Anode-supported solid oxide fuel cell with yttria-stabilized zirconia/gadolinia-doped ceria bilalyer electrolyte prepared by wet ceramic co-sintering process
Introduction
Decrease of the operating temperature of solid oxide fuel cells (SOFCs) from typically ∼1000 °C to an intermediate temperature range of 500 to 800 °C, offers many benefits, namely, low material degradation, high system reliability, long stack lifetime, short start-up time, and significant decreases in material requirements and fabrication costs due to the potential utilization of inexpensive and easily machinable metallic materials for the stack. Over the past few years, considerable efforts have been made to lower the operating temperature of SOFCs [1], [2], [3], [4], [5], [6]. Three approaches have been adopted: decreasing the electrolyte thickness, utilizing new electrolyte materials with high ionic conductivity at intermediate temperatures, and reducing the electrode polarization resistance. Due to its much higher ionic conductivity at intermediate temperatures than the conventional electrolyte material of yttria-stabilized zirconia (YSZ), doped ceria is regarded as one of the most promising electrolyte candidates for intermediate temperature SOFCs [7].
Many research groups have reported the development of intermediate-temperature SOFCs based on a thin-film electrolyte of doped ceria, which has been fabricated by various processes such as multi-layer tape casting [8], screen printing [9], [10], dry pressing [11], spray coating [12], [13], and spin coating [3]. Encouraging power densities have been achieved with ceria-based cells at temperatures below 650 °C. Doped ceria is prone, however, to reduction at low oxygen partial pressures [14]. The reduction of ceria from Ce4+ to Ce3+ will give rise to electronic conduction and thus result in a non-negligible loss in the open-circuit voltage (OCV) of the cell. It will also cause lattice expansion of the ceria electrolyte at the fuel side, and there by lead to mechanical stability problems with the cell or stack [15], [16]. To overcome the OCV loss and improve the chemical stability of doped ceria electrolytes in reducing atmospheres, one of the approaches proposed is to coat doped ceria with a very thin YSZ film to form a bilayer electrolyte. In this electrolyte, the YSZ film functions as an electronic blocking layer since YSZ is quite stable under reducing atmosphere conditions.
Since the ionic conductivity of YSZ is very low at intermediate temperatures, the YSZ film as an electronic blocking layer should be sufficiently thin so as to minimize its contribution to the overall resistance of the bilayer electrolyte. From a thermodynamic viewpoint, when the oxygen partial pressure across the ceria electrolyte is higher than the equilibrium value of the Ce2O3/CeO2 redox reaction at a certain temperature, the ceria electrolyte should be chemically stable. Hence, the stability of the bilayer electrolyte will be determined by the interfacial oxygen partial pressure between the YSZ film and the ceria substrate. Theoretical investigations have demonstrated [17], [18], [19] that the interfacial oxygen partial pressure is determined by the thickness ratio of the two electrolyte layers and that, generally, a YSZ film of a few μm in thickness should be sufficient to prevent the ceria electrolyte from reduction and thus overcome the OCV loss due to ceria reduction [17], [18], [19]. Towards this end, many techniques such as RF sputtering [20], [21], ion plating [22], [23], electrochemical vapour deposition (EVD) [21], metal–organic chemical vapour deposition (MOCVD) [24], DC sputtering [25] and sol–gel coating [26], [27], [28] have been employed to deposit a thin film of YSZ on doped-ceria electrolyte.
By reviewing experimental studies of the YSZ/doped-ceria bilayer electrolyte reported in the literature (see Table 1), four aspects can be summarized. First, except for the work reported by Tsai et al. [25], all the investigations were associated with a thin film of YSZ deposited on a relatively thick substrate of doped ceria (0.4–2 mm). Second, bilayer electrolytes exhibited an improvement in OCV compared with uncoated doped-ceria electrolyte, which demonstrates that YSZ deposition is practically effective in suppressing electronic conduction in the ceria electrolyte. Nevertheless, the improved OCVs are not exceptional as they still exhibit an obvious deviation from the theoretical electromotive forces (EMF), except for the bilayer electrolytes with a YSZ film deposited by ion plating [22], [23] or DC sputtering [25]. Third, the fact that the bilayer electrolyte with a 1.86 μm YSZ film deposited by the ion plating on a 1.825 mm samaria-doped ceria (SDC) substrate demonstrated an OCV of 1.08 V at 800 °C in H2/O2 atmospheres, which is quite close to the theoretical EMF, does indicate that the OCV loss for doped-ceria electrolyte cells can be satisfactorily avoided by introducing a YSZ thin film with good quality. Fourth, all the above methods used for YSZ film deposition involve either costly equipment (such as ion plating and RF sputtering) or complicated processes (such as sol–gel coating).
In view of these aspects, a wet ceramic process combined with co-sintering was selected to fabricate the thin film of YSZ/GDC bilayer electrolyte in this study because it is a very simple method and does not require any expensive equipment. With this powder-based wet ceramic process, 10-μm bilayer electrolyte thin films consisting of a 3 μm YSZ layer and a 7 μm GDC layer have been successfully obtained after co-sintering with the anode substrate at 1400 °C. Based on a high-quality thin film of YSZ/GDC bilayer electrolyte supported on Ni–YSZ cermet anode substrate, single cells have been assembled with a L0.8Sr0.2Co0.2Fe0.8O3–GDC (50:50 by weight) composite as the cathode and tested in humidified H2/air atmospheres.
Section snippets
Cell fabrication
Commercial powders of 8 mol% Y2O3/ZrO2 (YSZ, Tosoh, Japan) and NiO (J.T. Baker, USA) were used to prepare Ni–YSZ cermet anode supporting substrates. The powders were mixed in a composition of 40 wt.% YSZ and 60 wt.% NiO by ball milling in isopropanol, then the mixture was dried on a hot plate and milled manually using a mortar and a pestle to produce a homogeneously mixed anode powder. The resultant anode powder was compacted into discs under uniaxial pressure using a steel die of 24 mm in
Microstructure of electrolyte films
To serve effectively as an electronic blocking layer, the YSZ film is required to be dense and crack-free to avoid exposure of the GDC layer to the fuel. At the same time, the YSZ film should be deposited as thinly as possible to minimize the electrolyte resistance. Therefore, before the YSZ/GDC bilayer electrolyte film was prepared, attempts were made to deposit a thin monolayer YSZ film on the anode substrate by spray coating. After a number of trials, YSZ films with good quality and a
Conclusions
Thin films of YSZ (3 μm)/GDC (7 μm) bilayer electrolyte have been fabricated successfully on an anode substrate using a simple and cost-effective wet ceramic co-sintering process. Anode-supported single cells based on the bilayer electrolyte film have OCVs that are very close to the theoretical values. This implies that electronic conduction in the GDC electrolyte is effectively blocked by the YSZ film.
An anode-supported bilayer electrolyte cell exhibits severe diffusion polarization due to
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