U.S. patent application number 13/823084 was filed with the patent office on 2013-10-10 for oxygen concentration and method.
This patent application is currently assigned to Inotec AMD Limited. The applicant listed for this patent is Derek John Fray, Melvin Frederick Vinton. Invention is credited to Derek John Fray, Melvin Frederick Vinton.
Application Number | 20130264218 13/823084 |
Document ID | / |
Family ID | 43065127 |
Filed Date | 2013-10-10 |
United States Patent
Application |
20130264218 |
Kind Code |
A1 |
Vinton; Melvin Frederick ;
et al. |
October 10, 2013 |
OXYGEN CONCENTRATION AND METHOD
Abstract
An oxygen concentrator is for generating a flow of oxygen by
electrolysis of atmospheric humidity. It comprises a cathode (24)
and an anode (26) contacting opposite sides of a proton-conducting
membrane (12). A catalytic apparatus (14) comprises a diffusion
layer (28) which spaces a catalyst (30) from the cathode. The
cathode and the catalytic apparatus are contained within a cathode
chamber which comprises a ventilation means (44) for allowing a
controlled flow of air to the catalyst. In operation water is
electrolysed at the anode and hydrogen generated at the cathode
flows through the diffusion layer to the catalyst, where it reacts
with atmospheric oxygen to form water which flows back to the
proton-conducting membrane for further electrolysis.
Inventors: |
Vinton; Melvin Frederick;
(Cambridgeshire, GB) ; Fray; Derek John;
(Cambridge, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Vinton; Melvin Frederick
Fray; Derek John |
Cambridgeshire
Cambridge |
|
GB
GB |
|
|
Assignee: |
Inotec AMD Limited
Cambridge
GB
|
Family ID: |
43065127 |
Appl. No.: |
13/823084 |
Filed: |
September 13, 2011 |
PCT Filed: |
September 13, 2011 |
PCT NO: |
PCT/GB11/01343 |
371 Date: |
May 21, 2013 |
Current U.S.
Class: |
205/628 ;
204/228.1; 204/266 |
Current CPC
Class: |
C01B 13/0207 20130101;
C01B 5/00 20130101; Y02E 60/366 20130101; A61H 2033/143 20130101;
Y02E 60/36 20130101; C25B 1/10 20130101; A61M 35/30 20190501; A61H
2201/165 20130101 |
Class at
Publication: |
205/628 ;
204/266; 204/228.1 |
International
Class: |
C25B 1/10 20060101
C25B001/10 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 13, 2010 |
GB |
1015265.0 |
Sep 14, 2010 |
GB |
1015368.2 |
Claims
1. An oxygen concentrator comprising; a proton-conducting membrane;
a cathode contacting a first side of the membrane; an anode
contacting a second side of the membrane; a catalytic apparatus
comprising a catalyst and a diffusion layer, the diffusion layer
spacing the catalyst from the cathode; and a housing defining a
cathode chamber, the catalytic apparatus being contained within the
cathode chamber and the housing comprising a ventilation means for
allowing air to flow to the catalyst.
2. An oxygen concentrator according to claim 1, in which the
ventilation means is arranged to control the flow of air into or
through the cathode chamber.
3. An oxygen concentrator according to claim 1 or 2, in which the
ventilation means allows ventilation of the cathode chamber between
predetermined upper and/or lower limits.
4. An oxygen concentrator according to claim 3, in which
ventilation above the predetermined lower limit allows sufficient
atmospheric oxygen to reach the catalyst to react with more than
90%, preferably more than 95% and particularly preferably more than
99%, of hydrogen generated at the cathode during use of the
concentrator.
5. An oxygen concentrator according to claim 3 or 4, in which
ventilation below the predetermined upper limit allows less than
15%, preferably less than 5%, and particularly preferably less than
1%, of water generated at the catalyst by reaction of hydrogen with
atmospheric oxygen to be carried away from the catalyst by the
ventilating airflow.
6. An oxygen concentrator according to claim 3, 4, or 5, in which
ventilation below the predetermined upper limit allows more than
85%, preferably more than 95%, and particularly preferably more
than 99%, of water generated by reaction of hydrogen with
atmospheric oxygen at the catalyst to pass through the diffusion
layer to the cathode and the proton-conducting membrane.
7. An oxygen concentrator according to any preceding claim, in
which the ventilation means comprises one or more vents defined
through a wall of the housing.
8. An oxygen concentrator according to any preceding claim, in
which the area of the catalyst and of the cathode are between 150
mm.sup.2 and 2000 mm.sup.2, preferably between 300 mm.sup.2 and
1000 mm.sup.2, and particularly preferably between 400 mm.sup.2 and
600 mm.sup.2.
9. An oxygen concentrator according to any preceding claim, in
which the catalyst is substantially planar, and the cathode chamber
has a depth, measured perpendicular to the plane of the catalyst,
of between 0.4 mm and 10 mm, preferably between 0.5 mm and 7 mm,
and particularly preferably between 0.6 mm and 3 mm.
10. An oxygen concentrator according to any preceding claim, in
which the catalyst is substantially planar and has a lateral
dimension, and in which the cathode chamber has a depth, measured
perpendicular to the plane of the catalyst, of between 0.1 and
0.015 times, and preferably of between 0.04 and 0.02 times, the
lateral dimension.
11. An oxygen concentrator according to any preceding claim, in
which the catalyst has a lateral dimension of between 10 mm and 50
mm.
12. An oxygen concentrator according to any preceding claim, in
which the ventilation means comprises one or more vents defined
through a wall of the housing and in which the total area of the
vent or vents is between 7 mm.sup.2 and 80 mm.sup.2, preferably
between 10 mm.sup.2 and 40 mm.sup.2 and particularly preferably
between 12 mm.sup.2 and 20 mm.sup.2.
13. An oxygen concentrator according to any preceding claim, in
which the ventilation means comprises one or more vents defined
through a wall of the housing and in which the area of the catalyst
is between 10 and 70 times, preferably between 25 and 55 times, and
particularly preferably between 30 and 45 times, the total area of
the vent or vents.
14. An oxygen concentrator according to any preceding claim in
which the ventilation means comprises one or more vents defined
through a wall of the housing and in which the area of the cathode
is between 10 and 70 times, preferably between 25 and 55 times, and
particularly preferably between 30 and 45 times, the total area of
the vent or vents.
15. An oxygen concentrator according to any preceding claim, for
operation at a current density between 50 Am.sup.-2 and 250
Am.sup.-2, preferably between 75 Am.sup.-2 and 200 Am.sup.-2 and
particularly preferably between 100 Am.sup.-2 and 150
Am.sup.-2.
16. An oxygen concentrator according to any preceding claim, for
operation at a voltage of between 0.75V and 2V, preferably between
1V and 1.5V, and particularly preferably about 1.2V.
17. An oxygen concentrator according to any of claims 1 to 15, for
operation at a voltage of between 0.75V and 2V, preferably between
0.8V and 1.2V, and particularly preferably at about 1.0V.
18. An oxygen concentrator according to any preceding claim, for
producing a flow of oxygen gas from the anode of less than 30
ml/hour (measured at atmospheric or ambient pressure) for each 500
mm.sup.2 of the area of the anode.
19. An oxygen concentrator according to any preceding claim, in
which the cathode and/or the anode are in the form of layers coated
onto opposite sides of the proton-conducting membrane.
20. An oxygen concentrator according to any preceding claim, in
which the catalyst is in the form of a layer coated onto the
diffusion layer.
21. An oxygen concentrator according to claim 1, 19 or 20, in which
the catalyst, the diffusion layer, the cathode, the
proton-conducting membrane and the anode are in the form of two or
more layers pressed together.
22. An oxygen concentrator according to claim 21, in which the
layers are pressed together between a cathode-side conducting sheet
and an anode-side conducting sheet for the supply of electric
current to the layers.
23. An oxygen concentrator according to claim 21 or 22, in which a
pressure of more than 0.5 MPa, preferably more than 0.8 MPa, and
particularly preferably more than 0.9 MPa is applied to press the
layers together.
24. An oxygen concentrator according to any preceding claim, in
which the catalyst, the diffusion layer, the cathode, the
proton-conducting membrane and the anode area in the form of two or
more layers pressed together by a pressure which is applied by
retaining the layers within the housing, stacked together with a
compressed layer of a compressible, preferably resilient,
material.
25. An oxygen concentrator according to any preceding claim, in
which a continuous oxygen flow of at least 24 l/hr/m.sup.2 of the
area of the catalyst can be produced, measured at NTP, when a
constant current density is applied to the oxygen concentrator in
air of equal to or greater than 35% relative humidity.
26. An oxygen concentrator according to claim 25, in which the
constant current density is less than 150 Am.sup.-2 or 120
Am.sup.2, or preferably about 110 Am.sup.-2.
27. An oxygen concentrator according to any preceding claim, which
does not have a water reservoir.
28. An oxygen supply unit comprising an oxygen concentrator as
defined in any preceding claim, a power supply such as a
rechargeable battery, and an oxygen outlet coupled to an anode side
of the oxygen concentrator.
29. An oxygen supply unit according to claim 28, which is wearable
or ambulatory and is couplable to a hyperbaric dressing.
30. An oxygen supply unit according to claim 28 or 29, in which the
power supply supplies a predetermined current to the oxygen
concentrator and switches to a stand-by condition if the voltage
required to drive the predetermined current rises above a
predetermined voltage level.
31. An oxygen supply unit according to claim 30, in which the
predetermined current corresponds to a current density across the
catalyst or the MEA equal to or less than 150 Am.sup.-2 or 120
Am.sup.-2, or preferably of about 100 Am.sup.-2.
32. An oxygen supply unit according to claim 30 or 31, in which the
predetermined voltage level is 2.0V, or 1.5V, or 1.2V.
33. A method for concentrating oxygen from air, comprising the
steps of; providing a cathode and an anode on opposite sides of a
proton-conducting membrane, a diffusion layer adjacent to the
cathode and a catalyst spaced from the cathode by the diffusion
layer; allowing access of air to the catalyst through a ventilation
means; passing a current between the cathode and the anode to
electrolyse water derived from humidity in the air and from the
catalyst, to produce hydrogen at the cathode and oxygen at the
anode; and reacting the hydrogen with atmospheric oxygen at the
catalyst to produce water, to pass through the diffusion layer to
the cathode for further electrolysis; in which the ventilation
means controls the access of air to the catalyst between
predetermined upper and lower ventilation rates.
34. A method according to claim 33, for electrolysing water derived
only from humidity in the air and from the catalyst.
35. An oxygen concentrator substantially as described herein with
reference to the drawings.
36. An oxygen supply unit substantially as described herein with
reference to the drawings.
37. A method for concentrating oxygen substantially as described
herein with reference to the drawings.
Description
[0001] The invention relates to an oxygen concentrator and a method
for concentrating oxygen from air, such as ambient air.
[0002] There is evidence in the literature that the supply of
oxygen to a wound site can promote the healing of the wound. This
applies to both humans and animals. This technique is know as
topical oxygen therapy, and encourages the growth of fresh skin
tissue to close and heal wounds.
[0003] In most instances, the affected limb is placed in a chamber
(U.S. Pat. No. 4,003,371 and U.S. Pat. No. 3,744,491) or a bag
(U.S. Pat. No. 5,154,697 and U.S. Pat. No. 5,478,310) and oxygen is
fed to the chamber or bag from an oxygen cylinder. This is
impractical for many patients as it restricts the mobility of the
patient. To improve topical oxygen therapy, it would be desirable
to have a more convenient supply of oxygen.
[0004] In an alternative approach, as described in U.S. Pat. No.
5,578,022, U.S. Pat. No. 5,788,682 and U.S. Pat. No. 5,855,570,
electrolytic devices may be incorporated into or above bandages
that are placed over the wound. The cathode of the electrolytic
device is exposed to air to form hydrogen peroxide that dissolves
in a proton-conducting membrane adjacent the cathode. The hydrogen
peroxide diffuses through the membrane to an anode where it is
decomposed to form water and oxygen that is transmitted to the
wound. The presence of hydrogen peroxide is a significant problem
because it can kill healthy cells. It is therefore desirable to be
able to produce pure oxygen for healing of wounds without the use
of a hydrogen peroxide intermediate. Unfortunately, all known
proton-conducting membranes are highly acidic, and under these
conditions hydrogen peroxide is formed when oxygen is ionised at a
cathode.
[0005] An alternative approach to electrolytic oxygen generation is
to electrolyse water to produce hydrogen and oxygen, and to prevent
oxygen from contacting the cathode. This avoids the production of
hydrogen peroxide. WO 2006/092612 describes in one embodiment an
electrolytic oxygen generator in which water is supplied from a
source, or reservoir, of water to a proton-conducting membrane. The
water is electrolysed between a cathode and an anode on either side
of the membrane to form oxygen at the anode and hydrogen at the
cathode. A catalytic apparatus is provided adjacent to the cathode
in order to avoid releasing hydrogen from the oxygen generator into
the atmosphere, and in order to decrease the consumption of water
by the oxygen generation apparatus. The catalytic apparatus
comprises a catalyst layer spaced from the cathode by a
gas-permeable/liquid-permeable membrane. The catalyst layer is
exposed to air. Hydrogen generated at the cathode diffuses through
the gas-permeable/liquid-permeable membrane to the catalyst, where
it reacts with atmospheric oxygen to form water. Thus, the hydrogen
is not released into the atmosphere. At least some of the water
diffuses back through the gas-permeable/liquid-permeable membrane
to the cathode and the proton-conducting membrane, to supplement
the water provided from the water source.
[0006] The inventors have tested an electrolytic oxygen generator
of this type, implemented as a portable, ambulatory apparatus that
can be worn by a patient. The oxygen supply from the oxygen
generator is fed to a suitably-designed hyperbaric dressing
covering a wound for treatment. In these tests, the oxygen
generator has been powered by rechargeable batteries, and it has
been found that the water source needs to be refilled significantly
more often than the batteries need to be recharged.
[0007] It would be desirable for a portable oxygen generator to
operate for as long as possible without needing either the
batteries recharged or the water source replenished. The present
invention aims to address this problem.
SUMMARY OF INVENTION
[0008] The invention provides an oxygen concentrator, a method of
concentrating oxygen, and an oxygen supply unit as defined in the
appended claims, to which reference should now be made.
[0009] The invention provides an oxygen concentrator, rather than
an oxygen generator. This reflects the fact that the apparatus
according to the invention does not have, and does not need, a
water reservoir.
[0010] In a first aspect, the invention may thus advantageously
provide an oxygen concentrator comprising a proton-conducting
membrane; a cathode and an anode contacting first and second sides
of the membrane; a catalytic apparatus comprising a catalyst and a
diffusion layer, the diffusion layer spacing the catalyst from the
cathode; and a housing defining a cathode chamber. The catalytic
apparatus is contained within the cathode chamber and the housing
comprises a ventilation means for allowing air to flow to the
catalyst.
[0011] In use, a voltage is applied between the cathode and the
anode, to electrolyse water in the proton-conducting membrane. The
water is initially supplied through atmospheric humidity.
Electrolysis produces oxygen at the anode and hydrogen at the
cathode. The hydrogen passes through the diffusion layer (or
permeable layer) spacing the catalyst from the cathode. At the
catalyst, the hydrogen reacts with atmospheric oxygen within the
cathode chamber, to produce water. The water passes back through
the diffusion layer to the cathode and the proton-conducting
membrane for further electrolysis.
[0012] Surprisingly, the inventors have found that the oxygen
concentrator can operate continuously, with no water supply other
than atmospheric humidity, even at low humidity levels. In other
words, the oxygen concentrator advantageously does not need a water
reservoir and can electrolyse water derived only from humidity in
the air and from the catalyst. As described above, electrolytic
devices for concentrating oxygen from the atmosphere for topical
oxygen therapy have been proposed, but in these prior art devices
the cathode has been exposed to air to form hydrogen peroxide that
dissolves in a proton-conducting membrane adjacent to the cathode.
As described above, the creation of hydrogen peroxide is a
significant problem. By contrast, the present invention provides an
oxygen concentrator which operates by electrolysis of water, and in
which the cathode is advantageously not exposed to atmospheric
oxygen. The inventors have found that the provision of a catalyst
apparatus positioned very close to or directly adjacent to the
cathode converts the cathodically-generated hydrogen to water, and
recycles the water to the proton-conducting membrane, with such
little loss of water that the oxygen concentrator embodying the
invention can operate without a water reservoir. This is completely
contrary to the expectation and technical prejudice of the skilled
person, who would expect a water reservoir to be necessary for the
continuous generation of oxygen by electrolysis of water.
[0013] The performance of the oxygen concentrator is enhanced by
the action of the ventilation means allowing air to flow to the
catalyst, so that the cathodically-generated hydrogen can react
with atmospheric oxygen.
[0014] In a preferred embodiment which may improve the performance
of the oxygen concentrator still further, it is desirable that the
ventilation means is arranged to control the flow of air into or
through the cathode chamber in such a way that the ventilation of
the cathode chamber is between predetermined upper and lower
limits. The inventors have determined experimentally that a level
of ventilation, or a level of air flow or air diffusion, between
upper and lower limits enables enhanced operation of the
concentrator. It is found that if ventilation of the cathode
chamber falls below a predetermined lower limit or rises above a
predetermined upper limit, the resistance of the electrolytic cell
increases. But high efficiency of the oxygen concentrator may be
continuously maintained if the ventilation of the cathode chamber
is between predetermined upper and lower limits. Good electrical
efficiency to produce a desired oxygen flow is desirable to enable
an oxygen concentrator, such as a portable oxygen concentrator, to
operate for as long as possible from a self-contained source of
electrical power, such as a battery.
[0015] The electrochemical processes involved in the decrease in
efficiency of operation outside the optimum range of ventilation
are not fully understood. Nevertheless, the inventors believe that
the operation of the oxygen concentrator is as follows.
[0016] If ventilation of the cathode chamber is less than a
predetermined level, then it is believed that insufficient
atmospheric oxygen reaches the catalyst to react with all of the
hydrogen generated at the cathode. In that case, hydrogen may be
released into the atmosphere and recycling of water from the
catalyst to the proton-conducting membrane may be reduced.
Consequently, the electrolysis of water may slow down and
ultimately stop.
[0017] If the ventilation of the cathode chamber is above a
predetermined level, then substantially all of the hydrogen
generated at the cathode may react with atmospheric oxygen at the
catalyst to produce water. However, some of the water may enter (or
evaporate into) the air ventilating the cathode chamber and be
carried away from the catalyst into the atmosphere. This reduces
the amount of water recycled to the proton-conducting membrane, so
that the rate of electrolysis may slow down. It is believed that
this effect may vary depending on the humidity of the ambient air,
or ambient atmosphere. If ambient humidity is very high, then the
amount of water lost to the atmosphere may be reduced, and more of
the water present as humidity in the atmosphere may be drawn into
the diffusion layer and pass through the layer to the proton
conducting membrane. By contrast, if ambient humidity is very low,
then more water may be lost from the catalyst into the atmosphere,
and less humidity may be absorbed into the diffusion layer and
passed to the proton-conducting membrane.
[0018] Within the desired range of ventilation of the cathode
chamber, sufficient atmospheric oxygen may reach the catalyst to
react with substantially all of the hydrogen produced at the
cathode, and an advantageously large proportion of the water may
pass back through the diffusion layer to the proton-conducting
membrane for further electrolysis. Preferably none, or only a very
small fraction, of the water may be lost into the air ventilating
the cathode chamber.
[0019] It is believed that the catalytic apparatus may operate by
virtue of diffusion gradients across its thickness. Because
hydrogen is converted to water at the catalyst, hydrogen produced
at the cathode may diffuse down a diffusion gradient from the
cathode to the catalyst, and because water is electrolysed at the
proton-conducting membrane, water produced at the catalyst may
diffuse down a diffusion gradient towards the proton-conducting
membrane. The ventilation of the cathode chamber should therefore
be sufficiently high to allow reaction of substantially all of the
hydrogen reaching the catalyst, and should be sufficiently low that
water generated at the catalyst tends to diffuse towards the
proton-conducting membrane rather than being lost into the
atmosphere.
[0020] The performance of the oxygen concentrator might be expected
to vary depending on the humidity of the ambient air. In practice,
it has been found by the inventors that the oxygen generated by the
oxygen concentrator is at substantially the same humidity as the
ambient air. It is therefore understood that the combination of
ambient humidity entering the cathode chamber and the efficient
recycling of water by the catalytic apparatus are sufficient to
enable continuous oxygen generation at a very wide range of ambient
humidity. The concentrator may not be able to operate at extremely
low ambient humidity (for example in tests of one oxygen
concentrator embodying the invention, the inventors obtained
satisfactory operation down to 37% relative humidity, though design
improvements may allow operation at lower relative humidity, such
as 25% or even 10%), but it may be noted that an advantageous
application of the concentrator is to produce oxygen in an
ambulatory apparatus for topical oxygen therapy. The inventors have
found that if the oxygen concentrator is worn within clothing, then
the proximity to the wearer's body means that the humidity may be
sufficient to allow continuous oxygen generation at a predetermined
flow rate even if the ambient humidity level outside the clothing
is disadvantageously low.
[0021] Advantageously, because the oxygen concentrator operates by
electrolysing water and prevents the access of atmospheric oxygen
to the cathode (by virtue of the presence of the catalytic
apparatus), hydrogen peroxide is not formed.
[0022] The oxygen concentrator may in principle be of substantially
any size but its use is particularly advantageous in a portable
device. In a portable, or ambulatory, device the elimination of the
need to provide and replenish a water reservoir means that the
concentrator can operate advantageously for an extended period of
time. The period of operation is limited only by the power source
and it has been found that a conveniently-sized portable unit using
rechargeable batteries can operate for a week or more before the
batteries need to be recharged.
[0023] A portable oxygen concentrator may comprise a
proton-conducting membrane having an active area, between the
cathode and the anode, of between 150 mm.sup.2 and 2000 mm.sup.2,
preferably between 300 mm.sup.2 and 1000 mm.sup.2, and particularly
preferably between 400 mm.sup.2 and 600 mm.sup.2. For example, the
active area of the proton-conducting membrane may be a circle of 25
mm diameter. (The areas and shapes of the anode, cathode, diffusion
layer and catalyst may advantageously all be the same as the active
area and shape of the membrane.) Such a cell operating at a voltage
of about 1.2V and a current of 60 mA can produce oxygen
continuously at about 15 ml/hr. This is a convenient rate of oxygen
supply for provision to a suitable hyperbaric dressing covering a
wound.
[0024] The proton-conducting membrane, the cathode, the anode, the
catalyst and the diffusion layer may all be in the form of planar
layers, in contact with each other to form a stack of layers.
Advantageously, the cathode and the anode may be in the form of
layers coated on the first and second sides of the
proton-conducting membrane to form a Membrane Electrode Assembly
(MEA), and the catalyst may be in the form of a layer coated onto
one side of the diffusion layer. In the assembled oxygen
concentrator, the diffusion layer may then be pressed into contact
with the cathode.
[0025] It is necessary to provide a voltage between the cathode and
the anode. In a preferred embodiment, this may be achieved by
placing the stack of layers described above between a cathode-side
conducting sheet and an anode-side conducting sheet, to which a
supply of electricity can be connected. In this embodiment, the
diffusion layer must be electrically conducting in order to carry
the electric current from the cathode-side conducting sheet to the
cathode.
[0026] The cathode-side conducting sheet should be porous or
perforated in order to allow air to flow to the catalyst.
Similarly, the anode-side conducting sheet should be porous or
perforated in order to allow oxygen to flow away from the anode.
Both conducting sheets may conveniently be implemented as
perforated or foraminous metal sheets. These should be corrosion
resistant, for example stainless steel sheets.
[0027] Advantageously the cathode-side and anode-side conducting
sheets may be pressed against the opposite sides of the stack of
layers discussed above, and may be of a predetermined area and
shape corresponding to the active area and shape of the
proton-conducting membrane. The internal cross-sectional area and
shape of the cathode chamber may advantageously be substantially
the same as the area and shape of the active portion of the
membrane (with clearance provided so that the catalytic apparatus
and other components can fit into the cathode chamber). The depth
of the cathode chamber, measured perpendicular to the membrane,
should be sufficient to accommodate the catalytic apparatus and any
cathode-side conductive sheet, and to allow sufficient ventilation
such that air can flow to the catalyst.
[0028] The cathode chamber may therefore have a depth greater than
0.4 mm, 0.6 mm 1 mm or 2 mm, and less than 10 mm, 7 mm or 5 mm.
[0029] Hydrogen is generated across the entire active area of the
proton-exchange membrane, and may diffuse directly through the
diffusion layer from the cathode to the catalyst. Therefore, it is
important that the entire area of the catalyst should be
ventilated.
[0030] In order to allow this, the ventilation means may
advantageously comprise one or more vents defined through a wall of
the housing of the cathode chamber. The level of ventilation in the
cathode chamber may be related to the total area of the vent or
vents. For a typical oxygen concentrator embodying the invention,
the total area of the vent or vents may be between 7 mm.sup.2 and
80 mm.sup.2, preferably between 10 mm.sup.2 and 40 mm.sup.2 and
particularly preferably between 12 mm.sup.2 and 20 mm.sup.2.
[0031] In addition to the provision of a vent or vents, the cathode
chamber may be filled with a porous material, such as a foam or a
sintered material. This may advantageously allow air to flow or
diffuse through the cathode chamber but may act as a baffle to
moderate the ventilation to prevent the bulk flow of air, or
draughts of air, through the cathode chamber.
[0032] As described above, the oxygen concentrator may be
implemented in a range of different sizes. Thus, the ventilation
means may alternatively be defined in terms of the size of the
oxygen concentrator and, in particular, the active area of the
proton-conducting membrane. Thus, the active area of the membrane
may be between 10 and 70 times, preferably between 25 and 55 times,
and particularly preferably between 30 and 45 times the total area
of the vent or vents.
[0033] The ventilation of the cathode chamber may be affected by
its shape. Thus, for advantageous performance of the ventilation
means, the depth of the cathode chamber may be defined as being
proportional to a lateral dimension, or a maximum lateral
dimension, of the cathode chamber. For example, the cathode chamber
may have a lateral dimension which is between 10 and 70 times its
depth or preferably between 25 and 50 times its depth.
[0034] In a preferred embodiment, in order to achieve ventilation
across the entire area of the catalyst, the ventilation means may
be distributed across a lateral dimension of the cathode chamber.
For example the ventilation means may be in the form of two or more
vents spaced at different positions in a wall of the housing.
[0035] The inventors have found that optimum operation of the
oxygen concentrator is preferably achieved at an applied voltage of
between 0.75V and 2V, more preferably between 1V and 1.5V, and
particularly preferably about 1.2V. These are voltages applied to
the cell, for example between cathode-side and anode-side
conducting sheets as described above. The cell potential measured
between the cathode and the anode may be less than the applied cell
voltage but the inventors have measured cell efficiencies of more
than 80%, or more than 90%, and so it is believed that voltage
losses within the cell are small.
[0036] Corresponding to the voltages described above, the oxygen
concentrator may advantageously operate at a current density over
the active area of the proton-conducting membrane, of between 50
Am.sup.-2 and 250 Am.sup.-2, preferably between 75 Am.sup.-2 and
200 Am.sup.-2, and particularly preferably between 100 Am.sup.-2
and 150 Am.sup.-2.
[0037] It has been found that these electrical conditions may
advantageously produce a flow of oxygen gas from the anode of up to
30 ml/hour (measured at atmospheric or ambient pressure) for each
500 mm.sup.2 of the area of the anode. In practice, for topical
oxygen therapy, in a preferred embodiment oxygen may be produced at
a slightly hyperbaric pressure, for example at about 50 mbar above
atmospheric pressure. In more general terms, the anode chamber may
operate at a different pressure from the cathode chamber. The anode
chamber and the cathode chamber should therefore be hermetically
isolated from each other, so as to prevent flow of gas between
them. This provides the further advantage of preventing oxygen
generated at the anode from contacting the cathode, which could
disadvantageously lead to formation of hydrogen peroxide.
[0038] In order to enable operation without a water reservoir, the
efficiency of the oxygen concentrator is preferably as high as
possible, both in terms of electrical performance and in terms of
water recycling from the catalyst to the proton-conducting
membrane. In a preferred embodiment, the structure of the oxygen
concentrator may therefore be as follows. The cathode and/or the
anode may be in the form of layers coated onto opposite sides of
the proton-conducting membrane to form a MEA. The catalyst may be
in the form of a layer coated onto the diffusion layer.
Advantageously, the diffusion layer of the catalytic apparatus may
then be pressed against the cathode. In other words, the catalyst,
the diffusion layer, the cathode, the proton-conducting membrane
and the anode may advantageously be in the form of layers pressed
together. This may advantageously position the catalyst as close as
possible to, but separated from, the cathode.
[0039] The layers may be pressed together between a cathode-side
conducting sheet and an anode-side conducting sheet for the supply
of electric current to the layers. The inventors have found that a
high pressure urging the layers together may advantageously be
used. For example, a pressure of more than 0.5 MPa, preferably more
than 0.8 MPa, and particularly preferably more than 0.9 MPa may be
applied to urge the conducting sheets towards each other and to
compress the layers of the cell together. This means, for example,
that the layers of a cell in which the active area of the
proton-conducting membrane is about 500 mm.sup.2 should
particularly preferably be pressed together with a force of about
450 Newtons. In the inventor's experiments, it has been found that
at lower pressures, the electrical resistance of the cell is
increased, while at higher pressures little further decrease in
electrical resistance is obtained.
[0040] The layers may be loaded or pressed together in any
convenient manner. For example, an oxygen concentrator may
conveniently be housed within a rigid housing and resilient
elements positioned between the housing and the stack of layers to
apply the required pressure. Suitable resilient elements may
comprise O-rings or elastomeric foams, such as polyurethane foams.
Electrically-conductive foams may be used.
[0041] In the construction of an oxygen concentrator it is
important that the materials used should be sufficiently corrosion
resistant.
[0042] To generate oxygen, the oxygen concentrator should be
coupled to a suitable electrical power supply. In a preferred
embodiment, the oxygen concentrator may be operated in a
current-controlled mode. The applied current corresponds to the
rate of oxygen generation at the anode and so a user may set the
power supply to apply a predetermined constant current to the cell
depending on the desired oxygen flow rate. A corresponding voltage
is then required to drive the selected current. If, in use, the
oxygen concentrator fails to operate normally, its electrical
resistance will tend to rise, so that the required voltage to drive
the selected current will also rise. In a preferred embodiment, the
power supply is arranged so that if the required voltage rises
above a predetermined voltage threshold, the power supply switches
to a stand-by mode, in which the applied voltage is greatly reduced
or switched off. As described above, a failure mode of the cell may
be if the ambient atmosphere is of extremely low humidity. In that
case, the water content of the proton-conducting membrane may
progressively reduce and the resistance of the cell may rise.
[0043] A further aspect of the invention may thus provide an oxygen
supply unit comprising an oxygen concentrator, a battery and a
power supply for providing a predetermined current, for example a
user-selected current, to the oxygen concentrator. The power supply
may therefore be a constant-current power supply. The oxygen supply
unit may be portable, or wearable, for example for use for
ambulatory topical oxygen therapy.
[0044] As described above, an oxygen concentrator embodying the
invention advantageously comprises a cathode chamber having a
ventilation means which controls the flow or diffusion of air from
the atmosphere to the catalyst such that the concentrator can
produce a continuous flow of oxygen at a wide range of ambient
humidity. As noted above, the inventors' experiments have
demonstrated the continuous generation of oxygen down to 37%
relative humidity, or even 35% relative humidity. As also described
in this document, the ventilation means should provide a level of
ventilation between upper and lower limits, and if the ventilation
of the cathode chamber falls below a predetermined lower limit or
rises above a predetermined upper limit, the electrical resistance
of the electrolytic cell may disadvantageously increase. In more
general terms, this means that the resistance per unit area of the
cell (the area of the catalyst or of the Membrane Electrode
Assembly) may increase.
[0045] It is usually desirable to operate an oxygen concentrator so
as to generate a constant flow of oxygen at a desired flow rate.
The oxygen flow rate is related to the current flowing through a
cell and so it is commonly desirable to operate a cell at a
predetermined constant current. In this case, if the cell
resistance rises, a rise in the voltage applied to the cell is
observed. If the resistance of the cell increases, initially the
cell may continue to operate but at reduced efficiency, and if the
resistance rises too far, the oxygen output falls and eventually
the MEA may be damaged unless the applied current is reduced.
[0046] The electrical performance of an oxygen concentrator at a
known relative humidity level may thus provide a means for
assessing whether or not the level of ventilation at the catalyst
is between the upper and lower ventilation limits. In an oxygen
concentrator embodying this aspect of the invention, the
optimisation of the ventilation of the catalyst as described above
may advantageously increase, or maximise, the flow rate of oxygen
that can be produced continuously. As a result, the inventors have
found that oxygen concentrators having the following performance
have ventilation means that achieves ventilation of the catalyst
between the upper and lower limits identified above.
[0047] For such a cell in air of relative humidity equal to or
greater than 35%, a constant current density can be applied such
that, after any initial voltage transient, the cell produces a
continuous oxygen flow equal to or greater than 24 litres per hour
per m.sup.2 of the area of the catalyst (or of the cathode or MEA)
at NTP (normal temperature and pressure, 25 C and 1 atmosphere).
For experimental purposes a continuous oxygen flow may be
considered to be a flow maintained for 3 hours or more. An oxygen
concentrator achieving this performance must meet the requirement
that the ventilation of the catalyst is between the upper and lower
limits described above with reference to embodiments of the present
invention. If the ventilation were above the upper limit then
excessive water would be lost to the atmosphere, and if the
ventilation were below the lower limit then insufficient
atmospheric oxygen may reach the catalyst to convert to water all
of the H.sub.2 generated at the cathode. In either case the
performance of the cell would decrease over time until the oxygen
flow falls below the threshold level of 24 l/hr/m.sup.-2. At the
same time, the cell resistance would increase and the voltage
applied to the cell would correspondingly increase. An alternative
assessment of an unacceptable decrease in cell performance may thus
be made with reference to the increase in voltage required to drive
the predetermined constant current (after any initial voltage
transient period when the cell is switched on, which typically
lasts only 30 seconds or 1 minute at most). For example an increase
of the operating voltage of 20%, or 30%, for example within the
first 10 minutes or 20 minutes of operation, may be considered
unacceptable. Such an increase in operating voltage may therefore
indicate that the ventilation of the catalyst is outside the upper
and lower limits described above.
[0048] The value of the current density required to achieve the
oxygen flow equal to or greater than 24 l/hr per m.sup.2 of the
catalyst (or of the cathode or MEA), and the voltage applied to the
cell to achieve that current density, may vary depending on factors
other than the ventilation of the catalyst, such as the electrical
resistance of the cell and the thickness of components such as the
proton-conducting membrane. But in a cell having low electrical
resistance and an advantageously thin proton-conducting membrane,
and in which the distance between the catalyst and the cathode is
advantageously small (as described in the specific embodiments of
the invention hereinafter), then the minimum constant current
density to generate an oxygen flow of 24 l/hr/m.sup.2 at NTP is
about 110 Am.sup.-2 (based on the area of the cathode or the MEA,
or of the catalyst). This corresponds to a constant current of
about 55 mA applied to a cell having a circular catalyst of 25 mm
diameter (490 mm.sup.2) producing at least about 12 ml/hr of oxygen
at NTP.
[0049] As the skilled person would be aware, the Faraday equation
enables calculation of a maximum number of moles per unit time of
oxygen gas, and therefore a maximum oxygen flow rate at a given
temperature and pressure, such as at NTP, that can be produced by a
given current flowing through an oxygen concentrator. The
generation of 24 l/hr/m.sup.2 at 110 Am.sup.2 described above is
above about 95% of the maximum oxygen flow at this current, and so
such a cell may be considered to be more than 95% efficient. Oxygen
concentrators having ventilation of the catalyst within the desired
range described above may produce a continuous flow of oxygen at
more than 85%, preferably more than 90%, and particularly
preferably more than 92% or 95% efficiency.
[0050] Preferably, this performance is achieved at a cell voltage
of between 0.8V and 1.2V, or about 1.0V.
[0051] In a cell of this type, other metrics may also be used to
assess whether the ventilation of the catalyst is within the
prescribed range. For example the multiple of voltage times the
catalyst area (or the cathode or active MEA area), should not rise
above 6Vcm.sup.2, and/or the value of the voltage divided by the
current times the catalyst area (or the cathode or active MEA area)
should not rise above 120 Ohmcm.sup.2.
[0052] A cell having the preferred ventilation of the catalyst
described above may be able to produce a constant oxygen flow at a
lower relative humidity than mentioned above, such as at less than
37% or 35% relative humidity, but the flow may be less than 24
l/hr/m.sup.2 and the cell may only be able to operate continuously
at a reduced current.
[0053] The invention has been described in the context of topical
oxygen therapy, but as the skilled person would appreciate it may
be used in any application where a suitable flow of pure oxygen is
required.
[0054] Specific embodiments of the invention will now be described
by way of example, with reference to the accompanying drawings, in
which:
[0055] FIG. 1 is an exploded view of an oxygen concentrator
according to a first embodiment of the invention;
[0056] FIG. 2 is a three-quarter view of a cell base of the oxygen
concentrator of FIG. 1;
[0057] FIG. 3 is a plan view, from beneath, of the cell base;
[0058] FIG. 4 is a three-quarter view of a cell cover of the oxygen
concentrator of FIG. 1;
[0059] FIG. 5 is a plan view, from beneath, of the cell cover;
[0060] FIG. 6 is a partial exploded view of the oxygen concentrator
of FIG. 1, showing transverse sections of the cell base, the cell
cover, an O-ring and a MEA oriented for assembly;
[0061] FIGS. 7 and 8 are plan and side views of resilient
polyurethane foam layers of the oxygen concentrator of FIG. 1;
[0062] FIG. 9 is a plan view of a cathode-side conductive layer of
the oxygen concentrator of FIG. 1;
[0063] FIG. 10 is an enlarged partial view of the conductive layer
of FIG. 9;
[0064] FIG. 11 is a plan view of the membrane electrode assembly
(MEA) of the oxygen concentrator of FIG. 1;
[0065] FIG. 12 is an exploded view of an oxygen concentrator
according to a second embodiment of the invention;
[0066] FIG. 13 is a three-quarter view of the oxygen concentrator
of FIG. 12, in assembled form;
[0067] FIG. 14 is a transverse section of the assembled oxygen
concentrator of FIG. 13;
[0068] FIG. 15 is a diagrammatic section of a portion of the
membrane electrode assembly (MEA) and regeneration catalyst of an
oxygen concentrator showing the electrochemical processes of the
cell; and
[0069] FIG. 16 is a schematic diagram of an oxygen supply unit
embodying the invention.
[0070] FIG. 15 illustrates the electrochemical processes in the
cell of an oxygen concentrator. It shows a diagrammatic section of
part of the proton-conducting membrane which is in this case a
Nafion.RTM. membrane. An anode and a cathode are coated onto
opposite sides of the membrane and, on the cathode side, a
regeneration catalyst layer is spaced from the cathode by a
diffusion layer. Water in the membrane reacts at the anode to
produce gaseous oxygen and hydrogen ions. The hydrogen ions diffuse
through the membrane to the cathode where they combine with
electrons to form gaseous hydrogen. The hydrogen diffuses through
the diffusion layer to the catalyst layer, where it reacts with
atmospheric oxygen to form water. The water diffuses back through
the diffusion layer to the cathode and the Nafion membrane. The
water then diffuses through the membrane to the anode for
re-electrolysis.
[0071] FIG. 1 is an exploded view of an oxygen concentrator 2
according to a first embodiment of the invention. The concentrator
is constructed as a stack of substantially circular layers (as
described below) housed between a cell base 4 and a cell cover 6.
The cell base and the cell cover are machined from polymethyl
methacrylate, but could be fabricated from any suitable, inert
material. In the assembled concentrator the cover is secured to the
base by four bolts 8 which pass through holes in the cover and
screw into threaded holes 10 in the base.
[0072] FIGS. 2, 3, 4 and 5 show three-quarter and plan views of the
cell base and the cell cover.
[0073] When the oxygen concentrator is assembled, a
proton-conducting membrane 12 divides the space within the cell
into a cathode compartment 14 and an anode compartment 16, as
illustrated in FIG. 6. The membrane is of Nafion.RTM., type NRE212.
The membrane is circular, of 32 mm outside diameter, and seats on a
corresponding-sized step 18 of the cell base. A radially-outer
portion of the membrane is seated on the step 18 and is urged
against the step in the assembled cell by a nitrile O-ring 20. The
O-ring is urged against the membrane by a radially-outer flange
portion 22 of the cell cover and forms gas- and liquid-tight seals
between the radially-outer portion of the membrane and the step 18
of the cell base, between the radially-outer portion of the
membrane and the O-ring, and between the O-ring and the cell cover,
to define the cathode chamber 14 and the anode chamber 16.
[0074] A cathode 24 and an anode 26 are coated on opposite sides of
a central portion of the membrane to form a MEA. Each of the
cathode and the anode are in the form of a 25 mm circle, centrally
disposed on the circular membrane as shown in FIG. 11. The
thickness of the Nafion membrane is approximately 0.14 mm. The
cathode and the anode are sputtered. The cathode loading is at
least 0.3 mg of platinum per cm.sup.2 and the anode loading is at
least 1 mg of iridium per cm.sup.2. The MEA is marked with coloured
dots 25 to identify the anode and cathode sides.
[0075] On the cathode side of the membrane, the cathode chamber
contains a stack of three components (see FIG. 1). Adjacent the
cathode is a diffusion layer 28 coated on its side spaced from the
cathode with a sputtered platinum catalyst layer 30. The diffusion
layer may be of any suitable porous cloth, paper or composite (e.g.
plastic composites containing carbon, graphite, carbon nanotubes or
some combination thereof). It is electrically conductive and is
optionally hydrophobic. In the embodiment, the diffusion layer
comprises a 200 .mu.m non-organic woven 10% PTFE carbon layer. The
catalyst may be any suitable catalyst, or catalyst plus support
material, for adequately catalysing the reaction of hydrogen and
oxygen to form water. In the embodiment, the catalyst is a
sputtered layer of Pt, at a density of 2 mgcm.sup.-2. The diffusion
layer and catalyst are in the form of a 25 mm disc, corresponding
to the diameter of the anode and the cathode, which define the
active area of the MEA. Adjacent the catalyst is a conductive layer
in the form of a cathode-side perforated stainless steel disc 32.
This is illustrated in plan view in FIG. 9 and on an enlarged scale
in FIG. 10, to show that an array of small circular perforations 33
is formed through the disc. The perforations are 0.2 mm in diameter
and arranged in a close-packed array at 0.4 mm spacing. In the
assembled cell, the perforations allow access of air to the
catalyst 30. A tab 34 extends from the edge of the stainless steel
disc. In the assembled cell this emerges through a corresponding
slot 36 in the cell cover for the connection of an electrical
supply.
[0076] Finally, a disc of resilient, compressible, porous
polyurethane foam 38, again of 25 mm diameter, is positioned
between the stainless steel disc and a raised circular central
portion 40 of the cell cover, again of 25 mm diameter. The foam
disc is illustrated in FIGS. 7 and 8 and, before compression, is
approximately 1.4 mm in thickness. A portion of its edge 42 is cut
away to allow passage of the tab 34 of the stainless steel disc.
When the cell is assembled, the polyurethane foam disc is in a
compressed state, in order to press the layers of the cell
together. The polyurethane foam is porous, in order to allow access
of air to the catalyst. The foam may be conductive, for example
being a metallized, nickel over copper plated, polyurethane
foam.
[0077] Two circular holes 44 are formed through the cell cover,
through its thicker central portion 40, so as to open into the
cathode chamber. These holes provide ventilation for the cathode
chamber, and allow access of air, through the polyurethane foam and
the perforations of the stainless steel disc, to the catalyst
30.
[0078] In the anode chamber, a conductive, gas-permeable membrane
46 is positioned adjacent to the anode. This membrane is in the
form of a 25 mm disc, corresponding to the shape and size of the
anode. The material of the membrane may be the same as for the
diffusion layer 28 described above. An anode-side conductive layer,
in the form of a further perforated stainless steel disc 48,
contacts the side of the gas-permeable membrane 46 opposite to the
anode. The perforations are the same as for the cathode-side
stainless steel disc described above. A tab 50 extends from an edge
of the stainless steel disc and emerges through a corresponding
slot 52 in the cell base for the connection of an electricity
supply.
[0079] If desired, the membrane 46 may be omitted as long as
adequate electrical contact is then maintained between the anode
and the anode-side stainless-steel disc.
[0080] A resilient, compressible layer 54 of polyurethane foam is
positioned between the stainless steel disc and a central, recessed
circular portion 59 of the cell base. A small portion 56 of the
circular outer edge of the foam disc 54 is cut away to allow the
tab 50 of the stainless steel disc to pass through the slot 52 in
the cell base. The foam may optionally be electrically
conductive.
[0081] A gas outlet passage 58 leads radially outwardly from the
anode chamber 16 and terminates at a luer 60, which allows the
oxygen outlet to be coupled to a tube for connection to, for
example, a hyperbaric dressing covering a wound.
[0082] When the oxygen concentrator is assembled, the various
layers described above are placed in a stack between the cell base
and the cell cover. The screws 8 are then engaged with the cell
base and, as the screws are tightened, the resilient foam layers
38, 54 are compressed to apply a force to urge the layers of the
stack together. In addition, tightening the screws compresses the
O-ring 20 to seal the outer periphery of the cell and to separate
the cathode chamber from the anode chamber. The thickness and
compressibility of the foam layers, and the spacing between the
raised central portion 40 of the cell cover and the central
circular portion 59 of the cell base, are predetermined such that a
force of between 440 and 450 Newtons is applied to the layers
between the foam discs. This ensures good electrical contact
between the layers to minimise the electrical resistance of the
cell, and ensures close contact between the diffusion layer
supporting the catalyst and the adjacent cathode and
proton-conducting layer. The resilience of the foam layers also
advantageously accommodates any small variations in the dimensions
of the components of the oxygen concentrator, for example due to
manufacturing tolerances.
[0083] Before assembly of the cell, each foam layer has a thickness
of about 1.4 mm. In the assembled cell the cathode chamber,
containing one compressed layer of foam, has a depth of about 0.7
mm.
[0084] In an alternative embodiment, only one of the two foam
layers may be needed load the stack of layers in the cell
together.
[0085] In use, an electricity supply is connected to the tabs 34,
50 of the stainless steel discs and a current of approximately 60
mA is passed through the cell. The oxygen concentrator is
positioned so that the external openings of the ventilation holes
44 in the cell cover are open to the atmosphere. Under these
conditions, approximately 15 ml/hr of oxygen gas is generated at
the anode and emerges through the luer 60, at a pressure of
approximately 50 mbar above atmospheric pressure. A voltage of
approximately 1.2V is required to drive the 60 mA current. This is
a suitable voltage to obtain an advantageously long battery life
from a rechargeable battery.
[0086] The inventors have tested this cell at voltages above 1V and
at a range of currents up to 120 mA and found that, within this
range, oxygen is continually generated at the anode at a rate
proportional to the current. It is possible that higher currents
could be applied in order to generate higher rates of oxygen flow.
It is anticipated, however, that at high applied voltages, problems
of corrosion may arise within the cell.
[0087] The power supply for the cell is designed so that if the
cell resistance increases, so that the voltage required to drive
the predetermined or preselected current rises unacceptably, then
the power supply switches to a stand-by mode. In a preferred
embodiment the power supply switches off if the voltage required by
the cell rises above 2V. For example, in tests of the oxygen
concentrator it has been found that this may happen if one of the
vents 44 accidentally becomes blocked, so that ventilation of the
cathode chamber is reduced.
[0088] To investigate the performance of the cell of this
embodiment, and in particular the effect of ventilation of the
cathode chamber, cells of the same size and geometry to that shown
in FIGS. 1 to 11 were made, but having ventilation means comprising
different numbers of similar ventilation holes 44 in the cell
cover. The two ventilation holes 44 shown in FIG. 1 have a total
area of 13.2 mm.sup.2. A similar cell was made having one
ventilation hole, of area 6.6 mm.sup.2, and a similar cell was made
having four ventilation holes, of total area 26.4 mm.sup.2. Each
cell was supplied with a constant current of 55 mA at a temperature
of 22 (.+-.3).degree. C., relative humidity of 46 (+10/-5) % and at
atmospheric pressure of 1006 (.+-.10) mb. For the cell having two
ventilation holes, oxygen was continuously produced (after a
start-up transient lasting less than one minute) at between 9 and
11 ml/hr for the duration of the test, 30 hours. The cell voltage
was stable at about 1V. For the cells having one ventilation hole
or four ventilation holes, the voltage required to drive the 55 mA
current steadily rose over the first 20 minutes of operation to
above 1.5V, at which point the tests were terminated to prevent
damage to the cells. These test results illustrate the inventors'
understanding that the level of ventilation, or of access of the
catalyst to the atmosphere, is critical. Too little and the
catalyst is starved of atmospheric oxygen and fails to recycle
enough water to maintain humidity in the MEA. Too much and the
water is lost from the cell. In both cases an increase in
electrical resistance results due to the MEA reducing in
conductivity.
[0089] FIG. 12 is an exploded view of an oxygen concentrator
according to a second embodiment of the invention. FIG. 13 shows a
three-quarter view of the assembled cell and FIG. 14 shows the
assembled cell in transverse section. In the same way as in the
first embodiment, the oxygen concentrator comprises a stack of
layers housed between a cell base 100 and a cell cover 102. Each of
the layers is circular, of 25 mm diameter, except for a
proton-conducting membrane 104 which is of 32 mm diameter.
[0090] As in the first embodiment, the membrane 104 divides the
cell into a cathode chamber 106 and an anode chamber 108. The anode
chamber is contained within a blind cylindrical recess formed in
the cell base. The cylindrical recess is encircled by a step 110 on
which a radially-outer portion of the proton-conducting membrane
seats. A nitrile O-ring 112 of 26.5 mm internal diameter and 3 mm
section is positioned on top of the membrane and is compressed
against the membrane by the cell cover 102. Thus, when the cell
cover is in position on the cell base, the radially-outer portion
of the proton-conducting membrane 104 seals against the step 110 in
the cell base, and the O-ring 112 forms a seal between the membrane
and the cell cover.
[0091] In the assembled cell, the cell cover is held in position on
the cell base by six screws 114 received in corresponding threaded
holes 116 in the base.
[0092] As in the first embodiment, the proton-conducting membrane
is a Nafion membrane. A platinum anode 118 and a platinum cathode
120 are sputtered onto opposite sides of a central circular portion
of the membrane, of 25 mm diameter. This forms a Membrane Electrode
Assembly (MEA) and defines the working area of the membrane.
[0093] In the assembled cell, an electrically-conductive diffusion
layer 122 is positioned adjacent to and in contact with the
cathode. A platinum catalyst layer is sputtered onto the face
distant from the cathode. A porous metal disc 124 formed of
sintered metal particles, of AISL 316L stainless steel, is
positioned between the catalyst and the cell cover. A metal wire
126 is connected to a central point of the sintered disc and
emerges through an opening 128 in the cell cover for connection to
an electricity supply.
[0094] Adjacent to the anode is positioned an anode-side diffusion
layer, for example a gas-permeable membrane, 130, which allows the
passage of oxygen away from the anode. An anode-side sintered metal
disc 132 is positioned adjacent to the diffusion layer, within the
anode chamber 108, and seats on two concentrically-arranged O-rings
134, 136 seated on a base of the anode chamber formed in the cell
base. The sintered disc is of AISL 316L stainless steel. The
O-rings are respectively of 21 mm and 8 mm internal diameter, and
both are of 2 mm section. When the cell is assembled, the two
O-rings are resiliently compressed and so exert a force on the
anode-side sintered disc 132 which is substantially rigid. This in
turn presses the layers of the cell structure together, to ensure
good electrical contact between the layers. As in the first
embodiment, it is found that a compressive force of between 400N
and 450N is desirable.
[0095] A wire 133 is connected to a central portion of the
anode-side sintered disc and emerges though a hole 135 in the cell
base, for connection to an electricity supply.
[0096] As in the first embodiment, a hole is defined between the
anode chamber and an external surface of the cell base for
connection to a luer fitting 138.
[0097] The oxygen concentrator of the second embodiment is suitable
for connection to a power supply as described in relation to the
first embodiment, and its performance is similar to the oxygen
concentrator of the first embodiment.
[0098] Two 2.9 mm diameter holes 140 are formed through the cell
cover to ventilate the cathode chamber as described above. The
holes are spaced on opposite sides of the centre of the circular
cathode chamber, between the centre and the outside diameter of the
circular cathode chamber, to ensure even ventilation of the cathode
chamber across the entire area of the catalyst. Ventilation occurs
through the porous metal sinter, which acts as a baffle to prevent
bulk airflow, or draughts, through the cathode chamber as described
above.
[0099] FIG. 16 is a schematic diagram of an oxygen supply unit 200
embodying the invention. A rechargeable battery 202 is connected,
through a power supply unit 204, to an oxygen concentrator 206. The
battery, the power supply unit and the oxygen concentrator are
housed in a portable, wearable housing 208. An oxygen outlet 210 is
mounted on an outer surface of the housing for connection, for
example, to a tube leading to a hyperbaric dressing.
* * * * *