Identical Location Mapping of Pt in Polymer Electrolyte Fuel Cells before and after Heavy-Duty Accelerated Stress Test
Authors: Khedekar Kaustubh , Zaffora Andrea , Atanassov Plamen , Cheng Lei , Johnston Christina , Zenyuk Iryna V. 
Published: November 2022

Worldwide, governments and industries are pursuing the use of clean hydrogen to achieve zero emissions, especially for difficult to decarbonize sectors such as heavy-duty transportation, aviation, shipping and chemical manufacturing. Polymer electrolyte fuel cells (PEFCs) are an excellent candidate for heavy-duty vehicles (HDVs), particularly for their ability to scale range at a much smaller additional weight penalty1. However, initial system cost remains a significant challenge for large scale adoption mainly due to use of Platinum (Pt) electrocatalyst. Current approach to reduce 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 durability1. During a HDV 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 ripening2. The Pt ions can also diffuse towards the anode and get reduced near the membrane-cathode interface by the crossover hydrogen to form a Pt band2. In addition, the Pt ions can leave the system via effluent water. The complex balance between cost, performance and durability of PEFCs makes understanding degradation mechanisms a priority.

In this study, membrane electrode assemblies (MEAs) were subjected to accelerated stress tests (ASTs) to simulate heavy-duty lifetime. A simple method was developed to use micro-X-ray fluorescence spectroscopy to map identical locations of catalyst layer before and after the AST. The AST consisted of potential cycling between 0.6 V to 0.9 V (3s hold time each) for 90,000 cycles. Electrochemical characterization was performed at beginning of test, after 10k, 30k, 60k and 90k (end of test) AST cycles. The identical location maps revealed significant in-plane movement of Pt over the course of AST suggesting that electrochemical Ostwald ripening may not be a local effect. The in-plane movement of Pt led to development of local loading hotspots. A movement of Pt away from the cathode catalyst layer cracks was also observed. Such movement may be due to relative surface energy differences between the regions. A modified gas diffusion layer MEA was used to highlight the advantages and necessity of the method developed in this study. The modified MEA showed a ~13% change in/loss of Pt loading in the mapped region closer to the gas inlet. Identical location mapping allowed quantification of changes in the Pt loading caused by the AST. Lastly, identical location synchrotron micro-X-ray diffraction and florescence mapping was performed after the heavy-duty AST to identify correlation between Pt nanoparticle size growth and Pt loading. A direct correlation was observed, which developed only after the MEA was subjected to heavy-duty AST.

References

Cullen, D. A. et al. New roads and challenges for fuel cells in heavy-duty transportation. Nat. Energy
6, 462–474 (2021).

Khedekar, K. et al. Probing Heterogeneous Degradation of Catalyst in PEM Fuel Cells under Realistic Automotive Conditions with Multi-Modal Techniques. Adv. Energy Mater.
11, (2021).