Structural and electrochemical properties of a K2FeO4 cathode for rechargeable Li ion batteries

Ferrate is considered to be a potential cathode material for high-energy batteries, due to its high capacity based on three-electron transfer in electrochemical reactions. In this work, high-purity potassium ferrate (K₂FeO₄) was synthesized by a direct hypochlorite oxidation method. X-ray diffraction (XRD) and a charge-coupled device (CCD) were used to characterize the structure of the K₂FeO₄ as well as the channels for intercalation-deintercalation of Li ions. The one-dimension channel was observed in the direction of the a and b axes in the unit cell, with a radius 0.93 Å, which is beneficial for Li ion (radius = 0.76 Å) intercalation and deintercalation in K₂FeO₄. The experimental super-iron Li ion battery was assembled with 1 M LiPF₆ organic electrolyte (PC:EC:DMC = 1:3:6, v/v), a K₂FeO₄ cathode, and a metal lithium anode. The electrochemical performance of the K₂FeO₄ cathode was evaluated by a galvanostatic method and cyclic voltammetry (CV) in the potential range of 4.3-0.5 V at room temperature. It was demonstrated that one Li ion intercalates into the lattice of the K₂FeO₄ cathode along the channels of the a and b axes of the K₂FeO₄ unit cell, followed by a two-Li ion intercalation of isotropy in the initial discharge process. Amorphization of the K₂FeO₄ cathode is the main cause of its electrochemical performance decay.     

Adsorption of 2,4-D on Mg/Al–NO3 layered double hydroxides with varying layer charge density

Layered double hydroxides (LDHs) have high surface area and high anion exchange capacity, so they have been proposed to be an effective scavenger for contaminants. In this study, the adsorption of 2,4-dichlorophenoxyacetate (2,4-D) on Mg/Al–NO₃ LDHs with varying layer charge density was investigated with particular attention on the effect of the orientation of the interlayer nitrate. Three Mg/Al LDHs were synthesized with Al³⁺/(Al³⁺ + Mg²⁺) molar ratios of 3.3 (LDH3), 2.6 (LDH4) and 2.1 (LDH5). The results of adsorption experiments showed that LDH5 exhibited an S-type isotherm with a low 2,4-D adsorption capacity due to the low accessibility of 2,4-D to the interlayer space. The accessibility was restricted by the small basal spacing of LDH5 as a result of the parallel orientation of the interlayer nitrate with respect to the hydroxide sheet. Thus, the 2,4-D adsorption occurred mainly on the external surface of the material. On the contrary, LDH3, which has the highest layer charge density among the samples, contains nitrate with an orientation perpendicular to the hydroxide sheet of LDH3. The interlayer nitrate was readily exchanged by 2,4-D. Thus, in addition to the adsorption on the external surface, the replacement of the interlayer nitrate by 2,4-D contributed to a higher adsorbed amount of 2,4-D; the 2,4-D adsorption of LDH3 exhibited an L-type isotherm. For LDH4 that contained interlayer nitrate with both parallel and perpendicular orientations, the adsorption characteristics were between those of LDH3 and LDH5. This work has demonstrated the dependence of 2,4-D adsorption characteristics on the nitrate orientation in LDHs, as a consequence of changing layer charge density.     

Low temperature performance of copper/nickel modified LiMn2O4 spinels

This study presents an evaluation of structural changes resulting from cycling modified copper/nickel LiMn₂O₄ spinels at 263 K. In situ synchrotron XRD shows that cycling LiMn₂O₄ at 263 K resulted in the formation of mixed cubic and tetragonal phases with a consequent lower capacity. The differential capacitance profile normally exhibiting two peaks at 298 K is modified, showing only one oxidation peak at 263 K in the 4 V region. The changes observed are attributed to interactions between Jahn-Teller active Mn³⁺ species and Li⁺ ions. These changes are not observed once copper/nickel modified spinels are being evaluated, because of the decrease in Mn³⁺ population. All the observed changes are fully reversible once the material is cycled back at 298 K.     

Photocatalytic degradation of aqueous organocholorine pesticide on the layered double hydroxide pillared by Paratungstate A ion, Mg12Al6(OH)36(W7O24)·4H2O

Layered double hydroxide pillared by Paratungstate A ion, Mg₁₂Al₆(OH)₃₆(W₇O₂₄)·4H₂O, was prepared via anion exchange reaction of the synthetic precursor, Mg₄Al₂(OH)₁₂TA·xH₂O (TA²⁻=terephthalate), and [W₇O₂₄]⁶⁻ ion. Some physico-chemical properties were measured and the preparation conditions were studied. Trace aqueous organocholorine pesticide, hexachlorocyclohexane (HCH), was totally degraded and mineralized into CO₂ and HCl by irradiating a Mg₁₂Al₆(OH)₃₆(W₇O₂₄)·4H₂O suspension in the near UV area. Disappearance of trace HCH follows Langmuir–Hinshelwood first-order kinetics. The model and mechanism for the photocatalytic degradation of HCH on the Mg₁₂Al₆(OH)₃₆(W₇O₂₄)·4H₂O were proposed, indicating that the interlayer space is the reaction field, and that photogeneration of OH√ radicals are responsible for the degradation pathway.     

Ni0.5Zn0.5Fe2O4 nanoparticles and their magnetic properties and adsorption of bovine serum albumin

Ni_0.5Zn_0.5Fe₂O₄ nanoparticles were synthesized by the facile citrate-gel process and the preliminary measurement for adsorption of bovine serum albumin (BSA) protein on these nanoparticles was carried out. The gel precursor and resultant nanoparticles were characterized by TG-DSC, FTIR, XRD, TEM and VSM techniques and the BSA adsorption on the nanoparticles was analyzed by UV spectrophotometer at room temperature. The results show that the single phase of spinel Ni_0.5Zn_0.5Fe₂O₄ is formed at 400 °C. With increasing calcination temperature from 400 to 700 °C, the average grain size increases from about 14 to 45 nm and consequently, the specific saturation magnetization of Ni_0.5Zn_0.5Fe₂O₄ nanoparticles increases from about 46 to 68 Am²/kg. The coercivity initially increases and then decreases with increasing calcination temperature, with a maximum value 9.2 kA/m at 500 °C. The as-prepared Ni_0.5Zn_0.5Fe₂O₄ nanoparticles exhibit a good adsorbing ability for BSA and the optimized adsorption is achieved for the Ni_0.5Zn_0.5Fe₂O₄ nanoparticles calcined at 500 °C with grain size about 24 nm.     

Influence of halide ions on the adsorption of diphenylamine on iron in 0.5 M H2SO4 solutions

Halide ions are found to enhance the inhibition performance of amines due to enhanced adsorption of amines by the adsorbed halide ions on the metal surface. In this work, the synergistic action of halide ions on the corrosion inhibition of iron in 0.5 M H₂SO₄ by diphenylamine has been found out by electrochemical impedance and polarization methods. Analysis of impedance data has been made with equivalent circuit with constant phase angle element for calculation of double layer capacitance values. Experiments have been carried out in the concentration range of 100-1000 ppm of diphenylamine in the presence of 0.5-1.0 × 10⁻³ M of halide ions. Diphenylamine is found to be a cathodic inhibitor and the inhibition efficiency of about 65% is obtained at 1000 ppm. The anodic and cathodic Tafel slopes in the presence of diphenylamine alone and with halide ions are 65 ± 5 and 105 ± 5 mV, respectively. Diphenylamine inhibits corrosion by adsorption and the surface coverage values are increased considerably in the presence of halide ions. In the presence of iodide ions, the inhibition efficiency of diphenylamine at 100 ppm is increased to more than 90%. In the case of other halide ions, the inhibition efficiency of diphenylamine in increased to 80% at 1000 ppm. The order of synergism of halide ions is I⁻ ? Br⁻ > Cl⁻. The highest synergistic effect of iodide ions is due to chemisorption with metal surface due to its larger size and ease of polarizability.     

Study of the potentiometric response of the doped spinel Li1.05Al0.02Mn1.98O4 for the optimization of a selective lithium ion sensor

In this paper, we studied the development of a selective lithium ion sensor constituted of a carbon paste electrode modified (CPEM) with an aluminum-doped spinel-type manganese oxide (Li_1.05Al_0.02Mn_1.98O₄) for investigating the influence of a doping ion in the sensor response. Experimental parameters, such as influence of the lithium concentration in the activation of the sensor by cyclic voltammetry, pH of the carrier solution and selectivity for Li⁺ against other alkali and alkaline-earth ions were investigated. The sensor response to lithium ions was linear in the concentration range 5.62 × 10⁻⁵ to 1.62 × 10⁻³ mol L⁻¹ with a slope 100.1 mV/decade over a wide pH 10 (Tris buffer) and detection limit of 2.75 × 10⁻⁵ mol L⁻¹, without interference of other alkali and alkaline-earth metals, demonstrating that the Al³⁺ doping increases the structure stability and improves the potentiometric response and sensitivity of the sensor. The super-Nernstian response of the sensor in pH 10 can be explained by mixed potential arising from two equilibria (redox and ion-exchange) in the spinel-type manganese oxide.     

Selective oxidation of toluene on V2O5/TiO2/SiO2 catalysts modified with Te, Al, Mg, and K2SO4

The effect of the modification of vanadia catalysts supported on TiO₂/SiO₂ by the oxides of Al, Mg and Te, and K₂SO₄on the selective oxidation of toluene in the vapor phase has been studied. The catalysts were prepared by successive impregnation and characterized by BET surface measurements, XRD, XPS, and TPR. Addition of the second component decreased specific activity in all cases, except Al, mainly due to the decrease of surface area. Intrinsic activity was increased with addition of Te and Al, and decreased by that of Mg, while K₂SO₄ had little effect. These differences could be explained by the observed changes in either vanadium surface dispersion or reducibility. Selectivity to benzaldehyde increased markedly with addition of Te or K₂SO₄, that caused the formation of new oxide phases, V₃Ti₆O₁₇ and TiV₂O₆, in which vanadium is in a partially reduced state.     

Structural and magnetic properties of CoFe2O4 nanopowders,prepared using a modified Pechini method

Cobalt ferrite nanopowders were synthesized by means of the sol-gel method, using citric acid as a chelating agent, and various alcohols as gelling agent: ethanol (ET), ethylene glycol (EG), polyvinyl alcohol (PVA), 1,3 propanediol (PD) and a mixture of PVA and EG. The simultaneous TG/DTA analysis revealed different thermal behaviours of the synthesized gels, depending on the gelling agent. The powders obtained at 500 °C and annealed at 700 °C and 1000 °C contain a single CoFe₂O₄ phase. Scanning electron microscopy (SEM) revealed the influence of the gelling agent on the morphology of cobalt ferrite particles. The coercivity and the saturation magnetization of the powders obtained at 500 °C showed a strong dependence on the crystallite size, determined by the nature of the gelling agent.     

Synthesis and structural investigation of new graphite intercalation compounds containing the perfluoroalkylsulfonate anions C10F21SO3, C2F5OC2F4SO3, and C2F5(C6F10)SO3

The preparation of graphite intercalation compounds (GIC’s) of three perfluorinated alkylsulfonate anions, C₁₀F₂₁SO₃⁻, C₂F₅OC₂F₄SO₃⁻ and C₂F₅(C₆F₁₀)SO₃⁻ is described for the first time. Pure stage 2 GIC’s are obtained by chemical oxidation of graphite with K₂MnF₆ in a solution containing hydrofluoric and nitric acids for 72 h. One-dimensional electron density maps derived from powder diffraction data are fit to obtain models for the intercalate interlayer regions (galleries) structures: the structure models provide details on anion concentrations, orientations, and conformations. In all cases, anion bilayers are observed with anion sulfonate headgroups oriented towards graphene sheets. Compared with structures calculated for the isolated anions, the intercalated anion conformations show changes in dihedral angles, involving rotations about C-C or C-O bonds. For the GIC containing C₂F₅(C₆F₁₀)SO₃⁻, the anion conformation change is related to the more efficient packing of anions in the intercalate gallery.     

Effect of BaTiO3 addition on the microstructure and relaxor ferroelectric properties of Sr0.7Ba0.3Nb2O6 ceramics

A conventional solid-state reaction was used to synthesize (1-x) Sr_0.7Ba_0.3Nb₂O₆–xBaTiO₃ (0.00≤x≤0.10) ceramics. The phase structure, microstructure, and dielectric and relaxor ferroelectric properties of these ceramics were investigated. Tungsten bronze structure can be observed in ceramics, and addition of BaTiO₃ can make the grain size decrease and the porosity increase. The dielectric characteristics show diffuse phase transition phenomena for all samples, which were demonstrated by a linear fit of the modified Curie-Weiss law with γ varying between 1.54 and 1.88. As the BaTiO₃ content increases, the transition temperature (T_C) decreases gradually and has a minimum value of 37.53 °C at composition x=0.06, and the maximum dielectric constant (ε_max) increases gradually from 66 to 3309 and subsequently decreases to 1625 at x=0.10. In addition, the relaxor ferroelectric properties of these ceramics at x=0.8 are consistent with the Volgel-Fulcher relationship; polarization versus electric field (P-E) loops were measured at a different temperature.     

Effect of oriented defect-dipoles on the ferroelectric and piezoelectric properties of CuO-doped (K0.48Na0.52)0.96Li0.04Nb0.805Ta0.075Sb0.12O3 ceramics

The orientation characteristics of the defect dipoles formed by acceptor ions (${\mathrm{Cu}}_{\mathrm{Nb}}^{″\prime }$) and oxygen vacancies ($V_{\mathrm{O}}^{??}$) in CuO-doped (K_0.48Na_0.52)_0.96Li_0.04Nb_0.805Ta_0.075Sb_0.12O₃ piezoceramics were investigated. The ferroelectric and piezoelectric properties of the ceramics were obviously affected by the defect dipoles which are oriented along with the domains when poled under a dc electrical field of 3.5?kV/mm at the temperature near T_C. The poled ceramics with 1.0–3.0?mol% CuO displayed strong asymmetric P-E loops and a large internal bias field (E_i), 3.89–4.57?kV/cm, when polarized at the temperature near T_C. The enhanced piezoelectric coefficient d₃₃, 186–192?pC/N, was obtained for the ceramics poled near T_C when 0.02?≤?x?≤?0.03. The ceramics with oriented defect dipoles showed a relatively good thermal stability of d₃₃ until 180?°C.     

A comparative study of Ag/Al2O3 and Cu/Al2O3 catalysts for the selective catalytic reduction of NO by C3H6

The selective catalytic reduction (SCR) of NO by C₃H₆ in excess oxygen was evaluated and compared over Ag/Al₂O₃ and Cu/Al₂O₃ catalysts. Ag/Al₂O₃ showed a high activity for NO reduction. However, Cu/Al₂O₃ showed a high activity for C₃H₆ oxidation. The partial oxidation of C₃H₆ gave surface enolic species and acetate species on the Ag/Al₂O₃, but only an acetate species was clearly observed on the Cu/Al₂O₃. The enolic species is a more active intermediate towards NO + O₂ to yield—NCO species than the acetate species on the Ag/Al₂O₃ catalyst. The Ag and Cu metal loadings and phase changes on Al₂O₃ support can affect the activity and selectivity of Ag/Al₂O₃ and Cu/Al₂O₃ catalysts, but the formation of enolic species is the main reason why the activity of the Ag/Al₂O₃ catalyst for NO reduction is higher than that of the Cu/Al₂O₃ catalyst.     

Effects of argon sintering atmosphere on luminescence characteristics of Ca6BaP4O17:Sm3+ phosphors

In this paper, Ca₆BaP₄O₁₇:Sm³⁺ and Li⁺ co-doped Ca₆BaP₄O₁₇:Sm³⁺ phosphors were synthesized in air and argon atmospheres using a solid-state reaction method. The phosphor morphologies and crystal structure were studied using scanning electron microscopy and X-ray diffraction, respectively. The emission and absorption characteristics were investigated using photoluminescence emission spectroscopy and diffuse reflectance spectroscopy. The surface states and composition of phosphor were investigated using X-ray photoelectron spectroscopy. The emission integrated intensities of the phosphors sintered in an argon atmosphere increased 3.5 fold than the ones sintered in air atmosphere, with Li⁺ ions becoming embedded in the lattice of the Ca₆BaP₄O₁₇:Sm³⁺ phosphor. This occurs because there are fewer defect/oxygen vacancies and less of the secondary phase forms, leading to better Sm³⁺ emission. The results suggest that sintering a mixture of the raw materials of a phosphor in an argon atmosphere is a good approach for synthesizing Ca₆BaP₄O₁₇:Sm³⁺ phosphor powders. The color purity and CIE values of an optimized phosphor sample sintered in an argon atmosphere with an Li⁺ ion compensator were calculated to be ~ 99.6% and (0.612,0.386) in the orange–red region under 405-nm excitation, respectively. Moreover, the solid solubility of Sm³⁺ ions in the Ca₆BaP₄O₁₇ host can be enhanced by using an argon atmosphere in the synthesis process.     

Influence of porosity on the electrical properties of La9.33(SiO4)6O2 oxyapatite

Oxyapatites are very promising materials in terms of ionic conductivity. They can be considered as a potential electrolyte for fuel cells as SOFC. The influence of porosity on the electrical properties of La_9.33(SiO₄)₆O₂ oxyapatite is reported here. Hot pressed pellets with various densification ratios have been characterized by the complex impedance method. The high frequency response associated with the bulk contribution is much more affected by the porosity than the grain boundaries contribution: as a consequence, the electrical behaviour of the samples has been considered in assimilating the porous ceramics to composite materials made of apatite with various amounts of air inclusions. Thus, the porosity dependence of conductivity, activation energy and permittivity are reported here. A percolation threshold has been highlighted for porosity values greater than 30%, involving great lowering of the electrical performances.     

Electrochemical behaviour of the solid electrolyte Ag6I4WO4

Electrochemical investigation of the solid electrolyte Ag₆I₄WO₄ (0.8 AgI + 0.2 Ag₂WO₄ mixture) was performed. By means of cyclic voltammetry, normal pulse polarography and ac polarography methods the electrode processes and the capacity of electric double layer on electrode/electrolyte interface were investigated at several temperature levels (room temperature, 373 K, 386 K and 423 K). It was shown that on the electrode, either a Pt or Ag one, at direct and reverse cathodic polarization, Ag⁺ ? Ag redox process occurs, and for it a substantial exchange current value was found. Hence it is concluded that the redox process at all the observed temperatures is very fast and that the rate increases with a rise in temperature. Also the passivation of the electrode at anodic polarization affecting the electrolyte decomposition voltage level considerably was pointed out. The electric double-layer capacity values found show that the structure of the layer changes with temperature and dc potential, which is manifested on the capacity.     

Electrocatalyst materials for fuel cells based on the polyoxometalates—K7 or H7[(P2W17O61)Fe(H2O)] and Na12 or H12[(P2W15O56)2Fe4(H2O)2]

Free acids of the iron substituted heteropoly acids (HPA), H₇[(P₂W₁₇O₆₁)Fe^III(H₂O)] (HFe1) and H₁₈[(P₂W₁₅O₅₆)₂Fe^III₂(H₂O)₂] (HFe2) were prepared from the salts K₇[(P₂W₁₇O₆₁)Fe^III(H₂O)] (KFe1) and Na₁₂[(P₂W₁₅O₅₆)₂Fe^III₄(H₂O)₂] (NaFe4), respectively. The iron-substituted HPA were adsorbed on to XC-72 carbon based GDLs to form HPA doped GDEs after water washing with HPA loadings of ca. 1 μmol. The HPA was detected throughout the GDL by EDX. Solution electrochemistry of the free acids are reported for the first time in sulfate buffer, pH 1-3. The hydrogen oxidation reaction was catalyzed by KFe1 at 0.33 V, with an exchange current density of 38 mA/cm². Moderate activity for the oxygen reduction reaction was observed for the iron substituted HPA, which was dramatically improved by selectively removing oxygen atoms from the HPA by cycling the fuel cell cathode under N₂ followed by reoxidation to give a restructured oxide catalyst. The nanostructured oxide achieved an OCV of 0.7 V with a Tafel slope of 115 mV/decade. Cycling the same catalysts in oxygen resulted in an improved catalyst/ionomer/carbon configuration with a slightly higher Tafel slope, 128 mV/decade but a respectable current density of 100 mA/cm² at 0.2 V.     

Improving the high-temperature performances of a layered double hydroxide, [Ni4Al(OH)10]NO3, through calcium hydroxide coatings

In order to improve high-temperature performances, calcium hydroxide was coated onto [Ni₄Al(OH)₁₀]NO₃ through surface modification or mechanical blending. The existence of Ca(OH)₂ was revealed by XRD, and SEM which showed that there are Ca(OH)₂-rich blocks in the sample containing 13.0 wt% calcium prepared by surface modification. The addition of Ca(OH)₂ can retard the structural transformation from the layered double hydroxide [Ni₄Al(OH)₁₀]NO₃ to β-Ni(OH)₂. In addition, it improves charge/discharge performances and electrochemical reversibility by elevating the oxygen evolution potential, increasing the double layer capacity C_dl and decreasing the charge transfer resistance R_ct.     

Effect of annealing atmosphere on the structure and dielectric properties of unfilled tungsten bronze ceramics Ba4PrFe0.5Nb9.5O30

Unfilled tungsten bronze ceramics with the nominal formula Ba₄PrFe_0.5Nb_9.5O₃₀ were synthesized via the standard solid-state sintering route, and the effects of oxygen vacancies on the dielectric and electrical properties were investigated in addition to the structure. Room-temperature X-ray diffraction showed that the N₂-annealed sample had the largest cell volume. Low-temperature spectrum showed that N₂ annealing rendered the dielectric constant and dielectric loss more frequency dispersive, whereas O₂ annealing inhibited the frequency dispersion. The dc conductivity of all the samples originated from the electrons produced in the second ionization of oxygen vacancies and was most likely controlled by a mixed conduction mechanism of the electron and oxygen-vacancy ions. The N₂-annealed sample has the highest dc conductivity owing to its high concentration of oxygen vacancies. The broadening of the Raman lines and the decrease of Raman intensity for the N₂-annealed sample originated from a significant structural disorder. X-ray photoelectron spectra demonstrated that the increased oxygen vacancies caused by the change of valences of Fe and Pr ions contributed to the structural disorder.     

Structural and transport effects of doping perfluorosulfonic acid polymers with the heteropoly acids, H3PW12O40 or H4SiW12O40

A perfluorosulfonic acid (PFSA) polymer with pendant side chain -O(CF₂)₄SO₃H was doped with the heteropoly acids (HPAs), H₃PW₁₂O₄₀ and H₄SiW₁₂O₄₀. Infrared spectroscopy revealed a strong interaction between the HPA and the PFSA ionomer. Modes associated with the peripheral bonds of the HPA were shifted to lower wave numbers when doped into PFSA membranes. Small-angle X-ray scattering (SAXS) measurements showed the presence of large crystallites of HPA in the membrane with d spacings of ca. 10 Å, close to the lattice spacing observed in bulk HPA crystals. Under wet conditions the HPA was more dispersed and constrained the size of the sulfonic acid clusters to 20 Å at a 5 wt% HPA doping level, the same as in the vacuum treated ionomer samples. Under conditions of minimum hydration the HPA decreased the E_a for the self-diffusion of water from 27 to 15 kJ mol⁻¹. The reverse trend was seen under 100% RH conditions. Proton conductivity measurements showed improved proton conductivity of the HPA doped PFSAs at a constant dew point of 80 °C for all temperatures up to 120 °C and at all relative hummidities up to 80%. The activation energy for proton conduction generally was lower than for the undoped materials at RH ≤80%. Significantly the E_a was 1/2 that of the undoped material at RHs of 40 and 60%. A practical proton conductivity of 113 mS cm⁻¹ was observed at 100 °C and 80% RH.