Study of IrxRu1−xO2 oxides as anodic electrocatalysts for solid polymer electrolyte water electrolysis

Electrocatalysts of the general formula Ir_xRu_1−xO₂ were prepared using Adams’ fusion method. The crystallite characterization was examined via XRD, and the electrochemical properties were examined via cyclic voltammetry (CV) in, linear sweep voltammetry (LSV) and chronopotentiometry measurements in 0.5 M H₂SO₄. The electrocatalysts were applied to a membrane electrode assembly (MEA) and studied in situ in an electrolysis cell through electrochemical impedance spectroscopy (EIS) and stationary current density–potential relations were investigated. The Ir_xRu_1−xO₂ (x = 0.2, 0.4, 0.6) compounds were found to be more active than pure IrO₂ and more stable than pure RuO₂. The most active electrocatalyst obtained had a composition of Ir_0.2Ru_0.8O₂. With an Ir_0.2Ru_0.8O₂ anode, a 28.4% Pt/C cathode and the total noble metal loading of 1.7 mg cm⁻², the potential of water electrolysis was 1.622 V at 1 A cm⁻² and 80 °C.     

Highly oriented diamond growth on SixGe1−x (100) thin films

Highly oriented (100) diamond films have been successfully grown on Si_xGe_1−x (100) thin films by bias enhanced nucleation (BEN) in microwave plasma chemical vapor deposition (MPCVD) system. Raman spectra show the 1332 cm⁻¹ peak which proves the formation of diamond. Diamond nucleation density on Si_xGe_1−x substrate estimated by scanning electron microscopy is higher than 10⁹ cm⁻². The interface between diamond and Si_xGe_1−x substrate was characterized by transmission electron microscopy (TEM). About 20 nm decrease in thickness of the Si_xGe_1−x film was observed after bias enhanced nucleation step. TEM shows the existence of silicon carbide and heteroepitaxial diamond grains grown on Si_xGe_1−x substrate. Characterization from high-resolution TEM on the specimen of short time deposition reveals that a number of epitaxial diamond grains were directly nucleated on Si_xGe_1−x with {111} interplanar spacing ratio of diamond and Si_xGe_1−x of 2:3. The diamond nucleation is found to be preferred on the ridge position of the rough substrate surface. Diamond {100} facets were quickly developed in the early stage of growth.     

Structure and magnetic properties of Fe1−xCox nanowires in self-assembled arrays

Fe_1−xCo_x nanowires in self-assembled arrays with varying compositions were produced by the template-assisted pulsed electrochemical deposition method. The structural and magnetic properties of the arrays were investigated using several experimental techniques. TEM analyses indicated that the nanowires were regular, uniform, 8 μm in length and 50 nm in diameter. The results of X-ray diffraction indicated that the body-centered-cubic (bcc) (α), face-centered-cubic (fcc) (γ), and hexagonal-close-packed (hcp) () Fe–Co phases appeared in different compositions. Magnetic measurements showed that the coercivity and squareness of the hysteresis loops of the Fe_1−xCo_x changed with their compositions, which may be attributable to shape anisotropy. The room temperature ⁵⁷Fe Mössbauer spectra of the arrays of the Fe_1−xCo_x nanowires revealed strong shape anisotropy.     

Study of copper foam-supported Sn thin film as a high-capacity anode for lithium-ion batteries

Three-dimensional porous Sn thin film electrodes were prepared by electroless deposition on copper foam, then its morphology and electrochemical property were studied by means of scanning electron microscope (SEM), X-ray diffraction (XRD), electrochemical cycling test and cyclic voltammetry (CV). The porous framework and micro-holes have shown a great structure advantage in restricting severe volume changes when the Sn thin film was employed as anode for lithium-ion battery. The film electrode of sample C with an initial capacity of 676 mAh g⁻¹ showed good cycle performance displayed by retaining a capacity of 313 mAh g⁻¹ after 100 cycles.     

Thin-film CoB catalyst templates for the hydrolysis of NaBH4 solution for hydrogen generation

Thin-film CoB alloy catalysts were prepared on Ni-foam substrates using electroless as well as electroplating techniques. Electroless plating was carried out using cobalt (II) sulfate as the source of Co²⁺, sodium succinate as the complexing agent, and dimethyamine borane as the source of boron as well as the reducing agent. Electroplating was carried out using cobalt (II) sulfate and cobalt (II) chloride as the sources of cobalt, and boric acid as the source of boron. The thin-film CoB/Ni-foam templates were characterized using ICP, XRD and SEM techniques. The normalized B content was in the range of 1.0–1.30 and 0.20–0.60 wt.% for electroless and electroplated templates, respectively. The B content is less than that required for stoichiometric alloy formation, which indicates the simultaneous deposition of the Co metal along with CoB alloy. An optimum condition of 0.100 M L⁻¹ each of cobalt (II) sulfate heptahydrate Co(SO₄)·7H₂O, sodium succinate (Na₂C₄H₄O₄) and dimethylamine borane (CH₃)₂NHBH₃, at 60 °C with the pH value of 4–5 and a plating time of 1 h was identified for the preparation of the catalyst templates by electroless plating. Where as, 0.125 M L⁻¹ each of cobalt (II) chloride hexahydrate (CoCl₂·6H₂O), Co(SO₄)·7H₂O, 0.125 M L⁻¹ of boric acid at the current density range of 160–320 mA cm⁻² and a temperature of 60 °C was identified as the optimum condition for the electroplating method. Maximum H₂ generation rates of 1.64 and 0.30 L min⁻¹ g⁻¹ of catalyst were obtained with electroless and electroplated thin-film CoB/Ni-foam templates, respectively. The suitability of the electroless plated CoB/Ni-foam catalyst template for extended duration of hydrogen generation from NaBH₄ was studied up to 60 h. Activation energies of 44.47 and 54.89 kJ mol⁻¹ were calculated for electroless and electroplated CoB/Ni-foam catalyst templates, respectively.     

Carbon monoxide oxidation on Au(1 1 1) surface decorated by spontaneously deposited Pt

Platinum is deposited spontaneously on Au(1 1 1) surface from 1 mM H₂PtCl₆ + 1 M HClO₄ solution using multiple deposition procedure. X-ray photoelectron spectroscopy (XPS) analysis has shown that after immersion into the Pt containing solution and rinsing with water, Pt(OH)₂ resides on the Au(1 1 1) substrate. Consecutive depositions as well as in situ scanning tunneling microscopy (STM) and electrochemical measurements are performed on previously electrochemically reduced Pt/Au(1 1 1) surfaces. Only homogeneous distribution of thus deposited Pt islands is observed by in situ STM. With subsequent depositions, the width of deposited Pt islands increases, but stays lower than 10 nm, while a significant increase of Pt islands height is observed, leading to moderate increase of the coverage. Cyclic voltammetry (CV) profiles of obtained Pt/Au(1 1 1) surfaces, and CO stripping curves are recorded in 0.5 M H₂SO₄ solution. CO oxidation takes place only at higher potentials shifting negatively with increasing coverage. This is discussed with respect to Pt islands width and height distributions and to the influence of the Au(1 1 1) substrate surface.     

A mechanistic investigation of di-tert-butyl nitroxide using scanning electrochemical microscopy (SECM)

Cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) have been used to evaluate the electron-transfer mechanism of di-tert-butyl nitroxide (DTBN) in acetonitrile. The oxidation of DTBN is coupled to a rapid, irreversible chemical follow-up step that is difficult to characterize quantitatively with CV due to distortion of the voltammograms by solution resistance and mixed radial–linear diffusion within the scan rate region of interest. Collection efficiencies from the tip generation substrate collection (TGSC) mode of SECM were used to determine a rate constant of 21 s⁻¹ for the follow-up reaction. Collection efficiency versus distance plots obtained at 5 and 50 mM DTBN concentration are identical, confirming the first-order nature of the chemical reaction. Numerical simulation of linear scan voltammograms obtained at different tip/substrate distances provides a heterogeneous electron-transfer rate constant of 0.85 cm s⁻¹.     

High-performance Li4Ti5−xVxO12 (0 ≤ x ≤ 0.3) as an anode material for secondary lithium-ion battery

Powders of spinel Li₄Ti_5−xV_xO₁₂ (0 ≤ x ≤ 0.3) were successfully synthesized by solid-state method. The structure and properties of Li₄Ti_5−xV_xO₁₂ (0 ≤ x ≤ 0.3) were examined by X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electronic microscope (SEM), galvanostatic charge–discharge test and cyclic voltammetry (CV). XRD shows that the V⁵⁺ can partially replace Ti⁴⁺ and Li⁺ in the spinel and the doping V⁵⁺ ion does almost not affect the lattice parameter of Li₄Ti₅O₁₂. Raman spectra indicate that the Raman bands corresponding to the Li–O and Ti–O vibrations have a blue shift due to the doping vanadium ions, respectively. SEM exhibits that Li₄Ti_5−xV_xO₁₂ (0.05 ≤ x ≤ 0.25) samples have a relative uniform morphology with narrow size distribution. Charge–discharge test reveals that Li₄Ti_4.95V_0.05O₁₂ has the highest initial discharge capacity and cycling performance among all samples cycled between 1.0 and 2.0 V; Li₄Ti_4.9V_0.1O₁₂ has the highest initial discharge capacity and cycling performance among all samples cycled between 0.0 and 2.0 V or between 0.5 and 2.0 V. This excellent cycling capability is mainly due to the doping vanadium. CV reveals that electrolyte starts to decompose irreversibly below 1.0 V, and SEI film of Li₄Ti₅O₁₂ was formed at 0.7 V in the first discharge process; the Li₄Ti_4.9V_0.1O₁₂ sample has a good reversibility and its structure is very advantageous for the transportation of lithium-ions.     

In situ activated 3,5-dinitrobenzoic acid covalent attached to nanostructured platform for NADH electrooxidation

The activation of 3,5-dinitrobenzoic acid (3,5-DNBA) in situ is proposed for electrocatalytic detection of NADH at low overpotentials. Thus, 3,5-DNBA was strategically connected to a nanostructured surface based on MWCNTs and PEI (an amine rich polymer). Then, R–NO/R–NHOH was electrogenerated in situ by cycling the potential between 0.15 and −0.55 V versus SCE. The formation of an intermediate charge transfer complex is proposed for the reaction between NADH and redox mediator. The apparent Michaelis–Menten constant (K_M) obtained by RDE voltammetry and amperometry was in good agreement, around 10⁻⁵ mol L⁻¹. The k_obs for the catalytic reaction evaluated by chronoamperometry and RDE voltammetry, was in good agreement and was found to be around 10⁴ L mol⁻¹ s⁻¹. The sensor provides a linear response range for NADH from 4.0 up to 42.0 μmol L⁻¹ with a sensitivity of 71 nA L μmol⁻¹, detection and quantification limits of 1.2 and 4.0 μmol L⁻¹ respectively, with a response time of 0.16 s.     

Surface properties and catalytic performance of La1−xSrxCrO3 perovskite-type oxides for CO and C3H6 combustion

Catalytic CO oxidation and C₃H₆ combustion have been studied over La_1−xSr_xCrO₃ (x = 0.0–0.3) oxides prepared by solid-state reaction and characterised by X-ray diffraction (XRD), nitrogen adsorption (BET analysis) and X-ray photoelectron spectroscopy (XPS). The expected orthorhombic perovskite structure of the chromite is observed for all levels of substitution. However, surface segregation of strontium along with a chromium oxidation process, leading to formation of Cr⁶⁺-containing phases, is produced upon increasing x and shown to be detrimental to the catalytic activity. Maximum activity is achieved for the catalyst with x = 0.1 in which mixed oxide formation upon substitution of lanthanum by strontium in the chromite becomes maximised.     

Application of high-energy X-rays and Pair-Distribution-Function analysis to nano-scale structural studies in catalysis

We investigate the structure of supported Pt catalysts using high-energy X-ray scattering coupled with Pair-Distribution-Function (PDF) analysis. Recently, experimental approaches that enable the collection of PDF data in situ have been developed with time-resolution sufficient to study the structure of Pt nano-particles as they form. The differential PDF approach is utilized which allows the atom–atom correlations involving only Pt to be selectively recovered, enabling structural investigation of the supported particles and the mechanism of their formation. In parallel to the in situ analysis, we have examined samples prepared ex situ. Data collected on the ex situ samples show that the initial deposition of Pt⁴⁺ occurs as the PtCl₆²⁻ species which are retained even when annealed in an oxygen atmosphere. The Pt differential PDFs of the samples reduced in hydrogen at 200 and 500 °C indicated nano-crystalline face-centered-cubic (fcc) metallic Pt particles. The ex situ reduced samples also contain a weak correlations at 2.1 Å, which we assign to Pt–O interactions between the particles and the support surface. The in situ experiments, following the reduction of Pt⁴⁺ from 0 to 227 °C, indicate that the initial Pt nano-particles formed are ca. 1 nm in size, and become larger and more crystalline by 200 °C. The data suggest a particle growth mechanism where the initial particles that form are small (<1 nm), then agglomerate into ensembles of many small particles and lastly anneal to form larger well-ordered particles. Lastly, we discus potential future developments in operando PDF studies, and identify opportunities for synchronous application of complementary methods.     

Decolorization of methylene blue by heterogeneous Fenton reaction using Fe3−xTixO4 (0 ≤ x ≤ 0.78) at neutral pH values

In this work, a series of Fe_3−xTi_xO₄ (0 ≤ x ≤ 0.78) was synthesized using a new soft chemical method. The synthetic Fe_3−xTi_xO₄ were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Mössbauer spectroscopy, thermogravimetric and differential scanning calorimetry (TG–DSC) analyses. The results showed that they were spinel structures and Ti was introduced into their structures.Then, decolorization of methylene blue (MB) by Fe_3−xTi_xO₄ in the presence of H₂O₂ at neutral pH values was studied using UV–vis spectra, dissolved organic carbon (DOC) and element C analyses. Furthermore, the degradation products remained in reaction solution after the decolorization were identified using ionic chromatography (IC), ¹³C nuclear magnetic resonance spectra (NMR), liquid chromatography and mass spectrometry (LC–MS). Although small amounts of MB were mineralized, the aromatic rings in MB were destroyed completely after the decolorization. Decolorization of MB by Fe_3−xTi_xO₄ in the presence of H₂O₂ was promoted remarkably with the increase of Ti content in Fe_3−xTi_xO₄ due to the enhancement of both adsorption and degradation of MB on Fe_3−xTi_xO₄.     

La(1−x)SrxCo1−yFeyO3 perovskites prepared by sol–gel method: Characterization and relationships with catalytic properties for total oxidation of toluene

La_(1−x)Sr_xCo_(1−y)Fe_yO₃ samples have been prepared by sol–gel method using EDTA and citric acid as complexing agents. For the first time, Raman mappings were achieved on this type of samples especially to look for traces of Co₃O₄ that can be present as additional phase and not detect by XRD. The prepared samples were pure perovskites with good structural homogeneity. All these perovskites were very active for total oxidation of toluene above 200 °C. The ageing procedure used indicated good thermal stability of the samples. A strong improvement of catalytic properties was obtained substituting 30% of La³⁺ by Sr²⁺ cations and a slight additional improvement was observed substituting 20% of cobalt by iron. Hence, the optimized composition was La_0.7Sr_0.3Co_0.8Fe_0.2O₃. The samples were also characterized by BET measurements, SEM and XRD techniques. Iron oxidation states were determined by Mössbauer spectroscopy. Cobalt oxidation states and the amount of O⁻ electrophilic species were analyzed from XPS achieved after treatment without re-exposition to ambient air. Textural characterization revealed a strong increase in the specific surface area and a complete change of the shape of primary particles substituting La³⁺ by Sr²⁺. The strong lowering of the temperature at conversion 20% for the La_0.7Sr_0.3Co_(1−y)Fe_yO₃ samples can be explained by these changes. X photoelectron spectra obtained with our procedure evidenced very high amount of O⁻ electrophilic species for the La_0.7Sr_0.3Co_(1−y)Fe_yO₃ samples. These species able to activate hydrocarbons could be the active sites. The partial substitution of cobalt by iron has only a limited effect on the textural properties and the amount of O⁻ species. However, Raman spectroscopy revealed a strong dynamic structural distortion by Jahn–Teller effect and Mössbauer spectroscopy evidenced the presence of Fe⁴⁺ cations in the iron containing samples. These structural modifications could improve the reactivity of the active sites explaining the better specific activity rate of the La_0.7Sr_0.3Co_0.8Fe_0.2O₃ sample. Finally, an additional improvement of catalytic properties was obtained by the addition of 5% of cobalt cations in the solution of preparation. As evidenced by Raman mappings and TEM images, this method of preparation allowed to well-dispersed small Co₃O₄ particles that are very efficient for total oxidation of toluene with good thermal stability contrary to bulk Co₃O₄.     

EQCM study of electrodeposited PbO2: Investigation of the gel formation and discharge mechanisms

Lead dioxide thin films were electrodeposited on gold substrates and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mass change occurring upon immersion in a H₂SO₄ electrolyte and during electrochemical reduction was observed in situ by electrochemical quartz crystal microbalance (EQCM). A hydrated PbO₂ gel-type layer is formed at the surface of electrodeposited PbO₂. The concentration of the H₂SO₄ electrolyte does not affect the composition of the gel nor the amount of lead dioxide involved in the hydration process. It is established that 1.3 × 10⁻⁷ mol cm⁻² of β-PbO₂ are hydrated at the surface of an electrodeposited film and that the hydration reaction occurs according to the following reaction: PbO₂^(crystal) + xH₂O ↔ (PbO(OH)₂·(x − 1)H₂O)^(gel), where x = 8.1. The mass change occurring during the first and subsequent discharge of PbO₂ was recorded. It is shown that both PbO₂^(crystal) and PbO(OH)₂·(7.1)H₂O)^(gel) are reduced to PbSO₄ during the first discharge.     

Synthesis of LiFePO4/carbon composite from nano-FePO4 by a novel stearic acid assisted rheological phase method

LiFePO₄/carbon composite was synthesized at 600 °C for 4 h in an Ar atmosphere by a stearic acid assisted rheological phase method using amorphous nano-FePO₄ as the iron source. XRD, SEM and TEM observations show that the LiFePO₄/carbon composite has good crystallinity, ultrafine and well-dispersed particles of 60–150 nm size and in situ carbon coated on the surface of LiFePO₄ crystallites. The synthesized LiFePO₄/carbon composite shows a high discharge capacity of 160 mAh g⁻¹ and 155 mAh g⁻¹ at rates of 0.5 C and 1 C, respectively. Even at a high current density of 30 C, the material still presents a discharge capacity of 93 mAh g⁻¹ and exhibits an excellent cycling performance.     

Formation of linearly linked Fe clusters on Si(111)-7?×?7-C2H5OH surface

The Fe atoms were deposited on the Si(111)-7 × 7 surface, which has been saturated with the C₂H₅OH molecules. Then, the Fe clusters were formed on Si(111)-7 × 7-C₂H₅OH surface and in situ observed by the scanning tunneling microscopy (STM). The STM images showed that with the increase of Fe clusters, the size of clusters was about 5 nm and they self-assembled in straightly linked chain crossing the step to lower or upper terrace. X-ray photoelectron spectroscopy (XPS) was in situ carried out on the surface of Fe/Si(111)-7 × 7-C₂H₅OH samples before and after the introduction of thin air (4.5 × 10^-2 Langmuir) into the STM chamber. The XPS results showed that the Fe clusters are stable in the abovementioned thin air condition at room temperature. Based on the STM and XPS results, the driving force making one-dimensional straightly linked chain structure might be the magnetic force of the Fe clusters. The formation of straightly linked Fe clusters chains suggests the formation of single magnetic domain Fe clusters.

PACS

07.79.Cz, 81.15.-z, 75.75.Fk     

Synthesis and structural characterization of two novel 2D coordination polymers [PbCl(atza)]n and [PbCl(a4-ptz)]n

Reactions of Katza (atza⁻ = 5-aminotetrazole-1-acetato) or Ka4-ptz (a4-ptz⁻ = 5-[N-acetato(4-pyridyl)]tetrazole) with PbCl₂in aqueous solution, produced two new Pb(II) compounds, [PbCl(atza)]_n (1) and [PbCl(a4-ptz)]_n (2). Both compounds were structurally characterized by elemental analysis, IR spectroscopy and single-crystal X-ray diffraction. In 1 or 2, the Pb(II) centers are connected through the atza⁻ or a4-ptz⁻ bridging ligand to form a two-dimensional layered structure. The luminescence properties of 1 and 2 were investigated at room temperature in the solid state.     

Effect of electrolyte addition on activity of (Ga1−xZnx)(N1−xOx) photocatalyst for overall water splitting under visible light

Effects of electrolyte addition on photocatalytic activity of (Ga_1−xZn_x)(N_1−xO_x) modified with either Rh_2−yCr_yO₃ or RuO₂ nanoparticles as cocatalysts for overall water splitting under visible light (λ > 400 nm) are investigated. The cocatalyst Rh_2−yCr_yO₃ is confirmed to selectively promote the photoreduction of H⁺, while RuO₂ functions as both H₂ evolution sites and as efficient O₂ evolution sites. The activity of Rh_2−yCr_yO₃-loaded (Ga_1−xZn_x)(N_1−xO_x) is found to be suppressed in the presence of Cl⁻, which undergoes oxidation by photogenerated holes in the valence band of (Ga_1−xZn_x)(N_1−xO_x). Alkaline- and alkaline earth-metal cations in the reactant solution compensate the negative effect of Cl⁻ to a certain extent depending on the metal cation employed. Among the electrolytes examined, the addition of an appropriate amount of NaCl or A₂SO₄ (A = Li, Na, or K) to the reactant solution without pH control is found to increase activity by up to 75% compared to the case without additives. Direct splitting of seawater to produce H₂ and O₂ is also demonstrated using Rh_2−yCr_yO₃-loaded (Ga_1−xZn_x)(N_1−xO_x) catalyst under visible light.     

Hydrothermal preparation of octadecahedron Fe3O4 thin film for use in an electrochemical supercapacitor

A Fe₃O₄ film with regularly edge-affected cubic (octadecahedron) morphology was successfully prepared on stainless steel foil by a simple and benign hydrothermal process. The potential for the use of the film in a supercapacitor was tested by investigating the electrochemical behavior of the Fe₃O₄ film using cyclic voltammetry (CV) and galvanostatic charge/discharge tests. The Fe₃O₄ film showed a CV indicative of a typical pseudocapactive behavior in 1 mol L⁻¹ Na₂SO₃ solution. Furthermore, this film exhibited a specific capacitance of 118.2 F g⁻¹ at the current of 6 mA between −1 and 0.1 V with a capacity retention of 88.75% after 500 cycles.     

Preparation and electrochemical properties of functionalized ionic liquid self-assembled modified graphite electrode

A modified graphite electrode with functionalized ionic liquid (IL) pyridinium derivative of β-cyclodextrin ([CDbPy]BF₄) was prepared by layer-by-layer self-assembly technique. With ferrocene as probe, the characterization of the (CDIL/PDDA)_n/GE SAMs in the solution of phosphate (PBS, pH 7.0) was investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronocoulometry. The electrochemical behavior of p-chloronitrobenzene (p-CNB) at the modified electrode was studied. It was found that the modified electrode could catalyze the reduction of p-CNB and made the cathode peak move about 100 mV in positive direction in the solution of 0.1 mol/L PBS (pH 7.0). Differential pulse voltammetry (DPV) was applied to the determination of p-CNB in waste water with satisfactory results. The detection limit and the linear range of the concentration of p-CNB to the reduction peak current were 8.0 × 10⁻⁸ mol/L and 3.0 × 10⁻⁷–1.0 × 10⁻⁵ mol/L, respectively.