Direct internal reforming molten carbonate fuel cell with core–shell catalyst

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Direct internal reforming molten carbonate fuel cell with coreeshell catalyst Pengjie Wang a,b, Li Zhou a,*, Guanglong Li a,c, Huaxin Lin a, Zhigang Shao a,**, Xiongfu Zhang c, Baolian Yi a a

Fuel Cell System and Engineering Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning 116023, China b Graduate University of Chinese Academy of Sciences, Beijing 100049, China c School of Chemistry, Dalian University of Technology, Dalian, Liaoning 116012, China

article info

abstract

Article history:

A sort of coreeshell catalyst as a novel anti-alkali-poisoning concept was prepared, tested

Received 25 June 2011

and applied in the direct internal reforming molten carbonate fuel cell (DIR-MCFC). Results

Received in revised form

showed that the coreeshell catalyst possessed good alkali-poisoning resistance capacity,

21 September 2011

which was explained well by the micropore model of the catalyst. And the cell perfor-

Accepted 28 September 2011

mance could keep above 0.75V during 100 h test. When the steam carbon ratio was 2

Available online 23 November 2011

(S/C ¼ 2) and the current density was 150 mA cm2, the cell potential varied from 0.826 to 0.751 V and the voltage fluctuant phenomenon was explained specifically. Furthermore,

Keywords:

the short stack (three cells) was also assembled, and the maximum output power density

Molten carbon fuel cell

of the short stack was 338.4 mW cm2. The above results indicated that the coreeshell

Direct internal reforming

catalyst could be applied into the DIR-MCFC successfully.

Coreeshell catalyst

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Molten carbonate fuel cell (MCFC) is a kind of high temperature fuel cell with the operating temperature at 650  C. MCFC uses hydrogen as fuel, which can be obtained from the reforming gas of CH4 [1], C6H5OH [2] or landfill gas [3] etc. Recently, the reforming of steam and methane (SRM) are often combined with MCFC. There are three ways for the combination of MCFC and SRM technology: external reforming (ER), indirect internal reforming (IIR) and direct internal reforming (DIR). For DIR technology, the reforming catalyst is put in the anode chamber and the reforming reaction of steam and methane occurs in the anode chamber. As a result, the hydrogen and carbon monoxide from SRM reaction are

consumed directly by the electrochemical reaction. In addition, the heat produced from the electrochemical reaction is consumed by SRM reaction simultaneously. Therefore, the DIR-MCFC has an intrinsically high reforming efficiency. SRM is a widely practiced technology to produce hydrogen or synthesis gas [4] Intensive research efforts has been carried out on Ni-supported catalysts for steam reforming [4e7]. Roh et al. [4,5] have compared the catalytic activity of various Nibased catalysts, including Ni/Ce-ZrO2, Ni/ZrO2, Ni/MgAl2O4 and Ni/CeO2. Among of them, Ni/Ce-ZrO2 showed higher methane conversion w60% for CH4:H2O ¼ 1:3 at 750 CLaosiripojana et al. [6] compared the reactivity of methane stream reforming over Ni/Ce-ZrO2, Ni/CeO2 and Ni/Al2O3. At the condition of CH4:H2O:H2 ¼ 1:3:0.2 and 900  C,

* Corresponding author. Tel.: þ86 411 84379123; fax: þ86 411 84379185. ** Corresponding author. Tel.: þ86 411 84379153; fax: þ86 411 84379185. E-mail addresses: [email protected], [email protected] (L. Zhou), [email protected] (Z. Shao). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.09.151

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Ni/Al2O3 showed the highest initial methane conversion of 72%. Although Ni/Ce-ZrO2 presented better resistance toward carbon formation and more stable catalytic activity than Ni/ Al2O3, the former proved difficult to commercialize owing to the high cost [7]. So Ni/Al2O3 was commonly used due to low cost and easily available. However, the reforming catalyst of DIR-MCFC is more prone to be poisoned by alkali metal electrolytes [8] in DIRMCFC. Attentions of the scholars on MCFC have been attracted to study the mechanism of the alkali-poisoning of the catalyst. Covering and sintering of Ni active sites, formation of Ni-containing solid solution, and blocking of pore structure are suggested as major deactivation processes [9,10]. Therefore, the alkali-resistance catalysts are developed for application on DIR-MCFC. Park et al. [11] prepared Ni/MgAl2O4 by co-precipitation method, which showed good alkaliresistance performance. However, they didn’t evaluate the catalyst in cell. Li et al. [12] not only prepared the Ni/MgSiO3 catalyst, but also evaluated the catalyst in cell. This kind of catalyst showed alkali-resistance performance, and the cell with this catalyst operated very well. Our co-worker Zhou et al. [13] prepared the coreeshell nickel catalysts by encapsulating Ni/SiO2 and Ni/Al2O3 within zeolite shells. These catalysts showed better (Li/K)2CO3resistance in the out-of-cell test for 60h. In this study, the coreeshell Ni-based catalyst (Sil-1/Ni/ Al2O3) was prepared. Specifically, the core Ni/Al2O3 was prepared by us, which were encapsulated within zeolite shells by our co-worker. The coreeshell Ni-based catalyst was applied into the unit cell and short stack (three cells) of DIRMCFC. The cell performance was evaluated, including the initial cell performance and the durability of the cell for 100h operation.

2.

Experimental

2.1. Preparation of coreeshell Ni-based catalyst (Sil-1/ Ni/Al2O3) Certain proportion of Ni(NO3)2$6H2O(AR, Shantou Xilong Chemical Ltd.), were dissolved in deionized water, then the gAl2O3 beads (10e20 mess, Dalian Haixin Chemical Ltd) was immersed in the solution, and the catalyst loading the certain proportion of Ni was dried at 373 K for 12 h, followed by air calcinations in the furnace at 923 K for 4 h. These prepared catalysts were coated with zeolite shell with our co-workers’ help according to the procedure reported [13]. The catalysts were examined by scanning electron microscopy (SEM, KYKY-2000B). After 100h test, the coreeshell catalyst was also analyzed by energy-dispersive X-ray spectroscopy (EDX, JSM6360-LV). Moreover, the exit gas were analyzed during the test of the coreeshell catalysts by Gas Chromatography (GC-7890T).

2.2.

Preparation of aeLiAlO2 power and matrices

Coarse aeLiAlO2 powder was prepared by the calcinations of Li2CO3 (A.P., Xinhua Chemical Reagent Factory, Beijing, China) with aeAl2O3 (A.P. Chengdu Chemical Reagent Factory, Si

Chuan Province, China) in an equal molecular ratio at 973 K. Before the calcinations, Li2CO3 and aeAl2O3 were mixed sufficiently by ball-mill. Fine aeAl2O3 powder was prepared by “Chloride” synthesis method [14]. The aeLiAlO2 powder was mixed with deionized water solvent, polyvinyl alcohol (PVA) binder and other additives to form a homogeneous slurry. Then the matrices were fabricated by tape casting [15].

2.3.

Assemblage and assessment of DIR-MCFC

The physical parameters of matrix and electrode were listed in Table 1. The unit cell and short stack (three cells) were assembled. For each unit cell, 4 g catalyst was pre-stored in the anode chamber and 7.5g electrolyte (0.62Li2CO3þ0.38K2CO3, mol%) was put in the cathode chamber before the components were assembled into the cell. In order to ensure all the components contacts well, the assembling pressure of the unit cell and the short stacks were kept at 2.361t and 4.473t, respectively. When the temperature reached 650  C, gas tightness was examined by N2. The mixture gas of O2þCO2 (O2/CO2 ¼ 40/60) as oxidant and that of H2þCO2 (H2/CO2 ¼ 80/20) as fuel gas were fed to the fuel cell and flowed though the cathode and anode chambers, respectively. After the coreeshell catalyst in the anode was reduced for 8 h, the fuel gas was replaced with CH4þH2O, and the performance of the unit cell and short stack were tested with SUN-FEL10A electronic load (Dalian sunrise power limited-liability company).

3.

Results and discussion

3.1.

Out-of-cell test

The coreeshell Ni-based catalyst (Sil-1/Ni/Al2O3) was evaluated in the simulated electrolyte surrounding as in the cell by our co-worker and the results were presented in Fig. 1. As shown, the fresh Ni/Al2O3 catalyst exhibited high methane conversion of around 80%, but after 5h alkali vapor treatment in the apparatus (see Fig. 2), the methane conversion decreased to less than 1%. In the contrast, the alkali vapor treatment coreeshell catalyst (poisoned Sil-1/Ni/Al2O3) kept a stable methane conversion of around 62%, which presented the shell played an important role in protecting the core from alkali-poisoning. The principle of alkali-resistance and the shell structure of the Sil-1/Ni/Al2O3 catalyst were shown in Fig. 3, and SEM images of Sil-1 seeds layer of the catalyst was shown in Fig. 4. As shown in Fig. 3, the pore diameter of the

Table 1 e The physical parameters of matrix and electrode in DIR-MCFC. Component

Material

Area (cm2)

Thickness (mm)

Porosity (%)

Anode Cathode Matrix

Ni-Cr Ni aeLiAlO2

25.5 25.5 50.24

0.475 0.48 0.90e0.98

66.78 64.78 50.6

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˚ ) source at 40 kV and 40 mA. with CuKa radiation (l ¼ 1.54059 A Unfortunately, for its XRD analysis, no crystalline peaks (2q ¼ 5 w8 , 20 w25 ) of ZSM-5 zeolite were observed [17,18]. The main reason is that the amount of “shell” is not enough to detect, which implied that the “shell” was quite thin. The stable methane conversion of the coreeshell catalyst implied that the catalyst was suited to be applied in DIR-MCFC and was provided with better capability of anti-poison to alkali vapor. As mentioned above Knudsen diffusion could be key step in the mass transfer processes in which Graham’s law [19] could be applied. The law states that the ratio of effusion rate of gas 1 to that of gas 2 is inversely proportional to the square root of their molecular weight, written as follows Rate 1=Rate 2 ¼

Fig. 1 e Contrast of methane conversion of catalysts’ SRM reaction with different treatment methods at 923 K. (,) non-posioned Ni/Al2O3 (B) posioned Ni/Al2O3 (6) posioned sil-1/Ni/Al2O3.

˚ , which could be smaller than Sil-1 seeds layer was 5.1e5.6A the molecular dynamic diameters of CH4, H2O, H2, CO and CO2 ˚ , respectively. in this system (3.82, 2.7e3.2, 2.89, 3.76 and 3.3A Calculated using Chem 3D Ultra version 9.0 [16]). The Knudsen diffusion could be considered as the key step of the mass transfer in the catalytic reaction processes. The gases of methane and water molecules passed through the pores by hitting the walls of the pores which pushed the molecules progressively, arrived at catalytic sites and reacted. The reaction products of hydrogen, carbon monoxide and carbon dioxide moved out of the pores by the Knudsen diffusion. However, the diffusion rate of the reactive gases were slowed due to the exist of the shell. And the SRM reactive rate also became a little slower. That is to say, the shell slowed the SRM reactive rate while protected the core from alkali-poisoning. So the methane conversion decreased from 80% (no shell catalysts) to 62% (coreeshell catalysts) in Fig. 1. As shown in Fig. 4, the thickness of the shell was 3.5 um, and coated on the surface of the Ni-based catalyst tightly. In order to confirm the structures of the shell, the coreeshell catalysts powder X-ray diffraction (XRD) patterns was recorded using a PANalytical X’Pert PRO X-ray power diffractometer

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi M2=M1

(1)

Rate 1 is the effusion rate of gas 1; Rate 2 is the effusion rate of gas 2. M1 is the molar weight of gas 1; M2 is the molar weight of gas 2. In the test, the multi-component gas mixture was simplified as binary system in order to explain the alkali-poisoning resistance mechanism. The gas 1 was mixture gas (0.37H2 þ 0.10CH4 þ 0.40H2O þ 0.086CO2 þ 0.040CO), gas 2 was the vapor of molten carbonate (0.62Li2CO3 þ 0.38K2CO3), according to the Graham’s law, the calculated result being Rate 1/Rate 2 z 3. This result indicated that the effusion rate of alkali vapor through the micropores in the catalyst was much slower than that of the mixture gas, and the most alkali vapor was separated out of micropores in the catalyst. In addition, as known to all, the adsorption coefficient of silicate-1 zeolite to alkali vapor (gas 2) was bigger than the mixture gas (gas 1). If the adsorption coefficient was also considered, the Knudsen diffusion rate ratio of the two gases would become much bigger. Unfortunately, the adsorption coefficient of silicate-1 zeolite to alkali vapor was difficult to find. However, it could be also suggested that the coreeshell Ni-based catalyst showed good alkali-resistance performance.

3.2.

Unit cell test

In order to investigate the effect of electrolyte on the performance of DIR-MCFC, the single cell was assembled and its initial performance and durability of 100 h test were shown in Fig. 5 and Fig. 6 respectively.

Fig. 2 e Alkali vapor treatment apparatus of catalysts and coreeshell catalysts.

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Fig. 3 e The principle of alkali-resistance and the structure of coreeshell catalystethe micropore model of the catalyst.

As shown in Fig. 5, the open circuit potential of the cell rose from 1.138 to 1.167 V when the reactive gas pressure varied from 0.1 to 0.4 MPa. At 150 mA cm2, the cell voltage increased from 0.721 to 0.864 V, indicating that the increment of the reactive gas pressure promoted the cell performance. Li et al. [12] also evaluated the performance of DIR-MCFC unit cell. The output voltage kept about 0.77 V when the current density was 150 mA cm2. However, the operation pressure was not mentioned in the paper. We tested the performance of DIRMCFC unit cell at different operation pressures and found that the cell performance raised with the increment of operation pressure. This phenomenon could be explained as below. According to the Nernst Equation (2) of electrode reaction written as follow [20]

E ¼ E0 þ

1=2 RT pH2 pO2 RT pcCO þ ln a 2 ln pH2 O 2F 2F pCO2

(2)

E0dthe standard potential of fuel cell, V; Rdideal gas constant, its value equals to 8.314, J mol1 K1; Tdabsolute temperature, K; Fdfaraday constant, its value equals to 96500,

C mol1; pdabsolute pressure, Pa; superscript c represents cathode, a represents anode. According to the above Equation (2) of electrode reaction, the increment of reactive gas pressure was in favor of the cell potential. Further, the increment of reactive gas pressure was helpful for the mass transfer, which accordingly favored the processes of electrode reactions and increased the performance of DIR-MCFC. With the reactive gases at 0.2 MPa and steam to carbon ratio of 2:1(S/C ¼ 2), the 100 h durability test of DIR-MCFC was carried out at 150 mA cm2(see Fig. 6). As shown in Fig. 6, the cell voltage varied in the range between 0.751 and 0.826 V. The cell voltage experienced three stages: decreasing, increasing and stable stage. Specifically, the cell voltage varied from 0.782 to 0.750 V at the decreasing stage (0e4 h), from 0.750 to 0.826 V at the increasing stage(4e17 h), from 0.826 to 0.751 V quite slowly at the stable stage(17e100 h). This phenomenon was due to two primary factors: the change of the matrix structure and the slight alkali-poisoning of the catalyst. Specifically speaking, in the first stage, the hydrated water in the matrix was prone to lose at about 650  C by Equation (3) as fellows [21]

Fig. 4 e SEM images of Sil-1 seeds layer of the coreeshell Ni-based catalyst. (a) SEM image of Sil-1 seeds layer coated with the catalyst. (b) SEM image of the thickness of the Sil-1 seeds layer coated with the catalyst.

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1.2

180

1.2

160 1.1 140

1.1

Initially After 100h

120

0.9

0.8

100 80 60

1.0

U(V)

0.1MPa 0.2MPa 0.3MPa 0.4MPa

P(mW cm-2)

U(V)

1.0

0.9

0.8

40 0.7 20 0.6

0

20

40

60

80

0.7

0 100 120 140 160 180 200 220 240 260

0.6

j(mA cm-2)

0

Fig. 5 e The performance of the single cell with different pressures anode gas: CH4:H2O [ 100:200; cathode gas: O2:CO2 [ 80:120.

LiAlO2 $nH2 O/LiAlO2 þ nH2 O

(3)

0.9 0.8

U(V)

0.7 0.6 0.5 0.4 0.3 0.2 0.1 20

30

40

50

60

70

80

90

100

t(h) Fig. 6 e The durability of 100 h test of the single cell. P: 0.2 MPa; anode gas: CH4:H2O [ 100:200; cathode gas: O2:CO2 [ 80:120.

200

Fig. 7 e The EDX analysis of the coreeshell catalyst after 100 h test.

1.0

10

150

j(mA cm )

1.1

0

100

-2

Then the lost water reacted with Li2CO3 to produce Li2O whose melt point is higher than 1432  C [22], which blocked the pores in the matrix. Accordingly, the matrix conductivity decreased owing to the change of the electrolyte composition and some blocked pores in the matrix. Thus, the cell voltage dropped at the first stage. Simultaneously, the migrated amounts of the electrolyte from cathode to anode decreased due to the blocked pore, and also the amounts of alkali that the coreeshell catalyst adsorbed reduced. As a result of which, the cell showed stable performance. Along with the reaction, the CO2 accumulated at the surface of cathode and matrix, Li2O reacted with CO2 and changed back into Li2CO3 gradually [22], and the blocked pores were released. On the other hand, the electrolyte was well-distributed on the effect of reactive gas pressure, and the cell voltage rebounded to 0.826 V.

0.0

50

As shown in Fig. 7, the cell voltage changed from 0.788 to 0.755 V at 150 mA cm2 after the 100 h operation. The 33 mV difference suggested that the coreeshell Ni-based catalyst exhibited good alkali-resistance ability. The role mechanism of coreeshell Ni-based catalyst was demonstrated by the micropore model of the catalyst as shown in Fig. 3. According to Graham’s law, in the same manner managing the complicated system in DIR-MCFC as a simple one in which only two gases contained, gas1 was the mixture gas (H2 þ CH4 þ H2O þ CO2 þ CO), gas 2 was the vapor of molten carbonate (0.62Li2CO3 þ 0.38K2CO3). The calculated values of Rate 1/Rate 2 were shown in Table 2, as indicated in the Table, the calculated results being Rate 1/Rate 2 z 3 under the conditions of run on at 150 mA cm2 and open circuit of DIRMCFC. It illustrated obviously that the effusion rate of molten carbonate vapor was much slower than that of the mixture gas through the micropores in the catalyst through which the most vapor of molten carbonate couldn’t pass to arrive at the active site of the catalyst in which some zigzags enhanced the passing difficulty of molten carbonate vapor through the micropores in the catalyst because of its bigger molecules. The catalyst could suffer from slight poison with molten carbonate vapor, holding the comparatively higher and stable activity for the methane steaming reforming reaction. So the catalyst played an important role in alkali vapor resistance, which was also explained by micropore model of the catalyst as above. After 100 h test, the EDX analysis of the coreeshell catalyst in unit cell was preceded. As shown in Fig. 8, the amount of potassium atom was 0.47%, lithium atom was not found in the coreeshell catalyst. Finally, the voltage of the single cell began decreasing very slowly owing to the slight alkali-poison of the coreeshell catalyst and the slight loss of the electrolyte.

3.3.

Short stack test

A short stack (three cells) was assembled and the stack performance was shown in Fig. 9. The assembly of the short stack was different from that of the single cell owing to the

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Table 2 e The values of Rate 1/Rate 2 under the conditions of run on at 150 mA cmL2 and open circuit for DIR-MCFC. g1 (the mixture gases)

g2 (molten carbonate vapor)

0.48H2 þ 0.12CH4 þ 0.24H2O þ 0.074CO þ 0.086CO2 0.30H2 þ 0.17CH4 þ 0.43H2O þ 0.030CO þ 0.070CO2

M2/M1

0.62Li2CO3 þ 0.38K2CO3 0.62Li2CO3 þ 0.38K2CO3

Rate 1/Rate 2

Conditions

z3 z3

O.C.V 150 mA cm2

98/13 98/15

Fig. 8 e The performance comparison of the single cell initially and after 100 h. P: 0.2 MPa; anode gas: CH4:H2O [ 100:200; cathode gas: O2:CO2 [ 80:120.

increased number of matrix. The differences of the stacking pressures were shown in Fig. 10 during their first start-ups. As shown in Fig. 9, under the conditions that S/C was 2 and current density was 150 mA cm2, the output voltage varied from 2.079 to 2.256V when the reactive gas pressure rose from 0.1 to 0.3 MPa. Yoshiba et al. [23] assembled one 10 kW-class stack, which point out that the output voltage of the stacks was around w0.8 V at an operating current density of 150 mA/ cm2. Owing to the different fuel gases (H2) and higher operating pressure, the performance of the stacks is better than ours. The maximal output power density of our short stack was 338.4 mW cm2. The short stack performance also increased along with elevating the reactive gas pressure

3.4

350

3.2

300

3.0

250

6.0 5.5

200 150 100

2.4

6.5

50 2.2

20

40

60

80

100

120

140

160

4.5 4.0 3.5

0

3.0

-50

2.5

2.0 0

5.0

F(t)

U(V)

2.6

7.0

P(mW cm-2)

0.1MPa 0.2MPa 0.3MPa

2.8

owing to the same reasons as that in the unit cell. It was implied that the catalytic activity on the catalyst was as high as that in the unit cell. As shown in Fig. 10, at the beginning of the processes of burning matrices (the first start-ups), the stacking pressure of the single cell and the short stack were 47.0 and 89.0 kg cm2 respectively. The change of stacking pressure of the short stack along with the temperature was different from that of the single cell. For the short stack, the stacking pressure changed very slowly. However, the stacking pressure changed very quickly for the unit cell. The differences were resulted from the number of the matrix. During the first start-up of unit cell, the change of stacking pressure indicated above was affected by the shrink of

j(mA cm-2) Fig. 9 e The performance of the short stack with different pressures anode gas: CH4:H2O [ 300:600; cathode gas: O2:CO2 [ 200:300.

2.0

short stack single cell

0

100

200

300

400

500

600

700

Fig. 10 e The comparison of stacking pressures change of the single cell and the short stack along with temperature.

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the matrix brought about on burning matrix in the temperature range from room temperature to 400  C, and the thermal expansion of the steel components [24] in the temperature range from 400 to 650  C. As a result, the stacking pressure for the unit cell increased at the beginning.

3.4. The effect of reactive gas pressure on the performance of DIR-MCFC The SRM reaction and water-gas shift reaction equations were listed as follows [25] CH4 þ H2 O ¼ CO þ 3H2 CO þ H2 O ¼ CO2 þ H2

DH0298 ¼ 206kJ=mol DH0298 ¼ 41kJ=mol

(4) (5)

In the light of thermodynamic principle, the increment of reactive gas pressure went against the process of Equation (4). If carbon monoxide produced from Equation (4) became less, the process of Equation (5) would be affected. In one word, the increment of reactive gas pressure went against the SRM reaction. According to the Nernst Equation as above, the performances of the unit cell and the short stack were increased along with elevating reacting gas pressure, which indicated as above the increment of reactive gas pressure improved the processes of electrode reactions in the unit cell and the short stack, so their performances were increased in sequence. The SRM reaction was conjugated with the anodic reaction in the unit cell and the short stack worked on at higher current density especially, the increment of reactive gas pressure made for the processes of electrode reactions, significantly made for those of cathode reactions, at the same time, which could make for the processes of SRM reaction. Because a big amount of hydrogen was consumed by the unit cell and the short stack at higher current density, the SRM reactions could be promoted forward smoothly under higher reactive gas pressure, along with which, the performances of the unit cell and the short stack were getting higher. The results above also illustrated that the coreeshell Ni-based catalyst possessed good alkali-poisoning resistance.

4.

Conclusion

The sort of coreeshell catalyst as a novel anti-alkali-poisoning concept was proved feasible, which supplied a new way to solve the alkali-poisoning problem in DIR-MCFC. The coreeshell catalyst was applied to the unit cell and the short stack successfully. Under the conditions that pressure was 0.2 MPa and S/C was 2, the potential of the cell could keep above 0.75V at 150 mA cm2 during 100 h test, the initial maximum output power density was 165.25 mW cm2. The maximal output power density of the short stacks was 338.4 mW cm2.

Acknowledgments This work was financially supported by Chinese Ministry of Science (nos. 2007AA05Z137).

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