Optimization of the Morphology of Mesoporous Carbon Scaffolds and Pt Loading Methods for Use in PEMFC Cathodes

Proton exchange membrane fuel cells (PEMFCs) represent a promising technology for reaching net-zero goals and reducing fossil fuel dependence, especially in the automotive sector. However, PEMFCs still face challenges, including sluggish oxygen reduction (ORR) kinetics, durability issues, and high cost. Current PEMFCs utilize 40 wt% Pt/C with typical loadings of 0.3-0.5 mgPt/cm 2 . Along with the required proton conductor and binder (most commonly Nafion), this contributes significantly to the overall cost of the system. Additionally, the microporous nature of conventional carbon supports and the tortuosity of pathways between carbon particles can limit Pt nanoparticle (NP) utilization and add mass transport resistance. Also, surface-loaded Pt NPs are prone to agglomeration, dissolution, and poisoning by the Nafion binder, resulting in diminished stability and performance. To overcome these problems, our team initially focussed on the development of various types of ordered mesoporous carbons, including powders 1,2 . We then prepared first-time fully interconnected, mesoporous, binder-free, and self-supported carbon scaffolds (NCS) 3 . These are 100% carbon, have controllable monodisperse pores (5 to >100 nm in size), tunable thicknesses from 0.1 to >1000 µm, and area scalability up to >150 cm 2 , high porosity (~90%), ultra-low tortuosity, and high real surface areas (up to 600 m 2 /g). After loading these sheets with Pt NPs and Nafion, the ORR kinetics, measured in an MEA, were very good, but mass transport limitations at high current densities were still seen, likely due to challenges of proton, water, and oxygen transport through the pore necks, which are significantly smaller than the pore diameters. Therefore, a bimodal NCS film was designed and evaluated 4 , consisting of spheres (~1 um diameter) containing 3D interconnected, organized, and monodisperse 12 nm pores, with the spheres connected to each other by carbon nanofibers, leaving large secondary pores between them to enhance mass transport. Using ALD for Pt NP deposition, record-breaking ORR kinetics were observed, and a limiting current of 2.5 A/cm 2 was reached 4 . These results were somewhat surprising, as the Pt NPs are located inside the 12 nm pores inside the spheres, while Nafion is size-excluded from these pores due to their small 5 nm pore necks. However, the screening of Nafion from contacting Pt seems to prevent Nafion poisoning and yield better performance. However, mass transport limitations are still present, likely as the relatively large Pt-containing spheres present a long pathway for oxygen and protons to reach the Pt NPs in the absence of Nafion. To mitigate this issue, we are now preparing a range of bimodal NCS-12 scaffolds with significantly smaller spheres (200-500 nm) to decrease the diffusion length for oxygen and protons and improve accessibility to the catalytic Pt NPs. Using a hard-templating method, several different NCS12 materials have been made, with Pt NPs again loaded using ALD to achieve more control. The goal is to correlate structural differences of the bimodal carbon membrane with cathode performance in an MEA. The results indicate that the Pt ALD process does not alter the NCS-12 morphology, giving similar BET surface area, pore size and pore neck sizes before and after Pt deposition. TEM and FESEM analysis show that the Pt NP size and distribution is uniform throughout the NCS-12 thickness, ranging from 15-25 µm. Initial electrochemical testing in 0.5 M sulfuric acid solutions gave the real Pt surface area from Hupd (hydrogen underpotential deposition) and CO stripping measurements, using NCS-12 sheets with different Pt loadings. For ultra-low Pt loadings of < 2 wt%, confirmed by ICP and EDX analysis, unique CV profiles for both CO stripping and Hupd were observed, with HAADF-STEM analyses confirming that the Pt NPs are very small under these conditions. Interestingly, the ORR still exhibited a fairly impressive onset potential, suggesting that a highly active Pt state was achieved. Electrochemical performance testing is also being carried out using the ALD-Pt/NCS12 sheets at the cathode of an MEA, with the results compared to those obtained in aqueous media. Accelerated stress testing on the Pt NPs and the NCS-12 carbon support material was also performed following DOE protocols. Acknowledgements We thank Dr. Marwa Atwa, Dr. Anand Singh, and Dr. Scott Paulson for useful discussions and also acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada. References Li and M. Jaroniec, J. Am. Chem. Soc., 123, 9208–9209 (2001). Atwa et al., Chem. Mater. (2023). Atwa et al., Mater. Horiz., 8, 2451–2462 (2021). Atwa et al., ChemRxiv (2024).

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