Redox additives

Figure 42. Schematic representation of the shuttling occurring in an overcharged cell that is based on electrolytes containing redox additive as protection.

To maximize the current limit that could be shunted by redox additives so that the occurrence of such irreversible processes due to overflowing current could be more efficiently suppressed, the redox additive apparently should be present in the electrolyte at high concentrations, and both its oxidized and reduced forms should be very mobile species. Where the criteria for selecting potential redox additives are concerned, these requirements can be translated into higher solubility in nonaqueous media and lower molecular weight. In addition to solubility and diffusion coefficients, the following requirements should also be met by the potential redox additives (1) the formal potential of the redox couple [R]/[0] should be lower than the onset potential for major decom-... 

The cathode materials employed for the early lithium-based systems were 3.0 V class oxides or sulfides thus, the redox potential for the additive should be located in the neighborhood of 3.2—3.5 V. Accordingly, the first generation redox additive proposed by Abraham et al. was based on the iodine/ iodide couple, which could be oxidatively activated at the cathode surface at 3.20 V and then reduced at the lithium surface. " " " 2° For most of the ether-based solvents such as THF or DME that were used at the time, the oxidation potential of iodide or triiodide occurred below that of their major decompositions, while the high diffusion coefficients of both iodine and iodide in these electrolyte systems ( 3 x 10 cm s ) offered rapid kinetics to shuttle the overcharge current. Similarly, bromides were also proposed.Flowever, this class of halide-based additives were deemed impractical due to the volatility and reactivity of their oxidized forms (halogen). 

Figure 44. Voltage profile of overcharged Li/LiJV[n02 AA cells containing different substituted ferrocenes as redox additives in 1.0 M LiAsFe/EC/PC (A) reference (B) fer-rocenyl ketone (C) dimethylaminomethyFerrocene (D) ferrocene (E) n-butylferrocene. (Reproduced with permission from ref 429 (Figure 6). Copyright 1992 The Electrochemical Society.)...

Scheme 28. Effect of Methoxy Relative Position on the Stabilization of the Oxidized State of Anisole Redox Additives (a) 1,2-Methoxybenzene, Whose Oxidized Product Was Stabilized by the Neighboring Methoxy (b) 1,3-Methoxybenzene, in Which the Meta Methoxy Fails To Stabilize the Dianion...

Cr-Ni bimetallic catalyst-promoted redox addition of vinyl halides to aldehydes. 

The importance of the electron transfer reaction between RS" and an electron acceptor (Reactions 2 and 3) has been amply confirmed by the observation that the least acidic thiols are least resistant to oxidation (2), and by the enormously enhanced rate of reaction in the presence of redox catalysts, such as transition metal ions (13) or organic redox additives (14). In these latter cases, reactions of the type below become important,... 

N-type semiconductors can be used as photoanodes in electrochemical cells Q., 2, 3), but photoanodic decomposition of the photoelectrode often competes with the desired anodic process (1 4 5). When photoanodic decomposition of the electrode does compete, the utility of the photoelectrochemical device is limited by the photoelectrode decomposition. In a number of instances redox additives, A, have proven to be photooxidized at n-type semiconductors with essentially 100% current efficiency (1, 2, 3, 6>, ], 8, 9). Research in this laboratory has shown that immobilization of A onto the photoanode surface may be an approach to stabilization of the photoanode when the desired chemistry is photooxidation of a solution species B, where oxidation of B is not able to directly compete with the anodic decomposition of the "naked" (non-derivatized) photoanode (10, 11, 12). Photoanodes derivatized with a redox reagent A can effect oxidation of solution species B according to the sequence represented by equations (1) - (3) (10-15). 

A metal-carbon bond splitting is the first step in the sequence leading from methylcobalamin Im-Co(corrin)-CH3 to acetylcobalamin Im-Co(corrin)-COCH3 [117]. The radical CH3 formed in the primary photoredox step, associated with the reduction of Co(III) to Co(II), is trapped by a CO molecule and the redox addition of the radical CH3CO to the reduced pentacoordinated complex Co(II) results in the final Co(III) acetyl complex. 

Photochemical redox addition of a ligand L into an axial position can be schematized as follows... 

The irreversible electrochemical oxidation of [Ir(X)(CO)(PR3)2] (X = Cl, Br, I PR3 = PPh3, PPh2Et, PPhEt2, PEt3) on rotating Pt electrodes in Bu4NC104/CH2Cl2 reportedly proceeds at diffusion-controlled rates. In the redox addition process, atom transferability predominates over complex redox properties. Evidence indicates that the insoluble product of the one-electron oxidation of [Ir(X)(CO)(PR3)2] is a dimeric iridium(II) complex with an iridium-iridium bond.96... 

Fig. 43. Mechanism of overcharge protection (redox additive). After [586],...

The reactions of primary and secondary radicals with metal complexes, including bioinorganic compounds, are of the following types redox, addition and atom transfer. The products of the reactions include metal centers in unusual valency states and coordinated ligand-radicals ... 

RXN63 Preparation of Allylic Acetates from Alkynes by Tandem Redox-Addition... 

Fan, L. Q., J. Zhong, J. H. Wu, J. M. Lin, and Y. F. Huang. 2014. Improving the energy density of quasi-solid-state electric double-layer capacitors by introducing redox additives into gel polymer electrolytes. Journal of Materials Chemistry A 2 9011-9014. 

Senthilkumar, S. T., R. K. Selvan, and J. S. Melo. 2013. Redox additive/active electrolytes A novel approach to enhance the performance of supercapacitors. Journal of Materials Chemistry A 1 12386-12394. 

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