Four electron selective O2 reduction by a tetranuclear vanadium(IV/V)/hydroquinonate catalyst: application in the operation of Zn–air batteries

Abstract

The reduction of dioxygen plays a crucial role in both natural and artificial systems exhibiting peroxidase like activity, utilizing O₂ as a cheap and green oxidant in several applications including the development of new effective sources of clean energy. The exploration of new facile and cost-efficient electrocatalysts which promote the interconversion between H₂O and O₂ remains a crucial challenge. In this work, the applicability of peroxidase mimicking tetranuclear vanadium(IV/V) hydroquinonate, 1, as a proton-coupled electron transfer (PCET) oxygen reduction electrocatalyst to metal–air batteries is presented. Cyclic and rotating disk voltammetry studies show that the O₂ reduction is associated with the PCET mechanism. Measurements of the O₂ consumption vs. pH reveal that two molecules of 1 reduce one molecule of O₂ supporting a 4e⁻ reduction of O₂ to H₂O. ⁵¹V NMR spectroscopy used for H₂O₂ trap experiments supports the 4e⁻ reduction of O₂. Exhaustive electrolysis shows that the redox reactions of 1 are fully reversible in the presence of O₂. Compound 1 was used as a catalyst for the O₂ reduction in a Zn–air battery. Aqueous solutions of the complex were transformed into gels by the addition of a small quantity of sulfuric acid. Then, the complex in the form of gel was easily deposited on a carbon cloth electrode and was directly applied for the construction and operation of a Zn–air battery. The presence of the complex resulted in a large increase of the current and the power produced by the cell, particularly in an acidic electrolyte where the complex operates the best. The application of the biomimetic complex 1 in the operation of metal–air batteries opens a new and interesting route for applying such molecules in a wide range of high importance technological applications.