A new cell structure has been introduced into the SPE Water Electrolyzer which has improved overall characteristics significantly. Weight, reliability, and efficiency are the characteristics that are improved the most, with volume having a second order improvement. This paper discusses the capabilities of the new cell structure and the impact it would have in various space applications

Mcelroy, James F.

English

NASA Technical Reports Server

N94-23362
RECENT ADVANCES IN SPE ® WATER ELECTROLYZER
James F. McEIroy
HAMILTON STANDARD DIVISION
UNITED TECHNOLOGIES CORPORATION
WINDSOR LOCKS, CONNECTICUT 06096-1010
ABSTRACT
A new cell structure has been Introduced into the SPE Water Electrolyzer which has
Improved overall characteristics significantly. Weight, reliability, and efficiency are the
characteristics that are Improved the most, with volume having a second order improvement. This
paper discusses the capabilities of the new cell structure andthe impact it would have In various
space applications.
INTRODUCTION
In recent years, high pressure water electrolysis has been considered for several space
applications. These space applications include the generation of high pressure hydrogen and
oxygen for space station reboost propulsion, the generation of high pressure hydrogen and
oxygen for lunar base energy storage, an d the generation of.high pre..ss_eo_g.en fo.r on-orbit
space suit oxygen supply recharge. The technical approach taken wlm me _'_-.waler.|-ie.ctrol .yzer
has been to use the previously developed nuclear subm.adne S.PE ElectroJ_zer oesl.gn wnii.e.
reducing mass wherever possible. Because the basic nuclear suo.manne SPEElect.ron/zer ..celt
dasign is limited to approximately 400 psi differential pressures overooaro ano across me cell, me
stack of cells must be pressure balanced in a nitrogen-filled dome when operating above 400 psi.
This results in significant mass and system complexity.
Within the last two years, a new SPE Water Electrolyzer cell structure has been developed.
This cell structure, which has a lower mass than the 400 psi design, requires no pressure-balanced
dome when operating at elevated pressures (i.e., up to sever.al thousand psi)... This..ce.II structure
is configured to permit the hydrogen and oxygen compartments to operate at oinerendau pressures
equivalent to the maximum system pressure and, finally, this cell structure results in lower cell
voltage (i.e., higher efficiency).
BACKGROUND
SPE Water Electrolysis Technology Overview
The heart of the SPE Water Electrolyzer is the electrolysis cell which consists of an ion
exchange membrane with Teflon e bonded, finely divided metalelectrodes. Figure 1 shows this
arrangement alonQ-with the water electrolysis reactions. Since the fixed acid ion exchange.
membrane has neither a traditional bubble point nor free electrolyte, operating pressures ana
hydrogen-oxygen differentials are limited only by the surrounding structures. This affords a
significant safety factor in maintaining positive separation of the hydrogen and oxygen products.
SPE • is a reg_ Irademark of Hamilton Standard Division of United Technologies Corporation
Te(Ione is a registwed badema_ of E. I. DuPont De Nemours
Pi__ PAGE BLANK NOT FILMED
193
https://ntrs.nasa.gov/search.jsp?R=19940018889 2020-06-16T16:04:16+00:00Z
The introduction of the perfluorocarbon cation exchange membrane in the late 1960s
enabled the development of the SPE Electrolyzer Iii prior years, water electrolyzers made with
existing ion exchange membranes had useful lifetii_e s of only a few hundred hours. With the use
of perlluorocarbon ion exchange membranes, the SPE Water Eiectrolyzer cell life has been
demonstrated to be in excess of 14 years and projected to over 30 years depend,ing on operating
conditions.
In mostpractical appiicatiOns,- a number of cells are Stacked in a filter press arrangement
with as many as 100 cells electrically connected in series while the fluids are passed through the
cells in parallel. Without any free electrolyte, the parallel fluid flows can be co.nducted.wtthout fear
of shunt currents inducing stray water electrolysis and its potentially deleterious result of product
gas mixing. The purity of the product gases from SPE Water Electrolyzers is typically greater than
99.99%.
CATHODE
HYDROGEN
ELECTRODE '_
HYDROGEN
4H ÷ + 4e -1_ 2Hz
ION EXCHANGE
!
4H ÷
(-]_
I '1--
DCSOURCE
ANODE
_E OXYGEN
LECTRODE
(+)
PROCESS WATER
Fig. 1. - SPE Water Electrolyzer Reactions
Nuclear submarine :Oxygen _enerators ...... : ...............
The cell structures, developed during the 1970s for nuclear Submarine o_gen generation,
have a capability of operating continuously at up to 400 psi. Limited by stack compression and
cell frame strength, pressures higher than 400 psi are obtained by placing t_ entlre stack withln
a pressure vessel. Figure 2 shows a palr of I00 cell stacks, for 3000psi Oration, _h their
pressure vessel domes removed. The tl"ick lower end plates form one end ofthe pressure vessels.
194
The nuclear submarine SPE Oxygen Generator cell structure is also limited to about 400
psi differential pressure between oxygen and hydrogen. The significance of this design limit is that
a moderately complex pressure control system is employed at 3000 psi. The pressure control
system maintains oxygen and hydrogen pressures within 400 psi even during a power loss
depressurization.
Fig. 2. - 225 SCFH U.S. Navy SPE Oxygen Generator Modules
Space Station SPE Propellant Generator Demonstrator
_ Under contract to NASAJJSC, a 3000-psi hydrogen-oxygen generator based on the Naval
0.23 ftz SPE Water Electrolyzer cell configuration was designed and delivered. The purpose was
to demonstrate the feasibility of producing 3000 psi hydrogen and oxygen on orbit for periodic
rocket motor firing to maintain Space Station Freedom orbital altitude.
To reduce the mass and decrease the volume, as compared to the U.S. Navy design,
several configuration changes were made to the supporting pressure vessel and fluid manifold.
The resultant SPE propellant generator demonstrator is significantly smaller and lighter. In the SPE
propellant generator demonstrator, the pressure vessel =stwo torisphericai domes opposed on
either side of a central fluid plate. This configuration, with a rating of 4 pounds of water electrolyzed
per hour, is shown in Figure 3. The dimensions of the SPE propellantgenerator demonstrator are
13 inches across the domes and 13 inches in diameter at the dome flanges. The total weight of
the cell stack for space station propulsion is 193 Ibs.
195
 SPEi
Fig. 3. - SPE Propellant Generator, Domes Removed
Although the weight of the pressure vessel for the SPE Propellant Generator is significantly
reduced from the nuclear submarine SPE Oxygen Generator pressure vessel, the system
complexity remains the same. Figure 4 describes the SPE Propellant Generator system fluid
schematic. As shown on Figure 4, the pressure vessel is charged with nitrogen gas. This gas is
increased slowly while the system comes to pressure so as to prohibit any differential preSSure
from exceeding 400 psi. Both the product hydrogen and product oxygen are pressure regulated
using the pressure vessel nitrogen as a reference. An elaborate array of regulators, valves and
relief valves is required in each of the three gas systems to prevent any unsafe condition (i.e., no
two simultaneous and independent failures will produce a safety hazard). Since the cells are only
designed for 400 psi differential pressure, no two failures in the pressure control system can allow
pressure differentials to exceed 400 psi.
THE NEW SPE CELL STRUCTURE
In order to further r_uce Weighf ancl_sim_ the high pressureSPE EFectro[yzer system
pressure controls, a new SPE cell structure has been developed. This new cell structure reqUires
no pressure vessel enclosure in order to operate at elevated pressures. Demonstrated
characteristics of this new cell structure include:
• Leak free at greater than 3000 psi internal pressure
• Leak free at greater than 3000 psi oxygen chamber to hydrogen chamber differential pressure
• Pressure capability demonstrated with advanced thin membranes
• Over 30,000 operating cell hours demonstrated at 2000 psi differential pressure
196
STORAGE
H2
VENT
H2/H20[ p2/H208EP
_P
O2
VENT
SAMPLE
PORT
SUPPLY
VENT
SENSOR
COOLANT COOLANTRETURN
DG1022006ag
Fig. 4. - SPE Propellant Generator System Fluid Schematic
• Over 4000 operating hours on a single unit at 2000 psi differential pressure
• Three cell stack operated at 2000 psi differential pressure.
The basic construction materials for the new SPE cell structure are the same as used in the nuclear
submarine SPE Oxygen Generator system. Therefore, from a materials compatibility viewpoint,
the maturity of over 10 million SPE cell hours in nuclear submarine applications is maintained.
As previously mentioned, the weight of the domed stack in the Space Station SPE
Propellant Generator Demonstrator is 193 Ibs. In using the new SPE cell structure the stack weight
for the same total active cell area would be 30 Ibs. A minor volume improvement is also obtained
by elimination of the pressure vessel and a reduction of the cell spacing from seven cells per inch
in the nuclear submarine design to 10 cells per inch in the new structure.
The new SPE cell structure produces Improved electrolyzer efficiency by significantly
reducing internal component electrical resistance in the non-Omembrane components. Further,
ionic conductivity is improved since the new SPE cell structure is compatible with new high
performance thin membranes whereas the SPE nuclear submarine electrolyzer cell structure is
not. Figure 5 displays the performance improvement obtained with the new SPE cell structure.
197
2.5
I
tu
2.3
_, 1.9
1.7
TYPICAL SPE NUCLEAR SUBMARINE ELECTROLYZER PERFORMANCE
........ NEW' CELL STRUCTURE WITH SPE NUCLEAR SUBMARINE ELECTROLYZER MEMBRANE
•,, -- ,-, NEW CELL STRUCTURE WITH ADVANCED THIN MEMBRANE
120 ° F
150 PSIA
J
J
L_OOOI, eO #Q_°II°
_aw ,r
1.50 200 400 600 800 1000 1200 t400
CURRENT DENSITY -- AMPS/F'I "2 (ASF)
DG10220Olag
Fig. 5. - Single Cell Electrolyzer Performance
SYSTEMS IMPACT FROM THE NEW SPE CELL STRUCTURE
Apart from the aforementioned weight reductions from the new SPE c_structure, the
simplification and reliability improvement in resuitant systems is dramatic. In the case of the Space
Station SPE Propellant Generator, the nitrogen subsystem would be. altogether eliminated as
would the array of regulators, valves and relief valves nntended to maintain differential pressures
below 400 psi. A high pressure SPE Electrolyzer fluid schematic incorporating the characteristics
of the new cell structure is shown in Rgure 6. This figure depicts the r.eqluired SPE Elec{r01yzer
fluid schematic when it is desired to deliver both gaseous products at hagh pressure (i.e., high
pressure hydrogen and oxygen for reboost propulsion or lunar base energy storage). This Rgure
6 fluid schematic can be compared with the previously displayed Figure 4 to obtain a full
appreciation of the components eliminated by virtue of using the new SPE cell structure.
Further simplification and reliability improvements and weight reductions are obtained if
only one of the product gases is desired to be delivered at high pressure, l_n-uciear s;Jbrn_ine
SPE Oxygen Generator cell design both product gases are produced at high pressure when only
the oxygen is required to be at 3000 psi. The hydrogen, hav0ng been produced at about 3000 psi,
is simply regulated down to its desired pressure level. The new SPE cell structure allows each
product gas to be generated at its own desired pressure level.
=
l
198
H20 SUPPLY-=__
H_E_O
CONDUCTIVITY-_F
SENSOR "4--
O2/H20 SEP
COOLANT COOLANTRETURN
_O2
STORAGE
_) _'" O2VENT
OG1022005ag
Fig. 6 - High Pressure SPE Electrolyzer Fluid Schematic with the New Cell Structure
A lunar soil or material processing facility could require high pressure hydrogen from water
electrolysis while the product oxygen was discharged, to amb!ent for Ioc__ metal__olic_purp.os.es.
Such an electrolyzer with the new SPE cell structure is snown on Hgure /. Jne nJrmer
improvements in this configuration include:
• The high pressure water feed pump is eliminated.
• Oxygen pressure regulation components are eliminated.
• Rernaining oxygen system components can be designed for ambient pressure operation at
a significantly reduced weight.
• Additionally, the high pressure hydrogen/ambient pressure oxygen configuration is more effi-
cient due to reduced diffusion and the oxygen pressure Nernst effect.
199
STORAG
H2 VENT
H20 SUPPLY--_-__
Rg. 7.
H2/H20
SEP
CONDUCTIVITY_.l_
SENSOR
W CONT.
J VALVE
POWER ELECSUPPLY CONTROL
COOLANT COOLANTRETURN
DO1022004ag
- High Pressure Hydrogen SPE Electrolyzer Fluid Schematic with the
New Cell Structure
The last possible configuration with the new SPE cell structure is an electrolyzer producing
highpressure oxygen and ambient pressure or low pressure hydrogen. Such a configuration
could be used to recharge spacesuit oxygen supplies while on orbit. Figure u snows the fluid
schematic for high pressure oxygen and low pressure hydrogen. In this arr .angement press
water is introduced into the SPE Electrolyzer through the hydrogen cnamoers ot eacn cell.
Because water must diffuse through the cell membranes to the oxygen anodes before the
molecule is split into oxygen gas, electrons and protons, current densities are limite<l, to a
maximum of 500 amperes per square foot. However, the very major advantage is the elimination
of almost all of the high pressure components.
200
O2STORAGE "_),l::::J, _ i
02 VENT L_
ELECTROLYSIS
MODULE
CONDUCTIVITYSENSOR
SUPPLY
COOLANT
C
H20 SUPPLY _ (_
H2/H20 SEP
L.
_) CW PUMP
TEMP
CONT.
VALVE
ELEC
CONTROL
COOLANTRETURN .
H2 VENT
DG1022003ag
Fig. 8. - High Pressure Oxygen SPE Electrolyzer Fluid Schematic with the New Cell Structure
SUMMARY
The recently developed new SPE cell structure has allowed a much greater range of
capabilities in the SPE Electrolyzer product than with prior designs. Depending on the application,
it is expected that the simpler system will result in reliability improvements by at least a doubling.
of MTBF; system weight reductions up to 50%; an improved effictency or a oouodng oT current
density at a constant efficiency; and last but not least, system costs significantly reduced.
201
] $ 7Z ] {
Z
Section
Three
Advanced Concepts
for Space Power
PRK_G PAGE BLANK NOT FtL_EI_



Recent advances in SPE (tm) water electrolyzer

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