Electrode kinetics electrocatalysis measurement

Changes of A from one metal to another, for a given process (e.g. the HER), provide the principal basis for dependence of the kinetics of the electrode process on the metal and are recognized as the origin of electrocatalysis associated with a reaction in which the first step is electron transfer, with formation of an adsorbed intermediate. In the case of the HER, this effect is manifested in a dependence of the logarithm of the exchange current density, I o (i.e., the reversible rate of the process, expressed as A cm , at the thermodynamic reversible potential of the reaction) on metal properties such as 0 (Fig. 2) (14-16, 20). However, as was noted earlier, for reasons peculiar to electrochemistry, reaction rate constants cannot depend on under the necessary condition that currents must be experimentally measured at controlled potentials (referred to the potential of some reference... 

When the electroactive species or an intermediate adsorbs on the electrode surface, the adsorption process usually becomes an integral part of the charge transfer process and therefore cannot be studied without the interference of a faradaic current. In this situation, surface coverages cannot be measured directly and the role of an adsorbate must be inferred from a kinetic investigation. Tafel slopes and reaction orders will deviate substantially from those for a simple electron transfer process when an adsorbed intermediate is involved. Moreover the kinetic parameters, exchange current or standard rate constant, are likely to become functions of the electrode material and even the final products may change. These factors will be discussed further in the section on electrocatalysis (Section 1.4). 

There are numerous reports of the existence of electrocatalysis via an attached redox center on a SAM (see Sect. 4.3), but few reports in which the rates of electron transfer between the electrode and the attached redox molecule and between the attached redox molecule and the solution redox molecule are measured. It would be interesting to study the electron-transfer kinetics in SAMs with multiple redox molecules linked along a single tether (such as porphyrins [157] or metal-terpyridine complexes) [122]. From such a system, one could derive the rate of electron transfer between two redox molecules connected by a molecular bridge and check the considerable data available on intramolecular transfer obtained by other methods [254]. For a variety of applications, measurements of the rates of electron transfer between an electrode and metal nanoparticles tethered to the SAM are also of interest [255]. 

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