Fabrication of carbon paste electrode containing [PFeW11O39] polyoxoanion supported on modified amorphous silica gel and its electrocatalytic activity for H2O2 reduction

[PFeW₁₁O₃₉]⁴⁻ (PFeW₁₁) supported on the surface of 3-aminopropyl(triethoxy)silane modified silica gel was synthesized and used as a bulk modifier to fabricate a renewable three-dimensional chemically modified electrode. The electrochemical behavior of the modified electrode was investigated. Cyclic voltammetry studies showed that the PFeW₁₁ on the electrode surface sustained the same electrochemical properties as that of the PFeW₁₁ in solution. The preparation of chemically modified electrode is simple and quiet reproducible using inexpensive material. The modified electrode had high electrocatalytic activity toward H₂O₂ reduction and it was successfully applied as an electrochemical detector to monitor H₂O₂ in flow injection analysis (FIA). The electrocatalytic peak current was found to be linear with the H₂O₂ concentration in the range 10-200 μmol L⁻¹ with a correlation coefficient of 0.998 and a detection limit (3σ) of 7.4 μmol L⁻¹ H₂O₂. The electrode has the remarkable advantage of surface renewal owing to bulk modification, as well as simple preparation, good mechanical and chemical stability and reproducibility.     

Surface immobilisation of transition metal substituted Krebs type polyoxometalates, [X2W20M2O70(H2O)6] (X = Bi or Sb, M = Co or Cu), by the layer by layer technique

A series of transition metal (i.e. Cu²⁺ and Co²⁺) substituted Krebs type polyoxometalates (POMs), of the general formula [X₂W₂₀M₂O₇₀(H₂O)₆]ⁿ⁻, X = Sb or Bi, M = Co(II) or Cu(II), have been successfully immobilised onto carbon electrode surfaces through the employment of the layer-by-layer (LBL) technique. This involved the construction of alternating anionic POM, [X₂W₂₀M₂O₇₀(H₂O)₆]ⁿ⁻, layers and the cationic metallodendrimer, Ru(II)-metallodendrimer as the cationic layers, in addition to a [poly(diallyldimethylammonium chloride)] PDDA base layer. Stable multielectron redox couples associated with the W–O framework, for the Krebs type POMs, and the Ru(III/II) for the metallodendrimer, were clearly observed upon layer construction and redox switching within the pH domain of 2–6.5. The constructed multilayer assemblies exhibited pH dependent redox activity and thin layer behaviour up to 100 mV s⁻¹. The porosity and permeability of the individual multilayer assemblies towards an anionic probe were determined by AC impedance and cyclic voltammetry. The surface morphology of each multilayer was also determined by Atomic Force Microscopy (AFM).     

Preparation and characterization of negative thermal expansion Sc2W3O12/Cu core-shell composite

A composite of Sc₂W₃O₁₂/Cu where Sc₂W₃O₁₂, the core, is coated by the Cu shell was synthesized using simple electroless plating method. As-prepared Sc₂W₃O₁₂/Cu composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and thermomechanical analyzer (TMA) techniques. The study results show that the Pd-Sn activator was successfully formed on the surface of Sc₂W₃O₁₂ after the sensitization and activation. In the electroless plating process, Cu nanocrystals formed firstly, and then grew together to form a continuous coating. Sc₂W₃O₁₂/Cu core-shell composites exhibit a negative linear coefficient of thermal expansion CTE = −4.47 × 10⁻⁶ °C⁻¹ from room temperature to 200 °C.     

Surface properties of self-assembled monolayer films of tetra-substituted cobalt, iron and manganese alkylthio phthalocyanine complexes

Self-assembled monolayer (SAM) films of iron (SAM-1), cobalt (SAM-2) and manganese (SAM-3) phthalocyanine complexes, tetra-substituted with diethylaminoethanethio at the non-peripheral positions, were formed on gold electrode in dimethylformamide (DMF). Electrochemical, impedimentary and surface properties of the SAM films were investigated. Cyclic voltammetry was used to investigate the electrochemical properties of the films. Ability of the films to inhibit common faradaic processes on bare gold surface (gold oxidation, solution redox chemistry of [Fe(H₂O)₆]³⁺/[Fe(H₂O)₆]²⁺ and underpotential deposition (UDP) of copper) was investigated. Electrochemical impedance spectroscopy (EIS), using [Fe(CN)₆]^3−/4− redox process as a probe, offered insights into the electrical properties of the films/electrode interfaces. Surface properties of the films were probed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The films were employed for the electrocatalytic oxidation of the pesticide, carbofuran. Electrocatalysis was evidenced from enhanced current signal and less positive oxidation potential of the pesticide on each film, relative to that observed on the bare gold electrode. Mechanism of electrocatalytic oxidation of the pesticide was studied using rotating disc electrode voltammetry.     

Effect of synthesis condition on the structural and electrochemical properties of Li[Ni1/3Mn1/3Co1/3]O2 prepared by the metal acetates decomposition method

In order to get homogeneous layered oxide Li[Ni_1/3Mn_1/3Co_1/3]O₂ as a lithium insertion positive electrode material, we applied the metal acetates decomposition method. The oxide compounds were calcined at various temperatures, which results in greater difference in morphological (shape, particle size and specific surface area) and the electrochemical (first charge profile, reversible capacity and rate capability) differences. The Li[Ni_1/3Mn_1/3Co_1/3]O₂ powders were characterized by means of X-ray diffraction (XRD), charge/discharge cycling, cyclic voltammetry and SEM. XRD experiment revealed that the layered Li[Ni_1/3Mn_1/3Co_1/3]O₂ material can be best synthesized at temperature of 800 °C. In that synthesized temperature, the sample showed high discharge capacity of 190 mAh g⁻¹ as well as stable cycling performance at a current density of 0.2 mA cm⁻² in the voltage range 2.3-4.6 V. The reversible capacity after 100 cycles is more than 190 mAh g⁻¹ at room temperature.     

Structural and electrochemical behaviors of metastable Li2/3[Ni1/3Mn2/3]O2 modified by metal element substitution

Layered metastable lithium manganese oxides, Li_2/3[Ni_1/3−xMn_2/3−yM_x+y]O₂ (x = y = 1/36 for M = Al, Co, and Fe and x = 2/36, y = 0 for M = Mg) were prepared by the ion exchange of Li for Na in P2-Na_2/3[Ni_1/3−xMn_2/3−yM_x+y]O₂ precursors. The Al and Co doping produced the T^#2 structure with the space group Cmca. On the other hand, the Fe and Mg doped samples had the O6 structure with space group R-3m. Electron diffraction revealed the 1:2 type ordering within the Ni_1/3−xMn_2/3−yM_x+yO₂ slab. It was found that the stacking sequence and electrochemical performance of the Li cells containing T^#2-Li_2/3[Ni_1/3Mn_2/3]O₂ were affected by the doping with small amounts of Al, Co, Fe, and Mg. The discharge capacity of the Al doped sample was around 200 mAh g⁻¹ in the voltage range between 2.0 and 4.7 V at the current density of 14.4 mA g⁻¹ along with a good capacity retention. Moreover, for the Al and Co doped and undoped oxides, the irreversible phase transition of the T^#2 into the O2 structure was observed during the initial lithium deintercalation.     

Characterization of high tap density Li[Ni1/3Co1/3Mn1/3]O2 cathode material synthesized via hydroxide co-precipitation

Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode material for lithium ion batteries was prepared by mixing metal hydroxide, (Ni_1/3Co_1/3Mn_1/3)(OH)₂, with 6% excess LiOH followed by calcinations. The (Ni_1/3Co_1/3Mn_1/3)(OH)₂ with secondary particle of about 12 μm was prepared by hydroxide co-precipitation. The tap density of the obtained Li[Ni_1/3Co_1/3Mn_1/3]O₂ powder was 2.56 ± 0.21 g cm⁻³. The powder was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), particle size distribution (PSD) and galvanostatic charge-discharge cycling. The XRD pattern of Li[Ni_1/3Co_1/3Mn_1/3]O₂ revealed a well ordered hexagonal layered structure with low cation mixing. Secondary particles with size of 13-14 μm and primary particles with size of about 1 μm can be identified from the SEM observations. In the voltage range of 2.8-4.3 V, the initial discharge capacity of the Li[Ni_1/3Co_1/3Mn_1/3]O₂ electrode was 166.6 mAh g⁻¹, and 96.5% of the initial capacity was retained after 50 charge-discharge cycling.     

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.     

Synthesis and characterization of ZnxMg1 − xGa2O4:Co fabricated by low-temperature burning method

A series of Zn_xMg_1 − xGa₂O₄:Co²⁺ spinels (x = 0, 0.25, 0.5, 0.75, and 1.0) was successfully produced through low-temperature burning method by using Mg(NO₃)₂·4H₂O, Zn(NO₃)₂·6H₂O, Ga(NO₃)₃·6H₂O, CO(NH₂)₂, NH₄NO₃, and Co(NO₃)₂·6H₂O as raw materials. The product was characterized by X-ray diffraction, transmission electron microscopy, and photoluminescence spectroscopy. The product was not merely a simple mixture of MgGa₂O₄ and ZnGa₂O₄; rather, it formed a solid solution. The lattice constant of Zn_xMg_1 − xGa₂O₄:Co²⁺ (0 ≤ x ≤ 1.0) crystals has a good linear relationship with the doping density, x. The synthesized products have high crystallinities with neat arrays. Based on an analysis of the form and position of the emission spectrum, the strong emission peak around the visible region (670 nm) can be attributed to the energy level transition [⁴T₁(⁴P) → ⁴A₂(⁴F)] of Co²⁺ in the tetrahedron. The weak emission peak in the near-infrared region can be attributed to the energy level transition [⁴T₁(⁴P) → ⁴T₂(⁴F)] of Co²⁺ in the tetrahedron.     

Single crystal nanobelts of V3O7·H2O: A lithium intercalation host with a large capacity

In this study, single crystal V₃O₇·H₂O nanobelts were successfully synthesized using a simple hydrothermal route, in which templates or catalysts were absent. The synthesized V₃O₇·H₂O nanobelts are highly crystalline and have lengths up to several tens of micrometers. The width and thickness of the nanobelts are found to be about 30-50 and 30 nm, respectively. A lithium battery using V₃O₇·H₂O nanobelts as the positive electrode exhibits a high initial discharge capacity of 409 mAh g⁻¹, corresponding to the formation of Li_xV₃O₇·H₂O (x = 4.32). Such a high degree of electrochemical performance is attributed to the intrinsic properties of the single-crystalline V₃O₇·H₂O nanobelts.     

Electrochemical behavior of vanadium-substituted Keggin-type polyoxometalates in aqueous solution

Cyclic voltammetry was carried out on selected vanadium-substituted Keggin polyoxometalates in aqueous solution with a focus on K₃H₂[α-SiVW₁₁O₄₀] · 6H₂O (SiVW₁₁O₄₀) and K₄H₂[γ(1, 2)-SiV₂W₁₀O₄₀] · 4H₂O (SiV₂W₁₀O₄₀). The redox waves of the V-O and W-O framework for both polyanions are pH- and scan rate-dependent. The first vanadium-centered wave (V-wave) for SiVW₁₁O₄₀ shows a classical potential shift as a function of acidity at pH < 3.5, and then becomes pH-independent above this value. Above pH 3.5, the W-centers in SiVW₁₁O₄₀ and SiV₂W₁₀O₄₀ exhibit efficient electrocatalytic activity towards the reduction of nitrite and hydrogen peroxide while the V-waves also exhibit an electrocatalytic response in nitrite-containing solutions.     

Electrochemical capacitance of nanoporous hydrous RuO2 templated by anionic surfactant

The nanoporous RuO₂·3.38H₂O was synthesized with a surfactant template using sodium dodecyl sulfate. The surface area of the material amounted to 220 m² g⁻¹ while the maximum specific capacitance obtained was 870 Fg⁻¹ at a scan rate of 10 mV s⁻¹. The specific capacitance of nanoporous RuO₂·3.38H₂O electrode exhibits enhancement, compared with other porous RuO₂ materials synthesized by different methods. The nanoporous RuO₂·3.38H₂O is a very promising material for high performance capacitance.     

Influence of magnesia on the structure and properties of MgO-Al2O3-SiO2-F glass-ceramics

Effects of MgO content (13.4–31.4 mol%) on the structure and properties of MgO-Al₂O₃-SiO₂-F⁻ glass-ceramics were investigated by differential scanning calorimetry (DSC), X-ray diffractometry (XRD), infrared spectrophotometry (IR) and scanning electron microscopy (SEM). Results show that the main units of glass network structure are [SiO₄] and [AlO₄]. MgO contributes to the weakening of silica network and reduce the stability of glass structure. The main crystals of the MgO-Al₂O₃-SiO₂-F⁻ glass-ceramics are phlogopite, spinel, flur-pargrasite and forsterite. The increase of reheating temperature and MgO content are beneficial to the separation of phlogopite crystal, and cause an higher aspect ratio of the phlogopite phase, which improves the machinability of the glass-ceramics. Excellent machinability is obtained when MgO content is 31.4 mol% at the processing temperature of 1100 °C for 2 h.     

Sol–gel approach for controllable synthesis and electrochemical properties of NiCo2O4 crystals as electrode materials for application in supercapacitors

In this work, NiCo₂O₄ coral-like porous crystals, nanoparticles and submicron-sized particles have been systematically prepared via a facile sol–gel method, using citric acid as the chelating ligand and H₂O (H₂O-DMF) as solvent. The experimental results reveal that the initial molar concentration of reactants, reaction time and solvent species involved are crucial for preparing the target products. The as-obtained samples were characterized by means of XRPD, FESEM, HRTEM, EDS and SAED techniques. Finally, the electrochemical performances of NiCo₂O₄ crystals with distinct morphologies were evaluated by cyclic voltammetry, and galvanostatic charge–discharge cycling techniques. The results show that submicron-sized NiCo₂O₄ particles exhibit the best capacitive properties with high specific capacitance and excellent cycle stability. At a high mass loading (5.6 mg cm⁻²), specific capacitance value of submicron-sized NiCo₂O₄/Ni electrode reaches as much as 217 F g⁻¹, and 96.3% of which can be still maintained after 600 charge–discharge cycles. Therefore, NiCo₂O₄ crystal is very promising for real application in supercapacitors.     

Enhanced solid-state electrogenerated chemiluminescence of Au/CdS nanocomposite and its sensing to H2O2

Au nanoparticles (AuNPs) are good quenchers once they closely contact with luminophore. Here we reported a simple approach to obtain enhanced electrogenerated chemiluminescence (ECL) behavior based on Au/CdS nanocomposite films by adjusting the amount of AuNPs in the nanocomposite. The maximum enhancement factor of about 4 was obtained at an indium tin oxide (ITO) electrode in the presence of co-reactant H₂O₂. The mechanism of this enhancement was discussed in detail. The strong ECL emission from Au/CdS nanocomposites film was exploited to determine H₂O₂. The resulting ECL biosensors showed a linear response to the concentration of H₂O₂ ranging from 1.0 × 10⁻⁸ to 6.6 × 10⁻⁴ mol L⁻¹ with a detection limit of 5 nmol L⁻¹ (S/N = 3) and good stability and reproducibility.     

Electrical conductivity and redox stability of La2Mo2−xWxO9 materials

A series of compounds La₂Mo_2−xW_xO₉ (x = 0-2) were synthesized using a freeze-dried precursor method at relatively low temperatures (673-823 K). These materials were characterised by thermogravimetric and differential thermal analysis (TG/DTA), differential scanning calorimetric (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM) and dilatometric measurements. Oxygen stoichiometry was evaluated by coulometric titration and thermogravimetric analysis at 873-1273 K. The ionic and electronic conductivities of these materials were analysed by impedance spectroscopy and a Hebb-Wagner ion-blocking method under moderately reducing conditions. The presence of W⁶⁺ leads to an increase of the stability range (about 10⁻¹⁶ Pa for La₂Mo_0.5W_1.5O₉ at 1073 K) and prevents oxygen loss and amorphisation. Within the stability range, the electronic conductivity increases gradually as the temperature increases and as the oxygen partial pressure reduces. This indicates that the electronic transport is mainly n-type as a result of the oxygen-content decreasing in the molybdate lattice. Further reduction of the oxygen partial pressure gave rise to the decomposition of La₂Mo_2−xW_xO₉, leading to the formation of new phases with molybdenum in lower oxidation states, which further enhances the electronic conductivity. The results of the coulometric titration and the thermogravimetric studies under a dry 5% H₂/Ar flow suggest that tungsten doped lanthanum molybdate materials can be used as electrolyte only at low temperature and under moderate reducing conditions.     

Effect of viscosity and applied potential on oscillations at a Pt-Pt dual-electrode in a ferricyanide system

This paper outlines the effect of viscosity and applied potential on oscillations occurring at two platinum electrodes placed proximal to each other. Potential oscillations taking place on the primary electrode (WE1) under galvanostatic control in the ferricyanide system are affected by the solution viscosity as it modifies the convective feedback mechanism necessary for oscillations. Measured transition times correlate with those calculated using Sand equation thus allowing the estimation of current density windows for periodic oscillation for different solution viscosities for pre-determined transition times. Current oscillations on the secondary working electrode (WE2) - under potentiostatic control and induced by the potential oscillations on WE1 - can be tuned through the applied potential. At higher potentials the reaction is oxidation of [Fe(CN)₆]⁴⁻ and the coupling is primarily through the transfer of [Fe(CN)₆]⁴⁻ from WE1 to WE2 via H₂ evolution whilst at more cathodic potentials the reduction of [Fe(CN)₆]³⁻ takes place at both WE1 and WE2 and the convective feedback from WE1 refreshes the surface of both electrodes simultaneously.     

Effect of Ti substitution for Nb in double perovskite-type Ba3CaNb2O9 on chemical stability and electrical conductivity

This paper reports the synthesis, structure, chemical stability and electrical transport properties of Ti substituted Ba₃CaNb₂O₉ (BCN) to develop electrolytes for proton conducting solid oxide fuel cells (H-SOFCs). The powder X-ray diffraction (PXRD) of Ba₃CaNb_2−xTi_xO_9−δ (x = 0.1, 0.15, 0.2, 0.25 and 0.3) and Ba₃Ca_1.18Nb_1.82−xTi_xO_9−δ (x = 0.15 and 0.25) showed formation of double perovskite-like structure with lattice constant comparable to that of Ba₃Ca_1.18Nb_1.82O_9−δ (BCN18). Scanning electron microscopy (SEM) showed dense and pore-free microstructure for Ba₃CaNb_1.75Ti_0.25O_8.875. PXRD and Fourier transform infrared (FTIR) spectroscopy data confirmed long-term stability of Ba₃CaNb_2−xTi_xO_9−δ and Ba₃Ca_1.18Nb_1.82−xTi_xO_9−δ in boiling H₂O and in CO₂ at elevated temperatures. The AC impedance investigation showed contribution due to bulk, grain-boundary and electrode effect at low temperatures. The electrical conductivity of studied materials were measured in different medium including dry air, dry H₂, wet H₂, wet N₂ and D₂O. Increase in conductivity in wet N₂ and decrease in conductivity in D₂O confirmed the proton conduction in Ba₃CaNb_1.75Ti_0.25O_9-δ. Among Ti-substituted compounds investigated in this study, Ba₃Ca_1.18Nb_1.57Ti_0.25O_8.605 showed the highest conductivity of 3.5 × 10⁻⁴ S cm⁻¹ at 400 °C in wet N₂ (3%H₂O), which is comparable to reported values of Ba₂Ca_0.79Nb_0.66Ta_0.55O_6−δ and BCN18.     

Synthesis of spherical Li[Ni(1/3−z)Co(1/3−z)Mn(1/3−z)Mgz]O2 as positive electrode material for lithium-ion battery

Li[Ni_(1/3−z)Co_(1/3−z)Mn_(1/3−z)Mg_z]O₂ (z = 0, 0.04) positive electrode materials were synthesized via a co-precipitation method. These materials have α-NaFeO₂ () structure, as confirmed by X-ray diffraction (XRD) studies. Cation mixing in Li layer seemed to be decreased by Mg substitution as examined by Rietveld refinements of XRD data. Spherical morphologies were observed for the as-synthesized final products by scanning electron microscopy. Their electrochemical properties during charge and discharge were discussed. When magnesium ions are substituted, the initial reversible capacity reduced. However, the substitution for Mn sites in Li[Ni_1/3Co_1/3Mn_1/3]O₂ did not decrease the capacity because Mn sites substitution did not result in loss of electroactive elements in the compound. Differential scanning calorimetric studies showed the exothermic peaks of the charged electrode Li[Ni_(1/3−z)Co_(1/3−z)Mn_(1/3−z)Mg_z]O₂ (z = 0.04) were significantly smaller than that of Li[Ni_1/3Co_1/3Mn_1/3]O₂, which means that thermal stability was greatly improved by Mg substitution even at highly delithiated state.     

An α-K3PMo3W9O40 film loaded with silver nanoparticles: Fabrication, characterization and properties

A composite film consisting of the mixed-addenda Keggin-type polyoxometalate α-K₃PMo₃W₉O₄₀ (PMo₃W₉) and silver nanoparticles (AgNPs) was fabricated on quartz, silicon, and ITO by the layer-by-layer self-assembly method. The regular growth of the multilayer film was monitored by UV–vis spectroscopy, and the morphology was measured by atomic force microscopy (AFM). The multilayer film embedded by AgNPs exhibited the photo-luminescence ascribed to electronic transitions from excited states to d levels of the silver nanoparticles. The composite film also showed electrocatalytic activity towards reduction of NO₂⁻, H₂O₂, ClO₃⁻, BrO₃⁻, and IO₃⁻ attributed to tungsten-centered and molybdenum-centered redox processes of PMo₃W₉.