Advanced Materials Research Vols. 26-28

Abstract: For the regenerative fuel cell (RFC), water electrolysis cell performance using membrane electrode assembly (MEA) in polymer electrolyte fuel cell (PEMFC) were investigated. A part of Nafion had been secondary sprayed on the surface of catalytic layer and variation of cell performance was diminished. The conformation of stability, improvement of mechanical and electrical properties was accomplished by addition of PVDF, graphite and RuO2. With the addition of graphite power and RuO2, the voltage was decreased from 3.6V to 2.5V and 2.2V. The improvement of the mechanical properties was obtained by addition of PVDF. The electrolysis cell manufactured with MEA electrode was showed less decomposition voltage of 1.3V than with Nafion electrode at 10A of applied current. The stability of MEA was confirmed from 30 days of cell operation

Abstract: The decrease of material cost for manufacturing fuel cell stack is strongly required for overcoming commercial restriction. The epoxy based graphite bipolar plate (BP) for polymer electrolyte membrane fuel cells (PEMFC) has been prepared and electrical, mechanical and thermal properties were compared. The density of graphite composite bipolar plate showed from 1.67 to 2.54 as graphite content is increased from 60 to 80w/o and decreased as expanded graphite was added. The contact resistance between epoxy/graphite bipolar plate and gas diffusion layer (GDL) had lower value using carbon cloth than carbon paper for GDL. The conformation of thermal stability, improvement of mechanical and electrical properties was accomplished by addition of expanded graphite powder.

Abstract: Microstructures and hydrogen storage properties of Mg-Al super laminate composite alloys were investigated. The laminated sample was made by alternately stacking Mg films and Al films. The laminate was subjected to repetition of stacking and cold-rolling under an ambient condition (super lamination technique), combined with final heat treatment under an argon gas atmosphere. The number of films and thickness was nearly 15000 layers and about 50μm, respectivery. Their microstructures and hydrogen storage properties were studied by scanning electron microscopy, X-ray diffractometry and volumetric method by use of a Sieverts-type apparatus. In heat treatment process at 673K, Mg17Al12 intermetallic compound was formed by interdiffusion. This compound absorbed and desorbed hydrogen reversibly through the disproportionation and recombination react at 673K.

Abstract: The contact angles of pure Ni, Ni/7wt%Ni3Al, Ni/5wt%Ni3Al/ 5wt%Cr and Ni/10wt%Cr anodes for the MCFC were measured by means of the capillary rise method in 62mol%Li2CO3+ 38mol%K2CO3 and 52mol%Li2CO3+ 48mol% Na2CO3 electrolyte and at different atmosphere. Also surface fractal dimension (Ds), which could characterize pore structure of the anodes, was calculated from experimental data obtained by mercury porosimetry and nitrogen adsorption method. The surface fractal dimensions of the anode were in range from 2.75 to 2.81, because porosities of the anodes for MCFC were controlled regularly to about 62% during sintering. It was investigated from wetting-in experiment by capillary-rise method that the contact angles between the anodes and the carbonate electrolytes were relatively decreased at CO2 atmosphere rather than air atmosphere and the angles were also decreased in the 62mol%Li2CO3+38mol%K2CO3 electrolyte rather than that measured and in the 52mol%Li2CO3+48mol% Na2CO3 electrolyte.

Abstract: We have applied hybridization method to the surface modification by Ni coating on powder specimens of pure Mg and Mg-Mg2Ni eutectic alloy, and investigated the effect of the Ni coating on the hydrogenation kinetics in them. The hydrogenation kinetics at 350 degree C is greatly increased by the Ni coating for both the pure Mg and Mg-Mg2Ni eutectic alloy specimens. Thus, it is confirmed that the surface modification by the hybridization method is considerably effective to improve the hydrogenation kinetics in them. This improvement is thought to be a catalytic effect of the coated Ni. The hydrogenation kinetics of the Mg-Mg2Ni alloy specimen heat-treated at 470 degree C has increased compared with that of the as-cast alloy specimen. It is also confirmed that hydrogenation kinetics improves by the heat treatment to grow the eutectic structure consisted of Mg2Ni phase and Mg phase.

Abstract: Pure Ca(AlH4)2 is synthesized and its decomposition reactions are investigated in a fundamental manner. The XRD profile observed is in good agreement with the one reported by Fichtner et al. From TDS measurements, Ca(AlH4)2 appears to decompose into CaH2+2Al+3H2 in three steps. The peak temperature of the first reaction is about 470K which is higher than that for Mg(AlH4)2 (about 450K), indicating that Ca(AlH4)2 is more stable hydride than Mg(AlH4)2. In addition, the catalysis of TiCl3 is doped into Ca(AlH4)2 by ball milling under a hydrogen gas atmosphere and catalytic effects on the decomposition properties are investigated.

Abstract: The alloying effects of Pd on the hydrogen solubility and the resistance to hydrogen embrittlement are investigated for Nb-xmol%Pd-ymol%Zr (x=0~19; y=0, 1) alloys. The hydrogen solubility at 673 K is found to decrease with increasing Pd content in the alloys. Both pure Nb and Nb-Pd alloys possessed ductility in vacuum at 673 K. However, severe hydrogen embrittlement occurs in pure Nb when it is tested under the hydrogen pressure even as low as 0.01 MPa. In view of the small punch (SP) absorption energy, the addition of Pd into Nb improves the resistance to hydrogen embrittlement by decreasing the hydrogen solubility in the alloy, although brittle fracture is still observed in the Nb-15mol%Pd alloy tested under a hydrogen pressure of 0.015 MPa at 673 K.

Abstract: “Environmental cell” microscopy was applied for surveying gas reaction of hydrides in magnesium base alloys, which are candidates for hydrogen storage materials in advanced hydrogen energy systems. In order to clarify the mechanism of hydrogenation process, in-situ experiment has been carried out by using a 200 kV transmission electron microscope (TEM) equipped with a newly developed environmental cell, which is capable to 0.1 MPa in the temperature range between R.T. and 200°C. When hydrogen gas reacted with magnesium powders, straightening of surface steps (60~70 nm in height) was observed, indicating that volume expansion occurred. In addition, the formation of MgH2 was indicated in selected-area-diffraction patterns (SADP). The precise study on this in-situ experiment, as well as its improvement, will be continued, with using transparent films.

Abstract: The open cell Al (or Al foam) was employed in the alloy hydride system due to the rapid heat conductivity of Al. The hydrogen storage alloy powders were packed in this storage system made of Al open cell, and the changes of temperature and equilibrium pressure of hydrogen desorbed were measured. By applying the open cell Al in this hydrogen storage system, the reaction sensitivity of the temperature and equilibrium pressure was far rapidly increased than that without the open cell Al.

Abstract: Hydrogen storage materials are attracting much attention as media of storing hydrogen. High-pressure synthesis has been widely used for exploration of novel materials. We have reported that many new Mg-based hydrides or alloys have been synthesized by anvil-type apparatus under the pressure of GPa-order. In Mg - TM ( TM = Nb, Ta ) - H systems, it was reported that novel FCC-type hydride which is similar in crystal structure and composition to Mg7TiH13-16 was synthesized under 8 GPa. On the other hand, there is few reports of novel Ca-based hydrides to be synthesized under high pressure. However, the compressibility of calcium is higher than that of magnesium. Thus, there is a tendency for Ca compounds to be synthesized by lower pressure than Mg ones. This study describes the synthesis of new Ca-based hydrides by this high-pressure techniques. In Ca - TM ( TM = Ti, Hf, V, Nb and Ta ) systems, the influence of applied pressure on present phases were investigated. For the composition of CaH2 - 14.3 mol%ZrH2 in Ca - Zr - H system, novel hydride was synthesized at 1073 K for 2 h under 5 GPa. Crystal structure of the novel hydride was found to be FCC-type with a lattice parameter of a = 0.531 nm. In addition, the thermal stability and hydrogen contents of this novel hydride were investigated. In Ca - Hf - H system, the unknown phase was observed in the sample of CaH2 - 12.5 mol%HfH2 prepared 1073 K for 2 h under 5 GPa. This unknown phase is FCC structure with lattice parameter of a = 0.528(2) nm.