Irreversible electrode potentials decomposition potential

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. 

The potential window can be limited by the decomposition potential of a solute, not just a solvent. In particular, reactions of anodic oxidation of halides (Cl-, Br, and I-) on diamond are highly irreversible and have much higher overvoltage (for Cl, by 1 V) than on platinum or graphite electrodes [97, 123, 124], In all probability this is due to poor adsorption of intermediates, that is, Cl, Br, and I atoms, on the diamond electrode surface. We recall that the outer-sphere reactions discussed in Section 6.1 generally do not involve adsorption of intermediates and thus are not... 

Another important feature for lithium graphite intercalation compounds in Li -containing electrolytes is the formation of solid electrolyte interface (SEI) film. During the first-cycle discharge of a lithium/carbon cell, a part of lithium atoms transferred to the carbon electrode electrochemically will react with the nonaque-ous solvent, which contributes to the initial irreversible capacity. The reaction products form a Lb-conducting and electronically insulating layer on the carbon surface. Peled named this film as SEI. Once SEI formed, reversible Lb intercalation into carbon, through SEI film, may take place even if the carbon electrode potential is always lower than the electrolyte decomposition potential, whereas further electrolyte decomposition on the carbon electrode will be prevented. 

In thf the complexes Ni2 ( u.- j -PhC2R)Cp2 (including R = Ph, C CPh) undergo irreversible oxidation processes near -I-0.7 V (vs SCE, FcH /FcH " -l-O.ll V, FcH/FcH" " -I-0.56 V) which results in the formation of deposits on the electrode surface. The anodic sweep indicates the presence of a reversible reduction near — 1.30 V attributed to a Ni2-centered reduction and the formation of [Ni2(M-7 -PhC2R)Cp2] Further reduction results in decomposition of the complexes, and the liberation of the alkyne or diyne ligand, as evidenced by two characteristic alkyne/diyne reductions at very negative potentials. ... 

The cyclic voltammetric experiment can give a great deal of information about the redox activity of a compound and the stability and accessibility of its reduced or oxidised forms. For a fully chemically reversible process, ipa must equal rpc, i.e. all of the material oxidised at the electrode surface on the forward scan must be re-reduced on the reverse scan (or vice versa). If this condition does not hold true, then the process may be partially reversible (rpc < ipa) or irreversible (rpc = 0). Observation of processes that are not fully reversible implies decomposition or chemical reaction of the reduced or oxidised species and the ratio of ipa to /p(. will show a strong dependence on scan rate since the reverse current is related to the lifetime of the redox-generated material. Note that processes that are chemically reversible (in the sense that the reduced and oxidised species are both stable) may not be electrochemically reversible (a term that relates to the relative rates of forward and back electron transfer). Electrochemically reversible processes are characterised by a separation between the forward and reverse potential peaks of exactly 59 mV. 

The overvoltage or overpotential over is inserted in Eq. (3.20) to adjust for other processes that compete in the system and make electrodeposition less than ideally efficient. These processes are irreversible and include the effects of the decomposition of water, other solutes, and imperfections in the electrode surface. Because of these processes, a greater potential difference than calculated from the reference potential and the ionic concentration must be applied in order to achieve deposition. For the same reason, spontaneous deposition, inferred from a positive value of E°, may not occur if the overvoltage exceeds it. Overvoltage effects occur at both the cathode and the anode. 

XRR has been applied to the study of EEIs on several systems [201-205]. The technique was found to be sensitive not only to the formation of reaction layers but also to mass loss at the electrode surface due to processes of corrosion (dissolution) [201]. Of particular interest is the application of high energy synchrotron beams as sources, as their deep penetration capabilities enables the design of operando cells (Fig. 7.10a) [203], Therefore, uncertainty due to equilibration in the absence of an electrochemical potential is eliminated. The structural and chemical stability of EEIs during the lithium insertion/extraction processes have thus been evaluated (Fig. 7.10b) [201-204]. The dependence of these irreversible reactions on the crystal facet of the electrode material forming the EEI was established. It was found that electrolyte decomposition processes were coupled with the redox process occurring in the bulk of the electrode, which is a critical piece of information when designing materials that bypass such layer formation. 

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