In situ transmission electron microscopic observations of redox cycling of a Ni–ScSZ cermet fuel cell anode

In situ transmission electron microscopic observations of redox cycling of a Ni–ScSZ cermet... Abstract In situ transmission electron microscopy (TEM) observations of a Ni(O)–Sc2O3-stabilized ZrO2 (ScSZ; 10 mol% Sc2O3, 1 mol% CeO2, 89 mol% ZrO2) anode in a solid oxide fuel cell (SOFC) have been performed at high temperatures under a hydrogen/oxygen gas atmosphere using an environmental transmission electron microscope (ETEM); the specimens were removed from cross-sections of the real SOFC by focused ion beam milling and lifting. When heating the NiO–ScSZ anode under a hydrogen atmosphere of 3 mbar in ETEM, nano-pores were formed at the grain boundaries and on the surface of NiO particles at around 400°C due to the volume shrinkage accompanying the reduction of NiO to Ni. Moreover, densification of Ni occurred when increasing the temperature from 600 to 700°C. High-magnification TEM images obtained in the early stages of NiO reduction revealed that the (111) planes of Ni grew almost parallel to the (111) planes of NiO. In the case of heating Ni–ScSZ under an oxygen atmosphere of 3 mbar in ETEM, oxidation of Ni starting from the surface of the particles occurred above 300°C. All Ni particles became polycrystalline NiO after the temperature was increased to 800°C. Volume expansion/contraction by mass transfer to the outside/inside of the Ni particles in the anode during repeated oxidation/reduction seems to result in the agglomeration of Ni catalysts during long-term SOFC operation. We emphasize that our in situ TEM observations will be applied to observe electrochemical reactions in SOFCs under applied electric fields. in situ TEM, ETEM, solid oxide fuel cell, redox cycle, anode, Ni–ScSZ cermet Introduction Ni plays a catalytic role in the oxidation of hydrogen and methane [1,2] as well as in the decomposition of ammonia [3], and it is relatively cheap compared with precious metals; therefore, the oxidation and reduction mechanisms of Ni have been of great interest for a long time. In solid oxide fuel cells (SOFCs), composites of Ni and ZrO2-based ceramics exhibiting ionic conductivity are usually used as anode materials. Increasing the oxygen partial pressure in the fuel gas during cell operation and/or start/stop cycling at temperatures higher than 700°C causes oxidation and agglomeration of the Ni catalyst in SOFCs [4,5]. This results in a decrease in the anode activity and an increase in the anodic insulation resistance (IR) loss. The agglomeration of Ni and the reduction of the triple phase boundary (TPB) length in the anode during SOFC operation have been investigated quantitatively by focused ion beam scanning electron microscopy (FIB-SEM) 3D reconstruction [6–8]. Several in situ observation studies of the oxidation/reduction of Ni(O) using an environmental transmission electron microscope (ETEM) have been reported. Chenna et al. [9] described that the diffusion of Ni cations along grain boundaries and lattice defects promoted the oxidation and reduction of Ni(O) nanoparticles supported on SiO2 under O2/CH4 and H2 atmospheres, respectively. Regarding NiO powders, the reduction rate of NiO when heating to 600°C under a hydrogen atmosphere of 1.3 mbar in ETEM was calculated based on the change in the electron energy loss spectroscopy (EELS) peaks of Ni [10]. The pore generation and morphology of Ni particles at higher temperatures during reduction under a hydrogen atmosphere of 2 mbar have also been observed [11]. Jeangros et al. [12] investigated the structural change of a NiO–Y2O3-stabilized ZrO2 (YSZ) cermet anode while heating to 500°C under a hydrogen atmosphere of 1.4 mbar followed by heating to 550°C under an oxygen atmosphere of 3.2 mbar. They inferred that the diffusion and transfer of oxygen in NiO into the vacancy in YSZ triggered the reduction of NiO. In this paper, the microstructural evolution of Ni(O)–Sc2O3-stabilized ZrO2 (ScSZ; 10 mol% Sc2O3, 1 mol% CeO2, and 89 mol% ZrO2) accompanying reduction/oxidation under a hydrogen/oxygen atmosphere at 800°C, which is the operating temperature of SOFCs employing this material, has been investigated over sizes ranging from the sub-micron length scale to crystal-lattice scale using ETEM with aberration correction. From that, the reduction/oxidation mechanism of Ni particles and the effect of oxidation/reduction on the agglomeration of Ni catalysts during the SOFC operation have been elucidated. Since ScSZ exhibits higher ionic conductivity than YSZ at the operating temperature of SOFCs, we have employed the Ni–ScSZ for the standard anode of SOFCs. Moreover, in this study, we also developed the procedure for fabricating lamellas from cross-sections of the SOFC using FIB. The lamellas were prepared by FIB on the MEMS chip for the heating specimen holder of the ETEM. This procedure will be applied to the in situ observation of electrochemical reaction in SOFCs at high temperatures under applied electric fields. Methods SOFC preparation Electrolyte-supported cells employing ScSZ were used in this study. A mixture of either 56 wt% NiO and 44 wt% ScSZ or 80 wt% NiO and 20 wt% ScSZ was used for the anode, which was sintered at 1300°C for 3 h. Mixture of (La0.8Sr0.2)0.98MnO3 (LSM) and ScSZ in a weight ratio of 1:1 was used for the cathode, which was sintered at 1200°C for 5 h. NiO in the anode was reduced in a dry H2 gas at 900°C. The specimens for ETEM observations were taken from the cross section of the anode before and after this reduction treatment in a furnace using FIB milling and lifting, Versa 3D, FEI company. In situ TEM observation and sample preparation for the observation In situ TEM observations were performed using TitanTM ETEM G2, FEI company, with an accelerating voltage of 300 kV. This Titan ETEM has a differential pumping system, which enables us to introduce a reactive gas into the specimen chamber up to a pressure of 20 mbar and to maintain the high vacuum of the field-emission electron gun chamber at 5 × 10−6 Pa. A spherical aberration corrector for the objective lens is available for this Titan ETEM; the point resolution of a TEM image is <0.1 nm. In this experiment, an electron beam dose rate on the specimen was calculated to be 2–3 A/cm2 from the measured screen current and magnification (300 000× times). In STEM mode, the electron dose rate was 5–6 × 10−4 A/cm2 s. Furthermore, the contents of the gas exhausted from the specimen chamber can be analyzed by mass spectroscopy while introducing gas. A NanoEXTM-i/v sample holder and a MEMS microheater were used for heating the specimens. Figure 1 shows the procedure for preparing the specimens for TEM observations using FIB-SEM. Pt was deposited to fix the probe to the lamella, the lamella to a Cu grid, and the lamella on a MEMS microheater. It is necessary to set the lamella attached to the Cu grid horizontally when the lamella is transferred from the Cu grid to the MEMS (Fig. 1d and e). In the report by Duchamp et al. [13], the specimens for in situ TEM observations were prepared using FIB with a flip stage. In contrast, we used a specimen stub holder made from Mel-Build with the standard stage [14]. Fig. 1. View largeDownload slide Procedure for preparing a lamella of a SOFC anode by FIB for in situ TEM observation: (a) lifting the lamella from the cross section of the SOFC; (b) sticking the lamella to a Cu grid; (c) thinning to a thickness of around 100 nm; (d) transferring the lamella from the Cu grid to the MEMS; (e) placing the lamella at the hole position in the MEMS; and (f) standing the MEMS again, thinning the lamella to the thickness required for electron transparency, and cleaning. Fig. 1. View largeDownload slide Procedure for preparing a lamella of a SOFC anode by FIB for in situ TEM observation: (a) lifting the lamella from the cross section of the SOFC; (b) sticking the lamella to a Cu grid; (c) thinning to a thickness of around 100 nm; (d) transferring the lamella from the Cu grid to the MEMS; (e) placing the lamella at the hole position in the MEMS; and (f) standing the MEMS again, thinning the lamella to the thickness required for electron transparency, and cleaning. Before performing in situ ETEM observations of a NiO–ScSZ cermet, hydrogen gas was introduced into the specimen chamber and the partial pressure was increased to 3 mbar based on the vacuum of 1 × 10−6 mbar. Microstructural changes were then observed during heating from 30 to 800°C. The heating rate was 1°C/s from 200 to 800°C, and the temperature was held for 5 min each at 400, 500, 600 and 700°C, and for 10 min at 800°C. After the specimen had cooled to room temperature, the specimen chamber in ETEM was pumped for a few hours to obtain a vacuum of 1 × 10−6 mbar. Re-oxidation of Ni was then observed under an oxygen atmosphere of 3 mbar. In the case of Ni–ScSZ prepared by heat-treatment of NiO–ScSZ in a furnace with a dry H2 flow at 900°C, the microstructural changes of Ni-species catalysts accompanying oxidation/reduction were investigated during heating to 800°C under oxygen and hydrogen atmospheres, respectively. EELS analysis of the chemical composition of Ni(O) particles was carried out using a GIF Quantum Energy Filter, GATAN inc. Results and discussion In situ TEM observation of redox cycling of a NiO–ScSZ anode starting in a hydrogen atmosphere Figure 2 shows the bright field (BF)-scanning TEM (STEM) images of NiO–ScSZ taken during heating to 800°C under a hydrogen atmosphere of 3 mbar. As shown in these figures, nano-pores were generated at the grain boundaries and on the surface of NiO particles when the temperature was increased to 400°C. These pores spread to the inside of the NiO particles at temperatures above 400°C and were distributed homogeneously in the particles at 500°C. This pore formation was caused by the volume reduction of ~40% accompanying the phase transition from NiO to Ni: crystal structures of both Ni and NiO are face-centered cubic with the lattice parameters a of 0.3524 and 0.4177 nm, respectively. Crystal unit cells of both Ni and NiO contain 4 Ni atoms, therefore the volume reduction accompanying the phase transformation was calculated by the expression of (aNiO3 – aNi3) × 100/aNiO3. When increasing the temperature to 600°C, larger pores were generated due to the coarsening of small pores. Furthermore, densification of Ni occurred at 700°C: comparing the microstructure of Ni surrounded by white dotted rectangles in Fig. 2d–f, it is found that small pores disappear, only large pores remain, and the interfaces between Ni/Ni and Ni/ScSZ open as the temperature increases. Grain growth and coarsening of Ni particles were observed (see Supplementary Fig. S1) due to Ostwald ripening. The transformation from NiO to Ni after heating at 800°C was confirmed by high-resolution lattice images and EELS imaging. Fig. 2. View largeDownload slide BF-STEM images of the NiO–ScSZ anode taken during in situ observation while heating to 800°C in a hydrogen atmosphere of 3 mbar: (a) at room temperature before heating; (b) reduction first occurred at the grain boundaries and on the surface of NiO particles at 400°C; (c) pore formation occurred all over the NiO particles at 500°C; (d) larger pores were generated due to the coarsening of small pores at 600°C; (e) densification of Ni started at around 700°C; and (f) only large pores remained in the Ni particles after heating to 800°C, and the spaces at grain boundaries of Ni/Ni and Ni/ScSZ became large. Fig. 2. View largeDownload slide BF-STEM images of the NiO–ScSZ anode taken during in situ observation while heating to 800°C in a hydrogen atmosphere of 3 mbar: (a) at room temperature before heating; (b) reduction first occurred at the grain boundaries and on the surface of NiO particles at 400°C; (c) pore formation occurred all over the NiO particles at 500°C; (d) larger pores were generated due to the coarsening of small pores at 600°C; (e) densification of Ni started at around 700°C; and (f) only large pores remained in the Ni particles after heating to 800°C, and the spaces at grain boundaries of Ni/Ni and Ni/ScSZ became large. Most of the results described above are consistent with previous in situ TEM studies of the reduction of NiO particles and NiO–YSZ cermet. In particular, there seems to be no significant differences in microstructure evolution and reduction rate of NiO between NiO–YSZ in the previous studies and NiO–ScSZ in our study. This indicates that the NiO reduction process is not affected greatly by the oxide support. On the other hand, there are a few reports that high ionic conductivity of the electrolyte improved anode activity [15,16]. In order to investigate the interaction between oxide ionic conductor and Ni(O) particle, we have conducted more in situ TEM observation of redox cycling of NiO–ScSZ and high-magnification TEM observation of the NiO/ScSZ interface during the reduction. Figure 3 shows the BF-STEM images of NiO–ScSZ during redox cycling in ETEM. Figure 3b–d was taken after heating and keeping at 800°C for 10 min in a hydrogen, an oxygen, and a hydrogen atmosphere of 3 mbar, respectively. During the first reduction, the volume shrinkage of Ni-based grains occurred due to the transformation from NiO to Ni. The Ni particles were re-oxidized to form polycrystalline NiO when heating again to 800°C in an oxygen atmosphere, causing an associated volume expansion. These NiO particles again changed to Ni with corresponding densification during heating to 800°C in a hydrogen atmosphere. The volume changes of Ni-based particles during redox cycling resulted in a reduction of the surface areas of TPBs. In this experiment, no changes in the crystal structure and chemical composition of the ScSZ could be observed. Fig. 3. View largeDownload slide BF-STEM images of NiO–ScSZ during redox cycling while heating to 800°C: (a) at room temperature before the first reduction; (b) after the first reduction at 800°C in a hydrogen atmosphere of 3 mbar; (c) after re-oxidation at 800°C in an oxygen atmosphere of 3 mbar; and (d) after the second reduction at 800°C in a hydrogen atmosphere of 3 mbar. These images show that the volume of Ni-based particles decreases and the Ni particles become polycrystalline due to the redox cycling. This indicates that the redox cycling results in the decrease of TPBs in the SOFC anode. Fig. 3. View largeDownload slide BF-STEM images of NiO–ScSZ during redox cycling while heating to 800°C: (a) at room temperature before the first reduction; (b) after the first reduction at 800°C in a hydrogen atmosphere of 3 mbar; (c) after re-oxidation at 800°C in an oxygen atmosphere of 3 mbar; and (d) after the second reduction at 800°C in a hydrogen atmosphere of 3 mbar. These images show that the volume of Ni-based particles decreases and the Ni particles become polycrystalline due to the redox cycling. This indicates that the redox cycling results in the decrease of TPBs in the SOFC anode. Figure 4a–c shows high-magnification (observation magnification 300 000× times) images of the interface between NiO and ScSZ grains during heating to 800°C in a hydrogen atmosphere of 0.1 mbar. Fast Fourier Transform (FFT) images of ScSZ, NiO and Ni are inset in an each figure. In our ETEM configuration, the lattice images of the lamella fabricated by FIB can be taken at gas pressures of 0.1 mbar or lower. As shown in Fig. 4b, the high-magnification TEM images exhibit moiré fringes with the spacing of 1.3–1.4 nm at around the grain boundaries between NiO and ScSZ at 300°C. At a higher temperature of 400°C, pores were formed at the interface between NiO and ScSZ as shown in Fig. 4c. Ni grew toward the inside of the NiO particle with an almost specific orientation relationship: (111) of Ni is parallel to (111) of NiO and [200] of Ni to [200] of NiO. The spacing of the moiré fringes corresponds to the overlap of (111) planes in each Ni and NiO crystal. These results mean that only oxygen atoms were removed and the arrangement of Ni atoms remained in the early stage of the reduction. In the reduction process of NiO to Ni with lattice coincidence around 500–600°C, dislocations and twin boundaries were generated. However, approximately at 700°C, densification and grain growth of Ni started, the surface area of all the Ni particles decreased, and the lattice defects disappeared (see Supplementary Fig. S2). Fig. 4. View largeDownload slide High-magnification TEM images taken in the vicinity of the NiO/ScSZ interface during heating to 800°C in a hydrogen atmosphere of 0.1 mbar: (a) at room temperature before heating; (b) at 300°C; (c) at 400°C. Fast Fourier Transform (FFT) images inset are taken from the NiO particle and the ScSZ in (a), and from the regions surrounded by white dotted squares in (b) and (c), respectively. The red arrows in the FFT image of (b) indicate the spots from moiré fringes. Fig. 4. View largeDownload slide High-magnification TEM images taken in the vicinity of the NiO/ScSZ interface during heating to 800°C in a hydrogen atmosphere of 0.1 mbar: (a) at room temperature before heating; (b) at 300°C; (c) at 400°C. Fast Fourier Transform (FFT) images inset are taken from the NiO particle and the ScSZ in (a), and from the regions surrounded by white dotted squares in (b) and (c), respectively. The red arrows in the FFT image of (b) indicate the spots from moiré fringes. During both in situ observation of NiO–ScSZ redox cycling and the high-magnification TEM observation of the NiO/ScSZ interface, no change in the crystal structure and chemical composition of the ScSZ was observed. This might correspond to the thermal stability of ScSZ under reduction/oxidation conditions in these experiments. Regarding the interaction between Ni catalyst and oxide ionic conductor, Jeangros et al. [12] elucidated that the oxygen transfer from NiO to YSZ promoted the reduction reaction of NiO, although they did not observe microstructural changes of YSZ. Our high-magnification TEM observation of NiO reduction at the interface between NiO and ScSZ might suggest that hydrogen gas was dissociated to hydrogen atoms on the Ni surface, and reacted to oxygen which was released from NiO and was supplied through oxygen vacancy in the ScSZ. However, Ni metal was generated at the surface of NiO particles during the NiO reduction. This seems to result in decrease of the surface area of NiO/ScSZ grain boundaries and limitation of the oxygen diffusion from NiO to ScSZ. The effect of oxide ionic conductor on the Ni catalytic activity will be investigated while comparing with another conductors which have different crystal structures, and by in situ TEM observation of the redox cycling under applied electric fields. In situ TEM observation of redox cycling of the Ni–ScSZ anode starting in an oxygen atmosphere Figure 5 shows BF-STEM images of Ni–ScSZ during heating to 800°C in an oxygen atmosphere of 3 mbar. In this case, changes in the surface of the Ni particles were observed at around 400°C, and NiO with a lower density covered the surface of the Ni particles at 500°C. Over 500°C, NiO was grown on both the outside and inside of the Ni grains. In reference [10], oxidation of Ni was reported to proceed inhomogeneously. Therefore, the difference in the oxidation reaction rate caused differences in mass distribution and image contrast in the two-dimensional view. After heating at 800°C, Ni was converted to polycrystalline NiO as shown in Fig. 5f (and see Supplementary Fig. S3). This transformation from Ni to NiO was confirmed by high-resolution lattice images and EELS imaging. A temperature higher than 800°C is needed for densification and grain growth of NiO. Fig. 5. View largeDownload slide BF-STEM images of Ni–ScSZ taken during in situ observation while heating to 800°C in an oxygen atmosphere of 3 mbar: (a) at room temperature before heating; (b) structural changes in the Ni surface was noticed occurring before the temperature reached to 400°C; (c) the entire surface of Ni particles was oxidized at around 500°C; (d) large pores were visible inside Ni-based particles at around 600°C; (e) volume expansion of Ni-based particles was observed after heating to 700°C; and (f) each Ni particle became polycrystalline NiO after heating to 800°C. Fig. 5. View largeDownload slide BF-STEM images of Ni–ScSZ taken during in situ observation while heating to 800°C in an oxygen atmosphere of 3 mbar: (a) at room temperature before heating; (b) structural changes in the Ni surface was noticed occurring before the temperature reached to 400°C; (c) the entire surface of Ni particles was oxidized at around 500°C; (d) large pores were visible inside Ni-based particles at around 600°C; (e) volume expansion of Ni-based particles was observed after heating to 700°C; and (f) each Ni particle became polycrystalline NiO after heating to 800°C. Figure 6 shows high-magnification TEM images of the Ni surface in the Ni–ScSZ during heating to 800°C in an oxygen atmosphere of 0.001 mbar. The introduced oxygen gas pressure was reduced to the order of 102 in order to reduce the oxidation rate of Ni, although crystal-lattice images of Ni could be obtained under an oxygen atmosphere of 0.1 mbar. During this observation, when the temperature reached 300°C, the surface structure of Ni started to change: {111} cleavage planes of Ni appeared in a triangular shape. At temperatures higher than 300°C, these triangular planes became a film which covered the entire surface of the Ni, and the thickness of this amorphous oxide film increased to 5 nm at 400°C. In this stage, the change in the morphology of Ni started to be observed. This film was then removed from the surface of the Ni particle, and the thickness of the film was 10 nm at 500°C. Above 500°C, NiO crystals with facets grew on the outside of the particle, and oxidation proceeds into the inside of the Ni particle. As shown in these figures, in situ high-resolution TEM observation was also successfully done about the oxidation process of Ni particles in the SOFC anode under an oxygen atmosphere. Fig. 6. View largeDownload slide High-magnification TEM images of the surface of the Ni particle in Ni–ScSZ cermet during heating to 800°C in an oxygen atmosphere of 0.001 mbar: (a) at 300°C; (b) at 500°C; (c) at 510°C. An FFT image of the Ni particle is inset at the top of the right-hand side in (a), and an FFT image of the white dotted square region is at the bottom of the right-hand side in (b). Fig. 6. View largeDownload slide High-magnification TEM images of the surface of the Ni particle in Ni–ScSZ cermet during heating to 800°C in an oxygen atmosphere of 0.001 mbar: (a) at 300°C; (b) at 500°C; (c) at 510°C. An FFT image of the Ni particle is inset at the top of the right-hand side in (a), and an FFT image of the white dotted square region is at the bottom of the right-hand side in (b). Figure 7 shows BF-STEM images of Ni–ScSZ during redox cycling in ETEM. Figure 7b–d were taken after heating and keeping at 800°C for 10 min in an oxygen, a hydrogen, and an oxygen atmosphere of 3 mbar, respectively. As shown in these figures, a single Ni particle was divided into more than two Ni particles during redox cycling: in the process of oxidation at the maximum temperature of 800°C, a Ni particle became polycrystalline NiO. Following that polycrystalline NiO do not return to a Ni single crystal in the reduction at 800°C under this experimental condition, although densification occurs over 700°C. Moreover, these figures clearly indicate that the oxidation of Ni is accompanied by the volume expansion of the Ni particle. Regarding the ScSZ particles, no changes in the crystal structure and chemical composition could be observed in the same way as the redox cycling of NiO–ScSZ. One reason for the unchanged framework of ScSZ is that ScSZ is thermodynamically stable under the conditions of this experiment. Another reason might be the large surface area of the TEM samples, which enables Ni to expand from the free surface during oxidation. However, in the actual fuel cell, it is possibly that the volume expansion of Ni particles during redox cycling results in the agglomeration of Ni electrocatalysts. Fig. 7. View largeDownload slide BF-STEM images of Ni–ScSZ during redox cycling while heating to 800°C: (a) at room temperature before the first oxidation; (b) after the first oxidation at 800°C in an oxygen atmosphere of 3 mbar; (c) after reduction at 800°C in a hydrogen atmosphere of 3 mbar; and (d) after re-oxidation at 800°C in an oxygen atmosphere of 3 mbar. Fig. 7. View largeDownload slide BF-STEM images of Ni–ScSZ during redox cycling while heating to 800°C: (a) at room temperature before the first oxidation; (b) after the first oxidation at 800°C in an oxygen atmosphere of 3 mbar; (c) after reduction at 800°C in a hydrogen atmosphere of 3 mbar; and (d) after re-oxidation at 800°C in an oxygen atmosphere of 3 mbar. Figure 8 shows the BF-STEM images of a single Ni particle during redox cycling. Figure 8b–e were taken after heating and keeping at 800°C for 10 min in an oxygen, a hydrogen, an oxygen, and a hydrogen atmosphere of 3 mbar, respectively. These images clearly show that redox cycling of Ni particles accompanies not only volume changes, but also mass distribution. Mass transfer occurred to the outside of the particle during oxidation and to the inside during reduction. These changes in the volume and density distributions of Ni-based catalysts during redox cycling are considered to cause the agglomeration of Ni particles during SOFC operation. Fig. 8. View largeDownload slide BF-STEM images of a Ni single particle supported by oxide in the SOFC anode during redox cycling while heating to 800°C; (a) at room temperature before the first oxidation; (b) after the first oxidation at 800°C in an oxygen atmosphere of 3 mbar; (c) after reduction at 800°C in a hydrogen atmosphere of 3 mbar; (d) after re-oxidation at 800°C in an oxygen atmosphere of 3 mbar; and (e) after the second reduction at 800°C in a hydrogen atmosphere. These images show that the redox cycling is accompanied by the change in volume and mass distribution of the Ni-based particle. Fig. 8. View largeDownload slide BF-STEM images of a Ni single particle supported by oxide in the SOFC anode during redox cycling while heating to 800°C; (a) at room temperature before the first oxidation; (b) after the first oxidation at 800°C in an oxygen atmosphere of 3 mbar; (c) after reduction at 800°C in a hydrogen atmosphere of 3 mbar; (d) after re-oxidation at 800°C in an oxygen atmosphere of 3 mbar; and (e) after the second reduction at 800°C in a hydrogen atmosphere. These images show that the redox cycling is accompanied by the change in volume and mass distribution of the Ni-based particle. Concluding remarks Lamellas were prepared by FIB-SEM from the standard anode materials in electrolyte-supported fuel cell, and investigated during heating to 800°C in an oxygen or hydrogen atmosphere. The microstructural evolution over length scales ranging from a few microns to sub-nano scale was elucidated in an oxygen or a hydrogen atmosphere of 3 mbar. Moreover, crystal-lattice TEM images in the early stages of oxidation of Ni–ScSZ and reduction of NiO–ScSZ were taken under 0.001–0.1 mbar oxygen and hydrogen atmospheres, respectively. Changes in volume and mass distribution were observed during redox cycling, which are considered to result in Ni agglomeration during long-term cell operation. In future work, in situ TEM observation could be used to observe fuel cell materials under an applied electric field. Acknowledgements The author(s) gratefully acknowledge the support of the International Institute for Carbon Neutral Energy Research (WPI-I2CNER), sponsored by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT). This work was partially supported by Japan Science and Technology Agency (JST) through its ‘Center of Innovation Science and Technology based Radical Innovation and Entrepreneurship Program (COI Program)’. References 1 Choudhary R V , Rajput M A , and Mamman S A ( 1998 ) NiO-alkaline earth oxide catalysts for oxidative methane-to-syngas conversion: influence of alkaline earth oxide on the surface properties and temperature-programmed reduction/ reaction by H2 and methane . J. Catal. 178 : 576 – 585 . Google Scholar Crossref Search ADS 2 Bruce L , and Mathews F J ( 1982 ) The fischer-tropsch activity of nickel-zirconia . Appl. Catal. 4 : 353 – 370 . Google Scholar Crossref Search ADS 3 Simonsen B S , Chakraborty D , Chorkendorff I , and Dahl S ( 2012 ) Alloyed Ni-Fe nanoparticles as catalysis for NH3 decomposition . Appl. Catal. A Gen. 447–448 : 22 – 31 . Google Scholar Crossref Search ADS 4 Iwata T ( 1996 ) Characterization of Ni–YSZ anode degradation for substrate-type solid oxide fuel cells . J. Electrochem. Soc. 143 : 1521 – 1525 . Google Scholar Crossref Search ADS 5 Klemenso T , Appel C C , and Mogensen M ( 2006 ) In situ observations of microstructural changes in SOFC anodes during redox cycling . Electrochem. Solid-State Lett. 9 : A403 – A407 . Google Scholar Crossref Search ADS 6 Sumi H , Kishida R , Kim J-Y , Muroyama H , Matsui T , and Eguchi K ( 2010 ) Correlation between microstructural and electrochemical characteristics during redox cycles for Ni–YSZ anode of SOFCs . J. Electrochem. Soc. 157 : B1747 – B1752 . Google Scholar Crossref Search ADS 7 Kubota M , Muroyama H , Matsui T , and Eguchi K ( 2013 ) Microstructural change of Ni–YSZ anode under thermal cycles with redox treatments . ECS Trans. 57 : 589 – 597 . Google Scholar Crossref Search ADS 8 Joos J , Ender M , Rotscholl I , Menzler N H , and Ivers-Tiffee E ( 2014 ) Quantification of double-layer Ni/YSZ fuel cell anodes from focused ion beam tomography data . J. Power Sources 246 : 819 – 830 . Google Scholar Crossref Search ADS 9 Chenna S , Banerjee R , and Crozier A ( 2011 ) Atomic-scale observation of the Ni activation process for partial oxidation of methane using in situ environmental TEM . Chem. Cat. Chem. 3 : 1051 – 1059 . 10 Jeangros Q , Hansen W T , Wagner B J , Damsgaard D C , Dunin-Borkowski E R , Hebert C , Herle Van J , and Hessler-Wyser A ( 2013 ) Reduction of nickel oxide particles by hydrogen studied in an environmental TEM . J. Mater. Sci. 48 : 2893 – 2907 . Google Scholar Crossref Search ADS 11 Simonsen B S , Agersted K , Hansen V K , Jacobsen T , Wagner B J , Hansen W T , and Kuhn T L ( 2015 ) Environmental TEM study of the dynamic nanoscaled morphology of NiO/YSZ during reduction . Appl. Catal. A Gen. 489 : 147 – 154 . Google Scholar Crossref Search ADS 12 Jeangros Q , Faes A , Wagner B J , Hansen W T , Aschauer U , Herle Van J , Hessler-Wyser A , and Dunin-Borkowski E R ( 2010 ) In situ redox cycle of a nickel–YSZ fuel cell anode in an environmental transmission electron microscope . Acta Mater. 58 : 4578 – 4589 . Google Scholar Crossref Search ADS 13 Duchamp M , Xu Q , and Dunin-Borkowski E R ( 2014 ) Convenient preparation of high-quality specimens for annealing experiments in the transmission electron microscope . Microsc. Microanal. 20 : 1638 – 1645 . Google Scholar Crossref Search ADS PubMed 14 http://www.melbuild.com/. 15 Wagner T , Kirchheim R , and Ruhle M ( 1995 ) Chemical reactions at metal/ceramic interfaces duing diffusion bonding . Acta Metal. Mater. 43 : 1053 – 1063 . Google Scholar Crossref Search ADS 16 Uchida H , Suzuki H , and Watanabe M ( 1998 ) High-performance electrode for medium-temperature solid oxide fuel cells: effects of composition and microstructure on performance of ceria-based anodes . J. Electrochem. Soc. 145 : 615 – 620 . Google Scholar Crossref Search ADS © The Author(s) 2018. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Microscopy Oxford University Press

In situ transmission electron microscopic observations of redox cycling of a Ni–ScSZ cermet fuel cell anode

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Abstract

Abstract In situ transmission electron microscopy (TEM) observations of a Ni(O)–Sc2O3-stabilized ZrO2 (ScSZ; 10 mol% Sc2O3, 1 mol% CeO2, 89 mol% ZrO2) anode in a solid oxide fuel cell (SOFC) have been performed at high temperatures under a hydrogen/oxygen gas atmosphere using an environmental transmission electron microscope (ETEM); the specimens were removed from cross-sections of the real SOFC by focused ion beam milling and lifting. When heating the NiO–ScSZ anode under a hydrogen atmosphere of 3 mbar in ETEM, nano-pores were formed at the grain boundaries and on the surface of NiO particles at around 400°C due to the volume shrinkage accompanying the reduction of NiO to Ni. Moreover, densification of Ni occurred when increasing the temperature from 600 to 700°C. High-magnification TEM images obtained in the early stages of NiO reduction revealed that the (111) planes of Ni grew almost parallel to the (111) planes of NiO. In the case of heating Ni–ScSZ under an oxygen atmosphere of 3 mbar in ETEM, oxidation of Ni starting from the surface of the particles occurred above 300°C. All Ni particles became polycrystalline NiO after the temperature was increased to 800°C. Volume expansion/contraction by mass transfer to the outside/inside of the Ni particles in the anode during repeated oxidation/reduction seems to result in the agglomeration of Ni catalysts during long-term SOFC operation. We emphasize that our in situ TEM observations will be applied to observe electrochemical reactions in SOFCs under applied electric fields. in situ TEM, ETEM, solid oxide fuel cell, redox cycle, anode, Ni–ScSZ cermet Introduction Ni plays a catalytic role in the oxidation of hydrogen and methane [1,2] as well as in the decomposition of ammonia [3], and it is relatively cheap compared with precious metals; therefore, the oxidation and reduction mechanisms of Ni have been of great interest for a long time. In solid oxide fuel cells (SOFCs), composites of Ni and ZrO2-based ceramics exhibiting ionic conductivity are usually used as anode materials. Increasing the oxygen partial pressure in the fuel gas during cell operation and/or start/stop cycling at temperatures higher than 700°C causes oxidation and agglomeration of the Ni catalyst in SOFCs [4,5]. This results in a decrease in the anode activity and an increase in the anodic insulation resistance (IR) loss. The agglomeration of Ni and the reduction of the triple phase boundary (TPB) length in the anode during SOFC operation have been investigated quantitatively by focused ion beam scanning electron microscopy (FIB-SEM) 3D reconstruction [6–8]. Several in situ observation studies of the oxidation/reduction of Ni(O) using an environmental transmission electron microscope (ETEM) have been reported. Chenna et al. [9] described that the diffusion of Ni cations along grain boundaries and lattice defects promoted the oxidation and reduction of Ni(O) nanoparticles supported on SiO2 under O2/CH4 and H2 atmospheres, respectively. Regarding NiO powders, the reduction rate of NiO when heating to 600°C under a hydrogen atmosphere of 1.3 mbar in ETEM was calculated based on the change in the electron energy loss spectroscopy (EELS) peaks of Ni [10]. The pore generation and morphology of Ni particles at higher temperatures during reduction under a hydrogen atmosphere of 2 mbar have also been observed [11]. Jeangros et al. [12] investigated the structural change of a NiO–Y2O3-stabilized ZrO2 (YSZ) cermet anode while heating to 500°C under a hydrogen atmosphere of 1.4 mbar followed by heating to 550°C under an oxygen atmosphere of 3.2 mbar. They inferred that the diffusion and transfer of oxygen in NiO into the vacancy in YSZ triggered the reduction of NiO. In this paper, the microstructural evolution of Ni(O)–Sc2O3-stabilized ZrO2 (ScSZ; 10 mol% Sc2O3, 1 mol% CeO2, and 89 mol% ZrO2) accompanying reduction/oxidation under a hydrogen/oxygen atmosphere at 800°C, which is the operating temperature of SOFCs employing this material, has been investigated over sizes ranging from the sub-micron length scale to crystal-lattice scale using ETEM with aberration correction. From that, the reduction/oxidation mechanism of Ni particles and the effect of oxidation/reduction on the agglomeration of Ni catalysts during the SOFC operation have been elucidated. Since ScSZ exhibits higher ionic conductivity than YSZ at the operating temperature of SOFCs, we have employed the Ni–ScSZ for the standard anode of SOFCs. Moreover, in this study, we also developed the procedure for fabricating lamellas from cross-sections of the SOFC using FIB. The lamellas were prepared by FIB on the MEMS chip for the heating specimen holder of the ETEM. This procedure will be applied to the in situ observation of electrochemical reaction in SOFCs at high temperatures under applied electric fields. Methods SOFC preparation Electrolyte-supported cells employing ScSZ were used in this study. A mixture of either 56 wt% NiO and 44 wt% ScSZ or 80 wt% NiO and 20 wt% ScSZ was used for the anode, which was sintered at 1300°C for 3 h. Mixture of (La0.8Sr0.2)0.98MnO3 (LSM) and ScSZ in a weight ratio of 1:1 was used for the cathode, which was sintered at 1200°C for 5 h. NiO in the anode was reduced in a dry H2 gas at 900°C. The specimens for ETEM observations were taken from the cross section of the anode before and after this reduction treatment in a furnace using FIB milling and lifting, Versa 3D, FEI company. In situ TEM observation and sample preparation for the observation In situ TEM observations were performed using TitanTM ETEM G2, FEI company, with an accelerating voltage of 300 kV. This Titan ETEM has a differential pumping system, which enables us to introduce a reactive gas into the specimen chamber up to a pressure of 20 mbar and to maintain the high vacuum of the field-emission electron gun chamber at 5 × 10−6 Pa. A spherical aberration corrector for the objective lens is available for this Titan ETEM; the point resolution of a TEM image is <0.1 nm. In this experiment, an electron beam dose rate on the specimen was calculated to be 2–3 A/cm2 from the measured screen current and magnification (300 000× times). In STEM mode, the electron dose rate was 5–6 × 10−4 A/cm2 s. Furthermore, the contents of the gas exhausted from the specimen chamber can be analyzed by mass spectroscopy while introducing gas. A NanoEXTM-i/v sample holder and a MEMS microheater were used for heating the specimens. Figure 1 shows the procedure for preparing the specimens for TEM observations using FIB-SEM. Pt was deposited to fix the probe to the lamella, the lamella to a Cu grid, and the lamella on a MEMS microheater. It is necessary to set the lamella attached to the Cu grid horizontally when the lamella is transferred from the Cu grid to the MEMS (Fig. 1d and e). In the report by Duchamp et al. [13], the specimens for in situ TEM observations were prepared using FIB with a flip stage. In contrast, we used a specimen stub holder made from Mel-Build with the standard stage [14]. Fig. 1. View largeDownload slide Procedure for preparing a lamella of a SOFC anode by FIB for in situ TEM observation: (a) lifting the lamella from the cross section of the SOFC; (b) sticking the lamella to a Cu grid; (c) thinning to a thickness of around 100 nm; (d) transferring the lamella from the Cu grid to the MEMS; (e) placing the lamella at the hole position in the MEMS; and (f) standing the MEMS again, thinning the lamella to the thickness required for electron transparency, and cleaning. Fig. 1. View largeDownload slide Procedure for preparing a lamella of a SOFC anode by FIB for in situ TEM observation: (a) lifting the lamella from the cross section of the SOFC; (b) sticking the lamella to a Cu grid; (c) thinning to a thickness of around 100 nm; (d) transferring the lamella from the Cu grid to the MEMS; (e) placing the lamella at the hole position in the MEMS; and (f) standing the MEMS again, thinning the lamella to the thickness required for electron transparency, and cleaning. Before performing in situ ETEM observations of a NiO–ScSZ cermet, hydrogen gas was introduced into the specimen chamber and the partial pressure was increased to 3 mbar based on the vacuum of 1 × 10−6 mbar. Microstructural changes were then observed during heating from 30 to 800°C. The heating rate was 1°C/s from 200 to 800°C, and the temperature was held for 5 min each at 400, 500, 600 and 700°C, and for 10 min at 800°C. After the specimen had cooled to room temperature, the specimen chamber in ETEM was pumped for a few hours to obtain a vacuum of 1 × 10−6 mbar. Re-oxidation of Ni was then observed under an oxygen atmosphere of 3 mbar. In the case of Ni–ScSZ prepared by heat-treatment of NiO–ScSZ in a furnace with a dry H2 flow at 900°C, the microstructural changes of Ni-species catalysts accompanying oxidation/reduction were investigated during heating to 800°C under oxygen and hydrogen atmospheres, respectively. EELS analysis of the chemical composition of Ni(O) particles was carried out using a GIF Quantum Energy Filter, GATAN inc. Results and discussion In situ TEM observation of redox cycling of a NiO–ScSZ anode starting in a hydrogen atmosphere Figure 2 shows the bright field (BF)-scanning TEM (STEM) images of NiO–ScSZ taken during heating to 800°C under a hydrogen atmosphere of 3 mbar. As shown in these figures, nano-pores were generated at the grain boundaries and on the surface of NiO particles when the temperature was increased to 400°C. These pores spread to the inside of the NiO particles at temperatures above 400°C and were distributed homogeneously in the particles at 500°C. This pore formation was caused by the volume reduction of ~40% accompanying the phase transition from NiO to Ni: crystal structures of both Ni and NiO are face-centered cubic with the lattice parameters a of 0.3524 and 0.4177 nm, respectively. Crystal unit cells of both Ni and NiO contain 4 Ni atoms, therefore the volume reduction accompanying the phase transformation was calculated by the expression of (aNiO3 – aNi3) × 100/aNiO3. When increasing the temperature to 600°C, larger pores were generated due to the coarsening of small pores. Furthermore, densification of Ni occurred at 700°C: comparing the microstructure of Ni surrounded by white dotted rectangles in Fig. 2d–f, it is found that small pores disappear, only large pores remain, and the interfaces between Ni/Ni and Ni/ScSZ open as the temperature increases. Grain growth and coarsening of Ni particles were observed (see Supplementary Fig. S1) due to Ostwald ripening. The transformation from NiO to Ni after heating at 800°C was confirmed by high-resolution lattice images and EELS imaging. Fig. 2. View largeDownload slide BF-STEM images of the NiO–ScSZ anode taken during in situ observation while heating to 800°C in a hydrogen atmosphere of 3 mbar: (a) at room temperature before heating; (b) reduction first occurred at the grain boundaries and on the surface of NiO particles at 400°C; (c) pore formation occurred all over the NiO particles at 500°C; (d) larger pores were generated due to the coarsening of small pores at 600°C; (e) densification of Ni started at around 700°C; and (f) only large pores remained in the Ni particles after heating to 800°C, and the spaces at grain boundaries of Ni/Ni and Ni/ScSZ became large. Fig. 2. View largeDownload slide BF-STEM images of the NiO–ScSZ anode taken during in situ observation while heating to 800°C in a hydrogen atmosphere of 3 mbar: (a) at room temperature before heating; (b) reduction first occurred at the grain boundaries and on the surface of NiO particles at 400°C; (c) pore formation occurred all over the NiO particles at 500°C; (d) larger pores were generated due to the coarsening of small pores at 600°C; (e) densification of Ni started at around 700°C; and (f) only large pores remained in the Ni particles after heating to 800°C, and the spaces at grain boundaries of Ni/Ni and Ni/ScSZ became large. Most of the results described above are consistent with previous in situ TEM studies of the reduction of NiO particles and NiO–YSZ cermet. In particular, there seems to be no significant differences in microstructure evolution and reduction rate of NiO between NiO–YSZ in the previous studies and NiO–ScSZ in our study. This indicates that the NiO reduction process is not affected greatly by the oxide support. On the other hand, there are a few reports that high ionic conductivity of the electrolyte improved anode activity [15,16]. In order to investigate the interaction between oxide ionic conductor and Ni(O) particle, we have conducted more in situ TEM observation of redox cycling of NiO–ScSZ and high-magnification TEM observation of the NiO/ScSZ interface during the reduction. Figure 3 shows the BF-STEM images of NiO–ScSZ during redox cycling in ETEM. Figure 3b–d was taken after heating and keeping at 800°C for 10 min in a hydrogen, an oxygen, and a hydrogen atmosphere of 3 mbar, respectively. During the first reduction, the volume shrinkage of Ni-based grains occurred due to the transformation from NiO to Ni. The Ni particles were re-oxidized to form polycrystalline NiO when heating again to 800°C in an oxygen atmosphere, causing an associated volume expansion. These NiO particles again changed to Ni with corresponding densification during heating to 800°C in a hydrogen atmosphere. The volume changes of Ni-based particles during redox cycling resulted in a reduction of the surface areas of TPBs. In this experiment, no changes in the crystal structure and chemical composition of the ScSZ could be observed. Fig. 3. View largeDownload slide BF-STEM images of NiO–ScSZ during redox cycling while heating to 800°C: (a) at room temperature before the first reduction; (b) after the first reduction at 800°C in a hydrogen atmosphere of 3 mbar; (c) after re-oxidation at 800°C in an oxygen atmosphere of 3 mbar; and (d) after the second reduction at 800°C in a hydrogen atmosphere of 3 mbar. These images show that the volume of Ni-based particles decreases and the Ni particles become polycrystalline due to the redox cycling. This indicates that the redox cycling results in the decrease of TPBs in the SOFC anode. Fig. 3. View largeDownload slide BF-STEM images of NiO–ScSZ during redox cycling while heating to 800°C: (a) at room temperature before the first reduction; (b) after the first reduction at 800°C in a hydrogen atmosphere of 3 mbar; (c) after re-oxidation at 800°C in an oxygen atmosphere of 3 mbar; and (d) after the second reduction at 800°C in a hydrogen atmosphere of 3 mbar. These images show that the volume of Ni-based particles decreases and the Ni particles become polycrystalline due to the redox cycling. This indicates that the redox cycling results in the decrease of TPBs in the SOFC anode. Figure 4a–c shows high-magnification (observation magnification 300 000× times) images of the interface between NiO and ScSZ grains during heating to 800°C in a hydrogen atmosphere of 0.1 mbar. Fast Fourier Transform (FFT) images of ScSZ, NiO and Ni are inset in an each figure. In our ETEM configuration, the lattice images of the lamella fabricated by FIB can be taken at gas pressures of 0.1 mbar or lower. As shown in Fig. 4b, the high-magnification TEM images exhibit moiré fringes with the spacing of 1.3–1.4 nm at around the grain boundaries between NiO and ScSZ at 300°C. At a higher temperature of 400°C, pores were formed at the interface between NiO and ScSZ as shown in Fig. 4c. Ni grew toward the inside of the NiO particle with an almost specific orientation relationship: (111) of Ni is parallel to (111) of NiO and [200] of Ni to [200] of NiO. The spacing of the moiré fringes corresponds to the overlap of (111) planes in each Ni and NiO crystal. These results mean that only oxygen atoms were removed and the arrangement of Ni atoms remained in the early stage of the reduction. In the reduction process of NiO to Ni with lattice coincidence around 500–600°C, dislocations and twin boundaries were generated. However, approximately at 700°C, densification and grain growth of Ni started, the surface area of all the Ni particles decreased, and the lattice defects disappeared (see Supplementary Fig. S2). Fig. 4. View largeDownload slide High-magnification TEM images taken in the vicinity of the NiO/ScSZ interface during heating to 800°C in a hydrogen atmosphere of 0.1 mbar: (a) at room temperature before heating; (b) at 300°C; (c) at 400°C. Fast Fourier Transform (FFT) images inset are taken from the NiO particle and the ScSZ in (a), and from the regions surrounded by white dotted squares in (b) and (c), respectively. The red arrows in the FFT image of (b) indicate the spots from moiré fringes. Fig. 4. View largeDownload slide High-magnification TEM images taken in the vicinity of the NiO/ScSZ interface during heating to 800°C in a hydrogen atmosphere of 0.1 mbar: (a) at room temperature before heating; (b) at 300°C; (c) at 400°C. Fast Fourier Transform (FFT) images inset are taken from the NiO particle and the ScSZ in (a), and from the regions surrounded by white dotted squares in (b) and (c), respectively. The red arrows in the FFT image of (b) indicate the spots from moiré fringes. During both in situ observation of NiO–ScSZ redox cycling and the high-magnification TEM observation of the NiO/ScSZ interface, no change in the crystal structure and chemical composition of the ScSZ was observed. This might correspond to the thermal stability of ScSZ under reduction/oxidation conditions in these experiments. Regarding the interaction between Ni catalyst and oxide ionic conductor, Jeangros et al. [12] elucidated that the oxygen transfer from NiO to YSZ promoted the reduction reaction of NiO, although they did not observe microstructural changes of YSZ. Our high-magnification TEM observation of NiO reduction at the interface between NiO and ScSZ might suggest that hydrogen gas was dissociated to hydrogen atoms on the Ni surface, and reacted to oxygen which was released from NiO and was supplied through oxygen vacancy in the ScSZ. However, Ni metal was generated at the surface of NiO particles during the NiO reduction. This seems to result in decrease of the surface area of NiO/ScSZ grain boundaries and limitation of the oxygen diffusion from NiO to ScSZ. The effect of oxide ionic conductor on the Ni catalytic activity will be investigated while comparing with another conductors which have different crystal structures, and by in situ TEM observation of the redox cycling under applied electric fields. In situ TEM observation of redox cycling of the Ni–ScSZ anode starting in an oxygen atmosphere Figure 5 shows BF-STEM images of Ni–ScSZ during heating to 800°C in an oxygen atmosphere of 3 mbar. In this case, changes in the surface of the Ni particles were observed at around 400°C, and NiO with a lower density covered the surface of the Ni particles at 500°C. Over 500°C, NiO was grown on both the outside and inside of the Ni grains. In reference [10], oxidation of Ni was reported to proceed inhomogeneously. Therefore, the difference in the oxidation reaction rate caused differences in mass distribution and image contrast in the two-dimensional view. After heating at 800°C, Ni was converted to polycrystalline NiO as shown in Fig. 5f (and see Supplementary Fig. S3). This transformation from Ni to NiO was confirmed by high-resolution lattice images and EELS imaging. A temperature higher than 800°C is needed for densification and grain growth of NiO. Fig. 5. View largeDownload slide BF-STEM images of Ni–ScSZ taken during in situ observation while heating to 800°C in an oxygen atmosphere of 3 mbar: (a) at room temperature before heating; (b) structural changes in the Ni surface was noticed occurring before the temperature reached to 400°C; (c) the entire surface of Ni particles was oxidized at around 500°C; (d) large pores were visible inside Ni-based particles at around 600°C; (e) volume expansion of Ni-based particles was observed after heating to 700°C; and (f) each Ni particle became polycrystalline NiO after heating to 800°C. Fig. 5. View largeDownload slide BF-STEM images of Ni–ScSZ taken during in situ observation while heating to 800°C in an oxygen atmosphere of 3 mbar: (a) at room temperature before heating; (b) structural changes in the Ni surface was noticed occurring before the temperature reached to 400°C; (c) the entire surface of Ni particles was oxidized at around 500°C; (d) large pores were visible inside Ni-based particles at around 600°C; (e) volume expansion of Ni-based particles was observed after heating to 700°C; and (f) each Ni particle became polycrystalline NiO after heating to 800°C. Figure 6 shows high-magnification TEM images of the Ni surface in the Ni–ScSZ during heating to 800°C in an oxygen atmosphere of 0.001 mbar. The introduced oxygen gas pressure was reduced to the order of 102 in order to reduce the oxidation rate of Ni, although crystal-lattice images of Ni could be obtained under an oxygen atmosphere of 0.1 mbar. During this observation, when the temperature reached 300°C, the surface structure of Ni started to change: {111} cleavage planes of Ni appeared in a triangular shape. At temperatures higher than 300°C, these triangular planes became a film which covered the entire surface of the Ni, and the thickness of this amorphous oxide film increased to 5 nm at 400°C. In this stage, the change in the morphology of Ni started to be observed. This film was then removed from the surface of the Ni particle, and the thickness of the film was 10 nm at 500°C. Above 500°C, NiO crystals with facets grew on the outside of the particle, and oxidation proceeds into the inside of the Ni particle. As shown in these figures, in situ high-resolution TEM observation was also successfully done about the oxidation process of Ni particles in the SOFC anode under an oxygen atmosphere. Fig. 6. View largeDownload slide High-magnification TEM images of the surface of the Ni particle in Ni–ScSZ cermet during heating to 800°C in an oxygen atmosphere of 0.001 mbar: (a) at 300°C; (b) at 500°C; (c) at 510°C. An FFT image of the Ni particle is inset at the top of the right-hand side in (a), and an FFT image of the white dotted square region is at the bottom of the right-hand side in (b). Fig. 6. View largeDownload slide High-magnification TEM images of the surface of the Ni particle in Ni–ScSZ cermet during heating to 800°C in an oxygen atmosphere of 0.001 mbar: (a) at 300°C; (b) at 500°C; (c) at 510°C. An FFT image of the Ni particle is inset at the top of the right-hand side in (a), and an FFT image of the white dotted square region is at the bottom of the right-hand side in (b). Figure 7 shows BF-STEM images of Ni–ScSZ during redox cycling in ETEM. Figure 7b–d were taken after heating and keeping at 800°C for 10 min in an oxygen, a hydrogen, and an oxygen atmosphere of 3 mbar, respectively. As shown in these figures, a single Ni particle was divided into more than two Ni particles during redox cycling: in the process of oxidation at the maximum temperature of 800°C, a Ni particle became polycrystalline NiO. Following that polycrystalline NiO do not return to a Ni single crystal in the reduction at 800°C under this experimental condition, although densification occurs over 700°C. Moreover, these figures clearly indicate that the oxidation of Ni is accompanied by the volume expansion of the Ni particle. Regarding the ScSZ particles, no changes in the crystal structure and chemical composition could be observed in the same way as the redox cycling of NiO–ScSZ. One reason for the unchanged framework of ScSZ is that ScSZ is thermodynamically stable under the conditions of this experiment. Another reason might be the large surface area of the TEM samples, which enables Ni to expand from the free surface during oxidation. However, in the actual fuel cell, it is possibly that the volume expansion of Ni particles during redox cycling results in the agglomeration of Ni electrocatalysts. Fig. 7. View largeDownload slide BF-STEM images of Ni–ScSZ during redox cycling while heating to 800°C: (a) at room temperature before the first oxidation; (b) after the first oxidation at 800°C in an oxygen atmosphere of 3 mbar; (c) after reduction at 800°C in a hydrogen atmosphere of 3 mbar; and (d) after re-oxidation at 800°C in an oxygen atmosphere of 3 mbar. Fig. 7. View largeDownload slide BF-STEM images of Ni–ScSZ during redox cycling while heating to 800°C: (a) at room temperature before the first oxidation; (b) after the first oxidation at 800°C in an oxygen atmosphere of 3 mbar; (c) after reduction at 800°C in a hydrogen atmosphere of 3 mbar; and (d) after re-oxidation at 800°C in an oxygen atmosphere of 3 mbar. Figure 8 shows the BF-STEM images of a single Ni particle during redox cycling. Figure 8b–e were taken after heating and keeping at 800°C for 10 min in an oxygen, a hydrogen, an oxygen, and a hydrogen atmosphere of 3 mbar, respectively. These images clearly show that redox cycling of Ni particles accompanies not only volume changes, but also mass distribution. Mass transfer occurred to the outside of the particle during oxidation and to the inside during reduction. These changes in the volume and density distributions of Ni-based catalysts during redox cycling are considered to cause the agglomeration of Ni particles during SOFC operation. Fig. 8. View largeDownload slide BF-STEM images of a Ni single particle supported by oxide in the SOFC anode during redox cycling while heating to 800°C; (a) at room temperature before the first oxidation; (b) after the first oxidation at 800°C in an oxygen atmosphere of 3 mbar; (c) after reduction at 800°C in a hydrogen atmosphere of 3 mbar; (d) after re-oxidation at 800°C in an oxygen atmosphere of 3 mbar; and (e) after the second reduction at 800°C in a hydrogen atmosphere. These images show that the redox cycling is accompanied by the change in volume and mass distribution of the Ni-based particle. Fig. 8. View largeDownload slide BF-STEM images of a Ni single particle supported by oxide in the SOFC anode during redox cycling while heating to 800°C; (a) at room temperature before the first oxidation; (b) after the first oxidation at 800°C in an oxygen atmosphere of 3 mbar; (c) after reduction at 800°C in a hydrogen atmosphere of 3 mbar; (d) after re-oxidation at 800°C in an oxygen atmosphere of 3 mbar; and (e) after the second reduction at 800°C in a hydrogen atmosphere. These images show that the redox cycling is accompanied by the change in volume and mass distribution of the Ni-based particle. Concluding remarks Lamellas were prepared by FIB-SEM from the standard anode materials in electrolyte-supported fuel cell, and investigated during heating to 800°C in an oxygen or hydrogen atmosphere. The microstructural evolution over length scales ranging from a few microns to sub-nano scale was elucidated in an oxygen or a hydrogen atmosphere of 3 mbar. Moreover, crystal-lattice TEM images in the early stages of oxidation of Ni–ScSZ and reduction of NiO–ScSZ were taken under 0.001–0.1 mbar oxygen and hydrogen atmospheres, respectively. Changes in volume and mass distribution were observed during redox cycling, which are considered to result in Ni agglomeration during long-term cell operation. In future work, in situ TEM observation could be used to observe fuel cell materials under an applied electric field. Acknowledgements The author(s) gratefully acknowledge the support of the International Institute for Carbon Neutral Energy Research (WPI-I2CNER), sponsored by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT). This work was partially supported by Japan Science and Technology Agency (JST) through its ‘Center of Innovation Science and Technology based Radical Innovation and Entrepreneurship Program (COI Program)’. References 1 Choudhary R V , Rajput M A , and Mamman S A ( 1998 ) NiO-alkaline earth oxide catalysts for oxidative methane-to-syngas conversion: influence of alkaline earth oxide on the surface properties and temperature-programmed reduction/ reaction by H2 and methane . J. Catal. 178 : 576 – 585 . Google Scholar Crossref Search ADS 2 Bruce L , and Mathews F J ( 1982 ) The fischer-tropsch activity of nickel-zirconia . Appl. Catal. 4 : 353 – 370 . Google Scholar Crossref Search ADS 3 Simonsen B S , Chakraborty D , Chorkendorff I , and Dahl S ( 2012 ) Alloyed Ni-Fe nanoparticles as catalysis for NH3 decomposition . Appl. Catal. A Gen. 447–448 : 22 – 31 . Google Scholar Crossref Search ADS 4 Iwata T ( 1996 ) Characterization of Ni–YSZ anode degradation for substrate-type solid oxide fuel cells . J. Electrochem. Soc. 143 : 1521 – 1525 . Google Scholar Crossref Search ADS 5 Klemenso T , Appel C C , and Mogensen M ( 2006 ) In situ observations of microstructural changes in SOFC anodes during redox cycling . Electrochem. Solid-State Lett. 9 : A403 – A407 . Google Scholar Crossref Search ADS 6 Sumi H , Kishida R , Kim J-Y , Muroyama H , Matsui T , and Eguchi K ( 2010 ) Correlation between microstructural and electrochemical characteristics during redox cycles for Ni–YSZ anode of SOFCs . J. Electrochem. Soc. 157 : B1747 – B1752 . Google Scholar Crossref Search ADS 7 Kubota M , Muroyama H , Matsui T , and Eguchi K ( 2013 ) Microstructural change of Ni–YSZ anode under thermal cycles with redox treatments . ECS Trans. 57 : 589 – 597 . Google Scholar Crossref Search ADS 8 Joos J , Ender M , Rotscholl I , Menzler N H , and Ivers-Tiffee E ( 2014 ) Quantification of double-layer Ni/YSZ fuel cell anodes from focused ion beam tomography data . J. Power Sources 246 : 819 – 830 . Google Scholar Crossref Search ADS 9 Chenna S , Banerjee R , and Crozier A ( 2011 ) Atomic-scale observation of the Ni activation process for partial oxidation of methane using in situ environmental TEM . Chem. Cat. Chem. 3 : 1051 – 1059 . 10 Jeangros Q , Hansen W T , Wagner B J , Damsgaard D C , Dunin-Borkowski E R , Hebert C , Herle Van J , and Hessler-Wyser A ( 2013 ) Reduction of nickel oxide particles by hydrogen studied in an environmental TEM . J. Mater. Sci. 48 : 2893 – 2907 . Google Scholar Crossref Search ADS 11 Simonsen B S , Agersted K , Hansen V K , Jacobsen T , Wagner B J , Hansen W T , and Kuhn T L ( 2015 ) Environmental TEM study of the dynamic nanoscaled morphology of NiO/YSZ during reduction . Appl. Catal. A Gen. 489 : 147 – 154 . Google Scholar Crossref Search ADS 12 Jeangros Q , Faes A , Wagner B J , Hansen W T , Aschauer U , Herle Van J , Hessler-Wyser A , and Dunin-Borkowski E R ( 2010 ) In situ redox cycle of a nickel–YSZ fuel cell anode in an environmental transmission electron microscope . Acta Mater. 58 : 4578 – 4589 . Google Scholar Crossref Search ADS 13 Duchamp M , Xu Q , and Dunin-Borkowski E R ( 2014 ) Convenient preparation of high-quality specimens for annealing experiments in the transmission electron microscope . Microsc. Microanal. 20 : 1638 – 1645 . Google Scholar Crossref Search ADS PubMed 14 http://www.melbuild.com/. 15 Wagner T , Kirchheim R , and Ruhle M ( 1995 ) Chemical reactions at metal/ceramic interfaces duing diffusion bonding . Acta Metal. Mater. 43 : 1053 – 1063 . Google Scholar Crossref Search ADS 16 Uchida H , Suzuki H , and Watanabe M ( 1998 ) High-performance electrode for medium-temperature solid oxide fuel cells: effects of composition and microstructure on performance of ceria-based anodes . J. Electrochem. Soc. 145 : 615 – 620 . Google Scholar Crossref Search ADS © The Author(s) 2018. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

Journal

MicroscopyOxford University Press

Published: Oct 1, 2018

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