Electrodeposition of nickel-cobalt-zinc alloys from a borate bath

The electrodeposition has been studied of nickel-cobalt-zinc alloys from a borate bath containing nickel sulphate (120–140 g dm^–3), cobalt sulphate (30–46 g dm^–3), zinc sulphate (144–168 g dm^–3), boric acid (30 g dm^–3) and ammonium chloride (2 g dm^–3). The operating conditions were: current density, 2.0–5.0 A dm^–2; temperature, 30–40°C and pH, 2.4 to 5.4. Light grey, semibright, stressed films have been obtained. However, the deposits consist partially of black powder when the concentration of the various components is increased. The brightness is found to increase with decreasing temperature and pH of the solution. The total cathode efficiency increases when the pH and temperature of the solution decrease, whereas at any particular pH and temperature it first decreases, reaches a minimum and then increases with increasing current density.     

Role of Mo<Superscript>6+</Superscript> during nickel electrodeposition from sulfate solutions

The effect of Mo⁶⁺ on the current efficiency, deposit quality, surface morphology, crystallographic orientations and polarisation behaviour of the cathode during electrodeposition of nickel from sulfate solutions was investigated. Mo⁶⁺ did not have a significant effect on current efficiency over the concentration range 2–100 mg dm⁻³. However; a decrease in current efficiency by a magnitude of more than 20% was seen at 500 mg dm⁻³. The quality of the nickel deposit with reference to the visual appearance and contamination level varied with varying concentration of Mo⁶⁺; this was also reflected in the morphology and crystallographic orientations of the deposits. Addition of Mo⁶⁺ to the electrolyte introduced two new crystal planes i.e., (220) and (311). Depolarisation of the cathode was noted at lower concentrations of Mo⁶⁺ (2–40 mg dm⁻³) whereas polarisation of the cathode was observed at Mo⁶⁺ concentration >40 mg dm⁻³ .The effect of Mo⁶⁺ on parameters such as Tafel slope (b), transfer coefficient (α) and exchange current density (i ₀) were also determined.     

Influence of concentrations of copper,levelling agents and temperature on the diffusion coefficient of cupric ions in industrial electro-refining electrolytes

Few data are available for diffusion coefficients measured in industrial copper electrolytes. In the present work the influence of copper concentration (19.9–58.1 g dm⁻³), temperature (20–60°C) and concentrations levelling agents i.e. animal glue (0–5 mg dm⁻³) and thiourea (0–5 mg dm⁻³) on diffusion coefficients of copper was studied in industrial copper refinery electrolytes. Chronoamperometry at ultramicroelectrodes was used as an electrochemical technique. Apparent bulk diffusion coefficients were calculated on the basis of the theory of electrochemical nucleation on disc-shaped ultramicroelectrodes. Increasing copper concentration decreased the apparent bulk diffusion coefficient of copper and diffusion coefficients followed the Arrhenius temperature relationship. The experimental activation energy was 26.8 kJ mol⁻¹. The influence of levelling agents on diffusion coefficients was not strong in the studied concentration range of animal glue and thiourea.     

Simultaneous removal of organic and inorganic pollutants in tannery wastewater using electrocoagulation technique

Tannery wastewater can cause severe environmental problems related to its high chemical oxygen demand, high biochemical oxygen demand, high total suspended solids, high oil and grease contents together with the elevated chromium concentration and objectionable color. The one-step electrocoagulation process was carried out to simultaneously remove chromium and various pollutants from tannery wastewater at ambient temperature in the laboratory scale. Low-cost commercial iron plates were employed in this study as anodes and cathode materials. Effects of various parameters were investigated including types of electrode configuration, initial pH of wastewater (7–9), current density (15.7–24.6 Am⁻²) and circulating flow rate of wastewater (0–3.67 lmin⁻¹). The optimum condition was found by applying the mono-polar electrode in a parallel connection at the current density of 22.4 Am⁻², flow rate of wastewater of 3.67 lmin⁻¹ and 20 min electrolysis time. The initial pH of wastewater ranging from 7–9 provided the similar removal efficiency. At optimum condition, more than 95% of chromium and pollutants except TKN and TDS were eliminated from the wastewater and the properties of the treated wastewater met the standard and permitted to discharge into the environment. The required energy consumption at optimum condition was less than 0.13 kWhm⁻³ wastewater. In addition, the COD reduction was fit very well with the first-order kinetics model.     

Characterisation of a laboratory electrochemical ozonation system and its application in advanced oxidation processes

An electrochemical reactor for oxygen/ozone production was developed using perforated planar electrodes. An electroformed -PbO₂ coating, deposited on a platinised titanium substrate, was employed as anode while the cathode was a platinised titanium substrate. The electrodes were pressed against a solid polymer electrolyte to minimise ohmic drop and avoid mixing of the gaseous products (H₂ and O₂/O₃). Electrochemical ozone production (EOP) was investigated as function of current density, temperature and electrolyte composition. Electrochemical characterisation demonstrated ozone current efficiency, Φ_EOP, ozone production rate (g h⁻¹), , and grams of O₃ per total energy demand (g h⁻¹ W⁻¹), increase on decreasing electrolyte temperature and increasing current density. The best reactor performance for EOP was obtained with the base electrolyte (H₂SO₄ 3.0 mol dm⁻³) containing 0.03 mol dm⁻³ KPF₆. Degradation of reactive dyes used in the textile industry (Reactive Yellow 143 and Reactive Blue 264) with electrochemically-generated ozone was investigated in alkaline medium as function of ozone load (mg h⁻¹) and ozonation time. This investigation revealed ozonation presents very good efficiency for both solution decolouration and total organic carbon (TOC) removal.     

Electrochemical degradation of the Acid Blue 62 dye on a β-PbO<Subscript>2</Subscript> anode assessed by the response surface methodology

The electrochemical degradation of the anthraquinonic dye Acid Blue 62 in a filter-press reactor on a Ti/Pt/β-PbO₂ anode was investigated using the response surface methodology with the variables: current density, pH, [NaCl], and temperature. The system’s modeling was carried out with the charge required for 90% decolorization (Q ⁹⁰) and the chemical oxygen demand removal percentage after a 30 min electrolysis (COD ³⁰), with good correlations between predicted and observed values. Best conditions for decolorization were attained in acidic solutions (pH = 4) with medium to high [NaCl] (1.0–2.0 g L⁻¹) and lower temperature due to the prevalent oxidant species HOCl and Cl₂. Optimal conditions for COD ³⁰ removal were attained at high current densities in pH > 5 solutions with high [NaCl], when the prevalent oxidant species are HOCl and OCl⁻. The lowest charge per unit volume of the electrolyzed solution necessary for total mineralization was attained at pH 11.     

Electrochemical membrane reactor for the reduction of carbondioxide to formate

An electrochemical reactor with anode and cathode chambers separated by a composite perfluoro polymer cation exchange membrane was designed, fabricated and used for the reduction of dissolved carbon dioxide under ambient conditions to formate. The flow reactor enhanced the mass transfer of carbon dioxide compared to the batch reactor and maximum current efficiency of 93% for formate formation was obtained. A formate concentration of 1.5 × 10⁻² mol dm⁻³ was obtained. Experiments were conducted using two different perfluoro polymer membranes – Nafion 961 and Nafion 430. Optimum values of flow rate and current density were evaluated for the energy efficient formation of formate in aqueous phosphate buffer solutions.     

Synthesis and characterization of Ni–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite coatings containing different forms of alumina

Electrodeposited Ni–Al₂O₃ composite coatings were prepared using alumina powders synthesized from solution combustion method, precipitation method and a commercial source. Solution combustion synthesized alumina powder yielded α-phase; precipitation method yielded purely γ-phase; commercial alumina powder was a mixture of α-, δ- and γ-phases. A nickel sulfamate bath was used for electro-codeposition. The current densities (0.23 A dm⁻² for 20 h, 0.77 A dm⁻² for 6 h, 1.55 A dm⁻² for 3 h and 3.1 A dm⁻² for 1.5 h) and bath agitation speeds (100, 200, 600 and 1000 rpm) were varied. The pH variations of the bath were higher during the electrodeposition of combustion synthesized alumina. The effect of different forms of alumina particles on the microhardness and microstructure of the nickel composite coating was studied. Composite coating containing combustion synthesized alumina particles was found to have higher microhardness (550 HK). It was found that at lower agitation speed (100 rpm), bigger particles were incorporated and at higher agitation speed (1000 rpm), smaller particles were incorporated. The area fraction of alumina particles incorporated in nickel matrix was highest for commercial alumina (24%). This study shows that it is not suffice to take just the current density and stirring speeds into account to explain the properties of the coatings but also to take into account the source of particles and their properties.     

The use of a rotating cylinder electrode to selective recover palladium from acid solutions used to manufacture automotive catalytic converters

The reduction of palladium, rhodium and neodymium ions at concentrations of 0.94, 0.97 and 0.69 mol dm⁻³, respectively was studied in 1 mol dm⁻³ HNO₃ or 1 mol dm⁻³ HCl, at a stainless steel and a vitreous carbon electrode, at 25 °C. At a vitreous carbon electrode in a solution containing rhodium and palladium ions in 1 mol dm⁻³ HCl electrolyte, the reduction of metal ions occurred at a similar potential to the formation of hydrogen gas, which impeded the selective separation of the two metals. At a stainless steel cathode in 1 mol dm⁻³ HNO₃, palladium deposition occurred at a potential ≈0.35 V less negative than that of rhodium allowing the selective recovery of palladium. Neodymium ions were not electroactive in acidic chloride or nitrate media at pH 0. Using a solution obtained from a catalytic converter manufacturer containing palladium, rhodium and neodymium ions in 1 mol dm⁻³ HNO₃, palladium ions were preferentially removed at 0.15 V versus SHE at an average cumulative current efficiency of 57%.     

Manganese oxide nanocomposites with improved surface area prepared by one-pot surfactant route for electro catalytic and biosensor applications

Braunite phase manganese oxide is naturally available in manganese–silicate rocks with minor amount of silicate content. New synthetic route is attempted to prepare the manganese oxide nanoparticle and silica incorporated manganese oxide nanocomposite in the present study. XRD patterns reveal the braunite phase formation for as synthesized manganese oxide nanocomposite and silica incorporated MnO₂ nanocomposite materials. Improved BET surface area values are achieved by one step surfactant assisted method (i.e., 82 and 151 m²/g) compared to conventional route prepared manganese oxide nanomaterial. Flaky pastry type morphology was observed for as synthesized Si–MnO₂ nanocomposites. Cyclic voltammetry studies predict the electrocatalytic activity of manganese oxide nanoparticle and Si–MnO₂ nanocomposite in presence of electroactive redox couple. Si–MnO₂ nanocomposite modified glassy carbon (GC) electrode shows the effective electroactive response in presence of Fe²⁺/Fe³⁺ redox couple at 0.69 V with current density of 0.343 × 10⁻⁵ A/cm² compared to manganese oxide nanoparticle modified GC electrode. The biosensor responses for ascorbic acid have been tested in the present study and manganese oxide nanoparticle modified GC electrode shows effective response at low concentration of (1 × 10⁻⁵ M) ascorbic acid in phosphate buffer solution. Manganese oxide nanoparticle modified electrode shows the better response with current density value of 0.115 × 10⁻⁵ A/cm² compared to Si–MnO₂ nanocomposite.     

Electrodeposition and corrosion behaviour of a Ni–W–B amorphous alloy

Electrodeposition of Ni–W–B alloys from plating baths containing ammonia and citrate is reported. Optimum conditions for plating including current density, temperature, mechanical agitation and pH were studied. The corrosion resistance and amorphous character were also evaluated. The operational conditions for depositing the alloy with good corrosion resistance were: current density 35 mA cm⁻², bath temperature 40 °C, pH 9.0 and cathode rotation at 90 rpm. The alloy was deposited at 38% current efficiency, with an average composition of 73 wt% Ni, 27 wt% W and traces of boron and with E _corr −0.300 V and R _p 3.369×10⁴ Ω. The deposit obtained under these conditions had an amorphous character with the presence of some microcracks on its surface reaching down to the copper substrate. Electrochemical corrosion tests verified that the Ni–W–B alloy had better corrosion resistance than Co–W–B.     

Scale-up of the perforated bipole trickle-bed electrochemical reactor for the generation of alkaline peroxide

This paper reports work on the scale-up of a perforated bipole trickle-bed electrochemical reactor for the electro-synthesis of alkaline peroxide. The reactor uses a relatively simple cell configuration in which a single electrolyte flows with oxygen gas in a flow-by graphite felt cathode, sandwiched between a microporous polyolefin diaphragm and a nickel mesh/perforated Grafoil anode/bipole. Both one and two-cell reactors are scaled-up from cathode dimensions 120 mm high by 25 mm wide and 3.2 mm thick (reactor-A) to 630 mm high by 40 mm wide and 3.2 mm thick (reactor-B). The scale-up is achieved by the use of constrictions that prevent segregation of the 2-phase flow in the larger cell, combined with switching from a polypropylene to a polyethylene diaphragm with improved transport properties and raising the electrolyte feed concentration from 1 to 2 M NaOH.For the one-cell reactor-B with a polypropylene diaphragm, operating on a feed of 1 M NaOH and oxygen at 900 kPa(abs)/20 °C, the peroxide current efficiency at a superficial current density of 5 kA m⁻² increases from 27% (un-constricted cathode) to 57% with a constricted cathode. The corresponding current efficiencies at 3–5 kAm⁻² for reactor-A and the constricted reactor-B are respectively 69–64% and 66–57%. Under similar conditions at 3–5 kA m⁻² the one-cell constricted reactor-B with a polyethylene diaphragm gives current efficiencies of 88–64%, and changing to an electrolyte of 2 M NaOH raises this range to 90–80%. At 3–5 kA m⁻² the equivalent two-cell (bipolar) constricted reactor-B shows current efficiencies of 82–74% and at 5 kA m⁻² obtains 0.6 M peroxide in 2 M NaOH with specific energy 6.5 kWh per kg H₂O₂.     

The occurrence of bromate and perbromate on BDD anodes during electrolysis of aqueous systems containing bromide: first systematic experimental studies

Bromide electrolysis was carried out on laboratory-scale cells in the range of 1–1,005 mg [Br⁻] dm⁻³ using boron-doped diamond (BDD) anodes. These studies were part of fundamental research activities on drinking water electrolysis for disinfection. Synthetic water systems were mostly used in the experiments, which varied the temperature between 5 and 30 °C, the current density between 50 and 700 A m⁻², and the rotation rate of the rotating anode between 100 and 500 rpm (laminar regime). Hypobromite and bromate were found as by-products, as expected. Bromite was not detected. Higher bromate levels were formed at higher current density, but no clear relationship was observed between bromate concentration and the rotation rate or temperatures between 5 and 30 °C. Bromate yields higher than 90% were found at higher charge passed. Perbromate was found as a new potential synthesis or disinfection by-product (DBP), but no perbromate was detected at the lowest bromide concentrations and under drinking water conditions. The perbromate yield was about 1%, and somewhat lower when bromate was used as a starting material instead of bromide. At a temperature of 5 °C more perbromate was detected compared with experiments at 20°. Approximately 20 times more perchlorate was formed compared with perbromate formation in the presence of chloride ions of equimolar concentration. State of mechanistic considerations is presented and a mechanism for perbromate formation is proposed. The reaction from bromate to perbromate was found to be limited that is in contrast to the earlier studied chlorate-to-perchlorate conversion. In the measured concentration range, reduction processes at the mixed oxide cathode showed a much higher impact on the resulting concentration for perbromate than for bromate.     

Electrochemical pretreatment of distillery wastewater using aluminum electrode

Electrochemical (EC) oxidation of distillery wastewater with low (BOD₅/COD) ratio was investigated using aluminum plates as electrodes. The effects of operating parameters such as pH, electrolysis duration, and current density on COD removal were studied. At a current density of 0.03 A cm⁻² and at pH 3, the COD removal was found to be 72.3%. The BOD₅/COD ratio increased from 0.15 to 0.68 for an optimum of 120-min electrolysis duration indicating improvement of biodegradability of wastewater. The maximum anodic efficiency observed was 21.58 kg COD h⁻¹ A⁻¹ m⁻², and the minimum energy consumption observed was 0.084 kWh kg⁻¹ COD. The kinetic study results revealed that reaction rate (k) decreased from 0.011 to 0.0063 min⁻¹ with increase in pH from 3 to 9 while the k value increased from 0.0035 to 0.0102 min⁻¹ with increase in current density from 0.01 to 0.03 A cm⁻². This study showed that the COD reduction is more influenced by the current density. The linear and the nonlinear regression models reveal that the COD reduction is influenced by the applied current density.     

Electrodeposition of high purity manganese from low temperature (∼ −16° C) chloride electrolyte

Electrolytic manganese (99-98% purity and sulphur) is commercially produced by using sulphate electrolyte at 35° C with the addition of sulphur compounds at current efficiencies of about 60–65%. In the present investigations, adherent, compact and higher purity manganese was obtained by electrolysis in manganese chloride solutions at lower temperature (–16° C) without any additives such as sulphur or selenium compounds. A basic study was made to determine the current efficiency of deposition from low temperature chloride electrolytes. Factors affecting the process were: bath temperature, current density, ammonium salt concentration, pH and time of deposition. It was concluded that the electrodeposition of high purity manganese from low temperature chloride electrolyte was feasible. From a comparison between the MnCl₂-NH₄SO₃NH₂ and MnCl₂-NH₄Cl systems adopted, it was shown that the former had the advantage over the latter in its higher current efficiency (87% against 84%) below –8° C, while the latter was superior to the former with regard to its wider range of optimum current density (1–4 A dm^–2). The process may be used with advantage when high purity and ductile manganese free of impurities such as sulphur is required and when cooling of the bath is not an important and economic consideration.     

Optimization of process variables for a biosorption of nickel(II) using response surface method

The biosorption of nickel(II) was studied by using crab shell particles of diameter (d _p =0.012 mm) under different initial concentrations of nickel(II) in solution (0.01–5.0 g/l), temperature (20–40 °C), pH (2–6.5), and biosorbent dosages (0.5–10 g/l). The maximum removal of nickel(II) occurred at pH 6.5 and temperature 40 °C for a biosorbent dosage of 6 g/l. The results were modeled by response surface methodology (RSM), which determines the maximum biosorption of nickel(II) as a function of the above four independent variables, and the optimum values for the efficient biosorption of nickel(II) were obtained. The RSM studies were carried out using Box-Behnken design and the analysis of variance confirms the adequacy of the quadratic model with coefficient of correlation R² to be 0.9999. The quadratic model fitted the data well with Prob>F to be <0.0001, indicating the applicability of the present proposed model.     

Preparation and evaluation of RuO2–IrO2, IrO2–Pt and IrO2–Ta2O5 catalysts for the oxygen evolution reaction in an SPE electrolyzer

IrO₂–RuO₂, IrO₂–Pt and IrO₂–Ta₂O₅ electrocatalysts were synthesized and characterized for the oxygen evolution in a Solid Polymer Electrolyte (SPE) electrolyzer. These mixtures were characterized by XRD and SEM. The anode catalyst powders were sprayed onto Nafion 117 membrane (catalyst coated membrane, CCM), using Pt catalyst at the cathode. The CCM procedure was extended to different in-house prepared catalyst formulations to evaluate if such a method could be applied to electrolyzers containing durable titanium backings. The catalyst loading at the anode was about 6 mg cm⁻², whereas 1 mg cm⁻² Pt was used at the cathode. The electrochemical activity for water electrolysis was investigated in a single cell SPE electrolyzer at 80 °C. It was found that the terminal voltage obtained with Ir–Ta oxide was slightly lower than that obtained with IrO₂–Pt and IrO₂–RuO₂ at low current density (lower than 0.15 A cm⁻²). At higher current density, the IrO₂–Pt and IrO₂–RuO₂ catalysts performed better than Ir–Ta oxide.     

Removal of iron from drinking water by electrocoagulation: Adsorption and kinetics studies

The present study provides an electrocoagulation process for the removal of iron from drinking water with aluminum alloy as the anode and stainless steel as the cathode. The studies were carried out as a function of pH, temperature and current density. The adsorption capacity was evaluated with both the Langmuir and the Freundlich isotherm models. The results showed that the maximum removal efficiency of 98.8% was achieved at a current density of 0.06 A dm⁻², at a pH of 6.5. The adsorption of iron preferably fitting the Langmuir adsorption isotherm suggests monolayer coverage of adsorbed molecules. The adsorption process follows second-order kinetics. Temperature studies showed that adsorption was endothermic and spontaneous in nature.     

β-cyclodextrin as inverse phase transfer catalyst on the electrocatalytic hydrogenation of organic compounds in water

The optimum conditions for the electrocatalytic hydrogenation (ECH) of benzaldehyde in water, using a nickel sacrificial anode (SA) (referred to as ECH-SA) and β-cyclodextrin (β-CyD) as inverse phase transfer catalyst (IPTC) were determined. Four parameters were investigated: the morphology of the nickel deposited on the cathode matrix (Cu, Fe, Ni or Fe/Ni alloy (64:36)) during a pre-electrolysis, the size of the CyD cavity, the concentration of β-CyD, the supporting electrolyte concentration and the current density applied. The results showed that a Ni matrix together with ultrasound pre-electrolysis treatment allowed a nanostructured nickel deposit on the cathode surface. Under the best electrolysis conditions (2.8 mmol dm^?3 of β-CyD, 1.0 mol dm^?3 of NH₄Cl and a current density of 330 mA dm^?2), the yield of benzyl alcohol (99%) was 27% higher than that obtained under the same conditions but in the absence of β-CyD. Taking into account the hydrophobic character of the β-CyD, the best conditions of the ECH-SA method were applied to the hydrogenation of a variety of organic substrates. Excellent yields and current efficiencies were obtained with arylbenzaldehydes and acetophenone. ECH-SA of styrene gave moderate yield and current efficiency, and the hydrogenation of a terminal non-conjugated olefin (safrole) was not efficient.