Palladium hydride
Palladium hydride is metallic palladium that contains a substantial quantity of hydrogen within its crystal lattice. Despite its name, it is not an ionic hydride but rather an alloy of palladium with metallic hydrogen. At room temperature, palladium hydrides may contain two crystalline phases, α and β (sometimes called α'). Pure α phase exists at x < 0.017 whereas pure β phase is realised for x > 0.58; intermediate x values correspond to α-β mixtures.
Hydrogen absorption by palladium is reversible and therefore has been investigated for hydrogen storage. Palladium electrodes have been used in some cold fusion experiments, under the hypothesis that the hydrogen could be "squeezed" between the palladium atoms to help them fuse at lower temperatures than would otherwise be required. A great number of research labs in the United States, Italy, Japan, Israel, Korea, China and elsewhere claim to have observed cold fusion in palladium deuteride (heavy hydrogen version of palladium hydride).
The absorption of hydrogen gas by palladium was first noted by T. Graham in 1866 and absorption of electrolytically produced hydrogen, where hydrogen was absorbed into a palladium cathode, was first documented in 1939. Graham produced an alloy with the composition PdH0.75.
Palladium is sometimes metaphorically called a "metal sponge" (not to be confused with more literal metal sponges) because it soaks up hydrogen "like a sponge soaks up water". At room temperature and atmospheric pressure (standard ambient temperature and pressure), palladium can absorb up to 900 times its own volume of hydrogen. As of 1995, hydrogen can be absorbed into the metal-hydride and then desorbed back out for thousands of cycles. Researchers look for ways to extend the useful life of palladium storage.
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