Understanding the Electronic Structure of Ruthenium Oxides during Redox Cycling with High Energy Resolution X-ray Absorption Spectroscopy.

Ruthenium oxides are essential catalysts and charge storage materials whose performance across reactions, from the oxygen evolution reaction to the chlor-alkali process, depends critically on reversible Ru redox cycling and the associated electronic structure changes. Although the Ru K-edge X-ray absorption spectroscopy (XAS) is routinely used for in situ studies, it provides limited information due to the low spectral resolution. This work reveals direct insights on the electronic structure of crystalline and amorphous/hydrous ruthenium oxide under electrochemistry using high energy resolution fluorescence-detected (HERFD) XAS and first-principles density functional theory with the Bethe-Salpeter equation. The pre-edge peak, which is buried in conventional XAS, is well-resolved with HERFD-XAS, enabling the assignment of specific electronic transitions visualized in real and reciprocal space. Polarized HERFD-XAS measurements on single-crystal RuO2 are accurately reproduced by the first-principles calculations. Ultimately, in situ HERFD-XAS of hydrous ruthenium oxide at different electrochemical potentials shows the reversible filling and emptying of d-states accompanied by structural changes. Our work sheds light on the now accessible wealth of electronic structure information for the in situ characterization of 4d transition metals at their K edges via HERFD-XAS.