Water management in proton-exchange membrane fuel cells plays an important role in device operation, especially at lower temperatures where liquid water exists. Key to understanding water management is determining the mechanisms of water movement through the various fuel-cell components.  Traditionally, continuum-scale mathematical modeling has been used to optimize and understand fuel-cell water and thermal management. However, it is increasingly become apparent that there is a need for more detailed lower lengthscale information to feed into the models due to microscale and mesoscale heterogeneities. In this talk, we will examine these issues with a focus on liquid-water movement in fuel-cell diffusion media and in water and ion transport in fuel-cell membranes including both proton and alkaline varieties.

It is well known that continuum models use a volume-averaged approach to model liquid-water transport. This strategy is suitable for higher operating temperatures and homogeneous gas-diffusion-layer (GDL) morphologies, but, such structures are not realistic. To handle the complexity of the GDL morphology and access the expected capillary-fingering type of water transport in these porous media, pore-network models (PNMs) are utilized. However, while PNMs models capture the water transport through the GDL, they do not contain suitable descriptions of the other layers, complex nonlinear transport, and electrochemical phenomena. Here, we combine the advantage of both models in an iterative scheme integrating continuum and PN models to describe the multiphase and multiscale water transport properly. The coupling is achieved through an iterative scheme wherein the 2-D continuum PEFC model passes the calculated, heat and mass fluxes to the PNM, where they act as spatially varying source terms. Subsequently, the PNM returns to the continuum model a set of effective properties, such as thermal conductivity, effective diffusivity, and permeability, which are discretized along the electrode|GDL interface. This talk will discuss the work-flow between the coupled models including utilization of X-ray CT inputs and how such an approach can explain nonintuitive experimental data.

For an ion-exchange membrane, mathematical modeling is ideally suited to explore the genesis of the observed water profiles and subsequently the mechanisms of water movement.  Recently, such modeling has been receiving increased attention as new knowledge on the impact and control of the membrane/environment interface is elucidated.  To model such effects requires the use of a multiscale framework involving mesoscale networks and microscale ion-transport information. The linking between these scales will be discussed including appropriate ways to incorporate molecular-level information into a physically consistent framework that still allows for easy incorporation into cell-level fuel-cell models.    

Acknowledgement

I would like to acknowledge the various graduate students, postdocs, and collaborators who have worked on this research over the years. This work was supported by The Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.