A comparative evaluation on the anodic behavior of Cu and Ag electrodes in non-aqueous fluoride and fluoroborate media

The voltammetric responses of copper and silver had been extensively studied and compared in a variety of non-aqueous solvents such as acetonitrile (AN), propylene carbonate (PC) and sulfolane containing two different supporting electrolytes namely triethylaminetrishydrogen fluoride (TEA.3HF) and tetrabutylammonium tetrafluoroborate (TBABF₄). The dissolution rate and surface transformation on the electrode surfaces as a result of anodic polarization was investigated using atomic absorption spectroscopy (AAS) and scanning electron microscopy (SEM), respectively. In solvent-free TEA.3HF medium, the copper electrode shows high charge recovery ratio (Q _c/Q _a), and the difference between the initial anodic and cathodic potentials, obtained at a current density of 2 mA cm⁻², is around 0.11 V, suggesting that in this medium, Cu can certainly serve as reference electrode. On the other hand, on Ag electrode, substantial dissolution was observed leading to very high anodic (Q _a) and cathodic (Q _c) charges, and the surface morphology after the cyclic polarization results in roughened surface with large pores. The effects of incorporating AN and water as additives in TEA.3HF on the solubility and stability of these metal fluoride films are also reported. The dissolution pattern and film formation behavior of these two metals in the different solvents containing fluoride and fluoroborate ionic species have several qualitative similarities, as noted from cyclic voltammetry responses and SEM morphology. Anodic dissolution and precipitation process for both Cu and Ag depends significantly on the nature of supporting electrolytes as well as solvents. In AN containing 0.1 M TEA.3HF, the dissolution of Cu and Ag electrodes was very high. Fluoride salts of Cu show lesser solubility than Ag in those solvents, while fluoroborate salts exhibit the reverse trend. The AAS data suggest that for a particular salt, which may be either fluoride or fluoroborate of Cu and Ag, the relative solubility decreases in the order AN > PC > sulfolane.     

Anodic film formation on osmium electrodes: Part II. Galvanostatic and potentiostatic measurements

The formation and reduction of anodic films on Os electrodes in 2 M HCl and HClO₄ solutions were studied by anodic and cathodic charging curves. The galvanostatic oxidation of Os in HClO₄ shows the formation of OsO₂ as an intermediate step to OsO₄ that goes in the solution. The cathodic charging curves at Os electrodes previously oxidized at constant potential reveal the anodic film to be made up of a reversibly desorbed oxygen layer and an oxide phase reduced irreversibly. Both layers increase with time under potentiostatic conditions following a logarithmic equation until a constant value is reached. At all times, the content of OsO₂ in the anodic film at high potentials is larger than that of chemisorbed oxygen.In HCl solutions only the reversible reduction of an oxygen layer is observed. The growth of this film also complies with a direct logarithmic law before attaining a limiting coverage. The charge involved in the reduction increases linearly with the potential at a given time of formation. The results are discussed in terms of a Temkin-type isotherm and a place-exchange mechanism.     

Electrocatalysis of Nitrate Reduction at Copper‐Nickel Alloy Electrodes in Acidic Media

The cathodic reduction of NO in 1.0 M HClO₄ is investigated by voltammetry at pure Ni and Cu electrodes, and three Cu‐Ni alloy electrodes of varying composition, all configured as rotated disks. Voltammetric data obtained using these hydrodynamic electrodes demonstrate significantly improved activity for NO reduction at Cu‐Ni alloy electrodes as compared to the pure Ni and Cu electrodes. This observation is explained on the basis of the synergistic benefit of different surface sites for adsorption of H‐atoms, generated by cathodic discharge of H⁺ at Ni‐sites, and adsorption of NO at Cu‐sites on these binary alloy electrodes. Koutecky‐Levich plots indicate that the cathodic response for NO at a Cu₇₅Ni₂₅ electrode corresponds to an 8‐electron reduction, which is consistent with production of NH₃. In comparison, the cathodic response at Cu₅₀Ni₅₀ and Cu₂₅Ni₇₅ electrodes corresponds to a 6‐electron reduction, which is consistent with production of NH₂OH. Flow injection data obtained using Cu₅₀Ni₅₀ and Cu₂₅Ni₇₅ electrodes with 100‐μL injections exhibit detection limits for NO of ca. 0.95 μM (ca. 95 pmol) and 0.60 μM (ca. 60 pmol), respectively.     

On Copper(I) Fluorides,the Cuprophilic Interaction,the Preparation of Copper Nitride at Room Temperature,and the Formation Mechanism at Elevated Temperatures

Our attempts to synthesize the hitherto unknown binary copper(I) fluoride have led to first successes and a serendipitious result: By conproportionation of elemental copper and copper(II) fluoride in anhydrous liquid ammonia, two copper(I) fluorides were obtained as simple NH₃ complexes. One of them presents an example of ligand‐unsupported “cuprophilic” interactions in an infinite [Cu₂(NH₃)₄]²⁺ chain with alternating Cu–Cu distances. We discovered that both copper(I) fluorides can easily be converted into Cu₃N at room temperature, just by applying a vacuum. Additionally, we investigated the formation mechanism of the classical synthesis route of Cu₃N that starts with CuF₂ and flowing NH₃ in the temperature range between ambient and 290 °C by means of thermal analysis and in situ neutron diffraction. The reaction proceeds at elevated temperatures through the formation of a blue and amorphous ammoniate Cu(NH₃)₂F₂, the reformation of CuF₂, and finally the redox reaction to form Cu₃N.     

Combining Theory and Experiment to Characterize the Voltammetric Behavior of Nickel Anodes in the Simons Process

The Simons process, otherwise known as the electrochemical fluorination (ECF) method, is widely used in industry to electrolytically synthesize chemicals for various purposes. Even to this day, the exact mechanism of the ECF reaction remains unknown, but is believed to involve the formation of an anodic nickel fluoride film with highly oxidized nickel centers. In this study, experiments and density functional theory calculations are combined to characterize the initial anodic peak occurring at potentials typically required in an ECF cell. NiF₂ is believed to form a passivating layer at low potentials. The calculations show that a potential of +3.1 V is required to oxidize surface Ni²⁺ centers to Ni³⁺. This is in good agreement with the measured anodic peak at +3.57 V.     

Electrochemical fluorination of methanedisulfonyl fluoride

The electrochemical fluorination of methanedisulfonyl fluoride in anhydrous hydrogen fluoride to produce difluoromethanedisulfonyl fluoride, a precursor of difluoromethane disulfonic acid, was investigated. This study was carried out in a microprocessor-aided modified Simons' reactor system at constant anodic potential using a Cu/CuF₂ reference electrode. Product yields of 75–82%, current efftciencies in excess of 66%, and electrical energy efficiencies of at least 33% were obtained for the electrochemical fluorination step.Experimental results of this investigation are presented and discussed.     

The influence of monovalent cations on the stability of electrochemically formed nickel fluoride films

The stability of electrochemically formed NiF₂ film in 1.0 M perchloric acid containing monovalent fluorides namely, NH₄F, HF, NaF, KF and LiF, is investigated using cyclic voltammetry, chronoamperometry, atomic absorption spectroscopy and scanning electron microscopy. In addition to direct dissolution of nickel and dissolution through the oxide layer, a new mode of dissolution of NiF₂ film as NiF₃ ⁻ and NiF₄ ²⁻ through complex formation is proposed. This process is significantly influenced by the alkali metal fluorides. On a comparative basis the stability of NiF₂ decreases in the order NH₄F > HF > KF > LiF. Received: 29 July 1998 / Accepted: 3 November 1998     

Effect of the nature of conductive supported nickel electrocatalyst for salicylic acid oxidation in alkaline medium

The electrocatalytic activity of Ni films electrodeposited on glassy carbon (Ni/GC), titanium (Ni/Ti), and gold (Ni/Au) electrodes toward salicylic acid (SA) oxidation are investigated. The cyclic voltammetry studies show that the nature of substrate strongly influences the apparent electrocatalytic activities of the nickel over layer in basic medium. It is observed that the Ni/GC electrode has higher activity for SA oxidation compared to other electrodes. Effects of various parameters such as concentration of Ni²⁺, deposition time for Ni film growth, and deposition potential on the electrooxidation of SA are investigated. It is demonstrated that the Ni(OH)₂/NiOOH plays the key role in the electrooxidation of SA. The response to SA on the Ni/GC electrode is examined using chronoamperometry.     

Electrocatalytic Oxidation of Ethylene Glycol at Pt/Nanosized MOx/GC Composite Electrodes: SnO2 in Comparison to CeO2 and WO3

Stable metal oxides insoluble in acidic medium have been prepared and characterized. The influence of the type of metal oxide (MO_x) on the activity of Pt towards ethylene glycol oxidation in acidic medium has been examined. All modified Pt/MO_x/glassy carbon (GC) electrodes exhibited a better activity compared to Pt/GC. While Pt/SnO₂/GC electrode exhibited the highest activity, Pt/CeO₂/GC revealed the best tolerance against poisoning process.     

Electrochemical Behavior of Platinum-Modified Polyaniline Films in Sulfuric Acid Solutions of Carbon Monoxide

On the Pt–PAN–GC and PAN–Pt electrodes in H₂SO₄solutions that are first saturated with CO and then rid of it, during anodic scans of E _r, oxidation peaks are observed at 0.85–0.95 V, which disappear only after repeatedly cycling E _rin the interval 0.65 to 1.1 V. The effect is attributed to the capability of a PAN film to sorb considerable CO amounts. The sorption interaction is of a chemical nature. The electrooxidation of CO in the film is closely related to the PAN oxidation, which results in its destruction.     

One-step electrochemical synthesis of Pt–CeO2 composite thin films on a glassy carbon electrode

A Pt–CeO₂ composite thin film was prepared on a glassy carbon electrode by one-step electrochemical deposition technique. The film was constructed of Pt particles well dispersed and embedded in a porous CeO₂ substrate. The prepared Pt–CeO₂/GC electrode showed a better catalytic performance toward methanol electrooxidation compared with the bulk Pt electrode.     

Platinum Ultramicroelectrodes Modified with Electrogenerated Surfactant‐Templated Mesoporous Organosilica Films: Effect of Film Formation Conditions on Its Performance in Preconcentration Electroanalysis

Pt µdisc electrodes have been modified by mesoporous organosilica thin films by electrochemically assisted self‐assembly (EASA) of mercaptopropyltrimethoxysilane (MPTMS), tetraethoxysilane (TEOS), and the surfactant cetyltrimethylammonium bromide (CTAB). The EASA process involves the generation of hydroxide ions at the electrode/solution interface, upon the application of a cathodic current density, leading to TEOS and MPTMS polycondensation around the CTAB template and concomitant growing of a thiol‐functionalized mesoporous film onto the electrode surface. The experimental conditions (current density, deposition time, silane concentration and molar ratio between surfactant template and silane) were optimised to form a thin and permeable film likely to be used in preconcentration electroanalysis. The morphology of the film electrodes were characterised by scanning electron microscopy. The permeability properties of the modified Pt µdisc electrodes have been evidenced by cyclic voltammetry using Ru(NH₃)₆³⁺ as a redox probe. The best parameters identified for the film preparation are a current density of ? 8 mA cm^?2 applied for 15 s in a solution containing 110 mM of hydrolysed silane precursors and 70.4 mM of CTAB. Pt µdisc electrodes modified in these conditions were used for the open‐circuit preconcentration of Hg(II) species prior to their detection by anodic stripping voltammetry in a mercury‐free solution. In the optimized conditions, a sensitivity of 14.3 mA cm^?2 µM^?1 was obtained for the 0.02–0.08 µM concentration range. The analytical performance of such organosilica films could decay by up to two orders of magnitude for the materials prepared in conditions other than the optimized ones, highlighting the need for a fine control of the deposition parameters to elaborate sensors based on such modified ultramicroelectrodes.     

A new simple preparation of platinum-nickel alloy nanoparticles and their characterization as an electrocatalyst for methanol oxidation

Pt-Ni alloy nanoparticles were produced by casting 2 or 10 mM H₂PtCl₆ solutions on a Ni column. The apparent particle size for the resultant Pt-Ni alloys increased with the concentration of the H₂PtCl₆ solution, while the content of Pt in the alloy decreased. The potential sweeps of 5 cycles in an H₂SO₄ aqueous solution for Pt-Ni (2 mM)/Ni and Pt-Ni (10 mM)/Ni electrodes led to electrochemical behavior similar to a polycrystalline Pt electrode, suggesting the formation of a few thin Pt layers on each Pt-Ni alloy surface. In electrochemical measurements, both Pt-Ni/Ni electrodes showed more negative onset potential of methanol oxidation and slower degradation of oxidation current of methanol than the polycrystalline Pt electrode. X-ray photoelectron spectroscopy of both Pt-Ni/Ni electrodes showed the shift of Pt4f peaks to a higher binding energy, suggesting that the increase in the d vacancy in the balance band 5d orbital of Pt contributed to the improved electrocatalytic activity and durability of the Pt-Ni/Ni electrodes.     

Effect of polarization on the electrode behavior and microstructure of (La,Sr)MnO<Subscript>3</Subscript> electrodes of solid oxide fuel cells

The electrode behavior and microstructure of freshly prepared (La_0.8Sr_0.2)_0.9MnO₃ (LSM) electrodes were investigated under various polarization conditions. The original, large agglomerates in freshly prepared LSM electrodes were broken down into sphere-shaped grains when exposed to cathodic or anodic current passage of 200 mA cm^–2 at 800 °C in air for 3 h. Microstructural changes under cathodic polarization could be related to the pronounced diffusion and migration of oxygen vacancies and Mn ions on the LSM surface and lattice expansion, while lattice shrinkage under oxidation conditions most likely contributes to the structural changes under anodic polarization. Such morphological changes were irreversible and were found to be beneficial to the performance of freshly prepared LSM electrodes. Freshly prepared LSM electrodes behaved very differently with respect to the cathodic and anodic current passage treatment.     

On the mechanism of electrochemical dispersion of platinum under the action of alternating current

The behavior of platinum electrodes in alkali solutions under the action of pulse alternating current was studied. A mechanism of electrochemical dispersion of platinum was proposed. An important step of the mechanism is the discharge and cathodic insertion of alkali metal cations into the crystalline lattice of platinum followed by the decomposition of intermetallic compound Pt _m Na upon the interaction with water. Dispersion is facilitated by the presence of an oxide film on the platinum surface formed in the anodic period of pulses, a periodical alternation of electrode polarity resulting in the electrochemical injection of vacancies from the metal bulk to the surface, and hydrogen and oxygen formation in the corresponding periods.     

Electrogeneration and characterization of a poly(pyrrole–nickel (II) chlorin) electrode

We describe herein, the electrochemical properties of an hydroporphyrin of Ni(II) substituted by two pyrrole groups in organic media (THF and CH₂Cl₂). In the anodic region it has been shown that the electrochemical behaviour of this complex investigated by cyclic voltammetry and coulometry coupled with UV–visible spectroscopy is strongly dependent on the nature of the solvent.Furthermore, the electrochemical oxidation of the pyrrrole groups has led to the first example of an electrogenerated polypyrrole–metallochlorin film. The resulting modified electrodes exhibit the same electrochemical properties as the monomer free in solution. Preliminary experiments have also demonstrated the possibility to study electrochemically this polymeric film in CH₃CN contrary to the Ni(II) chlorin, which is insoluble in this medium.     

Nanostructured metal particlemodified electrodes for electrocatalytic and sensor applications

Nanotechnology has become one of the most exciting frontier fields in analytical chemistry. The huge interest in nanomaterials, for example in chemical sensors and catalysis, is driven by their many desirable properties. Although metal is a poor catalyst in bulk form, nanometre-sized particles can exhibit excellent catalytic activity due to their relative high surface area-to-volume ratio and their interface-dominated properties, which significantly differ from those of the bulk material. The integration of metal nanoparticles into thin film of permselective membrane is particularly important for various applications, for example in biological sensing and in electrocatalysis. We have already established different techniques to design permselective membrane-coated chemically modified electrodes with incorporated redox molecules for electrocatalytic, electrochromic and sensor applications. Recently, we have prepared nanostructured platinum and copper (represented M_nano, M = Pt and Cu) modified GC/Nafion electrodes (GC/Nf/M_nano) and characterized by using AFM, XPS, XRD and electrochemical techniques. The nanostructured M_nano modified electrodes were utilized for efficient electrocatalytic selective oxidation of neurotransmitter molecules in the presence of interfering species such as ascorbic acid (AA) and uric acid (UA). It has been also shown that the modified electrodes could be used as sensors for the detection of submicromolar concentrations of biomolecules with practical applications to real samples such as blood plasma and dopamine hydrochloride injection solution. The GC/Cu_nano electrode has been used for catalytic reduction of oxygen.     

Corrosion and corrosion inhibition characteristics of bulk nanocrystalline ingot iron in sulphuric acid

The corrosion and corrosion inhibition of bulk nanocrystalline ingot iron (BNII) fabricated from conventional polycrystalline ingot iron (CPII) by severe rolling was studied in 0.5 M H₂SO₄ solution using electrochemical impedance spectroscopy and potentiodynamic polarization techniques. The results indicate that BNII was more susceptible to corrosion in the acidic environment essentially because of an increase in the kinetics of the anodic reaction. An amino acid cysteine (cys) was employed as a corrosion inhibitor at concentrations of 0.001 and 0.005 M. Tests in inhibited solutions revealed that cys reduced the corrosion rates of both metal specimens by different mechanisms. For CPII cys inhibited the cathodic reaction but had a stimulating effect on the anodic process at low concentration and a trivial effect at higher concentration. For BNII, cys inhibited both the cathodic and the anodic reactions, although the former effect was more pronounced. Iodide ions improved the inhibitive effect of cys without altering the inhibition mechanism.     

Electroanalytical Study of Prussian Blue Modified Glassy Carbon Paste Electrodes

Two types of glassy carbon (GC) powder (i.e., Sigradur K and Sigradur G) have been mixed with mineral oil to obtain glassy carbon paste electrodes (GCPE's). The electrochemical behavior of such electrodes at different percentages of glassy carbon has been evaluated with respect to the electrochemistry of ferricyanide as revealed with cyclic voltammetry and the best paste composition was chosen. GC was then modified with Prussian Blue (PB), mixed at different percentages with unmodified GC and with a fixed amount of mineral oil in order to obtain PB modified glassy carbon paste electrodes (PB‐GCPE's). PB‐GCPE's with different percentages of GC modified with PB (PB‐GC) were compared and the dependence on the amount of PB on their performances was evaluated by studying the parameters of cyclic voltammetry (i.e., current peak, ΔE_p, anodic and cathodic current ratio, charge density) and the amperometric response to H₂O₂. Data interpretation based on the GC surface area is presented. GCPE's with a selected amount of PB‐GC were then tested as H₂O₂ probes and all the analytical parameters together with the dependence on pH were evaluated. Some preliminary experiments with these electrodes assembled as glucose, lysine and lactate biosensors are also reported.     

Protoelectrochemical properties of the single-crystal SrTiO3 doped in the surface region

The photoelectrochemical properties of the single crystal SrTiO₃, doped in the surface region are studied. It is found that the doped SrTiO₃ with Cr, Co, Pt and Rh give a relatively large photoresponse to visible light. The dark anodic currents which will be due to the resonance tunnelling or hopping mechanism are observed at the doped electrodes with the above metal cations. Therefore, it is concluded that the visible light response is mainly attributable to the formed impurity levels and/or structure defetcs by the doping metal cations near the conduction band of SrTiO₃. The above doped electrodes also bring the large cathodic photocurrent or the dark cathodic current due to the O₂ reduction, except for the Co doped electrode. This will show that the impurity levels act as the active site of O₂ reduction.