Decomposition of La2NiO4 in Sm0.2Ce0.8O2-La2NiO4 composites for solid oxide fuel cell applications

Solid State Ionics 300 (2017) 91–96

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Decomposition of La2NiO4 in Sm0.2Ce0.8O2-La2NiO4 composites for solid oxide fuel cell applications Deniz Cetin a, Sophie Poizeau b, John Pietras b, Srikanth Gopalan a,c,⁎ a b c

Division of Materials Science and Engineering, Boston University, Brookline, MA 02446, United States Saint Gobain Northborough R&D Center, Northborough, MA 01532, United States Department of Mechanical Engineering, Boston University, Boston, MA 02215, United States

a r t i c l e

i n f o

Article history: Received 7 September 2016 Received in revised form 21 November 2016 Accepted 26 November 2016 Available online xxxx Keywords: Lanthanum nickelate Samarium doped ceria Phase decomposition Solid oxide fuel cells Composite cathodes

a b s t r a c t The decomposition of La2NiO4 + δ (LNO) was studied in presence of a second phase, Sm0.2Ce0.8O2 − δ (SDC), for volume fractions of LNO below 55 vol%, after sintering at 1300 °C for 5 h. The details of the decomposition reaction were studied using a combination of experimental methods including X-ray diffractometry, dilatometry analysis and scanning electron microscopy imaging. The La2O3 resulting from the LNO decomposition gets incorporated into SDC phase leading to doubly doped ceria and the formation of free NiO. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Lanthanum nickelate, La2NiO4 + δ (LNO), is a mixed ionic and electronic conductor that has a combination of high surface exchange kinetics [1] and electrical conductivity [2]. It is therefore an excellent candidate for the oxygen electrode of solid oxide fuel cells (SOFCs) and solid oxide electrolyzer cells (SOECs), as well as oxygen permeation membranes and sensors. It has been previously shown that LNO with samarium doped ceria (SmxCe1 − xO2 − δ, SDC) as SOFC cathode materials lead to higher performance than standalone LNO [3,4]. However, the deployment of this composite system to SOFCs as the cathode material requires careful compositional tailoring in order to optimize the cathode phase stability and performance. Contradictions in the literature about the reactivity of LNO with ceria based electrolytes arise from a wide variety of preparation and processing routes of individual phases, differences in sintering conditions (temperature and time), and compositional variations from one study to another. The LNO synthesis methods impact the stability of the LNO phase [5] due to varying particle morphologies, which influence the contact area between LNO and the electrolyte material. In Table 1, a summary of the LNO synthesis routes, the processing conditions for the LNO-ceria composite cathodes, the reactivity of the phases, and formation of third phases is provided. ⁎ Corresponding author at: Division of Materials Science and Engineering, Boston University, Brookline, MA 02446, United States. E-mail address: [email protected] (S. Gopalan).

http://dx.doi.org/10.1016/j.ssi.2016.11.030 0167-2738/© 2016 Elsevier B.V. All rights reserved.

Montenegro-Hernandez et al. [5] prepared LNO by three different synthesis routes (Acetate-hexamethylenetetramine (HMTA) method, citrates method, and solid state reaction method), and the LNO reactivity with Gd0.1Ce0.9O2 − δ (GDC) electrolyte was tested. Mixing equal amounts of LNO with the electrolyte material for the reactivity tests, LNO prepared by HMTA method reacts to form decomposition products when mixed with the electrolyte materials at lower temperatures and for shorter times compared to the other methods. By contrast, LNO prepared via solid state reaction method in contact with GDC does not show any reactivity after annealing at 900 °C for 72 h. Montenegro-Hernandez et al. [6] reported the decomposition of LNO (prepared by Acetate-HMTA method) into its precursor oxides NiO and La2O3, and higher order Ruddlesden-Popper (RP) phases when sintered in contact with GDC at 900 °C. Sayers et al. [7] showed reactivity between LNO (prepared by sol-gel synthesis technique) and GDC. In their experiments, LNO transformed into higher order RP phases upon sintering 50:50 vol% LNO: GDC pellets at 900 °C for 24 h. No reactivity of LNO (prepared via nitrate-citrate procedure) [3] with dense ceria based electrolytes was observed at 1000 °C for 4 h. However, the same study reported that in the composite electrodes of LNO: SDC (67:33 by vol), higher order RP phases were formed. RP phase formation was also reported by Nicollet et al. [8] in LNO infiltrated GDC backbone. Higher order RP phases formed as a result of partial diffusion of lanthanum cations into the GDC backbone upon sintering at 900 °C. In a recent study with porous LNO layer on GDC, Fluora et al. [9] reported the appearance of La doped GDC and La deficient LNO at the LNO-GDC interface. TEM studies [10] on LNO prepared

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Table 1 Summary of the LNO (La2NiO4 + δ) reactivity with doped ceria. LNO preparation method

Fluorite phase

Sample type

Reaction temperature and time

Reaction phases

Reference

Acetate-HMTA Acetate-HMTA Sol-gel synthesis Nitrate-citrate route Nitrate-citrate route Infiltration method Solid state reaction Commercial LNO

GDC GDC GDC SDC, GDC SDC GDC GDC GDC

Pellet Porous layer on dense fluorite substrate Composite Pellet Porous layer on dense fluorite substrate Powder mix Infiltration on GDC backbone Powder mix Porous layer on porous GDC interlayer on dense YSZ

900 °C for 2 h 900 °C for 72 h 900 °C for 24 h 1000 °C for 4 h 1000 °C for 4 h 900 °C 900 °C for 72 h 1200 °C for 1 h

NiO and RP phases La2O3 and RP phases RP phases Not reported RP phases RP phases Not reported La-deficient LNO and La-doped GDC

[6] [6] [7] [3] [3] [8] [5] [9]

via HMTA method suggest that the region of compositional changes from the pristine material begins at the interface of LNO and GDC and then extends beyond this region. All previous studies were conducted with either equal amounts of LNO and ceria phase or more LNO than the ceria phase. However, the composition of the cathode, i.e. the volumetric fractions of each phase must be optimized to achieve high cathode performance and mechanical stability. Specifically, the requirement of achieving a match between the coefficients of thermal expansion (CTE) between the layers leads to a prescription of smaller volume fractions of the LNO phase in the composite. Thus, in this work, we investigate the suitability of Sm0.2Ce0.8O2 − δ: La2NiO4 + δ (SDC: LNO) composites, focusing on higher volume fractions of ceria than LNO. A study of these compositions through dilatometry analysis, scanning electron microscopy (SEM) imaging and X-ray diffractometry (XRD) as a function of sintering temperature and material composition provides insight into the instability of the LNO in the presence of SDC under SOFC cathodic conditions. 2. Experimental Sm0.2Ce0.8O2 (SDC) was purchased from Fuel Cell Materials and La2NiO4 (LNO) was synthesized through solid state reaction. To prepare LNO, stoichiometric mixtures of precursor powders La2O3 and NiO were ball milled using zirconia milling media in ethanol for 24 h. After drying in an oven at 70 °C, the mixed and milled precursors were calcined at 1200 °C for 4 h. The median particle size distribution of the LNO was determined to be 1.08 μm, and that of SDC 0.3 μm, via Horiba LA-910 Particle Size Analyzer. SDC: LNO mixtures with compositions between 34 and 54 vol% of LNO (density of LNO = 7.11 g/cm3, density of SDC = 7.15 g/cm3) were uniaxially pressed into disk shaped pellets under 45.6 kpsi pressure and were sintered at 1300 °C for 5 h in air. The XRD patterns of single-phase SDC and LNO powders and sintered pellets of the composites were obtained using Cu-Kα radiation by Bruker D8 Discover. Dilatometry tests were performed on a Linseis L75VD 1750 Vertical Dilatometer to measure the coefficient of thermal expansion of the SDC:LNO mixtures. The Linseis L75VD 1750 dilatometer system is equipped with dual alumina pushrods, and outer tubes and a type B (Pt/Rh) thermocouple. Alumina spacers were used during the test between the samples and pushrods. During the measurement, the pushrod is pressed against a sample material with a negligible load. As the sample changes dimension the displacement of the pushrod is recorded. During the dilatometry measurements a constant load of 225 mN was applied to each sample for the duration of the test. All measurements were conducted in an air atmosphere with a continuous flow rate of 4 L/min. For dilatometry tests, SDC: LNO mixtures with compositions between 34 and 64 vol% of LNO were binderized using poly(ethylene glycol) 400 and poly(vinyl alcohol) 205 as a binder system. Two samples were prepared for each composition for reproducibility. Each sample was prepared with 0.6 g of the binderized SDC: LNO mixtures pressed at room temperature under 23 kpsi in a 6 mm diameter die. The samples

were first sintered at 1280 °C for 30 min. Then, they were heated back up and down at 2 °C/min to measure the CTE up to 1200 °C. No additional sintering was observed during this cycle, the CTEs during the heat-up and cool-down cycle were the same, and no change in length of the sample before and after the cycle was recorded. Scanning electron microscopy (SEM) images of polished samples were obtained using Zeiss Supra 40 SEM. For the backscattering electron images, a Robinson detector was used. Energy dispersive X-ray spectroscopy (EDS) for elemental analysis and mapping was performed on Zeiss Supra 55VP Field Emission Scanning Electron Microscopy (FESEM). 3. Results Fig. 1 shows the coefficients of thermal expansion (CTE) of the composite samples along with the end components, namely single-phase SDC and single-phase LNO. Samples with high LNO volume fractions (54 and 64 vol.%) fit the rule-of-mixtures line, as expected from CTE measurements of mixtures of chemically equilibrated phases. However, for the composites with lower LNO volume fractions, namely 34 and 44 vol%, there is a large and anomalous deviation from the rule of mixtures line. The reasons for this will be discussed in the Section 4.4. X-ray diffraction patterns of the sintered pellets of 46:54, 56:44, and 66:34 SDC:LNO samples are presented in Fig. 2. All three diffractograms show formation of a new phase, namely, NiO. In the 66:34 SDC:LNO XRD scan, no LNO peaks are discernible suggesting the complete disappearance of the LNO phase. These results suggest complete decomposition of the LNO, even though no La2O3 peaks can be seen. The reasons for this will be discussed in the next section. 4. Discussion 4.1. Phase identification through X-ray diffraction Fig. 3 focuses on the major (111) peak of doped ceria. Fig. 3a is that of pure Sm0.2Ce0.8O2 − δ while Fig. 3b is that in the 66:34 SDC:LNO sample.

Fig. 1. Coefficients of thermal expansion of single-phase SDC (Sm0.2Ce0.8O2 − δ), singlephase LNO (La2NiO4 + δ), and SDC: LNO composites cooled from 1200 °C to 100 °C.

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Fig. 2. X-ray diffraction patterns of SDC: LNO composite pellets that were calcined at 1300 °C for 5 h.

From the XRD scans, the lattice parameter for the single-phase SDC and for the doped ceria phase in the 66:34 SDC:LNO composite were calculated to be 5.43 Å and 5.51 Å respectively. This shift in the lattice parameter calculated from the (111) peak of ceria signals a change in the composition of the doped ceria. The lattice expansion from 5.43 Å for Sm0.2Ce0.8O2 − δ to 5.51 Å for doped ceria in the 66:34 composite sample can be explained by the following sequence of steps. LNO decomposition occurs through the reaction La2NiO4 ➔ La2O3 + NiO, and the product La2O3 is incorporated by dissolution into the Sm0.2Ce0.8O2 − δ phase, thus rendering it a Sm and

La-co-doped ceria. The absence of La2O3 peaks combined with the appearance of NiO peaks in the 56:44 and 66:34 (SDC:LNO) XRD scans further corroborates this reaction mechanism. According to Hong and Virkar [11], the lattice parameter of rare earth oxide doped ceria can be calculated by using a simple model given in Eq.(1) 4 a ¼ pffiffiffi ð0:9971Þðr c þ r a Þ 3

ð1Þ

where rc and ra are the effective cation and anion radii, respectively.

Fig. 3. X-ray diffraction (111) peak of (3a) single-phase SDC; (3b) 66:34 (by vol.) SDC:LNO composite.

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Table 2 Ionic radii of the dopant and host species and oxygen vacancy. Ion

Shannon ionic radius (Å)

Reference

La3+ (VIII) Sm3+ (VIII) Ce4+ (VIII) O2– (IV) V∙∙O

1.16 1.079 0.97 1.38 1.164

[12] [12] [12] [12] [11]

4.2. Phase confirmation via electron microscopy imaging

Rare-earth dopants are known to be incorporated into the fluorite ceria phase according to the following defect reaction written in the usual Kröger-Vink notation. x CeO2 3x x RE2 O3 → x  RE0Ce þ  OXO þ V ∙∙O 2 2 2

ð2Þ

Thus, the incorporation of two rare-earth dopant cations produces one oxygen vacancy. We use the Hong and Virkar's model [11] to calculate the composition of the resulting rare-earth doped ceria phase. Denoting the number of La ions per formula unit in the resulting ceria phase as x, we can write the composition of the resulting phase as (LaxSm0.2Ce0.8 − x)O2 − δ. The number of oxygen vacancies per formula unit, δ, according to the point defect reaction above is given by, δ¼

1 ðx þ 0:2Þ 2

for singly rare-earth doped ceria phases. Moreover, the lattice parameter for highly rare earth doped ceria phases was previously found to be 5.5593 Å for 50 mol% La doped ceria [14]. Hence, our estimated composition of the final ceria phase is within 10% of the reported solubility limit of rare-earth doped cerias.

ð3Þ

Since there is one cation site and two anion sites per formula unit of ceria, the site fraction of oxygen vacancies is given by 1/4(x + 0.2). Assuming random distribution of cationic and anionic site occupants, we now write the anionic and cationic site radii as the radii of the occupants of the anionic and cationic sublattices weighted by their site fractions, i.e. r c ¼ x  r La þ 0:2 r Sm þ ð0:8−xÞ  r Ce

ð4Þ

r a ¼ ð0:95−0:25xÞ  r O þ ð0:05 þ 0:25xÞ  r V ∙∙O

ð5Þ

The formation of NiO phase in the 66:34 SDC:LNO sample (1300 °C for 5 h) shown in Fig. 2 has been further confirmed with the electron microscopy images (Fig. 4). In backscattered electron images, phases with higher atomic number appear lighter in color due to stronger scattering, whereas those with lower atomic number are darker. The NiO grains appear black in the backscattered electron microscope image in Fig. 4a. A secondary electron microscopy image of the same area is provided in Fig. 4b to distinguish the sample porosity from the NiO phases. In Fig. 5, the distribution of different phases on the surface of 46:54 and 66:34 SDC:LNO samples are presented using SEM-back scattered electron (BSE) imaging. The black, dark and light grey areas indicate the presence of NiO, LNO and doped ceria phases, respectively. EDS elemental analysis in Fig. 6 confirms these phases in the 66:34 SDC:LNO sample. NiO phases are present in both samples, which is consistent with the XRD patterns. BSE-SEM images (Fig. 5) show the presence of LNO phase in abundance in the 46:54 sample, but only in small amounts in the 66:34 sample. Even though the XRD pattern of 66:34 SDC:LNO (Fig. 2) does not show any LNO peaks, this seeming contradiction can be explained by the variance in detection limits of the two analysis methods (about 3% of sample for XRD and 0.5% for EDS). Elemental mapping by EDS around the precipitates in the 66:34 SDC:LNO sample (Fig. 6) confirms the precipitates to be the Ni-rich phases, presumably NiO, whereas cerium and lanthanum elements were distributed homogenously around them. Elemental mapping does not show regions rich in both La and Ni which would be a signature for LNO. 4.3. La-Sm-Ce-Ni-O phase equilibria

where the radii of the cations and anions are listed in Table 2 below. Substituting the ionic radii given in Table 2 into Eqs.(4) and (5) along with the calculated lattice parameter for doped ceria in 66:34 SDC: LNO sample (5.51 Å) into Eq.(1), the estimated composition of the Sm, La codoped ceria can be calculated as Sm0.2La0.23Ce0.57O2 − δ. Prior experimental studies [13,14] suggest that the solubility limit of rare earth dopants in ceria is around 50 mol% (while doped ceria phase retains its fluorite structure), with variations around that value depending on the size of the dopant ion. The present total dopant concentration of 0.43 per Ce-site is somewhat less than the solubility limit observed

The equilibrium phase mixture of the SDC: LNO composites is a solid solution of La2O3 and Sm2O3 co-doped ceria, NiO and LNO (depending on the overall composition). This is also consistent with the previously observed subsolidus phase equilibria in La2O3-CeO2-NiO system [15] at 1200 °C, also shown in the tentative phase diagram in Fig. 7. Hrovat et al. [14] presents a pseudo-binary tie line between La2NiO4 and La0.5Ce0.5O1.75 and concludes that LNO based cathode materials will react with ceria based solid solution to form of CeO2 solid solutions and NiO. Although the proposed phase diagram in Hrovat et al. [14] is that of a ternary, this phase equilibria can be pertinent to SDC:LNO system also, since the ceria lattice is able to dissolve many rare-earth

Fig. 4. (4a) BSE-SEM and (4b) SE-SEM images of 66:34 SDC:LNO pellet (calcined at 1300 °C for 5 h). The red circle is one of the NiO precipitates.

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Fig. 5. BSE-SEM images on (5a) 46:54 and (5b) 66:34 pellets (calcined at 1300 °C for 5 h).

cations of similar sizes, and the rare-earth cations exhibit very similar chemical properties. Additionally, the Gibbs phase rule can be applied to the SDC:LNO system to understand the observed phase equilibria. F ¼ C þ 2−P

ð6Þ

where F is the degree of freedom, C is the number of components and P is the number of phases. The number of components are five, namely,

Sm, La, Ce, Ni and O. Thus, F ¼ 5 þ 2−P or F ¼ 7−P

ð7Þ

We have four phases in equilibrium: these include a gas phase and three solid phases in the 56:44 and 66:34 SDC:LNO composites, namely, Sm2O3 and La2O3 co-doped ceria, NiO, and La deficient LNO. Thus, we have three degrees of freedom. Temperature and pressure take up two of the degrees of freedom, and since the composition of the three phases in equilibrium are fixed, namely the LNO, NiO, and the Sm2O3 and La2O3

Fig. 6. Elemental analysis on 66:34 SDC: LNO pellet by EDX. As indicated on the image: Cyan for Ni, Purple for Ce, Yellow for La. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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compositional readjustments of LNO: SDC cathodes resulting in highly doped ceria, NiO with disappearance of substantial amounts of LNO, pose long-term degradation risks. Previous work has focused on high LNO volume fractions (close to 55 vol%) where the effects of LNO decomposition are not pronounced. Even though prior workers have identified this composite material system as a promising option for SOFC applications, we conclude that integration of this material into SOFC systems as the cathode functional layer without substantial re-engineering of the compositions will likely result in lowering the SOFC performance for low LNO volume fractions.

Acknowledgements

Fig. 7. Subsolidus ternary phase equilibria in La2O3-CeO2-NiO system at 1200 °C in air, adapted from Hrovat et al. [15]

co-doped ceria (doped to the solubility limit of the ceria phase), the one degree of freedom left is the exact location in composition space within the three phase region, i.e. the overall composition which determines the relative amounts of each phase present. Further work is required to determine the exact compositional boundaries of the observed phases. 4.4. Coefficient of thermal expansion We are now in a position to explain the anomalous behavior of the CTE in Fig. 1. To begin with note that the CTE of the end components are 12.6 ± 0.1 ppm/K for the SDC phase and 14.1 ± 0.1 ppm/K for the LNO phase. Decomposition of LNO in samples with high LNO volume fractions does not greatly influence the overall volume fraction of phases and thereby the measured trend in CTE values is close the rule of mixtures line due to the abundance of the remnant LNO phase. By contrast, the smaller LNO volume fractions in the equilibrated 56:44 and 66:34 SDC:LNO composites resulting from the decomposition and the near disappearance of the LNO, lowers the overall CTE below the rule of mixtures line. Moreover, the CTE of the 66:34 SDC:LNO sample (12.1 ± 0.1 ppm/K) is less than that of single-phase SDC (12.6 ± 0.1 ppm/K) This suggests that the CTE of doped-ceria (the primary constituent in the equilibrated 66:34 SDC: LNO composite) is reduced to less than that of the end-component single-phase SDC when excess La2O3 formed due to LNO decomposition is incorporated into the ceria phase. 5. Conclusions Based on these results, mixtures of SDC (with less than 0.2 site fraction of Sm) and LNO, result in an equilibrium phase composition of La2O3-Sm2O3 co-doped CeO2 and NiO at 1300 °C. Therefore,

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