Scholarly article on topic 'Effect of grinding time of synthesized gadolinium doped ceria (GDC10) powders on the performance of solid oxide fuel cell'

Effect of grinding time of synthesized gadolinium doped ceria (GDC10) powders on the performance of solid oxide fuel cell Academic research paper on "Chemical sciences"

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Abstract of research paper on Chemical sciences, author of scientific article — Fatma Aydin, Ibrahim Demir, Mahmut Dursun Mat

Abstract Ceria-based materials are prospective electrolytes for low and intermediate temperature solid oxide fuel cells. In the present work, fully dense CeO2 ceramics doped with 10 mol% gadolinium (Gd0.1Ce0.9O1.95) were prepared with a sol–gel method and commercially purchased GDC10 electrolyte powders were processed. Particle sizes of synthesized electrolyte powders were minimized by ball-milling method. Grinding of the samples were performed in different times intervals (12 h, 15 h, 18 h, 20 h, 25 h, 30 h, 35 h, 40 h and 45 h). Then, these powders were prepared to obtain of solid oxide fuel cells (SOFCs). Performances of these cells having an active area of 1 cm2 were tested using a fuel cell test station that measured in different temperatures (650 and 700 °C). In the present study, gadolinium doped ceria (GDC10) synthesiszed powders were investigated by using XRD and SEM images. Performance values of synthesized GDC10's in different temperature were compared to by commercial GDC10. Commercial GDC10's performance at 650 °C were tested, and maximum current density of 0.413 W/cm2 and maximum current density of 0.949 A/cm2 were obtained. Commercial GDC10 at 650 °C has better result. However, synthesized GDC10's performance at 700 °C demonstrated better results than commercial GDC10's. The performance tests of samples which are 20 h mill showed that they have the maximum power density of was obtained as 0.480 W/cm2 and maximum current density of as 1.231 A/cm2.

Academic research paper on topic "Effect of grinding time of synthesized gadolinium doped ceria (GDC10) powders on the performance of solid oxide fuel cell"

Engineering Science and Technology, an International Journal xxx (2014) 1 —5

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Engineering Science and Technology, an International Journal

journal homepage: http://ees.elsevier.com/jestch/default.asp

Full length article

Effect of grinding time of synthesized gadolinium doped ceria (GDCio) powders on the performance of solid oxide fuel cell

Fatma Aydina'*, Ibrahim Demira, Mahmut Dursun Matb

a Department of Chemistry, Faculty of Science and Arts, University ofNigde, Nigde, Turkey

b Department of Mechanical Engineering, Faculty of Architectural and Engineering, University of Meliksah, Kayseri, Turkey

ARTICLE INFO

Article history: Received 28 November 2013 Received in revised form 6 February 2014 Accepted 21 February 2014

Keywords: Synthesis

Solid oxide fuel cells GDC

Electrolyte

Material development Sol—gel

ABSTRACT

Ceria-based materials are prospective electrolytes for low and intermediate temperature solid oxide fuel cells. In the present work, fully dense CeO2 ceramics doped with 10 mol% gadolinium (Gd01Ce09O195) were prepared with a sol—gel method and commercially purchased GDC10 electrolyte powders were processed. Particle sizes of synthesized electrolyte powders were minimized by ball-milling method. Grinding of the samples were performed in different times intervals (12 h, 15 h, 18 h, 20 h, 25 h, 30 h, 35 h, 40 h and 45 h). Then, these powders were prepared to obtain of solid oxide fuel cells (SOFCs). Performances of these cells having an active area of 1 cm2 were tested using a fuel cell test station that measured in different temperatures (650 and 700 °C). In the present study, gadolinium doped ceria (GDC10) synthesiszed powders were investigated by using XRD and SEM images. Performance values of synthesized GDC10's in different temperature were compared to by commercial GDC10. Commercial GDC10's performance at 650 °C were tested, and maximum current density of 0.413 W/cm2 and maximum current density of 0.949 A/cm2 were obtained. Commercial GDC10 at 650 °C has better result. However, synthesized GDC10's performance at 700 °C demonstrated better results than commercial GDC10's. The performance tests of samples which are 20 h mill showed that they have the maximum power density of was obtained as 0.480 W/cm2 and maximum current density of as 1.231 A/cm2.

Copyright © 2014, Karabuk University. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction

Solid oxide fuel cells (SOFCs) are electrochemical devices that convert the chemical energy of a fuel (including hydrogen, natural gas, coal-derived synthetic gas, reformed gasoline or diesel) into electrical energy by electrochemically combining fuel and oxidant gases across an ionic conducting ceramic without combustion, in a clean, cheap and efficient way [1]. However, the cost of the current SOFC systems have significantly effects on a wide range of commercial applications. In order to be economically competitive, the cost of the materials and the fabrication should be considerably

* Corresponding author. Tel.: +90 388 2254016. E-mail addresses: fatmaaydin@nigde.edu.tr, fatmaaydin07@gmail.com (F. Aydin).

Peer review under responsibility of Karabuk University

reduced. One way for cost reduction is to lower operating temperature so that interconnection, i.e. heat exchanges between the structural components will decrease [2,3]. Hence, working with lower temperatures provides utilization from comparatively cheap metal components in low and intermediate temperature SOFCs. For this purpose, by using high ionic conductivity electrolyte at low temperatures, operating temperatures can be reduced [4]. Most commonly, solid electrolyte materials which are used in SOFC are yttria stabilized zirconia, doped ceria (e.g. gadolinia doped and samaria doped ceria), stabilized Bi2O3 and strontium/magnesium doped lanthanum gallate [5]. YSZ-based SOFC is required to operate at high temperature of 800—1000 °C [6]. However, at high operating temperatures, some destructive factors such as thermal mismatch between materials, interfacial reactions at electrode/ electrolyte interfaces and electrode/interconnector interfaces, may occur [7,8]. For this reason the yttria stabilized zirconia electrolytes should be replaced with the alternative solid electrolytes having a higher conductivity at lower temperatures. Ceria based electrolytes having higher electrical conductivity than that of yttria-stabilized zirconia at lower operating temperatures (500—800 °C) have received much attention in recent years [9]. Ceria (CeO2)

http://dx.doi.org/10.1016/j.jestch.2014.02.003

2215-0986/Copyright © 2014, Karabuk University. Production and hosting by Elsevier B.V. All rights reserved.

compounds doped with rare-earths which are fluorite type oxides with oxygen vacancies show a higher oxide ionic conductivity than yttria stabilized zirconia in oxidizing atmospheres [10]. Moreover, doped ceria has lower cost in comparison with lanthanum gallate based electrolytes. Doping with R+3 ions (such as R = Gd, Sm, Yetc.) in the crystal structure of ceria has been found to increase the concentration of oxygen vacancy due to the reduction of Ce4+ to Ce3+. Doping with Gd3+ has been enhanced the ionic conductivity of ceria. Gd2O3 ion doped ceria has attracted much attention as a potential electrolyte for fuel cells at low operating temperatures

[11]. Among doped ceria compounds, especially Sm2O3 and Gd2O3 doped ceria compounds have remarked to possess the highest conductivity. Therefore, researchers have recently focused on the ceria based electrolytes. Since ceria based electrolytes have higher densities, they provide SOFCs to work under lower operation temperatures. There are several techniques for preparing Gd2O3 and Sm2O3 doped ceria nano powders such as citrate complexation

[12], combustion [13], hydrothermal synthesis [14,15], microwave-induced combustion [16], co-precipitation [17], pechini [18] and sol—gel [19]. The main advantage of sol gel technique has the capability of producing ultra fine powders with high purity and homogeneous phase composition at lower temperatures. Moreover, sol—gel method as synthesiszing technique has been used because of the advantages of less dangerous, raw materials more easily available, as well as can be prepared of pure and homogeny films at low temperatures, providing energy savings and no cause to contamination et. from other methods (glycine nitrate, solid state etc.) [20].

In this wise, synthesized powders of (10 mol%, Gd) gadolinia doped ceria (GDC10) were used as the electrolyte material in solid oxide fuel cell (SOFCs). GDC10 powders were synthesized via sol— gel technique. This method has advantages of low cost and relative simplicity [21]. The Crystallite structures and morphologies of synthesized powders were characterized by XRD and SEM techniques. Obtained to GDC10 electrolyte membrane was covered by NiO as anode and LSF40 (La060Sr040FeO3) as cathode. Values obtained from the performance of solid oxide fuel cell having an active area of 1 cm2 were measured with fuel cell test station at 650 and 700°, and finally compared with commercially used GDC10 [22,23].

Fig. 1. Single cell fabrication process.

down to room temperature. The initial thermal decomposition of the precursor was dried 110 °C for 24 h. Subsequently, obtained product was calcined in furnaces (PROTHERM) from room temperature to 600 °C at a heating rate of 1.5 °C min-1 for 24 h in air. In all cases, the components were ball-milled for 2 h at 150 rpm in a vessel with zirconia balls. Grinding process applied for different time intervals (12 h, 15 h, 18 h, 20 h, 25 h, 30 h, 35 h, 40 h and 45 h). Manufacturing procedures were schematically given in Fig. 1.

2.3. Slurry preparation

In this study, gadolinium (III) nitrate hexahydrate, cerium nitrate hexahydrate metal salts and citric acid were used as raw materials for sol—gel method. Obtained product was calcined at 600 °C for 24 h. After from calcination procedure, obtained powders were grinding (RETSCH PM 100) in different hours (12 h, 15 h, 18 h, 20 h, 25 h, 30 h, 35 h, 40 h and 45 h). After grinding, slurry of these powders were prepared by using polyvinyl butyral (PVB) as binder, polyethylene glycol (PEG-200) as plasticizer, menhaden fish oil as dispersant and ethyl methyl ketone as solvent for tape casting.

2.4. Fabrication process ofSOFC single cell

2. Experimental details

2.1. Materials

Cerium nitrate hexahydrate [Ce(NO3)3 6H2O], (99.9% purity, Alfa Aesar) gadolinium (III) nitrate hexahydrate [Gd(NO3)3.6H2O], (99.9% purity, Alfa Aesar) were used as metal precursors and citric acid (C6H8O7), (Alfa Aesar) was selected for the polymerization treatment.

2.2. Powder preparation

Cerium nitrate hexahydrate [Ce(NO3)3 6H2O] and gadolinium (III) nitrate hexahydrate [Gd (NO3)3 6H2O] were used as metal precursors and citric acid (C6H8O7) was selected for the polymerization treatment. Nitrates were dissolved in de-ionized water individually and then the solutions were mixed in a beaker. Anhydrous citric acid was dissolved in de-ionized water and then was added to the cation solution. The molar ratio of total oxide (TO):citric acid (CA) was selected as 2:3. After homogenization of this solution, temperature was raised to 80 °C and the solution was kept for 5 h at this temperature with magnetic stirring to remove excess water. During this time, the colour of the liquid turned from white to light brown. Then obtained light brown gel was cooled

Tape casting films were pressed by firstly hydraulic press and secondly by isostatic press at the 30 MPa. In this way the pressed electrolyte films were sintered at 1400 °C. Sintered electrolytes were coated by NiO as anode and LSF (La0.60Sr0.40FeO3) as cathode via screen printing method [18]. The electrochemical performances of prepared solid oxide fuel cells having an active area of 1 cm2 were measured in the fuel cell test station at 650 and 700 °C. The same procedures were also done for commercial GDC10.

Fig. 2. Experimental setup for testing SOFC.

2.5. Fuel cell tests

Fuel cell tests were performed in fuel cell test station (Scribner 855) in Fig. 2, using humidified 5% H2 + 95% N2 gas mixture, pure H2 as fuels and air as an oxidant.

3. Results and discussion

3.1. XRD measurements

XRD technique was used to determine the crystal structure of the crystalline phases. X-ray spectra of gadolinium-doped ceria powders were obtained over the 2 G. As it can be seen from Fig. 2, XRD analysis of the synthesis GDC10, coincides with the commercial. Therefore synthesis of GDC10 was successfully completed (Fig. 3).

3.2. Investigation of SEM photographs

Fig. 4 is illustrated by the SEM images both the synthesized GDC10 electrolyte by surface grinding 20 h and commercial GDC10 electrolyte surface. As seen in the Fig. 4a, particle size of the commercial GDC10 is highly growth and grains was quietly heterogeneous. Fig. 4b shows pore structure, but grains are highly small.

3.3. Electrochemical measurements

In order to prepare anode and cathode, samples grounded for different time intervals were coated on the electrolyte by using 66% NiO containing GDC and 50% LSF-40 containing GDC, respectively. Afterwards, the samples were sintered, and performance measurements were conducted on 1 cm2 area for anode and cathode. The performance of samples obtained through synthesis process was compared with commercially available samples at two temperatures, 650 and 700 °C, and the results are given in Figs. 5 and 6 respectively. Performance measurements showed that the studied

experimental parameters (i.e., temperature and grinding time) had a significant effect on the power density. Moreover, synthesized GDC10 exhibited different performances compared to commercially available GDC10. Maximum power densities for commercial GDC10 were recorded as 0.413 W/cm2 and 0.461 W/cm2 at 650 and 700 °C, respectively. Fig. 5 illustrates for the synthesis of GDC10 to current/ power density and current/voltage graphs at 650 °C. Maximum power density of commercial GDC10's at 650 °C was 0.413 W/cm2 and the maximum current density was 0.949 A/cm2. The best performance of fabricated cell with synthesized GDC10 was observed in the sample milled 20 h. Maximum power density of this sample was 0.391 W/cm2, the maximum current density was 0.899 A/cm2. After synthesizing, 18 and 15 h milled samples showed the performance values close to the commercial GDC10. These values were recorded as 0.327 W/cm2, 0.689 A/cm2 and 0.362 W/cm2, 0.895 A/cm2, respectively. Fig. 6 demonstrates the current/power density and current/voltage graphs of the synthesized GDC10 at 700 ° C. Commercial GDC10's maximum power density at 700 °C was 0.461 W/ cm2 and the maximum current density was 1.003 A/cm2. The best performance of fabricated cell with synthesized GDC10 was observed in the sample milled 20 h. Maximum power density of this sample was 0.480 W/cm2, the maximum current density was 1.231 A/cm2. After synthesizing, 18 h milled the samples showed the performance values close to the commercial GDC10. These values were recorded as 0.380 W/cm2, 0.854 A/cm2, respectively.

4. Summary (conclusion)

The membranes manufactured in house were coated by NiO as anode and LSF40 as cathode using screen printing technique.

Fig. 3. XRD results of synthesis and commercial GDCi0.

Fig. 4. SEM photographs of the electrolytes sintered at 1400 °C (a) Commercial GDC10 electrolyte, (b) 20 h grinding GDC10 electrolyte.

Fig. 5. The performance values (650 °C) of commercial and synthesizing the GDC10 electrolyte supported cell having an active area of 1 cm2.

Fig. 6. The performance values (700 °C) of commercial and synthesizing the GDCio electrolyte supported cell having an active area of 1 cm2.

Electrolyte supported solid oxide fuel cells having an active area of 1 cm2 were fabricated. Cell performances were tested electro-chemically using fuel cell test station at 650 and 700 ° C. Performance measurements of synthesized GDC10 were compared with

those of commercial GDCio at two different temperatures (i.e., 650 and 700 °C). Commercial GDC10 at 650 °C has maximum power density of 0.413 W/cm2 and maximum current density of 0.949 A/ cm2, that is, better from synthesized GDC10. However, synthesized GDC10 at 700 °C which are better performance than commercial GDC10. The samples 20 h mill, have maximum power density of 0.480 W/cm2 and maximum current density of 1.231 A/cm2. In this study, XRD and SEM images of synthesized GDC10 were investigated.

Acknowledgement

We would like to thank Research Projects Unit of Nigde University (The Project Code: FEB 2013/45) for financial support in this study. Moreover, we would like to thank Dr. Omer Selamet for proof reading process.

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