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Abstract:
The behaviour of a PEMFC with Pt anode under CO poisoning was experimentally investigated. Especially, the possibility and conditions for the occurrence of autonomous potential oscillation in galvanostatic operation were analysed [1]. A specific experimental setup, consisting of a differential fuel cell in H2/H2operation, was used in order to eliminate the masking effect of the cathode and to avoid spatial distributions along the channel. In galvanostatic operation, potential oscillations were observed (Fig. 1). In a first measurement series, the influence of CO concentration on the behaviour of the oscillations was analysed. Higher CO concentrations were found to increase the oscillation frequency because of enhanced CO adsorption [2].
In a second measurement series, the influence of the cell temperature was investigated. By means of activation energy analysis, an unequal influence of the temperature on the oscillation frequency has been found for Pt catalyst (EA = 40.5±0.6 kJ mol-1) compared to the previously reported case at PtRu (EA = 60.9 kJ mol-1) [3].
Additionally, oscillations were found in H2/O2 operation, too. They occur if the anode over¬potential is forced to increase until the onset potential for CO oxidation is reached. Nevertheless, in technical fuel cells operated with passive loads, the anode overpotential does usually not reach such high values, because a large fraction of the voltage loss is caused by cathodic and ohmic losses.
As an additional side effect, an increase of the Tafel slope at ηA≈350mV was observed in some of the galvanostatic polarisation curves. Two hypotheses for the occurrence of a second Tafel slope were drawn: on the one hand, the Heyrovski mechanism could enhance hydrogen adsorption at higher overpotentials; on the other hand, spatial distributions of the overpotential could lead to local CO oxidation and with this to enhanced hydrogen oxidation in parts of the reactor (e.g. under the ribs, where CO transport could be diffusion limited).
Finally, a model of Zhang and Datta [3], developed for PtRu and recently applied to PtPd [4], was adapted to fit the measurements. The adapted parameter set was compared to the original values and reasonable physical explanations for the adaptations were given. The main differences between Pt and PtRu were found to be a lower water dissociation constant and a lower CO oxidation constant.
