Transfer coefficient, electrode-electrolyte

This equation is that of a first-order reaction process, and thus the fraction of material electrolysed at any instant is independent of the initial concentration. It follows that if the limit of accuracy of the determination is set at C, = 0.001 C0, the time t required to achieve this result will be independent of the initial concentration. The constant k in the above equation can be shown to be equal to Am/ V, where A is the area of the pertinent electrode, V the volume of the solution and m the mass transfer coefficient of the electrolyte.20 It follows that to make t small A and m must be large, and V small, and this leads to the... 

In order to obtain a definite breakthrough of current across an electrode, a potential in excess of its equilibrium potential must be applied any such excess potential is called an overpotential. If it concerns an ideal polarizable electrode, i.e., an electrode whose surface acts as an ideal catalyst in the electrolytic process, then the overpotential can be considered merely as a diffusion overpotential (nD) and yields (cf., Section 3.1) a real diffusion current. Often, however, the electrode surface is not ideal, which means that the purely chemical reaction concerned has a free enthalpy barrier especially at low current density, where the ion diffusion control of the electrolytic conversion becomes less pronounced, the thermal activation energy (AG°) plays an appreciable role, so that, once the activated complex is reached at the maximum of the enthalpy barrier, only a fraction a (the transfer coefficient) of the electrical energy difference nF(E ml - E ) = nFtjt is used for conversion. 

Equation (80) relates the current density that flows through the electrode—electrolyte interface due to the electrode reaction to the overpotential in terms of two kinetic parameters, the exchange current density, j0, and the transfer coefficient, a. 

The exchange current and cathodic transfer coefficient for the PEVD system can be extracted from standard Tafel plots (Figure 40) as described in detail previously, and provide a measure of the nonpolarizability of the solid electrolyte/electrode interface. The values of Ig and at various temperatures are shown in Table 3. The activation energy of the exchange current can then be obtained from an Arrhenius... 

It has been shown very recently that l,3,5-tris(4-(ALphenyl-AL3-methylphenyl)phenyl) benzene (56), when studied in DMF and dichloromethane solutions, forms two and three reversible one-electron anodic waves, respectively92. The kinetics of heterogeneous electron transfers has recently been studied using the high speed microband channel electrode in solutions containing 0.1 M TBAP as electrolyte it was possible to find standard potentials E°, transfer coefficients a and, finally, standard rate constants k°. The obtained data, labeled by subscripts 1, 2 and 3 for the first, second and third electron transfer, respectively, are collected in Table 2. 

The apparent transfer coefficient of the cathodic reaction, ac, is a measure of the sensitivity of the transition state to the drop in electrostatic potential between electrolyte and metal [109,112]. According to Ref. 113, it is ac = 0.75 for the O2 reduction on Pt in aqueous acid electrolytes. In Ref. Ill the value ac = 1.0 was reported instead. Since the cathodic reaction is a complex multistep process, it might follow several reaction pathways, and the competition between them is affected by the operation conditions (rj, p, T). Therefore, different values of ac have been reported in different regimes of operation. Although in the simple reactions the transfer coefficient is a microscopic characteristic of the elementary act [112], for complex multistage reactions in fuel cell electrodes it is rather an empirical parameter of the model. The dependence of effective a for methanol oxidation on the catalyst layer preparation was recently studied [114]. 

Hfp,i (1/sec) are mass transfer coefficients for the transfer of solute f in process p (= 1 to designate an electrolytic jnocess) from the bulk of the soluticm to the electrode in reactor i. f = 1, p = 1 and i = 1 means the mass transfer coefFicient for the transfer of solute 1 in process 1 in reactor 1 f = 1, p = 1 and j = 2 stands for the mass transfer coefficient for the transfer of solute 1 in process 1 in reactor 2. kM is the mass transfer coefficient in m/sec, Ai and Vi are, respectively, the electrode area and the effective electrolyser vdume. ft is assumed [8 that the electrolytic process takes place under limiting current conditions, i.e. the solute concentrations on the surface of the electrode, C n = C i2 = 0. The probabilities are as follows ... 

For the common circumstance where the transfer coefficient, a, is approximately independent of the electrode potential, a single value of k or k , along with a serves to describe fully the electrochemical kinetics at a given temperature and system composition. As for the rate constants for homogeneous redox reactions, electrochemical rate parameters can be sensitive to electrolyte composition, largely as a result of variations in the structure of the interphasial region (electrochemical double layer) (see 12.3.7.3.). The influence of the electrode material is considered in 12.3.7.5. 

The partial current of CH4 formation is widely scattered when plotted against the potential. With an assumption that the partial current is proportional to proton activity, a linear Tafel relationship is obtained as shown in Fig. 25.The transfer coefficient is determined as 1.33. These facts indicate that the rate determining step of CH4 formation is involved with the second electron transfer to a hypothetical intermediate species such as COH in an electrochemical equilibrium with a CO adsorbed on the electrode and a proton from the electrolyte,... 

To counteract the (vexing) convection effects on kinetic experiments, Aogaki and co-workers, having developed a special electrode assembly to separate mass transport and kinetic effects, report a marked decrease in the exchange current density (about 25%) in magnetic fields imposed on a copper deposition cell. Virtually no effect on the transfer coefficient (a 0.44) was observed. Experimental results obtained in nickel-phosphorus alloy deposition, cupric ion reduction in ethylenediamine solutions, and the electrolytic reduction of acetophenone " are further demonstrations of the interaction of the magnetic fields with polarization characteristics, and point to the difficulty of fully eliminating the effect of convection and/or diffusion on electrode kinetics. 

Changes in the bnlk electrolyte velocity far from the electrode and/or variations in the electrolyte flow pattern (as might occnr if the cell geometry were altered) can be accounted for in Equation (26.73) by a change in the valne of 5, as is well known from bonndary-layer theory in nonelec-trochemical systems. For given valnes of and C , an increase in bulk electrolyte velocity will decrease 5, resulting in an increase in the current density. Frequently, 5 is related to the mass transfer coefficient (k, a common mass transfer parameter,... 

With a potentiostat the potential at the working electrode is linearly increased from 1.0 to 1.6 V and then decreased back to 0 V. In the first interval 1 is oxidized to the radical cation l+ with a peak potential of p.a = 1-38 V. 1 is stable in this solvent and is reduced in the reverse scan back to 1 at p,c = 1-32 V. The ratio of the current for reduction and oxidation ip c-ip.a = 1 indicates the stability of the radical cation. All of 1, that is formed by oxidation of 1 is reduced back to 1. This behavior is termed chemically reversible. Upon addition of 2,6-lutidine, the radical cation 1 reacts with the nucleophile to afford 2 , which is further oxidized to a dication, which yields the dication with 2,6-lutidine. This can be seen in the decrease of /p,c fp,a and an increase of due to the transition from an le to a 2e oxidation. From the variation of the ratio ip.c-ip,n with the scan rate, the reaction rate of the radical cation with the nucleophile can be determined [9]. This can also be aehieved by digital simulation of the cyclovoltammogram, whereby the current-potential dependence is calculated from the diffusion coefficients, the rate constants for electron transfer and chemical reactions of substrate and intermediates at the electrode/electrolyte interface [10]. With fast cyclovoltammetry [11] scan rates of up to 10 Vs- can be achieved and the kinetics of very short-lived intermediates thus resolved. 

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