Understanding the effects of internal air bleed, nitrogen dilution, and sulfur dioxide on the performance of PEM fuel cells

Data are presented to quantify how oxygen diffusing across the catalyst coated membrane (CCM) from the cathode side affects the performance of a PEMFC operating with low levels of CO in H2. The extent of CO poisoning, as measured by the anode overpotential was decreased when the cathode backpressure was increased from 101 to 303 kPa and when the CCM thickness was decreased from 25 mum to 5 mum. The data indicate that a lowering of the anode overpotential for 50, 100, and 500 ppm CO in H2 by the O2 diffusing through the membrane is a complicated function of CCM thickness, adsorption isotherms of H2, CO, and apparent rate constant of CO with O2. Under relatively high anode overpotential, the reaction of Ru-OH with adsorbed CO was the primary means of CO oxidation and the diffusing O2 reacts with H2. However, at cell voltage close to open circuit conditions, the O2 reacting with CO is kinetically limited. The diffused oxygen (also known as internal air bleed) was quantified with a model that includes O2 solubility and heterogeneous oxidation constants. The results indicate that the surface reaction of Pt-O and Pt-CO limits the effect of internal air bleed for thin CCMs.; Data are shown here that can be used to quantify the competition between CO and H2 adsorption on CCMs during PEMFC operation. These data are based on experiments with N2 dilution and this dilution might be expected due to H2 is consumed down the length of the channel or reformate concentration changes. The specific data are reported to examine the N2 dilution effect on CO poisoning. These data indicate, as expected, that the effect of CO poisoning is more pronounced when H2 is diluted at constant CO concentration. The unusual observation and the one that shows competitive CO/H2 adsorption for the first time in PEMFC literature is that the extent of poisoning (as measured by the anode polarization) is also more pronounced as H2 is diluted with N 2 even though the ratio of CO/H2 is held constant. Thus it is shown that the presence of N2 not only dilutes H2 but also makes the adsorbed CO more difficult to desorb from the surface of the anode catalyst.; The last part of this dissertation focuses on an air impurity SO 2. Transient data of SO2 in air are presented and an empirical relationship between cathode polarization loss and the concentration and dosage in mumole of SO2 in air was developed for the PRIMEA Series 56 CCM from W. L. Gore & Associates Inc. This relationship and the data are used to suggest a poisoning mechanism for SO2 in air and a mathematical model. Thus the transient behavior was modeled for the various SO2 concentrations and the model predictions based on the proposed poisoning mechanism was shown to agree with the experimental data very well. The goal is to present a methodology that aids the industry in their quest to characterize concentration and dosage effects on PEMFC performance. Experimentally this characterization is initiated by systematically decreasing the SO2 concentration from high levels to low levels approaching nominally low concentrations in unfiltered cathode feeds.