Sinterability, microstructures and electrical properties of Ni/Gd-doped ceria cermets used as anode materials for SOFCs
Introduction
The CeO2-based materials are an upcoming alternative solid electrolyte to yttria stabilized zirconia (YSZ) in solid oxide fuel cell (SOFC) applications. They have higher ion conductivity and lower ohmic losses than YSZ, and can also operate at lower temperatures (600–800 °C). These electrolytes based on ceria require special electrodes with a high performance and thermo-mechanically and chemically compatible.
Ni has been commonly used as anode material for SOFCs due to its high catalytic activity for the dehydrogenation of hydrocarbons as well as a highly electronic conductivity. The introduction of Ni into the doped ceria matrix would be a way to acquire sufficient electronic conductivity in order to prevent lateral electrical losses, therefore cermets based on doped ceria may be preferred for a high performance of the electrodes. The important functions of the CGO particles in the Ni–CGO cermet would be to: (a) supply oxide ions to the TPB, (b) suppress the sintering of Ni, and (c) match the thermal expansion coefficients between the cermet and the CGO electrolyte.
The electrical properties of a Ni–CGO cermet can be explained by percolation theory where the role of the microstructure it is very important for a satisfactory behavior of the anode material. Ni particles in the cermet should be connected to CGO particles (oxide ion supplier) and also exposed to H2 gas. Kawada et al. 1 have demonstrated that the preparation process is of importance in controlling the electrode stability and performance and it has been shown that optimization of the anode microstructure by decreasing the particle size of the Ni phase would increase the length of Ni–CGO triple phase boundaries, and thus improve the anodic performance. Recent studies2, 3, 4 on Ni/rare-earth-doped ceria cermet preparation are focused towards achievement of a uniform distribution of fine Ni particles in the ceramic matrix using different preparation techniques. In the present work, a polymeric organic complex solution method was used to prepare NiO/Ce0.9Gd0.1O1.95 (NiO–CGO) composites and the influence of Ni content in electrical conductivity was studied in order to identify the most suitable microstructure.
Section snippets
Experimental
NiO/Ce0.9Gd0.1O1.95 powders with three different proportions5 (50–50, 45–55 and 40–60 wt%) were prepared by the polymeric organic complex solution method. Aqueous solutions of corresponding nitrates were mixed by stirring with nitric acid and ethylenglycol. The as-obtained solutions were treated thermally in three steps, 80 °C for 24 h, 120 °C for 4 h and 220 °C for 24 h in oven to obtain a black resin embedding the NiO–CGO particles. After milling in an Agatha mortar, the resin was calcined at 800 °C
Characteristics of NiO–CGO powder
The simultaneous TG/DTA curves for the different prepared powders during heating of the dried polymeric gel between room temperature and 1000 °C, revealed the same thermal features in all cases and, for clarity, only that corresponding to the composite NiO–CGO 50 wt% is shown in Fig. 1. The TG curve shows a total weight loss of about 50% up to1000 °C, due to evolution of carbon compounds (CO or CO2) and elimination of nitrates (N2, NO and NO2). According to DTA curve it is found that in a small
Conclusion
Nanometric powders NiO–Ce0.9Gd0.1O1.95 with highly sinterizable characteristics were prepared by the polymeric organic complex solution method. Sintered composites NiO–CGO exhibit adequate microstructures and after reducing Ni–CGO cermets present a uniform distribution of spherical particles Ni surrounded by a pore space and by the CGO particles. Such microstructures could improve the TPB area between gas (H2), Ni (electronic conductor) and CGO (electrolyte with a high ionic conductivity). The
Acknowledgements
Financial support from the CICYT. MAT 2003-01163 Project and a grant from the Autonomous Community of Madrid are gratefully acknowledged.
References (7)
- et al.
Synthesis and properties of Ni–SDC cermets for IT-SOFC anode by co-precipitation
Solid State Ionics
(2004) - et al.
High performance electrodes for reduced temperature solid oxide fuel cells with doped lanthanum gallate electrolyte. I. Ni–SDC cermet anode
J. Power Sources
(2000) - et al.
Processing and characterisation of a fine nickel oxide/zirconia/composite prepared by polymeric complex solution synthesis”
J. Eur. Ceram. Soc.
(2003)
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