Fast Reduction Kinetics Research Articles

Chemical Strain and Oxidation-Reduction Kinetics of Epitaxial Thin Films of Mixed Ionic-Electronic Conducting Oxides Determined by X-Ray Diffraction

X-ray diffraction, at high T's and switching between N2/air atmospheres, was used to compare the chemical expansion due oxygen non-stoichiometry variations between epitaxial films of different mixed ionic-electronic conductors: La0.6Sr0.4CoO3−δ(LSC), Ba0.5Sr0.5Co0.8Fe0.2O3−δ(BSCF), LaNiO3−δ(LNO), La2NiO4+δ(L2NO) and GaBaCo2O5.5+δ(GBCO) and La0.7Sr0.3MnO3−δ(LSM). LSC and BSCF show the largest relative change in the cell parameter Δc/c = +0.5%, while L2NO and GBCO show negative Δc/c = −0.2% and −0.1%, respectively. LNO and LSM show either reduced or negligible chemical expansions. In all cases the values correspond to their particular defect equilibrium and degree of charge localization. The oxygen surface exchange kinetics was also evaluated from in-situ time-resolved analyses of the cell parameter variations. LSC, LNO and GBCO films show fast oxygen reduction kinetics, kchem = 5·10−6, 3·10−6, and 2·10−7 cm/s at 700°C, respectively, in relative agreement with reported values, while BSCF films show much slower kinetics than expected, below kchem = 10−7 cm/s at 650°C, related to the degradation process observed in the films.

Facile synthesis of morphology-controlled Co3O4 nanostructures through solvothermal method with enhanced catalytic activity for H2O2 electroreduction

Hydrogen peroxide (H2O2) replaced oxygen (O2) as oxidant has been widely investigated due to its faster reduction kinetics, easier storage and handling than gaseous oxygen. The main challenge of using H2O2 as oxidant is the chemical decomposition. In this article, by using different C2H5OH/H2O volume ratio as the solvent, Co3O4 with different morphologies (nanosheet, nanowire, ultrafine nanowire net, nanobelts, and honeycomb-like) direct growth on Ni foam are synthesized via a simple solvothermal method for the first time. Results show that the introduction of ethanol could obviously improve the catalytic performance toward H2O2 electroreduction. The sample prepared in the solution with the C2H5OH/H2O volume ratio of 1:2 shows the best catalytic performance among the five samples and a current density of 0.214 A cm−2 is observed in 3.0 mol L−1 KOH + 0.5 mol L−1 H2O2 at −0.4 V (vs. Ag/AgCl KCl), which is much larger than that on the other metal oxides reported previously, almost comparable with the precious metals. This electrode of Co3O4 directly grown on Ni foam has superior mass transport property, which combining with its low-cost and facile preparation, make it a promising electrode for fuel cell using H2O2 as the oxidant.

La0.6Sr0.4CoO3-δ (LSC) Thin Film Electrodes with Very Fast Oxygen Reduction Kinetics Prepared by a Sol-Gel Route

Abstract La0.6Sr0.4CoO3-δ (LSC) thin film electrodes of about 80 nm thickness were prepared via a sol-gel route on yttria-stabilized zirconia (YSZ) and gadolinium doped ceria (GDC) solid electrolytes. Impedance measurements on microelectrodes fabricated from these films revealed very low polarization resistances for electrochemical oxygen exchange at intermediate temperatures (500–600 ºC). Prerequisite for the very fast oxygen reduction kinetics was a limitation of the preparation (annealing) temperature to 600 ºC. Already after ca. 15 minutes annealing of the sol-gel prepared layer at this temperature, electrodes were electrochemically highly active despite low crystallinity (almost XRD amorphous). LSC electrodes on GDC showed much higher polarization resistances. However, this originated from an additional interfacial resistance rather than from kinetically slow LSC surfaces.

Mechanism for enhanced oxygen reduction kinetics at the (La,Sr)CoO3−δ/(La,Sr)2CoO4+δ hetero-interface

The recently reported fast oxygen reduction kinetics at the interface of (La,Sr)CoO3−δ (LSC113) and (La,Sr)2CoO4+δ (LSC214) phases opened up new questions for the potential role of dissimilar interfaces in advanced cathodes for solid oxide fuel cells (SOFCs). Using first-principles based calculations in the framework of density functional theory, we quantitatively probed the possible mechanisms that govern the oxygen reduction activity enhancement at this hetero-interface as a model system. Our findings show that both the strongly anisotropic oxygen incorporation kinetics on the LSC214 and the lattice strain in the vicinity of the interface are important contributors to such enhancement. The LSC214(100) surface exposed to the ambient at the LSC113/LSC214 interface facilitates oxygen incorporation because the oxygen molecules very favorably adsorb onto it compared to the LSC214(001) and LSC113(001) surfaces, providing a large source term for oxygen incorporation. Lattice strain field present near the hetero-interface accelerates oxygen incorporation kinetics especially on the LSC113(001) surface. At 500 °C, 4 × 102 times faster oxygen incorporation kinetics are predicted in the vicinity of the LSC113/LSC214 hetero-interface with 50% Sr-doped LSC214 compared to that on the single phase LSC113(001) surface. Contributions from both the anisotropy and the local strain effects are of comparable magnitude. The insights obtained in this work suggest that hetero-structures, which have a large area of (100) surfaces and smaller thickness in the [001] direction of the Ruddlesden–Popper phases, and larger tensile strain near the interface would be promising for high-performance cathodes.

New Insights into the Strain Coupling to Surface Chemistry, Electronic Structure, and Reactivity of La0.7Sr0.3MnO3

Effects of strain on the surface cation chemistry and the electronic structure are important to understand and control for attaining fast oxygen reduction kinetics on transition-metal oxides. Here we demonstrate and mechanistically interpret the strain coupling to Sr segregation, oxygen vacancy formation, and electronic structure on the surface of La0.7Sr0.3MnO3 (LSM) thin films as a model system. Our experimental results from X-ray photoelectron spectroscopy and scanning tunneling spectroscopy are discussed in light of our first principles-based simulations. A stronger Sr enrichment tendency and a more facile oxygen vacancy formation prevail for the tensile-strained LSM surface. At 500 °C in 10−3 mbar oxygen, both LSM film surfaces exhibit a metallic-like tunneling conductance, with a higher density of electronic states near the Fermi level on the tensile-strained LSM surface, contrary to the behavior at room temperature. Our findings illustrate the potential role and mechanism of lattice strain in tuning the reactivity of perovskite transition-metal oxides with oxygen in solid oxide fuel cell cathodes.

Aqueous room-temperature synthesis of Au–Rh, Au–Pt, Pt–Rh, and Pd–Rh alloy nanoparticles: fully tunable compositions within the miscibility gaps

Many binary late transition metal systems have large bulk miscibility gaps, and a variety of synthetic strategies have been developed to generate these non-equilibrium alloys as nanoparticles. While many of these methods strive to co-nucleate both elements by exploiting fast reduction kinetics or co-sequestration within a confined space, we show here that simple room-temperature borohydride co-reduction of appropriate aqueous metal salt solutions yields alloy nanoparticles in the bulk-immiscible Au–Rh, Au–Pt, Pt–Rh, and Pd–Rh systems. The compositions can be tuned across the entire Au1−xRhx, Au1−xPtx, Pt1−xRhx, and Pd1−xRhx solid solutions by varying the ratio of metal salt reagents, and they form in the presence of a variety of molecular and polymeric surface stabilizers. Reaction pathway studies on the model Au–Rh system suggest that the alloy nanoparticles form via a “conversion chemistry” mechanism: Au nanoparticle templates nucleate first, followed by diffusion of Rh to form homogeneous Au–Rh alloy nanoparticles. The alloy nanoparticles tend to be agglomerated, but this can be minimized by forming the nanoparticles directly on catalytically relevant high surface area carbon and biological supports, e.g. Vulcan carbon and wild-type M13 bacteriophage.

Structural and Chemical Investigations of (La, Sr)CoO3-δ Thin Film Electrodes Exhibiting Very Fast Oxygen Reduction Kinetics

The polarization resistance of La0.6Sr0.4CoO3-δ thin film electrodes prepared via pulsed laser deposition (PLD) is investigated by impedance spectroscopy for various film deposition temperatures. The electrodes were deposited on yttria stabilized zirconia (YSZ) single crystals and polycrystals and on polycrystalline gadolinia doped ceria (CGO). Structure-property relations emphasizing the correlation between crystallinity and performance are determined by additional XRD, SEM and TEM measurements. These experiments revealed that thin film electrodes of extraordinary low polarization resistance and little degradation may be produced on YSZ by optimizing the deposition conditions.

Rational design of novel cathode materials in solid oxide fuel cells using first-principles simulations

The search for clean and renewable sources of energy represents one of the most vital challenges facing us today. Solid oxide fuel cells (SOFCs) are among the most promising technologies for a clean and secure energy future due to their high energy efficiency and excellent fuel flexibility (e.g., direct utilization of hydrocarbons or renewable fuels). To make SOFCs economically competitive, however, development of new materials for low-temperature operation is essential. Here we report our results on a computational study to achieve rational design of SOFC cathodes with fast oxygen reduction kinetics and rapid ionic transport. Results suggest that surface catalytic properties are strongly correlated with the bulk transport properties in several material systems with the formula of La 0.5Sr 0.5BO 2.75 (where B = Cr, Mn, Fe, or Co). The predictions seem to agree qualitatively with available experimental results on these materials. This computational screening technique may guide us to search for high-efficiency cathode materials for a new generation of SOFCs.

Structural and Chemical Investigations of (La, Sr)CoO3-δ Thin Film Electrodes Exhibiting Extremly Fast Oxygen Reduction Kinetics

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Chemical and Electrochemical Characterization of a Novel Nanocomposite Formed from V 2 O 5 and Poly(N‐propane sulfonic acid aniline), a Self‐Doped Polyaniline

Synthesis and characterization of new nanocomposites of and a sulfonated, alkylated polyaniline derivative [poly(N‐propane sulfonic acid aniline), PSPAN] are described. Two types of nanocomposites have been produced, one by reaction of vanadium triisopropoxide solutions with N‐propane sulfonic acid aniline and one by addition of to these same solutions. Nanocomposites are characterized using thermal gravimetric analysis, differencial scanning calorimetry (DSC), cyclic voltammetry, chronopotentiometry, and impedance spectroscopy. The increase of the 001 reflection spacing, X‐ray diffraction results show that the polymer is intercalated into the interlayer region, consistent with the nanocomposite nature of the material. The DSC and infrared data suggest that water coordinated to vanadium sites exposed to the interlayer region is lost during formation of the nanocomposite. SEMs show that, in contrast to the relatively flat and featureless thin films, the nanocomposites are highly textured and have quite small feature sizes. Electrochemical results show that some of the nanocomposites have larger specific capacity (307 Ah/kg) and faster reduction kinetics than and similar cyclability. The relevance of these results to general approaches to the production of nanocomposites for Li secondary battery cathodes is discussed. © 2000 The Electrochemical Society. All rights reserved.

Importance of the Region Including Aspartates 57 and 60 of Ferredoxin on the Electron Transfer Complex with Photosystem I in the Cyanobacterium Synechocystis sp. PCC 6803

Ferredoxin reduction by photosystem I has been studied by flash-absorption spectroscopy. Aspartate residues 20, 57, and 60 of ferredoxin were changed to alanine, cystein, arginine, or lysine. On the one hand, electron transfer from photosystem I to all mutated ferredoxins still occurs on a microsecond time scale, with halftimes of ferredoxin reduction mostly conserved compared to wild-type ferredoxin. On the other hand, the total amplitude of the fast first-order reduction varies largely when residues 57 or 60 are modified, in apparent relation to the charge modification (neutralized or inverted). Substituting these two residues for lysine or arginine induce strong effects on ferredoxin binding (up to sixfold increase in KD), whereas the same substitution on aspartate 20, a spacially related residue, results in moderate effects (maximum twofold increase in KD). In addition, double mutations to arginine or lysine were performed on both aspartates 57 and 60. The mutated proteins have a 15- to 20-fold increased KD and show strong modifications in the amplitudes of the fast reduction kinetics. These results indicate that the acidic area of ferredoxin including aspartates 57 and 60, located opposite to the C-terminus, is crucial for high affinity interactions with photosystem I.

Reequilibration kinetics of the heterogeneous system NiO-NiCr2O4

Reequilibration kinetic data have been obtained for a model system consisting of an undoped NiO crystal covered with a thin layer of NiCr2O4. The data lead to following apparent chemical diffusion coefficients $$\tilde D^* $$ measured within the temperature range 900–1200°C when varying oxygen partial pressure from 1 to 10−5 atm: $$\begin{gathered} \tilde D^* _{red} = 5.28 \times 10^{ - 4} exp\left( {\frac{{19,160 \pm 870}}{{RT}}} \right); \hfill \\ \tilde D^* _{oxid} = 1.82 \times 10^{ - 2} exp\left( {\frac{{28,270 \pm 2,020}}{{RT}}} \right) \hfill \\ \end{gathered} $$ . Values of the apparent activation energies for the reequilibration processes were independent of the thickness of NiCr2O4 surface layer from about 0.01 to 0.05 mm. As determined by microprobe, the thickness of the NiCr2O4 surface layer changed only slightly during long reequilibration experiments. The change is in agreement with diffusion coefficients of Cr in NiO. Oxygen pressure dependence of electrical conductivity leads to the parameter 1/n = 1/5.55 being the same as those for undoped NiO crystals. This indicates the values of electrical conductivity taken at equilibrium are determined by the equilibrated undoped NiO crystal while the dissimilar reequilibration kinetic data result from the presence of the NiCr2O4 surface layer. The results indicate the slower oxidation kinetics to be surface-controlled and faster reduction kinetics to be bulk-diffusion-controlled.

Plastoquinone as a common link between photosynthesis and respiration in a blue-green alga

The role of plastoquinone in a thermophilic blue-green alga, Shynechococcus sp., was studied by measuring reduction kinetics of cytochrome 553 which was oxidized with red flash preferentially exciting photosystem I. Sensitivity of the cytochrome reduction to DBMIB Abbreviations: DCMU = 3-(3,4-dichlorophenyl)-1,1-dimethylurea; DBMIB = 2,5-dib romo-3-methyl-6-isopropyl-p-benzoquinone; HOQNO = 2-n-heptyl-4-hydroxyquinoline-N-oxide indicates that cytochrome 553 accepts electrons from reduced plastoquinone. Plastoquinone is in turn reduced in cells without electrons from photosystem II, since DCMU Abbreviations: DCMU = 3-(3,4-dichlorophenyl)-1,1-dimethylurea; DBMIB = 2,5-dib romo-3-methyl-6-isopropyl-p-benzoquinone; HOQNO = 2-n-heptyl-4-hydroxyquinoline-N-oxide , which inhibited methyl viologen photoreduction more strongly than DBMIB, failed to affect the cytochrome reduction. Participation of cyclic electron transport around photosystem I in cytochrome reduction in the presence of DCMU was excluded, because methyl viologen and antimycin A had no effect on the cytochrome kinetics. On the other hand, electron donation from endogenous substrates to plastoquinone was suggested from decreases in rate of the cytochrome reduction by dark starvation of cells and also from restoration of fast reduction kinetics by the addition of exogenous substrates to or by reillumination of starved cells.KCN, which completely suppressed respiratory O2-uptake, induced a marked acceleration of the cytochrome reduction in starved cells. The poison was less or not effective in stimulating the cytochrome reduction in more extensively starved or reilluminated cells.Results indicate that plastoquinone is functioning not only in the photosynthetic but also in the respiratory electron transport chain, thereby forming a common link between the two energy conservation systems of the blue-green alga.

Indirect evidence for a very fast recovery kinetics of chlorophyll-aII in spinach chloroplasts.

The 690 nm absorption change reflecting the turnover of the system-II-reaction center chlorophyll, Chl-aII (often referred to as P 680), has been investigated under different experimental conditions in spinach chloroplasts. A comparison was made with oxygen evolution and with absorption changes of Chl-aI measured at 703 nm, both indicating the number of electrons produced by system II. It was found: 1. The dependency on actinic flash intensity of the initial amplitudes of the measured 690 nm absorption change, deltaalpha0(Chl-aII) in Tris-washed chloroplasts is similar to that for the total amplutude of the 703 nm absorption change, deltaalpha0(Chl-aI) in normal chloroplasts, and can be described by an exponential function. On the other hand, deltaalphao(chl-aII) in normal chloroplasts exhibits a more complex biphasic dependency and much higher flash intensities are required for saturation. 3. Unver repetitive flash group excitation and in the presence of an ADRY(= acceleration of the deactivation reactions of the water-splitting enzyme system Y)-reagent the initial amplitude of the 690 nm absorption change oscillates in the same characteristic pattern as the oxygen evolution. 4. The initial amplitude of the 690 nm absorption change, deltaalpha0(Chl-aII), IN Tris-washed chloroplasts becomes significantly smaller (more than 50%) by the addition of system-II-electron donors (benzidine, p-phenylendiamine, tetraphenylboron), whereas the total amplitude of the 703 nm absorption change, detalalpha0)Chl-aI) increases 3-4-fold. In order to explain these results, the existance of a very fast reduction kinetics of Chl-aII+ is postulated, which is not detectable by our measuring equipment. The half time of this reaction is less than or equal to mus. Reaction centers with the very gast "undetected" Chl-aII+-reduction are photochemically transformed into slower one by double hit processes with a comparatively low quantum yield. Furthermore, it is inferred, that the dark recovery kinetics of Chl-aII is dependent on be charge accumulation state of the watersplitting enzyme system Y. This phenomenon is shown to explain also the oscillation pattern of delayed fluorescence. On the basis of the present results two alternative reaction schemes for the functional organization of the electron transport on the donor side of system II are discussed.

Reduction kinetics of cytochromes b

1. 1. Durohydroquinone (H 2DQ), a specific reductant for the second phosphorylation site, has been used in the study of the fast reduction kinetics of cytochromes b. 2. 2. In ubiquinone-extracted mitochondria, H 2DQ reduces cytochrome(s) b in a fast reaction, the rate and extent of which decreases upon reincorporation of ubiquinone. 3. 3. In cytochrome c-extracted mitochondria, H 2DQ reduces both cytochromes b. After ATP addition, with cytochrome b T reduced by endogenous substrate, H 2DQ reduces cytochrome b K at a slow rate whereas ubiquinone readily accepts reducing equivalents. In the presence of uncoupler, H 2DQ reduces cytochrome b K at a faster rate while ubiquinone behaves as a less efficient electron acceptor. Under these conditions, cytochrome b T is not reduced by H 2DQ. 4. 4. Cytochrome c 1 reduction can be resolved free of cytochrome c interference in cytochrome c-extracted mitochondria. Cytochrome c 1 reduction is 4 times faster in the presence of uncoupler than in the presence of ATP. 5. 5. Addition of cytochrome c to cytochrome c-extracted mitochondria stimulates the rate of cytochromes b reduction. 6. 6. In intact mitochondria, ATP addition causes 20 % reduction of cytochrome b T and speeds up the initial rate of cytochromes b reduction by a factor of 10. 7. 7. Addition of antimycin A to intact mitochondria results in 80–100 % reduction of cytochrome b K by endogenous substrate. ATP added to antimycin A-inhibited mitochondria produces oxidation of cytochrome b K and simultaneous and variable reduction of cytochrome b T. This latter effect is reverted by the addition of an uncoupler.