Effect Of H2S On Ni-YSZ SOFC Anodes

ECS Transactions, 58 (3) 21-36 (2013) 10.1149/05803.0021ecst ©The Electrochemical Society

Effect Of H2S On Ni-YSZ SOFC Anodes: A Combined In Situ Raman Spectroscopy - Impedance Spectroscopy Study

H. H. Mai Thi, N. Sergent, T. Pagnier LEPMI, CNRS – Grenoble-INP – Univ. de Savoie – UJF, BP75, F-38402 Saint Martin d’Hères France

A combined Raman spectroscopy-impedance spectroscopy study of a Ni-YSZ anode of a SOFC cell has ben performed at 500°C in open circuit potential and at 500 mV polarization. The effects of adding 200 ppm H2S to the Ar-3 %H2-3% H2O feeding gas have been observed. In OCP condition, the Raman spectrum of Ni3S2 is observed only 5.5 hours after the introduction of H2S, while the electrode impedance was reduced. The impedance grew only when the Raman signal of Ni3S2 saturated. At 500 mV polarization, Raman spectra of Ni3S2 were also observed together with a huge increase of the electrode impedance. For temperatures higher than 500°C, Ni3S2 shows no Raman spectrum, but high temperature optical spectroscopy shows that the growth of the Ni3S2 crystals can be observed at 715°C. Introduction Solid oxide fuel cell (SOFC) is a promising system to produce electricity since it can convert directly (or through external reforming) gasified biomass, coal gas, hydrocarbons due to its high working temperature (500-800°C for intermediate temperature SOFC) (1). However, it must be taken into account the presence of sulfide impurities in these fuels. Hydrogen sulfide is well known to have a detrimental impact on many SOFC components like the anode (2-5) as well as current collectors, heat insulators, and metallic separators. The poisoning mechanism of hydrogen sulfide has been studied for long in the catalysis field using conventional surface analysis techniques such as low energy electron diffraction (LEED), electron energy loss spectroscopy (EELS) (6). H2S adsorbs strongly and dissociatively on the nickel metal surfaces. At 450°C a surface coverage higher than 90% occurs at values of PH2S/PH2 of 0.1-1 ppm (7). The poisoning effect of H2S is in most cases determined through the changes in the cell performances, most often cell voltage at constant current (3, 8-10). The reaction products are generally characterized ex situ and post mortem, however Cheng and Liu (4) have characterized Ni-YSZ composite materials in situ by Raman spectroscopy. To our knowledge, there is no published result about a combination of electrochemical measurements and in situ structural characterization of the effect of H2S on SOFC anodes. The poisoning effect of H2S is proposed to be linked to sulphur adsorption, which blocks the H2 oxidation reaction, and/or to the formation of nickel sulfides at the surface

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ECS Transactions, 58 (3) 21-36 (2013)

of nickel grains. In situ Raman spectroscopy has proved to be able to characterize electrode reactions in SOFCs (11-13) and H2S reaction products (4, 14-15), as well as adsorbed species (16-18). Our objective was thus to combine electrochemical measurements with in situ Raman spectroscopy to obtain more knowledge about the poisoning mechanism of SOFC anodes. Experimental Commercial nickel-sulfur compounds (NiS Alfa Aesar, Ni3S2 Aldrich, NiS2 ABCR) were used to obtain reference Raman spectra. The powders were pressed into pellets before use. NiO-CGO powder was kindly provided by CEA. The powder was mixed for 20 hours in ethanol with zirconia balls in order to break agglomerates. The mixture was then put in an ultrasonic bath at 65°C to remove ethanol. Final drying was performed in an oven at 75°C. The resulting powder was then pressed uniaxially, then isostatically at 19.5 MPa. Sintering was performed at 715°C in air for 1 hour. NiO reduction was performed in Ar-3% H2 at 715°C for 3 hours. Half cells consisting of a 3% yttria-tetragonally stabilized zirconia electrolyte (thickness 150 or 300 µm, diameter 19 mm) and a NiO-cubic zirconia working electrode (WE) (thickness ~20µm, diameter 5 mm) were purchased from Kerafol. For electrochemical measurements, a counter (CE) and a reference (RE) electrode were made with Pt paste. Current collectors were made with Pt grids embedded in CE and WE and solely pressed on top of the WE (Figure 1).

Figure 1. Schematic view of the electrochemical cell. Raman spectra and electrochemical properties were measured in a lab-made cell allowing heating in a controlled atmosphere, 3-electrode measurements and Raman spectra collection (Figure 2). Raman spectra were obtained using a Renishaw InVia spectrometer in back-scattering micro-Raman configuration. The exciting source was the green (514.53 nm) line of an Ar laser delivering 11 mW on a 20 µm x 2 µm region of the sample surface. Laser light rejection was done with dielectric filters and the photon collector was a Peltier-cooled CCD. The laser light was focused on the sample surface with a x50 objective having a long working distance (8 mm). A line focus mode was selected, allowing the illumination of a 20 µm x 2 µm region.

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Figure 2. Schematic view of the measurement cell. Electrochemical measurements were performed with an Autolab PGstat30 potentiostat-impedancemeter. Results

Raman spectroscopy of nickel sulfide compounds at different temperatures

Raman intensity / a.u

NiS2 Ar

100

200

300

Raman shift / cm

400

500

-1

Figure 3. Raman spectra recorded at different positions on NiS2 pellet surface at room temperature. Spectra correspond to Ni3S2 (top), and a mixture of Ni3S2, NiS (middle, bottom).

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Figure 3 shows spectra from three different positions on NiS2 pellet surface at room temperature. Different Raman spectra can be observed, showing that the commercial product is not pure and consists in a mixture of several nickel sulfide compounds. This point was already stated by Wang et al (19). The same results were obtained with the two others commercial powders. The attribution of spectra was made using the data of (19). NiS2 pellet was then heated in Ar atmosphere. Raman spectra were recorded after 15 minutes stabilization at each temperature, at randomly chosen positions, and are shown in Figure 4. Since the powder is not pure, the randomly recorded spectra are very different; however the spectrum of Ni3S2 still predominates. It should be noted that no Raman band can be observed from 500°C and above, but a spectrum is again observed when decreasing temperature. This effect has also been observed by Cheng et al. (20) who attribute the disappearance of the Raman spectrum to a phase transition towards a Fm3m structure at ~560°C. According to numerous Ni-S phase diagrams (21-23), the high temperature Ni3S2 phase is not a define compound, but a solid solution which starts to appear near 515-520°C. Moreover, it is known that phase transition temperatures depend on the crystallite size. Finally, the transition is of second order, which means that the Raman spectrum can disappear experimentally before the true transition temperature. All these reasons can explain variations in the temperature at which spectra disappear, depending on different experimental conditions. NiS2 Ar

RT


100°C

300°C 500°C

300°C

100°C

RT 100

300 200 -1 Raman shift / cm

400

500

Figure 4. Raman spectra of NiS2 pellet recorded at various temperatures obtained during the heating and cooling processes (Raman intensity and signal/noise ratio depend on the microstructure and local composition and are thus not comparable among spectra).

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Formation of nickel sulfide crystals at 715°C observed by in situ optical microscope

Since Raman spectra of nickel sulfide cannot be recorded at high temperature (above 500°C), in situ optical imagery was employed to monitor the interaction between H2S and Ni at 715°C. Ni was used in the form of a mixture with gadolinium-doped cerium oxide pellet, which is known as Ni-CGO cermet. This cermet is a widely used material for SOFC anode. Figure 5 shows the optical images of Ni-CGO pellet at 715°C in a flow of 500 ppm H2S/ 3%H2/ Ar. After 5 hours in H2S, bright dots appear which gradually grow. No Raman spectra of these crystals can be obtained at 715°C. The whole system was then carefully cleaned by high flow rate of Ar during 1 hour before cooling fast in Ar (70°C/min) to room temperature. By this way, the risk for further attack of H2S to Ni during the cooling process is limited (4). No change in the appearance of the pellet surface can be seen by optical imagery during the cooling down to 50°C. As soon as the pellet sat at 50°C, Raman spectra of different bright crystals were taken, and are displayed in Figure 6. The Raman frequencies correspond well with Ni3S2. The intensity ratios between bands of crystal A are different from those of B, which reflects different crystallographic orientations.

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Figure 5. In situ optical images of the Ni-CGO surface at 715°C at different exposure times to 500 ppm H2S in 3%H2 in Ar. The white spots have been characterized at 50°C as being Ni3S2.

26



Figure 6. Raman spectra of two different crystals A, B on Ni-CGO surface. They are characteristic of Ni3S2.

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In situ Raman spectroscopy combined with electrochemical impedance measurements at OCP

As the electrochemical data depend on the electrochemical history of the sample (24, 25), we used a systematic procedure to collect them. The procedure is described in Table 1. It consists mainly in a polarization at increasing potentials prior to any measurement.

TABLE I. Experimental procedure for impedance measurements. Purpose

Standby condition

Applied voltage to measure

1.

Pretreatment at 715°C

715°C 3%H2/3%H2O/Ar OCP

715°C 3%H2/3%H2O/Ar Anodically polarized at 100, 300, 500 mV/ref (~3cycles)

2.

Pretreatment at 500°C

500°C 3%H2/3%H2O/Ar OCP

500°C 3%H2/3%H2O/Ar Anodically polarized at 100, 300, 500 mV/ref (~3cycles)

Operation w/o pollutant

500°C 3%H2/3%H2O/Ar OCP or 100 mV/ref polarization


4.

Operation in pollutant

500°C 3%H2/3%H2O/Ar + 200 ppm H 2S OCP or 100 mV/ref polarization

500°C 3%H2/3%H2O/Ar + 200 ppm H 2S OCP or 100 mV/ref polarization

5.

Operation after being poisoned



3.

The electrode impedance was measured every hour in the 105-10-2 Hz frequency range, with an AC voltage amplitude of 10 mV. For the half-cell kept at OCP condition, in situ Raman spectra of the anode surface were taken every 30 minutes after the introduction of 200 ppm H2S and were displayed in Figure 7. The quantity of Ni3S2 was determined using the area of the characteristic band at ~185 cm-1 (Figure 7) and is shown in Figure 8. Three stages S1 (0-5.5 h), S2 (5.5-11.4 h), S3 (from 11.4 h on) can be observed. During S1, the 185 cm-1 band is not yet observed. It may correspond to the nucleation of the first crystals of nickel sulfide, in which its quantity is too small to be detected, or to an adsorption phase. S2 reflects a fast accumulation of Ni3S2 on the anode surface. S3 indicates the saturation of Ni3S2 formed at the surface.

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3%H2 - 3%H2O - Ar 200 ppm H2S 500°C - OCP

47.3 h


25.1 h

21.2 h

19.2 h

17.3 h

15.3 h 11.3 h

7.4 h

1.5 h

160

200

240

280

320

360

Raman shift / cm

400

440

480

-1

Figure 7. Raman spectra of anode surface versus exposure time to 200 ppm H2S at 500°C. The arrow indicates the band used for Ni3S2 quantification.

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Introduction of H2S 11.4 h

5.5 h

60x10

3

S3 S1

S2

50

3%H2 - 3%H2O - Ar 500°C _ OCP

30

20


40

3%H2 - 3%H2O - Ar 200 ppm H2S 500°C _ OCP

10

0

180

185

190

195

200

205

210

215

220

225

230

235

time / hours

Figure 8. Evolution of the integrated intensity of the 195 cm-1 band of Ni3S2 as a function of H2S-exposure time at 500°C.

The three stages of Ni3S2 quantity variation are reflected in impedance spectra (Figure 9). Above 10 kHz, the impedance is characteristic of the ohmic drop between the reference and the working electrodes. This part remains unchanged. The major changes happen at a frequency range lower than 1 Hz. Slight changes occur also at intermediate frequencies (10 kHz-5 Hz). During S1, the low frequency arc in consecutive spectra varies continuously from capacitive to distorted inductive loop. During S2, the inductive loop becomes a round semi-circle, which decreases in size slowly with time. The spectrum at t = 12.9 h which corresponds to the beginning of Ni3S2 saturation has the smallest inductive loop. During the saturation period S3, from t = 12.9 h to 17.1 h, the inductive low frequency loop transforms continuously to a capacitive arc. From t = 18.5 h, it manifests itself as a complete 360° loop followed by another LF capacitive semi-circle.

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40 Hz

200 ppmH2S - 3%H2 - 3%H2O in Ar 500°C - OCP

5 Hz

10 kHz

8000 S1

7000

6000 -z"' /Ω Ω

5 Hz

before H2S 0.7 h 1.8 h 3.2 h 4.6 h

5000

1 Hz

5 Hz

1 Hz

1 Hz

4000

5 Hz

1 Hz

3000 5 Hz

2000

1000

1 Hz 10 kHz 40 Hz

1

2

3

4

5

6 7 Z' /Ω Ω

8


5500

-z"' /Ω Ω

40 Hz

5 Hz

6h 7.4 h 11.4 h 12.9 h

4500

4000

11 12x103

10

10 kHz

S2

5000

9

5 Hz

1 Hz

3500 5 Hz

3000

1 Hz

2500

2000

1500

1000

5 Hz 1 Hz

10 kHz

500 1 Hz

0

1000

2000

3000

4000 5000 Z' /Ω Ω

6000


7000

8000

9000

40 Hz

S3

5000 -z"' /Ω Ω

10 kHz

14.3 h 15.7 h 17.1 h 18.5 h 19.9 h 21.4 h 24.2 h

6000

4000

5 Hz 5 Hz 1 Hz

5 Hz 1 Hz

3000 5 Hz 1 Hz

2000 10 kHz

5 Hz

1000

1 Hz

40 Hz

1 Hz

5 Hz

1 Hz

0

1

2

3

4

5

6

7 Z' /Ω Ω

8

9

10 11x103

Figure 9. Impedance spectra for half-cell at OCP condition as a function of H2Sexposure time at 500°C.

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In situ Raman spectroscopy combined with electrochemical impedance measurements at 500 mV polarization

For half-cell polarized at 500 mV, the shape of low frequency part changed fast without going through many intermediate forms as in the case of OCP (see Figure 10). At t = 21 h a much larger size of spectrum was obtained as compared to OCP conditions. The poisoning effect of H2S is more severe under polarization, with a fast enlargement of spectrum size Raman spectra indicated the presence of Ni3S2, but with a much weaker intensity than in OCP conditions. This was perhaps due to the ageing of the microscope objective, since post mortem scanning electron microscopy analyses indicated a huge amount of sulphur in the sample. 3%H2 - 3%H2O - Ar H2S 200 ppm 500°C_500mV

0.03 h 1.3 h 2.5 h 3.7 h 4.9 h 6.2 h 7.4 h

-z"' /Ω Ω

2000

1500

40 Hz

79 Hz 10 kHz

time

40 Hz

1000

500

1.6 Hz 79 Hz 40 Hz

0

1.6 Hz 1.6 Hz

1000

2000

3000

4000

Z' /Ω Ω

3%H2 - 3%H2O - Ar H2S 200 ppm 500°C_500mV

-z"' /Ω Ω

2000

1500

10 Hz

10 kHz

1000

7.4 h 9.8 h 16 h 21 h

500

0

10 Hz

2

4

6 Z' /Ω Ω

8

3

10x10

Figure 10. Nyquist plots of half-cell polarized at 500 mV in function with H2S exposure time.

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Discussion Up to now, the H2 oxidation models are still under discussion (26-28). Impedance spectra in OCP conditions and under polarization suggest that a second order impedance is responsible for the low frequency (< 1 Hz) characteristics. The first electrode semicircle, between 10 kHz and 1 Hz, could be due to the charge-transfer resistance in parallel with a double-layer capacitance. Second-order impedances are known to appear in reactions following the VolmerHeyrowsky mechanism (29). In this model, two successive electrochemical steps share the same adsorption sites (equations [1]-[3]).

A + s + e A

[1]

A + A + e A

[2]

A → A + s

[3]

The concentration impedance of species s or As is described by a second order impedance of concentration with the numerator is first order in jω, and the denominator is second order in jω. In the general case, one of the concentration impedances dominates over the other, which means that the concentration impedance has the form Z =

(τ ω) (τ ω)(τ ω)

[4]

where x can be s or As, and τi time constants. A detailed model including elementary physical and electrochemical processes of diffusion, chemical reactions, charge transfer reactions has been proposed by Vogler et al. (26). The most probable mechanism was found to be very close to a Volmer-Heyrowsky mechanism. Their proposed model can be rewritten as

 1 H 2 + sNi → H Ni  2   OOx → O′′YSZ + VO••  1 → OH′YSZ + e′Ni + sNi  H Ni + O′′YSZ k  k2  H Ni + OH′YSZ  → H 2 O YSZ + e′Ni + s Ni  3 H 2 O YSZ k → H 2 Ogas + sYSZ  

[5]

The two first reactions are supposed to be very fast. Moreover, there is almost no concentration gradient for the adsorbed species on the Ni surface due to the fast diffusivity on Ni (26). Thus, it can be assumed that the concentration of HNi at the triple boundary points (TPB) remains constant. Then the kinetic equations will depend on the concentration of sYSZ, OH , and H O . This turns out to Volmer-Heyrovsky

33



mechanism. The impedance of the whole process will include charge transfer resistance and impedance concentration. So, we propose the following equivalent circuit: R1(R2Q2)(R3Q3)Zconc.

[6]

where (RiQi) means a resistance in parallel with a constant phase element. R1(R2Q2)(R3Q3) represent the connectors resistance R1, the electrolyte impedance R2Q2 and the charge transfer impedance R3Q3, with a characteristic frequency f3. Zconc. is the concentration impedance of adsorbed species observed at low frequencies. Figure 11 shows the diagram of fitted parameters including the characteristic frequency f3 of the medium frequency semicircle, the corresponding resistance and the concentration resistance of the LF impedance Zconc. A continuous degradation during 190 hours in clean fuel can be observed, with an increase of resistance R3, and a decrease of the peak frequency. As H2 lowered from 3% to 1%, R3 increases slightly. As soon as H2S was introduced, R3 fluctuated with time with slightly higher values than without H2S. But the most spectacular change is the decrease of the concentration resistance as soon as H2S is introduced, and its relatively small increase later. This low frequency arc was also reported by Primdahl et al. (30) and Kek et al.(31) to have relation with adsorption/diffusion of reactants/products. When H2S was introduced to the gas flow the nature of the LF arc changes immediately from capacitive to inductive. It is clear that H2S immediately interferes with the absorption of others species on the anode surface. From that moment on, the concentration resistance continues to increase to reach a plateau at ~220 hours, 15 hours later than the beginning of the saturation of Ni3S2 on the surface obtained by Raman spectroscopy.

500°C - OCP

1%H2

3%H2

200 ppm H2S

remove H2S

Peak frequency f3 / Hz

3%H2 - 3%H2O in Ar 80

6500

70

6000

60

5500

6000

4000

5000

50

4500

40

R3 Rconc. f3

30

4000

0 3500

60

80

100

120

140 160 time / hours

180

200

220

240

260

280

Rconc / Ω

40

R3 / Ω

20 20

2000

Figure 11. Evolution of fitted parameters (equivalent circuit equation [6]) with time for non-polarized cell with and without 200 ppm H2S at 500°C. The correlation between concentration resistance and the quantity of Ni3S2 is given in Figure 12. During the first 5 hours exposed to H2S, the adsorption properties change, which are reflected in the negative value of concentration resistances. As Ni3S2 accumulates more in the surface, the concentration resistance increases.

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Introduction of H2S 5.5 h 11.4 h 3

60x10

6000

S1

S3

S2

4000

3%H2 - 3%H2O - Ar

40

3%H2 - 3%H2O - Ar 200 ppm H2S 500°C _ OCP

500°C _ OCP 2000

30

20


Rconc /Ω

50

10

0

0

160

170

180

190

200

210

220

230

time / hours

Figure 12. The correlation between the concentration resistance and the Raman intensity of the Ni3S2 band. Conclusion We have been able to measure simultaneously the Raman spectrum and the impedance of a Ni-YSZ anode in a SOFC cell, at 500°C. The effect of adding 200 ppm H2S to Ar-3 % H2-3% H2O has been followed. Surprisingly, the initial step is a decrease of the electrode impedance. This decrease corresponds to the absence of the Ni3S2 Raman spectrum. When this Raman spectrum appears and grows, the electrode impedance is roughly stable. The impedance grows only when the Ni3S2 Raman spectrum saturates. All these effects are located in the low frequency part (< ~1 Hz) of the impedance spectrum, attributed to a concentration impedance. Under polarization, the effect of H2S on the electrode impedance was much stronger, multiplied by 10 in about 20 hours. However, the measured Ni3S2 Raman band intensity was low. This could be due to a setup problem. The Raman spectrum disappears above 500°C. However, the appearance and growth of Ni3S2 at the surface of the anode has been followed by in situ optical microscopy, using the microscope of the Raman spectrometer. This suggests that the same kind of study can be done at temperatures higher than 500°C. Acknowledgments This work was funded in part by Agence Nationale de la Recherche (Pile eau Biogaz project) and by Pôle de compétitivité Tenerrdis. CEA is kindly acknowledged for the NiCGO powder preparation. References 1. A. J. Jacobson, Chem. Mater., 22, 660 (2010). 2. Y. Matsuzaki , and I. Yasuda, Solid State Ionics, 132, 261 (2000).

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3. K. Sasaki, K. Susuki, A. Iyoshi, M. Uchimura, N. Imamura, H. Kusaba, Y. Teraoka, H. Fuchino, K. Tsujimoto, Y. Uchida , and N. Jingo, J. Electrochem. Soc., 153, A2023 (2006). 4. Z. Cheng, and M. Liu, Solid State Ionics, 178, 925 (2007). 5. E. Brightman, D. G. Ivey, D. J. L. Brett, and N. P. Brandon, J. Power Sources, 196, 7182 (2011). 6. C. H. Bartholomew, P. K. Agrawal, and J. R. Katzer, Adv. Catal., 31, 135 (1982). 7. C. H. Bartholomew, Appl. Catal. A, 212, 17 (2001). 8. Y. Shiratori, T. Oshima, and K. Sasaki, Int. J. Hydrog. Ener., 33, 6316 (2008). 9. J. F. B. Rasmussen, and A. Hagen, Fuel Cells, 10, 1135 (2010). 10. J. W. Yun, S. P. Yoon, S. Park, H. S. Kim, and S. W. Nam, Int. J. Hydrog. Ener., 36, 787 (2011). 11. R. C. Maher, L. F. Cohen, P. Lohsoontorn, D. J. L. Brett, and N. P. Brandon, J. Phys. Chem. A, 112(7), 1497 (2008). 12. R. C. Maher, V. Duboviks, G. J Offer, M. Kishimoto, N. P. Brandon, and L. F. Cohen, Fuel cells, 13(4), 455 (2013). 13. M. B. Pomfret, J. C. Owrutsky, R. A. Walker, ECS Trans., 11, 99 (2008). 14. T. Pagnier, M. Boulova, A. Galerie, A. Gaskov, and G. Lucazeau, J. Solid State Chem., 143, 86 (1999). 15. T. Pagnier, M. Boulova, A. Galerie, A. Gaskov, and G. Lucazeau, Sens. Actuators, B, Chem , 71, 134 (2000). 16. N. Sergent, M. Epifani, and T. Pagnier, J. Raman Spectrosc., 37, 1272 (2006). 17. A. Filtschew, D. Stranz, and C. Hess, Phys. Chem. Chem. Phys., 15, 9066 (2013). 18. O. Sze Nga Sum , E. Djurado, T. Pagnier, N. Rosman , C. Roux, and E. Siebert, Solid State Ionics, 76, 2599 (2005). 19. J.-H. Wang, Z. Cheng, J. L. Bredas, and M. Liu, J. Chem. Phys., 127, 214705 (2007). 20. Z. Cheng, H. Abernathy, and M. Liu, J. Phys. Chem. C, 111, 17997 (2007). 21. H. Rau, J. Phys. Chem. Solids, 37, 929 (1976). 22. M. Singleton, P. Nash, and K. J. Lee, Phase Diagrams of Binary Nickel Alloys, P. Nash ed., ASM International Materials Park, Ohio, p 277 (1991). 23. P. Waldner and A. D. Pelton, Z. Metallkunde, 95, 672 (2004). 24. R. J. Aaberg, R. Tunold, M. Mogensen, R. W. Berg , and R. Ødegaård, J. Electrochem. Soc., 145, 2244 (1998). 25. K. Vels Jensen, S. Primdah, I. Chorkendorff, and M. Mogensen, Solid State Ionics, 144, 197 (2001). 26. M. Vogler, A. Bieberle-Hütte, L. Gauckler, J. R. Warnatz , and W. G. Bessler, J. Electrochem. Soc., 156, B663 (2009). 27. W. G. Bessler, J. Warnatz, and D. G. Goodwin, Solid State Ionics, 177, 3371 (2007). 28. V.Sonn, A. Leonide, and E. Ivers-Tiffée, J. Electrochem. Soc., 155(7), B675 (2008). 29. J. P. Diard, B. L. Gorrec, C. Montella, and C. Montero-Ocampo, J. Electroanal. Chem., 352, 1 (1993). 30. S. Primdahl, and M. Mogensen, J. Electrochem. Soc., 144, 3409 (1997). 31. D. Kek, M. Mogensen, and S. Pejovnik, J. Electrochem. Soc., 148, A878 (2001).

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