Normal-state properties of the antiperovskite oxide Sr3−x_{3-x}SnO revealed by 119^{119}Sn-NMR

We have performed $^{119}$Sn-NMR measurements on the antiperovskite oxide superconductor Sr$_{3-x}$SnO to investigate how its normal state changes with the Sr deficiency. A two-peak structure was observed in the NMR spectra of all the measured samples. This suggests that the phase separation tends to occur between the nearly stoichiometric and heavily Sr-deficient Sr$_{3-x}$SnO phases. The measurement of the nuclear spin-lattice relaxation rate $1/T_1$ indicates that the Sr-deficient phase shows a conventional metallic behavior due to the heavy hole doping. In contrast, the nearly stoichiometric phase exhibits unusual temperature dependence of $1/T_1$, attributable to the presence of a Dirac-electron band.Comment: 5 pages, 4 figure

Stannites – a new promising class of durable electrocatalysts for efficient water oxidation

The oxygen evolution reaction (OER) through water oxidation is a key process for multiple energy storage technologies required for a sustainable energy economy such as the formation of the fuel hydrogen from water and electricity, or metal‐air batteries. Herein, we investigate the suitability of Cu2FeSnS4 for the OER and demonstrate its superiority over iron sulfide, iron (oxy)hydroxides and benchmark noble‐metal catalysts in alkaline media. Electrodeposited Cu2FeSnS4 yields the current densities of 10 and 1000 mA/cm2 at overpotentials of merely 228 and 330 mV, respectively. State‐of‐the‐art analytical methods are applied before and after electrocatalysis to uncover the fate of the Cu2FeSnS4 precatalyst under OER conditions and to deduce structure‐activity relationships. Cu2FeSnS4 is the first compound reported for OER among the broad class of stannite structure type materials containing multiple members with highly active earth‐abundant transition‐metals for OER.DFG, 390540038, EXC 2008: Cluster of Excellence UniSysCatTU Berlin, Open-Access-Mittel - 201

Boosting water oxidation through in situ electroconversion of manganese gallide: an intermetallic precursor approach

For the first time, the manganese gallide (MnGa4) served as an intermetallic precursor, which upon in situ electroconversion in alkaline media produced high‐performance and long‐term‐stable MnOx‐based electrocatalysts for water oxidation. Unexpectedly, its electrocorrosion (with the concomitant loss of Ga) leads simultaneously to three crystalline types of MnOx minerals with distinct structures and induced defects: birnessite δ‐MnO2, feitknechtite β‐MnOOH, and hausmannite α‐Mn3O4. The abundance and intrinsic stabilization of MnIII/MnIV active sites in the three MnOx phases explains the superior efficiency and durability of the system for electrocatalytic water oxidation. After electrophoretic deposition of the MnGa4 precursor on conductive nickel foam (NF), a low overpotential of 291 mV, comparable to that of precious‐metal‐based catalysts, could be achieved at a current density of 10 mA cm−2 with a durability of more than five days.DFG, 390540038, EXC 2008: UniSysCatTU Berlin, Open-Access-Mittel - 201

Understanding the formation of bulk- and surface-active layered (oxy)hydroxides for water oxidation starting from a cobalt selenite precursor

The urgent need for a stable, efficient, and affordable oxygen evolution reaction (OER) catalyst has led to the investigation of a vast amount of transition metal materials with multiple different anions.In situandpostcatalytic characterization shows that most materials transform during the harsh OER conditions to layered (oxy)hydroxides (LOH). Several open questions concerning thesein situformed LOH remain such as: an explanation for their strongly varying activities, or the effect of the precatalyst structure, leaching anions, and transformation conditions on the formed LOH. Herein, we report on a cobalt selenite precursor, which, depending on pH and potential, transforms irreversibly into two different LOH OER catalysts. Combining multiple electrochemical and analytical methodsexandin situ, we prove that one of these products is near-surface catalytically active and the other one throughout the bulk with anin situaverage cobalt oxidation state of 3.2. We deduce a detailed structural model explaining these differences and propose general concepts relating both the precatalyst structure and the transformation conditions to the final catalyst. Further, we apply these models to the most promising non-noble metal catalyst, NiFe LOH

In Situ Detection of Iron in Oxidation States ≥ IV in Cobalt‐Iron Oxyhydroxide Reconstructed during Oxygen Evolution Reaction

Cobalt‐iron oxyhydroxides (CoFeOOHx) are among the most active catalysts for the oxygen evolution reaction (OER). However, their redox behavior and the electronic and chemical structure of their active sites are still ambiguous. To shed more light on this, the complete and rapid reconstruction of four helical cobalt‐iron borophosphates with different Co:Fe ratios into disordered cobalt‐iron oxyhydroxides can be achieved, which are electrolyte‐penetrable and thus most transition metal sites can potentially participate in the OER. To track the redox behavior and to identify the active structure, quasi in situ X‐ray absorption spectroscopy is applied. Iron in high oxidation states ≥ IV (Fe4+) and its substantial redox behavior with an average oxidation state of around 2.8 to above 3.2 is detected. Furthermore, a 6% contraction of the Fe‐O bond length compared to Fe3+OOH references is observed during OER and a strong distortion of the [MO6] octahedra is identified. It is hypothesized that this bond contraction is caused by the presence of oxyl radicals and that di‐µ‐oxyl radical bridged cobalt‐iron centers are the active sites. It is anticipated that the detailed electronic and structural description can substantially contribute to the debate on the nature of the active site in bimetallic iron‐containing OER catalysts

In‐Liquid Plasma Modified Nickel Foam: NiOOH/NiFeOOH Active Site Multiplication for Electrocatalytic Alcohol, Aldehyde, and Water Oxidation

The oxygen evolution reaction (OER) and the value-added oxidation of renewable organic substrates are critical to supply electrons and protons for the synthesis of sustainable fuels. To meet industrial requirements, new methods for a simple, fast, environmental-friendly and cheap synthesis of robust, self-supported and high surface area electrodes are required. Herein, a novel in-liquid plasma (plasma electrolysis) approach for the growth of hierarchical nanostructures on nickel foam is reported on. Under morphology retention, iron can be doped into this high surface area electrode. For the oxidation of 5-(hydroxymethyl)furfural and benzyl alcohol, the iron-free, plasma-treated electrode is more suitable reaching current densities up to 800 mA cm$^{-2}$ with Faradaic efficiencies above 95%. For the OER, the iron-doped nickel foam electrode reaches the industrially relevant current density of 500 mA cm$^{-2}$ at 1.473 ± 0.013$_{VRHE}$ (60 °C) and shows no activity decrease over 140 h. The different effects of iron doping are rationalized using methanol probing and in situ Raman spectroscopy. Furthermore, the intrinsic activity is separated from the number of active sites, and, for the organic oxidation reactions, diffusion limitations are revealed. The authors anticipate that the plasma modified nickel foam will be suitable for various (electro)catalytic processes

Evolution of Superconductivity with Sr-Deficiency in Antiperovskite Oxide Sr3−x SnO

Abstract Bulk superconductivity was recently reported in the antiperovskite oxide Sr3−x SnO, with a possibility of hosting topological superconductivity. We investigated the evolution of superconducting properties such as the transition temperature T c and the size of the diamagnetic signal, as well as normal-state electronic and crystalline properties, with varying the nominal Sr deficiency x 0. Polycrystalline Sr3−x SnO was obtained up to x 0 = 0:6 with a small amount of SrO impurities. The amount of impurities increases for x 0 > 0.6, suggesting phase instability for high deficiency. Mössbauer spectroscopy reveals an unusual Sn4− ionic state in both stoichiometric and deficient samples. By objectively analyzing superconducting diamagnetism data obtained from a large number of samples, we conclude that the optimal x 0 lies in the range 0.5 < x 0 < 0.6. In all superconducting samples, two superconducting phases appear concurrently that originate from Sr3−x SnO but with varying intensities. These results clarify the Sr deficiency dependence of the normal and superconducting properties of the antiperovskite oxide Sr3−x SnO will ignite future work on this class of materials

Theoretical band structure of the superconducting antiperovskite Sr₃₋ₓ SnO

In order to investigate the position of the strontium deficiency in superconductive Sr₃₋ₓ SnO, we synthesized and measured X-ray-diffraction patterns of Sr₃₋ₓ SnO (x ~ 0.5). Because no clear peaks originating from superstructures were observed, strontium deficiency is most likely to be randomly distributed. We also performed first-principles band-structure calculations on Sr₃₋ₓ SnO (x = 0, 0.5) using two methods: full-potential linearized-augmented plane-wave plus local orbitals method and the Korringa-Kohn-Rostoker Green function method combined with the coherent potential approximation. We revealed that the Fermi energy of Sr₃₋ₓ SnO in case of x ~ 0.5 is about 0.8 eV below the original Fermi energy of the stoichiometric Sr₃SnO, where the mixing of the valence p and conduction d orbitals are considered to be small

Superconductivity in the antiperovskite Dirac-metal oxide Sr[3−x]SnO

マイナス金属イオンを含む酸化物で超伝導を発見 : 逆ペロブスカイト酸化物初の超伝導体. 京都大学プレスリリース. 2016-12-13.Investigations of perovskite oxides triggered by the discovery of high-temperature and unconventional superconductors have had crucial roles in stimulating and guiding the development of modern condensed-matter physics. Antiperovskite oxides are charge-inverted counterpart materials to perovskite oxides, with unusual negative ionic states of a constituent metal. No superconductivity was reported among the antiperovskite oxides so far. Here we present the first superconducting antiperovskite oxide Sr[3−x]SnO with the transition temperature of around 5 K. Sr[3]SnO possesses Dirac points in its electronic structure, and we propose from theoretical analysis a possibility of a topological odd-parity superconductivity analogous to the superfluid [3]He-B in moderately hole-doped Sr[3−x]SnO. We envision that this discovery of a new class of oxide superconductors will lead to a rapid progress in physics and chemistry of antiperovskite oxides consisting of unusual metallic anions