Thermal Gradients in Solid Oxide Fuel Cell Anodes: X-Ray Diffraction, Thermal Imaging and Model Prediction

Solid Oxide fuel cells (SOFC) are a widely discussed future power option due to their high energy efficiency and fuel flexibility in addition to low maintenance requirements.  However, the long term mechanical stability of the stack can be compromised due to simple operation as a result thermally mismatched materials (1).  The commonly used Ni / YSZ anode – electrolyte combination provides a high degree of electrochemical performance; however, the presence of the Ni catalyst which aids the performance of the cell results in a significant mismatch in the coefficient of thermal expansion (CTE) between the anode and electrolyte of cells (2).  Issues arising from mismatched CTE can be further exacerbated by the presence of thermal gradients within SOFCs.  Thermal gradients which can arise from simple operation, internal reforming and inhomogeneous current distribution amongst other factors (3-5) have been noted to significantly increase the prospect of cell failure (6). 

In this study the effect of thermal gradients on SOFC anodes is investigated using both experimental and computational methods.  Experiments conducted at Diamond Light Source using combined synchrotron X-ray diffraction and infrared thermal imaging (7)  are detailed highlighting the presence of non-uniform thermally derived stresses at operationally relevant temperatures within the Ni phase of the anode of cells.  Computationally derived thermal gradients have been generated using finite element method modelling in order to study the effect of cell operation utilising a three dimensional electrochemical model.  The results obtained from the model have been coupled with the experimental results in order to investigate the effect of various parameters upon the cell.  Operational conditions including cell polarization and fuel flow conditions and configurations are examined to highlight the importance of cell optimisation in minimising stresses within the cell anode to prolong cell lifetime whilst maximising performance.      

References

1.         S. Majumdar, T. Claar and B. Flandermeyer, J Am Ceram Soc, 69, 628 (1986).

2.         W. Z. Zhu and S. C. Deevi, Materials Science and Engineering: A, 362, 228 (2003).

3.         H. Apfel, M. Rzepka, H. Tu and U. Stimming, Journal of Power Sources, 154, 370 (2006).

4.         H. Severson and M. Assadi, Journal of Fuel Cell Science and Technology, 10, 061001 (2013).

5.         R. Clague, A. J. Marquis and N. P. Brandon, Journal of Power Sources, 210, 224 (2012).

6.         A. Nakajo, Z. Wuillemin, J. Van herle and D. Favrat, Journal of Power Sources, 193, 203 (2009).

7.         J. B. Robinson, L. D. Brown, R. Jervis, O. O. Taiwo, J. Millichamp, T. J. Mason, T. P. Neville, D. S. Eastwood, C. Reinhard, P. D. Lee, D. J. L. Brett and P. R. Shearing, Journal of Synchrotron Radiation, 21 (2014).