Improvement and characterization of polymer electrolyte membranes for fuel cell applications

The polymer electrolyte membrane is one of the major components of polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs). To improve the properties of polymer electrolyte membranes (PEMs), studies were carried out in three areas: (a) thermal oxidative stressing experiments of Nafion® membranes, (b) thermal gravimetric analysis (TGA) of three different types of PEMs under both nitrogen and oxygen purges, and (c) attempted minimization of methanol crossover via doping of Nafion ® membranes with bulky cations such as trimethylammonium, tetramethylammonium, and trifluoroethyl(tri-methyl)ammonium. These studies were aided by the use of TGA, X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and gas chromatography/mass spectrometry (GC/MS).; In order to improve upon the thermal stability Nafion® membrane samples were placed into an autoclave, and thermal oxidative stressing experiments were carried out under 2 atmosphere oxygen at 225°C for 24 hours. TGA studies indicate that thermal oxidative stressing of Nafion ® membranes can increase the onset of thermal decomposition by 10–25°C over that of unstressed material, without adversely affecting the proton conductivity.; Three different types of PEMs [perfluorinated, such as Nafion; partially fluorinated, such as IonClad R1010; and nonfluorinated, such as polybenzimidazole (PBI)] have been reported for use in PEMFCs and DMFCs. The thermal stabilities of all three types of PEMs have been investigated by thermal gravimetric analysis (TGA) under both nitrogen and oxygen purges. The temperature corresponding to the maximum rate of decomposition shifted significantly lower when TGA experiments were carried out with an oxygen rather than a nitrogen purge, even for perfluorinated membranes. Based on these findings, TGA with an oxygen purge may become a useful diagnostic test for evaluating new PEM materials in the future, as these conditions more clearly reproduce those in an operating fuel cell (cathode compartment).; In an attempt to minimize methanol crossover in DMFC, Nafion® membranes NF-117 was doped with trimethylammonium [(CH₃) ₃NH+], tetramethylammonium, [(CH₃)₄N +], and trifluoroethyl(trimethyl)ammonium [CF₃CH₂ N(CH₃)3+] cations. Diffusion measurements on these membranes indicate that doping with ammonium cations can reduce methanol crossover, but at the expense of dramatically reduced proton conductivity. Dipping the tetramethylammonium cation doped Nafion® NF-117 membrane back in 1 M nitric acid solution for 60 and 180 seconds improved the proton conductivity, but the ratio of conductivity to methanol permeability was not improved over that of the undoped or protonated form of Nafion ® NF-117.