Direct evidence of redox mediation between a poly(aniline-co-N-propanesulfonic acid aniline) and 2,5-dimercapto-1,3,4-thiadiazole by UV-visible reflectance spectroscopy

Journal of the Brazilian Chemical Society

Torresi, Roberto M.¹  University of Wyoming, Laramie, USA¹  Torresi, Susana I. Córdoba de¹  Buttry, Daniel A.¹  Universidade de São Paulo, São Carlos, Brazil¹  Universidade de São Paulo, São Paulo, Brazil¹ 

关键词: sulfonated polyaniline;    redox mediation;    DMcT;    lithium batteries;   

DOI  :  10.1590/S0103-50532002000400007

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

Several recent papers have described the electrochemical cycling of thin films of mixtures of polyaniline (PANI) and 2,5-dimercapto-1,3,4-thiadiazole (DMcT).1-5 The interest in this system arises from its possible use as a cathode material for Li secondary batteries,5 and is related to theoretical calculations suggesting that PANI/DMcT cathodes may have a very high energy density under the appropriate conditions.5 As part of a concerted effort to characterize the redox properties of this system, there has been a detailed study of the spectroscopic and microgravimetric changes that accompany the electrochemical polymerization and depolymerization of DMcT.6 There have also been studies of the redox behavior of DMcT and its derivatives in various solvent systems and of the spectroscopic changes that accompany the redox transformations of DMcT and its disulfide dimmer.7-10 Figure 1 shows the redox couple for DMcT, illustrating that it exists in a monomeric form in the reduced state and forms a polymer containing disulfide linkages after oxidation. The question of the protonation state of the reduced, monomeric material of DMcT in these films (i.e. dianion or monoprotonated monoanion, see Figure 2) remains unresolved at this time, although it has been speculated, based on the pKa values of DMcT and PANI, that DMcT probably exists as the monoanion under the conditions that prevail during redox cycling of PANI/DMcT films in non-aqueous solvents.7, 10 Because of this uncertainty, the issue of the protonation state of DMcT is not addressed in the present contribution.     Despite the reports of high energy density and good charge-discharge behavior of thin films of PANI/DMcT,5 a significantly unattractive feature of this system arises from the fact that PANI is a p-doped conducting polymer. Thus, under the conditions of its use in Li battery cathode the material is most likely to be neutral in its reduced (discharged) state and cationic in its oxidized (charged) state, a situation that will predominantly result in anion transport during redox cycling.11,12 Since it is desirable that cathode materials be capable both of charge storage and of accommodating Li+ produced by discharge of the anode, an investigation of the behavior of sulfonated derivatives of PANI in mixtures with DMcT was initiated. The motivation was to use a PANI derivative for which cation transport would be the dominant mode of achieving electroneutrality during redox cycling. Extensive previous work with "self-doped" conducting polymers suggested that sulfonated PANI derivatives would be suitable for this purpose.13 There also have been other reports of sulfonated derivatives of PANI.14 A particularly attractive system is poly(aniline-co-N-propanesulfonic acid aniline) (PAPSAH, Figure 3 shows the partially oxidized, deprotonated form), which has been produced in pure form, is easily dissolved in aqueous media, and has been thoroughly characterized.15-17   The redox and spectroscopic properties of PANI have been widely studied.18-20 The redox and spectroscopic behavior of the DMcT system and a few of its derivatives have also been thoroughly investigated.7,10 However, while there have been several studies of thin films of PANI/DMcT mixtures, to date there has not been a definitive spectroscopic demonstration of the mediation of DMcT redox chemistry by PANI or any of its derivatives. This report describes a comparison between the electrochemistry and UV-vis spectroscopy of PAPSAH and PAPSAH/DMcT thin films. As will be seen below, the reflectance and transmission spectroscopic results on PAPSAH/DMcT thin films demonstrate mediation of the redox process of DMcT by PAPSAH.  Experimental MaterialsPAPSAH was synthesized, isolated and purified following a literature procedure15 using the emeraldine salt of PANI (Aldrich) and propane sultone (Aldrich) as starting materials. Elemental analysis (C ¾ 44.8%, H ¾ 4.13%, N ¾ 5.83%, S ¾ 7.40%) revealed a molar ratio of N/S of 1.80, consistent with the formulation of the polymeric structure shown in Scheme 3 and reported previously.15,16 The oxidized polymer of DMcT was synthesized as previously described7 using 2,5-dimercapto-1,3,4-thiadiazole dipotassium salt (K2DMcT) (Aldrich) as starting material. All other materials were reagent grade or better and were used as received.Electrochemical measurementsElectrochemical measurements were made using conventional potentiostatic instrumentation. A Ag/AgCl reference electrode was used and all potentials are reported vs. this reference. For transmission spectroelectrochemistry, indium-doped tin oxide (ITO) electrodes were used (Delta Technologies, s < 20W). For reflectance spectroelectrochemistry, a locally designed and built (at USP) specularly reflective Au electrode was used. This electrode was cleaned by cycling between the solvent limits in aqueous 0.5 mol L-1 H2SO4 prior to use. Observation of a reproducible gold oxide electrochemical response was taken as indicative of cleanliness. Locally designed and built spectroelectrochemical cells were used for both transmission (at UW) and reflectance measurements (at USP). In the

【 授权许可】

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