Vacuum-annealed undoped polycrystalline CVD diamond: a new electrode material

Vacuum-annealing imparts conductivity to initially insulating undoped polycrystalline chemical-vapor-deposited diamond, thus turning it to a possible electrode material. The diamond film annealed at 1775 K appeared to be practically not conducting. With further increase in the annealing temperature above 1825 K, the film effective resistivity decreased from initial value of 10¹¹ to 10¹² Ω cm down to less than 0.1 Ω cm; the differential capacitance increased from ∼10⁻³ to ∼50 μF per 1 cm² of geometrical surface; the transfer coefficients for electrochemical reactions in the [Fe(CN)₆]^3−/4− redox solution increased from ∼0.2 to 0.5; and the degree of reversibility of the electrochemical reaction increased. The observed changes in the electrode properties are attributed to gradual change in the thickness and/or properties (first and foremost, conductivity) of the nondiamond carbon phase formed along the intercrystallite boundaries upon the annealing; the conducting phase is outcropping at the film surface as an array of microelectrodes (“active sites”).     

Control of the microparticle position in the channel based on dielectrophoresis

We report here the control of the microparticles position within fluid flow based on its size by using dielectrophoresis (DEP) with a microelectrode array consisted of rectangular features with the different size of width and gap. 3 μm- and 10 μm-diameter particles were introduced into the channel with 300 μm height at 30 μl/min. An AC electric field (20 V peak–peak and 2 MHz) was then applied to microelectrode arrays to form dielectrophoretic fluid cage, resulting in a formation of flow paths with low electric fields on the arrays. The microparticles separately flow in line streams along the paths formed between the rectangular features of the arrays, the 3 μm-diameter particles mainly flow through the narrow path and 10 μm-diameter particles through the wide path. These results indicated that positions of two types of microparticles in the fluidic channel were easily separated and controlled using the n-DEP.     

ELECTROOSMOTIC HELICAL FLOW PRODUCED BY COMBINED USE OF LONGITUDINAL AND TRANSVERSAL ELECTRIC FIELDS IN A RECTANGULAR MICROCHANNEL

We theoretically devise and simulate a microelectrode system that produces electroosmotic helical flow in a straight rectangular microchannel. In addition to a pair of primary electrodes that generate a longitudinal electric field, sets of secondary electrodes are installed to produce a transversal electric field. The secondary electrode system consists of point electrodes embedded along two edges of the bottom surface of the channel. The transversal electric field developed across the bottom surface causes the electroosmotic motion of fluid at the bottom of the channel along the transversal direction. As a combined effect of the primary electrode system that produces unidirectional longitudinal flow along the channel and the secondary electrode system that produces transversal flow across the bottom surface, the flow inside a rectangular microchannel follows a helical pattern. After simulating the electroosmotic helical flows developed in microchannels we analyze the mixing of sample liquid in such flow fields by calculating the trajectories of fluid particles.     

Electrochemical and morphological characterization of gold nanoparticles deposited on boron-doped diamond electrode

A novel two-step method was employed to synthesize gold nanoparticles dispersed on boron-doped diamond (BDD) electrode. It consisted of sputter deposition at ambient temperature of maximum 15 equivalent monolayers of gold, followed by a heat treatment in air at 600 °C. Gold nanoparticles with an average diameter between 7 and 30 nm could be prepared by this method on polycrystalline BDD film electrode. The obtained Au/BDD composite electrode appeared stable under conditions of electrochemical characterization performed using ferri-/ferrocyanide and benzoquinone/hydroquinone redox couples in acidic medium. The electrochemical behavior of Au/BDD was compared to that of bulk Au and BDD electrodes. Finally, the Au/BDD composite electrode was regarded as an array of Au microelectrodes dispersed on BDD substrate.     

The electrochemical redox processes in methacrylate-based polymer electrolytes II. - Study on microelectrodes

The electrochemical behaviour of ferrocene was studied in different gel polymer electrolytes based on methyl, ethyl and 2-ethoxyethyl methacrylate and compared to the liquid aprotic solution (propylene carbonate). Voltammetric and chronoamperometric measurements on microelectrodes were conducted in order to describe the qualitative as well as quantitative behaviour of ferrocene in different conditions. Heterogeneous electron-transfer rate constants and diffusion coefficients of ferrocene in polymer electrolytes were estimated to be 1.1-7.8 × 10⁻³ cm s⁻¹ and 4-13 × 10⁻⁸ cm² s⁻¹ depending on the electrolyte composition. The influence of the polymer polarity, ferrocene concentration and level of polymer cross-linkage on the kinetics of ferrocene oxidation and its transport was discussed. The electrolytes with poly(2-ethoxyethyl methacrylate) exhibit the highest ionic conductivity (2-4 × 10⁻⁴ S cm⁻¹) as well as diffusion coefficient of ferrocene (1.3 × 10⁻⁷ cm² s⁻¹) in their structure.     

Numerical computation of the Faradaic impedance of inlaid microdisk electrodes using a finite element method with anisotropic mesh adaptation

The Faradaic impedance of a microdisk electrode inlaid in an insulating surface is revisited by numerical computation using a finite element method (FEM) with anisotropic mesh adaptation. New features of the numerical results, as compared to previous works, are analyzed. A first attractive feature is that the diffusion impedance relative to a microdisk electrode, evaluated at the equilibrium potential of the electrode, depends both on electron-transfer and mass-transport kinetics, in contrast with the usual behaviour of uniformly accessible electrodes. Next, the domain of validity of the Fleishmann and Pons semi-analytical formulation of diffusion impedance is determined. Finally, the characteristic of impedance graphs, which are the diffusion resistance, the characteristic frequency at the apex of the Nyquist diagram and the imaginary part of the diffusion impedance at this apex, are studied as functions of a dimensionless parameter that compares the standard rate constant of electron transfer to the microelectrode diffusion constant. Closed form approximations are proposed for all quantities in order to help the analysis of experimental data.     

Screen printed recessed microelectrode arrays

The construction and characterisation of screen printed recessed microelectrode arrays are reported. This screen printing approach results in the fabrication of shallow recessed microelectrode arrays which are characterised by microscopy and cyclic voltammetry. The analytical utility of the shallow recessed microelectrode arrays is demonstrated with the cathodic stripping of manganese (II) allowing nano-molar levels to be readily detected exhibiting a superior performance over conventional carbon based electrodes. The limitations of using screen printing technology to fabricate microelectrode arrays are also discussed.     

Comparison of electrochemical methods for triiodide diffusion coefficient measurements and observation of non-Stokesian diffusion behaviour in binary mixtures of two ionic liquids

Results of diffusion coefficient measurements of triiodide in a mixture of two ionic liquids (1-methyl-3-propylimidazolium iodide and 1-butyl-3-methylimidazolium tetrafluoroborate) at 25 °C are described in this paper. Four electrochemical methods for measuring diffusion coefficients of triiodide were evaluated for their reliability and performance, including impedance spectroscopy and polarization measurements at thin layer cells as well as cyclic voltammetry and chronoamperometry at microelectrodes of different radii. Viscosities of the blends were measured to investigate the transport behaviour of triiodide ions used in Grätzel-type dye-sensitized solar cells.     

ELECTROOSMOTIC HELICAL FLOW PRODUCED BY COMBINED USE OF LONGITUDINAL AND TRANSVERSAL ELECTRIC FIELDS IN A RECTANGULAR MICROCHANNEL

We theoretically devise and simulate a microelectrode system that produces electroosmotic helical flow in a straight rectangular microchannel. In addition to a pair of primary electrodes that generate a longitudinal electric field, sets of secondary electrodes are installed to produce a transversal electric field. The secondary electrode system consists of point electrodes embedded along two edges of the bottom surface of the channel. The transversal electric field developed across the bottom surface causes the electroosmotic motion of fluid at the bottom of the channel along the transversal direction. As a combined effect of the primary electrode system that produces unidirectional longitudinal flow along the channel and the secondary electrode system that produces transversal flow across the bottom surface, the flow inside a rectangular microchannel follows a helical pattern. After simulating the electroosmotic helical flows developed in microchannels we analyze the mixing of sample liquid in such flow fields by calculating the trajectories of fluid particles.     

Activation behaviour of the Ni/MH batteries electrodic material Ni(OH)2 by single particle microelectrode technique

The activation process of Ni(OH)₂ used as the positive electrode active material of Ni/MH batteries was studied by a single particle microelectrode method thanks to an improved apparatus. The images of the Ni(OH)₂ particle during the charge process were collected. The electrochemical properties of Ni(OH)₂ were studied by cyclic voltammetry and galvanostatic charge/discharge of a single particle. The charge efficiency (η) of the single particle was as high as 94%. The normalized output rate (NOR) was proposed as a parameter to evaluate the output performance of the electrode material. The NOR value varied with the electrode potential value. But the NOR value remained constant at fixed electrode potential value during the activation process. This implies that the activation process did not improve the reaction rate of the particle, although the capacity kept increasing during the activation process. The intrinsic nature of the activation of Ni(OH)₂ was deduced as the formation of dispersed Ni(III) in the active mass. The Ni(III) phase was formed during the charge process and some remained unreduced during the discharge process. The remaining Ni(III) resulted in a much higher electronic conductivity of Ni(OH)₂.