Alkaline fuel cells (AFC) are competitors to proton-exchange membrane fuel cells for stationary applications [1]. Because many metals and metal oxides are stable at high pH [2], one may think that AFC electrocatalysts will be more stable in operation than in acidic medium. However, this was proven wrong for carbon-supported Pt and Pd nanoparticles (NPs) aged in liquid alkaline environment: these undergo severe electrochemical surface area losses even for a very mild potential cycling procedure in 0.1 M NaOH [3, 4]. Identical-location transmission electron microscopy (ILTEM) experiments revealed pronounced detachment of the Pt (and Pd) NPs from the carbon support, but minor degradation phenomena for the metal NPs and the carbon support itself. Additional experiments performed in various alkaline electrolytes (LiOH, NaOH, KOH, CsOH) coupled with in situ Fourier-transform infrared spectroscopy enabled to link this detachment to the formation of solid carbonates at the interface between the Pt (Pd) NPs and the carbon support, because the metal nanoparticles assist the local corrosion of the carbon support (firstly into CO₂, then CO₃^2- anions and finally into M₂CO₃, M = Li, Na, K or Cs).

Figure 1 demonstrates that the loss of Pt NPs is greatly decreased when the alkaline electrolyte is an anion-exchange membrane. In that case, the formation of solid carbonates is severely depreciated, because the counter-cation of the OH^- species are immobilized on the polymer backbone and can therefore not precipitate (as Na₂CO₃ does in NaOH aqueous electrolyte). Although the degradation of the Pt/C NPs is minored when the materials are aged in interface with an AEM, it is not suppressed: Ostwald ripening and Pt redeposition are observed, i.e. the mechanisms of degradation differ in solid versus liquid alkaline environment. Such differences between the fate of Pt-based electrocatalysts were also demonstrated for solid versus liquid acidic electrolytes [5, 6].

This work was partly supported by an ONR-global grant (project ONR N62909-16-1-2137).

References

[1] E.H. Yu, X. Wang, U. Krewer, L. Li, K. Scott, Direct oxidation alkaline fuel cells: from materials to systems, Energy Environ. Sci., 5 (2012) 5668.

[2] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, in, National Association of Corrosion Engineers, Houston, 1979, pp. 453.

[3] A. Zadick, L. Dubau, N. Sergent, G. Berthomé, M. Chatenet, Huge Instability of Pt/C Catalysts in Alkaline Medium, ACS Catal., 5 (2015) 4819.

[4] A. Zadick, L. Dubau, U.B. Demirci, M. Chatenet, Effects of Pd Nanoparticle Size and Solution Reducer Strength on Pd/C Electrocatalyst Stability in Alkaline Electrolyte, J. Electrochem. Soc., 163 (2016) F781.

[5] F.R. Nikkuni, L. Dubau, E.A. Ticianelli, M. Chatenet, Accelerated degradation of Pt₃Co/C and Pt/C electrocatalysts studied by identical-location transmission electron microscopy in polymer electrolyte environment, Appl. Catal. B: Environmental, 176-177 (2015) 486.

[6] F.R. Nikkuni, B. Vion-Dury, L. Dubau, F. Maillard, E.A. Ticianelli, M. Chatenet, The role of water in the degradation of Pt₃Co/C nanoparticles: An Identical Location Transmission Electron Microscopy study in polymer electrolyte environment, Appl. Catal. B: Environmental, 156–157 (2014) 301.