Date of Award

12-2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemical and Biomolecular Engineering

Advisor

Goodwin, Jr., James G.

Committee Member

Bruce , David A.

Committee Member

Kitchens , Christopher L.

Committee Member

Creager , Stephen E.

Abstract

It is believed that proton–exchange membrane fuel cells (PEMFCs) are going to be exploited as power generation devices for stationary, automotive, and portable applications in the near future because of their high performance and environmental friendliness. Up to the present, wide spread utilization of PEMFCs is not viable because of their cost and durability in the presence of fuel and oxidant stream impurities. These impurities include NH3, hydrocarbons, CO, CO2, halogenated compounds, cations, etc. The objective of this research was to investigate the effect of these harmful contaminants containing in the H2 fuel stream on the operation and durability of individual components in a fuel cell, to develop new diagnostic strategies to quantitatively and individually examine the conductivity of the ionomer and catalyst activity, and to suggest an approach to mitigate the effect of impurities and enhance fuel cell durability and performance.
It is known that NH3 is one of the most detrimental impurities to proton transport in the Nafion membrane and ionomer layers. In this study, the effect of NH4+ ions and NH3 on liquid–phase conductivity at ambient temperature and on gas–phase conductivity at typical fuel cell conditions (30–100%RH and 80°C) of Nafion membranes was investigated via electrochemical impedance spectroscopy. It was found that the liquid–phase and gas–phase conductivities decrease linearly and exponentially with an increase in the NH4+ ion composition in the membrane (yNH4+), respectively. In the presence of gas–phase NH3, the conductivity decreased with time–on–stream and reached steady–state. The kinetics of conductivity decrease were slower at higher humidities. The humidity and yNH4+ appear to have a combined effect on the conductivity of Nafion.
Experimental results suggested that proton conductivity in a Nafion membrane is affected not only by yNH4+ but also by its distribution. The NH4+ ions in the aqueous phase NH4+ ion–exchanged membranes were homogeneously distributed, while those in the gas phase NH3–poisoned membrane were non–uniformly distributed. It was found that at the same NH4+/NH3 uptake in Nafion membranes, the conductivities of these two kinds of membranes were not the same, except for the conductivity of membranes in the pure H+– and NH4+–forms. Therefore, the effect of NH4+ ion distribution on Nafion conductivity at conditions relevant to fuel cell environments and at room temperature in deionized (DI) water was investigated. The gas–phase conductivities of non–uniformly poisoned membranes were ca. 1.07–1.86 times larger than those of uniformly poisoned membranes, depending on humidity, contamination level, and NH4+ ion distribution. On the other hand, the effect of NH4+ ion distribution on liquid–phase conductivities cannot be examined due to rapid redistribution of ions in the Nafion under this condition.
It has been previously reported that during MEA preparation and/or fuel cell operation, foreign cationic ions such as Na+, Ca2+, Ni2+, Fe3+, etc., deriving from the gas supply, corrosion of materials, and other sources, accumulate in a PEMFC and cause a significant decrease in conductivity. In this study, the influence of nonproton–containing cations (Mn+ = Na+, Ca2+, and Fe3+) on conductivity of Nafion membranes was quantitatively investigated both in DI water at room temperature (25°C) and in the gas phase at 80°C under conditions similar to in a typical PEMFC. It was found that the impact of metal cations on liquid–phase conductivity was less severe than that on gas–phase conductivity. The conductivity of contaminated membranes in the Mn+–form was 12 times and 6–125 times less than the H+–form at 25°C in DI water and at 80°C in gas phase, respectively depending on humidity.
Even if a fuel cell is operated in neat H2 fuel and oxidant streams, it has been confirmed that the degradation of PEMFC performance can occur due to formation of H2O2 and material corrosion during fuel cell operations. Previous studies have reported that Nafion is not chemically stable under these severe conditions. A quantitative examination of properties and conductivities of degraded Nafion membranes at conditions relevant to a working fuel cell (30–100 %RH and 80°C) was performed. It was found that the degradation degree, water uptake, and conductivity of H2O2–treated membranes depended strongly on an Fe content and H2O2 treatment time. The properties and degradation level of H2O2–exposed Nafion observed in this study were compared with those reported in the literature and found to be in agreement considering differences in membrane treatment conditions and in membrane types.
Due to the lack of information about the effect of impurities on Nafion ionomer and the difficulty of conventional conductivity measurements in the catalyst layer of a PEMFC, an easy and convenient approach was initially developed in this work to quantitatively investigate the effect of impurities on proton availability and conductivity of Nafion components (membrane and ionomer in a catalyst layer). We developed and used an acid–catalysed reaction (esterification), since proton/acid sites in Nafion are crucial for both of proton transport and esterification. A thorough investigation of Nafion components as used in a general fuel cell was performed and quantitative results reported for the first time for physical and chemical properties, conductivity, and activation energy of a contaminated/degraded Nafion at conditions relevant to fuel cell. It was found that the new methodology with correlation could predict accurately the conductivity of Nafion membranes contaminated with other impurity species (i.e., NH3, Na+, etc.). Further investigation examined the proton availabilities of supported Nafion (Nafion on carbon and on Pt/C), as exists in the catalyst layer used in a PEMFC, in order to predict its conductivity. The effect of NH3 exposure on proton composition (yH+) of supported Nafion was similar to that of a Nafion membrane under the same conditions, indicating the possibility to use esterification to predict the conductivity in a catalyst layer. It was found that the predicted effective conductivities at practical fuel cell conditions of an NH4+–poisoned cathode catalyst layer obtained from the yH+ values (determined from esterification) and from the agglomerate model were found to correspond well with results available in the literature.
Another harmful impurity for PEMFC performance is CO. It significantly affects the hydrogen oxidation reaction (HOR) on Pt in the anode catalyst layer by interacting strongly to the Pt surface and blocking sites for H2 dissociation. The aim of this work was to quantitatively analyze the surface hydrogen concentration on Pt/C at conditions similar to a conventional fuel cell environment, since this parameter is crucial to fuel cell performance, especially in the presence of CO. A new diagnostic approach, H2–D2 switch with Ar purge (HDSAP), was used to investigate the amount of exchangeable adsorbed hydrogen on Pt surface during CO exposure. It was found that the presence of humidity decreases the kinetics of CO adsorption and increases in the amount of exchangeable surface hydrogen on Pt catalysts. This is because water competitively adsorbs on Pt sites, pore condensation hinders CO to diffuse to Pt sites, and dissociated water increases the concentration of exchangeable hydrogen on the Pt surface. The experimental results imply that water vapor helps diminish the effect of CO poisoning on Pt/C and better CO tolerance is expected at higher humidities.

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