The role of water in the electrochemical response of platinum

Abstract

The oxygen reduction reaction (ORR) and formation of oxide occur simultaneously on the surface of platinum catalysts in the range of the open circuit potential (OCP) of electrochemical gas sensors and fuel cells. The relationship between these processes relies on a fundamental understanding of the reactivity of water as a proton donor at the interface. The first part of this work used OCP measurements to study the parallel oxygen and oxide electrochemistry of platinum rotating disc electrodes. The OCP was shown to be controlled by the kinetics of the ORR, which depended on the nature and coverage of oxide in acid but was independent of oxides present in low quantities in alkaline solution. The formation of oxide at rest was found to be pH dependent, with the coverage being higher and the oxide more strongly bound in alkaline media than in acid. The oxides were gradually transformed into a more strongly coordinated form with time. The second part of this work controlled the availability of water in the ORR at microelectrodes by the systematic addition of water to nonaqueous electrolytes. Organic and alkali metal cations with different abilities to coordinate water were used to modify its presence in the double layer and its reactivity in proton transfer. Comparison of platinum and gold as electrode materials revealed that the ability of the metal to bind and dissociate water is an important descriptor for proton transfer in the ORR. It was shown that there is a balance between the affinity of the surface for H₂O and its site-blocking effect when coordinated to metal ions. The results emphasized the importance of the relative strengths of interactions between each of the species in the system, Pt, H₂O, M⁺ , and O_{2 -} , in determining the reactivity of water in the ORR.

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Identifiers

ORCID for Andrea Russell: orcid.org/0000-0002-8382-6443

ORCID for Guy Denuault: orcid.org/0000-0002-8630-9492

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