(Invited) Intrinsic Oxygen Surface Exchange Resistance Measurements on Mixed Ionic Electronic Conducting Infiltrate Nanoparticles

Abstract

Introduction 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 (RS ) values for infiltrated MIEC nano-particles, and 2) the large RS 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 (RP ) measurements and Finite Element Modeling (FEM) of 3D Focused Ion Beam- Scanning Electron Microscopy (FIB-SEM) reconstructions to back-calculate the RS values for a variety of infiltrated MIEC nano-particle compositions. Experimental Methods Following the procedures described in Burye and Nicholas,3-5 La0.6Sr0.4FeO3-x (LSF)-Ce0.9Gd0.1O1.95 (GDC), La0.6Sr0.4Fe0.8Co0.2O3-x (LSFC)-GDC, La0.6Sr0.4Co0.8Fe0.2O3-x (LSCF)-GDC, and La0.6Sr0.4CoO3-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 500-700oC NCC electrochemical impedance spectroscopy RP 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 RP calculations for various RS 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 RS values were then identified as those producing the observed RP values with a given, experimentally-measured infiltrate surface area. Results As an example, Figure 1 shows how the LSFC infiltrate RS values compare with bulk and thin film values reported in the literature.7-16 Conclusions All the MIEC infiltrate compositions tested here exhibited RS values that were within the range of RS values and activation energies previously reported in the literature. Analyses also confirmed that just as in bulk and thin film studies, the infiltrated RS values for LSC<LSCF<LSFC<LSF. Acknowledgements This work was supported by National Science Foundation award number CBET-1254453.