Soluble-phase redox couples

Liquid phase reactant chemistries originally developed for redox flow batteries can be exploited to great effect in microfluidic electrochemical cells. Most commonly, vanadium redox flow battery technology utilizes soluble vanadium redox couples in both half-cells for regenerative electrochemical energy storage units [53]. The combination of aqueous redox pairs in vanadium redox cells, and VO V... 

FIG. 11 General mechanism for the heterogeneous photoreduction of a species Q located in the organic phase by the water-soluble sensitizer S. The electron-transfer step is in competition with the decay of the excited state, while a second competition involved the separation of the geminate ion-pair and back electron transfer. The latter process can be further affected by the presence of a redox couple able to regenerate the initial ground of the dye. This process is commonly referred to as supersensitization. (Reprinted with permission from Ref. 166. Copyright 1999 American Chemical Society.)... 

The term redox catalysis is used when the redox couple P/Q (implying an electron carrier as mediator) merely plays the role of an electron carrier to substrate (X) mainly in homogeneous phase (without any formation of an adduct between the organic species and the catalyst). The electron exchange between the two soluble entities Q and X occurs only by means of a so-called outer sphere process (see Chapter 2). It is for example, the case for indirect reduction of aromatic halides by radical anions electrogenerated from a properly chosen mediator (for instance, an aromatic hydrocarbon or a ketone). 

Fig. 7 Driving force dependence of (fobs obtained for the reduction of ZnPor+ in benzene by various redox couples in the aqueous phase as probed by SECM in the presence of a full monolayer of ClO-lipid (a). As the driving force increases, fcobs increases in the presence of Fe(CN)6 (curve 1), but decreases for Co (II) sepalchrate (curve 2) and V + (curve 3). A similar analysis is presented in (b) but in the absence of the C-10 lipids and for substantially smaller concentration of the aqueous redox couple. The curve in (c) was obtained from photocurrent measurements at the polarizable water DCE interface in the presence of water-soluble porphyrin dimer (ZnTPPSiZnTMFVP) and ferrocene (Fc), dimethylferrocene (DMFc), butylferrocene (ButylFc), diferrocenylethane (DfcEt), and decamethylferrocene (DCMFc). (Figs, a, b and c were reprinted from Refs. [32, 34], respectively, with permission from the American Chemical Society.)...

Distinguished from true redox flow batteries, hybrid RFB systems employ partially soluble redox couples as active materials, either as a solid or a gas. Hybrid RFBs are more complicated than true RFBs because a new phase, different from the electrolytes, forms on the electrode. A zinc-bromine battery is considered as the prototypical hybrid RFB. 

The electroactive compounds are two soluble redox couples which are oxidized and reduced (section 2.2.4) in a reactor which forms the electrochemical cell. These two compounds have the ability to store electrical energy (during charge) and to restore this energy during the discharge phase. The core of the cell comprises two compartments separated by a proton exchange membrane (PEM), such as a Nafion membrane as... 

It should be noted also that the redox couple may be solution-soluble or may be immobilized within an electrode coating and that, in the former case, the chemical step may be carried out (1) with the organic substrate dissolved within the electrolyte (2) at the interface between the electrolyte and the substrate present as a separate phase or dissolved in a solvent immiscible with the electrolyte (3) within a solvent immiscible with the electrolyte, the redox reagent in its active oxidation state being extracted into the other solvent (maybe by phase-transfer catalysis) and (4) at the interface between the electrolyte and the organic substrate present as a solid. 

In mitochondria there are two types of mechanisms for coupling the electron transport to the movement of protons across the membrane. The first is based on anisotropic reduction and oxidation of a lipid-soluble quinone inside the membrane. The quinone, coenzyme Q, becomes protonated upon reduction and diffuses to an oxidation site on the other side of the membrane where removal of electrons leads to proton release. This is essentially a proton carrier system with the hydroquinone acting as the proton carrier in the lipid phase of the membrane. A further refinement of this system in mitochondria provides for a coenzyme Q redox cycle where the movement of one electron through the chain allows for two protons to cross the... 

A source of free radicals are needed for water-borne emulsion polymerization. The free radicals can be produced by thermal decomposition of peroxy compounds like persulfate [56], or by redox reactions like the persulfate/bisulfite couple [57], or by y-radiation [58,59]. The free-radical initiators can be either water or oil soluble, determining the preferred phase in which the free radicals will be produced. 

In general, experimenters performing recent hydrolysis studies have recognized some of the problems in earlier work (such as the need to work under an inert atmosphere in order to avoid carbonate contamination), and these studies should be more reliable, especially when coupled with direct spectroscopic methods for species determination. The major problems associated with solubility studies remain the attainment of steady-state conditions, the identification of solid phases, and control of the redox conditions. Measurements of hydrolysis reactions and solubilities at temperatures other than 25° C or ambient are almost nonexistent... 

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