Microelectrode Voltammetry Research Articles

Study on the Diffusion of Electroactive Molecules in Polymer Electrolyte by Microelectrode Voltammetry

Study on the Diffusion of Electroactive Molecules in Polymer Electrolyte by Microelectrode Voltammetry

Redox Polyether Hybrid Copper Bipyridine Complex Molten Salts.

The covalent attachment of poly(ethylene oxide) chains to metal bipyridine complexes produces viscous molecular melts with sufficient ionic conductivity to permit microelectrode voltammetry. This paper describes the use of this chemistry to prepare a new copper complex with "tailed" bipyridine ligands, [Cu(bpy(CO(2)MePEG-350)(2))(2)](ClO(4))(2), where MePEG-350 is methyl poly(ethylene glycol) with molecular weight 350 g mol(-)(1). The coupling of physical diffusion with electron hopping during voltammetry allows measurement of the electron self-exchange rate constant, k(EX), for the Cu(II/I) couple. Activation studies indicate the reaction to be nearly adiabatic, with an activation barrier larger than expected for purely inner-sphere rearrangements. This study also examines the depression of both electron transport and ionic conductivity caused by the addition of 1.4 M LiClO(4) to the undiluted room-temperature melt.

Voltammetry and Conductivity of a Polyether‐Pyridinium Room Temperature Molten Salt Electrolyte and of Its Polymer Electrolyte Solutions in Polydimethylsiloxane

This report describes the synthesis, microelectrode voltammetry, and ionic conductivity of a new room temperature molten salt N‐(methoxy(ethoxy)2ethyl)pyridinium p‐toluenesulfonate (abbreviated as ) and of its solution in a hydroxy‐terminated polydimethylsiloxane. Both ionically conductive liquids exhibit voltammetric potential windows of about 1.5 V. The negative potential limit is determined by the reduction of the pyridinium species, with subsequent radical coupling to form a voltammetrically observed viologen dimer. The estimated diffusivities of the species, of a diethyleneglycol‐tailed ferrocene redox solute studied, and by application of Nernst‐Einstein relation to the ionic charge carriers, all lie in the to range. Viscosities and glass transition thermal observations are reported as is the fit of the temperature dependencies of ionic conductivity in and in mixtures to Vogel‐Tamman‐Fulcher predictions.

Microelectrode Voltammetry and Electron Transport in an Undiluted Room Temperature Melt of an Oligo(ethylene glycol)-Tailed Viologen

This paper describes the synthesis, characterization, and voltammetry of a oligo(ethylene glycol) trimer derivative of 4,4‘-bipyridine, [N,N‘-(CH3(OCH2CH2)3)2-4,4‘-bipyridine][BF4]2 [V[E3M]2](BF4)2]. This “tailed” viologen is a room temperature molten salt. Its fast atom bombardment mass spectrum exhibits interesting aspects of cationation. Microelectrode voltammetry in the undiluted viologen melt shows the expected pair of reduction waves, but currents for the wave for the [V[E3M]2]2+/1+ are smaller than those for the [V[E3M]2]1+/0 couple. Analysis of the voltammetric behavior leads to the conclusion that the difference in currents arises from coupling of [V[E3M]2]1+/0 homogeneous electron exchanges to the physical diffusion of viologen species in the mixed-valent diffusion layer at the electrode interface. The physical diffusivity of the [V[E3M]2]2+ species, DPHYS, is much smaller than electron diffusivity through electron self-exchanges of the [V[E3M]2]1+/0 couple. kEX,1/0 is ∼106 M-1 s-1, or at least ...

Polyimide-polyether mixed conductors as switchable materials for electrochromic devices

A new family of redox-active polymers have been obtained by polycondensation of α,ω-diamino oligopolyethers with various aromatic tetracarboxylic acid anhydrides. The polyether blocks retain the usual cation co-ordination ability leading to solid-state ionic conduction while the relatively large electron affinity of the bis-imide moieties formed in the block polymers allows the reversible formation of stable radical anions (• − and •• 2−) in the range 2–2.6 V vs. Li°. Either slow-scan or microelectrode voltammetry indicate that the totality of the redox centres in such materials are readily accessible for all polyether spacer lengths tested (6–22 ether fragments), suggesting both a fast electron-exchange mechanism between anion radicals and a tendency for self assembly (stacking) of the planar aromatic groups. When increasing the number of atoms in the aromatic ring, the peak separation became smaller (benzene > naphthalene > perylene), resulting ultimately in simultaneous 2-electrons injection. These materials, due to the high coloration efficiency of the reduced state and their ready processability in thin films are excellent candidate for the realisation of laminated solid-state electrochromic devices.

Synthesis and voltammetry of a tetrathiafulvalene polymer electrolyte melt

The synthesis and electrochemical properties are described for a tetrathiafulvalene derivative with −S(CH2CH2O)3CH3 oligomers attached at each of its four corners (TTF[E3M]4). This material is a molecular melt at room temperature and will dissolve LiClO4 as a polymer electrolyte. Its radical cation salt, {TTF[E3M]}4+{(CF3SO−3)}, is also a room temperature melt in which LiClO4 is soluble. Microelectrode voltammetry in LiClO4 +TTF[E3M]4and in {TTF[E3M]4+}{CF3SO−3} shows that the self-diffusivities of these TTF derivatives in their melts are quite small and have room temperature values ranging from 8 × 10−9 to 5 × 10−12 cm2 s−1 depending on the LiClO4 content. The experimental diffusion coefficients derived from voltammetric waves for the TTF[E3M]0/+4 couple are larger than those for the TTF[E3M]+/2+4 couple by factors ranging from 2.5 to 13, a difference not understandable by migration effects alone. Consideration of coupling of homogeneous TTF[E3M]0/+4 electron self-exchange to diffusion during TTF[E3M]+4 reduction or TTF[E3M]4 oxidation leads to estimation of kEX,10 values in the 105 M−1 s−1 range and which increase with temperature and decrease with the addition of LiClO4 electrolyte.

Microelectrode voltammetry for studying host-guest complexation equilibria. An analysis of the possibilities of the method

The use of microelectrode voltammetry in the study of host-guest complexation is described. A digital simulation approach to describe the complexation as a second-order CE mechanism for any kinetic condition is proposed. For a completely mobile equilibrium, the constant can be determined by the very simple relationship ( I 1/ I 1,0) = (1 + D r K[L] b)/(1 + K[L] b), where I 1/ I 1,0 is the ratio of the limiting current in the presence and absence of the host species, D r is the ratio of the diffusion coefficients of the complex and the guest species, K is the equilibrium constant and [L] b is the bulk equilibrium concentration of the host species. Advantage is gained from the linear dependence of the limiting current on the diffusion coefficient of the reactant which allows an enlargement of the measurement range. The potential applicability of the method to electroinactive guest species was also explored by using it in a competitive procedure. Agreement was found between the theoretical results and those relative to the ferrocenecarboxylic acid + β-cyclodextrin and ferrocenecarboxylic acid + p-nitrophenol + β-cyclodextrin systems in aqueous solution at pH 9.2.

Methanol bonding to and electron-transfer reactivity of chloro(tetraphenylporphinato)manganese(III)

The ability of axial ligation to modulate redox potentials and electrode half-reactions of metalloporphyrins is illustrated by coordination of methanol to Mn(TPP)Cl (1) and its reduced and oxidized products in 1,2-dichloroethane. Visible spectrophotometry, spectroelectrochemistry, and cyclic and microelectrode voltammetry are used to characterize reactions whereby six-coordinate Mn(III) (Mn(TPP)(CH{sub 3}OH)Cl (3) and Mn(TPP)(CH{sub 3}OH){sub 2}{sup +} (4)) and five-coordinate Mn(II) (Mn(TPP)(CH{sub 3}OH)(6)) species are formed from 1. The axial ligand preferences of these two oxidation states lead to a biphasic behavior in the potential of Mn{sup III/II} reduction. At low CH{sub 3}OH concentrations, five-coordinate 1 is reduced to five-coordinate 6, causing E{sub 1/2} to shift to positive values with increasing (CH{sub 3}OH); at larger methanol concentrations, reduction of 4 to 6 predominates and the shift is in the opposite direction. In addition, the ring- and metal-centered oxidations of 1 at 1.22 and 1.56 V vs Ag/AgCl, respectively, coalesce into a single two-electron process at 1.31 V upon CH{sub 3}OH coordination. The negative shift in the metal-centered oxidation potential is an anticipated consequence of the greater donor strength of two CH{sub 3}OH ligands versus one Cl{sup {minus}} ligand. The positive shift in the ring-centered potential is believed to result from reduced deformationmore » of the porphyrin core and consequent stabilization of the porphyrin HOMO as axial coordination by CH{sub 3}OH returns the Mn atoms to an in-plate position from its significant out-of-plane displacement (0.27 {angstrom}) in 1.« less

The evaluation of rate constants for rapid electrode reactions using microelectrode voltammetry: virtues of measurements at lower temperatures

The prospects of utilizing electrochemical kinetic measurements at lower temperatures for evaluating reliably the standard rate constants k s of rapid redox couples in non-aqueous media, specifically using microelectrode voltammetry, have been examined by means of digital simulations together with experiment. Cyclic voltammetric measurements were made between 200 and 300 K at a gold microdisk, for ferrocenium-ferrocene (Fc +/0), o-nitrotoluene 0/− and nitromesitylene 0/− redox couples in acetone, propionitrile and butyronitrile, at a gold microdisk with scan rates from 100 to 10 4 V s −1. The latter two couples were selected as examples of systems that exhibit rapid but yet still unambiguously measurable k s values at ambient temperatures; the more facile Fc +/0 system has been suspected to yield immeasurably fast electrode kinetics using conventional techniques under these conditions. Sample simulated voltammograms were generated between 200 and 300 K for a sequence of activation enthalpies Δ H ex ‡ for electron exchange, accounting also for known temperature-dependent diffusion coefficients, solution resistance and double-layer capacitance. As long as Δ H ex ‡ > 10 kJ mol −1, the use of diminished temperatures in such solvents is predicted to facilitate the reliable evaluation of k s. This expectation is in harmony with experiment: k s for Fc +/0 is apparently evaluated readily at lower temperatures, yielding Δ H ex ‡ = 20 kJ mol −1. The temperature-dependent electrode kinetic data for Fc +/0 are compared briefly with some expectations of contemporary theory.

Electrochemical generation of 17-electron cation radicals from areneM(CO) 3 (M  Cr, Mo, W) and thiopheneCr(CO) 3 complexes in MeCN: Conventional cyclic voltammetric studies with digital simulation and microelectrode voltammetry in the absence of supporting electrolyte

One-electron electrochemical oxidation of areneM(CO) 3 (M  Cr, Mo, W) and thiopheneM(CO) 3 complexes in acetonitrile produces 17-electron cations (MeCN) n M(CO) 6— n . Thus oxidation of LCr(CO) 3, (L  C 6H 6, PhCl, PhNMe 2 thiophene, 3-methylthiophene) in acetonitrile leads to a rapid follow-up reaction in which the intermediate (MeCN) 3Cr(CO) 3 + is detected by cyclic voltammetry. Oxidations in MeCN are not affected by redox catalysis, confirming a simple substitution mechanism. Increasing the concentration of the complex has no effect on the yield of MeCN substituted product, again ruling out bimolecular decomposition pathways. The reaction is very rapid and quite distinct from photochemical or thermal substitution of these complexes. Microelectrode steady-state voltammetry shows that all the arene complexes undergo two successive one-electron transfers in MeCN. Reproducible microelectrode voltammograms could only be obtained in MeCN in the absence of supporting electrolyte; addition of supporting electrolyte rapidly diminished the electrode response owing to electrode coating.

Steady-state microelectrode voltammetry as a route to homogeneous kinetics

It is demonstrated that, under propitious circumstances, information concerning the kinetics of homogeneous reactions can be gained by studying how features of steady-state voltammograms depend on the size of the voltammetric microelectrode. Applications to CE, EC, ECE and EC' mechanisms are discussed.

Evaluation of a theory for the sensitivity and selectivity of a polymer diffusion-plasticization based solid-state electrochemical gas chromatography detector

The detector is based on the microelectrode voltammetry of an electroactive solute dissolved in a thin film of polymer electrolyte exposed to the column effluent gas

Microelectrode voltammetry of TCNQ in aprotic solvent at low concentrations of non-reducing and reducing salts

In aprotic solvent containing electrolyte, tetracyanoquinonedimethane (TCNQ) and its radical anion (TCNQ XXX) give well separated reversible reductions by cyclic voltammetry at conventional sized electrodes. However, without purposely added electrolyte in solvents such as tetrahydrofuran/acetonitrile (2:1) the reduction wave of TCNQ at microelectrodes was accompanied by only a barely detectable second wave. Addition of non-reducing salts such as tetrabutylammonium tetrafluoroborate (TBABF 4) increased the limiting current of the second wave. Results suggest an ECE pathway in which TCNQ is reduced to TCNQ XXX. Results are interpreted by assuming formation of an ion pair with the salt cation, e.g. (TGNQ XXX)(TBA +), which is reduced in the second wave. TBAI reduced TCNQ to TCNQ − in aprotic solvents in an equilibrium chemical redox step. Steady state voltammograms in these systems feature an anodic wave at −0.1 V for the catalytic oxidation of iodide, and a cathodic wave at about −0.8 V probably involving reduction of TCNQ XXX and I 2. Results showed that reduction of TCNQ to its anion radical by iodide, as well as ion pair formation involving TCNQ XXX, proceeds at rapid rates at room temperature and below.

Electrochemical cell design and experimental procedures for measuring electrode reaction kinetics at low and variable temperatures

AbstractAn electrochemical cell is described for measuring electrode reaction kinetics at low and variable temperatures in nonaqueous solvents. The cell contains a working electrode of conventional size (at which the standard heterogeneous electron transfer rate constant is determined by cyclic voltammetry) and a microelectrode (at which the diffusion coefficient is measured by steady‐state microelectrode voltammetry). Uncompensated solution resistance is minimized by a combination of close placement of the reference electrode salt bridge tip and the working electrode surface and instrumental positive feedback. Cell performance is evaluated by an examination of ferrocene oxidation and manganese(III) (tetraphenylporphyrin) chloride reduction at temperatures between 229 and 298 K. Comparison of the responses from these reversible and quasi‐reversible redox couples in the same solution demonstrates that the effects of uncompensated solution resistance are absent from the experimental measurements and that increases in the cyclic voltammetric peak potential separation with sweep rate are due entirely to the kinetics of the Mn(III)/Mn(II) electrode reaction.

Data-aquisition system for cyclic voltammetry at a microelectrode and application to fast electrode kinetics

A data-aquisition system for fast micro-electrode voltammetry is described. The computer-controlled system can be used at scan rates up to 10 000 V s −1. The response of the system was examined by monitoring fast electrochemical reactions. A heterogeneous rate constant of 1.1 cm s −1 was obtained for oxidation of ferrocene in acetonitrile. Oxidation of anthracene in acetonitrile was also studied; the enthalpy of activation was evaluated as 13 kJ mol −1 for the reaction of the anthracene radical cation with acetonitrile.

Theory of microelectrode voltammetry with little electrolyte

General theoretical relationships are derived that apply to steady-state voltammetry at hemispherical microelectrodes for any electrode reaction and any solution composition. These relationships are then applied to two typical systems: the oxidation of ferrocene and the reduction of a quinone. The effect of electrolyte concentration on these two systems is examined in detail. This study demonstrates why, in certain circumstances, small amounts of electrolyte behave as “excess supporting electrolyte.”