The effect of precursor solution desiccation or nano-ceria pre-infiltration on various La0.6Sr0.4FeyCo1-yO3-x infiltrate compositions

Here, electrochemical impedance spectroscopy, X-ray diffraction, and scanning electron microscopy were used to determine the effect of precursor solution desiccation and nano-ceria pre-infiltration on Solid Oxide Fuel Cell cathodes with various infiltrate compositions. The calcium chloride desiccation of citric acid containing La0.6Sr0.4FeO3-x (LSF), La0.6Sr0.4Fe0.8Co0.2O3-x (LSFC), La0.6Sr0.4Fe0.5Co0.5O3-x (LSCF55), La0.6Sr0.4Fe0.2Co0.8O3-x (LSCF), and La0.6Sr0.4CoO3-x (LSC) infiltrate precursor solutions reduced average infiltrate particle sizes from the ∼53 nm obtained with standard processing to ∼42 nm. Similarly, infiltration of the aforementioned La0.6Sr0.4FeyCo1-yO3-x precursor solutions into porous cathodes containing pre-infiltrated gadolinium doped ceria (GDC) particles reduced average La0.6Sr0.4FeyCo1-yO3-x infiltrate particle sizes from the ∼53 nm obtained with standard processing (i.e. in the absence of nano-GDC pre-infiltration) to ∼27 nm with nano-GDC pre-infiltration. These desiccation and nano-GDC pre-infiltration induced infiltrate particle size reductions resulted in improved cathode performance. For example, in comparison with the 650◦C operating temperatures required for standard LSC-GDC cathodes to achieve a polarization resistance of 0.1 cm2, identical cathodes subjected to desiccation or nano-GDC pre-infiltration achieved 0.1 cm2 at 630◦C and 570◦C, respectively. These promising results suggest that precursor solution desiccation or nano-GDC pre-infiltration may be useful for reducing the sizes of a variety of SOFC infiltrate materials. © The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0431609jes] All rights reserved.

have also been observed for LSCF precursor solutions utilizing citric acid, instead of Triton X-100, as an organic solution additive. 14,15 However, it is still unknown how these new processing techniques affect other MIEC infiltrate compositions. Therefore, the objective of the present work was to evaluate how precursor solution desiccation or nano-ceria pre-infiltration impact the microstructure, phase purity and electrochemical performance of nano-composite cathodes produced by infiltrating GDC scaffolds with La 0. 6

Experimental
The first step in the production of the symmetric cathodeelectrolyte-cathode cells analyzed here was to produce porous, wellnecked GDC cathode scaffolds on either side of dense GDC electrolyte pellets using the methods described in Burye and Nicholas. 14 Then, three different infiltration techniques (a standard, precursor solution desiccation, and a nano-GDC pre-infiltration technique) were used to introduce infiltrate nanoparticles into these porous cathode scaffolds.
The 0.5 M La 0.6 Sr 0.4 Fe y Co 1-y O 3-x (0 ≤ y ≤ 1) nitrate precursor solutions used for all three infiltration techniques were produced by dissolving 99.999% pure La(NO 3  For cells infiltrated in the standard manner (i.e no desiccation or nano-ceria pre-infiltration), La 0.6 Sr 0.4 Fe y Co 1-y O 3-x nitrate precursor solutions were pipetted into GDC cathode scaffolds, allowed to soak into the scaffold for five minutes, gelled in flowing air at 80 • C for 10 min, fired at 10 • C/min to 700 • C, held at 700 • C for 1 hr, and then cooled at >25 • C/min to room temperature. This process was repeated six times to produce cells with 12.0 vol% of infiltrate (defined as vol. infiltrate/(electrode geometric area * electrode thickness)).
For cells infiltrated and then desiccated, La 0.6 Sr 0.4 Fe y Co 1-y O 3-x nitrate precursor solutions were pipetted into porous GDC cathode scaffolds, allowed to soak into the scaffold for five minutes, and gelled in flowing air at 80 • C for 10 min, before being chemically desiccated with CaCl 2 for 8-10 hours. The infiltrated cathodes were then heated at 10 • C/min to 700 • C, held at 700 • C for 1 hr, and then cooled at >25 • C/min to room temperature. This process was repeated six times to produce cells with 12.0 vol% of infiltrate.
For nano-GDC pre-infiltrated cells, GDC nitrate solutions were first infiltrated into porous GDC scaffolds, allowed to soak for 5 minutes, gelled in flowing air at 80 • C for 10 minutes, heated at 10 • C/min to 700 • C, held at 700 • C for 1 hr, and then cooled at >25 • C/min to room temperature. This step was repeated two times to produce cathodes with 7.4 vol% nano-GDC oxide particles. Then, La 0.6 Sr 0.4 Fe y Co 1-y O 3-x precursor solutions were infiltrated, gelled and fired in the same manner as the samples processed with the standard technique. This step was repeated up to six times to achieve the desired La 0.6 Sr 0.4 Fe y Co 1-y O 3-x loading level of 12.0 vol% of infiltrate.
Finally, all cells were prepared for electrochemical testing by applying lanthanum strontium manganite (LSM) and gold current collectors using previously described methods. 13 All cells were then characterized via Electrochemical Impedance Spectroscopy (EIS) using previously described methods. 13 Specifically, cathode polarization resistance (R P ) values were determined by dividing the distance between x-axis intercepts on an EIS Nyquist plot by two (to account for the fact that each symmetric cell had two cathodes) and multiplying by the electrode's geometric area (0.5 cm 2 ). Based on the definition of resistance, ohmic resistivity (ρ Ohmic ) values for each cell were determined by dividing the high-frequency Nyquist plot x-axis intercept by the electrolyte thickness and multiplying by the electrode's geometric area (0.5 cm 2 ).
To help understand the observed performance trends, R P predictions were made using a previously-described 8,9,12 Simple Infiltrated Microstructure Polarization Loss Estimation (SIMPLE) model that accounts for electrical losses associated with oxygen exchange into the MIEC infiltrate and oxygen conduction through the GDC scaffold. Specifically, SIMPLE Model Rp predictions were made by inputting 1) La 0.6 Sr 0.4 Fe y Co 1-y O 3-x densities from the Joint Committee on Powder Diffraction Standards (JCPDS) database, 16 2) the MIEC infiltrate loading levels, cathode thicknesses, geometric cathode areas, cathode porosities, MIEC nano-particle diameters, and GDC scaffold column widths stated in Table I After EIS, Scanning Electron Microscopy was performed on fractured samples using previously described procedures. 13 Dedicated statistical software (Graph Pad Prism, La Jolla, CA) was used to determine if the infiltrate particle sizes produced via the different fabrication methods were statistically significantly different from one another. Specifically, a Shapiro Wilks normality test, a Kruskal-Wallis one-way analysis, and a post-hoc Dunn's test for multiple comparisons were used with significance set at P < 0.05. 19 X-Ray Diffraction was also performed on various samples using previously described procedures. 13 Table I provides a summary of the processing conditions and resulting microstructural characteristics for the symmetric cells analyzed here. Figure 1 shows representative 600 • C impedance curves for La 0.6 Sr 0.4 Fe y Co 1-y O 3-x -GDC symmetric cells with standard processing, precursor nitrate solution desiccation, and nano-GDC preinfiltration. In all cases, measurements taken with AC amplitudes of 25 and 50 mV yielded identical impedance curve shapes, and the general shape of the impedance curves were consistent with those observed previously in the literature for LSF-LSC infiltrated ceria. 9,[13][14][15]20,21 For electrodes fabricated with a particular infiltration method, Figure 1 shows that the electrode resistance systematically decreases as the infiltrate Co:Fe ratio increases. This behavior is consistent with previous literature studies showing that at typical SOFC operating temperatures the oxygen surface exchange resistance (Rs) of materials within the LSC-LSF solid solution decrease as the Co:Fe ratio increases, 17,18,[22][23][24] and suggests that oxygen surface exchange is a dominant source of resistance in these cathodes at 600 • C. Figure 1 shows that at 600 • C the performance of the GDC pre-infiltrated cells was better than the performance of the desiccated cells. In addition, Figure 1 shows that at 600 • C the performance of the standard cells was worse than either the GDC pre-infiltrated or desiccated cells. Figure 2 shows infiltrate polarization resistance vs. inverse temperature Arrhenius plots obtained from impedance curves such as those in Figure 1. Figure 2 shows that the trends of 1) GDC pre-infiltrated cells performing better than the desiccated cells, which, in turn, perform better than the standard cells, and 2) increased performance with increasing Co:Fe infiltrate composition also holds at all temperatures between 400 and 700 • C. Here, nano-GDC pre-infiltrated CAD LSC-GDC electrodes displayed the best performance, achieving 0.1 cm 2 (the resistance commonly defined as that needed for SOFC operation 25 ) at 570 • C. Although this performance is not as good as the 0.1 cm 2 at 540 • C displayed by the nano-GDC pre-infiltrated Triton X-100 derived LSCF-GDC electrodes reported in Burye and Nicholas, 15 (due to the larger size of LSCF infiltrate particles produced using citric acid instead of Triton X-100) it is still among the best performing infiltrated SOFC cathodes ever reported. 26,27 Figure 3 shows representative micrographs of LSF, LSFC, LSCF55, LSCF, and LSC infiltrated cells produced using either the standard, desiccated, or GDC pre-infiltration processing method. While only La 0.6 Sr 0.4 Fe y Co 1-y O 3-x infiltrate particles are shown in the standard and desiccated Figure 3 micrographs, both La 0.6 Sr 0.4 Fe y Co 1-y O 3-x infiltrate (at a 12.0 vol% loading level) and the nano-GDC pre-infiltrate (at a 7.4 vol% loading level) are present in the nano-GDC Figure 3 micrographs. Because the 25 nm average size of the nano-GDC pre-infiltrate particles 15 is close to the average infiltrate particle sizes shown in the nano-GDC Figure 3 micrographs, the average infiltrate particles sizes obtained from all Figure 3 micrographs were treated as those of the La 0.6 Sr 0.4 Fe y Co 1-y O 3-x infiltrate alone. As discussed in more detail in Burye and Nicholas,14,15 although additional experiments are needed, it is possible that a combination of evaporation induced self-assembly, infiltrate particle coarsening, or other mechanisms are responsible for particle size variations observed here. Figure 4, which summarizes infiltrate particle sizes obtained from micrographs such as those in Figure 3, shows that within a standard deviation of the observed infiltrate particle sizes, the size of the infiltrate particle sizes obtained with each of the three processing techniques was independent of the La 0.6 Sr 0.4 Fe y Co 1-y O 3-x Co:Fe ratio. Figure 4 also shows that across all infiltrate compositions, the infiltrate produced via nano-GDC pre-infiltration was significantly smaller (P < 0.001) than the infiltrate produced via desiccation, which, in turn, was significantly smaller (P < 0.02) than the infiltrate produced with standard processing. Smaller MIEC infiltrate particle sizes are desirable because they can lower cathode resistance by providing additional surface area for the incorporation of oxygen into the electrode. (  To ease comparison between impedance spectra, the ohmic offset resistance has been subtracted from each spectra. Also, for these plots raw impedance values have been multiplied by the 0.5 cm 2 cell geometric area and divided by two, so that the area specific resistance of one of the two cathodes in each symmetric cell can easily be determined from the difference in Figure 1 x-axis intercepts.

Results and Discussion
shown) had identical infiltrate particle sizes to those produced via nano-GDC pre-infiltration alone). Simple Infiltrated Microstructure Polarization Loss Estimation (SIMPLE) Model Rp predictions made using the average particle sizes from Figure 4 are shown in Figure 2. The good Figure 2 agreement between the experimentally measured and SIMPLE model predicted Rp values (except at temperatures of ∼700 • C where resistive processes, such as the gas concentration polarization known to occur in these cathodes at this temperature, not accounted for in the SIMPLE model result in differences) 13 suggest that processing induced changes in the La 0.6 Sr 0.4 Fe y Co 1-y O 3-x infiltrate particle size alone were responsible for the observed Rp differences. It is important to realize that infiltrate particle size was the only SIM-PLE model input variable that was altered when predicting Rp values for a given La 0.6 Sr 0.4 Fe y Co 1-y O 3-x infiltrate composition, and the La 0.6 Sr 0.4 Fe y Co 1-y O 3-x infiltrate Rs value was the only other variable altered with a change in La 0.6 Sr 0.4 Co x Fe 1-x O 3-δ infiltrate composition. Figure 5 compares the ohmic resistivity of each symmetric cell to the ionic resistivity of GDC. The good agreement between the measured ohmic resistivities and the ionic resistivity of GDC 9 in- dicates that the electrolyte layer was the dominant source of ohmic resistance in the symmetric cells, that electronic conduction within the electrodes was not a significant source of resistance, and that the processing-induced Rp differences shown in Figures 1 and 2 did not result from infiltrate electronic conductivity differences. Figure 6 shows X-ray diffractograms for La 0.6 Sr 0.4 Fe y Co 1-y O 3-x infiltrated cells with standard and desiccated processing. Unfortunately, attempts to obtain nano-GDC pre-infiltrated La 0.6 Sr 0.4 Fe y Co 1-y O 3-x XRD spectra (not shown) were unsuccessful due to overlap of the main 33 • and 47 • La 0.6 Sr 0.4 Fe y Co 1-y O 3-x peaks by those of GDC 28,29 and the fact that the strong XRD signal from the GDC overwhelmed the signal from the La 0.6 Sr 0.4 Fe y Co 1-y O 3-x . As shown in Figure 6, phase pure La 0.6 Sr 0.4 Fe y Co 1-y O 3-x was obtained for all compositions after firing at 700 • C, suggesting that the processing-induced Rp differences shown in Figures 1 and 2 did not result from infiltrate phase purity differences.

Conclusions
Taken together, the results presented in this paper indicate that 1) precursor gel desiccation or nano-GDC pre-infiltration reduces the size of citric acid derived LSF, LSFC, LSCF55, LSCF, and LSC infiltrate particles, and 2) that these particle size reductions alone are

Relative Intensity (CPS)
Two-Theta (deg) 20 Figure 6. X-ray diffractograms for La 0.6 Sr 0.4 Co y Fe 1-y O 3-x nano-particles produced by firing standard (top diffractograms) and desiccated (bottom diffractograms) citric acid-containing precursor gels outside GDC scaffolds at 700 • C. These diffractograms were referenced to LSF (JCPDS # 01-072-8133), 30 LSFC, 31 LSCF55, LSCF JCPDS # 00-048-0124, 28  responsible for the observed electrode performance improvements. Further, the infiltrate particle sizes produced via a specific fabrication approach are similar for all La 0.6 Sr 0.4 Fe y Co 1-y O 3-x Fe:Co ratios. Specifically, standard, precursor gel desiccation, and nano-GDC preinfiltration produced phase pure, infiltrate particles with average sizes of ∼53, 42, and 27 nm, respectively. Out of the compositions tested here, the LSC-GDC cathodes performed the best (achieving a 0.1 cm 2 polarization resistance at 630 • C via precursor solution desiccation and 570 • C via nano-GDC pre-infiltration) due to LSC having the lowest R S value of the infiltrate compositions tested here. The ability of the precursor gel desiccation and nano-GDC pre-infiltration techniques to modulate the size of the various infiltrate compositions studied here suggests that these techniques may be useful for producing nano-sized, multi-cation oxides for use in other devices and materials systems.

Acknowledgments
This work was supported by National Science Foundation (NSF) CAREER Award No. CBET-1254453. Profilometry work was conducted at the Michigan State Keck Microfabrication Facility. Microscopy work was conducted at the Michigan State Composites Center, which is supported by the NSF Major Instrumentation Program and Michigan State University.