Solid Metal Hydrides: Properties Relating to Their Application in Solar

15 Solid Metal Hydrides: Properties Relating to Their Application in Solar Heating and Cooling 1

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G. G. LIBOWITZ and Z. BLANK

Materials Research Center, Allied Chemical Corp., Morristown, N . J. 07960

Concepts for using solid metal hydrides for solar heating and cooling are described. In solar heating the enthalpy of formation of the metal hydride provides a means of stor ing solar thermal energy, while in cooling the endothermic dissociation of the hydride is used. The properties of metal hydrides required for these applications are reviewed, the most important properties being large enthalpies of forma tion (but relatively low thermal stabilities) and high hydro gen-to-metal ratios. There are two approaches to develop ing new hydrides to meet these requirements: (a) modifying the properties of known hydrides—examples based on the thermodynamics of solids are discussed in some detail; and (b) synthesizing new intermetallic-compound hydrides.

T ^ h e metal hydrides under consideration form b y direct combination of a t r a n s i t i o n m e t a l o r a l l o y w i t h h y d r o g e n as f o l l o w s :

M

+

| H

2

* ± M H *

(1)

T h e f o r m a t i o n of h y d r i d e Μ Η * is u s u a l l y a spontaneous e x o t h e r m i c r e a c t i o n w h i c h c a n b e r e v e r s e d e a s i l y b y a p p l y i n g heat.

T h e hydrogen

densities i n these m e t a l h y d r i d e s a r e e x t r e m e l y h i g h (e.g., t h e n u m b e r o f h y d r o g e n atoms p e r c m is greater t h a n i n l i q u i d h y d r o g e n ) ( J ) . F o r 3

this reason, a n d also because of t h e ease o f r e v e r s i b i l i t y of E q u a t i o n 1, 1

Current address: Corporate R&D Laboratories, The Singer Co., Fairfield, N. J. 07006. 271

Goodenough and Whittingham; Solid State Chemistry of Energy Conversion and Storage Advances in Chemistry; American Chemical Society: Washington, DC, 1977.

272

SOLID STATE

CHEMISTRY

these m a t e r i a l s a r e b e i n g w i d e l y i n v e s t i g a t e d ( 2 ) as a storage m e d i u m f o r h y d r o g e n i n its p o s s i b l e use as a f u e l ( 3 ) .

However, the applications

of m e t a l h y d r i d e s p r o p o s e d i n t h i s p a p e r use p r i m a r i l y t h e r e l a t i v e l y h i g h enthalpies o f formation of m e t a l hydrides rather t h a n their h y d r o g e n storage c a p a b i l i t y . Thermal

Storage

F i g u r e l a i l l u s t r a t e s t h e c o n c e p t p r o p o s e d f o r solar h e a t i n g (4, 5 ) , w h e r e b y m e t a l h y d r i d e s a r e u s e d f o r storage o f t h e r m a l energy.

A metal

h y d r i d e c o n t a i n e d i n a r e s e r v o i r i n t h e b a s e m e n t o f a h o u s e is h e a t e d b y Downloaded by UNIV OF LEEDS on June 18, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch015

solar e n e r g y v i a a heat t r a n s f e r m e d i u m s u c h as w a t e r o r a i r . T h e h y d r o g e n released b y the reverse of E q u a t i o n 1 is transferred t o a large storage r e s e r v o i r . B e c a u s e m a n y m e t a l h y d r i d e s h a v e h y d r o g e n d i s s o c i a t i o n pressures i n t h e r a n g e o f tens o f atmospheres ( a t t e m p e r a t u r e s o b t a i n a b l e f r o m solar h e a t ) , t h e h y d r o g e n gas c a n b e c o m p r e s s e d i n t h e storage t a n k w i t h o u t e m p l o y i n g a u x i l i a r y compressors.

H e a t is r e c o v e r e d

b y a l l o w i n g the s t o r e d h y d r o g e n t o flow b a c k t o t h e d e - h y d r i d e d m e t a l ( w h i c h is n o w u n d e r l o w pressure b e c a u s e i t is n o t b e i n g h e a t e d ) . T h e heat evolved b y E q u a t i o n 1 ( f o r w a r d reaction)

is used for hot water

a n d space h e a t i n g . A n a l t e r n a t i v e , i l l u s t r a t e d i n F i g u r e l b , w o u l d b e t o store t h e h y d r o g e n i n a s e c o n d a r y less stable h y d r i d e . T h i s c o n f i g u r a t i o n has t h e a d v a n -

To Hailing System ν Recycled to Colector

To Halting System or Recycled to Colector

Î

(a) Compressed Hydrogen Storage Figure

(b) Secondary Hydride Storage

I . Systems for storing solar energy using metal hydrides

thermal


15.

LJBowrrz AND BLANK Table I.

Hydride

367°K

t

0

Thermal Storage Capacity (J/g)

Dissociatiçn Pressure (atm)

Enthalpy (kJ/mol H)

2

0

273

Hydrides

Thermodynamic Properties of Hydrides for Solar Thermal Storage

System

VH .95**VH F e T i H o . i τ± F e T i H i .

Solid Metal

294°K

41 34

-40 -28

397 120

1.5 3.5

t a g e of r e q u i r i n g c o n s i d e r a b l y less v o l u m e t o store t h e h y d r o g e n .


b e c a u s e of

the endothermic

hydrogen w o u l d be evolved

n a t u r e of

Also,

the dissociation reaction,

if a leak developed,

less

thus i n c r e a s i n g t h e

safety factor. W i t h a s e c o n d a r y storage h y d r i d e t h e e q u i l i b r i u m d i s s o c i a t i o n p r e s sure of the p r i m a r y h y d r i d e at e l e v a t e d t e m p e r a t u r e s ( d u r i n g solar h e a t ing)

should be

h i g h e r t h a n t h a t of

t e m p e r a t u r e to p e r m i t spontaneous

the secondary

flow

h y d r i d e at

room

of h y d r o g e n gas f r o m t h e p r i

m a r y to t h e s e c o n d a r y h y d r i d e . C o n v e r s e l y , f o r the same r e a s o n i t w o u l d b e d e s i r a b l e f o r the e q u i l i b r i u m d i s s o c i a t i o n pressure of t h e

secondary

h y d r i d e to b e h i g h e r t h a n t h a t o f t h e p r i m a r y h y d r i d e at r o o m t e m p e r a ture. T h i s is i l l u s t r a t e d i n T a b l e I w h i c h lists t h e p r o p e r t i e s of t w o m e t a l h y d r i d e s t h a t c a n b e u s e d f o r t h e r m a l storage of solar energy.

Vanadium

d i h y d r i d e w h i c h has the h i g h e r t h e r m a l storage c a p a c i t y a n d e n t h a l p y of f o r m a t i o n c o u l d b e u s e d as t h e p r i m a r y h y d r i d e , w h i l e i r o n - t i t a n i u m h y d r i d e , w h i c h is less stable, c a n b e hydride.

u s e d as t h e s e c o n d a r y

T h e solar h e a t e d V H , w h i c h dissociates to t h e 2

V H , has a m u c h h i g h e r d i s s o c i a t i o n pressure ( 6 ) than iron-titanium hydride (7)

storage

monohydride

( 3 9 a t m at 3 6 7 ° K )

(3.5 a t m ) at r o o m t e m p e r a t u r e ( 2 9 4 ° K ) .

H o w e v e r , the d i s s o c i a t i o n pressure of V H a t m ) is less t h a n t h a t of F e T i H (3.5 a t m ) .

2

at r o o m t e m p e r a t u r e

(1.5

T h e relative pressure-tem

p e r a t u r e r e l a t i o n s h i p s w i l l b e d i s c u s s e d f u r t h e r i n t h e s e c t i o n o n solar cooling. Solar In

Cooling the

endothermic

application

of

d i s s o c i a t i o n of

metal

hydrides

the h y d r i d e

for

(reverse

solar of

cooling

E q u a t i o n 1)

the is

u s e d . T h e c o n c e p t is i l l u s t r a t e d i n F i g u r e 2. W a r m ( > 2 7 ° C ) a i r t o b e c o o l e d is passed o v e r a heat e x c h a n g e r c o n t a i n i n g a h y d r i d e ( H y d r i d e I ) w h i c h has a r e l a t i v e l y h i g h d i s s o c i a t i o n pressure ( ~ 1 0 - 2 0 a t m ) at a b o u t 27°C.

The

dissociating h y d r i d e removes heat f r o m the air, a n d

the

c o o l e d a i r is e j e c t e d i n t o the house. T h e h y d r o g e n e v o l v e d f r o m H y d r i d e I is a b s o r b e d b y a n a l l o y w h i c h f o r m s a m o r e stable h y d r i d e ( H y d r i d e



Return to solar collector

Hot water from solar collector 60-90°C

Figure 2.

TTTTTt

1·. Hydride

,1.1 I I I I

Cooling water at ambient £emp.

TTTT

Hydride

TTTTTT

Hydride.'.-

1 II IJ I

Solar cooling with metal hydrides

[XI • H2

Ij.j.l.H

Ambient air >27°C


Warm air to outside

into house

Cooled

15.

LiBOwiTZ

A N D

Solid Metal

B L A N K

275

Hydrides

I I ) w h i c h is c o o l e d b y w a t e r at a m b i e n t t e m p e r a t u r e . T h e r a t e of c o o l i n g is d e t e r m i n e d b y the rate of d i s s o c i a t i o n of H y d r i d e I w h i c h is c o n t r o l l e d b y the v a l v e s h o w n . T w o s u c h u n i t s operate s i m u l t a n e o u s l y . W h i l e one is i n a c o o l i n g c y c l e ( d e s c r i b e d a b o v e ) , t h e s e c o n d u n i t is i n a c h a r g i n g c y c l e , w h e r e b y H y d r i d e I I is h e a t e d b y solar e n e r g y so t h a t its d i s s o c i a t i o n p r e s s u r e b e c o m e s h i g h e r t h a n t h a t of H y d r i d e I. T h e h y d r o g e n is t h e n r e - a b s o r b e d b y the m e t a l o r a l l o y of H y d r i d e I , a n d the h e a t of this r e a c t i o n c a n b e e j e c t e d t o the outside. T h e g e n e r a l p r e s s u r e - t e m p e r a t u r e r e l a t i o n s h i p s of the t w o h y d r i d e s


a r e i l l u s t r a t e d i n F i g u r e 3. P o i n t A represents the t e m p e r a t u r e a n d p r e s sure (Pi)

of o p e r a t i o n of the c o o l i n g h y d r i d e ( I ) , a n d p o i n t Β shows the

c o r r e s p o n d i n g pressure ( P )

of H y d r i d e I I at t h e s a m e t e m p e r a t u r e .

n

S i n c e Ρ > P , the s e c o n d a r y a l l o y absorbs t h e h y d r o g e n e v o l v e d τ

n

from

H y d r i d e I t o f o r m H y d r i d e I I . T h e h e a t of r e a c t i o n tends to raise the t e m p e r a t u r e of H y d r i d e I I , a l t h o u g h i t is c o o l e d b y w a t e r at the a m b i e n t t e m p e r a t u r e . H o w e v e r , as seen i n F i g u r e 3, t h e t e m p e r a t u r e m a y a t t a i n a v a l u e c o r r e s p o n d i n g to p o i n t B ' before h y d r o g e n a b s o r p t i o n ceases because P

n

=

P

I t

I n t h e re-charge c y c l e H y d r i d e I I is h e a t e d t o a t e m p e r a t u r e c o r r e s p o n d i n g to p o i n t C ( 6 0 ° - 9 0 ° C ) , a n d h y d r o g e n flows f r o m H y d r i d e I I b a c k to t h e m e t a l of H y d r i d e I , p r o v i d e d H y d r i d e I does n o t r e a c h a temperature

corresponding

to p o i n t D .

A n efficient

heat

exchanger

I—

ι

~ 27°C

>60°C TEMPERATURE

Figure 3.

Pressure-temperature

rehtionships solar cooling

for metal hydrides used in


276

S O L I D

S T A T E

C H E M I S T R Y

w h i c h is sufficiently c o o l e d b y the a m b i e n t a i r w i l l p r e v e n t t h i s f r o m happening. I n m a n y m e t a l - h y d r o g e n systems there is a n hysteresis effect ( 8 ) i n t h e a b s o r p t i o n a n d d e s o r p t i o n of h y d r o g e n . I n s u c h cases t h e a b s o r p t i o n p r e s s u r e is a l w a y s h i g h e r t h a n t h e d e s o r p t i o n pressure. F i g u r e 3 is d r a w n u n d e r the a s s u m p t i o n t h a t t h e r e is n o hysteresis effect i n e i t h e r h y d r i d e . H o w e v e r , i f a p a r t i c u l a r h y d r i d e e x h i b i t s hysteresis, t w o p r e s s u r e - t e m perature curves must be d r a w n , one representing absorption a n d the o t h e r d e s o r p t i o n , f o u r curves i n a l l i f b o t h h y d r i d e s e x h i b i t hysteresis. P o i n t s Β ( Β ' ) a n d D w o u l d t h e n b e o n the a b s o r p t i o n c u r v e s , a n d p o i n t s


A a n d C o n the d e s o r p t i o n curves. S i n c e the slopes of t h e p r e s s u r e - t e m p e r a t u r e curves are a f u n c t i o n of the e n t h a l p y of f o r m a t i o n AH

t

of t h e h y d r i d e , the curves m u s t cross

at s o m e t e m p e r a t u r e (unless the A H

f

values of t h e t w o h y d r i d e s a r e

i d e n t i c a l ) . I n t h e case of F e T i H a n d V H i l l u s t r a t e d i n F i g u r e 4. H y d r i d e I, a n d V H

2

2

t h e y cross at a b o u t 7 9 ° C as

I n this case F e T i H is t h e p r i m a r y h y d r i d e ,

the secondary h y d r i d e , H y d r i d e I I . A t 2 7 ° C the

d i s s o c i a t i o n pressure of F e T i H is a b o u t t w i c e t h a t of V H

Figure

4.

2

(points A a n d

Pressure-temperature relationships for vanadium dihydride iron titanium hydride under conditions used in solar cooling


and

Solid Metal

277

Hydrides

15.

LIBOWITZ

B).

A s the v a n a d i u m absorbs h y d r o g e n , its t e m p e r a t u r e c a n r e a c h a

AND BLANK

v a l u e of a b o u t 41 ° C ( p o i n t Β ' ) before i t w o u l d stop a b s o r b i n g h y d r o g e n . I n t h e r e c h a r g e c y c l e , i f the V H

2

is h e a t e d to 60 ° C b y solar e n e r g y

( p o i n t C ) , the i r o n - t i t a n i u m h y d r i d e m u s t b e k e p t b e l o w 5 3 ° C D ) for r e c h a r g i n g to occur.

I f the V H

2

(point

is h e a t e d to 8 5 ° C ( p o i n t

C),

b e c a u s e of t h e cross-over of the curves, t h e i r o n - t i t a n i u m h y d r i d e c a n r e a c h a n e v e n h i g h e r t e m p e r a t u r e ( 8 8 ° C ) b e f o r e r e c h a r g i n g ceases. T h e curves i n F i g u r e 4 are d e s o r p t i o n curves. H o w e v e r , because of hysteresis i n the F e T i - H system ( 7 ) , p o i n t D s h o u l d a c t u a l l y b e o n t h e a b s o r p t i o n c u r v e ( 9 ) of this s y s t e m ; this w o u l d shift p o i n t D to the left


i n F i g u r e 4 t o a v a l u e of 3 1 ° C

rather than 53°C.

Consequently

the

v a n a d i u m d i h y d r i d e s h o u l d b e h e a t e d to a h i g h e r t e m p e r a t u r e , e.g., f o r point C

at 8 5 ° C a b s o r p t i o n of h y d r o g e n i n F e T i H

(point D )

would

o c c u r at 5 9 ° C . Properties of Metal

Hydrides

A l t h o u g h t w o specific h y d r i d e s w e r e m e n t i o n e d a b o v e , at the present t i m e there are n o k n o w n h y d r i d e s w h o s e properties are i d e a l l y s u i t e d for solar h e a t i n g a n d c o o l i n g . T o find n e w h y d r i d e systems i t is necessary to u n d e r s t a n d the f u n d a m e n t a l s o l i d state c h e m i s t r y of t r a n s i t i o n m e t a l h y d r i d e s , i n c l u d i n g t h e n a t u r e of t h e c h e m i c a l b o n d i n g , t h e e l e c t r o n i c a n d c r y s t a l structures, a n d t h e r m o d y n a m i c a n d t r a n s p o r t properties. Table II. 1. 2. 3. 4. 5.

Properties of Metal Hydrides for Solar Heating and Cooling

E n t h a l p y of f o r m a t i o n H i g h hydrogen-to-metal ratio Good thermal conductivity R a p i d rates of f o r m a t i o n a n d d i s s o c i a t i o n S t a b i l i t y t o w a r d s oxygen a n d moisture

T a b l e I I s u m m a r i z e s s o m e of the p r o p e r t i e s w h i c h w o u l d b e i m p o r t a n t i n u t i l i z i n g m e t a l h y d r i d e s i n the systems d i s c u s s e d a b o v e f o r solar heating a n d cooling. tion)

AH

t

S i n c e i t is t h e e n t h a l p y of f o r m a t i o n ( o r d i s s o c i a

of m e t a l h y d r i d e s w h i c h w i l l b e p r i m a r i l y u t i l i z e d i n t h i s

a p p l i c a t i o n , this p r o p e r t y m u s t b e c o n s i d e r e d the m o s t i m p o r t a n t . t h e r m a l figure of m e r i t , M h , m a y b e d e f i n e d ( 5 ) t

M

t

h

=

A

as f o l l o w s :

2 M W

w h e r e χ is t h e h y d r o g e n - t o - m e t a l r a t i o of t h e h y d r i d e as s h o w n i n E q u a t i o n 1, a n d M W is the m o l e c u l a r w e i g h t .

T h u s a h i g h v a l u e of χ is as

d e s i r a b l e as a h i g h absolute v a l u e of ΔΗ*. F o r t h e case of t h e s e c o n d a r y


278

SOLID S T A T E C H E M I S T R Y

o r storage h y d r i d e , the h y d r o g e n - t o - m e t a l r a t i o is most i m p o r t a n t , w h i l e i t is d e s i r a b l e to h a v e a l o w absolute v a l u e of Aff . f

F o r efficient heat transfer t h e t h e r m a l c o n d u c t i v i t i e s of the m e t a l hydrides should be high.

T h i s is g e n e r a l l y t r u e f o r t r a n s i t i o n m e t a l

h y d r i d e s because of t h e i r m e t a l l i c b o n d i n g ( J O ) . T h e k i n e t i c s of h y d r i d e f o r m a t i o n a n d d i s s o c i a t i o n m u s t b e

con

s i d e r e d b e c a u s e the rate of heat r e c o v e r y o r c o o l i n g w i l l d e p e n d these factors.

upon

T h e r e f o r e , the d i f f u s i o n rates of h y d r o g e n i n the m e t a l

( o r a l l o y ) , a n d i n s o m e cases i n t h e h y d r i d e phase, are i m p o r t a n t . ever, the a v a i l a b l e surface area of m e t a l or h y d r i d e m a y b e of

How greater


i m p o r t a n c e . B e c a u s e t h e t r a n s i t i o n metals e x p a n d o n h y d r i d e f o r m a t i o n , t h e c o r r e s p o n d i n g c r a c k i n g a n d s p a l l i n g w h i c h o c c u r increase the r a t e of h y d r i d e f o r m a t i o n . O n the other h a n d , most a l k a l i a n d a l k a l i n e e a r t h metals c o n t r a c t u p o n f o r m i n g t h e h y d r i d e , a n d this has a d e t r i m e n t a l effect o n t h e rate of r e a c t i o n

(II).

A l t h o u g h the h y d r i d e s w o u l d b e c o n t a i n e d i n a closed s y s t e m as i l l u s t r a t e d i n F i g u r e s 1 a n d 2, i t is d e s i r a b l e t h a t t h e y b e stable w i t h respect to o x y g e n a n d m o i s t u r e because of the p o s s i b i l i t y of leaks. M o s t m e t a l h y d r i d e s o x i d i z e easily, p a r t i c u l a r l y at e l e v a t e d temperatures. s o m e cases, h o w e v e r , o x i d a t i o n is n o t c o m p l e t e .

In

F o r e x a m p l e , i n t h e case

of F e T i , the surface of the a l l o y becomes c o a t e d w i t h a t h i n o x y g e n - r i c h film

( 1 2 ) w h i c h b l o c k s h y d r o g e n a b s o r p t i o n . H o w e v e r , the a l l o y c a n b e

r e - a c t i v a t e d b y h e a t i n g i n h y d r o g e n gas.

I t has also b e e n r e p o r t e d t h a t

some alloys w i l l a b s o r b h y d r o g e n i n the presence of o x y g e n or H 0

(13).

2

A n o t h e r r e q u i r e m e n t of m e t a l h y d r i d e s for these a p p l i c a t i o n s is t h a t t h e cost of t h e c o r r e s p o n d i n g m e t a l o r a l l o y be r e l a t i v e l y l o w .

T h i s is

the m a j o r d i s a d v a n t a g e of v a n a d i u m h y d r i d e . T h e d i s a d v a n t a g e of h i g h cost c a n be p a r t i a l l y o v e r c o m e b y finding a l l o y h y d r i d e s w h i c h h a v e h i g h v a l u e s of Aff

f

a n d h i g h h y d r o g e n - t o - m e t a l ratios, so t h a t M m is i n c r e a s e d

a n d less a l l o y is r e q u i r e d .

New Alloy Hydrides T h e r e are t w o approaches w h i c h c a n b e t a k e n i n the d e v e l o p m e n t of n e w h y d r i d e s : ( a ) the p r o p e r t i e s of k n o w n h y d r i d e s c a n b e m o d i f i e d b y appropriate alloying, and (b)

new intermetallic compounds w h i c h form

hydrides w i t h required properties c a n be synthesized. Property Modification by A l l o y i n g . A n example

of t h e first a p

p r o a c h is the m o d i f i c a t i o n of t h e r m o d y n a m i c p r o p e r t i e s . O n l y the p r o p erties of t h e p r i m a r y h y d r i d e s i n t h e a p p l i c a t i o n s p r o p o s e d

a b o v e are

considered

or

since t h e p r o p e r t i e s r e q u i r e d of t h e secondary

storage

h y d r i d e are the same as those n e e d e d w h e n s t o r i n g h y d r o g n as a f u e l , a n d this l a t t e r a p p l i c a t i o n has b e e n a m p l y d e s c r i b e d e l s e w h e r e ( 2 ,


14).

15.

L I B O W I T Z

As

A N D

B L A N K

Solid Metal

279

Hydrides

m e n t i o n e d a b o v e , f o r solar h e a t i n g a n d c o o l i n g t h e p r i m a r y

h y d r i d e m u s t h a v e a h i g h a b s o l u t e e n t h a l p y of f o r m a t i o n , w h i c h is a n i n d i c a t i o n of the b o n d s t r e n g t h i n a c o m p o u n d .

Consequently, if a par

t i c u l a r a l l o y h y d r i d e has satisfactory p r o p e r t i e s i n o t h e r respects, i t m a y b e p o s s i b l e to i n c r e a s e Δ ί / , b y a p p r o p r i a t e a l l o y i n g . A n e x a m p l e of this is t h e r e c e n t w o r k of V a n M a l et a l . ( 13) o n t h e i n t e r m e t a l l i c - c o m p o u n d hydride L a N i H . 5

7

T h e s e authors h a v e p r o p o s e d a " R u l e of

Reversed

S t a b i l i t y " w h i c h c a n b e s t a t e d as f o l l o w s : t h e less s t a b l e a n i n t e r m e t a l l i c c o m p o u n d , the greater t h e s t a b i l i t y of t h e c o r r e s p o n d i n g h y d r i d e . S i n c e t h e d i s s o c i a t i o n pressure of a h y d r i d e is a m e a s u r e of its s t a b i l i t y , a


decrease i n d i s s o c i a t i o n pressure w i l l i n d i c a t e a m o r e s t a b l e h y d r i d e a n d therefore a n increase i n t h e a b s o l u t e v a l u e of t h e e n t h a l p y of f o r m a t i o n as s h o w n b y the v a n ' t H o f f r e l a t i o n for E q u a t i o n 1:

lnP

H

= ^[(*H /RT)

-*S/R]

t

S i n c e E q u a t i o n 1 is a n e x o t h e r m i c r e a c t i o n , &H

t

increase i n a b s o l u t e v a l u e of &H

t

(2)

is n e g a t i v e , so t h a t a n

w i l l r e s u l t i n a decrease i n h y d r o g e n

d i s s o c i a t i o n pressure P , a c c o r d i n g t o E q u a t i o n 2. H

V a n M a i a n d co-workers (15)

a d d e d t h e a l l o y i n g elements c o b a l t ,

i r o n , a n d c h r o m i u m to t h e i n t e r m e t a l l i c c o m p o u n d L a N i

5

b y substituting

t h e m f o r 2 0 % of the n i c k e l present. A c c o r d i n g t o t h e e n t h a l p i e s of f o r mation Δ Η

Ι 0

of t h e i n t e r m e t a l l i c c o m p o u n d s

shown i n T a b l e III, the

s u b s t i t u t i o n of C o , F e , a n d C r f o r N i i n L a N i b i l i t y of the i n t e r m e t a l l i c c o m p o u n d L a N i s h o u l d h a v e the greatest effect since L a C r

8

5

5

s h o u l d decrease t h e sta

i n t h e o r d e r s h o w n , i.e., C r is t h e least stable ( h i g h p o s i

t i v e Δ ί / i c ) o f these c o m p o u n d s , w i t h F e a n d C o h a v i n g a lesser effect. S i n c e t h e s t a b i l i t y of t h e i n t e r m e t a l l i c c o m p o u n d is d e c r e a s e d b y a l l o y i n g , t h e s t a b i l i t i e s of t h e c o r r e s p o n d i n g h y d r i d e s s h o u l d b e i n c r e a s e d . T h i s w o u l d be i n d i c a t e d b y a decrease i n d i s s o c i a t i o n pressure P . H

The

results of the d i s s o c i a t i o n p r e s s u r e m e a s u r e m e n t s ( 1 5 ) are s h o w n b y t h e 40 ° C p r e s s u r e - c o m p o s i t i o n isotherms i n F i g u r e 5. T h e constant p r e s s u r e

Table III. Calculated Enthalpies of Formation of Some Intermetallic Compounds (17) Intermetallic Compound

&H

JC

(kcal/mol)

Journal of Loss-Common Matais



280

S O L I D

S T A T E

C H E M I S T R Y

Figure 5. Effect of alloying on the stability of lanthanum pentanickel hydride (adapted from Ref. 15) plateaus ( t w o p h a s e regions of t h e phase d i a g r a m ) represent the d i s s o c i a t i o n pressures of t h e h y d r i d e (16).

T h e h y d r i d e s of t h e C o , F e , a n d

C r s u b s t i t u t e d a l l o y s are m o r e stable t h a n the h y d r i d e of L a N i

5

i n the

o r d e r expected. A l t h o u g h i t is d e s i r a b l e t o increase t h e a b s o l u t e v a l u e o f Aff

f

as

m u c h as p o s s i b l e f o r t h e p r i m a r y h y d r i d e , as p o i n t e d o u t a b o v e , t h e d i s s o c i a t i o n pressure also is decreased, a n d this m a y r e s u l t i n a n e q u i U b r i u m h y d r o g e n pressure w h i c h is less t h a n t h a t of t h e

secondary

h y d r i d e u n d e r t h e c o n d i t i o n s of o p e r a t i o n . F u r t h e r m o r e , i f t h e h y d r o g e n pressure is too l o w , the r a t e of mass transfer of h y d r o g e n f r o m o n e p a r t of the c o o l i n g or h e a t i n g system to a n o t h e r becomes too l o w f o r effective operation. It c a n b e seen f r o m E q u a t i o n 2 t h a t t h e d e t r i m e n t a l effect of l a r g e values of AH

t

o n h y d r o g e n pressure m a y b e p a r t i a l l y offset b y d e c r e a s i n g

t h e e n t r o p y c h a n g e of the h y d r i d e f o r m a t i o n r e a c t i o n . T h i s c a n b e ac c o m p l i s h e d b y d e c r e a s i n g t h e v i b r a t i o n a l e n t r o p y of the h y d r i d e phase. A l o w e r v i b r a t i o n a l e n t r o p y is associated w i t h a h i g h e r v i b r a t i o n a l f r e q u e n c y a n d stronger b o n d i n g . H e n c e , a l l o y i n g elements w h i c h increase Δ ί / f b y s t r e n g t h e n i n g the c h e m i c a l b o n d s w i l l also t e n d t o h a v e a f a v o r a b l e effect o n the e n t r o p y c h a n g e . A n o t h e r p o s s i b i l i t y f o r d e c r e a s i n g AS of E q u a t i o n 1 is t o increase t h e e n t r o p y of t h e a l l o y phase b y f o r m i n g a r a n d o m a l l o y . A

possible


15.

LiBOwiTZ

A N D

Solid Metal

B L A N K

281

Hydrides

i l l u s t r a t i o n o f this effect c a n b e seen i n a n i n v e s t i g a t i o n of t h e h y d r i d e s of P d F e b y F l a n a g a n a n d c o - w o r k e r s 3

(J8)

i n w h i c h it was

observed

t h a t at 25 ° C the h y d r o g e n pressure i n e q u i h b r i u m w i t h the d i s o r d e r e d a l l o y w a s orders of m a g n i t u d e h i g h e r t h a n t h e pressure i n e q u i l i b r i u m w i t h t h e o r d e r e d a l l o y ( c u b i c C u A u s t r u c t u r e ) . T h e authors e x p l a i n e d 3

t h e greater s t a b i l i t y of the o r d e r e d h y d r i d e b y the existence of a l a r g e r n u m b e r of P d - H b o n d s t h a n i n t h e d i s o r d e r e d phase

(assuming

h y d r o g e n atoms enter t h e b o d y - c e n t e r e d site of the u n i t c e l l ) .

the

However,

a n o t h e r c o n t r i b u t i n g f a c t o r to the h i g h e r e q u i l i b r i u m pressure of the d i s o r d e r e d p h a s e h y d r i d e m a y h a v e b e e n the l a r g e r c o n f i g u r a t i o n a l e n t r o p y


of t h e d i s o r d e r e d a l l o y . T h e second

Synthesis of N e w Intermetallic Compounds.

approach

to d e v e l o p i n g n e w h y d r i d e s f o r solar h e a t i n g a n d c o o l i n g a p p l i c a t i o n s is to synthesize n e w i n t e r m e t a l l i c c o m p o u n d s

w h i c h w o u l d form hydrides

meeting the property requirements listed i n T a b l e II. I n general, interm e t a l l i c - c o m p o u n d h y d r i d e s a p p e a r to b e a r l i t t l e or n o r e s e m b l a n c e

to

the c o m p o n e n t m e t a l h y d r i d e s . T h i s is i l l u s t r a t e d b y t h e p r o p e r t i e s of the h y d r i d e s of the i n t e r m e t a l l i c c o m p o u n d Z i r c o n i u m forms

Z r N i listed i n Table

IV.

a d i h y d r i d e , a n d n i c k e l n o r m a l l y does n o t f o r m

a

h y d r i d e except u n d e r u n u s u a l c i r c u m s t a n c e s ( J O ) . H o w e v e r , t h e h y d r o gen-to-metal ratio i n Z r N i H

3

is h i g h e r t h a n w o u l d b e e x p e c t e d o n t h e

basis of t h e c o n s t i t u e n t metals Z r a n d N i . T h e structures (19) t w o h y d r i d e s are different.

T h e d i s s o c i a t i o n pressure (20)

of the

of Z r N i H

is

3

almost n i n e orders of m a g n i t u d e h i g h e r t h a n t h a t of Z r H , a l t h o u g h t h e 2

Z r - H distance i n Z r N i H

3

is less, w h i c h u s u a l l y i n d i c a t e s stronger b o n d i n g .

T h e R u l e of R e v e r s e d S t a b i l i t y ( J 5 ) has b e e n p r o p o s e d ( J 5 , 2 J ) as a m e t h o d of p r e d i c t i n g the h y d r i d e - f o r m i n g tendencies of i n t e r m e t a l l i c compounds.

A l t h o u g h this r u l e appears to b e of v a l u e i n p r e d i c t i n g the

effect of a l l o y i n g elements o n the p r o p e r t i e s of k n o w n h y d r i d e s as d i s c u s s e d a b o v e , f o r reasons p r e s e n t e d elsewhere (14) very limited applicability i n

finding

i t appears to h a v e

new intermetallic compound

hy

d r i d e s . A t the present t i m e t h e r e is n o o b v i o u s w a y of r e l i a b l y p r e d i c t i n g the p r o p e r t i e s of a h y d r i d e of a n i n t e r m e t a l l i c c o m p o u n d f r o m a k n o w l e d g e of t h e p r o p e r t i e s of the constituent m e t a l h y d r i d e s .

Consequently,

a n i n t e r m e t a l l i c - c o m p o u n d h y d r i d e s h o u l d b e v i e w e d as a p s e u d o - b i n a r y

Table IV.

Comparison of Properties of Z r H

D i s s o c . press, a t 2 5 0 ° C (torr) Z r - H distance ( Â )

and

ZrNiH

3

ZrH Tetragonal (distorted fluorite)

ZrNiH Orthorhombic

4 X 10 2.09

200 1.96

2

Structure

2

9

s


282

SOLID STATE

CHEMISTRY

metal h y d r i d e w i t h the intermetallic c o m p o u n d b e i n g considered a n e w metal.

T h e b e h a v i o r of the i n t e r m e t a l l i c c o m p o u n d t o w a r d s h y d r o g e n

t h e n d e p e n d s u p p n its p a r t i c u l a r c r y s t a l s t r u c t u r e a n d e l e c t r o n i c s t r u c ture ( 2 2 , 2 3 ) .

T h u s , to d e v e l o p n e w i n t e r m e t a l l i c - c o m p o u n d h y d r i d e s ,

b a s i c a l l o y t h e o r y m u s t b e u s e d , as w e l l as a f u n d a m e n t a l k n o w l e d g e of t h e n a t u r e of t h e m e t a l - h y d r o g e n b o n d s i n m e t a l h y d r i d e s .

Conclusion A r e v i e w of t h e k n o w n m e t a l a n d a l l o y h y d r i d e s reveals t h a t n o n e


meets a l l the r e q u i r e m e n t s l i s t e d i n T a b l e I I at a cost l o w e n o u g h m a k e t h e solar h e a t i n g a n d c c o l i n g concepts e c o n o m i c a l l y feasible.

to

d e s c r i b e d i n this p a p e r

T h e r e f o r e , f o r these concepts t o b e u t i l i z e d n e w

a l l o y h y d r i d e s m u s t be d i s c o v e r e d .

I n the d i s c u s s i o n o n m o d i f i c a t i o n of

h y d r i d e p r o p e r t i e s b y a l l o y i n g , t h e v a r i a t i o n of t h e r m o d y n a m i c p r o p e r ties is c o n s i d e r e d .

H o w e v e r , a l l o y i n g of k n o w n h y d r i d e s also m a y

be

e m p l o y e d to increase h y d r o g e n - t o - m e t a l ratios b y c h a n g i n g c r y s t a l s t r u c tures a n d i n c r e a s i n g l a t t i c e parameters (14) e l e c t r o n i c b a n d structures (22, 2 3 ) .

o r b y m o d i f i c a t i o n of t h e

Furthermore, electronic structure

m o d i f i c a t i o n s w i l l h a v e a n effect o n the t h e r m a l c o n d u c t i v i t y of a h y d r i d e . R a t e s of h y d r i d e f o r m a t i o n a n d d i s s o c i a t i o n also h a v e b e e n v a r i e d b y alloying

(24).

B e c a u s e m e t a l h y d r i d e systems p e r m i t i n d e f i n i t e h e a t storage

(5)

as o p p o s e d to other t h e r m a l storage m a t e r i a l s , t h e y offer a d i s t i n c t a d v a n tage to solar h e a t i n g a n d c o o l i n g i f a l l o y h y d r i d e s c a n b e f o u n d w h i c h w o u l d m a k e this c o n c e p t e c o n o m i c a l l y c o m p e t i t i v e . Literature

Cited

1. Libowitz, G. G., "The Solid State Chemistry of Binary Metal Hydrides," p. 47, Benjamin, New York, 1965. 2. World Hydrogen Energy Conf.Proc.,1st, Univ. of Miami, 1976, Session 8B. 3. Hagenmuller, P., ADVAN. C H E M . SER. (1977) 163, 1. 4. Libowitz, G. G., Intersoc. Energy Convers. Eng. Conf., 9th, 1974, 322-325. 5. Libowitz, G. G., Blank Z., Intersoc. Energy Convers. Eng. Conf., 11th, 1976, 673-680. 6. Reilly, J. J., Wiswall, R. H., Inorg. Chem. (1970) 9, 1678. 7. Reilly, J. J., Wiswall, R. H., Inorg. Chem. (1974) 13, 218. 8. Libowitz, G. G., "The Solid State Chemistry of Binary Metal Hydrides," pp. 83-86, Benjamin, New York, 1965. 9. Reilly, J. J., Johnson, J. R., World Hydrogen Energy Conf. Proc., 1st, 1976, 8B3-8B26. 10. Mueller, W. M., Blackledge, J. P., Libowitz, G. G., "Metal Hydrides," Academic, New York, 1968. 11. Libowitz, G. G., "The Solid State Chemistry of Binary Metal Hydrides," p. 13, Benjamin, New York, 1965. 12. Sandrock, G. D., Intersoc. Energy Convers. Eng. Conf. Proc., 11th, 1976, 967-971.


15. LIBOWITZ AND BLANK

Solid Metal Hydrides 283

13. Wiswall, R. H., Reilly, J. J., Intersoc. Energy Convers. Eng. Conf. Proc., 7th, 1972, 1342-1348.

14. Libowitz, G. G., in "Critical Materials Problems in Energy Production," C. Stein, Ed., Academic, 1976. 15. Van Mal, H. H., Buschow, K. H . J., Miedema, A. R., J. Less-Common Metals (1974) 35, 65.

16. Libowitz, G. G., "The Solid State Chemistry of Binary Metal Hydrides," pp. 5 0 - 5 5 , Benjamin, New York, 1976. 17. Miedema, A. R ., J. Less-Common Metals (1973) 32, 117. 18. Flanagan, T. B., Majchrzak, S., Baranowski, B., Philos. Mag. (1972) 25, 257. 19.

Peterson, S. W., Sadana, V. N., Korst, W. L., J. Phys. (Paris)

(1964)

25,

451. 20.

Libowitz, G. G., Hayes, H . F., Gibb, T. R. P., J. Phys. Chem. (1958) 62,


76.

21. Buschow, K. H . J., Van Mal, H . H., Miedema, A. R.,J.Less-Common Metals (1975) 42, 163.

Switendick, A. C., Solid State Commun. (1970) 8, 1463. Switendick, A. C., Int. J. Quant. Chem. (1971) 5, 459. 24. Douglass, D. L., Met. Trans. (1975) 6A, 2179.

22. 23.

RECEIVED July 27, 1976.


Solid Metal Hydrides: Properties Relating to Their Application in Solar

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