In Situ Fabrication of NafionâTitanate Hybrid Electrolytes for High

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In situ Fabrication of Nafion-Titanate Hybrid Electrolytes for High Temperature Direct Ethanol Fuel Cell Bruno Ribeiro Matos, Roberta A. Isidoro, Elisabete Inácio Santiago, Marcelo Linardi, Andre S. Ferlauto, Ana C. Tavares, and Fabio C Fonseca J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp405754v • Publication Date (Web): 25 Jul 2013 Downloaded from http://pubs.acs.org on July 30, 2013

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The Journal of Physical Chemistry

In situ Fabrication of Nafion-Titanate Hybrid Electrolytes for High Temperature Direct Ethanol Fuel Cell. Bruno R. Matos[a], Roberta A. Isidoro[a], Elisabete I. Santiago[a], Marcelo Linardi[a], Andre S. Ferlauto[b], Ana C. Tavares[c]and Fabio C. Fonseca[a]* [a] Instituto de Pesquisas Energéticas e Nucleares. Avenida Prof. Lineu Prestes, 2242. São Paulo, São Paulo, 05508000 (Brazil). [b] Universidade Federal de Minas Gerais, Departamento de Física, UFMG. Avenida Antônio Carlos, 6627, Belo Horizonte, Minas Gerais, 31270901 (Brazil) [c] Institut National de la Recherche Scientifique, Énergie, Matériaux et Télécommunications, INRS-EMT. 1650 Boulevard Lionel-Boulet.Varennes, Québec, J3X 1S2 (Canada) Keywords: Composite materials, Charge transport, Fuel Cells, Titanate Nanotubes. ABSTRACT: The synthesis and characterization of a novel Nafion-matrix nanocomposite by the in situ conversion of titania particles into titanate nanofilaments such as nanotubes and nanorods are reported. Titania nanoparticles grown inside Nafion hydrophilic domains were converted by a microwave-assisted hydrothermal reaction into the proton conducting titanate nanotubes and nanorods. Detailed characterization by Raman spectroscopy, X-ray diffraction, small angle X-ray scattering, and transmission electron microscopy evidenced an intimate interaction between titanate nanostructures and the ionomer hydrophilic phase. The favored localization of such high 1 ACS Paragon Plus Environment


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aspect ratio nanofilaments in the ionic phase of Nafion has a marked impact on the properties of the composites. The experimental data showed enhanced mechanical stability at high temperature (~130 °C) that was correlated to a strong temperature dependence of the proton conductivity in the same temperature range. Such properties contributed to a significant increase of the performance of direct ethanol fuel cells operating at high temperature (~130 °C) using hybrid Nafion-titanate electrolytes compared to commercial Nafion.

Introduction

Hybrid organic-inorganic composites are considered high-performance materials owing to the possibility of combining distinct functional phases to target a desired property.1,2 Nafion-based composites consisting of an ion-conducting matrix and hydrophilic ceramic nanoparticles are an important family of such nanostructured materials.3,4 Nafion-ceramic composite electrolytes are envisioned as a key component for clean and efficient energy conversion of biofuels in high temperature polymer electrolyte fuel cells PEMFCs (ca.130 °C).5,6 The direct use of alcohols is an advance with respect to H2/O2 PEMFCs in which the costs and technical issues associated with hydrogen production, purification, transport, and storage are eliminated.7 Amongst several biofuels, ethanol, with its high energy density, well-established and efficient production from renewable sources and ease of storage and transportation, has been praised as almost the ideal fuel for fuel cells.8,9 The direct ethanol fuel cells (DEFC) can contribute to a more environmentally sustainable model of energy conversion, storage, and portability.7,10 However, DEFC power density is at least two orders of magnitude lower than that of PEMFC using platinum electrocatalysts and requires further development before a practical application.

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In this context, along with the development of suitable electrocatalysts, the search for high performance electrolytes is crucial to further improve the DEFC performance and to push such device towards commercialization.5,11,12,13 Nafion-matrix composite electrolytes were found to increase the performance of direct alcohol fuel cells (DAFCs) operating at low (~40 - 60 °C) and high temperatures (in the 100-200 °C range).11-14 Nonetheless, DAFCs exhibit serious limitations associated with two major hurdles: i) the alcohol crossover across the Nafion membrane and ii) the sluggish ethanol/methanol oxidation reaction. Using Pt supported carbon electrocatalysts, the DEFC power density at 130 °C hardly reaches ~10 mW cm-2.15 The fabrication of hybrid membranes aims at reducing the ethanol permeability of the electrolyte and at sustaining the performance of Nafion at temperatures higher than the α-relaxation (Tα ~110 °C) above which the destabilization of Nafion ion conducting phase is observed.16 In fact, the enhanced performance of direct alcohol fuel cells (DAFC) at high temperature using composite electrolytes has been mostly attributed to a lower alcohol crossover and to their improved mechanical properties.12 Alcohol molecules can easily diffuse through the hydrophilic domains of the ionomer.17,18,19 Therefore, the permeation of alcohol molecules in composite membranes is greatly reduced if the inorganic phase is localized within Nafion hydrophilic phase.20,21 However, the localized growth of nanoparticles inside the Nafion phase-separated structure is critically dependent on the synthesis methodology. The casting methodology is often used for the preparation of Nafion composite membranes containing nanoparticles with diversified composition and morphology;22,23 nevertheless, controlling the particle distribution and localization in a cast polymer matrix is a hard task.24 Alternatively, the Nafion hydrophilic phase has been used as template for the in situ formation of inorganic nanoparticles by sol-gel 3 ACS Paragon Plus Environment


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routes, resulting in hybrid proton conducting membranes such as Nafion-titania, Nafion-silica, and Nafion-alumina.3,4 Although sol-gel composites have finely dispersed nanoparticles inside hydrophilic phase, relatively few studies have reported more controllable techniques to produce hybrid proton conducting membranes with nanostructured inorganic phases having high surface area or high length-to-diameter ratios such as porous frameworks, zeolites, and nanotubes.25,26 One example, is the in situ growth of acid functionalized zeolite framework into Nafion matrix via hydrothermal synthesis demonstrating the possibility of crystallizing intricate architectures within the polymer chains.14 The use of such Nafion-zeolite composite electrolyte resulted in significant increase in the power output of methanol-fueled DAFC as compared to Nafion.14 More recently, hydrogen and sodium titanate nanotubes (TN) have received a great deal of attention due to their facile synthesis and ability to conduct protons (σ ~ 10-2 Scm-1) at high T (~ 130 °C).26-28 The conversion of titania anatase nanoparticles into proton conducting titanate nanotubes by a simple alkaline hydrothermal reaction along with the advantage of using Nafion membranes as hosts to synthesize the precursor anatase nanoparticles 30 prompted us to fabricate Nafion-TN composites. Herein, the in situ transformation of nearly spherical anatase particles inside Nafion hydrophilic domains into titanate nanofilaments (nanotubes and nanorods) via a microwaveassisted alkaline hydrothermal reaction is reported (Figure 1).The nanotube phase should be preferred to anatase, since it has higher proton conductivity, higher specific surface area, higher aspect ratio and high water affinity. Detailed characterization of such composite electrolytes and DEFCs tests at 130 °C evidenced the enhanced properties and the superior performance of the hybrid electrolytes.


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Figure 1. Schematic representation of the synthetic route for the preparation of Nafion-titanate nanotube/nanorods hybrids. The pictorial representation of Nafion’s hydrophilic/hydrophobic interface is not accurate and aims only at illustrating the synthetic process.

Experimental Section Membrane Synthesis The methodology consisted of the controlled hydrolysis of titanium tetraisopropoxide (TIP) embedded in Nafion commercial membranes using a hydrogen peroxide solution.30 During the hydrolysis Ti-peroxy compounds are formed and act as stabilizing agents preventing the particles’ agglomeration. Nafion commercial films were immersed in anhydrous ethanol solvent for 1 h at room temperature to swell and kept inside closed glass bottles. A titanium isopropoxide solution in ethanol (1 mol L-1) was added to the bottle under N2 flow to prevent unwanted hydrolysis. The bottle was closed immediately after pouring the solution, and was kept for 1h at room temperature to allow the diffusion of the titanium precursor into the Nafion film prior to 5 ACS Paragon Plus Environment


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the hydrolysis and condensation of the nanoparticles. Hydrogen peroxide (30%) was then added to the solution with a H2O2:TIP molar ratio of 6:1.The bottles were stoppered and kept in an oven at 75 °C for 12 h. Afterwards, the membranes were removed from the solution and heat treated at 75 °C. The resulting Nafion-titania (N-T) composite membranes were washed and protonated by treatment in sulphuric acid solution at 80 °C.30 For the titanate nanofilaments (N-TN) fabrication, the N-T composite membranes were immersed in a concentrated basic solution (NaOH / 10 mol L-1) and placed inside a Teflon covered stainless steel reactor. The microwave-assisted hydrothermal process was performed by placing the reactor in a microwave oven at 140 °C for 180 min. The resulting Nafion-Titanate membranes in the sodium form (Na2Ti3O7.nH2O ― N-TN/Na) were washed with copious amounts of distilled water to remove the excess of sodium hydroxide. Ion exchange (HCl / 0.1 mol L-1) was carried out to obtain the protonated form of Nafion-Titanate composites (H2Ti3O7.nH2O ― N-TN/H) and subsequently washed with distilled water to remove the excess of chemicals. The relative volume fraction of TN is controlled by the amount of titanium precursor during the synthesis of N-T composites. As an example, the sample with 15 vol% of anatase resulted in 16 vol% of TN after conversion. In this study N-TN composite samples were synthesized with volume fraction ranging between 4 and 16 vol%. Membrane Characterization X-ray diffraction (XRD) measurements were performed with Rigaku-Miniflex II diffractrometer with Cuα radiation (λ = 1.54 Å) in the 2θ range of 5 - 80°. Raman measurements were performed in micro-Raman configuration using a Dilor XY triple monochromator and an Ar laser line (λ=514 nm) for Nafion and composite membranes; or a Delta-Nu system with λ=532 nm for TN/Na powder prepared ex situ. Transmission electron microscopy (TEM) images were recorded using a JEOL-2100F instrument with 200 kV of applied voltage on thin films (< 100 nm) 6 ACS Paragon Plus Environment

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prepared with an ultramicrotome in the plane direction of the film. Water and ethanol uptake measurements were performed by gravimetry at T = 25°C. Small-angle X-ray scattering (SAXS) experiments were carried out using synchrotron radiation at the SAXS beamline of the Brazilian National Synchrotron Light Laboratory (LNLS). Experiments were conducted with an incident wavelength λ = 1.488 Å in the range of the scattering vector q ~ 0.2 - 3.5 nm-1 (q = 4πsinθ/λ, being 2θ the scattering angle). The SAXS patterns of all the samples were collected with MarCCD detector and the intensity curves were corrected for parasitic scattering, integral intensity, and sample absorption. Dynamic mechanical analysis (DMA) was carried out with a TA instrument Q800 series with rectangular samples (15 x 6 mm2), with a heating rate of 3 °C min-1and an amplitude of 4 µm at low frequency (1Hz). The proton conductivity was measured by impedance spectroscopy (IS) using a frequency response analyzer (Solartron 1260) and a homemade Teflon sample holder with stainless steel terminal leads and a K-type thermocouple.31 For IS measurements, samples were sandwiched between two carbon cloth electrodes in order to facilitate the equilibrium with water vapor. The sample holder was inserted in an airtight stainless steel chamber containing distilled water to ensure ~100% relative humidity (RH) during the measurements. Two-probe (through-plane) IS measurements were performed in the 20-130 °C temperature range, over the 0.01 Hz to 30 MHz frequency interval and applied amplitude of 100 mV. Finally, the hybrid membranes were evaluated in a 5 cm2 single fuel cells fed with ethanol solution (1 mol L-1 / 5mL min-1) in the anode and pure oxygen in the cathode with cell temperature and oxygen humidifier heated between 80 °C and 130 °C. The anode was kept at atmospheric pressure (1 atm). The absolute oxygen pressure was 3 atm. The Pt/C and Nafion loadings were 1 mg cm-2 and 30 wt%, respectively, for both anode and



cathode. Polarization curves were obtained in duplicate experiments with estimated error of ~10%.

Results and Discussion Figure 2 shows the XRD patterns of Nafion-titania (N-T) precursor composite, Nafion-titanate composites (N-TN) in both Na+ (N-TN/Na) and H+ (N-TN/H) forms along with data of ex situ prepared titanate nanotubes. The diffraction pattern of N-T exhibits the characteristic haloes associated with Nafion, and the peaks corresponding to the anatase phase (JCPDS 21-1272) with an average crystallite size of ~6 nm as evaluated by the Scherrer equation. Upon the microwave-assisted hydrothermal conversion of the N-T precursor into N-TN composites, peaks

c

TN(020)

TN(110) TN(211)

d

TN(200)

corresponding to the anatase are no longer identified (Figs. 2b and 2c).

N N

10

20

30

A(204)

A(116) A(220) A(215) A(301)

A(105) A(211)

A(103) A(004)

a

A(200)

b A(101)

Intensity

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40 50 60 2θ / degree

70

80

Figure 2. XRD patterns of Nafion-titania (a) and Nafion-titanate nanofilament in the Na+ (b), H+ (c) forms, and ex situ prepared titanate nanotubes (d). “N” indicates the characteristic haloes of Nafion; “A” and “TN” stand for anatase and titanate, respectively. 8 ACS Paragon Plus Environment

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The XRD patterns of both N-TN/Na and N-TN/H display four characteristic peaks at 2θ ~10° (200), 24° (110), 28° (211), and 48° (020) corresponding to the titanate nanotubes, along with the haloes of the polymer matrix.30,32,33 But in the case of N-TN/Na composite, the (200) and (211) reflections have lower relative intensity than in the N-TN/H composite. Such effect was also observed in ex situ prepared NT powders and was attributed to a less ordered structure within the interlayer spacing promoted by the ion exchange.34 It was previously reported that titanate nanosheets (the precursor of nanotubes) and nanorods have XRD patterns similar to nanotubes.35,36 However, the peaks corresponding to the nanosheets are significantly broader than the ones of nanotubes, a feature associated with reduced dimensions (3 x 3 x 1 nm3) of the nanosheets. Similarly, the XRD of nanorods resemble to those of nanotubes except for the presence of an additional peak centered at 2θ ~ 8° 36, which is apparently absent in Fig. 2. The XRD patterns of the in situ converted titanate phase exhibit all the features of ex situ prepared titanate nanotubes (Fig.2d) and are a strong evidence that titanate nanotubes are present in the Nafion-matrix composites.34 Nonetheless, the similarity of XRD patterns with those of other titanate phases, such as nanorods, suggest that such nanostructures possibly coexist in the composite samples. Figure 3 shows the Raman spectra of Nafion, TN powder, and N-TN/Na composite. The Nafion spectrum has four distinct bands centered at ~ 292, 385, 731, and 805 cm-1; the first three bands are mainly associated with bending and stretching vibrations of perfluorocarbon groups (CF2) and the fourth is associated with the C-S symmetric stretching vibration.37,38 The titanate nanotube spectrum has four typical vibration bands located in the ~ 200 - 800 cm-1 range 9 ACS Paragon Plus Environment


assigned to the Ti-O-Na stretching (~ 278 and 670 cm-1) and to the Ti–O–Ti stretching modes in edge-shared TiO6 octahedra (~ 447 and 708 cm-1).39,40

c Intensity

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Ti-O

Ti-O-Na Ti-O

b CF2 CF2

CF2

C-S

a

300 400 500 600 700 800 900 -1 Raman Shift / cm

Figure 3. Raman spectra of Nafion (a), Na+ titanate nanotube (b) and Nafion-titanate nanofilaments composite in the Na+ form (c).

The Raman spectrum of N-TN/Na presents the characteristic bands of both organic and inorganic phases further confirming the coexistence of titanate phase and the Nafion matrix. However, it is not possible to accurately discriminate contributions arising from titanate nanotubes or nanorods. TEM analyses were carried out to follow the morphological changes resulting from the conversion of anatase into titanate in the Nafion-matrix, as shown in Figure 4.


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Figure 4. TEM images of the precursor Nafion-titania (4a), Nafion-titanate nanofilaments in the Na+ (4b and 4c).The insets of Figs. 4a and 4c show, respectively, a lower and a higher magnification detail of the inorganic phase. 11 ACS Paragon Plus Environment


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The images of the N-T precursor composite (Fig.4a) show a fine distribution of a relatively large volume fraction of titania nanoparticles with an average diameter of ~4 - 6 nm, in good agreement with the crystallite size estimated from XRD, and a typical interparticle distance of ~9 - 12 nm. The estimated diameters of anatase particles are compatible with the typical dimensions of Nafion hydrophilic phase (~4 nm), indicating that Nafion serves as host for the in situ growth of the inorganic phase.30,41 It was previously shown that titanate nanotubes consist typically of 3-5 multiple layers tubes with internal and external diameters ranging between 3-5 and

7-10 nm,

respectively,

and

characteristic

length

of

hundreds

of

nanometers

(100-300 nm).29,34 Both N-TN/Na and N-TN/H samples (Figs. 4c) show tubular nanostructures with characteristic dimensions compatible to the ones usually reported for titanate nanotubes in the powder form.34 Low magnification images of N-TN/Na and N-TN/H composites reveal a homogeneous distribution of nanotubes and nanorods in the polymeric matrix. Evidence of open ended tubes is depicted on the inset of the Fig. 4c, which shows higher magnification images of titanate nanotubes. The average diameter of titanate nanofilaments in the polymer matrix is roughly estimated to ~8 nm, in good agreement with previous reported values for titanate nanotube powder.34 It is possible to infer that the average length ~50 nm of the nanofilaments (Na+ form) in the Nafion matrix composites is shorter than the typical one of nanotubes prepared ex situ by the hydrothermal method.34 Such feature is possibly related to constraints imposed by the dimensions of Nafion’s hydrophilic clusters. On the other hand, nanorods produced by hydrothermal alkaline synthesis are obtained only at high temperatures (> ~ 180 °C) and the typical dimension of the nanorods cross section is around ~40 – 120 nm, which is considerably larger than that of the nanostructures found in the TEM micrographs. Nevertheless, TEM images 12 ACS Paragon Plus Environment

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show some filaments in which the characteristic contrast of the walls of nanotubes is not clearly observed, suggesting the formation of nanorods as well (Fig.4b and 4c). Similarly to ex situ synthesized titanate nanotubes, it is likely that a fraction of both non-converted anatase and titanate nanosheets remain in the composite samples. However, the combined results of XRD, Raman, and TEM provide compelling evidence for the successful conversion of anatase into the titanate tubular nanostructures inside the Nafion matrix. Nevertheless, a fraction of such nanotubes seem to have collapsed into the nanorod structure, as indicated by TEM images.28,33 Therefore, additional development of the synthesis methodology is required to increase the anatase/titanate nanotube conversion minimizing the presence of nanorods in the Nafion-matrix composites. Further relevant information concerning the morphology of the hybrid composites was obtained by small angle X-ray scattering (SAXS) analyses. The SAXS data for Nafion and for both N-T and N-TN composites are shown in Figure 5. The 2-D SAXS plot (Fig. 5 inset) of N-TN/H reveals uniform and concentric scattering rings as expected for isotropic composite samples. The SAXS pattern of Nafion reveals two characteristic maxima in accordance with previously reported data:42,43 the ionomer peak, assigned to the correlation of Nafion ionic domains, occurring at q ~2.3 nm-1 and the scattering of Teflon-like crystallites at q ~0.5 nm-1. The SAXS pattern of a cast composite (included for comparison) shows the ionomer peak practically unchanged by the TN/H inorganic phase, and a marked rise of the scattered intensity for q< 0.5 nm-1, which is typical of agglomerated particles. Such features are strong evidences of a weak interaction between the inorganic phase and the conducting phase of the matrix (ionic phase) in the cast samples.



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Figure 5. SAXS patterns of Nafion, N-T and N-TN/H composites. SAXS curve of N-TN/H prepared by casting is shown for comparison. Dashed line represents the fitting of a cylinder form factor. The inset shows the bidimensional SAXS plot of N-TN/H. “C” and “IP” stand for the crystalline and ionic clusters (ionomer peak) correlations for Nafion and “NP” indicates the nanoparticle correlation in the polymer matrix.

The SAXS data of the N-TN/H sol-gel composites displays distinct characteristics and clear differences as compared to that of commercial Nafion. Firstly, the SAXS patterns of composites are dominated by the scattering of the inorganic phase. The ionomer peak is absent in the case of N-TN composite with high filler content (16 vol%) and only a slender shoulder with a q similar to that of Nafion is observed for the less concentrated (4 vol%) N-TN sample. Secondly, the increase in the scattering intensity at q < 0.6 nm-1 for concentrated N-T and N-TN composites indicates the correlation of the inorganic phase domains in the polymer matrix.44 The 14 ACS Paragon Plus Environment

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correlation length of the inorganic particles and the decreased scattering intensity of the ionomer peak with increasing volume fraction of the inorganic phase add further evidence that the ionic domain of the Nafion matrix act as host for the in situ sol-gel synthesis of anatase nanoparticles and for their conversion into TN.3,4 The SAXS data of the N-TN/H (4 vol%) shown in Fig.5 was fitted in the 0.3 - 1.8 nm-1 q range by a form factor of cylindrical particles with ~7 nm diameter and 14 nm length, in excellent agreement with TEM analysis.45 It has been reported that both the swelling and the thermal history of Nafion affect the shape and position of the ionomer peak and result in strong morphological changes in the polymer matrix.46 However, the pronounced scattering of the inorganic phase hinders a more detailed evaluation of the characteristic scattering peaks of Nafion.24 The SAXS patterns of N-TN/H confirm the in situ synthesis of titanate nanofilaments within the hydrophilic phase of Nafion with a fine dispersion of the inorganic phase throughout the polymer matrix. Therefore, such nanotubes and nanorods are expected to have a marked influence on the mechanical and electrical properties of the composites. The temperature dependence of the dissipation factor, tanδ, measured by dynamic mechanical analysis (DMA) is shown in Figure 6. Nafion membranes have a pronounced relaxation process occurring at ~ 110 °C referred to as α-relaxation.16 Such relaxation was previously shown to be dependent on the electrostatic interactions among sulfonic groups and was associated to the long range motion of Nafion main and side chains via destabilization of the ionic interactions.16 The incorporation of inorganic particles into Nafion’s hydrophilic phase via sol-gel route was reported to decrease the intensity and displace the relaxation to higher temperatures.47 The increased Tα of sol-gel hybrids has been related to the interaction between the inorganic phase and the sulfonic groups of Nafion.47 As shown in the Fig. 6, the Nafion 15 ACS Paragon Plus Environment


α-relaxation identified at Tα ~ 110°C was essentially suppressed in the N-T composite. However, for the N-TN composite the α-relaxation was not observed up to ~ 220 °C. As compared to the titania nanoparticles, the marked influence of the TN on the mechanical properties of the ionomer matrix can be attributed to the high specific surface area and the high aspect ratio of the nanofilaments. The lower magnitude of the α-peak in the N-T and N-TN composites with respect to Nafion supports the hypothesis on the interaction between the Nafion ionic phase and the inorganic particles. It is important to mention that the swelling process in ethanol used for the preparation of the composites can substantially change the morphology of Nafion, thus making the direct comparison with swollen Nafion precursor membranes a hard task. However, the comparison of the composites’ electrical and mechanical properties with those of commercial Nafion, the reference electrolyte in numerous reports,4,5 is appropriate to put in evidence the benefits of the in situ synthesis of the titanate phase.

Nafion N-T N-TN/H

log tan δ

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30

60

90 120 150 180 210 T / °C

Figure 6. DMA curves (tanδ) as a function of temperature for commercial Nafion, N-T (15vol%) and N-TN/H (16vol%) composites.


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The temperature dependence of the proton conductivity (σ) of Nafion, N-T, and N-TN composites is shown in Figure 7. Although the proton conductivity of the composites with high volume fraction (≥ 15 vol%) of inorganic phase is lower than that of Nafion, the conversion of anatase into TN increased the conductivity of the composite samples, a feature possibly related to the proton conductivity of the TN. All samples display an Arrhenius-type behavior at low temperature (T< 100 °C) and the calculated activation energy Ea ~0.06 eV for Nafion is comparable to the values previously determined at high relative humidity.48 For both N-T and N-TN composites a similar Ea ~0.09 eV was calculated. Such higher Ea indicates that the inorganic phase affected the proton conduction of the composites as compared to Nafion. Nonetheless, the calculated Ea is within the typical range attributed to the Grotthuss mechanism, which is probably the main mechanism for proton conduction in both N-T and N-TN/H composites. The magnitude of σ is similar for N-TN/H cast and sol-gel samples. But, for T>100 °C the cast sample shows a less pronounced temperature dependence, similarly to Nafion, which further suggests a weaker interaction between the inorganic phase and the polymer matrix in this sample. For T> 100 °C, both sol-gel composites (N-T and N-TN/H) exhibit a rapid increase of σ with increasing T. The results of Fig. 7 indicate that the inorganic phase has a clear effect on the σ(T) behavior, a feature more evident for the sol-gel composites in which a strong interaction between the inorganic nanoparticles and the ionic phase of the matrix takes place. It is interesting to note that the temperature dependence of σ for both sol-gel composites is similar irrespective of the intrinsic transport properties of the inorganic phase (anatase or titanate nanotubes/nanorods). Such a behavior suggests that the proton conductivity of the composites is modulated by the



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matrix and possibly associated with the increased thermal stability of the composites at temperature above the α-relaxation of Nafion (Tα ~110°C), as shown in Fig. 6.49,50

Figure 7. Temperature dependence of the electrical conductivity of Nafion, N-T and N-TN/H composites at RH = 100%. Conductivity data for N-TN/H cast composite are also shown.

The composite samples were investigated as electrolytes in high temperature DEFC. The polarization curves showed in Figure 8a evidence for i < 50 mA cm-2 a large polarization loss attributed to the sluggish ethanol oxidation reaction. However, because the electrodes were the same in all fuel cell tests, the different features reported in Fig. 8 for i> 50 mA cm-2 can be attributed to the investigated electrolytes. The linear dependence of cell voltage on i is mainly due to the ohmic losses within the electrolyte.51 The open circuit voltage (OCV) for the composite electrolytes increased in the order Nafion (~ 530 mV) < N-T < N-TN/Hcast< N-TN/H (~ 660 mV). Such an increase of OCV is consistent with the ethanol uptake of the samples. The measured ethanol uptake for Nafion, N-T and N-TN/H is 82%, 35% and 30%, respectively. The 18 ACS Paragon Plus Environment

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higher OCV of N-TN hybrid is probably related to the high specific surface area and aspect ratio of the nanofilaments, which are more efficient to hinder the ethanol diffusion than nearly spherical titania nanoparticles. The DEFC with N-TN/H hybrid electrolyte has both the highest OCV and the best fuel cell performance (power density) at 130 °C.

Figure 8. Fuel cell polarization curves at 130 °C (a) and power density at 200 mV as a function of the operating temperature (b) for Nafion, N-T (15vol%), N-TN/H prepared by casting 19 ACS Paragon Plus Environment


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(15vol%) and in situ sol-gel (16vol%); with ethanol flux of 5 mL min-1 in the anode and O2 in the cathode.

The temperature dependence of the power density at 200 mV, shown in Figure 8b, reveals that the fuel cell with N-TN/H sol-gel electrolyte is the only one to exhibit a progressive increase of the performance with the temperature, in agreement with the thermomechanical properties and proton conductivity data shown in Figures 6 and 7, respectively. At 130 ºC the power density of Nafion-TN fuel cell (~ 17 mW cm-2) is almost two fold higher than that of the Nafion-based DEFC and the limiting current density of N-NT is similar to DEFC tests using optimized electrocatalysts.15 The enhanced DEFC performance is a balance between the temperature dependence of the proton conductivity, the absence of the α-transition at 110 °C, and the ethanol permeability of the electrolyte at high temperature.12,13 As compared to Nafion, all the composite electrolytes have lower ethanol crossover, as inferred from both the OCV values and the ethanol uptake (Fig.8). The higher conductivity of N-TN compared to the precursor N-T associated with lower ethanol permeability at high operating temperature can be attributed to both the intrinsic properties of the titanate nanotubes/nanorods and to the localization of the inorganic phase in the polymer matrix.

Conclusion A novel approach for the fabrication of Nafion-titanate nanotubes/nanorods composites by an in situ microwave-assisted hydrothermal treatment was demonstrated. Titania anatase homogeneously dispersed into the conducting phase of Nafion was converted in situ to proton conducting titanate nanofilaments, resulting in a hybrid material with improved properties at 20 ACS Paragon Plus Environment

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high temperature. A significant enhancement of ~70% of the maximum power density was demonstrated for direct ethanol fuel cell operating at 130 °C using Nafion-titanate hybrid electrolyte compared to a DEFC using commercial Nafion. Such improvement is associated with an effective interaction between organic and inorganic phases, which resulted in composite electrolytes with enhanced electrical and mechanical properties and reduced ethanol crossover.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ACKNOWLEDGMENT Thanks are due to Prof. M.A. Pimenta and Prof. L.G. Moura (Raman), Prof. J.F.Q. Rey (SAXS), Prof. D.J. Carastan (DMA). Thanks are also dueto the Canadian (NSERC and FQRNT) and Brazilian (CAPES, CNPQ, and CNEN) financial agencies.

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BRIEFS: It is possible to grow proton conducting titanate nanotubes directly inside Nafion ionomer membranes establishing an interface with the hydrophilic conducting phase. Table of Content Image :


In Situ Fabrication of NafionâTitanate Hybrid Electrolytes for High

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