Hydrothermal Synthesis and Characterization of an

DOI: 10.1002/ente.201700867

Hydrothermal Synthesis and Characterization of an Apatite-Type Lanthanum Silicate Ceramic for Solid Oxide Fuel Cell Electrolyte Applications Paramananda Jena,[a] Pankaj K. Patro,*[b] Amit Sinha,[b] Raja K. Lenka,[b] Akhilesh K. Singh,*[a] Tarasankar Mahata,[b] and Pankaj K. Sinha[b] Apatite-type lanthanum silicate (La10Si6O27; LS) nanopowders are synthesized using a hydrothermal process and used for the electrolyte applications in solid oxide fuel cells (SOFCs). The synthesized nanopowders are characterized by Rietveld refinement of the X-ray diffraction (XRD) data, SEM–energy-dispersive X-ray spectroscopy (EDS), TEM, and dialotometry. The prepared nanopowders can be sintered to near theoretical density at sintering temperature of 1500 8C. Reactivity studies between the synthesized La10Si6O27 material and cathode materials such as La0.6Sr0.4Co0.2Fe0.8O3d (LSCF), Nd2NiO4 (NNO), and

Pr2NiO4 (PNO), are carried out by XRD analysis revealing that they do not react. Symmetric cells are fabricated and characterized electrochemically. The area specific resistances for LSCF, NNO, PNO, and PNO-LS (1:1 (wt %)) measured at 900 oC, are 100, 2.2, 1.8, and 0.5 W cm2, respectively. EDS analysis shows that interdiffusion of cations at the cathode– electrolyte interface is not detected for LS/PNO, which is attributed to the low area-specific resistance values. The results demonstrate that Pr2NiO4 and the composites are promising cathode materials for the apatite-type lanthanum silicate electrolyte for SOFC applications.

Introduction In recent years, the uncertainty in the hydrocarbon reserve in the earths crust and its depleting trend have compelled researchers, scientists, and entrepreneurs to look for sustainable, energy efficient, low cost, eco-friendly alternative energy sources and technologies. The alternative energy technology devices such as fuel cells, supercapacitors, and lithium-ion batteries are the most suitable electrochemical energy sources for conversion and storage.[1] Among these, fuel cells, in particular solid oxide fuel cells (SOFCs), are the most promising for future markets due to their high efficiency energy conversion, fuel adaptability, reliability, modular construction, and nonpolluting nature (zero emissions).[1–5] However, to date, most of the SOFCs are manufactured using yttriastabilized zirconia (YSZ) as the electrolyte due to its pure ionic conducting nature over a wide oxygen partial pressure range and ease to fabricate highly dense ceramics.[6–9] Apart from YSZ, several families of oxide-ion-conducting materials have been developed as solid electrolytes for SOFCs application; these include fluorite type (stabilized CeO2 and dBi2O3), pervoksite type (LaGaO3, Na0.5Bi0.5TiO3), brownimillerite type (Ba2Ln2O3), aurivillus type (BIMEVOX), pyrochlore type (Gd2Zr2O7), and apatite-type oxides.[10–13] Among these, apatite-type lanthanum silicates oxide materials have attracted much attention due to their high oxide ionic conductivity, which is comparable to the conventional oxide ion conductors such as YSZ.[13–16] Although apatite-type lanthanum silicate shows higher conductivity compared to the state of art YSZ electrolyte, it has not been fully explored for the study of electrolyte applications. Apatite-type lanthanum silicates have hexagonal crystal structure with space group P63/ Energy Technol. 2018, 6, 1 – 9

m, consisting of isolated tetrahedral SiO4 units with rareearth cations located in the seventh and the ninth coordinated cavity sites of the crystal. The remaining oxygen ions reside in 1D channels associated through the structure. As a result, high conduction has been reported via an interstitial pathway mechanism in which the interstitial oxygen ions pass through cavities located between La channels and isolated SiO4 tetrahedra along the c-axis. This is in contrast to the oxide ion vacancy mechanism observed for perovskites and fluorite-type oxide ion conductors.[17–23] Additionally, solid state 29Si NMR studies of apatite-type silicates have confirmed the presence of interstitial oxygen sites that are correlated with the Si environment and the ionic conductivity.[24] On the other hand, preparation of highly electrical conducting lanthanum silicate samples with the phase purity and sintering temperature lower than 1700 oC remains a challenge to date. In a conventional solid-state route, the issues associated with synthesis of lanthanum silicate electrolytes are [a] Dr. P. Jena, Dr. A. K. Singh School of Material Science and Technology Indian Institute of Technology Banaras Hindu University Varanasi, 221005, U.P. (India) E-mail: [email protected] [b] Dr. P. K. Patro, Dr. A. Sinha, Dr. R. K. Lenka, Dr. T. Mahata, Dr. P. K. Sinha Powder Metallurgy Devision, Materials Group Bhabha Atomic Research Center Vashi Complex Navi Mumbai, 400703, Maharashatra (India) E-mail: [email protected] Supporting Information for this article can be found under: https://doi.org/10.1002/ente.201700867.

2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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higher sintering temperature (> 1700 8C) and mixed phase formation. In recent years, it has been observed in nanostructured ceramics that the presence of the large fraction of grain boundaries can lead to remarkably enhanced electrical, mechanical, magnetic, optical sensing, and biomedical properties.[25, 26] In order to synthesize pure single phase nanocrystalline lanthanum silicate powders with better sinterability at lower temperatures, many efforts have been made via wetchemical routes, including sol–gel,[27–29] citric-nitric (modified sol–gel),[30, 31] and freeze-drying[32] methods. However, not only the high relative density and high conductivity but also the chemical compatibility and low area specific resistance are the most important parameters for an electrolyte in the device fabrication and high performance for the SOFCs. It is still a challenging task to develop compatible cathode materials for the apatite-type lanthanum silicates electrolytes, although many efforts have been made in past.[33] The present work focuses on synthesis of single-phase La10Si6O27 nanopowders using hydrothermal process. The synthesized La10Si6O27 nanopowders are characterized and sinterability studies are carried out. The electrical conductivity, compatibility, and area specific resistance (ASR) measurements with different state of the art cathode materials are studied from the viewpoint of the application of La10Si6O27 as the electrolyte for SOFCs. The current–voltage (I–V) characteristics of fuel cells utilizing La10Si6O27 as the electrolyte material are studied.

Results and Discussion X-ray diffraction (XRD) patterns of the prepared La10Si6O27 powders calcined at three different temperatures, 400 8C, 600 8C, and 800 8C for 3 h along with the International Centre for Diffraction Data (ICDD) reference data are shown in Figure 1 a. A comparison of the observed XRD patterns with standard ICDD data (00-053-0291) confirms the formation of pure crystalline apatite-type single phase with hexagonal structure at 800 8C. From Figure 1 a it is observed that broadening of the XRD peaks decreases with increasing calcination temperature due to increase of the crystallite size. The average crystallite size of the La10Si6O27 powder sample calcined at 800 8C is found to be 18 nm, using the Scherrer equation, t = 0.9 l/bcosq, where t is the average crystallite size, l is the wavelength of incident X-rays from CuKa radiation, b is the full width at half maxima (corrected for instrumental broadening) of the diffraction peak, and q is the Bragg diffraction angle. The XRD pattern of the LS powder calcined at 800 8C was analyzed by Rietveld structure refinement using hexagonal structure with P63m space group. The Rietveld fit between the observed diffraction data with the calculated peak profile is shown in Figure 1 b, which is very good, confirming the hexagonal phase for La10Si6O27 powder. The agreement factors profile factor(Rp), weighted profile factor (Rwp), expected weighted profile factor(Rexp) and reduced chi-square (c2) are found to be 10.5, 13.9, 10.81, and 1.65, respectively, which further confirms the results of the structural analysis. The unit cell parameters and unit cell Energy Technol. 2018, 6, 1 – 9

Figure 1. a) XRD patterns along with ICDD reference data for the La10Si6O27 samples calcined at different temperatures 400, 600, and 800 8C for 3 h. b) Experimentally obtained (red dots), Rietveld calculated (continuous black line), their difference (continuous blue bottom line) profiles and Bragg peak positions (vertical black tick marks) obtained after Rietveld analysis of the XRD data using hexagonal structure with space group P63/m for La10Si6O27 sample calcined at 800 8C.

volume obtained from Rietveld structure refinement are a = b = 9.7405(5) (), c = 7.1864(4) (), V = 590.496 (3), which are in agreement with the ICDD standard (00-053-0291). The schematic representation of apatite-type La10Si6O27 hexagonal unit cell and La site coordination environment is shown in Figure S1 (Supporting Information). It can be observed that the hexagonal crystal structure of the apatite type La10Si6O27 consists of La1 (6 h), Si (6 h) as alternating layer triangles and a La2 (4f) site is in between them. The oxide ions O 1 (6 h), O 2 (6 h), and O 3 (12i) are located close to the Si site and O 4 (2a) is located at the corner of the unit cell.[34–36] Figure 2 a shows the high-resolution transmission electron microscopy (HRTEM) image of the prepared La10Si6O27 powder obtained after calcination at 800 8C. From the image shown in Figure 2 a, it is observed that the particles are agglomerated and the size ranges from 100 to 250 nm. Further these are acicular in shape having aspect ratio of 2.9. The high-resolution TEM image shown in Figure 2 b indicates that the interplanar distances of 2.95, 3.35, and 4.2 corre-


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Figure 2. a) HRTEM, b) d spacing, and c) SAED pattern images of the La10Si6O27 sample calcined at 800 8C.

spond to the inter-lattice planes (211), (102), and (200) of the La10Si6O27 sample. The selected area electron diffraction (SAED) pattern of the particle shown in Figure 2 c shows the diffraction along the zone axis of (118). This further certifies the phase purity of La10Si6O27 powder synthesized using the hydrothermal technique.

Shrinkage behavior of La10Si6O27 green pellet sample with temperature is shown in Figure 3 a. From the dilatometric shrinkage curve of the green compact pellet prepared from powder calcined at 800 8C, it is observed that the shrinkage starts from 1200 8C and ends at a temperature close to 1500 8C. Based on the shrinkage behavior data the sintering temperature was chosen between 1400 8C to 1500 8C. Figure 3 b shows the relative densities of La10Si6O27 pellets sintered at 1400 8C, 1450 8C, and 1500 8C. La10Si6O27 can be sintered to 95 % of the theoretical density when sintered at 1500 8C for 8 h. Tao and Irvine have reported 72 % density of La10Si6O27 sample when sintered at 1400 8C for 72 h for samples synthesized using a sol–gel method.[27] Moreover, 90 % of the theoretical density is reported for lanthanum silicate prepared using a sol–gel combustion route and sintered at 1500 8C.[37] The higher densification of La10Si6O27 sample synthesized via a hydrothermal route may be due to high degree of crystallinity and surface energy at low calcination temperature favoring faster diffusion in the weakly agglomerated nanoparticles. The surface morphology of the La10Si6O27 pellet sintered at 1500 8C for 8 h is shown in Figure 3 c. This scanning electron microscopy (SEM) image corroborates the observed higher density ( 95 % of 1Th) after sintering. The estimated grain size is < 2 mm. The energy-dispersive X-ray spectroscopy (EDS) data of the sintered sample shown along with the SEM image, closely matches with the standard stoichiometry of La10Si6O27.

Figure 3. a) Dilatometric shrinkage, curve for the green pellet La10Si6O27. b) Relative densities of the La10Si6O27 pellets sintered at 1400, 1450, and 1500 8C. c) SEM image of the La10Si6O27 pellet sintered at 1500 8C for 8 h along with elemental compositions obtained from EDS. d) Coefficient of thermal expansion (CTE) of the La10Si6O27 pellet sintered at 1500 8C for 8 h.

Energy Technol. 2018, 6, 1 – 9


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sponds to the grain boundary (Rgb) contribution. The two depressed semicircles observed in the impedance plot can be fitted to a parallel combination of a resister with a capacitor (C)/constant phase element (CPE). The total equivalent circuit for the sample is inserted in Figure 4 a and the obtained circuit parameters are tabulated in Table A (Supporting Information).[47] Total impedance of the equivalent circuit can be analyzed by the following mathematical equation: Z* ¼ ½Rb 1 þ jwCb 1 þ ½Rgb 1 þ jwCgb 1

ð1Þ

where subscripts b and gb represent the bulk (grain interior) and grain boundary contributions, respectively, and w represents the angular frequency. The impedance spectrum obtained at 500 8C, shown in Figure 4 b, exhibits part of a depressed semicircle at higher frequencies and another depressed semicircle at lower frequencies. The first one can be correlated to the grain boundary phenomena, whereas the low frequency second one relates to the electrode phenomena. It is clearly observed that with an increase in temperature, the semicircles in the spectrum shift to the higher frequency region and, as a result, total resistance decreases and conductivity increases. The total resistance (R) is the sum of the grain interior (Rgi) and grain boundary (Rgb) resistances obtained from the impedance plot, analyzed using the WinFIT software. Conductivity of the sample is calculated by the relationship s = L/RA, where L is the thickness of the pellet and A is the area of the pellet. Figure 4 c shows the Arrhenius plot (log (sT) vs. 1000/T) of LS1400 8C and LS1500 8C. The Arrhenius equation for oxide ion conduction is given by:

Figure 4. Impedance (Z’ vs. Z’’) plots at a) 200 8C and b) 500 8C of the La10Si6O27 pellet sintered at 1500 8C for 8 h. c) log (sT) vs. 1000/T plots of the La10Si6O27 pellets sintered at 1400 and 1500 8C for 8 h.

Figure 3 d shows the variation of the coefficient of thermal expansion (CTE) with temperature for the sintered La10Si6O27 pellet. The average CTE is found to be 11 106 K1, which is well within the fuel cell operating temperature region, indicating the suitability of this material for many state of art materials used for SOFCs with CTE in the range 10 to 12 106 K1.[38–46] Figure 4 a,b shows the typical impedance plots (imaginary (Z’’) vs. real (Z’)) measured at 200 8C and 500 8C, respectively, for LS pellet samples sintered at 1500 8C for 8 h. Numbers in the impedance plots represent data collected in different decades from 10 Hz to 106 Hz. Two depressed semicircles are observed in the impedance curve at 200 8C, which can be assigned to grain interior and grain boundary contributions. The red solid points represent the experimentally observed impedance data and the black continuous line represents the fitted value. The first depressed semicircle in the higher frequency region corresponds to the grain interior (Rgi) contribution and the second depressed semicircle in the lower frequency region correEnergy Technol. 2018, 6, 1 – 9

sT ¼ s0 ExpðEa =KTÞ

ð2Þ

where Ea and s0 are activation energy and temperature independent conductivity, respectively. In the Arrhenius plot, the experimental data are fitted to a straight line. The obtained conductivity is compared with the literature reports as shown in Table B (Supporting Information). Conductivity is comparable with the reported data at lower sintering temperatures. The literature reveals higher ionic conductivity of La10Si6O27 compared to conventional YSZ electrolyte in the intermediate temperature region and is well reflected from the conductivity data tabulated in Table B (Supporting Information).[15, 27, 37] A few reports suggest improvement in the ionic conductivity of La10Si6O27 by doping at La and Si sites.[44–46, 48–50] There is still the possibility to improve the conductivity value through the hydrothermal route approach by suitable doping in La10Si6O27 for using a promising solid electrolyte for intermediate temperature SOFCs. The chemical reactivity between the electrolyte and electrode materials is one of important concerns for SOFC performance. The reactivity between the electrolyte and electrode materials should be negligible to avoid the formation of any third phase that could increase the ohmic resistance


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Figure 5. XRD patterns of a) LS, LSCF, and LS-LSCF (1:1 wt %); b) LS, NNO, and LS-NNO (1:1 wt %); and c) LS, PNO, and LS-PNO (1:1 wt %) at room temperature (RT) and after heat treatment at 1200 C for 12 h.

of the cell and reduce the electrochemical reaction sites at the triple phase boundary (TPBs) interface. Therefore, it is essential to know the chemical reactivity between the cell components and the optimum sintering temperature to avoid possible chemical reactions at the electrode–electrolyte interfaces during cell operation to achieve better performance of the cell. We have investigated the chemical reactivity of the prepared La10Si6O27 sample with a few electrode materials such as La0.6Sr0.4Co0.2Fe0.8O3d (LSCF), Nd2NiO4 (NNO), and Pr2NiO4 (PNO) purchased from Marion Technologies. XRD patterns of the pure La10Si6O27, pure electrode materials La0.6Sr0.4Co0.2Fe0.8O3d (LSCF), Nd2NiO4 (NNO), Pr2NiO4 (PNO), and the mixtures of both in equal quantities (1:1 wt %), LS-LSCF, LS-NNO, LS-PNO at room temperature before and after heat treatment at 1200 8C for 12 h are shown in Figure 5. No additional peak is observed in the diffraction pattern for the powder mixtures of electrolyte and electrode materials, even after heat treatment for 12 h for 1200 8C. Le Bail refinements were carried out for the obtained XRD patterns of the mixtures (1:1 wt %) LS-LSCF, LSNNO, LS-PNO, and heat treated at 1200 8C for 12 h shown in Energy Technol. 2018, 6, 1 – 9

Figure S2 (Supporting Information). The obtained unit cell parameters, unit cell volume, and c2 data are summarized in Table C (Supporting Information). From Table C it is observed that there is variation in the lattice parameters as well as cell volume of both phases after heat treatment at 1200 8C for 12 h. In the case of LS-LSCF, the lattice parameters a = b in the apatite phase (LS) show shrinkage while c expands after heat treatment. In contrast, the lattice parameters a = b of LSCF phase expand while c shows shrinkages. However, overall cell volume expands for both the phases after heat treatment. In case of LS-NNO, LS-PNO the lattice parameters a = b, c of the apatite phase shows shrinkage after heat treatment while the lattice parameters a shrink and b, c expand for NNO, PNO phases. In addition, the cell volume of the apatite phase shrinks and NNO, PNO phases expand. The small variation of cell volumes of both the phases indicates that the reactivity of the three electrode materials with the LS electrolyte is not very significant. Considering the low chemical reactivity between the silicate electrolyte and few high-performance cathode materials, even at high firing temperatures, one may expect low area specific resistance of the cell. To investigate further, ASR of the electrode–electrolyte combinations in the form of symmetric cells is studied and EDS analysis at the interface of the symmetrical cells is carried out to determine the influence of the interfacial reaction between the two. Impedance spectra at 900 8C of the symmetric cells using three different electrodes, LSCF, NNO, PNO, and a composite of LS-PNO (1:1 (wt %)), on a La10Si6O27 pellet sintered at 1500 8C 8 h are shown in Figure 6. ASR has been calculated from the impedance plots after normalizing with the sample dimension. The ASR values of LSCF, NNO, PNO, and a composite of LS-PNO at 900 8C are 100, 2.2, 1.8, and 0.5 W cm2, respectively. The ASR values at 700 8C, 850 8C, and 900 8C are given in Table 1 together with the cell sintering temperatures. From the vast variation in the ASR values, interaction of different electrodes with LS electrolyte is expected. The measured ASR values indicate reactivity level of different electrodes with LS electrolyte in the following order LSCF > NNO > PNO > PNO-LS. The degree of interfacial reaction seems to be negligible for the PNO-based electrode. Further confirmation can be inferred from the EDS analysis of the electrode–electrolyte interface of the symmetric cell. The fractured cross-section of the electrolyte–electrode interface of different symmetric cells after ASR measurements is shown in Figure S3 (Supporting Information). From Figure S3, it is observed that all the electrode layers have adequate porosity and good adherence with the electrolyte layer. The coating thickness of the cathode materials varies in the range 15–40 mm on the lanthanum silicate pellets. The SEM–EDS at different points across the electrode– electrolyte interface of the LS/LSCF, LS/NNO, and LS/PNO symmetrical cells are shown in Figure 7 a–c, along with the compositional distributions of different atom % at the electrode–electrolyte interface. From the Figure 7 a of the EDS, it is observed that the cobalt, strontium, and iron diffusion


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interface into the bulk of the electrolyte while silicon diffuses from the interface into the bulk of the cathode. From Figure 7 c of the EDS, it is observed that there is no diffusion of praseodymium into the electrolyte or silicon into the electrode. In fact, only very negligible nickel was detected at the interface towards the electrolyte. From the obtained ASR values and the atom % compositions from EDS analysis at the interface, it appears that the nickel-based cathode materials, particularly Pr2NiO4 and its composites, are the most compatible and suitable materials for apatite-type La10Si6O27 electrolyte for fabrication of SOFCs. Current-to-voltage response of a LS electrolyte supported cell using LS-NiO as the anode and LS-PNO (1:1 (wt %)) as the cathode is shown in Figure 8 a. Impedance plot of the cell (LS-NiO/LS/LS-PNO) is given in Figure 8 b to show the total resistance with temperature. The high open-circuit voltage (OCV) achieved during fuel cell testing at 600 8C is 1.0 V, indicating the gas tightness of the cell. This OCV decreased with increases in the operating temperature, which is in accordance with the thermodynamics of the fuel cell. The maximum powers drawn from the cell were 4, 5, and 7 mW cm2 and the corresponding current densities were 8, 12, and 15 mA cm2 at 750, 800, and 850 8C. The power densities obtained for the single cell LS-NiO/LS/LS-PNO in our work are higher than the reported values of NiO-LS/LS/LSCF of the La10Si6O27 electrolyte supported cells.[49] This is attributed to the lower polarization resistance of the LS-Pr2NiO4 composite cathode. It can be inferred that ohmic resistance of the cell is very high compared to the total resistance due to the use of an electrolyte layer of 1 mm. The powder density can be further improved using a thin electrolyte layer and by tailoring the microstructure of the electrodes.

Conclusions Figure 6. Impedance spectra plots at 900 C for the symmetric cells: a) LS/ LSCF, b) LS/NNO, c) LS/PNO, and d) LS/PNO-LS (1:1 wt %).

Table 1. Sintering temperatures of the different cathode materials on La10Si6O27 pellet sintered at 1500 8C for 8 h and obtained ASR values at 700, 850, and 900 8C. Symmetrical cells

Sintering temp. [8C]/ sintering time [h]

Obtained ASR [Wcm2] 700 8C 850 8C 900 8C

LSCF-LS-LSCF NNO-LS-NNO PNO-LS-PNO LS/PNO-LS-PNO/LS [1:1 (wt %)]

1000/1.5 1120/1.5 1150/1.5 1150/1.5

700 17 15 3

210 7 2.5 0.9

100 2.2 1.8 0.5

occurs from electrode into bulk of the electrolyte, while silicon has diffused from the interface into the bulk of the cathode. Similarly, from Figure 7 b of the EDS, it is observed that the neodymium and very minute nickel diffusion occurs from Energy Technol. 2018, 6, 1 – 9

Apatite-type single phase La10Si6O27 nanopowder samples were synthesized by hydrothermal method. The nanopowders obtained from hydrothermal synthesis exhibit high densification with negligible porosity for the pellet sintered at 1500 8C for 8 h. The calculated total electrical conductivity of La10Si6O27 pellet was found to be 1.09 104 S cm1 at 500 8C, which is comparable to reported values. The calculated ASR values at 900 8C from the obtained impedance spectra of the fabricated symmetrical cells such as LSCF/LS/LSCF, NNO/ LS/NNO, PNO/LS/PNO, and PNO-LS/LS/PNO-LS (1:1 (wt %)) were 100, 2.2, 1.8, and 0.5 W cm2, respectively. The higher ASR values for LSCF- and NNO-based cathode materials may be attributed to interfacial diffusion between the respective electrode and the lanthanum silicate electrolyte material. In the case of PNO/LS/PNO, EDS analysis confirmed that inter-diffusion of cations is negligible at the electrolyte–electrode interface. The current–voltage curve for the cell with LS electrolyte and PNO cathode resulted in Pmax of 7 mW cm2. The present study shows that Pr2NiO4 (PNO) and LS-PNO composite are promising cathode materials for the apatite-type lanthanum silicate (La10Si6O27) electrolyte for SOFCs applications.


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Figure 7. SEM–EDS at different points and compositional distribution of different atom % across the electrode/electrolyte interface of the symmetrical cells a) LS-LSCF, b) LS-NNO, and c) LS-PNO.

These two solutions were mixed together under continuous stirring to form a clear transparent homogenous solution, followed by addition of dropwise diethyl amine (DEA). After a few minutes ( 5 min), nucleation of the corresponding hydroxide occurred, resulting in the formation of white precipitate. Then, the solution containing the white precipitate was transferred into a stirred autoclave reactor having material of construction titanium grade-2 (Chemito Technologies, India) and a sufficient amount ( 1.25 L) of deionized water was added for hydrothermal treatment. The hydrothermal synthesis was carried out at a temperature of 240 8C and 45 bar pressure for 24 h under a continuous stirred condition. The obtained polymeric precipitate was washed with deionized water and subsequently dried at 60 8C for 12 h followed by calcinations at different temperatures.

Basic characterization

Figure 8. a) Power density and current–voltage (I–V) response (linear plots) of La10Si6O27 electrolyte supported cell (NiO-LS/LS/PNO-LS). b) Impedance plot for the cell.

Experimental Section Powder synthesis Analar grade precursor chemicals such as lanthanum nitrate hexahydrate (La(NO3)3·6 H2O) [Loba, 99 %], tetraethyl orthosilicate (TEOS; Si(OC2H5)4) [Sigma–Aldrich 99.997 %], diethyl amine ((C2H5)2NH) [Merck, 99.9 %], absolute ethanol (Thomas Baker), and de-ionized (DI) water were used for the synthesis. The chemicals were used as received, without further purification. The required stoichiometric amounts of precursor chemicals such as lanthanum nitrate and tetraethyl orthosilicate (TEOS) were dissolved separately in deionized water and ethanol, respectively. Energy Technol. 2018, 6, 1 – 9

The XRD measurements on calcined powder samples were carried out by using Rigaku Miniflex/600 powder diffractometer with a CuKa source. The data were collected in the 2q range 108– 908 with a step size of 0.028. The diffraction data were analyzed by Rietveld structure refinement using the FULLPROF suit. Morphology and chemical composition of the as-prepared La10Si6O27 powder sample was carried out using SEM–EDS (ZEISS-EVO18, Germany) and TEM (Model FEI Tecnai G2F30 with 300 kV operating voltage). Sintering and thermal expansion studies were carried out using a thermomechanical analyzer (Setsys Evolution, Setram Instruments, France) in flowing air atmosphere at 1600 and 900 8C, respectively. For the sintering study, green compact of 6 mm diameter and 8 mm length samples were prepared using uniaxial die pressing and for thermal expansion a similar sample was sintered in a furnace at 1500 8C for 8 h. The grain morphology of the sintered La10Si6O27 pellet was investigated using SEM. The electrical conductivity data were collected for the sintered La10Si6O27 pellet using a high-performance frequency response analyzer (Novocontrol Alpha A) in the fre-


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quency range 1 MHz to 0.2 Hz and temperature range 200– 700 8C. The chemical reactivity of the prepared La10Si6O27 electrolyte sample with different electrode materials was investigated by XRD. For this, mixtures of prepared La10Si6O27 electrolyte sample and electrode powders were prepared in a 1:1 (wt %) ratio, ground in an agate mortar, pelletized, and then fired at 1200 8C for 10 h. For area-specific resistance measurements, electrodes were coated on both sides (symmetric cell) of the dense La10Si6O27 pellets using a slurry prepared with the different electrode powders and 2.5 wt % of ethyl cellulose terpineol oil as a binder. The symmetrical cells were fired depending on the electrode composition, between 1000–1150 8C in air at different dwelling times. Afterwards, a Pt-based ink was applied onto the electrodes to obtain a current collector layer and fired at 900 8C for 2 h. The ASR values were measured in air atmosphere with a two-electrode configuration. Further, SEM–EDS of the symmetrical cells were obtained to analyze the microstructure and reactivity between electrode/electrolyte layers.

Acknowledgements The authors would like to thank the Central Instrument Facility (CIF), Indian Institute of Technology (Banaras Hindu University), Varanasi-221005, U.P., India for providing SEMEDX facilities. P.J. gratefully acknowledges SMST, IIT (BHU), Government of India, for postdoctoral fellowship. A.K.S. acknowledges the Center for Energy and Resources Development (CERD), IIT (BHU) for financial assistant.

Conflict of interest The authors declare no conflict of interest.

Keywords: area specific resistance · electrolytes hydrothermal synthesis · impedance · solid oxide fuel cells

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Manuscript received: November 21, 2017 Revised manuscript received: December 22, 2017 Accepted manuscript online: January 9, 2018 Version of record online: && &&, 0000


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FULL PAPERS Phase-pure La10Si6O27 (LS) ceramics are synthesized using a hydrothermal process. La10Si6O27 could be sintered to 95 % of the theoretical density and the reactivity of LS with several state-ofthe-art cathode materials is investigated. Diffusion of cations is not detected at cathode-electrolyte interface of PNO/LS. Pr2NiO4 (PNO), and LSPNO are shown to be promising cathodes for LS electrolytes.


P. Jena, P. K. Patro,* A. Sinha, R. K. Lenka, A. K. Singh,* T. Mahata, P. K. Sinha && – && Hydrothermal Synthesis and Characterization of an Apatite-Type Lanthanum Silicate Ceramic for Solid Oxide Fuel Cell Electrolyte Applications


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