BaCe0.85Y0.15O2.925 dense layer by wet powder spraying as electrolyte for SOFC/SOEC applications
Introduction
Fuel cells and hydrogen have been recognized as important technologies to reach the low carbon economy planned by the European Union for 2050 [1], especially if carbon free hydrogen produced through renewable, nuclear or carbon capture and sequestration (CCS) methods will be widely available. In the last 20 years studies on solid oxide fuel cells (SOFCs) have increased exponentially. The reasons which make high-temperature solid oxide fuel cells very promising in a new energy scenario are their very high efficiencies, low pollutant emissions, and operation flexibility, independently from sizes (high production power plants as well as in domestic production) [2]. High temperature working conditions result in very high energetic efficiency with low cost materials even when they operate in reverse polarization, as solid oxide electrolysers (SOECs). The capability to work in reverse mode makes SOFCs suitable for integration in hydrogen production units powered by renewable energy sources in both autonomous and grid-connected systems [3].
As regards the operation of reversible or regenerative cells, the advancements made on the use of proton-conducting ceramics such as barium cerate-based oxides instead of the oxide-conducting electrolytes (gadolinia-doped ceria and yttria-stabilized zirconia) are noteworthy [4], [5]. These ceramics exhibit protonic conductivity of the order of 10− 2 S cm− 1 at intermediate temperatures, in the range of 500–650 °C, and advantageously both in SOFC and SOEC mode hydrogen is not water vapor diluted in the fuel electrode, thus simplifying the steam electrolysis plant or enhancing the fuel utilization factor in SOFCs. The opportunity to work at lower temperatures reduces compatibility, sealing and durability issues arising from long-term interactions of different component materials, these problems being the key bottleneck that nowadays prevents the penetration of such fuel cells into the market. BaCe0.85Y0.15O2.925 is a highly conductive electrolyte, but its poor chemical stability limits its practical application [6]. To overcome this difficulty some authors [7], [8] have recently investigated the performances of BCY electrolyte layers sandwiched between two other layers that are chemically more stable (Y-doped BaZrO3, BZY and (Pr, Y)-doped BaZrO3, BZPY), with very little losses in electrochemical performances. They used co-pressing [7] and pulsed laser deposition [8] to produce the BCY layer and suggest that the key-factor is the protective layer thickness. Several techniques have been proposed for the fabrication of dense layers in the few μm thickness range, including wet coating methods like sol–gel dip-coating and spin-coating [9], pulsed laser deposition (PLD) [8], [10], physical vapor deposition (PVD) [11] and chemical vapor deposition (CVD) [12].
Wet powder spraying (WPS) is a relatively cost-effective process, originally used for preparing porous ceramic layers [13], [14], [15]. The method is suitable for different types of surfaces (planar, curve, corrugated) and is appropriate also for producing dense layers [16], by means of adapting some process parameters (distance between the nozzle and the substrate, spray velocity/pressure, angle of the spray cone and viscosity of the suspension itself).
In this work the deposition of a dense and thin BaCe0.85Y0.15O2.925 (BCY) layer by WPS was investigated, with the aim of optimizing the fabrication procedure of a thin and dense ceramic layer through a simple and scaling-up technology, suited to be used also with different ceramic materials and electrolyte multilayers (e.g. BZY/BCY/BZY). Moreover, attention was paid to the influence of the substrate morphology on the densification of the sprayed film.
Section snippets
Experimental
The WPS of BCY was carried out on a porous Ni-BCY anode substrate, in order to produce a solid oxide anode supported cell. The anode was made of two different layers, i.e. a gas distributor layer with high porosity and thickness (600 μm), having a mechanical supporting function, connected to a less porous anode functional layer (AFL), as a substrate for spraying and having the purpose of increasing the triple-phase boundary length at the electrolyte/anode interface. Then two batches with same
Anode preparation
The optimum microstructure of a porous Ni-based cermet anode strongly depends on the starting powders and on the processes used to mix the components [21], [22]. Ni/BCY cermet is commonly fabricated by ball milling of NiO and BCY powders [23], [24], followed by the anode layer preparation technique (e.g. cold pressing and tape-casting). NiO reduction upon exposure to hydrogen gas constitutes the last process after the electrolyte and cathode deposition. The very similar value of densities of
Conclusions
The aim of this work was the fabrication of a thin BCY electrolyte layer by wet powder spraying (WPS), which is a relatively simple and cost-effective method easily suitable in view of scaling up. Two main problems have to be faced to obtain thin dense layers through this technique. The first is related to process parameters, i.e. rheology and stability of the sprayed suspension, and the second one is connected with the substrate morphology. Most of all, it was concluded that the suspension
Acknowledgements
The financial support of the Italian Ministry of Education, University and Research (MIUR) (Protocol Number 2010KHLKFC_005), National PRIN Project “BIOITSOFC” is acknowledged.
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