Proton Diffusion Coefficient in Electrospun Hybrid ... - ACS Publications

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Proton Diffusion Coefficient in Electrospun Hybrid Membranes by Electrochemical Impedance Spectroscopy Leslie Dos Santos,†,‡,§ Christel Laberty-Robert,*,† Manuel Maréchal,∥ Hubert Perrot,‡,§ and Ozlem Sel‡,§ †

Downloaded by TEXAS A&M INTL UNIV on September 7, 2015 | http://pubs.acs.org Publication Date (Web): September 4, 2015 | doi: 10.1021/acs.langmuir.5b02171

Sorbonne Université, UPMC Univ. Paris 06, UMR7574, Laboratoire de Chimie de la Matière Condensée de Paris, UPMCUMR7574, Collège de France, 11, place Marcelin Berthelot, 75005 Paris, France ‡ Sorbonne Université, UPMC Univ. Paris 06, UMR 8235, Laboratoire Interfaces et Systèmes Electrochimiques, F-75005, Paris, France § CNRS, UMR 8235, LISE, F-75005, Paris, France ∥ Université Grenoble Alpes, CNRS/CEA-INAC-SPrAM, F-38000 Grenoble, France ABSTRACT: Electrochemical Impedance Spectroscopy (EIS) was, for the first time, used to estimate the global transverse proton diffusion coefficient, DHEHM , in electrospun hybrid + conducting membranes (EHMs). In contrast to conventional impedance spectroscopy, EIS measurements were performed at room temperature with a liquid interface. In this configuration, the measure of the bulk proton transport is influenced by the kinetics of the transfer of proton at the solid/liquid interface. We demonstrated that the use of additives in the process of the membrane impacts the organization of the hydrophilic domains −7 and also the proton transport. The DEHM cm2 H+ is close to 1.10 −1 −7 2 −1 s (± 0.1.10 cm s ) for the EHMs without additive, whereas it is 4.10−6 cm2 s−1 (± 0.4.10−6 cm2 s−1) for EHMs with additives. enhance the proton conductivity.8,9 This paper deals with the evaluation of the proton diffusion coefficient (DEHM H+ ) using specific EIS measurement for electrospun hybrid membranes with different compositions when the membrane is in contact with liquid water. Indeed, in conventional EIS measurements, the material resistance is estimated when the EHM is inserted between two blocking electrodes, and the main contribution is related to the migration effect in this case. Moreover, the kinetics of transfer of the species at the interface are not evaluated.10,11 In our specific configuration, diffusion is mainly due to concentration change effect. This result is important, as a membrane may exhibit fast proton transport in the bulk and poor surface proton conductivity that is detrimental for the fuel cell application.12,13 In our configuration, the kinetics of the transfer of protons in the membrane is considered. In our method, an electroactive intermediate film is required between the working electrode and the EHMs. Gabrielli and Perrot et al. showed by an ac-electrogravimetry study that electroactive polypyrrole thin film doped with heteropolyanions (Ppy− HPA) can specifically exchange protons in an electrolyte (0.5 M HClO4) under polarization.14 Kim et al. has introduced a new methodology to study proton transport properties of classical HM thin films by ac-electrogravimetry in a bilayer configuration using Ppy−HPA as a mediator film.14 This work

1. INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) have attracted great interest for future energy supply due to their high efficiency and low emissions.1,2 An important part in PEMFCs is the membrane where the reference level is a perfluorosulfonic acid polymer, called Nafion. It exhibits excellent chemical and mechanical properties and high proton conductivity under high relative humidity.1 However, Nafion has several drawbacks such as high cost, relatively low operational temperatures (below 100 °C), and high methanol permeability, preventing its commercial viability.3,4 Recently, our group has introduced electrospun hybrid membranes (EHMs) as alternatives to Nafion.5 The interest is the intimate mixing of two components, PVDF-HFP polymer (poly(vinylidene fluoride-hexafluoropropylene) and functionalized silica network, with antagonist chemical properties such as hydrophilicity and hydrophobicity, which recreate the nano segregation between hydrophobic and hydrophilic domains at nanoscale.5 This phase separation is comparable to what exists in Nafion, and it is the key parameter for its high performance when hydrated.6,7 Accordingly, the EHM presents proton conductivity values that are equivalent to, or even slightly higher than that obtained for Nafion at 80 °C and high relative humidity.5 The addition of the polyethylene oxide (PEO) and PVDF-2-hydroxy type additives to the EHM fabrication and their influence over the proton conductivity is still an unresolved issue. It was thought that the additives may influence the proton diffusion coefficient (DEHM H+ ) and therefore © XXXX American Chemical Society

Received: June 15, 2015 Revised: August 24, 2015

A

DOI: 10.1021/acs.langmuir.5b02171 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. (a) Scheme of the PPy−HPA−EHM bilayer. (b) CV of PPy−HPA (black) and PPy−HPA−EHM (red) in HClO4 (0.5 M) between 0.05 and 0.4 V vs SCE. Picture of (c) PPy−HPA (1 × 2.5 cm) and (d) PPy−HPA−EHM on gold substrate deposited. (e) PPy−HPA cross-section and( f) EHM SEM-FEG images.

phenomena in EHMs in a configuration where the proton transfer in the membrane is taken into account. To the best of our knowledge, no information regarding this aspect has been reported previously in the literature. Furthermore, special attention was given to the effect of the additives over the values of the EHMs, as it slightly changes the membrane structure.

highlighted the importance of the insertion phenomena and proton diffusion in the membrane and allowed the DEHM H+ to be estimated as a function of the applied potential. This methodology was also applied to estimate the DEHM in Nafion H+ −6 thin films.15 A DEHM value between 0.97 × 10 to 0.37 × 10−6 + H 2 −1 cm s in the potential range of 0.125−0.350 V versus SCE was estimated. These values agree with pulse-field-gradient NMR (PFG−NMR) and simulation studies.16−18 In this study, the methodology previously reported to study the proton transport in EHM by EIS with a bilayer configuration was modified. In contrast to the previous work,14,15 this processing approach allows the synthesis of membrane with thickness of 30 μm. Unfortunately, finer membranes (below 30 μm) are not homogeneous. Thus, acelectrogravimetry was not used, as the EHMs are too thick for the microbalance studies. The estimation of the macroscopic transverse diffusion coefficient (DEHM H+ ) in EHM is solely reported. The bilayer working electrode is composed of a gold substrate on which Ppy−HPA film is electropolymerized and then EHM is electrospun. The advantage of this bilayer system is to induce proton transfer by potential control. Ppy−HPA is necessary to play a role of “proton pump” through the EHM layer, which indicates that the main force remains the concentration gradient inside the EHM membrane. Therefore, the driving force for proton insertion/expulsion is diffusion. In fact, when a potential is applied, electronic transfer occurs only between the gold/mediator film interface since EHM is only an ionic conductor. Under a cathodic polarization, HPA is reduced. This reaction needs two protons for charge compensation purposes, causing a proton concentration decrease in EHM (eqS 1 and 2).

2. EXPERIMENTAL SECTION 2.1. Ppy−HPA Films. Silicomolybdic acid solution (H4SiMo12O40) and pyrrole monomer were purchased from Aldrich. Gold foils ((1 × 2.5) cm, 0.05 mm thickness, 99.95% purity (Goodfellow)) are used as substrates. A 3.57 mL portion of α-H4SiMo12O40 solution was mixed with 25 mL of water (0.02M), and 16.6 μL of pyrrole was added to this solution. Mediator film was obtained under galvanostatic regime at a current value of 150 μA during 625 s (Figure 1 c). An equivalent film thickness of 300 nm was estimated by FEG-SEM (Figure 1 e). Before the EHM deposition, the mediator film was equilibrated by cyclic voltammetry (CV), between 0.05 and 0.4 V vs SCE in HClO4 (0.5M) at 20 mV s−1. The details of the synthesis are given elsewhere.14 2.2. EHM Solution. Six hundred milligrams of PVDF-HFP (Solvay chemicals) was dissolved in 7 mL of DMF (Sigma-Aldrich). Then two silica precursors (tetraethyl orthosilicate (TEOS) (Sigma-Aldrich and 2-(4-chlorosulfonylphenyl) ethyltrichlorosilane) (CSPTc) (ABCR)) and two additives (340 mg of polyethylene glycol (Mw: 100 × 106 g mol−1) (Sigma-Aldrich) and 312 mg of PVDF-2-hydroxy (Specific Polymers) were added. The inorganic weight percentage of the solution was 55%, and the TEOS/CSPTc molar ratio was 1/2. The resulting solution was stirred for 3 h at 70 °C before electrospinning. This composition gives EHM membranes with IEC values higher (1.56 mmol g−1) than Nafion (0.9 mmol g−1). 2.3. Ppy−HPA−HM Bilayer Film. EHM solution was then electrospun on Ppy−HPA film for 20 min at 0.1 mL/min and at 26 kV, which resulted in 32 μm thick membranes. The bilayer sample was dried at 70 °C during 24 h to enhance the condensation of silica network (Figure 1d,f). PPy−HPA and Ppy−HPA−EHM were characterized by CV. Between 0.05 and 0.4 V vs SCE, two redox peaks were observed due to the oxidation−reduction reactions of HPA (eqs 1 and 2) (Figure 1b). 2.4. Electrochemical Impedance and Conductivity Measurements. A frequency response analyzer (Solartron 1254) and a potentiostat (SOTELEM) were used for EIS measurements. The modified WE was polarized at a given potential and a sinusoidal potential perturbation with a small amplitude (10 mV rms) was superimposed between 65 kHz and 0.01 Hz. The measurements were performed on three different membranes for each composition. Five

SiMo VI12O40 4 − + 2H+ + 2e− ⇆ H 2SiMo VI10Mo V 2O40 4 − (1) H 2SiMo VI10Mo V 2O40 4 −

+

−

+ 2H + 2e ⇆ H4SiMo

VI

Mo 4 O40 4 − V

8

(2)

This modification will be compensated by proton diffusion through the membrane. A schematic illustration of the different reactions occurring at various interfaces is given in Figure 1a. The aim of this work is the introduction of the new experimental methodology for studying proton transport B


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Figure 2. SEM-FEG images of (a) EHM with and (b) without additives. (c) SANS of EHM with (red) and without (blue) additives. (d) Proton conductivity values of EHM with (red) and without (blue) additives and Nafion at 80 °C.

Figure 3. EIS spectra of gold/PPy−HPA/EHM at 270 mV vs SCE: EHM (a) with and (b) without additives. successive EIS measurements were done on each membrane. The evaluated diffusion coefficient corresponds to the average of the 15 EIS measurements. EHM proton conductivities (in-plane-4-electrode) were measured using a BT-512 In−Plane Membrane Conductivity Test System (BekkTech LLC) at 80 °C between 20 and 100% relative humidity (RH). The dry dimensions of the membrane were considered for calculations (thickness = 32 μm). 2.5. Small-Angle Neutron Scattering (SANS). SANS experiments were carried out with the PAXY spectrometer in the Leon Brillouin Laboratory (Saclay, France). Three sample-to-detector distances (SDDs) and neutrons wavelengths (λ) were used to cover magnitudes of the scattering vector modulus q from 0.04 to 4 nm−1 (λ = 12 Å, SDD = 6.75 m; λ = 12 Å, SDD = 3 m and λ = 5 Å, SDD = 1 m). The thickness of the sample was 30 μm, and the sample diameter was about 13 mm.

Q units, demonstrating the condensation of the silica network (not shown here). SANS measurements were performed to study the various interfaces in EHMs at the nanometer scale.19 Additives are suspected to either structure the silica network or to change the chemistry of silica/PVDF−HFP interfaces. For EHM with additives, SANS data reveal on large scattering q, a maximum at ∼2 nm corresponding to an average distance of ∼3 nm. Analogously to previous work,1,20,21 this feature is attributed to the correlation length characteristic of the “ionomer” peak. This correlation distance between sulfonic sites is affected by the presence of additives or not (Figure 2c). This correlation peak moves to higher distance, indicating a smaller distance for the sulfonic acid group in membranes without additives. Due to selective affinity of additives to either hydrophobic or hydrophilic components, additives bring on EHM a certain organization to the inorganic phase. Indeed, the average distance between the sulfonic groups is extended to 6.1 nm, demonstrating that chemical additives mainly structure the functionalized SiO2 network differently. They play the role of surfactant. At low scattering vector q, the q−3.8 power law is the signature of a not-very-well-defined interface between the polymer and the silica components; they are intermingled. This power law is observed for membranes with(out) additives. Accordingly, additives seem to not influence the polymer/ hydrophilic interfaces; they solely influence the microstructure of the silica network.

3. RESULTS AND DISCUSSION EHMs are composed of AN interconnected network of hybrid fibers with “mille-feuilles” morphology. Precisely, the micronsized hybrid fibers consist of an alternation of a dense and hydrophobic perfluorinated phase and a hydrophilic functionalized silica network.5 PEO and PVDF-2-hydroxy are used as additives. Additives influence the phase separation (hydrophilic versus hydrophobic) and the chemistry of interfaces. This does directly impact proton pathways in the EHMs. FEG-SEM analyses of EHMs with(out) additives do not demonstrate structural difference at this scale (submicron scale and micron scale) (Figure 2a,b). 29Si spectra indicate the presence of T and C


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Figure 4. EIS spectra of gold/PPy−HPA/EHM at 270 mV vs SCE with additives in (a) HClO4 and (b) H2SO4.

noted that the DHM H+ is not affected by a modification of the nature of the electrolyte. If a small amount of SO42− and ClO4− anions interact with H+, they may reduce its transport in a small way. This result suggests that the method developed mainly provides information on bulk proton transport. Previous studies combining mass response and EIS on HM elaborated by spray have shown that the response of the counterion was very small, confirming that the diffusion coefficient is mainly due to proton.15 EHMs with additives show DHM H+ values close to those measured for Nafion: 0.97 × 10−6 to 0.37 × 10−6 cm2 s−1,15 but higher than those obtained in the absence of additives. This highlights that additives have an impact on proton transport. The difference in DHM H+ might be interpreted in terms of different bulk proton pathway in the membrane with(out) additives. One possible explanation is the size of the ionic domains that is smaller for membranes without additives according to the SANS study and their connectivity that is less important for membrane without additives, limiting the proton transport. Indeed, the PEO permits controlling the ionic domain size and their connectivity, creating pathways for protons. The proton conductivity of EHMs is further estimated to be 117 mS/cm at RT for fully hydrated membranes taking into estimated and by applying in f irst account the D HHM + approximation the Nernst−Einstein equation (σ = (F2 × DHM H+×CH+)/(R × T)) with CH+ is the concentration of proton in the membrane (the value is estimated to be 0.002 mmol in the volume of the studied membrane), F is the Faraday constant, T is the temperature, and R is the gas constant. The same approach was used for EHM without additives, and conductivity values are very small (3 mS/cm) compared to the experimental ones of 40 mS/cm. However, these results are not directly comparable, as the BekkTech apparatus allows determining in-plane conductivity value. In EIS configuration, bulk conductivity is determined. These comparisons highlight the difficulty in comparing the proton transport in these membranes determined through different approaches as the microstructure of the materials is not fully isotropic.

In-plane conductivity measurements determined by BT-512 were performed at 80 °C and various % RH. At RH higher than 60%, EHMs synthesized with additives have conductivity values higher than EHMs without additives (Figure 2d). Sorption experiments were performed for membranes with(out) additives. They exhibit the same behavior. This indicates that the water content in the membrane is comparable to the RH%. The enhancement in the proton conductivity might then be related to the difference in the organization of the sulfonic function in the silica domains. EHMs synthesized with additives have conductivity values comparable to Nafion for RH ≥ 90%. At RH lower than 80%, the conductivity values are clearly lower than that of Nafion. The observed differences might be due to the hydrophobic character of the EHM compared to Nafion. We then evaluated DEHM for EHM with(out) additives by H+ EIS measurements. To do this, bilayer electrodes of PPy− HPA/EHM were studied by EIS. EIS data of the EHMs, with(out) additives, were measured at 270 mV vs SCE (HPA reduction peak in Figure 1f) in 0.5 M HClO4 (Figure 3). The EIS spectra show a quasi-semicircle at high frequencies characterizing the electrical double layer capacitance, which is followed by straight lines. The medium frequency response corresponds to the proton diffusion, while the low frequency response is related to the ionic transfer at the Ppy−HPA/EHM interface (Figure 1e and Figure 3). The EIS was performed several times14,15 at the same potential and showed reasonable reproducibility. Then, the experimental data were fitted to the theoretical (ΔE/ΔI)(ω) given in eq 3, which allows the estimation of DHHM Details of the theoretical approach are described + . elsewhere.15,16 ⎛ ⎛ ⎞⎞ ⎜ tan⎜ −L jωHM ⎟ ⎟ D H+ ⎠ ⎝ ΔE 1 ⎜ ⎟ (ω) = 1 − M HHPA/HM + ⎟ HPA/HM ⎜ HM ΔI FG H+ + jωD H ⎜⎜ ⎟⎟ ⎝ ⎠ −

K HHPA/HM + jωG HHPA/HM +

(3)

4. OUTLOOK This report presents an original configuration to determine the proton diffusion coefficient (DHM H+ ) by EIS. In contrast to conventional studies, the membrane is in contact with a liquid. To do so, the electrospun membranes were directly deposited onto gold electrodes. Electrospinning was used, as this method allows mixing the intrinsic properties of (in)organic components without creating a microphase separation. The DHM H+ is

The good agreement between the experimental and fitted results is evident for EHMs with(out) additives (Figure 3) in terms of shape and frequency distribution. The DHM H+ is close to 1.10−7 cm2/s at 270 mV vs SCE for an EHM without additive, whereas it is 4.10−6 cm2/s with additives. Before discussing the relevance of these data, similar experiences were performed for EHM with additives in different electrolytes (Figure 4). We D


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determined to be 4.10−6 cm2/s in this EHM, close to the Nafion value, the state of the art. Further, we demonstrated that the developed technique is sensitive enough to measure different DHM H+ . In particular, we found that additives modified DHM H+ because they structure the silica network in different ways. The developed technique can be extended to the estimation of diffusion coefficient of other cations such as Li+ and Na+. This is very important for the devolpment of Li or Na−air battery. In Li−air battery, there is a need to develop new Li-ion condutor to replace the current Li-ion conductor ceramic that protects the Li-metal electrode from water. This protecting layer increased the ohmic resistance and then limited the performances of the system.The EHMs appear to be an interesting candidate as the mixing of the two components (inorganic and organic components) might be a good approach to fulfill all the criteria required to achieve high performance. We are currently extending the method to determine the Li+ diffusion coefficient for EHM developed in the context of aqueous Li−air battery.

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Membrane for Fuel Cells Studied by Current-Sensing Atomic Force Microscopy. Electrochim. Acta 2014, 143, 383−389. (13) Hara, M.; Daiki, H.; Inukai, J.; Hara, M.; Miyatake, K.; Watanabe, M. Reversible/irreversible Increase in Proton-Conductive Areas on Proton-Exchange-Membrane Surface by Applying Voltage Using Current-Sensing Atomic Force Microscope. J. Electroanal. Chem. 2014, 716, 158−163. (14) 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 (38), 13746−13757. (15) Sel, O.; To Thi Kim, L.; Debiemme-Chouvy, C.; Gabrielli, C.; Laberty-Robert, C.; Perrot, H. Determination of the Diffusion Coefficient of Protons in Nafion Thin Films by Ac-Electrogravimetry. Langmuir 2013, 29 (45), 13655−13660. (16) Ohkubo, T.; Kidena, K.; Takimoto, N.; Ohira, A. Molecular Dynamics Simulations of Nafion and Sulfonated Poly Ether Sulfone Membranes II. Dynamic Properties of Water and Hydronium. J. Mol. Model. 2012, 18 (2), 533−540. (17) Devanathan, 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. (18) 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 (5), 1529−1537. (19) Gebel, G.; Diat, O. Neutron and X-Ray Scattering: Suitable Tools for Studying Ionomer Membranes. Fuel Cells 2005, 5 (2), 261− 276. (20) Mosdale, R.; Gebel, G.; Pineri, M. Water Profile Determination in a Running Proton Exchange Membrane Fuel Cell Using SmallAngle Neutron Scattering. J. Membr. Sci. 1996, 118 (2), 269−277. (21) Alkan Gürsel, S.; Gubler, L.; Gupta, B.; Scherer, G. G. Radiation Grafted Membranes. Fuel Cells I 2008, 215, 157−217.

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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REFERENCES

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