Oxygen evolution on electrodeposited cobalt oxides

This paper concerns the preparation of cobalt oxides through anodic deposition from Co(NO3)2 aqueous solutions on different substrates. The electroformed oxide films exhibit good chemical stability and lower oxygen overvoltages, irrespective of the substrate material. Electrocatalytic properties are investigated through polarisation curves and impedance measurements, while the active surface area is estimated by cyclic voltammetry. Experimental data are analysed in terms of a possible reaction mechanism.     

Oxygen evolution and hypochlorite production on Ru-Pt binary oxides

The electrocatalytic activities of Ru-Pt binary oxide electrodes prepared by thermal decomposition for both oxygen and chlorine evolution reactions (OER and CER) were investigated by cyclic voltammetry (CV) and log i/E relationships (Tafel study). Both CV and Tafel studies revealed that the electrodes from the coating solutions with 60 and 20 mol % Pt content possessed the maximum apparent activity for OER and optimal apparent activity for CER/hypochlorite production, respectively. The specific activity (i/q ^*) revealed that mixing of the RuO₂ and PtO _x had no synergistic effects for OER due to the occurrence of phase segregation, which was revealed by element mapping/surface morphologies and Auger electron spectroscopy. Lower current efficiencies for hypochlorite production were found on the freshly prepared binary electrodes (type I electrodes) than on those having been treated by repetitive CVs (type II electrodes). Stability testing of both type I and II electrodes was measured in 0.5 m NaCl solution at 300 mA cm^–2 for 480 h, indicating that both type I and II electrodes are quite stable under the above conditions.     

氧化锰电催化析氧反应及其电极界面特性

析氧半反应是速控步骤。氧化锰（MnO_x）具有多价态,可形成多个子步骤降低活化能。二氧化钛（TiO₂）是抗氧化性半导体。考察两种氧化锰催化剂析氧性能,即滑动弧等离子体合成MnO_x（pM）和商业化（cM）,与商业化TiO₂ （cT）比较。通过理想极化电极系统（IPE）解析,表达电极界面特性,揭示三相界面与活性的关联。结果表明：对两种MnO_x 催化剂,pM比cM性能更优。碱性环境中,前者起始电位低180 mV,Tafel斜率近半。酸性中,均表现阶跃式极化,析氧性能相近。cM比cT催化剂析氧性能更优,起始电位低420 mV。溶液电阻R_s与活性变化趋势一致。对相同催化剂,考察I/C、担量、单双层结构,电容压降占比f_Cd与活性一致。对不同催化剂,考察cM, cT或pM, cM,f_Cd与活性不一致,这与多价态锰参与非法拉第过程有关。     

Mesoporous semiconducting oxides for gas sensor application

A fabrication procedure of thermally stable mesoporous SnO₂ and TiO₂ powders has been overviewed along with their gas-sensing properties. Treatment of an as-prepared composite material of a supramolecule surfactant and SnO₂, i.e. a self-assembly of the surfactant fringed with a SnO₂ thin wall, with phosphoric acid enabled us to fabricate thermally stable ordered mesoporous SnO₂ powder having a d₁₀₀ value of 3.2 nm, a crystallite size of 2.0 nm and a large specific surface area of 305 m² g⁻¹ even after calcination at 600 °C for 5 h. A thick film sensor fabricated with the ordered mesoporous SnO₂ powder exhibited higher sensing performance than that fabricated with SnO₂ powder prepared by a conventional method and therefore having a lower specific surface area. Surface modification of the conventional SnO₂ powder with a mesoporous SnO₂ layer was also found to be effective for improving the sensing properties. Mesoporous TiO₂ powder could be prepared by employing a modified sol-gel method with Ti(NO₃)₄ and polyethylene glycol having different molecular weights. Higher sensitivity was achieved with a disc-type sensor fabricated with mesoporous TiO₂ powder, in comparison with one fabricated with commercially available TiO₂ powder in the same form, but its sensing properties needed to be further modified.     

Oxygen evolution anodes composed of anodically deposited Mn-Mo-Fe oxides for seawater electrolysis

Preparation of anodes for oxygen evolution in seawater electrolysis was carried out. Manganese-molybdenum double oxides, Mn_1−xMo_xO_2+x, prepared by anodic deposition from MnSO₄-Na₂MoO₄ solutions showed the 100% oxygen evolution efficiency at a current density of 1000 A m⁻² in 0.5 M NaCl at 30 °C and pH 12, but an increase in solution temperature resulted in dissolution of the oxides as molybdate and permanganate ions. In order to increase the stability of the electrodes at higher temperatures the addition of iron to the manganese-molybdenum oxides was performed by anodic deposition in MnSO₄-Na₂MoO₄-FeNH₄(SO₄)₂ solutions. The electrodes thus prepared showed the 100% oxygen evolution efficiency at 1000 A m⁻² in 0.5 M NaCl at 30-90 °C, when proper amounts of molybdenum and iron were contained. The iron addition also enhanced the oxygen evolution efficiency. The electrodes were not composed of oxide mixtures but triple oxides, Mn_1−x−yMo_xFe_yO_2+x−0.5y, consisting of Mn⁴⁺, Mo⁶⁺ and Fe³⁺. The formation of the triple oxides seemed responsible for enhancement of both oxygen evolution efficiency and stability.     

Oxygen reduction electrocatalysis on some manganese oxides

Cathodic oxygen reduction on some manganese oxides in alkaline solution was examined by using a rotating ring−disk electrode (rrde) and a Teflon-bonded electrode technique. The activity of γ-MnO₂ for cathodic oxygen reduction was larger than that of γ-MnOOH at the potential below —200 mV vs Hg/HgO (rrde). The electrochemical activity of carbon-supported γ-MnOOH Teflon-bonded electrode, on the contrary, was extremely high compared with that of γ-MnO₂. It was found that the γ-MnOOH in the Teflon-bonded electrode was thermally decomposed mainly to Mn₃O₄ and Mn₅O₈ in the course of the electrode preparation. These facts suggest that one of the causes of the high activity of the Teflon-bonded electrode containing γ-MnOOH is manganese oxides with distortion which have been produced in the electrode by the thermal decomposition of γ-MnOOH.     

Solid state ceramic gas sensors based on interfacing ionic conductors with semiconducting oxides

Solid-state ceramic NO_x sensors based on interfacing an ionic conductor (NASICON) with semiconducting oxides (rare earth perovskite-type oxides) were investigated. NASICON powders were pressed into thimbles 12 mm long with 3 mm inner diameter and 4 mm outer diameter, then sintered at 1270°C in air. A Pt wire was attached to the outer surface of the tubes using a platinum paste. A uniform Au/Pd (60 wt.%) coating, permeable to oxygen but not to NO_x, was sputtered for 40 min on the sensor external surface to allow the exposure of both electrodes to the gas atmosphere without using reference air. Windowless energy-dispersive spectroscopy (EDS) was used to evaluate the chemical composition of the Au–Pd layer before and after the performance of sensing tests. Sodalite powder as an auxiliary phase was tightly packed into the NASICON thimbles with a Pt lead for the electrical contact. To get an in-situ NO conversion to NO₂, a Pt-loaded alumina powder was used as a catalyst and incorporated with the sensor on the top of the auxiliary phase. Nano-sized and chemically-pure rare earth perovskite-type oxide (LaFeO₃, SmFeO₃, NdFeO₃ and LaCoO₃) powders, prepared by the thermal decomposition of the corresponding hexacyanocomplexes, were also used in the electrochemical cells. Each of the tested oxides was packed into the thimbles replacing the sodalite and the Pt-loaded alumina catalyst. Tests were performed also using only the perovskitic oxides. The microstructure of the materials tested was evaluated using scanning electron microscopy (SEM). The NO₂ sensing properties of the prototype sensors were investigated at controlled temperature (in the range 300–600°C) by measuring the electromotive force (EMF) at different NO₂ concentrations (in the range 2–2000 ppm in air). Some measurements were performed at various NO concentrations diluted with Ar. The results obtained showed a promising NO₂ sensing performance when ferrites were used. SmFeO₃ has a lower catalytic effect on NO oxidation than the Pt-loaded alumina catalyst, and has a similar effect to sodalite when used as auxiliary phase. The perovskite-type oxides are more preferable as auxiliary phase than sodalite because they improve the stability of the electrochemical sensor performances.     

Photocatalytic oxidation of Auramine O in the presence of semiconducting oxides

The photocatalytic degradation of Auramine O, a diarylmethane fluorescent dye, has been investigated in aqueous heterogeneous solutions containing semiconductor oxides as photocatalysts. The disappearance of the organic molecule follows approximately a pseudo‐first order kinetics according to the Langmuir–Hinshelwood model. CO₂ and NH₄⁺ ions have been identified as mineralization products. Various commercial photocatalysts are compared with respect to their decolorization efficiency, as well as the production of CO₂. The effect of pH and H₂O₂ on the reaction rate is ascertained. © 2000 Society of Chemical Industry     

Impedance spectroscopic investigation on electrical conduction and relaxation in manganese substituted pyrochlore type semiconducting oxides

The effect of simultaneous substitutions of Mn in both A and B sites of the pyrochlore type semiconducting oxides: (CaCe_1−2xMn_2x)(Sn_1−xMn_xINb)O_7−δ (x=0, 0.1, 0.2, 0.3 and 0.4) on the electrical conduction and relaxation was studied in detail using impedance analysis as a function of frequency over a wide range of temperature. Impedance and modulus analysis clearly explain the relaxation in these materials and its dependence on Mn concentration. Grain boundary dominant electrical characteristics have been observed with progressive Mn substitution. Correlated barrier hopping model was successfully applied for explaining the conduction mechanism in these compounds. Variation of hopping parameters with Mn substitution in these materials indicates strong dependence on the grain and grain boundary contributions. This insight to the conduction mechanism of the system offers in tuning the electrical properties for desired applications such as NTC thermistors.     

Oxygen evolution reaction on transition metal borides

The oxygen evolution reaction on the transition metal borides (mainly Co_xB and Ni_xB) was investigated with various composition ratio of BMetal and sintering temperature. The activity of oxygen evolution reaction became maximum at the composition range of BCo =$\text{1}\text{3}$ and BNi ? $\text{1}\text{3}$, respectively. The sintering temperature also affected largely the activity of oxygen evolution reaction and the metal borides with the composition ratio BMetal = $\text{1}\text{3}$ gave the highest activity at the sintering temperature lower than 300°C for nickel borides and 400 ～ 500°C for cobalt borides. The oxygen evolution reaction on iron boride and lanthanum hexaboride was also briefly discussed, and the order of the activity of oxygen evolution reaction was as follows:

The Tafel slope of oxygen evolution reaction on metal borides was varied from ca. 40 mV for Ni (ca 70 mV for Co) to ca. 120 mV with the increase in the activity of oxygen evolution reaction. The oxide formation became easier with the increase in the activity of oxygen evolution reaction and the differential capacity of the oxide formation in the range more cathodic than the oxygen evolution increased with the decrease in the apparent activation energy of oxygen evolution reaction at 0.60 V (HgHgO).     

Photodegradation of the textile dye Reactive Black 5 in the presence of semiconducting oxides

The photocatalytic degradation of Reactive Black 5, a textile azo dye, has been investigated in aqueous heterogeneous solutions containing semiconductor oxides as photocatalysts. The disappearance of the organic molecule follows approximately a pseudo‐first kinetic order according to the Langmuir–Hinshelwood model. CO ₂, NO₃⁻, and SO₄ ²⁻ ions have been identified as mineralization products. Various commercial photocatalysts were compared with respect to their decolorization efficiency, as well as the production of CO₂. The effect of pH and H₂O₂ on the reaction rate was ascertained. © 1999 Society of Chemical Industry     

Oxygen evolution on electrochemically generated cobalt spinel coating

The oxygen evolution reaction (OER) has been investigated in 40 wt % KOH at 80°C on a thick cobalt oxide coating obtained by potential cycling of a cobalt electrode for 0, 2, 4, 10 and 17h in the same electrolyte with and without dissolved strontium. The kinetic parameters of the OER were determined after preanodization for 1 h at 1 A cm^–2. Improved electrocatalytic activity for the OER, better mechanical strength of the coating and lower variation of the oxygen overpotential with time were noticed up to 70 h of polarization as the coating built up in the presence of dissolved strontium in the electrolyte. The beneficial effect of dissolved strontium on the electrocatalytic activity is ascribed to the accumulation of Co₃O₄, which results in a lower Tafel slope for the OER.     

Oxygen evolution on NiCo2O4 electrodes

NiCo₂O₄ was investigated as anode material for alkaline water electrolysis. This catalyst was prepared by thermal decomposition of metal salts and this rapid and simple technique gives reproducible results. A study of the preparation parameters shows that factors such as decomposition temperature, duration of the heat treatment and catalyst loading, determine the morphology of the oxide layer and so influence the performance of the catalyst. The conductivity of the oxide layer was found to change markedly with the final heat treatment. It is shown that alternative Teflon-bonded NiCo₂O₄ electrode structures give approximately the same activity.     

Stabilization of polymorphic phases in oxides. Oxygen vacancies

Polymorphic transitions in Al₂O₃, tungsten bronzes, 2CaO · SiO₂, and ZrO₂ are considered. Considering that at high temperatures oxides usually have excess oxygen vacancies, a further increase in their concentration due to introduction of corresponding additives slows diffusion, impedes the polymorphic transition, and stabilizes the phase. Introduction of additives that result in an increased concentration of cation vacancies, on the other hand, accelerates diffusion and facilitates the polymorphic transition. Translated from Steklo i Keramika, No. 3, pp. 19–22, March, 1999.     

Oxygen evolution reaction on thick hydrous nickel oxide electrodes

The electrocatalytic characteristics of thick hydrous oxide films obtained by repetitive square-wave potential perturbations for the oxygen evolution reaction in alkaline solutions are studied. Two kinds of films, fresh and aged, which can be related to the α and β nickel oxide structures respectively were used. The results show a marked difference between their Tafel slopes. Besides, a linear dependence of the apparent electrocatalytic activity upon the amount of oxide present in the film is obtained, which is explained on the basis of an increase in the roughness factor.     

Oxygen evolution reaction on thermally treated iridium oxide films

The properties of electrochemically grown and thermally treated oxide films on iridium were examined by cyclic voltammetry and potentiostatic polarization at potentials of the oxygen evolution reaction in 0.5 mol dm^–3 sulphuric acid. The oxide was grown by square wave pulses from –0.25 to +1.25 V vs SCE, a procedure much faster in comparison with potentiodynamic activation at the same frequency. The activated electrode, exhibiting low corrosion resistance during oxygen evolution, was subsequently stabilized by heat treatment. Optimal conditions between stability and electrocatalytic activity have been determined to be between 200 and 300°C.     

Defect engineering in semiconducting oxides: Control of ZnO surface potential via temperature and oxygen pressure

The technological usefulness of a semiconductor often depends on the types, concentrations, charges, spatial distributions, and mobilities of the atomic‐scale defects it contains. For semiconducting metal oxides, defect engineering is relatively new and involves complex transport and reaction networks. Surface‐based methods hold special promise in nanostructures where surface‐to‐volume ratios are high. This work uses photoreflectance augmented by X‐ray photoelectron spectroscopy to show that the surface potential V_S for Zn‐terminated ZnO(0001) can be manipulated over a significant range 54.97–79.08 kJ/mol (0.57–0.82 eV) via temperature and the partial pressure of O₂. A defect transport model implies this variation in V_S should affect the injection rate of oxygen interstitials by a factor of three. Such injection plays an important role in controlling the concentrations of oxygen vacancies deep in the bulk, which often prove troublesome as trapping centers in photocatalysis and photovoltaics and as parasitic emitters in light‐emitting devices. © 2015 American Institute of Chemical Engineers AIChE J, 62: 500–507, 2016     

Oxygen transfer in mixed oxides in the case of CO oxidation over oxides containing Cu, Mo and V

The combined measurement of reaction rate and oxygen activity in the catalyst by solid electrolyte potentiometry (SEP) can be used in principle to examine the concept of oxygen transfer in mixed oxides, often used to explain synergetic effects between different solid phases. This is shown in the case of copper molybdate and a Mo-V-O_x mixed oxide, using the oxidation of CO as a test reaction. Both the pure oxidic phases and a physical mixture are used in the study. The rate measurements show that the addition of Mo-V-O_x, inactive in CO oxidation, enhances the catalytic activity of CuMoO₄. On the other hand, SEP measurements indicate the existence of a gradient of oxygen activity between the two phases as the driving force of oxygen transfer from the Mo-V oxide to the copper molybdate. This revised version was published online in July 2006 with corrections to the Cover Date.     

氧化石墨烯生长钨基氧化物酸环境电催化析氢

以磷钨酸为钨源,单层氧化石墨烯(GO)为载体,引入碳纳米管(CNTs),通过一步水热法和在空气中煅烧制备了纳米异质结(WO3-rGO-CNTs).通过FTIR、XRD、XPS、TG、SEM、EDS、TEM和BET对其进行了表征,利用线性伏安法和循环伏安法对其进行了电催化析氢(HER)反应测试.结果表明,WO3晶体粒子均...