Lanthanum nickel hydride-hydrogen/metal oxide cell

A sealed electric fuel cell of the type utilizing a reoxidizable compound as the positive electrode and a hexagonal nickel-rare earth metal hydride as the negative electrode is disclosed. The energy released by the electrochemical oxidation-reduction reaction of the anode appears in the cell as electrical energy.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to rechargeable fuel cells and more particularly to 
an improved cell in which the positive electrode contains the oxidizing 
agent and the negative electrode is formed from a hexagonal intermetallic 
compound of the composition AB.sub.5 where A represents a rare-earth metal 
and B represents nickel or cobalt. 
2. Prior Art 
Considerable interest and attention has been directed to the development of 
fuel cells and much research has centered around problems of safety due, 
primarily to the use of high pressure hydrogen in lightweight pressure 
vessels. Optimizing the cells while maintaining or increasing efficiency 
has been the subject of continuing study with the development of a type of 
cell using a gas as one member of the electrochemical couple and a 
chemically active solid state material as the other member. A survey of 
contemporary research efforts in this area is found in a 1973 publication 
by NASA entitled, "Fuel Cells", SP-5 NASA SP-5115. 
As disclosed in U.S. patent application Ser. No. 259,524, filed on June 5, 
1972 now U.S. Pat. No. 3,867,199 and entitled "Nickel Hydrogen Fuel Cell" 
(and assigned to the assignee of this invention), one type device is a 
fuel cell wherein at the negative electrode is a chemically oxidizable and 
ionizable gas such as hydrogen and at the positive electrode is an 
electrochemically reducible metal compound typified by nickel hydroxide. 
In accordance with the above referenced invention, the positive electrode 
is nickel hydroxide on a conductive support and the negative electrode 
comprises a catalytic layer of platinum or palladium on a conductive 
support. Between these electrodes is a separator wetted with an 
electrolyte such as an aqueous solution of KOH. 
Cells of this type can operate over a wide range of ambient temperatures 
and can be constructed in various configurations with inherent overcharge 
and overdischarge protection. The cell, however, must be hermetically 
sealed after filling with hydrogen and typically operates at pressures 
ranging from 100 to 600 psia at room temperature. Accordingly, special 
design considerations are present in this type of cell to effectuate 
operation in this high pressure realm. Furthermore, since hydrogen is 
stored as a gas, extreme care must be exercised to avoid explosions caused 
by hydrogen leakage. Some of the operating problems and conditions for 
these cells are discussed in Earl and Dunlop, "Chemical Storage of 
Hydrogen in Ni/H.sub.2 Cells", COMSAT TECHNICAL REVIEW, Fall, 1973. 
Accordingly, studies have tended in the direction of attempting to define 
systems which will store hydrogen as a reduced compound rather than as a 
gas at higher pressures. Some hexagonal intermetallic compounds of the 
generalized composition AB.sub.5, where A represents a rare-earth metal 
and B represents nickel or cobalt, are known to easily absorb and desorb 
large quantities of hydrogen gas under relatively small pressures at room 
temperature. The ability of these compounds to absorb hydrogen is 
described in van Vucht et al. "Reversible Room-Temperature Absorption of 
Large Quantities of Hydrogen by Intermetallic Compounds", Philips Research 
Reports, Vol 25, pp. 133-140 (1970). This property of hexagonal 
nickel-rare earth metal compounds was utilized in U.S. patent application 
Ser. No. 506,086 now U.S. Pat. No. 3,959,018, "Low Pressure Nickel 
Hydrogen Cell", (assigned to the same assignee of the present application) 
which describes and claims the use of LaNi.sub.5 for the chemical storage 
of hydrogen in Ni/H.sub.2 cells. As shown in that patent application, the 
hydrogen absorbing compound is stored in a hermetically sealed pressure 
resistant chamber comprising the cell and is separated from the electrode 
stack. Cells constructed in the manner taught by that application operate 
in a pressure range of 15 to 30 psia at room temperature with a maximum 
pressure in the order of 45 psia. It is evident that such reduced 
pressures make the design of the cell itself easier as well as eliminating 
the major safety hazard, that of high pressure gaseous hydrogen since it 
is now stored as a reduced compound instead of as a gas. 
Despite the improvements represented by the above referenced patent 
application, such a cell when chemically storing hydrogen requires an 
intermediate absorption or desorption step prior to discharge or charging. 
In the case of lanthanum nickel hydride, the reactions are represented by 
the equation: 
##STR1## 
The hydrogen gas upon reaching the surfaces of the catalyst of the 
negative cell plates dissociates by the action of the catalyst to 
monatomic form and from this point the reactions shown below are 
conventional to fuel cells. 
##STR2## 
The present invention eliminates this principle disadvantage of the prior 
art by using LaNi.sub.5 as the negative electrode in the fuel cell. The 
prior art, such as represented by two patents to Dilworth, U.S. Pat. Nos. 
3,405,008 and 3,405,009 discloses the use of intermetallic compounds as 
fuel cell electrodes but neither patent teaches the use of an 
intermetallic hydride. The U.S. Pat. No. 3,405,008 discloses the use of a 
generalized compound MNi.sub.5 (wherein M is a rare earth) and the U.S. 
Pat. No. 3,405,009 teaches a compound M'Ni.sub.3 wherein M' is a 
transition metal. Similarly, U.S. Pat. No. 3,669,745 to Beccu discloses an 
accumulator electrode comprising nickel and a mixture of titanium hydride 
or zirconium hydride and the hydrides of the rare earths (Col. 2, lines 
52-56). There is no suggestion in this patent that the electrode comprises 
an intermetallic compound, although in Col. 3, lines 53 et seq. it is 
indicated that there is some alloying between the metal hydride and the 
activating material. 
Accordingly, it is an object of this invention to use an intermetallic 
hydride as an electrode in a fuel cell. 
It is another object of this invention to contain the hydrogen fuel in a 
compact solid hydride form thereby eliminating the need for high pressure 
vessels to contain the fuel cell. 
It is still another object of this invention to replace the platinum 
electrode currently being used for the negative electrode with one of a 
series of intermetallic hydride electrodes. 
A further object of this invention is to eliminate the intermediate step of 
absorption or desorption required where an intermetallic hydride is stored 
separately from the fuel cell electrodes. 
Yet another object of this invention is to reduce the volume of a fuel cell 
as compared with either nickel-hydrogen or nickel-cadmium cells. 
SUMMARY OF THE INVENTION 
These and other objects of this invention are realized in a preferred 
embodiment where a positive electrode such as nickel hydroxide, silver 
oxide or manganese dioxide is utilized. The negative electrode is an 
intermetallic composition such as LaNi.sub.5 in place of the 
conventionally used platinum. This substitution represents a significant 
cost reduction over currently used fuel cells, and this reduction in cost 
is further enhanced because conventional case design and construction 
methods are employed. This is possible as a result of the low pressure 
mode of operation due to the elimination of gaseous hydrogen storage at 
high pressures. 
Rather than utilize a separate storage means for hydrogen, thereby 
dictating an intermediate step of absorption or desorption, the LaNi.sub.5 
composition electrode oxidizes the hydrogen stored in it directly on 
discharge and stores hydrogen as a hydride during charging.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, a schematic of the simplified embodiment of the 
invention is shown. The cell 10 is shown wherein the container 12 is 
fabricated from stainless steel by conventional techniques. It is 
important to note initially that the lower operating pressures of a cell 
made in accordance with this invention allows substantial reductions in 
both the size and the strength requirements of the container. High 
pressure cells require cylindrical vessels made of high strength 
materials, i.e., Inconel alloys or beryllium nickel with the attendant 
considerations of cost and size. For cells operating at the lower 
pressures in accordance with this invention, a variety of other materials 
easier to fabricate and lower in cost can be used such as 304L stainless 
steel. Additionally, prismatic design techniques may be employed to take 
into consideration space requirements and unusual mounting locations, such 
as in spacecraft utilization of such cells. It is believed that the size 
of a particular cell can be reduced by a factor of 2.5 over high pressure 
designs for a given desired output. The electrode stack inside the 
container is held in place by an insulated rod 14 which utilizes two 
plastic compression plates 16, 18 that sandwich the electrodes. The plates 
are firmly held in position by nuts 20, 22. Disposed in contact with 
plates 16 and 18 are the LaNi.sub.5 negative electrodes 24, 26. 
Before activation, lanthanum pentanickel commercially obtained is in a 
chunky granular form. Activation is by high pressure absorption of 
hydrogen with vacuum desorption cycles at room temperature. The technique 
is described by Reilly and Wiswall. "The Reaction of Hydrogen with Alloys 
of Magnesium and Copper", Inorganic Chemistry, Vol. 6, pg. 2220 (1967). 
High pressure adsorption is generally done at a hydrogen pressure of 40 
Atm. for 2-3 hours followed by a vacuum desorption. This cycle is repeated 
several times. Once activated, the LaNi.sub.5 becomes a fine powder 
suitable for formation into an anode or negative electrode. 
Two methods are described for fabricating the LaNi.sub.5 negative 
electrodes. In one method the activated LaNi.sub.5 powder is mixed with a 
binder, such as Teflon 30 and water until it becomes dough-like in 
substance. The resulting mixture, normally about 30% TFE solids by weight 
is rolled over the entire surface of a nickel screen. The electrode is 
dried in a vacuum oven for approximately 5 hours. The dry electrode is 
then sintered in an inert atmosphere at about 275.degree. C for 
approximately 30 minutes. As an alternative, a small amount of platinum 
black (less than 10%) may be added to the dough-like mix prior to 
spreading. The advantage of using platinum is to increase the rate at 
which hydrogen can be formed directly on the electrode. This monatomic 
hydrogen then moves by surface diffusion directly to the LaNi.sub.5. 
The second method involves spraying an aqueous hydride-Teflon dispersion 
onto a current collecting screen. The mix is prepared by weighting the 
hydride sufficiently for desired loading plus as additional amount for 
waste. Water is added at the ratio of approximately 18 ml/3 g. hydride 
and the teflon dispersion mixed in sufficient quantity to yield 30% TFE 
solids by weight. As in the prior example, a small amount of platinum 
black (less than 10%) may be added to the mix. A thin TFE sheet is applied 
as a backing to the screen and the mixture is sprayed in thin layers over 
the entire surface of the screen. When a sufficient thickness is built up, 
the dry electrode is sintered in an inert gas atmosphere oven for about 30 
minutes at a temperature of approximately 275.degree. C. 
The plates which comprise the positive electrode 36 are made by 
manufacturing methods known in the prior art as described in a survey 
publication, "Alkaline Storage Batteries", Copyright 1969 by John Wiley & 
Son, Inc. 
Disposed adjacent to the positive electrodes are separators 32, 34 which 
are fabricated from nylon, potassium titanate or any other insulating 
composition which remains relatively inert in the environment of the cell. 
Centered in the electrode stack is the positive electrode 36 made from any 
conventional oxidizing agent as previously described to form the 
electrochemical couple. Typical examples are nickel hydroxide, silver 
oxide, manganese dioxide and mercuric oxide. The electrode 36 is 
electrically coupled to positive terminal 38b. 
Negative terminal 28 is electrically connected to the negative electrodes 
24, 26 by the tab 30 and similarly positive terminal 38 is electrically 
connected to the positive electrode 36 by tab 40. 
In assembly of the cell, a quantity of electrolyte, typically about 30% by 
weight solution of potassium hydroxide is placed in the casing 12 via fill 
tube 42 after the cell stack is in place. The quantity of electrolyte 
placed in the cell is limited to the amount needed to completely wet the 
electrode stack while at the same time allowing for adequate oxygen 
recombination on overcharge. 
Although aqueous KOH is the preferred electrolyte, KOH may be replaced by 
or mixed with other alkaline salts, e.g., sodium or lithium hydroxides or 
mixtures thereof. Once the electrolyte has been added, hydrogen may be 
introduced into the chamber to charge it, generally about 1 atmosphere. 
The fill tube 42 may then be pinched to seal the cell. 
In operation, it may be assumed that the cell has been fully charged and is 
connected for use through an exterior circuit. FIG. 2 shows the 
performance data for the test cell, where cell pressure and voltage are 
plotted as functions of time for a complete discharge. During discharge, 
the pressure within the cell tends to remain constant, as shown in FIG. 2 
and the reaction taking place is represented as: 
at the anode: 
EQU 6.7e.sup.- + 6.7 NiO(OH) + 6.7 H.sub.2 O .fwdarw. 6.7 Ni(OH).sub.2 + 
6.7(OH).sup.- 
at the cathode: 
EQU 6.7 (OH).sup.- + LaNi.sub.5 H.sub.6.7 .fwdarw. 6.7 H.sub.2 O + LaNi.sub.5 
+ 6.7e.sup.- 
the net reaction is: 
EQU 6.7 NiO(OH) + LaNi.sub.5 H.sub.6.7 .fwdarw. 6.7 Ni(OH).sub.2 + LaNi.sub.5 
On discharge the average voltage is about 1.2 volts as seen in FIG. 2. On 
charging, the net reaction is reversed and the action at the cathode is 
characterized by the reformation of the hexagonal intermetallic hydride. A 
major advantage as taught by this invention is the substantial reduction 
of the heat dissipated during cell discharge. The conventional Ni--H.sub.2 
cell generates about 9 Kcal of heat per mole of hydrogen gas consumed on 
discharge at a slow quasi-reversible rate. This heat generation is due to 
the irreversibility of the cell. During a normal discharge, additional 
heat would be generated due to cell polarization. 
The thermal advantage of using the lanthanum nickel hydride negative 
electrode is that during cell discharge, it absorbs heat at the rate of 
7.2 Kcal/mole of hydrogen. Thus the heat generated by the cell on 
discharge is reduced by 7.2 Kcal/mole of hydrogen. 
While LaNi.sub.5 has been used as the active material, it is apparent other 
hexagonal intermetallic compounds of the composition AB.sub.5 where A 
represents a rare-earth metal and B represents nickel or colbalt may be 
used. If other hydrides are used, with a larger heat reaction, the heat 
production during discharge could be reduced nearly to zero. In situations 
where the cycling regime consists of long charge and short discharge 
periods, typified by use in synchronous satellite eclipse operation, the 
heat generation rate during charge would still be small. For applications 
using such cycling regimes. the use of metal-hydride batteries would 
therefore be highly beneficial in terms of reducing thermal constraints on 
the design and location of the battery. 
Several practical embodiments of the test cell in FIG. 1 are shown in FIGS. 
3-7. FIGS. 3 and 4 show the construction of a prismatic cell. In FIG. 3, a 
side view, the container 50 has a top cap 52 welded in place at the point 
of junction with the container. The weld 54 takes place once the cell is 
completely loaded with active elements. Disposed around the inside walls 
of the container is an insulating layer 56 to shield the electrode stack 
from the container material, normally stainless steel. This insulating 
material can be potassium titanate, nylon, asbestos or a variety of other 
well-known insulators. A series of hydride electrodes 58 alternating with 
conventional positive electrodes 60 are disposed in a tightly packed 
arrangement between separators 62 inside the cell. The hydride or negative 
electrode may be built-up on nickel grids 64 in a manner previously 
described. The positive electrode can be any conventional electrode 
containing the oxidizing agent to form the couple, such as nickel 
hydroxide, silver oxide, manganese dioxide and mercuries oxide. The 
separators 62 may be joined to one side of each electrode during 
fabrication thereof, or inserted as the stack is built-up. Each electrode 
has a projection or tab 66 which electrically connects the electrode to 
the respective conductor 68. Although, as shown, each tab 66 is bent to 
overlap and join the conductor 68, it is apparent that a bus bar 
arrangement (not shown) can be used to shorten the lengths of each tab and 
thereby reduce problems of cracking or breaking of these tabs. The 
conductor 68 is embedded in a plastic compression seal 70 having a metal 
outer body 72 that is either welded or brazed to the top cap 52. A fill 
tube, not shown, may be employed for charging the cell with hydrogen or 
filling the casing with electrolyte. 
A second form of the invention, a stacked cylindrical cell, is shown in 
FIGS. 5 and 6. In this embodiment, a cylinder 80 has a cap 82 welded to it 
to form a bottom cap. An insulating liner 84 surrounds the cylinder and 
bottom cap walls. This liner may be a cup insert or directly bonded to the 
walls and is of a material previously described in the FIGS. 2 and 4 
embodiment. An electrode stack comprising hydride electrodes 86, 
separators 88 and positive electrodes 90 is built-up of wafers having a 
shape 92 as shown in FIG. 5. The electrodes are generally circular with 
clipped portions 94 to accommodate electrode tabs. For the positive 
electrodes, the tabs 96 are joined to a bus bar 98 by welding or other 
electrical coupling. The bus bar 98 is then connected to the conductor 
100, the conductor being constructed in a manner similar to the FIG. 4 
embodiment having a plastic compression seal 101. The hydride electrodes 
86 have tabs 102, similar to tabs 96 and are connected to bus bar 104. 
This bus bar is welded at a convenient place to the wall of the cylinder. 
A pair of retaining springs 106, 108 are used to provide a measure of 
compression to the electrode stack and hold it firmly in place inside the 
cell. Retainer spring 106 is placed on end cap 87, projecting upward and 
the compressive forces are generated by the placement of top cap 110 on 
the cell and seam welding it in place as shown at location 112. 
Electrolyte may be added or utilized in a manner described for the FIG. 1 
test cell embodiment and a fill tube (not shown) employed as needed. 
A third embodiment is shown in FIG. 7 in which the electrodes are spiral 
wrapped as helices in a so-called "jelly-roll" structure. In this 
embodiment, shown in a cut-away top view, the cell 120 is cylindrical 
having a liner 122 similar to that shown in FIG. 6. A composite, 
comprising layers of separators 124, 126, a positive electrode 128 and a 
hydride electrode 130 is tightly wound in a spiral fashion to fit inside 
the cell. The construction of the electrodes is accomplished in a manner 
identical to that described herein and they are sized such that the 
tightly wound bundle will fit into the cell. Conductor pick-offs are at 
the ends of the bundle, normally, one conductor at the center of the cell 
and a second along the circumference. 
It is to be understood that the above-identified and described embodiments 
are merely illustrative of numerous and varied other arrangements which 
may form applications of the principles of the invention. Other 
embodiments may readily be devised by those skilled in the art without 
departing from the spirit and scope of the invention.