Dr. Changrong Xia
Professor, Department of Materials Science and Engineering, University of Science and Technology of China, China

Speech Title: Direct CO₂ electrolysis in solid oxide electrolysis cell using ceramic cathodes based on molybdenum doped strontium ferrite

Abstract: The ever-increasing consumption of fossil fuels has caused substantial increase in atmospheric CO₂ concentration that has led to undesirable climate changes and more frequent natural disasters. Solid oxide electrolysis cell (SOEC) can efficiently convert CO₂ to CO and oxygen that are important chemicals and building blocks for industrial applications. Consequently, SOEC is a promising technology to utilize CO₂ and store electricity from renewable resources. However, the use of SOECs for direct CO₂ electrolysis has been hampered from the absence of a stable, highly catalytic active and cost effective cathode material that can demonstrate long term cell performance under high current densities for commercial applications. This report presents our efforts on developing ceramic porous composites based on redox-stable Sr₂Fe_1.5Mo_0.5O_6-δ (SFM) electronic-ionic mixed conductors as the electrocatalysts to electrolyze and convert 100% CO₂ to CO without using any safe gases like H₂ and CO. Samaria-doped ceia (SDC) and metal were used as the second phase to the composite. The SFM and SDC powders were synthesized by soft-chemical routes such as glycine-nitrate combustion method while NiFe nanoparticles were formed by in situ exsolvation . Full single cells and half single cells were prepared for electrochemistry analysis. The full cells were supported on LSGM (La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ electrolytes with LSCF (La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ oxygen electrodes. LDC (La_0.4Ce_0.6O_2-δ was used as the interlayer between LSGM electrolyte and fuel electrode to prevent the mutual element diffusion between LSGM and SFM in the cell fabrication and operation processes. Comparing with those reported for the typical oxide ceramic electrodes, high electrochemical performance has be demonstrated for single phase SFM cathode using 100% CO₂ as the feeding gas. For example, a current density of 0.71 Acm^-2 was obtained using a full cell supported on LSGM electrolyte operated at 800⁰C and an applied voltage of 1.5 V. The current density, 0.71 Acm^-2 is higher than almost all of the reports, from 0.05 to 0.47 Acm^-2 for electrolyzing pure CO₂ at 800 ^oC using redox stable oxide electrode materials. Meanwhile, the interfacial polarization resistance was reduced from 0.239 Ωcm² to 0.190 Ωcm². The SFM-SDC cathode demonstrated good durability when the cell was operated at 1.5 V and 800 °C for more than 100 h under harsh conditions of 1 Acm^-2 current density and using 100% CO₂ as the feeding gas. The electrolysis performance was improved by using SFM-SDC composite cathode, and the current density increased to 1.09 Acm^-2 under the same operation conditions. The performance is further improved by using SFM-NiFe composite with homogenously dispersed nano-sized NiFe alloy nanoparticles, which can enhance chemical adsorption and surface reaction kinetics for the CO₂ reduction reaction. SOECs with the NiFe nanoparticle enhanced SFM electrodes have demonstrated the highest electrochemical activity to date amongst all the reported electrodes towards direct CO₂ electrolysis without addition of reducing gas, with a peak current density of 2.16 Acm^-2 using an applied voltage of 1.5 V (vs. Pt reference electrode) at 800 ^oC. Furthermore, this novel cathode shows excellent sintering and coking resistance during 500 hrs operation under harsh conditions (current density above 1 A cm^-2 under an applied voltage of 1.3 V at 800 ^oC). In conclusion, SFM based electrocatalysts show promising performance as the electrode materials for direct CO₂ conversion in SOECs.

Acknowledgements: This work was supported by the National Natural Science Foundation of China (91645101)