Journal of Power Sources 192 (2009) 367–371 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour Short communication Unbiased characterization of three-phase microstructure of porous lanthanum doped strontium manganite/yttria-stabilized zirconia composite cathodes for solid oxide fuel cells using atomic force microscopy and stereology S. Zhang, M. Lynch, A.M. Gokhale ∗ , M. Liu School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, GA 30332-0245, United States article info Article history: Received 4 March 2009 Received in revised form 20 March 2009 Accepted 23 March 2009 Available online 31 March 2009 Keywords: Porous composite cathode Microstructure Triple phase boundaries Atomic force microscope abstract Microstructural characteristics of porous LSM/YSZ composite cathodes greatly influence the performance of solid oxide fuel cells. The triple phase boundaries, for example, account for a significant portion of the electrochemically active sites in these porous composite cathodes. Nonetheless, experimental character- ization of the relevant microstructural attributes has been problematic due to lack of suitable microscopy techniques for simultaneous observations of all three phases (i.e., LSM, YSZ, and porosity) needed for identification and unbiased characterization of the triple phase boundaries. In this contribution it is shown that a combination of chemical etching and atomic force microscopy clearly reveals all three phases and the triple phase junctions in the microstructural sections. Further, stereological techniques based on the geometric probabilities of stochastic geometry enable unbiased statistical estimation of total triple phase boundary length per unit volume and other microstructural attributes from simple counting measurements performed on representative microstructural sections. © 2009 Elsevier B.V. All rights reserved. 1. Introduction and background Lanthanum doped strontium manganite (LSM) is the most widely used cathode for solid oxide fuel cells (SOFC) based on yttria-stabilized zirconia (YSZ) electrolyte because of its excellent chemical and thermal compatibility with YSZ. Nonetheless, the catalytic activity of LSM is severely limited by its poor ionic con- ductivity, especially at low operating temperatures. Consequently, in the LSM/YSZ cathodes, the electrochemically active sites for oxy- gen reduction reactions are mostly at the triple phase boundaries (TPB), which are the lineal regions common to the LSM, YSZ, and air (porosity). As a result, porous composites containing LSM and an ionic conductor such as YSZ have been used to increase the active reaction sites for oxygen reduction reactions via an increase in the total length of the TPB per unit volume in the microstruc- ture [1–4]. It follows that the electrochemical performance of such porous composite cathodes is microstructure sensitive. The LSM/YSZ composite cathodes are typically fabricated using powder processing techniques involving printing or spraying and sintering of a powder mix consisting of YSZ and LSM powders. It has been reported that the meso-scale microstructure of the porous composite cathodes is sensitive to the processing parameters such as the mean sizes of the initial powders [5,6] and sintering tem- ∗ Corresponding author. Tel.: +1 404 894 2887; fax: +1 404 894 9140. E-mail address: arun.gokhale@mse.gatech.edu (A.M. Gokhale). perature [7,8], and the microstructure in turn affects the properties such as the polarization resistance and ohmic resistance that dictate the performance of the cathode. The key microstructural parame- ter that affects the electro-chemical response of porous LSM/YSZ cathodes is the total length of the LSM–YSZ–pores triple phase boundaries (strictly speaking, triple lines), i.e., TPB, in the three- dimensional (3D) microstructure per unit volume (i.e., the length density) because O 2 reduction and incorporation of O 2- into the electrolyte take place at these sites [9]. Accordingly, development of quantitative relationships among the processing parameters, microstructural geometry, and electro-chemical response of porous composite cathode materials is vital to the effective optimization of electrode performance. Clearly, to establish such quantitative correlations, it is imperative to observe all three microstructural constituents, namely, LSM, YSZ, and pores, simultaneously in the microstructure so that the microstructural features such as triple phase boundaries can be unambiguously identified and quanti- tatively characterized in an unbiased manner. Unfortunately, due to the microstructural length scales and the chemical nature of the phases in these microstructures, the conventional optical and scanning electron microscopy (SEM) techniques are not useful for simultaneous observations of all the three microstructural phases of interest in these materials. Optical microscopy is not useful due to sub-micron length scales of the grains, and the conventional SEM techniques are not useful because they do not provide sufficient contrast between YSZ and LSM phases due to their comparable average atomic numbers [10,11]. Although a high-resolution elec- 0378-7753/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2009.03.043