Over the past decade, the Solid Oxide Fuel Cell community has developed high-performance Nano-Composite Cathodes (NCCs) by infiltrating, gelling, and thermally decomposing mixed ionic electronic conducting (MIEC) nitrate precursor solutions inside porous ion-conducting (IC) scaffolds.^{1, 2} Unfortunately, attempts to identify optimal infiltrate compositions and predict optimal NCC microstructures has been complicated by 1) a lack of intrinsic oxygen surface exchange resistance (R_S) values for infiltrated MIEC nano-particles, and 2) the large R_S variations observed in the literature for bulk and thin film MIEC samples with nominally identical compositions. The present work addresses these problems by using NCC polarization resistance (R_P) measurements and Finite Element Modeling (FEM) of 3D Focused Ion Beam- Scanning Electron Microscopy (FIB-SEM) reconstructions to back-calculate the R_Svalues for a variety of infiltrated MIEC nano-particle compositions.

Experimental Methods

Following the procedures described in Burye and Nicholas,^3-5 La_0.6Sr_0.4FeO_3-x (LSF)-Ce_0.9Gd_0.1O_1.95 (GDC), La_0.6Sr_0.4Fe_0.8Co_0.2O_3-x (LSFC)-GDC, La_0.6Sr_0.4Co_0.8Fe_0.2O_3-x (LSCF)-GDC, and La_0.6Sr_0.4CoO_3-x (LSC)-GDC NCCs were prepared through multiple nitrate solution infiltrations into porous GDC scaffolds using either a standard (S), desiccation (D) or nano-GDC pre-infiltration (P) approach. Open-circuit 500-700^oC NCC electrochemical impedance spectroscopy R_P measurements in air were then taken, and the infiltrate surface areas were determined via SEM. 3D NCC microstructural reconstructions were obtained by epoxy infiltrating non-infiltrated GDC scaffolds in epoxy, using the FIB-SEM to collect 2D serial sections, and using the MIMICS computer program (Materialise) to produce a 3D finite element meshes of the cathode and electrolyte. Following the procedures discussed in Nicholas and Barnett,⁶ open-circuit NCC R_P calculations for various R_S and infiltrate surface area combinations were performed by placing an arbitrary voltage across the reconstructed volume (1 V) and solving Laplace’s Equation inside the GDC scaffold. Infiltrate R_S values were then identified as those producing the observed R_Pvalues with a given, experimentally-measured infiltrate surface area.

Results

As an example, Figure 1 shows how the LSFC infiltrate R_S values compare with bulk and thin film values reported in the literature.^7-16

Conclusions

All the MIEC infiltrate compositions tested here exhibited R_S values that were within the range of R_S values and activation energies previously reported in the literature. Analyses also confirmed that just as in bulk and thin film studies, the infiltrated R_Svalues for LSC<LSCF<LSFC<LSF.

Acknowledgements

This work was supported by National Science Foundation award number CBET-1254453.

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Figure 1. The intrinsic surface resistance of infiltrated LSFC (solid symbols) compared to bulk and thin film R_S values reported in the literature.^7-16 Data from thin film samples are denoted by an asterix after the legend citation.