Title

Field-structured proton exchange membranes for fuel cells

Date of Completion

January 2007

Keywords

Chemistry, Polymer

Degree

Ph.D.

Abstract

Fuel cells offer promising energy solutions that are reliable and available. Since conventional fuel cell types such as solid-oxide fuel cells require expensive materials extreme operating conditions, proton exchange membrane-based fuel cells (PEMFC) have attracted the attention of industry, especially for transportable applications. The goal of this research was to develop and study the properties of novel polymer membranes for possible application in direct methanol fuel cells (DMFC). Of particular interest were membranes based on proton-conducting micro and nano particles dispersed in a curable or solvent-based matrix. To this end, nanoparticles of sulfonated crosslinked polystyrene (SXLPS) were synthesized by emulsion and emulsifier-free emulsion polymerizations. Particles with ion exchange capacity (IEC) of up to 2.2 meq/g were obtained and used in the fabrication of PEMs. The morphology of the membrane was modified by aligning the SXLPS particles in the matrix using electric field. Direct proton-transport routes were formed which in turn enhanced the proton conductivity of the membranes. While inexpensive and straightforward, electric fields have practical and fundamental limitations which suggested the use of magnetic fields. For particle alignment in magnetic field, composite nanoparticles were synthesized which had reasonable ion exchange capacity as well as magnetic susceptibility. The morphologies observed in the composite particles were explained using a synthesis model. Membranes were fabricated by aligning the acidic SXLPS particles with an external magnetic field of 0.1 Tesla, which is easily achievable in an industrial scale.^ Typical properties of fuel cell membranes including proton conductivity, water and methanol absorption and permeability, and state of water were studied. It was observed that at SXLPS loading of more than 35 wt%, the membranes had proton conductivities comparable to the standard material, Nafion® 112. However, the methanol permeability was lower than Nafion ® 112 for up to 40 wt% SXLPS loading. Low methanol permeability is beneficial for DMFC application. Finally, the effect of particle size, loading, ion exchange capacity and membrane microstructure on the fuel cell properties were studied and analyzed. ^