Performance And Degradation Mechanism Of Flat-tube Solid Oxide Electrolysis Cells For Co-electrolysis

High-temperature solid oxide electrolysis cell（SOEC）is an efficient energy conversion device that can convert electrical energy into chemical energy.In recent years,syngas production from H₂O/CO₂ co-electrolysis using SOEC technology has received a lot of attention because it can efficiently realize CO₂ conversion and utilization.During the long-term operation of the cell,the performance of the cell often gradually declined due to various factors,especially the high temperature and high humidity environment,which raised the requirement for electrode stability.Consider of the disadvantages of the structure of traditional flat SOEC.,the investigation of SOECs with suitable structures and good long-term stability had become the key to achieve large-scale commercialization of SOECs.In this work,the co-electrolysis performance and long-term stability of SOEC were investigated based on the flat-tube SOEC,and the behavior of Ni in high temperature and high humidity environment was studied by using Ni patterned electrode.The main results were as follows:（1）The effects of operating conditions（temperature,fuel gas components,current density,loading voltage,etc.）on the electrochemical performance,product composition and short-term durability of SOEC were investigated,and the electrode process of flat-tube SOEC was systematically analyzed.It was shown that the increase of operating temperature and the increase of water vapor proportion in the cathode gas contributed to the electrolysis performance of SOEC.And the five electrode processes of flat-tube SOEC were distinguished by electrochemical impedance spectroscopy and distribution of relaxation time methods.Short-term constant-current electrolysis experiments of SOEC at different current densities demonstrated the stability of the cell.At the same time,water vapor in the cathode compared to carbon dioxide could effectively mitigate the electrolytic performance decay,thus improving the stability of the cell.（2）Based on the investigation of the electrochemical performance of flat-tube SOEC for H₂O/CO₂ co-electrolysis,the durability of flat-tube SOEC under different operating modes was further investigated.The results showed that at a constant current of 300 m A cm^-2 at 750°C,the cell was operated for more than 1003 h with 40%H₂O+40%CO₂+20%H₂ atmosphere,with an electrolysis voltage degradation rate of 10.69%kh^-1(122 m V kh^-1).While run in charge/discharge cyclic mode for 24 cycles（576 h）,the electrolysis voltage degradation rate was higher,reached 32.36%kh^-1(342 m V kh^-1).In both test modes,the cell with charge/discharge cycles exhibited a higher degradation than the cell with constant current electrolysis.Electrochemical analysis showed that both cathodes and anodes showed degradation,but the degradation of the cathode was significantly greater.Microstructural analysis showed that the cathode degradation was attributed to Ni sintering and loss,and delamination was found at the anode/electrolyte interface of the cell after charge/discharge cycles.（3）To investigate the sintering and loss of Ni,a Ni film was plated on an yttrium oxide stabilized zirconia substrate（YSZ）by magnetron sputtering to form a three-phase boundary,and the sintering and migration behavior of Ni under different atmospheres as well as polarization conditions were investigated with this patterned electrode.At 800°C,Ni in H₂O atmosphere exhibited a faster sintering rate than that in H₂ atmosphere,and CO₂ replacing part of the H₂O slowed down the sintering rate of Ni.Under polarization conditions,the applied electrolytic voltage accelerated the sintering of Ni and caused the migration of the Ni on the surface of YSZ.