Cathodic Sweep Research Articles

Carbon Surface Conditioning Produces an Anode Suitable for Heavy-Duty Discharge in Li Secondary Batteries

ABSTRACTCarbon fibers prepared at 1000 °C from mesophase pitch were treated with trimethyl silazane for modifying surface -OH group into trimethyl silyl group. The irreversible reduction current on the first cathodic sweep was diminished. Low-temperature carbon fibers prepared at 700°C and active carbon fibers were evaluated for electrochemical lithium doping and undoping reaction in propylenecarbonate(PC)/ethylene carbonate or PC/dimethyl carbonate mixtures(l:l) containing 1 M LiClO4 at room temperature.Pristine samples prepared at 700 °C gave very poor reversibility and capacity on cyclic voltammograms(cv), but a constant current charge/discharge with a very low current gave a capacity over 400 mAh/g. After prolonged heating at 700°C, the cv characteristics were gradually improved. Partial oxidation in a mild oxidizing atmosphere at 700 °C for a few minutes was found to be quite effective in improving the cv characteristics remarkably. The charge/discharge capacity, however, was very similar to that obtained for the pristine sample at a very low consatant current. This indicates that the heat treatments effectively increase the electrochemical reaction rate of lithium doping and undoping, but have no effect on the intrinsic capacity.Active carbon fibers were found to have considerable capacity after repeated oxidation/reduction cycles in solution. Partial oxidation treatment was also effective in increasing the charge/discharge current. The capacity after the treatment was over 400mAh/g, which is far larger than that of the double layer capacity of the sample.The improvement due to the heat treatment is elucidated based on the removal of a surface inhibiting layer through the treatment.

In Situ X‐Ray Absorption Fine Structure Study on Structure Transformation and Electronic State of Various Pt Particles on Carbon Electrode

The structure and electronic state of fine Pt particles on the carbon electrode were investigated by XANES and extended x‐ray absorption fine structure (EXAFS) spectroscopies. The white line intensities were processed into the parameter representing the density of unoccupied 5d state. They were described as kinds of voltammograms. It was different between the cathodic sweep and the anodic sweep even in the same potential. The small particle electrode showed large hysteresis of the d‐density change. These changes by potential sweep are discussed in relation to the structure of the particle on the basis of the parameters of EXAFS. The large change in the small particle was explained by the ratio of the surface atoms in the whole particle. The thickness of the oxide layer and lattice disorder in subsurface was estimated from these structural parameters.

An electrochemical study of the atmospheric corrosion of iron—II. Cathodic and anodic processes on uncorroded and pre-corroded iron

Redox processes occurring on uncorroded iron, and on iron pre-corroded by atmospheric exposure for 30 days, have been studied by voltammetry utilising a thin-film electrolyte in the likely potential and pH ranges present during atmospheric corrosion. On pre-corroded iron, oxidation of Fe(OH) 2 or Fe 2+ to FeO(OH), depending on surface pH, occurs as a passivation-type phenomenon and stifles further anodic activity at relatively anodic potentials (> −160 mV(SCE) at pH 5.5 and ≈ 60 mV(SCE) at pH 3.5) such as occur towards the final stages of drying in a wet-dry atmospheric cycle. On uncorroded iron, free Fe 2+ species were found during cathodic sweeps after anodic polarisation. These were absent on pre-corroded iron electrodes confirming their consumption during the production of a solid oxide. On uncorroded iron, no passivation was found up to 0 mV(SCE) which suggests either that pre-existing solid oxides are required to initiate passivation either by taking part in the reaction or by acting as nucleation sites for growth of new oxide during passivation. No evidence was found for redox cycles involving Fe 3O 4 and FeO(OH). The results suggest that Fe(OH) 2 may be oxidised to Fe 3O 4 or Fe 2O 3. These data largely confirm Strattman's modification to the Evans' scheme for cyclic oxidation during wetting and drying during atmospheric corrosion of iron. The importance of reduction of FeO(OH) supporting anodic dissolution during wet periods is limited by the volume of rust available for reaction at the iron/rust interface. A primary limitation of these redox processes is likely to be the electrical conductivity of the rust or the electrolyte.

A combined piezoelectric and EQCM study of underpotential deposition of silver on gold electrodes

The piezoelectric detection of change in surface energy combined with the use of electrochemical quartz crystal microbalance (EQCM) was applied to underpotential deposition ( upd) of silver on gold electrode. The upd of silver on gold was performed with a potentiodynamic method (10 mV s −1) in a deaerated solution of 0.1 mol dm −3 Na 2SO 4 + 10 −3 mol dm −3 H 2SO 4 + 5 × 10 −5 mol dm −3 Ag 2SO 4. The amplitude, | A|, and phase angle, φ, of the piezoelectric signals changed depending on the adsorbed amount of silver in the potential range of 1.10-0.56 V ( she). Particularly, two characteristic potentials at which | A| took a minimum value and φ changed by about 140°, appeared in both anodic and cathodic potential sweep. The characteristic potentials, E 1 = 0.62 V, and E 2 = 0.90 V ( she) correspond to the potentials of electrocapillary minimum and maximum, respectively. The presence of two characteristic potentials during the upd was explained by taking into account the changes in adsorption state of HSO − 4 or SO 2− 4 ions. The EQCM results indicated that the complete charge transfer of silver ions, z = 1 takes place during the upd. The piezoelectric detection of change in surface energy responded sensitively to the structural changes in the upd adsorbate as compared with the EQCM.

Ex-situ IRAS and LEED studies of underpotentially deposited copper on a Pt(111) electrode in a sulfuric acid solution: layer exchanges of anions with copper

The electrodeposition of copper on a Pt(111) electrode in a 0.5M H 2SO 4 solution was studied by means of voltammetry, low energy electron diffraction and ex-situ infrared reflection absorption spectroscopy. At any potential between 0.25 and 0.85 V versus the standard hydrogen electrode, the same diffraction pattern of (√3 × √3)-R30° was observed due to a top-layer structure of bisulfate or sulfate before and after the copper deposition. The voltammetric peak at 0.59 V was due to copper adsorption while that at 0.49 V originated from a bisulfate to sulfate conversion on copper during a cathodic sweep. At an electrode potential for underpotential deposition of copper, a bisulfate becomes weakly bound to platinum atoms allowing copper deposition; simultaneous adsorption of bisulfate on the copper layer prohibits further copper deposition.

In Situ Scanning Tunneling Microscopy Studies of the Formation and Reduction of a Gold Oxide Monolayer on Au(111)

An in situ scanning tunneling microscopy study of morphological changes during formation and reduction of an oxide monolayer on Au(111) in 0.1M is presented. During oxidation of a freshly prepared surf ace, the arrangement of metal atoms persisted through the hydroxide peak at 1.35 V vs. reversible hydrogen electrode (RHE). At the peak potential for formation (1.55 V vs. RHE), a slightly roughened layer propagated across terraces starting from edges. An ordered oxidized surface structure was not resolved and the surface roughness is approximately . This is ascribed tentatively to the formation of a monolayer of oxide by place exchange. During a slow cathodic sweep, the oxidized surface restructured into wormlike islands of monatomic height at the peak potential for oxide reduction at 1.15 Vvs. RHE. These islands coalesced to form flat terraces with monatomic pits in the surface at potentials within the double‐layer region. Pits eventually fused with terrace ledges to restore the original terrace morphology.

Experimental study of the alkalization model for the PbSO 4 anodic layer on lead/acid positive-plate grids

A lead electrode has been prepolarized anodically in sulfuric acid solution and then subjected to a cathodic potential sweep with simultaneous illumination. The photocurrents produced by PbO change from anodic to cathodic values pass through zero as the potential sweep continues. Under the present conditions, the potential at zero photocurrent is a fixed value, viz., −0.65 V, (versus Hg/Hg 2SO 4), and is equal to the flat-band potential of PbO. From this, the pH in the PbO layer is calculated to be 9. This finding provides strong support for the alkalization model of the PbSO 4 anodic layer.

The kinetics of the reduction processes of PbO film in H 2SO 4—VI. Effects of PbSb alloy electrodes

The electrochemical behaviour of PbO film on PbSb electrodes has been studied by the scanning electron microscope (SEM), X-ray diffraction and the electrochemical technique. In the cathodic potential sweep, the easy transformation of tetra-PbO → 3PbO · PbSO 4 · H 2O → PbO · PbSO 4 is caused by the many micropores produced in the dissolution of Sb, and the reduction peaks of these three compunds are observed simultaneously on the linear sweep voltammograms. It is found that the rates of their transformation and reduction are controlled by the diffusion of the OH − ion through the anodic film. Also the effects of Sb on the electrochemical behaviour of the tetra-PbO and basic lead sulfate layers are discussed.

Unusual Postmonolayer Oxide Behavior of Gold Electrodes in Base

Gold electrodes in base exhibit an unusual type of reaction in potential sweep experiments in the region between completion of the monolayer oxide film and the onset of regular oxygen gas evolution. A large peak, which was particularly well resolved at elevated temperature, was observed on the anodic sweep; the process involved, which had no counterpart on the cathodic sweep, was attributed to catalytic eyolution of oxygen, the unusually high activity being of a transitory character. The effect was attributed to the high activity of gold cations emerging through defects in the monolayer oxide film; the subsequent loss of activity was postulated to be due to dimerization of the hydrated cations leading to accumulation of less active hydrous Au(III) oxide species. A remarkable feature of the effect was the degree to which the charge associated with this unusual peak was influenced by prepolarizing the electrode at potentials below ca. 0.1 V (reference hydrogen electrode). Certain similarities in behavior between gold and platinum in this area were pointed out.

Corrosion of NdFeB permanent magnet materials

NdFeB is an important class of new magnetic materials, however corrosion resistance is a serious concern and literature on the electrochemical behavior of NdFeB is scarce. This paper reports the results of an electrochemical investigation of the corrosion behavior of sintered NdFeB magnets obtained from three manufacturers. Linear polarization (cyclic voltammetry) experiments were conducted in aqueous solutions ranging in pH from 0.7 to 13.5. A limited degree of passivation was observed in all solutions which is believed to be due to the formation of a complex Fe-Nd oxide and/or hydroxide film. The presence of a small amount of chloride ion, 10 to 100 ppm, shows only a slight effect, but higher concentrations (1000 ppm) cause a total breakdown in passivity and a dramatic increase in anodic current. The cathodic potential sweep shows an abrupt and unusual oxidation process, giving rise to an oxidation peak not commonly seen. This peak may result from dissolution of the film or preferential attack of intergranular phases.

Polarographic determination of fluoride using the adsorption wave of the Ce(III)-alizarin complexone-fluoride complex

A very sensitive electrochemical method for trace measurement of fluoride in water is discussed. The complex of cerium(III) with alizarin complexone (ALC) and fluoride ion is adsorbed at the dropping mercury electrode. In cathodic sweeps, the peak height is directly proportional to the concentration of fluoride over the range 8 × 10 −8-5 × 10 −6 M (1.5 × 10 −9–9.5 × 10 −8 g/ml), and the detection limit is 5 × 10 −8 M (9.5 × 10 −10 g/ml). The proposed method was applied to the determination of fluoride in water.

The Study of Electrochemical Properties of PbO Film on Lead in H 2 SO 4

Potentiostatic oxidations and potentiodynamic sweep of lead in were performed in the potential range between −1200 and +600 mV (with respect to the electrode). It has been found that three peaks of cathodic reduction on the cathodic sweep curve appear at −1000, −870, and about −800 mV, respectively. The peak at −800 mV is the reduction peak of tetragonal which is formed at the potential positive to −550 mV. After the electrode is passivated, the oxidation of Pb to tetragonal becomes the main process in the growth of anodic layer. The peak at −870 mV is the reduction peak of basic lead sulfates. This peak and the peak of oxidizing Pb to in the cathodic sweep overlap. The reduction of basic lead sulfates may be performed by a mechanism of the dissolution‐deposition.

Application of the hydrous oxide mediation model of electrocatalysis to reactions at noble metal anodes in base—II. Au, Ru, and Ir

The oxidation of four compounds, hydrazine, the formate anion, formaldehyde and methanol, was investigated, using cyclic voltammetry techniques, at polycrystalline Au, Ir and Ru metal anodes in base (the behaviour at three other metals, Pt, Pd and Rh, in base was described in Part I of this series). The results were discussed in terms of the incipient hydrous oxide mediation model of electrocatalysis. In the case of Au it was demonstrated that the onset (anodic sweep)—termination (cathodic sweep) potential for different compounds varied—but the values noted coincided with earlier reported minor peaks for gold in base that corresponded to adatom—incipiet hydrous oxide transitions at quite low coverage sites on the surface. The behaviour of Ir and Ru was complicated by the fact that the potential ranges for hydrogen adsorption and monolayer oxide formation (both of which can exert an inhibiting influence) overlap, ie the region where the adatom—incipient hydrous oxide transition can occur without complications is extremely limited in the case of these two metals. Hydrous oxide transitions may be observed with the latter, but these tend to be oxide—oxide, rather than adatom-oxide reactions. For electrocatalytically demanding reactions involving an interfacial redox cycle the adatom—incipient hydrous oxide couples seem preferable as the reduced state (the adatom) is then more likely to enhance the activation of the dissolved reactant via chemisorption effects.

Silver ion-exchanged zeolite modified electrodes: observation of electrochemically distinct silver ions

In this paper the electrochemical response of electrodes modified with silver ion exchanged Y type zeolites is examined. The cyclic voltammograms for the fully exchanged materials resemble closely those observed previously for silver mordenite zeolites. At low exchange levels (<10%, i.e., <5.6 silver ions per unit cell) cathodic linear sweep voltammetry indicates the presence of two electrochemically distinct silver ions

The Effect of Chromate Addition on Cathodic Reduction of Hypochlorite in Hydroxide and Chlorate Solutions

The cathodic reduction of chromate and its effect on the reduction of hypochlorite have been studied by cyclic voltammetry, using a rotating disk electrode of platinum in solution as well as in an electrolyte with a composition and temperature which were typical for industrial chlorate synthesis. It was found that in both electrolytes a thin film of presumably , with a thickness of only one or two molecular layers, is formed. After formation of this film in a cathodic sweep of the potential, the hypochlorite reduction reaction is almost completely inhibited in the reversed sweep, up to the potential region for dissolution of the formed film. From the experimental results and a theoretical analysis of the mass transfer rate of hypochlorite ions to the cathode surface, it is concluded that the addition of chromate to the electrolyte in the industrial chlorate process leads to the formation of a thin film on the cathode surface, which hinders electron transfer at the reduction of hypochlorite.

Use of Raman Spectroscopy and Rotating Split Ring Disk Electrode for Identification of Surface Layers on Iron in 1M NaOH

In situ Raman spectroscopy and rotating split‐ring disk electrode were used to identify products formed on iron in at 22°C at various peaks of cyclic voltammograms. The peak at the most active potential of the cathodic reverse sweep has been ascribed to the reduction of to Fe2+ species: this peak suggested that started to form at the first peak of anodic sweep and that it built up at nobler potentials. The Raman spectroscopy has revealed the formation of in the wide range of potentials, of and/or at peak III of anodic sweep, and of after prolonged polarization at a potential of 0.27 V (SCE). Rapid cycling or long exposure resulted in the formation of and . It is suggested that the passivating film on iron in is composed of an inner layer and of an outer layer containing other products.

Some Unique Aspects of Electrochemical Processes at the Group Ib Metal/Aqueous Solution Interface

It has been known for some time that the group Ib metals, copper, silver, and gold, are unique with regard to the magnitude of the surface‐enhanced Raman spectroscopy response recorded, especially for organic bases, at the metal/solution interface. Another unique feature of these three metal electrode systems is described here; they display an adatom (M*)/hydrous oxide transition in which the cation is in the univalent (M+) state. It is suggested that these are the only metals known to date that readily yield relatively insoluble, surface‐bound, univalent hydrous oxide species that behave as strong bases. The redox transition in question, M*/M(1) hydrous oxide, displays an unusual shift in potential upon altering the solution pH; this shift (previously reported for gold and silver) was monitored in the present work by recording the change in onset (anodic sweep)/termination (cathodic sweep) potential for dimethylamine borane oxidation, a reaction induced by the oxidized form of the couple acting as a mediator. It was pointed out that the two unique features of these three metals have the same basis, i.e., an array of univalent cations attached to the electrode surface.

The Participation of Interfacial Hydrous Oxide Species in Some Anodic Reactions at Copper Electrodes in Base

The anodic behavior of copper is surprisingly complex as this element can exhibit 4 (0, +1, +2, +3) oxidation states and most of its oxides can exist in two distinct forms, anhydrous and hydrated. Hydrous oxides in general have been widely ignored, but an appreciation of their properties is essential for a complete understanding of the behavior of metals. In the present case attention was focused on the behavior of copper in base at low potentials where a minor Cu adatom/Cu(I) hydrous oxide transition seemed to occur. Evidence for the latter included the observation that an oxide reduction peak occurred under certain conditions at ca. 0.05V (RHE); this is significantly lower than the usual value for ,, or reduction. Furthermore, the same value was observed for the onset (anodic sweep)/termination (cathodic sweep) potential for other reactions on copper in base, e.g., formaldehyde oxidation or copper dissolution in the presence of CN− ions. As in the case of previous work with other metals, the response for low‐coverage hydrous oxide formation on copper in base is not obvious on the anodic sweep in cyclic voltammetry experiments; however, prior data from ac work provide evidence for the type of effect postulated here. The behavior of copper, as may be expected, bears a distinct similarity to that of silver and gold; similar proposals were made recently for the latter metals. The catalytic activity of copper in base at higher potentials is attributed to mediation of interfacial processes by other redox transitions in this system.

Determination of Nitrite by Single-Sweep Polarography

Abstract A new method for determining nitrite is proposed. the mechanism for the polarographic waves and the condition for determining nitrite are discussed in this paper. In cathodic sweeps, the peak height is directly proportional to the concentration of nitrite over the range 5×10−9−6×10−7 g/ml, the detection limit is 4×10−9g/ml. the experiments showed that this polarographic wave is an adsorption wave, and useful in determination of nitrite in water sample.

Intermediates and Products of Ethanol Oxidation on Platinum in Acid Solution

AbstractThe adsorbate formed during ethanol oxidation in acidic solutions is studied by Fourier transform infrared reflection‐absorption spectroscopy (FTIR) at constant potential, on‐line (DEMS) and thermal desorption mass spectroscopy (ECTDMS). – As in the case of methanol adsorbate COad was identified by FTIR and in addition COHad by ECTDMS. On‐line MS experiments showed the formation of methane during the cathodic sweep of the voltammogram. Main products of ethanol oxidation from the bulk are carbon dioxide and acetaldehyde.