Skip to main content
eScholarship
Open Access Publications from the University of California

Scholarly Works (6 results)

  • Thesis
  • Peer Reviewed
UC Irvine Electronic Theses and Dissertations (2022)

Governments and industries are pursuing the use of green hydrogen to achieve zero emissions, especially for difficult to decarbonize sectors such as transportation, aviation, shipping and chemical manufacturing. Polymer electrolyte fuel cells (PEFCs) are an excellent candidate for light and heavy-duty vehicles (LDVs and HDVs) particularly for their ability to scale range at a much smaller additional weight penalty. However, initial system cost remains a barrier for large scale adoption mainly due to use of Platinum (Pt) electrocatalyst. Reducing initial cost by utilizing highly dispersed Pt nanoparticles (2-3 nm) adversely affects the system lifetime. The smaller nanoparticle size does result in improved Pt dispersion, which enhances performance and reduces Pt loading and cost. But smaller nanoparticles also tend to degrade faster due to higher surface energy, which negatively impacts durability. During vehicle drive cycle, Pt nanoparticle surface undergoes repeated oxidation-reduction which leads to dissolution of Pt ions causing loss in electrochemical surface area. The dissolved Pt ions can redeposit on nearby larger nanoparticles. This effect is known as electrochemical Ostwald ripening. The Pt ions can also diffuse towards the anode and get reduced at the membrane-cathode interface by the crossover hydrogen to form a Pt band. In addition, the Pt ions can completely leave the system via effluent water. The critical balance between cost, performance and durability makes understanding PEFC degradation a priority. Thus, implementing advanced electrochemical and analytical techniques, this dissertation addresses the challenge to decarbonize transportation sector by improving the durability of hydrogen powered net-zero emission PEFC systems.

The results of this dissertation can be distinguished into four chapters. In chapter 1, novel use of μ-X-ray diffraction (μ-XRD) was accomplished to analyze Pt nanoparticle size growth after accelerated stress tests were performed on MEAs using flow fields with different land-channel geometries. Two dimensional μ-XRD maps of the flow field inlet and outlet regions showed heterogeneity with higher electrocatalyst degradation near the inlet caused by increased local relative humidity. For flow field with land-channel dimensions 1 mm, higher electrochemical Ostwald ripening was observed under the lands when compared to channels due to differences between heat and water management. This land-channel heterogeneity disappeared for flow fields with land-channel dimension below 0.5 mm. However, the inlet-outlet heterogeneity stayed. In chapter 2, using μ-X-ray computed tomography (μ-XCT), morphological differences between commercially available gas diffusion layers (GDLs) like SGL 22BB, Freudenberg H23C6 and AvCarb MB30 designed to be used in high RH and high current density conditions were elucidated. Accelerated stress tests (ASTs) performed using the GDLs highlighted distinct effect of morphology and microporous layer cracks (via water management) on Pt dissolution. CFD simulations of water transport in GDLs (using Lattice Boltzmann method) were also used to explain the observed phenomenon. In chapter 3, using focused-ion beam scanning electron microscopy, non-dispersive infrared spectroscopy, μ-XRD and μ-X-ray fluorescence (μ-XRF) in tandem with oxygen mass transport resistance experiments a precise chronology of cathode catalyst layer degradation (two phases) during carbon corrosion was revealed. In the initial phase, amorphous carbon in contact with Pt nanoparticles oxidized fast. Rapid carbon loss and catalyst layer thinning occurred, but pore structure did not change significantly. Pt nanoparticles detached from the support and ECSA decreased drastically. In the second phase, carbon corrosion slowed down, but severe pore structure collapse was observed explaining the apparent lag observed between carbon loss and the cathode catalyst layer pore structure collapse. In the last chapter, an experimental protocol was developed to enable μ-XRF mapping of membrane electrode assemblies subjected to HDV lifetime. Heavy in-plane movement of Pt in the cathode catalyst layer was revealed for the first time suggesting that electrochemical Ostwald ripening may not be a local effect. Successive synchrotron μ-XRD and μ-XRF discovered a previously unknown correlation between nanoparticle size growth and loading which develops only after HDV lifetime. The results provide short-term system mitigation strategies and long-term direction for durable catalyst materials development to enable zero emission PEFC vehicles.

    Cover page: Understanding Degradation in Polymer Electrolyte Fuel Cells for Light and Heavy-duty Vehicle Applications
    • Article
    • Peer Reviewed
    UC Irvine Previously Published Works (2022)
    Gas diffusion layers (GDLs) play a crucial role in heat transfer and water management of cathode catalyst layers in polymer electrolyte fuel cells (PEFCs). Thermal and water gradients can accelerate electrocatalyst degradation and therefore the selection of GDLs can have a major influence on PEFC durability. Currently, the role of GDLs in electrocatalyst degradation is poorly studied. In this study, electrocatalyst accelerated stress test studies are performed on membrane electrode assemblies (MEAs) prepared using three most commonly used GDLs. The effect of GDLs on electrocatalyst degradation is evaluated in both nitrogen (non-reactive) and air (reactive) gas environments at 100% relative humidity. In situ electrochemical characterization and extensive physical characterization is performed to understand the subtle differences in electrocatalyst degradation and correlated to the use of different GDLs. Overall, no difference is observed in the electrocatalyst degradation due to GDLs based on polarization curves at the end of life. But interestingly, MEA with a cracked microporous layer (MPL) in the GDL exhibited a higher electrocatalyst loading loss, which resulted in a lower and more heterogeneous increase in the average electrocatalyst nanoparticle size.
      Cover page: Effect of Commercial Gas Diffusion Layers on Catalyst Durability of Polymer Electrolyte Fuel Cells in Varied Cathode Gas Environment
      • Article
      • Peer Reviewed
      UC Irvine Previously Published Works (2020)
      Pt catalysts in polymer electrolyte fuel cells degrade heterogeneously as the catalyst particles are exposed to local variations throughout the catalyst layer during operation. State-of-the-art analytical techniques for studying degradation of Pt catalysts do not possess fine spatial resolution to elucidate such non-uniform degradation behavior at a large electrode level. A new methodology is developed to spatially resolve and quantify the heterogeneous Pt catalyst degradation over a large area (several cm2) of aged MEAs based on synchrotron X-ray microdiffraction. PEFC single cells are aged using voltage cycling as an accelerated stress test and the degradation heterogeneity at a micrometer length scale is visualized by mapping Pt catalyst particle size after voltage cycling. It is demonstrated in detail that the Pt catalyst particle size growth is non-uniform and follows the flow field geometry. The Pt particle size growth is greater in the area under the flow field land, while it is minimal in the area under the flow field channel. Additional non-uniformity is observed with the Pt particle size increasing more rapidly at the air outlet area than the Pt particle size at the inlet area.
        Cover page: Mapping of Heterogeneous Catalyst Degradation in Polymer Electrolyte Fuel Cells
        • Article
        • Peer Reviewed
        UC Irvine Previously Published Works (2023)
        Fuel cell heavy-duty vehicles (HDVs) require increased durability of oxygen-reduction-reaction electrocatalysts, making knowledge of realistic degradation mechanisms critical. Here identical-location micro-X-ray fluorescence spectroscopy was performed on membrane electrode assemblies. The results exposed heavy in-plane movement of electrocatalyst after HDV lifetime, suggesting that electrochemical Ostwald ripening may not be a local effect. Development of local loading hotspots and preferential movement of electrocatalyst away from cathode catalyst layer cracks was observed. The heterogeneous degradation exhibited by a modified cathode gas diffusion layer membrane electrode assembly after HDV lifetime was successfully quantified by the identical-location approach. Further synchrotron micro-X-ray diffraction and micro-X-ray fluorescence experiments were performed to obtain the currently unknown correlation between electrocatalyst nanoparticle size increase and loading change. A direct correlation was discovered which developed only after HDV lifetime. The work provides a route to engineer immediate system-level mitigation strategies and to develop structured cathode catalyst layers with durable electrocatalysts. [Figure not available: see fulltext.]
          Cover page: Revealing in-plane movement of platinum in polymer electrolyte fuel cells after heavy-duty vehicle lifetimeCreative Commons 'BY-NC-SA' version 4.0 license
          • Article
          • Peer Reviewed
          UC Irvine Previously Published Works (2021)
            Cover page: Carbon Corrosion in Polymer Electrolyte Fuel Cells: A Complex Interplay between Morphological Changes and Electrochemical Performance
            • Article
            • Peer Reviewed
            UC Irvine Previously Published Works (2021)
            The heterogeneity of polymer electrolyte fuel cell catalyst degradation is studied under varied relative humidity and types of feed gas. Accelerated stress tests (ASTs) are performed on four membrane electrode assemblies (MEAs) under wet and dry conditions in an air or nitrogen environment for 30 000 square voltage cycles. The largest electrochemically active area loss is observed for MEA under wet conditions in a nitrogen gas environment AST due to constant upper potential limit of 0.95 V and significant water content. The mean Pt particle size is larger for the ASTs under wet conditions compared to dry conditions, and the Pt particle size under land is generally larger than under the channel. Observations from ASTs in both conditions and gas environments indicate that water content promotes Pt particle size growth. ASTs under wet conditions and an air environment show the largest difference in Pt particle size growth for inlet versus outlet and channel versus land, which can be attributed to larger water content at outlet and under land compared to inlet and under channel. From X-ray fluorescence experiments Pt particle size increase is a local phenomenon as Pt loading remains relatively uniform across the MEA.
              Cover page: Probing Heterogeneous Degradation of Catalyst in PEM Fuel Cells under Realistic Automotive Conditions with Multi‐Modal Techniques
              Top