NASA Lewis Research Center is currently involved in advanced cell component development for nickel-hydrogen cells and batteries. Long life, high energy density, improved performance and reliability are required for energy storage systems in future space missions. Commercially available as well as experimental hydrogen electrodes were assessed and copared to the state-of-the-art hydrogen electrode that is currently being used in nickel-hydrogen batteries. These electrodes were evaluated by scanning electron microscopy and standard electrochemical polarization measurements. Production variables such as Teflon content and platinum catalyst loading were considered in order to assess various hydrogen electrods with regard to the different electrode manufacturing processes

Charleston, J. A.

English

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NASA Technical Memorandum 87264
Assessment of Commercially Available and
Experimental Hydrogen Electrodes
HC .s:2/
CSCl
JG6-23J35
Uac.las
Jo Ann Charleston
Levra Research Center
Cleveland, Ohio
Prepared for the
32nd International Power Sources Symposium
sponsored by the Department of the Army
Cherry Hill, New Jersey, June 9-12, 1986
NASA
https://ntrs.nasa.gov/search.jsp?R=19860013564 2020-03-20T15:02:40+00:00Z
ASSESSMENT OF COMMERCIALLY AVAILABLE AND EXPERIMENTAL
HYDROGEN ELECTRODES
Jo Ann Charleston
National Aeronautics and Space Administration
Lewis Research C.enter
Cleveland, Ohio 44135
SUMMARY
NASA Lewis Research Center 1s currently Involved 1n advanced cell compo-
nent development for nickel-hydrogen cells and batteries. Long life, high
energy density, Improved performance and reliability are required for energy
storage systems 1n future space missions. Commercially available as well as
experimental hydrogen electrodes were assessed and compared to the state-of-
the-art hydrogen electrode that 1s currently being used 1n the nickel-hydrogen
batteries. These electrodes were evaluated by scanning electron microscopy
and standard electrochemical polarization measurements. Production variables
such as Teflon content and platinum catalyst loading were considered 1n order
to assess various hydrogen electrodes with regard to the different electrode
manufacturing processes.
INTRODUCTION
Advanced cell component development 1s being performed at NASA Lew,1s to
achieve Improved performance and life for nickel-hydrogen batteries. The
state-of-the-art (SOA) hydrogen electrodes used 1n nickel-hydrogen cells con-
sist of a Teflon-platinum catalyst mixture, bonded to a pure nickel photo-
chemlcally etched screen substrate. The substrate 1s backed with a porous
Teflon (Cortex) membrane which 1s gas permeable (ref. 1). The catalyst loading
1s 7±2 mg/cm2 fuel cell grade platinum black. This electrode 1s manufactured
under contract by Hughes Aircraft Company for the A1r Force. Commercially
available hydrogen electrodes and experimental hydrogen electrodes were evalu-
ated and compared with the SOA hydrogen electrode. The commercially available
hydrogen electrodes selected .for study were made by Energy Research Corporation
(ERC) and Life Systems, Inc. (LSI). The ERC hydrogen electrodes consisted of a
Teflon-platinum catalyst mixture, (2 mg/cm2 fuel cell grade platinum black
catalyst), bonded to a nickel substrate with a porous Teflon backing. The LSI
(fuel cell type) electrodes consisted of a platinum-Teflon mixture deposited
on a porous gold-plated nickel screen. The catalyst loading was greater than
10 mg/cm2. The manufacturing process and electrode properties are proprietary.
Experimental hydrogen electrodes were made for NASA under contract by LSI accord-
Ing to Hughes specifications at various catalyst loadings (0.5 mg/cm2-10 mg/cm2).
CHARACTERIZATION OF HYDROGEN ELECTRODES
Hydrogen electrodes usually consist of three different materials: cata-
lyst, Teflon and fine-meshed metal screen. They are fabricated by applying a
mixture of catalyst and Teflon on a metal screen which serves as a current
collector and as mechanical support for the catalyst mixture. In order to
produce acceptable hydrogen electrodes on a production scale, the following
variables must be defined: Teflon content, platinum-catalyst loading and sin-
tering temperature. Teflon 1s used as a binder to give the electrode a degree
of hydrophobldty, which makes 1t possible for gaseous reactants to diffuse
readily to reaction sites. Platinum black 1s used as the catalyst because of
Its high exchange current density 1n alkaline electrolyte. Since the catalyst
layer of an electrode consists of a mixture of very small hydrophlUc platinum-
black particles and hydrophoblc Teflon particles, their arrangement 1n the
layer will have a great Influence on the wettablHty characteristic of the
electrode. In order for a porous gas diffusion hydrogen electrode to be effec-
tive, there must be an appropriate amount of catalyst which, after being wetted
by electrolyte, provide reaction sites for the heterogeneous electrochemical
reactions to take place. The Teflon-catalyst mixture must allow uniform rapid
recombination of oxygen which 1s generated during overcharge 1n nickel-hydrogen
batteries. Too high a Teflon content will cause the catalyst to form a discon-
tinuous matrix, so that the catalyst particles will exist as Isolated "Islands."
Permanent dry regions of the electrocatalyst would not be utilized because of
Inadequate electrolyte channels (ref. 2). This would cause very limited utili-
zation of the hydrogen electrode which 1n turn would lead to a sharp decrease
1n electrode efficiency. Too low a Teflon content may cause "flooding" of the
electrochemical active sites.
An Improper sintering temperature-time profile or the use of an aged
Teflon suspension 1n the electrode mixture can result 1n hydrogen electrodes
which cannot adequately catalyze the electrochemical oxidation and reduction
reaction.
Hydrogen electrodes are affected by electrode flooding, which reduces the
necessary hydrogen transfer to and from the active sites on the electrode.
Flooded electrodes show a higher polarization than those that are not flooded.
While Investigating the cause of flooding, 1t was found that a substantial
residue of Triton X-100 (wetting agent) was left 1n these electrodes after
sintering.
This paper will discuss the results of an assessment of commercially
available and experimental hydrogen electrodes for nickel-hydrogen batteries.
These electrodes were evaluated by scanning electron microscopy and standard
electrochemical polarization measurements. Production variables such as Teflon
content and platinum catalyst loading were considered 1n order to assess vari-
ous hydrogen electrodes with regard to the different electrode manufacturing
processes. Sintering temperatures were not considered since this Information
was proprietary.
EXPERIMENTAL
Hydrogen electrodes obtained from commercial suppliers along with the SOA
hydrogen electrode were evaluated by scanning electron microscopy and polari-
zation measurements.
A. Scanning Electron Microscopy - The scanning electron microscopy tech-
nique was used to Investigate the detailed structure of Teflon and catalyst
with respect to various electrode manufacturing processes. This technique
provided Information on the size, shape, and mlcrostructure of the catalyst-
Teflon mixture and on the arrangement and distribution of catalyst aggregates.
Photomicrographs were taken with an Amray 1200B Scanning Electron Microscope
(SEM). Samples were coated with a Polaron E5100 "Cool" Sputter Coater. After
the samples were coated they were placed Into the vacuum chamber of the scan-
ning electron microscope. Photomicrographs were taken of the catalyst side
and the Teflon backing of each hydrogen electrode at magnification ranges of
50-1KX.
B. Polarization Measurements - Standard electrochemical measurements were
made using the "floating half cell" method (ref. 3).
SAMPLE PREPARATION
A rectangular sample measuring 1.0 by 1.2 cm was cut from the electrode,
leaving the white Gortex membrane on the sample. A 0.2 by 1.0 cm strip of
membrane was carefully peeled off from the sample to expose the platinum black
and the screen. A nickel wire was spot welded to the exposed screen of the
sample. The wire was bent 90° where 1t joined the sample, so that the plane
of the sample was perpendicular to the nickel wire with the membrane side up.
The test apparatus 1s shown 1n figure 1.
TEST PROCEDURE
1. The working and the reference electrodes were floated on the surface
on the KOH solution such that rising oxygen bubbles from the nickel counter
electrode made minimal or no contact with the electrode surface. Before taking
any measurements, the test cell was purged with nitrogen for at least 5 m1n,
then with hydrogen at a rate of 10 bubbles per second until the voltage reading
neared zero.
2. Before applying current to the cell, the hydrogen rate was reduced to
2 to 4 bubbles per second.
3. Current was passed between the working and counter electrode, and the
voltage of the working electrode was measured with respect to the hydrogen
Pt/Au reference electrode. Polarization measurements were made from 5 to
50 mA for both the anodic and cathodlc reaction.
4. When the polarization measurements were completed, the apparatus was
flushed with nitrogen before removing the cap of the test cell.
RESULTS AND DISCUSSION
Commercially available and experimental hydrogen electrodes were examined
with an Amray 1200B Scanning Electron Microscope (SEM). Figure 2 shows SEM
photomicrographs of three hydrogen electrodes with similar platinum catalyst
loadings. These electrodes were fabricated using different electrode manufac-
turing processes. The Hughes SOA hydrogen electrode showed a relatively uni-
form cluster of catalyst-Teflon mixture, LSI experimental and LSI fuel cell
type electrodes showed a series of void channels between the platinum-Teflon
mixture throughout the surface of the electrode. Figure 3 shows SEM photo-
micrographs of hydrogen electrodes manufactured at reduced catalyst loadings.
The LSI experimental hydrogen electrode with a loading of 0.6 mg/cm^ showed
the platinum-Teflon mixture being randomly distributed with most of the nickel
screen substrate being exposed. The LSI experimental electrode with a catalyst
3
loading of 2 mg/cm2 also Indicated a nonunlform distribution of catalyst mix-
ture but of less Intensity. The ERC commercial hydrogen electrode also had a
catalyst loading of 2 mg/cm2. The Teflon-catalyst mixture of this electrode
consists of a series of long channels. The electrode structure 1s very uniform.
The SEN photomicrographs clearly Indicate that each electrode's morphological
structure differs depending upon the manner 1n which the Teflon-platinum cata-
lyst mixture was applied to the substrate material. The difference may be
caused by variations 1n Teflon content and sintering temperatures.
Standard polarization measurements were made 1n order to determine how
effective the catalyst structure was 1n promoting the required electrochemical
reaction. Figure 4 shows anodic and cathodlc polarization plots of various
hydrogen electrodes. LSI (fuel cell type) electrodes showed lower polarization
than any of the other electrodes tested. Figure 5 shows polarization plots of
electrodes that were fabricated at a reduced catalyst loading. The LSI experi-
mental (0.6 and 2 mg/cm2) electrodes anodic polarization measurement showed
a significant Increase 1n voltage. This Indicated that these electrodes were
not as reversible. The high polarization measurements may be attributed to
the fact that the Teflon-catalyst mixture was not uniformly dispersed as shown
1n the SEM photomicrographs 1n figure 3. The ERC commercial hydrogen elec-
trode, which had a more uniform catalyst distribution, showed a more reversible
anodic and cathodlc polarization measurement.
CONCLUSION
Commercially available and experimental hydrogen electrodes have been
assessed and evaluated. Only electrodes manufactured at catalyst loadings
greater than 10 mg/cm2 gave slightly lower polarization measurements when
compared to the SOA hydrogen electrode. For electrodes manufactured at reduced
catalyst loadings, the morphological structure of the hydrogen electrode must
be optimized to best utilize the characteristics of the catalyst-Teflon mixture
for optimum electrochemical activity. Electrode manufacturing processes must
be better defined to produce hydrogen electrodes that are comparable to the SOA
for Improved performance and reliability.
REFERENCES
1. Charleston, J. A.: Nickel-Hydrogen Component Development. NASA TM-83487,
1983.
2. Lee, M.C.: Transport Processes 1n Teflon-Bonded Fuel Cell Electrodes.
University of Microfilm International, Ann Arbor, Michigan, 1976.
3. G1ner, J.; and Smith, S.: A Simple Method for Measuring the Polarization
of Hydrophoblc Gas Diffusion Electrodes. Electrochem. Technol.,
5(l-2):59-61, Jan.-Feb. 1967.
TEST ELECTRODE
(PHOTOCHEMICALLY ETCHED
Ni SCREEN/Pt CATALYST)
REFERENCE
ELECTRODE
(Pt/Au SCREEN)
RESISTOR u- "BOX e-
TEST
CELL 1
j
A
NITROGEN
SOURC
-HYDROGEN
SOURCE
'
DIGITAL
ELECTROMETEI
POWER
SUPPLY
WATER BUBBLER
Figure 1. - Polarization test cell and polarization test schematic diagram.
ORIGINAL PAGE IS
OF POOR QUALITY
Figure 2. - SEM photomicrographs of commercially available and experimental hydrogen
electrodes.
Figure 3. - SEM photomicrographs of commercially available and experimental hydrogen
electrodes at reduced catalyst loadings.
ANODIC POLARIZATION CATHODIC POLARIZATION
70
60
> 50
z 40
o
§30
s 202 10
A LSKEXP)
O HUGHES (SOA)
0 LSI (FUEL CELL TYPE)
7 mg/cm'
7 mg/cm' 5
>10 mg/cm2 £j
I I I
10 20 30 40 50 60 70
0
10
20
30
40
50
60
70
0
A LSI (EXP)
O HUGHES (SOA)
O LSI (FUEL CELL TYPE)
>10 mg/cm'
7 mg/cm'-
7 mg/cm^
J I L_l
10 20 30 40 50 60 70
CURRENT DENSITY, mA/cm'
Figure 4.- Polarization of commercially available and experimental hydrogen electrodes.
o
70
60
50
40
30
20
10
0
ANODIC POLARIZATION
,_0.6 mg/cm2
CATHODIC POLARIZATION
A LSKEXP.)
O LSKEXP.)
a ERC
0
10
20
30
40
50
60
70
10 20 30 40 50 0
A LSI (EXP.)
O LSKEXP.)
D ERC
,— 2 mg/cm^
2 mg/cm'
10 20 30 40
CURRENT DENSITY, mA/cm''
Figure 5. - Anodic and cathodic polarization as a function of Pt catalyst loading.
50
1. Report No.
NASA TH-87264
2. Government Accession No. 3. Recipient's Catalog No.
4. Title and Subtitle 5. Report Date
Assessment of Commercially Available and
Experimental Hydrogen Electrodes 6. Performing Organization Code
506-41-21
7. Authors)
Jo Ann Charleston
8. Performing Organization Report No.
E-2958
10. Work Unit No.
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
11. Contract or Grant No.
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546
13 Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
Prepared for the 32nd International Power Sources Symposium sponsored by the
Department of the Army, Cherry H111, New Jersey, June 9-12, 1986.
16 Abstract
NASA Lewis Research Center 1s currently Involved 1n advanced cell component
development for nickel-hydrogen cells and batteries. Long life, high energy
density, Improved performance and reliability are required for energy storage
systems 1n future space missions. Commercially available as well as experimental
hydrogen electrodes were assessed and compared to the state-of-the-art hydrogen
electrode that 1s currently being used 1n the nickel-hydrogen batteries. These
electrodes were evaluated by scanning electron microscopy and standard electro-
chemical polarization measurements. Production variables such as Teflon content
and platinum catalyst loading were considered 1n order to assess various hydrogen
electrodes with regard to the different electrode manufacturing processes.
17. Key Words (Suggested by Authors))
Hydrogen electrode; Catalyst;
Polarization; Teflon
19. Security Classif. (of this report)
Unclassified
18. Distribution Statement
Unclassified
STAR Category
20. Security Classif. (of this page)
Unclassified
- unlimited
44
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