Langzeitstabilität der Kathoden-Katalysatorschicht in Polymerelektrolyt-Brennstoffzellen (English shop)

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This work investigates the long-term stability of the cathode catalyst layer of polymer electrolyte fuel cells in automotive applications. The aim was to identify the site of attack and the mechanism of the governing degradation processes and to evaluate countermeasures based on materials engineering and operating strategy.

The performance loss during fuel cell operation could be attributed, across the entire current density range, to the decrease in the active catalyst surface area of the cathode. The surface area loss is caused by oxidation of the carbon support – so-called carbon corrosion – and by platinum dissolution – so-called Ostwald ripening. Since the chemical membrane degradation, which ultimately limits service life because it leads to pinhole formation, could be traced back to intermediates of the oxygen reduction reaction formed on the cathode side, the cathode is the site of attack and of origin of all ageing processes during fuel cell operation.

By means of potentiodynamic corrosion measurements, various oxidation mechanisms of the catalyst support could be identified. The corrosion rate, which increases with the cathode potential according to Butler-Volmer, is further enhanced in certain potential ranges by the superposition of dynamically initiated processes. Thus, the reduction of the platinum oxide layer during a cathodic potential change leads to increased corrosion of the catalyst support through reaction of the desorbing oxygen species with the carbon.

In addition to the oxidation of the carbon support, the process of Ostwald ripening could be identified as a cause of the loss of active catalyst surface area. Here, the decrease in platinum surface area is attributable to the growth of the catalyst particles and to the loss of platinum into the electrolyte membrane as a consequence of dissolution and precipitation processes. The alternation between oxidising and reducing potentials in dynamic operation increases catalyst degradation. Moreover, the oxidation of the carbon support substantially accelerates the agglomeration processes by destabilising the platinum-carbon interfaces.

In addition to the use of more corrosion-stable graphitised support materials, the use of agglomeration-resistant catalyst materials should be sought. With platinum-cobalt alloys, in addition to the improved cathode stability under constant-potential operation, an increased oxygen reduction activity together with a reduced hydrogen peroxide formation rate could be demonstrated. In dynamic operation, however, the high activity of the platinum-cobalt catalyst led to increased support corrosion and thus to reduced stability compared with pure platinum catalysts.

Since the stability requirements of automotive applications are not yet met by today’s cathode materials, measures relating to operating strategy are indispensable, beyond the materials engineering approach, for increasing the long-term stability of polymer electrolyte fuel cells. In addition to avoiding supersaturated conditions and high temperatures in order to minimise carbon corrosion, the definition of an anodic potential limit is particularly necessary. Limiting the cell voltage reduces the maximum corrosion and agglomeration rates occurring and additionally prevents the dynamically initiated oxidation of the catalyst support. When a voltage window of 650-850 mV was maintained, Ketjenblack-supported cathodes in dynamic operation achieved a long-term stability comparable to that at a constant 850 mV.