The Research Of The Two-phase Transport Behavior In Proton Exchange Membrane Fuel Cells

As a highly effective and clean power generating device, proton exchange membrane fuel cell(PEMFC) has obtained more and more support, with the emphasis on the environment and the consumption of the traditional fossil fuels. It has good potential in the applications of space science, submarine, mobile power sources and small power station, it is the most suitable candidate for the new generation motors of the vehicles, especially. Many stations and motor companies have spent huge fund and human resources on it. But, before it can be widely commercially used, the issues of life and cost must be solved. Water is the key issue in PEMFCs, which is a significant technical challenge. On one hand, sufficient water is needed in the membrane to maintain sufficiently high proton conductivity, but on the other hand, excess liquid water in the electrode can cause 'flooding', and hinder the transport of reactant from the gas channel to the catalyst layer. Liquid water can also make the hydrophobicity loss in the gas diffusion layer then decrease the lifetime. Modeling two-phase behavior in PEMFCs can describe the phenomena in PEMFCs theoretically and can help to optimize the design, save the test cost, improve the operating condition and then increase the lifetime. So, it has the very important meanings. In this study, a two-fluid model is applied to simulate the two-phase behavior in PEMFCs, and a fractal method is developed to predict the permeability and the water relative permeability of the gas diffusion layer(GDL). We also manufactured the single PEMFC and compared the test performance of PEMFC and the predicted performance.First, a two-fluid model is applied to simulate the two-phase behavior in PEMFCs for the first time, it can describe the detachment of the droplet on the GDL surface, which can't be included in the multiphase mixture model. The mass transfer of species, electrochemical reaction in catalyst layer, proton conservation, multicomponent diffusion and mass exchange between gas phase, liquid phase and polymer phase are all included in the model. In addition, a simplified model is developed to predict the detachment diameter of the droplet on GDL surface, and the results show that the detachment diameter decreases with the increase of gas velocity, gas viscosity, gas density and GDL surface contact angle, as well as the decrease of the contact angle hysteresis. The effect of the operating parameters and structure parameters on the two phase behavior are mainly investigated, the numerical results by the two-fluid model show that (1)The water saturation in the GDL is determined by two process, i.e. the transport of the the liquid in GDL by capillary flow and the transport of the droplet on the GDL surface and in the gas channel; (2) The small detachment diameter of the droplet on the GDL surface is advantageous to water removal in the cathode side; (3) The dry air in the cathode side can drag more droplets for it's high viscosity compared with the viscosity of the saturated air, but it will also increase the pressure loss, so it's suitable for the pressurized operation; (4)The ambient pressure operation has better water removal behavior than pressurized operation for the operating current density and stoichiometry, so, the pressurized operation need more hydrophilic gas channel walls; (5)The water removal will be promoted by increasing the contact angle of the gas diffusion layer for it not only increases the capillary force in the GDL but also decreases the droplet detachment diameter on the GDL surface; (6)The circle shape of the pore in the GDL surface is most suitable for the droplet detachment, so it has the best water removal function.Second, a fractal model is developed to predict the permeability and liquid water relative permeability of GDL (TGP-H-120 carbon paper) in PEMFCs for hydrophilicity case and hydrophobicity case. Area dimension, tortuosity dimension, maximum pore size, thickness of GDL, and saturation as well as hydrophilic pore fraction in GDL are involved in this model. The area dimension and tortuosity dimension are obtained by box-counting method based on the micrographs (in-plane and through-plane) of TGP-H-120. The maximum pore size in the GDL is obtained according to the structure of the GDL. The model is validated by the comparison between the predicted results and experimental data. The water relative permeability obtained by the fractal model without the consideration of the hydrophobicity is close to that of effective media theory. The results with consideration of hydrophobicity indicate: (1)The water relative permeability of hydrophobicity case is much higher than that of hydrophilicity case. (2)To effectively remove the water in cathode side of PEMFCs, hydrophobic carbon paper is better than hydrophilic carbon paper; (3)As for the anode side, hydrophilic carbon paper or the carbon paper with a certain fraction of hydrophilic pores will be preferable; (4)Water relative permeability increases with increase of tortuosity dimension for hydrophobicity case but decreases for hydrophilicity case. (5)In the view of the carbon paper design, the nonuniform pore size distribution is more suitable for the application of PEMFCs than uniform pore size distribution...