Solid oxide electrolysis cell (SOEC) could efficiently and directly electrolyze CO2 and thus a potential technique toward carbon neutral cycle. The electrolysis efficiency is limited by CO2 reduction reaction at the cathode. This report presents our efforts on developing ceramic cathodes based on redox-stable Sr2Fe1.5Mo0.5O6-δ (SFM) electronic-ionic mixed conductors. Full cells were fabricated with LSGM (La0.9Sr0.1Ga0.8Mg0.2O3) electrolyte substrates and LSCF(La0.6Sr0.4Co0.2Fe0.8O3)-SDC (Samaria-doped ceia) oxygen electrodes for electrochemistry analysis. When single phase SFM was used as the cathode, a current density of 0.71 A×cm-2 was obtained at 800 °C and an applied voltage of 1.5 V using CO2 as the feeding gas. The electrolysis performance was increased to 1.09 A×cm-2 improved by using SFM-SDC composite cathode. The performance was 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. A 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 showed 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). Doping to SFM could also improve the performance for CO2 reduction reaction. For example, the substitution of oxygen by fluorine increases surface oxygen species concentration by 11%, CO2 adsorption by a factor of ~2, bulk oxygen vacancy concentration by 35% at 800 oC, and consequently enhances the surface reaction rate constant for CO2 reduction reaction and chemical bulk diffusion coefficient by factors of 2-3. Furthermore, the single cell achieved current density of 1.36 A cm-2 at 1.5 V and superior stability over 120 hours at 800 oC under harsh conditions. The theoretical computations further prove that fluorine doping is energetically favorable to CO2 adsorption and dissociation.

Carbon dioxide reduction via molybdenum doped strontium ferrite cathode in solid oxide electrolysis cell

2019 Fall National Meeting

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