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Remartinez, V. Celorrio
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article
Electrocatalytic Site Activity Enhancement via Orbital Overlap in A 2MnRuO 7(A = Dy 3+, Ho 3+, and Er 3+) Pyrochlore Nanostructures
Abstract
Oxygen electrocatalysis at transition metal oxides is one of the key challenges underpinning electrochemical energy conversion systems, involving a delicate interplay of bulk electronic structure and surface coordination of the active sites. In this work, we investigate for the first time the structure-activity relationship of A2RuMnO7 (A = Dy3+, Ho3+, Er3+) nanoparticles, demonstrating how orbital mixing of Ru, Mn, and O promotes high density of states (DOS) at the appropriate energy range for oxygen electrocatalysis. The bulk and surface structure of these multicomponent pyrochlores are investigated by high-resolution transmission electron microscopy, X-ray diffraction, X-ray absorption (XAS), X-ray emission (XES) and X-ray photoemission (XPS) spectroscopies. The materials exhibit high phase purity (cubic fcc with a space group Fd3 ̅m), in which variations in M-O bonds length are less than 1% upon replacing the A-site lanthanide. XES and XPS show that the mean oxidation state at the Mn-site as well as the nanoparticle surface composition were slightly affected by the lanthanide. The pyrochlore nanoparticles are significantly more active than the binary RuO2 and MnO2 towards the 4-electron oxygen reduction reaction (ORR) in alkaline solutions. Interestingly, normalization of kinetic parameters by the number density of electroactive sites concludes that Dy2RuMnO7 shows twice higher activity than benchmark materials such as LaMnO3. Analysis of the electrochemical profiles supported by DFT calculations reveals that the origin of the enhanced catalytic activity is linked to the mixing of Ru and Mn d-orbitals and O p-orbitals at the conduction band which strongly overlap with the formal redox energy of O2 in solution. The activity enhancement strongly manifests in the case of Dy2RuMnO7 where Ru/Mn ratio is closer to 1 in comparison with the Ho3+ and Er3+ analogs. These electronic effects are discussed in the context of the Gerischer formalism for electron transfer at the semiconductor/electrolyte junctions.