Unravelling the Degradation Mechanisms of Metal-Based Carbon-Supported Electrocatalysts in Alkaline Medium: Similarities and Differences Compared to Acidic Medium

Abstract

Direct liquid alkaline fuel cells (DLAFC), e.g. using performant anion-exchange polymers [1], may viably compete with H2-fed proton-exchange membrane fuel cells for the powering of portable or automotive devices: non-Pt electrodes can be used without detrimental loss of performances and liquid fuel storage is easy versus compressed H2 gas. The common belief considers that AFC electrocatalysts are stable in alkaline environment, owing to the better thermodynamics stability of many metal and oxides at high pH [2]. However, this belief does not apply to carbon-supported Pt nanoparticles (NPs), which undergo massive degradations even for a very mild potential cycling procedure in 0.1 M NaOH [3]. Identical-location transmission electron microscopy (ILTEM) experiments demonstrate that the very pronounced loss of electrochemical surface area (ECSA) is linked to the detachment of the Pt NPs from the carbon support; surprisingly, the latter does not undergo any consequent corrosion, and the Pt NPs seem very stable from a dissolution/redeposition view point (Ostwald ripening). Similar ILTEM experiments demonstrate that carbon-supported palladium NPs (Pd/C) are more stable than Pt/C but still suffer NP detachment from the carbon [4]. The degradation of Pd/C NP is slightly larger when the alkaline electrolyte contains strong reducing species (e.g. hydrogen or hydrazine borane), and this effect is enhanced for small Pd/C NP than for large ones. In contrast with acidic medium, this set of results suggests that the propensity of the metal NPs to be reduced (nobleness) enhances their instability. This hypothesis was further confirmed for non-noble Ni3Co/C NPs, which are more durable than Ni3Ag/C, Ni3Pd/C, Pd/C and Pt/C ones: the link between the carbon support and the NPs is more easily destroyed in liquid alkaline electrolytes when the NPs are containing a noble metal.