(91d) SEM and TEM Studies of Electrode Structure in PEM Fuel Cells

In the search for sustainable fuels, hydrogen remains of interest for long-range vehicular applications. There are numerous technical challenges that must be overcome in order to achieve significant market penetration. The interface between the electrode and electrolyte remains one of the areas of greatest ignorance with regard to a molecular-level understanding of fuel cell structure and transport processes. A catalyst nanoparticle in the anode of the Membrane Electrode Assembly (MEA) must fulfill not only its role in catalyzing the dissociation of molecular hydrogen at its surface, but it must also participate in three transport process. The molecular hydrogen must be able to reach the catalyst nanoparticle surface. Once molecular hydrogen is dissociated into electrons and protons, the electrons must have a path to the electrode and the protons must have a path to the electrolyte. The structure of the MEA at both the mesoscopic and molecular level plays a crucial role in the determination of the relative ease or difficulty with which each of these transport properties can occur. In order to understand the relationships between MEA structure and the transport properties, it is important to be able to characterize the structure and measure the property of interest (conductivity in this case) at a fine resolution. In the work described here, we have measured the in-plane conductivity of a series of fuel cell electrodes using a nano-scale four probe method. This allows local conductivity to be determined with a spatial resolution of a few micrometers. Using this information a network analysis can then be performed to trace the conduction circuit pathways. Combined with images that reveal factors such as catalyst size and distribution, the local conductivity measurements lead to a generation efficiency that can be evaluated accordingly. In-plane conductivities are compared on the basis of particle size and distribution. In order to maximize proton and electron flux, details on the structure and property relationships are also needed. Nano-scale force sensitive manipulators, inside the SEM and operated while imaging the area of interest, can be used to measure the adhesion of the catalyst particles to the substrate by determining the force required to dislodge them. Relationships between catalyst size and distribution along with mechanical property of the solid interface and generation efficiency are explored based on techniques discussed above.

Acknowledgments

This work was supported by the STAIR program at the University of Tennessee, funded by NSF under agreement number: DGE 0801470.

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