Cu-Doped La0.3Sr1.7Fe1.5Mo0.5O6-σ Decorated with Exsolved Nanoparticles for the Improvement of Syngas Production in Solid Oxide Electrolysers
Authors: Błaszczak Patryk , Ducka Agata , Kwaśnica Aleksander , Wang Sea-Fue , Bochentyn Beata , Jasiński Piotr 
Published: December 2024

A major concern within modern society is the need to substitute the fossil fuels with more renewable sources of the energy. Chemical industry is one of major players in the field of the usage of the crude oil to produce more complex chemicals. Despite of being good source of carbon species, a dropping amount of the coal and oil is raising the questions for future solutions. Carbon dioxide can be promising and sustainable source of carbon, but its high C-O bond energy (806 kJmol-1) poses general problems in efficient utilization. The most common feedstock in chemical synthesis is the syngas, a mixture of H2 and CO, which can be simply processed into more complex molecules like e.g., methanol, dimethyl ether, or methane.

Highly inert CO2 can be transformed into more valuable CO via, so called, Reverse Water-Gas Shift Reaction (RWGS). Even though there are couple proposed CO2 activation mechanisms, and knowledge on the reaction is quite high, the common issue is still proper selection of the catalyst for the reaction. In the surface redox mechanism Cu metal is being oxidized by CO2 to form Cu2O with the release of CO. After that, Cu+ ions are getting reduced back to metallic copper using surrounding H2. Solid Oxide Cells can become good source of ‘green’ hydrogen and, at the same time, perform direct electrolysis or chemical reduction of CO2. High temperature, at which SOECs are working, is beneficial from the point of Faradaic efficiency and CO2 conversion rate. Steam electrode of SOEC contains copious amount of nickel. It is also a well-known catalyst for many reactions, but in this case the electrodes are struggling under carbon-rich atmosphere. Therefore, an addition of different catalytic materials is needed. Commonly used are mixed catalytic heterogenous systems composed of Cu supported on oxide materials e.g., FexOy, MoO3, La2O3, and many more.

In this study a series of catalysts for SOECs were fabricated based on the Cu-doped double perovskite structure able to exsolve intermixed Cu or Cu-Fe nanoparticles supported on the matrix lattice of La0.3Sr1.7Fe1.5Mo0.5O6-σdenoted as LSFM. The usage of LSFM compound as a matrix for Cu exsolution was based on the idea that it already consists of the oxides, which are currently utilized as the catalysts in RWGS. On the other hand, quite high mixed ionic-electronic conductivity may lead to even more enhanced electrochemical activity of the compound.

The samples with varying Cu dopant level (5-20 mol.%) were synthesized via modified Pechini method maintaining fixed Fe:Mo ratio. Selected powders were processed into inks and screenprinted onto Ni-YSZ electrode. During the startup procedure of the SOEC, the catalysts were activated and the exsolution of metallic nanoparticles occurred. The powders and catalytic layers were characterized using XRD, SEM, and XPS techniques in both oxidized and reduced states. Doped compounds were able to exsolve metallic nanoparticles with the coverage density and size depending on the initial Cu amount in the lattice and the temperature of the reduction. Additional Temperature-Programmed Reduction and Oxidation (TPR/TPO) tests were performed to better understand the formation mechanism of surficial nanoparticles.

SOECs equipped with additional layer of LSFM-Cu on the fuel electrode were examined for the performance in coelectrolysis mode under flowing H2O-CO2 with subsequent measurement of the outlet gases concentration using GC-MS. New layer altered the production of syngas concerning H2:CO ratio, yield, selectivity and conversion rates. The anchoring effect of the exsolution process causes higher integration of the nanoparticles compared to conventional catalysts produced via simple deposition and most likely limits the evaporation and surface diffusion of copper. LSFM-Cu can be promising material for increasing the efficiency of the SOECs.

Acknowledgements

This work was supported by a project funded by National Science Centre Poland, based on decision UMO-2021/43/B/ST8/01831 (OPUS 22).