Molecular Simulation of Polymer Membranes in Fuel Cells

Abstract

Polymer membranes are a proven electrolyte material in fuel

cells that operate at low temperatures and high power densities. Many membranes with similar architectures and chemistries have been synthesized over the past 50 years, yet the market continues to be dominated by one specific type, Nafion. Some key advantages and

disadvantages of Nafion have been identified in the last decade using

experimental measurements of macroscopic properties. Lacking microscopic resolution, these experiments cannot fully explain how properties of Nafion are related to its precise chemical structure and

morphology. Understanding these structure/property relationships could be a viable path towards superior membranes. In this thesis, structure/property relationships of Nafion are investigated using molecular simulation.

Previous experiments have shown that Nafion begins to conduct protons at a remarkably low humidity compared to other polymer electrolytes. This low threshold of humidity was hypothesized to be a sign of highly aspherical water clusters percolating to form a continuous network of hydrogen bonds. This hypothesis is consistent with simulations, reported in this thesis, that show rod-like water clusters at low to moderate humidity. These simulations are validated by computing the water sorption isotherm and comparing with experiments.

Water permeation experiments have demonstrated that water

transport is bottled-necked by the membrane/water-vapor interface, but not the membrane/liquid-water interface. These interfacial phenomena are qualitatively reproduced by simulations in this thesis. The slow transport at the membrane/water-vapor interface is attributed to a local enrichment of hydrophobic functional groups on the polymer.

Previous experiments have revealed that water can act as both an antiplasticizer and a plasticizer in Nafion. Mechanical stiffness has also been shown by experiments to strongly depend on counterion type. Both of these trends were observed in this thesis, and correlate with changes in microscopic structure.

This thesis demonstrates that experimentally relevant properties of fuel cell membranes are accessible with modern computing resources. These properties, when combined with the atomistic resolution of simulations, are a valuable guide for interpreting experiments. The next step forward would be to repeat these types of calculations on membranes other than Nafion to more fully understand how membrane performance depends on polymer chemistry.