Outer-sphere Electron-transfer Reactions Research Articles

Evaluation of the Solid-Electrolyte Interphase Formation in a Bis(fluorosulfonyl)amide Based Ionic Liquid in the Presence Lithium Ion Using Different Redox Probes

The SEI formation in 0.5 M LiFSA/BMPFSA was investigated using different redox probes. The redox reaction of a metallocene and organic compound on a Pt electrode were investigated by cyclic voltammetry in BMPFSA and 0.5 M LiFSA/BMPFSA. Cyclic voltammograms were taken after holding the electrode at different potentials for 1 h. The cyclic voltammograms were unchanged in BMPFSA, indicating that BMPFSA was stable in the measured potential range and that the redox reaction was not affected by a change in the electric double layer structure. On the other hand, peak shifts were observed in the presence of Li+ at potentials more negative than –1.1 V vs. Ag|Ag(I), attributed to a decrease in the electron transfer rates and indicating that the SEI formation promoted by the interaction between Li+ and FSA– affects the redox reactions not only of metallocene but also of organic compounds that undergo outer sphere electron transfer reactions.

Redox Reactions of Ag(I)/Ag and Ferrocenium/Ferrocene in 1-Butyl-1-methylpyrrolidinium Bis(fluorosulfonyl)amide Ionic Liquid

The electrode reactions of Ag(I)/Ag and ferrocenium/ferrocene (Fc+/Fc) were investigated in an ionic liquid, 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA). The potential of Ag(I)/Ag depended on the logarithm of the concentration of Ag(I), as predicted by the Nernst equation, indicating the Ag(I)/Ag can be used as a reference electrode reaction in BMPFSA. The reversible electrode reaction of Fc+/Fc was observed in BMPFSA by cyclic voltammetry. The donor number of BMPFSA was estimated to be 13 from the difference in the formal potentials of Ag(I)/Ag and Fc+/Fc, indicating the coordination ability of FSA– was slightly stronger than that of bis(trifluoromethylsulfonyl)amide (TFSA–). The diffusion coefficients (D) of Fc and Fc+ were (5.7 ± 0.7) and (3.3 ± 0.2) × 10–7 cm2 s–1, respectively. The ratio of D of Fc+ against that of Fc was smaller than those in TFSA–-type ionic liquids, reflecting the higher charge density of FSA–. The standard rate constant (k 0) of Fc+/Fc was estimated to be (5.4 ± 1.1) × 10–3 cm s–1. The apparent activation energy for k 0 was close to the activation energy for D, suggesting the electrode reaction of Fc+/Fc can be regarded as the outer sphere electron transfer reaction with a very small reorganization energy.

Communication—Determination of the Formation Potential of Solid-Electrolyte Interphase in Amide-Type Ionic Liquids Containing Lithium Salts

The formation potentials of solid-electrolyte interphase (SEI) were investigated in 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing LiFSA and LiTFSA, respectively, by monitoring the redox reaction of ferrocene with high sensitivity. The SEI was found to affect the electrode kinetics of the outersphere electron transfer reaction. The SEIs with the thickness of less than a few nm formed in LiFSA/BMPFSA and LiTFSA/BMPTFSA at 2.4 and 2.5 V vs Li∣Li(I), respectively. The cathodic decomposition of FSA– and TFSA– was considered to be promoted by the interaction with Li+ based on the ab-initio calculation.

Electroprecipitation of Nanometer-Thick Films of Ln(OH)3 [Ln = La, Ce, and Lu] at Pt Microelectrodes and Their Effect on Electron-Transfer Reactions.

We report investigations of the deposition of nanometer-thick Ln(OH)3 films (Ln = La, Ce, and Lu) and their effect on outer-sphere and inner-sphere electron-transfer reactions. Insoluble Ln(OH)3 films are deposited from aqueous solutions of LaCl3 onto the surface of 12.5 μm radius Pt microdisk electrodes during water or oxygen reduction. Both reactions produce interfacial OH-, which complexes with Ln3+, resulting in the precipitation of Ln(OH)3. Surface analyses by scanning electron microscopy (SEM), SEM-energy-dispersive X-ray spectroscopy, and atomic force microscopy indicate the formation of a 1-2 nm thick uniform film. Outer-sphere electron-transfer reactions (Ru(NH3)63+ reduction, FcMeOH oxidation, and Fe(CN)64-/3- oxidation/reduction) were investigated at Ln(OH)3-modified electrodes of different film thicknesses. The results demonstrate that the steady-state transport-limited current for these reactions decreases with an increase in the film thickness. Moreover, the degree of blockage depends upon the redox species, suggesting that the Ln(OH)3 films are free from pinholes greater than the size of the redox molecules. This suggests that the films are either ionically conducting or that electron tunneling occurs across these thin layers. A similar blocking effect was observed for the inner-sphere reductions of H2O and O2. We further demonstrate that the thickness of La(OH)3 films can be controlled by anodic dissolution. Additionally, we show that La3+ lowers the supersaturation of dissolved H2 required to nucleate a stable nanobubble.

Enhanced electrochemical performance of supercritical fluid treated N, P co-doped graphene by dual redox-additives

The low specific energy of carbon-based electrodes in aqueous electrolytes hampers the commercial application of supercapacitors (SCs). Redox-additive addition with conventional electrolyte is an efficient strategy to enhance the specific energy of SC. Here, we demonstrate the constructive synergistic effect and enhance the electrochemical performance by the addition of [Fe(o-phen)3]2+ with [Fe(CN)6]3-/H2SO4. The increase in the specific capacitance (Cs) of N, P co-doped graphene (NPGO) from 290 F g−1 in H2SO4 to 685 F g−1 by the addition of redox-additive [Fe(CN)6]3- with H2SO4. Further enhance in the Cs to 865 F g−1 has been observing by the addition of second redox-additive [Fe(o-phen)3]2+ with [Fe(CN)6]3-/H2SO4. The specific energy of flexible supercapacitor (FSC) uplift 3.6-times using [Fe(o-phen)3]2+(0.63 mM)+[Fe(CN)6](10 mM)/PVA/H2SO4 electrolyte than 6.8 Wh kg−1 of PVA/H2SO4. The enhance in Cs and specific energy has been noticing after the addition of [Fe(o-phen)3]2+ with [Fe(CN)6]3-/H2SO4 due to the outer sphere electron transfer reaction between [Fe(o-phen)3]2+ and [Fe(CN)6]3- with H2SO4.

Voltammetric Mapping of Hydrogen Evolution Reaction on Pt Locally via Scanning Electrochemical Cell Microscopy.

The advancement in nanoscale electrochemical tools has offered the opportunity to better understand heterogeneity at electrochemical interfaces. Scanning electrochemical cell microscopy (SECCM) has been increasingly used for revealing local kinetics and the distribution of active sites in electrocatalysis. Constant-contact scanning and hopping scanning are the two commonly used modes. The former is intrinsically faster, whereas the latter enables full voltammetry at individual locations. Herein, we revisit a less used mode that combines the advantages of hopping and constant-contact scan, resulting in a faster voltammetric mapping. In this mode, the nanodroplet cell in SECCM maintains contact with the surface during the scanning and makes intermittent pauses for local voltammetry. The elimination of frequent retraction and approach greatly increases the speed of mapping. In addition, iR correction can be readily applied to the voltammetry, resulting in more accurate measurements of the electrode kinetics. This scanning mode is demonstrated in the oxidation of a ferrocene derivative on HOPG and hydrogen evolution reaction (HER) on polycrystalline Pt, serving as model systems for outer-sphere and inner-sphere electron transfer reactions, respectively. While the kinetics of the inner-sphere reaction is consistent spatially, heterogeneity is observed for the kinetics of HER, which is correlated with the crystal orientation of Pt.

Cation-Dependent Interfacial Structures and Kinetics for Outer-Sphere Electron-Transfer Reactions

The kinetics of aqueous outer-sphere electron-transfer (ET) reactions are determined in large part by noncovalent electrostatic interactions that originate from the surrounding electrolyte solution. In this work, we examine the role of spectator cations in modifying the rate of heterogeneous ET for an [Fe(CN)6]3–/[Fe(CN)6]4– redox pair. We combine the results of electrochemical measurement, in situ surface-enhanced infrared absorption spectroscopy (SEIRAS), classical molecular dynamics simulation, and theoretical modeling to demonstrate how changing the identity of the spectator cation species over a series that includes Li+, Na+, K+, Rb+ to Cs+ influences the solvation properties and ET kinetics of the redox species. By analyzing the results in the context of the Marcus–Hush–Chidsey (MHC) theory, we find that the solvent reorganization energy increases systematically as the cationic radius decreases. The trend can be attributed to cation-dependent coordination environments of the redox species, whereby more cations of less charge density such as Cs+ than Li+ are present in the redox solvation shell in bulk and at the electrified interface, promoting weaker hydrogen bonds and lowering the effective interfacial static dielectric constant. We discuss the implications of these findings for enabling the tunability of reaction thermodynamics and rates in electrochemical processes.

Influence of defects in graphene on electron transfer kinetics: The role of the surface electronic structure

The kinetics of an outer-sphere nonadiabatic electron transfer reaction at the defective graphene is theoretically investigated. Different intrinsic and extrinsic point defects were analyzed. The band structure from DFT, in combination with the Gerischer model, was used for the predictions. Calculations were done taking into account the quantum capacitance of the surface for the correct prediction of alignment and occupation of graphene electronic states upon contact with an electrolyte. We have shown that an electrochemical property of graphene largely depends on defect type due to differences in the electronic structure induced by defects. Due to small graphene quantum capacitance, the electron transfer kinetics can be modulated by changes in the double-layer capacitance. The deviations of the potential dependence of the rate constant from the Butler-Volmer kinetics are predicted.

Unified Mechanism of Oxygen Atom Transfer and Hydrogen Atom Transfer Reactions with a Triflic Acid-Bound Nonheme Manganese(IV)–Oxo Complex via Outer-Sphere Electron Transfer

Outer-sphere electron transfer from styrene, thioanisole, and toluene derivatives to a triflic acid (HOTf)-bound nonheme Mn(IV)-oxo complex, [(N4Py)MnIV(O)]2+-(HOTf)2 (N4Py = N, N-bis(2-pyridylmethyl)- N-bis(2-pyridyl)methylamine), has been shown to be the rate-determining step of different types of redox reactions such as epoxidation, sulfoxidation, and hydroxylation of styrene, thioanisole, and toluene derivatives, respectively, by [(N4Py)MnIV(O)]2+-(HOTf)2. The rate constants of HOTf-promoted epoxidation of all styrene derivatives with [(N4Py)MnIV(O)]2+ and electron transfer from electron donors to [(N4Py)MnV(O)]2+ exhibit a remarkably unified correlation with the driving force of outer-sphere electron transfer in light of the Marcus theory of electron transfer. The same electron-transfer driving force dependence is observed in the oxygen atom transfer from [(N4Py)MnIV(O)]2+-(HOTf)2 to thioanisole derivatives as well as in the hydrogen atom transfer from toluene derivatives to [(N4Py)MnIV(O)]2+-(HOTf)2. Thus, mechanisms of oxygen atom transfer (epoxidation and sulfoxidation) reactions of styrene and thioanisole derivatives and hydrogen atom transfer (hydroxylation) reactions of toluene derivatives by [(N4Py)MnIV(O)]2+-(HOTf)2 have been unified for the first time as the same reaction pathway via outer-sphere electron transfer, followed by the fast bond-forming step, which exhibits the singly unified electron-transfer driving force dependence of the rate constants as outer-sphere electron-transfer reactions. In the case of the epoxidation of cis-stilbene by [(N4Py)MnIV(O)]2+-(HOTf)2, the isomerization of cis-stilbene radical cation to trans-stilbene radical cation occurs after outer-sphere electron transfer from cis-stilbene to [(N4Py)MnIV(O)]2+-(HOTf)2 to yield trans-stilbene oxide selectively, which is also taken as evidence for the occurrence of electron transfer in the acid-catalyzed epoxidation.

An insight into the outer- and inner-sphere electrochemistry of oxygenated single-walled carbon nanohorns (o-SWCNHs)

The electrocatalysis/interference of single-walled carbon nanohorns (o-SWCNHs) in relation to outer-sphere and inner-sphere electron transfer reactions.

Redox Reaction of 2,2,6,6-Tetramethylpiperidine-1-Oxyl (TEMPO) in Lithium Bis(trifluoromethylsulfonyl)Amide-Tetraglyme Solvate Ionic Liquid

Electrode reactions of the redox couples dissolved in liquid electrolytes are able to be utilized for energy storage devices. Vanadium redox flow battery is one of the typical large-scaled energy storage devices using aqueous solutions for both anolye and catholyte.[1] The cell voltages of such aqueous redox flow batteries are often limited by the electrochemical potential window of water. In order to enlarge the cell voltage and increase energy density, it is necessary to use aprotic electrolytes having wider electrochemical potential windows. Typical aprotic electrolytes used for non-aqueous batteries like lithium-ion batteries are composed of organic solvents, which are volatile and flammable. However, such organic electrolytes are unfavorable especially for the large-scaled batteries requiring large amount of electrolytes because of safety in case of accident. Therefore, it is necessary to select less flammable electrolytes for non-aqueous redox flow batteries. The equimolar mixture of lithium bis(trifluoromethylsulfonly)amide (LiTFSA) and tetraglyme (G4) has been known to form the solvate ionic liquid, which is composed of a lithium-G4 complex, [Li(G4)]+, and TFSA–, and exhibits such properties as less volatility, less flammability, and wide electrochemical potential window, similar to those of ionic liquids.[2,3] The high concentration of lithium species in the solvate ionic liquid is also advantageous to the batteries using lithium ion as a charge carrier. Since the cathodic limit of the solvate ionic liquid is determined by deposition and dissolution of lithium, it is possible to construct the batteries combining a lithium anode with the redox reactions dissolved in the solvate ionic liquid using a lithium-ion conductive solid-state electrolyte as a separator between the anolyte and catholyte. We have already proposed the battery using the redox reaction of tris(2,2'-bipyridine)iron(III/II) complexes in the solvate ionic liquid with the cell voltage of about 4 V.[4] In the present study, the redox reaction of a stable free radical, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), was investigated in the solvate ionic liquid, [Li(G4)]TFSA, for application to the redox flow battery. Both TEMPO and its oxidized form, TEMPO+, were found to be soluble in the solvate ionic liquid at ambient temperature. The redox reaction of TEMPO+/TEMPO was observed in the cyclic voltammetry of a glassy carbon electrode in [Li(G4)]TFSA containing TEMPO. The diffusion of TEMPO was found to be faster than that of TEMPO+, probably due to coulombic interaction of TEMPO+ with the ions composing the solvate ionic liquid. The standard rate constant was determined by electrochemical impedance spectroscopy. The temperature dependence of the standard rate constant suggested the redox reaction of TEMPO+/TEMPO can be regarded as an outer-sphere electron transfer reaction with a small reorganization energy. The redox battery was constructed using a lithium anode and the catholyte of the solvate ionic liquid containing TEMPO with a thin lithium-ion conductive solid-state electrolyte membrane. The redox battery could be operated successfully with the average discharge voltage of 3.4 V.

Measurements and Simulations of the Acidity Dependence of the Kinetics of the Iron-Catalyzed Belousov-Zhabotinsky Reaction: Proton-Catalysis in the Electron Transfer Reaction Involving the [Fe(phen)3]3+ Species.

The acidity dependence of the iron-catalyzed bromate-malonic acid Belousov-Zhabotinsky reaction was studied in the range 0.36 M < [H2SO4]0 < 1.20 M, and the temporal evolutions of the oscillation patterns were analyzed. The experimental results show that the period times PT i decrease exponentially with increasing acidity and that the period times parallel the decrease of the reduction times RT with increasing acidity. Simulations using the reactions of the commonly accepted core reaction mechanism failed to match the measurements even in a qualitative fashion. However, we found that compelling agreement between the experiments and the simulations can be achieved over the entire range with the inclusion of second-order proton-catalysis of the oxidation of bromomalonic acid (BrMA) by the [Fe(phen)3]3+ species in the reaction identified in this paper as reaction 9 (R9), and this [H+] dependence is informative about the species involved in the outer sphere electron transfer reaction. The trication [Fe(phen)3]3+ species is stabilized by ion pairing and solvation, and one may anticipate the presence of [Fe(phen)3(HSO4) n(H2O) m](3- n)+ species ( n = 0-3). Our results suggest that the removal of aggregating HSO4- ions by protonation creates a better oxidant and facilitates the approach of the reductant BrMA, and the second-order [H+] dependence further suggests that BrMA is primarily oxidized by a doubly charged [Fe(phen)3(HSO4)1(L) k]2+ species. Considering the complexity of the BZ system and the uncertainties in the many reaction rate constants, we were somewhat surprised to find this high level of agreement by (just) the replacement of R9 by R9'. In fact, the near-quantitative agreement presents a powerful corroboration of the core reaction mechanism of the BrMA-rich BZ reaction, and the replacement of R9 by R9' extends the validity of this core reaction mechanism to acidities above and below the typical acidity of BZ reactions ([H+] ≈ 1 M).

Fabrication and characterization of porous, conductive, monolithic Ti4O7 electrodes

Highly porous and conductive monolithic electrodes have found various electrochemical applications in water treatment and energy storage and conversion. In this work we have successfully synthesized monolithic Ti4O7 electrodes by two production methods—mechanical pressing of Ti4O7 powders and gel casting of TiO2 powders followed by sintering and reduction to Ti4O7. The electrodes were characterized by X-ray diffraction (XRD), Hg porosimetry, scanning electron microscopy, thermal gravimetric analysis, Mott-Schottky analysis, and sweep voltammetry. The removal of oxygen atoms from the TiO2 precursor resulted in n-type doping behavior, and XRD measurements confirmed the electrodes consisted of high purity Ti4O7. Porosimetry analyses indicated the electrodes had high porosities (44–48%) and low bulk densities (∼2.0 g cm−3), which allowed for high permeate fluxes of up to 801 ± 81 LMH bar−1, when operated in a flow-through mode. The efficient mass transfer achieved in the porous electrodes resulted in high rate constants for the oxidation of Fe(CN)64− (kobs = 3.0–4.5 × 10−4 m s−1). The electrodes also showed high rates of removal for oxalic acid and terephthalic acid, which were probes for direct oxidation and OH production, respectively. Chemical etching of the electrodes increased the electroactive surface area by approximately 2-fold, which increased the capacitance (30 mF cm−2) and reactivity of the electrodes towards outer sphere electron transfer reactions and the production of OH by the same factor. These results indicated that the Ti4O7 monolithic electrodes could enhance charge transfer reactions in various electrochemical applications.

The Role of Structure Making/Breaking Ions in Solvation Shell and Redox Reaction Entropy of Outer Sphere Electron Transfer Reactions

Marcus theory1 of electron transfer is the dominant theory to describe the rate constant of electron transfer reactions both in chemistry and biology during last several decades. The general treatment on the solvation free energy of reactants is largely based on Born model2 that the solvent is assumed as a dielectric continuum and each reactant is treated as a sphere where the first solvation layer is dielectrically saturated. However, these assumptions could shield the fact of non-covalent interactions within the solvation shell of reactants, including reactant nuclei-solvent, reactant nuclei-inert ion, inert ion-solvent interactions and hydrogen bonding of solvent molecules. The entropy change of an electron transfer reactions, , is the difference of standard molar entropy between the reduced and oxidized forms of a redox couple. The reaction entropy of outer sphere self-exchange can be qualitatively described by Born model even though the Born prediction differs from the experimental data of some redox couples, according to previous studies3. It was shown that the reaction entropy of a given redox couple increases with decreasing in solvent acceptor number. This dependence of upon the solvent nature could be associated with the degree of “internal order” of solvent molecules. Besides solvent-solvent molecule interaction, the “internal order” of solvent could be disrupted by dissolved charged species, including the reactant nuclei of redox couples and inert ions either from the counter-ion of redox couples or from the supporting electrolyte. Depending on its effects on water structure and vice versa, ions are usually categorized in either “structure maker” or “structure breaker”4, which could affect surrounding hydrogen bonds and water molecular arrangement around itself5. In addition to solvent-solvent and ion-solvent interactions, the coulombic interaction between the redox couples and the ions with opposite sign of charge to those of redox couples has been suggested6. It could form an ionic atmosphere and, thereby affects to the reaction entropy of redox couples. In aqueous electrolyte, a simple description of this coulombic interaction by an electrostatic energy necessary for forming an ionic atmosphere by surrounding the species with the counter ions seems not to be prudent due to the fact of ternary interactions of redox nuclei, counter ions and solvent molecules. However, the non-covalent interactions have not, to the best of our knowledge, previously been considered in terms of the entropy change of outer sphere self-exchange electron transfer reactions, and it is the impact of structure making/breaking ions on the structure of the solvation shell of the reactant nuclei that constitutes the focus of this work. Here, we studied reaction entropy of two outer sphere reactions: [Fe(H2O)6]2+/3+ and [Fe(CN)6]3-/4- upon the nature and the concentration of anions and cations, respectively. For both positively and negatively charged redox couples, it was shown that structure maker ions enhance the “order” in vicinity of redox nuclei while structure breaker ions promote “disorder”. We suggest also molecular level models for redox nuclei solvation by taking into account reactant nuclei-solvent, reactant nuclei-inert ion, inert ion-solvent interactions and hydrogen bonding of solvent molecules. References (1) Marcus, R. A. J. Chem. Phys. 1956, 24, 966. (2) Born, M. Z. Für Phys. 1920, 1 (1), 45–48. (3) Hupp, J. T.; Weaver, M. J. Inorg. Chem. 1984, 23 (22), 3639–3644. (4) Marcus, Y. Chem. Rev. 2009, 109 (3), 1346–1370. (5) Miller, D. J.; Lisy, J. M. J. Am. Chem. Soc. 2008, 130 (46), 15381–15392. (6) Yamato, Y.; Katayama, Y.; Miura, T. J. Electrochem. Soc. 2013, 160 (6), H309–H314.

Penicillamine and captopril: mechanistic exploration of defensive actions of thiol drugs against a metal bound-superoxo complex

In acid-media ([H+] = 0.01–0.06 M), each of the thiol compounds, D-penicillamine (PEN, LPH2) and captopril (CAP, LCH2) exist in several proton-dependent forms which can reduce the superoxo complex [(en)(dien)CoIII(O2)CoIII(en)(dien)]5+ (1) to the corresponding peroxo [(en)(dien)CoIII(O2)CoIII(en)(dien)]4+ (2) or the hydroperoxo complex [(en)(dien)CoIII(OOH)CoIII(en)(dien)]5+ (3). The observed first-order rate constants, ko,P and ko,C for PEN and CAP increase with the increase in [TPEN] and [TCAP] (which are the analytical concentrations of the respective thiols) but decrease with the increase in the media-acidity ([H+]) and the media ionic strength (I). The protolytic equilibria in aqueous solution allow several potentially reducing forms to coexist for both PEN (LPH3+, LPH2, LPH−, and LP2−) and CAP (LCH2, LCH−, LC2−) but the kinetic analyses reveal that the order of reactivity for the species are LPH3+ ~ LPH2 <<< LPH− and LCH2 < LCH− <<< LC2−, respectively. The predominance and higher reactivities of the anionic species, LPH− and LC2− are supported by the negative slopes of the plots of ko,P or ko,C versus I. Moreover, a large value of kH/kD for PEN suggests an inner-sphere electroprotic reaction pathway while the absence of such effect for CAP strongly supports an outer-sphere electron transfer reaction. These propositions are supported by the structural features of LPH− and LC2−.

Advanced Electrochemistry of Individual Metal Clusters Electrodeposited Atom by Atom to Nanometer by Nanometer.

Metal clusters are very important as building blocks for nanoparticles (NPs) for electrocatalysis and electroanalysis in both fundamental and applied electrochemistry. Attention has been given to understanding of traditional nucleation and growth of metal clusters and to their catalytic activities for various electrochemical applications in energy harvesting as well as analytical sensing. Importantly, understanding the properties of these clusters, primarily the relationship between catalysis and morphology, is required to optimize catalytic function. This has been difficult due to the heterogeneities in the size, shape, and surface properties. Thus, methods that address these issues are necessary to begin understanding the reactivity of individual catalytic centers as opposed to ensemble measurements, where the effect of size and morphology on the catalysis is averaged out in the measurement. This Account introduces our advanced electrochemical approaches to focus on each isolated metal cluster, where we electrochemically fabricated clusters or NPs atom by atom to nanometer by nanometer and explored their electrochemistry for their kinetic and catalytic behavior. Such approaches expand the dimensions of analysis, to include the electrochemistry of (1) a discrete atomic cluster, (2) solely a single NP, or (3) individual NPs in the ensemble sample. Specifically, we studied the electrocatalysis of atomic metal clusters as a nascent electrocatalyst via direct electrodeposition on carbon ultramicroelectrode (C UME) in a femtomolar metal ion precursor. In addition, we developed tunneling ultramicroelectrodes (TUMEs) to study electron transfer (ET) kinetics of a redox probe at a single metal NP electrodeposited on this TUME. Owing to the small dimension of a NP as an active area of a TUME, extremely high mass transfer conditions yielded a remarkably high standard ET rate constant, k0, of 36 cm/s for outer-sphere ET reaction. Most recently, we advanced nanoscale scanning electrochemical microscopy (SECM) imaging to resolve the electrocatalytic activity of individual electrodeposited NPs within an ensemble sample yielding consistent high k0 values of ≥2 cm/s for the hydrogen oxidation reaction (HOR) at different NPs. We envision that our advanced electrochemical approaches will enable us to systematically address structure effects on the catalytic activity, thus providing a quantitative guideline for electrocatalysts in energy-related applications.

Kinetic Modulation of Outer-Sphere Electron Transfer Reactions on Graphene Electrode with a Sub-surface Metal Substrate

The atomic thickness of graphene electrodes makes them a suitable platform to explore interfacial interactions with dissimilar materials and their impact on the rates and mechanisms of electrochemical reactions. Here, we report on the modulation of the electrochemical kinetics of outer-sphere redox mediators by metal electrodes buried in the sub-surface of continuous double layer graphene electrodes. Graphene was obtained via chemical vapor deposition and a near-full coverage of graphene over the metal surface was inspected via microscopy and by the use of electrochemically active surface redox probes. Enhancements in the rate of electron transfer on three reactive species, ferrocyanide, ferricyanide, and ferrocene, were observed on graphene coated metal deposits using scanning electrochemical microscopy in the feedback mode. Our results strongly suggest that the electron transfer kinetics to outer sphere redox species across the graphene/electrolyte interface is significantly enhanced by the use of subsurface metals. We show that buried Au deposits exhibit ca. 5 times enhancement of the electrochemical rate constant. These enhancements are likely a consequence of increased electronic density of states due to a donating effect from the substrate.

Electrodeposition of Single Nanometer-Size Pt Nanoparticles at a Tunneling Ultramicroelectrode and Determination of Fast Heterogeneous Kinetics for Ru(NH3)6(3+) Reduction.

We studied extremely fast kinetics of an outer-sphere heterogeneous electron transfer (ET) reaction at a single Pt nanoparticle (NP) using the newly adapted Kotecký-Levich (K-L) method. In this work, an electrode was prepared by nucleating and growing a single Pt NP on a tunneling ultramicroelectrode (TUME) that produces 1-40 nm or greater dimensions. Such a small-size electrode greatly enhances the mass transfer rate, thus enabling us to reliably determine ET kinetic parameters for fast ET reactions. Based on the recently demonstrated K-L model for a general UME, ET kinetic information could be measured by constructing a plot of 1/current density vs 1/mass transfer rate from the series of steady-state voltammograms obtained using Pt NP-deposited TUMEs. For this K-L plot, we altered the mass transfer rates by varying the electrode size, i.e., the Pt NP size in this work. The determined standard rate constant, k(0), of heterogeneous reduction reaction for Ru(NH3)6(3+) was unprecedentedly high, at 36 ± 4 cm/s, confirmed by theoretical simulation. Extended applications to various electrocatalytic reactions with different types of electrodeposited metal NPs will show the versatility of our approach. Particularly, this novel fabrication of a nanometer-sized electrode and its application to fast ET kinetic study with simple instrumentation should be useful in studies of particle size and structure effects on given catalytic reactions.

A kinetic study of the oxidations of 2-mercaptoethanol and 2-mercaptoethylamine by heteropoly 11-tungsto-1- vanadophosphate in aqueous acidic medium

The kinetics of oxidation of 2-mercaptoethanol and 2-mercaptoethylamine by the heteropoly 11-tungsto-1- vanadophosphate anion, [PVVW11O40]4−, have been studied spectrophotometrically in aqueous perchloric acid at 25 °C. EPR and optical studies show that [PVVW11O40]4− is reduced to the one-electron reduced heteropoly blue, [PVIVW11O40]5−, whilst the thiols are oxidized to the corresponding disulphides, RSSR. Spectrophotometric titrations show that the stoichiometry of both reactions is 1:1. At constant pH, the reactions show simple second-order kinetics with first-order dependence of rate on both [oxidant] and [thiol]. At constant [thiol], the rate of the reaction increases with increasing pH. Plots of kobs/[thiol]t versus 1/[H+] are linear with finite intercepts, showing that both the undissociated thiol (RSH) and thiolate ion (RS−) are reactive species. Generation of RS· from RSH proceeds via a separated-concerted proton–electron transfer mechanism. The reaction of thiolate ion is a simple outer-sphere electron transfer reaction. By applying the Marcus theory, the self-exchange rate constants for the couples HOCH2CH2S·/HOCH2CH2S− and H3N+CH2CH2S·/H3N+CH2CH2S− were evaluated as 3 × 109 and 2.2 × 108 M−1 s−1, respectively, at 25 °C.

An experimental and ab initio study on the abiotic reduction of uranyl by ferrous iron

It is important to understand the mechanisms controlling the removal of uranyl from solution from an environmental standpoint, particularly whether soluble Fe(II) is capable of reducing soluble U(VI) to insoluble U(IV). Experiments were performed to shed light into discrepancies of recent studies about precipitation of U-containing solids without changing oxidation states versus precipitation/reduction reactions, especially with respect to the kinetics of these reactions. To understand the atomistic mechanisms, thermodynamics, and kinetics of these redox processes, ab initio electron transfer (ET) calculations, using Marcus theory, were applied to study the reduction of U(VI)aq to U(V)aq by Fe(II)aq (the first rate-limiting ET-step). Outer-sphere (OS) and inner-sphere (IS) Fe–U complexes were modeled to represent simple species within a homogeneous environment through which ET could occur.Experiments on the chemical reduction were performed by reacting 1mM Fe(II)aq at pH 7.2 with high (i.e., 0.16mM) and lower (i.e., 0.02mM) concentrations of U(VI)aq. At higher U concentration, a rapid decrease in U(VI)aq was observed within the first hour of reaction. XRD and XPS analyses of the precipitates confirmed the presence of (meta)schoepite phases, where up to ∼25% of the original U was reduced to U4+ and/or U5+-containing phases. In contrast, at 0.02mM U, the U(VI)aq concentration remained fairly constant for the first 3h of reaction and only then began to decrease due to slower precipitation kinetics. XPS spectra confirm the partial chemical reduction U associated with the precipitate (up to ∼30%). Thermodynamic calculations support that the reduction of U(VI)aq to U(IV)aq by Fe(II)aq is energetically unfavorable. The batch experiments in this study show U(VI) is removed from solution by precipitation and that transitioning to a heterogeneous system in turn enables the solid U phase to be partially reduced.Ab initio ET calculations revealed that OS ET is strongly kinetically inhibited in all cases modeled. OS ET as a concerted proton-coupled ET reaction (ferrimagnetic spin configuration) is thermodynamically favorable (−35kJ/mol), but kinetically inhibited by concurrent proton-transfer (10−19s−1). OS ET as a sequential proton-coupled ET reaction is thermodynamically unfavorable (+102kJ/mol) as well as kinetically inhibited, where ET is the rate-limiting step (10−12s−1). In contrast, the reduction of U(VI)aq to U(V)aq by Fe(II)aq as an IS ET reaction is both thermodynamically favorable (−16kJ/mol) and kinetically rapid (108s−1); the IS ET rate is several orders of magnitude faster than the OS ET rate. Thus, reduction of U(VI)aq to U(V)aq by Fe(II)aq in a homogenous system could occur if an IS Fe–U complex can be achieved. However, the formation of IS Fe–U complexes in an homogeneous solution is predicted to be low; considerable thermodynamic and kinetic barriers exist to proceed from an OS ET reaction to an IS ET reaction, a process that needs to overcome dehydration of the first solvation shell (+96kJ/mol) and hydrolysis of Fe(II)aq. The computational results complement and further substantiate experimental results where the reduction of U(VI)aq by Fe(II)aq does not occur.