Co-electrolysis of steam and CO2 in full-ceramic symmetrical SOECs: a strategy for avoiding the use of hydrogen as a safe gas

Abstract

The use of cermets as fuel electrodes for solid oxide electrolysis cells requires permanent circulation of reducing gas, e.g. H₂ or CO, so called safe gas, in order to avoid oxidation of the metallic phase. Replacing metallic based electrodes by pure oxides is therefore proposed as an advantage for the industrial application of solid oxide electrolyzers. In this work, full-ceramic symmetrical solid oxide electrolysis cells have been investigated for steam/CO₂ co-electrolysis. Electrolyte supported cells with La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ reversible electrodes have been fabricated and tested in co-electrolysis mode using different fuel compositions, from pure H₂O to pure CO₂, at temperatures between 850–900 °C. Electrochemical impedance spectroscopy and galvanostatic measurements have been carried out for the mechanistic understanding of the symmetrical cell performance. The content of H₂ and CO in the product gas has been measured by in-line gas micro-chromatography. The effect of employing H₂ as a safe gas has also been investigated. Maximum density currents of 750 mA cm⁻² and 620 mA cm⁻² have been applied at 1.7 V for pure H₂O and for H₂O : CO₂ ratios of 1 : 1, respectively. Remarkable results were obtained for hydrogen-free fuel compositions, which confirmed the interest of using ceramic oxides as a fuel electrode candidate to reduce or completely avoid the use of safe gas in operation minimizing the contribution of the reverse water shift reaction (RWSR) in the process. H₂ : CO ratios close to two were obtained for hydrogen-free tests fulfilling the basic requirements for synthetic fuel production. An important increase in the operation voltage was detected under continuous operation leading to a dramatic failure by delaminating of the oxygen electrode.