Microstructural evolution of nanosized Ce0.8Gd0.2O1.9/Ni infiltrate in a Zr0.84Y0.16O1.92-Sr0.94Ti0.9Nb0.1O3- based SOFC anode under electrochemical evaluation

W. Zhang, L. Theil Kuhn, T. Ramos, P.S. Jørgensen, B.R. Sudireddy, J.J. BentzenDepartment of Energy Conversion and Storage, Risø Campus - Technical University of Denmark

References[1] Mogensen, M., Sammes, N.M. & Tompsett, G.A. (2000) Solid State Ionics 129, 63-94.[2] Muecke, U.P., Graf, S., Rhyner, U. & Gauckler, L.J. (2008) Acta Mater. 56, 677-687.[3] Iwanschitz, B., Sfeir, J., Mai, A. & Schutze, M. (2010) J. Electrochem. Soc. 157, B269-B278.[4] Thurber, A. et al. (2007) Phys. Rev. B 76, 165206.[5] Shan, W., Luo, M, Ying, P., Shen, W. &  Li, C. (2003) Appl. Catal. A 246, 1-9.

IntroductionCeO2-based materials have received intensive attention as they have a lot of important physical, chemical and electrochemical properties [1]. Recently, Gd-doped CeO2 (CGO)/Ni infiltrate was found to be an effective electrocatalyst, greatly enhancing the electrocatalytic activity for fuel oxidation in solid oxide fuel cells (SOFCs) [2,3]. How stable is the structure of infiltrated nano-sized electrocatalysts under electrochemical operation? This issue is usually addressed by evaluating electrode performance without detailed structural investigations. However, the behavior of electrocatalysts are of paramount importance for performance and performance stability. Therefore an accurate understanding of the microstructure evolution during

electrochemical operation will facilitate evaluating performances of SOFC anodes, and in turn optimize its design.Here we report a wealth of microstructural investigations of Ce0.8Gd0.2O1.9/Ni (hereafter CGO/Ni)-infiltrated Zr0.84Y0.16O1.92 composited Sr0.94Ti0.9Nb0.1O3- (STN94/8YSZ) anode in a symmetric cell design under a short electrochemical evaluation test (fingerprint test), applying electrochemical impedance spectroscopy (EIS) at mild 3% H2O/H2 and harsh 50% H2O/H2 environment at temperature up to 850 C.

Schematic of a symmetric cell with anode.

Backscattered electron images of the fracture surface of the sample (a) before and (b) after the fingerprint test. The CGO/Ni infiltrates are indicated by the arrows.

TEM images of the sample (a) before and (b) after the fingerprint test. The right part shows the SAED pattern and its indexing of the egions A and B in the left TEM images, respectively.

HRTEM image of the composite anode sample before the test.

(a) HRTEM image and (b) filtered counterpart of the sample after the test. The inset in (a) is the low-magnification TEM image.

Bright-field (BF) STEM EDX elemental maps of the sample (a) before and (b) after the test. For each series, electronic image, Ce, Ni, Sr and Zr maps are listed, respectively.

BF-STEM EDX line scan profiles of the sample (left) before and (right) after the test, and the middle two images showing the corresponding regions.

Formed networks of CGO and Ni after the phase separation. (a) BF STEM images showing the existence of both CGO and Ni phases, which are confirmed by EDX line scan profiles. b) Schematic illustration of microstructure evolution during the fingerprint test.

The statistics distribution of Ce/Gd and Ni/Ce elemental ratios of the infiltrate (a) before and (b) after the test, by using heights of the peaks of Ni and CGO in EDX line scan profiles in this view.

Conclusions

Results and Discussion

AcknowledgementsThe authors wish to thank E. Abdellahi for TEM sample preparation and S. Primdahl for discussions. W. Zhang would like to thank A. Mohammed Hussain, J.R. Bowen and M. Chen for some fruitful discussion. Financial support from the European project SCOTAS-SOFC (FCH-JU 256730) is gratefully acknowledged.

* Corresponding author: Wei Zhang, wzha@dtu.dk

Applying comprehensive microstructural investigations allowed for the correlation between performance degradation and microstructural changes. In terms of both phase and composition, a) the phase separation from CGO/Ni to CGO and Ni daughter phases; b) both CGO and Ni phases have grown up with the final size ranging from several to tens of nanometers as well as some above 100 nm, in comparison with their parent ~ 5 nm CGO/Ni infiltrates. However, the formed networks of CGO and Ni are identified.

The involved mechanism of such infiltration instability was correlated with the performance of anode for SOFC. The microstructure instability resulted in a slight adverse effect on the electrochemical performance of SOFC.

Considering that CGO keeps the fluorite structure (CaF2 type) of CeO2 lattice (space group Fm-3m) by replacing Ce4+ sites using Gd3+, the present Ni is well dispersed in the CGO phase, through either substituting Ce4+/Gd3+ or at the interstitial sites according to the large difference of ionic radius of between Ni2+ (0.69Å) and Ce4+/Gd3+ (0.92/0.938Å).

The analysis is also in agreement with previous reports that nickel was successfully doped to nanocrystalline ceria [4]. Ascribed to the strong metal-support interaction (SMSI) effect between nickel and ceria, nickel can be incorporated into the lattice of ceria nanoparticles.According to density functional theory calculations, oxygen vacancies are likely to be formed in the pure or doped ceria (particularly during

the reduction), which is important for anchoring of metal elements.

The occurrence of the main phase separation route can be expressed as CGO/Ni CGO+Ni. It was reported that it is difficult to reduce the nickel intermediated ceria solid solution [5] with the fluorite structure to some extent. The environment of high temperature and heavy steam as well as electric current may be the main reason for such microstructure evolution.

The tiny Ni clusters can react with oxygen and H2O to form Ni(OH)2 in the presence of local higher oxygen activity; Ni is then recondensed when Ni(OH)2 is reduced at the sites of local lower oxygen activity. Overview of polarisation (Rp) and series resistances (Rs) as a function of test temperature and H2O

content during the fingerprint test (see the data and for the tested sample in this work).Degradation of both Rp and Rs occurred for all samples, but the present sample is at the lower level, which is associated with the formation of networks CGO and Ni although they were grown.

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