Photogalvanic cells: Part X. The transparent disc electrode and the iron-thionine system

The kinetics of the iron-thionine system have been investigated using the transparent rotating disc electrode and flash electrolysis. Using the transparent disc electrode, from the results for the variation of photocurrent with rotation speed and irradiance, we can find the quantum efficiency for the production of semithionine, the efficiency for producing leucothionine from semithionine and the rate constant for the back reaction of leucothionine with Fe(III). This rate constant is also determined from the flash electrolysis experiments. The values of the kinetic parameters are found to be in good agreement with values determined by analysis of the photostationary state, flash photolysis and ac modulation at a transparent disc electrode. The relative merits of the different techniques for investigating photogalvanic systems are discussed.     

Photogalvanic cells: Part VIII. The theory of the transparent rotating disc electrode

The theory of the transparent rotating disc electrode is first extended to cover the case where two photogenerated species react with second-order kinetics and where one of the species is electroactive and the other is not. Secondly the theory is developed to describe the case where the solution is bleached close to the electrode. Thirdly theory is presented for experiments where the light source is modulated and the phase shift and amplitude of the resulting modulated photocurrent are measured.     

The light-dependent oxygen reduction by monolayers of hydrated chlorophyll a oligomer

The phenomenon of light-dependent O₂ uptake by monolayers of hydrated chlorophyll a oligomer deposited by the Langmuir-Blodgett technique on an SnO₂ optically transparent electrode has been observed. Spectra of cathodic photocurrents coincided with the absorption spectrum of hydrated oligomer of chlorophyll a. In the presence of an artificial electron donor, hydroquinone, and an oxygen electron acceptor, both the cathodic and the anodic photocurrent caused by dry and wet chlorophyll a molecules of monolayer were measured under illumination in the range 400–800 nm. The effects of electrode potentials and redox reagents on the magnitude of solar energy conversion by the chlorophyll a monolayer at the optically transparent electrode are discussed. ESR and circular dichroism spectra show that hydrated oligomer of chlorophyll a consists of six molecules of chlorophyll a bonded with water molecules.     

Properties of semiconductor electrodes coated with living films of cyanobacteria

IntactPhormidium sp. cells, immobilized on a SnO₂ semiconductor electrode, are capable of transferring electrons to SnO₂ in a light-dependent reaction. Drying a “wet” algal electrode at 50°C for 60 min increases photocurrent output capacity by 100-fold. We have studied the effect of various parameters on photocurrent generation. The magnitude of the photocurrent increased with increasing light intensity and depended on the nature of the electrolyte solution. The output, about 8 μA 10 μg Chl^?1 cm⁺², was obtained using 50 mM H₃BO₃?Na₂CO₃?KCl buffer as an electrolyte, an irradiance (>460 nm) of 250 J/m², and potentiostatic conditions (the algal working electrode was poised at +0.6 V vs a saturated calomel electrode). The yield was more than doubled upon addition of an electron carrier, such as methyl viologen, benzyl viologen, or Vitamin K₃, to the electrolyte solution. Maximum photocurrent was obtained at around pH 8 and 45°C, which are optimal conditions for growth of the cyanobacterium. Furthermore, DCMU, an inhibitor of photosynthetic electron flow, drastically decreased the yield, as did heat treatment of the electrode at 110°C for 15 min. The photocurrent action spectrum peak coincided well with the absorption peak of the light-harvesting pigment, phycocyanin. These results support the idea that electron transfer can occur across algal cell walls from the source of the light-induced reactions located within the lamellar membranes to the semiconductor electrode.     

Photoelectrochemical behavior of a novel composite In0.15Ga0.85As/GaAs|GaAs/Al0.3Ga0.7As multiple quantum well electrode

A novel composite In_xGa_1−xAs/GaAs|GaAs/Al_xGa_1−xAs multiple quantum well material with different well widths was studied as a new kind of photoelectrode in a photoelectrochemical cell. The photocurrent spectrum and photocurrent–electrode potential curve were measured in ferrocene nonaqueous solution. Pronounced quantization effects and strong exciton absorption were observed in the photocurrent spectrum. The effects of surface states and interfacial states on the photocurrent–electrode potential curve are discussed.     

Spectroelectrochemical Measurements at Colloidal Au Multilayer Optically Transparent Electrodes

Application of thin colloidal Au multilayers on glass substrates as optically transparent electrodes is reported herein. Evaluation of the electrode by scanning electron microscopy and as an optically transparent thin layer electrode (OTTLE) illustrate its use for spectroelectrochemical applications. A spectroelectrochemical Nernst plot acquired for the oxidation of o‐tolidine yielded a value of E°=0.605 V (vs. Ag/AgCl) and n=1.88. Both of these values are in excellent agreement with previously published values obtained using a traditional Au minigrid OTTLE. The colloidal Au multilayer electrode is expected to offer many advantages over the minigrid OTTLE in specific applications. These advantages include direct control over microscopic surface topography, large electrochemical surface area, applicability to many different electrode geometries, and increased microscopic surface roughness for SERS and SPR applications.     

Predictability of properties in ternary n-alkane/butanone/poly(dimethylsiloxane) systems from flory-huggins binary interaction parameters and inversion point in preferential solvation

An empirical procedure has been developed, based on the Flory-Huggins theory as generalized by Pouchlý, which permits the calculation of preferential sorption coefficient, λ, and the total sorption term, Y, from previous information on the binary interaction parameters, χ_13′⁰, χ₂₃⁰ and g₁₂(φ₁₀) and of the mixture composition at which the sign of λ inverts. The obtained expressions were applied to seven cosolvent polymer systems, n-alkane/butanone/poly(dimethylsiloxane), for which experimental values of λ and Y are known. In all the studied systems, the theoretical predictions are in fair accord with the experimental data.     

Signal-to-noise enhancement using carbon fiber electrodes in the photoelectroanalytical detection of tris(2,2′-bipyridine)ruthenium(II)

Photoelectroanalytical chemistry (PEAC) is a sensitive and selective technique for the detection and quantitation of tris(2,2′-bipyridine)ruthenium(II). Currently, the detection limit of this method is restricted by background photocurrent due to photochemical processes which occur on the electrode surface. Since these processes increase as the illuminated electrode area increases, experiments were carried out using a cylindrical carbon fiber as the working electrode in a non-flowing system. Results were compared to both a glassy carbon and a platinum macroelectrode. Determination of the signal photocurrent density for the fiber electrode showed it to be of the same order of magnitude as the larger electrodes, while the background photocurrent density was more than two orders of magnitude lower than at the larger electrodes. The signal-to-noise ratio for the microelectrode was also much higher than for either macroelectrode. On the basis of these results, a theoretical detection limit of approximately 10⁻⁹M is possible using a carbon fiber electrode in an ordinary (static) electrochemical cell.     

The acidity functions of trifluoroethanol and hexafluoroisopropanol,and their mixtures with water

Acidity measurements in trifluoroethanol and hexafluoroisopropanol as solvents, and in their mixtures with water, are reported. The hydrogen electrode and the glass electrode were used for pH measurements. The “experimental” autoprotolysis constants were determined from measurements in strongly acidic (trifluoromethanesulfonic) media and in strongly basic (alcoholate) media: for trifluoroethanol, pK = -log K/mol² l^-2 = 15 and for hexafluoroisopropanol, pK = 14.8. Evaluation of the pH-indicator potential systems with reference to the ferrocene/ferricinium couple gives the acidity function R₀(H). The values obtained are compared to the H₀ values evaluated in the same conditions.     

Photoelectrochemical starch-O<Subscript>2</Subscript> biofuel cell consisting of chlorophyll derivative-sensitized TiO<Subscript>2</Subscript> anode and enzyme-based cathode

A new photoelectrochemical starch-O₂ biofuel cell has been developed, consisting of chlorin-e₆ (Chl-e₆) adsorbed on a TiO₂ layer onto an optical transparent conductive glass electrode as a photoanode, bilirubin oxidase (BOD)-modified electrode as a cathode, and a solution containing starch, glucoamylase, glucose dehydrogenase and NAD⁺ as fuel. The short-circuit photocurrent and the open-circuit photovoltage of this cell are 9.0 μA cm^?2 and 530 mV, respectively. The maximum power, FF and \(\eta\) values are estimated to be 1.7 μW cm^?2, 0.36 and 0.0017 %, respectively. Thus, this new type of the photochemical starch-O₂ biofuel cell has been developed by using the visible light photosensitization of Chl-e₆ on a TiO₂ film photoanode.     

Apparent molar volumes and apparent molar heat capacities of aqueous barium nitrate at temperatures from (278.15 to 393.15) K and at the pressure 0.35 MPa

Apparent molar volumes V_φ and apparent molar heat capacities C_p,φ were determined for aqueous solutions of barium nitrate Ba(NO₃)₂ at molalities m=(0.0025 to 0.2) mol · kg⁻¹, at T=(278.15 to 393.15) K, and at the pressure 0.35 MPa. Our V_φ values were calculated from densities obtained using a vibrating-tube densimeter, and our C_p,φ values were obtained using a twin fixed-cell, power-compensation, differential-output, temperature-scanning calorimeter. Our results were fitted to functions of m and T and compared with values from the literature.     

Modelling of reaction–diffusion processes: the theory of catalytic electrode processes at hemispheroidal ultramicroelectrodes

The transient chronoamperometric current for a catalytic electrode reaction (EC′ reaction) at a hemispheroidal Ultramicroelectrode is reported. This simple new approximate multidimensional analytical expression is valid for all values of time and all values of the rate constant. It is more accurate for all times and large K than for short times and small K. The analytical results are compared with recently presented digital simulation results (Galceran and co-workers, J. Electroanal. Chem 466 (1999) 15) for a disc electrode and are found to be in good agreement.     

Electrochemical approach to the development of a photoelectrode on the basis of photosynthetic reaction centers

Experimental approaches to the coupling of photoinduced charge separation in reaction centers (RCs) of the photosynthetic bacterium Rhodobacter sphaeroides R-26 with electron transfer to an electrode are discussed. Exogenous quinones are used as an electron transfer mediator. With the use of photopolarography it is shown that water-soluble ubiquinone can serve as a diffusionally mobile mediator of electron transfer. Some methods of quinone immobilization at an electrode have been developed to obtain a non-diffusional mediator of electron transfer. Quasi-reversible electrochemical kinetics were observed for aminonaphthoquinone immobilized as a monolayer at a Pt electrode. The ubiquinone-depleted RCs were subjected to affinity immobilization at these chemically modified electrodes probably due to the insertion of the immobilized quinone into the primary quinone Q_A binding site. The quantum efficiency of photocurrent formation was ca. 5% for the photoelectrode obtained. The electrochemical process of the immobilized quinone is shown to be the stage that limits electron transfer from RCs to the electrode.     

Voltammetric studies of the activity of an electrode made of a graphite-epoxy composite in redox reactions of ascorbic acid and hydroquinone

The potentials of the anodic peak of ascorbic acid oxidation and the potential differences of anodic and cathodic peaks (ΔE _p) of the hydroquinone/benzoquinone redox system at an electrode made of a graphite-epoxy composite are determined in weakly acidic and neutral supporting electrolytes by direct and cyclic voltammetry. The results obtained are compared with thermodynamic values and with the available values of these parameters at different solid electrodes for the above-mentioned redox systems. The effect of aging of the surface of electrodes made of graphite-epoxy composites on the potentials and peak currents of the anodic oxidation of ascorbic acid are studied. It is demonstrated that the regeneration of the electrode surface by mechanically cutting thin layers is important for reducing the δE _p value of the hydroquinone/benzoquinone redox system down to 28–30 mV in supporting electrolytes with pH 2.0 and 7.0. This value is typical of thermodynamically reversible electrode reactions involving two-electron transfer at 20–25°C.     

Apparent molar volumes and apparent molar heat capacities of aqueous lead nitrate at temperatures from 278.15 K to 393.15 K and at the pressure 0.35 MPa

Apparent molar volumes V_φ and apparent molar heat capacities C_p,φ were determined for aqueous solutions of lead nitrate [Pb(NO₃)₂] at m=(0.02 to 0.5) mol · kg⁻¹, at T=(278.15 to 393.15) K, and at the pressure 0.35 MPa. Our V_φ values were calculated from densities obtained using a vibrating-tube densimeter, and our C_p,φ values were obtained using a twin fixed-cell, power-compensation, differential-output, temperature-scanning calorimeter. Our results were fitted to functions of m and T and compared with results from the literature.     

The oxidation of ascorbate at copolymeric sulfonated poly(aniline) coated on glassy carbon electrodes

Self-doped poly(aniline)s as electrode coatings to catalyze ascorbate oxidation are revisited in this article. Sulfonated poly(aniline) (SPAN) was deposited on glassy carbon electrodes as a copolymer of aniline and its sulfonated derivative, 2-aminobenzenesulfonic acid (2-ABSA). The resulting deposits are reproducible and show good stability and electroactivity at pH > 7, enabling studies at typical physiological pH values. Calibration curves were obtained using a rotating disc electrode at a sampling potential of 0.2 V, displaying linear dependence in the region 0–20 mM ascorbate. A kinetic model based on the Michaelis–Menten reaction mechanism, previously validated for poly(aniline) composites, was used to analyse the form of the calibration curve leading to values of the effective reaction constants K_ME and k′_ME. New calibration curves constructed for different sampling potentials were used to elucidate the rate limiting step at saturated kinetics. Rotating disc voltammetry performed at increasing pH (from pH 2 to 9) showed a dramatic decrease in the limiting current, without any evidence for a change in the reaction mechanism.     

Development of a versatile rotating ring-disc electrode for in situ pH measurements

There are some electrocatalytic reactions in which the key parameter explaining their behavior is a local change in pH. Therefore, it is of utter importance to develop an electrode that could quantify this parameter in situ, but also be customizable to be used in different systems. The purpose of this work is to build a versatile rotating ring/disc electrode (RRDE) with IrO_x deposited on a glass tube as a ring and any kind of material as disc. As the IrO_x is sensitive to pH variation, the reactions promoted on the disc can trigger proportional pH shifts on the ring. In such assembly, the IrO_x ring presents a fast response time even during the pH transients due to the small thickness of the ring (approximately 10 μm), which enables the detection of interfacial pH changes. The ring electrode was tested toward the interfacial pH shift observed during the electrolytic reduction of water on the disc and also characterized by acid–base titration to determine the response time. As the main conclusions, fast response and durable RRDE were obtained, and this assembly could be used to revisit many electrocatalytic reactions in order to test the importance of local pH on the process.     

Electrophoretic deposition of ZnO photoanode for plastic dye-sensitized solar cells

Plastic dye-sensitized solar cells have been fabricated based on an organic dye (D 149) and ZnO photoanode prepared via room temperature electrophoretic deposition (EPD) to yield a conversion efficiency of 4.17% under 100 mW cm^?2 AM 1.5 illumination. Intensity modulated photocurrent spectroscopy analyses reveal that the fabricated ZnO electrodes have adequate interparticle connection, even in the absence of any post-treatment. This study demonstrates that EPD is a convenient method for photoanode fabrication and ZnO photoelectrodes obtained via EPD are promising for efficient plastic solar cells.     

铁钝化膜半导体特性的光电化学研究

The photoelectrochemical behavior of the passive film on iron in pH=8.4 borate buffer solution was investigated. The mechanism considered for the generation of the photocurrent in the passive film, and the principle of the measurement of photocurrent were discussed. The band model of noncrytalline semiconductor was used to demonstrate the depedence of photocurrent on electrode potential and light wavelength. Results indicate that the passive film formed on iron in pH=8.4 borate solution is a noncrystalline semiconductor, its flatband potential (φ_(fb)) is -0.30 V(SCE), band gap energy (E_g) is affected by measuring potential (φ_m), When +0.30 V<φ_m<+0.80 V (SCE), E_g is 1.9 eV.     

Volumetric properties and volumetric interaction parameters for the CsCl–monosaccharide (d-galactose,d-xylose and d-arabinose)–water systems at T=298.15 K

Density measurements have been carried out at T=298.15 K for the CsCl–monosaccharide (d-galactose, d-xylose and d-arabinose)–water systems. The apparent molar volume of saccharides V_φ,S in the ternary solutions, the corresponding infinite dilution apparent molar volume V_φ,S^∘, and the standard transfer volume Δ_tV_φ,S^∘ of saccharides from water to aqueous CsCl solutions have been determined. The McMillan–Mayer theory was employed to relate the excess thermodynamic functions to a series of interaction parameters to obtain the volumetric interaction parameters of CsCl with monosaccharide in water. These parameters are interpreted by the group additivity principle and the stereochemistry of these monosaccharide molecules.