In-Situ Exsolution of Metal Nanoparticles in Solid Oxide Cells for Efficient Syngas Generation from Steam and Carbon Dioxide
Authors: Kyriakou Vasileios , Dimitrakopoulos Georgios , Wang Jiayue , Neagu Dragos , Tsampas Michalis N , Yildiz Bilge 
Published: May 2020

The global sustainability targets require technologies to provide clean chemical and fuel feedstock molecules on a large scale often by employing renewable electricity as the energy input. Nowadays, syngas (carbon monoxide and hydrogen), the key intermediate for synthetic fuels through Fischer-Tropsch, is primarily generated from fossil fuels, which are responsible for gigantic energy consumption and green-house gases emissions. The co-electrolysis of carbon dioxide and steam in solid oxide electrolysis cells (SOECs) constitutes an emerging route for syngas feedstock production and thus, for storing the intermittent renewable electricity in the form of chemical bonds1.
In SOECs, the Ni-based cermets are the most commonly employed materials as fuel electrodes due to their high electro-catalytic activity. Ni-based cermets, however, exhibit significant drawbacks, such as Ni coarsening under redox conditions and coking in the electrochemical active zone during CO2 electrolysis3. Perovskite oxides (ABO3) are the most promising alternatives due to their exceptional redox stability, extensive range of functionalities and the concept of exsolution. During exsolution, metallic nanoparticles of reducible species (usually from the B-site) grow at the surface of the perovskite oxide scaffold at reducing atmospheres, i.e. hydrogen-rich gas phase3 or imposition of high cathodic overpotentials4. Hence, perovskite oxide electrodes can be decorated with uniformly dispersed, anchored, coke and oxidation-resistant metal nanoparticles that can dramatically enhance the electrocatalytic performance and stability of the cell4.
Compared to the slower exsolution via thermochemical reduction by hydrogen, the exsolution driven by electrochemical poling require only some minutes under high applied bias (~2.0 V) to form considerably higher population densities of metal nanoparticles on the oxide surface. Therefore, this method does not only offer, new pathways to electrode nanostructuring but also simplifies remarkably their preparation procedure4.
Along these lines, here, we employ A-site deficient perovskites capable of exsolution, ((La,Ca)(Ti,M)O3 (LCT-M, M = Ni, Fe, Rh), as fuel electrodes (cathode) in ZrO2-based electrolyte supported cells for the high temperature steam and/or carbon dioxide electrolysis process. To achieve nucleation of the metal nanoparticles, reduction by both hydrogen and bias is investigated. Additionally, the effect of the imposed overpotential and gas atmosphere on the population, nature and shape of the particles is explored and it is correlated to the apparent cell performance during electrolysis.

References

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[4] J. Myung, D. Neagu, D. N. Miller, J. T. S. Irvine, Nature 2016, 537, 528.