Electrodes energy barriers

When a neutral molecule settles onto an electrode bearing a positive charge, the electrons in the molecule are attracted to the electrode surface and the nuclei are repelled (Figure 5.2), viz., the electric field in the molecule is distorted. If the electric field is sufficiently intense, this distortion in the molecular field reduces the energy barrier against an electron leaving the molecule (ionization). A process known... 

Fig. 20.17 Potential energy-distance curves for a cathodic reaction showing how the potential energy barrier is lowered by when E < p,z.c. The barrier is assumed to be symmetrical so that /S => yi, where 5 is the distance of the O.H.P. from the surface of the electrode. Full curve—no field across double layer dashed curve-potential diflcrence is E and is negative...

In Section 1.4 it was assumed that the rate equation for the h.e.r. involved a parameter, namely the transfer coefficient a, which was taken as approximately 0-5. However, in the previous consideration of the rate of a simple one-step electron-transfer process the concept of the symmetry factor /3 was introduced, and was used in place of a, and it was assumed that the energy barrier was almost symmetrical and that /3 0-5. Since this may lead to some confusion, an attempt will be made to clarify the situation, although an adequate treatment of this complex aspect of electrode kinetics is clearly impossible in a book of this nature and the reader is recommended to study the comprehensive work by Bockris and Reddy. ... 

In the absence of either surface states, which may pin the Fermi level at the interface between the dielectric and the electrode, the energy barriers, which in turn... 

Electroosmotic flow, 195 End column detection, 89 Energy barrier, 16 Enzyme electrodes, 172, 174 Enzyme immunoassays, 185 Enzyme inhibition, 181 Enzyme reconstitution, 178 Enzyme wiring, 178 Equilibrium potential, 15 Ethanol electrodes, 87, 178 Exchange current, 14... 

The additional potential required to cause some electrode reactions to proceed at an appreciable rate. The result of an energy barrier to the electrode reaction concerned, it is substantial for gas evolution and for electrodes made of soft metals, e.g. Hg, Pb, Sn and Zn. It increases with current density and decreases with increasing temperature, but its magnitude is variable and indeterminate. It is negligible for the deposition of metals and for changes in oxidation state. 

Since the proton is transferred from a position right in front of the electrode, the assumptions made in the phenomenological derivation of the Butler-Volmer equation may not be valid furthermore, a proton can tunnel through a potential energy barrier in the reaction path. Nevertheless, an empirical law of the form ... 

A positive potential difference between the electrode and the solution will increase the speed of the anodic reaction ( Equation 1 ) as the energy barrier is lowered A becomes A—anFE (F is Faraday constant, 96847 CMole-x), E is the potential (in Volts) and a is a fraction (0[Pg.6]

In studying interfacial electrochemical behavior, especially in aqueous electrolytes, a variation of the temperature is not a common means of experimentation. When a temperature dependence is investigated, the temperature range is usually limited to 0-80°C. This corresponds to a temperature variation on the absolute temperature scale of less than 30%, a value that compares poorly with other areas of interfacial studies such as surface science where the temperature can easily be changed by several hundred K. This "deficiency" in electrochemical studies is commonly believed to be compensated by the unique ability of electrochemistry to vary the electrode potential and thus, in case of a charge transfer controlled reaction, to vary the energy barrier at the interface. There exist, however, a number of examples where this situation is obviously not so. 

When the work functions of the contact electrodes are not well matched to the bands of the EL polymers, energy barriers are formed at the respective interfaces. The height of the barrier for hole injection is determined by the difference between the work function of... 

If one now sets the potential of the working electrode more positive than that of equilibrium, the oxidation process is facilitated (as seen in Figure 5). Thus, the profile of the free energy curves becomes that illustrated in Figure 12, in which the energy barrier for the oxidation is lower than that of reduction. 

Utilizing Eqs. (34) to (39) in Eq. (33), the potential energy surface for the iodide ion-iodine system as a function of distance x from the electrode and the normalized solvent coordinate qig was determined as given in Fig. 15 as a contour plot. It is observed that far from the electrode surface, the ionic and the atomic states are separated by an energy barrier... 

Figure 8-1 shows the potential energy barrier for the transfer reaction of redox electrons across the interface of metal electrode. On the side of metal electrode, an allowed electron energy band is occupied by electrons up to the Fermi level and vacant for electrons above the Fermi level. On the side of hydrated redox particles, the reductant particle RED is occupied by electrons in its highest occupied molecular orbital (HOMO) and the oxidant particle OX, is vacant for electrons in its lowest imoccupied molecular orbital (LUMO). As is described in Sec. 2.10, the highest occupied electron level (HOMO) of reductants and the lowest unoccupied electron level (LUMO) of oxidants are formed by the Franck-Condon level sphtting of the same frontier oihital of the redox particles... 

The plane of closest approach of hydrated ions, the outer Helmholtz plane (OHP), is located 0.3 to 0.5 run away from the electrode interface hence, the thickness of the interfacial compact layer across which electrons transfer is in the range of 0.3 to 0.5 nm. Electron transfer across the interfacial energy barrier occurs through a quantum tunneling mechanism at the identical electron energy level between the metal electrode and the hydrated redox particles as shown in Fig. 8-1. 

Fig. 8-1. Potential energy barrier for tunneling transfer of electrons across an interface of metal electrode (a) cathodic electron transfer from an occupied level of metal electrode to a vacant level of l drated oxidant particles, (b) anodic electron transfer fiom an occupied level of hjrdrated reductant particles to a vacant level of metal electrode. M. = electrode surface OHP = outer Helmholtz plane cfuh = Fermi level of electnms in metal electrode. [From Gerischer, I960.]...

Figure 9-1 illustrates the energy barrier to the transfer of metallic ions across the electrode interface these energy barriers are represented by two potential energy curves, and their intersection, for surface metal ions in the metallic bond and for hydrated metal ions in aqueous solution. As described in Chaps. 3 and 4, the energy level (the real potential, a. ) of interfadal metal ions in the metallic bonding state depends upon the electrode potential whereas, the energy level (the real potential, of hydrated metal ions is independent of the electrode potential. 

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