Electrochemical characterization of different screen-printed gold electrodes

Voltammetric characterization of electrochemical systems is very important with the aim of optimising a required application. In this case, different gold screen-printed electrodes have been evaluated. Although the use of carbon has widespread, gold is still in the beginning. Two different substrates: polycarbonate and alumina have been evaluated. Differences in morphology are clear when observed by SEM. Potassium ferricyanide, p-aminophenol, indigo carmine, silver nitrate and ferrocene have been chosen as model analytes. Potential scans can be made at scan rates as high as 2000 mV s⁻¹ which could result very advantageous. Proportionality between peak current and concentration maintains at this scan rate as demonstrated with ferricyanide. Sensitivity is increased and linear dynamic ranges are widened when alternative electrochemical techniques (SWV and ACV) are employed. In the silver system in basic media, catalysis-mediated reduction is only possible on alumina electrodes since the potential window is very narrow in polymeric electrodes. The precision (intra and interelectrode) has been thoroughly evaluated. Reusability is possible since adequate RSDs are obtained when measurements are performed in different drops without cleaning in-between. On the other hand, low cost of fabrication makes disposability of electrodes an advantageous characteristic that avoids tedious cleaning treatments.     

Electrochemical noise analysis on multiple dissimilar electrodes: Theoretical analysis

The electrochemical potential and current noise generated by multiple electrodes are analyzed by considering the current flowing across each electrode and the electrochemical potential of the electrode array. By introducing the concept of a “virtual electrode”, the analysis of the electrochemical noise generated by an array of electrodes is reduced to the case of two dissimilar electrodes. For each electrode, an apparent impedance, Z_ρ*, can be determined as the square root of the power spectral density of potential divided by the power spectral density of the individual electrode current. When two dissimilar pairs of nominally identical electrodes are used and it is possible to assume that the pair of electrodes corroding more produces higher noise levels and displays lower impedance, the actual electrode impedance can be obtained with acceptable precision from the value of the nominal impedance. Further, the asymmetry between the two dissimilar pairs can also be quantitatively evaluated.     

Electrochemical reduction of perchlorate ions on platinum-activated nickel

The electrochemical reduction of perchlorate anion has been studied in a cell with a nickel working electrode and a platinum counter electrode in concentrated solutions of HClO₄. Significant reduction of perchlorate to chloride ions occurs on the nickel cathode in the potential region where hydrogen evolution occurs. It is shown that the mechanism of this process involves platinum deposited in small amounts on the cathode as a result of oxidation of the platinum anode in the perchloric acid solution. The reduction of perchlorate is accompanied by oxidation of the nickel cathode, which is attributed to chlorate ions formed in the initial step of perchlorate reduction. Changes in the surface structure of the nickel electrode have been followed using atomic force microscopy.     

Electrochemical studies of pyrite oxidation and reduction using freshly-fractured electrodes and rotating ring-disc electrodes

Oxidation and reduction processes on coal- and mineral-pyrite surfaces have been investigated to better understand the reactions that control the hydrophobicity and flotation behavior of pyrite. The incipient oxidation and reduction reactions were studied using fresh surfaces of pyrite that were created by in situ fracturing electrodes potentiostated at a predetermined potential. Chronoamperometry immediately after fracture and subsequent cyclic voltammetry have established that fresh fracture surfaces of pyrite instantaneously assume a unique potential (referred to as the “stable” potential) at which neither oxidation nor reduction takes place. For Peruvian and Chinese pyrites, the stable potential is −0.28 V (standard hydrogen electrode, SHE) at pH 9.2 and 0 V at pH 4.6. The initial oxidation of pyrite begins at potentials slightly positive of the stable potential and is believed to produce a hydrophobic sulfur-rich species, most likely a polysulfide or metal-deficient sulfide. A rotating ring-disc electrode (RRDE) was employed to study the kinetics and mechanisms of surface reactions on pyrite over moderate potential ranges. Two distinct soluble reduction products (ferrous hydroxide and HS⁻) and one distinct soluble oxidation product (ferrous hydroxide) were observed on pyrite in alkaline solutions. It is concluded that the initial oxidation of pyrite and the oxidation of ferrous to ferric hydroxide occur in a similar potential range. When the electrode is oxidized, e.g. by polishing, prior to experiments, the initial oxidation of pyrite is masked by the oxidation of ferrous hydroxide, making it difficult to study the oxidation of pyrite itself.     

Nitrate reduction on single-crystal platinum electrodes

The structure sensitivity of the reduction of nitrate has been studied on a series of single-crystal platinum electrodes by cyclic voltammmetry and in situ FTIRAS in sulfuric and perchloric acid solutions. The nitrate reduction is a structure-sensitive reaction on single-crystal platinum electrodes. However, this structure sensitivity is essentially controlled by other species (hydrogen, sulfate) that interact strongly with the electrode surface rather than by a structure-sensitive nitrate adsorption, dissociation or reduction.Voltammetric and spectroscopic data point to adsorbed nitric oxide (NO) as the main stable intermediate of the nitrate reduction to ammonia. No evidence for the formation of N₂O was found. On surfaces with sufficiently wide (1 1 1) terraces in the absence of specifically adsorbed sulfate, an oxidizable nitrate reduction product is detected voltammetrically, which may tentatively be attributed to the formation of a small amount of hydroxylamine earlier during the voltammetric scan.     

Electrochemical reduction of dichlorodifluoromethane on silver and lead electrodes

The electrochemical conversion of dichlorodifluoromethane (CFC-12) was studied on silver and lead electrodes. The main products detected were CHClF₂, CH₂F₂, CH₃F and CH₄. Cyclic voltammetry and constant potential electrolytic experiments in acetonitrile showed that on silver the reduction mechanism starts with the elimination of one chloride ion at –1.05 V. The current density and the product distribution strongly depend on the nature of the electrolyte used. Higher current densities were observed in methanol and acetonitrile, whereas in propylene carbonate (PC) the current density was found to be 13 times lower than that in acetonitrile. This difference was mainly attributed to the differing diffusion coefficient of CFC-12 in various solvents. A consecutive reaction mechanism was proposed to explain the experimental results.     

Electrochemical characterization of SnO2 electrodes doped with Ru and Pt

Antimony-platinum doped tin dioxide electrodes supported on titanium have been prepared by thermal decomposition. The effect of the progressive replacement of Sb with Ru (x = 0.00; 3.25; 6.50; 13.00 at.%) on their electrochemical response in acid medium has been analysed by cyclic voltammetry. The morphology of the coatings was observed by scanning electron microscopy. Ti/SnO₂-Sb-Pt electrodes without Ru presented a cracked-mud structure, typical of oxide electrodes prepared by thermal decomposition. The introduction of Ru in the oxide layer modified the coating morphology. The roughness increased and passed through a maximum with the increase of Ru content. A relation between the surface morphology, the roughness factor, voltammetric charge and the electrochemical activity has been established. The mechanism and electrocatalytic activity towards the oxygen evolution reaction has been studied from Tafel measurements. The progressive introduction of Ru in the electrodes increased their electrocatalytic activity for the oxygen evolution reaction with a change on the mechanism from non-active to active electrodes. The electrocatalytic activity mainly depends on electronic factors.     

Electrochemical reduction of dilute chromate solutions on carbon felt electrodes

Carbon felt is a potential material for electrochemical reduction of chromates. Very dilute solutions may be efficiently treated due to its large specific surface area and high porosity. In this work, the up-scaling of this technology is investigated using a new type of separated cell and once-through flow of industrial rinse water. A significant enhancement of the process is obtained due to copper deposition during long-term operation. The co-deposition and re-solution of copper occurs depending on the inlet chromate concentration. When previously deposited copper is present a current-free reduction of chromate takes place resulting in current efficiencies apparently above 100%. Very high space time yields are obtained even for effluents at low concentration and optimised conditions (high flow rates and pH 2). The economic feasibility of the technology is also considered. Continuous, single-pass operation results in lower energy requirements than batch processing. The economic potential of the process is also evaluated in comparison with chemical detoxification of chromate. The operating costs for the electrochemical treatment of very dilute effluents on a carbon felt electrode are 30% lower than for the chemical method.     

Electrochemical reduction of cerium oxide into metal

The Fray Farthing and Chen (FFC) and Ono and Suzuki (OS) processes were developed for the reduction of titanium oxide to titanium metal by electrolysis in high temperature molten alkali chloride salts. The possible transposition to CeO₂ reduction is considered in this study. Present work clarifies, by electro-analytical techniques, the reduction pathway leading to the metal. The reduction of CeO₂ into metal was feasible via an indirect mechanism. Electrolyses on 10 g of CeO₂ were carried out to evaluate the electrochemical process efficiency. Ca metal is electrodeposited at the cathode from CaCl₂–KCl solvent and reacts chemically with ceria to form not only metallic cerium, but also cerium oxychloride.     

On the electrochemistry of 2,4,6-triphenylnitrobenzene and related compounds

Cyclic voltammetry at a glassy carbon electrode of 2,4,6-triphenylnitrobenzene (1) in DMF shows two reversible one-electron reductions. Preparative reduction of 1 yields 2,4,6-triphenylaniline under both acidic and alkaline conditions and in DMF in the presence of acetic anhydride. On reduction in dry DMF is formed a stable, violet radical anion with a broad maximum at 682 nm; in acetonitrile 1 is reported to yield an unstable bluish radical which is transformed to a yellow-purple radical with a maximum at 520 nm. Solid-state ¹³C NMR confirmed the existence of two stereoisomers in the crystals found by X-ray structure determination, whereas ¹³C NMR in chloroform at ambient temperature indicated one isomer in solution. 2,4,6-Triphenylaniline shows in CV in acetonitrile a reversible oxidation; in acetonitrile containing pyridine it is oxidized to azo(2,4,6-triphenylbenzene). This compound is in acidic aqueous DMF reduced to 2,4,6-triphenylaniline. The three phenyl groups act as electron donating groups during reductions akin to p-hydroxy or p-amino groups.     

Nanoparticles produced by borohydride reduction as precursors for metal hydride electrodes

The performance of electrodes, prepared from amorphous CoxByHz nanoparticles without additives, in 20% KOH solution was tested by means of cyclic voltammetry and chronopotentiometry. Peaks, assigned to hydrogen adsorption and desorption, are observed in the cyclic voltammograms. After charging, hydrogen atoms occupy different types of sites in the substrate, from which electrochemical desorption occurs. An increase in hydrogen content as a result of repeated cycling was established. Discharge capacity of the electrodes, estimated from the chronopotentiometric discharge curves obtained, is about 250mAhg–1. The observed changes in hydrogen and boron content, due to electrochemical treatment, indicate that the electrode material is an active participant in the whole electrochemical process. Hypotheses for the reaction mechanism are proposed.     

Electrochemical promotion of bimetallic RhAg/YSZ catalysts for the reduction of NO under lean burn conditions

At positive overpotentials, RhAg bimetallic electrodes interfaced with yttria-stabilised zirconia (YSZ) exhibit strong electrochemical promotion in the catalytic reduction of NO by propene, even in the presence of a large excess of gaseous oxygen. Compared with the unpromoted system, at positive overpotentials, the N₂ production rate can be accelerated by a factor of eight: this is the key figure of merit. In addition the N₂ selectivity improves from 28 to 55%. The effect on reaction rates is strongly non-faradaic and persists to a significant degree in the absence of applied potential. At negative overpotentials the nitrogen-containing products exhibit faradaic behaviour, possibly reflecting self-poisoning of the system due to excessive adsorption of oxygen. The overall performance of the RhAg/YSZ bimetallic system is substantially better than that of Rh/YSZ, suggesting that the former are promising candidates for use in novel approaches to NO_x reduction under fuel lean conditions.     

Electrochemical reduction of oxygen on thin-film Pt electrodes in acid solutions

The electrochemical reduction of oxygen on thin-film platinum electrodes in 0.1 M HClO₄ and 0.05 M H₂SO₄ solutions has been investigated using the rotating disk electrode (RDE) method. Thin films of Pt (0.25-20 nm thick) were prepared by vacuum evaporation onto glassy carbon substrate. The surface morphology of Pt films was examined by transmission electron microscopy (TEM). The specific activity of O₂ reduction was higher in HClO₄ and decreased with decreasing film thickness. In H₂SO₄, the specific activity was lower and appeared to be independent of the Pt loading. The values of Tafel slopes close to −120 mV dec⁻¹ in high current density range and −60 mV dec⁻¹ in low current density range were obtained for all electrodes in both solutions, indicating that the mechanism of O₂ reduction is the same for thin-film electrodes as for bulk Pt. The number of electrons transferred per O₂ molecule was close to four for all thin Pt films studied.     

Oxygen reduction on platinum electrodes in base: Theoretical study

Electrode-potential-dependent activation energies for electron transfer have been calculated using a local reaction center model and constrained variation theory for the oxygen reduction reaction on platinum in base. Results for four one-electron transfer steps are presented. For the first, O₂(ads) is predicted to be reduced to adsorbed superoxide, O₂⁻(ads), which dissociates with a low activation barrier to O(ads) + O⁻(ads). Then a proton transfer form H₂O(ads) to O⁻(ads) takes place, forming OH(ads) + OH⁻(aq). The second electron transfer reacts O(ads) with H₂O(aq) to form a second OH(ads) + OH⁻(aq). The third and fourth electron transfers react the two OH(ads) with two H₂O(aq) to form two H₂O(ads) + two OH⁻(aq). All three different surface reduction reactions are predicted to have reversible potentials in the −0.24 V(SHE) to −0.29 V(SHE) range for 0.1 M base and activation energies for the superoxide formation step are close to the experimentally observed range in 0.1 M base for the overall four-electron to water over the three low index (1 1 0) (1 0 0) and (1 1 1) surfaces: 0.38-0.49 eV at 0.35 eV respectively at 0.88 V(RHE). Predicted reversible potentials for forming O₂⁻(ads) are compared with estimates from the experimental literature. The difference between the acid mechanism, where the peroxyl radical, OOH(ads) is the first reduction intermediate, and the base mechanism, where superoxide, O₂⁻(ads) is the first reduction intermediate, is discussed.     

Electroanalytical application of modified diamond electrodes

Metal-modified diamond electrodes were fabricated by using ion implantation method for electroanalytical applications. Nickel and copper ion were implanted at a different film of boron-doped diamond (BDD) with a dose of 5×10¹⁴ cm⁻² for each type of ion. The electrochemical behavior has been studied for glucose oxidation in alkaline media by using cyclic voltammetry and flow injection analysis. Those electrodes exhibited high catalytic activity and excellent electrochemical stability with low background current even after strong ultrasonication in the cleaning process. The results indicate that metal-implanted method could be a promising method for controlling the electrochemical properties of diamond electrodes.     

Well-defined core-shell structures based on silsesquioxane microgels: Grafting of polystyrene via ATRP and product characterization

Silsesquioxane microgel nanoparticles characterized by low diameters (below 30 nm) and reduced polydispersity can be produced in an acid-catalyzed sol-gel process in an aqueous microemulsion. Suitable surface modification of such structures leads to macroinitiators for atom-transfer radical polymerization (ATRP). This polymerization method was applied in order to graft polystyrene chains onto the surface of the microgels. Well-defined structures exhibiting a core-shell architecture were produced with the M_w of grafted polymers ranging from 8.5 to about 30 kg/mol. The products were extensively characterized with light scattering, X-ray scattering, thermal analysis (TGA/DSC) and microscopy (TEM/SEM) to obtain information on parameters characterizing polystyrene brush. Polymer-grafted nanoparticles will be used for the modification of homopolymer and block copolymer matrices.     

Electrochemical reduction of 2,4-dinitrophenol on nanocomposite electrodes modified with mesoporous silica and poly(vitamin B1) films

Electrochemical reduction of 2,4-dinitrophenol was investigated on a glassy carbon electrode modified with a nanocomposite Santa Barbara Amorphous silica (SBA-15) film and poly(vitamin B₁) film. For sensitive and selective detections, vanadium pentoxide and cerium oxide nanoparticles were incorporated into the matrix of SBA-15. 2,4-Dinitrophenol was reduced on the modified electrode at −0.39 and −0.25 V, corresponding to the reduction of 4-dinitrophenol and 2-dinitrophenol, respectively. Both cathodic peak currents were controlled by the diffusion of 2,4-dinitrophenol. The amplitude of the peak currents was proportional to the 2,4-dinitrophenol concentration in the range of 3.0–30 μM. The modified electrode demonstrated a long lifetime for the detection of 2,4-dinitrophenol. The detection limit of 2,4-dinitrophenol was 0.5 μM. Moreover, the modified electrode was used successfully to detect 2,4-dinitrophenol in lake water.     

Ultrasonic-assisted preparation of AlON from alumina/carbon core-shell nanoparticle

An aluminium oxynitride (AlON) powder was synthesized by carbothermal reduction nitridation (CRN) method. For this purpose, first Al₂O₃/C core-shell nanoparticles were prepared by the pyrolysis of Al₂O₃/polyacrylonitrile (PAN) nanocomposite precursor at 800?°C for 2?h in an argon atmosphere. Alumina/PAN precursor was prepared by ultrasonic method at room temperature. Then, by two-step thermal treatment of Al₂O₃/C core-shell nanoparticles at 1500–1600?°C for 2?h, followed by subsequent heating at 1750?°C for 1?h in N₂ flow, AlON powder was synthesized. The sample was investigated via Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and CHNS elemental analysis.     

Electrochemical reactivity at graphitic micro-domains on polycrystalline boron doped diamond thin-films electrodes

This paper deals with the electrochemical reactivity of boron doped diamond (BDD) electrodes. A comparative study has been carried out to show the influence of the presence of graphitic micro-domains upon the surface of these films. Those graphitic domains are sometimes present on as-grown boron doped diamond electrodes. The effect of doping a pure Csp³ diamond electrode is established by highly oriented pyrolytic graphite (HOPG) abrasion onto the diamond surface. In order to establish the effect of doping on a pure Csp³ diamond electrode, the amount of graphitic domains was increased by means of HOPG crystals grafted onto the BDD surface. Indeed that method allows the enrichment of the Csp² contribution of the electrode.The presence of graphitic domains can be correlatively associated with the presence of kinetically active redox sites. The electrochemical reactivity of boron doped diamond electrodes shows a distribution of kinetic constants on the whole surface of the electrode corresponding to different active sites. In this paper, we have studied by cyclic voltammetry and electrochemical impedance spectroscopy the kinetics parameters of the ferri/ferrocyanide redox couple in KCl electrolyte. A method is proposed to diagnose the presence of graphitic domains on diamond electrodes, and an electrochemical “pulse cleaning” procedure is proposed to remove them.     

Nanostructured carbon electrodes for laccase-catalyzed oxygen reduction without added mediators

Reduction of dioxygen catalyzed by laccase was studied at carbon electrodes without any added mediators. On bare glassy carbon electrode (GCE) the catalytic reduction did not take place. However, when the same substrate was decorated with carbon nanotubes or carbon microcrystals the dioxygen reduction started at 0.6 V versus Ag/AgCl, which is close to the formal potential of the laccase used. Four different matrices: lecithin, hydrophobin, Nafion and lipid liquid-crystalline cubic phase were employed for hosting fungal laccase from Cerrena unicolor. The carbon nanotubes and nanoparticles present on the electrode provided electrical connectivity between the electrode and the enzyme active sites. Direct electrochemistry of the enzyme itself was observed in deoxygenated solutions and its catalytic activity towards dioxygen reduction was demonstrated. The stabilities of the hosted enzymes, the reduction potentials and ratios of catalytic to background currents were compared. The boron-doped diamond (BDD) electrodes prepolarized to high anodic potentials exhibited behavior similar to that of nanotube covered GCE pointing to the formation of nanostructures during the anodic pretreatment. BDD is a promising substrate in terms of potential of dioxygen reduction, however the catalytic current densities are not large enough for practical applications, therefore as shown in this paper, it should be additionally decorated with carbon particles being in direct contact with the electrode surface.