Electrode surface, mass transport

Sonoelectrochemistry has also been used for the efficient employment of porous electrodes, such as carbon nanofiber-ceramic composites electrodes in the reduction of colloidal hydrous iron oxide [59], In this kind of systems, the electrode reactions proceed with slow rate or require several collisions between reactant and electrode surface. Mass transport to and into the porous electrode is enhanced and extremely fast at only modest ultrasound intensity. This same approach was checked in the hydrogen peroxide sonoelectrosynthesis using RVC three-dimensional electrodes [58]. 

OXftmik of solution) > (electrode surface) MASS TRANSPORT O felectrode surface) "t" riQ, - - (electrode surface) ELECTRON TRANSFER... 

Hydrodynamic voltammetry — is a voltammetry technique featuring an electrolyte solution which is forced to flow at a constant speed to the electrode surface. -> mass transport of a redox species enhanced in this way results in higher current. The forced flow can be accomplished either by agitation of the solution (solution stirring, or channel flow), or the electrode (electrode rotation, see -> rotating disk electrode or vibration,... 

Fig. 3 Concentration profile for the RDE normal to the electrode. Reaction of O at the RDE produces a concentration [O]o at the electrode surface. Mass transport to the electrode occurs by diffusion across zd- Atz > zd, the solution is well-stirred and the concentration of O is the bulk concentration[0]co-...

On the other hand, bulk concentrations are required for estimation of the respective surface concentrations that are the terms of kinetic equations. To obtain the data for the solution layer adjacent to the electrode surface, mass transport of chemically interacting species should be considered. Quantitative formulation of this problem is based on differential equations representing Pick s second law and supplemented with the respective kinetic terms. It turns out that some linear combinations of these equations make it possible to eliminate kinetic terms. So produced common diffusion equations involve total concentrations of metal, ligand and proton donors (cj j, c, and Cj4, respectively) as functions of time and space coordinates. It follows from the relationships obtained that the total metal concentration varies in the same manner as the concentration of free metal ions in the absence of ligand. Simultaneously, the total ligand concentration remains constant within the whole region of the diffusion layer. This proposition also remains valid for proton donors and acceptors. 

This section and the next are dedicated to the basics of the silicon-electrolyte contact with focus on the electrolyte side of the junction and the electrochemical reactions accompanying charge transfer. The current across a semiconductor-electrolyte junction may be limited by the mass transport in the electrolyte, by the kinetics of the chemical reaction at the interface, or by the charge supply from the electrode. The mass transport in the bulk of the electrolyte again depends on convection as well as diffusion. In a thin electrolyte layer of about a micrometer close to the electrode surface, diffusion becomes dominant The stoichiometry of the basic reactions at the silicon electrode will be presented first, followed by a detailed discussion of the reaction pathways as shown in Figs. 4.1-4.4. 

As mentioned earlier, CB is prone to oxidation, the so-called carbon corrosion, which results in the loss of surface area, changes in the pore structure and finally also leads to sintering of the supported nanoparticles and eventually their loss from the support surface. This affects both the kinetics of the reaction and the electrode s mass transport behavior resulting in a significant loss of performance with operation time. Consequently, carbon support durability is considered to be a major barrier for the successful commercialization of fuel cell technology in the automotive sector. So much so, during the last decade, more than 60 publications dealt with carbon corrosion mechanisms in fuel cell apphcation [82]. 

We note from Eqn. 11 that the thickess of the diffusion layer is inversely proportional to Therefore as rotation speed is increased, the diffusion layer thickness decreases, and the rate of mass transfer to the electrode (the mass transport flux) increases. Hence we see that the diffusion layer thickness can be accurately controlled by means of the rotation speed. We also note from Eqn. 11 that the diffusion layer thickness is independent of the radial coordinate r (see Fig. 2.2). This means that the diffusion layer is uniformly thick over the entire surface of the electrode. In technical terms the electrode is said to be uniformly accessible, and as a consequence the current density is uniform over the entire surface of the disk. 

Linear-scan voltammograms generally have a sigmoid shape and are called voltammetric waves. The constant current beyond the steep rise is called the diffusion-limited current, or simply the limiting current ii because the rate at which the reactant can be brought to the surface of the electrode by mass-transport processes limits the current. Limiting currents are usually directly proportional to reactant concentration. Thus, we may write... 

For electrode (conductor/semiconductor) surfaces, mass transport can be controlled with a variety of experimental protocols and the interfacial flux is measured directly via the current response (measured as a function of potential, time, etc.) [1], This is not true of other interfaces, such as minerals and many biomimetic surfaces in contact with the solution. In these instances, fluxes have often been deduced in a convoluted time- and space-averaged manner by determining the accumulation/loss of material in a bulk phase as a function of time. This leads to a considerable loss of dynamic resolution. Furthermore, in some systems, mass transport between the bulk and the interface is difficult to estimate, leading to incorrect mechanistic interpretation, with major implications for practical applications, whether this concerns drug transport across cell membranes or the growth of crystals. 

Diflfiision, convection and migration are the fonns of mass transport that contribute to the essential supply and removal of material to and from the electrode surface [1, 2, 3 and 4],... 

The great advantage of the RDE over other teclmiques, such as cyclic voltannnetry or potential-step, is the possibility of varying the rate of mass transport to the electrode surface over a large range and in a controlled way, without the need for rapid changes in electrode potential, which lead to double-layer charging current contributions. 

Of course, in order to vary the mass transport of the reactant to the electrode surface, the radius of the electrode must be varied, and this unplies the need for microelectrodes of different sizes. Spherical electrodes are difficult to constnict, and therefore other geometries are ohen employed. Microdiscs are conunonly used in the laboratory, as diey are easily constnicted by sealing very fine wires into glass epoxy resins, cutting... 

Walton D J, Phull S S, Chyla A, Lorimer J P, Mason T J, Burke L D, Murphy M, Compton R G, Ekiund J C and Page S D 1995 Sonovoltammetry at platinum electrodes surface phenomena and mass transport processes J. Appl. Electrochem. 25 1083... 

The flux of material to and from the electrode surface is a complex function of all three modes of mass transport. In the limit in which diffusion is the only significant means for the mass transport of the reactants and products, the current in a voltammetric cell is given by... 

Influence of the Kinetics of Electron Transfer on the Faradaic Current The rate of mass transport is one factor influencing the current in a voltammetric experiment. The ease with which electrons are transferred between the electrode and the reactants and products in solution also affects the current. When electron transfer kinetics are fast, the redox reaction is at equilibrium, and the concentrations of reactants and products at the electrode are those specified by the Nernst equation. Such systems are considered electrochemically reversible. In other systems, when electron transfer kinetics are sufficiently slow, the concentration of reactants and products at the electrode surface, and thus the current, differ from that predicted by the Nernst equation. In this case the system is electrochemically irreversible. 

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