Nafion® and nanocellulose: A partnership for greener polymer electrolyte membranes

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Nafion® and nanocellulose: A partnership for greener polymer electrolyte membranes Tiago D.O. Gadim a , Carla Vilela b , Francisco J.A. Loureiro a , Armando J.D. Silvestre b , Carmen S.R. Freire b,∗ , Filipe M.L. Figueiredo a,∗ a b

CICECO—Aveiro Institute of Materials, Department of Materials and Ceramic Engineering, University of Aveiro, 3810-193 Aveiro, Portugal CICECO—Aveiro Institute of Materials, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal

a r t i c l e

i n f o

Article history: Received 30 October 2015 Received in revised form 11 January 2016 Accepted 13 January 2016 Available online xxx Keywords: Bacterial cellulose Nafion® Nanostructured composite membrane Protonic conductivity Membrane electrode assembly

a b s t r a c t This paper reports the characterization of a Nafion® /bacterial cellulose (BC) nanocomposite prepared by impregnation of a nanofibrillar BC membrane with Nafion® . Such nanocomposite membrane is obtained crack-free and with thickness close to 100 ␮m. Dynamic mechanical analysis reveals a storage modulus more than 1 order of magnitude higher than for pure Nafion® , with BC effectively limiting the viscous flow of the nanocomposite up to 125 ◦ C. The in-plane protonic conductivity is 0.14 S cm−1 at 94 ◦ C and 98% relative humidity, which is lower than for pure Nafion® , but in line with the lower concentration of acid groups of the composite (0.60 vs. 0.92 mmol [H+ ] g−1 for Nafion® ). The application of the Nafion® /BC membrane in an air/hydrogen fuel cell is demonstrated. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Perfluorosulfonic acid-based membranes consisting of a hydrophobic fluorocarbon backbone and hydrophilic sulfonic pendant side chains, e.g., Nafion® from DuPontTM , are the most common proton exchange membranes (PEM) for PEM fuel cells (PEMFC) (Hoogers, 2003). Indeed, these materials present an interesting combination of thermal stability, mechanical and transport properties (Mauritz and Moore, 2004). However, the cost, production difficulties, and environmental problems associated with fluorinated compounds and formation of toxic intermediates are disadvantages (Hickner et al., 2004; Zhang and Shen, 2012) that should be minimized in the materials for the next generation of devices. Research of eco-friendly alternatives to Nafion® is seldom due to the difficulty in finding a material that meets all the requirements for application as a membrane separator in PEMFC, including conductivity, adequate mechanical performance, gas tightness, dimensional and chemical stability against strong acidic and oxidative conditions, and all in a compressed stack at temperatures up to 100 ◦ C (desirably higher than 120 ◦ C) and under highly variable humidity conditions (from dry to saturation).

∗ Corresponding authors. E-mail addresses: [email protected] (C.S.R. Freire), [email protected] (F.M.L. Figueiredo).

Examples of eco-friendly alternatives to Nafion® include materials based on chitosan (Ma and Sahai, 2013), vegetable cellulose (Guilminot et al., 2008; Rooke et al., 2011; Seo et al., 2009) and bacterial cellulose (BC) (Evans et al., 2003; Gadim et al., 2014; Jiang et al., 2012; Lin et al., 2013; Yang et al., 2009). Among them, BC is the most attractive solution for the development of bio-based PEMFC components due to the combination of good thermal stability and mechanical properties, and the ability to be produced in the form of membranes (Klemm et al., 2005). This is demonstrated in our recent report on the preparation of proton conducting membranes of BC and poly(4-styrene sulfonic acid) (PSSA) with high storage modulus and protonic conductivity (Gadim et al., 2014). Jiang et al. (2015) reported the preparation of BC/Nafion® membranes by the incorporation of homogenized BC pulp into a Nafion® dispersion followed by casting. Although the systematic analysis of the conductivity of these membranes under variable temperature and relative humidity was not reported, the performance in both PEMFC and direct methanol fuel cells (DMFC) seems to be satisfactory. Here we follow a different approach to obtain Nafion® /BC nanocomposites. The underlining concept is similar to our previous study on PSSA/BC membranes (Gadim et al., 2014), but instead of promoting the in situ polymerization, the protonic conductor is simply diffused into the BC tridimensional network. In this case, the conducting polymer cannot be soluble in water since there is no physical connection to the BC and, hence, it would naturally

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leach out of the wet membrane. Nafion® is suitable to test this approach because it is insoluble in water, while it will also greatly benefit from the excellent viscoelastic behavior of BC at temperatures above 80 ◦ C. The preparation and detailed characterization of the physico-chemical properties of such Nafion® /BC nanocomposite membranes is reported, and their use in an air/hydrogen fuel cell is demonstrated. 2. Materials and methods 2.1. Chemicals and materials Nafion® perfluorinated resin (Mn 1100, Aldrich) was used as received without any further purification. Other chemicals and solvents were of laboratory grades. Bacterial cellulose (BC) (tridimensional network of nano and microfibrils with 10–200 nm width), in the form of wet membranes, was produced in our laboratory using the Gluconacetobacter sacchari bacterial strain (Trovatti et al., 2011) and following established culture procedures (Hestrin and Schramm, 1954). 2.2. Preparation of pure Nafion® and Nafion® /BC membranes A pure Nafion® membrane was prepared by casting on a petri dish (5 cm in diameter) a commercial Nafion® dispersion, previously dried at 45 ◦ C to evaporate the solvent, and re-dispersed in N,N-dimethylacetamide. The casted membrane was washed with de-ionized water, then with a 3 vol.% aqueous hydrogen peroxide and again with de-ionized water, during 1 h under boiling conditions for all steps. Then, the membrane was protonated in boiling aqueous sulfuric acid (0.5 M) for 2 h, and finally washed with deionized water, being kept in the same media until use. The typical thickness of these membranes in the dry state is slightly in excess of 50 ␮m. The membrane was then re-dispersed in a mixture of ethanol (2.5%), n-propanol (55.0%) and water (42.5%) (24 mL of solvent/400 mg of Nafion® ) at 50 ◦ C during 2 h. This dispersion was poured into an Erlenmeyer containing a drained wet BC membrane (∼400 mg dry weight, 8 cm diameter) and left overnight at room temperature with stirring to facilitate the polymer migration into the BC tridimensional network. The membrane was drained, washed three times with the ethanol/n-propanol/water mixture, and finally dried at 40 ◦ C in a ventilated oven for 12 h and kept in a desiccator. The experiments were made in triplicate. These experimental conditions were selected based on preliminary experiments carried out in order to obtain a composite membrane with the composition of at least 50 wt.% Nafion® and 50 wt.% BC in order to ensure percolation of the ionomer.

Elemental analysis (CHNS) was carried out in a Leco Truspec 2061 series 4555 equipment, using up to 10 mg of sample, a combustion furnace temperature of 1075 ◦ C and an after burner temperature of 850 ◦ C. All samples were crushed, frozen in liquid nitrogen and then lyophilized for 72 h before the analysis of a minimum of 2 replicas per sample. The ion exchange capacity (IEC) of the membranes previously soaked in 0.1 M NaCl aqueous solution for 24 h was estimated according to IEC (mmol g−1 ) = (VNaOH × MNaOH )Wd −1 , where VNaOH is the volume at the equivalence point of a 0.005 M NaOH solution used to titrate the exchanged NaCl solution, MNaOH is the molar concentration of this solution and Wd is the initial weight of the dry membrane. Thermogravimetric analysis (TGA) was carried out with a Shimadzu TGA 50 analyzer equipped with a platinum cell. Samples were heated at a constant rate of 10 ◦ C min−1 from room temperature up to 800 ◦ C under a nitrogen flow of 20 mL min−1 . The thermal decomposition temperature was taken as the onset of approximately 0.5% weight-loss, after the initial evaporation of moisture. The dynamic mechanical analysis (DMA) curves of rectangular pieces of membrane with 30 × 5 mm2 were obtained on a Tritec 2000 DMA (Triton Technologies) operating in tension mode (single strain) at 1 Hz and with 0.005 mm displacement. The temperature was sweep from −30 to 180 ◦ C with a constant heating rate of 2 ◦ C min−1 . Before the measurements, the materials were kept in a conditioning cabinet at 30 ◦ C and 50% relative humidity (RH) for 72 h. 2.4. Protonic conductivity The in-plane protonic conductivity () of the membranes was determined by electrochemical impedance spectroscopy (Agilent 2980A LCR meter) under variable temperature (40–100 ◦ C) and RH (20–98%) conditions in a climatic chamber (ACS Discovery DY110). The measurements were carried out on rectangular pieces of membrane with ca. 1.5 × 0.5 cm2 on which two stripes of silver paste (Agar Scientific) were applied separated by approximately 1 cm. These membranes were placed in a tubular sample holder designed to ensure fully exposure of the membrane surface to the surrounding atmosphere, and providing the necessary electrical contact between the sample and the LCR meter through 4 platinum wires. The impedance spectra were recorded between 20 Hz and 2 MHz with test signal amplitude of 100 mV (in some cases also with 500 mV) and analyzed with the ZView software (Version 2.6b, Scribner Associates) in order to assess the ohmic resistance (R) of the membrane. The conductivity was then calculated using  = L(RA)−1 , where L is the thickness and A the area of the cross section of the dry membrane.

2.3. Characterization methods 2.5. Preparation of membrane electrode assemblies (MEAs) FTIR-ATR spectra were taken with a Perkin Elmer FT-IR System Spectrum BX spectrophotometer equipped with a single horizontal Golden Gate ATR cell, over the range of 600–4000 cm−1 at a resolution of 4 cm−1 averaged over 32 scans. Scanning electron microscopy (SEM) images of the membrane surface and cross-section were obtained on a HR-FESEM SU-70 Hitachi microscope equipped with an EDS Bruker QUANTAX 400 detector operating at 4 kV. The samples were previously coated with a carbon film. The X-ray diffraction (XRD) analysis was carried out on a Phillips X’pert MPD diffractometer using Cu K␣ radiation ( = 1.541 Å) with a scan rate of 0.05 ◦ s−1 in 2 scale. The patterns were collected in reflection mode with the membranes placed on a Si wafer to provide mechanical support and to avoid the bending of the membrane.

A commercial Pt:C electro catalyst powder (nominally 40% on carbon black, HiSPEC 4000, Johnson Matthey) was mixed with Nafion® ionomer suspension (1100 EW 20 wt.%, D2021, Ion Power) and isopropanol in 1:3:4.5 weight proportions, respectively. This suspension was ultra-sonicated for 30 min and sprayed directly onto a gas diffusion layer (GDL, Sigracet 34BC, SGL Group) using a nitrogen spray gun. The Pt loading was adjusted to 1 mg cm−2 by multiple spraying steps with intermediate drying and weighting. After drying overnight in air, a small amount of Nafion® dispersion was brushed on the catalyzed surface of the GDL. Two 2.7 × 2.7 cm2 pieces of the catalyzed GDL were then placed aligned on both sides of a membrane, thus forming the membrane electrode assembly (MEA). The MEA was compressed in situ at 0.4 N m−1 between the interconnector plates (interdigitated flow field design, Pragma

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Fig. 1. Snapshot of a) dry BC membrane, b) dry Nafion® /BC nanocomposite membrane, c) Nafion® /BC nanocomposite MEA on top of the bipolar plate and d) Nafion® /BC MEA after the fuel cell test (the tear in the membrane was formed when removing the cell from the contact with the Viton o’ring).

Fig. 2. FTIR-ATR spectra of Nafion® /BC membrane and of the corresponding individual components.

Industries) of the fuel cell setup, and heated up to 80 ◦ C for 1 h to consolidate the assembly before the measurements.

ductivity, was performed. Moreover, the membrane was assembled with electrodes and tested in hydrogen/air fuel cell configuration.

2.6. Single cell tests

3.1. Compositional, structural and morphological characterization

Prior to test the cell performance, a run-in stage was carried out under atmospheric pressure at room temperature by feeding a humidified flow rate of both H2 (25 sccm) and air at 0.6 V, during 2 h. The performance of the single cell was evaluated through current potential curves measured in potentiostactic mode (PAR VersStat 4). The cell resistance was investigated during the polarization by electrochemical impedance spectroscopy (EIS, PAR VersStat 4) between 10 mHz and 1 MHz with an applied AC amplitude of 10–50 mV.

3. Results and discussion A Nafion® /BC nanocomposite membrane was obtained by a simple and fast procedure, viz. the diffusion of Nafion® into the BC tridimensional network of cellulose fibrils. The ensuing membrane was homogeneous, indicating a good dispersion of the ionomer inside the BC network and was clearly more translucent than a pristine BC membrane, as illustrated by comparison of Fig. 1a and b. A thorough characterization in terms of composition, structure, morphology, thermal and viscoelastic properties, and protonic con-

According to the elemental analysis, the Nafion® /BC nanocomposite membrane presents a composition of 50 wt.% of Nafion® and 50 wt.% of BC with a sulfur content of 0.50 mmol g−1 , which agrees with the nominal composition (Nafion® :BC = 1:1). The IEC obtained for the pristine Nafion® membrane (0.92 mmol [H+ ] g−1 ) is in agreement with values reported elsewhere (DuPontTM , 2015; Rosero-Navarro et al., 2014a,b). In the case of the nanocomposite membrane, its IEC value of 0.60 mmol [H+ ] g−1 is lower than pure Nafion® , but is in tune with the lower concentration of acid groups of the composite. Fig. 2 shows the infrared spectra of Nafion® , pristine BC and Nafion® /BC membrane. The FTIR-ATR spectrum of the nanocomposite is consistent with the expected chemical structure and composition due to the presence of the characteristic absorption bands of BC at 3200–3400 cm−1 (O H stretching), 2900 cm−1 (C H stretching), 1310 cm−1 (O H bending) and 1030 cm−1 (C O stretching) (Pecoraro et al., 2008), together with those of Nafion® at about 1200 cm−1 (CF2 and SO3 − asymmetric stretching), 1142 cm−1 (CF2 symmetric stretching), and 1056 cm−1 (SO3 − symmetric stretching) (Gruger et al., 2001; Jiang et al., 2015; Kunimatsu et al.,

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Fig. 3. X-ray diffractograms of Nafion® /BC membrane and of the corresponding individual components.

2010). This points out that the incorporation of Nafion® into the BC membrane was indeed successfully accomplished. The effect of the introduction of Nafion® inside the BC network on the crystallinity of the obtained membrane was assessed by X-ray diffraction. According to Fig. 3, the XRD pattern of the perfluorosulfonyl fluoride copolymer is characteristic of an amorphous material with a broad diffraction peak at about 2 18◦ (Xu et al., 2005), whereas BC exhibited a diffractogram typical of cellulose I (native cellulose) (Hon, 1996), with main diffraction peaks at 2 14.3◦ , 15.9◦ and 22.6◦ . As expected, the pattern of the nanocomposite membrane combines the features of both components. Still, the crystallinity of BC itself seems to be mostly retained given the rather sharp diffraction peaks, which is a good indication that BC is not degraded by the presence of the acidic Nafion® . This is fundamental to provide the composite Nafion® /BC with the superior mechanical properties of BC, as will be later discussed. The surface morphology of the nanocomposite membrane was investigated by SEM and EDX, with the results illustrated in Fig. 4. The micrographs provide evidence of the homogeneous distribution of both phases in the composite with Nafion® totally filling the characteristic nanofibrillar network of BC. This was further corroborated by the EDX mapping of sulfur showing that the elemental distribution is homogeneous, as observed in Fig. 4c) for the crosssection of the membrane. 3.2. Thermal stability and viscoelastic properties The thermal degradation profile of Nafion® (Fig. 5) follows a three-step pathway including water evaporation below 100 ◦ C, decomposition of the sulfonic groups in the range of 290–380 ◦ C and the oxidative destruction of the perfluorinated matrix from 400 ◦ C to 580 ◦ C (Lage et al., 2004; Surowiec and Bogoczek, 1988). The pristine BC, in addition to the dehydration at ca. 100 ◦ C (loss of

about 7 wt.%), has a typical single step degradation profile at 348 ◦ C, where the sample loses about 70% of the weight as the result of the pyrolysis of the cellulose skeleton (Wang et al., 2007), leaving a residue (at 800 ◦ C) corresponding to about 13% of the initial mass. Besides the evaporation of adsorbed water below 100 ◦ C, the TGA tracing of the nanocomposite membrane shows three weightloss steps with maximum decomposition temperatures at about 160, 345 and 485 ◦ C. The loss is about 15 wt.% in the first stage (from 125 to 195 ◦ C), which is similar to the loss expected due to the decomposition of the BC assuming a Nafion® /BC of 50:50 weight ratio. The apparent degradation of BC at lower temperatures when in the composite might result from the catalytic effect of the sulfonic groups of Nafion® , case analogous to the effect of sulfuric acid documented elsewhere (Roman and Winter, 2004). The other two stages occur in the same temperature range observed for Nafion® , thus suggesting the same origin. The second stage between 295 and 375 ◦ C with a weight loss of ∼13% is associated with the loss of the sulfonic groups of Nafion® , and the third weight loss (∼40% from 400 to 580 ◦ C) is due to the oxidative destruction of the perfluorinated matrix (Surowiec and Bogoczek, 1988). The ratio of the residues corresponding to each individual component indicates 51 wt.% of BC in the composite, which is in excellent agreement with that estimated from the elemental analysis. Although the composite starts to decompose in N2 atmosphere at temperatures lower than pure BC or Nafion® , it is still thermally stable under the standard operating temperature of PEMFC (<100 ◦ C). Dynamic mechanical analysis (DMA) was performed to assess the viscoelastic properties of the nanocomposite membrane, as typified in Fig. 6. For neat Nafion® the storage modulus (E’) decreases with increasing temperature up to 30 ◦ C, then with a severe decay from 70 ◦ C up to ∼140 ◦ C (Fig. 6, top). This is accompanied by a maximum of the elastic loss at about 115 ◦ C (Fig. 6, bottom) assigned to the glass ␣-transition temperature and a minimum at about 20 ◦ C allocated to the glass ␤-transition temperature (Cele and Ray, 2009). For the pristine BC membrane, the variation of E and loss tangent (tan ı) as a function of temperature display a broad phase relaxation between −20 and 40 ◦ C (Fig. 6), ascribed to the release of water molecules, known to act as plasticizers (Lacerda et al., 2013). The Nafion® /BC storage modulus is more than 1 order of magnitude higher than for pure Nafion® throughout the entire temperature range, confirming the role of the BC network in reducing the membrane deformation upon heating. Worth noting is the fact that, even though E’ drops from 3.4 GPa at –30 ◦ C to 10 MPa at 180 ◦ C, the composite membrane still maintains a satisfactory mechanical integrity. The slight change in E’ observed between 0 and 30 ◦ C is paired to one tan ı peak in the same temperature range, which might be associated with the plasticizing effect of water on BC. Moreover, a broad peak from 82 to 173 ◦ C is also visible with a steep drop at 125 ◦ C which is not observed for either BC or Nafion® (Fig. 6). This has an obvious correlation with the weight-loss observed at the same temperature by thermogravimetry (Fig. 5), indicating a partial degradation of BC macromolecules possibly caused by the acidic character of the sulfonic acid groups of Nafion® (Roman and Winter, 2004). The comparison of these data with the results reported by Jiang et al. (2015) points out a similar trend for E’ and tan ı. Nonetheless, the present membrane presents higher E’ values, viz. better viscoelastic properties, which are associated with the direct use of BC as membrane without being homogenized in a food blender and then casted, as proposed by Jiang et al. (2015).

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Fig. 4. SEM micrographs of Nafion® /BC nanocomposite membrane showing: a) surface and b) cross-section views, and c) EDX mapping.

3.3. Protonic conductivity and fuel cell test Fig. 7 displays the Arrhenius plot of the conductivity of the Nafion® /BC nanocomposite membrane measured under variable temperature (40–100 ◦ C) and RH (30–98%) conditions. It is immediately apparent that the in-plane conductivity is strongly dependent on RH, and less on temperature. For instance, an augment of RH from 30 to 98% at 94 ◦ C increases the conductivity by 2 orders of magnitude (from 0.0014 to 0.14 S cm−1 , respectively), while increasing the temperature from 40 to 94 ◦ C at 60% RH increases the conductivity by less than 1 order of magnitude (from 0.0052 to 0.011 S cm−1 , respectively). Data from Jiang et al. (2015) for an equivalent membrane with BC:Nafion® of 1:1 in weight is lower, thus suggesting better percolation of Nafion® in the membranes prepared in this work. Although the conductivity of the Nafion® /BC membrane is lower than for pure Nafion® , it is in tune with the lower concentration of acid groups in the composite, and still within the limits necessary for application. The activation energy of 10–20 kJ mol−1 is similar to the values usually reported for Nafion® , thus confirming that proton transport in the composite indeed occurs through the Nafion® . It is worth noting that after the electrical measurements at 94 ◦ C with 98% RH in the presence of metallic silver electrodes, the membranes preserved the mechanical integrity and displayed no signs of chemical modification. This is a relevant feature for PEMFC application, in view of the demanding strong oxidizing conditions resulting from the combination of high humidity and high acid concentration, further catalyzed by the platinum catalyst nanoparticles of the electrode. The Nafion® /BC MEA was assembled using conventional Pt/C electrodes on carbon GDLs, as displayed in Fig. 1c. The power curve obtained for such MEA at room temperature under humidified hydrogen-air gradients is shown in Fig. 8. The open-circuit voltage (OCV) reached 0.9 V, which confirms the gas tightness of the membrane. The maximum power density was 16 mW cm−2 at a current density of 40 mA cm−2 . These values are 5–6 times lower

Fig. 5. Thermograms of Nafion® /BC composite and of the corresponding individual components.

than the ones reported by Jiang et al. (2015) for a BC/Nafion® (1:7) membrane. The difference is most likely due to the poor electrode performance of our cell, as suggested by the drop in OCV for small current densities. It could also be due to the lower conductivity of our membrane, which has lower ionomer content. In fact, we measured (by AC impedance) 0.04 S cm−1 at 40 ◦ C/98% RH for our BC:Nafion® = 1:1 composite, while Jiang et al. (2015) measured 0.06 S cm−1 at 30 ◦ C/100% RH for a BC:Nafion® = 1:7 sample. Differences can actually be higher since we measured in-plane conductivity and those authors measured the through-plane component, which is usually lower for Nafion® . Possible degradation of the membrane during the fuel cell tests should be ruled out since the membrane kept the properties after the tests, including transparency and mechanical integrity, as displayed in Fig. 1d. Alternatively, one may compare the DC conductivity estimated from the linear range of the I–V curves. This is approximately 1.3 × 10−3 S cm−1 from our data and 3.7 × 10−3 S cm−1 for the

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Fig. 8. Polarization and power curves of a Nafion® /BC-based MEA collected at room temperature under humidified hydrogen/air gradient.

4. Conclusions

Fig. 6. Storage modulus (E , top) and loss tangent (tan ı, bottom) as a function of temperature for the Nafion® /BC composite and the corresponding individual components. The inset is a zoomed view of BC storage modulus.

The present work aimed to explore the possible partnership between Nafion® and BC for the production of proton conducting membranes. A study was carried out to assess the thermal stability, viscoelastic properties and protonic conductivity of Nafion® /BC nanocomposite membranes. The simple diffusion of a dispersion of Nafion® into the BC tridimensional network proved to be an easy and rapid approach to prepare homogeneous and translucent membranes with good thermal and viscoelastic properties. Conversely, their protonic conductivity was lower than for pure Nafion® , but in line with the equally lower concentration of acid groups. Moreover, the application of these membranes in a lab-scale fuel cell was successfully accomplished, demonstrating the effectiveness of the proposed processing route. Acknowledgments This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER007679 (FCT Ref. UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when appropriate cofinanced by FEDER under the PT2020 Partnership Agreement, CelFuelCel (EXPL/CTM-ENE/0548/2012), HyPEM (PTDC/CTMCER/109843/2009). The Portuguese Foundation for Science and Technology (FCT) is also acknowledged for a post-doctoral grant (SFRH/BPD/84168/2012) to Carla Vilela, contract under Investigador FCT 2012 (IF/01407/2012) to Carmen S. R. Freire and contract under Investigador FCT 2013 (IF/01174/2013) to Filipe M. L. Figueiredo. Thermal Analysis Laboratory was funded by FEDER Funds through Programa Operacional Factores de Competitividade—COMPETE and by National Funds through FCT under the project REEQ/515/CTM/2005.

Fig. 7. Arrhenius plot of the in-plane conductivity of Nafion® /BC membrane at relative humidity of 30, 60, 80 and 98% (increasing on the direction of the arrows). The straight lines are linear fits to the Arrhenius model.

BC:Nafion® = 1:7 membrane of Jiang et al. (2015), both values being about 1 order of magnitude lower than the corresponding AC estimates. This still cannot fully explain the somewhat low fuel cell performance, thus confirming the electrode polarization also as a limiting factor of the power output of our cell. Nevertheless, the present result demonstrates the functional properties of the Nafion® /BC nanocomposite membrane for fuel cells or related technologies where a proton separator is needed.

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Please cite this article in press as: Gadim, T.D.O., et al., Nafion® and nanocellulose: A partnership for greener polymer electrolyte membranes. Ind. Crops Prod. (2016), http://dx.doi.org/10.1016/j.indcrop.2016.01.028