A Kinetic Hypothesis to Explain the Function of Electrons in by Noyes W. A.

By Noyes W. A.

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It has been considered that, over the stack lifetime (~5,500 h), there will be 300,000 large voltage cycles. However, it is expected that activity degradation rates will have been established by 30,000 cycles and can be extrapolated to 300,000 cycles. A catalyst-derived degradation rate of <3 μV h–1 is considered the acceptable target. It has also been estimated that the fuel cell stack will be subject to 30,000 start–stop transients during its lifetime. 2 V during a start–stop transient due to a local cell reversal event as H 2 replaces air or air replaces H2 in the anode.

Two mechanisms have been proposed: Ligand effect: CO adsorption is lowered by alloying, thus decreasing CO coverage and increasing sites available for H 2 adsorption/ dissociation and oxidation. Bifunctional effect: CO oxidized by the alloying element is effective at dissociating H2O and providing OH to react with CO adsorbed on Pt and thus decreasing CO coverage. It is likely that both mechanisms are active and dependent on potential. At low potentials (<200 mV) on PtRu, the bifunctional mechanism is not active because Ru is unable to dissociate adsorbed H2O to produce OH.

However, after a number of years of development, the DoE announced in 2003 that it was cancelling its fuel processor research program for automotive applications. It is now accepted that automotive fuel cell systems will run on pure H2 as a fuel and, as a consequence, emphasis on developing effective H2 storage materials is much higher. Because the use of reformate as fuel is still favored for small, stationary fuel cell systems, the need to achieve reformate tolerance is still critical. The definition of reformate tolerance is that, compared to running on pure H2, a fuel cell stack can run on reformate and show no change in performance, apart from that expected for dilution effects (of H2 due to CO2, N2, H2O).

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