Mass transport, in electrochemical cells

These two equations (4.2) and (4.3) together with (4.4) (continuity equation for incompressible fluids) and with the boundary conditions of the particular reactor define the convective mass transport in electrochemical cells. It is important to take in mind that this exhaustive description is frequently used in electrochemical engineering, especially in cases such as the electroplating processes where the current distribution becomes a key factor in the performance of the process. 

J. S. Newman, The fundamental principles of current distribution and mass transport in electrochemical cells, in Electroanalvtical Chemistry - A Series of Advances. Vol. 6, A. J. Bard, editor, Marcel Dekker, New York, 1972, pp. 187-352. 

Electrode Processes in Solid Electrolyte Systems, Douglas O. Raleigh The Fundamental Principles of Current Distribution and Mass Transport in Electrochemical Cells, John Newman... 

The analysis of mass transfer in electrochemical cells requires the use of equations that describe the condition of electroneutrality (which applies for the entire elecnolyte outside the double layer at an electrode), species fluxes, mass conservation, current density, and fluid hydrodynamics. Often, mass transport events are rate limiting, as compared to kinetics processes at the electrode surface, in which case the overall electrode reaction rate is solely dependent on species mass transfer (e.g., during high-rate electroplating of some metals and for those elecnochemical reactions where the concentration of reactant in solution is low). 

Mass transport in electrochemical reactions is defined as the transport of reactants and products which does not include the transport of electric or ionic charges. In the case of polymer electrolyte fuel cells (PEFCs), mass transport includes the following phenomena ... 

In a typical spectroelectrochemical measurement, an optically transparent electrode (OTE) is used and the UV/vis absorption spectrum (or absorbance) of the substance participating in the reaction is measured. Various types of OTE exist, for example (i) a plate (glass, quartz or plastic) coated either with an optically transparent vapor-deposited metal (Pt or Au) film or with an optically transparent conductive tin oxide film (Fig. 5.26), and (ii) a fine micromesh (40-800 wires/cm) of electrically conductive material (Pt or Au). The electrochemical cell may be either a thin-layer cell with a solution-layer thickness of less than 0.2 mm (Fig. 9.2(a)) or a cell with a solution layer of conventional thickness ( 1 cm, Fig. 9.2(b)). The advantage of the thin-layer cell is that the electrolysis is complete within a short time ( 30 s). On the other hand, the cell with conventional solution thickness has the advantage that mass transport in the solution near the electrode surface can be treated mathematically by the theory of semi-infinite linear diffusion. 

It follows from Equation 6.12 that the current depends on the surface concentrations of O and R, i.e. on the potential of the working electrode, but the current is, for obvious reasons, also dependent on the transport of O and R to and from the electrode surface. It is intuitively understood that the transport of a substrate to the electrode surface, and of intermediates and products away from the electrode surface, has to be effective in order to achieve a high rate of conversion. In this sense, an electrochemical reaction is similar to any other chemical surface process. In a typical laboratory electrolysis cell, the necessary transport is accomplished by magnetic stirring. How exactly the fluid flow achieved by stirring and the diffusion in and out of the stationary layer close to the electrode surface may be described in mathematical terms is usually of no concern the mass transport just has to be effective. The situation is quite different when an electrochemical method is to be used for kinetics and mechanism studies. Kinetics and mechanism studies are, as a rule, based on the comparison of experimental results with theoretical predictions based on a given set of rate laws and, for this reason, it is of the utmost importance that the mass transport is well defined and calculable. Since the intention here is simply to introduce the different contributions to mass transport in electrochemistry, rather than to present a full mathematical account of the transport phenomena met in various electrochemical methods, we shall consider transport in only one dimension, the x-coordinate, normal to a planar electrode surface (see also Chapter 5). 

Wang15 investigated heat and mass transport and electrochemical kinetics in the cathode catalyst layer during cold start, and identified the key parameters characterizing cold-start performance. He found that the spatial variation of temperature was small under low current density cold start, and thereby developed the lumped thermal model. A dimensionless parameter, defined as the ratio of the time constant of cell warm-up to that of ice... 

Several cell configurations are common in electrochemical research and in industrial practice. The rotating disk electrode is frequently used in electrode kinetics and in mass-transport studies. A cell with plane parallel electrodes imbedded in insulating walls is a configuration used in research as well as in chemical synthesis. These are two examples of cells for which the current and potential distributions have been calculated over a wide range of operating parameters. Many of the principles governing current distribution are illustrated by these model systems. 

In the first part, Chapters 2-6, some fundamentals of electrode processes and of electrochemical and charge transfer phenomena are described. Thermodynamics of electrochemical cells and ion transport through solution and through membrane phases are discussed in Chapter 2. In Chapter 3 the thermodynamics and properties of the interfacial region at electrodes are addressed, together with electrical properties of colloids. Chapters 4-6 treat the rates of electrode processes, Chapter 4 looking at fundamentals of kinetics, Chapter 5 at mass transport in solution, and Chapter 6 at their combined effect in leading to the observed rate of electrode processes. 

Perry ML, Newman J, Cairns EJ (1998) Mass transport in gas-diffusion electrodes a diagnostic tool for fuel-cell cathodes. J Electrochem Soc 145 5-15... 

Many modes of convection have been implemented in electrochemical cells in terms of stirring, rotation, vibration, and so forth. They may be divided into (wo main groups according to our ability to describe the mass transport mathematically. If a magnetic bar is used to stir the solutions as in preparative electrolysis (PE), the... 

Mass transport in an electrochemical reactor occurs by three mechanisms migration in the electrical field, film diffusion, and convection. The first of these is a special feature of electrochemical reactions, whereas the other two are common to all reactions that have a solid phase. However, where an inertsupporting electrolyte is used, the effect of migration can be neglected. With this assumption, let us consider a single electrode reaction given by reaction 21.3. When a finite current is passed through the cell and conditions are perfectly reversible, the concentration overpotential can be expressed as (Pickett, 1979)... 

CFD models of DEFC have been also proposed [188]. Suresh and Jayanti developed an one-dimensional, single phase, isothermal mathematical model for a liquid-feed DEFC, taking into account mass transport and electrochemical phenomena on both the anode side and the cathode side [189]. Tafel kinetics expressions have been used to describe the electrochemical oxidation of ethanol at the anode and the simultaneous ethanol oxidation and ORR at the cathode. The model in particular accounts for the mixed potential effect caused by ethanol cross-over at the cathode and is validated using the data from the literature. Model results show that ethanol crossover can cause a significant loss of cell performance. 

This work presents electrochemistry from a macroscopic viewpoint, and is divided into 4 parts the thermodynamics of electrochemical cells, electrochemical kinetics, transport processes, and finally current distribution and mass transfer in electrochemical systems (including porous electrodes and semiconducting electrodes). Problems to solve are presented at the end of each chapter, without the answers. 

J.R.P. Jayakody, S.H. Chung, L. Durantino, H. Zhang, L. Xiao, B.C. Benicewicz, NMR studies of mass transport in high-acid-content fuel cell membranes based on phosphoric acid and polybenzimidazole, J. Electrochem. Soc. 154 (2) (2007) B242-B246. 

A few electrochemical analysis methods which can be used to characterize the impact of mass transport limitations on cell operation are listed here. These methods are sometimes used in combination with water visualization to draw... 

Fuel cell engineers refer to jo/mpt as the mass activity. From an electrochemical perspective. Figure 1.6a represents a Tafel plot. However, the voltage Eceii (jo) does not only account for a single plain electrified interface, but also depends on electron flux, ion flux, and mass transport in all components and across all interfaces of the MEA. 

The overall performance of microfluidic electrochentical cells is generally dictated by reactant mass transport limitations. It is also the unique mass transport characteristics of microfluidic cells that distinguish than from the conventional MEA-based cell designs. Therefore, mass transport in nuCTofluidic electrochemical cells is both an interesting and useful subject for research and has spearheaded the contributions towards enhanced cell performance over the past several years. 

---