Variable Scan Rate Cyclic Voltammetry Research Articles

Understanding How Organosilicon Additives Delay Electrolyte Decomposition on LiNi0.8Mn0.1Co0.1O2 (NMC 811) Cathodes Using in Situ Techniques

Cathode materials such as LiNi0.8Mn0.1Co0.1O2 (NMC811) are of great interest for Li-ion battery (LIB) systems. By increasing the Ni content, the price decreases since Co is an expensive transition metal. In addition, NMC811 offers higher capacity and operating at high voltages, compared to other cathode materials (i.e., LCO, LMO, etc.). However, this cathode material suffers degradation from different sources. One of the main detrimental processes is oxygen loss from the lattice, which can cause decomposition of electrolyte components such as cyclic carbonates and further generate undesirable gassing in LIB systems. Several different organosilicon (OS) compounds have been reported to decrease gassing during storage with NMC622/Gr multi-layer pouch cells. Besides reducing gassing, OS compounds have been reported to increase the thermal stability of PF6 - and reduce HF, which is detrimental for LIB systems. Currently, the exact mechanism for how OS additives alleviate these undesired electrolyte degradation reactions is not well known. In this study we performed a mechanistic investigation on the effects of OS3 (organosilicon additive) during Li the intercalation/deintercalation processes on NMC811. First, we explored how OS3 affects the electron transfer kinetics by performing variable scan rate cyclic voltammetry. Second, we studied the mass accumulated on NMC811 surfaces during the first cycle as a function of applied potential using in situ electrochemical quartz crystal microbalance (EQCM). Finally, we analyzed the chemical composition of the cathode surface layer and how it evolves as a function of potential using in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) at stepwise points during the first cycle. Together, these techniques provide a comprehensive study of the reactions occurring at the NMC811 electrode surface during the first cycle, including lithium intercalation kinetics, mass deposition and dissolution, and the chemical composition of surface decomposition species as a function of potential, it was found that OS3 delays the onset of mass accumulation and therefore delays electrolyte degradation during the first cycle on NMC811 cathode materials as confirmed by the presence of new absorbance features on in situ ATR-FTIR. We successfully identified a potential mechanism of action of how organosilicon additives help to prevent the carbonate-based electrolyte to decompose on NMC811 surfaces. These studies further provide new insights into the mechanisms of performance enhancement observed with OS additives.

(Invited) Spectroelectrochemical Sensors: The Quest for Selectivity

Introduction Exceptional selectivity is needed for sensors to accurately measure analytes in complex samples such as nuclear waste and natural water that contain many potentially interfering substances. Spectroelectrochemistry offers a unique means for providing selectivity by electrochemically modulating the optical signal used for quantitation. The modulated signal can be distinguished from the constant signals of potential interferences. An additional level of selectivity can be provided by adding a very thin film to the transparent electrode to first preconcentrated the analyte while rejecting interferences. Sensors using both visible absorption and fluorescence modes of detection have been developed for detection of organic and inorganic species including ferrocyanide [1] and technetium [2] complexes in nuclear waste and natural water. We have recently extended this research to the spectroelectrochemical investigation of molten salt systems, [3] including lanthanide and actinide measurements related to pyroprocessing [4] and molten salt reactor applications. Thin layer fluorescence-based spectroelectrochemical sensor A planar optically transparent electrode chip was fabricated and characterized for multimode sensing of target species. The chip was comprised of an indium tin oxide (ITO) working electrode, a platinum auxiliary electrode, and a Ag/AgCl reference electrode. Fluorescence was used for detection because limits of detection are typically several magnitudes lower than for absorption spectroscopy and selectivity improved by having adjustable wavelengths for both excitation and emission. A fluorescence spectroelectrochemical measurement using this chip incorporated into an optically transparent thin layer electrochemical (OTTLE) cell was demonstrated using the modulated emission of [Ru(bpy)3]2+. Molten salt absorbance-based spectroelectrochemistry Molten salt systems are of growing interest for a variety of applications, including meeting future energy needs such as molten salt reactors (MSRs). These reactors are of interest for several factors, including the ability to be operated at atmospheric pressures even at extremely high temperatures. The molten salts serve several functions, including heat transfer fluid, reactor coolant, and ability to contain high concentrations of dissolved fuel (plutonium, uranium, thorium). While the benefits of molten salt reactors have been identified, the chemistry of the molten salts is still not understood. Spectroelectrochemistry can provide the tools to characterize, monitor, and control the chemical behavior of fuel species (U, Pu), fission, and corrosion products throughout the molten salt reactor loop. Several of these species are electroactive and have a unique spectroscopic fingerprint that can be optically detected and modulated using electrochemistry. Spectroelectrochemical characterization can provide fundamental information such as concentration, oxidation state, and speciation at any point in the molten salt reactor system. The absorbance-based spectroelectrochemical modulation of uranium was demonstrated in the molten LiCl-KCl eutectic. The unique spectroscopic signatures for U(III) and U(IV) were identified and monitored as it was electrochemically modulated between the U(III)/U(IV) redox states. Based on variable scan-rate cyclic voltammetry, the diffusion coefficients of U(III) and U(IV) were measured.

Properties of electrochemically copolymerized aniline and melamine on functionalized multiwalled‐carbon nanotube film electrodes

AbstractThe electrochemical properties of polymerized aniline (PANI) and polymerized melamine (PMEL) that were electrochemical copolymerized (PANIMEL) on a glassy carbon electrode (GCE) that had been coated with functionalized multiwalled carbon nanotubes (fMWCNT) to form a PANIMEL/fMWCNT/GCE film electrode were studied, with an aim toward electrochemical energy storage (EES). A number of factors, such as the choice of working electrode, electrolyte, switching potential, applied scan rate, and type of fMWCNTs, were initially investigated and evaluated during the individual electrochemical polymerization of aniline and melamine via successive potential cycling. The electrochemical copolymerisation of aniline and melamine was then studied with an ideal monomeric ratio of 1:3 that gave an optimal ratio of the voltammetric peak current heights with distinguishable redox peak potentials. Variable scan rate cyclic voltammetry (CV) of the electrosynthesized copolymer film electrode confirmed the dominance of the surface‐confined electron transfer process at the electrode. The electrochemical stability of the copolymer film electrode was also assessed and revealed a limited cyclability of the daughter polymeric melamine, which was hypothesized to be due to an excessive nitrogen content combined with a low porosity that led to a poor ion intercalation‐deintercalation mechanism. Electrochemical impedance spectroscopy (EIS) was performed to evaluate the electrochemical performance of the copolymerized film electrode with other control electrodes. The corresponding EIS results suggested that the copolymerized film electrode was electrochemically superior to the PMEL/fMWCNT/GCE film electrode but was inferior to the PANI/fMWCNT/GCE film electrode.

Preparation and electrochemical performance of sodium manganese oxides as cathode materials for aqueous Mg-ion batteries

In recent years, the cathode materials of magnesium ion batteries have become a hot point of research, and the improvement of high-rate performance and cycle stability has become the main research goal. In this paper, sodium manganese oxide (Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O) nanomaterial with a blended structure of nanowires and nanosheets is prepared by the hydrothermal method. The structure and morphology of the material are analyzed by X-ray diffraction and scanning electron microscopy. The variable rate charge-discharge curves and variable scan rate cyclic voltammetry curves are obtained by a battery tester and electrochemical workstation, respectively. The results show that the hydrothermal reaction time has significant effects on phase structure and morphology composition of the material. The nanosheets and nanowires in the sample form a closely blend by 72-h hydrothermal reaction (NMO-72), and the nanosheets effectively fill into the intersecting space of the nanowires. In this way, the tap density of the material is improved. More importantly, NMO-72 has higher discharge specific capacity and rate cycling performance. At a current density of 50 mA·g<sup>–1</sup>, the discharge specific capacity of NMO-72 reaches 229.1 mAh·g<sup>–1</sup>. At a current density of 1000 mA·g<sup>–1</sup>, the discharge specific capacity of the NMO-72 stabilizes at 81 mAh·g<sup>–1</sup>. When the current density returns to 50 mA·g<sup>–1</sup> again, the discharge specific capacity remains stable at 164.7 mAh·g<sup>–1</sup>. Besides, the cyclic voltammetry test shows that the NMO-72 material has more excellent magnesium ion diffusion kinetic performance than other materials. Therefore, the NMO-72 material has more excellent reversible specific capacity, high rate performance and cycling stability.

(Invited) Spectroelectrochemical Sensors: The Quest for Selectivity

Introduction Exceptional selectivity is needed for sensors to accurately measure analytes in complex samples such as nuclear waste and natural water that contain many potentially interfering substances. Spectroelectrochemistry offers a unique means for providing selectivity by electrochemically modulating the optical signal used for quantitation. The modulated signal can be distinguished from the constant signals of potential interferences. An additional level of selectivity can be provided by adding a very thin film to the transparent electrode to first preconcentrated the analyte while rejecting interferences. Sensors using both visible absorption and fluorescence modes of detection have been developed for detection of organic and inorganic species including ferrocyanide [1] and technetium [2] complexes in nuclear waste and natural water. We have recently extended this research to the spectroelectrochemical investigation of molten salt systems, [3] including lanthanide and actinide measurements related to pyroprocessing [4] and molten salt reactor applications. Thin layer fluorescence-based spectroelectrochemical sensor A planar optically transparent electrode chip was fabricated and characterized for multimode sensing of target species. The chip was comprised of an indium tin oxide (ITO) working electrode, a platinum auxiliary electrode, and a Ag/AgCl reference electrode (Figure 1A,B). Fluorescence was used for detection because limits of detection are typically several magnitudes lower than for absorption spectroscopy and selectivity improved by having adjustable wavelengths for both excitation and emission. A fluorescence spectroelectrochemical measurement using this chip incorporated into an optically transparent thin layer electrochemical (OTTLE) cell was demonstrated using the modulated emission of [Ru(bpy)3]2+. Molten salt absorbance-based spectroelectrochemistry Molten salt systems are of growing interest for a variety of applications, including meeting future energy needs such as molten salt reactors (MSRs). These reactors are of interest for several factors, including the ability to be operated at atmospheric pressures even at extremely high temperatures. The molten salts serve several functions, including heat transfer fluid, reactor coolant, and ability to contain high concentrations of dissolved fuel (plutonium, uranium, thorium). While the benefits of molten salt reactors have been identified, the chemistry of the molten salts is still not understood. Spectroelectrochemistry can provide the tools to characterize, monitor, and control the chemical behavior of fuel species (U, Pu), fission, and corrosion products throughout the molten salt reactor loop. Several of these species are electroactive and have a unique spectroscopic fingerprint that can be optically detected and modulated using electrochemistry. Spectroelectrochemical characterization can provide fundamental information such as concentration, oxidation state, and speciation at any point in the molten salt reactor system. Absorbance-based spectroelectrochemical modulation of uranium was demonstrated in the molten LiCl-KCl eutectic. The unique spectroscopic signatures for U(III) and U(IV) were identified and monitored as it was electrochemically modulated between the U(III)/U(IV) redox states. Based on variable scan-rate cyclic voltammetry, the diffusion coefficients of U(III) and U(IV) were measured. The spectroelectrochemistry of lanthanides and other actinides in chloride melts is being explored for sensor development.

Electrocatalytic Hydrogen Production by a Nickel Complex Containing a Tetradentate Phosphine Ligand

A nickel complex has been synthesized using the P4N2 ligand, a tetradentate phosphine ligand with two pendant amines in the ligand backbone. The rigidly square pyramidal [Ni(P4N2)(CH3CN)]2+ complex is an electrocatalyst for the reduction of protons to hydrogen. Using N,N-dimethylformamidium ([DMF(H)+]) as the acid in an acetonitrile/water solution, [Ni(P4N2)(CH3CN)]2+ displays a turnover frequency of 1.6 × 106 s–1, which is among the fastest rates reported for any molecular electrocatalyst. This high catalytic rate comes at the cost of a 1200 mV overpotential at the catalytic half-wave potential. The Ni(II) state of the catalyst was found to be stable under strongly acidic conditions, with only trace decomposition observed over 2 weeks in the presence of 0.1 M trifluoromethanesulfonic acid (HOTf). The catalyst was less stable in the Ni(0) state due to the inability of the rigid P4N2 ligand to adopt a tetrahedral geometry. Using variable scan rate cyclic voltammetry, a first-order rate constant of ∼1 s–1 w...

Electrochemical Detection of Transient Cobalt Hydride Intermediates of Electrocatalytic Hydrogen Production.

A large variety of molecular cobalt complexes are used as electrocatalysts for H2 production, but the key cobalt hydride intermediates are frequently difficult to detect and characterize due to their high reactivity. We report that a combination of variable scan rate cyclic voltammetry and foot-of-the-wave analysis (FOWA) can be used to detect transient Co(III)H and Co(II)H intermediates of electrocatalytic H2 production by [Co(II)(P(tBu)2N(Ph)2)(CH3CN)3](2+) and Co(II)(dmgBF2)2(CH3CN)2. In both cases, reduction of a transient catalytic intermediate occurs at a potential that coincides with the Co(II/I) couple. Each reduction displays quasireversible electron-transfer kinetics, consistent with reduction of a Co(III)H intermediate to Co(II)H, which is then protonated by acid to generate H2. A bridge-protonated Co(I) species was ruled out as a catalytic intermediate for Co(II)(dmgBF2)2(CH3CN)2 from voltammograms recorded at 1000 psi of H2. Density functional theory was used to calculate Co(III)-H and Co(II)-H bond strengths for both catalysts. Despite having very different ligands, the cobalt hydrides of both catalysts possess nearly identical heterolytic and homolytic Co-H bond strengths for the Co(III)H and Co(II)H intermediates.

Radically Enhanced Molecular Switches

The mechanism governing the redox-stimulated switching behavior of a tristable [2]rotaxane consisting of a cyclobis(paraquat-p-phenylene) (CBPQT(4+)) ring encircling a dumbbell, containing tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) recognition units which are separated from each other along a polyether chain carrying 2,6-diisopropylphenyl stoppers by a 4,4'-bipyridinium (BIPY(2+)) unit, is described. The BIPY(2+) unit acts to increase the lifetime of the metastable state coconformation (MSCC) significantly by restricting the shuttling motion of the CBPQT(4+) ring to such an extent that the MSCC can be isolated in the solid state and is stable for weeks on end. As controls, the redox-induced mechanism of switching of two bistable [2]rotaxanes and one bistable [2]catenane composed of CBPQT(4+) rings encircling dumbbells or macrocyclic polyethers, respectively, that contain a BIPY(2+) unit with either a TTF or DNP unit, is investigated. Variable scan-rate cyclic voltammetry and digital simulations of the tristable and bistable [2]rotaxanes and [2]catenane reveal a mechanism which involves a bisradical state coconformation (BRCC) in which only one of the BIPY(•+) units in the CBPQT(2(•+)) ring is oxidized to the BIPY(2+) dication. This observation of the BRCC was further confirmed by theoretical calculations as well as by X-ray crystallography of the [2]catenane in its bisradical tetracationic redox state. It is evident that the incorporation of a kinetic barrier between the donor recognition units in the tristable [2]rotaxane can prolong the lifetime and stability of the MSCC, an observation which augurs well for the development of nonvolatile molecular flash memory devices.

Ground-State Thermodynamics of Bistable Redox-Active Donor–Acceptor Mechanically Interlocked Molecules

Fashioned through billions of years of evolution, biological molecular machines, such as ATP synthase, myosin, and kinesin, use the intricate relative motions of their components to drive some of life's most essential processes. Having control over the motions in molecules is imperative for life to function, and many chemists have designed, synthesized, and investigated artificial molecular systems that also express controllable motions within molecules. Using bistable mechanically interlocked molecules (MIMs), based on donor-acceptor recognition motifs, we have sought to imitate the sophisticated nanoscale machines present in living systems. In this Account, we analyze the thermodynamic characteristics of a series of redox-switchable [2]rotaxanes and [2]catenanes. Control and understanding of the relative intramolecular movements of components in MIMs have been vital in the development of a variety of applications of these compounds ranging from molecular electronic devices to drug delivery systems. These bistable donor-acceptor MIMs undergo redox-activated switching between two isomeric states. Under ambient conditions, the dominant translational isomer, the ground-state coconformation (GSCC), is in equilibrium with the less favored translational isomer, the metastable-state coconformation (MSCC). By manipulating the redox state of the recognition site associated with the GSCC, we can stimulate the relative movements of the components in these bistable MIMs. The thermodynamic parameters of model host-guest complexes provide a good starting point to rationalize the ratio of GSCC to MSCC at equilibrium. The bistable [2]rotaxanes show a strong correlation between the relative free energies of model complexes and the ground-state distribution constants (K(GS)). This relationship does not always hold for bistable [2]catenanes, most likely because of the additional steric and electronic constraints present when the two rings are mechanically interlocked with each other. Measuring the ground-state distribution constants of bistable MIMs presents its own set of challenges. While it is possible, in principle, to determine these constants using NMR and UV-vis spectroscopies, these methods lack the sensitivity to permit the determination of ratios of translational isomers greater than 10:1 with sufficient accuracy and precision. A simple application of the Nernst equation, in combination with variable scan-rate cyclic voltammetry, however, allows the direct measurement of ground-state distribution constants across a wide range (K(GS) = 10-10(4)) of values.

Electrochemically Induced Chemically Reversible Proton-Coupled Electron Transfer Reactions of Riboflavin (Vitamin B2)

The electrochemical behavior of the naturally occurring vitamin B(2), riboflavin (Fl(ox)), was examined in detail in dimethyl sulfoxide solutions using variable scan rate cyclic voltammetry (ν = 0.1 - 20 V s(-1)) and has been found to undergo a series of proton-coupled electron transfer reactions. At a scan rate of 0.1 V s(-1), riboflavin is initially reduced by one electron to form the radical anion (Fl(rad)(•-)) at E(0)(f) = -1.22 V versus Fc/Fc(+) (E(0)(f) = formal reduction potential and Fc = ferrocene). Fl(rad)(•-) undergoes a homogeneous proton transfer reaction with the starting material (Fl(ox)) to produce Fl(rad)H(•) and Fl(ox)(-), which are both able to undergo further reduction at the electrode surface to form Fl(red)H(-) (E(0)(f) = -1.05 V vs Fc/Fc(+)) and Fl(rad)(•2-) (E(0)(f) = -1.62 V vs Fc/Fc(+)), respectively. At faster voltammetric scan rates, the homogeneous reaction between Fl(rad)(•-) and Fl(ox) begins to be outrun, which leads to the detection of a voltammetric peak at more negative potentials associated with the one-electron reduction of Fl(rad)(•-) to form Fl(red)(2-) (E(0)(f) = -1.98 V vs Fc/Fc(+)). The variable scan rate voltammetric data were modeled quantitatively using digital simulation techniques based on an interconnecting "scheme of squares" mechanism, which enabled the four formal potentials as well as the equilibrium and rate constants associated with four homogeneous reactions to be determined. Extended time-scale controlled potential electrolysis (t > hours) and spectroscopic (EPR and in situ UV-vis) experiments confirmed that the chemical reactions were completely chemically reversible.

Solution-Phase Mechanistic Study and Solid-State Structure of a Tris(bipyridinium radical cation) Inclusion Complex

The ability of the diradical dicationic cyclobis(paraquat-p-phenylene) (CBPQT(2(•+))) ring to form inclusion complexes with 1,1'-dialkyl-4,4'-bipyridinium radical cationic (BIPY(•+)) guests has been investigated mechanistically and quantitatively. Two BIPY(•+) radical cations, methyl viologen (MV(•+)) and a dibutynyl derivative (V(•+)), were investigated as guests for the CBPQT(2(•+)) ring. Both guests form trisradical complexes, namely, CBPQT(2(•+))⊂MV(•+) and CBPQT(2(•+))⊂V(•+), respectively. The structural details of the CBPQT(2(•+))⊂MV(•+) complex, which were ascertained by single-crystal X-ray crystallography, reveal that MV(•+) is located inside the cavity of the ring in a centrosymmetric fashion: the 1:1 complexes pack in continuous radical cation stacks. A similar solid-state packing was observed in the case of CBPQT(2(•+)) by itself. Quantum mechanical calculations agree well with the superstructure revealed by X-ray crystallography for CBPQT(2(•+))⊂MV(•+) and further suggest an electronic asymmetry in the SOMO caused by radical-pairing interactions. The electronic asymmetry is maintained in solution. The thermodynamic stability of the CBPQT(2(•+))⊂MV(•+) complex was probed by both isothermal titration calorimetry (ITC) and UV/vis spectroscopy, leading to binding constants of (5.0 ± 0.6) × 10(4) M(-1) and (7.9 ± 5.5) × 10(4) M(-1), respectively. The kinetics of association and dissociation were determined by stopped-flow spectroscopy, yielding a k(f) and k(b) of (2.1 ± 0.3) × 10(6) M(-1) s(-1) and 250 ± 50 s(-1), respectively. The electrochemical mechanistic details were studied by variable scan rate cyclic voltammetry (CV), and the experimental data were compared digitally with simulated data, modeled on the proposed mechanism using the thermodynamic and kinetic parameters obtained from ITC, UV/vis, and stopped-flow spectroscopy. In particular, the electrochemical mechanism of association/dissociation involves a bisradical tetracationic intermediate CBPQT((2+)(•+))⊂V(•+) inclusion complex; in the case of the V(•+) guest, the rate of disassociation (k(b) = 10 ± 2 s(-1)) was slow enough that it could be detected and quantified by variable scan rate CV. All the experimental observations lead to the speculation that the CBPQT((2+)(•+)) ring of the bisradical tetracation complex might possess the unique property of being able to recognize both BIPY(•+) radical cation and π-electron-rich guests simultaneously. The findings reported herein lay the foundation for future studies where this radical-radical recognition motif is harnessed particularly in the context of mechanically interlocked molecules and increases our fundamental understanding of BIPY(•+) radical-radical interactions in solution as well as in the solid-state.

RF-sputtering preparation of gold-nanoparticle-modified ITO electrodes for electrocatalyticapplications

The preparation of gold-nanoparticle(AuNPs)-modified indium tinoxide (ITO) electrodes (AuNPs/ITO) was performed by radio-frequency (RF) sputtering from Ar plasmas at temperatures as low as60 °C, tailoringthe AuNP morphology and content as a function of the sole sputtering time. The latterparameter was varied from 5 to 20 min in order to investigate the influence of goldamount and distribution on the electrochemical performances of the resultingAuNPs/ITO systems. The electrodes were characterized using field emission-scanning electronmicroscopy (FE-SEM), UV–vis absorption and x-ray photoelectron spectroscopies (XPS);moreover variable scan rate cyclic voltammetry (CV) studies were performed to examinetheir electrochemical behavior. The electrocatalytic activity of the nanostructuredAuNPs/ITO electrodes toward methanol oxidation was investigated and compared with a continuous gold film(Aufilm/ITO). The catalyticefficiency of the AuNPs/ITO systems was found to increase with the gold content and theAuNPs–support boundary region in the corresponding samples. For the longest sputtering time(i.e. 20 min) the performances of the nanostructured electrode were better than theAufilm/ITO reference, despite the much lower catalyst amount. Furthermore, conversely from theAuNPs/ITO samples,in the Aufilm/ITO case the gold film displayed a poor adhesion to the substrate and the electrode could beused only for a limited number of electrochemical cycles.

The role of low levels of water in the electrochemical oxidation of α-tocopherol (vitamin E) and other phenols in acetonitrile

The phenol, α-tocopherol, can be electrochemically oxidised in a -2e(-)/-H(+) process to form a diamagnetic cation that is long-lived in dry organic solvents such as acetonitrile and dichloromethane, but in the presence of water quickly reacts to form a hemiketal. Variable scan rate cyclic voltammetry experiments in acetonitrile with carefully controlled amounts of water between 0.010 M-0.6 M were performed in order to determine the rate of reaction of the diamagnetic cation with water. The water content of the solvent was accurately determined by Karl Fischer coulometric titrations and the voltammetric data were modelled using digital simulation techniques. The oxidation peak potential of α-tocopherol measured during cyclic voltammetry experiments was found to shift to less positive potentials as increasing amounts of water (0.01-0.6 M) were added to the acetonitrile, which was interpreted based on hydrogen-bonding interactions between the phenolic hydrogen atom and water. Several other phenols were examined and they displayed similar voltammetric features to α-tocopherol, suggesting that interactions of phenols with trace amounts of water were a common occurrence in acetonitrile. The H-bonding interactions of α-tocopherol with water were also examined via NMR and UV-vis spectroscopies, with the voltammetric and spectroscopic studies extended to include other coordinating solvents (dimethyl sulfoxide and pyridine).

Variable Scan Rate Cyclic Voltammetry and Theoretical Studies on Tocopherol (Vitamin E) Model Compounds

Variable scan rate (0.1-500 V s(-1)) cyclic voltammetry experiments were performed on a series of model tocopherol (vitamin E) compounds with differing degrees of methyl substitution around the aromatic (phenolic) ring. alpha-Tocopherol, with a fully methylated aromatic ring, produced stable phenoxonium cations upon oxidation in CH3CN, and was modeled via an ECE mechanism (where "E" represents an electron transfer and "C" a chemical step). Compounds with less methyl substitution around the aromatic ring were more reactive following oxidation, and formed additional oxidation products (hemiketals and p-quinones), and were modeled according to a more complicated ECECC mechanism. The equilibrium and rate constants associated with the chemical steps were estimated by digital simulations of the variable scan rate data over a range of temperatures ( T = 253-313 K) in acetonitrile containing 0.5 M Bu4NPF6 as the supporting electrolyte. The relative lifetimes of the phenoxonium cations of tocol and the tocopherols were compared with theoretical results obtained from molecular orbital calculations.

Energetics of Concerted Two-Electron Transfer and Metal−Metal Bond Cleavage in Phosphido-Bridged Molybdenum and Tungsten Carbonyl Complexes

The kinetics and thermodynamics of concerted two-electron transfer and metal−metal bond cleavage in the binuclear phosphido-bridged complexes [M2(μ-PPh2)2(CO)8]0/2- [M = Mo (10/2-), W(20/2-)] have been determined by variable scan-rate cyclic voltammetry in 0.3 M TBAPF6/acetone. The reductions of 1° and 2° are accompanied by an increase of 1.08 Å in M−M distance and expansion and contraction by 29° of the M−P−M and P−M−P angles, respectively, within an intact M2(μ-PPh2)2 unit. The one-electron electrode potentials of these systems are highly inverted: ΔE°‘ = − = +0.17 V for 10/2- and +0.18 V for 20/2-, and the rate constant of the second heterogeneous electron-transfer reaction is smaller than the first: ksh,2/ksh,1 = 0.1 for Mo and 0.018 for W. Results are consistent with progressive cleavage of the metal−metal bond in two one-electron steps, of which the second is rate-limiting, because it is accompanied by a larger part of the structural change. EHMO calculations reveal that the redox-active orbital is a metal−metal antibonding (σ*) orbital with substantial bridging-ligand character that decreases markedly in energy on passing from the metal−metal bonded M(I)2 state to nonbonded M(0)2. Despite this feature, electron-transfer thermodynamics and kinetics are not significantly metal-dependent. Rather, comparisons with structurally similar sulfido-bridged complexes reveal that electron-transfer energetics are influenced more extensively by the bridging ligand, with more positive potentials and larger electron-transfer rates observed for RS- versus R2P- bridged species.

Fabrication and Characterization of Nanoelectrode Arrays Formed via Block Copolymer Self-Assembly

A novel approach to highly ordered and modular nanoelectrode arrays (NEAs) has been developed using block copolymer self-assembly. Variable scan rate cyclic voltammetry studies were performed to characterize the NEA. At low scan rates, the NEA behaves similar to a macroelectrode, while at high scan rates the nanoelectrodes act independently. This is an important feature for real-time in vivo sensing and other electroanalytical applications.

Electrochemical Studies on the Competition Between Intramolecular Electron Transfer and Intermolecular Protonation of Nitroaryl Radical Anions in the Reductions of Apical Carboxylic Acid Substituents of 2- and 3-Nitro- 9,10-dihydro-9,10-ethanoanthracene-9-carboxylic Acids

The results obtained from variable scan rate cyclic voltammetry (c.v.) on 2-nitro- and 3-nitro-9,10- dihydro-9,10-ethanoanthracene-9-carboxylic acids [(4) and (5), respectively], combined with simulations of various c.v. responses, are consistent with reduction of a benzylic acid group being facilitated by an intramolecular electron transfer process. This intramolecular process involves a one-electron reduction of the nitroaromatic group, followed by a rapid and irreversible π*(ArNO2)•- → π*(RCO2H)•- intramolecular electron transfer to the carboxylic acid group at a benzylic bridgehead position of the acids (4) and (5). The reduction potentials of the acid groups are shifted more than 0·3 V to positive potentials at slow scan rates (20-100 mV s-1) compared with the unnitrated acid derivative (6). The reduction potentials and the relative peak currents for the reductions of the nitro and acid groups for each of compounds (4) and (5) are dependent on the concentrations of the reactants. At concentrations of substrate >1 mM, reduction of the acid moiety is increasingly dependent on complex intermolecular processes. These intermolecular processes compete with intramolecular electron transfer from the nitroaryl anion to the apical acid group at the benzylic bridgehead position. Digital simulations of the voltammetric data were confined to substrate concentrations <1 mM, and show that the intramolecular reductions of the apical carboxylic acid protons of (4) and (5) are complicated by competing intermolecular electron transfer and intermolecular self-protonations of the nitro radical anions. The value of the intramolecular electron transfer rate constant for the meta compound is an order of magnitude larger than that for the para compound, which is the opposite reactivity pattern to that generally found in the SRN1 reactions of m- and p-nitrobenzyl halides. This indicates that there is likely to be an important contribution from an intramolecular through-space electron transfer mechanism for the former reaction