Evaluation of SOFC Anode Performance and Degradation Considering Carbon Formation: Modeling and Simulation Study

Solid oxide fuel cells (SOFC) are a promising technology for supplying electrical energy for future demands. Due to many advantages such as high efficiency, low emissions, low noise, reliability, and fuel flexibility, it is expected that SOFC will possess a major role in future energy conversion technologies. One of the biggest advantages of an SOFC is its ability of direct utilization of various fuel types, i.e. H₂/H₂O, CO/CO₂and hydrocarbons. It is, however, well known that operation of an SOFC based upon various fuels could cause various types of cell degradation. Therefore, understanding the origin and evolution of degradation processes on micro(nano)-scale is essential to develop long-term operating SOFC technology.

 In the present contribution we investigate performance and degradation behavior of porous Ni/YSZ SOFC anode considering solid carbon formation. For the modeling and simulation study we use a 1D+1D model of a planar SOFC, which includes coupled electrochemistry and transport through membrane-electrode assembly and 1D gas channels. Anode and cathode chemistry is described using elementary reaction kinetic approach.¹ Current-voltage relationships are modeled by directly solving for the electric-potential distribution in the electrodes and electrolyte taking into account the electrical double layer capacitance. The gaseous species transport within porous Ni/YSZ anodes phase is represented by two parallel transport pathways: Stefan-Maxwell flow and Darcy viscous flow. The transport of gaseous reactants to and products from surface takes place through convection and diffusion in the channel parallel to the electrode surface, described by Navier-Stokes equations.¹ The evolution of carbon inside/on the anode is quantified by a multi-phase modeling approach,²which is based upon the volume fractions change of all phases as a function of time and spatial location inside the cell, by taking into account the chemical rate laws of all involved reactions and overall mass conservation. Using this approach we solve computationally both steady-state and transient problems.

 The formation of solid carbon at the Ni/YSZ anode depends on many factors, e.g. types of fuel, anode morphology, steam-to-carbon ratio, operating conditions etc. In our work we consider two main carbon types which are formed on Ni surface, i.e., film and pyrolytic. The formation of these carbon types is modeled by elementary reactions of ordinary surface-adsorbed carbon species on Ni surface. Their thermodynamic data (enthalpy and entropy) and kinetics (pre-exponential factor and activation energy) are derived from literature available temperature-programmed desorption and oxidation (TPD/TPO) measurements.³ This data together with a full mechanism of methane reforming, which consist of 21 surface reactions and electrochemical charge-transfer processes, are used to analyze electrochemical experiments of Chen et al.⁴ These experiments were conducted for Ni/YSZ anode-supported SOFC and cover an extended range of operating temperatures (923 K ≤ T ≤ 1023 K) and various syngas compositions (CH₄, CO, CO₂, H₂, H₂O) used for impedance and voltage stability test measurements. Using our approach we have modeled latter experimental results at OCV (upper panle in Fig. 1) and at constant current density of 2300 A·cm^‒2 (lower panel in Fig. 1). The obtained good agreement between electrochemical experimental and simulated results indicates the validity of the developed cell degradation model due to carbon formation. In addition, lifting of Ni particles (dusting) which occur at high temperature and low current density and causes the main performance decrease has been assessed.

Figure 1. Upper panel - Comparison between experimental and simulated cell stability at OCV and temperature of 923 K and 1023 K; Lower panel - Comparison between experimental and simulated voltage cell stability at 2300 A·cm^‒2and different temperatures. The experimental results are taken from Ref. [4].

References

1. W. G. Bessler, S. Gewies, and M. Vogler, Electrochim. Acta, 53, 1782–1800 (2007).

2. J. P. Neidhardt et al., J. Electrochem. Soc., 159, A1528–A1542 (2012).

3. V. Alzate-Restrepo and J. M. Hill, Appl. Cat. A: General, 342, 49–55 (2008).

4. T. Chen, W. G. Wang, H. Miao, T. Li, and C. Xu, J. Power Sources, 196, 2461–2468 (2011).