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Please use this identifier to cite or link to this item: https://dspace.lboro.ac.uk/2134/11031

Title: Rapid prototyping of electrode materials for fuel cells.
Authors: van Dijk, Nicholas J.
Issue Date: 2005
Publisher: © N.J. van Dijk
Abstract: This thesis concerns the optimisation of a redox flow cell known commercially as the Regenesys® system. The core chemistry involves the coupling of two redox couples, namely polysulfide/sulfide and bromide/tribromide, which together generate a cell voltage of 1.5V. A major goal was to develop a method to catalyse the redox reactions of the aqueous polysulfide/sulfide couple, which are presently the most inefficient parts of the system. This was successfully achieved. The main tool used throughout this study was screen-printing. This had the advantage that it permitted the emulation of full-sized industrial electrodes on the laboratory scale. In a significant breakthrough, we showed that it was possible to screen-print high porosity activated carbon electrodes with high reproducibility (<10% coefficient of variation). This allowed us to optimise numerous electrode properties such as binder loading, particle size, electrode thickness, pore size distribution and catalyst loading. After optimising the electrode properties it became possible to carry out electrode kinetic studies on high surface area electrodes (7 m2 per geometric square centimetre) having exceptionally short rise times « 5 seconds). These studies revealed, for the first time, that the polysulfide reduction current in the Regenesys® system was limited by a slow chemical step in solution. Highly accurate measurements using random assemblies of microelectrodes (RAMTM electrodes) then established that the activation energy of this chemical step was 73 ± 5 kJ morl and corresponded to the dissociation of s~- into S; and S;. This mechanism was confirmed by comparison of electrochemical reaction rates with chemical speciation diagrams. A large-scale screening programme of commercial activated carbons allowed us to identify three carbon properties that strongly influenced the reduction of polysulfide species. These were mesoporosity, backbone conductivity, and wettability. The ideal activated carbon was shown to have a median pore diameter of circa 10 nm, a highly graphitised backbone, and a wettable interior. The commercially available activated carbons that come closest to these properties were Chemviron APA, Norit RO, and Sutcliffe Speakman SRD/333/2. When tested, these gave a truly remarkable five-fold increase in reduction current compared with the present industry standard. In a second large scale screening programme, more than eighty compounds were tested for catalytic activity towards the reduction of polysulfide species. It was found that Cobalt (11) Phthalocyanine was the best catalyst in 1.0 M NazS4.6, and Cobalt(lI) Bis(salicylaldehyde) was the best catalyst in 1.0 M NazS3.4. The use of both compounds were patented. In the case of Cobalt (IT) Phthalocyanine it was found that the overpotential for the reduction of polysulfide species could be decreased by more than 500mV. Finally, the mechanism of catalysis was shown to be one of electronic superexchange. The positively charged cobalt centres in the catalysts were able to screen the electrostatic repulsion between the negatively charged electrodes and the negatively charged polysulfide ions, while the d 2 orbitals acted as conduits for electron z tunnelling. The existence of such through-molecule electron tunnelling probably explains why planar transition metal complexes are a common motif in biological systems.
Description: A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.
Version: Closed access
URI: https://dspace.lboro.ac.uk/2134/11031
Appears in Collections:Closed Access PhD Theses (Chemistry)

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