Abstract
Accelerated stress tests are commonly applied in order to obtain a prediction of fuel cell life time within a short testing period. The stress test in this work considers a fuel cell which is constantly under load with frequent periods of very high current. Four different cells were operated each with a specific load profile. As a result severe performance degradation was observed in the region of high current densities. A short overview over the phenomena which may contribute to this specific fuel cell degradation is compiled from literature. Based on this overview major degradation modes are identified and combined with a simple polarization curve model. The modeling results allow for two different interpretations. Carbon corrosion reactions can explain the observed effects if cell voltage is assumed as the driving force for degradation. A better fit was obtained by using the overall heat flux as degradation criterion. In this case an increase in local temperature could lead to redistribution and loss of phosphoric acid from the MEA. The results are supported by the fact that the model yields consistent time dependend results for four different and non-periodic load cycles.
Highlights
The expected lifetime of a polymer electrolyte fuel cell (PEFC) ranges from 5 000 h for mobile application to 40 000 h for stationary application.[1,2] In order to gain information about time dependent degradation in advance accelerated test protocols are used.[3,4] Depending on the design of these tests valuable information about the nature of the main degradation mode under the applied operation conditions can be obtained
The purpose of this paper is to present the results of a specific accelerated degradation test for high temperature polymer electrolyte fuel cells (HT-PEFC) with phosphoric acid doped polybenzimidazole (PBI/H3PO4) membranes
On the other hand long term investigations of HT-PEFC show that the phosphoric acid loss through evaporation is very small
Summary
Catalyst degradation.— Catalyst degradation is the result of a complex interplay of platinum dissolution, agglomeration, platinum particle detachment and carbon corrosion.[6,11,12,17,18,19,20] As a result inhomogeneous degradation behavior for different catalyst locations can be observed by identical location transmission electron microscopy,[21] which arise from the fact that fuel cell electrode layers are composite materials with inherent inhomogeneities at the level of catalyst. The underlying mechanism seem to be different for ’mild’ (local) starvation and ’severe’ (overall) starvation.[11] In case of overall fuel starvation, the cell voltage reaches negative values the cause being an increase of the anode potential to values larger than 1.0 V while the cathode potential remains at ’normal’ working potential smaller than 1.0 V.60 Under these conditions severe catalyst corrosion was observed at the anode and to a lesser extend at the cathode[60] and carbon dioxide[61] and oxygen[62] are observed in the anode off gas. A detailed discussion of degradation phenomena in start/ stop procedures can be found here.[13]
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