So₂ Adsorption On Carbon-Supported Pt Electrocatalyst

<p>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.</p> <p>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.</p> <p>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 O₂after 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.</p> <p>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.</p>

Name thesis:degree_name

PhD

Level thesis:degree_level

Campus Access Dissertation

Discipline thesis:degree_discipline

Chemical Engineering

Year

2009

Author dc:creator

• Punyawudho, Konlayutt

Contributors dc:contributor

• John W. Van Zee

• Chemical Engineering
• Engineering
• PEM fuel cell contamination
• Pt/C catalyst
• SO2 adsorption
• SO2 contamination

Statement dc:rights

• © 2009, Konlayutt Punyawudho

Repository record dc:identifier

https://scholarcommons.sc.edu/etd/83

OAI identifier oai:identifier

oai:scholarcommons.sc.edu:etd-1084