- Email: [email protected]
Enhanced electrical conductivity of LaNbO4 by A-site substitution
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 x x x ( 2 0 1 6 ) 1 e7
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Enhanced electrical conductivity of LaNbO4 by A-site substitution Yong Cao a,b, Nanqi Duan a, Dong Yan a,*, Bo Chi a, Jian Pu a, Li Jian a a
Center for Fuel Cell Innovation, School of Materials Science and Engineering, State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science & Technology, Wuhan, Hubei 430074, China b Institute of Electronic Engineering, China Academy of Engineering Physics, Mianyang, Sichuan, China
article info
abstract
Article history:
In LaNbO4, Sm, Gd and Yb were investigated as substitutes to enhance its ion conductivity,
Received 22 May 2016
for its application in solid oxide fuel cell. La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) was prepared via a
Received in revised form
solidestate reaction route and single phase was attained by calcining the constituent ox-
7 August 2016
ides at 1200 C for 5 h. The total conductivity of LaNbO4 was increased by partly replaced La
Accepted 9 August 2016
with Sm, Gd and Yb, due to the muti-valences of the substitutions. The highest conduc-
Available online xxx
tivity (1.76 104 S cm1) was achieved for La0.9Sm0.1NbO4 at 800 C in wet air, more than one order higher compared to that of LaNbO4. The introduction of Sm, Gd or Yb also slightly
Keywords:
increased the ion transport number of LaNbO4, indicating that the increase of the total
LaNbO4
conductivity was mainly contributed by the enhanced ion conductivity, which was bene-
A-site substitution
ficial for its application as a pure ion conductor.
Crystal structure
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Electrical conductivity
Introduction Fuel Cell is developed to concert the chemical energy in hydrogen or hydrocarbon to electric power, which can be classified into Proton Exchange Membrane Fuel Cell (PEMFC), Phosphorous Acid Fuel Cell (PAFC), Molten Carbonate Fuel Cell (MCFC), Solid Oxide Fuel Cell (SOFC) [1e3] on the basis of its electrolyte. Each type of Fuel Cell has its own advantages/ disadvantages. For example, PEMFC is more popular and prominent than others at low temperature, and attracts lots of attention [4e9]. For this contribution, it focuses on the high temperature SOFC due to its wide range of fuel sources [10e14]. Conventionally SOFC uses the oxygen ion conductor as the electrolyte [15], and products such as water generate on the anode side, diluting the fuel and deteriorating the
traditional Ni-anode [16]. Recently, the development of SOFC based on the high temperature proton conductors (P-SOFC) [17e25], with water produced on the cathode side, provides an effective way to avoid the disadvantages. Lanthanum orthoniobate LaNbO4, exhibiting obvious proton conduction in the wet atmospheres, is developed as an electrolyte for P-SOFC [26e28]. Its protonic conductivity can be increased by doping with dopants, such as Ca, Sr, Zn at A-site [26,27,29e32], Mn, Ga, Ge, Si, B, Ti, Zr, Al at B-stie [31,33,34], but the solubility of the dopants in LaNbO4 is quite low and the improvement is limited. Ce, Pr or CeeYb substitution similarly increases the conductivity of LaNbO4, with the contribution of the enlarged electron conductivity, and reduces the open circuit voltage [34,35]. Replacing Nb by Mo or W also improves the conductivity of LaNbO4, but equally worsens its stability due to elements volatilization [36,37]. V and Ta are
* Corresponding author. Fax: þ86 27 87558142. E-mail address: [email protected] (D. Yan). http://dx.doi.org/10.1016/j.ijhydene.2016.08.056 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Cao Y, et al., Enhanced electrical conductivity of LaNbO4 by A-site substitution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.056
2
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 x x x ( 2 0 1 6 ) 1 e7
furthermore taken, demonstrating adverse effect on the conductivity [38,39]. In this contribution, lanthanide elements Sm, Gd and Yb are selected as the partial substitutes, aiming to raise the ion conductivity of LaNbO4, which has not been explored so far. Ions of Sm, Gd, Yb propose the comparable ion radius with that of La and multi-valences of þ2/þ3 at high temperature [40e43], which can replace La to large contents and introduce oxygen vacancy into LaNbO4 (described by reaction (1e3)) to increase its oxide ion conductivity at the same time. The increased oxygen vacancy concentration can also promote the proton formation via Equation (4) [30,31,44,45], and transportation of protons through the Grotthuss mechanism [46], to enhance the proton conductivity. The results in the present study also indicate that taking Sm, Gd and Yb as substitutions can increase the ion conductivity of LaNbO4 obviously. 1 2SmxLa þ OxO ¼ 2Sm0La þ V o þ O2 2
(1)
1 x 0 2GdLa þ OxO ¼ 2GdLa þ V o þ O2 2
(2)
1 x 0 2YbLa þ OxO ¼ 2YbLa þ V o þ O2 2
(3)
x H2 O þ V O þ OO /2OH
(4)
Experimental Material and specimen preparation Powders of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) were prepared via a solidestate reaction route, using La2O3, Sm2O3, Gd2O3, Yb2O3, Nb2O5 (Sinopharm Chemical Reagent Co., Ltd) as the raw materials. The constituent oxides were calcined at 900 C for 3 h to remove the absorbed water and carbon dioxide before weighting, and mixed as the ratios through ball-milling with ethanol as solvent and zirconia balls as medium for 48 h with a speed of 180 rpm. After being dried in oven at 80 C for 6 h, the mixtures were fired at 1200 C for 5 h, and ball-milled for 48 h to achieve fine powders. The powders were subsequently cold pressed into bars and pellets with the pressure of 100 MPa, and sintered at 1500 C for 5 h, prepared for the total conductivity (st ) and open circuit voltage (OCV) measurement. The dimensions of the sintered bars and pellets were 14 5 4 mm and 22 1.1 mm, respectively.
Material characterization and electrical measurement The synthesized powders were subjected to X-ray diffraction (XRD, X'Pert Pro), within the 2q range of 10e110 , for phase identification and Rietveld analysis by using the X'Pert Highscore Plus and GSAS software, respectively. Microstructure of the sintered specimens was characterized with a field emission scanning electron microscope (F-SEM, Sirion 200). Thermal expansion behaviors of the specimens were examined by dilatometry (TMA Q400EM) in N2 with a heating rate of 20 C min1.st of the specimens was measured between 400 and 800 C in wet (3 vol.% H2O, controlled by a bubbler) air and
5 vol.% H2eN2., using the four-probe direct current method. Cell with the configuration as Pt/La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb)/Pt was prepared for the OCV measurement, with wet air and 5 vol.% H2eN2 supplied on both sides. Current and voltage for the st and OCV measurement were recorded by an Agilent B2901A source meter.
Results Phase and microstructure identification Fig. 1 shows the XRD patterns of the synthesized La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) powders. All the peaks can be indexed by the monoclinic LaNbO4 (JCPDS file no.01-078-0157) (Fig. 1a), but the positions of the peaks shift slightly to the higher angles by M (M ¼ Sm, Gd, Yb) substituted (Fig. 1b). The degree of peak shift (D(2q)) for La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) is in the order as La0.9Sm0.1NbO4 < La0.9Gd0.1NbO4 < La0.9Yb0.1NbO4. Fig. 2 illustrates the Rietveld refinement result of the XRD patterns of La0.9Sm0.1NbO4, taking as an example for La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb). Good fitting is achieved by using the monoclinic LaNbO4 as the model for La0.9Sm0.1NbO4, and similar results are achieved for La0.9Gd0.1NbO4 and La0.9Yb0.1NbO4. Fig. 3 presents the variety of the cell parameters of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) with the ion radius. The magnitude of the lattice parameters follows the order as LaNbO4 > La0.9Sm0.1NbO4 > La0.9Gd0.1NbO4 > La0.9Yb0.1NbO4, same order as the ion radius of La, Sm, Gd and Yb. The ion radii of La3þ, Sm3þ, Gd3þ, Yb3þ with 8-fold coordination are 1.160, 1.079, 1.053, 0.985 Å, respectively, as reported in Ref. [47]. Fig. 4 shows the SEM images of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) sintered at 1500 C for 5 h. All the specimens are quite dense and pore-free, with similar grain sizes in the range of 2e5 mm. Fig. 5 shows the thermal expansion curves of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb), measured in N2 between 300 and 900 C during the heating process. A break is observed for each curve, corresponding to the phase transformation from the monoclinic to tetragonal structure. The phase transformation temperature of LaNbO4 derived from the thermal expansion curve is around 520 C, quite similar with the results reported in Ref. [48,49]. La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) proposes slightly higher phase transformation temperature compared to that of LaNbO4. As SmNbO4 [50], GdNbO4, YbNbO4 [51] have much higher phase transformation temperature compared to that of LaNbO4, it is quite reasonable that the variety of the crystal structure leads to the slightly higher phase transformation temperature in La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb).
Conductivity of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) Fig. 6 illustrates the st of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) measured in wet air. A break is observed for all the measured curves, ascribed to the phase transformation from the monoclinic to tetragonal structure, as proved by the thermal expansion curves (Fig. 5). The st of LaNbO4 reaches ~105 S cm1 at 800 C, similar with that reported in Refs. [30,36], but higher compared to that described in Ref. [52]. The
Please cite this article in press as: Cao Y, et al., Enhanced electrical conductivity of LaNbO4 by A-site substitution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.056
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 x x x ( 2 0 1 6 ) 1 e7
3
Fig. 1 e XRD patterns of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb): (a) 2q~20e80 and (b) 2q~25e35 . Fig. 7 exhibits the activation energy (Ea) of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) varied with the ion radius, derived by fitting the st with the Arrhenius equation (Equation (5)). La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) has smaller Ea value compared to that of LaNbO4, attributed to the increased oxygen vacancy concentration. Ea of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) for the monoclinic structure is higher compared to that with tetragonal structure, as the later one proposes higher crystal structure symmetry [53]. The value of Ea follows the order as La0.9Gd0.1NbO4 > La0.9Sm0.1NbO4 > La0.9Yb0.1NbO4 for the monoclinic structure and La0.9Sm0.1NbO4 > La0.9Yb0.1NbO4 > La0.9Gd0.1NbO4 for the tetragonal structure. Ea st ¼ s0 exp RT
Fig. 2 e Rietveld refinement result of the XRD pattern of La0.9Sm0.1NbO4. difference becomes bigger at low temperature. For instance, at 400 C, the st of LaNbO4 reported here is almost one order lower than the magnitude shown in Refs. [30,36,52]. The difference between the results is mainly due to the inconsistent specimen preparation routes as well as measurement methods. When the A.C. impedance method is used to measure the conductivity, only small oscillation voltage is applied, and the influence on the grain conductivity is quite small. But for the four-probe direct current method, a current utilized is necessary to generate voltage, and the value of the voltage varied with the direction as well as the magnitude of the current, as the electrical resistance on the grain boundary is quite sensitive. In this paper, the conductivity of the specimens is measured under the same conditions for simplicity. Although the discrepancy exists, but it still can provide enough information to discuss the effect of Sm, Gd, or Yb substitution on LaNbO4. As shown in Fig. 6, La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) exhibits higher st compared to that of pure LaNbO4 in the research region, following the order as: La0.9Sm0.1NbO4 > La0.9Gd0.1NbO4 > La0.9Yb0.1NbO4 > LaNbO4. st of La0.9Sm0.1NbO4 reaches 1.76 104 S cm1 at 800 C, more than one order higher than that of LaNbO4.
(5)
where s0 is the pre-exponential factor, R is the gas constant. Additionally, the st of La0.9Sm0.1NbO4 is also measured in wet 5 vol.% H2eN2, for comparison with the st in the oxidized atmosphere, as shown in Fig. 8. The st in wet 5 vol.% H2eN2 is higher compared to that measured in wet air in the research regions, but the difference diminishes at high temperature. In 5 vol.% H2eN2, partial of Sm3þ might be reduced to Sm2þ [40], increasing the Sm2þ concentration, as well as the electron conductivity by promoting the electron transfer between Sm3þ and Sm2þ via the reversible reaction (6). The increased ion conductivity (si ) might also contribute to the enlargement of st . At high temperature, the electron conductivity (se ) in La0.9Sm0.1NbO4, related with the oxygen partial pressure, starts to become considerable, leading to the gradually decreased difference. Sm0La /SmxLa þ e0
(6)
Fig. 9 shows the OCV of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) varied with temperature, with wet air and 5 vol.% H2eN2 supplied on both sides between 600 and 900 C. The values of the OCV decrease with increasing temperature, and follow the order as La0.9Sm0.1NbO4 > La0.9Gd0.1NbO4 > La0.9Yb0.1NbO4 > LaNbO4 in the research range. The ion transport number (ti ), as described by Equation (7), decreases from 0.96, 0.94, 0.92, 0.90 to 0.84, 0.83, 0.80, 0.79 as temperature increases from 600 to 900 C for La0.9Sm0.1NbO4, La0.9Gd0.1NbO4, La0.9Yb0.1NbO4, LaNbO4, respectively, indicating that the ion conduction is
Please cite this article in press as: Cao Y, et al., Enhanced electrical conductivity of LaNbO4 by A-site substitution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.056
4
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 x x x ( 2 0 1 6 ) 1 e7
Fig. 3 e Variety of the cell parameters of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) as a function of the ion radius.
Fig. 4 e SEM images of La0.9M0.1NbO4 (M ¼ (a) Sm, (b) Gd, (c) Yb) as well as (d) LaNbO4 sintered at 1500 C for 5 h.
dominated in La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) and LaNbO4. The increased OCV of LaNbO4 also confirms that the enlarged st is mainly contributed by the enhanced si . By combining the results shown in Figs. 6 and 9, it indicates that the magnitude of the si follows the order as La0.9Sm0.1NbO4 > La0.9Gd0.1NbO4 > La0.9Yb0.1NbO4 > LaNbO4. ti ¼
si si DVmea ¼ ¼ st si þ se DVcal
(7)
where DVmea and DVcal represent OCV measured in experiment and calculated by Nernst equation, respectively.
Discussion In La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb), reactions (8e10) can happen, as well as reactions (11e12), due to the transfer of the electrons between the ions with the varied valences, and the
Please cite this article in press as: Cao Y, et al., Enhanced electrical conductivity of LaNbO4 by A-site substitution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.056
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 x x x ( 2 0 1 6 ) 1 e7
5
Fig. 5 e Thermal expansion curves of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb), measured in N2 between 300 and 900 C. Fig. 8 e st of La0.9Sm0.1NbO4 in wet 5 vol.% H2eN2 and air.
Fig. 6 e st of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) in wet air.
Fig. 9 e Dependence of the OCV of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) on temperature, measured with wet air and 5 vol.% H2eN2 supplied on both sides.
existence of the oxygen vacancy. Therefore, charge carriers as proton, oxygen vacancy/ion, electron and hole are existed, and contribute to the st of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb). 0
x
0
x
GdLa /GdLa þ e0 YbLa /YbLa þ e0
(9)
La0La /LaxLa þ e0
(10)
1 2 x V O þ O /OO þ 2h 2
(11)
h þ e0 /null
Fig. 7 e Variety of the Ea of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) with the ion radius.
(8)
(12)
As the proton, oxide ion and electron conductivity are related with the oxygen vacancy, its concentration will exhibit big influence on the st . The oxygen vacancy concentration is
Please cite this article in press as: Cao Y, et al., Enhanced electrical conductivity of LaNbO4 by A-site substitution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.056
6
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 x x x ( 2 0 1 6 ) 1 e7
essentially depended on the concentration of the ions with the valence of þ2 at A-site, which is closely connected with the third ionization potential (TIP) of the ions. The TIP of the ion corresponds to the energy required for the ion with the valence of þ2 oxidized to that with the valence of þ3, reflecting the degree of difficulty to remove one electron from the ion, as described by reactions (6, 8e10), for Sm, Gd, Yb, La, respectively. The TIP of Sm, Gd, Yb and La is 23.4, 20.6, 25.1 and 19.2 eV, respectively [54], making the order of the stability of the ions as Yb2þ > Sm2þ > Gd2þ > La2þ. From the point of the TIP, it is reasonable that the concentration of the ions with the valence of þ2 follows the same order as the magnitude of their TIP. Therefore, the oxygen vacancy concentration in La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) introduced by A-site substitution follows the order as La0.9Yb0.1NbO4 > La0.9Sm0.1NbO4 > La0.9Gd0.1NbO4 > LaNbO4. The increased oxygen vacancy concentration in LaNbO4 results in the enhancement of the st (shown in Fig. 6). Also, higher oxygen vacancy concentration makes the distance for the ion transportation shorter, bringing about the decrease of the magnitude of the Ea by increasing the oxygen vacancy concentration roughly (Fig. 7). But the Ea of La0.9Gd0.1NbO4 for the tetragonal structure is the smallest, which can't be explained in the same way and is not clear so far. Except the charge carrier concentration, the space for the movement of the ions also intimately affects the st . The cell lattice parameters of La0.9Yb0.1NbO4 are much smaller compared to that of La0.9Sm0.1NbO4 and La0.9Gd0.1NbO4, leading to a smaller s0 (Equation (5)) than the other two, which is the main reason for its low st among La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb). Higher oxygen vacancy concentration and larger space for the movement of the charge carriers, make the st of La0.9Sm0.1NbO4 higher than that of La0.9Gd0.1NbO4. At the same time, the electrons transfer between the ions with different valences (illustrated by reactions (6, 8e10)) can contribute electron conduction to the st , which is also related with the concentration of the ions with the valence of þ2. Higher concentration of the ions with the valence of þ2 will generate higher se . Moreover, the existence of the oxygen vacancy can introduce the electrons into La0.9M0.1NbO4 via reactions (11e12). Therefore, the introduction of Sm, Gd, Yb can increase the si and se of LaNbO4 simultaneously. If the degree of the increase of the si is higher compared to that of the se , the ion transport number will be increased; Otherwise not. The competition between the enhancement of the si and se , leads to the order of the magnitude of the OCV shown in Fig. 9.
Conclusions Single phase La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) is prepared successfully via a solidestate reaction route, by calcining the constituent oxides at 1200 C for 5 h. The introduction of Sm, Gd or Yb can increase the oxygen vacancy concentration in LaNbO4 due to its muti-valences, and improve the total conductivity of LaNbO4 obviously. The highest conductivity, 1.76 104 S cm1, is achieved for La0.9Sm0.1NbO4 at 800 C in wet air, more than one order higher compared to that of LaNbO4. The oxygen vacancy concentration as well as the
crystal structure affects the electrical properties of LaNbO4 significantly, causing the conductivity of La0.9Sm0.1NbO4 > La0.9Gd0.1NbO4 > La0.9Yb0.1NbO4 > LaNbO4. Taking Sm, Gd, or Yb as a substitute at A-site also slightly increases the open circuit voltage of LaNbO4, which is beneficial for its application as a pure ion conductor. Sm substituted LaNbO4 exhibits the highest ion conductivity and open circuit voltage, making it the best candidate among La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) for the application as the electrolyte of P-SOFC.
Acknowledgments This research was financially supported by the National Natural Science Foundation of China under the project 51172082. The XRD, SEM characterizations were assisted by the Analytical and Testing Center of Huazhong University of Science and Technology.
references
[1] Haile SM. Fuel cell materials and components. Acta Mater 2003;51:5981e6000. [2] Joon K. Fuel cells - a 21st century power system. J Power Sources 1998;71:12e8. [3] Steele BC, Heinzel A. Materials for fuel-cell technologies. Nature 2001;414:345e52. [4] Alavijeh AS, Venkatesan S, Khorasany R, Kim W, Kjeang E. Ex-situ tensile fatigue-creep testing: a powerful tool to simulate in-situ mechanical degradation in fuel cells. J Power Sources 2016;312:123e7. [5] Alavijeh AS, Khorasany RM, Habisch A, Wang GG, Kjeang E. Creep properties of catalyst coated membranes for polymer electrolyte fuel cells. J Power Sources 2015;285:16e28. [6] Khorasany RM, Alavijeh AS, Kjeang E, Wang G, Rajapakse R. Mechanical degradation of fuel cell membranes under fatigue fracture tests. J Power Sources 2015;274:1208e16. [7] Khorasany RM, Goulet M-A, Alavijeh AS, Kjeang E, Wang GG, Rajapakse R. On the constitutive relations for catalyst coated membrane applied to in-situ fuel cell modeling. J Power Sources 2014;252:176e88. [8] Silberstein MN, Boyce MC. Constitutive modeling of the rate, temperature, and hydration dependent deformation response of Nafion to monotonic and cyclic loading. J Power Sources 2010;195:5692e706. [9] Khorasany RMH, Singh Y, Sadeghi Alavijeh A, Kjeang E, Wang GG, Rajapakse RKND. Fatigue properties of catalyst coated membranes for fuel cells: ex-situ measurements supported by numerical simulations. Int J Hydrogen Energy 2016;41:8992e9003. [10] Liang F, Zhou W, Chi B, Pu J, Jiang SP, Jian L. Pd-YSZ composite cathodes for oxygen reduction reaction of intermediate-temperature solid oxide fuel cells. Int J Hydrogen Energy 2011;36:7670e6. [11] Chen X, Hua B, Pu J, Li J, Zhang L, Jiang SP. Interaction between (La, Sr)MnO3 cathode and NieMoeCr metallic interconnect with suppressed chromium vaporization for solid oxide fuel cells. Int J Hydrogen Energy 2009;34:5737e48. [12] Li M, Hua B, Jiang SP, Pu J, Chi B, Jian L. BaZr0.1Ce0.7Y0.1Yb0.1O3-d as highly active and carbon tolerant anode for direct hydrocarbon solid oxide fuel cells. Int J Hydrogen Energy 2014;39:15975e81.
Please cite this article in press as: Cao Y, et al., Enhanced electrical conductivity of LaNbO4 by A-site substitution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.056
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 x x x ( 2 0 1 6 ) 1 e7
[13] Wang J, Yan D, Pu J, Chi B, Jian L. Fabrication and performance evaluation of planar solid oxide fuel cell with large active reaction area. Int J Hydrogen Energy 2011;36:7234e9. [14] Liu Y, Chen K, Zhao L, Chi B, Pu J, Jiang SP, et al. Performance stability and degradation mechanism of La0.6Sr0.4Co0.2Fe0.8O3-d cathodes under solid oxide fuel cells operation conditions. Int J Hydrogen Energy 2014;39:15868e76. [15] Wachsman ED, Lee KT. Lowering the temperature of solid oxide fuel cells. Science 2011;334:935e9. [16] Jiao Z, Takagi N, Shikazono N, Kasagi N. Study on local morphological changes of nickel in solid oxide fuel cell anode using porous Ni pellet electrode. J Power Sources 2011;196:1019e29. [17] Zhang L, Yang W. High-performance low-temperature solid oxide fuel cells using thin proton-conducting electrolyte with novel cathode. Int J Hydrogen Energy 2012;37:8635e40. [18] Yang L, Wang S, Blinn K, Liu M, Liu Z, Cheng Z. Enhanced sulfur and coking tolerance of a mixed ion conductor for SOFCs: BaZr0.1Ce0.7Y0.2-xYbxO3-d. Science 2009;326:126e9. [19] Bao JX, Ohno H, Kurita N, Okuyama Y, Fukatsu N. Proton conduction in Al-doped CaZrO3. Electrochim Acta 2011;56:1062e8. [20] Zhao F, Chen F. Performance of solid oxide fuel cells based on proton-conducting BaCe0.7In0.3-xYxO3-d electrolyte. Int J Hydrogen Energy 2010;35:11194e9. [21] Katahira K, Kohchi Y, Shimura T, Iwahara H. Protonic conduction in Zr-substituted BaCeO3. Solid State Ionics 2000;138:91e8. [22] Guo Y, Ran R, Shao Z. A novel way to improve performance of proton-conducting solid-oxide fuel cells through enhanced chemical interaction of anode components. Int J Hydrogen Energy 2011;36:1683e91. [23] Norby T, Christiansen N. Proton conduction in Ca- and Srsubstituted LaPO4. Solid State Ionics 1995;77:240e3. [24] Tande C, Perez-Coll D, Mather GC. Surface proton conductivity of dense nanocrystalline YSZ. J Mater Chem 2012;22:11208e13. [25] Guo Y, Ran R, Shao Z. Fabrication and performance of a carbon dioxide-tolerant proton-conducting solid oxide fuel cells with a dual-layer electrolyte. Int J Hydrogen Energy 2010;35:10513e21. [26] Haugsrud R, Norby T. Proton conduction in rare-earth orthoniobates and ortho-tantalates. Nat Mater 2006;5:193e6. [27] Haugsrud R, Norby T. High-temperature proton conductivity in acceptor-doped LaNbO4. Solid State Ionics 2006;177:1129e35. [28] Magraso A, Fontaine ML, Larring Y, Bredesen R, Syvertsen GE, Lein HL, et al. Development of proton conducting SOFCs based on LaNbO4 electrolyte - status in Norway. Fuel Cells 2011;11:17e25. [29] Fjeld H, Kepaptsoglou DM, Haugsrud R, Norby T. Charge carriers in grain boundaries of 0.5% Sr-doped LaNbO4. Solid State Ionics 2010;181:104e9. ~ a-Martı´nez J, Kim J-H, Bridges CA, Huq A, Hodges JP, [30] Bi Z, Pen et al. Effect of Ca doping on the electrical conductivity of the high temperature proton conductor LaNbO4. Int J Hydrog Energy 2012;37:12751e9. [31] Huse M, Norby T, Haugsrud R. Effects of A and B site acceptor doping on hydration and proton mobility of LaNbO4. Int J Hydrog Energy 2012;37:8004e16. [32] Cao Y, Tan Y, Yan D, Chi B, Pu J, Jian L. Electrical conductivity of Zn-doped high temperature proton conductor LaNbO4. Solid State Ionics 2015;278:152e6. [33] Brandao AD, Gracio J, Mather GC, Kharton VV, Fagg DP. B-site substitutions in LaNb1-xMxO4-d materials in the search for
[34]
[35] [36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45] [46] [47]
[48]
[49]
[50]
[51] [52]
[53]
[54]
7
potential proton conductors (M¼Ga, Ge, Si, B, Ti, Zr, P, Al). J Solid State Chem 2011;184:863e70. Solı´s C, Serra JM. Adjusting the conduction properties of La0.995Ca0.005NbO4-d by doping for proton conducting fuel cells electrode operation. Solid State Ionics 2011;190:38e45. Cao Y, Chi B, Pu J, Jian L. Effect of Ce and Yb co-doping on conductivity of LaNbO4. J Eur Ceram Soc 2014;34:1981e8. Li C, Bayliss RD, Skinner SJ. Crystal structure and potential interstitial oxide ion conductivity of LnNbO4 and LnNb0.92W0.08O4.04 (Ln ¼ La, Pr, Nd). Solid State Ionics 2014;262:530e5. Cao Y, Duan N, Wang X, Chi B, Jian L. Enhanced electrical conductivity of Mo-doped LaNbO4. J Eur Ceram Soc 2015;35:1979e83. Bi ZH, Bridges CA, Kim JH, Huq A, Paranthaman MP. Phase stability and electrical conductivity of Ca-doped LaNb1xTaxO4-d high temperature proton conductors. J Power Sources 2011;196:7395e403. Brandao AD, Antunes I, Frade JR, Torre J, Kharton VV, Fagg DP. Enhanced low-temperature proton conduction in Sr0.02La0.98NbO4-d by scheelite phase retention. Chem Mat 2010;22:6673e83. Mikhail P, Hulliger J. Sm2þ in oxide lattices: an unexplored chapter of crystal chemistry. Comments Inorg Chem 1999;21:263e83. MacDonald MR, Bates JE, Ziller JW, Furche F, Evans WJ. Completing the series of þ2 ions for the lanthanide elements: synthesis of molecular complexes of Pr2þ, Gd2þ, Tb2þ, and Lu2þ. J Am Chem Soc 2013;135:9857e68. Yoshida T, Tanaka T, Yoshida S, Hikita S, Baba T, Ono Y. EXAFS study of Yb species encapsulated in potassium Y-type zeolite. Solid State Commun 2000;114:255e9. Tanaka T, Hanada T, Yoshida S, Baba T, Ono Y. Valence variation of Yb encapsulated in the supercage of Y-Type zeolite. Jan J Appl Phy 1993;32:481e3. A, Haugsrud R, Einarsrud M-A, Syvertsen GE, Magraso Grande T. The effect of cation non-stoichiometry in LaNbO4 materials. Int J Hydrog Energy 2012;37:8017e26. Norby T. Proton conduction in solids: bulk and interfaces. MRS Bull 2009;34:923e8. Agmon N. The Grotthuss mechanism. Chem Phys Lett 1995;244:456e62. Shannon RD. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr 1976;32:751e67. Jian L, Wayman C. Monoclinic-to-tetragonal phase transformation in a ceramic rare-earth orthoniobate, LaNbO4. J Am Ceram Soc 1997;80:803e6. Jian L, James RD. Prediction of microstructure in monoclinic LaNbO4 by energy minimization. Acta Mater 1997;45:4271e81. Rooksby H, White E. The structures of 1: 1 compounds of rare earth oxides with niobia and tantala. Acta Crystallogr 1963;16:888e90. an V. High-temperature transitions in rare-earth Stubic niobates and tantaIates. J Am Ceram Soc 1964;47:55e8. pez D, Ruiz-Morales JC, Savvin S, Amsif M, Marrero-Lo ~ ez P. Low temperature sintering of LaNbO4 proton Nu´n conductors from freeze-dried precursors. J Eur Ceram Soc 2012;32:1235e44. Fjeld H, Toyoura K, Haugsrud R, Norby T. Proton mobility through a second order phase transition: theoretical and experimental study of LaNbO4. Phys Chem Chem Phys 2010;12:10313e9. Shvartsburg AA. Gas-phase metal trications in protic solvent complexes. J Am Chem Soc 2002;124:7910e1.
Please cite this article in press as: Cao Y, et al., Enhanced electrical conductivity of LaNbO4 by A-site substitution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.056
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Enhanced electrical conductivity of LaNbO4 by A-site substitution Yong Cao a,b, Nanqi Duan a, Dong Yan a,*, Bo Chi a, Jian Pu a, Li Jian a a
Center for Fuel Cell Innovation, School of Materials Science and Engineering, State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science & Technology, Wuhan, Hubei 430074, China b Institute of Electronic Engineering, China Academy of Engineering Physics, Mianyang, Sichuan, China
article info
abstract
Article history:
In LaNbO4, Sm, Gd and Yb were investigated as substitutes to enhance its ion conductivity,
Received 22 May 2016
for its application in solid oxide fuel cell. La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) was prepared via a
Received in revised form
solidestate reaction route and single phase was attained by calcining the constituent ox-
7 August 2016
ides at 1200 C for 5 h. The total conductivity of LaNbO4 was increased by partly replaced La
Accepted 9 August 2016
with Sm, Gd and Yb, due to the muti-valences of the substitutions. The highest conduc-
Available online xxx
tivity (1.76 104 S cm1) was achieved for La0.9Sm0.1NbO4 at 800 C in wet air, more than one order higher compared to that of LaNbO4. The introduction of Sm, Gd or Yb also slightly
Keywords:
increased the ion transport number of LaNbO4, indicating that the increase of the total
LaNbO4
conductivity was mainly contributed by the enhanced ion conductivity, which was bene-
A-site substitution
ficial for its application as a pure ion conductor.
Crystal structure
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Electrical conductivity
Introduction Fuel Cell is developed to concert the chemical energy in hydrogen or hydrocarbon to electric power, which can be classified into Proton Exchange Membrane Fuel Cell (PEMFC), Phosphorous Acid Fuel Cell (PAFC), Molten Carbonate Fuel Cell (MCFC), Solid Oxide Fuel Cell (SOFC) [1e3] on the basis of its electrolyte. Each type of Fuel Cell has its own advantages/ disadvantages. For example, PEMFC is more popular and prominent than others at low temperature, and attracts lots of attention [4e9]. For this contribution, it focuses on the high temperature SOFC due to its wide range of fuel sources [10e14]. Conventionally SOFC uses the oxygen ion conductor as the electrolyte [15], and products such as water generate on the anode side, diluting the fuel and deteriorating the
traditional Ni-anode [16]. Recently, the development of SOFC based on the high temperature proton conductors (P-SOFC) [17e25], with water produced on the cathode side, provides an effective way to avoid the disadvantages. Lanthanum orthoniobate LaNbO4, exhibiting obvious proton conduction in the wet atmospheres, is developed as an electrolyte for P-SOFC [26e28]. Its protonic conductivity can be increased by doping with dopants, such as Ca, Sr, Zn at A-site [26,27,29e32], Mn, Ga, Ge, Si, B, Ti, Zr, Al at B-stie [31,33,34], but the solubility of the dopants in LaNbO4 is quite low and the improvement is limited. Ce, Pr or CeeYb substitution similarly increases the conductivity of LaNbO4, with the contribution of the enlarged electron conductivity, and reduces the open circuit voltage [34,35]. Replacing Nb by Mo or W also improves the conductivity of LaNbO4, but equally worsens its stability due to elements volatilization [36,37]. V and Ta are
* Corresponding author. Fax: þ86 27 87558142. E-mail address: [email protected] (D. Yan). http://dx.doi.org/10.1016/j.ijhydene.2016.08.056 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Cao Y, et al., Enhanced electrical conductivity of LaNbO4 by A-site substitution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.056
2
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 x x x ( 2 0 1 6 ) 1 e7
furthermore taken, demonstrating adverse effect on the conductivity [38,39]. In this contribution, lanthanide elements Sm, Gd and Yb are selected as the partial substitutes, aiming to raise the ion conductivity of LaNbO4, which has not been explored so far. Ions of Sm, Gd, Yb propose the comparable ion radius with that of La and multi-valences of þ2/þ3 at high temperature [40e43], which can replace La to large contents and introduce oxygen vacancy into LaNbO4 (described by reaction (1e3)) to increase its oxide ion conductivity at the same time. The increased oxygen vacancy concentration can also promote the proton formation via Equation (4) [30,31,44,45], and transportation of protons through the Grotthuss mechanism [46], to enhance the proton conductivity. The results in the present study also indicate that taking Sm, Gd and Yb as substitutions can increase the ion conductivity of LaNbO4 obviously. 1 2SmxLa þ OxO ¼ 2Sm0La þ V o þ O2 2
(1)
1 x 0 2GdLa þ OxO ¼ 2GdLa þ V o þ O2 2
(2)
1 x 0 2YbLa þ OxO ¼ 2YbLa þ V o þ O2 2
(3)
x H2 O þ V O þ OO /2OH
(4)
Experimental Material and specimen preparation Powders of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) were prepared via a solidestate reaction route, using La2O3, Sm2O3, Gd2O3, Yb2O3, Nb2O5 (Sinopharm Chemical Reagent Co., Ltd) as the raw materials. The constituent oxides were calcined at 900 C for 3 h to remove the absorbed water and carbon dioxide before weighting, and mixed as the ratios through ball-milling with ethanol as solvent and zirconia balls as medium for 48 h with a speed of 180 rpm. After being dried in oven at 80 C for 6 h, the mixtures were fired at 1200 C for 5 h, and ball-milled for 48 h to achieve fine powders. The powders were subsequently cold pressed into bars and pellets with the pressure of 100 MPa, and sintered at 1500 C for 5 h, prepared for the total conductivity (st ) and open circuit voltage (OCV) measurement. The dimensions of the sintered bars and pellets were 14 5 4 mm and 22 1.1 mm, respectively.
Material characterization and electrical measurement The synthesized powders were subjected to X-ray diffraction (XRD, X'Pert Pro), within the 2q range of 10e110 , for phase identification and Rietveld analysis by using the X'Pert Highscore Plus and GSAS software, respectively. Microstructure of the sintered specimens was characterized with a field emission scanning electron microscope (F-SEM, Sirion 200). Thermal expansion behaviors of the specimens were examined by dilatometry (TMA Q400EM) in N2 with a heating rate of 20 C min1.st of the specimens was measured between 400 and 800 C in wet (3 vol.% H2O, controlled by a bubbler) air and
5 vol.% H2eN2., using the four-probe direct current method. Cell with the configuration as Pt/La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb)/Pt was prepared for the OCV measurement, with wet air and 5 vol.% H2eN2 supplied on both sides. Current and voltage for the st and OCV measurement were recorded by an Agilent B2901A source meter.
Results Phase and microstructure identification Fig. 1 shows the XRD patterns of the synthesized La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) powders. All the peaks can be indexed by the monoclinic LaNbO4 (JCPDS file no.01-078-0157) (Fig. 1a), but the positions of the peaks shift slightly to the higher angles by M (M ¼ Sm, Gd, Yb) substituted (Fig. 1b). The degree of peak shift (D(2q)) for La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) is in the order as La0.9Sm0.1NbO4 < La0.9Gd0.1NbO4 < La0.9Yb0.1NbO4. Fig. 2 illustrates the Rietveld refinement result of the XRD patterns of La0.9Sm0.1NbO4, taking as an example for La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb). Good fitting is achieved by using the monoclinic LaNbO4 as the model for La0.9Sm0.1NbO4, and similar results are achieved for La0.9Gd0.1NbO4 and La0.9Yb0.1NbO4. Fig. 3 presents the variety of the cell parameters of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) with the ion radius. The magnitude of the lattice parameters follows the order as LaNbO4 > La0.9Sm0.1NbO4 > La0.9Gd0.1NbO4 > La0.9Yb0.1NbO4, same order as the ion radius of La, Sm, Gd and Yb. The ion radii of La3þ, Sm3þ, Gd3þ, Yb3þ with 8-fold coordination are 1.160, 1.079, 1.053, 0.985 Å, respectively, as reported in Ref. [47]. Fig. 4 shows the SEM images of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) sintered at 1500 C for 5 h. All the specimens are quite dense and pore-free, with similar grain sizes in the range of 2e5 mm. Fig. 5 shows the thermal expansion curves of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb), measured in N2 between 300 and 900 C during the heating process. A break is observed for each curve, corresponding to the phase transformation from the monoclinic to tetragonal structure. The phase transformation temperature of LaNbO4 derived from the thermal expansion curve is around 520 C, quite similar with the results reported in Ref. [48,49]. La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) proposes slightly higher phase transformation temperature compared to that of LaNbO4. As SmNbO4 [50], GdNbO4, YbNbO4 [51] have much higher phase transformation temperature compared to that of LaNbO4, it is quite reasonable that the variety of the crystal structure leads to the slightly higher phase transformation temperature in La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb).
Conductivity of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) Fig. 6 illustrates the st of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) measured in wet air. A break is observed for all the measured curves, ascribed to the phase transformation from the monoclinic to tetragonal structure, as proved by the thermal expansion curves (Fig. 5). The st of LaNbO4 reaches ~105 S cm1 at 800 C, similar with that reported in Refs. [30,36], but higher compared to that described in Ref. [52]. The
Please cite this article in press as: Cao Y, et al., Enhanced electrical conductivity of LaNbO4 by A-site substitution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.056
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 x x x ( 2 0 1 6 ) 1 e7
3
Fig. 1 e XRD patterns of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb): (a) 2q~20e80 and (b) 2q~25e35 . Fig. 7 exhibits the activation energy (Ea) of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) varied with the ion radius, derived by fitting the st with the Arrhenius equation (Equation (5)). La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) has smaller Ea value compared to that of LaNbO4, attributed to the increased oxygen vacancy concentration. Ea of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) for the monoclinic structure is higher compared to that with tetragonal structure, as the later one proposes higher crystal structure symmetry [53]. The value of Ea follows the order as La0.9Gd0.1NbO4 > La0.9Sm0.1NbO4 > La0.9Yb0.1NbO4 for the monoclinic structure and La0.9Sm0.1NbO4 > La0.9Yb0.1NbO4 > La0.9Gd0.1NbO4 for the tetragonal structure. Ea st ¼ s0 exp RT
Fig. 2 e Rietveld refinement result of the XRD pattern of La0.9Sm0.1NbO4. difference becomes bigger at low temperature. For instance, at 400 C, the st of LaNbO4 reported here is almost one order lower than the magnitude shown in Refs. [30,36,52]. The difference between the results is mainly due to the inconsistent specimen preparation routes as well as measurement methods. When the A.C. impedance method is used to measure the conductivity, only small oscillation voltage is applied, and the influence on the grain conductivity is quite small. But for the four-probe direct current method, a current utilized is necessary to generate voltage, and the value of the voltage varied with the direction as well as the magnitude of the current, as the electrical resistance on the grain boundary is quite sensitive. In this paper, the conductivity of the specimens is measured under the same conditions for simplicity. Although the discrepancy exists, but it still can provide enough information to discuss the effect of Sm, Gd, or Yb substitution on LaNbO4. As shown in Fig. 6, La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) exhibits higher st compared to that of pure LaNbO4 in the research region, following the order as: La0.9Sm0.1NbO4 > La0.9Gd0.1NbO4 > La0.9Yb0.1NbO4 > LaNbO4. st of La0.9Sm0.1NbO4 reaches 1.76 104 S cm1 at 800 C, more than one order higher than that of LaNbO4.
(5)
where s0 is the pre-exponential factor, R is the gas constant. Additionally, the st of La0.9Sm0.1NbO4 is also measured in wet 5 vol.% H2eN2, for comparison with the st in the oxidized atmosphere, as shown in Fig. 8. The st in wet 5 vol.% H2eN2 is higher compared to that measured in wet air in the research regions, but the difference diminishes at high temperature. In 5 vol.% H2eN2, partial of Sm3þ might be reduced to Sm2þ [40], increasing the Sm2þ concentration, as well as the electron conductivity by promoting the electron transfer between Sm3þ and Sm2þ via the reversible reaction (6). The increased ion conductivity (si ) might also contribute to the enlargement of st . At high temperature, the electron conductivity (se ) in La0.9Sm0.1NbO4, related with the oxygen partial pressure, starts to become considerable, leading to the gradually decreased difference. Sm0La /SmxLa þ e0
(6)
Fig. 9 shows the OCV of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) varied with temperature, with wet air and 5 vol.% H2eN2 supplied on both sides between 600 and 900 C. The values of the OCV decrease with increasing temperature, and follow the order as La0.9Sm0.1NbO4 > La0.9Gd0.1NbO4 > La0.9Yb0.1NbO4 > LaNbO4 in the research range. The ion transport number (ti ), as described by Equation (7), decreases from 0.96, 0.94, 0.92, 0.90 to 0.84, 0.83, 0.80, 0.79 as temperature increases from 600 to 900 C for La0.9Sm0.1NbO4, La0.9Gd0.1NbO4, La0.9Yb0.1NbO4, LaNbO4, respectively, indicating that the ion conduction is
Please cite this article in press as: Cao Y, et al., Enhanced electrical conductivity of LaNbO4 by A-site substitution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.056
4
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 x x x ( 2 0 1 6 ) 1 e7
Fig. 3 e Variety of the cell parameters of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) as a function of the ion radius.
Fig. 4 e SEM images of La0.9M0.1NbO4 (M ¼ (a) Sm, (b) Gd, (c) Yb) as well as (d) LaNbO4 sintered at 1500 C for 5 h.
dominated in La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) and LaNbO4. The increased OCV of LaNbO4 also confirms that the enlarged st is mainly contributed by the enhanced si . By combining the results shown in Figs. 6 and 9, it indicates that the magnitude of the si follows the order as La0.9Sm0.1NbO4 > La0.9Gd0.1NbO4 > La0.9Yb0.1NbO4 > LaNbO4. ti ¼
si si DVmea ¼ ¼ st si þ se DVcal
(7)
where DVmea and DVcal represent OCV measured in experiment and calculated by Nernst equation, respectively.
Discussion In La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb), reactions (8e10) can happen, as well as reactions (11e12), due to the transfer of the electrons between the ions with the varied valences, and the
Please cite this article in press as: Cao Y, et al., Enhanced electrical conductivity of LaNbO4 by A-site substitution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.056
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 x x x ( 2 0 1 6 ) 1 e7
5
Fig. 5 e Thermal expansion curves of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb), measured in N2 between 300 and 900 C. Fig. 8 e st of La0.9Sm0.1NbO4 in wet 5 vol.% H2eN2 and air.
Fig. 6 e st of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) in wet air.
Fig. 9 e Dependence of the OCV of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) on temperature, measured with wet air and 5 vol.% H2eN2 supplied on both sides.
existence of the oxygen vacancy. Therefore, charge carriers as proton, oxygen vacancy/ion, electron and hole are existed, and contribute to the st of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb). 0
x
0
x
GdLa /GdLa þ e0 YbLa /YbLa þ e0
(9)
La0La /LaxLa þ e0
(10)
1 2 x V O þ O /OO þ 2h 2
(11)
h þ e0 /null
Fig. 7 e Variety of the Ea of La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) with the ion radius.
(8)
(12)
As the proton, oxide ion and electron conductivity are related with the oxygen vacancy, its concentration will exhibit big influence on the st . The oxygen vacancy concentration is
Please cite this article in press as: Cao Y, et al., Enhanced electrical conductivity of LaNbO4 by A-site substitution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.056
6
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 x x x ( 2 0 1 6 ) 1 e7
essentially depended on the concentration of the ions with the valence of þ2 at A-site, which is closely connected with the third ionization potential (TIP) of the ions. The TIP of the ion corresponds to the energy required for the ion with the valence of þ2 oxidized to that with the valence of þ3, reflecting the degree of difficulty to remove one electron from the ion, as described by reactions (6, 8e10), for Sm, Gd, Yb, La, respectively. The TIP of Sm, Gd, Yb and La is 23.4, 20.6, 25.1 and 19.2 eV, respectively [54], making the order of the stability of the ions as Yb2þ > Sm2þ > Gd2þ > La2þ. From the point of the TIP, it is reasonable that the concentration of the ions with the valence of þ2 follows the same order as the magnitude of their TIP. Therefore, the oxygen vacancy concentration in La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) introduced by A-site substitution follows the order as La0.9Yb0.1NbO4 > La0.9Sm0.1NbO4 > La0.9Gd0.1NbO4 > LaNbO4. The increased oxygen vacancy concentration in LaNbO4 results in the enhancement of the st (shown in Fig. 6). Also, higher oxygen vacancy concentration makes the distance for the ion transportation shorter, bringing about the decrease of the magnitude of the Ea by increasing the oxygen vacancy concentration roughly (Fig. 7). But the Ea of La0.9Gd0.1NbO4 for the tetragonal structure is the smallest, which can't be explained in the same way and is not clear so far. Except the charge carrier concentration, the space for the movement of the ions also intimately affects the st . The cell lattice parameters of La0.9Yb0.1NbO4 are much smaller compared to that of La0.9Sm0.1NbO4 and La0.9Gd0.1NbO4, leading to a smaller s0 (Equation (5)) than the other two, which is the main reason for its low st among La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb). Higher oxygen vacancy concentration and larger space for the movement of the charge carriers, make the st of La0.9Sm0.1NbO4 higher than that of La0.9Gd0.1NbO4. At the same time, the electrons transfer between the ions with different valences (illustrated by reactions (6, 8e10)) can contribute electron conduction to the st , which is also related with the concentration of the ions with the valence of þ2. Higher concentration of the ions with the valence of þ2 will generate higher se . Moreover, the existence of the oxygen vacancy can introduce the electrons into La0.9M0.1NbO4 via reactions (11e12). Therefore, the introduction of Sm, Gd, Yb can increase the si and se of LaNbO4 simultaneously. If the degree of the increase of the si is higher compared to that of the se , the ion transport number will be increased; Otherwise not. The competition between the enhancement of the si and se , leads to the order of the magnitude of the OCV shown in Fig. 9.
Conclusions Single phase La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) is prepared successfully via a solidestate reaction route, by calcining the constituent oxides at 1200 C for 5 h. The introduction of Sm, Gd or Yb can increase the oxygen vacancy concentration in LaNbO4 due to its muti-valences, and improve the total conductivity of LaNbO4 obviously. The highest conductivity, 1.76 104 S cm1, is achieved for La0.9Sm0.1NbO4 at 800 C in wet air, more than one order higher compared to that of LaNbO4. The oxygen vacancy concentration as well as the
crystal structure affects the electrical properties of LaNbO4 significantly, causing the conductivity of La0.9Sm0.1NbO4 > La0.9Gd0.1NbO4 > La0.9Yb0.1NbO4 > LaNbO4. Taking Sm, Gd, or Yb as a substitute at A-site also slightly increases the open circuit voltage of LaNbO4, which is beneficial for its application as a pure ion conductor. Sm substituted LaNbO4 exhibits the highest ion conductivity and open circuit voltage, making it the best candidate among La0.9M0.1NbO4 (M ¼ Sm, Gd, Yb) for the application as the electrolyte of P-SOFC.
Acknowledgments This research was financially supported by the National Natural Science Foundation of China under the project 51172082. The XRD, SEM characterizations were assisted by the Analytical and Testing Center of Huazhong University of Science and Technology.
references
[1] Haile SM. Fuel cell materials and components. Acta Mater 2003;51:5981e6000. [2] Joon K. Fuel cells - a 21st century power system. J Power Sources 1998;71:12e8. [3] Steele BC, Heinzel A. Materials for fuel-cell technologies. Nature 2001;414:345e52. [4] Alavijeh AS, Venkatesan S, Khorasany R, Kim W, Kjeang E. Ex-situ tensile fatigue-creep testing: a powerful tool to simulate in-situ mechanical degradation in fuel cells. J Power Sources 2016;312:123e7. [5] Alavijeh AS, Khorasany RM, Habisch A, Wang GG, Kjeang E. Creep properties of catalyst coated membranes for polymer electrolyte fuel cells. J Power Sources 2015;285:16e28. [6] Khorasany RM, Alavijeh AS, Kjeang E, Wang G, Rajapakse R. Mechanical degradation of fuel cell membranes under fatigue fracture tests. J Power Sources 2015;274:1208e16. [7] Khorasany RM, Goulet M-A, Alavijeh AS, Kjeang E, Wang GG, Rajapakse R. On the constitutive relations for catalyst coated membrane applied to in-situ fuel cell modeling. J Power Sources 2014;252:176e88. [8] Silberstein MN, Boyce MC. Constitutive modeling of the rate, temperature, and hydration dependent deformation response of Nafion to monotonic and cyclic loading. J Power Sources 2010;195:5692e706. [9] Khorasany RMH, Singh Y, Sadeghi Alavijeh A, Kjeang E, Wang GG, Rajapakse RKND. Fatigue properties of catalyst coated membranes for fuel cells: ex-situ measurements supported by numerical simulations. Int J Hydrogen Energy 2016;41:8992e9003. [10] Liang F, Zhou W, Chi B, Pu J, Jiang SP, Jian L. Pd-YSZ composite cathodes for oxygen reduction reaction of intermediate-temperature solid oxide fuel cells. Int J Hydrogen Energy 2011;36:7670e6. [11] Chen X, Hua B, Pu J, Li J, Zhang L, Jiang SP. Interaction between (La, Sr)MnO3 cathode and NieMoeCr metallic interconnect with suppressed chromium vaporization for solid oxide fuel cells. Int J Hydrogen Energy 2009;34:5737e48. [12] Li M, Hua B, Jiang SP, Pu J, Chi B, Jian L. BaZr0.1Ce0.7Y0.1Yb0.1O3-d as highly active and carbon tolerant anode for direct hydrocarbon solid oxide fuel cells. Int J Hydrogen Energy 2014;39:15975e81.
Please cite this article in press as: Cao Y, et al., Enhanced electrical conductivity of LaNbO4 by A-site substitution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.056
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 x x x ( 2 0 1 6 ) 1 e7
[13] Wang J, Yan D, Pu J, Chi B, Jian L. Fabrication and performance evaluation of planar solid oxide fuel cell with large active reaction area. Int J Hydrogen Energy 2011;36:7234e9. [14] Liu Y, Chen K, Zhao L, Chi B, Pu J, Jiang SP, et al. Performance stability and degradation mechanism of La0.6Sr0.4Co0.2Fe0.8O3-d cathodes under solid oxide fuel cells operation conditions. Int J Hydrogen Energy 2014;39:15868e76. [15] Wachsman ED, Lee KT. Lowering the temperature of solid oxide fuel cells. Science 2011;334:935e9. [16] Jiao Z, Takagi N, Shikazono N, Kasagi N. Study on local morphological changes of nickel in solid oxide fuel cell anode using porous Ni pellet electrode. J Power Sources 2011;196:1019e29. [17] Zhang L, Yang W. High-performance low-temperature solid oxide fuel cells using thin proton-conducting electrolyte with novel cathode. Int J Hydrogen Energy 2012;37:8635e40. [18] Yang L, Wang S, Blinn K, Liu M, Liu Z, Cheng Z. Enhanced sulfur and coking tolerance of a mixed ion conductor for SOFCs: BaZr0.1Ce0.7Y0.2-xYbxO3-d. Science 2009;326:126e9. [19] Bao JX, Ohno H, Kurita N, Okuyama Y, Fukatsu N. Proton conduction in Al-doped CaZrO3. Electrochim Acta 2011;56:1062e8. [20] Zhao F, Chen F. Performance of solid oxide fuel cells based on proton-conducting BaCe0.7In0.3-xYxO3-d electrolyte. Int J Hydrogen Energy 2010;35:11194e9. [21] Katahira K, Kohchi Y, Shimura T, Iwahara H. Protonic conduction in Zr-substituted BaCeO3. Solid State Ionics 2000;138:91e8. [22] Guo Y, Ran R, Shao Z. A novel way to improve performance of proton-conducting solid-oxide fuel cells through enhanced chemical interaction of anode components. Int J Hydrogen Energy 2011;36:1683e91. [23] Norby T, Christiansen N. Proton conduction in Ca- and Srsubstituted LaPO4. Solid State Ionics 1995;77:240e3. [24] Tande C, Perez-Coll D, Mather GC. Surface proton conductivity of dense nanocrystalline YSZ. J Mater Chem 2012;22:11208e13. [25] Guo Y, Ran R, Shao Z. Fabrication and performance of a carbon dioxide-tolerant proton-conducting solid oxide fuel cells with a dual-layer electrolyte. Int J Hydrogen Energy 2010;35:10513e21. [26] Haugsrud R, Norby T. Proton conduction in rare-earth orthoniobates and ortho-tantalates. Nat Mater 2006;5:193e6. [27] Haugsrud R, Norby T. High-temperature proton conductivity in acceptor-doped LaNbO4. Solid State Ionics 2006;177:1129e35. [28] Magraso A, Fontaine ML, Larring Y, Bredesen R, Syvertsen GE, Lein HL, et al. Development of proton conducting SOFCs based on LaNbO4 electrolyte - status in Norway. Fuel Cells 2011;11:17e25. [29] Fjeld H, Kepaptsoglou DM, Haugsrud R, Norby T. Charge carriers in grain boundaries of 0.5% Sr-doped LaNbO4. Solid State Ionics 2010;181:104e9. ~ a-Martı´nez J, Kim J-H, Bridges CA, Huq A, Hodges JP, [30] Bi Z, Pen et al. Effect of Ca doping on the electrical conductivity of the high temperature proton conductor LaNbO4. Int J Hydrog Energy 2012;37:12751e9. [31] Huse M, Norby T, Haugsrud R. Effects of A and B site acceptor doping on hydration and proton mobility of LaNbO4. Int J Hydrog Energy 2012;37:8004e16. [32] Cao Y, Tan Y, Yan D, Chi B, Pu J, Jian L. Electrical conductivity of Zn-doped high temperature proton conductor LaNbO4. Solid State Ionics 2015;278:152e6. [33] Brandao AD, Gracio J, Mather GC, Kharton VV, Fagg DP. B-site substitutions in LaNb1-xMxO4-d materials in the search for
[34]
[35] [36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45] [46] [47]
[48]
[49]
[50]
[51] [52]
[53]
[54]
7
potential proton conductors (M¼Ga, Ge, Si, B, Ti, Zr, P, Al). J Solid State Chem 2011;184:863e70. Solı´s C, Serra JM. Adjusting the conduction properties of La0.995Ca0.005NbO4-d by doping for proton conducting fuel cells electrode operation. Solid State Ionics 2011;190:38e45. Cao Y, Chi B, Pu J, Jian L. Effect of Ce and Yb co-doping on conductivity of LaNbO4. J Eur Ceram Soc 2014;34:1981e8. Li C, Bayliss RD, Skinner SJ. Crystal structure and potential interstitial oxide ion conductivity of LnNbO4 and LnNb0.92W0.08O4.04 (Ln ¼ La, Pr, Nd). Solid State Ionics 2014;262:530e5. Cao Y, Duan N, Wang X, Chi B, Jian L. Enhanced electrical conductivity of Mo-doped LaNbO4. J Eur Ceram Soc 2015;35:1979e83. Bi ZH, Bridges CA, Kim JH, Huq A, Paranthaman MP. Phase stability and electrical conductivity of Ca-doped LaNb1xTaxO4-d high temperature proton conductors. J Power Sources 2011;196:7395e403. Brandao AD, Antunes I, Frade JR, Torre J, Kharton VV, Fagg DP. Enhanced low-temperature proton conduction in Sr0.02La0.98NbO4-d by scheelite phase retention. Chem Mat 2010;22:6673e83. Mikhail P, Hulliger J. Sm2þ in oxide lattices: an unexplored chapter of crystal chemistry. Comments Inorg Chem 1999;21:263e83. MacDonald MR, Bates JE, Ziller JW, Furche F, Evans WJ. Completing the series of þ2 ions for the lanthanide elements: synthesis of molecular complexes of Pr2þ, Gd2þ, Tb2þ, and Lu2þ. J Am Chem Soc 2013;135:9857e68. Yoshida T, Tanaka T, Yoshida S, Hikita S, Baba T, Ono Y. EXAFS study of Yb species encapsulated in potassium Y-type zeolite. Solid State Commun 2000;114:255e9. Tanaka T, Hanada T, Yoshida S, Baba T, Ono Y. Valence variation of Yb encapsulated in the supercage of Y-Type zeolite. Jan J Appl Phy 1993;32:481e3. A, Haugsrud R, Einarsrud M-A, Syvertsen GE, Magraso Grande T. The effect of cation non-stoichiometry in LaNbO4 materials. Int J Hydrog Energy 2012;37:8017e26. Norby T. Proton conduction in solids: bulk and interfaces. MRS Bull 2009;34:923e8. Agmon N. The Grotthuss mechanism. Chem Phys Lett 1995;244:456e62. Shannon RD. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr 1976;32:751e67. Jian L, Wayman C. Monoclinic-to-tetragonal phase transformation in a ceramic rare-earth orthoniobate, LaNbO4. J Am Ceram Soc 1997;80:803e6. Jian L, James RD. Prediction of microstructure in monoclinic LaNbO4 by energy minimization. Acta Mater 1997;45:4271e81. Rooksby H, White E. The structures of 1: 1 compounds of rare earth oxides with niobia and tantala. Acta Crystallogr 1963;16:888e90. an V. High-temperature transitions in rare-earth Stubic niobates and tantaIates. J Am Ceram Soc 1964;47:55e8. pez D, Ruiz-Morales JC, Savvin S, Amsif M, Marrero-Lo ~ ez P. Low temperature sintering of LaNbO4 proton Nu´n conductors from freeze-dried precursors. J Eur Ceram Soc 2012;32:1235e44. Fjeld H, Toyoura K, Haugsrud R, Norby T. Proton mobility through a second order phase transition: theoretical and experimental study of LaNbO4. Phys Chem Chem Phys 2010;12:10313e9. Shvartsburg AA. Gas-phase metal trications in protic solvent complexes. J Am Chem Soc 2002;124:7910e1.
Please cite this article in press as: Cao Y, et al., Enhanced electrical conductivity of LaNbO4 by A-site substitution, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.056