2245-5
Joint ICTP-IAEA Advanced School on the Role of Nuclear Technology in Hydrogen-Based Energy Systems
13 - 18 June 2011
J. Huot Universite du Quebec a Trois-Rivieres Canada & Institute for Energy Technology Norway
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, 13 – 18 June 2011 History
• T. Graham (1866) – Metal palladium absorbs hydrogen – Hydrogen can permeate Pd-membranes
• Reilly and Wiswall (1968)
– Mg2Ni, FeTi
• Van Vucht, Kuijpers and Bruning (1970)
– LaNi5 Applications of MH
• Hydrogen storage • Purification/separation • Isotope separation • Hydrogen getters • Hydrogen compression • Heat storage • Heat pumps and refrigerators • Temperature sensors and actuators
• Liquid H2 (boil-off losses) • Batteries electrodes • Permanent magnet production • Neutron moderators • Switchable Mirrors Hydrogen storage
System Hydrogen Hydrogen -3 mol H2 dm wt.% Gas (273K, 1 bar) 0.045 100 pressure (150 bar) 6.7 1.2
LH2 (20K) 35 100
MgH2 55 7.7
LaNi5H6 52 1.4
Metal Hydrides • Advantages ¾ High volumetric density ¾ Low pressure ¾ Endothermic reaction (desorption)
• Disavantages ¾ Temperature of operation ¾ Hydrogen sorption kinetic ¾ Cost ¾ Pyrophoricity 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 CaH2. – high conductivities just below or at the melting point.
– Complex ionic hydrides LiAlH4, NaAlH4
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 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.
– TiH2 and ThH2. – LaNi5H6,FeTiH2
Note this division should not be taken too literally. Most hydrides are a mixture of different bonding. Example: LiH mainly ionic but partly covalent. A. Züttel (2004) Schematic of formation Formation
• H2 2H • Oxide layer • Solid solution
• PH2 H concentration • Nucleation of phase (hydride) • H on octahedral or tetrahedral site • Lattice expansion • Symmetry reduction Thermodynamics Reaction x M H MH Q 2 2 x
Q is the heat of reaction
Low concentration (x<<1) : phase
Hydride : phase Thermodynamics
Phase rule (Gibbs) F = C – P + 2
Components = 2 (H + Metal)
Phases: (x<<1) 2 ( , H2)
when nucleation of 3 ( , , H2) Degree of freedom: (x<<1) F =2 ((P and c varies) when nucleation of F =1 (plateau)
Pressure-Composition Isotherm (PCT) Low concentration • hydrogen randomly distributed in the metal host lattice • concentration varies slowly with temperature.
Condition for thermodynamic equilibrium. 1 H H p( T, H) H p( T, c, H ) 2 2 Ideal gas: 0H TS0 RTln p H 2 H 2 H 2 H 2 H 2
Chemical potential of a dissolved H atom: c H TSid RT ln H H H H b - c HH : enthalpy
id Sh : non-configurational part of entropy b: number of interstitial sites per atom
Ln(c/b-c): configurational part of entropy
Low concentration Seivert law 1/ 2 p 2H K S
• H2 is an ideal gas
• H2 is dissociated • Higher concentration transition H: enthalpyy S: entropy constant
A. Züttel (2004) Züttel, A., Materials for hydrogen storage. Materials Today, 2003. 6(9): p. 24-33. Dependence of H on concentration
H-H interactions H H V H
x V x x V
Elastic ‘Electronic’ Elastic contribution2 H V v H 0 K u els V x x 0 v
v 0 : Atomic volume v 2 H : Volume increases/H K 0 : Bulk modulus constant Elastic contribution constant • Elastic contributions electronic contribution
• Elastic = attractive
• Energy ~ few hundredths of eV
• long range 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. Palladium
http://arxiv.org/ftp/cond-mat/papers/0304/0304307.pdf Pd PdH
http://www.nat.vu.nl/CondMat/griessen/ http://www.nat.vu.nl/CondMat/griessen/ Hydrogen in alloys Practical applications specific properties Intermetallic hydrides have a wider range of hydride stability Alloys of: – A: hydride forming – B: non-hydride forming Types
AB5 (LaNi5, CaNi5), AB2 (ZrMn2, ZrV2), AB (FeTi) and A2B (Mg2Ni). Stability
A + (x/2)H2 ↔ AHx
(P’, GA)
The alloy ABn reacts with hydrogen as
ABn + (x/2)H2 ↔ AHx + nB 2 G P P'exp A xRT
P>P’ Destabilization Stability Heat of formation
H n(AB2 x H ) H x(AH ) Hn (Bx H ) H n(AB )
Miedema’s rule of reversed stability
Less stable alloys form more stable hydrides Crystal structure
Formation of hydride – Expansion of the lattice (2-3 Å3) – 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 Interstitial sites Octahedral Tetrahedral fcc
hcp
bcc Crystal structure Hydrogenation – Lattice expansion, distortion – Same crystal structure
Structure Type
Cubic Ti2Ni, MgCu2, CaF2, Th6Mn23, CsCl, Cr3Si
Hexagonal CaCu5, MgZn2, Mg2Ni, AlB2, PuNi3, Pd15P2
Tetragonal TiCu, CuAl2, MoSi2, Nd2Fe14B
Orthorhombic CrB, Fe3C
Monoclinic Pd6P TiFe
: cubic, Octahedral sites
: Orthorhombic, Distorted octahedral
: Distorted Orthorhombic Geometry
• Minimum hole size: 0.4Å • Minimum bond distance: 2.1Å
Stability of hydride increases with size of interstice. 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 Amorphous material Dynamics
• Proton vibrating on interstitial site (1014Hz) • Jump to neighbour site (109Hz) • Rapid diffusion • Diffuse faster in open structures (BCC) than in closed packed structures (FCC) • Low activation energy (Arrhenius)
D 0 D exp( Ea /kT )
• Effect of structure PdCu alloy
• Reverse isotope effect for Pd
http://www.nat.vu.nl/CondMat/griessen/ 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 Rate-limiting step Growth m m dimensionality constant constant nuclei sites nucleation rate 11/23/2
Diffusion 2 1 2 33/25/2
112
Interface 223 transformation 334 MgH2 + TiVMN bcc alloy Hydrogen desorption 573 K
0
-1 MgH 40 hrs 2 %% tt MgH -2% mol. BCC 2 hrs -2 2 ww , tt MgH -2% mol. BCC 20 hrs 2 nn
ee -3 tt MgH -2% mol. BCC 40 hrs
nn 2
co MgH -2% mol. BCC 80 hrs -4 2 n ee g o r -5 dd yy H -6
-7 0 100 200 300 400 500 600 700 800 900 1000 Desorption time, sec Rate limiting step
1.2 1 - (1-f)1/3 = kt f = kt 1 - (1-f)1/2 = kt [-In(1-f)]1/3 = kt ss 1.0 1/2
nn [-In(1-f)] = kt
oo 2/3 ii 1-(2f/3)-(1-f) = kt tt
aa Best linear fit uu 0.8 qq ee ff oo 0.6 ee dd ii ss tt ff 0.4 ee LL 0.2
0.0 0 50 100 150 200 250 300 350 Time, sec
Bulk nucleation and growth Destabilization
• Chemical – Formation of a new compound
• Size effect
• Excess enthalpy and strain at the grain boundary
• Recrystallization Bond strength Chemical destabilization Effect of cluster size 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