(Invited) Intrinsic Oxygen Surface Exchange Resistance Measurements on Mixed Ionic Electronic Conducting Infiltrate Nanoparticles
Authors: Burye Theodore E. , Nicholas Jason D. 
Published: February 2020

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.

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Figure 1.

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

Figure 1