Surface and Charge Transport Characterization of Polyaniline

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Surface and Charge Transport Characterization of Polyaniline-Cellulose Acetate Composite Membranes Asif A. Qaiser,* Margaret M. Hyland, and Darrell A. Patterson Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019 Auckland, New Zealand ABSTRACT: This study elucidates the charge transport processes of polyaniline (PANI) composite membranes and correlates them to the PANI deposition site and the extent of PANI surface layering on the base microporous membranes. PANI was deposited either as a surface layer or inside the pores of cellulose acetate microporous membranes using various in situ chemical polymerization techniques. The extent of PANI layering at the surface of the base membrane and its oxidation and doping states were characterized using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). PANI deposition on the membranes showed a strong dependence on the polymerization technique and polymerization time within a single technique. In XPS, the deconvolution of C 1s and N 1s core-level spectra of the composite membranes was used to quantify the extent of PANI layering at the surface along with its oxidation and doping states. PANI incompletely covered the surface of the base microporous membranes for all the employed techniques. However, the extent of the layering increased with the polymerization time in a particular technique. The charge transport through the bulk membrane and charge transfer at the membrane/ electrode interface were studied by electrochemical impedance spectroscopy (EIS). The data were analyzed using the equivalent circuit modeling technique. The modeling parameters revealed that PANI deposition at the surface enhanced the interfacial charge transfer but the process depended on the extent of the surface coverage of the membrane. In addition, the charge transport in the bulk membrane depended on the PANI intercalation level, which varied depending on the polymerization technique employed. In addition, the EIS of electrolyte-soaked membranes was also conducted to evaluate the effects of PANI deposition site on charge transport in the presence of an electrolyte. PANI layering at the pore walls of the base membrane from diaphragmatic polymerization in a two-compartment cell showed that charge transport processes were strongly affected by the interaction of the electrolyte with the PANI layer at the pore surface. This study successfully showed the dependence of charge transport mechanisms of PANI composite membranes on the PANI deposition site and extent of surface layering at the membrane surface.

1. INTRODUCTION Films and membranes comprising intrinsically conducting polymers (ICPs) such as polyaniline (PANI), polypyrrole (PPY), and polythiophene (PTh) have been the focus of study in recent years because of their superior charge transport properties, which can be tuned for specific applications such as sensors, charge storage, and membranes.1,2 These properties can be adjusted by varying the oxidation and doping states of the ICP either chemically or electrochemically. This property change imparts significant changes in the morphology, hydrophilicity, and electronic and ionic conductivities of the membranes.3-7 Among the commonly used ICPs, polyaniline (PANI) has gained special attention because of its high electrical conductivity, good environmental stability, simple and low-cost synthesis, unique redox chemistry, and acid dopability.8,9 Various techniques have been employed to synthesize pristine PANI and PANI composite membranes, including electrochemical and chemical polymerizations, solution blending and subsequent film casting, and in situ polymerization on a base membrane.1 The chemical oxidative polymerization of aniline has been the subject of specific focus r 2011 American Chemical Society

because of its simplicity and low cost.10 However, this polymerization technique has the disadvantage of poor control of deposition level on the base membrane as compared to the electrochemical polymerization technique. In addition to the oxidation and doping states of polyaniline, the deposition site and deposition level also affect the electrochemical performance of the membranes.11 Ion-exchange membranes have been modified with polyaniline using various in situ chemical polymerization techniques. PANI deposition in these membranes has changed their permselectivity for different counterions. These techniques include solution-phase (dipping) polymerization and diaphragmatic polymerization in a two-compartment permeation cell.7,12-15 The exchangeability of the ion-exchange membrane and the nature and concentration of the oxidant were used to control PANI intercalation site in the base membrane. PANI was either layered at the surface or incorporated inside the bulk Received: October 1, 2010 Revised: December 22, 2010 Published: February 2, 2011 1652

dx.doi.org/10.1021/jp109455m | J. Phys. Chem. B 2011, 115, 1652–1661

The Journal of Physical Chemistry B membrane. Sata et al.7 deposited PANI films on a single face or both faces of a cation-exchange membrane. These surface layers blocked the permeation of divalent ions over monovalent ions because of the strong electrostatic interaction of PANI with the ions. The composite membranes showed the highest level of permselectivity at an optimum polymerization time of 1 h. At longer polymerization times, conductive emeraldine was oxidized to insulating pernigraniline, which dramatically reduced the permselectivity. In addition to this study, the effects of PANI deposition site on the permselectivity of PANI-cation-exchange membranes have also been studied.12-15 The deposition site was varied by changing the oxidant,15 the oxidant concentration,14 or the contacting pattern of monomer and oxidant.13 The extent of PANI layering at the surface and the PANI oxidation and doping states were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy, and elemental analysis. The deconvolution of the core-level XPS spectra showed the extent of PANI layering at the surface of the base homogeneous membrane.14 The layering extent increased with increasing polymerization time in the two-compartment cell polymerization. Permselectivity was measured by electrodialyzing a mixture of various cations (Hþ, Naþ, Ca2þ, Cu2þ, Zn2þ). It was shown that a homogeneous but thin surface layer was required to effectively block the permeation of multivalent cations as compared to that of monovalent ions.13 The PANI deposition site also influenced the permselectivity of these composite membranes. PANI deposition in the bulk of the base membrane blocked the permeation of divalent cations,14 whereas, in contrast, a thin surface layer improved the permselectivity of the base polystyrene sulfonic acid membrane.15 PANI deposition developed hydrophilic ionic channels in the base hydrophobic membrane, hence improving the permselectivity of the membrane. In the electrochemical application of the membranes, in addition to the charge transport inside the bulk membrane, interfacial transfer processes at the membrane/electrolyte interface have been found to play an important role.16 Electrochemical impedance spectroscopy (EIS) has been employed as a dynamic technique to study these charge transport and -transfer processes in ionexchange membranes.16,17 Because PANI shows Hþ-coupled transport of anions,18,19 EIS has been employed extensively to investigate the electrochemical processes arising from the electronic and ionic charge transportation in PANI films and membranes. The electronic conduction mechanism based on the two-domain theory has been elaborated by assigning different conductivities for amine and imine sites in pristine PANI films.20,21 In addition to the EIS studies of pristine PANI membranes, the effects of PANI intercalation levels on the charge transport characteristics of the composite membranes have also been elaborated.22,23 These studies have focused on the effects of PANI intercalation level and doping state on anion-coupled transport processes in the electrolyte-soaked/bathed membranes. However, the effects of PANI deposition site using EIS have not been presented so far in the literature. In our previous study,24 we showed the variation of PANI deposition site in microporous mixed cellulose ester (ME) membranes using various in situ chemical polymerization techniques. PANI was deposited mainly at the surface of the base membrane using solution-phase and vapor-phase polymerizations. In contrast, two-compartment cell polymerization yielded PANI deposition on the pore walls of the base microporous membrane, along with some surface layering. The PANI deposition site and

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the extent of surface layering were characterized using scanning electron microscopy (SEM), FTIR spectroscopy, and dc conductivity measurements. In the present study, we deposited PANI on cellulose acetate (CA) membranes using various previously developed polymerization techniques. The extent of PANI layering at the surface along with its oxidation and doping states were characterized, quantitatively, using XPS and FTIRATR spectroscopy. Cellulose acetate was used as the base membrane in this study because of its ability to swell and hence act as a weak cation-exchange membrane during diaphragmatic polymerization in a two-compartment cell. Moreover, because CA functionalities comprise only C, H, and O, CA yields a clear interpretation of PANI deposition at the membrane surface in terms of C-N and N-H bonds when studied using XPS and FTIR spectroscopy. For the potential applications of these membranes as electrochemically active porous membranes, EIS was used to characterize the charge transfer processes at the membrane/electrode interface and in the bulk of the composite membranes. Equivalent circuit modeling of the EIS data showed a strong dependence of interfacial charge transfer on the composite surface of the membranes. The process was found to depend strongly on the extent of PANI layering at the surface. In addition, the contribution of the bulk membrane to the charge transfer process depended on the PANI intercalation level in the bulk membrane. EIS of electrolyte- (HCl-) impregnated membranes was also conducted and showed pronounced effects of PANI deposition site in the presence of the electrolyte in the membranes.

2. EXPERIMENTAL SECTION Microporous cellulose acetate membranes (pore size = 0.45 μm, thickness = 115 μm) were acquired from Whatman and were modified by depositing polyaniline using various in situ chemical polymerization techniques, as described in our earlier work.24 Briefly, in the solution-phase polymerization, CA membranes were dipped in a polymerizing solution of aniline and the oxidant for various times. For vapor-phase polymerization, base membranes were first soaked with aniline that was subsequently polymerized with ammonium persulfate vapors in a closed environment. In the two-compartment cell polymerization, bare CA membrane separated aniline and oxidant (FeCl3) solutions in the two compartments of the cell, and the monomer and oxidant were allowed to counter-diffuse through the membrane. The concentrations of various reagents used in this study are listed in Table 1. FTIR spectra were obtained using a Perkin-Elmer Spectrum 100 FTIR spectrometer with an ATR accessory (diamond crystal). XPS spectra were obtained on AXIS Ultra DLD spectrometer (Kratos Analytical Ltd.). Survey and core-level spectra were obtained with an Al KR monochromatic X-ray source (1486.69 eV) operated at 15 kV and 10 mA. The residual pressure in the analysis chamber during scans was kept below 10-8 Torr. Corelevel C 1s and N 1s spectra, after background subtraction, were curve-resolved using a Gaussian line shape with a Lorentzian broadening function. The lowest C 1s binding energies of unmodified CA membrane and PANI-CA were adjusted to 285.4 and 284.6 eV, respectively, to compensate for sample charging effects.25,26 Alternating-current (ac) impedance spectroscopy was conducted by sandwiching either dry or (1 M) HCl-soaked (for >12 h) membranes between two inert metallic discs of 10-mm diameter 1653

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The Journal of Physical Chemistry B

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Table 1. Concentrations of Monomer and Oxidant Solutions Used in Polymerization Techniques polymerization technique

monomer solution

oxidant solution

solution-phase

aniline (0.8 M) þ 0.4 M HCl

vapor-phase


FeCl3 3 6H2O (0.3 M) þ 0.4 M HCl ammonium persulfate (0.3 M) þ 3 M HCl

two-compartment cell


FeCl3 3 6H2O (0.3M) þ 0.4 M HCl

Figure 1. FTIR-ATR spectra of CA and PANI-CA membranes: (a) CA, unmodified; (b) CA, Vap, APS; (c) CA, Poly, 5 days; (d) CA, P1, Fe, 22 h; (e) CA, P1, Fe, 6 h; and (f) CA, P1, Fe, 3 h.

each. A Princeton Applied Research (PAR) VersaSTAT potentiostat was used for the application of an ac perturbation to the electrodes and for the acquisition of impedance data. Impedance studies were conducted in potentiostatic mode with an ac wave magnitude of 100 mV. The EIS results were analyzed using ZSimpWin software. It appeared that a minimum PANI deposition time was required to obtain a smooth EIS curve. Unmodified CA and PANI-CA synthesized by solution-phase polymerization for e48 h and by two-compartment-cell polymerization for e3 h showed highly scattered data with high levels of impedance (>1012 Ω). These spectra were not included in the analysis. In the following discussion, CA and PANI-CA membranes are specified according to a nomenclature that incorporates the polymerization technique (P1, two-compartment cell; Poly, solution phase; Vap, vapor phase), oxidant (APS, ammonium persulfate; Fe, FeCl3 3 6H2O); and polymerization time (in days or hours).

3. RESULTS AND DISCUSSION FTIR-ATR spectroscopy and XPS were used in this study to characterize the composite surface of PANI-CA membranes. FTIR-ATR spectroscopy yields information of the chemical functionalities present at the surface and in the subsurface (∼200-nmdeep) region, whereas XPS limits the analysis to the surface only (∼10-nm depth). The former technique is mainly used for qualitative studies of chemical species, whereas the latter has the advantage of being a semiquantitative technique. PANI deposition at the surface and inside the bulk membrane converted the highly insulating CA membrane to an electrochemically active composite membranes. The electrochemical properties of the composite membranes depend on the PANI deposition level

on the base membrane, along with the PANI oxidation and doping states. Second, because XPS quantifies the chemical functionalities at the surface, the interfacial phenomena in the electrochemical characterization (e.g., EIS) can be explained in terms of the surface chemical character of the membranes. SEM images of the surface and the cross section of PANI-ME composite membranes presented in our earlier work showed that the solution-phase and vapor-phase polymerizations deposited PANI as a surface layer up to a depth of few micrometers in the bulk membrane.24 In the two-compartment cell polymerization, PANI was deposited on the internal pore walls of the base membrane without blocking the pores. 3.1. FTIR Spectroscopy. The FTIR-ATR spectra of unmodified CA and PANI-CA composite membranes are shown in Figure 1. Cellulose acetate is identified by the strong characteristic bands at 1739 cm-1 (CdO stretching in ester), 1368 cm-1 (CH3 bending), 1230 cm-1 (C-O), and 1064 cm-1 (C-O in ether linkage).27,28 The presence of PANI at the surface of the base membrane is identified by the appearance of a strong doublet at 1570 cm-1 (quinonoid) and 1485 cm-1 (benzenoid). Moreover, the oxidation and doping states of surface-deposited PANI are indicated by IR absorption peaks arising from the polaronic structure of PANI at 1306 cm-1 (π-electron delocalization), 1235 (C-Nþ stretching) and 1152 cm-1 (-NHþd stretching).29 The presence of the C-O peak at 1230 cm-1 interferes with the PANI peak at 1242 cm-1. However, a comparatively broad peak at ∼1230 cm-1 for PANI composite membranes as compared to that of unmodified CA indicates an overlap of multiple peaks at this wavenumber. These polaronic peaks indicate PANI in its emeraldine-doped state. The emeraldine oxidation state is also indicated by the almost equal areas of quinonoid and benzenoid 1654



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Figure 2. C 1s and N 1s core-level spectra of (a) unmodified CA membrane; (b) CA, P1, 6 h; and (c) CA, Vap, APS.

peaks. The extent of PANI layering on the surface can be qualitatively evaluated by the relative intensities of the quinonoid and benzenoid peaks in relation to the CdO peak at 1739 cm-1. The progression of the PANI characteristic peaks and the relative suppression of the CdO peak indicate the extent of PANI layering at the surface (Figure 1a-f). FTIR spectra of the membranes show that PANI incompletely covered the surface of the CA membrane even after prolonged polymerization by different polymerization techniques. This can be attributed to the fibrous nature of the base membrane, which yields a highly

irregular porous surface. Uniform layering at this surface is nearly impossible, and only a discontinuous patterned layer could be achieved. Aniline polymerization in the two-compartment cell clearly showed the effect of polymerization time on PANI deposition levels in the membranes (Figure 1d-f). Furthermore, a shorter polymerization time (22 h) in the two-compartment cell polymerization yielded a surface deposition comparable to that obtained after 5 days using the solution-phase polymerization technique (Figure 1c,d). The shoulder at 1612 cm-1 represents a high concentration of benzene at prolonged polymerization times. 1655



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Table 2. N 1s Deconvolution Results of Relative Atomic Concentrations (%) N 1s (at. %)

membrane

Figure 3. Structure of cellulose acetate repeat unit for BE assignments.

The interaction of COO groups with PANI in the two-compartment cell polymerization is also shown by a significant shift in the position of the CdO peak from 1739 to 1739, 1731, and 1725 cm-1 after 3, 6, and 22 h of polymerization, respectively. This shift can be attributed to the electrostatic interaction between the positively charged PANI chains and the COO functionality of the base membrane. The general trends of PANI deposition on the surface of the base membrane are similar to those of PANI deposition on mixed cellulose ester (ME) membranes presented in our previous study.24 However, in contrast to ME, which consists of cellulose nitrate as the major component, PANI characteristic peaks in PANI-CA membranes are clearer because of the lack of interference of CA peaks in the spectra. In addition, the FTIR spectrum of a CA membrane subjected to prolonged solution-phase polymerization (5 days) is also included in Figure 1. The spectrum shows prominent PANI peaks that indicate significant PANI deposition at the surface from the prolonged polymerization times. At a comparatively shorter polymerization time (i.e., 22 h), the FTIR results showed incomplete PANI layering at the surface.24 3.2. X-ray Photoelectron Spectroscopy. The presence of various species at the surface of the PANI-CA composite membranes were studied by deconvoluting the C 1s and N 1s corelevel XPS spectra. The C 1s spectrum of unmodified CA was deconvoluted into five subpeaks at various binding energies (BEs) (Figure 2) representing various carbon bonds (numbered in Figure 3) in the structure of cellulose triacetate. These peaks are assigned to C-H (#1) at 285.4 eV, C-C (#2) at 286.2, C-C-O (#3) at 287.4, O-C-O (#4) at 288.6, and O-CdO (#5) at 289.9.25,26,29,30 The PANI coating introduced additional subpeaks in the C 1s spectra of the membranes (Figure 2). The C 1s spectra of the PANI-CA composite membranes were decomposed into 284.6 eV (aromatic C-H), 285.4 eV (C-C/C-N/ CdN), 286.2 eV (CdNþ), 287.4 eV (C-Nþ/CdO), 288.6 eV (O-C-O), and 289.9 eV (O-CdO) components. The N 1s spectrum was deconvoluted into four component peaks at 398.5 eV (-Nd), 399.4 eV (-NH-), and >400 eV (protonated nitrogen).13-15,30-33 The protonated nitrogen originated, preferentially, from the imine nitrogen of PANI because the relative magnitude of the dissociation constant was higher than that of amine (pKa = 5.5 for -NHþd versus 2.5 for -NH2).14,34 The appearance of multiple peaks for positively charged nitrogen depends on the localization/delocalization of the positive charge on nitrogen because of the variable association of doping Cl- ions. The lower-BE peak was assigned to the delocalized nitrogen in protonated imine, whereas the higher-BE peak can be

398.5 eV

399.4 eV

400.5 eV

201-403 eV

(-Nd)

(NH-)

(polar Nþ)

(loc Nþ)

CA, P1, 6 h

-

43

43

14

CA, Vap, APS

9

32

48

11

attributed to more localized positively charged nitrogen.31,32,35 Neoh et al.30 assigned the latter component to the protonated amine of PANI in the presence of a high concentration of anions. The oxidation and doping states of surface-deposited PANI can be quantified by resolving N 1s spectra of the membranes. The deconvolution of the N 1s spectra of the membranes synthesized using the two-compartment cell polymerization shows totally protonated imine nitrogen as a subpeak at 400.5 eV (Table 2). This emphasizes PANI deposition in the emeraldine salt state. In contrast, the N 1s spectrum of the vapor-phasemodified membrane shows unprotonated imine (398.4 eV) in addition to the protonated component (400.5 eV). Nevertheless, the total imine component adds up to ∼50% in this case, too, indicating an emeraldine salt. The presence of unprotonated imine exhibits the limitation of vapor-phase polymerization for complete imine protonation. However, the presence of protonated amine (401.3 eV) in a significant percentage indicates the simultaneous (but less preferred) doping of amine nitrogen at the high concentration of acid in the polymerization reaction. These trends are generally in line with the PANI oxidation states observed in an earlier study on the hydrolytic degradation of PANI and X-ray degradation of cellulose nitrate in the base membranes.36 The clear interpretation of the C 1s peaks is difficult because of the overlapping of oxygen-containing functionalities of the base membrane with the C-N/CdN functionalities in PANI. The presence of prominent O-C-O (288.6 eV) and O-CdO (289.9 eV) peaks from the two-compartment-cell polymerization shows incomplete PANI layering at the surface of the base membrane. In addition, the O-CdO peak, when studied in conjunction with the high O/C ratio from the survey-level spectrum (not shown here), indicates PANI hydrolysis to benzoquinone and hydroquinone after prolonged polymerization in the two-compartment cell.36,37 On the other hand, the large C-C/C-H peak (284.6 eV) in membranes synthesized using vapor-phase polymerization shows a high aromatic content at the surface that arises from the presence of unreacted aniline at the surface. Aniline might have diffused out to the surface during the polymerization reaction. The presence of delocalized charge on PANI chains is evidenced by the C 1s peak at 286.2 eV emerging from the polaronic PANI structure.35 3.3. EIS Characterization of PANI Composite Membranes. The Nyquist (or Cole-Cole) plots of dry membranes are shown in Figure 4, where the imaginary component of the impedance (-Z00 ) is plotted against its real component (Z0 ). The frequency is also shown on the plots, which decreases going from left to right on the Z0 scale. The data in Figure 4 clearly show the effect of the polymerization technique on the conductivity level of the membranes. Polymerization time in a particular polymerization technique emerged as another influential factor. For the composite membranes synthesized by the two-compartment-cell polymerization, impedance decreases by about four orders of magnitude from 3- to 6-h polymerization, whereas it decreases to 1656



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Figure 4. Nyquist plots of EIS data for PANI-CA membranes: (a) CA, P1, 3 h; (b) CA, P1, 6 h; (c) CA, P1, 30 h; and (d) CA, Vap, APS.

one-half of its value from 6- to 30-h polymerization. This indicates a possible “saturation” intercalation at about 6 h of polymerization. The EIS data of the membranes synthesized using the vapor-phase technique show an intermediate conductivity level, emphasizing the strong effects of the employed polymerization technique in terms of the PANI layering extent on the membrane. Furthermore, the shape of the curves indicates the electrochemical processes taking place in the membranes. The curves generally appear in the shape of distorted semicircles, which indicates both the capacitive and resistive characteristics of the charge transport process.38 For prolonged polymerization in the two-compartment cell, the curves bent down at the low-frequency end, showing resistive charge transference across the membrane. The spectrum of CA, P1, 3 h shows an upward rise at low frequencies that is characteristic of a dominant capacitive process in the membrane. The sandwiching of the membranes between two electrodes constitutes a metal/polymer/metal configuration that consists of the following charge transfer and -transport processes.38,39 • The electronic charge from metal electrodes is transferred to the composite membrane at metal/polymer interfaces. The electrodes are ion-blocking in this case, and only electrons are transferred across the interface. The surface of the composite membrane plays an important role in this transfer process

Figure 5. Equivalent circuit representing the composite membrane synthesized by the two-compartment cell polymerization (3 h).

because of the presence of electronically conductive PANI at the surface. However, the extent of the surface layering depends on the particular polymerization technique and the time. • The charge transport processes in the bulk membranes are highly dependent on the PANI content in the membrane. The charge transference can be attributed to the delocalized conjugated structure of PANI where inter- and intrachain hopping of electronic and ionic charge play an important role. Moreover, the distribution of conductive PANI in the insulating CA was considered in the modeling, along with the spatial variation of the oxidation and doping states in the membranes. These processes in PANI composite membranes can be modeled using the equivalent circuit (EC) technique. In ECs, 1657



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Table 3. Equivalent Circuit Parameters for HCl-Doped Dry Membranes membrane CA, P1, 3 h

EC model (QR)[Q(RW)] χ = 5.5 10 2

CA, P1, 6 h

χ = 1.7 10 CA, P1, 30 h

χ = 1.0 10 CA, Poly, 5 days

(QR)Q

CA, Vap, APS

χ = 1.9 10 (QR)(QR) 2

R1 (Ω)

(Y0)CPE2S-sR

R2

R2 (Ω)

YwS.s0.5

2.5 10-9

0.83

1.2 107

8.5 10-11

0.88

1.2 107

1.2 10-8

7.5 10-6

0.60

123

2.3 10-5

0.60

7.2 103

-

2.6 10-5

0.47

938

5.0 10-5

0.69

1.7 103

-

8.5 10-9

0.61

4.6 104

1.3 10-5

0.20

-

-

6.6 10-9

1.00

1.2 103

3.2 10-6

0.50

4.8 104

-

-3

(QR)(QR) 2

R1

-3

(QR)(QR) 2

(Y0)CPE1S-sR

-3

-4

χ2 = 3.1 10-3

simple electrical elements such as resistors (R), capacitors (C), and inductors (I) correspond to the various physical processes in an electrochemical system. The Nyquist plot of the composite membrane synthesized by 3-h two-compartment polymerization shows a composite curve with multiple regions in various frequency ranges. An equivalent circuit shown in Figure 5 can represent the electrochemical processes of this composite membrane.40,41 The R-C elements in this EC correspond to two distinctive relaxation processes at high and low frequencies. The first R-C element indicates the high-frequency resolution of impedance that arises from the electronic charge transport in the bulk membrane. R1 represents the resistance of the electronic path made up of PANI-coated strands of the membrane skeleton, whereas C1 is the capacitance in the charge transport process. The charge transport in PANI takes place as a mixed electronic-ionic conduction through polaron/bipolaron transition, as shown by the following equilibrium structure.9,19,21

The charge transport process involves inter- and intrachain hopping of anions on PANI molecular chains. The impedance resolution at low frequency can be attributed to the comparatively slower charge transfer processes taking place at the surface.42,43 The resistance (R2) represents the interfacial charge transfer resistance, whereas C2 represents the interfacial capacitance that arises from the double-layer charging at the surface. In the case of the PANI layer at the surface, C2 might represent the pseudocapacitance of the redox transition in the PANI layer. The Warburg element (defined later) is included in the second R-C element to model diffusion-controlled transport processes in the membrane. The overall impedance of the equivalent circuit (Z(ω)) is represented by " # " # R1 R2 ZW ZðωÞ ¼ þ ð1Þ 1 þ jωCR1 R2 þ ZW þ jωC2 ðR2 ZW Þ where ZW represents the impedance of a Warburg element. A parallel R-C combination models the relaxation process with a time constant of τ = RC that involves a kinetically controlled process in the presence of a capacitive charging. The Warburg element characterizes a diffusion process that originates

from the restricted one-dimensional diffusion in the bulk membrane. The impedance of a Warburg process is modeled as44 1 ð2Þ ZW ðωÞ ¼ pffiffiffiffiffi Yw jω where Yw is the admittance of the Warburg element. In Figure 4, the resolution of the impedance does not exhibit a proper semicircle; rather, it shows a depressed semicircle, indicating inhomogeneous charge transfer processes probably because of the spatial distribution of conducting polyaniline in the composite membranes. This inhomogenity arises because of either incomplete and nonuniform oxidation of PANI chains or incomplete surface and in-bulk PANI coating. In this case, the capacitive part of the response cannot be modeled by a pure capacitive element and it is replaced by a constant-phase element (Q) for which the admittance (Y = 1/Z) is written as23 1 ZðωÞ ¼ ðjωÞ - R ð3Þ Y0 where R denotes the distribution index, which varies from 0 to 1, and the CPE gives a pure capacitance for R = 1. Y0 (= 1/Z0) represents the admittance at ω = 1 (rad/s). The results obtained by fitting the equivalent circuit to the EIS data of PANI composite membranes are reported in Table 3. For the membranes modified by the prolonged polymerizations in the solution- and vapor-phase and two-compartment cell techniques, the EC model of Figure 5 is reduced to only two R-C (or Q) elements connected in series with each other. The low χ2 values (i.e., 10-4-10-3) indicate the good quality of the model fitting. The first (high-frequency) R-Q element represents charge transport process through the bulk membrane where R1 represents electronic charge transfer resistance, and the capacitive component (Y0) emerges from the polaron/bipolaron transitions involving electrons and doping anions. The high Y0 values coupled with the significantly low resistance values arise from the charge transport dominantly through PANI in the bulk membrane that resulted from the prolonged polymerization in the two-compartment cell. The solution- and vapor-phase polymerizations show almost 4 orders of magnitude lower capacitance and higher resistance values, signifying low PANI deposition in the bulk membranes. In this case, PANI was mainly deposited at the surface of the membrane. However, the resistance values in these membranes are 2-3 orders of magnitude lower compared to that from 3 h polymerization in the two-compartment cell (CA, P1, 3 h). This shows the effects of the comparatively higher contents of PANI deposition in the membranes from the vapor-phase and prolonged solution-phase polymerizations. 1658



Figure 6. Nyquist plots of PANI composite membranes impregnated with 1 M HCl: (a) ME, Poly, Fe, 22 h; (b) ME, P1, 22 h; and (c) ME, Vap, APS.

The second R-Q element can be assigned to a long-range process that involves the interfacial charge transfer between the electrodes and membrane surface.43 The resistance represents the rate of charge transfer, whereas the capacitance represents a double layer charging at the surface. This double layer depends on the quality of the contact with the electrodes and the surface chemistry of the membrane. The values of the resistance and the capacitance strongly depend on the extent of PANI layering and the PANI oxidation and doping states as studied by FTIR spectroscopy and XPS. The capacitive admittance (Y0) increases for prolonged polymerization in the two-compartment-cell technique because of better surface coverage with PANI. The origin

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of this capacitive behavior can be attributed to the redox processes in the surface-deposited PANI layer in addition to the development of a double layer at the metal/polymer interface. In this case, R departs significantly from the value for a pure capacitance (R = 1), indicating a highly distorted and nonuniform surface layering. The progressive decrease in the resistance with the polymerization time also indicates improved PANI layering at the surface. In the two-compartment-cell polymerization for shorter times (i.e., for 3 h), the inclusion of an additional Warburg element in series with the resistance of the low-frequency loop shows diffusion control of the charge transfer process at the interface. It can be attributed to the restricted diffusion of electrons from the metal electrode to the highly insulating composite surface in the presence of air gaps, as also indicated by the high resistance value.43 The higher resistance value from the vapor-phase polymerization indicates a discontinuous layer and highly uneven surface probably making a poor contact with the electrodes. In the case of solution-phase polymerization, the higher PANI content at the surface eliminates the resistance of the charge transfer, and the interfacial process is represented by a distributed capacitance (Q) only. The higher admittance value of this CPE also shows the interfacial charge transfer through a dominant PANI surface layer at the interface. Pristine PANI films in the dry state are visualized as composed of highly ordered metallic islands surrounded by the disordered regions.19,20 The electronic conductivity of the films has been explained in terms of the high and low conductivities of these two separate phases, respectively. The addition of a liquid electrolyte enhances the conductivity of the films up to several orders of magnitude. This increase is attributed to the high protonic conductivity of otherwise less conductive disorderly organized chains.19 To elucidate the effects of the electrolyte on the charge transport properties of the membranes, EIS of the electrolyte-soaked (1 M HCl) membranes was also conducted. The composite membranes synthesized by the two-compartment-cell polymerization for comparatively shorter polymerization time, (i.e., < 6 h) show highly scattered data with very high impedance values (∼1012 Ω). Similar trends were also observed for the solutionphase-modified membranes for less than 22-h polymerization times. It shows the role of electrolyte retention in the composite membranes with small PANI intercalation levels. However, the effects of the soaking level on the charge transport characteristics of the membranes need further study. The Nyquist plots of electrolyte-soaked PANI composite membranes are shown in Figure 6. The composite membranes synthesized by the solution-phase polymerization showed impedance resolution at low frequencies, whereas the membranes synthesized using vapor-phase and two-compartment-cell polymerizations showed the resolution at high frequencies. These plots, respectively, show the diffusionally and kinetically controlled processes of the charge transport through the membranes. The latter also emphasizes the swift interfacial transference because of the presence of PANI and the electrolyte layer at the membrane surface. In fact, the fast charge transfer in the PANI phase dominates the double-layer capacitance effects by the fast discharging of the electronic charge at the interface. The data were successfully fitted by a single R-Q element connected with a series resistance Ro (Figure 7). The results of the model fitting are shown in Table 4. In the case of solution-phase polymerization, Ro corresponds to the electronic resistance of a two-phase system comprising the membrane and the pore-entrapped electrolyte. The low-frequency R-Q element shows diffusionally 1659



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Figure 7. Equivalent circuit for PANI composite membranes soaked in 1 M HCl.

Table 4. Equivalent Circuit Parameters for PANI Composite Membranes Soaked in 1 M HCl R

R0 (Ω)

R1 (Ω)

Y0(S-s )

R

36.70

4.1 104

7.7 10-4

0.81

ME, Vap, APS χ2 = 6.1 10-4

0.01

5.4 104

1.7 10-9

0.77

ME, P1, 22 h

0.80

1.00

1.6 10-2

0.71

membrane ME, Poly, 22 h χ = 2.05 10 2

χ = 2.7 10 2

-3

-4

controlled slow charge transference at the interface involving anion inclusion in the membrane. The semicircle at high frequencies from the two-compartment-cell polymerization shows charge transfer involving the redox transformations of the PANI phase in the membrane. In fact, the significantly small resistance and high level of capacitance (admittance) indicate the faradictype reaction between the anions in the pores and the PANI layer at the pore walls of the composite membrane. The impedance resolution at high frequencies with high levels of resistance can be attributed to the charge conduction through anilinium (Ph-NH2þ) ions because of prolonged impregnation of the base membrane prior to the polymerization with APS vapors.

4. CONCLUSIONS Electronic and ionic charge conduction mechanisms of electrochemical membranes comprising polyaniline and cellulose acetate have been elaborated in this study in relation to the surface characterization of the membranes. PANI was deposited on microporous cellulose acetate membranes using various in situ chemical polymerization techniques that include solutionphase and vapor-phase polymerizations and diaphragmatic polymerization using a two-compartment permeation cell. The extent of PANI layering at the surface and the PANI oxidation and doping states depending on the employed polymerization technique were characterized by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The deconvolution of the C 1s and N 1s core-level spectra yielded semiquantitative information on the extent of PANI deposition on the surface. Electrochemical impedance spectroscopy (EIS) was used to analyze the charge transport and interfacial charge transfer properties of the composite membranes. The equivalent circuit modeling of the EIS data showed pronounced effects of the PANI deposition site and extent of surface layering on the electrochemical characteristics of the membranes. In-pore PANI deposition by the two-compartment-cell technique showed low resistance coupled with high capacitance because of the swift charge transport in the bulk membrane. PANI layering at the

membrane surface facilitated the electronic charge transfer from the metal electrodes to the composite membrane. Further, the electrolyte (HCl) soaking of the composite membranes showed prominent effects of the PANI deposition site in the base membrane. The pore electrolyte seemed to interact with the PANI layer at the pore surface, hence affecting the charge transport in the bulk membrane. This study successfully correlated the charge transport processes of the PANI-CA composite membranes to the deposition site, extent of PANI layering at the surface, and presence of electrolyte in the membranes.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ64 211086087. Fax: þ64 9 373 7463. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors would like to thank Mr. Stephen Cawley of CACM, The University of Auckland, for his technical support in EIS and Professor David Williams (Chemistry) for his help in the interpretation of EIS results. Moreover, the financial support of Higher Education Commission (HEC) in terms of Ph.D scholarship is gratefully acknowledged. ’ REFERENCES (1) Sairam, M.; Nataraj, S. K.; Aminabhavi, T. M.; Roy, S.; Madhusoodana, C. D. Sep. Purif. Rev. 2006, 35, 249. (2) Schultze, J. W.; Karabulut, H. Electrochim. Acta 2005, 50, 1739. (3) Anderson, M. R.; Mattes, B. R.; Reiss, H.; Kaner, R. B. Synth. Met. 1991, 41, 1151. (4) Pile, D. L.; Hillier, A. C. J. Membr. Sci. 2002, 208, 119. (5) Pile, D. L.; Zhang, Y.; Hillier, A. C. Langmuir 2006, 22, 5925. (6) Ariza, M. J.; Otero, T. F. J. Membr. Sci. 2007, 290, 241. (7) Sata, T.; Ishii, Y.; Kawamura, K.; Matsusaki, K. J. Electrochem. Soc. 1999, 146, 585. (8) Blinova, N. V.; Stejskal, J.; Trchova, M.; Ciric-Marjanovic, G.; Sapurina, I. J. Phys. Chem. B 2007, 111, 2440. (9) Wallace, G. G.; Spinks, G. M.; Kane-Maguire, L. A. P.; Teasdale, P. R. Conductive Electroactive Polymers: Intelligent Materials Systems, 2nd ed.; CRC Press: Boca Raton, FL, 2003. (10) Pud, A.; Ogurtsov, N.; Korzhenko, A.; Shapoval, G. Prog. Polym. Sci. 2003, 28, 1701. (11) Sata, T.; Sata, T.; Yang, W. J. Membr. Sci. 2002, 206, 31. (12) Tan, S.; Viau, V.; Cugnod, D.; Belanger, D. Electrochem. SolidState Lett. 2002, 5, 55. (13) Tan, S.; Laforgue, A.; Belanger, D. Langmuir 2003, 19, 744. (14) Tan, S.; Belanger, D. J. Phys. Chem. B 2005, 109, 23480. (15) Tan, S.; Tieu, J. H.; Belanger, D. J. Phys. Chem. B 2005, 109, 14085. (16) Park, J.-S.; Choi, J.-H.; Woo, J.-J.; Moon, S.-H. J. Colloid Interface Sci. 2006, 300, 655. (17) Park, J.-S.; Choi, J.-H.; Yeon, K.-H.; Moon, S.-H. J. Colloid Interface Sci. 2006, 294, 129. (18) Wen, L.; Kocherginsky, N. M. J. Membr. Sci. 2000, 167, 135. (19) Stejskal, J.; Bogomolova, O. E.; Blinova, N. V.; Trchova, M.; Sedenkova, I.; Prokes, J.; Sapurina, I. Polym. Int. 2009, 58, 872. (20) Afzal, A. B.; Akhtar, M. J.; Nadeem, M.; Hassan, M. M. J. Phys. Chem. C 2009, 113, 17560. (21) Lvovich, V. F. J. Electrochem. Soc. Interface 2009, Spring 2009, 62. (22) Deka, M.; Nath, A. K.; Kumar, A. J. Membr. Sci. 2009, 327, 188. (23) Compan, V.; Riande, E.; Fernandez-Carretero, F. J.; Berezina, N. P.; Sytcheva, A. A. R. J. Membr. Sci. 2008, 318, 255. (24) Qaiser, A. A.; Hyland, M. M.; Patterson, D. A. J. Phys. Chem. B 2009, 113, 14986. 1660



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Surface and Charge Transport Characterization of Polyaniline

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