Page 242 - Materials Chemistry, Second Edition

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3.5. Reversible Hydrogen Storage

Table 3.7. Comparison of Metals and Alloys for Hydrogen Storage
Metal/alloy (MH 2(ads) H 2 concentration Decomposition Reversible H 2
compound) stored (wt%) temp. ( C) adsorption/

desorption?
Pd (PdH 0.6 ) 0.6 25 Yes
“AB 2 ” a 1.7–3.3 <100 Yes
LaNi 5 (LaNi 5 H 6 ) 2.5 25 Yes
FeTi (FeTiH 1.7 ) 2.5 25 Yes
BaRe (BaReH 9 ) 3.5 <100 Yes
Mg 2 Ni(Mg 2 NiH 4 ) 3.6 25 Yes
Na (NaH) 4.2 425 Yes
LaMg 12 (LaH 3 , MgH 2 ) 4.6 290 Yes
Ca (CaH 2 ) 4.8 600 Yes
NaAl:Ti (NaAlH 4 : TiO 2 ) 5.5 125 Yes
Li 2 N(Li 2 NH) 6.7 285 Yes
Mg (MgH 2 ) 7.6 330 No
LiAl (LiAlH 4 ) 8.0 180 No
Li 3 Be 2 (Li 3 Be 2 H 7 ) 8.7 300 Yes
LiB:Si (LiBH 4 : SiO 2 ) 9.0 200–400 No
NaB (NaBH 4 :H 2 O) 9.2 25 No
Al (AlH 3 ) 10.0 150 No
Al:N ((NH 3 )AlH 3 ) 12 150 No
Li (LiH) 12.6 720 No
NaB (NaBH 4 ) 13.0 400 No
LiB:N (LiBH 4 :NH 4 F) 13.6 25 No
Be (BeH 2 ) 18.2 250 No
LiB (LiBH 4 ) 19.6 380 No
BeB 2 (Be(BH 4 ) 2 ) 20.6 40 No
Note: This table does not include important nonmetals such as carbon allotropes or boron nitride
compounds; These materials will be discussed in subsequent chapters.
a
A ¼ V, Ti; B ¼ Zr, Ni. Also includes complex combinations (e.g., ZrNi 1.2 Mn 0.48 Cr 0.28 V 0.13 ).

Ternary hydrides of the general formula (MH x ) a (EH y ) b , where E is either a metal
or nonmetal, are also important candidates for hydrogen storage applications. For
example, some of the highest wt% storage values are exhibited by reducing agents
such as sodium metal or lithium borohydride – NaBH 4 and LiBH 4 , respectively.
Relevant for materials design, the decomposition temperature of ternary hydrides
y+
may be altered through choice of E . For example, the T dec of LiGaH 4 is ca.50 C

higher than that of the binary GaH 3 ; on the other hand, the T dec of BeH 2 is ca. 225 C

greater than that of Be(BH 4 ) 2 .
The trend in T dec may be rationalized by the relative difference in electronegativ-
x+ y+ 3+ +
ities of M and E species. For LiGaH 4 ,Ga is a stronger Lewis base than Li ,
indicating that electron density will preferentially flow away from the gallium
+
center, forming ionic Ga–H bonds. This causes a strengthening of the Li ·H
+

interactions through donation of H to Li , resulting in a higher overall T dec .In

contrast for Be(BH 4 ) 2 , the B–H bonds are covalent in nature, which causes H to be
2+
withdrawn from Be . This results in a relatively low T dec value that approaches
room temperature.
A number of molecular transformations take place during the formation of metal
hydrides. Once hydrogen gas is adsorbed on the metal surface, the diatomic