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Evaluation of the Electrospun Inorganic/Organic Composite Membrane for High Temperature PEMFC
High temperature fuel cell has been highlighted for its use in the wide range of the applications because of its simplified system due to easier integration and high energy efficiency.[1-2] There are several fascinating reasons for operating PEMFC at high temperature and low related humidity such as electrochemical kinetics, CO poisoning of Pt/C catalyst and water management. Also, the lifetime targets (5000 h stated objective by DOE) must be achieved while the costs of the fuel cell systems must be reduced concurrently.[1] In order to ameliorate the durability and lifetime of PEMFCs, a better analyzing of failure tools and corresponding mitigation methods are urgently required.[2] 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.[3] Some of the composite membranes with particulate additives have shown improved fuel cell performance at high temperature and low humidity because of improved hydroscopic and thermal stability. Nevertheless, in practice, composite film casting by solvent evaporation or extrusion is often challenging due to particle agglutination. Recently, ceramic nanofibers have been prepared via electrospinning.[4] 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 membrane that could not be achieved by particulate additives.
In this study, we prepared inorganic nanofibrous web supported polymer electrolyte membrane to increase operating temperature of the pristine polymer electrolyte membrane. Accelerated life-time tests (ALT) by changing the voltage sweep were developed to have better understanding of the degradation mechanisms in the fuel cell system. Further, factors of degradation were studied by conducting the electrochemical methods and characterizations.
Gravimetric swelling by water of the Aquivion-impregnated inorganic nanofibrous webs as a membrane was higher than Aquivion cast film (32.8% 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 inorganic nanofibrous webs uniformly distributed in the membrane. Therefore, no areal expansion of membrane is expected in the actual fuel cell test that would be beneficial to mechanically stable operation. A single cell with Aquivion/phosphate modified inorganic nanofiber composite membrane showed the best performance at partially humidified condition of 120 °C / 40% RH. It was expected that the higher water retention of the Aquivion/ phosphate modified inorganic nanofiber composite membrane played a beneficial role that the phosphate functionality enhanced proton conductivity by enhancing proton transfer through the Grotthus mechanism at higher temperature and the lower humidity conditions.
References
[1] B. Wahdame et al., Journal of Power Sources 182 (2008) 429–440
[2] S. Zhanga, et al., International journal of hydrogen energy 34 (2009) 388–404
[3] P. Costamagna, et al., Electrochimica Acta 47 (2002) 1023-1033.
[4] Yao, Y et al.. Electrochemistry Communications 13 (2011) 1005-1008.