smbaco2o5+ as high efficient oxygen electrode of solid oxide electrolysis cells

SmBaCo2O5+δδδδ as High Efficient Oxygen Electrode of Solid Oxide Electrolysis Cells

Bo Weia,b, Kongfa Chena, Ling Zhaoa, Na Aia, Zhe Lüb and San Ping Jianga

a Fuels and Energy Technology Institute, Curtin University, Perth, WA 6102, Australia b Department of Physics, Harbin Institute of Technology, Harbin, 150080, China

Oxygen electrode is a key component for solid oxide electrolysis cells (SOECs). Double perovksite oxide SmBaCo2O5+δ (SBCO) exhibits high electrical conductivity and fast oxygen transport properties, herein we proposed SBCO as a novel high efficient SOEC oxygen electrode based on Ce0.9Gd0.1O1.9 (GDC) electrolyte. Preliminary study showed that an area specific resistance of 0.024 Ω cm2 was achieved under OCV condition at 900°C and a stable operation of 20 h was obtained at an anodic polarization current density of 200 mA⋅cm-2.

Introduction The development of clean and efficient hydrogen production methods is of great importance for the coming hydrogen economy. As a promising alternative to methane-steam reforming, high temperature solid oxide electrolysis cells (SOECs) are particularly favorable due to the thermodynamic and kinetics advantages (1, 2). Operation at elevated temperature involves lower energy consumption than traditional low temperature steam electrolysis and reduced overpotential at both oxygen and fuel electrodes. In recent years, SOECs have received considerable attention for (i) the continued growth of intermittent renewable sources like wind and solar, and (ii) an expanded range of applications including H2 or syngas production, energy storage, etc. In principle, SOECs are reversibly operated solid oxide fuel cells (SOFCs). When functioned as SOECs for H2 production, steam is fed to the fuel electrode (cathode) where it is reduced to H2 (H2O+2e-→ O2-+H2). At the oxygen electrode (anode) side, oxygen ions immigrated from electrolyte are oxidized to oxygen gas (O2- → 1/2O2+2e-). Similar to that in SOFC, (La,Sr)MnO3 (LSM) is so far the most widely used oxygen electrodes in SOEC (3, 4). To improve the anode performance, mixed conductive La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) with high oxygen diffusion and surface exchange kinetics exhibited improved electrochemical performance, which is thus considered as a very promising alternative to LSM (5). Furthermore, many other commonly used SOFC cathodes like La0.8Sr0.2FeO3-δ (LSF) (3), Ba0.5Sr0.5Co0.2Fe0.8O3-δ (BSCF)(6), Ba0.9Co0.5Fe0.4Nb0.1O3-δ (BCFN) (7) and SrCo1−xMoxO3-δ (8) have been investigated as alternative oxygen electrodes with reasonable performance. Recently, A–site cation ordered REBaCo2O5+δ (RE = Gd Pr, Nd, Sm) double perovskite oxides have received tremendous attention due to fast transport properties (9-13). The ideal structure can be represented as the stacking sequence of REOδCoO2BaOCoO2. The layered structure with alternating lanthanide and alkali earth planes provides disorder-free channels for ionic motion, thereby increasing the oxygen diffusivity by many orders of

10.1149/05701.3189ecst ©The Electrochemical SocietyECS Transactions, 57 (1) 3189-3196 (2013)

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magnitude. Although high electrochemical performance for SOFC were identified in these layered compounds, the application of double perovskite oxides on SOEC oxygen electrodes has not been reported. In this paper, we presented our preliminary study on the electrochemical properties of SmBaCo2O5+δ (SBCO) electrode under SOEC anodic condition.

Experimental

Powder Synthesis

SmBaCo2O5+δ was prepared by a modified sol-gel process using both

ethylenediamine tetraacetic acid (EDTA) and citrate as complexing agents. Analytical grade of Sm2O3, Ba(NO3)2, Co(NO3)2·H2O, EDTA, NH3·H2O and citric acid were used as raw materials. A mole ratio of 1:1:2 for total metal ions:EDTA:citric acid was used. Firstly, Sm2O3 was dissolved in nitric acid in a water bath at 80°C. EDTA-NH3·H2O solution, rest metal nitrates and citric acid were added subsequently and mixed under continuous stirring. Then, the pH value of the solution was adjusted to ~7 using ammonia water. With the evaporating of water, a purple viscous gel was obtained which was pre-heated at 200°C in an oven for 10h and followed by calcination at 1000°C for 4 h to yield the oxide powder.

Half-cell Fabrication

Gadolinia-doped ceria (Gd0.1Ce0.9O1.95, GDC) was used as solid electrolyte. GDC

powder was pressed into disks using a Φ=22mm stainless steel mould and followed by sintering at 1500°C for 5h. As-obtained SBCO powder was thoroughly mixed with organic ink (Fuel Cell Materials, USA) at a weight ratio of 1:1 to form the electrode ink. Then it was painted onto the centre of GDC surface with an effective area of 0.5cm2. Pt ink (Fuel Cell Materials) was brushed symmetrically to the opposite side of SBCO electrode, acting as the counter electrode. Pt ink was also coated as a ring near the counter electrode to serve as the reference electrode. The gap between counter electrode and reference electrode was ~4mm. These electrodes were sintered at 1050°C for 4h with a heating and cooling rates of <3°C⋅min-1.

Characterizations

The phase purity of as-prepared SBCO was determined by X-ray diffraction

measurement with Cu-Kα radiation (XRD, Bruker D8 Advance, Germany). The result confirmed the formation of single phase with an orthorhombic structure. Using a three-electrode configuration, electrochemical performance of SBCO anode was measured by a Potentiostat (Gamry Reference 3000, USA) between 650°C and 900°C with 50°C interval. Polarization behaviour of the SBCO electrodes was examined at a constant anodic current density of 200 mA⋅cm-2 at 900°C for 20h. Current passage was interrupted from time to time to collect the impedance spectra under open circuit condition, which were carried out in the frequency range of 100 kHz ~0.1 Hz with an amplitude of 10 mV.

ECS Transactions, 57 (1) 3189-3196 (2013)




Results and Discussion

Electrochemical Property It has been reported that double perovskite oxides can easily react with YSZ electrolyte, but they demonstrate good chemical compatibility with GDC electrolyte. In this study, we investigated the electrochemical performance of SBCO electrode based on GDC electrolyte. Figure 1a shows the impedance spectra collected under open-circuit voltage condition. Good performance was obtained with specific polarization resistances (Rp) as low as 0.024 Ω⋅cm2 at 900 °C and 0.13 Ω⋅cm2 at 800°C. Figure 1b shows the Arrhenius plots of the polarization resistance of the SBCO electrode. The dependence is nearly linear, and the activation energies (Ea) calculated from the slope of linear fit is 123.23 kJ⋅mol-1, which is larger than that of pervious reported SBCO (107.09 kJ⋅mol-1, sintered at 1000°C) (14). Such difference can be caused by the different fabrication condition like powder synthesis method, sintering temperature etc.

2.0 2.5 3.0 3.5-1.5

-1.0

-0.5

0.0

0.5

-Z ''

/ Ω

Z ' / Ω

900 oC 850 oC 800 oC 750 oC

(a)

0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0(b)

ln R

p /

Ω c

m2

1000/T (K -1)

Ea=123.23kJ mol-1

Figure 1. (a) Typical impedance spectra of SBCO anode obtained under OCV condition. Active area of electrode is 0.5 cm2 (b) Arrhenius plot of the SBCO anode.





-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Ove

rpot

entia

l / V

Current Density / A cm-2

900 oC 850 oC 800 oC 750 oC SOFC

SOEC

Figure 2. Current–overpotential curves of the SBCO electrode measured in both SOFC and SOEC modes in air.

Figure 2 shows the current–overpotential curves of SBCO oxygen electrode measured in both SOFC and SOEC modes in air. The iR loss was automatically subtracted during testing via a current interrupt technique. Between 750°C ~900°C, all curves ended at a current density of 800 mA⋅cm-2. At 900°C, the overpotentials are 15mV and 36 mV for anodic and cathodic polarization, respectively. While at 800°C, the corresponding values decreases to 54 mV and 80 mV. It is clear that SBCO electrode performs better under anodic polarization condition than that in cathodic polarization, indicating SBCO is more efficient for oxygen evolution reaction. Marina et. al have compared the reversible performance of La0.8Sr0.2FeO3-δ, La0.6Sr0.4Co0.2Fe0.8O3-δ and La0.6Sr0.4Cu0.1Fe0.9O3-δ electrodes, which showed worse performance in SOEC mode than in SOFC mode (3). This can be explained by the fact that oxygen reduction reactions can increase the oxygen vacancy concentration, whereas, an opposite trend occurs during oxygen evaluation reaction. The behaviour of SBCO oxygen electrode is not fully understood and need further studies. However, there are differences between SBCO and LSCF in terms of the oxygen deficiency and oxygen transport properties. Very high concentration of disordered oxygen vacancy is confirmed in these materials. According to Kim et. al, the oxygen nonstoichiometry, δ of SBCO at room temperature is ~0.7 in air, which decreases to about 0.3 at 900°C (15). Frontera et. al also found that the that δ can be tailored from δ ≈0.2 to δ ≈ 0.9 in PrBaCo2O5+δ cobaltite (16). As a comparison, the δ

value in simple perovskite-type La0.6Sr0.4Co0.2Fe0.8O3-δ is as low as 0.05 at 900°C (17). For cobalt-free compositions of La0.8Sr0.2FeO3-δ and La0.6Sr0.4Cu0.1Fe0.9O3-δ, it can be easily estimated that the oxygen vacancy concentrations are even lower. Moreover, in ordered REBaCo2O5+δ oxides, oxygen vacancies are mainly localized into [REOδ] layers, which provide disorder-free channels for ionic motion and greatly increases the oxygen mobility. In fact, the oxygen transport and surface exchange coefficients of double perovskite oxides are remarkably higher than of La0.5Sr0.5CoO3-δ (13, 18). Backing to the oxygen evolution reaction, although it can consume the oxygen vacancy, high concentration of deficiency still exists in SBCO which is still abundant for anodic reaction with no negative effect.





2.8 3.0 3.2 3.4 3.6 3.8 4.0-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Anodic Polarization, 750 oC 20mV 10mV 5mV OCV

-Z''

/ Ω

Z' /Ω

(a)

2.8 3.0 3.2 3.4 3.6 3.8 4.0-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

-Z''

/ Ω

Z' /Ω

Cathodic Polarization, 750 oC -20mV -10mV -5mV OCV

(b)

Figure 3. Impedance spectra of SBCO electrode under different biases, (a) anodic polarization and (b) cathodic polarization. Figure 3 a-b show the impedance spectra of SBCO electrode that collected at 750°C and under various anodic and cathodic biases. The specific polarization resistance obtained at OCV is 0.245 Ω⋅cm2. Under anodic polarization, the polarization resistance decreases with the increasing of overpotential and the Rp value at 20 mV is reduced to 0.18 Ω⋅cm2. On the contrast, the Rp is increased when the electrode is cathodically polarized. Similar results were reported in SrCo0.9Mo0.1O3-δ electrode (8). In SOFC mode, the oxygen reduction is affected by the oxygen diffusion and surface adsorption. While in SOEC condition, the oxygen evolution is mainly affected by the bulk oxygen diffusion, because electrons are easily lost from oxygen ions (8).





Short-Term Stability To study the stability of SBCO electrode, a short-term durability measurement was

carried out at 900°C for 20 h. Figure 4 depicts the polarization curve with an anodic current density of 200 mA⋅cm−2. A relatively stable performance can be observed. The initial overpotential is about 4mV, which finally reaches to 5mV. Figure 5 gives the typical impedance spectra of SBCO anode during the polarization treatment at 900°C. These curves are characterized by a large high-frequency inductance tail and two depressed semi-circulars for oxygen evolution reaction. The polarization resistance almost remains constant with a value of 0.025 Ω⋅cm2 after anodic current passage for 20h.

0.000

0.002

0.004

0.006

0.008

0.010

Ove

rpot

entia

l / V

SBCO 900oC Anodic current 200mA cm-2

0 2 4 6 8 10 12 14 16 18 20

Time / h

Figure 4. Polarization curves of SBCO anode measured at 200 mA⋅cm−2 for 20h.

2.12 2.16 2.20

-0.04

0.00

-Z''

(Ω)

Z' (Ω )

20h 16h 8h 0h

Figure 5. Impedance spectra of SBCO anode measured after different polarization time.





Conclusions In conclusion, double perovskite oxide SBCO was proposed and studied as a novel efficient oxygen electrode for SOEC. At 900 °C and 800°C, specific polarization resistances are 0.024 Ω⋅cm2 and 0.13 Ω⋅cm2, respectively. SBCO electrode exhibits better performance in anodic polarization condition than that in cathodic polarization, indicating SBCO is more efficient for oxygen evolution. A relative stable polarization is obtained at 900°C for 20 h with an anodic current density of 200 mA⋅cm−2.

Acknowledgments

The work is supported by Curtin Research Fellow Program, Curtin University, Australian Research Council (LP110200281) and National Natural Science Foundation of China (20901020, U1134001). The authors acknowledge the facilities, scientific and technical assistance of the Curtin X-Ray Laboratory, which is partially funded by the University, State and Commonwealth Governments.

References

1. A. Hauch, S. D. Ebbesen, S. H. Jensen and M. Mogensen, J. Mater.Chem., 18, 2331 (2008).

2. M. Ni, M. K. H. Leung and D. Y. C. Leung, Int. J. Hydrog. Energy, 33, 2337 (2008).

3. O. A. Marina, L. R. Pederson, M. C. Williams, G. W. Coffey, K. D. Meinhardt, C. D. Nguyen and E. C. Thomsen, J. Electrochem. Soc., 154, B452 (2007).

4. R. Knibbe, M. L. Traulsen, A. Hauch, S. D. Ebbesen and M. Mogensen, J. Electrochem. Soc., 157, B1209 (2010).

5. F. Tietz, D. Sebold, A. Brisse and J. Schefold, J. Power Sources, 223, 129 (2013). 6. P. Kim-Lohsoontorn, D. J. L. Brett, N. Laosiripojana, Y. M. Kim and J. M. Bae,

Int. J. Hydrog. Energy, 35, 3958 (2010). 7. Z. B. Yang, C. Jin, C. H. Yang, M. F. Han and F. L. Chen, Int. J. Hydrog. Energy,

36, 11572 (2011). 8. A. Aguadero, D. Perez-Coll, J. A. Alonso, S. J. Skinner and J. Kilner, Chem.

Mater., 24, 2655 (2012). 9. K. Zhang, L. Ge, R. Ran, Z. P. Shao and S. M. Liu, Acta Mater., 56, 4876 (2008). 10. A. A. Taskin, A. N. Lavrov and Y. Ando, Applied Physics Letters, 86 (2005). 11. D. Parfitt, A. Chroneos, A. Tarancon and J. A. Kilner, J. Mater.Chem., 21, 2183

(2011). 12. S. J. Lee, D. S. Kim, P. Muralidharan, S. H. Jo and D. K. Kim, J. Power Sources,

196, 3095 (2011). 13. G. Kim, S. Wang, A. Jacobson, L. Reimus, P. Brodersen and C. Mims, J. Mater.

Chem., 17, 2500 (2007). 14. J. H. Kim, Y. Kim, P. A. Connor, J. T. Irvine, J. Bae and W. Zhou, J. Power

Sources, 194, 704 (2009). 15. J. H. Kim, L. Mogni, F. Prado, A. Caneiro, J. Alonso and A. Manthiram, J.

Electrochem. Soc., 156, B1376 (2009).





16. C. Frontera, A. Caneiro, A. Carrillo, J. Oró-Solé and J. García-Muñoz, Chem. Mater., 17, 5439 (2005).

17. J. Stevenson, T. Armstrong, R. Carneim, L. Pederson and W. Weber, J. Electrochem. Soc., 143, 2722 (1996).





smbaco2o5+ as high efficient oxygen electrode of solid oxide electrolysis cells

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