Enhanced oxygen evolution at hydrous nickel oxide electrodes via electrochemical ageing in alkaline solution

The effect of electrochemically ageing hydrous nickel oxide films via slow repetitive potential multi-cycling across the main nickel (II/III) redox peak was investigated in an aqueous base environment using cyclic voltammetry and steady state polarisation curves in the oxygen evolution reaction (OER) region. Similarities between hydrous nickel oxide films and electroprecipitated ‘battery type’ nickel oxide were shown due to their similar change in redox and oxygen evolving properties as a result of film ageing. This ageing method was found to significantly enhance the OER performance of the hydrous nickel oxide electrode with the OER overpotential decreasing by 60 ± 2 mV and experiencing a 10 fold increase in OER rate for a fixed overpotential over that of an un-aged electrode. The OER turnover frequency for an aged electrode was found to be 1.16 ± 0.07 s^− 1 in comparison to 0.05 ± 0.003 s^− 1 for a hydrous nickel oxide electrode not subjected to ageing.     

Enhanced electrochemical evolution of oxygen by using nanoflowers made from a gold and iridium oxide composite

We report on the synthesis of a composite made from iridium oxide and gold that has a flower-like morphology. The ratio of iridium oxide to gold can be controlled by altering the concentrations of the metal precursors or the pH of the solution containing the reductant citrate. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and laser confocal micro-Raman spectroscopy were applied to characterize the structures of the nanoflowers, and a mechanism of their formation was deduced. The nanoflowers display an electrocatalytic activity in an oxygen evolution reaction (OER) that is significantly enhanced compared to bare iridium oxide nanoparticles. The highest turnover frequency for the OER of the new nanoflowers is 10.9?s^?1, which is almost one order of magnitude better than that of the respective nanoparticles. These attractive features are attributed to the high oxidation states of iridium in the nanoflowers which is caused by the transfer of electronic charge from metal oxides to gold, and also to the flower fractal structure which is thought to provide a much more accessible surface than suspensions of the respective nanoparticle.

Revealing the correlation between structure evolution and electrochemical performance of high-voltage lithium cobalt oxide

Lithium cobalt oxide(LCO)is the dominating cathode materials for lithium-ion batteries(LIBs)deployed in consumer electronic devices for its superior volumetric energy density and electrochemical performances.The constantly increasing demands of higher energy density urge to develop high-voltage LCO via a variety of strategies.However,the corresponding modification mechanism,especially the influence of the long-and short-range structural transitions at high-voltage on electrochemical performance,is still not well understood and needs further exploration.Based on ss-NMR,in-situ X-ray diffraction,and electrochemical performance results,it is revealed that the H3 to H1-3 phase transition dictates the structural reversibility and stability of LCO,thereby determining the electrochemical performance.The introduction of La and Al ions could postpone the appearance of H1-3 phase and induce various types of local environments to alleviate the volume variation at the atomic level,leading to better reversibility of the H1-3 phase and smaller lattice strain,and significantly improved cycle performance.Such a comprehensive long-range,local,and electronic structure characterization enables an in-depth understanding of the structural evolution of LCO,providing a guiding principle for developing high-voltage LCO for high energy density LIBs.     

High performance gold-supported platinum electrocatalyst for oxygen reduction

High performance gold-supported Pt electrocatalyst for the reduction of oxygen was prepared by replacing Cu adlayers, deposited potentiostatically on Au, with Pt at open-circuit potential in a 0.1 M HCl solution containing K₂PtCl₆. Auger Electron Spectroscopy and Atomic Force Microscopy reveal the surface modification. The kinetics of oxygen reduction on this platinum modified electrode was studied by the rotating-disc electrode technique. The activity of the electrode is lower than the activity of a smooth Pt electrode in the negative potential scan, but it is significantly higher in the positive scan.     

Electrochemical sensing chemical oxygen demand based on the catalytic activity of cobalt oxide film

Cobalt oxide sensing film was in situ prepared on glassy carbon electrode surface via constant potential oxidation. Controlling at 0.8 V in NaOH solution, the high-valence cobalt catalytically oxidized the reduced compounds, decreasing its surface amount and current signal. The current decline was used as the response signal of chemical oxygen demand (COD) because COD represents the summation of reduced compounds in water. The surface morphology and electrocatalytic activity of cobalt oxide were readily tuned by variation of deposition potential, time, medium and Co²⁺ concentration. As confirmed from the atomic force microscopy measurements, the cobalt oxide film, that prepared at 1.3 V for 40 s in pH 4.6 acetate buffer containing 10 mM Co(NO₃)₂, possesses large surface roughness and numerous three-dimensional structures. Electrochemical tests indicated that the prepared cobalt oxide exhibited high electrocatalytic activity to the reduced compounds, accompanied with strong COD signal enhancement. As a result, a novel electrochemical sensor with high sensitivity, rapid response and operational simplicity was developed for COD. The detection limit was as low as 1.1 mg L⁻¹. The analytical application was studied using a large number of lake water samples, and the accuracy was tested by standard method.     

Electropolymerization of cobalt porphyrins and corroles for the oxygen evolution reaction

Developing large-scale electrocatalysts using molecular complexes for the oxygen evolution reaction (OER) is of great importance. Herein, four cobalt porphyrins and corroles are deposited on electrode substrates using a simple and fast electropolymerization method. Our results showed that Co-1-P@CC, formed by electropolymerizing Co tetrakis(p-N-pyrrolylphenyl)porphyrin (Co-1-P) on carbon cloth (CC), is the most active OER catalyst in the examined Co porphyrins and corroles in alkaline aqueous solutions by displaying an onset overpotential of 380 mV. Long-term electrolysis tests confirmed the stability of these electropolymerized films by functioning as OER electrocatalysts.     

Improving biomass-derived carbon with cobalt/cobalt oxide doping for oxygen reduction reaction

Journal of Solid State Electrochemistry - Biomass-derived carbons from cypress leave, vegetable sponge, and pine needle were successfully prepared. They present three dimensional honeycomb...     

Electrocatalytic oxygen evolution reaction of hierarchical micro/nanostructured mixed transition cobalt oxide in alkaline medium

Journal of Solid State Electrochemistry - In this work, different cobalt-based mixed transition metal oxides (MTMOs) were fabricated by hydrothermal process followed by calcination. Doping Co3O4...     

Simple chemical route for nanorod-like cobalt oxide films for electrochemical energy storage applications

We used a simple chemical synthesis route to deposit nanorod-like cobalt oxide thin films on different substrates such as stainless steel (ss), indium tin oxide (ITO), and microscopic glass slides. The morphology of the films show that the films were uniformly spread having a nanorod-like structure with the length of the nanorods shortened on ss substrates. The electrochemical properties of the films deposited at different time intervals were studied using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS). The film deposited after 20 cycles on ss gave the highest specific capacity of 67.6 mAh g^?1 and volumetric capacity of 123 mAh cm^?3 at a scan rate 5 mV s^?1 in comparison to 62.0 mAh g^?1 and 113 mAh cm^?3 obtained, respectively, for its counterpart on ITO. The film electrode deposited after 20 cycles on ITO gave the best rate capability and excellent cyclability with no depreciation after 2000 charge–discharge cycles.     

Facile synthesis of cobalt oxide/reduced graphene oxide composites for electrochemical capacitor and sensor applications

Reduced graphene oxide sheets decorated with cobalt oxide nanoparticles (Co₃O₄/rGO) were produced using a hydrothermal method without surfactants. Both the reduction of GO and the formation of Co₃O₄ nanoparticles occurred simultaneously under this condition. At the same current density of 0.5 A g⁻¹, the Co₃O₄/rGO nanocomposites exhibited much a higher specific capacitance (545 F g⁻¹) than that of bare Co₃O₄ (100 F g⁻¹). On the other hand, for the detection of H₂O₂, the peak current of Co₃O₄/rGO was 4 times higher than that of Co₃O₄. Moreover, the resulting composite displayed a low detection limit of 0.62 μM and a high sensitivity of 28,500 μA mM⁻¹cm⁻² for the H₂O₂ sensor. These results suggest that the Co₃O₄/rGO nanocomposite is a promising material for both supercapacitor and non-enzymatic H₂O₂ sensor applications.     

Improved activity of SnO for the photocatalytic oxygen evolution

SnO prepared by soft chemistry exhibits a black color and semiconducting properties. The X-ray diffraction indicates a tetragonal symmetry (SG: P4/nmm) with nano crystallites of an average size of 85 nm. The forbidden band, determined from the diffuse reflectance is found to be 1.46 eV. The electrical conductivity occurs by polaron hopping and follows an Arrhenius type law with activation energy of 0.21 eV, the change in the slope at 526 K is attributed to the oxidation to SnO₂. The photo-electrochemical study shows n type conduction with a flat band potential of ?0.45 V, close to the photocurrent onset potential (?0.40 V). The electrochemical impedance spectroscopy shows the bulk contribution of SnO (R_b = 1.7 kΩ cm²) and decreases down to 1.89 kΩ cm² under illumination. The photocatalytic properties have been evaluated for the first time for to the oxygen evolution. The valence band, deriving from Sn²⁺: 5p orbital with a potential (?0.80 V_SCE/5.55 eV), is suitably positioned with respect to O₂/H₂O level (～0.6 V_SCE), leading to water oxidation under visible light. The best performance occurs at pH ～ 7 with an oxygen liberation rate of 23 µmol mL h^?1 (mg catalyst)^?1 and a quantum efficiency of 1.2%. An improvement of ～13% is observed on the system SnO/clay.     

Enhanced electrochemical activity of redox-labels in multi-layered protein films on indium tin oxide nanoparticle-based electrode

Facile electrical communication between redox-active labeling molecules and electrode is essential in the electrochemical detection of bio-affinity reactions. In this report, nanometer-sized indium tin oxide (ITO) particles were employed in the fabrication of porous thick film electrodes to enhance the otherwise impeded electrochemical activity of redox labels in multi-layered protein films, and to enable quantitative detection of avidin/biotin binding interaction. To carry out the affinity reaction, avidin immobilized on an ITO electrode was reacted with mouse IgG labeled with both biotin and ruthenium Tris-(2,2′-bipyridine) (Ru-bipy). The binding reaction between avidin and biotin was detected by the catalytic voltammetry of Ru-bipy in an oxalate-containing electrolyte. On sputtered ITO thin film electrode, although a single layer of Ru-bipy labeled avidin exhibited substantial anodic current, attaching the label to the outer IgG layer of the avidin/biotin-IgG binding pair resulted in almost complete loss of the signal. However, electrochemical current was recovered on ITO film electrodes prepared from nanometer-sized particles. The surface of the nanoparticle structured electrode was found by scanning electron microscopy to be very porous, and had twice as much surface binding capacity for avidin as the sputtered electrode. The results were rationalized by the assumption of different packing density of avidin inner layer on the two surfaces, and consequently different electron transfer distance between the electrode and Ru-bipy on the IgG outer layer. A linear relationship between electrochemical current and IgG concentration was obtained in the range of 40-4000 nmol L⁻¹ on the nanoparticle-based electrode. The approach can be employed in the electrochemical detection of immunoassays using non-enzymatic redox labels.     

Size-dependent catalytic activity of cobalt phosphides for hydrogen evolution reaction

Transition metal phosphides are a class of promising electrocatalysts for hydrogen evolution reaction(HER) to replace noble metals.In this work,we for the first time synthesize carbon supported CoP nanoparticles with the average particle sizes from 3.3 to 9.2 nm,via a solvothermal process followed by low-temperature topological phosphorization,and the size-dependent HER activity of the CoP is investigated by virtue of TEM,XRD,XPS and the electrochemical techniques.It is discovered that the 9.2nm-CoP particles possess high intrinsic HER catalytic activity as compared to the 3.3nm-CoP,although the smaller one displays a high mass activity due to the large surface area.Detailed studies manifest that the small CoP particles suffer from serious oxidation once exposing to air.In contrast,most cobalt remains in the quasi-metallic state in the relatively large CoP particles,which is beneficial for the desorption of Hads,the rate determining step of the HER process over CoP surface.In addition,the low charge transfer resistance across the liquid/solid interfaces also contributes to the excellent HER activity of the relatively large CoP particles.     

Influence of addition of cobalt oxide on microstructure and electrochemical capacitive performance of nickel oxide

Ni–Co oxide nanocomposite was prepared by thermal decomposition of the precursor obtained via a new method—coordination homogeneous coprecipitation method. The synthesized sample was characterized physically by X-ray diffraction, scanning electron microcopy, energy dispersive spectrum, transmission electron microscope, and Brunauer–Emmett–Teller surface area measurement, respectively. Electrochemical characterization of Ni–Co oxide electrode was examined by cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance measurements in 6-mol L⁻¹ KOH aqueous solution electrolyte. The results indicated that the addition of cobalt oxide not only changed the morphology of NiO but also enhance its electrochemical capacitance value. A specific capacitance value of 306 F g⁻¹ of Ni–Co oxide nanocomposite with n _Co = 0.5 (n _Co is the mole fraction of Co with respect to the sum of Co and Ni) was measured at the current density of 0.2 A g⁻¹, nearly 1.5 times greater than that of pure NiO electrode. Lower resistance and better rate capability can also be observed.     

Role of rhodium doping into lanthanum cobalt oxide (LaCoO3) perovskite and the induced bifunctional activity of oxygen evolution and reduction reactions in alkaline medium

Transition metal oxides, especially perovskites, have been considered effective electrocatalysts for the oxygen evolution (OER) and oxygen reduction (ORR) reactions in an alkaline solution. Here, a series of lanthanum cobalt rhodium oxide perovskites with the chemical formula LaCo_1-xRh_xO₃ (LCRO, 0.1 ≤ x ≤ 0.70) were prepared through the approach of solid-phase synthesis and their bifunctional electrocatalytic activity was assessed for both the OER and ORR. The crystallinity, morphology, surface, and electrocatalytic features of the LCRO were significantly correlated with the rhodium content. The LaCo_0.7Rh_0.3O₃ electrocatalysts with x = 0.3 showed enhanced electrocatalytic bifunctional performance with a substantially lower OER/ORR onset potential of 1.38/0.73 V vs HRE, smaller Tafel slope (116/90 mV/dec), and low charge-transfer resistance, which is the most efficient catalyst among the other studied ratios and superior to the pristine lanthanum cobalt oxide benchmark electrocatalysts. The LaCo_0.7Rh_0.3O₃ electrode exhibit good bifunctional electrocatalytic behavior and long-term durability with an OER and ORR onset potential gap (ΔE = E_OER ? E_ORR) of only 0.65 V, which could be credited to the enriched oxygen vacancies, lattice expansion and the improved electrical conductivity upon the doping of larger size of Rh ions. The LaCo_1-xRh_xO₃ catalysts are obtained from abundant materials that have the potential of highly-active bifunctional OER and ORR electrocatalysts.     

Engineering morphologies of cobalt oxide/phosphate-carbon nanohybrids for high-efficiency electrochemical water oxidation and reduction

Active non-noble metal catalysts plays a decisive role for water electrolysis,however,the rational design and development of cost-efficient electrocatalysts with Pt/IrO2-like activity is still a challenging task.Herein,a facile one-step electrodeposition route in deep eutectic solvents(DESs) is developed for morphology-controllable synthesis of cobalt oxide/phosphate-carbon nano hybrids on nickel foam(CoPO@C/NF).A series of CoPO@C/NF nanostructures including cubes,octahedrons,microspheres and nanoflowers are synthesized,which show promising electrocatalytic properties toward oxygen and hydrogen evolution reactions(OER/HER).Such surface self-organized microstructure with accessible active sites make a significant contribution to the enhanced electrochemical activity,and hybridizing cobalt oxide with cobalt pyrophosphates and carbon can result in enhanced OER performance through synergistic catalysis.Among all nanostructures,the obtained microspherical CoPO@C/NF-3 catalyst exhibits excellent catalytic activities for OER and HER in 1.0 M KOH,affording an anodic current density of 10 mA cm^-2 at overpotentials of 293 mV for OER and 93 mV for HER,with good long-time stability.This work offers a practical route for engineering the high-performance electrocatalysts towards efficient energy conversion and storage devices.     

Z型复合催化剂g-C3N4/Vo-ZnO光催化活性的研究

随着科学技术的不断进步和经济的快速发展,人类对自然资源的需求量越来越大,在开发利用自然资源的同时,大量的有机污染物也随之进入自然环境.这些物质不仅污染环境、破坏生态,更对人类的生活和健康带来了巨大的威胁.研究证实,半导体光催化剂在光照条件下可以破坏有机污染物的分子结构,最终将其氧化降解成CO2、H2O或其它不会对环境产生二次污染的小分子,从而净化水质.近年来,有关光催化降解有机污染物的报道日益增多. ZnO作为一种广泛研究的光催化降解材料,因其无毒、低成本和高效等特点而具有一定的应用前景.但是ZnO较大的禁带宽度(3.24 eV)导致其只能吸收紫外光部分,而对可见光的吸收效率很小,极大地制约了其实际应用.除此之外, ZnO受光激发产生的电子-空穴分离效率较低、光催化过程中的光腐蚀严重也是制约其实际应用的重要因素.为了提高ZnO的光催化活性和稳定性,本文合成了用g-C3N4修饰的氧空位型ZnO(g-C3N4/Vo-ZnO)复合催化剂,在有效调控ZnO半导体能带结构的同时,通过负载一定量的g-C3N4以降低光生电子-空穴对的复合速率和反应过程中ZnO的光腐蚀,增强催化剂的光催化活性和稳定性.本文首先合成前驱体Zn(OH)F,然后焙烧三聚氰胺和Zn(OH)F的混合物得到g-C3N4/Vo-ZnO复合催化剂,并采用电子顺磁共振波谱(EPR)、紫外-可见光谱(UV-vis)、高分辨透射电镜(HRTEM)和傅里叶变换红外光谱(FT-IR)等表征了它们的结构及其性质. EPR结果表明,ZnO焙烧后具有一定浓度的氧空位,导致其禁带宽度由3.24 eV降至3.09 eV,因而提高了ZnO对可见光的吸收效率. UV-vis结果显示, Vo-ZnO复合g-C3N4后对可见光的吸收显著增强. HRTEM和FT-IR结果均表明, g-C3N4纳米片和Vo-ZnO颗粒之间通过共价键形成了强耦合,这对g-C3N4/Vo-ZnO复合催化剂中光生载流子的传送和光生电子-空穴对的有效分离起到重要作用.可见光催化降解甲基橙(MO)和腐殖酸(HA)的实验进一步证明, g-C3N4/Vo-ZnO复合材料具有较好的光催化活性,优于单一的g-C3N4或Vo-ZnO材料.同时还发现, g-C3N4的负载量对光催化活性有显著影响,当氮化碳的负载量为1 wt%时,所制材料具有最高的光催化活性：可见光照射60 min后,MO降解率可达到93%, HA降解率为80%.复合材料光催化活性的增强一方面是因为氧空位的形成减小了ZnO的禁带宽度,使得ZnO对可见光的吸收能力大大增强;另一方面, g-C3N4和Vo-ZnO的能带符合了Z型催化机理所需的有效能带匹配,使得光生电子-空穴对得到了有效的分离,从而提高了光催化活性.降解MO的循环实验表明, g-C3N4/Vo-ZnO催化剂具有很好的稳定性且不容易发生光腐蚀.与此同时,我们对比了用不同方法制备的g-C3N4/ZnO材料的催化性能.结果显示,本文制备的g-C3N4/Vo-ZnO复合材料具有更好的降解效率.总体而言,对于降解有机污染物, g-C3N4/Vo-ZnO可能是一个更为有效可行的催化体系.此外,本文也为设计与制备其他新型光催化剂提供了一条新的思路.     

The effect of aluminium oxide on the reduction of cobalt oxide and thermostabillity of cobalt and cobalt oxide

During precipitation and calcination at 200°C nanocrystalline Co₃O₄ was obtained with average size crystallites of 13 nm and a well developed specific surface area of 44 m² g^?1. A small addition of a structural promoter, e.g. Al₂O₃, increases the specific surface area of the cobalt oxide (54 m² g^?1) and decreases the average size of crystallites (7 nm). Al₂O₃ inhibits the reduction process of Co₃O₄ by hydrogen. Reduction of cobalt oxide with aluminium oxide addition runs by equilibrium state at all the respective temperatures. The apparent activation energy of the recrystallization process of the nanocrystalline cobalt promoted by the aluminium oxide is 85 kJ mol^?1. Aluminium oxide improves the thermostability of both cobalt oxide and the cobalt obtained as a result of oxide phase reduction.      

Tuning size and catalytic activity of nano-clusters of cobalt oxide

Cobalt oxides were prepared by three different methods: (1) by reacting cobalt nitrate with oxalic acid, (2) co-precipitating cobalt nitrate with sodium carbonate, and (3) using sodium dodecyl sulphate as organic surfactant. All three samples were characterized before and after calcination by solvent extraction and the resulting products examined by IR spectroscopy. In the case of method 3, the removal of surfactant was followed by TGA studies. Products from all three methods were identified by XRD. Peaks in low angle XRD indicate the porous nature of the oxides. The morphology of the pores was studied by transmission electron microscopy. Some irregular pore structures were obtained for samples from methods 1 and 2, with an average size of 4–6 nm. Only the product from method 3 using SDS as template showed ordered structure and optimum size, and Brunauer-Emmet-Teller surface areas of the as-prepared, as well as the treated samples, exhibited H3 type hysteresis. The samples from the three methods were used as catalysts in the oxidation reaction of cyclohexane under mild conditions and the catalytic efficiency of the cobalt oxide was comparable with mesoporous cobalt oxides.