Characterization of Structural and Electronic Transitions During Reduction and Oxidation of Ru(acac)₃ Flow Battery Electrolytes by using X‐ray Absorption Spectroscopy

Abstract

Metal acetylacetonates possess several very attractive electrochemical properties; however, their cyclabilities fall short of targets for use in nonaqueous redox flow batteries. This paper describes structural and compositional changes during the reduction and oxidation of ruthenium(III) acetylacetonate [Ru(acac)3], a representative acetylacetonate. Voltammetry, bulk electrolysis, and in situ X‐ray absorption spectroscopy (XAS) results are complemented by those from density functional theory (DFT) calculations. The reduction of Ru(acac)3 in acetonitrile is highly reversible, producing a couple at −1.1 V versus Ag/Ag+. In situ XAS and DFT indicate the formation of [Ru(acac)3]− with Ru−O bonds lengthened relative to Ru(acac)3, nearly all of the charge localized on Ru, and no ligand shedding. The oxidation of Ru(acac)3 is quasireversible, with a couple at 0.7 V. The initial product is likely [Ru(acac)3]+; however, this species is short‐lived, converting to a product with a couple at −0.2 V, a structure that is nearly identical to that of Ru(acac)3 within 3 Å of Ru, and approximately 70 % of the charge extracted from Ru (balance from acetylacetone). This non‐innocence likely contributes to the instability of [Ru(acac)3]+. Taken together, the results suggest that the stabilities and cyclabilities of acetylacetonates are determined by the degree of charge transfer to/from the metal.

Elucidating the Role of Reduction Kinetics in the Phase-Controlled Growth on Preformed Nanocrystal Seeds: A Case Study of Ru

“…We used DFT-based calculations to elucidate the atomic-scale interactions that govern the phase-selective growth observed experimentally. Previous studies indicate that RuO 2 is formed as an intermediate during the early stages of Ru­(acac) 3 decomposition in the presence of oxygen or oxygen-containing species, − serving as a transition point between the Ru­(III) precursor and Ru atoms. As such, it is not unreasonable to focus our analyses on interactions involving RuO 2 rather than Ru­(acac) 3 .…”

Tris(β-ketoiminato)ruthenium(III) complexes: Electrochemical and computational chemistry study

“…Furthermore, Ru-complexes containing these N,N bidentate ligands such as (2,2'bipyridine, 1-10-phenanthroline) are used as dye-sensitizers for solar cells [ 5 ]. On the other hand, metal complexes containing O,O bidentate ligands (such as acetyl acetone) are used in catalysis [ 6 ] and as electrolyte in non-aqueous redox flow batteries [ 7 ]. All these complexes have attracted a lot of interest because of their favourable photophysical and electrochemical properties, which can be systematically fine-tuned to achieve optimal customized material characteristics.…”