The present study investigates CO poisoning effect of Nafion and PBI membrane fuel cells. The CO kinetic model is developed and extended to simulate transient characteristics of the CO poisoning process. The transportation of hydrogen, CO, oxygen, proton, liquid and vapor water are all discussed. This work can accurately predict fuel cell performance under various fuel compositions and realize transient degradations of fuel cell performance, thus giving the following conclusions:
First, the catalyst layer is treated as a thin film instead of an interface to investigate the response time interval required to reach the steady state under different conditions.
Even more current density distribution, reactant gas distribution, and coverage across the anode catalyst layer are also investigated. As a result, the θH decreases with increasing CO concentration, which in turn, causes a low hydrogen electro-oxidation.
The time interval needed to reach steady state tss is strongly influenced by CO concentrations. This is due to the fact that the CO electro-oxidation is insufficient to free up the catalyst sites. Therefore, it is easy to accumulate on the Pt catalyst site with a high level CO concentration and then decrease the hydrogen oxidation, which in turn, cause a decrease in the response time interval tss. A better cell performance can be obtained for a system with a higher overpotential or gas porosity, especially at low level CO concentration. This is because that for a case with a high porosity, the hydrogen fuel can be easily fed into the catalyst layer and a high anode overpotential can free up the catalyst sites for hydrogen oxidation by bringing about the great CO oxidation. Finally, effects of the CO levels have a significant impact on the response time interval, especially for the low level ppm CO.
Next, we modified and extended our previous study from single-phase to two-phase model. A much more comprehensive mathematical model was developed to gain a
further understanding of CO poisoning process. The transportation of hydrogen, carbon monoxide, proton, vapor and liquid water were all considered in this work across the MEA of the PEMFC. The hydrogen coverage and liquid water saturation decreases as the CO concentration and the dilute of hydrogen increase. Increasing the amount of CO and hydrogen dilution also drop the gradient of the liquid water distribution across the membrane and fall the loss of ionic potential. The distribution of liquid water depends more strongly on the CO concentration than on dilution of hydrogen in the MEA of the fuel cell. The theoretical results indicate that a large dropping rate of the current density is observed in the range between 10-50 ppm CO.
In this study, increasing the amount of pure hydrogen can drastically increase the cell current density for a wide range 10~100 ppm of CO, promoting the tolerance for CO of the fuel cell.
In order to realize the transient nature of poisoning from carbon monoxide across the MEA of the PEMFC, we modified the previous steady state two-phase model.
Platinum catalyst has a strong affinity for CO, inhibiting the electro-oxidation of hydrogen. The hydrogen coverage and liquid water saturation declines as the reaction proceeded. The gradient of the liquid water distribution across the membrane and the ionic potential also fell with time. The distribution of liquid water depends more strongly on the CO concentration than on dilution of hydrogen in the MEA of the fuel cell. The theoretical results indicate that a higher CO concentration results in large drop in the time to reach steady state tss. In this study, increasing the amount of pure hydrogen drastically increases tss for a wide range 10~100 ppm of CO contents. At 100 ppm CO, the cell voltage does not clearly affect tss. A large time tss can be achieved by increasing the amount of hydrogen. At 10 ppm CO, the influence of hydrogen dilution on the time tss is weak at cell voltage below 0.6 V. Thereafter, the
decay of fuel cell performance under various fuel compositions was predicted accurately. The theoretical results showed good agreement with experiments.
High temperature PBI membrane is a potential option to solve water management and tolerance of CO. The present study develops a transient, one-dimensional mathematical model to analyze PBI membrane fuel cells. Various fuel compositions are considered to realize the effect of fuel composition such as CO concentration and hydrogen dilution of the fuel cell. Chemical bonding is much stronger between CO and the Pt catalyst than hydrogen. Thus, hydrogen coverage rapidly declines with time.
Higher CO content and hydrogen dilution also cause significant increase on CO coverage. Ionic potential loss decreases with time during the CO poisoning process.
The polarization curves of PBI membrane fuel cell were measured at temperature 120~180oC. Simulation results show good agreement with experimental data. The simulation from this work can accurately predict fuel cell performance under various fuel compositions and realize transient degradations of fuel cell performance, thus providing sufficient information for the designing reformer and fuel cell system.
Different types of alloy catalyst such as Pt/Ru, Pt/Mo should be considered in the future work. Chemical kinetics of alloy catalyst is still absent to investigate the CO tolerance under various fuel compositions. Theoretical studies are required to gain a further understanding of chemical kinetics of anode composite electrocatalysts. A complete set of experimental data under various RHs, temperatures and fuel compositions of PBI membrane fuel cells is also absent. These studies are definitely required for the designing fuel cell systems. In addition, an integration of PBI membrane fuel cells with methanol steam reformer is desired in the future works.
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List of Publications
1. S. K. Wu, C. P. Wang, H. S. Chu, “Size Effects of the Heat Transfer for a Two-Layer Concentric Circular Tube with Interface Thermal Resistance,” J.
Chinese Society of Mechanical Engineers, 2004, 25, 115-123.
2. H. S. Chu, C. P. Wang, W. C. Liao, W. M. Yan, “Transient Behaviors of CO Poisoning in the Anode Catalyst Layer of PEM Fuel Cell,” J. Power Sources, 2006, 159, 1071-1077.
3. C. P. Wang, H. S. Chu, “Transient Analysis of Multicomponent Transport with Carbon Monoxide Poisoning Effect of a PEM Fuel Cell,” J. Power Sources, 2006, 159, 1025-1033.
4. C. P. Wang, H. S. Chu, “Two-Phase Modeling of a PEMFC with CO Poisoning Effect Using Dilute Hydrogen Feed,” ASME Fuel Cell Conference 2006-97208, Irvine, CA, 19-21, June, 2006.
5. C. P. Wang, H. S. Chu,, Y. Y. Yan, K. L. Hsueh, “Transient Evolution of Carbon Monoxide Poisoning Effect of PBI Membrane Fuel Cells”, J. Power Sources, 2007, (Published).