Date of Award

1-1-2009

Document Type

Campus Access Dissertation

Department

Chemical Engineering

First Advisor

John W. Van Zee

Abstract

This dissertation considers sulfur dioxide (SO2) as a contaminant in the air fed to the cathode in Proton Exchange Membrane Fuel Cells (PEMFCs). Since the mechanism of SO2 contaminant is complicated, experimental data was obtained for isolated effects. For example at Open Circuit Voltage (OCV) conditions, accumulations of SO2 in PEMFCs were studied and the data showed that the primary accumulation was on the Pt/C electrocatalysts. Therefore, adsorption isotherms were obtained for SO2 on Pt/C electrocatalysts.

Accumulations are quantified with material balances on the inlet and the outlet SO2 stream during OCV. SO2 measurements were performed by oxidizing the SO2 in H2O2 solutions and analyzing those solutions with a pH electrode and ion chromatography. SO2 concentrations, exposure dosages, and Membrane Electrode Assembly (MEA) treatments were investigated. The data showed the accumulations were independent of concentration, but dependent on dosage. The treatment of "hydrated" and "un-hydrated" MEAs were characterized by Electrochemical Impedance Spectroscopy (EIS) to relate high frequency resistance to water content. The hydrated MEA showed greater accumulations than the un-hydrated MEA. And a hydrated MEA exposed to a wed feed stream (i.e., 50%RH at anode, 0% RH cathode) showed accumulations exceeding the available Pt sites. With an un-hydrated MEA, the SO2 adsorbed only when the electrocatalyst was presented.

To study the isolated Pt/C electrocatalyst, Temperature Programmed Desorption (TPD) was used to quantify the adsorption of SO2. First SO2 concentrations in N2 were varied from 5 ppm to 1% (vol) and adsorption isotherms were determined at 25, 50, and 80°C. Oxygen assisted (O-assisted) desorption experiments (i.e., successive TPD experiments following exposure to room temperature O2 after the first TPD event) produced an additional SO2 peak at a temperature higher than the initial SO2 peak. These two types of SO2 adsorption were identified as weakly-adsorbed SO2 species desorbed between 140 and 200 ºC, depending on concentration, and a strongly-adsorbed, dissociated species. For the strongly-adsorbed, dissociative species, 18O2 isotope introduction during O-assisted desorption yielded ratios of 50%, 36% and 14% for SO2 masses of 64, 66 and 68, respectively. The activation energy and kinetic constant of desorption are reported for weakly adsorbed SO2 at 1% and 20 ppm SO2 using the Polanyi-Wigner equation.

As a second step in isolating the adsorption on Pt/C electrocatalysts, TPD was used to study SO2 adsorption in the presence of O2. These results showed that in the presence of O2, the amount of adsorption SO2 was much larger than those in the absence of O2 (i.e., SO2 in N2). The results also showed that Pt was required for these large amounts of adsorption and the amount of adsorbed SO2 was about 75 times smaller with only the carbon support. Amounts that exceed monolayer coverage on Pt correspond to a spillover on the carbon support. The spillover of SO2 was examined by varying the Pt loading and particle size to distinguish desorption temperatures. X-ray Photoelectron Spectroscopy (XPS) indentified Pt-S, C-S, C-SOx and Pt-SO4 as adsorbed species on the platinum and the carbon support. Both TPD and XPS show that Pt is necessary for the spillover. The bonding of sulfur/sulfur oxide adsorbed on carbon support was strong and stable so that the SO2 did not diffuse back on to the platinum surface, once the Pt-SO2 species was removed.

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