Martha Greenblatt

Oxide ion conducting solid electrolytes based on Bi2O3

The high oxide ion conductivity of solid solutions of bismuth oxide was initially discovered by Takahashi and coworkers. The bismuth oxide based compounds are much better solid electrolytes than the well-known stabilized zirconia. The only difficulty which has prevented their use in high temperature fuel cells and gas sensors up to now is their instability against reduction at low oxygen partial pressures. In this article, we review the structural properties, thermal expansion, electrical conductivity, thermodynamic stability, and surface properties of bismuth oxide and solid solutions of bismuth oxide with face centred cubic, rhombohedral, tetragonal or layer structures.

Photochemical Water Oxidation by Crystalline Polymorphs of Manganese Oxides: Structural Requirements for Catalysis

Manganese oxides occur naturally as minerals in at least 30 different crystal structures, providing a rigorous test system to explore the significance of atomic positions on the catalytic efficiency of water oxidation. In this study, we chose to systematically compare eight synthetic oxide structures containing Mn(III) and Mn(IV) only, with particular emphasis on the five known structural polymorphs of MnO2. We have adapted literature synthesis methods to obtain pure polymorphs and validated their homogeneity and crystallinity by powder X-ray diffraction and both transmission and scanning electron microscopies. Measurement of water oxidation rate by oxygen evolution in aqueous solution was conducted with dispersed nanoparticulate manganese oxides and a standard ruthenium dye photo-oxidant system. No Ru was absorbed on the catalyst surface as observed by XPS and EDX. The post reaction atomic structure was completely preserved with no amorphization, as observed by HRTEM. Catalytic activities, normalized to surface area (BET), decrease in the series Mn2O3 > Mn3O4 ≫ λ-MnO2, where the latter is derived from spinel LiMn2O4 following partial Li(+) removal. No catalytic activity is observed from LiMn2O4 and four of the MnO2 polymorphs, in contrast to some literature reports with polydispersed manganese oxides and electro-deposited films. Catalytic activity within the eight examined Mn oxides was found exclusively for (distorted) cubic phases, Mn2O3 (bixbyite), Mn3O4 (hausmannite), and λ-MnO2 (spinel), all containing Mn(III) possessing longer Mn-O bonds between edge-sharing MnO6 octahedra. Electronically degenerate Mn(III) has antibonding electronic configuration e(g)(1) which imparts lattice distortions due to the Jahn-Teller effect that are hypothesized to contribute to structural flexibility important for catalytic turnover in water oxidation at the surface.

Nanocrystalline Ni5P4: a hydrogen evolution electrocatalyst of exceptional efficiency in both alkaline and acidic media

Producing hydrogen (H2) by splitting water with fossil-free electricity is considered a grand challenge for developing sustainable energy systems and a carbon dioxide free source of renewable H2. Renewable H2 may be produced from water by electrolysis with either low efficiency alkaline electrolyzers that suffer 50–65% losses, or by more efficient acidic electrolyzers with rare platinum group metal catalysts (Pt). Consequently, research has focused on developing alternative, cheap, and robust catalysts made from earth-abundant elements. Here, we show that crystalline Ni5P4 evolves H2 with geometric electrical to chemical conversion efficiency on par with Pt in strong acid (33 mV dec−1 Tafel slope and −62 mV overpotential at −100 mA cm−2 in 1 M H2SO4). The conductivity of Ni5P4 microparticles is sufficient to allow fabrication of electrodes without conducting binders by pressing pellets. Significantly, no catalyst degradation is seen in short term studies at current densities of −10 mA cm−2, equivalent to ∼10% solar photoelectrical conversion efficiency. The realization of a noble metal-free catalyst performing on par with Pt in both strong acid and base offers a key step towards industrially relevant electrolyzers competing with conventional H2 sources.

Hydrothermal synthesis and low temperature conduction properties of substituted ceria ceramics

Abstract Rare earth or alkaline earth oxide-substituted ceria (CeO 2 ) powders were successfully prepared via hydrothermal synthetic route. Using these powders after freeze-drying, highly dense ceramics of Ce 1− x M x O 2 with M = Ca 2+ , Sr 2+ , Sm 3+ , Gd 3+ and Y 3+ and x = 0.2 were prepared by sintering at much lower temperatures than 1700 °C, common for CeO 2 preparation by conventional ceramic techniques. For example, 20 mol% samaria-substituted ceria with 98% of the theoretical density was sintered at 1450 °C. The conductivities determined by an ac impedance method were approximately 100 times higher than that of yttria-stabilized zirconia (YSZ). The conductivity was optimal with Sm 3+ substitution ( σ (210 °C ) ~10 −6 S/cm). The analysis of the impedance spectra indicate that the observed impedance at the relatively low temperatures of measurement (300–600 °C) is predominantly dependent on grain boundary effects.

Structural Requirements in Lithium Cobalt Oxides for the Catalytic Oxidation of Water

The development of water oxidation catalysts (WOCs) to replace costly noble metals in commercial electrolyzers and solar fuel cells is an unmet need that is preventing the global development of hydrogen fuel technologies. Two of the main challenges in realizing catalytic water splitting are lowering the substantial overpotential that is required to achieve practical operating current densities in the O2-evolving halfreaction at the anode, and the use of earth-abundant elements for the fabrication of inexpensive electrodes that are free from noble metals. To meet these challenges, molecular catalysts that are based upon the cubic CaMn4Ox core within photosystem II in photosynthetic organisms, which is the gold standard of catalytic efficiency, have begun to appear. Among solid-state materials, several noble-metal oxides, which include IrO2 and RuO2, are already in use in industrial electrolyzers, but are not globally scalable. Aqueous solutions of cobalt phosphate form water-oxidation catalysts under electrolysis and photolysis that are suitable for the fabrication of noncrystalline electrode materials. Nanocrystalline spinel-phase metal oxides (AM2O4, M= transition metals) that are comprised of M4O4 cubical subunits and are active water oxidation catalysts have been developed. The catalytic activity of the spinel Co3O4 has been reported for Co3O4 nanorods that are incorporated into SBA-15 silica, as well as Co3O4 nanoparticles that are adsorbed onto Ni electrodes. NiCo2O4 spinel also oxidizes water when the nanoparticles are electrophoretically deposited onto a Ni electrode. Reports that examined the effect of lithium doping on the surface of Co3O4 electrodes in solutions of KOH attributed the higher evolution rate of O2 to better electrical conductivity. However, the oxidation of water by Co3O4 was strongly dependent on crystallite size and surface area and frequently necessitates high overpotentials and alkaline conditions to accelerate the rate of reaction. In contrast, we recently reported that the catalytically inert spinel LiMn2O4 gives spinel l-MnO2, which is an active water oxidation catalyst, upon topotactic delithiation. Thus, the importance of removing the A-site lithium for catalysis by the cubic Mn4O4 core of spinels was revealed. [11]

Effect of precursors on the structure of phosphosilicate gels 29Si and 31P MAS-NMR study

Gels were prepared using phosphoric acid, triethyl phosphate and trimethyl phosphite as the precursors of phosphorus. The upper limit for P 2 O 5 for gel formation is 90 mol% for gels using phosphoric acid and trimethyl phosphite, and 70 mol% for gels using triethyl phosphate. Gels prepared from phosphoric acid with more than 10 mol% of P 2 O 5 partially crystallize to Si 5 O(PO 4 ) 6 after heat treatment at 200°C for 10 h. Under the same heat treatment condition, all the gels prepared from trimethyl phosphite crystallize. Gels prepared from triethyl phosphate are amorphous even after heat treatment at 800°C for 10 h. However, chemical analysis indicates that most of the phosphorus in the triethyl-phosphate-prepared gels is driven off during the heating process; no more than 8 mol% of the initial amount of P 2 O 5 is retained in the gels. The effects of phosphorus precursors on the phosphosilicate gels were studied by X-ray diffraction, and 29 Si and 31 P magic angle spinning (MAS)-nuclear magnetic resonance (NMR). 29 Si and 31 P spectra show that Si O P and P O P linkages do not form in the xerogels (i.e. gels heat treated to ∼ 60°C). The gels heat treated at 200°C or above (those that did not crystallize) show evidence of Si O P and P O P network formation.

Physics and Chemistry of Low-Dimensional Inorganic Conductors

Materials and Chemistry: An Introduction to the Design of Lowdimensional Solids J. Rouxel. Molybdenum and Tungsten Bronzes: Lowdimensional Metals with Unusual Properties M. Greenblatt. Nanostructures of Lowdimensional Oxides: Nature and Role of Defects M. Hervieu. Misfit Layer Compounds (MX)1+x(TX)2m M=Pb, Bi, Sn T=Ti, V, Cr, Ta, Nb X=S, Se, 0.08 < GBPY/YGBP < 0.28 n = 1, 2, 3 J. Rouxel, A. Meerschaut. Clusters in Reduced Molybdenum and Tungsten Oxideand Sulfide Systems R.E. McCarley. New Materials from Reactions in Intermediate Temperature Molten Salts: Synthetic Methodologies for Multinary Solid State Chalcogenides M.G. Kanatzidis. Introduction to Charge Density Waves, Spin Density Waves and Wigner Crystals: Density Waves in Solids G. Gruner. Charge Density Waves in Quasi Two Dimensional Conductors: Molybdenum and Phosphate Tungsten Bronzes C. Schlenker. Organic Superconductors and Spin Density Waves D. Jerome. Wigner Crystals P.B. Littlewood et al. Structural Studies: Structural and Dynamical Aspects of the Charge Density Wave Instability J.P. Pouget. Incommensurately Modulated Structuresof Inorganic Charge Density Wave Compounds S. van Smaalen. Scanning Tunneling and Atomic Force Microscopy on Charge Density Wave and Related Materials R. Wiesendanger. Interpreting Scanning Tunneling and Atomic Force Microscopy Images M.H. Whangbo, J. Ren. Electronic Structures: Theory and Experiment: The Electronic Structure of Transition Metal Oxides and Chalcogenides P.A. Cox. Fermi Surface Nesting and ElectronicInstabilities in Transition Metal Oxides and Bronzes E. Canadell, M.H. Whangbo. Structural and Electronic Instabilities of Transition Metal Chacogenides M.H. Whangbo, et al. Photoemission Studies in Transition Metal Chalcogenides D.Malterre, et al. Angle Resolved Photoemission and Resonant Photoemission Studies of Quasi Lowdimensional Oxide Conductors: Fermi Surface and Defects K.E. Smith, et al. Fermi and Luttinger Liquids in Lowdimensional Metals C. Castellani, C. DiCastro. Thermal Properties and Excitations: Thermal Lattice Fluctuations in Quasi Onedimensional Conductors: OpticalExperiments L. Degiorgi, et al. Elastic and Thermal Properties of Lowdimensional Conductors J.W. Brill. Glassy Behavior and Metastability in the Charge or Spin Density Wave State K. Biljakovic. Dynamics of Charge and Spin Density Waves: Nonlinear Transport, Dislocations, and Plasticity: Introduction to Charge Density Wave Transport: Basic Phenomena and Models P. Monceau. Theory of Charge and Spin Density Wave Transport W. Wonneberger. Dislocations in the Charge Density Wave State of Nobium Triselenide: I Phase Slip Properties: II Bulk Charge Density Wave Transport J.C. Gill. Charge Density Wave Dynamics in the Blue Bronzes A0.30MoO3 (A = K, Rb) J. Dumas. Photoinduced Properties of Charge Density Wave Materials G. Mihaly. 2 additional articles. Index.

Ionic conductivities of Bi4V2−xMxO11−x2 (M = Ti, Zr, Sn, Pb) solid solutions

Abstract Bi 4 V 2−x M x O 11− x 2 (M = Ti, Zr, Sn, Pb) was prepared by solid state reactions. All the substituted phases retain the α-phase structure for x x > 0.20, the high-oxygen-ionic conducting γ-phase is stabilized at room temperature. The highest ionic conductivity was observed in Bi 4 V 1.70 Ti 0.30 O 10.85 with σ 500 k = 4 × 10 −4 S/cm. The higher conductivity of substituted phases compared with the parent compound Bi 4 V 2 O 11 is correlated with increased oxygen vacancies generated by aliovalent substitutions. The highest ionic conductivity in Ti substituted system is attributed to its lower activation energy.

Quasi two-dimensional electronic properties of the lithium molybdenum bronze, Li0.9Mo6o17

Abstract Four probe electrical resistivity measurements between 0.3K and 300K were made on single crystals of the violet-red bronze Li 0.9 Mo 6 O 17 grown by a temperature gradient flux technique. The temperature variation of the resistivity shows metallic conductivity and highly anisotropic behavior similar to K 0.9 Mo 6 O 17 . The room temperature resistivity, measured in the direction parallel to the plate axis, is 9.5×10 -3 Ωcm and 2.47Ωcm perpendicular to that axis. A phase transition observed at ∼24K is possibly related to the onset of a charge density wave. A transition to the superconducting state is observed at Tc ∼ 1.9K.

Preparation and superconducting properties of (Tl,Bi)Sr2CaCu2Oy

The high-Tc superconducting compound (Tl,Bi)Sr2CaCu2Oy (y∼7, TlBi∼1 and Tc∼95 K) was prepared by solid state reaction using Sr2Tl2O7 as a precursor Tl-containing material. (Tl,Bi)Sr2CaCu2Oy forms with a tetragonal unit cell a=3.796(1), c=12.113(2) A and is isostructural with (Tl,Pb)Sr2CaCu2Oy, adopting the so-called 1212-type structure in the new class of high-Tc superconducting oxides. Substitution of Bi for Tl in this compound leads to the formation of single (Tl,Bi)-O layers. The Tc of (Tl,Bi)Sr2CaCu2Oy is higher than these of TlBa2CaCu2Oy and (Tl,Pb)Sr2CaCu2Oy which also have 1212-type structure.