Scholarly article on topic 'Influence of Calcination on the Properties of Nickel Oxide-Samarium Doped Ceria Carbonate (NiO-SDCC) Composite Anodes'

Influence of Calcination on the Properties of Nickel Oxide-Samarium Doped Ceria Carbonate (NiO-SDCC) Composite Anodes Academic research paper on "Materials engineering"

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{"Nickel oxide" / "samarium doped ceria carbonates" / "composite anode"}

Abstract of research paper on Materials engineering, author of scientific article — K.H. Ng, S. Lidiyawati, M.R. Somalu, A. Muchtar, H.A. Rahman

Abstract Apart from its composition, the starting powder properties such as particle size potentially affect the triple phase boundary and the electrochemical performance. Calcination process has been identified as one of the factors that influence the particle size of the composite anode powders. This study investigates the correlation between calcination temperature and properties (i.e., chemical, physical, and thermal) of NiO–samarium-doped ceria carbonate (SDCC) composite anodes. NiO–SDCC composite anode powder was prepared with NiO and SDCC through high-energy ball milling. The resultant composite powder was subjected to calcination at various temperatures ranging from 600°C to 800°C. Characterizations of the composite anode were performed through X-ray diffraction (XRD), Fourier transform infrared spectroscopy, energy dispersive spectroscopy, field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), dilatometry, and porosity measurements. The composite anodes exhibited good chemical compatibility during XRD after calcination and sintering. The FTIR result verified the existence of carbonates in all the composite anodes. The increment in calcination temperature from 600°C to 800°C resulted in the growth of nanoscale particles, as evidenced by the FESEM micrographs and crystallite size. Nonetheless, the porosity obtained remained within the acceptable range for a good anodic reaction (20% to 40%). The TGA results showed gradual mass loss in the range of 400°C to 600°C (within the low-temperature solid oxide fuel cell region). The composite anodes calcined at 600°C and 700°C revealed a good thermal expansion coefficient that matches that of the SDCC electrolyte.

Academic research paper on topic "Influence of Calcination on the Properties of Nickel Oxide-Samarium Doped Ceria Carbonate (NiO-SDCC) Composite Anodes"

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Procedia Chemistry 19 (2016) 267 - 274

5th International Conference on Recent Advances in Materials, Minerals and Environment (RAMM) & 2nd International Postgraduate Conference on Materials, Mineral and Polymer

(MAMIP), 4-6 August 2015

Influence of Calcination on the Properties of Nickel OxideSamarium Doped Ceria Carbonate (NiO-SDCC) Composite Anodes

K.H. Nga, S. Lidiyawatia, M.R. Somalub, A. Muchtarb, H.A. Rahman,a*

aDepartment of Design and Materials Engineering, Faculty of Mechanical Engineering and Manufacturing, Universiti Tun Hussein Onn

Malaysia,86400,Johor, Malaysia bFuel Cell Institute, Universiti Kebangsaan Malaysia, 43700, Bangi, Selangor, Malaysia

Abstract

Apart from its composition, the starting powder properties such as particle size potentially affect the triple phase boundary and the electrochemical performance. Calcination process has been identified as one of the factors that influence the particle size of the composite anode powders. This study investigates the correlation between calcination temperature and properties (i.e., chemical, physical, and thermal) of NiO-samarium-doped ceria carbonate (SDCC) composite anodes. NiO-SDCC composite anode powder was prepared with NiO and SDCC through high-energy ball milling. The resultant composite powder was subjected to calcination at various temperatures ranging from 600 °C to 800 °C. Characterizations of the composite anode were performed through X-ray diffraction (XRD), Fourier transform infrared spectroscopy, energy dispersive spectroscopy, field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), dilatometry, and porosity measurements. The composite anodes exhibited good chemical compatibility during XRD after calcination and sintering. The FTIR result verified the existence of carbonates in all the composite anodes. The increment in calcination temperature from 600 °C to 800 °C resulted in the growth of nanoscale particles, as evidenced by the FESEM micrographs and crystallite size. Nonetheless, the porosity obtained remained within the acceptable range for a good anodic reaction (20 % to 40 %). The TGA results showed gradual mass loss in the range of 400 °C to 600 °C (within the low-temperature solid oxide fuel cell region). The composite anodes calcined at 600 °C and 700 °C revealed a good thermal expansion coefficient that matches that of the SDCC electrolyte.

© 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-reviewunderresponsibility of Schoolof Materialsand MineralResourcesEngineering,Universiti SainsMalaysia

* Corresponding author. Tel.: +607-453-7335; fax: +607-453-6080. E-mail address: hamimah@uthm.edu.my

1876-6196 © 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia doi:10.1016/j.proche.2016.03.104

Keywords: Nickel oxide, samarium doped ceria carbonates, composite anode

1. Introduction

SOFC is an environmental friendly electrochemical device that efficiently converts chemical energy from fuel into electricity. SOFC has received considerable attention as a new-generation power system. A typical SOFC requires high operating temperatures (600°C to 1000 °C), which result in detrimental issues, such as cell degradation, thermal expansion mismatches, induced chemical instability, and expensive sealing and interconnecting material. These disadvantages eventually result in high operational costs. Reduction of the operating temperature is important to address these technical issues. However, the conventional combination of composite anodes, such as nickel-yttria-stabilized zirconia (Ni-YSZ) suffers from undesirable polarization resistance caused by low electrode kinetics and poor ionic conductivity at reduced temperatures1,2,3.

Much research attention has been devoted to the development of high-performance composite anode materials for reduced-temperature applications. A potential strategy is to incorporate highly conductive doped ceria electrolytes, such as samarium-doped ceria (SDC) and gadolinia-doped ceria (GDC), into the highly electronic and catalytic constituent of Ni4,5. Progressive studies on Ni-SDC and Ni-GDC mostly yielded a promising power output at intermediate temperatures (600°C to 800 °C). However, doped ceria electrolytes suffer from chemical instability caused by the partial reduction of Ce4+ to Ce3+, thereby deteriorating cell performance in the low-temperature region (400°C to 600 °C). The development of a novel composite electrolyte of samarium-doped ceria carbonates (SDCC) through the introduction of a small amount of alkaline salts shown a breakthrough for low-temperature SOFCs (LT-SOFCs) application. SDCC electrolytes exhibit distinctive performance and offer better chemical and mechanical stability than other pristine-doped ceria electrolytes performing at low temperatures6.

NiO-SDCC is considered a new combination of composite anode materials. Jarot et al. (2011) reported the influence of different carbonate contents and pellet fabrication pressures on the physical and chemical properties of NiO-SDCC7. NiO-SDCC exhibited promising performance at a low temperature as a single cell with SDCC electrolyte and lanthanum strontium cobalt ferrite-SDCC cathode in the low-temperature region. Despite exhibiting good cell performance, studies on NiO-SDCC powder processing are still limited. Calcination is a common powder processing technique that has been found to influence powder properties. Consequently, selection of a suitable calcination temperature is important because powder characteristics, such as particle size, potentially affect the triple phase boundary (TPB), microstructure, and eventually the electrochemical performance8.

In the present study, NiO-SDCC composite anodes were prepared by using ball milling process. The composite powders were subjected to calcination temperatures of 600, 700 and 800 °C. The composite anode properties (chemical, physical and thermal) were investigated in relation to calcination temperatures.

Nomenclature

SOFC Solid oxide fuel cell

NiO Nickel oxide

SDC Samarium doped ceria

YSZ Yytria stabilised zirconia

GDC Gadolium doped ceria

SDCC Samarium doped ceria carbonate

2. Experimental procedure

2.1. Materials and specimen preparation

Commercially available SDC (Kceracell, Korea), Li2CO3, and Na2CO3 (Sigma-Aldrich, USA) powders were used as the starting materials to prepare SDCC composite electrolyte. Subsequently, a mixture of 20 wt.% binary carbonates (67 mol.% Li2CO3 : 33 mol.% Na2CO3) and 80 wt.% SDC powder were ball milled in ethanol. The mixing process was carried out by using ball milling machine (Pulverisette 6-Fristch, Germany) at 150 rpm for 24 h. The resultant SDCC slurry was oven-dried overnight, ground in agate mortar and calcined in an electric furnace at 680 °C for 1 h. Commercial NiO powder (Kceracell, Korea) was mixed with the aforementioned SDCC electrolyte to prepare NiO-SDCC composite anode powders at the weight ratio of 50:50 respectively. The composite anode powder was ball milled at 550 rpm for 2 h in ethanol using zirconium oxide balls and bowl. The mixture was subsequently dried overnight and thoroughly ground in an agate mortar. The composite powders were calcined at 600, 700 and 800°C for 1 h with heating rate of 5 °C min-1 in air. Henceforth, the composite anode powders are referred to as NiO-SDCC600, NiO-SDCC700, and NiO-SDCC800. The calcined composite anode powders were grounded again in agate mortar and uniaxially compacted into pellets (13 mm diameter, ~0.7 mm thick) and cylindrical rods (6 mm diameter, ~20 mm long). These samples were sintered at 600 °C for 1 h with a heating rate of 2 °C min-1 in air.

2.2. Characterisation methods

The phase purity and crystalline phases of the calcined composite anode powders and pellets were examined by using an X-ray diffractometer, XRD (D8 Advance-Bruker, Germany) with radiation source of Cu Ka (X=1.5406A) and 20 ranging from 10° to 80°. Fourier transform infrared spectroscopy (FTIR) was performed via Perkin Elmer Spectrum 100 FT-IR spectrometer, USA. The IR measurement was achieved by using attenuated total reflectance (ATR) sampling method operated over the spectra range of 4000 to 550 cm-1. The morphologies of the composite anode powders and pellets were evaluated through field emission scanning electron microscopy (JSM 6700F-JEOL, Japan). The average composite particle size was attained by measuring of 50 particles via Image J 1.48 software. Thermogravimetric analysis (TGA) was performed to study the weight loss over the range temperature of 30 to 900 °C in air at a heating rate of 10 °C min-1 (Thermobalance-Linseis, Germany). Thermal expansion coefficient (TEC) measurement of the composite anodes was conducted with a cylindrical rod. The analysis was performed from 30 to 900 °C by using a dilatometer (L75H-Linseis, Germany) at a heating rate of 5 °C min-1 with alumina as the standard reference. The EDS spectra and mapping were obtained by using scanning electron microscopy coupled with energy dispersion spectroscopy (SEM-EDS) (JSM 6380LA-JEOL, Japan). Porosity and density measurements of the sintered pellets were conducted by using Archimedes method (Density Kit XP-Mettler Toledo, USA). Ethanol was utilized as immersion medium instead of water due to solubility of carbonates in water.

3. Results and Discussion

3.1. XRD analysis

Fig.1 shows the XRD spectra of raw NiO, SDCC and NiO-SDCC calcined at various temperatures. The XRD patterns display only two distinguished cubic crystalline phases resembling NiO (JCPDS No: 47-1049) and SDC (JCPSD No: 75-0158) after powder calcination and pellet sintering. No distinct carbonates peaks were detected due to the amorphous carbonates coating on SDC particles without altering the structures of composite anodes. Besides that, no remarkable secondary and new phases were detected within the sensitivity of XRD, thereby demonstrating good chemical compatibility after mixing, calcination and sintering. These characteristics are important for composite anodes for maintaining its own individual functions as electronic and ionic conductor7,9.

Sherrer's equation was applied to determine the crystallite size10. The average crystallite size (Table 1) were obtained by using three major peaks of the XRD pattern corresponding to crystalline phase of NiO at the planes of 111, 220 and 311 (28.49°, 47.34°, 56.21°,respectively) and SDC at the planes of 111, 200 and 220 (37.25°, 43.28° and 62.88°,respectively). Most of the crystallite sizes of SDC were found slightly larger compared to NiO that may be attributed to the carbonate coating on the SDC particles7. At increasing calcination temperatures, the growth of NiO was comparable with SDC since NiO tends to grow faster. Similar finding was reported previously by Skalar et al. (2014) on Ni-SDC11. By contrast, SDC grows steadily as carbonate shields SDCC from energetic growth12. A small increment in crystallite size was observed for sintered pellets as compared with the calcined powders because of the relatively low sintering temperature of 600 °C.

(3) Nio-sDccaco VNiO • SDC ï Y ï I I»! i fct?

NÍO-SDCC700 Il 1 1 ft . A - A -

Ni0-SDCC600 1 k

SDCC J_ I i * , Ax

N¡0 ,1 J 1 1 A A

( i i ■ ■ i ■

<b) NiosDcceoo ï í IÍ VNiO •SDC t

Ni0-SDCC700 ■

NtO-SDCC&OO uJ_

SDCC JL \ 11 .

NiO J l! k A A

i _i_ 1 _' '

26/ Degree

40 50 60

Z6/ Degree

Fig 1. XRD pattern of (a) calcined and (b) sintered NiO-SDCC composite anodes

3.2. Morphologies of the composite anode powder and pellet

The FESEM morphologies of the calcined composite anode powders were in nano-submicron size, as shown in Fig. 2. A significant increment in particle size was observed with the increase of calcination temperatures as shown in Fig. 2 (a-c). The findings are consistent with the increment of crystallite size (DNio and Dsdc) from XRD broadening and the increase of measured average composite particle size (Dparticle) as shown in Table 1. These results obey the calcination theory, which states the particle growth increases with the increase in calcination temperatures3,13. Similar findings were observed for Ni-SDC and Ni-GDC at increasing calcination temperatures5,11. Fig. 3 shows that slight increment in the average pellet grain size (Dgrain) compared to Dparticle was mainly due to low sintering temperature. Overall, the calcined composite anode powders and pellet grain sizes remained in the nano-submicron scale (< 1 |m) that is essential for maximizing the TPB length for high cell performance14.

Fig. 2. The FESEM micrographs of calcined (a) Ni0-SDCC600, (b) Ni0-SDCC700 and (c) Ni0-SDCC800 composite anode powders

Fig.3. The FESEM micrographs of (a) NiO-SDCC6QQ (b) NiO-SDCC7QQ (c) NiO-SDCC800 sintered pellets

Table 1. Average crystallite size, composite particle and grain size of NiO-SDCC

Crystallite size of Crystallite size of Average size of Average grain

calcined powders sintered pellets composite anode size of sintered

Sample DNi0 DSDC DNi0 DSDC particles pellets

(nm) (nm) (nm) (nm) Dparticle Dgrain

(nm) (nm)

Ni0-SDCC600 25.51 ± 5.12 40.77 ± 2.29 28.30 ± 4.19 37.80 ± 3.66 97.62 107.42

Ni0-SDCC700 30.84 ± 5.65 42.25 ± 6.02 36.38 ± 2.13 43.75 ± 3.69 103.78 113.50

Ni0-SDCC800 44.97 ± 5.52 49.55 ± 7.81 42.82 ± 9.45 48.81 ± 3.63 114.05 127.65

3.3. FTIR and EDS mapping

FTIR spectra (Fig. 4) proved the existence of carbonates in the all of the composite anode powders, with CO32" absorption bands ranging from 1437 to 1430 cm-1 (intense and broad peak) and 862 to 861 cm-1 (narrow peak). The presence of carbonates in the composite anodes are important to provide co-ionic (O2-/H+) conductions that may improve the cell performance. High calcination temperature such as 800 °C that is above the theoretical eutectic melting temperature of carbonates (578 °C), results in remarkably weaker CO32- intensity. This condition may be attributed to the starting of carbonate decomposition8,15.

Ni0-SDCC800

Ni0-SDCC700

- Ni0-SDCC600

Ni0-SDCC

0 3500 3000 2500 2000 1500 1000 5

Wavenumber (cm"1)

Fig 4. FTIR spectra of as-prepared and calcined NiO-SDCC composite anode powders

Fig. 5 shows the EDS spectra and elemental mapping obtained from the cross section of the NiO-SDCC800 composite anode pellets. The results comfirmed the elements of the composite anodes were in good homogeneity with four well-distributed elements detected, namely nickel (Ni), samarium (Sm), ceria (Ce) and sodium (Na). Lithium (Li) was not detected due to its low atomic mass16.

Fig. 5. The EDS mapping of the NiO-SDCC800 composite anode pellet.

3.4. Porosity and density of sintered pellets

Fig. 6 shows the porosity and density of sintered composite anode pellets obtained via Archimedean principle. The open porosity of the pellet displayed reduction in porosity with the increase of density and calcination temperatures. The reduction of porosity may be associated with the increase of particle size of the calcined powders. Similar porosity reduction trend was reported by Jarot et al.(2011) on high carbonate content of SDCC on NiO that induce by larger particle size7. Large grain size also tends to achieve high densification, thereby reducing the porosity. All composite anodes adequately satisfied the porosity requirement of anode in the range of 20 % to 40% for gaseous transport17. It is also expected to achieve higher porosity due to reduction of NiO to Ni on hydrogen fuel prior to cell operation.

Fig. 6. Porosity and density of NiO-SDCC composite anodes calcined at various temperatures

3.5. Thermal analysis

Fig. 7 shows the thermogravimetric (TG) curve that was used to determine the weight loss of the NiO, SDCC, as-prepared NiO-SDCC and calcined NiO-SDCC powders. NiO-SDCC800 and as-prepared NiO-SDCC exhibited the lowest and the highest total mass loss of 3.27 % and 5.17 % respectively. This result is due to the fact that most of the mass loss occurred during calcinations of as-prepared NiO-SDCC that includes desorption of water, organic and inorganic volatile substance removal from ethanol at a temperature ranging from 30 °C to 400 °C. A significant mass loss of approximately 1 % to 2% was observed within 700°C to 900°C for SDCC, as-prepared and calcined NiO-SDCC composite anode powders. The degradation may be attributed to the starting of the carbonates decomposition that is slightly above the eutectic point of the carbonates3,15. Meanwhile, gradual mass loss can be observed within 400 to 700°C. This result indicates that the composite anode powders were able to maintain thermal stability within the selected calcination temperatures (600°C to 800 °C) and sintering temperature (600 °C). Notably, LT-SOFC targets the operating temperature of 400°C to 600°C.

94-|-,-1-,-1-1-1-,-1-1-

30 200 400 600 800

Temperature/°C

Fig. 7. TG curves of NiO, SDCC, as-prepared NiO-SDCC and NiO-SDCC calcined at different temperatures

The average thermal expansion coefficient (TEC) values were calculated at a temperature range of 30 °C to 600 °C as shown in Table 2. Obtaining suitable thermal matching with the electrolyte component is important to avoid delamination and residual stresses while maintaining good mechanical compatibility3. The TEC trend exhibits a gradual increment with the increase in calcination temperature; this increment may be attributed to the increase in composite particle size18. According to Chen et al. (2008), the acceptable TEC difference limit is within 15 % to 20 % for attaining good thermodynamic compatibility19. Therefore, NiO-SDCC600 and NiO-SDCC700 exhibited promising thermal matching with TEC difference of 7.77 % and 13.59 % respectively.

Table 2. Average TEC value over temperature range of (30 °C to 600°C) for SDCC and NiO-SDCC composite anodes

Sample Average TEC (10-6 K-1) TEC Difference (%)

SDCC 10.30 ±0.86 -

NiO-SDCC600 11.10 ± 0.07 7.77

NiO-SDCC700 11.70 ± 1.40 13.59

NiO-SDCC800 12.70 ± 0.83 23.30

4. Conclusion

In conclusion, NiO-SDCC composite anode powders were calcined at various temperatures. The effects of calcination on the chemical, physical and thermal properties of composite anodes were investigated. All composite anodes exhibited the absence of secondary peaks on XRD, thereby signifying good chemical compatibility. FTIR analysis confirmed the existence of carbonate bonding in all composite anode powders. The composite powder size and sintered pellet grain sizes increased with the increase in calcination temperatures. Meanwhile, all composite anodes exhibited sufficient amount of porosity (20 % to 40 %) for good cell performance. The TGA results revealed gradual mass loss in the region of 400 °C to 600 °C, indicating that the calcination temperatures (600 °C to 800 °C) and sintering temperature (600 °C) were suitable for cell fabrication. Among all composite anodes, NiO-SDCC600 and NiO-SDCC700 demonstrated permissible TEC matching with the electrolyte in the range of 15 % to 20 %. A more detailed investigation will be performed in future to determine the effects of calcination based on the electrochemical characterization and single cell performance testing.

Acknowledgements

The authors gracefully acknowledge Universiti Tun Hussein Onn Malaysia (UTHM) and the Malaysian Government under RACE research grant scheme for research sponsorship.

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