Performance And Long-Term Durability Of Direct-Methane Flat-Tube Solid Oxide Fuel Cells With Symmetric Double-Sided Cathodes

One of the key strengths of solid oxide fuel cell（SOFCs）systems is their excellent fuel flexibility,especially the potential to utilize directly natural gas,biogas,or other hydrocarbon fuels without employing expensive external reforming processes.Direct internal reforming（DIR）SOFCs that the methane is directly fed and reformed at the anode to produce hydrogen for power generation,such as the steam and CO₂ reforming of methane,have higher overall system efficiency.Unfortunately,the main issue is the coking formation by the following reactions:（i）cracking reaction of CH₄,（ii）CO disproportionation and（iii）reverse water gas reaction when using the most widely-used Ni-YSZ cermet as the anode for direct internal-reforming SOFCs with methane-based fuels.In this study,we investigated the performance and stability of a newly developed symmetric flat-tube solid oxide fuel cells with double-sided cathodes（DSC cells）which was directly fueled with methane（natural gas,biogas）.The effect of steam/carbon（S/C）ratio,CO₂ to CH₄ ratio,temperature,and current density on the performance,and long-term stability of the DSC as well as the carbon deposition of the anode were investigated in details.The main works and consequences are listed below:（1）When the DSC cell was using pure H₂ fuel,the power density of the cell at 0.8 V reached560 mW.cm^-2 at 750℃and the corresponding fuel utilization was calculated to be 57.5%.Such performance is between the YSZ-based anode-supported planar and tubular SOFCs as expected.The cell experienced ca.2030 h discharge operation,4 times of thermal cycles,and short periods of gas depletion accidents,and the degradation rate is calculated to be ca.10%per thousand hours for the discharge period between each two Stops.The creep behavior of the steel screws,the change in the microstructure,and the failure of gas sealing parts are considered to be responsible for the performance degradation.However,it is still in lack of evidence to determine which factor contributed the most to the performance degradation.（2）When the DSC cell was directly fueled with humidified methane,the thick anode support and inner channels of the DSC formed an efficient microreactor for steam-reforming of methane,resulting in high conversion rate of methane and CO selectivity.In particular,when the S/C was 2,the conversion of CH₄ at 750°C achieved 100%in the DSC and no carbon deposition was observed.Moreover,the voltage of DSC with was stable throughout 190 h under a discharge current density of 0.257 A cm^-2.（3）When the DSC cell was directly fueled with methane and CO₂,the thick anode support and inner channels of the DSC formed an efficient microreactor for dry-reforming of methane,resulting in high conversion rate of methane and CO selectivity.The anode functional layer did not suffer from any carbon deposition during the internal dry-reforming of methane in regardless of the CO₂/CH₄ ratio.In particular,the DSC was stably operated for 500 h in CO₂/CH₄=2 with a constant current density of 0.2 Acm^-22 at 750°C,suggested that the poor-quality biogas（low levels of methane constitution）can be used efficiently for power generation and the production of synthesis gas（CO+H₂）by SOFC with direct internal dry reforming.For CO₂/CH₄=0.5 feed,the stable operation of the DSC within the catalyst in the channels（¹5h stable operation）was to some extent improved compared to that without catalyst（⁵h stable operation）.The addition of the catalysts as the in-cell reformer in the internal fuel channels and the multi-stage process,potentially eliminating the risk of carbon deposited on the support anode,which may be a feasible and facile method to prolong cell life in the carbon deposition conditions.