Solid oxide fuel cells (SOFCs) can be fueled with various gases, including carbon-containing compounds. High operating temperatures, exceeding 600 °C, and the presence of a porous, nickel-based SOFC anode, might lead to the formation of solid carbon particles from fuels such as carbon monoxide and other gases with hydrocarbon-based compounds. Carbon deposition on fuel electrode surfaces can cause irreversible damage to the cell, eventually destroying the electrode. Soot formation mechanisms are strictly related to electrochemical, kinetic, and thermodynamic conditions. In the current study, the effects of carbon deposition on the lifetime and performance of SOFCs were analyzed in-operando, both in single-cell and stack conditions. It was observed that anodic gas velocity has an impact on soot formation and deposition, thus it was also studied in depth. Single-anode-supported solid oxide fuel cells were fueled with gases delivered in such a way that the initial velocities in the anodic compartment ranged from 0.1 to 0.7 m/s. Both cell operation and post-mortem observations proved that the carbon deposition process accelerates at higher anodic gas velocity. Furthermore, single-cell results were verified in an SOFC stack operated in carbon-deposition regime by dry-coupling with a downdraft 150 kWth biomass gasifier.

The degradation of solid oxide electrolysis (SOE) cells with different anode sweep gases was studied in 1000 h-long measurements in order to investigate the impact of sweep gas composition on cell performance. Cathode-supported electrolysis cells with an La_0.6Sr_0.4Co_0.2Fe_0.8O₃ air electrode (active area of 4 × 4 cm²) were tested under a constant current (−0.25 A/cm²) in the electrolysis mode while supplying the cathode side with 70% H₂O–30% H₂ mixtures at 800 °C and using oxygen, nitrogen, and steam as sweep gases. It was demonstrated that the degradation of the anode in steam conditions resulted in more than a 2-fold increase in both, polarization and ohmic resistance (from 0.20–0.25 to 0.6–0.65 Ω cm² compared to relatively stable values of 0.15–0.2 Ω cm² for N₂), as a consequence of the phase decomposition. Strontium played an important role in steam-induced degradation, migrating from the volume of the electrode layer to the surface of the electrolyte. As a result, the Sr-enriched layer demonstrated susceptibility to Cr poisoning. The cell purged with N₂ demonstrated enhanced performance, while the use of oxygen led to degradation originating from the well-described delamination process. DRT analysis demonstrated some similarity of the spectra for steam and N₂, namely the presence of a slow process at $\tau \approx 0.5$ s, which might be associated with hindered oxygen transport due to point defect association in the perovskite structure. The results of this study showed that Sr-containing materials likely cannot be used as an SOE anode in high humidity conditions.

The elevated operating temperature of solid oxide fuel cell (SOFC) makes it possible to internally reform the incoming hydrocarbonaceous fuel directly on surface of the anode in presence of catalytic material such as nickel cermet. Thus, except hydrogen and carbon monoxide, SOFCs can be fuelled with various organic compounds in gaseous state like methane, petroleum gases, light alcohols, and others. The presence of carbon-based compounds in the fuel might result in the formation and deposition of soot (or coke) on the surface of SOFC anode. The operation of solid oxide fuel cells with a solid carbon layer on the anode surface might lead to several undesirable effects, such as the partial or complete loss of the electrochemical activity. It is commonly known that the process of carbon deposition in the anodic compartment of SOFC depends on the thermodynamic and kinetic conditions. The growth of the deposit is driven by factors such as the steam-to-carbon ratio (S/C), temperature, catalytic properties of the material of the anode, and the current density of the cell. Carbon deposition is intensified by the increase of S/C ratio or by the rise of current density. The higher the temperature, the longer period of time is also required for the solid carbon particles to deposit on the porous surface. The rate of soot formation may be strongly augmented by the presence of nickel. Nickel serves as an excellent catalyst for the carbon deposition process, such as methane cracking (Eq. (1)), reduction of carbon monoxide (Eq. (2)) and disproportionation of carbon monoxide, also known as Boudouard reaction (Eq. (3)). The thermodynamic boundaries of the deposition process are defined by carbon formation isotherms, presented in tertiary diagrams commonly known as the Gibbs diagrams (Fig. 1). They graphically define the equilibrium composition for various gas mixtures composed of carbon, hydrogen and oxygen atoms [1]. The Gibbs diagram aids in estimation of the values of parameters and compositions which allow safe operation without sooting. However, in real scenario of SOFC operation the conditions may not fit in the safe domains of Gibbs diagrams. Such cases require an individual attention due to crucial role in SOFC stack degradation. In the current study it was assumed that one of the parameters that also affects the rate of the carbon deposition process is the anodic gas velocity. The flow parameters influence the kinetics of the multiphase reaction, especially on the porous surfaces. Additionally, gradual increase of the thickness of the carbon deposit can disturb the flow pattern, resulting in significant local disturbances of anode gas or in formation of the stagnant diffusion-controlled zones. The effect of the gas velocity on formation of the carbon deposits was experimentally investigated using 5 cm x 5 cm anode supported SOFCs. The carbon deposition was observed during the electrochemical measurements and confirmed by the post mortem analysis of the cells. The correlation between the gas velocity and sooting-driven anode deactivation was derived using analysis of the data collected by electrochemical impedance spectroscopy (EIS) and the measurements of the polarization of the cell. In frame of the study, an attempt to propose the first approach to experimentally determine the influence of gas velocity on the carbon deposition phenomenon was made. Acknowledgment This work was financially supported by the National Science Centre, Poland, Grant No. 2015/19/N/ST8/01876. References [1] Motylinski K., Naumovich Y., Numerical model for evaluation of the effects of carbon deposition on the performance of 1 kW SOFC stack – a proposal, E3S Web of Conferences 2017, In press. Figure 1

This article discusses the experimental characterization of a 50 mm x 50 mm anode supported solid oxide fuel cell with active area of 16 cm² which was operated in direct internal reforming conditions over an extended period of time. The cell with a given composition of electrodes and electrolyte (NiO-YSZ/YSZ/GDC/LSCF) was subject to complete internal reforming of methane for 1,000 hours. Analysis of the performance included electrochemical characterization (j-V curves and EIS) and measurement of the composition of gases exiting the anodic compartments of the cell. This approach was used to observe the degradation of performance and to determine the effect of the state of the cell on the molar fraction of gases in the outlet. Microstructure studies provided insight into the changing structure of the electrodes after the degradation was observed. The results are quantified and discussed.

The integration of solid oxide fuel cells (SOFCs) with biomass gasification reactors raises the possibility of solid particle contamination of the gaseous fuel entering the cell. Technical specifications from SOFC manufacturers, among other sources, claim that SOFCs do not tolerate the presence of solid particles in fuel. However, there is very limited literature on the experimental investigation of feeding SOFCs with particulate matter aerosols. In this study, a standard 5 × 5 cm anode-supported SOFC was fueled by two types of aerosols, namely, (1) inert powder of grain sizes and concentration equivalent to gasifier fly ash and (2) a real downdraft gasifier fly ash, both suspended in a gaseous fuel mixture. For reference, cells were also investigated with a dust-free fuel gas of the same composition. A straightforward negative influence of the inert powder aerosol could not be confirmed in experiments with a duration of 6 days. That said, the introduction of carbonaceous fly ash aerosol caused slow but irreversible damage to the SOFC. The degradation mechanisms were studied, and the presence of carbon-containing particles was found to clog the pores of the SOFC anode. The maximum measured power density of the SOFC equaled 855 mW/cm² (850 °C, reference fuel). Feeding inert aerosol fuel caused no rapid changes in power density. A moderate drop in performance was observed throughout the experiment. The contamination of fuel with fly ash resulted in an initial performance gain and a ca. 25% performance drop longer term (43 h of contamination). Post-mortem analysis revealed contamination on the walls of the gas channels, with some visible alumina or fly ash spots in the anode area.