Modeling On Electro-Chemical Reactions And Transport Processes In Fuel Cell Cathodes

Fuel Cells have emerged as one of the most promising technologies for the power source of the future. The fuel cell is an electrochemical device that converts energy into electricity and heat without combustion. Some types of fuel cells approach commercial feasibility. Fuel cell technology is now becoming applicable for a large variety of technical areas. These include, inter alia, stationary power supplies and electro-traction. In order to optimize and improve fuel cell design, repeated experimentation must be carried out, which may be costly and time consuming. One viable alternative is to use a mathematical model to uncover the present design limitations and to determine where improvements can be made.In this paper all kinds of fuel cell, the current research situation and working principle of proton exchange membrane fuel cell(PEMFC) are outlined. Mathematical models in relation to electrochemistry reaction and transfer process in direct methanol fuel cell(DMFC) are compared and analyzed. Latest progress of membrane materials and instrument analysis technique in DMFC has also been described. DMFC is considered as a superior one so far, with cheerful prospects to be expected.The Thin-Film and Flooded-Agglomerate(TFFA) model assumes that the oxygen diffusion electrode made of a gas-diffusion layer and a reaction layer possesses a double-scales of porous structure in the reaction layer. Oxygen diffusion transport and electrochemical reaction in the oxygen cathode of alkaline fuel cell(AFC) and PEMFC have been expressed with the TFFA model, including oxygen transferring in gas channel of gas diffusion layer and reaction layer, oxygen dissolution and diffusion in thin-film of reaction layer, oxygen reaction and diffusion in flooded-agglomerate of reaction layer and the conduction of electron and ion. Numeric algorithm of the model equations is also obtained. The TFFA model calculation has been carried out to investigate the effects of structural parameters of the AFC cathode and PEMFC cathode on the electrode performance, which is respectively represented by the curve of cathode overpotential versus current density and the curve of cathode potential versus current density. Among these parameters, as far as the AFC cathode is concerned, much attention has been paid to the porosity of the reaction layer, the thickness of the thin-film on the flooded-agglomerate, the radius of the flooded-agglomerate, the volume fraction of the flooded-agglomerates in the reaction layer. The calculation results show that the radius of the gas channels in the reaction layer and the porosity and average pore radius of the gas-diffusion layer seem to be less important for the performance of the electrode. For the PEMFC cathode, besides above the parameters, the effects of the thickness of the reaction layer, the ion conductivity of Nafion electrolyte and the electronicconductivity in carbon phase of the reaction layer on the electrode performance have also been investigated. The research results show that the radius of the gas channels and the electronic conductivity in carbon phase of the reaction layer, and the porosity of the gas diffusion layer are relatively less important.In this paper the anode porous diffusion layer model of DMFC for mass transfer has been described on the basis that all components are gaseous and well mixed. The purpose of the model is to determine the variation in concentration of species in the diffusion layer and thus the concentration of the reactive species at the edge of the reaction layer.At present, methanol crossover from the anode to the cathode has appeared to be a major limitation for DMFC development. For this reason, as one of the alternatives, membrane technology tries to obtain new methanol-impermeable polymer electrolytes. In this sense, it would be interesting to develop easy methods to check the new materials in relation to methanol transport. Finally, a simple model has been described which permits to estimate easily the methanol diffusion coefficient of the membrane of...