Effects of the Gas Velocity on Formation of the Carbon Deposits on Fuel Electrode of AS-SOFC

Abstract

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.