- Email: [email protected]
ceramic substrate structures for solid oxide fuel cells
Thin Solid Films 519 (2010) 650–657
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Synthesis and characterization of electrolyte-grade 10%Gd-doped ceria thin film/ceramic substrate structures for solid oxide fuel cells M.G. Chourashiya a, S.R. Bharadwaj b, L.D. Jadhav c,⁎ a b c
Department of Physics, Shivaji University, Kolhapur 416 004, India Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India Department of Physics, Rajaram College, Kolhapur 416 004, India
a r t i c l e
i n f o
Article history: Received 4 November 2009 Received in revised form 19 July 2010 Accepted 17 August 2010 Available online 27 August 2010 Keywords: Ceramic thin films Spray pyrolysis 10%Gd-doped ceria Solid oxide fuel cell Phase-contacts
a b s t r a c t In the present research, spray pyrolysis technique is employed to synthesize 10%Gd-doped ceria (GDC) thin films on ceramic substrates with an intention to use the "film/substrate" structure in solid oxide fuel cells. GDC films deposited on GDC substrate showed enhanced crystallite formation. In case of NiO–GDC composite substrate, the thickness of film was higher (~ 13 μm) as compared to the film thickness on GDC substrate (~ 2 μm). The relative density of the films deposited on both the substrates was of the order of 95%. The impedance measurements revealed that ionic conductivity of GDC/NiO–GDC structure was of the order of 0.10 S/cm at 500 °C, which is a desirable property for its prospective application. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cell (SOFC) is a well-known and highly efficient power generation device (operates at 1000 °C). SOFC converts gaseous fuel electrochemically into electricity in a highly efficient (N60%) and eco-friendly (virtually zero pollution) manner. However, the commercialization of SOFCs has not been fully demonstrated due to its initial cost of fabrication, degradation in reliability and durability during its prolonged operation. All these problems have originated from the required operating temperature (TOP) of these SOFCs, which is higher than 900 °C. Therefore now-a-days researchers are focusing on the development of intermediate or low temperature (IT or LT) SOFCs (operated below 700 °C) instead of high temperature (HT-) SOFCs. IT/ LT–SOFCs have the inherent characteristics to reduce the fabricationcost. As the operating temperature is reduced, fabrication of IT/LT SOFCs would allow the use of standard quality sealant, interconnects, etc. instead of high quality (high cost) state-of-the-art materials. As there would be less or no degradation in cell components due to thermal strains at reduced operating temperatures, it is expected to offer better reliability for extended operational life. Therefore, these advantages of IT and LT–SOFC (TOP b 700 °C) over HT–SOFCs (TOP N 900 °C) are expected to speed up the commercialization of SOFC technology.
⁎ Corresponding author. E-mail addresses: [email protected] (M.G. Chourashiya), [email protected] (L.D. Jadhav). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.110
Several approaches are mentioned in literatures [1–3] to overcome the bottle-neck of high operating temperature of SOFCs. (i) Instead of conventional solid electrolyte e.g. yttria stabilized zirconia (YSZ), use of alternate solid electrolyte materials, which have high ionic conductivity at relatively low temperatures, should be preferred. (ii) The thickness of electrolyte should be decreased to lower the ohmic losses during the ionic conduction in solid electrolyte. However, to avoid the fuel crossover across the electrolyte, the thickness of electrolyte must be optimized. Thin film electrolyte with relative density of the order of 95% and having the thickness of 10–20 μm would suffice the purpose. (iii) It is observed from earlier research [3] that the reduction of TOP increases the electrode potentials across the electrode/electrolyte interface and this leads to degradation in cell performance. The kinetics of electrode processes can be improved by identifying newer electrode materials that can perform well at lower operating temperatures. During the search of an alternate material for conventional solid electrolyte, 10%Gadolinia-doped ceria (GDC) was an exciting discovery due to its high ionic conductivity with comparatively lower activation energies at relatively low operating temperatures. These properties of GDC make it an ideal candidate for IT–SOFCs, operating at 550–650 °C [2,4–8]. In order to increase the ionic conductivity at further reduced temperatures, reduction in the thickness of the GDC electrolyte (10– 20 μm) is reported to be advantageous [9]. Lower operating temperature would also be helpful to avoid the well-known transition of GDC from ionic to electronic conductor (by transition of valency of cerium from Ce4+ to Ce3+), in a typical fuel cell environment [10]. In order to employ the thin film electrolyte in SOFCs, it has to be supported by some kind of substrate
M.G. Chourashiya et al. / Thin Solid Films 519 (2010) 650–657
and if the substrate itself acts as an electrode for SOFC, it would be an added advantage. In the present investigation, an attempt has been made to employ these approaches simultaneously using spray pyrolysis technique (SPT) to prepare GDC thin films on electrode-grade ceramic substrate. Apart from the application of GDC thin films as solid electrolyte for IT– SOFCs, it can also be used as a protective layer in conventional HT– SOFCs. The commercialization of conventional HT–SOFCs is mainly obstructed by the non-reliability issues of electrode interfaces with the conventional solid electrolytes i.e. YSZ. When YSZ solid electrolyte is operated with lanthanum–strontium–magnate (LSM), as cathode, in typical fuel cell conditions, it forms undesirable phases, such as SrZrO3, LaZrO7, etc., at the interface. These insulating and undesirable phases lead to degradation in the performance of the cell. Thus, to avoid this, an ultrafine protective coating layer (~ 100–500 nm) between electrolyte and cathode materials is essential. The protective layer separates two cell components and thereby avoids the undesirable phase formation by keeping them apart. However, it must have the comparable electrical properties as one of either components and should not form any undesirable phase with either of cell component. Dense (or gastight) microstructure is not a mandatory criterion for protective layer, as the electrolyte-membrane would provide the gas-tightness for actual operation of the fuel cell. When GDC is used as a protective layer, it remains stable (particularly on cathode side) and avoids the formation of undesirable phases either with the electrolyte (YSZ) or with the cathode (LSM) in HT– SOFCs. Hence, to deal with this aspect, SPT was also employed to deposit the GDC thin film on electrolyte-grade ceramic substrates. The preparation of dense ceria and GDC thin films onto sapphire (highly dense and smooth) substrate using SPT with thicknesses ranging from 30 to 400 nm has been well demonstrated by Rupp et al. [11]. SPT is a well-known method to prepare tailor-made metal oxide thin films on either conducting or non-conducting substrates. Doping of host film-material with any element in any proportion can be achieved by simply adding it in some form to the spray solution. The versatility of the SPT and requirement of fabrication of dense, thin solid electrolyte film on functional ceramic substrates makes SPT a versatile technique for development of electrode/electrolyte structures (half-cells) [12–18]. In the present investigation, GDC thin films were deposited on two types of ceramic substrates namely GDC (electrolyte-grade) and NiO–GDC (electrode-grade composite). The GDC ceramic sample as electrolytegrade substrate was selected due to its availability for experiments. These substrates were prepared by conventional ceramic route. In general, the chemical deposition methods are sensitive to the physical properties (particularly surface properties) of the substrates. Therefore, the preparative parameters of SPT were separately optimized for electrolyte-grade (single phase with highly dense and smooth surfaced) and electrode-grade (composite with comparatively porous and rough surfaced) ceramic substrates. In the present work, micro-structural and electrical properties of these “film/substrate” structures and that of bare substrates were investigated.
651
Similarly, for synthesis of 30 mol%NiO–70 mol%GDC (hereinafter referred to as NiO–GDC) ceramic substrates, NiO (Extra pure, AR grade from HIMEDIA Inc.) and calcined GDC (at 1200 °C) powders were mixed, pelletized and sintered to obtain the desired composite phase. The selection of ceramic composition of NiO–GDC (precursor composite for Ni–GDC anode) substrate was done on the basis of their prospective application as anode for SOFC [20,21]. While preparing the NiO–GDC composite samples, the binder removal step was excluded and the final sintering was carried out at 1400 °C/8 h in air. The binder removal step and high temperature sintering are generally employed to obtain dense ceramic bodies. The dimensions of the sintered substrates were 0.12 cm in thickness and 2.5 cm in diameter. For surface modification of NiO– GDC substrates, it was heat treated in reducing atmosphere of 5%H2–95% Ar with gas flow rate of 500 ml/min at 900 °C for 5 h. The reduction treatment leads to porous and rough surfaced Ni–GDC composite (hereinafter referred to as Ni–GDC) ceramic substrates. To prepare GDC thin films, cerium nitrate (Ce(NO3)3 6H2O, 99.9% Pure; ALFA AESAR) and gadolinium nitrate (Gd(NO3)3 6H2O, 99.9% Pure; ALFA AESAR) were dissolved separately in double distilled water to form solution with same concentrations. The equi-molar aqueous solution of cerium nitrate and gadolinium nitrate was mixed in desired proportion (i.e. 9:1) to form the spray solution. This solution was then sprayed by air-blast type of nozzle on ceramic substrates. The substrates were kept on a hot plate with controlled preset temperature. The optimized deposition (hot plate) temperature for GDC and Ni–GDC substrates were 280 and 250 °C, respectively. The detailed optimization procedure for the deposition of ceria and Gd-doped ceria on glass substrate is reported elsewhere [22]. Our initial attempts to deposit GDC thin films on NiO–GDC ceramic substrate resulted in non-adherent films, while, the GDC films deposited on surface-modified NiO–GDC, i.e. Ni–GDC, ceramic substrate were adherent and thick. The rough surfaced Ni–GDC substrates might have assisted relatively more nucleation centers than that of comparatively dense NiO–GDC ceramic substrates. Moreover, the adherent GDC film on Ni–GDC was only achieved by “precipitative deposition” (discussed later). The optimized preparative parameters of SPT for deposition of GDC films on glass (from ref. [22]), GDC and Ni–GDC are listed in Table 1. The post heat treatment at 450 °C/2 h was employed for both the "film/substrate" structures. Further, crystallization of GDC films and improvement in "film/ substrate" interface were achieved by subsequent heat treatment of the GDC/GDC and GDC/Ni–GDC structures at 1000 °C/8 h in air. This heat treatment transformed the Ni–GDC phase of composite ceramic substrate to NiO–GDC (herein after referred to as NiO–GDC*) phase. 2.2. Characterizations The phases formed after deposition of GDC over GDC and Ni–GDC, i.e. GDC/GDC and GDC/NiO–GDC*, were studied using PHILIPS X-ray diffractometer (PW-3710) with Cu–Kα radiation source (in Bragg– Brentano parafocusing geometry). The surface and fractured morphologies of the samples were analyzed using Scanning Electron Micrographs (SEM) which was imaged by scanning electron
2. Experimental 2.1. Synthesis of GDC/GDC and GDC/NiO–GDC structures For the synthesis of GDC ceramic substrate, commercially available Gd2O3 (AR grade, 99.9%) and CeO2 (AR grade, 99.9%) powders from HIMEDIA Inc., was mixed in 1:9 proportion with the help of agate mortar-and-pestle. The mixed powder was then calcined at 1200 °C/4 h in air. The calcined powder was reground and mixed with organic binder (poly-vinyl alcohol). The binder added powder was pelletized and subsequently heat treated at 800 °C/2 h for binder removal. After the binder removal step, the samples were finally sintered at 1500 °C/8 h in air [19].
Table 1 The optimized preparative parameters of spray pyrolysis technique (SPT) for deposition of 10%Gd doped ceria (GDC) films on different substrates. Substrates → Preparative parameters ↓
Glass [21]
GDC
Ni–GDC
Solution concentration (M) Solution flow rate (ml/min) Deposition time (h) Substrate temperature (°C) Post-annealing to avoid peel-off (°C) Sintering to improve the interface with the substrate (°C)
0.4 3 ± 0.2 0.5 300 450 –
0.4 3 ± 0.2 0.5 280 450 1000
0.4 3 ± 0.2 0.5 250 450 1000
652
M.G. Chourashiya et al. / Thin Solid Films 519 (2010) 650–657
1260 Impedance Analyzer; 1 MHz–32 MHz). Impedance measurements were obtained as function of frequency (1 Hz–10 MHz) for various temperatures ranging from 250 °C to 500 °C. Prior to measurements, the samples were coated with platinum paste (MaTeck GmbH, Germany) and heat treated at 200 °C for 2 h. The platinum coated sample was sandwiched between two platinum electrodes. The collected impedance data were analyzed by impedance analysis software (ZView Version 2.4a) and used to calculate ac conductivities. The calculated ac conductivities were then fitted to the Arrhenius relation for thermally activated conduction [23]. 3. Results and discussion 3.1. Structural and morphological characterizations of ceramic substrates Fig. 1. XRD pattern of ceramic substrates. (a) 10%Gd doped ceria (GDC), (b) NiO–GDC, (c) Ni–GDC and (d) NiO–GDC*.
microscope (make: JEOL-JSM-6360, Japan; operating voltage = 20 kV) in Secondary Electron (SE) mode. In order to distinguish the different phases in the composite samples, the Back Scattered Electron (BSE) mode was employed. The morphology of the grain growth was examined using “Atomic Force Microscopy (AFM), Nanoscope E” of “Digital Instruments, USA” in contact mode, with “V” shape silicon nitride cantilever of length 100 micrometer and spring constant of 0.58 N/m. The surface roughness was determined from the AFM images. The electrical characterization was carried out by ac impedance measurements using impedance analyzer (SOLARTRON
The ceramic substrates i.e. GDC, NiO–GDC and Ni–GDC were characterized by X-ray Diffractogram (XRD). The XRD patterns were compared with JCPDS PDF nos. 75-0161 (10%Gd-doped ceria), 04-0850 (Cubic Ni) and 78-0643 (Cubic NiO) to confirm the individual phase peaks and were accordingly indexed in Fig. 1. Absence of peaks corresponding to Gd2O3 and comparatively larger lattice parameter of formed phase (~5.419 Å) than that of host lattice of ceria (5.410 Å) confirms the dissolution of Gd2O3 in ceria lattice, to form GDC [24]. In the XRD pattern of NiO–GDC and NiO–GDC* substrates, the presence of peaks corresponding to NiO phase in addition to GDC phase (Fig. 1b and d) confirms the formation of composite substrate. Moreover, XRD pattern of Ni–GDC revealed no peaks corresponding to NiO indicating complete reduction of NiO to Ni (Fig. 1c). The average lattice parameters calculated
Fig. 2. SEM image of (a) GDC, (b) NiO–GDC, (c) Ni–GDC and (d) NiO–GDC* ceramic substrate. The SEM images shown in (b), (c) and (d) are taken in BSE mode to distinguish the different phases in the samples.
M.G. Chourashiya et al. / Thin Solid Films 519 (2010) 650–657
653
for GDC, NiO and Ni phases are 5.419 Å, 4.169 Å and 3.540 Å, respectively, which are in good agreement with the standard (JCPDS) values. Fig. 2 shows the SEM images of the ceramic substrates collected by operating the scanning electron microscope in SE/BSE mode. SEM (in SE mode) of GDC ceramic substrate (Fig. 2a) showed the homogeneous and dense surface comprising of well-grown grains, which is attributed to the high sintering temperature employed during its fabrication. The estimated relative density of GDC ceramic substrate was 98%. The surface morphology (SEM collected in BSE mode) of the NiO–GDC (sintered at 1400 °C/8 h) is comparatively porous (Fig. 2b). In Fig. 2b, the clear existence of two different phases of NiO (black grains) and GDC (white grains) can be seen, supporting the composite nature of samples obtained from XRD (Fig. 1). These porous NiO–GDC ceramic samples, upon reduction in H2 atmosphere at 900 °C, turns into more porous Ni–GDC substrate (Fig. 2c). The increased porosity is attributed to the fact that the heating the NiO–GDC in reducing environment eliminates the oxygen from the NiO phase by forming H2O. The probable reaction during this process could be 5%H2 −95% Ar at 900∘ C
NiO → Ni + H2 O↑
ð1Þ
GDC thin films were deposited on this porous Ni–GDC substrate followed by annealing at 1000 °C for 8 h in air. This leads to reoxidation of Ni into NiO phase, confirmed by XRD analysis (Fig. 1d), in Air at 1000∘C
2Ni + O2 ðfrom airÞ → 2NiO
ð2Þ
Fig. 4. The typical XRD patterns of (a) NiO–GDC* ceramic substrate and (b) GDC film on NiO–GDC* ceramic substrate.
3.2. Structural and morphological characterizations of GDC/GDC and GDC/NiO–GDC* structures It is a common practice to use the grazing incidence x-ray for characterization of thin films having thicknesses of the order of few hundred nanometers. But, as the thicknesses of films for GDC/GDC and
The SEM image (in BSE mode) of NiO–GDC* is shown in Fig. 2d. The grains of NiO phase are comparatively larger in NiO–GDC* than that of NiO–GDC. The grains of NiO in NiO–GDC (Fig. 2b) sample undergo decrease in size after reduction treatment and can be seen as clusters of comparatively smaller Ni grains embedded in GDC grains. Further, when the Ni–GDC composite is heat treated in air at 1000 °C, the transformation of Ni to NiO leads to large grains of NiO (Fig. 2d). Moreover, in NiO–GDC* the agglomeration of grains to a comparatively large extent takes place due to relatively mobile Ni species and leads to formation of comparatively large clusters and pores in the sample. Apart from these morphological dissimilarities, the relative densities (determined using standard Archimedes method) of composite samples varied with change of compositions. The Ni–GDC sample possesses relative density of 71%, while that of NiO–GDC and NiO–GDC* possesses approximately same relative density of 79%.
Fig. 3. The typical XRD patterns of (a) GDC ceramic substrate and (b) GDC film on GDC ceramic substrate. I111 showing the intensity counts of (111) peak in XRD pattern.
Fig. 5. SEM of GDC films deposited on (a) GDC and (b) Ni–GDC substrates and annealed at 450 °C for 3 h in air. Insets: Fractured morphology showing interface.
654
M.G. Chourashiya et al. / Thin Solid Films 519 (2010) 650–657
GDC/NiO–GDC* structures are greater than 2 μm; the grazing incidence x-rays was not employed for characterizations of the thin films. During the characterization of these samples by XRD, the instrumental parameters were kept fixed so that the comparison of XRDs of samples could be done under identical conditions. In Fig. 3, the XRD pattern of GDC thin film deposited on GDC substrate (annealed at 1000 °C) is compared with that of bare GDC substrate. The intensities of all the peaks of bare GDC substrate have been doubled for GDC films deposited on GDC substrate, which could be due to enhanced density of Bragg planes. These observations reveal that in case of GDC/GDC structure, GDC substrate promotes the crystal growth of GDC film (“substrate effect”), which is obvious in chemical deposition techniques. Since the lattice parameter of depositing material and the substrate is same, the free energy exchange between them facilitates more nucleation centers along substrate crystallites and hence the rate of crystallite formation is enhanced during film deposition (thickness ~ 2 μm). Now, it would be an obvious thought that as NiO–GDC/Ni–GDC composite substrate contains 70%GDC, similar effect of enhanced crystallite formation should be observed. But in NiO–GDC/Ni–GDC composite substrate, 30% of NiO/Ni is homogeneously distributed and therefore the probability of formation of identical nucleation sites in the vicinity, as in case of GDC substrate, might be rarely achieved. Therefore, the added rate of crystallite formation has not been observed for the case of GDC/NiO–GDC* structure (Fig. 4). Due to use of “precipitative deposition” for Ni–GDC substrate, higher grain growth than that of conventional grain growth of spray
pyrolysis is observed which has resulted in higher thickness (~13 μm) of the film. In precipitative deposition, the films are deposited at deposition temperatures (here 250 °C) less than that of decomposition temperature of precursor solution (280 °C). The film at “asdeposited” state is a layer of precipitate with entrapped solvent. Thus, to form the film with desired phase and to evaporate the solvent, the “as-deposited” films were subsequently heat treated at 450 °C for 3 h, as thermo-gravimetric analysis of precursor showed no weight loss above 450 °C [22]. Sufficient precaution had to be taken while evaporating the solvent, as there is large probability of warping of films in this step. However, while depositing the GDC on GDC substrate the optimum pyrolysis of sprayed droplets was observed (as deposition and decomposition temperature is same) and therefore the films formed at “as-deposited” state had the desired phase. In this case, as the droplet reaches to the substrate, it becomes completely dry. Fraction of these dry particles (not having optimum momentum to reach the substrate) may flyaway due to thermophoretic forces at the surface of substrates. The particles reach the substrate and then try to find the site or form the sites for its stabilizations (formation of nucleation centers). As the GDC substrate has the large grains and comparatively uniform surface than Ni–GDC, the formation of nucleation sites is comparably inefficient and therefore the delayed grain growth take place which finally results in comparatively less thickness. However, one order higher thickness of GDC film in case of Ni–GDC composite substrate than that of GDC substrate is mainly attributed to the precipitative deposition. In precipitative deposition, comparatively higher fraction of sprayed droplets (heavier and wet
Fig. 6. SEM of GDC films deposited on (a) GDC and (b) NiO–GDC* substrate and sintered at 1000 °C in air. Insets: Fractured morphology showing interface. AFM images of GDC film deposited on (c) GDC and (d) NiO–GDC* substrate and sintered at 1000 °C in air.
M.G. Chourashiya et al. / Thin Solid Films 519 (2010) 650–657
particles) reaches and get attached to the substrate, which results in large thickness (~ 13 μm) of GDC film on NiO–GDC* substrate after annealing. The GDC film, thus formed, completely screens the XRD peaks originating from NiO–GDC* substrate (Fig. 4b). The films deposited on ceramic substrates were annealed to improve crystallization of films and also the quality of film/substrate interface. For an annealing temperature of 450 °C, the films showed “less adhered” interface with the substrate and the surface grains were discontinuous (Fig. 5a–b). Such interface and discontinuous grains could lead to additional interfacial resistance to the total structure. High temperature sintering of GDC thin films could be employed to improve film/substrate interface. Hence, to avoid interfacial resistance in the total system and improve the film/ substrate interface, the post heat treatment at 1000 °C in air was carried out. This post heat treatment leads to uniform and adherent film/substrate interface in addition to improved surface morphology of the film (depicted in Fig. 6a–b). Besides this, the relative density of films were determined by comparing the fractured SEM of films and that of GDC ceramic substrate sintered at 1500 °C (98%), which is of the order of 97 (±2)%. The high density of the films achieved at 1000 °C was attributed to the presence of nano-granules (avg. grain size 83–85 nm), as can be seen in the respective AFM images of films of GDC/GDC and GDC/NiO–GDC* structures in Fig. 6c–d, respectively. After post heat treatment, GDC film on GDC showed discontinuous and fractured grain morphology, while GDC film on NiO–GDC* showed the better grain growth and grain connectivity. If the cross section of GDC/GDC is carefully observed it can be seen that the average roughness of the GDC substrates is roughly same as that of thickness of GDC film (~2 μm). Therefore, the presence of cracks and discontinuities is an obvious consequence. Hence, to achieve the crack-free, ultra-thin and dense films using SPT it is mandatory to polish the substrate surface to a level of roughness, which should be a fraction of desired thickness of films. However, it should be noted here that the GDC films on GDC substrate are deposited with the intention of forming the protective layer on electrolyte grade substrate. Nonetheless, AFM showed reasonably smooth surfaced grains of GDC films deposited on both the substrates. For GDC/GDC structure, there were granules with size as large as 250 nm compared to their smaller counterparts in the range of 30–40 nm for GDC/NiO–GDC* structure. The size distribution of granules in the GDC films deposited on GDC substrate is comparatively wider than that of on NiO–GDC* substrate (Table 2).
655
Fig. 7. Fractured SEM of platinum coated film/substrate interface; Inset: SEM of uniform coating platinum onto the surface of GDC film.
370 °C respectively are shown in Fig. 8. It showed usual trend of three semicircles in complex impedance plots. In principle, one would have different electrical responses due to grains of two kinds (NiO and GDC) and grain contacts of three kinds (GDC–GDC, NiO–NiO and GDC–NiO). In addition, the conductivity across GDC (ionic) and NiO grains (electronic-holes) would add different responses to impedance spectra. The situation would be much more difficult for the film/substrate structures (where the contacts may behave differently than into the bulk ceramic). Consequently, to assign the contributions originating from all these sources or to extract all this information from the obtained impedance spectra would require a well-dedicated study (out of scope of present research). Thus, here, we simply assign three semicircles (from right) observed in impedance plots to correspond to slow, intermediate, and fast electrical processes. Further, the analysis revealed that the parts of spectra assigned to intermediate and fast electrical processes possesses the capacitance values of the order of 10− 8 and 10− 11 F/ cm2, respectively. These typical values of capacitances, in general, originates from grain boundary (GB) polarization in a polycrystalline
3.3. Electrical characterizations of GDC/GDC and GDC/NiO–GDC* structures The electrical characterization of structures and bare substrates were carried out using impedance measurements (1 Hz–10 MHz) in the temperature range of 250–500 °C. The typical fractured SEM image of platinum coated structure, imaged after the impedance measurements is shown in Fig. 7. Inset of Fig. 7 shows the uniform coating of platinum paste over the surface of GDC films. It can be seen in Fig. 7 that the platinum paste has remained on the surface of the film and has not penetrated/diffused into the film to short with substrate surface. The typical complex impedance (Nyquist) plots for GDC/GDC and GDC/NiO–GDC* structure measured at 340 °C and
Table 2 Surface roughness and average grain size estimated from AFM images of 10%Gd doped ceria (GDC) thin films on different substrates. GDC films on
Surface roughness (nm)
Avg. grain size (nm)
Min. grain size (nm)
Max. grain size (nm)
Grain size Distribution
GDC NiO–GDC*
18.64 8.86
83 85
30–40 60–70
250 150
Wider Moderate
Fig. 8. Typical complex impedance (Nyquist) plot of GDC film on (a) GDC substrate and (b) NiO–GDC* substrate.
656
M.G. Chourashiya et al. / Thin Solid Films 519 (2010) 650–657
Fig. 10. Schematic of micro-structural phase contacts for GDC/GDC and GDC/NiO–GDC* structures.
Fig. 9. Variation of ln(σacT) as function of 1000/T for (a) GDC/GDC and (b) GDC/NiO– GDC* structures.
material (~10− 8 F/cm2) and that of from dielectric relaxation of bulk (grain interior (GI)) material (~10− 11 F/cm2). Hence, this analogy is further extended and these processes are referred to as GI (fast) and GB (intermediate) processes, and the respective conductivities as GI and GB conductivities. Complex impedance plots for structures and substrates showed the gradual decrease in impedances originating from all the three contributions at higher temperatures. The semi-circle originating due to GI impedance contribution disappeared at 440–460 °C for all four
Table 3 Values of grain interior (GI) and grain boundary (GB) conductivities at 500 °C in air along with respective activation energies. Samples ↓
Pair
σGI (S/cm)
σGB (S/cm)
EaGI (eV)
EaGB (eV)
GDC substrate GDC/GDC NiO–GDC* substrate GDC/NiO–GDC*
G–G
0.035 0.027 0.107 0.103
0.0017 0.0028 0.0009 0.0005
0.77 1.07 0.81 1.02
1.09 1.40 0.90 0.93
N–G
studied samples. The impedance data were analyzed by impedance analysis software (ZView Version 2.4a) and used to extract the GI (Rg) and GB impedances (Rgb). The values of Rg and Rgb obtained from the fit parameters were used to calculate ac conductivities and were further fitted to the Arrhenius relation (Fig. 9). The values of GI and GB conductivity and respective activation energies of samples are listed in Table 3. For both the structures and substrates, the magnitudes of GI conductivity remained higher than that of GB conductivity. Comparatively decreased electrical conduction observed for GDC/GDC and GDC/NiO–GDC* structure than that of respective bare substrates could be assigned to the existence various phase contacts in the samples. Various phase contacts are feasible among micron sized grains of GDC, NiO and voids from the substrates and nano sized GDC grains in the film. It is reasonable to assume that the homo phase-contacts offer less opposition than that of hetero phase-contacts for a particular process. The possible micro-structural phase contacts for GDC/GDC and GDC/NiO–GDC* structures are schematically illustrated in Fig. 10 and the resulting number of total contacts are listed in Table 4. The possible phase-contacts in GDC/GDC structure could be only of “nano/micron grains of GDC (NG–MG)” type at film/substrate interface, as the presence of voids (rel. density N 98%) and other Table 4 Various phase contacts for different systems. Samples ↓
Probable phase contacts
Number of phase contacts
GDC substrate GDC/GDC structure NiO–GDC* substrate
MG–MG MG–MG; NG–NG; MG–NG; MG–MG; MN–MN; MG–MN; MV–MG; MN–MV; MG–MG; MG–MN; MG–MV; MG–NG; MN–MN; MN–MV; MN–NG; MV–NG; NG–NG;
1 3 5
GDC/NiO–GDC* structure
MG — micron sized grains of GDC from the substrates. MN — micron sized grains of NiO from the substrates. MV — micron sized voids from the substrates. NG — nano sized grains of GDC from films.
9
M.G. Chourashiya et al. / Thin Solid Films 519 (2010) 650–657
657
Table 5 Calculations of the apparent thickness (t) and actual GB (T) ratios (0.8–1.2% error). The calculation assumes isotropic grain boundaries throughout the specimen.
GDC GDC film on GDC substrate NiO–GDC* GDC film on NiO–GDC* substrate
# pair
Thickness (t) (μm)
Avg. grain size (d) (μm)
GB per unit length (D = 1/d) (μm)
Total GB T=A×t×D
Apparent “t” ratio
Actual GB (T) ratio
G–G
1200 2 1200 13
11 0.083 2.5 0.085
0.0909 12.048 0.4 11.76
=109.0909 a =24.09639 a =480 b =152.9412 b
600
4.527273
92
3.138462
N–G
Where, “a” and “b” are surface area (A) of the samples, which was same for the G–G and G–N pair respectively.
impurities could be ruled out in this case. Thus, inability of GDC/GDC structure to surmount the GI conductivity of bare substrate could be attributed to the only existing phase-contact of NG–MG type, since the grain size distributions of film and substrate are different. According to Ruiz-Trejo et al. [25], nano-ceramics have conduction paths along the grain boundaries. Bellino and co-workers [26] emphasize the fact that in nano-ceramics of doped ceria, and suggested that oxygen migration can take place along the grain boundaries resulting in an enhanced GB ionic conductivity. In agreement with their observation, our data showed enhancement in GB conductivity for GDC/ GDC structure (Table 3). It should be noted that though apparently the thickness ratio of the substrate and the top layer is high; the actual grain boundary length could vary for nano-ceramics. The ratio of GB of substrate to GB of film is calculated (Table 5) and is found to be only 3 to 4. This determines the contribution of GB from nano-ceramics to the GB conductivity of total structure of GDC/GDC. The GI conductivity of NiO–GDC* is found to be higher than that of GDC, which is explained by an additional electron-hole conductivity offered by NiO [27]. Similar results were also observed for GI conductivity of GDC/NiO–GDC structure. However, the slight decrease in GI conduction than that of bare substrate could be attributed to added phase-contact of “NG–MG” type. The GB conductivities of NiO–GDC* and GDC/NiO–GDC* structures are comparably lower than that GDC and GDC/GDC structure, respectively. Therefore, by observing Table 3 and relating it with the chart of phase contacts (Table 4), one can say that as the number of hetero-contacts in a system is increased an increase in overall impedance of the system is observed.
with a layer of suitable cathode material, could find promising application in low temperature-SOFCs. Further research in this direction is being planned in our laboratory and the results will be communicated shortly.
4. Conclusions and further scope of research
[13] [14]
In the present investigation, the potential of formation of solid electrolyte thin film on to ceramic substrates using spray pyrolysis technique (SPT) has been demonstrated. Analysis of GDC/GDC structure revealed that electrolyte-grade substrates could be coated with ultra-thin protective layer using SPT to avoid the interfacial reactions. Besides, the studies on GDC films deposited on NiO–GDC substrate depicted that the electrolyte-grade GDC films could also be deposited using SPT. However, some modification, as mentioned, should be employed in the process. Further, the choice of material for solid electrolyte film i.e. GDC and NiO–GDC as substrate (precursor composite ceramic anode) would enable us to utilize the prepared structure (GDC/NiO–GDC) for fabrication half-cell in low temperature-SOFCs. Electrical characterization of GDC/NiO–GDC structure showed that there is only a nominal decrease in overall ionic conduction of the structure over the bare substrate, indicating good quality film/substrate interface. Such synthesized structures along
[15] [16]
Acknowledgements The authors are very much thankful to DRDO, New Delhi for their financial support. We would like to acknowledge Dr. D. Phase, UGCDAE IUC Indore for extending SEM characterization facilities to us and to Mr. Ahire, Sr. Engineer, UGC-DAE IUC Indore for his co-operation during the SEM characterizations. Dr. M. G. Chourashiya is thankful to CSIR, New Delhi for a senior research fellowship.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[17] [18]
[19] [20] [21] [22] [23] [24] [25]
[26] [27]
S.C. Singhal, Solid State Ionics 135 (2000) 305. B.C.H. Steele, Solid State Ionics 129 (2000) 95. M. Mogensen, N.M. Sammes, G.A. Tompett, Solid State Ionics 129 (2000) 63. S. Wang, T. Kato, S. Nagata, T. Kaneko, N. Iwashita, T. Honda, M. Dokiya, Solid State Ionics 152–152 (2002) 477. B.C.H. Steele, Solid State Ionics 134 (2000) 3. T.S. Zhang, J. Ma, L.B. Kong, P. Hing, J.A. Kilner, Solid State Ionics 167 (2004) 191. Y.J. Leng, S.H. Chan, S.P. Jiang, K.A. Khor, Solid State Ionics 170 (2004) 9. C. Hatchwell, N.M. Sammes, I.W.M. Brown, Solid State Ionics 126 (1999) 201. T. Fukui, K. Murata, S. Ohara, H. Abe, M. Naito, K. Nogi, J. Power Sources 125 (2004) 17. J.B. Goodenough, Annu. Rev. Mater. Res. 33 (2003) 91. J.L.M. Rupp, T. Drobek, A. Rossi, L.J. Gauckler, Chem. Mater. 19 (2007) 1134. T. Setoguchi, T. Inoue, H. Takebe, K. Eguchi, K. Morinaga, H. Arai, Solid State Ionics 37 (1990) 217. E.M. Kelder, O.C.J. Nijs, J. Schoonman, Solid State Ionics 68 (1994) 5. A.A. Van Zomeren, E.M. Kelder, J.C.M. Marijnissen, J. Schoonman, J. Aerosol Sci. 25 (1994) 1229. C.H. Chen, A.A.J. Buysman, E.M. Kelder, J. Schoonman, Solid State Ionics 80 (1995) 1. K.L. Choy, in: W.E. Lee (Ed.), British Ceramic Proc., Ceramic Films, Coatings, The Institute of Materials, 54, 1995, p. 65. K.L. Choy, W. Bai, B.C.H. Steele, in: A.J. Mcevoy, K. Nisancioglu (Eds.), Materials, Processes, 10th SOFC Workshop, Les Diablerets, Int. Energy Agency, 1997, p. 233. N.H.J. Stelzer, C.H. Chen, Rij Van, L.N.J. Schoonman, in: A.J. Mcevoy, K. Nisancioglu (Eds.), Materials, Processes, 10th SOFC Workshop, Les Diablerets, Int. Energy Agency, 1997, p. 236. M.G. Chourashiya, J.Y. Patil, S.H. Pawar, L.D. Jadhav, Mater. Chem. Phy. 109 (2008) 39. C.R. Xia, F.L. Chen, M.L. Liu, Electrochem. Solid-State Lett. 4 (2001) A52. C. Xia, M. Liu, Adv. Mater. 14 (2002) 521. M.G. Chourashiya, S.H. Pawar, L.D. Jadhav, App. Surf. Sci. 254 (2008) 3431. J.A. Kilner, B.C.H. Steele, in: O.T. Sorensen (Ed.), Non-Stoichiometric Oxides, Academic Press, New York, 1981. R.V. Wandekar, M. Ail (Basu), B.N. Wani, S.R. Bharadwaj, Mater. Chem. Phys. 99 (2006) 289. E. Ruiz-Trejo, J. Santoyo-Salazar, R. Vilchis-Morales, A. Benitz-Rico, F. GomexGarcia, C. Flores-Morales, J. Chavez-Carvayar, G. Tavizon, J. Solid State Chem. 180 (2007) 3093. M.G. Beliino, D.G. Lamas, N.E. Walsoe de Reca, Adv. Funct. Mater. 16 (2006) 107. Y.M. Park, G.M. Choi, Solid State Ionics 120 (1999) 265.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Synthesis and characterization of electrolyte-grade 10%Gd-doped ceria thin film/ceramic substrate structures for solid oxide fuel cells M.G. Chourashiya a, S.R. Bharadwaj b, L.D. Jadhav c,⁎ a b c
Department of Physics, Shivaji University, Kolhapur 416 004, India Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India Department of Physics, Rajaram College, Kolhapur 416 004, India
a r t i c l e
i n f o
Article history: Received 4 November 2009 Received in revised form 19 July 2010 Accepted 17 August 2010 Available online 27 August 2010 Keywords: Ceramic thin films Spray pyrolysis 10%Gd-doped ceria Solid oxide fuel cell Phase-contacts
a b s t r a c t In the present research, spray pyrolysis technique is employed to synthesize 10%Gd-doped ceria (GDC) thin films on ceramic substrates with an intention to use the "film/substrate" structure in solid oxide fuel cells. GDC films deposited on GDC substrate showed enhanced crystallite formation. In case of NiO–GDC composite substrate, the thickness of film was higher (~ 13 μm) as compared to the film thickness on GDC substrate (~ 2 μm). The relative density of the films deposited on both the substrates was of the order of 95%. The impedance measurements revealed that ionic conductivity of GDC/NiO–GDC structure was of the order of 0.10 S/cm at 500 °C, which is a desirable property for its prospective application. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cell (SOFC) is a well-known and highly efficient power generation device (operates at 1000 °C). SOFC converts gaseous fuel electrochemically into electricity in a highly efficient (N60%) and eco-friendly (virtually zero pollution) manner. However, the commercialization of SOFCs has not been fully demonstrated due to its initial cost of fabrication, degradation in reliability and durability during its prolonged operation. All these problems have originated from the required operating temperature (TOP) of these SOFCs, which is higher than 900 °C. Therefore now-a-days researchers are focusing on the development of intermediate or low temperature (IT or LT) SOFCs (operated below 700 °C) instead of high temperature (HT-) SOFCs. IT/ LT–SOFCs have the inherent characteristics to reduce the fabricationcost. As the operating temperature is reduced, fabrication of IT/LT SOFCs would allow the use of standard quality sealant, interconnects, etc. instead of high quality (high cost) state-of-the-art materials. As there would be less or no degradation in cell components due to thermal strains at reduced operating temperatures, it is expected to offer better reliability for extended operational life. Therefore, these advantages of IT and LT–SOFC (TOP b 700 °C) over HT–SOFCs (TOP N 900 °C) are expected to speed up the commercialization of SOFC technology.
⁎ Corresponding author. E-mail addresses: [email protected] (M.G. Chourashiya), [email protected] (L.D. Jadhav). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.110
Several approaches are mentioned in literatures [1–3] to overcome the bottle-neck of high operating temperature of SOFCs. (i) Instead of conventional solid electrolyte e.g. yttria stabilized zirconia (YSZ), use of alternate solid electrolyte materials, which have high ionic conductivity at relatively low temperatures, should be preferred. (ii) The thickness of electrolyte should be decreased to lower the ohmic losses during the ionic conduction in solid electrolyte. However, to avoid the fuel crossover across the electrolyte, the thickness of electrolyte must be optimized. Thin film electrolyte with relative density of the order of 95% and having the thickness of 10–20 μm would suffice the purpose. (iii) It is observed from earlier research [3] that the reduction of TOP increases the electrode potentials across the electrode/electrolyte interface and this leads to degradation in cell performance. The kinetics of electrode processes can be improved by identifying newer electrode materials that can perform well at lower operating temperatures. During the search of an alternate material for conventional solid electrolyte, 10%Gadolinia-doped ceria (GDC) was an exciting discovery due to its high ionic conductivity with comparatively lower activation energies at relatively low operating temperatures. These properties of GDC make it an ideal candidate for IT–SOFCs, operating at 550–650 °C [2,4–8]. In order to increase the ionic conductivity at further reduced temperatures, reduction in the thickness of the GDC electrolyte (10– 20 μm) is reported to be advantageous [9]. Lower operating temperature would also be helpful to avoid the well-known transition of GDC from ionic to electronic conductor (by transition of valency of cerium from Ce4+ to Ce3+), in a typical fuel cell environment [10]. In order to employ the thin film electrolyte in SOFCs, it has to be supported by some kind of substrate
M.G. Chourashiya et al. / Thin Solid Films 519 (2010) 650–657
and if the substrate itself acts as an electrode for SOFC, it would be an added advantage. In the present investigation, an attempt has been made to employ these approaches simultaneously using spray pyrolysis technique (SPT) to prepare GDC thin films on electrode-grade ceramic substrate. Apart from the application of GDC thin films as solid electrolyte for IT– SOFCs, it can also be used as a protective layer in conventional HT– SOFCs. The commercialization of conventional HT–SOFCs is mainly obstructed by the non-reliability issues of electrode interfaces with the conventional solid electrolytes i.e. YSZ. When YSZ solid electrolyte is operated with lanthanum–strontium–magnate (LSM), as cathode, in typical fuel cell conditions, it forms undesirable phases, such as SrZrO3, LaZrO7, etc., at the interface. These insulating and undesirable phases lead to degradation in the performance of the cell. Thus, to avoid this, an ultrafine protective coating layer (~ 100–500 nm) between electrolyte and cathode materials is essential. The protective layer separates two cell components and thereby avoids the undesirable phase formation by keeping them apart. However, it must have the comparable electrical properties as one of either components and should not form any undesirable phase with either of cell component. Dense (or gastight) microstructure is not a mandatory criterion for protective layer, as the electrolyte-membrane would provide the gas-tightness for actual operation of the fuel cell. When GDC is used as a protective layer, it remains stable (particularly on cathode side) and avoids the formation of undesirable phases either with the electrolyte (YSZ) or with the cathode (LSM) in HT– SOFCs. Hence, to deal with this aspect, SPT was also employed to deposit the GDC thin film on electrolyte-grade ceramic substrates. The preparation of dense ceria and GDC thin films onto sapphire (highly dense and smooth) substrate using SPT with thicknesses ranging from 30 to 400 nm has been well demonstrated by Rupp et al. [11]. SPT is a well-known method to prepare tailor-made metal oxide thin films on either conducting or non-conducting substrates. Doping of host film-material with any element in any proportion can be achieved by simply adding it in some form to the spray solution. The versatility of the SPT and requirement of fabrication of dense, thin solid electrolyte film on functional ceramic substrates makes SPT a versatile technique for development of electrode/electrolyte structures (half-cells) [12–18]. In the present investigation, GDC thin films were deposited on two types of ceramic substrates namely GDC (electrolyte-grade) and NiO–GDC (electrode-grade composite). The GDC ceramic sample as electrolytegrade substrate was selected due to its availability for experiments. These substrates were prepared by conventional ceramic route. In general, the chemical deposition methods are sensitive to the physical properties (particularly surface properties) of the substrates. Therefore, the preparative parameters of SPT were separately optimized for electrolyte-grade (single phase with highly dense and smooth surfaced) and electrode-grade (composite with comparatively porous and rough surfaced) ceramic substrates. In the present work, micro-structural and electrical properties of these “film/substrate” structures and that of bare substrates were investigated.
651
Similarly, for synthesis of 30 mol%NiO–70 mol%GDC (hereinafter referred to as NiO–GDC) ceramic substrates, NiO (Extra pure, AR grade from HIMEDIA Inc.) and calcined GDC (at 1200 °C) powders were mixed, pelletized and sintered to obtain the desired composite phase. The selection of ceramic composition of NiO–GDC (precursor composite for Ni–GDC anode) substrate was done on the basis of their prospective application as anode for SOFC [20,21]. While preparing the NiO–GDC composite samples, the binder removal step was excluded and the final sintering was carried out at 1400 °C/8 h in air. The binder removal step and high temperature sintering are generally employed to obtain dense ceramic bodies. The dimensions of the sintered substrates were 0.12 cm in thickness and 2.5 cm in diameter. For surface modification of NiO– GDC substrates, it was heat treated in reducing atmosphere of 5%H2–95% Ar with gas flow rate of 500 ml/min at 900 °C for 5 h. The reduction treatment leads to porous and rough surfaced Ni–GDC composite (hereinafter referred to as Ni–GDC) ceramic substrates. To prepare GDC thin films, cerium nitrate (Ce(NO3)3 6H2O, 99.9% Pure; ALFA AESAR) and gadolinium nitrate (Gd(NO3)3 6H2O, 99.9% Pure; ALFA AESAR) were dissolved separately in double distilled water to form solution with same concentrations. The equi-molar aqueous solution of cerium nitrate and gadolinium nitrate was mixed in desired proportion (i.e. 9:1) to form the spray solution. This solution was then sprayed by air-blast type of nozzle on ceramic substrates. The substrates were kept on a hot plate with controlled preset temperature. The optimized deposition (hot plate) temperature for GDC and Ni–GDC substrates were 280 and 250 °C, respectively. The detailed optimization procedure for the deposition of ceria and Gd-doped ceria on glass substrate is reported elsewhere [22]. Our initial attempts to deposit GDC thin films on NiO–GDC ceramic substrate resulted in non-adherent films, while, the GDC films deposited on surface-modified NiO–GDC, i.e. Ni–GDC, ceramic substrate were adherent and thick. The rough surfaced Ni–GDC substrates might have assisted relatively more nucleation centers than that of comparatively dense NiO–GDC ceramic substrates. Moreover, the adherent GDC film on Ni–GDC was only achieved by “precipitative deposition” (discussed later). The optimized preparative parameters of SPT for deposition of GDC films on glass (from ref. [22]), GDC and Ni–GDC are listed in Table 1. The post heat treatment at 450 °C/2 h was employed for both the "film/substrate" structures. Further, crystallization of GDC films and improvement in "film/ substrate" interface were achieved by subsequent heat treatment of the GDC/GDC and GDC/Ni–GDC structures at 1000 °C/8 h in air. This heat treatment transformed the Ni–GDC phase of composite ceramic substrate to NiO–GDC (herein after referred to as NiO–GDC*) phase. 2.2. Characterizations The phases formed after deposition of GDC over GDC and Ni–GDC, i.e. GDC/GDC and GDC/NiO–GDC*, were studied using PHILIPS X-ray diffractometer (PW-3710) with Cu–Kα radiation source (in Bragg– Brentano parafocusing geometry). The surface and fractured morphologies of the samples were analyzed using Scanning Electron Micrographs (SEM) which was imaged by scanning electron
2. Experimental 2.1. Synthesis of GDC/GDC and GDC/NiO–GDC structures For the synthesis of GDC ceramic substrate, commercially available Gd2O3 (AR grade, 99.9%) and CeO2 (AR grade, 99.9%) powders from HIMEDIA Inc., was mixed in 1:9 proportion with the help of agate mortar-and-pestle. The mixed powder was then calcined at 1200 °C/4 h in air. The calcined powder was reground and mixed with organic binder (poly-vinyl alcohol). The binder added powder was pelletized and subsequently heat treated at 800 °C/2 h for binder removal. After the binder removal step, the samples were finally sintered at 1500 °C/8 h in air [19].
Table 1 The optimized preparative parameters of spray pyrolysis technique (SPT) for deposition of 10%Gd doped ceria (GDC) films on different substrates. Substrates → Preparative parameters ↓
Glass [21]
GDC
Ni–GDC
Solution concentration (M) Solution flow rate (ml/min) Deposition time (h) Substrate temperature (°C) Post-annealing to avoid peel-off (°C) Sintering to improve the interface with the substrate (°C)
0.4 3 ± 0.2 0.5 300 450 –
0.4 3 ± 0.2 0.5 280 450 1000
0.4 3 ± 0.2 0.5 250 450 1000
652
M.G. Chourashiya et al. / Thin Solid Films 519 (2010) 650–657
1260 Impedance Analyzer; 1 MHz–32 MHz). Impedance measurements were obtained as function of frequency (1 Hz–10 MHz) for various temperatures ranging from 250 °C to 500 °C. Prior to measurements, the samples were coated with platinum paste (MaTeck GmbH, Germany) and heat treated at 200 °C for 2 h. The platinum coated sample was sandwiched between two platinum electrodes. The collected impedance data were analyzed by impedance analysis software (ZView Version 2.4a) and used to calculate ac conductivities. The calculated ac conductivities were then fitted to the Arrhenius relation for thermally activated conduction [23]. 3. Results and discussion 3.1. Structural and morphological characterizations of ceramic substrates Fig. 1. XRD pattern of ceramic substrates. (a) 10%Gd doped ceria (GDC), (b) NiO–GDC, (c) Ni–GDC and (d) NiO–GDC*.
microscope (make: JEOL-JSM-6360, Japan; operating voltage = 20 kV) in Secondary Electron (SE) mode. In order to distinguish the different phases in the composite samples, the Back Scattered Electron (BSE) mode was employed. The morphology of the grain growth was examined using “Atomic Force Microscopy (AFM), Nanoscope E” of “Digital Instruments, USA” in contact mode, with “V” shape silicon nitride cantilever of length 100 micrometer and spring constant of 0.58 N/m. The surface roughness was determined from the AFM images. The electrical characterization was carried out by ac impedance measurements using impedance analyzer (SOLARTRON
The ceramic substrates i.e. GDC, NiO–GDC and Ni–GDC were characterized by X-ray Diffractogram (XRD). The XRD patterns were compared with JCPDS PDF nos. 75-0161 (10%Gd-doped ceria), 04-0850 (Cubic Ni) and 78-0643 (Cubic NiO) to confirm the individual phase peaks and were accordingly indexed in Fig. 1. Absence of peaks corresponding to Gd2O3 and comparatively larger lattice parameter of formed phase (~5.419 Å) than that of host lattice of ceria (5.410 Å) confirms the dissolution of Gd2O3 in ceria lattice, to form GDC [24]. In the XRD pattern of NiO–GDC and NiO–GDC* substrates, the presence of peaks corresponding to NiO phase in addition to GDC phase (Fig. 1b and d) confirms the formation of composite substrate. Moreover, XRD pattern of Ni–GDC revealed no peaks corresponding to NiO indicating complete reduction of NiO to Ni (Fig. 1c). The average lattice parameters calculated
Fig. 2. SEM image of (a) GDC, (b) NiO–GDC, (c) Ni–GDC and (d) NiO–GDC* ceramic substrate. The SEM images shown in (b), (c) and (d) are taken in BSE mode to distinguish the different phases in the samples.
M.G. Chourashiya et al. / Thin Solid Films 519 (2010) 650–657
653
for GDC, NiO and Ni phases are 5.419 Å, 4.169 Å and 3.540 Å, respectively, which are in good agreement with the standard (JCPDS) values. Fig. 2 shows the SEM images of the ceramic substrates collected by operating the scanning electron microscope in SE/BSE mode. SEM (in SE mode) of GDC ceramic substrate (Fig. 2a) showed the homogeneous and dense surface comprising of well-grown grains, which is attributed to the high sintering temperature employed during its fabrication. The estimated relative density of GDC ceramic substrate was 98%. The surface morphology (SEM collected in BSE mode) of the NiO–GDC (sintered at 1400 °C/8 h) is comparatively porous (Fig. 2b). In Fig. 2b, the clear existence of two different phases of NiO (black grains) and GDC (white grains) can be seen, supporting the composite nature of samples obtained from XRD (Fig. 1). These porous NiO–GDC ceramic samples, upon reduction in H2 atmosphere at 900 °C, turns into more porous Ni–GDC substrate (Fig. 2c). The increased porosity is attributed to the fact that the heating the NiO–GDC in reducing environment eliminates the oxygen from the NiO phase by forming H2O. The probable reaction during this process could be 5%H2 −95% Ar at 900∘ C
NiO → Ni + H2 O↑
ð1Þ
GDC thin films were deposited on this porous Ni–GDC substrate followed by annealing at 1000 °C for 8 h in air. This leads to reoxidation of Ni into NiO phase, confirmed by XRD analysis (Fig. 1d), in Air at 1000∘C
2Ni + O2 ðfrom airÞ → 2NiO
ð2Þ
Fig. 4. The typical XRD patterns of (a) NiO–GDC* ceramic substrate and (b) GDC film on NiO–GDC* ceramic substrate.
3.2. Structural and morphological characterizations of GDC/GDC and GDC/NiO–GDC* structures It is a common practice to use the grazing incidence x-ray for characterization of thin films having thicknesses of the order of few hundred nanometers. But, as the thicknesses of films for GDC/GDC and
The SEM image (in BSE mode) of NiO–GDC* is shown in Fig. 2d. The grains of NiO phase are comparatively larger in NiO–GDC* than that of NiO–GDC. The grains of NiO in NiO–GDC (Fig. 2b) sample undergo decrease in size after reduction treatment and can be seen as clusters of comparatively smaller Ni grains embedded in GDC grains. Further, when the Ni–GDC composite is heat treated in air at 1000 °C, the transformation of Ni to NiO leads to large grains of NiO (Fig. 2d). Moreover, in NiO–GDC* the agglomeration of grains to a comparatively large extent takes place due to relatively mobile Ni species and leads to formation of comparatively large clusters and pores in the sample. Apart from these morphological dissimilarities, the relative densities (determined using standard Archimedes method) of composite samples varied with change of compositions. The Ni–GDC sample possesses relative density of 71%, while that of NiO–GDC and NiO–GDC* possesses approximately same relative density of 79%.
Fig. 3. The typical XRD patterns of (a) GDC ceramic substrate and (b) GDC film on GDC ceramic substrate. I111 showing the intensity counts of (111) peak in XRD pattern.
Fig. 5. SEM of GDC films deposited on (a) GDC and (b) Ni–GDC substrates and annealed at 450 °C for 3 h in air. Insets: Fractured morphology showing interface.
654
M.G. Chourashiya et al. / Thin Solid Films 519 (2010) 650–657
GDC/NiO–GDC* structures are greater than 2 μm; the grazing incidence x-rays was not employed for characterizations of the thin films. During the characterization of these samples by XRD, the instrumental parameters were kept fixed so that the comparison of XRDs of samples could be done under identical conditions. In Fig. 3, the XRD pattern of GDC thin film deposited on GDC substrate (annealed at 1000 °C) is compared with that of bare GDC substrate. The intensities of all the peaks of bare GDC substrate have been doubled for GDC films deposited on GDC substrate, which could be due to enhanced density of Bragg planes. These observations reveal that in case of GDC/GDC structure, GDC substrate promotes the crystal growth of GDC film (“substrate effect”), which is obvious in chemical deposition techniques. Since the lattice parameter of depositing material and the substrate is same, the free energy exchange between them facilitates more nucleation centers along substrate crystallites and hence the rate of crystallite formation is enhanced during film deposition (thickness ~ 2 μm). Now, it would be an obvious thought that as NiO–GDC/Ni–GDC composite substrate contains 70%GDC, similar effect of enhanced crystallite formation should be observed. But in NiO–GDC/Ni–GDC composite substrate, 30% of NiO/Ni is homogeneously distributed and therefore the probability of formation of identical nucleation sites in the vicinity, as in case of GDC substrate, might be rarely achieved. Therefore, the added rate of crystallite formation has not been observed for the case of GDC/NiO–GDC* structure (Fig. 4). Due to use of “precipitative deposition” for Ni–GDC substrate, higher grain growth than that of conventional grain growth of spray
pyrolysis is observed which has resulted in higher thickness (~13 μm) of the film. In precipitative deposition, the films are deposited at deposition temperatures (here 250 °C) less than that of decomposition temperature of precursor solution (280 °C). The film at “asdeposited” state is a layer of precipitate with entrapped solvent. Thus, to form the film with desired phase and to evaporate the solvent, the “as-deposited” films were subsequently heat treated at 450 °C for 3 h, as thermo-gravimetric analysis of precursor showed no weight loss above 450 °C [22]. Sufficient precaution had to be taken while evaporating the solvent, as there is large probability of warping of films in this step. However, while depositing the GDC on GDC substrate the optimum pyrolysis of sprayed droplets was observed (as deposition and decomposition temperature is same) and therefore the films formed at “as-deposited” state had the desired phase. In this case, as the droplet reaches to the substrate, it becomes completely dry. Fraction of these dry particles (not having optimum momentum to reach the substrate) may flyaway due to thermophoretic forces at the surface of substrates. The particles reach the substrate and then try to find the site or form the sites for its stabilizations (formation of nucleation centers). As the GDC substrate has the large grains and comparatively uniform surface than Ni–GDC, the formation of nucleation sites is comparably inefficient and therefore the delayed grain growth take place which finally results in comparatively less thickness. However, one order higher thickness of GDC film in case of Ni–GDC composite substrate than that of GDC substrate is mainly attributed to the precipitative deposition. In precipitative deposition, comparatively higher fraction of sprayed droplets (heavier and wet
Fig. 6. SEM of GDC films deposited on (a) GDC and (b) NiO–GDC* substrate and sintered at 1000 °C in air. Insets: Fractured morphology showing interface. AFM images of GDC film deposited on (c) GDC and (d) NiO–GDC* substrate and sintered at 1000 °C in air.
M.G. Chourashiya et al. / Thin Solid Films 519 (2010) 650–657
particles) reaches and get attached to the substrate, which results in large thickness (~ 13 μm) of GDC film on NiO–GDC* substrate after annealing. The GDC film, thus formed, completely screens the XRD peaks originating from NiO–GDC* substrate (Fig. 4b). The films deposited on ceramic substrates were annealed to improve crystallization of films and also the quality of film/substrate interface. For an annealing temperature of 450 °C, the films showed “less adhered” interface with the substrate and the surface grains were discontinuous (Fig. 5a–b). Such interface and discontinuous grains could lead to additional interfacial resistance to the total structure. High temperature sintering of GDC thin films could be employed to improve film/substrate interface. Hence, to avoid interfacial resistance in the total system and improve the film/ substrate interface, the post heat treatment at 1000 °C in air was carried out. This post heat treatment leads to uniform and adherent film/substrate interface in addition to improved surface morphology of the film (depicted in Fig. 6a–b). Besides this, the relative density of films were determined by comparing the fractured SEM of films and that of GDC ceramic substrate sintered at 1500 °C (98%), which is of the order of 97 (±2)%. The high density of the films achieved at 1000 °C was attributed to the presence of nano-granules (avg. grain size 83–85 nm), as can be seen in the respective AFM images of films of GDC/GDC and GDC/NiO–GDC* structures in Fig. 6c–d, respectively. After post heat treatment, GDC film on GDC showed discontinuous and fractured grain morphology, while GDC film on NiO–GDC* showed the better grain growth and grain connectivity. If the cross section of GDC/GDC is carefully observed it can be seen that the average roughness of the GDC substrates is roughly same as that of thickness of GDC film (~2 μm). Therefore, the presence of cracks and discontinuities is an obvious consequence. Hence, to achieve the crack-free, ultra-thin and dense films using SPT it is mandatory to polish the substrate surface to a level of roughness, which should be a fraction of desired thickness of films. However, it should be noted here that the GDC films on GDC substrate are deposited with the intention of forming the protective layer on electrolyte grade substrate. Nonetheless, AFM showed reasonably smooth surfaced grains of GDC films deposited on both the substrates. For GDC/GDC structure, there were granules with size as large as 250 nm compared to their smaller counterparts in the range of 30–40 nm for GDC/NiO–GDC* structure. The size distribution of granules in the GDC films deposited on GDC substrate is comparatively wider than that of on NiO–GDC* substrate (Table 2).
655
Fig. 7. Fractured SEM of platinum coated film/substrate interface; Inset: SEM of uniform coating platinum onto the surface of GDC film.
370 °C respectively are shown in Fig. 8. It showed usual trend of three semicircles in complex impedance plots. In principle, one would have different electrical responses due to grains of two kinds (NiO and GDC) and grain contacts of three kinds (GDC–GDC, NiO–NiO and GDC–NiO). In addition, the conductivity across GDC (ionic) and NiO grains (electronic-holes) would add different responses to impedance spectra. The situation would be much more difficult for the film/substrate structures (where the contacts may behave differently than into the bulk ceramic). Consequently, to assign the contributions originating from all these sources or to extract all this information from the obtained impedance spectra would require a well-dedicated study (out of scope of present research). Thus, here, we simply assign three semicircles (from right) observed in impedance plots to correspond to slow, intermediate, and fast electrical processes. Further, the analysis revealed that the parts of spectra assigned to intermediate and fast electrical processes possesses the capacitance values of the order of 10− 8 and 10− 11 F/ cm2, respectively. These typical values of capacitances, in general, originates from grain boundary (GB) polarization in a polycrystalline
3.3. Electrical characterizations of GDC/GDC and GDC/NiO–GDC* structures The electrical characterization of structures and bare substrates were carried out using impedance measurements (1 Hz–10 MHz) in the temperature range of 250–500 °C. The typical fractured SEM image of platinum coated structure, imaged after the impedance measurements is shown in Fig. 7. Inset of Fig. 7 shows the uniform coating of platinum paste over the surface of GDC films. It can be seen in Fig. 7 that the platinum paste has remained on the surface of the film and has not penetrated/diffused into the film to short with substrate surface. The typical complex impedance (Nyquist) plots for GDC/GDC and GDC/NiO–GDC* structure measured at 340 °C and
Table 2 Surface roughness and average grain size estimated from AFM images of 10%Gd doped ceria (GDC) thin films on different substrates. GDC films on
Surface roughness (nm)
Avg. grain size (nm)
Min. grain size (nm)
Max. grain size (nm)
Grain size Distribution
GDC NiO–GDC*
18.64 8.86
83 85
30–40 60–70
250 150
Wider Moderate
Fig. 8. Typical complex impedance (Nyquist) plot of GDC film on (a) GDC substrate and (b) NiO–GDC* substrate.
656
M.G. Chourashiya et al. / Thin Solid Films 519 (2010) 650–657
Fig. 10. Schematic of micro-structural phase contacts for GDC/GDC and GDC/NiO–GDC* structures.
Fig. 9. Variation of ln(σacT) as function of 1000/T for (a) GDC/GDC and (b) GDC/NiO– GDC* structures.
material (~10− 8 F/cm2) and that of from dielectric relaxation of bulk (grain interior (GI)) material (~10− 11 F/cm2). Hence, this analogy is further extended and these processes are referred to as GI (fast) and GB (intermediate) processes, and the respective conductivities as GI and GB conductivities. Complex impedance plots for structures and substrates showed the gradual decrease in impedances originating from all the three contributions at higher temperatures. The semi-circle originating due to GI impedance contribution disappeared at 440–460 °C for all four
Table 3 Values of grain interior (GI) and grain boundary (GB) conductivities at 500 °C in air along with respective activation energies. Samples ↓
Pair
σGI (S/cm)
σGB (S/cm)
EaGI (eV)
EaGB (eV)
GDC substrate GDC/GDC NiO–GDC* substrate GDC/NiO–GDC*
G–G
0.035 0.027 0.107 0.103
0.0017 0.0028 0.0009 0.0005
0.77 1.07 0.81 1.02
1.09 1.40 0.90 0.93
N–G
studied samples. The impedance data were analyzed by impedance analysis software (ZView Version 2.4a) and used to extract the GI (Rg) and GB impedances (Rgb). The values of Rg and Rgb obtained from the fit parameters were used to calculate ac conductivities and were further fitted to the Arrhenius relation (Fig. 9). The values of GI and GB conductivity and respective activation energies of samples are listed in Table 3. For both the structures and substrates, the magnitudes of GI conductivity remained higher than that of GB conductivity. Comparatively decreased electrical conduction observed for GDC/GDC and GDC/NiO–GDC* structure than that of respective bare substrates could be assigned to the existence various phase contacts in the samples. Various phase contacts are feasible among micron sized grains of GDC, NiO and voids from the substrates and nano sized GDC grains in the film. It is reasonable to assume that the homo phase-contacts offer less opposition than that of hetero phase-contacts for a particular process. The possible micro-structural phase contacts for GDC/GDC and GDC/NiO–GDC* structures are schematically illustrated in Fig. 10 and the resulting number of total contacts are listed in Table 4. The possible phase-contacts in GDC/GDC structure could be only of “nano/micron grains of GDC (NG–MG)” type at film/substrate interface, as the presence of voids (rel. density N 98%) and other Table 4 Various phase contacts for different systems. Samples ↓
Probable phase contacts
Number of phase contacts
GDC substrate GDC/GDC structure NiO–GDC* substrate
MG–MG MG–MG; NG–NG; MG–NG; MG–MG; MN–MN; MG–MN; MV–MG; MN–MV; MG–MG; MG–MN; MG–MV; MG–NG; MN–MN; MN–MV; MN–NG; MV–NG; NG–NG;
1 3 5
GDC/NiO–GDC* structure
MG — micron sized grains of GDC from the substrates. MN — micron sized grains of NiO from the substrates. MV — micron sized voids from the substrates. NG — nano sized grains of GDC from films.
9
M.G. Chourashiya et al. / Thin Solid Films 519 (2010) 650–657
657
Table 5 Calculations of the apparent thickness (t) and actual GB (T) ratios (0.8–1.2% error). The calculation assumes isotropic grain boundaries throughout the specimen.
GDC GDC film on GDC substrate NiO–GDC* GDC film on NiO–GDC* substrate
# pair
Thickness (t) (μm)
Avg. grain size (d) (μm)
GB per unit length (D = 1/d) (μm)
Total GB T=A×t×D
Apparent “t” ratio
Actual GB (T) ratio
G–G
1200 2 1200 13
11 0.083 2.5 0.085
0.0909 12.048 0.4 11.76
=109.0909 a =24.09639 a =480 b =152.9412 b
600
4.527273
92
3.138462
N–G
Where, “a” and “b” are surface area (A) of the samples, which was same for the G–G and G–N pair respectively.
impurities could be ruled out in this case. Thus, inability of GDC/GDC structure to surmount the GI conductivity of bare substrate could be attributed to the only existing phase-contact of NG–MG type, since the grain size distributions of film and substrate are different. According to Ruiz-Trejo et al. [25], nano-ceramics have conduction paths along the grain boundaries. Bellino and co-workers [26] emphasize the fact that in nano-ceramics of doped ceria, and suggested that oxygen migration can take place along the grain boundaries resulting in an enhanced GB ionic conductivity. In agreement with their observation, our data showed enhancement in GB conductivity for GDC/ GDC structure (Table 3). It should be noted that though apparently the thickness ratio of the substrate and the top layer is high; the actual grain boundary length could vary for nano-ceramics. The ratio of GB of substrate to GB of film is calculated (Table 5) and is found to be only 3 to 4. This determines the contribution of GB from nano-ceramics to the GB conductivity of total structure of GDC/GDC. The GI conductivity of NiO–GDC* is found to be higher than that of GDC, which is explained by an additional electron-hole conductivity offered by NiO [27]. Similar results were also observed for GI conductivity of GDC/NiO–GDC structure. However, the slight decrease in GI conduction than that of bare substrate could be attributed to added phase-contact of “NG–MG” type. The GB conductivities of NiO–GDC* and GDC/NiO–GDC* structures are comparably lower than that GDC and GDC/GDC structure, respectively. Therefore, by observing Table 3 and relating it with the chart of phase contacts (Table 4), one can say that as the number of hetero-contacts in a system is increased an increase in overall impedance of the system is observed.
with a layer of suitable cathode material, could find promising application in low temperature-SOFCs. Further research in this direction is being planned in our laboratory and the results will be communicated shortly.
4. Conclusions and further scope of research
[13] [14]
In the present investigation, the potential of formation of solid electrolyte thin film on to ceramic substrates using spray pyrolysis technique (SPT) has been demonstrated. Analysis of GDC/GDC structure revealed that electrolyte-grade substrates could be coated with ultra-thin protective layer using SPT to avoid the interfacial reactions. Besides, the studies on GDC films deposited on NiO–GDC substrate depicted that the electrolyte-grade GDC films could also be deposited using SPT. However, some modification, as mentioned, should be employed in the process. Further, the choice of material for solid electrolyte film i.e. GDC and NiO–GDC as substrate (precursor composite ceramic anode) would enable us to utilize the prepared structure (GDC/NiO–GDC) for fabrication half-cell in low temperature-SOFCs. Electrical characterization of GDC/NiO–GDC structure showed that there is only a nominal decrease in overall ionic conduction of the structure over the bare substrate, indicating good quality film/substrate interface. Such synthesized structures along
[15] [16]
Acknowledgements The authors are very much thankful to DRDO, New Delhi for their financial support. We would like to acknowledge Dr. D. Phase, UGCDAE IUC Indore for extending SEM characterization facilities to us and to Mr. Ahire, Sr. Engineer, UGC-DAE IUC Indore for his co-operation during the SEM characterizations. Dr. M. G. Chourashiya is thankful to CSIR, New Delhi for a senior research fellowship.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[17] [18]
[19] [20] [21] [22] [23] [24] [25]
[26] [27]
S.C. Singhal, Solid State Ionics 135 (2000) 305. B.C.H. Steele, Solid State Ionics 129 (2000) 95. M. Mogensen, N.M. Sammes, G.A. Tompett, Solid State Ionics 129 (2000) 63. S. Wang, T. Kato, S. Nagata, T. Kaneko, N. Iwashita, T. Honda, M. Dokiya, Solid State Ionics 152–152 (2002) 477. B.C.H. Steele, Solid State Ionics 134 (2000) 3. T.S. Zhang, J. Ma, L.B. Kong, P. Hing, J.A. Kilner, Solid State Ionics 167 (2004) 191. Y.J. Leng, S.H. Chan, S.P. Jiang, K.A. Khor, Solid State Ionics 170 (2004) 9. C. Hatchwell, N.M. Sammes, I.W.M. Brown, Solid State Ionics 126 (1999) 201. T. Fukui, K. Murata, S. Ohara, H. Abe, M. Naito, K. Nogi, J. Power Sources 125 (2004) 17. J.B. Goodenough, Annu. Rev. Mater. Res. 33 (2003) 91. J.L.M. Rupp, T. Drobek, A. Rossi, L.J. Gauckler, Chem. Mater. 19 (2007) 1134. T. Setoguchi, T. Inoue, H. Takebe, K. Eguchi, K. Morinaga, H. Arai, Solid State Ionics 37 (1990) 217. E.M. Kelder, O.C.J. Nijs, J. Schoonman, Solid State Ionics 68 (1994) 5. A.A. Van Zomeren, E.M. Kelder, J.C.M. Marijnissen, J. Schoonman, J. Aerosol Sci. 25 (1994) 1229. C.H. Chen, A.A.J. Buysman, E.M. Kelder, J. Schoonman, Solid State Ionics 80 (1995) 1. K.L. Choy, in: W.E. Lee (Ed.), British Ceramic Proc., Ceramic Films, Coatings, The Institute of Materials, 54, 1995, p. 65. K.L. Choy, W. Bai, B.C.H. Steele, in: A.J. Mcevoy, K. Nisancioglu (Eds.), Materials, Processes, 10th SOFC Workshop, Les Diablerets, Int. Energy Agency, 1997, p. 233. N.H.J. Stelzer, C.H. Chen, Rij Van, L.N.J. Schoonman, in: A.J. Mcevoy, K. Nisancioglu (Eds.), Materials, Processes, 10th SOFC Workshop, Les Diablerets, Int. Energy Agency, 1997, p. 236. M.G. Chourashiya, J.Y. Patil, S.H. Pawar, L.D. Jadhav, Mater. Chem. Phy. 109 (2008) 39. C.R. Xia, F.L. Chen, M.L. Liu, Electrochem. Solid-State Lett. 4 (2001) A52. C. Xia, M. Liu, Adv. Mater. 14 (2002) 521. M.G. Chourashiya, S.H. Pawar, L.D. Jadhav, App. Surf. Sci. 254 (2008) 3431. J.A. Kilner, B.C.H. Steele, in: O.T. Sorensen (Ed.), Non-Stoichiometric Oxides, Academic Press, New York, 1981. R.V. Wandekar, M. Ail (Basu), B.N. Wani, S.R. Bharadwaj, Mater. Chem. Phys. 99 (2006) 289. E. Ruiz-Trejo, J. Santoyo-Salazar, R. Vilchis-Morales, A. Benitz-Rico, F. GomexGarcia, C. Flores-Morales, J. Chavez-Carvayar, G. Tavizon, J. Solid State Chem. 180 (2007) 3093. M.G. Beliino, D.G. Lamas, N.E. Walsoe de Reca, Adv. Funct. Mater. 16 (2006) 107. Y.M. Park, G.M. Choi, Solid State Ionics 120 (1999) 265.