ZrO2 Nanofibrous

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Phosphate modified TiO2/ZrO2 nanofibrous web composite membrane for enhanced performance and durability of high temperature PEM fuel cells Chanmin Lee, Jeong Ho Park, Yukwon Jeon, Joo-Il Park, Hisahiro Einaga, Yen B. Truong, Ilias Louis Kyratzis, Isao Mochida, Jonghyun Choi, and Yong Gun Shul Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00941 • Publication Date (Web): 14 Jun 2017 Downloaded from http://pubs.acs.org on June 18, 2017

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Phosphate modified TiO2/ZrO2 nanofibrous web composite membrane for enhanced performance and durability of high temperature PEM fuel cells Chanmin Leea,b†, Jeongho Parkc†, Yukwon Jeona, Joo-Il Parkd, Hisahiro Einagae, Yen B. Truongf, Illias L. Kyratzisf, Isao Mochidag, Jonghyun Choih*, Yong-Gun Shula,c*

a

Department of Chemical and Biomolecular Engineering, Yonsei University, Yonsei-ro

50, Seodaemun-gu, Seoul 120-749 Republic of Korea b

Center of Advanced Instrumental Analysis, Kyushu University, Kasuga, Fukuoka, 816-8580,

Japan c

Graduate Program in New Energy and Battery Engineering, Yonsei University, Yonsei

-ro 50, Seodaemun-gu, Seoul 120-749 Republic of Korea d

Petroleum Research Center, Kuwait Institute for Scientific Research (KISR), P.O. Box

24885 Safat, 13109 Kuwait e

Department of Energy and Material Sciences, Kyushu University, Kasuga, Fukuoka 816-

8580 Japan f

Commonwealth Scientific and Industrial Research Organisation (CSIRO), Bayview Av

e, Clayton, Victoria 3168 Australia g

Kyushu Environmental Evaluation Association, Matsukadai, Fukuoka 813-0004, Japan

h

The New Zealand Institute for Plant and Food Research, 3 Bisley Rd, Hamilton, W

aikato 3214 New Zealand

ACS Paragon Plus Environment

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Corresponding authors, Emails: [email protected] (J.C.) and [email protected] (Y.G.S.) † These authors have contributed equally to this work.

Keywords: TiO2/ZrO2, nanofiber, phosphoric acid, electrospinning, durability, fuel cell

Abstract Aquivion®/titanium zirconium oxide nanofibrous web composite membrane was prepared and tested as a proton exchange membrane in hydrogen/air fuel cell. The incorporation of a small dose (9 wt.% of membrane) of uniformly distributed electrospun titanium zirconium oxide (TiO2/ZrO2, Ti : Zr = 1 : 1 atomic ratio) nanofibrous web significantly improved hydromechanical stability of the composite membranes which exhibited approximately 2 times higher water retention and 30 times lower dimensional change than pristine Aquivion membrane under in-water membrane hydration condition. Phosphate functionalities were successfully added onto the nanofiber surface as confirmed by XPS analysis. The added phosphate functionality resulted in higher proton conductivity of the prepared composite membrane compared to the non-modified TiO2/ZrO2 nanofibrous web composite membrane (e.g., 0.027 S cm-1 vs. 0.021 S cm-1 at 120 °C and 40 % RH). A single cell test also showed the effect of added TiO2/ZrO2 nanofibrous web. A single cell with Aquivion/TiO2/ZrO2 nanofibrous web composite membrane outperformed a single cell with pristine Aquivion membrane in fully humidified conditions (100 % RH at 75 °C and 90 °C). The Aquivion/phosphate modified TiO2/ZrO2 nanofibrous web composite membrane showed the best single cell performance at all four testing conditions including the fully humidified


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medium temperature conditions (e.g., Pmax = 1.18 W cm-2 at 75 °C and 100% RH and Pmax = 0.97 W cm-2 at 90 °C and 100% RH) and partially humidified high temperature conditions (Pmax = 0.45 W cm-2 at 120 °C and 40% RH and Pmax = 0.21 W cm-2 at 140 °C and 20 % RH). The composite membrane also displayed the excellent durability evidenced by the accelerated life time (ALT) test results. Overall, the phosphate modified TiO2/ZrO2 nanofibrous web composite membrane enhanced the electrical properties and durability of the fuel cell especially at high temperatures (> 120 °C).

1. Introduction Proton exchange membrane (or polymer electrolyte membrane) fuel cells (PEMFCs) are one of the most promising energy conversion devices that could replace fossil fuel internal combustion engines (IECs). Their simple system design, environmentally-friendly operation and high energy efficiency (approximately 60%) make PEMFCs attractive for use in automotive applications [1]. However, there are still technical and manufacturing costreduction challenges that hinder the complete practical use of PEMFC systems. One of the most obvious and significant technical challenges is developing membranes for high temperature operation. Among the three major components (i.e., anode, membrane and cathode) of PEMFC systems, the membrane is the component which acts as a selective proton transport medium from the anode to the cathode. Thus, a key property of the membrane is high proton conductivity (under a wide range of temperature and humidity conditions). Currently, perfluorosulfonic acid (PFSA) ionomers (e.g., Nafion®) are commonly used in PEMFCs as polymer electrolyte membranes because of their reliable proton conductivity and mechanical stability especially under medium temperature (up to 80 °C) and humid conditions [2]. However, at high temperatures and low humidity conditions (e.g., T >>


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80 °C and RH < 100% RH), the PFSA membranes often unsatisfactorily perform particularly due to weakened mechanical stability and membrane dehydration which causes a significant loss in proton conductivity led to substantial reduction of power generated by the PEMFC system [3]. The ability to operate PEMFCs under higher temperature and lower relative humidity conditions is beneficial as it could resolve the carbon monoxide (CO) poisoning of catalysts from impurities of H2 fuel and relieve water flooding problems [4]. In the development of membranes that can effectively operate under higher temperature and lower humidity conditions, researchers have suggested organic-inorganic composite membranes with inorganic particulate additives including titania [5-7], zirconium phosphate [8-11], zeolite [12], and silica [5,13]. Some of the composite membranes with particulate additives have shown improved fuel cell performance at high temperature and low humidity because of improved hygroscopic and thermal stability [9,14]. Nevertheless, in practice, composite film casting by solvent evaporation or extrusion is often challenging mainly due to particle agglutination [15]. Over the last decade, ceramic nanofibers (generally in the form of non-woven fiber webs) prepared via electrospinning followed by thermal treatments have been widely reported [1617]. Such electrospun ceramic nanofibers have been applied to areas such as catalysts, sensors, filter membranes and electrode materials for energy conversion devices (e.g., photovoltaic cells, batteries and fuel cells) in the R&D level [17]. Inorganic nanofibrous porous webs have also been embedded in polymeric matrices such as sulfated ZrO2 fibers in Nafion matrix [18] and SiO2/sulfonated poly ether ether ketone composite fibers in Nafion matrix [19]. The uniformly dispersed 3-dimensional foam-like inorganic nanofibrous web completely resolved the agglomeration issue of particulate additives. The aspect ratio of the fibers was extremely high thereby providing additional mechanical strength to the composite


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membrane that could be unlikely achieved by particulate additives. Proton conduction is governed by two mechanisms: vehicular mechanism and the Grotthuss (or hopping) mechanism [15]. In general, under fully humidified conditions, protons form complex molecules with water molecules such as H3O+, H5O2+, and H9O4+ and they are transferred through water channels in hydrated membranes (vehicular mechanism) [20]. On the other hand, especially at high temperature and partially or non-humidified conditions, hydrogen bonding sites of phosphoric acid acting as proton accepting media promotes proton transport by the Grotthus (or hopping) mechanism [21-23]. For this reason, phosphoric acid has been incorporated into proton conducting membranes mainly for high temperature and low humidity applications. Phosphoric acid doped polybenzimidazole (PBI) systems have been widely studied [24,25] and the acid doped PBI membranes in general exhibited reasonable conductivity under dry conditions attributed to Grotthus mechanism of proton conduction by using phosphoric acid medium [26-28]. In this study, we prepared inorganic (TiO2/ZrO2) nanofibrous web supported polymer electrolyte membrane to increase operating temperature and durability of the pristine polymer electrolyte membrane. Highly conductive Aquivion PFSA (commercially available) was selected and used as the polymer electrolyte matrix mainly due to its better performance compared with Nafion PFSA above 110 °C [29]. In order to improve proton transport through the membrane as well as mechanical strength of the membrane, the surface of electrospun TiO2/ZrO2 nanofibers was functionalized by phosphoric acid. The phosphate modified TiO2/ZrO2 nanofibrous web was impregnated with Aquivion to turn the mat into a dense membrane. The actual fuel cell performance and durability (accelerated life time test) of membrane electrode assembly (MEA) with the phosphate modified TiO2/ZrO2 nanofibrous web/Aquivion composite membrane was evaluated and compared with un-modified TiO2/ZrO2 nanofibrous web/Aquivion composite and pristine Aquivion membranes.


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2. Experimental 2.1 Materials Polyvinylpyrrolidone (PVP, MW=1,300,000), titanium (IV) isopropoxide (TiP, 97% in isopropanol), zirconium (IV) propoxide (ZrP, 70% in n-propanol), ethanol (99.9%), acetic acid (99%) and phosphoric acid (85%) were purchased from Sigma-Aldrich. Ethanol and acetic acid were used as a solvent and an additive in TiO2/ZrO2 precursor solution, respectively. The Aquivion® dispersion (E83-24B, 810 – 850 g/eq.) was obtained from Solvay Plastics and used in the preparation of composite and pristine Aquivion membranes.

2.2 Preparation of TiO2/ZrO2 nanofibrous web The metal oxide precursor solution for electrospinning was prepared in the following manner. 0.83 g PVP was added to 7 g ethanol. The mixture was stirred for 1 h to completely dissolve PVP in ethanol. To the PVP/ethanol solution, 0.35 g acetic acid, 0.92 g TiP and 0.88 g ZrP were added (1:1 molar ratio of TiP and ZrP) and vigorously stirred at room temperature for 5 min. The solution was then electrospun under the following conditions: (i) 14 kV applied voltage, (ii) 15 cm spinneret tip-to-collector distance, (iii) 30 µL min-1 flow rate, (iv) spinneret (30G gauge number, 0.15 mm inner diameter) and (v) at 20 °C and 25% RH. Collected nanofibrous webs were calcined in an air atmosphere at 500, 700 or 1000 °C for 6 h with 1 °C min-1 heating rate.

2.3 Surface modification of TiO2/ZrO2 nanofibers After the calcination, TiO2/ZrO2 nanofibrous web was immersed in 30 mL of 85 %


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phosphoric acid for 24 h at room temperature. The modified nanofibrous web was rinsed with deionized water multiple times until the rinsed water reached pH 6 to completely remove any un-reacted phosphoric acid. Phosphate modified TiO2/ZrO2 nanofibrous web was dried in a convection oven at 80 °C for 12 h.

2.4 Membrane preparation The 24 wt% Aquivion dispersion was diluted to 12 wt% by adding an additional amount of the mixed solvent (n-propanol, isopropanol and water (1:1:0.1 molar ratio)). Dried porous nanofibrous web was immersed in 12 wt% Aquivion dispersion: 9 mg of 12 wt% Aquivion dispersion per 1 cm2 nanofibrous web. The impregnated membrane was dried in the oven at 60 °C for 12 h and then further dried in the oven at 80 °C for 12 h to remove residual solvent. The thickness of the nanofibrous web before impregnation was 45 µm and 55 – 60 µm after impregnation.

2.5 Nanofibrous web and composite membrane characterization The fiber morphology and fiber diameter of electrospun, calcined and phosphate modified TiO2/ZrO2 nanofibrous webs; and the impregnated polymer composite membrane (surface and cross-section) were observed by using a scanning electron microscope (JSM-6701F, JEOL). The surface area (BET model) and pore diameter (BJH model) were measured using nitrogen adsorption/desorption method with Belsorp II (BEL Japan, Inc.). Prior to the Nitrogen adsorption/desorption experiment, the sample was degassed under vacuum at 100 °C for 3 h. X-ray crystal structure of the TiO2/ZrO2 nanofibers was examined by using xray diffraction (XRD; Miniflex, Rigaku, Japan) at a rate of 2 θ min-1. Thermogravimetric analysis (TGA) of nanofibrous web was performed on TGA/DSC1 system (Mettler Toledo)


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from 30 °C to 1000 °C with heating rate of 5 °C min-1 under an air atmosphere. The X-ray photoelectron spectroscopy (XPS) analysis was performed by a K-alpha spectrometer (Thermo Scientific) with a monochromated Al Kα X-ray source (Al Kα line at 1486.6 eV) at a power of 36 W (12 kV, 3 mA). The pressure in the vacuum chamber during analysis was 4.9 × 10-9 mbar. Once existing elements were identified from the survey spectra, high resolution spectra were obtained from individual peaks at 50 eV pass energy with a resolution of 0.78 eV. The width of analyzed sample was 400 µm. The binding energies were calibrated based on C 1s peak at 284.8 eV to minimize charge effect. The spectra were fitted via the Fityk software using the Shirley background subtraction and Gaussian function options. Gravimetric and areal change of membranes upon water uptake was calculated from the measured values of the dry and wet membranes. Equilibrium wet weight and area of membranes were measured after 24 hr soaking in water at 25 °C and 80 °C, respectively.

2.6 Membrane electrode assembly (MEA) preparation and Single cell performance test A catalyst slurry was prepared by mixing 40 wt% Pt/C (Johnson Matthey) with 24 wt% Aquivion dispersion, DI water and isopropanol (1:1.6:4:12 weight ratio). The Aquivion was used for a binder of the catalyst layer. The catalyst slurry was sprayed on both sides of the membrane using the catalyst-coated membrane (CCM) method [30]. The Pt loading was 0.4 mg cm-2 on both sides of membrane. The single cell stack was assembled with end plates, collector plates, graphite plates, Teflon gaskets, gas diffusion layers (SGL 10BC, Carbon paper) and the MEA. The humidified H2 and O2 gases were fed to anode and cathode continuously in a stoichiometric ratio of 1.0:1.5. To evaluate the performance of single cells, polarization curves were recorded using a DC electrode load (KFM2030, Kikusui electronics, Japan) under four testing conditions (75 °C and 100% RH, 90 °C and 100% RH, 120 °C and


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40% RH, and 140 °C and 20% RH). Electrochemical impedance was then measured using AC impedance spectroscopy (VSP, BioLogic) over the frequency range of 0.1 – 10,000 Hz under the four testing conditions and the impedance values were used to calculate proton conductivity of membranes.

2.7 Durability test To evaluate durability of single cells with varied membranes, accelerated life time (ALT) test was performed by sweeping voltage from 0.6 to 1.0 V at 120 °C and 40 % RH. Current density was measured for 500 cycles (approximately 35 hours) and one cycle of ALT test was carried out with repetitive step (15 seconds per one step) having 0.05 V of interval sweeping voltage for 4.5 min [31,32]. Finally, to compare with initial and final electrochemical conditions, polarization curves and impedance spectra were evaluated before and after the ALT test.

3. Results and discussion 3.1 TiO2/ZrO2 nanofibrous webs preparation and characterization A mixture solution of Ti and Zr precursors (TiP and ZrP), and PVP (as a guide polymer and a viscoelasticity enhancer) was electrospun under carefully controlled conditions to avoid formation of beads and/or droplets as described in the Experimental Section 2.2. The electrospun fibers were thermally treated in an air atmosphere to completely remove the organics components of electrospun fibers and produce TiO2/ZrO2 nanofibrous webs. Upon calcination, fiber diameter decreased from an average of 497 nm (20.9% RSD (relative standard deviation), n = 72) to an average of 344 nm (15.1% RSD, n = 131) (see Fig. 1). The fibrous web has noticeably shrunk in both x- and y-directions (35%) and the thickness of the


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nanofibrous web was also slightly reduced from 55 µm (before calcination) to 50 µm (after calcination) as expected once the PVP was removed. A weight loss (approximately 60%) by calcination was observed as shown in the corresponding TGA results (Fig. 2(a)). It is expected that TiO2/ZrO2 inter-particulate distance in the fiber was significantly reduced and the particles were fused at the contact points due to the removal of organic components of electrospun fibers. The Ti:Zr ratio of the calcined TiO2/ZrO2 nanofibrous web was expected to be 1:1 (atomic ratio) and confirmed by XRD analysis. The XRD patterns of the nanofibrous web calcined at 500, 700 and 1000 °C for 6 h are shown in Fig. 2(b). While no crystalline peaks were detected for the TiO2/ZrO2 nanofibrous web calcined at 500 °C (X-ray amorphous), the crystalline peaks for the nanofibrous web calcined at 700 and 1000 °C matched the XRD pattern of Srilankite (Ti0.5Zr0.5O2) which indicates 1:1 atomic ratio of Ti:Zr. The X-ray amorphous TiO2/ZrO2 nanofibrous web was used for further study because (i) the TiO2/ZrO2 nanofibrous web is not necessarily crystalline for our application (inorganic additive skeleton for polymer composite membrane) and (ii) X-ray amorphous metal oxide generally exhibits higher surface area than crystalline metal oxide in the same composition so the X-ray amorphous metal oxide might provide more efficient reaction sites for surface modification [33]. The nitrogen adsorptiondesorption curve exhibits a type IV hysteresis loop, a typical response of mesoporous materials [34]. The pore size was approximately 10 nm (calculated with adsorption branch of the isotherm) and surface area (BET) was 75 m2 g-1. It should be noted that the measured mesopores originated from inter-particulate space in the fibers but not the inter-fiber space.

3.2 Phosphate modification of TiO2/ZrO2 nanofiber surface


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Although the TiO2/ZrO2 nanofibrous web is an effective additive to improve the mechanical and thermal properties of the polymer composite, they do not have proper functional groups to enhance proton transport. In order to enhance proton conductivity and to increase compatibility of the inorganic nanofiber surfaces with the polymer matrix, the calcined TiO2/ZrO2 nanofibrous web was phosphate modified. X-ray photoelectron spectroscopy (XPS) was used to confirm the phosphate modification of the TiO2/ZrO2 nanofiber surfaces. In Fig. 3, The XPS survey scan spectra indicated the presence of phosphorus after the phosphoric acid treatment while no phosphorus was present before the acid treatment. The O 1s spectra of non-modified TiO2/ZrO2 can be deconvoluted into two peaks: one at 530.35 eV which corresponds to O-M (M = Ti or Zr) bonding [35], and the other broader peak at 532.8 eV which is related to O-H bonding [36]. For phosphate modified TiO2/ZrO2 nanofibers, on the other hand, new peak was observed at 531.36 eV (Fig. 3(b)). This new peak (at 531.36 eV) corresponded to a different type of oxygen bonding present on the surface of nanofiber attributed to phosphorous functional groups [37] and associated with HPO2-4 groups [38,39]. Two distinct peaks of phosphorus (P 2p) were observed for phosphate modified TiO2/ZrO2 nanofiber surfaces while no such peaks appeared for non-modified TiO2/ZrO2 nanofiber surfaces. The main peak was at 133.96 eV which corresponds to the phosphate state [40]. A shoulder peak is also observed at 135.08 eV which is related to the bidentate bonding of phosphate species to the TiO2/ZrO2 nanofiber surfaces [41]. It was clearly shown that phosphate functionalization on the TiO2/ZrO2 nanofibers surface was performed via mainly monodentate and bidentate bondings.

3.3. Nanofiber composite membrane formation and evaluation


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Prepared composite membranes consisted of the non-modified or phosphate modified TiO2/ZrO2 nanofibrous web and a short-side chain PFSA, Aquivion ionomer [29,42] and were tested in single fuel cells. Fig. 4 shows a dense film formed by filling inter-fiber spaces with the Aquivion ionomer. The nanofibrous webs were uniformly distributed in all 3 dimensions: x-, y- and z-axis (Fig. 4(b)). Gravimetric swelling in water of the Aquivionimpregnated TiO2/ZrO2 membranes (non-modified and phosphate modified) was higher than Aquivion cast film (32.8% and 33.8% vs. 17.5%) but more importantly, no length change in x- and y-axis was observed in wet state due to the presence of rigid TiO2/ZrO2 nanofibrous webs uniformly distributed in the membrane (Table 1). Therefore, no areal expansion of membrane is expected in the actual fuel cell test that would be beneficial to mechanically stable operation and durability.

3.4 Single cell performance Fig. 5 describes the polarization (i vs. V) and power density curves of single cells with Aquivion impregnated TiO2/ZrO2 composite membrane, Aquivion impregnated phosphate modified TiO2/ZrO2 composite membrane and Aquivion only membrane at various temperatures and relative humidity (75 °C / 100% RH , 90 °C / 100% RH , 120 °C / 40% RH, and 140 °C / 20% RH). A single cell with Aquivion/TiO2/ZrO2 nanofiber composite membrane outperformed a single cell with Aquivion membrane at fully humidified conditions (75 °C / 100% RH and 90 °C / 100% RH). The maximum power densities of those were 1120 and 917 mW cm-2 compared to 695 and 694 mW cm-2 for Aquivion membrane at 75 °C / 100% RH and 90 °C / 100% RH, respectively (see Fig. 5a and b). However, at higher temperature and lower humidity conditions (120 °C / 40% RH and 140 °C / 20% RH), maximum power density of Aquivion only membrane was higher than Aquivion/TiO2/ZrO2 nanofiber composite membrane (about 362 and 167 mW cm-2). At such conditions, it was


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expected that water retention of TiO2/ZrO2 nanofibers significantly low due to too low humidity levels (40 or 20% RH) and the nanofibers are considered as diluents of proton conducting materials. On the other hand, a single cell with Aquivion/phosphate modified TiO2/ZrO2 nanofiber composite membrane showed the best performance at all four conditions including fully humidified (75 °C / 100% RH and 90 °C / 100% RH) and partially humidified (120 °C / 40% RH / 140 °C and 20% RH). In addition, the proton conductivities of Aquivion/phosphate modified TiO2/ZrO2 nanofiber composite membrane were 0.027 S cm-1 and 0.0080 S cm-1 at 120 °C / 40% RH and 140 °C / 20% RH, respectively, as shown in Fig. 6. These values are higher than that of Aquivion only membrane (0.025 S cm-1 and 0.0073 S cm-1) at the same conditions. It was expected that the higher water retention of the Aquivion/phosphate modified TiO2/ZrO2 nanofiber composite membrane played a beneficial role under fully humidified conditions (75 °C and 90 °C) and the phosphate functionality enhanced proton conductivity by enhancing proton transfer through the Grotthus mechanism at higher temperature and the lower humidity conditions (120 °C / 40% RH and 140 °C / 20% RH).

3.5 Durability test and electrochemical analysis Fig. 7 shows the result of accelerated life time (ALT) test, and before- and after-ALT electrochemical analysis. The current density reduction rate of Aquivion membrane was – 11.74 mA cm-2 h-1 while that of Aquivion/phosphate modified TiO2/ZrO2 nanofiber composite membrane was – 8.57 mA cm-2 h-1 (Fig. 7a). It is also noted that the current density of Aquivion membrane was drastically decreased especially after 15 hour. The better cell durability of Aquivion/phosphate modified TiO2/ZrO2 nanofiber composite membrane is probably due to the thermally and mechanically stable inorganic fibrous component of the


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membrane and its phosphate functionality which can result in superior proton conductivity at high temperature and low humidity than the pristine Aquivion membrane. Additionally, for comparison, polarization curves, electrochemical impedance spectra and linear sweep voltammetry (LSV) curves were recorded before and after ALT test. The cell performance for both Aquivion and Aquivion/phosphate modified TiO2/ZrO2 membranes dropped during the ALT test. However, it is shown that the Aquivion/phosphate modified TiO2/ZrO2 membrane drew more current (50 mA cm-2 (before-ALT) and 90 mA cm-2 (after-ALT) at 0.6 V) than Aquivion membrane. It is noted that the Aquivion/phosphate modified TiO2/ZrO2 membrane outperformed the Aquivion membrane even more after ALT test than before ALT test (Fig. 7b). Impedance spectra showed the similar trend and the Aquivion/phosphate modified TiO2/ZrO2 membrane exhibited lower charge transfer resistance than the Aquivion membrane both before and after ALT test (Fig. 7c). To examine hydrogen crossover before and after ALT test, LSV (linear sweep voltammetry) was conducted. As shown Fig. 7d, current density of Aquivion/phosphate modified TiO2/ZrO2 membrane slightly changed from 0 to 0.21 mA cm-2 whereas Aquivion rapidly increased from 2.22 to 6.78 mA cm-2. The current density of both Aquivion/phosphate modified TiO2/ZrO2 and Aquivion was still below 10 mA cm-2 even after ALT test indicating no obvious hydrogen crossover through the membrane. On the other hand, the relatively higher current density of Aquivion membrane is suspected of the potential damage of the membrane which may be associated with material degradation or pinhole formation [43].

4. Conclusions We prepared phosphate-modified TiO2/ZrO2 nanofibrous web by electrospinning and surface functionalization. Then, the nanofibrous web was composited with a short-side chain PFSA,


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Aquivion. Unlike other particulate inorganic fillers, the TiO2/ZrO2 nanofibrous web did not require any special effort to avoid agglomeration in the polymer matrix and was naturally evenly dispersed in the polymer matrix. Membrane characterizations and fuel cell tests were conducted to compare the properties and performances of three different membranes: Aquivion/phosphate modified TiO2/ZrO2 nanofibrous composite, Aquivion/TiO2/ZrO2 nanofibrous composite and pristine Aquivion membranes. The Aquivion membrane composited with and mechanically supported by inorganic nanofibrous web significantly improved the resistance to swelling compared to the pristine Aquivion membrane. For proton conductivity and single cell performance in both fully and partially humidified conditions the Aquivion/phosphate modified TiO2/ZrO2 nanofibrous composite membrane outperformed the Aquivion/non-modified TiO2/ZrO2 nanofibrous composite and Aquivion only membranes. The ALT test was also conductive to evaluate the membrane durability and the superior durability of Aquivion/phosphate modified TiO2/ZrO2 nanofibrous composite membrane was observed at the high temperature and partially humidified condition (120 °C and 40% RH).

Acknowledgements This research was supported by the Technology Innovation Industrial Program funded by the Ministry of Trade, Industry and energy (MOTIE), Republic of Korea (grant number 10052823)

References [1] Steele, B.C.; Heinzel, A. Materials for fuel-cell technologies. Nature 2001, 414, 345–352.


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[19] Lee, C.; Jo, S.; Choi, J.; Baek, K.Y.; Truong, Y.; Kyratzis, I.L.; et al. SiO2/sulfonated poly ether ether ketone (SPEEK) composite nanofiber mat supported proton exchange membranes for fuel cells. J Mater Sci 2013 48, 3665–3671. [20] Kreuer, K.D.; Rabenau, A.; Weppner, W. Vehicle Mechanism, A New Model for the Interpretation of the Conductivity of Fast Proton Conductors. Angew Chemie Int Ed English 1982, 21, 208–209. [21] Kreuer, K.D. Proton Conductivity: Materials and Applications. Chem Mater 1996, 8, 610–641. [22] Agmon, N. The Grotthuss mechanism. Chem Phys Lett 1995, 244, 456–462. [23] Eikerling, M.; Kornyshev, A.A.; Kuznetsov, A.M.; Ulstrup, J.; Walbran, S. Mechanisms of Proton Conductance in Polymer Electrolyte Membranes. J Phys Chem B 2001, 105, 3646–3662. [24] Wainright, J.S. Acid-Doped Polybenzimidazoles: A New Polymer Electrolyte. J Electrochem Soc 1995, 142, L121- L123. [25] Li, Q,; Jensen, J.O.; Savinell, R.F.; Bjerrum, N.J. High temperature proton exchange membranes based on polybenzimidazoles for fuel cells. Prog Polym Sci 2009, 34, 449– 477. [26] Dippel, T.; Kreuer, K.D.; Lassègues, J.C.; Rodriguez, D. Proton conductivity in fused phosphoric acid; A 1H/31P PFG-NMR and QNS study. Solid State Ionics 1993, 61, 41– 46. [27] Schuster, M.F.H.; Meyer, W.H.; Schuster, M.; Kreuer, K.D. Toward a New Type of Anhydrous Organic Proton Conductor Based on Immobilized Imidazole. Chem Mater 2004, 16, 329–337. [28] Vilčiauskas, L.; Tuckerman, M.E.; Bester, G.; Paddison, S.J.; Kreuer, K.D. The mechanism of proton conduction in phosphoric acid. Nat Chem 2012, 4, 461–466.


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Fig. 1 SEM images of electrospun (a) TiO2/ZrO2/PVP nanofibrous web and (b) calcined (at 500 °C) TiO2/ZrO2 nanofibrous web, and fiber diameter distribution for (c) TiO2/ZrO2/PVP nanofibrous web and (d) TiO2/ZrO2 nanofibrous web.


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Fig. 2 Characterization of TiO2/ZrO2 nanofibrous web: (a) TGA results of the electrospun (before calcination) nanofibrous web and calcined TiO2/ZrO2 nanofibrous web, (b) XRD patterns of TiO2/ZrO2 nanofibrous webs calcined at different temperatures (500, 700 and 1000 °C), (c) N2 adsorption-desorption isotherm and (d) pore size distribution (BJH model) of TiO2/ZrO2 nanofibrous web calcined at 500 °C.


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Fig. 3 XPS results of TiO2/ZrO2 (Ti:Zr = 1:1 atomic ratio) nanofiber surfaces before and after phosphate modification: (a) survey spectra, (b) oxygen 1s spectra and (c) phosphorous 2p spectra.


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Fig. 4 SEM images of surface (a) and cross-sectional view (b)) of Aquivion-impregnated TiO2/ZrO2 nanofiber composite membrane. The magnified view (the insert in the (b)) indicated where the nanofibers present.


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Fig. 5 Polarization and power density curves of the phosphate modified TiO2/ZrO2 nanofiber/Aquvion composite, TiO2/ZrO2 nanofiber/Aquivion composite and Aquivion membranes at four different testing conditions: (a) 75 °C and 100 % RH, (b) 90 °C and 100 % RH, (c) 120 °C and 40 % RH and (d) 140 °C and 20 % RH.


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Fig.

6

Proton

conductivity

comparison

of

the

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phosphate

modified

TiO2/ZrO2

nanofiber/Aquvion composite, TiO2/ZrO2 nanofiber/Aquivion composite and Aquivion membranes at 120 °C / 40 % RH and 140 °C / 20 % RH.


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Fig. 7 Variation of current density of TiO2/ZrO2/Phosphate/Aquivion and Aquivion during ALT test at 0.6 V under 120 °C, 40 % RH and atmospheric pressure (a), initial and final electrochemical analysis; polarization curve (b) and impedance spectra (c) and LSV curve (d).


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Table 1 Membrane swelling in water (a measured at 25 °C. b measured at 80 °C) Gravimetric Areal Membrane swellinga (%) swellingb (%) Aquivion/TiO2/ZrO2

32.8

ZrO2 Nanofibrous

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