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Synthesis and characterization of 10%Gd doped ceria (GDC) deposited on NiO-GDC anode-grade-ceramic substrate as half cell for IT-SOFC
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Synthesis and characterization of 10%Gd doped ceria (GDC) deposited on NiO-GDC anode-grade-ceramic substrate as half cell for IT-SOFC M.G. Chourashiya a,*, L.D. Jadhav b a b
Department of Physics, Shivaji University, Kolhapur 416 004, India Department of Physics, Rajaram College, Kolhapur 416 004, India
article info
abstract
Article history:
In the present research work spray pyrolysis technique (SPT) is employed to synthesize
Received 1 August 2010
GDC (10%Gd doped ceria) thin films on anode-grade-ceramic substrate (porous NiO-GDC).
Received in revised form
The film/substrate structure was characterized for their micro-structural and electrical
29 November 2010
properties along with their interfacial-quality. By optimization of preparative parameters
Accepted 18 December 2010
of SPT and modification of surface of anode-grade ceramic substrate, we were able to
Available online 26 January 2011
prepare the GDC films having thickness of the order of 13 mm on NiO-GDC substrate. Further to improve the interfacial quality and densification of film, annealing of structure
Keywords:
at 1000 C for 8 h was carried out which leads to fully dense (>96%) GDC films, forming
GDC thin films
a gas-tight interface with substrate. Impedance measurements revealed that grain interior
Spray pyrolysis
conductivity for GDC/NiO-GDC half-cell was of the order of 0.1S/cm at 500 C which is the
Impedance spectroscopy
desired conductivity for successful operation of IT-SOFC. The activation energy for grain
IT-SOFC
interior and grain-boundary conduction estimated for GDC/NiO-GDC was 1.07 eV and 0.93 eV, respectively. Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Solid oxide fuel cells (SOFCs) have received much attention as promising power generation device because of their high conversion efficiency of chemical energy to electric power. Comparable to other types of fuel cells, SOFCs have the advantage of fuel flexibility i.e. are able to work with hydrogen, hydrocarbon reformate and, in some conditions, with hydrocarbon fuels directly. Commercial solid electrolyte materials for SOFCs are based on stabilized zirconia which requires to be operated at 900e1000 C to ensure the sufficient ionic conductivity. However, the high temperature operation of SOFC demands stringent requirements for other cell components and must be fabricated from high cost ceramic materials
to resist its thermal decomposition at these temperatures. If the operating temperature could be reduced to intermediate temperatures (500e700 C), less expensive materials, such as steel, could be used in these auxiliary components. Electrolytes based on ceria are good candidates for intermediate temperature (IT-) SOFC, due to their inherent property of higher ionic conductivity at these temperatures [1]. Ceria-based electrolytes have relatively large unit cells compared to systems based on zirconia and, as a consequence, ceria-based systems have larger channels through which oxygen ions can pass during the conduction. However, Bevan and Summerville [2] demonstrated using lattice parameter data, that the introduction of the Gdþ3 ions into the CeO2 lattice minimizes unit cell expansion or contraction.
* Corresponding author. E-mail address: [email protected] (M.G. Chourashiya). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.12.083
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This fact was further confirmed by Kilner [3] and concluded that the CeO2eGd2O3 system is one of the highest ionically conducting, ceria-based binary systems. Since, the substitution of larger dopants (than Gd) into ceria crystal lattices blocks the path of the migrating oxygen ions more effectively, thus increasing the ion-migration enthalpy [4]. However, even having an excellent ionic conduction behavior, Gd doped ceria has a relatively narrow region of oxide ion conductivity as an electronic component is introduced on reduction [5], producing a short circuit in the cell. When the electrolyte is subjected to partial pressure of oxygen typically associated with the anode side of a fuel cell, its electronic contribution to the overall conductivity increases, due to the reduction of Ce4þ to a Ce3þ state, resulting in failure of the electrolyte material [6]. Under these condition the doped ceria is partially reduced and its lattice parameter increases leading to the generation of stress in the electrolyte. Atkinson [7] calculated these stresses for a range of different parameters which includes doping level (Gd), temperature, oxygen activity, etc. and indicated that the maximum ‘safe’ operating temperature for Gd doped Ceria at an anode is about 750 C. To address the significant problem with ceria based materials of poor stability at low partial pressure of oxygen, a number of approaches have been taken, such as finding the ideal doping level and ion to balance stability with adequate oxide ion conductivity, decreasing the operating temperature and improving materials processing etc. In such an effort to minimize the instability of Gd doped ceria at lower oxygen partial pressures, other ceria based materials of the general formula Ce1-xMxO2-d, where M ¼ Gd, Sm, Ca, Mg have been studied [8,9]. However, Gd0.1Ce0.9O1.9 was the preferred choice for the electrolyte, with a compromise between higher stability to reduction and good oxide ion conductivity at 600 C; as higher Gd concentration materials are more readily reduced, though the oxide ion conductivity is increased. Doshi et al. have shown that Ce0.8Gd0.2O1.9 is purely an ionic conductor in fuel environments only below 450 C [10]. Decreasing the temperature of operation for Gd doped ceria to 600 C appears to minimize the reduction of the electrolyte; however, lower temperatures result in the greater loss of power density due to the decrease in the ionic conductivity. There are two main approaches to overcome these problems: the first is to decrease the electrolyte thickness [11,12] and the second is to use electrolyte materials with high ionic conductivity at low temperature such as doped ceria [13]. In addition, reduction of the electrode polarization resistance [14] also would be helpful in reducing the operating temperature. The reduced thickness of dense electrolyte minimizes the internal ohmic losses [15] leading to higher conductivity at lower temperatures. Thus, thin film technology could enable Gd doped ceria electrolytes to be operated at further reduced temperature [16]. In such cases, according to Xia and Liu [17], partial internal shorting arising from the electronic conduction of Gd doped ceria becomes insignificant. Moreover, according to Atkinson [7], thin film technology not only helps to reduce the operating temperature of cell but also minimize the risk of fracture in ceria based electrolytes during thermal cycling of cell. Among the different thin film fabrication techniques spray pyrolysis technique (SPT) seems to satisfy
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the desired criteria for preparation of thin solid electrolyte [18,19]. The versatility of the SPT and requirement of fabrication of dense and thin solid electrolyte films on functional ceramic substrates (non-conducting at deposition temperatures) makes SPT a most suitable technique for in-situ fabrication of electrode/electrolyte half-cell for solid oxide fuel cells. Apart from this, unlike many other film deposition techniques, SPT represents a very simple and relatively costeffective method. In present research, all the three approaches [1,16,20], i.e. (a) implementation of thin film electrolyte (spray pyrolysis), (b) use of high ion-conducting electrolyte (10%Gd doped ceria e referred as GDC hereinafter) and (c) lowering the electrode/electrolyte polarizations (depositing the electrolyte on electrode substrate), are emphasized. The preparative parameters of spray pyrolysis technique (SPT) such as precursor solution concentration and substrate temperature were optimized to deposit dense and adherent films of GDC on NiO-GDC (anode-grade) substrate. The anode-grade ceramic substrates of NiO-GDC were prepared by conventional ceramic route. The structural, morphological (surface and fractured) and electrical characterization were carried out to investigate the competence of fabrication technique and fabricated halfcell for its application in IT-SOFCs.
2.
Experimental
2.1.
Ceramic substrate preparation
The powders of Gd2O3 (AR grade, 99.9%) and CeO2 (AR grade, 99.9%) from HIMEDIA Inc., were mixed in 1:9 proportion with an agate mortar. The mixed powder was then calcined at 1200 C/4 h in air, to obtain the GDC powder. The calcined GDC powder was then reground and mixed with NiO (Extra pure, AR grade from HIMEDIA Inc.) in desired proportion to obtain the composite phase of (NiO)30-(GDC)70 e referred as NiO-GDC hereinafter. The selection of ceramic composition of NiO30GDC70 (precursor composite for Ni-GDC anode) substrate was decided on the basis of their viability in prospective use as an anode of half-cell for IT-SOFC [21,22]. The mixed powder was then again reground with organic binder (Poly-Vinyl Alcohol e PVA). The binder added powder was pelletized with the help of hydraulic press machine. The green samples then sintered at 1400 C for 8 h in air. The pre-sintering (binder removal step) of samples were intentionally excluded, as the binder removal step and high sintering temperature (>1500 C) is usually employed to obtain dense ceramic bodies and here we are expecting for porous structured (anode-grade) NiO-GDC substrates. This aspect is discussed in results and discussion section. The dimensions of the sintered substrates were 0.12 cm in thickness and 2.5 cm in diameter. To modify the surface properties of the substrates, NiO-GDC samples were heat treated in reducing atmosphere of 5%H2e95%Ar with gas flow rate of 500 ml/min at 900 C for 5 h. The reduction treatment leads to formation of comparably more porous structured and rough surfaced Ni-GDC composite (referred as Ni-GDC hereinafter) ceramic substrates. As the SPT is surface sensitive technique, this rough surfaced (Ni-GDC) substrate showed the positive effects during film synthesis (discussed later).
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Moreover, to keep the surface of substrates flat, during the heat treatments an alumina (Al2O3) sheet is placed over the surface of NiO-GDC samples.
2.2.
Thin film synthesis
Aqueous solution of cerium nitrate and gadolinium nitrate (99.9% Pure; ALFA AESAR) were mixed in desired proportion to obtain the (10%Gd doped ceria) GDC thin films. The mixed solution was then sprayed by glass nozzle with air as carrier gas on ceramic substrates (kept at preset temperature). The optimization of preparative parameters of SPT was carried out to form dense and good quality GDC thin films on ceramic substrates. The similar optimization procedure as that was applied to obtain GDC thin films on glass substrate is employed here, which is reported elsewhere [23]. Due to nonadherence of GDC films on NiO-GDC substrate, we deposited the GDC thin films on Ni-GDC substrate. The deposition of GDC thin films onto anode-grade ceramic substrate (Ni-GDC) were carried out using optimized preparative parameters of SPT (i.e. precursor solution with concentration of 0.04M and substrate temperature of 250 C). Moreover, the faultlessness in interface of GDC film with ceramic substrate was achieved by subsequent heat treatment at 1000 C for 8 h in air. The post heat treatment resulted in dense films along with gas-tight interface; however the NiGDC phase of substrate again transformed to NiO-GDC. The schematic of workflow which was followed during the present work is shown in Fig. 1.
2.3.
Characterizations
The morphological and structural characterizations of deposited GDC films, and composite ceramic substrates at various
stages were studied using SEM (JEOL-JSM-6360, Japan) and XRD (PHILIPSePW-3710) with Cu-Ka radiation source, respectively. In order to distinguish the different phases in the composite samples, the Back Scattered Electron (BSE) mode was employed to obtain the BSE image. The electrical characterization of GDC/ NiO-GDC structure were performed using ac impedance measurements (SOLARTRONe1260; impedance analyzer) for temperatures ranging from 250 C to 500 C. The grain interior and grain boundary impedances were extracted by analyzing the collected impedance data using ZView Version 2.4a e freeware software. 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 mm and spring constant of 0.58 N/m. The surface roughness was determined from the AFM images.
3.
Results and discussion
3.1. Synthesis and characterizations of NiO-GDC composite ceramic substrates A general procedure for fabrication of ceramic samples includes 5 major steps, namely, (1) raw ceramic powder categorizations, (2) synthesis of ceramic powders, (3) green body formation, (4) pre-sintering and (5) final sintering. Therefore initially, to prepare raw GDC powder, CeO2 (ceria) and Gd2O3 (as starting materials) were characterized for their structural properties. Fig. 2 shows the XRD pattern of starting and calcined powders. These XRD patterns were compared with their respective JCPDS files as,-PDF No. 81-0792 for CeO2 and 76-0155 for Gd2O3 and indexed accordingly. Both the powders possesses cubic lattice structure with lattice
Fig. 1 e The schematic of work-flow which was followed during the present work. Left: Ceramic substrate synthesis. Right: Thin films synthesis.
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Fig. 2 e XRD patterns of commercially available CeO2, Gd2O3 and GDC powders calcined at 1200 C/4 h.
˚ for CeO2 (std. ‘a’ ¼ 5.410 A ˚ ) and 10.80 A ˚ for parameter of 5.411 A ˚ ). The crystallite size calculated by Gd2O3 (std. ‘a’ ¼ 10.79 A Scherer’s formula for both powders was around 35e45 nm. In ceramic processing, starting chemicals having the narrow particle size distribution is advantageous, as it facilitates uniform sintering of ceramic bodies. Further, these solid phase reactant of CeO2 and Gd2O3 in calculated amount of weights (9: 1) were mixed in agate mortar. The homogenization was confirmed by observing the change in color of powder which turned into yellowish white after thorough mixing. The color of starting powder of ceria (CeO2) was yellow while that of Gd2O3 was white. After proper mixing of the starting chemicals, the mixture was calcinated in furnace at 1200 C for 4 h in air. This heat treatment leads to formation of GDC phase which was confirmed by XRD (Fig. 2). Now to prepare composite ceramic substrate (NiO-GDC), the calcined GDC powder was mixed with NiO in an agate mortar. After homogenization the PVA is added to mixture and was again grinded in agate mortar. The binder added powders were pressed in circular disk shaped pellets (green sample). However, before pressing the powders in pellet form, uniform tapping of the die (filled with powders) was carefully adapted. Since in dry pressing, tapping increases the packing density and eliminate any in-homogeneities in the packing. If the die (mold) is not filled homogeneously, the final pressed shape will be very different from that of the die. The fourth (i.e. pre-sintering) step is usually followed to improve the densification rate during final sintering step. But for the anode-grade ceramic substrate it is desirable to have the porous structured morphology. Thus, this step was not followed here. The green bodies (pressed samples) consists a mass of ceramic powder held together by organic polymer e binder (PVA), which are distributed at the particleeparticle contacts and gave the strength to the green body. When necessary heat of evaporation for polymer drying and the heat of reaction, either exothermic or endothermic, for polymer thermal decomposition were supplied, the evaporation and thermal decomposition of PVA in green body starts. Here, this occurs during the sintering step of NiO-GDC green samples. Both evaporation and thermal decomposition of polymer gives off huge volumes of gases. These gases diffuse through the porous network of the green body and exert different types
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of stresses, which includes thermal gradients, gas pressure gradient, liquid pressure gradient, etc. These stresses act to warp or crack the green body. Hence it is very important to control the heating rate of green body in furnace. Here, the heating rate of 3 C/min was kept for all the ceramic samples. These highly pressurized evolved gases escape to ambiance keeping the pores in the NiO-GDC substrates. The density of NiO-GDC samples were estimated using standard Archimedes principle and its estimated relative density was 79%. The relative density of prepared samples were estimated using the relation,% density ¼ ddthm 100 , where, dm is the density of samples measured using Archimedes principle and dth the theoretical density given as, dth ¼ NA4a3 ½ð1 xÞMCe þ x,MGd þð2 12,xÞMO , where, 0.1 is gadolinium content, a the lattice constant at room temperatures, Na the Avogadro number (6.023 1023), and M refers to the respective atomic weights. As mentioned above, in the primary stage of work it was planned to deposit GDC on NiO-GDC to form GDC/NiO-GDC precursor half-cell, which were further planned to reduce insitu to get electrolyte/anode (GDC/Ni-GDC) half-cells. However, an attempt to deposit GDC films onto NiO-GDC substrate results into non-adherent films with thickness less than 2 mm. An electrolyte is supposed to have the thickness of the order of 10 mm to avoid the gas-cross overs during its operation. Thus, to improve the film thickness and adherence with substrate it was proposed to reduce the NiO-GDC substrate to obtain porous with rough surfaced sample and then to deposit GDC film on it. Fig. 3 shows the XRD patterns of NiO-GDC at various stages. In all three stages of processing of NiO-GDC substrates, the absence of peaks corresponding to Gd2O3 and comparatively ˚ ) than that of the host lattice larger lattice parameter (w5.419 A ˚ ) confirms the complete dissolution of Gd2O3 in of ceria (5.410 A ceria lattice, to form GDC. The XRD pattern of as-synthesized NiO-GDC was compared with JCPDS-PDF no. 75-0161 (GDC) and 78-0643 (Cubic NiO) to confirm the individual phase peaks and was accordingly indexed in Fig. 3a. All the reflection peaks could be either indexed to NiO or GDC, confirming formation of composite without any undesirable phases. In addition to that the lattice parameters calculated for each of phase are in well agreement with that of respective standard values. The ˚ average lattice parameters for GDC and NiO phase are 5.419 A ˚ , respectively. These observations affirm that NiO and 3.540 A phase remains separate and does not diffuse into the GDC
Fig. 3 e XRD patterns of ceramic substrates at different stages. (a) as-synthesized NiO-GDC, (b) Ni-GDC and (c) after-oxidation NiO-GDC.
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lattice to form the undesirable phase. The average crystallite size estimated for GDC and NiO phases are 581 nm 343 nm, respectively. The XRD pattern of reduced sample i.e. of Ni-GDC (Fig. 3b) was compared with JCPDS-PDF no. 75-0161 (GDC) and 04-0850 (Cubic Ni). The peaks are indexed accordingly in the Fig. 3b and no peaks corresponding to NiO-phase were observed, confirming the complete reduction of sample under investigation. The average lattice parameters calculated for GDC and ˚ and 4.169 A ˚ , respectively. The average Ni phases are 5.418 A crystallite size calculated using Scherrer’s formula for GDC and Ni phases are 575 nm 403 nm, respectively. These reduced samples employed to synthesize the GDC/Ni-GDC half-cell and further heat treated in air for improvement of crystallinity in the film and quality of the interface. Upon heat treatment, the Ni-GDC substrates were oxidized and become NiO-GDC again, which was confirmed by comparing its XRD pattern with JCPDS files (Fig. 3c). No reflection peak due to Ni was observed, which confirms the complete oxidation of Ni to NiO. The calculated lattice parameters for each phase were in agreement with that of respective standard values from JCPDS. The average lattice parameters for GDC and NiO phases ˚ and 3.541 A ˚ , respectively. Apart from the similarare 5.419 A ities in lattice parameters of as-synthesized NiO-GDC and after-oxidation NiO-GDC substrate, the crystallite size of NiO phase in later case is higher (610 nm) than as-synthesized NiO-GDC (443 nm) samples. Fig. 4 shows the SEM and BSE images (insets) of the NiOGDC ceramic substrates at different stages. The surface morphology of the NiO-GDC (sintered at 1400 C/8 h) is porous (Fig. 4a). The porous nature of NiO-GDC samples is attributed to the procedure employed for its synthesis (without 4th-step). One can also attribute the same to its composition, as the composition involves two phases (composite) either of which avoids the lattice diffusion (which is the major process of densification) at particular sintering temperature. These porous NiO-GDC (green in color) ceramic samples, upon reduction in H2 atmosphere at 900 C, turns into more porous Ni-GDC (dark black in color) samples (Fig. 4b). 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. 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 re-oxidation of Ni into NiO phase, confirmed by XRD (Fig. 3c). The after-oxidation NiO-GDC showed comparatively larger grains than as-synthesized NiO-GDC. The inspection SEM and BSE images of composite substrates (Fig. 4aec) clearly reveals that there are two phase grains, as expected, one is of GDC phase and other is of NiO/Ni phase. The grains of NiO in assynthesized NiO-GDC (Fig. 4a) sample undergo decrease in size after reduction treatment and can be clearly seen as clusters of comparatively smaller Ni grains embedded in GDC grains (Fig. 4b). Further when the Ni-GDC composite is heat treated in air at 1000 C, the transformation of Ni to NiO, again leads to increase in grain size of NiO (Fig. 4c). In ‘after-oxidation’ NiO-GDC samples, the re-agglomeration of grains takes place and leads to formation of comparatively large clusters and pores in the sample. Apart from these morphological dissimilarities, the relative densities of composite samples
varied with change in compositions. The Ni-GDC sample possesses least relative density of 71%, while that of assynthesized and after-oxidation NiO-GDC possesses approximately same relative density of 79% [24,25]. The complex impedance plot of after-oxidation NiO-GDC sample recorded at 290 C is shown in Fig. 5a. It shows the usual trend of three semicircles in complex impedance plots corresponding to electrode, grain boundary (GB) and grain interior (GI) impedance contributions to total impedance of the system. In line with other polycrystalline samples complex impedance plots for substrates showed the gradual decrease in impedances with an increase in measurement temperature. The criterion for ‘how the half-circles in impedance plots are assigned to GI and GB’ is discussed in next section. The impedance data were further analyzed by impedance analysis software to extract the grain interior (Rg) and grain boundary impedances (Rgb). While analyzing the impedance data, the data points corresponding to electrode contribution was skipped to avoid the complexities in extraction of Rg and Rgb. In accordance with Duncan and Laxia [26], the use of 2CPE circuit is employed here to obtain the best fit for observed impedance data, which adequately allowed us to determine the resistance of system under investigation. This fitting cannot be considered for interpretation of the exact physical processes that would occur in such systems. The values of Rg and Rgb obtained from the fit parameters were used to calculate ac conductivities and were further fitted to the modified Arrhenius relation. The modified Arrhenius a relation is given as,sT ¼ s0 expðE kT Þ , where Ea is the activation energy for the conduction process, T the temperature, s0 the pre-exponential factor and k the Boltzmann constant. Fig. 5b shows the Arrhenius plot for after-oxidation NiO-GDC substrate. The values of GI and GB conductivity measured at 500 C for after-oxidation NiO-GDC substrate are 0.107S/cm and 0.0009 S/cm. respectively. The activation energies calculated from Arrhenius plots for after-oxidation NiO-GDC substrate for GI and GB conductivity are 0.81 eV and 0.9 eV, respectively.
3.2. Synthesis and characterizations of GDC thin films on NiO-GDC ceramic substrate There are number of processes occur either sequentially or simultaneously during film formation by spray pyrolysis. The mechanism of thin film deposition by spray pyrolysis can be divided into three main steps: atomization of the precursor solution, transport of the resultant aerosol and decomposition of the precursor on the substrate. The first step in spray pyrolysis is generation of spray with the help of atomizers. In atomizers the filled precursor solution is transformed into fine droplets and forced or directed towards the substrates. There are three types (air blast, ultrasonic and electrostatic) of atomizers which are usually employed in SPT which have their own parameter to control the droplet distribution. Here we have employed air-blast type of atomizer which is relatively simple and cost-effective than the others. The morphological properties of the films directly depend upon these droplet distributions. If the droplet distribution is narrow the films are observed to have uniform grain sizes. In air-blast type atomizer the atomizing medium is air and the
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Fig. 4 e SEM of NiO-GDC ceramic samples at different stages. (a) as-synthesized NiO-GDC, (b) Ni-GDC and (c) after-oxidation NiO-GDC. Insets shows BSE images of respective samples.
resultant droplet distribution of solution with air is referred as “Aerosol”. After atomization this aerosol is transported to substrate where the deposition of the metal salt in thin film form occurs. The energy required for transport of aerosol is gained from the driving force of atomizing medium. During the transport of aerosol towards the heated substrate, initially aerosol gets warmed and undergoes vaporization of solvent and leads to a size reduction (shrinkage) of the droplet and lead to the development of a concentration gradient within
the droplet. As the droplet moves to heated substrate, continuous vaporization of solvent takes place and consequently leads to formation of precipitate. The precipitation of the precursor in droplet occurs as the concentration exceeds the solubility limit. When the precipitate reaches closer to the substrate, the sudden increase in air temperature leads to pyrolytic decomposition of the precipitate and forms the solid particles. The mechanism of aerosol transition as function of temperature gradient is shown in Fig. 6. These various
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Fig. 5 e (a) Complex impedance plot of after-oxidation NiO-GDC substrate measured at 290 C and (b) variation of ln(sacT ) as function of 1000/T for after-oxidation NiO-GDC substrate.
processes during the aerosol transport are mainly responsible for the structural and morphological properties of spray deposited films. The sufficient temperature gradient is thus necessary for complete pyrolysis of the spray. If the temperature is not optimized the evaporation of solvent may be in premature (wet) or exceedingly matured (completely dried) state. The earlier case (i.e. premature state) is deliberately
Fig. 6 e a) Temperature gradient as function of distance from substrate. Inset: Transition of aerosol as function of temperature gradient produced by heated substrate, during aerosol transport.
used for preparation of films on non-sticky substrates while later case (i.e. exceedingly matured state) is used for preparation of ultrafine powders. When the precipitate reaches the substrate, the precipitate undergoes decomposition and then nucleation and growth of thin films on the substrate take place. Many processes occur simultaneously when a droplet hits the surface of the substrate, namely, evaporation of residual solvent, spreading of the droplet and salt decomposition. After decomposition of the precursor on the substrate the subsequent nucleation and grain growth results in the formation of film. A droplet hitting the surface may stick, rebound, spread or splash on the surface. After droplet impact, the physical and chemical properties of the substrate play an impotent role to influence the film nucleation and growth. The surface of substrate provides the nucleation site for film growth. During the deposition of GDC thin films on NiO-GDC ceramic substrate, it was observed that the deposited films were non-adherent to substrate, which is undesirable for its prospective application. This non-adherence of the film with substrate could lead to faulty-cell fabrication which eventually may lead to degradation in cell performance or in worst case, failure of fuel cell. The non-adherence of the film may be attributed to the relatively dense and smooth surface of assynthesized NiO-GDC substrates. As mentioned earlier SPT is sensitive to surfaces of substrates, it was planned to modify the surface by heating the substrate samples in reducing atmosphere. The heating of substrates in reducing atmosphere served the dual purposes. Firstly, this heat treatment in
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Fig. 7 e Surface and fractured morphology of GDC thin film deposited on Ni-GDC and annealed at 450 C/4 h. reducing atmosphere leads to transfer of NiO-GDC substrate to Ni-GDC, which is anode for ceria based fuel cell. Secondly, the substrate surfaces become rough and porous. The transformation of substrate on one hand gave more nucleation sites for growth of film by the virtue of rough and porous surfaces, but on other hand the metallic species (Ni) from substrate are
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also exposed to surfaces. In fact, it is quite difficult to deposit the thin films, using SPT, on metallic substrates. It is observed fact that the presence of metallic species in the substrate (e.g. Ni in cease of Ni-GDC) at elevated temperatures (greater than room temperatures) hinders the stabilization of sprayed droplets on to the substrate. To get the films deposited on to such substrates, a method of wet-film-formation during spray pyrolysis was employed. In this method, the films are deposited at temperatures lower than decomposition temperature (i.e. at 250 C < decomposition temperature ¼ 280 C [21]) of precursor and subsequently heat treated to a higher temperature to complete the decomposition of deposited materials. These wet films then consequently heated to complete decomposition of entrapped solvent in the films. This is not a good practice to prepare the films by SPT as the films contains high amount of solvent (water in our case) and which might lead to cracking or peeling-off of the films during heat treatment. Therefore the deposited films were heat treated at 450 C with a controlled and slow heating rate (1 C/min). This heat treatment not only dries deposited films but also improves crystallinity and avoids warping/cracking of films with time span [23]. On one hand wet film formation depicts the danger of peeling-effect in deposited films (if kept un-heat treated) but on the other hand it results in higher thickness in a single run. The wet droplets pile up to comparably higher thickness,
Fig. 8 e (a) Surface and fractured morphology of GDC thin film deposited on NiO-GDC and post-annealed at 1000 C/8 h. (b) AFM image of GDC thin film.
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which can be attributed to higher thickness (w13 mm) obtained for single deposition (Figs. 7 and 8a). The decreased substrate temperature for highly thermal conducting (e.g. metallic) substrate is also reported in literature [27]; however, its theoretical understanding is not clear. Formation of nano-crystallites during thin film deposition is the characteristic of SPT. The sprayed droplet when approaches to substrate, experience high thermal-gradient at substrate (Fig. 6) and get rapidly pyrolized. After the pyrolysis, the decomposed solid particle, in general, possess the dimensions of few nanometers. These accumulated nanometer sized particles undergo sintering and partial grain growth at deposition temperature (Fig. 7). Further crystallization and grain growth in films occurs upon the post-annealing treatment (Fig. 8a), but the grain remains in nanometer size with improved compactness (Fig. 8b). In our case, we carried out the post-annealing treatment at 1000 C for 8 h. This postannealing heat treatment leads to uniform and adherent interface between film and substrate. Moreover the surface morphology of the film was also observed to improve (Fig. 8a). If the relative density of fractured SEM of films is compared with that of ceramic substrates sintered at 1400 C (79%), it can be estimated that the relative density of the films are quite higher. In the cross-section of film, there are virtually no pores as that can be seen in the fractured SEM of ceramic substrate. This type of highly compact and dense microstructure is one of the most desirable characteristics for its application as solid electrolyte. In case of bulk sample, such high density for GDC can only be achieved at sintering temperature greater than 1500 C while it was achieved here (in the case of thin films) only at 1000 C. This higher densification rates in thins films are attributed to the presence of nano-granules having average grain size of 85 nm (Fig. 8b). Further analysis of surface morphology of films using AFM revealed that film possesses the surface roughness of 8.86 nm. Fig. 9 shows the XRD pattern of GDC film deposited on afteroxidation NiO-GDC substrate prepared with optimized preparative parameters of SPT. The thickness of film was large enough (w13 mm) to suppress the NiO peaks originating from subs˚ , which trate. The lattice parameter of the GDC film was 5.420 A is nearly same as that of standard lattice parameter for GDC. To estimate the competence of the GDC film deposited on anode grade (after-oxidation) NiO-GDC ceramic substrate as
a solid electrolyte and electrode assembly (half-cell), the halfcell was tested for its electrochemical properties using Electrochemical Impedance Spectroscopy (EIS). However, thin films having uneven thickness/pores could lead to failure of half-cell testing by shortening the current-collector with electrode via thin electrolyte. This doubt is clearly depicted in its schematic (Fig. 10a). The fractured SEM image of platinum coated half-cell, imaged after the impedance measurements (Fig. 10b) shows that the platinum paste remained on the surface of the film and has not penetrated/diffused into the film to short with substrate surface. Inset of Fig. 10b shows the uniform coating of platinum paste over the surface of GDC films. EI spectra of half-cell showed typical trend of three semicircles of a polycrystalline ionic conductors (Fig. 11a). As the half-cell involves various interfaces, even within their individual microstructures, extracting the information of the corresponding to various physical processes involved within it is a quite complex task. Moreover, the conductivity across GDC (ionic) and NiO grains (electronic-holes) would add different responses to impedance spectra leading to increased complexities. Therefore for the sake of estimation of competence of prepared half-cell, we employed a simple approach. Here, we simply assign three semicircles (from right) observed in impedance plots to correspond to sluggish, intermediary, 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 108 and 1011 F/cm2, respectively. These typical values of capacitances, in general, originate from GB polarization in a polycrystalline material (w108F/cm2) and that of from dielectric relaxation of bulk (GI) material (w1011 F/cm2). Hence, this resemblance is further stretched and these processes are referred as GI (fast) and GB (intermediate) processes, and the respective conductivities as GI and GB conductivities. The impedance data were analyzed by impedance analysis software (ZView Version 2.4a) and used to extract the grain interior (Rg) and grain boundary impedances (Rgb). The typical fit result from the analysis, for GDC film deposited on after-oxidation NiO-GDC substrate is shown in Fig. 11b. 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 to estimate the activation energies.
Fig. 9 e The XRD pattern of GDC film deposited on after-oxidation NiO-GDC substrate.
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Fig. 10 e (a) Schematic of system formed by ‘Pt’ electrode/film-substrate/‘Pt’ electrode for impedance measurement. Right part shows the doubtful penetration of ‘pt’ paste. (b) SEM of tri-layer of ‘pt’ paste/film/substrate. Inset: ‘Pt’ paste coated onto surface of substrate.
Fig. 11 e (a) Complex impedance plot of half-cell and (b) The typical fit result from the analysis, for GDC/NiO-GDC half-cell substrate.
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The GI and GB conductivity for GDC film deposited on afteroxidation NiO-GDC substrate at 500 C are 0.103 S/cm and 0.0005 S/cm, respectively. The decrease in these values compared to those for substrates are attributed to the increased number of interfaces. The calculated activation energies for GI and GB conductivity are 1.02 eV and 0.93 eV, respectively. For substrate as well as half-cell, the GI conductivity is constantly dominated the GB conductivity. The amount of GI dominance in synthesized half-cell was two times than that of after-oxidation NiO-GDC substrate. As the GB processes, particularly in polycrystalline ionic conductors, are influenced by the presence of impurities, defects, concentration gradients, double layer capacitances across the grain boundaries, etc., the GB conductivities are always relatively smaller than that of GI conductivities. Also it is known fact for ceria systems that the “grain boundary effect” was greatest for dilute solid solutions (e.g. 10%Gd doping, as it appears here) and the activation energy for conductivity along the GB were higher than that of GI conductivity [28,29]. This is observed to be true for after-oxidation NiO-GDC substrate, however, the difference in activation energies are small. Comparatively higher Ea for GI conductivity than that of GB conductivity observed for synthesized half-cell is due to the interface between the GDC thin film and the substrate. However, it should be noted that the ac conductivity of bare substrate and that of structure are of the same order, revealing that the impedance from the interface is of negligible level. Also, the grain interior (GI) conductivity for synthesized halfcell was of the order of 0.1S/cm at 500 C. This is the desired ionic conductivity for successful operation of SOFC at intermediate temperature. These observations are in favor of implementation of synthesized half-cell for IT-SOFCs.
4.
Conclusions
The half cell (GDC/NiO-GDC) prepared in this study reveals that the GDC film on anode-grade ceramic substrate has thickness of 13 mm with relative density of 98%. Also the electrical characterization revealed that the interface is also of negligible level. All these properties obtained for GDC/NiOGDC half-cell are the desirable characteristics for its implication in IT-SOFCs. The grain interior conductivity of such a half cell is w0.1S/cm at 500 C. Distinctly, the SPT synthesized highly sinterable GDC films prepared on anode with less impeding interface, offers an added advantage of co-sintering at as low as 1000 C. This ability of co-sintering at lower temperature avoids the electrode/electrolyte interfacial problems during fabrication of complete cell and thereby would allow better and reliable electrical performance during its operational life.
Acknowledgements The authors are very much thankful to UGC-DAE IUC Indore for providing the SEM characterization facilities. Dr. M.G. Chourashiya is thankful to CSIR, New Delhi for a senior research fellowship.
references
[1] Fuentes RO, Baker RT. Synthesis and properties of Gadolinium-doped ceria solid solutions for IT-SOFC electrolytes. Int J Hydrogen Energ 2008;33:3480e4. [2] Bevan DJM, Summerville E. In: Gschneider KA, Eyring L, editors. Handbook on the physics, chemistry of rare-earths. Amsterdam: North Holland; 1979. p. 4. [3] Kilner JA. Fast anion transport in solids. Solid State Ionics 1983;8:201e7. [4] Stafford RJ, Rothman SJ, Routbort JL. Effect of dopant size on the ionic conductivity of cubic stabilised ZrO2. Solid State Ionics 1989;37:67e72. [5] Sammes NM, Cai Z. Ionic conductivity of ceria/yttria stabilized zirconia electrolyte materials. Solid State Ionics 1997;100:39e44. [6] J. Petterson, Alfred University Doctoral Thesis, Alfred Ny. 2003. [7] Atkinson A. Chemically-induced stresses in gadoliniumdoped ceria solid oxide fuel cell electrolytes. Solid State Ionics 1997;95:249e58. [8] Steele BCH. Appraisal of Ce1yGdyO2y/2 electrolytes for ITSOFC operation at 500 C. Solid State Ionics 2000;129:95e110. [9] Hatchwell C, Sammes NM, Brown IWM. Effect of nonstoichiometry on the protonic and oxygen-ionic conductivity of Sr2(ScNb)O6: a complex perovskite. Solid State Ionics 1999; 126:201e7. [10] Doshi R, Richards VL, Carter JD, Wang X, Krumpelt M. Development of solid-oxide fuel cells that operate at 500 C. J Electrochem Soc 1999;146:1273e8. [11] De Souza S, Visco SJ, De Jonghe LC. Thin-film solid oxide fuel cell with high performance at low-temperature. Solid State Ionics 1997;98:57e61. [12] Kim JW, Virkar AV, Fung KZ, Mehta K, Singhal SC. Polarization effects in intermediate temperature, anode-supported solid oxide fuel cells. J Electrochem Soc 1999;146:69e78. [13] Xia C, Liu Simple MA, Approach Cost-Effective. To fabrication of dense ceramic Membranes on porous substrates. J Am Ceram Soc 2001;84:1903e5. [14] Jiang SP, Leng YJ, Chan SH, Khor KA. Development of (La, Sr) MnO3-based cathodes for intermediate temperature solid oxide fuel cells. Electrochem Solid-State Lett 2003;6:A67e70. [15] Song HZ, Wang HB, Zha SW, Peng DK, Meng GY. Aerosolassisted MOCVD growth of Gd2O3-doped CeO2 thin SOFC electrolyte film on anode substrate. Solid State Ionics 2003; 156:249e54. [16] Yonghong Kong, Bin Hua, Jian Pu, Bo Chi, Li Jian. A costeffective process for fabrication of metal-supported solid oxide fuel cells. Int J Hydrogen Energ 2010;35:4592e6. [17] Xia C, Liu M. Microstructures, conductivities, and electrochemical properties of Ce0.9Gd0.1O2 and GDCeNi anodes for low-temperature SOFCs. Solid State Ionics 2002; 152:423e30. [18] Choy KL. In: Lee WE, editor. British ceramic proceedings, ceramic films and coatings, vol. 54. The Institute of Materials; 1995. p. 65. [19] Choy KL, Bai W, Steele BCH. In: Mcevoy AJ, Nisancioglu K, editors. Materials, processes. 10th SOFC Workshop, Les Diablerets, Int. Energy Agency; 1997. p. 233. [20] Fu CJ, Liu QL, Chan SH, Ge XM, Pasciak G. Effects of transition metal oxides on the densification of thin-film GDC electrolyte and on the performance of intermediatetemperature SOFC. Int J Hydrogen Energy 2010;35:11200e7. [21] Xia CR, Chen FL, Liu ML. Reduced-temperature solid oxide fuel cells fabricated by screen printing. Electrochem SolidState Lett 2001;4:A52e4.
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[22] Xia C, Liu M. Novel cathodes for low-temperature solid oxide fuel cells. Adv Mater 2002;14:521e3. [23] Chourashiya MG, Pawar SH, Jadhav LD. Synthesis and characterization of Gd0.1Ce0.9O1.95 thin films by spray pyrolysis technique. App Surf Sci 2008;254:3431e5. [24] Aruna ST, Muthuraman M, Patil KC. Synthesis and properties of Ni-YSZ cermet: anode material for solid oxide fuel cells. Solid State Ionics 1998;111:45e51. [25] Badwal SPS, Drennan J. Yttria-zirconia: effect of microstructure on conductivity. J Mater Sci 1987;22: 3231e9.
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[26] Duncan H, Laxia A. Influence of the electrode nature on conductivity measurements of gadolinia-doped ceria. Solid State Ionics 2005;176:1429e37. [27] Rajpure KY, Bhosale CH. A study of substrate variation effects on the properties of n-Sb2S3 thin film/polyiodide/C photoelectrochemical solar cells. Mater Chem Phys 2000; 64:14e9. [28] Wang DY, Nowick AS. The grain boundary effect in doped ceria solid electrolytes. J Solid State Chem 1980;35:325e33. [29] Adham KE, Hammou A. “Grain boundary effect” on ceria based solid solutions. Solid State Ionics 1983;9/10:905e12.
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Synthesis and characterization of 10%Gd doped ceria (GDC) deposited on NiO-GDC anode-grade-ceramic substrate as half cell for IT-SOFC M.G. Chourashiya a,*, L.D. Jadhav b a b
Department of Physics, Shivaji University, Kolhapur 416 004, India Department of Physics, Rajaram College, Kolhapur 416 004, India
article info
abstract
Article history:
In the present research work spray pyrolysis technique (SPT) is employed to synthesize
Received 1 August 2010
GDC (10%Gd doped ceria) thin films on anode-grade-ceramic substrate (porous NiO-GDC).
Received in revised form
The film/substrate structure was characterized for their micro-structural and electrical
29 November 2010
properties along with their interfacial-quality. By optimization of preparative parameters
Accepted 18 December 2010
of SPT and modification of surface of anode-grade ceramic substrate, we were able to
Available online 26 January 2011
prepare the GDC films having thickness of the order of 13 mm on NiO-GDC substrate. Further to improve the interfacial quality and densification of film, annealing of structure
Keywords:
at 1000 C for 8 h was carried out which leads to fully dense (>96%) GDC films, forming
GDC thin films
a gas-tight interface with substrate. Impedance measurements revealed that grain interior
Spray pyrolysis
conductivity for GDC/NiO-GDC half-cell was of the order of 0.1S/cm at 500 C which is the
Impedance spectroscopy
desired conductivity for successful operation of IT-SOFC. The activation energy for grain
IT-SOFC
interior and grain-boundary conduction estimated for GDC/NiO-GDC was 1.07 eV and 0.93 eV, respectively. Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Solid oxide fuel cells (SOFCs) have received much attention as promising power generation device because of their high conversion efficiency of chemical energy to electric power. Comparable to other types of fuel cells, SOFCs have the advantage of fuel flexibility i.e. are able to work with hydrogen, hydrocarbon reformate and, in some conditions, with hydrocarbon fuels directly. Commercial solid electrolyte materials for SOFCs are based on stabilized zirconia which requires to be operated at 900e1000 C to ensure the sufficient ionic conductivity. However, the high temperature operation of SOFC demands stringent requirements for other cell components and must be fabricated from high cost ceramic materials
to resist its thermal decomposition at these temperatures. If the operating temperature could be reduced to intermediate temperatures (500e700 C), less expensive materials, such as steel, could be used in these auxiliary components. Electrolytes based on ceria are good candidates for intermediate temperature (IT-) SOFC, due to their inherent property of higher ionic conductivity at these temperatures [1]. Ceria-based electrolytes have relatively large unit cells compared to systems based on zirconia and, as a consequence, ceria-based systems have larger channels through which oxygen ions can pass during the conduction. However, Bevan and Summerville [2] demonstrated using lattice parameter data, that the introduction of the Gdþ3 ions into the CeO2 lattice minimizes unit cell expansion or contraction.
* Corresponding author. E-mail address: [email protected] (M.G. Chourashiya). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.12.083
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This fact was further confirmed by Kilner [3] and concluded that the CeO2eGd2O3 system is one of the highest ionically conducting, ceria-based binary systems. Since, the substitution of larger dopants (than Gd) into ceria crystal lattices blocks the path of the migrating oxygen ions more effectively, thus increasing the ion-migration enthalpy [4]. However, even having an excellent ionic conduction behavior, Gd doped ceria has a relatively narrow region of oxide ion conductivity as an electronic component is introduced on reduction [5], producing a short circuit in the cell. When the electrolyte is subjected to partial pressure of oxygen typically associated with the anode side of a fuel cell, its electronic contribution to the overall conductivity increases, due to the reduction of Ce4þ to a Ce3þ state, resulting in failure of the electrolyte material [6]. Under these condition the doped ceria is partially reduced and its lattice parameter increases leading to the generation of stress in the electrolyte. Atkinson [7] calculated these stresses for a range of different parameters which includes doping level (Gd), temperature, oxygen activity, etc. and indicated that the maximum ‘safe’ operating temperature for Gd doped Ceria at an anode is about 750 C. To address the significant problem with ceria based materials of poor stability at low partial pressure of oxygen, a number of approaches have been taken, such as finding the ideal doping level and ion to balance stability with adequate oxide ion conductivity, decreasing the operating temperature and improving materials processing etc. In such an effort to minimize the instability of Gd doped ceria at lower oxygen partial pressures, other ceria based materials of the general formula Ce1-xMxO2-d, where M ¼ Gd, Sm, Ca, Mg have been studied [8,9]. However, Gd0.1Ce0.9O1.9 was the preferred choice for the electrolyte, with a compromise between higher stability to reduction and good oxide ion conductivity at 600 C; as higher Gd concentration materials are more readily reduced, though the oxide ion conductivity is increased. Doshi et al. have shown that Ce0.8Gd0.2O1.9 is purely an ionic conductor in fuel environments only below 450 C [10]. Decreasing the temperature of operation for Gd doped ceria to 600 C appears to minimize the reduction of the electrolyte; however, lower temperatures result in the greater loss of power density due to the decrease in the ionic conductivity. There are two main approaches to overcome these problems: the first is to decrease the electrolyte thickness [11,12] and the second is to use electrolyte materials with high ionic conductivity at low temperature such as doped ceria [13]. In addition, reduction of the electrode polarization resistance [14] also would be helpful in reducing the operating temperature. The reduced thickness of dense electrolyte minimizes the internal ohmic losses [15] leading to higher conductivity at lower temperatures. Thus, thin film technology could enable Gd doped ceria electrolytes to be operated at further reduced temperature [16]. In such cases, according to Xia and Liu [17], partial internal shorting arising from the electronic conduction of Gd doped ceria becomes insignificant. Moreover, according to Atkinson [7], thin film technology not only helps to reduce the operating temperature of cell but also minimize the risk of fracture in ceria based electrolytes during thermal cycling of cell. Among the different thin film fabrication techniques spray pyrolysis technique (SPT) seems to satisfy
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the desired criteria for preparation of thin solid electrolyte [18,19]. The versatility of the SPT and requirement of fabrication of dense and thin solid electrolyte films on functional ceramic substrates (non-conducting at deposition temperatures) makes SPT a most suitable technique for in-situ fabrication of electrode/electrolyte half-cell for solid oxide fuel cells. Apart from this, unlike many other film deposition techniques, SPT represents a very simple and relatively costeffective method. In present research, all the three approaches [1,16,20], i.e. (a) implementation of thin film electrolyte (spray pyrolysis), (b) use of high ion-conducting electrolyte (10%Gd doped ceria e referred as GDC hereinafter) and (c) lowering the electrode/electrolyte polarizations (depositing the electrolyte on electrode substrate), are emphasized. The preparative parameters of spray pyrolysis technique (SPT) such as precursor solution concentration and substrate temperature were optimized to deposit dense and adherent films of GDC on NiO-GDC (anode-grade) substrate. The anode-grade ceramic substrates of NiO-GDC were prepared by conventional ceramic route. The structural, morphological (surface and fractured) and electrical characterization were carried out to investigate the competence of fabrication technique and fabricated halfcell for its application in IT-SOFCs.
2.
Experimental
2.1.
Ceramic substrate preparation
The powders of Gd2O3 (AR grade, 99.9%) and CeO2 (AR grade, 99.9%) from HIMEDIA Inc., were mixed in 1:9 proportion with an agate mortar. The mixed powder was then calcined at 1200 C/4 h in air, to obtain the GDC powder. The calcined GDC powder was then reground and mixed with NiO (Extra pure, AR grade from HIMEDIA Inc.) in desired proportion to obtain the composite phase of (NiO)30-(GDC)70 e referred as NiO-GDC hereinafter. The selection of ceramic composition of NiO30GDC70 (precursor composite for Ni-GDC anode) substrate was decided on the basis of their viability in prospective use as an anode of half-cell for IT-SOFC [21,22]. The mixed powder was then again reground with organic binder (Poly-Vinyl Alcohol e PVA). The binder added powder was pelletized with the help of hydraulic press machine. The green samples then sintered at 1400 C for 8 h in air. The pre-sintering (binder removal step) of samples were intentionally excluded, as the binder removal step and high sintering temperature (>1500 C) is usually employed to obtain dense ceramic bodies and here we are expecting for porous structured (anode-grade) NiO-GDC substrates. This aspect is discussed in results and discussion section. The dimensions of the sintered substrates were 0.12 cm in thickness and 2.5 cm in diameter. To modify the surface properties of the substrates, NiO-GDC samples were heat treated in reducing atmosphere of 5%H2e95%Ar with gas flow rate of 500 ml/min at 900 C for 5 h. The reduction treatment leads to formation of comparably more porous structured and rough surfaced Ni-GDC composite (referred as Ni-GDC hereinafter) ceramic substrates. As the SPT is surface sensitive technique, this rough surfaced (Ni-GDC) substrate showed the positive effects during film synthesis (discussed later).
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Moreover, to keep the surface of substrates flat, during the heat treatments an alumina (Al2O3) sheet is placed over the surface of NiO-GDC samples.
2.2.
Thin film synthesis
Aqueous solution of cerium nitrate and gadolinium nitrate (99.9% Pure; ALFA AESAR) were mixed in desired proportion to obtain the (10%Gd doped ceria) GDC thin films. The mixed solution was then sprayed by glass nozzle with air as carrier gas on ceramic substrates (kept at preset temperature). The optimization of preparative parameters of SPT was carried out to form dense and good quality GDC thin films on ceramic substrates. The similar optimization procedure as that was applied to obtain GDC thin films on glass substrate is employed here, which is reported elsewhere [23]. Due to nonadherence of GDC films on NiO-GDC substrate, we deposited the GDC thin films on Ni-GDC substrate. The deposition of GDC thin films onto anode-grade ceramic substrate (Ni-GDC) were carried out using optimized preparative parameters of SPT (i.e. precursor solution with concentration of 0.04M and substrate temperature of 250 C). Moreover, the faultlessness in interface of GDC film with ceramic substrate was achieved by subsequent heat treatment at 1000 C for 8 h in air. The post heat treatment resulted in dense films along with gas-tight interface; however the NiGDC phase of substrate again transformed to NiO-GDC. The schematic of workflow which was followed during the present work is shown in Fig. 1.
2.3.
Characterizations
The morphological and structural characterizations of deposited GDC films, and composite ceramic substrates at various
stages were studied using SEM (JEOL-JSM-6360, Japan) and XRD (PHILIPSePW-3710) with Cu-Ka radiation source, respectively. In order to distinguish the different phases in the composite samples, the Back Scattered Electron (BSE) mode was employed to obtain the BSE image. The electrical characterization of GDC/ NiO-GDC structure were performed using ac impedance measurements (SOLARTRONe1260; impedance analyzer) for temperatures ranging from 250 C to 500 C. The grain interior and grain boundary impedances were extracted by analyzing the collected impedance data using ZView Version 2.4a e freeware software. 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 mm and spring constant of 0.58 N/m. The surface roughness was determined from the AFM images.
3.
Results and discussion
3.1. Synthesis and characterizations of NiO-GDC composite ceramic substrates A general procedure for fabrication of ceramic samples includes 5 major steps, namely, (1) raw ceramic powder categorizations, (2) synthesis of ceramic powders, (3) green body formation, (4) pre-sintering and (5) final sintering. Therefore initially, to prepare raw GDC powder, CeO2 (ceria) and Gd2O3 (as starting materials) were characterized for their structural properties. Fig. 2 shows the XRD pattern of starting and calcined powders. These XRD patterns were compared with their respective JCPDS files as,-PDF No. 81-0792 for CeO2 and 76-0155 for Gd2O3 and indexed accordingly. Both the powders possesses cubic lattice structure with lattice
Fig. 1 e The schematic of work-flow which was followed during the present work. Left: Ceramic substrate synthesis. Right: Thin films synthesis.
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Fig. 2 e XRD patterns of commercially available CeO2, Gd2O3 and GDC powders calcined at 1200 C/4 h.
˚ for CeO2 (std. ‘a’ ¼ 5.410 A ˚ ) and 10.80 A ˚ for parameter of 5.411 A ˚ ). The crystallite size calculated by Gd2O3 (std. ‘a’ ¼ 10.79 A Scherer’s formula for both powders was around 35e45 nm. In ceramic processing, starting chemicals having the narrow particle size distribution is advantageous, as it facilitates uniform sintering of ceramic bodies. Further, these solid phase reactant of CeO2 and Gd2O3 in calculated amount of weights (9: 1) were mixed in agate mortar. The homogenization was confirmed by observing the change in color of powder which turned into yellowish white after thorough mixing. The color of starting powder of ceria (CeO2) was yellow while that of Gd2O3 was white. After proper mixing of the starting chemicals, the mixture was calcinated in furnace at 1200 C for 4 h in air. This heat treatment leads to formation of GDC phase which was confirmed by XRD (Fig. 2). Now to prepare composite ceramic substrate (NiO-GDC), the calcined GDC powder was mixed with NiO in an agate mortar. After homogenization the PVA is added to mixture and was again grinded in agate mortar. The binder added powders were pressed in circular disk shaped pellets (green sample). However, before pressing the powders in pellet form, uniform tapping of the die (filled with powders) was carefully adapted. Since in dry pressing, tapping increases the packing density and eliminate any in-homogeneities in the packing. If the die (mold) is not filled homogeneously, the final pressed shape will be very different from that of the die. The fourth (i.e. pre-sintering) step is usually followed to improve the densification rate during final sintering step. But for the anode-grade ceramic substrate it is desirable to have the porous structured morphology. Thus, this step was not followed here. The green bodies (pressed samples) consists a mass of ceramic powder held together by organic polymer e binder (PVA), which are distributed at the particleeparticle contacts and gave the strength to the green body. When necessary heat of evaporation for polymer drying and the heat of reaction, either exothermic or endothermic, for polymer thermal decomposition were supplied, the evaporation and thermal decomposition of PVA in green body starts. Here, this occurs during the sintering step of NiO-GDC green samples. Both evaporation and thermal decomposition of polymer gives off huge volumes of gases. These gases diffuse through the porous network of the green body and exert different types
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of stresses, which includes thermal gradients, gas pressure gradient, liquid pressure gradient, etc. These stresses act to warp or crack the green body. Hence it is very important to control the heating rate of green body in furnace. Here, the heating rate of 3 C/min was kept for all the ceramic samples. These highly pressurized evolved gases escape to ambiance keeping the pores in the NiO-GDC substrates. The density of NiO-GDC samples were estimated using standard Archimedes principle and its estimated relative density was 79%. The relative density of prepared samples were estimated using the relation,% density ¼ ddthm 100 , where, dm is the density of samples measured using Archimedes principle and dth the theoretical density given as, dth ¼ NA4a3 ½ð1 xÞMCe þ x,MGd þð2 12,xÞMO , where, 0.1 is gadolinium content, a the lattice constant at room temperatures, Na the Avogadro number (6.023 1023), and M refers to the respective atomic weights. As mentioned above, in the primary stage of work it was planned to deposit GDC on NiO-GDC to form GDC/NiO-GDC precursor half-cell, which were further planned to reduce insitu to get electrolyte/anode (GDC/Ni-GDC) half-cells. However, an attempt to deposit GDC films onto NiO-GDC substrate results into non-adherent films with thickness less than 2 mm. An electrolyte is supposed to have the thickness of the order of 10 mm to avoid the gas-cross overs during its operation. Thus, to improve the film thickness and adherence with substrate it was proposed to reduce the NiO-GDC substrate to obtain porous with rough surfaced sample and then to deposit GDC film on it. Fig. 3 shows the XRD patterns of NiO-GDC at various stages. In all three stages of processing of NiO-GDC substrates, the absence of peaks corresponding to Gd2O3 and comparatively ˚ ) than that of the host lattice larger lattice parameter (w5.419 A ˚ ) confirms the complete dissolution of Gd2O3 in of ceria (5.410 A ceria lattice, to form GDC. The XRD pattern of as-synthesized NiO-GDC was compared with JCPDS-PDF no. 75-0161 (GDC) and 78-0643 (Cubic NiO) to confirm the individual phase peaks and was accordingly indexed in Fig. 3a. All the reflection peaks could be either indexed to NiO or GDC, confirming formation of composite without any undesirable phases. In addition to that the lattice parameters calculated for each of phase are in well agreement with that of respective standard values. The ˚ average lattice parameters for GDC and NiO phase are 5.419 A ˚ , respectively. These observations affirm that NiO and 3.540 A phase remains separate and does not diffuse into the GDC
Fig. 3 e XRD patterns of ceramic substrates at different stages. (a) as-synthesized NiO-GDC, (b) Ni-GDC and (c) after-oxidation NiO-GDC.
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lattice to form the undesirable phase. The average crystallite size estimated for GDC and NiO phases are 581 nm 343 nm, respectively. The XRD pattern of reduced sample i.e. of Ni-GDC (Fig. 3b) was compared with JCPDS-PDF no. 75-0161 (GDC) and 04-0850 (Cubic Ni). The peaks are indexed accordingly in the Fig. 3b and no peaks corresponding to NiO-phase were observed, confirming the complete reduction of sample under investigation. The average lattice parameters calculated for GDC and ˚ and 4.169 A ˚ , respectively. The average Ni phases are 5.418 A crystallite size calculated using Scherrer’s formula for GDC and Ni phases are 575 nm 403 nm, respectively. These reduced samples employed to synthesize the GDC/Ni-GDC half-cell and further heat treated in air for improvement of crystallinity in the film and quality of the interface. Upon heat treatment, the Ni-GDC substrates were oxidized and become NiO-GDC again, which was confirmed by comparing its XRD pattern with JCPDS files (Fig. 3c). No reflection peak due to Ni was observed, which confirms the complete oxidation of Ni to NiO. The calculated lattice parameters for each phase were in agreement with that of respective standard values from JCPDS. The average lattice parameters for GDC and NiO phases ˚ and 3.541 A ˚ , respectively. Apart from the similarare 5.419 A ities in lattice parameters of as-synthesized NiO-GDC and after-oxidation NiO-GDC substrate, the crystallite size of NiO phase in later case is higher (610 nm) than as-synthesized NiO-GDC (443 nm) samples. Fig. 4 shows the SEM and BSE images (insets) of the NiOGDC ceramic substrates at different stages. The surface morphology of the NiO-GDC (sintered at 1400 C/8 h) is porous (Fig. 4a). The porous nature of NiO-GDC samples is attributed to the procedure employed for its synthesis (without 4th-step). One can also attribute the same to its composition, as the composition involves two phases (composite) either of which avoids the lattice diffusion (which is the major process of densification) at particular sintering temperature. These porous NiO-GDC (green in color) ceramic samples, upon reduction in H2 atmosphere at 900 C, turns into more porous Ni-GDC (dark black in color) samples (Fig. 4b). 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. 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 re-oxidation of Ni into NiO phase, confirmed by XRD (Fig. 3c). The after-oxidation NiO-GDC showed comparatively larger grains than as-synthesized NiO-GDC. The inspection SEM and BSE images of composite substrates (Fig. 4aec) clearly reveals that there are two phase grains, as expected, one is of GDC phase and other is of NiO/Ni phase. The grains of NiO in assynthesized NiO-GDC (Fig. 4a) sample undergo decrease in size after reduction treatment and can be clearly seen as clusters of comparatively smaller Ni grains embedded in GDC grains (Fig. 4b). Further when the Ni-GDC composite is heat treated in air at 1000 C, the transformation of Ni to NiO, again leads to increase in grain size of NiO (Fig. 4c). In ‘after-oxidation’ NiO-GDC samples, the re-agglomeration of grains takes place and leads to formation of comparatively large clusters and pores in the sample. Apart from these morphological dissimilarities, the relative densities of composite samples
varied with change in compositions. The Ni-GDC sample possesses least relative density of 71%, while that of assynthesized and after-oxidation NiO-GDC possesses approximately same relative density of 79% [24,25]. The complex impedance plot of after-oxidation NiO-GDC sample recorded at 290 C is shown in Fig. 5a. It shows the usual trend of three semicircles in complex impedance plots corresponding to electrode, grain boundary (GB) and grain interior (GI) impedance contributions to total impedance of the system. In line with other polycrystalline samples complex impedance plots for substrates showed the gradual decrease in impedances with an increase in measurement temperature. The criterion for ‘how the half-circles in impedance plots are assigned to GI and GB’ is discussed in next section. The impedance data were further analyzed by impedance analysis software to extract the grain interior (Rg) and grain boundary impedances (Rgb). While analyzing the impedance data, the data points corresponding to electrode contribution was skipped to avoid the complexities in extraction of Rg and Rgb. In accordance with Duncan and Laxia [26], the use of 2CPE circuit is employed here to obtain the best fit for observed impedance data, which adequately allowed us to determine the resistance of system under investigation. This fitting cannot be considered for interpretation of the exact physical processes that would occur in such systems. The values of Rg and Rgb obtained from the fit parameters were used to calculate ac conductivities and were further fitted to the modified Arrhenius relation. The modified Arrhenius a relation is given as,sT ¼ s0 expðE kT Þ , where Ea is the activation energy for the conduction process, T the temperature, s0 the pre-exponential factor and k the Boltzmann constant. Fig. 5b shows the Arrhenius plot for after-oxidation NiO-GDC substrate. The values of GI and GB conductivity measured at 500 C for after-oxidation NiO-GDC substrate are 0.107S/cm and 0.0009 S/cm. respectively. The activation energies calculated from Arrhenius plots for after-oxidation NiO-GDC substrate for GI and GB conductivity are 0.81 eV and 0.9 eV, respectively.
3.2. Synthesis and characterizations of GDC thin films on NiO-GDC ceramic substrate There are number of processes occur either sequentially or simultaneously during film formation by spray pyrolysis. The mechanism of thin film deposition by spray pyrolysis can be divided into three main steps: atomization of the precursor solution, transport of the resultant aerosol and decomposition of the precursor on the substrate. The first step in spray pyrolysis is generation of spray with the help of atomizers. In atomizers the filled precursor solution is transformed into fine droplets and forced or directed towards the substrates. There are three types (air blast, ultrasonic and electrostatic) of atomizers which are usually employed in SPT which have their own parameter to control the droplet distribution. Here we have employed air-blast type of atomizer which is relatively simple and cost-effective than the others. The morphological properties of the films directly depend upon these droplet distributions. If the droplet distribution is narrow the films are observed to have uniform grain sizes. In air-blast type atomizer the atomizing medium is air and the
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Fig. 4 e SEM of NiO-GDC ceramic samples at different stages. (a) as-synthesized NiO-GDC, (b) Ni-GDC and (c) after-oxidation NiO-GDC. Insets shows BSE images of respective samples.
resultant droplet distribution of solution with air is referred as “Aerosol”. After atomization this aerosol is transported to substrate where the deposition of the metal salt in thin film form occurs. The energy required for transport of aerosol is gained from the driving force of atomizing medium. During the transport of aerosol towards the heated substrate, initially aerosol gets warmed and undergoes vaporization of solvent and leads to a size reduction (shrinkage) of the droplet and lead to the development of a concentration gradient within
the droplet. As the droplet moves to heated substrate, continuous vaporization of solvent takes place and consequently leads to formation of precipitate. The precipitation of the precursor in droplet occurs as the concentration exceeds the solubility limit. When the precipitate reaches closer to the substrate, the sudden increase in air temperature leads to pyrolytic decomposition of the precipitate and forms the solid particles. The mechanism of aerosol transition as function of temperature gradient is shown in Fig. 6. These various
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Fig. 5 e (a) Complex impedance plot of after-oxidation NiO-GDC substrate measured at 290 C and (b) variation of ln(sacT ) as function of 1000/T for after-oxidation NiO-GDC substrate.
processes during the aerosol transport are mainly responsible for the structural and morphological properties of spray deposited films. The sufficient temperature gradient is thus necessary for complete pyrolysis of the spray. If the temperature is not optimized the evaporation of solvent may be in premature (wet) or exceedingly matured (completely dried) state. The earlier case (i.e. premature state) is deliberately
Fig. 6 e a) Temperature gradient as function of distance from substrate. Inset: Transition of aerosol as function of temperature gradient produced by heated substrate, during aerosol transport.
used for preparation of films on non-sticky substrates while later case (i.e. exceedingly matured state) is used for preparation of ultrafine powders. When the precipitate reaches the substrate, the precipitate undergoes decomposition and then nucleation and growth of thin films on the substrate take place. Many processes occur simultaneously when a droplet hits the surface of the substrate, namely, evaporation of residual solvent, spreading of the droplet and salt decomposition. After decomposition of the precursor on the substrate the subsequent nucleation and grain growth results in the formation of film. A droplet hitting the surface may stick, rebound, spread or splash on the surface. After droplet impact, the physical and chemical properties of the substrate play an impotent role to influence the film nucleation and growth. The surface of substrate provides the nucleation site for film growth. During the deposition of GDC thin films on NiO-GDC ceramic substrate, it was observed that the deposited films were non-adherent to substrate, which is undesirable for its prospective application. This non-adherence of the film with substrate could lead to faulty-cell fabrication which eventually may lead to degradation in cell performance or in worst case, failure of fuel cell. The non-adherence of the film may be attributed to the relatively dense and smooth surface of assynthesized NiO-GDC substrates. As mentioned earlier SPT is sensitive to surfaces of substrates, it was planned to modify the surface by heating the substrate samples in reducing atmosphere. The heating of substrates in reducing atmosphere served the dual purposes. Firstly, this heat treatment in
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Fig. 7 e Surface and fractured morphology of GDC thin film deposited on Ni-GDC and annealed at 450 C/4 h. reducing atmosphere leads to transfer of NiO-GDC substrate to Ni-GDC, which is anode for ceria based fuel cell. Secondly, the substrate surfaces become rough and porous. The transformation of substrate on one hand gave more nucleation sites for growth of film by the virtue of rough and porous surfaces, but on other hand the metallic species (Ni) from substrate are
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also exposed to surfaces. In fact, it is quite difficult to deposit the thin films, using SPT, on metallic substrates. It is observed fact that the presence of metallic species in the substrate (e.g. Ni in cease of Ni-GDC) at elevated temperatures (greater than room temperatures) hinders the stabilization of sprayed droplets on to the substrate. To get the films deposited on to such substrates, a method of wet-film-formation during spray pyrolysis was employed. In this method, the films are deposited at temperatures lower than decomposition temperature (i.e. at 250 C < decomposition temperature ¼ 280 C [21]) of precursor and subsequently heat treated to a higher temperature to complete the decomposition of deposited materials. These wet films then consequently heated to complete decomposition of entrapped solvent in the films. This is not a good practice to prepare the films by SPT as the films contains high amount of solvent (water in our case) and which might lead to cracking or peeling-off of the films during heat treatment. Therefore the deposited films were heat treated at 450 C with a controlled and slow heating rate (1 C/min). This heat treatment not only dries deposited films but also improves crystallinity and avoids warping/cracking of films with time span [23]. On one hand wet film formation depicts the danger of peeling-effect in deposited films (if kept un-heat treated) but on the other hand it results in higher thickness in a single run. The wet droplets pile up to comparably higher thickness,
Fig. 8 e (a) Surface and fractured morphology of GDC thin film deposited on NiO-GDC and post-annealed at 1000 C/8 h. (b) AFM image of GDC thin film.
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which can be attributed to higher thickness (w13 mm) obtained for single deposition (Figs. 7 and 8a). The decreased substrate temperature for highly thermal conducting (e.g. metallic) substrate is also reported in literature [27]; however, its theoretical understanding is not clear. Formation of nano-crystallites during thin film deposition is the characteristic of SPT. The sprayed droplet when approaches to substrate, experience high thermal-gradient at substrate (Fig. 6) and get rapidly pyrolized. After the pyrolysis, the decomposed solid particle, in general, possess the dimensions of few nanometers. These accumulated nanometer sized particles undergo sintering and partial grain growth at deposition temperature (Fig. 7). Further crystallization and grain growth in films occurs upon the post-annealing treatment (Fig. 8a), but the grain remains in nanometer size with improved compactness (Fig. 8b). In our case, we carried out the post-annealing treatment at 1000 C for 8 h. This postannealing heat treatment leads to uniform and adherent interface between film and substrate. Moreover the surface morphology of the film was also observed to improve (Fig. 8a). If the relative density of fractured SEM of films is compared with that of ceramic substrates sintered at 1400 C (79%), it can be estimated that the relative density of the films are quite higher. In the cross-section of film, there are virtually no pores as that can be seen in the fractured SEM of ceramic substrate. This type of highly compact and dense microstructure is one of the most desirable characteristics for its application as solid electrolyte. In case of bulk sample, such high density for GDC can only be achieved at sintering temperature greater than 1500 C while it was achieved here (in the case of thin films) only at 1000 C. This higher densification rates in thins films are attributed to the presence of nano-granules having average grain size of 85 nm (Fig. 8b). Further analysis of surface morphology of films using AFM revealed that film possesses the surface roughness of 8.86 nm. Fig. 9 shows the XRD pattern of GDC film deposited on afteroxidation NiO-GDC substrate prepared with optimized preparative parameters of SPT. The thickness of film was large enough (w13 mm) to suppress the NiO peaks originating from subs˚ , which trate. The lattice parameter of the GDC film was 5.420 A is nearly same as that of standard lattice parameter for GDC. To estimate the competence of the GDC film deposited on anode grade (after-oxidation) NiO-GDC ceramic substrate as
a solid electrolyte and electrode assembly (half-cell), the halfcell was tested for its electrochemical properties using Electrochemical Impedance Spectroscopy (EIS). However, thin films having uneven thickness/pores could lead to failure of half-cell testing by shortening the current-collector with electrode via thin electrolyte. This doubt is clearly depicted in its schematic (Fig. 10a). The fractured SEM image of platinum coated half-cell, imaged after the impedance measurements (Fig. 10b) shows that the platinum paste remained on the surface of the film and has not penetrated/diffused into the film to short with substrate surface. Inset of Fig. 10b shows the uniform coating of platinum paste over the surface of GDC films. EI spectra of half-cell showed typical trend of three semicircles of a polycrystalline ionic conductors (Fig. 11a). As the half-cell involves various interfaces, even within their individual microstructures, extracting the information of the corresponding to various physical processes involved within it is a quite complex task. Moreover, the conductivity across GDC (ionic) and NiO grains (electronic-holes) would add different responses to impedance spectra leading to increased complexities. Therefore for the sake of estimation of competence of prepared half-cell, we employed a simple approach. Here, we simply assign three semicircles (from right) observed in impedance plots to correspond to sluggish, intermediary, 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 108 and 1011 F/cm2, respectively. These typical values of capacitances, in general, originate from GB polarization in a polycrystalline material (w108F/cm2) and that of from dielectric relaxation of bulk (GI) material (w1011 F/cm2). Hence, this resemblance is further stretched and these processes are referred as GI (fast) and GB (intermediate) processes, and the respective conductivities as GI and GB conductivities. The impedance data were analyzed by impedance analysis software (ZView Version 2.4a) and used to extract the grain interior (Rg) and grain boundary impedances (Rgb). The typical fit result from the analysis, for GDC film deposited on after-oxidation NiO-GDC substrate is shown in Fig. 11b. 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 to estimate the activation energies.
Fig. 9 e The XRD pattern of GDC film deposited on after-oxidation NiO-GDC substrate.
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Fig. 10 e (a) Schematic of system formed by ‘Pt’ electrode/film-substrate/‘Pt’ electrode for impedance measurement. Right part shows the doubtful penetration of ‘pt’ paste. (b) SEM of tri-layer of ‘pt’ paste/film/substrate. Inset: ‘Pt’ paste coated onto surface of substrate.
Fig. 11 e (a) Complex impedance plot of half-cell and (b) The typical fit result from the analysis, for GDC/NiO-GDC half-cell substrate.
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The GI and GB conductivity for GDC film deposited on afteroxidation NiO-GDC substrate at 500 C are 0.103 S/cm and 0.0005 S/cm, respectively. The decrease in these values compared to those for substrates are attributed to the increased number of interfaces. The calculated activation energies for GI and GB conductivity are 1.02 eV and 0.93 eV, respectively. For substrate as well as half-cell, the GI conductivity is constantly dominated the GB conductivity. The amount of GI dominance in synthesized half-cell was two times than that of after-oxidation NiO-GDC substrate. As the GB processes, particularly in polycrystalline ionic conductors, are influenced by the presence of impurities, defects, concentration gradients, double layer capacitances across the grain boundaries, etc., the GB conductivities are always relatively smaller than that of GI conductivities. Also it is known fact for ceria systems that the “grain boundary effect” was greatest for dilute solid solutions (e.g. 10%Gd doping, as it appears here) and the activation energy for conductivity along the GB were higher than that of GI conductivity [28,29]. This is observed to be true for after-oxidation NiO-GDC substrate, however, the difference in activation energies are small. Comparatively higher Ea for GI conductivity than that of GB conductivity observed for synthesized half-cell is due to the interface between the GDC thin film and the substrate. However, it should be noted that the ac conductivity of bare substrate and that of structure are of the same order, revealing that the impedance from the interface is of negligible level. Also, the grain interior (GI) conductivity for synthesized halfcell was of the order of 0.1S/cm at 500 C. This is the desired ionic conductivity for successful operation of SOFC at intermediate temperature. These observations are in favor of implementation of synthesized half-cell for IT-SOFCs.
4.
Conclusions
The half cell (GDC/NiO-GDC) prepared in this study reveals that the GDC film on anode-grade ceramic substrate has thickness of 13 mm with relative density of 98%. Also the electrical characterization revealed that the interface is also of negligible level. All these properties obtained for GDC/NiOGDC half-cell are the desirable characteristics for its implication in IT-SOFCs. The grain interior conductivity of such a half cell is w0.1S/cm at 500 C. Distinctly, the SPT synthesized highly sinterable GDC films prepared on anode with less impeding interface, offers an added advantage of co-sintering at as low as 1000 C. This ability of co-sintering at lower temperature avoids the electrode/electrolyte interfacial problems during fabrication of complete cell and thereby would allow better and reliable electrical performance during its operational life.
Acknowledgements The authors are very much thankful to UGC-DAE IUC Indore for providing the SEM characterization facilities. Dr. M.G. Chourashiya is thankful to CSIR, New Delhi for a senior research fellowship.
references
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