Photo-induced electron transfer of a chlorophyll—liquid crystal electrode to water

A platinum plate was coated with a thin layer of chlorophyll and liquid crystal in order to make it photo-excitable. The chlorophyll—liquid crystal electrode immersed in water was found to shift its electrode potential to a more positive value on light irradiation. Furthermore light irradiation increased cathodic current. Liquid crystals such as N-(p-methoxybenzylidene)-p′-butylaniline markedly enhanced a photo-response in electrode potential and current. A prominent photo-response of the electrode appeared in an acidic solution around pH 4. A possibility of a direct electron transfer from a photo-excited chlorophyll—liquid crystal electrode to water, ie, photo-decomposition of water, is suggested.     

Electrochemical processes in the photoelectrolysis of nicotinamide adenine dinucleotide on the chlorophyll electrodes

The photoelectrolysis, in which redox compounds are electrolysed on a pair of photo-excitable electrodes by supplying photo—energy in place of electric energy, has been performed. The photo—excitable electrodes were prepared by coating a platinum plate with a thin layer of a chlorophyll—quinone composite. These electrodes were called chlorophyll electrodes. The chlorophyll electrode of chlorophyll—naphthoquinone composite worked as a cathode and that of chlorophyll—anthrahydroquinone composite as an anode when they were illuminated. The chlorophyll electrode of chlorophyll—naphthoquinone composite was characterized by an electrochemical behavior of p-type semiconductor electrode. Reduction of nicotinamide adenine dinucleotide (NAD⁺) was carried out on the chlorophyll electrode under illumination at various controlled electrode potentials. NAD was reduced at extremely noble electrode potentials are compared with the reductive potential of NAD⁺ to NADH. Electron transfer accompanied with the photoelectrochemical reactions is discussed.     

Photoelectrochemical oxygen evolution from water by a manganese chlorophyll—liquid crystal electrode

In order to accomplish the photoelectrochemical evolution of oxygen from water, manganese chlorophyll was incorporated in a thin liquid crystal membrane attached to a platinum plate. The manganese chlorophyll—liquid crystal electrode reacted to water on visible light irradiation. The potential of the platinum markedly depended on the states of the immobilized chlorophyll. Visible light irradiation induced a negative shift in potential. The incorporation of a nematic liquid crystal was found to considerably enhance the potential shift on light irradiation. The negative potential shift accounted for the light-induced electron flow from water to the platinum phase. The photoelectrochemical reaction of the immobilized manganese chlorophyll was performed in parallel with the direct determination of the oxygen evolution. The molecular oxygen evolution was associated with the photocurrent.     

Mechanism of the chlorine evolution on a ruthenium oxide/titanium oxide electrode and on a ruthenium electrode

The mechanism of the chlorine evolution on titanium electrodes coated with a layer of ruthenium oxide and titanium oxide under different experimental conditions, and on a ruthenium electrode, both in acidic chloride solution, has been investigated. Potentiodynamic current density—potential curves were recorded as a function of the time anodic pre-polarisation, the composition of the solution and the temperature. Moreover, potential decay curves were determined. Theoretical potential decay curves were deduced for both the Tafel reaction (2 Cl_ad→Cl₂) and the Heyrovsky reaction (Cl^? + Cl_ad → Cl₂ + e^?) as a rate determining step in the formation of molecular chlorine. They were compared with those found experimentally. The influence of possible diffusion of atomic chlorine out of the electrode was also taken into consideration. It was found that for all the electrodes investigated, molecular chlorine is formed both at anodic polarisation and on open circuit according to the Volmer—Heyrovsky mechanism, where the Heyrovsky reaction is the rate-determining step. The transfer coefficient is 0.5 for the chlorine evolution at an “ideal” ruthenium oxide titanium oxide electrode and at a ruthenium electrode.     

Radiation polymerization of methyl methacrylate in the presence of liquid crystals

Radiation polymerization of methyl methacrylate (MMA) was studied in the presence of two liquid crystals: N-(p-methoxybenzylidene)-p-butylaniline (MBBA, nematic) and cholesteryl 2-(ethoxy—ethoxy) ethyl carbonate (CHEECO, cholesteric). For comparison, polymerization was also carried out in the presence of benzene and cholesterol. Tacticities of the resultant poly(methyl methacrylate) were determined from nuclear magnetic resonance spectroscopy. Enhancement of syndiotacticity of the polymer obtained in the presence of cholesteric liquid crystal was observed. The rates of polymerization of pure MMA and MMA with different additives at various concentrations were studied. The viscosity-average molecular weights at one dose were also determined. Replica polymerization in the presence of preformed isotactic poly(methyl methacrylate) [i-PMMA] or syndiotactic poly(methyl methacrylate) [s-PMMA] with and without CHEECO was studied. Enhancement of syndiotacticity was observed in the presence of i-PMMA with and without CHEECO.     

Electrochemical deposition of fine Pt particles on n-Si electrodes for efficient photoelectrochemical solar cells

Fine platinum (Pt) particles were deposited electrochemically on n-type silicon (n-Si) electrodes from an aqueous hexachloroplatinic acid(IV) solution by the single potential step (SPS) and double potential step (DPS) methods. The distribution density of the Pt particles on n-Si was 10⁸ cm⁻² for the SPS method, whereas it increased from 10⁹ to 10¹⁰ cm⁻² by a shift of the pulse potential at the initial step of the DPS method from −1.0 to −4.0 V versus SCE and remained nearly constant at more negative potentials. The size of the Pt particles enlarged with the charge density passing across the electrode surface at a potential of −0.70 V versus SCE, which was applied throughout for the SPS method and at the second step for the DPS method. Photoelectrochemical (PEC) solar cells equipped with Pt-electrodeposited n-Si electrodes generated open-circuit photovoltages (V_OC) of 0.51–0.61 V, much higher than those for n-Si electrodes coated with continuous Pt layers (ca. 0.2–0.3 V). Solar cell characteristics changed with the pulsed potential and charge density passing across the electrode surface which changed the size and distribution density of the Pt particles. The characteristics were explained well by our previous theory on metal-dot coated n-Si electrodes.     

Etude electrochimique a l'etat solide du couple anthraquinone—9.10—anthraquinol,en tant que matiere active d'electrode de generateur secondaire

The—9.10 anthraquinone—anthraquinol couple has the proper characteristics to be used, in the solid state, in negative electrode reactions of secondary electrochemical generators in the presence of an aqueous electrolyte.Our research work covered the conditions of electrochemical formation of anthraquinol, of little stability in the presence of oxygen, within the electrode and starting with anthraquinone. The electrochemical characteristics of such electrodes were then measured.The equilibrium potential of the electrochemical system was about 80 mV/enh in a normal sulphuric medium and the actual capacities recorded were measured with respect to different operation conditions of the electrodes. The variation of these characteristics, according to electrode composition, medium acidity and temperature, were studied.This research led to the development of an experimental battery supplying a voltage of approx 0,5 V and having a mass energy ratio of about 25 Wh/k of electrode.     

A study of iodine adlayers on polycrystalline gold electrodes by in situ electrochemical Rutherford backscattering (ECRBS)

Iodine adsorption on a polycrystalline gold electrode was studied by in situ electrochemical Rutherford backscattering (ECRBS) using an ultrahigh vacuum (UHV)-electrochemical cell comprising of a thin-film silicon nitride window. The depth resolution of RBS allowed for measurement of nuclide concentration of the diffuse double-layer, electrode surface and near-surface regions. ECRBS measurements on the gold electrode, initially exposed to −500 mV vs. a platinum pseudo-reference electrode, in a potassium iodide solution, showed an increase in the 2.07 MeV iodine peak indicative of iodine adsorption. The surface concentration of the iodine adlayer was directly measured by ECRBS to be 1.3 ± 0.3 nmol/cm². ECRBS measurements on a gold electrode exposed to 1.5 V vs. a platinum pseudo-reference electrode, in a potassium iodide solution display a decrease in the 2.16 MeV gold peak and a shift to lower energies. Scanning electron microscopy images of electrodes studied by ECRBS displayed roughened surfaces consistent with gold dissolution. This work demonstrates the potential for in situ ECRBS using thin-film silicon nitride windows to become a powerful tool for the investigation of a wide range of electrochemical processes in areas such as corrosion, electrodeposition and electrocatalysis.     

Current distribution at electrode surfaces as simulated by finite element method

In this article, an approach of finite element method has been introduced to predict the distribution of potential and current on electrode surfaces. In this method, the bulk of electrolyte solution and the main body of solid electrodes are modeled to be normal conductive media; while the surface of the electrode, whose current-potential (I-V) curve satisfies experimental data, is modeled to be special nonlinear boundary conditions. This modeling approach for electrochemistry system is successfully verified by experimental data, and would contribute to a better understanding of the distribution of electrical potential and current at electrode surfaces in practical systems with high voltage coupling effect.     

Electrocatalysis and the chlorine evolution reaction

The chlorine evolution reaction has been investigated on D.S.A. type electrodes in 5.13 M NaCl solution. A large amount of potentiostatic current—potential and impedance—potential data has been collected over a wide potential and frequency range. The current—potential and impedance—potential data has been reduced by curve fitting to a series of a parameter curves. The impedance—potential data were analysed by two methods: an equivalent circuit and the parameter curves C_dlE, R_ctE, R_ωE reported, and a reaction mechanism and the parameter curves C_dlE, kSHE, R_ωE reported. The two ways of analysing the impedance data are equivalent. However the analysis into a reaction parameter has the advantage that all the data, current—potential and impedance—potential, can be tested using a single set of parameters (kSH, R_ω, a, b, D_A, D_B, δ).In this work the data is compared to that expected for a redox reaction. Any deviation from the expected behaviour of a perfect redox reaction is ascribed to Cl₂ bubble formation and to electrode structure effects. It is suggested that the parameter curves are related also to bubble formation characteristics and the ‘real’ area of the electrode.     

The mechanism of the chlorine evolution on different types of graphite anodes during the electrolysis of an acidic NaCl solution

The behaviour of the electrode and the formation of chlorine were studied practically for porous graphite used in the industrial electrolysis of an aqueous NaCl solution only.In this investigation this was done for Acheson graphite electrodes and for highly orientated pyrolytic graphite electrodes of which the edge plane or the basal plane served as electrode surface. Voltammograms were determined by continuous scanning with different rates of the potential between a minimum and maximum value. From the experimental results it follows that two different surface compounds, the lp oxygen and the hp oxygen are formed on all the graphite electrodes investigated. In the stationary state lp oxygen is present at low potential and the hp oxygen at high potentials. The formation and the removal of each compound depends on the electrolytic conditions, especially on the maximum potential of the sweep. The conditions of the sweep determine whether these oxides are really present on the electrode surface during the anodic or the cathodic sweep. Chlorine is formed according to the Volmer-Heyrovsky mechanism on all the three types of electrodes. The current density, the transfer coefficient and the activation energy for the edge plane electrode and the Acheson graphite electrode are of the same order of magnitude, while for the basal plane electrode they deviate strongly.     

The variation of current density and electrode potential with electrode resistance and its role in cell design

The potential drop along parallel electrodes of finite resistance has been calculated (a) assuming a pure resistive representation of the cell and (b) by considering the polarization characteristics of an irreversible electrode reaction. It was found that a much more uniform potential (and also current) distribution was obtained with the current feeders at opposite ends of the electrodes rather than at the same ends. With this configuration it is advantageous to have electrodes of equal resistance. Equations are given that enable the potential and current density distribution to be easily calculated. A factor I_tot/I_max was derived which is a measure of the effective utilization of the electrode area. It may be used for calculating the required electrode area with non-uniform cd distribution.     

Electrochemical behavior of glassy carbon electrodes modified by multi-walled carbon nanotube/surfactant films in a buffer solution and an ionic liquid

The electrochemical behavior of glassy carbon (GC) electrodes coated with multi-walled carbon nanotube (MWCNT)/surfactant films was studied in an ionic liquid and a phosphate buffer solution (pH = 6.86), using cyclic voltammetry. The dispersion of MWCNTs in different media was investigated by scanning and transmission electron microscopy. Cast films of MWCNT/zwitterionic dodecyldimethylamine oxide on a GC electrode show a typical redox couple in phosphate buffer solution, which is better than that of MWCNT/anionic sodium dodecyl sulfate and cationic alkyltrimethylammonium bromide. However in the ionic liquid, 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]), the GC electrode modified by MWCNT/cationic surfactant films shows a well-defined irreversible reduction of MWCNTs. The cyclic voltammograms clearly show that the surfactant hydrophilic group plays an important role in the electrochemical behavior of the MWCNTs. The electrolytes also have an important effect. In an ionic liquid, the strong binding of the ionic liquid cations with the MWCNTs may change the structure of the modified films and lead to changes of electrochemical behavior.     

Bismuth oxide-coated (La,Sr)MnO3 cathodes for intermediate temperature solid oxide fuel cells with yttria-stabilized zirconia electrolytes

Taking Y₂O₃ stabilized Bi₂O₃ (YSB) as an example, bismuth oxide-added (La,Sr)MnO₃ (LSM) is evaluated as a cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs) with 8 mol% Y₂O₃ stabilized ZrO₂ (YSZ) electrolytes. YSB was added to LSM cathodes using an impregnation method, dramatically improving the electrode performance. The interfacial polarization resistance R_p, at 700 °C for the electrode coated with 50 wt.% of YSB is 0.14 Ω cm², which is only 0.2% of the value for a pure LSM electrode. The high oxygen ionic conductivity and the catalytic activity of YSB, as well as the favorable electrode microstructure are likely reasons for the dramatic reduction of R_p. The YSB-added LSM cathodes also exhibited lower overpotential and higher exchange current density than the pure LSM cathode. Moreover, these electrodes show much lower R_p than that of parallel-fabricated LSM electrodes with samaria-doped-CeO₂ as well as other LSM-based electrodes reported in the literature, demonstrating the superiority of the of YSB as the ionic conduction component in composite LSM electrodes. The superior performance of the single cell further demonstrates that the bismuth oxide-added LSM cathode is an excellent candidate for IT-SOFCs.     

Influence of mass transport on the performance of carbon gas-diffusion air electrodes in alkaline solution

The effect of mass transport on the electrochemical behaviour of carbon gas-diffusion air electrodes in alkaline solution was investigated on the basis of ΔE(i) curves. These curves are obtained by subtraction of potential values for an electrode operating with airE _air(i) from potential values for the same electrode operating with pure oxygenE _oxygen(i) at the same current densityi. Three different regions on these curves connected with different modes of mass transport are recognized. A model of the gas-diffusion air electrode which takes into account the diffusion of the gas, diffusion of the dissolved gas, electrochemical reaction and IR drop is used to explain the experimental results.     

Etude de mecanismes ec sur electrode a pate de graphite

A theoretical analysis of EC mechanisms on graphite paste electrode (GPE) in linear sweep voltammetry is proposed for the following E_iC_j mechanisms. reversible electrochemical reactions E_i: Ox + ne ? Red (E₁), or: Ox + e ? S, S + e ? Red, 2S ? Ox + red (E₂); irreversible chemical reaction C_j: p Ox → A (C₁), or: p Ox → A + m Red (C₂), or: p Ox + m Red → A (C₃).Current—potential relationships are similar to those obtained by Laviron for both layer electrodes and film covered electrodes when the differential capacitance C and the circuit and electrode resistance R are negligible. For large values of the kinetic contants, it is possible to establish a set of relationships between the mechanism parameters and the experimental characteristics of the voltammograms.On a practical point of view, two types of mathematical analysis are proposed.     

Elektroanalytische untersuchungen—I. Die auswirkung von kanteneffekten auf die zahl der peaks in cyclischen voltammogrammen

The pathways of the electrochemical oxidation of 2,6-di-tert-butyl-4-(4-dimethylaminophenyl)-phenol (1), 2,6-di-tert-butyl-4-methoxy-aniline (2) and a mixture of N, N-dimethylaniline and 2,6-di-tert-butyl-4-phenyl-phenol (3) are examples for reactions which show a dependence of the number of appearing peaks on the electroactive areaA of the electrode during cyclic voltammetric investigations. This is discussed in terms of edge effects and, in addition, the barrier of an inhibiting substance — protons in this case — near the electrode. Two electrodes with different electroactive areas and a “shielded” electrode have been used.     

Cathodic polarization of variously corroded iron electrodes in acid perchlorate solutions

The steady-state cathodic reaction taking place on iron electrodes in deaerated, acid perchlorate solutions free of iron salts in the hydrogen evolution reaction. However, reduction of corrosion products formed during preceding anodic polarization will also occur in various potential regions. Cathodic polarization diagrams recorded with rotating disc electrodes (rde) show three reaction regions. In region I, starting below the corrosion potential, the reaction is influenced by pH of the solution and by potential. This is followed, at lower potentials, by region II, where the reaction is limited by diffusion of hydrogen ions to the electrode. The limiting cd is determined by pH of the solution and by the speed of rotation of the rde. At still lower potentials, in region III, there is a further increase of cd due to discharge of water molecules. This reaction is influenced by potential, but not by pH or rde rotation rate.Experiments lasting a few minutes demonstrate that the behaviours in the regions I and II depend on the type of preceding anodic polarization; the alternative effects will be cathodic peak in region I, attributed to the reduction of a previously formed oxidic iron compound approaching the composition of magnetite, or an additional contributon to the cd due to reduction of a different iron compound and in excess of the quantity accounted for the hydrogen evolution.A detailed, rather novel and powerful type of analysis of the reaction recorded in region I indicates a low and constant coverage by adsorbed hydrogen atoms giving rise to a parallel combination of a Volmer—Tafel mechanism (the minor part) and a Volmer—Heyrovsky mechanism (the major part) for the hydrogen evolution reaction.     

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.     

The pH-dependence of oxygen reduction on multi-walled carbon nanotube modified glassy carbon electrodes

The pH-dependence of oxygen electroreduction has been investigated on multi-walled carbon nanotube (MWCNT) modified glassy carbon (GC) electrodes. Various surfactants were used in the electrode modification: dihexadecyl hydrogen phosphate, cetyltrimethylammonium bromide, sodium dodecyl sulfate and Triton X-100. Electrochemical experiments were carried out in 0.5 M H₂SO₄ solution, acetate buffer (pH 5), phosphate buffers (pH 6, 7 and 8), borate buffer (pH 10), 0.01 M KOH, 0.1 M KOH and in 1 M KOH solution, using the rotating disk electrode (RDE) method. The oxygen reduction behaviour of MWCNT-modified GC electrodes at different pHs was compared. The RDE results revealed that the half-wave potential (E_1/2) of oxygen reduction was higher in solutions of high pH. At lower pHs (pH < 10) the value of E_1/2 did not essentially depend on the solution pH. A comparison with previous studies on bare GC showed that the pH-dependence of the half-wave potential of oxygen reduction on MWCNT-modified GC electrodes follows a similar trend to that observed for bare GC.