Voltammetric Determination of the Iodide/Iodine Formal Potential and Triiodide Stability Constant in Conventional and Ionic Liquid Media

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• 作者：Cameron L. Bentley ; Alan M. Bond ; Anthony F. Hollenkamp ; Peter J. Mahon ; Jie Zhang
• 刊名：Journal of Physical Chemistry C
• 出版年：2015
• 出版时间：October 1, 2015
• 年：2015
• 卷：119
• 期：39
• 页码：22392-22403
• 全文大小：536K
• ISSN：1932-7455

文摘

The iodide/triiodide/iodine (I^{鈥?/sup>/I₃鈥?/sup>/I₂) redox system has been the subject of electrochemical investigations for well over half a century and remains a contemporary research interest due to the integral role of the I鈥?/sup>/I₃鈥?/sup> couple in dye-sensitized solar cell (DSSC) technology. In this study, we have calculated the formal potential (E0鈥?/sup>) of the I鈥?/sup>/I₂ process and the stability constant (K_stab) of I₃鈥?/sup> in two protic solvents (water and ethanol), two aprotic solvents (acetonitrile and propylene carbonate), eight aprotic ionic liquids (AILs), and one protic ionic liquid (PIL) using the voltammetric methodology developed herein. Furthermore, using 1-ethyl-3-methylimidazlium bis(trifluoromethanesulfonyl)imide (abbr. [C₂mim][NTf₂]) as a 鈥渕odel鈥?ionic liquid-based DSSC electrolyte system, we have also investigated the influence of three common additives/impurities in DSSCs (i.e., tert-butylpyridine, Li+, and water) on the parameters E0鈥?/sup>(I鈥?/sup>/I₂) and K_stab and characterized two analogous redox systems, Br鈥?/sup>/Br₃鈥?/sup>/Br₂ and SeCN鈥?/sup>/(SeCN)₃鈥?/sup>/(SeCN)₂. E0鈥?/sup>(I鈥?/sup>/I₂) and K_stab(I₃鈥?/sup>) increase in the order ethanol 鈮?acetonitrile < propylene carbonate < AILs < PIL < water; and water < ethanol 鈮?PIL < acetonitrile 鈮?AILs < propylene carbonate, respectively. In the presence of the additives/impurities (see above), E0鈥?/sup>(I鈥?/sup>/I₂) and K_stab increase in the order 0.5 M tert-butylpyridine < neat [C₂mim][NTf₂] 鈮?0.3 M Li+ < 2 wt % water and 0.5 M tert-butylpyridine 鈮?2 wt % water < 0.3 M Li+ 鈮?neat [C₂mim][NTf₂], respectively. Finally, E0鈥?/sup>(X鈥?/sup>/X₂) and K_stab(X₃鈥?/sup>) increase in the order SeCN鈥?/sup>/(SeCN)₂ 鈮?I鈥?/sup>/I₂ < Br鈥?/sup>/Br₂ and (SeCN)₃鈥?/sup> 鈮?Br₃鈥?/sup> < I₃鈥?/sup>, respectively, in [C₂mim][NTf₂]. The trends in the (pseudo)halide/(pseudo)halogen formal potentials and tri(pseudo)halide stability constants have been rationalized in terms of the physicochemical parameters (i.e., polarity, Gutmann donor/acceptor numbers, ionic strength, etc.) of the respective solvent/ionic liquid media.}