Charge Transfer Rate Constant Research Articles

O₂(X³Σg⁻) and O₂(a¹Δg) charge exchange with simple ions.

We present theory and experiments which describe charge transfer from the X³Σg⁻ and a(1)Δg states of molecular oxygen and atomic and molecular cations. Included in this work are new experimental results for O2(a(1)Δg) and the cations O(+), CO(+), Ar(+), and N₂⁺, and new theory based on complete active space self-consistent field method calculations and an extended Langevin model to calculate rate constants for ground and excited O2 reacting with the atomic ions Ar(+), Kr(+), Xe(+), Cl(+), and Br(+). The T-shaped orientation of the (X - O2)(+) potential surface is used for the calculations, including all the low lying states up to the second singlet state of the oxygen molecule b¹Σ(g)⁺. The calculated rate constants for both O₂(X³Σg⁻) and O2(a(1)Δg) show consistent trends with the experimental results, with a significant dependence of rate constant on charge transfer exothermicity that does not depend strongly on the nature of the cation. The comparisons with theory show that partners with exothermicities of about 1 eV have stronger interactions with O2, leading to larger Langevin radii, and also that more of the electronic states are attractive rather than repulsive, leading to larger rate constants. Rate constants for charge transfer involving O2(a(1)Δg) are similar to those for O₂(X³Σg⁻) for a given exothermicity ignoring the electronic excitation of the O2(a(1)Δg) state. This means (and the electronic structure calculations support) that the ground and excited states of O2 have about the same attractive interactions with ions.

Investigation of Charge Transfer Kinetics of Polyaniline Supercapacitor Electrodes by Scanning Electrochemical Microscopy

Scanning Electrochemical Microscopy (SECM) is introduced as a promising technique to probe localized interfacial kinetics at the interface of electrolyte/supercapacitor electrode based on polyaniline (PANI) by measuring approach curves from which heterogeneous charge transfer rate constants (k eff) are extracted. The values correlate with the effectiveness of the electrode material for supercapacitor application. Specifically, measurements on PANI films of different thicknesses show that potential‐dependent rate constants are observed only for PANI films of up to 5 μm thickness. In addition to the thickness of PANI, k eff is also found to be affected by the applied potential and surface morphology of PANI electrodes. These findings correlate with the macroscopic electrochemical performance of PANI electrodes which shows enhanced specific charge storage ability when their thickness is below 5 μm. Under these conditions, they deliver a specific capacitance of 486 F g−1 and a rate capability of 89%. The observed correlation between microscopic kinetic data determined by SECM and macroscopic device characteristics provides rational guidelines for the optimization of the physical and structural properties of high performance supercapacitor electrodes.

Electrochemical and catalytic investigations of epinephrine, acetaminophen and folic acid at the surface of titanium dioxide nanoparticle-modified carbon paste electrode

In the present paper, the use of a carbon paste electrode modified with 1-(4-(1, 3-dithiolan-2-yl)-6, 7-dihydroxy-2-methyl-6, 7-dihydrobenzofuran-3-yl)ethanone (DDE) and TiO2 nanoparticles prepared by a simple and rapid method was described. The modified electrode showed excellent properties for electrocatalytic oxidization of epinephrine (EP), acetaminophen (AC) and folic acid (FA). The apparent charge transfer rate constant, k s = 1.14 s−1, and transfer coefficient, α = 0.54, for electron transfer between the modifier and carbon paste electrode were calculated. It has been found that under optimum condition (pH = 7.0) in cyclic voltammetry, the oxidation of EP occurs at a potential about 280 mV less positive than that of an unmodified carbon paste electrode. The values of transfer coefficients (α = 0.46), catalytic rate constant (k = 1.2 × 104 M−1 s−1) and diffusion coefficient (D = 2.70 × 10−5 cm2 s−1) were calculated for EP. Differential pulse voltammetry (DPV) exhibited two linear dynamic ranges of 0.5 to 50.0 μM and 50.0 to 1,000 μM for EP. This modified electrode is quite effective not only for the detection of EP, AC and FA but also for the simultaneous determination of these species in a mixture. The limit of detection for EP, AC and FA is 0.10, 1.80 and 2.36 μM, respectively.

Nanostructured Base Electrochemical Sensor for Simultaneous Quantification and Voltammetric Studies of Levodopa and Carbidopa in Pharmaceutical Products and Biological Samples

AbstractA novel carbon paste electrode modified with carbon nanotubes and 5‐amino‐2′‐ethyl‐biphenyl‐2‐ol (5AEB) was fabricated. The electrochemical study of the modified electrode, as well as its efficiency for electrocatalytic oxidation of levodopa (LD) and carbidopa (CD), is described. Cyclic voltammetry (CV) was used to investigate the redox properties of this modified electrode at various scan rates. The apparent charge transfer rate constant, ks, and transfer coefficient, a, for electron transfer between 5AEB and CPE were calculated as 17.3 s−1 and 0.5, respectively. Square wave voltammetry (SWV) exhibits a linear dynamic range from 2.5×10−7 to 2.0×10−4 M and a detection limit of 9.0×10−8 M for LD.

Electrochemical and thermodynamic properties of Ln(III) (Ln = Eu, Sm, Dy, Nd) in 1-butyl-3-methylimidazolium bromide ionic liquid.

The electrochemical behavior and thermodynamic properties of Ln(III) (Ln = Eu, Sm, Dy, Nd) were studied in 1-butyl-3-methylimidazolium bromide ionic liquid (BmimBr) at a glassy carbon (GC) electrode in the range of 293–338 K. The electrode reaction of Eu(III) was found to be quasi-reversible by the cyclic voltammetry, the reactions of the other three lanthanide ions were regarded as irreversible systems. An increase of the current intensity was obtained with the temperature increase. At 293 K, the cathodic peak potentials of −0.893 V (Eu(III)), −0.596 V (Sm(III)), −0.637 V (Dy(III)) and −0.641 V (Nd(III)) were found, respectively, to be assigned to the reduction of Ln(III) to Ln(II). The diffusion coefficients (D o), the transfer coefficients (α) of Ln(III) (Ln = Eu, Sm, Dy, Nd) and the charge transfer rate constants (k s) of Eu(III) were estimated. The apparent standard potential (E 0*) and the thermodynamic properties of the reduction of Eu(III) to Eu(II) were also investigated.

A study of the electrochemical behavior of the Ti(IV)/Ti(III) redox couple in the NaCl-KCl-NaF-K2TiF6 melt in order to optimize the electrorefining of titanium and synthesis of alloys based on this metal

Temperature dependence of the standard charge-transfer rate constant was obtained in a study of the redox reaction Ti(IV) + e ⇆ Ti(III) in a (NaCl-KCl)equim-NaF(10 wt %)-K2TiF6 melt by cyclic voltammetry. The processes of titanium refining and high-temperature electrochemical syntheses of alloys on its basis can be optimized by using this dependence.

Electrochemical behaviour of ferrocene in the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate, EMIMBF4, at 298 K

Electrochemical oxidation of ferrocene (Fc) at platinum and glassy carbon electrodes in the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate, EMIMBF4, has been studied by cyclic voltammetry (CV), convolutive potential sweep voltammetry (CPSV), chronopotentiometry (CP) and chronoamperometry (CA).On both electrodes, the electro-oxidation of ferrocene to ferrocenium (Fc+) takes place in a quasi-reversible electrochemical mode. The intrinsic rate constant of charge transfer, k0, and the charge transfer coefficient, α, have been calculated for the first time in the mentioned ionic liquid, by simulation of the cyclic voltammograms, logarithmic analysis of the convoluted curves and from the steady-state current potential curves by applying the Gauss–Newton non-linear square method. The diffusion coefficient of Fc has been also calculated by CV, CPSV, CP and CA. A two-way analysis of variance, ANOVA, has shown that with a 95% confidence level, there are no significant differences between the diffusion coefficients obtained with both electrodes by the different techniques.

Electrochemical investigation of the redox couple Sm(III)/Sm(II) on a tungsten electrode in molten LiF–CaF2–SmF3

This article is focused on the electrochemical investigation (cyclic voltammetry and related studies) of the redox couple Sm(III)/Sm(II) in an eutectic LiF–CaF2 melt containing SmF3. The first step of reduction for Sm(III) ions involving one electron exchange in soluble/soluble Sm(III)/Sm(II) system was found on a tungsten electrode. The study of the Sm(II)/Sm(0) electrode reaction was not feasible, due to insufficient electrochemical stability of LiF–CaF2. The first step was found reversible at temperatures 1,075 and 1,125 K up to polarization rate 1 V/s and at temperature 1,175 K the process was reversible at all sweep rates applied in this study. The diffusion coefficients (D) of Sm(II) and Sm(III) ions were determined by cyclic voltammetry, showing that D decreases when oxidation state increase, while the activation energy of diffusion (Ea) increases. The standard rate constants of charge transfer (ks) were calculated for the redox couple Sm(III)/Sm(II) at 1,075 and 1,125 K based on the data of cyclic voltammetry.

Redox-Active Molecular Wires Derived from Dinuclear Ferrocenyl/Ruthenium(II) Alkynyl Complexes: Covalent Attachment to Hydrogen-Terminated Silicon Surfaces

After their brief characterization, a voltammetric study of the Fe(II)/Ru(II) heterobinuclear organometallic complexes [Fe]C≡C-1,4-(C6H4)C≡C[Ru]C≡C-1,4-(C6H4)C≡CH (2), FcC≡C[Ru]C≡C-1,4-(C6H4)C≡CH (3), and PhC≡C[Ru]C≡C-1,1′-Fc(C≡CH) (4) ([Fe] = Fe(κ2-dppe)(η5-C5Me5), [Ru] = trans-Ru(κ2-dppe)2; Fc = ferrocenyl; dppe =1,2-bis(diphenylphosphino)ethane) is reported in solution. These complexes which possess pendant ethynyl groups have then been grafted onto hydrogenated silicon surfaces using a mild photochemical protocol, to yield redox-active functional interfaces (Si-2/Si-3/Si-4) that were characterized by X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry. The resulting interfaces Si-2, Si-3, and Si-4 possess surface coverages and electrochemical responses that differ significantly and that depend on the nature of the grafted molecule. While the coverages calculated for the more rigid of these systems (Si-2 and Si-3) are similar (around 10–10 mol cm–2), the significantly lower coverage achieved for Si-4 (4.3 × 10–11 mol cm–2) can be ascribed to the presence of a flexible linker between the redox sites in 4. The two grafted redox centers can be electrochemically addressed in a stepwise fashion, the species exhibiting apparent charge transfer rate constants with the Si surface that are considerably higher than those reported for many related systems (>200 s–1). Comparison between Si-2 and Si-3 reveals that the use of the ferrocenyl group instead of a “Fe(κ2-dppe)(η5-C5Me5)” moiety improves the kinetic stability of the first oxidized state and its apparent charge transfer rate constant, while also increasing the first oxidation potential. However, an enhanced fragility is observed for Si-3 and even more so for Si-4 at higher oxidation potentials; this may be related to the larger spin density present on the ferrocenyl terminus of the grafted dications 32+ and 42+, as evidenced by molecular modeling calculations. Several ways to solve this important issue are discussed in the conclusion.

Electrochemically Grown Self-Organized Hematite Nanotube Arrays for Photoelectrochemical Water Splitting

Hematite nanostructures were electrochemically grown by ultrasound-assisted anodization of iron substrates in an ethylene glycol based medium. These hematite nano-architectures can be tuned from a 1-D nanoporous layer to a self-organized nanotube one if the grown is done onto a bare iron foil substrate or onto an electrochemical pretreated one, respectively. Depending upon the pre-treatment conditioning, the self-organized nanotube layer consists of nanotube arrays with a single tube inner diameter of approximately 40–50 nm and wall thickness of 20–30 nm. Their morphological, structural and optoelectronic properties are studied. The photoelectrochemical properties of the resulting hematite nanostructures are studied from the point of view of their application as photoanodes in splitting of water. Through the photocurrent transients for the three nanostructured hematite type electrodes under study, the rate constants ktr and krec corresponding to the rate constant of charge transfer and recombination processes have been determined. In all cases, the potential value where ktr > krec was attained at more negative values than the reversible potential of water oxidation, indicating a photocatalytic effect. All samples show a maximum IPCE value between 350 and 375 nm, being the samples pretreated at −1.0 V which shows the highest IPCE value: 45% at 375 nm.

Electrochemical characterization of the encapsulated polyoxometalates (POMs) into the zeolite

The electrochemical characterization of polyoxometalates encapsulated into the zeolite (POM@Z) is presented for the first time. A stable film comprising POM@Z and graphite powder was prepared on glassy carbon electrode (POM@Z-GCE) by dip-coating. The cyclic voltammograms of the modified electrode showed three well-defined pairs of redox peaks due to the redox reaction of the encapsulated POMs. The values of charge transfer coefficient (α) and the heterogeneous charge transfer rate constant (kapp) for three pairs of redox peaks were evaluated. The electrochemical studies revealed that the encapsulated POMs show very slower electrochemical responses than those of un-encapsulated POMs arising from the restraint in the zeolite pores towards diffusion of charge-balancing electrolytes. According to the obtained evidences an intra-zeolite charge transfer mechanism for the encapsulated POMs can be concluded. The POM@Z-GCE showed the advantages of excellent electrocatalytic activity towards reduction of H2O2.

Theoretical Study of Charge Transfer in Styryl Thiazilo Quinoxaline Dyes STQ-1, STQ-2,and STQ-3 in Organic Media System

Charge transfer in styryl dyes STQ-1, STQ-2,and STQ-3 with organic media system has been studied theoretically depending on the Franck- Condon rule and continuum dielectric model . The reorientation energies (eV) were evaluated theoretically depending on dipole momentum, dielectric constant , and refrective index n. The rate constant of charge transfer has been calculated depending on the reorientation energy (eV) ,effective free energy , potential height barrier , and coupling coefficient . A matlap program has been written to calculated the rate constant of charge transfer and other parameter. The results of calculations show that STQ-2 dye is more reaction for charge transfer compare with STQ-1 and STQ-3 dyes

Theoretical Study of Charge Transfer in Styryl Thiazilo Quinoxaline Dyes STQ-1, STQ-2,and STQ-3 in Organic Media System

Charge transfer in styryl dyes STQ-1, STQ-2,and STQ-3 with organic media system has been studied theoretically depending on the Franck- Condon rule and continuum dielectric model . The reorientation energies (eV) were evaluated theoretically depending on dipole momentum, dielectric constant , and refrective index n. The rate constant of charge transfer has been calculated depending on the reorientation energy (eV) ,effective free energy , potential height barrier , and coupling coefficient . A matlap program has been written to calculated the rate constant of charge transfer and other parameter. The results of calculations show that STQ-2 dye is more reaction for charge transfer compare with STQ-1 and STQ-3 dyes

Quantifying the Relationship between the Maximum Achievable Voltage and Current Levels in Low-Bandgap Polymer Photovoltaics

A critical problem in the design of materials for organic photovoltaics is quantifying the driving force needed for efficient charge separation without losses associated with a large overpotential. Here, we directly measured the effect of the molecular driving force on the charge transfer rate in films of low-bandgap push–pull type polymers mixed with a series of fullerene-based molecular acceptors using broadband near-infrared transient absorption spectroscopy. By systematically tuning the absolute energy levels of the donor and acceptor, as well as the relative offset between them, we determine the minimum voltage loss required to achieve a high short circuit current. A molecular donor–acceptor framework provides a quantitative description of the charge transfer rate constants in our system and describes the scaling of the photogenerated current with S1–LUMO energy offset. These results point to potential efficiency gains for high performing polymer devices through recovery of additional voltage without sacrificing current output.

Electrocatalytic oxidation of methanol by ZSM-5 nanozeolite-modified carbon paste electrode in alkaline medium

Development of a novel modified electrode for electrocatalytic oxidation of methanol in order to decrease overvoltage is importance. In this paper, carbon paste electrode (CPE) was modified by ZSM-5 nanozeolite. The average diameter of used nanozeolite was 97 nm. Ni2+ ions were incorporated to the nanozeolite by immersion of the modified electrode in a 0.1 M nickel chloride solution. Then, electrochemical studies of this electrode were performed by using cyclic voltammetry(CV) in alkaline medium. This modified electrode was used as an anode for the electrocatalytic oxidation of methanol in 0.1 M of NaOH solution. The obtained data demonstrated that ZSM-5 nanozeolite at the surface of CPE improves catalytic efficiency of the dispersed nickel ions toward methanol oxidation. The values of electron transfer coefficient, charge-transfer rate constant, and the electrode surface coverage are obtained 0.61, 0.2342 s−1, and 4.33 × 10−8 mol cm−2, respectively. Also, the mean value of catalytic rate constant between the methanol and redox sites of electrode and diffusion coefficient were found to be 2.54 × 104 cm3 mol−1 s−1 and 1.85 × 10−8 cm2 s−1, respectively. Obtained results from both CV and chronoamperometric techniques indicated that the electrode reaction is a diffusion-controlled process.

Fourier‐Transformed Large‐Amplitude AC Voltammetric Study of Tetrathiafulvalene (TTF): Electrode Kinetics of the TTF0/TTF.+ and TTF.+/TTF2+ Processes

AbstractLarge‐amplitude Fourier‐transformed (FT) AC voltammetry was used at high frequency to study the electrode kinetics of tetrathiafulvalene (TTF), its radical cation TTF.+ and that of the dication TTF2+ in acetonitrile (0.1 M Bu4NPF6). The two oxidized forms were prepared by quantitative bulk electrolysis of TTF under dry‐box conditions. A high frequency of 233 Hz was used at glassy carbon (GC) and platinum (Pt) macro disk electrodes to enhance the kinetic sensitivity and hence upper limit of the measurable heterogeneous charge transfer rate constant (k0). Difficulties associated with the use of high frequencies at macro disk electrodes are discussed. Comparison of experimental data with simulations is used to extract k0 and other parameters for each of the TTF0/.+ and TTF.+/2+ processes from both reduction and oxidation perspectives. k0 values for TTF0/.+and TTF.+/0 processes are ≥1.0 cm s−1 and consequently regarded as reversible under the AC conditions used. However, the significantly slower quasi‐reversible behaviour observed for the TTF.+/2+ redox couple, with k0 being 0.3±0.05 cm s−1, can be precisely estimated.

Simultaneous determination of droxidopa and carbidopa using a carbon nanotubes paste electrode

A novel carbon paste electrode modified with carbon nanotube and 5-amino-2’-ethyl -biphenyl-2-ol (5AEB) was fabricated. The electrochemical study of the modified electrode, as well as its efficiency for electrocatalytic oxidation of droxidopa and carbidopa, is described. Cyclic voltammetry was used to investigate the redox properties of this modified electrode at various scan rates. The apparent charge transfer rate constant, ks, and transfer coefficient, α, for electron transfer between 5AEB and carbon nanotubes paste electrode were calculated as 17.3±0.1s−1 and 0.5, respectively. The electrode was also employed to study the electrocatalytic oxidation of droxidopa, using cyclic voltammetry, chronoamperometry, square wave voltammetry and electrochemical impedance spectroscopy as diagnostic techniques. It has been found that the oxidation of droxidopa at the surface of modified electrode occurs at a potential of about 355mV less positive than that of an unmodified CPE. The diffusion coefficient, electron transfer coefficient, and heterogeneous rate constant, for oxidation of droxidopa at the modified electrode surface were also determined. Square wave voltammetry exhibits a linear dynamic range from 1.2×10−7 to 2.25×10−4M and a detection limit of 50.0nM for droxidopa. Finally this modified electrode was used for simultaneous determination of droxidopa and carbidopa for the first time.

Oxidized multiwalled carbon nanotubes for improving the electrocatalytic activity of a Schiff base modified electrode in determination of isoprenaline

In this paper, the cyclic voltammetry technique was used to investigate the redox properties of a carbon paste electrode (CPE) modified by a synthesized Schiff base (PNH) and oxidized multiwall carbon nanotubes (OCNTs). The effect of oxidization of carbon nanotubes on the charge transfer rate constant of the PNH was studied. Also the electrocatalytic activity of the modified electrode for oxidation of isoprenaline (IP) via EC′ mechanism was described. The oxidation of IP at modified electrode occurs at a potential about 315mV less positive than at unmodified carbon paste electrode. The value of catalytic rate constant was calculated for IP, using chronoamperometry. Based on square wave voltammetry, the oxidation of IP exhibited a dynamic range between 1.0 and 1800.0μM and a detection limit (3σ) of 0.3μM. Also, simultaneous determination of IP and captopril (CA) was described at the modified electrode. Finally, this electrode was used for the determination of these substances in synthetic solutions and IP injections.

Developing a sensor for the simultaneous determination of adrenaline, uric acid, and tryptophan

A chemically modified electrode is constructed based on a coumestan derivative and multiwall carbon nanotubes modified carbon paste electrode (CMWCNT-CPE). The surface charge transfer rate constant, ks, and the charge transfer coefficient, α, for the electron transfer between coumestan and MWCNT-CPE were estimated. CMWCNT-CPE presents a highly catalytic activity for adrenaline (AD) electrooxidation. The results show that the peak potential of AD at the CMWCNT-CPE surface shifted by about 145 mV toward negative values compared with that at the MWCNT-CPE surface. Differential pulse voltammetry exhibited three linear ranges and a detection limit of 0.2 μM for AD. For a mixture containing AD, uric acid (UA), and tryptophan (Trp), three signals corresponding to the analytes could well separate them from each other. Moreover, CMWCNT-CPE was used to determine AD in an adrenaline injection solution and UA in a human urine sample with satisfactory results. To confirm the proposed method, the AD injection solution and the urine sample were spiked with different certain amounts of AD, UA, and Trp.

Biochips with Nano-Ensembles: Integration with Nanoelectrode Ensembles for Biodetection Applications

Nanoelectrode ensemble (NEE) films have attracted increasing interest in recent years due to their unique combination of electrochemical (EC) characteristics, including ultrafast electron transfer reactions, high signal-to-noise (S/N) ratios, and enhanced detection sensitivity [1]-[3]. NEEs are nanotechology-based electroanalytical tools that find application in a variety of fields ranging from sensors to electronics and from energy storage to magnetic materials [4]. The characteristics of NEEs are 1) extreme sensitivity to the kinetics of the charge transfer process [5], which means they are able to measure very high charge transfer rate constants [6], and 2) that they dramatically reduce capacitive currents, which act as noise in voltammetry detection procedures [7], [8].