Accelerated fuel cell tests of anodic Pt/Ru catalyst

The rapid growth of fuel cell application witnessed today is among others a result of the catalyst design. It determines the long-term stability, lifetime and the performance of the cells. Various catalysts systems are available for various types of fuel cells. For example, polymer-electrolyte-membrane fuel cells use catalysts systems based on platinum nanoparticles which in most cases are spread on the carbon support. Although there are several fuel cells degradation mechanisms taking place during their operation, particle dissolution and particles agglomeration are considered the main one. Degradation affects both of the dispersed nanoparticles and the support materials. Therefore, it is important to understand the occurrence of the various particle degradation mechanisms as well as tracking the changes taking place in the single particles.

A group of researchers at Max Planck Institute Researchfür Eisenforschung in Germany: Dr. Katharina Hengge, Thomas. Gänsler, Dr. Enrico. Pizzutilo, Professor Karl Mayrhofer and Professor. Christina Scheu in collaboration with Dr. Christoph Heinzl from their industry partner in München and Michael Beetz from Ludwig-Maximilians-Universität München investigated the structure, chemical composition and the stability of the Pt/Ru catalysts distributed on high-surface-area carbon. The catalyst was applied at the anode of polymer electrolyte fuel cells. The choice of the Pt/Ru system was due to its capabilities of enhancing the carbon monoxide tolerance compared to pure platinum. During the experiment, the authors used identical location transmission electron microscopy (TEM) to analyse changes of singles the catalysts particles during the potential cycling. Their work is currently published in the journal, International Journal of Hydrogen Energy.

The authors observed that the general performance of the fuel cell greatly relies on the stability of the catalyst. For instance, a higher fuel cell stability was achieved when the maximum potential value decreased thus the Pt/Ru catalysts showed some less degradation behavior. Furthermore, it was realized that during the potential cycling, the primary degradation mechanisms were dissolution and dealloying. This was however intense at the initial stages of the electrochemical treatments.

The german researchers noted that the presence of Ru in the Pt/Ru catalysts allowed elimination of the CO from the Pt surface at a lower voltage. This was, however, irrespective of the fact that Ru is generally less stable especially during the electrochemical treatment process. Apart from the dissolution and dealloying, the effects of Ostwald ripening and agglomeration were also observed. Additionally, to understand the cell performance deterioration, it is necessary to evaluate the specific operation related changes of the surface areas of the catalysts. This can be performed in the as-prepared state as well as after the potential cycling. The authors used electron tomography experiments for the local catalyst nanoparticles analysis and CO stripping measurements for the evaluation of global degradation effects. The authors are optimistic that their study can be extended to investigating the stability of the fuel cells not only under the start-stop operation conditions but also during the regular operations.