Stability of Haynes 242 as metallic interconnects of solid oxide fuel cells (SOFCs)

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Stability of Haynes 242 as metallic interconnects of solid oxide fuel cells (SOFCs) Y. Liu a,*, J. Zhu b a

Department of Chemical, Materials and Biomolecular Engineering, Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, USA b Department of Mechanical Engineering, Tennessee Technological University, Cookeville, TN 38505, USA

article info

abstract

Article history:

Haynes 242 is a unique Ni based alloy for interconnect application of solid oxide fuel cells

Received 27 March 2010

(SOFCs), due to its low CTE (coefficient of thermal expansion) value which matches other

Received in revised form

fuel cell components over its peer of other Ni based alloys, together with very excellent

6 May 2010

oxidation resistance, and electrical conductive nature of the thermally grown oxide scales.

Accepted 8 May 2010

A NieMo rich intermetallic phase was observed to precipitate in this alloy during the

Available online 11 June 2010

thermal exposure in the SOFC environments and cause stability issue. This paper examined the stability of Haynes 242 in terms of oxidation resistance, electrical property, and

Keywords:

oxidation kinetics as interconnects of solid oxide fuel cells. Results indicated that the

SOFC

precipitation helped significantly towards formation of NiO to reduce the Cr evaporation,

Interconnect

and influenced little on oxidation resistance and electrical property.

Oxidation

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

Haynes 242 Stability

1.

Introduction

With rising energy consumption, diminishing quantities of fossil fuels, and the more and more severe environment pollution, the demand for more efficient and environment-friendly devices is growing rapidly. The problem that has been presented to the scientific community is to develop technology to allow the power generation industries to use a variety of clean fuels with higher conversion efficiencies and low emissions than is currently possible. Solid oxide fuel cell (SOFC) has been extensively studied as a promising power generation system that can directly convert the chemical energy of various fuels into electricity without combustion and mechanical processes, and has pretty high efficiency, low emission level, and flexibility of fuel choices [1e5]. Many of the technical challenges in the development of SOFC are materials related [6e12].

Recent progress in development of the solid oxide fuel cells has led to the reduction of its operation temperature to the range of 500e800
C, which makes metallic alloys feasible for SOFC interconnects application [13e18]. Metallic interconnects are advantageous over traditional ceramic interconnects due to their low cost, high electronic and thermal conductivity, good manufacturability, and improved mechanical strength, etc. Current materials of metallic interconnect are Fe and Ni based alloys. While more attention has been given to the Fe-based alloys due to their appropriate CTE (coefficient of thermal expansion) value, Ni based alloys generally exhibit superior oxidation resistance with much thinner oxide scale and better conductivity but unacceptably high CTE value over their Fe based competitors[19e26]. In the family of Ni based alloys, Haynes 242 is a unique one that possesses low CTE value matching other fuel cell

* Corresponding author. Tel.: þ1 860 486 8960; fax: þ1 860 486 4745. E-mail addresses: [email protected], [email protected] (Y. Liu). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.05.053

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components, together with very excellent oxidation resistance, and the electrical conductive nature of the thermally grown oxide scales. As shown in Fig. 1, the CTE value of Haynes 242 is nearly the same with that of Fe based alloys such as Ebrite and Crofer 22 APU, and match very well with other typical cell components such as cathode LSM and anode Ni þ YSZ [27,28]. Moreover, the alloy Haynes 242 is low Crcontaining and the Cr evaporation can be alleviated significantly by formation of protective NiO outer layer which separates the inter Cr2O3 layer from direct contact with the SOFC cathode environment. In our previous work [29], Haynes 242 was systematically studied both in cathode and anode environments and this alloy exhibited high potential as SOFC interconnect with very excellent oxidation and conducting performances. However, stability concern arises for this alloy in that a NieMo rich intermetallic phase was observed to be precipitated out in the alloy during the thermal exposure to either oxidizing (cathode) or reducing (anode) atmosphere. To the best of our knowledge, the study on stability issue of the alloy Haynes 242 as interconnect of solid oxide fuel cells has not been reported so far. In this paper, the stability of Haynes 242 was examined in terms of its oxidation resistance, electrical property, and oxidation kinetics as interconnects of solid oxide fuel cells. The effect of the precipitation on the oxide scale morphology was investigated, and the potential of this alloy as SOFC interconnect was discussed with respect to its stability issue.

2.

Experimental

Alloys of Haynes 242 were purchased from Haynes International, Inc. and the nominal chemical composition is Ni-25.0Mo-8.0Cr2.5*Co-2.0*Fe-0.8*Mn-0.8*Si-0.5*Al-0.5*Cu-0.03*C-0.006*B (wt. %, *maximum). In the standard heat treatment, the alloy is annealed at a temperature between 1065 and 1095
C and then water quenched. The aging treatment is carried out at 650
C for 24 h. The as-received alloy is subjected to above conventional heat treatment by the vender. The microstructure of Haynes 242 after standard heat treatment consists of a fine dispersion of ordered, metastable, lenticular, approx. 20-nm-long Ni2(Mo,Cr) domains/precipitates with a body

16 Cathode LSM Anode Ni+YSZ Ebrite Crofer 22 APU Haynes 242

14

-6

CTE, 10 K

-1

12 10 8 6 4 2 0 0

200

400

600

800

1000

o

Temperature, C Fig. 1 e Comparison of CTE (coefficient of thermal expansion) values of the Haynes 242 alloy with Fe based alloys and other typical components of SOFCs.

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centered orthorhombic crystal structure in a face centered cubic g matrix [30]. Circular samples with a diameter of 25 mm and a thickness of 1 mm were cut from the alloy sheets by electric discharge machining (EDM). Each sample was drilled a hole with a diameter of 1 mm on the upper center and then polished to 800 grits using SiC sand paper, ultrasonically cleaned in acetone, and dried immediately before further treatment. In order to investigate the stability and its influence on the performances of this alloy as SOFC interconnect, the asprepared circular samples were divided into four groups, and encapsulated into quartz glass tubes for various annealing treatment. The quartz glass tubes were filled with Argon shield gas after vacuuming to prevent the oxidation of samples during the annealing process. The annealing treatment was carried out at a constant temperature of 800
C (simulating SOFC working temperature) in a box furnace for 0 h, 100 h, 500 h, and 1000 h, respectively, to give various extent of precipitation/growth of NieMo rich intermetallic phases. The quartz glass tubes with samples inside were removed from the furnace after annealing, and cooled down in air to room temperature. Then the glass tubes were broken and the samples were taken out of them for next steps of experiments. Long-term cyclic oxidation test was conducted in dry air for four groups of alloys to evaluate their oxidation performance and scale spallation resistance. Each cycle consisted of isothermal holding at 800
C in air for 100 h and followed by air quenching to room temperature, with a total of 10 cycles. The cumulative oxidation time for cyclic oxidation test was, therefore, 1000 h. The phase constitutes in the oxide scales formed on each group of alloys were identified with X-ray diffraction (XRD). The surface morphologies and cross sections of the oxidized samples were observed using scanning electron microscopy (SEM) with a JEOL 6335F field-emission instrument (Tokyo, Japan) and an energy-dispersive X-ray analysis (EDX). The oxide scale surface was covered by the Pt paste for protection purpose before mechanical polishing was conducted to make above cross section samples. To clearly reveal the NieMo rich precipitation, the cross section of the alloys was etched by the Kalling’s solution before SEM observation. Cross sections without etching were also observed to check the oxide scale and its vicinity area. The morphologies of the oxide scale formed on each group of alloys were studied, compared, and analyzed to investigate how the structure of substrate alloys influences the oxide scale structure and its formation. Electrical resistance of each group of samples after 1000 h cyclic oxidation was measured using the 2-probe 4-point method in air from 500
C to 800
C with a step size of 50
C. Fig. 2 shows the schematic of the experimental setup for electrical resistance measurement. The upper and lower oxide surfaces were covered with Pt paste and Pt meshes with four Pt leads for current supply and voltage drop measurement. The variation of voltage across the samples under different currents ranging from 1 to 10 mA was verified to obey the Ohm’s Law exactly, which indicated that the interfacial polarization was negligible within the applied current range. A constant current of 10 mA was used in all the measurements. A widely accepted parameter for scaling the electrical resistance of the oxide scales, area specific resistance (ASR), was reported here.

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Oxide Scale

Pt

V

Haynes 242

I

Pt

Pt Paste V

Fig. 2 e 2-probe 4-point method for electrical property measurement.

ASR reflected both the electrical conductivity and the thickness of the oxide scale. At each temperature, the ASR was calculated according to the following equation, ASR ¼ AV I , where V, I, and A are voltage, current, and Pt paste covered area, respectively.

3.

Results

As shown in Fig. 3, after 1000 h oxidation, precipitations were observed in all alloys with and without annealing treatment. The average size of the precipitates, measured by a contrast based image analysis software, increased slightly with increasing annealing time, and much larger than the finely dispersed nanometer scale Ni2(Mo,Cr) domains in as-received alloys before oxidation. It is noteworthy that nearly no precipitate was observed in the vicinity area under the oxide scale for all groups of alloys. Outward diffusion of related elements during high temperature oxidation changed the local composition and consequently changed precipitation behavior in these areas. This indicated that the growth of oxide scale might be influenced by the near-surface microstructure evolution/stability of the alloys. X-ray diffraction patterns, as shown in Fig. 4, identified the precipitates as Ni3(Mo,Cr). Ni2(Mo,Cr) was reported to be a metastable phase [30], and it transformed into Ni3(Mo,Cr) in the alloys during annealing and long-term oxidation via high temperature diffusion of related elements. EDX analysis also confirmed the composition of the precipitates. Figs. 4 and 5 are the X-ray diffraction pattern and morphology for the oxide scale formed on Haynes 242 with various annealing treatments, respectively. XRD results for

Relative Intensity, arb. units

I

500

400

Metal Substrate: a (Mn,Cr,Ni) 3 O4 : b a Ni 3 (Mo,Cr): c c e Cr2 O3 : d NiO: e c d cc cb d c de cd

300

c c c

d

200

d cde cd ccc b

cb

bd d

0 20

d c c c b dcde cd

30

40

a d e

c e

c

c

d c

Annealing 100 hrs

a bd d

da

ce

100 c

c

Annealing 500 hrs bd d

d d cde c

a d

c e

a

c c c

d

Annealing 1000 hrs a

c

c

No Annealing a

c bd d

50

c

60

a d e

e

70

c

80

c

90

2 Fig. 4 e XRD patterns of the Haynes 242 alloys after cyclic oxidation at 800
C for 1000 h with various annealing conditions.

alloys with various annealing conditions displayed the same phase constitutes: (1) Cr2O3, due to its highly negative energy of formation, Cr2O3 preferentially formed at the beginning of the oxidation stage and stayed at bottom as a diffusion barrier to prevent further direct attack to inner alloys; (2) NiO, Ni from the Ni based matrix formed a stable NiO phase in the SOFC cathode environment; (3) M3O4 spinel (M ¼ Mn, Cr, Ni), it was formed via diffusion of Mn, Cr, and Ni species through the Cr2O3 layer and mixed in Cr2O3. No spallation was observed for the oxide scales formed on these alloys. Though with same phase constitutes, the oxide scale morphologies were dramatically different for the alloys with various annealing conditions. As shown in Fig. 5, numerous small NiO particles (small and white color ones) were observed on the oxide surface of the alloy without annealing, and less small NiO particles were formed on the oxide surface of the alloy with 100 h annealing. For alloys annealed for 500 and 1000 h, nearly no NiO small particles can be observed on the oxide scales. Views with more dramatic difference were shown in Fig. 5e, f, respectively, on its high magnification. The above difference on oxide scale morphology is consistent with the relative

Fig. 3 e SEM images of the cross section of Haynes 242 after cyclic oxidation at 800
C for 1000 h with various annealing conditions: no annealing (a) and annealing 1000 h (b).

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Fig. 5 e SEM images of the oxide scale surface formed on Haynes 242 after cyclic oxidation at 800
C for 1000 h with various annealing conditions: no annealing (a); annealing 100 h (b); annealing 500 h (c); annealing 1000 h (d); no annealing on high magnification (e); annealing 1000 h on high magnification (f).

intensity of the strongest NiO peak measured in XRD pattern. The intensity of the peak around 43
decreases with increasing annealing time, changing from higher, comparable, to lower than its right neighbor peak from Ni3(Mo,Cr) precipitates. Above relative intensity change can be regarded

as primarily from the change of NiO content (as observed in Fig. 5) because no difference of the amount of Ni3(Mo,Cr) precipitates was observed in the vicinity area under the oxide scales formed on all groups of alloys. It is interesting to observe some rose-like large oxide product (big and white

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4.

Discussion

The Ni3(Mo,Cr) precipitates in the near-surface area under the oxide scale significantly influenced the oxide scale morphologies. As shown in Fig. 8, the rose-like oxide product was ubiquitous on the oxide scales for all alloys, and under the rose-like oxide product, circular and porous internal oxidation regions were observed. The rose-like oxide product is primarily NiO with small amount of Cr2O3 at the bottom, indicating that the circular and porous internal oxidation

0.30 No Annealing Annealing 100 hrs Annealing 500 hrs Annealing 1000 hrs

Mass Gain, mgcm

-2

0.25 0.20 0.15 0.10 0.05 0.00 0

200

400

600

800

1000

Oxidation Time, hrs Fig. 6 e Oxidation resistance of the Haynes 242 alloys with various annealing conditions.

-1

-3.0

2

Log(ASR/T), cm K

color ones), as shown in Fig. 5aed, on each group of alloys with and without annealing treatment. These rose-like oxides are mainly NiO with small amount of Cr2O3 confirmed with EDX analysis. They played a very important role for the stability of Haynes 242 as SOFC interconnect, and we will discuss it later. Fig. 6 displayed the oxidation resistance of Haynes 242 with various annealing conditions. Haynes 242 displayed very excellent oxidation resistance with extremely low mass gain (which is basically one order of magnitude less than its Fe based competitor Crofer 22 APU [3]) at various extent of precipitation before oxidation test. The mass gain did not vary much with various annealing conditions. The alloys annealed longer with more extent of precipitation exhibited slightly worse oxidation resistance. Fig. 7 displayed the area specific resistance (ASR) of Haynes 242 with various annealing conditions after oxidation for 1000 h. All groups of Haynes 242 alloys exhibited low area specific resistance, and the values are very close with each other. Alloys without annealing and with annealing 100 h before oxidation displayed almost same values. This indicated that the precipitation of Ni3(Mo,Cr) intermetallic phase does not influence much on the overall electrical conductivity of Haynes 242. The measured values for all alloys are almost linear with changing temperatures, indicating that the measured resistance is primarily from the oxide scales. The activation energy of the electrical conduction, calculated by the slope of the fitted line, is 0.75 eV for alloys without annealing. There is an obvious increasing of the activation energy of the electrical conduction with increasing annealing time. The activation energies of the electrical conduction for alloys annealed for 500 h and 1000 h were determined to be 0.79 eV and 0.83 eV, respectively.

-3.5 -4.0

No Annealing Annealing 100 hrs Annealing 500 hrs Annealing 1000 hrs

-4.5 -5.0 -5.5 0.9

1.0

1.1

1000/T, K

1.2

1.3

-1

Fig. 7 e Area specific resistance (ASR) and the fitting line of the Haynes 242 alloys after cyclic oxidation at 800
C for 1000 h with various annealing conditions.

regions under rose-like oxides contributed to the formation of NiO and small amount of Cr2O3. Further EDX analysis found that the Ni and Cr contents in the rose-like oxides are close to their contents in the Ni3(Mo,Cr) precipitates. During the annealing and oxidation process, Ni3(Mo,Cr) precipitated, grew, and oxidized. These rose-like oxides were formed by selective oxidation of these intermetallic Ni3(Mo,Cr) precipitates into NiO, MoO3, and Cr2O3. Cr2O3 is not much because Ni3(Mo,Cr) is NieMo riched. Due to highly negative energy of formation and its stability at pretty low oxygen pressure, small amount of Cr2O3 formed and stabilized at the bottom. Significant amount of NiO formed on the top of the Cr2O3. MoO3 is highly volatile at the temperature over 700
C [31,32], and it evaporated out of the oxide scale and leave the Ni3(Mo, Cr) precipitate regions as porous internal oxidation regions under rose-like oxides. Therefore, those rose-like oxide products are actually formed by being blowing out of NiO/Cr2O3 when MoO3 was evaporated out of the alloy and oxide scale. With the understanding that it is the selective oxidization of Ni3(Mo,Cr) precipitate that resulted in formation of NiO, it is not difficult to understand the difference of oxide scale morphology formed on Haynes 242 with various annealing conditions. For Haynes 242 without annealing before oxidation, nanometer scale Ni2(Mo,Cr) dispersed in the g matrix. During the thermal exposure to the air at 800
C, these Ni2(Mo,Cr) phases were metastable and transformed into Ni3(Mo,Cr). Ni3(Mo,Cr) precipitate tended to grow larger as the total energy could be reduced by reducing the surface area to reach a more stable state. Such growth of the precipitates could be realized kinetically by diffusion of related elements such as Ni, Mo and Cr from the surrounding g matrix. As Ni3(Mo,Cr) grew, Ni3(Mo,Cr) precipitates combined with each other and the number of Ni3(Mo,Cr) precipitates became less. As the Ni3(Mo,Cr) precipitates grew, the Ni3(Mo,Cr) intermetallic precipitates on or near the surface oxidized the same time. The oxidation of Ni3(Mo,Cr) resulted in the formation of numerous NiO small particles on the surface. As oxidation time went on, the Ni3(Mo,Cr) precipitates further grew to be large enough. Then large amount of MoO3 vapor evaporated out from these large Ni3(Mo,Cr) precipitates to form the roselike NiO regions as mentioned above. For Haynes 242 annealed

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Fig. 8 e Surfaces and cross sections of the oxide scale on Haynes 242 showing “bubbles” where MoO3 evaporated out: no annealing (a,b); annealing 1000 h (c,d).

for 1000 h, before oxidation Ni3(Mo,Cr) precipitation has been nearly completed during annealing process. They will not experience the gradual growth process. Therefore, nearly no NiO small particles were observed on the oxide scale. During the oxidation, NiO only formed on these Ni3(Mo,Cr) large precipitates. MoO3 evaporated out from these regions to leave underlying porous internal oxidation areas. Based on above analysis, Fig. 9 summarized the oxide scale formation process on Haynes 242 after conventional heat treatment (without annealing, as-received condition from the vender) and microstructure evolution of the substrate alloy. Before oxidation, the microstructure consisted of fine dispersion of Ni2(Mo,Cr) domains/precipitates in g matrix. During oxidation, Cr2O3 layer was preferentially established on the surface due to its highly negative energy of formation, metastable Ni2(Mo,Cr) was starting to transform into Ni3(Mo,Cr) and grew bigger. At the same time, Ni3(Mo,Cr) oxidized to give formation of numerous NiO particles on the Cr2O3 layer. Meanwhile, Mn, Cr, and Ni species diffused from the g matrix through the Cr2O3 layer. (Mn,Cr,Ni)3O4 spinel formed and mixed in Cr2O3 layer. Finally, large precipitates of Ni3(Mo,Cr) were formed and its oxidation gave rose-like oxide products (bubbles). Ni3(Mo,Cr) precipitations helped significantly for the formation of top NiO layer. With less precipitation rate and precipitate growth rate, the NiO particles are expected to have smaller size and the NiO layer is expected to be more continuous. A continuous NiO top layer is expected to be

helpful for reducing the Cr evaporation as various protective coatings on Cr2O3-forming alloys do [8,18]. It separates the Cr2O3 layer from direct contact with the SOFC working environments. If continuous with reasonable density, it might also act as a diffusion barrier to significantly slow the outward diffusion of the Cr species. Cr evaporation is reported to poison the SOFC by the mechanism of deposition/reduction onto the cathode part [33]. No report was found on whether volatile MoO3 will be reduced on the cathode of the SOFC, however, it is possible that the volatile Mo species from Haynes 242 may deposit onto other cell components such as cathode during the cooling process after the SOFC was shut down. And it might influence the performance stability of the solid oxide fuel cells in later runs, such as by blocking the cathode that ought to be permeable. Compared with the Cr evaporation, the evaporation of the volatile MoO3 is relatively more severe based on the observation of very porous regions where MoO3 evaporated out. It is reported that the vapor pressure of MoO3 can reach as high as 10.13 Pa at 661
C in air, and it increases very rapidly at high temperature [31]. The good thing is more MoO3 will not readily form after the oxide scales are well established because the Mo species are in the alloy substrate and the oxygen pressure there is not high enough for MoO3 formation. The volatility of MoO3 is expected to be milder below 700
C [32,34], and meanwhile the oxide scale on top of it could be helpful to alleviate its evaporation significantly. Thus Haynes 242 may be more suitable to be

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Fig. 9 e Schematics of microstructure evolution and oxide scale formation for Haynes 242 without annealing treatment (not on scale): small and big pictures in d showed the surface and cross section of oxide scale, respectively.

considered to use at lower temperature range such as 500e700
C as SOFC interconnect. At this temperature range, the oxidation resistance would be better, and the conductivity of the oxide scale is expected to still stay in a reasonable range according to our measurements in Fig. 7. Also Haynes 242 has better CTE match in this temperature range, as indicated in Fig. 1. Haynes 242 with various annealing conditions showed slight difference of performances, and this can be explained based on above discussion on the morphology difference of the oxide scales and microstructure evolution in the alloys. For alloys with annealing 500 h and 1000 h before oxidation, Ni3(Mo,Cr) precipitation is nearly completed and the large Ni3(Mo,Cr) precipitate on the surface is directly exposed to air and subjected to oxidation at the beginning. Ni3(Mo,Cr) should be not very oxidation resistant based on the observation of porous internal oxidation regions and much thicker oxide scale on top in Fig. 8. In contrast, for the alloy without annealing, Ni3(Mo,Cr) precipitation formed and grew gradually with Cr2O3 layer formation, the established Cr2O3 layer (though maybe not much) have protective effect to alleviate the oxidation of Ni3(Mo,Cr) precipitates. Consequently, Haynes 242 with annealing exhibited slightly worse oxidation resistance compared with the one without annealing. As for the conducting performance of the oxide scale, the alloy without annealing has a nearly continuous NiO layer on top of

it. NiO has better conductivity over Cr2O3 at SOFC working temperatures [35], and it helps to have lower activation energy of electrical conduction. With increasing annealing time, less continuous NiO formed on the oxide scale, and the activation energy of electrical conduction increases. Another factor that varied the activation energy of electrical conduction may be related to the stoichiometry of M3O4 (M ¼ Mn, Cr, Ni) spinel in the oxide scales. In the crystal structure of M3O4 (M ¼ Mn, Cr, Ni) spinel, the oxygen ion occupies the face centered cubic lattice, cations fill either octahedral or tetrahedral interstice. The electronic conduction of M3O4 (M ¼ Mn, Cr, Ni) spinel is via the electron hopping by exchanging electrons between Mn2þ and Mn3þ [36,37]. Filling more Mn ions into the interstices will improve the electrical conductivity of the M3O4 (M ¼ Mn, Cr, Ni) spinel. EDX analysis observed slightly more Mn species on the oxide scale formed on Haynes 242 without annealing, and the M3O4 (M ¼ Mn, Cr, Ni) spinel in the oxide scale may be more conductive compared with the one formed on the alloy with annealing treatment, and resulted in slightly lower activation energy of electrical conduction.

5.

Conclusion

Ni2(Mo,Cr) in the alloy Haynes 242 transformed into Ni3(Mo,Cr) precipitates and grew larger during the thermal exposure to

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the SOFC cathode environment. The precipitation of Ni3(Mo, Cr) intermetallic phase helped significantly towards formation of the NiO top layer, and influenced little on oxidation resistance and electrical property of Haynes 242. Though Ni3(Mo,Cr) precipitated out in Haynes 242, The alloy has good performance stability as SOFC interconnects in terms of keeping very excellent oxidation resistance and pretty low area specific resistance. Due to the potential harm of the volatile MoO3 and its high volatility over 700
C, It might be better to consider the alloy Haynes 242 to be used at lower temperature such as 500e700
C as interconnect of solid oxide fuel cells.

Acknowledgements Authors gratefully acknowledge supports of this work from the Institute of Materials Science, University of Connecticut. Special thanks are given to Oak Ridge National Lab (ORNL) and Tennessee Technological University for their technical support and facility access.

references

[1] Haile SM. Fuel cell materials and components. Acta Mater 2003;51:5981e6000. [2] Jiang TL, Chen MH. Thermal-stress analyses of an operating planar solid oxide fuel cell with the bonded compliant seal design. Int J Hydrogen Energy 2009;34:8223e34. [3] Yang ZG. Recent advances in metallic interconnects for solid oxide fuel cells. Int Mater Rev 2008;53:39e54. [4] Wu JW, Johnson C, Gemmen RS, Liu X. The performance of solid oxide fuel cells with MneCo electroplated interconnect as cathode current collector. J Power Sources 2009;189: 1106e13. [5] Otani M, Tsukui S, Yoshida K, Umezaki Y, Mukai T. Fabrication of Gd0.5Sr0.5CoO3 film for SOFC cathode by pulsed laser deposition. Solid State Ionics 2010;180:1667e71. [6] Gannon P, Gorokhovsky VI, Deibert M, Smith RJ, Kayani AN, White PT, et al. Enabling inexpensive metallic alloys as SOFC interconnects: an investigation into hybrid coating technologies to deposit nanocomposite functional coatings on ferritic stainless steel. Int J Hydrogen Energy 2007;32:3672e81. [7] Yang Z, Singh P, Stevenson JW, Xia G. Investigation of modified NieCreMn base alloys for SOFC interconnect applications. J Electrochem Soc 2006;153:1873e9. [8] Liu Y, Chen DY. Protective coatings for Cr2O3 forming interconnects of solid oxide fuel cells. Int J Hydrogen Energy 2009;34:9220e6. [9] Huczkowski P, Christiansen N, Shemet V, Niewolak L, PironAbellan J, Singheiser L, et al. Growth mechanisms and electrical conductivity of oxide scales on ferritic steels proposed as interconnect materials for SOFC’s. Fuel Cells 2006;2:93e9. [10] Akhtar N, Decent SP, Kendall K. Structural stability of silver under single-chamber solid oxide fuel cell conditions. Int J Hydrogen Energy 2009;34:7807e10. [11] Yang ZG, Xia GG, Li XS, Stevenson JW. (Mn, Co)3O4 spinel coatings on ferritic stainless steels for SOFC interconnect applications. Int J Hydrogen Energy 2007;32:3648e54.

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[12] Tucker MC, Lau GY, Jacobson CP, Visco SJ, De Jonghe LC. Cu-YSZ cermet solid oxide fuel cell anode prepared by hightemperature sintering. J Power Sources 2010;195:3119e23. [13] Chen X, Hou PY, Jacobson CP. Protective coating on stainless steel interconnect for SOFCs: oxidation kinetics and electrical properties. Solid State Ionics 2005;176:425e33. [14] Wu JW, Johnson CD, Jiang Y, Gemmen RS, Liu X. Pulse plating of MneCo alloys for SOFC interconnect applications. Electrochimica Acta 2008;54:793e800. [15] Chu CL, Lee J, Lee TH, Chen YN. Oxidation behavior of metallic interconnect coated with LaeSreMn film by screen painting and plasma sputtering. Int J Hydrogen Energy 2009; 34:422e34. [16] Froitzheim J, Meier GH, Niewolak L, Ennis PJ, Hattendorf H, Singheiser L, et al. Development of high strength ferritic steel for interconnect application in SOFCs. J Power Sources 2008; 178:163e73. [17] Wu JW, Jiang YL, Johnson CD. DC electrodeposition of MneCo alloys on stainless steels for SOFC interconnect application. J Power Sources 2008;177:376e85. [18] Huang WH, Gopalan S, Pal UB, Basu SN. Evaluation of electrophoretically deposited CuMn1.8O4 spinel coatings on Crofer 22 APU for solid oxide fuel cell interconnects. J Electrochem Soc 2008;155:B1161e7. [19] Church BC, Sanders TH, Speyer RF, Cochran JK. Thermal expansion matching and oxidation resistance of FeeNieCr interconnect alloys. Mater Sci Eng A 2007;452:334e40. [20] Lins VFC, Castro MMR, Domingues RZ, Matencio T. High temperature cyclic oxidation resistance of iron chromium base alloys. Chem Eng Technol 2010;33:334e40. [21] Hua B, Pu J, Lu FS, Zhang JF, Chi B, Jian L. Development of a FeeCr alloy for interconnect application in intermediate temperature solid oxide fuel cells. J Power Sources 2010;195: 2782e8. [22] Jablonski PD, Cowen CJ, Sears JS. Exploration of alloy 441 chemistry for solid oxide fuel cell interconnect application. J Power Sources 2010;195:813e20. [23] Hua B, Lu FS, Zhang JF, Kong YH, Pu J, Chi B, et al. Property of a Ni-based alloy in SOFC anode environment. J Electrochem Soc 2009;156:B1261e6. [24] Chen XB, Hua B, Pu J, Li J, Zhang L, Jiang SP. Interaction between (La, Sr)MnO3 cathode and NieMoeCr metallic interconnect with suppressed chromium vaporization for solid oxide fuel cells. Int J Hydrogen Energy 2009;34:5737e48. [25] Balland A, Gannon P, Deibert M, Chevalier S, Caboche G, Fontana S. Investigation of La2O3 and/or (Co, Mn)3O4 deposits on Crofer22APU for the SOFC interconnect application. Surf Coating Tech 2009;203:3291e6. [26] Fontana S, Chevalier S, Caboche G. Metallic interconnects for solid oxide fuel cell: effect of water vapour on oxidation resistance of differently coated alloys. J Power Sources 2009; 193:136e45. [27] Qudakkers WJ, Greiner H, Kock W, Bossel V. In: Proceedings of the first Eur Solid Oxide Fuel Cell Forum. 1994;1:p. 525, [Lucerne, Switzerland]. [28] Comparison of thermal expansion characteristics, http://www. haynesintl.com/Haynes242Alloy/242HaynesAlloyCTEC.htm. [29] Liu Y. Performance evaluation of several commercial alloys in a reducing environment. J Power Sources 2008;179:286e91. [30] Dymek S, Wrobel M, Dollar M, Blicharski M. Influence of plastic deformation and prolonged aging time on microstructure of a Haynes 242 alloy. J Microsc 2006;224:24e6. [31] Vorontsov V, An W, Luo JL, Sanger AR, Chuang KT. Performance and stability of composite nickel and molybdenum sulfide-based anodes for SOFC utilizing H2S. J Power Sources 2008;179:9e16.

7944

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[32] Han Q. Analysis on molybdenum and molybdenum alloys of oxidization protection and application at high temperature. China Molybdenum Ind 2002;26:32e6. [33] Hilpert K, Dos D, Miller M, Peck DH, Weiss R. Chromium vapor species over solid oxide fuel cell interconnect materials and their potential for degradation processes. J Electrochem Soc 1996;143:3642e7. [34] Liu Y, Wang HM. Elevated temperature wear behaviors of a CoeMoeSi ternary metal silicide alloy. Scripta Mater 2005; 52:1235e40.

[35] Choi JS, Yo CH. A study on the conductivity of polycrystalline semiconductor nickel oxide. J Korean Chem Soc 1968;12:39e43. [36] Sergiol L, Umberto R, Ntonio DG. Crystal chemistry and cation distribution in some Mn-rich natural and synthetic spinels. Eur J Mineral 1997;9:31e42. [37] Natsuko S, Teruhisa H, Yue PX, Katsuhiko Y, Haruo K, Manuel EB, et al. Structure and transport property of manganeseechromiumeiron oxide as a main compound in oxide scales of alloy interconnects for SOFCs. Solid State Ionics 2005;176:681e6.