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.