Abstract
Among the existing fuel-cell types, alkaline-exchange-membrane fuel cells (AEMFC) have intriguing features as compared to proton-exchange-membrane fuel cells (PEMFC). Their major advantage is the possibility of using non-noble catalysts due to faster oxygen-reduction reaction (ORR) kinetics in alkaline than in acidic media. However, water management is a more serious concern in AEMFCs because OH- conductivity is more highly dependent on water content and the ORR consumes water. Compared to PEMFCs, the lower performance of AEMFCs is mostly caused by extremely nonuniform distribution of water in the ionomer phase between the anode and cathode as well as the increased overpotential for the hydrogen oxidation reaction. In this presentation, we will discuss the performance-limiting mechanisms specific to different operating conditions (e.g. varying inlet relative humidity (RH)) based on a cell-level mathematical model. For example, anode flooding can be a critical issue at 100% RH, whereas at lower RH high ionic transport resistance in the cathode dominates due to dehydrated ionomer phase at high current, where the impact of water-consuming ORR kinetic polarization is also critical. Low AEMFC performance at high current is not simply due to mass transport issues with vapor/membrane water, but a consequence of poor water distribution leading to sluggish OH- conduction and ORR kinetics. A sensitivity analysis of design parameters including the humidifying condition and membrane property is performed to identify the most significant factors controlling performance. Overall, water management and ionic/mass transport characteristics of an AEMFC assembly are discussed in detailed. The developed model will be used to examine and elucidate performance bottlenecks and enable strategies to overcome them, significantly increasing the possibility of AEMFC commercialization. Acknowledgements This work was funded by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, of the U. S. Department of Energy under contract number DE-AC02-05CH11231, program manager David Peterson.
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