In-Situ Raman Spectroscopy Probing of Solid Oxide Electrolysis Cells

Abstract

Solid oxide electrolysis cell (SOEC) technology is a promising route for sustainable recycling of carbon dioxide and steam into synthetic fuels and commodity chemicals via syngas (H2+ CO) production. This technology is based on the reverse polarity of the prominent solid oxide fuel cell (SOFC) and commonly consists of a porous Ni/YSZ cathode, dense YSZ electrolyte and porous LSM/YSZ anode.High temperature co-electrolysis of CO2 and H2O is a complicated process and has not yet been fully understood. This is due to a number of reactions that take place simultaneously: water electrolysis, carbon dioxide electrolysis and the reverse water-gas shift reaction (rWGS CO2 + H2 ↔ CO + H2O). Other possible reactions driven by the equilibrium may include the Bosch reaction (CO + H2 ↔ C(s) + H2O) and Boudouard reaction (2CO ↔ CO2 + C(s)).Carbon deposition on the nickel-cermet surface of SOECs can severely degrade the performance of the cell by deactivating the reaction sites. Recent work by Tao et al. has shown that, at high current densities and conversion efficiencies of CO2 and H2O to CO and H2, carbon formation on the surface of Ni/YSZ fuel electrode is likely to occur [1].SOECs are typically analyzed in-situ using electrochemical impedance spectroscopy and current-voltage characteristics and ex-situ using scanning electron microscopy to obtain information about cell’s surface topography and composition. However, these techniques are not able to provide conclusive information about the processes occurring during operation of the cell.In this work we aim to use in-situ Raman spectroscopy alongside traditional electrochemical techniques to probe the surface species of the SOEC electrodes for carbon impurities deposition and provide insight into the fundamental processes and reaction pathways that occur during high temperature (700 – 850 oC) co-electrolysis of CO2 and steam. In-situ Raman spectroscopy is also applied to characterize structural changes and material transformations during SOEC operation.The particular area of interest is the triple phase boundary (TPB), where the electrochemical reactions take place. Accessing the TPB is challenging due to high operation temperatures and the depth of laser penetration into the sample.The experimental setup with optical access for the Raman microscope will be discussed in detail as well as the correlation of in-situ spectroscopic data with the electrochemical information obtained from the SOECs.