Synthesis and photocatalytic properties of CdS-intercalated metal oxides

Photocatalytic properties for hydrogen production were investigated on layered titanium compounds intercalating CdS in the interlayer, which were prepared by direct cation exchange reactions and sulfurization processes. The photocatalytic activity of the compounds intercalating CdS was superior to those of simple CdS and the physical mixture of CdS and metal oxides. The improvement might be attributed to the formation of microheterjunctions between the CdS nanoparticles and the layers of oxides. The activity was also enhanced by the substitution of Nb for partial Ti in K₂Ti₄O₉, which led to the increase in CdS content in the interlayer and the rapid transport of photogenerated electrons through the layer of metal oxide.     

Synthesis and electrochemical properties of layered LiNi2/3Sb1/3O2

LiNi_2/3Sb_1/3O₂ has been prepared by ion-exchange from NaNi_2/3Sb_1/3O₂. Powder X-ray diffraction (XRD) and single crystal electron diffraction by transmission electron microscopy (TEM) of the material indicate that it is isostructural with α-NaFeO₂. Electrochemical test shows that the reversible capacity for Li intercalation and deintercalation drops rapidly with the number of times cycled. Both experimental evidences and first principles calculations point to the migration of nickel as the reason for the poor capacity retention.     

Catalytic transformation of H2S for H2 production

Hydrogen sulfide (H₂S) gas is a by-product from natural gas refining, hydrodesulfurization of various fossil fuels, and syngas cleaning from pyrolysis and gasification. Catalytic pyrolysis of H₂S provides an alternative and effective pathway to recover both H₂ and sulfur. Catalysts from hydrotalcite of ZnAl, ZnNiAl, and ZnFeAl were employed for H₂S pyrolysis and compared with TiO₂ and MoS₂ at atmospheric pressure and temperatures in the range of 923–1123 K. Kinetic analysis was carried out in a packed bed reactor which revealed the effect of H₂S partial pressures to be of the order of 0.8–1 with respect to H₂S. The developed novel catalysts showed improved performance with significantly reduced activation energy compared to TiO₂ by 30 kJ/mol as well as higher H₂S conversion during pyrolysis (17% at 1173 K) than with MoS₂ catalyst, even at high H₂S partial pressure which is necessary for viable hydrogen production. The new approach showed an alternate economical and efficient pathway of catalyst design to obtain high activity and stability for simultaneous H₂ energy and pure sulfur recovery from unwanted H₂S resources.     

Enhancing the electrochemical properties of NiFe2O4 anode for lithium ion battery through a simple hydrogenation modification

Nanocrystal NiFe₂O₄ (NFO) octahedron with a spinel structure has been successfully synthesized by a one-step hydrothermal method. The effects of hydrogenation on the crystal structure, morphology, surface structure, and the electrochemical performance of NFO are comprehensively investigated for the first time. After hydrogenation, the well-defined octahedron morphology of NFO disappears and a small fraction of metallic Ni and some oxygen vacancies are generated after hydrogenation which has been characterized by X-ray diffraction (XRD), X-ray photoelectronic spectrometer (XPS) and Positron annihilation lifetime spectroscopy (PALS). Compared to the pristine NFO or the annealed NFO in air, the hydrogenated samples exhibit much better capacity retention (60% higher than un-hydrogenated NFO at 50th cycle) and rate capability (3 times higher at 1 A/g), which can be largely attributed to the synergetic effect of the conductive metallic Ni and oxygen vacancies resulting from H₂ reduction. Furthermore, this facile hydrogenation modification method may also be applied to improve the electrochemical performances of other transition metal oxides electrodes.     

Hydrothermal synthesis of SnO2–V2O5 mixed oxide and electrochemical screening of carbon nano-tubes (CNT), V2O5, V2O5–CNT,and SnO2–V2O5–CNT electrodes for supercapacitor applications

Nano-scaled SnO₂–V₂O₅ mixed oxide is synthesized by a hydrothermal method in an autoclave. For comparative evaluation, V₂O₅ single oxide is prepared by a conventional process from ammonium vanadate. The capacitive behaviour of the following electrodes is studied by cyclic voltammetry in 0.1 M KCl solutions: carbon nano-tubes (CNT), V₂O₅, V₂O₅–CNT, and SnO₂–V₂O₅–CNT. At a scan rate of 100 mV s⁻¹, the SnO₂–V₂O₅–CNT electrode provides the best performance, viz., 121.4 F g⁻¹. The nano-scaled mixed oxide is characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Raman spectra.     

Structure characteristics of CdS/H1.9K0.3La0.5Bi0.1Ta2O7 and photocatalytic activity for hydrogen evolution under visible light

Visible-light-driven CdS/HKLBT photocatalyst was prepared by ion exchange of Cd²⁺ in aqueous Cd(CH₃COO)₂ solutions, then by sulfurization in aqueous N₂H₈S solutions. The characterization by XRD, SEM, HRTEM and XRS revealed that CdS nanoparticles exist both on the surface and in the interlayer of HKLBT. The composite CdS/HKLBT showed higher photocatalytic activity for hydrogen evolution (504.2 μmol/h) than that of pure CdS (187.3 μmol/h), even than that of 0.5 wt%Pt/CdS (496.0 μmol/h) under visible light (λ > 400 nm) in the presence of lactic acid as sacrificial reagent. The enhancement of photocatalytic activity is attributed to the strong contact between CdS and HKLBT in CdS/HKLBT as well as the effective separation of photogenerated carrier in CdS through electron rapid injection into CB of HKLBT.     

Impedance analysis of PLD LiNi0.8Co0.2O2 film electrode

The kinetics of intercalation are discussed using a pulsed laser deposition (PLD) film electrode and electrochemical impedance analysis. Films of LiNi_0.8Co_0.2O₂, deposited on single crystal substrates, were used for the study. The films have intercalative or blocking orientations on different crystal surfaces of the substrates. Impedance spectra show that there are at least three elemental processes in intercalation. Two processes at higher frequencies suggest that they occur at the electrode surface and are influenced by the orientation of the film. The third process appearing at low frequencies below 1 Hz indicates lithium motion in the bulk structure and shows the largest resistance among the three processes. This lithium conduction in a thin PLD film shows a semicircular response and is considered to be influenced more by the structure due to the nanometer-scale thickness.     

Surface modification of LiNi0.5Mn1.5O4 by ZrP2O7 and ZrO2 for lithium-ion batteries

The spinel LiNi_0.5Mn_1.5O₄ has been surface modified separately with 1.0 wt.% ZrO₂ and ZrP₂O₇ for the purpose of improving its cycle performance as a cathode in a 5-V lithium-ion cell. Although the modifications did not change the crystallographic structure of the surface-modified samples, they exhibited better cyclability at elevated temperature (55 °C) compared with pristine LiNi_0.5Mn_1.5O₄. The material that was surface modified with ZrO₂ gave the best cycling performance, only 4% loss of capacity after 150 cycles at 55 °C. Electrochemical impedance spectroscopy demonstrated that the improved performance of the ZrO₂-surface-modified LiNi_0.5Mn_1.5O₄ is due to a small decrease in the charge transfer resistance, indicating limited surface reactivity during cycling. Differential scanning calorimetry showed that the ZrO₂-modified LiNi_0.5Mn_1.5O₄ exhibits lower heat generation and higher onset reaction temperature compared to the pristine material. The excellent cycling and safety performance of the ZrO₂-modified LiNi_0.5Mn_1.5O₄ electrode was found to be due to the protective effect of homogeneous ZrO₂ nano-particles that form on the LiNi_0.5Mn_1.5O₄, as shown by transmission electron microscopy.     

Influence of multiple oxide (Cr2O3/Nb2O5) addition on the sorption kinetics of MgH2

The influence of multiple additions of two oxides, Cr₂O₃ and Nb₂O₅, as additives on the hydrogen sorption kinetics of MgH₂ after milling was investigated. We found that the desorption kinetics of MgH₂ were improved more by multiple oxide addition than by single addition. Even for the milled MgH₂ micrometric size powders, the high hydrogen capacity with fast kinetics were achieved for the powders after addition of 0.2 mol% Cr₂O₃ + 1 mol% Nb₂O₅. For this composition, the hydride desorbed about 5 wt.% hydrogen within 20 min and absorbed about 6 wt.% in 5 min at 300 °C. Furthermore, the desorption temperature was decreased by 100 °C, compared to MgH₂ without any oxide addition, and the activation energy for the hydrogen desorption was estimated to be about 185 kJ mol⁻¹, while that for MgH₂ without oxide was about 206 kJ mol⁻¹.     

Effects of H2O2 addition to the cell balance and self-discharge of Ni/MH batteries with AB5 and A2B7 alloys

In this paper we have compared nickel/metal hydride batteries made from AB₅ and Nd-only A₂B₇ alloys with or without addition of hydrogen peroxide (H₂O₂). The biggest advantages Nd-only A₂B₇ alloys have over AB₅ alloys are: a higher positive electrode utilization rate, lower initial internal resistance and less resistance increase after a 60 °C storage, and higher capacity and resistance degradation during cycling. The hydrogen peroxide was used as an oxidation agent and was added into the electrolyte before closing the cells. The H₂O₂ can oxidize both Co(OH)₂ in the positive electrode and MH alloy in the negative electrode. From the test results, H₂O₂ oxides the MH alloy preferentially over the Co(OH)₂ in the case of AB₅ alloy. This preferential oxidation is reversed in the case of the A₂B₇ alloy in which Co(OH)₂ is oxidized first. In cells made from both alloys, the addition of H₂O₂ prevented the venting of cells during formation, increased the utilization of positive electrode, improved the 60 °C charge retention, and increased the mid-point voltage after 300 cycles. Additionally the H₂O₂ also improved the cell balance for A₂B₇ alloy by decreasing the over-discharge reservoir in the negative electrode and reducing the capacity degradation in A₂B₇ alloy. However, the addition of H₂O₂ in cells made with AB₅ alloy deteriorated the cell balance by increasing the over-discharge reservoir in the negative electrode. The different cell balance and failure mechanisms for the two alloy compositions and H₂O₂ additive were compared and discussed.     

Cation ordering in the layered Li1+x(Ni0.425Mn0.425Co0.15)1 − xO2 materials (x = 0 and 0.12)

The layered Li_1+x(Ni_0.425Mn_0.425Co_0.15)_1 − xO₂ (x = 0 and 0.12) materials were prepared by a coprecipitation method. Their structure was investigated using the combination of X-ray and electron diffraction experiments. For both materials (x = 0 and 0.12), the electron diffraction patterns revealed an in-plane √3a_hex. × √3a_hex. superstructure in agreement with the ordering of the Li⁺, Ni²⁺, Ni³⁺, Mn⁴⁺ and Co³⁺ ions in the transition metal layers. The stoichiometry of these materials was not in agreement with an ideal ordering: the possible presence of point defects or of a domain microstructure was thus discussed. Electron diffraction also revealed that these ordered layers were slightly correlated along the c_hex. axis for both materials.     

Structural and thermal stabilities of layered Li(Ni1/3Co1/3Mn1/3)O2 materials in 18650 high power batteries

The structural and thermal stabilities of the layered Li(Ni_1/3Co_1/3Mn_1/3)O₂ cathode materials under high rate cycling and abusive conditions are investigated using the commercial 18650 Li(Ni_1/3Co_1/3Mn_1/3)O₂/graphite high power batteries. The Li(Ni_1/3Co_1/3Mn_1/3)O₂ materials maintain their layered structure even when the power batteries are subjected to 200 cycles with 10 C discharge rate at temperatures of 25 and 50 °C, whereas their microstructure undergoes obvious distortion, which leads to the relatively poor cycling performance of power batteries at high charge/discharge rates and working temperature. Under abusive conditions, the increase in the battery temperature during overcharge is attributed to both the reactions of electrolyte solvents with overcharged graphite anode and Li(Ni_1/3Co_1/3Mn_1/3)O₂ cathode and the Joule heat that results from the great increase in the total resistance (R_cell) of batteries. The reactions of fully charged Li(Ni_1/3Co_1/3Mn_1/3)O₂ cathodes and graphite anodes with electrolyte cannot be activated during short current test in the fully charged batteries. However, these reactions occur at around 140 °C in the fully charged batteries during oven test, which is much lower than the temperature of about 240 °C required for the reactions outside batteries.     

A comparative study of electrodes comprising nanometric and submicron particles of LiNi0.50Mn0.50O2, LiNi0.33Mn0.33Co0.33O2, and LiNi0.40Mn0.40Co0.20O2 layered compounds

In this paper we compare the behavior of LiNi_0.5Mn_0.5O₂, LiNi_0.33Mn_0.33Co_0.33O₂ (NMC) and LiNi_0.4Mn_0.4Co_0.2O₂ as cathode materials for advanced rechargeable Li-ion batteries. These materials were prepared by a self-combustion reaction (SCR) from the metal nitrates and sucrose, followed by calcination at elevated temperatures. The temperature and duration of calcination enabled the adjustment of the average particle size and size distribution. It was established that the annealing temperature (700–900 °C) of the as-prepared oxides influences strongly the crystallite and particle size, the morphology of the material, and the electrochemical performance of electrodes in Li-cells. Capacities up to 190, 180 and 170 mAh g⁻¹ could be obtained with Li[NiMn]O₂, LiNi_0.4Mn_0.4Co_0.2O₂ and LiNi_0.33Mn_0.33Co_0.33O₂, respectively. In terms of rate capability, the order of these electrodes is NMC < LiNi_0.4Mn_0.4Co_0.2O₂ ? Li[NiMn]O₂. Many hundreds of cycles at full DOD could be obtained with Li[NiMn]O₂ and NMC electrodes in Li-cells, at room temperature. All of these materials develop a unique surface chemistry that leads to their passivation and stabilization in standard electrolyte solutions (alkyl carbonates/LiPF₆). The surface chemistry was studied by FTIR, XPS and Raman spectroscopy and is discussed herein.     

Eliminating the irreversible capacity loss of high capacity layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2 cathode by blending with other lithium insertion hosts

The large irreversible capacity loss generally encountered with the high capacity layered oxide solid solutions between layered Li[Li_1/3Mn_2/3]O₂ and LiMO₂ (M = Mn, Ni, and Co) has been reduced by blending layered Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂, which is a solid solution between Li[Li_1/3Mn_2/3]O₂ and Li[Mn_1/3Ni_1/3Co_1/3]O₂, with spinel Li₄Mn₅O₁₂ or LiV₃O₈. The irreversible capacity loss decreases from 68 to 0 mAh g⁻¹ as the Li₄Mn₅O₁₂ content increases to 30 wt.% in the Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂-Li₄Mn₅O₁₂ composite and the LiV₃O₈ content increases to 18 wt.% in the Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂-LiV₃O₈ composite. The decrease in irreversible capacity loss is due to the ability of Li₄Mn₅O₁₂ or LiV₃O₈ to insert the extracted lithium that could not be inserted back into the layered Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ during the first cycle. The Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂-LiV₃O₈ composite with ∼18 wt.% LiV₃O₈ exhibits a high capacity of ∼280 mAh g⁻¹ with little or no irreversible capacity loss and good cyclability.     

Synthesis and performance of layered perovskite-type H-ABi2Ta2O9 (A = Ca,Sr, Ba,K0.5La0.5) for photocatalytic water splitting

Novel photocatalysts, protonated layered perovskite oxides H-ABi₂Ta₂O₉ (A = Ca, Sr, Ba, K_0.5La_0.5) for overall water splitting were synthesized by ion exchange with acid treating. The characterization by XRD, HRTEM indicated that all of ABi₂Ta₂O₉ (A = Ca, Sr, Ba, K_0.5La_0.5) were able to form new single-phase protonated layered oxides. The measurement of photocatalytic activity showed every protonated layered oxide could overall split water into H₂ and O₂, although the ratios of H₂ and O₂ are unstoichiometric. The sequence of photocatalytic H₂ production is HCBT(107.0 μmol/h) < HBBT(119.5 μmol/h) < HSBT(162.7 μmol/h) < HKLBT(189.3 μmol/h). The difference of ionic radius of cations in interlayer influenced the band gaps, and resulted in the distinction of photocatalytic activity. Pt loading enhanced apparently the photocatalytic activity. Among all of photocatalysts in this study, 0.1 wt%Pt/HSBT showed the highest photocatalytic activity for H₂ evolution, reaching 491 μmol/h.     

Synthesis and characterization of doped Li[Mn0.5−x/2Ni0.5−x/2Cox]O2 positive electrode materials

Layered positive electrode materials for rechargeable lithium-ion batteries of the general formula Li[Mn_0.5−x/2Ni_0.5−x/2Co_x]_1−yM_yO₂ (x ≤ 1/3, 0 ≤ y ≤ 0.05) were synthesized by a solid state route. The effect of doping elements M on the electrochemical performance was investigated. It was found that doping with niobium or tantalum has a positive effect on the cycling stability compared to the undoped parent compounds (y = 0). High discharge capacities, excellent cycling stabilities and high rate capabilities were achieved.     

Fabrication of all-solid-state lithium battery with lithium metal anode using Al2O3-added Li7La3Zr2O12 solid electrolyte

Li₇La₃Zr₂O₁₂ (LLZ) solid electrolyte is one of the promising electrolytes for all-solid-state battery due to its high Li ion conductivity and stability against Li metal anode. However, high calcination temperature for LLZ preparation promotes formation of La₂Zr₂O₇ impurity phase. In this paper, an effect of Al₂O₃ addition as sintering additive on LLZ solid electrolyte preparation and electrochemical properties of Al₂O₃-added LLZ were examined. By the Al₂O₃ addition, sintered LLZ pellet could be obtained after 1000 °C calcination, which is 230 °C lower than that without Al₂O₃ addition. Chemical and electrochemical properties of the Al₂O₃-added LLZ, such as stability against Li metal and ion conductivity, were comparable with the LLZ without Al₂O₃ addition, i.e. σ_bulk and σ_total were 2.4 × 10⁻⁴ and 1.4 × 10⁻⁴ S cm⁻¹ at 30 °C, respectively. All-solid-state battery with Li/Al₂O₃-added LLZ/LiCoO₂ configuration was fabricated and its electrochemical properties were tested. In cyclic voltammogram, clear redox peaks were observed, indicating that the all-solid-state battery with Li metal anode was successfully operated. The redox peaks were still observed even after one year storage of the all-solid-state battery in the Ar-filled globe-box. It can be inferred that the Al₂O₃-added LLZ electrolyte would be a promising candidate for all-solid-state battery because of facile preparation by the Al₂O₃ addition, relatively high Li ion conductivity, and good stability against Li metal and LiCoO₂ cathode.     

Microemulsion preparation and electrochemical characteristics of LiNi1/3Co1/3Mn1/3O2 powders

Layer-structured LiNi_1/3Co_1/3Mn_1/3O₂ was successfully synthesized via a reverse microemulsion (RμE) route. Well-crystallized nanosized (around 45 nm) powders were obtained with calcination at 800 °C. The Rietveld refinement data revealed low degree of cationic displacement in the obtained powders. Within the voltage range of 2.5–4.5 V, the microemulsion-derived LiNi_1/3Co_1/3Mn_1/3O₂ delivered 187.2 and 195.5 mAh g⁻¹ at room temperature and 55 °C, respectively. The prepared powders were found to exhibit low irreversible capacity and good capacity retention. Microemulsion-derived LiNi_1/3Co_1/3Mn_1/3O₂ demonstrated better rate capability than the solid-state derived samples, owing to the reduced particle size and increased surface area. Once the upper cut-off voltage reached 4.6 V, the capacity faded more rapidly than in other operation potential ranges. In this study, the microemulsion process effectively improved the electrochemical characteristics of LiNi_1/3Co_1/3Mn_1/3O₂. This soft chemical route possesses a great potential for synthesizing other types of cathode materials with multiple cations.     

Mechanistic study on lithium intercalation using a restricted reaction field in LiNi0.5Mn0.5O2

Structure changes of LiNi_0.5Mn_0.5O₂ were detected at the electrode/electrolyte interface of lithium cell using synchrotron X-ray scattering and two-dimensional model electrodes. The electrodes were constructed by an epitaxial film of LiNi_0.5Mn_0.5O₂ synthesized by pulsed laser deposition (PLD) method. The orientation of the film depends on the substrate plane; the 2D layer of LiNi_0.5Mn_0.5O₂ is parallel to the SrTiO₃(1 1 0) substrate ((1 1 0) LiNi_0.5Mn_0.5O₂//(1 1 0) SrTiO₃), while the 2D layer is perpendicular to the SrTiO₃(1 1 1) substrate ((0 0 3) LiNi_0.5Mn_0.5O₂//(1 1 1) SrTiO₃). The in situ X-ray diffraction of LiNi_0.5Mn_0.5O₂(0 0 3) confirmed three-dimensional lithium diffusion through the two-dimensional transition meal layers. The intercalation reaction of LiNi_0.5Mn_0.5O₂ will be discussed.     

Evolution of physicochemical and electrocatalytic properties of NiCo2O4 (AB2O4) spinel oxide with the effect of Fe substitution at the A site leading to efficient anodic O2 evolution in an alkaline environment

Within the framework of this work, spinel-type ternary transition metal oxides of nickel, cobalt and iron with the composition Fe_xNi_1−xCo₂O₄ (0 ≤ x ≤ 1) were prepared and tested as promising electrocatalysts for the oxygen evolution reaction (OER) in alkaline water electrolysis. The hydroxide precipitation method was used for the synthesis. The morphology, structure and specific surface area of the prepared electrocatalysts were determined by means of scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, the Brunauer Emmet Teller method and X-ray photo electron spectroscopy. The electrochemical properties were tested by thin-film technique on a rotating disk electrode and in a single-cell laboratory water electrolyzer coupled with electrochemical impedance spectroscopy. The OER studies indicate that substitution of Ni by Fe increases the electrocatalytic activity of the resulting material significantly. The highest activity was achieved for x = 0.1. Whereas the current density obtained using a pure nickel anode in the water electrolysis test was 54 mA cm⁻² at a cell voltage of 1.85 V, in the case of the anode modified with NiCo₂O₄ catalyst the value was 87 mA cm⁻². Using ternary transition metal oxides in the water electrolysis test and under identical conditions, the catalyst with the highest activity displayed a current density of 115 mA cm⁻².