The effect of SO2 on the Ni-YSZ electrode of a solid oxide electrolyzer cell operated in co-electrolysis

In this work, the effect of sulphur poisoning of the Ni-YSZ electrode of an SOEC operated in co-electrolysis mode was investigated. Short-term tests with exposure up to 5 ppmv of SO2 were performed at OCV and under polarization (0.25 A cm−2). The two-stages degradation pattern observed consisted of an initial fast voltage increase followed by a slower voltage increase similar to that of an SOFC exposed to H2S. Electrochemical impedance spectroscopy and the analysis of the distribution of relaxation times showed that both the catalytic and electrochemical reactions were affected by SO2. After extended periods in SO2-free reactant, only a partial recovery of the performances was observed even when exposure amounted to only 0.5 ppmv of SO2 independently on the current density. A durability test at a constant polarization of 0.5 A cm−2 showed a voltage ‘runaway’ behavior during successive exposures to 1 ppmv and 2 ppmv of SO2. This behavior originated from a drastic increase of the serial resistance, which almost completely recovered when the SO2 supply was cut. This behavior was not observed during exposure to 0.5 ppmv of SO2, suggesting that, in these test conditions, the voltage ‘runaway’ behavior could be avoided at a sub-ppmv level. Successive exposure-recovery cycles were found to weaken the SOEC tolerance to SO2 and a low frequency pseudo-inductive arc was observed in the impedance response during and after the second exposure to SO2.

Poisoning effects of chlorine on a solid oxide cell operated in co-electrolysis

Stefan Diethelm, Guillaume Jeanmonod, Jan Van Herle

Although the effects of impurities on the Ni-YSZ electrode of a solid oxide cell in fuel cell operation have been studied, reports on their effects during electrolysis operation are limited. Here, short-term experiments at various current densities and a durability test (1400 h) are performed to investigate the effects of HCl on the Ni-YSZ electrode of a solid oxide electrolysis cell (SOEC) operated in co-electrolysis mode. Without current bias, exposure to 10 ppmv of HCl was tolerated whereas significant degradation was observed when the SOEC was exposed to 5 ppmv of HCl under polarization, mostly related to an increase of the resistance associated with the charge-transfer processes. Prolonged exposure to HCl revealed an initial steady degradation before an increase by a factor of >10 caused by polarization resistance increases, thereby emphasizing the importance of prolonged poisoning tests. Stopping the HCl supply and operating the SOEC in an HCl-free atmosphere allowed for voltage stabilization but no performance recovery was observed. An increased serial resistance indicates that the desorption of Cl can also be detrimental. A low-frequency pseudo-inductive hook appeared in the electrochemical impedance spectra, possibly related to the increase of the charge-transfer processes' resistance and indicating modification to the electrochemical pathways.

ELSEVIER2021

Alternative anode materials for methane oxidation in solid oxide fuel cells

Joseph Sfeir

Fuel Cells are electrochemical devices that are able to directly convert chemical energy to electrical energy, without any Carnot limitation. Hence, their energy efficiencies are relatively high. Among the various types of fuel cells, solid oxide fuel cells (SOFC) are operated at high temperatures and in principle can run on various fuels such as natural gas and hydrogen. As natural gas is sought to become one of the main fuels of the next decades, its direct feed to a SOFC is desirable as the reaction free energy is high CH4 + 2O2 = CO2 + 2H2O, ΔrG°800°C = -800.8kJ/mol At present, conventional SOFC are operated with pure hydrogen or partially to fully reformed natural gas implying an efficiency penalty. SOFC anodes are generally made of electrocatalytically active Ni-YSZ cermet. Pure CH4 is not fed directly to the anode, because of problems associated with the anode deactivation and coking as CH4 leads to the detachment of Ni particles from the YSZ support and their encapsulation by carbon. To overcome this problem, partially oxidized or reformed CH4 is used. For SOFCs running on direct CH4 feed, alternative anode materials to Ni-YSZ, or new anode formulations are necessary. However, in this case, several parameters influence the anode performance and stability. The anodes should withstand reduction at a low PO2 of around 10-24 atm, be compatible with the SOFC electrolyte, possess an acceptable conductivity and thermal expansion coefficient, and appropriate catalytic and electrocatalytic properties along with low coking activity. LaCrO3 and CeO2-based materials as well as some metal catalysts were studied here for their potential application as anodes in direct oxidation of CH4. This choice of materials was made as lanthanum chromite and ceria-based materials are mixed conductors known to resist quite well to the very reducing conditions in SOFC as they are commonly used for interconnect and electrolyte materials respectively. Moreover, these compounds possess some catalytic activity for CH4 activation and combustion. LaCrO3 and CeO2-based materials show rather low electrical conductivity (on the order of 1 S/cm in reducing conditions), necessitating the application of an extra material for proper current collection. Nevertheless, these oxides possess many of the stringent characteristics cited above. For SOFC anode purposes, LaCrO3-based compounds, substituted with Ca, Sr, Mg, Mn, Fe, Co and Ni, were synthesized by a modified citrate route from nitrate precursors. An optimal calcination temperature of 1100°C was found to be necessary to obtain XRD-pure yet sinteractive compounds. Conductivity measurements in air and in reducing atmospheres of 10-21 atm showed that the La A-site substituents (Ca and Sr) influenced the conductivity more than the Cr B-site substituents (Mg, Mn, Fe, Co and Ni). In humidifed H2, the total conductivity was maximally of 6.5 S/cm. Surface reaction with YSZ electrolyte was evidenced by SEM, XPS and SIMS, in reducing conditions, especially under current load, when these powders were applied as anodes. A current induced demixing effect was noted for heavily substituted LaCrO3. Ceria-based compounds, doped with Y, Nb, Pr and Gd, were synthesized through 3 different techniques: the solid-state method, coprecipitation and the NbCl5 route. The coprecipitation route was found to be most appropriate for Y, Pr and Gd, whereas for Nb a modified precipitation and the NbCl5 technique were the most efficient. Co-doping of Nb and Gd or Pr was studied in order to increase the electronic as well as the ionic conductivity in CeO2. Among all dopants, Nb was found to promote most the adhesion to YSZ electrolyte sheets. Adding Nb to Gd-doped ceria improved the electrode adhesion on YSZ sheets, but deteriorated its conductivity in oxidizing conditions. No interfacial reaction with YSZ was observed by HRTEM when Nb-doped ceria anodes were sintered on YSZ at 1200°C/4h. Catalytic measurements were undertaken with these materials in CH4 rich atmospheres. Different reaction mixtures were chosen to simulate the various SOFC operating conditions: partial oxidation, CO2 reforming by recycling and H2O reforming. Among the different elements, Sr and Ni were found to be the most active substituents for LaCrO3, as they promote all three types of reactions. The combination of both elements is favorable not only for catalytic use, but also for electrode application as they tend to increase the electronic conductivity of the material. Temperature programmed oxidation (TPO) and TEM made on the catalysts after runs in CH4, showed a very low carbon coverage over these oxides, except in the case of Fe substitution, over which carbon deposition was promoted. XPS analysis showed also that all B-site substituted compounds did not segregate after the catalytic runs whereas Ca and Sr (A-site substituents) tended to segregate in H2 and H2O rich atmospheres. Thermodynamic calculations, made using correlations developed in literature, show that Ca and Sr are expected to segregate in H2 and H2O rich atmospheres as they tend to form volatile hydroxyl species. Also, the thermodynamic stability of the LaCrO3 was observed to depend much on the substitution. For CeO2-based oxides, the activity for steam reforming increased from Nb

EPFL2002

Model-assisted identification of solid oxide cell elementary processes by electrochemical impedance spectroscopy measurements

Priscilla Caliandro, Stefan Diethelm, Arata Nakajo, Jan Van Herle

Electrochemical Impedance Spectroscopy (EIS) is extensively used to characterize Solid Oxide Cells (SOCs) to extract information on the elementary loss mechanisms. However, these individual mechanisms usually overlap in the frequency domain, requiring dedicated data processing for unambiguous identification. A powerful method for discriminating process contributions is the analysis by the Distribution of Relaxation Times (DRT). The de-convoluted spectrum of SOC generally presents, six peaks from mHz to hundreds of kHz. DRT peak-to-process attribution is often obtained by experimental sensitivity analysis. In the study, six parameters have been systematically varied: temperature, current density, partial pressure of O-2 at the oxygen electrode, partial pressure of steam at the fuel electrode, total flow rates and fuel composition. The effect of wires inductance and different cell geometries has also been analyzed. The study provides detailed information about the contribution of the elementary processes to the total losses over a wide range of operation regimes. It further refines peak-to process attributions by using a dynamic numerical model that includes gas and solid phase transport coupled with charge transfer and chemical reactions. Moreover, the non-univocal literature attribution of processes in the middle frequency range is clarified: strongly overlapping peaks cannot be separated even by DRT.

ELSEVIER2019

Alternative anode materials for methane oxidation in solid oxide fuel cells

Joseph Sfeir

EPFL2002