Redox mediators for thermoelectrochemical cells and electroanalysis of quinones in protic ionic liquids

Greatorex-Davies, Peter (2022) Redox mediators for thermoelectrochemical cells and electroanalysis of quinones in protic ionic liquids. PhD thesis, University of Nottingham.

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Abstract

Thermoelectrochemical cells offer promising prospects for future harvesting of waste heat in many areas, such as; power stations, geothermal energy and in devices utilising human body heat. There are two main sets of devices currently used in these areas. The most widely used devices are solid-state thermoelectric cells, but these suffer from low Seebeck coefficients, expensiveness of parts and lack of flexibility. Thermoelectrochemical devices utilising aqueous electrolytes are also used, however, under normal conditions these are limited to operating temperatures below 75°C.

In this report investigations have been conducted for several redox mediators dissolved in aqueous or ionic liquid electrolyte for use in thermoelectrochemical cells. Both aqueous [Fe(CN)6]3−/4− and I−/[I3]− redox couples showed reasonably similar results to the literature, and in both cases a good power output was obtained similar to that found in the literature. The Results for the non-aqueous I−/[I3]− solutions showed a decreased performance both in terms of Seebeck coefficient and maximum power output.

For the Q/QH2 redox couple this trend was the same with the aqueous results again providing a better performance than for the IL solutions. This is in spite of the lower analyte concentration due to the limit in solubility of Q in water.

Investigations for Fe(acac)2/3, V(acac)3/VO(acac)2 have also been carried out in triethylammonium trifluoroacetate ([tea][TFAc]). The Fe(acac)2/3 solution showed the best Se of 1.53 mV K−1 at temperatures up to 30 °C, which is comparable to that of the benchmark system, aqueous [Fe(CN)6]3−/4−, at the same concentrations. However, instabilities of the species led to very poor performance when attempting to generate useful power outputs.

The electrochemistry of quinones has been investigated in a wide variety of solutions and even in water, shows significant complexity. The variation in voltammetric response lies in the key role of protons in the reduction of Q to QH2. In well buffered water this reduction reaction is a simple reversible two electron wave. However, in unbuffered water and other non-aqueous media insufficient protons in the system result in other deprotonated forms becoming present.

Here investigations of the Q/QH2 mechanism in Protic Ionic Liquids (PILs) reveal different processes occurring in PILs synthesised with a strong acid, such as diethylmethylammonium triflate ([dema][TfO]) and PILs synthesised with a weaker acid, such as [tea][TFAc]. In [dema][TfO] the several Q/QH2 redox waves are observed corresponding to different electrochemical processes. These separate waves are concluded to be due the protons being sourced from the acid, hydronium ions, and the base, with QH2 and QH− being products of the Q reductions. By adding acid, the waves eventually merge to form a single, two-electron reduction of Q to QH2.

For [tea][TFAc], the results behave as a buffered aqueous system, and when adding either acid or base the voltammetry does not change significantly and remains a two-electron reduction to QH2.

These results demonstrate the usefulness of Q/QH2 voltammetry in characterising the acid content in various PILs with even small changes in the systems resulting in significant observable changes in the observed redox reactions.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Walsh, Darren
Dupont, Jairton
Keywords: Thermoelectrochemical cells, Redox mediators, Electroanalysis, Quinones, Protic ionic liquids
Subjects: Q Science > QD Chemistry
Faculties/Schools: UK Campuses > Faculty of Science > School of Chemistry
Item ID: 69019
Depositing User: Greatorex-Davies, Peter
Date Deposited: 02 Aug 2022 04:40
Last Modified: 02 Aug 2022 04:40
URI: http://eprints.nottingham.ac.uk/id/eprint/69019

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