Pore-scale Study Of Gas/liquid Transport In Carbon Paper Gas Diffusion Layer Of A Proton Exchange Membrane Fuel Cell

With rapid progress of the researches on alternative and clean energy sources, proton exchange membrane fuel cells (PEMFCs) have become a promising energy conversion device for hydrogen applications. Among various and complicated transport phenomena occur in PEMFCs, reactant gas diffusion and water management are two very important issues which are greatly related to the performance and durability of the PEMFCs in order to become a commercial reality. Macroscopic models, however, are deficient to study these transport phenomena in PEMFCs. In this thesis, the single and two phase transport in the gas diffusion layer (GDL) and gas flow channel of PEMFCs are investigated using pore-scale simulations, thus providing more fundamental understandings of these transport problems.Due to the limitations of macroscopic models and existing experimental methods at the present time, the lattice Boltzmann method (LBM), which is based on mesoscale kinetic theory, has been used in present work to perform quantitative investigations of gas and water transport within the very thin carbon paper GDL with actual pore structure and wettability distribution taken into consideration. The major contents in this thesis include the investigations of single and two phase transport processes in carbon paper GDL, relative permeability and capillary pressure-saturation relationship of carbon paper GDL and water droplet dynamics on the surface of carbon paper GDL in a gas channel. The details are described as follows:1. Anisotropic permeability of carbon paper GDL. The multiple-relaxation-time (MRT) LBM is used to study the anisotropic permeabilities of carbon paper GDLs. The porous carbon paper is numerically reconstructed using the stochastic method by taking into consideration of various porosities and microstructures to imitate different GDL compression ratios and PTFE contents. The detail flow field of single phase fluid in the GDL microstructure is presented and the resulting permeability and tortuosity are calculated, which show anisotropic characteristics of the reconstructed carbon paper with in-plane permeability higher than through-plane and in-plane tortuosity lower than through-plane. The simulated and measured permeabilities are compared with the Kozeny-Carman and∧-base relations, which predict permeabilities as a function of porosity accurately for GDL with different compression ratios, but inaccurately for GDL with different PTFE contents. The Bruggeman equation is faulty to predict the anisotropic tortuosities. The fitted relations of tortuosity and porosity are obtained from simulation results and are used in a fractal model, which indicates that the fractal model only provides good predictions on through-plane permeabilities of carbon paper GDL.2. Two-phase transport in carbon paper GDL. Water transport dynamic behaviors and water distribution in the microstructure of carbon paper GDL is investigated using the multiphase free-energy LBM. The relative permeabilities and capillary pressure-saturation relations of carbon paper with different wettability distributions and PTFE contents are obtained from pore-scale simulations, and are compared with empirical relationships and experimental measurements. The results show that the wettability plays a significant role on water saturation distribution in water transport in GDL. For highly hydrophobicity, the water transport falls in the regime of capillary fingering, while for neutral wettability, water transport exhibits the characteristic of stable displacement, although both processes are capillary force dominated flow with same capillary numbers. In addition, the introduction of hydrophilic paths in the GDL leads the water to flow through the hydrophilic pores preferentially, which would facilitate the removal of liquid water more effectively, thus alleviating flooding in catalyst layer (CL) and GDL. In addition, the pore-scale simulated capillary pressure-saturation relationships of carbon paper with different PTFE contents are in good agreement with experimental results, indicating the coexistence of both hydrophilic and hydrophobic properties in the PTFE treated GDL. The fitted capillary pressure curves in present works could provide more accurate predictions of the effects of two-phase flow in PEMFC models than using the standard Leverett-Udell empirical relationship for sand.3. Water droplet dynamic behavior on the hydrophobic GDL surface. The dynamic behaviors of water droplet emergence, growth, detachment and subsequent movement on the GDL surface are presented. The detached mode or detached size of the droplet under the influence of gas flow velocity and GDL surface wettability is investigated. The results confirm that the emerging water droplet holding on the GDL surface is attributed to the surface tension force from the connecting of the water in the GDL pore and the emerged part on the GDL surface. It shows that liquid water removal is facilitated by a high gas flow velocity on a more hydrophobic GDL surface. Furthermore, a simplified analytical model based on force balance is presented to predict the droplet detachment size and moving velocity, and the predicted results are in good agreement with the mesoscopic simulation results.The results of the present work provide physical insight for understanding the reactant gas and product water transport in the porous GDL and water droplet dynamics on the interface of GDL and gas flow channel. The results are helpful for design of optimal reactant diffusion and water management strategies for PEMFCs, and also provide theoretical basis for the development of system models of PEMFCs.