ABSTRACT
Polymer electrolyte membrane fuel cells (PEMFCs) fueled by hydrogen are considered to be most suitable for automotive applications owing to their fast dynamics and high power densities. However, the performances are limited primarily by processes in PEMFCs appeared in the catalyst layers (CLs), which highly dependon themicroscopic structure-related properties.TheCLs at both the anode and cathode have complex and multi-phase porous structures, as shown in Figure 3.1. Platinum (Pt) nano-particles are supported by larger carbonaceous substrates, constructing the framework of the agglomerate with the primary pores of 3-10 nm. The Pt clusters range 2-5 nm and the most commonly employed carbon support, Vulcan XC-72, has a diameter of 20-30 nm. The spaces between the agglomerates are the secondary pores with sizes in the range of 10-50 nm (Lim et al., 2008). The ionomer in the CLs serves as both a binder and the pathway for the protons generated/consumed in the electrochemical reactions. In order to reduce the risk of water flooding, polytetrafluoroethylene (PTFE) is commonly used to bind the catalyst particles and form a hydrophobic path (Buryachenko et al., 2005). The typical thickness of a CL ranges from 10 to 30 microns and is a function of the material composition and the fabrication processing. There is no single, perfect, and all-comprising model for predicting fuel cell properties on
all length-and time scales. As shown in Figure 3.2, the density functional theory (DFT) can be applied at the atomistic scale (10−10 m) chemical reactions in the three-phase boundary (TPB); the molecularDynamics (MD) andMonteCarlo (MC)methods, based on classical force fields, can be employed to describe individual atoms or clusters of catalyst materials at the nano-/micro-scale (10−7−10−9 m); the particle-based methods (e.g. DPD) or mesh-based methods, for example Lattice-Boltzmann (LB), are used to solve the complex fluid flows in the porous media at the meso-scopic scale (10−6−10−3 m); and at the macroscopic scale (>10−3 m), continuum models are available for structural and coupled fluid-structural simulation in the optimum design for PEMFC. Thus, due to the limit of computational efficiency, it is important to make sure which phenomena and properties one is primarily interested in. The properties are usually determined by structural hierarchies and processes on very different length-and associated timescales in PEMFC. An efficient modeling technique of the PEMFC requires special consideration of the relevant physicochemical processes at the respective length and time scales.
