Characterization of reactant gases, water and heat distributions in proton-exchange-membrane fuel cells.

摘要

Proper reactant gas distribution and water and heat management are essential for obtaining high-power-density performance at high energy efficiency for proton-exchange-membrane (PEM) fuel cells. An along-the-channel model has been developed to evaluate the effects of various design and operating parameters on the performance of PEM fuel cells. The model accounts for water transport across membrane by electro-osmosis, diffusion, and convection, temperature distribution in the solid phase along the flow channel, and heat removal by natural convection and coflow and counterflow heat exchanger. Results from the model show that the performance of a PEM fuel cell could be improved by anode humidification and positive differential pressure between the cathode and anode to increase the back transport rate of water across the membrane. Results also show that effective heat removal is necessary for preventing excessive temperatures, which could lead to local membrane dehydration. For optimal heat removal and distribution, the counterflow heat exchanger is most effective.; A gas-diffusion model was developed to study the effect of the design parameters of the gas distributor plates. Laplace's equation with boundary conditions that account for the shoulder area design was used to describe the gas transport through the cathode electrode layer. From the results, it was found that the current generated in the cathode side can be limited as much as 45% when compared with a gas distributor design which has no shoulder area.; To improve the mass transport rates of the reactants from the flow channels to the catalysts of the porous electrode, an interdigitated gas distributor design has been developed. This design reduces the electrode-flooding problem in the cathode. To understand how these effects contribute to the cell performance, experiments were performed. Simultaneously, two mathematical models for the cathode electrode were also developed. The first model, a multi-component, single-phase cathode model, describes the two-dimensional flow patterns and the distributions of the gaseous species in the porous electrode and predicts the current density generated at the reaction interface as a function of various operating conditions and design parameters. Results from the model show that the flow-through conditions created by the interdigitated gas distributor greatly reduces the diffusion layer in the electrode. This model has been expanded to a two-phase cathode model which includes the presence of liquid water in the electrode. The flow of the liquid water in the electrode is described as convection due to the gas flow and capillary diffusion due to the difference of capillary pressures of liquid water. The modeling results showed that a higher differential pressure between the inlet and outlet channels is favored. Moreover, more channels and thinner shoulder width are preferred in designing the interdigitated gas distributor. Results also show that the electrode thickness needs to be optimized to get optimal performance because a thinner electrode results in reduced reactant flow rates, and a thicker electrode results in an increased diffusion layer thickness.