Open-circuit Potential Transients Research Articles

Galvanic displacement of copper adatoms on platinum in solutions

The kinetics of interaction of copper adatoms Cuad on polycrystalline Pt (pc Pt) with PtCl42- (0.5 M H2SO4 supporting electrolyte) at open circuit is studied. Hidden currents of Cuad displacement (ihidCu) are compared with exchange currents of Cu2++2e⇄Cuad reaction determined by the impedance method and also with hidden currents of platinum deposition on pc Pt under open-circuit conditions. The substantial excess of ihidCu high θCu over the Pt deposition currents (in absence Cuad) is attributed to the “direct” chemical interaction between PtCl42- and Cuad. The anomalous increase in ihid with increasing potential at small θCu is explained by the involvement of adsorbed oxygen forms in the displacement process. CVA and transients of open-circuit potential are measured on the Pt(1 1 1) electrode. For measurements “in the solution meniscus”, the displacement process on Pt(1 1 1) and pc Pt was shown to be largely diffusion controlled.

The effect of hydrocarbon chain length in normal aliphatic alcohols on their reaction with oxygen adsorbed on polycrystalline platinum electrode

The reaction of adsorbed oxygen (Oads) with aliphatic alcohols n-CnH2n + 1OH with n = 2–5 is studied by the method of transients of open-circuit potential in combination with potentiodynamic pulses. It is shown that these alcohols react with Oads by a mechanism the same as for CH3OH. Kinetic parameters of these reactions are determined in ranges of high and medium surface coverages with Oads. These data together with analogous results obtained earlier for CH3OH were studied with the aim of elucidating how the length of the hydrocarbon chain affects the kinetics of interaction of alcohols with Oads. The complex variations of the reaction rate with n (with a maximum) are explained by several factors among which the energy of the C–H bond at α-carbon atom and the degree hydration of alcohols should be singled out.

Enhanced Carrier Transport of N-Doped TiO2 for Photoelectrochemical Cells

The carrier transport kinetics of the TiO2 film doped with N (TiO2:N) were investigated by measuring the current and open circuit potential transients under light on/off illumination. These measurements were compared to an undoped film. The N in TiO2 not only shifted the light absorption into a longer wavelength region (known effect) but also enhanced the carrier transport. The combination of these two effects improved the photogeneration of the electron–hole pairs and suppressed their recombination, resulting in much better photoelectrochemical performance, compared to that of the undoped TiO2.

A voltammetric study on the corrosion of prestressed steel in saturated Ca(OH)2 solution containing chloride ions

An electrochemical study of prestressed steel in saturated Ca(OH)2 aqueous solutions (pH 12) was carried out in the absence and in the presence of chloride ions, in such a concentration that simulates the composition of seawater. Cyclic voltammetry, linear sweep voltammetry, open circuit potential transients, atomic absorption spectroscopy and scanning electron microscopy coupled to electron diffraction spectroscopy were employed. The linear polarisation curves analysis led to the determination of polarisation resistance, R p, corrosion potential, E corr, corrosion current density, j corr , Tafel slopes, breakdown potential, E b and repassivation potential E repass. A linear dependence of the breakdown potential, E b, on the square root of scan rate was obtained, according to the Point Defect Model (PDM). A crossover characteristic of the nucleation processes was observed in the presence of chloride ions. SEM/EDS studies revealed, as expected, a strong influence of the presence of chloride ions observed in the transpassive and the active regions. In conclusion, chloride ions contribute to enhance the corrosion of steel, most probably due to their adsorption on both the active and the passive electrode surfaces.

Formaldehyde interaction with adsorbed oxygen on open-circuit platinum electrodes in aqueous sulfuric acid solutions

The open circuit potential transients and cathodic potentiodynamic pulses were measured upon formaldehyde (methylene glycol) interaction with pre-adsorbed oxygen (Oads) on Pt/Pt and pc Pt electrodes in aqueous sulfuric acid solutions. The slowest interaction of CH2(OH)2 with Oads was observed in the high coverage range of the electrode surface (θO ∼ 0.2 0.8 to 1). The process rate in this range is determined by the direct reaction of Oads with CH2(OH)2 molecules from the bulk solution. In the middle surface oxygen coverage range (θO 0.2 to 0.8), CH2(OH)2 interaction with Oads takes place by the mechanism of “conjugated reactions”. The kinetic parameters of reactions for CH2(OH)2, HCOOH, and CH3 OH were compared. The rate of CH2(OH)2-Oads interaction on Pt electrodes in the high oxygen coverage range was found higher by an order of magnitude than that of HCOOH and by two orders of magnitude than in the case of CH3OH.

Mechanism of interaction of molecular hydrogen with adsorbed oxygen on platinum electrodes under open circuit conditions

Transients of open-circuit potential observed at the reaction of hydrogen molecules with oxygen preliminarily adsorbed (Oads) on the smooth polycrystalline (pc Pt) and platinized platinum (Pt/Pt) electrodes are measured under conditions of controlled stirring of solution (0.5 M H2 SO4). The dependence of the surface coverage with Oads(θO) on the potential in the cause of the potential decay on pc Pt are determined. It is found that for Pt/Pt, the reaction kinetics is largely determined by diffusion of H2. For pc Pt in the range of high θO, the Eley-Ridiel mechanism is realized. For medium θO, the regions where the reaction obeys the mechanisms of Eley-Ridiel, “conjugated reactions”, and diffusion control are observed to overlap (even at the most intense stirring possible). The rate of H2 reaction with Oads is substantially higher compared with analogous reactions of CO, HCOOH, and CH3OH.

The Location of Events Producing Potential Transients During Pitting of Freely Corroding Al and Its Alloys

The behavior of a number of Al alloys have been studied using open circuit potential transients in conjunction with current density mapping, imaging, and time-delay of arrival of potential changes. Generally, potential transients are analyzed as noise relating to initiation and repassivation of metastable pits, a behavior demonstrated for stainless steels. However, measurements on pure Al, using segmented electrodes and imaging methods, show that most transients emanate from growing pits. Comparing behaviors of alloys shows that AA1100 and AA2024 are similar to pure Al. AA7075 shows similar transients but only a thin highly deformed surface layer is locally attacked along the abrasion direction. AA5083 in contrast first shows transients dominated by metastable pitting and only after hours of exposure do events within growing pits dominate the production of transients.

Factors controlling the time evolution of the corrosion potential of aluminum in alkaline solutions

The mechanism of corrosion of 99.99% purity aluminum in alkaline solutions was investigated, through detailed examination of open-circuit potential transients. These transients displayed a characteristic time dependence, in which the potential first decreased over a few seconds to a minimum of −1.7 to −1.9 V vs. Ag/AgCl, and then slowly increased over a period of hours. The value of the minimum potential of electropolished foils, along with its dependence on pH and aluminate ( Al ( OH ) 4 - ) ion concentration, indicated that it was determined by the Nernst potential for the oxidation of surface aluminum hydride (AlH 3). This finding supports the direct role of hydride in the dissolution process. The increase of anodic polarization after the minimum potential occurred in two stages, the first correlated with the buildup of surface hydride, and the second with surface enrichment of Cu and Fe impurities.

Special features of methanol interaction with adsorbed oxygen at platinized platinum electrode: Transients of the open-circuit potential

Open-circuit potential transients are measured under the conditions of methanol interaction with the pre-adsorbed oxygen at platinized platinum electrode. The time necessary for complete removal of the adsorbed oxygen monolayer appeared being shorter by a factor of ∼1.5 as compared with smooth polycrystalline platinum. The dependence of platinum surface coverage with adsorbed oxygen on the potential during its decay is found. It was shown that the reaction of methanol with the adsorbed oxygen is most slow at a high coverage (1–0.8). It is suggested that at these coverages, like the case of polycrystalline platinum, the adsorbed oxygen directly interacts with the methanol molecules from the solution. At moderate coverages (0.8–0.2), the reaction of the adsorbed oxygen with methanol at the platinized platinum is better described by the “conjugated reactions” mechanism. The specific rates of the methanol dissociative adsorption at the platinized platinum turned out to be close to those observed earlier for the polycrystalline platinum.

Effects of Particulate Silica Coatings on Localized Corrosion Behavior of AISI 304SS under Atmospheric Corrosion Conditions

The effects of a coating of silica particles on the localized corrosion behavior of AISI 304 stainless steel (304SS) during drying of thin electrolyte layers in controlled relative humidity environments were investigated by measurements of the transients of open-circuit potential (OCP) and galvanic current. The silica coatings were composed of spherical silica particles and were deposited on 304SS by cathodic electrophoretic deposition. It was confirmed that the silica layer worked very well as a host layer to soak up electrolyte solutions and that it remained intact under wet and dry conditions. OCP and galvanic current transients indicated that the silica layer affected propagation more than initiation of pitting corrosion. The propagation of pitting corrosion for silica-coated samples was slower than for uncoated 304SS.

Electroless silver deposition on polypyrrole and poly(3,4-ethylenedioxythiophene): The reaction/diffusion balance

Processes of spontaneous metallic silver deposition from AgNO 3 solutions on electrodes coated by semi-oxidised conducting polymers: polypyrrole and poly(3,4-ethylenedioxythiophene) were studied. Kinetic information was obtained from open circuit potential transients, showing that for slightly oxidised polymers in quiescent solution the rate of Ag(I) reduction is dependent on Ag(I) ion diffusion in solution. In stirred solutions or for more oxidised polymers the initial step of the process is controlled by new phase (Ag on conducting polymer) formation, as shown using rotating disk electrode voltammetry. The influence of Ag(I) concentration, conditioning time, kind of polymer and its thickness on the amount of accumulated silver was analysed. The reaction-diffusion balance determines where the silver is deposited in the polymer. Although both polymers can be penetrated by Ag + ions, the highest amount of Ag is detected on the polymer surface, as laser ablation coupled with mass spectrometry experiments has shown.

Open circuit stability of underpotentially deposited Pb monolayer on Cu(1 1 1)

The stability of underpotentially deposited (UPD) Pb layer on Cu(1 1 1) is investigated by conventional electrochemical techniques in perchlorate solution at open circuit potential (OCP). In situ scanning tunneling microscopy (STM) is employed to monitor and ascertain structural and morphological changes at characteristic potentials. A corrosion-like mechanism associated with UPD layer stripping powered by reduction processes is found to operate in the system of interest in absence of potential control. OCP transients suggest strong dependence of the Pb layer stability upon the concentration of oxidizing agents, such as oxygen and/or nitrate ions, present in the solution. It is found that the increase of the oxidizing agent concentration results in a proportional decrease of the Pb UPD layer stripping time at OCP. The concentration of the dissolved oxygen is found to affect the UPD layer behavior in the entire range of underpotentials in accordance with the strong affinity of the Pb 2+/Cu(1 1 1) system to the oxygen reduction reaction (ORR). In contrast to oxygen, nitrate ions appear to play active role only in the potential range positive to the UPD peaks where mostly bare Cu surface is in contact with the solution. Specifically adsorbing Cl − ions are examined as a possible inhibitor of the reduction processes operating in the Pb underpotential range. Concentrations of Cl − ions as low as 1 × 10 −4 M are found to stabilize the Pb UPD layer by a factor of 2.5.

Interaction of HCO-substances with adsorbed oxygen on platinum electrodes: Open-circuit transient reactions of HCOOH and CO

Transients of open-circuit potential are measured for the interaction of HCOOH (in 0.5 M H 2SO 4) with oxygen preadsorbed on a Pt/Pt electrode. A monolayer of O ads (ML O ads) in formic acid solutions is removed much more slowly as compared with CO solutions of equal concentrations. Potentiodynamic pulses show that the reduction of ML O ads proceeds most slowly in the regions of great O ads coverages ( θ O ≥ 0.8) for HCOOH solutions and at medium coverages in CO solutions. The difference in the behavior of HCOOH and CO is explained by the fact that chemisorption is dissociative for HCOOH and associative for CO. In the region of medium θ O, E versus τ dependences for both compounds studied are well described by an equation derived for the mechanism of “conjugated reactions” under the assumption that adsorption of HCO-compounds on sites unoccupied by O ads is the limiting stage.

Electrical Double Layer Structure on Electrodes of Platinum Metals: Effect of the Carbon Monoxide Adsorption

Results on the paper's theme are reviewed. Attention is focused mainly on the use of transients of current and open-circuit potential for elucidating changes in the structure of the electrical double layer (EDL) caused by the CO adsorption. It is shown that the high binding energy of CO with surfaces of platinum metals leads to a strong suppression of the total differential capacitance of the electrode, the recharge of the surface, the displacement of strongly adsorbed atoms formed during the adsorption of ions with a total charge transfer, and to other alterations in the EDL plates facing both solution and metal. The importance of allowing for “double-layer” effects when solving problems of electrocatalysis on platinum metals is emphasized.

Development of novel process for Ru CMP using ceric ammonium nitrate (CAN)-containing nitric acid

In the present study, electrochemical and surface analytical techniques were employed to study the mechanism of chemical mechanical planarisation (CMP) process for highly inert noble ruthenium (Ru) film in ceric ammonium nitrate (CAN) additives-containing acidic slurry. The steady-state polarisation curves with surface abrasion exhibited increment in the corrosion potential and cathodic current, suggesting that the partially exposed bare Ru acts as a cathodic site for CAN reduction on Ru 2O 3-covered Ru film as validated by X-ray photoelectron spectroscopy (XPS). Based upon the combined results of open-circuit potential (OCP) and anodic current transients, with scanning electron microscopy (SEM), we may assert that the mechanical removal of Ru 2O 3 enhances the local formation rate of RuO 2 or RuO 4 film, resulting in a loss of Ru by successive galvanic corrosion at the anodic sites. Using CAN-containing acidic slurry, Ru CMP process for manufacturing a bottom electrode for capacitors in DRAM device was satisfactorily accomplished as validated by SEM and atomic force microscopy (AFM).

Copper corrosion in buffered and non-buffered synthetic seawater: a comparative study

The electrochemical behaviour of copper in neutral buffered and non-buffered synthetic seawater and in pure chloride solutions has been studied by cyclic voltammetry, weight loss measurements, open circuit potential and scanning electron microscopy (SEM). Values of the repassivation potentials of Cu in non-buffered and buffered synthetic seawater, at 50 mV s−1, were 0.12 and 0.46 V vs. SCE, respectively. The sharpness, heights and location of the different peaks as well as their charges were shown to be influenced by the composition of the solution, buffering conditions, deoxygenation, polarization potential and time. High chloride concentrations lead to higher oxidation charges. The anodic and the cathodic charges were shown to increase as the chloride concentration increases. The open circuit potential transients of copper in non-deoxygenated, non-buffered synthetic seawater indicate pitting from the beginning of the exposure, while in buffered solutions the pitting appeared only after a quite long exposure period, i.e. after 40 days. Corrosion rates of Cu samples after 3 months of immersion were higher in solutions of pure chloride (0.5 M) than in synthetic seawater. After six months the differences were even more noticeable. SEM images have showed a somewhat higher density of pits on copper samples immersed in the chloride solution (0.5 M), in comparison with those in synthetic seawater.

Open circuit potential transients and electrochemical quartz crystal microgravimetry measurements of dissolution of copper in acidic sulfate solutions

It is demonstrated that open circuit potential transients measurements are able to give the realistic and fast information on the rate of metal corrosion. The results of the studies of the rate of copper corrosion in naturally aerated unstirred acidic sulfate solutions obtained by this method are close to the results of electrochemical quartz crystal microgravimetry measurements. It was found that within the concentration range CuSO 4: 0.001–0.01 M, H 2SO 4: 0.005–0.5 M, independently of whether 0.1 M K 2SO 4 is present in the solution or not, the rate of corrosion of thin (about several tens monolayers) and fresh deposits of copper does not depend on the solution composition and is limited by the transport of the dissolved oxygen. The rate of corrosion of such deposits sometimes exceeds the rate of corrosion of compact copper samples.

Experimental check-up of the relationship between transients of current and open circuit potential for strong adsorption of neutral species and ions on a hydrogen electrode

Transients of current and open circuit potential are measured during carbon monoxide adsorption on Pt/Pt, Rh/Pt, and Ir/Pt electrodes from sulfuric acid and hydrochloric acid solutions, as well as during the adsorption of iodine and iodide ions on Pd/Pt from sulfuric acid solutions. The integral values of transients are shown to agree with those expected theoretically. The possibility of estimating open circuit potential transients based on the data on current transients is analyzed. A combination of transient measurements with other adsorption methods makes it possible to show that in a number of systems the electrode surface is recharged as a result of formation of adsorbed monolayers and to reveal differences in the nature of adlayers formed in solutions of I 2 and I −.

Adsorption Peculiarities of Carbon Monoxide on Iridium Electrodeposits

The transients of current and open-circuit potential observed at CO adsorption on Ir/Pt electrodes in solutions of sulfuric and hydrochloric acids are shown to adequately agree with those theoretically expected. From voltammetric curves of electrooxidation of chemisorbed CO, the values of its adsorption are estimated by making a “proper double-layer correction.” It is concluded that CO is preferentially adsorbed in the bridge form. During CO adsorption from sulfuric acid solutions on an electrode with a preliminarily accumulated monolayer of copper adatoms (Cuad), small negative current are observed and Cuad are displaced by ∼50%. To explain these results, it is assumed that CO is reduced by copper adatoms.

Transients of the Current and Open-Circuit Potential Occurring during the Carbon Monoxide Adsorption on a Rhodium Electrode

Transients of the current and open-circuit potential are measured after bringing carbon monoxide in contact with an Rh/Pt electrode in 0.5 M H2SO4 and 1 M HCl solutions. To interpret these transients from positions of theoretical views on the transients of the current and open-circuit potential that were developed previously for the adsorption of neutral species on a hydrogen electrode, curves of dependences of the total electrode charge on the electrode potential are plotted in the presence and absence of chemisorbed CO on the surface. Good agreement of theory with experiment is established. In 1 M H2SO4, values of the potential of a zero total charge (PZTC) in the absence and presence of COads and values of the “reverse” potential happen to be close, whereas the PZTC in 1 M HCl perceptibly shifts in the positive direction as a result of the CO adsorption, while the reverse potential, conversely, falls in the region of potentials of hydrogen evolution on Rh. As a result of the latter, in 1 M HCl, throughout the entire range of potentials where restrictions of the employed modeling notions are obeyed, in accordance with theory, integrated values of the current transients have only a negative sign, and those of the open-circuit potential transients have only a positive sign.