80 K.H.J. BUSCHOW
absorption and desorption, which reflect the presence of hysteresis in the temperature composition relationship. Suda et al. derived an empirical expression for the thermal conductivity, comprising a contribution proportional to the H/M ratio and pressure- dependent contributions of the type ( l n p ) n, where n ranges from 1 to 3.
The effect of hydrogen absorption on superconductive properties was studied on relatively few compounds. These comprise Th7Fe 3 (Malik et al., 1978a), CeCo2 (Buschow and Sherwood, 1978), several La-Ni compounds (Oesterreicher et al., 1976) and several Th-R alloys (Oesterreicher et al., 1977). In all cases it was found that the absorption of hydrogen gas leads to disappearance or lowering of the superconducting state.
of the powder becomes reduced by brittle fracture due to elastic stresses (see section 3.1). This leads to a considerable increase of the surface area and the corresponding catalytic activity, so that after the attainment of a critical grain diameter chemical reaction kinetics ceases to control the flow rate. This stage is reached after only one or two cycles in LaNi» and TiMnl.5; in TiFe-base intermetallics some more cycles are required. Since the equilibrium plateau pressure of the ternary hydride strongly depends on the temperature, the pressure level desired can be adjusted by heating the ternary hydride via an internal or external heating system attached to the steel container.
As an example fig. 52 shows results of the storage source described by McCue (1980). The device is relatively compact, portable and rechargeable and is based on encapsulated LaNi 5, able to store more than 200 litres of hydrogen. An internal heating system allows the pressure and flow rate to be increased. Refilling can, for instance, take place "overnight" by means of an electrolytic hydrogen generator since low pressures and low flow rates can be applied during recharging. All storage devices based on ternary hydrides have in common that the supply pressure remains virtually constant during almost the total delivery period. Other advantages over conventional pressurized gas vessels are the substantial reduction in volume and some reduction in weight, and the fact that the safety risk is considerably reduced. A sudden rupture of the container vessel will only lead to a rather gradual decharging, the large recoil momentum associated with the rupture of a high pressure vessel being absent. A disadvantage is the deterioration of the ternary hydride power as a result of poisoning (section 3.4) or as a result of phase separation (section 4.4). In most cases, however, restoration of the hydrogen sorption properties can be achieved by gently
2/.0 I 200 d~
--~ 160 120
£1.
c -
80
o
( m 3 ) .
1 2 3
... °~,?,~ i
La Ni s H x . 100 C
[
I I I i~
O0 2 4 6 8 10
volume delivered (fr 3) 15
10
~g
Fig. 52. Hydrogen pressure versus volume delivered (at STP) for a compact portable hydrogen storage device described by McCue (1980). The temperature of the internal heating system equals 100°C; pressure switch control 200 lbf/in2; open circles represent performance with a flow rate of 17 E/h (total delivery period 13.5 h) full circles represent performance with a flow rate of 170 E/h (total delivery period 1.5 h).
82 K.H.J. BUSCHOW
heating in vacuum. Further points to note in the conduction and operation of such storage systems are the large volume expansion of the metallic host upon charging (A V~ V = 0.25 in the case of LaN%) and the exponential pressure increase during accidental heating.
6.2. Hydrogen purißcation and hydrogen getters
Heated membranes of palladium-silver alloys have been commonly used (Fromm and Hörz, 1980) to obtain hydrogen gas of very high purity. Only the hydrogen gas is capable of diffusing through these membranes. The fact thät palladium-silver sheets are less susceptible to embrittlement than most metals reacting with hydrogen makes them suitable for use in membranes. Such diffusion cells are fairly expensive and the process is rather slow. Moreover, constant supervision of the palladium-silver membranes is required to prevent malfunctioning arising from leaks.
As an alternative one may consider hydrogen purification by means of absorption- desorption cycles based on metal hydrides, operating at room temperature. Wenzl and Klatt (1978) showed that by selective absorption of hydrogen in FeTi, hydrogen gas of technical purity (99.9~) can be converted into hydrogen of ultra-high purity (99.9999~). At the same time a hydrogen gas source with a certain storage capacity is created. The pressure of the very pure hydrogen gas can be varied by changing the temperature.
A somewhat different application was reported by Reilly and Wiswall (1972), who noted that RM» compounds are still able to form ternary hydrides when brought into contact with gas mixtures having a composition like that resulting from the steamreforming of hydrocarbons followed by CO shifts and methanation steps. They found that CO» even when present in large concentrations, does not interfere with the H2 uptake. On the other hand, small amounts of CO have an inhibiting effect (see also section 3.4). The inhibition can be reduced somewhat by raising the temperature or substituting Cu partly for Ni in LaN%. It will be clear that these poisoning effects set a limit to large scale applications of this method of hydrogen purification when operations on a continuous reaction basis are required.
Materials that can rapidly bind gases liberated from the interior of an evacuated vessel after it has been sealed oft are called getters. Such materials are widely used in the field of vacuum tube fabrication. Examples are Th-Ce-A1 (van Vucht, 1963) and Zr(V~_xFex)2 (Mendelsohn and Gruen, 1980).
6.3. Heat pumps
The coupling of the absorption-desorption cycles of two different hydrides (I and II) can be used to transfer heat from a low-temperature reservoir (T = Te) to a high temperature reservoir (T = Th). The principle of this heat pump is illustrated by fig.
53. In the first stage (a) the two metal hydrides I and II are in open communication with each other. As can be seen from the top part of fig. 53, hydride II is less stable than the hydride I. It reaches a given equilibrium pressure P2 at a substantially lower
D.
T
®
\ i m \ Im -rmV,
T -1
Hydride ]I l Tl 1
Hydride I 1 Th 1
(G) (b) (c)
Fig. 53. Principle of a heat pump based on two hydrides. Hydride I is more stable than hydride II. The lower part of the figure shows the flow of hydrogen gas (see the arrows inside the closed hydride systems) when hydride I is exposed to a high temperature source (situation b) or when hydride II is brought into contact with a low temperature source (situation c). In both situations the heat of absorption is dissipated to the environment at T = T m (T~ < T m < Th). This can be done by cooling with water or air, indicated by the smaller arrows in the Tm system. A description of these processes in terms of lnp versus I/T
characteristics is schematically given in the upper part of the figure. Open circles represent hydrides in decharged condition, full circles represent hydrides i n recharged condition. The situations in the upper part of the figure correspond to those in the lower part after completion of the hydrogen transfer shown by the arrows within the closed hydride systems in the lower part.
temperature (Tm) than hydride I (Th). When hydride I, being saturated with hydrogen, is heated to Th the hydrogen pressure rises to P2 and the hydrogen desorbs.
The hydrogen is absorbed again by the hydride II, which therefore increases in temperature until it reaches T m corresponding to P2. This situation is represented in fig. 53b. The heat of absorption AHII is dissipated at Tm. When nearly all the hydrogen frorn hydride I has been desorbed, thermal contact between system I and the heat source at Th is broken. The hydride I is brought now into contact with the environment having a temperature Tm. The temperature in system I now decreases and hydrogen is re-absorbed (fig. 53c). The hydrogen pressure drops to Pl and the desorption from system II causes its temperature to drop to Te. During the desorption process system II takes up heat from the reservoir at T« while the heat of reaction A H I corresponding to the absorption by system I is dissipated to the environment at Tm. When all the hydrogen has been re-absorbed by I, system I is coupled again to the heat source at Th and the cycle is repeated.
The total heat dissipated to the environment at T m is equal to A H I + AHII. The heat input at Th is A H I . The efficiency is therefore
= 1 + A H n / A H [ .
It can be shown that this expression can be re-written as q = (1 - Te~Th)(1 - T«/Tm),
84 K.H.J. BUSCHOW
Carnot efficiency being attained when T~ = T2m/T« (van Mal, 1976). Temperature gradients needed for the heat exchange at the various levels were taken to be infinitely small in the above discussion. More sophisticated heat pumps based on more than two different hydrides have been discussed by van Mal (1976) and Buschow and van Mal (1982).
In the above discussion it was tacitly assumed that the heat of reaction AH is temperature independent. If follows from the results described in section 4.1 that this is often not the case. A further complication is the presence of a sorption hysteresis in the isotherms and the absence of really flat portions in the sorption isotherm.
Important factors affecting the efficiency are undoubtedly the thermal conductivity and the heat transfer of the hydride powder particles (Lynch, 1980; Töpler et al.,
1980a,b). Since the thermal conductivity of the hydrid e particles is rather low and therefore the heat transfer is rather limited, large temperature gradients have to be employed. Considerable improvement can be achieved by using hydrides that have been compacted to form porous solids supported by a thin metal matrix. The technical feasibility of this was shown by Ron et al. (1980) on LaNi»-15~oA1 compacts. These authors reported high thermal conductivities and rapid sorption.
kinetics. Further studies in this field were made by Suda et al. (1983a, b).
A chemical heat pump based on two hydrides designed for the storage and recovery of thermal energy for heating, cooling and evergy conversion (HYCSOS) was developed at the Argonne National Laboratory (Sheft et al., 1980; Gruen et al., 1978). The two hydrides used are those of the compounds LaNi» and CaNi» or pseudobinary compounds based on them. The system has been tested for several years and is reported to compare favourably with a unit combining solar cooling with direct solar heating.
6.4. Energy storage
Energy storage systems based on metal hydrides were described by several authors (Wakao et al., 1983; Yonezu et aI., 1983; Kawamura et al., 1983a, b).
An interesting application of metal hydrides as energy storage systems is utility load levelling or peak shaving: electrical energy produced during off-peak hours is stored for use during peak demand hours. An engineering model of such a peak shaving system was described by Reilly (1978b). The hydrogen generated electro- lytically in off-peak hours is slightly compressed and stored in a hydrogen-absorbing intermetallic compound. During peak hours the hydrogen is released and fed into a fuel cell to generate electricity.
Metal hydride systems can also be used as storage media for heat and fuel in automotive applications (Töpler et al., 1980). The use of hydrogen in vehicles with internal combustion engines has the advantages of giving the engine a large thermal efficiency and producing exhaust gases virtually free from pollutants. A disadvantage is the weight penalty of the hydride tank, which is 10-20 times heavier than a filled petrol tank. This still compares favourably, however, with a lead-acid acctlmulator for electrically propelled vehicles. The feasibility of using hydrides in motor vehicles has been studied by Daimler-Benz in Berlin and Stuttgart on hydrogen-driven passenger cars and small buses (Töpler et al., 1980a, b; Buchner, 1978a).
6.5. Electrochemical cells
Ternary hydrides can also be used for the chemical storage of hydrogen (Justi et al., 1970; Gutjahr et al., 1973; Ewe et al., 1973; Earl and Dunlop, 1974; Buchner, 1976; Bronoel et al., 1976; Markin et al., 1978; van Rijswick, 1978; Videm, 1978;
Holleck et al., 1980). A schematic representation of a Ni-H2 cell is given in fig. 54.
Here the ternary hydride is seen to be compeltely isolated from the electrolyte in a separate hydride compartment (E). The access of H2 gas is made possible via holes in the compartment which are covered by a microporous membrane such as teflon.
Oxygen can be prevented from reaching the ternary hydride by passing the H 2 gas over a large catalytic surface (platinum black) to recombine 02 and H2 to H20. The reaction in the electrochemical cell ean be represented by
NiOOH + ½H2~~-Ni(OH)2,
where the upper and lower arrows pertain to decharging and charging, respectively.
During the charging period the hydrogen pressure would normally increase from 3 to 33 atm, but because the hydrogen is stored in a suitable ternary hydride the operating pressure does not exceed a few atmospheres. The hydrogen desorbs from the ternary hydride during electrical discharge and the nickel hydroxide is regen- erated. The electromotive force (emf) between the two electrodes is larger the less firmly the hydrogen is bonded in the ternary hydride. For a hydride having an absorption equilibrium pressure near 1 atm at room temperature the emf with a NJ(OH)2 counter electrode is about 1.35 V. Each factor of 10 in the equilibrium
/ /
Z / / / / ù - /
~ X
,~FA
i / %\ ' / /
\
\ \ , ' / ,
\ " / /
>,'/z
b l z
A B C D E
Fig. 54. Schematic representation o f a N i - H 2 cell. A: Ni oxide electrode, B: separating intermediate layer, C: H 2 electrode, D: gas space, E: ternary hydride (after Holleck et al., 1980).
86 K.H.J. BUSCHOW
pressure of the metal-gas sorption reaction corresponds to a change of 30 mV in the emf of the cell.
Selection criteria for ternary hydrides are: (i). they must be capable of operating over a convenient pressure range (0.5-5 atm), and (ii) they must be sufficiently corrosion-resistant to make the application of the KOH electrolyte possible. In addition the presence of small amounts of oxygen generated if overcharging occurs must not lead to deterioration. The hydride LaNi»Hx seems to meet these require- ments reasonably weil. An advantage of the described cell based on LaNi»Hx is its relatively high electrochemical capacity (0.37 A h/g). Somewhat smaller values are found for the hydrides TiNiHx and Ti2NiHx (0.25 A h/g). A disadvantage of the NJ-H2 cells is the continuous decrease in their storage capacity (Holleck et al., 1980).
6.6. Thermal compressors and heat engines
A number of favourable properties of LaNi»H x, comprising the fast rate of the sorption reactions, the constancy of the Hz pressure during charging and decharging at a given temperature, the large absorption capacity, and the favourable pressure range of LaNi» hydride have been exploited by van Mal (1976) and Nomura (1983) to build a thermal absorption compressor for hydrogen gas.
The working principle of the compressor can be sketched as follows: Hydrogen gas is absorbed at a low temperature level (20°C) where the pressure is relatively low (1.5 atm). The hydride is then heated to 140°C, leading to a pressure of about 50 atm.
After desorption the pressure drops and recharging at the lower temperature is necessary. Despite the dead volume (at least 60~o for a container with LaNi» hydride in powdered form) a relatively high pressure ratio can still be reached owing to the large absorption capacity of LaNi5.
In a prototype three containers were used and mounted in a vacuum enclosure (van Mal, 1976). Each of these containers was equipped with its own electric heating and water cooling. This multiple system, operated with the appropriate differences in phase between the charging and decharging modes, was used to obtain a nearly steady flow of 10 mg/s of hydrogen at 45 atm and 160°C with a heat input of about 1 kW. Since thermal energy is used for the work of compression, the compressor may be driven by waste heat or solar energy. A hydride compressor in conjunction with highly efficient power conserving systems was also suggested by Powel et al. (1975).
Closely related to the working principle of the hydrogen compressor is the working principle of a heat engine based on metal hydrides. A water pump operating as a heat engine was devised by Northrup and Heckes (1980). This device is schematically shown in fig. 55. The system operates between two heat reservoirs R h and Re. A ternary hydride in a storage container C is decharged by the heat supplied by Ph.
The generated hydrogen gas expands a flexible bulk B. Its expansion presses the water out of the container and lifts it to the height desired. This water can be used as a low-temperature heat reservoir to cool the hydride in C. Hydrogen is reabsorbed and the container around B becomes filled with water again, one-way valves V preventing the backflow of water.
Rh
C
hof
cold
V \
RI
Fig. 55. Schematic representation of a hydrogen-actuated water pump, lifting the water from an underground reservoir R« to a higher level. The ternary hydride in C is decharged by the heat provided by the heat source R h and becomes charged again after cooling via the relatively cold water originating from Re. The pumping operation proceeds by means of the H2 gas inside the flexible bulb B and one-way valves (V). This schematic representation is similar to that given by Wenzl (1982).
In practical cases one can use the heat provided by a solar collector (Rh) able to heat the hydride to about 80°C, which is sufficiently above the temperature of underground water (about 20°C) to bring about the large H2 pressure difference needed for the pumping (0.4 to 3 atm when CaNisHx is used). F o r more details the reader is referred to the papers by N o r t h r u p and Heckes (1980) and Wenzl (1982).
6.7. H and D isotope separation
Owing to the large difference in mass between hydrogen and deuterium atoms pronounced isotope effects may occur with hydrogen in metals. The difference in free energy upon absorption o f H2 and D 2 can be regarded as an algebraic sum of a number of contributions pertaining to the differences in enthalpy and entropy of the two gases and those o f the hydride (deuteride) phase. Isotopic separation experiments using ternary hydrides based on Ti and Mg were reported by Wiswall et al. (1977), T a n a k a et al. (1978a, b) and Buchner (1978a, b). In TiNi the deuteride is significantly
88 K.H.J. BUSCHOW
less stable than the hydride, leading to a difference of a factor of 10 in the plateau pressure. This difference can be used to obtain a gas phase enriched in deuterium.
In LaNi» too, the deuteride is less stable than the hydride. Here the plateau pressure difference is comparatively small (van Mal, 1976; Biris et al., 1976).
6.8. Neutron moderators and neutron generators
The extremely large hydrogen density in combination with the high thermal stability of several hydrides makes these materials suitable for applications as moderators in nuclear fission reactors (Keinert, 1971; Mueller et al., 1968). A small-scale application of metal hydrides is found in neutron generators. Here a thin layer of the metal hydride saturated with deuterium or tritium is used as a target for accelerated D and T ions, where the DT reaction leads to a high neutron flux (Reifenschweiler, 1972).