(609d) Layer-by-Layer Assembly to Design Novel Membrane Architectures for Fuel Cells
The global need for clean and sustainable energy sources has led to a renewed interest in fuel cells, especially hydrogen and methanol powered fuel cells. The alternating adsorption of complementary functionalized species, known as the layer-by-layer (LBL) assembly, is an elegant and robust method of fabricating solid-state electrolytes and polymer composites. This approach presents strong advantages allowing the incorporation of many different functional materials within a single film at a full range of compositions with exceptional homogeneity. The process is also flexible; by altering assembly conditions such as deposition pH, additives (salts or surfactants), and co-solvent choice, it is possible to greatly affect the final thin film composition and properties. These systems have allowed the formation of a range of electrochemical devices including electrolytes for batteries and proton exchange membranes (PEMs) for fuel cells. We have utilized this method to develop PEMs targeted for portable power applications such as direct-methanol fuel cells (DMFCs). This work describes the optimization of multilayer assembled membranes for use in low temperature fuel cells. We have recently reported the highest ionic conductivity value ever obtained from an LBL assembled membrane. By pairing a highly sulfonated and water-soluble aromatic polyether (poly2,6-dimethyl 1,4-phenylene oxide) with an amine based polycation, we have obtained proton conductivity values of up to 70.0 mS/cm at 25°C. These membranes also exhibit low liquid methanol permeability and have high chemical and mechanical stability to provide applications in DMFCs. We have demonstrated that coating traditional fuel cell membranes with LBL films improves the power output of DMFCs by over 50%. We are also developing standalone LBL films that are thick enough to peel off from a low surface energy substrate. This is accomplished by using a home-built, automated sprayer setup that reduces the total LBL fabrication time by 40x. The optimization of assembly parameters such as polymer concentration, spray time, spray pressure, and the air/solution ratio are shown to obtain high quality membranes.