Light weight energy storage systems for future space missions are investigated. One of the systems being studied is the nickel hydrogen battery. This battery is designed to achieve longer life, improve performance, and higher energy densities for space applications. The nickel hydrogen component development is discussed. Test data from polarization measurements of the hydrogen electrode component is presented

Charleston, J. A.

English

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NICKEL-HYDROGEN COMPONENT DEVELOPMENT
Jo Ann Charleston
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
SUMMARY
The NASA Lewis Research Center is currently involved in the research and
development of light weight energy storage systems for future space missions.
One of the systems now being studied is the nickel-hydrogen battery. This
battery is designed to achieve longer life, improve performance, and higher
co
energy densities for space applications. This paper will discuss the recent
4J
	nickel-hydrogen component development. Test data from polarization measure-
ments of the hydrogen electrode compont^R_ will be presented.
INTRODUCTION
Future NASA space missions will require long life, high power and light
weight energy storage systems which are °--fond the capability of existing sys-
tems. Therefore, the NASA Lewis Research Center (Lewis) is currently involved
in research in the area of electrochemical energy conversion and storage for
space applications. The objective of this work is to achieve improved per-
formance, higher energy density, and longer life for electrochemical devices
for space applications. One of the systems being studied is the nickel-
hydrogen battery.
The Air Force has spent considerable time and money developing a workable
nickel-hydrogen cell and attempting to establish a commercial supplier of com-
ponents and light weight cells. In industry, COMSAT has also expended con-
siderable effort on the nickel-hydrogen system for communication satellites.
NASA Lewis is complementing this work by evaluating the cell designs and opti-
mizing cell components to achieve improved performance and life. Advanced
cell component development is necessary, particularly for low-earth orbit
applications which require 5-yr life at deep depths-of-discharge. This means
that most of the battery capacity is discharged and recharged in each low-
earth orbit cycle, which is about 90 min. Advanced technology development
work is being performed on the nickel electrode, the separator, and the hydro-
gen electrode.
Chemistry and Electrochemistry of Nickel Hydrogen Cells
During charge, as shown in table 1, the nickel hydroxide is converted to
the more charged form of nickel hydroxide. At the hydrogen electrode, water
is electrolyzed to form hydrogen during charge. During the discharge cycle,
the evolved hydrogen gas is consumed by oxidation at the hydrogen electrode,
the nickel electrode is discharged and electrical energy is produced.
Cell Components
A typical nickel-hydrogen cell is shown in figure 1. This is the Air
Force design which is cylindrically shaped with hemispherical end caps. The
pressure vessel is made of Inconel 718. The inrer surface is coated with a
thin layer of zirconium oxide to provide a wick to maintain proper distribu-
tion of electrolyte.
Figure 2 illustrates the individual stack assembly which consists of
nickel and hydrogen electrodes, separators and a gas screen, whi,.h are shaped
in a "pineapple slice" pattern. These components are supported on a central
core and two rigid end plates attached to an Inconel weld ring which is subse-
quently welded at the cylinder dome interface. The nickel electrodes used in
the nickel-hydrogen cells are made with a nickel screen substrate and a sin-
tered carbonyl nickel powder plaque, which is electrochemically impregnated
with nickel-hydroxide active material.
Acceptance of the nickel-hydrogen cell as a viable energy storage system
for space application depends on its cycle life. An important factor limiting
the cycle life of the nickel-hydrogen cell is degradation of the nickel elec-
trode. Some of the modes of degradation are as follows: (1) change in pore
size distribution during cycling; (2) extraction of electrolyte from the sepa-
rator by the increasing number of small pores in the nickel electrod(; (3)
density changes of the active material during charge and discharge; .nd (4)
active material flaking or being extruded from the electrode. Through con-
tractual efforts with Hughes Aircraft Company, these problems are being
addressed. Two types of separator material are used as state-of-the-art sepa-
rators in nickel-hydrogen cells; they are asbestos and zircar (zirconium oxide
felt). Separator selection is known to affect the voltage characteristics of
the cell and affects the oxygen recombination rate. The rapid oxygen recombi-
nation on hydrogen electrodes is referred to as "popping" and is believed to
be caused by channeling of evolved oxygen through paths available in the sepa-
rator. Asbestos separators do not allow popping, but life tests have shown
that cells with asbestos separators give a slightly lower end-of-discharge
voltage. Zircar separators exhibit some popping effects, but give a better
cell performance during long term testing. We are presently working on devel-
oping separator materials that are superior to either asbestos or zircar.
The gas screen provides stiffness and adequate gas flow. It is made of a
monofilament polypropylene screen.
The hydrogen electrode used in nickel-hydrogen cells consists of a Teflon-
platinum catalyst mixture bonded to a pure nickel photochemically etched foil
substrate or a screen substrate. The electrode is backed with a porous Teflon
(Goretex) hydrophobic membrane which is gas permeable. The nickel foil or
screen substrate has good mechanical stability and acts as the current collec-
tor. This electrode has demonstrated a remarkable degree of stability with
respect to electro-oxidation; huwever, some problems have been encountered
with these electrodes. One problem is flooding, which blocks the pores of the
electrode preventing the free flow of gases within the cell. Another problem
is the high cost of the platinum electrocatalyst. The high cost of the plati-
num electrocatalyst can be decreased by reducing the catalyst loading. Hydro-
gen electrodes have been fabricated with various catalyst loadings. These
electrodes are evaluated using polarization test data.
In this paper, experimental results of the polarization measurements and
future electrode development work for the hydrogen electrodes will be presen-
ted and discussed.
Experimental
The polarization test described in this paper is designed to provide an
indication that the hydrogen electrodes were properly made and handled before
cycle testing. The polarization, or overvoltaac (i.e., the change in voltage
from the equilibrium voltage when current fluws through an electrode) at cur-
rents below the mass transfer limiting current (point where the rate of access
of hydrogen to the catalyst becomes the limiting factor), is a measure of how
effective the electrode catalyst structure is in promoting the reaction:
charge
	
H 2O + e	 OA + 112 H2.
discharge
The hydrogen electrodes are affected by electrode flooding (a gradual or
sudden increase in the volume of electrolyte in an electrode) which reduces
the necessary hydrogen transfer to and from the electrodes. F;^oded electrodes
show higher polarizations than those that have not been flooded. Polarization
of the hydrogen electrodes is measured before and after cycling.
Sample Preparation
All of the hydrogen electrodes were prewashed with 50-percent ethanol and
50-percent trichloroethylene before testing.
A rectangular sample measuring 1.0 x 1.2 cm was cut from the electrode,
leaving the white Goretex membrane on the sample. A line was lightly cut into
the backing so that a 1.0 x 1.0 cm 2
 area was outlined. The 0.2 x 1.0 cm
strip of membrane was carefully peeled off from the sample to expose the plat-
inum black and the screen. A 6-in. length of nickel wire was spot welded to
the exposed screen of the sample. The wire was bent 90' where it joined the
sample, so that the plane of the sample is perpendicular to the nickel wire
with 'he membrane side up. The test apparatus is shown in figure 3.
Test Procedure (refs. 1 and 2)
1. The working and the reference electrodes are floated on the surface
of the KOH solution such that rising oxygen bubbles from the nickel counter
electrode make minimal or no contact with them. Before taking any measure-
ments, the test cell is purged with nitrogen for at least 5 min, then with
hydrogen at a rate of 10 bubbles per second until the voltage reading is near
zero.
2. Before applying current to the cell, the hydrogen rate is reduced to
2 to 4 bubbles per second.
Lf
3
3. Current is passed between the working and counter electrode, and the
voltage of the working electrode is compared to the voltage of the reference
electrode, which has no current passing through it. Polarizations are meas-
ured at 5 to 50 mA for oxidation and reduction.
4. When the polarization measurements are completed, the apparatus is
flushed with nitrogen before removing the cap of the test cell.
RESULTS
The hydrogen electrodes which were tested were manufactured by Hughes
Aircraft Company and Life Systems, Inc. (LSI). LSI hydrogen electrodes were
made according to Hughes' specifications. The polarization test results are
shown in Table 2. The standard loading for the hydrogen elFtrode used in the
Air Force nickel-hydrogen cell design is from 8 to 10 mg/cm . Table 2 com-
pares the olarization characteristics of an electrode withh a catalyst loading
of 7 mg/cm- with an electrode having a loading of 10 mQ cm2 . (For cost-
effectiveness the desired p;atinum loading is 0.6 mg/cm .) Under contrac,
LSI has fabricated hydrogen electrodes at loadings from 10.0 to 0.6 mg/cm .
The polarization test r sults are shown in Table 3. The electrode which was
fabricated at 0.6 mg/crrK showed much hi gher polarization than the electrode
fabricated at 10.0 mg/cm2.
CONCLUSION
In order to fabricate a hydrogen electrode that is similar to the present
state-of-the-art hydrogen electrode with a reduction in catalyst loading, it
can be concluded that the structure of the hydrogen electrode has to be opti-
mized regarding the characteristics of the catalyst and the Teflon content
(ref. (3). Too low a Teflon content may cause "flooding" of the active sites
and an increase in liquid phase diffusion losses. Too high a Teflon content
can cause permanent dry regions of the electrocatalyst which will then not be
utilized. It is important to have a proper distribution of catalyst and Teflon
so as to establish a three dimensional network with microporous regions of
hydrophobic teflon phase for efficient gas diffusion and a continuous phase of
wettable electrocatalyst with just a thin electrolyte film which will maintain
the electrochemical reaction for the required current densities and voltages.
Future research will be directed toward further optimization of the hydrogen
electrode.
REFERENCES
1. Giner, J.; and Smith, S., Jan.-Feb. 1967. "A simple method for measuring
the polarization of hydrophobic gas diffusion electrodes," Electrochem.
Technol. 5(1-2):59-61.
2. Giner, J.; Parry, J. M.; Smith, S.; and Turcha% M., Dec. 1969. "Methods
for characterizing the structure and electrochEmical behavior of
Teflon-bonded Pt electrodes," j. Electrochem. Sic.- 116(12): 1692-1696.
4
3.	 Giner, J.; and Hunter, C., Aug. 1969, "The mechanism of operation of the
Teflon-bonded gas diffusion electrode: a mathematical model," J.
Electrochem. Soc. 116(8): 1124-1130.
5
Table 1. - CHEMISTRY AND ELECTROCHEMISTRY OF Ni-H2 Cells
Charge - Discharge:
Ni(OH) 2 + OH	 Ni00H2+ H 2O + e -	 Nickel electrode
H2O + e	 112 H2 + OH -	 Hydrogen electrode
Ni(OH) 2 	--^	 Ni00H + 112 H2 -	 Overall cell reaction
Overcharge:
20H —►
 112 02 + H2O + 2e -	 Nickel electrode
2H20 + 2e --► H2 + 20H -	 Hydrogen electrode
-	
Overall net reactionH 2O —► H2 + 112 02
H2 + 112 02 —o H2O -	 Resultant chemical recombination
TABLE 2. - POLARIZATION OF HUGHES
AND LSI ELECTRUUESa
Electrode Current
densi^y,
Polarization
RED, OXY,
mA/cm mV mV
Hughes 0 -0.0 +0.7
7 mg/cm2 5 -4.2 +3.1
(25 percent 10 -7.8 +6.7
teflon) 25 -17.1 +17.0
35 -24.6 +94.2
50 -35.0 +35.0
LSI 0 -0.0 +0.1
10 mg/cm2 5 -7.2 +7.5
(25 percent 10 -14.6 +16.3
teflon) 25 -35.6 +43.9
35 -50.4 +63.8
50 -71.1 +94.3
aAll electrodes are presoaked in 31
percent KOH for 24 hours.
a
7
E_
TABLE 3. - POLARIZATION OF LSI ELECTRODES
AT DIFFERENT LOADINGSa
Platinum loading,
mg/cm2
Current
density,
Polarization
RED, OXY,
mA/cm mV mV
10 mg/cm2 5 -7.2 +1.7
(25 percent 10 -14.7 +16.5
teflon) 25 -36.0 +42.8
35 -50.7 +62.6
50 -74.0 +93.0
0.6 mg1cm2 5 -31.4 +59.7
(25 percent 10 -46.8 +159.0
teflon) 25 -85.9 +169.8
35 -115.9 +176.4
50 --
1	
--
aAll electrodes are presoaked in 31 percent
KOH for 24 hours.
ORIGINIAL P"!E 19
OF POOR QUAL14Y
8
i^
ORIGINAL PAGE 1;j
OF POOR QUALITY
r 
ELECTkODE
POSITIVE LEADS
	
WELD RING ASSEMBLY / i STACK	 WASHER
p	 ASSEMBLY	 i
i
\	 j	 t SEAL r BELLE VILLE
POSITIVE
	
i WASHERS t4
\ ThER MINAL	 ^1	 ---	 I
	^ 	 NUT
_- ----- -------- i
L NEGATIVE
	
---------------------------- 	
i	 i	 i	 ^^	 TERMINALi
`WASHER
INSULATING
	
i	 WASHER
DOME	 CYLINDER	 ^,'
(CERAMIC)
1 - NUT	 PROTECTIVE
—FII L TUBF
	
L CORE
	 END PLATE
	 L NEGATIVE WASHER
LEADS
FigU re 1.	 5OA-hr nickel hydrogen cell.
Figure 2. - Stack assembly.
ORIGINAL PAGE IS
OF POOP QUALITY
TEST ELECTRODE
(PHOTOCHENIICALLY ETCHED
Ni SCREEN/Pt CATALYST)—'
0	 ^I 
II	
II `REFERENCE ELECTRODE
ELECTRODE
DE ^
	
IPt/Au SCREEN)
(Ni WIRE ) --/
NITROCEN
SOURCE
	 :,	 , :1
HYDROGEN
SOURCE
	
L 80 ml. 31% KOH
WATER BUBBLER
(a) Polarization test cell.
Figure 3.
(b) Polarization test schematic diagram.
Figure 3. - Concluded.
1


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