Semiconductor properties and redox responses at a-C:N thin film electrochemical electrodes

The semiconductor capacitances of the nitrogen-doped amorphous carbon (a-C:N) materials with different sp³/sp² C ratios were studied as a function of electrode potential in a-C:N/aqueous electrolyte systems. This dependence of capacitance on electrode potential in aqueous 0.1 M NaOH shows that the investigated a-C:N materials are intrinsic semiconductors. The space-charge layers inside the a-C:N electrodes behave similar to a Helmholtz layer because of the presence of surface states when the electrolytes contain O₂ or anions other than OH⁻. The lower density and mobility of carriers of materials with a higher sp³ C fraction within the a-C:N material causes a suppression of redox reactions, and the lower density of carriers contributes to a lower capacitance.     

Correlation of film structure and molecular oxygen reduction at nitrogen doped amorphous carbon thin film electrochemical electrodes

Nitrogen doped amorphous carbon (a-C:N) thin film electrodes with a range of film structures have been deposited using a filtered cathodic vacuum arc system. The correlation between film structure and electro-reduction of molecular oxygen in aqueous media at the electrodes has been explored. In aqueous 0.1 M NaOH, dioxygen reduction is inhibited at all the a-C:N electrodes compared with that at glassy carbon electrodes. The potential of the dioxygen reduction current peak shifts negatively at a-C:N electrodes as the sp³ C fraction in the a-C:N materials increases, while the current peak height decreases simultaneously. The a-C:N electrodes possess high sensitivity for investigating the mechanism of dioxygen reduction. It was found that the catalytic H₂O₂ reduction to H₂O on carbon materials is attributed to oxygen species at sp² C sites.     

Correlation between the gel layer properties and the electrochemical behaviour of hydrogen selective glass electrodes

Some pH-responsive glass electrodes have been characterised with respect to gel layer thickness, durability and ion exchange capacity. Glasses with a low durability exhibited a thick gel layer and a high ion exchange capacity in comparison with high durability glasses. The electrochemical properties were judged by measuring the asymmetry potential, the acid and alkaline errors and the kinetic pattern obtained in going from base to acid. It was concluded that the thinner the gel layer the more ideal the electrode function. The results indicate that the gel layer properties rather than the bulk glass properties should be considered in predicting the electrochemical behaviour.     

The electrochemical behaviour of oxygen and hydrogen peroxide on silver electrodes

The reduction of oxygen and the reduction and oxidation of hydrogen peroxide at silver electrodes is examined, with special reference to the side reactions involving the metal and its corrosion products.     

Kryptate complexes—adsorption at electrodes and their potential electrochemical use

The adsorption of the K⁺ complex of Kryptofix 222² or 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8,8,8]hexacosan, at the dropping mercury electrode was examined by ac polarography and drop-time measurements in 1 M potassium fluoride. The maximum surface coverage and diffusion coefficient of the complex were measured. A summary of potential uses in electrochemistry of the complex is given, and its usefulness as a cationic adsorbate demonstrated by its promotion, at rather low concentration, of the cathodic dissolution of lead as tetraethyl lead in a partly aqueous solution.     

Bubble behaviour during oxygen and hydrogen evolution at transparent electrodes in KOH solution

For oxygen and hydrogen evolving transparent nickel electrodes in KOH solutions, parameters characterizing the behaviour of bubbles which are adhered to the electrode surface during gas evolution, have been determined in dependence on current density, i, velocity of solution flow, v, pressure, p, temperature, T, and concentration of KOH. Based on experimental data a new basic bubble parameter, J, has been introduced, which accounts for the bubble behaviour. It has been found that J = a₁i^h₁ and J/(J₀?J) = a₂v^h₂ where J₀ = J at v = 0 ms^?1 and a₁,a₂h₁ and h₂ are empirical constants; some of these depend on nature of gas evolved. Moreover, the parameter J is almost proportional to the KOH concentration, increases in a decreasing rate with increasing pressure and increases linearly with the reciprocal of the absolute temperature.     

Electrolytic resistance of solution layers at hydrogen and oxygen evolving electrodes in alkaline solution

During alkaline water electrolysis, additional energy losses occur owing the presence of bubbles in the solution, particularly close to both the gas-evolving electrodes.For both hydrogen and oxygen-evolving disc electrodes (diameters from 0.2 to 2.0 mm) in KOH solutions, the reduced increase in ohmic resistance, ΔR*, has been determined by the alternating current—impedance method.It has been found that, for hydrogen-evolving electrodes, log ΔR* = 1₁ + b log i, where the exponent b at 0.1 A cm^?2 < i < 5 A cm^?2 does not depend on the diameter, position and material of the electrode, pressure and temperature but does significantly depend on KOH concentration. The factor a₁, however, being dependent on the position, height and material of electrode, temperature and KOH concentration. ΔR* cannot be expressed for the oxygen-evolving electrode by a general equation, due to the coalescence behaviour of oxygen bubbles.Moreover, it has been established that the Bruggemann equation is useful to determine the ohmic resistance of a solution layer containing gas bubbles of different size at which each bubbles adheres to the electrode surface.     

Preparation and electrochemical property of three-phase gas-diffusion oxygen electrodes for metal air battery

Three-phase gas-diffusion oxygen electrodes for metal air battery were prepared and characterized. Nano-structured γ-MnO₂ catalysts were synthesized by solid state redox reaction of two compounds, Mn(CH₃COO)₂·4H₂O and C₂H₂O₄·2H₂O. Their crystal phase, morphologies and particle size were characterized by XRD, TEM, respectively. The electrochemical property of three-phase gas-diffusion oxygen electrodes composed of nano-structured γ-MnO₂ catalysts for oxygen reduction was examined by using the linear polarization method in a neutral solution. Besides, the surface morphologies of the catalytic layer of three-phase gas-diffusion oxygen electrodes were also investigated by SEM. Experimental results revealed that these kinds of three-phase gas-diffusion oxygen electrodes have excellent electrochemical performance. The optimal proportion of nano-structured γ-MnO₂ catalysts in the catalytic layer was 60 wt.%. Three-phase gas-diffusion oxygen electrodes composed with nano-structured γ-MnO₂ catalysts appear to be a highly possible candidate for applications in neutral solution metal air battery.     

Relationships between surface character and electrochemical processes on diamond electrodes: dual roles of surface termination and near-surface hydrogen

The electrochemical behavior of as-deposited, electrochemically oxidized and oxygen plasma-treated semiconducting polycrystalline diamond films was examined. The AD surface was much more active than the oxidized surfaces for solvent discharge reactions (H₂ and O₂ evolution) as well as for electron transfer involving the ferro/ferricyanide couple. The capacitance as a function of potential was used to calculate acceptor concentration profiles, which showed an enriched layer of approximately 15 nm in depth for the as-deposited surface, possibly due to loosely bound hydrogen. These additional carriers appear to enhance electron transfer rates.     

Mechanisms for the evolution and ionization of oxygen at platinum electrodes

By considering published data on the kinetics of the oxygen electrode at a platinum surface it is suggested that the electrode process follows the same path in both acid and alkaline solution. This path is identified, and attention is drawn to certain restrictions on the use of diagnostic criteria.     

Development of electrochemical oxidase biosensors based on carbon nanotube-modified carbon film electrodes for glucose and ethanol

Functionalised multi-walled carbon nanotubes (MWCNTs) were cast on glassy carbon (GC) and carbon film electrodes (CFE), and were characterised electrochemically and applied in a glucose-oxidase-based biosensor. MWCNT-modified carbon film electrodes were then used to develop an alcohol oxidase (AlcOx) biosensor, in which AlcOx-BSA was cross-linked with glutaraldehyde and attached by drop-coating. The experimental conditions, applied potential and pH, for ethanol monitoring were optimised, and ethanol was determined amperometrically at −0.3 V vs. SCE at pH 7.5. Electrocatalytic effects of MWCNT were observed with respect to unmodified carbon film electrodes. The sensitivity obtained was 20 times higher at carbon film/MWCNT-based biosensors than without MWCNT.     

Direct electron transfer between PQQ dependent glucose dehydrogenases and carbon electrodes: An approach for electrochemical biosensors

Glucose biosensors based on direct electron transfer (DET) between two types of PQQ dependent glucose dehydrogenases: soluble (s-PQQ-GDH) and membrane-bound (m-PQQ-GDH) and modified carbon black electrodes were created. The behaviour of both enzymes on carbon paste electrodes was investigated and some differences were elucidated. It was demonstrated that for the achievement of DET between the adsorbed s-PQQ-GDH or m-PQQ-GDH and the carbon paste electrodes particular orientation of the enzymes is required. The stability and the selectivity of the biosensors were studied as well. The calculated inactivation constants for s-PQQ-GDH and m-PQQ-GDH-based biosensors were 0.001 and 0.0001 min⁻¹, respectively. Furthermore, the possibility to detect lactose concentration in dairy products was shown.     

Reactivity at the film/solution interface of ex situ prepared bismuth film electrodes: A scanning electrochemical microscopy (SECM) and atomic force microscopy (AFM) investigation

Bismuth film electrodes (BiFEs) prepared ex situ with and without complexing bromide ions in the modification solution were investigated using scanning electrochemical microscopy (SECM) and atomic force microscopy (AFM). A feedback mode of the SECM was employed to examine the conductivity and reactivity of a series of thin bismuth films deposited onto disk glassy carbon substrate electrodes (GCEs) of 3 mm in diameter. A platinum micro-electrode (? = 25 μm) was used as the SECM tip, and current against tip/substrate distance was recorded in solutions containing either Ru(NH₃)₆³⁺ or Fe(CN)₆⁴⁻ species as redox mediators. With both redox mediators positive feedback approach curves were recorded, which indicated that the bismuth film deposition protocol associated with the addition of bromide ions in the modification solution did not compromise the conductivity of the bismuth film in comparison with that prepared without bromide. However, at the former Bi film a slight kinetic hindering was observed in recycling Ru(NH₃)₆³⁺, suggesting a different surface potential. On the other hand, the approach curves recorded by using Fe(CN)₆⁴⁻ showed that both types of the aforementioned bismuth films exhibited local reactivity with the oxidised form of the redox mediator, and that bismuth film obtained with bromide ions exhibited slightly lower reactivity. The use of SECM in the scanning operation mode allowed us to ascertain that the bismuth deposits were uniformly distributed across the whole surface of the glassy carbon substrate electrode. Comparative AFM measurements corroborated the above findings and additionally revealed a denser growth of smaller bismuth crystals over the surface of the substrate electrode in the presence of bromide ions, while the crystals were bigger but sparser in the absence of bromide ions in the modification solution.     

Electrochemical and spontaneous deposition of ruthenium at platinum electrodes for methanol oxidation: an electrochemical quartz crystal microbalance study

PtRu electrodes with Ru surface concentration ranging from 20 to 50% were prepared by electrolysis of Ru(NO)(NO₃)₃ at a constant potential and/or by spontaneous Ru deposition performed at open circuit potential from a RuCl₃ solution. The amount of either spontaneously or electrochemically deposited ruthenium on the platinum electrode was determined by means of an electrochemical quartz crystal microbalance (EQCM). The effect of the Ru surface concentration on the rate of methanol electrooxidation was also investigated and correlated to the EQCM measurements.     

Use of gas-diffusion electrodes for high-rate electrochemical reduction of carbon dioxide. I. Reduction at lead,indium- and tin-impregnated electrodes

The use of metal-impregnated polytetrafluoroethylene-bonded carbon gas-diffusion electrodes for the electrochemical reduction of carbon dioxide in aqueous solution has been investigated over a wide range of pH (1 to 5). High rates of reduction of carbon dioxide to formic acid were demonstrated. Lead-impregnated electrodes operated at 115 mA cm^–2 in an aqueous acidic electrolyte (pH 2) selectively produced formic acid with a current efficiency of nearly 100% at aniR-corrected potential of approximately –1.8V versus saturated calomel electrode. Electrodes impregnated with either indium or tin produced formic acid at rates comparable with those containing lead. However, in addition to formic acid, small quantities of carbon monoxide were also produced and the simultaneous production of hydrogen by the reduction of water was more significant. Thus, it appears that the electrocatalytic activity for the electrochemical reduction of carbon dioxide to formic acid is lead>indiumtin.     

Microwave activation of electrochemical processes: High temperature phenol and triclosan electro-oxidation at carbon and diamond electrodes

The electrochemical oxidation of phenolic compounds in aqueous media is known to be affected by the formation of electro-polymerized organic layers which lead to partial or complete electrode blocking. In this study the effect of high intensity microwave radiation applied locally at the electrode surface is investigated for the oxidation of phenol and triclosan in alkaline solution at a 500 μm diameter glassy carbon or at a 500 μm × 500 μm boron-doped diamond electrode. The temperature at the electrode surface and mass transport enhancement are determined by calibration with the Fe(CN)₆^3−/4− redox system in aqueous 0.3 M NaOH and 0.2 NaCl (pH 12) solution. The calibration shows that strong thermal and mass transport effects occur at both glassy carbon and boron-doped diamond electrodes. The average electrode temperature reaches up to 390 K and mass transport enhancements of more than 20-fold are possible. For the phenol electro-oxidation at glassy carbon electrodes and at a concentration below 2 mM a multi-electron oxidation (ca. 4 electrons) occurs in the presence of microwave radiation. For the electro-oxidation of the more hydrophobic triclosan only the one-electron oxidation occurs. Although currents are enhanced in presence of microwave radiation, rapid blocking of the electrode surface in particular at high phenol concentrations still occurs.