Corrosion behaviour of nitrided ferritic stainless steels for use in solid oxide fuel cell devices

Plasma nitriding was applied to ferritic stainless steel substrates to improve their performances as interconnects for solid oxide fuel cell devices. The samples underwent electrical conductivity test and SEM/EDS, TEM/EDS, environmental-SEM analyses. The first stages of corrosion were recorded in-situ with the e-SEM. Nitriding is effective in limiting the undesired chromium evaporation from the steel substrates and accelerates the corrosion kinetics, but its influence of the electrical conductivity is ambiguous. No intergranular corrosion is found in the steel substrate after long time operation. Nitriding helps commercially competitive porous coating to improve chromium retention properties of metal interconnects

Damage of Siloxanes on Ni-YSZ Anode Supported SOFC Operated on Hydrogen and Bio-Syngas

This work presents the poisoning effect of organic oxo-silicon compounds (siloxanes) which are generally found in sewage biogas. Lifetime and durability associated with solid oxide fuel cell (SOFC) technology is strongly related to the amount of contaminants that reach the stack. Several experiments on Ni anode-supported (AS) single cells are performed in order to clarify the mechanism of degradation and also the possibility of cell performance recovery. Three experiments focus on the degradation and recovery of AS Ni-YSZ fed with H-2, co-feeding 5 ppm D4-siloxane (octamethylcyclotetrasiloxane, C8H24O4Si4) as representative compound for the organic silicon species, at 800 degrees C. A fourth experiment focuses on the durability of the AS Ni-YSZ cell with variable concentrations of the impurity (0-5ppm), during steady state polarization (0.25Acm(-2)) for 250 h, using simulated biogas-reformate fuel H-2/CO/CO2/H2O: irreversible degradation was observed with the D4-impurity feed in the anode gas. Post-test scanning electron microscopy (SEM) results indicate formation of SiO2(s) deposits, which block pores and reduce the TPB length

In-situ Observation of Co-Ce Coated Metallic Interconnect Oxidation Combined with High-Resolution Post Exposure Analysis

Oxidation of Sanergy SSHT steel with Ce-Co coating, used as interconnect material, was observed in-situ in a modified environmental scanning electron microscope (ESEM) at nominal temperature between 800°C and 900°C under 33Pa of oxygen, for different durations (up to 60h). After these in-situ observations, lamellas of the observed target areas were prepared using a focused ion beam (FIB). This allowed the evaluation in the cross section of the final composition of the oxidation layers of the steel by scanning transmission electron microscopy (STEM) and energy dispersive x-ray analysis (EDX), of the same zone observed directly on the surface by ESEM. Results show the diffusion of manganese and iron towards the cobalt coating. Underneath, chromia scale forms. Under the applied conditions, the very thin cerium layer does not prevent the diffusion of other elements from the steel. Niobium (with silicium) and titanium form oxidized precipitates in the steel just below the chromia scale

Visualizing the importance of oxide-metal phase transitions in the production of synthesis gas over Ni catalysts

Synthesis gas, composed of H2 and CO, is an important fuel which serves as feedstock for industrially relevant processes, such as methanol or ammonia synthesis. The efficiency of these reactions depends on the H2: CO ratio, which can be controlled by a careful choice of reactants and catalyst surface chemistry. Here, using a combination of environmental scanning electron microscopy (ESEM) and online mass spectrometry, direct visualization of the surface chemistry of a Ni catalyst during the production of synthesis gas was achieved for the first time. The insertion of a homebuilt quartz tube reactor in the modified ESEM chamber was key to success of the setup. The nature of chemical dynamics was revealed in the form of reversible oxide-metal phase transitions and surface transformations which occurred on the performing catalyst. The oxide-metal phase transitions were found to control the production of synthesis gas in the temperature regime between 700 and 900 °C in an atmosphere relevant for dry reforming of methane (DRM, CO2: CH4 =0.75). This was confirmed using high resolution transmission electron microscopy imaging, electron energy loss spectroscopy, thermal analysis, and C18O2 labelled experiments. Our dedicated operando approach of simultaneously studying the surface processes of a catalyst and its activity allowed to uncover how phase transitions can steer catalytic reactions