Letter pubs.acs.org/Langmuir
Determination of the Diffusion Coefficient of Protons in Nafion Thin Films by ac-Electrogravimetry Ozlem Sel,*,†,‡ L. To Thi Kim,†,‡ Catherine Debiemme-Chouvy,†,‡ Claude Gabrielli,†,‡ Christel Laberty-Robert,§ and Hubert Perrot†,‡ †
UPR 15 du CNRS, LISE, 4 place Jussieu, 75252 Paris, France Université Pierre et Marie Curie, LISE, 4 place Jussieu, 75252 Paris, France § LCMCP-CNRS-UMR-7574-Collège de France, Bat. C-D, 11, place Marcelin Berthelot, 75231 Paris, France ‡
S Supporting Information *
ABSTRACT: This letter deals with an adaptation of the ac-electrogravimetry technique to extract separately the dynamic properties of H+ and water in Nafion nanometric thin films (average thickness of 400 nm). An original theoretical approach was developed to extract the representative parameters from ac-electrogravimetry data. The concentration change of the exchanged species and the diffusion coefficient of the protons in a Nafion nanometric thin film (D = 0.5 × 10−6 cm2 s−1 at 0.3 V vs SCE) were estimated for the first time according to the applied potential. The conductivity value of Nafion thin films was calculated from the Nernst−Einstein equation using diffusion coefficients and concentration values extracted from ac-electrogravimetry data. The calculated conductivity results agree well with the experimental proton conductivity values of Nafion thin films.
1. INTRODUCTION The proton-exchange membrane (PEM) plays a central role as a medium for proton transport in fuel cell performances. Nafion, the most attractive polymer electrolyte developed so far, shows excellent proton conductivity but requires entrapped liquid water for efficient H+ transport.1 Many studies on H+ diffusion in Nafion membranes were conducted to understand its mechanism and eventually to help in designing alternative PEMs with improved properties. Pulsed field gradient (PFG) NMR2−4 provided a convenient means for measuring H+ diffusion coefficients for Nafion at various levels of hydration,5 conditioned at different temperatures,6 and for Nafion/SiO2 composites,2 as well as to determine proton diffusion in proton-conducting ionic liquids.7 The utility of quasi-elastic neutron scattering (QENS) for investigating the dynamic behavior of water within Nafion has already been demonstrated on a completely saturated membrane8 and for a range of hydration levels between dry and saturated states.9 Besides experimental methods, classical molecular dynamic simulations have been used to study H+ dynamics,10 and recent advancements in modeling and simulation studies provided theoretical methods to account for structural diffusion, which © 2013 American Chemical Society
are capable of describing changes in the bonding topology between water and mobile protons.11−14 However, the experimental methods used to determine dynamic properties (i.e., the diffusion coefficient of H+) are limited to studying bulk materials, but they are not available to study thin film properties. However, the procedures used to fabricate gas diffusion electrodes for PEM fuel cells (PEMFCs) include making nanometric thin ionomer films (i.e., an electrode ink including the catalyst and Nafion is formulated and painted), giving rise to the electrocatalytic layer. Understanding the microstructure and the properties of Nafion on the nanoscale is crucial to minimizing activation and mass-transfer losses in PEMFCs. Therefore, investigating the properties of Nafion thin films is of great importance for designing electrode/membrane interfaces in PEMFCs as well as miniaturizing energy conversion devices (i.e., microfuel cells). To do this, the adaptation of sophisticated coupled electrochemical techniques for proton-conducting membrane characterization is required. Received: April 18, 2013 Revised: September 26, 2013 Published: October 16, 2013 13655
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Figure 1. (a) Schematic illustration of the working electrode coated with PPy-HPA/Nafion. (b) FEG−SEM image of the PPy-HPA/Nafion bilayer. = 1.40 × 10−3 cm s−1, GHPA/Nafion = 2.91 × 10−5 mol s−1 cm−2 V−1, (c) (ΔE/ΔI)(ω), with L = 600 nm (Nafion), d = 200 nm (PPy-HPA), KHPA/Nafion H+ H+ −3 −1 −7 2 −1 Nafion/sol −3 = 8.16 × 10 cm s , and D = 5.8 × 10 cm s . (d) (Δm/ΔE)(ω), with K = 1.30 × 10 cm s−1, GNafion/sol = 8.71 × 10−6 mol MHPA/Nafion + + H H H+ Nafion/sol Nafion/sol −1 −2 −1 −4 −1 −6 −1 −2 −1 Nafion/sol −4 −1 Nafion/sol s cm V , KH2O = 9.99 × 10 cm s , GH2O = 1.73 × 10 mol s cm V , KClO4− = 1.89 × 10 cm s , GClO4− = −1.33 × 10−8 −1 −2 −1 + −1 mol s cm V , MW(H ) = 1 g mol , MW(H2O) = 18 g mol−1, and MW(ClO4−) = 99 g mol−1 of the PPy-HPA/Nafion bilayer at 0.275 V/SCE in 0.5 M HClO4 electrolyte.
potential perturbation with a small amplitude (ΔE).15 The advantage of combining such transfer functions is the possibility of a fair separation of the different electrochemical processes, which concomitantly involve the mass and charge changes. A schematic illustration of the studied bilayer architecture is provided in Figure 1a. Electronic transfer occurs at the gold electrode/mediator film interface. For a cathodic potential increase, the protons enter the Nafion from the solution that constitutes the Nafion/solution interface and subsequently diffuse in the membrane up to the PPy-HPA/Nafion interface, where they are inserted. The derivation of the final transfer function, (ΔE/ΔI)(ω), for the model taking into account the transport phenomenon is detailed in ref 18. The key reactions are summarized below. At the PPy-HPA/Nafion interface (at x = d, Figure 1a), protons, are inserted or expelled from the mediator film
ac-electrogravimetry (i.e., the simultaneous measurements of the usual electrochemical impedance, (ΔE/ΔI)(ω), and the mass/potential transfer function, (Δm/ΔE)(ω)) allows the change in mass related to unit charge passing through the electrode/film/solution interfaces to be determined. Therefore, all of the species involved in the charge-compensation process occurring during the redox process of the electroactive thin films can be separately identified by their atomic mass.15 Here, we report on the adaptation of ac-electrogravimetry to the study of diffusion coefficients and concentration changes of H+ in Nafion117 as a function of the applied potential. Because Nafion is only an ionic conductor, a mediator film (polypyrrole doped with heteropolyanions SiMo12O404− (PPy-HPA)16−20) is introduced between the working electrode (which is gold coated on the quartz crystal) and Nafion. This mixed-conductor mediator film is necessary to provide H+ transfer between the different interfaces and therefore to study the proton transport inside Nafion, which is only an ionic conductor. The diffusion coefficients and concentration changes of exchanged species are estimated as a function of applied potential. To the best of our knowledge, this is the first report on the dynamic properties of H+ in nanometric thin film Nafion.
k2
⟨⟩HPA + H+Nafion + e− ⇄ ⟨H⟩HPA k1
(1)
where H+Nafion is the proton diffusing in the Nafion thin film, ⟨ ⟩HPA represents the available sites for ion insertion in the mediator film, ⟨H⟩HPA is the concentration of the protons inserted in the mediator film, and ki = k0i exp(biE) are the classical Tafel kinetic rate constants. Then, the flux of protons, JH+(d), entering the mediator film is equal to
2. THEORETICAL BASIS ac-electrogravimetry consists of coupling electrochemical impedance measurements with a fast-response quartz crystal microbalance (QCM) used in ac mode. It allows the response in current ((ΔE/ΔI) (ω)), via the electrical transfer function, and in mass ((Δm/ΔE) (ω)), via the mass potential transfer function, to be obtained simultaneously owing to a sinusoidal
HPA Nafion HPA + )c + + JH+ (d) = k 2(cmax − c HHPA (d) − k1(c HHPA − cmin ) H
(2) 13656
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Figure 2. (a) (Δci/ΔE)bilayer and (b) relative concentration change, Δcbilayer , for H+, H2O, and ClO4−. (c) Estimated diffusion coefficients (D) from i (IEC) are obtained by the fitting of the proposed model (section 2). (d) Proton conductivity calculated by using D (from panel c) and the molesNafion H+ using the ion-exchange capacity (IEC) of Nafion1100 (0.9 mequiv/g) and the thin film dimensions.1
where cHPA H+ is the proton concentration in the PPy-HPA film, Nafion cH+ is the concentration of the protons in Nafion close to the HPA PPy-HPA/Nafion interface (x = d), and cHPA max and cmin are the maximum and minimum concentrations of the protons in the PPy-HPA film. Therefore, for a small potential perturbation, ΔE, the change in the proton flux, ΔJH+ at x = d, is equal to
⎛ ⎛ ⎞⎞ jω ⎜ ⎟⎟ ⎟ tanh⎜⎜− L Nafion D + ⎜ ⎝ ⎠⎟ H ΔE 1 HPA/Nafion ⎜1 − M H+ ⎟ (ω) = HPA/Nafion Nafion ΔI FG H+ jωD H+ ⎜ ⎟ ⎜ ⎟ ⎝ ⎠ −
jωdFG HHPA/Nafion +
(4)
where L is the Nafion thin film thickness and cNafion is the H+ diffusion coefficient of the protons in Nafion. Equation 4 was used to fit the experimental electrochemical impedance data in this study. In our basic approach, to simplify the analytical model, only diffusion is taken into account; the migration effect has been considered to be a minor contribution. This can be justified by the fact that without a mediator film, which acts as a proton pump, the electrochemical response is drastically different under the same experimental conditions. The electrogravimetric transfer function, (Δm/ΔE)(ω) was calculated by assuming the PPy-HPA/Nafion bilayer to be a single compact film14 and taking into account the protons, solvent, and possible ClO4− transfer occurring at the Nafion/ solution interface:
+ ΔJH+ (d) = G HHPA/Nafion ΔE + K HHPA/Nafion Δc HHPA + + + + M HHPA/Nafion Δc HNafion (d ) +
K HHPA/Nafion +
(3)
where + K HHPA/Nafion = k1 + k 2c HNafion (d ) +
HPA Nafion + )c + G HHPA/Nafion = b2k 2(cmax − c HHPA (d ) + H HPA + − b1k1(c HHPA − cmin ) HPA and MHPA/Nafion = k2(cHPA max − cH+ ) are the constants related to H+ the transfer at the PPy-HPA/Nafion interface. The equation describing the faradaic electrochemical impedance (ΔE/ΔI)(ω), which takes into account the diffusion phenomenon (jωΔCHPA/Nafion = D((∂2ΔCHPA/Nafion)/(∂x2))) is given in ref 18 and is equal to
⎛ G HNafion/sol + GSNafion/sol Δm = − ⎜⎜mH+ + ms Nafion/sol ΔE jω(d + L)K H+ jω(d + L)KSNafion/sol ⎝ + mClO−4
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Nafion/sol − GClO 4
jω(d +
⎞ ⎟
Nafion/sol ⎟ − L)K ClO ⎠ 4
(5)
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where GNafion/sol , KNafion/sol , GHNafion/sol , KHNafion/sol , GNafion/sol , and H+ H+ ClO−4 2O 2O Nafion/sol are the transfer coefficients between Nafion and the KClO−4 electrolyte. Equation 5 does not include the terms related to the diffusion phenomena. Because there is no experimental data in the frequency range available for (Δm/ΔE)(ω) as a result of an instrument limitation (where the diffusion is observed in the (ΔE/ΔI)(ω) response), eq 5 was used to simulate the (Δm/ ΔE)(ω) responses.
Δcibilayer ΔE
=− ω→ 0
GiNafion/sol K iNafion/sol
(GiNafion/sol and KiNafion/sol values are obtained from the simulation of the data in Figure 1c).19 Figure 2b shows the + bilayer relative concentration change, Δcbilayer H+ , for H and that, ΔcH2O , for H2O in the bilayer with respect to the potential calculated from the previous derivative functions. The global proton concentration change given in Figure 2b is related to the PPyHPA/Nafion bilayer, and the electroneutrality is maintained. Indeed, each proton that flows through the Nafion is “consumed” by the HPA electrochemical response located in the PPy matrix. The contribution of H2O is calculated to ∼20% in molar and ∼81% in gravimetric terms for the total species involved during the charge-compensation process. Similar mass changes were observed in sulfonic acid functional silica and fluorinated polymer hybrid membranes.19 A small amount of the anion (ClO4−) was also detected, but its contribution to the electrochemical process is rather weak. The fitting of (ΔE/ΔI)(ω) using the model described in section 2 provided parameters G, K, and M for protons. The model used to simulate the experimental data takes into account the diffusion phenomena in the Nafion layer. On the basis of the rotating disk electrode experiments (Figure S1 in the Supporting Information), with the same film configuration (gold/PPy-HPA/Nafion, same electrolyte), the effect of bulk diffusion of protons is small and therefore was neglected over the diffusion of protons in the Nafion layer. In our previous papers, with mixed electroactive films of about 150 nm thickness, the diffusion effect was not observed.15 In the present study, Nafion, which is only an ionic conductor, has a thickness that is 3 times greater, and for this reason, diffusion in Nafion is considered to be preponderant. The diffusion coefficient of the protons (D) in Nafion thin films under saturated conditions was estimated as a function of the applied potential by using eq 4 (Figure 2c). In the potential range of 0.125−0.350 V versus SCE, diffusion coefficients vary slightly from 0.97 × 10−6 to 0.37 × 10−6 cm2 s−1, which may be due to some structural changes in the environment of the diffusing proton accompanied by water. The D values are in agreement with the self-diffusion coefficient of protons in the Nafion bulk membrane reported in recent PFG-NMR11 and simulation studies (1 × 10−6 cm2 s−1).13 Furthermore, the estimated DNafion and the ion-exchange H+ capacity (IEC) of Nafion provide a calculation of the proton conductivity through the Nernst−Einstein relation25
3. EXPERIMENTAL SECTION The details of the fabrication of the mediator film (PPy-HPA) on gold electrodes of QCM are detailed elsewhere.18,19 The PPy-HPA films on gold electrodes were placed in a laboratory-made mask, and a 4 μL portion of a Nafion 1100 suspension (Aldrich, a 5% mixture of lower aliphatic alcohols and water) was deposited on the mediator films. A schematic illustration and a field-emission gun scanning electron microscopy (FEG-SEM, Zeiss, Supra 55) image of the bilayers can be seen in Figure 1a,b. The bilayers are electroacoustically thin enough (average thickness of 600 nm) that the Sauerbrey equation applies.21 The electrochemical experiments were carried out in a classical three-electrode cell in 0.5 M HClO4 as the electrolyte. The reference electrode was a saturated calomel electrode (SCE), the counter electrode was a platinum grid, and the working electrode was modified gold-patterned quartz. ac-electrogravimetry experiments were performed using a laboratory-made 9 MHz oscillator acting as a microbalance and quartz resonators (TEMEX, France) with a fundamental resonance frequency of 9 MHz. For ac-electrogravimetry, a four-channel frequency response analyzer (FRA, Solartron 1254) and a potentiostat (SOTELEM-PGSTAT) were used. The QCM was used in a dynamic regime,22 the modified working electrode (0.2 cm2) was polarized at a selected potential, and a sinusoidal small-amplitude potential perturbation (30 mV rms) was superimposed. The microbalance frequency change, Δf m, corresponding to the mass response, Δm, of the modified working electrode was measured simultaneously with the ac response, ΔI, of the electrochemical system. The resulting signals were sent to the four-channel FRA, which allowed the electrogravimetric transfer function (Δm/ΔE)(ω) and the electrochemical impedance (ΔE/ΔI)(ω) to be obtained simultaneously at a given potential.15
4. RESULTS AND DISCUSSION Figure 1c,d present transfer functions (ΔE/ΔI)(ω) and (Δm/ ΔE)(ω) for the PPy-HPA/Nafion bilayer studied. The experimental and the simulated data from the model (Theoretical Basis) are reported on the same graph, and good agreement between the two is evident. Figure 1c shows that the low-frequency part of the electrochemical impedance (ΔE/ΔI)(ω) of the bilayer is identical to that obtained for the PPy-HPA film18−20 alone, which represents a vertical line corresponding to the insertion/expulsion mechanism. At high frequencies (inset in Figure 1c), the electrochemical impedance of the bilayer is characterized by the transport of the protons. The simulation of (Δm/ΔE)(ω) reveals that two main species participate in the mechanism: the loop at high frequencies is related to the protons, and that at low frequencies, to free water (Figure 1d). Because the loops are in the same quadrant of the diagram, the flux directions are the same for proton and freesolvent transfers. This simultaneous insertion/expulsion of water may indicate that protons are accompanied by water during their diffusion. This finding is in agreement with the proton-transport phenomena in proton-conducting membranes.23,24 To quantify the role of each species, ((Δcbilayer )/(ΔE))|ω→0 i has been estimated as a function of the potential (Figure 2a). Indeed, at low frequencies, ((Δcbilayer )/(ΔE))|ω→0 becomes i
σ = F2
+ + D HNafion c HNafion (E ) RT
(6)
as a function of potential (Figure 2d). The average σ value for a Nafion thin film (400 nm) is ∼20 mS cm−1 at RT in the immersed state, calculated by using DNafion and the ionH+ exchange capacity of Nafion. The σ value of a classical Nafion bulk membrane (N117) determined by in-plane four-electrode measurements is ∼60 mS cm−1 at 30 °C and 100% RH. This value is slightly higher than our calculated value. However, Siroma et al. reported ∼27 mS/cm for a Nafion thin film (70 nm) at RT under saturated conditions,26 and Karan et al. measured ∼18 mS/cm at 96% RH at RT for a Nafion thin film (50 nm).27 Our calculated σ value of the Nafion thin film is in good agreement with the results in refs 26 and 27. The authors claimed that the activation energy for proton conduction 13658
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increases with the decrease in film thickness, and possible differences in microstructure and water uptake behavior of the Nafion thin film may result in a decrease in proton conductivity.26,27 In our study, the diffusion coefficient value agrees well with the self-diffusion coefficient of proton in Nafion bulk membranes. However, the conductivity values are lower with respect to those of the bulk material. Thus, the lower conductivity values suggest that the concentration of protons might be lower as a result of the morphological and structural changes in Nafion when the thickness is decreased to the nanoscale. Indeed, a recent GISAXS (grazing-incidence small-angle scattering) study by Eastman et al. showed that the nanostructure of Nafion is greatly affected by the film thickness.28 Additionally, a small contribution of ClO4− (ac electrogravimetry data, Figure 2) and its electrostatic interaction with H+ may reduce the transport of H+ in the water channels of the Nafion thin film and may play an additional role in decreasing the proton conductivity.
(3) Noto, V. D.; Boaretto, N.; Negro, E.; Stallworth, P. E.; Lavina, S.; Giffin, A. G.; Greenbaum, S. G. Inorganic Organic Membranes Based on Nafion, [(ZrO2)(HfO2)0.25] and [(SiO2)(HfO2)0.28] Nanoparticles. Part II: Relaxations and Conductivity Mechanism. Int. J. Hydrogen Energy 2012, 37, 6215−6227. (4) Ochi, S.; Kamishima, O.; Mizusaki, J.; Kawamura, J. Investigation of Proton Diffusion in Nafion117 Membrane by Electrical Conductivity and NMR. Solid State Ionics 2009, 180, 580−584. (5) Zawodzinski, T. A.; Neeman, M.; Sillerud, L. O.; Gottesfeld, S. Determination of Water Diffusion Coefficients in Perfluorosulfonate Ionomer Membranes. J. Phys. Chem. 1991, 95, 6040−6044. (6) Kidena, K.; Ohkubo, T.; Takimoto, N.; Ohira, A. PFG-NMR Approach to Water Transport Mechanism in Polymer Electrolyte Membranes Conditioned at Different Temperature. Eur. Polym. J. 2010, 46, 450−455. (7) Iojoiu, C.; Hana, M.; Molmeret, Y.; Martinez, M.; Cointeaux, L.; ElKissi, N.; Teles, J.; Lepretre, C. J.; Judeinstein, P.; Sanchez, J. Y. Ionic Liquids and Their Hosting by Polymers for HT-PEMFC Membranes. Fuel Cells 2010, 10, 778−789. (8) Volino, F.; Pineri, M.; Dianoux, A. J.; De Geyer, A. Water Mobility in a Water-soaked Nafion Membrane − A High Resolution Neutron Quasi-Elastic Study. J. Polym. Sci., Polym. Phys. 1982, 20, 481−496. (9) Pivovar, A. M.; Pivovar, B. S. Dynamic Behavior of Water within a Polymer Electrolyte Fuel Cell Membrane at Low Hydration Levels. J. Phys. Chem. B 2005, 109, 785−793. (10) Devenathan, R.; Venkatnathan, A.; Dupuis, M. J. Atomistic Simulation of Nafion Membrane. I. Effect of Hydration on Membrane Nanostructure. J. Phys. Chem. B 2007, 111, 8069−8079. (11) Ohkuba, T.; Kidena, K.; Takimoto, N. Molecular Dynamics Simulations of Nafion and Sulfonated Poly Ether Sulfone Membranes II. Dynamic Properties of Water and Hydronium. J. Mol. Model. 2012, 18, 533−540. (12) Selvan, M. E.; Keffer, D. J.; Cui, S. Reactive Molecular Dynamics Study of Proton Transport in Polymer Electrolyte Membranes. J. Phys. Chem. C 2011, 115, 18835−18846. (13) Devenathan, R.; Dupuis, M. Insight from Molecular Modelling: Does the Polymer Side Chain Length Matter for Transport Properties of Perfluorosulfonic Acid Membranes? Phys. Chem. Chem. Phys. 2012, 14, 11281−11295. (14) Jorn, R.; Voth, G. A. Mesoscale Simulations of Proton Transport in Proton Exchange Membranes. J. Phys. Chem. C 2012, 116, 10476− 10489. (15) Gabrielli, C.; Garcia-Jareno, J. J.; Keddam, M.; Perrot, H.; Vicente, F. ac-Electrogravimetry Study of Electroactive Thin Films. I. Application to Prussian Blue. J. Phys. Chem. B 2002, 106, 3182−3191. (16) Debiemme-Chouvy, C.; Deslouis, C.; Cachet, H. Investigation by EQCM of the Electrosynthesis and the Properties of Polypyrrole Films Doped with Sulphate Ions and/or a Keggin-Type Heteropolyanion, SiMo12O404-. Electrochim. Acta 2006, 51, 3622−3631. (17) Debiemme-Chouvy, C.; Rubin, A.; Perrot, H.; Deslouis, C.; Cachet, H. ac-Electrogravimetry Study of Ionic and Solvent Motion in Polypyrrole Films Doped with an Heteropolyanion, SiMo12O44−. Electrochim. Acta 2008, 53, 3836−3843. (18) To Thi Kim, L.; Sel, O.; Debiemme−Chouvy, C.; Gabrielli, C.; Laberty-Robert, C.; Perrot, H.; Sanchez, C. Proton Transport Properties in Hybrid Membranes Investigated by ac-Electrogravimetry. Electrochem. Commun. 2010, 12, 1136−1139. (19) Sel, O.; To Thi Kim, L.; Debiemme−Chouvy, C.; Gabrielli, C.; Laberty-Robert, C.; Perrot, H.; Sanchez, C. Proton Insertion Properties in a Hybrid Membrane/Conducting Polymer Bilayer Investigated by ac Electrogravimetry. J. Electrochem. Soc. 2010, 157, F69−F76. (20) To Thi Kim, L.; Debiemme−Chouvy, C.; Gabrielli, C.; Perrot, H. Redox Switching of Heteropolyanions Entrapped in Polypyrrole Films Investigated by ac Electrogravimetry. Langmuir 2012, 28, 13746−13757.
5. CONCLUSIONS ac-electrogravimetry shows that H+ transfer in Nafion thin films occurs with a contribution of H2O, and it is possible that H+ diffusion is accompanied by water. The diffusion coefficient (DNafion ) and concentration change of protons in the PPy-HPAH+ Nafion bilayer thin film were estimated from ac-electrogravimetry data for the first time. Previously, via techniques such as PFG-NMR, diffusion coefficients were accessible only for bulk materials. The D values obtained agree well with the self-diffusion coefficient of protons in a classical Nafion bulk membrane.11,13 Conductivity values for the Nafion thin film, calculated from the Nernst−Einstein equation using acelectrogravimetry data, are slightly lower than the measured H+σ of bulk Nafion membranes. This slight difference can be attributed to the possible differences in nanostructure and water uptake behavior of a Nafion thin film compared to those of the bulk and to the presence of a small number of ClO4− ions and their electrostatic interaction with H+. The diffusion coefficients at different temperature and at different λ (amount of water per sulfonic acid group) on very thin layers (∼10 nm) will be the topic of a future study.
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ASSOCIATED CONTENT
S Supporting Information *
The rotating disk electrode experiments with the same film configuration (gold/PPy-HPA/Nafion in 0.5 M HClO4). Effect of the rotation rate over the EIS response of PPy-HPA/Nafion on a bare gold electrode at 275 mV versus SCE in 0.5 M HClO4. This material is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Mauritz, K. A.; Moore, R. B. State of Understanding Nafion. Chem. Rev. 2004, 104, 4535−4585. (2) Ye, G.; Hayden, C. A.; Goward, G. R. Proton Dynamics of Nafion and Nafion/SiO2 Composites by Solid State NMR and Pulse Field Gradient NMR. Macromolecules 2007, 40, 1529−1537. 13659
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(21) Sauerbrey, G. The Use of a Quartz Crystal Oscillator for Weighing Thin Layers and Microweighing Applications. Z. Phys. 1959, 155, 206−222. (22) Gabrielli, C.; Perrot, H.; Rose, D. New Frequency/Voltage Convertors for ac-Electrogravimetric Measurements Based on Fast Quartz Crystal Microbalance. Rev. Sci. Instrum. 2007, 78, 074103−1. (23) Kreuer, K. D.; Paddison, S. J.; Spohr, E.; Schuster, M. Transport in Proton Conductors for Fuel-Cell Applications: Simulations, Elementary Reactions, and Phenomenology. Chem. Rev. 2004, 104, 4637−4678. (24) Mauritz, K. A.; Moore, R. B. State of Understanding of Nafion. Chem. Rev. 2004, 104, 4535−4585. (25) Prentice, G. Electrochemical Engineering Principles, Prentice-Hall: Engle-wood Cliffs, NJ, 1991. (26) Siroma, Z.; Ioroi, T.; Fujiwara, N.; Yasuda, K. Proton Conductivity along Interface in Thin Cast of Nafion. Electrochem. Commun. 2002, 4, 143−145. (27) Paul, D. K.; Fraser, A.; Karan, K. Towards the Understanding of Proton Conduction Mechanism in PEMFC Catalyst Layer: Conductivity of Adsorbed Nafion Films. Electrochem. Commun. 2011, 13, 774−777. (28) Eastman, S. A.; Sangcheol, K.; Page, K. A.; Rowe, B. W.; Kang, S.; Soles, C. L. Effect of Confinement on Structure, Water Solubility, and Water Transport in Nafion Thin Films. Macromolecules 2012, 45, 7920−7930.
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