Oxidation states study of nickel in solid oxide fuel cell anode using x-ray full-field spectroscopic nano-tomography Yu-chen Karen Chen-Wiegart, William M. Harris, Jeffrey J. Lombardo, Wilson K. S. Chiu, and Jun Wang Citation: Appl. Phys. Lett. 101, 253901 (2012); doi: 10.1063/1.4772784 View online: http://dx.doi.org/10.1063/1.4772784 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v101/i25 Published by the American Institute of Physics.

Related Articles Designing a miniaturised heated stage for in situ optical measurements of solid oxide fuel cell electrode surfaces, and probing the oxidation of solid oxide fuel cell anodes using in situ Raman spectroscopy Rev. Sci. Instrum. 83, 053707 (2012) Biogas fuel reforming for solid oxide fuel cells J. Renewable Sustainable Energy 4, 023106 (2012) Electric-field-induced current-voltage characteristics in electronic conducting perovskite thin films Appl. Phys. Lett. 100, 012101 (2012) Control of oxygen delamination in solid oxide electrolyzer cells via modifying operational regime Appl. Phys. Lett. 99, 173506 (2011) Three-dimensional mapping of nickel oxidation states using full field x-ray absorption near edge structure nanotomography Appl. Phys. Lett. 98, 173109 (2011)

Additional information on Appl. Phys. Lett. Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

APPLIED PHYSICS LETTERS 101, 253901 (2012)

Oxidation states study of nickel in solid oxide fuel cell anode using x-ray full-field spectroscopic nano-tomography Yu-chen Karen Chen-Wiegart,1 William M. Harris,2 Jeffrey J. Lombardo,2 Wilson K. S. Chiu,2 and Jun Wang1,a) 1

Photon Science Directorate, Brookhaven National Laboratory, Upton, New York 11973, USA Department of Mechanical Engineering, University of Connecticut, Storrs, Connecticut 06269, USA

2

(Received 15 October 2012; accepted 6 December 2012; published online 20 December 2012) Identifying the chemical state and coupling with morphological information in three dimensions are of great interest in energy storage materials, which typically involve reduction-oxidation cycling and structural evolution. Here, we apply x-ray nano-tomography with multiple x-ray energies to study oxidation states of nickel (Ni) and nickel oxide phases in Ni-yttria-stabilized zirconia (YSZ), a typical anode material of solid oxide fuel cells (SOFC). We present a method to quantitatively identify the nickel-based oxides from Ni-YSZ anode composite, and obtain chemical mapping as well as associated microstructures at nanometer scale in three dimensions. NiO particles manually placed on a Ni-YSZ composite anode were used for validation of the method, while no nickel oxides were found to be present within the electrode structure as remnants of the cell fabrication process. The application of the method can be widely applied to energy storage materials including SOFCs, Li-ion batteries, and supercapacitors, as well as other systems for oxidation and reduction C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772784] study. V Revealing the underlying electrochemical mechanisms in many energy storage materials such as fuel cells, batteries, and supercapacitors is crucial for improving their performance and for ensuring safe operation. To achieve this goal, it is essential to obtain a thorough understanding regarding the oxidation state change of the energy storage materials during reduction-oxidation (redox) cycling. For materials used in energy storage, the change of oxidation state during cycling in some situations is a natural result of operation, occurring specifically on the active sites. In other cases, the redox reaction is undesirable and may only occur locally. Nickel-yttriastabilized zirconia (Ni-YSZ), a widely used anode material in solid oxide fuel cells (SOFC), uses Ni as the catalyst for hydrogen oxidation.1 However, nickel oxide (NiO) may be left as residuals of cell fabrication, in which mixed NiO and YSZ powder is purposely subjected to high temperature to reduce NiO to metallic Ni and form the composite anode structure.2 Also, Ni oxidation may occur undesirably during operation to reform NiO. As a result, the un-reduced and reformed NiO in either case would prohibit the electrocatalytic reaction of hydrogen oxidation via Ni. Moreover, large volume changes have also been reported during NiO/Ni redox, with percent changes as large as 40.9% for reduction and 69.2% for oxidation.2 Therefore, it is critical to characterize the distribution of NiO/Ni redox process in three dimensions (3D) to shed the light on the effects of potential NiO existence to the SOFC operations. Transmission x-ray microscopy (TXM), a full-field, high resolution hard x-ray imaging technique, has been demonstrated to be an effective tool to study SOFCs,3 Li-ion battery electrodes,4 and nano-porous metals used for electrode substrates in batteries and supercapacitors.5 By combining with x-ray absorption near edge structure (XANES) a)

Author to whom correspondence should be addressed. Electronic mail: [email protected]. Telephone: þ1-631-344-2661. Fax: þ1-631-344-3238.

0003-6951/2012/101(25)/253901/4/$30.00

spectroscopy, the TXM is able to perform spectroscopic imaging. The two dimensional (2D) full-field spectroscopic imaging can be achieved by collecting a series of TXM projections across an element x-ray absorption edge. This XANES imaging results in contrast variation between images as a function of x-ray energies, corresponding to the elemental absorption spectrum. Furthermore, 3D spectroscopic nano-tomography can be attained by collecting tomographic data at multiple energies across an element absorption edge. Therefore, a spatial distribution of an element within the sample and oxidation state of the chosen element can be directly extracted from the reconstructed 3D images.6 Spectroscopic nano-tomography offers unprecedented sub-50 nm spatial 3D resolution, non-destructive imaging, and chemical and elemental sensitivity which provide a unique path for our study.7 The feasibility to study the Ni oxidation by XANES imaging using the TXM has previously been shown with a sample consisting of Ni foil and NiO powder to simulate a composite material.8 Oxidation of Ni in Ni-YSZ was also reported using the TXM,2 where the Ni-YSZ was heated ex situ to induce oxidation and high temperature annealing. Imaging at single x-ray energy above the Ni K-edge revealed a NiO film on the Ni surface as well as an un-reduced residual NiO core in the Ni-YSZ.2 Here, we report an oxidation state study of Ni in a Ni-YSZ anode using x-ray full-field XANES spectroscopic nano-tomography. The XANES imaging data were collected at multiple energies to allow a quantitative and precise identification of potential existence of NiO within the Ni-YSZ sample. The experiments were carried out on X8C beamline at National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL). The recently developed TXM at NSLS has unique capabilities to overcome some difficulties with older designs.7 One of the distinctive capabilities is the

101, 253901-1

C 2012 American Institute of Physics V

253901-2

Chen-Wiegart et al.

ability to keep the same magnification for all x-ray images collected at multiple x-ray energies for XANES imaging. Since the image distance is proportional to the magnification and the focal length of the objective zone plate which depends on the x-ray energy, as the x-ray energy changes, the detector should be adjusted accordingly to maintain the same magnification for all images. As a result, the best resolution can be preserved, eliminating the need for images collected at different magnifications to be re-sampled to achieve consistent pixel size.7 This is particularly critical when a large energy change is required during an experiment. Figure 1 demonstrates a XANES imaging of a Ni-YSZ sample performed at the X8C beamline. A series of TXM 2D projections were collected on the Ni-YSZ sample across the Ni absorption K-edge. Notice that the contrast variation of the Ni phase corresponds to the x-ray absorption spectrum of Ni. The Ni-YSZ sample was prepared by tape-casting and sintering procedure with details described elsewhere.9 A section of the Ni-YSZ was fractured from the bulk sample to create a sharp feature, and was glued to the tip of a steel pin. NiO powders were manually placed on the surface of the NiYSZ sample under an optical microscope. The NiO powder serves as unbiased reference later during data analysis to assist in accurate identification of un-reduced NiO within the Ni-YSZ sample. Before performing the spectroscopic nano-tomography, XANES spectra of Ni foil (Alfa Aesar, 99.95% pure), NiO powder (Fuel Cell materials, 99.99% pure), and Ni2O3 powder (Inco Limited, F-Grade) were collected at beamline 6-2 C at the Stanford Synchrotron Radiation Lightsource (SSRL) using a conventional x-ray absorption technique. The spectra shown in Figure 2 agree well with the published data.8 Based on the measured spectra, three x-ray energies 8300 eV, 8348 eV, and 8375 eV indicated in Figure 2 were chosen for collecting nano-tomography datasets on the NiYSZ sample with the reference of NiO powders. A lens-coupled scintillator with a 2048  2048 pixels camera detector was used to record images. A total of 181 projections of the sample were collected over a 180 angular

FIG. 1. A series of representative full-field XANES images from the TXM at NSLS demonstrate the unique TXM capability. The magnification at various energies is kept constant via automated adjustment of the detector position and therefore preserves the best resolution of this instrument.

Appl. Phys. Lett. 101, 253901 (2012)

FIG. 2. The absorption spectra of the Ni, NiO, and Ni2O3 near the Ni absorption K-edge (8333 eV). Three tomography measurements were carried out at 8300, 8348, and 8375 eV, labeled in the figure as Tomo1: E8300, Tomo 2: E8348, and Tomo 3:E8375, respectively.

range for each tomography dataset, using a camera binning of 2  2. Background normalization was achieved separately. 20 background images were collected and then averaged with the sample removed from the TXM field of view. The intensity of each pixel in the sample projections was then divided by the intensity of the corresponding pixel in the averaged background image to yield the transmission signal. The alignment of the x-ray projections was performed automatically using a run-out correction system designed for the TXM at NSLS which is one of the unique capabilities of this microscope.7 A standard filtered backprojection reconstruction algorithm was used to reconstruct the 3D structure.10 The same byte-scaling was applied to reconstruct the three distinctive datasets (E8300, E8348, and E8375) collected at different x-ray energies. The three tomographic datasets were spatially registered using AVIZO software package (version 6.2, VSG) for voxel to voxel comparison. Figure 3 shows the TXM 2D projections of the NiOdecorated Ni-YSZ sample taken at the three x-ray energies. The NiO reference powders decorated on the surface of the Ni-YSZ sample are indicated by arrows in the figures, where the gray scale represents the x-ray attenuation. The 3D reconstruction of the Ni-YSZ sample with one of the decorating NiO powders at 8348 eV is shown in Figure 4(a). The virtual cross-section images of the NiO powder from three distinctive x-ray energies are shown in Figures 4(b)–4(d). The contrast of the NiO between three x-ray energies can be clearly identified. Below the Ni absorption edge 8300 eV [Figure 4(b)], NiO absorbs the least compared with its absorptions at the other two energies. At x-ray energy of 8348 eV [Figure 4(c)], NiO displays peak absorption and shows relatively less absorption in the post-edge region at 8375 eV [Figure 4(d)]. This behavior is consistent with the absorption spectra of NiO shown in Figure 2, and is distinctly different from that of Ni, which maintains an approximately constant level of absorption in the post-edge region. Therefore, the contrast of NiO between E8348 and E8375 can be used to identify NiO within the Ni-YSZ composite by searching for this contrast in the Ni-YSZ 3D structure at these two distinctive energies. The internal structure of the Ni-YSZ pore composite is revealed and shown through its virtual cross-section images at the three energies in Figure 5. The contrast of Ni below

253901-3

Chen-Wiegart et al.

Appl. Phys. Lett. 101, 253901 (2012)

FIG. 3. TXM 2D projections of the NiOdecorated Ni-YSZ sample taken at the x-ray energies of (a) 8300, (b) 8348, and (c) 8375 eV. The NiO reference powders are indicated by arrows in the figures.

FIG. 4. (a) 3D reconstruction of the Ni-YSZ anode sample from E8375 with the decorated NiO powder, indicated by an arrow (red online). The crosssection of the NiO powder is shown at the location indicated by the dash line (red online) at various x-ray energies: (b) 8300, (c) 8348, and (d) 8375 eV.

FIG. 5. The reconstructed virtual cross-section image of the SOFC sample is shown at the location indicated in Figure 4(a) by the solid line (green online) at various x-ray energies. (a) E8300, (b) E8348, (c) E8375 eV, and (d) the difference between E8348 and E8375 shows merely noise levels.

8300 eV (Figure 5(a)) and above (Figures 5(b) and 5(c)) the Ni K-edge 8348 eV and 8375 eV is evident. However, there is no apparent contrast between the E8348 (Figure 5(b)) and E8375 (Figure 5(c)) images. These two reconstructions are nearly identical, with the exception of some minor noise attributable to imperfect background normalization, imaging noise, or reconstruction artifacts. This lack of contrast was further verified and shown in Figure 5(d) by subtracting Figure 5(c) from Figure 5(b). Note there is no clear identifiable features in the subtracted image Figure 5(d) to confirm the existence of un-reduced NiO phase in the internal structure, suggesting that all nickel oxides were reduced to Ni during initial cell operation following the fabrication process described in Ref. 9. This observation is somewhat different than that previously reported in the literature,2 in which a different cell fabrication procedure was used. The discrepancy could possibly be attributed to variations in starting nickel oxide powder size, cell fabrication, or operational conditions. By calculating the size-dependent contrast of a single NiO feature between E8348 and E8375, the method used in this work is sensitive to detect NiO particles as small as about 140 nm. For NiO particles with size smaller than 140 nm, the contrast in TXM projections between E8348 and E8375 would be smaller than the background noise level 1.6%, which is determined by the TXM instrument resolution as quantified previously in Ref. 7, where the power spectrum density at the cutoff frequency is equivalent to 1.6% contrast. The methodology reported here, using nanotomography at multiple energies rather than a single energy, allows quantitatively and precisely identifying NiO phase induced by redox cycle in Ni-YSZ anode with a sensitivity of 140 nm particle size. In summary, XANES spectroscopic nano-tomography was performed on a porous Ni-YSZ solid oxide fuel cell anode to quantitatively identify any nickel oxides inside. No evidence was found to suggest un-reduced NiO core remained in the Ni-YSZ anode indicating the NiO has been fully reduced to the catalytic metallic Ni. The XANES spectroscopy combined with nano-tomography provides an opportunity to quantitatively characterize oxidation state of composite materials. This methodology allows unambiguous identification of NiO phase from Ni, YSZ, and pore phases of the Ni-YSZ composite anode, and can be widely applied to reduction and oxidation study on energy storage and conversion materials including SOFCs, Li-ion batteries, and supercapacitors. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic

253901-4

Chen-Wiegart et al.

Energy Sciences, under Contract No. DE-AC02-98CH10886. W. M. Harris, J. J. Lombardo, and W. K. S. Chiu acknowledge financial support from an Energy Frontier Research Center on Science Based Nano-Structure Design and Synthesis of Heterogeneous Functional Materials for Energy Systems (HeteroFoaM Center) funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (Award No. DE-SC0001061). Ni-YSZ samples used in this study were provided by Professor Meilin Liu of the Georgia Institute of Technology and Dr. Jan Van herle of Ecole Polytechnique Federale de Lausanne (EPFL). 1

J. R. Wilson, W. Kobsiriphat, R. Mendoza, H. Y. Chen, J. M. Hiller, D. J. Miller, K. Thornton, P. W. Voorhees, S. B. Adler, and S. A. Barnett, Nature Mater. 5(7), 541 (2006). 2 P. R. Shearing, R. S. Bradley, J. Gelb, F. Tariq, P. J. Withers, and N. P. Brandon, Solid State Ionics 216, 69 (2012). 3 Y.-c. K. Chen-Wiegart, J. S. Cronin, Q. Yuan, K. J. Yakal-Kremski, S. A. Barnett, and J. Wang, J. Power Sources 218, 348 (2012); G. J. Nelson,

Appl. Phys. Lett. 101, 253901 (2012) K. N. Grew, J. R. Izzo, J. J. Lombardo, W. M. Harris, A. Faes, A. HesslerWyser, J. Van herle, S. Wang, Y. S. Chu, A. V. Virkar, and W. K. S. Chiu, Acta Mater. 60, 3491 (2012); P. R. Shearing, R. S. Bradley, J. Gelb, S. N. Lee, A. Atkinson, P. J. Withers, and N. P. Brandon, Electrochem. Solid State Lett. 14(10), B117 (2011). 4 Y.-c. K. Chen-Wiegart, P. Shearing, Q. Yuan, A. Tkachuk, and J. Wang, Electrochem. Commun. 21, 58 (2012). 5 Y. C. K. Chen, Y. S. Chu, J. Yi, I. McNulty, Q. Shen, P. W. Voorhees, and D. C. Dunand, Appl. Phys. Lett. 96(4), 043122 (2010); Y.-c. K. ChenWiegart, S. Wang, Y. S. Chu, W. Liu, I. McNulty, P. W. Voorhees, and D. C. Dunand, Acta Mater. 60, 4972 (2012). 6 F. Meirer, J. Cabana, Y. J. Liu, A. Mehta, J. C. Andrews, and P. Pianetta, J. Synchrotron Radiat. 18, 773 (2011). 7 J. Wang, Y.-c. K. Chen, Q. Yuan, A. Tkachuk, C. Erdonmez, B. Hornberger, and M. Feser, Appl. Phys. Lett. 100(14), 143107 (2012). 8 G. J. Nelson, W. M. Harris, J. R. Izzo, K. N. Grew, W. K. S. Chiu, Y. S. Chu, J. Yi, J. C. Andrews, Y. J. Liu, and P. Pianetta, Appl. Phys. Lett. 98(17), 173109 (2011). 9 A. Faes, A. Hessler-Wyser, D. Presvytes, C. G. Vayenas, and J. Van Herle, Fuel Cells 9(6), 841 (2009). 10 F. Natterer, The Mathematics of Computerized Tomography (Wiley, New York, 1986).