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

Abstract

The multi-faceted significance of the anion upon the thermoelectrochemistry of the iron(ii/iii) redox couple has been extensively explored.

Using iron sulphate to form both n-type and p-typepseudo-thermoelectrics: non-hazardous and ‘second life’ thermogalvanic cells

“…Acidified solutions of [Fe(SO 4 )] have previously been used in thermogalvanic cells. 4,15,16,22 Specifically the addition of H 2 SO 4 to [Fe(SO 4 )] thermocell solutions significantly increases the S e (with reported values between +0.50 and +0.90 mV K −1 , depending upon concentration and pH), 4,15,22 although notably this is below the S e observed for un-acidified hexaaqua Fe 2+/3+ solutions (between 1.35 and 1.66 mV K −1 ). 16,22 The S e observed for hexaaqua Fe 2+/3+ solutions are also slightly increased by acidification.…”

“…This is slightly lower than previously reported values, with three similar un-acidified solutions also reporting S e in the range of +0.29 to +0.34 mV K −1 . 4,16,22 This is likely due to this study using a higher concentration of [Fe(SO 4 )] (which will reduce the S e ) and also due to not correcting the applied temperature difference to the experienced temperature difference (explained in the Experimental section). The values measured here are referred to as the 'apparent Seebeck coefficient' (aS e ) because they are only valid for the precise temperatures used (T c , T h and ΔT; explained in more detail below).…”

“…21 Besides this, other reported thermocell devices employ inherently or deliberately acidified aqueous solutions of Fe 2+/3+ (S e(tg) up to +1.76 mV K −1 ) in conjunction with aqueous solutions of [Fe(CN) 6 ] 3−/4− (S e(tg) up to −1.43 mV K −1 ). 4,[14][15][16][17] Fundamental studies have recently probed the Fe 2+/3+ system 22 and [Fe(CN) 6 ] 3−/4− system 23 in detail, as well as the fundamentals of their combinations electrically in-series and in-parallel. 4 These in-series thermocell devices have also been applied, e.g.…”

“…36 Despite this, reported thermocell works note using [Fe(CN) 6 ] 3−/4− solutions in close conjunction with a heat source and Fe 2+/3+ solutions containing up to 10 wt% HCl, 17 and even non-acidified aqueous solutions of Fe 2+/3+ are strongly inherently acidic due to water-of-hydration hydrolysis. 22 Table 1 summarises five reported works which used Fe 2+/3+ and [Fe(CN) 6 ] 3−/4− to prepare in-series thermocell devices; the produced electrical power values are reported, but for each a 'worst case scenario' is also explored, based upon the electrolytes in the reported devices accidentally mixing. It was assumed that the [Fe(CN) 6 ] 4− evolved HCN (g) , and the resulting HCN (g) concentration was calculated for five sealedenvironment scenarios.…”

The immobilisation and reactivity of Fe(CN)63−/4− in an intrinsically microporous polyamine (PIM-EA-TB)

“…1). Ferrocyanide has previously been employed as a redox mediator in electro-organic transformations [1], as an electron shuttle for respirating living cells [2], in thermoelectrochemical devices [3], in the electroanalytical determination of ascorbate [4], nitrite [5], hydrogen sulphide [6,7] or (via electrochemiluminescence) in the determination of thiols [8]. Ferrocyanide has been employed also as heterogeneous or polymer-immobilised redox catalyst for sulphide detection [9] and for nitrite detection [10].…”