Investigation of CO Tolerance in Proton Exchange Membrane Fuel Cells

Zhang, Jingxin

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Investigation of CO Tolerance in Proton Exchange Membrane Fuel Cells

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The need for an efficient, non-polluting power source for vehicles in urban environments has resulted in increased attention to the option of fuel cell powered vehicles of high efficiency and low emissions. Of various fuel cell systems considered, the proton exchange membrane (PEM) fuel cell technology seems to be the most suitable one for the terrestrial transportation applications. This is thanks to its low temperature of operation (hence, fast cold start), and a combination of high power density and high energy conversion efficiency. Besides automobile and stationary applications (distributed power for homes, office buildings, and as back-up for critical applications such as hospitals and credit card centers), future consumer electronics also demands compact long-lasting sources of power, and fuel cell is a promising candidate in these applications. The goal of a cost effective and high performance fuel cell has resulted in very active multidisciplinary research. Although significant progress has been made on PEM fuel cells over the last twenty years, further progress in fuel cell research is still needed before the commercially viable fuel cell utilization in transportation, potable and stationary applications. A chief goal among others is the design of PEM fuel cells that can operate with impure hydrogen containing traces of CO, which has been the objective of this research. Standard Pt and PtRu anode catalyst has been studied systematically under practical fuel cell conditions, in an attempt to understand the mechanism and kinetics of H2/CO electrooxidation on these noble metal catalysts. In the study of Pt as anode catalyst, it was found that the fuel cell performance was strongly affected by the anode flow rate and cathode oxygen pressure. A CO electrooxidation kinetic model was developed taking into account the CO inventory in the anode, which can successfully simulate the experimental results. It was found that there is finite CO electrooxidation even on Pt anode with H2/CO as anode feed. Thus, anode overpotential and outlet CO concentration is a function of anode inlet flow rate at a constant current density. The on-line monitoring of CO concentration in PEM fuel cell anode exit has proved that the ~{!0~}ligand mechanism~{!1~} and ~{!0~}bifunctional mechanism~{!1~} coexist as the CO tolerance mechanisms for PtRu anode catalyst. For PtRu anode catalyst, sustained potential oscillations were observed when the fuel cell was operated at constant current density with H2/CO as anode feed. Temperature was found to be the key bifurcation parameter besides current density and the anode flow rate for the onset of potential oscillations. The anode kinetic model was extended further to unsteady state which can reasonably reproduce and adequately explain the oscillatory phenomenon. The potential oscillations are due to the coupling of anode electrooxidation of H2 and CO on PtRu alloy surface, on which OHad can be formed more facile, preferably on top of Ru atoms at lower overpotentials. One parameter bifurcation and local linear stability analysis have shown that the bifurcation experienced during the variation of fuel cell temperature is a Hopf bifurcation, which leads to stable potential oscillations when the fuel cell is set at constant current density. It was further found that a PEM fuel cell operated in an autonomous oscillatory state produces higher time-averaged cell voltage and power density as compared to the stable steady-state operation, which may be useful for developing an operational strategy for improved management of power output in PEM fuel cells with the presence of CO in anode feed. Finally, an Electrochemical Preferential Oxidation (ECPrOx) process is proposed to replace the conventional PrOx for cleaning CO from reformate gas, which can selectively oxidized CO electrochemically while generating supplemental electrical power without wasting hydrogen.

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