Electrochemical Performance And Deterioration Mechanism Of Flat-tube Solid Oxide Fuel Cells Operated With Methane

Solid oxide fuel cells（SOFCs）have a wide range of fuel options,including methane,which can be utilized in various ways,including dry reforming of methane（DRM）,methane steam reforming（MSR）,and partial oxidation of methane（POM）.In-situ monitoring is useful for assessing working conditions and understanding degradation mechanisms in cell performance and guiding the adjustment of SOFC operating conditions to improve cell performance and durability.However,the real-time and in-situ monitoring method for large-scale SOFC single cells is still needed.Therefore,in this paper,a real-time monitoring method for a large-scale flat-tube SOFC was designed and invented.By this method,the degradation mechanism of cell electrochemical performance and catalytic activity during long-term operation with methane dry reforming were investigated.The feasibility of power generation using hythane was explored based on the results of SOFC operating with DRM.Electrochemical performance,catalytic performance,and decay mechanism in these scenarios were also inquired about.Carbon deposition and Ni sintering were found in the cells operated with DRM and hythane.Therefore,patterned Ni-film-anode button cells were prepared and used for the investigation of the mechanism of Ni sintering and migration.The main work and corresponding conclusions of each part of the thesis are shown as follows.（1）A method for real-time and in situ monitoring of gas composition concentration and temperature field distribution in the fuel channel of SOFC anode was designed and invented.The method was applied to a cell operated with DRM over 1000 h.A gradual decrease in the catalytic activity of the anode Ni-YSZ for DRM was found to occur along the fuel flow direction starting from the cell inlet region.The decrease in catalytic activity brought about a change in the catalytic and electrocatalytic reaction regions,and the consequent change in the thermal effect of the cell anode,as reflected by the change in anode temperature.The morphological characterization of different regions of the anode and the laser-confocal Raman spectroscopy of the working cell revealed a decreasing distribution of carbon accumulation and Ni particle sintering along the direction of fuel flow.The performance degradation mechanism of SOFC for long-term DRM power generation is discussed by combining in-situ monitoring results with post-hoc analysis.（2）The feasibility,electrochemical performance,and anode catalytic performance of SOFC for power generation using hythane were investigated in this section.Then the degradation mechanism was discussed by morphological characterization and elemental analysis of the working cell as well.The CO₂ content of the fuel mixture was gradually reduced to the minimum level which equals that of biohythane composition.A stable discharge of 150 h with a CO₂/CH₄ molar ratio of less than 2was achieved.Compared with the DRM,carbon accumulation inside the cell was reduced but significant carbon accumulation was observed in the inlet gas pipes.The sintering of Ni particles near the inlet port was obvious,similar to that of the long-term DRM operating cell.（3）The sintering and migration behavior of Ni under methane reforming discharge conditions was studied in a simplified test environment using patterned Ni-film-anode button cells.The H₂discharge process was selected as a reference.Then,the sintering behavior of Ni film under DRM and MSR conditions was compared.As a result,the disconnection between Ni film and the YSZ substrate under current and steam was found.The comparison also revealed that the high steam concentration in MSR conditions promotes Ni sintering more significantly,a phenomenon that may limit the durability and stability of SOFC in long-term operation.