Modeling and Simulation the Influence of Solid Carbon and Nickel Oxide Formation on SOFC Performance and Degradation

Vitaliy Yurkiva,b, Jonathan P. Neidhardta,b and Wolfgang G. Besslerc
a German Aerospace Centre (DLR), Institute of Technical Thermodynamics, 70569 Stuttgart, Germany
b Institute of Thermodynamics and Thermal Engineering (ITW), University of Stuttgart, 70550 Stuttgart, Germany
c Offenburg University of Applied Sciences, 77652 Offenburg, Germany
E-mail: Vitaliy.Yurkiv@dlr.de

Abstract:

Solid oxide fuel cells (SOFC) are promising alternative energy sources for a wide application range from mobile technology to stationary power plants due to their high energy efficiency and low pollutant emission. One of the biggest advantages of an SOFC is its ability of direct utilization of various fuel types, i.e. H2/H2O, CO/CO2 and 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 work we have developed an elementary kinetic model of various degradation processes coupled with (electro)-chemistry and (gas, porous)-phase transport in an SOFC. The model is applied to support the development and evaluation of a consistent mechanism describing the formation of solid carbon and nickel oxide (NiO) at Ni/YSZ anodes. 
Regarding nickel oxide formation, thermodynamic calculations are used to predict the local conditions for the formation of nickel oxide. Simulation results reveal a big gap between local condition inside the cell and global operating parameters. Simulations allow the prediction of global conditions for ‘safe’ cell operation. A map of safe operation points according to the global parameters cell voltage and fuel utilization indicates that for low fuel utilization even under very low voltages no NiO is formed.
The simulations of cells operated on partially reformed hydrocarbons show that performance and degradation are influenced significantly by the operation temperature and applied potential. Specifically, at OCV and high temperature (1023 K), a surface carbon layer is formed which covers Ni surface and Ni three-phase boundary, blocking heterogeneous and charge-transfer reactions. However, at lower temperature (923 K) carbon growth mainly proceeds inside of anode porous phase leading to significant diffusion polarization. On the other hand, under applied potential simulation results reveal the appearance of Ni dusting, which leads to a breakdown of a cell. Literature experimental data is used for validation of the model.