The anode material in a conventional design of solid oxide fuel cell (SOFC) operating above 1123 K is typically made from NiO/Zirconia. NiO/Zirconia anodes are known to perform well in hydrogen but exhibit difficulties when natural gas is used as a fuel. Natural gas is much cheaper than hydrogen and widely available but causes carbon deposition and deactivation of the NiO/Zirconia SOFC anode. One objective of this work was to prepare and characterize NiO/Zirconia anodes both as powders and as applied to extruded zirconia tubes. The problem of carbon deposition when NiO/Zirconia anodes operate in methane, the main component of natural gas, was investigated. Another aim was to address the problem of coking with an effort to moderate carbon deposition by using additives to the NiO/Zirconia anode. Temperature programmed reduction (TPR) was used to study the reaction characteristicso f NiO/Zirconia anodes.T he carbon depositedo n thesea nodesa fter methane decomposition and reforming was characterized using temperature programmed oxidation (TPO). The anodes were placed in a reactor (stainless-steel, alumina or zirconia) tube in a test assembly developed for an extruded tubular SOFC. The reactor inlet was connected to a flexible gas handling system and the exhaust to a continuously sampling mass spectrometer. This system also allowed simultaneous study of electrical and catalytic measurementsThe various conditions for methane reforming have been shown to influence the character of carbon deposited and the quantity deposited. Conditions such as anode calcination temperature, anode reduction regime, reforming temperature and reforming time have all been shown to influence the reactions occuring on the anode including carbon deposition, subsequently characterized by TPO. NiO/Zirconia powders have also been shown to behave differently from NiO/Zirconia anodes adhered to zirconia tubes. An alkali metal additive has been shown to moderate carbon deposition and improve cell performance. Small additions of Li, typically 1 mol %, to the Ni/Zirconia anode cause a decrease in carbon deposition after reforming at temperatures of 1123 K and 1173 K. The activation energy of surface carbon removed by oxygen is lowered by approximately 50 kJ mot' for the 1 mol % Li doped Ni/Zirconia anode compared to the undoped powder. Anodes doped with Li displayed greater cell performances. The improvements seen with these additives show that their use could offer a viable alternative to conventional anodes in current SOFC systems. Tubular solid oxide fuel cells have been tested in a custom built rig whereby electronic and catalytic measurements can be sampled simultaneously. This was used to monitor the influence of drawing current on the reactions occurring on the anode. The presence of alkali Li on the doped Ni anode surface has been shown to interfere with surface reactions under electrochemical load/steam reforming
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:343244 |
| Date | January 2000 |
| Creators | Coe, Neil J. |
| Publisher | Keele University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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