Vanessa K. Peterson

Selective Binding of O2 over N2 in a Redox–Active Metal–Organic Framework with Open Iron(II) Coordination Sites

The air-free reaction between FeCl(2) and H(4)dobdc (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) in a mixture of N,N-dimethylformamide (DMF) and methanol affords Fe(2)(dobdc)·4DMF, a metal-organic framework adopting the MOF-74 (or CPO-27) structure type. The desolvated form of this material displays a Brunauer-Emmett-Teller (BET) surface area of 1360 m(2)/g and features a hexagonal array of one-dimensional channels lined with coordinatively unsaturated Fe(II) centers. Gas adsorption isotherms at 298 K indicate that Fe(2)(dobdc) binds O(2) preferentially over N(2), with an irreversible capacity of 9.3 wt %, corresponding to the adsorption of one O(2) molecule per two iron centers. Remarkably, at 211 K, O(2) uptake is fully reversible and the capacity increases to 18.2 wt %, corresponding to the adsorption of one O(2) molecule per iron center. Mössbauer and infrared spectra are consistent with partial charge transfer from iron(II) to O(2) at low temperature and complete charge transfer to form iron(III) and O(2)(2-) at room temperature. The results of Rietveld analyses of powder neutron diffraction data (4 K) confirm this interpretation, revealing O(2) bound to iron in a symmetric side-on mode with d(O-O) = 1.25(1) Å at low temperature and in a slipped side-on mode with d(O-O) = 1.6(1) Å when oxidized at room temperature. Application of ideal adsorbed solution theory in simulating breakthrough curves shows Fe(2)(dobdc) to be a promising material for the separation of O(2) from air at temperatures well above those currently employed in industrial settings.

Comprehensive study of carbon dioxide adsorption in the metal–organic frameworks M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn)

Analysis of the CO2 adsorption properties of a well-known series of metal–organic frameworks M2(dobdc) (dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate; M = Mg, Mn, Fe, Co, Ni, Cu, and Zn) is carried out in tandem with in situ structural studies to identify the host–guest interactions that lead to significant differences in isosteric heats of CO2 adsorption. Neutron and X-ray powder diffraction and single crystal X-ray diffraction experiments are used to unveil the site-specific binding properties of CO2 within many of these materials while systematically varying both the amount of CO2 and the temperature. Unlike previous studies, we show that CO2 adsorbed at the metal cations exhibits intramolecular angles with minimal deviations from 180°, a finding that indicates a strongly electrostatic and physisorptive interaction with the framework surface and sheds more light on the ongoing discussion regarding whether CO2 adsorbs in a linear or nonlinear geometry. This has important implications for proposals that have been made to utilize these materials for the activation and chemical conversion of CO2. For the weaker CO2 adsorbents, significant elongation of the metal–O(CO2) distances are observed and diffraction experiments additionally reveal that secondary CO2 adsorption sites, while likely stabilized by the population of the primary adsorption sites, significantly contribute to adsorption behavior at ambient temperature. Density functional theory calculations including van der Waals dispersion quantitatively corroborate and rationalize observations regarding intramolecular CO2 angles and trends in relative geometric properties and heats of adsorption in the M2(dobdc)–CO2 adducts.

Direct Evidence of Concurrent Solid-Solution and Two-Phase Reactions and the Nonequilibrium Structural Evolution of LiFePO4

Lithium-ion batteries power many portable devices and in the future are likely to play a significant role in sustainable-energy systems for transportation and the electrical grid. LiFePO(4) is a candidate cathode material for second-generation lithium-ion batteries, bringing a high rate capability to this technology. LiFePO(4) functions as a cathode where delithiation occurs via either a solid-solution or a two-phase mechanism, the pathway taken being influenced by sample preparation and electrochemical conditions. The details of the delithiation pathway and the relationship between the two-phase and solid-solution reactions remain controversial. Here we report, using real-time in situ neutron powder diffraction, the simultaneous occurrence of solid-solution and two-phase reactions after deep discharge in nonequilibrium conditions. This work is an example of the experimental investigation of nonequilibrium states in a commercially available LiFePO(4) cathode and reveals the concurrent occurrence of and transition between the solid-solution and two-phase reactions.

Dynamic solubility limits in nanosized olivine LiFePO4.

Because of its stability, nanosized olivine LiFePO(4) opens the door toward high-power Li-ion battery technology for large-scale applications as required for plug-in hybrid vehicles. Here, we reveal that the thermodynamics of first-order phase transitions in nanoinsertion materials is distinctly different from bulk materials as demonstrated by the decreasing miscibility gap that appears to be strongly dependent on the overall composition in LiFePO(4). In contrast to our common thermodynamic knowledge, that dictates solubility limits to be independent of the overall composition, combined neutron and X-ray diffraction reveals strongly varying solubility limits below particle sizes of 35 nm. A rationale is found based on modeling of the diffuse interface. Size confinement of the lithium concentration gradient, which exists at the phase boundary, competes with the in bulk energetically favorable compositions. Consequently, temperature and size diagrams of nanomaterials require complete reconsideration, being strongly dependent on the overall composition. This is vital knowledge for the future nanoarchitecturing of superior energy storage devices as the performance will heavily depend on the disclosed nanoionic properties.

Hydrogen adsorption in HKUST-1: a combined inelastic neutron scattering and first-principles study.

Hydrogen adsorption in high surface area nanoporous coordination polymers has attracted a great deal of interest in recent years due to the potential applications in energy storage. Here we present combined inelastic neutron scattering measurements and detailed first-principles calculations aimed at unraveling the nature of hydrogen adsorption in HKUST-1 (Cu3(1,3,5-benzenetricarboxylate)2), a metal-organic framework (MOF) with unsaturated metal centers. We reveal that, in this system, the major contribution to the overall binding comes from the classical Coulomb interaction which is not screened due to the open metal site; this explains the relatively high binding energies and short H2-metal distances observed in MOFs with exposed metal sites as compared to traditional ones. Despite the short distances, there is no indication of an elongation of the H-H bond for the bound H2 molecule at the metal site. We find that both the phonon and rotational energy levels of the hydrogen molecule are closely similar, making the interpretation of the inelastic neutron scattering data difficult. Finally, we show that the orientation of H2 has a surprisingly large effect on the binding potential, reducing the classical binding energy by almost 30%. The implication of these results for the development of MOF materials for better hydrogen storage is discussed.

Evaluation of undoped and M-doped TiO2, where M = Sn, Fe, Ni/Nb, Zr, V, and Mn, for lithium-ion battery applications prepared by the molten-salt method

The molten-salt method was used to synthesize a series of transition-metal containing titanium dioxides. Some of the transition metals were found to substitute into the TiO2 lattice, such as (Ti0.9Fe0.1)O2, (Ti0.9Zr0.1)O2, (Ti0.9V0.1)O2, and (Ti0.9Mn0.1)O2, while others were formed as composite electrodes (in addition to relatively minor substitutions), namely 0.1SnO2–0.9TiO2 and 0.05NiO–0.1Nb2O5–0.9TiO2. Although identical synthesis-conditions were used the different transition metals yielded different phases. A comparative study of the electrodes relating surface area and composition (via X-ray photoelectron spectroscopy, XPS), and electrochemical behaviour is presented in this work. Among the substituted single phase electrodes, (Ti0.9Zr0.1)O2 exhibited the best reversible capacity of ∼160 mA h g−1, at the end of the 60th cycle in the voltage range 1.0–2.6 V, with a capacity fade of 24% from the 2nd to the 60th cycle. Among the composite electrodes, 0.05NiO–0.1Nb2O5–0.9TiO2 shows the best performance which is comparable to pure TiO2 but with a slower capacity-fade on extended cycling. The worst performing electrode is (Ti0.9V0.1)O2 with a reversible capacity of only ∼70 mA h g−1 at the end of 70 cycles with a current density of 130 mA g−1 in the voltage range 1.0–2.6 V and a capacity drop of 52% from the 2nd to the 70th cycle. The composite 0.1SnO2–0.9TiO2 features the highest irreversible capacity-loss. Zr-substitution into TiO2 gives the best electrochemical performance.

In situ neutron powder diffraction studies of lithium-ion batteries

Neutron powder diffraction (NPD) offers many advantages in the analysis of battery materials. Understanding the relationship between the structural transformations of electrode materials and their electrochemical performance within lithium-ion batteries is crucial for further development of these technologies and is the overall goal of in situ NPD experiments. In this work, we present NPD data of electrode materials within batteries that are collected in situ during electrochemical cycling, including the commercially available materials LiCoO2, LiMn2O4, LiFePO4 and graphite and the YFe(CN)6 and FeFe(CN)6 materials that are not commercially available. Using these data, we illustrate the experimental approach and requirements for the collection of in situ NPD data of sufficient quality for detailed structural analyses of the electrode components of interest within batteries.

Variation in structure and Li+-ion migration in argyrodite-type Li6PS5X (X = Cl, Br, I) solid electrolytes

All-solid-state rechargeable lithium-ion batteries (AS-LIBs) are attractive power sources for electrochemical applications due to their potentiality in improving safety and stability over conventional batteries with liquid electrolytes. Finding a solid electrolyte with high ionic conductivity and compatibility with other battery components is a key factor in raising the performance of AS-LIBs. In this work, we prepare argyrodite-type Li6PS5X (X = Cl, Br, I) using mechanical milling followed by annealing. X-ray diffraction characterization reveals the formation and growth of crystalline Li6PS5X in all cases. Ionic conductivity of the order of 7 × 10−4 S cm−1 in Li6PS5Cl and Li6PS5Br renders these phases suitable for AS-LIBs. Joint structure refinements using high-resolution neutron and laboratory X-ray diffraction provide insight into the influence of disorder on the fast ionic conductivity. Besides the disorder in the lithium distribution, it is the disorder in the S2−/Cl− or S2−/Br− distribution that we find to promote ion mobility, whereas the large I− cannot be exchanged for S2− and the resulting more ordered Li6PS5I exhibits only a moderate conductivity. Li+ ion migration pathways in the crystalline compounds are modelled using the bond valence approach to interpret the differences between argyrodites containing different halide ions.

A niobium and tantalum co-doped perovskite cathode for solid oxide fuel cells operating below 500 °C

The slow activity of cathode materials is one of the most significant barriers to realizing the operation of solid oxide fuel cells below 500 °C. Here we report a niobium and tantalum co-substituted perovskite SrCo0.8Nb0.1Ta0.1O3−δ as a cathode, which exhibits high electroactivity. This cathode has an area-specific polarization resistance as low as ∼0.16 and ∼0.68 Ω cm2 in a symmetrical cell and peak power densities of 1.2 and 0.7 W cm−2 in a Gd0.1Ce0.9O1.95-based anode-supported fuel cell at 500 and 450 °C, respectively. The high performance is attributed to an optimal balance of oxygen vacancies, ionic mobility and surface electron transfer as promoted by the synergistic effects of the niobium and tantalum. This work also points to an effective strategy in the design of cathodes for low-temperature solid oxide fuel cells.

Negative Thermal Expansion in LnCo(CN)6 (Ln=La, Pr, Sm, Ho, Lu, Y): Mechanisms and Compositional Trends†

Negative thermal expansion (NTE) is a comparatively rare phenomenon that is found in a growing number of materials. The discovery of new NTE materials and the elucidation of mechanisms underpinning their behavior is important both in extending the field and enabling tailored thermal expansion properties. NTE has been found throughout a broad family of cyanide coordination frameworks, arising from thermal population of low-energy transverse vibrations of the cyanide bridges, which reduce the average metal–metal distances, and thus the lattice parameters, with increasing temperature. More complex mechanisms have been established in metal– organic framework materials, in which both local and longrange modes contribute to NTE. The low-energy dynamics of metal-based materials are often modeled in terms of rigid unit modes (RUMs), wherein the metal-centered polyhedra are treated as rigid, with only the linkage being flexible. Most NTE cyanide frameworks are members of two cubic structural types: Zn(CN)2 analogues, [2a,b,5] containing tetrahedral metal centers in the diamondoid topology; and Prussian blue analogues, with octahedral metal centers in the a-Po topology. NTE has recently been observed in a framework of a different structural type: ErCo(CN)6, [7] possessing hexagonal symmetry (P63/mmc) owing to the combination of ErN6 trigonal prisms alternating with CoC6 octahedra. ErCo(CN)6 displays near-isotropic NTE with axial coefficients of thermal expansion (CTEs) aa= da/adT= 8 10 6 K , ac= 9 10 6 K 1 and effective linear CTE, al= 1/3 dV/VdT= 9 10 6 K . Herein we probe in detail the novel mechanism for NTE in this structure type through a comprehensive approach combining synthesis, structural and dynamic analysis, and modeling. Substitution of other trivalent lanthanoids for Er yields an extended series, LnCo(CN)6, of which representative members have been selected for characterization (Ln= La, Pr, Sm, Ho, Lu, and Y). Topotactic dehydration of the parent framework hydrates LnCo(CN)6·nH2O (n= 4, 5) yields an extended isostructural series with the trigonal prismatic LnN6 coordination geometry (Figure 1a, inset), which is a rare example of an isostructural