Electrospun Bipolar Membranes for Portable Fuel Cells

Shifting to high pH fuel cell operating environments could potentially lower the overall fuel cell costs by reducing or replacing noble metals with other electrocatalytically-active non-noble metal materials. Anion-exchange membranes (AEMs) with mobile OH^- ions are one way to shift to higher pH environments, and have received attention for use in alkaline fuel cells.[1]  Most AEMs studied to-date are organic and exhibit low chemical stabilities and low ionic conductivities (i.e. mobile transport ions are OH^-, CO₃^2- or mixed ion form) after prolonged exposure to high pH media and temperatures greater than 60°C.[2]  A number of advancements have been made for increasing the ion-exchange group stability for reasonable ion-conductivities by using phenyl guanidinium-[3], imidazolium-[4] and other metal-cation exchange groups[5].  Park et al. recently demonstrated a novel approach for creating AEM membranes by electrospinning.[6]  The electrospinning approach allows for short- and long-range fabrication, which is useful for proton- and/or hydroxide-transport membranes.

In addition to the electrocatalyst and membrane materials concerns, fuel cells commonly employee auxiliary components to assist in balancing the water production and consumption at the anode and cathode compartments.[7]  The auxiliary components cause the total fuel cell device (i.e. composed of stack and other components) to increase in size and weight.  One way to reduce the logistics burden and overall balance of plant (BOP) is to design bipolar membranes that are comprised of an acid membrane component spatially separated from an alkaline membrane component. The bipolar membrane concept was recently demonstrated by Unlu et al.[8]  The reduction in the BOP would therefore reduce the overall cost and size of the fuel cell device. 

In this study, we synthesize AEM and PEM precursors and subsequently co-electrospin these precursors into a bipolar membrane.  The bipolar membrane produced may be potentially used for portable hybrid acid-alkaline fuel cells.  The electrospun PEM component is primarily made of Nafion embedded in an inert matrix layer.  The electrospun AEM component is comprised of a synthesized and subsequently modified polysulfone derivative embedded in an inert matrix layer.  We provide physical characterizations such as TEM, SEM, BET, swelling and dimensional change measurements to understand each electrospun component at the PEM and AEM compartments.  Moreover, we perform ex-situ ion-transport measurements for bulk conductivities of the isolated and combined components.  Figure 1 shows a schematic of the electrospinning process whereby a rotating roll configuration with multiple high voltage power supplies is utilized for fabricating a co-electrospun bipolar membrane.  An SEM image is shown for the AEM fiber mat component, which is typically placed at the cathode compartment.   

Acknowledgements

We gratefully acknowledge the fuel cell team at the U.S. Army Research Laboratory.  We acknowledge the U.S. Department of the Army and Army Materiel Command for funding and support.         

References

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[2]  Y.-J. Wang, J. Qiao, R. Baker, J. Zhang, Chemical Society Reviews 42 (2013) 5768.

[3]  D.S. Kim, C.H. Fujimoto, M.R. Hibbs, A. Labouriau, Y.-K. Choe, Y.S. Kim, Macromolecules 46 (2013) 7826.

[4]  F. Gu, H. Dong, Y. Li, Z. Si, F. Yan, Macromolecules 47 (2014) 208.

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[6]  A.M. Park, P.N. Pintauro, Electrochemical and Solid-State Letters 15 (2011) B27.

[7]  K.N. Grew, D. Chu, ECS Transactions 50 (2013) 103.

[8]  M. Ünlü, J. Zhou, P.A. Kohl, Angewandte Chemie International Edition 49 (2010) 1299.