 F of electropolymerization charge (Qp) is needed for synthesis of 1 mol of the polymer unit (2 F for electrooxidation and 0.25 F for doping process). The values of film thickness were calculated assuming the validity of the following proportions: 1000 mC cm-2-4.2 µm for PPy30 and 1000 mC cm-2-7.7 µm for PEDOT.31 It is worth keeping in mind that the thickness of conducting polymer films can be varied substantially even in the case when the same electrolytes are chosen, but different electrochemical methods of electropolymerization are applied or different solvents are used (for sample: 0.06 M Py + 0.05 M TEAPF6 in PC, -40 °C, Iconst ) 0.125 mA cm-2, 1000 mC cm-2 - 6.9 µm;32 0.1 M Py + 0.1 M TBAPF6 in PC, -27 °C, a potentiostatic method, 1000 mC cm-2-4.7 µm;33 0.1 M Py + 0.1 M LiClO4 in ACN, Iconst ) 0.125 mA cm-2, 1000 mC cm-2-2 µm;34 0.06 M Py + 0.1 MTEABF4 in ACN, 1000 mC cm-2 - 4.2 µm30). Values of the mass of the samples shown in Table 2 indicate only the mass of the polymeric phase without including the mass of doping anions.

Lithium Ion Exchange Behavior of Polypyrroles Freshly deposited layers of conducting polymers were thoroughly rinsed with pure and deoxygenated propylene carbonate to wash the residuals of easily soluble oligomeric species or monomeric species off. Then, the samples were placed in deoxygenated solutions of LiClO4 (1 M) or LiTFSI (0.87 M) in PC. In the first stage of electrochemical investigations, PPy and PEDOT films were electrochemically cycled at 10 mV s-1 in the potential window (-1.5 V, 0.0 V) until stable voltammograms were obtained (usually 10 scans). Directly after this pretreatment, next voltammetric measurements were done in the same potential window with varying the potential scan rate from the slowest values (5 mV s-1) to the highest one (200 mV s-1). Only the third scan recorded at each potential scan rate was put into analysis. Afterward, impedance spectra were collected (using 10 frequency data points per decade) in the potentiostatic mode applying a perturbation sine which frequency was altered from 20 kHz to a few millihertz and which amplitude was equal to (5 mV. For these EIS measurements, equilibration of progressively dedoped polymeric samples was necessary beforehand at each dc steady potential value for 20 min. The samples deposited on EQCM electrodes were not monitored by EIS methods. Thin films of TiO2 (an4 and plur1) were electrodeposited on platinum substrates (0.020 cm2) with the use of the procedure described previously by us.29,35 Shortly, platinum electrodes were cycled (12 or 50 times) from -0.3 to -0.8 V vs SCE in the regime of the cyclic voltammetry method keeping a constant potential scan rate (10 or 20 mV s-1) and using electrolytic solutions with the following compositions: solution A, 4.2 × 10-3 M TiCl3, 7.8 × 10-3 M H2C2O4, 9.6 × 10-2 M NH4NO2 in water, pH ) 4.1 (adjusted with a diluted NH3aq solution); or solution B, 3.7 × 10-3 M TiCl3, 6.9 × 10-3 M H2C2O4, 8.6 × 10-2 M NH4NO2, 6.9 × 10-3 M NH3, 8 cm3 of the 10% solution of Pluronic L-31 (M ) 1100 g mol-1; Aldrich), 3 cm3 of EtOH (the total volume of solution was 20 cm3) respectively for the samples an4 and plur1 (annealing temperature: 600 °C in air). The mass of these electroactive TiO2 films was estimated at about 0.66 µg (an4) and 0.22 µg (plur1). 3. Results and discussion 3.1. Electropolymerization Process. Representative voltammograms featuring the process of electrodeposition of various polypyrrole films are show in Figure 1. In cases that are not shown there, the currents of electropolymerization never exceeded 5 mA cm-2 and usually were lower than 3 mA cm-2. The highest value of the electrodeposition potential was 0.3 V vs Ag/Ag+, which is about 0.8 V vs SCE. It is worth remarking that partially conjugated polypyrrole PPy(III), synthesized galvanostatically in acetonitrile, is produced at current densities exceeding 12-19 mA cm-2 and cation-exchanging PPy(II) is produced at current densities lower than 0.01-0.06 mA cm-2.23 According to that, we may not expect to obtain PPy(III) under the current conditions measured during the synthesis done in our work. Moreover, the observed electropolymerization current did not decrease with an increasing number of the potential cycles. The decrease is a characteristic behavior during the electrodeposition of PPy with only partly conjugated chains. Additionally, it is predicted that the PPy(I) variant is forming at the intermediate current range.23 Electropolymerization of pyrrole in propylene carbonate was found similar to the process in acetonitrile. Propylene carbonate added to acetonitrile (20% of PC in the mixed solvent) had not influenced considerably the electropolymerization process of pyrrole.26

J. Phys. Chem. B, Vol. 114, No. 21, 2010 7161

Figure 1. Initial CV scans (25 mV s-1) of the PPy electrodeposition procedure. Compositions of the cell solutions are described in Table 1. A scan number is shown in the parentheses in the legend. To see: (i) effects of a substrate electrode, compare ncpy1, ncpy1eq, and ncpy1Ti; (ii) effects of water, compare ncpy1 and tpy1; (iii) effects of a potential window, compare ncpy1 and ncpy2.

A significant catalytic effect of water, added to the aprotic electrolyte, on the electropolymerization process was observed (compare the CV curves for the ncpy1 and ncpy4 films, Figure 1). A similar catalytic effect is observed in the presence of p-toluenesulfonate (compare ncpy1 and tpy1).36 The catalytic effect is interpreted, in both cases, as due to a faster deprotonation of the σ-dimers of the oligomeric radical cation intermediates. However tosylate anions are more active proton scavengers than water molecules36 that should induce the composition of PPy films and change the proportion between PP(I) and Ppy(II) (see the next section, Figure 3, bottom panel for PPy(TOS) (tpy1) and PPy(ClO4-) (ncpy4)). Effects of the different substrates of the polymer deposit are significant and seen clearly even for the metal electrodes: the polycrystalline Pt and the Au layer on quartz (see inset of Figure 1). In the latter case always one observes a decrease of the oxidation current of pyrrole as well as 3,4-ethylenedioxythiophene. Moreover, the electropolymerization on the Pt electrode modified with the nanostructured titanium(IV) oxide begins at significantly more positive potentials and prolongs in a broader potential range after changing the polarization direction as compared with the process taking place on the naked Pt electrode. It is probably due to a higher number of nucleation sites available at the TiO2 surface leading to a faster deposition process in subsequent cycles of the CV procedure. However, for the tripled amount of the nanostructured TiO2 (0.66 µg of oxide/0.020 cm2) as compared to the plur1 sample, the electropolymerization was hindered considerably and practically ineffective. Representative CV curves (25 mV s-1) of the electropolymerization process of PPy (film ncpy1) and the comparison of dependencies of the capacitive currents of the selected PPy films on the anodic charge used for their deposition are shown in Figure 2. When the thickness of a film increases in a course of electrodeposition, some of the potentially exchangeable anions are immobilized in the film. A straight proportionality between the polymerization charge/thickness and the capacitive current

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Figure 2. (a) Representative CV curves (25 mV s-1) of the electropolymerization process of PPy (film ncpy1) and (b) the voltammetric response (Is´r(i) values determined at the potential equal to about -0.2 V and presented in the function of the used electrodeposition charges) of continuously growing PPy films (selected samples); more details in Tables 1 and 2.

of the films is observed only for the initial cycles of the CV deposition. The increase in the current vanishes at different stages of the film formation depending on physical conditions of electropolymerization (potential window, scan rate) and composition of the electrolyte (Figure 2b). The immobilization effect was occurring at lower values of the polymerization charge when the process was performed in the narrowest potential range, i.e. (-0.6 V, +0.2 V) (ncpy2-3). It is most probably due to the highest density of PPy layers formed under those conditions. Moreover, those PPy films were very resistant to removal from the platinum surface by a mechanical rubbing. Furthermore, one can expect that ion-exchange properties of PPy films on electrodes shall be dependent not only on their thickness but also on the value of the negative limit of the potential window, Ev2, employed in the CV deposition procedure. A specific feature of 4-toluenesulfonate anions is that they are nearly not ejected from PPy (as opposed to PEDOT) and the corresponding redox current of growing PPy does not increase during the electrodeposition process. To determine the oxidation degree, x, of PPy doped with tosylate, a method based on the quartz resonant frequency change accompanying the growth of the polymer, -∆f, was invoked [eqs 7 and 9]. Namely, the EQCM response recorded for successive cycles of the deposition (for E equal to about -0.3 V) is related to a sum of masses due to PPy and the tosylate anions immobilized in a PPy structure. Assuming 100% yield of the electrodeposition process, relation 9 between the deposit mass, mti, and the total polymerization charge, Qpi, holds (171 g mol-1 and 65.07 g mol-1 are molar masses of tosylate ion and polypyrrole mer, respectively, and x is the oxidation degree of PPy):

mti /ng )

[(

)

(

) ]

Qpi × 10-3 Qpi × 10-3 65.07 + x 171 × 109 (2 + x)F (2 + x)F (9)

Assuming validity of the theoretical relation (eq 7), ∑mti ) -1.106 ng · Hz1- × ∑(-∆fti) (where i ) 1, 2, ..., n ) final cycle number), a mean value of the oxidation degree for the two films tpy1eq and tpy1eqtf is 0.26. Results obtained from the analysis of the electropolymerization process allowed to explain the differences in the ionexchange properties of various PPy layers and confirmation of

the absence of viscoelastic properties that is essential for correctness of the EQCM analysis based on Sauerbrey’s equation for the quartz crystal microbalance. 3.2. Ion-Exchange Properties of PPy Films Studied Using CV and EIS Methods. The voltammograms of various PPy samples, shown in Figures 3 and 4, evidence that even small modifications of electrodeposition conditions can result in significant differences of electrochemical properties. In general, the PPy layers, that have been deposited, are chemically inhomogeneous, which is featured as complex characteristics of the CV peaks (Figure 3). The anodic CV peaks of the PPy films, observed at 5 mV s-1, are positioned at three values of E: -0.95 (-0.42), -0.85 (-0.32) and -0.5 V (+0.03 V vs Ag/AgCl/3 M KCl). The cathodic CV peaks, observed at the same scan rate, are located at two values of E: -1.0 and -0.75 V. When V ) 100 mV s-1, most of the anodic peaks are situated in the (-0.76, -0.53 V vs Ag/Ag+) range, which corresponds to (-0.23, 0 V vs Ag/ AgCl/3 M KCl). The peaks observed at the lowest potentials can be attributed to the cation exchanger PPy(II). According to Zhou,23 PPy(I) is characterized by sharp anodic-cathodic CV peaks of similar heights (Ep,a ) 0 V, Ep,c ) -0.28 V vs Ag/ AgCl/3 M KCl), PPy(II) generates a sharp anodic peak (Ep,a ) -0.23 V vs Ag/AgCl/3 M KCl) accompanied by a lower and a broader cathodic peak (Ep,c ) -0.37 V vs Ag/AgCl/3 M KCl), whereas PPy(III) gives two badly shaped but similar CV redox peaks located at the most positive potentials. Unfortunately, we were unable to ascertain any correlations between values of electrodeposition currents, electrodeposition potentials (potential windows) and the chemical composition identities of polypyrrole (PPy(I), PPy(II), PPy(III)) with the related ion-exchange property CV features (compare Figures 3, 4). Moreover, it is impossible to differentiate the chemical structures of PPy for the thicker films, and the task is at least difficult for the higher potential scan rates.37 The delocalization of the polaronic and bipolaronic states, the chain length or the structural order of the PPy phase is also a factor influencing the position of voltammetric redox peaks.38 It seems that the cation-exchange properties of PPy are determined not only by the chemical composition of the polymer, denoted as PPy(I), PPy(II) and PPy(III), but also by many factors related to conditions of the polymer redox process (a substrate and morphology of the films, an electrolyte

Lithium Ion Exchange Behavior of Polypyrroles


Figure 3. Comparisons of cyclic voltammograms shown in the pseudocapacitance-voltage format, i.e., I(E) v-1 m-1 ) f(E), where V and m stand for potential scan rate and mass of the sample, respectively, and their dependencies on the scan rate for selected PPy films performing the redox switching in 1 M LiClO4-PC (series I); in the labels: potential scan rate/mV s-1; for details see Tables 1 and 2.

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Figure 4. First (black) and 10th (green) scans of CV recorded for selected PPy films. The insets show names of PPy samples, positive limit potentials of their electrodeposition Ev1, composition of the electrolyte used in CV measurements, and substrate electrodes. Potential scan rate )10 mV s-1. Further details on the PPy samples can be found in Tables 1 and 2.

composition, a potential window, a time window). Electropolymerization processes of the studied PPy films were performed at relatively low potentials (E e +0.3 V vs Ag/Ag+, that approximately corresponds to E e +0.8 V vs SCE) that should generate PPy with mixed ion-exchange properties39 (mixture of PPy(I) and PPy(II)). All the PPy films showed the expected behavior, which is illustrated in Figure 4. The first cycle effect shown there is due always to the cation transport accompanying the PPy redox process. Another piece of evidence for the cationexchange properties of the polymer films was the observed redox activity and inter/deintercalation of lithium ions of the TiO2 substrate underlying PPy.29 Even in the case of thick layers of PPy(ClO4-) revealing the least favorable cation-exchange activity, the electrochemical activity of the oxide was still observed (see Figure A2 in the Supporting Information). A partial immobilization of the counterions (perchlorate anions) in thicker PPy films also contributes to the cation exchange between PPy and contacting phases (see Figure 2). The concept of “electrochemical porosity”,40,41 closely related with morphology of an electrode surface, might be suitable to interpret results collected in Figures 3-5. “Electrochemical porosity” was introduced to characterize (by means of the CV method) fast redox surface processes associated with the pseudocapacitance of many transition metal oxides. This quantity cannot be identified with the morphology being for sample viewed with the SEM microscopy. Despite the fact that this morphological factor cannot be defined and determined in

the same way for the conducting polymer films, there is no conceptual obstacle to using this term to explain how the molecular packing of their structure and the kind of the secondary electrolyte influences their ionic conductivity response to the electrochemical switching. A strong correlation has been found between morphology and ion-exchange properties of PPy films on electrodes studied by EQCM and AFM methods.42 In general, a higher active surface area of a conducting polymer should cause its higher and faster pseudocapacitive response to the electrical stimulus. An amount of the electric charge, involved in the electrochemical process, depends little on the potential scan rate in the case of the most porous layers. Usually porous and sensitive to the concentration of an electrolyte, layers of poly(3,4-ethylenedioxythiophene) can be considered as solid evidence of the morphology effects.31,43,44 PEDOT films, examined in the range (-1.5 V, 0 V), have conducted reversibly the larger amounts of the electric charge during the electrochemical process as compared to the case of PPy (see the corresponding CV curves in Figure A1 in the Supporting Information). The parameter xef (Figure 5) can be considered as a measure of capability of a one mer of a polymer to store a unit ionic charge. The xef values were determined using eq 10 with anodic (Qa), cathodic (Qc) and polymerization (Qp) charges determined for the redox performance and electropolymerization CV curves. It should be noted that xef corresponds to the effective charge quantities taking part in the actual redox process of the polymer


Figure 5. Analysis of CV curves (voltammetric series I; (-1.5, 0 V) potential range; Figure 3 and other data), recorded in 0.87 M LiTFSI-PC (tpy1eqtf) or 1 M LiClO4-PC (all the other samples), for different PPy films; xef ) 2QCV/(Qp - QCV), where QCV ) (1/2)(|Qc| + Qa). Meanings of colors and lines: PPy(ClO4) cycled in 1 M LiClO4-PC (green curves); PPy(TOS) cycled in 1 M LiClO4-PC (black curves); PPy(TOS) cycled in 0.87 M LiTFSI (blue curves); the reference systems for the analysis of polymerization conditions or the nature of the polymer (dotted and thicker lines, respectively); data for PEDOT (tfped4) shown for a comparison; the symbol of primary anion electrolyte, the maximum potential value of an electropolymerization process and other parameters changed during electrodeposition were included in the labels.

involving the actually exchangeable ions and it can differ from the oxidation degree of the polymer attained in the electrodeposition process.

xef ) 2QCV/(Qp - QCV), where QCV ≡ |Qc | ≡ Qa ≡ (1/2)(|Qc | + Qa) (10) The xef values were in the range of (0.35, 0.40), decreasing with the time window of the redox process (5, 200 mV s-1) in the monitored potential range (-1.5, 0 V), for the porous layer of PEDOT(TFSI). PEDOT(ClO4) and PEDOT(TOS) behaved similarly in this respect (data not shown in Figure 5). The high electrochemical activity concerns also the PPy(TOS) and PPy(ClO4) films but only ones that were synthesized in the presence of water in PC (1% v/v) (ncpy4) or electrodeposited on rough surfaces (ncpy1Ti, ncpy1eq). The xef values of PPy(TOS) have not changed markedly even when 1 M LiClO4 was substituted by 0.87 M LiTFSI (compare samples tpy1eq and tpy1eqtf, Figure 5 and Figure A3 in the Supporting Information) that can be considered as a result of a high porosity of the cation-exchangeable PPy. The most significant differences of the properties revealed polypyrrole doped with perchlorate as the primary counterion (ncpy1, ncpy1Ti, ncpy2, ncpy3 and ncpy1eq), especially when the substrate electrode was changed.

J. Phys. Chem. B, Vol. 114, No. 21, 2010 7165 A rougher surface of the substrate always resulted in less coherent and more porous layers of PPy(ClO4) that subsequently exhibited more visible pairs of the CV redox peaks situated at potentials about -0.95 V (V ) 5 mV s-1) and corresponded to the coupled transport of the solvated lithium ions as evidenced by additional EQCM studies (see later on in section 3.3). Moreover, the more porous layers, characterized by the high xef values apparently independent of the scan rate, exhibited the stronger first cycle effect illustrated in Figure 4. The effect concerns in particular the ncpy1eq sample of PPy(ClO4) resembling the performance of PPy(TOS). A similar behavior was observed for the ncpy4 film, showing also the high xef values, produced at currents that were 5 times higher than used for preparing the ncpy1eq film. However, the ncpy2 and ncpy3 samples, electrodeposited at similar current conditions as the ncpy1eq film, but on the smooth surfaces, featured low cation exchange and low CV redox peaks at about -0.95 V. In this case, the low porosity of PPy causes strongly solvated lithium ions to penetrate the polymer phase very slowly, which results in slow changes of the polymer phase morphology and ionexchange activity upon prolonged potential switching (Figure A4 in the Supporting Information). In all the other cases, except the latter one, the stable voltammetric response had been already attained after 10 initial cycles of polarization. This explanation of the low electrochemical activity of samples ncpy2-3 can be in accordance with another experimental fact: if the formation of screened anions (P•+A-) due to steric reasons is also possible, conducting polymer films are characterized with the suppressed ability to exchange anions as well as cations45 (very low xef values). It should be remembered that the first scan effect is very sensitive to a solvent and the secondary electrolyte.46 For example, the effect was not seen when PC (used in the electropolymerization procedure) was changed for H2O (used in the electrochemical switching).21 Moreover, in the course of electropolymerization of ncpy4 (performed in 1 M LiClO4-PC + 1% v/v H2O), perchlorate anions were exchanging easily between the growing polymer phase and the electrolyte up to the polymerization charge equal to about 5 mC (Figure 2 and Figure A5 in the Supporting Information). Afterward, however, perchlorate ions were very difficult to eject from that PPy(ClO4) film in a dry PC, their behavior being quite similar to that observed for PPy(TOS) (Figure 4). The impact of the morphology on the redox properties of studied PPy films was confirmed further by electrochemical impedance measurements done after an initial series of voltammetric studies (Figures 6 and 7). It is well-known that even the first voltammetric cycle, recorded directly after preparation of a PPy layer, can change the polymer irreversibly, especially when cations can diffuse to the polymer phase.21,49 All the samples of PPy(TOS), already after the first redox performance cycle, became very porous and resembled poly(3,4-ethylenedioxythiophene) layers with regard to a very fast process of the ion exchange. The process is characterized by very low values of the ion transfer resistance at the polymer/electrolyte interface as well as high values of the diffusion coefficients of ions in the polymer (of the order of 10-6-10-7 cm2 s-1). It is worthy of note that we did not take into consideration the effect being due to molecular packing (electrochemical porosity) of the films which could lead to the substantial overestimation of the actual D-coefficient values by several orders of magnitude in the case of the most porous or nanostructured samples.48 Hence, the highest values of the effective diffusion coefficient may also prove the porous nature of PEDOT(doped with different anions)

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Figure 6. Complex plane impedance spectra of thin PPy films: ncpy1 (a), ncpy3 (b), tpy1 (c) and ncpy1Ti (d), collected at E ) -0.25 and -1.0 V; for estimations of (1) substrate effect, compare (a) and (d); (2) potential window effect, compare (a) and (b); (3) doping anion effect, compare (a) and (c); EIS measurements done after the voltammetric series I; electrolyte, 1 M LiClO4-PC; electrode surface area, 0.018 cm2. More detailed EIS spectra can be found in the Supporting Information (see Figure A6).

Figure 7. Ion transfer resistances at the polymer/electrolyte interface (a) and the ion diffusion coefficients (b) for selected PPy and PEDOT films at different redox states of the polymers set by the electrode potential E. The impedance/admittance spectra were analyzed in terms of the equivalent electrical circuit models, i.e. Re(Qdl[RctT]) or Re(Qdl[Rct(QT)]), using EQIVALENT CIRCUIT v. 4.51 software.47 Symbols: Re, Qdl, Rct, T, Q indicate solution resistance, double layer impedance, charge transfer resistance, diffusion impedance, impedance due to an additional transport path, respectively. Detailed descriptions of analytical procedures can be found in refs 29, 48.

and PPy(TOS). In the potential range where the lithium ionexchange processes are the most prevailing (the range of corresponding CV peaks), values of the Rct parameter decrease almost up to 0 Ω and the effective diffusion coefficients reach there the highest values. In comparison, the diffusion coefficients determined for the PPy(ClO4) films deposited on the smooth electrodes are nearly 3 orders of magnitude lower in value. The most compact layers of PPy (ncpy2 and ncpy3) are characterized by the highest ion transfer resistances, Rct > 8 kΩ. The EIS results are in agreement with conclusions, found in the literature, on the foremost importance of the porous structure of PPy, as well as other conducting polymers, for the transport of small ions having the highest solvation radii, like Li+.50 3.3. Ion-Exchange Properties of the Polypyrrole Films Investigated with the EQCM Technique. The transport processes of ionic and neutral species taking place in the electrochemically switched polypyrrole films were studied in the function of the kind of the electrolyte used during electrosynthesis (0.1 M LiClO4 or 0.05 M TBAT; Figure 8, Table

3) or used during subsequent electrochemical testing (1 M LiClO4 or 0.87 M LiTFSI; Figure 10, Table 4). All synthesized PPy samples were characterized with a mixed ionic conductivity (anionic-cationic) irrespectively of the used primary or secondary electrolyte (PPy(ClO4-) cycled in a LiClO4-PC solution; PPy(TOS) cycled in a LiClO4-PC or LiTFSI-PC solution). However, some significant differences between their transport behaviors were observed. Perchlorate anions are more weakly bound to PPy chains than p-toluenesulfonate anions (compare Figure 8a with Figure 8b and Figure 5f with Figure 5h). In the first-measured voltammogram, there is almost undetectable mass change of the EQCM electrodes covered with the PPy(TOS) film in the potential window from 0.0 to -1.0 V (see Figure 8b, first scan). At the same time, the voltammetric current is approaching the value of 0 µA (see the related CV curve in the Figure 5f). However, a negligible amount of tosylate, perchlorate or TFSI ions is exchangeable. In the case of tpy1eqtf sample (PPy(TOS) cycled in 0.87 M LiTFSI-PC), the determined values of the Mef parameter at E ) -0.2 V are equal to 98



Figure 8. Changes of -∆f and the Mef parameter during measurements of the first ten voltammetric curves (V ) 10 mV s-1) (see Figure 5f,h) in 1 M LiClO4-PC solution or 0.87 M LiTFSI-PC solution for the freshly deposited (uncycled before) ncpy1eq film (PPy(ClO4-)) (a, c) or tpy1eqtf film (PPy(TOS)) (b, d), respectively; direction of the potential sweeping, -0.2 V (or -0.3 V) f 0.0 V f -1.5 V f -0.2 V (or -0.3 V); film thickness, 581 nm (ncpy1eq) and 460 nm (tpy1eqtf); E vs Ag/Ag+.

(doping) and -262 g mol-1 (dedoping process) (see Figure 8d). It was experimentally stated later that the parameter Mef amounted to 228, 284, 314, 333, 331, 348 g mol-1 (electrochemical doping) and -267, -284, -283, -289, -274, -262 (electrochemical dedoping) at the same potential in the second, third, fourth, sixth, eighth and tenth voltammetric scans (10 mV s-1, see Figure 8d), and for the same sample. In this case, solvent molecules were initially transported in the opposite direction to the flux of TFSI anions (280 g mol-1) (first 2 cycles) and then in the same direction. In the 1.0 M LiClO4-PC electrolyte, PPy(TOS) and PPy(ClO4-) samples exchanged mainly perchlorate anion in the most positive potential region (E > -0.6 V, see Figure 8c, Mef ≈ ( 100 g mol-1). It is worth noticing that quantitative analysis of -∆f changes accompanying electrodeposition process (with using eq 9) or voltammetric charge associated with the origin of the irreversible cathodic peak below E ) -1.0 V in the first-measured scans (see Figure 5) (with using eq 10) gives very similar values of the effective oxidation level of the PPy(TOS) films, respectively 0.26 and 0.31 for the tpy1eq film (PPy(TOS) cycled in 1.0 M LiClO4-PC) or 0.26 and 0.28 for the tpy1eqtf film (PPy(TOS) cycled in 0.87 M LiTFSI-PC). It suggests that the PPy(TOS)

films incorporate Li+ ions during the performance of the first dedoping cycle in the quantity equivalent to the unreleased (unexpelled) tosylate anion content. Furthermore, PPy(TOS) films are able to exchange Li+ cations in a persistent way when their repetitive electrochemical switching is performed in the used electrolytic media. This feature is dissimilar for PPy(ClO4-) samples. More distinct and global (connected) analysis of the EQCM data from Figure 8b and related CV curves have revealed that a total amount of transported solvent molecules, but not a total amount of exchanged lithium ions, is gradually lowered as the cycle number is increased continuously (see Table 3). PPy(TOS) films very slowly reached their equilibrium state of solvation in comparison with PPy(ClO4-) samples (compare Figures 8c and 8d). The latter samples of PPy are able to incorporate substantially bigger quantities of PC molecules in the first dedoping process despite the fact that they are regarded (by us) as worse cation exchangers than PPy(TOS) layers. Probably, the solvent transport behavior of the PPy films is dependent on different factors like their hydrophobic8 or mechanical properties as well as morphology. It was confirmed in the literature many times51 that polypyrrole films obtained in the presence of small inorganic anions, like ClO4- and BF4-,

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TABLE 3: Semiquantitative Analysis of the EQCM Data from Figure 8ba Li+ ion intercalation (dedoping of PPy) scan number

potential range/V: E1, -1.5, E2 -0.983, -1.132 -1.025, -1.203 -1.007, -1.275 -0.965, -1.316 -0.953, -1.233 -0.888,-1.185 -0.938,-1.420

1 2 4 7 10 tpy1eq, 9 ncpy1eq,10

|∆f|/Hz

∆m(Li+PC)/ng

|∆Q|/mC

∆m(Li)/ng

∆m(PC)/ng

nLi/nPC

9555 5350 4602 3270 3003 5875 3738

10568 5917 5090 3617 3321 6498 4134

3.34 0.87 0.76 0.77 0.84 1.63 1.65

240 63 55 55 60 117 119

10328 5908 5035 3562 3261 6381 4015

1:2.9 1:6.4 1:6.2 1:4.4 1:3.7 1:3.7 1:2.3

Li+ ion deintercalation (doping of PPy) scan no.

% Lib

potential range/V: E2,E3

|∆f|/Hz

∆m(Li+PC)/ng

|∆Q|/mC

∆m(Li)/ng

∆m(PC)/ng

nLi/nPC

1 2 4 7 10

30 100 115 100 92

-1.132, -0.834 -1.203, -0.858 -1.275, -0.846 -1.316, -0.864 -1.233, -0.858

4687 4483 3507 2860 2603

5184 4958 3879 3163 2879

1.01 0.87 0.88 0.77 0.77

73 63 63 55 55

5111 4895 3816 3108 2824

1:4.8 1:5.3 1:4.1 1:3.8 1:3.5

a Changes of the transported masses of Li+ ions and propylene carbonate molecules during initial electrochemical cycling (10 mV s-1, 0.87 M LiTFSI) of the tpy1eqtf film (PPy(TOS), h ) 460 nm); ∆m(Li+PC): a total sum of the fluxed mass of PC and Li+ species, calculated on the basis of Sauerbrey’s assumption (eq 7) by using the changes of |∆f| values indicated by the EQCM microbalance. ∆m(Li): the Li+ ions mass calculated from voltammetric charges |∆Q| (integration of the CV curves was done in the same potential windows as the analysis of the appropriate -∆f(E) curves; values of the E2 potential were chosen for I ) 0 µA in the reverse scans just after the polarization direction was changed). For comparison, some extra information was included for the ncpy1eq and tpy1eq samples. b The percentage reversibility of the intercalation process of Li+ ions (charge recovery during the expulsion process of Li+ ions).

TABLE 4: The Influence of the Scan Rate (W) on the Mass Transport Processes Taking Place in Selected PPy Samplesa V/mV s-1

Mef+/g mol-1

Mef-/g mol-1

5 439 -638 10 318 -265 25 246 -191 50 219 -149 200 180 -77 PPy(TOS) tpy1eqtf in 0.87 M LiTFSI 5 97 -160 10 65 -98 25 53 -71 50 46 -55 PPy(TOS) tpy1eq in 1 M LiClO4 5 103 -133 10 83 -91 25 69 -77 50 54 -72 PPy(ClO4) ncpy1eq in 1 M LiClO4

Mef+ b +1a +1a +1a +1a +1a

+1.6s +0.4s -0.3s -0.6s -1.0s

+1a +1a +1a +1a

+0.0s -0.3s -0.5s -0.5s

+1a +1a +1a +1a

+0.0s -0.2s -0.3s -0.4s

Mef- b

-∆fa+/Hz

-∆fa-/Hz

-∆fc+/Hz

-∆fc-/Hz

∆(∆fc)/Hz

-1a -3.5s -1a +0.1s -1a +0.9s -1a +1.3s -1a +2.0s -∆f(5-50) -1a -0.6s -1a +0.0s -1a +0.3s -1a +0.4s -∆f(5-50) -1a -0.6s -1a +0.1s -1a +0.2s -1a +0.3s -∆f(5-50)

4429 4229 4065 3976 3504 10.2% 836 806 763 701 16.1% 1166 1127 1099 1068 8.4%

-4801 -4614 -4440 -4327 -3781 9.9% -1939 -1860 -1795 -1703 12.2% -2213 -2146 -2075 -1999 9.6%

2945 2597 2264 2038 1593 30.8% 5958 5587 5158 4803 19.4% 3452 3330 3259 3198 7.4%

-2573 -2212 -1889 -1687 -1316 34.4% -4855 -4533 -4126 -3801 21.7% -2405 -2311 -2283 -2267 5.7%

372 385 375 351 277 1103 1054 1032 1002 1047 1019 976 931

a The changes in the values of the parameter Mef determined at E ) -0.2 V (from Figure 10) and expressed with using the unit g mol-1 or expressed as the sum of molar masses (multiplied by adequate experimental coefficients) of anions (“a”) and solvent molecule (“s”); total changes in the mass electrode (expressed in terms of EQCM frequency response observed during different periods of measurements of the each of voltammetric curves) associated with the insertion process of Li+ cations (-∆fc+; the segment CD of the E-Mef curve, see for sample Figure 10a) or anions (-∆fa+, the segment AB) and the expelling process of cations (-∆fc-, the segment DA) or anions (-∆fa-, the segment BC); the end points A, B, C and D on the E-Mef curves correspond to Mef ) 0 g mol-1; -∆f(5-50), a percentage decrease in the value of a given parameter, determined when a potential scan rate was increased 10 times from 5 to 50 mV s-1; ∆(∆fc) ) |-∆fc+| - |-∆fc-|; the superscript “+” and the superscript “-” indicates a case of insertion or expelling processes, respectively. b a, anion; s, PC.

are more brittle and less resistant to mechanical damage than these doped initially with bigger organic anions (for sample: tosylate). The brittleness can be conductive to the swelling process and changes in the morphology (surface roughening) connected with the solvent and cation ingress upon cycling to a great extent. Additionally, it was ascertained that the average number of PC molecules per one inserted or expelled Li+ ion was equal to 2.9 or 4.8 respectively, in the situation when the tpy1eqtf sample was electrochemically cycled for the first time (see Figures 5f and 8b and Table 3). Furthermore, only approximately 30% out of the total amount of Li+ cations inserted for the first time into a PPy(TOS) lattice moves into/out of the films once again during further electrochemical switching. This feature is

stable on the time scale of the measurement of a few tens of cyclic voltammograms at 5 mV s-1 in the potential window (-1.5 V, 0.0 V). The results give an additional proof of the lack of exchangeability (permanent immobilization) of tosylate ions incorporated into PPy films in propylene carbonate. The accuracy of this analysis was dependent on the mass of exchanged anions and was connected with the problem of assignation of the potential window in which particular mass fluxes (solvated cations or anions) were prevailing. More accurate calculations were possible to carry out when a LiClO4-PC electrolyte was substituted by a LiTFSI-PC electrolyte for electrochemical testing because of more pronounced mass sensitivity of PPy-covered EQCM electrode to the detection of anion movement.



Figure 9. Voltammograms (10 mV s-1, third scans) (a) and related E-Mef curves (b), measured for the tpy1eqtf sample (PPy(TOS)) in 0.87 M LiTFSI-PC, starting potential E ) -1.4 V.

Despite the fact that an average number of PC molecules accompanying Li+ motion in PPy films is very close to the solvation number of Li+ ion in propylene carbonate (being equal to 3-4;52 see Table 3), some retardations in the transport process of PC molecules are detectable by the use of the EQCM technique. It is unbelievable that the values of Li+/PC molar ratio could be as high as about (8 (Mef ≈ ( 800 g mol-1; see Figures 8 and 9). Presumably, lithium ions do not cross the PPy/liquid electrolyte interface with the whole of the inner solvation shell. However, the accumulation of unshelled lithium cations within PPy films gives rise to the induction of the osmotic pressure difference between the polymeric phase and the electrolyte solution, resulting in the coupled transport process of solvent molecules. Simultaneously, enormous volume changes in the polypyrrole films are occurring. Osmotic forces cause solvent molecules to move into/out of the polymer during electrochemical switching in quantities far in excess of those in the inner solvation shell of the mobile ion.53,54 Stronger kinetic limitations on the transport process are usually found in the case of solvent species than ions.55 Especially, solvent molecules belonging to the outer solvation shell of transferred ions are slowly moving through the polymer.56 The hysteresis appearing in the frequency/potential diagrams in the case of PPy(SiF62-) and PPy(AlF63-) was also assigned to the kinetic limitations of water and ion transport in the polymer phase.6,57 Summing up the above considerations, the ion-exchange properties of PPy(TOS) can be described with two following equations (11, 12):

(P)nd(Tos-)dC+ - ye- T (P)ny+d(Tos-)(d-y)C+ + yC+ (11) (P)ny+d(Tos-)(d-y)C+ - ze- + zA- T (P)n(y+z)+d(Tos-)(d-y)C+zA-

(12)

where n ) 1, d ≈ 0.26, y + z ) xef, and y ≈ 25-30% of xef (when V ) 10 mV s-1). Presence of defects in PPy chains and the processes of solvent transport were not included in these equations. Quasistable (P)nd(Tos-)dC+ species are formed when first dedoping scan is performed, but the coefficient d is decreased only as a result of long-lasting cycling (aging processes). As it can be deduced from Figure 9, lithium ion

intercalation is not strongly affected by the exchange processes of secondary anions (TFSI-) when the potential scan rate is not too high (V e 10 mV s-1). When the potential window, in which the PPy(TOS) films are switched, is gradually narrowed ((-1.5 V, 0 V) f (-1.5 V, -0.7 V)), any serious changes in the E-Mef curve are observable (see Figure 9b). However, some decrease in the measured currents is determined at the same time (see Figure 9a), which can suggest that anionic transport is not entirely eliminated even after narrowing the potential window down to (-1.5 V, -0.9 V). Nevertheless, anionic (eq 12) and cationic (eq 11) conductivity of PPy(TOS) can be considered as independent of each other in some potential ranges if the potential scan rate V is not too high. It is a direct consequence of permanent anchoring of tosylate anions in PPy matrix. Therefore, it was possible to look into individual mass fluxes of solvated cations and anions. The effect of the used scan rate on the mass transport behavior of the PPy samples is shown in Figure 10 and Table 4. In general, more solvent molecules are involved in a Li+ intercalation process than deintercalation process (compare parameters: -∆fc+ and -∆fc-, Table 4). The formed excess of PC molecules (and probably other neutral species like ionic pairs) is however further removed from PPy films during the both (insertion and deinsertion) exchange processes of anions (eq 12), particularly when V g 10 mV s-1. Therefore, a wide potential regime of cycling of the PPy films supports the complete removal of all neutral species. More and more distinct diminishment of the parameter Mef+ is detectable when the potential scan rate is increased continuously (see Figure 10 and Table 4). If the used potential scan rate does not exceed the value of 5 mV s-1, the expelling process of the remaining PC molecules proceeds mainly alongside with the deinsertion process of anionic dopand. As the potential scan rate is increased, ion and solvent transport processes become more complicated and more dependent upon each other. Under such circumstances, the exchange processes of cations and anion, which proceed in the opposite directions, can not be considered as parallel or consecutive electrochemical reactions (the case when V e 5 mV s-1), but competitive reactions. Especially, the huge kinetic limitations of the transport process of TFSI anions (see Figure 10c, V ) 200 mV s-1) is a major hindrance to the influx of Li+ and PC molecules into the PPy(TOS) films. The increase in the potential scan rate V leads to attenuation of the PPy film ability to exchange Li+ ions. Additionally, the decrease in the mass of transported solvent

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Dziewon´ski and Grzeszczuk (anodic and cathodic) through the PPy-modified EQCM electrodes are not strongly altered by the change of the potential scan rate (see Figure 4, the case of the tpy1eq, tpy1eqtf ncpy1eq sample). On the other hand, the exchange processes of anions are also influenced by the deintercalation processes of solvated Li+ ions if V g 10 mV s-1. The anion doping process of the PPy films cannot be finished before the inversion of the potential ramp (the segment AB of CV curves, Figure 10a) and must be continued after reaching the vertex potential E ) 0.0 V (the segment BC). Probably, it is a major reason of extremely low values of the parameter Mef manifested in the fragment BC of the Mef-E curves. Therefore, it would be more correct to explain the experimental values of the parameter Mef- in line with the assumption that the anion intake is still a dominant process after the reversal of the potential scan if this parameter (V) takes values bigger than 5 mV s-1 (which should induce the use of the positive sign of the coefficient by which the parameter “a” (the molar mass of anions) is multiplied; see Table 4). Ion exchange properties of the polypyrrole films doped with perchlorate anions (the ncpy1eq film) are more complex than the discussed PPy(TOS) sample because of the instability of cationic defects in the PPy(ClO4) phase and faster disappearance of its ability of cation transport as the switching time is prolonged. 4. Conclusions

Figure 10. Calculated E-Mef curves (related to adequate stationary voltammograms) by using eq 6 for the ncpy1eq film (a), tpy1eq film (b) and tpy1eqtf film (c) electrochemically cycled with different scan rates in 1 M LiClO4-PC (a, b) or 0.87 M LiTFSI-PC (c); more information concerning studied samples in Table 1 and Table 2.

and/or neutral species is evidenced markedly (in particular, compare the parameter -∆f(5-50) calculated for -∆fc+ and -∆fc- quantities, Table 4) despite the fact that passed charges

Polypyrrole reveals various ion-exchange properties depending on conditions of the electrochemical synthesis. Dependencies between the mechanism of electropolymerization and the chemical composition of the polymer and its ability to transport cations have been described in the literature. However, a majority of the reports concern polypyrrole in acetonitrile media. Properties of polypyrrole prepared and performing in propylene carbonate are not recognized thoroughly by now. For example, a much higher viscosity of propylene carbonate can impact the dynamics of the reacting system considerably. In this work, different morphologies of polypyrrole layers on electrodes were generated depending on the chemical composition of electropolymerization media, the surface state of electrode substrates and the electrical conditions of cyclic voltammetry procedures applied to perform electrolytic processes. Porosity of the polymer phase, considered in the molecular scale using the concept of electrochemical porosity, was usually found to increase under consecutive cycles of the redox switching of PPy that is coupled to the exchange of ions between the polymer and other phases of the system. Ion-exchange properties of PPy(TOS) or PPy(ClO4) were found to be independent of current densities of their electrodeposition as well as the predicted chemical composition of the polymer (i.e., chain length distribution, chain branching). On the contrary, (1) porosity of the polymer layer, (2) solvent used in electropolymerization and redox switching processes, (3) a nature of counterions, and (4) thickness of the polymer layer are the factors of foremost importance. Cation-exchange properties of PPy are evidenced by the CV redox peaks or humps at potentials about -1.0 V vs Ag/Ag+. Although tosylate ions are practically immobilized in the polypyrrole phase (i.e., nearly not ejecting from PPy), the estimated contribution of the cation exchange in the total ion exchange of PPy(TOS) is only about 25-30% (the polymer process in 1 M LiClO4-PC or 0.87 M LiTFSI) despite usually lower values of diffusion coefficients of anions. The Li+ intercalation process seems to be independent of the width of the potential window. The Li+ intercalation/deinter-

Lithium Ion Exchange Behavior of Polypyrroles calation can be considered as occurring in parallel to the anion exchange. However, when observed in the long time scale (the low potential scan rates, V), the two processes can be treated independently as each of them prevails at different redox states of the polymer. In a shorter time scale (V g 10 mV s-1), the two processes compete to fulfill interaction forces imposed by the redox state of PPy. Then, the rate of ejection of anions determines how many solvated lithium ions can be located in the PPy structure. The facility of PPy films to exchange cation ions, when an aqueous electrolyte is selected, was reported to be more manifested after increasing the potential scan rate (see for example ref 55). This behavior is utterly converse for the PPy(TOS) layers described in this paper, which means that lithium ion transport is mainly determined by persistently immobilized tosylate ions and to a lesser extent determined by the transport of exchangeable secondary anions. Potential scan rate is the parameter which considerably affects the slow transport processes of solvent molecules and shifts the potential window in which these processes are proceeding. Especially at the highest scan rates, removal of the excess of neutral species (solvent and probably electrolyte salt) is more effective alongside the influx and efflux of secondary anions. Probably in the aftermath of osmotic strength creation within the samples during their electrochemical switching, solvent transport processes are invoked too, which is in accordance with the very high Mef values referring to lithium ion intercalation/ deintercalation. This result is not associated with the viscoelasticity of the PPy samples obtained during experiments.29 Supporting Information Available: Additional figures as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Heeger, A. J. ReV. Mod. Phys. 2001, 73, 681–700. (2) Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007, 91, 954–985. (3) Lange, U.; Roznyatovskaya, N. V.; Mirsky, V. M. Anal. Chim. Acta 2008, 614, 1–26. (4) Malinauskas, A. Synth. Met. 1999, 107, 75–83. (5) Nova´k, P.; M”uller, K.; Santhanam, K. S. V.; Haas, O. Chem. ReV. 1997, 97, 207–281. (6) Ke¸pas, A.; Grzeszczuk, M. Electrochim. Acta 2006, 51, 4167–4175. (7) Khalkhali, R. A. Russ. J. Electrochem. 2005, 41, 950–955. ¨ ; pik, A.; Forse`n, O. Electrochim. Acta 2003, 48, (8) Syritski, V.; O 1409–1417. (9) Ren, X.; Pickup, P. G. J. Phys. Chem. 1993, 97, 5356–5362. (10) Tamm, J.; Alumaa, A.; Hallik, A.; Johanson, U.; Tamm, L.; Tamm, T. Russ. J. Electrochem. 2002, 38, 182–187. (11) Inzelt, G.; Kerte´sz, V.; Nyba¨ck, A.-S. J. Solid State Electrochem. 1999, 3, 251–257. (12) Paik, W.; Yeo, I.-H.; Suh, H.; Kim, Y.; Song, E. Electrochim. Acta 2000, 45, 3833–3840. (13) Ke¸pas, A.; Grzeszczuk, M. J. Electroanal. Chem. 2005, 582, 209– 220. (14) Vidanapathirana, K. P.; Careem, M. A.; Skaarup, S.; West, K. Solid State Ionics 2002, 154-155, 331–335. (15) Skaarup, S.; Bay, L.; Vidanapathirana, K.; Thybo, S.; Tofte, P.; West, W. Solid State Ionics 2003, 159, 143–147. (16) Raudsepp, T.; Marandi, M.; Tamm, T.; Sammelselg, V.; Tamm, J. Electrochim. Acta 2008, 53, 3828–3835.

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JP100796A

Impact of the Electrochemical Porosity and Chemical Composition on

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