A Novel Phosphotungstic Acid Impregnated Meso-Nafion

Journal of Membrane Science 427 (2013) 101–107 Contents lists available at SciVerse ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci A novel phosphotungstic acid impregnated meso-Nafion multilayer membrane for proton exchange membrane fuel cells Jin Lin Lu a, Qing Hong Fang b, Sheng Li Li a, San Ping Jiang c,n a School of Materials and Metallurgy, University of Science and Technology Liaoning, Anshan, 114051, China b School of Materials Science and Engineering, Shenyang University of Chemical Technology, Shenyang, 110142, China c Fuels and Energy Technology Institute & Department of Chemical Engineering, Curtin University, Perth, WA 6102, Australia article info abstract Article history: A novel mesoporous Nafion (meso-Nafion) based multilayer membrane impregnated with phospho- Received 16 January 2012 tungstic acid (HPW) is developed for the operation of proton exchange membrane fuel cells (PEMFCs) at Received in revised form elevated high temperature and low humidity conditions. Very different to the pristine Nafion 19 September 2012 membrane, the HPW-impregnated meso-Nafion multilayer membrane possesses a much higher Accepted 22 September 2012 conductivity under both high and low humidity conditions. For example, the proton conductivity is Available online 2 October 2012 0.072 S cmÀ1 at 80 1C under 40% RH, which is 4 times better than 0.015 S cmÀ1 measured on pristine Keywords: Nafion 1135 membrane under identical conditions. Moreover, the new multilayer structure effectively Mesoporous Nafion alleviates the leaching problem of impregnated HPW. The fuel cells assembled with the multilayer Multilayer structured membrane membrane show good performances and much low degradation when operated at reduced humidity Phosphotungstic acid impregnation and elevated temperatures of 120 1C, as compared to the cells with pristine Nafion membranes. The Proton exchange membrane fuel cells results indicate that the multilayer membrane can significantly widen the application range of PEMFCs under low relative humidity (RH) and elevated high temperatures. & 2012 Elsevier B.V. All rights reserved. 1. Introduction shrinkage and consequent poor contact between membranes and electrodes [12,13]. Therefore, a fully hydrated membrane at elevated Proton exchange membrane fuel cells (PEMFCs) are one of the temperatures is desirable for efficient proton conduction in the most promising clean energy technologies under development. membranes, which requires complicated water management [14].It Owing to their inherently high energy efficiency, high power remains a critical challenge to maintain proper hydration of the density, rapid start-up, PEMFCs are extremely attractive for use as membranes for the operation of the PEMFCs at elevated high power sources in applications ranging from portable devises to temperature and/or low humidity conditions. stationary power [1–4]. Current PEMFCs generally operate at Heteropolyacids (HPAs) are well known as superionic conduc- relatively low operation temperatures of 80 1C. However, one tors in their fully hydrated states, particularly those based on the of the major issues of low temperature operation is the poisoning Keggin structure [15]. Thus, HPAs have been used to improve the of Pt-based anode catalysts by the trace amount of CO present in water retention and/or to enhance resistance to methanol cross- fuels [5,6]. Increasing the operating temperature to over 100 1C over properties of Nafion membranes [16,17]. Ramani et al. brings important benefits with respect to alleviated problems of [18–20] prepared different Nafion/HPA composite membranes CO poisoning, improved reaction kinetics in anode and cathode using recasting method with different HPAs, such as HPW, [7–9]. On the other hand, the low humidity operation of PEMFCs silicotungstic acid (STA), phosphomolybdic acid (PMA), and also presents many advantages such as reduced water management, silicomolybdic acid (SMA). When the additive particle size was simplified system design and reduced cost, etc [10,11]. However, the in the order of 1–10 mm, no conductivity improvement was state-of-the-art membrane currently used for PEMFCs, such as observed as compared to Nafion at low humidity conditions. Dupont’s NafionTM or perfluorosulfonic acid (PFSA) polymers, shows Reducing the additive particle size enhanced the proton conduc- a significant loss in conductivity due to the dehydration of the tivity of the composite membrane. Malhotra et al. [21] impreg- membranes when operating at high temperatures and/or low nated HPW dissolved in different solvents into Nafion 117 humidity conditions. The dehydration also results in the membrane membranes. A power output of 450 mW cmÀ2 was achieved on the cell with Nafion/HPW membrane (using tetra-n-butylammo- nium chloride as solvent) at 120 1C. Shao et al. [22,23] synthe- n Corresponding author. Tel.: þ61 8 9266 9804; fax: þ61 8 9266 1138. sized the hybrid Nafion–silicon oxide-HPW membrane for high E-mail address: [email protected] (S.P. Jiang). temperature operation of PEMFCs. The HPW molecules were first 0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.09.041 102 J.L. Lu et al. / Journal of Membrane Science 427 (2013) 101–107 supported on a silica gel and the as-obtained powders were then recast with Nafion ionomer. Similarly Sacc et al. [24] prepared the Nafion-based membrane using HPW supporting on a nanopow- À2 dered ZrO2 as new filler. A peak power of 300 mW cm was obtained at 90 1Cwithdrygases.Cuietal.[25] successfully modified Nafion membranes by casting polyelectrolyte complexes of chitosan and HPW and the new chitosan/HPW modified Nafion composite membranes shows better performance for direct methanol fuel cells (DMFCs). However, HPW molecules are prone to agglomerate to form large particles or clusters during the recasting process and cause the phase separation between HPW and Nafion matrix, which in turn would lower the conductivity and performance of the composite membranes. In addition, HPW may leach out during the operation of fuel cell, which would lead to the instability of the composite membranes. Recently, a meso-Nafion membranes were successfully synthesized in which highly ordered mesopores were introduced into Nafion via a novel micelle templating method [26,27]. In this method, nonionic surfactant, PEO127–PPO48–PEO127 (Pluronic F108) was used to control the micelle size in the polymer mixture. The surfactant embedded in the synthesized micelle/Nafion precursor was then removed by reflux with hot water, forming well-ordered mesoporous structures in Nafion. Here a novel multilayer membrane based on the meso-Nafion impregnated with HPW will be reported for the high temperature and/or low humidity operation of Fig. 1. Fabrication procedure for the HPW-impregnated meso-Nafion multilayer membranes. PEMFCs. It is known that HPW has the highest stability and strongest acidity among the Keggin-type HPAs. The purpose of HPW impregnation is to further improve the proton conductivity, stability and water retention properties of the meso-Nafion mem- in Milli-Q water at 80 1Cfor30min,andinan8wt%H2SO4 solution branes, especially under elevated high temperature and/or low at 80 1C for 30 min. After each treatment, the membranes were humidity conditions. rinsed in Milli-Q water three times to remove traces of H2O2 and H2SO4. 2. Experimental section 2.2. Physical properties and structure characterization 2.1. Synthesis of multilayer membrane The surface and cross-sections of the membranes were exam- The multilayer membrane consists of a central meso-Nafion ined using scanning electron microscopy (SEM, JEOL JSM-6360). layer impregnated with HPW (H3PW12O40) sandwiched with two The composition and elemental mapping were obtained by side block layers. First, a meso-Nafion membrane was fabricated energy dispersive spectroscopy (EDS, Oxford, UK). The cross using a novel soft-templating method [26,27]. In this method, a sectional specimens of the multilayer membrane were prepared TM non-ionic block copolymer, Pluronic F108 (Mw¼14,600, BASF by breaking the membrane under liquid nitrogen. The samples Corp.) and Nafion 520 ionomers (EW¼1000, 5 wt% Nafion, were Au-sputtered under vacuum before SEM examination. The DuPont, USA) were mixed together by a constant weight ratio. characteristics of the HPW-impregnated multilayer membranes The above mixture was poured into Petri dishes for heating were also examined by a Fourier transform infrared spectroscopy- treatment. The F108 surfactant was removed by treating the attenuated total reflectance (FTIR-ATR, Perkin Elmer FTIR Spec- membranes in boiled water. The as-obtained meso-Nafion trum) with a resolution of 2 cmÀ1. A diamond crystal was used as membrane was then immersed into HPW solutions with different the ATR plate. The thermal properties of the membranes were concentrations, heated at 80 1C for 2 h, followed by heating the determined using a thermogravimetric analyzer (Perkin-Elmer membrane at 130 1C for 1 h, then cooled down to ambient TGA 7) at a heating rate of 5 1C minÀ1 from room temperature to temperature. The HPW-impregnated meso-Nafion was then sand- 1000 1C under a continuous air flow with a flow rate of 60 wiched with two Nafion layers to prevent the leaching of standard-state cubic centimeter per minute (SCCM). The impregnated HPW. First, Nafion ionomers were transferred to a membranes were first placed under 80% RH environment at room s N,N-Dimethylformamide (DMF, Sigma-Aldrich ) solution by temperature for 48 h prior to the TGA tests. The mechanical distilling until the solution temperature reached 153 1C to remove strength of the membranes was measured with a mechanical water and solvent. The quantitative Nafion/DMF mixture was tester (Instron Model 5543) at room temperature. A 100 N load sprayed onto the as-prepared HPW-impregnated meso-Nafion cell was used and the strain rate was 20 mm minÀ1. membrane to form the surface layers, followed by heating at Water uptake measurements of the membranes were 90 1C for 2 h and 160 1C for 1 h.

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