Year

2014

Degree Name

Doctor of Philosophy

Department

Intelligent Polymer Research Institute

Abstract

A possible solution to the worlds growing energy problem is to improve the efficiency of energy conversion processes by harnessing waste heat. Thermogalvanic systems, also known as thermocells, allow the direct conversion of thermal to electrical energy. A device consists of two electrodes operating at different temperatures and placed in contact with a redox-based electrolyte. The temperature dependence of the redox reactions generates a potential difference such that when the thermocell is connected to an external circuit, current and power may be extracted.

The initial part of this work focused on an investigation of K3Fe(CN)6/K4Fe(CN)6 concentration (in aqueous media) as the electrolyte for thermocell applications with carbon based electrodes. Thermocell performance is maximised through the use of 0.4 M K3Fe(CN)6/K4Fe(CN)6 due to the low thermal conductivity and high ionic conductivity of said solution.

Various thermocell designs were developed to understand how electrodes should be positioned to achieve increased power output. With electrodes placed far apart, a large temperature difference between electrodes may be attained. However, this results in a lower current density because of the larger distance that ions need to diffuse in the thermocell. Electrodes also need to be oriented such that reaction products from the cathode only need to travel in a straight path to get to the anode, and vice versa. A Tshaped thermocell produced the highest power density due to the optimum electrode position.

Composite electrodes of single-walled carbon nanotubes (SWNT) and reduced graphene oxide (rGO) for thermocell applications were developed and tested. The best performing electrode had a composition of 90 % SWNT and 10 % rGO by weight and a thickness of 4.5 μm. This optimised electrode favours rapid electrolyte diffusion and more efficient access to the electroactive surface area. Further enhancements in thermocell performance were realised through the use of a novel stacked electrode configuration, which consists of 10 alternating layers of the optimised SWNT-rGO film and stainless steel. Using the stacked electrode, a power conversion efficiency relative to a Carnot engine (Φr) of 2.63 % was attained. To date this Φr is the highest reported value in thermocells.

The optimised SWNT-rGO composite was incorporated into carbon cloth in order to investigate the effect of vascular electrode structures on thermocell performance. The maximum power density (585 mW/m2) is obtained by dip coating the carbon cloth with 5 layers of the SWNT-rGO composite. Excessive amounts of the nano-carbon composite blocks the pores in the carbon cloth and reduces the performance of the vascular structure.

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