Fabrication and structural properties of porous Cu–YSZ cermets for solid oxide fuel cells

    Fabrication and structural properties of porous Cu-YSZ cermets for solid oxide fuel cells C.M. Kim, J. Kim, K. Park PII: DOI: Reference:

S0032-5910(14)00016-3 doi: 10.1016/j.powtec.2014.01.007 PTEC 9935

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Powder Technology

Received date: Revised date: Accepted date:

21 March 2013 30 November 2013 6 January 2014

Please cite this article as: C.M. Kim, J. Kim, K. Park, Fabrication and structural properties of porous Cu-YSZ cermets for solid oxide fuel cells, Powder Technology (2014), doi: 10.1016/j.powtec.2014.01.007

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Fabrication and structural properties of porous Cu-YSZ cermets for solid

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C. M. Kim, J. Kim, K. Park*

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oxide fuel cells

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Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University,

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Seoul 143-747, Korea

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Abstract

In this study, porous Cu-YSZ cermets with various Cu contents are fabricated and their

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crystal structure and morphological features are investigated by X-ray diffraction and scanning

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electron microscopy. Carbon black-containing CuO/Y2O3-stabilized ZrO2 (YSZ) composites

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exhibit a highly porous structure due to the evolution of CO and CO2 gases during sintering. The porosity of the CuO/YSZ composites decreases with increases of the sintering temperature and CuO content. Highly porous Cu/YSZ cermets are fabricated by reducing the porous CuO/YSZ composites in (Ar + 6% H2) atmosphere. Higher CuO content and higher sintering temperature reduce the porosity of Cu/YSZ cermets after reduction treatment. The microstructure and porosity of Cu/YSZ cermets as an anode material are remarkably affected by the CuO content, sintering temperature, and carbon black content. We believe that the porous Cu/YSZ cermets are a promising anode material for high performance of SOFC.

Keywords: solid state reaction; sintering; microstructure; porosity; SOFC

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Tel: 82-2-3408-3777; fax: 82-2-3408-4342; E-mail: [email protected]

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*

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Introduction

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Solid oxide fuel cells (SOFCs) have been considered for alternate energy source because of the environmental contamination associated with the usage of fossil fuels and their limited

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availability. The SOFC is an electrochemical device that converts the chemical energy of a fuel

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into electrical energy in a clean, cheap, and efficient way. The electrical energy is obtained by the reduction of O2 to O2- anions at the cathode, the diffusion of the O2- anions through an electrolyte,

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and finally the oxidation of the fuel by O2- at the anode [1]. The electrolyte is an electronic

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insulator and possesses high ionic conductivity that conducts O2- ions. The yttria-stabilized

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zirconia (YSZ) is conventionally used in the electrolyte. Its electronic conductivity must be kept as low as possible to prevent losses from leakage currents. Cathode and anode materials are electronically conductive. The coefficient of thermal expansion (CTE) of cathode and anode materials is similar to that of the electrolyte to minimize stresses built up because of their CTE mismatch.

Ni/YSZ cermet is generally used for the anode. Ni in the anode makes the anode an electronic conductor and is used as a hydrocarbon reforming catalyst [2]. However, Ni causes a serious coking problem for Ni/YSZ cermet under high steam-to-hydrocarbon ratios. In dry CH4, Ni forms carbon fibers above 700 °C, in which these fibers can completely fill the anode compartment. At the same time, other problems of Ni include redox instability and poisoning by fuel impurities such as sulfur [3]. Thus, Gorte et al. [2] used Co and Fe instead of Ni in the Ni/YSZ cermet. They exhibited similar catalytic properties to that of Ni and suffered similar

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carbon formations. As a result, recently several alternative anode materials have been proposed to fabricate high-performance SOFCs. Among them, Cu-based cermet is one of the most promising

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anode materials. It is considered that Cu is an excellent current collector with a poor catalytic

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activity towards hydrocarbon oxidation [4]. It has a good oxidation resistance in the cermet and is

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also relatively inexpensive [2,5]. From the viewpoint of the operation of SOFCs, the operation at high temperatures (900–1000 °C) leads to serious problems, including a chemical reaction

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between components, thermal degradation of materials, and cracking during cycles owing to the

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CTE mismatch [6]. It is thus necessary to lower the operating temperature of SOFCs. A great deal of attention has been paid to intermediate temperature SOFC which typically operates

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between 550 and 800 °C [5].

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A network of pores in the anode is crucial to provide a diffusion path for gaseous reactants

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and products. These functions depend strongly on the morphological features of the anode and the size of pores [7,8]. An appropriate size of the grains and pores in the anode is needed in allowing rapid diffusion of gases to the active reaction area [9]. High diffusion rate provides the anode with greater access to and from the active three-phase boundary (TPB) regions where the reactions occur and therefore reduces polarization due to the gas diffusion resistance. A finesized grain in the anode significantly increases the reaction area. In addition, small pores substantially reduce the gas diffusion and limit the overall reaction rate. Thus, to increase the pore size and porosity, pore formers such as graphitic carbon, rice or corn starch, graphite, short carbon fibers, and polymer spheres were used [9,10]. The pore formers used were oxidized over 600 °C, remaining pores inside the anode after sintering [11]. However, too high porosity and too large pore reduce the amount of total TPB area available for reaction. To improve the stability and performance of the anode in SOFC applications, it is thus necessary to fabricate the anode

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with an appropriate porosity and pore size. In this study, porous Cu/YSZ cermets with various Cu contents are fabricated by reducing

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CuO/YSZ composites, considering the numerous advantages of Cu discussed previously.

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Subsequently, the crystal structure and morphological features of the Cu/YSZ cermets are

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investigated, depending on the CuO content, sintering temperature, and carbon black content.

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Experimental

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In the present study, CuO (99.9% purity, Kojundo Chemical, Japan), yttria-stabilized zirconia (YSZ; Tosoh Co.), and carbon black (Alfa Aesar Johnson Matthey Co.) powders were

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used as starting materials. The weight of the CuO, YSZ, and carbon black powders used is given

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in Table 1. The powders were weighed in specific proportions and mixed with ethanol for 5 h at

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350 rpm using a planetary mill (Fritsch Pulverisette 6) and ZrO2 balls. The mixture was dried at 60 °C in an oven for 12 h. The planetary-milled powders were then carefully ground in mortar and passed through a 200-mesh sieve. Subsequently, the sieved powders were pressed at a uniaxial pressure of 70 MPa to prepare green pellets of 3 mm-thick and 10-mm in diameter. The pellets were sintered at two different temperatures (1050 and 1100 °C) for 3 h in air, and then furnace cooled. The sintered samples were reduced at 800 °C in (Ar + 6% H2) atmosphere for 3 h to fabricate Cu-YSZ cermets. The crystal structure of the sintered and reduced samples was analyzed with an X-ray diffractometer (XRD; Rigaku DMAX-2500) using Cu Kα radiation at 40 kV and 100 mA. The microstructure of the sintered and reduced samples was investigated with a field emission scanning electron microscope (FE-SEM, Hitachi S-4700). The porosity of the sintered and the reduced samples was measured by the Archimedes method.

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Results and discussion Fig. 1 shows the XRD patterns of the mixed powders of CuO and YSZ. The mixed powders

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consist of the CuO and YSZ without any impurities [12-14]. CuO crystallizes in a monoclinic

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structure (a=4.682 Å, b=3.424 Å, and c=5.127 Å) along with a space group of C12/c1. The CuO

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structure can be expressed as a complex of ordered chain fragments. Cu atoms in chains have an octahedral distorted oxygen coordination: four nearest oxygen atoms are located at the distance of

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1.96 Å and two at 2.78 Å [14]. YSZ crystallizes in a cubic structure (a=5.139 Å). Zirconia has

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three different polymorphs: monoclinic (m-ZrO2; space group P21/c), tetragonal (t-ZrO2; space group P42/nmc), and cubic (c-ZrO2; space group Fm3m) [15]. As the CuO content in the mixed

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powders increases, as expected, the intensity of CuO peaks increases, whereas that of YSZ peaks

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decreases. The XRD patterns of the mixed powders of CuO, YSZ, and carbon black are shown in

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Fig. 2. As was mentioned in Fig. 1, with increasing the CuO content in mixed powders, the intensity of CuO peaks increases, while that of YSZ peaks decreases. No impurity peaks are detected in Fig. 2.

A typical FE-SEM image and particle diameter of planetary-milled sample 50AC powders are shown in Figs. 3(a) and (b), respectively. The powders of CuO and YSZ show a smooth surface and spherical-like morphology. However, there is a distinct difference in the sizes of the two powders, i.e., 660 and 100 nm for CuO and YSZ powders, respectively. In particular, YSZ powders show a narrow size distribution. The densification of the green pellets, consisted of CuO and YSZ, during sintering is considered to first occur in the CuO powders and the YSZ powders are densified later because of the difference in the densification temperatures of CuO and YSZ powders [16]. The YSZ suppresses the grain growth of CuO powders during sintering. The characteristics of other planetary-milled samples are nearly the same as those of planetary-milled

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sample 50AC. Fig. 4 shows the XRD patterns from the carbon black-free CuO/YSZ composites sintered at

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1100 °C in air. The XRD peaks are sharpened after sintering at 1100 °C, indicating an increased

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crystallite size [17]. YSZ in the sintered samples crystallizes in both the cubic and the monoclinic

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structures. The presence of monoclinic YSZ phase is due to the fact that some of the YSZ phase is transformed from the cubic structure to the monoclinic structure by introducing CuO into the The

formation

of monoclinic YSZ phase

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zirconia lattice.

was

reported

for the

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YSZ/(La0.8Sr0.2)0.98Fe0.98Cu0.02O3-δ, YSZ/CuNb2O6, and YSZ/CuO composite systems, which was attributed to the reaction of two components in the composite systems [4]. The reaction of YSZ

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with the Cu-containing oxides at relatively low temperatures (900-1000 °C) for 2-3 h yielded a

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monoclinic YSZ phase. A possible reason for the formation of monoclinic YSZ phase is as

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follows: CuO interacts with Y3+ to produce m-ZrO2 from the cubic YSZ [4]. This means that CuO is responsible for the formation of monoclinic YSZ. The quantity of monoclinic zirconia formed did not depend on the CuO content above 5 wt%. The sintering condition is below 900 °C under oxidizing atmospheres to inhibit YSZ destabilization. The m-ZrO2 exhibits a very low conductivity and degrades the mechanical properties due to the phase transformation to tetragonal structure at high temperatures or the hydrothermal ageing at low temperatures [4]. XRD patterns of the CuO/YSZ composites sintered at 1050 °C are the same as those at 1100 °C, except for the peak intensity (not shown here). The CuO/YSZ composites sintered at 1050 °C show more broad peaks in comparison with those at 1100 °C. The crystal structure of CuO/YSZ composites with 5 wt% carbon black is basically equivalent to that of carbon black-free CuO/YSZ composites (Fig. 5).

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Fig. 6 exhibits the XRD patterns of samples 30AC, 40AC, 50AC, 60AC, and 70AC sintered at 1100 °C and then reduced at 800 °C in (Ar + 6% H2) atmosphere. No peaks corresponding to

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the CuO are detected. This means that by means of the reduction treatment, the CuO phase

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disappears and Cu is formed [18]. It is likely that the Cu is formed by the reduction of CuO to

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Cu2O and then Cu2O to Cu, with Cu2O as an intermediate of the reduction [19]. The onset of reduction occurs at about 250 °C and this is finished at 430-560 °C, depending on the Cu content

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in titania-doped yttria stabilized zirconia (Y0.2Ti0.18Zr0.62O1.9) cermet. The upper temperature

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increases for higher Cu contents. The oxidation process is not completed until the temperature approaches 600 °C owing to the slower diffusion of oxygen into Cu in comparison with the

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diffusion of oxygen from CuO [20]. YSZ in the reduced samples, as with the sintered ones, has

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the cubic and monoclinic structures. As the CuO content increases, the amount of metallic Cu

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increases, while that of cubic YSZ and monoclinic ZrO2 decreases. Fig. 7 shows FE-SEM images of the carbon black-free CuO/YSZ composites sintered at 1100 °C in air. The density of the sintered CuO/YSZ composites increases with an increase in CuO content. A well-defined, open, and uniform porous microstructure can be observed in a wide range. In this figure, the CuO phase remains as largely isolated particles after sintering at this temperature. The CuO is molten so that it wets on the YSZ particles. The size of the CuO phase is greatly increased due to the diffusion of CuO during the sintering process. It has been previously reported that in addition to the volume and grain boundary diffusions during sintering, other two mechanisms of material transport, i.e., evaporation and condensation of atoms and surface diffusion, exist [21]. The condensation of atoms takes place preferentially on the concave surface, i.e., on the pore, where the vapor pressure is low, while the surface diffusion occurs from the convex to the concave surface, where the particle contacts initiate [22]. The morphological

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features of carbon black-containing CuO/YSZ composites are basically the same as those of carbon black-free CuO/YSZ composites. However, the carbon black-containing CuO/YSZ

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composites exhibit more porous structure compared to carbon black-free CuO/YSZ composites

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(not shown here). This is due to the CO and CO2 gases extracted by the exothermic reactions of

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carbon black and oxygen during sintering [23].

The apparent porosities of the CuO/YSZ composites as functions of CuO content, carbon

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black content, and sintering temperature are shown in Fig. 8. At a given sintering temperature, by

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increasing the CuO content, the porosity of the CuO/YSZ composites tends to decrease with CuO content, e.g., 50.6, 48.4, 47.2, 41.5, and 33.1% for samples 30AC, 40AC, 50AC, 60AC, and

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70AC sintered at 1050 °C, respectively. With the same CuO content, the porosity of the

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CuO/YSZ composites sintered at 1050 °C is higher than that at 1100 °C. In addition, with the

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same CuO content and sintering temperature, the porosity of CuO/YSZ composites with 5 wt% carbon black is higher than that of carbon black-free CuO/YSZ composites. This means that the use of carbon black as a pore-former is a highly effective way to produce porous CuO/YSZ composites. For example, the porosities of samples 30AC and 30BC sintered at 1100 °C are 44.5 and 57.1%, respectively. The apparent porosities of CuO/YSZ composites with various carbon black contents, as a function of CuO content and sintering temperature, are summarized in Table 2. Fig. 9 exhibits the surface morphology of the porous Cu/YSZ cermets sintered at 1100 °C and then reduced at 800 °C in (Ar + 6% H2) atmosphere. The porous structure can provide sufficient TPB length and be beneficial for the transport of fuel gas and the diffusion of the exhaust gas, therefore promoting the reaction rate [24]. At a fixed amount of CuO, the apparent porosity of the Cu-YSZ cermets is higher than that of the CuO-YSZ composites because of the

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oxygen extraction by means of the change of CuO to Cu during the reduction treatment [7,25]. The isolated Cu is observed due to the non-wetting interaction between the Cu and YSZ network.

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The Cu and YSZ provide continuous electronic and ionic pathways in SOFC application,

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respectively. The apparent porosity of the Cu-YSZ cermets, as with the CuO-YSZ composites,

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decreases with an increase in CuO content. The carbon black-containing Cu/YSZ cermets show similar morphology as the carbon black-free Cu/YSZ cermets, but exhibit more porous structure

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compared to carbon black-free Cu/YSZ cermets. Higher porosity was obtained by reducing the

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CuO/YSZ cermets sintered at lower temperature (1050 °C) (not shown here). Fig. 10 shows the apparent porosity of the reduced Cu/YSZ cermets without carbon black.

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The porosity of the Cu/YSZ cermets depends strongly on the CuO content and sintering

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temperature. The reduction treatment leads to an increase in the porosity due to the oxygen

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extraction in CuO during reduction treatment [7,25]. A similar behavior was reported for the composite based on CuO and Y0.2Ti0.18Zr0.62O1.9. The porosity of the CuO/Y0.2Ti0.18Zr0.62O1.9 composite sintered at 1000 °C for 10 h was 40% and the porosity was 50% after reduction [19]. Higher CuO content yields lower porosity mainly because the CuO/YSZ composites with higher CuO content have lower porosity. At a fixed amount of CuO, samples sintered at 1050 °C show higher porosity by approximately 12% than those sintered at 1100 °C. In general, it is known that a suitable porosity for SOFC applications is approximately 40% for the anode [25]. In this respect, sample 60AC sintered at 1100 °C as well as sample 70AC sintered at 1050 °C are desirable for anode application. Furthermore, it was found that the reduced Cu/YSZ cermets with 5 wt% carbon black were highly porous and contained cracks. Therefore, the porosity of the Cu/YSZ cermets with 5 wt% carbon black was not measured. Table 3 shows the apparent porosities of the Cu/YSZ cermets, considering the CuO content and sintering temperature. The porosity of

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Cu/YSZ cermets can be controlled by adjusting the CuO content, sintering temperature, and

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carbon black content.

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4. Conclusions

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We presented the crystal structure and microstructural properties of porous Cu-YSZ cermets, prepared by reducing CuO/YSZ composites, as an anode material in SOFC. Porous CuO/YSZ

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composites were fabricated via the planetary milling of CuO and YSZ powders followed by the

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sintering of CuO/YSZ green pellets in air. The carbon black-containing CuO/YSZ composites showed higher porosity compared to carbon black-free CuO/YSZ composites due to the evolution

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of CO and CO2 gases during sintering. Also, the porosity of the CuO/YSZ composites became

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increased with a decrease in sintering temperature. Highly porous Cu/YSZ cermets were

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fabricated by reducing the CuO/YSZ composites upon exposure to (Ar + 6% H2) gas. The high porosity was obtained by the oxygen extraction by means of the reduction of CuO to Cu during reduction treatment. The porosity of the Cu/YSZ cermets decreased with increasing the CuO content. It was found that the CuO content, sintering temperature, and carbon black content substantially affected the microstructural features and porosity of Cu/YSZ cermets. We obtained appropriate CuO content and sintering temperature for fabricating porous Cu/YSZ cermets. The porous Cu/YSZ cermets fabricated in this study are a promising anode material for high performance of SOFC.

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P.E. Strizhak, Structure and state of copper oxide species supported on yttria-stabilized zirconia, J. Phys. Chem. C 113 (2009) 21368–21375. [15] V. Ramaswamy, M. Bhagwat, D. Srinivas, A.V. Ramaswamy, Structural and spectral features of nano-crystalline copper-stabilized zirconia, Catalysis Today 97 (2004) 63-70. [16] M.G. Catherine, G.G. Richard, B.G. Javier, Control of microstructure, sinterability and performance in Co-precipitated Ni-YSZ, Cu-YSZ and Co-YSZ SOFC anodes, Journal of Materials Chemistry 16 (2006) 885-897. [17] S. Bandyopadhyay, H. Dutta, S.K. Pradhan, XRD, HRTEM characterization of mechanosynthesized Ti0.9W0.1C cermet, Journal of Alloys and Compounds 581 (2013) 710– 716.

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[18] D.G. Kim, K.W. Lee, S.T. Oh, Y.D. Kim, Preparation of W-Cu nanocomposite powder by hydrogen-reduction of ball-milled W and CuO powder mixture, Materials Letters 58 (2004)

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[21] F. Thümmler, R. Oberacker, An Introduction to Powder Metallurgy, The Institute of

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[22] A.M. Maliska, H.C. Pavanati, A.N. Klein, J.L.R. Muzart, The influence of ion energy bombardment on the surface porosity of plasma sintered iron, Materials Science and Engineering A 352 (2003) 273-278. [23] K. Park, K.U. Jang, Response characteristics to reducing gases in BaTiO3-based thick films, Journal of Alloys and Compounds 391 (2005) 123-128. [24] K. Chen, Z. Lü, X. Chen, N. Ai, X. Huang, B. Wei, J. Hu, W. Su, Characteristics of NiO/YSZ anode based on NiO particles synthesized by the precipitation method, Journal of Alloys and Compounds 454 (2008) 447-453. [25] J.-H Lee, H Moon, H.-W Lee, J Kim, J.-D Kim, K.-H Yoon, Quantitative analysis of microstructure and its related electrical property of SOFC anode, Ni–YSZ cermet, Solid State Ionics 148 (2002) 15-26.

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Tables

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Table 1.Weight of the CuO, YSZ, and carbon black powders used. Table 2. Porosity of CuO/YSZ composites as a function of CuO content, carbon black content,

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and sintering temperature.

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Table 3. Porosity of Cu/YSZ cermets as a function of CuO content and sintering temperature.

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Figures

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Fig. 1. XRD patterns of the commercial (a) CuO and (b) YSZ powders and of the mixed powders of samples (c) 30AC, (d) 40AC, (e) 50AC, (f) 60AC, and (g) 70AC.

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Fig. 2. XRD patterns of the commercial (a) CuO and (b) YSZ powders and of the mixed powders

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of samples (c) 30BC, (d) 40BC, (e) 50BC, (f) 60BC, and (g) 70BC.

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Fig. 3. (a) FE-SEM image and (b) particle size distribution of mixed powders of sample 50AC. Fig. 4. XRD patterns of the carbon black-free CuO/YSZ composites sintered at 1100 °C for 3 h: samples (a) 30AC, (b) 40AC, (c) 50AC, (d) 60AC, and (e) 70AC.

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Fig. 5. XRD patterns of the carbon black-containing CuO/YSZ composites sintered at 1100 °C for 3 h: samples (a) 30BC, (b) 40BC, (c) 50BC, (d) 60BC, and (e) 70BC.

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Fig. 6. XRD patterns of the Cu/YSZ cermets sintered at 1100 °C for 3 h and then reduced at 800 °C for 3 h: samples (a) 30AC, (b) 40AC, (c) 50AC, (d) 60AC, and (e) 70AC.

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Fig. 7. SEM images of the carbon black-free CuO/YSZ composites sintered at 1100 °C for 3 h: samples (a) 30AC, (b) 40AC, (c) 50AC, (d) 60AC, and (e) 70AC. Fig. 8. Porosity of the carbon black-free CuO/YSZ composites sintered at (a) 1050 and (b) 1100 ℃ and of the carbon black-containing CuO/YSZ composites sintered at (c) 1050 and (d) 1100 ℃.

Fig. 9. SEM images of the Cu/YSZ cermets sintered at 1100 °C for 3 h and then reduced at 800 °C for 3 h: samples (a) 30AC, (b) 40AC, (c) 50AC, (d) 60AC, and (e) 70AC. Fig. 10. Porosity of the Cu/YSZ cermets sintered at (a) 1050 and (b) 1100 °C and then reduced at 800 °C.

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Table 1.Weight of the CuO, YSZ, and carbon black powders used. Composition (wt%)

Sample 30AC

CuO:YSZ=30:70

Sample 40AC

CuO:YSZ=40:60

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Sample

CuO:YSZ=50:50

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Sample 50AC

CuO:YSZ=60:40

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Sample 60AC Sample 70AC

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Sample 30BC Sample 40BC

CuO:YSZ:C*=30:70:5 CuO:YSZ:C*=40:60:5 CuO:YSZ:C*=50:50:5 CuO:YSZ:C*=60:40:5

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Sample 60BC

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Sample 50BC

CuO:YSZ=70:30

CuO:YSZ:C*=70:30:5

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Sample 70BC

C* indicates carbon black.

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Table 2. Porosity of CuO/YSZ composites as a function of CuO content, carbon black content,

Sample

Sintering temperature (°C)

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1050 Sample 30AC 1050

44.5 48.4 44.1 47.2

1100

43.2

1050

41.5

1100

35.1

1050

33.1

1100

26.7

1050

61.7

1100

57.1

1050

57.4

1100

55.3

1050

55.5

1100

54.1

1050

51.3

1100

45.2

1050

43.4

1100

37.5

D

AC CE P

Sample 30BC

TE

Sample 70AC

50.6

1050 Sample 50AC

Sample 60AC

Porosity (%)

MA

NU

1100

SC

1100 Sample 40AC

PT

and sintering temperature.

Sample 40BC

Sample 50BC

Sample 60BC

Sample 70BC

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Reduction temperature (°C)

1050

800

1100

800

RI

50.3

1050

800

57.6

800

50.1

800

55.2

800

49.3

800

47.1

1100

800

42.8

1050

800

42.3

1100

800

34.1

Sample 30AC

Sample 40AC

NU

1100 1050 1100 1050

AC CE P

Sample 70AC

D

Sample 60AC

MA

Sample 50AC

~ 18 ~

PT

Sintering temperature (°C)

SC

Sample

TE

Table 3. Porosity of Cu/YSZ cermets as a function of CuO content and sintering temperature. Porosity (%) 59.8

TE

D

MA

NU

SC

RI

PT

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AC CE P

Fig. 1. XRD patterns of the commercial (a) CuO and (b) YSZ powders and of the mixed powders of samples (c) 30AC, (d) 40AC, (e) 50AC, (f) 60AC, and (g) 70AC.

~ 19 ~

TE

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC CE P

Fig. 2. XRD patterns of the commercial (a) CuO and (b) YSZ powders and of the mixed powders of samples (c) 30BC, (d) 40BC, (e) 50BC, (f) 60BC, and (g) 70BC.

~ 20 ~

AC CE P

TE

D

MA

NU

SC

RI

PT

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Fig. 3. (a) FE-SEM image and (b) particle size distribution of mixed powders of sample 50AC.

~ 21 ~

TE

D

MA

NU

SC

RI

PT

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AC CE P

Fig. 4. XRD patterns of the carbon black-free CuO/YSZ composites sintered at 1100 °C for 3 h: samples (a) 30AC, (b) 40AC, (c) 50AC, (d) 60AC, and (e) 70AC.

~ 22 ~

TE

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC CE P

Fig. 5. XRD patterns of the carbon black-containing CuO/YSZ composites sintered at 1100 °C for 3 h: samples (a) 30BC, (b) 40BC, (c) 50BC, (d) 60BC, and (e) 70BC.

~ 23 ~

TE

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig. 6. XRD patterns of the Cu/YSZ cermets sintered at 1100 °C for 3 h and then

AC CE P

reduced at 800 °C for 3 h: samples (a) 30AC, (b) 40AC, (c) 50AC, (d) 60AC, and (e) 70AC.

~ 24 ~

AC CE P

TE

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig. 7. SEM images of the carbon black-free CuO/YSZ composites sintered at 1100 °C for 3 h: samples (a) 30AC, (b) 40AC, (c) 50AC, (d) 60AC, and (e) 70AC.

~ 25 ~

TE

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC CE P

Fig. 8. Porosity of the carbon black-free CuO/YSZ composites sintered at (a) 1050 and (b) 1100 ℃ and of the carbon black-containing CuO/YSZ composites sintered at (c) 1050 and (d) 1100 ℃.

~ 26 ~

AC CE P

TE

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig. 9. SEM images of the Cu/YSZ cermets sintered at 1100 °C for 3 h and then reduced at 800 °C for 3 h: samples (a) 30AC, (b) 40AC, (c) 50AC, (d) 60AC, and (e) 70AC.

~ 27 ~

TE

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC CE P

Fig. 10. Porosity of the Cu/YSZ cermets sintered at (a) 1050 and (b) 1100 ℃ and then reduced at 800 ℃.

~ 28 ~

AC CE P

TE

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Graphical abstract

~ 29 ~

ACCEPTED MANUSCRIPT

Highlights

PT

▶ Porous CuO-YSZ composites are fabricated by sintering the CuO/YSZ green pellets. ▶ Highly

RI

porous Cu-YSZ cermets are fabricated by reducing the CuO/YSZ composites. ▶ The porous Cu-

AC CE P

TE

D

MA

NU

SC

YSZ cermet is a promising anode material for high performance of SOFC.

~ 30 ~