Electroless plated Cu–Ni anode catalyst for natural gas solid oxide fuel cells

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Electroless plated Cu–Ni anode catalyst for natural gas solid oxide fuel cells Azadeh Rismanchian a , Jelvehnaz Mirzababaei b , Steven S.C. Chuang a,∗ a b

Department of Polymer Science, The University of Akron, Akron, OH 44325-3909, USA Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH 44325-3906, USA

a r t i c l e

i n f o

Article history: Received 13 December 2013 Received in revised form 20 April 2014 Accepted 12 May 2014 Available online xxx Keywords: CH4 –SOFC Cu–Ni alloy Anode stability Faraday resistance Carbon type Raman spectroscopy

a b s t r a c t A Ni/YSZ anode and a Ni/YSZ modified by Cu electroless plating were investigated for direct utilization of CH4 , the major component in natural gas. The catalytic activity of the anodes toward H2 and CH4 oxidation reactions were investigated by comparing the Faraday resistance, RF , of the cells obtained from impedance spectroscopy. The RF ratio of Cu–Ni/YSZ in CH4 to H2 was greater than that of Ni/YSZ, indicating low catalytic activity of Cu–Ni/YSZ anode toward CH4 oxidation. The addition of Cu to the Ni/YSZ anode decreased the catalytic activity, but increased the long-term stability of the anode in CH4 fuel. The Cu–Ni/YSZ anode showed long-term stability of 138 h in dry CH4 at 750 ◦ C. The Raman spectra of the fuel cell cross-section showed a change in type of carbon as a function of Cu concentration. The Cu rich surface showed more disordered carbon as opposed to graphitic carbon on Cu deficient areas in which Cu concentration gradually decreased toward the interlayer. Graphitic carbon produced on highly active Ni surface is known as a precursor to coking. Thus, optimizing the distribution of Cu in the Ni/YSZ anode will be required to develop a stable and high performance anode catalyst for direct CH4 utilization. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Abundance of natural gas in the United States has generated significant interest in its utilization. One potential application of natural gas, comprising more than 95% of methane, is its direct utilization as a fuel for the solid oxide fuel cell (SOFC). The critical issue in the development of a natural gas SOFC is the stability of the conventional Ni/YSZ anode. The main approaches to enhance the Ni/YSZ stability is the use of (i) high oxygen ion conductive materials, such as samarium-doped ceria (SDC), to accelerate the oxidation rate of deposited carbon [1,2], (ii) mixed ionic and electronic conductive perovskites [2,3], and (iii) a metal with low catalytic activity for C H dissociation which can form alloy with Ni such as Sn or Cu [2,4–6]. The low stability of the Ni/YSZ anode in the direct natural gas SOFC is due to the high catalytic activity for C H bond dissociation in hydrocarbons and catalyzing the C C bond formation, known as coking [7]. The C C and strong M C (metal–carbon) bond formation occur on the highly active step sites of Ni catalyst surface [8–10]. The mechanism by which the addition of a second metal suppresses the carbon formation has been suggested to be the

∗ Corresponding author. Tel.: +1 330 972 6993. E-mail address: [email protected] (S.S.C. Chuang).

deposition of second element at the step sites of Ni while the terrace sites remain free for the catalytic activity [9,11]. Cu has been selected as a second metal in Ni/YSZ anode in many studies. Cu has low catalytic activity for C H dissociation and C C formation and high electronic conductivity to maintain the fuel cell performance [12]. A number of methods have been reported for the addition of Cu to the Ni/YSZ anode including impregnation of Cu nitrate precursors [13], Cu electroplating [14], microwave irradiation from a Cu nitrate solution [15], and vacuum-assisted Cu electroless plating [16]. Electroless plating is a low cost technique to prepare a uniformly deposited layer of metals. The concentration of Cu solution and pH value could control the deposition rate, varying Cu particle size and the thickness of Cu layer [17]. Electroless plating has been used for deposition of Ni on YSZ for anode fabrication [18] and Ag on cathode to increase the power density of the SOFC [19]. In this study Cu was added to the Ni/YSZ anode by Cu electroless plating. The advantages of electroless plating over other techniques of Cu addition are using alkaline solution to limit the dissolution of Ni and deposition of Cu in reduced state. This approach not only eliminates the calcination step [15,16], but also deposit Cu on the step sites of Ni. The activity and stability of the Cu–Ni/YSZ anode was studied in CH4 fuel with an emphasis on determination of Faraday resistance and the type of carbon deposited on the surface and cross-section of the anode.

http://dx.doi.org/10.1016/j.cattod.2014.05.012 0920-5861/© 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: A. Rismanchian, et al., Electroless plated Cu–Ni anode catalyst for natural gas solid oxide fuel cells, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.05.012

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1.0 Cu-Ni/YSZ H2

794.33 Hz

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Fig. 1. Voltage–current curves and impedance spectra of a Ni/YSZ anode before and after Cu electroless plating, flowing He/H2 (100 sccm, 50 vol% H2 ) at 750 ◦ C.

2. Experimental 2.1. Fuel cell fabrication The anode-supported solid oxide fuel cells comprised of a 640 ␮m NiO/YSZ (70 wt% NiO) anode support layer, a 30 ␮m NiO/YSZ (50 wt% NiO) anode interlayer, a 32 ␮m YSZ electrolyte layer, a 20 ␮m LSM/YSZ (60 wt% LSM) cathode interlayer, and a 15 ␮m LSM cathode current collector layer. The anode support, anode interlayer, and the electrolyte were fabricated by tape casting of the slip of each layer. The slips were prepared by mixing the binder, plasticizer and metal oxide powders, NiO (AEE Co.) and YSZ (TZ-8Y, Tosoh) in ethanol as a solvent. The anode support slip contained microcrystalline cellulose pore former (10 wt% PH-301). The tapes were cut into 23 mm discs and fired at 1400 ◦ C. The cathode interlayer, LSM/YSZ (60 wt% LSM, Heraeus) and cathode current collector (LSM, Heraeus) layer with active area of 1 cm2 were screen-printed on the electrolyte surface followed by sintering at 1200 and 1100 ◦ C, respectively. The Cu–Ni/YSZ anode was prepared by reducing Ni/YSZ anode fuel cell and electroless plating of the copper on the anode surface. Electroless plating of Cu on the reduced Ni/YSZ anode support was carried out by (i) acid cleaning, (ii) surface sensitization by 20 g/l SnCl2 and 40 ml/l HCl solutions, (iii) surface activation by 0.25 g/l PdCl2 and 0.5 ml/l HCl solutions, (iv) Cu deposition with a 0.04 M CuSO4 ·5H2 O precursor. Ethylenediaminetetraacetic acid (0.1 M EDTA·2Na) and formaldehyde (37 wt% HCHO) were used as complexing and reducing agent, respectively. The pH 12.5 was kept constant during the deposition process by adding NaOH solution.

2.2. Electrochemical performance testing The anode of the fuel cell was connected to a tubular stainless steel reactor serving as an anode current collector and fuel chamber. An alloy strip was used as cathode current collector. Silver paste was used to improve the connection of the alloy strip to the cathode surface. The reactor was heated to 750 ◦ C with a heating rate of 5 ◦ C/min. The Cu electroless plated anodes were reduced in He/H2 (100 sccm, 50 vol% H2 ) at 750 ◦ C for 24 h prior to testing in order to ensure the formation of Cu–Ni alloy. The electrochemical performance of the cells in He/H2 (100 sccm, 50 vol% H2 ) and He/CH4

(100 sccm, 50 vol% CH4 ) at 750 ◦ C were tested with a potentiostat and a frequency response analyzer (Solarton cell test system 1470 E and 1400). Impedance spectra were collected at the open circuit voltage (OCV). 2.3. Characterization The microstructure of the Cu–Ni/YSZ anode and the distribution of Cu was characterized by scanning electron microscopy (SEM, Quanta 200 FEI) coupled with energy dispersive X-ray spectroscopy (EDS). The EDS mappings revealed that Cu was deposited on the surface of the Ni particles in Ni/YSZ anode support. The amount of Cu and its distribution along the anode cross-section was analyzed by X-ray fluorescence (XRF, microEDX-1300 SHIMADZU) with a 5 ␮m resolution. The formation of Cu–Ni alloy after reducing the Cu electroless plated cell in H2 was investigated by X-ray diffractometer (XRD, Philips APD 3700). Ni crystal size and composition of the Cu–Ni alloy particle was calculated from XRD patterns. The type of the carbon formed on the surface of the cells after exposure to CH4 was characterized by Raman spectroscopy using a Thermo Scientific DXR confocal Raman microscope with 780 nm laser. Samples were irradiated with 20 mW laser. The porosity measurements were performed by Archimedes method before and after Cu electroless plating. 3. Results and discussion 3.1. Fuel cell performance in H2 and CH4 A Ni/YSZ anode SOFC and a Ni/YSZ anode modified with Cu electroless plating have been tested. Electroless plating decreased the porosity of the Ni/YSZ from 46% to 35%. The particle size after Cu–Ni alloy formation was 2–5 ␮m. The detailed results of the microstructure characterization will be discussed in Section 3.3. Fig. 1 shows voltage–current curves and impedance spectra of a Ni/YSZ anode before and after Cu electroless plating in He/H2 (100 sccm, 50 vol% H2 ) at 750 ◦ C. The maximum current density of the Ni/YSZ anode decreased from 887 mA/cm2 to 715 mA/cm2 after Cu electroless plating. The lower performance of the Cu–Ni/YSZ anode can be attributed to the reduction of Ni active sites due to the formation of Cu–Ni alloy on the surface as well as an increase in ohmic

Please cite this article in press as: A. Rismanchian, et al., Electroless plated Cu–Ni anode catalyst for natural gas solid oxide fuel cells, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.05.012

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Since cathodes used in this study were kept with the same composition and structure, the variation in RF can be attributed to the change in anode structure/composition or the reaction with different fuels. A large RF reflects a highly resistive electrochemical reaction with a high activation energy and low number of active sites. Active sites on the SOFC anode are in the form of three phase boundary where O2− in the YSZ phase, electrons in the Ni phase, and hydrogen on the surface of Ni meet, as shown in Fig. 2. The larger Faraday resistance of Ni was consistent with a sharper slope of the

Voltage (V)

Ni/YSZ

H2

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200

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(t0+0.2 h)

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0

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Ni/YSZ

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7.94 Hz Hz

CH4 0.1 Hz

1.0 12589 125.89 Hz

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0.1 Hz

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2

0

0 900

Fig. 4 shows the voltage–current curves and impedance spectra of Cu–Ni/YSZ anode at 750 ◦ C in He/H2 (100 sccm, 50 vol% H2 ) and He/CH4 (100 sccm, 50 vol% CH4 ). The maximum current density was 680 mA/cm2 in H2 and 227 mA/cm2 in CH4 . The cell operated in CH4 under a constant load of 0.4 V for 180 h. The current density that was recorded after 138 h showed a slight increase compared to the initial performance in CH4 . This is in contrast to the rapid decline in the current density of Ni/YSZ anode. The performance of this Cu–Ni/YSZ anode exhibits one of the most stable anodes reported for the direct CH4 fuel cells in literature. The increase in

Power density (mW/cm )

300

1.2

(3)

3.2. Cu–Ni anode stability in dry CH4

)

(2)

2

CH4 + 4O2− → CO2 + 2H2 O + 8e−

-Z" ( .cm

(1)

0

1

2 3 2 Z' ( .cm )

2.0 )

H2 + O2− → H2 O + 2e−

CH4 → Cad + 4Had

2

resistance. The formation of Cu–Ni alloy through the diffusion of Cu into Ni lattice could increase the metal particle size, decreasing Ni active surface area and lowering available three phase boundary (TPB) for electrochemical oxidation reaction [14,20,21]. The ohmic and Faraday (polarization) resistance of the anodes were obtained from the impedance spectra. The ohmic resistance is the intercept of the high frequency arc with the X-axis. The Faraday resistance is the difference between the intercept of low frequency and high frequency arc with the X-axis. The low current density of the Cu–Ni/YSZ anode was also attributed to the increase of ohmic resistance from 0.28  cm2 for Ni/YSZ to 0.74  cm2 . The increase in ohmic loss could be related to the contact resistance between current collectors and the electrode surfaces [22]. Faraday resistance, RF , in H2 was 1.79  cm2 for Ni/YSZ and 0.74  cm2 for Cu–Ni/YSZ anode. It should be noted that Faraday resistance is related to the activation energy of an electrochemical reaction [23]. Electrochemical oxidation of H2 and CH4 with O2− on the anode can be expressed as reactions (1) and (2).

V–I curve at low current density region, where the electrode overpotential is dominated by activation polarization [24]. The larger Faraday resistance (i.e., high activation energy for the reaction) in initial Ni/YSZ anode can be attributed to the insufficient reducing time in H2 prior to Cu electroless plating. Insufficient reduction would not convert all of the Ni oxide to the reduced Ni sites to serve as three phase boundary for the reaction. Fig. 3 shows the voltage–current curves and impedance spectra of Ni/YSZ and Cu–Ni/YSZ anodes at 750 ◦ C in He/H2 (100 sccm, 50 vol% H2 ) and He/CH4 (100 sccm, 50 vol% CH4 ). Switching the fuel from H2 to CH4 changed the Faraday resistance mainly at the low frequency region of impedance spectra suggesting that the low frequency arc is related to anode processes [6]. Faraday resistance of the Cu–Ni/YSZ anode is lower than that of Ni/YSZ anode in both H2 and CH4 fuels. The decrease in the absolute value of RF was due to the longer reduction time of anode in H2 after Cu electroless plating. However, the ratio of Faraday resistance in CH4 to H2 for Cu–Ni/YSZ anode was greater than that for Ni/YSZ anode. Assuming that the absolute value of cathode polarization is constant, this ratio shows a change in the activity of the anode catalyst for CH4 oxidation relative to that of H2 . Table 1 summarizes the performance and the RF (CH4 )/RF (H2 ) for each anode. The larger ratio for Cu–Ni/YSZ anodes suggested that Cu–Ni had lower activity for CH4 oxidation than Ni. Since Cu is an inactive catalyst toward CH4 cracking, reaction (3), the main reaction path for CH4 oxidation would be the direct electro-oxidation of CH4 rather than oxidation of H2 and carbon as CH4 cracking products [25].

-Z" ( .cm

Fig. 2. Schematic of the three phase boundary for the fuel cell anode where O2− in the YSZ phase, electrons in the Ni phase, and hydrogen on the surface of Ni meet.

3

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H2 CH4

12.58 Hz

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4

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0

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Fig. 3. Voltage–current curves and impedance spectra of Ni/YSZ and Cu–Ni/YSZ anode prepared by Cu electroless plating, flowing He/H2 (100 sccm, 50 vol% H2 ) and He/CH4 (100 sccm, 50 vol% CH4 ) at 750 ◦ C (t0 : time at which CH4 entered the anode chamber).

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Table 1 Performance summary of the Ni/YSZ and Cu–Ni/YSZ fuel cells. Cell no.

Operating condition (◦ C)

Anode

OCV (V)

Imax (mA/cm2 )

Pmax (mW/cm2 )

Rohm ( cm2 )

RF ( cm2 )

RF (CH4 )/RF (H2 )

Figures

1

H2 -750 H2 -750 CH4 -750 H2 -750 CH4 -750 H2 -750 CH4 -750

Ni/YSZ Cu–Ni/YSZ Cu–Ni/YSZ Ni/YSZ Ni/YSZ Cu–Ni/YSZ Cu–Ni/YSZ

0.950 0.908 0.820 0.980 0.972 1.05 0.990

887 715 207 815 366 680 227

164 161 48 218 106 155 57

0.279 0.74 0.70 0.33 0.37 0.98 1.6

1.79 0.8 3.10 1.49 3.82 1.05 2.9

3.10/0.8 = 3.87

Figs. 1 and 3

3.82/1.49 = 2.56

Fig. 3

2.9/1.05 = 2.76

Fig. 4

than active sites, i.e., three phase boundary, for electrochemical reactions [32]. The cross-section mapping shows the distribution of carbon formed on the anode surface. The surface of the anode was more prone to coking than the bulk of the anode. The flux of the O2− anion transferred from the cathode side may not sufficiently oxidize all the surface carbon [33]. Carbon formation strongly depends on the Ni crystallite size. Small Ni crystallite size may have high resistance toward carbon formation due to small step edges on the surface [8,34]. The average Ni crystallite size in the anode structure of the cell in this study was 45 nm calculated from Scherrer equation. The concentration of Cu in the Cu–Ni alloy is another effective parameter on the resistance of the anode toward coke formation. The higher Cu concentration has been reported to form less carbon [35]. The Cu–Ni alloy had the composition of Ni0.56 Cu0.44 calculated from Vegard’s law. The lattice parameter was 3.56 A˚ which ˚ The lattice paramis intermediate of aNi = 3.52 A˚ and aCu = 3.65 A. eter was calculated based on the fcc structure of the Cu–Ni alloy [36]. Fig. 7 shows the Raman spectra of the surface of Ni/YSZ anode and Cu–Ni/YSZ anode exposed to CH4 at 750 ◦ C. It should be noted that both anodes were not subjected to the same period of long term studies. The spectra show the characteristic peaks of graphitic carbon (G) at 1581 cm−1 and disordered carbon (D) at 1316 cm−1 and 1615 cm−1 . The difference in the peak intensities and ID /IG ratio suggests the presence of different types of carbon [26]. The ID /IG ratio for the Ni/YSZ anode was decreased after 240 h exposure to CH4 , suggesting the higher formation rate of graphitic carbon.

The SEM/EDS characterization was performed before and after the performance test of Cu–Ni/YSZ anode. Fig. 5 shows the SEM micrographs and EDS elemental mapping of Ni, Zr, and Cu for the Ni/YSZ and Cu–Ni/YSZ anode before the performance test. The EDS mappings revealed that the Cu was located on the Ni particles rather than YSZ. Cu was selectively deposited on the Ni particles and formed Cu–Ni alloy upon reducing in H2 at 750 ◦ C. The Ni particle size has grown after the formation of Cu–Ni alloy. A Cu–Ni alloy network was created on the surface. A similar observation was reported for a deposited Pt film on YSZ film [31]. Fig. 6 shows the carbon mapping on the surface and crosssection of Cu–Ni/YSZ anode exposed to CH4 for 180 h. Carbon mapping on the surface showed local carbonization has occurred inside the pores. The discontinuity between the Ni particles around pore region made the Ni particles serve as cracking catalyst rather

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current density is attributed to the extended TPB via connection of the isolated metal particles by deposited carbon [5]. The low level of deposited carbon improves the electron transfer to the external circuit. Further deposition of carbon would decrease gas diffusion and cause the mass transfer polarization by blocking the pores [26–28]. The impedance spectra were recorded at OCV condition which corresponds to the low current region of the V–I curve, representing the activation polarization. The increase in the Faraday resistance resulting from anode deactivation was consistent with the increase in the slope of the V–I curve at low current region [29,30].

2512 Hz

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3

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0.79 Hz 79.43 Hz

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5011 Hz

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Fig. 4. Voltage–current curves and impedance spectra of Cu–Ni/YSZ flowing He/H2 (100 sccm, 50 vol% H2 ) and He/CH4 (100 sccm, 50 vol% CH4 ) at 750 ◦ C (t0 : time at which CH4 entered the anode chamber).

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Fig. 5. SEM image and EDS mapping of Ni, Zr, and Cu on the surface of Ni/YSZ anode before and after Cu electroless plating.

Graphitic carbon has been reported as the carbon type that forms on Ni at temperatures above 725 ◦ C and causes the catalyst deactivation [1,37–39]. The reason for low intensity of the D and G peaks for the Ni/YSZ anode compared to the Cu–Ni/YSZ anode is that the Ni/YSZ anode was maintained at dry reforming condition (CH4 + CO2 ) to allow its long-term operation [33,40]. Comparing

the Raman spectra of Cu–Ni/YSZ anode after 2 h and 180 h exposure to CH4 revealed that both graphitic and disordered carbon were formed at the initial stage and grew at different rates. Long time exposure of Cu–Ni/YSZ to CH4 produced more disordered carbon, evidenced by increased intensity of D peaks at 1316 and 1615 cm−1 .

Fig. 6. SEM image and EDS mapping of Cu–Ni/YSZ anode after 180 h exposure to CH4 fuel at 750 ◦ C.

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1581 G

Less Cu/Ni ratio across the anode makes the catalyst prone to the formation of graphitic carbon.

1316 D

100

1615 D

Fig. 8. Raman spectra of carbon at cross-section of the Ni/YSZ after 2 h and Cu–Ni/YSZ anode after 180 h exposure to CH4 fuel at 750 ◦ C. The XRF elemental mapping of Cu shows the gradient in the Cu concentration in wt% along the anode cross-section.

4. Conclusions

Intensity (a.u.)

Ni/YSZ ( 2 h CH4) Ni/YSZ (240 h CH4/CO2) Cu-Ni/YSZ (2 h CH4)

Cu-Ni/YSZ (180 h CH4)

2000

1500 -1 Raman shift (cm )

1000

Fig. 7. Raman spectra of carbon formed on the surface of Ni/YSZ and Cu–Ni/YSZ anode after exposure to CH4 fuel at 750 ◦ C.

The electroless plated Cu–Ni/YSZ anode exhibited significantly higher stability, but a greater RF (CH4 )/RF (H2 ) ratio than Ni/YSZ. The Raman spectra of carbon revealed that Cu–Ni alloy produced graphitic carbon at a lower rate than Ni alone in CH4 –SOFC. The results suggest that electroless plated Cu can be effective in blocking the formation of graphitic carbon. Further improvement of Ni/YSZ anode stability in CH4 may be achieved by controlling the Cu distribution. Acknowledgments The authors acknowledge the financial support received for this research from the Department of Energy (DE-FC36 06GO86055), Ohio Coal Development Office, and FirstEnergy Corporation. The authors gratefully acknowledge Dr. Yu-Wen Chen for careful proofreading and Mr. Long Zhang for his help in collecting the Raman spectra. References

Fig. 8 shows the Raman spectra of the cross-section of the Ni/YSZ anode after 2 h and Cu–Ni/YSZ anode after 180 h exposure to CH4 at 750 ◦ C. It should be noted that Ni/YSZ anode was rapidly deactivated in 2 h. Further CH4 exposure led to cracking of the anode. The spectra for both anodes showed a decrease in carbon intensity with increasing the scanning depth below the anode surface. The disordered carbon (ID ) and graphitic carbon (IG ) at 30 ␮m depth for Ni/YSZ anode with 2 h CH4 exposure and Cu–Ni/YSZ anode with 180 h CH4 exposure exhibited the same intensity, indicating that electroless plated Cu significantly decreased the formation rates of both types of carbon. The ID /IG ratio remained almost constant along the cross-section of Ni/YSZ anode. The ID /IG ratio decreased across the cross-section of Cu–Ni/YSZ anode suggesting the presence of graphitic carbon in regions far away from the anode surface. The XRF elemental mapping for Cu at the same cross-section shows a gradual decrease in Cu concentration from the anode surface down to 100 ␮m of the bulk anode. The bulk anode contains less Cu than the surface anode.

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Please cite this article in press as: A. Rismanchian, et al., Electroless plated Cu–Ni anode catalyst for natural gas solid oxide fuel cells, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.05.012