A multi-scale hierarchical approach for understanding the structure of the polymer electrolyte membrane fuel cell (PEMFC) electrodes - from nanoparticles to composites.

摘要

Polymer Electrolyte Membrane Fuel Cell (PEMFC) electrodes are the most important component of fuel cell membrane electrode assemblies (MEAs). They consist of catalytic nanoparticles dispersed on the surface of an electron conducting support such as carbon, mixed with a proton conducting ionomer material in an interpenetrating three dimensional matrix. We present a multi-scale hierarchical approach to understand the structures and properties of these individual components. The interaction energies between the different components are estimated, from their individual optical spectra, using a Lifshitz formulation to determine the effective Hamaker coefficients. Constant temperature molecular dynamics simulations are used to estimate the effect of the interaction energy between the support and the nanoparticle on the properties of the nanocatalyst such as structure, stability, utilization and durability. Possible explanations for the observed durability and activity properties of the nanoparticles are provided. The use of the model as a tool for design and development of new catalytic materials is demonstrated. A stochastic Monte Carlo process approach is developed to effectively model the evolution of the structure of both the supported catalyst and composite electrodes based on their synthesis methods. Properties such as dispersion and loading for the supported catalysts have been analyzed for both the model and experimental systems. Challenges involved with the development of new materials for supported catalysts are addressed. Experiments were developed to compare the simulated structures with the actual structures observed for these systems. Transmission electron microscopy and atomic force microscopy techniques are used to analyze the structures of supported catalysts and composite electrodes respectively. A percolation theory based approach is developed to estimate the effective proton and electron conductivities of the simulated electrodes. Optimum compositions for the composite electrodes are determined based on both the structure and transport properties for the components in the electrode. Finally, a simple geometric approach to estimate the "true" utilization of the different components is provided. Application of the model to the overall understanding of the behavior of the fuel cell components is demonstrated, and the challenges involved with the improvement of the existing system are addressed.