Intermetallic-based hydrides

Other Metal and Intermetallic-based Hydrides New Developments... 

A brief general overview of simple metal hydrides and intermetallic-based hydrides including their historical development has already been given in Sect. 1.1 and 1.2. Early research results on these hydrides have been summarized in a book by MueUer... 

The first intermetallic compound that could reversibly absorb hydrogen (ZrNi) was reported by Libowitz et al. [60]. A magnesium-based hydride with relatively high capacity (Mg2Ni) was reported by Reilly and Wiswall in 1968 [61]. Later, room-temperature hydrides such as the ternary hydrides TiFeH2 and LaNisHfi were... 

More general is the rule of reversed stability (Miedema model) the more stable an intermetallic compound, the less stable is the corresponding hydride, and the other tvay around [36]. This model is based on the fact that hydrogen can only participate on a bond with a neighboring metal atom if the bonds between the metal atoms are at least partially broken (Figures 5.25 and 5.26). 

In the case of ZrNi and ZrCo intermetallics, the preformed hydrides are seen to loose hydrogen at open circuit in acid solutions, after which the intermetallic matrix dissolves [554]. A relatively higher stability of the hydride of ZrCo has been attributed to the presence of higher Co oxides which are more resistant to acids. Studies of this kind have relevance to the possible open circuit behaviour of cathodes based on intermetallic compounds. 

Abstract. In the present work it is for the first time proposed to use a photoelectrochemical cell with a GaAs photoanode and a metal hydride cathode based on intermetallic alloys LaNi5.xCox, where 0hydrogen conversion with possibility to storage hydrogen in a cathode material. The characteristics of photoanode and cathodes have been investigated and results obtained are discussed. 

Lead-acid, nickel-iron (Ni-Fe), nickel-cadmium (NiCd), and nickel-metal hydride (NiMH) batteries are the most important examples of batteries with aqueous electrolytes. In lead-acid batteries, the overall electrochemical reaction upon discharge consists of a comproportionation of Pb° and Pb4+ to Pb2+. All nickel-containing battery reactions are based on the same cathodic reduction of Ni3+ to Ni2+, but utilize different anodic reactions providing the electrons. Owing to toxicity and environmental concerns, the formerly widely used Cd°/Cd2+ couple (NiCd cells) has been almost entirely replaced by H/H+, with the hydrogen being stored in a special intermetallic compound (NiMH). 

Sandrock, G. D. Reilly, J. J. and Johnson, J. R., Interrelations Between Phase Diagrams and Hydriding Properties for Alloys Based on the Intermetallic Compound FeTi, "Applications of Phase Diagrams in Metallurgy and Ceramics", National Bureau of Standards SP-496, 1977, 483. 

Sandrock, G. D., "The Interrelations Among Composition Microstructure, and Hydriding Behavior for Alloys Based on the Intermetallic Compound FeTi", International Nickel Co., Suffern, NY, 10901, Final Report for Contract BNL 352410S, June 30, 1976. 

F-treated intermetallics exhibit extremely high reactivity and selectivity to H2, even in CO and CO2 atmospheres. The treatment behaves as a curing effect of the external surface and creates a fluoride-rich layer with a Ni-enriched subsuperficial layer. The hydriding behavior is increased by the presence of microcracks running through the external layer the initial activation process is enhanced, even for Mg-based alloys. In addition, a protective effect is observed since the samples can be safely stored in air for long time without degradation or surface reactivity [116]. 

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