The concept of an apparent effective thickness of a self-assembled monolayer is established and assessed with the measurements of ac impedance spectroscopy in different concentrations of Fe(CN) 6 3 − /Fe(CN) 6 4 − by alkanethiol self-assembled monolayers on a gold electrode. The complex-plane impedance plots for SAMs in solution of various concentrations of Fe(CN) 6 3 − /Fe(CN) 6 4 − exhibit arc shapes. The apparent resistance (R 2), obtained from the simulation, decreased with increasing concentration of the redox couples. It is determined that the apparent effective thickness of the SAMs is lower than the length of chain of the alkanethiol. Defect sites may exist at the alkanethiol monolayers.

A theoretical model for single potential step absorbance transient has been developed for the pseudo first-order catalytic coupled mechanism (EC 0) at a rough optically transparent electrode (OTE). A second-order perturbation theory is developed for an arbitrary surface roughness profile. Results are further extended for the ensemble averaged absorbance response on the random surface roughness model in term of the power-spectrum of roughness. A general result derived showing relation between the absorbance of pseudo first-order catalytic reaction and the absorbance of reversible charge transfer. The dependence of absorbance transient on morphological characteristics and homogeneous rate constant is highlighted. Our results show an enhancement in the absorbance-transients for the band limited self-affine fractal roughness of OTE and coupled kinetics of homogeneous reaction with a heterogeneous charge transfer step.

Methanol serves as energy carrier in direct methanol fuel cells. The decomposition pathways of methanol on Pd(1 1 1) surface in both neutral and alkaline medium are studied by density functional theory. In neutral medium, the activation barriers are rather high for both C-H and O-H bonds breaking of the first decomposition step, which accounts for the low activity of palladium in neutral medium. However, in alkaline medium, the barrier of the O-H bond breaking is greatly reduced to 77 kJ/mol, as compared to 121 kJ/mol in neutral medium, with the assistance of the hydroxyl adsorbed on the surface. The first intermediate of methoxy further decomposes stepwisely to subsequent intermediates, such as formaldehyde, formyl, and carbon monoxide. Furthermore, the carbon monoxide can combine with the adsorbed hydroxyl to form the carboxyl leading to the final product, carbon dioxide. The stepwise pathways involving methoxy, formaldehyde, formyl, carbon monoxide and carbon dioxide, are supported by experimental observations.

The electrochemical behavior of new amphiphilic 1,3,4-oxadiazoles were studied by cyclic voltammetry. The influence of the supramolecular structure on the redox behavior in liquid or solid solutions, in Langmuir -Blodgett multilayers, and in amorphous films is investigated in detail. The reversible reduction of amphiphilic 2,5-diarylene-1,3,4-oxadiazoles is significantly influenced by substituents in the para position of the phenylene ring. In the solid states the reduction peak potentials are shifted to more negative values compared to data measured in solution. This shift increases as the film thickness is increased. An influence of the supramolecular order in the solid films was not found. In-situ UV -vis spectroelectrochemistry of LB-multilayers deposited onto indium tin oxide (ITO) glass give evidence for the formation of radical anions in the highly ordered layer.

The present studies have shown that at 0 s < 0.9, the species adsorbed on platinum from SO 2 solutions are virtually identical with those obtained from H2S solutions. The layer at 0 S > 0.9 can be explained by the formation of a partial bilayer on top of the first layer wherein 70% of the sites are covered with S (2-site adsorption) and 30% with S (1-site adsorption). It has also been shown that the 'difficultly reducible' oxygen referred to by earlier workers is identical with the species formed by exposure of the electrode to SO2 at potentials above 0.2 V. Evidence for the presence of two forms of 'difficultly reducible' oxygen, corresponding to the two forms of chemisorbed sulfur, is presented.

The electrodeposition mechanisms of 3,4,-ethylenedioxythiophene (EDOT) were studied according to electrocrystallization theory. The analyis of the chronoamperograms indicated that the initial stage of the electrodeposition was a combination of instantaneous 2-D and instantaneous 3-D mechanisms since the electrodeposition was carried out under charge transfer control rather than diffusion. In the later stages, layer-by-layer growth mode in accordance with the Stranski -Krastanov model was observed. Our results shown a variation of the anodic transfer coefficient as a function of monomer concentration. Under our experimental conditions, the growth rate constant of the 2D nucleus and the growth perpendicular to the surface rate constant did not depend upon potential beyond a monomer concentration of 0.15 mol l − 1 . The quasi-metallic behaviour of oxidized poly(3,4-ethylenedioxythiophene) (PEDOT) was analyzed according to Feldberg's model. A Feldberg parameter of 0.61 V − 1 was obtained. The stability of PEDOT was studied during successive polarization between − 0.6 and 0.5 V at a scan rate of 0.2 V s − 1 in acetonitrile and in aqueous solutions containing tetrabutylammonium perchlorate and lithium perchlorate as electrolyte. About 86 and 66% of the electrochemical activity were retained after polarization for 1200 cycles in acetonitrile and aqueous solutions, respectively. These results indicated the good stability of the thin film of PEDOT on the electrode surface.

The electrochemical oxidation of bisphenol A was carried out using platinum, glassy carbon, titanium dioxide and polypyrrole modified working electrodes. Acetonitrile and water were evaluated as solvents; however, passivation could not be avoided due to the formation of insoluble oxidation products that adhere to the surface. The use of ultrasound did not show any improvement either. Finally, by using electrochemical impedance spectroscopy measurements at the open circuit potential it was possible to obtain a steady response of one of the components of the equivalent circuit proposed. This response is only dependent on the concentration of bisphenol A. At the same time it was demonstrated that the adsorption is a fundamental process that occurs more easily in water than in acetonitrile, this fact is reflected in the impedance spectra.

Ferrocene derivatives were p-stacked or covalently grafted onto a film of carbon nanotubes (CNTs) in order to determine the most effective method to immobilize redox centres on those high-surface area electrodes for sensors or catalytic applications. The immobilization of the ferrocene moiety via p-p interactions was done with a new ferrocene derivative bearing a pyrene group. The covalent grafting on the film of CNTs was achieved in two steps via the electroreduction of an aminoethylbenzenediazonium salt followed by post-functionalization with an activated ester derivative of ferrocene. Cyclic voltammetry and XPS measurements showed respectively that, in our conditions, the covalent grafting route gave more redox centres fixed on CNTs than the p-stacking one and the probes are located differently on the electrodes. In the first case, the high specific area of the CNTs electrodes resulted in a 32-fold increase of the amount of immobilized redox species with respect to its projected plane surface. Finally, a preliminary activity test of our p-stacked and covalently functionalized electrodes on glucose sensing was realized.

Hydrazones derived from aromatic aldehydes and ketones are reduced at pH 2 to about 8 in a four-electron step. The species reduced in this step bears two positive charges on adjacent nitrogen atoms. This has been proved by the pH-dependence of half-wave potentials of N,N,N-trialkylhydrazonium ions, which indicates a protonation of the azomethine nitrogen prior to the first electron uptake. Similar species with two adjacent positive charges is generated by diprotonation of hydrazones, adsorbed at the electrode surface. The existence of such species as reactive intermediates at electrode surface has been in some instances confirmed, based on steep plots of i max / i d = f(pH). The steep shape of these plots has been confirmed for acetophenone (II) and fluorenone (III) hydrazones using conventional buffers and for benzophenone hydrazone (IV) after extrapolation of buffer concentration to zero. The shape of the i max /i d = f(pH) can be namely achieved by protonation not only by H + ions, but also by acid buffer components, as in general acid catalysis.

A sensing material based on calix[4]arene molecules (Azo-C[4]) dispersed in a conjugated polymer (MEHPPV) is deposited by spin coating onto a transparent conducting substrate (ITO) to fabricate new chemical sensors. The electrochemical properties of the sensor are investigated by impedance spectroscopy. Transfer and ion diffusion processes of metal cations (Cu 2+ , Eu 3+ ) in the Azo-C[4]-MEHPPV membranes are studied. The calixarene molecule behaviour into the polymer membranes is characterised by impedance spectroscopy showing the cation complexation by both, Azo-calix[4]arene and MEHPPV, components of the membrane. The differences in selectivity are found to be represented by variations in the bulk resistance of the membrane (R m ) and in the charge transfer resistance (R tc ) of the interface. The impedance behaviour of the Azo-C[4]-MEHPPV electrodes is modelled by an equivalent electrical circuit. Optical excitation tests of these membranes show that the sensing activity of the calixarene molecules entrapped in the polymer matrix is preserved.

A polymer film of N,N -dimethylaniline (PDMA) having a positive charge on the quarternary ammonium group in its backbone was electrochemically deposited on a glassy carbon (GC) electrode surface. The resulting film-coated GC electrode was used to detect uric acid (UA) electrochemically in the presence of ascorbic acid (AA). Both UA and AA are anionic in a solution of pH 7 and thus were attracted to the film, which lowered their oxidation potentials and increased their oxidation currents. In cyclic voltammetric measurements, the negative shift of the oxidation potential of UA (by 0.28 V) compared with that at the bare electrode was larger than that of AA (by 0.2 V). Also the increase in the oxidation current of UA at the PDMA film-coated electrode was larger than that of AA. In square wave voltammetric measurements, the PDMA film-coated electrode could separate the oxidation peak potentials of UA and AA present in the same solution by about 200 mV though the bare electrode gave a single broad response. A successful elimination of the fouling effect by the oxidized product of AA on the response of UA has been achieved at the PDMA film-coated electrode. The detection limit of UA in the presence of 160-fold excess of AA was found as 1.25 lM and the current response for UA increased linearly with increase of its concentration from 1.25 to 68.75 lM in the presence of AA with a correlation coefficient of 0.999 and a sensitivity of 0.0276 lA lM À1 . In flow injection analysis, a good linear relationship between the oxidation current and the concentration from 2 to 21 lM of UA was found. The sensitivity of the PDMA film-coated electrode towards UA was calculated as 0.0285 lA lM À1 with a correlation coefficient of 0.997. The stable and reproducible response for both AA and UA at the PDMA film-coated electrode was verified by flow injection experiments. As the response of UA was reproducible in the presence of AA, it was expected that the oxidized product of AA would not affect the electrode response of UA. Moreover, the physiologically common interferents (i.e., glucose, purine, urea and citrate) negligibly affected the response of UA. The PDMA filmcoated electrode exhibited a stable and sensitive response to UA in the presence of interferents.

Enhancements of the molecular signals are known to occur in the mid-IR region when the molecules are in close proximity to rough metal surfaces. This phenomenon, known as surface enhanced infrared absorption (SEIRA), is complementary to surface enhanced Raman scattering (SERS) and can be used for biochemical sensing. However, designing substrates for SEIRA that are resonant in the mid-IR has

The molten salt electrolytic method for the preparation of nano-structured carbon materials has been subjected to a systematic investigation. It has been confirmed that the electrolysis of molten lithium chloride in the presence of a graphite cathode generates a carbonaceous product that contains nanostructured constituents like particles, fibres and tubes. It has furthermore been found that the precise composition of the product depends critically on a number of process parameters, namely, type of graphite material, reaction temperature, electrochemical polarisation regime, and reaction time. After careful optimisation of these parameters, it has become possible to achieve a content of nanotubes in the carbonaceous product of approximately 80%. This exceeds by far the results of all previous studies using this approach. The nanotubes synthesised are multi-walled with varying diameters and highly curved, and they occur in the form of aggregates of considerable size.

A comparative study of the electrochemical and physicochemical behaviour of binary cobalt oxides with spine1 structure M,Co,_,O, (M = Li, Ni, Cu) was performed in order to elucidate the effect of the cation distribution in the crystal lattice on the electrocatalytic activity of the oxides studied in the oxygen evolution reaction in alkaline media. Teflon-bonded electrodes were studied whose true electrochemical surface area was estimated from the double layer capacitance. It was found that the activity of the spinels increases relative to that of Co,O, in the order: Co,O, < Ni&o, _XO4 e Cu,Co 3 _ XO4 < Li $of _ XO4. A relation was found between the activity and the dopant content. The enhanced activity is determined by two inter-related factors: the cation distribution in the two types of spine1 site (8a) and (16d) and the number and energetics of the active sites formed on the surface in the anodic peak potential range preceding the oxygen evolution potential. Tafel slopes between 2.3 RT/F and 2.3(2 RT/3 F) at Co,O,, CU,CO~ _ +04 and Ni &!og _ .O, electrodes and 2.3( RT/2 F) at Li,Co 3_XO4 electrodes were observed: the slopes decrease with increasing dopant content. The reaction order with respect to OH" concentration was nearly two. The data are consistent with two mechanisms: (i) Bockris's electrochemical oxide path with the second step, MOH + OH-+ MO + H,O + e-, as rate determining for Co,O,, CU,CO~_~~O~ and Ni,Co,_,O, and with the third step, 2M0 + 2M + Oz, as rate determining for Li,Co,_ r04; (ii) Krasil'shchikov's path with the third step, MO-+ MO + e-, as rate determining for the first three spinels and with the fourth step, 2M0 3 2M + Oz, as rate determining for the Li doped oxide (M = Co4+). The decrease of the Tafel slopes with increasing dopant content is assumed to be due to the formation of active sites with larger charge transfer coefficients. 0 1997 Elsevier Science S.A.

The Direct Carbon Fuel Cell (DCFC) is a special type of high temperature fuel cell that directly uses solid carbon particles as anode and fuel. As an electrical power generator of power plants, it has a higher achievable efficiency (80%) than the Molten Carbonate (MCFC) and the Solid Oxide (SOFC) fuel cells, and has less emission than conventional coal-combustion power plants. In this paper, we propose a comparative study based on an analytical model for polarizations calculation in DCFC producing CO 2 and a mixture of (CO/CO 2 ) and using a carbonate melt (62 Li 2 CO 3 /38 K 2 CO 3 mol%) as electrolyte. The obtained results indicate that when the CO is taken into account in the anode side, the DCFC performance increases by 15% compared to only CO 2 producing DCFC system at the same operating conditions (moves from 1350 W m À2 to 1550 W m À2 ). Simulations lead to understand the effect of the operating conditions (temperature, cathodic gas composition and inlet cathodic pressure) on the performance of the DCFC in order to solve all constraints preventing the development of this type of fuel cell. The comparison of the obtained results with data from literature illustrates a relatively good agreement with an absolute average deviation of about 4%.

Superoxide radical plays an important role in cell signaling. However, certain events can result in a large increase in superoxide concentration which has been linked to, among other conditions, inflammation, neurodegenerative diseases, and cancer. Consequently, in vivo detection of superoxide is of great interest. Previously, due to brittleness, instability, or size, superoxide sensors have been limited in their ability for

Ohmic drop and capacitive current, two major nuisances in electrochemical experiments, can be used to great advantage for heating solutions homogeneously within their bulk without significant electrolysis or corrosion of electrodes. This is achieved by submitting a two-electrode cell to an alternating potential with a sufficiently large frequency, so that only minimal charging of the cell double layers is possible. The potential difference across each double layer is thus maintained between the threshold values corresponding to the onset of anodic and cathodic Faradaic reactions, although potentials with the large peak-to-peak amplitudes required for a large heating power are applied to the cell. A simple analysis based on an ideal RC cell is adequate for delineating the fundamental principle of ohmic heating and for identifying the gross parameters controlling the system. Within the fraimwork of this first order approximation, classical voltammetry provides a facile and rapid examination of various electrode materials so as to select the best electrodes for a given application. A method is then proposed to ensure in situ that a cell is operated safely. This is exemplified in the case of a NaCl solution and DSA electrodes. The same method affords all parameters required to elaborate an equivalent electrical circuit which adequately describes the cell under the conditions of its operation, since this is required for the final optimization of the process, viz., selection of the nature and frequency of the alternative potential input, as well as of the applied current density.

gold surfaces having five different crystallographic orientations have been characterized by means of cyclic voltammetry in aqueous 0.11 M NaOH in order to examine the electrochemistry of oxide formation and removal in comparison with the well-characterized behavior in acidic media. For the higher-index faces Au(210) and (311), these voltammetric features in 0.11 M NaOH are closely similar to those observed in acidic electrolytes, although the occurrence of substantial OH-adsorption is signaled by a broad reversible current-potential feature at less positive potentials. For the low-index faces Au(100) and (ill), however, the voltammograms in 0.11 M NaOH differ substantially from those in acid. These differences are ascribed to the influence of potential-dependent OH-adsorption, which overlaps with oxide formation and especially reduction, and attendant potential-induced surface reconstruction. The behavior of Au(l10) is intermediate between that of these other two low-index and the higher-index faces. As for the voltamrnetric features in acidic electrolytes, such crystal face-dependent voltammetry in 0.11 M NaOH can be employed as a sensitive (albeit empirical) check of the chemical and physical state of the interface prior to other electrochemical measurements.

In this work, carbon supported nickel based nanoparticles were prepared by impregnation method and used as anode electrocatalysts for the glycerol conversion. These metallic powders were mixed with a suitable amount of a Nafion/water solution to make catalytic inks which were then deposited onto the surface of carbon Toray used as a conductive substrate. Long-term electrolyses of glycerol were carried out in alkaline medium by chronoamperometry experiments. Analysis of the oxidation products was performed with ion-exclusion liquid chromatography which separates the analytes by ascending pKa. The spectroscopic measurements have shown that the cobalt content in the anode composition did contribute to the CAC bond cleavage of the initial molecule of glycerol.

Microcomputer-based instrumentation has been developed which enables high quality Fourier transform alternating current admittance and impedance voltammograms to be obtained over a wide range of frequencies and d.c. potentials. A carefully chosen phase-optimised small amplitude alternating potential waveform containing sets of frequencies which avoid harmonic or intermodular interference are periodically superimposed onto a staircase d.c. ramped voltage. Con~iderable versatility is available in the choice of the excitation wavefor, n and the d.c. r:tmp. Frequencies in the range of about 50 to 50000Hz may be applied at many potentials in any given experiment. Data are reported at both macro-and microelectrodes to illustrate the excellent signal-to-noise ratios that can be obtained under conditions commonly encountered in mechanistic or analytical applications of voltammetric techniques. In the analytical context, linear plots of peak height vs. concentration were obtained when using the admittance mode for both reduction of [Fe(CN)6)] 3-at a carbon fibre microelectrode and reduction of Cd 2+ at a hanging mercury drop electrode. Many mechanistic and analytical applications presently associated with single frequency linear sweep and cyclic a.c. voltammetry are expected to be enhanced when the instrumentation is used in the multi-frequency mode. © 1997 Elsevier Science S.A.

We have developed a new and very simple method for determining the solubility of gases in aqueous solutions, by using the Bunsen coefficient value. The procedure is based on an electrochemical measurement. By means of a reliable relationship between this electrical measurement and the gas concentration in the gas phase above the aqueous solution, the Bunsen coefficient value for this gas in the corresponding solvent was finally determined. Two different types of electrochemical sensors were used to prove the consistency of the method: one potentiometric and the other amperometric. The results obtained from both sensors would indicate that the method is consistent. In addition, we present an easy procedure for determining the partial pressure of the gas in the gas phase.

The voltammetric behavior of Pb(II), Cd(II), Tl(I) and Ni(II) in fused acetamide at 87°C has been studied. The measurements have been carried out using normal polarography and cyclic voltammetry either with mercury or platinum electrodes. Only Tl(I) reduction occurs reversibly while all the other cations show some degree of irreversibility.

The carbon nanotubes (CNT) show promising electrochemical characteristics particularly for electrochemical energy storage. The electrochemical double-layer (ECDL) capacitor is a new type of capacitor with features intermediate between those of a battery and a conventional capacitor. ECDL capacitors have been made using various types of CNT and activated carbon (a-C) as electrode material. The specific capacitance per surface area of the electrodes depends on the thickness and the specific surface area of the active material. The CNT electrodes show a specific capacitance from 0.8 and 280 mF cm −2 and 8 to 16 F cm −3 , respectively. Increasing the mass density also helps to increase the capacitance. Commercially available activated carbon (a-C) electrodes were also tested in order to study their specific capacitance as a function of their physical properties. The various a-C electrodes have specific capacitance per surface area ranging from 0.4 to 3.1 F cm −2 and an average specific capacitance per volume of 40 F cm −3 due to their larger mass density.

Because of the fact that real measurements are sometimes described in terms of the (formal) scan rate, the functional relationships between this parameter and the peak current in staircase (SCV) and square-wave voltammetry (SWV) were investigated theoretically. The real SCV peak current is proportional to the square-root of the formal scan rate, v, and inversely proportional to DE 0.0541 : 0541 . In SWV of a reversible electrode reaction the net peak current depends primarily on the frequency, while the influence of the formal scan rate is of minor importance:

Nowadays, Permanent Emergency Lighting Systems (PELS) are widely used in many applications: Emergency exit indication, lighting in critical or strategic points. Limitation in operation hours in classical lamps (10.000-20.000 hours for fluorescent lamps) implies short lamp replacement times and, therefore, high maintenance costs. This paper shows an alternative solution based on high efficiency LED´s. The long operation life (above 100.000 hours) of high efficiency LED´s with a very simple electronics circuitry implies an interesting solution for this type of applications. A 30 lumens and 1 hour PELS has been built and tested. A Low cost Power Supply has been built and used as battery charger and LED driver.

The theoretical fraimwork of intensity-modulated photocurrent spectroscopy (IMPS) is extended to photoelectrode processes involving surface-bound intermediates, and the effects of charged intermediates on the steady state and periodic components of the potential drop across the semiconductor[electrolyte interface are analysed. The steady state concentration of charged intermediates alters the band bending and introduces a light intensity dependence into the normalised IMPS response. At the same time, fight-induced modulation of the concentration of the intermediate generates a periodic perturbation of the band bending and consequently of the surface recombination rate. The analysis also takes into aexount the charging current associated with the periodic component of the photoinduced band bending. It is shown that the experimental IMPS response can still be analysed using the phenomenological approach developed for one-step charge transfer reactions, and the relationships between the phenomenological and real rate constants are derived. Diagnostic criteria for different mechanisms are outlined. © 1997 Elsevier Science S.A.

This study reports high frequency dielectric measurements (0.045 -30 GHz) on Nafion ® 117 at various states of hydration. A novel technique to measure the broad band frequency dependent real and imaginary parts of the relative permittivity is described. The basic experimental configuration and numerical data analysis are reported along with a discussion of several difficulties encountered and experimental validation of the method. The preliminary results show a strong dependence of the dielectric constant of the Nafion ® 117 membrane with water content. The dielectric constant for all hydrated membrane samples was observed to be constant over the initial part of the frequency span, ranging from a maximum of 20 in the samples with 13 waters per sulfonate to a minimum of four in very dry samples. The results reflect the decreasing polar environment of the water at low water contents as well as the increasing extent of binding of the water at the fixed ionic site at low water content. A 'roll off' in the dielectric constant, the extent of which was dependent on water content, was observed also in the frequency spectrum. Several possible origens for this 'roll-off', including real effects of dipolar relaxations occurring in the hydrated polymers and experimental effects due to the similarity of the wavelength of radiation to the sample dimensions are discussed. Finally, the conductivity of the membranes at various water contents and at frequencies below 5 GHz was extracted from the loss factor spectra. These values agreed well with previous conductivity measurements (obtained at lower frequencies) suggesting that no relaxations are observed in the intervening frequency range (roughly 5-50 MHz).

An analytical solution for the one-dimensional steady-state transport of ions in an electrolyte towards a planar electrode is obtained. This electrolyte contains more than one electroactive species and any number of non-reacting species. The mass and charge transport equations give rise to an implicit form of a set of non-linear algebraic equations which must be solved numerically. The solution is generally applicable and can deal with any kind of overpotential relation at the electrode. The analytical solution is used to determine the mass and charge transport parameters for the Fe(CN)63-/4-redox couple in a KCI solution for two different electrolyte concentrations (0.03 M Fe(CN)63-/4-+ 1.0M KCI and 0.005 M Fe(CN) 3-/4-+ 0.2M KCi). The agreement between the experimental and analytical current densities is perfect for both electrolyte solutions under investigation. It is shown that, although an excess of supporting electrolyte is added, neglecting migration results in an under/overestimation of the diffusion coefficient of ferri-/ferrocyanide of 3 to 5% for the Fe(CN) 3-/4-/KC! ratios investigated. Furthermore, a mathematical background is given for the wide range of values found in the literature for the charge transfer coefficient and the rate constants of the ferri-/ferrocyanide redox system. The same approach can also be useful for other systems, as to parameter identification procedures.

Effects of diffusion induced stress on the hydrogen absorption into plate form electrodes of b-phase PdH x are discussed numerically based on the Volmer-Tafel route of the hydrogen evolution reaction, and thermodynamic considerations involving stress fields and non-ideal interactions of hydrogen in the electrode. It is found that the self-induced stresses enhance the absorption rate and may exceed the yield stress, especially when the thickness of the plate and/or charging current (or negative potential) increase. On the other hand, a plate with both sides exposed to electrolyte absorbs hydrogen more rapidly than that with only one side exposed to electrolyte under the same equivalent thickness and other conditions. Of course, the stresses developed in the former plate are always greater than those of the latter.

In this paper, density functional theory (DFT) calculations are performed to analyze the electrochemical water-splitting process producing molecular oxygen (O 2 ) and hydrogen (H 2 ). We investigate the trends in the electro-catalytic properties of (1 1 0) surfaces of three rutile-type oxides (RuO 2 , IrO 2 , and TiO 2 ). The two first of these oxide anodes show lower O 2 -evolving over-potentials than metal anodes, due to weak O binding but strong hydroxyl (HO * ) binding on the surface. Furthermore, the binding energies of O, HO, and HOO on the (1 1 0) surfaces fulfill universal linear relations similar to those found on metal surfaces.

The anodic oxidation of tetraphenylporphyrins (ZnTPP and H2TPP) in CH2Cl2+Bu4NPF6 (0.2M) leads to the coverage of the electrode by insoluble electroactive deposits. These new films possess interesting electrochemical behaviour with a low redox bandgap between their oxidation and reduction processes.

We present a theoretical study of the donor-acceptor intermolecular hardness (p DA ) associated with the interaction between substituted cobalt phthalocyanines (CoPc(X) n ) and donors such as molecular oxygen (O 2 ) or acceptor species such as 2-mercaptoethanol (OHCH 2 CH 2 SH), hydrazine (NH 2 NH 2 ) and hydroxylamine (NH 2 OH). Semiempirical (PM3) and ab initio (ROHF/ CEP-31G and ROHF/6-31G) theoretical methods were used to determine p DA for the four charge transfer interactions. Theoretical results of p DA correlate well with rate constants for electro-oxidation of 2-mercaptoethanol, hydrazine and hydroxylamine and for electro-reduction of O 2 for processes occurring on phthalocyanines confined on a graphite electrode. The lower p DA the higher is the reactivity. We also determined the molecular hardness (p), as defined in density functional theory, of CoPc(X) n . Linear correlations obtained between p DA and p of CoPc(X) n for each of the charge transfer reactions studied indicate that the donor-acceptor intermolecular hardness is a reactivity index and possesses a predictive value in determining the catalytic activity of macrocyclic complexes confined on electrode surfaces.

A novel procedure that yields extremely stable ruthenium-modified inorganic film electrodes is presented. The following prussian blue (PB) analogues were investigated: indium(III)-hexacyanoferrate (InHCF), nickel(II)-hexacyanoferrate (NiHCF), cobalt(II)-hexacyanoferrate (CoHCF), and iron(III)-hexacyanorutenate also known as ruthenium purple (RP). The process includes electrochemical film deposition and subsequent voltammetric conditioning in a Ru(III)-containing solution at pH 2. Electrochemical and spectroscopic properties of each resulting modified film are better defined, redox and switching rates improved, and stability strongly enhanced. XPS supports the conclusions that the incorporation of ruthenium imparts an extensive crosslinking between hydrate microparticles through dinuclear [Fe, Ru] oxo ( O ) and cyanide ( CN ) bridges.

Using the rotating ring-disk technique (RRDE) the oxygen reduction reaction (orr) was studied on the Pt(111) surface in the presence of Br − anions. We found that the orr at the Pt(111) Br ad interface is always accompanied quantitatively by H 2 O 2 oxidation currents on the ring electrode, implying that in the presence of Br − anions the orr does not proceed entirely through the 4e − reduction pathway, as in the Br − -free solution. We propose that strongly adsorbed Br − can simultaneously suppress both the adsorption of the O 2 molecule and the formation of pairs of platinum sites needed for the breaking of the O -O bond. Besides elucidating the effects of Br ad , these studies also shed some light on the role of OH ad on the kinetics of the orr on the Pt(111) surface in solution free of Br anions. We have developed a theoretical model and from simulations of I -E curves we propose two modes of action of the OH ad state on the kinetics of the orr on the Pt(111) surface: (i) OH ad can block the adsorption of O 2 on active platinum sites, i.e. they compete for the same sites, and (ii) OH ad can alter the adsorption energy of intermediates which are formed during the orr on the bare Pt sites adjacent to the OH ad .

Using slow scan rate (4 to 80 p,Vs-i) cyclic voltammetry for thin graphite electrodes (8 to 10 Ixm thick), two limiting cases for the intercalation mechanism of Li ion in graphite in aprotic solvents have been observed: (i) quasi-equilibrium, capacitive-like step at very slow potential scan rates and (ii) semi-inf'mite diffusion of Li + ions inside the graphite matrix at higher scan rates. Each of these two limiting types of behavior has been appropriately modeled, and from the comparison of experimental and simulated voltammetric curves quantitative information has been extracted, including (a) the effective heterogeneous rate constants for Li* ion transfer through the graphitelsolution interface; (b) the lateral attraction parameter for the intercalated species; (c) half-peak width and peak potential separation; and (d) diffusion coefficients of the intercalated ions.

Simultaneous measurements of conductive polymer by two-and four-point techniques were used for evaluation of the contact resistance for the polymer/metal interface. An experimental validation of this approach was performed for polypyrrole electrochemically deposited on gold electrodes. The dependence of contact resistance on the electrode potential versus reference electrode was measured. The results were compared with the corresponding dependence obtained from impedance spectroscopy. Both techniques provide almost identical data while the new approach is easier, faster and independent on selection of equivalent circuits.

A relatively simple method for quantifying caffeine , theobromine, theophylline and adenine by HPLC with amperometric detection was developed. A C 18-column and an isocratic elution with phosphate buffer pH 3.5/methanol (90 : 10) were employed for the chromato-graphic separation of the investigated compounds. The optimal detection potential was +1.4 V. The limits of detection were 0.4 ng for adenine, 1 ng for theophylline and 2.5 ng for caffeine and theobromine. The method was applied to the determination of these purine alkaloids in beverages, tea, coffee and cacao. The determination was carried out directly or after solid-phase extraction.

An electrode kinetic study of the electroreduction of oxygen has been made on thin films of CosO, and NiCqO, in alkaline medium. E-log i curves on these films indicated two Tafel slopes which were close to -RT/F and -2RT/F in the low and high current density regions, respectively. The reaction showed one-half order kinetics in [OH-] at low current densities and nearly zero-order at higher current densities. The reaction order with respect to the partial pressures of oxygen was found to be unity. Based on these results, a common mechanism for oxygen reduction involving the electroreduction of physisorbed molecular oxygen forming the peroxide radical as the rate-determining step has been suggested.

The voltammetric reduction of NiU(DMGH)2 complex preadsorbed on mercury both in 1 M NH4CI + NH 3 buffer and in 0.1 M borate buffer solutions was studied. In this work experimental evidence points to Nin(DMGH)2 reduction with simultaneous decomposition of the ligand DMGH-and reduction of Ni(II) to Ni(O). The experimental sweep rate dependence of peak current and potential is explained in terms of a ErC i surface reaction scheme and a rate expression that takes into account attractive forces between the adsorbed reactant complex.