Soft-templating of nanostructured materials for thermoelectric power harvesting and catalysis

Abstract

The ever increasing demand for energy along with climate change has stimulated much research in the field of green energy technology. Thermoelectric generators are one form of green energy technology that generates electrical power from waste heat harvesting. Current fabrication techniques for thermoelectric materials are costly and the efficiency of materials is low. This work uses electrodeposition as a cheap fabrication that allows for control of a large number of variables to optimise materials.

In the literature the doping of bismuth tellurium selenide has shown improvements in thermoelectric performance. Bismuth telluride and bismuth tellurium selenide have been electrodeposited before, but the electrochemical doping of bismuth tellurium selenide has not received much interest in the literature. In Chapter 3 bismuth tellurium selenide was doped with Cu to higher levels than ever reported before, by the addition of Cu(NO3)2 into the electrolyte. The level of Cu doping was shown to be linear with increasing Cu(NO3)2 concentration in the electrolyte, as determined by EDX. Cu doping was shown to exponentially decay the electrical conductivity, whilst concentrations between 1.25 mM and 1.75 mM Cu(NO3)2 were shown to increase the magnitude of the Seebeck coefficient, from -55 μV K-1 in the absence of Cu to a peak value of -390 μV K-1 when using an electrolyte concentration of 1.50 mM Cu(NO3)2. Whilst a slight improvement in power factor was seen for one sample made with 1.50 mM Cu(NO3)2 present in the electrolyte, this was not reproducible and on average Cu doping was not shown to improve thermoelectric performance. The electrochemical doping of bismuth tellurium selenide with Ag is reported for the first time and was shown to be linear with increasing Ag(NO3) concentration in the electrolyte. The addition of 0.25 mM Ag(NO3) in the electrolyte was shown to lower electrical conductivity to 296 S cm-1 from 564 S cm-1 in the absence of Ag(NO3). Further addition of Ag(NO3) into the electrolyte was shown to not diminish the electrical conductivity further. Ag doping also diminished the magnitude of the Seebeck coefficient. The most striking observation was the use of a SMSE reference electrode instead of a SCE reference electrode yielded the greatest improvement in thermoelectric performance of bismuth tellurium selenide films, producing a power factor of 0.33 mW m-1 K-1, compared to a value of 0.13 mW m-1 K-1 when using an SCE reference. The effect of different reference electrodes on the electrodeposition of bismuth telluride based materials has not been reported in the literature. The improvement in power factor is believed to be caused by the removal of Cl- ions, which may have been causing a detrimental effect on the electrodeposition of bismuth tellurium selenide films for thermoelectric applications.

Reduced dimensionality offers a strategy for increasing the efficiency of thermoelectric materials primarily by lowering lattice thermal conductivity. In Chapter 4 a lyotropic liquid crystal is used for the first time to template bismuth telluride. The use of a phytantriol template during bismuth telluride electrodeposition was shown to produce a disordered nanostructure containing nanowires with diameters as low as 6 nm. Prior to this work the smallest bismuth telluride nanowires reported in the literature were 15 nm. The composition of these nanowires was controlled by altering the composition of the electrolyte. A Seebeck coefficient of -88 μV K-1 was measured. The films were seen to oxidise, with oxygen content of 60 atomic % measured by EDX. This oxidation resulted in a low electrical conductivity of 19 S cm-1 being measured.

Whilst bismuth telluride is the best performing room temperature thermoelectric material, the rarity and toxicity of tellurium restricts the commercial viability of thermoelectric generators manufactured with bismuth telluride. Sulphur is an earth abundant element that is considered non-toxic and like tellurium sulphur is a group 16 element. This makes bismuth sulphide a potential alternative to bismuth telluride. In Chapter 5 planar bismuth sulphide films were successfully electrodeposited at a potential of -0.4 V vs SCE form an electrolyte created using 100 mM Bi(NO3)3, 100 mM Na2S2O3 and 200 mM EDTA in deionised water. A room temperature Seebeck coefficient of -29.8 μV K-1 was recorded. The use of a phytantriol template resulted in the production of a single diamond nanostructure with a lattice parameter of 139.6 (± 3.2) Å, as shown by SAXS and TEM. The electrodeposit was the first ever single diamond phytantriol templated semiconductor material. A lower Seebeck coefficient of -12.3 μV K-1 was recorded for the nanostructure.

Fuel cells are promising technologies for transport and portable power generation due their ability to continually generate electricity when supplied with fuel. Current fuel cells are electrocatalysed by expensive Pt based materials. Pd is a more earth abundant and cheaper material with the potential to be used instead of Pt for the electrocatalyst in fuel cells. In Chapter 6 a phytantriol template was used for the first time to electrodeposit Pd with a high surface area per mass of 30.8 m2 g-1, and a high surface area per volume of 3.66 × 106 cm2 cm-3. The large surface area was due to a single diamond nanostructure being formed with a lattice parameter of 140.0 (± 4.6) Å, as determined by SAXS. This high surface area Pd nanostructure allowed for greater electrooxidation of methanol, ethanol and glycerol per gram of Pd when compared to Pd black and a film deposited in the absence of phytantriol.

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ORCID for Iris S. Nandhakumar: orcid.org/0000-0002-9668-9126

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