Enhancing thermoelectrochemical properties by tethering ferrocene to the anion or cation of ionic liquids: altered thermodynamics and solubility

Abstract

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.

The thermoelectrochemistry of the aqueous iron(ii)/iron(iii) redox couple: significance of the anion and pH in thermogalvanic thermal-to-electrical energy conversion

“…The trend in S e values across the redox couples can be rationalised in terms of acidbase equilibrium and ion pairing. The size and valence of the redox centre is highly influential in determining the S e value, 16 and an ideal Fe 2+ and Fe 3+ redox couple is expected to have a high S e value by virtue of their small size and high ionic charge. 11 However, it's well established that such iron salts undergo association with water and anions which will reduce their overall charge and therefore their redox entropy change; 11 this is especially true with Fe 3+ and anions such as [SO 4 ] 2-, which can even form oligomeric [Fe(SO 4 ) n ] 3-2n complexes.…”

“…As shown in Figure 8( Figure 6); this effect is therefore dominated by the increase in the S e due to changes in speciation. Since an increased S e exerts a very significant effect upon the efficiency of the heat-to-current transfer, 16 it results in a higher current as well as a higher potential, both boosting power output. In the case of FeCF 3 SO 3 and FeNO 3 , acidification reduces both R S and R ET ; further work is required to identify which of the many components involved in acidification (changed speciation, reduced ohmic drop, changes in surface electrocatalytic sites, etc.)…”

“…12,14 The thermoelectrochemistry of a number of ferrocene/ferroceniumbased systems have been reported, all in ionic liquids, with S e values ranging from -0.088 ± 0.02 mV K -1 to +1.67 ± 0.05 mV K -1 . 12,[14][15][16] The 'un-complexed' Fe 2+ /Fe 3+ system, investigated as their [NTf 2 ] --based salts in an [NTf 2 ] --based ionic liquid, gave an S e of +0.96 ± 0.04 mV K -1 . 12 Therefore the S e for aqueous Fe 2+ / Fe 3+ is expected to reach between ca.…”

Thermoelectricity and Thermodiffusion in Magnetic Nanofluids: Entropic Analysis

“…To be in-line with this trend, here we loosely employ the term "Seebeck" to delineate the thermoelectric potential generation across two electrodes in thermogalvanic cells, due to both the oxidoreduction reactions and the internal thermoelectric field (see next section for more precision)) effect (in the order of 1 to 10 mV/K) in complex liquids. (See for example: ionic liquids ([ 3 , 4 , 5 , 6 , 7 ]), aqueous ([ 8 , 9 , 10 , 11 ]) and mixed-electrolytes ([ 12 , 13 ]), and gelled electrolytes ([ 14 , 15 ])).…”

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“…In another study conducted by Abraham, IL electrolyte was achieved using Co II/III tris(bipyridyl) redox couple in 1-ethyl-3methylimidazolium tetracyanoborate IL that showed the highest power density (240 W m . It was showed that tethering a fixed charge to a desirable redox centre could simultaneously improve solubility and entropic changes [106]. Black, et al showed that lithium bis(trifluoromethylsulfonyl)imide (Li[NTf2])/tetraglyme(G4) mixing resulted in thermoelectrochemical electrolytes when combined with lithium metal electrodes [107].…”