Rechargeable Metal Hydrides: A New Concept in Hydrogen Storage

15 Rechargeable Metal Hydrides: A New Concept in Hydrogen Storage, Processing, and Handling G. D. SANDROCK

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 22, 2015 | http://pubs.acs.org Publication Date: March 26, 1980 | doi: 10.1021/bk-1980-0116.ch015

The International Nickel Company, Inc., Inco Research & Development Center, Sterling Forest, Suffern, NY 10901 E. SNAPE

ERGENICS Division, MPD Technology Corporation, 4 William Demarest Place, Waldwick, NJ 07463 Hydrogen has been traditionally stored, transported, and used in the form of compressed gas or cryogenic liquid. The purpose of this paper is to discuss a third alternative, namely the use of rechargeable metal hydrides. In the most elementary sense, a rechargeable metal hydride is a metal powder that can act as a solid "sponge" for hydrogen. In a chemical and practical sense, of course, the concept is a bit more complicated than that. In this paper we will present an introduction into the science and applications of rechargeable metal hydrides. It will consist of three parts: (a) a review of the fundamentals and practical properties of metal hydrides, (b) a survey of the main families of rechargeable hydrides that are commercially available and, (c) a brief summary of the potential applications of hydrides within the existing hydrogen industry. Fundamentals of Rechargeable Metal Hydrides Basic Chemistry and Thermodynamics. The key to the understanding and use of rechargeable metal hydrides is the simple reversible reaction of a solid metal Me with gaseous H2 to form a solid metal hydride MeHx:

Me + —2 H2 t MeHx

(Eq. 1)

Not all metals react directly with gaseous H2, and of those that do, some are not readily reversible (i.e., the reverse reaction to liberate gaseous H2 cannot be readily performed) . Fortunately, there are a number of metals that do react directly and reversibly in the manner of Eq. 1, and do so at practical temperatures and pressures (e.g., room temperature and near atmospheric pressure). Such metals include elements, solid-solution alloys, and especially intermetallic compounds. We call the hydrides of these metals "rechargeable metal hydrides". In effect, the metal becomes a solid "sponge" for hydrogen that can be repeatedly charged and discharged at will. They bear a physical analogy

0-8412-0522-l/80/47-116-293$07.50 © 1980 American Chemical Society In Hydrogen: Production and Marketing; Smith, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


294

HYDROGEN: PRODUCTION AND MARKETING

with a water sponge and chemical analogy with a rechargeable electric battery. Rechargeable metal hydrides offer a number of advantages over compressed gas and cryogenic liquid for the storage and handling of hydrogen in the commercial sector. A primary advantage of hydrides is their extremely high volumetric packing density for hydrogen ( 1) . As shown in Table I, the volumetric density of H in typical hydrides is many times that of high pressure gas and even significantly greater than that of liquid hydrogen.

This is a result of the fact that H-atoms are chem-

ically bound very compactly within the hydride crystal lattice. A second advantage of hydride storage is the low pressures that are required, a factor that has safety implications to be discussed in more detail later. A third advantage, especially relative to liquid hydrogen, is the high energy efficiency of hydride storage. This, again, will be discussed in more detail later in this paper. Disadvantages of hydrides, such as cost and weight, will also be discussed in the appropriate context. Table I . Hydrogen Content of Various Media ( 1) . (Does not include container weight or void volumes) Wt.% H Medium Volumetric (Atoms H/ml,Density x 10"22) Nh H2, liquid

100

4.2

H2, gas at 100 atm MgH2

100 7.6

0.5 6.7 8.3

UH3

1.3

TiH2

4.0

9.1

VH2

2.1

11.4

Mg2NiH4.2

3.8

5.9

FeTiHi.74 ¦* FeTiHo.m

1.5

5.5

LaNi5H6.7

1.5

7.6

The phenomenological and thermodynamic aspects of the reaction shown in Eq. 1 should be discussed in more detail.

Typically, the absorption and desorption properties of metals are determined from pressure composition (P-C) isotherms, an idealized form of which is shown in Figure 1. If we start with the metal phase at Point 1, maintain a constant temperature and slowly increase the H2 pressure, relatively little happens at first. As the H2 pressure increases, a small amount of hydrogen goes into solution into the metal phase. At some pressure (Point 2) , the hydriding reaction (Eq. 1) begins and the sample starts to absorb large quantities of hydrogen at nearly constant

pressure. This pressure, Pp, is called the "plateau pressure".

The plateau 2-3 then corresponds to a two-phase mixture of metal

Me and metal hydride MeHx. At Point 3, the sample has been completely converted to the hydride phase and a further increase

In Hydrogen: Production and Marketing; Smith, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


15. sandrock and snape

Rechargeable Metal Hydrides

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Figure 1. Ideal absorption and desorption isotherm for a metal-hydrogen system


296


in applied H2 pressure to Point 4 results only in a small additional pickup of hydrogen in solution in the hydride phase. In principle this curve is reversible. As H2 is extracted from the gas phase in contact with the sample, the hydride phase will dissociate (dehydride) to Me + H2 gas and attempt to maintain the equilibrium plateau pressure until it is fully dissociated (back


to Point 2) .

Although the ideal behavior shown in Figure 1 is occasionally observed in practice, there are usually slight deviations from this ideality. To illustrate this, isotherms obtained for a practical nicke 1-aluminum-mischmetal compound (2) are shown in Figure 2 . Compared to the ideal curve (Figure 1) , the plateau is often sloped slightly and the plateau limits are often not as sharp. In addition, there is almost always some pressure hysteresis between absorption and desorption (see the 25 °C curves of Figure 2) . The hysteresis in this example is relatively small, although clearly measurable. Figure 2 shows the strong dependence of temperature on the plateau pressure. The higher the temperature, the higher is the plateau pressure. This is an important thermodynamic consequence of the heat of reaction AH associated with Eq. 1. The hydriding reaction (->) is exothermic and the dehydriding reaction («-) is

endothermic. The plateau pressure Pp is related to the absolute temperature T by the Van't Hoff equation

lnPp = - |£ + C P

x

RT

(Eq. 2)

where x is defined in Eq. 1, Ah is the enthalpy change (heat) of the hydriding reaction, R is the universal gas constant, and C is a constant related to the entropy change of the hydriding reaction. Thus, from a series of experimental isotherms such as

shown Figure 2, a Van't Hoff plot of In Pp vs. 1/T can be made

and the value of AH for a particular material can be readily determined from the slope of that plot.

This is done in Figure 3, using the H/M =0.5 values of Pp

taken from the desorption isotherms in Figure 2. The resultant value of AH is -6.7 kcal/mol H2 for this particular alloy. This is the heat that is generated during the hydriding reaction and must be supplied during the dehydriding reaction. Note that the heat involved in this case is only about 12% of the lower heating value of the hydrogen involved (-57.8 kcal/mol) and represents a "low-grade" form of heat. An inspection of Figure 2 or Figure 3 shows that room temperature (or even ice water temperature) "waste" heat would be capable of dissociating MNii+.sAlo.s hydride to provide H2 at pressures of 1 atm absolute or more. Engineering Properties. In addition to the simple principles discussed above, there are a number of engineering properties that bear on practical applications of hydrides in the hydrogen storage, processing, and handling fields. Most of these are




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At temperatures on the order of 200°C, where diffusion of the metal atoms becomes significant, the latter reaction tends to begin and results in a loss of reversible capacity (i.e., irreversible Eq. 4 tends to predominate over reversible Eq. 3) . Although most of the intermetallic compounds tend to be thermodynamically unstable relative to disproportionation, their actual tendency to do so varies markedly from system to system. For example, LaNi5 is clearly much more resistant to disproportionation than CaNi5 . For applications requiring excursions to high temperatures (see the compressor described later) , the material must be chosen with consideration of the chemical

stability. Thermal conductivity and specific heat are properties that should be considered in heat exchanger design of hydride beds.



304


Figure 6. M et alio graphic cross section of activated FeTi particle: magnification, 200X.

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Figure 7. Schematic reaction curves showing loss of kinetics and capacity as a result of cycling with impure H2.





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Specific heats of metals and hydrides are easily determined and typically fall in the range of 0.1-0.2 cal/g°C. Thermal conductivity is a little more difficult to determine. The conductivity of the metal or hydride phase is not sufficient; the effective conductivity of the bed must be determined. This depends on alloy, particle size, packing, void space, etc. Relatively little data of an engineering nature is now available and must be generated for container optimization. Techniques to improve thermal conductivity of hydride beds are needed. As pointed out earlier, good heat exchange is the most important factor in rapid cycling. The inherent safety of hydrides (13,14) gives them an advantage over compressed gas and liquid hydrogen storage and handling. The small void space and low pressures involved mean there is little gaseous H2 immediately available for catastrophic release in a tank rupture situation. The endothermic, self limiting, nature of the desorption reaction also tends to limit the rate of accidental discharge after rupture. However, some unique care must be taken with hydride storage. Active hydride powders can be mildly pyrophoric on sudden exposure to air. For example, the AB5 compounds will begin to glow like coal a few minutes after being suddenly exposed to air in the activated condition. As discussed earlier, it is extremely important to avoid the expansion problem. Finally, because of the nature of the Van't Hoff plot, very high pressures can be generated if a charged hydride is accidentally heated too much. All hydride containers should have adequate pressure relief devices for fire and other potential accident situations.

Hydriding alloys represent a new class of engineering materials requiring the learning of new metallurgical production factors. Historically, we have been especially active in this

area(^-8_) .

We feel the state of production quality control is

now in a reasonably good condition. As will be discussed later, we now produce and market a wide variety of hydriding alloys. As these materials find more and more widespread application, we must concentrate our efforts even more intensely on potential cost problems and the long term availability of raw materials. Families of Rechargeable Metal Hydrides A brief review of the main classes of available rechargeable metal hydrides will be given in this section. Most of the practical (near room temperature) hydride formers are intermetallic compounds consisting of at least one element A that has a very high affinity for hydrogen and at least one element B that has a relatively low affinity for hydrogen. Commonly they fall into three classes of intermetallic compounds: AB, AB5 and A2B. Van't Hoff plots of representative examples are shown in Figure 4.


306

HYDROGEN: PRODUCTION AND MARKETING AB Compounds.

The most well known of the AB compounds is

FeTi , developed about 1969 at Brookhaven National Lab ( 15_) .

A

hysteresis loop for FeTi is shown in Figure 8. Note there are effectively two plateaus representing approximately

FeTi + 1/2 H2 Z FeTiH

(Eq. 5)

FeTiH + 1/2 H2 t FeTiH2

(Eq. 6)


and

FeTi is the lowest cost room temperature hydride presently available, its principle advantage. Relative to other hydrides, it has a few disadvantages: (a) high hysteresis (see Figure 8) , (b) low "poisoning" resistance, at least to 02 (16) and, (c) a heating requirement for activation. In addition, it is sensitive to a

number of production variables (4_, 5^, £,7_,8_) .

However, carefully

handled, it is an effective H-storage medium and has been used in a number of prototype storage tanks. Like many of the hydride systems, various partial ternary substitutions can be made in FeTi, providing changes in plateau

pressure and greatly increasing its versatility (5_,l/7_) .

Examples

of elements that can be partially substituted into FeTi are Mn, Cr, Co, Ni and V. Ni is very effective at lowering the plateau pressure. Mn is an especially attractive substitution because (Fe,Mn)Ti can be activated at room temperature (16) , eliminating the need for heating the container. AB 5 Compounds.

The classic AB5 hydrogen storage compound is

LaNi5, developed at the Philips Laboratories around 1969 (jLO) .

A

25°C hysteresis loop for LaNi5 is shown in Figure 9. It shows classic absorption/desorption behavior and has very attractive hydrogen storage properties: convenient plateau pressures, low hysteresis, excellent kinetics, easy activation, and relatively good resistance to poisoning. The primary disadvantage of LaNi5 is its high cost. We have had a major program aimed at lowering the cost of the AB5 compounds ( 2) . The philosophy of this program is shown in Figure 10 . By substituting the low cost rare-earth mixture mischmetal (M) for expensive La, a much lower cost hydrogen storage compound results. However, MNi5 has impractically high plateau pressures and hysteresis so that further substitutions of Ca, Cu, Mn, Fe or Al must be made. Al is especially potent in lowering the plateau pressure. As shown in Figure 11, plateau pressure can be varied over a wide range to suit the requirements of the intended application. Some loss of capacity may result, but on the basis of raw materials cost per unit of hydrogen storage capacity, we have developed a number of AB5 ternary compounds that are substantially cheaper than LaNi5 (Table III) .

CaNi5 (3_) is also a useful storage compound where the avail-

able hydrogen pressure is only 1 atmosphere at room temperature (Figure 4) .

CaNi5 and MNi5 form a continuous solid solution so




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Rechargeable Metal Hydrides: A New Concept in Hydrogen Storage

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