Screening and Characterization of Ternary Oxides for High-Temperature Carbon Capture

Carbon capture and storage (CCS) is increasingly being accepted as a necessary component of any effort to mitigate the impact of anthropogenic climate change, as it is both a relatively mature and easily implemented technology. High-temperature CO2 absorption looping is a promising process that offers a much lower energy penalty than the current state of the art amine scrubbing techniques, but more effective materials are required for widespread implementation. This work describes the experimental characterisation and CO2 absorption properties of several new ternary transition metal oxides predicted by high-throughput DFT screening. One material reported here, Li5SbO5, displays reversible CO2 sorption, and maintains 72 % of its theoretical capacity out to 25 cycles. The results in this work are used to discuss major influences on CO2 absorption capacity and rate, including the role of the crystal structure, the transition metal, the alkali or alkaline earth metal, and the competing roles of thermodynamics and kinetics. Notably, this work shows the extent and rate to which ternary metal oxides carbonate is driven primarily by the identity of the alkali or alkaline earth ion and the nature of the crystal structure, whereas the identity of the transition ion carries little influence in the systems studied here

Gd12_{12}Co5.3_{5.3}Bi and Gd12_{12}Co5_{5}Bi, Crystalline Doppelgänger with Low Thermal Conductivities

Attempts to prepare Gd$_{12}$Co$_{5}$Bi, a member of the rare-earth (RE) intermetallics RE$_{12}$Co$_{5}$Bi, which were identified by a machine-learning recommendation engine as potential candidates for thermoelectric materials, led instead to formation of the new compound Gd$_{12}$Co$_{5.3}$Bi with a very similar composition. Phase equilibria near the Gd-rich corner of the Gd-Co-Bi phase diagram were elucidated by both lab-based and variable-temperature synchrotron powder X-ray diffraction, suggesting that Gd$_{12}$Co$_{5.3}$Bi and Gd$_{12}$Co$_{5}$Bi are distinct phases. The higher symmetry structure of Gd$_{12}$Co$_{5.3}$Bi (cubic, space group Im3̅, Z = 2, a = 9.713(6) Å), as determined from single-crystal X-ray diffraction, is closely related to that of Gd$_{12}$Co$_{5}$Bi (tetragonal, space group Immm). Single Co atoms and Co-Co dumbbells are disordered with occupancies of 0.78(2) and 0.22(2), respectively, in Gd$_{12}$Co$_{5.3}$Bi, but they are ordered in Gd$_{12}$Co$_{5}$Bi . Consistent with this disorder, the electrical resistivity shows less dependence on temperature for Gd$_{12}$Co$_{5.3}$Bi than for Gd$_{12}$Co$_{5}$Bi . The thermal conductivity is low and reaches 2.8 W m$^{-1}$ K$^{-1}$ at 600 °C for both compounds; however, the temperature dependence of the thermal conductivity differs, decreasing for Gd$_{12}$Co$_{5.3}$Bi and increasing for Gd$_{12}$Co$_{5}$Bi as the temperature increases. The unusual trends in thermal properties persist in the heat capacity, which decreases below 2R, and in the thermal diffusivity, which increases at higher temperatures.This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) (A.O.O. and A.M). M.W.G. is grateful for support from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant No. 659764. T.D.S. also acknowledges resources from the DARPA SIMPLEX program N66001-15-C-4036. We thank Dr. S. Lapidus for assistance with the high-resolution synchrotron XRD experiments, made possible through the mail-in powder diffraction service, at 11-BM at the Advanced Photon Source at Argonne National Laboratory. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357

Low-dimensional quantum magnetism in Cu(NCS)2: A molecular framework material

Low-dimensional magnetic materials with spin-$\frac{1}{2}$ moments can host a range of exotic magnetic phenomena due to the intrinsic importance of quantum fluctuations to their behavior. Here, we report the structure, magnetic structure and magnetic properties of copper(II) thiocyanate, Cu(NCS)$_2$, a one-dimensional coordination polymer which displays low-dimensional quantum magnetism. Magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, $^{13}$C magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy, and density functional theory (DFT) investigations indicate that Cu(NCS)$_2$ behaves as a two-dimensional array of weakly coupled antiferromagnetic spin chains ($J_2 = 133(1)$ K, $\alpha = J_1/J_2 = 0.08$). Powder neutron-diffraction measurements confirm that Cu(NCS)$_2$ orders as a commensurate antiferromagnet below $T_\mathrm{N} = 12$ K, with a strongly reduced ordered moment (0.3 $\mu_\mathrm{B}$) due to quantum fluctuations

Metal-Organic Nanosheets Formed via Defect-Mediated Transformation of a Hafnium Metal-Organic Framework

We report a hafnium-containing MOF, hcp UiO-67(Hf), which is a ligand-deficient layered analogue of the face-centered cubic fcu UiO-67(Hf). hcp UiO-67 accommodates its lower ligand:metal ratio compared to fcu UiO-67 through a new structural mechanism: the formation of a condensed "double cluster" (Hf$_{12}$O$_{8}$(OH)$_{14}$), analogous to the condensation of coordination polyhedra in oxide frameworks. In oxide frameworks, variable stoichiometry can lead to more complex defect structures, e.g., crystallographic shear planes or modules with differing compositions, which can be the source of further chemical reactivity; likewise, the layered hcp UiO-67 can react further to reversibly form a two-dimensional metal-organic framework, hxl UiO-67. Both three-dimensional hcp UiO-67 and two-dimensional hxl UiO-67 can be delaminated to form metal-organic nanosheets. Delamination of hcp UiO-67 occurs through the cleavage of strong hafnium-carboxylate bonds and is effected under mild conditions, suggesting that defect-ordered MOFs could be a productive route to porous two-dimensional materials.M.J.C. was supported by Sidney Sussex College, Cambridge; M.J.C., J.A.H., and A.L.G. were supported by the European Research Council (279705); and J.L., A.C.F., E.C.-M., and C.P.G. were supported by the Engineering and Physical Sciences Research Council (U.K.) under the Supergen Consortium and Grant (EP/N001583/1). D.F.-J. thanks the Royal Society for funding through a University Research Fellowship. The Diamond Light Source Ltd. (beamlines I11 (EE9940, EE15118), I12 (EE12554), and I15 (EE13681, EE13843) is thanked for providing beamtime. Via our membership of the UK’s HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202), this work used the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk). Part of this work was performed using the Darwin Supercomputer of the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk/), provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council

A highly reactive precursor in the iron sulfide system

Iron sulfur (Fe–S) phases have been implicated in the emergence of life on early Earth due to their catalytic role in the synthesis of prebiotic molecules. Similarly, Fe–S phases are currently of high interest in the development of green catalysts and energy storage. Here we report the synthesis and structure of a nanoparticulate phase (FeSnano) that is a necessary solid-phase precursor to the conventionally assumed initial precipitate in the iron sulfide system, mackinawite. The structure of FeSnano contains tetrahedral iron, which is compensated by monosulfide and polysulfide sulfur species. These together dramatically affect the stability and enhance the reactivity of FeSnano

Study of Defect Chemistry in the System La<inf>2- x</inf>Sr<inf>x</inf>NiO<inf>4+δ</inf> by ¹⁷O Solid-State NMR Spectroscopy and Ni K-Edge XANES

The properties of mixed ionic-electronic conductors (MIECs) are most conveniently controlled through site-specific aliovalent substitution, yet few techniques can report directly on the local structure and defect chemistry underpinning changes in ionic and electronic conductivity. In this work, we perform high-resolution17O (I = 5/2) solid-state NMR spectroscopy of La2-xSrxNiO4+δ, an MIEC and prospective solid oxide fuel cell (SOFC) cathode material, showing the sensitivity of17O hyperfine (Fermi contact) shifts and quadrupolar coupling constants due to local structural changes arising from Sr substitution (x). Previously, we resolved resonances from three distinct oxygen sites (interstitial, axial, and equatorial) in the unsubstituted x = 0 material (Halat et al., J. Am. Chem. Soc. 2016, 138, 11958). Here, substitution-induced changes in these three spectral features indirectly report on the ionic conductivity, local octahedral tilting, and electronic conductivity, respectively, of the (substituted) materials. In particular, the intensity of the17O resonance arising from mobile interstitial defects decreases, and then disappears, at x = 0.5, consistent with reports of lower bulk ionic conductivity in Sr-substituted phases. Second, local distortions among the split axial oxygen sites diminish, even on modest incorporation of Sr (x < 0.1), which is also accompanied by faster spin-lattice (T1) relaxation of the interstitial17O resonances, indicating increased mobility of the associated sites. Finally, the hyperfine shift of the equatorial oxygen resonance decreases due to conversion of Ni2+(d8) to Ni3+(d7) by charge compensation, a mechanism associated with improved electronic conductivity in the Sr-substituted phases. Valence and coordination changes of the Ni cations are further supported by Ni K-edge X-ray absorption near-edge structure (XANES) measurements, which show a decrease in the Jahn-Teller distortion of the Ni3+sites and a Ni coordination change consistent with the formation of oxygen vacancies. Ultimately, these insights into local atomic and electronic structure that rely on17O solid-state NMR spectroscopy should prove relevant for a broad range of aliovalently substituted functional paramagnetic oxides

Interface Instability in LiFePO4–Li3+xP1–xSixO4 All-Solid-State Batteries

All-solid-state batteries (ASSBs) based on non-combustible solid electrolytes are promising candidates for safe and high energy storage systems, but it remains a challenge to prepare systems with stable interfaces between the various solid components that survive both the synthesis conditions and electrochemical cycling. We have investigated cathode mixtures based on a carbon-coated LiFePO4active material and Li3+xP1-xSixO4solid electrolyte for potential use in all-solid-state batteries. Half-cells were constructed by combining both compounds into pellets by spark plasma sintering (SPS). We report the fast and quantitative formation of two solid solutions (LiFePO4-Fe2SiO4and Li3PO4-Li2FeSiO4) for different compositions and ratios of the pristine compounds, as tracked by powder X-ray diffraction and solid-state nuclear magnetic resonance; X-ray absorption near edge spectroscopy confirms the formation of iron silicates similar to Fe2SiO4. Scanning electron microscopy and energy dispersive X-ray spectroscopy reveals diffusion of iron cations up to 40 μm into the solid electrolyte even in the short processing times accessible by SPS. Electrochemical cycling of the SPS treated cathode mixtures demonstrates a substantial decrease in capacity following the formation of the solid solutions during sintering. Consequently, all-solid-state batteries based on LiFePO4and Li3+xP1-xSixO4would necessitate iron ion blocking layers. More generally, this study highlights the importance of systematic studies on the fundamental reactions at the active material-solid electrolyte interfaces to enable the introduction of protective layers for commercially successful ASSBs

Mechanical properties of the ferroelectric metal-free perovskite [MDABCO](NH4)I3

The metal-free hybrid organic-inorganic perovskite [MDABCO](NH4)I3 (with MDABCO = N-methyl-1,4-diazabicyclo[2.2.2]octane) was recently discovered to exhibit an excellent ferroelectric performance, challenging established ceramic ferroelectrics. We here probe the mechanical properties of [MDABCO](NH4)I3 by combining high pressure single crystal X-ray diffraction and nanoindentation, underlining the exceptional role and opportunities that come with the use of sustainable, metal-free perovskite ferroelectrics