Effect of fabrication method on properties and performance of bimetallic Ni0.75Fe0.25 anode catalyst for solid oxide fuel cells

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Effect of fabrication method on properties and performance of bimetallic Ni0.75Fe0.25 anode catalyst for solid oxide fuel cells Yuzhou Wu a,b, Chao Su a, Wei Wang a, Huanting Wang b, Zongping Shao a,b,* a

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry & Chemical Engineering, Nanjing University of Technology, No.5 Xin Mofan Road, Nanjing 210009, PR China b Department of Chemical Engineering, Monash University, Clayton 3800, Australia

article info

abstract

Article history:

Ni/Fe alloy-based anodes have attracted much attention recently due to their potential for

Received 8 February 2012

improving anodic activity and suppressing carbon deposition when operating on carbon-

Received in revised form

containing fuels. However, some inconsistent results about the iron alloying effect were

5 March 2012

reported in literature. In the present work, we systematically studied the influence of

Accepted 9 March 2012

synthesis method on properties and cell performances of a Ni0.75Fe0.25 þ SDC (60:40 v/o)

Available online 7 April 2012

alloy-ceramic anode for solid oxide fuel cells. Three different methods, i.e. physical mixing

Keywords:

solegel route (C-GNP), were used. Samples were analysed by X-ray diffraction,

Solid oxide fuel cell

temperature-programmed reduction/oxidation, scanning electron microscopy and elec-

Alloying

trochemical impedance spectroscopy. It was revealed that the phase structure of anode

Carbon deposition

components, chemical interaction between nickel and iron, and the electrode micro-

Anode

structure were strongly dependent on the synthesis method. The coking resistance was

route (PMR), simultaneous glycine nitrate process/solegel route (S-GNP) and combined GNP

found to be more sensitive to anode phase structure and chemical binding between Ni and Fe phases, whereas the cell power output was mainly determined by the electrode microstructure. As a result, the iron content of the NiFe-based anode should be carefully controlled in different preparation methods to achieve high cell performances. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Due to the rising energy shortage and decreasing reserves of fossil fuels, there are considerable research efforts worldwide on developing solid oxide fuel cells (SOFCs) for practical applications because SOFCs possess several outstanding features such as high energy conversion efficiency, low emissions, low noises, and fuel flexibility [1e4]. Since SOFCs are mostly operated based on oxygen anion conducting electrolytes, a wide range of fuels such as hydrocarbons and coal

can be potentially applied to SOFCs directly, without the process of external reforming into H2 [5e7]. Electro-oxidation of hydrocarbon fuels can be either through direct way, or indirect way. For the direct electro-oxidation of hydrocarbon fuels, the intermediate of synthesis gas (a mixture of CO and H2), is formed through the internal reforming of fuels over the anode [8]. Currently significant research efforts are required to improve tolerability of the anode towards carbon deposition, lower operating temperature, and optimize electrode

* Corresponding author. Nanjing University of Technology, College of Chemistry & Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, No.5 Xin Mofan Road, Nanjing 210009, PR China. Tel.: þ86 25 8317 2256; fax: þ86 25 83172242. E-mail addresses: [email protected], [email protected] (Z. Shao). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.03.034

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microstructure for higher power output, before SOFCs can be widespread commercialized [9e11]. Although the cathode and electrolyte are in general the main sources of cell polarization, a large anodic polarization resistance may also be experienced at reduced temperature [11e14], even operating on hydrogen fuel. In addition, coke is easily formed over the conventional nickel cermet anode when hydrocarbons are applied as the fuels [15e17]. Therefore, there is a growing interest in developing alternative anode materials with high catalytic activity for fuel electro-oxidation, high coking resistance and low cost. R.J. Gorte et al. reported a new anode composed of Cu as an electronic conducting phase and CeO2 as an anode catalyst, for both H2 and hydrocarbon fuels [18]. A much improved anticarbon deposition capability was demonstrated for hydrocarbon fuels as compared to the conventional nickel cermet anode. However, the power outputs were much smaller than those generated from cells with nickel anode because of poor electrochemical activity of Cu and CeO2. Over the past several years, introducing other transition metals into nickel to form bimetallic or trimetallic alloys for anode have received much attention [19e25]. Among the various transition metals, iron has been mostly studied because of its low cost, abundance, and high effectiveness in improving the performance of nickel anode. Ishihara and co-workers reported NieFe bimetallic anode prepared by impregnation of iron nitrate to NiO for ITSOFC utilizing thin-film LaGaO3 electrolyte [26,27]. They observed a dramatic improvement on anodic activity when a small amount (5 wt.%) of Fe was added to Ni, and as low as 34 mV of the anodic over potential was achieved at 0.1 A cm2 at 600  C, which was around half that of pure nickel-based anode. Zhu et al. investigated the iron alloying effect on the performance of nickel-based anode in hydrogen fuel, and they found that single cell with YSZ þ Ni0.75Fe0.25 anode exhibited the highest performance when the anodes with different Ni/ Fe ratios were compared [28]. Similarly, Liu et al. studied YSZ þ Ni1xFex as potential anode material for SOFCs, and demonstrated the cells with YSZ þ Ni0.9Fe0.1 anode had the highest power output [29]. In addition to improving the electrocatalytic activity for fuel oxidation, a proper amount of iron added into nickelbased electrode may also enhance the coking resistance when operating on hydrocarbon fuels [30,31]. Lee et al. observed the enhanced stability of Ni-Fe þ GDC anodes operating on dry methane fuel [30]. A cell with conventional Ni þ GDC cermet anode failed after the operation on dry methane for 12 h at a current density of 0.2 A cm2 at 650  C, while a cell with Ni0.9Fe0.1 þ GDC anode showed no performance degradation for over 50 h of continuous operation under similar conditions. Wang et al. demonstrated that the introduction of iron into nickel-based anode can effectively suppress the coke formation of an SOFC operating on dimethyl ether fuel [31]. However, some inconsistent results were also reported in literature about the iron alloying effect. For example, Zhu et al. reported that the anodic over potential decreased by introducing 20e25% Fe into nickel anode [28,32], while Ishihara et al. just gave the opposite conclusion [26,27]. They attributed such discrepancy to the different fabrication processes. Fiuza et al. recently applied Fe-Ni/YSZ - GDC as electrocatalysts for ethanol steam reforming reaction, they found that in

addition to the iron content, the preparation method of the catalysts also had big influence on the coking resistance and catalytic activity for ethanol steam reforming of the catalysts [33]. There is a need for a detailed investigation into the fabrication method on the performance of alloy electrode, and development of high performance anode for SOFCs. It is reported that the physical mixing method, the Pechini method and the glycine nitrite process (GNP) have been used to prepare alloy electrodes [29,33]. The traditional physical mixing method is widely used in commercial process due to its low cost and convenient process. However, since it is based on the solid-state reaction, the reaction will only occur on the solidesolid phase boundary area with limited contact. The distribution of the components is hard to achieve atomic level as compared with the solution-based methods. Thus in the final product, the yield of the generated catalyst after solidstate reaction is relatively low. Glycine nitrite process (GNP) is one of the widely used solution-based routes for powder preparation [34,35]. It is derived by the self-combustion reaction of glycine, and could generate fine particles. As compared with traditional physical mixing method, glycine nitrate process could achieve higher yield for the final product with the rapid self-combustion assistance. However, due to the fine powder generated via GNP, the shrinkage caused by crystal agglomeration during high temperature sintering is another consideration for commercial use of GNP in powder preparation for SOFC. In this paper, physical mixing method, glycine nitrate process (simultaneously), as well as a combined route of physical mixing method and GNP were applied to synthesize the starting powder for NiFe alloy anodes. In the present work, we systematically examined the influence of synthetic method on the properties and performances of the specific Ni0.75Fe0.25 þ SDC (60:40 v/o) alloyceramic anode. The results demonstrated clearly that the resistance of the anode towards coke formation as well as single cell performance are both closely related to the synthesis technique; an optimal synthesis method for the Ni0.75Fe0.25 þ SDC alloy-ceramic anode was proposed.

2.

Experimental section

2.1.

Synthesis of Fe2O3eNiO/SDC powders

Various Ni0.75Fe0.25O1.125  SDC powders were prepared via the following three routes: physical mixing route (PMR), simultaneous glycine nitrate process (S-GNP) and combined glycine nitrate process (C-GNP). All chemicals we used in this paper are analytical reagent grade chemicals. The metal nitrates, Fe2O3, glycine, PVB were obtained from Sinopharm Chemical Reagent Co., Ltd, whereas the NiO powder was from Chengdu Shudu Nano-Science Co., Ltd. The schematic diagram of the three routes was shown in Fig. 1. For the PMR, 6.636 g of NiO, 2.364 g of Fe2O3, 6 g of SDC (via EDTA-CA solegel method) were mixed thoroughly in ethanol medium for around 10 min with the help of a mortar and pestle and then dried at 80  C. For the S-GNP, 25.735 g of Ni(NO3)2$6H2O, 30.627 g of Fe(NO3)3,9H2O, 3.090 g of Sm(NO3)3$6H2O and 12.090 g of Ce(NO3)2$6H2O were first dissolved in de-ionized water, and then w35 g of glycine (NH2eCH2eCOOH) was added as a fuel into the solution (a

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Fig. 1 e Schematic diagram of the three preparation routs for Ni0.75Fe0.25O1.125 L SDC powders: physical mixing route (PMR), simultaneous glycine nitrate process (S-GNP) and combined glycine nitrate process (C-GNP).

molar ratio of glycine to the total metallic cations is 2). The solution was heated to evaporate excess water until a gel precursor was formed. After the gel was placed in an electric oven at 250  C for several hours, a primary solid precursor was produced via combustion, which was further calcined at 850  C for 5 h in static air to obtain the Ni0.75Fe0.25O1.125 þ SDC powder. For the C-GNP, 25.735 g of Ni(NO3)2$6H2O, 30.627 g of Fe(NO3)3,9H2O and w27 g of glycine were dissolved in deionized water, and then heated until the formation of a gel precursor. Similarly, 3.090 g of Sm(NO3)3$6H2O,12.090 g of Ce(NO3)2$6H2O and w8 g of glycine were dissolved to yield a mixed solution and then heated until the formation of a gel precursor. After the combustion in the electric oven at 250  C, both SDC and Ni0.75Fe0.25O1.125 solid precursors were calcined separately at 850  C for 5 h. Ni0.75Fe0.25O1.125 þ SDC powder was obtained by mixing 9 g of Ni0.75Fe0.25O1.125 and 6 g of SDC in ethanol and then dried at 80  C.

2.2.

Characterization

The phase structure of the unreduced and hydrogen reduced anode samples (sintered at 1400  C for 5 h) were analyzed by an X-ray diffractometer (XRD, Bruker D8 Advance) equipped with Cu Ka radiation (l ¼ 0.1541 nm). The crystallite sizes of Ni species were obtained on the basis of the broadening of XRD peaks using the Scherrer formula D ¼ kl/B cosq, where D is the crystallite size (nm), l is the wavelength of Cu Ka radiation, and B is the calibrated width of diffraction peak at the halfmaximum intensity. The lattice parameters and the unit cell volume of the crystals were calculated via Unit Cell program according to the typical diffraction peaks of the materials.

The morphologies of anodes before and after reduction were examined by environmental scanning electron microscopy (ESEM, QUANTA-200). All data in this study were plotted by Origin 7.0, with a Peak Fit Module 71. Carbon deposition experiment was carried out as follows. About 0.3 g of anode powder sintered at 1400  C for 5 h was dropped into a flow-through quartz tube with an inner diameter of 8 mm. The anode powder was first reduced by pure hydrogen at 750  C for 30 min at a flow rate of 20 ml min1 [STP]. After the reduction, the reactor temperature was decreased to 600  C under the same hydrogen atmosphere, then the atmosphere was switched to methane flow at 20 ml min1 [STP] and the temperature was kept at 600  C for 30 min. The sample was then cooled to room temperature under the protection of flowing helium. Hydrogen temperature-programmed reduction (H2-TPR) was performed to identify the interaction of NieFe oxide with SDC as well as the interaction between Ni and Fe. Around 0.03 g of anode powder (after the sintering at 1400  C for 5 h in air) was placed at the bottom of a mica U-tube reactor. The reactor was flushed with argon at a flow rate of 30 ml min1 [STP] for 30 min before the atmosphere was changed to 10 vol % H2eAr gas mixture. The temperature was ramped at a rate of 10  C min1 from room temperature up to 930  C. The effluent gases were monitored by an in-situ TCD detector in a BELCAT-A apparatus. Oxygen temperature-programmed oxidation (O2-TPO) was conducted to get information about the carbon deposition properties of different anode materials. The experimental procedure was similar to the process for H2-TPR. Approximately 0.03 g of methane-treated sample (as-described in the

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carbon deposition experiment) was put into the bottom of a Utype reactor, and the reactor temperature was increased from room temperature to 930  C under flowing pure O2 at a flow rate of 15 ml min1 [STP]. During the heating process, the carbon deposited over the anode catalyst through the methane-treatment was oxidized into gaseous CO2, which was detected by an on-line Mass Spectrometer (MS, Hiden, QIC-20).

2.3.

Fuel cell fabrication and cell test

Ba0.5Sr0.5Co0.8Fe0.2O3d (BSCF) and Sm0.2Ce0.8O1.9 (SDC) powders were synthesized by a conventional EDTA-citrate complexing solegel process. A final calcination at 950  C and 850  C for 5 h for BSCF and SDC was selected, respectively, for the purpose of ash-removing and crystalline phase formation. Disk-shaped half cells with anode-supported thin-film SDC electrolyte configuration were fabricated by die-pressing, followed by high temperature sintering at 1400  C for 5 h in stagnant air [36]. The cathode layer was formed by spray deposition, followed by sintering at 1000  C for 2 h. Silver was used as the current collector for both electrodes. The fuel cell was sealed onto a quartz tube reactor with the cathode exposed to ambient air and anode flowing with 3% water humidified hydrogen at a flow rate of 40 ml min1 [STP]. Voltage vs. current density polarization curves were obtained by a Keithley 2420 source meter based on the four-terminal configuration. The overall resistance of the cell was obtained by EIS using a Solartron 1260 frequency response analyzer in combination with a Solartron 1287 potentiostat.

3.

Results and discussion

3.1.

Phase structure

Fig. 2a shows XRD patterns of the unreduced Ni0.75Fe0.25O1.125 þ SDC anode powders prepared from the three different methods, all samples were pre-sintered at 1400  C in air for 5 h, which is similar to those in real fuel cell fabrication. For comparison, the XRD patterns of sintered NiO, Fe2O3 and SDC (1400  C) are also shown separately in Fig. 2b. Crystalline SDC phase was detected in all three samples with the corresponding diffraction peaks indexed in the figure.

There was no detectable peak shift in the XRD results of SDC, suggesting the solid phase reaction between Ni0.75Fe0.25O1.125 and SDC was not that serious to be detected by the XRD equipment after the calcinations at 1400  C. In addition to the SDC phase, a phase related to nickel oxide (NiO) in the cubic form (JCPDS 78-0643) was also observed, alongside with the appearance of a mixed oxide solution of nickel and iron in a spinel-type cubic structure, similar to the NiFe2O4 phase (JCPDS 74-2081). Qualitative observation of the relative intensity of the diffraction peaks for the bimetallic samples suggested that there were considerable amounts of both the nickel oxide-related phase and the NiFe2O4erelated phase in the samples. However, the relative intensities of the diffraction peaks for SDC (diffraction plane (200) for example) and NiFe2O4 (diffraction plane (311) for example) varied from sample to sample. Since the actual amount of SDC is the same in all three samples, the only explanation for such a variation is the different NiFe2O4 concentration in the samples. It implies iron did not fully react with nickel oxide to form the NiFe2O4erelated phase. Since no free Fe2O3 was detected in the samples, it is reasonable to assume that the Fe2O3 was incorporated into nickel oxide with the formation of a nickele rich solid solution with the same lattice structure to NiO. The ˚ , while calculated lattice parameter for the pure NiO is 4.1778 A ˚ for the the lattice parameters are 4.1779, 4.1765, and 4.1740 A nickel oxides in the Ni0.75Fe0.25O1.125 þ SDC samples prepared from PMR, S-GNP and C-GNP, respectively. The lattice parameter of NiO prepared via PMR is almost the same as that of the pristine NiO. However, as compared with the pristine phase, an obvious decrease of the lattice parameters was observed for the samples prepared via S-GNP and C-GNP. Considering the larger ionic radius of Ni (II, 83.0 pm) than Fe (III, 69.0 pm) in 6-coordination with oxygen [37], some iron was likely incorporated into the nickel oxide lattice for the samples synthesized from S-GNP and C-GNP with the formation of solid solutions with similar lattice structure to NiO but a smaller lattice parameter. Clearly, the actual composition of the NiFe2O4-related phase in the samples prepared from the three different methods differed somewhat from each other. Since this kind of incorporation is realized by the strong interatomic bond between nickel atom and iron atom, the sample prepared via C-GNP represents the strongest interaction between nickel and iron, whereas the sample prepared via PMR exhibits the least interaction.

Fig. 2 e (a) X-ray diffraction patterns of Ni0.75Fe0.25O1.125 e SDC samples made from a: PMR, b: S-GNP and c: C-GNP, (b) X-ray diffraction patterns of NiO, SDC and Fe2O3 before hydrogen reduction. (All samples were pre-calcined at 1400  C for 5 h).

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Shown in Fig. 3 are XRD patterns of the various anode samples after the reduction in a hydrogen atmosphere at 800  C for 5 h. In all samples, metallic nickel was crystallized as a face centred cubic (FCC) structure (JCPDS 87-0712) with no observation of metallic iron phase. As discussed in Fig. 2, we have demonstrated that the iron oxide formed solid solution and NiFe2O4-related spinel phase with nickel oxide in the unreduced samples, it seems the metallic iron still formed solid solution with metallic nickel after the hydrogen reduction [38]. Due to the insertion of iron into the nickel crystal ˚ 3 in the structure, the unit cell volume increased from 43.76 A 3 ˚ in the FCC FCC nickel structure to 44.82, 44.85 and 44.75 A NieFe alloy structure for the samples prepared from PMR, SGNP and C-GNP, respectively, as calculated based on the (111), (200) and (220) diffraction planes of nickel. This is well ˚) understood because of the larger atomic radius of Fe (1.40 A ˚ ) [39]. Lattice parameters for the nickeleiron than Ni (1.35 A solid solution phase calculated by (111), (200) and (220) diffraction planes of XRD patterns shown in Fig. 3 were 3.552, ˚ for the samples prepared by PMR, S-GNP and 3.553 and 3.550 A C-GNP, respectively. The lattice parameters fitted in the range ˚ . It suggests that the composition of of a ¼ 3.545e3.560 A nickelerich NiOeFe2O3 solid solutions in the various samples changes in the composition range of Ni2Fe to Ni3Fe according to Awaruite mineral data [40]. Table 1 shows crystallite sizes of the nickel and iron compounds calculated using the Scherrer equation for unreduced and reduced samples. It shows that the Ni crystallites in the sample prepared from SGNP are much smaller than those in the samples obtained using both PMR and C-GNP. The SDC phase thus likely acted as a dispersant, which effectively suppressed the sintering of the NiO/Fe2O3 phases during the synthesis of the samples prepared from S-GNP.

Table 1 e Crystallite size of nickel species calculated from XRD patterns via the Scherrer formula. Sample

PMR S-GNP C-GNP

Mean crystallite size (nm) NiO of calcined samplesa

Ni of reduced samplesb

51.94 60.06 54.83

49.25 27.59 56.80

a Average calculated value of (111), (200) and (220) NiO planes. b Average calculated value of (111), (200) and (220) Ni and NieFe planes.

Nickel is the main electrocatalyst in the SOFC anode, and the interaction between nickel and iron could affect the catalytic

activity of nickel for fuel oxidation and also coking resistance when operating on hydrocarbon fuels. As H2-TPR was an effective technique to probe the chemical interactions between different components, it was used to analyze various samples to get information about the interaction between iron oxide and nickel oxide. The H2-TPR profiles of NiO, Fe2O3 and Ni0.75Fe0.25O1.125 prepared from GNP are shown in Fig. 4, and all samples were pre-calcined at 1400  C for 5 h. The Gaussian deconvolution method was used to separate the reduction peaks. The percentage of the hydrogen consumption for each separated peak to total amount of hydrogen consumption during the H2-TPR process for above materials is shown in Table 2. For pure NiO phase, a very small peak for hydrogen consumption appears at around 300  C, followed by a large peak at around 400  C. The small peak can be assigned to the reduction of surface oxygen over nickel oxide. It suggests that the reduction of nickel oxide mainly occurs in a single step, i.e. NiO þ H2 ¼ Ni þ H2O. The peak temperature for the reduction of the NiO phase is at around 390  C in this study, which is slightly higher than those for free NiO or NiO species with very weak interactions with the support phase of w330  C reported in the literature [41e43]. It may be explained by the large grain size of NiO phase formed due to the high sintering temperature in our case. As to Fe2O3, multiple reduction peaks are

Fig. 3 e X-ray diffraction patterns of reduced Ni0.75Fe0.25 L SDC samples prepared via various methods: (a) PMR, (b) S-GNP, (c) C-GNP.

Fig. 4 e H2-TPR profiles of NiO, Fe2O3 and Ni0.75Fe0.25O1.125: (d) experimental profiles; (- - -) Gaussian deconvolution of the TPR profiles.

3.2.

Interaction between anode components

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Table 2 e The area percentage of each separated peak to the total H2-TPR peak according to H2-TPR profiles of NiO-SDC, Ni0.75Fe0.25O1.125 L SDC and Fe2O3-SDC prepared via S-GNP, as well as pure NiO, Ni0.75Fe0.25O1.125 and Fe2O3 in Figs. 4 and 5. Sample

NiO NiO-SDC Ni0.75Fe0.25O1.125 Ni0.75Fe0.25O1.125  SDC Fe2O3 Fe2O3-SDC

Total H2-TPR peak area 2.36E6 1.41E6 2.58E6 1.54E6 3.35E6 1.88E6

Area percentage of each peak to the total H2-TPR peak Peak 1

Peak 2

Peak 3

Peak 4

Peak 5

4.77 24.8 9.63 41.8 1.37 21.0

68.5 32.1 12.5 32.2 22.2 32.4

3.13 32.5 68.0 1.55 65.6 23.2

23.6 10.6 6.52 18.0 4.73 11.1

e e 3.35 6.45 6.14 12.3

*The temperature range from peak 1 to peak 5 represented from low to high.

observed, and the reduction starts at around 400  C and completes at around 930  C. Fiuza et al. also investigated the H2-TPR profile of monometallic iron oxide calcined in air at 900  C [33]. They observed the similar complexity of reduction curve of iron oxides, although the curves differed slightly from ours, which may be related to the different processing of the oxides. For the pristine Fe2O3 phase, the reduction curves can be separated into five peaks via Gaussian deconvolution, with the peak temperatures of around 484, 583, 703, 766 and 855  C, respectively. The two main reduction peaks are at 583 and 703  C. The multi reduction peaks suggest that the reduction of iron oxide is fairly complicated. The percentage of the hydrogen consumption for each reduction peak to total amount of hydrogen consumed during the H2-TPR process is shown in Table 2. As to the bimetallic sample Ni0.75Fe0.25O1.125, the reduction curves can also be separated into five peaks at 432, 486, 592, 673 and 703  C, respectively. It was found that its reduction took place at the temperature range higher than NiO while lower than Fe2O3. It suggests a strong chemical interaction likely occurs between NiO and Fe2O3 phases, increasing the chemical stability of NiO while decreasing the stability of Fe2O3. To improve the anode performance, SDC phase was usually introduced to Ni0.75Fe0.25O1.125 to form a percolative mixture with both ionic and electronic conductivity. The interaction between NiFe alloy and SDC may also have significant effect on the anode performance. Previously, it was revealed by XRD that there was no serious reaction between SDC and Ni/Fe oxides. Fig. 5 shows the H2-TPR profiles of NiOSDC, Ni0.75Fe0.25O1.125SDC and Fe2O3SDC prepared from S-GNP, where all samples were sintered at 1400  C for 5 h. For comparison, the results of NiO, Ni0.75Fe0.25O1.125 and Fe2O3 are also presented. It was found that the reduction process becomes smoother when SDC was introduced to form composite with Ni0.75Fe0.25O1.125, NiO or Fe2O3. It suggests that SDC did affect the reduction of NiO/Fe2O3 oxides. For the NiOSDC composite, the reduction temperature increased as compared with pristine NiO with the main reduction temperature at around 395  C. The reduction curves of pure Fe2O3 and Fe2O3-SDC presented in a similar trend, but the first three reduction peaks shifted from 484, 583 and 703  C to 582, 654 and 698  C, respectively, after the incorporation of SDC phase. It further suggests that certain interaction did exist between SDC and the transition metal oxide(s). It is interesting that the SDC phase in some cases also facilitated the

reduction of transition metal oxide(s). For example, the two high temperature reduction peaks for Fe2O3 shifted from around 766 and 855  C respectively to 728 and 804  C after the incorporation of SDC; in some cases, SDC likely acted as a dispersant to prevent the iron oxide grains from agglomeration. Consequently, the reduction peaks caused by the diffusion phenomena, which appeared at the high reduction temperature, were effectively decreased. Interestingly, the shoulder reduction peak appeared at around 500e700  C in the H2-TPR profile of Ni0.75Fe0.25O1.125, disappeared in the reduction profile of Ni0.75Fe0.25O1.125  SDC, which can also be explained by the suppressing effect of SDC for the sintering of the Ni0.75Fe0.25O1.125 grains [33]. As shown in Table 2, by comparing with samples before and after the incorporation with SDC, the reduction process of each sample was clearly revealed. This indicates that SDC acted as a dispersant to resist the agglomeration of iron oxide grains. To determine the effect of synthesis method on the interaction between cell components, the H2-TPR profiles of Ni0.75Fe0.25O1.125  SDC composites prepared by the three different methods were also measured, and the results are shown in Fig. 6. The Gaussian deconvolution was used again

Fig. 5 e H2-TPR profiles of NiO-SDC, Ni0.75Fe0.25O1.125 L SDC and Fe2O3-SDC prepared via S-GNP, as well as pure NiO, Ni0.75Fe0.25O1.125 and Fe2O3,: (d) experimental profiles; (- - -) Gaussian deconvolution of the TPR profiles.

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well with the conclusion derived previously from the XRD results shown in Fig. 2. Such a difference could be simply attributed to the different strength of interaction between nickel oxide and iron oxide since the interaction between SDC and the Ni/Fe oxides was less significant. The reduction of sample prepared from C-GNP was shown to be the hardest, while the sample prepared via PMR was the easiest. It demonstrated that the preparation methods had a big influence on the chemical interaction between nickel and iron, and the sample prepared via C-GNP had the strongest interaction while the sample prepared from the PMR had the weakest interaction.

3.3.

Fig. 6 e (a) H2-TPR profiles of various Ni0.75Fe0.25O1.125 L SDC anodes prepared from different methods: (d) experimental profiles; (- - -) Gaussian deconvolution of the TPR profiles.

to study the difference in reduction behaviour, and the results are shown in Table 3. By comparing the results shown in Table 3, it was found that various samples prepared from PMR, S-GNP and C-GNP had different reduction behaviour. As to the reduction capability, nickel oxide is much easier than iron oxide. Due to the interaction of iron oxide to nickel oxide, however, the reduction temperature of nickel oxide goes up. According to the reduction temperature and the reduction process at each temperature stage as shown in Fig. 6 and Table 3, the main reduction happens at the lowest temperature of w419  C for PMR powder while the highest of w584  C for CGNP powder. Interestingly, the reduction curve for C-GNP powder presented a shoulder peak caused by the diffusion phenomena from 500  C to 700  C, which has been observed in the TPR curve of Ni0.75Fe0.25O1.125 as well. It illustrates the reduction behaviour of C-GNP powder is much similar to that of Ni0.75Fe0.25O1.125, whereas the PMR and S-GNP powder is not that obvious. Thus it indicates the interaction between nickel and iron in C-GNP powder should be close to that in Ni0.75Fe0.25O1.125. For the first reduction peak in the three samples with reduction temperature below 450  C, it could be regarded as the reduction of the weakly interacted NiO. As shown in Table 3, the amount of weakly interacted NiO in the sample prepared from PMR is the highest, while it is the lowest for the sample prepared from C-GNP, agreeing pretty

Electrode microstructure

To investigate the effect of synthesis method on the electrode microstructure, the anode material with the composition of Ni0.75Fe0.25O1.125 þ SDC (Ni0.75Fe0.25O1.125 to SDC weight ratio of 60 to 40) was prepared by the aforementioned three different methods; some PVB was added as a pore former in the fabrication of the anode. Specifically, i): Ni0.75Fe0.25O1.125 þ SDC was synthesized from PMR, i.e. by simply hand-grinding the mixture of NiO, Fe2O3, SDC and PVB in ethanol medium for w10 min (anode-1); ii): Ni0.75Fe0.25  SDC powder synthesized from S-GNP was hand-ground with the addition of PVB in ethanol medium for 10 min, (anode-2); and iii): Ni0.75Fe0.25O1.125 powder prepared from GNP was physically mixed with SDC and PVB in ethanol medium by hand-grinding for 10 min (anode-3). The as-obtained materials were then fabricated into disk-shaped anodes by dry pressing and sintering at 1400  C in air. Fig. 7 shows the SEM images of various sintered anode disks before and after the hydrogen reduction. The reduction of the disk-shaped anodes was carried out in pure hydrogen at 700  C for 2 h. As shown in Fig. 7b, anode-2 prepared by S-GNP showed a highly porous structure and the pores were almost in round shape. The grain sizes of the anode were less than 2 mm. After the reduction, the porosity increased somewhat, while the grains were still very uniformly distributed with the sizes of less than 1.5 mm. For anode-3 shown in Fig. 7c, the electrode before the reduction was dense with no clearly visible grain boundaries. After the reduction, the grain boundary became clear and the grains were in irregular shape. Though the non-uniform particle distribution was found in the reduced anode, the unique morphology of the coral-like Ni phase could be seen in the anode-3 after the reduction. For anode-1 as shown in Fig. 7a, the electrode before reduction exhibited poor microstructure, and its grain distribution was

Table 3 e The area percentage of each separated peak to the total H2-TPR peak according to H2-TPR profiles of various Ni0.75Fe0.25O1.125 L SDC anodes prepared from different methods in Fig. 6. Ni0.75Fe0.25O1.125  SDC

PMR S-GNP C-GNP

Total H2-TPR peak area 1.52E6 1.54E6 1.56E6

Area percentage of each peak to the total H2-TPR peak Peak 1

Peak 2

Peak 3

Peak 4

Peak 5

62.3 41.8 9.89

10.1 32.2 55.7

14.0 1.55 17.8

8.53 18.0 12.7

5.04 6.45 3.91

*The temperature range from peak 1 to peak 5 represented from low to high.

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interaction between iron and nickel in the NiFe2O4erelated phase in those two samples before the reduction, as indicated in the XRD results in Fig. 2b and the H2-TPR results in Fig. 6. It was further confirmed by the different carbon removal temperature as shown in Fig. 8a. The temperature required for the carbon removal was the lowest for the sample prepared from S-GNP, while it was the highest for the sample prepared from C-GNP. The temperature difference for the carbon removal reflected the existing of distinct carbon species including the relative amount of soft and hard carbons. The soft carbon was preferred for its easier elimination. As shown in Fig. 8b and c, it was astonishing that more coke was observed over the iron/SDC catalyst than that over the nickel/ SDC catalyst for the samples prepared from both PMR and SGNP, in particular for the catalysts prepared from the PMR. Generally, more carbon was deposited on the sample prepared from S-GNP, which may be explained by the smaller crystalline size of the metallic nickel phase. By introducing iron into nickel, only slight improvement in coking resistance was observed, and the improved resistance towards coke formation was more pronounced for the catalysts prepared from the GNP methods. However, the coke formation was suppressed much more significantly for the sample when the 25 wt.% Fe was introduced through the S-GNP, rather than the one which was prepared through the PMR. As a result, the effect of alloying nickel with iron on the coking resistance is highly dependent on its microstructure created via the synthesis method. Fig. 7 e SEM images of anode-1 (a), anode-2 (b), anode-3 (c) before and after the hydrogen reduction.

non-uniform with large grain sizes up to ten microns; in addition the pores were in irregular shape. We further measured the sintering shrinkage of cells. The cell with anode-2 prepared from S-GNP showed the largest shrinkage of 46.4%, followed by the cell with anode-3 prepared from C-GNP of 40.2%, while the cell with anode-1 prepared from PMR had the smallest shrinkage of 37.4%.

3.4.

Coking resistance

It is well known that coke formation is a practical problem for conventional nickel cermet anode operating on hydrocarbon fuels. The coking resistance of NiFe-SDC anode powders prepared by the three methods was analysed by O2-TPO technique with the results shown in Fig. 8. For comparison, the results of NiO, Fe2O3 and Ni0.75Fe0.25O1.125 (all fired at 1400  C) were also presented. It was very interesting that the Ni0.75Fe0.25O1.125  SDC prepared via S-GNP showed much larger CO2 peak area than those prepared via the PMR and CGNP, which may be explained by the much smaller crystalline size of the metallic nickel phase obtained via S-GNP, as indicated in Table 1. However, although the crystalline size of nickel in samples prepared via PMR and C-GNP was very close to each other, the carbon deposition of the sample via C-GNP was twice that of the sample prepared via PMR. The possible explanation for such a difference is the different chemical

3.5.

Fuel cell performance

Fig. 9 shows the IeV polarization curves of the cells with BSCF cathode, SDC electrolyte with a thickness of around 20 mm and SDC þ Ni0.75Fe0.25O1.125 anodes prepared from the three different methods. The OCV was between w0.77 and w0.88 V at temperatures between 650 and 450  C for the cell with anode-1 prepared from PMR, and the maximum power output was w80 and w600 mW cm2 at 450 and 650  C, respectively. The maximum power density at 650  C was w430 mW cm2 for the cell made of anode-2 and w564 mW cm2 for the cell made of anode-3, while their OCVs are similar to the cell made of anode-1. Although the microstructure of reduced anode-2 was quite attractive, its corresponding cell presented the lowest performance. It could be explained by their different anode porosity, which proved to have a significant influence on cell performance. Considering the sintering shrinkage of cell-2 was the highest among all three cells, the lowest porosity of anode-2 thus led to the lowest cell output. The electrochemical impedance spectra (EIS) of the as-tested cells are shown in Fig. 10. The high-frequency and low-frequency depressed arcs are due to the electrode polarization process. As both the electrolyte and cathode are the same for all three cells, the difference in the arcs could be simply attributed to the anode polarization resistance. The arcs in EIS of the cell with anode-2 are larger than that of the cells with anode-1 and anode-3 at respective temperatures as shown in Fig. 10. It suggests the anode-2 has a relatively larger polarization resistance than the others, agreeing pretty well with the lowest anode porosity as suggested from the cell shrinkage results.

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Fig. 8 e O2-TPO profiles in CO2 signal of studied materials after the methane-treatment: (a) Ni0.75Fe0.25 L SDC prepared from the three different methods; Ni-SDC, Ni0.75Fe0.25 L SDC and Fe-SDC prepared from PMR (b), and S-GNP (c).

Fig. 9 e Voltage and power density vs current density plots for various cells at different temperatures for anode-supported cells with anode-1 (a), anode-2 (b), anode-3 (c).

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Fig. 10 e Impedance spectra of the various cells with different anodes at selected temperatures: anode-1 (a), anode-2 (b), anode-3 (c).

4.

Conclusions

As to bimetallic Ni/Fe anode in SOFCs, three synthesis methods of PMR, S-GNP and C-GNP were used to examine their influence on the anode properties and cell performance. The synthesis methods affected the anode in three aspects: phase structure, chemical interaction between cell components and electrode microstructure. The smallest Ni crystal sizes were obtained for the sample prepared from S-GNP. Iron was incorporated into the NiO phase to form a solid solution with the lattice structure of NiO. The chemical interactions between nickel and iron as well as between NiFe and SDC phases highly depended on synthesis method, and the sample prepared via C-GNP and PMR showed the strongest and the worst chemical interaction respectively. The non-uniform particle distribution was found in both reduced anode-1 (PMR powder) and reduced anode-3 (C-GNP powder). The cell with anode-2 (S-GNP powder) showed the most uniform pore structure and had the largest shrinkage of 46.5% after sintering. The coking resistance was revealed to be much more sensitive to the phase structure and the chemical interactions between cell components than the electrode microstructure. The carbon deposition was the most severe over anode-2 (S-GNP powder) owing to the smallest crystal size of the

metallic species, while the amount of hard carbon was the highest in anode-3 (C-GNP powder), likely due to the strongest interaction between its anode components. However, the cell performance relied more significantly on the electrode microstructure. The cells with anode-1 and anode-3 presented similar performances, whereas the cell with anode-2, which had a largest sintering shrinkage, presented the worst performance. Hence, through affecting phase structure, chemical interaction and electrode microstructure, the resistance of the anode towards coke formation as well as single cell performance are strongly synthesis method dependent. The optimal ratio of iron to nickel for the NieFe based alloyceramic anode to achieve maximum cell performance is thus highly case-dependent.

Acknowledgements This work was supported by the “National Science Foundation for Distinguished Young Scholars of China” (under contract No. 51025209), the “Outstanding Young Scholar Grant at Jiangsu Province” (under contract No. 2008023) and the program for New Century Excellent Talents (2008). Z. Shao also acknowledges ARC for a Future Fellowship.

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references

[1] Singhal SC. Solid oxide fuel cells for stationary, mobile, and military applications. Solid State Ion 2002;152:405e10. [2] Minh NQ. Ceramic fuel-cells. J Am Ceram Soc 1993;76:563e88. [3] Atkinson A, Barnett S, Gorte RJ, Irvine JTS, McEvoy AJ, Mogensen M, et al. Advanced anodes for high-temperature fuel cells. Nat Mater 2004;3:17e27. [4] Yang L, Choi Y, Qin WT, Chen HY, Blinn K, Liu MF, et al. Promotion of water-mediated carbon removal by nanostructured barium oxide/nickel interfaces in solid oxide fuel cells. Nat Commun; 2011. 2. [5] Zhan ZL, Barnett SA. An octane-fueled solid oxide fuel cell. Science 2005;308:844e7. [6] Jain SL, Lakeman JB, Pointon KD, Marshall R, Irvine JTS. Electrochemical performance of a hybrid direct carbon fuel cell powered by pyrolysed MDF. Energy Environ Sci 2009;2: 687e93. [7] Cimenti M, Hill JM. Thermodynamic analysis of solid oxide fuel cells operated with methanol and ethanol under direct utilization, steam reforming, dry reforming or partial oxidation conditions. J Power Sources 2009;186:377e84. [8] Shao ZP, Zhang CM, Wang W, Su C, Zhou W, Zhu ZH, et al. Electric power and synthesis gas co-generation from methane with zero waste gas emission. Angew Chem Int Edit 2011;50:1792e7. [9] Yang L, Wang SZ, Blinn K, Liu MF, Liu Z, Cheng Z, et al. Enhanced sulfur and coking tolerance of a mixed ion conductor for SOFCs: BaZr0.1Ce0.7Y0.2xYbxO3d. Science 2009; 326:126e9. [10] Hibino T, Hashimoto A, Inoue T, Tokuno J, Yoshida S, Sano M. A low-operating-temperature solid oxide fuel cell in hydrocarbon-air mixtures. Science 2000;288:2031e3. [11] Suzuki T, Hasan Z, Funahashi Y, Yamaguchi T, Fujishiro Y, Awano M. Impact of anode microstructure on solid oxide fuel cells. Science 2009;325:852e5. [12] Baek SW, Bae J. Anodic behavior of 8Y2O3-ZrO2/NiO cermet using an anode-supported electrode. Int J Hydrogen Energy 2011;36:689e705. [13] Chan SH, Khor KA, Xia ZT. A complete polarization model of a solid oxide fuel cell and its sensitivity to the change of cell component thickness. J Power Sources 2001;93:130e40. [14] Ivers-Tiffe´e E, Virkar AV. Chapter 9: electrode polarizations. In: Singhal SC, Kendall K, editors. High temperature solid oxide fuel cells: fundamentals, design, and applications. Oxford, UK: Elsevier B.V.; 2003. p. 229e60. [15] Kim T, Liu G, Boaro M, Lee SI, Vohs JM, Gorte RJ, et al. A study of carbon formation and prevention in hydrocarbon-fueled SOFC. J Power Sources 2006;155:231e8. [16] Murray EP, Harris SJ, Jen HW. Solid oxide fuel cells utilizing dimethyl ether fuel. J Electrochem Soc 2002;149. A1127-A31. [17] Su C, Wu YZ, Wang W, Zheng Y, Ran R, Shao ZP. Assessment of nickel cermets and La0.8Sr0.2Sc0.2Mn0.8O3 as solid-oxide fuel cell anodes operating on carbon monoxide fuel. J Power Sources 2010;195:1333e43. [18] Gorte RJ, Vohs JM. Novel SOFC anodes for the direct electrochemical oxidation of hydrocarbons. J Catal 2003;216: 477e86. [19] Lu ZG, Zhu JH, Bi ZH, Lu XC. A Co-Fe alloy as alternative anode for solid oxide fuel cell. J Power Sources 2008;180:172e5. [20] Ringuede A, Labrincha JA, Frade JR. A combustion synthesis method to obtain alternative cermet materials for SOFC anodes. Solid State Ion 2001;141:549e57. [21] An W, Gatewood D, Dunlap B, Turner CH. Catalytic activity of bimetallic nickel alloys for solid-oxide fuel cell anode reactions from density-functional theory. J Power Sources 2011;196:4724e8.

9297

[22] Ringuede A, Fagg DP, Frade JR. Electrochemical behaviour and degradation of (Ni, M)/YSZ cermet electrodes (M¼Co, Cu, Fe) for high temperature applications of solid electrolytes. J Eur Ceram Soc 2004;24:1355e8. [23] Huang B, Wang SR, Liu RZ, Wen TL. Preparation and performance characterization of the Fe-Ni/ScSZ cermet anode for oxidation of ethanol fuel in SOFCs. J Power Sources 2007;167:288e94. [24] Park HC, Virkar AV. Bimetallic (Ni-Fe) anode-supported solid oxide fuel cells with gadolinia-doped ceria electrolyte. J Power Sources 2009;186:133e7. [25] Wang SZ, He Q, Liu ML. Promising Ni-Fe-LSGMC anode compatible with lanthanum gallate electrolyte. Electrochim Acta 2009;54:3872e6. [26] Ishihara T, Shinagawa M, Kawakami A, Matsumoto H. Ni-Fe bimetallic anode for intermediate temperature solid oxide fuel cells using LaGaO3 based oxide electrolyte. Mater Sci Forum 2007;539-543:1350e5. [27] Ishihara T, Yan J, Enoki M, Okada S, Matsumoto H. Ni-Fe alloy-supported intermediate temperature SOFCs using LaGaO3 electrolyte film for quick startup. J Fuel Cell Sci Technol; 2008:5. [28] Lu XC, Zhu JH, Bi ZH. Fe alloying effect on the performance of the Ni anode in hydrogen fuel. Solid State Ion 2009;180:265e70. [29] Ding J, Liu J, Guo WM. Fabrication and study on Ni1-xFexOYSZ anodes for intermediate temperature anode-supported solid oxide fuel cells. J Alloy Compd 2009;480:286e90. [30] Kan H, Lee H. Enhanced stability of Ni-Fe/GDC solid oxide fuel cell anodes for dry methane fuel. Catal Commun 2010;12:36e9. [31] Wang SZ, Gao J. High performance Ni-Fe-lanthanum gallate composite anodes for dimethyl ether fuel cells. Electrochem Solid State Lett 2006;9. A395-A8. [32] Lu XC, Zhu JH. Ni-Fe þ SDC composite as anode material for intermediate temperature solid oxide fuel cell. J Power Sources 2007;165:678e84. [33] Fiuza RD, da Silva MA, Boaventura JS. Development of Fe-Ni/ YSZ-GDC electrocatalysts for application as SOFC anodes: XRD and TPR characterization and evaluation in the ethanol steam reforming reaction. Int J Hydrogen Energy 2010;35:11216e28. [34] Chick LA, Pederson LR, Maupin GD, Bates JL, Thomas LE, Exarhos GJ. Glycine nitrate combustion synthesis of oxide ceramic powders. Mater Lett 1990;10:6e12. [35] Patil KC, Aruna ST, Ekambaram S. Combustion synthesis. Curr Opin Solid State Mat Sci 1997;2:158e65. [36] Lin Y, Ran R, Guo YM, Zhou W, Cai R, Wang J, et al. Protonconducting fuel cells operating on hydrogen, ammonia and hydrazine at intermediate temperatures. Int J Hydrogen Energy 2010;35:2637e42. [37] Shannon RD. Revised effective ionic-radii and systematic studies of interatomic dictances in halides and chalcogenides. Acta Crystallogr Sect A 1976;32:751e67. [38] Massalski TB, Baker H, Murray JL, Bennett LH. Binary alloy phase diagrams. 2nd ed. ASM International; 1990. [39] Slater JC. Atomic radii in crystals. J Chem Phys 1964;41:3199. [40] Bose P, Bid S, Pradhan SK, Pal M, Chakravorty D. X-ray characterization of nanocrystalline Ni3Fe. J Alloy Compd 2002;343:192e8. [41] Robertson SD, McNicol BD, De Baas JH, Kloet SC, Jenkins JW. Determination of reducibility and identification of alloying in copper-nickel-on-silica catalysts by temperatureprogrammed reduction. J Catal 1975;37:424e31. [42] Takenaka S, Umebayashi H, Tanabe E, Matsune H, Kishida M. Specific performance of silica-coated Ni catalysts for the partial oxidation of methane to synthesis gas. J Catal 2007; 245:392e400. [43] Wang W, Ran R, Su C, Shao ZP, Jung DW, Seo S, et al. Effect of nickel content and preparation method on the performance of Ni-Al2O3 towards the applications in solid oxide fuel cells. Int J Hydrogen Energy 2011;36:10958e67.