Materials Science Forum Vols. 561-565

Abstract: Super-laminates have been attracting attention since co-authors Ueda et al. reported that Mg/Cu super-laminates showed reversible hydrogenation and dehydrogenation at 473K. The Mg/Cu super-laminates were prepared by a repetitive fold and roll method. Initial activation at 573 K led the super-laminates to absorb hydrogen at 473K. TEM observations of micro/nano-structures in the super-laminates and Mg2Cu powder were performed in order to clarify the process of hydrogenation and dehydrogenation at 473K. The as-rolled Mg/Cu super-laminates have laminated structures in size of sub-micrometer thickness composed of Mg and Cu layers with dense lattice defects. The super-laminates after initial activation keep laminated structure and have uniformly distributed pores with a sub-micrometer diameter. It is considered that these micro/nano-structures of Mg/Cu super-laminates lead to lower dehydrogenation temperature and better kinetics.

Abstract: Direct evidences of hydrogen loosely trapped between graphene layers in nanostructured graphite prepared by mechanical milling in a hydrogen atmosphere are presented, based on a combinational study of FT-IR, electron diffraction (ED) and electron energy-loss spectroscopy (EELS). The FT-IR spectrum of nanostructured graphite exhibited a new broad absorption band at very low frequencies around 660 cm-1, which almost disappeared by annealing up to 800 K. ED and plasmon peaks in EELS detected the unusual shrinkage and subsequent expansion of the fragmented graphene interlayer distance by hydrogen incorporation and desorption with annealing, well correlated with the change in intensity of the 660 cm-1 IR band. All the present results support our previous studies [S. Muto et al., Jpn. J. Appl. Phys. 44, 2061 (2005); T. Kimura et al, J. Alloys and Compounds 413, 150 (2006).].

Abstract: Pt4ZrO2/C was prepared and compared with commercial Pt/C (46.6 wt.% TKK) in terms of the durability as cathode catalyst in a high temperature proton exchange membrane fuel cell (PEMFC) based on H3PO4 doped polybenzimidazole (PBI) by a potential sweep test. The catalysts before and after the potential sweep test were characterized by RDE, XRD, TEM and ICP-AES. After 3000 cycles potential sweep test, the overall performance loss of the Pt4ZrO2/C membrane electrode assembly (MEA) was less than that of the Pt/C MEA. In brief, the preliminary results indicate that Pt4ZrO2/C catalyst is a good candidate of Pt/C catalyst in high temperature PEMFC based on H3PO4 doped PBI for achieving longer cell life-time and higher cell performance.

Abstract: Mg-based laminate composites prepared by a repetitive-rolling method have a large hydrogen capacity as well as good kinetics and reversibility for hydrogen reaction. Mg/Pd laminate composite (Mg/Pd=6) can absorb a large amount of hydrogen up to 1.47 H/M (4 mass%), and desorb hydrogen reversibly at 573K. In this study, we investigated the mechanism of phase transition of the Mg/Pd laminate composites during hydrogenation/dehydrogenation by means of in-situ XRD measurements. In Mg/Pd laminate composite (Mg/Pd=6), the XRD profile of as-rolled sample indicated the existence of Mg and Pd within the detection ability of our XRD equipment. Mg6Pd intermetallic compound are formed during initial activation process. The Mg6Pd intermetallic compound can store hydrogen reversibly through the disproportionation and recombination processes.

Abstract: Ammonia formation activity was studied on the Ru/(MgO-CeO2) catalyst loaded on a Ag-Pd hydrogen permeable membrane. The ammonia formation rate for the membrane reactor was more increased than that based on the conventional flow-type one due to the high reactivity of atomic hydrogen supplied from the Ag-Pd membrane. In addition, other on-site hydrogen generation type membrane reactor was also constructed, where the Ag-Pd alloy membrane was not only atomic hydrogen supplier but also an electrode for water electrolysis. Ammonia was effectively formed even at 373 K on this membrane reactor with the electrodeposited Ru metal powder catalyst.

Abstract: Sorption of ten different quantities of D2 within the porous coordination metal framework Cu3(1,3,5-benzenetricarboxylate)2 were studied using neutron powder diffraction. D2 sorption in the porous material reached saturation (at 25 K) at an amount equivalent to ~6.5 wt. % H2. Rietveld analyses revealed that at saturation there are two saturated small pore and three saturated large pore sites, in addition to the primary site at the coordinatively unsaturated Cu. We reveal that the linked bimodal pore system provides up to nine distinct sites for D2, at moderate D2 concentrations up to saturation. Intermediate sites are attributed to local D2-D2 interaction within the confined pore spaces. Within the small and large pores one, and two, intermediate sites are identified, respectively. The population of D2 at each site was found to affect the host lattice.

Abstract: The kinetics of hydrogen reaction (absorption and desorption) on the MgH2 have been reported to be improved significantly by addition of transition metal oxides as catalysts. Among the oxides reported previously, Cr2O3 seems to improve hydrogen absorption kinetics and Nb2O5 for desorption kinetics. The catalytic effect of addition of more than one oxide, however, has not been reported yet. We investigated the hydrogen reaction kinetics of ball milled MgH2 powders added with either Cr2O3 or ZnO together with Nb2O5. In absorption reaction, the hydrogen contents reached 6 wt% and 5.3 wt% in 5 min for the powders added with 1 mol% ZnO + 1 mol% Nb2O5 and with 1 mol% Cr2O3 + 1 mol% Nb2O5, respectively. Those powders desorbed hydrogen up to about 4.5 wt% in 20 min. The significant improvement was not expected if one of the oxides was added separately. The combination of two kinds of oxides might play an important role for improvement of reaction kinetics.

Abstract: In order to improve the hydrogenation property of Mg/Ti laminate composite, Ni was added as a third element. Hydrogen storage properties of the laminated Mg/Ti/Ni alloy films were studied. Laminated Mg/Ti/Ni multilayer alloy films were prepared by cold rolling of stacked Mg, Ti and Ni under ambient conditions. The stacked foils were subjected to repetition of rolling and stacking, resulting in super-laminated foils with the thickness less than 0.3mm, containing more than 15000 layers. Microstructures of the super-laminates were studied by scanning electron microscopy and X-ray diffractometry. Their hydrogenation behaviors were investigated by use of a Sieverts type apparatus. The hydrogen storage capacity (H/M) of the laminate with composition Mg/Ti/Ni=9.0/0.9/0.1 amounted H/M=1.6 at 573K, 0.4MPa. Initial activation property was improved by controlling the amount of Ni appropreately.

Abstract: Ti-Cr-V alloys with body-centered cubic (BCC) structure were investigated on hydrogen absorbing-desorbing cyclic durability by using H2 without and with several concentrations of CO (Hereafter, they are described as pure H2 and n ppm CO, respectively.). The alloy which offers the excellent durability except the early stages of cyclic absorption-desorption under pure H2 atmosphere gave good durability also in a flow of low concentration of CO. However, capacities’ degradation curves under high CO concentration reflected that duration and concentration effects competed with each other. The results suggests to be able to express each degradation by intrinsic and extrinsic factors as an aggregation of several prime functions.

Abstract: This study is intended to reduce the difference of thermal expansion coefficient between metallic interconnector and solid electrolyte of SOFC (Solid Oxide Fuel Cell) without sacrificing of electrical conductivity. Fe-Cr alloys have been chosen as candidate materials due to its merit of low cost and high temperature oxidation resistance. Different amount of alloys element and compositions have been varied to optimize the properties by method of alloys design with aid of thermodynamics software Thermal-Cal. Phase diagrams of multi-components alloys have been drawn to predict the possible stable phases formed in the investigated metals. An arc melter and plasma melting furnace were used to melt the investigated alloys. The measurements of thermal expansion coefficients and electrical conductivities are carried out with TMA and ASR resistance instrument. The results indicate that the Fe-10Cr alloy exhibits the smallest thermal expansion coefficient among the alloys, while Fe-16Cr has a lowest electrical resistance .