Electrochemical cycling

Electrogenerated conducting polymer films incorporate ions from the electrolyte medium for charge compensation (182). Electrochemical cycling in an electrolyte solution results in sequential doping and undoping of the polymer film. In the case of a -doped polymer, oxidation of the film results in the... 

The relative stability of the anions derived from cyclopropene and cyclopentadiene by deprotonation is just the reverse of the situation for the cations. Cyclopentadiene is one of the most acidic hydrocarbons known, with a of 16.0. The plCs of triphenylcyclo-propene and trimethylcyclopropene have been estimated as 50 and 62, respectively, from electrochemical cycles. The unsubstituted compound would be expected to fall somewhere in between and thus must be about 40 powers of 10 less acidic than cyclopentadiene. MP2/6-31(d,p) and B3LYP calculations indicate a small destabilization, relative to the cyclopropyl anion. Thus, the six-7c-electron cyclopentadienide ion is enormously stabilized relative to the four-7c-electron cyclopropenide ion, in agreement with the Hixckel rule. 

Recent ECQM work and X-ray diffraction have confirmed the conversion of the a/y cycle to the / // cycle up on electrochemical cycling in concentrated alkali. Earlier ECQM studies of a- Ni(OH)2 films had shown a mass inversion in the microgravi metric curve after prolonged cycling [64] there is a mass decrease in charge instead of a mass increase. More recent work has confirmed that this mass... 

The good cycling stability of the tin in TCO is quite unusual, because the electrochemical cycling of Li ASn and also of other Li alloy electrodes is commonly associated with large volume changes in the... 

The charge transport and optical properties of the [Si(Pc)0]-(tos)y)n materials as y=0 -+ 0.67 are reminiscent of the [Si(Pc)0]-(BF4)y)n system, but with some noteworthy differences. Again there is an insulator-to-metal transition in the thermoelectric power near y 0.15-0.20. Beyond this doping stoichiometry, the tosylates also show a continuous evolution through a metallic phase with decreasing band-filling. However, the transition seems somewhat smoother than in the BF4 system for y)>0.40, possibly a consequence of a more disordered tosylate crystal structure. Both [Si(Pc)0]-(tos)y)n optical reflectance spectra and four-probe conductivities are also consistent with a transition to a metal at y 0.15-0.20. Repeated electrochemical cycling leads to considerably more decomposition than in the tetrafluoroborate system. 

Polythiophene films can be electrochemically cycled from the neutral to the conducting state with coulombic efficiencies in excess of 95% [443], with little evidence of decomposition of the material up to + 1.4 V vs. SCE in acetonitrile [37, 54, 56, 396,400] (the 3-methyl derivative being particularly stable [396]), but unlike polypyrrole, polythiophene can be both p- and n-doped, although the n-doped material has a lower maximum conductivity [444], Cyclic voltammetry shows two sets of peaks corresponding to the p- and n-doping reactions, with E° values at approximately + 1.1 V and — 1.4 V respectively (vs. an Ag+/Ag reference electrode)... 

It is known that in the process of electrochemical cycling of tin oxide-based thin films versus lithium metal there are at least two reactions in standard alkylcarbonate electrolyte based on lithium salts [1], The first of them leads to formation of metallic tin on the first cycle ... 

Sco et al (1987) employed the piezoelectric surface stress technique to study the behaviour of gold on electrochemical cycling in aqueous sulphate solution. They obtained plots of u and vs. E at a range of pH from 0.3 to 14 and a typical example of these is shown in Figure 2.12 which shows the plots obtained at pH 3. It should be emphasised that the data in Figure 2.12 were obtained dynamically by simultaneously scanning the potential in the... 

Two types of potential step experiments were carried out in this work those in which the working electrode was electrochemically cycled in acid electrolyte alone (type A) before methanol was admitted with the electrode held at a very low potential, and those in which the electrode was cycled in acid electrolyte in the presence of methanol (type B) before the spectroscopic... 

Figure 3.80 Values of n, k and thickness L obtained t ia three parameter fits to the A, T and intensity data obtained during the growth of a polypyrrole film on a sputtered Pt electrode in N2-saturated I M NaCIO4/0,l M pyrrole. The potential was stepped from OV to 0.8 V vs, SCE for 15 s, and readings taken every 20 ms. Reprinted from tkarochimica Acia, 36. P,A. Christensen and A. Hamnett, In situ Spectroscopic Investigations of the Growth, Electrochemical Cycling and Overoxidation of Polypyrrolc in Aqueous Solution, pp. 1263-1286 (1991), with kind permission from Pergamon Press Ltd., Headington Hill Hall. Oxford 0X3 OBW, UK.

Other common techniques have been applied to the assembly of layers or films of TTF-derived molecular conductors. Compound 29 is an example of an amphiphilic TTF derivative. It forms conducting Langmuir-Blodgett charge transfer films with the acceptor TCNQF4 (30).98 Self-assembly of compound 31 on gold by electrochemical methods yielded an electroactive monolayer which was remarkably stable to electrochemical cycling.99... 

Two examples from literature illustrate this approach nicely. Moore et al.114 assembled thiol-terminated long-chain S204-crown TTF onto Au and Pt surfaces by thiolato-metal bonds (see Figure 12). In the presence of various cations, most successfully Ag+, small differences were observed in the first oxidation potential (typically 60-80 mV). Similar responses were observed in solution state experiments with the same materials. The SAMs were stable when electrochemically cycled over the first oxidation wave but unstable when scanned beyond this point. Liu et al.115,116 prepared SAMs of 45 and 46 on Au substrate. Anchored to the solid surface by four Au S bonds per molecule, these SAMs were stable for hundreds of cycles over the full oxidation range. In response to the presence of Na+ both the TTF oxidation waves were shifted anodically by 55-60 mV. This observation was ascribed to either surface aggregation or cooperativity of neighbouring crown rings. 

For more sophisticated spectroelectrochemical analysis, electrochemical cycling is coupled with X-ray absorption spectroscopy (XAS) methods, which probe electronic and local atomic structures with element specificity.266-234 Because XAS techniques are compatible with in situ investigations and do not require long-range structural order for analysis, they are... 

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