Peter G. Khalifah

Mixed close-packed cobalt molybdenum nitrides as non-noble metal electrocatalysts for the hydrogen evolution reaction.

A two-step solid-state reaction for preparing cobalt molybdenum nitride with a nanoscale morphology has been used to produce a highly active and stable electrocatalyst for the hydrogen evolution reaction (HER) under acidic conditions that achieves an iR-corrected current density of 10 mA cm(-2) at -0.20 V vs RHE at low catalyst loadings of 0.24 mg/cm(2) in rotating disk experiments under a H2 atmosphere. Neutron powder diffraction and pair distribution function (PDF) studies have been used to overcome the insensitivity of X-ray diffraction data to different transition-metal nitride structural polytypes and show that this cobalt molybdenum nitride crystallizes in space group P63/mmc with lattice parameters of a = 2.85176(2) Å and c = 10.9862(3) Å and a formula of Co0.6Mo1.4N2. This space group results from the four-layered stacking sequence of a mixed close-packed structure with alternating layers of transition metals in octahedral and trigonal prismatic coordination and is a structure type for which HER activity has not previously been reported. Based on the accurate bond distances obtained from time-of-flight neutron diffraction data, it is determined that the octahedral sites contain a mixture of divalent Co and trivalent Mo, while the trigonal prismatic sites contain Mo in a higher oxidation state. X-ray photoelectron spectroscopy (XPS) studies confirm that at the sample surface nitrogen is present and N-H moieties are abundant.

Thermally stable N2-intercalated WO3 photoanodes for water oxidation.

We describe stable intercalation compounds of the composition xN(2)·WO(3) (x = 0.034-0.039), formed by trapping N(2) in WO(3). The incorporation of N(2) significantly reduced the absorption threshold of WO(3); notably, 0.039N(2)·WO(3) anodes exhibited photocurrent under illumination at wavelengths ≤640 nm with a faradaic efficiency for O(2) evolution in 1.0 M HClO(4)(aq) of nearly unity. Spectroscopic and computational results indicated that deformation of the WO(3) host lattice, as well as weak electronic interactions between trapped N(2) and the WO(3) matrix, contributed to the observed red shift in optical absorption. Noble-gas-intercalated WO(3) materials similar to xN(2)·WO(3) are predicted to function as photoanodes that are responsive to visible light.

Cobalt Molybdenum Oxynitrides: Synthesis, Structural Characterization, and Catalytic Activity for the Oxygen Reduction Reaction

Here, we report the synthesis and characterization of CoxMo1 xOyNz compounds supported on carbon black as potential cathode catalysts for ORR. They were prepared by a conventional impregnation method. Their ORR activities in both acid and alkaline electrolytes were evaluated via half-cell measurements. The synthesis temperature and sample composition both strongly impacted their physical and chemical properties. Factors influencing their crystal structures, morphologies and ORR activities will be discussed based on the results of structural and spectroscopic studies.

Degradation and (de)lithiation processes in the high capacity battery material LiFeBO3

Lithium iron borate (LiFeBO3) is a particularly desirable cathode material for lithium-ion batteries due to its high theoretical capacity (220 mA h g−1) and its favorable chemical constituents, which are abundant, inexpensive and non-toxic. However, its electrochemical performance appears to be severely hindered by the degradation that results from air or moisture exposure. The degradation of LiFeBO3 was studied through a wide array of ex situ and in situ techniques (X-ray diffraction, nuclear magnetic resonance, X-ray absorption spectroscopy, electron microscopy and spectroscopy) to better understand the possible degradation process and to develop methods for preventing degradation. It is demonstrated that degradation involves both Li loss from the framework of LiFeBO3 and partial oxidation of Fe(II), resulting in the creation of a stable lithium-deficient phase with a similar crystal structure to LiFeBO3. Considerable LiFeBO3 degradation occurs during electrode fabrication, which greatly reduces the accessible capacity of LiFeBO3 under all but the most stringently controlled conditions for electrode fabrication. Comparative studies on micron-sized LiFeBO3 and nanoscale LiFeBO3–carbon composite showed a very limited penetration depth (∼30 nm) of the degradation phase front into the LiFeBO3 core under near-ambient conditions. Two-phase reaction regions during delithiation and lithiation of LiFeBO3 were unambiguously identified through the galvanostatic intermittent titration technique (GITT), although it is still an open question as to whether the two-phase reaction persists across the whole range of possible Li contents. In addition to the main intercalation process with a thermodynamic potential of 2.8 V, there appears to be a second reversible electrochemical process with a potential of 1.8 V. The best electrochemical performance of LiFeBO3 was ultimately achieved by introducing carbon to minimize the crystallite size and strictly limiting air and moisture exposure to inhibit degradation.

Molybdenum nitrides as oxygen reduction reaction catalysts: structural and electrochemical studies.

Monometallic (δ-MoN, Mo5N6, and Mo2N) and bimetallic molybdenum nitrides (Co0.6Mo1.4N2) were investigated as electrocatalysts for the oxygen reduction reaction (ORR), which is a key half-reaction in hydrogen fuel cells. Monometallic hexagonal molybdenum nitrides are found to exhibit improved activities over rock salt type molybdenum nitride (γ-Mo2N), suggesting that improvements are due to either the higher molybdenum valence or a more favorable coordination environment in the hexagonal structures. Further enhancements in activity were found for hexagonal bimetallic cobalt molybdenum nitride (Co0.6Mo1.4N2), resulting in a modest onset potential of 0.713 V versus reversible hydrogen electrode (RHE). Co0.6Mo1.4N2 exhibits good stability in acidic environments, and in the potential range lower than 0.5 V versus RHE, the ORR appears to proceed via a four-electron mechanism based on the analysis of rotating disc electrode results. A redetermination of the structures of the binary molybdenum nitrides was carried out using neutron diffraction data, which is far more sensitive to nitrogen site positions than X-ray diffraction data. The revised monometallic hexagonal nitride structures all share many common features with the Co0.6Mo1.4N2 structure, which has alternating layers of cations in octahedral and trigonal prismatic coordination, and are thus not limited to only trigonal prismatic Mo environments (as was originally postulated for δ-MoN).

Effects of laser energy and wavelength on the analysis of LiFePO4 using laser assisted atom probe tomography

The effects of laser wavelength (355 nm and 532 nm) and laser pulse energy on the quantitative analysis of LiFePO₄ by atom probe tomography are considered. A systematic investigation of ultraviolet (UV, 355 nm) and green (532 nm) laser assisted field evaporation has revealed distinctly different behaviors. With the use of a UV laser, the major issue was identified as the preferential loss of oxygen (up to 10 at%) while other elements (Li, Fe and P) were observed to be close to nominal ratios. Lowering the laser energy per pulse to 1 pJ/pulse from 50 pJ/pulse increased the observed oxygen concentration to nearer its correct stoichiometry, which was also well correlated with systematically higher concentrations of (16)O₂(+) ions. Green laser assisted field evaporation led to the selective loss of Li (~33% deficiency) and a relatively minor O deficiency. The loss of Li is likely a result of selective dc evaporation of Li between or after laser pulses. Comparison of the UV and green laser data suggests that the green wavelength energy was absorbed less efficiently than the UV wavelength because of differences in absorption at 355 and 532 nm for LiFePO₄. Plotting of multihit events on Saxey plots also revealed a strong neutral O2 loss from molecular dissociation, but quantification of this loss was insufficient to account for the observed oxygen deficiency.

Synthesis and Characterization of Visible Light Absorbing (GaN)1–x(ZnO)x Semiconductor Nanorods

Although the (GaN)(1-x)(ZnO)x solid solution is one of the most effective systems for driving overall solar water splitting with visible light, its quantum yield for overall water splitting using visible light photons has not yet reached ten percent. Understanding and controlling the nanoscale morphology of this system may allow its overall conversion efficiency to be raised to technologically relevant levels. We describe the use a Ga2O3(ZnO)16 precursor phase in the synthesis of this phase which naturally results in the production of arrays of nanorods with favorable diameters (∼100 nm) and band gaps (∼2.5 eV). Substantial absorption within the band gap is observed, part of which is found to follow the E(-3) scaling characteristic of free carriers scattered by ionized impurity sites. Compositional analysis suggests that a substantial quantity of cation vacancies (∼3%) may be present in some samples. The typical nanorod growth direction and dominant {1011} facet for powders in this system have been identified through electron microscopy methods, leading to the conclusion that polarity may play an important role in the high photoactivity of this family of wurtzite semiconductors.

Structures of Delithiated and Degraded LiFeBO3, and Their Distinct Changes upon Electrochemical Cycling

Lithium iron borate (LiFeBO3) has a high theoretical specific capacity (220 mAh/g), which is competitive with leading cathode candidates for next-generation lithium-ion batteries. However, a major factor making it difficult to fully access this capacity is a competing oxidative process that leads to degradation of the LiFeBO3 structure. The pristine, delithiated, and degraded phases of LiFeBO3 share a common framework with a cell volume that varies by less than 2%, making it difficult to resolve the nature of the delithiation and degradation mechanisms by conventional X-ray powder diffraction studies. A comprehensive study of the structural evolution of LiFeBO3 during (de)lithiation and degradation was therefore carried out using a wide array of bulk and local structural characterization techniques, both in situ and ex situ, with complementary electrochemical studies. Delithiation of LiFeBO3 starts with the production of LitFeBO3 (t ≈ 0.5) through a two-phase reaction, and the subsequent delithiation of this phase to form Lit-xFeBO3 (x < 0.5). However, the large overpotential needed to drive the initial two-phase delithiation reaction results in the simultaneous observation of further delithiated solid-solution products of Lit-xFeBO3 under normal conditions of electrochemical cycling. The degradation of LiFeBO3 also results in oxidation to produce a Li-deficient phase D-LidFeBO3 (d ≈ 0.5, based on the observed Fe valence of ∼2.5+). However, it is shown through synchrotron X-ray diffraction, neutron diffraction, and high-resolution transmission electron microscopy studies that the degradation process results in an irreversible disordering of Fe onto the Li site, resulting in the formation of a distinct degraded phase, which cannot be electrochemically converted back to LiFeBO3 at room temperature. The Li-containing degraded phase cannot be fully delithiated, but it can reversibly cycle Li (D-Lid+yFeBO3) at a thermodynamic potential of ∼1.8 V that is substantially reduced relative to the pristine phase (∼2.8 V).

Substitutional mechanism of Ni into the wide-band-gap semiconductor InTaO4 and its implications for water splitting activity in the wolframite structure type.

The mechanism of Ni substitution into the oxide semiconductor InTaO(4) has been studied through a combination of structural and spectroscopic techniques, providing insights into its previously reported photoactivity. Magnetic susceptibility and X-ray absorption near-edge spectroscopy (XANES) measurements demonstrate that nickel is divalent within the host lattice. The combined refinement of synchrotron X-ray and neutron powder diffraction data indicates that the product of Ni doping has the stoichiometry of (In(1-x)Ni(2x/3)Ta(x/3))TaO(4) with a solubility limit of x ≈ 0.18, corresponding to 12% Ni on the In site. Single-phase samples were only obtained at synthesis temperatures of 1150 °C or higher due to the sluggish reaction mechanism that is hypothesized to result from small free energy differences between (In(1-x)Ni(2x/3)Ta(x/3))TaO(4) compounds with different x values. Undoped InTaO(4) is shown to have an indirect band gap of 3.96 eV, with direct optical transitions becoming allowed at photon energies in excess of 5.1 eV. Very small band-gap reductions (less than 0.2 eV) result from Ni doping, and the origin of the yellow color of (In(1-x)Ni(2x/3)Ta(x/3))TaO(4) compounds instead results from a weak (3)A(2g) → (3)T(1g) internal d → d transition not associated with the conduction or valence band that is common to oxide compounds with Ni(2+) in an octahedral environment.

Photocatalytic hydrogen evolution using nanocrystalline gallium oxynitride spinel

Photocatalytic hydrogen evolution from water was observed over nanocrystalline gallium oxynitride spinel under simulated solar light irradiation (320 nm < λ < 800 nm). Up to 8 μmol h−1 of H2 was evolved without co-catalyst loading. The photocatalyst was synthesized by the ammonolysis of gallium nitrate hydrate (Ga(NO3)3·xH2O). Optical measurements indicate an indirect gap (Eg) in the visible region (Eg = 2.50 eV) which is ascribed to photoexcitations from the N 2p valence states. A direct gap has an onset at ultraviolet energies (Eg = 3.69 eV), which is ascribed to photoexcitations from lower energy O 2p valence states.