Operando Studies on High-Temperature CO2 Electrolysis to Fuels

High temperature CO2 electroreduction of one of the most promising technologies for converting CO2 to fuels. Electroreduction can typically result in various reduced products but interms of energy efficiency a electrolytic reduction to CO followed by Fischer Tropsch (along with H2) based conversion to hydrocarbon fuels is most preferred. Nickel-Yttria Stabilized Zirconia cermet (Ni/YSZ) is one of the most common electrodes used in solid oxide fuel cells. Lately, various groups have essentially repurposed the same electrode for CO2 electrolysis. However, it suffers from catalyst degradation and poor activity. Successful operation of these electrodes requires presence of H2 in the CO2 stream. In presence of hydrogen CO2 undergoes reverere water gas shift reaction leading to production of CO (non electrochemically) in combination with electrochemical water splitting. Using operando Raman and online Mass Spectroscopy we have followed the evolution of catalyst from preatreatment in various atmospheres to its performance under conditions of operation. The mentioned electrodes were operated at current densities between 100-400 mA/cm2. The operando studies show that Ni active sites oxidize to NiOx in the presence of pure CO2. The poor electronic conductivity of NiO adversely affects the activity of the electrode. The failure is rather drastic at higher current densities. Apart from this, mass spectroscopy confirmed the formation of coke during CO2 electrolysis through Bouduard reaction. The absence of any Raman signal for carbon indicated that the reaction only proceeds to a small extent and the primary deactivation occurs through oxidation of Ni. Small amounts of H2 (~5%) in the mixture prevented the deactivation, but the reaction seemed to proceed through Reverse Water Gas Shift reaction followed by Water electrolysis. Furthermore, we have explored the performance of Ni(Mx)/YSZ type cathodes for CO2 reduction. The electrode performances were compared using three electrode configuration based measurements. Our results indicate that catalysts development in situ is drastically different for mixed metal oxides often resulting in different type of spatial distribution of the metals compared to the pure Ni/YSZ. Many electrodes show promising behavior in terms of both enhanced activity and catalyst stability. Figure 1