Chemical Activity of the Peroxide/Oxide Redox Couple: Case Study of Ba5Ru2O11 in Aqueous and Organic Solvents

Abstract

The finding that triggering the redox activity of oxygen ions within the lattice of transition metal oxides can boost the performances of materials used in energy storage and conversion devices such as Li-ion batteries or oxygen evolution electrocatalysts has recently spurred intensive and innovative research in the field of energy. While experimental and theoretical efforts have been critical in understanding the role of oxygen nonbonding states in the redox activity of oxygen ions, a clear picture of the redox chemistry of the oxygen species formed upon this oxidation process is still missing. This can be, in part, explained by the complexity in stabilizing and studying these species once electrochemically formed. In this work, we alleviate this difficulty by studying the phase Ba₅Ru₂O₁₁, which contains peroxide O₂^2- groups, as oxygen evolution reaction electrocatalyst and Li-ion battery material. Combining physical characterization and electrochemical measurements, we demonstrate that peroxide groups can easily be oxidized at relatively low potential, leading to the formation of gaseous dioxygen and to the instability of the oxide. Furthermore, we demonstrate that, owing to the stabilization at high energy of peroxide, the high-lying energy of the empty σ* antibonding O-O states limits the reversibility of the electrochemical reactions when the O₂^2-/O^2- redox couple is used as redox center for Li-ion battery materials or as OER redox active sites. Overall, this work suggests that the formation of true peroxide O₂^2- states are detrimental for transition metal oxides used as OER catalysts and Li-ion battery materials. Rather, oxygen species with O-O bond order lower than 1 would be preferred for these applications.

Figure 1

Schematic representation of the oxygen redox chemistry in transition metal oxides with the transition from oxide ion (O^2–) to peroxo-like (O₂)ⁿ⁻ groups and peroxide (O₂)^2–, all the way to gaseous oxygen release.

Figure 2

(a) Structure of Ba₅Ru₂O₁₁ showing alternance of BaO₃ layers and Ba₂O₂ layers. (b) Peroxide groups in Ba₂O₂ layers statistically distributed over three possible configurations (shown in gray), one of them being shown in red. (c) Ba₂O layers replacing the Ba₂O₂ layers to form Ba₅Ru₂O₁₀.

Figure 3

Rietveld refinement of the neutron diffraction patterns recorded with λ = 1.29 Å and λ = 2.53 Å. The top panel shows the refinement obtained from the Ba₅Ru₂O₁₀ structural model, while the bottom one is performed from the Ba₅Ru₂O₁₁ model. The red crosses, black continuous line, and bottom green line represent the observed, calculated, and difference patterns, respectively. Vertical blue tick bars mark the reflection positions.

Figure 4

Experimental [101̅0] (a) HAADF-STEM and (b) ABF-STEM images of the pristine Ba₅Ru₂O₁₁ compound investigated in this work, together with theoretical images calculated using structure model for the (c and d) Ba₅Ru₂O₁₁ and (e and f) Ba₅Ru₂O₁₀ compounds. In the experimental images, perovskite blocks are highlighted with chevrons made up of 3 BaO (marked with circles) and 2 Ru columns; the Ba₂O₂ layers between the perovskite blocks are marked with asterisks. Note the presence of additional dots in ABF-STEM images (compared to HAADF-STEM) next to the Ru columns, corresponding to the O columns. Together with the O atoms in the BaO, these O positions form projection of the RuO₆ octahedra.

Figure 5

(a) Ru K-edge XANES spectra of Ba₅Ru₂O₁₁ compared to Ru⁵⁺-containing perovskite La₂LiRuO₆, Ru⁵⁺-containing rock salt Li₃RuO₄, Ru⁴⁺-containing layered compound Li₂RuO₃, and Na₂Ru⁶⁺O₄. (b) Magnitude of Fourier transform (FT) of k³-weighted EXAFS oscillations (solid line) of Ba₅Ru₂O₁₁ along with fitting results (○). In inset shows the filtered EXAFS oscillations in the q space. The FT is not corrected for phase shifts.

Figure 6

(a) Atom-projected density of states (pDOS) computed from DFT for Ba₅Ru₂O₁₁ where the oxide ions and ruthenium cations from the RuO₆ octahedra (red and blue, respectively) are differentiated from peroxide (O₂)^2– groups in the Ba₂O₂ planes (plain orange). (b) Negative and positive Fukui functions rendering the electronic density upon electron removal or addition, respectively, where Ru–O* orbitals are polarized upon reduction (positive Fukui) and Ru–O* as well as O–O π* states are polarized upon oxidation (negative Fukui). (c) X-ray absorption spectrum at the O K-edge in the fluorescence mode for the pristine Ba₅Ru₂O₁₁ compared with the pDOS for oxygen and peroxo-groups.

Figure 7

Aqueous electrochemistry of Ba₅Ru₂O₁₁ at different pH and under positive or negative polarization. (a) Under Ar-saturated solution at pH 13, starting under cathodic polarization, (b) under O₂-saturated solution at pH 13 starting in anodic polarization, (c) under O₂-saturated solution at pH 13 starting in cathodic polarization, and (d) in O₂-saturated solution at pH 7 starting in anodic polarization. E°(H₂O/O₂) at pH 13 is equal to 0.463 V versus NHE and at pH 7 equal to 0.817 V versus NHE. Arrows are guides for the initial polarization of each curve.

Figure 8

Electrochemistry of Ba₅Ru₂O₁₁ in organic electrolytes EC:DMC (1:1) 1 M LiPF₆, with (a) a charge at C/10 (red) with no initial discharge, compared to a GITT curve of the processes initiated by reduction of the pristine phase, with pulses corresponding to C/10 over 0.2 Li⁺ followed by a 6 h relaxation time in discharge then charge (blue). (b) A full discharge over 10 Li⁺ equivalents corresponding to the full conversion into metallic ruthenium and its corresponding derivative curve dQ/dV, (c) in situ XRD for Ba₅Ru₂O₁₁ in EC:DMC (1:1) 1 M LiPF₆, with a first cycle starting in discharge, limited to 2 Li⁺ equivalents, followed by a second discharge over 4 Li⁺ equivalents. A contour plot for the most intense peak at around 30° is provided on the left.

Figure 9

HAADF-STEM images taken along the [101́0] zone axis of Ba₅Ru₂O₁₁ after (a) electrochemical reduction and (b) oxidation in nonaqueous medium. In image (a), lines highlight corrugation of the perovskite blocks due to swelling of the Ba₂O₂ layers; and the inset shows the corresponding ED pattern with characteristic tangential smearing of the Bragg reflections. Image (b) is a representative low magnification image. It demonstrates that a large portion of the particle is missing (marked with an asterisk); and the inset shows a high resolution HAADF-STEM image, confirming that the crystal structure of the material is preserved in the area unaffected by the dissolution.

Figure 10

(a) Ru K-edge XANES spectra for pristine Ba₅Ru₂O₁₁ and the samples reduced with 1 Li⁺, 2 Li⁺, or oxidized for the equivalent of 1 Ba²⁺. (b) Ru K-edge EXAFS oscillations weighted by k³ of the pristine Ba₅Ru₂O₁₁ and ex situ samples. (c) O K-edge XAS spectra for pristine Ba₅Ru₂O₁₁ and ex situ samples. (d) Evolution upon cycling of the integrated areas for the absorption peak at about 526 eV (orange) and about 529 eV (blue) normalized compared to the peaks for the pristine compound.