Polymer Electrolyte Fuel Cells (PEFCs) play a crucial role in achieving carbon neutrality and the Sustainable Development Goals (SDGs). PEFCs face several challenges, including diffusion polarization, material transport efficiency, and durability. Our research focuses particularly on the operation of PEFCs under the harsh conditions of subzero temperatures. Many regions worldwide experience cold climates, making the resolution of these issues vital for the further popularization of PEFCs. In such environments, the transport efficiency of materials within the polymer electrolyte membrane (PEM), including protons crucial for fuel cell operation, decreases due to the potential freezing of water molecules and a significant reduction in kinetic energy. Nevertheless, previous research[1] elucidated that proton hopping contributes proton diffusion in ice. Therefore, this phenomenon is critical to the context of this research. In this study, we conducted a detailed analysis of the relationship between changes in water clusters and proton diffusion properties within PEM. We performed ReaxFF[2] MD simulations, which can treat proton hopping. The parameter set, which comprehensively defines the empirical coefficients, was modified from the one developed by D. Fantauzzi et al.[3] This modification includes adjusting the ionization degree to theoretical value and preventing the abnormal bond which occurred in Nafion chains. Based on the transport analysis, both water and hydronium ions reduced diffusion coefficients under subzero conditions, with the decrease in water, which is more pronounced at higher water contents. In contrast, the diffusion coefficient of hydronium ions decreased less significantly. To investigate this discrepancy, proton transport by proton hopping was analyzed, revealing that proton hopping increased under subzero conditions. This increase likely mitigated the reduction in hydronium ion diffusivity compared to water. Structural analysis further indicated that the hydrogen-bonding network among water molecules becomes more enhanced under subzero conditions, leading to the formation of larger water clusters around sulfonic acid groups. This structural change is presumed to have facilitated additional proton hopping pathways, thereby enhancing proton transport.

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