Effect of electrode materials on the kinetics of the electro-reduction of peroxyacetic acid

Electrochemical behavior of peroxyacetic acid (PAA) and hydrogen peroxide (H₂O₂) was examined at various metal and carbon electrodes (i.e., Au, Ag, Cu, Pt, Pd, Rh, Ti, W, Hg, Ni, Fe, glassy carbon (GC), and basal-plane pyrolytic graphite (BPG)) in 0.1 M acetate buffer solution (pH 5.5) using potentiostatic (i.e., cyclic voltammetry and rotating disk electrode voltammetry) and galvanostatic techniques. It was found that the electro-reductions of PAA and H₂O₂ are highly sensitive to electrode material. Both species were found to be electrochemically and separately reduced at Au, Ag, Cu, Pt, Pd, GC, and BPG electrodes. On the other hand, at Fe, Ni, Hg, Rh, Ti, and W electrodes, voltammetric response for the PAA reduction was not obviously observed. The kinetics of electro-reduction of PAA in 0.1 M acetate buffer solution was studied at Au, Ag, and GC electrodes in details, and the relevant kinetic parameters (i.e., the exchange current density, j₀, the standard rate constant, k⁰, and cathodic transfer coefficient, α_c) were estimated from the Tafel plots. The cyclic voltammetric reduction peak potentials obtained for the PAA reduction at Au, Ag, and GC electrodes were compared with those calculated using the kinetic and thermodynamic parameters obtained under the same experimental conditions. The measured and calculated reduction peak potentials at each electrode were found to be in agreement with each other, indicating that the evaluated values of kinetic parameters for the reduction of PAA at Au, Ag, and GC electrodes are reasonable.     

Oxygen reduction at Au nanoparticles electrodeposited on different carbon substrates

The electrocatalytic reduction of molecular oxygen (O₂) has been performed in O₂-saturated 0.5 M KOH solution at Au nanoparticles electrodeposited onto two different carbon substrates, namely glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG). Cyclic voltammetry (CV) technique has been used in this investigation. The electrocatalytic activity of the Au nanoparticle-based electrodes is inherently related to its electrodeposition conditions (i.e., the absence or presence of some additives) as well as the nature of the substrate. For instance, Au nanoparticles electrodeposited onto GC (nano-Au/GC) from K[AuBr₄] in the presence of 25 μM cysteine showed a high electrocatalytic activity towards the oxygen reduction reaction (ORR) as demonstrated by the largest positive shift of the cathodic peak potential (at ca. −0.165 V versus Ag/AgCl/KCl (sat)). On the other hand, two well-separated successive reduction peaks corresponding to the 2-step 4-electron reduction of oxygen were observed at the different nano-Au/HOPG electrodes. The relative ratio of the two peak current heights changed significantly depending on the electrodeposition conditions of the Au nanoparticles. The morphology of the different Au nanoparticles electrodeposited onto the different substrates was depicted by scanning electron microscope (SEM) technique.     

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

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

Electrochemical modification of glassy carbon electrode by poly-4-nitroaniline and its application for determination of copper(II)

Electrochemical modification of glassy carbon (GC) electrode by poly-4-nitroaniline (P4NA), electrochemical reduction of P4NA and applicability of electrode modified in this way for determination of copper(II) (Cu(II)) is reported in this study. Electrochemical surface modification was performed by cyclic voltammetry in the potential range between +0.9 V and +1.4 V vs. Ag/Ag⁺ (in 10 mM AgNO₃) at the scan rate of 100 mV/s by 100 cycles in non-aqueous media. In order to provide electrochemical reduction of nitro groups on the P4NA-modified GC electrode surface (P4NA/GC), the cyclic voltammograms inducing/evidencing the reduction of nitro groups were performed in the potential range between −0.1 V and −0.8 V vs. Ag/AgCl/(sat.KCl) at the scan rate of 100 mV/s. The reduced P4NA/GC surfaces (Reduced-P4NA/GC) were treated with aqueous solution of nitrilotriacetic acid. The sensitivity of GC electrode modified in described way towards Cu(II) was investigated in Britton-Robinson buffer solution, pH 5.0. The potentiometric generic pulse technique was applied as innovative electrochemical method for detection of analytical signal. It was shown that GC electrodes modified in here described way will be suitable for the determination of Cu(II) in technological waste water and/or some other solutions containing Cu(II) ions.     

Electrochemically reduced titanocene dichloride as a catalyst of reductive dehalogenation of organic halides

We have studied a reaction between the reduced form of titanocene dichloride (Cp₂TiCl₂) and a group of organic halides: benzyl derivatives (4-XC₆H₄CH₂Cl, X = H, NO₂, CH₃; 4-XC₆H₄CH₂Br, X = H, NO₂, PhC(O); 4-XC₆H₄CH₂SCN, X = H, NO₂) as well as three aryl halides (4-NO₂C₆H₄Hal, Hal = Cl, Br; 4-CH₃O-C₆H₄Cl). It has been shown that the electrochemical reduction of Cp₂TiCl₂ in the presence of these benzyl halides leads to a catalytic cycle resulting in the reductive dehalogenation of these organic substrates to yield mostly corresponding toluene derivatives as the main product. No dehalogenation has been observed for aryl derivatives. Based on electrochemical data and digital simulation, possible schemes of the catalytic process have been outlined. For non-substituted benzyl halides halogen atom abstraction is a key step. For the reaction of nitrobenzyl halides the complexation of Ti(III) species with the nitro group takes place, with the electron transfer from Ti(III) to this group (owing to its highest coefficient in LUMO of the nitro benzyl halide) followed by an intramolecular dissociative electron redistribution in the course of the heterolytic CHal bond cleavage.The results for reduced titanocene dichloride centers immobilized inside a polymer film showed that the catalytic reductive dehalogenation of the p-nitrobenzyl chloride does occur but with a low efficiency because of the partial deactivation of the film due to the blocking of the electron charge transport between the electrode and catalytic centers.     

Kinetic study of nitrate reduction on Pt(1 1 0) electrode in perchloric acid solution

The kinetics of electrocatalytic reduction of nitrate on Pt(1 1 0) in perchloric acid was studied with cyclic voltammetry at a very low sweep rate of 1 mV s⁻¹, where pseudo-steady state condition was assumed to be achieved at each electrode potential. Stationary current-potential curves in perchloric acid in the absence of nitrate showed two peaks at 0.13 V and 0.23 V (RHE) in the so-called adsorbed hydrogen region. The nitrate reduction proceeded in the potential region of the latter peak in the pH range studied. The reaction orders with respect to NO₃⁻ and H⁺ were observed to be close to 0 and 1, respectively. The former value means that the adsorbed NO₃⁻ at a saturated coverage is one of the reactants in the rate-determining step (rds). The latter value means that hydrogen species is also a reactant above or on the rds. The Tafel slope of nitrate reduction was −66 mV per decade, which is taken to be approximately −59 mV per decade, indicating that the rds is a pure chemical reaction following electron transfer. We discuss two possible reaction schemes including bimolecular and monomolecular reactions in the rds to explain the kinetics and suggest that the reactants in the rds are adsorbed hydrogen and adsorbed NO₃⁻ with the assistance of the results in our recent report for nitrate reduction on Pt(S)[n(1 1 1) × (1 1 1)] electrodes: the nitrate reduction mechanism can be classified within the framework of the Langmuir-Hinshelwood mechanism.     

Stability of iodine on ruthenium during copper electrodeposition and its effects on the nucleation behavior of electrodeposited copper

Cyclic voltammetry, current-time-transient measurements, and X-ray photoelectron spectroscopy (XPS) have been used to study the nucleation behavior of electrochemically deposited Cu films on Ru substrates as a function of Ru pre-treatment. Pre-treatment consisted of cathodic polarization in either 1 M H₂SO₄ or in 1 M H₂SO₄ + 1 mM KI, followed by sample emersion and placement in a 1 M H₂SO₄ + 50 mM CuSO₄ plating bath. XPS measurements confirmed the presence of adsorbed I on the Ru surface following pre-treatment in the KI/H₂SO₄ solution. Cyclic voltammogram (CV) data for electrodes either as-received or pre-reduced in H₂SO₄ and then immersed in the plating solution exhibited a broad peak in the overpotential region consistent with oxide reduction followed by Cu deposition. No underpotential deposition (UPD) feature was observed for these electrodes. In contrast, the sample pre-reduced in I-containing electrolyte exhibited a narrow Cu deposition peak in the overpotential region and a UPD Cu feature centered at 80 mV vs. Ag/AgCl. Current-time-transient (CTT) measurements of Cu deposition on as-received electrodes or electrodes pre-reduced in I-free solution exhibited potential-independent kinetics that are not well described by either progressive or instantaneous nucleation models and which at long times indicate a combination of diffusion and kinetic control. In contrast, CTT measurements of deposition kinetics for samples reduced in I-containing electrolyte exhibited complex, potential-dependent behavior and that at long times indicates diffusion control. XPS results also indicated that the iodine adlayer on Ru reduced in I-containing electrolyte is stable upon polarization to at least −200 mV vs. Ag/AgCl. These data indicate that a protective I adlayer may be deposited on an air-exposed Ru electrode as the oxide surface is electrochemically reduced, and that this layer will inhibit reformation of an oxide during the Cu electroplating process. Therefore, electrochemical pre-treatment in I-containing electrolyte may be of practical utility under industrial conditions for Cu electroplating.     

Electrocatalytic reduction of chlorophenoxy acids at palladium-modified glassy carbon electrodes

Palladium species can be immobilized on a glassy carbon electrode by voltage cycling between 0.0 and −0.4 V versus SCE in solutions containing 0.5 mM Na₂PdCl₆ in order to facilitate the electrocatalytic reduction of chlorophenoxycarboxylic acids in solutions buffered at pH 7. Cyclic voltammetry, measurements at the rotating disc electrode (RDE) and chronoamperometric techniques were used in order to investigate the electrochemical behaviour of the modified electrodes (GC/Pd) towards the catalytic reduction of chlorophenoxycarboxylic acids. A reaction mechanism is proposed and discussed. A probable scheme for the electroreduction of chlorophenoxycarboxylic species in neutral medium involves a simultaneous and competitive adsorption of the organic molecules and hydrogen atoms on the catalytic sites, followed by an irreversible hydrodechlorination reaction. 2,4-Chlorophenoxyacetic acid can be dehalogenated to a chlorine-free product in neutral aqueous solutions at relatively low cathodic polarizations and at ambient temperature using a GC/Pd electrode.     

THE SOLVENT EXTRACTION OF ZINC FROM LITHIUM CHLORIDE BY ALIQUAT 336 CHLORIDE,BROMIDE AND IODIDE IN CHLOROFORM: AN ANALYTICAL INVESTIGATION

ABSTRACT

The extraction of zinc by Aliquat CI in chloroform is studied as a function of LiCl concentration. A plot of log D versus total [Cl] shows that log D increases sharply from 0-2 M LiCl, then remains constant over a considerable range of concentrations ( 2-8 M ), before shooting up again from 8-14 M. Further work is carried out at 4 M LiCl to determine the extraction equation, and quantitative aspects determined such as a plot of log D versus log [Aliquat Cl]_free, giving a ratio of 2:1 for extractant: metal, and the ratio [Cl / Zn]_extracted, which is 2.

In order to follow the exchange of halides across the interface, Aliquat Br and Aliquat I are used as extractants in chloroform to study the extraction of zinc chloride from 4 M LiCL Organic and aqueous phases are analysed for halides and zinc, and the ratio [ΔCl] / [Zn] in the organic phase determined to be 3 for both Aliquat Br and I at 4M LiCl, indicating extraction of ZnCl₃,' and not ZnCl₄ ² at this concentration. Further work with these extractants based on plots of log D versus log [Aliquat X]_free ( X = Br, I ) is presented and extraction equations are proposed for these extractants, which fit the data.

Extraction of zinc at 4 M LiCl by Aliquat Cl is then considered in terms of the extraction of the trichlorozincate (II) species, ZnCl₃,“, and the extraction constant determined.     

Electrocatalytic reduction of NO2: Platinum modified glassy carbon electrode

Platinum particles were electrochemically deposited over glassy carbon (GC) to prepare GC-Pt electrodes. The electrocatalytic behaviors of this electrode have been compared with that of an ordinary polycrystalline(OPC) Pt and GC electrode in reducing NO₂⁻ at neutral medium. The as prepared GC-Pt electrode reduced NO₂⁻, exhibiting double-peak reduction waves. The reduction performance of this electrode was noticed at least 7.8 times higher than that of an OPC Pt electrode. The sensitivity of the GC-Pt electrode was found to be enhanced by the temperature rise. A consecutive mechanism, NO₂⁻ → NO → NH₄⁺, over the as prepared GC-Pt electrode has been investigated.     

Electrochemical behaviour of glassy carbon electrodes modified with aryl groups

The electrochemical modification of the glassy carbon (GC) electrode surface with biphenyl, 1-naphthyl, 2-naphthyl, 4-bromophenyl, 4-decylphenyl and 4-nitrophenyl groups was performed by the diazonium reduction method. The blocking behaviour of aryl films grafted by three different procedures was compared. Oxygen reduction was studied on these modified GC electrodes using the rotating disk electrode (RDE) method. The highest blocking efficiency for O₂ reduction was observed for 4-bromophenyl groups. The barrier properties of aryl-modified GC surfaces were also characterised using Fe(CN)₆³⁻ and dopamine redox probes. Electrochemical measurements were carried out in 0.1 M K₂SO₄ containing 1 mM K₃Fe(CN)₆ and in 0.1 M H₂SO₄ containing 1 mM dopamine using cyclic voltammetry (CV). The blocking action varied significantly depending on the surface modifier used and the solution based redox species studied.     

Mechanisms of electrochemical reduction and oxidation of nitric oxide

A summary is given of recent work on the reactivity of nitric oxide on various metal electrodes. The significant differences between the reactivity of adsorbed NO and NO in solution are pointed out, both for the reduction and the oxidation reaction(s). Whereas adsorbed NO can be reduced only to hydroxylamine and/or ammonia, it takes NO in solution to produce N₂O and N₂. From the reduction of NO on a series on stepped single-crystal Pt electrodes, it is concluded that NO_ads reduction is not a structure sensitive process. The protonation of the adsorbed NO is rate-determining; neither the NO adsorption strength nor the NO bond breaking play a significant role in its reduction rate. Whereas adsorbed NO on polycrystalline Pt can only be oxidized to nitrate, in the presence of NO in solution nitrous acid HNO₂ may also be formed, in a potential region where adsorbed NO is otherwise stable. Interestingly, on Pt(1 1 1) and Pt(5 5 4) NO_ads may be oxidized to HNO₂ in a surface-bonded redox couple. Whereas surface oxides appear to catalyze the oxidation of solution NO to HNO₂, the further oxidation to nitrate seems to be inhibited by the presence of surface oxides. Both the reduction and oxidation of solution NO appear to be not very metal-dependent reactions, as they take place with approximately equal rate on all electrode metals studied, including gold. This suggests the involvement of weakly adsorbed intermediates, and the relatively unimportant role of surface-bonded NO in the bulk NO reduction and oxidation activity.     

Electrochemical study of chitosan films deposited from solution at reducing potentials

We report the electrochemical characterization of chitosan films deposited at gold electrodes from an acidic solution at reducing potentials. Cyclic voltammetry was used to characterize the deposition and electroactivity of chitosan coated gold electrodes. Chitosan films were found to deposit at gold electrodes at potentials more negative than −1.0 V versus Ag/AgCl, a potential associated with the onset of water reduction and increase in pH near the electrode. The chitosan films are electrochemically inactive; similar background charging currents are observed at bare gold and chitosan coated electrodes. The chitosan films are permeable to both cationic [Ru(NH₃)₆^3+/2+] and anionic [Fe(CN)₆^3−/4−] redox couples, but anionic complexes are retained in the chitosan film. Semiintegral analysis was used to examine adsorbed redox species at the chitosan coated electrode surface. Electrochemical parameters, including apparent diffusion coefficients for the redox probes at the electrodeposited chitosan modified electrodes are presented and are comparable to values reported for cast chitosan films.     

Electrochemical hydrodehalogenation of polychloromethanes at silver and carbon electrodes

The reductive dehalogenation of CCl₄, CHCl₃, CH₂Cl₂ and CH₃Cl has been investigated by cyclic voltammetry and controlled-potential electrolysis at Ag, glassy carbon (GC) and graphite electrodes in dimethylformamide (DMF) + 0.1 M Et₄NClO₄ in the absence and presence of a proton donor. In particular, the study was focused in the evaluation of the intermediates and final products of the reduction process and how their distribution could be affected by tuning relevant chemical and electrochemical parameters. In general, depending on the value of the applied potential, all polychloromethanes (PCMs) can be partially or completely dechlorinated, methane being exclusively formed in the latter case. The nature of the electrode material and the proton availability of the medium affect drastically the distribution of reduction products. The results point out that at both types of electrode, reduction of PCMs takes place through two competing reaction pathways both leading to methane. One reaction route involves a sequence of reductive dehalogenation steps, with the removal of one chlorine atom at a time, whereas the other is based on hydrogenolysis of carbenes and bypasses the intermediacy of partially dechlorinated PCMs. The presence of a proton source affects substantially the hydrodehalogenation efficiency, enhancing the concentration of intermediate PCMs and the final yield of methane. The silver electrode exhibits an extraordinary electrocatalytic effect resulting in remarkable positive shifts of the reduction potentials of all PCMs with respect to GC. The Ag surface strongly affects the kinetics of the dissociative electron transfer to CH_nCl_(4−n) (n = 0–3) as well as the reactivity of the intermediate radicals, carbanions and carbenes.     

A laccase-glucose oxidase biofuel cell prototype operating in a physiological buffer

Here we report on the design and study of a biofuel cell consisting of a glucose oxidase-based anode (Aspergillus niger) and a laccase-based cathode (Trametes versicolor) using osmium-based redox polymers as mediators of the biocatalysts’ electron transfer at graphite electrode surfaces. The graphite electrodes of the device are modified with the deposition and immobilization of the appropriate enzyme and the osmium redox polymer mediator. A redox polymer [Os(4,4′-diamino-2,2′bipyridine)₂(poly{N-vinylimidazole})-(poly{N-vinylimidazole})₉Cl]Cl (E⁰′ = −0.110 V versus Ag/AgCl) of moderately low redox potential is used for the glucose oxidizing anode and a redox polymer [Os(phenanthroline)₂(poly{N-vinylimidazole})₂-(poly{N-vinylimidazole})₈]Cl₂ (E⁰′ = 0.49 V versus Ag/AgCl) of moderately high redox potential is used at the dioxygen reducing cathode. The enzyme and redox polymer are cross-linked with polyoxyethylene bis(glycidyl ether). The working biofuel cell was studied under air at 37 °C in a 0.1 M phosphate buffer solution of pH range 4.4-7.4, containing 0.1 M sodium chloride and 10 mM glucose. Under physiological conditions (pH 7.4) maximum power density, evaluated from the geometric area of the electrode, reached 16 μW/cm² at a cell voltage of 0.25 V. At lower pH values maximum power density was 40 μW/cm² at 0.4 V (pH 5.5) and 10 μW/cm² at 0.3 V (pH 4.4).     

Study of the electrochemical deposition of Sn-Ag-Cu alloy by cyclic voltammetry and chronoamperometry

The electrochemical deposition of Sn-Ag-Cu alloy from weakly acidic baths onto glassy carbon electrodes (GCE) was studied by cyclic voltammetry (CV) and chronoamperometry (CA). The properties of the electrodeposits were characterized by scanning electron microscopy (SEM), energy-dispersive spectrometery (EDS) and X-ray diffraction (XRD). Test results indicate that the two cathodic peaks in the CV curves, at −0.6 V and −0.85 V during the forward scan towards the negative potentials, correspond to the irreversible deposition of a solid solution of tin, silver and copper. The underpotential deposition (UPD) of Sn occurs at −0.6 V during the cathodic period and the amount of Ag and Cu in the Sn-Ag-Cu alloy decreases with increasingly negative cathodic potentials. During the forward scan, towards the positive potentials used in CV testing, cathodic peaks at −0.85 V appear in the CV curves for baths containing mixtures of tin salts and triethanolamine (TEA). This corresponds to a reduction of transient complex ions [Sn(TEA)_x]²⁺ on the surface of the cathode. Furthermore, the formation and reduction of [Sn(TEA)_x]²⁺ is a diffusion controlled process. On the surface of the GCE, the actual nucleus growth mechanism of the Sn-Ag-Cu alloy is represented by the progressive nucleation model.     

Redox behaviour of cerium oxychloride in molten MgCl2-NaCl-KCl eutectic

The electrochemical behaviour of cerium oxychloride in MgCl₂-NaCl-KCl ternary eutectic was investigated by cyclic voltammetry at 823 K. The cyclic voltammogram of UO₂Cl₂-CeOCl in MgCl₂-NaCl-KCl eutectic shows two peaks during the cathodic sweep as well as anodic sweep. The reduction of UO₂²⁺ is by a single step two-electron transfer and that of CeO⁺ is by a single step one-electron transfer. The reduction of CeO⁺ was found to be quasi-reversible.The reduction potentials of UO₂²⁺/UO₂ and CeO⁺/CeO versus Ag(I)/Ag reference electrode at 823 K are 0.103 and −0.299 V, respectively. The diffusion coefficient of CeO⁺ at 823 K is in the range of (1.7-1.9) × 10⁻⁵ cm² s⁻¹. The cyclic voltammogram for 0.015 mol% CeOCl shows an additional peak during the anodic sweep at −0.056 V, which is being attributed to monolayer dissolution of CeO at the glassy carbon working electrode. Electrochemical impedance data of 0.015 mol% CeOCl in MgCl₂-NaCl-KCl eutectic at the open circuit potential was fitted to a Randles cell from which the heterogeneous rate constant was estimated. X-ray photoelectron spectroscopy was used to confirm that the oxidation state of cerium in the eutectic is +3.     

Electroreduction of oxygen on glassy carbon electrodes modified with in situ generated anthraquinone diazonium cations

The electrochemical reduction of oxygen on glassy carbon (GC) electrodes modified with in situ generated diazonium cations of anthraquinone (AQ) has been studied using the rotating disk electrode (RDE) technique. The electrografting of the GC electrodes was carried out in two different media: in acetonitrile and in an aqueous acidic solution (0.5 M HCl). 1- and 2-Aminoanthraquinone were used as starting compounds for the formation of the corresponding diazonium derivatives. The anthraquinone diazonium cations were generated by reaction of the aminoanthraquinones with tert-butyl nitrite and sodium nitrite in acetonitrile and in 0.5 M HCl, respectively. For comparison purposes, the previously synthesised and crystallised diazonium tetrafluoroborates of anthraquinone were used for the GC surface modification. Cyclic voltammetry was employed to determine the surface concentration of AQ in O₂ free 0.1 M KOH. The electrocatalytic behaviour towards O₂ reduction was similar for all the AQ-modified electrodes studied. The kinetic parameters of oxygen reduction were determined using a surface redox catalytic cycle model. The rate constant of the reaction between the semiquinone radical anion of AQ and molecular oxygen was virtually independent of the point of attachment of the quinone to the electrode surface.     

Electrochemical performance of silver-modified Ba0.5Sr0.5Co0.8Fe0.2O3−δ cathodes prepared via electroless deposition

Silver-modified Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) cathodes for intermediate-temperature solid-oxide fuel cells (IT-SOFCs) were prepared by an electroless deposition process using N₂H₄ as the reducing agent at room temperature. This fabrication technique together with tailored electrode porosity, modified the BSCF electrodes with silver content that varied from 0.3 to 30 wt.% without damaging the electrode microstructure. Both the Ag loading and firing temperatures were found to have a significant impact on the electrode performance, which could facilitate or block the electrochemical processes of the BSCF-based cathodes, processes that include charge-transfer, oxygen adsorption and oxygen electrochemical reduction. At an optimal Ag loading of 3.0 wt.% and firing temperature of 850 °C, an area specific resistance of only 0.042 Ω cm² at 600 °C was achieved for a modified BSCF cathode.     

Experimental and theoretical studies of l-cysteine adsorbed at Ag(1 1 1) electrodes

We have investigated l-cysteine adsorbed on Ag(1 1 1) electrodes under different conditions. We have employed experimental and theoretical approaches to obtain a better understanding of the adsorbed layer. An estimation of the coverage from charge measurements and the second harmonic response shows C_3v symmetry for the interface indicating a (√3 × √3)R°30 overlayer. The theoretical calculations show a variety of different structures with local adsorption energy minima. Particularly, under special initial conditions, zwitterionic structures adsorbed at different sites have been found. This can account for the multiplicity of redox processes observed experimentally below the potential of zero charge. The presence of an external field produces the stabilization of the zwitterion by interaction of the amino/carboxylic groups with the substrate.