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Tailoring Mixed Ionic Electronic Conducting Nano-Particle Size through Desiccation and/or Doped Ceria Oxide Pre-Infiltration
Mixed Ionic and Electronic Conducting (MIEC) multi-cation oxide nano-particles have been used to enhance the performance of SOFC electrodes [1]. These SOFC MIEC nano-particles are often produced by thermally decomposing gelled MIEC precursor solutions infiltrated into porous ionic conducting (IC) scaffolds. Average SOFC nano-particle sizes produced by this thermal decomposition technique are typically 55 nm in diameter [1, 2], and result in SOFC operating temperatures (defined as the lowest temperature where the electrode polarization resistance, (RP), is equal to 0.1 Ωcm2) in excess of 600°C. Since MIEC particle size directly correlates with the surface area available for oxygen incorporation (and hence SOFC electrode performance), the objective of this study was to determine whether it was possible to reduce MIEC nano-particle size through desiccation and/or sequential infiltration.
Experimental Methods
Heavily-infiltrated nano-composite La0.6Sr0.4Co0.8Fe0.2O3 (LSCF) on Ce0.9Gd0.1O1.95 (GDC) and LSCF-GDC on GDC nano-composite cathodes were prepared through multiple nitrate solution infiltrations into porous GDC scaffolds. For the LSCF on GDC samples, precursor LSCF solutions were gelled inside a GDC scaffold, desiccated, and fired at 700°C. For the LSCF-GDC on GDC samples, GDC precursor solutions were gelled inside a GDC scaffold and fired at 700°C prior to LSCF precursor solution infiltration, gelation, and firing at 700°C.
Results
As shown in Figure 1, average nano-particle sizes of 20 nm ± 5 nm (determined through scanning electron microscopy) were achieved through both desiccation (using CaCl2 as a desiccant) and sequential infiltration (using 8.0 vol % of pre-existing nano-GDC as a decomposition catalyst). By altering the amount of desiccation it is possible to controllably adjust the LSCF nano-particle size from 20 nm ± 5 nm ( note the ± refers to the standard deviation of the actual particle sizes and is not a reflection of the scanning electron microscopy (SEM) resolution, which is ± 1 nm) to 60 nm ± 18 nm. Similarly, sequential infiltration can be used to adjust average nano-particle size from 20 nm ± 5 nm to 50 nm ± 18 nm.
As shown in Figures 2 & 3, the desiccation and sequential infiltration approaches progressively change precursor solution decomposition temperature peak positions. Figure 2 shows that desiccation shifts the strontium nitrate peak position to lower temperatures with stronger desiccants. Figure 3 demonstrates that as the nano-GDC loading level increases, the decomposition peak positions shift from 800°C with 0 vol % nano-GDC to 200°C with 50 vol % nano-GDC. (Volume percentage is the volume % of the IC scaffold porosity occupied by nano-particles). Through the adjustment of precursor gel decomposition peak position to lower temperatures smaller nano-particle sizes are achieved.
As shown in Figure 4, NCCs infiltrated with LSCF with 20 nm nano-particle diameters have RP values of 0.1 Ωcm2 at 565°C using drying/desiccation and NCCs using LSCF + GDC with 20 nm nano-particle diameters have RP values of 0.1 Ωcm2 at 540°C using sequential infiltration.
Conclusions
Two different approaches have been developed to influence precursor solution decomposition temperature and reduce average nano-particle sizes. Average MIEC nano-particle size were reduced to 20 nm and SOFC operating temperatures (i.e. the temperature at which RP ≤ 0.1 Ωcm2) are 565°C with desiccation and 540°C with sequential infiltration.
Acknowledgements
This work was made possible through a Michigan State University faculty startup grant.
References
[1] J.D. Nicholas, S.A. Barnett, J. Electrochem. Soc, 157, B536-B541, 2010.
[2] Z. Zhan, D. Han, T. Wu, X. Ye, S. Wang, T. Wen, S. Cho, S.A. Barnett, R. Soc. Chem. Adv, 2, 4075-4078, 2012.
[3] J.D. Nicholas, L. Wang, A.V. Call, S.A. Barnett, Phys. Chem. Chem. Phys, 14, 15379-15392, 2012.