Polymer Electrolyte Membrane Fuel Cells

Collaboration between General Motors and Materials and Process Simulation Center on study polymer electrolyte membrane fuel cell (PEM FC) materials and processes was started in April, 2002. To present, four main directions can be distinguished within this project:

  • Nanostructure and transport in Nafion
  • Electrode processes in PEM FCs
  • Electrode/electrolyte interfaces
  • Alternative membranes

    Establishment of structure/system conditions–property relationships in PEM is one of the major goals of the project. Varying a set of system conditions we then analyze how this affects properties of the membrane. Such variables as blockiness of the polymer, water content, counter ions, equivalent weight of the polymer, temperature, acidity of the ionomer, backbone flexibility and hydrophobicity, length, polarity and architecture of the side chains, electric field are supposed to be the most significant for the physical properties of PEM. We have extensively analyzed the effect of blockiness, water content and counter ions on the water and hydronium diffusion, nanophase segregation, patchiness of the interface and cation solvation and have started studying the microscopic dynamics of water and cations in the membrane to find mechanisms of their migration that could be helpful for increasing the liquid state stability of water in the membrane. Modeling of Platinum/Nafion Interfaces

    Modeling of Platinum/Nafion Interfaces

    The electrode/electrolyte interface is the area where many important physico-chemical processes occur, for instance, protons approach and generate water reacting with oxygen gas independently supplied. Therefore, it is essential to understand the morphology of the nanophase segregation in this area. In order to investigate the structure and dynamics of the electrode/polymer interface, we have optimized a first-principle based force field and run MD simulations to characterize the water distribution and polymer conformation at the interface and to analyze the density profile as a function of the distance from the electrode. The presence of the electrode surface seems to result in extraction of the water that forms a water monolayer on the surface comparable to that obtained in quantum mechanical calculations for the 2/3ML water coverage. For the low water content there is a direct attachment of the polymer chains to the electrode, resulting in hydrophilic regions. However, increasing the water content prevents this attachment and almost the whole polymer is separated from the electrode by the water monolayer. The dynamics of the water, proton, and oxygen gas diffusion at the interface are planning to be analyzed as well. This will help us to understand characteristics of the interface in comparison with the bulk phase of the membrane and establish a guideline for designing a new molecular architecture that might provide a better performance.

    Figure 1: Model for simulating the Platinum/Nafion Interface.

    Personnel: Dr. Seung Soon Jang, Dr. Timo Jacob, and Dr. Boris Merinov.