Study On CO₂/H₂O Co-electrolysis By Symmetric Flat-tube Solid Oxide Cells

The high-temperature co-electrolysis not only realizes the conversion and utilization of CO₂,reducing the environmental problems caused by the greenhouse efficiently,but also improves the efficiency of its use with renewable energy,which is helpful to the development of new energy industry and the solution of energy problems.However,the progress of co-electrolysis based on the solid oxide cell is very complicated.It is operated in harsh environment such as high temperature and humidity for a long time,so the durability needs to be further improved.The research group proposed and prepared symmetrical flat-tube solid oxide fuel cells with symmetric cathodes（DSC）,which have better mechanical properties than the flat plate structure and meet certain electrochemical performance.Therefore,in this thesis,the co-electrolysis experiments are carried out on these DSCs.The effects of various operating conditions,such as temperature,fuel electrode reactants,current,and voltage on the electrochemical performance of co-electrolysis were discussed.The electrochemical performance of co-electrolysis in the range of650℃-800℃is grasped.The dynamics of the fuel electrode at 750℃,such as the competition between vapor and CO₂ as well as the effect of voltage on impedance spectrum are given.The durability at different fuel electrode reactants is performed at750℃.DSC fails due to the oxidation of nickel-based electrodes at high reactant content,but the decay rate is 14.7%/138 h at low reactant content.The feasibility of DSC was verified during 11 cycles of electrolysis/discharge mode.The attenuation of electrolytic mode was significantly higher than that of discharge mode.At the eleventh cycle,the electrolytic decay rate reached 18.75%/10h.The attenuation of discharge mode was affected by electrolysis.The application research of reverse water-gas shift reaction（RWGS）over catalyst-Cu Fe/Al₂O₃ in co-electrolysis of DSC was carried out.At 700℃,the DSC-RWGS system has improved CO₂ conversion compared to DSC without catalyst at 0.0 A/cm² and 0.036 A/cm²,which still retain considerable electrochemical performance.The effects of temperature and fuel electrode reactants on the co-electrolysis performance of DSC-RWGS system were mastered.The addition of Cu Fe/Al₂O₃ catalyst does not change the competitive relationship between vapor and CO₂ at 700℃.The DSC-RWGS system has stable electrical and catalytic properties at open circuit voltage（OCV）for 18 h as well as 0.11 A/cm² for 101 h.The electrical performance deteriorates severely at 0.18A/cm².In particular,the decay rate is 14.29%/11.5 h and CO₂ conversion decreases slightly.Cracking of the sealant at the fuel electrode site causes voltage decay.However,whether higher current results in attenuation of electrolytic stability requires further verification.The attenuation mechanism of（La,Sr）（Co,Fe）O₃（LSCF）was analyzed at 750℃.The effects of temperature,uniaxial compressive stress,and contact material type on the electrochemical performance of LSCF were mastered.The results show that applying a certain uniaxial compressive stress to the LSCF electrode helps to improve the stability of the electrical properties of LSCF and inhibits the formation of SrO on the surface.The contact between YSZ and LSCF is more likely to cause Sr enrichment on the surface of LSCF.Electrolytic polarization at 0.2 A/cm² can delay the attenuation of LSCF or even improve its electrical performance,while inhibiting the enrichment of Sr on the surface of LSCF.