Grant: Discovery Grants Program - Individual

Polymer electrolyte membranes (PEMs) are used in hydrogen fuel cells as proton conductive media. Their operation is based on structural diffusion of protons in water-filled nanopores, facilitated by an ion exchange mechanism between acidic groups and water. The conductivity depends critically on the pore morphology, the concentration of free protons and their mobility, and the structure of the underlying pore network which emerges from phase separation of the PEM into hydrophobic and hydrophilic domains. This research aims to widen the fundamental understanding of PEMs, including their thermodynamic properties under equilibrium and non-equilibrium conditions. At the heart of this program is the study of the interplay between pore formation and transport of water and protons at the nano-scale, using continuum electro-kinetic and thermodynamic models. A deeper knowledge of these phenomena is required to understand the functionality of PEM fuel cells and to enhance their applicability at elevated temperature by maximizing proton conductivity, mechanical and chemical stability, and water retention capability. Several properties of the membrane will be investigated, including its morphology. Water absorption will be described as an interfacial process at the pore level, balancing capillary, electro-osmotic and elastic pressures. The controversial question whether the geometry of PEM nano-pores can be described accurately by the minimization of a free energy forms a major part of this program. An extension to non-equilibrium states, as found under fuel cell operation, will assess corresponding changes of equilibrium shapes. Pore network models are going to link results at the microscopic scale to macroscopic transport properties. For these purposes, Poisson-Nernst-Planck and Stokes equations will be employed to simulate charge and mass flow in nano-pores, and validated against experimental data (e.g. conductivity, water drag, water sorption). Given the computational expense of discrete models, continuum models, when valid, have the potential to play a major role in nano-science, providing more intuitive knowledge of fundamental physical processes.