Electrochemical CO2 reduction on perovskite catalysts

Abstract

Carbon Conversion and Utilization (CCU) is likely to play a predominant role in the transformation towards a sustainable energy production in the future. It has the potential of storing excess energy from renewable sources by converting CO2 into synthetic fuels, for instance, via the Fischer-Tropsch reaction. A promising method for CO2 splitting is direct CO2 electrolysis using Solid Oxide Electrolysis Cells (SOECs). They show excellent energy efficiency which is caused by their high operation temperature up to 900 °C1. Often, the research goal is to reduce the operating temperature for longer lifetime, which, however, increases the likelihood of coke formation, especially on the state-of-the-art Ni-YSZ cermet cathodes. Perovskite-type cathodes are a promising alternative, where detailed mechanistic understanding of the CO2 reduction reaction is required to further improve the cathode materials. In the work at hand, novel perovskite-type thin film electrodes with different A-site compositions, namely La0.6Ca0.4FeO3-δ (LCF), Nd0.6Ca0.4FeO3-δ (NCF) and Pr0.6Ca0.4FeO3-δ (PCF), were prepared using Pulsed Laser Deposition (PLD) and photolithography. These cathode materials were investigated with respect to their electro-catalytic performance with Electrochemical Impedance Spectroscopy (EIS) and Direct Current (DC) measurements. By combining EIS and DC measurements with an in-situ and operando, lab-based Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS) setup, the simultaneous determination of the chemical composition on the surfaces of the electrodes was possible at temperatures between 500 °C to 800 °C in a CO/CO2 gas mixture at a pressure of 1 mbar. The results showed that LCF exhibits the best electro-catalytic performance across the entire temperature range. This led to the conclusion that the change of A-site cation to Nd and Pr has no positive effect on the electro-catalytic activity of the materials for direct CO2 splitting. Results from NAP-XPS measurements confirmed the presence of a carbonate intermediate, which is converted fastest by LCF. The current-voltage characteristics of all materials showed an exponential behavior, which is most likely caused by the higher electron concentration on the surface with increased cathodic overpotential. In addition, metallic Fe appears under sufficiently high cathodic bias. In a detailed analysis of the current-voltage characteristics in combination with NAP-XPS measurements, a hysteresis was observed which seems to be correlated to the formation of metallic Fe0-nanoparticles upon cathodic polarization. Since the current density is reproducibly decreased as soon as Fe0 is present on the surfaces of the electrodes, the conclusion is drawn that Fe0-exsolution can be seen as a degradation phenomenon in the case of direct CO2 splitting.