Chem. Mater. 1993,5, 767-769
767
A New Route to Metal Hydrides D. W. Murphy,' S. M. Zahurak, B. Vyas, M. Thomas, M. E. Badding, and W.-C. Fang AT&T Bell Laboratories Murray Hill, New Jersey 07974-2070 Received March 23, 1993 Revised Manuscript Received April 30, 1993
An extensive body of knowledge exists on hydrides of intermetallic compounds.112 Hydrides have long been of interest because they are capable of reversible storage of hydrogen with greater volumetric density than liquid hydrogen, suggesting their utility in a variety of energystorage and heat-transfer systems. The majority of potential applications involve dihydrogen gas and hence the chemistry of metal hydrides has correspondingly concentrated on formation (and reversibility) using Hz. The emerging commercialization of secondary nickel metal hydride (Ni/MH) batteries3 merits a reexamination of metal hydrides under conditions similar to those in the batteries. These batteries have the same positive electrode (NiOOH) and electrolyte (30?4 KOH) as nickel-cadmium batteries but have an intermetallic negative electrode capable of forming a metal hydride by electrochemical reduction of water. There are several key differences between aqueous electrochemical hydride formation and hydride formation with dihydrogen gas: (1)To function in aqueous electrochemicalapplications, intermetallics and their hydrides must be stable to the electrolyte or form a surface that is electronically insulating but permits proton diffusion. Corrosion reactions prevent the use of many known hydrides. (2) For electrochemical applications diffusion of hydrogen across the metal/electrolyte interface and within the metal must be fast enough to allow for complete reaction within a few hours at ambient temperature, whereas elevated temperature and/or pressure are common for reactions with Hz. (3)Reactions with H2 require H-H bond breaking. This may lead to an activation energy which does not arise in electrochemical reactions since hydride formation occurs directly from hydrogen ions. To facilitate the search for electroactive metal hydrides, we have sought hydride-forming reactions that would mimic the electrochemical conditions. In this paper we report that borohydride in aqueous solution is a convenient reagent that satisfies these conditions. Several previous studies have reported chemical alternatives to dihydrogen gas leading to metal hydrides. These include reduction of intermetallics by alcohols at mildly elevated temperatures (300-450 KI4 or by hydrazine in aqueous KOH.5 These reactions have only been reported for materials previously activated by hydrogen gas and we did not find them to be effective on unactivated metals. (1) Hydrogen in Intermetallic Compounds I; Topics in Applied Physics; Schlapbach, L.,Ed.; Springer: New York, 1988;Vol. 63. (2) Hydrogen in Intermetallic Compounds ZI; Topics in Applied Physics; Schlapbach, L.,Ed.; Springer; New York, 1992;Vol. 67. (3) (a)HydrogenStorageMaterials,Batteriesand Electrochemistry, Corrigan,A., Srinivasan, S., Eds.; The Electrochemical Society Sympceium Proceedings Series; Pennington, NJ, 1992. (b) Ovshinsky, S.R.; Fetcenko, M. A.; Ross, J. Science 1993, 260, 176. (4) (a) Imamura, H.; Yamada, K.; Nukui, K.; Tsuchiya, S. J. LessCommon Met. 1986,123, L1. (b) Imamura, H.;Takada, T.; Tsuchiya, S. Int. J.Hydrogen Energy 1988, 13, 11. (5) Wakao, S.;Nakano, H.; Chubachi, S. J.Less-Common Met. 1984, 104, 385.
("I)
LaNi,,Al, (HS-207)
(200)
15
20
25
30
35 40 45 5 0 2-Theta (Cu Ka)
55
60
65
70
Figure 1. Powder X-ray diffraction for LaNir.,Alo.s (bottom) and @-LaNid,&,3H, obtained by reaction with B&- (top). The pattern was taken in a focusing Guinier geometry with the product on a piece of tape and moist with the BH4- solution. Asterisks indicate impurity peaks.
In a recent report6 Nd2Fel4BH6 was formed by reaction of water with a mixture of NdzFeuB, Ca, and CaC12, which had been melted together. To find a reaction that would be general and simulate electrochemicalhydride formation, we sought a reagent that would be sufficiently reducing (1-0.8V vs SHE), a source of hydrogen, and compatible with basic, aqueous solutions. These considerations suggested that BH4- might be suitable. In a typical reaction, a large excess of NaBH4 (5.0 g in 50 mL of water) was added to 1.0 g of LaNi4.7Ab.3 (as received from Aldrich, random pieces with maximum size ~ 1 .mm 5 on edge) and stirred overnight in an open Nalgene beaker. No immediate reaction is apparent, but after several hours the LaNi4.7Ab.3becomes powdered and gas evolution is observed. X-ray powder diffraction patterns of the product immediately followingisolation by filtration confirmed formation of the hydride rB-LaNi4.7Ab.3Hx. Because of the substantial hydrogen vapor pressure @ H ~ ) and the ease of oxidation of the hydride in air, the X-ray pattern reverts to that of LaNi4.7A10.3over about 1h. The X-ray powdered diffraction shown in Figure 1was taken in situ with the powder moist with the BH4- solution. To test the generality of the reaction, we have used a representative group of materials known to form hydrides ~, that exhibit a wide range of stoichiometries, p ~ p land reported activation conditions as summarized in Table I. A range of reaction conditions (BH4- concentration, pH, and temperature) was explored. Hydride formation occurred even in strong base (30% KOH) where BH4hydrolysis is negligible, but it was most convenient to simply use water for which hydrolysis rapidly gives a pH of 9-10. Mildly elevated temperatures (40-50 "C)were found to speed the slower reactions (FeTi), but BHAhydrolysis is also greatly accelerated, necessitating frequent renewal of the reagent. A large excess of BH4-speeds initiation of the reaction, but much lower concentrations are effective thereafter. Lattice parameters of the starting (6) Ram, S.;Joubert, J. C. Appl. Phys. Lett. 1992, 61, 613.
0897-4756/93/2805-076~~04.00/0 0 1993 American Chemical Society
768 Chem. Mater., Vol. 5, No. 6, 1993
Communications
Table I. Hydrides Prepared with Borohydride hydride phases
PH*(at%)*
-
standard activationC PH,(atm) 68 (0 "C)
lattice parametersd (intermetallic) (hydride) a = 4.925(3), c = 4.002(3)8 a,a = 4.947(3), c = 4.001(4)8 a = 4.887(7), c = 4.005(6)s a,a = 4.90(2), c = 4.02(1)8 a = 4.924(5), c = 3.998(5)8 a,a = 4.950(4), c = 3.990(3)s a = 4.91(2), c = 3.99(2)8 j3,a = 5.32(3), c = 4.16(2)8 a = 2.984(1)k 7 , a 6.618(5)kJ
intermetallie MmNis (HS 204)'
f
(atm) 120 (a j3)
CeNish
f
48 (a
j3)'
Cao.zMmo.aNi5 (HS-203)e
f
37 (a
8)
68
MmNi4,l$'eo,s (HS-209)'
f
13 (a
8)
34-68
FeTih
0 c a c 0.lJ 1.0 c j3 c 1.2 1.7 C y C 2.0
5b-+B)i 10 (j3 y)
1-7 (450 OC), 60-70
MmNi4,&.5 (HS-208)'
f
4.25 (a
20
a
LaNi4 (HS-205)'
f
2.0 (a
20
j3, a = 5.29(1), c = 4.21(1)s a = 5.020(5). c = 3.979(5)s 8, a = 5.4iSi7), c = 4 . 2 i S ( w
CaNis (HS-201)'
0 C a C 0.4m 0.8 C a' C 1.2 4.5 C B C 5.6 6.0 C y
0.05 (a a ' ) m 0.6 (a' j3) 20-30 ( B -Y)
10
a
f
0.5 (45 OC)
-
-
j3)
--- . +
0)
j3
.I
--
(a
8)
f
0.44 (a
j3)
10
MgzNi (HS-301)'
0 < a C 0.3 j3= 4.0 0 c a c 0.10 0.6 C j3 C 1.0 a c 59
le (a
8)
20-30 (300 "C)
103 (a
j3)O
Pd NdzFeD'
= 4.947(3), c = 3.943(8)8
+ y (mixed phase)
a = 4.95(6), c = 4.05(5)8 j3, a = 5.25(1), c = 4.20(1)s
LaNir.7Al0.s(HS-207)'
-
= 4.955(7), c = 4.043(6)s
= 5.025(1).c = 4.009(l)g j3,a = 5.389(1), c = 4.278(1)8 a = 5.23(1), c = 13.54(3)8 a,a = 5.202(3), c = 13.219(7)8 a =3.8w j3, a = 4.042(1)k a = 8.806(6),c = 12.204(9)' a,a = 8.972(5),c = 12.362(5)' a
*
a Mm is Mischmetal, a mixture of rare earths. Values from ref 14 unless otherwise indicated. Values from Aldrich Technical Information Bulletin Number AL-146 and for room temperature unless otherwise indicated. d X-ray powder patterns for hydrides measured i n situ with intermetallic in contact with BH4- solution. e HY-STOR purchased from Aldrich. f a and j3 phases exist for most AB& with composition ranges of the order of 0 C a < 1 and 5 C j3 C 6 . 8 Hexagonal unit cell. Prepared by RF melting of elements. From ref 15. From ref 11. These values are somewhat dependent on Fe/Ti. Cubic unit cell. I A small impurity of unhydrided material was present. From ref 7. Purchased from Santoku. 0 From ref 16. p Purchased from Permag. g From ref 6. Tetragonal unit cell.
materials and products of reaction with borohydride are given in Table I and are in good agreement with literature values. Insight into both thermodynamic and kinetic aspects of the reaction can be obtained from the information in Table I. Many hydride forming systems form phases with distinct hydrogen stoichiometry ranges separated by a miscibility gap. Phases with progressively more hydrogen are denoted a,p, y, etc. The corresponding pH2-composition relationship for the case of three such hydride phases is schematically illustrated in Figure 2. The a phases generally exhibit small volume changes compared to the hosts ( < 5 % ) whereas /3 and higher phases can have expansions of 220%. Examination of the results shows that the hydriding power of BH4- is the equivalent of 2030 atm of H2. This value is arrived at by noting that systems with plateau pressures below this value go to the higher hydride phases (e.g., /3-MmNi4.15Feo.%H,)whereas only a phases are formed for materials with higher plateau pressures (e.g., CeNi5). For CaNi5' the p y plateau is sloped from 20 to 30 atm rather than flat, and both B and y phases were observed by diffraction. The example of Mg2Ni illustrates the importance of kinetic factors. MgzNi forms a-Mg2NiH0.3 containing hydrogen in interstitial sites of Mg2Ni and a more stable (once formed) p-MgzNiH4 with a completely different metal atom arrangement.8~9Even though the thermodynamic arguments above suggest that BH4- should afford
-
(7) Yoshikawa, A.; Mataumoto, T. J. Less-CommonMet. 1982, 84, 263. (8) Reilly, J. J.; Wiswall, R. H. Inorg. Chem. 1968, 7 , 2554.
Hydrogen Content
Figure 2. Schematic intermetallic-hydrogen phase diagram illustrating single phase regions separated by two phase regions with a constant plateau pressure.
the phase, only the a phase is formed on reaction with BH4-. The activation energy necessary for the metal atom rearrangement is undoubtedly responsible. The room temperature reaction can not supply the activation energy needed for the structural rearrangement and reaction stops (9) Gama, Z.; Mintz, M. H.; Kimmel, G.; Hadri, Z. Inorg. Chem. 1979, 18, 3595.
Chem. Mater., Vol. 5, No. 6, 1993 769
Communications
1 A
1
Particle Size (microns) Figure 3. Particle size distributions obtained by sequential sieving for MmNia.&b.&oo.? (mechanically ground starting material) (A), MmNi&b.&00.7 after 15 hydride/dehydride cycles with Hz gas (A), MmNi3.&.&oo~ (BH4-reaction) (w), NdzFe14B (BH4-reaction)(e),LaNi5 (BH~rreaction)(v),LaNi4.7Ab.3(BHdreaction) (v),and CaNi5 (BH4- reaction) (0).
at the CY phase. Formation of the p phase with H2 also requires elevated temperatures as noted by the standard activation condition noted in Table I. The standard activation condition for FeTi also involves high temperatures. In this case, however, no large structural rearrangement is involved, but rather a surface modification is thought to convert a passivating (Fe,Ti)0, film to a nonpassivating mixture of Ti02 and elemental Fe.10 Reactivity differences between FeTi purchased from Aldrich (little hydride formation even after 2 weeks with BH4-) and FeTi synthesized in our laboratory by RF melting and minimal subsequent exposure to air (about two weeks for 90% conversion to y-FeTiH,) suggest that interface oxides are important in determining the rate of this reaction. I t is interesting to note that only unreacted FeTi and yFeTiH, are observed during the course of this reaction with none of the intermediate p-FeTiH,." The permanent magnet material NdzFelrB is known to form a hydride with H2 gas12J3or by reaction of water with an intimate mixture of NdzFelrB, Ca and CaC12.'jWe (10)Tatarchuk, B. J.; Dumesic, J. J. Catalysis 1981, 70, 308. (11) Reilly, J. J.; Wiswall, R. H., Jr. Inorg. Chem. 1974, 13, 218. (12) I'Heritier, P.;Chaudouet, P.; Madar, R.; Rouault, A.; Senateur, J. P.; Fruchart, R. C.R. Acad. Sci. 1984,299,849. (13) Oesterreicher,K.;Oesterreicher, H. Phys. Status Solidi A 1984, 85, K61. (14) Huston, E.L.; Sandrock, G. D. J. Less-Common Met. 1980, 74, 435.
observed hydride formation with lattice parameters close to those reported in these studies. The hydride formed by borohydride is stable to air and vacuum as reported for the Ca derived hydride. Hydriding has a minimal effect on the saturation magnetization, but dramatically lowers the coercive force from 3.33 KOe in NdzFelrB to 0.075 kOe in the hydride. The overall reaction of BH4- with intermetallics to form metal hydrides is envisioned as proceeding via a metal assisted borohydride hydrolysis as in eq 1, although the M
+ BH, + 3H20
-
M-H
+ H2BO< + 3.5H2
(1)
detailed mechanism is likely complex. In particular, it is not known whether BH4- or a partially hydrolyzed BHpn(OH),- species is the active hydriding agent and whether or not the reagent is adsorbed on the surface prior to the reaction. The facts that hydrides with p~~ > 1atm can be prepared by the reaction and that the reactions can occur in strong base where unassisted BH4- hydrolysis is negligible lead to the conclusion that hydride formation does not occur simply by reaction with dihydrogen formed by spontaneous BH4- hydrolysis. An important practical aspect of the reaction is the production of free-flowingpowders of hydrideable metals. Decrepitation is a well-known means of producing particles for applications such as powder metallurgy or battery electrode preparation. The particle size distributions of some of the intermetallics following treatment with BH4are shown in Figure 3. For MmNi3.5Alo.&oo.7 the mean size distribution of the starting material of -300 pm was reduced to -35 pm. The same material after 15 cycles hydriding with dihydrogen gas (7.1atm, room temperature) and dehydriding Torr, room temperature) gave a median size of 25 pm. Other ABS'sexamined gave similar product size distributions regardless of the initial size. For NdzFelrB 65% of the product fell into the narrow band of 6-17 pm. In summary, it has been shown that aqueous BH4- is a general and effective reagent for hydriding intermetallics that have P H ~ S120-30 atm. The size distribution of the products are comparable to those obtained after several hydrideldehydride cycles with dihydrogen gas. We believe that the ease of this method and the similarity to conditions in NiIMH batteries will facilitate the discovery and evaluation of metal hydride electrode materials.
-
Acknowledgment. The authors are indebted to T. Siegrist and R. M. Fleming for help with X-ray diffraction, M. T. McCormack for preparation of CeNitj, T. H. Tiefel and S. Jin for magnetic measurements, and J. H. Wernick for helpful discussions. ~~~
(15) Lundin, C. E.;Lynch, F. E.; Magee, C. B. J.Less-CommonMet. 1977, 56, 19. (16) Frieske, H.;Wicke, E. Ber.Bunsen-Ges. Physik. Chem. 1973,77, 50.
