Selected Review of the Degradation of Pt and Pd-based Carbon-supported Electrocatalysts for Alkaline Fuel Cells: Towards Mechanisms of Degradation

It is usually believed that carbon-supported electrocatalysts are stable in alkaline environment, owing to the better thermodynamics stability of many metals and oxides at high pH. By focusing on a selected literature review concerning Pt/C and Pd/C nanoparticles, and in particular from identical-location transmission electron microscopy (ILTEM), it is demonstrated that this common knowledge is erroneous in aqueous alkaline electrolytes: both Pt/C and Pd/C suffer pronounced loss of electrochemical surface area (ECSA), and the latter is linked to the detachment of the metal nanoparticles from the carbon support. Raman and X-ray photoelectron spectroscopy show that these severe degradations are neither linked to massive corrosion of the carbon support nor to an overall change in carbon chemistry, but instead to a very localized corrosion of the carbon in the vicinity of the metal nanoparticles, leading to nucleation and growth of solid carbonate (when the electrolyte contains alkali metal cations), which expels the metal nanoparticles from their support. The mechanisms and extent of degradation depend on the nature of the metal nanoparticles, but also on their texture and on the nature of the support onto which they are immobilized.