BaCe0.7Zr0.1Y0.2-xNdxO3- Proton Conducting Electrolyte for Intermediate Temperature Solid Oxides Fuel Cells

Jie-Yuan Lin1,2, Xian-Zhu Fu2, Gui-Hua Zhou2, Jing-Li Luo2,*, Karl T. Chuang2, Alan R. Sanger2,

Ru-An Chi1 1School of Chemical Engineering & Pharmacy, Wuhan Institute of Technology, Wuhan, Hubei 430205, P.R. China 2Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada

E-mail: Jingli.Luo@ualberta.ca

Abstract: The sinterability, conductivity and chemical stability in CO2 atmosphere are compared for a series of doped barium cerate perovskite oxides BaCe0.7Zr0.1Y0.2-xNdxO3- (0 ≤ x ≤ 0.2) which synthesized using a solid state reaction method. Among the series of electrolytes doped with various amounts of Nd and Y, BaCe0.7Zr0.1Y0.15Nd0.05O3- has the optimal combination of sinterability and conductivity, and shows high CO2 resistance. A solid oxide fuel cell using the BaCe0.7Zr0.1Y0.15Nd0.05O3- proton conducting electrolyte at 650-700 oC efficiently co-produces electrical power and value-added ethylene from ethane.

Key words: Electricity, solid oxide fuel cell, proton conductor, conversion of ethane to ethylene, barium ce-rate

1. Introduction

Solid oxide fuel cells (SOFCs) have attracted much at-tention in recent years since they can produce electrical energy with high efficiency and they are more environ-mentally friendly when compared to conventional elec-tricity generation from fossil fuels. Proton conducting electrolytes have higher ionic conductivity at lower tem-peratures compared to oxide ion conducting electrolytes, since protons have lower activity energy of transporta-tion than oxide ions. To date, Y doped BaCeO3- exhibits the highest proton conductivity among the reported perovskite oxides [1, 2]. However, Y doped BaCeO3- is unstable in CO2 containing atmospheres, thus limiting its application as electrolyte material for SOFCs [3-5]. Herein we report Zr, Y and Nd co-doped BaCeO3- perovskite oxides as proton conducting electrolytes for IT-SOFC for conversion of ethane selectively to ethylene.

2. Experimental

2.1 Electrolytes preparation

Compounds having the generalized formula BaCe0.7Zr0.1Y0.2-xNdxO3-, where x ranges between 0 and 0.2, were prepared by solid state reactions. Starting ma-terials BaCO3, CeO2, ZrO2, Y2O3, and Nd2O3 were ball-milled in stoichiometric ratio for 24 h. The pressed mixtures then were calcined at temperatures in the range from 1100 oC to 1400 oC for 10 h to determine the opti-mum temperature. After a single perovskite phase was obtained, the resulted materials were ball-milled again for 24 h, pressed at 5 t.in-2 in a stainless steel mold to form discs, and sintered at 1500 oC for 10 h with heating and cooling rates of 1 oC.min-1. Then gold paste was screen printed onto each side of polished pellets, calcined at 900 oC for 30 min to obtain membrane electrode as-

semblies (MEA). Instead of gold, platinum paste was used to prepare

porous electrodes on proton conducting electrolytes of MEA for SOFC performance tests, as Pt is an active catalyst for conversion of ethane to ethylene and reduc-tion of oxygen.

2.2 Characterization of electrolytes

The phase structure and the crystal lattice parameters for each electrolyte were determined at room temperature using a Rigaku Rotaflex powder X-ray diffractometer (XRD) with CuKα radiation. The morphology was in-vestigated using a Hitachi S-2700 scanning electron mi-croscopy (SEM).

The proton conductivity measurements were carried out using a Solartron 1287 electrochemical interface to-gether with 1255B frequency response analysis instru-mentation.

The chemical stability of sintered electrolytes in CO2 atmosphere was determined using a TA SDT Q600 thermal gravity analysis (TGA) in a CO2/He flow (CO2 = 15 mL.min−1; He = 100 mL.min−1) from room tempera-ture to 1200 oC at a heating rate of 10 oC.min−1.

2.3 SOFC fabrication and test

The SOFC was fabricated by placing the

Pt/electrolyte/Pt MEA between concentric pairs of alumina tubes in a vertical Thermolyne F79300 tubular furnace. After the SOFC reached the prescribed operat-ing temperature, ethane and oxygen were supplied as anode and cathode feed gas, respectively. All the elec-trochemical tests were performed using a Solartron 1287 electrochemical interface together with 1255B frequency response analysis instrumentation. The outlet gas from

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the anode chamber was analyzed using a Hew-lett-Packard model HP5890 GC having a thermal con-ductivity detector. The ethane conversion and ethylene selectivity were calculated according to the previously reported method [6].

3. Results and Discussion

3.1 Phase structure

Pressed stoichiometric mixtures of BaCO3, CeO2, ZrO2, Y2O3, and Nd2O3 precursors for preparation of BaCe0.7Zr0.1Y0.15Nd0.05O3- electrolyte were heated at 1100 oC, 1200 oC, 1300 oC, and 1400 oC for 10 h in air(Fig. 1). Compared to the samples calcines at 1100 oC and1200 oC, XRD of samples calcined at 1300 oC and 1400 oC showed only the perovskite phase.

20 30 40 50 60 70 80 90

1100 oC

1200 oC

1300 oC

inte

nsit

y

2(degree)

1400 oC

Figure 1. XRD patterns of BaCe0.7Zr0.1Y0.15Nd0.05O3- precursor calcined at different temperatures for 10 h in air.

3.2 Sinterability of the doped barium cerate

Figure 2. shows the surface SEM images of BaCe0.7Zr0.1Y0.2-xNdxO3- (0 ≤ x ≤ 0.2) discs after sinter-ing at 1500 oC for 10 h in air. When the Nd content was in the range of 5~15 mol % (mol % corresponds to the cation content in B position), very few pores were ob-served, and the grains size did not change significantly (12~15 μm). It's reasonable to say that Y and Nd co-dopping appeared to improve the sinterability of the samples. SEM showed that the surfaces of Y and Nd co-doped samples sintered at 1500 oC were very dense compared to the samples containing no Y or Nd.

Figure 2. Surface SEM images of the discs sintered at 1500 oC for 10 h in air: (a) BaCe0.7Zr0.1Y0.2O3-, (b) BaCe0.7Zr0.1Y0.15Nd0.05O3-, (c) BaCe0.7Zr0.1Y0.125Nd0.75O3-, (d) BaCe0.7Zr0.1Y0.1Nd0.1O3-, (e) BaCe0.7Zr0.1Y0.05Nd0.15O3-, (f) BaCe0.7Zr0.1Nd0.2O3-.

3.3 Conductivity of doped barium cerate

0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30

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1.0

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1.6

1.8

1000/T, K-1

L

og

/ m

S.c

m-1

a b c d

Figure 3. Conductivity of BaCe0.7Zr0.1Y0.2-xNdxO3- in humid hy-drogen atmosphere (5 % H2 balance with Ar) at 500-800 oC: (a) BaCe0.7Zr0.1Y0.15Nd0.05O3-, (b) BaCe0.7Zr0.1Y0.125Nd0.75O3-, (c) BaCe0.7Zr0.1Y0.1Nd0.1O3-, (d) BaCe0.7Zr0.1Y0.05Nd0.15O3-.

The conductivities of the doped barium cerate in hu-

midified hydrogen atmosphere at temperature in the range 500-800 oC are compared in Fig.3. Increasing the Nd content in the samples, the conductivity of BaCe0.7Zr0.1Y0.2-xNdxO3- increases first then decreases. At 700 oC, the value of the BaCe0.7Zr0.1Y0.15Nd0.05O3- is 29 mS.cm-1, which is the highest among the samples at the same test temperature.

3.4 Stability of the doped barium cerate

It is well known that doped-BaCeO3- perovskite ox-ide is unstable in CO2 atmosphere at elevated tempera-ture because the material reacts readily with CO2 and so decomposes to ceria and BaCO3.

[7]. Fig. 4 shows the

c d

e f

b a

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TGA curves of BaCe0.85Y0.15O3- (BCY) and BaCe0.7Zr0.1Y0.15Nd0.05O3- (BCZYN) in CO2 atmosphere. When the temperature was above 500 oC there was large weight uptake to a maximum of 10.1 % for BCY. In contrast, there was negligible weight uptake when BCZYN was heated in CO2, showing that Zr doping im-proved the chemical stability of barium cerate in CO2 atmosphere at high temperature [8].

0 200 400 600 800 1000 120098

100

102

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110

112

114

Temperature / oC

Wei

ght /

%

a

b

Figure 4. Chemical stability of sintered (a) BaCe0.85Y0.15O3- and (b) BaCe0.7Zr0.1Y0.15Nd0.05O3- determined using TGA in CO2 atmos-phere.

3.5 Performance of the IT-SOFC

Since BaCe0.7Zr0.1Y0.15Nd0.05O3- demonstrated the best combination of conductivity and sinterability, as well as with good chemical stability, it was used to fabricate SOFC with porous Pt anodes. Fig.5 shows the electro-chemical performance of the cell. The maximum power density of the cell was 54 mW.cm-2 and the correspond-ing current density was 146 mA.cm-2 at 650 oC. When the operating temperature was increased to 700 oC the maximum power density of the cell increased to 132 mW.cm-2 at current density 302 mA.cm-2.

Ethylene and electrical power were co-produced by conversion of ethane in the cell. Ethane conversion was 18.7 % at 650 oC, and ethylene selectivity was high to 90 %. The by-products in the anode chamber were mainly methane, with traces of H2, CO and CO2. At 700 oC eth-ane conversion inceased to 38.1 %. However, ethylene selectivity decreased to 89 % and methane selectivity increased. Increased conversion of ethane to methane at higher operating temperatures showed that cracking of ethane increased, in competition with ethane dehydroge-nation. Nevertheless, ethylene selectivity was much higher comparing to conventional oxidative dehydroge-nation of ethane to ethylene [9].

4. Conclusions

Among Zr, Y and Nd co-doped barium cerate perovskite

oxides synthesized using solid state reactions, BaCe0.7Zr0.1Y0.15Nd0.05O3- had the best combination of conductivity, sinterability, and good chemical stability in the presence of CO2. An intermediate temperature SOFC with BaCe0.7Zr0.1Y0.15Nd0.05O3- as proton conducting electrolyte exhibited excellent performance for conver-sion of ethane selectively to ethylene with co-generation of electrical energy.

0 100 200 300 400 500 600 700 800 900 10000.0

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er density / mW

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tage

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Figure 5. Current density-voltage and power density curves of ethane/oxygen solid oxide fuel cells with BaCe0.7Zr0.1Y0.15Nd0.05O3- as proton conducting electrolyte at 650 and 700 oC. The flow rates of ethane and oxygen each are 150 mL.min-1.

Acknowledgements

This work was supported by Natural Sciences and Engi-neering Research Council of Canada/NOVA Chemicals CRD Grant, Alberta Innovates Energy and Environment Solutions and Micro Systems Technology Research In-stitute.

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