Junko Matsuda

Ni-Fe Nitride Nanoplates on Nitrogen-Doped Graphene as a Synergistic Catalyst for Reversible Oxygen Evolution Reaction and Rechargeable Zn-Air Battery

Obtaining bifunctional electrocatalysts with high activity for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is a main hurdle in the application of rechargeable metal-air batteries. Earth-abundant 3d transition metal-based catalysts have been developed for the OER and ORR; however, most of these are based on oxides, whose insulating nature strongly restricts their catalytic performance. This study describes a metallic Ni-Fe nitride/nitrogen-doped graphene hybrid in which 2D Ni-Fe nitride nanoplates are strongly coupled with the graphene support. Electronic structure of the Ni-Fe nitride is changed by hybridizing with the nitrogen-doped graphene. The unique heterostructure of this hybrid catalyst results in very high OER activity with the lowest onset overpotential (150 mV) reported, and good ORR activity comparable to that for commercial Pt/C. The high activity and durability of this bifunctional catalyst are also confirmed in rechargeable zinc-air batteries that are stable for 180 cycles with an overall overpotential of only 0.77 V at 10 mA-2 .

Mechanism of activation of TiFe intermetallics for hydrogen storage by severe plastic deformation using high-pressure torsion

TiFe, a potential candidate for solid-state hydrogen storage, does not absorb hydrogen without a sophisticated activation process because of severe oxidation. This study shows that nanostructured TiFe becomes active by high-pressure torsion (HPT) and is not deactivated even after storage for several hundred days in the air. Surface segregation and formation of Fe-rich islands and cracks occur after HPT. The Fe-rich islands are suggested to act as catalysts for hydrogen dissociation and cracks and nanograin boundaries act as pathways to transport hydrogen through the oxide layer. Rapid atomic diffusion by HPT is responsible for enhanced surface segregation and hydrogen transportation.

Synthesis and crystal structure of a Pr5Ni19 superlattice alloy and its hydrogen absorption-desorption property.

The intermetallic compound Pr(5)Ni(19), which is not shown in the Pr-Ni binary phase diagram, was synthesized, and the crystal structure was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Two superlattice reflections with the Sm(5)Co(19)-type structure (002 and 004) and the Pr(5)Co(19)-type structure (003 and 006) were observed in the 2θ region between 2° and 15° in the XRD pattern using Cu Kα radiation. Rietveld refinement provided the goodness-of-fit parameter S = 6.7 for the Pr(5)Co(19)-type (3R) structure model and S = 1.7 for the Sm(5)Co(19)-type (2H) structure model, indicating that the synthesized compound has a Sm(5)Co(19) structure. The refined lattice parameters were a = 0.50010(9) nm and c = 3.2420(4) nm. The high-resolution TEM image also clearly revealed that the crystal structure of Pr(5)Ni(19) is of the Sm(5)Co(19) type, which agrees with the results from Rietveld refinement of the XRD data. The P-C isotherm of Pr(5)Ni(19) in the first absorption was clearly different from that in the first desorption. A single plateau in absorption and three plateaus in desorption were observed. The maximum hydrogen storage capacity of the first cycle reached 1.1 H/M, and that of the second cycle was 0.8 H/M. The 0.3 H/M of hydrogen remained in the metal lattice after the first desorption process.

The nanostructure and hydrogenation reaction of Mg50Co50 BCC alloy prepared by ball-milling

Mg50Co50 alloy before and after hydrogenation was investigated by means of transmission electron microscopy (TEM). Mg50Co50 alloy before hydrogenation was found to contain crystals not larger than 5 nm in size. Selected-area electron diffraction patterns (SAEDPs) revealed that these nanocrystals have a body-centered cubic (BCC) structure with a lattice parameter of about 0.3 nm. Distribution of Mg and Co elements in the Mg50Co50 alloy was uniform, indicated by energy dispersive x-ray spectroscopy (EDS) analysis. Crystallization and decomposition occurred in the Mg50Co50 alloy during hydrogenation. A large number of crystals larger than 10 nm were observed in the hydrogenated sample. The SAEDPs showed polycrystalline rings corresponding to the BCC phase and the Co metal phase. The existence of Mg-rich Mg-Co crystals and Co particles was also confirmed by TEM-EDS analysis.

High-Jc thick YBCO coated conductors by TFA-MOD process

The TFA-MOD process was applied to fabricate YBCO films on PLD-CeO/sub 2//IBAD-Gd/sub 2/Zr/sub 2/O/sub 7//Hastelloy substrates. In order to obtain higher Ic performance, thicker films maintaining high J/sub c/ values are required. J/sub c/ depends strongly on the YBCO crystal alignment. Dependences of J/sub c/ on /spl Delta//spl phi/ of the CeO/sub 2/ substrates has been investigated. It was found that the J/sub c/ value increased with improvement of the crystal alignment of the CeO/sub 2/ buffer layers. Also, the J/sub c/ value depended strongly on the P/sub H2O/ during the crystallization. The pore size in the film was smaller in the high J/sub c/ films fabricated under medium P/sub H2O/ and becomes larger in the low-J/sub c/ films under low and high P/sub H2O/. Furthermore, crack formation was observed in thick films crystallized at high P/sub H2O/. The large pore causes local reduction of current paths and additionally introduces the concentration of electric fields. It was found that both the porosity and crack formations limit the J/sub c/ properties. Finally, a YBCO film with 2.05 /spl mu/m in thickness was fabricated on a CeO/sub 2//Gd/sub 2/Zr/sub 2/O/sub 7/ layer buffered Hastelloy substrate with /spl Delta//spl phi//spl sim/4/spl deg/. A J/sub c/ value of 2.02 MA/cm/sup 2/ and transport I/sub c/ value of 413 A at 77 K, self-field were obtained.

Progress in R&D for coated conductors by TFA-MOD processing

Recent progress in the research and development of the TFA-MOD processing for the coated conductors in Japan was reviewed. For the higher Ic performance, the optimizations both in the calcinations and crystallization steps have taken place. Consequently, the extremely high Ic value of 413 A was achieved by the combination of the information from the above investigations and the use of highly textured CeO/sub 2/ cap layered substrates. Concerning the development of long tapes, a 8.6-m-long tape with its end-to-end Ic value of 119 A was successfully grown by a reel-to-reel system. Additionally, improvement of the production rate in the calcination step was realized by designing a multi-turn system.

In situ observation on hydrogenation of Mg-Ni films using environmental transmission electron microscope with aberration correction

In situ transmission electron microscopy (TEM) was performed to observe the hydrogenation of Mg-Ni films in a hydrogen atmosphere of 80–100 Pa. An aberration-corrected environmental TEM with a differential pumping system allows us to reveal the Angstrom-scale structure of the films in the initial stage of hydrogenation: first, nucleation and growth of Mg2NiH4 crystals with a lattice spacing of 0.22 nm in an Mg-rich amorphous matrix of the film occurs within 20 s after the start of the high-resolution observation, then crystallization of MgH2 with a smaller spacing of 0.15 nm happens after approximately 1 min. Our in situ TEM method is also applicable to the analysis of other hydrogen-related materials.

High magnetic field properties of critical current density in Y_1Ba_2Cu_3O_7 coated conductor fabricated by improved TFA-MOD process

Current transport characteristics in a Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) coated conductor fabricated by a trifluoroacetates-metal organic deposition (TFA-MOD) process were investigated over a wide range of temperature and magnetic field up to 25 T. Improved TFA-MOD process was successfully introduced for the better property of critical current density, J/sub c/, at high magnetic fields. The J/sub c/ and critical current, I/sub c/, values of multi-coated film with 1.2 /spl mu/m thickness and 1 cm width were 2.1 MA/cm/sup 2/ and 251 A at 77 K in self-field. Additionally, the superior J/sub c/ properties remained even at high magnetic fields over 20 T and lower temperature, e.g. J/sub c/ value at 30 K in 25 T was 1.0 MA/cm/sup 2/. The J/sub c/ value was about 2.5 times higher than those of previous TFA-MOD process at wide range of magnetic field. Moreover, the statistical distribution of J/sub c/ in the conductor was also estimated within a framework of the percolation model. The uniformity in the YBCO coated conductor was improved by optimizing the TFA-MOD process.

Effects of heating rate in calcination process on microstructures of Y123 precursor and final films formed by advanced TFA-MOD method

We have investigated the effects of heating rates in the calcination process on microstructures of Y123 precursor and final films. As a result, it was found that there is CuO segregation in the vicinity of the Y123 precursor film surface, when the precursor is prepared at a rapid heating rate in the calcination. In the case of heat-treating this precursor film in the crystallization process, many large pores and misoriented Y123 crystals tend to remain in the Y123 final film. It is important to control the heating rate in the calcination, in order to attain higher J/sub C/ of the Y123 final films.

Mixing effects of Cr2O3-PrBaMn2O5 for increased redox cycling properties of Fe powder for a solid-oxide Fe-air rechargeable battery

Large capacity rechargeable batteries are now strongly required for generating electric power from renewable energy, like solar cells or wind power generators. For this purpose, solid oxide Fe–air rechargeable batteries have been studied. In this study, for increasing the stability of the redox cycling properties of Fe powder at 623 K, mixing effects of Cr2O3 and PrBaMn2O5 catalysts were investigated and it was found that Fe powder mixed with 3 wt% Cr2O3–3 wt% PrBaMn2O5 showed excellent stability against redox cycling at 623 K. When Fe powder mixed with Cr2O3–PrBaMn2O5 was used, stable charge and discharge capacity could be achieved over 50 cycles by using the cell consisting of Ni–Fe/La0.9Sr0.1Ga0.8Mg0.2O3/Ba0.6La0.4CoO3. The discharge capacity of the cell achieved was larger than 770 mA h gFe−1 at 623 K. Increased capacity and cycle performance could be assigned to the deep redox degree of Fe powder.