(Invited) Quantifying Water Distribution and Connectivity in Ionomers: Comparison of Hydrated Nafion and Nafion/Polybenzimidazole Systems

Thursday, 5 October 2017: 08:00
National Harbor 14 (Gaylord National Resort and Convention Center)
H. Liu (University of Tennessee, Knoxville), S. Cavaliere, D. J. Jones, J. Rozière (CNRS - ICGM - AIME - University of Montpellier), and S. J. Paddison (University of Tennessee)
The design of ionomers for electrochemical and fuel cell applications would greatly benefit from a fundamental understanding of how chemical structure determines hydrated morphology. Ion transport through ionomer membrane is essentially governed by the connectivity of ‘tortuous’ water network. Quantification of the connectivity of the water domains poses a challenge in the design of highly conductive ionomers. Blending of different ionomers into a single membrane through casting or electrospinning should permit the bias of a thin film with a specific morphology exhibiting high conductivity. We are currently seeking to understanding how the electrospinning of either fibers or particles of polybenzimidazole PBI) with Nafion determines the hydrated morphology of the composite membrane. In the present work we implement dissipative particle dynamics (DPD) simulations, a coarse-grained scheme, to investigate hydrated Nafion and hydrated Nafion/PBI. Quantitative water cluster analysis confirms that increasing hydration level lead to ever-growing water domains, finally a percolating water channel for both systems. A percolating conductive network of the Nafion/PBI composite system emerges at a hydration level of 4 H2O/SO3H , which is less than that observed in a neat Nafion system. Cluster size distribution and accompanying informative configuration snapshots present a clear advantage over the widely-used radial distribution function method to characterize the ion conductive water channel. This proposed analysis can be readily applied to atomistic and various coarse grained systems. We hope an extensive usage of this quantitative tool will greatly deepen our understanding on the structure-and-function relationship and facilitate the rational design of optimal ionomers.