U.S. patent application number 10/360583 was filed with the patent office on 2003-10-23 for high-temperature low-hydration ion exchange membrane electrochemical cell.
Invention is credited to McElroy, James Frederick, Sokoloski, Darren Scott.
Application Number | 20030196893 10/360583 |
Document ID | / |
Family ID | 29218814 |
Filed Date | 2003-10-23 |
United States Patent
Application |
20030196893 |
Kind Code |
A1 |
McElroy, James Frederick ;
et al. |
October 23, 2003 |
High-temperature low-hydration ion exchange membrane
electrochemical cell
Abstract
This invention relates to an electrochemical cell having a
high-temperature low-hydration (HTLH) ion exchange membrane serving
as an electrolyte layer. The membrane may be a non-fluorinated
ionomer membrane, such as an acid-doped polybenzimidazole (PBI)
membrane. The HTLH membrane is sandwiched by an anode having a
hydrogen-carrying fluid feed chamber with an inlet for receiving a
hydrogen-carrying fluid, and a cathode having a hydrogen product
chamber with an outlet for discharging a hydrogen product gas. The
anode and cathode are electrically couplable to an electric current
source for powering the electrochemical cell to produce hydrogen
gas in a reduction reaction at the cathode. The hydrogen-carrying
fluid may be water, in which case the electrochemical cell serves
as an electrolyzer; or, the hydrogen carrying fluid may be a
hydrogen gas, in which case the cell serves as a hydrogen pump.
Inventors: |
McElroy, James Frederick;
(Suffield, CT) ; Sokoloski, Darren Scott;
(Burnaby, CA) |
Correspondence
Address: |
Paul F. Rusyn, Esq.
DORSEY & WHITNEY LLP
Suite 3400
1420 Fifth Avenue
Seattle
WA
98101
US
|
Family ID: |
29218814 |
Appl. No.: |
10/360583 |
Filed: |
February 6, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60375200 |
Apr 23, 2002 |
|
|
|
Current U.S.
Class: |
204/266 ;
204/296 |
Current CPC
Class: |
C25B 9/73 20210101; C25B
1/04 20130101; Y02E 60/36 20130101; C25B 1/02 20130101 |
Class at
Publication: |
204/266 ;
204/296 |
International
Class: |
C25B 009/00; C25B
013/08 |
Claims
1. An electrochemical cell comprising (a) an anode comprising a
hydrogen-carrying fluid feed chamber with an inlet for receiving a
hydrogen-carrying fluid; (b) a cathode comprising a hydrogen
product chamber with an outlet for discharging a hydrogen product
gas; and (c) a high-temperature low-hydration membrane sandwiched
between the anode and the cathode; the anode and the cathode being
electrically couplable to an electric current source for powering
the electrochemical cell to produce hydrogen gas in a reduction
reaction at the cathode.
2. The electrochemical cell of claim 1 wherein the high-temperature
low-hydration membrane is a non-fluorinated ionomer membrane.
3. The electrochemical cell as claimed in claim 2 wherein the
membrane is an acid-doped polybenzimidazole membrane.
4. The electrochemical cell as claimed in claim 3 wherein the
polybenzimidazole membrane is doped with an acid selected from the
group of H.sub.2SO.sub.4 and H.sub.3PO.sub.4.
5. The electrochemical cell of claim 4 wherein the
hydrogen-carrying fluid feed chamber is a water feed chamber for
receiving feed water and comprises an outlet for discharging
reaction product and unreacted water, and the electrochemical cell
is an electrolyzer that produces hydrogen gas in a reduction
reaction at the cathode, and oxygen gas in an oxidation reaction at
the anode.
6. The electrochemical cell of claim 4 wherein the
hydrogen-carrying fluid feed chamber is a hydrogen feed chamber for
receiving hydrogen-carrying feed gas, and the electrochemical cell
is a pump that produces hydrogen gas in a reduction reaction at the
cathode.
7. An electrochemical electrolyzer comprising (a) an anode
comprising a water feed chamber with an inlet for receiving feed
water and an outlet for discharging unreacted water and product
oxygen gas; (b) a cathode comprising a hydrogen product chamber
with an outlet for discharging product hydrogen gas; and (c) a
high-temperature low-hydration membrane sandwiched between the
anode and the cathode; the anode and the cathode being electrically
couplable to an electric current source for powering the
electrolyzer to produce hydrogen gas in a reduction reaction at the
cathode, and oxygen gas in an oxidation reaction at the anode.
8. The electrolyzer as claimed in claim 7 wherein the membrane is a
non-fluorinated ionomer membrane.
9. The electrolyzer as claimed in claim 8 wherein the membrane is
an acid-doped polybenzimidazole membrane.
10. The electrolyzer as claimed in claim 9 wherein the
polybenzimidazole membrane is doped with an acid selected from the
group of H.sub.2SO.sub.4 and H.sub.3PO.sub.4.
11. An electrolyzer system comprising (a) the electrolyzer of claim
7; (b) a feed water stream in fluid flow communication with the
water feed chamber inlet of the electrolyzer; (c) a hydrogen
product stream in fluid flow communication with the hydrogen
product chamber outlet of the electrolyzer; and (d) a heat
exchanger thermally coupled to the electrolyzer and in fluid flow
communication with the water feed stream upstream of the
electrolyzer, such that heat produced by the electrolyzer is used
to heat the feed water stream.
12. The electrolyzer system of claim 11 further comprising a water
vaporizer in fluid flow communication with the water feed stream
upstream of the electrolyzer and downstream of the heat exchanger,
for vaporizing the feed water.
13. The electrolyzer system of claim 11 wherein the heat exchanger
is a vaporizer and comprises a pair of thermally conductive
separator plates, a water vaporizing channel in between the
separator plates having an inlet for receiving a liquid water feed
stream and an outlet for discharging a water vapor feed stream, the
vaporizer being thermally coupled to the electrolyzer by at least
one of the separator plates being in thermal contact with the
electrolyzer.
14. The electrolyzer system of claim 13 wherein the feed chamber
outlet discharges a water vapor and oxygen gas stream, and the
system further comprises a water recirculation circuit that
comprises (a) a condensing heat exchanger having an inlet in fluid
flow communication with the water vapor and oxygen discharge
stream, and an outlet for discharging a liquid water stream
condensed by the heat exchanger; and (b) a water tank in fluid flow
communication with the liquid water stream discharged from the heat
exchanger outlet, a liquid water make-up stream, and the feed water
stream upstream of the vaporizer, such that liquid water recovered
by the condensing heat exchanger is returned to the water feed
stream.
15. The electrolyzer system of claim 12 wherein the feed chamber
outlet discharges a water vapor and oxygen gas stream and the
system further comprises a water recirculation circuit comprising a
gas/water separator in fluid flow communication with the water
vapor and oxygen gas discharge stream downstream of the heat
exchanger, and having an oxygen gas discharge outlet, and a water
discharge outlet fluidly coupled to the feed water stream upstream
of the heat exchanger.
16. The electrolyzer system of claim 15 further comprising a
hydrogen storage chamber in fluid flow communication with the
hydrogen product stream discharged from the electrolyzer.
17. An electrochemical pump comprising (a) an anode comprising a
hydrogen feed chamber with an inlet for receiving
hydrogen-containing feed gas; (b) a cathode comprising a hydrogen
product chamber with an outlet for discharging product hydrogen
gas; (c) a high-temperature low-hydration membrane sandwiched
between the electrodes; the anode and cathode being electrically
couplable to an electric current source for powering the
electrochemical pump to produce the product hydrogen gas in a
reduction reaction at the cathode.
18. The pump as claimed in claim 17 wherein the membrane is a
non-fluorinated ionomer membrane.
19. The pump as claimed in claim 18 wherein the membrane is an
acid-doped polybenzimidazole membrane.
20. The pump as claimed in claim 19 wherein the polybenzimidazole
membrane is doped with an acid in the group of H.sub.2SO.sub.4 and
H.sub.3PO.sub.4.
21. The pump as claimed in claim 20 wherein the hydrogen feed
chamber further comprises an outlet for discharging unreacted
hydrogen-containing feed gas.
22. An electrochemical pump system comprising (a) the
electrochemical pump of claim 21; (b) a hydrogen feed stream in
fluid flow communication with the hydrogen feed chamber inlet; (c)
a hydrogen discharge stream in fluid flow communication with the
hydrogen feed chamber outlet and the hydrogen and water feed
stream; (d) a heat exchanger in fluid flow communication with the
hydrogen discharge stream downstream of the pump and upstream of
the hydrogen feed stream, and in fluid flow communication with a
coolant stream, such that the discharge stream can be cooled in the
heat exchanger before joining with the feed stream; and, (e) a
recirculation pump in fluid flow communication with the discharge
stream.
23. An electrochemical pump system comprising (a) the
electrochemical pump of claim 21; (b) a hydrogen and water feed
stream in fluid flow communication with the hydrogen feed chamber
inlet; (c) a hydrogen discharge stream in fluid flow communication
with the hydrogen feed chamber outlet; (d) an electrolyzer having a
hydrogen discharge outlet in fluid flow communication with the
hydrogen feed stream upstream of the pump, and an inlet in fluid
flow communication with a water feed stream, for producing hydrogen
from the water feed stream.
24. The pump system of claim 23 wherein the electrolyzer is an
electrochemical electrolyzer comprising an anode, a cathode and a
high-temperature low-hydration membrane sandwiched between the
anode and the cathode.
25. The pump system of claim 24 further comprising a heat
recirculation circuit comprising a (a) a water vaporizer in fluid
flow communication with the water feed stream upstream of the
electrolyzer; and (b) a heat exchanger in fluid flow communication
with the water feed stream upstream of the vaporizer and thermally
coupled to at least one of the electrolyzer and the pump, such that
heat generated by at least one of the electrolyzer and the pump is
transferable to the water feed stream.
26. The pump system of claim 25 further comprising a water
recirculation circuit comprising (a) an oxygen and water discharge
stream in fluid flow communication with the electrolyzer; (b) a
gas/water separator having an inlet in fluid flow communication
with the oxygen and water discharge stream downstream of the
electrolyzer, and a water discharge outlet fluidly coupled to the
feed water stream upstream of the heat exchanger, and an oxygen
vent.
27. An electrochemical pump system comprising: (a) the
electrochemical pump of claim 17; and, (b) a natural gas reformer
comprising a hydrogen gas outlet in fluid flow communication with
the hydrogen feed chamber inlet of the pump and a natural gas inlet
in fluid flow communication with a natural gas source.
28. The electrochemical pump system of claim 27 further comprising
a water vaporizer comprising a water feed inlet in fluid flow
communication with a liquid water feed source, and a water
discharge outlet in fluid flow communication with a water discharge
stream that is in turn in fluid flow communication with the
reformer.
29. The electrochemical pump system of claim 28 further comprising
a thermal recirculation circuit comprising a thermal conduction
conduit thermally coupling the vaporizer and at least one of the
pump and reformer, such that heat generated by the reformer or the
pump is transferable to the vaporizer.
30. An electrochemical pump system comprising (a) the
electrochemical pump of claim 17; and, (b) a vaporizer in fluid
flow communication with a water feed stream, and being thermally
coupled to the pump such that heat generated by the pump is
transferable to the water feed stream in the vaporizer.
31. The pump system of claim 30 wherein the vaporizer comprises a
pair of thermally conductive separator plates, a water vaporizing
channel in between the separator plates, a water feed inlet and
water vapor outlet, the vaporizer being thermally coupled to the
pump by one of the separator plates being in thermal contact with
the pump.
32. The electrochemical pump as claimed in claim 17 wherein the
hydrogen feed chamber further comprises a contaminant discharge
outlet downstream of the inlet, such that the contaminants in the
feed gas are filtered by operation of the electrochemical pump and
discharged via the contaminant discharge outlet.
33. An electrochemical filter comprising (a) an anode comprising a
hydrogen feed chamber with an inlet for receiving an unfiltered
feed gas comprising hydrogen and contaminants, and an outlet
downstream of the inlet and for discharging the contaminants; (b) a
cathode comprising a hydrogen product chamber with an outlet for
discharging a hydrogen product gas; (c) a high-temperature
low-hydration membrane sandwiched between the electrodes; and the
anode and cathode being electrically couplable to an electric
current source for powering the electrochemical filter to produce
the hydrogen product gas in a reduction reaction at the cathode,
thereby separating the contaminants from the hydrogen in the feed
gas.
Description
RELATED APPLICATIONS
[0001] This application incorporates by reference and claims
priority from U.S. provisional application No. 60/375,200 entitled
High Temperature Electrochemical Pump Using Low Hydration Ion
Exchange Membrane filed Apr. 23, 2002.
TECHNICAL FIELD
[0002] The present invention relates to the use of High-Temperature
Low-Hydration (HTLH) ion exchange membranes as an electrolyte in an
electrochemical cell.
BACKGROUND OF THE INVENTION
[0003] An electrochemical cell has multiple uses, including use as
a fuel cell to generate electricity by electrochemically reacting a
hydrogen fuel stream and an oxidant stream, as an electrolyzer to
separate water into its constituent elements, and as a pump to
increase the pressure of a particular gas, e.g. hydrogen. An
electrochemical cell typically comprises an electrolyte layer
sandwiched by an anode layer and a cathode layer. A feed stream
supplies a hydrogen-containing fluid stream to the anode, wherein
it is electrochemically reacted to produce electrons, protons, and
other reaction products. The protons conduct through the
electrolyte, which is proton conductive, but substantially
impermeable to reactant gas. The anode and the cathode are
connected to an electric circuit to create an electric potential
between them and allow electric current to flow from one to the
other. The protons that have conducted through the electrolyte to
the cathode combine with the electrons to produce hydrogen gas,
which can be at a higher pressure than the feed stream. In an
electrolyzer, the feed stream is water (separated at the anode into
oxygen, protons and electrons), and in a pump, the feed stream may
be hydrogen gas or a hydrogen-containing gas.
[0004] Among conventional electrochemical cell electrolyzers and
pumps, many use solid electrolyte technology. In solid electrolyte
electrochemical electrolyzers and pumps, a solid or quasi-solid
material that selectively conducts anions or cations is used as an
electrolyte. Typical electrolyte materials used include fluorinated
polymers, of which the polymer marketed under the trademark
Nafion.RTM. is a good example. This class of material incorporates
acid groups into the polymer chain and requires high levels of
hydration with water in order to conduct protons. It is a
characteristic of these membranes that the mechanism of proton
conduction also generates a tendency for water to migrate from one
side of the membrane to the other. This effect is often referred to
as "electro-osmotic drag" or "protonic pumping". When used in
electrochemical electrolyzers or pumps, these membranes have the
following drawbacks:
[0005] Hydrogen produced at the higher pressure cathode side of the
cell diffuses back through the membrane to the lower pressure anode
side of the cell thereby reducing the pumping efficiency. At the
pressures being considered for vehicle hydrogen fueling systems,
the losses due to diffusion are on the order of 10 to 20%. The
mechanism for this diffusion is generally understood to be the
diffusion of gases through the water contained in the fluorinated
type membrane in its hydrated state.
[0006] Water migrates across the membrane in the direction of
proton flow. The amount of water that migrates to the high pressure
hydrogen side of the cell is significant. Subsequent processing of
the hydrogen is usually required to separate the liquid water and
to dry the hydrogen before it enters a storage device.
[0007] The membrane must be kept hydrated. Typically this is
achieved by introducing water or water vapor in with the hydrogen
feed stream at the anode side of the cell.
[0008] It is therefore desirable to provide an electrochemical cell
that does not suffer from the problems or disadvantages associated
with electrochemical cells having a fluorinated polymer electrolyte
membrane. In particular, it is desirable to provide an
electrochemical pump and electrolyzer that are improvements over
fluorinated polymer electrolyte type electrochemical pumps and
electrolyzers.
SUMMARY
[0009] According to one aspect of the invention, there is provided
an electrochemical cell comprising (a) an anode comprising a
hydrogen-carrying fluid feed chamber with an inlet for receiving a
hydrogen-carrying fluid; (b) a cathode comprising a hydrogen
product chamber with an outlet for discharging a hydrogen product
gas; and (c) a high-temperature low-hydration (HTLH) membrane
sandwiched between the anode and cathode; the anode and cathode
being electrically couplable to an electric current source for
powering the electrochemical cell to produce hydrogen gas in a
reduction reaction at the cathode.
[0010] The fluid feed chamber may also include an outlet for
discharging unreacted hydrogen-carrying fluid and/or reaction
products; the discharge may be controlled by a purge valve at the
outlet. The feed fluid for the fluid feed chamber may be water, and
in such case, the electrochemical cell is an electrolyzer that
produces hydrogen gas in a reduction reaction at the cathode, and
oxygen gas in an oxidation reaction at the anode. The feed water
may be in liquid or vapor form. Alternatively, the feed fluid for
the fluid feed chamber may be hydrogen gas at or near ambient
pressure, and the electrochemical cell in such a case is a pump
that produces hydrogen gas in a reduction reaction at the cathode,
at a pressure that is higher than the hydrogen feed gas.
[0011] In the electrolyzer or in the pump, the HTLH membrane may be
a non-fluorinated ionomer membrane, such as an acid-doped
polybenzimidazole (PBI) membrane. In particular, the PBI membrane
may be doped with an acid in the group of H.sub.2SO.sub.4 and
H.sub.3PO.sub.4.
[0012] According to another aspect of the invention, there is
provided an electrolyzer system comprising the electrolyzer
described above, a feed water stream in fluid flow communication
with the water feed chamber inlet of the electrolyzer, a hydrogen
product stream in fluid flow communication with the hydrogen
product chamber outlet of the electrolyzer; and a heat exchanger
thermally coupled to the electrolyzer and in fluid flow
communication with the water feed stream upstream of the
electrolyzer, such that heat produced by the electrolyzer is used
to heat the feed water stream.
[0013] The electrolyzer system may further comprise a water
vaporizer that is in fluid flow communication with the water feed
stream upstream of the electrolyzer and downstream of the heat
exchanger, such that feed water entering the vaporizer has been
previously heated by the heat exchanger, thereby reducing the
amount of energy needed to vaporize the feed water. Alternatively,
the heat exchanger may be the vaporizer and if so comprises: a pair
of thermally conductive separator plates, a water vaporizing
channel in between the separator plates having an inlet for
receiving a liquid water feed stream and an outlet for discharging
a water vapor feed stream. The vaporizer is thermally coupled to
the electrolyzer by at least one of the separator plates being in
thermal contact with the electrolyzer.
[0014] The fluid feed chamber outlet may discharge a water vapor
and oxygen gas stream, and the system may further comprise a water
recirculation circuit that comprises: a condensing heat exchanger
having an inlet in fluid flow communication with the water vapor
and oxygen discharge stream and an outlet for discharging a liquid
water stream condensed by the heat exchanger; and a water tank in
fluid flow communication with (a) the liquid water stream
discharged from the heat exchanger outlet, (b) a liquid water
make-up stream, and (c) the feed water stream upstream of the
vaporizer, such that liquid water recovered by the condensing heat
exchanger is returned to the water feed stream. Furthermore, the
system may further comprise a water recirculation circuit
comprising a gas/water separator with an inlet in fluid flow
communication with the water vapor and oxygen gas discharge stream
downstream of the heat exchanger. The separator has an oxygen gas
discharge outlet, and a water discharge outlet fluidly coupled to
the feed water stream upstream of the heat exchanger.
[0015] The electrolyzer system may also have a hydrogen storage
chamber in fluid flow communication with the hydrogen product
stream discharged from the electrolyzer.
[0016] According to another aspect of the invention, there is
provided an electrochemical pump system comprising: (a) the
electrochemical pump described above, (b) a hydrogen feed stream in
fluid flow communication with the hydrogen feed chamber fluid
inlet, (c) a hydrogen discharge stream in fluid flow communication
with the hydrogen feed chamber fluid outlet and the hydrogen feed
stream, (d) a heat exchanger in fluid flow communication with the
hydrogen discharge stream downstream of the pump and upstream of
the hydrogen feed stream, and in fluid flow communication with a
coolant stream, such that the discharge stream can be cooled in the
heat exchanger before joining with the feed stream; and (e) a
recirculation pump in fluid flow communication with the discharge
stream.
[0017] According to yet another aspect of the invention, there is
provided an electrochemical pump system comprising: (a) the
electrochemical pump described above, (b) a hydrogen feed stream in
fluid flow communication with the hydrogen feed chamber fluid
inlet, (c) a hydrogen discharge stream in fluid flow communication
with the hydrogen feed chamber fluid outlet, and (d) an
electrolyzer having a hydrogen discharge outlet in fluid flow
communication with the hydrogen feed stream upstream of the pump,
and an inlet in fluid flow communication with a water feed stream,
the electrolyzer being used to produce hydrogen from the water feed
stream. The electrolyzer in this case may be an electrochemical
electrolyzer comprising an HTLH membrane sandwiched between an
anode and a cathode. In this pump system there may also be a heat
recirculation circuit that comprises a water vaporizer in fluid
flow communication with the water feed stream upstream of the
electrolyzer; and a heat exchanger in fluid flow communication with
the water feed stream upstream of the vaporizer and thermally
coupled to at least one of the electrolyzer and the pump, such that
heat generated by at least one of the electrolyzer and the pump is
transferable to the water feed stream. The pump system may also
include a water recirculation circuit that comprises: an oxygen and
water discharge stream in fluid flow communication with the
electrolyzer; and a gas/water separator having an inlet in fluid
flow communication with the oxygen and water discharge stream
downstream of the electrolyzer, and a water discharge outlet
fluidly coupled to the feed water stream upstream of the heat
exchanger, and an oxygen vent.
[0018] According to another aspect of the invention, there is
provided an electrochemical pump system comprising: the
electrochemical pump described above; and, a natural gas reformer
comprising a hydrogen gas outlet in fluid flow communication with
the hydrogen feed chamber fluid inlet of the pump and a natural gas
inlet in fluid flow communication with a natural gas source. The
electrochemical pump may also include a water vaporizer comprising
a water feed inlet in fluid flow communication with a liquid water
feed source, and a water discharge outlet in fluid flow
communication with a water discharge stream that is in turn in
fluid flow communication with the reformer. The electrochemical
pump system may also include a thermal recirculation circuit
comprising a thermal conduction conduit thermally coupling the
vaporizer and at least one of the pump and reformer, such that heat
generated by the reformer or the pump is transferable to the
vaporizer.
[0019] According to another aspect of the invention, there is
provided an electrochemical pump system comprising: the
electrochemical pump described above; and, a vaporizer in fluid
flow communication with a water feed stream, and being thermally
coupled to the pump such that heat generated by the pump is
transferable to the water feed stream in the vaporizer. The
vaporizer of this system may comprise (a) a pair of thermally
conductive separator plates, (b) a water vaporizing channel in
between the separator plates, and (c) a water feed inlet and water
vapor outlet. The vaporizer is thermally coupled to the pump by at
least one of the separator plates being in thermal contact with the
pump.
[0020] According to yet another aspect of the invention, there is
provided an electrochemical filter comprising: (a) an anode
comprising a hydrogen feed chamber with an inlet for receiving an
unfiltered feed gas comprising hydrogen and contaminants, and an
outlet downstream of the inlet and for discharging the
contaminants; (b) a cathode comprising a hydrogen product chamber
with an outlet for discharging a hydrogen product gas; and (c) a
high-temperature low-hydration membrane sandwiched between the
electrodes. The anode and cathode are electrically couplable to an
electric current source for powering the electrochemical filter to
produce the hydrogen product gas in a reduction reaction at the
cathode, thereby separating the contaminants from the hydrogen in
the feed gas.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIGS. 1(a) and (b) are schematic illustrations of a hydrogen
electrochemical pump (1(a)) and electrolyzer (1(b)) each having an
HTLH electrolyte membrane.
[0022] FIG. 2 is a schematic illustration of a system for pumping
hydrogen utilizing an HTLH ion exchange membrane electrochemical
pump.
[0023] FIG. 3 is a schematic illustration of a system for pumping
hydrogen utilizing an HTLH ion exchange membrane electrochemical
pump in which the hydrogen is generated from a high temperature
water electrolyzer.
[0024] FIG. 4 is a schematic illustration of a system for pumping
hydrogen utilizing an HTLH ion exchange membrane electrochemical
pump in which the hydrogen is generated from a natural gas steam
reformer.
[0025] FIG. 5 is a schematic illustration of an integrated
vaporizer cell and HTLH ion exchange membrane electrochemical pump
cell.
[0026] FIG. 6 is a schematic illustration of a system for
generating hydrogen utilizing an HTLH ion exchange membrane
electrolyzer.
[0027] FIG. 7 is a schematic illustration of an integrated
vaporizer cell and HTLH ion exchange membrane electrolyzer
cell.
DETAILED DESCRIPTION
[0028] High-temperature low-hydration (HTLH) membranes are herein
defined as ion exchange membranes that exhibit proton conductivity
and operate at higher temperatures and require lower levels of
hydration than fluorinated polymer type membranes (referred
generally as hydrated membranes). In particular, HTLH membranes are
operable at temperature ranges in the order of 100 to 200.degree.
C. and with a hydration level of less than or equal to 20% by
weight.
[0029] One example of a suitable HTLH membrane is a non-fluorinated
ionomer membrane, such as an acid-doped polybenzimidazole (PBI)
membrane. Other types of proton-conducting HTLH membranes include
those made with sulfonated polyphenylene oxide;
polydimethylphenylene oxide phosphoric acid; polyether ketone;
imidazole doped polyether ketone; a blend of sulfonated
polyphenylene oxide and polyvinylidene fluoride; a blend of
ortho-sulfone-sulfonated polyethersulfone and polybenzimidazole;
and 1-butyl 3-methyl imidazolium trifluoromethane sulfonate doped
Nafion.RTM..
[0030] Some of the characteristics reported for HTLH membranes are:
thermal stability in simulated fuel cell environments up to
temperatures of 200.degree. C., ability to operate at temperatures
up to 200.degree. C. in a fuel cell, good proton conductivity at
low water activities, low crossover of methanol in direct methanol
fuel cell applications, and slow oxygen reduction kinetics when
used in a fuel cell membrane-electrode assembly (MEA).
[0031] An acid-doped PBI membrane was prepared by soaking a thin
film of the PBI polymer in an acid solution. A typical suitable
acid is H.sub.2SO.sub.4 or H.sub.3PO.sub.4. Preparation of
acid-doped PBI membranes is known in the art; for example, the
article by Savadogo et al., entitled "Hydrogen/oxygen polymer
electrolyte membrane fuel cell (PEMFC) based on acid-doped
polybenzimidazole (PBI)" Journal of New Materials Electrochemical
Systems 3, pp. 345-349 (2000) discloses the preparation of
sulphuric-acid-doped PBI and phosphoric-acid-doped PBI for use in
fuel cells. The resulting sulphuric or phosphoric acid-doped PBI
membrane is comprised of acid ions bound ionically to the polymer
backbone. Savadogo et al report that the phosphoric acid-doped PBI
membrane is thermally stable up to the test temperature of
185.degree. C.
[0032] In the Encyclopedia of Polymer Science and Engineering,
2.sup.nd edition, Buckley et al indicates that a phosphoric-acid
doped PBI membrane retains short-term resistance to heat at
temperatures as high as 500.degree. C.
[0033] In the article entitled "Acid-Doped Polybenzimidazoles: A
New Polymer Electrolyte" in the Journal of The Electrochemical
Society, Volume 142, Wainright et al showed that phosphoric-acid
doped PBI has high conductivity (5.times.10.sup.-2 Scm) at the test
temperature of 190.degree. C. They also found that phosphoric-acid
doped PBI has low water content and low methanol crossover.
[0034] In the article entitled "Electro-osmotic Drag Coefficient of
Water and Methanol in Polymer Electrolytes at Elevated
Temperatures" in the Journal of The Electrochemical Society, Volume
143, No. 4, April 1996, Weng et al reported that the water
electro-osmotic drag coefficient of phosphoric-acid doped PBI is
essentially zero at the tested temperatures up to 200.degree.
C.
[0035] The HTLH membrane is incorporated into an electrochemical
cell system that is connected to an electric current source. Upon
the supply of current and a hydrogen-containing gas stream to the
cell, an electrochemical process occurs: the hydrogen-containing
gas is oxidized at the anode to produce protons that conduct
through the membrane, and the conducted protons are reduced at the
cathode to produce hydrogen product gas at higher pressure. Unlike
a fuel cell, the electrochemical reduction occurs substantially
free of oxygen, and therefore, the product fluid produced at the
cathode is hydrogen gas. Such an HTLH electrochemical cell can be
used for several purposes, such as hydrogen pumping, hydrogen
filtering, and electrolysis.
[0036] The HTLH membrane has been found to have high proton
conductivity at low water activity relative to hydrated membranes.
This characteristic is expected to reduce the inefficiencies
associated with the diffusion of gas species across the membrane.
In conventional hydrated-membrane type electrochemical cell systems
generating high-pressure gas on one side of the membrane, these
inefficiencies can be very high. For example, in a typical
electrochemical electrolyzer cell using a 0.01 inch thick
Nafion.RTM. membrane and producing 6000 psig hydrogen and near
ambient pressure oxygen at one (1) amp/in.sup.2, the loss of
current efficiency (diffusional losses) has been found to be in the
order of 10 to 30%. The primary mechanism for diffusion in ion
exchange membranes generally is understood in the art to be related
to the diffusivity of the gas species through the water phase of
the membrane, which is a function of the partial pressure
differential across the membrane. Therefore, in order to maintain a
reasonable current efficiency, electrolyzer cells using hydrated
membranes operate at relatively low pressure differentials. In
contrast, an HTLH ion exchange membrane retains very little water
relative to hydrated membranes. Thus, there is very little
diffusion of gas species across the HTLH membrane via the water
phase of the membrane, making it easier to operate at higher
differential partial pressures, with little loss in current
efficiency associated with gas diffusion. For example, an
electrolyzer cell having a 0.01 inch thick HTLH membrane operating
at 1 amp/in.sup.2 @ 6000 psig hydrogen gas is expected to have an
efficiency of around 99%.
[0037] In contrast to hydrated membranes, HTLH ion exchange
membranes do not require the presence of significant amounts of
water phase in the membrane for proton conduction. This results in
reduced electro-osmotic drag of water across the membrane. Since
the HTLH membrane is operable at lower levels of hydration than
hydrated membranes, the product hydrogen gas is correspondingly
drier. This has an important system benefit in that it reduces the
requirement for dewatering the hydrogen gas produced in
electrochemical cells. In typical electrochemical pump systems or
electrolyzers using hydrated membranes, the dewatering equipment
for drying the hydrogen gas comprises a significant portion of the
system volume and weight. Using HTLH ion exchange membranes
minimizes or eliminates the volume and weight of the dewatering
equipment.
[0038] An electrochemical cell system using HTLH ion exchange
membranes requires minimal or no water vapor feed for adequate
membrane hydration. Furthermore, any water vapor required can be
made using the waste heat of the electrochemical cell. The reduced
hydration requirement has important system benefits in that it
minimizes water handling equipment extraneous to the cell.
[0039] Electrochemical Pump
[0040] Electrochemical pumping is used to filter hydrogen from
carrier gases and for compressing hydrogen to higher pressures.
This process is based upon an electrochemical cell in which an
anode and a cathode are connected to an electric circuit, which
creates an electric potential between them and allows electrons to
flow from anode to cathode. The anode and cathode are also
connected by an electrolyte that allows either anions or cations to
migrate from anode to cathode. An electrochemical pump based on
acid-doped PBI could potentially operate in the temperature range
of 100 to 200.degree. C.
[0041] FIGS. 1 to 5 illustrate an electrochemical pump and systems
that utilize HTLH ion exchange membranes for the purpose of pumping
hydrogen. The characteristics of this class of membranes, as
exemplified by the acid doping of PBI thin film with sulphuric or
phosphoric acid, solve a number of the problems that arise from
using hydrated membranes as exemplified by Nafion.RTM..
[0042] An HTLH electrochemical pump can be integrated with a
hydrocarbon reforming process to filter and purify the hydrogen
rich reformate. Because of the higher operating temperature, the
electrochemical pump is not particularly susceptible to carbon
monoxide (CO) contamination. Carbon monoxide contamination is a
typical problem with lower temperature electrochemical hydrogen
oxidation half cells. For example, it is expected that an HTLH
electrochemical pump operating at 160.degree. C. can use a hydrogen
feed stream having CO up to 10,000 ppm. In contrast, a Nafion-based
pump operating at below 100.degree. C. is limited to a CO content
of about 10 ppm. A high temperature, CO-tolerant electrochemical
pump also reduces or eliminates the need for a selective oxidation
stage in the reformer. This stage is typically used to reconvert CO
to carbon dioxide (CO.sub.2) as the reformate from the reformer
stage is cooled. The water vapor in the reformate is used to
hydrate the HTLH membrane in the electrochemical pump, thereby
eliminating or reducing the need for a separate membrane hydration
system.
[0043] An operating temperature range of 100 to 200.degree. C.
presents opportunities for thermal integration with other systems.
The waste thermal energy from the electrochemical pump process
could be integrated with systems using an endothermic process for
hydrogen storage, such as the process disclosed by Millennium Cell,
which requires heat and hydrogen to reformulate sodium borate to
sodium borohydride.
[0044] The waste thermal energy from the electrochemical pump
process could also be integrated with a vaporization process to
vaporize liquid water for other processes. Examples of other
processes are water vaporization for steam reforming and water
vaporization for high temperature electrolysis.
[0045] The high temperature of an HTLH electrochemical pump allows
for more compact heat rejection systems due to the higher
temperature difference between the process waste thermal energy and
the ambient environment used as a heat sink.
[0046] Referring now to FIG. 1(a) and according to one embodiment
of the invention, an electrochemical cell 10a (with the half
reactions for the oxidation and reduction of hydrogen labeled on
the figure) serves as a hydrogen gas pump and has an HTLH ion
exchange membrane 12 for the membrane electrolyte; a suitable
membrane is PBI thin film with imbibed phosphoric acid. The
electrolyte 12 is sandwiched between a pair of electrodes, namely a
hydrogen feed electrode 14a, and a hydrogen product electrode 16.
The hydrogen feed electrode 14a comprises a separator plate 18, and
a hydrogen feed chamber 23a, which includes a fluid inlet 17a, and
a fluid outlet 21a. The hydrogen product electrode 16 comprises a
separator plate 20, a hydrogen product chamber 25 and a fluid
outlet 19. The electrodes 14a, 16 are electrically coupled by an
electric circuit 22. The circuit 22 is electrically coupled to an
electric current source 24. In operation, low pressure hydrogen
hydrated with at least enough water vapor to hydrate the HTLH
membrane is fed into the fluid inlet 17a of the hydrogen feed
chamber 23a, wherein some of the hydrogen is catalyzed and
disassociated into protons and electrons. The unreacted hydrogen
and water is discharged via the fluid outlet 21a, and may be
recovered in a recirculation loop (not shown). The protons conduct
through the HTLH membrane 12 and combine with electrons at the
hydrogen product electrode 16 to produce hydrogen product gas;
electrons are moved through the circuit 22 by the current source
24. Product gas is discharged through fluid outlet 19 of the
hydrogen product chamber 25; a control valve (not shown) is
installed at the fluid outlet 19 and operated to control the
pressure of the hydrogen product gas. The hydrogen feed electrode
14a may be supplied hydrogen feed gas at near ambient pressure and
the hydrogen product electrode 16 may operate to discharge product
hydrogen gas at elevated pressure. For example, the hydrogen
product electrode 16 may operate at pressures up to in the order of
6000 psig or even higher depending on the mechanical strength of
the cell components and the ability of the current source to supply
adequate electrons.
[0047] While the hydrogen feed chamber 23a is shown to have a fluid
outlet 21a in FIG. 1(a) such that a hydrogen feed stream passes
through the hydrogen feed chamber, the pump may operate
"dead-ended" at the feed chamber side and have a purge valve (not
shown) attached to the feed chamber fluid outlet 21a. In a
dead-ended operation, the hydrogen feed gas is substantially
completely reacted in the hydrogen feed chamber 23a, and the purge
valve remains closed during normal pump operation. The purge valve
may be opened from time to time to discharge any contaminants or
excess water that may have accumulated during operation.
[0048] Referring to FIG. 2, a system 30 for pumping hydrogen using
an HTLH membrane electrochemical cell 10a is shown. A humidifier 32
feeds low pressure hydrogen feed gas and water vapor to the
electrochemical cell 10a via feed line 44. The water vapor is used
to hydrate the membrane and is typically in the order of 20% by
weight or less. In this system 30, some of the low pressure
hydrogen feed and water vapor is recirculated through a heat
exchanger 34 to carry away waste heat from the electrochemical cell
10a. In this connection, the low pressure hydrogen and water vapor
is coupled to the electrochemical cell 10a such that hydrogen and
water vapor is fed into the hydrogen feed chamber 23a via fluid
inlet 17a and unreacted hydrogen and water vapor is discharged from
the fluid outlet 21a into fluid conduit 38. The discharged hydrogen
and water vapor absorbs heat from the electrochemical reaction and
is transmitted from the electrochemical cell 10a to the heat
exchanger 34 by a circulation pump 36 fluidly coupled to the
electrochemical cell 10a via the fluid conduit 38. Heat from the
hydrogen and water vapor stream is transferred in the heat
exchanger 34 to a coolant stream 40 passing through the heat
exchanger 34, and the cooled hydrogen and water vapor stream is
recirculated back into feed line 44 by fluid conduit 42. Hydrogen
is output from the electrochemical cell 10a by way of hydrogen
output line 48.
[0049] Alternatively, a coolant other than hydrogen may be directly
circulated through the electrochemical cell 10a. Possible coolants
are air, water, a water/glycol solution, etc. As a further
alternative, coolant may be transmitted directly through a fluidly
isolated coolant loop (not shown) in the electrochemical cell 10a
as is known in the art.
[0050] Referring to FIG. 3, a system 50 for pumping hydrogen using
the HTLH membrane electrochemical cell 10a (pump) uses hydrogen
generated by an electrolyzer cell 10b. The electrolyzer cell 10b
may use an HTLH ion exchange membrane, which is described in
further detail below. In this system 50, feed water is fed via a
cold water feed conduit 52 into a water tank 54. Feed water is then
supplied from the water tank 54 via another cold water feed conduit
56 to a heat exchanger 58. Thermal transfer conduits are used to
transmit waste heat from the electrolysis and pumping processes to
the heat exchanger 58, which heats up the feed water. The heated
feed water is then sent via heated water feed conduit 68 to a water
vaporizer 70, wherein thermal input means 72 (e.g. electric heater)
may be applied if needed to vaporize the water, and water vapor is
transferred to the electrolyzer cell 10b for hydrogen production
via water vapor feed line 74. If required, water vapor can be used
to heat and/or hydrate the hydrogen produced by electrolyzer cell
10b via bleed line 78 connecting water vapor feed line 74 directly
to a hydrogen inlet line 80 of the electrochemical cell 10a.
Product oxygen gas and excess water vapor is discharged from the
electrolyzer cell 10b via thermal conduction conduit 82 fluidly
connected to the electrolyzer cell 10b and thermally coupled to the
heat exchanger 58, for heat transfer to feed water passing through
the heat exchanger 58. The oxygen gas and excess water is then
discharged from the heat exchanger 58 to a gas/water separator 62
via discharge conduit 60 for separation into an oxygen exhaust
stream 64 and water stream, which is recycled back into water tank
54 via water conduit 66.
[0051] Low pressure hydrogen gas produced by the electrolyzer cell
10b and water vapor is fed into the electrochemical cell 10a via
the inlet line 80. The electrochemical pumping process can be
controlled to produce high pressure hydrogen product gas; the
pumping process generates heat and the heat is collected and
transferred back to the heat exchanger 58 via a thermal conduction
conduit 81, which may be for example a closed water or other
suitable fluid loop thermally coupled to the electrochemical cell
10a and heat exchanger 58. The electrochemical cell 10a discharges
hydrogen product gas through gas line 84 into a hydrogen storage
system 86. The storage system 86 may be heated by a suitable
thermal input means 88, if desired.
[0052] Alternatively, a thermal energy transfer could occur
directly to the water stream being vaporized by passing the water
directly through the electrolyzer cell 10b and or the
electrochemical cell 10a.
[0053] Referring to FIG. 4, a system 90 for pumping hydrogen
includes the HTLH membrane electrochemical cell 10a and uses
hydrogen generated by a natural gas steam reforming process. In
this system 90, feed water is fed via a cold water feed conduit 92
into a water tank 94, and then to a water vaporizer 98 via another
cold water feed conduit 96. The water in the water vaporizer 98 is
heated by a suitable thermal input means 100. Water vapor produced
by the vaporizer 98 is fed into a natural gas reformer 104 via
water vapor feed line 102, and is used in the reforming process to
produce hydrogen reformate from natural gas supplied to the
reformer 104 via natural gas feed line 105. An additional reformer
stage 108, such as a shift stage, may be connected in series by
fluid line 106 to operate to reform natural gas at a lower
temperature stage. Reformate and water vapor then are discharged
from second reformer 108 to the electrochemical cell 10a by way of
fluid line 110. A thermal conduction conduit 114 thermally coupled
to the electrochemical cell 10a, first and second reformers 104,
108 and vaporizer 98 is used to transmit waste heat from the
reforming and pumping processes to the water vaporization process
by a suitable fluid in the conduit 114. Thermal energy means (e.g.
electric heater) may be used by the vaporizer 98 if needed to
vaporizer water. The hydrogen is discharged from the
electrochemical cell 10a as high pressure hydrogen to a hydrogen
storage system 118 via high pressure hydrogen line 116. The storage
system 118 may be heated by a suitable thermal input means 120, if
desired.
[0054] Alternatively a thermal energy transfer could occur directly
to the water stream being vaporized by passing the water directly
through the reformer(s) 104, 108 and/or electrochemical cell
10a.
[0055] Referring now to FIG. 5, a vaporizer cell 130 is combined
with an HTHL membrane electrochemical cell 10a to form an
integrated vaporizer/pump assembly 131; multiple vaporizer/pump
assemblies can be combined to form a vaporizer/pump assembly stack
(as shown by repeating lines in FIG. 5). The electrochemical cell
10a is thermally coupled to the vaporizer cell 130 to allow direct
transfer of thermal energy from the electrochemical pumping process
to the vaporization process. The vaporizer cell 130 comprises a
pair of electrically and thermally conductive separator plates 134,
136 sandwiching a current conducting heat transfer plate 132. The
electrochemical cell 10a comprises a pair of flow distribution and
current conducting feed and product electrode layers 140, 142
sandwiching a HTHL ion exchange membrane electrolyte 143. The
electrodes 140, 142 are connected to a current source via an
external circuit (not shown). Low pressure hydrogen gas is fed
through an inlet 144 of the feed electrode 140, and high pressure
hydrogen gas is discharged out of an outlet 147 in the product
electrode 142 by the electrochemical process previously described.
Heat produced by the pumping process is thermally transferred
through the electrodes 140, 142 to the separator plates in adjacent
vaporizer cells 130. The heat is used in the vaporization process,
wherein liquid feed water is fed through an inlet 148 of the
vaporizer 130 and vaporized; water vapor is discharged from an
outlet 146 of the vaporizer 130.
[0056] In the electrochemical pump, the water for membrane
hydration may be purified by a vaporization process and the thermal
energy of the electrochemical pump process may be integrated with
the vaporization process. In the present electrochemical pump
system, the thermal energy of the electrochemical pump process may
be integrated with any endothermic process for storing the hydrogen
produced from said electrochemical pump system. For example, the
thermal energy of the electrochemical pump process may be
integrated with the endothermic process for converting sodium
borate to sodium borohydride. 17.
[0057] Electrolyzer
[0058] According to another embodiment of the invention and
referring to FIGS. 1(b), 6 and 7, an electrochemical electrolyzer
and system having an HTLH ion exchange membrane are provided.
[0059] Referring to FIG. 1(b) in particular, an electrolysis cell
10b with the half reactions for the electrolysis of water (as
labeled in FIG. 1(b)) serves as an electrolyzer cell and has an
HTLH ion exchange membrane 12 for the membrane electrolyte; a
suitable membrane is PBI thin film with imbibed phosphoric acid.
The electrolyte 12 is sandwiched between a pair of electrodes,
namely a water feed electrode 14b, and a hydrogen product electrode
16. The water feed electrode 14b comprises a separator plate 18,
and a water feed chamber 23b, which includes a fluid inlet 17b and
a fluid outlet 21b. The hydrogen product electrode 16 comprises a
separator plate 20, a hydrogen product chamber 25 and a fluid
outlet 19. The electrodes 14b, 16 are electrically coupled by an
electric circuit 22. The circuit 22 is electrically coupled to an
electric current source 24. In operation, low pressure water vapor
is fed into the inlet of the fluid inlet 17b of the water feed
chamber 23b, wherein some of the water vapor is catalyzed and
disassociated into oxygen molecules, protons and electrons. Product
oxygen gas and unreacted water vapor are discharged via the fluid
outlet 21b. The protons conduct through the HTLH membrane 12 and
combine with electrons at the hydrogen product electrode 16 to
produce hydrogen product gas; electrons are moved through the
circuit 22 by the current source 24. Hydrogen product gas is
discharged through fluid outlet 19 of the hydrogen product chamber
25; a control valve (not shown) attached to the fluid outlet 19 can
be operated to control the pressure of the product gas. The water
feed electrode 14b may operate at near ambient pressure and the
hydrogen product electrode 16 may operate at elevated pressure. The
product electrode 16 may be operated at up to around 6000 psig or
higher, depending on the mechanical strength of the electrode 16
and the ability of the current source to supply adequate
electrons.
[0060] This HTLH electrochemical cell 10a is suitable for use as
the electrolyzer cell 10b in FIG. 3.
[0061] Preferably, the HTLH membrane electrolyzer cell 10b is fed
vaporized water. This prevents the washout of acid that occurs when
such membranes are washed or soaked in liquid water. The
vaporization of the water feed has the added benefit of inherently
separating ionic contaminants from the water vapor. In typical
electrolyzers operating below the boiling point of water a system
for de-ionizing the liquid water is generally used. Therefore, an
advantage provided by an HTLH membrane electrolyzer cell 10b is
that the need for a liquid water de-ionization system is
eliminated.
[0062] Further, liquid water is typically a means for carrying
contaminants into these types of electrochemical cells. Again the
use of water vapor in the HTLH membrane electrolyzer cell reduces
the influx of potential contaminants to the cell.
[0063] In conventional electrolyzers operating at higher pressures
the separator plates and electrodes are designed to provide maximum
support to the electrolyte membrane. The designs typically utilize
very small flow channels for the liquid water feed that results in
water flow rate and circulation requirements many times the rate of
water electrolysis to ensure complete distribution of the water
within the flow field. The use of water vapor feed increases the
mobility of the water molecules within the electrode/reactant
diffusion layer and reduces the flow circulation for distribution
requirements to a minimum.
[0064] An electrolyzer cell using an acid-doped PBI membrane
electrolyte is expected to be operable in the temperature range of
100 to 200.degree. C. This presents opportunities for thermal
integration with other systems. The waste thermal energy from the
electrolysis process is integrable with systems using an
endothermic process for hydrogen storage, such as the process
disclosed by Millennium Cell, which requires heat and hydrogen to
reformulate sodium borate to sodium borohydride. The waste thermal
energy from the electrolysis process could also be integrated with
the vaporization process to vaporize a liquid water feed. Examples
of such integrated systems are shown in FIGS. 6-8 and discussed
below.
[0065] Referring to FIG. 6, a system 150 for generating hydrogen
uses the HTLH membrane electrolyzer cell 10b discussed previously
and has a heat exchanger 158 that uses heat produced by the
electrolyzer cell 10b. In this system 150, feed water is fed via a
cold water feed conduit 152 into a water tank 154. Feed water is
then supplied from the water tank 154 via another cold water feed
conduit 156 to the heat exchanger 158. Thermal conduction conduits
159 are used to transmit waste heat from the electrolysis process
to the heat exchanger 158, which heats up the feed water. The
heated feed water is then sent via heated water feed conduit 168 to
a water vaporizer 170, wherein thermal input means 172 (e.g.
electric heater) may be applied if needed to vaporize the water,
and water vapor is transferred to the electrolyzer cell 10b for
hydrogen production via water vapor feed line 174. Product oxygen
gas and excess water vapor is discharged from the electrolyzer cell
10b via thermal conduction conduit 159 fluidly connected to the
electrolyzer cell 10b and thermally coupled to the heat exchanger
158, for heat transfer to feed water passing through the heat
exchanger 158. The oxygen gas and excess water is then discharged
from the heat exchanger 158 to a gas/water separator 162 via
discharge conduit 160 for separation into an oxygen exhaust stream
164 and water stream, which is recycled back into water tank 154
via water conduit 166. The hydrogen is discharged from the
electrolyzer cell 10b as hydrogen to a hydrogen storage system 177
via high pressure hydrogen line 175. The storage system 177 may be
heated by a suitable thermal input means 179, if desired.
[0066] Alternatively, a thermal energy transfer could occur
directly to the water stream being vaporized by passing the water
directly through the electrolyzer cell 10b.
[0067] Referring now to FIG. 7, a vaporizer cell 180 is combined
with an HTHL membrane electrochemical electrolyzer cell 10b to form
an integrated vaporizer/pump assembly 184; multiple
vaporizer/electrolyzer assemblies 184 can be combined to form a
vaporizer/electrolyzer assembly stack (as shown by repeating
lines). The electrolyzer cell 10b is thermally coupled to the
vaporizer cell 180 to allow direct transfer of thermal energy from
the electrolysis process to the vaporization process. The vaporizer
cell 180 comprises a pair of electrically and thermally conductive
separator plates 186, 188 sandwiching a current conducting heat
transfer plate 190. The electrolyzer cell 10b comprises a pair of
flow distribution and current conducting water feed and hydrogen
product electrode layers 192, 194 sandwiching a HTHL ion exchange
membrane electrolyte 196. The electrodes 192, 194 are connected to
a current source via an external circuit (not shown). Low pressure
water vapor is fed through an inlet 198 of the water feed electrode
192, and hydrogen gas is discharged out of an outlet 200 in the
product electrode 194 by the electrolysis process previously
described. Heat produced by the electrolysis is thermally
transferred through the electrodes 192, 194 to the separator plates
in adjacent vaporizer cells 180. The heat is used in the
vaporization process, wherein liquid feed water is fed from a water
tank 202, through a circulating pump (not shown), through an inlet
204 of the vaporizer 180 via a water feed line 206, and vaporized;
water vapor is discharged from an outlet 208 of the vaporizer 180.
Unreacted water and oxygen gas is discharged from the electrolyzer
cell 10b and through a heat exchanger 210 to reject excess heat and
return the excess water or water vapor via Water return line 212
back into the water tank 202. Alternatively, the water return line
212 can bypass the water tank 202 and couple to the water feed line
206, and the heat exchanger 210 or heat exchangers can be located
on either or both of the water lines 212, 206, as appropriate for
water and heat balance, as is known in the art.
[0068] It is to be understood that even though various embodiments
and advantages of the present invention have been set forth in the
foregoing description, the above disclosure is illustrative only,
and changes may be made in detail, and yet remain within the broad
principles of the invention. For example, some of the components
described above may be implemented using a variety of different
compounds and different structures. Therefore, the present
invention is to be limited only by the appended claims.
* * * * *