Performance Of An Air-Breathing Proton Exchange Membrane Fuel Cell And Optimum Open Ratio Of Cathode Channel

The proton exchange membrane fuel cell (PEMFC) working at low temperature has become one of the most interesting alternative for clean power production. In addition to fuel cells for stationary or automotive applications, fuel cells also has the potential to complement or to substitute batteries in portable applications, due to its high electrical efficiency, rapid start-up, flexibility with respect to power and capacity, long lifetime and good ecological balance. These attributes make it attracts lots of researchers' attention. However, the typical PEMFC system with its heavy reliance on subsystems for cooling, humidification and air supply would not be practical in small applications. How to achieve high fuel and energy efficiency, energy and power density with low cost and small size is the key factor for the PEMFC to be used as portable generator replacing rechargeable batteries.The air-breathing proton membrane fuel cell, which operates at ambient temperature without humidifier and air compressor, is one of the best candidates for miniature fuel cells. However, the performance of the air-breathing PEMFC decreased due to oxygen transport limitation, membrane dehydration, and unreasonable designed parameters of the cells. All these factors are strongly influenced by fluid dynamics, heat and mass transfer inside the cell. Therefore, in this study, analysis of transport phenomena inside the cell were made by numerical modeling, and the solutions of the model were verified by experiments. The results of study provide deeper understanding of the effects of above mentioned transport parameters on the cell performances. Hence the results show how to optimize the channel structure of the cell and subsequently improve the cell performances itself.A complex 3-dimensional mathematical model of an air-breathing PEMFC with real geometry was developed. The theoretical analysis with this numerical model and experiments showed that the flow, heat and mass transfer inside the cell was coupled, and gave strong effects on the performance of the cell. With the development of this model, it was possible to analyze the transport phenomena in an air-breathing PEMFC in 3- dimension with real geometry, and hence to more clearly understand the parameters that decide the performances of the cell. Therefore, this model provides a useful tool for comprehensive understanding,optimum design, and improvement of the air-breathing PEMFC.Results of modeling and numerical analysis showed that the anode side of membrane could sufficiently be hydrated by diffusion from cathode when a relatively thin membrane was used. The anode can operate without humidification in the studied case. Therefore concentration losses caused by oxygen transport limitation plays major role in the cell performance reduction. The concentration loss is due to the insufficient air flux from cathode natural convection, and water blockage in the gas channel. It was also shown that configuration of the cathode channel has strong effect on the transport of oxygen and water to anode.In the air-breathing PEMFC, the concentration over-potential on the function of mass transfer coefficient of oxygen in the catalyst layer was calculated for the first time by the author. The oxygen transfer coefficients were calculated by solving the complete N-S equation. In addition, a formula between Sherwood number and Grashof number has been obtained through dimensionless analysis of heat and mass transfer in cathode natural convection. This equation is significant to reveal the nature of concentration losses of oxygen at the anode in the air-breathing PEMFC.To study the effect of channel configuration on the performance of the air-breathing PEMFC, a numerical model has been built for different channel and rib width. The calculation results showed that cell performance could be improved by optimum channel configuration. The optimum channel width of 3mm, the rib width of lmm, and the associated cathode channel open ratio of 75.9% have been obtained from the study. Finally, an original design idea of cathode channel structure is put forward for the first time.An experiment to test the model has been designed to investigate the temperature distribution and performance of the cell in operation. The experiment results show that the relative humidity of ambient air also has significant effect on the cell performance. The liquid water transport could be ignored, and the ohmic losses in membrane and oxygen transport limitation during air natural convection are the key factors for the performance drop of the air-breathing PEMFC when ambient humidity is 53%. On the other hand, when ambient humidity is 73%, liquid water was observed accumulating in the channel and diffusion layer. The liquid water blocked the gas channel and caused serious oxygen transportlimitation that resulting in the concentration losses increase and the performance decrease. In conclusion, the suitable ambient humidity in these three has been decided to be 63% for the air-breathing PEMFC of this study.