Browsing by Subject "Degradation mechanisms"
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Item Understanding PEMFCs by 3D-identical location TEM and EELS(2018-12-07) Yu, Kang (Ph. D. in materials science and engineering); Liu, Yuanyue; Ferreira, Paulo Jorge; Mullins, Charles Buddie; Yu, Guihua; Xie, JianProton exchange membrane fuel cells are among the most promising power sources for both automotive and stationary applications, due to their cleanness and high efficiency. A critical factor which strongly affects the performance of these devices is the ionomer distribution and coverage on the carbon support, since the catalysts, which are typically Pt and/or Pt-alloy nanoparticles, must be located at the carbon support/ionomer interface to be active and perform the oxygen reduction reaction at the cathode. Yet, the characterization and identification of the ionomer distribution within the cathode has been very challenging. In this regard, the first part of the dissertation proposes a novel approach to identify and differentiate the ionomer, using the carbon signal produced by STEM-EELS is reported. In this fashion, not only the ionomer distribution, but also the carbon support distribution, can be probed. In addition, the proposed approach allows us to identify ionomer-rich and carbon-support rich regions, which are quite challenging to determine using other methods. The development of PEMFCs is also restricted by the degradation Pt or Pt-alloy nanoparticles during potential cycling. However, the degradation mechanisms are not fully understood. In this regard, the role of the various degradation mechanisms during a durability test is investigated by identical location transmission electron microscope (IL-TEM). In addition, by combining a low accelerating voltage and STEM-LAADF imaging, STEM tomography of Pt nanoparticles on carbon support is clearly resolved. The combination of IL TEM and STEM-tomography reveals few changes in the carbon support during potential cycling. In addition, Pt migration on the carbon support is rarely observed. In the last part of this work, a systematic study of functionalized carbon support by TEM is conducted. Positively charged functionalization of the carbon support leads to a more uniform Pt distribution and smaller pore sizes in the MEA.