Materials Science Forum Vols. 654-656

Abstract: It is well known that catalytic additives and mechanical milling are effective in improving hydrogen desorption kinetics of MgH2. In this study, the effect of catalytic additives including BaCa1-xNdxO3-δ (BCN) on the desorption behavior of MgH2 was investigated. It was found that BCN can improve the desorption kinetics, but not as effective as other known additives such as Nb2O5. The effect of milling temperature was also studied. It was found that the cryogenic milling is not as effective as room temperature milling primarily due to the inhomogeneous particle size distribution.

Abstract: Alloys of the hypo-eutectic Mg-Mg2Ni system can be modified by trace elemental additions which change the microstructure and the functional properties. The modified microstructure results in improvements to the hydrogen storage properties. In alloys of optimal composition, it has been shown that the reversible storage of 6.5-7wt% H2 is possible at a rate of reaction that is realistic for industrial applications. This paper discusses the mechanism of hydrogen release in air at atmospheric pressure with in-situ crystallographic phase transformation analysis obtained by synchrotron radiation X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS).

Abstract: MgH2-LiBH4 system is one of the promising hydrogen storage materials. In the system, it was found that there was a mutual interaction between the two hydrides, but its mechanism has not been clarified yet. In this work, we found an “H-D” exchange between MgD2 and LiBH4 during heating. IR absorption spectroscopy revealed that the peak of “B-D” vibration appeared at 275 °C below the melting and hydrogen desorption of the system, indicating that this exchange proceeded even in solid phases. The hydrogen desorption properties of the composite of catalyst-doped MgH2 and LiBH4 under inert gas were investigated by mass spectroscopy. The results showed that the hydrogen desorption temperature of the first step was over 300 °C, in spite of catalyst-doped MgH2 can release hydrogen at 200 °C. The above results might suggest the hydrogen desorption from catalyst-doped MgH2 is somehow suppressed by the exchange effect between MgH2 and LiBH4.

Abstract: The silver decoration method was applied to the Mg-11.3mol%Ni eutectic alloy to investigate hydrogen distribution in it. The plate specimens of the alloy were electrochemically hydrogen charged using NaOH aqueous solution at ambient temperature. The specimens were silver decoration treated using K[Ag(CN)2] solved KOH aqueous solution to visualize the hydrogen location as the location of the silver grain. An effect of Pt coating on the decoration process was also examined. The Pt coating has drastically decreased the undesirable precipitation of the silver grain that caused by the direct galvanic reaction between the specimen and the decoration solution. The observed hydrogen location on the hydrogen input side has not been the Mg phase but the Mg2Ni phase in the eutectic structure. The hydrogen location on the hydrogen output side has been also the Mg2Ni phase. It has been thought that hydrogen is preferentially absorbed in the Mg2Ni phase and penetrates into the specimen through the Mg2Ni phase.

Abstract: Magnesium based hydrogen storage materials were prepared by a conventional melting and casting technique. Characterisation of microstructure and hydrogen sorption properties of the alloys was carried out. Additions of Al, Cu and Ni lead to the formation of eutectic mixtures, Mg-Mg17Al12, Mg-Mg2Cu and Mg-Mg2Ni, respectively, with an inter-lamellar spacing of a few hundred nanometers. 3d and 4d transition metals were also added to Mg based alloys and were found to form intermetallic compounds that were homogeneously dispersed in the alloys. The dehydrogenation rate of the Mg alloys was quantitatively analysed in order to determine the rate-limiting step for the hydrogen desorption kinetics. The catalysing role of each intermetallic compound for the hydrogen desorption kinetics is further discussed.

Abstract: We carried out in-situ observation on the catalytic effect of Nb2O5 in MgH2 by using a high voltage transmission electron microscope (HVEM). We synthesized two kinds of samples, which were prepared by ball milling and by hand mixing. In milled sample, Nb2O5 was not confirmed from high resolution images, however, NbO was identified. As the temperature increased, the decomposition of MgH2 proceeded, while Mg formed and grew. It suggested that NbO had the catalytic effect to promote the dehydrogenation of MgH2. In mixed sample, which was prepared to clearly observe the boundary between the catalyst and Mg phase, it was revealed that the dehydrogenation started from the boundary of MgH2 and Nb2O5. This result suggested that the dehydrogenation could proceed with hydrogen diffusion from MgH2 through Mg phase to the boundary.

Abstract: This paper compares proton diffusion through plasma-polymerised proton-exchange membranes (PEMs) produced using traditional wet-chemical methods (Nafion®) and those produced using plasma-polymerisation. Using quasielastic neutron scattering and a simple model of proton motion we find the measured diffusion-rate of protons in the plasma-polymerised material and Nafion® is the same (within 1 standard error) even though the plasma-polymerised membrane has 80 % less water than the Nafion®. We attribute this result to the highly cross-linked structure of the plasma-polymerised membrane.

Abstract: Solid oxide fuel cells (SOFCs) enable environmentally friendly energy to be produced with high efficiency. The market entry of SOFC systems depends on the functionality of the components and on the costs. The SOFC has not yet reached market maturity. This presentation focuses on the possibilities for manufacturing SOFCs with high power outputs and long-term durability by using manufacturing technologies feasible in industry. For the past 15 years, FZ Jülich has been developing large-size so-called anode-supported SOFCs (up to 200 x 200 mm²) with reproducibly high power output (> 2 A/cm² at 800°C). Novel technologies for high-capacity manufacturing such as tape casting and roller coating have been introduced. Additionally, newly developed thin-film techniques have led to power outputs of more than 3 A/cm² at 800°C and more than 1.5 A/cm² below 700°C. These high power densities open up new possibilities for the operation of SOFCs at low temperatures to ensure low degradation and therefore long lifetimes.

Abstract: A micro-DMFC is a promising power source to substitute the lithium battery and used in the portable electronic devices. In this study, the effect of four types of flow fields of bipolar plates in a micro-direct methanol fuel cells (micro-DMFCs) on their performance were experimentally investigated. MEMS technology was applied in fabrication of micro-channel for the micro-DMFCs. The effect of the type of flow field design on the performance of micro-DMFCs was electrochemically evaluated for the four types of flow fields. The micro-DMFCs with double-channel serpentine presented the highest maximum power density and the micro-fuel cells with mixed multi-channel serpentine with narrow channels had the lowest maximum power density.

Abstract: LiFe1-xNdxPO4 /C cathode material has been synthesized by solid-state reaction. The structure and electrochemical performances of LiFe1-xNdxPO4 /C(x = 0 - 0.08) have been studied by XRD, FE-SEM, EIS and galvanostatic charge-discharge. The magnetization curve of LiFe1-xNdxPO4/C sample is measured by SQUID (Superconducting Quantum Interference Device) to examine if Fe3+ ion exists in LiFe1-xNdxPO4/C. The results show that a small amount of aliovalent Nd3+-dopant substitution on Fe+2 can effectively reduce the particle size of LiFePO4/C. The cell parameters of LiFe1-xNdxPO4 (x = 0.04 - 0.08) are calculated on Si internal standard, and LiFe1-xNdxPO4/C has the same olivine structure as LiFePO4, and delivers the highest discharge capacity of 165.2 mAh•g−1 at 0.2 C rate and the capacity retention rate is 92.8 % after 100 cycles. The charge transfer resistance decreases with adding glucose and Nd3+ ion. The measured theoretical capacity of 168.65 mAh∙g-1 is obtained. All the results imply that aliovalent doping substitution on Fe site in LiFePO4 is tolerant.