A new neodymium-doped BaZr0.8Y0.2O3−δ as potential electrolyte for proton-conducting solid oxide fuel cells

Journal of Membrane Science 415–416 (2012) 391–398

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A new neodymium-doped BaZr0.8Y0.2O3  d as potential electrolyte for proton-conducting solid oxide fuel cells Yu Liu, Youmin Guo, Ran Ran, Zongping Shao n State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry & Chemical Engineering, Nanjing University of Technology, No. 5 Xin Mofan Road, Nanjing 210009, PR China

a r t i c l e i n f o

abstract

Article history: Received 1 March 2012 Received in revised form 10 May 2012 Accepted 11 May 2012 Available online 2 June 2012

Chemically stable BaZr0.8Y0.2O3  d (BZY) oxide is limited to applications as an electrolyte for solid oxide fuel cells (SOFCs) because of its poor sintering behavior. This study attempts to improve the sinterability and conductivity of BZY using the partial substitution of Zr4 þ in BZY with Nd3 þ . An oxide with the nominal composition of BaZr0.7Nd0.1Y0.2O3  d (BZNY) is specifically investigated. Results from X-ray diffraction (XRD) demonstrate Nd3 þ is successfully doped into the lattice as anticipated and transmission electron microscopy (TEM) characterizations verify the morphology and crystal structure of the BZNY powder. Dilatometric measurement and scanning electron microscopy (SEM) observations provide verification that the sinterability of the oxide is effectively improved by introducing Nd3 þ . XRD and CO2-TPD results demonstrate that BZNY is relatively stable with respect to the CO2 atmosphere. The total conductivity of BZNY in wet H2 is 2.76  10  3 S cm  1 at 600 1C. An anode-supported thin-film BZNY electrolyte ( 30 mm) cell is fabricated, and the electrolyte layer is found to be well densified after co-sintering with the anode substrate at 1450 1C for 5 h. The cell delivers a peak power density of 142 mW cm  2 at 700 1C, higher than the reported values for a similar cell with BZY electrolyte. & 2012 Elsevier B.V. All rights reserved.

Keywords: Solid oxide fuel cells Proton conductor Yttrium-doped barium zirconate Sinterability

1. Introduction Air pollution and the global warming effect caused by the excessive and inefficient burning of fossil fuels using conventional combustion technologies have become a serious problem in our modern society [1]. To realize a sustainable future, people have become increasingly concerned about the development of newer and cleaner energy materials and advanced energy conversion technologies with improved efficiency and reduced emissions [2]. In this regard, solid oxide fuel cells (SOFCs), which electrochemically convert chemical energy to electric power with the overall efficiency potentially exceeding 70% and zero emission with hydrogen fuel, have been considered one of the most promising energy conversion systems for the future [3]. Based on the diffusion mechanism of the electrolyte, SOFCs can be divided into oxygen-ion-conducting SOFCs (O2  -SOFCs) and proton-conducting SOFCs (H þ -SOFCs) [4]. Traditional O2  -SOFCs with thick yttria-stabilized zirconia electrolytes typically operate at 850–1000 1C to achieve sufficient power output with practical attractiveness [5]. The high operating temperatures, however, introduce numerous drawbacks that make temperature the main obstacle for the commercialization of SOFC technology [6]. To

n

Corresponding author. Tel.: þ86 25 83172256; fax: þ 86 25 83172242. E-mail address: [email protected] (Z. Shao).

0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.05.062

realize their practical application, it is now generally accepted that the operating temperature of SOFCs should be reduced to the intermediate range (500–800 1C) [7]. Because of its much smaller size, a proton (H þ ) has much higher mobility than an oxygen ion (O2 ); consequently, H þ -SOFCs with a high-temperature proton conductor (HTPC) as the electrolyte may potentially deliver higher power output than O2 -SOFCs at intermediate or lower temperatures (400–700 1C) [8]. Typically, the HTPCs in H þ -SOFCs are perovskite-type oxides, which have an activation energy for ion diffusion of 0.3–0.6 eV [9], lower than the oxygen-ion-conducting electrolyte of 0.8–1.5 eV [10]. As a result, selected HTPC materials exhibit a higher ionic conductivity than most oxygen-ion-conducting oxides at intermediate temperatures [11]. To be a potential electrolyte material in H þ -SOFCs, the HTPC must meet three basic requirements: high ionic conductivity, high chemical stability and good sinterability. Unfortunately, most of the available HTPCs fail to meet all the three requirements simultaneously. For example, BaCeO3-based proton conductors show high proton conductivity and good sintering behavior [12], but they can easily react with CO2 and H2O atmospheres, destroying the perovskite structure [13], which is essential to the maintenance of high protonic conductivity. In contrast, BaZrO3-based proton conductors (BaZr0.8Y0.2O3 d, BZY), which show perfect chemical stability in CO2 and H2O atmospheres [14], have poor sinterability [15]. Currently, there is considerable research on methods to increase the sinterability of BaZrO3-based proton conductors by

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adopting various strategies [16,17]. Among them, the application of sintering aids was the most widely used. Single oxides like ZnO, Al2O3 and NiO have been widely tested [18–20]. Although dense pellets were successfully obtained at relatively low sintering temperatures, these sintering aids often gave rise to the reduction of total conductivity of the electrolyte [21,22]. In addition, sintering aids could react with the parent material to form unwanted impurity phase(s). For example, introducing CuO as a sintering aid to BaZr0.85Y0.15O3  d led to the formation of a Ba2YCu3Ox impurity phase [23]. The reduction of the sintering aid is also a major concern for practical application. For example, a CuO sintering aid can be reduced to metallic copper under cell operating conditions, which will increase the partial electronic conductivity in the electrolyte and thus decrease the cell open circuit voltage (OCV) [24]. The simplest perovskite has the nominal composition of ABO3, where A represents an alkaline earth element in twelve-coordination with an oxygen anion, and B represents a transition metal element in six-coordination with an oxygen anion [25]. To enhance protonic conductivity, it is common to dope the B-site with proper trivalent elements such as Y, Nd, Sm, Yb, In, Eu and Gd [26–28]. Such doping will create oxygen-ion vacancies in the oxide lattice, and thus increase the protonic conductivity of the oxide. Interestingly, the sintering properties of some perovskite oxides are also affected by the dopant. Previously, we demonstrated that doping Zn2 þ into the lattice structure of BaZr0.4 Ce0.4Y0.2O3  d was the most effective way to achieve improved sintering without obviously impairing the mechanical properties of the electrolyte [29]. Rare earth cations have been shown to be promising elements to modify the properties of proton-conducting oxides [30,31]. Recently, Fabbri et al. reported that Pr and Y co-doped BZY oxide has improved both sinterability and highly protonic conductivity [32]. However, as the authors noted, Pr is an element with lower electronegativity than Zr, which can enhance the alkalinity of the oxide and result in a higher reactivity towards acid gases. Previously, many reports have proposed that Nd is a promising dopant for BaCeO3-based ceramics with respect to high proton conductivity [33,34]. In this study, we report the application of Nd as a dopant for BZY to successfully enhance its total conductivity and also sintering capability. BaZr0.8Y0.2O3  d and BaZr0.1Ce0.7 Y0.2O3  d (BZCY) were also synthesized and compared with BaZr0.7Nd0.1Y0.2O3  d (BZNY) in the context of sintering behavior and chemical stability. Finally, BZNY was applied in real anodesupported single cells for power generation investigation.

weight 30,000–45,000) was also added to the anode mixture as a pore-former during the ball milling process. Ba0.5Sr0.5Co0.8 Fe0.2O3  d (BSCF) was applied as a cathode, which was also synthesized by the EDTA–citrate complexing sol–gel method. 2.2. Cell fabrication To prepare the BZNY electrolyte substrates for symmetrical cells, which were applied for the conductivity investigation, a specified amount of BZNY powder was added into a stainless steel die with a diameter of 15 mm and pressed under a hydraulic pressure of 300 MPa for 1 min. To prepare anode-supported duallayer single cells for I–V characterization, a dual dry pressing technique was applied, where the appropriate amount of NiOþBZNY anode powder was first dry-pressed as a substrate with 150 MPa, then approximately 0.02 g of brick-red BZNY powder was uniformly distributed over the anode substrate and pressed again with a pressure of 250 MPa. The green electrolyte substrates and dual-layer cells were then sintered at 1450 1C for 5 h at a heating rate of 2 1C min  1 to allow the densification of the electrolyte layers. BSCF slurry was painted onto the central surface of the dense BZNY electrolyte membrane with a geometric area of 0.48 cm2, and then fired at 1000 1C for 2 h to obtain complete single cells. For the symmetric cells, both surfaces of the BZNY electrolyte were deposited with silver paste and silver wires served as the current collector and current lead, respectively. 2.3. Electrochemical characterization The performance of an anode-supported single cell was tested in a home-developed cell testing system. Humidified H2 (  3% H2O), which acted as the fuel gas, was fed into the anode side at a flow rate of 80 ml min  1 [STP] while the cathode side was exposed to ambient air as the oxidant. I–V polarization of the cell was measured from 700 1C to 500 1C, at 50 1C intervals, using a Keithley 2420 digital sourcemeter in a 4-probe mode. The electrochemical impedance spectra (EIS) of the cells were measured via an AC impedance method using an electrochemical workstation composed of a Solartron 1260A frequency response analyzer and a Solartron 1287 potentiostat. The applied frequency range was from 0.1 Hz to 100 kHz, and the signal amplitude was 10 mV under OCV conditions. The electrical conductivity of the electrolyte was tested in air, wet air, H2 and wet H2 from 750 to 500 1C at 50 1C intervals using the same electrochemical workstation for EIS measurements. 2.4. Other characterizations

2. Experimental section 2.1. Materials processing The BZNY, BZY and BZCY powders were synthesized via an EDTA–citrate complexing sol–gel method. Using the synthesis of BZNY as an example, stoichiometric amounts of Ba(NO3)2, Zr(NO3)4  5H2O, Nd(NO3)3  nH2O (n ¼1.2, as determined by thermogravimetric analysis) and Y(NO3)3  6H2O were dissolved in deionized water, then EDTA and citric acid with a ratio of 1:2:1 to the total metal cations, acting as complexing agents, were added. NH4OH was used to adjust the pH value of the solution between 6 and 8. After the solution became transparent, it was heated at 80 1C with stirring to evaporate the water and obtain a gel. Afterwards, the gel was pretreated at 240 1C for 6 h and then heated to 1100 1C for 10 h to obtain a brick-red BZNY powder. The NiOþBZNY anode powder was prepared by mixing 50 wt% NiO and 50 wt% BZNY with high-energy ball milling (Pulverisette 6) using ethanol as a solvent. Polyvinyl butyral (PVB, molecular

To study the chemical stability of the various electrolyte materials, specified amounts of BZNY, BZY and BZCY electrolyte powders were introduced into a quartz tube and treated in CO2 gas, H2 or wet H2 atmosphere with a flow rate of 40 ml min  1 [STP] at 650 1C for 2 h. After the treatment, 0.05 g samples were added to a U-type quartz tube and heated from room temperature to 930 1C at a rate of 10 1C under pure argon atmosphere at a flow rate of 15 ml min  1 [STP]. The mass peaks of CO, CO2, O2 and argon were specifically monitored during the experiment. The amount of formed carbonate over the samples after treatment in a CO2 atmosphere was calculated from the CO2 peak area using SrCO3 as a standard substance. The structural phases of the fresh and treated electrolyte powders were examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD patterns were recorded at room temperature using an ARL X’TRA diffractometer with Cu Ka radiation. The diffractometer was operated at 40 kV and 35 mA within a 2y scanning range of 10–901 at a step size of

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0.021. Specified amounts of BZNY powders were dissolved in isopropyl alcohol, which acts as a dispersing agent, and ultrasonically treated for 6 h. Next, 4–6 drops of the solution were placed on carbon films and dried for TEM (JEOL JEM-2100) examination. Specific surface area of the samples was characterized by N2 adsorption at the temperature of liquid nitrogen using a BELSORP II instrument. Before analysis, the samples were treated at 250 1C for 2 h in vacuum to remove the weakly adsorbed species on the surface. Dilatometric curves (DIL 402C dilatometer manufactured, NETZSCH) were measured to evaluate the sintering behavior of the BZNY, BZY and BZCY electrolyte pellets. The relative densities of the three electrolyte pellets were calculated by the Archimedes method. The micrograph of sintered electrolyte pellets and the cross-sectional microstructures of the tested single cell were investigated by scanning electron microscopy (SEM, Model QUANTA-200).

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3. Results and discussion 3.1. Phase analysis Fig. 1 shows the XRD pattern of BZNY powder prepared from the complexing method, after calcination at 1100 1C in air for 10 h. All Bragg peaks can be indexed based on a cubic perovskite-type structure with space group Pm3m. Compared to the reflection lines of BZY, the diffraction peaks of BZNY were shifted towards lower Bragg angles, indicating the lattice expansion. This result is consistent with ˚ with respect to that of Y3 þ (0.900 A) ˚ and the larger cation size of Nd3 þ (0.983 A) ˚ It suggests that Nd3 þ was successfully incorporated into the B-site Zr4 þ (0.720 A). of the perovskite structure as intended. Fig. 2a shows the low-magnification TEM image of the as-synthesized BZNY powder. Primary particles are in a cubic shape with a size of 15–25 nm, which were agglomerated to form secondary particles with a size of 100–500 nm as a result of the high calcination temperature. It is known that the size of the particles partly determines the conductivity of the materials. In addition, the presence of agglomerated particles has a harmful effect on the final state of sintering and heterogeneities in the microstructure after calcination. [35] Most of the cubicshaped crystals present a polycrystalline crystal structure, which was verified using XRD and HR-TEM technologies. Fig. 2b and c shows the HR-TEM images of the BZNY particle with lattice fringes. The particle has relatively well-defined crystalline facets with two fringes detected. One is with an interplanar spacing of d¼ 0.283 nm, coinciding with the (110) diffraction plane of BZNY perovskite, the other is with an interplanar spacing of d ¼0.234 nm, corresponding to the (111) plane of BZNY perovskite. Fig. 2d shows the selected area electron diffraction (SAED) patterns of the BZNY particles, which indicate that the BZNY is a polycrystalline crystal, and the diffraction rings consist of discrete diffraction spots. The interplanar spacings of d ¼0.232, 0.284 and 0.201 nm were calculated, which correspond to the diffraction planes (111), (110) and (200), respectively.

3.2. Sintering behavior

Fig. 1. X-ray diffraction patterns of BZNY and BZY powder after sintering at 1100 1C for 10 h.

To study the sintering behavior of BZNY, BZY and BZCY, green pellets were prepared by dry-pressing and then sintering at 1400, 1450 and 1500 1C for 5 or 10 h, respectively. As shown in Table 1, the linear shrinkage of BZNY and BZY after sintering at 1500 1C for 10 h is approximately 16.10% and 11.85%, respectively. The linear shrinkage of BZNY sintered at 1400 1C for 10 h approached that of the BZY pellet sintered at 1500 1C for 10 h. This finding indicates that Nd3 þ -doping promoted the sintering of the BZY electrolyte. We further studied the sintering behavior of the various samples by dilatometric measurement with the curves shown in Fig. 3. The BZNY, BZCY and BZY pellets began to shrink at 970, 960 and

Fig. 2. Low-magnification TEM image of BZNY powder after sintering at 1000 1C for 10 h (a), HR-TEM images of the particles with lattice fringes (b) and (c), and SAED of selected BZNY microspheres (d).

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Table 1 Linear shrinkages of BZY, BZCY and BZNY pellets after sintering at different temperatures for 5 h (or 10 h). Samples

Linear shrinkage (%) 1400 1C

BZY BZCY BZNY

1450 1C

1500 1C

5h

10 h

5h

10 h

5h

10 h

7.62 15.36 11.47

7.98 16.06 12.05

9.15 15.86 13.42

10.11 16.19 13.97

10.53 16.25 14.91

11.85 16.35 16.10

Fig. 4. Surface morphologies of (a) BZY, (b) BZNY, and (c) BZCY pellets after sintering at 1500 1C for 10 h, as observed by SEM.

Table 2 Relative densities of BZNY, BZY and BZCY pellets after sintering at different temperatures for 5 h (or 10 h, 20 h, 40 h). Samples Fig. 3. Dilatometric curves of the BZNY, BZY and BZCY green pellets from 200 to 1400 1C in air atmosphere. 1120 1C, respectively. The shrinkage of BZNY reached approximately 13% at 1400 1C, which was almost twice that of BZY (7%) and approached that of BZCY (14%) at the same temperature, further demonstrating that the partial substitution of Zr in BZY by Nd enhanced the sinterability of the oxide. Fig. 4 shows SEM images from the surface and cross-sectional views of BZNY, BZY and BZCY pellets sintered at 1500 1C for 10 h. The BZNY pellet sintered better than the BZY pellet, although some surface defects appeared on the BZNY pellet. The relative densities of the pellets sintered at 1500 1C for 10 h, as measured by the Archimedes method, are presented in Table 2, which clearly indicates the improved sintering after Nd3 þ -doping. To investigate the effect of sintering time on the microstructure of the BZNY pellet, the pellet was sintered at 1500 1C for 10, 20 and 40 h. SEM images of the three pellets are shown in Fig. 5. From the surface view, the samples that were sintered for 10 or 20 h were still not fully densified, while the pellet sintered for 40 h displayed a relatively dense surface. From the sectional view, the number of enclosed pores was decreased with the increase of sintering time. This result suggests the sintering of BZNY at 1500 1C was strongly kinetically controlled. Therefore, the sintering temperature was further increased to 1600 1C to speed up the sintering rate. As shown in Fig. 5g and h, a dense surface and interior was achieved after sintering for a relatively short period of 10 h. 3.3. Chemical stability High chemical stability against CO2- and H2O-containing atmospheres is critical for a proton conductor as an electrolyte in H þ -SOFCs to ensure long-term stability when operating on carbon-containing fuels, such as reforming gas of hydrocarbons. BZY was reported to have strong chemical stability in a CO2 atmosphere [36]. To investigate the effect of Nd3 þ doping on the chemical stability of BZY, exact amounts of BZY, BZNY and BZCY were separately treated in a CO2, H2 and wet H2 atmosphere at 650 1C for 2 h and then subjected to XRD characterization, and the result is shown in Fig. 6. Following treatment in a dry or wet H2 atmosphere, all three samples maintained the initial perovskite lattice structure, implying they can stably operate under a dry hydrogen atmosphere and humidified atmosphere. After the treatment in the CO2 atmosphere, the basic perovskite structure was destroyed for the BZCY sample with the appearance of some diffraction peaks related to carbonate, which suggests that BZCY was not stable enough under a CO2-containing atmosphere. This result is consistent with other reports in the literature [37]. However, both BZNY and BZY samples

BZNY BZY BZCY

Relative density (%) 1400 1C

1450 1C

1500 1C

1600 1C

5h

10 h

5h

10 h

5h

10 h

20 h

40 h

10 h

60.5 56.7 92.4

61.1 56.9 93.7

68.9 60.1 95.7

69.1 60.4 96.1

74.3 62.1 96.5

82.7 63.1 96.7

85.1 n/a n/a

90.7 n/a n/a

94.1 n/a n/a

maintained the original perovskite lattice structure, which suggests that BZNY was significantly more chemically stable than BZCY against CO2. To further compare the chemical stability of BZNY, BZY and BZCY against CO2, the three samples were also subjected to CO2-TPD analysis following the treatment in the CO2 atmosphere, with the corresponding profiles shown in Fig. 7. The BZCY sample showed a strong CO2 peak near 900 1C, which was attributed to the decomposition of a formed bulk carbonate [38]. Both BZNY and BZY samples showed a weak CO2 desorption peak between 500 and 800 1C, which was attributed to the surface-adsorbed CO2. Unlike BZY, a weak peak between 800 and 900 1C was also observed for BZNY, which can be attributed to the decomposition of the bulk carbonate. The bulk carbonate formation rates were calculated by taking into account the specific surface area of the powders, and the results are listed in Table 3. Although the carbonate formation rate of BZNY was twice that of BZY, it was still a satisfactory value with respect to BZCY, which was four times larger than that of BZNY. These results demonstrated that BZNY still possessed favorable chemical stability against CO2, and it is promising as a SOFC electrolyte with respect to chemical stability.

3.4. Conductivity Although BZY potentially has very high proton conductivity, poor sintering behavior results in a very large grain boundary resistance. Consequently, the BZY electrolyte usually shows low apparent conductivity [39], which hinders a high power output for a cell with a BZY electrolyte. It is well known that the conductivity of oxide conductors is very sensitive to the dopant [40]. The conductivity of BZNY in air, wet air, H2 and wet H2 was measured by the AC impedance method within the temperature range of 450–750 1C. To minimize the effect of enclosed pores and surface defects on the electrical conductivity, the BZNY pellets were sintered at 1600 1C for 10 h. It was found that the impedance arc related to the proton bulk diffusion was larger than that related to the proton grain boundary diffusion at both 500 and 600 1C. The small grain boundary arc

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Fig. 5. SEM images from surface view of BZNY pellets sintered at 1500 1C for (a) 10 h, (c) 20 h, (e) 40 h and (g) 1600 1C for 10 h and fracture images of BZNY pellets sintered at 1500 1C for (b) 10 h, (d) 20 h, (f) 40 h, and (h) 1600 1C for 10 h. confirmed that the BZNY pellets were well densified after sintering at 1600 1C for 10 h. With the decrease in operating temperature, both the resistance associated with grain boundary diffusion and its ratio to the total resistance were increased. The bulk conductivity of BZNY in wet air was calculated to be 5.13  10–4 and 8.50  10–4 S cm  1 at 600 and 500 1C, respectively, whereas the corresponding grain boundary conductivity was 3.83  10  3 and 5.70  10–4 S cm  1, respectively. Fig. 8a and b shows the Arrhenius plots of the temperature dependence of conductivity of BZNY in air, wet air, H2 and wet H2. Fig. 8c reports the impedance spectra for BZNY at 500 1C and 600 1C in air. In general, the total conductivity of BZNY in air was much larger than that in H2 and wet H2. For example, the total conductivity of BZNY at 600 1C in air, wet air, H2 and wet H2 was 4.15  10  3, 4.64  10  3, 1.08  10  3 and 2.76  10  3 S cm  1, respectively. This finding implies that the contribution of p-type electronic conductivity to the total conductivity was relatively large under an air atmosphere; however, such p-type electronic conduction was likely effectively suppressed by introducing water vapor or hydrogen [41]. The conductivity of BZNY at 600 1C was as twice as the reported value of 1.10  10  3 S cm  1 for a BaZr0.9Y0.1O3  d pellet in wet H2 atmosphere [42]. The higher conductivity of BZNY than BZY can be explained by the decreased grain boundary resistance after Nd3 þ doping. The activation energies (Ea) of the electrical conduction for the BZNY electrolyte under various atmospheres were calculated according to the following Arrhenius equation:

sT ¼ A expðEa=kTÞ

ð1Þ

where T, A, Ea and k represent the absolute temperature, the pre-exponential factor, the activation energy and the Boltzmann constant, respectively. Below 600 1C, the activation energy in wet air was approximately 0.38 eV, which was consistent with that of typical proton conduction (approximately 0.3–0.6 eV) [25] and which suggests that the proton served as the major charge carrier. However, the activation energy increased to 0.58 eV with the further increase in temperature, implying that mixed electron–hole/proton conductivity became predominant in this circumstance. The activation energy in air showed the same trend. The possible explanation for the lower activation energies in H2 and wet H2 than in air is that the protons are the main charge carriers in the hydrogen atmosphere while oxygen-ionic conduction likely also plays an important role in air.

substrate promoted sintering of the electrolyte membranes. A similar observation was also reported by Bi et al. [43]. Fig. 10 shows the I–V curves for the cell measured from 700 to 500 1C using 3% H2O-humidified hydrogen as fuel and ambient air as the cathode atmosphere. The OCVs reached 0.90, 0.99, and 1.05 V at 700, 600 and 500 1C, respectively, which were comparable to those reported for similar H þ -SOFCs with different electrolyte materials [32]. The high OCV values imply that Nd3 þ doping did not obviously increase the partial electronic conductivity under the fuel cell operating conditions. Peak power densities reached 142, 105 and 69 mW cm  2 at 700, 600 and 500 1C, respectively, which are comparable to the reported values for a SOFC with a BZPY electrolyte of approximately 20 mm thickness [44] and higher than similar cells with a BZY electrolyte [38]. EIS of the single cell was also measured under the OCV condition with the results shown in Fig. 11. The intercept of the impedance spectra at high frequency with the real axis represents the ohmic resistance, whereas the intercept at low frequency with the real axis corresponds to the total resistance of the cell. The difference between the intercepts at high and low frequencies is the electrode polarization resistance, which is a sum of the resistance of the two interfaces, i.e., the cathode/electrolyte interface and the anode/electrolyte interface. At 700 1C, the ohmic resistance was the main contribution to the total resistance. In contrast, the polarization resistance of the cell increased more quickly with decreasing temperature; it was twice the ohmic resistance at 500 1C. Thus, the reduction of cell output at lower temperature was largely because of the high polarization resistance. The total cell ohmic resistance was approximately 1.46 O cm2 at 600 1C. If it is assumed that the ohmic resistance was solely from the electrolyte layer, the conductivity of the BZNY thin-film electrolyte is then calculated from the ohmic resistance and the membrane thickness and is 3.57  10  3 S cm  1, which is comparable to the value of a self-supported BZY membrane at the same temperature [43] but is much lower than the conductivity of a self-supported BZNY membrane of 4.64  10  3 S cm  1. This result implies that the interfacial reaction between the cathode and the electrolyte was not negligible, which may cause a significant increase in the ohmic resistance of the cells [45]. Therefore, developing a more suitable cathode material or reducing the thickness of the electrolyte membrane could help to further enhance the performance of the BZNYbased fuel cell.

3.5. Cell performance

4. Conclusions From the above studies, BZNY was established as a potential electrolyte for H þ -SOFCs with improved conductivity and sinterability compared to a BZY proton conductor. The BZNY was then applied to an actual single cell for a power generation test. Here, the electrolyte thin film layer and the anode were cosintered at 1450 1C in air. Fig. 9 shows an SEM image from the cross-sectional view of the single cell with reduced anode. A tri-layer structure consisting of a rather porous anode, a dense electrolyte layer with thickness of 30 mm and a porous cathode layer were observed. As demonstrated previously, there were still some enclosed pinholes in the self-supported BZNY electrolyte membrane even after sintering at 1500 1C; however, the thin-film electrolyte in the anode-supported single cell was better densified after sintering at 1450 1C, although some enclosed pores were still observed. This finding suggests that the NiO-based anode

In this study, we proposed Nd3 þ -doped BZY (BZNY) as a potential electrolyte for proton-conducting SOFCs. The as-synthesized BZNY oxide had a cubic perovskite oxide structure. The Nd3 þ doping was found to improve the sinterability of BZY, and dense BZNY pellets were obtained after sintering at 1500 1C for 40 h or 1600 1C for 10 h. By applying an anode-supported configuration, BZNY was densified at 1450 1C because of the enhanced sintering effect of the anode substrate. The conductivity of the self-supported BZNY electrolyte reached 4.15  10  3, 4.64  10  3,

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Table 3 BET specific surface areas of the as-prepared BZNY, BZY and BZCY samples and their carbonate formative rates under CO2 atmosphere. Samples

Specific surface area (m2 g  1)

Carbonate formative rate (10  6 mol m  2 min  1)

BZCY BZNY BZY

12.6(7) 13.3(1) 15.4(5)

1.48 0.28 0.14

Fig. 6. XRD patterns of (a) BZY, (b) BZNY and (c) BZCY powders after treatment in H2, wet H2 and CO2 atmospheres.

Fig. 8. Temperature dependence of electrical conductivity of BZNY in Arrhenius plots BZNY under (a) dry and wet air and (b) dry and wet H2 atmospheres; (c) the impedance spectra of BZNY at 500 1C and 600 1C in air atmosphere.

Fig. 7. CO2-TPD profiles of the BZNY, BZY and BZCY powders after treatment in a CO2 atmosphere at 650 1C for 2 h.

1.08  10  3 and 2.76  10  3 S cm  1 in air, wet air, H2 and wet H2 at 600 1C, respectively, which is higher than the reported value for BZY. The Nd3 þ doping also increased the carbonate formation rate; however, BZNY still possesses favorable chemical stability in a CO2 atmosphere. An anode-supported thin film BZNY electrolyte cell with a BSCF cathode delivered a peak power density and an OCV of 105 mW cm  2 and 0.99 V at 600 1C, respectively. Therefore, BZNY is highly promising as a new potential electrolyte for proton-conducting SOFCs with good sinterability, chemical stability and protonic conductivity.

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and the ‘‘Outstanding Young Scholar Grant at Jiangsu Province’’ under Contract no. 2008023.

References

Fig. 9. SEM image from the cross-sectional view of the tested BZNY-based single cell.

Fig. 10. I–V and I–P curves of a single cell with BZNY electrolyte from 500 to 700 1C.

Fig. 11. Electrochemical impedance spectra of the single cell with BZNY-based electrolyte under open-circuit condition.

Acknowledgments This work was supported by the National Science Foundation for Distinguished Young Scholars of China under Contract no. 51025209, the Program for New Century Excellent Talents (2008)

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