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Porous NiO–YSZ and Ni–YSZ composites fabricated using NiO–YSZ composite nanopowders and WO3 additive
Materials Characterization 160 (2020) 110113
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Porous NiO–YSZ and Ni–YSZ composites fabricated using NiO–YSZ composite nanopowders and WO3 additive J. Kima, G.H. Kimb, K. Parka, a b
T
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Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 05006, Republic of Korea Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 13679, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Composites Microstructure Porosity Electron microscopy Amode SO FC
NiO–YSZ composite nanopowders were synthesized through a hydrothermal process without an additional mixing of NiO and YSZ nanopowders. Porous Ni–yttria stabilized zirconia (YSZ) composites, Ni–YSZ composites, with high porosity (25.0–32.8%) were fabricated by reducing NiO-YSZ composites in (94% Ar + 6% H2) gas. The electrical conductivity (14,221 cm−1 at 30 °C) and bending strength (130 MPa) of Ni–YSZ composites were enhanced by the addition of WO3 (0.1 wt% WO3) to NiO–YSZ composite nanopowders. Furthermore, the Y2(WO4)3 with a negative coefficient of thermal expansion (CTE) reduced the CTE of Ni–YSZ composites. The CTE values of 0 and 0.6 wt% WO3–added Ni–YSZ composites were 13.8 × 10−6 and 12.4 × 10−6 °C−1, respectively. We demonstrate that the fabrication of NiO–YSZ composite nanopowders and the addition of WO3 to the NiO–YSZ composite nanopowders are highly effective methods for the fabrication of high-quality Ni–YSZ anode materials in solid oxide fuel cells.
1. Introduction
extend the applications of Ni–YSZ composites as anode materials of SOFCs, the current research focuses on enhancing the electrical conductivity and bending strength of Ni–YSZ composites and on reducing the coefficient of thermal expansion (CTE) of Ni–YSZ composites through the addition of WO3 to (65 wt% NiO + 35 wt% YSZ) composite nanopowders (hereafter referred to as NiO–YSZ composite nanopowders). In this work, we directly synthesized NiO–YSZ composite nanopowders through a hydrothermal process without an additional mixing of NiO and YSZ nanopowders. Furthermore, we added WO3 powders to the NiO–YSZ composite nanopowders as a sintering aid. WO3 was previously utilized as a sintering aid in NiCuZn ferrites [12,13]. Here, we investigated the microstructure, electrical conductivity, bending strength, and CTE properties of porous Ni–YSZ composites prepared by reducing the NiO–YSZ composites. The addition of WO3 (0.1 wt% WO3) is highly effective for increasing the electrical conductivity and bending strength of Ni–YSZ composites and for reducing the CTE of Ni–YSZ composites.
Solid oxide fuel cells (SOFCs) can directly convert the chemical energy of fuels, such as hydrogen, methane, and methanol, into electrical energy [1–3]. SOFCs are efficient power–generating systems with high energy conversion efficiency, high fuel flexibility, and environmentally friendly nature in comparison with other fuel cell systems [4–6]. The energy conversion efficiency of SOFCs depends strongly on the microstructural and electrical properties of the cathode, electrolyte, and anode [7,8]. Among these components, the anode plays the most significant role in oxidizing fuels to generate power [5]. Ni–yttria stabilized zirconia (YSZ) composites, Ni–YSZ composites, are the most widely used materials for anodes due to its excellent physical, thermal, and mechanical compatibility with other SOFC components, high electrocatalytic activity for H2 oxidation, and low fabrication cost [5,7,9]. Traditionally, Ni–YSZ composites have been prepared by two different processes: (1) mechanical mixing of large–sized Ni and YSZ powders followed by a sintering of Ni–YSZ green bodies and (2) mechanical mixing of large–sized NiO and YSZ powders followed by a reduction of NiO–YSZ composites [6,10,11]. The large–sized Ni and YSZ grains in Ni–YSZ composites fabricated by the aforementioned two processes were not uniformly distributed, thereby yielding insufficient reactive sites and a corresponding low conversion efficiency. To further ⁎
Corresponding author. E-mail address: [email protected] (K. Park).
https://doi.org/10.1016/j.matchar.2019.110113 Received 15 October 2019; Accepted 27 December 2019 Available online 28 December 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
2. Experimental Composite nanopowders of NiO and 8 mol% YSZ (65 wt% NiO + 35 wt% YSZ) were prepared by a hydrothermal process. Ni (NO3)2·6H2O, ZrOCl3·8H2O, and YCl3·6H2O were used as starting
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[14,15]. The prepared NiO–YSZ composite nanopowders are composed of NiO and YSZ nanopowders without any impurities [Fig. 1(B)]. Both the NiO and YSZ nanopowders at room temperature crystallize in the cubic crystal structure [16,17]. The crystallite size (D) of NiO–YSZ composite nanopowders can be calculated from the Scherrer formula [18]: D = (0.9λ) / (βcosθ), where λ is the wavelength of radiation, β is the corrected full width at half maximum of the diffraction peak (in radian), and θ is the angle of the diffraction peak. The calculated crystallite size of NiO–YSZ composite nanopowders is 53 nm. Fig. 1(C) shows the XRD patterns from the sintered NiO–YSZ composites. The NiO–YSZ composites with ≤0.3 wt% WO3 consist of only NiO and YSZ phases. The two phases have a cubic crystal structure at room temperature [16,17], as in the NiO–YSZ composite nanopowders. No un-reacted oxides are detected. On the other hand, the NiO–YSZ composites with ≥0.4 wt% WO3 contain the NiWO4 secondary phase along with the NiO and YSZ phases. The NiWO4 may be formed by the following reaction [19]: NiO + WO3 → NiWO4. The compounds formed by the reaction of WO3 and YSZ are not detected. Fig. 1(D) exhibits the XRD patterns of Ni–YSZ composites fabricated by reducing the NiO–YSZ composites in (94% Ar + 6% H2) atmosphere. No XRD peaks derived from NiO are detected. This means that the NiO phase disappears through the reduction treatment and that Ni is formed. In the Ni–YSZ composites, Ni has the face–centered cubic (fcc) crystal structure, and YSZ has the cubic crystal structure. NiWO4 phase is not observed in the XRD patterns. FE–SEM images of the sintered NiO–YSZ composites are shown in Fig. 2(A). The NiO–YSZ composites show porous structures and small grains. The exothermic reaction of carbon black powders and oxygen during sintering releases CO and CO2 gases, thereby forming porous structure [20,21]. The porous structure enhances the diffusion of reduction gas within the composites during the subsequent reduction treatment. The sintered NiO–YSZ composites consist of small grains, which are attributed to the fact that the hydrothermal–synthesized NiO–YSZ composite nanopowders are used to prepare the composites. YSZ grains in the composites suppress the grain growth of NiO grains during sintering. The grain size and density of WO3–added NiO–YSZ composites are larger than those of WO3–free NiO–YSZ composites since the added WO3 acts as a sintering aid [12,13]. The measured porosities of sintered NiO–YSZ composites are in the range of 0.3%–10.3% (Table 1). The effective diffusion rate enhances through the WO3 addition, thereby allowing pores to shrink and to stay at grain boundaries during grain growth and consequently enhances the densification and grain growth during sintering. The sintered NiO–YSZ composites with ≥0.3 wt% WO3 contain ultra–small YSZ grains (~700 nm), and the formation of ultra–small YSZ grains is pronounced with increasing WO3 content. For the WO3–free NiO–YSZ composite, NiO and YSZ grains are homogeneously distributed (Supplementary Fig. S1). The elemental maps show that Y has a strong preference for Zr in comparison with the Ni. Furthermore, O is relatively homogeneously distributed, whereas Zr, Y, and Ni are heterogeneously distributed. In addition, for the WO3–added NiO–YSZ composites, O and W are relatively homogeneously distributed, whereas Zr, Y, and Ni are heterogeneously distributed, as in the WO3–free NiO–YSZ composite. Fig. 2(B) exhibits the SEI and associated elemental maps from the surface of the NiO–YSZ composite with 0.4 wt% WO3. Y, Zr, and O maps represent that ultra–small grains in SEI correspond to YSZ. The FE–SEM images of Ni–YSZ composites fabricated by reducing NiO–YSZ composites in (94% Ar + 6% H2) gas are displayed in Fig. 3(A). The measured porosity of the Ni–YSZ composites is much higher than that of the sintered NiO–YSZ composites because of oxygen extraction through the change of NiO to Ni during reduction treatment [5,20,22]. The measured porosities of Ni–YSZ composites are in the range of 25.0%–32.8% (Table 1). The porosity of WO3–added Ni–YSZ composites is smaller than that of WO3–free Ni–YSZ composites. The porous Ni–YSZ composites are desirable for anode application in SOFCs
materials. Initially, ZrOCl3·8H2O and YCl3·6H2O were separately dissolved in distilled water and then thoroughly mixed together. The reaction of the mixtures was carried out at 130 °C for 72 h with stirring. Subsequently, a mixed solution of Ni(NO3)2·6H2O, NH3, and NaOH was added to the obtained mixed solution. The resultant solution was heated at 150 °C for 24 h. After the hydrothermal reaction, the synthesized powders were washed with distilled water several times and were then dried at 120 °C for 12 h. The dried powders were heated at 900 °C for 6 h in air atmosphere to prepare NiO–YSZ composite nanopowders. To fabricate NiO–YSZ composites, the hydrothermal–synthesized NiO–YSZ composite nanopowders were weighed in specific proportions and then mixed with WO3 powders, carbon black powders, binder (polyvinyl butyral; PVB), dispersant (KD–1), and ethyl alcohol for 1 h at a speed of 350 rpm with a planetary monomill (Fritsch Pulverisette 6) and ZrO2 balls. Carbon black powders were added as a pore former to increase the porosity of NiO–YSZ composites, and WO3 powders were added as a sintering aid to improve the sinterability of NiO–YSZ composites. NiO–YSZ composites were fabricated by drying and pressing of planetary–milled mixed powders followed by a sintering of green pellets at 1400 °C in air. Subsequently, porous Ni–YSZ composites were fabricated by reducing sintered NiO-YSZ composites in (94% Ar + 6% H2) atmosphere. Further details on the sample preparation may be found elsewhere [11]. To analyze the microstructural properties of NiO–YSZ and Ni–YSZ composites, an X–ray diffraction (XRD) system (Rigaku DMAX–2500) and field emission scanning electron microscope (FE–SEM; Hitachi S–4700) were utilized. To investigate the distribution of constituent elements in NiO–YSZ and Ni–YSZ composites, secondary electron images (SEI) and elemental maps were obtained using an electron probe analyzer installed on the FE-SEM. The porosity of NiO–YSZ and Ni–YSZ composites was measured by the Archimedes method. Transmission electron microscopy (TEM) samples were prepared by cutting, grinding, polishing, and then Ar ion milling. The detailed microstructure of ionmilled samples was examined with a Titan 80–300 G1 microscope (FEI, Einthoven, The Netherlands) operated at 300 kV. The chemical compositions of the prepared samples were examined using an energy dispersive spectroscopy (EDS) attached to a Talos F200X TEM (FEI, Einthoven, The Netherlands) operated at 200 kV. Four windowless EDS detectors simultaneously collected the characteristic X-rays from the nano-scale region for quantitative compositional analysis and provided elemental maps to reveal the distribution of specific elements in the samples. For the measurements of electrical conductivity of Ni–YSZ composites, the Ni–YSZ composites were cut with a diamond saw in the form of rectangular bars of 3 mm × 3 mm × 16 mm and polished with SiC papers. The electrical conductivity of Ni–YSZ composites was measured in the temperature range of 30–950 °C under reducing Ar atmosphere using the direct current four–probe method. The CTE of the composites was measured from 40 to 1100 °C in Ar atmosphere with a dilatometer (NETZSCH DIL 402C) at a heating rate of 10 °C min−1. The bending strength of NiO–YSZ and Ni–YSZ composites with a dimension of 3 mm × 4 mm × 25 mm was measured at room temperature in air using a four–point bending machine at a crosshead speed of 0.5 mm min−1. The inner and outer span lengths of the samples were 10 and 20 mm, respectively. 3. Results and discussion An FE–SEM image and XRD pattern of hydrothermal–synthesized NiO–YSZ composite nanopowders are shown in Fig. 1(A) and (B), respectively. In Fig. 1(A), the YSZ powders show a smooth surface and a spherical-like shape, whereas the NiO powders show a sharp-edged surface and polyhedral shape. The size of the composite powders ranges from 100 to 500 nm. These powder characteristics are favorable for preparing NiO–YSZ composites with small grains. The hydrothermal synthesis route is beneficial for producing highly crystallized nanopowders with high purity without any heat treatment in a short time 2
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Fig. 1. (A) FE–SEM image and (B) XRD pattern of hydrothermal–synthesized NiO–YSZ composite nanopowders. XRD patterns of the (C) NiO–YSZ and (D) Ni–YSZ composites with (a) 0, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.4, and (f) 0.6 wt% WO3.
because porous structure provides diffusion paths for gaseous reactants and products and allows more access to and from active three–phase boundary (TPB) sites, thereby reducing polarization due to gas diffusion resistance [23]. Ultra–small YSZ grains, indicated by arrows, exist in the Ni–YSZ composites with ≥0.3 wt% WO3. The formation of ultra–small YSZ grains is pronounced with increasing WO3 content, and ultra–small YSZ grains partially cover the Ni grains. Fig. 3(B) exhibits the SEI and associated elemental maps from the surface of the Ni–YSZ composite with 0.6 wt% WO3. This figure reveals that the Ni–YSZ
Table 1 Porosity of the NiO–YSZ and Ni–YSZ composites with various WO3 contents. Amount of WO3 (wt%)
NiO–YSZ composites (%)
Ni–YSZ composites (%)
0 0.1 0.2 0.3 0.4 0.6
10.3 0.6 0.5 0.5 0.3 0.3
32.8 26.7 26.5 25.0 26.0 25.8
Fig. 2. (A) FE–SEM images of the sintered NiO–YSZ composites with (a) 0, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.4, and (f) 0.6 wt% WO3. (B) SEI and associated elemental maps from the surface of sintered NiO–YSZ composite with 0.4 wt% WO3. 3
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Fig. 3. (A) FE–SEM images of the Ni–YSZ composites with (a) 0, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.4, and (f) 0.6 wt% WO3. (B) SEI and associated elemental maps of the Ni–YSZ composite with 0.6 wt% WO3.
composite with 0.1 wt% WO3 is shown in Fig. 4(A). The selected area electron diffraction (SAED) patterns from Ni and YSZ are shown in Fig. 4(B) and (C), respectively. Ni and YSZ form the fcc crystal structure (a = 0.3523 nm) and cubic crystal structure (a = 0.5152 nm), respectively [25]. These results are consistent with the previous XRD data [Fig. 1(D)], confirming the crystal structure of Ni–YSZ composites analyzed by XRD. The overlay elemental map of the Ni–YSZ composite
composites contain Ni and YSZ grains. The main elements (Ni and Zr) of the Ni–YSZ composites are uniformly distributed, thereby providing suitable electronic networks and reducing the possibility of cracking and delamination in the Ni–YSZ composites. Ni and YSZ grains provide continuous electronic and ionic pathways, respectively [9,24]. More detailed structural information of the Ni–YSZ composites is obtained through TEM studies. A TEM bright-field image of the Ni–YSZ
Fig. 4. (A) TEM bright-field image of the Ni–YSZ composite with 0.1 wt% WO3. (B) [110] SAED pattern from Ni in the Ni–YSZ composite with 0.1 wt% WO3. (C) [101] SAED pattern from YSZ in the Ni–YSZ composite with 0.1 wt% WO3. (D) Overlay elemental map of the Ni–YSZ composite with 0.1 wt% WO3. 4
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Fig. 5. (A) TEM bright-field image of the Ni–YSZ composite with 0.3 wt% WO3. SAED patterns from (B) “Ni + WO2.92” region, indicated by “large circle” in (A), and from (C) Y2(WO4)3 of the Ni–YSZ composite with 0.3 wt% WO3. (D) Overlay elemental map of the Ni–YSZ composite with 0.3 wt% WO3. Note that the Y2(WO4)3 phase, indicated by an arrow in (D), penetrates into YSZ grain.
with 0.1 wt% WO3 is shown in Fig. 4(D). The chemical compositions of Ni and YSZ grains are analyzed by EDS. The amount of W in YSZ grain is larger than that in Ni grain (Supplementary Fig. S2). This means that the solubility of WO3 in YSZ is larger than that in Ni. The elemental maps of the Ni–YSZ composite with 0.1 wt% WO3 are shown in Supplementary Fig. S3. A TEM bright-field image of the Ni–YSZ composite with 0.3 wt% WO3 is shown in Fig. 5(A). Spherical- and needle-like WO2.92 exists inside Ni grain, and Y2(WO4)3 exists at the interface between Ni and YSZ grains. The SAED pattern of “Ni + WO2.92” region indicated by “large circle” in Fig. 5(A) is shown in Fig. 5(B). The strong and weak spots in Fig. 5(B) correspond to Ni and WO2.92, respectively. The WO2.92 crystallizes in the monoclinic structure (a = 1.193 nm, b = 0.382 nm, c = 5.972 nm, and β = 98.30°) [26]. The [100] SAED pattern of Y2(WO4)3 at the interface between Ni and YSZ grains, indicated by “small circle” in Fig. 5(A), is represented in Fig. 5(C), indicating the orthorhombic structure (a = 1.0070 nm, b = 1.3937 nm, c = 0.9980 nm) [27]. It is important to note that the Y2(WO4)3 has a negative CTE, i.e., αa = −10.35 × 10−6 °C−1, αb = −3.06 × 10−6 °C−1, and αc = −7.62 × 10−6 °C−1 in the temperature range of 20–800 °C [28]. The negative CTE of Y2(WO4)3 in the Ni–YSZ composites is effective for reducing the CTE of Ni–YSZ composites, thereby improving the thermal expansion compatibility between the Ni–YSZ composite and YSZ. In addition, the Y2(WO4)3 has very low electrical conductivity (2.51 × 10−4 cm−1) [29], so it decreases the electrical conductivity of Ni–YSZ composites. These results will be discussed in detail in the following paragraph (Fig. 8). Fig. 5(D) shows an overlay elemental map of the Ni–YSZ composite with 0.3 wt% WO3. This figure reveals that the WO2.92 and Y2(WO4)3 exist inside Ni grains and at the interface between Ni and YSZ grains, respectively. The penetration of the Y2(WO4)3
into YSZ grain is observed in Fig. 5(D). The elemental maps of the Ni–YSZ composite with 0.3 wt% WO3 are shown in Supplementary Fig. S4. It is observed that the Ni grains in the Ni–YSZ composite with 0.3 wt % WO3 often contain micro-twins, as shown in Fig. 6(A) and (B). Fig. 6(A) and (B) exhibit TEM bright– and dark–field images of the Ni–YSZ composite with 0.3 wt% WO3, respectively that contains microtwins in Ni grain. The [011] SAED pattern from the micro-twinned Ni grain, indicated by “circle” in Fig. 6(B), is shown in Fig. 6(C). Nickel forms a micro-twin structure due to its low stacking fault energy [30]. A considerable amount of plastic deformation can be accommodated by the formation and movement of twins [31]. The WO2.92 is also observed inside Ni grains [Fig. 6(D)]. Fig. 7(A) and (B) exhibit a bright-field image and overlay elemental map of the Ni–YSZ composite with 0.6 wt% WO3, respectively. The elemental maps of the Ni–YSZ composite with 0.6 wt% WO3 are shown in Supplementary Fig. S5. As shown in these figures, the amount of Y2(WO4)3 in the Ni–YSZ composite with 0.6 wt% WO3 is larger than that in Ni–YSZ composite with 0.3 wt% WO3, and the Y2(WO4)3 in the Ni–YSZ composite with 0.6 wt% WO3 more significantly penetrates into YSZ grains than that in the Ni–YSZ composite with 0.3 wt% WO3. These indicate that the penetration of the Y2(WO4)3 into YSZ grains in the Ni–YSZ composites is pronounced with increasing WO3 content. The penetration of the Y2(WO4)3 into YSZ grain in the composites leads to the formation of ultra–small YSZ grains, which decreases the electrical conductivity and bending strength. Fig. 8(A) represents the electrical conductivity of Ni–YSZ composites with various WO3 contents. The electrical conductivity decreases with increasing temperature, indicating a metallic behavior. The electrical conductivity of the studied WO3–free Ni–YSZ composite is significantly higher than that of other Ni–YSZ composites fabricated by 5
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Fig. 6. TEM (A) bright- and (B) dark-field images of the Ni–YSZ composite with 0.3 wt% WO3 that contains micro-twinned Ni grain. (C) [011] SAED pattern from the micro-twinned Ni grain indicated by “circle” in (B). (D) Overlay elemental map of the Ni–YSZ composite with 0.3 wt% WO3.
It is important to note that the electrical conductivity of the Ni–YSZ composite with 0.1 wt% WO3 (e.g., 14,221 cm−1 at 30 °C) is higher than that of the WO3–free Ni–YSZ composite (e.g., 12,256 cm−1 at 30 °C). The higher electrical conductivity is mainly attributed to the lower porosity through the WO3 addition, which increases the time between scattering events of charge carriers and a corresponding increase in carrier mobility. On the other hand, when the amounts of WO3 are equal to or higher than 0.3 wt%, the electrical conductivity decreases with increasing WO3 content due to the formation of Y2(WO4)3
previous studies [5]. For example, the electrical conductivity at 950 °C for the studied WO3–free Ni–YSZ composite is 1850 cm−1, and those at 950 °C for the (56.4 wt% Ni + 43.6 wt% YSZ) and (66.9 wt% Ni + 33.1 wt% YSZ) composites prepared by Lee et al. [5] were ~450 and ~1020 cm−1, respectively [5]. The observed metallic behavior and high electrical conductivity of the studied Ni–YSZ composites imply that the electrical conduction dominantly occurs through the metallic Ni grains. The electrical conductivity of Ni measured at 1000 °C is more than five orders of magnitude larger than that of YSZ [22].
Fig. 7. (A) TEM bright-field image and (B) overlay elemental map of the Ni–YSZ composite with 0.6 wt% WO3. Note that the Y2(WO4)3, indicated by an arrow in (B), penetrates into YSZ grain. 6
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the Ni–YSZ composites with 0 and 0.1 wt% WO3 are 59 and 130 MPa, respectively, indicating a 220% increase in bending strength through the WO3 addition. In spite of the high porosity (26.7%) of the Ni–YSZ composite with 0.1 wt% WO3, its bending strength is substantially higher than that of the Ni–YSZ composite (~12% porosity) reported earlier [32], which is attributed to the Small-Sized grains. The bending strength of the Ni-YSZ composite (~12% porosity) fabricated by Matula et al. [32] was ~54 MPa [32]. When the amount of WO3 is equal to or higher than 0.2 wt%, the measured bending strength of the Ni–YSZ composites decreases with the increase in WO3 content probably because of the larger amount of ultra–small YSZ grains, which are formed by the penetration of Y2(WO4)3 into YSZ grains, as discussed in Fig. 5(D). Since Ni possesses remarkably higher CTE than YSZ, a significant expansion mismatch between the Ni–YSZ anode and the YSZ electrolyte in SOFCs develops large amounts of residual stress, which can induce the formation of cracks or delamination during the fabrication and operation [22]. The CTE values of Ni and 8 mol% YSZ were reported as 16.9 × 10−6 and 10.5 × 10−6 °C−1, respectively [33,34]. Therefore, the thermal expansion mismatch between the Ni–YSZ anode and the YSZ electrolyte is an important factor in designing SOFC components, and reducing the CTE of Ni–YSZ composites for high thermal expansion compatibility between them is necessary. Fig. 8(C) shows the CTE of Ni–YSZ composites with various WO3 contents measured at 40–1100 °C, and the obtained CTE values are summarized in Table 2. The CTE of WO3–free Ni–YSZ composite is comparable with that of (68.7 vol% Ni + 31.3 vol% YSZ) composites (13.5 × 10−6 °C−1) studied previously [34]. The obtained CTE of Ni–YSZ composites decreases with increasing WO3 content, i.e., 13.8 × 10−6, 13.1 × 10−6, and 12.4 × 10−6 °C−1 for the Ni–YSZ composites with 0, 0.1, and 0.6 wt% WO3, respectively. The decrease in CTE is mainly attributed to the fact that the higher WO3 content yields the larger amount of Y2(WO4)3 with negative CTE [28]. The addition of WO3 to NiO–YSZ composite nanopowders is thus effective for improving the thermal expansion compatibility between the Ni–YSZ composite and YSZ.
4. Conclusions NiO–YSZ composite nanopowders (100–500 nm) were synthesized by a hydrothermal process without an additional mixing of NiO and YSZ nanopowders. Porous small–grain NiO–YSZ composites were prepared by solid-state reaction, using the hydrothermal–synthesized NiO–YSZ composite nanopowders and carbon black powders. Highly porous Ni–YSZ composites with porosities of 25.0%–32.8% were fabricated by reducing the NiO–YSZ composites in (94% Ar + 6% H2) gas. The addition of WO3 (0.1 wt%) enhanced the electrical conductivity and bending strength of Ni–YSZ composites. The electrical conductivities of 0 and 0.1 wt% WO3–added Ni–YSZ composites measured at 30 °C were 12,256 and 14,221 cm−1, respectively, and the bending strengths of 0 and 0.1 wt% WO3–added Ni–YSZ composites were 59 and 130 MPa, respectively, indicating a 220% increase in bending strength through the WO3 addition. Furthermore, the WO3 addition reduced the CTE of Ni–YSZ composites due to the formation of Y2(WO4)3 with negative CTE. The CTE values of the Ni–YSZ composites with 0 and 0.6 wt % WO3 were 13.8 × 10−6 and 12.4 × 10−6 °C−1, respectively. On the basis of the electrical conductivity, bending strength, and CTE properties of the prepared Ni–YSZ composites, the fabrication of NiO–YSZ
Fig. 8. (A) Electrical conductivity of Ni–YSZ composites. (B) Bending strength of NiO–YSZ and Ni–YSZ composites. (C) Thermal expansion (ΔL/L) of Ni–YSZ composites.
with very low electrical conductivity (2.51 × 10−4 Scm−1) and ultra–small YSZ grains [29]. Ultra–small grains yield large grain boundary areas that act as carrier scattering sites, thereby decreasing the carrier mobility and electrical conductivity. Fig. 8(B) exhibits the bending strength of NiO–YSZ and Ni–YSZ composites with various WO3 contents measured at room temperature. The bending strength of NiO–YSZ composites is much higher than that of Ni–YSZ composites. The bending strength of NiO–YSZ composites significantly becomes large with the increase in WO3 content, for instance, 84 and 215 MPa for the NiO–YSZ composites with 0 and 0.6 wt % WO3, respectively, indicating a 256% increase in bending strength. Notably, the addition of a small amount of WO3 (0.1 wt% WO3) to NiO–YSZ composite nanopowders enhances the bending strength of the Ni–YSZ composites due to the lower porosity. The bending strengths of
Table 2 CTE of the Ni–YSZ composites with various WO3 contents.
7
Amount of WO3 (wt%)
CTE (°C−1)
0 0.1 0.6
13.8 × 10−6 13.1 × 10−6 12.4 × 10−6
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composite nanopowders and the addition of WO3 to the NiO–YSZ composite nanopowders can be applied for the fabrication of highquality Ni–YSZ anode materials in SOFCs.
fabricated by the solid-state reaction method, Ceram. Int. 39 (2013) 7467–7474. [12] H. Su, H. Zhang, X. Tang, Y. Jing, Effects of MoO3 and WO3 additives on densification and magnetic properties of highly permeable NiCuZn ferrites, Mater. Chem. Phys. 102 (2007) 271–274. [13] H. Su, H. Zhang, X. Tang, Effects of Bi2O3–WO3 additives on sintering behaviors and magnetic properties of NiCuZn ferrites, Mater. Sci. Eng. B 117 (2005) 231–234. [14] T. Zhang, C.G. Jin, T. Qian, X.L. Lu, J.M. Bai, X.G. Li, Hydrothermal synthesis of single-crystalline La0.5Ca0.5MnO3 nanowires at low temperature, J. Mater. Chem. 14 (2004) 2787–2789. [15] J.S. Zhu, J.B. Liu, H. Wang, M.K. Zhu, H. Yan, Synthesis of SrMnO3 powders with different morphologies by hydrothermal method, Cryst. Res. Technol. 42 (2007) 241–246. [16] JCPDS file # 30–1468. [17] JCPDS file # 47–1049. [18] K. Park, M.H. Heo, K.Y. Kim, S.J. Dhoble, Y. Kim, J.Y. Kim, Photoluminescence properties of nano-sized (Y0.5Gd0.5)PO4:Eu3+ phosphor powders synthesized by solution combustion method, Powder Technol. 237 (2013) 102–106. [19] W. Noh, B.S. Park, K. Hong, J. Kim, I. Kim, J. Park, Electrical properties and microstructures of NiO-doped WO3 thick films, J. Mater. Sci. Lett. 21 (2002) 1271–1274. [20] A.R. Hanifi, M.A. Laguna-Bercero, N.K. Sandhu, T.H. Etsell, P. Sarkar, Tailoring the microstructure of a solid oxide fuel cell anode support by calcination and milling of YSZ, Sci. Rep. 6 (2016) 27359. [21] K. Park, K.U. Jang, Response characteristics to reducing gases in BaTiO3-based thick films, J. Alloys Compd. 391 (2005) 123–128. [22] H. Itoh, T. Yamamoto, M. Mori, T. Horita, N. Sakai, H. Yokokawa, M. Dokiya, Configurational and electrical behavior of Ni-YSZ cermet with novel microstructure for solid oxide fuel cell anodes, J. Electrochem. Soc. 144 (1997) 641–646. [23] C.M. Kim, J. Kim, K. Park, Fabrication and structural properties of porous Cu–YSZ cermets for solid oxide fuel cells, Powder Technol. 254 (2014) 425–431. [24] S.T. Aruna, M. Muthuraman, K.C. Patil, Synthesis and properties of Ni-YSZ cermet: anode material for solid oxide fuel cells, Solid State Ionics 111 (1998) 45–51. [25] JCPDS file # 81-1551. [26] PDF file # 01-71-0070. [27] PDF file # 01-070-4480. [28] D.A. Woodcock, P. Lightfoot, C. Ritter, Negative thermal expansion in Y2(WO4)3, J. Solid State Chem. 149 (2000) 92–98. [29] S. Khaliullin, A.S. Khaliullina, A.Y. Neiman, High-temperature conductivity and structure of Y2(WO4)3 ceramics, Russian J. Phys. Chem. B 10 (2016) 62–68. [30] C.B. Thomson, V. Randle, A study of twinning in nickel, Scr. Mater. 35 (1996) 385–390. [31] N.P. Daphalapurkar, J.W. Wilkerson, T.W. Wright, K.T. Ramesh, Kinetics of a fast moving twin boundary in nickel, Acta Mater. 68 (2014) 82–92. [32] G. Matula, T. Jardiel, R. Jimenez, B. Levenfeld, A. Várez, Microstructure, mechanical and electrical properties of Ni-YSZ anode supported solid oxide fuel cells, Archives of Mater. Sci. Eng. 32 (2008) 21–25. [33] H. Hayashi, T. Saitou, N. Maruyama, H. Inaba, K. Kawamura, M. Mori, Thermal expansion coefficient of yttria stabilized zirconia for various yttria contents, Solid State Ionics 176 (2005) 613–619. [34] T.-S. Lee, J. Ko, K.-S. Lee, B.-H. Kim, Characterization of Ni-YSZ cermet anode for SOFC prepared by glycine nitrate process, J. Korean Cryst. Growth Cryst. Technol. 21 (2011) 21–26.
Acknowledgments We acknowledge the financial support by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry & Energy (No.20184030202260). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.matchar.2019.110113. References [1] B.C.H. Steele, A. Heinzel, Materials for fuel-cell technologies, Nature 414 (2001) 345–352. [2] H. Ding, Z. Tao, S. Liu, J. Zhang, A high-performing sulfur-tolerant and redox-stable layered perovskite anode for direct hydrocarbon solid oxide fuel cells, Sci. Rep. 5 (2015) 18129. [3] R. Lan, P.I. Cowin, S. Sengodan, S. Tao, A perovskite oxide with high conductivities in both air and reducing atmosphere for use as electrode for solid oxide fuel cells, Sci. Rep. 6 (2016) 31839. [4] L. Zhang, S.P. Jiang, W. Wang, Y. Zhang, NiO/YSZ, anode-supported, thin-electrolyte, solid oxide fuel cells fabricated by gel casting, J. Power Sources 170 (2007) 55–60. [5] 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, Solid State Ionics 148 (2002) 15–26. [6] S.K. Pratihar, A.D. Sharma, H.S. Maiti, Electrical behavior of nickel coated YSZ cermet prepared by electroless coating technique, Mater. Chem. Phys. 96 (2006) 388–395. [7] J.C. Ruiz–Morales, J. Canales–Vázquez, D. Marrero–López, J.T.S. Irvine, P. Núñez, Improvement of the electrochemical properties of novel solid oxide fuel cell anodes, La0.75Sr0.25Cr0.5Mn0.5O3−δ and La4Sr8Ti11Mn0.5Ga0.5O37.5−δ, using Cu–YSZ-based cermets, Electrochim. Acta 52 (2007) 7217–7225. [8] J.C. Ruiz–Morales, J. Canales–Vázquez, D. Marrero–López, J. Peña–Martínez, A. Tarancón, J.T.S. Irvine, P. Núñeza, Is YSZ stable in the presence of Cu?, J. Mater. Chem. 18 (2008) 5072–5077. [9] D. Skarmoutsos, P. Nikolopolous, A. Tsoga, Titania doped YSZ for SOFC anode Nicermet, Ionics 5 (1999) 455–459. [10] C.M. Grgicak, R.G. Green, J.B. Giorgi, Control of microstructure, sinterability and performance in Co-precipitated NiYSZ, CuYSZ and CoYSZ SOFC anodes, J. Mater. Chem. 16 (2006) 885–897. [11] J. Kim, K.H. Cho, I. Kagomiya, K. Park, Structural studies of porous Ni/YSZ cermets
8
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Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Porous NiO–YSZ and Ni–YSZ composites fabricated using NiO–YSZ composite nanopowders and WO3 additive J. Kima, G.H. Kimb, K. Parka, a b
T
⁎
Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 05006, Republic of Korea Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 13679, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Composites Microstructure Porosity Electron microscopy Amode SO FC
NiO–YSZ composite nanopowders were synthesized through a hydrothermal process without an additional mixing of NiO and YSZ nanopowders. Porous Ni–yttria stabilized zirconia (YSZ) composites, Ni–YSZ composites, with high porosity (25.0–32.8%) were fabricated by reducing NiO-YSZ composites in (94% Ar + 6% H2) gas. The electrical conductivity (14,221 cm−1 at 30 °C) and bending strength (130 MPa) of Ni–YSZ composites were enhanced by the addition of WO3 (0.1 wt% WO3) to NiO–YSZ composite nanopowders. Furthermore, the Y2(WO4)3 with a negative coefficient of thermal expansion (CTE) reduced the CTE of Ni–YSZ composites. The CTE values of 0 and 0.6 wt% WO3–added Ni–YSZ composites were 13.8 × 10−6 and 12.4 × 10−6 °C−1, respectively. We demonstrate that the fabrication of NiO–YSZ composite nanopowders and the addition of WO3 to the NiO–YSZ composite nanopowders are highly effective methods for the fabrication of high-quality Ni–YSZ anode materials in solid oxide fuel cells.
1. Introduction
extend the applications of Ni–YSZ composites as anode materials of SOFCs, the current research focuses on enhancing the electrical conductivity and bending strength of Ni–YSZ composites and on reducing the coefficient of thermal expansion (CTE) of Ni–YSZ composites through the addition of WO3 to (65 wt% NiO + 35 wt% YSZ) composite nanopowders (hereafter referred to as NiO–YSZ composite nanopowders). In this work, we directly synthesized NiO–YSZ composite nanopowders through a hydrothermal process without an additional mixing of NiO and YSZ nanopowders. Furthermore, we added WO3 powders to the NiO–YSZ composite nanopowders as a sintering aid. WO3 was previously utilized as a sintering aid in NiCuZn ferrites [12,13]. Here, we investigated the microstructure, electrical conductivity, bending strength, and CTE properties of porous Ni–YSZ composites prepared by reducing the NiO–YSZ composites. The addition of WO3 (0.1 wt% WO3) is highly effective for increasing the electrical conductivity and bending strength of Ni–YSZ composites and for reducing the CTE of Ni–YSZ composites.
Solid oxide fuel cells (SOFCs) can directly convert the chemical energy of fuels, such as hydrogen, methane, and methanol, into electrical energy [1–3]. SOFCs are efficient power–generating systems with high energy conversion efficiency, high fuel flexibility, and environmentally friendly nature in comparison with other fuel cell systems [4–6]. The energy conversion efficiency of SOFCs depends strongly on the microstructural and electrical properties of the cathode, electrolyte, and anode [7,8]. Among these components, the anode plays the most significant role in oxidizing fuels to generate power [5]. Ni–yttria stabilized zirconia (YSZ) composites, Ni–YSZ composites, are the most widely used materials for anodes due to its excellent physical, thermal, and mechanical compatibility with other SOFC components, high electrocatalytic activity for H2 oxidation, and low fabrication cost [5,7,9]. Traditionally, Ni–YSZ composites have been prepared by two different processes: (1) mechanical mixing of large–sized Ni and YSZ powders followed by a sintering of Ni–YSZ green bodies and (2) mechanical mixing of large–sized NiO and YSZ powders followed by a reduction of NiO–YSZ composites [6,10,11]. The large–sized Ni and YSZ grains in Ni–YSZ composites fabricated by the aforementioned two processes were not uniformly distributed, thereby yielding insufficient reactive sites and a corresponding low conversion efficiency. To further ⁎
Corresponding author. E-mail address: [email protected] (K. Park).
https://doi.org/10.1016/j.matchar.2019.110113 Received 15 October 2019; Accepted 27 December 2019 Available online 28 December 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
2. Experimental Composite nanopowders of NiO and 8 mol% YSZ (65 wt% NiO + 35 wt% YSZ) were prepared by a hydrothermal process. Ni (NO3)2·6H2O, ZrOCl3·8H2O, and YCl3·6H2O were used as starting
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[14,15]. The prepared NiO–YSZ composite nanopowders are composed of NiO and YSZ nanopowders without any impurities [Fig. 1(B)]. Both the NiO and YSZ nanopowders at room temperature crystallize in the cubic crystal structure [16,17]. The crystallite size (D) of NiO–YSZ composite nanopowders can be calculated from the Scherrer formula [18]: D = (0.9λ) / (βcosθ), where λ is the wavelength of radiation, β is the corrected full width at half maximum of the diffraction peak (in radian), and θ is the angle of the diffraction peak. The calculated crystallite size of NiO–YSZ composite nanopowders is 53 nm. Fig. 1(C) shows the XRD patterns from the sintered NiO–YSZ composites. The NiO–YSZ composites with ≤0.3 wt% WO3 consist of only NiO and YSZ phases. The two phases have a cubic crystal structure at room temperature [16,17], as in the NiO–YSZ composite nanopowders. No un-reacted oxides are detected. On the other hand, the NiO–YSZ composites with ≥0.4 wt% WO3 contain the NiWO4 secondary phase along with the NiO and YSZ phases. The NiWO4 may be formed by the following reaction [19]: NiO + WO3 → NiWO4. The compounds formed by the reaction of WO3 and YSZ are not detected. Fig. 1(D) exhibits the XRD patterns of Ni–YSZ composites fabricated by reducing the NiO–YSZ composites in (94% Ar + 6% H2) atmosphere. No XRD peaks derived from NiO are detected. This means that the NiO phase disappears through the reduction treatment and that Ni is formed. In the Ni–YSZ composites, Ni has the face–centered cubic (fcc) crystal structure, and YSZ has the cubic crystal structure. NiWO4 phase is not observed in the XRD patterns. FE–SEM images of the sintered NiO–YSZ composites are shown in Fig. 2(A). The NiO–YSZ composites show porous structures and small grains. The exothermic reaction of carbon black powders and oxygen during sintering releases CO and CO2 gases, thereby forming porous structure [20,21]. The porous structure enhances the diffusion of reduction gas within the composites during the subsequent reduction treatment. The sintered NiO–YSZ composites consist of small grains, which are attributed to the fact that the hydrothermal–synthesized NiO–YSZ composite nanopowders are used to prepare the composites. YSZ grains in the composites suppress the grain growth of NiO grains during sintering. The grain size and density of WO3–added NiO–YSZ composites are larger than those of WO3–free NiO–YSZ composites since the added WO3 acts as a sintering aid [12,13]. The measured porosities of sintered NiO–YSZ composites are in the range of 0.3%–10.3% (Table 1). The effective diffusion rate enhances through the WO3 addition, thereby allowing pores to shrink and to stay at grain boundaries during grain growth and consequently enhances the densification and grain growth during sintering. The sintered NiO–YSZ composites with ≥0.3 wt% WO3 contain ultra–small YSZ grains (~700 nm), and the formation of ultra–small YSZ grains is pronounced with increasing WO3 content. For the WO3–free NiO–YSZ composite, NiO and YSZ grains are homogeneously distributed (Supplementary Fig. S1). The elemental maps show that Y has a strong preference for Zr in comparison with the Ni. Furthermore, O is relatively homogeneously distributed, whereas Zr, Y, and Ni are heterogeneously distributed. In addition, for the WO3–added NiO–YSZ composites, O and W are relatively homogeneously distributed, whereas Zr, Y, and Ni are heterogeneously distributed, as in the WO3–free NiO–YSZ composite. Fig. 2(B) exhibits the SEI and associated elemental maps from the surface of the NiO–YSZ composite with 0.4 wt% WO3. Y, Zr, and O maps represent that ultra–small grains in SEI correspond to YSZ. The FE–SEM images of Ni–YSZ composites fabricated by reducing NiO–YSZ composites in (94% Ar + 6% H2) gas are displayed in Fig. 3(A). The measured porosity of the Ni–YSZ composites is much higher than that of the sintered NiO–YSZ composites because of oxygen extraction through the change of NiO to Ni during reduction treatment [5,20,22]. The measured porosities of Ni–YSZ composites are in the range of 25.0%–32.8% (Table 1). The porosity of WO3–added Ni–YSZ composites is smaller than that of WO3–free Ni–YSZ composites. The porous Ni–YSZ composites are desirable for anode application in SOFCs
materials. Initially, ZrOCl3·8H2O and YCl3·6H2O were separately dissolved in distilled water and then thoroughly mixed together. The reaction of the mixtures was carried out at 130 °C for 72 h with stirring. Subsequently, a mixed solution of Ni(NO3)2·6H2O, NH3, and NaOH was added to the obtained mixed solution. The resultant solution was heated at 150 °C for 24 h. After the hydrothermal reaction, the synthesized powders were washed with distilled water several times and were then dried at 120 °C for 12 h. The dried powders were heated at 900 °C for 6 h in air atmosphere to prepare NiO–YSZ composite nanopowders. To fabricate NiO–YSZ composites, the hydrothermal–synthesized NiO–YSZ composite nanopowders were weighed in specific proportions and then mixed with WO3 powders, carbon black powders, binder (polyvinyl butyral; PVB), dispersant (KD–1), and ethyl alcohol for 1 h at a speed of 350 rpm with a planetary monomill (Fritsch Pulverisette 6) and ZrO2 balls. Carbon black powders were added as a pore former to increase the porosity of NiO–YSZ composites, and WO3 powders were added as a sintering aid to improve the sinterability of NiO–YSZ composites. NiO–YSZ composites were fabricated by drying and pressing of planetary–milled mixed powders followed by a sintering of green pellets at 1400 °C in air. Subsequently, porous Ni–YSZ composites were fabricated by reducing sintered NiO-YSZ composites in (94% Ar + 6% H2) atmosphere. Further details on the sample preparation may be found elsewhere [11]. To analyze the microstructural properties of NiO–YSZ and Ni–YSZ composites, an X–ray diffraction (XRD) system (Rigaku DMAX–2500) and field emission scanning electron microscope (FE–SEM; Hitachi S–4700) were utilized. To investigate the distribution of constituent elements in NiO–YSZ and Ni–YSZ composites, secondary electron images (SEI) and elemental maps were obtained using an electron probe analyzer installed on the FE-SEM. The porosity of NiO–YSZ and Ni–YSZ composites was measured by the Archimedes method. Transmission electron microscopy (TEM) samples were prepared by cutting, grinding, polishing, and then Ar ion milling. The detailed microstructure of ionmilled samples was examined with a Titan 80–300 G1 microscope (FEI, Einthoven, The Netherlands) operated at 300 kV. The chemical compositions of the prepared samples were examined using an energy dispersive spectroscopy (EDS) attached to a Talos F200X TEM (FEI, Einthoven, The Netherlands) operated at 200 kV. Four windowless EDS detectors simultaneously collected the characteristic X-rays from the nano-scale region for quantitative compositional analysis and provided elemental maps to reveal the distribution of specific elements in the samples. For the measurements of electrical conductivity of Ni–YSZ composites, the Ni–YSZ composites were cut with a diamond saw in the form of rectangular bars of 3 mm × 3 mm × 16 mm and polished with SiC papers. The electrical conductivity of Ni–YSZ composites was measured in the temperature range of 30–950 °C under reducing Ar atmosphere using the direct current four–probe method. The CTE of the composites was measured from 40 to 1100 °C in Ar atmosphere with a dilatometer (NETZSCH DIL 402C) at a heating rate of 10 °C min−1. The bending strength of NiO–YSZ and Ni–YSZ composites with a dimension of 3 mm × 4 mm × 25 mm was measured at room temperature in air using a four–point bending machine at a crosshead speed of 0.5 mm min−1. The inner and outer span lengths of the samples were 10 and 20 mm, respectively. 3. Results and discussion An FE–SEM image and XRD pattern of hydrothermal–synthesized NiO–YSZ composite nanopowders are shown in Fig. 1(A) and (B), respectively. In Fig. 1(A), the YSZ powders show a smooth surface and a spherical-like shape, whereas the NiO powders show a sharp-edged surface and polyhedral shape. The size of the composite powders ranges from 100 to 500 nm. These powder characteristics are favorable for preparing NiO–YSZ composites with small grains. The hydrothermal synthesis route is beneficial for producing highly crystallized nanopowders with high purity without any heat treatment in a short time 2
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Fig. 1. (A) FE–SEM image and (B) XRD pattern of hydrothermal–synthesized NiO–YSZ composite nanopowders. XRD patterns of the (C) NiO–YSZ and (D) Ni–YSZ composites with (a) 0, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.4, and (f) 0.6 wt% WO3.
because porous structure provides diffusion paths for gaseous reactants and products and allows more access to and from active three–phase boundary (TPB) sites, thereby reducing polarization due to gas diffusion resistance [23]. Ultra–small YSZ grains, indicated by arrows, exist in the Ni–YSZ composites with ≥0.3 wt% WO3. The formation of ultra–small YSZ grains is pronounced with increasing WO3 content, and ultra–small YSZ grains partially cover the Ni grains. Fig. 3(B) exhibits the SEI and associated elemental maps from the surface of the Ni–YSZ composite with 0.6 wt% WO3. This figure reveals that the Ni–YSZ
Table 1 Porosity of the NiO–YSZ and Ni–YSZ composites with various WO3 contents. Amount of WO3 (wt%)
NiO–YSZ composites (%)
Ni–YSZ composites (%)
0 0.1 0.2 0.3 0.4 0.6
10.3 0.6 0.5 0.5 0.3 0.3
32.8 26.7 26.5 25.0 26.0 25.8
Fig. 2. (A) FE–SEM images of the sintered NiO–YSZ composites with (a) 0, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.4, and (f) 0.6 wt% WO3. (B) SEI and associated elemental maps from the surface of sintered NiO–YSZ composite with 0.4 wt% WO3. 3
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Fig. 3. (A) FE–SEM images of the Ni–YSZ composites with (a) 0, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.4, and (f) 0.6 wt% WO3. (B) SEI and associated elemental maps of the Ni–YSZ composite with 0.6 wt% WO3.
composite with 0.1 wt% WO3 is shown in Fig. 4(A). The selected area electron diffraction (SAED) patterns from Ni and YSZ are shown in Fig. 4(B) and (C), respectively. Ni and YSZ form the fcc crystal structure (a = 0.3523 nm) and cubic crystal structure (a = 0.5152 nm), respectively [25]. These results are consistent with the previous XRD data [Fig. 1(D)], confirming the crystal structure of Ni–YSZ composites analyzed by XRD. The overlay elemental map of the Ni–YSZ composite
composites contain Ni and YSZ grains. The main elements (Ni and Zr) of the Ni–YSZ composites are uniformly distributed, thereby providing suitable electronic networks and reducing the possibility of cracking and delamination in the Ni–YSZ composites. Ni and YSZ grains provide continuous electronic and ionic pathways, respectively [9,24]. More detailed structural information of the Ni–YSZ composites is obtained through TEM studies. A TEM bright-field image of the Ni–YSZ
Fig. 4. (A) TEM bright-field image of the Ni–YSZ composite with 0.1 wt% WO3. (B) [110] SAED pattern from Ni in the Ni–YSZ composite with 0.1 wt% WO3. (C) [101] SAED pattern from YSZ in the Ni–YSZ composite with 0.1 wt% WO3. (D) Overlay elemental map of the Ni–YSZ composite with 0.1 wt% WO3. 4
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Fig. 5. (A) TEM bright-field image of the Ni–YSZ composite with 0.3 wt% WO3. SAED patterns from (B) “Ni + WO2.92” region, indicated by “large circle” in (A), and from (C) Y2(WO4)3 of the Ni–YSZ composite with 0.3 wt% WO3. (D) Overlay elemental map of the Ni–YSZ composite with 0.3 wt% WO3. Note that the Y2(WO4)3 phase, indicated by an arrow in (D), penetrates into YSZ grain.
with 0.1 wt% WO3 is shown in Fig. 4(D). The chemical compositions of Ni and YSZ grains are analyzed by EDS. The amount of W in YSZ grain is larger than that in Ni grain (Supplementary Fig. S2). This means that the solubility of WO3 in YSZ is larger than that in Ni. The elemental maps of the Ni–YSZ composite with 0.1 wt% WO3 are shown in Supplementary Fig. S3. A TEM bright-field image of the Ni–YSZ composite with 0.3 wt% WO3 is shown in Fig. 5(A). Spherical- and needle-like WO2.92 exists inside Ni grain, and Y2(WO4)3 exists at the interface between Ni and YSZ grains. The SAED pattern of “Ni + WO2.92” region indicated by “large circle” in Fig. 5(A) is shown in Fig. 5(B). The strong and weak spots in Fig. 5(B) correspond to Ni and WO2.92, respectively. The WO2.92 crystallizes in the monoclinic structure (a = 1.193 nm, b = 0.382 nm, c = 5.972 nm, and β = 98.30°) [26]. The [100] SAED pattern of Y2(WO4)3 at the interface between Ni and YSZ grains, indicated by “small circle” in Fig. 5(A), is represented in Fig. 5(C), indicating the orthorhombic structure (a = 1.0070 nm, b = 1.3937 nm, c = 0.9980 nm) [27]. It is important to note that the Y2(WO4)3 has a negative CTE, i.e., αa = −10.35 × 10−6 °C−1, αb = −3.06 × 10−6 °C−1, and αc = −7.62 × 10−6 °C−1 in the temperature range of 20–800 °C [28]. The negative CTE of Y2(WO4)3 in the Ni–YSZ composites is effective for reducing the CTE of Ni–YSZ composites, thereby improving the thermal expansion compatibility between the Ni–YSZ composite and YSZ. In addition, the Y2(WO4)3 has very low electrical conductivity (2.51 × 10−4 cm−1) [29], so it decreases the electrical conductivity of Ni–YSZ composites. These results will be discussed in detail in the following paragraph (Fig. 8). Fig. 5(D) shows an overlay elemental map of the Ni–YSZ composite with 0.3 wt% WO3. This figure reveals that the WO2.92 and Y2(WO4)3 exist inside Ni grains and at the interface between Ni and YSZ grains, respectively. The penetration of the Y2(WO4)3
into YSZ grain is observed in Fig. 5(D). The elemental maps of the Ni–YSZ composite with 0.3 wt% WO3 are shown in Supplementary Fig. S4. It is observed that the Ni grains in the Ni–YSZ composite with 0.3 wt % WO3 often contain micro-twins, as shown in Fig. 6(A) and (B). Fig. 6(A) and (B) exhibit TEM bright– and dark–field images of the Ni–YSZ composite with 0.3 wt% WO3, respectively that contains microtwins in Ni grain. The [011] SAED pattern from the micro-twinned Ni grain, indicated by “circle” in Fig. 6(B), is shown in Fig. 6(C). Nickel forms a micro-twin structure due to its low stacking fault energy [30]. A considerable amount of plastic deformation can be accommodated by the formation and movement of twins [31]. The WO2.92 is also observed inside Ni grains [Fig. 6(D)]. Fig. 7(A) and (B) exhibit a bright-field image and overlay elemental map of the Ni–YSZ composite with 0.6 wt% WO3, respectively. The elemental maps of the Ni–YSZ composite with 0.6 wt% WO3 are shown in Supplementary Fig. S5. As shown in these figures, the amount of Y2(WO4)3 in the Ni–YSZ composite with 0.6 wt% WO3 is larger than that in Ni–YSZ composite with 0.3 wt% WO3, and the Y2(WO4)3 in the Ni–YSZ composite with 0.6 wt% WO3 more significantly penetrates into YSZ grains than that in the Ni–YSZ composite with 0.3 wt% WO3. These indicate that the penetration of the Y2(WO4)3 into YSZ grains in the Ni–YSZ composites is pronounced with increasing WO3 content. The penetration of the Y2(WO4)3 into YSZ grain in the composites leads to the formation of ultra–small YSZ grains, which decreases the electrical conductivity and bending strength. Fig. 8(A) represents the electrical conductivity of Ni–YSZ composites with various WO3 contents. The electrical conductivity decreases with increasing temperature, indicating a metallic behavior. The electrical conductivity of the studied WO3–free Ni–YSZ composite is significantly higher than that of other Ni–YSZ composites fabricated by 5
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Fig. 6. TEM (A) bright- and (B) dark-field images of the Ni–YSZ composite with 0.3 wt% WO3 that contains micro-twinned Ni grain. (C) [011] SAED pattern from the micro-twinned Ni grain indicated by “circle” in (B). (D) Overlay elemental map of the Ni–YSZ composite with 0.3 wt% WO3.
It is important to note that the electrical conductivity of the Ni–YSZ composite with 0.1 wt% WO3 (e.g., 14,221 cm−1 at 30 °C) is higher than that of the WO3–free Ni–YSZ composite (e.g., 12,256 cm−1 at 30 °C). The higher electrical conductivity is mainly attributed to the lower porosity through the WO3 addition, which increases the time between scattering events of charge carriers and a corresponding increase in carrier mobility. On the other hand, when the amounts of WO3 are equal to or higher than 0.3 wt%, the electrical conductivity decreases with increasing WO3 content due to the formation of Y2(WO4)3
previous studies [5]. For example, the electrical conductivity at 950 °C for the studied WO3–free Ni–YSZ composite is 1850 cm−1, and those at 950 °C for the (56.4 wt% Ni + 43.6 wt% YSZ) and (66.9 wt% Ni + 33.1 wt% YSZ) composites prepared by Lee et al. [5] were ~450 and ~1020 cm−1, respectively [5]. The observed metallic behavior and high electrical conductivity of the studied Ni–YSZ composites imply that the electrical conduction dominantly occurs through the metallic Ni grains. The electrical conductivity of Ni measured at 1000 °C is more than five orders of magnitude larger than that of YSZ [22].
Fig. 7. (A) TEM bright-field image and (B) overlay elemental map of the Ni–YSZ composite with 0.6 wt% WO3. Note that the Y2(WO4)3, indicated by an arrow in (B), penetrates into YSZ grain. 6
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the Ni–YSZ composites with 0 and 0.1 wt% WO3 are 59 and 130 MPa, respectively, indicating a 220% increase in bending strength through the WO3 addition. In spite of the high porosity (26.7%) of the Ni–YSZ composite with 0.1 wt% WO3, its bending strength is substantially higher than that of the Ni–YSZ composite (~12% porosity) reported earlier [32], which is attributed to the Small-Sized grains. The bending strength of the Ni-YSZ composite (~12% porosity) fabricated by Matula et al. [32] was ~54 MPa [32]. When the amount of WO3 is equal to or higher than 0.2 wt%, the measured bending strength of the Ni–YSZ composites decreases with the increase in WO3 content probably because of the larger amount of ultra–small YSZ grains, which are formed by the penetration of Y2(WO4)3 into YSZ grains, as discussed in Fig. 5(D). Since Ni possesses remarkably higher CTE than YSZ, a significant expansion mismatch between the Ni–YSZ anode and the YSZ electrolyte in SOFCs develops large amounts of residual stress, which can induce the formation of cracks or delamination during the fabrication and operation [22]. The CTE values of Ni and 8 mol% YSZ were reported as 16.9 × 10−6 and 10.5 × 10−6 °C−1, respectively [33,34]. Therefore, the thermal expansion mismatch between the Ni–YSZ anode and the YSZ electrolyte is an important factor in designing SOFC components, and reducing the CTE of Ni–YSZ composites for high thermal expansion compatibility between them is necessary. Fig. 8(C) shows the CTE of Ni–YSZ composites with various WO3 contents measured at 40–1100 °C, and the obtained CTE values are summarized in Table 2. The CTE of WO3–free Ni–YSZ composite is comparable with that of (68.7 vol% Ni + 31.3 vol% YSZ) composites (13.5 × 10−6 °C−1) studied previously [34]. The obtained CTE of Ni–YSZ composites decreases with increasing WO3 content, i.e., 13.8 × 10−6, 13.1 × 10−6, and 12.4 × 10−6 °C−1 for the Ni–YSZ composites with 0, 0.1, and 0.6 wt% WO3, respectively. The decrease in CTE is mainly attributed to the fact that the higher WO3 content yields the larger amount of Y2(WO4)3 with negative CTE [28]. The addition of WO3 to NiO–YSZ composite nanopowders is thus effective for improving the thermal expansion compatibility between the Ni–YSZ composite and YSZ.
4. Conclusions NiO–YSZ composite nanopowders (100–500 nm) were synthesized by a hydrothermal process without an additional mixing of NiO and YSZ nanopowders. Porous small–grain NiO–YSZ composites were prepared by solid-state reaction, using the hydrothermal–synthesized NiO–YSZ composite nanopowders and carbon black powders. Highly porous Ni–YSZ composites with porosities of 25.0%–32.8% were fabricated by reducing the NiO–YSZ composites in (94% Ar + 6% H2) gas. The addition of WO3 (0.1 wt%) enhanced the electrical conductivity and bending strength of Ni–YSZ composites. The electrical conductivities of 0 and 0.1 wt% WO3–added Ni–YSZ composites measured at 30 °C were 12,256 and 14,221 cm−1, respectively, and the bending strengths of 0 and 0.1 wt% WO3–added Ni–YSZ composites were 59 and 130 MPa, respectively, indicating a 220% increase in bending strength through the WO3 addition. Furthermore, the WO3 addition reduced the CTE of Ni–YSZ composites due to the formation of Y2(WO4)3 with negative CTE. The CTE values of the Ni–YSZ composites with 0 and 0.6 wt % WO3 were 13.8 × 10−6 and 12.4 × 10−6 °C−1, respectively. On the basis of the electrical conductivity, bending strength, and CTE properties of the prepared Ni–YSZ composites, the fabrication of NiO–YSZ
Fig. 8. (A) Electrical conductivity of Ni–YSZ composites. (B) Bending strength of NiO–YSZ and Ni–YSZ composites. (C) Thermal expansion (ΔL/L) of Ni–YSZ composites.
with very low electrical conductivity (2.51 × 10−4 Scm−1) and ultra–small YSZ grains [29]. Ultra–small grains yield large grain boundary areas that act as carrier scattering sites, thereby decreasing the carrier mobility and electrical conductivity. Fig. 8(B) exhibits the bending strength of NiO–YSZ and Ni–YSZ composites with various WO3 contents measured at room temperature. The bending strength of NiO–YSZ composites is much higher than that of Ni–YSZ composites. The bending strength of NiO–YSZ composites significantly becomes large with the increase in WO3 content, for instance, 84 and 215 MPa for the NiO–YSZ composites with 0 and 0.6 wt % WO3, respectively, indicating a 256% increase in bending strength. Notably, the addition of a small amount of WO3 (0.1 wt% WO3) to NiO–YSZ composite nanopowders enhances the bending strength of the Ni–YSZ composites due to the lower porosity. The bending strengths of
Table 2 CTE of the Ni–YSZ composites with various WO3 contents.
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Amount of WO3 (wt%)
CTE (°C−1)
0 0.1 0.6
13.8 × 10−6 13.1 × 10−6 12.4 × 10−6
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composite nanopowders and the addition of WO3 to the NiO–YSZ composite nanopowders can be applied for the fabrication of highquality Ni–YSZ anode materials in SOFCs.
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