Study of Oxygen Dissolution in Molten Tin: A Novel SOFC Anode
Creators: name: Agbede, Oluseye Omotoso
Publisher: name: Imperial College London
Year: 2014

description: Conventional power plants for the conversion of fossil fuels to electricity have low efficiencies and produce large amount of carbon dioxide, a greenhouse gas, which contribute to climate change. Hence, a molten tin reformer and methane-fuelled SOFC with molten tin anode (Sn(l)-SOFC) for easier CO2 capture and higher power efficiency were investigated. Both systems involved oxygen dissolution in molten tin and methane reaction with the dissolved oxygen, as well as gas bubbling, so oxygen dissolution and methane reaction at bubble | molten tin interface were investigated. Oxygen was separated successfully from a 10%O2-He blend through gas bubbling and dissolution in molten tin which suggests that oxygen may be separated from air in the molten tin reformer by bubbling air through molten tin in the first stage of the periodic process. An LSM-YSZ/LSM double-layered reference electrode and YSZ electrolyte potentiometric oxygen sensor was used to measure the concentration of dissolved oxygen in molten tin; hence, enabling derivation of the solubility limit and Gibbs energy change for the formation of SnO which was in equilibrium with oxygen at the solubility limit. The solubility of oxygen in molten tin in equilibrium with SnO in the temperature range 973 – 1123 K was ca. 0.019 - 0.107 atom%. The rate of oxygen dissolution in molten tin when 10%O2-He blend was bubbled through it was controlled by chemical reaction at the bubble | molten tin interface; the mechanism involved a first step of chemisorption to molten tin at the bubble | molten tin interface, forming SnO as the absorbed intermediate. The second step of the mechanism involved the dissociation of SnO to molten tin and oxygen atom incorporated in the molten tin. The rate limiting step was the dissociation of SnO into molten tin and oxygen atom. Likewise, the rate of deoxygenation of molten tin by 10%CH4-He was not limited by the diffusion of oxygen atoms in the molten tin but might be limited by surface reaction at the bubble | molten tin interface. The performance of the molten tin reformer and methane-fuelled Sn(l)-SOFC depends on bubble size and behaviour, so bubbles generated in molten tin were characterized by determining the sizes, shape, velocities, and behaviour under different operating conditions of nozzle diameter, gas flow rates and temperatures. A pressure pulse technique which incorporates a differential pressure transducer was employed successfully in the measurement of frequencies of bubble formation in molten tin at high temperatures in the range 973 -1173 K while the bubbles were approximated as oblate spheroids which wobbled. LSM cathodes were deposited on micro-tubular YSZ electrolytes and the microstructures and electrical conductivities characterized by scanning electron microscopy (SEM) and four-point probe resistance measurement, respectively. SEM micrographs showed the densification of LSM cathodes with increased sintering temperature, which resulted in increased electrical conductivities. Potential difference-current density data and impedance spectra were determined for a methane-fuelled SOFC with molten tin anode. A peak power density of about 100 W m-2 at a current density of 222 A m-2 and potential difference of 0.45 V was obtained for the methane-fuelled SOFC with molten tin anode at 850 oC. Impedance spectra showed that ohmic potential losses controlled the reactor performance, with about half of those arising from the inherent difficulty in achieving a low resistance contact at the (Ag wire) Ag wool current collector | LSM cathode interface.