Rotating Ring Disk Voltammetry Research Articles

A new spin on electrochemistry in the undergraduate lab

Abstract The growing interest in electrochemistry over recent years has sparked an increase in the popularity of various electrochemical techniques, including more advanced methods, that have previously been overlooked in academia and industry. This makes comprehensive hands-on experience in electrochemistry a highly demanded addition to chemistry graduates. However, many students do not receive sufficient training in the theory and experimental design to confidently use and apply various electrochemical techniques throughout their undergraduate, and sometimes even in graduate studies. Here we summarize the theory and practical applications for both rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) techniques. The different modes of operation of rotating ring disk voltammetry, methodologies of data analysis and interpretation as well as the scope of the information that can be extracted from the RDE/RRDE are discussed. Proposed modifications of the laboratory curriculum will allow students to examine and learn valuable information about the reactions on the surface of the electrode/liquid interface. This information will allow chemists to confidently use RDE and RRDE techniques for a wide range of research and development targets. Furthermore, incorporating these techniques into existing chemistry laboratories will help chemistry educators to enrich the undergraduate chemistry curriculum and improve students’ learning outcomes.

(Invited) Investigating the Photoelectrocatalytic Activity of Titania for Nitrogen Fixation Using Rotating Ring-Disk Electrode Voltammetry

Reports of photocatalytic nitrogen fixation on titania have surfaced for nearly eight decades [1]. Demonstrations have occurred in both the gas and aqueous phase, and with various environmental conditions (relative humidity, temperature, pressure). Nearly all experimental observations suggest that abiotic nitrogen photofixation may be possible at ambient temperature and pressure [2,3]. This is impactful as it promotes the possibility for nutrient based fertilizer production from environmentally abundant materials (minerals) using only the sun as a source of energy. However, the reaction pathway that results in ammonia production is not well understood. Furthermore, the wide band gap mineral coupled with the location of the band edges, suggest that nitrogen reduction not likely. Here, we aim to investigate photo-catalytic nitrogen fixation by titania in an aqueous environment through a series electrocatalytic rotating ring disk voltammetry experiments. We demonstrate that this experimental approach is effective at discerning the low-level ammonium concentration intrinsic to photocatalytic nitrogen fixation experiments through monitoring the ammonia oxidation peak at the ring electrode. Results show that that no ammonium is detected without illumination with both a rutile and mix-phased titania photocatalyst. With illumination ammonium was detected with the rutile phase titania, but not with the mixed phase photocatalyst. This suggest that mineral phase plays an important role in promoting nitrogen photofixation. Rotating ring disk electrode experiments may provide a new avenue to attain a higher degree of precision for detecting ammonia at low levels from both photo and electrocatalyst. [1]N. Dhar, E. Seshacharyulu and N. Biswas, Proc. Natl. Acad. Sci., India, 1941,7, 115–131 [2]A. J. Medford and M. C. Hatzell, ACS Catal., 2017, 2624–2643. [3]G. Schrauzer and T. Guth, J. Am. Chem. Soc. , 1977, 99, 7189–7193.

Electrodeposition of Indium from a Phosphonium-Based Ionic Liquid

Electrochemical deposition of indium metal from aqueous electrolytes is challenging as it occurs simultaneously with hydrogen evolution. The evolution of hydrogen can compromise cathodic current efficiency and can cause porosity of the deposited metal, but it can be avoided by using ionic liquid electrolytes. The high thermal stability of ionic liquids allows for high-temperature electrodeposition on a liquid cathode, offering several advantages over a solid electrode, such as easier separation of electrolysis products and the possibility to design a continuous electrowinning process. In this paper, the electrochemical behavior of indium in the ionic liquid trihexyl(tetradecyl)phosphonium chloride (Cyphos IL 101) was studied at 120 °C and 180 °C. Indium metal was electrodeposited on platinum and molybdenum substrates and on a liquid indium cathode at 180 °C. Impurities present in commercial Cyphos IL 101 interfered with the measurement of the electrochemical window, so that it had to be carefully purified. The electrochemistry of indium in Cyphos IL 101 was studied by cyclic voltammetry, QMCB, rotating disk and rotating ring disk voltammetry. InCl3 concentration between 10 and 400 mM in Cyphos IL 101 were used in this study. Therefore, the major indium(III) species in solution corresponds to the [InCl6]3– complex. Using the Levich equation, a diffusion coefficient of (7.5 ± 0.1) 10–12 m2 s–1 at 120 °C was calculated for [InCl6]3–. It was found that indium(III) is the most stable oxidation state in the ionic liquid. This ion is reduced via a two-step process, going from indium(III) to indium(I) and subsequently to indium(0). The disproportionation of indium(I) to indium(III) and indium(0) was clearly illustrated. During cyclic voltammograms, the anodic and cathodic peak potentials was found to depend on the scan rate. As the scan rate increases, the separation between the anodic and cathodic peaks becomes greater, indicating that the deposition of indium is irreversible or quasi-reversible. The indium electrodeposits were characterized by SEM-EDS. When depositing indium on molybdenum substrates, the deposition of spherical indium droplets was observed, even at temperatures substantially below the melting point of indium (see figure). The probable mechanism behind this deposition morphology will be explained. Figure 1

Development of Nanostructured-Carbon-Supported Gold Nanoparticles As Catalysts for Electroreduction of Oxygen and Carbon Dioxide

There has been growing interest in the field of oxygen electroreduction, particularly with respect to potential applications in the science and technology of low-temperature fuel cells.. Obviously, many efforts have been made to develop suitable alternative electrocatalysts efficient enough to replace electrocatalysts based on scarce strategic elements such as platinum-group metals. Despite intensive research in the area, there are still a number of fundamental problems to be resolved, and the practical oxygen reduction catalysts still utilize systems based on platinum. Various homogeneous and heterogeneous catalytic systems have been proposed to induce the CO2 reduction and, depending on reaction conditions (applied potential, strength of the electrolyte, local pH, local concentration of CO2 and the catalyst applied), various products can be obtained. Because the CO2 molecule is very stable, its electroreduction would be characterized by large over-potentials. In this respect, behavior of carbon dioxide resembles that of dioxygen. In both cases, the limiting steps involve protonation of the respective species. The present study refers to a novel and unique approach of fabrication and deposition of gold nanoparticles on the surfaces of reduced graphene oxide and multi-walled carbon nanotubes. Among important issues is application of inorganic (rather than organic) capping ligand, heteropolymolybdate, to modify and stabilize (as well as probably also to link with the oxygen or hydroxyl groups on graphene surfaces) gold nanoparticles. During operation in alkaline medium or neutral media, polyoxometallates disappear but catalytically highly active gold remains and it exhibits excellent stability. The resulting material has occurred to show highly potent electrocatalytic properties toward electroreductions of carbon dioxide in neutral medium and oxygen in alkaline solution. What is even more important is that both carbon nanotubes and graphene have occurred to act effectively as carriers for gold nanostructures. Mutual activating interactions are feasible. The conclusions are reached on the basis of diagnostic electrochemical (e.g. rotating ring disk voltammetry), spectroscopic (FTIR) and microscopic (SEM, TEM) experiments. A series of comparative experiments with different carbon carriers and model catalytic materials (e.g. Vulcan-supported platinum) have also been performed. With respect to oxygen reduction, our diagnostic experiments at different concentrations of H2O2, support a view that the effect of the fast following chemical (H2O2-reductive-decomposition) reaction could be the dominating factor in explaining the observed positive potential shift observed during the oxygen reduction. The fact, that the optimum graphene-based catalytic system produced the oxygen reduction peak current comparable to that observed at the model platinum containing catalyst, would imply the efficient four-electron-type reduction mechanism.

Highly-Acidic Mixed-Metal-Oxides (WO3-ZrO2) As Active Supports for Dispersed Metal Centers: Enhancement of Electrocatalytic Reduction of Oxygen and Carbon Dioxide

Application of metal oxides as active matrices in electrocatalysis is particular importance. The hydrous behavior, which favors proton mobility and affects overall reactivity, reflects not only the oxide’s chemical properties but its texture and morphology as well. What is of importance to electrochemical science and technology, certain nonstoichiometric mixed-valence oxides could exhibit pseudo-metallic conductivity and possess appreciable catalytic activity. Recently, mixed oxide systems stabilized through Zr-O-W bonds have been demonstrated to be very attractive acid catalysts exhibiting high catalytic activities and good stabilities in many demanding industrial reactions. For electrocatalytic applications, mixed-valent tungsten(VI,V) oxide and zirconium(IV) oxide have been sequentially deposited and integrated through voltammetric potential cycling to form sub-microstructured films on glassy carbon electrode. The mixed WO3/ZrO2 systems are characterized by fast charge (electron, proton) propagation during the system’s redox transitions. By dispersing metallic Pt or Cu electrocatalytic nanoparticles over such active WO3/ZrO2 supports, the electrocatalytic activities of the respective systems toward the reduction of oxygen or carbon dioxide have been enhanced even at decreased loadings in acid media. The enhancement effects should be attributed to features of the mixed metal oxide support such as porosity and high population of hydroxyl groups (due to presence of ZrO2), high Broensted acidity of sites formed on mixed WO3/ZrO2, fast electron transfers coupled to unimpeded proton displacements (e.g. in HxWO3), as well as strong metal-support interactions between nanosized metals (Pt or Cu) and the metal (W, Zr) oxo species. The fact that WO3/ZrO2 nanostructures are in immediate contact with the metallic catalytic sites leads to the specific interactions (via the surface hydroxyl groups) with the reaction intermediates (H2O2 or CO adsorbates). Further mechanistic studies involving rotating ring disk voltammetry and gas-diffusion electrode measurements are planned in addition to spectroscopic and XPS measurements. The mixed-metal systems will also be considered together with nanostructured carbon carriers such as reduced graphene oxide, carbon nanotubes or carbon black.

Anchoring of ultrafine Co3O4 nanoparticles on MWCNTs using supercritical fluid processing and its performance evaluation towards electrocatalytic oxygen reduction reaction

Anchoring ultrafine Co3O4 nanoparticles on MWCNTs using a supercritical fluid showed high ORR performance.

Carbon-Supported Gold and Silver Nanoparticles As Catalysts for Electroreduction of Oxygen in Alkaline Media

The present study refers to a novel and unique approach of fabrication and deposition of gold nanoparticles on the surfaces of reduced graphene oxide and multi-walled carbon nanotubes. Among important issues is application of inorganic (rather than organic) capping ligand, heteropolymolybdate, to modify and stabilize (as well as probably also to link with the oxygen or hydroxyl groups on graphene surfaces) gold nanoparticles. During operation in alkaline medium or neutral media, polyoxometallates disappear but catalytically highly active gold remains and it exhibits excellent stability. The resulting material has occurred to show highly potent electrocatalytic properties toward electroreductions of carbon dioxide in neutral medium and oxygen in alkaline solution. What is even more important is that both carbon nanotubes and graphene have occurred to act effectively as carriers for gold nanostructures. Mutual activating interactions are feasible. The conclusions are reached on the basis of diagnostic electrochemical (e.g. rotating ring disk voltammetry), spectroscopic (FTIR) and microscopic (SEM, TEM) experiments. A series of comparative experiments with different carbon carriers and model catalytic materials (e.g. Vulcan-supported platinum) have also been performed. With respect to oxygen reduction, our diagnostic experiments at different concentrations of H2O2, support a view that the effect of the fast following chemical (H2O2-reductive-decomposition) reaction could be the dominating factor in explaining the observed positive potential shift observed during the oxygen reduction. The fact, that the optimum graphene-based catalytic system produced the oxygen reduction peak current comparable to that observed at the model platinum containing catalyst, would imply the efficient four-electron-type reduction mechanism. Graphene and other distinct carbon nanostructures are explored as supports (carriers) for dispersed metallic silver nanocenters which exhibit electrocatalytic activity toward reduction of oxygen in alkaline media. To facilitate fabrication, immobilization and distribution of Ag(0) nanoparticles, various types of graphene (e.g. reduced graphene oxide, RGO, and CFx graphene with carbon black) have been modified with the silver analogue of polynuclear Prussian Blue, namely with ultra-thin silver(I) hexacyanoferrate(II,III) layers. Following the heat-treatment step at temperatures as high as 400-600oC, some thermal decomposition of the cyanometallate network occurs and, consequently, metallic silver sites are generated. Their formation and distribution are facilitated by the voltammetric potential cycling in KOH electrolyte. The most promising electrocatalytic results with respect to the reduction of oxygen (the highest currents and the most positive electroreduction potentials) have been obtained when graphene nanostructures are combined or intermixed with Vulcan XC-72R nanoparticles. What is even more important that, due to the presence of the polynuclear cyanoferrate modifier or linker, the amounts of the undesirable hydrogen peroxide intermediate are significantly decreased.

Probing the Lithium-Sulfur Redox Reactions By Rotating-Ring Disk Electrode Measurements

Introduction Rechargeable lithium-sulfur batteries promise to provide a 2 to 3 times higher specific energy than conventional lithium rechargeable batteries. Detailed mechanistic understanding of sulfur redox reactions is critical for developing efficient and stable lithium-sulfur batteries. It is proposed that the solvent´s ability to solvate the primary polysulfide reduction products will influence the reduction/oxidation process.1 However, in lots of studies, the maximum reduction charge obtained is ~4 e-/S8 for potentials as low as +1 to +1.5 V vs. Li/Li+, i.e., significantly lower than the 12 to 16 e-/S8 which are obtained for long-term discharge curves (fraction of hours to hours) in battery cells using electrodes based on high-surface area carbons.2-5 Here, we examine the differences in sulfur charge/discharge behavior in low (DOL:DME) vs. high-dielectric solvents (DMSO) and over short and long time scales using rotating ring disk voltammetry (RRDE) on flat model electrode6 and galvanostatic characterization in a specially designed Li-sulfur catholyte battery cell, respectively. Results and Discussion Figure 1 shows the ring (Fig. 1a), disk (Fig. 1b) current and kinetic current (Fig. 1c) of sulfur reduction in DMSO and DOL:DME(1:1). By using Levich-Koutecky slopes, the number of electrons exchanged during the reaction can be obtained. For DMSO (blue lines/symbols in Fig. 1), the first reduction wave (~ -0.9 to ~ -1.5 VFc corresponds to a 2-electron reduction of S8 to S8 2- and has a Tafel slope of ~90 mV/decade. Considering the similar Tafel slope observed for the 1st and 2nd reduction waves, we hypothesize that the 2nd reduction wave is likely the series of two 2-electron processes with the second step being rate limiting (i.e., S8 + 2 e- --> S8 2- followed by S8 2- + 2 e- --> S8 4- --> 2 S4 2-). For DOL:DME (red lines/symbols in Fig. 1) only one reduction step corresponding to ~5 e-/S8 is observed. This could be related to the solvation ability of the solvent. The two well-separated reduction waves observed in DMSO can be rationalized by strongly solvated and thus energetically stabilized S8 2- anions so that their further electrochemical reduction requires a significantly more cathodic potential. On the contrary, if S8 2- is weakly solvated and its formation is thus energetically unfavorable, its formation might be delayed until more cathodic potentials are reached at which point multiple electron transfer reactions might occur. The influence of the solvent’s solvation power on the sulfur reduction processes and the implications for Li-S batteries will be discussed Acknowledgments The authors are grateful to Dr. Juan Herranz-Salaner for RRDE cell design and scientific discussion, Dr. Himendra Jha for experimental assistance and scientific discussion, and Johannes Hattendorff for the measurement of solvent viscosity. Financial support of this research by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology performed under the auspices of the EEBatt project. Figure 1. Capacitively, non-Ohmically corrected ring (a) and disk (b) currents recorded at 50 mV/s in Ar-saturated 4 mM S8 – 0.2 M LiClO4 DMSO (Blue) and 4 mM S8 – 1.0 M LiTFSI DOL:DME(1:1) (Red) at rotation rates between 100 and 1600 rpm and continuously holding the Au ring electrode at 0.494 VFc for DMSO and at 0.557 VFc for DOL:DME. (c) Kinetic currents measured in DMSO and DOL:DME. 60 mV/dec and 120 mV/dec lines are provided as a guide-to-the-eye for comparison. Note that 0 VFc = +3.756 VLi in DMSO and 0 VFc = +3.283 VLi in DOL:DME.

Enhancement of Electrocatalytic Activity of Novel Ruthenium Based Nanocatalyst Supported on Nitrogen Doped Carbons for Oxygen Reduction

Herein, ruthenium nanoparticles modified with mixed selenium and nitrogen overlayers (RuSexNy) were prepared via chelatation method onto the N-doped and undoped carbons (Norit and MWCNTs) under analogous experimental conditions. . The crystalographic data reveals the same type of crystallographic phases in RuSexNy crystallites, regardless of the doping of carbon support, nevertheless, as transmission electron microscopy measurements have shown the nanoparticles fabricated onto the N-doped carbons were found to be of smaller diameters. The studies of oxygen reduction reaction in acid (0.5 M H2SO4) medium using rotating ring disk voltammetry and impedance spectroscopy show that RuSexNy nanoparticles supported on the N-doped carbons exhibit higher ORR catalytic activity with generation of lower amounts of hydrogen peroxide. The spillover effect due to N- doping of carbon support was suggested to be responsible for improved decomposition of adsorbed ORR intermediates in carbon supported RuSexNy materials.

Electrocatalytic Activity toward Oxygen Reduction of RuSxNy Catalysts Supported on Different Nanostructured Carbon Carriers

Ruthenium admixed or nanoparticles modified with mixed selenium and nitrogen overlayers (RuSexNy) were fabricated onto different carbon nanostructured supports (that include multiwalled carbon nanotubes, Vulcan and Norit) under analogous experimental conditions. It is apparent from X-ray measurements that both Vulcan and Norit supported RuSexNy crystallites are characterized by the same type of crystallographic phases. Important parameters that influence electrocatalytic activity include the nature of functionalization of carbon support, in addition to its active surface area and morphology that affect distribution and size of active RuSexNy centers. Despite the same loading of ruthenium (40 μg cm−2), diameters of RuSexNy nanoparticles have somewhat differed and ranged from 2 to 6 nm, as it has been evident from transmission electron microscopy measurements. The systems have been evaluated with respect to oxygen reduction reaction in acid (0.5 M H2SO4) medium using rotating ring disk voltammetry and impedance spectroscopy. Having in mind such parameters as the half-wave reduction potential, the electroreduction current density, the heterogenous rate constant, the percent formation of the hydrogen peroxide undesirable intermediate as well as the overall charge transfer resistance appearing during oxygen reduction, the RuSexNy-based system utilizing functionalized Vulcan is characterized by the highest electrocatalytic activity.

Manganese dioxide nanotube and nitrogen-doped carbon nanotube based composite bifunctional catalyst for rechargeable zinc-air battery

Abstract A composite bifunctional catalyst (MnO2–NCNT) was prepared from manganese dioxide (MnO2) nanotubes and nitrogen-doped carbon nanotubes (NCNT) for the purpose of oxygen reduction (ORR) and evolution (OER) catalysis in the rechargeable zinc-air battery. From the half cell test, the MnO2–NCNT composite illustrated excellent activities towards ORR and OER in alkaline conditions. Based on the battery test, the composite catalyst displayed outstanding discharge and charge performance while maintaining good stability. In both cases, the marked performance improvements from MnO2–NCNT compared favourably to the NCNT and MnO2, which are the constituents of the composite. In particular, MnO2–NCNT exhibited improved half wave potential by 220 mV compared to MnO2 and much superior OER stability compared to NCNT based on the rotating ring disk voltammetry results. According to battery test, MnO2–NCNT decrease the battery resistance by 34% and concurrently improved the durability, discharge and charge performance in comparison to the MnO2 nanotubes.

Electrochemical Study on Corrosion Inhibition of Copper in Hydrochloric Acid Medium and the Rotating Ring-Disc Voltammetry for Studying the Dissolution

Dissolution characteristics of copper in hydrochloric acid medium and the effect of 4-amino 1,2,4-triazole (ATA) on the corrosion process have been studied using conventional electrochemical techniques and rotating ring-disc electrodes (RRDEs). Corrosion potential () and corrosion current density () were obtained by Tafel extrapolation methods. Charge transfer resistance () and double-layer capacitance () were obtained from the electrochemical impedance spectroscopy (EIS). ATA was shown to be an effective inhibitor for the copper-corrosion inhibition in acid medium. The corrosion rate was retarded in presence of inhibitors mainly because of the adsorption of the inhibitor on the electrode surface. Adsorption of the inhibitor on the metal surface was found to follow the Langmuir adsorption isotherm. Standard free energy change of the adsorption process () was calculated to be −54.3 kJ mol−1; such a large negative value of suggests the prescence of a chemisorption process.

Modification of Pt nanoparticles with polyoxometallate monolayers: Competition between activation and blocking of reactive sites for the electrocatalytic oxygen reduction

Surfaces of bulk platinum and unsupported (Vulcan-free) Pt nanoparticles, that are modified and stabilized with such Keggin type heteropolyacids of molybdenum and tungsten as H 3PMo 12O 40 and H 3PW 12O 40, have been characterized using cyclic voltammetry, FTIR (by reflectance), as well as transmission and scanning electron microscopies. The presence of the polyoxometallate monolayer on platinum results in the partial suppression of the interfacial formation of PtOH/PtO oxides. Both molybdates and tungstates seem to interact with Pt surface via their corner oxygen atoms. The existence of spacious, largely hydrated, polyoxometallate monolayers on platinum does not block access of reactant (oxygen) to the catalytic Pt sites. The electrocatalytic properties of H 3PMo 12O 40 and H 3PW 12O 40 modified Pt nanoparticles towards reduction of oxygen in acid medium have been examined and compared using rotating ring–disk voltammetry. Reactivity of Pt-free H 3PMo 12O 40 and H 3PW 12O 40 with respect to reduction of hydrogen peroxide has also been considered. Our results clearly show that modification of Pt nanoparticles with PW 12 (but not with PMo 12) results in the enhancement of the electrocatalytic reduction of oxygen.

金属種を複合したポリピロール錯体担持炭素粒子の酸素還元触媒活性

We reported that carbon black modified with cobalt and various transition metal ions (M) incorporated in a polypyrrole film (Co+MPPy/C) were good electrocatalysts for the reduction of oxygen (O2). After heat treatment of the catalysts at 600 to 650°C under argon atmosphere, which were prepared using with iron and iridium ions as M, the resulted catalysts gave rise to the reduction of O2 at a remarkably positive potential (Ep>0.40 V vs SCE). Rotating ring disk voltammetry (RRDV) revealed that Co+IrPPy/C reduced O2 completely with four electrons (nav=4.0). X-ray photoelectron spectroscopy and X-ray diffraction indicated that the cobalt and M ions were coordinated with nitrogen as the donor atoms and the active sites were maintained even after the heat treatment under argon atmosphere without aggregation of metals and formation of alloys.

Preparation and electrocatalytic properties of the TBO/nafion chemically-modified electrodes

Abstract The glassy carbon electrode modified with nafion and toluidine blue O (TBO) was prepared and their electrochemical properties were investigated. The TBO molecules were strongly and irreversibly adsorbed on the nafion-modified glassy carbon surface. The electrochemical behavior and mechanism for interactions of the TBO molecules with the nafion film were investigated through cyclic voltammetric and UV–vis spectra methods. The electrocatalytic reductions of nitric oxide and oxygen were performed at this modified electrode by cyclic voltammetry, rotating ring disk voltammetry and chronoamperometry as diagnostic techniques. The nafion membrane played a duel role as a matrix for the TBO immobilization and also helped to partitioning the NO from the solution phase. The diffusion coefficients of nitric oxide and oxygen at the TBO/nafion/GC modified electrode were calculated using chronoamperometric technique and rotating ring disk voltammetry, respectively. The ‘ D ’ value for nitric oxide and oxygen obtained by the above methods were found as 1.2798 × 10 −6 and 1.4 × 10 −5 cm 2 s −1 , respectively. From the intercept value of the Koutecky–Levich plot, the catalytic reaction rate constant, k , was calculated for O 2 reduction and found as 6.6 × 10 4 M −1 S −1 .

Surface-Enhanced Resonance Raman Scattering and Voltammetry Study of the Electrocatalytic Reduction of Oxygen by the μ-Oxo Dimer of Iron(III) Tetra-4-N-methylpyridylporphyrin

In this study, surface-enhanced resonance Raman scattering, SERRS, and voltammetric techniques (cyclic voltammetry, rotating disk electrochemistry, and rotating ring disk electrochemistry) are used to elucidate the mechanism of oxygen reduction on a silver electrode with iron(III) tetra-4-N-methylpyridylporphyrin. The results indicate that after the oxidation reduction cycle at pH 10 and pH 4, the iron porphyrin is adsorbed on the silver surface as a high-spin, five-coordinated μ-oxo-bridged dimer. On the forward voltage scan, this dimer undergoes two successive one-electron reductions at about −0.3 and −0.5 V to form a high-spin fully reduced Fe(II) μ-oxo-bridged dimer. SERRS spectra show that the first electron transfer forms a mixed-valence Fe(II)−O−Fe(III) μ-oxo-bridged dimer system. At pH 2, only the high-spin reduced Fe(II) monomer is adsorbed on the surface. At pH 4 and pH 10, a shift in the reduction of the oxygen overpotential by approximately +150 mV is observed and the active catalyst in this process is attributed to the partially reduced μ-oxo dimer. A value of n ≈ 4 was calculated for the number of electrons involved in this catalytic reaction, and rotating ring disk voltammetry showed that only 10% stable hydrogen peroxide species is produced on the dimer-modified surface. These results are used to formulate a mechanism for the catalytic reduction of oxygen.

The transpassive dissolution mechanism of highly alloyed stainless steels: I. Experimental results and modelling procedure

The transpassive dissolution of austenitic stainless steels (AISI 316L, AISI 904L, 254SMO and 654SMO) in a 0.5 M sulphate solution with pH 2 was studied by conventional and rotating ring–disc voltammetry, as well as electrochemical impedance spectroscopy. The main process in the transpassive potential region was found to be the release of soluble Cr(VI), while small amounts of lower-valency Cr or Mo species are released as well. Secondary passivation readily occurs for AISI 316L, whereas the remaining highly alloyed steels dissolve at high current densities in the whole potential range studied. The dissolution rate was found to increase in the order AISI 904L<254SMO<654SMO. Thus it can be correlated to the increase in the Cr and especially Mo content of the steel substrate. The impedance spectra contain contributions from the transpassive dissolution of Cr and secondary passivation, probably due to enrichment of Fe in the outermost layer of the surface film. A kinetic model of the process is proposed, including a two-step transpassive dissolution of Cr via a Cr(VI) intermediate and the dissolution of Fe(III) through the anodic film. The model was found to be in quantitative agreement with steady state current vs. potential curves and electrochemical impedance spectra. The kinetic parameters of transpassive dissolution were determined and the relevance of their values is discussed.

The stability of the passive state of iron–chromium alloys in sulphuric acid solution

The passivation and the transpassive dissolution of Fe–Cr alloys (12% and 25% Cr) was studied with a combination of electrochemical techniques—conventional and rotating ring–disk voltammetry, impedance spectroscopy and the contact electric resistance (CER) technique developed to measure the dc resistance of surface films. Rotating ring–disk studies indicated that both soluble Cr (VI) and Fe (III) are released from the alloys in the transpassive region. The electronic resistance of the transpassive anodic film was found to decrease as Cr (VI) is released from the outermost layers adjacent to the interface and to increase subsequently due to the formation of a Fe (III) rich secondary passive film. Impedance spectra of the Fe–25% Cr alloy were found to include contributions from both the film growth and transpassive dissolution reactions, whereas the corresponding spectra of the Fe–12% Cr alloy reflected mainly the contribution of the film. On the basis of the experimental results, a generalized model of the transpassivity of Fe–Cr alloys is proposed. The model represents the anodic film as a highly doped n-type semiconductor–insulator–p-type semiconductor (n–i–p) structure. Injection of negative defects at the film/solution interface results in their accumulation as a negative surface charge. It alters the non-stationary film growth rate controlled by the transport of positive defects (oxygen vacancies). The transpassive dissolution reaction is assumed to be a two-stage process featuring a Cr (IV) intermediate. The relaxation of the Fe fraction in the outermost cation layer of the film is taken into account as well. Fitting of the experimental data on the basis of equations derived for the steady state and impedance response enable the determination of the kinetic parameters of transpassive dissolution.

Electrochemical Catalytic Reduction of Oxygen by a Self-Doped Polyaniline–Co-Porphyrin Complex-Modified Glassy Carbon Electrode

The four-electron reduction of oxygen to water takes place on a glassy carbon electrode by electrocatalysis using the Co-porphyrin–polyaniline complex. The four-electron transfer processes were confirmed by electrochemical measurements such as cyclic voltammetry, rotating disk voltammetry (RDV), and rotating ring-disk voltammetry (RRDV). The number of electrons transferred was determined on the basis of the Koutecky–Levich plot in RDV. Large amounts of H2O2 formation were not detected by RRDV. RRDV revealed that the direct four-electron reduction of oxygen proceeded without the formation of H2O2. The process obeys a mechanism in which the four-electron reduction takes place through a multiple electron injection from the polyaniline to the cobalt complex after electron transfer from Co to oxygen. The four-electron reduction of oxygen also proceeds in a pseudo-neutral medium (pH 6.2). Poly(2,3-dicarboxyaniline) acts as an excellent electron mediator from the electrode to the Co-porphyrin matrix and shows stable redox activity over a wide pH range due to the self-dopable carboxy substituents in the polymer chain.

Electrochemically modulated complexation process for gas removal and concentration

AbstractAn electrochemically modulated complexation process was designed to extract and concentrate a gas‐phase solute. The process was based on flowthrough electrolysis cells that electrochemically modulate a liquid‐phase mass‐transfer agent between high and low solute affinity forms. The liquid phase in the high‐affinity form contacts a feed gas phase for the extraction and contacts the receiving gas phase in the low‐affinity form. The chemical system used to demonstrate the general process was a Cu(II)/Cu(I) redox couple in an acidic chloride electrolyte to both concentrate carbon monoxide against a pressure gradient and selectively separate it from a mixture with nitrogen. Experimental results and modeling of this process are reported. The kinetics of the electrolysis reactions in the flow cells are discussed within the context of rotating ring disk voltammetry experiments.