Abstract
Nanostructured carbons were fluorinated and used as supports for Pt nanoparticles deposition using a modified polyol route. The resulting materials exhibited similar Pt nanoparticles sizes, but different agglomeration rates. The electrocatalysts were tested toward the oxygen reduction reaction, and their stability was investigated in simulated load cycling (0.6 < E < 1.0 V vs. RHE) or start-up/shutdown (1.0 < E < 1.5 V vs. RHE) protocols. Irrespective the support material, the former protocol caused Pt nanocrystallites dissolution/redeposition via Ostwald ripening, mildly decreasing the electrochemically-active surface area and ORR activity. In contrast, the carbon supports were strongly corroded after the start-up/shutdown protocol, resulting in pronounced detachment/agglomeration of Pt nanocrystallites, albeit in absence of significant particle-size growth. Fluorination had different effects on the stability of structurally-ordered and structurally-disordered carbons: beneficial effects were observed for the latter whereas the former was affected negatively. “Free” dangling groups present in structurally-disordered carbon, known to be prone to preferential oxidation in PEMFC environment, combine with the fluorine precursors upon fluorination, leading to formation of more robust C-F bonds versus oxidation than original C-O bonds. In contrast, fluorination of structurally-ordered carbon creates structural disorder (C-C bonds are broken), leading to promotion of electrochemical corrosion.
Highlights
This higher resistance to corrosion does not grant durability in operation: Castanheira et al.[12,39] and Linse et al.[19] demonstrated that in presence of Pt nanoparticles in contact with the carbon support, the tougher behavior of graphitic carbon is only a reality above 1.0 V vs. RHE; below this value, carbon corrosion is assisted by the catalytic role of Pt, and hardly any difference in the rate of corrosion is witnessed between nanostructured carbons of various levels of graphitization/organization
The present results show that, this does not modify the fate of the YS-supported electrocatalysts in the load-cycle accelerated stress test (AST), it severely depreciates the resistance of its surface to corrosion during the start-stop AST
The bare carbon supports were strongly corroded after the start-up/shutdown protocol, resulting in pronounced detachment of the Pt nanocrystallites but no growth in their size
Summary
This depreciated ORR mass activity essentially originates from the decrease in SPt,CO in these conditions, mostly associated to Ostwald ripening and the Pt-catalyzed carbon support corrosion.[12,39] the Pt nanoparticles are rounder and larger after the base-load protocol (see the representative TEM micrographs of Figure S2 and Figure S3) than they were before (see Figure 2 and Figure 3). The EN-F-24% sample maintains its ORR activity upon AST as well as the graphitized carbon YS did (see Figure 7B), but with the clear advantage to maintain a good dispersion of Pt nanoparticles owing to the much higher BET surface area of the EN-based carbon than for YS.
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