Electrode kinetics reactant transport

A model of such structures has been proposed that captures transport phenomena of both substrates and redox cosubstrate species within a composite biocatalytic electrode.The model is based on macrohomo-geneous and thin-film theories for porous electrodes and accounts for Michaelis—Menton enzyme kinetics and one-dimensional diffusion of multiple species through a porous structure defined as a mesh of tubular fibers. In addition to the solid and aqueous phases, the model also allows for the presence of a gas phase (of uniformly contiguous morphology), as shown in Figure 11, allowing the treatment of high-rate gas-phase reactant transport into the electrode. 

This is the famous Butler-Volmer (B-V) equation, the central equation of phenomenological electrode kinetics, valid under conditions where there is a plentiful supply of reactant (e.g., the Ag+ ions) by easy diffusion to and from electrodes in the solution, so that the rate of the reaction is indeed controlled by the electric charge transfer at the interface, and not by transport of ions to the electrode or away from it. 

In deriving eqn. (80), limitations due to mass transport at the interface were not considered. Strictly speaking, this is not realistic and as the reaction rate increases with overpotential in each direction a variation of the concentrations of reactant and product at the surface operates and concentration polarization becomes more important. Each exponential expression in eqn. (80) must be multiplied by the ratio of surface to bulk concentrations, ci s/ci b. The effect of mass transfer in electrode kinetics has been discussed in Sect. 2.4. 

Figure 12(b) shows the local current distribution of first and second order reactions and applied over potentials ° for the coupled anode model without the mass transfer parameter y. The figure also shows the effect of a change in the electrode kinetics, in terms of an increase in the reaction order (with respect to reactant concentration) to 2.0, on the current distribution. Essentially a similar variation in current density distribution is produced, to that of a first order reaction, although the influence of mass transport limitations is more severe in terms of reducing the local current densities. 

An aim of the model is to determine the influence of the various mass transport parameters and show how they influence the polarization behavior of three-dimensional electrodes. In the model we have adopted relatively simple electrode kinetics, i.e., Tafel type, The approach can also be applied to more complicated electrode kinetics which exhibit non-linear dependency of reaction rate (current density) on reactant concentration. 

The electrolysis measurements were conducted at three flow rates i) anolyte 3.4 ml/min and catholyte 4.4 ml/min ii) anolyte 11.8 ml/min and catholyte 11 ml/min) in) anolyte 22 ml/min and catholyte 27 ml/min). The tests were run at ambient temperature and pressure. Linear sweep voltammetry data obtained for the AHA and Nafion 115 membranes indicated very little effect of the flow rate on the electrode kinetics as long as the mass transport limitation is not reached. Apparently, the higher flow rates of reactants passing through the electrodes do not speed up the electrochemical conversion rates in the electrolyser used in this study. 

The rate at which an electrochemical process proceeds is governed by the intrinsic electrode kinetics or by mass-transport processes. If reactants are readily available at an electrode surface, then mass-transport limitations do not govern the overall rate in this section we shall consider this case, in which sluggish kinetics govern the rate. 

Electrocatalytic surface reactions may involve convective or diffusive transport of reactants and products external to the electrode surface or in the porous structure. If the rate of mass transport is comparable to or slower than the surface rate, the electrode kinetic and selectivity behavior will be altered (48a, 60-62, 407). 

The key factors that control the rate of electrodeposition and the structure, physical properties, uniformity, and composition of electrodeposited metals and alloys are (1) thermodynamics (where the electric potential is based on the standard electromotive series) (2) electrode kinetics (which may vary with the structure of the electrodeposit) and (3) mass transport (which is important at high current densities, where the delivery of reactant to the cathode surface affects the local deposition rate and the structure of the deposit). 

Tertiary current distribution. This method of analysis applies to those systems where there is significant mass transport and electrode polarization effects. Electrode kinetics is considered, with electrode surface concentrations of reactant and/or products that are no longer equal to those in the bulk electrolyte due to finite mass transfer resistance. The analysis of tertiary current distributions is complex, involving the solution of coupled... 

ELECTRODE KINETICS OF ELECTRON-TRANSFER REACTION AND REACTANT TRANSPORT IN ELECTROLYTE SOLUTION... 

Effect of Reactant Transport on the Electrode Kinetics of Electron-Transfer Reaction 57... 

Besides the kinetics of electron-transfer reaction discussed above, the process of reactant transport near an electrode... 

Reactant transport can not only affect the kinetics of the electron transfer but also affect the thermodynamics of the electrode reaction. In this section, we will discuss these two effects separately. 

Kinetics of Reactant Transport Near and within Porous Matrix Electrode Layer ... 

In the above sections, we have presented the electrode kinetics of electron-transfer reaction and reactant transport on planar electrode. However, for practical application, the electrode is normally the porous electrode matrix layer rather thtin a planner electrode siuface because of the inherent advantage of large interfacial area per unit volume. For example, the fuel cell catalyst layers are composed of conductive carbon particles on which the catalyst particles with several nanometers of diameter are attached. On the catalyst particles, some proton or hydroxide ion-conductive ionomer are attached to form a solid electrolyte, which is uniformly distributed within the whole matrix layer. Due to the electrode layer being immersed into the electrolyte solution, this kind of electrode layer is called the flooded electrode layer . 

Here we are intending to give a sense as to how to treat the reactant transports within the electrode layer roughly as well as what is its effect on the electron-transfer kinetics. Figure 2.12(A) shows the schematic of such an electrochemical electrode system consisted of the current collector, matrix electrode layer, and the electrolyte. 

For a research on the electrode reaction mechanism and kinetics, particularly those of oxygen reduction reaction (ORR) (O2 + 4H+ + 4e -> 2H2O in acidic solution, or 02 + 2H20 + 4e -> 40H in alkaline solution), it is necessary to design some tools that could control and determine the reactant transportation near the electrode surface and its effect on the electron-transfer kinetics. A popular method, called the rotating disk electrode (RDE) technique has heen widely used for this purpose, particularly for the ORR. 

This first chapter to Volume 2 Interfadal Kinetics and Mass Transport introduces the following sections, with particular focus on the distinctive feature of electrode reactions, namely, the exponential current-potential relationship, which reflects the strong effect of the interfacial electric field on the kinetics of chemical reactions at electrode surfaces. We then analyze the consequence of this accelerating effect on the reaction kinetics upon the surface concentration of reactants and products and the role played by mass transport on the current-potential curves. The theory of electron-transfer reactions, migration, and diffusion processes and digital simulation of convective-diffusion are analyzed in the first four chapters. New experimental evidence of mechanistic aspects in electrode kinetics from different in-situ spectroscopies and structural studies are discussed in the second section. The last... 

The functions of porous electrodes in fuel cells are 1) to provide a surface site for gas ionization or de-ionization reactions, 2) to provide a pathway for gases and ions to reach the catalyst surface, 3) to conduct water away from the interface once these are formed, and 4) to allow current flow. A membrane electrode assembly (MEA) forms the core of a fuel cell and the key electrochemical reactions take place in the MEA. MEA performance is severely affected by electrode composition, structure, and geometry, and especially by cathode structure and composition, due to poor oxygen reduction kinetics and transport liniitations of the reactants in the cathode catalyst layer. 

In a fuel cell, the difference in reactant gas compositions at the two electrodes leads to the formation of a difference in Galvani potential between anode and cathode, as discussed in the section Electromotive Force. Thereby, the Gibbs energy AG of the net fuel cell reaction is transformed directly into electrical work. Under ideal operation, with no parasitic heat loss of kinetic and transport processes involved, the reaction Gibbs energy can be converted completely into electrical energy, leading to the theoretical thermodynamic efficiency of the cell. 

As evidenced at the beginning of the treatment of the electrode kinetics, the Butler-Volmer equation only accounts for the kinetics of the charge transfer, once supposing that the supply of electroactive species at the electrode occurs at infinite rate, i.e., assuming that infinite rate of mass transfer is operative. This is out of the reality a finite rate characterizes the mass transport. As a consequence of the charge transfer, in fact, mass transfer of reactant and product to and from the electrode surface, respectively, is induced. 

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