- Email: [email protected]
Preparation and evaluation of new composites as anodes for intermediate-temperature solid oxide fuel cells
Solid State Ionics 181 (2010) 1366–1371
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Preparation and evaluation of new composites as anodes for intermediate-temperature solid oxide fuel cells Weitao Bao ⁎, Jihai Cheng, Zhanyong Hu, Sijia Jin Department of Chemistry and Materials Engineering, Hefei University, Hefei, 230022, China
a r t i c l e
i n f o
Article history: Received 18 August 2008 Received in revised form 12 July 2010 Accepted 22 July 2010 Keywords: SOFCs Anode Doped LaCrO3
a b s t r a c t Composites, which consist of Sr-doped LaCrO3, CeO2, and NiO, were prepared according to a general formula (La0.7Sr0.3)1−xCexCr1−xNixO3−δ (x = 0–0.6) and were assessed as anodes for intermediate-temperature solid oxide fuel cells (ITSOFCs) in terms of phase structure, electrical conductivity, thermal expansion coefficient, and fuel cell performance. Results showed, for x N 0.2, CeO2 and NiO precipitated after being calcined at 800– 1450 °C and dispersed on a doped LaCrO3 support. At 800 °C, the electrical conductivities of the composites (for x = 0.2, 0.4 and 0.6) in a humidified 3% H2O hydrogen atmosphere were 0.72, 1.22 and 4.89 Scm− 1, respectively. The average values of the thermal expansion coefficients of the composites (in a temperature range of 30–800 °C) were 11 × 10− 6 (x = 0.2) and 11.5 × 10− 6 K− 1 (x = 0.6). SOFCs with different anodes were characterized by current–voltage measurements and electrochemical impedance spectroscopy. The peak power density of a single cell with different composite anodes increased with increasing x. At 800 °C, the maximum power density was 81.5 mW cm− 2, and the area specific resistances of the single cells under open circuit condition were 3.1 (x = 0.2) and 1.6 Ω cm− 2 (x = 0.6) using humidified (3% H2O) hydrogen as fuel and ambient air as oxidant. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are promising, clean, and efficient power sources, particularly for stationary applications [1,2]. In the popular Ni/YSZ (yttria-stabilized zirconia, 8 mol% Y2O3) cermet anode, there are still some problems although nickel has excellent catalytic activity for hydrogen oxidation and good electrical conductivity [3]. The major disadvantage of Ni cermet anodes is the anode degradation due to the large Ni―NiO volume change under repeated oxidation–reduction cycles. Such redox cycle behavior may occur accidentally and cause catastrophic failure in SOFC systems. Less critical issues with Ni/YSZ anodes include the possibility of Ni sintering at the operating temperatures of SOFCs, resulting in a decrease of anode performance. Therefore, many novel Ni-free or reduced-Ni SOFC anode materials have been developed [4–6]. Perovskite type doped LaCrO3 and SrTiO3 are also potential anode materials for SOFCs due to the relatively good stability in both reducing and oxidizing atmospheres at high temperatures [7,8]. Doping at the A- and B-site in LaCrO3 and SrTiO3 with varied elements has been done to improve the electrochemical properties of the materials [7–12]. However, the power density of the single cell with doped LaCrO3 or doped SrTiO3 as anodes is very low comparing to that achieved with Ni/YSZ cermets as anodes [8,13]. In this study, we
⁎ Corresponding author. Tel.: + 86 551 2158439; fax: + 86 551 2158436. E-mail address: [email protected] (W. Bao). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.07.028
report results for SOFC composite anodes based on Sr-doped LaCrO3 containing varying amounts of NiO and CeO2, and the effect of calcination temperature. Results on anode microstructure and phases
Fig. 1. XRD patterns of samples (x = 0.4) calcined at different temperatures for 5 h.
W. Bao et al. / Solid State Ionics 181 (2010) 1366–1371
1367
2. Experimental
Fig. 2. XRD patterns of samples (x = 0.2, 0.4 and 0.6) calcined at 1450 °C for 5 h.
present are detected. In order to elucidate the role of Ni and CeO2 on electrochemistry, power density and area specific resistance were measured with the anodes in humidified hydrogen fuel.
Composites were prepared according to a general formula (La0.7Sr0.3)1−xCexCr1−xNixO3−δ (x = 0–0.6). Synthesis process was as follows: Stoichiometric amounts of Ce(NO3)3, La(NO3)3, Cr(NO3)3, Sr(NO3)2 and Ni(NO3)2 nitrates (analytical reagent grade) were dissolved in distilled water to form a solution. Glycine (NH 2 ―CH 2 ― COOH) was then added to the solution. The solution was subsequently heated on a hot plate until it was ignited, producing metal-oxide ‘ash’. The ash was finally heated at 700 °C to remove any possible carbon residues and to form as-prepared powder. The powders calcined at 800–1500 °C for 2–5 h in air were characterized by an X-ray diffractometer (XRD) (D/max-rA, Rigaku). The samples were analyzed by a scanning electron microscope (JEOL JSM-6400) equipped with EDX for compositional analysis. Energydispersive X-ray (EDX)/SEM mapping was performed on an environmental scanning electron microscope (XT30 ESEM-TEP, Philips). Thermal expansion characteristics were obtained using a Dilatometer (Netzsch, DIL 402C) in air at an increase rate of 10 °C/min. Electrical conductivity was measured using standard DC four-probe techniques (34401 multimeter, H.P.) in a temperature range of 500–800 °C. For this measurement, the as-prepared powders were pressed at 200 MPa to form a rectangular bar with a size of 4.1 × 4.1 × 37 mm after being calcined at 1450 °C for 5 h in air. Performances of composite anodes were characterized based on a single cell with YSZ electrolyte support. Commercial YSZ powder was pressed into pellets under 200 MPa and then were calcined in air at
Fig. 3. EDX analysis of a sample (x = 0.4) calcined at 1450 °C for 5 h and then reduced at 900 °C for 24 h.
1368
W. Bao et al. / Solid State Ionics 181 (2010) 1366–1371
Fig. 4. EDX mapping of La, Sr, Ce, Cr and Ni elements in the composites (x = 0.4) calcined at 1200 °C for 5 h in air.
1400–1500 °C for 5 h to obtain about 300 μm thick YSZ pellets with a relative density of 92%. A slurry consisting of as-prepared powder, ethylene glycol, and ethanol in a ratio of 1:1.0–2.0:0.5–2.0 by weight was applied to one side of a YSZ electrolyte pellet, which was subsequently fired at 1350 °C in air for 2 h. The obtained anode has an effective area of 1.0 cm2 with a thickness of about 50 μm. La0.8Sr0.2MnO3 (LSM) prepared by glycine nitrate process mixing with YSZ in a ratio of 60:40 (wt.) was used for cathode powder. Using the similar method, LSM/YSZ cathode with same effective area and thickness was prepared at the other side of the YSZ electrolyte pellet and then was calcined at 1150 °C for 2 h. The cell was sealed on an alumina tube with a silver paste (DAD-87, China). The silver paste was coated on each electrode as current collector. Electrochemical impedance
spectroscopy (EIS) measurements were taken using an impedance analyzer (IM6ex, Zahner). Humidified (3% H2O) hydrogen was used as fuel and ambient air as oxidant. 3. Results and discussion Shown in Fig. 1 is XRD patterns of a sample (x = 0.4) calcined at different temperatures. CeO2 and SrCrO4 as second phases were observed after calcination at 1100 °C for 5 h. The well-crystalline doped LaCrO3, CeO2 and NiO phases formed after calcination at 1450 °C for 5 h. Shown in Fig. 2 is XRD patterns of samples (x = 0.2, 0.4 and 0.6) calcined at 1450 °C for 5 h. It can be seen that CeO2 as second phase precipitates when x = 0.2, and no NiO and SrO was
W. Bao et al. / Solid State Ionics 181 (2010) 1366–1371
Fig. 5. SEM images of the composites (x = 0.4) calcined at (a) 1200 °C and (b) 1450 °C for 5 h in air.
detected, indicating that the Ni and Sr were doped into LaCrO3 lattice completely. CeO2 precipitating revealed that the solubility of CeO2 in Sr- and Ni-doped LaCrO3 was exceeded. With CeO2
Fig. 6. Expansion characteristics of the composites in air, (a) x = 0.2, (b) x = 0.6.
1369
precipitating, the defect structure of the doped LaCrO3 was formed. Theoretically, the defect structure has excess chromium and nickel and thermodynamically tends to precipitate as Cr2O3 or NiO. However, no other phases were detected when x = 0.2 as shown in Fig. 2. For x = 0.4, NiO starts to precipitate. An EDX analysis of a sample(x = 0.4) calcined at 1450 °C for 5 h and then reduced at 900 °C for 24 h is shown in Fig. 3. For the region rich in La, Sr and Cr, corresponding to the Sr-doped LaCrO3 phase, there is 1.51 at.% Ni in the doped LaCrO3 phase, where Ce is not detected. Accordingly, we can speculate that the doped Ni and Ce element almost completely precipitates as NiO and CeO2 when x = 0.4. This result indicates that, when x = 0.4, (La0.7Sr0.3)1−xCexCr1−xNixO3−δ decomposes into La0.7Sr0.3Cr1O3−δ, NiO and CeO2. Shown in Fig. 4 is the La, Sr, Ce, and Ni X-ray peak intensities vs position at x = 0.4 after calcination at 1200 °C for 5 h. The Ni and Ce appeared to be uniformly distributed in the 1200 °C-calcined sample, probably indicating that superfine NiO and CeO2 particles were too small to be individually resolved. Fig. 5 shows SEM images after calcination at (a) 1200 °C and (b) 1450 °C. Fig. 5a shows about 100 nm-sized particles dispersed on the support. In Fig. 5b, the high temperature led to the formation of large CeO2 (0.5–1.0 μm) and NiO (0.5–2.0 μm) particles. That is, the NiO and CeO2 particles were very small and were uniformly distributed after calcination at 1200 °C as shown in Figs. 4 and 5a, but obviously coarsened after being calcined at 1450 °C. The fine and highly dispersed NiO and CeO2 have higher free surface area, which plays an electrocatalytic role in the composite anode. However, at 1200 °C, the fine particles were acquired as shown in Fig. 5, but at the same time the second phases, such as SrCrO4, was detected as shown in Fig. 4. Therefore the calcining temperatures in this experiment are selected in the range of 1300–1400 °C in order to eliminate impurity phases. The optimizing calcination temperature remains to be further studied. Obviously, in this material's design, with the help of the decomposition of doped LaCrO3 phase, the active oxide precipitates from the solid phase during calcining. Consequently, the unique microstructure with finely dispersed and stable oxides on the support surface was obtained. Shown in Fig. 6 is the thermal expansion characteristics of samples (x = 0.2, 0.6) after pre-calcination at 1450 °C for 5 h. The linear thermal expansion rate (dL/L0) increases from 8.81 × 10− 3 (x = 0.2) to 9.15 × 10− 3 (x = 0.6). The average values of thermal expansion coefficients of the composites (in a temperature range of 30–800 °C) are 11 × 10− 6 and 11.5 × 10− 6 K− 1 at x = 0.2 and 0.6, respectively. Shown in Fig. 7 is the temperature dependence of the electrical conductivity of the composites with x = 0.2, 0.4 and 0.6 in air and humidified H2. In air, it is clear that the electrical conductivities of the composites decrease with increasing x. The change is mainly affected by the increases of low conductive NiO and CeO2 phases. However, the electrical conductivities of the composites increase with increasing x in the reduction atmosphere and reach a maximum conductivity of 4.89 Scm− 1 at 800 °C. The increase of the electrical conductivity is mainly attributed to the increases of conductive metallic Ni. Cell voltage and power density for a single cell based on YSZ electrolyte of about 300 μm thickness as a function of current density are shown in Fig. 8. At 800 °C, peak power density of the single cell with different composite anodes is 12.6, 32, 48.2 and 81.5 mW cm− 2 at x = 0, 0.2, 0.4 and 0.6, respectively. By comparison the value of power density around 124 mW cm− 2 is usually recorded at the same condition when Ni/YSZ is used as anode. The peak power density increases with increasing x. To better evaluate the composite anodes, electrochemical impedance spectroscopy measurements under open circuit conditions at 800 °C using a two-electrode configuration are shown in Fig. 9. All single cells in this study have the same cathode and processing conditions. The impedance spectra arc represents area specific resistance of a single cell. From Fig. 9, area specific resistance of the single cell for x = 0.6 is 1.6 Ω cm2, which is obviously lower
1370
W. Bao et al. / Solid State Ionics 181 (2010) 1366–1371
Fig. 7. Temperature dependence of the electrical conductivity of the composites (x = 0.2, 0.4 and 0.6) in air (a) and humidified H2 (b).
than 3.1 Ω cm2 for x = 0.2. These results are in good agreement with the power densities as shown in Fig. 8, indicating that Ni and CeO2 largely modify the electrode performance and enhance the power density of the single cell. For the presence of CeO2, previous studies [14] indicated that CeO2, is beneficial to the catalytic properties of the anode. The main reasons include that it becomes a mixed conductor in the reducing fuel environment, a condition which should expand the reaction zone beyond three-phase boundaries. Nickel is an excellent catalyst for the electrochemical oxidation of hydrogen with high electrical conductivity [3]. Earlier investigation argued that the addition of NiO was crucial to the electrochemical performance of La0.8Sr0.2Cr0.98O3−δ―Ce0.9Gd0.1O1.95―Ni anodes [15]. Therefore, it is crucial for the presence of a metallic Ni and CeO2 phase to enhance power density and provide low area specific resistance for a single cell. 4. Conclusion The new materials system has been investigated as anodes for SOFCs by characterizing its crystal structure, electrical conductivity, thermal expansion coefficients and power density of single cells. The unique microstructure with Ni and CeO2 phases dispersed on a Srdoped LaCrO3 support was obtained. In air, the electrical conductivity of the composites decreased with increasing x; In humidified H2, the electrical conductivity of the composites increased with increasing x
Fig. 8. Voltage and power density vs current density measured in humidified H2 and air at 800 °C for fuel cells with composite anode, bulk YSZ electrolyte support, and LSM/YSZ cathode.
and the maximum electrical conductivity at 800 °C was 4.89 Scm− 1 at x = 0.6. The average values of thermal expansion coefficients of the composites (in a temperature range of 30–800 °C) were 11 × 10− 6 and 11.5 × 10− 6 K− 1 at x = 0.2, 0.6, respectively. Peak power densities of single cells with the composite anodes increased with increasing x, the maximum power density at 800 °C was 81.5 mW cm− 2. At 800 °C, area specific resistances of cells with different composition anodes under open circuit condition were 3.1 and 1.6 Ω cm2 at x = 0.2, 0.6, respectively. Much work is to be done to enhance the performances of single cells with the composite anodes and research the characteristics of carbon deposition using CH4 + 3% H2O as fuel.
Acknowledgment This work was supported by the natural science foundation of education department of Anhui province under contract No. KJ2007A125ZC.
Fig. 9. Impedance spectroscopy results at OCV for fuel cells with composite anode using humidified H2 as fuel and ambient air as oxidant at 800 °C, (a) x = 0.2 and (b) x = 0.6.
W. Bao et al. / Solid State Ionics 181 (2010) 1366–1371
References [1] N.Q. Minh, Solid State Ionics 174 (2004) 271. [2] Y.F. Yi, A.D. Rao, J. Brouwer, G.S. Samuelsen, J. Power Sources 144 (2005) 67. [3] A. Atkinson, S. Barnett, R.J. Gorte, J.T.S. Irvine, A.J. Mcevoy, M. Mogensen, S.C. Singhal, J. Vohs, Nat. Mater. 3 (2004) 17. [4] S. Park, J.M. Vohs, R.J. Gorte, Nature 404 (2000) 265. [5] S.W. Tao, J.T.S. Irvine, Nat. Mater. 2 (2003) 320. [6] O.A. Marina, N.L. Canfield, J.W. Stevenson, Solid State Ionics 149 (2002) 21. [7] H. Kurokawa, L.M. Yang, C.P. Jacobson, L.C.D. Jonghe, S.J. Visco, J. Power Sources 164 (2007) 510.
1371
[8] S. Hui, A. Petric, J. Euro. Ceram. Soc. 22 (2002) 1673. [9] H.P. He, Y.Y. Huang, J.M. Vohs, R.J. Gorte, Solid State Ionics 175 (2004) 171. [10] J. Sfeir, P.A. Buffat, P. Mockli, N. Xanthopoulos, R. Vasquez, H.J. Mathieu, J.V. Herle, K.R. Thampi, J. Catal. 202 (2001) 229. [11] A.L. Sauvet, J. Fouletier, J. Power Sources 101 (2001) 259. [12] A.L. Sauvet, J. Fouletier, F. Gaillard, M. Primet, J. Catal. 209 (2002) 25. [13] I. Yasuda, M. Hishinuma, Solid State Ionics 80 (1995) 141. [14] E.P. Murray, T. Tsai, S.A. Barnett, Nature 400 (1999) 649. [15] B.D. Madsen, S.A. Barnett, J. Electrochem. Soc. 1546 (2007) B501.
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Preparation and evaluation of new composites as anodes for intermediate-temperature solid oxide fuel cells Weitao Bao ⁎, Jihai Cheng, Zhanyong Hu, Sijia Jin Department of Chemistry and Materials Engineering, Hefei University, Hefei, 230022, China
a r t i c l e
i n f o
Article history: Received 18 August 2008 Received in revised form 12 July 2010 Accepted 22 July 2010 Keywords: SOFCs Anode Doped LaCrO3
a b s t r a c t Composites, which consist of Sr-doped LaCrO3, CeO2, and NiO, were prepared according to a general formula (La0.7Sr0.3)1−xCexCr1−xNixO3−δ (x = 0–0.6) and were assessed as anodes for intermediate-temperature solid oxide fuel cells (ITSOFCs) in terms of phase structure, electrical conductivity, thermal expansion coefficient, and fuel cell performance. Results showed, for x N 0.2, CeO2 and NiO precipitated after being calcined at 800– 1450 °C and dispersed on a doped LaCrO3 support. At 800 °C, the electrical conductivities of the composites (for x = 0.2, 0.4 and 0.6) in a humidified 3% H2O hydrogen atmosphere were 0.72, 1.22 and 4.89 Scm− 1, respectively. The average values of the thermal expansion coefficients of the composites (in a temperature range of 30–800 °C) were 11 × 10− 6 (x = 0.2) and 11.5 × 10− 6 K− 1 (x = 0.6). SOFCs with different anodes were characterized by current–voltage measurements and electrochemical impedance spectroscopy. The peak power density of a single cell with different composite anodes increased with increasing x. At 800 °C, the maximum power density was 81.5 mW cm− 2, and the area specific resistances of the single cells under open circuit condition were 3.1 (x = 0.2) and 1.6 Ω cm− 2 (x = 0.6) using humidified (3% H2O) hydrogen as fuel and ambient air as oxidant. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are promising, clean, and efficient power sources, particularly for stationary applications [1,2]. In the popular Ni/YSZ (yttria-stabilized zirconia, 8 mol% Y2O3) cermet anode, there are still some problems although nickel has excellent catalytic activity for hydrogen oxidation and good electrical conductivity [3]. The major disadvantage of Ni cermet anodes is the anode degradation due to the large Ni―NiO volume change under repeated oxidation–reduction cycles. Such redox cycle behavior may occur accidentally and cause catastrophic failure in SOFC systems. Less critical issues with Ni/YSZ anodes include the possibility of Ni sintering at the operating temperatures of SOFCs, resulting in a decrease of anode performance. Therefore, many novel Ni-free or reduced-Ni SOFC anode materials have been developed [4–6]. Perovskite type doped LaCrO3 and SrTiO3 are also potential anode materials for SOFCs due to the relatively good stability in both reducing and oxidizing atmospheres at high temperatures [7,8]. Doping at the A- and B-site in LaCrO3 and SrTiO3 with varied elements has been done to improve the electrochemical properties of the materials [7–12]. However, the power density of the single cell with doped LaCrO3 or doped SrTiO3 as anodes is very low comparing to that achieved with Ni/YSZ cermets as anodes [8,13]. In this study, we
⁎ Corresponding author. Tel.: + 86 551 2158439; fax: + 86 551 2158436. E-mail address: [email protected] (W. Bao). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.07.028
report results for SOFC composite anodes based on Sr-doped LaCrO3 containing varying amounts of NiO and CeO2, and the effect of calcination temperature. Results on anode microstructure and phases
Fig. 1. XRD patterns of samples (x = 0.4) calcined at different temperatures for 5 h.
W. Bao et al. / Solid State Ionics 181 (2010) 1366–1371
1367
2. Experimental
Fig. 2. XRD patterns of samples (x = 0.2, 0.4 and 0.6) calcined at 1450 °C for 5 h.
present are detected. In order to elucidate the role of Ni and CeO2 on electrochemistry, power density and area specific resistance were measured with the anodes in humidified hydrogen fuel.
Composites were prepared according to a general formula (La0.7Sr0.3)1−xCexCr1−xNixO3−δ (x = 0–0.6). Synthesis process was as follows: Stoichiometric amounts of Ce(NO3)3, La(NO3)3, Cr(NO3)3, Sr(NO3)2 and Ni(NO3)2 nitrates (analytical reagent grade) were dissolved in distilled water to form a solution. Glycine (NH 2 ―CH 2 ― COOH) was then added to the solution. The solution was subsequently heated on a hot plate until it was ignited, producing metal-oxide ‘ash’. The ash was finally heated at 700 °C to remove any possible carbon residues and to form as-prepared powder. The powders calcined at 800–1500 °C for 2–5 h in air were characterized by an X-ray diffractometer (XRD) (D/max-rA, Rigaku). The samples were analyzed by a scanning electron microscope (JEOL JSM-6400) equipped with EDX for compositional analysis. Energydispersive X-ray (EDX)/SEM mapping was performed on an environmental scanning electron microscope (XT30 ESEM-TEP, Philips). Thermal expansion characteristics were obtained using a Dilatometer (Netzsch, DIL 402C) in air at an increase rate of 10 °C/min. Electrical conductivity was measured using standard DC four-probe techniques (34401 multimeter, H.P.) in a temperature range of 500–800 °C. For this measurement, the as-prepared powders were pressed at 200 MPa to form a rectangular bar with a size of 4.1 × 4.1 × 37 mm after being calcined at 1450 °C for 5 h in air. Performances of composite anodes were characterized based on a single cell with YSZ electrolyte support. Commercial YSZ powder was pressed into pellets under 200 MPa and then were calcined in air at
Fig. 3. EDX analysis of a sample (x = 0.4) calcined at 1450 °C for 5 h and then reduced at 900 °C for 24 h.
1368
W. Bao et al. / Solid State Ionics 181 (2010) 1366–1371
Fig. 4. EDX mapping of La, Sr, Ce, Cr and Ni elements in the composites (x = 0.4) calcined at 1200 °C for 5 h in air.
1400–1500 °C for 5 h to obtain about 300 μm thick YSZ pellets with a relative density of 92%. A slurry consisting of as-prepared powder, ethylene glycol, and ethanol in a ratio of 1:1.0–2.0:0.5–2.0 by weight was applied to one side of a YSZ electrolyte pellet, which was subsequently fired at 1350 °C in air for 2 h. The obtained anode has an effective area of 1.0 cm2 with a thickness of about 50 μm. La0.8Sr0.2MnO3 (LSM) prepared by glycine nitrate process mixing with YSZ in a ratio of 60:40 (wt.) was used for cathode powder. Using the similar method, LSM/YSZ cathode with same effective area and thickness was prepared at the other side of the YSZ electrolyte pellet and then was calcined at 1150 °C for 2 h. The cell was sealed on an alumina tube with a silver paste (DAD-87, China). The silver paste was coated on each electrode as current collector. Electrochemical impedance
spectroscopy (EIS) measurements were taken using an impedance analyzer (IM6ex, Zahner). Humidified (3% H2O) hydrogen was used as fuel and ambient air as oxidant. 3. Results and discussion Shown in Fig. 1 is XRD patterns of a sample (x = 0.4) calcined at different temperatures. CeO2 and SrCrO4 as second phases were observed after calcination at 1100 °C for 5 h. The well-crystalline doped LaCrO3, CeO2 and NiO phases formed after calcination at 1450 °C for 5 h. Shown in Fig. 2 is XRD patterns of samples (x = 0.2, 0.4 and 0.6) calcined at 1450 °C for 5 h. It can be seen that CeO2 as second phase precipitates when x = 0.2, and no NiO and SrO was
W. Bao et al. / Solid State Ionics 181 (2010) 1366–1371
Fig. 5. SEM images of the composites (x = 0.4) calcined at (a) 1200 °C and (b) 1450 °C for 5 h in air.
detected, indicating that the Ni and Sr were doped into LaCrO3 lattice completely. CeO2 precipitating revealed that the solubility of CeO2 in Sr- and Ni-doped LaCrO3 was exceeded. With CeO2
Fig. 6. Expansion characteristics of the composites in air, (a) x = 0.2, (b) x = 0.6.
1369
precipitating, the defect structure of the doped LaCrO3 was formed. Theoretically, the defect structure has excess chromium and nickel and thermodynamically tends to precipitate as Cr2O3 or NiO. However, no other phases were detected when x = 0.2 as shown in Fig. 2. For x = 0.4, NiO starts to precipitate. An EDX analysis of a sample(x = 0.4) calcined at 1450 °C for 5 h and then reduced at 900 °C for 24 h is shown in Fig. 3. For the region rich in La, Sr and Cr, corresponding to the Sr-doped LaCrO3 phase, there is 1.51 at.% Ni in the doped LaCrO3 phase, where Ce is not detected. Accordingly, we can speculate that the doped Ni and Ce element almost completely precipitates as NiO and CeO2 when x = 0.4. This result indicates that, when x = 0.4, (La0.7Sr0.3)1−xCexCr1−xNixO3−δ decomposes into La0.7Sr0.3Cr1O3−δ, NiO and CeO2. Shown in Fig. 4 is the La, Sr, Ce, and Ni X-ray peak intensities vs position at x = 0.4 after calcination at 1200 °C for 5 h. The Ni and Ce appeared to be uniformly distributed in the 1200 °C-calcined sample, probably indicating that superfine NiO and CeO2 particles were too small to be individually resolved. Fig. 5 shows SEM images after calcination at (a) 1200 °C and (b) 1450 °C. Fig. 5a shows about 100 nm-sized particles dispersed on the support. In Fig. 5b, the high temperature led to the formation of large CeO2 (0.5–1.0 μm) and NiO (0.5–2.0 μm) particles. That is, the NiO and CeO2 particles were very small and were uniformly distributed after calcination at 1200 °C as shown in Figs. 4 and 5a, but obviously coarsened after being calcined at 1450 °C. The fine and highly dispersed NiO and CeO2 have higher free surface area, which plays an electrocatalytic role in the composite anode. However, at 1200 °C, the fine particles were acquired as shown in Fig. 5, but at the same time the second phases, such as SrCrO4, was detected as shown in Fig. 4. Therefore the calcining temperatures in this experiment are selected in the range of 1300–1400 °C in order to eliminate impurity phases. The optimizing calcination temperature remains to be further studied. Obviously, in this material's design, with the help of the decomposition of doped LaCrO3 phase, the active oxide precipitates from the solid phase during calcining. Consequently, the unique microstructure with finely dispersed and stable oxides on the support surface was obtained. Shown in Fig. 6 is the thermal expansion characteristics of samples (x = 0.2, 0.6) after pre-calcination at 1450 °C for 5 h. The linear thermal expansion rate (dL/L0) increases from 8.81 × 10− 3 (x = 0.2) to 9.15 × 10− 3 (x = 0.6). The average values of thermal expansion coefficients of the composites (in a temperature range of 30–800 °C) are 11 × 10− 6 and 11.5 × 10− 6 K− 1 at x = 0.2 and 0.6, respectively. Shown in Fig. 7 is the temperature dependence of the electrical conductivity of the composites with x = 0.2, 0.4 and 0.6 in air and humidified H2. In air, it is clear that the electrical conductivities of the composites decrease with increasing x. The change is mainly affected by the increases of low conductive NiO and CeO2 phases. However, the electrical conductivities of the composites increase with increasing x in the reduction atmosphere and reach a maximum conductivity of 4.89 Scm− 1 at 800 °C. The increase of the electrical conductivity is mainly attributed to the increases of conductive metallic Ni. Cell voltage and power density for a single cell based on YSZ electrolyte of about 300 μm thickness as a function of current density are shown in Fig. 8. At 800 °C, peak power density of the single cell with different composite anodes is 12.6, 32, 48.2 and 81.5 mW cm− 2 at x = 0, 0.2, 0.4 and 0.6, respectively. By comparison the value of power density around 124 mW cm− 2 is usually recorded at the same condition when Ni/YSZ is used as anode. The peak power density increases with increasing x. To better evaluate the composite anodes, electrochemical impedance spectroscopy measurements under open circuit conditions at 800 °C using a two-electrode configuration are shown in Fig. 9. All single cells in this study have the same cathode and processing conditions. The impedance spectra arc represents area specific resistance of a single cell. From Fig. 9, area specific resistance of the single cell for x = 0.6 is 1.6 Ω cm2, which is obviously lower
1370
W. Bao et al. / Solid State Ionics 181 (2010) 1366–1371
Fig. 7. Temperature dependence of the electrical conductivity of the composites (x = 0.2, 0.4 and 0.6) in air (a) and humidified H2 (b).
than 3.1 Ω cm2 for x = 0.2. These results are in good agreement with the power densities as shown in Fig. 8, indicating that Ni and CeO2 largely modify the electrode performance and enhance the power density of the single cell. For the presence of CeO2, previous studies [14] indicated that CeO2, is beneficial to the catalytic properties of the anode. The main reasons include that it becomes a mixed conductor in the reducing fuel environment, a condition which should expand the reaction zone beyond three-phase boundaries. Nickel is an excellent catalyst for the electrochemical oxidation of hydrogen with high electrical conductivity [3]. Earlier investigation argued that the addition of NiO was crucial to the electrochemical performance of La0.8Sr0.2Cr0.98O3−δ―Ce0.9Gd0.1O1.95―Ni anodes [15]. Therefore, it is crucial for the presence of a metallic Ni and CeO2 phase to enhance power density and provide low area specific resistance for a single cell. 4. Conclusion The new materials system has been investigated as anodes for SOFCs by characterizing its crystal structure, electrical conductivity, thermal expansion coefficients and power density of single cells. The unique microstructure with Ni and CeO2 phases dispersed on a Srdoped LaCrO3 support was obtained. In air, the electrical conductivity of the composites decreased with increasing x; In humidified H2, the electrical conductivity of the composites increased with increasing x
Fig. 8. Voltage and power density vs current density measured in humidified H2 and air at 800 °C for fuel cells with composite anode, bulk YSZ electrolyte support, and LSM/YSZ cathode.
and the maximum electrical conductivity at 800 °C was 4.89 Scm− 1 at x = 0.6. The average values of thermal expansion coefficients of the composites (in a temperature range of 30–800 °C) were 11 × 10− 6 and 11.5 × 10− 6 K− 1 at x = 0.2, 0.6, respectively. Peak power densities of single cells with the composite anodes increased with increasing x, the maximum power density at 800 °C was 81.5 mW cm− 2. At 800 °C, area specific resistances of cells with different composition anodes under open circuit condition were 3.1 and 1.6 Ω cm2 at x = 0.2, 0.6, respectively. Much work is to be done to enhance the performances of single cells with the composite anodes and research the characteristics of carbon deposition using CH4 + 3% H2O as fuel.
Acknowledgment This work was supported by the natural science foundation of education department of Anhui province under contract No. KJ2007A125ZC.
Fig. 9. Impedance spectroscopy results at OCV for fuel cells with composite anode using humidified H2 as fuel and ambient air as oxidant at 800 °C, (a) x = 0.2 and (b) x = 0.6.
W. Bao et al. / Solid State Ionics 181 (2010) 1366–1371
References [1] N.Q. Minh, Solid State Ionics 174 (2004) 271. [2] Y.F. Yi, A.D. Rao, J. Brouwer, G.S. Samuelsen, J. Power Sources 144 (2005) 67. [3] A. Atkinson, S. Barnett, R.J. Gorte, J.T.S. Irvine, A.J. Mcevoy, M. Mogensen, S.C. Singhal, J. Vohs, Nat. Mater. 3 (2004) 17. [4] S. Park, J.M. Vohs, R.J. Gorte, Nature 404 (2000) 265. [5] S.W. Tao, J.T.S. Irvine, Nat. Mater. 2 (2003) 320. [6] O.A. Marina, N.L. Canfield, J.W. Stevenson, Solid State Ionics 149 (2002) 21. [7] H. Kurokawa, L.M. Yang, C.P. Jacobson, L.C.D. Jonghe, S.J. Visco, J. Power Sources 164 (2007) 510.
1371
[8] S. Hui, A. Petric, J. Euro. Ceram. Soc. 22 (2002) 1673. [9] H.P. He, Y.Y. Huang, J.M. Vohs, R.J. Gorte, Solid State Ionics 175 (2004) 171. [10] J. Sfeir, P.A. Buffat, P. Mockli, N. Xanthopoulos, R. Vasquez, H.J. Mathieu, J.V. Herle, K.R. Thampi, J. Catal. 202 (2001) 229. [11] A.L. Sauvet, J. Fouletier, J. Power Sources 101 (2001) 259. [12] A.L. Sauvet, J. Fouletier, F. Gaillard, M. Primet, J. Catal. 209 (2002) 25. [13] I. Yasuda, M. Hishinuma, Solid State Ionics 80 (1995) 141. [14] E.P. Murray, T. Tsai, S.A. Barnett, Nature 400 (1999) 649. [15] B.D. Madsen, S.A. Barnett, J. Electrochem. Soc. 1546 (2007) B501.