Anodically formed oxide films and oxygen reduction on electrodeposited ruthenium in acid solution

The impedance of the anodically formed hydrous Ru oxide in the system Ru|oxide film|1 M HClO₄ solution has been studied in the range of potentials where the electrode process occurs by a double electron and proton exchange between the oxide film and the solution. The results allowed us to clearly distinguish between the surface process at higher frequency and the bulk process at lower frequency. The high-frequency charging is found to be coupled to Faradaic charging at the film/solution interface. Evaluation of the impedance data at lower frequency, using diffusion equations for the finite boundary conditions, yields an effective proton diffusion coefficient to be 10⁻¹⁰ to 10⁻¹¹ cm² s⁻¹.Oxygen reduction on the spontaneously oxidized ruthenium electrode was discussed on the basis of a rotating ring-disk voltammetry.     

Anodic deposition of hydrous ruthenium oxide for supercapaciors: Effects of the AcO concentration, plating temperature, and oxide loading

Effects of the sodium acetate (NaCH₃COO, denoted as NaAcO) concentration, plating temperature, and oxide loading on the pseudocapacitive characteristics of hydrous ruthenium oxide (denoted as RuO₂·xH₂O) films anodically plated from aqueous RuCl₃·xH₂O media were systematically investigated in this work. The electrochemical behavior of RuO₂·xH₂O with annealing in air at 200 °C for 2 h is approximately independent of the NaAcO concentration and plating temperature although a negative shift in the onset and peak potentials of deposition with rising the plating temperature is found. The morphologies and adhesion of RuO₂·xH₂O deposits strongly depend on the deposition rate which is obviously influenced by varying the above two deposition variables. The specific capacitance of RuO₂·xH₂O is monotonously decreased from 760 to 505 F g⁻¹ when the oxide loading is gradually increased from 0.34 to 1.0 mg cm⁻², due to the longer pathways of both electrons and protons during the redox transitions. The XRD and Raman spectroscopic analyses reveal the extremely localized crystalline nature of as-deposited RuO₂·xH₂O. All RuO₂·xH₂O deposits show the ideal pseudocapacitive characteristics, definitely illustrating the merits of RuO₂·xH₂O prepared by anodic deposition without considering the advantages of its simplicity, one-step, reliability, low cost, and versatility for electrode preparation.     

Effect of hydrothermal treatment on the performance of RuO2-Ta2O5/Ti electrodes for use in supercapacitors

Hydrothermal treatment (HTT) of RuO₂-Ta₂O₅/Ti electrode, as a method for improving their performance, for use in supercapacitors was investigated.The results show that HTT significantly enhances the stability of the electrodes. The specific capacitance of electrodes, subject to HTT in the temperature range 180-250 °C remains unchanged after 1000 CV cycles between −0.2 and 1.1 V vs. SCE; without HTT a decay to 97% of the initial is observed. The results also show that HTT decreases the activity of the electrodes for O₂ and H₂ evolution and increases the voltage window by 56-135 mV for supercapacitors, but with a specific capacitance decrease of 7-27%. XPS analyses show the existence of more hydroxides after the HTT, which leads to a little increase in the interplanar distance as indicated in the XDR results. Contact angle measurements show the presence of a more hydrophilic surface after HTT.     

Degradation of azo dye from aqueous solutions using nano-SnO2/Ti electrode prepared by electrophoretic deposition method: Experimental Design

In this work, degradation of C.I. Acid Red 33 (AR33) in aqueous solutions was investigated. The combined electrolysis–ozone (ECO) process optimized based on SnO₂ nanoparticles electrode (nano-SnO₂/Ti) as anode using response surface methodology (RSM) involving a five-level central composite design (CCD). The nano-SnO₂/Ti electrode was prepared using electrophoretic deposition (EPD) method. The electrode was characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and cyclic voltammetry (CV). The initial pH, current density, reaction time and electrolyte concentration were selected as independent variables in central composite design while color removal efficiency was considered as the response function. Based on analysis of variance (ANOVA), the coefficient of determination value (R² = 0.981) was high. In optimum conditions, maximum color removal efficiency (93.2%) was obtained after 16 min; and the removal of chemical oxygen demand (COD) was reduced to 57.1% after 60 min.     

Electrophoretic deposition (EPD) of hydrous ruthenium oxides with PTFE and their supercapacitor performances

The effect of PTFE addition was investigated for the electrophoretic deposition (EPD) of hydrous ruthenium oxide electrodes. Mechanical stability of electrode layers, together with deposition yield, was enhanced by using hydrous ruthenium oxide/PTFE dispersions. High supercapacitor performance was obtained for the electrodes prepared with 2% PTFE and 10% water. When PTFE content was higher, the rate capability became poor with low electronic conductivity; higher water content than 10% resulted in non-uniform depositions with poor cycleability and power capability. When electrodes were heat treated at 200 °C for 10 h, the specific energy was as high as 17.6 Wh/kg based on single electrode (at 200 W/kg); while utilizable energy was lower with heat treatment time of 1 and 50 h, due to the high resistance and gradual crystallization, respectively. With PTFE addition and heat treatment at 200 °C for 10 h, the specific capacitance was increased by 31% (460 → 599 F/g at ca. 0.6 mg/cm²) at 10 mV/s, and the deposition weight was increased up to 1.7 mg/cm² with initial capacitance of 350 F/g.     

Preparation and characterization of PbO2 electrodes doped with different rare earth oxides

PbO₂ electrodes doped with rare earth oxides (Re-PbO₂), including Er₂O₃, Gd₂O₃, La₂O₃ and CeO₂, were prepared by anodic codeposition in order to investigate the effect of rare earth oxide dopants on the properties of PbO₂ electrodes. The physicochemical properties of the Re-PbO₂ electrodes were analyzed by spectral methods and electrochemical measurements. The surface morphology of the Re-PbO₂ electrodes held the characteristics of the dopants and the crystal grain of PbO₂. The crystal structure of the PbO₂ electrodes was also influenced by doping with different rare earth oxides. The presence of Er₂O₃ and La₂O₃ in the PbO₂ films could enhance the direct anodic oxidation, which was helpful to mineralize 4-chlorophenol. The 4-chlorophenol decay on the Re-PbO₂ electrodes was analyzed and good fitting was found using the relation for the pseudo-first order reaction. Of the electrodes examined, the Er-PbO₂ electrode exhibited the best performance for the degradation of 4-chlorophenol. The removal rates of COD and 4-chlorophenol during the 9 h electrolysis at a current density of 20 mA cm⁻² were 80.7 and 100%, respectively, with the current efficiency being 16.0-10.1%.     

Pitting corrosion of lead in sodium carbonate solutions containing NO3 ions

Pitting corrosion of Pb in Na₂CO₃ solutions (pH=10.8) containing NaNO₃ as a pitting corrosion agent has been studied using potentiodynamic anodic polarization, cyclic voltammetry and chronoamperometry techniques, complemented with scanning electron microscopy (SEM) examinations of the electrode surface. In the absence of NO₃⁻, the anodic voltammetric response exhibits three anodic peaks prior to oxygen evolution. The first anodic peak A₁ corresponds to the formation of PbCO₃ layer and soluble Pb²⁺ species in solution. The second anodic peak A₂ is due to the formation of PbO beneath the carbonate layer. Peak A₂ is followed by a wide passive region which extends up to the appearance of the third anodic peak A₃. The later is related to the formation of PbO₂. Addition of NO₃⁻ to the carbonate solution stimulates the anodic dissolution through peaks A₁ and A₂ and breaks down the dual passive layer prior to peak A₃. The breakdown potential decreases with an increase in nitrate concentration, temperature and electrode rotation rate, but increases with an increase in carbonate concentration and potential scan rate. Successive cycling leads to a progressive increase in breakdown potential. The current/time transients show that the incubation time for passivity breakdown decreases with increasing the applied anodic potential, nitrate concentration and temperature.     

Oxygen reduction reaction on spontaneously and potentiodynamically formed Au/TiO2 composite surfaces

From aqueous solutions, gold was deposited on freshly polished titanium surface either by displacement reaction or by potentiodynamic polarization. Both of these procedures resulted in the formation of Au/TiO₂ composite layer over the titanium surface, although with quite different Au content. The electrochemical behaviour of these composite layers was examined in 0.1 M NaOH solution, both oxygen free and oxygen saturated, using rotating disc technique. In the potential region of oxygen reduction reaction, the Au/TiO₂ layer obtained by potentiodynamic polarization, displayed the voltammetric curve similar to that observed on (1 0 0) plane of Au monocrystal. The Koutecky-Levich analysis evidenced that 4e⁻ route of oxygen reduction dominates on both type of composite layers.     

Influencing factors on electrochromic hysteresis performance of ruthenium purple produced by a WO3/Tris(2,2′-bipyridine)ruthenium(II)/polymer hybrid film

A [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine)/WO₃ hybrid (denoted as Ru-WO₃) film was prepared as a base layer on an indium tin oxide electrode by electrodeposition from a colloidal solution containing peroxotungstic acid, [Ru(bpy)₃]²⁺ and poly(sodium 4-styrenesulfonate). A ruthenium purple (RP, Fe^III₄[Ru^II(CN)₆]₃, denoted as Fe^III-Ru^II) layer was electrodeposited on a neat WO₃ film or a Ru-WO₃ film from an aqueous RP colloid solution to yield a WO₃/RP bilayer film or a Ru-WO₃/RP bilayer film, respectively. The spectrocyclic voltammetry measurement reveals that Fe^II-Ru^II is oxidized to Fe^III-Ru^II by a geared reaction of [Ru(bpy)₃]^2+/3+ and Fe^III-Ru^II is reduced by a geared reaction of H_xWO₃/WO₃ in the Ru-WO₃/RP film. These geared reactions produced electrochromic hysteresis of the RP layer. However, the absorbance change in the hysteresis was smaller than that for the Ru-WO₃/Prussian blue bilayer film reported previously, resulting from the lower electroactivities of any redox component for the Ru-WO₃/RP film. The lower electroactivities could be explained by the specific interface between the Ru-WO₃ and RP layers. It might contribute to either an increase of the interfacial resistance between the Ru-WO₃ and RP layers, or formation of the physically precise interface between the layers to make it difficult for counter ions to be transported in the interfacial liquid phase involved in the redox reactions in the film. The specific interface at the Ru-WO₃ and RP layers could be formed possibly by the electrostatic interaction between [Ru(bpy)₃]²⁺ and terminal [Ru(CN)₆]⁴⁻ moieties of RP. It could be suggested by the decreased redox potential of [Ru(bpy)₃]²⁺ in the Ru-WO₃ layer from 1.03 to 0.61 V by formation of the RP layer.     

Formation of nitrogen oxides from fuel-N through HCN and NH3: a model-compound study

The conversion of fuel-nitrogen to HCN and NH₃ and to nitrogen oxides was studied with nitrogen-containing model compounds, chosen to represent the main nitrogen and oxygen functionalities in fossil fuels. Two kinds of experiments were performed in an entrained-flow reactor at 800 °C. The conversion of model-compound-N to HCN and NH₃ was determined under inert conditions, and the formation of NO, N₂O and NO₂ was determined under oxidizing conditions. In inert atmosphere, oxygen-containing functional groups had an important effect on the ratio of HCN to NH₃. In particular, OH groups bound directly in the ring structure increased the conversion of nitrogen to NH₃. In oxidizing atmosphere, the conversions of model-compound-N to N₂O were high, but the substituent groups had no well-defined effect on the ratio of N₂O to NO. The formation of NO₂ was insignificant.     

Anodic dissolution of tantalum and niobium in acetone solvent with halogen additives for electrochemical synthesis of Ta2O5 and Nb2O5 thin films

Tantalum(V) and niobium(V) oxide films, which are typically difficult to prepare by electrochemical methods using aqueous solutions, are easily fabricated in an acetone bath using Ta and Nb anodes as the metal sources and a metal-free solvent containing halide ions as the supporting electrolyte. At the initial stage of electrolysis, anodic oxidation of the metal anode proceeds in the presence of water as an impurity in the acetone solvent. Subsequently, pitting corrosion of the oxide film on the metal anode occurs as a result of the action of halide ions. In this stage, anodic corrosion proceeds only in the presence of Br₂, and not in acetone containing I₂. Finally, Ta or Nb species are deposited directly on the cathode surface via the reactions with cathodically generated hydroxide ions, and the films need to be annealed at high temperature to effect crystallization. In these processes, the metal plate acts as a soluble anode with respect to Br⁻ and as a metal source for electrodeposition. The coating on a stainless steel substrate prepared by the present technique acts as an effective barrier against electrolytic corrosion.     

Study of a gold electrode modified by trans-[Ru(NH3)4(Ist)SO4] to produce an electrochemical sensor for nitric oxide

The modification of a gold electrode surface by electropolymerization of trans-[Ru(NH₃)₄(Ist)SO₄]⁺ to produce an electrochemical sensor for nitric oxide was investigated. The influence of dopamine, serotonin and nitrite as interferents for NO detection was also examined using square-wave voltammetry (SWV). The characterization of the modified electrode was carried out by cyclic voltammetry, electrochemical quartz crystal microbalance (EQCM) and SERS techniques. The gold electrode was successfully modified by the trans-[Ru(NH₃)₄(Ist)SO₄]⁺ complex ion using cyclic voltammetry. The experiments show that a monolayer of the film is achieved after ten voltammetric cycles, that NO in solution can coordinate to the metal present in the layer, that dopamine, serotonin and nitrite are interferents for the detection of NO, and that the response for the nitrite is much less significant than the responses for dopamine and serotonin. The proposed modified electrode has the potential to be applied as a sensor for NO.     

Electrophoretic deposition of BaTiO3 thin films from stable colloidal aqueous solutions

Polycrystalline BaTiO₃ (BTO) thin films were electrodeposited on titanium foils from colloidal aqueous solutions of BTO nanoparticles which were synthesized by a hydrothermal process, under autogenous conditions. Two kinds of reactants have been studied: one involves an aqueous solution of titanium tetrachloride and barium hydroxide octahydrate without any added solvent, whereas the second was obtained by dispersing amorphous titanium oxide in different volumes of barium hydroxide aqueous solutions. The nanoparticles were characterized by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). The particle size distribution (PSD) and the zeta potential measurements indicate that the colloids in the solution are polydisperse when using the first synthesis method, and they tend to be quasi monodisperse in the case of the second synthesis route. A porous microstructure of the electrodeposited films was detected by SEM. Finally, the dielectric properties of the BTO thin films were investigated.     

La0.6Ca0.4CoO3, La0.1Ca0.9MnO3 and LaNiO3 as bifunctional oxygen electrodes

A series of perovskite catalysts was investigated for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline electrolyte and at room temperature, supplied by oxygen or air. A meniscus cell was used to screen-test candidate catalysts for their bifunctionality and assess their activity for ORR at 3 mm depth of immersion (DOI) in the electrolyte. Based on the meniscus data LaNiO₃, La_0.1Ca_0.9MnO₃ and La_0.6Ca_0.4CoO₃ were selected for further assessment in microelectrode and half-cell studies. Activity tests for the ORR and OER, Tafel slopes at high current densities and apparent activation energies for the ORR were determined using a microelectrode technique on samples of the selected perovskites, La_0.1Ca_0.9MnO₃, La_0.6Ca_0.4CoO₃ and LaNiO₃ with and without graphite support. Tafel slopes of ca. 120 mV per decade and apparent activation energies of approximately 18 kcal mol⁻¹ were measured at high cathodic current densities. Cycle-life and performance of La_0.1Ca_0.9MnO₃, La_0.6Ca_0.4CoO₃ and LaNiO₃-based gas-diffusion electrodes in half-cell configurations were tested at a constant current density of 25 mA cm⁻² with subsequent and intermittent polarizations. Similar activities resulted in the ORR, while increased numbers of cycles were observed for the La_0.1Ca_0.9MnO₃-based electrode. Furthermore, electrode material compositions, especially PTFE contents were optimized to conform to the establishment of the three phase interactions of the electrode structure. Transmission Electron microscopy (TEM) and BET-surface area analyses were carried out in order to find out the morphological and surface properties of the perovskite materials.     

Microstructures and ferroelectric properties of PbTiO3/PbZrO3 superlattices deposited by pulse laser deposition

In general, the strain effect and the electrostatic effect are important factors influencing the electrical properties of ferroelectric superlattices. However, the interfacial diffusion may also greatly influence the electrical properties of ferroelectric superlattices. Here, we deposited PbTiO₃/PbZrO₃ (PTO/PZO) superlattices on Nb-doped SrTiO₃ (NSTO) single-crystal substrates by pulse laser deposition with the same deposition processes but different cooling processes to explore the effects of interface diffusion on the structural and electrical properties of the superlattices. The experimental results showed that with increasing the holding time in the cooling process after deposition, the ferroelectric and dielectric properties were enhanced, meanwhile, the leakage current density was reduced. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) results showed that the PTO/PZO interfaces became blurred with increasing cooling time. Therefore, the increase in ferroelectric and dielectric properties and the decrease in leakage current density may be due to the interdiffusion of Ti and Zr ions at the PTO/PZO interfaces resulting in the formation of Pb(Zr_xTi_1-x)O₃ (PZT) and the decrease of interface defects at the PTO/PZO interfaces. Our results demonstrate that besides the strain and electrostatic effects, the interdiffusion of the elements at the interfaces is also an important factor that influences the electrical properties of ferroelectric superlattices.     

Ketone-amine based suspensions for electrophoretic deposition of Al2O3 and ZrO2

Stable suspensions based on methylethylketone (MEK), n-butylamine and nitrocellulose were developed for the electrophoretic deposition (EPD) of Al₂O₃ and ZrO₂ powder. Deposits with a high green density, smooth surface and high deposition yield were obtained upon adding 10–15 vol.% n-butylamine in combination with 1 wt.% nitrocellulose. The influence of the reaction between MEK and n-butylamine, forming water and imines, on the electrophoretic deposition behaviour was investigated. Experimental results revealed that the zeta potential is not a straightforward indication of the stability of these suspensions, since the maximum absolute zeta potential did not correspond with a maximum suspension stability, due to the additional electrosteric stabilisation of the adsorbed charged nitrocellulose.     

Effect of rare-earth element oxides (La2O3, Ce2O3) on the structural and physico-chemical characteristics of Pd/Al2O3 monolithic catalysts of nitrogen oxide reduction by methane

It is shown that introduction of additives of rare-earth element oxides (La₂O₃, CeO₂) enables regulating the structural and functional characteristics of Pd/Al₂O₃-catalysts (applied on ceramic monoliths of honeycomb structure) of nitrogen oxide reduction by methane. Modifying additives provide increase of thermal stability of porous structure of both highly dispersed Al₂O₃, as the second support, and the catalyst as a whole.

Contribution of La₂O₃ and CeO₂ in increasing the thermal stability is of an additive nature, and lanthanum oxide shows the higher efficiency than cerium one. According to X-ray phase analysis data, stabilizing action is conditioned by occurrence of rare-earth element oxides into lattice of Al₂O₃, which retards diffusional processes leading to phase transitions of low-temperature crystalline modifications of alumina into high-temperature ones with a low specific surface. For the catalyst samples modified with La₂O₃ an effect of thermal activation is observed, which is revealed by increase in catalytic activity as a result of annealing at 850 °C. Such a phenomenon, as shown by means of X-ray photoelectron spectroscopy technique, can be explained via stabilization of palladium in singly charged state in the form of groups of Pd⁺O₂⁻ and corresponding increase in concentration of active centers.     

Effect of oxygenation on electrocatalysis of La0.6Ca0.4CoO3−x in bifunctional air electrode

A vacuum-annealed La_0.6Ca_0.4CoO_3−x was consecutively oxygenated in air at temperatures decreasing from 800 to 100 °C, and its electrocatalytic activities for oxygen reduction and evolution were then measured as a function of the oxygenation temperature. The valence of Co cation, changing between +2 and +3, was found susceptible to annealing either in vacuum or air. The catalytic activities initially decrease monotonically as the oxygenation temperature was decreased from 800 to 300 °C, as a result of increasing oxygen content, and then rise abruptly with the oxygen reduction activity reaching a maximum at 200 °C and the oxidation activity at 150 °C. X-ray photoelectron spectroscopy analysis indicated that the enhancements by the low-temperature oxygenation involved increased OH coverage and less charged cations at surface. The results clearly reveal the importance of the post-calcination annealing process for optimizing the performance of La_0.6Ca_0.4CoO_3−x in air electrode applications.     

Effect of hydrodynamics on the deposition of CaSO4 scale on stainless steel

Laboratory experiments were conducted using a rotating cylinder electrode apparatus to study the effect of solution hydrodynamics on the deposition of calcium sulfate scale on stainless steel substrate. The results indicate that the solution hydrodynamics has a significant bearing on the deposition rate of calcium sulfate. The results are in agreement with the theoretical prediction of a diffusion controlled process. Furthermore, scanning electron microscopy revealed the secondary nucleation growth of prismatic needle- or rod-like crystals branching out randomly over the already existing primary crystals of CaSO₄ on the SS-316 substrate.     

CO oxidation catalysts based on copper and manganese or cobalt oxides supported on MgF2 and Al2O3

The catalytic activity of a mixed phase of copper–cobalt and copper–manganese oxides supported on magnesium fluorine or alumina has been studied in low temperature CO oxidation at 30 °C. During calcination, the oxides studied partially react to form different type spinels depending on the calcination temperature. These spinels have different effect on the catalytic activity. In low temperature CO oxidation the copper–manganese catalysts are more active than the copper–cobalt ones.