(558s) 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 Perovskites

Global average concentration of CO₂ in the atmosphere is rising at a rapid rate [1]. One attractive way to address this issue is to electrolyze CO₂ using a solid oxide electrolysis cell (SOEC) powered by a renewable source [2,3]. In fact, a mixture of H₂O and CO₂ can be efficiently co-electrolyzed in an SOEC to produce syngas (H₂+CO), which can further be used as raw material in a Fischer-Tropsch plant or an oxo process to produce liquid hydrocarbons and valuable chemicals. Due to the high operating temperature of SOEC, the electrical energy input required for electrolysis is lower than the low-temperature electrolyzers, which makes this process economically more feasible [3].

The SOEC used in this study consists of an yttria-stabilized zirconia (YSZ) solid oxide electrolyte that can conduct O^2- ions at elevated temperatures. The anode consists of a commercially available mixture of lanthanum strontium manganite (La_0.8Sr_0.2MnO₃) and YSZ, whereas the cathode consists of an inhouse developed doped lanthanum strontium ferrite perovskite oxide. H₂O and CO₂ get electrolyzed at the cathode to produce O^2- ions; these ions then travel through the YSZ electrolyte to the anode where they combine to form molecular oxygen. The state-of-the-art Ni-YSZ cathode often deteriorates in performance under such conditions [3]. Perovskite oxide materials, due to their high ionic and electronic conductivity, structural stability and catalytic tunability, can be promising cathode catalysts [3]. In this study, a Ni and Co doped La_0.7Sr_0.2FeO₃ (LSF) perovskite oxide synthesized by a sol-gel process is investigated as a cathode for CO₂ and H₂O co-electrolysis at 800°C. The reasoning behind using an A-site deficient LSF perovskite oxide is that an increase in A-site leads to formation of La₂O₃ and SrO on the surface as shown by XPS, and these oxides being alkaline in nature adsorb CO₂ more strongly, thereby causing formation of coke as indicated by XRD, XPS and SEM-EDX on post-reaction cathodes. The doped material shows high activity for production of syngas with tunable H₂/CO ratio [2]. Such a control over the syngas composition is necessary as production of different chemicals need H₂ and CO in different ratios.

Doping a fraction of the Fe-sites with Ni and Co leads to modification of the physical and chemical properties of LSF perovskite. In-situ XRD experiments under air and He environment show that the Ni- and Co-doped LSF materials are stable at the electrolyzer operating temperature of 800 °C. They have an orthorhombic perovskite structure at room temperature, which changes to rhombohedral at 400°C and becomes cubic at 800°C. A distortion in the oxygen sub-lattice was also detected in the doped materials by Raman spectroscopy, which affects the oxygen mobility and conductivity. Doping with Ni or Co or both improves the oxygen mobility as well as electronic conductivity of these ferrite materials. An increase in the Fe⁴⁺/Fe³⁺ charge carrier concentration is be the reason for such an improvement in electronic conductivity.

Faradaic efficiency for production of H₂ and CO decreased after doping with Co, whereas it increased to about 100% after doping with Ni or Ni+Co. This decrease in activity for the Co-doped sample can possibly be attributed to two reasons, namely (a) formation of coke as shown by post-reaction temperature-programmed oxidation and Raman spectroscopy, and (b) oxidation of the Co ions under current application as shown by in-situ XANES. Deposition of coke is not observed over the Ni-doped cathodes. Moreover, the composition of the produced syngas was found to be a function of the type and level of the dopant. In general, H₂/CO ratio in the syngas decreased with increase in Co amount in the cathode whereas it increased with an increase in the Ni amount. This difference in H₂/CO ratio is due to the stronger interaction of the Co-doped sample with CO₂ compared to the Ni-doped sample, as shown by in situ DRIFTS experiments during CO₂+H₂O adsorption-desorption. A long-term co-electrolysis test performed for more than 100 hours over a La_0.7Sr_0.2Ni_0.1Co_0.1Fe_0.8O₃ cathode showed stable electrochemical performance.

In conclusion, this study shows that SOEC can be a promising technology for conversion of CO₂ into fuels and chemicals. Doping of LSF cathode with Ni or Co not only enhances the electrochemical activity for CO₂ and H₂O co-electrolysis, but also helps to tune the H₂/CO ratio of the produced syngas as a function of the dopant level.

References

[1] E. S. R. Laboratory, Global Monitoring Division, Trends in Atmospheric CO₂ (2018).

[2] Deka, D. J., Gunudz, S, Fitzgerald, T., Miller, J.T., Co, A., Ozkan, U. S., Applied Catalysis B: Environmental 248 (2019) 487–503.

[3] Gunduz, S., Deka, D.J., and Ozkan, U.S., Advances in Catalysis (Chunshan Song) Vol. 62 p. 113. Elsevier, Cambridge, 2018.