Polymer Electrolyte Fuel Cells (PEFCs) are essential technologies for achieving a carbon-neutral society and fulfilling the Sustainable Development Goals (SDGs). One critical issue in PEFC operation is the significant reduction in the mass transport properties of polymer electrolyte membrane (PEM) under subzero conditions. In this study, we performed a molecular-level investigation of the transport properties and internal structure in PEM using reactive force field molecular dynamics (ReaxFF MD), which can capture both chemical reactions and dynamics processes. Our results revealed that at subzero temperatures, the hydrogen-bond network within the PEM becomes stronger, leading to the formation of larger water clusters. These structural changes increase the number of effective proton hopping events and diversify the proton transport pathways. Consequently, while the diffusivity of water molecules decreases significantly under high water contents, the diffusivity of proton remains comparable to that under low water contents. These findings indicate that proton diffusivity is less sensitive to temperature-induced structural changes due to the contribution of proton hopping. This molecular-level understanding may help improve the performance of PEMs under subzero conditions.
Nishizawa et al. (Wed,) studied this question.