Operando Raman Spectroscopy and Impedance Spectroscopy, a Powerful Combination to Better Understand Fuel Electrodes in Solid Oxide Cells

Abstract

Solid oxide cells are based on an oxygen ion conducting ceramics and thus can be used to convert a large variety of fuels into electricity and vice versa. As solid oxide fuel cells, hydrocarbon fuels such as methane can be converted to electricity forming water and carbon dioxide. However, the reversed reaction the reduction of water and carbon dioxide to hydrogen and carbon monoxide (the so called syngas) is also of interest e.g. for the synthesis of high value fuels in subsequent methanation or Fischer-Tropsch reactions. The state-of-the-art electrode for both solid oxide fuel cells and electrolysis cells are so called Nickel cermet electrodes, which are prone to carbon formation due to the high amount of Ni (usually above 33vol%).An alternative fuel electrode concept is based on an electronically conducting ceramic and using infiltrated metals as electrocatalytically active elements. This concept is versatile as it allows the use of various electrocatalysts, not limited to Ni. Based on a comparative study, Fe, Co in addition to Ni have been identified as promising electrocatalyst materials and have been investigated as regards their behavior in carbon containing gas atmospheres [1]. Raman spectroscopy in combination with impedance spectroscopy and especially during operation of the cell is a powerful tool to investigate electrodes in solid oxide cells [2,3].In this study Raman spectroscopy has been applied either in-situ (at open circuit voltage) or operando (under load in fuel cell mode) to investigate carbon formation and electrochemical performance. The results show a clear decrease in carbon formation when Co was present in the electrode. In-situ experiments confirmed in general an electrode deactivation when carbon was built up, while operando studies revealed an electrode activation when carbon was observed. This latter effect is tentatively attributed to an improvement of the electrical contact within the ceramic electrode. The influence on the impedance as well the detailed impact of operation parameters will be discussed.