Rotating Disc Electrode

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 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.

Platinum based electrocatalysts for the oxygen reduction reaction were prepared by electrodeposition of Pt particles in a polyaniline film acting as a convenient matrix to achieve low Pt loadings (from 11 to 600 mg cm − 2 ). Using a rotating disc electrode or a rotating ring disc electrode allowed us to separate the different contributions in the overall reaction : diffusion of molecular oxygen in the electrolyte solution, diffusion inside the PAni film, adsorption process, and electron transfer. Koutecky-Levich analysis made possible to evaluate the kinetics parameters (total number of exchanged electrons, limiting current density, Tafel slope and exchange current density). These parameters vary with the Pt loading, displaying a steep increase at around 200 mg.cm − 2 , above which the behavior of bulk platinum is found. At lower Pt loadings, the kinetics is controlled by a Temkin isotherm, and a large amount of hydrogen peroxide is formed.

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.

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. #

Electrochemical reduction of hydrogen peroxide is studied on a sand-blasted stainless steel (SSS) electrode in an aqueous solution of NaClO 4 . The cyclic voltammetric reduction of H 2 O 2 at low concentrations is characterized by a cathodic peak at -0⋅40 V versus standard calomel electrode (SCE). Cyclic voltammetry is studied by varying the concentration of H 2 O 2 in the range from 0⋅2 mM to 20 mM and the sweep rate in the range from 2 to 100 mV s -1 . Voltammograms at concentrations of H 2 O 2 higher than 2 mM or at high sweep rates consist of an additional current peak, which may be due to the reduction of adsorbed species formed during the reduction of H 2 O 2 . Amperometric determination of H 2 O 2 at -0⋅50 V vs SCE provides the detection limit of 5 µM H 2 O 2 . A plot of current density versus concentration has two segments suggesting a change in the mechanism of H 2 O 2 reduction at concentrations of H 2 O 2 ≥ 2 mM. From the rotating disc electrode study, diffusion co-efficient of H 2 O 2 and rate constant for reduction of H 2 O 2 are evaluated.

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 .

High performance gold-supported Pt electrocatalyst for the reduction of oxygen was prepared by replacing Cu adlayers, deposited potentiostatically on Au, with Pt at open-circuit potential in a 0.1 M HCl solution containing K 2 PtCl 6 . Auger Electron Spectroscopy and Atomic Force Microscopy reveal the surface modification. The kinetics of oxygen reduction on this platinum modified electrode was studied by the rotating-disc electrode technique. The activity of the electrode is lower than the activity of a smooth Pt electrode in the negative potential scan, but it is significantly higher in the positive scan. Ó

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.

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 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.

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,

This work was carried out to study the inhibition mechanism of a carbon steel in a 200 mg l À1 NaCl solution by a non-toxic multi-component inhibitor used for water treatment in cooling circuits. The inhibitive formulation was composed of 50 mg l À1 fatty amines associated with 200 mg l À1 phosphonocarboxylic acid salts. Steady-state current-voltage curves, obtained with a rotating disc electrode, revealed that the properties of the protective layer were dependent on the electrode rotation rate and on the immersion time. The cathodic process of oxygen reduction was not modified in the presence of the inhibitive mixture. As expected, the current densities increased when the rotation rate was increased. In the anodic range, original behaviour was observed: the current densities decreased when the electrode rotation rate increased. The morphology and the chemical composition of the inhibitive layers allowed the electrochemical results to be explained. Two distinct surface areas were visualised on the metal surface and the ratio between the two zones was dependent on the flow conditions. This behaviour was attributed to a mechanical effect linked to centrifugal force. XPS analysis 0010-938X/$ -see front matter Ó (N. Pébère).

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.

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

The reaction between the cyclopentadienyliridium complexes Cp*Ir(L)CI 2 (where L= PMe 3, PPh 3, CN-t-Bu) and disodium maleonitriledithiolate (Na2mnt) yields the corresponding rant-substituted compounds Cp* Ir(PMe3)(mnt) (1), Cp* Ir(PPh3)(mnt) (2) and Cp* Ir(CN-t-Bu)(mnt) (3). All of these new compounds have been isolated in high yield and characterized in solution by IR and NMR spectroscopy. The solid state structures of 2 and 3 were determined by single-crYostal X-ray diffraction analysis. 2 crystallizes in the triclinic space ~roup PI with a = 10.4557 A, b = 10.630(1) ,~, c = 14.894(1) A, o~ = 91.382(8) °, /3 = 91t. 141(6) °, 3/= 118.182(7) °, V = 1458.5(3) A 3 and Z = 2. Full-matrix least-squares refinement yielded R = 0.0281 for 3425 (I > 3o-(1)) reflections. 3 crystallizes in the monoclinic space group P2Jn with a = 8.9609 ,~, b = 20.343(1) A, c = 11.9115(9) ,~, /3 = 94.168(6) °, V= 2165.6(3) ~3 and Z = 4. Full-matrix least-squares refinement yielded R = 0.0245 for 2266 (1 > 3o-(1)) reflections. The redox properties of 1-3 have been explored by cyclic and rotating disc electrode voltammetric techniques in MeCN and CH 2C12 solvents. Each of these compounds exhibit a well-defined one-electron oxidation wave that is assigned to the 0/+1 redox couple, along with a + 1/+ 2 redox wave, the reversibility of which is highly dependent on the nature of the ancillary two-electron donor ligand (L) and the solvent. Extended Hiickel calculations have been performed on the model compounds Cplr(PH 3)(mnt) and Cplr(CNH)(mnt), with the results used in a discussion of the nature of the HOMO and LUMO levels in 1-3.

In this investigation the effect of surface treatments on the corrosion resistance of a commercial NdFeB sintered magnet has been investigated. A solution of 10 g L − 1 NaH 2 PO 4 , acidified to pH 3.8 has been used for phosphating this magnet. The corrosion resistance of the phosphated magnet was investigated in a 0.10 mol L − 1 Na 2 SO 4 solution by electrochemical impedance spectroscopy and cyclic voltammetry with rotating disc electrode. The obtained results reveal that the resistance decreases with exposure time due to the development of pores and/or defects in the conversion coating exposing the substrate to corrosive attack. The effect of tungstate incorporation into the phosphate conversion coating resulting from a phosphating treatment prior to immersion in the tungstate solution was evaluated. The proposed treatment consists of re-immersing the phosphated samples in a 0.1 mol L − 1 Na 2 WO 4 solution during 72 h at the open circuit potential (OCP). Under these conditions, the corrosion resistance of the magnet was improved and this was attributed to the formation of a protective layer due to the adsorption of tungstate anions at the metallic substrate exposed in the coating, decreasing metal dissolution.

Poly(4-vinylpyridine) (PVP)-based anion exchange polymers are not studied as much as cation exchange polymers Nafion and Eastman Kodak AQ for electroanalytical applications. Similarly, octacyanomolybdate [Mo(CN)8 4−] has not been studied much as a redox mediator. This communication presents results from examinations of the behaviour of Mo(CN)8 4−-doped PVP ionomer film electrode to highlight the opportunities for realization of the application of this composite electrode for l-ascorbic acid (AH2) estimation via electrocatalytic mediation in acidic medium. The modified electrodes were characterized by cyclic voltammetry and rotating disc electrode voltammetry. PVP coatings possess strong anion-binding capacity for Mo(CN)8 4− mediator with an extraction coefficient of 990, and electrostatically cross-linked PVP films offer insignificant resistance to permeation of AH2, facilitating a cross-exchange reaction between the substrate and the mediator in the entire film volume. They show ef...

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.

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 N 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.

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 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 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).

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.

Micro/mesoporous conducting nitrogen-containing carbon nanorods/nanotubes with high surface area, and excellent electrocatalytic activity for the oxygen reduction reaction, were synthesized by the carbonization of self-assembled polyaniline (PANI) 5-sulfosalicylate nanorods/nanotubes. Carbonization was carried out by means of gradual heating in a nitrogen atmosphere up to 800 • C. The morphology of the PANI 5-sulfosalicylate was preserved after carbonization. Carbonized PANI nanostructures containing ∼10 wt% of nitrogen had a conductivity of 0.8 S cm −1 . The influence of the carbonization on the porosity and specific surface area of the nanostructured PANI has been analyzed. The morphology, elemental composition, molecular structure, surface area, and electrical characteristics of novel carbonized nanostructured PANI were investigated by scanning and transmission electron microscopies, elemental microanalysis, FTIR and Raman spectroscopies, nitrogen adsorption-desorption and conductivity measurements, respectively. The electrocatalytic activity of carbonized PANI 5-sulfosalicylate nanorods/nanotubes material towards oxygen reduction reaction in alkaline conditions has been studied by the voltammetry with the rotating disc electrode.

AbstractÐAn Os hydrogel based on the covalent attachment of Os(byp) 2 ClPyCHO to PAA-NH 2 was cross-linked with glucose oxidase (GOx), lactate oxidase (LOx) and horseradish peroxidase (HRP) on GC and Au electrodes by PEG-400 bifunctional reagent. Single layer monoenzyme (GOx or LOx) and bienzyme (HRP-GOx) single layer modi®ed electrodes were prepared with the Os moieties acting as``electron wires or electron shuttles''. Cyclic voltammetry showed diusional charge propagation in the gel which resulted more stable than similar ferrocene based gels reported before. In solutions containing the substrates, catalytic currents were obtained due to enzyme catalysis for the oxidation of glucose and lactate by the respective enzymes mediated by the Os polymer either by detecting directly the anodic current in a single enzyme electrode or indirectly by further reducing the peroxide formed in the aerobic enzymatic cycle at the Os-wired HRP. A rotating disc electrode (RDE) and a wall jet electrode (WJE) were employed as hydrodynamic electrodes in order to correct the amperometric response for substrate concentration polarization in the external electrolyte. #

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.

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.

Platinum based nanocatalyst at home made Nb-TiO 2 support was synthesized and characterized as the catalyst for oxygen reduction reaction in 0.1 mol dm −3 NaOH, at 25 • C. Nb doped TiO 2 catalyst support, containing 5% of Nb, has been synthesized by modified acid-catalyzed sol-gel procedure in non-aqueous medium. BET and X-ray diffraction (XRD) techniques were applied for characterization of synthesized supporting material. XRD analysis revealed only presence of anatase TiO 2 phase in synthesized support powder. Existence of any peaks belonging to Nb compounds has not been observed, indicating Nb incorporated into the lattice.

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 electroreduction kinetics of hexaamminecobalt(III) cations on electrochemically polished Bi planes has been studied by cyclic voltammetry and rotating disc electrode methods. The rate constant for the heterogeneous electroreduction reaction of the [Co(NH 3 ) 6 ] 3þ cation on the Bi(0 0 1), Bi(1 1 1) and Bi(0 1 1) planes has been established and the results have been compared with the data for the Ag(1 1 0), Au(h k l) and Hg electrodes [Electrocatalysis, 171]. In the region of zero charge potential, the value of the experimental transfer coefficient a exp obtained depends on the crystallographic structure of the Bi plane. a exp is only slightly higher than 0.5 for the Bi(1 1 1) and Bi(0 0 1) planes but somewhat lower than 0.5 for the electrochemically most active Bi(0 1 1) plane. For more concentrated base electrolyte solutions, the values of the apparent transfer coefficient a app corrected for the double layer effect are practically independent of the base electrolyte concentration and depend slightly on the electrode polarisation. Noticeably different values of the double layer corrected rate constant and a app for the Bi(0 0 1), Bi(0 1 1), Bi(1 1 1), Hg and Ag(1 1 0) electrodes from the data for the Au(h k l) planes have been explained by the different inner layer structure, i.e. by the different water and ClO À 4 adsorption energies at the metals under discussion.

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.

We have studied the electrocatalytic activity of RuO2-PVC film electrodes, fabricated using RuO2 powders prepared at five different temperatures, viz., 300, 400, 500, 600 and 700°C, for the oxidation of glucose in high alkaline media, 1 to 3 M NaOH. The RuO2-PVC film electrodes have been first characterized in 1 to 3 M NaOH solution by cyclic voltammetry (CV) and rotating disc electrode (RDE) techniques in a wide potential range −1,100 to 450 mV (SCE), and three redox pairs representing Ru(IV)/Ru(III), Ru(VI)/Ru(IV) and Ru(VII)/Ru(VI) transitions have been identified. The voltammetric peaks at low sweep rates have been analyzed using surface activity theory formulated for interacting electroactive adsorption sites, and interaction terms have been evaluated. The total voltammetric surface charges have been analyzed as per Trassatti’s formalism with respect to their dependence on potential sweep rate, and charges associated with less accessible and more accessible surface sites have been calculated. For glucose oxidation, the results have indicated that RuO2 (700°C)-PVC electrode shows two oxidation peaks in contrast to RuO2 (300°C)-PVC electrode. Also, RuO2 (700°C)-PVC electrode exhibits higher intrinsic electrocatalytic activity than the 300°C electrode, although the former possesses lower electrochemically active surface area. Additionally, kinetic analyses made from RDE results with reference to Michealis–Menten (MM) enzyme catalysis has shown that RuO2 (700°C) electrode possesses extended glucose-sensing range in terms of MM kinetic constant, K M , compared to other electrodes. Possible reasons for such differences in the behavior of the electrodes of different temperatures towards glucose oxidation are identified from studies on oxidation of glucose in solutions of different pH, oxidation of different glucose derivatives, and also from physicochemical results from BET, XRD, SEM, DTGA, XPS analysis of RuO2 powder samples.

Composites consisting of multi-walled carbon nanotubes (MWCNTs) and iron–nitrogen containing compounds as catalysts for the electroreduction of oxygen in acidic media were directly prepared on a glassy carbon (GC) electrode in a bottom-up synthesis. In a first step, MWCNTs were drop-coated in form of an ink onto the electrode. Afterwards the nanotubes were modified with catalytically active films of iron porphyrin (FeTMPP-Cl) or iron phenanthroline (Fe(phen)3) through a pulsed potential deposition technique. Finally the prepared electrodes were heat-treated in an inert gas atmosphere.By employing cyclic voltammetry and rotating disc electrode measurements it is shown that the activity for the oxygen reduction reaction (ORR) at such composites increases progressively with every applied synthesis step showing the possibility for direct synthesis of a catalyst on an electrode. The activities of FeTMPP-Cl/MWCNT and Fe(phen)3/MWCNT composites prepared by this technique are higher than that of similar electrocatalysts prepared by wet impregnation and heat treatment. The presented approach opens possibilities for systematic tuning of electrode structures, for example by stepwise build-up of gas diffusion electrodes.

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.

The electrocatalytic activity of carbonized polyaniline nanostructures (Carb-nanoPANI) towards oxygen reduction reaction (ORR), estimated in 0.1 mol dm −3 KOH solution, was significantly improved upon a hydrothermal treatment in 1 mol dm −3 KOH solution. Namely, the onset of ORR was shifted by ∼70 mV to more positive potentials, and the number of electrons consumed per O 2 molecule was enhanced in comparison to the original material. The number of electrons involved in ORR depended on loading, and with a loading of 0.5 mg cm −2 , for the potentials lower than −0.5 V vs SCE, the number of electrons approached 4. For this material, high stability of electrochemical behavior and resistance to the poisoning by ethanol was evidenced by potentiodynamic cycling.

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).

In this paper, electrochemical polymerization of pyrrole on 316L stainless steel substrates was accomplished using a rotating disc electrode (RDE). By applying various current densities and disc rotation speeds, coatings were produced with the aid of galvanostatic technique. Experiments were performed in an aqueous solution containing 0.2 M pyrrole and 0.1 M oxalic acid. Current densities and disc rotation speed ranges were from 0.05 to 1.0 mA/cm 2 and from 0 to 1500 rpm, respectively. Using a scanning electron microscope (SEM), the morphology of polypyrrole coatings was studied, and the morphology diagram was determined. The results obtained showed that various morphologies were obtained by changing the current density and/or disc rotation speed. These results also showed that apart from conventional morphologies of polypyrrole coatings reported in the literature, a new semicrystalline morphology was obtained under the conditions of very low current density (0.05 mA/cm 2 ) and disc rotation speed (≤50 rpm). The degree of crystallinity of this morphology was estimated to be 68% by grazing-incidence small-angle X-ray scattering (GISAXS). The elemental analysis (CHN) revealed the ratio of semicrystalline polypyrrole to oxalic acid dopant to be 4:1.

The dissolution behavior of gold and silver from Au/Ag alloys in aerated cyanide solutions has been investigated using rotating disc electrodes. The variables studied included concentration of cyanide, oxygen partial pressure, and rotating speed of the disc. The dissolution potential and the rate of dissolution were obtained in view of the anodic and cathodic current-potential relationships. The results were discussed in terms of the mixed potential theory. The results showed that the dissolution rate of gold and silver from the alloys was partially controlled by chemical reaction but largely controlled by transport of either oxygen or cyanide, depending on their relative concentrations under the experimental conditions employed in this study. The diffusion coefficient of free cyanide, DcN-, was found to be (1.25 + 0.05) • 10-5 cm2/s. The diffusion coefficient of oxygen, Do2 , was calculated to be (1.29 + 0.02) • 10-5 cm2/s.

The reduction of the silver cation, Ag + , was studied in dilute aqueous Na2S2O3/NaHSO3 solution at a platinum and stainless steel rotating electrode in the absence and presence of ultrasound. Under silent conditions, electrode potential measurements indicate that silver deposition is the main reaction occurring at the cathode during the electrolysis of a model photographic processing solution. Under insonation conditions at 20 kHz the reduction wave shifts anodically with increasing ultrasonic power. Similarly the discharge of hydrogen shifted anodically with increasing power. It is also shown that below 1000 r.p.m. insonation leads to improved limiting currents when compared to the silent system. As the rotation speed of the electrode increases above 1000 r.p.m., the limiting current decreases as the ultrasonic power increases. 7 1997 Published by Elsevier Science Ltd

Porous electrodes are required to achieve satisfactory performance of the aqueous sulphide/polysulphide redox couple in energy conversion and storage applications. A flow cell for testing flow-through porous electrodes was constructed and operated. The effects of electrode material, temperature, flow rate, and electrolyte composition were studied. Catalytic electrode surface layers of Co and MoS2 demonstrated performances which were more than adequate to meet a design goal of 10-20 mA cm -2 at less than 50 mV overpotential. Flow rate variation had only a small effect on the current density-overpotential behaviour, whereas raising the temperature and/or adding dimethylformamide to the electrolyte had much larger effects. These observations are consistent with steadystate results obtained on rotating disc electrodes.

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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.

The diffusional problem of a rotating porous electrode has been analysed based on the mass transfer equations for a partially blocked electrode. It is shown that the porous geometry leads to a dependence of the current on rotation rate identical to that corresponding to a coupled diffusion-activated electron transfer mechanism. The apparent rate constant, however, is related only to the geometry of the porous surface. It is shown that the characteristic diffusional length corresponds to the pore dimension modified by a term including the transition from linear to spherical diffusional geometry at the pore entrance. The theory is compared with experimental results for the reduction of hexacyanoferrate(III) and oxygen on platinum electrodes covered by an insulating film of nickel hydroxide. It is shown that the limiting currents from Koutecký Á/Levich plots give a much larger blocking factor than that derived from the analysis of a semilogarithmic current function. A comparison of these two methods for calculating the blocking factor provides information on the tortuosity coefficient of the film. #

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.

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.

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.

In our previous works, the corrosion inhibition properties of propylamine (PA), iso-propylamine (i-PA), butylamine (BA), tert-butylamine (t-BA), hexylamine (HA), ethylenediamine (EDA), diethanolamine (DEA), 3-amino-1-propanol (3-AP), 2-dimethylethanolamine (2-DEA), cyclohexylamine (CHA), N-methylcyclohexylamine (N-MCA) and dicyclohexylamine (DCHA) have been investigated by means of electrochemical impedance spectroscopy (EIS) with a carbon steel rotating disc electrode in some petroleum/water corrosive mixtures containing acetic acid and NaCl at 25°C. In this work, the Kramers-Kronig transformations (KKTs) were applied to evaluate the validity of the impedance data of these amine compounds in hydrodynamic condition. The results obtained showed that experimental impedance data did not satisfy in K-K relations completely viz. indicated some errors. These errors were related to stray capacitances, external transmission line effects, pits formation and change in interface during data acquisition at high and low frequencies regions. In addition, the selection of a suitable equivalent circuit strongly emphasized to better interpret of EIS data which in turn cause better resulted KKTs spectra. With considering obtained transformations, more effective inhibitor was selected with regard to its charge transfer resistance, the corrosion capacitance and well satisfying in K-K relations.

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.