Instability Of Commercial Pt/C And Pd/C Electrocatalysts In Alkaline Media
Electrochemical measurements, combined with Identical-Location Transmission Electron Microscopy (ILTEM) allowed to demonstrate that the catalysts degradation in 0.1 M NaOH at 25°C is extremely severe for a mild accelerated stress tests (AST – 150 cycles, at 100 mV sec-1 and 25°C, between 0.1 and 1.23 V vs.RHE).
The AST performed directly on TEM grids (Gold + Lacey carbon) for Pt/C in Identical-Location Transmission Electron Microscopy (ILTEM) imaging (Fig.1.a-b) demonstrated extensive nanoparticles loss (either by dissolution or detachment from the carbon surface) and non-negligible extent of agglomeration. These observations were further bridged with the results obtained with the CO-stripping technique (Fig.1.c), which clearly highlight the appearance of a pre-peak associated to the Pt agglomerates, features that are generally observed for much harsher and/or longer degradation protocols and at higher temperature value in acidic media1,2,3.
As a result, the electrochemical surface area (ECSA) losses are about 3 times worse in alkaline than in acidic media (60 % of ECSA loss vs. ca. 20% in H2SO4 and HClO4, Fig.1.d) for such a “mild” AST.
The effect of the electrocatalyst material nature (Pd or Pt) and size effects on the degradation profile and rate have also been investigated.
In these harsh degradation in alkaline medium, extensive carbon corrosion has been ruled out, according to Raman spectroscopy measurements that show nearly no sign of structural changes of the carbon, although such effect is often observed in acidic media4. Therefore, we attributed the huge changes of Pt/C and Pd/C morphology upon AST in alkaline medium to the modification of the carbon surface carbon chemistry, in agreement with XPS measurements, which simply destroys the anchoring sites of the Pt (resp. Pd) nanoparticles and favors their simple detachment from the carbon surface. We also cannot rule out cathodic corrosion of the Pt (resp. Pd) nanoparticles at the lower vertex potential of the AST, as experienced by the group of Koper 5.
(1) F. Nikkuni, E. Ticianelli, L. Dubau, M. Chatenet, Electrocatal., 4 (2013) 104-116.
(2) L. Dubau, L. Castanheira, G. Berthomé, F. Maillard Electrochim. Acta, 110 (2013) 273– 281.
(3) Meier, J. C., Galeano, C., Katsounaros, I., Witte, J., Bongard, H. J., Topalov, A. a, Mayrhofer, K. J. J. (2014). Beilstein Journal of Nanotechnology, 5, 44–67.
(4) Dubau, L., Castanheira, L., Chatenet, M., Maillard, F., Dillet, J., Maranzana, G., (2014). International Journal of Hydrogen Energy, 39(36), 21902–21914.
(5) Yanson, A. I.; Rodriguez, P.; Garcia-Araez, N.; Mom, R. V.; Tichelaar, F. D.; Koper, M. T. M. Angew. Chem. Int. Ed. 2011, 50, 6346.