Homogeneous Electrocatalytic Oxygen Reduction Reaction by a Versatile Polyoxometalate in an Aqueous Solution

In this work, we have explored a well-known water-soluble versatile polyoxometalate, Na₆[V₁₀O₂₈]∙18H₂O (1), for electrocatalytic homogeneous oxygen reduction reaction (ORR). Both RDE and RRDE experiments evident that compound 1 is actually involved in a two-electron reaction mixed with a four-electron reaction. Compound 1 exhibits remarkable stability in solution, remaining intact without decomposition/electrodeposition during electrocatalysis.

Abstract

The electrochemical production of H₂O₂ via a two-electron (2e⁻) oxygen reduction reaction (ORR) presents a compelling alternative to the traditional anthraquinone oxidation process used in industry, as it deals with limited H₂O₂ generation. This work describes the first paradigm of a polyoxometalate (POM) compound, Na₆V₁₀O₂₈·18H₂O (1), that per se exhibits aqueous homogeneous electrocatalytic oxygen reduction reaction (ORR). Compound 1 is not only a well-known and versatile POM compound but has also been extensively studied as far as its biological activities are concerned. However, it was never known that it has the ability to perform electrocatalytic ORR. In literature, a limited number of studies exist regarding POM-based materials that have been immobilized within various nanomaterials and employed as heterogeneous electrocatalysts for the ORR. Compound 1 demonstrates remarkable electrochemical stability in solution, as confirmed by a prolonged (10-hour) constant potential electrolysis. Throughout this extended electrolysis, compound 1 maintains its integrity without undergoing decomposition or electrodeposition, which has been rigorously verified through various spectroscopic and microscopic analyses. The K-L plot derived from the RDE experiment reveals that the number of electrons transferred for this ORR is 2.7 (at a potential of −0.5 V vs. NHE). The use of rotating ring-disc electrode (RRDE) experiments and spectrophotometry techniques have confirmed the electrochemical generation of H₂O₂. Compound 1 is involved in a two-electron transfer reaction mixed with a four-electron transfer reaction, generating 53% H₂O₂ by two-electron oxygen reduction.

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