SOFC cathodic layers using wet powder spraying technique with self synthesized nanopowders
Introduction
Fuel cells are energy conversion devices that directly convert the chemical energy of a fuel into electrical power and heat. Among the fuel cell types, solid oxide fuel cells (SOFC) have attracted much attention due to their advantages such as higher electrical conversion efficiency, long-term stability, fuel flexibility and environmental friendliness [1], [2], [3], [4], [5].
The two main configurations of SOFCs are tubular and planar [6], [7], [8] ], [9], [10], [11]. Among the different configurations, this study is focused in planar SOFCs, which have a much simpler manufacturing process and lower fabrication cost than those of their tubular counterparts [12], [13], [14]. Regarding planar configuration, two types are preferred: electrode (mainly at anode) supported cells and electrolyte supported cells [3], [15], [16]. Although electrolyte supported cells may exhibit higher ohmic losses, they are more robust and have demonstrated much better stability during the reduction-oxidation processes and thermal cycles [17], [18].
The main drawbacks for the industrial implementation of SOFCs are their manufacturing cost and high degradation rate [14], [19], [20], [21]. The aim is to replace the expensive and complex processing for the cell manufacturing with cheaper, simpler and industrially scalable techniques. Within these techniques, spray coating is a cost effective deposition method for electrode layers, showing good quality and thickness control. In particular, this method present a highly reproducible process for covering planar surfaces [22], [23], [24], [25].
In the development of a planar SOFC stack, each repeating unit is composed of an anode, electrolyte, cathode and interconnect [26], [27]. Among the typical material choices, yttria stabilized zirconia (YSZ) is the most commonly used electrolyte for SOFCs because of its low cost, high ionic transference number in oxidizing and reducing atmospheres, and good chemical and mechanical properties [28], [29], [30]. The mechanical properties of the YSZ allow it to withstand the residual stresses from cell fabrication processes as well as the stresses from the operational conditions. Samarium doped ceria (SDC) or gadolinium doped ceria (GDC) can also be used as electrolytes [31], [32], [33], or as a protective barrier between the YSZ electrolyte and commonly used cathode materials, preventing the formation of poorly conducting secondary phases, such as La2Zr2O7 or SrZrO3, which are deleterious for as cathode performance [34], [35], [36], [37], [38]. Iron containing perovskites such as La0.6Sr0.4FeO3 (LSF) or La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) are good candidates as SOFC cathodes showing high mixed conductivity and good catalytic activity for the oxygen reduction reaction (ORR) [39], [40], [41], [42]. The use of cathode contact layers such as LaNi0.6Fe0.4O3 (LNF) and LaNi0.6Co0.4O3 (LNC) have been demonstrated to improve the electron transfer through the contact interface from interconnect to the cathode layer. In addition, it means that the ORR of the TPB in the cathode receives more electrons from the interconnector, increasing the performance of the cell [43], [44], [45].
In this research, the deposition of the cathode side components (SDC as protective layer, LSF as cathode and LNF as contact layer) by wet powder spraying (WPS) deposition has been studied with the aim of optimizing the fabrication process using self-made materials, using a cheap, simple and scalable technique. Furthermore, special attention has been paid to the influence of the ball milling process and the layer thickness in determining the morphology and stability of the layers. The systems were characterized by X-ray diffraction (XRD) Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDX). Electrochemical impedance spectroscopy (EIS) measurements were performed in a button cell test rig (NorECs) and a Zanher Zennium workstation, in air at 700 and 800 °C.
Section snippets
Synthesis and system fabrication
All the SOFC component powders were prepared by a glycine nitrate (GN) process. Stoichiometric amounts of the corresponding metal nitrates were dissolved in deionized water. For all compositions, glycine was then added into the nitrate aqueous solution (Glycine-nitrate 1:1) under continuous stirring. The resulting viscous liquid was auto-ignited by heating up to approximately 455 °C and the obtained powders were calcined around 600 °C for 5 h to remove carbon residues. In the case of LSF and
Initial structural study of the compounds
The purity of the samples was analyzed by X-ray diffraction. All the materials (YSZ, SDC and LSF) prepared through the GN combustion route show the desired final phases. The signal identification for all XRD patterns was in good agreement with the Powder Diffraction File database (PDF). All the samples were pure as can be observed in Fig. 3 for different batches. For the LNF compound, the appearance of extra shoulders in the experimental profile indicated a possible phase segregation to give
Conclusions
Uniform and well-adhered coatings of LSF, SDC and LNF have been deposited by wet powder spraying onto an YSZ electrolyte support, with self-synthesized materials. The microstructure of the system has been shown to be unstable at thicknesses greater than 10 μm due to stresses caused by different TECs, while layers below 3 μm thickness lacked homogeneity. The best results have been obtained with the inks ball milled at 270 rpm, avoiding the appearance of coarsening in the layers after sintering.
Acknowledgements
This research has been funded by the Ministerio de Economía, Industria y Competitividad [MAT2016-76739-R and MAT2015-68078-R] [AEI/FEDER, UE] and Dpto. Educación of the Basque Government [IT-630-13]. The authors thanks the support received by the European Regional Development Fund (ERDF). The technical support of SGIker of UPV/EHU is gratefully acknowledged. A. Wain-Martin thanks Ministerio de Economía y Competitividad for funding his work [BES-2014-068433].
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