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Accelerated Stress Test of Pt/C Nanoparticles in an Interface with an Anion-Exchange MembraneAn Identical-Location Transmission Electron Microscopy Study

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journal contribution
posted on 04.01.2018, 00:00 by Clémence Lafforgue, Marian Chatenet, Laetitia Dubau, Dario R. Dekel
The durability of a state-of-the-art Pt/C electrocatalyst was assessed by accelerated stress test (AST) procedures conducted in liquid alkaline electrolyte (0.1 M NaOH) and in solid anion exchange polymer electrolyte using a “dry cell”, i.e. in absence of liquid electrolyte. In a liquid environment, the positive and negative vertex potential values have a great influence on the extent and on the magnitude of the degradations: the loss of electrochemical surface area observed for a wide potential range (0.1 < E < 1.23 V vs RHE) is ascribed to detachment of the Pt nanoparticles from their support. The Pt nanoparticles assist the local corrosion of the carbon support material, eventually yielding solid alkali–metal carbonates that mechanically expel them from their support. Such corrosion is linked to the propensity of the Pt nanoparticles to (i) accept carbon surface groups (COad-like species) when their surface is free of oxides (“reduced” metal state, for E < 0.6 V vs RHE) and then to (ii) electro-oxidize the COad species into CO2 in the well-known Langmuir–Hinshelwood CO-stripping reaction, possible if OHad species do form (“oxidized” metal state, for E > 0.6 V vs RHE). As a result, when the AST is performed between 0.1 < E < 0.6 V vs RHE or between 0.6 < E < 1.23 V vs RHE, i.e. when the Pt nanoparticles are either mostly reduced or oxidized, respectively, the degradation processes at stake are less intense and different: Ostwald ripening proceeds in the former case and Pt nanoparticle agglomeration in the latter. In contrast to the case of liquid electrolyte, when the most severe AST (0.1 < E < 1.23 V vs RHE) is performed in the dry cell, the magnitude and main mechanisms of degradation significantly change. Because there is no excess water to dissolve the Ptz+ species formed by corrosion of the Pt nanoparticles, 3D Ostwald ripening and local redeposition on existing particles become more likely: the anion-exchange ionomer better traps the Ptz+ species and prevent their diffusion away from the active layer. In addition, the absence of free alkali metal cation avoids the precipitation of solid carbonates, and therefore, the detachment of the Pt nanoparticles from their support is not favored. This shows that the degradation processes of a given electrocatalyst not only depends on its nature but also on the vertex potential values scanned in the AST and, importantly, on the nature of the electrolyte medium investigated. Finally, the very dramatic degradations experienced in liquid electrolyte for Pt/C nanoparticles are somewhat mitigated in solid alkaline electrolyte, which harbors hope to develop durable AEM-based fuel cells and electrolyzers.