Oxygen Activation and Dissociation on Transition Metal Free Perovskite Surfaces

Density functional theory and low energy ion scattering spectroscopy were applied to study the mechanism of oxygen dissociation on the SrO-terminated surfaces of strontium titanate (SrTiO₃) and iron-doped strontium titanate (SrTi_1–<i>x</i>Fe_<i>x</i>O_3−δ). Our study reveals that while O₂ dissociation is not favored on the SrO-terminated perovskite surface, oxygen vacancies can act as active sites and catalyze the O–O bond cleavage. Electron transfer from lattice oxygen atoms to the O₂ molecule, mediated by the subsurface transition metal cations, plays an important role in the resulting formation of surface superoxo species. The O₂ molecule dissociates to produce oxygen ions, which are incorporated into the perovskite lattice, and highly active oxygen radicals on the perovskite surface, which further recombine to O₂ molecules. Our focus on the SrO-terminated surface, rather than the TiO₂ layer, which is presumed to be more catalytically active, was driven by experimental observation using low energy ion scattering spectroscopy, which reveals that the surface of SrTiO₃ after high temperature heat treatment is SrO-terminated, and hence this is the surface that is technologically relevant for devices such as solid oxide fuel cells (SOFCs). Our study demonstrates that although the more active BO₂-perovskite layer is not exposed at the gas–solid interface, the SrO-terminated surfaces also actively participate in oxygen exchange reaction

Dual Substitution Strategy to Enhance Li⁺ Ionic Conductivity in Li₇La₃Zr₂O₁₂ Solid Electrolyte

Solid state electrolytes could address the current safety concerns of lithium-ion batteries as well as provide higher electrochemical stability and energy density. Among solid electrolyte contenders, garnet-structured Li₇La₃Zr₂O₁₂ appears as a particularly promising material owing to its wide electrochemical stability window; however, its ionic conductivity remains an order of magnitude below that of ubiquitous liquid electrolytes. Here, we present an innovative dual substitution strategy developed to enhance Li-ion mobility in garnet-structured solid electrolytes. A first dopant cation, Ga³⁺, is introduced on the Li sites to stabilize the fast-conducting cubic phase. Simultaneously, a second cation, Sc³⁺, is used to partially populate the Zr sites, which consequently increases the concentration of Li ions by charge compensation. This aliovalent dual substitution strategy allows fine-tuning of the number of charge carriers in the cubic Li₇La₃Zr₂O₁₂ according to the resulting stoichiometry, Li_{7–3<i>x</i>+y}Ga_<i>x</i>La₃Zr_2–<i>y</i>Sc_<i>y</i>O₁₂. The coexistence of Ga and Sc cations in the garnet structure is confirmed by a set of simulation and experimental techniques: DFT calculations, XRD, ICP, SEM, STEM, EDS, solid state NMR, and EIS. This thorough characterization highlights a particular cationic distribution in Li_6.65Ga_0.15La₃Zr_1.90Sc_0.10O₁₂, with preferential Ga³⁺ occupation of tetrahedral Li_24<i>d</i> sites over the distorted octahedral Li_96<i>h</i> sites. ⁷Li NMR reveals a heterogeneous distribution of Li charge carriers with distinct mobilities. This unique Li local structure has a beneficial effect on the transport properties of the garnet, enhancing the ionic conductivity and lowering the activation energy, with values of 1.8 × 10^–3 S cm^–1 at 300 K and 0.29 eV in the temperature range of 180 to 340 K, respectively