Instability Of Commercial Pt/C And Pd/C Electrocatalysts In Alkaline Media

Alkaline fuel cells are interesting alternatives to proton-exchange membrane fuel cells, owing to the possibility to feed them directly with complex fuels (such as ethanol, methanol, ammonia, hydrazine, sodium borohydride, etc.) with acceptable performances. In that frame, studying the stability of carbon-supported catalysts nanoparticles in alkaline media is mandatory, these properties having largely been investigated in acidic electrolytes in the literature so far. Whether some works have focused on general aspects of carbon stability in alkaline media, to date none of them have investigated the stability of metal nanoparticles on carbon supports. Here, we studied the degradation of commercial (E-TEK) Pt and Pd nanoparticle electrocatalysts supported on Vulcan XC-72 (noted Pt/C and Pd/C) in NaOH electrolyte solutions, and compared the fate of the electrocatalysts to that in H₂SO₄, HClO₄electrolyte solutions.

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 media^1,2,3.

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.

References

(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.