Joint ICTP-IAEA Advanced School on the Role of Nuclear Technology in Hydrogen-Based Energy Systems June Basics of metal hydrides

Transcription

1 Joint ICTP-IAEA Advanced School on the Role of Nuclear Technology in Hydrogen-Based Energy Systems June 2011 Basics of metal hydrides J. Huot Universite du Quebec a Trois-Rivieres Canada & Institute for Energy Technology Norway

2 Basics of metal hydrides J. Huot Université du Québec à Trois-Rivières Present address: Institute for Energy Technology, Norway Joint ICTP-IAEA Advanced School on the Role of Nuclear Technology in Hydrogen-Based Energy Systems Trieste Italy, June 2011

3 History T. Graham (1866) Metal palladium absorbs hydrogen Hydrogen can permeate Pd-membranes Reilly and Wiswall (1968) Mg 2 Ni, FeTi Van Vucht, Kuijpers and Bruning (1970) LaNi 5

4 Applications of MH Hydrogen storage Purification/separation Isotope separation Hydrogen getters Hydrogen compression Heat storage Heat pumps and refrigerators Temperature sensors and actuators Liquid H 2 (boil-off losses) Batteries electrodes Permanent magnet production Neutron moderators Switchable Mirrors

5 Hydrogen storage System Hydrogen mol H 2 dm -3 Hydrogen wt.% Gas (273K, 1 bar) pressure (150 bar) LH 2 (20K) MgH LaNi 5 H

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7 Metal Hydrides Advantages High volumetric density Low pressure Endothermic reaction (desorption) Disavantages Temperature of operation Hydrogen sorption kinetic Cost Pyrophoricity

8 Classes of hydrides Ionic or saline hydrides Formed by alkali and alkaline earth metals. hydrogen is a negatively charged ion (H-) Typical binary ionic hydrides are sodium hydride NaH and calcium hydride CaH 2. high conductivities just below or at the melting point. Complex ionic hydrides LiAlH 4, NaAlH 4 Covalent hydrides compounds of hydrogen and nonmetals. atoms of similar electronegativities share electron pairs. low melting and boiling points. (most of them are liquid or gaseous at room temperature) weak van der Waals forces. water (H2O), hydrogen sulfide (H2S), silane (SiH4), aluminum borohydride Al(BH4)3, methane (CH4) and other hydrocarbons. Complex chemical reactions should be used to synthesize them

9 Classes of hydrides Metallic hydrides Formed by transition metals including rare earth and actinide series. hydrogen acts as a metal and forms a metallic bond. wide variety stoichiometric and nonstoichiometric compounds. formed by direct reaction of hydrogen with the metal or by electrochemical reaction. TiH 2 and ThH 2. LaNi 5 H 6,FeTiH 2 Note this division should not be taken too literally. Most hydrides are a mixture of different bonding. Example: LiH mainly ionic but partly covalent.

10 A. Züttel (2004)

11 Schematic of formation

12 Formation H 2 2H Oxide layer Solid solution P H2 H concentration Nucleation of phase (hydride) H on octahedral or tetrahedral site Lattice expansion Symmetry reduction

13 Thermodynamics Reaction x M 2 H 2 MH x Q Q is the heat of reaction Low concentration (x<<1) : phase Hydride : phase

14 Thermodynamics Phase rule (Gibbs) F = C P + 2 Components = 2 (H + Metal) Phases: (x<<1) 2 (, H 2 ) when nucleation of 3 (,, H 2 ) Degree of freedom: (x<<1) F =2 (P and c varies) when nucleation of F =1 (plateau) Pressure-Composition Isotherm (PCT)

15 Low concentration hydrogen randomly distributed in the metal host lattice concentration varies slowly with temperature. Condition for thermodynamic equilibrium. 1 2 ( p, T) ( p, T, c H 2 H H ) Ideal gas: 0 0 H 2 H ln H TS 2 H RT 2 p H 2 Chemical potential of a dissolved H atom: H H H TS id H RT ln c b - c

16 H H : enthalpy S id h : non-configurational part of entropy b: number of interstitial sites per atom Ln(c/b-c): configurational part of entropy Low concentration Seivert law p / 2 K H S 1 2 H 2 is an ideal gas H 2 is dissociated

17 Higher concentration transition H: enthalpy S: entropy constant A. Züttel (2004)

18 Züttel, A., Materials for hydrogen storage. Materials Today, (9): p

19 Dependence of H on concentration H-H interactions H x H V x V x H x V Elastic Electronic

20 Elastic contribution H V x V x K 0 v v 2 H 0 u els v 0 : Atomic volume 2 v H : Volume increases/h K 0 : Bulk modulus constant Elastic contribution constant

21 Elastic contributions electronic contribution Elastic = attractive Energy ~ few hundredths of ev long range

22 Electronic contribution Expansion of lattice modification of the symmetry of electronic states and reduction of the width of the bands Appearance of a metal-hydrogen bonding band below the metal d-band. New attributes in the lower portion of the density of states due to H-H interaction.

23 Palladium

24 Pd PdH

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27 Practical applications Hydrogen in alloys specific properties Intermetallic hydrides have a wider range of hydride stability Alloys of: A: hydride forming B: non-hydride forming Types AB 5 (LaNi 5, CaNi 5 ), AB 2 (ZrMn 2, ZrV 2 ), AB (FeTi) and A 2 B (Mg 2 Ni).

28 Stability A + (x/2)h 2 AH x (P, G A ) The alloy AB n reacts with hydrogen as AB n + (x/2)h 2 AH x + nb P P'exp 2 G xrt A P>P Destabilization

29 Stability Heat of formation H( ABnH 2 x ) H( AH x ) H( BnH x ) H( ABn) Miedema s rule of reversed stability Less stable alloys form more stable hydrides

30 Crystal structure Formation of hydride Expansion of the lattice (2-3 Å 3 ) Volume expansion (30vol.%) Reduction of symmetry Hydrogen occupy specific sites Octahedral (O), Tetrahedral (T) fcc low concentration O site hcp T and O sites distorted bcc T and O sites greatly distorted

31 Interstitial sites Octahedral Tetrahedral fcc hcp bcc

32 Hydrogenation Lattice expansion, distortion Same crystal structure Crystal structure Structure Type Cubic Ti 2 Ni, MgCu 2, CaF 2, Th 6 Mn 23, CsCl, Cr 3 Si Hexagonal CaCu 5, MgZn 2, Mg 2 Ni, AlB 2, PuNi 3, Pd 15 P 2 Tetragonal TiCu, CuAl 2, MoSi 2, Nd 2 Fe 14 B Orthorhombic Monoclinic CrB, Fe 3 C Pd 6 P

33 TiFe : cubic, Octahedral sites : Orthorhombic, Distorted octahedral : Distorted Orthorhombic

34 Geometry Minimum hole size: 0.4Å Minimum bond distance: 2.1Å Stability of hydride increases with size of interstice.

35 Amorphous material Produced by: Rapid quenching Sputtering Ball milling Hydrogen sites presents a distribution of energy states Hydrogen enters successively higher energy sites Hydrogen occupies distorted tetrahedral on fourfold coordinated sites

36 Amorphous material

37 Dynamics Proton vibrating on interstitial site (10 14 Hz) Jump to neighbour site (10 9 Hz) Rapid diffusion Diffuse faster in open structures (BCC) than in closed packed structures (FCC) Low activation energy (Arrhenius) D D exp( E / kt 0 a )

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39 Effect of structure PdCu alloy Reverse isotope effect for Pd

40 Kinetics Must take into account nucleation and growth process. Johnson-Mehl-Avrami -ln{ln(1-f)} = ln(b) + m ln(t) f : reacted fraction m : constant (rate-limiting step) B = parameter that depends only on T and P

41 Rate-limiting step Growth dimensionality m constant nuclei sites m constant nucleation rate 1 1/2 3/2 Diffusion /2 5/ Interface transformation

42 MgH 2 + TiVMN bcc alloy Hydrogen desorption 573 K 0 Hydrogen content, wt% MgH 2 40 hrs MgH 2-2% mol. BCC 2 hrs MgH 2-2% mol. BCC 20 hrs MgH 2-2% mol. BCC 40 hrs MgH 2-2% mol. BCC 80 hrs Desorption time, sec

43 Rate limiting step Left side of equations (1-f) 1/3 = kt f = kt 1 - (1-f) 1/2 = kt [-In(1-f)] 1/3 = kt [-In(1-f)] 1/2 = kt 1-(2f/3)-(1-f) 2/3 = kt Best linear fit Time, sec Bulk nucleation and growth

44 Destabilization Chemical Formation of a new compound Size effect Excess enthalpy and strain at the grain boundary Recrystallization

45 Bond strength

46 Chemical destabilization

47 Effect of cluster size

48 Conclusion MH have many practical applications and may be the solution for hydrogen storage problems MH are also ideal systems for fundamental understanding of: Physics Chemistry Metallurgy Surface science Nanotechnology Clusters