Oxygen Redox Reactions Research Articles

A Broad Stability Investigation of Nb-Doped SrCoO2.5+δ as a Reversible Oxygen Electrode for Intermediate-Temperature Solid Oxide Fuel Cells

The present work reports a systematic study on the structural, thermal, electrical and electrochemical stability of SrCo1−xNbxO2.5+δ series as a potential reversible oxygen-electrode for intermediate-temperature solid oxide fuel cells. The identified best composition is x = 0.10, which exhibits a stable pseudo primitive cubic structure at <700°C and a reversible oxygen redox reaction at 350°C. The conductivity of this material is p-type and also exhibits a peak at 350°C, implying that the electron hole conduction is closely associated with the oxygen nonstoichiometry. Electrochemical impedance spectroscopy analysis indicates a low polarization resistance rate-limited by a slower surface O2 dissociation step. Overall, the material is thermally stable and oxygen redox reversible below 700°C, above which a catalytically less active brownmillerite SrCoO2.5 is formed.

Surface studies of lithium–oxygen redox reactions over HOPG

The O2/Li2O2 electrode reaction has been studied on low surface area HOPG electrodes in 0.1M LiPF6 in dimethyl sulfoxide (DMSO) electrolyte. Studies were performed using electrochemical cells coupled to a XPS spectrometer and to an AFM microscope. AFM images after electrochemical treatment at cathodic potentials exhibited 20 to 100nm in height features, whereas anodic treatment showed a thin film of about 1nm thickness deposited over the HOPG electrode. XPS spectra after electrochemical treatment showed surface species due to DMSO and LiPF6 decomposition. These findings indicate the high reactivity of oxygen reduction products towards the electrolyte and the solvent. The unwanted deposits formed under electrochemical operation cannot be completely eliminated from the surface even after applying high anodic potentials. This highlights the known loss of capacity of Li-air batteries, issue that must be overcome for successful applications.

Boron and Nitrogen Co-Doped Porous Carbon Materials Derived from Orange Peels As an Electrocatalyst for the All-Vanadium Redox Flow Batteries

Nowadays, intensive efforts have been achieved to overcome the drawbacks of carbon felts by using surface functionalities for abundant active sites or increasing electron conductivities. In this regard, various catalyst materials have been reported, which is summarized by metal- or carbon-based materials. The metal-based catalysts (Pt, Bi, and Ir) have demonstrated improved cycle life and catalytic activity towards the vanadium redox reactions. Pacific Northwest National Laboratory (PNNL) has reported novel electrocatalysts that employ a bismuth nanoparticle or niobium oxide nanorod. In contrast, carbon-based catalysts have been highlighted because of its stability in concentrated-acidic condition and high electrical conductivity. Modified multi-walled carbon nanotubes (MWCNTs), carbon nanotubes/nanofibers (CNTs/CNFs) composite, N-doped CNTs, N-doped graphene, graphene oxide (GO), GO/MWCNT have been developed. However, most of them use expensive materials such as CNTs, which is not cost-effective.Cost-effective catalysts derived from biomass for energy storage materials such as ORR catalysts, supercapacitors, and VRFBs have been reported using peanut shell, coconut shell, walnut shell, bamboo, sunflower seed shell and so on. Recently, varieties of porous carbon materials that are derived from plant biomass have been synthesized. For example, Chen et al. used Typha orientalis to synthesize nitrogen-doped nanoporous carbon nanosheets for oxygen redox reaction (ORR) catalyst. Gao et al. presented nitrogen-doped carbon by using amaranthus for ORR catalyst. Madhavi et al. used coconut shell as the mesoporous carbon electrode for VRFBs. Yet, the orange peels have not been introduced as a bio-derived carbon precursors. Orange processing in the United States produces ~700,000 tons of peel wastes annually. A orange peel has unique heterogeneous structure, including cellulose, hemicelluloses, lignins, pectins, free sugars, proteins. Lignin can play a key role in order to convert orange peel into porous carbons through pyrolysis. Proteins such as ceramide in orange peels also exclude the need for external nitrogen sources such as NH3gas. Heteroatom doping methods have been widely used for various applications such as ORR catalysts or supercapacitors electrodes. In general, heteroatom doping on carbon framework enhances electron conductivity, electrolyte permeability, and ion-accessibility on the electrode surface. In case of nitrogen atoms, the defective active sites and charge delocalization in carbon atoms can be formed by doped nitrogen atoms. As for boron atoms, the catalytic activity of the carbon-based catalysts can be attributed to the asymmetric spin density of carbon atoms induced by electron-deficient boron doping. This positively charged boron atom is favorable to capture of the reactants. In particular, the co-doping with two elements (e.g. B, P, or S with N) enhances much more catalytic activities than singly-doped catalysts. Difference in electronegativity (x) induces asymmetric spin density of atoms, enhancing the catalytic activities. Thus, the synergetic effects among different heteroatoms can be expected by using boron atoms with lower electronegativity (x = 2.04) and/or nitrogen atoms with higher electronegativity (x = 3.04) than carbon atoms (x= 2.55). To find optimized conditions for boron and nitrogen co-doping as an electrocatalyst for vanadium redox reactions, we suggest environmentally friendly and cost effective method for porous carbon catalysts by a self-contained nitrogen in orange peels with additional boron co-doping. This heteroatom co-doped porous carbon coated with the ketjenblack nanoparticles (KB-BNPC) includes defect-rich active sites. Improved catalytic activities toward vanadium redox couples of V2+/V3+ and VO2 +/VO2+ were demonstrated, exhibiting increased redox onset potentials and peak current. Notably, we controlled the doping ratio of B and N atoms, suggesting that B/N ratio of 0.65 gives rise to synergistic effect of the heteroatoms for vanadium redox reactions. We believe that the optimized porous structures and defects in the KB-BNPC catalyst facilitate the absorption and desorption of the vanadium ions by enhancing electron and mass transfer kinetics.

Effect of core and shell compositions on MeOx@LaySr1−yFeO3 core–shell redox catalysts for chemical looping reforming of methane

The chemical looping reforming (CLR) process converts methane into syngas through cyclic redox reactions of an active lattice oxygen (O2−) containing redox catalyst. In CLR, methane is partially oxidized to CO and H2 using the active lattice oxygen of a redox catalyst. In a subsequent step, the oxygen-deprived redox catalyst is regenerated by air. Such a process can eliminate the need for steam and/or oxygen in reforming, thereby improving methane conversion efficiency. A number of perovskite-structured mixed metal oxides are known to be active for CLR. However, the oxygen storage capacity of perovskites tends to be low, limiting their practical application in chemical looping. In contrast reducible metal oxides such as cobalt and iron oxides can store up to 30wt.% lattice oxygen but are less selective for syngas generation. We explore oxygen carriers that utilize the advantages of both perovskites and first-row transition metal oxides by integrating a transition metal oxide core with a mixed ionic–electronic conductive (MIEC) perovskite support/shell. MIEC perovskites facilitate countercurrent conduction of O2− and electrons, allowing facile O2− transport though the solid. It is proposed that this conduction allows rapid oxygen transport to and from the transition metal oxide cores irrespective of the porosity of the redox catalyst. In this work, we show that MeOx@LaySr1−yFeO3 can be an excellent model catalyst system for CLR. The activity, selectivity, and coke resistance of the core–shell system can be tuned by changing the ratio of La to Sr in the perovskite shell and the type of transition metal oxide in the core. Our studies indicate that lower Sr loadings can improve activity and selectivity of the catalyst for methane partial oxidation, but make the LSF shell less resistant to decomposition during the reduction step.

Pd-Co/CNF Composite Material as Electro-Catalysts for Lithium-Oxygen Batteries

Lithium-Oxygen (Li-O2) batteries have attracted much attention owing to their high energy densities compared to conventional lithium ion batteries, and are one of the promising lithium secondary batteries for electric vehicles (EVs) and hybrid electric vehicles (HEVs). The theoretical specific energy density of the Li-O2 battery with the discharge reaction of 2Li++O2+2e-→Li2O2 is 3505 Wh kg-1. However, the low energy efficiency, poor rate capability, and limited cycle life of these batteries cause problems for their commercialization. The sluggish oxygen redox reaction results in low power and high overpotentials. Also, the pores of oxygen cathode can easily be blocked by insoluble discharge products, which cause incomplete discharge, inhibiting the flow of oxygen to the reaction sites. Therefore, the design of cathodes having oxygen diffusion pathway and uniform distribution of catalysts is the key role which can enhance the performances of Li-O2 batteries. This work focuses on the electrochemical effects of palladium-cobalt (Pd-Co)/carbon nanofiber (CNF) composite catalyst for a cathode. The nanosized Pd-Co catalysts are embedded evenly into CNFs. The high catalytic activity of Pd-Co catalysts reduce the overpotentials, which can improve the energy efficiency. Furthermore, the interconnected pore network structure creates effective gas diffusion channels, thus improving specific capacity and cycle life for Li-O2 cell. PdCo/CNF composite catalysts are synthesized by using polyol method and electrospinning method. Cells were tested at the current densities of 100 mA g-1 and 200 mA g-1with limited voltage of 2.0 to 4.5V. The morphology of the samples was observed on a field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). The structure of the samples was characterized by X-ray diffraction (XRD) spectroscopy and Inductively coupled plasma atomic emission spectrometry (ICP-AES). The result was analyzed by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV).[1] Y.C. Lu, B.M. Gallant, D.G. Kwabi, J.R. Harding, R.R. Mitchell, M.S. Whittingham, Y. Shao-Horn, Energy Environ. Sci., 6, 750 (2013).[2] L.J. Hardwick, P.G. Bruce, Curr. Opin. Solide. St. M., 16, 178 (2012).[3] M.A. Rahman, X. Wang, C. Wen, J. Appl Electrochem, 44, 5 (2014)[4] W. Wang, D. Zheng, C. Du, Z. Zou, X. Zhang, B. Xia, H. Yang, D.L. Akins, J. Power Sources, 167, 243 (2007)[5] B.W. Huang, X.Z. Liao, H. Wang, C.N. Wang, W.S. He, Z.F. Ma, J. Electrochem. Soc. 160, A1112 (2013)

Invited Presentation: Ionic Liquid-Based Lithium/oxygen Battery

In the last few years, much research effort is being focused on rechargeable lithium–air (O2) batteries which feature a theoretical specific energy comparable to that of gasoline. While these cutting-edge batteries are at their early stage of development with many challenges to be overcome, they are the most promising candidates to power electric vehicles with long driving range. In the frame of the European LABOHR Project we are investigating the use of hydrophobic ionic liquid (IL)-based electrolytes in these batteries. Basic studies of the oxygen redox reaction in N-butyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide at home-made catalyst-free porous carbon electrode demonstrated that O2mass transport in IL becomes crucial at the fastest battery discharge rates, which are of interest for automotive applications. The study also demonstrated that it is possible to overcome this limitation by new flow-cell designs, and the main achievements are reported and discussed. Acknowledgement Work funded by the European Commission in the 7th Framework Programme FP7-2010-GC-ELECTROCHEMICAL STORAGE, under contract no. 265971 “Lithium-Air Batteries with split Oxygen Harvesting and Redox processes” (LABOHR). All the partners of the LABOHR Project are acknowledged for the fruitful discussions on Li/O2 batteries with ionic liquids.

Materials challenges in rechargeable lithium-air batteries

Abstract

A Study on the Oxygen Redox Reduction in Ionic Liquid-Based Electrolytes for Li/O2 Battery Application

Low volatility, thermal and electrochemical stability and good conductivity of ionic liquids (ILs) make them “key electrolytes” for the development of Li/O2 batteries which are considered the most promising high-energy systems for a market success of electric vehicles of long driving range. Particularly, hydrophobic pyrrolidinium salts of bis(trifluoromethanesulfonyl)imide feature a high stability toward superoxide that is formed during the discharge/recharge of Li/O2 batteries. Under the European LABOHR Project, the oxygen redox reaction (ORR) and O2 diffusion coefficient and solubility, which are crucial parameters for Li/O2 battery operation, have been investigated in such ILs and related to the physical-chemistry properties of the electrolytes. It has also been demonstrated that ILs provide a real possibility for the development of rechargeable Li/O2 batteries that can safely operate even above room temperature. The main achievements of this study are here reported and discussed Acknowledgement Work funded by the European Commission in the 7th Framework Programme FP7-2010-GC-ELECTROCHEMICAL STORAGE, under contract no. 265971 “Lithium-Air Batteries with split Oxygen Harvesting and Redox processes” (LABOHR). All the partners of the LABOHR Project are acknowledged for the fruitful discussions on Li/O2 batteries with ionic liquids.

Determination of the “NiOOH” charge and discharge mechanisms at ideal activity

Optimization of electrodeposition conditions produced Ni(OH)2 deposits chargeable up to 1.84±0.02 e− per Ni on and the resulting nickel oxide/hydroxide active material could subsequently deliver 1.58±0.02 e− per Ni ion (462mAh/g) over a potential range <0.2V. The ability of the “NiOOH” active material to deliver an approximately ideal charge and discharge facilitated a coulometric and thermodynamic analysis through which the charge/discharge mechanisms were determined from known enthalpies of formation. The (dis)charge states were confirmed with in situ Raman spectroscopy. The mechanisms were additionally evaluated with respect to pH and potential dependence, charge quantities, hysteresis, and fluoride ion partial inhibition of the charge mechanism. The results indicate that the “NiOOH” (dis)charges as a solid-state system with mechanisms consistent with known nickel and oxygen redox reactions. A defect chemistry mechanism known for the LiNiO2 system also occurs for “NiOOH” to cause both high activity and hysteresis. Similar to other cation insertion nickel oxides, the activity of the “NiOOH” mechanism is predominantly due to oxygen redox activity and does not involve the Ni4+ oxidation state. The “NiOOH” was produced from cathodic electrodeposition of Ni(OH)2 from nickel nitrate solutions onto highly oriented pyrolytic graphite at ideal electrodeposition current efficiencies and the deposition mechanism was also characterized.

Influence of Hydrocarbon and CO2 on the Reversibility of Li–O2 Chemistry Using In Situ Ambient Pressure X-ray Photoelectron Spectroscopy

Identifying fundamental barriers that hinder reversible lithium–oxygen (Li–O2) redox reaction is essential for developing efficient and long-lasting rechargeable Li–O2 batteries. Addressing these challenges is being limited by parasitic reactions in the carbon-based O2–electrode with aprotic electrolytes. Understanding the mechanisms of these parasitic reactions is hampered by the complexity that multiple and coupled parasitic reactions involving carbon, electrolytes, and Li–O2 reaction intermediates/products can occur simultaneously. In this work, we employed solid-state cells free of carbon and aprotic electrolytes to probe the influence of surface adventitious hydrocarbons and carbon dioxide (CO2) on the reversibility of the Li–O2 redox chemistry using in situ synchrotron-based ambient pressure X-ray photoelectron spectroscopy. Direct evidence was provided, for the first time, that surface hydrocarbons and CO2 irreversibly react with Li–O2 reaction intermediates/products such as Li2O2 and Li2O, forming...

An electrochemical study of oxygen reduction in pyrrolidinium-based ionic liquids for lithium/oxygen batteries

The oxygen redox reaction (ORR) in pyrrolidinium-based ionic liquids (ILs) without and with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is investigated at different temperatures for lithium/oxygen battery application. Results obtained by cyclic voltammetry, rotating disk electrode, and ultramicroelectrode potential steps at glassy carbon are reported and discussed in relation to the physical–chemical properties of the investigated ILs. Diffusion coefficients and solubility of oxygen in the ILs and tentative values of ORR heterogeneous rate constant in ILs without LiTFSI are reported. The effect of lithium salt on the ORR in IL is also discussed in view of lithium/oxygen battery application.

Effect of lithium ions on oxygen reduction in ionic liquid-based electrolytes

The effect of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) on the oxygen redox reaction (ORR) in several pyrrolidinium-based ionic liquids (ILs) is investigated by cyclic voltammetry. The results are compared to those found with LiTFSI-tetraglyme and LiTFSI-1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidone solutions. The effect of the addition of tris(pentafluorophenyl) borane (TPFPB) to IL-LiTFSI solutions is also investigated. The results demonstrate that the presence of 0.1 M Li + renders the ORR similar in the investigated ILs, lowers the reduction voltammetric currents and makes ORR electrochemically irreversible. The addition of TPFPB to IL-TFSI solutions favours the solubility of oxygen-based products of Li + and should positively affect the performance of IL-based lithium-air batteries.

Bipolar Switching Characteristics and Scalability in NiO Layers Made by Thermal Oxidation of Ni

This paper deals with the bipolar switching properties of Ni\\NiO\\Ni cells for which the NiO layer is made by thermal oxidation of Ni layers. Physical characterization of the NiO layer revealed a strong deficiency in oxygen and a defective crystal structure. This microstructure allowed reproducible bipolar switching, which we attributed to the drift of oxygen anionic species and redox reactions at the anode. Conductive atomic force microscopy (C-AFM) experiments on Ni\\NiO surfaces showed the scaling potential of the bipolar switching. C-AFM and macroscopic electrical measurements showed similarities and consistent results related to oxygen movement effects at the nanometer scale through relaxation processes and switching events. A discussion is conducted on the origin of the change in resistance, observed either at a microscopic or at a macroscopic scale. A distinction is made between electrochemical redox reactions taking place along conductive channels near the Ni\\NiO\\Ni top electrode and local changes of oxygen-vacancy densities.

Nanocrystalline semiconductor materials for solar water-splitting

The production of hydrogen from water using solar light is very promising for generation of an ecologically pure energy carrier. Semiconductor photocatalysts are used to harvest the light energy and induce the hydrogen and oxygen redox reactions. Different types of TiO 2 aerogels have been successfully synthesized and the sample with the highest BET surface area was morphologically characterized with the help of SEM measurements. To improve the photoelectrochemical (PEC) efficiency of the material, it is desirable to red-shift the PEC onset to also include the less energetic but more intense visible part of the solar spectrum. The most efficient and commonly used dyes in dye-sensitized TiO 2 solar cells are based on ruthenium complexes. A series of ZnO/TiO 2 aerogels were prepared by the sol–gel method followed by supercritical drying. By varying the ZnO quantity (1–10%) and the nature of Zn precursors–zinc(II) nitrate hexahydrate and zinc(II) acetylacetonate, an extensive study has been performed.

In situ CCD video and voltammetric studies on enhanced cathodic peak observed at a hanging mercury drop electrode during consecutive two one-electron redox reactions in aprotic solutions

We have extensively studied the enhancement of the second reduction peak current I p c 2 of a stepwise one-electron redox reaction at a hanging mercury drop electrode (HMDE) in aprotic solution on the basis of cyclic voltammetric and in situ CCD video measurements. The cyclic voltammograms of the redox reactions of molecular oxygen, 1,4-dinitrobenzene, benzoquinone, 2,3,5,6-tetramethyl benzoquinone, azobenzene, methylviologen dichloride and 9,10-diphenylanthracene (DPA) at HMDE and glassy carbon (GC) electrode in dimethyl sulfoxide solutions containing 0.1 M tetraethylammonium perchlorate (TEAP) were compared. Generally, the I p c 2 was found to be greater than the first reduction peak current I p c 1 (i.e., I p c 2 / I p c 1 > 1 ) at HMDE, while the opposite result (i.e., I p c 2 / I p c 1 < 1 ) was obtained at GC electrode. Changing the state (and shape) of the mercury electrode from the liquid (and spherical) (i.e., HMDE) to semi-solid (and planar) (e.g., mercury film-coated gold electrode), increasing the concentration of TEAP and the use of poly(vinylchloride) as a surfactant were found to effectively allow the value of I p c 2 / I p c 1 to be closer to 1, indicating that the so-called streaming phenomenon of HMDE results in I p c 2 / I p c 1 > 1 . From the CCD video of HMDE captured during the electrochromic redox reaction of DPA, it was clarified that the upward streaming of the HMDE surface causes a continuous supply of the redox species from the bulk of the solution to the electrode surface, resulting in the enhanced I p c 2 .

The Direct Electron Transfer of Iron‐Containing Superoxide Dismutase (Fe‐SOD) and its Catalysis for the Oxygen Reduction Reaction (ORR) in Room Temperature Ionic Liquids (RTILs) on a Gold Electrode

AbstractIn this work, for the first time, the direct electron transfer of iron‐containing superoxide dismutase (Fe‐SOD) was observed by cyclic voltammetry on a gold (Au) electrode in three RTILs, i.e., 1‐ethyl‐3‐methylimidazolium tetrafluoroborate (EMIBF4), 1‐n‐propyl‐3‐methylimidazolium tetrafluoroborate (PMIBF4) and 1‐n‐butyl‐3‐methylimidazolium tetrafluoroborate (BMIBF4). And the results demonstrate that when the scan rate was as low as 1 mV/s, a pair of well‐defined quasi‐reversible peaks of Fe‐SOD was presented, while as the potential scan rate was above 10 mV/s, the reduction peak of Fe‐SOD disappeared though its oxidation peak could be clearly observed even as the potential scan rate was up to 400 mV/s, strongly indicating that these CVs we observed were attributable to Fe‐SOD rather than the impurities in RTILs. Its catalysis for oxygen reduction reaction (ORR) was directly verified by the shifting of formal potential, E0′, of ORR, to the positive direction though the value of standard rate constant, κ0, corresponding to ORR, was not much enhanced. In PMIBF4, for the multi‐walled carbon nanotubes (MWCNTs)‐modified gold electrode, both the reduction peak current and oxidation peak current for oxygen redox reaction were all dramatically enhanced compared to the case of a bare gold electrode, and the value of κ0 was also increased from 3.1 × 10−3 cm s−1 for the bare gold electrode, to 17.5 × 10−3 cm s−1. Hence, in the presence of Fe‐SOD in RTILs, MWCNTs, showing catalysis for the electron transfer process of ORR, coupled with Fe‐SOD, leading to the shifting of formal potential corresponding to ORR to the positive direction, presented us a satisfactory catalysis for ORR in RTILs. Some reasons available for this catalysis behavior stemming from Fe‐SOD, and MWCNTs as well, for ORR are discussed based on the previously developed proposition.

Effect of microstructure on the electrochemical behavior of Pt/YSZ electrodes

Two types of O2,Pt/YSZ electrode preparation (Pt/YSZ cermet and sputtered platinum film) have been characterized by SEM and by cyclic voltammetry and chronoamperometry at 450 °C in 20 kPa oxygen. Cyclic voltammetry on the cermet and on the as-sputtered non-porous film electrode evidenced the characteristics of the PtO x /Pt couple. The corresponding redox reaction occurs at the metal/electrolyte interface and it manifests itself by an anodic wave and one of more cathodic peaks in the voltammogram. Heat treatment of the sputtered electrode at 700 °C in oxygen atmosphere resulted in a porous structure by coalescence of the film. Cyclic voltammetry of the porous film electrode featured the characteristics of the O2/O2− couple, i.e. the redox reaction of gaseous oxygen occurring at the tpb. Chronoamperometry at anodic potentials showed similar features for both electrode preparations: an initial inhibition, a current peak and a slow activation, the latter being related to the phenomenon of electrochemical promotion of catalysis.

Current Oscillatory Phenomena Based on Redox Reactions at a Hanging Mercury Drop Electrode (HMDE) in Dimethyl Sulfoxide

A systematic and comprehensive study on cyclic voltammetric anodic current oscillation (CVACO) at a hanging mercury drop electrode (HMDE) was carried out for the redox reactions of molecular oxygen (O2), nitrobenzene (NB), 1,4-dinitrobenzene (DNB), benzoquinone (BQ), 2,3,5,6-tetramethylbenzoquinone (TMBQ), benzophenone (BP), azobenzene (AB), 2,1,3-benzothiadiazole (BTD), 7,7,8,8-tetracyanoquinodimethane (TCNQ), methyl viologen dichloride (MV2+), and tris(2,2‘-bipyridine)ruthenium(II) dichloride [Ru(bpy)32+] in dimethyl sulfoxide (DMSO) solutions containing 0.1 M tetraethylammonium perchlorate (TEAP). From the electrocapillary curve (ECC) obtained using a dropping mercury electrode as well as the capacitance versus potential curves measured using electrochemical impedance technique, the value of the potential of zero charge (PZC) was estimated to be −0.27 V versus Ag|AgCl|NaCl (sat.) in a DMSO solution containing 0.1 M TEAP. CVACO was found to occur only for the redox couples (i.e., BP0/BP•-, O20/O2•-, AB0/AB•-, Ru(bpy)32+/Ru(bpy)3+, BTD0/BTD•-, NB0/NB•-, DNB0/DNB•-, DNB•-/DNB2-, TMBQ0/TMBQ•-, MV2+/MV•+, and BQ•-/BQ2-) having the formal potentials (E0‘ values) more negative than the PZC. CVACO was largely dependent on the concentrations of redox species and TEAP; for example, in the case of BTD the intensity of CVACO increased with increasing concentration, and CVACO ceased at high concentrations of TEAP (≥0.5 M). Furthermore, CVACO was not observed for the BQ0/BQ•- redox couple having E0‘ (= −0.31 V) near the PZC, and a pronounced cathodic maximum was observed for the TCNQ•-/TCNQ2- redox couple with E0‘ (= −0.16 V) more positive than the PZC. These observations and the factors governing the CVACO are discussed on the basis of the theory presented for the polarographic maxima of the first kind. The observed CVACO and the cathodic maximum obtained for the TCNQ•-/TCNQ2- redox couple could be explained in terms of the so-called streaming effect.

The Distribution and Redox Chemistry of Iron in the Pettaquamscutt Estuary

A series of high resolution (10cm) vertical profiles of iron were determined across the oxic/anoxic boundary in the Lower Pond of the Pettaquamscutt Estuary. Selective chemical treatments and multiple analytical methods were used to detemine the oxidation state and lability of iron across the oxic/anoxic boundary. The vertical distributions of dissolved and total iron were determined by atomic absorption spectroscopy, and dissolved Fe(II) and reducible iron were determined using a modified Ferrozine spectrophotometric method. Well-developed maxima of total dissolved iron ≈7·5μM occurred within the oxic/anoxic transition zone. Analysis of Fe(II) by the FZ method indicates that more than 95% of the dissolved iron determined by atomic absorption spectroscopy within the maximum is in the form of Fe(II). The concentration of dissolved Fe(II) ranged from <4nM in oxygenated surface waters to between 7 and 8μM at the total dissolved iron maximum.Both dissolved and total iron samples were treated with ascorbic acid to quantify the fraction of iron that was reducible in this system. Dissolved iron is quantitatively reduced to Fe(II) by 3·5m depth, and particulate iron was almost completely dissolved by 6m. Thermodynamic speciation calculations indicate that the dominant species of Fe(II) in the anoxic waters is the Fe(HS)+complex. In addition, the concentration of Fe(II) in the anoxic zone appears to be controlled by precipitation of a sulfide phase, the ion activity product for waters below 7m is in good agreement with the solubility product of mackinawite.The vertical distribution of oxidation states of the metals indicates non-equilibrium conditions due to microbiological and chemical processes occurring in the redox transition zone. A one-dimensional vertical, eddy diffusion model is presented that incorporates redox reactions of iron, sulfide and oxygen. The modeling suggests the maximum in Fe(II) can be achieved through inorganic oxidation and reduction reactions, however the depth at which the maximum occurs is sensitive to sulfide oxidation, which appears to be dominated by biological oxidation. The magnitude of the Fe(II) maximum depends on the flux of iron into the basin, and reductive dissolution of particulate iron.

Oxygen Reduction at the Interface YSZ / LaMnO3 prepared by a Vapor Phase Processes

Dense and thin LaMnO3 and YSZ was deposited on porous La0.85(Sr)0.15MnO3 substrate in order to study oxygen redox behavior at an interface of LaMnO3/YSZ. YSZ film was deposited by the CVD - EVD method on porous substrate at first, LaMnO3 film was deposited on the YSZ film by the EVD method. Results of the AC impedance measurements showed that electrode interface conductivity (σe) was directly proportional to the surface area, while σe v. s. TPB (Three Phase Boundary) length plot did not show linear relationship. It was concluded that LaMnO3 surface as well as TPB region, is an active site for oxygen redox reaction when the LaMnO3 film electrode is very thin (about few μm).