Stable Anion Exchange and Bipolar Membranes for Direct Borohydride Fuel Cells for Unmanned Underwater Vehicles

摘要

Research and development efforts directed towards chemically stable anion exchange membranes (AEMs) for solid alkaline fuel cells have increased substantially over the past 5 to 10 years. AEMs also find possible applications in other energy storage and conversion technologies (e.g. metal-air batteries and redox flow batteries). These AEMs need to demonstrate high performance and adequate chemical durability in taxing chemical environments. Currently, the United States Navy is seeking to develop lightweight, durable, and high performing fuel cells for unmanned underwater vehicle (UUV) propulsion. UUVs implementing such devices are constrained to non-flammable fuels and oxidants and non-gaseous reactants and products. A direct sodium borohydride fuel cell (DBFCs) using hydrogen peroxide (H2O2) as a liquid oxidant is a device that meets these constraints and offers high potential to achieve the desired energy and power objectives. Since sodium borohydride (NaBH4) is only stable in alkaline solutions and the parasitic reactions involving H2O2 are minimized in acidic environments, it is critical to maintain pH control of the respective anolyte and catholyte compartments. A DBFC with a monopolar membrane (either AEM or cation exchange membrane (CEM)) will lead to undesired transport of acid or base across the membrane, resulting in a pH change in the catholyte and anolyte compartments. A solution to this problem is to employ a bipolar membrane configuration that splits water at the membrane junction, which prevents the mixing of acids and bases between the two compartments. This presentation will describe our group's efforts to solve two challenging material problems in employing a bipolar membrane DBFC configuration. They include: i.) Developing AEMs that are stable in alkaline environments at operating temperatures, and ii.) Construction of high performance bipolar membranes to minimize the overpotential associated with splitting water at the bipolar junction.