Development of electrocatalysts/electrodes for their application in H2O (or H2O + CO2) co-electrolysis processes in high temperature solid oxide electrolysis cells

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Ιωαννίδου, Ευαγγελία
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In the present thesis targeted modifications with Au, MoOx and FexOy were applied on commercially available NiO/GDC (65 wt.% NiO – 35 wt.% Ce0.9Gd0.1O2-x) cermet, by means of deposition precipitation and deposition co-precipitation. Extensive physicochemical characterization was performed for the prepared materials, in the form of powders and as electrode films, with various surface and bulk techniques in order to extract information about their surface and bulk structure. The oxidation properties of the powders were examined in the presence of H2O and CO2, in the TGA, at 650−800 oC. Furthermore, there are reports on comparative electrocatalytic measurements of the developed materials as fuel electrodes under high temperature (800−900 oC) Η2Ο electrolysis. The single SOECs comprised a circular shaped, planar − electrolyte (8YSZ) with GDC10 (Gd0.10Ce0.90O2-x) │ LSCoF (La0.6Sr0.4Co0.8Fe0.2O3-δ) as oxygen electrode. In regards to the H2O/CO2 co-electrolysis process, it is widely accepted that the fuel solid oxide electrodes meet a complex environment, where catalytic reactions, such as the Reverse Water Gas Shift (RWGS) reaction, are coupled with electrochemical processes, such as Η2Ο and CO2 electrolysis. The extent of each reaction determines the composition of the products, i.e. H2/CO ratio. Up today, there are various H2O/CO2 co-electrolysis scenarios about the extent of CO production, resulting from the RWGS reaction or/and the CO2 electrochemical reduction. This research topic was another key part of the presented thesis. In this respect, the examined modified 3 wt.% Au-Ni/GDC, 3 wt.% Mo-Ni/GDC, 3 wt.% Au – 3 wt.% Mo-Ni/GDC and 2 wt.% Fe-Ni/GDC electrocatalysts were also investigated, in the form of half-electrolyte supported cells, for their performance in the RWGS reaction through catalytic-kinetic measurements at 800−900 oC. The samples were tested at open circuit potential conditions (OCP), in order to elucidate their catalytic activity towards the production rate of CO (rco), which is one of the products of the H2O/CO2 co-electrolysis reaction. Through the latter approach a reference profile for the catalytic performance of the candidate electrodes was created, by applying co-electrolysis feed conditions. In continuation to the catalytic investigation, this research focused on further elucidating the extent of the occurring electro-catalytic processes during H2O/CO2 co-electrolysis. For this reason, the electrolyte supported Ni-Ce0.9Gd0.1O2-x||ZrO2(8 mol% Y2O3)||Gd0.10Ce0.90O2-x|La0.6Sr0.4Co0.8Fe0.2O3-δ SOC was examined in Η2Ο/CO2 co-electrolysis mode at 800−900 oC, by applying various pΗ2Ο/pCO2 feed ratios, in the range of 0 ≤ pΗ2Ο/pCO2 ≤ 1 and two pΗ2 values (2 and 21 kPa). The main objective was to discriminate the occurrence of individual CO2 electrolysis during polarization and the contribution of the RWGS reaction on the production rate of CO (rCO).
Solid oxide electrolysis cells, H2O electrolysis, H2O/CO2 co-electrolysis, Au-Mo-Fe-Ba-Ni/GDC electrocatalysts, H2O/CO2 oxidation, Reverse water gas shift reaction