The strong catalytic effect of Pb(II) on the oxygen reduction reaction on 5 nm gold nanoparticles.

Citrate-capped gold nanoparticles (AuNPs) of 5 nm in diameter are synthesized via wet chemistry and deposited on a glassy carbon electrode through electrophoresis. The kinetics of the oxygen reduction reaction (ORR) on the modified electrode is determined quantitatively in oxygen-saturated 0.5 M sulphuric acid solution by modelling the cathode as an array of interactive nanoelectrodes. Quantitative analysis of the cyclic voltammetry shows that no apparent ORR electrocatalysis takes place, the kinetics on AuNPs being effectively the same as on bulk gold. Contrasting with the above, a strong ORR catalysis is found when Pb(2+) is added to the oxygen saturated solution or when the modified electrode is cycled in lead alkaline solution such that lead dioxide is repeatedly electrodeposited and stripped off on the nanoparticles. In both cases, the underpotential deposition of lead on the gold nanoparticles is found to be related to the catalysis.

A kinetic study of oxygen reduction reaction and characterization on electrodeposited gold nanoparticles of diameter between 17 nm and 40 nm in 0.5 M sulfuric acid.

Kinetic and mechanistic studies of the oxygen reduction reaction (ORR) in oxygen saturated 0.5 M sulfuric acid at 298 K at a gold macroelectrode and at an electrodeposited gold nanoparticle-modified glassy carbon electrode are reported. The conditions of electrodeposition are optimized to obtain small nanoparticles of diameter from 17 nm to 40 nm. The mechanism and kinetics of ORR on the gold macroelectrode are investigated and compared with those obtained for nanoparticle-modified electrodes. The mechanism for this system includes two electron and two proton transfers and hydrogen peroxide as the final product. The first electron transfer step corresponding to the reduction of O2 to O2(-)˙ is defined as the rate determining step. No significant changes are found for the nanoparticles here employed: electron transfer rate constant (k0) is k0,bulk = 0.30 cm s(-1) on the bulk material and k0,nano = 0.21 cm s(-1) on nanoparticles; transfer coefficient (α) changes from αbulk = 0.45 on macro-scale to αnano = 0.37 at the nano-scale.

Nanomaterial modified electrodes: evaluating oxygen reduction catalysts.

Intense current research is directed at the evaluation of nanomaterials as catalysts for the oxygen reduction reaction. This is commonly undertaken by means of voltammetric measurements supported on an electrode surface presumed inert other than for providing electrical contact. At their basis these factors usually involve measurement of a current or current density, measured at a fixed potential. However we now report that the current/current density at a fixed potential can vary with the surface coverage of the nanoparticles in the catalyst, without any change in fundamental kinetic or thermodynamic parameters, even though the voltammetric signal shows that the reduction is fully transport controlled. This finding leads us to the conclusion that caution should be expressed when comparing catalysts in this way. In particular the essential need is emphasised for characterising the coverage, porosity and particle size, when inferring inherent electrochemical activity and using a suitable physical model to extract catalytic parameters.

Fabrication of disposable gold macrodisc and platinum microband electrodes for use in room-temperature ionic liquids.

We report a simple and facile methodology for constructing gold macrodisc and platinum microband electrodes for use in room temperature ionic liquids (RTILs). To validate the use of gold macrodisc electrodes, the voltammetry of Ru(NH3)6(3+) was studied in 0.1 M aqueous KCl. The Randles-Sevcík equation was used to calculate the diffusion coefficient, giving excellent agreement with literature values, suggesting that the gold macrodisc electrode is capable of performing quantitative electroanalysis in aqueous media. Gold macrodisc electrodes were used to study oxidation of ferrocene in N-butyl-N-methylpyrrolidinium bis(fluoromethylsulfonyl)imide ([C4mpyrr][NTf2]) using cyclic voltammetry. The diffusion coefficient of ferrocene, (2.43 ± 0.07) × 10(-11) m(2) s(-1), was obtained. This value is very close to the literature value, indicating good performance of gold electrodes in RTILs. Platinum microband electrodes were tested in 1-propyl-3-methylimidazolium bis-trifluoromethylsulfonylimide ([Pmim][NTf2]) containing decamethylferrocene. Diffusion coefficients and electron transfer rates were obtained by fitting relevant simulations to the experimental data. For comparison, analogous experiments and analyses were performed on a commercial platinum microdisc, where the results obtained from both microdisc and microband agree well, further suggesting that the platinum microband electrode is suitable to be used in RTILs. Finally, gold macrodisc and platinum microband electrodes were used for oxygen detection. Gold macrodisc electrodes were used to find the peak currents of oxygen at each volume percentage analysed. Platinum microband electrodes showed steady-state currents of different volumes of oxygen. These two results are compared which resulted in excellent agreement. This is further confirmed by studying Henry's law constants obtained from both electrodes. The excellent behaviour of these two fabricated electrodes suggests that they are suitable for quantitative measurements and practicable for real world applications.

Changed reactivity of the 1-bromo-4-nitrobenzene radical anion in a room temperature ionic liquid.

Radical anions of 1-bromo-4-nitrobenzene (p-BrC6H4NO2) are shown to be reactive in the room temperature ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, ([C4mPyrr][NTf2]), by means of voltammetric measurements. In particular, they are shown to react via a DISP type mechanism such that the electrolysis of p-BrC6H4NO2 occurs consuming between one and two electrons per reactant molecule, leading to the formation of the nitrobenzene radical anion and bromide ions. This behaviour is a stark contrast to that in conventional non-aqueous solvents such as acetonitrile, dimethyl sulfoxide or N,N-dimethylformamide, which suggests that the ionic solvent promotes the reactivity of the radical anion, probably via stabilisation of the charged products.

A joint experimental and computational search for authentic nano-electrocatalytic effects: electrooxidation of nitrite and L-ascorbate on gold nanoparticle-modified glassy carbon electrodes.

The investigation of electrocatalytic nanoeffects is tackled via joint electrochemical measurements and computational simulations. The cyclic voltammetry of electrodes modified with metal nanoparticles is modeled considering the kinetics of the electrochemical process on the bulk materials of the different regions of the electrode, that is, the substrate (glassy carbon) and the nanoparticles (gold). Comparison of experimental and theoretical results enables the detection of changes in the electrode kinetics at the nanoscale due to structural and/or electronic effects. This approach is applied to the experimental assessment of electrocatalytic effects by gold nanoparticles (Au NPs) in the electrooxidation of nitrite and L-ascorbate. Glassy carbon electrode is modified with Au NPs via seed-mediated growth method. Divergence between the kinetics of these processes on gold macroelectrodes and gold nanoparticles is examined. Whereas claimed catalytic effects are not observed in the electrooxidation of nitrite, electrocatalytic nanoeffects are verified in the case of L-ascorbate. This is probably due to that the electron transfer process follows an adsorptive mechanism. The combination of simulation with experiments is commended as a general strategy of authentification, or not, of nanoelectrocatalytic effects.

The theory of cyclic voltammetry of electrochemically heterogeneous surfaces: comparison of different models for surface geometry and applications to highly ordered pyrolytic graphite.

The cyclic voltammetry at electrodes composed of multiple electroactive materials, where zones of one highly active material are distributed over a substrate of a second, less active material, is investigated by simulation. The two materials are assumed to differ in terms of their electrochemical rate constants towards any given redox couple. For a one-electron oxidation or reduction, the effect on voltammetry of the size and relative surface coverages of the zones as well as the rate constant of the slower zone are considered for systems where it is much slower than the rate constant of the faster zones. The occurrence of split peak cyclic voltammetry where two peaks are observed in the forward sweep, is studied in terms of the diffusional effects present in the system. A number of surface geometries are compared: specifically the more active zones are modelled as long, thin bands, as steps in the surface, as discs, and as rings (similar to a partially blocked electrode). Similar voltammetry for the band, step and ring models is seen but the disc geometry shows significant differences. Finally, the simulation technique is applied to the modelling of highly-ordered pyrolytic graphite (HOPG) surface and experimental conditions under which it may be possible to observe split peak voltammetry are predicted.

Dynamic theory of membrane potentials.

An accurate understanding of the dynamics of membrane potential formation underpins modern electrophysiology and much of cell biochemistry. Computer simulations using a Nernst-Planck-Poisson (NPP) finite difference method are used to model the dynamic evolution of a series of membrane systems in which two reservoirs of electrolyte solution are separated by a thin membrane which is impermeable to selected species. Two specific examples are considered in detail. The first ("type 1") is the case in which the solutions are monophasic but of unequal concentration, and the second ("type 2") is the case in which the solutions are of equal concentrations but different phase, with a common impermeant ion (a bi-ionic membrane). The validity of the Goldman equation for membrane potential, as applied to each case, is investigated. The type 1 case is shown to reach a steady state, and strong agreement with the Donnan equation for potential difference is observed. For the type 2 case, it is shown that the potential difference consists of two separable components: a localized, Donnan-type potential that reaches a pseudosteady state and a dynamically expanding diffuse component, with properties similar to those of a liquid junction potential, that does not reach a steady state but rather discharges at constant potential difference. This is contrary to the classical interpretation of a static diffuse layer, due to Planck, Henderson, and Goldman.

Dynamic theory of type 3 liquid junction potentials: formation of multilayer liquid junctions.

A Nernst-Planck-Poisson finite difference simulation is used to model the dynamic evolution of a series of liquid junctions of the type A(+)X(-)|B(+)Y(-), in which all ionic species are monovalent and present in equal concentration (a subset of Lingane's type 3), from a nonequilibrium initial condition to a condition of steady-state potential difference. Simulations are performed in a linear space without constrained diffusion. Analysis of the dynamics shows very good agreement with recently presented revisions for the type 1 and 2 cases [ J. Phys. Chem. B 2010 , 114 , 187 - 197 ] Considerable deviation of the value of the limiting liquid junction potential from that predicted by the classical Henderson equation [ Z. Phys. Chem. 1907 , 59 , 118 - 127 ] is shown in many cases and investigated as a function of the size of the various diffusion coefficients. Significantly, the formation of a "multilayer liquid junction", characterized by the existence of more than one instantaneous point of electroneutrality and thus more than one stationary point in the electric field (in a finite range of space), is inferred for the first time in a number of cases. The conditions for such a multilayer liquid junction are determined.