Analysis of Transport Characteristics of Protons in Polymer Electrolyte Membranes in PEFC below Freezing Temperatures
Authors: Nishizawa Hiroki , Mabuchi Takuya , Tokumasu Takashi 
Published: December 2024

Fuel cells have been attracted to achieve carbon neutralization and Sustainable Development Goals (SDGs) worldwide. Polymer electrolyte fuel cells (PEFC) offer the advantage of starting up at normal temperatures, enabling miniaturization and lightweight design. PEFCs are utilized in various sectors including fuel cell vehicles and household stationary power sources. Challenges include variations in water characteristics between the catalyst layer and polymer electrolyte membrane (PEM)1, as well as wastewater freezing below freezing temperatures. Decreased proton transport characteristics are significant due to their impact on power generation efficiency. The Grotthuss mechanism2, in which a proton diffuses through the hydrogen bond network of water molecules, is considered a dominant factor for proton diffusion below freezing temperatures because the kinetic energy decreases significantly. Sulfonate groups in each polymer chain act as hydrophilic functional groups, water molecules and hydronium ions to localize in solvation areas, where proton hopping occurs. This study aims to investigate the proton hopping characteristics within PEM below freezing temperatures.

This research employs reactive force field molecular dynamics (ReaxFF3,4) to investigate proton hopping characteristics in PEM in that environment. ReaxFF MD enables the reproduction of bond switching, facilitating the consideration of chemical reactions. A parameter set, which we improved regarding ionization degree and the abnormal bond between the oxygen of Nafion chain and the hydrogen of the hydronium ions, based on the work of D. Fantauzzi et al.5, was used in this study. ReaxFF MD was performed with large-scale atomic/molecular massively parallel simulator (LAMMPS6). Nafion was employed as the polymer material of PEM in the simulations.

Figure 1. (a) shows the detail of solvation around the sulfonate groups. The black line shows the solvation areas. The area enclosed by the solid line where two solvation regions overlap is referred to as the "overlapped area" 7. Figure 1. (b) shows the result of radial distribution function (RDF) between sulfur of functional groups and oxygen of water molecules at different temperatures of 300 K (black) and 200 K (red), respectively. The first caves of these RDFs are about 4.3 Å, and thus the radius of solvation areas is 4.3 Å in all cases. The similarity of these results to previous studies7 demonstrates the capability to accurately analyze the structures around solvation areas and overlapped areas. Further investigation will compare the relationship between proton hopping frequency and proton diffusion coefficients at different temperatures, as well as evaluate the correlation between overlapped areas and proton hopping positions under various conditions.

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Figure 1