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Research Papers

Evaluation of Porous Ni-YSZ Cermets With Ni Content of 0–30 Vol. % as Insulating Substrates for Segmented-in-Series Tubular Solid Oxide Fuel Cells

[+] Author and Article Information
Zhenwei Wang

 Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka-shi, Kanagawa-ken 240-0196, Japan

Masashi Mori1

 Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka-shi, Kanagawa-ken 240-0196, Japanmasashi@criepi.denken.or.jp

Takanori Itoh

 AGC Seimi Chemical Co. Ltd., 3-2-10 Chigasaki, Chigasaki-shi, Kanagawa-ken 253-8585, Japan

1

Corresponding author.

J. Fuel Cell Sci. Technol 9(2), 021004 (Mar 19, 2012) (7 pages) doi:10.1115/1.4005607 History: Received August 29, 2011; Revised September 22, 2011; Published March 07, 2012; Online March 19, 2012

Nickel was added to a substrate composed of porous Y2 O3 -stabilized ZrO2 (YSZ) in order to minimize anode damage during redox cycling in segmented-in-series tubular solid oxide fuel cells (SOFCs) with YSZ electrolytes. In this study, the electrical insulating and thermal properties of these materials were evaluated for their suitability as substrates in the tubular SOFCs. When the Ni content was ≤20 vol. %, the porous cermets showed an electrical resistance of ≤67 Ω cm at 900 °C, indicating that the theoretical open circuit voltage for the tubular SOFCs could be achieved. However, the cermet with 20 vol. % Ni was destroyed during the first heating cycle in air because of large isothermal expansion. However, no obvious cracks were observed for cermets with ≤10 vol. % Ni. From the viewpoint of thermogravimetric measurement, this suggests that there are two redox mechanisms for Ni particles in the substrate. They were reduced/oxidized by both the gases and the oxide-ions passing through the YSZ framework. Based on the insulating and thermal properties of the substrate, the optimal composition was found to be approximately 10 vol. % Ni.

Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Porosities of the NiO-YSZ and Ni-YSZ samples as a function of nickel volume content. The broken line represent the minimum porosity allowed easy diffusion and transportation of the reactant and product gases.

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Figure 2

Lattice parameters of the Ni-doped YSZ fluorites before and after reducing at 1000 °C as a function of nickel content. The closed symbols indicate where the second phase, consisting of fluorite and NiO or Ni, was observed.

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Figure 3

SEM observation (a) and the distribution of Ni (b) and Zr (c) elements for the 30 vol. % Ni-YSZ cermet. The gray and light gray parts represent YSZ and Ni, respectively.

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Figure 4

Electrical properties of the porous Ni-YSZ samples as a function of Ni volume fraction in a 30% H2 -N2 reducing atmosphere. (a) Electrical resistance as a function of Ni volume fraction. The broken line represents the resistance of the dense YSZ at 900 °C, (b) Arrhenius plots of electrical conductivity. ----: YSZ, ○: 5 vol. % Ni-YSZ, ▵: 10 vol. % Ni-YSZ, □: 20 vol.% Ni-YSZ, ●: 30 vol. % Ni-YSZ.

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Figure 5

Equivalent circuit of segmented-in-series tubular SOFC

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Figure 6

Output voltage of single cell in segmented-in-series tubular SOFC as a function of resistant of substrate. The closed circle and triangle represent the OCVs of 20 vol. % Ni- and 30 vol. % Ni-YSZ substrates, respectively.

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Figure 7

TEC values of the porous NiO-YSZ composites in the H2 atmosphere and Ni-YSZ cermets in air during the redox cycling measurement as a function of nickel mole fraction. ○: in the H2 atmosphere during the first heating cycle, ▵: in air during the first heating cycle, ●: in the H2 atmosphere during the second heating cycle, ▴: in air during the second heating cycle.

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Figure 8

Differential TEC curves of the composites in the H2 atmosphere. The broken line represents the result of YSZ. (a) 8 vol. % NiO-YSZ, (b) 29 vol. % NiO-YSZ.

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Figure 9

Weight changes of the composites in the H2 atmosphere as a function of temperature. The broken lines represent the TG curves of the unfired samples. (a) 29 vol. % NiO-YSZ and 41 vol. % NiO-YSZ. Solid line : YSZ, ○: 29 vol. % NiO-YSZ, - - - : unfired 29 vol. % NiO-YSZ, ▵: 41 vol. % NiO-YSZ, - - - - : unfired 41 vol. % NiO-YSZ. (b) ○: YSZ, ▵: 8 vol. % NiO-YSZ, - - - : unfired 8 vol. % NiO-YSZ, □: 15 vol. % NiO-YSZ, ----- : unfired 15 vol. % NiO-YSZ.

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Figure 10

TG curves of the fired NiO, NiO-ZrO2 , and NiO-Y2 O3 samples in the H2 atmosphere. (a) NiO-ZrO2 , Solid line : NiO, - - - : 2 mol%ZrO2 , ---- : 4 mol%ZrO2 , (b) NiO-Y2 O3 , Solid line : NiO, - - - : 2 mol%YO1.5 , ----- : 4 mol%YO1.5 .

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Figure 11

Lattice parameters of NiO with dopants (Y2 O3 or ZrO2 ) after firing at 1400 °C for 6 h

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Figure 12

XRD patterns of the unfired and fired 15 vol. % NiO-YSZ samples after reduction at 500 °C for 5 min. (a) Unfired sample and (b) fired sample heat-treated at 1400 °C for 6 h

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Figure 13

REM micrographs of sintered 15 vol. % NiO-YSZ composite before (a) and after (b) reducing at 1000 °C in the H2 atmosphere. The gray and dark gray parts show the YSZ and NiO or Ni particles, respectively.

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Figure 14

Weight change of the Ni-YSZ samples in air and the broken lines represent the weight change of the unfired samples. (a) Weight change, — — — : 5 vol. % Ni-YSZ, - - - : 10 vol. % Ni-YSZ, ----- : 20 vol. % Ni-YSZ, (b) differential weight change of the fired samples, (c) differential weight change of the unfired samples.

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Figure 15

Differential TEC curves of the cermets in air. The broken line represents the results of YSZ. (a) 5 vol. % Ni-YSZ, (b) 20 vol. % Ni-YSZ.

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Figure 16

Maximum TEC values of the cermets during TEC measurements in air

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