Nickel hydride

Nickel hydride is either an inorganic compound of the formula NiHx or any of a variety of coordination complexes.

For Nickel–hydride battery, see Nickel–hydrogen battery.

"The existence of definite hydrides of nickel and platinum is in doubt".[1] This observation does not preclude the existence of nonstoichiometric hydrides. Indeed, nickel is a widely used hydrogenation catalyst. Experimental studies on nickel hydrides are rare and principally theoretical.

Hydrogen hardens nickel (as it does most metals), inhibiting dislocations in the nickel atom crystal lattice from sliding past one another. Varying the amount of alloying hydrogen and the form of its presence in the nickel hydride (precipitated phase) controls qualities such as the hardness, ductility, and tensile strength of the resulting nickel hydride. Nickel hydride with increased hydrogen content can be made harder and stronger than nickel, but such nickel hydride is also less ductile than nickel. Loss of ductility occurs due to cracks maintaining sharp points due to suppression of elastic deformation by the hydrogen, and voids forming under tension due to decomposition of the hydride.[2] Hydrogen embrittlement can be a problem in nickel in use in turbines at high temperatures.[3]

In the narrow range of stoichiometries adopted by nickel hydride, distinct structures are claimed. At room temperature, the most stable form of nickel is the face-centred cubic (FCC) structure α-nickel. It is a relatively soft metallic material that can dissolve only a very small concentration of hydrogen, no more than 0.002 wt% at 1,455 °C (2,651 °F), and only 0.00005% at 25 °C (77 °F). The solid solution phase with dissolved hydrogen, that maintains the same structure as the original nickel is termed the α-phase. At 25 °C 6kbar of hydrogen pressure is needed to dissolve in b=nickel, but the hydrogen desorbs at pressures below 3.4 kbar.[4]

Surface

Hydrogen dissociates on nickel surfaces. The dissociation energies on a Ni(111), Ni(100), and Ni(11O) are respectively 46, 52, and 36 kJ/mol. The H2 dissociates from each of these surfaces at distinct temperatures: 320–380, 220–360, and 230–430 K.[4]

High pressure phases

Crystallographically distinct phases of nickel hydride are produced with high pressure hydrogen gas at 600 MPa.[4] Alternatively it can be produced electrolytically.[5] The crystal form is face-centred cubic or β-nickel hydride. Hydrogen to nickel atomic ratios are up to one, with hydrogen occupying an octahedral site.[6] The density of the β-hydride is 7.74 g/cm3. It is coloured grey.[6] At a current density of 1 Amp per square decimeter, in 0.5 mol/liter of sulfuric acid and thiourea a surface layer of nickel will be converted to nickel hydride. This surface is replete with cracks up to millimeters long. The direction of cracking is in the {001} plane of the original nickel crystals. The lattice constant of nickel hydride is 3.731 Å, which is 5.7% more than that of nickel.[5]

The near-stoichiometric NiH is unstable and loses hydrogen at pressures below 340 MPa.[4]

Molecular nickel hydrides

A large number of nickel hydride complexes are known. Illustrative is the complex trans-NiH(Cl)(P(C6H11)3)2.[7]

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References
  1. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  2. Xu, Xuejun; Mao Wen; Zhong Hu; Seiji Fukuyama; Kiyoshi Yokogawa (2002). "Atomistic process on hydrogen embrittlement of a single crystal of nickel by the embedded atom method". Computational Materials Science. Elsevier. 23 (1–4): 131–138. doi:10.1016/s0927-0256(01)00217-8.
  3. Xu, Xuejen; Mao Wen; Seiji Fukuyama; Kioshi Yokogawa (2001). "Simulation of Hydrogen Embrittlement at Crack Tip in Nickel Single Crystal by Embedded Atom Method" (PDF). Materials Transactions. 42 (11): 2283–2289. doi:10.2320/matertrans.42.2283. ISSN 1345-9678.
  4. Shan, Junjun (11 November 2009). On the formation and decomposition of a thin NiHx layer on Ni(111) (PDF). The Interaction of Water and Hydrogen with Nickel Surfaces. Leiden: Universiteit Leiden. p. 94. ISBN 9789085704171. Retrieved 11 February 2013.
  5. Takano, Noriyuki; Shinichirou Kaida (2012). "Crack Initiation by Cathodic Hydrogen Charging in Nickel Single Crystal". ISIJ International. 52 (2): 263–266. doi:10.2355/isijinternational.52.263.
  6. Travares, S. S. M.; A. Lafuente; S. Miraglia; D. Fruchart; S. Pairis (2003). "SEM Characterization of Hydrogenated Nickel". Acta Microscopia. 12 (1).
  7. Eberhardt, N. A.; Guan, H. (2016). "Nickel Hydride Complexes". Chemical Reviews. 116 (15): 8373–8426. doi:10.1021/acs.chemrev.6b00259. PMID 27437790.CS1 maint: multiple names: authors list (link)

See also
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