We explore ex situ and in situ fuel cell catalyst degradation test methods and the impact of catalyst degradation on fuel cell performance. A series of platinum-loaded carbons with two different carbon supports are aged ex situ in an isothermal oven and in situ using a 1.2-V fuel cell accelerated test. The ex situ combustion test and in situ 1.2V accelerated fuel cell test both show that the rate and extent of carbon combustion for samples with the same platinum loading increases as the surface area of the carbon increases, presumably because platinum deposits as smaller particles, covering more of the carbon surface. In addition, the rate of reaction/loss of carbon is shown to increase significantly if humidity is introduced into the environment, a concern for long-term polymer electrolyte membrane fuel cell (PEMFC) operation because PEMFCs operate in a hot, humidified, oxygenated environment. Finally, graphitized carbon black supports with the same surface area and platinum loading as ungraphitized supports show much greater stability in both the ex situ and in situ tests, suggesting that carbon surface chemistry plays a strong role in oxidative stability under fuel cell conditions.

This study focuses on changes in the structure of ionomer membranes, provided by the 3M Fuel Cells Component Group, as a function of the equivalent weight (EW) and the relationship between the structure and the properties of the membrane. Wide-angle X-ray diffraction results showed evidence of both non-crystalline and crystalline ordered hydrophobic regions in all the EW membranes except the 700 EW membrane. The spectral changes evident in the vibrational spectra of the 3M membranes can be associated with two major phenomena: (1) dissociation of the proton from the sulfonic acid groups even in the presence of small amounts of water; and (2) changes in the conformation or the degree of crystallinity of the poly(tetrafluoroethylene) hydrophobic domains both as a function of EW and membrane water content. All the membranes, regardless of EW, are thermally stable up to 360 °C. The wet membranes have conductivities between 7 and 20 mS/cm at 125 °C. In this condition, the conductivity values follow VTF behavior, which suggests that the proton migration occurs via proton exchange processes between delocalization bodies (DBs) that are facilitated by the dynamics of the host polymer. The conductivity along the interface between the hydrophobic and hydrophilic domains makes a larger contribution in the smaller EW membranes likely due to the existence of a greater number of interfaces in the membrane. The larger crystalline domains present in the higher EW membranes provide percolation pathways for charge migration between DBs, which reduces the probability of charge transfer along the interface. Therefore, at higher EWs although there is charge migration along the interface within the hydrophobic-hydrophilic domains, the exchange of protons between different DBs is likely the rate-limiting step of the overall conduction process.

An extensive SAXS investigation of the 3M perfluorinated sulfonic acid ionomer was performed to investigate the morphological changes that occur during and after annealing at temperatures above the T α. The effect of film thickness in the range studied, 11–45 μm, was found to be negligible. These properties were studied as a function of equivalent weight from 700 to 1100 and correlated with the water uptake as measured by dynamic vapor sorption. Isoscattering points were observed in dynamic annealing experiments of the unboiled annealed films at q = 0.023, 0.096 Å–1. On initial water uptake these films also showed isoscattering points at q = 0.024, 0.220 Å–1; q = 0.029, 0.223 Å–1; and q = 0.030, 0.211 Å–1 at 50, 80, or 95 °C, respectively, indicating a decrease in the symmetry of the scattering objects in these size regimes. Isoscattering points were absent in similar water uptake experiment for the films after boiling.

Various durability properties of a nanostructured thin film based catalyst system (3M's NSTF) which does not contain carbon or additional ionomer, are compared with conventional carbon supported dispersed Pt catalysts. The NSTF catalysts are shown to have mass losses by TGA at 170 o C that are two orders of magnitude less. The NSTF whisker support material in combination with the large-grained polycrystalline thin film catalysts that encapsulate the whiskers give significantly greater durability under normal fuel cell operating conditions and also under accelerated high voltage conditions, whether scanning between 0.6 and 1.2 volts at 20mV/sec under H 2 /N 2 , or under steady state conditions of 1.5 volts. At 1.5 volts, NSTF catalysts lose no surface area, specific activity or fuel cell performance over periods as long as 3 hours, whereas the Pt/C based electrodes lose both large amounts of surface area, activity and performance in just 30 minutes at 1.5 volts.

This work describes the modification of the sulfonyl fluoride precursor of perfluorinated sulfonic acids (PFSAs) by the conversion of the sulfonyl fluoride group to a bis sulfonylimide group. The usefulness of these new polymers for fuel cell applications includes novel proton exchange membranes, catalyst additives or tie layers designed to be thermally and chemically robust while operating within a fuel cell's harsh environment at higher temperatures.

Recent work at 3M has focused on the development of solvent cast proton exchange membranes (PEM's) for use in PEM fuel cells. These new membranes are a perfluorinated sulfonic acids based on a low molecular weight perfluorinated monomer and they exhibit excellent mechanical properties and chemical stability and high ionic conductivity. The low molecular weight of the monomer allows membranes with equivalent weight as low as 800 g/mole to have good mechanical properties when hydrated. Stabilizing additives in these membranes have been shown to improve the oxidative stability in Fenton's tests. Physical property, conductivity and fuel cell tests have been performed. When incorporated into membrane electrode assemblies, these new membranes have provided excellent performance and a greater than 15-fold increase in durability under accelerated fuel cell test conditions, compared with similar commercial PEM's.

Highly conducive to high conductivity: Polyoxometalates were incorporated in the backbone of a hydrocarbon polymer to produce proton-conducting films. These first-generation materials contain large, dispersed clusters of polyoxometalates. Although the morphology of these films is not yet optimal, they already demonstrate practical proton conductivities and proton diffusion within the clusters appears to be very high.

Membranes based on 3M's perfluoro imide acid (PFIA) ionomer have been studied using the open circuit voltage (OCV) hold accelerated stress test. For these samples, a decay in the OCV potential within the first 200 hours was observed along with an increase in cell resistance that rapidly grows near the end of life. A multi-membrane electrode assembly (MEA) technique was employed to study the origins of these phenomena. Effluent water collected at the beginning of life and later, during a recovery protocol, was analyzed using liquid chromatography -mass spectroscopy (LC-MS). A variety of small molecule fragments were detected that could be traced to the ionomer side chain. The center layer of a three-membrane MEA was separated at the end of life and analyzed by 19 F nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy. Peaks associated with the bis(sulfonyl)imide group of the PFIA ionomer were observed to have reduced in intensity and new peaks, assigned to a perfluorosulfonamide side chain, appeared. The combination of the fragments detected in the effluent water, along with the spectral changes in the membranes after aging, point to one or more degradation processes involving the PFIA ionomer side chain. This decomposition is likely analogous to reactions described for perfluorosulfonic acid (PFSA) ionomers but with new consequences owing to the greater number of potential fragments.