Understanding Catalyst Layer Morphologies and Degradation and their Impact on Critical Oxygen Transport in PEMFC Cathodes
thesisposted on 07.07.2021, 19:01 by Jonathan BraatenJonathan Braaten
Polymer electrolyte membrane fuel cells or PEMFCs are a promising technology to address the need for electrification of the US and world automotive fleets. They are well-suited to vehicles requiring longer range and higher power operation like semis, delivery trucks, and traditional fleet vehicles as a result of fuel cells' innate decoupling of energy conversion from storage, allowing the power generating fuel cell stack and fuel tank to be scaled independently as needed for range and/or power. However, PEMFC technology remains more expensive than the incumbent internal combustion engine as a result of PEMFCs' reliance on platinum (Pt) for the catalysts in their anodes and cathodes, making up roughly 40% of the overall stack cost at high production volumes. Pt does not benefit from economies of scale due to its pure raw material costs and thus there have been widespread efforts to reduce Pt loadings to levels consistent with the Pt usage in catalytic converters for internal combustion engine vehicles. With these efforts to reduce Pt, significant performance losses have been observed due to hindered reactant transport, specifically oxygen (O2), and catalyst degradation and
ultimately Pt loss has occurred. In this dissertation, we outline five efforts aimed at better understanding and evaluating Pt-based catalyst degradation and the impacts on performance as well as addressing the need for improved O2 transport to the catalyst surface that has traditionally hindered high power performance. Specifically, we studied Pt and Pt-alloy catalyst and carbon support degradation for state-of-the-art catalyst layers (CLs) using our nanoscale X-ray computed tomography (nano-CT) imaging technique. We then evaluated the impacts of leached cations
from unstable and degraded catalysts as well as other common contaminants on the O2 transport properties of the ionomer that covers the catalyst surface and studied the migration and accumulation of such cations within the CL in-operando using a novel X-ray conducive PEMFC in a hydrogen (H2) pump configuration. In the final efforts of this dissertation, we examined the fundamental transport of O2 through water-filled nanopores that are common in state-of-the-art carbon supports and integrated a novel high oxygen permeable ionomer (HOPI) with state-of-the-art CLs to improve PEMFC performance at low and high power,
corresponding to improvement in heavy duty PEMFC vehicle efficiency and total available power.
- Doctor of Philosophy (PhD)