The dispersing solvent used for fuel cell catalyst ink preparation plays a vital role in establishing the resulting morphology of the electrode layers, which in turn will impact the performance of proton exchange membrane (PEM) fuel cells. In this study, we report the impact of various ionomer dispersion solvents on PEM fuel cell performance and durability; two aqueous (1-propanol/water and 2-propanol/water) and several non-aqueous dispersing solvents (ethylene glycol and 1,2-butanediol) are compared. The cathode catalyst layer (CCL) fabricated using inks prepared with 1-propanol/water (3:1, w/w) exhibited the best initial performance followed by the CCL prepared using ethylene glycol. The CCLs made from non-aqueous ethylene glycol and 1,2-butanediol exhibited the best durability upon accelerated stress testing. Scanning transmission electron microscopy combined with energy dispersive X-ray spectroscopy indicated that, after the stress test, the distribution of both the Nafion ionomer and Pt nanoparticles within the CCLs prepared with non-aqueous ionomer dispersions underwent less change than those prepared with aqueous dispersions, which is responsible for the improved durability.

For proton exchange membrane fuel cells to be cost‐competitive in light‐ and heavy‐duty vehicle applications, their Pt content in the catalyst layers needs to be lowered. However, lowering the Pt content results in voltage losses due to high local oxygen transport resistances at the ionomer–Pt interface. It is therefore crucial to use ionomers that have higher oxygen permeability than Nafion. In this work, novel high oxygen permeability ionomers (HOPIs) are presented, with up to five times higher oxygen permeability than Nafion, synthesized by copolymerization of perfluoro‐2,2‐dimethyl‐1,3‐dioxole (PDD) with perfluoro(4‐methyl‐3,6‐dioxaoct‐7‐ene) sulfonyl fluoride (PFSVE). PDD is the source of higher permeability due to its open ring structure, while PFSVE provides ionic conductivity. Optimization of PDD content and equivalent weight enables increased fuel cell performance, mainly at high current densities, where HOPIs can achieve power densities >1.25 W cm−2 and exceed the 0.8 A cm−2 U.S. Department of Energy durability target by losing only 4.5 mV, which is over six times less than 30 mV. The interactions between HOPI and SO3− groups with a PtCo/C catalyst are also elucidated here at a fundamental level.

In a proton exchange membrane (PEM) fuel cell, the local oxygen transport across the ionomer film in the catalyst layer has a significant impact on electrode performance especially at high current density.1 It is therefore crucial to use ionomers that have higher oxygen permeability than the baseline Nafion. In this work, novel ionomers with increased oxygen permeability have been synthesized by copolymerization of perfluoro-2,2-dimethyl-1,3-dioxole (PDD) with perfluoro(4-methyl-3,6-dioxaoct-7-ene) sulfonyl fluoride (PFSVE) and a ter-monomer. PDD is the main source of higher permeability due to its bulky structure, PFSVE provides ionic conductivity and the amount of the ter-monomer is adjusted to achieve high yields without compromising the equivalent weight (EW). The ring structure of PDD creates additional open space within the ionomer structure for improved gas permeability. Some of the newly developed ionomers have up to five times higher permeability than Nafion, which should result in a significant improvement of fuel cell performance, mainly at high current densities. Ionomers with different PDD content and equivalent weight have been studied to establish the correlation between ionomer properties and MEA performance. Local oxygen resistance, ionomer sheet resistance, ionomer coverage, and SO3 - group coverage will be evaluated and correlated to electrode performance. The performance and durability of the electrodes using high permeability ionomer will be correlated to the ionomer interaction with catalyst particle. This work will provide a comprehensive understanding of interactions among Pt, carbon, ionomer and their impact on the electrode structure and fuel cell performance and durability. The attained information will be used to improve fuel cell electrode design. Acknowledgement: The project is financially supported by the Department of Energy’s Fuel Cell Technology Office under the Grant DE-SC0018597. References: 1. Baker et al, J. Electrochem. Soc. 156, B991 (2014).

The development of low cost and high performance cathode for the oxygen reduction reaction (ORR) remains a grand challenge for transportation applications of polymer electrolyte membrane fuel cells (PEMFCs). Fe and Co-based platinum group metal (PGM)-free catalysts have been previously studied, however facing poor long-term durability and may potentially accelerate membrane degradation caused by oxygen radicals1. To overcome the above weakness, Li et al.2 has developed Mn based PGM-free catalyst which can mitigate membrane degradation, meanwhile showing a competitive catalyst activity compared to Fe based catalyst. The activity of the Mn-N-C catalysts measured using rotating disk electrodes (RDEs) in acidic electrolytes approaches state-of-the-art Fe-N-C catalysts3. More importantly, the Mn-N-C catalysts have demonstrated enhanced stability using potential cycling (0.6-1.0 V) in O2-saturated acidic electrolytes. In addition to the Mn-based PGM-free catalyst development, the electrode design is also critical for the MEA performance. The oxygen reduction reaction (ORR) may occur at different interface in the catalyst layers for PGM and PGM-free catalysts. For PGM catalysts, the ORR takes place on the Pt/ionomer interface, while Pt is on the surface of the catalyst support. However, since the Mn active sites are embedded inside the metal organic framework (MOF), it is more difficult to establish the catalyst/ionomer interface PGM-free catalyst. Hereby, the ionomer with lower equivalent weight (EW=830) has been proposed to replace the conventional Nafion ionomer (EW=1100). The shorter chain of the low EW ionomer can penetrate into the micro-pores in the catalyst layer easier, which may promote their interaction with the catalyst active sites. In addition to the low EW ionomer, the ionomer to carbon (I/C) ratio also has a great impact on the proton, gas and water transports that can further affect the MEA performance. The particle size of the catalyst also affects the MEA performance. In the RDE studies, the Mn catalyst with a smaller average particle size (50 nm) shows a better ORR activity than the one with a larger average particle size (80nm). However, in the MEA studies, catalysts with larger average particle size seems to demonstrate better performance, likely due to optimal pore structures of electrodes. This work will provide a systematical understanding and guidance of the MEA design not only for Mn-based catalyst but also for other PGM-free catalysts. Acknowledgement: The project is financially supported by the Department of Energy’s Fuel Cell Technology Office under the Grant DE-EE0008075. Reference: Wang, X. X., Prabhakaran, V., He, Y., Shao, Y., Wu, G., Adv. Mater. 2019, 1805126. Li, J.; Chen, M.; Cullen, D. A.; Hwang, S.; Wang, M.; Li, B.; Liu, K.; Karakalos, S.; Lucero, M.; Zhang, H.; Lei, C.; Xu, H.; Sterbinsky, G. E.; Feng, Z.; Su, D.; More, K. L.; Wang, G.; Wang, Z.; Wu, G., Nature Catalysis 2018, 1 (12), 935-945. Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P., Science 2011, 332 (6028), 443-447.