- Email: [email protected]
Effect of Sm-doping on the hydrogen permeation of Ni–La2Ce2O7 mixed protonic–electronic conductor
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 4 5 0 8 e4 5 1 1
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
Effect of Sm-doping on the hydrogen permeation of NieLa2Ce2O7 mixed protoniceelectronic conductor Litao Yan, Wenping Sun, Lei Bi, Shumin Fang, Zetian Tao, Wei Liu* CAS Key Laboratory of Energy Conversion Materials, University of Science and Technology of China (USTC), Hefei 230026, PR China
article info
abstract
Article history:
The cermet consisting of electronic conductor Ni and proton conductor La2Ce2O7 (LDC)
Received 28 January 2010
shows good chemical stability but poor hydrogen permeability. In order to improve the
Received in revised form
hydrogen permeability, novel NieLa2xSmxCe2O7 (x ¼ 0, 0.025, 0.05, 0.075, 0.1 and 0.2)
25 February 2010
cermets were developed for hydrogen separation. The results show that Sm element
Accepted 27 February 2010
doping of LDC can affect the rate of hydrogen permeation, with NieLa1.95Sm0.05Ce2O7
Available online 1 April 2010
possessing the highest hydrogen permeation fluxes. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Keywords: Hydrogen separation Cermet Proton conductor
1.
Introduction
In view of the energy crisis and global climate change, clean hydrogen is expected to substitute for oil-based fuel in the future [1]. High-purity hydrogen separated by membrane from products of the reforming and partial oxidation of natural gas is promising as a low-cost energy source [2]. Compared with traditional Pd-alloy membranes, composite membranes consisting of electronic conducting Ni and proton conducting BaCe0.7Zr0.1Y0.2O3-d (BCZY) or La2Ce2O7 (LDC) are more costeffective [3,4]. Zuo et al. reported the cermet NieBCZY showed rather high performance and reasonable stability in H2O and CO2, but the hydrogen permeability greatly decreased in CO2-containing atmospheres [3e5]. In previous works, we found that the cermet NieLDC can tolerate CO2 completely; unfortunately, the performance is poor. Recently Anderson et al. reported their quantum mechanical calculation results suggesting the relationship between defect association and
ionic conductivity in doped ceria. They suggested doping with a mixture of trivalent cations could result in higher ionic conductivity than single doping [6]. In order to improve the hydrogen permeability of NieLDC, we synthesized La2xSmxCe2O7 (x ¼ 0, 0.025, 0.05, 0.075, 0.1 and 0.2) powders via a citrate-nitrate gel combustion method. These powders were mixed with Ni and sintered in a reducing atmosphere to form cermets. The effect of trivalent cation co-doping on the hydrogen permeation fluxes of these cermets was studied.
2.
Experimental procedure
La2xSmxCe2O7(x ¼ 0, 0.025, 0.05, 0.075, 0.1 and 0.2) powders were prepared by citrateenitrate gel combustion method. Firstly, Ce(NO3)3$6H2O was dissolved in distilled water in a beaker. Secondly, appropriate amounts of La2O3 and Sm2O3
* Corresponding author. Tel.: þ86 551 3602940; fax: þ86 551 3601592. E-mail address: [email protected] (W. Liu). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.02.134
4509
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 4 5 0 8 e4 5 1 1
x=0.2 x=0.1 x=0.075 Intensity (a.u.)
were dissolved in diluted nitric acid in another beaker. The solutions were mixed, stirred and heated. Then citric acid was added. The molar ratio of citric acid to metal ions was 1.5:1. Ammonia was added to the solution until the pH value was about 8. The solution was heated for hours to form a viscous gel. The gel was heated with an electric furnace until ignition. The obtained powders were calcined at 800 C for 3 h. After required phase compositions were confirmed from XRD patterns, the powders were mixed with Ni powder (>99.9%) in a volume ratio of 3:2. The mixed powders were ball-milled in alcohol for 24 h. The powders were dried and pressed into pellets with a diameter of 15 mm, and sintered at 1430 C for 5 h in 4% H2 balanced with nitrogen. The density of the pellets was measured in mercury by Archimedes method. The hydrogen permeation flux measurement was carried out in a self-built apparatus. The sintered pellets were polished with 120, 240, 600-grit SiC sandpaper to make both sides flat and parallel. The thickness of each pellet was about 0.6 mm to make the results comparable. The pellets were sealed on an alumina tube with a glass ring sealant at 990 C for 60 min. The feed gas was acquired by mixing 20 ml/min high purity H2 and 80 ml/min N2 followed by bubbling through deionized water at 25 C. The sweep gas was high purity Ar (99.99%) with a flow rate of about 20 ml/min. The permeated effluent gas was analyzed by GC-14C (Shimadzu) using high purity Ar as a carrier gas. In our experiments, glass ring was used for sealing; a small amounts of nitrogen and hydrogen would leak through incomplete sealing, so the permeated effluent gas includes hydrogen, Ar and nitrogen, and at the side of feed gas, the volume ratio of nitrogen and hydrogen was 1:4. The hydrogen leakage rates were checked by leaked nitrogen and were below 5% during our measurements. The phase composition was analyzed by X-ray diffraction (XRD, Philips X’Pert Pro Super, Cu Ka1, 40 kV, 50 mA).
x=0.05 x=0.025
x=0
20
30
40
50
:: Ln2 O3 / 2Ln0Ce þ 3O o þ Vo
Ln ¼ Sm; Y; La; Gd..
Due to the high dopant concentration (w50%) in our samples, the oxygen vacancies are ordered in the crystal lattice. It is noticeable that the site of oxygen vacancy caused by La ion
3.00E-008
Ni−La1.9Sm0.1Ce2O7 dry Ni−La1.95Sm0.05Ce2O7 dry
2.50E-008
Ni−La1.9Sm0.1Ce2O7 wet Ni−La1.95Sm0.05Ce2O7 wet
2.00E-008
2
Shown in Fig. 1 are XRD patterns of La2xSmxCe2O7 powders (x ¼ 0, 0.025, 0.05, 0.075, 0.1 and 0.2). All the samples show fluorite structure. As the Sm content increases, the peak position shifts to higher angles, indicating that the fluorite unit cell volume decreases as the smaller Sm3þ ion replaced the larger La3þ. Fig. 2 shows the temperature dependence of hydrogen permeation through the NieLa1.95Sm0.05Ce2O7 (NieLSC05) and the NieLa1.9Sm0.1Ce2O7 (NieLSC10) cermet membranes between 700 and 900 C. The appreciable hydrogen permeability suggests that the Sm-doped LDC is a proton conducting ceramic. The hydrogen flux increased as the temperature increased in both dry and wet conditions (in the wet condition, w3% water vapor was obtained by bubbling the feed gas through deionized water at room temperature). The highest flux of 2.86 108 mol cm2 s1 at 900 C was obtained using NieLSC05 cermet in wet feed gas. In comparison with the NieLSC05 sample, NieLSC10 sample had lower fluxes in both dry and wet conditions. Fig. 2 also shows that the fluxes in wet atmosphere were much higher than that in dry condition at the same temperature for both samples, which indicated that the presence of water vapor in the feed gas could enhance the
80
proton conductivity as well as the surface catalytic activity of the cermet [3], leading to higher hydrogen permeation performance. Fig. 3 shows the temperature dependence of hydrogen flux though all of the NieLa2xSmxCe2O7 (x ¼ 0, 0.025, 0.05, 0.075, 0.1 and 0.2) membranes under wet conditions. The hydrogen fluxes increased with temperature increasing, and the highest hydrogen flux was 2.86 108 mol cm2 s1 at 900 C when x ¼ 0.05. The hydrogen fluxes of all the Sm-doped samples except x ¼ 0.2 was greater than that of the undoped sample. Trivalent cation ions dope CeO2 would have this defect equation:
J (mol/cm s)
Results and discussion
70
Fig. 1 e X-ray diffraction (XRD) patterns of synthesized ceramic powders of La2LxSmxCe2O7 (x [ 0, 0.025, 0.05, 0.075, 0.1 and 0.2).
CeO2
3.
60
2 theta (degree)
1.50E-008
1.00E-008
5.00E-009 700
750
800
850
900
o
Temperature ( C)
Fig. 2 e Hydrogen permeation flux through NieLa2LxSmxCe2O7 (x [ 0.05, 0.1) membranes under wet and dry atmospheres.
4510
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 4 5 0 8 e4 5 1 1
3.20E-008 o
900 C o 850 C o 800 C o 750 C o 700 C
2.80E-008
2.00E-008
2
J (mol/cm s)
2.40E-008
1.60E-008
1.20E-008
Fig. 5 e Schematic process of water incorporation into oxygen vacancies in NNN position of La ions.
8.00E-009
4.00E-009 0.00
0.05
0.10
0.15
0.20
Sm-content (x)
Fig. 3 e Hydrogen permeation flux through NieLa2LxSmxCe2O7 (x [ 0, 0.025, 0.05, 0.075, 0.1 and 0.2) membranes under wet atmospheres.
was different from that caused by Sm ion. The quantum mechanical calculation results of Andersson et al. [6] suggest that oxygen vacancies formation energy in next nearest neighbor position (NNN) of La ion is lower than that in the nearest neighbor (NN) position. In the case of Sm ion, it is opposite. Thus the oxygen vacancy occupies the NNN position of La ion but the NN position of Sm ion, which is shown in Fig. 4. A small amount of Sm-doping destroys the oxygen vacancy sequence, which may facilitate proton transport in the LDC, but proton transport may become difficult again as a new sequence is formed with larger concentrations of Sm cations. Thus the hydrogen permeability of NieLa2xSmxCe2O7 (x ¼ 0.075, 0.1 and 0.2) was smaller than that of NieLa1.95Sm0.05Ce2O7 at the same temperature. Another explanation is that excessive Sm-doping would also decrease the surface reaction rate. When the sample is exposed in moisture, water molecular will be incorporated in the oxygen vacancy. The reaction is as follows: H2 O þ V::o þ Oxo /2OH$o As the oxygen vacancy caused by La ion occupies the NNN position of La ion, oxygen ion in adsorbed water molecule occupies the oxygen vacancy in the neighbor of Ce ions. At the same time, two protons are formed around this oxygen
ion. These protons suffer electrostatic repulsion from central cation (Ce4þ or La3þ). As the repulsion between La3þ and proton is lower than that between Ce4þ and proton, these protons have higher affinity to the La ions so they move to the oxygen ions in NN positions. This process is shown in Fig. 5. For oxygen vacancies caused by Sm ions, two protons are always present near the same oxygen ion after water molecule is incorporated into the oxygen vacancy in the NN position of Sm ions. Considering the unit cell volume decreases with Sm-doping in La sites as confirmed by Fig. 1, the repulsion between Sm3þ and proton is bigger than that between La3þ and proton. Thus it would be harder for the incorporation of water into oxygen vacancies in NN positions of Sm ions. It should be mentioned that the CeeO interatomic distances also decrease upon doping with smaller trivalent (e.g. Sm3þ) [6], thus the incorporation of water into oxygen vacancies in NNN positions of Sm ions is even harder. Generally, the doping of La with Sm can decrease the surface reaction rate. Why trivalent cation ions co-doped in CeO2 can improve the proton conductivity and a precise explanation of why the excessive Sm doped sample would decrease the proton conductivity is still under investigation.
4.
Conclusions
Hydrogen membranes based on Ni and lanthanum-doped ceria are successfully prepared. Sm-doping in LDC can affect the rate of hydrogen permeation. Sm-doping with a small amount is beneficial for hydrogen permeability. However, excess Sm-doping can decrease it. Among all the compositions investigated, NieLa1.95Sm0.05Ce2O7 has the highest hydrogen permeability. Excess Sm-doping reduces the cell volume and decreases the surface reaction rate.
Acknowledgements
Fig. 4 e Schematic description of defect configuration in CeO2.
We gratefully acknowledge the support of National High-tech R&D Program of China (no. 2007AA05Z157), Key Program of Chinese Academy of Sciences (no. KJCX1.YW07), and the CAS Special Grant for Postgraduate Research, Innovation and Practice.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 4 5 0 8 e4 5 1 1
references
[1] Dicks AL. Hydrogen generation from natural gas for the fuel cell systems of tomorrow. J Power Sources 1996;61: 113e24. [2] Zhang G, Dorris S, Balachandran U, Liu M. Interfacial resistances of NieBCY mixed-conducting membranes for hydrogen separation. Solid State Ionics 2003;159:121e34. [3] Zuo CD, Dorris SE, Balachandran U, Liu ML. Effect of Zr-doping on the chemical stability and hydrogen permeation of the
4511
NieBaCe0.8Y0.2O3d mixed protoniceelectronic conductor. Chem Mater 2006;18:4647e50. [4] Zuo CD, Lee TH, Dorris SE, Balachandran U, Liu ML. Composite NieBa(Zr0.1Ce0.7Y0.2)O3 membrane for hydrogen separation. J Power Sources 2006;159:1291e5. [5] Zuo CD, Lee TH, Song SJ, Chen L, Dorris SE, Balachandran U, et al. Hydrogen permeation and chemical stability of cermet NieBa[Zr0.8xCexY0.2]O3 Membranes. Electrochem Solid State Lett 2005;8:J35. [6] Andersson David A, Simak Sergei I, Skorodumova Natalia V, Abrikosov Igor A, Johansson B. Optimization of ionic conductivity in doped ceria. Proc Natl Acad Sci USA 2006;103:3518.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
Effect of Sm-doping on the hydrogen permeation of NieLa2Ce2O7 mixed protoniceelectronic conductor Litao Yan, Wenping Sun, Lei Bi, Shumin Fang, Zetian Tao, Wei Liu* CAS Key Laboratory of Energy Conversion Materials, University of Science and Technology of China (USTC), Hefei 230026, PR China
article info
abstract
Article history:
The cermet consisting of electronic conductor Ni and proton conductor La2Ce2O7 (LDC)
Received 28 January 2010
shows good chemical stability but poor hydrogen permeability. In order to improve the
Received in revised form
hydrogen permeability, novel NieLa2xSmxCe2O7 (x ¼ 0, 0.025, 0.05, 0.075, 0.1 and 0.2)
25 February 2010
cermets were developed for hydrogen separation. The results show that Sm element
Accepted 27 February 2010
doping of LDC can affect the rate of hydrogen permeation, with NieLa1.95Sm0.05Ce2O7
Available online 1 April 2010
possessing the highest hydrogen permeation fluxes. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Keywords: Hydrogen separation Cermet Proton conductor
1.
Introduction
In view of the energy crisis and global climate change, clean hydrogen is expected to substitute for oil-based fuel in the future [1]. High-purity hydrogen separated by membrane from products of the reforming and partial oxidation of natural gas is promising as a low-cost energy source [2]. Compared with traditional Pd-alloy membranes, composite membranes consisting of electronic conducting Ni and proton conducting BaCe0.7Zr0.1Y0.2O3-d (BCZY) or La2Ce2O7 (LDC) are more costeffective [3,4]. Zuo et al. reported the cermet NieBCZY showed rather high performance and reasonable stability in H2O and CO2, but the hydrogen permeability greatly decreased in CO2-containing atmospheres [3e5]. In previous works, we found that the cermet NieLDC can tolerate CO2 completely; unfortunately, the performance is poor. Recently Anderson et al. reported their quantum mechanical calculation results suggesting the relationship between defect association and
ionic conductivity in doped ceria. They suggested doping with a mixture of trivalent cations could result in higher ionic conductivity than single doping [6]. In order to improve the hydrogen permeability of NieLDC, we synthesized La2xSmxCe2O7 (x ¼ 0, 0.025, 0.05, 0.075, 0.1 and 0.2) powders via a citrate-nitrate gel combustion method. These powders were mixed with Ni and sintered in a reducing atmosphere to form cermets. The effect of trivalent cation co-doping on the hydrogen permeation fluxes of these cermets was studied.
2.
Experimental procedure
La2xSmxCe2O7(x ¼ 0, 0.025, 0.05, 0.075, 0.1 and 0.2) powders were prepared by citrateenitrate gel combustion method. Firstly, Ce(NO3)3$6H2O was dissolved in distilled water in a beaker. Secondly, appropriate amounts of La2O3 and Sm2O3
* Corresponding author. Tel.: þ86 551 3602940; fax: þ86 551 3601592. E-mail address: [email protected] (W. Liu). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.02.134
4509
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 4 5 0 8 e4 5 1 1
x=0.2 x=0.1 x=0.075 Intensity (a.u.)
were dissolved in diluted nitric acid in another beaker. The solutions were mixed, stirred and heated. Then citric acid was added. The molar ratio of citric acid to metal ions was 1.5:1. Ammonia was added to the solution until the pH value was about 8. The solution was heated for hours to form a viscous gel. The gel was heated with an electric furnace until ignition. The obtained powders were calcined at 800 C for 3 h. After required phase compositions were confirmed from XRD patterns, the powders were mixed with Ni powder (>99.9%) in a volume ratio of 3:2. The mixed powders were ball-milled in alcohol for 24 h. The powders were dried and pressed into pellets with a diameter of 15 mm, and sintered at 1430 C for 5 h in 4% H2 balanced with nitrogen. The density of the pellets was measured in mercury by Archimedes method. The hydrogen permeation flux measurement was carried out in a self-built apparatus. The sintered pellets were polished with 120, 240, 600-grit SiC sandpaper to make both sides flat and parallel. The thickness of each pellet was about 0.6 mm to make the results comparable. The pellets were sealed on an alumina tube with a glass ring sealant at 990 C for 60 min. The feed gas was acquired by mixing 20 ml/min high purity H2 and 80 ml/min N2 followed by bubbling through deionized water at 25 C. The sweep gas was high purity Ar (99.99%) with a flow rate of about 20 ml/min. The permeated effluent gas was analyzed by GC-14C (Shimadzu) using high purity Ar as a carrier gas. In our experiments, glass ring was used for sealing; a small amounts of nitrogen and hydrogen would leak through incomplete sealing, so the permeated effluent gas includes hydrogen, Ar and nitrogen, and at the side of feed gas, the volume ratio of nitrogen and hydrogen was 1:4. The hydrogen leakage rates were checked by leaked nitrogen and were below 5% during our measurements. The phase composition was analyzed by X-ray diffraction (XRD, Philips X’Pert Pro Super, Cu Ka1, 40 kV, 50 mA).
x=0.05 x=0.025
x=0
20
30
40
50
:: Ln2 O3 / 2Ln0Ce þ 3O o þ Vo
Ln ¼ Sm; Y; La; Gd..
Due to the high dopant concentration (w50%) in our samples, the oxygen vacancies are ordered in the crystal lattice. It is noticeable that the site of oxygen vacancy caused by La ion
3.00E-008
Ni−La1.9Sm0.1Ce2O7 dry Ni−La1.95Sm0.05Ce2O7 dry
2.50E-008
Ni−La1.9Sm0.1Ce2O7 wet Ni−La1.95Sm0.05Ce2O7 wet
2.00E-008
2
Shown in Fig. 1 are XRD patterns of La2xSmxCe2O7 powders (x ¼ 0, 0.025, 0.05, 0.075, 0.1 and 0.2). All the samples show fluorite structure. As the Sm content increases, the peak position shifts to higher angles, indicating that the fluorite unit cell volume decreases as the smaller Sm3þ ion replaced the larger La3þ. Fig. 2 shows the temperature dependence of hydrogen permeation through the NieLa1.95Sm0.05Ce2O7 (NieLSC05) and the NieLa1.9Sm0.1Ce2O7 (NieLSC10) cermet membranes between 700 and 900 C. The appreciable hydrogen permeability suggests that the Sm-doped LDC is a proton conducting ceramic. The hydrogen flux increased as the temperature increased in both dry and wet conditions (in the wet condition, w3% water vapor was obtained by bubbling the feed gas through deionized water at room temperature). The highest flux of 2.86 108 mol cm2 s1 at 900 C was obtained using NieLSC05 cermet in wet feed gas. In comparison with the NieLSC05 sample, NieLSC10 sample had lower fluxes in both dry and wet conditions. Fig. 2 also shows that the fluxes in wet atmosphere were much higher than that in dry condition at the same temperature for both samples, which indicated that the presence of water vapor in the feed gas could enhance the
80
proton conductivity as well as the surface catalytic activity of the cermet [3], leading to higher hydrogen permeation performance. Fig. 3 shows the temperature dependence of hydrogen flux though all of the NieLa2xSmxCe2O7 (x ¼ 0, 0.025, 0.05, 0.075, 0.1 and 0.2) membranes under wet conditions. The hydrogen fluxes increased with temperature increasing, and the highest hydrogen flux was 2.86 108 mol cm2 s1 at 900 C when x ¼ 0.05. The hydrogen fluxes of all the Sm-doped samples except x ¼ 0.2 was greater than that of the undoped sample. Trivalent cation ions dope CeO2 would have this defect equation:
J (mol/cm s)
Results and discussion
70
Fig. 1 e X-ray diffraction (XRD) patterns of synthesized ceramic powders of La2LxSmxCe2O7 (x [ 0, 0.025, 0.05, 0.075, 0.1 and 0.2).
CeO2
3.
60
2 theta (degree)
1.50E-008
1.00E-008
5.00E-009 700
750
800
850
900
o
Temperature ( C)
Fig. 2 e Hydrogen permeation flux through NieLa2LxSmxCe2O7 (x [ 0.05, 0.1) membranes under wet and dry atmospheres.
4510
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 4 5 0 8 e4 5 1 1
3.20E-008 o
900 C o 850 C o 800 C o 750 C o 700 C
2.80E-008
2.00E-008
2
J (mol/cm s)
2.40E-008
1.60E-008
1.20E-008
Fig. 5 e Schematic process of water incorporation into oxygen vacancies in NNN position of La ions.
8.00E-009
4.00E-009 0.00
0.05
0.10
0.15
0.20
Sm-content (x)
Fig. 3 e Hydrogen permeation flux through NieLa2LxSmxCe2O7 (x [ 0, 0.025, 0.05, 0.075, 0.1 and 0.2) membranes under wet atmospheres.
was different from that caused by Sm ion. The quantum mechanical calculation results of Andersson et al. [6] suggest that oxygen vacancies formation energy in next nearest neighbor position (NNN) of La ion is lower than that in the nearest neighbor (NN) position. In the case of Sm ion, it is opposite. Thus the oxygen vacancy occupies the NNN position of La ion but the NN position of Sm ion, which is shown in Fig. 4. A small amount of Sm-doping destroys the oxygen vacancy sequence, which may facilitate proton transport in the LDC, but proton transport may become difficult again as a new sequence is formed with larger concentrations of Sm cations. Thus the hydrogen permeability of NieLa2xSmxCe2O7 (x ¼ 0.075, 0.1 and 0.2) was smaller than that of NieLa1.95Sm0.05Ce2O7 at the same temperature. Another explanation is that excessive Sm-doping would also decrease the surface reaction rate. When the sample is exposed in moisture, water molecular will be incorporated in the oxygen vacancy. The reaction is as follows: H2 O þ V::o þ Oxo /2OH$o As the oxygen vacancy caused by La ion occupies the NNN position of La ion, oxygen ion in adsorbed water molecule occupies the oxygen vacancy in the neighbor of Ce ions. At the same time, two protons are formed around this oxygen
ion. These protons suffer electrostatic repulsion from central cation (Ce4þ or La3þ). As the repulsion between La3þ and proton is lower than that between Ce4þ and proton, these protons have higher affinity to the La ions so they move to the oxygen ions in NN positions. This process is shown in Fig. 5. For oxygen vacancies caused by Sm ions, two protons are always present near the same oxygen ion after water molecule is incorporated into the oxygen vacancy in the NN position of Sm ions. Considering the unit cell volume decreases with Sm-doping in La sites as confirmed by Fig. 1, the repulsion between Sm3þ and proton is bigger than that between La3þ and proton. Thus it would be harder for the incorporation of water into oxygen vacancies in NN positions of Sm ions. It should be mentioned that the CeeO interatomic distances also decrease upon doping with smaller trivalent (e.g. Sm3þ) [6], thus the incorporation of water into oxygen vacancies in NNN positions of Sm ions is even harder. Generally, the doping of La with Sm can decrease the surface reaction rate. Why trivalent cation ions co-doped in CeO2 can improve the proton conductivity and a precise explanation of why the excessive Sm doped sample would decrease the proton conductivity is still under investigation.
4.
Conclusions
Hydrogen membranes based on Ni and lanthanum-doped ceria are successfully prepared. Sm-doping in LDC can affect the rate of hydrogen permeation. Sm-doping with a small amount is beneficial for hydrogen permeability. However, excess Sm-doping can decrease it. Among all the compositions investigated, NieLa1.95Sm0.05Ce2O7 has the highest hydrogen permeability. Excess Sm-doping reduces the cell volume and decreases the surface reaction rate.
Acknowledgements
Fig. 4 e Schematic description of defect configuration in CeO2.
We gratefully acknowledge the support of National High-tech R&D Program of China (no. 2007AA05Z157), Key Program of Chinese Academy of Sciences (no. KJCX1.YW07), and the CAS Special Grant for Postgraduate Research, Innovation and Practice.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 4 5 0 8 e4 5 1 1
references
[1] Dicks AL. Hydrogen generation from natural gas for the fuel cell systems of tomorrow. J Power Sources 1996;61: 113e24. [2] Zhang G, Dorris S, Balachandran U, Liu M. Interfacial resistances of NieBCY mixed-conducting membranes for hydrogen separation. Solid State Ionics 2003;159:121e34. [3] Zuo CD, Dorris SE, Balachandran U, Liu ML. Effect of Zr-doping on the chemical stability and hydrogen permeation of the
4511
NieBaCe0.8Y0.2O3d mixed protoniceelectronic conductor. Chem Mater 2006;18:4647e50. [4] Zuo CD, Lee TH, Dorris SE, Balachandran U, Liu ML. Composite NieBa(Zr0.1Ce0.7Y0.2)O3 membrane for hydrogen separation. J Power Sources 2006;159:1291e5. [5] Zuo CD, Lee TH, Song SJ, Chen L, Dorris SE, Balachandran U, et al. Hydrogen permeation and chemical stability of cermet NieBa[Zr0.8xCexY0.2]O3 Membranes. Electrochem Solid State Lett 2005;8:J35. [6] Andersson David A, Simak Sergei I, Skorodumova Natalia V, Abrikosov Igor A, Johansson B. Optimization of ionic conductivity in doped ceria. Proc Natl Acad Sci USA 2006;103:3518.