Composite Nafion-Titania membranes for low relative humidity applications in a PEFC

Polymer Electrolyte Fuel Cells (PEFCs) systems represent a highly efficient and environmentally clean electric power supply for stationary and mobile applications. In recent times, the need to develop PEFC operating at low or without any humidification with water vapour partial pressures below 1.5 abs. bar has been gradually increasing, especially for the automotive sector. A typical PEFC device utilizing polymer membranes based on perfluorosulfonic acid (PFSA), such as Nafion® as an electrolyte, supplies an excellent proton conduction and chemical stability when the membrane operates in a water-swollen state since the PSFA proton conductivity proceeds thank to hydronium ions presence. In these conditions, the proton conductivity is very high, but this aspect contemporarily becomes the main obstacle for a PEFC when a low relative humidity (RH) is requested. In-fact, the proton conductivity of Nafion membranes using a 30%RH is less than 10% of the value obtained with an 80%RH [1]. Hence, in drastic low RH conditions, it is highly desirable to enhance the proton conductivity maintaining an appropriate amount of water inside the membrane to achieve an appreciable electrochemical performance. For these reasons, the introduction of hygroscopic and/or proton conductor inorganic compounds (as received and differently functionalised/supported) such as SiO2, TiO2, ZrO2 [2-4], etc. into the polymeric Nafion matrix represents the most commonly used solution. In-fact, such fillers are able to retain water in the molecular framework, to protect the membrane from the swelling assuring the mechanical properties of the polymer chain and to prevent the degradation mechanism. In particular, TiO2 nanoparticles have been often employed as fillers to improve the mechanical strength of the membranes [3]. It has been demonstrated the TiO2/water interface is positively charged under acidic conditions and characterized by a very high concentration of adsorbed protons [5]. Adjemian et al. [6] suggested that a chemical interaction between the metal oxide surface and the Nafion polymer occurs. In-fact, the metal oxide particles act to crosslink the Nafion polymer chains with good electrochemical results at reduced RH levels for titanium composite membranes. Starting from previous investigations [7], in this work, Nafion® polymer matrix modification was investigated using a commercial treated anatase TiO2. Composite Nafion-titania (N-TiO2) membranes with a large area (20cmx25cm) for stack applications cast through a standardised procedure using the Doctor-Blade casting technique [7] were developed with three different TiO2 loadings (5, 10, 15wt.%). They were characterised in terms of structural, morphological and chemical-physical properties (water uptake, Ion Exchange Capacity, lambda, H+ concentration and mobility) with the aim of reducing the humidification levels for a PEFC stack and investigating their behaviour as a function of filler content. The influence of filler was studied, resulting in a swelling reduction of the composite membranes if compared to a pristine recast Nafion membrane (Nrecast), cast through the same method and used as a reference. A good proton conductivity was observed for all N-TiO2 membranes with values ranging 1.3-1.9·10-1 Scm-1 at 60°C and 50%RH against a value of 1.5·10-1 Scm-1 related to the reference membrane. The electrochemical characterization on all samples was carried out with different humidification levels (50, 75%, 100%RH) at 60 and 80°C as cell temperatures evaluating the results in terms of polarisation curves. A screening was performed among the composite developed membranes and the polymeric film containing a 10wt.% (NTiO10) of filler has supplied a comparable performance respect to the reference Nrecast supplying about 0.654V @ 0.65Acm-2 at low RH level (60°C, 50%RH). Moreover, an Accelerated Stress Test (AST) cycling the current between 0.2 up to 0.8A cm-2 was carried out in drastic conditions (60°C, 50%RH) with the aim of verifying the mechanical stability of the composite membranes, in particular, Nrecast and NTIO10 during the AST were compared: a higher stability and durability was verified for the selected composite membrane (221 hrs. supplied by NTiO10 against 155hrs. by Nrecast) highlighting the improved mechanical properties. After AST, I-V curves and H2 crossover measurements were performed to have a confirmation about the improved mechanical resistance of the membranes. Encouraging results were obtained in terms of power density supplied with a value of about 0.435 W cm-2 for NTIO10 against a value of about 0.390 W cm-2 for Nrecast, both @0.65Acm-2. Moreover, a reduced crossover for NTiO10 was evidenced if compared to Nrecast, probably thank to the filler capability to reduce the swelling maintaining a good stiffness of the polymeric matrix. Such promising results have permitted the realization of a 250W PEFC stack with non-commercial components for marine applications. Acknowledgments The authors acknowledge the financial support of MIUR for the project "High Efficiency Technologies for On-board Environmental and Sustainable Energy use (TESEO)", in the framework of National Operational Programme for Research and Competitiveness 2007-2013. References [1]G. Alberti, M. Casciola, Solid State Ionics 145 (2001) 3-16 [2] A. Alvarez, C. Guzmán, A. Carbone, A. Saccà, I. Gatto, R. Pedicini, E. Passalacqua, R. Nava, R. Ornelas, J. Ledesma-García, L.G. Arriaga, J. Power Sources 196 (2011) 5394-5401 [3] M.B. Satterfield, P.W. Majsztrik, H. Ota, J.B. Benziger, A.B. 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