Production of syngas with controllable H2/CO ratio by high temperature co-electrolysis of CO2 and H2O over Ni and Co- doped lanthanum strontium ferrite perovskite cathodes

Abstract

Conversion of CO2 and H2O into synthesis gas (a mixture of H2 and CO) in a high temperature solid oxide electrolysis cell (SOEC) is an attractive route for CO2 utilization. Depending on the composition of the syngas, it can be directly used as a fuel or fed to an oxo process or a Fischer-Tropsch process to produce value added chemicals. Designing an efficient and stable cathode for an SOEC that can yield syngas with controllable H2/CO ratio is of fundamental interest. In the current study, Ni and Co- doped A-site deficient perovskite materials of the form La0.7Sr0.2NixCoyFe1-x-yO3 (x,y = 0; x = 0, y = 0.2; x = 0.1, y = 0.1; x = 0.2, y = 0) are studied as SOEC cathodes for co-electrolysis of CO2 and H2O at 800 °C. Modifications of the surface and bulk properties of these materials due to doping were investigated using in-situ XRD, XPS, Raman spectroscopy, electronic conductivity measurements, oxygen mobility test, in-situ DRIFTS and XANES. The Co- doped perovskite La0.7Sr0.2Co0.2Fe0.8O3 showed the lowest Faradaic efficiency and the ones doped with Ni showed nearly 100% Faradaic efficiency for total production of H2 and CO where the H2/CO ratio in the produced syngas increased with increasing Ni content in the cathode material. This ratio could be controlled by tuning the B-site dopant levels, cell voltage and H2O/CO2 ratio in the cathode feed stream. In-situ XANES studies showed that during CO2 and H2O co-electrolysis, the Co ions in La0.7Sr0.2Co0.2Fe0.8O3 may get oxidized, thus possibly reducing the number of oxygen vacancies in the material and hence lowering the electrochemical activity. Moreover, post-electrolysis analyses show that the Co- doped cathode forms graphitic carbon, which lowers the Faradaic efficiency for syngas production. No graphitic carbon formation was observed on the Ni- doped cathodes. A long-term co-electrolysis test performed for approximately 110 h on a La0.7Sr0.2Ni0.1Co0.1Fe0.8O3 cathode shows good stability of these materials in terms of electrochemical performance and coke resistance.