Direct electrochemical reduction of Dy<Subscript>2</Subscript>O<Subscript>3</Subscript> in CaCl<Subscript>2</Subscript> melt

The electrochemical reduction of Dy₂O₃ in CaCl₂ melt was studied. The cyclic voltammetry, chronoamperometry, AC impedance and constant voltage electrolysis were employed. A single cathodic current peak in the cyclic voltammogram and one response semicircle in the AC impedance spectrum were observed, supporting a one-step electrochemical reduction mechanism of Dy₂O₃. No intermediates were observed by XRD, which confirmed the following electrochemical reduction sequence: Dy₂O₃ → Dy. The charge transfer resistances and the activation energies involved in the electrochemical reduction step of Dy₂O₃ were obtained by simulating the AC impedance spectra with equivalent circuits. The electrochemical reduction reaction of Dy₂O₃ is controlled by the charge transfer process at a low voltage range and by the diffusion process at a high voltage range.     

Study of electrochemical properties and the charge/discharge mechanism for Li<Subscript>4</Subscript>Mn<Subscript>5</Subscript>O<Subscript>12</Subscript>/MnO<Subscript>2</Subscript>-AC hybrid supercapacitor

Spinel Li₄Mn₅O₁₂ was prepared by a sol–gel method. The manganese oxide and activated carbon composite (MnO₂-AC) were prepared by a method in which KMnO₄ was reduced by activated carbon (AC). The products were characterized by XRD and FTIR. The hybrid supercapacitor was fabricated with Li₄Mn₅O₁₂ and MnO₂-AC, which were used as materials of the two electrodes. The pseudocapacitance performance of the Li₄Mn₅O₁₂/MnO₂-AC hybrid supercapacitor was studied in various aqueous electrolytes. Electrochemical properties of the Li₄Mn₅O₁₂/MnO₂-AC hybrid supercapacitor were studied by using cyclic voltammetry, electrochemical impedance measurement, and galvanostatic charge/discharge tests. The results show that the hybrid supercapacitor has electrochemical capacitance performance. The charge/discharge test showed that the specific capacitance of 51.3 F g⁻¹ was obtained within potential range of 0–1.3 V at a charge/discharge current density of 100 mA g⁻¹ in 1 mol L⁻¹ Li₂SO₄ solution. The charge/discharge mechanism of Li₄Mn₅O₁₂ and MnO₂-AC was discussed.     

Preparation and properties of Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanorods as supercapacitor material

Co₃O₄ nanorods have been successfully synthesized by thermal decomposition of the precursor prepared via a facile and efficient microwave-assisted hydrothermal method, using cetyltrimethylammonium bromide (CTAB) with ordered chain structures as soft template for the first time. The obtained Co₃O₄ was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrochemical measurements. The results demonstrate that the as-synthesized nanorods are single crystalline with an average diameter of about 20 to 50 nm and length up to several micrometers. Preliminary electrochemical studies, including cyclic voltammetry (CV), galvanostatic charge–discharge, and electrochemical impedance spectroscopy (EIS) measurements, are carried out in 6 M KOH electrolyte. Specific capacitance of 456 F g⁻¹ for a single electrode could be achieved even after 500 cycles, suggesting its potential application in electrochemical capacitors. This promising method could provide a universal green chemistry approach to synthesize other low-cost and environmentally friendly transition metal hydroxide or oxide.     

High surface area LaNiO<Subscript>3</Subscript> electrodes for oxygen electrocatalysis in alkaline media

LaNiO₃ coatings on nickel-foam supports were prepared by brush painting. The electrochemical properties were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Comparative studies were performed with LaNiO₃-pelleted electrodes. The roughness factors were determined by CV and found to be 5,208 ± 350 and 4,037 ± 250 for the pelleted and coated electrodes, respectively. EIS measurements confirm the results obtained by CV. Values lower than 0.3 were calculated for the morphology factors for both electrodes, indicating low electrochemical porosity. The experimental method used in this work to synthesise the oxide coupled with the use of Ni foam as support has proved to be very effective in producing oxide electrodes with surface areas higher than those referred to in relevant literature.     

Electrochemical performance of La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>Cr<Subscript>0.9</Subscript>M<Subscript>0.1</Subscript>O<Subscript>3</Subscript> perovskites as SOFC anodes in CO/CO<Subscript>2</Subscript> mixtures

The performance of La_0.75Sr_0.25Cr_0.9M_0.1O₃ (M = Mn, Fe, Co, and Ni) perovskitic materials as anodes was studied for a CO-fueled solid oxide fuel cell. The electrocatalytic performance and the tolerance to carbon deposition were investigated, while electrochemical characterization was carried out via AC impedance spectroscopy and cyclic voltammetry. The La_0.75Sr_0.25Cr_0.9Fe_0.1O₃ perovskite showed the best anode performance at temperatures above 900 °C; while at temperatures below 900 °C, the best performance was achieved with the La_0.75Sr_0.25Cr_0.9Co_0.1O₃ material. AC impedance spectroscopy was used for a semi-quantitative analysis of the LSC-M_0.1 anodes performance in view of total cell and charge transfer resistance. All anode materials exhibit high electronic conductivity and presumably do not substantially contribute to the overall cell resistance and concomitant ohmic losses.     

A study of the lithium intercalation into nanoparticles of LiCoO<Subscript>2</Subscript> from an aqueous solution

Nanoparticles of lithium cobalt oxide (LiCoO₂) were synthesized by means of a citrate sol–gel combustion route. The particles were characterized by scanning and transmission electron microscopies (SEM and TEM), energy-dispersive X-ray spectroscopy, and X-ray diffraction (XRD) measurements. Near spherical nanoparticles of around 100 nm were observed in SEM and TEM micrographs. XRD data indicated that the as-prepared nanoparticles presented pure phase of LiCoO₂ with R-3m symmetry. The kinetics of electrochemical intercalation of lithium into the nanoparticles were investigated by means of cyclic voltammetry (CV), chronoamperometry, and electrochemical impedance spectroscopy (EIS) with special emphasis on the application potential as cathode material for aqueous rechargeable lithium batteries. CV studies of the nanoparticles at slow scan rate of 0.1 mV s⁻¹ between 600 and 820 mV versus Ag/AgCl, demonstrated that the nanoparticles represented well-defined reversible peaks. The non-linear chemical diffusion of lithium into the nanoparticles was explored by EIS. In this regards, the results were discussed based on an equivalent circuit, distinguishing the kinetic properties of lithium intercalation. The kinetic parameters of lithium intercalation were obtained using the equivalent circuit, which were in good agreement with the experimental results. The changes of kinetic parameters of lithium intercalation with potential were also discussed in detail.     

Improved performances of mechanical-activated LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript>/MWNTs cathode for aqueous rechargeable lithium batteries

LiMn₂O₄/multi-walled carbon nanotubes (MWNTs) composite was synthesized by mechanical activation reaction followed by a heat-treatment (500 °C). The LiMn₂O₄ and LiMn₂O₄/MWNTs as cathodes were investigated in 1 M Li₂SO₄ by cyclic voltammetry (CV), galvanostatic charge/discharge (GC), and electrochemical impedance spectroscopy (EIS). The LiMn₂O₄/MWNTs cathode delivered higher discharge capacity (117 mAh g⁻¹) than LiMn₂O₄ (84.6 mAh g⁻¹). Furthermore, the results from EIS showed that LiMn₂O₄/MWNTs had a faster kinetic process for lithium ion intercalation/de-intercalation than LiMn₂O₄. Besides, LiMn₂O₄/MWNTs had better cycling stability and rate capability than LiMn₂O₄, which was confirmed by GC testing. SEM images showed that a three-dimensional network structure was formed during the mechanical activation, giving a decrease of particle size.     

Electrochemical behavior of K<Subscript>4</Subscript>Fe(CN)<Subscript>6</Subscript> in [bmim]PF<Subscript>6</Subscript>/TX-100/H<Subscript>2</Subscript>O based microemulsions

The electrochemical behavior of potassium ferrocyanide [K₄Fe(CN)₆] at Pt/ionic liquid (IL) microemulsion interfaces was investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). H₂O/TX-100/bmimPF₆ was used to prepare three IL microemulsions: water in 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF₆) (W/IL), bicontinuous (WIL) and bmimPF₆ in water (IL/W). The results show that the IL microemulsion systems have relatively narrower potential windows compared with the pure IL system. The redox potential gap is about 100 mV in the pure water and the three IL microemulsions. The redox potentials of K₄Fe(CN)₆/K₃Fe(CN)₆ and the redox peak currents decrease in the order pure water, IL/W, WIL, W/IL. Furthermore, the peak currents increase linearly with the square root of the scan rate, while the diffusion coefficient increased in the order W/IL, WIL, IL/W. The Nyquist plots obtained in the WIL and IL/W systems show capacitive resistance arcs at high frequencies and 45° straight lines at low frequencies, implying that the electrochemical reactions are controlled by charge transfer and diffusion steps. For the W/IL system there is only a 45° straight line in the Nyquist plot, indicating that diffusion is the controlling step at all frequencies.     

Electrochemical properties of microwave-assisted reflux-synthesized Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles in different electrolytes for supercapacitor applications

The nanosized Mn₃O₄ particles were prepared by microwave-assisted reflux synthesis method. The prepared sample was characterized using various techniques such as X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FT-IR), Raman analysis, and transmission electron microscopy (TEM). Electrochemical properties of Mn₃O₄ nanoparticles were investigated using cyclic voltammogram (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge analysis in different electrolytes such as 1 M KCl, 1 M Na₂SO₄, 1 M NaNO₃, and 6 M KOH electrolytes. XRD pattern reveals the formation of single-phase Mn₃O₄ nanoparticles. The FT-IR and Raman analysis also assert the formation of Mn₃O₄ nanoparticles. The TEM image shows the spherical shape particles with less than 50 nm sizes. Among all the electrolytes, the Mn₃O₄ nanoparticles possess maximum specific capacitance of 94 F g⁻¹ in 6 M KOH electrolyte calculated from CV. The order of capacitance obtained by various electrolytes is 6 M KOH > 1 M KCl > 1 M NaNO₃ > 1 M Na₂SO₄. The EIS and galvanostatic charge–discharge results further substantiate with the CV results. The cycling stability of Mn₃O₄ electrode reveals that the prepared Mn₃O₄ nanoparticles are a suitable electrode material for supercapacitor application.     

Voltammetric behavior and the determination of quercetin at a flowerlike Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles modified glassy carbon electrode

Flowerlike Co₃O₄ nanoparticles were used as a modifier on the glassy carbon electrode to fabricate a quercetin (Qu) sensor. The morphology and crystallinity of the prepared Co₃O₄ material were investigated by scanning electron microscopy and X-ray diffraction. Electrochemical behavior of Qu at the sensor was studied by cyclic voltammetry and semi-derivative voltammetry. Results suggested that the modified electrode exhibited a strong electrocatalytic activity toward the redox of Qu. The electron transfer coefficient (α), the number of electron transfer (n), and the diffusion coefficient (D) of Qu at the sensor were calculated. Under the optimum conditions, the catalytic peak currents of Qu were linearly dependent on the concentrations of Qu in the range from 5.0 × 10⁻⁷ to 3.3 × 10⁻⁴ M, with a detection limit of 1.0 × 10⁻⁷ M. This proposed method was successfully applied to determine the quercetin concentration in Ginkgo leaf tablet and human urine samples.     

Preparation and Characterization of LiNi<Subscript>0.8</Subscript>Co<Subscript>0.2</Subscript>O<Subscript>2</Subscript> Synthesized by an Emulsion Drying Method

A layered LiNi_0.8Co_0.2O₂ solid solution, which is a promising cathode material for secondary lithium batteries, was successfully synthesized by an emulsion drying method. Because electrochemical properties significantly depend on the conditions of the synthesis, the calcination temperature was carefully determined on the basis of X-ray diffraction and TG studies. The prepared cathodes were characterized by means of SEM, BET, X-ray diffraction, Rietveld refinement, cyclic voltammetry and a charge-discharge experiment. From the Rietveld analysis, it was found that powder calcined at 800 °C for 12 h exhibits a well ordered and lower cation mixed layered structure than the others. The cyclic voltammetry experiment shows that phase transformation can be suppressed considerably by increasing the calcination temperature to 800 °C. The highest discharge capacity of 188.4 mA h g⁻¹ was obtained from the sample prepared at 800 °C. Furthermore, a high capacity retention ratio of 88.1% was found for the initial value after 50 cycles at a constant current density of 40 mA g⁻¹ between 2.7 V_Li/Li⁺ and 4.3 V_Li/Li⁺. In the rate capability test, the cathode delivered a higher discharge capacity of 153.1 mA h g⁻¹ at a 4 C (800 mA g⁻¹) rate.     

Synthesis and characterization of palladium nanoparticles immobilized on ZrO<Subscript>2</Subscript> nanotubes as a new highly active electrode for methanol electro-oxidation

Palladium nanoparticles/zirconium oxide nanotubes/zirconium (Pd-NPs/ZrO₂-NTs/Zr) electrodes were prepared by electroless plating method. ZrO₂-NTs consisting of individual nanotubes of about 60–80 nm diameters were fabricated by anodizing Zr foil in electrolyte solution containing dimethylformamide, glycerol, water and ammonium fluoride. The morphology and surface characteristics of ZrO₂-NTs and Pd-NPs/ZrO₂-NTs/Zr electrodes studied using scanning electron microscopy as well as energy dispersive X-ray spectroscopy. The results revealed that Pd-NPs were homogeneously deposited on the surface of ZrO₂-NTs. Moreover, electrocatalytic properties of Pd-NPs/ZrO₂-NTs/Zr and flat Pd electrodes toward methanol oxidation were investigated using cyclic voltammetry, electrochemical impedance spectroscopy and chronoamperometry. The results indicated that Pd-NPs/ZrO₂-NTs/Zr electrode represents an enhanced electrocatalytic activity and better stability for methanol oxidation compared to flat Pd. Thus, the Pd-NPs/ZrO₂-NTs/Zr catalyst is expected to be a promising electrode material for direct methanol fuel cells. Additionally, the effects of scan rate, methanol concentration and temperature dependency of methanol oxidation on Pd-NPs/ZrO₂-NTs/Zr electrode have been explored in this study.     

Electrochemical synthesis of ammonia from water and nitrogen catalyzed by nano-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> and CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> suspended in a molten LiCl-KCl-CsCl electrolyte

Nano-Fe₂O₃ and CoFe₂O₄ were suspended in molten salt of alkali-metal chloride (LiCl-KCl-CsCl) and their catalytic activity in electrochemical ammonia synthesis was evaluated from potentiostatic electrolysis at 600 K. The presence of nanoparticle suspension in the molten chloride resulted in improved production of NH₃, recording NH₃ synthesis rate of 1.78×10^?10 mol s^?1 cm^?2 and 3.00×10^?10 mol s^?1 cm^?2 with CoFe₂O₄ and Fe₂O₃, which are 102% and 240% higher than that without the use of a nanocatalyst, respectively. We speculated that the nanoparticles triggered both the electrochemical reduction of nitrogen and also chemical reaction between nitrogen and hydrogen that was produced from water electro-reduction on cathode. The use of nanocatalysts in the form of suspension offers an effective way to overcome the sluggish nature of nitrogen reduction in the molten chloride electrolyte.     

Gd<Superscript>3+</Superscript> doped Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles with proper magnetic and supercapacitive characteristics: A novel synthesis platform and characterization

A novel electrochemical procedure was developed for the facile preparation of Gd-doped iron oxide nanoparticles (GdIO-NPs). A simple galvanostatic deposition (i=10 mA cm^-2) was done in an additive-free aqueous solution containing FeCl₂·4H₂O, Fe(NO₃)₃·9H₂O and GdCl₃·6H₂O. The XRD, FE-SEM, EDS and TEM characterizations showed that the product is composed of 15% GdIO-NPs with 10 nm in size. VSM analysis proved that the GdIO-NPs are superparamagnetic. The cyclic voltammetry and charge-discharge tests showed that the prepared GdIO-NPs are capable to deliver specific capacity as high as 190.1 F g^-1 at 0.5A g^-1 and capacity retention of 95.1% after 2000 cycling. Based on the results, it was concluded that the developed electrochemical strategy acts as an efficient procedure for the preparation of lanthanide doped MNPs with proper magnetic and supercapacitive characters.     

The enhancement effect of phosphomolybdic acid (H<Subscript>3</Subscript>PMo<Subscript>12</Subscript>O<Subscript>40</Subscript>) on Pd/C catalyst for the electroreduction of hydrogen peroxide

A Pd/C electrode modified by H₃PMo₁₂O₄₀ was prepared and its catalytic performance for H₂O₂ electroreduction in acidic medium was investigated by cyclic voltammograms. Pd nanoparticles supported on Vulcan XC-72 carbon were prepared by chemical reduction of PdCl₂ in aqueous solution using NaBH₄ as the reducing agent. X-ray diffraction analysis indicated that the particle size of Pd is around 9.7 nm. The modified electrode was prepared by cyclic voltammograms in H₂SO₄ solution containing H₃PMo₁₂O₄₀. The results showed that H₃PMo₁₂O₄₀ can efficiently enhance the electrocatalytic activity for H₂O₂ electroreduction on Pd/C. The effect of H₃PMo₁₂O₄₀ content on the electrocatalytic activity of the catalyst was also investigated by CV. The best results appeared at the concentration of H₃PMo₁₂O₄₀ = 0.5 mmol L⁻¹.     

Facile synthesis of nano-Li<Subscript>4</Subscript> Ti<Subscript>5</Subscript>O<Subscript>12</Subscript> for high-rate Li-ion battery anodes

One of the most promising anode materials for Li-ion batteries, Li₄Ti₅O₁₂, has attracted attention because it is a zero-strain Li insertion host having a stable insertion potential. In this study, we suggest two different synthetic processes to prepare Li₄Ti₅O₁₂ using anatase TiO₂ nanoprecursors. TiO₂ powders, which have extraordinarily large surface areas of more than 250 m² g^-1, were initially prepared through the urea-forced hydrolysis/precipitation route below 100°C. For the synthesis of Li₄Ti₅O₁₂, LiOH and Li₂CO₃ were added to TiO₂ solutions prepared in water and ethanol media, respectively. The powders were subsequently dried and calcined at various temperatures. The phase and morphological transitions from TiO₂ to Li₄Ti₅O₁₂ were characterized using X-ray powder diffraction and transmission electron microscopy. The electrochemical performance of nanosized Li₄Ti₅O₁₂ was evaluated in detail by cyclic voltammetry and galvanostatic cycling. Furthermore, the high-rate performance and long-term cycle stability of Li₄Ti₅O₁₂ anodes for use in Li-ion batteries were discussed.     

Ni<Superscript>2+</Superscript> reduction under solar irradiation over CuFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/TiO<Subscript>2</Subscript> catalysts

The photo-electrochemical characterization of the hetero-system CoFe₂O₄/TiO₂ was undertaken for the Ni²⁺ reduction under solar light. The spinel CoFe₂O₄ was prepared by nitrate route at 940 °C and the optical gap (1.66 eV) was well matched to the sun spectrum. The flat band potential (-0.21 V_SCE) is more cathodic than the potential of Ni²⁺/Ni couple (-0.6 V_SCE), thus leading to a feasible nickel photoreduction. TiO₂ with a gap of 3.2 eV is used to mediate the electrons transfer. The reaction is achieved in batch configuration and is optimized with respect to Ni²⁺ concentration (30 ppm); a reduction percentage of 72% is obtained under sunlight, the Ni²⁺ reduction is strongly enhanced and follows a first order kinetic with a rate constant of 4.6×10^-2 min^-1 according to the Langmuir-Hinshelwood model.     

Synthesis,structural, and electrochemical performance of V<Subscript>2</Subscript>O<Subscript>5</Subscript> nanotubes as cathode material for lithium battery

Various vanadium oxide nanostructures are currently drawn interest for the potential applications of Li batteries, super capacitors, and electrochromic display devices. In this article, the synthesis of V₂O₅ nanotubes by hydrothermal method using 1-hexadecylamine (HDA) and PEO as a template and surface reactant were reported, respectively. The structural properties and electrochemical performances of these nanostructures were investigated for the application of Li batteries. Structure and morphology of the samples were investigated by XRD, FTIR, SEM, and TEM analysis. The battery with V₂O₅ nanotubes electrode showed initial specific capacity of 185 mAhg⁻¹, whereas the PEO surfactant V₂O₅ nanotubes exhibited 142 mAhg⁻¹. It was found that PEO surfactant V₂O₅ nanotubes material showed less specific capacity at initial stages but better stability was exhibited at higher cycle numbers when compared to that of V₂O₅ nanotubes. The cyclic performance of the PEO surfactant material seems to be improved with the role of polymeric component due to its surface reaction with V₂O₅ nanotubes during the hydrothermal process.     

Proton diffusivity in the BaZr<Subscript>0.9</Subscript>Y<Subscript>0.1</Subscript>O<Subscript>3−δ</Subscript> proton conductor

The thermally activated proton diffusion in BaZr_0.9Y_0.1O_3−δ was studied with electrochemical impedance spectroscopy (IS) and quasi-elastic neutron scattering (QENS) in the temperature range 300–900 K. The diffusivities for the bulk material and the grain boundaries as obtained by IS obey an Arrhenius law with activation energies of 0.46 eV and 1.21 eV, respectively. The activation energies obtained by IS for the bulk are 0.26 eV above 700 K and 0.46 eV, below 700 K. The total diffusivity as obtained by IS is by one order of magnitude lower than the microscopic diffusivity as obtained by QENS. The activation energies obtained by QENS are 0.13 eV above 700 K and 0.04 eV, below 700 K. At about 700 K, the diffusion constants for IS and QENS have a remarkable crossover, suggesting two processes with different activation energies.     

Low-Temperature Preparation of Superparamagnetic CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Microspheres with High Saturation Magnetization

Based on a low-temperature route, monodispersed CoFe₂O₄ microspheres (MSs) were fabricated through aggregation of primary nanoparticles. The microstructural and magnetic characteristics of the as-prepared MSs were characterized by X-ray diffraction/photoelectron spectroscopy, scanning/transmitting electron microscopy, and vibrating sample magnetometer. The results indicate that the diameters of CoFe₂O₄ MSs with narrow size distribution can be tuned from over 200 to ~330 nm. Magnetic measurements reveal these MSs exhibit superparamagnetic behavior at room temperature with high saturation magnetization. Furthermore, the mechanism of formation of the monodispersed CoFe₂O₄ MSs was discussed on the basis of time-dependent experiments, in which hydrophilic PVP plays a crucial role.