On Diffusion Transport Properties in Fuel Cell Gas Diffusion Layers Using the Lattice Boltzmann Method

Abstract: The polymer electrolyte fuel cell (PEFC) is considered as one of the most promising devices for providing efficient, clean and noiseless conversion of chemical energy to electrical energy. This device can provide electrical and thermal energy for transport, mobile and stationary applications ranging in a wide range of power requirements. However, in spite of its promising potential and increasing presence during recent years, the PEFCs are still not widely commercialized around the world. The competition with current technologies is hard, especially due to the high cost involved in the PEFC production and degradation issues.The energy conversion within the PEFCs is maintained by different multi-physics and multi-chemical phenomena that occur at different length and time scales. The reactant gases and the electrons, products of the electrochemical reactions, flow through complex and anisotropic geometries which make their description difficult, especially when a pore-scale analysis is considered. As part of a PEFC, the gas diffusion layer (GDL) plays an important role in the energy conversion process, giving mechanical support to the cell and providing a structure to which the reactant and product fluids can flow; as well as it allows the flow of electrons from the active sites to the current collectors and vice versa.A complete understanding of the diffusion transport properties, considering the morphological configuration at the pore-scale level, can give an insight to improve certain characteristics of the GDLs and eventually enhance the behavior of the whole system. Considering that a pore-scale and in-situ experiment represents a considerable investment of resources, computational tools to describe the different transport phenomena through the GDLs offer a unique opportunity to study the diffusion transport phenomena and estimate the properties of the GDLs.Two- and three-dimensional models representing GDLs have been developed to analyze the impact of morphological configurations on certain diffusion transport properties, as well as the fluid behavior and mass transport through the mentioned layers when they are subjected to different conditions of compression, morphological configurations or inflow. Due to the complexity of the GDLs, the Lattice Boltzmann method (LBM) was chosen as the computational tool to describe and analyze the fluid flow behavior and the mass transport phenomena through the GDLs. This methodology can be applied not only to the mentioned layer in PEFCs, but also to the porous media found in other type of FC such as solid oxide fuel cells (SOFCs). The GDLs are stochastically created and the diffusion transport parameters such as porosity, gas-phase tortuosity, permeability, inertial coefficient and normalized diffusivitiy are analyzed from a pore-scale point of view. This thesis not only provides insightful information about the different diffusion transport GDL parameters, but also offers an analysis of the effects of morphological configurations on the mentioned properties. Several correlations for gas-phase tortuosity, permeability and diffusibility among others, are proposed to predict the behavior of the mentioned parameters. The computation of the parameters is supported by single-phase Lattice Boltzmann models which allow a deep analysis of the fluid behavior and the mass transport phenomena through the digitally created GDLs. The GDL generation and the LB models are completely developed by the author.