U.S. patent application number 10/470419 was filed with the patent office on 2004-08-05 for electrochemical cell stacks.
Invention is credited to Graydon, John W, Kirk, Donald W., Thorpe, Steven J.
Application Number | 20040151953 10/470419 |
Document ID | / |
Family ID | 4168259 |
Filed Date | 2004-08-05 |
United States Patent
Application |
20040151953 |
Kind Code |
A1 |
Kirk, Donald W. ; et
al. |
August 5, 2004 |
Electrochemical cell stacks
Abstract
An electrochemical cell stack comprising stack walls and a
plurality of electrolytic cells within the stack walls, each cell
comprising cell members selected from an anode a cathode; a
membrane separator frame (14) formed of a non-conductive material
and having a frame first planar peripheral surface; a frame second
planar peripheral surface; and a central portion defining a
membrane-receiving aperture (18); a membrane (20) within the
aperture to provide an anolyte circulation chamber and a catholyte
circulation chamber distinct one from the other within the frame,
an impermeable cell end wall (12) formed of a non-conductive
material between the anode and cathode and the anodes and cathodes
of adjacent cells of said stack; wherein each of said anode, said
cathode, said separator frame and said end wall has a portion
defining an anolyte flow inlet channel (30), a catholyte flow inlet
channel (32), a spent anolyte channel (36) and a spent catholyte
channel(34); said anolyte flow inlet channel and said spent anolyte
channel are in communication with said anolyte circulation chamber,
said catholyte flow inlet channel and said spent catholyte channel
are in communication with said catholyte circulation chamber. The
cell stack is of greatly reduced footprint, operable at relatively
high temperatures and pressures and which is stable under current
load.
Inventors: |
Kirk, Donald W.; (Toronto,
CA) ; Thorpe, Steven J; (Toronto, CA) ;
Graydon, John W; (Toronto, CA) |
Correspondence
Address: |
MANELLI DENISON & SELTER
2000 M STREET NW SUITE 700
WASHINGTON
DC
20036-3307
US
|
Family ID: |
4168259 |
Appl. No.: |
10/470419 |
Filed: |
January 24, 2004 |
PCT Filed: |
January 30, 2002 |
PCT NO: |
PCT/CA02/00118 |
Current U.S.
Class: |
429/446 ;
429/455; 429/457; 429/469; 429/501; 429/516 |
Current CPC
Class: |
C25B 15/02 20130101;
C25B 9/73 20210101 |
Class at
Publication: |
429/012 |
International
Class: |
H01M 008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 1, 2001 |
CA |
2,333,859 |
Claims
1. An electrochemical cell stack (10) comprising stack walls
(54,56) and a plurality of electrolytic cells within the stack
walls (54,56), each cell comprising cell members selected from an
anode (38,48); a cathode (40,46); a membrane (20); a membrane
separator frame (14) formed of a nonconductive material and having
a first side and a second side opposite to the first side; said
frame (14) having a. a frame first planar peripheral surface on
said first side; and b. a frame second planar peripheral surface on
said second side; an impermeable cell end wall (12) formed of a
non-conductive material between said anode (48) and said cathode
(46) and the anodes (38) and cathodes (40) of adjacent cells of
said stack (10); characterized in that (i) said frame (14) has a
central portion defining a membrane-receiving aperture (18); (ii)
said membrane (20) is within said aperture (18) to provide an
anolyte circulation chamber (22) and a catholyte circulation
chamber (23), distinct one from the other, within said frame; and
(iii) wherein each of said anode (38,48), said cathode (40,46),
said separator frame (14) and said end wall (12) has a portion
defining an anolyte flow inlet channel (30), a catholyte flow inlet
channel (32), a spent anolyte channel (36) and a spent catholyte
channel (34); and wherein said anolyte flow inlet channel (30) and
said spent anolyte channel (36) are in communication with said
anolyte circulation chamber (22), said catholyte flow inlet channel
(32) and said spent catholyte channel (34) are in communication
with said catholyte circulation chamber (23) within said frame
(14).
2. A cell stack as defined in claim 1 wherein said anode has an
anode first planar surface which abuts said frame first planar
peripheral surface as to define with said member said anolyte
circulation chamber within the confines of said frame, and said
cathode has an cathode second planar surface which abuts said frame
second planar peripheral surface as to define with said member said
catholyte circulation chamber within the confines of said
frame.
3. A cell stack as defined in claim 1 or claim 2 wherein said anode
in whole or in part is disposed within said anolyte circulation
chamber and said cathode in whole or in part is disposed within
said catholyte circulation chamber.
4. A cell stack as defined in any one of claims 1-3 wherein said
anode is in contact with said membrane within said anode
circulation chamber and said cathode is in contact with said
membrane within said cathode circulation chamber.
5. A cell as defined in any one of claims 1-4 wherein said anode is
formed as a laminate with, or coating on said membrane and said
cathode is formed as a laminate with, or coating on said
membrane.
6. A cell stack as defined in any one of claims 1-5 wherein said
anolyte circulation chamber has a lower portion defining an
inverted triangle having an apex defining an anolyte entry port in
communication with said anolyte flow inlet channel, and an upper
portion defining a triangle having an apex defining an anolyte exit
port in communication with said spent anolyte channel; and said
catholyte circulation chamber has a lower portion defining an
inverted triangle having an apex defining a catholyte entry port in
communication with said catholyte flow inlet channel, and an upper
portion defining a triangle having an apex defining a catholyte
exit port in communication with said spent catholyte channel.
7. A cell stack as defined in claim 6 wherein said anolyte entry
port is central of said frame; said anolyte exit port is adjacent a
first periphery of said frame; said catholyte entry port is central
of said frame; and said catholyte exit port is adjacent the
periphery remote from said first periphery.
8. A cell stack as defined in any one of claims 1-7 further
comprising a plurality of compressible sealing members dispersed
between adjacent cell members selected from said anode, said
cathode, said frame and said cell wall, at the peripheries thereof
and adjacent said anolyte and catholyte flow inlet channels and
said spend anolyte and catholyte channels.
9. A cell stack as defined in claim 8 wherein said compressible
sealing member is an o-ring, and said cell members have portions
defining o-ring receiving recesses.
10. A cell stack as defined in any one of claims 1-9 wherein said
frame and said cell end wall are formed of a polymeric, engineered
plastics material.
11. A cell stack as defined in any one of claims 1-10 wherein said
anode and said cathode are in the form of a metallic foil having a
thickness selected from 0.05-0.1 mm.
12. A cell stack as defined in any one of claims 1-11 in a
monopolar filter press arrangement.
13. A cell stack as defined in any one of claims 1-11 in a bipolar
arrangement.
14. A cell stack as defined in any one of claims 1-13 wherein said
anolyte and catholyte comprise an alkaline aqueous solution for the
electrolytic production of hydrogen and oxygen.
15. A cell stack as defined in any one of claims 1-14 wherein said
stack walls are subjectable to a cell stack external pressure; said
anode operably produces oxygen at an oxygen pressure within said
anolyte chamber; said catholyte operably produces hydrogen at a
hydrogen pressure within said catholyte chamber; means for
providing each of said oxygen pressure and said hydrogen pressure
with a positive pressure differential greater than said cell stack
external pressure; and said membrane separator frames and said
impermeable cell end wall are formed of a structural plastics
material.
16. A cell stack as defined in claim 15 wherein said external
pressure is provided by air at atmospheric pressure.
17. A cell stack as defined in claim 16 further comprising means
for providing said positive pressure differentials selected from
2-6 atmospheres.
18. An electrochemical stack as defined in any one of claims 1-17
further comprising a spent anolyte solution having an anolyte
liquid level and hydrogen gas above said anolyte liquid level; a
spent catholyte solution having a catholyte liquid level and
hydrogen gas above said catholyte liquid level; means for detecting
said anolyte and said catholyte liquid levels; valve means for
releasing said oxygen gas from above said anolyte level when said
catholyte liquid level is detected; means for releasing said
hydrogen gas from above said catholyte level when said anolyte
liquid level is detected; wherein said detection of said anolyte
level comprises means for irradiating said anolyte liquid level
with incident infrared radiation at an angle to effect scattering
of said radiation; and wherein said detection of said catholyte
level comprises means for irradiating said catholyte liquid level
with incident infrared radiation at an angle to effect scattering
of said radiation.
Description
FIELD OF THE INVENTION
[0001] This invention relates to electrochemical cell stacks,
particularly, to monopolar filter press cell stacks, and more
particularly to internally pressurized monopolar water electrolytic
cells for the production of hydrogen and oxygen.
BACKGROUND TO THE INVENTION
[0002] Electrosynthesis is one example of an electrochemical
process comprising a method for the production of chemical
reaction(s) that is electrically driven by passage of an electric
current, typically a direct current (DC), in an electrochemical
cell through an electrolyte between an anode electrode and a
cathode electrode from an external power source. The rate of
production is proportional to the current flow in the absence of
parasitic reactions. For example, in a liquid alkaline water
electrolysis cell, the DC current is passed between the two
electrodes in an aqueous electrolyte to split water, the reactant,
into component product gases, namely, hydrogen and oxygen where the
product gases evolve at the surfaces of the respective
electrodes.
[0003] Water electrolysers have typically relied on membranes or
separators between the two halves of an electrolysis cell to ensure
that the two gases, namely, oxygen and hydrogen produced in the
electrolytic reaction are kept separate and do not mix. Each of the
separated gases must be discharged from the cell at essentially the
same pressure since membranes and separators fail with pressure
differential. Simple pressure control systems, such as a small
water column of several centimeters in height for each gas and
discharge to atmospheric pressure are used to control the pressure
within this pressure differential.
[0004] In the conventional monopolar cell design in wide commercial
use today, one cell or an array of cells in parallel is contained
within one functional electrolyser, cell compartment, or individual
tank. Each cell is made up of an assembly of electrode pairs in a
separate tank where each assembly of electrode pairs connected in
parallel acts as a single electrode pair. The connection to the
cell is through a limited area contact using an interconnecting bus
bar such as that disclosed in Canadian Patent No. 3,02,737, issued
to A. T. Stuart (1930). The current in the form of a flow of
electrons is taken from the cathode bus bar via an electrical
connection to a portion of a cathode in one cell, then through the
electrolyte in the form of ions to the anode of that cell and then
to the anode bus bar using a similar electrical connection. The
current is usually taken off one electrode at several points and
the connection made by means of clamps, soldered joints, mechanical
screw connections and the like.
[0005] Electrolysis apparatus having pressurized external cell
structures are known for producing hydrogen. For example, U.S. Pat.
No. 5,665,211, issued 1997, describes a pressurized container
within which is an electrolytic cell. There is no integration of
the cell itself as the pressure containment device, and, thus, the
apparatus is bulky and heavy. U.S. Pat. No. 3,652,431, issued 1972,
describes an electrolysis cell where external pressure from a
liquid such as water is used to support a container in which
pressurized electrolysis is conducted. U.S. Pat. No. 4,042,481,
issued 1977, describes a pressurized tank containing cylindrical
porous anode and cathode tubes which allow escape of the oxygen and
hydrogen produced. However, the apparatus requires the need for a
tank to house cells and, thus, this does not represent efficient
use of overall space or footprint. There is also the potential for
mixing of oxygen and hydrogen produced if gas does not diffuse
through the porous electrode tubes. The cylindrical configuration
of the anodes and cathodes present fabrication challenges and the
spacing of these electrodes will require substantial room to
prevent non-uniform currents if multiple cells are used. U.S. Pat.
No. 5,733,422, issued 1998, describes a tank with a header box
wherein the top is screwed onto the side wall plates. Again, this
is clearly not a design suitable for lightweight and inexpensive
polymeric materials.
[0006] There is, therefore, a need for electrolytic cells,
particularly, water electrolysers, which do not suffer from the
aforesaid disadvantages.
SUMMARY OF THE INVENTION
[0007] The present invention provides an electrolytic cell stack
having a beneficial novel relationship of cell components involving
the inverse structural role of some components, through the use of
a single electrolyte circulation and membrane frame within each
cell. There is no need for end-boxes and compressible elastomeric
materials which, however, may be optional. The circulation and
membrane frame preferably, is formed of a structural plastics
material, which can provide support to thin foil electrodes if the
latter are used. In the absence of gaskets and compressible
elastomeric frames, advantageous higher operating temperatures can
be readily attained. This is particularly so when the cell stack is
pressurized as hereinafter described.
[0008] In one aspect, the invention provides an electrochemical
cell stack comprising stack walls and a plurality of electrolytic
cells within the stack walls, each cell comprising cell members
selected from an anode; a cathode; a membrane separator frame
formed of a non-conductive material and having a first side and a
second side opposite to said first side;
[0009] (a) a frame first planar peripheral surface on said first
side;
[0010] (b) a frame second planar peripheral surface on said second
side; and
[0011] (c) a central portion defining a membrane-receiving
aperture;
[0012] a membrane within the aperture which provides an anolyte
circulation chamber and a catholyte circulation chamber distinct
one from the other within said frame; an impermeable cell end wall
formed of a non-conductive material between said anode and said
cathodes and the anodes and cathodes of adjacent cells of the
stack; wherein each of said anode, said cathode, said separator
frame and said end wall has a portion defining an anolyte flow
inlet channel, a catholyte flow inlet channel, a spent anolyte
channel and a spent catholyte channel; and wherein said anolyte
flow inlet channel and said spent anolyte channel are in
communication with said anolyte circulation chamber, and said
catholyte flow inlet channel and said spent catholyte channel are
in communication with said catholyte circulation chamber.
[0013] In one preferred embodiment, the invention provides a cell
stack as hereinabove defined wherein said anode has an anode first
planar surface which abuts said frame first planar peripheral
surface as to define with said member said anolyte circulation
chamber within the confines of said frame, and said cathode has an
cathode second planar surface which abuts said frame second planar
peripheral surface as to define with said member said catholyte
circulation chamber within the confines of said frame.
[0014] In another preferred embodiment, the invention provides a
cell stack as hereinabove defined wherein said anode in whole or in
part is disposed within said anolyte circulation chamber and said
cathode in whole or in part is disposed within said catholyte
circulation chamber. In this later defined cell stack, one or both
of the anode and the cathode are in contact with the membrane, on
opposite sides thereof, within the respective electrolyte
circulation chamber, as for example, a laminate with or coating on
the membrane.
[0015] The production of a bilayer or trilayer porous assembly
offers the distinct advantages of minimal electrode/membrane
thickness and, hence, inter-electrode cell resistance, as well as
ease of processing on a continuous basis by the integration of
separate parts, namely, current carrier+activation+membrane, using
known, suitable processing methods.
[0016] The production of such a bi or trilayer composite structure
can be carried out, for example, by utilizing a core membrane
material and metallizing this externally, wherein the membrane may
be either polymeric or ceramic in nature, formed by, for example,
weaving, felting, tape casting, sintering and the like. The
metallizing process can be selected, but not limited to one of
plasma vapour deposition, chemical vapour deposition, plasma
spraying, electrodeposition and the like. In an alternative and
inverse process, a membrane material, can be deposited on an
existing porous electrode structure. These two processes can be
used either separately, or, in combination to produce a trilayer
structure.
[0017] The aforesaid herein defined cell stacks are more preferred
wherein said anolyte circulation chamber has a lower portion
defining an inverted triangle having an apex defining an anolyte
entry port in communication with said anolyte flow inlet channel,
and an upper portion defining a triangle having an apex defining an
anolyte exit port in communication with said spent anolyte channel;
and said catholyte circulation chamber has a lower portion defining
an inverted triangle having an apex defining a catholyte entry port
in communication with said catholyte flow inlet channel, and an
upper portion defining a triangle having an apex defining a
catholyte exit port in communication with said spent catholyte
channels.
[0018] More preferably, the cell stack as hereinabove defined has
the anolyte entry port central of the frame; the anolyte exit port
is adjacent a first periphery of the frame; the catholyte entry
port is central of the frame; and the catholyte exit port is
adjacent the periphery remote from the first periphery.
[0019] The cell stack as previously defined further comprises a
plurality of compressible sealing members disposed between adjacent
cell members selected from the anode, the cathode, the frame and
the cell wall, at the peripheries thereof and adjacent the anolyte
and catholyte flow inlet channels and the spent anolyte and
catholyte channels which are not facing the upper portion defining
a triangle having an apex defining a catholyte exit port or anolyte
exit port channel. It should be noted that the offset pairs of
seals at the peripheries contains the large pressure differential
with the exterior while the seals at the inlet manifolds contain a
small pressure differential of about 1 psi to allow differential
flow rates in the anolyte and catholyte flow channels and the seals
at the spent electrolyte ports have essentially no pressure
differential to contain but must prevent electrolyte from mixing
ports with the preferred design having thin foil electrodes, it is
not possible to provide a seal against the unsupported electrodes
which are facing the upper portion of the frame member defining a
triangle having an apex defining a catholyte exit port or anolyte
exit port channel. Since there is essentially zero pressure drop
between the circulation chambers, it is only necessary to prevent
the electrolyte from flowing behind the electrode to the opposing
electrolyte circulation chamber by providing one seal at the exit
ports per circulation frame member. This seal is easily achieved on
the side of the circulation frame member which does not have the
open triangle exhaust port. Thus only one half of the exit ports
are sealed and the seals alternate between one outlet. In another
configuration (bipolar) described later, this alternating seal
configuration is not sufficient for preventing electrolyte mixing
and a stiffer electrode must be used to allow for a seal on one
side of the electrode at the exit port.
[0020] Most preferably, the compressible sealing members are
o-rings, and the cell members have portions defining o-ring
receiving recesses.
[0021] The frames and cell end walls are most preferably formed of
a structural plastics material. The anodes and cathodes are most
preferably in the form a metallic foil or the like having a
thickness, preferably selected from 0.05-0.10 mm. In the case of a
bipolar cell stack design, hereinafter described, the electrodes
have a sufficient stiffness to hold a seal with an o-ring. Thus,
stiffer electrodes will be preferred in that configuration.
[0022] Although not limiting, the invention is particularly of
benefit in electrochemical processes that produce one or more
gaseous products, such as chlorine, hydrogen and oxygen, the latter
two from the electrolysis of aqueous potassium hydroxide
electrolyte solutions, particularly in a monopolar filter press
structural arrangement.
[0023] In a further aspect, the invention provides an improved
process for providing hydrogen from a monopolar electrolytic cell
having cell walls under an external pressure; anolyte solution
having an anolyte liquid level; catholyte solution having a
catholyte liquid level; the process comprising generating oxygen at
an oxygen pressure within the cell above the anolyte; generating
hydrogen at a hydrogen pressure within the cell above said
catholyte; the improvement wherein each of the oxygen pressure and
the hydrogen pressures provide a positive pressure differential
greater than the external pressure.
[0024] The external pressure is provided most generally by air, and
at an ambient pressure of 1 atmosphere.
[0025] The positive pressure is readily attainable within the cell
up to about 8 atmospheres, but a practical pressure is preferably
selected from 2-6 absolute atmospheres.
[0026] The production of hydrogen from prior art monopolar cell
stacks has been limited to current densities of less than about 500
mA/cm.sup.2 at steady state operation. The primary problem is that
at high current density the volume of gas produced at the electrode
surfaces becomes so great that cell resistance rises dramatically,
liquid contact with the electrode surface is reduced and parts of
the electrode may cease to function, unsteady liquid and gas flows
develop and energy efficiency decreases dramatically. Resistive
heating exacerbates the problems causing electrode heating and
damage to cell components which renders the cell dangerous to
operate.
[0027] A multicell stack according to the invention avoids these
problems and allows current densities of greater than 750
mA/cm.sup.2 to be run at steady state almost indefinitely (100's of
hours). Surprisingly, these high current densities have been run
with smooth, non-activated planar electrodes which are in thin foil
form and with "zero gap" cells (5.25 mm) and plastic frame members.
It would be expected that at high current density (>500
mA/cm.sup.2) resistive heating in the thin foil electrodes and from
the resistance in the electrolyte due to gas formation, would cause
this type of cell to fail. In fact, this cell has been run for
hundreds of hours at current densities up to 600 mA/cm.sup.2
without damage. The operation clearly demonstrates that the
resistance buildup at high current density has been overcome. A
larger power supply and thicker electrodes would allow even higher
current densities to be run.
[0028] The accomplishment means that small electrolysis cells using
this design are able to produce large amounts of hydrogen required
for industrial regenerative and personal fuel appliance
applications and space limited configurations. A side benefit is
that pressurized hydrogen is available for direct storage in a
vessel or media, such as metal or chemical hydride or as an
economical source of pre-pressurized hydrogen for mechanical or
electrical compressors for high-pressure use.
[0029] The invention is of particular, but not limiting, value in
monopolar electrolyte cell stacks, and, accordingly, in a further
aspect the invention provides a monopolar electrolytic cell stack
having a cell stack as hereinabove defined wherein said stack walls
are subjectable to a cell stack external pressure; said anode
operably produces oxygen at an oxygen pressure within said anolyte
chamber; said catholyte operably produces hydrogen at a hydrogen
pressure within said catholyte chamber; means for providing each of
said oxygen pressure and said hydrogen pressure with a positive
pressure differential greater than said cell stack external
pressure; and said membrane separator frames and said impermeable
cell end walls as formed of a structural plastics material.
[0030] In one embodiment, the cell stack according to the
invention, is based on the monopolar electrolysis cell stack design
having the anodes and cathodes in a folded configuration known as a
"double electrode plate", but of relatively very narrow thickness
in the form of a metal foil. The cell containment is by means of a
thin polymer plate of "Noyel".RTM. or like engineering plastic
having an electrolyte slot and flow channels for both liquid inlet
and gas outlet. A diaphragm or membrane is used to separate anolyte
and catholyte. Sealing of the electrolyte within the cell is
achieved by means of O-rings in grooves set in the plate to provide
a leak free condition at all operating values of current. The
electrolyte is pumped into the cells through flow constricted
channels in each cell compartment.
[0031] The high current operation is achieved by valving off the
oxygen and hydrogen outlet channels until a pressure of up to 6
atmospheres is achieved. In this mode, current densities of up to 1
A/cm.sup.2 can be run at steady operation without damaging
resistive heating effects. Without valving, the cell could not be
run at more than 250 mA/cm.sup.2 before excessive gassing caused
unstable operation and damaging resistive heating. Prior art cells
operating at steady state at 1 A/cm.sup.2 with smooth planar
electrodes are known, but do not have the advantages of the unitary
electrolyte circulation and membrane frame according to the present
invention.
[0032] The present invention provides, in one aspect, a process and
apparatus for producing hydrogen gas and oxygen gas at an elevated
pressure by electrolysis in an alkaline aqueous solution.
Maintaining the hydrogen pressure above the catholyte liquid level
within the cell offers the following advantages.
[0033] 1. The drastic reduction in the volume of evolved gas lowers
the electrical resistance of the mixture of electrolyte and gas
bubbles within the cell compartment to produce a higher energy
efficiency.
[0034] 2. The smaller volume of gas also allows operation at higher
current densities than is the case at lower pressures where the
large volume of gas would form plugs within the cell compartment
resulting in unstable operation or preventing electrolysis
altogether.
[0035] 3. The rate of flow of electrolyte required through the cell
is much less. This decreases the size of the electrolyte channels
required, the capacity of the pump, and the erosion and wear on all
components of the cell caused by the high linear velocity of
flowing electrolyte.
[0036] 4. Separation of gas and liquid is easier since the
volumetric flow rates of electrolyte and especially of the gases
are much lower.
[0037] 5. The hydrogen gas contains much less water vapour that may
require subsequent removal.
[0038] 6. Much less heat is lost from the stack due to the smaller
amount of water that is evaporated.
[0039] 7. Electrolysis can be carried out at temperatures above
100.degree. C. for greater energy efficiency.
[0040] 8. Pressurized hydrogen is directly available for moderate
pressure applications. Alternatively, the pressure can easily be
raised further using a single-stage compressor that is much cheaper
to buy and operate than the compressor required if the same amount
of hydrogen was at atmospheric pressure.
[0041] The pressurized cell stack when pressurized up to 8
atmospheres is substantially constructed of a polymeric material,
such as, for example, "NOYEL".TM. structural plastics material.
[0042] The stack, in one embodiment, is essentially supported only
by the cell end walls and wherein the polymer frames are designed
with sufficient width at the top, bottom and sides and in
connection with the electrode members as to withstand the internal
gas pressures without other internal support and metallic end stack
members. In an alternative embodiment, internal support is provided
by the frame members and not the cell end walls, which may be in
the form of a non-conductive, electrolyte impermeable film or the
like.
[0043] In the aspect of the present invention involving a cell
stack capable of being operable at greater than ambient pressures,
in the issue of pressure in the cell stack, there are three
directions to be considered, namely, the ends, the sides and the
top of the stack. The sides and top, although not quite of the same
dimensions, can be considered together because they resist the
pressure in the same way, i.e., by the stiffness of the structural
plastics material and their contact with the electrode members. The
ends resist pressure by being thick and stiff and by having, in the
preferred embodiment, tie rods connecting the two metallic end
plates. These ends must resist the pressure for two reasons,
namely, one is simple containment and the second is to keep the
plates in good contact with the O-ring seals.
[0044] The following describes, firstly, the end plate pressure
requirements, and secondly the sidewall pressure issues.
[0045] At the terminal ends of the cell stack, one plate has
atmospheric pressure on its outer side, while on the inside is the
pressure of the cell contents. Either the end wall per se or the
end wall plus additional support for the wall must be of sufficient
strength to resist any pressure differential. Thus, the last
electrode of the stack can be made thick enough to resist the
pressure or it can be the same as all of the other internal
electrodes, but being supported by a polymeric plate or metal plate
of sufficient strength to resist the pressure. A first attempt used
a 0.635 cm steel plate in addition to a glass-filled polyphenylene
oxide backing plate but this was of insufficient stiffness to allow
complete sealing to take place within the cell stack. In a
preferred embodiment of the present invention, a 0.953 cm stainless
steel plate was then used which was sufficient to provide good
sealing. It was found that some degree of deflection of the end
walls was due to the pressure of the gases and fluids within the
electrolyte frame chambers pushing outward on the end and the
inward force applied by tie rods holding the two end plates
together.
[0046] With reference now to the cell walls and pressure, the
internal pressure must also be resisted by the frame members and
peripheral o-ring seals. The pressure is from the inside of the
electrolyte circulation chambers to the outside atmosphere and is
contained by the frame wall and seals. Surprisingly, we have found
that gasket seals which are conventionally used are less preferred
in preventing leakage of electrolytes even under very small
internal pressures of less than one psi. Both gaskets and o-rings
are made of elastomers which distort easily under stress and thus
provide no structural strength for pressure resistance. In the
present cell stack, the structural strength is provided by the
frame walls and by their contact with the electrodes and end plate
members. The tie rods thus provide the important function of
compressing the seals for leak prevention but also provide
structural strength by pressing the frame members and electrodes
together with the end walls. With gaskets of compressible elastomer
material used as structural frame members in plate and frame
designs, such as by U.S. Pat. No. 6,080,290, disclosed, for
example, in U.S. Pat. No. 6,080,290, issued 2000, the cell
generally cannot resist internal pressures greater than a few psi
because the gaskets or frame members will expand outward. If
sufficient tension from the tie rods is applied to allow friction
to hold the gasket or frame member in place, then the gasket or
frame member becomes so compressed that their structural function
is lost. In the embodiments according to the invention, it is
believed that the gaskets would require so much tie rod tension to
effect satisfactory sealing, that even the rigid frame members
would be susceptible to cracking or compressive failure. In sharp
contrast, in the preferred practice of the invention O-ring sealing
provides a number of benefits and has been demonstrated to provide
a solution to this problem. An O-ring, by virtue of having a much
smaller contact area with the frame members, electrodes and end
walls than a gasket, does not require excessive tension from tie
rods. By locating the O-rings in channels, the compression forces
the frame member walls to contact directly with the metal
electrodes and, thus, transfers strength to the frame member.
Metals, generally, have much higher strengths than polymeric
materials. In addition to the strengthening aspect, the cell stack
was more rigid than it would have been with a rubbery gasket
sandwiched between the cell components. Furthermore, in the
preferred embodiment of the invention, the O-ring is prevented from
"slipping out" from between the electrodes and frame members by the
O-ring channels. Gasket seals, particularly with slippery caustic
solutions, are quite susceptible to seal "bowout" if the stack
pressure rises. Thus, the current embodiments provides a
significant safety advantage.
[0047] To conserve linear dimensional space, i.e. to make the cell
stack as short as possible, the supporting frame is preferably thin
and the O-ring channels offset one from the other within the frame
so as not to weaken the frame at directly opposite locations.
[0048] We have found that the dimensions of the frame and cell wall
structure formed of a structural polymeric material must be of a
sufficient longitudal and peripheral thickness as not to expand
with pressure and cause leakage or crack.
[0049] Thus, the pressurized cell stack aspect of the present
invention allows the individual walls and frames to be formed of a
structurally rigid plastics material, and optionally supported by
metal end plates. The stack may be, most advantageously, operated
at an electrolyte solution temperature of at least up to
110.degree. C.
[0050] The polymer frames are designed with sufficient width at
their upper, lower and side portions as to withstand the internal
pressure without additional support. The membranes may be readily
bonded by, for example, adhesives or heat and pressure while
maintaining precise dimensions.
[0051] The most preferred embodiment comprises integration of
manifolds, flow control channels, fluid distributors and collectors
across the electrolyte circulation chambers and membrane within a
single frame. Only four electrolyte ports, namely, anolyte and
catholyte input and output sources are required in the end
wall.
[0052] In most preferred embodiments, the electrodes are relatively
thin, e.g. of the order of 0.05-0.1 mm. These makes the stack more
compact and comprises monolithic, impermeable, single sheets of
foil with no perforations that do not need a second central
element, i.e. no sharp, distinct electrode frame to act as a
current carrier.
[0053] In a further aspect, the invention provides a novel pressure
control system of particular value in offering improved pressure
regulation at elevated temperatures and relatively high current
density which produces relatively high volumes of hydrogen and
oxygen gases within the confines of a compact cell and cell stack
according to the invention. It is a distinct advantage that such a
pressure control system provides for pressure differentials of less
than 12 mm water between the anolyte and catholyte circulation
chambers and maintains steady this pressure differential to avoid
mechanical stress on the separator membrane diaphragm and to avoid
electrolyte crossover from anolyte to catholyte or vice versa.
[0054] The pressure control system of use in the present invention
provides robust and steady control of pressure with a high degree
of safety. In consequence of the electrolysis cell reactions, the
hydrogen gas flow rate is twice that of the oxygen flow rate. The
main features compensate for the differential gas volume generation
within the cell, piping and gas liquid separators. This
differential in flow rate is not so important in wide gap cells,
but in the compact cell stacks according to the present invention,
the gas and electrolyte flows experience significant hydraulic
resistance due to the narrow gap between walls and constricted flow
at exit apertures which must fit within the compact stack
structure. Increase in operating temperature, which is important in
reducing voltage loss, further requires good pressure control and
gas volume management, since the volumes of gas rise directly
proportional to the temperature in the cell. Passive cell design
control features include a reduction of the electrolyte flow to the
anolyte compartment by one-half of the catholyte flow as described
hereinafter, in order to achieve the same gas to liquid fraction in
both circulating chambers. This provides the same fluid density in
both sides, which is important for achieving the same constant
liquid level head in the gas liquid separators. The outlet
manifolds and tubing are kept as large as possible to minimize
friction. The tubing length from the cell stack to the gas liquid
separator for the catholyte and hydrogen gas is reduced relative to
the anolyte and oxygen gas to compensate for the higher electrolyte
plus hydrogen gas flow rate.
[0055] The level of the liquid in the, respective, gas liquid
separator is used as the first indicator of the pressure
differential in the cell stack. The pressure control of the system
must allow the liquid levels to stay equal or within some small
level of tolerance supported by the membrane. Although electronic
control systems may be used to adjust pressure regulators a more
direct method is, preferred, involving a matched pair of back
pressure regulators--one for oxygen and one for hydrogen. These
back pressure regulators are controlled by a common, single
compressed gas pressure source. This common single compressed gas
always provides the same release pressure to both regulators
automatically regardless of the control pressure. With this device
the primary pressure balance control is assured. Operation of the
cells with this configuration shows exceptionally stable pressure
balance during many hours of operation, even at high current
density >600 mA/cm.sup.2 and temperatures up to 100.degree. C.
This control of pressure balance is achieved without the continuous
control adjustment that is required by electronic control systems.
Furthermore, no sensor input is required for this control and,
thus, adds a high level of security to the system.
[0056] With all control systems, there is a long term drift that
must be addressed. In much of the testing work, occasional manual
adjustment of the regulator was required to keep the system in
balance. In industrial practice, manual adjustment is not desirable
for continuous operation.
[0057] Thus, in a further aspect, the invention provides an
improved process for providing hydrogen and oxygen gases from an
electrolytic cell stack having
[0058] a spent anolyte solution having an anolyte liquid level and
oxygen gas above said anolyte liquid level;
[0059] a spent catholyte solution having a catholyte liquid level
and hydrogen gas above said catholyte liquid level;
[0060] the improvement comprising
[0061] detecting at least one of said anolyte and said catholyte
liquid levels; releasing said oxygen gas from above said anolyte
level when said catholyte liquid level is detected;
[0062] or releasing said hydrogen gas from above said catholyte
level when said anolyte liquid level is detected;
[0063] wherein said detection of said anolyte level comprises
irradiating said anolyte liquid level with incident infrared
radiation at an angle to effect scattering of said radiation;
and
[0064] wherein said detection of said catholyte level comprises
irradiating said catholyte liquid level with incident infrared
radiation at an angle to effect scattering of said radiation.
[0065] In a further aspect the invention provides an
electrochemical stack as hereinabove defined further
comprising:
[0066] a spent anolyte solution having an anolyte liquid level and
hydrogen gas above said anolyte liquid level;
[0067] a spent catholyte solution having a catholyte liquid level
and hydrogen gas above said catholyte liquid level;
[0068] means for detecting said anolyte and said catholyte liquid
levels;
[0069] valve means for releasing said oxygen gas from above said
anolyte level when said catholyte liquid level is detected;
[0070] means for releasing said hydrogen gas from above said
catholyte level when said anolyte liquid level is detected;
[0071] wherein said detection of said anolyte level comprises means
for irradiating said anolyte liquid level with incident infrared
radiation at an angle to effect scattering of said radiation;
and
[0072] wherein said detection of said catholyte level comprises
means for irradiating said catholyte liquid level with incident
infrared radiation at an angle to effect scattering of said
radiation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0073] In order that the invention may be better understood, a
preferred embodiment will now be described by way of example only
with reference to the accompanying drawings wherein
[0074] FIG. 1 is a diagrammatic, exploded perspective view of a
monopolar, filter press, four-cell stack embodiment, in open-form
to enhance understanding, according to the invention;
[0075] FIG. 2 is a schematic flow diagram of the process operation
with a two cell stack electrolyser embodiment according to the
invention;
[0076] FIG. 3 is a diagrammatic elevational view of a separator
frame and membrane of use in a cell stack according to the
invention;
[0077] FIG. 4 is a perspective view of a separator frame and
membrane of use in a cell stack according to the invention;
[0078] FIG. 5 is a diagrammatic elevational view of a cell wall of
use in a cell stack according to the invention;
[0079] FIGS. 6, 7 and 8 are diagrammatic, exploded perspective
views of alternative embodiments of cell-stacks, according to the
invention; and
[0080] FIG. 9A and FIG. 9B are diagrammatic representations of
coated membranes of use in the practice of the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0081] With reference to FIG. 1, this shows generally as 10, a cell
stack of a plate and frame, filter press design comprising multiple
subunits. Each sub-unit consists of one side of each of two
non-conductive end wall 12 and a non-conductive separator frame 14.
Walls 12 and frames 14 are each rectangularly shaped and are formed
of a glass fiber-filled polyphenylene oxide (NOREL.TM., GFN-3,
General Electric Company) and measure 15.0 cm wide.times.22.7 cm
long.
[0082] End walls 12 are 0.40 cm thick, solid in their middle
regions, serve to divide one cell from its adjacent neighbor and to
support electrodes 16. Separator frames 14 are 0.63 cm thick and
have large rectangularly-shaped apertures 18 within which is a thin
(1.2 mm) porous, hydrophilic liquid-conductive separator membrane
20 made of felted polyphenylene sulphide (Ryton.TM. of Phillips
Petroleum) to divide aperture 18 into anolyte circulation chamber
22 and catholyte circulation chamber 23. Membrane 20 prevents
intermixing of the electrolysis hydrogen and oxygen product gas
bubbles formed on opposite sides, respectively, of membrane 20, and
also minimizes the flow of gas-saturated electrolyte from one side
to the other. Membrane 20 provides an exposed area of 10
cm.times.10 cm and is held into frame 14 by thermal pressure
bonding to strips of polyphenylene oxide which are then solvent
welded to separator frame 14. Tight control of dimensional
tolerances, which are essential to a properly positioned flat
separator, is achieved by the use of aluminum jigs in both bonding
steps.
[0083] Beneath aperture 18, shallow inverted triangular shelves 24
are machined into both sides of frames 14 to distribute the
incoming electrolyte in a uniform, vertical flow past the surfaces
of electrode 16. Similarly, above aperture 18, shallow triangular
shelves 26 are machined into both sides to form collectors that
conduct the spent anolyte and catholyte electrolyte and gas
mixtures out of the respective circulation chambers 22, 23 while
maintaining a uniform vertical flow past electrodes 16 and, also,
promoting some bubble coalescence. As seen in FIG. 3 a slot is
machined between each inlet manifold hole and the bottom of the
appropriate flow distributor shelf at its apex wherein a small
polymer bar with a precisely machined hole 61 along its axis is
bonded into this slot to control the rate of flow of the incoming
electrolyte, which in FIG. 3 is anolyte inlet manifold hole 30.
[0084] Each of end walls 12 and membrane frames 14 has four holes
drilled through it--two at the bottom to form the anolyte and
catholyte manifold inlet 30, 32, respectively, and two at the top
to form the spent catholyte and hydrogen, and the anolyte and
oxygen, gas manifold outlets 34, 36, respectively.
[0085] The two single terminal electrodes, anode 38 and cathode 40,
are partially annealed (1/8 hard), commercially pure nickel (Nickel
200). Each is 0.0074 cm thick with a satin finish. Double
electrodes 16 are cut from nickel sheet of the same type and four
manifold holes 44 and four locating holes are drilled in each half.
The sheet is folded in a U shape around one side of each end wall
12. One half acts as a cathode 46 and one half as an anode 48, and
are thus electrically connected through the central bend portion of
the sheet. This double electrode plate arrangement results in a
stack of monopolar cells that are connected in series. In the
assembled stack, electrolysis occurs on an area 10.times.10 cm
square located in the middle of each electrode on the side adjacent
to the central cavity of the separator frame. No surface treatment
or other procedure was used to activate the electrodes for
increased energy efficiency.
[0086] The description hereinabove of the components of stack 10 is
modified slightly for the components at each end. End electrodes
38, 40 are single and terminate outside stack 10 in tabs 50, 51,
respectively. These tabs are sandwiched between two strips of
copper 0.1 cm thick (not shown), which are compressed into the
nickel electrode by small bolts (not shown). This connection
provides a good, low resistance electrical contact that ensures an
even current distribution in end electrodes 38, 40 to minimize
resistive heating at the copper-nickel contact. The two
non-conductive end backing plates 54, 56, respectively, at the ends
of the stack are thicker (1.2 cm) than internal end walls 12 so
that they can be drilled and tapped at the ends of the inlet and
outlet manifolds to accept o-ring seal connectors (not shown)
through which electrolyte will enter and leave the stack along with
the hydrogen and oxygen gases.
[0087] The apparatus of the present invention preferably provides
electrolyte and gas sealing by the use of o-rings, preferably,
formed of ethylene-polypropylene rubber, and located within the
stack as follows.
[0088] A pair of concentric, offset o-ring grooves are machined
around the holes that form the inlet manifolds on both sides of the
frame or plate. The holes forming the outlet manifolds each have
only one concentric o-ring groove (the opposite side of the plate
or frame is the outlet channel). Manifold o-rings are inserted into
the grooves of the frames and plates prior to their assembly into
the stack. A pair of offset o-ring grooves are machined into both
sides of the plates and frames around the periphery of the cell
into which the wall o-rings are inserted. Finally, four locating
holes are drilled into the four corners of the plates and frames to
ensure precise alignment during the compression of the stack after
assembly.
[0089] FIG. 4 represents a membrane frame 14, shown split apart
down a central vertical plane in order to better illustrate the
frame features on the backside.
[0090] With reference to FIGS. 3 and 4 in order to seal stack 10,
an arrangement of o-rings are located in grooves 60, 62 in each
frame 14 and end wall 12. Since these o-rings must seal against
flexible electrode 16, the mating o-rings on either side of
electrode 16 are offset from each other so that the this nickel
foil is backed by the adjacent frame to provide support. Inlet
manifold seal 64 must maintain only a small pressure differential,
while the outlet manifold seal 66 has essentially no pressure
differential. In contrast, the peripheral wall seal 60 must sustain
the entire internal pressure of up to 100 psig and so its
performance is critical to the integrity of the stack. The precise
alignment of frames 14 and endwalls 12 in the stack that is
necessary to ensure that the seals will be offset and function
properly is achieved by the insertion of hardened steel pins into
the four locator holes in each corner of each component during
assembly. Manifold conduits 61 and 63 represent control channels
for anolyte input flow and catholyte input flow, respectively.
[0091] External Support.
[0092] In order to support the end walls and to maintain the
integrity of the o-ring seals between adjacent frames within the
stack against the internal pressure, in this embodiment the entire
stack is confined between two stainless steel terminal end plates
0.953 cm thick adjacent to the end frames 14. These end plates are
held together by six 7 mm or 1 cm connecting rods with threaded
ends that pass close to but do not touch the top, bottom and sides
of the stack. When put under tension using a torque wrench, these
rods provide 6000 lb of compressive force to the end plates. The
end plates measure 19 cm.times.27 cm and have holes 38 mm in
diameter to accommodate the manifold connectors that are attached
to the end frames. Thin 0.8 mm rubber sheets are interposed between
the end plates and the end frames. These accommodate the small
dimensional changes in the end plates and stack as the rods are put
under tension, and as the stack expands during heating and
increasing internal hydrostatic pressure and, thus, maintain an
even, constant compressive force on the stack. Without these
cushions, the small movements that occur when the stack is
initially compressed and when it goes through the temperature and
internal pressure changes during the operating cycle may overstress
the frames and lead to cracking.
[0093] In operation, in the embodiment shown, the electrolyte is a
solution of 30% by weight, commercial-grade potassium hydroxide
dissolved in deionized water and is driven through stack 10 by a
pump (not shown). The electrolyte enters stack 10 through the two
inlet manifolds 30, 32 and is distributed to cell compartments 22,
23 from each manifold via flow control channels 61 and 63, flow
control channel 61 supplying anode chamber 22 and the other, 63,
supplying cathode chamber 23. The flow control channels 61 are
designed to ensure an equal flow of electrolyte into each cell and
in the proper anolyte to catholyte flow ratio of 1:2. This is
achieved by constructing flow control channels 61 and 63 of the
proper length and diameter to ensure a pressure drop of about 1 psi
when electrolyte is pumped through stack 10 at the proper total
flow rate and desired flow ratio. The spent electrolytes, now
containing hydrogen or oxygen gas bubbles, leaving each cell
compartment is directed into two outlet manifolds 34, 36,
respectively, past shelves 26 at the top of each circulation
chamber. This exit system allows the gas-bearing electrolytes to
flow freely with a minimum pressure drop.
[0094] With reference also to FIG. 2 in operation, electrolyte is
pumped into stack 100 via the inlet manifold connectors. The
gas-bearing electrolytes leaving the two outlet manifold connectors
are conducted via separate pipes 102, 104 to two gas-liquid
separators 106, 108, respectively, that remove the hydrogen and
oxygen and return bubble-free electrolyte solutions to pump 110 for
recirculation to stack 100. It is important to maintain the same
pressure in the two outlet manifolds to minimize movement of
electrolyte from one compartment to the other through membrane 20
and minimize hydrostatic stress across the membranes in the
cells.
[0095] With further reference to FIG. 2, this shows in more detail
in operation, a centrifugal pump 110 that delivers 30% w/w
potassium hydroxide aqueous electrolyte at a rate of 0.8 L/min per
cell into cell stack 100, via the inlet manifold connectors. The
hydrogen and oxygen gas-bearing spent electrolytic solutions leave
the stack as described aforesaid. Inside separators 108, 106, the
hydrogen and oxygen bubbles are separated from the electrolyte
solution by gravity. The bubble-free electrolytes then leave the
bottom of each separator and re-circulate to stack 100 via a common
return 112. Alternatively, the electrolyte may also be returned to
the stack in separate streams instead of being combined into a
common stream.
[0096] The separated oxygen and hydrogen gases pass upward through
a packed column 114, 116, respectively, that serve to cool the
gases and remove electrolyte mist and condensed water vapour from
them. The cool, demisted gases then pass through two back-pressure
regulators 118, 120, respectively, that maintain the desired system
pressure as indicated by two pressure regulator gauges 122, 124,
respectively. The rate of outflow of dry gas at atmospheric
pressure from regulators 122, 124 is monitored by two flow meters
126, 128, respectively. The oxygen is generally vented to
atmosphere and the hydrogen passed to storage 130. Visual
monitoring of the liquid levels in the separators is by means of
sight glasses 133 attached to them. This allows maintenance of a
pressure differential between the two outlet streams of less than
75 mm of water. This function may be automated by attaching an
infrared level sensor 132 to each of sight glasses 133, each of
which controls its respective solenoid valve 134. By thus venting
gas either from the hydrogen gas/liquid separator or the oxygen
gas/liquid separator it is possible to achieve even tighter control
of the pressure differential.
[0097] Although the pressure in the entire cell stack and
supporting system is high (up to 100 psig), in this embodiment, the
pressure drop due to flow through the system is only a few psi, so
the power of pump 110 required is quite small. The system pressure
may be generated, initially, either by an external, inert gas
supply 140 or by the internally generated hydrogen and oxygen
gases. It is maintained during operation by the evolving gases and
back pressure regulators 118, 120. The same external inert gas
supply may be also used to maintain an identical controlling
pressure in both domes of regulators 128, 129 via lines 142. Stack
100 is maintained at the desired operating temperature by either
heating or cooling the electrolyte externally, or by placing
insulation or cooling fans on stack, 100, per se. The need for
heating or cooling is determined by the heat balance of the entire
system during operation. The electrolysis process generates a
substantial amount of heat, depending on its efficiency.
[0098] A direct electric current is provided from a Xantrax.TM.
power supply by 8 gauge copper cables that are fastened by bolts to
the copper strips (all not shown) that are attached to the two end
electrodes 144, 146. Sufficient voltage is applied to drive the
desired current of 50 or 60 amps through the stack. This provides a
current density on the electrodes of 500 or 600 mA/cm.sup.2.
[0099] A small amount of deionized water 148 may be continuously or
periodically pumped into the system by a diaphragm pump 150 to
replace the water consumed by electrolysis and leaving as water
vapour with the hydrogen and oxygen gases. In the present
embodiment, the potassium hydroxide electrolyte is added to the
system through inlet conduit 152 or drained from it through outlet
conduit 154, when the system is not operating and is at atmospheric
pressure.
[0100] The stack is designed to operate either intermittently or
continuously at pressures between 60 and 100 psig and temperatures
between 60 and 100.degree. C. At a current of 50 amps, the
production rate of hydrogen gas on a dry basis is 913 standard
liters per hour for a stack of 40 cells.
[0101] With reference to FIG. 6, this shows a cell stack 600 of a
plate and frame, filter press design comprising four subunits
according to the invention, wherein each anode 648 and cathode 646
is a single electrode plate. Each sub-unit consists of one side of
each of two non-conductive end walls 612 and a non-conductive
separator frame 614. Walls 612 and frames 614 are each
rectangularly shaped and formed of a glass fibre-filled
polyphenylene oxide and measure 15.0 cm wide by 22.7 cm long, as
described hereinbefore with reference to FIG. 1.
[0102] For very high current density applications, it may be
appropriate to use thicker cathodes 646 and anodes 648. Electrodes
646, 648 are single, rectangular, planar members which conveniently
extend from stack 600 on the front side for 648 anodes and on the
back side for cathodes 646. This configuration allows connection to
an anode bus bar on the front side and a cathode bus bar on the
back side, and has the minimum voltage requirement, since each cell
in the stack is operating in parallel. Other features of the cell
are as hereinbefore described in the details of FIG. 1.
[0103] FIG. 5 shows an elevational view of an end wall 80 of a cell
within a cell stack according to the invention having inlet anolyte
channel 30, inlet catholyte channel 32, outlet spent anolyte
channel 36 and outlet spent catholyte channel 34. Each cell wall on
each side has an o-ring receiving peripheral groove 60 and circular
o-ring receiving recesses 62 for o-rings 64.
[0104] It is clear from FIG. 6, that since each unit is independent
of the other, each internal cell could be reversed such that the
electrode connections could be anode cathode**cathode anode**anode
cathode**. The advantage of this latter configuration is that the
pairs of anode terminals and pairs of cathode terminals are joined
before being connected to the bus bar. This is an advantage for
thin foil electrodes, which do not have much stiffness. Paired
connections would be more robust in an industrial setting.
[0105] With reference to FIG. 7, this shows a similar filter press
cell stack design comprising seven subunits, each having a
non-conductive separator frame 714 similar to that in FIG. 6.
However, FIG. 7 does not include non-conductible end cell-wall
members 612 of FIG. 6, and, accordingly, each anode 748 and cathode
746 functions with both of their sides active. A single anode 738
and a single unitized separator frame 714 mates to the first anode
by means of the O-ring seating described earlier. A single cathode
746 mates to the facing side of the separator frame and completes
the first subunit cell. The obverse side of the single cathode is
also the cathode for the second subunit and mates to the catholyte
flow channel side of the next unitized separator frame 714. The
second anode mates to the anolyte flow channel side of that
unitized separator frame 714 and completes the second sub-unit. The
obverse side of the second anode is the anode for the start
sub-unit 3 and mates to the anolyte flow channel side of the third
unitized separator frame. This connection of cells continues until
the terminal anode which is active on one side only. The advantage
in this configuration is the elimination of the cell end walls. In
comparing FIG. 6 and FIG. 7 it is clear that the design of FIG. 7
provides a seven cell stack in the same space that only four cells
were provided by the design in FIG. 6.
[0106] FIG. 8 shows unitized separator frames in a bipolar cell
stack configuration. Only the end electrodes 800, 810 are connected
to the power supply. The internal electrodes 850 operate in the
bipolar state with one side being anodic and the other side being
cathodic. End electrode 800 is connected to the positive high
voltage side of a dc power supply, while end electrode 810 is
connected to the negative high voltage side of the dc power supply.
The internal 850 electrodes are electrically isolated and do not
need tabs for connection to a bus bar. As is well known, the
advantage of bipolar configuration over a conventional monopolar
configuration is that only two electrical connections are needed
per cell stack at the expense of a higher voltage requirement for
the stack. The voltage requirements for this arrangement depend on
the number of cells in the stack. The seven subunit cell stack
shown in FIG. 8 requires a power supply with about 14V
capability.
[0107] The advantages of the unitized frame separator are equally
conveyed to bipolar cell stack configurations. The combination of
the flow channels and membrane separators as shown as 814 allows a
more compact cell design and reduces the number of seals required.
For example the end wall members and their respective seals are not
required in this configuration. Furthermore because the current is
delivered to the cell stack from the end electrodes and current is
passed through the stack via ions to each electrode face rather
than from the external edge connectors as in monopolar designs
there is less electrolyte resistive heating. Furthermore since the
electrical current passes perpendicularly through the electrode
rather than along its width to the external busbar there is less
metal resistance heating. Less resistive heating in the electrodes
means that higher current densities can be applied to the cell
stack before heating of the polymeric walls and frames in contact
with the electrodes becomes a limitation. The advantages of the
bipolar cell configuration over monopolar configurations with
respect to resistances are well known. The advantage of the
unitized frame separator in this configuration is that it provides
a single compact unit having gas flow balanced flow channels,
membrane separator and inlet and outlet ports that can be readily
combined in a stack. It should be obvious to one skilled in the art
that in order to prevent the mixing of anolyte and catholyte in the
exit manifolds in this bipolar configuration, that a seal must be
provided on one side of each exit manifold as was described in FIG.
1. For the bipolar configuration however there is no endwall frame
member to support the seal. Therefore the electrode member must be
of sufficient stiffness to provide the seal against the unitized
frame separator exit manifold O-ring. For example a slightly
thicker nickel electrode material than that described in FIG. 1 and
which had been hardened could be used for this purpose.
[0108] The aforesaid most preferred monopolar filter press cell
stack is of an overall external rectilinear shape which conserves
space, being more compact, as compared to the usual prior art
cylindrical design. It also provides the following advantages in
consequence of the shapes and configurations used in the cell stack
design.
[0109] The end walls and frames are selected to be as thin as
possible, consistent with the depth required by the O-ring grooves,
to conserve space and increase cell efficiency by more closely
spacing the anode and cathode and, thus, providing for lower cell
resistance.
[0110] The O-rings on either side of the electrodes are offset from
one another to seal against the thin electrode, where it will be
supported by the substance of the plate or frame. Large O-rings are
positioned to include all pressurized parts of the cell within them
so that these are the only seals that must withstand and seal
against any high pressure differential.
[0111] The present invention allows of a small diameter of inlet
manifold to conserve space, yet still functions as a header to
provide equal pressure at the flow control channel to each cell.
This is achieved because the flow control channels have a much
smaller diameter than the inlet manifold. It also allows of flow
control channel diameters and lengths that are sized to produce
desired pressure drops and relative flow rates to the anode and
cathode compartments. It further allows for the stream of
electrolyte leaving the flow control channel to impinge on a
perpendicular surface that diffuses the incoming jet and, thus,
ensures a uniform vertical upward flow of electrolyte over the
electrode surface. A gradual widening of the inlet shelf, in the
general form of an inverted triangle, which is cut as deeply as
possible into the frame also ensures the uniform upward flow of
electrolyte over each of the electrode surfaces within each of the
anolyte and catholyte circulation chambers. In this preferred
embodiment, the gas-evolving area has an aspect ratio of unity to
minimize bubble content of electrolyte, for maximum energy
efficiency, while still ensuring an even flow of electrolyte over
the electrode. The outlet shelves are as large as possible and
empty into the outlet manifolds through apertures that are as large
as possible to minimize any variation in back pressure in the
electrolysis chambers that would result in a differential pressure
across the separators. Similarly, the outlet manifold diameters are
a large as possible.
[0112] The significantly relative thinness of electrodes conserve
space, conserve metal and allow easy assembly with no residual
stresses on the other components of the assembled stack. The nickel
foil electrodes are as large as possible in order to conduct
current from one gas-evolving area to the next with minimum
resistive losses.
[0113] With reference again to FIG. 2, the robust and steady
pressure differential control system of use in the practice of the
present invention is now described.
[0114] The fine control of this pressure system is achieved by use
of an IR light sensor 132 on each of the liquid level site tubes
133. The efficacy of this light detection method is unexpected
since the electrolyte is transparent and, accordingly, blockage of
the light by the electrolyte was not anticipated. When the
sensitivity of the detector was reduced sufficiently to record the
difference between light passing through the empty tube and the
electrolyte-filled tube, ordinary lighting would interfere with the
sensor. However, we discovered that if the light source is angled
sufficiently from the detector, and the detector sensitivity
adjusted, then the electrolyte allows passage of the light beam
when the site glass was filled, but would reflect the light beam if
the liquid meniscus passed through the site glass. With this
configuration for the detector and light source, if the liquid
level fell below the level at the light sensor, then the infrared
beam from the light source was reflected and tripped a relay to
energize the appropriate throttled solenoid valve 134 to cause
bleeding of a requisite small amount of gas from the high pressure
side. The liquid level would then rise and return to the normal
operating level. One advantage of this type of fine control over
that described in the prior art, is that the sensor is outside of
the electrolyte and, thus, not susceptible to corrosion or
blockage. Further, the sensor and controller are easily inspected,
tested and replaced if necessary, without the need to shut down the
system. Testing can be done by blockage of the light beam with a
piece of paper between the light beam and sensor and listening for
the bleed valve activation. The pressure control sensitivity does
not depend on the sensitivity of the sensor, since it is a simple
on-off system. The level detector is easily adjusted by
mechanically moving the light source and detector to the desired
position of the liquid site tube. The high/low signal system is
well understood and has reliable mechanical relays. Fine control of
long term drift is thus easily achieved.
[0115] Thus, the aforesaid pressure differential control system
provides a unique pressure control system up to 6 atmospheres
absolute with a pressure differential of less than 12 mm of
electrolyte, safely and reliably and inexpensively.
[0116] FIG. 9A shows a membrane assembly shown generally as 900
having a ceramic porous body 910 with a nickel anode coating 912 on
one side and a nickel cathode coating 914 on the other to provide a
tricomponent membrane.
[0117] FIG. 9B shows an analogous bicomponent membrane 920 of a
membrane body 922 having a nickel anode coating 924, in combination
with an adjacent planar nickel plate cathode 926.
EXAMPLE
[0118] A partial stack according to FIG. 1 has been operated
typically at 50 A, 67 psig and 95.degree. C. with an electrolyte
flow rate of 0.8 L/min per cell. The voltage required is 2.15 V per
cell indicating an energy efficiency of 69.4% based on the higher
heating value of hydrogen. With a common electrolyte return, the
current efficiency is 97.5% and the hydrogen production rate is
22.2 standard L/h per cell.
[0119] Although this disclosure has described and illustrated
certain preferred embodiments of the invention, it is to be
understood that the invention is not restricted to those particular
embodiments. Rather, the invention includes all embodiments which
are functional or mechanical equivalents of the specific
embodiments and features that have been described and
illustrated.
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