Working electrodes concentration polarization

Voltammetry comprises a ivap of electro-analytical methods in which information about the analyte is obtained by measuring current as a function of applied potential under conditions that promote polarization of an indicator, or working, electrode. IVhen current proportional to analytic concentration is monitored at fixed potential, the technique, is called amperomeiry. Generally, to enhance polarization, working electrodes in voltammetry and anqret onietry have surface areas of a few squan> millimeters at the most and. in some applications, a few. square micrometers or less. 

For isolating the overpotential of the working electrode, it is common practice to admit hydrogen to the counter-electrode (the anode in a PEMFC the cathode in a direct methanol fuel cell, DMFC) and create a so-called dynamic reference electrode. Furthermore, the overpotential comprises losses associated with sluggish electrochemical kinetics, as well as a concentration polarization related to hindered mass transport ... 

The polarization of the measuring (working) electrode, which is typically a rotating platinum disk embedded in a Teflon sheath, is held constant at some value at which the analyte reduces or oxidizes. The solution is stirred due to the rotation of the electrode. The resulting current is then measured as the titrant is added. The titrant reacts with the analyte, removing it from the solution, thus decreasing its concentration. The measured current therefore also decreases. When all of the analyte has reacted with the titrant, the decrease will stop, signaling the end point. 

Fulleride anions are often more soluble, especially in more polar solvents, than the parent fullerenes. For example, in bulk electrolysis experiments with tetra-n-butylammonium perchlorate (TBACIO4) as supporting electrolyte, carried out in acetonitrile where Cjq is completely insoluble, fairly concentrated, dark red-brown solutions of 50 can be obtained [81]. Upon reoxidation, a quantitative deposition of a neutral Cjq film on the surface of a gold/quartz crystal working electrode takes place. This Cjq film can be stepwise reductively doped with TBA, leading to (Cjo )... 

Concentration polarization and overpotential can both occur at the working and auxiliary electrodes. There is an ohmic potential drop between working and auxiliary electrodes. To obtain the best measurement of the working electrode potential, the reference electrode should be placed as close as possible to the working electrode (Figure 17-4). 

The lithium polymer battery (LPB), shown schematically in Fig. 7.21, is an all-solid-state system which in its most common form combines a lithium ion conducting polymer separator with two lithium-reversible electrodes. The key component of these LPBs is the polymer electrolyte and extensive work has been devoted to its development. A polymer electrolyte should have (1) a high ionic conductivity (2) a lithium ion transport number approaching unity (to avoid concentration polarization) (3) negligible electronic conductivity (4) high chemical and electrochemical stability with respect to the electrode materials (5) good mechanical stability (6) low cost and (7) a benign chemical composition. 

A two-electrode configuration also can be used in a voltammetric or polar-ographic cell in which the current is measured as a function of the applied potential. In this case the working-electrode potential will be less than the applied potential because of the iR drop in the cell. In addition, the current passing through the reference electrode may cause its potential to deviate from its equilibrium (zero-current) value, due to changes in concentration of the electroactive species at the metal-solution interface. Both of these effects act to reduce the potential of the working electrode ... 

Under working conditions, with a current density j, the cell voltage E(J) decreases greatly as the result of three limiting factors the overvoltages r a and r c at both electrodes due to a rather low reaction rate of the electrochemical processes (activation polarization), the ohmic drop RJ in the electrolyte and interface resistance Re, and mass transfer limitations for reactants and products (concentration polarization). 

Figure 4-7 Concept of electrochemical reaction increasing the diffusion layer thickness (concentration polarization) of analyte via a reduction (or oxidation) at the surface of the working electrode. As time (t) increases, the diffusion layer thickness grows quickly to a value that is determined by degree of convection in the sample solution.

Electrochemical cells employed to carry out voltammet-ric or amperometric measurements can involve either a two or three electrode configuration. In the two electrode mode, the external voltage is applied between the working and a reference electrode, and the current monitored. Since the current must also pass through the reference electrode, such current flow can potentially alter the surface concentration of electroactive species that poises the actual half-cell potential of the reference electrode, changing its value by a concentration polarization process. For example, if an Ag/AgCl reference electrode were used in a cell in which a reduction reaction for the analyte occurs at the working electrode, then an oxidation reaction would take place at the surface of the reference electrode ... 

Such concentration polarization of the reference electrode is prevented by maintaining the current density (J amperes/cm ) very low at the reference electrode. This is achieved in practice by making sure that the area of the working electrode in the electrochemical cell is much smaller than the surface area of the reference electrode hence the total current flow will be limited by this much smaller area, and J values for the reference wiU be very small, as desired, to prevent concentration polarization. 

The driving force for all electrochemical processes at conventional electrodes (i.e. mass transport to electrodes and electrochemical reactions) is the difference between the electrode potential at operation and at equilibrium, which is the overpotential rj. It is theoretically a difficult term to work with because it combines the effects of many physical processes (e.g. Stern layer potential gradient change and concentration polarization) into a single quantity. Yet it is accessible experimentally, and hence a practical quantity for the better understanding of complicated electrochemical systems such as clays. 

Fig. 2.23 (a) Comparison of the CO stripping of the three colloidal catalysts, (b) Steady-state polarization curves along with simulated curves for methanol oxidation of the three different 30wt.% PtRu/Vulcan XC-72 catalysts (symbols) at two different temperatures (22 and 60°C). Conditions fixed delay of 5 min, methanol concentration 1 M in the working electrode compartment, flow rate 10Lh . ... 

Within its working window, an electrode can be depolarized by electroactive substances which are dissolved in the electrolyte. The electrochemical reaction on the electrode surface causes concentration gradients perpendicular to the electrode surface. The current is proportional to these concentration gradients. This relationship depends on the electrode geometry, on the hydrodynamic conditions in the solution (whether it is stirred, or not) and on the voltammetric technique. However, in all cases, the current reaches a maximum, or a limiting value, which is proportional to the bulk concentration of the reactant. This is called the concentration polarization of the working electrode. It is the basis of all analytical applications of voltammetry. 

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