Invited: Advanced Solid-State NMR Spectroscopy Studies for Improved Performance in PEM-FCs

The membrane electrode assembly (MEA) installed in proton exchange membrane fuel cells (PEM-FCs) is critical to the power density and lifetime of these devices.  Catalyst-coated membranes (CCMs), essential components of state-of-the-art MEA designs, provide an optimal structure for catalyst function, a wide range of operational temperatures, and optimal electronic performance.¹ Furthermore, efforts have been made to increase the ionic conductivity and mechanical performance of these membranes through composition with various solid acids.^{2, 3}Unfortunately, due to the complexity of PEM-FC devices, there are several operational challenges to overcome, including catalyst poisoning, low proton conductivity at ambient humidities, and ionomer or membrane degradation.

This work aims, in part, to investigate the incorporation of functionalized graphene oxide (GO) into PEM-FCs. The membrane within these devices is most commonly made of Nafion^®, a proprietary fluoropolymer, due to its high ionic conductivity and chemical stability. However, the material is expensive and not ideal for every fuel cell application. GO, a derivative of the super-material graphene, has intrinsic proton conductivity which is comparable to Nafion^®. This makes GO and GO/composite membranes viable candidates for use in PEM-FCs.^4-6Little work has been published on structural relationships or the mechanisms of ion conductivity within these composite materials.

To date, novel functionalized GO samples have been reliably synthesized. Sulfonic acid groups have been chemically grafted onto GO sheets, connected by organic linker groups of varying length and rigidity. These materials have been studied using various ¹H and ¹³C solid state NMR techniques. ¹³C NMR spectra show clearly the functional group modification of GO samples after the grafting of alkylsulfonic acid groups. Dehydration of these samples allows the collection of ¹H spectra with resolved acid proton / water peaks, despite the rapid exchange between these two proton populations. This lecture will describe current results in studying the synthesis and modification of sulfonated GO samples, as well as the proton dynamics and electrochemical performance in polymer electrolyte membranes either composed of or infused with functionalized GO.

Additionally, ¹H Double Quantum NMR is a well-established probe of local dynamics.⁷ Here for the first time, we extend this concept to characterize the fluorinated ionomer backbone. ¹⁹F double quantum SSNMR methods are applied to industrially-relevant ionomer materials to quantify and compare local dynamics of the ionomer side chains and backbones. ¹⁹F Double Quantum Filtered NMR experiments have been performed to investigate proton dynamics as well as local dynamics of perfluorinated polymer backbone and side chains. It has been shown that the backbone ¹⁹F has a steeper dipolar coupling build-up curve compared to the side chain, indicating a noticeable difference in rigidity. Future studies on the effect of relative humidity on ¹⁹F dynamics will provide a measure of the trends in local mobility at the molecular level, distinguishing side-chain from backbone contributions.

References:

1) Bessarabov, D.; Hitchcock, A. Membrane Technology 2009, 12,  6-12. 2) Yan, Z. B.; De Almeida, N. E.; Traer, J. W.; Goward, G. R. Phys. Chem. Chem. Phys. 2013, 15(41)  17983-17992. 3) Lee, Y. J.; Bingöl, B.; Murakhtina, T.; Sebastiani, D.; Meyer, W. H.; Wegner, G.; Spiess, H. W. J. Phys. Chem. B 2007, 111(33)  9711-9721. 4) Kumar, R.; Scott, K. Chem. Commun. 2012, 48(45)  5584-5586. 5) Tseng, C. Y.; Ye, Y. S.; Cheng, M. Y.; Kao, K. Y.; Shen, W. C.; Rick, J.; Chen, J. C.; Hwang, B. J. Adv. Energy Mater. 2011, 1(6)  1220-1224. 6) Zarrin, H.; Higgins, D.; Jun, Y.; Chen, Z.; Fowler, M. J. Phys. Chem. C 2011, 115(42)  20774-20781. 7) Ghassemzadeh, L.; Kreuer, K.; Maier, J.; Müller, K. J. Power Sources 2011, 196 (5)  2490-2497.