Moving forward towards in-situ proton conduction dynamics in an operating fuel cell Sourav Maiti 1 , Ivan Scivetti 2 , Amy Edmeades 1 , C. D. M. Hutchison 1 , G. M. Greetham 1 , Gilberto Teobaldi 3 and Paul. M. Donaldson 1 1 Central Laser Facility, Harwell Science and Innovation Campus, UK, 2 Scientific Computing Department, Daresbury Laboratory, UK, 3 Scientific Computing Department, Rutherford Appleton Laboratory, UK Proton exchange membranes (PEMs) such as Nafion are an integral part of the H 2 /O 2 fuel cell allowing proton conduction from anode to cathode. There is therefore a drive to understand proton transport dynamics in Nafion, especially under operating conditions. Transport of the excess proton transport through aqueous media is known to be interlinked with the structural dynamics of water. 1 For this reason, measuring the ultrafast dynamics of PEM water molecules plays an essential role in investigating proton transport in Nafion. Previous studies have substituted the protons in Nafion with sodium ions (Na + ), simplifying the infrared spectra to allow transient vibrational experiments 1 , however, the resulting system is no longer a proton conductor, thus impeding in-situ measurements of proton transport in an operating fuel cell.
Figure 1. Representative 2DIR spectra (parallel polarization) of (a) OD-stretch in HOD (8%) in protonated Nafion and (b) proton continuum [H + (H 2 O) n ] at 150 fs population time. (c) Comparison of anisotropy decay in protonated Nafion and HOD in Nafion. Our work aims to address the following questions: (i) Can we study the water dynamics in the protonated-Nafion through vibrational spectroscopy? (ii) Can polarization selective vibrational spectroscopy of protonated-Nafion provide insight into the proton transport process especially to distinguish between bulk and interfacial transport? (iii) Can molecular dynamics simulations with density functional theory (MD-DFT) help to unravel the complex interplay between proton transport and water orientation at the Nafion interfaces? We show that (i) studying water dynamics in Na + -Nafionis possible, the first step towards in-situ studies (Figure 1a). O-D dynamics in protonated Nafion are similar to those in Na + -Nafion. The anisotropy decay of water indicates the co-existence of interfacial and bulk water components, thus suggesting slower water dynamics and proton transport at the interface. The 2DIR spectra of protonated Nafion (Figure 1b) show the off-diagonal peaks very different from bulk acid. (ii) the anisotropy decay of protonated water (H + (H 2 O)n) in Nafion conveys information about the proton transport dynamics which has a closer time scale to spectral diffusion (Figure 1c). 2,3 . (iii) although MD-DFT results significantly depend on the adopted DFT approximation and the atomistic models, computed orientational and proton-transfer correlation functions allow identifying bulk and interfacial water with distinct anisotropic responses. The qualitative agreement with previous force-field studies 4 for the rotational dynamics demonstrates the benefit and potential of DFT to investigate the highly-reactive chemistry of proton- conducting membranes. References
1. Roget, S. A., et al., J Phys Chem B , 2019 , 123, 9408. 2. Sofronov, O. O., et al., ACS Cent Sci 2020 , 6 , 1150. 3. Carpenter, W. B., et al., J Phys Chem B 2018 , 122 , 2792. 4. Savage, J. et al. J Phys Chem C , 2014, 118, 17436-17445
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