Modeling of Degradation of Solid Oxide Fuel Cell (SOFC) at Stack-Scale
Authors: Babaie Rizvandi Omid , Miao Xing-Yuan , Frandsen Henrik Lund 
Published: May 2020

Solid Oxide Fuel Cells (SOFC) converts chemical energy of a fuel (hydrogen or reformed hydrocarbons) to electricity directly with high efficiency. High-temperature operation of the SOFC makes it a fuel-flexible device; however, the high temperatures induce some degradation mechanisms. Degradation of the SOFC is a barrier against its widespread use and commercialization. Therefore, a thorough analysis is necessary to investigate mitigation strategies for reducing the degradation rate and so improving performance.

Degradation experiments are time consuming and costly. Modeling of the SOFC stack is therefore a good alternative for studying degradation mechanisms and mitigation techniques. However, computational cost of numerical simulations of an SOFC is high because of the multiphysics modeling approach that is needed to take care of the interactions of physics such as flow distribution, mass transport, charge and energy transfer. The high computational cost of the numerical models hinders 3D modeling of the SOFC even at a single-cell scale. Therefore, 1D and 2D models with all physics included have been developed, or 3D models with simplifying assumptions, such as isothermal approach, have been developed. To the best of the authors’ knowledge, all the degradation models of the SOFC are developed at a single-cell scale, even most of them concentrate on a part of the electrodes and electrolyte.

Here, a homogenized model of a full stack in 3D is used to investigate the degradation with a reasonable computational time. The homogenized model considers flow in manifolds, and in the active domain it considers all relevant physics, i.e. currents, gas flows, heat transport, mass balances and mechanical stresses. It does so by use of a homogenized description of all physics, such that it correctly represent the physics without describing the geometrical detail but rather represent the response of all layers effectively. It was shown that the model predicts the overall responses of the stack in a realistic manner, while it is about two orders of magnitude faster than conventional stack models where the geometry inside the stack is explicitly represented.

The model includes transport equations of mass, momentum, species, charges, and energy. In this study, the following degradation mechanisms are added to the model: nickel particles coarsening (agglomeration) which decreases triple-phase boundary (TPB) length and electric conductivity of anode electrode, chromium poisoning at the cathode side which reduces TPB length of cathode electrode, and corrosion of interconnect which increases ASR (area specific resistance) through the resistance of oxide scale against electric charge transfer.

The transient model takes less than a day to simulate tens of thousands of hours of degradation on a workstation with Intel Xeon 3.7 GHz 6-core processor. The peak RAM (random access memory) is about 10 GB for the simulations. The volume averages of the aforementioned degradation in the active domain over time are qualitatively consistent with common trends reported in the literature. Moreover, the model resolves distributions of the degradation mechanisms over the active domain. Therefore, it can be used to modify the stack design to minimize degradation rates. Moreover, cells that experience high degradation rates could be exchanged with the ones at safe regions of the stack to improve its lifetime.

Figure 1