Electrochemical process, mass transport

Figure 3.9 illustrates the electrochemical and mass transport events that can occur at an electrode modified with a interfacial supramolecular assembly [9]. For monolayers in contact with a supporting electrolyte, the principal process is heterogeneous electron transfer across the electrode/monolayer interface. However, as discussed later in Chapter 5, thin films of polymers [10] represent an important class of interfacial supramolecular assembly (ISA) in which the properties of the redox center are affected by the physico-chemical properties of the polymer backbone. To address the properties of these thin films, mass transfer and reaction kinetics have to be considered. In this section, the properties of an ideally responding ISA are considered. 

In general, electrochemical systems are heterogeneous and involve at least one (or both) of the fundamental processes - mass transport and an electron-transfer reaction. Moreover, electrochemical reactions involve charged species, so the rate of the electron-transfer reaction depends on the electric potential difference between the phases (e.g. between the electrode surface and the solution). The mass transport processes mainly include diffusion, conduction, and convection, and should be taken into account if the electron-transfer reaction properties are to be extracted from the experimental measurements. The proper control of the mass transport processes seems to be one of the main problems of high-temperature electrochemical studies. 

The major error with Tafel extrapolation is the effect of concentration polarization introduced when the applied current is large. The theoretical basis of this method is only valid for the corrosion systems with an electrochemical control. If the applied current is significant, some corrosion systems may come under mass transport control, or at least under mixed electrochemical and mass transport control. This will make it very difficult to define the Tafel region from the polarization curve, introducing errors in the extrapolation process. Tafel extrapolation is not applicable in highly resistive environments for example, glacial acetic acid. 

The main objective of this chapter is to introduce students to the processes and their explanations when a current passes through an electrochemical cell. This chapter will cover both electrochemical processes and transport phenomena, which occur in the electrolyte and at the electrolyte/electrode interface due to the current in the electrochemical cell. Homogeneous electrochemical reactions are not considered in this book. Overpotentials of the electrochemical cell and half-reactions are introduced. Mechanisms of the charge and mass transfer processes are considered, and corresponding equations describing them are presented and analyzed. Some simplifications and generalizations of the fundamental equations are also given. 

An important feature of SECCM for the characterization of interfaces is that qualitative and quantitative information can be extracted from experimental data, making the technique particularly beneficial for understanding and interpreting (electro)chemical processes at the nanoscale. This requires a careful analysis of electrochemical and mass-transport phenomena occurring at the substrate as well as within the barrels of the double-barrel pipette, which necessitates the solution of... 

An equivalent-circuit model describing the impedance response of a typical lubricant has to take into account processes divided spatially into bulk lubricant and interfacial electrochemical (adsorption, mass transport, and charge transfer) zones [6]. Separation between these two zones can be achieved following the distributed impedance analysis in Section 6-3. The bulk resistance BULK represents a lossy part of the overall bulk-media relaxation mechanism [3,4], always existing in parallel with a bulk-media capacitance as ... 

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... 

Oxidation of Adsorbed CO The electro-oxidation of CO has been extensively studied given its importance as a model electrochemical reaction and its relevance to the development of CO-tolerant anodes for PEMFCs and efficient anodes for DMFCs. In this section, we focus on the oxidation of a COads monolayer and do not cover continuous oxidation of CO dissolved in electrolyte. An invaluable advantage of COads electro-oxidation as a model reaction is that it does not involve diffusion in the electrolyte bulk, and thus is not subject to the problems associated with mass transport corrections and desorption/readsorption processes. 

For small K, i.e., K = 0.5 in Fig. 17, the response of the equilibrium to the depletion of species Red] in phase 1 is slow compared to diffusional mass transport, and consequently the current-time response and mass transport characteristics are close to those predicted for hindered diffusion with an inert interface. As K is increased, the interfacial process responds more rapidly to the electrochemical perturbation in phase 1. The transfer of the target species across the interface generates an enhanced flux to the UME, causing... 

Many of the electrochemical techniques described in this book fulfill all of these criteria. By using an external potential to drive a charge transfer process (electron or ion transfer), mass transport (typically by diffusion) is well-defined and calculable, and the current provides a direct measurement of the interfacial reaction rate [8]. However, there is a whole class of spontaneous reactions, which do not involve net interfacial charge transfer, where these criteria are more difficult to implement. For this type of process, hydro-dynamic techniques become important, where mass transport is controlled by convection as well as diffusion. 

In this chapter, we describe some of the more widely used and successful kinetic techniques involving controlled hydrodynamics. We briefly discuss the nature of mass transport associated with each method, and assess the attributes and drawbacks. While the application of hydrodynamic methods to liquid liquid interfaces has largely involved the study of spontaneous processes, several of these methods can be used to investigate electrochemical processes at polarized ITIES we consider these applications when appropriate. We aim to provide an historical overview of the field, but since some of the older techniques have been reviewed extensively [2,3,13], we emphasize the most recent developments and applications. 

In the literature we can now find several papers which establish a widely accepted scenario of the benefits and effects of an ultrasound field in an electrochemical process [13-15]. Most of this work has been focused on low frequency and high power ultrasound fields. Its propagation in a fluid such as water is quite complex, where the acoustic streaming and especially the cavitation are the two most important phenomena. In addition, other effects derived from the cavitation such as microjetting and shock waves have been related with other benefits reported for this coupling. For example, shock waves induced in the liquid cause not only an enhanced convective movement of material but also a possible surface damage. Micro jets of liquid, with speeds of up to 100 ms-1, result from the asymmetric collapse of cavitation bubbles at the solid surface [16] and contribute to the enhancement of the mass transport of material to the solid surface of the electrode. Therefore, depassivation [17], reaction mechanism modification [18], surface activation [19], adsorption phenomena decrease [20] and the mass transport enhancement [21] are effects derived from the presence of an ultrasound field on electrode processes. We have only listed the main phenomena referring to the reader to the specific reviews [22, 23] and reference therein. 

Having defined our near electrode region, we turn now to consider the various techniques that can be employed in the in situ investigation of the reactions that occur within it. The various methods that can be employed will each provide different types of information on the processes occurring there. As has already been discussed, cyclic voltammetry is the most common technique first employed in the investigation of a new electrochemical system. However, in contrast to the LSV and CV of adsorbed species, the voltammetry of electroactivc species in solution is complicated by the presence of an additional factor in the rate, the mass transport of species to the electrode. Thus, it may be more useful to consider first the conceptually more simple chronoamperometry and chronocoulometry techniques, in order to gain an initial picture of the role of mass transport. 

If the electrochemical kinetics of the process are facile then the overall process will be dominated entirely by mass transport. Kinetic parameters such as the exchange current cannot, therefore, be obtained from such a system by analysis of the cyclic voltammetric response. Systems which satisfy this condition are normally referred to as reversible . This is slightly unfortunate... 

As suggested before, the role of the interphasial double layer is insignificant in many transport processes that are involved with the supply of components from the bulk of the medium towards the biosurface. The thickness of the electric double layer is so small compared with that of the diffusion layer 8 that the very local deformation of the concentration profiles does not really alter the flux. Hence, in most analyses of diffusive mass transport one does not find any electric double layer terms. For the kinetics of the interphasial processes, this is completely different. Rate constants for chemical reactions or permeation steps are usually heavily dependent on the local conditions. Like in electrochemical processes, two elements are of great importance the local electric field which affects rates of transfer of charged species (the actual potential comes into play in the case of redox reactions), and the local activities... 

Mass transfer rate processes, 25 279 Mass-transfer resistance, 11 808 external, 25 290—293 Mass-transfer theory, 10 761 Mass transport, electrochemical cell, 9 658-659... 

Formation of porous silicon is an anodic dissolution process, which consists of carrier transport in the semiconductor, electrochemical reactions at the interface, and mass transport of the reactants and reaction products in the electrolyte. There are a... 

Influence of Mass Transport on Charge Transfer. Electrochemically Reversible and Irreversible Processes... 

It must, however, be taken into account that the concept of electrochemical reversibility or irreversibility of an electron transfer is relative. In fact, to accelerate the redox processes one can act either on the mass transport (by stirring the solution) or on kRed and k0x (by changing the electrode potential, as seen in Section 4.1.1). 

From hereafter, we will neglect the electrochemical aspects of redox processes of adsorbed proteins, in order to consider in more detail redox processes governed by diffusion of the protein in solution. As mentioned in Chapter 2, Section 1.6, the case of adsorbed species is in some ways easier to treat, given the absence of the mathematical laws of mass transport, but in some cases it may be complicated by anomalous currents or electrode poisoning. 

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