In situ TEM study of reduction and reoxidation of NiO/ceramic composites

Nickel/ceramic solid oxide fuel cell anodes exhibit a dimensional instability when experiencing a reduction-oxidation cycle. As fuel is supplied on the anode side, the as-sintered nickel oxide phase (NiO) reduces to metallic Ni and then remains in this state during operation. Yet, several factors may lead to an accidental reoxidation of the Ni, which may rupture parts of the cell, hence degrading its performance. The mechanisms behind the dimensional instability of Ni/yttria-stabilised zirconia (YSZ) anodes are investigated here through an innovative environmental transmission electron microscopy assessment. NiO particles and NiO/YSZ composites are reduced and reoxidised in the microscope in a few mbar of hydrogen and oxygen, respectively, up to 500-850 °C. Images, diffraction patterns, electron energy-loss spectra and energy-filtered micrographs are acquired, usually at constant temperature intervals during the reactions, to capture in situ the nanostructure, crystallography and chemistry. The reaction kinetics are retrieved from both the changes in shapes of the Ni L23 edges in energy-loss spectra and from energy-filtered images (with nm-resolution), analysed to provide quantitative data and correlated to the structure. Complementary data include post-exposure microscopy, in situ X-ray diffraction and density functional theory computations. While the surface nucleation of Ni domains, their growth and impingement control the reduction of NiO particles, the results reveal a modification of the mechanisms in the presence of yttria-stabilised zirconia, with the transfer of oxygen from NiO to the oxygen vacancies of the YSZ ceramic now triggering the reaction. Intragranular voids form in both cases as oxygen is removed. The final Ni structure at high temperature is then observed to coarsen as it minimises its surface energy, with the percolation of the Ni phase influenced by the symmetry of its grain boundaries. The reoxidation of Ni is controlled mainly by the outward diffusion of Ni ions through the grain boundaries of the growing NiO film. While some NiO inward growth occurs through the formation of oxide film cracks, the Ni2+ outward diffusion process remains unbalanced and voids form in the NiO phase. These internal voids are responsible for the dimensional instability of the composite along with Ni coarsening at high temperature. Several parameters for improved performance and redox tolerance are then identified based on these results

Correlation of fluorescence microscopy, electron microscopy, and NanoSIMS stable isotope imaging on a single tissue section.

Correlative light and electron microscopy allows localization of specific molecules at the ultrastructural level in biological tissue but does not provide information about metabolic turnover or the distribution of labile molecules, such as micronutrients. We present a method to directly correlate (immuno)fluorescent microscopy, (immuno)TEM imaging and NanoSIMS isotopic mapping of the same tissue section, with nanometer-scale spatial precision. The process involves chemical fixation of the tissue, cryo sectioning, thawing, and air-drying under a thin film of polyvinyl alcohol. It permits to effectively retain labile compounds and strongly increases NanoSIMS sensitivity for 13C-enrichment. The method is illustrated here with correlated distribution maps of a carbonic anhydrase enzyme isotype, β-tubulin proteins, and 13C- and 15N-labeled labile micronutrients (and their anabolic derivates) within the tissue of a reef-building symbiotic coral. This broadly applicable workflow expands the wealth of information that can be obtained from multi-modal, sub-cellular observation of biological tissue

Measurements of local chemistry and structure in Ni(O)-YSZ composites during reduction using energy-filtered environmental TEM

Energy-filtered transmission electron microscopy images are acquired during the reduction of a NiO–YSZ composite in H2 up to 600 °C. Temperature-resolved quantitative information about both chemistry and structure is extracted with nm spatial resolution from the data, paving the way for the development of detailed reduction models

In Situ Reduction and Oxidation of Nickel from Solid Oxide Fuel Cells in a Titan ETEM

Environmental transmission electron microscopy was used to characterize in situ the reduction and oxidation of nickel from a Ni/YSZ solid oxide fuel cell anode support between 300-500°C. The reduction is done under low hydrogen pressure. The reduction initiates at the NiO/YSZ interface, then moves to the center of the NiO grain. At higher temperature the reduction occurs also at the free NiO surface and the NiO/NiO grain boundaries. The growth of Ni is epitaxial on its oxide. Due to high volume decrease, nanopores are formed during reduction. During oxidation, oxide nanocrystallites are formed on the nickel surface. The crystallites fill up the nickel porosity and create an inhomogeneous structure with remaining voids. This change in structure causes the nickel oxide to expand during a RedOx cycle

In situ Reduction and Oxidation of Nickel from Solid Oxide Fuel Cells in a Transmission Electron Microscope

Environmental transmission electron microscopy was used to characterize in situ the reduction and oxidation of nickel from a Ni/YSZ solid oxide fuel cell anode support between 300-500°C. The reduction is done under low hydrogen pressure. The reduction initiates at the NiO/YSZ interface, then moves to the center of the NiO grain. At higher temperature the reduction occurs also at the free NiO surface and the NiO/NiO grain boundaries. The growth of Ni is epitaxial on its oxide. Due to high volume decrease, nanopores are formed during reduction. During oxidation, oxide nanocrystallites are formed on the nickel surface. The crystallites fill up the nickel porosity and create an inhomogeneous structure with remaining voids. This change in structure causes the nickel oxide to expand during a RedOx cycle

Hydrogen plasma treatment for improved conductivity in amorphous aluminum doped zinc tin oxide thin films

Improving the conductivity of earth-abundant transparent conductive oxides (TCOs) remains an important challenge that will facilitate the replacement of indium-based TCOs. Here, we show that a hydrogen (H-2)-plasma post-deposition treatment improves the conductivity of amorphous aluminum-doped zinc tin oxide while retaining its low optical absorption. We found that the H-2-plasma treatment performed at a substrate temperature of 50 degrees C reduces the resistivity of the films by 57% and increases the absorptance by only 2%. Additionally, the low substrate temperature delays the known formation of tin particles with the plasma and it allows the application of the process to temperature-sensitive substrates. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License

Quantification of hydrogen in nanostructured hydrogenated passivating contacts for silicon photovoltaics combining SIMS-APT-TEM : A multiscale correlative approach

Multiscale characterization of the hydrogenation process of silicon solar cell contacts based on c-Si/SiOx/nc-SiCx(p) has been performed by combining dynamic secondary ion mass-spectrometry (D-SIMS), atom probe tomography (APT), and transmission electron microscopy (TEM). These contacts are formed by high-temperature firing, which triggers the crystallization of SiCx, followed by a hydrogenation process to passivate remaining interfacial defects. Due to the difficulty of characterizing hydrogen at the nm-scale, the exact hydrogenation mechanisms have remained elusive. Using a correlative TEM-SIMS-APT analysis, we are able to locate hydrogen trap sites and quantify the hydrogen content. Deuterium (D), a heavier isotope of hydrogen, is used to distinguish hydrogen introduced during hydrogenation from its background signal. D-SIMS is used, due to its high sensitivity, to get an accurate deuterium-to-hydrogen ratio, which is then used to correct deuterium profiles extracted from APT reconstructions. This new methodology to quantify the concentration of trapped hydrogen in nm-scale structures sheds new insights on hydrogen distribution in technologically important photovoltaic materials

Multimodal Microscale Imaging of Textured Perovskite-Silicon Tandem Solar Cells.

Halide perovskite/crystalline silicon (c-Si) tandem solar cells promise power conversion efficiencies beyond the limits of single-junction cells. However, the local light-matter interactions of the perovskite material embedded in this pyramidal multijunction configuration, and the effect on device performance, are not well understood. Here, we characterize the microscale optoelectronic properties of the perovskite semiconductor deposited on different c-Si texturing schemes. We find a strong spatial and spectral dependence of the photoluminescence (PL) on the geometrical surface constructs, which dominates the underlying grain-to-grain PL variation found in halide perovskite films. The PL response is dependent upon the texturing design, with larger pyramids inducing distinct PL spectra for valleys and pyramids, an effect which is mitigated with small pyramids. Further, optimized quasi-Fermi level splittings and PL quantum efficiencies occur when the c-Si large pyramids have had a secondary smoothing etch. Our results suggest that a holistic optimization of the texturing is required to maximize light in- and out-coupling of both absorber layers and there is a fine balance between the optimal geometrical configuration and optoelectronic performance that will guide future device designs

Amorphous gallium oxide grown by low-temperature PECVD

Owing to the wide application of metal oxides in energy conversion devices, the fabrication of these oxides using conventional, damage-free, and upscalable techniques is of critical importance in the optoelectronics community. Here, the authors demonstrate the growth of hydrogenated amorphous gallium oxide (a-GaOx:H) thin-films by plasma-enhanced chemical vapor deposition (PECVD) at temperatures below 200 °C. In this way, conformal films are deposited at high deposition rates, achieving high broadband transparency, wide band gap (3.5-4 eV), and low refractive index (1.6 at 500 nm). The authors link this low refractive index to the presence of nanoscale voids enclosing H2, as indicated by electron energy-loss spectroscopy. This work opens the path for further metal-oxide developments by low-temperature, scalable and damage-free PECVD processes