Electrospun Composite Anion Exchange Membranes

Abstract

Fuel cells are an important component of a renewable energy-based economy, with applications in transportation, portable, and stationary power. Alkaline anion exchange membrane fuel cells in particular are of interest because they potentially allow the use of non-precious metals as electrode catalysts. In this dissertation, a new series of anion exchange membranes for alkaline fuel cells has been fabricated by a dual fiber electrospinning process. Two fibers were simultaneously electrospun: a halomethylated precursor to a polyelectrolyte (chloromethylated polysulfone, chloro/iodomethylated polysulfone, or brominated poly(phenylene oxide)) and an uncharged, reinforcing polymer (polyphenylsulfone). Dual fiber mats were transformed into a dense and defect-free anion exchange membranes by: (i) compressing the mat at 5,000 psi for ~20 seconds, (ii) exposing to solvent vapor which softened the polyphenylsulfone and allowed it to flow and fill void space around polyelectrolyte precursor fibers, and (iii) immersion in free base (such as trimethylamine) to convert remaining halomethyl side groups to fixed charge cations. Some polyelectrolyte precursor fibers were crosslinked to prevent solubility of the polyelectrolyte in water. The best membrane was found to be a diamine-crosslinked poly(phenylene oxide)-based polyelectrolyte with benzyl trimethylammonium fixed charge groups, which had a membrane ion exchange capacity of 2.0 mmol/g, high hydroxide ion conductivity in water at room temperature (66 mS/cm), reasonable gravimetric water swelling (97%), robust mechanical properties (16 MPa stress-at-break when fully hydrated), and good chemical stability (little/no degradation over 1 week immersed in 1.0 M KOH at 60°C. This membrane was employed in a membrane electrode assembly for an alkaline fuel cell, which demonstrated high power at 60°C under H2/O2 gas flow and with a platinum loading of 0.5 mg/cm2 for anode and cathode.