Understanding the Acid-Alkaline Bipolar Membrane Electrolyte for Fuel Cell Applications (2015)

One of the challenges confronting fuel cell technologies is their integration into a practical, robust system. The Balance of Plant (BoP) components needed to manage the fuel and oxidant, humidification and/or water removal, and thermal loads are often cumbersome to integrate. These issues can present unique requirements and are persistent across most types of fuel cell applications (e.g., wearable and portable, transportation, backup and primary power, etc.) and fuel cell types (e.g., polymer electrolyte, direct methanol, reformed methanol, solid oxide, and so forth). The research and development efforts presented in this work are specifically geared toward wearable and portable power applications. In this configuration, a compact, lightweight, and cost effective system with a high specific energy (density) is desired. Polymer electrolyte fuel cells with energy dense liquid fuels such as methanol are often used in attempt to fill this need. Direct methanol fuel cell (DMFC) systems have shown that they can reliably provide large energy density values over a wide range of environmental and operational conditions; however, DMFCs size/footprint and cost continue to be a drawback. The size and complexity of present DMFC systems are largely dictated by the fuel/water and thermal management components. Costs are often attributed to those associated with the membrane electrode assembly (MEA) and specifically platinum-based electrocatalysts, but other factors such as packaging and BoP also play a role. In this talk, we will present our efforts to develop bipolar membrane fuel cell (BMFC) technologies to address some of these shortcomings. The BMFC uses a hybrid of proton exchange membrane (PEM) and anion exchange membrane (PEM) materials such that there is at least one internal acid-alkaline membrane junction, or interface. Depending on the respective configuration, water generation or consumption will occur at the junction. Paul Kohl’s group has demonstrated how this junction can be (i) used to improving hydrogen/oxygen MEA performance at low relative humidity such that the need for external humidification in reduced, and (ii) integrated into low temperature DMFC MEAs [1-3]. Bipolar membrane configurations may additionally be used to adjust the respective anode and cathode chemistries (i.e., acid or alkaline) such that the electrochemical kinetics may be improved and/or the amount of platinum-based electrocatalysts may be reduced. The focus of this talk will be the modeling of the bipolar membrane electrolyte materials and the BMFC system that complement our ongoing experimental materials and systems development efforts. A specific focus will be placed on understanding the nature of the bipolar membrane junction and its influence on transport phenomena within the BMFC. The insights from this modeling effort on the utilization of this technology for the development of a BMFC based DMFC with improved water/fuel and thermal management will be discussed. Acknowledgments: KNG, JPM, and DC gratefully acknowledge support from the U.S. Department of the Army, U.S. Army Materiel Command, Research Development and Engineering Command, and U.S. Army Research Laboratory. JMA, LL, and PAK gratefully acknowledge support from the U.S. Department of the Army, Deputy Assistant Secretary of the Army (DASA) International Collaboration Program through contract no. W911NF-14-2-0008. References: 1. M. Unlu, J. Zhou, and P.A. Kohl, J. Electrochem. Soc. , 157 (10), B1391 (2010). 2. M. Unlu, J. Zhou, and P.A. Kohl, Fuel Cells , 10 (1), 54 (2010). 3. J. Zhou, K. Joseph, J.M. Ahlfield, D.-Y. Park, and P.A. Kohl, J. Electrochem. Soc. , 160 (6), F573 (2013).