Understanding the thermo-mechanical behaviour of solid oxide fuel cell anodes using synchrotron X-ray diffraction

Abstract

The formation of cermet materials via the addition of electrolyte ceramic to metal-based electrodes has become widely adopted in solid oxide fuel cell fabrication due to its benefits in maximizing triple-phase boundary densities, while mitigating bulk thermal expansion mismatch between electrode and electrolyte layers. This work improves thermo-mechanical understanding via examination of nickel-based anode materials using synchrotron X-ray diffraction; two cermet materials are studied: Ni-YSZ and Ni-GDC, with comparison to a ceramic-free Ni sample. Findings conclude that although the ceramic addition has minor effects on the cubic Ni structure within isothermal environments, stress induced by the different thermal properties within the cermet materials results in a shifted Ni thermal expansion peak on passing the Curie point. Moreover, extended cycling of the Ni-YSZ sample suggests that low-temperature operation (ca. 600 ℃) may require several thermal cycles, or extended dwell times, to alleviate residual Ni stresses, this has potential implications for SOFC design and operation strategies. Highlights  The effects of ceramic addition to the expansion of the cubic structure of Ni in isothermal environments are minor over a single thermal cycle.  Accelerated stress tests revealed that lower temperature operation, i.e. 600 ℃ rather than higher 1000 ℃, results in deformation of the lattice parameters at both, low and high temperatures, likely due to a combination of two mechanisms: shock-induced tensile stress and delayed relaxation hysteresis.  High and low temperature deformation has been found to be divergent resulting in a greater relative deformation in the cold lattice parameters.  The thermal expansion non-linearity on passing the Ni Curie temperature is not suppressed; however, it is shifted to a higher temperature, a temperature that is dependent upon the ceramic employed.

Developments in X-ray tomography characterization for electrochemical devices

“…Therefore, thermal ramp-rates are typically limited to below 5 ºC min -1 in order to prevent delamination and cracking. Whilst this has been combatted by the addition of ceramic to the anode to reduce undesirable macroscopic thermal expansion arising from mismatch between the constituent layers [48], microscopic interactions in both isothermal [49] and non-isothermal [50] environments are still thought to be problematic. The ceramic skeleton is also thought to impede agglomeration mechanisms in both electrodes [51,22].…”

“…Furthermore, many chemistries are emerging as next-generation electrode materials such as mixed-ionic-electronic-conductors (MIECs) [182], which may also benefit from such studies. Finally, there are many possibilities for employing XRD-CT to investigate SOFC materials; although investigations using point-and powder-XRD into SOFCs, particularly with focus on the anode [49,50], have proven very valuable, extending such studies to become spatially resolved would further elucidate the origins and developments of cell constituents. For example, the ability to spatially resolve the multi-scale, structural, chemical and mechanical developments through temporally and spatially resolved crystal-, nano-, micro-and macro-structural mapping can provide unprecedented insight [183].…”

In situ compression and X-ray computed tomography of flow battery electrodes

“…Transport properties of porous materials are strongly linked to their microstructure, and detailed investigation of these materials in 3D is required for a full understanding of many electrochemical systems [25,26] . Recent advances in X-ray computed tomography (CT) have led to the technique being employed in electrochemical systems such as lithiumion batteries [27][28][29][30][31][32][33][34][35][36][37] , polymer electrolyte membrane fuel cells [38][39][40][41][42] , solid oxide fuel cells [43][44][45][46][47][48][49] and supercapacitors [50] . Recently, X-ray CT has also been applied to flow battery systems for both commercially available felts and electrospun carbonised fibres [51][52][53][54][55][56] .…”

X‐ray Nano Computed Tomography of Electrospun Fibrous Mats as Flow Battery Electrodes

“…The recent advances in X‐ray computed tomography (CT) have allowed characterisation of the microstructure of many electrochemical devices such as lithium‐ion batteries, solid oxide fuel cells, polymer electrolyte fuel cells, supercapacitors, and, more recently, RFBs . The application of tomographic images in the study of porous media has the benefit of allowing numerical simulations to be conducted directly on the actual structures represented in the images, and therefore allowing prediction of the material's performance based on its microstructural properties.…”