UC Irvine

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.