Carbon electrodes of nanometer size have been fabricated by electrochemical etching of carbon fibers followed by deposition of electrophoretic paint. These electrodes have been characterized with steady-state voltammetry using a range of redox probes. These redox probes were chosen because they show a range of formal potentials and kinetic rate constants (k0) and are charged to different extents. The voltammetric response of these electrodes in both the presence and absence of supporting electrolyte has been investigated. In the presence of supporting electrolyte, well-defined steady-state voltammograms of sigmoidal shape have been obtained on electrodes with effective radii as small as 1 nm. In the absence of supporting electrolyte, however, the voltammetric behavior varies with the redox system. Deviation from the expected migration-diffusion response is observed when the electrode is smaller than 20 nm for the reduction of the multicharged cationic species hexammineruthenium(III) (Ru(NH3)6 3+). Deviation from the ideal behavior of migration-coupled diffusion is exhibited at electrodes even of micrometer size during the reduction of the multicharged hexacyanoferrate-(III) anion (Fe(CN)6 3-), which has a median reduction potential and relatively low k0. Such an effect is also observed for the oxidation of the hexacyanoferrate(II) anion (Fe(CN)6 4-). In comparison, the hexachloroiridate-(IV) anion (IrCl6 2-), with a higher k0 and more positive reduction potential, shows deviations similar to those seen for hexaammineruthenium(III); i.e., no deviation from expected behavior is seen until the electrode size is less than ca. 20nm. It is argued that the dynamic diffuse double-layer effect rather than the Frumkin effect is the major source of the observed nonideal behavior. The results indicate that the dynamic diffuse doublelayer effect can function even when the electrode reaction is reversible or quasi-reversible and becomes more pronounced at very small electrodes. The nature of size effects on the voltammetric response at nanometer size electrodes are discussed.

The effect of the mass transport coefficient of reactant and product species during the oxygen reduction reaction, orr, on platinum in an acidic electrolyte has been experimentally examined and kinetically modeled. By using carbon electrodes having electroactive radii on the nanometer scale it is possible to produce single Pt particles having effective radii ranging from several micrometers to several tens of nanometers. As the mass transport coefficient is directly related to the size of these platinum particles, it is possible to examine the effect of mass transport on the orr in regions inaccessible to other experimental techniques. At the smallest of these Pt particles, mass transport coefficients equivalent to a rotating disk electrode at rotation rates (ö) of greater than 108 rpm are obtainable. Under low mass transport conditions equivalent to those obtainable using the normal rotating disk technique (i.e, ω<10000rpm), oxygen reduction is seen to proceed via a four-electron reduction to water as has been reported in the general literature. Under high mass transport conditions about 75% of reactant oxygen molecules are reduced to water with the balance being only reduced as far as hydrogen peroxide. The production of peroxide which this result implies may be an important aspect within the cathode catalyst layer of solid polymer electrolyte fuel cells, as these layers are inherently designed to provide high mass transport coefficients. The oxygen reduction reaction on single catalyst particles is modeled according to the parallel reaction mechanism originally introduced by Wroblowa et al. [Wroblowa, H. S.; Pan, Y. C.; Razumney, J. J. Electroanal. Chem. 1976, 69, 195]. A general expression is derived to predict the effects of the mass transport rate, surface blockage, and potential on the effective electron-transfer number, neff, which reflects the average number of electrons produced during the reduction of each dioxygen molecule. It is shown that a pure series (or indirect) reaction mechanism for the four-electron reduction of oxygen on Pt electrodes in sulfuric acid solution is consistent with the experimental results. The kinetics of the orr is analyzed using both Tafel plots and the half-wave potential method. The kinetic parameters extracted from the half-wave potential method are in very good agreement with those extrapolated from the Tafel curves with a -120 mV per decade slope. The complexities involved in the orr kinetics are discussed according to the results obtained on these small single-particle electrodes. Specifically, the effect of the double-layer structure and the role of anion adsorption are considered. It is argued that the electrocatalytic reduction of oxygen involves inner-sphere electron-transfer steps and that its kinetics are affected by the potential profile within the compact double layer, especially inside the inner Helmholtz plane (IHP). Anion adsorption may perturb the orr in a way much more complex than simply blocking the surface sites and may significantly change the potential near the IHP, thereby changing the effective driving potential that the reactant and reaction intermediates experience. This may be partly responsible for the variable apparent values of the transfer coefficient of the orr in different potential regions. A new mechanism for the size effects of catalyst nanoparticles on their electrocatalytic properties toward oxygen reduction is proposed in terms of the particle size tunable structure of the double layer.

The mechanism and kinetics of the hydrogen oxidation reaction (hor) has been investigated using carbonsupported single particles of Pt electrocatalyst with radii as small as 40nm. The high mass transport rates on such small particles enable us to investigate the rapid kinetics of the hor in the absence of diffusion limitations. Surface kinetic controlled polarization curves during the electrochemical oxidation of hydrogen molecules in acid solution have been obtained in the entire H UPD region, showing features obviously different from those obtained on normal micrometer electrodes or in RDE experiments. For instance, two current plateaus rather than one are seen during the steady-state polarization of the hor on electrodes made of small particles. Upon decreasing the size of the Pt particles, the two current plateaus show greater separation and become better defined. A theoretical model for the steady-state polarization of the hor has been developed in which UPD H atoms of various states are considered as the reactive intermediates and the Frumkin adsorption mode is assumed for the atomic H on Pt electrodes. It is shown that the first current plateau represents the limiting reaction rate under adsorption or combined adsorption-diffusion control while the second plateau current corresponds to the limiting diffusion-controlled reaction rate. It is pointed out that Tafel plots that have been frequently used for kinetics analysis in the hor are meaningless, especially in the potential region below 0.05V vs RHE. The polarization curves are fitted with a general polarization equation derived according to our model. The fitting shows that the hor on Pt proceeds most likely via the Tafel-Volmer reaction mechanism rather than the Heyrovsky-Volmer mechanism. These results have significant implications on the understanding and modeling of the reactions in solid polymer electrolyte fuel cells.

Results for the electrodeposition of platinum on carbon electrodes of nanometer size are presented. It is shown that electrodes with very small electroactive areas simplify the study of the nucleation and growth mechanism involved in electrodeposition. Reducing the electroactive area of the substrate easily controls the number of nucleation sites. With the use of substrates having electroactive radii of a few nanometers, it is possible to form only one single growth center and to allow that center to grow independently. The current transient associated with the growth of such a single nucleus provides both kinetic and mechanistic information about the electrodeposition process. A mathematical formula for the current transient under combined electrokinetic and mass-transport control based on the work of Fletcher [J. Cryst. Growth 1983, 62, 505] and Kruijt et al. [J. Electroanal. Chem. 1994, 371, 13] is used to fit the transients to extract the exchange current density and diffusion coefficient of the reactants. For the Pt on carbon deposition process at low overpotentials, for which the electron-transfer steps control the overall deposition process, single nucleation is observed when the electrode is smaller than about 5 nm in size. It is found that the single nucleation and growth processes can also occur at relatively large electrodes (100nm in size) when a high overpotential is applied so that a diffusion-controlled deposition process is established. Such a phenomenon is analyzed in terms of the depletion layer of electroactive species around the growing nucleus, and the effect that this has on the nucleation rate on the surrounding electrode surface.

A dynamic diffuse double-layer model is developed for describing the electrode/electrolyte interface bearing a redox reaction. It overcomes the dilemma of the traditional voltammetric theories based on the depletion layer and Frumkin's model for double-layer effects in predicating the voltammetric behavior of nanometersized electrodes. Starting from the Nernst-Planck equation, a dynamic interfacial concentration distribution is derived, which has a similar form to the Boltzmann distribution equation but contains the influence of current density. Incorporation of the dynamic concentration distribution into the Poisson and Butler-Volmer equations, respectively, produces a dynamic potential distribution equation containing the influence of current and a voltammetric equation containing the double-layer effects. Computation based on these two equations gives both the interfacial structure (potential and concentration profiles) and voltammetric behavior. The results show that the electrochemical interface at electrodes of nanometer scales is more like an electricdouble-layer, whereas the interface at electrodes larger than 100 nm can be treated as a concentration depletion layer. The double-layer nature of the electrode/electrolyte interface of nanometer scale causes the voltammetric responses to vary with electrode size, reactant charge, the value of formal redox potential, and the dielectric properties of the compact double-layer. These voltammetric features are novel in comparison to the traditional voltammetric theory based on the transport of redox molecules in the depletion layer.