Outer-Sphere Redox Systems

In screening electrolyte redox systems for use in PEC the primary factor is redox kinetics, provided the thermodynamics is not prohibitive, while consideration of properties such as toxicity and optical transparency is important. Facile redox kinetics provided by fast one-electron outer-sphere redox systems might be well suited to regenerative applications and this is indeed the case for well-behaved couples that have yielded satisfactory results for a variety of semiconductors, especially with organic solvents (e.g., [21]). On the other hand, many efficient systems reported in the literature entail a more complicated behaviour, e.g., the above-mentioned polychalcogenide and polyiodide redox couples actually represent sluggish redox systems involving specific interactions with the semiconductor... 

Figure 11. Plot of the thermal activation energy corresponding to non-equilibrium solvation [E(p — E ] of the oxidized and reduced species (horizontal axis) as a function of the ionization energy [ERedip) Eo i p)]. The plot is derived from Fig. 4.1. (see section 4.2). 2 is the reorganization energy of the outer sphere redox system.

Figure 9.14 Dependence of the built-in voltage Uu) of ( ) n-InP and ( ) n-InP-CH2CgH4CF3 electrodes on the nemstian potential U(PJK ) of solution contacts with the one-electron, outer-sphere redox systems l,r-dimethylferrocene and decamethylferrocene °. Nemstian potentials have been referenced with respect to a methanolic SCE. The mean f/bj value of every electrode/electrolyte contact was determined using at least 10 AC input voltage frequencies, and the means and standard deviations of these mean f/w values for at least five separate electrodes are depicted at each value of U(A/A ). Linear regression for the two sets of data yields slopes of 0.85 and 0.86 for the etched and BrCH2CgH4Cp3-modified InP, respectively. Source Prokopuk and Lewis (2004).

Here a value of AG = 0.7 eV is found, which leads to A = 2.8 eV [66]. This rather large value is mainly due to an inner sphere reorganization (see Section 6.1.2). Much smaller values are obtained with pure outer sphere redox systems, for instance metallocenes. In the latter cases AG values in the order of 0.2-0.26 eV have been reported [67], i.e. values which correspond to A = 0.7-1 eV. There are other cases such [Fe(CN)6] where one would also expect an outer sphere reorganization but rather high values have been found (AG = 0.55 eV A = 2.2 eV) [67]. In this context it should also be mentioned that modern theories on electron transfer at electrodes have shown that the A values also depend on the distance of the electron acceptor or donor molecules from the electrode surface [65]. 

Clean PPF films possess a low 5-10 pF cm and exhibit reladvely rapid electron-transfer kinetics for several outer-sphere redox systems. For instance, in the low to mid 10 cm sec range is observed for Ru(NH3)6 chlorpromazine andF CbOs (62-64). 

Diamond electrodes tend to be quite active for electron transfer without pretreatment, at least for some outer-sphere redox systems. Exposure to the laboratory atmosphere does not deactivate this electrode like it does other sp carbons (39). As deposited diamond... 

The field of modified electrodes spans a wide area of novel and promising research. The work dted in this article covers fundamental experimental aspects of electrochemistry such as the rate of electron transfer reactions and charge propagation within threedimensional arrays of redox centers and the distances over which electrons can be transferred in outer sphere redox reactions. Questions of polymer chemistry such as the study of permeability of membranes and the diffusion of ions and neutrals in solvent swollen polymers are accessible by new experimental techniques. There is hope of new solutions of macroscopic as well as microscopic electrochemical phenomena the selective and kinetically facile production of substances at square meters of modified electrodes and the detection of trace levels of substances in wastes or in biological material. Technical applications of electronic devices based on molecular chemistry, even those that mimic biological systems of impulse transmission appear feasible and the construction of organic polymer batteries and color displays is close to industrial use. 

In another study, which extended the understanding of the behavior of both dissolved and tethered redox probes, the electrochemistry of a structurally related series of outer sphere redox complexes was compared at Au-S(CH2)nOH mono-layers (System 5). As bipyridyls replace cyano ligands in Fe(bpy) (CN)6 2 - "... 

The observed standard rate constants, k , for one-electron, outer-sphere redox couples vary over wide ranges . Couples such as [RuCNHj) ] , [Fe bipy)j] and [Cr(bipy)j) (bipy = 2,2 -bipyridine) each exhibit values of k that approach the experimentally observable limit of a few cm s . Because the differ. ice, a, in the metal-ligand bond lengths in the oxidized and reduced stetes are mi - jt S <0.5 nm) for each of these systems , the major component of AG arises frjm the outer-shell (solvent) component AG, which is predicted to be insensitive to the chemical nature of the reactant. Much smaller values of are observed for redox couples that involve greater structural differences between the oxidized and reduced forms, as expected because the inner-shell (metal-ligand distortion) component AG will increase rapidly for larger values of a. 

Redox systems such as ferrocene (Fc ) and cobaltocene (CoCp2 ) and their derivatives are usually assumed to be suitable non-adsorbing outer sphere redox couples for use in non-aqueous solutions such as methanol or acetonitrile. One example is... 

The observation of differing responses when using varied inner- and outer- sphere redox probes allows insights to be deduced regarding the state of the surface structure of the electrode material in question. McCreery [62, 87] has provided a road map for commonly utilised redox probes, as shown in Fig. 3.23, which allows researchers to clarify from experimental observations the redox systems and how they are affected. 

In the case of other systems in which one or both of the reactants is labile, no such generalization can be made. The rates of these reactions are uninformative, and rate constants for outer-sphere reactions range from 10 to 10 sec b No information about mechanism is directly obtained from the rate constant or the rate equation. If the reaction involves two inert centers, and there is no evidence for the transfer of ligands in the redox reaction, it is probably an outer-sphere process. 

The outer sphere mechanism may take place in all redox active systems while inner sphere mechanism requires substitutionally labile reactants and products. 

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