Investigating the effect of thermal gradients on stress in solid oxide fuel cell anodes using combined synchrotron radiation and thermal imaging

James B Robinson,Leon D Brown,Rhodri Jervis,Oluwadamilola O Taiwo,Thomas M.M Heenan,Jason Millichamp,Thomas J Mason,Tobias P Neville,Ralph Clague,David S Eastwood,Christina Reinhard,Peter D Lee,Daniel J.L Brett,Paul R Shearing

doi:10.1016/j.jpowsour.2015.04.104

Abstract

Thermal gradients can arise within solid oxide fuel cells (SOFCs) due to start-up and shut-down, non-uniform gas distribution, fast cycling and operation under internal reforming conditions. Here, the effects of operationally relevant thermal gradients on Ni/YSZ SOFC anode half cells are investigated using combined synchrotron X-ray diffraction and thermal imaging. The combination of these techniques has identified significant deviation from linear thermal expansion behaviour in a sample exposed to a one dimensional thermal gradient. Stress gradients are identified along isothermal regions due to the presence of a proximate thermal gradient, with tensile stress deviations of up to 75 MPa being observed across the sample at a constant temperature. Significant strain is also observed due to the presence of thermal gradients when compared to work carried out at isothermal conditions.

Highlights

Solid oxide fuel cells (SOFCs) are considered to be a promising future power technology due to their high efficiency and fuel flexibility [1]; the widespread adoption of such devices has been inhibited by, amongst other things, the long term mechanical instability of the stack [2e5]
Thermal imaging has previously been performed on pellet SOFCs by Brett et al [8] who utilised the technique to evaluate heat transfer coefficients, and Pomfret et al [9,10] who used a filtered CCD camera to investigate the carbon formation caused by employing methanol and ethanol as direct fuels for SOFCs
Non-uniform thermal expansion has been observed in Ni/YSZ SOFC anode half cells due to the presence of operationally relevant thermal gradients

Summary

Introduction

Solid oxide fuel cells (SOFCs) are considered to be a promising future power technology due to their high efficiency and fuel flexibility [1]; the widespread adoption of such devices has been inhibited by, amongst other things, the long term mechanical instability of the stack [2e5]. Celik et al [34] utilised FEA analysis to investigate the degradation of SOFCs at relevant operating temperatures; the authors conclude that stress can be decreased by increasing the YSZ or Ni content and the porosity of the anode; this results in reduced mechanical strength in the sample [35,36]. Focused ion beam tomography has been utilised in order to conduct microstructural stress analysis on SOFC anodes [48]; this work indicated that the stresses experienced by the Ni phase within the anode exceeded the yield strength, resulting in plastic deformation at temperatures approaching 800 C This highlights the need for fundamental studies in order to fully capture the effects of operationally relevant phenomena on SOFCs in operando. The results highlight the significant effect thermal gradients have upon both internal stresses and strains in SOFCs, while showing the effects of a gradient located close to an isothermal region on the strain within the cell

Experimental

Results and discussion

Conclusions

Methods