Experimental Development of Alkaline and Acid-Alkaline Bipolar Membrane Electrolytes

Anion-exchange membranes (AEMs) have received attention for use in alkaline fuel cells and bipolar membrane applications [1] The bipolar membrane concept was recently demonstrated by Unlu et al. as a way to reduce fuel cell costs (i.e., Pt loading) and the overall balance of plant (BOP).[2-3]  Bipolar membranes are typically comprised of an H⁺-conducting membrane adjacent to an OH^--conducting membrane with at least one internal acid-alkaline junction. However, most AEMs studied to-date are organic and exhibit low chemical stabilities and low ionic conductivities after prolonged exposure to high pH media and temperatures greater than 60°C.[4]  A number of advancements have been made to increase the ion-exchange group stability by using phenyl guanidinium-[5], imidazolium-[6] and other metal-cation exchange groups[7], among others; all of which provide potential routes to circumvent stability issues.        

In this study, we prepared AEM and PEM precursors which are further developed into bipolar membranes.  The combination of our precursors and our electrospinning process proves useful for preparing H⁺- and OH^--conducting membranes. In addition to the aforementioned AEM stability issues, bipolar membranes can suffer from material compatibility issues at the AEM/PEM junction.  We demonstrate an approach to create bipolar membranes by, for example, co-electrospinning that allows for short- and long-range fabrication with desirable AEM/PEM interfacial binding.  For example, Park et al. recently demonstrated a novel approach for creating AEM membranes by electrospinning, which we extend specifically to develop bipolar membranes.[8]  As a proof of concept, we chose PEM and AEM components made primarily of Nafion and modified polysulfone or modified poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) derivatives, respectively.  We present 3 different approaches for conjoining the AEM and PEM layers to create the bipolar membranes.      

Ex-situ ion-transport measurements are presented for bulk in-plane ionic conductivities (σ) of the isolated membranes in OH^--, H⁺- and CO₃^2—forms and are compared to the through-plane σ for the bipolar membranes.  Physical characterizations such as SEM, BET, and gravimetric swelling measurements are presented for each isolated and combined membranes.  For select membranes, we show polarization (i-V) curves for a single MEA operating under H₂/O₂.  Figure 1 shows a schematic of the electrospinning process whereby a rotating roll configuration is utilized for fabricating a co-electrospun bipolar membrane with a typical SEM image shown for an AEM fiber mat.

Acknowledgments:

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

References:

[1]  S. Malkhandi, P. Trinh, A.K. Manohar, K.C. Jayachandrababu, A. Kindler, G.S. Prakash, S.R. Narayanan, Journal of The Electrochemical Society 160 (2013) F943.

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

[3]  K.N. Grew, D. Chu, J. Electrochem. Soc., 161(1) F1037 (2014).

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

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

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

[7]  Y. Zha, M.L. Disabb-Miller, Z.D. Johnson, M.A. Hickner, G.N. Tew, Journal of the American Chemical Society 134 (2012) 4493.

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