Exchange current density impedance

Figure 5. Measurement and analysis of steady-state i— V characteristics, (a) Following subtraction of ohmic losses (determined from impedance or current-interrupt measurements), the electrode overpotential rj is plotted vs ln(i). For systems governed by classic electrochemical kinetics, the slope at high overpotential yields anodic and cathodic transfer coefficients (Ua and aj while the intercept yields the exchange current density (i o). These parameters can be used in an empirical rate expression for the kinetics (Butler—Volmer equation) or related to more specific parameters associated with individual reaction steps.(b) Example of Mn(IV) reduction to Mn(III) at a Pt electrode in 7.5 M H2SO4 solution at 25 Below limiting current the system obeys Tafel kinetics with Ua 1/4. Data are from ref 363. (Reprinted with permission from ref 362. Copyright 2001 John Wiley Sons.)...

Erdey-Gruz, 1048, 1306 1474 Erschler, 1133, 1134, 1425 Ethylene oxidation, anodic, 1052 1258 Exchange current density, 1049, 1066 correction of, 1069 definition, 1053 electrocatalysis and, 1278 impedance and, 1136 interfacial reaction, 1047 and partly polarizable interface, 1056 Excited states, lifetime, 1478 Exothermic reaction, 1041 Ex situ techniques, 785, 788, 1146... 

Calculating Exchange Current Densities and Rate Constants from Impedance Plots. If one takes the Butler-Volmer equation (7.24) under the reversible condition, i.e that in which the overpotential, rj, tends to zero, then,... 

J. C. B. Randies, Discuss. Faraday Soc. 1 11 (1947). A simple derivation of the exchange current density from impedance measurements. 

Thus, Thonstad et al. [135,136] used the relaxation method with galvano-static perturbation and electrochemical impedance spectroscopy to study the kinetics of the A1(III)/A1 electrode reaction in cryolite-alumina melts and found that the exchange current densities were of the order of 5-15 A cm 2. The general electrode reaction scheme may be written... 

The effective area of the OTS-coated PtO electrode can be derived if the charge transfer resistance (K ) is known. Rct can be obtained from impedance data measured at a potential near the reversal potential (37, 33) Rct = RT/(nFAI0), where R is the universal gas constant, T is absolute temperature, n is the number of electrons transferred per molecule of TONE, F is Faraday s constant, I0 is the exchange current density, and A is the effective surface area. Because the impedance spectra of the PtO and PtO-OTS electrodes were measured under the same conditions, the value of Rct may be assumed to be affected only by the effective surface area. In Figure 3, the impedance data are replotted as 2 versus 1 /a)1 2, where a) is the angular frequency (2 tt/). Rct is estimated from the intercept on the Z axis by extrapolation. The Rct values are 95 and 980 fl for PtO and PtO-OTS, respectively. An OTS coverage factor, 0, can then be estimated from (1 — 0) = ct(Pto)/ ct(Pto-OTS> In is case 0 = 0.9. 

The internal contact between the membrane and the preamplifier represents a parasitic impedance. Let us consider two limiting cases concerning the values of the equivalent resistances in Fig.2. A good ion-selective electrode has high exchange current density and therefore low value of the charge transfer resistance Ri (io = 10 A/cm, i.e., Ret = 25 II... 

To check further on the extent to which electrode reactions on the basal plane may be impeded by semiconductor effects, the Fe(CN)6 -Fe(CN)6 redox couple has been examined and found to have an exchange current on the basal plane which is 1/3 of that on the edge plane. This difference may be caused by a difference in the ratio of true-to-apparent surface area or ionic double-layer effects (different point of zero charge) as well as semiconductor effects but is certainly far less than the two orders of magnitude difference in the exchange current densities for the O2 reduction on the basal and edge planes. 

Figure 18. Dependence of the exchange current density on step density on an Ag (100) face intersected by screw dislocations ( 5 x 10 disl. cm ). Before each series of impedance measurements for the evaluation of the exchange current density, the face was grown at the indicated overpotential IR corrected) until a steady state profile is obtained. Step densities Lg calculated from the overpotential of growth according to Eq. (38)/26.34,35)...

In order to evaluate specific electrochenucal characteristics, such as exchange current density io = R- Tin F Rct -S, estimation of the particles surface area S is necessary. Note that the experimental estimation of the surface boundary between electronically and ionically conductive media in a composite material consisting of multiple particles has not been successful to date. BET surface area estimation usually tends to severely overestimate this surface area. It correlates well with irreversible capacity loss during first intercalation (lijima et al. [1995]) but not with the surface impedance of materials (Aurbach et al. [2001]), which indicates that loosely electrically connected microparticles make a major contribution to BET surface area but not to the electrochemically active area. In order to estimate only the area that is electrically accessible and also to use the same value of S for both diffusion and surface kinetics, it is common to use a summary geometric area of the particles of the active material as an estimate for surface area. See further details in the bringing it aU together section. 

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