Influence of anode material on electrochemical decomposition of urea

Voltammetric investigations into the process of anodic decomposition of urea using Ti/Pt, Ti/(Pt-Ir)_70:30, Ti/RuO₂, Ti/(RuO₂-TiO₂)_40:60, Ti/(RuO₂-TiO₂-IrO₂)_20:60:20, Ti/(Ta₂O₅-IrO₂)_70:30 electrodes have been carried out. Two anodes namely (Ti/(Pt-Ir)_70:30 and Ti/(Ta₂O₅-IrO₂)_70:30) were found to be stable enough and indicated the required activity to produce non-toxic N₂ and CO₂ in place of nitrites and nitrates, the commonly reported electrochemical urea oxidation products. The kinetics of the process in question using the above two electrodes was further examined in a periodic electrolyzer. The effect of the anodic current density (j = 2-10 A/dm²), initial urea concentration (c_0M = 1-10 g/dm³) and the concentration of sodium chloride (c_NaCl = 1-10 g/dm³) on basic process indices (average rate of urea decomposition, a; current efficiency, W_p, and unit DC power consumption, Z_j) was discussed.     

Synergies between electrochemical oxidation and activated carbon adsorption in three-dimensional boron-doped diamond anode system

In order to enhance the performance of boron-doped diamond (BDD) anode system, activated carbon is added into BDD anode system to construct a three-dimensional electrode system. The degradation rates of p-nitrophenol and COD were significantly improved by 2–7 times compared to two-dimensional BDD anode system. More importantly, the synergy between electrochemical oxidation and activated carbon adsorption was observed. Investigations revealed that the synergy was not only resulted from direct electrochemical oxidation at activated carbon, but also from electro-catalysis of activated carbon to indirect electrochemical oxidation mediated by hydroxyl radicals. Although oxygen was reduced to hydrogen peroxide at the activated carbon surface, but the oxidation of hydrogen peroxide was not the main reason for the improvement of three-dimensional electrode system due to its relative weak oxidation capacity. Instead, the decomposition of hydrogen peroxide to hydroxyl radicals at catalysis of activated carbon played an important role on the enhancement of three-dimensional electrode system.     

Electrochemical treatment of aqueous solutions containing urea

Investigations on the anodic decomposition of urea using Ti/Pt and Ti/(RuO₂–TiO₂)_40:60 electrodes were carried out. The kinetics of the process were examined in a periodic electrolyser. The effect of anodic current density, initial urea concentration, and sodium chloride concentration on the effectiveness of the basic process (average rate of urea decomposition, current efficiency, and unit power consumption) is discussed. When a Ti/Pt electrode is applied for urea removal from aqueous solution urea is not decomposed directly at the surface of the electrode, but rather in the bulk of the solution by hypochlorite formed during the process. When the Ti/(RuO₂–TiO₂)_40:60 electrode is used for the removal of urea from aqueous solutions, the reaction of urea with chlorine adsorbed at the electrode predominates. In both cases non-toxic products of urea decomposition (N₂, CO₂,) are formed. Comparison of the effectiveness of anodic decomposition of urea for the Ti/Pt and Ti/(RuO₂–TiO₂)_40:60 electrodes in the periodic electrolyser at optimum process parameters has revealed that the former electrode is more favorable.     

Scale-up of B-doped diamond anode system for electrochemical oxidation of phenol simulated wastewater in batch mode

Scale-up of boron-doped diamond (BDD) anode system is critical to the practical application of electrochemical oxidation in bio-refractory organic wastewater treatment. In this study, the scale-up of BDD anode system was investigated on batch-mode electrochemical oxidation of phenol simulated wastewater. It was demonstrated that BDD anode system was successfully scaled up by 121 times without performance deterioration based on the COD and specific energy consumption (E_sp) models in bath mode. The COD removal rate and E_sp for the scaled-up BDD anode system through enlarging the total anode area while keeping similar configuration, remained at the similar level as those before being scaled up, under the same area/volume value, current density, retention time and wastewater characteristics. The COD and E_sp models used to describe the smaller BDD anode system satisfactorily predicted the performance of the scaled-up BDD anode system. Under the suitable operating conditions, the COD of phenol simulated wastewater was reduced from 540 mg l⁻¹ to 130 mg l⁻¹ within 3 h with an E_sp of only 34.76 kWh m⁻³ in the scaled-up BDD anode system. These results demonstrate that BDD anode system is very promising in practical bio-refractory organic wastewater treatment.     

Generation of oxysterols formed by free radicals and enzymes by electrochemical oxidation

It is commonly accepted that cholesterol oxide derivatives, also named oxysterols, are 27 carbon‐atom molecules deriving either from enzymatic and non‐enzymatic oxidation of cholesterol. Most of these compounds can be synthesized by more or less difficult and time consuming chemical reactions, and some of them have been discovered before the identification of the enzymes [mainly cytochrome P450 enzymes (CYP enzymes)] involved in their biosynthesis. A wide range of biological activities depends on oxysterols. Some oxysterols are also involved in the synthesis of cholesterol metabolites which have various properties. The paper by Weber et al. in this issue of European Journal of Lipid Science and Technology is of interest because it reports that an electrochemical oxidation of cholesterol rapidly generates numerous oxysterols which are usually generated either by enzymatic processes in vivo or under the action of different chemical and physical agents (free radicals, sun light, …) in vivo or not. This has a crucial importance and suggests that the border between enzymatic and non‐enzymatic generation of oxysterols should be reconsidered, at least for some of these compounds. Consequently, these observations may have major chemical and physiopathological significance.     

Degradation of azo dye by three clean advanced oxidation processes: Wet oxidation, electrochemical oxidation and wet electrochemical oxidation—A comparative study

This work compared the degradation behaviors of an azo dye, cationic red X-GRL, by three clean advanced oxidation processes: wet oxidation (WO), electrochemical oxidation (EO) and wet electrochemical oxidation (WEO). It was found that the WO process was readily to remove color while the EO process was prone to decompose pollutants. Integrated with the advantage of two processes, the WEO process posed synergetic effects for both efficient removal of color and COD. Even at the low temperature of 120 °C, the color could be almost completely removed at 60 min, and the COD removal was about 43.2% at 120 min, which was greatly protomted from that by WO at similar conditions (about 7.8%). And it demonstrated a better performance under a reduced temperature and a wider dye concentration range, representing the most promising alternative for environmental application among the three processes. The main degradation products for three processes were detected and a simplified five-stages degradation pathway was suggested, which indicated that the WEO process mineralized much more completely than the other two processes.     

Influence of oxidation level on capacitance of electrochemical capacitors fabricated with carbon nanotube/carbon paper composites

Gaseous oxidation of carbon papers (CPs) decorated with carbon nanotubes (CNTs) with varying degrees of oxidation was conducted to investigate the influence of surface oxides on the performance of electrochemical capacitors fabricated with oxidized CNT/CP composites. The oxidation period was found to significantly enhance the O/C atomic ratio on the composites, and the increase in oxygen content upon oxidation is mainly contributed by the formation of CO and C-O groups. The electrochemical behavior of the capacitors was tested in 1 M H₂SO₄ within a potential of 0 and 1 V vs. Ag/AgCl. Both superhydrophilicity and specific capacitance of the oxidized CNT/CP composites were found to increase upon oxidation treatment. A linearity increase of capacitance with O/C ratio can be attributed to the increase of the population of surface oxides on CNTs, which imparts excess sites for redox reaction (pseudocapacitance) and for the formation of double-layer (double-layer capacitance). The technique of ac impedance combined with equivalent circuit clearly showed that oxidized CNT/CP capacitor imparts not only enhanced capacitance but also a low equivalent series resistance.     

Electrophoretic deposition of network-like carbon nanofibers as a palladium catalyst support for ethanol oxidation in alkaline media

A network-like carbon nanofiber (CNF) film with an open porous structure formed by the open space between entangled CNFs is fabricated by electrophoretic deposition. The performance of the CNF film as an electrocatalyst in the presence of electrodeposited Pd nanoparticles for ethanol oxidation in alkaline media is investigated. Cyclic voltammetric analyses show the electrocatalyst has a good electrocatalytic activity toward ethanol oxidation in KOH solution. This is believed to be due to the high dispersion of Pd on the CNF film with a three-dimensional network structure which can provide a large number of available Pd active sites for ethanol oxidation, and to the structural and electrical properties of the film.     

Synthesis of multi-branched porous carbon nanofibers and their application in electrochemical double-layer capacitors

Multi-branched carbon nanofibers with a porous structure have been synthesized on a Cu catalyst doped with Li, Na, or K. The products were characterized by field emission scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy and Raman spectroscopy. Using this new type of nanofiber as polarized electrodes, an electrochemical double-layer capacitor with a specific capacitance of ca. 297 F/g was obtained using 6 M KOH as the electrolyte.     

Optimization of Pt-Ir on carbon fiber paper for the electro-oxidation of ammonia in alkaline media

Plating bath concentrations of Pt(IV) and Ir(III) have been optimized as well as the total catalytic loading of bimetallic Pt-Ir alloy for the electro-oxidation of ammonia in alkaline media at standard conditions. This was accomplished using cyclic voltammetry, scanning electron microscopy (SEM), energy dispersive X-ray (EDX), and statistical optimization tools. Concentrations of Pt(IV) and Ir(III) of the plating bath strongly influence electrode surface atomic compositions of the Pt-Ir alloy directly affecting the electro-oxidation behavior of ammonia. Several anode materials were studied using cyclic voltammetry, which demonstrated that Pt-Ir was the most active catalyst for the electro-oxidation of ammonia. Criteria for optimization were minimizing the climatic oxidation overpotential for ammonia and maximizing the exchange current density. Optimized bath composition was found to be 8.844 ± 0.001 g L⁻¹ Pt(IV) and 4.112 ± 0.001 g L⁻¹ Ir(III) based on electrochemical techniques. Physical characterization of the electrodes by SEM indicates that the plating bath concentrations of Pt and Ir influence the growth and deposition behavior of the alloy.     

Nickel-carbonate nanowire array: An efficient and durable electrocatalyst for water oxidation under nearly neutral conditions

It is highly attractive but still remains a great challenge to develop an efficient electrocatalyst for oxygen evolution reaction under nearly neutral conditions. In this work, we report the transformation of Ni₃S₂ nanowire array on nickel foam into the amorphous nickel carbonate nanowire array on nickel foam (NiCO₃/NF). The resulting NiCO₃/NF shows high electrocatalytic activity towards water oxidation and affords current density of 50 mA·cm⁻² at overpotential of 395 mV in 1.0 mol·L⁻¹ KHCO₃. Moreover, this NiCO₃/NF is also durable with a long-term electrochemical durability of 60 h. This catalyst electrode achieves a high turnover frequency of 0.21 mol O₂·s⁻¹ at the overpotential of 500 mV.     

Influence of anode material on the electrochemical oxidation of 2-naphthol: Part 1. Cyclic voltammetry and potential step experiments

The anodic oxidation of 2-naphthol has been studied by cyclic voltammetry and chronoamperometry, using a range of electrode materials such as Ti-Ru-Sn ternary oxide, lead dioxide and boron-doped diamond (BDD) anodes. The results show that polymeric films, which cause electrode fouling, are formed during oxidation in the potential region of supporting electrolyte stability. IR spectroscopy verified the formation of this organic film. While the Ti-Ru-Sn ternary oxide surface cannot be reactivated, PbO₂ and BDD can be restored to their initial activity by simple anodic treatment in the potential region of electrolyte decomposition. In fact, during the polarization in this region, complex oxidation reactions leading to the complete incineration of polymeric materials can take place on these electrodes due to electrogenerated hydroxyl radicals. Moreover, it was found that BDD deactivation was less pronounced and its reactivation was faster than that of the other electrodes.     

Electrode passivation by reaction products of the electrochemical and enzymatic oxidation of p-phenylenediamine

The electrochemical and enzymatic oxidation of p-phenylenediamine (PPD) was studied under various conditions to evaluate its reversibility and stability when used as a laccase redox mediator. In accordance with published results, PPD oxidation during cyclic voltammetry showed that a passivation occurred with cycling in static systems without laccase. Such passivation was observed both in McIlvaine and acetate buffers on glassy carbon and platinum electrodes. Our results suggest that the oxidised form of PPD reacts with other PPD molecules in their reduced state to form a polymer on the surface of the electrode. When laccase is present in solution, PPD is only found in its oxidised state and another behaviour is observed at the electrode, with the appearance of a redox couple with an E’ centered at about −25 mV vs. Ag/AgCl in 0.1 M acetate buffer. These redox waves are attributed to the formation of soluble PPD oligomers. RRDE experiments showed no passivation, meaning that in order to form a film, the reaction products from the first oxidation of PPD must be further oxidised in a slow, homogeneous reaction. Results from these experiments have a significant importance in the design of efficient enzyme-modified electrodes since PPD diffusion in the thick polymer layers used to immobilise enzymes is a much slower process than in free solution, which allows enough time for the oxidation products to further react at the electrode and to polymerise on its surface. Passivation can therefore be observed with modified electrodes, even under hydrodynamic conditions, while similar conditions will provide a reversible system on bare electrodes. An example of this consequence of the reactivity of PPD oxidation products on the mediator's reversibility is given through experiments with a modified glassy carbon RDE functionalised with a thick layer of poly(ethyleneimine) microcapsules. In such case, the cyclic voltammograms showed that the oxidation and reduction processes are broader than at a bare electrode and that higher scan rates or rotation rates must be used to avoid passivation.     

Facile and clean electrochemical synthesis of new acetaminophen derivatives through electrochemical oxidation of acetaminophen in the presence of thiouracil derivatives

The electrochemical oxidation of acetaminophen (1a) is carried out in the presence of thiouracil derivatives (3a–c), as nucleophiles, in an acetate buffer solution (0.15?M, pH 5) mixed with Dimethylformamide (DMF) using cyclic voltammetry and coulometry under a constant potential. The results obtained indicate that N-acetyl-p-benzoquinone-imine derived from acetaminophen participates in a 1,4-Michael-type addition reaction with thiouracils to form the corresponding acetaminophen derivatives (4a–c) in good yields and with high purities using a facile, catalyst-free, and one-pot electrochemical method using three carbon electrodes in an undivided cell under mild conditions. The products obtained were characterized after purification by IR, ¹H NMR, and ¹³C NMR spectroscopies, and by the elemental analysis method.     

Electrochemical oxidation of ethanol in acid media on titanium nitride supported fuel cell catalysts

In the present study, titanium nitride, TiN that possesses good electronic conductivity, high corrosion resistance combined with the ability to support metallic particles, has been used to anchor Pt catalysts and subsequently used for ethanol oxidation. Platinum deposited on TiN (Pt–TiN) surface is contrasted with the conventional support material, Vulcan carbon for the electrochemical oxidation of ethanol in acidic medium. Though the comparison is not straight forward due to different morphology/particle size of the Pt catalyst on the two supports, the present investigations reveal that the TiN support lead to surface Ti–OH type functional groups that help in reducing the accumulation of carbon monoxide on the catalyst surface. The Tafel slopes are similar but the exchange current density on Pt–TiN is approximately twice that of the value observed on Pt–C. X-ray photoelectron spectroscopy data support the long term stability and electrocatalytic activity of Pt–TiN electrocatalyst.     

Model interpretation of electrochemical impedance spectroscopy and polarization behavior of H2/CO mixture oxidation in polymer electrolyte fuel cells

A mathematical model is proposed based on electrode kinetics analysis for the oxidation of 2% CO+H₂ mixture in polymer electrolyte fuel cells. Successful simulation of the polarization curve and experimental impedance spectra for Pt/C electrode system confirm the validity of the model which shows that the impedance is strongly dependent on electrode potential. With the increase of potential, an inductive behavior will occur. It is believed that the appearance of this inductive pattern can be used as a criterion for the onset of CO oxidation by the coincidence of the potential at which inductive behavior occurs with the ignition potential for CO oxidation. The effects of change of CO oxidation rate constant and CO adsorption equilibrium constant on impedance pattern, as well as on CO surface coverage and Faradaic current are also delineated with the use of the proposed model.     

Fabrication of novel porous Pd particles and their electroactivity towards ethanol oxidation in alkaline media

Novel porous Pd particles (nanoPd-PEG, nanoPd-PEG-EDTA, nanoPd-HCHO-EDTA, nanoPd-EG, nanoPd-HCHO and nanoPd-EG-EDTA) were synthesized by a hydrothermal method using different reduction agents in the absence and presence of EDTA and investigated as electrocatalysts for ethanol oxidation in alkaline solutions. Results showed that PdCl₂ was hydrothermally reduced to nano-scale palladium particles and a three-dimensional texture was formed for Pd particles. Presence of EDTA was favorable for the formation of Pd nanoparticles with small sizes of ca. 70 nm. Ethanol oxidation on the present Pd catalysts took place at a more negative anodic potential in 1 M NaOH solution. Among the electrocatalysts investigated, the electrocatalytic activity of the nanoPd-HCHO-EDTA was the greatest, which was characterized by the largest anodic peak current density of 151 mA cm⁻² and lowest onset oxidation potential of −0.788 V (vs. SCE) for the positive scan. Very low charge transfer resistances on the nanoPd-HCHO-EDTA in 1 M NaOH containing various concentrations of ethanol were obtained according to the analysis for electrochemical impedance spectra (EIS). The prepared porous Pd catalysts were promising alternatives to Pt electrodes applied in alkaline direct alcohol fuel cells.     

Mechanism study of the ethanol oxidation reaction on palladium in alkaline media

In this work, the mechanism of the ethanol oxidation reaction (EOR) on a palladium electrode was studied using the cyclic voltammetry method. The dissociative adsorption of ethanol was found to proceed rather quickly and the rate-determining step was the removal of the adsorbed ethoxi by the adsorbed hydroxyl on the Pd electrode. The Tafel slope was found to be 130 mV dec⁻¹ at lower potentials, which suggests that the adsorption of OH⁻ ions follows the Temkin-type isotherm on the Pd electrode. In comparison, the Tafel slope increased gradually to 250 mV dec⁻¹ at higher potentials. The change in the Tafel slope indicated that, at higher potentials, the kinetics is not only affected by the adsorption of the OH⁻ ions, but also by the formation of the inactive oxide layer on the Pd electrode.     

Exponential growth of electrochemical double layer capacitance in glassy carbon during thermal oxidation

The evolution of the electrochemical double layer capacitance of glassy carbon during thermochemical gas phase oxidation was studied with electrochemical impedance spectroscopy. Particular attention was paid to the initial oxidation stage, during which the capacitance grows exponentially. This stage could be experimentally assessed by lowering the reaction temperature and oxidant partial pressure. After a specific oxidation time the capacitance growth experiences a cross-over to a logistic growth.