Fourier Transformed Ac Voltammetry Research Articles

Unravelling the effect of Cl- on alkaline saline water electrooxidation on NiFe (oxy)hydroxides

One of the major problems for seawater electrolysis is chloride corrosion arising from the chlorine oxidation reaction (ClOR). Although researchers have endeavored to improve the OER activity (overpotentials below 490 mV) to avoid the ClOR, an OER deactivation effect of chloride anions still looms large even before the ClOR becomes a serious issue, and its mechanism remains unclear. Herein using a simple Fourier transform ac voltammetry (FTacV) method, we unveil a hidden fact about the detrimental effect of Cl- over an exemplar catalyst NiFe (oxy)hydroxide: it increases the formation potential of Ni(IV) thus lowering the OER performance. This is borne out by theoretical calculations, which reveal changes in the electronic environment of Ni sites induced by replacing the bridge OH- species with Cl- and the resulting difficulty to form high-valence Ni(IV). The consequence is a large increase in energy barrier of the OH* deprotonation step that lowers the OER activity. The poison effect of Cl- can be restrained by adding carbonate ions, which restores the facile formation of Ni(IV) and stabilizes the electrode for over 140-h saline water oxidation reaction at the current density of 100 mA cm−2. This work helps to guide the design of active and durable seawater-splitting electrocatalysts.

Oxygen vacancies and surface reconstruction on NiFe LDH@Ni(OH)2 heterojunction synergistically triggering oxygen evolution and urea oxidation reaction

The combination of heterostructure and oxygen vacancy is one of the effective strategies to enhance the performance of oxygen evolution reaction (OER) and urea oxidation reaction (UOR). Herein, NiFe LDH@Ni(OH)2-z p-n heterostructure with abundant oxygen vacancies was successfully fabricated by multi-step strategy as electrodeposition and in-situ etching. NiFe LDH@Ni(OH)2-z p-n heterostructure can modulate the d-band center to optimize the adsorption energies of hydroxides, while oxygen vacancies also contributed to increasing the amount of active sites and reducing charge transfer resistance during OER and UOR. In-situ UV–vis spectroscopy and Fourier transform a.c. voltammetry (FTACV) unveiled the surface reconstruction of NiFe LDH to generate Ni/FeOOH which facilitated the adsorption of the reaction intermediates and accelerated charge transfer during OER and UOR. As a result, NiFe LDH@Ni(OH)2-z presented the low overpotential of 250/290 mV at 100/200 mA cm−2, small Tafel slope (43.6 mV dec−1) and excellent stability. The reaction order of OER with nearly unity indicated the fast adsorption process of OH-. NiFe LDH@Ni(OH)2-z also shows excellent electrocatalytic performance for UOR with 1.44 V at 100 mA cm−2 and Tafel slope of 37 mV dec−1. This work offers a novel approach for the formation of p-n heterostructure with rich oxygen vacancies to engineer the electronic modulation and surface reconstruction for synergistically triggering OER and UOR.

A Spotter’s guide to dispersion in non-catalytic surface-confined voltammetry experiments

• Effects of thermodynamic and kinetic dispersion in surface confined voltammetry. • Distinguishing dispersed from non-dispersed cases by looking at voltammetry data. • Thermodynamic dispersion is usually resolvable for all experiments. • Kinetic dispersion is only resolvable for certain systems under defined conditions. Dispersion, or the distribution of kinetic and thermodynamic parameter values describing a redox reaction, has long been acknowledged as a complicating factor in the analysis of both solution and surface-confined voltammetry experiments. In this tutorial paper, we show how varying levels of dispersion can affect the appearance of the experimental current, mainly illustrating the concept with reference to surface-confined reversible one electron transfer. We focus on three voltammetric techniques, ramped Fourier Transform AC Voltammetry, Purely Sinusoidal Voltammetry and Direct Current Voltammetry, showing the effect the dispersion parameters have on the appearance of the resulting current. We have also implemented an interactive web-based simulation package for comparison of the effects of the various parameters, available here. As modelling dispersion significantly increases the computational burden of simulating voltammetry experiments, making well-informed choices about when to include this effect is essential. To facilitate this, we discuss the intuition for when to include dispersion when fitting experimental voltammetry data, again with reference to the three techniques described above.

Fourier Transform Alternating Current Voltammetry (FTacV) As a Tool for in-Operando Study of PGM-Free ORR Catalysts' Durability in Pemfcs

The activity of Pt-group metal-free (PGM-free) ORR catalysts, has been increasing significantly in the past decade, and reached a level where it is comparable to Pt-based catalysts, proving their viability. Nowadays, the advancement in their development matured to the point where their stability and durability under fuel cells operating conditions must be determined, and the activity of the catalyst must be studied in-situ in-operando, in order to monitor the catalyst performance. While the advancement in activity is impressive, the next necessary improvement, durability, is impeded by the lack of a procedure that is specific towards the catalyst’s activity, and informative with respect to its degradation rate during fuel cell testing, undermining the ability of understanding durability issues. Currently, the degradation of the catalyst is described quantitatively by using the Tafel region of polarization curves. While an indispensable measurement, it is not specific towards the processes that the catalyst undergoes, as its output is influenced by many factors, such as pore structure, membrane conductivity, and especially, the activity of various active sites. This lowers the usefulness of this method, as it is not specific towards any active site in particular, limiting any inferences on possible routes that may be taken towards understanding and mitigating the catalyst’s degradation. Hence, there is a critical need to develop new analytical tools that have high specificity towards different aspects of the catalyst’s performance.With PGM catalysts, this problem issue was easily solved using an electrochemical activity descriptor that is specific to the active site: electrochemical active surface area (ECSA). This descriptor, which can be determined during an accelerated stress test (AST), helps quantify the degradation of Pt-based active sites, and has been playing an important role in the development of durable PGM ORR catalysts.In this work, Fourier-Transform Alternating Current Voltammetry (FTacV), was used as a method for describing the PGM-free ORR catalyst degradation. FTacV has shown great applicability in unraveling complex mechanisms and in discerning low signal faradaic reactions from high capacitive currents and noise. It is backed by an extensive theoretical background. Recently, FTacV has been used to probe the electron transfer in surface bound catalysts and enzymes, providing invaluable information on the electron transfer mechanisms that underpin the electrocatalytic reactions, thus, enabling further development of the catalysts and better understanding of these reaction.In this talk we will present the use of Fourier Transform ac voltammetry (FTacV) as an electrochemical, catalyst-specific, method for following the PGM-free degradation during fuel cell operation. For the first time, FTacV was employed during the operation of a polymer electrolyte membrane fuel cell before, during, and after stability tests. FTacV was found to be a highly sensitive technique to follow the activity of electrochemically active species, opening the path towards the accurate quantification of electrochemical active site density (EASD) of PGM-free ORR catalysts.

Determination of kinetic and thermodynamic parameters from large amplitude Fourier transform ac voltammetry of immobilized electroactive species

In the present work, the derivation of the standard rate constant k0, the transfer coefficient a and the standard (or formal) potential E0 of immobilized electroactive species, with the use of large amplitude Fourier transform alternating current cyclic voltammetry (FTacV) is examined. Both the kinetic equation of the coverage and the current integral equation are solved. The kinetic regions of the current response are distinguished, the chief observables in the case of both fast and slow kinetics are showcased and finally based on these, a methodology to extract k0, a and E0 is introduced.

Insights into the Electron Transfer Kinetics, Capacitance and Resistance Effects of Implantable Electrodes Using Fourier Transform AC Voltammetry on Platinum

The charge transfer mechanism at the electrode-solution interface was assessed by Fourier transform AC voltammetry (FTACV). The faradaic reactions that occur within the safe potential window on platinum had slow electron transfer kinetics. The charge transfer mechanisms during short chronopotentiometric stimulation of cells, is most likely dominated by capacitance. Impedance was modelled with a single time constant. FTACV was fit with a 2-component equivalent circuit comprising a series capacitor and resistor. Capacitance and resistance varied with electrode potential, area, topography, surface functionality and solution composition. Capacitance correlated with charge storage capacity measured by voltammetry. Increased capacitance reduced the change in potential during chronopotentiometry. Increased resistance resulted in uncompensated resistance, and a larger change in potential during chronopotentiometry. Uncompensated resistance in tissue may lead to the measured potential of an electrode being considerably higher than its true potential, leading to a conservative estimate of the safe operating potential window. An impedance test is used to assess electrode performance in vivo. The impedance test is a function of capacitance, faradaic charge and resistance. Impedance test results allow electrode comparison, indicating changes in electrode-tissue interface, electrode failure and power usage, however use of Ohm's law to calculate an impedance value is not valid.

Voltammetric and electrosynthetic triggered gel formation

An electrochemical method has been introduced to trigger the gelation process between catechol and chitosan. The process is based on the initial heterogeneous oxidation of catechol at an electrode surface with subsequent homogeneous polymerisation with chitosan. It is characterised using square-wave voltammetry (SWV) and large amplitude Fourier Transformed ac voltammetry (FTACV). The first employment of FTACV on this crosslinking process indicates a consistent response as SWV with a viscous gel being formed at the electrode surface. The application of bulk electrolysis at a carbon foam electrode, for scale up to form a homogeneous gel has been subsequently demonstrated. The resulting gel properties have been characterised by several techniques with its thermal stability at ca. 250 °C proving it an appealing candidate for various conditions.

Facile electrochemical co-deposition of metal (Cu, Pd, Pt, Rh) nanoparticles on reduced graphene oxide for electrocatalytic reduction of nitrate/nitrite

Monodispersed metal (Cu, Pd, Pt, Rh) nanoparticles have been anchored on reduced graphene oxide (rGO) by a facile electrochemical co-deposition method. Scanning electron microscopic images reveal the presence of homogeneously dispersed metal nanoparticles on the rGO nanosheets. The as-prepared nanocomposites demonstrate excellent electrocatalytic activity for electroreduction of NO3− and NO2−. The mechanisms associated with the best performed Rh-rGO were studied in detail by cyclic voltammetry, X-ray photoelectron spectroscopic (XPS) and Fourier transformed ac voltammetry. It is proposed that the electrocatalytic reduction of NO3− on Rh-rGO is catalyzed by an adsorbed H* (H+aq + e− → H*) species to form NO2− which is further catalyzed by H* to form a surface confined NO* intermediate. Rh-rGO displays high electrochemical stability for NO3− reduction over the pH range of 4–9, which could be a suitable candidate for applications in remediation of NO3− contamination.

Probing biological redox chemistry with large amplitude Fourier transformed ac voltammetry.

Biological electron-exchange reactions are fundamental to life on earth. Redox reactions underpin respiration, photosynthesis, molecular biosynthesis, cell signalling and protein folding. Chemical, biomedical and future energy technology developments are also inspired by these natural electron transfer processes. Further developments in techniques and data analysis are required to gain a deeper understanding of the redox biochemistry processes that power Nature. This review outlines the new insights gained from developing Fourier transformed ac voltammetry as a tool for protein film electrochemistry.

Parameterization of Water Electrooxidation Catalyzed by Metal Oxides Using Fourier Transformed Alternating Current Voltammetry.

Detection and quantification of redox transformations involved in water oxidation electrocatalysis is often not possible using conventional techniques. Herein, use of large amplitude Fourier transformed ac voltammetry and comprehensive analysis of the higher harmonics has enabled us to access the redox processes responsible for catalysis. An examination of the voltammetric data for water oxidation in borate buffered solutions (pH 9.2) at electrodes functionalized with systematically varied low loadings of cobalt (CoOx), manganese (MnOx), and nickel oxides (NiOx) has been undertaken, and extensive experiment-simulation comparisons have been introduced for the first time. Analysis shows that a single redox process controls the rate of catalysis for Co and Mn oxides, while two electron transfer events contribute in the Ni case. We apply a "molecular catalysis" model that couples a redox transformation of a surface-confined species (effective reversible potential, Eeff0) to a catalytic reaction with a substrate in solution (pseudo-first-order rate constant, k1f), accounts for the important role of a Brønsted base, and mimics the experimental behavior. The analysis revealed that Eeff0 values for CoOx, MnOx, and NiOx lie within the range 1.9-2.1 V vs reversible hydrogen electrode, and k1f varies from 2 × 103 to 4 × 104 s-1. The k1f values are much higher than reported for any water electrooxidation catalyst before. The Eeff0 values provide a guide for in situ spectroscopic characterization of the active states involved in catalysis by metal oxides.

Electrochemistry of Molten Sulfides: Copper Extraction from BaS-Cu2S

The electrolytic extraction of liquid copper at 1105°C from a molten sulfide electrolyte composed of 57 wt% BaS and 43 wt% Cu2S was investigated. DC cyclic voltammetry, Fourier transformed AC voltammetry, and galvanostatic electrolysis revealed that the electrodeposition of copper is possible in the selected molten sulfide electrolyte. The half wave potential for the reaction on graphite was determined, and liquid copper of high purity was obtained by galvanostatic electrolysis. These preliminary results confirm that molten sulfides free of alkaline elements could be used as an electrolyte for faradaic applications, despite the semi-conducting nature of the melt. In addition to demonstrating the need for enhanced understanding of the transport properties of such electrolyte, the results show the critical impact of the cell design to improve the process faradaic efficiency.

Redox Levels of a closo-Osmaborane: A Density Functional Theory, Electron Paramagnetic Resonance and Electrochemical Study.

A closo-type 11-vertex osmaborane [1-(η(6)-pcym)-1-OsB10H10] (pcym = para-cymene) has been synthesized and characterized by single-crystal X-ray diffraction and elemental analysis, as well as by (11)B and (1)H NMR, UV-visible, and mass spectrometry. The redox chemistry has been probed by dc and Fourier transformed ac voltammetry and bulk reductive electrolysis in CH3CN (0.10 M (n-Bu)4NPF6) and by voltammetry in the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (Pyrr1,4-NTf2), which allows the oxidative chemistry of the osmaborane to be studied. A single-crystal X-ray diffraction analysis has shown that [1-(η(6)-pcym)-1-OsB10H10] is isostructural with other metallaborane compounds of this type. In CH3CN (0.10 M (n-Bu)4NPF6), [1-(η(6)-pcym)-1-OsB10H10] undergoes two well-resolved one-electron reduction processes with reversible potentials separated by ca. 0.63-0.64 V. Analysis based on a comparison of experimental and simulated ac voltammetric data shows that the heterogeneous electron transfer rate constant (k(0)) for the first reduction process is larger than that for the second step at GC, Pt, and Au electrodes. k(0) values for both processes are also larger at GC than metal electrodes and depend on the electrode pretreatment, implying that reductions involve specific interaction with the electrode surface. EPR spectra derived from the product formed by one-electron reduction of [1-(η(6)-pcym)-1-OsB10H10] in CH3CN (0.10 M (n-Bu)4NPF6) and electron orbital data derived from the DFT calculations are used to establish that the formal oxidation state of the metal center of the original unreduced compound is Os(II). On this basis it is concluded that the metal atom in [1-(η(6)-pcym)-1-OsB10H10] and related metallaboranes makes a 3-orbital 2-electron contribution to the borane cluster. Oxidation of [1-(η(6)-pcym)-1-OsB10H10] coupled to fast chemical transformation was observed at 1.6 V vs ferrocene(0/+) in Pyrr1,4-NTf2. A reaction scheme for the oxidation involving formation of [1-(η(6)-pcym)-1-OsB10H10](+), which rearranges to an unknown electroactive derivative, is proposed, and simulations of the voltammograms are provided.

Optimisation of windowing for harmonic recovery in large-amplitude Fourier transformed a.c. voltammetry

The Fourier transform (FT)/band filtering (windowing)/inverse Fourier transform (iFT) sequence of operations with rectangular or ‘bell-shaped’ window functions is widely used for signal processing in physical chemistry and, in particular, in the technique of FT a.c. voltammetry (FTACV). However, the use of the rectangular window introduces ‘ringing noise’ (oscillations) into the iFT data, which may mask the faradaic a.c. signal of interest in the higher order harmonics that are available in large amplitude FTACV, while ‘bell-shaped’ functions may distort the magnitude and shape of the recovered a.c. harmonic components. The application of a new adjustable hybrid window based on the convolution of a rectangular and a Gaussian profile is now introduced to diminish the magnitude of the ‘ringing’. The use of such a window is shown to significantly reduce both the amplitude and propagation time of the ‘ringing’ and, at the same time preserves the magnitude and shape of the recovered harmonic components. These attributes facilitate the accurate recovery of the faradaic signal. The advantages provided by application of the hybrid window when estimating thermodynamic and kinetic parameters from optimised comparisons of experimental and theoretical FTACV data are demonstrated.

New Insights into the Analysis of the Electrode Kinetics of Flavin Adenine Dinucleotide Redox Center of Glucose Oxidase Immobilized on Carbon Electrodes

New insights into electrochemical kinetics of the flavin adenine dinucleotide (FAD) redox center of glucose-oxidase (GlcOx) immobilized on reduced graphene oxide (rGO), single- and multiwalled carbon nanotubes (SW and MWCNT), and combinations of rGO and CNTs have been gained by application of Fourier transformed AC voltammetry (FTACV) and simulations based on a range of models. A satisfactory level of agreement between experiment and theory, and hence establishment of the best model to describe the redox chemistry of FAD, was achieved with the aid of automated e-science tools. Although still not perfect, use of Marcus theory with a very low reorganization energy (≤0.3 eV) best mimics the experimental FTACV data, which suggests that the process is gated as also deduced from analysis of FTACV data obtained at different frequencies. Failure of the simplest models to fully describe the electrode kinetics of the redox center of GlcOx, including those based on the widely employed Laviron theory is demonstrated, as is substantial kinetic heterogeneity of FAD species. Use of a SWCNT support amplifies the kinetic heterogeneity, while a combination of rGO and MWCNT provides a more favorable environment for fast communication between FAD and the electrode.

Studies on the Nuances of the Electrochemically Induced Room Temperature Isomerization of cis-Stilbene in Acetonitrile and Ionic Liquids

Electrochemical reduction of cis-stilbene occurs by two well-resolved one-electron reduction steps in acetonitrile with (n-Bu)4NPF6 as the supporting electrolyte and in N-butyl-N-methylpyrrolidinium (Pyrr1,4(+)) and (trimethylamine)(dimethylethylamine)-dihydroborate bis(trifluoromethylsulfonyl)amide (NTf2(-)) ionic liquids (ILs). Mechanistic details of the electroreduction have been probed by dc and Fourier transformed ac voltammetry, simulation of the voltammetry, bulk electrolysis, and EPR spectroscopy. The first one-electron reduction induces fast cis to trans isomerization in CH3CN and ILs, most likely occurring via disproportionation of cis-stilbene radical anions and fast transformation of the cis-dianion to the trans-configuration. The second reduction process is chemically irreversible in CH3CN due to protonation of the dianion but chemically reversible in highly aprotic ILs under high cis-stilbene concentration conditions. Increase of the (n-Bu)4NPF6 supporting electrolyte concentration (0.01-1.0 M) in CH3CN induces substantial positive shifts in the potentials for reduction of cis-stilbene, consistent with strong ion pairing of the anion radical and dianion with (n-Bu)4N(+). However, protection by ion pairing against protonation of the stilbene dianions or electrochemically induced cis-trans-stilbene isomerization is not achieved. Differences in electrode kinetics and reversible potentials for cis-stilbene(0/•-) and trans-stilbene(0/•-) processes are less pronounced in the Pyrr1,4-NTf2 ionic liquid than in the molecular solvent acetonitrile.

The Observation of Dianions Generated by Electrochemical Reduction of trans-Stilbenes in Ionic Liquids at Room Temperature

Three highly aprotic bis(trifluoromethylsulfonyl)amide (NTf2(-)) based ionic liquids (ILs) containing the cations trihexyl(tetradecyl)phosphonium (P6,6,6,14(+)), N-butyl-N-methylpyrrolidinium (Pyrr4,1(+)), and (trimethylamine)(dimethylethylammine)dihydroborate ((N111)(N112)BH2(+)) have been examined as media for room temperature voltammetric detection of highly basic stilbene dianions electrochemically generated by the reduction of trans-stilbene (t-Stb) and its derivatives (4-methoxy-, 2-methoxy-, 4,4'-dimethyl-, and 4-chloromethyl-). Transient and steady-state data in the ILs were compared with results obtained in the molecular solvent acetonitrile. In all media examined, the t-Stb(0/•-) process is chemically and electrochemically reversible with a heterogeneous charge transfer rate constant in CH3CN of 1.5 cm s(-1), as determined by Fourier transformed AC voltammetry. However, further reduction to the dianion was always irreversible in this molecular but weakly acidic solvent. On the other hand, a substantial level of chemical reversibility for the reduction of t-Stb(•-) to t-Stb(2-) on the time scale of cyclic voltammetry is achieved when the concentration of trans-stilbene, [t-Stb], appreciably exceeds the concentration of adventitious water or other proton sources. In particular, these conditions are met when [t-Stb] ≥ 0.1 M in thoroughly dehydrated and purified ILs, while in the presence of CH3CN, t-Stb(2-) still suffers fast irreversible protonation under these stilbene concentration conditions. The E0/•-(0) values (vs Fc(0/+)) for substituted trans-stilbenes in acetonitrile and (N111)(N112)BH2-NTf2 do not differ substantially, nor do the E0/•-(0) and E•-/2-(0) differences or other aspects of the voltammetric behavior.

Attributes of Large‐Amplitude Fourier Transformed Alternating Current Voltammetry at Array and Single Carbon Fiber Microdisk Electrodes

AbstractThe attributes of large‐amplitude Fourier transformed AC voltammetry have been assessed at electrodes constructed from either a single carbon fiber microdisk or a random assembly of carbon fiber microdisks (RAM). Factors that govern the limits of detection for the FcMeOH and uric acid analytes have been established. For the reversible FcMeOH oxidation process, the RAM electrode provides the lower limit of detection through substantial amplification of the peak current magnitude, and hence superior faradaic current‐to‐noise ratio. In contrast for the overall two‐electron oxidation of uric acid, the much faster electron transfer rate achieved at the electrochemically treated single carbon fiber microelectrode provides the lower detection limit, which is again optimal in the sixth harmonic.

Estimation of electrode kinetic and uncompensated resistance parameters and insights into their significance using Fourier transformed ac voltammetry and e-science software tools

In transient forms of voltammetry, quantitative analysis of electrode kinetics and parameters such as uncompensated resistance (Ru) and double layer capacitance (Cdl) are usually undertaken by comparing experimental and simulated data. Commonly, the skill of the experimentalist is heavily relied upon to decide when a good fit of simulated to experimental data has been achieved. As an alternative approach, it is now shown how data analysis can be based on implementation of e-science software tools. Previously, a standard heuristic data analysis approach applied to the oxidation of ferrocene in acetonitrile (0.1M Bu4NPF6) at a glassy carbon electrode using higher order harmonics available in Fourier transformed ac voltammetry implied that the heterogeneous charge transfer rate constant k0 is ⩾0.25cms−1 with the charge transfer coefficient (α) lying in the range of 0.25–0.75. Application of e-science software tools to the same data set allows a more meaningful understanding of electrode kinetic data to be provided and also offers greater insights into the sensitivity of the IRu (Ohmic drop) on these parameters. For example, computation of contour maps based on a sweep of two sets of parameters such as k0 and Ru or α and k0 imply that α is 0.50±0.05 and that k0 lays in the range 0.2–0.4cms−1 with Ru around 130Ohm. Quantitative evaluation of k0, α and Ru for the quasi-reversible [Fe(CN)6]3-+e⇌[Fe(CN)6]4- process at a glassy carbon electrode in aqueous media is also facilitated by use of e-science software tools. In this case, when used in combination with large amplitude Fourier transformed ac voltammetry, it is found for each harmonic that k0 for the electrode process lies close to 0.010cms−1, α is 0.50±0.05 and Ru is ⩽10Ohm.

Detailed Electrochemical Studies of the Tetraruthenium Polyoxometalate Water Oxidation Catalyst in Acidic Media: Identification of an Extended Oxidation Series using Fourier Transformed Alternating Current Voltammetry

The electrochemistry of the water oxidation catalyst, Rb(8)K(2)[{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(γ-SiW(10)O(36))(2)] (Rb(8)K(2)-1(0)) has been studied in the presence and absence of potassium cations in both hydrochloric and sulfuric acid solutions by transient direct current (dc) cyclic voltammetry, a steady state dc method in the rotating disk configuration and the kinetically sensitive technique of Fourier transformed large-amplitude alternating current (ac) voltammetry. In acidic media, the presence of potassium ions affects the kinetics (apparent rate of electron transfer) and thermodynamics (reversible potentials) of the eight processes (A'/A to H/H') that are readily detected under dc voltammetric conditions. The six most positive processes (A'/A to F/F'), each involve a one electron ruthenium based charge transfer step (A'/A, B'/B are Ru(IV/V) oxidation and C/C' to F/F' are Ru(IV/III) reduction). The apparent rate of electron transfer of the ruthenium centers in sulfuric acid is higher than in hydrochloric acid. The addition of potassium cations increases the apparent rates and gives rise to a small shift of reversible potential. Simulations of the Fourier transformed ac voltammetry method show that the B'/B, E/E', and F/F' processes are quasi-reversible, while the others are close to reversible. A third Ru(IV/V) oxidation process is observed just prior to the positive potential limit via dc methods. Importantly, the ability of the higher harmonic components of the ac method to discriminate against the irreversible background solvent process allows this (process I) as well as an additional fourth reversible ruthenium based process (J) to be readily identified. The steady-state rotating disk electrode (RDE) method confirmed that all four Ru-centers in Rb(8)K(2)-1(0) are in oxidation state IV. The dc and ac data indicate that reversible potentials of the four ruthenium centers are evenly spaced, which may be relevant to understanding of the water oxidation electrocatalysis. A profound effect of the potassium cation is observed for the one-electron transfer process (G/G') assigned to Ru(III/II) reduction and the multiple electron transfer reduction process (H/H') that arise from the tungstate polyoxometalate framework. A significant shift of E°' to a more positive potential value for process H/H' was observed on removal of K(+) (~100 mV in H(2)SO(4) and ~50 mV in HCl).

Large amplitude Fourier transformed ac voltammetry at a rotating disc electrode: a versatile technique for covering Levich and flow rate insensitive regimes in a single experiment

The theory for large amplitude Fourier transformed ac voltammetry at a rotating disc electrode is described. Resolution of time domain data into dc and ac harmonic components reveals that the mass transport for the dc component is controlled by convective-diffusion, while the background free higher order harmonic components are flow rate insensitive and mainly governed by linear diffusion. Thus, remarkable versatility is available; Levich behaviour of the dc component limiting current provides diffusion coefficient values and access to higher harmonics allows fast electrode kinetics to be probed. Two series of experiments (dc and ac voltammetry) have been required to extract these parameters; here large amplitude ac voltammetry with RDE methodology is used to demonstrate that kinetics and diffusion coefficient information can be extracted from a single experiment. To demonstrate the power of this approach, theoretical and experimental comparisons of data obtained for the reversible [Ru(NH(3))(6)](3+/2+) and quasi-reversible [Fe(CN)(6)](3-/4-) electron transfer processes are presented over a wide range of electrode rotation rates and with different concentrations and electrode materials. Excellent agreement of experimental and simulated data is achieved, which allows parameters such as electron transfer rate, diffusion coefficient, uncompensated resistance and others to be determined using a strategically applied approach that takes into account the different levels of sensitivity of each parameter to the dc or the ac harmonic.