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NiO-GDC-BCY composites as an anode for SOFC
Accepted Manuscript NiO-GDC-BCY composites as an anode for SOFC S.T. Jadhav, V.R. Puri, L.D. Jadhav PII:
S0925-8388(16)31592-4
DOI:
10.1016/j.jallcom.2016.05.243
Reference:
JALCOM 37763
To appear in:
Journal of Alloys and Compounds
Received Date: 17 March 2016 Revised Date:
10 May 2016
Accepted Date: 22 May 2016
Please cite this article as: S.T. Jadhav, V.R. Puri, L.D. Jadhav, NiO-GDC-BCY composites as an anode for SOFC, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.05.243. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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NiO-GDC-BCY composites as an anode for SOFC S. T. Jadhav1, V. R. Puri1, L. D. Jadhav2* 1Department of Physics, Shivaji University, Kolhapur - 416 004, Maharashtra, India
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2Department of Physics, Rajaram College, Kolhapur – 416 004, Maharashtra, India
Graphical Abstract
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(B)- Biogas production plant (taken from internet) (C)- High temperature furnace used for the open circuit voltage measurement
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(A) Schematic presentation of setup for cell testing 1- Pellet (Sample), 2-Silver foils 3-Cu wire for connections, 4-Glass Setup 5 – Path for gas flow
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NiO-GDC-BCY composites as an anode for SOFC S. T. Jadhav1, V. R. Puri1, L. D. Jadhav2* 1Department of Physics, Shivaji University, Kolhapur - 416 004, Maharashtra, India 2Department of Physics, Rajaram College, Kolhapur – 416 004, Maharashtra, India
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Corresponding [email protected]
Abstract
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NiO-Ce0.9Gd0.1O1.95-BaCe0.8Y0.2O3-δ (NiO-GDC-BCY) anode based solid oxide fuel cell was fabricated, in which the composite powders of NiO-GDC and BCY were synthesized by
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solution combustion method and mixed by physical mixing. In the present case, the composite anode material was prepared with varying amount of GDC (2 wt% to 8 wt%) with respect to NiO (38 wt% to 32wt%) and studied their structural, morphological and electrical properties. The single cell was fabricated comprising NiO-GDC-BCY (34:6:60) anode, LSCF (cathode) and
atmospheres. 1 Introduction
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BaCe0.8Y0.2O3-δ (electrolyte). The open circuit voltage was measured in hydrogen and biogas
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Nickel based ceramic composite materials have been used due to their capability of electro catalysis. Also, chemical and mechanical stability, catalytic activity and cost make nickel
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a strong candidate as compared to other possible metals [1, 2]. Pure Ni has thermal expansion coefficient (TEC) higher than normally used oxide electrolytes [3-5]. Therefore, electrolyte phase is added to Ni to form a cermet (ceramic+metal) with TEC acceptably close to those of the other cell components and to inhibit coarsening of the metallic particles. Performance of Ni-BCY composite as an anode is determined by its electrical properties as well as by the homogeneity of anode microstructure. Also, the porosity of the anode must be tailored with regard to mass transport considerations as well as mechanical strength by employing the appropriate preparation
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method. In addition, anode must possess sufficiently high electronic and ionic conductivity in the reducing environment at the operating temperature for efficient triple phase boundary operations and to reduce the polarization losses as well.
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Another possibility is to use redox phase such as doped ceria and ceria-containing mixed oxides in order to improve catalyst stability. This support plays an important role on the mechanism of carbon removal during hydrocarbon redox [6-8]. The use of ceria-based oxides
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(such as gadolinium-doped ceria GDC) in the composition of SOFC anodes has been also investigated [9-15]. Ceria exhibits a mixed ionic and electronic conductivity due to the reduction
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of Ce4+ to Ce3+. This improves the electrocatalytic activity of the anode due to the increase of the reaction zone over the three-phase boundary. In addition, higher conductivities are obtained when ceria is doped with trivalent cations, such as Gd3+. Furthermore, the high oxygen storage capacity of ceria provides a high resistance to carbon deposition during ethanol reforming [7].
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Thus the present study explores the triple phase composite i.e. NiO-GDC-BCY as an anode for SOFC. In this composite, proton conducting electrolyte (BCY) has been synthesized by solution combustion technique with optimum oxidant to fuel ratio. Similarly, NiO and GDC
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are also synthesized by solution combustion technique. The nature of a catalyst has significant influence on carbon formation. Ni is electronic conducting as well as Ni is known to be an
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excellent catalyst towards cracking of hydrocarbons, so that carbon species are more liable to deposit on the surfaces of Ni. In order to use hydrocarbons as a fuel with anode, extensive efforts have been made to suppress carbon deposition, such as introducing oxides [16], so in present case GDC is added, which supports NiO to enhance catalytic activity of anode. The effect of GDC concentration on phase stability, microstructure and electrical properties is studied by
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keeping electrolyte phase constant (BaCe0.8Y0.2O3-δ). The variation of GDC concentration helps to get high performance anode for proton conducting SOFC. 2 Experimental
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NiO-Ce0.9Gd0.1O1.95-BaCe0.8Y0.2O3-δ (NiO-GDC-BCY) triple phase composites were prepared by physical mixing of these three phases. Single phase BCY powder was prepared by solution combustion synthesis as reported in ref. [17]. Similarly the NiO [18] and GDC [19]
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were also synthesized by solution combustion synthesis technique; Samples were prepared such that the reduced NiO-GDC-BCY composites would contain 40, 38, 36, 34 and 32 wt% of NiO
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and 0, 2, 4, 6 and 8 wt% of GDC. To increase porosity, 3% of polyvinyl alcohol (PVA) as pore former was also added in each mixture. After proper mixing and grinding, composite powders were pressed into circular pellets of 1.5 cm diameter. All pellets were heated at 700 oC to remove the PVA, followed by sintering at 1300 oC. These pellets were then reduced at 700 o
GDC-BCY cermet anode.
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C under constant flow of a 3% of hydrogen plus 97% argon to convert NiO-GDC-BCY into Ni-
2.1 Fabrication of cell (NiO-GDC-BCY-Anode)
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Anode supported single cell comprises of LSCF (La0.6Sr0.4Co0.8Fe0.2O3-δ, cathode), BCY (BaCe0.8Y0.2O3-δ, electrolyte) and NiO-GDC-BCY (anode). NiO-GDC-BCY powder containing
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6 % GDC has been chosen since it shows highest dc conductivity in both hydrogen and biogas atmospheres. NiO-GDC-BCY powder containing 5% of starch (0.3 g) was pressed in 15 mm diameter die and sintered at 1300 oC for about 5 h. Then electrolyte was deposited over the anode by spray deposition technique (100 mL solution with 0.5 M concentration); followed by sintering at 1000 oC. The details of spray pyrolysis can be found in [20]. The cathode material, LSCF, was synthesized by solution-combustion route, followed by calcinations at 600 oC for 5 h. Slurry of
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LSCF was prepared with terpineol and brush painted on top of the electrolyte layer. The cell thus prepared was co-fired at 1300 oC for 5 h. Silver paste is then employed on both anode and cathode sides followed by heating at 600 oC for 2 h [21].
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The open circuit voltage was measured using the set-up indigenously in our lab. The set up was made up of glass, which has facility to supply hydrogen/biogas at anode side and air at cathode side. The opposite surfaces of the cell are covered by silver, in order to provide the
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required best contact for the voltage measurements. The cells are placed between the two perforated silver foils kept fixed on the opposite surface of the sample by stainless steel clamps
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and is then fixed in the set up. Before carrying out the OCV measurements, the temperature inside the cell is slowly raised (1 °C/min) up to about 600 °C in order to assure the softening of silver foils. After this thermal treatment, hydrogen/biogas and oxygen can be fluxed, respectively, at the anode side and at the cathode side without any gas loss.
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The structural properties of prepared composite pellet were characterized by X-ray diffractometer with (PHILIPS PW-3710) Cu Kα as radiation source. Morphological properties were analyzed using field emission scanning electron microscope (FE-SEM, Hitachi S-4200).
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Elemental analysis of composites was carried out by EDAX (Hitachi S-4200) The dc conductivity was measured in the temperature range of 450-600 °C.
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3 Results and discussion
3.1 X-ray diffraction studies XRD patterns of NiO-GDC-BCY composites heated at 1300 oC in air are shown in Fig.
1. It shows corresponding to NiO (JCPDS 01-075-0161), GDC (JCPDS 82-2372) and BCY (ICSD card No- 01 082-2372 for BaCe0.9Y0.1O2.95) only, without any other impurity peaks. The
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major phase in the composites is BCY, as observed in Fig. 1, obviously due to high concentration of BCY in composite. The (002) peak at 32.51o from GDC is observed only for the 32:8:60 and it is not detected
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by XRD for other compositions. Whilst, no shift in peak positions is observed indicating that GDC has not dissolved in the BCY. The crystallite sizes (D) of the samples are calculated from the Scherer’s formula [22-24] for the most intense peak. The crystallite sizes of the NiO ((200)
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plane) and BCY ((002) plane) phases in NiO-GDC-BCY-composites are also given in Table 1. The crystallite size is observed to increase up to 4 wt% of GDC and then after decreased.
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Fig. 2 shows X-ray diffraction patterns of Ni-GDC-BCY composites after reduction in 3% hydrogen plus argon atmosphere. In this case, the NiO powder is reduced separately in hydrogen plus argon atmosphere. It is then mixed thoroughly with required amounts of GDC and BCY; followed by grinding, calcinations and pelletization. The Ni-GDC-BCY pellets are then
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sintered at 1300 oC and further reduced in hydrogen plus argon atmosphere. In this process, NiO is reduced completely to Ni and hence no NiO peaks are observed in the XRD patterns of NiGDC-BCY compositions (Fig. 2). While peaks at 44.6 and 52.3o represent (111) and (200)
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planes of Ni, crystallized in cubic structure with lattice parameters, a=b=c=3.52 Ao. The reduction eventually leads to increased porosity.
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3.2 FE-SEM studies
Fig. 3 shows FE-SEM images of sintered NiO-GDC-BCY composites before (A1 to E1)
reduction. All samples show compact morphology with some porosity, which is due to removal of PVA during heating. As NiO/GDC amounts change, shape and size of the grains change. It is also observed that as GDC amount increases, densification increases. The relative density of NiO-GDC is 90%, which increases up to 95 % for composite with 32:8:60 (Table 2). This
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indicates that GDC helps in sintering and hence densification of the composites. However, it is very difficult to distinguish the separate particles of NiO, GDC and BCY phases due to lower GDC amount and therefore, observed morphology is mainly attributed to BCY and NiO phases.
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After reduction in hydrogen, porosity of composites is increased due to reduction of the surface NiO as shown in Fig. 3 (A2 to E2). The porosity is fundamental to facilitate the gas to reach the TPB both within the whole composite electrode and at the electrolyte/electrode interface. Still,
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with increase in GDC content, densification is observed to increase. The relative density of NiOGDC is 80%, which increases up to 88 % for composite with 32:8:60.
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Fig. 4 shows mapping of NiO-GDC-BCY composite. The area under observation is 3 µm in which all the elements are observed to be homogeneously distributed. The performance of the anode cermet is critically dependent on the microstructure and the distribution of Ni phases in the cermet [25].
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3.3 DC conductivity
Fig. 5 represents the conductivity of NiO-GDC-BCY composites measured in 3% hydrogen plus argon atmosphere in the temperature range of 450-600 oC. From Fig. it is
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observed that conductivity increases with increases in GDC amount up to 6 % and then after decreased for 8 % (Table 2). This is assigned to change in morphology from porous to dense
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with increase in GDC amount. As GDC amount increases density of the composite increases and the triple phase boundary regions are decreases. The 32:8:60 composition is relatively dense than other samples; this causes the less diffusion of gas through the bulk and hence conductivity decreases and activation energy increases. The composite with the composition 34:6:60 shows high conductivity. Also the activation energies are 0.45, 0.44, 0.42, 0.41 and 0.51 eV respectively for 0, 2, 4, 6 and 8 wt% GDC.
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Fig. 6 shows variation of conductivity with respect to temperature of composites in biogas. The conductivity values are 2.3×10-3, 2.63×10-3, 2.78×10-3, 3.67×10-3 and
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3.2×10-3 S/cm for 38:2:60, 36:4:60, 34:6:60 and 32:8:60 composites, respectively. From this data it is observed that, as GDC content increases, the conductivity of the composite increases, because as GDC amount increases the density of the sample increases so there
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is less probability of gas diffusion. But after 6 wt% GDC i.e. for 8 wt% GDC conductivity is observed to decrease. This decrease in conductivity of the composite is
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attributed to the dense morphology. The corresponding activation energies are listed in Table 2. A little higher conductivity in biogas is assigned to enhanced catalytic activity for dry reformation of methane. 3.4 Cell characteristics
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Fig. 7a shows the cross-sectional view of the single cell. It shows BCY electrolyte sandwiched by a porous anode (top layer) and a porous cathode (bottom layer). Sufficient porosity in electrodes is important in order to allow rapid transport of gaseous reactants and to
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provide abundant sites for electrochemical reactions in properly designed electrodes [26]. Porous structure of anode layer (Fig. 7b) is obtained as a result of decomposition of starch which was
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added as a pore former [27]. This morphology of anode is similar to that reported for 34:6:60 composite in Fig. 3 (D1 and D2). Electrolyte, BCY layer is observed to be dense without any voids or cracks, which can be attributed to the excellent compatibility of electrolyte BCY with cathode and anode materials. Good adhesion can also be seen at both the anode-electrolyte and cathode-electrolyte interfaces, which has been produced by co-firing at high temperatures. Thickness of cathode, electrolyte and anode layer is found to be ∼75 µm, ∼45 µm and ∼1000 µm, respectively.
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3.5 Open circuit voltage measurement Fig. 8 shows schematic representation of set up used for Open circuit voltage measurement. The figure also shows schematic of biogas plant. Fig. 9 (A) shows variation of
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open circuit voltage as a function of temperature in 3% hydrogen plus argon and in biogas atmospheres. The open circuit voltage in both atmospheres is observed to increase with increase in operating temperature. With an increase in operating temperature, polarizations at each
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electrode/electrolyte interface start to decrease and lead to an increase in open circuit voltage. The OCV of cell with NiO-BCY anode is 0.53 and 0.39 V in hydrogen and biogas respectively.
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While it is 0.55 and 0.36 V for NiO-GDC-BCY. This shows that the addition of GDC improves ionic conductivity of anode [28, 29] in hydrogen atmosphere. The lower open circuit voltage in biogas is assigned to the formation of barium carbonate. Further, carbon deposition may be another reason.
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3.6 Stability of anode in biogas
The composite is exposed to biogas atmosphere for two hours at 600 oC. Then these composites are characterized by XRD, FE-SEM and EDX.
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Fig. 10 (A) shows XRD pattern of NiO-GDC-BCY exposed to biogas for 2 h at 600 oC to study the chemical stability of composite material in carbon containing atmosphere. It shows
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shows peak matching with NiO (JCPDS 01-078-0643), GDC (JCPDS 82-2372) and BCY (JCPDS 82-2372 for BaCe0.9Y0.1O2.95) and barium carbonate (JCPDS-00-005-0378) Carbon and hydrogen are formed by Boudouard reaction (eq. 3) during methane decomposition. This carbon then reacts with barium from BCY to form barium carbonate. Fig. 10 (B) shows FE-SEM image of sample exposed to biogas at 600 oC. After exposure to biogas, particle clusters resembling cauliflower are observed on the surface of the composite shown in Fig. 10 (B). These clusters are
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detected by EDX. This is carbon deposited on the surface of composite as shown in Fig. 10 (D). The carbon deposited is 4.56 wt%. This carbon deposition is caused by the Boudouard reaction since hydrocarbon pyrolysis rarely occurs below 700 oC. As the amount of CO formed within the
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cell increases, the Boudouard reaction also increases, and carbon accumulates on the catalyst. 4. Conclusions
The XRD patterns of all NiO-GDC-BCY composites show the presence of NiO and
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BCY and no intermediate phase formation. The conductivity is observed to increase with GDC content up to 6 wt% in both hydrogen and biogas. The highest conductivity in hydrogen is
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3.4×10-3 S/cm at 600 oC. Similarly, the highest conductivity in biogas is 3.67×10-3 S/cm at 600 o
C. The complete cells (with NiO-BCY and NiO-GDC-BCY anodes) are tested in hydrogen and
biogas. The open circuit voltages are 0.53 and 0.55 V, respectively for NiO-BCY and NiO-GDCBCY in hydrogen. Similarly the open circuit voltages are 0.36 and 0.39 V, respectively for NiO-
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BCY and NiO-GDC-BCY in biogas. The low open circuit voltage is due to presence of barium
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carbonate and carbon deposition over the anode surface.
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Table 1 Lattice parameters of the composite before and after reduction Lattice parameters (Ao) after reduction BCY
b 6.23 6.22 6.24
c 6.22 6.14 6.12
a 8.71 8.73 8.73
34:6:60 8.73
6.22
6.13
8.72
32:8:60 8.73
6.21
6.14
8.72
b 6.22 6.23 6.21
c 6.12 6.14 6.14
6.22
6.13
6.24
6.12
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a 40:0:60 8.71 38:2:60 8.72 36:4:60 8.71
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BCY
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Lattice parameters (Ao) before reduction
Table 2 Crystallite densities of the composites before reduction
Hydrogen
dc conductivity (S/cm) 2.2 2.7 3.0 3.4 1.4
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6.42 6.38 6.34 6.31 6.20
90.03 90.10 91.60 92.20 94.03
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40:0:60 38:2:60 36:4:60 34:6:60 32:8:60
X-Ray Relative density density
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Composition
Activation energy (eV) 0.45 0.44 0.42 0.41 0.51
Biogas dc conductivity (S/cm) 2.37 2.63 2.78 3.67 3.2
Activation energy (eV) 0.43 0.46 0.44 0.40 0.42
Table 3 Crystallite size of composite before and after exposure to biogas.
Sample NiO BCY
Crystallite size (nm) Before exposure After exposure 65 79
59 68
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Figure Captions Fig. 1 X-ray diffraction patterns of NiO-GDC-BCY composites before reduction. Fig.2 X-ray diffraction patterns of Ni-GDC-BCY composites after reduction in hydrogen.
E2) reduction. Fig. 4 EDX mapping of NiO-GDC-BCY composite after reduction.
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Fig.3 FE-SEM micrographs of NiO-GDC-BCY composites before (A1 to E1) and after (A2 to
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Fig.5 The temperature dependence of DC conductivities of NiO-GDC-BCY composites in hydrogen.
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Fig. 6 The temperature dependence of DC conductivities of NiO-GDC-BCY composites in biogas.
Fig. 7 FE-SEM microstructures of (a) cross section of SOFC and (b) anode layer. Fig. 8 Schematic representation of (A) set up for OCV measurement, (B) biogas plant.
in biogas.
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Fig. 9 Open circuit voltage of NiO-BCY and NiO-GDC-BCY (34:6:60) in (A) 3% hydrogen (B)
Fig. 10 (A) XRD pattern (B) FE-SEM image (C) EDX spectrum of NiO-GDC-BCY
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exposed to biogas for 2 h at 600 oC.
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* *O
*
*
*
*O
*
*
*
*O
*
*
*O
*
*
*
*
* 34:6:60
* 36:4:60
* 38:2:60
* *O
40:0:60
20
30
40
50
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2θ (degree)
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Fig. 1
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#
*
*
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Intensity (a.u.)
32:8:60
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*- BCY #- GDC o- NiO
60
*
70
80
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*- BCY #- GDC o- Ni
*
O
*O
*
*
O
*O
*
*
O
*O
*
O
*O
*
O
*O
* 34:6:60
* 36:4:60
* 38:2:60
* 40:0:60
20
30
40
50
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2θ (degree)
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*
*
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#
*
*
*
*
*
*
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Intensity (a.u.)
32:8:60
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*
Fig. 2
60
70
80
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(A2)
(A1)
(B1)
(B2)
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B-38:2:60
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B-40:0:60
(C1)
(C2)
(D1)
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B-36:4:60
(D2)
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B-34:6:60
(E1)
(E2)
B-32:8:60
Fig. 3
Ce
Ni
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Ba
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3 µm
Gd
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Y
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Fig. 4
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Fig. 5
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Fig. 6
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(a)
(b)
Anode layer
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Anode
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Electrolyte
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Cathode
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Fig. 7
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(A)
5
4
3
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(B)
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(A) Schematic presentation of setup for cell testing 1- Pellet (Sample), 2-Silver foils 3-Cu wire for connections, 4-Glass Setup 5 – Path for gas flow
1
2
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(B)- Biogas production plant (taken from internet)
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Fig. 8
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Open circuit voltage (V)
Open circuit voltage (V)
0.38
0.50 0.45 0.40 0.35 0.30
NiO-BCY NiO-GDC-BCY
0.36 0.34
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NiO-BCY NiO-GDC-BCY
0.55
0.32 0.30
(B)
(A)
0.25
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Fig. 9
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NiO-GDC-BCY composites as an anode for SOFC S. T. Jadhav1, V. R. Puri1, L. D. Jadhav2* 1Department of Physics, Shivaji University, Kolhapur - 416 004, Maharashtra, India
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2Department of Physics, Rajaram College, Kolhapur – 416 004, Maharashtra, India
Highlight
1. NiO-GDC-BCY composite anode is successfully synthesized by physical mixing of powders
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prepared by solution combustion.
2. NiO-GDC-BCY anode based cell is fabricated comprising LSCF as cathode and BCY as an
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electrolyte.
3. The open circuit voltage is measured in hydrogen and biogas atmosphere.
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4. Stability of NiO-GDC-BCY is studied in biogas atmosphere.
S0925-8388(16)31592-4
DOI:
10.1016/j.jallcom.2016.05.243
Reference:
JALCOM 37763
To appear in:
Journal of Alloys and Compounds
Received Date: 17 March 2016 Revised Date:
10 May 2016
Accepted Date: 22 May 2016
Please cite this article as: S.T. Jadhav, V.R. Puri, L.D. Jadhav, NiO-GDC-BCY composites as an anode for SOFC, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.05.243. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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NiO-GDC-BCY composites as an anode for SOFC S. T. Jadhav1, V. R. Puri1, L. D. Jadhav2* 1Department of Physics, Shivaji University, Kolhapur - 416 004, Maharashtra, India
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2Department of Physics, Rajaram College, Kolhapur – 416 004, Maharashtra, India
Graphical Abstract
(B)
(A)
(C)
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5
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4
3
(B)- Biogas production plant (taken from internet) (C)- High temperature furnace used for the open circuit voltage measurement
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(A) Schematic presentation of setup for cell testing 1- Pellet (Sample), 2-Silver foils 3-Cu wire for connections, 4-Glass Setup 5 – Path for gas flow
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NiO-GDC-BCY composites as an anode for SOFC S. T. Jadhav1, V. R. Puri1, L. D. Jadhav2* 1Department of Physics, Shivaji University, Kolhapur - 416 004, Maharashtra, India 2Department of Physics, Rajaram College, Kolhapur – 416 004, Maharashtra, India
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Corresponding [email protected]
Abstract
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NiO-Ce0.9Gd0.1O1.95-BaCe0.8Y0.2O3-δ (NiO-GDC-BCY) anode based solid oxide fuel cell was fabricated, in which the composite powders of NiO-GDC and BCY were synthesized by
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solution combustion method and mixed by physical mixing. In the present case, the composite anode material was prepared with varying amount of GDC (2 wt% to 8 wt%) with respect to NiO (38 wt% to 32wt%) and studied their structural, morphological and electrical properties. The single cell was fabricated comprising NiO-GDC-BCY (34:6:60) anode, LSCF (cathode) and
atmospheres. 1 Introduction
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BaCe0.8Y0.2O3-δ (electrolyte). The open circuit voltage was measured in hydrogen and biogas
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Nickel based ceramic composite materials have been used due to their capability of electro catalysis. Also, chemical and mechanical stability, catalytic activity and cost make nickel
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a strong candidate as compared to other possible metals [1, 2]. Pure Ni has thermal expansion coefficient (TEC) higher than normally used oxide electrolytes [3-5]. Therefore, electrolyte phase is added to Ni to form a cermet (ceramic+metal) with TEC acceptably close to those of the other cell components and to inhibit coarsening of the metallic particles. Performance of Ni-BCY composite as an anode is determined by its electrical properties as well as by the homogeneity of anode microstructure. Also, the porosity of the anode must be tailored with regard to mass transport considerations as well as mechanical strength by employing the appropriate preparation
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method. In addition, anode must possess sufficiently high electronic and ionic conductivity in the reducing environment at the operating temperature for efficient triple phase boundary operations and to reduce the polarization losses as well.
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Another possibility is to use redox phase such as doped ceria and ceria-containing mixed oxides in order to improve catalyst stability. This support plays an important role on the mechanism of carbon removal during hydrocarbon redox [6-8]. The use of ceria-based oxides
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(such as gadolinium-doped ceria GDC) in the composition of SOFC anodes has been also investigated [9-15]. Ceria exhibits a mixed ionic and electronic conductivity due to the reduction
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of Ce4+ to Ce3+. This improves the electrocatalytic activity of the anode due to the increase of the reaction zone over the three-phase boundary. In addition, higher conductivities are obtained when ceria is doped with trivalent cations, such as Gd3+. Furthermore, the high oxygen storage capacity of ceria provides a high resistance to carbon deposition during ethanol reforming [7].
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Thus the present study explores the triple phase composite i.e. NiO-GDC-BCY as an anode for SOFC. In this composite, proton conducting electrolyte (BCY) has been synthesized by solution combustion technique with optimum oxidant to fuel ratio. Similarly, NiO and GDC
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are also synthesized by solution combustion technique. The nature of a catalyst has significant influence on carbon formation. Ni is electronic conducting as well as Ni is known to be an
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excellent catalyst towards cracking of hydrocarbons, so that carbon species are more liable to deposit on the surfaces of Ni. In order to use hydrocarbons as a fuel with anode, extensive efforts have been made to suppress carbon deposition, such as introducing oxides [16], so in present case GDC is added, which supports NiO to enhance catalytic activity of anode. The effect of GDC concentration on phase stability, microstructure and electrical properties is studied by
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keeping electrolyte phase constant (BaCe0.8Y0.2O3-δ). The variation of GDC concentration helps to get high performance anode for proton conducting SOFC. 2 Experimental
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NiO-Ce0.9Gd0.1O1.95-BaCe0.8Y0.2O3-δ (NiO-GDC-BCY) triple phase composites were prepared by physical mixing of these three phases. Single phase BCY powder was prepared by solution combustion synthesis as reported in ref. [17]. Similarly the NiO [18] and GDC [19]
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were also synthesized by solution combustion synthesis technique; Samples were prepared such that the reduced NiO-GDC-BCY composites would contain 40, 38, 36, 34 and 32 wt% of NiO
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and 0, 2, 4, 6 and 8 wt% of GDC. To increase porosity, 3% of polyvinyl alcohol (PVA) as pore former was also added in each mixture. After proper mixing and grinding, composite powders were pressed into circular pellets of 1.5 cm diameter. All pellets were heated at 700 oC to remove the PVA, followed by sintering at 1300 oC. These pellets were then reduced at 700 o
GDC-BCY cermet anode.
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C under constant flow of a 3% of hydrogen plus 97% argon to convert NiO-GDC-BCY into Ni-
2.1 Fabrication of cell (NiO-GDC-BCY-Anode)
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Anode supported single cell comprises of LSCF (La0.6Sr0.4Co0.8Fe0.2O3-δ, cathode), BCY (BaCe0.8Y0.2O3-δ, electrolyte) and NiO-GDC-BCY (anode). NiO-GDC-BCY powder containing
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6 % GDC has been chosen since it shows highest dc conductivity in both hydrogen and biogas atmospheres. NiO-GDC-BCY powder containing 5% of starch (0.3 g) was pressed in 15 mm diameter die and sintered at 1300 oC for about 5 h. Then electrolyte was deposited over the anode by spray deposition technique (100 mL solution with 0.5 M concentration); followed by sintering at 1000 oC. The details of spray pyrolysis can be found in [20]. The cathode material, LSCF, was synthesized by solution-combustion route, followed by calcinations at 600 oC for 5 h. Slurry of
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LSCF was prepared with terpineol and brush painted on top of the electrolyte layer. The cell thus prepared was co-fired at 1300 oC for 5 h. Silver paste is then employed on both anode and cathode sides followed by heating at 600 oC for 2 h [21].
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The open circuit voltage was measured using the set-up indigenously in our lab. The set up was made up of glass, which has facility to supply hydrogen/biogas at anode side and air at cathode side. The opposite surfaces of the cell are covered by silver, in order to provide the
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required best contact for the voltage measurements. The cells are placed between the two perforated silver foils kept fixed on the opposite surface of the sample by stainless steel clamps
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and is then fixed in the set up. Before carrying out the OCV measurements, the temperature inside the cell is slowly raised (1 °C/min) up to about 600 °C in order to assure the softening of silver foils. After this thermal treatment, hydrogen/biogas and oxygen can be fluxed, respectively, at the anode side and at the cathode side without any gas loss.
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The structural properties of prepared composite pellet were characterized by X-ray diffractometer with (PHILIPS PW-3710) Cu Kα as radiation source. Morphological properties were analyzed using field emission scanning electron microscope (FE-SEM, Hitachi S-4200).
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Elemental analysis of composites was carried out by EDAX (Hitachi S-4200) The dc conductivity was measured in the temperature range of 450-600 °C.
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3 Results and discussion
3.1 X-ray diffraction studies XRD patterns of NiO-GDC-BCY composites heated at 1300 oC in air are shown in Fig.
1. It shows corresponding to NiO (JCPDS 01-075-0161), GDC (JCPDS 82-2372) and BCY (ICSD card No- 01 082-2372 for BaCe0.9Y0.1O2.95) only, without any other impurity peaks. The
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major phase in the composites is BCY, as observed in Fig. 1, obviously due to high concentration of BCY in composite. The (002) peak at 32.51o from GDC is observed only for the 32:8:60 and it is not detected
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by XRD for other compositions. Whilst, no shift in peak positions is observed indicating that GDC has not dissolved in the BCY. The crystallite sizes (D) of the samples are calculated from the Scherer’s formula [22-24] for the most intense peak. The crystallite sizes of the NiO ((200)
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plane) and BCY ((002) plane) phases in NiO-GDC-BCY-composites are also given in Table 1. The crystallite size is observed to increase up to 4 wt% of GDC and then after decreased.
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Fig. 2 shows X-ray diffraction patterns of Ni-GDC-BCY composites after reduction in 3% hydrogen plus argon atmosphere. In this case, the NiO powder is reduced separately in hydrogen plus argon atmosphere. It is then mixed thoroughly with required amounts of GDC and BCY; followed by grinding, calcinations and pelletization. The Ni-GDC-BCY pellets are then
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sintered at 1300 oC and further reduced in hydrogen plus argon atmosphere. In this process, NiO is reduced completely to Ni and hence no NiO peaks are observed in the XRD patterns of NiGDC-BCY compositions (Fig. 2). While peaks at 44.6 and 52.3o represent (111) and (200)
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planes of Ni, crystallized in cubic structure with lattice parameters, a=b=c=3.52 Ao. The reduction eventually leads to increased porosity.
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3.2 FE-SEM studies
Fig. 3 shows FE-SEM images of sintered NiO-GDC-BCY composites before (A1 to E1)
reduction. All samples show compact morphology with some porosity, which is due to removal of PVA during heating. As NiO/GDC amounts change, shape and size of the grains change. It is also observed that as GDC amount increases, densification increases. The relative density of NiO-GDC is 90%, which increases up to 95 % for composite with 32:8:60 (Table 2). This
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indicates that GDC helps in sintering and hence densification of the composites. However, it is very difficult to distinguish the separate particles of NiO, GDC and BCY phases due to lower GDC amount and therefore, observed morphology is mainly attributed to BCY and NiO phases.
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After reduction in hydrogen, porosity of composites is increased due to reduction of the surface NiO as shown in Fig. 3 (A2 to E2). The porosity is fundamental to facilitate the gas to reach the TPB both within the whole composite electrode and at the electrolyte/electrode interface. Still,
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with increase in GDC content, densification is observed to increase. The relative density of NiOGDC is 80%, which increases up to 88 % for composite with 32:8:60.
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Fig. 4 shows mapping of NiO-GDC-BCY composite. The area under observation is 3 µm in which all the elements are observed to be homogeneously distributed. The performance of the anode cermet is critically dependent on the microstructure and the distribution of Ni phases in the cermet [25].
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3.3 DC conductivity
Fig. 5 represents the conductivity of NiO-GDC-BCY composites measured in 3% hydrogen plus argon atmosphere in the temperature range of 450-600 oC. From Fig. it is
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observed that conductivity increases with increases in GDC amount up to 6 % and then after decreased for 8 % (Table 2). This is assigned to change in morphology from porous to dense
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with increase in GDC amount. As GDC amount increases density of the composite increases and the triple phase boundary regions are decreases. The 32:8:60 composition is relatively dense than other samples; this causes the less diffusion of gas through the bulk and hence conductivity decreases and activation energy increases. The composite with the composition 34:6:60 shows high conductivity. Also the activation energies are 0.45, 0.44, 0.42, 0.41 and 0.51 eV respectively for 0, 2, 4, 6 and 8 wt% GDC.
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Fig. 6 shows variation of conductivity with respect to temperature of composites in biogas. The conductivity values are 2.3×10-3, 2.63×10-3, 2.78×10-3, 3.67×10-3 and
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3.2×10-3 S/cm for 38:2:60, 36:4:60, 34:6:60 and 32:8:60 composites, respectively. From this data it is observed that, as GDC content increases, the conductivity of the composite increases, because as GDC amount increases the density of the sample increases so there
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is less probability of gas diffusion. But after 6 wt% GDC i.e. for 8 wt% GDC conductivity is observed to decrease. This decrease in conductivity of the composite is
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attributed to the dense morphology. The corresponding activation energies are listed in Table 2. A little higher conductivity in biogas is assigned to enhanced catalytic activity for dry reformation of methane. 3.4 Cell characteristics
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Fig. 7a shows the cross-sectional view of the single cell. It shows BCY electrolyte sandwiched by a porous anode (top layer) and a porous cathode (bottom layer). Sufficient porosity in electrodes is important in order to allow rapid transport of gaseous reactants and to
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provide abundant sites for electrochemical reactions in properly designed electrodes [26]. Porous structure of anode layer (Fig. 7b) is obtained as a result of decomposition of starch which was
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added as a pore former [27]. This morphology of anode is similar to that reported for 34:6:60 composite in Fig. 3 (D1 and D2). Electrolyte, BCY layer is observed to be dense without any voids or cracks, which can be attributed to the excellent compatibility of electrolyte BCY with cathode and anode materials. Good adhesion can also be seen at both the anode-electrolyte and cathode-electrolyte interfaces, which has been produced by co-firing at high temperatures. Thickness of cathode, electrolyte and anode layer is found to be ∼75 µm, ∼45 µm and ∼1000 µm, respectively.
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3.5 Open circuit voltage measurement Fig. 8 shows schematic representation of set up used for Open circuit voltage measurement. The figure also shows schematic of biogas plant. Fig. 9 (A) shows variation of
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open circuit voltage as a function of temperature in 3% hydrogen plus argon and in biogas atmospheres. The open circuit voltage in both atmospheres is observed to increase with increase in operating temperature. With an increase in operating temperature, polarizations at each
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electrode/electrolyte interface start to decrease and lead to an increase in open circuit voltage. The OCV of cell with NiO-BCY anode is 0.53 and 0.39 V in hydrogen and biogas respectively.
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While it is 0.55 and 0.36 V for NiO-GDC-BCY. This shows that the addition of GDC improves ionic conductivity of anode [28, 29] in hydrogen atmosphere. The lower open circuit voltage in biogas is assigned to the formation of barium carbonate. Further, carbon deposition may be another reason.
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3.6 Stability of anode in biogas
The composite is exposed to biogas atmosphere for two hours at 600 oC. Then these composites are characterized by XRD, FE-SEM and EDX.
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Fig. 10 (A) shows XRD pattern of NiO-GDC-BCY exposed to biogas for 2 h at 600 oC to study the chemical stability of composite material in carbon containing atmosphere. It shows
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shows peak matching with NiO (JCPDS 01-078-0643), GDC (JCPDS 82-2372) and BCY (JCPDS 82-2372 for BaCe0.9Y0.1O2.95) and barium carbonate (JCPDS-00-005-0378) Carbon and hydrogen are formed by Boudouard reaction (eq. 3) during methane decomposition. This carbon then reacts with barium from BCY to form barium carbonate. Fig. 10 (B) shows FE-SEM image of sample exposed to biogas at 600 oC. After exposure to biogas, particle clusters resembling cauliflower are observed on the surface of the composite shown in Fig. 10 (B). These clusters are
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detected by EDX. This is carbon deposited on the surface of composite as shown in Fig. 10 (D). The carbon deposited is 4.56 wt%. This carbon deposition is caused by the Boudouard reaction since hydrocarbon pyrolysis rarely occurs below 700 oC. As the amount of CO formed within the
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cell increases, the Boudouard reaction also increases, and carbon accumulates on the catalyst. 4. Conclusions
The XRD patterns of all NiO-GDC-BCY composites show the presence of NiO and
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BCY and no intermediate phase formation. The conductivity is observed to increase with GDC content up to 6 wt% in both hydrogen and biogas. The highest conductivity in hydrogen is
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3.4×10-3 S/cm at 600 oC. Similarly, the highest conductivity in biogas is 3.67×10-3 S/cm at 600 o
C. The complete cells (with NiO-BCY and NiO-GDC-BCY anodes) are tested in hydrogen and
biogas. The open circuit voltages are 0.53 and 0.55 V, respectively for NiO-BCY and NiO-GDCBCY in hydrogen. Similarly the open circuit voltages are 0.36 and 0.39 V, respectively for NiO-
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BCY and NiO-GDC-BCY in biogas. The low open circuit voltage is due to presence of barium
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carbonate and carbon deposition over the anode surface.
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Table 1 Lattice parameters of the composite before and after reduction Lattice parameters (Ao) after reduction BCY
b 6.23 6.22 6.24
c 6.22 6.14 6.12
a 8.71 8.73 8.73
34:6:60 8.73
6.22
6.13
8.72
32:8:60 8.73
6.21
6.14
8.72
b 6.22 6.23 6.21
c 6.12 6.14 6.14
6.22
6.13
6.24
6.12
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a 40:0:60 8.71 38:2:60 8.72 36:4:60 8.71
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BCY
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Lattice parameters (Ao) before reduction
Table 2 Crystallite densities of the composites before reduction
Hydrogen
dc conductivity (S/cm) 2.2 2.7 3.0 3.4 1.4
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6.42 6.38 6.34 6.31 6.20
90.03 90.10 91.60 92.20 94.03
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40:0:60 38:2:60 36:4:60 34:6:60 32:8:60
X-Ray Relative density density
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Composition
Activation energy (eV) 0.45 0.44 0.42 0.41 0.51
Biogas dc conductivity (S/cm) 2.37 2.63 2.78 3.67 3.2
Activation energy (eV) 0.43 0.46 0.44 0.40 0.42
Table 3 Crystallite size of composite before and after exposure to biogas.
Sample NiO BCY
Crystallite size (nm) Before exposure After exposure 65 79
59 68
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Figure Captions Fig. 1 X-ray diffraction patterns of NiO-GDC-BCY composites before reduction. Fig.2 X-ray diffraction patterns of Ni-GDC-BCY composites after reduction in hydrogen.
E2) reduction. Fig. 4 EDX mapping of NiO-GDC-BCY composite after reduction.
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Fig.3 FE-SEM micrographs of NiO-GDC-BCY composites before (A1 to E1) and after (A2 to
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Fig.5 The temperature dependence of DC conductivities of NiO-GDC-BCY composites in hydrogen.
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Fig. 6 The temperature dependence of DC conductivities of NiO-GDC-BCY composites in biogas.
Fig. 7 FE-SEM microstructures of (a) cross section of SOFC and (b) anode layer. Fig. 8 Schematic representation of (A) set up for OCV measurement, (B) biogas plant.
in biogas.
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Fig. 9 Open circuit voltage of NiO-BCY and NiO-GDC-BCY (34:6:60) in (A) 3% hydrogen (B)
Fig. 10 (A) XRD pattern (B) FE-SEM image (C) EDX spectrum of NiO-GDC-BCY
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exposed to biogas for 2 h at 600 oC.
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* *O
*
*
*
*O
*
*
*
*O
*
*
*O
*
*
*
*
* 34:6:60
* 36:4:60
* 38:2:60
* *O
40:0:60
20
30
40
50
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2θ (degree)
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Fig. 1
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#
*
*
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Intensity (a.u.)
32:8:60
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*- BCY #- GDC o- NiO
60
*
70
80
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*- BCY #- GDC o- Ni
*
O
*O
*
*
O
*O
*
*
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*O
*
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*
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*O
* 34:6:60
* 36:4:60
* 38:2:60
* 40:0:60
20
30
40
50
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2θ (degree)
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*
*
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#
*
*
*
*
*
*
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Intensity (a.u.)
32:8:60
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*
Fig. 2
60
70
80
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(A2)
(A1)
(B1)
(B2)
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B-38:2:60
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B-40:0:60
(C1)
(C2)
(D1)
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B-36:4:60
(D2)
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B-34:6:60
(E1)
(E2)
B-32:8:60
Fig. 3
Ce
Ni
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Ba
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3 µm
Gd
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Y
AC C
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Fig. 4
TE D
M AN U
SC
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AC C
EP
Fig. 5
TE D
M AN U
SC
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AC C
EP
Fig. 6
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(a)
(b)
Anode layer
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Anode
SC
Electrolyte
M AN U
Cathode
AC C
EP
TE D
Fig. 7
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(A)
5
4
3
M AN U
SC
(B)
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(A) Schematic presentation of setup for cell testing 1- Pellet (Sample), 2-Silver foils 3-Cu wire for connections, 4-Glass Setup 5 – Path for gas flow
1
2
TE D
(B)- Biogas production plant (taken from internet)
AC C
EP
Fig. 8
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Open circuit voltage (V)
Open circuit voltage (V)
0.38
0.50 0.45 0.40 0.35 0.30
NiO-BCY NiO-GDC-BCY
0.36 0.34
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NiO-BCY NiO-GDC-BCY
0.55
0.32 0.30
(B)
(A)
0.25
0.28 420
440
460
480
500
400
420
440
460
480
500
o
o
Temperature ( C)
Temperature ( C)
TE D
M AN U
Fig. 9
SC
400
(A)
biogas exposure
#- BCY *- GDC +- NiO @- BaCO3
EP
#
Intensity (a.u.)
(B)
Carbon
#
#
*
@
30
40
AC C
20
#
+
+
# #
+
50
60
2θ (degree)
70
(B)
80
Fig. 10
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NiO-GDC-BCY composites as an anode for SOFC S. T. Jadhav1, V. R. Puri1, L. D. Jadhav2* 1Department of Physics, Shivaji University, Kolhapur - 416 004, Maharashtra, India
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2Department of Physics, Rajaram College, Kolhapur – 416 004, Maharashtra, India
Highlight
1. NiO-GDC-BCY composite anode is successfully synthesized by physical mixing of powders
SC
prepared by solution combustion.
2. NiO-GDC-BCY anode based cell is fabricated comprising LSCF as cathode and BCY as an
M AN U
electrolyte.
3. The open circuit voltage is measured in hydrogen and biogas atmosphere.
AC C
EP
TE D
4. Stability of NiO-GDC-BCY is studied in biogas atmosphere.