Iodine/iodide redox couple

Zhao Y, Wang L, Byon HR (2013) High-performance rechargeable lithium-iodine batteries using triiodide/iodide redox couples in an aqueous cathode. Nat Commun 4 1896... 

The E°i(ads) values obtained here indicate that, upon surface coordination, the redox potential of the iodine/iodide couple is shifted in the negative direction by about 0.90 V on Au, 0.76 V on Pt, and 0.72 V on Ir. These chemisorption-induced redox potential shifts can be employed to estimate the ratio of the formation constants for surface coordination of iodine and iodide ... 

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). 

Another specialized form of potentiometric endpoint detection is the use of dual-polarized electrodes, which consists of two metal pieces of electrode material, usually platinum, through which is imposed a small constant current, usually 2-10 /xA. The scheme of the electric circuit for this kind of titration is presented in Figure 4.1b. The differential potential created by the imposition of the ament is a function of the redox couples present in the titration solution. Examples of the resultant titration curve for three different systems are illustrated in Figure 4.3. In the case of two reversible couples, such as the titration of iron(II) with cerium(IV), curve a results in which there is little potential difference after initiation of the titration up to the equivalence point. Hie titration of arsenic(III) with iodine is representative of an irreversible couple that is titrated with a reversible system. Hence, prior to the equivalence point a large potential difference exists because the passage of current requires decomposition of the solvent for the cathode reaction (Figure 4.3b). Past the equivalence point the potential difference drops to zero because of the presence of both iodine and iodide ion. In contrast, when a reversible couple is titrated with an irreversible couple, the initial potential difference is equal to zero and the large potential difference appears after the equivalence point is reached. 

Iodine — Iodine, L, is a halogen which occurs naturally mainly as iodide, I- [i]. Iodine (Greek ioeides for colored violet ) is a black solid with a melting point of 113.6 °C which is readily undergoing sublimation to form a violet gas. Iodine occurs in the oxidation states -1,0, +1, +3, +5, +7 and it possesses a rich redox chemistry [ii]. In aqueous solution the formation of I2 from I- occurs with a standard potential of 0.621V vs. SHE and this oxidation process is preceded by the formation of I3 with a standard potential of 0.536 V vs. SHE. For the reaction I2(cryst) + 2e - 21 E = 0.535 V. The I—/I3 redox couple is employed, for example, in solar cells [iii] and in long-lived lithium-iodine battery systems. The oxidation of I2 in organic solvents results formally in I+ intermediates which is a powerful oxidant and useful, for example, in electro-synthetic chemical processes [ii]. 

A bimetal redox couple, zinc/cobaloxime, promotes hydropcrfluoroalkylation of electron-deficient alkenes, such as acrylates, acrylonitrile and methyl vinyl ketone, by perfluoroalkyl iodides and bromides, hydrogen replacing iodine or bromine. A typical reaction is the formation of2. ... 

The mid-point potential of the redox couple is given by the Nemst equation, and is therefore dependent on the relative concentrations of iodide and iodine. The concentrations of these species required for efficient device function are in turn constrained by kinetic requirements of dye regeneration at the working electrode, and iodide regeneration at the counter electrode, as discussed below. Typical concentrations of these species are in the range 0.1-0.7 M iodide and 10-200 mM iodine, constraining the mid-point potential of this electrolyte to -0.4 V vx. NUE. It should... 

Finally, we carried out measurements on complete functional devices. We measured the incident photon-to-current conversion efficiency (IPCE) spectra for devices sensitized with AR25 using as electrolyte a solution containing the redox couple iodine/iodide (see Experimental Section). Figure 4 illustrates the IPCE spectra for an AR25/DSSC. 

The second electrode (cathode) often called counter electrode also has an electron-conducting sublayer of FTO on which a thin layer of platinum was deposited. The electrolyte was a solution of iodine in a potassium iodide KI solution, forming 13 ions, and thus, an U/Ij" redox couple. 

The oxidation numbers of oxygen and hydrogen have not changed in the course of the reaction. However, that of iodine has changed. It went from +V (iodate ion) and -I (iodide ion) to 0 (iodine). The two redox couples are... 

The Ross reference electrode differs from the others and consists of a platinum wire immersed in a solution containing tri-iode and iodide ions. The Pt electrode responds to the redox potential established by the iodine(tri-iodide)-iodide couple. This solution is separated from the sample by a bridge electrolyte, which is 3 M KCl. 

Most kinetic determinations of anions involve the iodide ion, which exhibits a strong catalytic effect on the reaction between cerium(IV) and arsenic(III) and a few others as a result of the redox properties of the I2/ I couple. Other anions that can be determined using their intrinsic catalytic effect include sulfur-containing species such as sulfite, sulfide, and thiosulfate, which are quantified by means of the iodine/sodium azide system, and phosphates, which are measured through their effect on the formation of molybdenum blue. Table 5 gives illustrative examples of determinations for these anions and a few others. 

Most redox systems exhibit an apparent standard potential that decreases when the pH increases (for example, this is the case of the couples Mn04 /Mn + and Cr207 /Cr see Chap. 20). By comparison with these couples, iodine becomes a strong oxidant in slightly acidic and weakly alkaline media. We must recall that in alkaline media (about pH = 9), iodine disproportionates into hypoiodous acid (or into hypoiodites) and into iodide ions, these species being in false equilibrium. Moreover, the second time, iodine disproportionates into iodate and iodide ions that are in true equilibrium. Hence, the potential of the couple I2/I is independent of pH in the approximate range 0 < pH < 9. 

Generally, methods are based on solvent extraction of the additive followed by analysis for the extracted additive by a suitable physical technique such as visible spectrophotometry of the coupled antioxidant, redox spectrophotometric methods, ultraviolet spectroscopy, infrared spectroscopy, gas chromatography, thin-layer chromatography or column chromatography. In general, direct chemical methods of analysis have not foimd favour. These include potentiometric titration with standard sodium isopropoxide in pyridine medium or reaction of the antioxidant with excess standard potassium bromide-potassium bromate (ie. free bromine) and estimation of the unused bromine by addition of potassium iodide and determination of the iodine produced by titration with sodium thiosulphate to the starch end-point. ... 

---