Cell Current Efficiency Research Articles

Effect of additives and electrode roughness on the production of hypochlorite sanitizer by the electrolysis of NaCl solution in a batch recycle reactor

The present work aims at studying the effect of some of the variables such as solution flow rate, electrode surface roughness, current density, NaCl concentration and the presence of additives such as drag reducing polymers and surfactants on the performance of an electrochemical reactor which produces sodium hypochlorite by the electrolysis of NaCl solution. A rectangular undivided cell with a graphite anode and Ni plated steel cathode was used to conduct the present study. The performance of the batch recycle reactor was assessed by determining: (i) hypochlorite yield (ii) cell current efficiency and (iii) electrical energy consumption in kW•h/kg. The results revealed that the energy consumption of hypochlorite production increases by increasing current density and solution flow rate while it decreases by increasing NaCl concentration and electrode surface roughness. Electrode roughness improved the performance of the cell and decreased the energy consumption by an amount ranging from 23 to 39% compared to the smooth electrode surface depending on the operating conditions. The presence of additives such as drag reducing polymers or surfactants were found have beneficial effects on the cell performance and decreases the electrical energy consumption especially with rough electrodes.

Process Engineering Challenges for the Development of Electrolytic Reduction of Uranium Oxide in Molten LiCl-Li2O

Electrolytic uranium oxide reduction has the potential to be used for recycling spent fuel to a range of nuclear reactors from commercial light water reactors to advanced nuclear reactors—including molten salt reactors and sodium cooled reactors. However, several process engineering-related problems have been identified that need to be addressed to support efficient, cost-effective commercial implementation. Two keys are cathode product purity and cell current efficiency. High product purity relies upon the effective removal of LiCl-Li2O from the cathode basket and reduced uranium. Removal of the salt from the basket can be achieved via high speed spinning at 650oC. But entrained Li2O will remain and should be minimized during the reduction process. Methods for removing hydroxide impurities from the salt are reported that improve cell-current efficiency. The extent of this reduction can be determined via three distinct techniques (thermogravimetric analysis, elemental oxygen analysis, and bromine dissolution) and is key in helping determine the efficiency of the electrochemical cell.

Impact of supporting electrolytes on the stability of TiO2–Ti counter electrode during H2O2 electrogeneration

This work focuses on the role of common supporting electrolytes (SEs) in the electro-chemical inertness of Ti-based materials employed for the anodic (direct) oxidation coupled with H2O2 electro-generation at the graphite cathode for the concurrent decomposition of organic contaminants. SEs are added to boost up the ionic conductivity of solution but a question always remains on the effect of SEs on the stability of anode materials. The use of ClO 4 − is encouraged in the electro-Fenton process as it does not form complexes with Fe2+/Fe3+; however, it is found that ClO 4 − corroded the TiO2 coated Ti (TiO2–Ti) anode very fast (>60 min) and, Ti4+ ions formed a yellow color complex (λmax = 380 nm) with H2O2. The influence of Cl–, NO 3 − and SO 4 2− was insignificant on the stability of TiO2–Ti. The cell current efficiency of H2O2 formation dropped sharply with in the case of TiO2–Ti anode. The TiO2–Ti corrosion also reduced the mass transfer co-efficient of DO transport from bulk to the cathode surface because of Ti4+ adsorption on graphite.

Fabrication of reduced graphene oxide-graphite paste electrode for H2O2 formation and its implication for ciprofloxacin degradation

A multilayer reduced graphene oxide (rGO)-impregnated graphite electrode was fabricated to improve the production efficiency of H2O2. Graphene oxide was prepared using the improved Hummer's method and, it was then reduced to multilayer rGO using hydrazine hydrate. The XRD peak at 2θ = 24.1° and, the Raman shift with D, G and low-intensity 2D bands at 1333, 1591 and 2641 cm−1, respectively, confirmed the formation of multilayered rGO structure. The performance of the electrode was tested through cyclic voltammetry and H2O2 generation studies at different pH. H2O2 generation was about 22.41mg/L in 60min of electrolysis using the graphite/rGO-graphite (anode/cathode combination) system. The cell current efficiency was 15.54% more using the rGO-modified electrode as compared to the graphite cathode. The degradation potential of electro-generated H2O2 was tested for ciprofloxacin, an antibiotic drug, during electro-Fenton and UVA/H2O2 oxidation processes in an undivided cell.

Impact of the Usage of a Slotted Collector Bar on Thermoelectric Field in a 300-kA Aluminum Reduction Cell

The horizontal current in a metal pad is critical because of its effect on the aluminum reduction cell current efficiency and energy consumption. A type of slotted collector bar was considered to have great potential to reduce the horizontal current. The effects of the slotted collector bar on the horizontal current in the metal pad, current, and temperature distribution in the cathode carbon and collector bar were simulated using the finite-element method. The results show that the maximum current at the middle of the metal pad decreases from 11,940 A m−2 to 9490 A m−2 and the peak of current density (the maximum current density) shifts toward the cell side. Moreover, the maximum horizontal current and average horizontal current at the middle of the metal pad in the cell with slotted collector bar decreases by ~50% and 50.9%, respectively. However, the cathode voltage in the cathode with the slotted collector bar is ~53 mV higher than that in the conventional cell, and the temperature in the slotted collector bar is higher than that in the conventional cathode. The results of this study may provide the database in understanding the effect of the slotted collector bar on cell.

Metal Instabilities and its Effect on Cell Performance during Aluminium Smelting

The Hall-Heroult process for the production of aluminium is based on the electrochemical reduction of alumina (Al2O3) dissolved in a cryolite (Na3AlF6) based electrolyte. Instability in cell voltage is referred to as noise. Normal voltage noise is inevitable due to bubble evolution and it has little effect on performance parameters such as, current efficiency and power consumption. Metal rolling noise (wavy noise) is caused by the disturbances in cell magnetic field and it affects the cell current efficiency adversely. Investigating the causes of the cell instability in the aluminium smelting cells can lead to better cell performance. Understanding the variation in cell voltage is critical for cells, because magnitude of voltage determines the energy consumption pattern in the process and hence, any saving on voltage can save energy. Voltage affects the current efficiency of the cell and an optimum cell voltage leads to higher current efficiency without compromising on energy consumption. Magnetic, current distribution, heat loss and voltage at zero current measurements along with online current and voltage signal can help to identify the problems and their combined effects on the performance of the cells. In order to estimate the loss in current efficiency of the aluminum electrolysis cells due to metal instabilities, measurements were performed and data analyzed. The present paper analyses the effect of voltage fluctuations (noise) during metal instability along with cause of instability and its effect on current efficiency of the cell. Measurements carried out to estimate the deviations from the normal cell operations are also discussed.

Cell Voltage Noise in Aluminium Smelting

Aluminium reduction process is a nonlinear, time varying electrochemical process, in which alumina (Al2O3) dissolved in cryolite (Na3AlF6) is electrolysed to produce molten aluminium. In industrial practice the term noise refers to unsteady voltage signal in the reduction cell or random and/or uncontrolled fluctuations in the cell voltage; it can be used to identify problems in the cell operation. The main objectives of the reduction process is to achieve as high a current efficiency as possible with minimum energy consumption. Noises of certain frequencies are generally considered unfavorable to cell current efficiency and energy consumption. Therefore, noise control is an important issue in commercial cell automatic control system. The present paper analyses noise from different sources during cell operation and their possible causes with help of Fast Fourier Transformation techniques in terms of frequency and amplitudes of the fluctuations. An advanced data acquisition system was used for collecting the data required for the noise studies. The characterization of the three types of noise generated is discussed. Real-time algorithms have been implemented to allow on-line estimation of the noise sources and intensities.

Perovskite chromates cathode with resolved and anchored nickel nano-particles for direct high-temperature steam electrolysis

Doped lanthanum chromates are now commonly used to prepare composite cathodes for direct steam electrolysis. However, the limitation of electrochemical performances and cell current efficiency by insufficient electro-catalytic activity is a major challenge. This paper reports the use of reversibly exsolved catalytic metallic Ni nano-particles on A-site deficient and B-site excess perovskite (La0.75Sr0.25)0.95(Cr0.8Ni0.2)0.95Ni0.05O3−δ (LSCNNi), in order to achieve an activity-enhanced composite cathode via high-temperature reduction under reducing atmospheres. The electrical properties of the ceramic are investigated methodically and correlated with the performances of the composite electrodes in symmetric and electrolysis cells. XRD, SEM, EDS and XPS results, confirm that the exsolution or dissolution of nano metallic catalyst is reversible in redox treatment cycles. The Faradic efficiency reaches approximately 80% for the LSCNNi cathode with the flow of 5%H2/Ar. Ultimately, the unified effect of metallic catalyst and redox-stable ceramic produce striking redox stability as well as electrochemical performances of the titled composite cathode. The results signify that the composite cathode with exsolved nickel nano-particles, is a good potential fuel electrode for direct steam electrolysis in an oxygen-ion conducting solid oxide electrolyzer.

Colour diminution and COD reduction in treatment of coloured effluent by Electrocoagulation

This work experimentally investigates the colour and Chemical Oxygen Demand (COD) removal of effluents containing: Trypan Blue (TB); Orange G (OG) dye using the two most common electrode materials – Mild Steel (MS) and Aluminium (Al). Effects of the time of electrotreatment, solution pH, electrode material, current density and initial dye concentration on decolourisation and COD reduction are reported. MS electrodes show better colour removal efficiency event at lower cell current efficiency compared with Al electrodes and are able to achieve up to 90–98% removal in less than 60 min. Removal efficiency is higher in the case of TB compared with OG irrespective of electrode material due to higher electrostatic attraction between TB molecules and metal hydroxide sludge. Higher current density enhances colour removal even though pH elevation (approaching close to zero point charge) rate is higher. COD reduction is found to be about 10-20% lower than colour diminution measured spectrophotometrically suggesting different mechanisms of dye removal involved during EC.

Removal of Cr 3+ by electrocoagulation with multiple electrodes: Bipolar and monopolar configurations

Removal of Cr 3+ from aqueous solutions by electrocoagulation (EC) with multiple electrodes was studied with both bipolar and monopolar configurations. The influence of chloride ions, pH, initial Cr 3+ concentration and cell current on removal performance was investigated in a batch stirred cell. It was noted that, the enhancement of solubility of Cr(OH) 3 and co-precipitated [Cr, Fe](OH) 3 in presence of chloride ions results in reduction of removal efficiency. Removal of Cr 3+ improved at higher pH due to combined effect of precipitation, co-precipitation as solid solution like species, adsorption, cathodic reduction and sweep coagulation. The maximum Cr 3+ removal per unit charge loading was found to be increasing with increase in initial Cr 3+ concentration. The cell current efficiency was less than that predicted from Faraday's law of electrolysis for both configurations. The operating cost was estimated from the power cost and cost of electrode material. The former was 40% and the later was 43% higher for bipolar configuration compared to monopolar configuration for achieving final chromium discharge limit of 2.0 mg/l.

Fluoroethylene carbonate electrolyte and its use in lithium ion batteries with graphite anodes

The electrolyte decomposition during the first lithiation of graphite is reduced to 85 mA h/g in an electrolyte containing equal volumes of fluoroethylene carbonate (Fluoro-EC) and of a co-solvent propylene carbonate (PC). The volume fraction of Fluoro-EC can be further reduced to 0.05 in a tri-solvent system with a co-solvent containing equal volumes of ethylene carbonate (EC) and PC. A lithium ion cell containing Fluoro-EC PC and EC shows a long cycle life. The capacity decreases by 37% from the initial value in over 200 cycles. Cell current efficiency is 100%, thus solving the poor cell current efficiency when chloroethylene carbonate (Chloro-EC) is used in place of Fluoro-EC.

Electrochemical regeneration of ceric sulphate in an undivided cell

Ceric sulphate (0–0.5 m) was generated electrochemically from cerous sulphate slurries (0.5–0.8 m total cerium) in 1.61 m sulphuric acid, at 50 °C, using a bench scale differential area undivided electrochemical cell with an anode to cathode ratio of eleven. A cell current efficiency for Ce(IV) of 90% was obtained at an anode current density of 0.25 A cm−2. An empirical model illustrates an increase in overall current efficiency for Ce(IV) with an increase in electrolyte velocity, an increase in total cerium concentration, and a decrease in the cell current. From separate kinetic studies on rotating electrodes, both, anode and cathode kinetics were found to be affected by cerium sulphate adsorption processes. Anode adsorption of cerous sulphate species leads to inhibited mass transfer and negatively affected current efficiencies for Ce(IV). Cathode adsorption of cerium sulphate is thought to be responsible for high cathode current efficiencies for hydrogen (93–100%). The dissolved cerous sulphate concentration increased with increasing ceric sulphate and total cerium sulphate concentrations resulting in slurries with a stable dissolved cerous sulphate concentration of as high as 0.851 m in 1.6 m H2SO4 at room temperature.

Sorption and transport behavior of perfluorinated ionomer membranes in concentrated NaOH solution

The NaOH electrolyte uptake of Nafion-901 membrane has been measured. The membrane exhibits a strong Donnan exclusion effect. It is found that a significant change of sorption behavior occurs at concentrations higher than 9 M. The activity coefficient ratio and various ion transport properties also show dramatic change at 9 M. The membrane conductivity has been measured for a wide range of caustic concentration and temperature. It is observed that the transport mechanism in the carboxylate layer is different from that in the sulfonate layer. The relation between the membrane properties and cell current efficiency is discussed.