Jonathan Quinson1 Melanie Röefzaad1 Davide Deiana2 Thomas Hansen2 Jakob Wagner2 Markus Nesselberger1 Andrew Crampton3 Claron Ridge3 Florian Schweinberger3 Ueli Heiz3 Matthias Arenz1 4

1, University of Copenhagen, Copenhagen, , Denmark
2, Technical University of Denmark, Lyngby, , Denmark
3, Technical University of Munich, Munich, , Germany
4, University of Bern, Bern, , Switzerland

The degradation of size-selected Pt nanoclusters is studied under electrochemical conditions. Insight into the early stage of degradation mechanism is given by scanning transmission electron microscopy (STEM). Size selected clusters catalyst mimic carbon supported Pt nanoclusters and nanoparticles typically employed in proton exchange membrane fuel cells (PEMFCs).

In contrast to common assumptions, it is demonstrated that even extremely small Pt clusters made of 22 to 68 atoms exhibit a remarkable stability under electrochemical conditions.
In mild accelerated degradation tests conditions, a main effect is particle migration without agglomeration.
At higher potential limits (in the typical operation range of PEMFCs) particle detachment and/or minor Pt dissolution is observed.
By investigating mixed cluster samples it is clearly demonstrated that no preferential dissolution of Pt22 by Ostwald ripening - usually held responsible to be the main mechanism for activity loss in Pt fuel cell catalysts - is observed.

The results indicate that subnanometer Pt clusters catalysts indeed might be a feasible option as extremely high-dispersed PEMFCs as long as the operation conditions are carefully adjusted and the active phase is effectively immobilized onto the carbon support.