Electrochemical recovery of silver from waste aqueous Ag(I)/Ag(II) redox mediator solution used in mediated electro oxidation process

The paper presents a process for the electrochemical recovery of silver(Ag) by electro deposition on the electrode surface from the waste solutions of Ag(I)/Ag(II) redox system in nitric acid medium used for the mediated electrochemical process. Electrochemical recovery was carried out in an undivided cell with DSA-O₂ electrodes at room temperature condition. At an optimized current density of 12 A/dm², 99% of Ag recovery efficiency was achieved with high yield and low energy consumption. Experimental runs were made in order to observe the performance of the Ag recovery process. The operating conditions like current density, temperature and Ag(I) concentration of the electrolyte, the acid concentrations, agitation rate and inter-electrode distance were optimized.     

Electrochemical behavior of ruthenium (III), rhodium (III) and palladium (II) in 1-butyl-3-methylimidazolium chloride ionic liquid

Electrochemical behavior of ruthenium (III), rhodium (III) and palladium (II) in 1-butyl-3-methylimidazolium chloride (bmimCl) and their ternary and binary solutions in bmimCl was studied at various working electrodes at 373 K by cyclic voltammetry and chronoamperometry. Ruthenium (III) chloride forms a stable solution with bmimCl and the cyclic voltammogram of ruthenium (III) in bmimCl recorded at glassy carbon electrode consisted of several redox waves due to the complex nature of ruthenium to exist in several oxidation states. Electrolysis of ruthenium (III) chloride in bmimCl at the cathodic limit of bmimCl (−1.8 V (vs. Pd)) did not result in ruthenium metal deposition. However, it was deposited from bmimPF₆ and bmimNTf₂ room temperature ionic liquids at −0.8 V (vs. Pd). The electrochemical behavior of ruthenium (III) in bmimCl in the presence of palladium (II) and rhodium (III) was studied by cyclic voltammetry. The presence of palladium (II) in bmimCl favors underpotential deposition of ruthenium metal. The nuclear loop at −0.5 V (vs. Pd) was observed in all solutions when palladium (II) co-existed with other two metal ions. Nucleation and growth of the metal on glassy carbon working electrode was investigated by chronoamperometry. The growth and decay of chronocurrents has been found to follow the instantaneous nucleation model with three-dimensional growth of nuclei.     

Electrochemical gold deposition from sulfite solution: application for subsequent polyaniline layer formation

Electrochemical gold deposition from sulfite solutions was studied by means of voltammetry, EIS and EQCM. A gold film electrode was used for polyaniline layer formation by electrochemical oxidation of aniline. The standard electrochemical reduction potential of the reaction [Au(SO₃)₂]³⁻ + e⁻ = Au + 2 SO₃²⁻ was determined, and is equal to 0.116 V (vs. NHE). Both solution stirring and temperature increase accelerate the electrochemical reduction of gold, when the electrode potential is below −0.55 V. When the potential is above −0.55 V the electrochemical reduction proceeds via passive layer formation. Our study suggests that the passive layer consists of chemically adsorbed sulfite ions and sulfur. The gold film deposited from sulfite solution is a high quality substrate suitable for conducting polymer layer formation. This technique, where a polymer layer electrode is prepared by thin gold film deposition onto a metal surface and by subsequent polymer layer formation, can be applied in sensor research and technology.     

Reduction of chromate in dilute solution using hydrogen in a GBC-cell

Reduction of metal ions in dilute solutions is of great interest for purification of waste waters and process liquids. Hydrogen gas is a very attractive reductant, since its use gives no additional pollution. In this paper the reduction of chromate in a sulphuric acid medium has been studied. A new electrochemical cell, a GBC-cell, which is a combination of a gas-diffusion electrode in direct contact with a packed bed of carbon particles, is introduced. Hydrogen gas flows along the hydrophobic side of the gas-diffusion electrode and a chromate solution is pumped upwards through the bed. Experiments were carried out with H₂SO₄ solutions initially containing 70 mol m^–3 chromate at various temperatures, solution flow rates, H₂SO₄ concentrations and bed thicknesses. Experimental results for the chromate reduction are described by an empirical relation. It has been found that the reduction of chromate is a first-order reaction in chromate and the apparent rate constant for the chromate reduction increases with decreasing chromate concentration and increasing temperature, H₂SO₄ concentration and bed thicknesses and is practically independent of the flow rate of the solution. It is concluded that the new GBC-cell is very attractive for the reduction of chromate in dilute solutions and for industrial abnlication on a large scale.     

Electrophoretic deposition and properties of strontium-doped sodium potassium niobate thick films

We have processed strontium-modified potassium sodium niobate (KNNSr) piezoelectric thick films on metalized alumina by electrophoretic deposition and sintering for energy-harvesting applications. The deposition yield, the electric field in the close vicinity of the planar working electrode (calculated with a finite-element method) and the thickness profiles of the as-deposited layers were examined for ethanol-based suspensions with different conductivities, deposition times, inter-electrode distances and sizes of the counter electrode. Uniformly thick and defect-free, as-deposited KNNSr layers were processed from a suspension with a conductivity of 21.2 μS/cm, deposited for 90 s, for an inter-electrode distance of 3 mm and a diameter of the counter electrode similar to the sample’s dimensions. KNNSr layers sintered at 1100 °C with a thickness of 30 μm and a relative density of 75% exhibited a dielectric permittivity of 325, dielectric losses of 0.04 at 10 kHz and room temperature, and a d₃₃ coefficient of 60 pC/N.     

Characterization of ultramicroelectrode arrays combining electrochemical techniques and optical microscopy imaging

The fabrication of chemical transducers using standard Si/SiO₂/metal microelectronic technology has given rise to a widespread development of ultramicroelectrode arrays (UMEAs) as analytical tools. The electrochemical behaviour and performance of UMEAs depend on their geometry, radius and inter-electrode distance. Therefore, a suitable design and fabrication process is required. Provided that UMEA fabrication processes are not totally efficient so far, a small fraction of the electrodes on-chip always remain passivated. In this work, Pt and Au UMEAs with three different geometries were designed and fabricated. A complete electrode characterization in terms of fabrication efficiency and analytical response is carried out by combining electrochemical techniques, transmission light and fluorescence optical imaging. Voltammetric measurements performed in ferrocyanide solutions used as a model electroactive molecule, always results in a reproducible sigmoidal response for different potential scan rate intervals depending on the electrode geometry. Optical visualization of the electrodes using transmission light microscopy relies on either electrochemical oxidation of the electrode surface or electrodeposition of gold nanoparticles. Alternatively, the anchorage of fluorescent oligonucleotide conjugates on the gold nanoparticle surface enables electrode imaging by fluorescence microscopy. Results obtained with the different techniques proposed show an excellent correlation among them. Differences in data turned out to be non-statistically relevant. It is demonstrated that less than 20% of the individual electrodes within one chip remained passivated.     

Production of syngas plus oxygen from CO2 in a gas-diffusion electrode-based electrolytic cell

Most of the research published in electrochemical CO₂ reduction has been reported for half-cells, with little consideration of the overall system. However, it is necessary to consider the eventual involvement of full cells. We conducted CO₂ reduction and water oxidation in a CO₂-reducing full cell with larger geometric surface area (2×2 cm²) and with a relatively small inter-electrode gap (1-2 mm) in order to minimize ohmic losses. The result was an ca. 1:1 CO/H₂ (v/v) gas ratio at a current density of 10 mA cm⁻² and a cell voltage of 3.05 V, producing O₂ at the counter electrode. Based on an enthalpic voltage of 1.36 V, this constitutes an overall energy efficiency of 44.6%.     

Electrocatalytic oxidation of methanol on the nickel–cobalt modified glassy carbon electrode in alkaline medium

In this work, Ni–Co alloy coating on the surface of glassy carbon (GC) electrode was performed by cyclic voltammetry. The results showed that the deposition of Ni–Co is an anomalous process. The deposition bath was prepared according to the metal ion Ni/Co ratio of 4:1 using NiSO₄·7H₂O and CoSO₄·8H₂O, and the total concentration of all solutions was 40.0 mM. The pH of the bath solution was adjusted at 2.0 using boric acid at room temperature. The modified electrode was conditioned by potential recycling in a potential range of 100–700 mV (vs. Ag/AgCl) by cyclic voltammetric method in an alkaline solution. The Ni–Co modified electrode showed a higher activity towards methanol oxidation in the Ni (III) and Co (IV) oxidation states. Cyclic voltammetry was used for the electrochemical characterization of the Ni–Co modified electrode and the mechanism of methanol oxidation is proposed. The result of double steps chronoamperometry shows that the methanol electrooxidation is an irreversible reaction. Moreover, the effects of various parameters such as mole ratio of Ni–Co in the alloy in modification step, potential scan rate, methanol concentration and solution temperature on the electro-oxidation of methanol have also been investigated.     

Nanoscale surface modification of diamond for enhanced electrochemical sensing: Electrochemical characteristics of the surface modification

Nanoscale modification of the surface characteristics of boron-doped diamond has been demonstrated by local electrical discharge in electrolyte solutions. The nano-spots were created on (100)-oriented and oxygen-terminated surface of single crystal diamond using a Pt–Ir tip close to the surface. The created nano-spots caused an increase of the electrochemical activity of the diamond surface in 0.1 M H₂SO₄ electrolyte observed in the anodic potential range. The presence of the nano-spots was identified by selective deposition of gold clusters on the processed surface in a gold galvanic solution.     

Spherical polymer brushes with vinylimidazolium-type poly(ionic liquid) chains as support for metallic nanoparticles

We present here the synthesis of spherical polyelectrolyte brushes by photo-emulsion polymerization with a solid polystyrene core (diameter ～ 100 nm) onto which chains of vinylimidazolium-type poly(ionic liquid)s have been densely grafted. The as-synthesized brush particles were employed as nanoreactors for the generation and immobilization of metal nanoparticles. Transmission electron microscopy characterization shows that Au and Pd nanoparticles with diameter of 2.1 ± 0.2 nm and 2.5 ± 0.3 nm are homogeneously embedded inside the PIL brushes, respectively. The study of swelling behavior of the brush particles before and after metal deposition indicates obviously ion-specific effect. The composite particles exhibit a long-term colloidal stability in aqueous solutions as well as in ionic liquids. Catalytic activity of the as-synthesized metal nanocomposite particles is investigated by using the reduction of 4-nitrophenol with NaBH₄ as a model reaction, which can be compared directly with reported systems. In addition, it is found that the rate constant k_app of PIL-metal nanocatalyst could be modulated by salt concentration.     

Effects of non-linear kinetics on free convection in an electrochemical cell with a porous separator

The spatial evolution of the ionic concentration of an electrolyte in an isothermal electrochemical cell with a porous separator between the electrodes was investigated for large values of Rayleigh number. The reaction kinetics were described by the Butler–Volmer equation. The full problem, involving the coupled partial differential equations describing the velocity field, the ionic concentration, and the electric potential, was reduced by means of regular and singular perturbation theory, to a simplified evolution equation, coupled with a transcendental function for the ionic concentration and electric potential; the solution was found to agree well with the numerical solution of the full problem. In the limit of large and small cell voltages, closed analytical solutions were secured for the concentration, potential, and overall current density.     

Effects of a small addition of Mn on the corrosion behaviour of Zn in a mixed solution

The effects of a small addition of Mn (0.4 wt%) on the corrosion behaviour of pure Zn (99.995 wt%) in a mixed solution (0.1 M NaCl + 0.1 M Na₂SO₄ + 0.01 M NaHCO₃, pH 8.4) were investigated by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and X-ray photoelectron spectroscopy (XPS). The electrochemical impedances of both Zn and Zn–0.4Mn have been successfully fitted with a suitable EIS equivalent circuit model. Fitted impedance results revealed that 0.4 wt% Mn improved both the pore resistance and charge transfer resistance of Zn in the mixed solution. As a result, both anodic and cathodic reaction rates were reduced. X-ray photoelectron spectroscopy (XPS) analysis showed that the corrosion films formed in the mixed solution consisted of zinc oxide (ZnO), zinc hydroxide (Zn(OH)₂) and zinc hydrozincite (Zn₅(CO₃)₂(OH)₆). The role of small addition of Mn is that it promotes the precipitation of hydrozincite in the pores of corrosion film. An “alleviation of local acidification” mechanism is proposed to explain the investigated results.     

III. Co-electrodeposition/removal of copper and nickel in a spouted electrochemical reactor

Results are presented of an investigation of co-electrodeposition of copper and nickel from acidic solution mixtures in a cylindrical spouted electrochemical reactor. The effects of solution pH, temperature, and applied current on metal removal/recovery rate, current efficiency, and corrosion of the deposited metals from the cathodic particles were examined under galvanostatic operation. The quantitative and qualitative behavior of co-electrodeposition of the two metals from their mixtures differs significantly from that of the individual single metal solutions. This is primarily attributed to the metal displacement reaction between Ni(0) and Cu(2+). This reaction effectively reduces copper corrosion, and amplifies that for nickel (at least at high concentrations). It also amplifies the separation of the deposition regimes of the two metals in time, which indicates that the recovery of each metal as a relatively pure deposit from the mixture is possible. It was also shown that nitrogen sparging considerably increases the observed net electrodeposition rates for both metals - considerably more so than from solutions with just the single metals alone. A numerical model of co-electrodeposition, corrosion, metal displacement, and mass transfer in the cylindrical spouted electrochemical reactor is presented that describes the behavior of the experimental copper and nickel removal data quite well.     

微流控反应器中苯酚在Ti/SnO2-Sb2O5电极上的阳极氧化分析

目前在电化学氧化处理法降解苯酚废水的研究过程中,研究者多将重心放在活性电极的探索及制备上,而对于反应器的开发鲜有报道。就这一问题,本文研究了新型微流控反应器中苯酚的电化学降解效果。电化学氧化实验在装有Ti/SnO₂-Sb₂O₅阳极的微型流通池中操作进行,实验对循环体系的体积流率Φ_V、电极间距h的影响进行了考察。结果表明,当流通电解槽中的阴阳极间距采用微米级尺寸时,苯酚的阳极氧化反应取得了较快的氧化速度。在 em= 20 mA/cm²、Φ_V= 0.54 mL/min的电解条件下,电解2~3h苯酚去除率即可达到90%以上,相同流速下电极间距h越小降解速率越快。且由数据回归得到了苯酚的一系列随h的减小而增大的准一级反应的反应速率常数。这一结论表明微流控电解槽内的苯酚降解过程主要传质控制过程。     

Microscopic and electrochemical characterization of lead film electrode applied in adsorptive stripping analysis

Lead film electrodes (PbFEs) deposited in situ on glassy carbon or carbon paste supports have recently found application in adsorptive stripping voltammetric determination of inorganic ions and organic substances. In this work, the PbFE, prepared in ammonia buffer solutions, was investigated using scanning electron microscopy, atomic force microscopy and various voltammetric techniques. The microscopic images of the lead films deposited on the glassy carbon substrate showed a considerable variability in microstructure and compactness of the deposited layer depending on the selected experimental conditions, such as the concentration of Pb(II) species, the nucleation and deposition potential, and the time applied. The catalytic adsorptive systems of cobalt and nickel in a solution containing 0.1 ammonia buffer, 2.5 × 10⁻⁵ M nioxime and 0.25 M NaNO₂ were employed to investigate the electrochemical characteristics and utility of the in situ prepared lead films.The optimal parameters, i.e. the lead concentration in the solution, the procedure of film removal, and the time and potential of lead nucleation and film deposition for the adsorptive determination of metal traces, were selected, resulting in the very good reproducibility (RSD = 4.2% for 35 scans) of recorded signals. The voltammetric utility of the lead film electrode was compared to that of glassy carbon, mercury film and bismuth film electrodes, and was subsequently evaluated as superior.     

Desulfurization of gasoline by a new method of electrochemical catalytic oxidation

A new deep desulfurization process for gasoline was obtained by means of electrochemical catalytic oxidation with an electrochemical fluidized-bed reactor on particle group anode. The particle group anode was activated carbon-supported cerium dioxide (CeO₂/C), and the electrolyte was aqueous cerium nitrate solution, and copper pillar was cathode in the electrochemical reactions. The CeO₂/C particle group anode could remarkably accelerate the electrochemical reaction rate and promote the electrochemical catalysis performance for the electrochemical desulfurization reaction. Thermodynamics feasibility analysis clarified that the theoretical decomposition voltage ranged from 0.1–0.5 V in pure acid electrolyte system and desulfurization reactions could not spontaneously carry out, but the reactions were spontaneous when aqueous cerium nitrate solution serves as electrolyte. And the rule of the gasoline desulfurization by the means of electrochemical catalytic oxidation was investigated. The experimental results indicated that the optimal desulfurization conditions were as follows: the cell voltage, concentration of the Ce³⁺ ions, feed volume flow rate and the CeO₂ percentage by weight were 3.2 V, 0.08 mol l⁻¹, 300 ml min⁻¹ and 5.0 wt%, respectively. Under these conditions the concentration of sulfur in gasoline was reduced from 310 to 50 parts per million by weight (ppmw). Based on these experimental results, a mechanism of indirect electrochemical oxidation was also proposed.     

The initial stages of palladium deposition onto Pt(1 1 1)

The electrodeposition of Pd onto Pt(1 1 1) from PdCl₂ and PdSO₄ containing solutions was studied by cyclic voltammetry and in situ scanning tunnelling microscopy. Pd deposition starts by forming a pseudomorphic monolayer in both cases. While in the presence of chloride this monolayer is deposited at underpotentials, its formation in chloride-free solution is kinetically hindered to such an extent that the deposition peak is shifted negative of the equilibrium potential. Detailed structure information has been obtained from STM data about the Pd layer as well as the co-adsorbed anions. Bulk deposition from the chloride-containing solution proceeds via a quasi layer-by-layer fashion. However, the particular electrochemical properties of the first Pd monolayer disappear only after deposition of the equivalent of four more Pd layers. The electrochemical behaviour of such films is similar to that of a rough Pd(1 1 1) surface. Pd bulk deposition from chloride-free solution leads to the formation of three-dimensional clusters from the very beginning. About 10 ML equivalents are needed in that case to completely cover the first Pd monolayer.     

Deposition of tin oxide,iridium and iridium oxide films by metal-organic chemical vapor deposition for electrochemical wastewater treatment

In this research, the specific electrodes were prepared by metal-organic chemical vapor deposition (MOCVD) in a hot-wall CVD reactor with the presence of O₂ under reduced pressure. The Ir protective layer was deposited by using (Methylcyclopentadienyl) (1,5-cyclooctadiene) iridium (I), (MeCp)Ir(COD), as precursor. Tetraethyltin (TET) was used as precursor for the deposition of SnO₂ active layer. The optimum condition for Ir film deposition was at 300 °C, 125 of O₂/(MeCp)Ir(COD) molar ratio and 12 Torr of total pressure. While that of SnO₂ active layer was at 380 °C, 1200 of O₂/TET molar ratio and 15 Torr of total pressure. The prepared SnO₂/Ir/Ti electrodes were tested for anodic oxidation of organic pollutant in a simple three-electrode electrochemical reactor using oxalic acid as model solution. The electrochemical experiments indicate that more than 80% of organic pollutant was removed after 2.1 Ah/L of charge has been applied. The kinetic investigation gives a two-step process for organic pollutant degradation, the kinetic was zero-order and first-order with respect to TOC of model solution for high and low TOC concentrations, respectively.     

Influence of cell construction on the electrochemical reduction of nitrate

The electrochemical reduction of nitrates in the weakly alkaline electrolyte, simulating spent solution after regeneration of a strongly basic anion exchanger was studied. Copper was used as the cathode material. The influence of the electrolyser construction, cathode form and process parameters on the current efficiency of the nitrate reduction was followed. Four types of cell construction were used: a plate-electrode cell, a cell with a fluidised bed of inert particles in the inter-electrode space, a packed bed cathode cell and a vertically moving particle bed cell. The highest current efficiency with respect to the nitrate reduction was observed for the vertically moving particle bed reactor. On the other hand, unexpectedly high efficiency was also observed for the simple plate-electrode cell, the disadvantage being very low current density (below 40 A m⁻²) resulting in suitable behaviour. From the point of view of cell efficiency coupled with simplicity of construction and operation, the optimal cell seems to be the cell with the fluidised bed of inert particles in the inter-electrode space.     

Electrodeposition and Capacitive Behavior of Films for Electrodes of Electrochemical Supercapacitors

Polypyrrole films were deposited by anodic electropolymerization on stainless steel substrates from aqueous pyrrole solutions containing sodium salicylate and tiron additives. The deposition yield was studied under galvanostatic conditions. The amount of the deposited material was varied by the variation of deposition time at a constant current density. SEM studies showed the formation of porous films with thicknesses in the range of 0–3 μm. Cyclic voltammetry data for the films tested in 0.5 M Na₂SO₄ solutions showed capacitive behavior and high specific capacitance (SC) in a voltage window of 0.9 V. The films prepared from pyrrole solutions containing tiron showed better capacitive behavior compared to the films prepared from the solutions containing sodium salicylate. A highest SC of 254 F g⁻¹ was observed for the sample with a specific mass of 89 μg cm⁻² at a scan rate of 2 mV s⁻¹. The SC decreased with an increasing film thickness and scan rate. The results indicated that the polypyrrole films deposited on the stainless steel substrates by anodic electropolymerization can be used as electrodes for electrochemical supercapacitors (ES).