Oxygen evolution on aged IrO x /Ti electrodes in alkaline solutions

O₂ evolution from 1 mol dm⁻¹ NaOH aqueous solution was studied on IrO _x /Ti electrodes already used for more than 3 years (aged). IrO _x was prepared by thermal decomposition of the chloride in the temperature range from 330 to 500 °C. Half of the electrodes were stored in air between experiments, the other half in water. The state of the electrode surface was monitored by recording voltammetric curves in a potential region prior to O₂ evolution before and after each group of experiments. O₂ evolution was studied by measuring quasistationary current–potential curves. Tafel slopes were derived using two different approaches. The reaction order with respect to OH⁻ was also determined and found to be fractional. Results show that the reaction mechanism does not depend on either the calcination temperature or the storage conditions. However, stability appears to be higher for electrodes calcined at higher temperatures and stored in air. Dedicated to Professor Algirdas Vaškelis on the occasion of his 70th birthday.     

Electrocatalytic evolution of hydrogen on amorphous Fe-Nb-B-Rare-Earth-Metal electrodes from alkaline solutions

Cathodic processes for electrochemical generation of hydrogen in aqueous KOH solutions in the presence of active electrodes made of Fe-Nb-B-based amorphous metallic alloys alloyed with rare-earth metals (Y, Gd, Dy) were studied. All the amorphous alloys exhibit a catalytic effect in the electrochemical evolution of hydrogen from 5 M aqueous KOH solutions. The electrocatalytic system with Fe₈₂Nb₂B₁₄Gd₂ and Fe₈₂Nb₂B₁₄Dy₂ electrodes exhibiting a prolonged autocatalytic effect can be recommended for practical application.     

Enhanced oxygen evolution at hydrous nickel oxide electrodes via electrochemical ageing in alkaline solution

The effect of electrochemically ageing hydrous nickel oxide films via slow repetitive potential multi-cycling across the main nickel (II/III) redox peak was investigated in an aqueous base environment using cyclic voltammetry and steady state polarisation curves in the oxygen evolution reaction (OER) region. Similarities between hydrous nickel oxide films and electroprecipitated ‘battery type’ nickel oxide were shown due to their similar change in redox and oxygen evolving properties as a result of film ageing. This ageing method was found to significantly enhance the OER performance of the hydrous nickel oxide electrode with the OER overpotential decreasing by 60 ± 2 mV and experiencing a 10 fold increase in OER rate for a fixed overpotential over that of an un-aged electrode. The OER turnover frequency for an aged electrode was found to be 1.16 ± 0.07 s^− 1 in comparison to 0.05 ± 0.003 s^− 1 for a hydrous nickel oxide electrode not subjected to ageing.     

Effect of methylviologen on the hydrogen evolution at mercury electrodes in aqueous buffer solutions

Methylviologen reduces the hydrogen overvoltage and gives a catalytic hydrogen wave at the dropping mercury electrode in aqueous buffer solutions (pH 3–7). The electrochemical behaviour of the catalytic hydrogen evolution was studied by dc and pulse polarography, potential-step chronoamperometry, and constant potential electrolysis. A possible reaction scheme has been proposed to explain the experimental results, in which the reduced neutral form of methylviologen MV⁰ is assumed to be oxidized to a monocation radical MV^·+ by the weak acid component in the buffer solution. Generation of MV^·+ during the hydrogen evolution is supported by the visual observation of a blue colour appearing at the surface and also in the vicinity of the electrode when a large catalytic current is flowing in dc polarography and constant potential electrolysis.     

Recent progress in electrochemical oxidation of saccharides at gold and copper electrodes in alkaline solutions

This article reviews the progress made in the past 10 years, on electrochemical oxidation of saccharides in alkaline media for gold and copper electrodes. The mechanism and processes associated with the electrochemical oxidation of saccharides at native and surface coated electrodes continues to be of great interest. Despite the effort and various mechanisms proposed, still the need for an electrochemically active material that understands the complexity associated with saccharides continues to increase as their detection poses a challenge for bioanalytical chemistry and liquid chromatography.     

Surface poisoning during electrocatalytic monosaccharide oxidation reactions at gold electrodes in alkaline medium

In the present study, the surface poisoning of electrocatalytic monosaccharide oxidation reactions at gold electrodes were investigated. In the cyclic voltammetric studies, the electrocatalytic oxidation of aldohexose and aldopentose type monosaccharides, aminosugars, acetyl-glucosamine and glucronamide were observed at gold plate electrodes in alkaline medium. However, in controlled-potential electrolytic studies ranging −0.3 to −0.2 V in reaction solutions, current flows during electrolyses decreased quickly with time, except when glucosamine was used as a substrate.Results from surface enhanced infrared adsorption (SEIRA) spectroscopic measurements at an evaporated gold electrode for the electrocatalytic oxidation of glucose in 0.1 mol dm⁻³ NaOH at −0.3 V and Gaussian simulated spectra indicated that the gluconic acid as a 2-electron oxidation product and/or its analogs adsorbed onto the gold surface. Electrochemical quartz crystal microbalance (EQCM) measurement results, along with surface adsorption results from surface poisoning at the gold electrode during electrolytic reactions, suggested that gluconic acid and/or its analogs adsorbed vertically onto electrode surfaces in a full monolayer packing-like conformation. In the case of the electro oxidation of glucosamine in 0.1 mol dm⁻³ NaOH at −0.2 V, the obtained SEIRA spectra and EQCM results, clearly indicated that the glucosaminic acid as a 2-oxidation glucosamine product did not strongly bind onto the gold electrode surface.     

Platinum catalysed nanoporous titanium dioxide electrodes in H2SO4 solutions

The preparation and electrochemistry of dispersed Pt metal on nanoporous titanium dioxide coatings is described. It is shown that photocatalytic deposition of Pt centres on a nanoporous titanium dioxide layer fabricated from TiO₂ nanoparticles leads to high surface area electrocatalysts. The reactions investigated are the evolution of hydrogen and the oxidation of carbon monoxide and methanol.     

Effect of surface compound formation on the photo-induced reaction at polycrystalline TiO2 semiconductor electrodes

The oxidation of methanol at the surface of illuminated polyerystalline TiO₂ semiconductor electrodes was investigated by measuring the current-doubling effect. Compounding TiO₂ with MoO₂ or MoS₂ increases the efficiency of alcohol oxidation. This compounding effect is discussed with reference to the electrode characteristics.     

Triple ion formation in solutions of alkaline sulfocyanides

Through the infrared and Raman spectra of alkaline sulfocyanides in high-and low-polarity media, nitromethane (CH₃NO₂) and tetrahydrofuran (THF), triple cations and triple anions may be characterized. The formation of these species at the expense of ion pairs or more aggregated species such as dimers is discussed taking into account cation solvation, anion solvation, and the influence of cation-anion interaction.     

Fractional reaction orders in oxygen evolution from acidic solutions at ruthenium oxide anodes

Oxygen evolution at RuO₂ anodes from acidic solutions exhibits a fractional reaction order with respect to H⁺. This is close to −1 for a set of “compact” film electrodes, and close to −2 for a set of “cracked” film electrodes. The difference in mechanism is related to the different defect structure of the surface. Possible reasons for the observed fractional reaction orders are discussed. It is suggested that the experimental observation may be explained in terms of the reaction proceeding with different mechanisms in parallel on different patches of the surface whose structure is governed by the acid-base properties of the oxide.     

Hydrogel-based flexible micro-reference electrodes for use in alkaline and neutral pH solutions

Two types of nonbreakable, flexible micro-reference electrodes filled with gel-electrolytes were prepared for use in solutions with alkaline and neutral pH. The electrodes are intended for electrochemical measurements, in which chloride-free conditions are important. Due to the flexible, bendable construction of the electrodes, electrochemical experiments at locations difficult to access with common reference electrodes are enabled. Hg|HgO-type electrodes were prepared from amalgamated Au wires, followed by oxidation of the amalgam, which is mounted in a PTFE tube filled with 0.1M NaOH solution immobilized in a PAA-g-PEO gel. The potential of this type of electrode was found to be 0.162?±?0.002 V (SHE) at room temperature. Cu|CuSO₄ electrodes, consisting of a Cu wire immersed in a saturated CuSO₄ solution jellied with gelatin, showed a stable open-circuit potential of 0.312?±?0.001 V (SHE). Further characterization of the electrodes was performed in terms of electrochemical impedance spectroscopy and micro-polarization measurements. As an alternative to the flexible electrodes, rigid electrodes in glass enclosure were fabricated in analogy to the flexible-type electrodes.     

Phenomenological aspects related to the electrochemical behaviour of smooth palladium electrodes in alkaline solutions

The voltammetric behaviour of smooth palladium electrodes in 1 M NaOH is studied in the potential range related to the thermodynamic stability of water. The electrosorption of H atoms on bulk Pd appears as a reversible reaction coupled to a diffusion process which occurs within bulk Pd. The voltammetric electrodesorption of H from bulk Pd is a process under mixed control, i.e. the diffusion from the bulk and the surface oxidation of H atoms. Fast pseudocapacitive reactions are detected in the range 0.2–0.4 V associated with the adsorption of H atoms at the submonolayer level. The initial stages of Pd oxide layer formation, at ca. 0.68 V, involves two reversible stages. The Pd oxide monolayer formation is achieved at 1.25 V/RHE and is followed by the formation of a third reversible system. This system is enhanced by an excursion in the potential range of the oxygen evolution reaction. This reversible system is probably a redox system involving Pd(II)/Pd(IV) species. The voltammetric electroreduction of the Pd oxide film shows rather irreversible behaviour. Inhibition effects on the reversible adsorption of H atoms due to residual oxide species were observed as well as inhibition on loading the Pd electrode with hydrogen to form the (α + β)-PdH phase. Rotating ring-disc experiments demonstrate that Pd electrodissolution in basic solutions is much smaller than in acid solutions. However, soluble palladium species are detected, especially during the formation of the fast redox systems, in the potential range related to Pd oxide layer growth.     

Mechanism of oxygen reactions at porous oxide electrodes. Part 2--Oxygen evolution at RuO2, IrO2 and Ir(x)Ru(1-x)O2 electrodes in aqueous acid and alkaline solution

The kinetics and mechanism of the oxygen evolution reaction at a series of RuO(2)/IrO(2) mixed oxides in aqueous acid and alkaline solution has been examined using a variety of electrochemical methods. Factors affecting the electrocatalytic activity have been elucidated and novel oxygen evolution mechanisms in terms of reactive oxyruthenium and oxyiridium surface groups are proposed.     

Amperometric detection of simple carbohydrates at platinum electrodes in alkaline solutions by application of a triple-pulse potential waveform

The response of ten simple carbohydrates was investigated voltammetrically at platinum electrodes in 0.10 M sodium hydroxide by application of a conventional linear sweep waveform and a triple-pulse waveform. Linear-sweep data were interpreted to suggest that electrochemical reactions of the carbohydrates involve oxidation of adsorbed hydrogen atoms produced by surface-catalyzed dehydrogenation of the adsorbed carbohydrate. The triple-pulse measurement technique was evaluated for a flow-injection system by introducing 100-μl samples into a stream of 0.1 M NaOH with a flow rate of 0.375 ml min^-1, and measuring the peak current. Peak currents for ten carbohydrates at 0.5 mM ranged from 17 to 42 μa and a detection limit of 0.01 mM was evaluated for dextrose. Calibration plots of reciprocal peak current (I^-1_p) vs. reciprocal of concentration (C^-1) were linear for dextrose and sorbitol concentrations between 0.1 and 1.0 mM.     

Structural evolution during the reaction of Li with nano-sized rutile type TiO2 at room temperature

Rutile type TiO₂ nanoparticles (10 nm × 200 nm in size) were prepared using a precipitation method in aqueous solution. Their lithium reactivities were followed using both classical galvanostatic insertion and in situ XRD measurements, and compared to that of a bulk and commercial nano-sized (50 nm) rutile TiO₂ sample so as to stress the interlink between Li insertion electrochemical capacity and crystallite size. For the highly divided material, we obtained a reversible capacity of 0.5 Li ion per formula unit after a first reduction step during which, the material is irreversibly transformed. Such a reduction step is shown to enlist two solid solution domains followed by the formation of a rocksalt type phase LiTiO₂. Such a specific reactivity of nano-sized rutile TiO₂ is explained in terms of better lattice strains accommodation during the insertion of lithium.     

Cathodic electrochemiluminescence in aqueous solutions at bismuth electrodes

The high hydrogen evolution overpotential of a bismuth electrode makes it a powerful electrode for cathodic electrochemiluminescence studies in aqueous solutions.     

The formation of highly excited H in the reaction H2+(v) + H2 → H + H

A quasiclassical trajectory surface hopping method has been used to study H(v) + H₂ → H + H for v = 0, 3, 7, 10, 13, and 17 with an emphasis on determining the H internal energy and angular momentum distributions for high v. For v = 13 and 17, significant cross sections are found for producing H at energies above its dissociation energy. An average metastable H lifetime of 11.5 ps for v = 13 and 4.7 ps for v = 17 is found, but there is also a much longer lived component to the lifetime distributions that is more important for v = 13 than for v = 17. Some of the longer lived metastables correspond to high angular momentum orbiting states of H, but other sources of metastability are also present.     

Probing silver nanoparticles during catalytic H2 evolution

Employing silver nanoparticles from a recently developed synthesis [Evanoff, D. D.; Chumanov, G. J. Phys. Chem. B 2004, 108, 13948] and a well-studied probe molecule, p-aminothiophenol, we follow changes at the surface of the particles during the conditioning and eventually the catalytic production of hydrogen from water using strongly reducing radicals. Injection of electrons into the particles causes pronounced variations in the intensity of the surface enhanced Raman scattering (SERS) spectrum of the probe molecule. These spectral changes are caused by changes in the Fermi-level energy that are in turn caused by changes in the silver ion concentrations or in the pH, or by changes in electron density in the particle. This correlation highlights the effect of the chemical potential on the SERS enhancement at the end of the particles synthesis. The intensity of the SERS spectra increases in the presence of the silver ions when excitation at 514 nm is utilized. When the Ag(+) ions in the colloidal suspension are completely reduced by the radicals and the particles operate in the catalytic mode, the SERS spectrum is too weak to record, but it can reversibly be recovered upon the addition of Ag(+). The effect of pH on the SERS intensity is similar in nature to that of the silver ions but is complicated by the pKa of the aminothiol and the point of zero charge (pzc) of the particles. It is hypothesized that as the particles cross the pzc (around neutral pH) the electrostatic interaction between the protonated amine headgroup of the probe and the positively charged surface increases the density of probe molecules in the perpendicular orientation at the expense of a competing species. This conversion results in enhanced SERS signals and is observable during the preconditioning stage of the particles. Indeed, adsorption isotherms of the probe indicate the presence of two species. In analogous previous observations these two species have been attributed to perpendicular and flat adsorption orientations of the deprotonated probe molecule relative to the particle surface. However, preliminary density functional calculations on relevant prototypes raise the possibility that the two species may be the probe molecule and a cationic form produced by charge transfer in the ground state from the chemisorbed probe to the metal. These two forms of the probe have differing electronic structures and vibrational frequencies, with perhaps differing orientations relative to the surface. Whichever is the correct interpretation, a neutral molecule in a flat orientation or a radical cation, this species is easier to replace than the other in competitive adsorption by ethanethiol.