Azzam N. Mansour

Electroless deposition of conformal nanoscale iron oxide on carbon nanoarchitectures for electrochemical charge storage.

We describe a simple self-limiting electroless deposition process whereby conformal, nanoscale iron oxide (FeO(x)) coatings are generated at the interior and exterior surfaces of macroscopically thick ( approximately 90 microm) carbon nanofoam paper substrates via redox reaction with aqueous K(2)FeO(4). The resulting FeO(x)-carbon nanofoams are characterized as device-ready electrode structures for aqueous electrochemical capacitors and they demonstrate a 3-to-7 fold increase in charge-storage capacity relative to the native carbon nanofoam when cycled in a mild aqueous electrolyte (2.5 M Li(2)SO(4)), yielding mass-, volume-, and footprint-normalized capacitances of 84 F g(-1), 121 F cm(-3), and 0.85 F cm(-2), respectively, even at modest FeO(x) loadings (27 wt %). The additional charge-storage capacity arises from faradaic pseudocapacitance of the FeO(x) coating, delivering specific capacitance >300 F g(-1) normalized to the content of FeO(x) as FeOOH, as verified by electrochemical measurements and in situ X-ray absorption spectroscopy. The additional capacitance is electrochemically addressable within tens of seconds, a time scale of relevance for high-rate electrochemical charge storage. We also demonstrate that the addition of borate to buffer the Li(2)SO(4) electrolyte effectively suppresses the electrochemical dissolution of the FeO(x) coating, resulting in <20% capacitance fade over 1000 consecutive cycles.

Investigation of the Lithiation and Delithiation Conversion Mechanisms of Bismuth Fluoride Nanocomposites

and LiF during lithiation and the reformation of BiF3 during delithiation. It has been shown that only the high-pressure tysonite phase of BiF3 reforms during the oxidation sweep and that no bismuth fluoride compound with an oxidation state of the bismuth lower than 3 is formed as intermediate during the lithiation or delithiation reactions. Finally, it has been demonstrated that the different plateaus or pseudo plateaus observed on the lithiation and delithiation voltage profiles stem from polarization changes brought about by the dramatic structural changes occurring in the nanocomposite upon cycling. A model, based on the variation of the electronic and ionic transport mechanisms as a function of the state of completion of the conversion and reconversion reactions, is proposed to explain those polarization changes.

Structure, bonding, and catalytic activity of monodisperse, transition-metal-substituted CeO2 nanoparticles.

We present a simple and generalizable synthetic route toward phase-pure, monodisperse transition-metal-substituted ceria nanoparticles (M0.1Ce0.9O2-x, M = Mn, Fe, Co, Ni, Cu). The solution-based pyrolysis of a series of heterobimetallic Schiff base complexes ensures a rigorous control of the size, morphology and composition of 3 nm M0.1Ce0.9O2-x crystallites for CO oxidation catalysis and other applications. X-ray absorption spectroscopy confirms the dispersion of aliovalent (M(3+) and M(2+)) transition metal ions into the ceria matrix without the formation of any bulk transition metal oxide phases, while steady-state CO oxidation catalysis reveals an order of magnitude increase in catalytic activity with copper substitution. Density functional calculations of model slabs of these compounds confirm the stabilization of M(3+) and M(2+) in the lattice of CeO2. These results highlight the role of the host CeO2 lattice in stabilizing high oxidation states of aliovalent transition metal dopants that ordinarily would be intractable, such as Cu(3+), as well as demonstrating a rational approach to catalyst design. The current work demonstrates, for the first time, a generalizable approach for the preparation of transition-metal-substituted CeO2 for a broad range of transition metals with unparalleled synthetic control and illustrates that Cu(3+) is implicated in the mechanism for CO oxidation on CuO-CeO2 catalysts.

In situ XAS investigation of the oxidation state and local structure of vanadium in discharged and charged V2O5 aerogel cathodes

We have examined the evolution of the oxidation state and atomic structure of vanadium(V) in discharged and charged nanophase vanadium pentoxide (V2O5) aerogel cathodes under in situ conditions using X-ray absorption spectroscopy (XAS). We show that the oxidation state of V in V2O5 aerogel cathode heated under vacuum (100 mTorr) at 220 8C for 20.5 h is similar to that of V in a commercially obtained sample of orthorhombic V2O5. In addition, lithium (Li) insertion during the first cycle of discharging leads to the reduction of V(V) to V(IV) and V(IV) to V(III) in a manner consistent with the stoichiometry of the sample (i.e. Lix V2O5). Li extraction during charging leads to oxidation of V(III) to V(IV) and then V(IV) to V(V). Furthermore, the oxidation state of V in fully charged cathodes remains unchanged with cycling (upto at least the 16th cycle) from that of V in the control V2O5 aerogel cathode. However, the average oxidation state of V in discharged V2O5 cathodes increased with cycling. Moreover, the local structure of V in the discharged state has a higher degree of symmetry than that of the fully charged state. A significant change in the structure of the V/V correlation of discharged cathodes is observed with cycling indicating the formation of electrochemically irreversible phases. # 2002 Elsevier Science Ltd. All rights reserved.

An In Situ X‐Ray Absorption Spectroscopic Study of Charged Li ( 1 − z ) Ni ( 1 + z ) O 2 Cathode Material

We have measured in situ the Ni K-edge X-ray absorption spectra of Li (1-z) Ni (1+z) O 2 cathode material charged in a nonaqueous cell. The material was charged to various states of charge (i.e., Li content) which corresponded to x = 0.0, 0.12, 0.24, 0.37, 0.49, and 0.86 in Li (1-x-z) Ni (1+z) O 2 . We have determined variations in the Ni-O and Ni-Ni coordination numbers, bond lengths, and local disorders as well as the Ni K-edge energies as a function of Li content. We show that in the pristine state, the composition of the material can be described by the formula Li 0,86 Ni 1.14 O 2 (i.e., x = 0 and z = 0.14). That is, the material consists of Ni 2+ (25%) and Ni 3 + (75%) with half the Ni 2+ atoms residing in Li sites and the other half in the NiO 2 slabs. Upon charging, initially Ni 2+ is oxidized to Ni 3+ up to a state of charge which corresponds to x = 2z. Upon further charging to states corresponding to 2z < x ≤ 1 - z, Ni 3+ is oxidized to Ni 4+ with fractions being dependent on the values of x and z. Analysis of the edge energies for NiO, stoichiometric LiNiO 2 , and KNiIO 6 as reference compounds for Ni 2+ , Ni 3+ , and Ni 4+ , respectively, shows a quadratic dependence for edge energy vs. oxidation state. This type of correlation is consistent with variations observed in earlier studies for some Mn reference compounds in the same range of oxidation states. Oxidation-state determination of Ni in Li (1-x-z) Ni (1+z) O 2 as a function of state of charge (i.e., Li content or x) on the basis of edge energies yielded results which are in excellent agreement with oxidation state determinations made on the basis of the mole fractions for Ni 2+ , Ni 3+ , and Ni 4+ extracted from extended X-ray absorption fine structure spectra.

In Situ X‐Ray Absorption Spectroscopy Study of Li ( 1 − z ) Ni ( 1 + z ) O 2 ( z ≤ 0.02 ) Cathode Material

In situ X‐ray absorption spectroscopy study of phases shows that the Ni K edge continuously shifts to higher energies with a decrease in Li content in a manner consistent with oxidation of Ni(III) to Ni(IV) The Ni K‐edge energy for is consistent with that observed for chemically prepared quadrivalent Ni in . Variations in the coordination numbers, bond lengths, and disorders as a function of state of charge (i.e., the value of x) are consistent with the following facts: (i) a Jahn‐Teller distortion for Ni(III), (ii) an undistorted environment for Ni(IV), and (iii) an electrochemical oxidation of Ni(III) to Ni(IV).

An in situ X-ray absorption spectroscopic study of charged Li{sub (1{minus}z)}Ni{sub (1+z)}O{sub 2} cathode material

We have measured in situ the Ni K-edge X-ray absorption spectra of Li (1-z) Ni (1+z) O 2 cathode material charged in a nonaqueous cell. The material was charged to various states of charge (i.e., Li content) which corresponded to x = 0.0, 0.12, 0.24, 0.37, 0.49, and 0.86 in Li (1-x-z) Ni (1+z) O 2 . We have determined variations in the Ni-O and Ni-Ni coordination numbers, bond lengths, and local disorders as well as the Ni K-edge energies as a function of Li content. We show that in the pristine state, the composition of the material can be described by the formula Li 0,86 Ni 1.14 O 2 (i.e., x = 0 and z = 0.14). That is, the material consists of Ni 2+ (25%) and Ni 3 + (75%) with half the Ni 2+ atoms residing in Li sites and the other half in the NiO 2 slabs. Upon charging, initially Ni 2+ is oxidized to Ni 3+ up to a state of charge which corresponds to x = 2z. Upon further charging to states corresponding to 2z < x ≤ 1 - z, Ni 3+ is oxidized to Ni 4+ with fractions being dependent on the values of x and z. Analysis of the edge energies for NiO, stoichiometric LiNiO 2 , and KNiIO 6 as reference compounds for Ni 2+ , Ni 3+ , and Ni 4+ , respectively, shows a quadratic dependence for edge energy vs. oxidation state. This type of correlation is consistent with variations observed in earlier studies for some Mn reference compounds in the same range of oxidation states. Oxidation-state determination of Ni in Li (1-x-z) Ni (1+z) O 2 as a function of state of charge (i.e., Li content or x) on the basis of edge energies yielded results which are in excellent agreement with oxidation state determinations made on the basis of the mole fractions for Ni 2+ , Ni 3+ , and Ni 4+ extracted from extended X-ray absorption fine structure spectra.

Solid-state activation of Li2O2 oxidation kinetics and implications for Li–O2 batteries

As one of the most theoretically promising next-generation chemistries, Li–O2 batteries are the subject of intense research to address their stability, cycling, and efficiency issues. The recharge kinetics of Li–O2 are especially sluggish, prompting the use of metal nanoparticles as reaction promoters. In this work, we probe the underlying pathway of kinetics enhancement by transition metal and oxide particles using a combination of electrochemistry, X-ray absorption spectroscopy, and thermochemical analysis in carbon-free and carbon-containing electrodes. We highlight the high activity of the group VI transition metals Mo and Cr, which are comparable to noble metal Ru and coincide with XAS measured changes in surface oxidation state matched to the formation of Li2MoO4 and Li2CrO4. A strong correlation between conversion enthalpies of Li2O2 with the promoter surface (Li2O2 + MaOb ± O2 → LixMyOz) and electrochemical activity is found that unifies the behaviour of solid-state promoters. In the absence of soluble species on charge and the decomposition of Li2O2 proceeding through solid solution, enhancement of Li2O2 oxidation is mediated by chemical conversion of Li2O2 with slow oxidation kinetics to a lithium metal oxide. Our mechanistic findings provide new insights into the selection and/or employment of electrode chemistry in Li–O2 batteries.

Structural and Electrochemical Properties of amorphous and crystalline molybdenum oxide aerogels

Abstract The structure and chemical differences among amorphous, crystalline and nanocrystalline molybdenum oxide aerogels were determined using extended X-ray absorption fine structure (EXAFS), Fourier transform infrared (FTIR) analysis and powder X-ray diffraction (XRD). These different forms of the same nominal material are produced by heat treatment. The influence of the structural differences on electrochemical properties was examined using stepped cyclic voltammetry. The most interesting material was the MoO 3 aerogel heated to 300 °C. The material was found to be nanocrystalline; there are no XRD peaks but the EXAFS were virtually identical to orthorhombic MoO 3 . The electrochemcial response of the nanocrystalline material contains characteristics of both the amorphous and crystalline forms, but with better lithium capacity than either one.

Electrochemical Li-ion storage in defect spinel iron oxides: the critical role of cation vacancies

Rechargeable lithium-ion batteries are the preferred power source for consumer electronic devices, but the cost and toxicity of many cathode materials limit their scale-up. Worldwide research efforts are addressing this concern by transitioning from conventional Co- and Ni-based intercalation hosts towards Fe- and Mn-based alternatives. The unfavorable energetics of the Fe2+/3+redox couple and limited Li-insertion capacities render the use of iron oxides impractical. We address this limitation with the defect spinel ferrite γ-Fe2O3 as a model structure for Li-ion insertion by replacing a fraction of the Fe3+ sites with highly oxidized Mo6+ to generate cation vacancies that shift the onset of Li-ion insertion to more positive potentials as well as increase capacity. In the present study, native and Mo-substituted iron oxides are synthesized via base-catalyzed precipitation in aqueous media, yielding nanocrystalline spinel materials that also exhibit short-range disorder characteristic of a proton-stabilized structure. The Mo-substituted ferrite reported herein is estimated to have ∼3× as many cation vacancies as γ-Fe2O3 with a corresponding increase in the Li-ion capacity to >100 mA h g−1 between 4.1 and 2.0 V vs.Li/Li+. This dual enhancement in capacity and insertion potential will enable these and related defect spinel ferrites to be explored as positive electrode materials for lithium batteries, while retaining the cost advantages of a material whose metal composition is still predominately iron based.