Modeling, Simulation and Characterization of Atomic Force Microscopy Measurements for Ionic Transport and Impedance in PEM Fuel Cells

The polymer electrolyte membrane fuel cell is a power source with the potential for reducing green-house gas emissions. Characterizing the electrolyte of a fuel cell is an important procedure for assessing the performance of the entire device. Atomic force microscopy (AFM) is one of the major instruments for such characterization, since it can be used for determining the surface potential and/or charge distributions in a fuel cell electrolyte. In order to understand the mechanisms involved in the AFM imaging of the membrane material, we have been developing detailed models that are able to describe the contributions from sample properties and AFM probe tip geometry to the AFM images. In the past year, we have extended our initial model to take into account fuel cell electrolyte materials that are dielectric with spatial variation in charge distributions. This development allows us to accurately predict AFM images based on knowledge of the sample charge distribution and to characterize the resolution of the AFM. The proposed numerical method developed in this project also provides insights for the development of new algorithms for reconstructing sample charge distributions from AFM images, which would provide a comprehensive quantitative evaluation of fuel cell devices. Introduction Solid polymer fuel cells promise to be an efficient power source for mobile and stationary applications with the potential for a greatly reduced environmental impact. However, the ionic diffusion behavior under current load conditions in the ion-selective membranes of proton exchange membrane fuel cells (PEMFCs) is not completely understood. Further understanding of ion behavior at the Nernst diffusion layer of the membrane surface could enable development of new classes of solid polymer fuel cell membranes with increased mass transport. In this project we are examining and characterizing the properties of solid polymer membranes through analytical and numerical modeling of ion transport, impedance, diffusion and atomic force microscopy imaging. To address the challenges of enabling new solid polymer fuel cell membranes, the project has four primary objectives: 1. Atomic force microscope imaging. Nanoscale AFM surface imaging of dielectric or polarizable fuel cell materials involves complex AFM tip interactions with unknown surface and space charges resulting in electrostatic and van der Waals forces. Nonlinear modeling will be employed to solve the inverse problem of finding the charge distributions from AFM images with the goal of elucidating the mechanisms of charge motion in fuel cell and biological media relevant for environmental cleanup. 2. Simulation of impedance measurements. Modeling and simulation of AFM impedance measurements in fuel cells to shed light on the effects of probe tip geometry and surface roughness and topography with goal of developing scaling laws for fuel cell performance. 3. Modeling of ionic transport in PEMFC’s. Analytical and numerical modeling of ionic diffusion in fuel cells taking into account electrochemical kinetics, current distribution, hydrodynamics and multi-component transport with the goal of elucidating mass transport characteristics. 4. Particle diffusion modeling. In order to further understand mechanisms of ion and solvent transport in hydrated PEMFC membranes, modeling that accounts for local molecular information will be employed. Our primary efforts have been directed to the first objective. Figure 1: Schematic of the AFM tip (without the cantilever) and the sample