In polymer electrolyte membrane fuel cell research, shaped nanoparticles have attracted much attention in the last years. For the cathode, octahedral PtNi nanoparticles show promise in their high oxygen reduction reaction (ORR) activities due to their extended availability of (111) facets. This group of materials has been studied extensively including methods like XPS, TEM, EDX and EELS to understand their growth and degradation^1-4. However, nanoscale analysis of their degradation has not been observed in real time.

Here, we present a novel in situ TEM electrochemistry study in acidic media on octahedral Pt-Ni nanoparticles synthesized via a solvo-thermal route. With real time imaging we were able to track and observe changes in the catalyst structure and particle shape during different electrochemical treatments⁵. For each electrochemical treatment, various potential sequences were applied in the TEM cell. We were able to confirm strong corrosive consequences on the catalyst structure and particle shape of an uncontrolled high open circuit potential (~ 1.6 V vs. RHE) after controlled electrochemical cycling. Additionally the impact on the material of stepwise increase of the upper potential (≤ 1.4 V vs. RHE) during electrochemical cycling in comparison with equivalent chronoamperometric profile was studied, showing increased degradation rates when high potentials were held. Furthermore we were able to observe the dissolution of nickel catalyst particles, even with relatively cautious electrochemical treatment (0.0 – 1.0 V vs. RHE). While electrochemical cycling up to 1.2 V vs. RHE is not harmful to the octahedral particle shape, it was still possible to observe limitations of catalyst stability by motion on the catalyst support.

Our study allows us to gain deeper insight of the real time behavior and degradation of shaped PtNi electrocatalysts, and develop understanding of the parameters that will optimize the lifetime of the catalyst.

1. Arán-Ais, R. M.; Yu, Y.; Hovden, R.; Solla-Gullón, J.; Herrero, E.; Feliu, J. M.; Abruña, H. D. Journal of the American Chemical Society 2015,137, (47), 14992-14998.

2. Choi, S.-I.; Xie, S.; Shao, M.; Odell, J. H.; Lu, N.; Peng, H.-C.; Protsailo, L.; Guerrero, S.; Park, J.; Xia, X.; Wang, J.; Kim, M. J.; Xia, Y. Nano Letters 2013,13, (7), 3420-3425.

3. Gan, L.; Cui, C.; Heggen, M.; Dionigi, F.; Rudi, S.; Strasser, P. Science 2014,346, (6216), 1502-1506.

4. Beermann, V.; Gocyla, M.; Willinger, E.; Rudi, S.; Heggen, M.; Dunin-Borkowski, R. E.; Willinger, M.-G.; Strasser, P. Nano Letters 2016, 16, (3), 1719-1725.

5. Holtz, M. E.; Yu, Y.; Gunceler, D.; Gao, J.; Sundararaman, R.; Schwarz, K. A.; Arias, T. A.; Abruña, H. D.; Muller, D. A. Nano Letters 2014, 14, (3), 1453-1459.