A Strategy to Mitigate Grain Boundary Blocking in Nanocrystalline

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C: Energy Conversion and Storage; Energy and Charge Transport

A Strategy to Mitigate Grain Boundary Blocking in Nanocrystalline Zirconia Arseniy Bokov, Jeffery A. Aguiar, Matthew L. Gong, Alexey Nikonov, and Ricardo H. R. Castro J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08877 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on November 10, 2018

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The Journal of Physical Chemistry

A Strategy to Mitigate Grain Boundary Blocking in Nanocrystalline Zirconia Arseniy Bokov1, Jeffery A. Aguiar2,3, Matthew L. Gong4, Alexey Nikonov5, Ricardo H. R. Castro1* (1) Department of Materials Science and Engineering, University of California, Davis, Davis CA, USA, 95616; (2) Nuclear Materials Department, Idaho National Laboratory, Idaho Falls ID, USA, 83415; (3) Department of Materials Science and Engineering, University of Utah, Salt Lake City UT, USA, 84112; (4) Scientific Computing Imaging Institute, University of Utah, Salt Lake City UT, USA, 84112; (5) Institute of Electrophysics, Ural Division of the Russian Academy of Sciences, Yekaterinburg, Russia, 620016

Abstract A major challenge in the application of nanostructured electrolytes in solid oxide electrochemical cells is grain boundary blocking originated from unsatisfied atomic bonding and coordination. The resulting increase in grain boundary resistivity works against the expected benefits from the enhanced ion exchange rates enabled by the extensive interfacial network in nanocrystalline materials. This study addresses this challenge by demonstrating that a reduction in the grain boundary excess energies increases the net ionic conductivity as directly measured by impedance electrical spectroscopy in nanocrystalline yttria-stabilized zirconia (YSZ). The reduced grain boundary energy was designed by doping the system with lanthanum, leading to local excess energy reduction due to segregation of La to boundaries as observed by scanning transmission electron microscopy based energy dispersive spectroscopy (STEM-EDS). The results suggest rare earth ions with favorable grain boundary segregation enthalpy can smooth out the energy landscape across grain boundaries and thus facilitate ion mobility in the nanocrystalline electrolyte.

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Introduction Solid oxide electrolytes are the core components of many electrochemical devices such as solid oxide electrolysis cells and fuel cells (SOEC and SOFC).1–3 Despite their relatively high efficiency, practical applications of these technologies are still limited due to high operation temperatures.4 For instance, the most common electrolyte found in SOFC, cubic yttria stabilized zirconia (YSZ), can only ensure reasonable performance at temperatures above 800 °C.5 Such a requirement narrows down the list of suitable materials for the remaining of the cell components (i.e. for cathodes, anodes, and interconnects) and leads to fast degradation of the device as a whole.6 Recent studies have demonstrated that nanostructures can play a major role in the race to lower the operation temperatures of electrochemical cells. For instance, nanostructured electrolytes provide a large triple-phase-boundary area, which can decrease polarization losses due to an increased rate of oxygen surface exchange and fuel oxidation.7 However, the ionic conductivity of electrolytes may experience a rather reverse trend. As the resistivity of grain boundaries is generally greater than the bulk of the grain,8–10 a large network of boundaries inevitably hinders the transport of oxygen ions. Although this effect is not pronounced at elevated temperatures due to high oxygen mobilities, it does result in significant Ohmic losses in the lower temperature range since grain boundaries act as internal barrier for ionic transport.11–13 The blocking effect of grain boundaries arises from a combination of structural and electrostatic effects even in pure materials.14–19 The crystallographic mismatch naturally favors the accumulation of oxygen vacancies in the grain boundary core, and this effect can be further promoted by the segregation of dopants with lower charge. As a result, adjacent layers in the vicinity of the boundary become vacancy-depleted, thereby limiting the flux of oxygen ions across neighboring grains. Additionally, depending on the chemistry of the system, the boundary core


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may also generate particular electronic states within the band gap – effectively creating a negatively charged barrier for intergranular oxygen transport.20,21 To address this fundamental issue of grain boundary blocking, it is important to appreciate the co-existence of different boundary misorientations within the same polycrystalline material. The boundaries with high excess energy and large number of dangling bonds often form deep potential wells, while the boundaries with low excess energy can ensure a smoother transition from one grain to another.22 Not surprisingly, grain boundary energies inversely correlate with the rate of ionic transport, i.e. oxygen diffusion is much faster across interfaces with low excess energy.23,24 Therefore, one can potentially address the blocking effect of grain boundaries by smoothing out the energetic landscape and reducing the number of high energy interfaces.25,26 The energetic inhomogeneity across grain boundary networks can be decreased through thermodynamically driven dopant segregation. For example, rare-earth and transition metal ions have demonstrated a tendency to accumulate at the grain boundaries and lower the excess boundary energy of the host in some oxides.27,28 Such energetic stabilization can be associated with the increase in coordination number or with elongating of cation-oxygen bond at the interface regions.29,30 Moreover, the thermodynamic segregation is an inherently anisotropic event, as dopants mainly affect the high energy boundaries and do not alter the low energy ones to the same extent.31 This implies that, on average, excess grain boundary energy is lowered by decreasing the number of high energy interfaces. The later effect can be especially beneficial since it might be used for mitigating the blocking effect and improving the grain boundary conductivity. The goal of this study is to address the correlation of the grain boundary energy, segregation and ionic conductivity in nanocrystalline fully dense YSZ. The grain boundary energy was purposely modified by introducing lanthanum as a dopant. The direct measurements by microcalorimetry revealed the decrease in grain boundary energy attributed to the segregation of


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La3+, as demonstrated by scanning electron microscopy and electron dispersion spectroscopy (STEM-EDS). The reduction of the grain boundary energy was accompanied by a significant increase in ionic conductivity measured by impedance spectroscopy, suggesting the efficacy of this approach to mitigate the blocking effect of grain boundaries.

Experimental Synthesis of nanocrystalline powders. The nanopowders of yttria stabilized zirconia (YSZ) were synthesized from the following starting reagents: Zirconium oxynitrate hydrate (Sigma Aldrich, 99%), Yttrium nitrate hexahydrate (Alfa Aesar, 99.9%), and Lanthanum nitrate hexahydrate (Alfa Aesar, 99.9%). The Zr and Y ions were to form the host cubic structure, while La was chosen as a dopant based on previous reports on favorable enthalpy of segregation in the zirconia lattice.32,33 The required proportions of the chosen nitrate salts were dissolved in deionized water (0.75M) and slowly dripped into 5M excess solution of NH4OH. The reference composition consisted of ZrO2 + 10 mol. % of Y2O3 (10YSZ), while some molar fraction of Y was substituted by La in the doped compositions (0.5La9.5YSZ, 1La9YSZ, 1.5La8.5YSZ) with the goal of maintaining the concentration of trivalent ions in the system. The precipitates, which nucleated in the ammonium hydroxide solution, were washed with ethanol by centrifuging three times and then dried at 80 °C for 48 hours. The resulting hydroxide precursors were crushed in a mortar and then calcined at 450 °C in ambient air for 2 hours to form oxide nanoparticles. Densification of nanocrystalline oxide. The obtained nanopowders were degassed and transferred to a dry-nitrogen glovebox. Each composition was loaded into the Diamond-SiC composite inner die with 4 mm inner diameter and then inserted into the outer graphite die of 19 mm inner diameter. The resulting assemblies were transported from the glovebox to a spark plasma sintering setup SPS 825S (Syntex, Japan) in sealed containers in a similar design, as proposed by


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Muche et al.,34 but replacing inner die with Diamond-SiC.26 The spark plasma sintering experiments were performed in a medium vacuum (10-2 Torr). The maximum temperature was varied in the range of 960-1070 °C, depending on the amount of La in the sample. The heating profile consisted of two ramps: about 120 °C/min until 700-810 °C and then 25 °C/min until the final temperature. The load was applied during the second ramp at 340 MPa/min to a final load of 1.7 GPa. There was no isothermal exposure upon reaching final temperatures, i.e. all samples were sintered during the thermal ramp. The detailed conditions used for SPS experiments are provided in Table S1 (Supporting Information). Note that all samples were heat-treated after SPS in ambient air for 1 hour at 800-950 °C which is 150 °C below the final sintering temperature. Although this is designed to re-oxidize the material and eliminate carbon residues, at this temperature and time, enough diffusion is expected to relax interfacial atoms to stable positions. This guarantees no special SPS condition exists at the grain boundaries. The maximum sintering temperature was varied in order to assure that all studied compositions were fully dense (as measured by Archimedes method)35 and showed similar grain sizes, since La tends to slow down the grain growth.33 The resulting pellets were about 1mm tall and 4mm in diameter. The reoxidation of partially reduced zirconium and burn off of residual carbon was performed in a box furnace under ambient air at 150 °C below the maximum sintering temperature for 1 hour. The appearance of the resulting pellets is shown in Figure S1 (Supporting Information). Sample preparation for electron microscopy. Electron transparent TEM thin foils were prepared from the sintered YSZ pellets using a FEI Helios Dual Beam focus ion beam (FIB) instrument. The samples were coated with a layer of carbon to improve conductivity and minimize drift inside the FIB. Inside the FIB, an additional thin layer of fine to coarsened grained Pt was deposited over 15 µm by 3 µm rectangular area to a total thickness of 3 µm to protect and demark


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a region for lift-out. A thin foil lift-out proceeded over this rectangular area and resulted in 13x5 µm lamellae measuring less than 100 nm in thickness mounted to a copper Omniprobe TEM grid. A final clean-up was performed using 5 kV Ga-beam with the beam current of 12 pA to remove material deposited during FIB preparation and reduce damage from the initial milling. Care was taken to minimize the ion beam interaction with the exposed surface throughout the TEM sample preparation. This was done by depositing multiple layers of gold and platinum, as well as minimizing exposure to the Ga ion beam. Transmission Electron Microscopy. Analytical transmission electron microscopy was performed on a JEOL 2800 operated in TEM and STEM modes at 80 kV and equipped with dual high solid angle 30 mm2 windowless Si drift detectors. In STEM mode, a nanometer-size analytical probe with the beam current of less than 110 pA was used to image and characterize the samples. Both high angle annular dark field (HAADF) and annular bright field imaging was used to resolve the sample microstructure and chemistry. A typical ADF STEM image of the nanocrystalline YSZ electrolytes produced in this study is shown in Figure S1 (Supporting Information). Under these same STEM beam and imaging conditions, electron dispersive X-ray spectral (EDS) chemical imaging was utilized to acquire the Zr-K, O-K, La-L, and Y-K edges with the best achievable spatial and energy resolution for the microscope. The acquisition parameters to resolve quantitative EDS spectral images were comprised of multiple 5 second scans over a 512 by 512 pixel area. The initial quantification of the collected EDS spectra utilized Cliff-Lorimer thin film correction and a nominal thin film thickness of 95 nm to calculate the weighted atomic percent and net count spectral maps using the calibrated Thermo Scientific software. K-mean clustering. In addition to composition analysis and mapping with STEM EDS, overall cluster analysis was performed to capture additional partitioning and elemental trends in the data using unsupervised machine learning method k-means clustering. Based on the collective


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compositions reported at each pixel, the grouping of elemental pairs, including Zr/O, Y/O, Zr/Y, La/Zr, La/Y, and La/O, were further partitioned into four groupings, constituting individual cell fractions per comparison. Convergence in the final partitioning using k-means clustering was set to twenty iterations in all cases. Following convergence, visualizing the data was performed in Matlab to evaluate for direct or null comparisons between elements, as well as weighted trends within the groups. Direct measurements of grain boundary excess energy. The sintered YSZ pellets of all compositions were analyzed using Differential Scanning Microcalorimetry (Netzsch DSC404, Netzsch GmbH, Germany) to directly measure their grain boundary energies.36 The samples were heated up in argon at 20°C/min up to 1150°C followed by 20 minutes holding time to activate grain growth. As there were no phase transitions or redox reactions expected, the measured heat signal could be exclusively associated with the release of the excess grain boundary energy. The observed heat signal was then normalized by the change in grain boundary area to calculate the grain boundary energy following validated protocols reported in the literature.37 The initial and final grains sizes, i.e. before and after the heat release, were analyzed with X-ray Powder Diffraction (Bruker-AXS D8 Advance diffractometer, Bruker Inc.) and Scanning Electron Microscopy (FEI Nova NanoSEM430). The grain boundary areas were calculated from the grain sizes using a tetrakaidekahedron grain shape. The surface and porosity contributions were neglected due to small grain sizes and full density of the obtained samples. Electrical properties of nanocrystalline ceramics. Sintered pellets of all compositions were investigated by Electrochemical Impedance Spectroscopy to evaluate their electrical properties and determine activation energies for charge transport. Before measurements, silver paste was placed onto the top and bottom surfaces of the YSZ pellets and annealed at 600 °C for 30 min. The choice of Ag was motivated by low sintering temperature of the samples, so small


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grain sizes could be retained. The impedance was measured by using a Solartron Sl-1260/1287 in the range of 300-550 °C under ambient air with Pt mesh electrodes. The frequency range of the probing signal was set from 1 MHz to 0.1 Hz with the amplitude of 300 mV (300-450 °C) and 100 mV (450-500 °C).

Results and Discussion The X-ray diffraction patterns for the as-synthesized powders and sintered pellets of 10YSZ and 1.5La8.5YSZ are shown in Figure 1. The peaks are relatively broad for the powders, which is consistent with grain sizes in the nanoscale. The sintered pellets show narrower peaks as compared to the as-synthesized nanoparticles, implying that some grain growth has occurred during sintering. The whole profile fitting resulted in 6 nm and 20 nm average crystallite size for powders and for sintered samples, respectively.


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Figure 1. The XRD patterns for as-synthesized powders and sintered pellets of 10YSZ and 1.5La8.5YSZ samples. The superscripts at the top of the image represent hkl indices for the corresponding peaks.

It can be noticed that the XRD profiles in Figure 1 demonstrate only reflections of the cubic fluorite phase even for the case of La-doping. This indicates that dopant atoms do not induce any noticeable distortion for the host zirconia lattice and do not initiate the formation of second phase. Such structural analysis, however, does not provide any insights on the relative distribution of the host and dopant ions (to the limit of the X-ray diffraction). In order to further evaluate this, the sintered nanocrystalline pellets of 10YSZ and 1.5La8.5YSZ were comparatively analyzed with STEM-based EDS imaging. As it can be seen in Figure 2a-b, the signals for O, Zr, and Y ions are homogeneous throughout the assessed area for both 10YSZ and 1.5La8.5YSZ samples.


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a

b

Figure 2. STEM-EDS based chemical maps for 10YSZ (a) and 1.5La8.5YSZ (b) samples reported in total net counts.

Due to the small size of the grains and dimensions of microscopy samples, the counts for all elements were on the same order as the uncertainty of the analysis technique. Therefore, k-means clustering was performed to partition the chemical signals more accurately, i.e. to plot the net counts for different elements versus each other. Figure 3a-b shows that Zr and O atoms demonstrate a normal centered distribution in both 10YSZ and 1.5La8.5YSZ compositions. The clustering implies that the zirconium and oxygen ions co-exist across the entire cross-section of the studied samples, in equivalent portions.


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a

b

c

d

Figure 3. The k-type clustering for Zr/O pair in 10YSZ (a) and in 1.5La8.5YSZ (b). The ktype clustering for Zr/Y pair in 10YSZ (c) and 1.5La8.5YSZ (d).

The k-clustering for the pair of Zr and Y ions are also similar for both 10YSZ and 1.5La8.5YSZ compositions (Figure 3c-d). These distributions, however, are not symmetric. There is a clear shift towards the Zr-axis due to a molar predominance of Zr over Y in both compounds. It means that there are some regions where only Zr atoms are


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present and no Y atoms are observed. At the same time, there is only a negligible number of points where Y exist irrespectively of Zr – implying that yttrium is not prone to segregate to grain boundaries at such small grain sizes. This is rather unexpected outcome since yttrium has been reported to demonstrate some degree of segregation in the zirconia lattice of the coarsegrained samples.38

a

c

b

d


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Figure 4. K-means clustering for La/Zr (a) and for La/Y (b). Dark field STEM image of 1.5La8.5YSZ sample (c). The probability map for La segregation (d) reported in normalized units.

The La/Zr and La/Y distributions in 1.5La8.5YSZ show a much higher degree of asymmetry comparing to Zr/Y maps (Figure 4a-b). There are regions where Zr and La are both present, but there are also numerous regions with La detected irrespectively to Zr. The La/Y map demonstrates no correlation among these elements so that they can be both present independently from each other. The calculated correlations between La and Zr/Y allowed for the construction of the dopant segregation profile. The condition for segregation is that La must be present at a given point while Zr and Y do not. The resulting segregation map (Figure 4d) for the corresponding STEM image (Figure 4c) scales from 0 to 1 representing the least and the most likely positions of segregation. It can be seen that the segregation of La to the grain boundaries is homogenous with the exception of a few enriched triple-junctions. The dopant distribution profile derived from the STEM-based EDS analysis is very consistent with the concept of thermodynamically driven segregation. The result implies that La atoms might be capable of stabilizing the grain boundaries and thus, facilitate cross-grain oxygen transport by reducing the interfacial potential well. To verify this assumption, the nanocrystalline sintered pellets were subjected to a combined assessment, which involved calorimetric measurements and impedance spectroscopy. ACS Paragon Plus Environment

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The energetic properties of the produced nanoceramics were directly analyzed with Differential Scanning Microcalorimetry. As indicated in the experimental section, all samples were subjected to grain growth in a narrow range of temperatures in order to release the excess grain boundary energies. The obtained heat effects were then normalized by the change in grain boundary area so the integrals of such signals would directly represent the average grain boundary energy in the material (Figure 5a-d). The detailed calculations can be found in Table S2 (Supporting Information). The grain boundary energy for the reference 10YSZ composition (0.89 J.m-2) is slightly lower than it has been previously reported.33,36 The literature values are higher possibly due to differences in yttrium content or due to neglected effect of residual porosity on the calorimetric calculations.33,36 Despite the difference, such value of interfacial energy still remains very high, so it is not surprising that this material demonstrates a strong tendency for grain boundary blocking. The addition of La, however, incrementally decreases the grain boundary energy in the YSZ systems down to 0.50 J.m-2 for 1.5La8.5YSZ.


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Figure 5. The normalized heat signal associated with the release of excess grain boundary energy in 10YSZ (a), 0.5La9.5YSZ (b), 1La9YSZ (c), and 1.5La8.5YSZ (d). The integral value is shown in the top-right corner of these graphs. The electrochemical impedance spectra for 10YSZ (e), 0.5La9.5YSZ (f), 1La9YSZ (g), and 1.5La8.5YSZ (h) at 400 °C. The superscripts indicate frequencies of the probing signal.

Such energetic stabilization can be attributed to the thermodynamic segregation and dopant effect on the bonding environment at the grain boundary.30 Even a small amount of La can induce a considerable redistribution of oxygen so that O atoms can better accommodate the position of the ACS Paragon Plus Environment

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rest Zr and Y atoms.26 As a result, the presence of dopant atom provides the energetic stabilization of the boundaries, given that structural incompatibility is one of the main contributions to the value of excess grain boundary energy.39 Combining it with the anisotropic nature of segregation, it is therefore possible to decrease the number of sharp interfaces in nanocrystalline materials.26 As thermodynamic segregation and the associated energetic stabilization of grain boundaries are verified, it is important to understand how these effects can impact the transport properties of nanocrystalline electrolytes. To investigate that, a set of samples with the same composition as those for the calorimetric measurements was subjected to Electrochemical Impedance Spectroscopy. As shown by Nyquist plots in Figure 5e-h, all compounds demonstrate three distinct semi-circles at 400 °C. The first corresponds to resistivity of the bulk (grain interior), the second reflects resistivity of the grain boundaries, and the third represents processes on the electrodes.40 It can be seen that the electrode contribution is expectedly similar for all compounds since the same silver paste was placed onto each specimen. The resistivity for the bulk of the grains also does not differ across the studied compositions. However, the resistivity of grain boundaries is highly dependent on the concentration of dopant: higher La contents correspond to lower resistance. While the results imply that the reduction in grain boundary energy can decrease the grain boundary resistance, the corresponding activation energy should be assessed to get a better understanding of the underlying mechanism. Therefore, the first two semi-circles of the impedance spectra (bulk and grain boundary contributions) obtained at various temperatures were analyzed using the equivalent electric circuit. The circuit consisted of two consecutive blocks in which a resistor and a constant-phase element were connected in parallel.40–42 The results of such data analysis are listed in Table 1 and represented by Arrhenius plots in Figure 6 (see Table S3 in Supporting Information for more details).


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Figure 6. (a) The ionic conductivity of the bulk b and grain boundaries gb for 10YSZ and 1.5La8.5YSZ samples measured in the range of 300-550 °C, showing that bulk values coincide but grain boundary values significantly differ. (b) The ionic conductivity of the grain boundaries gb shows incremental increase with La content.


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Table 1. Grain size, activation energy and ionic conductivity for 10YSZ, 0.5La9.5YSZ, 1La9YSZ, 1.5La8.5YSZ Grain size, nm

Grain boundary energy, J/m2

10YSZ

19.9 ± 1.0

0.5La9.5YSZ

Sample

Activation energy

Bulk conductivity at 450 °C, S

cm-1

Grain boundary conductivity at 450 °C, S cm-1

Total conductivity at 450 °C, S cm-1

Eb, eV

Egb, eV

Etot, eV

0.89 ± 0.02

1.02 ± 0.01

1.26 ± 0.02

1.22 ± 0.02

(1.86 ± 0.11) x 10-4

(5.02 ± 0.12) x 10-5

(3.96 ± 0.09) x 10-5

20.2 ± 1.1

0.72 ± 0.03

1.00 ± 0.02

1.24 ± 0.01

1.19 ± 0.01

(1.66 ± 0.10) x 10-4

(5.30 ± 0.13) x 10-5

(4.02 ± 0.10) x 10-5

1La9YSZ

20.3 ± 1.6

0.61 ± 0.03

0.98 ± 0.01

1.20 ± 0.01

1.15 ± 0.01

(1.79 ± 0.06) x 10-4

(6.30 ± 0.11) x 10-5

(4.66 ± 0.08) x 10-5

1.5La8.5YSZ

20.2 ± 2.1

0.50 ± 0.03

0.98 ± 0.01

1.20 ± 0.01

1.14 ± 0.01

(1.80 ± 0.11) x 10-4

(7.35 ± 0.21) x 10-5

(5.22 ± 0.15) x 10-5


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The bulk ionic conductivity and associated activation energy for 10YSZ (Figure 6a) is in a very good agreement with the literature data for microcrystalline samples within the range of studied temperatures,15,16 suggesting that nano-sized grains do not alter these values as previously suggested.13 The bulk conductivity for 1.5La8.5YSZ is similar to 10YSZ, while the corresponding activation energies slightly differ: 0.98±0.1 eV and 1.02±0.1 eV, respectively. The activation energy for grain boundary conductivity of 10YSZ in this study (1.26±0.02 eV for 20 nm) is similar to the samples with 50-900 nm grain size reported by Dura et al.,9 but higher than for the case of 17 nm. The observed difference is likely a result of residual porosity, as Dura et al. reported 84% density, while samples in this study are fully dense – as measured by Archimedes method (relative density >99.9%) and confirmed by observing multiple STEM and SEM images showing no porosity and by noticing high levels of optical transparency (see Figure S1). As seen in Figure 6b, the grain boundary conductivity in La-containing YSZ is generally higher, demonstrating a small incremental increase proportional to the dopant concentration. The activation energy shows a reverse trend, slightly decreasing from 1.26±0.2 eV for 10YSZ to 1.20±0.1 eV for 1.5La8.5YSZ (Table 1). The resulting activation energies for the total conductivity are 1.22±0.2 eV and 1.14±0.1 eV for 10YSZ and 1.5La8.5YSZ, respectively. The obtained results suggest a correlation between the thermodynamically-driven segregation and grain boundary blocking. Those are linked by the grain boundary excess energy which is a measure of the number of unsatisfied bonds at the grain boundary. In truth, the energy released in the DSC experiments is a measure of excess energies that in this case can be directly related to the grain boundary area alone. On the hand, a large excess energy corresponds to significant bonding discontinuities existing at the grain boundaries, creating deep potentials that are likely to trap crossing ion movement.


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It has been shown by Monte Carlo simulation that La doping of YSZ does not increase the number of bonds at the grain boundaries,26 but the long La-O bond length allows for redistribution of oxygen ions that can accommodate coordination and chemical bonds, reducing the excess energy. It is known that oxygen vacancies are depleted in the space-charge zones at the grain boundaries,14 resulting in a high resistivity to ion transport. The segregation of La causes a reduction in the height of the inherently formed potential barrier at the grain boundaries that reduces the mobility of the ions. In highly doped ionic conductors, such as 10YSZ studied here, the grain boundary width is expected to be small,43 but the barrier height shall depend on the excess energies. Such argument is consistent with very small changes measured for the activation energy, which is a combination of energies of formation and migration of vacancies. The data suggests that La is not facilitating the formation of new vacancies, but simply creating a smoother path for ion transferring across grain boundaries. As an addition consideration, the data from STEM-EDS did not show signals other than Zr, Y, O, La, suggesting contaminations are very minimal. Guo has reported on the role of alumina in zirconia demonstrating Si accumulation at grain boundaries that increased conductivity.44 However, even if traces of contaminants such as SiO2 did exist, a very high grain boundary area of the nanocrystalline samples should be taken into account. Therefore, if contaminates segregate, the net concentration at a given grain boundary is certainly negligible and would not affect the conductivity or eclipse the effects of La segregated to grain boundaries. Finally, the thermodynamic segregation and associated structural and energetic changes should not be confused with the formation of intergranular films. In the present study, the dopant atoms occupy the cationic lattice sites of the host material and homogenize the interfacial zone.26 This was achieved by a very short sintering time and low dopant concentration so that dopant atoms could not diffuse to a long distance, and therefore, could not form a distinct phase.


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Conclusions The present study demonstrates that thermodynamic design of interfaces can significantly mitigate the grain boundary blocking and improve transport properties in solid oxide electrolyte materials. The results further suggest the improvement originated from the segregation of dopant atoms that decrease the grain boundary energy and smooth out structural and electrostatic fluctuations between neighboring grains. Therefore, the fraction of low energy boundaries can be increased, which leads to facilitated cross-grain oxygen transport. The functionality of the proposed approach is verified on the model system, cubic yttria stabilized zirconia with La-enriched boundaries.

Associated content Supporting Information Photograph and ADF STEM image of 10YSZ and 1.5La8.5YSZ (Figure S1). Spark Plasma Sintering conditions used for 10YSZ and La-YSZ (Table S1). Calculations of grain boundary energy for 10YSZ and La-YSZ (Table S2). Parameters of the equivalent circuit for the impedance spectra of 10YSZ and La-YSZ (Table S3).

Author information Corresponding author *Email: [email protected] (R. H. R. C.) ORCID 0000-0002-7574-7665 - Ricardo H. R. Castro 0000-0003-1485-6048 - Alexey Nikonov 0000-0002-2657-3160 - Matthew Gong


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0000-0001-6101-4762 - Jeffery A. Aguiar 0000-0002-0293-8225 - Arseniy Bokov Notes The authors declare no competing financial interest.

Acknowledgments Army Research Office Grant W911NF-17-1-0026 and W911NF-18-1-0361 are acknowledged by RC and AB. AN acknowledges funding within the state task 0389-2015-0025 to perform measurements of ionic conductivity. JAA and ML acknowledge the INL Laboratory Directed Research & Development (LDRD) Program under DOE Idaho Operations Office Contract DEAC07-05ID14517. This work was performed in part at the Utah Nanofab sponsored by the College of Engineering, Office of the Vice President for Research, and the Utah Science Technology and Research (USTAR) initiative of the State of Utah. The authors appreciate the support of the staff and facilities that made this work possible at the University of Utah.

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A Strategy to Mitigate Grain Boundary Blocking in Nanocrystalline

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