Design of Active Sites on Nickel in the Anode of Intermediate-Temperature Solid Oxide Fuel Cells using Trace Amount of Platinum Oxides.

Invited for this month's cover is the group of Prof. Dr. Toshiyuki Mori at National Institute for Materials Science (NIMS), Japan. The front cover picture shows the formation of new active sites on Ni in the anode of a solid oxide fuel cell (SOFC), which displays high performance at intermediate temperature. The combination of processing route design, microanalysis, and surface atomistic simulation provides us with a new design paradigm for fabrication of high-performance SOFCs. Read the full text of the article at 10.1002/cplu.201800170.

Design of Active Sites on Nickel in the Anode of Intermediate-Temperature Solid Oxide Fuel Cells using Trace Amount of Platinum Oxides.

In recent years, the lowering of the operation temperature of solid oxide fuel cells (SOFCs) has attracted much attention owing to the trade-off between the best performance and the life span of SOFCs. For this challenge, new active sites on the Ni surfaces in a Nickel-Yttria-Stabilized Zirconia (Ni-YSZ) cermet anode of SOFCs have been created by deposition of trace amounts of platinum oxide (PtOx ) followed by an activation step of the anode at 1073 K in a hydrogen flow. The internal resistance (IR) free value (185 mA cm-2 at 0.8 V) observed for the single cell with an anode sputtered with a trace amount of PtOx (Pt content in anode: from 9 to 91 ppm) at 973 K is conspicuously higher than that of a similar single cell with a nonsputtered cermet anode (85 mA cm-2 ) at 0.8 V and 1073 K. Transmission electron microscopy microanalysis shows that the defect structure is formed on a partially oxidized Ni surface by active Pt species. Also, surface atomistic simulation on NiO (111) predicts the formation of Frenkel defect clusters with Pt cations, which partially cover the Ni surface. The formation of Frenkel defect clusters on the partially oxidized Ni surface (i.e., creation of new active sites for formation of water molecules) promotes the anode reaction, resulting in improvements in the anode performance of SOFC single cells at 973 K. Design of the aforementioned new active sites on Ni through sputtering of trace amounts of PtOx provides a great opportunity for "radical innovation" in the design of intermediate-temperature SOFCs.

Sparse modeling of EELS and EDX spectral imaging data by nonnegative matrix factorization.

Advances in scanning transmission electron microscopy (STEM) techniques have enabled us to automatically obtain electron energy-loss (EELS)/energy-dispersive X-ray (EDX) spectral datasets from a specified region of interest (ROI) at an arbitrary step width, called spectral imaging (SI). Instead of manually identifying the potential constituent chemical components from the ROI and determining the chemical state of each spectral component from the SI data stored in a huge three-dimensional matrix, it is more effective and efficient to use a statistical approach for the automatic resolution and extraction of the underlying chemical components. Among many different statistical approaches, we adopt a non-negative matrix factorization (NMF) technique, mainly because of the natural assumption of non-negative values in the spectra and cardinalities of chemical components, which are always positive in actual data. This paper proposes a new NMF model with two penalty terms: (i) an automatic relevance determination (ARD) prior, which optimizes the number of components, and (ii) a soft orthogonal constraint, which clearly resolves each spectrum component. For the factorization, we further propose a fast optimization algorithm based on hierarchical alternating least-squares. Numerical experiments using both phantom and real STEM-EDX/EELS SI datasets demonstrate that the ARD prior successfully identifies the correct number of physically meaningful components. The soft orthogonal constraint is also shown to be effective, particularly for STEM-EELS SI data, where neither the spatial nor spectral entries in the matrices are sparse.

Determination of three-dimensional strain state in crystals using self-interfered split HOLZ lines.

An experimental method to measure the strain through the thickness of a crystal is demonstrated. This enables the full three-dimensional stress-strain state of a crystal at the nanoscale to be determined taking the current practice from two-dimensional strain state determination. Knowing the 3D strain state is desired by crystal growers in order to improve their crystal's quality. This method involves combining electron diffraction with electron interferometry in a transmission electron microscope. The electron diffraction uses a split higher order Laue zone (HOLZ) line and the electron interferometry uses an electron biprism.

Quantitative evaluation of annular bright-field phase images in STEM.

A phase reconstruction method based on multiple scanning transmission electron microscope (STEM) images was evaluated quantitatively using image simulations. The simulation results indicated that the phase shift caused by a single atom was proportional to the 0.6th power of the atomic number Z. For a thin SrTiO3 [001] crystal, the reconstructed phase at each atomic column increased according to the specimen thickness. The STEM phase images can quantify the oxygen vacancy concentration if the thickness is less than several nanometers.

Phase reconstruction in annular bright-field scanning transmission electron microscopy.

A novel technique for reconstructing the phase shifts of electron waves was applied to Cs-corrected scanning transmission electron microscopy (STEM). To realize this method, a new STEM system equipped with an annular aperture, annularly arrayed detectors and an arrayed image processor has been developed and evaluated in experiments. We show a reconstructed phase image of graphite particles and demonstrate that this new method works effectively for high-resolution phase imaging.

Observation of the potential distribution in GaN-based devices by a scanning electron microscope.

Mapping of the potential distribution using a scanning electron microscope (SEM) has been reported in recent years [1,2] for semiconductors such as Si, GaAs and InP. But, there are no such studies on GaN-based devices, to our knowledge. In this study, we observed two types of GaN-based devices by SEM to see if there is a condition that the contrast matches the potential distribution of the devices. The first device we studied was GaN p-n junction (p, n ∼5 × 10(17) cm(-3)). The device was cut, and polished from the cross-section to a flat surface. The cross-section was observed by SEM. Fig. 1(a) shows an SEM image taken at 3 kV. The p-region appears bright and the n-region appears dark. The image intensity changes at the position of p-n junction, for which we used electron beam induced current (EBIC) technique to determine the p-n junction position. Fig. 1(b) is a line profile across the p-n junction (broken line) of the SEM image together with a calculated potential distribution (solid line) using p and n concentrations. It can be seen that the contrast profile matches the potential distribution very well. The SEM observations were carried out for several accelerating voltages. But, best result was obtained at 3 kV. For lower accelerating voltages, the image seemed to reflect the surface potential. On the other hand, higher accelerating voltages resulted in blurred images. The second sample was a light emitting diode structure based on AlN where a multiple quantum well (MQW) structure was sandwiched by p- and n-AlGaN materials. In this case, the sample was obliquely polished from the surface (∼10°) to improve the lateral resolution. The SEM image could reveal the structure of MQW.jmicro;63/suppl_1/i22/DFU051F1F1DFU051F1Fig. 1.(a) SEM image of p-n GaN. (b) Comparison of line profile across the p-n junction (broken line) and a calculated potential distribution (solid line). AcknowledgementWe thank professor H. Amano (Nagoya University) for providing the samples.

In situ transmission electron microscopy of ionic conductivity and reaction mechanisms in ultrathin solid oxide fuel cells.

Solid oxide fuel cells (SOFCs) are promising candidates for use in alternative energy technologies. A full understanding of the reaction mechanisms in these dynamic material systems is required to optimize device performance and overcome present limitations. Here, we show that in situ transmission electron microscopy (TEM) can be used to study redox reactions and ionic conductivity in SOFCs in a gas environment at elevated temperature. We examine model ultrathin half and complete cells in two environmental TEMs using off-axis electron holography and electron energy-loss spectroscopy. Our results from the model cells provide insight into the essential phenomena that are important for the operation of commercial devices. Changes in the activities of dopant cations in the solid electrolyte are detected during oxygen anion conduction, demonstrating the key role of dopants in electrolyte architecture in SOFCs.

Restoration of singularities in reconstructed phase of crystal image in electron holography.

Off-axis electron holography can be used to measure the inner potential of a specimen from its reconstructed phase image and is thus a powerful technique for materials scientists. However, abrupt reversals of contrast from white to black may sometimes occur in a digitally reconstructed phase image, which results in inaccurate information. Such phase distortion is mainly due to the digital reconstruction process and weak electron wave amplitude in some areas of the specimen. Therefore, digital image processing can be applied to the reconstruction and restoration of phase images. In this paper, fringe reconnection processing is applied to phase image restoration of a crystal structure image. The disconnection and wrong connection of interference fringes in the hologram that directly cause a 2π phase jump imperfection are correctly reconnected. Experimental results show that the phase distortion is significantly reduced after the processing. The quality of the reconstructed phase image was improved by the removal of imperfections in the final phase.

A simple way to obtain backscattered electron images in a scanning transmission electron microscope.

We have fabricated a simple detector for backscattered electrons (BSEs) and incorporated the detector into a scanning transmission electron microscope (STEM) sample holder. Our detector was made from a 4-mm(2) Si chip. The fabrication procedure was easy, and similar to a standard transmission electron microscopy (TEM) sample thinning process based on ion milling. A TEM grid containing particle objects was fixed to the detector with a silver paste. Observations were carried out using samples of Au and latex particles at 75 and 200 kV. Such a detector provides an easy way to obtain BSE images in an STEM.

Image processing for phase imperfections in electron holography.

In electron holography, an abrupt reversal of contrast from white to black may occur in a digitally reconstructed phase image, potentially resulting in inaccurate phase information. There are two types of such abrupt reversals. One is a phase jump of 2 π during the digital reconstruction process; in this case, the phase can be unwrapped and smoothly connected for such a jump. The other is caused by the disconnection of interference fringes due to weak electron-wave amplitude in some areas of the specimen. We propose a searching technique for finding the disconnection points based on mathematical morphology and then correcting the interference fringes. Finally, a comparatively accurate phase information image is reconstructed from the corrected interference fringes.

Development of advanced electron holographic techniques and application to industrial materials and devices.

The development of a transmission electron microscope equipped with a field emission gun paved the way for electron holography to be put to practical use in various fields. In this paper, we review three advanced electron holography techniques: on-line real-time electron holography, three-dimensional (3D) tomographic holography and phase-shifting electron holography, which are becoming important techniques for materials science and device engineering. We also describe some applications of electron holography to the analysis of industrial materials and devices: GaAs compound semiconductors, solid oxide fuel cells and all-solid-state lithium ion batteries.

Development of an environmental high-voltage electron microscope for reaction science.

Environmental transmission electron microscopy and ultra-high resolution electron microscopic observation using aberration correctors have recently emerged as topics of great interest. The former method is an extension of the so-called in situ electron microscopy that has been performed since the 1970s. Current research in this area has been focusing on dynamic observation with atomic resolution under gaseous atmospheres and in liquids. Since 2007, Nagoya University has been developing a new 1-MV high voltage (scanning) transmission electron microscope that can be used to observe nanomaterials under conditions that include the presence of gases, liquids and illuminating lights, and it can be also used to perform mechanical operations to nanometre-sized areas as well as electron tomography and elemental analysis by electron energy loss spectroscopy. The new instrument has been used to image and analyse various types of samples including biological ones.

Mapping of minority carrier lifetime distributions in multicrystalline silicon using transient electron-beam-induced current.

We have used transient electron-beam-induced current (EBIC) to map minority carrier lifetime distributions in multicrystalline Silicon (mc-Si). In this technique, the electron beam from a scanning transmission electron microscope was on-off modulated while the sample was scanned. The resulting transient EBIC was analyzed to form a lifetime map. An analytical function was introduced as part of the analysis in determining this map. We have verified this approach using numerical simulations and have reproduced a lifetime map for an mc-Si wafer.

In situ analytical electron microscopy studies of redox reactions at a YSZ/Pt interface.

Redox reactions were studied at a single yttria-stabilized zirconia (YSZ)/Pt electrode interface, in parallel with pure YSZ with no catalyst electrode, by in situ analytical electron microscopy at elevated temperatures and in an oxygen atmosphere. In situ electron holography showed that the oxide underwent reduction at elevated temperatures in a vacuum and was consequently reoxidized upon exposure to an oxygen flux at the same temperature. In situ energy loss spectroscopy measurements were in agreement with in situ electron holography observations and indicated that the oxidation state of the host cation zirconium was altered in the reduced state of the YSZ to the metastable state Zr(3+).

In situ off-axis electron holography of metal-oxide hetero-interfaces in oxygen atmosphere.

Off-axis electron holography has been extended to in situ observations in gas atmospheres. The Yttria-stabilized zirconia (YSZ)-Pt hetero-interface was characterized by electron holography at high temperature in a vacuum and in an oxygen atmosphere. Analysis of the phase shift profiles revealed high mobility of anions in the oxide in the vicinity of the interface in the oxygen atmosphere. This would compensate for any increase in the number of oxygen vacancies in YSZ through the metal interface.

Coherent electron interference from amorphous TEM specimens.

For the first time, the electron intensity on the diffraction plane from amorphous transmission electron microscope (TEM) specimens has been found to have sufficient coherence to produce fringes in interferograms that were created using a wavefront splitting method of diffracted beam interferometry. The fringes were found to exist from low to high electron-scattering angles. Their spatial frequency depended on the angular overlap of the interfering beams, which was controlled by an electron biprism. From these interferograms, phase information of amorphous materials, which is information now lacking and required for determining their atomic structures, was obtained. An immediate application of this interference is a new method to determine the spatial resolution of the TEM that occurs at the shear angle for fringe disappearance.

Improvement of windowed type environmental-cell transmission electron microscope for in situ observation of gas-solid interactions.

We have developed an improved, windowed type environmental-cell (E-cell) transmission electron microscope (TEM) for in situ observation of gas-solid interactions, such as catalytic reactions at atmospheric pressure. Our E-cell TEM includes a compact E-cell specimen holder with mechanical stability, resulting in smoother introduction of the desired gases compared with previous E-cell TEMs. In addition, the gas control unit was simplified by omitting the pressure control function of the TEM pre-evacuation chamber. This simplification was due to the successful development of remarkably tough thin carbon films as the window material. These films, with a thickness of <10 nm, were found to withstand pressure differences >2 atm. Appropriate arrangement of the specimen position inside the E-cell provided quantitatively analyzable TEM images, with no disturbances caused by the windowed films. As an application, we used this E-cell TEM to observe the dynamic shape change in a catalytic gold nanoparticle supported on TiO(2) during the oxidation of CO gas.

Development of a real-time stereo transmission electron microscope.

A new transmission electron microscope that allows for stereoscopic observations in real time at a video rate has been developed. In order to make stereo pairs at a high speed, illumination of a specimen from two directions instead of specimen tilting is adopted. Two electrostatic deflectors make it possible to alternate the illumination direction within a blank period of 1.2 ms between two adjacent video fields. Three dimensional displays give observers a stereo impression without the need for special glasses. By considering the circle of least confusion due to spherical aberration, a lateral resolution <1 nm and a longitudinal resolution of 6.3 nm are possible with a proper defocused condition. This improvement of the resolution is confirmed for the image of Au fine particles. The motion of objects in the lateral direction can be detected up to a speed of 8.3 nm s(-1). Using this microscope fine particles of ZnO were observed.