Microelectrode Voltammetry Research Articles

Elucidating the Effects of Temperature on Nonaqueous Redox Flow Cell Cycling Performance

The impact of cell temperature is a relatively underexplored area within the burgeoning field of nonaqueous redox flow batteries (NAqRFBs). Here, we investigate the effect of elevated temperature on the performance of nonaqueous redox electrolytes and associated flow cells. Using a model compound, N-(2-(2-methoxyethoxy)-ethyl)phenothiazine (MEEPT), in a propylene-carbonate-based electrolyte, we experimentally measure the temperature dependence of relevant physicochemical properties (i.e., electrolyte conductivity, viscosity, diffusivity) and electrochemical characteristics (i.e., chemical and electrochemical reversibility) across a temperature range of 30 °C to 70 °C. We then perform flow cell studies, finding that while ohmic and mass transport resistances decrease significantly with increases in temperature for the MEEPT/MEEPT+· redox couple, accessible electrolyte capacity gradually reduces at temperatures 50 °C. Ex-situ, post-test characterization using microelectrode voltammetry suggests that this capacity fade is due to instability of the MEEPT radical cation. Finally, using MEEPT as a posolyte and a model viologen negolyte (bis(2-(2-methoxyethoxy)ethyl)viologen), we assemble a full cell and perform polarization analyses, observing a 2× increase in the peak power density when the operating temperature is increased from 30 °C to 70 °C. Broadly, this work highlights opportunities for systematic engineering of nonaqueous electrolytes and flow cells for higher power operation at elevated temperatures.

Protolysis studies and quantification of acids and bases in aqueous solutions by microelectrode voltammetry

Protolysis studies and quantification of acids and bases in aqueous solutions by microelectrode voltammetry

An automated and lightweight framework for electrolyte diagnostics using quantitative microelectrode voltammetry

Voltammetry is a ubiquitous electroanalytical method that can be used to help probe sustainable electrochemical technologies. When conducted with a microelectrode (radius ca. μm), voltammetry enables special interrogation of electrolyte solutions by minimizing distortions and facilitating near-steady-state measurements. Methodologies aimed to evaluate the behavior of redox-active species often leverage well-established, physically-grounded expressions that can be extended to examine electrolyte solutions under non-ideal conditions (e.g., signal convolution from multiple redox events) by simulating the entire voltammogram. To characterize these analyte systems, we first develop closed-form expressions—building on previous work that utilizes oblate spheroidal coordinates—and establish a framework for rapidly evaluating electrolyte composition. We subsequently apply finite difference transient voltammogram models to assess the performance of this workflow. We then validate our findings using model, deterministically-prepared nonaqueous electrolyte solutions containing N-[2-(methoxyethoxy)ethyl]phenothiazine. Overall, we show the toolkit is particularly adept at rapidly (< 1 min) estimating the degree to which an electrolyte solution is charged (its “state-of-charge”) and remains intact (its “state-of-health”). Finally, we highlight potential extensions of this method towards advancing in situ or operando diagnostic methods within operating electrochemical devices.

Berberine isolation from Coscinium fenestratum: optical, electrochemical, and computational studies.

Berberine was extracted from Coscinium fenestratum (tree turmeric) and purified by column chromatography. The UV-Vis absorption spectroscopy of berberine was studied in acetonitrile and aqueous media. TD-DFT calculations employing the B3LYP functional were found to reproduce the general features of the absorption and emission spectra correctly. The electronic transitions to the first and second excited singlet states involve a transfer of electron density from the electron donating methylenedioxy phenyl ring to the electron accepting isoquinolium moiety. An estimate of the electrochemical gap (2.64 V) was obtained from microelectrode voltammetry and good agreement was found with quantum chemical calculations using the cc-pVTZ basis set and the B3LYP, CAM-B3LYP and wB97XD functionals. The calculations indicate spin density of the radical dication is delocalised over the molecule. These basic data are useful for assessment of the synthesis of donor-acceptor polymeric materials employing oxidative polymerization or co-polymerisation of berberine.

Assessment of non-aqueous solvents in the electrooxidation of resorcinol, phloroglucinol, pyrogallol and role of co-solvent in determination of pyrogallol with microelectrode voltammetry

Assessment of non-aqueous solvents in the electrooxidation of resorcinol, phloroglucinol, pyrogallol and role of co-solvent in determination of pyrogallol with microelectrode voltammetry

Methanol Oxidation at Platinum in Alkaline Media: A Study of the Effects of Hydroxide Concentration and of Mass Transport

The hydroxide ion concentration dependence of the methanol oxidation reaction at Pt was studied using microelectrode voltammetry and rotating disk electrode voltammetry. Both methods suggest that the rate of methanol oxidation is limited by hydroxide mass transport at low hydroxide concentrations, while it is inhibited by hydroxide adsorption at high concentrations. It was possible to shift from the transport-limited regime to the inhibitory regime by varying the bulk concentration of hydroxide or by varying mass transport to the electrode. Rotating ring-disk electrode voltammetry was employed to qualitatively assess changes in the diffusion layer pH. The results indicated a decrease in the surface pH during methanol oxidation, as expected, but also that the pH reached a steady state during hydroxide transport limited methanol oxidation.

Analytical and Numerical Modeling of Microelectrode Voltammetry in Oblate Spheroidal Coordinates

Voltammetry is a ubiquitous electroanalytical technique used to examine electroactive compounds for a range of purposes, such as component characterization of an electrochemical device or mechanistic analyses of electrode reactions.1–3 Although most commonly performed on a macroelectrode (radius ~mm), voltammetry conducted on a microelectrode (radius ~μm) enables interrogation of electrolyte compositions inaccessible to macroelectrodes, as the smaller radius minimizes ohmic distortions, reduces charging currents, and enables near-steady-state measurements at slower scan rates (e.g., 10 mV/s).4 These characteristics also mean that microelectrodes hold promise for the examination of concentrated multi-component electrolytes, potentially enabling in situ or operando analysis in practical embodiments.2,4 Methodologies aimed to evaluate electrolyte behavior in electrochemical cells leverage well-established expressions,1–4 and while physically grounded, they do not account for conditions where migration or non-idealities impact the voltammetric response.3 In these instances, more extensive modeling, often framed in cylindrical coordinates,1,5 is needed to estimate the features of interest (e.g., bulk concentrations) with sufficient accuracy. These methods involve partial differential equations with piecewise boundary conditions, most often resulting in expressions of limited adoptability due to either numerical complexity or lack of physicality.5 The mathematical treatment may be strengthened by obviating the piecewise boundary conditions through the use of oblate spheroidal coordinates. Indeed, with this coordinate transform, closed form steady-state and numerical transient voltammograms are obtainable with greater ease.6,7 Although this change has simplified modeling, to the best of our knowledge, existing treatments using both cylindrical and oblate spheroidal coordinates are only applicable to a restricted set of conditions (e.g., equal diffusion coefficients, no bulk product present) rarely encountered in real devices,5–7 limiting their ability to study practical electrolytes.To extend the applicability of this approach, we use oblate spheroidal coordinates to derive steady-state and transient microelectrode voltammogram models capable of accounting for different electron transfer kinetics, dissimilar diffusion coefficients, and the presence of both reactant and product in the bulk solution (illustrated in the abstract figure). In this presentation, we will discuss the derivation of the closed form steady-state solutions and their evaluation using cases previously described in literature.5,6 We will then describe the development of the finite difference transient models capable of simulating voltammograms over a broader range of conditions (e.g., varying scan rates), which, in turn, are validated using COMSOL®. Finally, we will discuss the application of this procedure to evaluate electrolyte behavior in electrochemical cells, such as examining whether more complex physical phenomena (e.g., migration-induced transport) can be integrated into this framework and whether closed form steady-state solutions can accurately estimate relevant parameters from transient voltammograms.Acknowledgments: This work was funded by the National Science Foundation under Award Number 1805566. B.J.N. and K.M.T. both gratefully acknowledge the National Science Foundation Graduate Research Fellowship Program under grant no. 1122374. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

Electrochemically Induced Mesomorphism Switching in a Chlorpromazine Hydrochloride Lyotropic Liquid Crystal

The discovery of electrochemical switching of the Lα phase of chlorpromazine hydrochloride in water is reported. The phase is characterized using polarizing microscopy, X-ray scattering, rheological measurements, and microelectrode voltammetry. Fast, heterogeneous oxidation of the lyotropic liquid crystal is shown to cause a phase change resulting from the disordering of the structural order in a stepwise process. The underlying molecular dynamics is considered to be a cooperative effect of both increasing electrostatic interactions and an unfolding of the monomers from “butterfly”-shaped in the reduced form to planar in the oxidized form.

A Method for Evaluating Soluble Redox Couple Stability Using Microelectrode Voltammetry

Soluble, redox-active organic materials hold promise as charge-storage species for flow batteries; however, chemical stability during extended operation remains a key challenge. While a number of spectroscopic and electrochemical techniques are currently used to probe these complex and often ill-defined decay pathways, these techniques have limitations, including lack of accessibility and potentially cumbersome preparatory steps. Here, we use microelectrode voltammetry to observe nonaqueous flow battery electrolytes directly, simultaneously identifying the rate of self-discharge (reversible material loss) and material decay (irreversible material loss). We validate this technique using ferrocene as a stable model redox couple, examine and address sources of error, and, finally, demonstrate its capability by assessing the decay of a well-studied and moderately-stable substituted dialkoxybenzene [2,5-di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene]. These results suggest that microelectrodes may have utility for rapid assessment of redox electrolyte state-of-charge and state-of-health, both in operando and post mortem.

Peak potential shift of fast cyclic voltammograms owing to capacitance of redox reactions

Cyclic voltammograms of a ferrocenyl derivative in aqueous solution showed diffusion-controlled peak, of which currents deviated downward from the proportionality of the square-root of potential scan rates, v, and of which potentials were shifted with an increase in v. The dependence of the peak on v can be attributed conventionally to sluggish charge transfer rates and the solution resistance. This redox species has been proved to exhibit extremely fast transfer rates by microelectrode voltammetry. The solution resistance was suppressed by using small currents less than 3 μA at a thin cylindrical electrode even at scan rates of 8 V s−1. The dependence was inconsistent with the theory for the heterogeneous kinetics. It can be explained in terms of the negative capacitance associated with the redox reaction. Electrochemical reactions occur in the direction of relaxing the applied electric field, as for the double layer charging currents. However, the electrochemically generated charge enhances the electric field, so that the negatively charging current should flow. The negative capacitance was close to ca. −64 μF cm−2 for 1 mM redox concentration. The values varied linearly with the concentration.

Differentiating between ageing times of typical Chinese liquors by steady-state microelectrode voltammetry

A typical Chinese liquor was characterised using a triple-electrode system with clean and simple design. In this approach, liquors were analysed with no other pre-treatment than mixing the liquor with a potassium chloride solution to ensure sufficient conductivity. Two broad reduction peaks at [−0.3 V, −0.5 V] and [−0.7 V, −0.8 V] were present in the cyclic voltammetry, demonstrating the feasibility of the gold electrode for liquor characterisation. Both the competing reactions in the liquor samples contributed to the reduction current. Thus, the steady-state microelectrode voltammetry affords an opportunity to estimate the total concentration of the redox-active species in the mixed compounds according to the relationship between the total concentration and the value of limiting diffusion current. Further, the discrimination of liquors at different ageing times was realised by the conjunction of the electrochemical results and principal component analysis.

Electrochemical Characteristics of a Triphenylamine Derivative by Microelectrode Voltammetry

AbstractWith the objective of understanding the kinetic redox properties of triphenylamine derivatives in association with chemical reactions, for their future application in functional organic semiconductor devices, the electrochemical characteristics of 4‐(2,2‐diphenylethenyl)‐N,N‐bis(4‐methylphenyl)‐benzenamine (TPA) were evaluated. Based on cyclic voltammograms of TPA on Pt disk electrodes with diameters of 300 μm and 10 μm at slow and fast scan rates in an acetonitrile solution, the TPA.+ is stable, while the TPA2+ is unstable. Importantly, the unstable TPA2+ appears to break down by a subsequent chemical reaction. A Cottrell plot analysis from chronoamperometry of a solution containing TPA reveals that both the first and second oxidations are one‐electron reactions. Concerning the stabilization mechanism of the first oxidation state of TPA, the results of molecular orbital calculations indicate that the electrons of the HOMO level are distributed in the triphenylamine group, which induces a resonance‐stabilized TPA.+. Based on these results, TPA/TPA.+ is suggested to have a sufficient stability for further application in organic semiconductor devices.

Correlating Ionic Conductivity, Oxygen Transport and ORR with Structure of Dialkylacetamide-Based Electrolytes for Lithium-Air Batteries

Chemical structures of lithium and tetrabutylammonium (TBA) salt solutions in N,N-dimethylacetamide (DMAc) and N,N-diethylacetamide (DEAc), two high Donor Number organic solvents, have been studied. In LiX salt solutions (where X = PF6−, CF3SO3−, ClO4− and NO3−), solvation occurs when the Li+ bonds with the solvent's carbonyl group forming Li+[O=C(CH3)N(CH3)2]nX− ion pairs. Infrared and 13C-NMR spectra are consistent with the ion pair being solvent-separated when the anion is PF6−, ClO4− or NO3−, and a contact ion pair in the case of CF3SO3−. Chemical interactions between TBA+ and the solvents to form conducting solutions appeared to be dipolar in nature. Ionic conductivities of TBA+ and Li+ electrolytes were measured and correlated with their viscosities. In 0.1M TBAPF6/DMAc, the O2 solubility and diffusion coefficient (3.09 × 10−6 mol/cm−3 and 5.09 × 10−5 cm2s−1, respectively) measured using microelectrode technique are typical of values measured in several TBA+ solutions. Microelectrode voltammetry revealed steady-state limiting current behavior for oxygen reduction reactions (ORR) in TBAX/DMAc electrolytes indicating a reversible ORR process. Conversely, microelectrode current-voltage data for ORR in LiX/DMAc electrolytes revealed irreversible behavior mainly ascribed to the blockage of the electrode surface by insoluble ORR products. The ORR in DMAc correlated with its high Donor Number and the overall process conformed to the Hard-Soft Acid-Base theory.

Understanding the Influence of the Electrochemical Behavior of Potassium Hexacyanoferrate(II/III) on Acetonitrile/Water Binary Mixtures: An Experimental and Molecular Dynamics Study

Microelectrode voltammetry was employed to gain information on the behavior of potassium hexacyanoferrate(II/III) in acetonitrile/water binary mixtures in order to understand the bulk properties of the mixture and the electrode/binary solution interface properties. The first analysis of the obtained data demonstrated that the diffusion coefficient values of hexacyanoferrate(III) apparently do not respect the Stokes–Einstein equation, considering the hydrodynamic radius does not change with acetonitrile content. Another interesting observation is the adsorption of electrogenerated potassium hexacyanoferrate(II) onto the electrode surface in binary mixtures with 50 mol % acetonitrile or higher. Molecular dynamics simulations showed that acetonitrile molecules accumulate on the surface of an electrode model and also that ionic species tend to form clusters in acetonitrile-rich mixtures. Since potassium hexacyanoferrate(II) is not soluble in acetonitrile, the adsorption of the iron complex onto the electrode is related to the formation of the acetonitrile layer on the surface of the electrode. The diffusion coefficient of hexacyanoferrate(III) in the mixtures decreases because of the ion clustering and the formation of an acetonitrile layer on the surface of the electrode, which is an obstacle that the ions have to overcome to reach and leave the electrode.

Microelectrode Studies of the Critical Breakdown and Spatial Bifurcation in Additive Derived S-NDR Copper Electrodeposition

Copper electrodeposition is widely used in integrated circuit manufacturing, ranging from damascene plating of sub-100 nanometer features to deposition of micron-scale through-silicon-via (TSV) interconnects used for 3D-chip stacking.1 Electrodeposition of void-free copper structures in high aspect ratio cavities is achieved with various suppressing, accelerating, and leveling organic additives; enabling superconformal, bottom-up filling of these recessed features. Recently, the use of a single suppressing organic additive has been demonstrated and modeled in a variety of different metal systems (Cu, Ni, Co, Au) for TSV-scale feature filling.2-5 In these systems, bottom-up fill is derived from an S-shaped negative-differential-resistance (S-NDR), generating active plating regions at the bottom of the TSVs and passive regions along the walls and surface. This active/passive bifurcation is driven by adsorption/desorption kinetics of the suppressing polymer and chloride complexes, mass-transport of the chemical species, and the TSV geometry. Additional studies of S-NDR systems demonstrate that in the absence of any electrode geometric constraints the bifurcation occurs in random Turing-like patterns.2 In other reaction systems, such bifurcating active-passive reactions are found to be dependent on the electrode size such that for reduced electrode dimensions the ability to bifurcate is frustrated and a homogeneous surface reactions occurs across the electrode.6 In this work, microelectrodes are used to probe the dependence of bath composition on the critical breakdown behavior in single organic additive, acid copper sulfate electrolytes. The well-defined, high mass-transport rates and negligible IR-drop inherent in microelectrodes provides new insight into the chemical behavior of copper S-NDR systems at length scales relevant to industrial applications. Moreover, voltammetry at microelectrodes with dimensions less than the critical dimensions of the spatial instability in an S-NDR system provides more precise kinetic relationships; a result enabled by the homogeneity of the reaction across the electrode surface. However, one challenge of electrodeposition on microelectrodes is significant shape and area change during electrodeposition. Thus, our cyclic voltammetry and galvanodynamic linear sweeps are designed to ensure copper deposition is less than 10% of the microelectrode diameter, maintaining the disk-shaped geometry. Microelectrode experiments that vary chloride concentration demonstrate a clear dependence of the critical breakdown potential (a more negative shift) with increased chloride concentration. This dependence is far more apparent at microelectrodes (12.5 & 25 μm diameter), where an increase in chloride concentration from 10 μM to 100 μM shifts the breakdown potential by 80 mV more negative. At macroscopic electrodes (0.5 cm diameter), the breakdown potential shifts by only 10-20 mV. This observation and its interplay with transport and the polymer-chloride adlayer is currently being further studied in more detail. Similar experiments varying the supporting electrolyte concentration (sulfuric acid in this case) show little dependence on the breakdown potential and overall shape in the cyclic voltammetry hysteresis, validating that microelectrodes indeed have negligible solution IR contributions. In addition to individual microelectrode voltammetry, microelectrode arrays are used to explore the effect of electrode size on the active-passive bifurcation and subsequent pattern formation. References T. P. Moffat, J. E. Bonevich, W. H. Huber, A. Stanishevsky, D. R. Kelly, G. R. Stafford, and D. Josell, “Superconformal Electrodeposition of Copper in 500-90 nm Features”, J. Electrochem. Soc., 147 (2000) 4524.T. P. Moffat and D. Josell, “Extreme Bottom-Up Superfilling of Through-Silicon-Vias by Damascene Processing: Suppressor Disruption, Positive Feedback and Turing Patterns”, J. Electrochem. Soc., 159 (2012) D208.D. Josell and T. P. Moffat, “Bottom-Up Nickel Deposition in High Aspect Ratio Through Silicon Vias”, J. Electrochem. Soc., 163 (2016) D322.D. Josell, M. Silva, and T. P. Moffat, “Superconformal Bottom-Up Cobalt Deposition in High Aspect Ratio Through Silicon Vias”, J. Electrochem. Soc., 163 (2016) D809.D. Josell and T. P. Moffat, “Superconformal Bottom-Up Gold Deposition in High Aspect Ratio Through Silicon Vias”, J. Electrochem. Soc., 164 (2017) D327.S. Bozdech, K. Krischer, D. A. Crespo-Yapur, E. Savinova, and A. Bonnefont, “1/f2 noise in bistable electrocatalytic reactions on mesoscale electrodes”, Faraday Discuss., 193 (2016) 187.

Microelectrode voltammetry of multi-electron transfers complicated by coupled chemical equilibria: a general theory for the extended square scheme.

A very general and simple theoretical solution is presented for the current-potential-time response of reversible multi-electron transfer processes complicated by homogeneous chemical equilibria (the so-called extended square scheme). The expressions presented here are applicable regardless of the number of electrons transferred and coupled chemical processes, and they are particularized for a wide variety of microelectrode geometries. The voltammetric response of very different systems presenting multi-electron transfers is considered for the most widely-used techniques (namely, cyclic voltammetry, square wave voltammetry, differential pulse voltammetry and steady state voltammetry), studying the influence of the microelectrode geometry and the number and thermodynamics of the (electro)chemical steps. Most appropriate techniques and procedures for the determination of the 'interaction' between successive transfers are discussed. Special attention is paid to those situations where homogeneous chemical processes, such as protonation, complexation or ion association, affect the electrochemical behaviour of the system by different stabilization of the oxidation states.

(Keynote) Redox Active Polymers: A Size Selective Solution for Nonaqueous Redox Flow Batteries

Non-aqueous redox flow batteries (NRFBs) are a potentially viable alternative to their aqueous counterparts as they offer a wider range of redox active species and electrolytes available for their design.[1] For instance, the larger electrochemical windows of organic solvents can be used to search for redox molecules with higher energy densities. However, NRFBs suffer from the general lack of suitable membranes. The low ionic conductivity observed when non-dedicated ion-exchange membranes are used in organic solvents has slowed down their wide-scale implementation. We have hypothesized that separating the redox active species in the electrolyte compartments by size-exclusion while allowing free flow of the supporting electrolyte presents a potentially powerful alternative to the use of poorly-performing ionexchange membranes. In our case, this approach is made possible by the careful design and evaluation of soluble Redox Active Polymers (RAPs), together with porous commercial off-the-shelf separators, as exemplified by the strategy depicted in the Figure below. RAPs consist of a polymeric unconjugated backbone that is decorated with high energy density and highly stable redox active pendants that display facile electron transfer. In addition to an attractive reactivity, RAPs must have solubilities in the M range and display low cross-over characteristics across porous separators. We will present our advances towards implementing such strategy using RAPs with different chemical functionalities, for both catholytes and anolytes with the aim of creating an “ALL RAP” non-aqueous redox flow battery. Our first studied system, based on viologen RAPs with molecular weight between 21 and 318 kDa exhibited size dependent transport across the porous separator.[2] This was achieved with RAPs that displayed a high solubility of up to 2.7 M, reversible electron transfer, and 94-99% charge/discharge efficiency. In the study of RAPs we have introduced new measurement techniques, including micro- and nano-electrodes for the steady state characterization of this species. Microelectrode voltammetry allowed to prove facile electron transfer even in highly concentrated solutions (+1.0 M). Preliminary testing using a stirred cell showed the possibility of integrating RAPs and porous separators into a working device that achieved stable charge/discharge and only 2% crossover over several hours. We are working towards the development of guidelines that aid our understanding of RAP electrochemistry.The application of new electrochemical and ionic imaging methods based on scanning electrochemical microscopy using micro- and nano- electrodes developed in our group [3] has allowed us to better understand the careful balance between RAP size and ion and electron mobility. We will discuss how these guidelines have significantly aided our molecular design and how they impact the performance of this new type of flow batteries. Acknowledgement. Work supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. [1] R.M. Darling, K.G. Gallagher, J.A. Kowalski, S. Ha, F.R. Brushett. Pathways to Low-Cost Electrochemical Energy Storage: a Comparison of Aqueous and Nonaqueous Flow Batteries. Energy Environ. Sci. 7 (2014) 3459.[2] N. Gavvalapalli, J. Hui, K. Cheng, T. Lichtenstein, M. Shen, J.S. Moore and J. Rodríguez-López. J. Impact of Redox Active Polymer Molecular Weight on the Electrochemical Properties and Transport Across Porous Separators in Non-Aqueous Solvents. J. Am. Chem. Soc. 136 (2014) 16309.[3] Z. J.Barton, and J. Rodríguez-López. Lithium Ion Quantification Using Mercury Amalgams as In Situ Electrochemical Probes in Nonaqueous Media. Anal. Chem. 86 (2014) 10660. Figure 1

Microelectrode Voltammetry of Dioxygen Reduction in a Phosphonium Cation-Based Room-Temperature Ionic Liquid: Quantitative Studies

Microelectrode voltammetry is used to study the electrochemical reduction of dioxygen, O2, in the room-temperature ionic liquid trihexyl(tetradecyl)phosphonium trifluorotris(pentafluoroethyl)phosphate [P6,6,6,14][FAP]. The nature of the unusual voltammetric waves is quantitatively modeled via digital simulation with the aim of clarifying apparent inconsistencies in the literature. The reduction is shown to proceed via a two-electron reaction and involve the likely capture of a proton from the solvent system. The oxidative voltammetric signals seen at fast scan rates are interpreted as resulting from the reoxidation of HO2•–. In the presence of large amounts of dissolved carbon dioxide the reductive currents decrease by a factor of ca. two, consistent with the trapping of the superoxide radical, O2•–, intermediate in the two-electron reduction process.

Effect of pH and dye concentration on the n-octanol/water distribution ratio of phenothiazine dyes: a microelectrode voltammetry study

Microelectrode voltammetry was employed to gain information on the n-octanol/water partition ratio of phenothiazine dyes. The data were obtained as a function of pH and dye concentration in aqueous solutions. All the dyes present hydrophobic behavior at pH above 7.0. The analysis of the results shows not only the formation of neutral species in alkaline solution but the association of cationic form of these dyes with OH− play important roles in the partitioning of molecules from aqueous solution to n-octanol.

A Simple Approach to Calculate the Micelle Aggregation Numbers of Ionic Liquid-Based Surfactants: Electrochemical Behavior of Aggregate-Solubilized Ferrocene Studied by Microelectrode Voltammetry

Micellar diffusion coefficient (DMDC), micellar hydrodynamic radius (RH) and average aggregation numbers (Nagg) of 1-alquil-3- methylimidazolium chloride (CxMeImCl x = 12, 14, 16) surfactants in aqueous solution have been determined by using voltammetric information obtained with disk microelectrodes. Ferrocene was used as redox-active electrochemical probe. DMDC values determined by extrapolation of the plot of diffusion coefficient versus surfactant concentration to the critical micelle concentration were found to be in good agreement with the literature values, based on static light scattering and theoretical calculations.