The Design And Synthesis Of High Performance Polyolefins For Use In Alkaline Anion Exchange Membrane Fuel Cells

Abstract

Fuel cells are devices that convert the chemical energy stored in a fuel directly into electricity and have the potential to serve as a highly efficient and environmentally sustainable power generation technology for stationary and mobile applications. Within a fuel cell, the polymer electrolyte membrane serves as the ion conducting medium between the anode and cathode, making it a central, and often performance-limiting component of the fuel cell. The most common polymer electrolyte membrane fuel cells operate under acidic conditions and are therefore proton conducting. Although proton exchange membrane (PEM) fuel cells are well developed and can offer excellent performance, they rely almost exclusively on platinum, a very expensive and scarce noble metal. This dependence on platinum has severely hindered wide scale commercialization of PEM fuel cell technologies. By comparison, alkaline fuel cells that employ hydroxide conducting alkaline anion exchange membranes (AAEMs) are relatively unexplored. A major advantage of alkaline fuel cells, when compared to acidic fuel cells, is their enhanced reaction kinetics for oxygen reduction, permitting the use of less costly, non-noble metal catalysts (e.g. Ni). Therefore, high performance AAEMs could significantly advance fuel cell technologies. We have been working to develop new polymeric materials that can serve as effective AAEMs. Prior work in this area has mainly focused on re-engineering existing materials to access AAEMs. In contrast, we approached this problem from a synthetic perspective by designing and synthesizing materials from the ground up. Herein, the synthesis of two separate AAEM systems that are synthesized via ring-opening metathesis polymerization are described. The first route involves the copolymerization of a tetraalkylammonium-functionalized norbornene with dicyclopentadiene. The crosslinked thin films generated are mechanically strong and exhibit exceptional methanol tolerance. The second route involves the synthesis of a solvent processable, tetraalkylammonium-functionalized polyethylene for use as an AAEM. The membranes are insoluble in both pure water and aqueous methanol but exhibit excellent solubility in a variety of other aqueous alcohols. These solubility characteristics extend the utility of this system for use as both an AAEM and ionomer electrode material from a single polymer composition. The AAEMs generated are mechanically strong and exhibit high hydroxide conductivities. Lastly, we have developed a standardized procedure for measuring the alkaline stability of a benzyltrimethylammonium (BTMA) model compound and a BTMA functionalized polyethylene. The procedure is broadly applicable and should serve as a testing method to better understand other systems, specifically those based on novel cations. Applying this procedure should facilitate the discovery of AAEMs with increased base stability, thus enabling high temperature AAEM fuel cell operation.