Date of Award

1-1-2013

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Xingjian Xue

Abstract

Multi-physicochemical models are developed for solid oxide fuel cells and electrolysis cells. The models describe the complicated transport processes of charge (electron/ion) conservation, mass/species conservation, momentum conservation, and energy conservation. Transport processes are coherently coupled with chemical reforming processes, surface elementary reaction processes, as well as electro-oxidation processes of both hydrogen and carbon monoxide. The models are validated with experimental data and utilized for fundamental mechanism studies of SOFCs fueled with different type of fuels, such as hydrogen, hydrocarbon, e.g., methane, H2S, and their mixtures. The fundamental mechanisms associated with syngas generation using electrolysis cell are also extensively investigated using the developed model.

The simulation results of SOFCs show that that the Nernst potential EH2 shows a strong correlation with the cell voltage, increasing with increasing the cell voltage. The ECO shows a weak dependence on the cell voltage, especially at the anode/electrolyte interface. Suitable H2S content in CH4 fuel is beneficial to improve the reforming process of CH4 and SOFC electrochemical performance particularly H2-H2O electro-oxidation process. The adsorbed surface species are very sensitive to the variations of the supplied hydrogen and oxygen as well as the cell voltage. To mitigate potential surface carbon deposition, one may: (1) suitably increase H2O content in the fuel; (2) reduce the content of CH4, CO, CO2 in the supplied fuel; (3) increase the operating temperature; (4) increase the cell operating current; (5) improve exchange current density of electrodes.

The simulation results of electrolyzer cell indicate that: (1) the intensity of surface electrolysis processes appears to be strong at the H2 electrode/electrolyte interface even though the composite electrode is assumed; (2) the surface electrolysis processes of CO2 and H2O are pretty much independent with each other; (3) the carbon coking effect is mainly determined by the fraction of CO2 in the H2 electrode; (4) high cell voltage conditions may cause the enhancement of the surface coverage of C(s) and the deposition of carbon on the surface of Ni catalyst; (5) high operating temperature may effectively improve adsorption/desorption rate, and enhance surface electrolysis process as well as potentially mitigate carbon deposition on Ni surface.

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