Heterogeneous Charge Transfer Kinetics Research Articles

(Invited) Manipulating Interfacial Electrochemistry with Moiré Superlattices

At electrode–electrolyte interfaces, crystallographic defects are frequently implicated as active sites that mediate interfacial electron transfer (ET) by introducing high densities of localized electronic states (DOS). However, conventional defects are challenging to deterministically synthesize and control at an atomic level, hindering the direct study of how electronic localization impacts interfacial reactivity. Azimuthal misalignment of atomically thin layers produces moiré superlattices and alters the electronic band structure, in a manner that is systematically dependent on the interlayer twist angle. Using van der Waals nanofabrication of two-dimensional heterostructures, scanning electrochemical cell microscopy measurements, and four-dimensional scanning transmission electron microscopy, we report a strong twist angle dependence of heterogeneous charge transfer kinetics at twisted bilayer and trilayer graphene electrodes with the greatest enhancement observed near the ‘magic angles’. These effects are driven by the angle-dependent engineering of moiré flat bands that dictate the electron transfer processes with the solution-phase redox couple, and the structure of the relaxed moiré superlattice. Moiré superlattices therefore serve as an unparalleled platform for systematically interrogating and exploiting the dependence of interfacial ET on local electronic structure.

Field-Effect Tuning of Electrochemistry at 2D Semiconductor Electrodes: Charge Transfer Kinetics Can be Modulated with a Back Gate Voltage

Semiconductor electrodes are widely used in electrocatalytic reactions. The ability to improve and modulate the heterogeneous charge transfer kinetics on semiconducting electrodes is a major challenge for the electrochemical application of these materials. Instead of changing the electronic occupation of the semiconductor by chemical doping, we adopt an electronic method to modulate band edge alignment and solution electrochemistry at 2D ZnO working electrodes.In our approach, a field-effect transistor with ultrathin ZnO semiconductor (3-5 nm) is fabricated, the heart of which is a metal-insulator-semiconductor (MIS) stack. A high-dielectric-constant material (HfO2) is used as the insulator to realize low-voltage tunability of the device. The ZnO layer of the device functions as the working electrode (WE) for electrochemical reactions. By applying a back gate voltage to this MIS field-effect transistor, charge carriers are electrostatically induced and the ZnO band edges shift at the front face of the ZnO in contact with electrolyte. These effects can be exploited to manipulate the charge transfer kinetics on the ZnO electrode at a given electrode potential. We demonstrate this by focusing on outer-sphere redox chemistry. In order to eliminate mass transfer effects, microfluidic flow cells are coupled to the ZnO devices. Steady state kinetic analysis is carried out to extract the electron transfer rate constant of the redox process as a function of both electrochemical and back gate potentials. The heterogeneous electron transfer rate can be tuned by over an order of magnitude using the back gate voltage. The work demonstrates that a back gate provides another degree of freedom in determining electrochemical reaction kinetics at ultrathin semiconductor working electrodes. The method also opens possibilities for control over a wider range of semiconductor electrochemistry, such as electrocatalytic reactions. Figure 1

Tunable angle-dependent electrochemistry at twisted bilayer graphene with moiré flat bands.

Tailoring electron transfer dynamics across solid-liquid interfaces is fundamental to the interconversion of electrical and chemical energy. Stacking atomically thin layers with a small azimuthal misorientation to produce moiré superlattices enables the controlled engineering of electronic band structures and the formation of extremely flat electronic bands. Here, we report a strong twist-angle dependence of heterogeneous charge transfer kinetics at twisted bilayer graphene electrodes with the greatest enhancement observed near the 'magic angle' (~1.1°). This effect is driven by the angle-dependent tuning of moiré-derived flat bands that modulate electron transfer processes with the solution-phase redox couple. Combined experimental and computational analysis reveals that the variation in electrochemical activity with moiré angle is controlled by a structural relaxation of the moiré superlattice at twist angles of <2°, and 'topological defect' AA stacking regions, where flat bands are localized, produce a large anomalous local electrochemical enhancement that cannot be accounted for by the elevated local density of states alone.

Operando Raman Shift Replaces Current in Electrochemical Analysis of Li-ion Batteries: A Comparative Study.

Li-rich and catalytically active - (x = 1.48) was investigated as a cathode for its heterogeneous charge transfer kinetics. Using a specially designed two-electrode system lithium half cell, Butler–Volmer analysis was performed, and Raman spectra were acquired in 18 mV intervals. A direct correlation was observed between the Raman shift of the active modes ,, , and , and the development of the Faraday current at the working electrode. The Raman intensity and the Raman shift were implemented to replace the current in a Tafel plot used for the analysis of Butler–Volmer kinetics. Striking similarities in the charge transfer proportionality constants were found for current and Raman-based analysis. The potential of this new method of Raman-aided electrochemical detection at the diffraction limit is discussed.

(Invited) Analysis of the Heterogeneous Charge-Transfer Kinetics for Adsorbed Redox Monolayers on Semiconductor

This presentation will describe our latest results in comprehensive modeling of the voltammetric response of redox active monolayers on semiconductor electrodes. The motivation is to provide more utility in voltammetric data that pertain to redox molecules adsorbed on semiconductors, as common in dye-sensitized photoelectrochemistry and artificial photosynthesis systems. First, a background on the prior art on kinetic analysis of redox monolayers on metal and semiconductor electrodes will be provided. Second, the mathematical model used in this work will be detailed, including a brief primer on the derivation of the expression that relates semiconductor surface energetics, surface traps, electrostatics, charge-transfer kinetics, and Marcus theory. Third, the voltammetric responses of several redox-active species physisorbed and chemisorbed on Si electrodes will be presented. The focus will be on responses collected in the dark and in non-aqueous electrolytes. Fourth, the extent of kinetic information that can be extracted from individual and sets of voltammograms will be described.

Modeling Faradaic Reactions and Electrokinetic Phenomena at a Nanochannel-Confined Bipolar Electrode

We present the most comprehensive two-dimensional numerical model to date for a nanoconfined bipolar electrochemical system. By accounting for the compact Stern layer and resolving the diffuse part of the electrical double layer at the bipolar electrode (BPE) surface and channel walls, our model captures the impact of surface polarization and ionic charge-screening effects on the heterogeneous charge-transfer kinetics, as well as nonlinear electrokinetic transport phenomena such as induced-charge electroosmosis and concentration polarization. We employ the Poisson-Nernst-Planck and Stokes flow system of equations, unified with generalized Frumkin-Butler-Volmer reaction kinetics, to describe water electrolysis reactions and the resulting transport of ions and dissolved gases in the confined BPE system. Our results demonstrate that under a sufficiently large applied electric field, the rapid reaction kinetics on our Pt BPE dynamically transition from charge-transfer-limited to mass-transfer-limited temporal regimes as regions depleted of redox species form and propagate outward from the respective BPE poles. This phenomenon was visualized experimentally with a pH-sensitive fluorescein dye and showed excellent phenomenological agreement with our numerical calculations, providing a foundation for further understanding and developing bipolar electrochemical processes in confined geometries. We introduce two prospective applications arising from our work: (1) a hybrid hydrodynamic/electrochemical peristaltic pump and (2) deducing information about chemical kinetics through simulation.

Field Effect Modulation of Heterogeneous Charge Transfer Kinetics at Back-Gated Two-Dimensional MoS2 Electrodes.

The ability to improve and to modulate the heterogeneous charge transfer kinetics of two-dimensional (2D) semiconductors, such as MoS2, is a major challenge for electrochemical and photoelectrochemical applications of these materials. Here we report a continuous and reversible physical method for modulating the heterogeneous charge transfer kinetics at a monolayer MoS2 working electrode supported on a SiO2/p-Si substrate. The heavily doped p-Si substrate serves as a back gate electrode; application of a gate voltage (VBG) to p-Si tunes the electron occupation in the MoS2 conduction band and shifts the conduction band edge position relative to redox species dissolved in electrolyte in contact with the front side of the MoS2. The gate modulation of both charge density and energy band alignment impacts charge transfer kinetics as measured by cyclic voltammetry (CV). Specifically, cyclic voltammograms combined with numerical simulations suggest that the standard heterogeneous charge transfer rate constant (k0) for MoS2 in contact with the ferrocene/ferrocenium (Fc0/+) redox couple can be modulated by over 2 orders of magnitude from 4 × 10-6 to 1 × 10-3 cm/s, by varying VBG. In general, the field effect offers the potential to tune the electrochemical properties of 2D semiconductors, opening up new possibilities for fundamental studies of the relationship between charge transfer kinetics and independently controlled electronic band alignment and band occupation.

Facile and versatile approaches to enhancing electrochemical performance of screen printed electrodes

Screen printed carbon electrodes provide attractive opportunity for the development of miniaturized low cost electrochemical sensors. However, the electrodes display very low level of electrochemical performance due to the nonelectroactive components in the ink formulation and relatively low graphitic carbon content imposed by the constraints of screen-printing. In this work, selective etching action of a judiciously selected organic solvent was envisaged to improve the electrochemical characteristics of the electrodes. The procedure involves soaking the electrode strips in N,N-dimethylformamide (DMF) for just minutes of etching followed by drying. Five minutes DMF treated electrode, for instance, showed a 100-fold increase in the heterogeneous charge transfer kinetics compared to the untreated electrodes. The effective area of the solvent treated electrode was 57-fold of its apparent geometric area. The solvent treated electrodes also showed a dramatically decreased charge transfer resistance from ca. 18,000Ω to 180Ω. The results of surface examination using scanning probe microscopy and electrochemical impedance spectroscopy revealed enhancement in the porosity of the solvent treated electrodes. The procedure is versatile and applicable to other screen-printed electrodes such as gold, silver or silver/silver chloride to improve the performances characteristics including stability. The ability to activate several electrodes simultaneously and multiple re-usability of the solvent makes this approach cost effective and very promising to facilitate mass production of miniaturized and disposable electrochemical sensors.

Direct Voltammetric Detection of DNA and pH Sensing on Epitaxial Graphene: An Insight into the Role of Oxygenated Defects

In this paper, we carried out detailed electrochemical studies of epitaxial graphene (EG) using inner-sphere and outer-sphere redox mediators. The EG sample was anodized systematically to investigate the effect of edge plane defects on the heterogeneous charge transfer kinetics and capacitive noise. We found that anodized EG, consisting of oxygen-related defects, is a superior biosensing platform for the detection of nucleic acids, uric acids (UA), dopamine (DA), and ascorbic acids (AA). Mixtures of nucleic acids (A, T, C, G) or biomolecules (AA, UA, DA) can be resolved as individual peaks using differential pulse voltammetry. In fact, an anodized EG voltammetric sensor can realize the simultaneous detection of all four DNA bases in double stranded DNA (dsDNA) without a prehydrolysis step, and it can also differentiate single stranded DNA from dsDNA. Our results show that graphene with high edge plane defects, as opposed to pristine graphene, is the choice platform in high resolution electrochemical sensing.

Understanding the Electrochemical Reactivity of Bamboo Multiwalled Carbon Nanotubes: the Presence of Oxygenated Species at Tube Ends May not Increase Electron Transfer Kinetics

AbstractThe result of acid washing multiwalled carbon bamboo nanotubes is explored with voltammetry and X‐ray Photoelectron Spectroscopy; it is found that the presence of oxygenated species formed as a result at the ends of the nanotubes does not measurably change the heterogeneous charge transfer kinetics.

Stepwise hapticity changes in sequential one-electron redox reactions of indenyl-molybdenum complexes: combined electrochemical, ESR, X-ray, and theoretical studies.

Reduction of the dication [(eta5-Ind)(Cp)Mo[P(OMe)3]2]2+ (1(2+)) and oxidation of the neutral complex (eta3-Ind)(Cp)Mo[P(OMe)3]2 (1) proceed through a one-electron intermediate, 1+. The structures of 1(2+) and 1 have been determined by X-ray diffraction studies, which show the slip-fold distortion angle, Omega, of the indenyl ring increasing from 4.1 degrees in 1(2+) to 21.7 degrees in 1. Cyclic voltammetry and bulk electrolysis were employed to define the thermodynamics and heterogeneous charge-transfer kinetics of reactions 1(2+) + e(-) <==> 1+ and 1+ + e(-) <==> 1: DeltaE1/2 = 113 mV in CH3CN and 219 mV in CH2Cl2/0.1 M [NBu4][PF6]; k(s) = 0.4 cm x s(-1) for 1(2+)/1+ couple, 1.0 cm x s(-1) for 1+/1 couple in CH3CN. ESR spectra of 1+ displayed a surprisingly large hyperfine splitting (7.4 x 10(-4) x cm(-1)) from a single 1H nucleus, and spectra of the partially deuterated indenyl analogue confirmed assignment of a(H) to the H2 proton of the indenyl ring. The related eta5 18-electron complexes [(eta5-Ind)(Cp)Mo(dppe)]2+ (2(2+)) (dppe = diphenylphosphinoethane) and (eta5-Ind)(Cp)Mo(CN)2 (3) may also be reduced in two successive one-electron steps; ESR spectra of the radicals 2+ and 3- showed a similarly large a(H2) (8.7 x 10(-4) and 6.4 x 10(-4) x cm(-1), respectively). Molecular orbital calculations (density functional theory, DFT, and extended Hückel, EH) predict metal-indenyl bonding in 1+ that is approximately midway between that of the eta5 and eta3 hapticities (e.g., Omega = 11.4 degrees ). DFT results show that the large value of a(H2) arises from polarization of the indenyl-H2 by both inner-sphere orbitals and the singly occupied molecular orbital (SOMO) of 1+. The measured ks values are consistent with only minor inner-sphere reorganizational energies being necessary for the electron-transfer reactions, showing that a full eta5/eta3 hapticity change may require only small inner-sphere reorganization energies when concomitant with a pair of stepwise one-electron-transfer processes. The indenyl ligand in 1+ is best described as donating approximately four pi-electrons to Mo by combining a traditional eta3 linkage with two "half-strength" Mo-C bonds.

A Comparison of Simulated and Experimental Voltammograms Obtained for the [Fe(CN)6]3-/4- Couple in the Absence of Added Supporting Electrolyte at a Rotating Disk Electrode

The voltammetric behavior of the [Fe(CN)6]3-/4- couple at a glassy carbon rotating macrodisk electrode without added supporting electrolyte is shown to be in close to ideal agreement with the theory presented over a wide range of electrode rotation and scan rates when the concentration of electroactive species used is 50 mM. The influences of migration, uncompensated resistance, heterogeneous charge-transfer kinetics, and inhomogeneous diffusion are shown to be well modeled by a finite difference simulation scheme that affords excellent agreement between experiment and theory. The use of the rotating disk electrode, even when only moderate rotation rates are employed, is shown to eliminate the problem with natural convection that exists when using stationary electrodes (macro- or microdisk) and that is enhanced in the absence of added supporting electrolyte. Consequently, it has been concluded that the rotating disk electrode method represents an almost ideal technique for conducting studies without added...

Voltammetry of azobenzene microcrystals

Azobenzene solid particles have been mechanically attached to a graphite electrode and measured by cyclic staircase voltammetry. Well-developed and widely separated cathodic and anodic peaks were observed. Redox reaction is controlled by both the heterogeneous charge transfer kinetics and the mass transfer. Its mechanism is explained by the surface diffusion model. Reaction starts at the three-phase boundary, where electrons are transferred from the electrode surface to the attached azobenzene molecules. The electrons are then propagated over the surface of microcrystals by a series of exchange reactions between hydrazobenzene and azobenzene molecules, with the participation of proton donors from the solution. The apparent mass transfer occurs in this way. In the crystal lattice the transmission of protons is not possible, and consequently there is no propagation of electrons.

The roles of cesium ions in regulating photoinduced interfacial charge transfer at the n-type cadmium chalcogenide/ferrocyanide interface

Addition of Cs + ions to electrolytes utilized with n-type cadmium chalcogenide photoanodes has previously been reported to have major effects on the stability and in some cases the electrochemical output parameters of such cells. The observed behavior has been attributed both to chemical effects occurring in the electrolyte and to semiconductor surface modification based on the adsorption of Cs + ions. The present study evaluates the effects of Cs + on n-CdX/ferri-ferrocyanide (X= S, Se) photo- electrochemical cells. Two effects are observed: the intercalation of Cs + into [CdFe(CN) 6] 2− 1− overlayers which are intrinsic to the semiconductor surface, and the specific chemisorption of Cs + onto CdSe surfaces. The former effect is found to reorganize the overlayer microstructure allowing for enhanced interfacial charge transport, while the latter process leads to unfavorable heterogeneous charge-transfer kinetics.

Electrochemical kinetic discrimination of the single-electron-transfer events of a two-electron-transfer reaction: cyclic voltammetry of the reduction of the bis(hexamethylbenzene)ruthenium dication

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTElectrochemical kinetic discrimination of the single-electron-transfer events of a two-electron-transfer reaction: cyclic voltammetry of the reduction of the bis(hexamethylbenzene)ruthenium dicationDavid T. Pierce and William E. GeigerCite this: J. Am. Chem. Soc. 1992, 114, 15, 6063–6073Publication Date (Print):July 1, 1992Publication History Published online1 May 2002Published inissue 1 July 1992https://doi.org/10.1021/ja00041a026RIGHTS & PERMISSIONSArticle Views909Altmetric-Citations88LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Solvent effect on the kinetics of the electrooxidation of phenothiazine

One-electron oxidation of phenothiazine to the corresponding radical cation was used as a model system to study the solvent effect on the heterogeneous charge-transfer kinetics. The standard rate constant of the studied electrode reaction was found to depend considerably on the nature of the solvent. The experimental kinetic data has been interpreted in terms of the dielectric dynamic properties of the solvent. The linear relationships between the standard rate constant and the reciprocal of the longitudinal dielectric relaxation time of a given solvent have been found in the aprotic case as well as for hydrogen-bonded solvents. The difference between both groups of solvents may be explained by the different dielectric relaxation behaviour or by the difference in the reaction site in respect to the electrode surface.

The Effect of Tetraalkylammonium Ions on the Heterogeneous Charge Transfer Kinetics for the Reduction of Benzonitrile and Dinitromesitylene in DMF.

The Effect of Tetraalkylammonium Ions on the Heterogeneous Charge Transfer Kinetics for the Reduction of Benzonitrile and Dinitromesitylene in DMF.

Normalized potential sweep voltammetry Part II. Application to heterogeneous charge transfer kinetics

Plotting theoretical voltammetric electrode potential data for Nernstian charge transfer on the X-coordinate, that for quasi-reversible charge transfer on the Y-coordinate and normalized current (I/Ip) on the Z-coordinate revealed that projections on to the X−Y plane resulted in: (E−Ep/2)quasi=m(E−Ep/2)Nern where the slopes m are directly related to the normalized heterogeneous rate constant Λ (Λ=ks(DnF/RT)−1/2ν−1/2). Working curves, m−1 vs. 1/Λ, dependent upon the value of the transfer coefficient, α, were calcualted for the determination of ks. For the experimental evaluation of ks, values of m−1 are obtained as a function of the voltage sweep rate, ν, which allows both ks and α to be obtained from the working curves. The method is applicable to processes with ks<1 cm s−1 and is most suitable for processes having rate constants ranging from 10−3 to 10−1 cm s−1. After obtaining the rate constant and transfer coefficient from the working curves, the appropriate theoretical current-voltage curve can be calculated and used in the three-dimensional analysis of the experimental data. An experimental verification of the method is presented.

Normalized potential sweep voltammetry: Part II. Application to heterogeneous charge transfer kinetics

Plotting theoretical voltammetric electrode potential data for Nernstian charge transfer on the X-coordinate, that for quasi-reversible charge transfer on the Y-coordinate and normalized current ( I/I p) on the Z-coordinate revealed that projections on to the X−Y plane resulted in: ( E− E p/2) quasi= m( E− E p/2) Nern where the slopes m are directly related to the normalized heterogeneous rate constant Λ (Λ= k s( DnF/RT) −1/2ν −1/2). Working curves, m−1 vs. 1/Λ, dependent upon the value of the transfer coefficient, α, were calcualted for the determination of k s. For the experimental evaluation of k s, values of m−1 are obtained as a function of the voltage sweep rate, ν, which allows both k s and α to be obtained from the working curves. The method is applicable to processes with k s<1 cm s −1 and is most suitable for processes having rate constants ranging from 10 −3 to 10 −1 cm s −1. After obtaining the rate constant and transfer coefficient from the working curves, the appropriate theoretical current-voltage curve can be calculated and used in the three-dimensional analysis of the experimental data. An experimental verification of the method is presented.

The Effect of Heterogeneous Charge Transfer Kinetics on Electrode Measurements.

The Effect of Heterogeneous Charge Transfer Kinetics on Electrode Measurements.