Understanding and Controlling Wettability of Carbon Powders for PEM Fuel Cell Applications
Date
2015-03-30
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Abstract
The wettability of catalyst layers (CLs), especially of the cathode, is critical to enhancing the performance and lifetime of polymer electrolyte membrane fuel cells (PEMFCs). As the CLs are composed of Pt nanoparticles supported on carbon powders, bound together with Nafion®, the main focus of this work was to understand and control the wettability of carbon materials for their use in PEMFCs.
The droplet impacting (DI) method was first used to study the wettability of conventional, microporous Vulcan® carbon (VC)/Nafion composites, showing that the wettability varies with VC content (from hydrophilic to superhydrophobic) and is significantly affected by the presence of Nafion. This is attributed primarily to the orientation and coverage of the Nafion molecular chains on the carbon surface, as well as the roughness of the composite surfaces.
Both contact angle kinetics (CAK) and water vapor sorption (WVS) measurements were then used to examine the relative wettability of several other types of carbon powders, but in the absence of Nafion, and using VC as a benchmark material. It was found that colloid-imprinted carbon (CIC) powders, synthesized using silica colloid templates of varying sizes, are much more hydrophilic than VC, due to their high surface oxygen group density. It was also shown that heat treatment at 1500 °C in N2 makes the CICs much more hydrophobic by removing the surface oxygen groups.
Ordered mesoporous carbons (OMCs), synthesized using hexagonal mesoporous silica templates but with different carbon precursors, were also found to contain hydrophilic mesopores, but the anthracene-derived OMC contains more large mesopores and is more hydrophilic than the sucrose-derived OMC, which may have a very thin carbon coating on its surface. These differences were explained as being due to the different formation mechanisms of the two OMCs.
Both CIC and VC surfaces were also surface modified with pentafluorophenyl groups, using an in-situ diazonium reduction reaction. The fluorination was confirmed to increase the hydrophobicity of these carbons, but the reaction was also found to introduce nitro groups onto the carbon surfaces.
Overall, this thesis work has demonstrated that the three methods used here for wettability assessment, DI, CAK and WVS, can all contribute to the understanding of the properties of carbon powder surfaces. The as-synthesized mesoporous carbons have been shown to be very hydrophilic, and either heat treatment or surface fluorination converts them to a hydrophobic state.
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Energy, Engineering--Environmental, Materials Science
Citation
Li, X. (2015). Understanding and Controlling Wettability of Carbon Powders for PEM Fuel Cell Applications (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/28556