Atomistic Modeling in Study of Polymer Electrolyte Fuel Cells – A Review

Abstract

Polymer electrolyte fuel cell (PEFC) is considered as one of the most promising power sources for futurist’s hydrogen economy. As shown in Fig. 1, operation of a Nafion-based PEFC is dictated by transport processes and electrochemical reactions at catalyst/polymer electrolyte interfaces and transport processes in the polymer electrolyte membrane (PEM), in the catalyst layers consisting of precious metal (Pt or Ru) catalysts on porous carbon support and polymer electrolyte clusters, in gas diffusion layers (GDLs), and in flow channels. Specifically, oxidants, fuel, and reaction products flow in channels of millimeter scale and diffuse in GDL with a structure of micrometer scale. Nafion, a sulfonic acid tetrafluorethylene copolymer and the most commonly used polymer electrolyte, consists of nanoscale hydrophobic domains and proton conducting hydrophilic domains with a scale of 2–5 nm. The diffusivities of the reactants (O2, H2, and methanol) and reaction products (water and CO2) in Nafion and proton conductivity of Nafion strongly depend on the nanostructures and their responses to the presence of water. Polymer electrolyte clusters in the catalyst layers also play a critical role in the catalysis of the nano-sized Pt catalysts. Electrochemical reactions occur at the interfaces between catalysts (Pt or Pt/Ru) and Nafion. The catalytic activity of the Pt catalysts is believed to be dictated by transport processes, adsorption/desorption, and charge transfer in the interfacial area. While transport processes may occur in an area of a few nanometers, adsorption/desorption and charge transfer occur within a region of a few angstroms from the surface of a nano-particulate catalyst. Thus, modeling or simulation of PEFC is a multiscale problem.