Rotating Disc Electrode

The brakes are one of the most important aspects of a vehicle, since it fulfills all the stopping functions and requirements. As trivial brakes may appear to be, many issues surround their heating characteristics when it comes to their development; including material choice, contact region properties, development of hotspots and associated physical geometry. This project seeks to develop experimental test rig applicable to thermal analysis of disk brakes. For the dimensioning of test bench, calculation of theoretical possible brake torque assessment of disc, heat transfer from a rotating disc, plays an important role. The development of Brake rotor[1], which can be used to check the hot spots, is described. For the test, calculations of theoretically possible disc torques, measurement of temperature using non-contact temperature measuring device have been done. A special focus is laid on hot spots and temperature distribution[2] in serial tests. The test protocol is checked using graphs. This paper focuses on quantification of thermal performance of different brake disc under various operating parameters; including constant load braking, various operating speed and development of empirical relation generalizing the temperature pattern in brake discs. This paper also presents the various solutions related to materials, design changes and help to dissipate the heat and eliminate the associated problems.

The aim of this report is to scrutinize the thin-film rotating disc electrode (TF-RDE) method for investigating the electrocatalytic activity of high surface area catalysts. Special emphasis is given to the oxygen reduction reaction (ORR) on carbon-supported platinum catalysts. On the basis of measurements on four different Pt catalyst samples with various average particle sizes, it is demonstrated in detail how the intrinsic properties of the catalyst, i.e., the mass activity (A/g Pt ) and the specific activity (A/m 2 Pt ), are evaluated. The potential sources of error are critically discussed and guidelines for the measurements are given. Furthermore, the specific ORR activities determined for the different catalyst samples are analyzed and compared to polycrystalline Pt. The previously reported effect of the particle size on the specific activity for the ORR is interpreted on the basis of the shift in the potential of zero total charge and the concomitant alteration of the adsorption properties.

The kinetics of Ni-Mo alloy electrodeposition on steel substrates from an aqueous citrate-ammonia complex baths has been investigated by means of steady-state polarisation measurements in a system with a rotating disc electrode (RDE). Partial current densities for discharge of Ni(II) and Mo(VI) ions and hydrogen evolution as a function of molybdate concentration in the bath, cathode potentials and the rate of mass transport were determined. It has been shown thatunder all investigated conditions -Ni-Mo alloy deposition is more favourable than pure nickel and the cathodic process is strongly influenced by the Mo(VI) content in the solution. The Ni(II) electroreduction rate initially increases, as the cathode potential shifts towards more negative values and the concentration of molybdate grows in the solution. However, for the highest examined MoO 4 2 − content, a considerable decrease in the rate of the process is subsequently observed at certain limit potentials, the values of which depend on molybdate concentration and hydrodynamic conditions. This effect, related to the formation of intermediate molybdenum oxides (characterised by very low overvoltage for hydrogen evolution), becomes less pronounced when the RDE rotation speed is increased. Hydrogen evolution is strongly associated with molybdenum deposition. An increase of the molybdate ions concentration in the bath, as well as an increase in the rate of mass transport, leads to an increase in Mo content in deposits and to the reduction of current efficiency. The Ni-Mo coatings electrodeposited from the designed bath (with the current efficiency of about 70%) containing about 30 wt.% Mo, are characterised by a shiny-grey appearance and good adhesion to the steel substrate. They are characterised by column growth and amorphous microstructure with randomly distributed nanocrystallites of the MoNi 4 intermetallic phase.

The oxygen reduction reaction (ORR) was studied at carbon supported MoO x -Pt/C and TiO x -Pt nanocatalysts in 0.5 mol dm −3 HClO 4 solution, at 25 • C. The MoO x -Pt/C and TiO x -Pt/C catalysts were prepared by the polyole method combined by MoO x or TiO x post-deposition. Home made catalysts were characterized by TEM and EDX techniques. It was found that catalyst nanoparticles were homogenously distributed over the carbon support with a mean particle size about 2.5 nm. Quite similar distribution and particle size was previously obtained for Pt/C catalyst. Results confirmed that MoO x and TiO x post-deposition did not lead to a significant growth of the Pt nanoparticles.

The aim of this report is to scrutinize the thin-film rotating disc electrode (TF-RDE) method for investigating the electrocatalytic activity of high surface area catalysts. Special emphasis is given to the oxygen reduction reaction (ORR) on carbon-supported platinum catalysts. On the basis of measurements on four different Pt catalyst samples with various average particle sizes, it is demonstrated in detail how the intrinsic properties of the catalyst, i.e., the mass activity (A/g Pt ) and the specific activity (A/m 2 Pt ), are evaluated. The potential sources of error are critically discussed and guidelines for the measurements are given. Furthermore, the specific ORR activities determined for the different catalyst samples are analyzed and compared to polycrystalline Pt. The previously reported effect of the particle size on the specific activity for the ORR is interpreted on the basis of the shift in the potential of zero total charge and the concomitant alteration of the adsorption properties.

The electrochemical oxygen reduction reaction (ORR) was studied at Pt and Pt alloyed with 30 atom% Ni in 1 M H 2 SO 4 and in 1 M H 2 SO 4 /0.5 M CH 3 OH by means of rotating disc electrode. In pure sulphuric acid, the overpotential of ORR at 1 mA cm (2 is about 80 mV lower at Pt 70 Ni 30 than at pure Pt. It was found that in methanol containing electrolyte solution the onset potential for oxygen reduction at PtNi is shifted to more positive potentials and the alloy catalyst has an 11 times higher limiting current density for oxygen reduction than Pt. Thus, PtNi as cathode catalyst should have a higher methanol tolerance for fuel cell applications. On the other hand, no significant differences in the methanol oxidation on both electrodes was found using cycling voltammetry, especially regarding the onset potential for methanol oxidation. During all the measurements no significant electrochemical activity loss was observed at Pt 0.7 Ni 0.3 . Ex-situ XPS investigations before and after the electrochemical experiments have revealed Pt enrichment in the first surface layers of the PtNi. #

Data has been obtained on the levelling effect of hydroxyethylated-2-butyne-1, 4-diol (bFerasineQ) in the copper electrodeposition from acidic sulfate electrolytes using cyclic voltammetry, hydrodynamic quasi steady-state voltammetry at rotating disc electrodes (RDE) and electrochemical impedance measurements. These are corroborated with morphological and structural analysis of copper deposits leading to the conclusion that finer grained deposits are produced, promoting a [110] texture. The additive moderately hinders the mass transfer of the copper ions from the bulk solution to the outer limit of the electrode double layer. Due to its adsorption on the electrode surface, Ferasine inhibits the charge transfer and slightly changes the copper electrodeposition mechanism. It affects the electrocrystallization step, impeding the crystal growth process and promoting the nucleation of copper. D

The hydrogen evolution reaction (HER) occurring on steel in an oxygen-free aqueous solution containing dissolved CO 2 was investigated. This reaction was modelled by taking into account the two dissociation reactions of dissolved CO 2 . The mathematical problem was solved numerically using a finite element method (FEM). A fair agreement between the measurements performed on a steel rotating disc electrode and the theoretical calculations was obtained. Thus, the cathodic behaviour of steel in CO 2 -containing solutions can be fully explained by the buffering effect induced by the presence of CO 2 . Some interfacial pH measurements performed on a gold electrode also support this conclusion.

For decades electrochemical technology has contributed successfully to environmental protection. In addition, environmental electrochemistry is capable of giving a huge contribution to further reach this goal by (1) application and improvement of existing technology, and (2) research, development and implementation of new technology. There is a tendency in the metal finishing and metal processing industries to redesign processes as closed loop systems. Therefore, we think especially the purification of dilute heavy metal ion-containing process liquids needs much of our attention. Electrochemical processes relevant to this purpose are considered and examples of (possible) industrial applications are discussed. Moreover, a critical evaluation of the usefulness of different types of electrodes is given. Generally, the two-dimensional electrode is the most favourable electrode configuration: the static flat electrode at high flow rates and in combination with mass transfer promoters, and the rotating disc electrode at high peripheral velocity. In both cases turbulent flow condition at the electrode are preferred to enhance the mass transfer. For redox systems where both the reactant and the product are soluble, the porous electrode is favourable due to its high specific surface area.

Electrocatalytic activities of various carbon-supported platinum–chromium alloy electrocatalysts towards oxygen reduction in 1 mol l−1 H2SO4 and in 1 mol l−1 H2SO4/1–3 mol l−1 CH3OH, were investigated by means of rotating disc electrode experiments and in solid polymer electrolyte direct methanol fuel cells. The activity of these electrocatalysts for methanol oxidation was evaluated using cyclic voltammetry. It was found that Pt9Cr/C prepared by reduction with NaBH4 exhibits the lowest activity for methanol oxidation and the highest activity for oxygen reduction in the presence of methanol, in comparison to commercial Pt/C, Pt3Cr/C and PtCr/C electrocatalysts.

Xerogel carbon support Graphene Carbon nanotubes a b s t r a c t Non-precious metal catalysts for the oxygen reduction reaction (ORR) in proton-exchangemembrane fuel cells (PEMFCs) were obtained by pyrolysis of iron citrate and polyacrylonitrile on mesoporous xerogel carbon support. Chemical-physical characterizations, electrochemical studies by the rotating disc electrode, and electrochemical tests in a PEMFC configuration demonstrated that the porosity of the pristine carbon promotes the formation of graphene and carbon nanotube structures featuring ORR catalytic activity. (M. Mastragostino).

The electrochemical oxidation of borohydride has been studied by means of voltammetry at Pt and Pt-Ni, Pt-Co bimetallic rotating disc electrodes (RDEs). The bimetallic catalysts are prepared by means of a galvanic replacement method (whereby electrodeposited Ni and Co layers are partially replaced by Pt when immersed in a chloroplatinic solution) and are shown to have a Pt shell-bimetallic alloy core. The effects of electrode history, potential scan direction, rotation speed and electrode material on borohydride oxidation have been investigated. For all Pt-based catalysts tested a gradual decrease of the voltammetric current from its initial value to a steady state response is observed in the kinetic control potential region, irrespective of scan direction and rotation speed. The initial deactivation of the catalyst at low overpotentials as well as the shape of the initial and steady-state voltammograms in that region point to the heterogeneous hydrolysis of borohydride and the subsequent oxidation of its products. As the catalytic activity for the hydrolysis reaction decreases the voltammograms shift to more positive potentials where direct borohydride oxidation dominates. The bimetallic Pt-Ni and Pt-Co catalysts exhibit a more negative open circuit potential and higher oxidation currents at low overpotentials than Pt, but lower apparent number of electrons transferred in the mass transport control region. These findings can be interpreted by the lowering of the d-band energy level of Pt in the presence of Ni and Co and the associated decrease in its adsorption affinity.

Quasi-steady state hydrodynamic voltammetry at a rotating-disc electrode and electrochemical impedance spectroscopy were used to investigate the influence of triethyl-benzyl-ammonium (TEBA) chloride on the kinetics of copper electrodeposition from sulphate acidic electrolytes. SEM and X-ray diffraction analysis were used to examine the morphology and the structure of copper deposits. The kinetic parameters (i0, ac, k0), obtained by both Tafel and

The voltammetric behaviour of (ferrocenylmethyl)trimethylammonium hexafluorophosphate, 2-ferrocenylacrylic acid, N-ferrocenylmethyl-N%-methyl-4,4%-bipyridinium hexafluorophosphate and 9-ferrocenylmethyladenine was studied in aqueous and/or acetonitrile solutions by means of cyclic voltammetry and rotating-disc electrode voltammetry. The influence of b-cyclodextrin on the voltammetric behaviour of the compounds was also studied in aqueous solutions. (Ferrocenylmethyl)trimethylammonium hexafluorophosphate and 2-ferrocenylacrylic acid, studied in aqueous solutions at pH 9.2, exhibit typical reversible one electron transfer behaviour, whereas the redox reaction of 9-ferrocenylmethyladenine, studied in acetonitrile, is strongly affected by adsorption. N-ferrocenylmethyl-N%-methyl-4,4%-bipyridinium hexafluorophosphate gives voltammetric responses for both ferrocene and viologen redox centres in acetonitrile. In aqueous solutions it is hydrolyzed to ferrocenylmethylalcohol and N-methylbipyridyl, a process facilitated by the neighbouring group participation of the iron atom. In all cases b-cyclodextrin causes a decrease in the currents and shifts the potentials positively. The formation constants of the inclusion complexes of compounds with b-cyclodextrin were determined by quantitative evaluation of E 1/2 values and possible explanations about their differentiation are discussed. In the case of N-ferrocenylmethyl-N%-methyl-4,4%-bipyridinium hexafluorophosphate it was found that b-cyclodextrin decreases the rate of its hydrolysis substantially.

Electrochemical behavior of tungsten in concentrated strong alkaline solutions (10 À3 -10 À1 M KOH, 11 < pH < 13) was studied by means of rotating disc electrode technique. Contrary to the case of less concentrated alkaline solutions, the formation of solid anodic passive film was not evidenced. Chronoamperometric measurements evidenced a fast establishment of steady state conditions in the surface region. In spite of the complexity of overall surface electrochemistry, a linear relationship between the limiting anodic currents and the concentration of OH À ions was evidenced, which may be used for easy and accurate determination of OH À ion concentration at high pH values where the glass electrode loses its high reliability.

This paper studies the effect of solution speed and sulfide ion concentration present as a pollutant in 3.5% NaCl solution on the electrochemical behavior of mild steel. The weight loss/gain due to the interaction of the materials with the surrounded environments was calculated after rotating cylindrical- shape mild steel samples in sulfide polluted NaCl solution for 24 hrs at a

The limiting current from Fe(CN)z-/Fe(CN)z-redox in dilute aqueous KCI at a platinum electrode appears to be little affected by the frequency of simultaneous ultrasonic irradiation in the range 20-800 kHz, and is not influenced by choice of bath or probe as sonic source, provided measurements are made at constant ultrasonic intensity. The limiting current is dependent on ultrasonic intensity at constant frequency. In contrast, the peak potential of the redox couple varies with ultrasonic frequency in the same range, but does not vary with ultrasonic intensity at constant frequency. Application of ultrasound to a rotating disc electrode provides an increase in limiting current across the frequency range, the magnitude of which varies with both concentration and rotation speed. The average maximum increase is some l.5-fold which corresponds to an effective rotation rate of 1OOOOOrpm (1667 Hz) for a silent solution in order lo achieve the same transport rates. It is shown that changes in macroscopic temperature throughout the experiment are insufficient to cause the observed enhanced diffusion. Copyright 0 1996

The effect of a new additive (IT-85) representing a mixture of triethyl-benzyl-ammonium chloride (TEBA) and hydroxyethylated-2-butyne-l,4-diol ('Ferasine') on the kinetics of copper electrodeposition from sulphate acidic electrolytes, as well as on the morphology and structure of copper deposits was investigated and compared with those exerted by its two components, TEBA and Ferasine. Quasi-steady state hydrodynamic voltammetry at a rotating-disc electrode and electrochemical impedance spectroscopy were performed in order to obtain information about the kinetics of the cathodic process. SEM and X-ray diffraction analysis were used to examine the morphology and the structure of copper deposits. The kinetic parameters (a c , k 0 ) obtained by both Tafel and Koutecky-Levich interpretations showed that all tested organic additives have an inhibiting effect on copper electrodeposition. Unlike IT-85 or Ferasine, TEBA acts only as a blocking agent in the copper discharge process, without changing the reaction pathway corresponding to the absence of additives. Comparison of the inhibiting effects exerted by IT-85 and its components on the electrodeposition process pointed to the existence of a beneficial complementarity of TEBA and Ferasine when they are used in mixture.

During electrowinning of copper from wastes, it was observed that Zn2+ impurities had some effects on the aspect of the deposits although the codeposition of these two metals is impossible under normal conditions. Cyclic voltammetry, impedance analysis and rotating disc electrode techniques were used to characterize the effect of the presence of small amounts of Zn2+ on the copper electrodeposition

Mass transport to rotating cylinder electrodes (radius 0.5cm and height 1.2cm) fabricated from reticulated vitreous carbon (RVCRCE) was investigated using linear sweep voltammetry in a 0.5 M NazSO 4 + l mM CuSO4 electrolyte at pH2. At a fixed cupric ion concentration the limiting current was found to be dependent upon velocity to the power 0.55 to 0.71 depending upon the porosity grade of the carbon foam. The product of mass transport coefficient and specific electrode area, kmAe, was found to be approximately 0.51s -1 at 157rads -1 (corresponding to 1500rpm) for the 100ppi material. The experimental data are compared to the predicted performance of a hydrodynamically smooth rotating disc electrode (RDE) and rotating cylinder electrode (RCE). Nomenclature V Ve A electrode area (cm 2) x Ae active electrode area per unit volume z (cm -1) bulk copper concentration (mol cm -3) concentration at t = 0 (mol cm -3) concentration at time t (mol cm -3) diffusion coefficient (cm 2 s -1) Faraday constant (96 485 A s mo1-1) height of rotating cylinder electrode (cm) limiting current (A) limiting current at an RDE (A) limiting current at an RCE (A) limiting current at a rotating RVCRCE (A) mass transport coefficient (cm s -i) radius of RCE (cm) electrolyte velocity (cm s -1) CB CO Ct D F h /c IL, RDE IL, RCE IL, RVC km r U reactor volume (cm 3) overall volume of electrode (cm 3) characteristic length (cm) number of electrons Greek symbols 7 /J 0,2 ratio of limiting current at an RVCRCE relative to an RDE of same diameter ratio of limiting current at an RVCRCE relative to an RCE of same overall volume thickness of the diffusion layer (cm) electrolyte viscosity (cm 2 s -1) rotation speed (rad s -x) Dimensionless groups Re = Ux/v Reynolds number Sc = v/D Schmidt number Sh = kmx/D Sherwood number

The limiting current is an important parameter for the characterization of mass transport in electrochemical systems operating under convective-diffusion control. Four methods to determine the limiting current from current (I) vs. potential (E) plots are considered. Strategies to determine the limiting current values include: (1) direct measurement from I vs. E curves, (2) estimation from the current value at E L = DE/2 where DE is the length of the limiting current plateau, (3) evaluation of the first derivative dI/dE in the I vs. E curve and (4) from plots of E/I vs. I À1 . The electrode reactions chosen to demonstrate the different strategies are: Cu(II) ? Cu(I) and Cu(I) ? Cu(0) in 1.5 mol dm À3 NaCl (pH 2) at a platinum rotating disc electrode and FeðCNÞ 3À 6 ! FeðCNÞ 4À 6 in 1 mol dm À3 NaOH at a 60 ppi reticulated vitreous carbon electrode (RVC).

Studies are presented of the kinetics and mechanism of oxygen electroreduction reaction on CoPd catalysts synthesized on carbon black XC72. As shown both in model conditions and in the tests within the cathodes of hydrogen-oxygen fuel cells with proton conducting electrolyte, CoPd/C system features a higher activity, as compared to Co/C. The highest activity in the oxygen reduction reaction is demonstrated by the catalysts with the Pd:Co atomic ratio being 7:3 and 4:1. The structural studies (XPS and XRD, and also the data of CO desorption measurements) evidence the CoPd alloy formation, which is reflected in the negative shift of the bonding energy maximum as compared to Pd/C and in the appearance of the additional CO desorption maximums on the voltammograms. It is found by means of structural research that CoPd alloy is formed in the course of the catalyst synthesis which features a higher catalytic activity of the binary systems. Besides, CoPd/C catalyst is more stable in respect to corrosion than Pd supported on carbon black. The measurements on the rotating disc electrode and rotating ring-disc electrode evidence that CoPd/C system provides the predominant oxygen reduction to water in the practically important range of potentials (E > 0.7 V). The proximity of kinetic parameters of the oxygen reduction reaction on CoPd/C and Pt/C catalysts points to the similar reaction mechanism. The slow step of the reaction is the addition of the first electron to the adsorbed and previously protonated O 2 molecule. The assumptions are offered about the reasons causing the higher activity and selectivity of the binary catalyst towards oxygen reduction to water, as compared to Co/C. The studies of the most active catalysts within the fuel cell cathodes are performed.

A comparative study of the influence of a new additive (IT-85), based on ethoxyacetic alcohol and triethyl-benzyl-ammonium chloride, and thiourea on copper electrodeposition from acidic sulfate solutions has been performed in order to obtain information about the kinetics of the cathodic process. In spite of their different chemical nature, both additives were found to be efficient as leveling agents, leading

The kinetics of the bioelectrocatalytic reduction of hydrogen peroxide has been studied at gold electrodes modified with different forms of horseradish peroxidase (HRP). Native HRP, wild type recombinant HRP (rec-HRP) and its two mutant forms containing a six-histidine tag at the C-or N-terminus, C His rec-HRP and N His rec-HRP, respectively, have been used for an adsorptive modification of the gold electrodes. The histidine sequences, i.e. histidine tags, were introduced into the peroxidase structure by genetic engineering of non-glycosylated rec-HRP using an Escherichia coli expression system. Experiments with a gold rotating disc electrode demonstrated that electrodes with the adsorbed rec-HRP forms exhibited high and stable current response to H 2 O 2 due to its bioelectrocatalytic reduction based on direct (mediatorless) ET between gold and the active site of HRP. The heterogeneous ET rate constants were evaluated to be in the order of 20 or 33 s − 1 between rec-HRP or its histidine mutants and gold, respectively, in 0.01 M phosphate buffer (pH 7.4) containing 0.15 M NaCl. The increase in the heterogeneous ET rate found for C His rec-HRP and N His rec-HRP is probably due to the interaction of the histidine tag with the electrode surface. The kinetic data demonstrate that new possibilities for enhancing the catalytic activity of the enzyme at the electrode solution interface can be achieved by genetic engineering design of the enzyme molecules.

The electrochemical properties of Au electrodes grafted with 4-nitrophenyl and 4-decylphenyl groups have been studied. The electrografting of gold electrode surface with aryl groups was carried out by electroreduction of the corresponding diazonium salts in acetonitrile. The nitrophenyl film growth on gold was examined by atomic force microscopy, electrochemical quartz crystal microbalance and X-ray photoelectron spectroscopy. These measurements showed that a multilayer film of nitrophenyl groups was formed. Cyclic voltammetry was used to study the blocking properties of aryl-modified gold electrodes towards the Fe(CN) 6 3−/4− redox system. The reduction of oxygen was strongly suppressed on these electrodes as evidenced by the rotating disc electrode results.

Oxidation of ferrocene at a poly(3-octylthiophene) (POT) f'dm electrode has been investigated by using cycfic voltammetry, rotating disc electrode voltammetry and el~hemical impedance spectroscopy. The POT film electrode was prepared by galvanostatic electropolymerization of 3-octylthiophene on a rotating Pt disc electrode. In the medium used, i.e. 0.1 M LiBF 4 + propylene carbonate, the formal redox potentials of fetrocene and POT are 0.4 and 1.1V vs. AgLAgCI respectively. This makes it possible to study the oxidation of ferrocene at both undoped (semiconducting) POT and at doped (electronically conducting) POT. The voltammetric data show that the oxidation of ferrocene is kinetically controlled at undoped or lightly doped POT while ferrocene is oxidized at a diffusion limited rate at highly doped POT. Impedance dat~ on the POT film electrode in solutions containing different concentrations of supporting electrolyte and ferrocene indicate that the ferrocene oxidation occurs in parallel with the doping process, resulting in a competition between ferrocene and doping ions for the available 'sites' at the polymer[solution interface. The kinetics of electron transfer at the polymerlsolution interface are discussed based on the Butler-Volmer formulation of electrode kinetics. © 1997 Elsevier Science S.A.

Low loading sulfided rhodium catalysts supported on carbon nanotubes (CNTs) were prepared from RhCl 3 by deposition-precipitation using hydrogen peroxide, followed by an exposure to hydrogen sulfide and an additional thermal treatment in the range from 400 • C to 900 • C. Hydrogen sulfide was generated online from hydrogen and sulfur vapor over molybdenum disulfide as catalyst. By elemental analysis, the Rh loading of the prepared catalysts was found to be 1.4-1.8 wt%. Morphology and composition of the resulting catalysts were characterized by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), and X-ray photoelectron spectroscopy (XPS). Nanoparticles were found to be highly dispersed on the CNTs with an average diameter as small as 1.0 nm determined by TEM. Sintering occurred during heat treatments at 650 • C and 900 • C in helium, as evidenced by XRD, TEM, and XPS. The treatment with hydrogen sulfide significantly enhanced the activity of the supported rhodium catalysts for the oxygen reduction reaction (ORR) in hydrochloric acid, as determined by rotating disc electrode measurements. The sulfided catalyst annealed at 650 • C with a particle size of about 2.5 ± 1.0 nm showed the best performance for the ORR, which is discussed based on the presence of a more stable rhodium sulfide layer on the metallic rhodium particles.

A hydrogen peroxide sensitive interface based on a self-deposited redox polyelectrolyte-peroxidase assembly was used as model system for investigation of the electron transfer between chemically modified horseradish peroxidase and the redox active centers of poly[(vinylpyridine)OsðbpyÞ 2 Cl] redox polymer. The assembly was built up by sequential adsorption of the redox polymer and the peroxidase on gold electrodes modified with a self-assembled monolayer of 3-mercapto-1-propane sulfonic acid. Making use of a kinetic model, formulated for steady-state conditions, and of the rotating disc electrode technique it was possible to estimate: (i) the second order rate constant of reaction between the modified peroxidase and hydrogen peroxide (2:74 Â 10 5 M À1 s À1 ) and (ii) the apparent turnover number of the oxidized enzyme regeneration by the osmium redox center (2:14 s À1 ), i.e., two parameters that reflect the efficiency of the electrical communication within the investigated interface.

The swelling and diffusion behaviour of a Nafion ® 117 ion-exchange membrane was investigated in mixed water-methanol solutions. The membrane porosity was found as a function of the methanol content in solution. The water and methanol uptake was determined by Raman spectrometry. The permeation experiments were realized in conditions where a membrane initially separated a water-methanol electrolyte solution from a solution prepared with pure water, both solutions containing either 0.1 M HCl or 0.5 M HCl. Different rotation speeds of stirrer rods were applied in the range of 70-700 rpm. A mathematical model for water-methanol permeation through a membrane taking into account the transfer through adjacent diffusion boundary layers (DBLs) and the dependence of the methanol diffusion coefficient on the methanol concentration was developed. The DBL thickness, ı, and the methanol permeability, P*, were determined by numerical fitting of experimental data. It was found that the rotating stirrers used in the cell were almost as efficient as rotating electrodes or membranes: we have found that ı ≈ 1.2ı Levich , where ␦ Levich was calculated by the well-known Levich equation for rotating disc electrodes. However, the contribution of diffusion resistance of DBL into the overall resistance of the three-layer system was not negligible, even at high rotation speeds, due to high methanol permeation through Nafion ® 117. When taking into account the DBLs, the "true" methanol permeability of Nafion ® 117 in the presence of 0.5 M HCl in the upstream and downstream compartments (at 25 • C) was equal to 0.44 × 10 −5 cm 2 s −1 . If the contribution of DBLs was not considered, the apparent methanol permeability at rotation speed of 70 rpm (ı ≈ 76 m) was equal to 0.35 × 10 −5 cm 2 s −1 . Even at 650 rpm (ı ≈ 25 m), the apparent methanol permeability, (0.41 × 10 −5 cm 2 s −1 ) was still lower than its "true" value. An approximate equation relating the apparent (efficient) non-electrolyte permeability with the "true" membrane permeability and the diffusion layer thickness was deduced following the approach proposed by Helfferich. (L. Dammak). losses in cell potential . At low current densities, chemical water-methanol diffusion is the main mechanism of the solvent transport, while at high current densities, the "cross-over" is essentially controlled by electro-osmotic water drag .

The ammonia-treated carbon-supported cobalt tungsten (Co-W/C) was prepared by a reaction employing a temperature program in a stream of NH 3 between 773 and 1073 K. The effects of the NH 3 heat-treatment temperature, Co-W ratio and the preparation method were investigated. The activity of Co-W/C for the oxygen reduction reaction (ORR) was evaluated using a rotating disc electrode and single fuel cell measurements. The Co-W/C prepared by the impregnation method with the same atomic ratio of Co and W, and NH 3 heat treated at 823 K, exhibited the highest ORR activity with an onset potential of 0.74 V (vs. RHE at 5 A cm −2 ). The XRD and temperature-programmed measurements revealed that the catalyst active species were ascribed to the presence of CoW oxynitride and Co nitride. The catalyst surface was characterized as nitrided metal (accommodation of the N-atom in the host Co-W lattice resulting from a sufficient distance between the Co-W atoms and N) and N-containing carbon (Co surrounded by N-atoms and attached to the C support) by X-ray photoelectron spectroscopy. The Co-W oxynitrides, Co nitride and pyrrolic-type nitrogen of the N-containing carbons are likely responsible for the good ORR activity.

Xerogel carbon support Graphene Carbon nanotubes a b s t r a c t Non-precious metal catalysts for the oxygen reduction reaction (ORR) in proton-exchangemembrane fuel cells (PEMFCs) were obtained by pyrolysis of iron citrate and polyacrylonitrile on mesoporous xerogel carbon support. Chemical-physical characterizations, electrochemical studies by the rotating disc electrode, and electrochemical tests in a PEMFC configuration demonstrated that the porosity of the pristine carbon promotes the formation of graphene and carbon nanotube structures featuring ORR catalytic activity. (M. Mastragostino).

Xerogel carbon support Graphene Carbon nanotubes a b s t r a c t Non-precious metal catalysts for the oxygen reduction reaction (ORR) in proton-exchangemembrane fuel cells (PEMFCs) were obtained by pyrolysis of iron citrate and polyacrylonitrile on mesoporous xerogel carbon support. Chemical-physical characterizations, electrochemical studies by the rotating disc electrode, and electrochemical tests in a PEMFC configuration demonstrated that the porosity of the pristine carbon promotes the formation of graphene and carbon nanotube structures featuring ORR catalytic activity. (M. Mastragostino).

The oxygen reduction reaction (ORR) has been studied using different types of carbon supported Pt-Co alloys and (Pt-Co)-Cs 2.5 H 0.5 PW 12 O 40 composite electrodes. The composite electrodes, prepared using different methods, have been characterized by RDE voltammetry in O 2 saturated acidic media. The composite electrodes show better performances toward ORR than the commercial pristine catalyst in terms of mass activities and half wave potentials. The composite prepared using the in house Pt-Co alloy shows the higher catalytic activity. The analysis of XRD spectra of the home-made Pt-Co alloy reveals the presence of different phases (fct and fcc) that probably justify the better catalytic properties. (S. Dsoke).

The electrochemical behaviour of a Cu rotating disc electrode in neutral aerated NaC1 solution was investigated in the cathodic and anodic ranges and at the corrosion potential. In the cathodic range, where the reduction of oxygen takes place, reduction peaks allow the identification and quantitative evaluation of insoluble corrosion products (CuC1 and Cu20 ). In the anodic range Cu is dissolved, most likely as CuC12. A new mechanism for the anodic dissolution is proposed after comparing our data with previously published mechanisms. Corrosion currents were found to decrease with time and to be a function of the rotation rate of the electrode. Both the mixed kinetics of the anodic partial reaction and diffusion through a porous layer seem to be relevant in controlling/corr.

The electrochemical reduction of trichloroacetic acid (TCAA) in water has been analyzed through voltammetric studies with a copper rotating disc electrode supported by controlled-potential bulk electrolysis and electrochemical surface-enhanced Raman spectroscopy (SERS) experiments. The influences of the mass-transport conditions and concentration were studied. It has been pointed out that the electrochemical reduction of trichloroacetic acid takes place prior to the massive hydrogen evolution. Strong adsorption was observed on the electrode surface, and dichloroacetic acid (DCAA), monochloroacetic acid (MCAA), and chloride anions were detected as reduction products. The SERS experiments point to a secondary mechanism in which dissociative adsorption of the trichloroacetic acid gives rise to the adsorption of CO, and therefore, to the production of 1C molecules as by-products of the electrochemical reduction.

The electroreduction of the peroxodisulfate anions on the electrochemically polished (EP) Cd(0 0 0 1), Bi(1 1 1) and Bið0 1 1Þ planes has been studied by cyclic voltammetry, rotating disc electrode and impedance methods. The rate constant values of the heterogeneous electroreduction reaction of the S 2 O 2À 8 anion on the EP Cd(0 0 0 1) and Bi(h k l) planes dependent on electrode polarisation and base electrolyte concentration have been calculated. The values of apparent transfer coefficient a app corrected for the double layer effect (a app 6 0.15) only very weakly depend on the electrode potential and base electrolyte concentration. The very low values of the apparent charge transfer coefficient for S 2 O 2À 8 electroreduction show that the activationless charge transfer mechanism is valid for EP Cd(0 0 0 1) and EP Bi(h k l) electrodes in various surface inactive base electrolyte solutions. This conclusion is in a good agreement with the theoretical models for the high hydrogen overvoltage metals based on the diabatic charge transfer mechanism from the metal to an ion [R.R. Nazmutdinov,

Electrochemical deposition of gold from citrate solution, with and without thallium, has been studied using a rotating disc electrode (RDE) and linear sweep voltammetry. RDE was made from a mono-crystal silicon wafer, coated with a thin film of gold, deposited by a vapor deposition process (VDP), serving as a substrate for the electrochemical deposi- tion of gold. Limiting current density is achieved neither in citrate nor in thallium free electrolyte. The rotation speed significantly inf- luences the process rate only at lower temperatures. At elevated temperatures gold deposition occurs in a kinetic domain. Tafel slopes confirmed that the electrochemical reaction occurs in two steps, depending on both the electrode potential and the presence of thallium ions in the electrolyte. The reaction kinetics changes in the presence of thallium ions as well. The mechanism of gold deposition is predominantly dependent on the electrolyte composition. In the citrate, thal- lium free solution, ...

A special electrochemical pretreatment was employed in the case of finely polished glassy carbon electrode surfaces to increase the adsorption of the redox mediator Meldola blue (7-dimethylamino-1,2-benzophenoxazine). The chemically modified glassy carbon electrode holding the adsorptive mediator layer was used to measure the NADH concentration amperometrically. The interfering effect of non-ionic surfactants was studied for NADH measurements using a rotating disc electrode technique and flow injection measurements. The glassy carbon electrode response was less affected and could be regenerated much more easily than the response of a similar spectrographic graphite based electrode, studied earlier. Surface active agents can influence the apparent coverage of the surface by the mediator.

Oxidation of ferrocene at a poly(3-octylthiophene) (POT) f'dm electrode has been investigated by using cycfic voltammetry, rotating disc electrode voltammetry and el~hemical impedance spectroscopy. The POT film electrode was prepared by galvanostatic electropolymerization of 3-octylthiophene on a rotating Pt disc electrode. In the medium used, i.e. 0.1 M LiBF 4 + propylene carbonate, the formal redox potentials of fetrocene and POT are 0.4 and 1.1V vs. AgLAgCI respectively. This makes it possible to study the oxidation of ferrocene at both undoped (semiconducting) POT and at doped (electronically conducting) POT. The voltammetric data show that the oxidation of ferrocene is kinetically controlled at undoped or lightly doped POT while ferrocene is oxidized at a diffusion limited rate at highly doped POT. Impedance dat~ on the POT film electrode in solutions containing different concentrations of supporting electrolyte and ferrocene indicate that the ferrocene oxidation occurs in parallel with the doping process, resulting in a competition between ferrocene and doping ions for the available 'sites' at the polymer[solution interface. The kinetics of electron transfer at the polymerlsolution interface are discussed based on the Butler-Volmer formulation of electrode kinetics. © 1997 Elsevier Science S.A.

AbstractÐThe anodic dissolution of aluminium in organic AlCl 3 /dimethylsulfone electrolytes was investigated by measuring anodic potentiostatic polarization curves and AC impedance responses on a rotating disc electrode. Mass transport limited current plateaux are obtained. They are related to the limitation of the dissolution rate by the diusion of Al 3+ . The variations of the ac impedance responses with rotation rate and applied potential are interpreted with models that assume the presence at the surface, of an aluminium chloride salt layer. The salt layer has a duplex structure composed of an inner compact layer through which aluminium ions migrate by high-®eld conduction and an outer porous layer where ionic conduction occurs in the electrolyte ®lling the pores. #

The initial stages of gold electrodeposition on a gold electrode were studied in a proprietary bath (Renovel N) using linear sweep voltammetry and chronoamperometry. Tafel plots with two different slopes were obtained, indicating that the mechanism for gold deposition ...

The galvanic reaction of metallic copper in Cr(VI)-laden aqueous solutions of varying pH was examined by in situ u.v.–visible spectrophotometry, rotating disc electrode chronopotentiometry and cyclic voltammetry. The galvanic reaction in 0.2 M H2SO4 solutions was pseudo first order in Cr(VI) concentration. Experiments with both magnetically stirred solutions and a copper mesh or a copper film in a rotating disc electrode configuration revealed the reaction to be diffusion-controlled with respect to Cr(VI) transport to the copper surface. Finally, cyclic voltammetry data in Cr(VI)-laden media of varying pH underline the important role of protons in the galvanic reaction.

In this work a method is set-up to extract the mass and charge transfer parameters quantitatively from linear sweep voltammetry with a rotating disc electrode (LSV/RDE) experiments. It requires the proposition of an appropriate mechanism for the reaction under study and its mathematical translation into an expression that analytically describes the voltammogram. Powerful parameter estimation algorithms (maximum likelihood combined with Gauss-Newton and Levenberg-Marquardt minimization methods) are used to adjust the values of these model parameters, in order to obtain a good agreement between experimental and modeled data. The values of the model parameters that give rise to the best match, characterize the system quantitatively. Moreover, the method provides error estimates of the obtained parameter values. However it is only after a statistical evaluation of the obtained results, that it is decided whether the model is able to describe the experiments or not. Compared to others, this method offers the advantage that it uses one integrated expression that accounts for mass and charge transfer steps, and this without simplifying their mathematical expressions. Other elementary reaction steps (like chemical reactions, migration, adsorption) can also be included, as long as their influence on the polarization curve is mathematically formulated. Another plus-point is that the whole polarization curve is considered, rather then just some part in which only mass or charge transfer are supposed to be rate determining. This approach has never before been used in electrochemical studies and the coupling of statistically founded parameter estimation techniques with LSV/RDE experiments is an important innovative point of this work. In the first article of this series the theoretical background is described. Furthermore, an evaluation of the proposed method on simulated experiments is given at the end of this manuscript.

Two new phenothiazine derivatives, 3,7-di(m-aminophenyl)-10-ethyl-phenothiazine (DAEP) and 3,7di(m-hydroxyphenyl)-10-ethyl-phenothiazine (DHEP), were used to modify solid graphite electrodes by adsorption from DMSO solution. Cyclic voltammetry and rotating disc electrode measurements, performed under different experimental conditions (pH, potential scan rate, rotation speed and NADH concentration) showed that: (i) both compounds undergo a quasi-reversible one electron / one proton redox process (k s = 0.73 s À1 for DAEP and 0.38 s À1 for DHEP), occurring at a formal standard potential which is not affected by the nature of substituent at positions 3 and 7; (ii) the pK a values of the adsorbed mediators are slightly different (6.9 for DAEP and 7.8 for DHEP); (iii) the second order rate constant for NADH electrocatalytic oxidation is significantly larger for DAEP (k obs,[NADH]=0 = 6014 M À1 s À1 ) than for DHEP (k obs,[NADH]=0 = 134 M À1 s À1 ). Considering the mediators' structure, we made an attempt to explain the higher electrocatalytic efficiency of DAEP mediator.

In this study the oxygen reduction reaction (ORR) is investigated on a nanoparticulate silver electrocatalyst in alkaline solution. The catalytic activity of the catalyst is determined both in terms of mass activity as well as specific activity and turn over frequency, respectively. It is demonstrated that the established mass activities are independent of the applied catalyst loading, an essential requirement for a reasonable analysis. The determination of the electrochemically active surface area (ECA) or the number of electrochemically accessible sites (NECAS), respectively, is performed by the underpotential deposition of lead. The obtained value of the activity is compared to activities of polycrystalline silver and platinum measured in the same electrolyte, as well as to literature data.

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The work presents a study on the influence of the sputtering pressure on the surface structure and morphology of low Pt loaded electrodes and their electrochemical behaviour toward oxygen reduction reaction (orr) in sulphuric acid solution and polymer electrolyte membrane (Nafion 117). Pt was deposited as thin film upon hydrophobic carbon paper substrates at sputtering pressure varied in the range 2-13 Pa. The test samples are analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The catalytic activity is assessed by applying the methods of linear sweep voltammetry (LSV) on rotating disc electrode (RDE) and cyclic voltammetry (CV). The results obtained show strong influence of the sputtering pressure on the surface structure and crystal orientation which in turn, affects the orr efficiency. The best electrode performance in both electrolytes used is obtained for the Pt film deposited at pressure of 9 Pa. The results obtained in Nafion 117 show that catalyst utilisation in this electrode exceeds significantly the one for a commercial ELAT electrode at the same operation conditions. The research demonstrated that by simple variations in the sputter regime it is possible to optimise the catalysts morphology in order to increase the catalytic activity toward the electrochemical reaction of interest at the same time controlling precisely the required precious metal loading.

Oxygen reduction at oxide-derived Pd electrodes is investigated by using the rotating disc electrode technique. On the basis of these experiments along with further data analysis (ie reaction orders, Tafel slopes, etc.), a mechanism of oxygen reduction on an oxide-derived Pd electrode is proposed to explain the observed rate equation. Copyright 0 1996 Elsevier Science Ltd

Detecting and enumerating fecal coliforms, especially Escherichia coli, as indicators of fecal contamination, are essential for the quality control of supplied and recreational waters. We have developed a sensitive, inexpensive, and small-volume amperometric detection method for E. coli b-galactosidase by beadbased immunoassay. The technique uses biotin-labeled capture antibodies (Ab) immobilized on paramagnetic microbeads that have been functionalized with streptavidin (bead-Ab). The bead-Ab conjugate captures E. coli from solution. The captured E. coli is incubated in Luria Bertani (LB) broth medium with the added inducer isopropyl b-D-thiogalactopyranoside (IPTG). The induced b-galactosidase converts p-aminophenyl b-Dgalactopyranoside (PAPG) into p-aminophenol (PAP), which is measured by amperometry using a gold rotating disc electrode. A good linear correlation (R 2 =0.989) was obtained between log cfu mL À1 E. coli and the time necessary to product a specific concentration of PAP. Amperometric detection enabled determination of 2·10 6 cfu mL À1 E. coli within a 30 min incubation period, and the total analysis time was less than 1 h. It was also possible to determine as few as 20 cfu mL À1 E. coli under optimized conditions within 6-7 h. This process may be easily adapted as an automated portable bioanalytical device for the rapid detection of live E. coli.