Investigation of surface properties of Ru-based oxide electrodes containing Ti, Ce and Nb

Mixed oxide electrodes with nominal composition Ti/[Ru_(0.3)Ti_(0.6)Ce_(0.1−x)]O₂[Nb₂O₅]_(x), (0≤x≤0.1) were prepared through a process of thermal decomposition (450 °C) of chloride precursor mixture solutions. Composition effects on the surface properties were investigated and the oxide electrodes were characterized in acid solution: ex situ by XRD and in situ by open circuit potential measurements (E_oc), cyclic voltammetry (CV), the double-layer capacitance (CV and electrochemical impedance spectroscopy techniques) and roughness factor (CV). Except the E_oc, all parameters varied with the electrode composition. It was shown that the presence of CeO₂ increases the electrochemical active area, but it makes the coating less stable when submitted to continuous potential cycles because of cathodic dissolution of this oxide. By adding Nb₂O₅ this instability can be lowered. The maximum activity (voltammetric charge) was observed with the [Ru_(0.3)Ti_(0.6)Ce_(0.07)]O₂[Nb₂O₅]_(0.03) composition.     

氧化石墨烯生长钨基氧化物酸环境电催化析氢

为了解决商用铂碳电极在电催化析氢反应（HER）领域生产成本高、催化剂使用周期短等问题，利用磷钨酸提供钨源，以单层氧化石墨烯（GO）作为载体，引入利于电子传输轨道的碳纳米管（CNTs），通过一步水热法和空气中煅烧制备得到WO3-rGO-CNTs纳米异质结电催化剂。利用FT-IR、XRD、XPS、TG、SEM、EDAX、TEM和BET对WO3-rGO-CNTs的化学结构和物理形貌进行了表征。结果表明，在单层GO表面均匀生长WO3晶体，并引入CNTs后，纳米异质结WO3-rGO-CNTs在酸性电解质中表现出优异的HER催化活性。利用线性伏安法（LSV）和循环伏安法（CV）对WO3-rGO-CNTs进行HER测试，当电流密度为10 mA/cm2时，其过电势为218 mV；塔菲尔斜率为130.5 mV/dec。当过电势为-0.5 V vs. RHE时，其阻抗值为8.2 Ω。同时，WO3-rGO-CNTs纳米异质结可以在218 mV（电流密度为10 mA/cm2）过电势下，保持50 h的稳定性和耐久性；其双层电容值为1.2 mF/cm2。电化学数据表明，WO3与GO和CNTs间由于异质结构的存在，产生了协同效应：GO为WO3晶体提供了广阔的金属反应活性位点，而CNTs则提供了利于电子传输的活性轨道。     

Characterization of nanocrystalline manganese oxide powder prepared by inert gas condensation

Nanocrystalline manganese oxide powders were synthesized by an inert gas condensation technique. The manganese oxide powders, prepared from vaporized metallic manganese in a helium pressure of 10 mbar, then oxidized under the oxygen pressures of 50 and 100 mbar, exhibited a mixture of β-Mn, MnO and Mn₃O₄ phases. X-ray diffractometry (XRD) analysis identified the predominant oxide formation of the as-prepared powder to be the phase of MnO. In situ synchrotron XRD and differential scanning calorimetry (DSC) data showed that synthesized manganese powder converts from β-Mn to MnO in the temperature range of 150–250 °C and subsequently converts to Mn₃O₄ above 250 °C. Transmission electron microscopy examinations confirmed that oxidation starts from the surface of the condensed β-Mn particles. Detailed valance variations of manganese of the resulting powders were investigated by synchrotron X-ray absorption spectroscopy techniques.     

Influence of the plastic strain on the hydrogen evolution reaction on polycrystalline nickel electrodes in H2S04

The influence of plastic strain on the hydrogen evolution reaction (HER) on polycrystalline nickel, in H₂SO₄ electrolyte at 293 K, is studied a cathodic potential range. Linear polarisation curves and electrochemical impedance spectra show that the HER is described by a mechanism of Volmer-Heyrovsky, which parameters do not depend on plastic strain. The plastic strain modifies the density and the distribution of dislocations. These evolutions are well understood and lead to changes in the number of hydrogen additional adsorption sites, and then to changes in the current density developed on pre-strained samples. From 0 to 2.4% of plastic strain, the current density increases to reach a maximum value at 2.4%. This current density increase is directly linked to the dislocation density increase. Over 2.4%, the distribution of dislocations is modified and the current density decreases but keeps higher than the current density on an unstrained sample.     

Effect of composition on the surface and electrocatalytic properties of Ti/IrO x +RhO x electrodes: H2 evolution from acidic solution

IrO _x +RhO _x mixed oxide layers on a Ti support were prepared by thermal decomposition at 450 °C over the whole composition range. The temperature range 450–600 °C was explored for the composition 30 mol% RhO _x . Samples were characterized by means of SEM, XPS, cyclic voltammetry and polarization curves. Their electrocatalytic properties were tested for the H₂ evolution reaction. The following experimental parameters were scrutinized: voltammetric charge, Tafel slope, reaction order (H⁺), electrical resistance of electrocatalysts. The electrocatalytic properties were evaluated at constant potential as a function of temperature as well as of composition. The electrode stability was assessed by comparing CV curves before and after groups of experiments. A reaction mechanism has been proposed. RhO _x is more active than IrO _x , its effect showing up for compositions >30 mol%. In honour of Professor G. Kreysa on the occasion of his 60th birthday. On leave from Institute for Nuclear Research, Pitesti, Romania.     

The positive effect of hydrogen on the reaction of nitric oxide with carbon monoxide over platinum and rhodium catalysts

The effect of adding 330–4930 ppm hydrogen to a reaction mixture of NO and CO (2000 ppm each) over platinum and rhodium catalysts has been investigated at temperatures around 200–250°C. Hydrogen causes large increases in the conversion of NO and, surprisingly, also of CO. Oxygen atoms from the additional NO converted are eventually combined with CO to give CO₂ rather than react with hydrogen to form water. This reaction is described by CO + NO +3/2H₂ CO₂ + NH₃ and accounts for 50–100% of the CO₂ formed with Pt/Al₂O₃ and 20–50% with Rh/Al₂O₃. With the latter catalyst a substantial amount of NO converted produces nitrous oxide. Comparison with a known study of unsupported noble metals suggests that isocyanic acid (HNCO) might be an important intermediate in a reaction system with NO, CO and H₂ present.     

Morphology,thermal and mechanical properties of epoxy adhesives containing well-dispersed graphene oxide

Graphene oxide (GO) is prepared and introduced into epoxy resins through a wet-transfer migration technique using a three-roll mill. The results of TEM, XRD and digital microscope observation show that good dispersion of GO is achieved without using any additives. The mechanical and thermal properties of GO/epoxy (GO/EP) adhesives are enhanced with GO incorporated. A 10.2% increase in Young's modulus and a 56.3% increase in elevated-temperature (120 °C) lap shear strength (LSS) was observed on addition of 1.0 wt% GO, compared to the neat epoxy adhesive. Increased glass transition temperature and improved thermal stability of the GO/EP adhesives are also observed in the DMA and TG analysis. Moreover, the toughness of the GO/EP adhesives is improved and much rougher fracture surface can be observed compared with the neat epoxy adhesive. No GO agglomeration can be observed in the SEM images of GO/EP adhesive with 1.0 wt% loading.     

Electrocatalytic oxygen reduction and hydrogen evolution reactions on phthalocyanine modified electrodes: Electrochemical, in situ spectroelectrochemical, and in situ electrocolorimetric monitoring

This study describes electrochemical, in situ spectroelectrochemical, and in situ electrocolorimetric monitoring of the electrocatalytic reduction of molecular oxygen and hydronium ion on the phthalocyanine-modified electrodes. For this purpose, electrochemical and in situ spectroelectrochemical characterizations of the metallophthalocyanines (MPc) bearing tetrakis-[4-((4′-trifluoromethyl)phenoxy)phenoxy] groups were performed. While CoPc gives both metal-based and ring-based redox processes, H₂Pc, ZnPc and CuPc show only ring-based electron transfer processes. In situ electrocolorimetric method was applied to investigate the color of the electrogenerated anionic and cationic forms of the complexes. The presence of O₂ in the electrolyte system influences both oxygen reduction reaction and the electrochemical and spectral behaviors of the complexes, which indicate electrocatalytic activity of the complexes for the oxygen reduction reaction. Perchloric acid titrations monitored by voltammetry represent possible electrocatalytic activities of the complexes for hydrogen evolution reaction. CoPc and CuPc coated on a glassy carbon electrode decrease the overpotential of the working electrode for H⁺ reduction. The nature of the metal center changes the electrocatalytic activities for hydrogen evolution reaction in aqueous solution. Although CuPc has an inactive metal center, its electrocatalytic activity is recorded more than CoPc for H⁺ reduction in aqueous solution.     

Design and electrochemical study of SnO2-based mixed oxide electrodes

For the electrochemical treatment of wastewater, it is critical to develop electrodes with a high activity for the oxidation of pollutants, long lifetimes, and low cost. In the present study, we have fabricated four different SnO₂-based electrodes (Ti/SnO₂-Sb₂O₅, Ti/SnO₂-Sb₂O₅-PtO_x, Ti/SnO₂-Sb₂O₅-RuO₂ and Ti/SnO₂-Sb₂O₅-IrO₂) using the thermal decomposition method and, for the first time, systemically studied their stability and electrocatalytic activity towards the degradation of 2-nitrophenol (2-NPh), 3-nitrophenol (3-NPh) and 4-nitrophenol (4-NPh). Scanning electron microscope (SEM) and X-ray energy dispersive spectrometry (EDS) were used to characterize the morphology and composition of the four different SnO₂-based electrocatalysts. Lifetime tests show that doping IrO₂ or RuO₂ greatly improves the stability of the SnO₂-based electrodes. The electrochemical activities of the prepared SnO₂-based electrodes were characterized using the degradation of 2-NPh, 3-NPh and 4-NPh. In situ UV/vis spectroscopy was used to monitor the concentration changes of the nitrophenols with time showing that the rate constants for the electrochemical oxidation of the nitrophenols decrease in the order of: 2-NPh > 4-NPh > 3-NPh. The effect of the applied current densities and initial concentrations of nitrophenols have also been investigated. Our study has shown that the fabricated Ti/SnO₂-Sb₂O₅-IrO₂ electrodes are very promising for the electrochemical treatment of wastewater.     

Structure evolution,thermal properties and sintering resistance of promising thermal barrier coating material La2(Zr0.75Ce0.25)2O7

Rare-earth doped zirconates are promising candidate materials for high-performance thermal barrier coatings (TBCs). The phase and microstructure stability is an important issue for the materials that must be clarified, which is related to the long-term stable work of TBCs at high temperatures. In this work, La₂(Zr_0.75Ce_0.25)₂O₇ (LCZ) ceramic coatings prepared by atmospheric plasma spraying present a metastable fluorite phase, which can transform into stable pyrochlore under high-temperature annealing. The detailed structure evolution of the ceramic coatings is characterized systematically by SEM, XRD and Raman. The associated thermal properties of LCZ ceramics were also reported. Results show that LCZ ceramic has an ultralow thermal conductivity (0.65 W/m·K, 1200 °C), which is only 1/3 of that of yttria-stabilized zirconia (YSZ). The thermal expansion coefficients of LCZ ceramic increase from 9.68 × 10^-6 K^-1 to 10.7 × 10^-6 K^-1 (300 - 1500 °C), which are relatively larger than those of La₂Zr₂O₇. Besides, Long-term sintering demonstrates that LCZ ceramic coating has preferable sintering resistance at 1500 °C, which is desirable for TBC applications.     

Hydrogen evolution reaction on Ni-P alloys: The internal stress and the activities of electrodes

The hydrogen evolution reaction (HER) was studied on Ni-P electrodes prepared by electrodeposition at temperatures varying from 23 °C to 65 °C. The activities for the HER of the electrodes first decreased slowly with the increasing temperature of the Ni-P preparation. A sudden decrease in the HER activity occurred on the electrode prepared at 65 °C. Similar dependence was found for the variation of the amounts of absorbed hydrogen with the electrodeposition temperature of the Ni-P electrodes. The behavior of Ni-P electrodes prepared from Ni-P powders was quite different. (Ni-P powder was prepared by peeling off the Ni-P layer and by milling the leaves of the Ni-P alloy in a vibrating ball mill.) The Ni-P powder electrodes displayed little activity independently of the temperature of the Ni-P powder preparation. It was followed from the results that the high activity for the HER of the layer Ni-P electrodes prepared at T ≤ 53 °C was caused by the internal stress in the layer. The stress originated during the electrodeposition of the Ni-P layer by co-deposited and absorbed hydrogen.     

Analysis of the use of voltammetric results as a steady state approximation to evaluate kinetic parameters of the hydrogen evolution reaction

The use of the voltammetric response j^vol(η) of a potentiodynamic sweep at a slow scan rate vs in place of a steady state polarization curve j^ss(η) for the determination of the kinetic parameters of the hydrogen evolution reaction is analyzed. It is proposed to consider jvol(η,vs)≅jss(η) when the condition 0.99≤jvol(η,vs)/jss(η)≤1.01 is verified in the overpotentials range η ≤ −0.05 V. It has been also established a simple relationship between the maximum admissible scan rate and the equilibrium polarization resistance R_p. Finally, the application of this criterion on different electrodes is described and discussed.     

In-situ testing of surface evolution of SiC during thermal ablation: Mechanisms of formation,flowing and growth of liquid silica beads

In this work we use real-time and in-situ observation technique at high temperature to capture the surface evolution of SiC (silicon carbide) specimen subjected to oxyacetylene torch flame. The obtained real-time images reveal clearly the nucleation, flowing and growth of the liquid silica beads on the surface of the specimen during thermal ablation process. A detailed model is developed to qualitatively analyze and interpret the mechanisms of the nucleation, flowing and the growth of the liquid silica beads. The result is expected to provide a better understanding of the ablation mechanism of SiC materials and serve as a preparatory work for the design and modification of the surface properties and topography of the materials to improve the thermal ablation resistance.     

Temperature dependence of surface composition and morphology in polymer blend film

Thin films of poly(methyl methacrylate) (PMMA) and poly(styrene-ran-acrylonitrile) (SAN) blend can phase separate upon heating to above its critical temperature. Temperature dependence of the surface composition and morphology in the blend thin film upon thermal treatment was studied using in situ X-ray photoelectron spectroscopy (XPS) and in situ atomic force microscopy (AFM). It was found that in addition to phase separation, the blend component preferentially diffused to and aggregated at the surface of the blend film, leading to the variation of surface composition with temperature. At 185 °C, above the critical temperature, the amounts of PMMA and SAN phases were comparable. At lower temperatures PMMA migrated to the surface, leading to a much higher PMMA surface content than in the bulk. The migration and preferential segregation of a blend component in thin films demonstrated here are responsible for the great difference between in situ and ex situ experimental (not real quenching or annealing) results of polymer blend films, and help explain the slow kinetics of surface phase separation at early stage for blend thin films reported in literature. This is significant for the control of surface properties of polymer materials.     

Fundamentals of hydrogen evolution reaction and its implications on near-neutral pH stress corrosion cracking of pipelines

In this work, electrochemical techniques were utilized to investigate the hydrogen evolution reaction on X-70 pipe steel and the hydrogen permeation through the steel in near-neutral pH environmental condition. The results demonstrate that the steel has always been in an active-dissolution state in near-neutral pH solution and there is no film formed on the steel surface. Hydrogen evolution is inhibited by anodic polarization of the steel, which is attributed to the alternation of hydrogen evolution mechanism and kinetics on the anodially polarized steel. Combined with slow strain rate tensile tests, it is found that the high susceptibility of steel to stress corrosion cracking (SCC) is always associated with a high hydrogen permeation current. A thermodynamic model was developed, by analyzing the change in free-energy of the steel in the presence and absence of hydrogen and stress, to determine the interactions of hydrogen, stress and anodic dissolution at the crack-tip. The role of hydrogen involvement in pipeline near-neutral pH SCC could be determined quantitatively by characterizing the effect of hydrogen concentration on the dissolution rate of steel and the synergism of hydrogen and stress to promote crack growth.     

Electrocatalysis by nanoparticles: Optimization of the loading level and operating pH for the oxygen evolution at crystallographically oriented manganese oxide nanorods modified electrodes

The electrocatalytic evolution of oxygen gas is investigated at manganese oxide nanorods (nano-MnO_x) modified Au, Pt and GC electrodes in a wide range of pH values, ranging from highly acidic to highly basic. Morphological investigation has been carried out by a scanning electron microscopy (SEM), which revealed the deposition of nano-MnO_x in a nanorod morphology. A significant enhancement of the electrocatalytic activity of the Au, Pt and GC electrodes towards the oxygen evolution reaction (OER) was observed upon the electrodeposition of nano-MnO_x onto the aforementioned electrodes. The effect of the surface coverage of the manganese oxide and the pH of the electrolyte was investigated to seek an optimization. The highest cathodic shift in the onset potential of the OER was obtained in 0.5 M KOH irrespective of the substrate whereas the optimum loading (surface coverage) was about ca. 52%. The origin of the enhancement of the OER is addressed with the assistance of an X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) techniques. The preferential electrodeposition of crystallographically oriented nano-MnOx (in the manganite phase, γ-MnOOH) is thought to play the primary role in the observed enhancement.     

The physical–chemical properties and electrocatalytic performance of iridium oxide in oxygen evolution

An IrO₂ anode catalyst was prepared by using the Adams method for the application of a solid polymer electrolyte (SPE) water electrolyzer. The effect of calcination temperature on the physical–chemical properties and the electrochemical performance of IrO₂ were examined to obtain a low loading and a high catalytic activity of oxygen evolution at the electrode. The physical–chemical properties were studied via thermogravimetry–differential scanning calorimetry (TG–DSC), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The electrochemical activity was investigated by using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronopotentiometry in 0.1 mol L⁻¹ H₂SO₄ at room temperature. The optimum condition was found to be at the calcination temperature of 500 °C, where the total polarization reached a minimum at high current densities (>200 mA cm⁻²). The optimized catalyst was also applied to a membrane electrode assembly (MEA) and stationary current–potential relationships were investigated. With an optimized catalytic IrO₂ loading of 1.5 mg cm⁻² and a 40% Pt/C loading of 0.5 mg cm⁻², the terminal applied potential difference was 1.72 V at 2 A cm⁻² and 80 °C in a SPE water electrolysis cell.     

Amperometric detection of hydrogen peroxide at nano-nickel oxide/thionine and celestine blue nanocomposite-modified glassy carbon electrodes

A simple procedure was developed to prepare a glassy carbon (GC) electrode modified with nickel oxide (NiOx) nanoparticles and water-soluble dyes. By immersing the GC/NiOx modified electrode into thionine (TH) or celestine blue (CB) solutions for a short period of time (5–120 s), a thin film of the proposed molecules was immobilized onto the electrode surface. The modified electrodes showed stable and a well-defined redox couples at a wide pH range (2–12), with surface confined characteristics. In comparison to usual methods for the immobilization of dye molecules, such as electropolymerization or adsorption on the surface of preanodized electrodes, the electrochemical reversibility and stability of these modified electrodes have been improved. The surface coverage and heterogeneous electron transfer rate constants (k_s) of thionin and celestin blue immobilized on a NiOx-GC electrode were approximately 3.5 × 10⁻¹⁰ mol cm⁻², 6.12 s⁻¹, 5.9 × 10⁻¹⁰ mol cm⁻² and 6.58 s⁻¹, respectively. The results clearly show the high loading ability of the NiOx nanoparticles and great facilitation of the electron transfer between the immobilized TH, CB and NiOx nanoparticles. The modified electrodes show excellent electrocatalytic activity toward hydrogen peroxide reduction at a reduced overpotential. The catalytic rate constants for hydrogen peroxide reduction at GC/NiOx/CB and GC/NiOx/TH were 7.96 (±0.2) × 10³ M⁻¹ s⁻¹ and 5.5 (±0.2) × 10³ M⁻¹ s⁻¹, respectively. The detection limit, sensitivity and linear concentration range for hydrogen peroxide detection were 1.67 μM, 4.14 nA μM⁻¹ nA μM⁻¹ and 5 μM to 20 mM, and 0.36 μM, 7.62 nA μM⁻¹, and 1 μM to 10 mM for the GC/NiOx/TH and GC/NiOx/CB modified electrodes, respectively. Compared to other modified electrodes, these modified electrodes have many advantages, such as remarkable catalytic activity, good reproducibility, simple preparation procedures and long-term stabilities of signal responses during hydrogen peroxide reduction.     

Structural parameters,energy states and magnetic properties of the novel Se-doped NiFe2O4 ferrites as highly efficient electrocatalysts for HER

This study is devoted to NiFe₂O₄ with different masses of Se (NFO + x%Se) (x = 0.0–4.0%) spinel ferrite nanoparticles production and investigation. The results of the crystal structure, microstructure and magnetic properties are presented as a function of the chemical content of the NFO + x%Se. Superparamagnetic (at 300 K) and ferrimagnetic (at 10 K) states are observed for all samples in the wide magnetic field range. The field dependencies of the magnetization show that Se-substitution does not change the main magnetic characteristics when x<2.0%. We observe a non-linear dependence of magnetic parameters for sample with x ≥ 2.0% (for NFO+2%Se, we determine the increase of the main magnetic parameters for 20% of the average values and the minimum values belong to the NFO+3%Se). The undoped sample and NiFe₂O₄+x%Se are soft magnets and characterized by the low coercivity (varying in the range 560–647 Oe). At T = 10 K squareness ratio (Sq. = Mr/Ms) is in a range of (0.216–0.318). This indicates a preferable single-domain state of crystallites, which differs from the magnetic structure at T = 300 K. Furthermore, the NFO + x%Se (x = 2.0) have a low overpotential of about ?327 mV, and a small Tafel slope of 91 mV/dec, which makes it a better for HER (hydrogen evolution reaction) catalyst than the undoped NiFe₂O₄.