Enabling pure CO2 reduction on Ni-YSZ electrodes: Operando studies indicate an oxide-mediated mechanism

Ni-YSZ-based electrodes are well-established in the field of solid oxide technologies. Ni/YSZ-based architectures have well-known performance and mechanical properties besides well-established manufacturing processes. Solid oxide-based CO2 electrolysis on Ni-YSZ at high temperature requires the presence of H2 in the CO2 inlet stream. It is believed that in pure CO2 streams the reaction fails due to oxidation of the Ni-YSZ electrode. Using operando Raman spectroscopy and online mass spectroscopy, we have shown that CO2 can in fact be reduced on the Ni-YSZ surface. Our measurements, reveal that Ni-YSZ oxidizes to NiOx-YSZ within a pure CO2 stream. CO2 electrolysis is possible on this oxide surface via a surface oxygen and vacancy-mediated mechanism similar to those observed within other oxide cathodes such as CeOx. The deactivation of the electrode coincides with strongly reducing conditions and at current densities > 400 mA/cm2, NiOx is completely reduced coinciding with complete stoppage of CO production. Cu-impregnation into Ni-YSZ was demonstrated to mitigate the deactivation issue by forming a more stable surface oxide on Ni which continued to carry out CO2 reduction under strongly reducing conditions. The new electrode demonstrated improved kinetics and stability against carbon deposition via Bouduard reaction.