Amorphous Ru1−yCryO2 loaded on TiO2 nanotubes for electrochemical capacitors

Amorphous Ru_1−yCr_yO₂/TiO₂ nanotube composites were synthesized by loading different amount of Ru_1−yCr_yO₂ on TiO₂ nanotubes via a reduction reaction of K₂Cr₂O₇ with RuCl₃·nH₂O at pH 8, followed by drying in air at 150 °C. Cyclic voltammetry and galvanostatic charge/discharge tests were applied to investigate the performance of the Ru_1−yCr_yO₂/TiO₂ nanotube composite electrodes. For comparison, the performance of amorphous Ru_1−yCr_yO₂ was also studied. The results demonstrated that the three dimensional nanotube network of TiO₂ offered a solid support structure for active materials Ru_1−yCr_yO₂, allowed the active material to be readily available for electrochemical reactions, and increased the utilization of active materials. A maximum specific capacitance 1272.5 F/g was obtained with the proper amount of Ru_1−yCr_yO₂ loaded on the TiO₂ nanotubes.     

Fabrication and characterization of Li1+x−yNb1−x−3yTix+4yO3 substrates using aqueous tape casting process

An aqueous system for tape casting Li_1+x−yNb_1−x−3yTi_x+4yO₃ (LNT) ceramics was developed using poly(vinyl alcohol) (PVA) binder, ethylene alcohol (EG) plasticizer and ammonium salt of polycarboxylate (PCA-NH₄) dispersant. The zeta potential measurement showed that the isoelectric point of the LNT particles moved slightly toward more acid region after the dispersant absorbed on the particles, while the zeta potential increased significantly. The rheological test indicated that the ceramic slurry exhibited a typical pseudoplastic behavior without thixotropy. The effect of solid loading on the properties of the green tapes was investigated. The increase in the solid loading increased the tensile strength and the green density of the tapes. TGA analysis indicated that the organic additives in the green tapes can be completely removed by heat treatment at 600 °C. SEM micrographs showed that the microstructure of the green and sintered tapes was homogeneous.     

Electrodeposition of P-type BixSb2−xTey thermoelectric film from dimethyl sulfoxide solution

The electrochemical behaviors of Bi(III), Te(IV), Sb(III) and their mixtures in DMSO solutions were investigated using cyclic voltammetry and linear sweep voltammetry measurements. On this basis, Bi_xSb_2−xTe_y film thermoelectric materials were prepared by potentiodynamic electrodeposition technique from mixed DMSO solution, and the compositions, structures, morphologies as well as the thermoelectric properties of the deposited films were also analyzed. The results show that Bi_xSb_2−xTe_y compound can be prepared in a very wide potential range by potentiodynamic electrodeposition technique in the mixed DMSO solutions. After anneal treatment, the deposited film prepared in the potential range of −200 to −400 mV shows the highest Seebeck coefficient (185 μV/K), the lowest resistivity (3.34 × 10⁻⁵ Ω m), the smoothest surface, the most compact structure and processes the stoichiometry (Bi_0.49Sb_1.53Te_2.98) approaching to the Bi_0.5Sb_1.5Te₃ ideal material most. This Bi_0.49Sb_1.53Te_2.98 film is a kind of nanocrystalline material and (0 1 5) crystal plane is its preferred orientation.     

Effects of B-site ion (M) substitution on the ionic conductivity of (Li3xLa2/3−x)1+y/2(MyTi1−y)O3 (M=Al, Cr)

Effect on the ionic conductivity of B-site ion (M) substitution in (Li_3xLa_2/3−x)_1+y/2M_yTi_1−yO₃ (M=Al, Cr) has been investigated. It has been found that partial substitution of smaller Al³⁺ for Ti⁴⁺ is effective to enhance the ionic conductivity of Li_3xLa_2/3−xTiO₃. At 300 K, the maximum bulk conductivity of (1.58±0.01)×10⁻³ S cm⁻¹ is observed from the composition of (Li_0.39La_0.54)_1−y/2Al_yTi_1−yO₃ with y=0.02 (x=0.13), that is the highest yet reported for known perovskite solutions at room temperature. The conductivity enhancement is interpreted as being due to the substitution-induced bond-strength change rather than due to bottleneck size change for Li migration, TiO₆-octahedron tilting or A-site cation ordering.     

Preparation and cycling performance of Aland F co-substituted compounds Li4AlxTi5−xFyO12−y

Li₄Al_xTi_5−xF_yO_12−y compounds were prepared by a solid-state reaction method. Phase analyses demonstrated that both Al³⁺ and F⁻ ions entered the structure of spinel-type Li₄Ti₅O₁₂. Charge-discharge cycling results at a constant current density of 0.15 mA cm⁻² between the cut-off voltages of 2.5 and 0.5 V showed that the Al³⁺ and F⁻ substitutions improved the first total discharge capacity of Li₄Ti₅O₁₂. However, Al³⁺ substitution greatly increased the reversible capacity and cycling stability of Li₄Ti₅O₁₂ while F⁻ substitution decreased its reversible capacity and cycling stability slightly. The electrochemical performance of the Al³⁺-F⁻-co-substituted specimen was better than the F⁻-substituted one but worse than the Al³⁺-substituted one.     

One-step electrochemical preparation of the ternary (BixSb1−x)2Te3 thin films on Au(1 1 1): Composition-dependent growth and characterization studies

This study reports on the synthesis of ternary semiconductor (Bi_xSb_1−x)₂Te₃ thin films on Au(1 1 1) using a practical electrochemical method, based on the simultaneous underpotential deposition (UPD) of Bi, Sb and Te from the same solution containing Bi³⁺, SbO⁺, and HTeO₂⁺ at a constant potential. The thin films are characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), energy dispersive spectroscopy (EDS) and reflection absorption-FTIR (RA-FTIR) to determine structural, morphological, compositional and optic properties. The ternary thin films of (Bi_xSb_1−x)₂Te₃ with various compositions (0.0 ≤ x ≤ 1.0) are highly crystalline and have a kinetically preferred orientation at (0 1 5) for hexagonal crystal structure. AFM images show uniform morphology with hexagonal-shaped crystals deposited over the entire gold substrate. The structure and composition analyses reveal that the thin films are pure phase with corresponding atomic ratios. The optical studies show that the band gap of (Bi_xSb_1−x)₂Te₃ thin films could be tuned from 0.17 eV to 0.29 eV as a function of composition.     

Electrochemical and thermal studies of Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 (x = 1/12, 1/4, 5/12, and 1/2)

Samples of the layered cathode materials, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ (x = 1/12, 1/4, 5/12, and 1/2), were synthesized at 900 °C. Electrodes of these samples were charged in Li-ion coin cells to remove lithium. The charged electrode materials were rinsed to remove the electrolyte salt and then added, along with EC/DEC solvent or 1 M LiPF₆ EC/DEC, to stainless steel accelerating rate calorimetry (ARC) sample holders that were then welded closed. The reactivity of the samples with electrolyte was probed at two states of charge. First, for samples charged to near 4.45 V and second, for samples charged to 4.8 V, corresponding to removal of all mobile lithium from the samples and also concomitant release of oxygen in a plateau near 4.5 V. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples with x = 1/4, 5/12 and 1/2 charged to 4.45 V do not react appreciably till 190 °C in EC/DEC. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples charged to 4.8 V versus Li, across the oxygen release plateau, start to significantly react with EC/DEC at about 130 °C. However, their high reactivity is similar to that of Li_0.5CoO₂ (4.2 V) with 1 μm particle size. Therefore, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples showing specific capacity of up to 225 mAh/g may be acceptable for replacing LiCoO₂ (145 mAh/g to 4.2 V) from a safety point of view, if their particle size is increased.     

Evolution of the local structure and electrochemical properties of spinel LiNixMn2−xO4 (0 ≤ x ≤ 0.5)

A series of Ni substituted spinel LiNi_xMn_2−xO₄ (0 ≤ x ≤ 0.5) have been synthesized to study the evolution of the local structure and their electrochemical properties. X-ray diffraction showed a few Ni cations moved to the 8a sites in heavily substituted LiNi_xMn_2−xO₄ (x ≥ 0.3). X-ray photoelectron spectroscopy confirmed Ni²⁺ cations were partially oxidized to Ni³⁺. The local structures of LiNi_xMn_2−xO₄ were studied by analyzing the and A_1g Raman bands. The most compact [Mn(Ni)O₆] octahedron with the highest bond energy of Mn(Ni)O was found for LiNi_0.2Mn_1.8O₄, which showed a Mn(Ni)O average bond length of 1.790 Å, and a force constant of 2.966 N cm⁻¹. Electrolyte decomposition during the electrochemical charging processes increased with Ni substitution. The discharge capacities at the 4.1 and 4.7 V plateaus obeyed the linear relationships with respect to the Ni substitution with the slopes of −1.9 and +1.9, which were smaller than the theoretical values of −2 and +2, respectively. The smaller slopes could be attributed to the electrochemical hysteresis and the presence of Ni³⁺ in the materials.     

Synthesis and tunable thermal expansion properties of Sc2−xYxW3O12 solid solutions

A new series of rare earth solid solutions Sc_2−xY_xW₃O₁₂ was successfully synthesized by the conventional solid-state method. Effects of doping ion yttrium on the crystal structure, morphology and thermal expansion property of as-prepared Sc_2−xY_xW₃O₁₂ ceramics were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TG), field emission scanning electron microscope (FE-SEM) and thermal mechanical analyzer (TMA). Results indicate that the obtained Sc_2−xY_xW₃O₁₂ samples with Y doping of 0≤x≤0.5 are in the form of orthorhombic Sc₂W₃O₁₂-structure and show negative thermal expansion (NTE) from room temperature to 600 °C; while as-synthesized materials with Y doping of 1.5≤x≤2 take hygroscopic Y₂W₃O₁₂·nH₂O-structure at room temperature and exhibit NTE only after losing water molecules. It is suggested that the obvious difference in crystal structure leads to different thermal expansion behaviors in Sc_2−xY_xW₃O₁₂. Thus it is proposed that thermal expansion properties of Sc_2−xY_xW₃O₁₂ can be adjusted by the employment of Y dopant; the obtained Sc_1.5Y_0.5W₃O₁₂ ceramic shows almost zero thermal expansion and its average linear thermal expansion coefficient is −0.00683×10⁻⁶ °C⁻¹ in the 25–250 °C range.     

Hydrothermal synthesis of LiNixCo1−xO2 cathode materials

Ultrafine powders of LiCoO₂, nonstoichiometric LiNiO₂ and LiNi_0.9Co_0.1O₂ were prepared under mild hydrothermal conditions. The influence of the molar ratio of Li/Co, Li/Ni and Li/(Ni + Co) was studied. The final products were investigated by XRD, TEM and EDS. To synthesize a stoichiometric LiNiO₂ under mild hydrothermal conditions was found to be a big challenge. Transmission electron microscopies (TEM) revealed the formation of well-crystallized LiCoO₂ and LiNi_0.9Co_0.1O₂ with average size of 100 nm and 10 nm, respectively.     

Study on the rapid synthesis of LiNi1−xCoxO2 cathode material for lithium secondary battery

LiNi_1−xCo_xO₂ (x = 0, 0.1, 0.2) cathode materials were successfully synthesized by a rheological phase reaction method with calcination time of 0.5 h at 800 °C. All obtained powders are pure phase with α-NaFeO₂ structure (R-3m space group). The samples deliver an initial discharge capacity of 182, 199 and 189 mAh g⁻¹ (25 mA g⁻¹, 4.35-3.0 V), respectively. The reaction mechanism was also discussed, which consists of a series of defect reactions. As a result of these defect reactions, the reaction of forming LiNi_1−xCo_xO₂ takes place in high speed.     

Preparation, characterization and electrical conductivity studies of NdYb1−xGdxZr2O7 (0 ≤ x ≤ 1.0) ceramics

Several compositions of NdYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 1.0) ceramics were prepared by pressureless-sintering method at 1973 K for 10 h in air. The relative density, microstructure and electrical conductivity of NdYb_1−xGd_xZr₂O₇ ceramics were analyzed by the Archimedes method, X-ray diffraction, scanning electron microscopy and impedance plots measurements. NdYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 0.3) ceramics have a single phase of defect fluorite-type structure, and NdYb_1−xGd_xZr₂O₇ (0.7 ≤ x ≤ 1.0) ceramics exhibit a single phase of pyrochlore-type structure; however, the NdYb_0.5Gd_0.5Zr₂O₇ composition shows mixed phases of both defect fluorite-type and pyrochlore-type structures. The measured values of the grain conductivity obey the Arrhenius relation. The grain conductivity of each composition in NdYb_1−xGd_xZr₂O₇ ceramics gradually increases with increasing temperature from 673 to 1173 K. NdYb_1−xGd_xZr₂O₇ ceramics are oxide-ion conductor in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The highest grain conductivity value obtained in this work is 1.79 × 10⁻² S cm⁻¹ at 1173 K for NdYb_0.3Gd_0.7Zr₂O₇ composition.     

Effect of Pt addition on the isomerization properties of MoO2 − x(OH)y deposited on TiO2

Isomerization of n-hexane and n-pentane were studied using equivalent 5 monolayers of MoO₃ deposited on TiO₂. Addition of 2.5% Pt by weight of MoO₃ on the Mo catalyst resulted in an increase in the catalytic activity of the system in favor of hydrocracking products. Surface characterization by XPS-UPS and ISS reveal that the sample surface contains Oxygen, Molybdenum, Platinum and Titanium. Apparently, the metallic properties of the deposited Pt favors the hydrocracking reactions and becomes dominant at reaction temperatures higher than 623 K. Balanced metal-acid functions in MoO_2 − x(OH)_y phase seems to be in optimized condition toward the hydroisomerization process. The contribution of Platinum addition to this catalytic reaction is not obvious. Combination of surface XPS-UPS, ISS and catalytic reactions carried out at similar experimental conditions enabled us to have better insight concerning the catalytic activities of the different chemical species present on the sample surface.     

Optical characterisation of anodic sulphide films on Hg1−xCdxTe (MCT) grown by the potential step method

The growth of native sulphide films on Hg_1−xCd_xTe (MCT) by potential steps to two different electrochemical growth regions has been studied by in-situ ellipsometry, photocurrent spectroscopy and second harmonic generation (SHG) rotational anisotropy. Films grown at −0.4 V versus SCE were porous and consisted mainly of cubic close packed (ccp) CdS. The photocurrent spectrum of the film exhibited a cut-off which closely conformed to the expected bandgap of CdS. The gradual increase in SH intensity as the sulphide film thickened was due to a contribution to the overall SH signal from the CdS film itself. No evidence of any hcp species was apparent in the rotational anisotropy patterns, with the four-fold pattern expected for the vicinal MCT surface being maintained even after film growth. When the potential was stepped to −0.3 V, the film grown absorbed the ellipsometer radiation once a thickness of 86 nm was reached. Photocurrent measurements showed a tail extending into the red region of the spectrum. These phenomena have been attributed to the incorporation of ccp HgS into the sulphide film as at this potential, the electrochemical reaction of the HgTe component of MCT can occur. SHG rotational anisotropy confirmed the four-fold symmetry of the ccp surface film.     

The kinetics of hydrogen transport through amorphous Pd82−yNiySi18 alloys (y=0-32) by analysis of anodic current transient

Hydrogen transport through amorphous Pd_82−yNi_ySi₁₈ alloys (y=0-32) was investigated in 0.1 M NaOH solution by analysis of the anodic current transient. It was found that the anodic current transient shows the non-Cottrell behaviour, but its shape and value remain nearly constant regardless of the hydrogen discharging potential. From the coincidence of the anodic current transient theoretically calculated with that experimentally measured, it is suggested that the change in surface concentration of hydrogen with time is uniquely given by the rate of hydrogen transfer from absorbed state at the electrode sub-surface to adsorbed state on the electrode surface. This means that neither the ‘constraint of constant concentration’ nor the ‘constraint by Butler-Volmer behaviour’ is effective at the electrode surface during hydrogen extraction. On the basis of the theoretical current-time relation under the ‘constraint by hydrogen transfer of absorbed state to adsorbed state’, the hydrogen diffusivity was determined to have an almost constant value of (1.3±0.4)×10⁻⁸ cm² s⁻¹, irrespective of the Ni content and in the absence of Ni. On the other hand, it is inferred that the rate constant of hydrogen transfer decreases markedly with increasing Ni content due to the Ni(OH)₂ layer formed on the electrode surface.     

An investigation of cell-impedance-controlled lithium transport through LiCoO2/Li1−δMn2O4 bilayer film electrode prepared by rf magnetron sputtering

Lithium transport through LiCoO₂/Li_1−δMn₂O₄ bilayer film electrode prepared by radio-frequency (rf) magnetron sputtering was investigated in a 1 M solution of LiClO₄ in propylene carbonate. From the analyses of the AC-impedance spectra experimentally measured from the Li_1−δMn₂O₄ single-layer and LiCoO₂/Li_1−δMn₂O₄ bilayer film specimens, the internal cell resistance of the LiCoO₂/Li_1−δMn₂O₄ bilayer film electrode was determined to be smaller in value than that of the Li_1−δMn₂O₄ single-layer film electrode over the whole potential range, which can be accounted for by the kinetic facility for the interfacial charge-transfer reaction in the presence of the more conductive LiCoO₂ surface film. Moreover, from the analyses of the anodic current transients measured from both the film specimens, it was suggested that the cell-impedance-controlled constraint at the electrode surface is changed to the diffusion-controlled constraint simultaneously characterised by the large potential step and the small amount of lithium transferred during lithium transport. In addition, in the case of the LiCoO₂/Li_1−δMn₂O₄ bilayer film electrode, it was found that the critical value of the applied potential step needed for the mechanism transition is reduced, which strongly indicates that the internal cell resistance plays a significant role in determining the cell-impedance-controlled lithium transport. Furthermore, from the comparison of the cathodic current transients measured on the Li_1−δMn₂O₄ single-layer film specimens with various thicknesses, it was experimentally verified that the diffusion resistance is explicitly distinguished from the internal cell resistance.     

Electrochemical and ex situ XRD investigations on (1−x)LiNiO2·xLi2TiO3 (0.05≤x≤0.5)

An attempt to understand the unusual electrochemical behaviors in (1−x)LiNiO₂·xLi₂TiO₃ (0.05≤x≤0.5), an excess initial charge capacity exceeding the oxidation of transitional metal to +4 accompanying the appearance of an irreversible initial charge plateau when x reached 0.075, was performed. The decreased charge-discharge polarization after charging to 4.6 and 4.8 V and increased columbic reversibility after charging to 4.6 V typically for x=0.1 and 0.2, in contrast to charging to 4.4 V, suggested that the excess initial charge capacity possibly did not come mainly from electrolyte decomposition; while ex situ XRD results in the sample with x=0.2 confirmed that Li⁺ were really extracted at the stage of the charge plateau, ruling out the possibility that electrolyte decomposition mainly accounted for the unusual electrochemical behaviors. It was inferred that the species responsible for charge compensation for the excess charge capacity must be oxygen ions in these materials, considering that Ni⁴⁺ and Ti⁴⁺ are generally impossible to be oxidized to a higher valence. Various electrochemical cycling experiments demonstrated that the sample for x=0.05 with high resistant ability to high voltage and temperature is very promising cathode material in view of observed capacity and cycleability from a viewpoint of application.     

Oxygen reduction reaction on electrodeposited Pt100−x−yNixPdy thin films

The kinetics of the oxygen reduction reaction (ORR) were examined on a series of Pt_100−x−yNi_xPd_y ternary alloys. Films were produced by electrodeposition that involved a combination of underpotential and overpotential reactions. For Pt-rich Pt_100−x−yNi_xPd_y alloy films (x < 0.65) Ni co-deposition occurred at underpotentials while for Ni-rich films (x > 0.65) deposition proceeded at overpotentials. Rotating disk electrode (RDE) measurements of the ORR kinetics on Ni-rich Pt_100−x−yNi_xPd_y thin films revealed up to ∼6.5-fold enhancement of the catalytic activity relative to Pt films with the same Pt mass loading. More than half of the electrocatalytic gain may be attributed to surface area expansion due to Ni dealloying. Surface area normalization based on the H_upd charge reduced the enhancement factor to a value less than 2. The most active ternary alloy film for ORR was Pt₂₅Ni₇₃Pd₂. Comparison of the ORR on Pt, Pt₂₀Ni₈₀, Pt₂₅Ni₇₃Pd₂ thin films indicate that the binary alloy is the most active with a H_upd normalized ORR enhancement factor of up to 3.0 compared to 1.6 for the ternary alloy.