Abstract
A strategy for drastic improvement in the durability of a Pt/C catalyst and a PtCo/C alloy catalyst for the oxygen reduction reaction (ORR) by melamine surface modification was demonstrated. The durability of the catalysts was evaluated by an accelerated durability test (ADT) using a square wave potential cycling of 0.6–1.0 V vs RHE performed in Ar-saturated 0.1 mol dm–3 HClO4 containing a very small amount of melamine (10 μmol dm–3) at 80 °C for 10,000 cycles. Transmission electron microscopy (TEM) observations showed that melamine addition suppressed a size increase in the catalyst nanoparticles (NPs) during the ADT caused by the Ostwald ripening and/or migration-coalescence, which mitigated a loss in the electrochemical surface area of the catalysts. More importantly, TEM-EDX compositional analysis revealed that melamine addition suppressed Co dealloying of the PtCo NPs during the ADT and the alloy catalyst retained a high ORR mass activity of 1019 A g–1-Pt at 0.9 V vs RHE even after the ADT. The durability improvement is considered to arise from the suppression of Pt-oxygenated species formation caused by destabilization of a stable water molecule network formed on the catalyst surface and selective adsorption of melamine molecules at low-coordination surface Pt sites. Our results open up an alternative avenue for drastic improvement in the durability of the Pt-based catalysts for the ORR in polymer electrolyte fuel cells.
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