Aldous Leigh, Black Jeffrey J, Elias Maximo C, Gélinas Bruno, Rochefort Dominic
Department of Chemistry, King's College London, London, SE1 1DB, UK.
Phys Chem Chem Phys. 2017 Sep 13;19(35):24255-24263. doi: 10.1039/c7cp04322h.
Entropic changes inherent within a redox process typically result in significant temperature sensitivity. This can be utilised positively or can be a detrimental process. This study has investigated the thermoelectrochemical properties (temperature-dependant electrochemistry) of the ferrocenium|ferrocene redox couple in an ionic liquid, and in particular the effect of covalently tethering this redox couple to fixed positive or negative charges. As such, the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide was employed to dissolve ferrocene, as well as cationic-tethered ferrocene (the 1-ethyl-3-(methylferrocenyl)imidazolium cation) and anionic-tethered ferrocene (the ferrocenylsulfonyl(trifluoromethylsulfonyl)imide anion). These systems were characterised in terms of their voltammetry (apparent formal potentials, diffusion coefficients and electron transfer rate constants) and thermoelectrochemistry (temperature coefficients of the cell potential or 'Seebeck coefficients', short circuit current densities and power density outputs). The oxidised cationic species behaved like a dicationic species and was thus 6-fold more effective at converting waste thermal energy to electrical power within a thermoelectrochemical cell than unmodified ferrocene. This was almost exclusively due to a significant boost in the Seebeck coefficient of this redox couple. Conversely, the oxidised anionic species was formally a zwitterion, but this zwitterionic species behaved thermodynamically like a neutral species. The inverted entropic change upon going from ferrocene to anion-tethered ferrocene allowed development of a largely temperature-insensitive reference potential based upon a mixture of acetylferrocene and ferricenyl(iii)sulfonyl(trifluoromethylsulfonyl)imide.
氧化还原过程中固有的熵变通常会导致显著的温度敏感性。这既可以被积极利用，也可能是一个有害的过程。本研究调查了二茂铁鎓/二茂铁氧化还原对在离子液体中的热电化学性质（温度依赖电化学），特别是将该氧化还原对共价连接到固定正电荷或负电荷的影响。因此，使用离子液体1-乙基-3-甲基咪唑双（三氟甲基磺酰基）亚胺来溶解二茂铁，以及阳离子连接的二茂铁（即1-乙基-3-（甲基二茂铁基）咪唑阳离子）和阴离子连接的二茂铁（即二茂铁基磺酰基（三氟甲基磺酰基）亚胺阴离子）。这些体系通过其伏安法（表观形式电位、扩散系数和电子转移速率常数）和热电化学（电池电位的温度系数或“塞贝克系数”、短路电流密度和功率密度输出）进行表征。氧化态的阳离子物种表现得像二价阳离子物种，因此在热电化学电池中将废热能转化为电能的效率比未修饰的二茂铁高6倍。这几乎完全是由于该氧化还原对的塞贝克系数显著提高。相反，氧化态的阴离子物种形式上是两性离子，但该两性离子物种在热力学上表现得像中性物种。从二茂铁到阴离子连接的二茂铁时熵变的反转使得基于乙酰基二茂铁和二茂铁基（III）磺酰基（三氟甲基磺酰基）亚胺的混合物开发出了一种对温度基本不敏感的参比电位。
