Charge-transfer polarization curves

Fig. 3.1 1 Experimental charge-transfer polarization curves, E vs. log liexl, for positive and negative overpotentials...

The linear relationships shown for E as a function of log i are frequently observed for only small deviations from equilibrium. It is shown subsequently that the linear relationship corresponds to an upset in the mechanism of transfer of the ions between the metal and the solution and is termed charge-transfer polarization. As the potential is changed progressively from E, the curves deviate from linearity (Fig. 3.2). Along the reduction branch, Ered M becomes more negative than the linear relationship would indicate. This additional deviation is due to removal of metal ions from the solution in the vicinity of the interface at a rate such that diffusion of the ions in the solution toward the inter-... 

Fig. 3.1 Polarization curves illustrating charge-transfer polarization (Tafel ° behavior) for a single half-cell reaction, (a) Anodic polarization,...

The linear E versus log i curve, reflecting Tafcl-typc behavior, is referred to as charge-transfer polarization because it is associated with the actual separation of charge at the electrode interface. In the case of a metal, charge transfer involves either transfer of a metal ion into the solution and an electron(s) into the metal (oxidation or corrosion) or the combination of a metal ion in solution with an electron(s) to form an ef-... 

The constants characterizing the electrode reaction can be found from this type of polarization curve in the following manner. The quantity k"e is determined directly from the half-wave potential value (Eq. 5.4.27) if E0r is known and the mass transfer coefficient kQx is determined from the limiting current density (Eq. 5.4.20). The charge transfer coefficient oc is determined from the slope of the dependence of In [(yd —/)//] on E. 

This chapter is based on the thermodynamic theory of membrane potentials and kinetic effects will be considered only in relation to diffusion potentials in the membrane. The ISE membrane in the presence of an interferent can be thought of as analogous to a corroding electrode [46a] at which chemically different charge transfer reactions proceed [15, 16]. Then the characteristics of the ISE potentials can be obtained using polarization curves for electrolysis at the boundary between two immiscible electrolyte solutions [44[Pg.35]

Let us now look at the conversion of Curve A to Curve B. What has happened there is that a small charge-transfer resistor has been added in parallel to the doublelayer capacitor, through which electrons can shuttle between Pt and the redox couple (5.6). In other words, the addition of the redox couple has converted the polarized interface (Fig. 5.2a) to a nonpolarized interface (Fig. 5.2b). 

In the polarization curve, three parts can be observed kinetic, ohmic, and mass transfer. In the kinetic part, the cell voltage drop is due to the charge-transfer kinetics, i.e., the 02 reduction and H2 oxidation rate at the electrode surface, which is dominated by the kinetic I-rj equation (Equation 1.37). In the ohmic part, the cell voltage drop is mainly due to the internal resistance of the fuel cell, including electrolyte membrane resistance, catalyst layer resistance, and contact resistance. In the mass transfer part, the voltage drop is due to the transfer speed of H2 and 02 to the electrode surface. 

In the circuit, Rs is the electrolyte resistance, CPE indicates the double-layer capacitance, Rc, is the methanol oxidation charge-transfer resistance, while R1 and Cl are the mass transfer related resistance and capacitance (mainly due to methanol adsorption or CO coverage). The physical expression of these parameters can be deduced from the reaction kinetics. In the methanol oxidation reaction, the overall charge transfer rate is the sum of each charge-transfer step (rct). The Faradaic resistance (Rj) equals the inverse of the DC polarization curve slope ... 

At the transition between the two current density ranges, the polarization curve for Cu deposition starts diverging from the calculated Tafel curve. This divergence was attributed to the transition from charge transfer to concentration overvoltage control of the copper reduction. It was concluded from these results that the reduction at the cathode surface of metal ions adsorbed on the particles plays a fundamental role in the codeposition mechanism. 

Fig. 12. Upper part Schematic drawing according to calculated gas phase potential energy curves for the twist coordinate in dimethylaminocyano-biphenyl (left) and dimethylaminocyano-phenylan-thracene (right) [50], The lowest singlet state Sj is of locally excited nature (small dipole moment), S3 is of charge transfer nature with pure TICT character at 90° but without the usual energy minimum for the biphenyl derivative. Solvation (lower part) preferentially lowers the CT state. It becomes Sj in sufficiently polar solvents, and the energetic lowering is biased towards 90 because the dipole moment possesses a maximum for the perpendicular conformation...

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