Improving Catalytic Activity of Solid Oxide Electrolysers for the Electrosplitting of Water Vapor and CO2 by the Addition of Affordable and Eco-Friendly Promoter Metals
Authors: Błaszczak Patryk , Ducka Agata , d'Acapito Francesco , Matlak Krzysztof , Wolanin Barbara , Amati Matteo , Wang Sea-Fue , Bochentyn Beata , Jasiński Piotr 
Published: December 2024

Recently, Solid Oxide Electrolysis Cells (SOECs) can be considered a solution for the elimination of atmospheric CO2 and limiting its negative environmental impact. Thanks to high operation temperatures, full-solid construction and high Faradaic efficiency it is a promising method for a production of great amounts of relatively cheap and pure hydrogen. Furthermore, these ceramic cells can be set to perform CO2 electroreduction along with water vapor. Thanks to that, it is possible to simultaneously supply various chemical processes with H2-CO feedstock using one, relatively simple setup. In that sense, the SOECs can provide ‘green’ hydrogen and reduce the CO2 amount by delivering the valuables such as syngas or other chemicals. Recently, high temperature H2O/CO2 coelectrolysis in SOECs became reasonable alternative to other technologies of syngas production such as steam reforming of biogas or coal gasification, which both release huge amount of the greenhouse gases.
The main constituent of water-hydrogen electrode is bifunctional composite of NiO and Zr0.84Y0.16O2-δ, which upon the reduction forms metallic network of Ni within the scaffold of ionic conductor. This type of electrode is quite well established for working with water vapor in electrolysis mode but struggles under the atmosphere containing CO and/or CO2. Ni grains are considered active, but not the most efficient towards RWGS as well as electrochemical splitting of H2O/CO2. Over the years other transition metals - Cu, Fe, or Co - as well as their oxides - MnO, ZnO - were found to be more prominent and used in catalytic reactors. Further increase of the activity and the efficiency can be achieved thanks to promoter metals and their oxides e.g., K2O, Na2O, Rb2O, MgO, CaO. The addition of the transition metals can be promising alternative to the usage of the noble metals considering mostly their much lower price. On the other hand, the addition of alkaline metal oxides can increase the adsorption rate of CO2 and prolong the retention time in the proximity of the electrode. As the SOECs are generally non-pressurized systems, the efficiency of the electrode surface catalysts should be pushed to maximum in order to provide valuable yield of the outlet gases.
A series of modified SOECs was prepared by the addition of 5 mol.% of the transition metal (Co, Cu, Mn, Fe) or alkaline metal oxide (Na, K, Ca, Mg, Rb, Li, Sr) nanoparticles. The samples have been fabricated by simple wet impregnation method. The SOECs were tested for the coelectrolysis of CO2/H2O mixture with an addition of H2 for electrode protection against the reoxidation. The electrical tests were considering the utilization factor of water vapor and electrochemical behavior of the cells under various CO2:H2:H2O ratios to better understand the mechanisms of syngas formation.
It was found out that depending on the reducibility of the metals they got dissolved into Ni grains (e.g., Co, Fe) or formed the secondary phases on the top of the grains (e.g. MnO). It was observed as an accumulation of the metal ions inside Ni-rich grains or dissipation of the ions all over the imaging area as seen in STXM images. Alkaline metal oxides formed a mixture of the phases all over Ni and YSZ grains. A series of in-situ XAS measurements at the ESRF (France) was performed to observe the formation of various mixed compounds and the response of electrode's components to CO2-rich atmosphere at high temperature. The surficial changes of the oxidation states were determined using in-situ XPS imaging of modified electrode structure using SPEM at the Elettra (Italy). The addition of secondary metal alters the energy levels on Ni surface and increases the tendency to form carbonate-like compounds, what prolongs the retention time of CO2. The addition of guest metal highly increased CO2 conversion and selectivity towards CO production as well as the electrical efficiency due to the alterations of basic-acid sites and introduction of active redox couples.

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). The XAS and XPS experiments were supported by CERIC-ERIC consortium and performed at the European Synchrotron Radiation Facility (ESRF) and the Elettra Sincrotrone Trieste, respectively. The access to ESRF was financed by the Polish Ministry of Education and Science – decision no. 2021/WK/11.