Original Article
Scaling up aqueous processing of A-site deficient strontium titanate for SOFC anode supports

https://doi.org/10.1016/j.jeurceramsoc.2017.11.057Get rights and content

Abstract

All ceramic anode supported half cells of technically relevant scale were fabricated in this study, using a novel strontium titanate anode material. The use of this material would be highly advantageous in solid oxide fuel cells due to its redox tolerance and resistance to coking and sulphur poisoning. Successful fabrication was possible through aqueous tape casting of both anode support and electrolyte layers and subsequent lamination. Screen printing of electrolyte layers onto green anode tapes was also attempted but resulted in cracked electrolyte layers upon firing. Microstructural, electrical and mechanical properties of anode supports and half cells will be discussed. The use of two different commercial titanate powders with nominal identical, but in reality different stoichiometries, strongly affect electrical and mechanical properties. Careful consideration of such variations between powder suppliers, and batches of the same supplier, is critical for the successful implementation of ceramic anode supported solid oxide fuel cells.

Introduction

Conventional state-of-the-art Ni cermet anodes for solid oxide fuel cells (SOFCs) perform well in hydrogen and high steam reformate fuels [1], but still suffer from a number of drawbacks. Their poor redox stability, tendency for coking in hydrocarbon fuels and low sulphur tolerance has led many researchers to look for alternative anode materials. One promising approach is to replace the cermet with an electronically conducting ceramic to provide both structural support and current collection, whereas electrocatalytic activity can be obtained through impregnation of precursors solutions of electrocatalytically active materials into the porous scaffold. A-site deficient strontium titanates show high n-type conductivity and therefore offer promise as the electronically conducting backbone. Previous work has shown promising results for a La and Ca co-doped SrTiO3 anode in an electrolyte supported cell (ESC) configuration, into which nickel and ceria were impregnated to serve as fuel oxidation catalysts. Fuel cell performances in hydrogen were similar to those for Ni-cermet anodes, but with much improved redox stability [2], [3], [4].

The A-site deficient La0.20Sr0.25Ca0.45TiO3 (LSCTA-) shows reasonable mechanical strength [5] and may therefore also be suitable as anode support material in an anode supported cell (ASC) design, allowing for SOFC operation at lower temperatures. To this end, an aqueous tape casting process was developed, as an environmentally friendly alternative to the organics based process [6], [7]. Also, the aqueous processing allows for using a wider range of polymeric pore formers that would otherwise dissolve in the solvent. This in turn allows for a more precise control of the microstructure of the porous scaffold for optimal electrode performance. Although strontium titanates have been used previously in tape cast electrodes, the resulting cells are commonly small (<1 cm2) and the anode supports are typically composites of stabilised zirconia, which provides structural support and ionic conductivity [8], [9].

In this study we report on the very first scale-up attempt of single phase LSCTA- anode supported SOFCs, from the laboratory scale casting process at the University of St Andrews to the pre-pilot scale environment at the Technical University of Denmark, DTU (Roskilde) [10]. Apart from enabling fabrication of larger cells, the optimised environment at DTU should allow for better control of the process, ultimately leading to superior ceramics. Half cells are prepared by depositing thin electrolyte layers onto tape cast anode supports through either screen printing or lamination. Resulting ceramics were analysed for their microstructure and mechanical properties. The electrical properties of the ceramics, prepared by both small and large scale processing, were finally assessed and compared with previous reports.

The ultimate aim of the work was to produce all-ceramic half cells at an industrially relevant scale using industrially relevant processing techniques, as part of a proof-of-concept project. The time constraints imposed by such projects forced the employment of a very pragmatic approach, rather than a fully systematic bottom-up strategy. The results reported in this manuscript will reflect this and should therefore be regarded as a report on the practical development of full scale strontium titanate supported SOFC, the novelty of which is of interest in itself. To our knowledge, the largest reported cells of this kind are 5 × 5 cm2, produced by the less scalable process of warm pressing, rather than tape casting [11], [12].

Section snippets

Powders

Two LSCTA- (La0.20Sr0.25Ca0.45TiO3) powders were used in this study. Powder A was synthesised by Topsoe Fuel Cells A/S (TOFC), Denmark, using the drip pyrolysis method [13]. The second, powder B, was purchased from CerPoTech AS, Norway, who use a comparable solution based method, namely spray pyrolysis. The resulting powders were nanosized, high surface area powders (∼40 m2/g, measured by BET, Tristar II 3020, Micromeritics, UK). The powders were calcined in air for 5 h at 1200–1300 °C prior to

Powder (Phase analysis)

Both LSCTA- powders were characterised by X-ray diffraction after air calcination at 1300 °C; the results are shown in Fig. 1. Powder A could be indexed using the orthorhombic spacegroup P b n m, with lattice parameters a = 5.464 Å, b = 5.465 Å, c = 7.734 Å, which is in good agreement with previously reported data by Aljaberi and Irvine [15], despite slightly larger lattice parameters. There are some minor additional reflections, suggesting the presence of a secondary phase. In powder B, however, even

Conclusions

The transfer of ceramic formulations and methods developed in the lab to an industrial environment still forms a serious barrier to the successful implementation of many novel materials into commercial SOFC and other applications. In this study it has been proven possible to fabricate anode supported half cells on a technically relevant scale, using such a novel ceramic anode material. Moreover, the paper discusses processing techniques, which are industrially relevant and suitable for further

Acknowledgements

The authors gratefully acknowledge funding from the Fuel Cells and Hydrogen Joint Undertaking under grant agreement n° 256730. Angus Calder and Sylvia Williamson are also acknowledged for carrying out XRF and BET measurements, respectively.

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