U.S. patent application number 14/564910 was filed with the patent office on 2015-10-15 for gas permeable electrodes and electrochemical cells.
The applicant listed for this patent is University of Wollongong. Invention is credited to Stephen Thomas BEIRNE, Jun CHEN, Gerhard Frederick SWIEGERS, Caiyun WANG.
Application Number | 20150292094 14/564910 |
Document ID | / |
Family ID | 49757324 |
Filed Date | 2015-10-15 |
United States Patent
Application |
20150292094 |
Kind Code |
A1 |
SWIEGERS; Gerhard Frederick ;
et al. |
October 15, 2015 |
GAS PERMEABLE ELECTRODES AND ELECTROCHEMICAL CELLS
Abstract
An electrode for a water splitting device, the electrode
comprising a gas permeable material, a second material, for example
a further gas permeable material, a spacer layer positioned between
the gas permeable material and the second material, the spacer
layer providing a gas collection layer and a conducting layer. The
conducting layer can be provided adjacent to or at least partially
within the gas permeable material. The gas collection layer is able
to transport gas internally in the electrode. The gas permeable
materials can be gas permeable membranes. Also disclosed are
electrochemical cells using such an electrode as the cathode and/or
anode, and methods for bringing about gas-to-liquid or
liquid-to-gas transformations, for example for producing
hydrogen.
Inventors: |
SWIEGERS; Gerhard Frederick;
(Woonona, AU) ; CHEN; Jun; (Balgownie, AU)
; BEIRNE; Stephen Thomas; (Farmborough Heights, AU)
; WANG; Caiyun; (Mangerton, AU) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
University of Wollongong |
Wollongong |
|
AU |
|
|
Family ID: |
49757324 |
Appl. No.: |
14/564910 |
Filed: |
June 11, 2013 |
PCT Filed: |
June 11, 2013 |
PCT NO: |
PCT/AU2013/000617 |
371 Date: |
December 9, 2014 |
Current U.S.
Class: |
204/282 |
Current CPC
Class: |
C25B 11/035 20130101;
Y02E 60/36 20130101; C25B 1/10 20130101; C25B 11/00 20130101; C25B
9/08 20130101; C25B 1/04 20130101; Y02E 60/366 20130101 |
International
Class: |
C25B 11/00 20060101
C25B011/00; C25B 1/10 20060101 C25B001/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 12, 2012 |
AU |
2012902448 |
Claims
1. An electrode for a water splitting device, comprising: a gas
permeable material; a second material; a spacer layer positioned
between the gas permeable material and the second material, the
spacer layer providing a gas collection layer; and, a conducting
layer.
2. The electrode of claim 1, wherein the conducting layer is
provided adjacent to or at least partially within the gas permeable
material.
3. The electrode of claim 1, wherein the conducting layer is
deposited on the gas permeable material.
4. The electrode of claim 1, wherein the gas permeable material is
deposited on the conducting layer.
5. The electrode of claim 1, wherein the gas collection layer is
able to transport gas internally in the electrode.
6. The electrode of claim 5, wherein the gas collection layer is
able to transport gas internally in the electrode to at least one
gas exit area positioned at or near an edge or an end of the
electrode.
7. The electrode of claim 1, wherein the gas permeable material and
the second material are separate layers.
8. The electrode of claim 1, wherein the second material is a gas
permeable material.
9. The electrode of claim 1, wherein the second material is a gas
permeable material and a second conducting layer is provided
adjacent to or at least partially within the second material.
10. The electrode of claim 1, wherein the second material is a gas
permeable material and a second conducting layer is deposited on
the second material.
11. The electrode of claim 1, wherein the gas permeable material is
a gas permeable membrane.
12. The electrode of claim 1, wherein the second material is a
further gas permeable membrane.
13. The electrode of claim 1, wherein the electrode is formed of
flexible layers.
14. The electrode of claim 13, wherein the electrode is at least
partially wound in a spiral.
15. The electrode of claim 1, wherein the conducting layer includes
one or more catalysts.
16. The electrode of claim 1, wherein the spacer layer is
positioned adjacent to an inner side of the gas permeable
material.
17. The electrode of claim 1, wherein the conducting layer is
positioned adjacent to, on or partially within an outer side of the
gas permeable material.
18. The electrode of claim 1, wherein the gas permeable material
and the second material includes PTFE, polyethylene or
polypropylene.
19. The electrode of claim 15, wherein at least a portion of the
conducting layer is between the one or more catalysts and the gas
permeable material.
20. The electrode of claim 1, wherein the spacer layer is in the
form of a gas channel spacer.
21. The electrode of claim 1, wherein the spacer layer includes
embossed structures on an inner surface of the gas permeable
material and/or the second material.
22. The electrode of claim 1, wherein the gas permeable material
and the spacer layer are contiguous.
23. The electrode of claim 22, wherein the gas permeable material
and the second material are contiguous.
24-32. (canceled)
33. The electrode of claim 1, wherein the gas collection layer is
at least partially filled with the spacer layer that allows gases
to pass through the spacer layer.
34-49. (canceled)
Description
TECHNICAL FIELD
[0001] The present invention generally relates to electrochemical
devices or cells, electrodes, methods of manufacture thereof,
and/or methods for electrochemical or electrolytic reactions or
processes. In particular aspects, the present invention relates to
devices, cells, electrodes and/or methods for bringing about
gas-to-liquid or liquid-to-gas transformations and, for example, to
water electrolysis cells or electrodes that achieve
water-splitting. In other examples, the present invention relates
to methods of manufacturing electrodes and/or electrochemical
devices or cells including the electrodes.
BACKGROUND
[0002] The electrolytic splitting of water into hydrogen gas and
oxygen gas is generally achieved by applying a current to two,
closely located electrodes, typically made of platinum, each of
which are in contact with an intermediate water solution. At one
electrode--the anode--water is typically oxidized according to the
half-reaction given in equation (1). At the other electrode--the
cathode--protons (H.sup.+) are typically reduced according to the
half reaction shown in equation (2). The overall reaction at the
two electrodes is given in equation (3):
2H.sub.2O.fwdarw.O.sub.2+4H.sup.++4e.sup.- (anode) (1)
4e.sup.-+4H.sup.+.fwdarw.2H.sub.2 (cathode) (2)
2H.sub.2O.fwdarw.O.sub.2+2H.sub.2 (overall reaction) (3)
[0003] Numerous devices for splitting water electrolytically, known
as water electrolysers, are commercially available. A common
problem with commercially-available water electrolysers is that
they are generally inefficient in their ability to convert
electrical energy into energy within the hydrogen that they
generate. That is, they display low energy efficiency in the
transformation of water into hydrogen. Hydrogen is, of course, a
fuel that could in the future supplant fossil fuels like gasoline
and diesel. Moreover, it is potentially a non-polluting fuel since
the only product of combusting hydrogen is water.
[0004] One kilogram of hydrogen contains the equivalent of 39 kWh
of electrical energy within it (by its Higher Heating Value, or
HHV, measure). However commercial electrolysers typically require
substantially more electrical energy than 39 kWh to generate 1 kg
of hydrogen. For example, the Stuart IMET 1000 electrolyser
requires, on average, 53.4 kWh of electrical energy to generate 1
kg of hydrogen, giving it an overall energy efficiency for the
conversion of water into hydrogen (HHV) of 73%. That is,
approximately one quarter of the electrical energy fed into the
electrolyser is wasted (largely as heat) and not harnessed to make
hydrogen.
[0005] Similarly the Teledyne EC-750 electrolyser requires 62.3 kWh
of electrical energy to make 1 kg of hydrogen (63% energy
efficiency HHV). The Proton Hogen 380 electrolyser requires 70.1
kWh/kg of hydrogen (56% energy efficiency, HHV), while the Norsk
Hydro Atmospheric type No. 5040 (5150 AmpDC) requires 53.5 kWh/kg
of hydrogen generated (73% energy efficiency, HHV). The
AvalenceHydrofiller 175 requires 60.5 kWh of electrical energy to
generate 1 kg of hydrogen (64% energy efficiency, HHV).
[0006] In summary therefore, current commercially-available water
electrolysers are relatively wasteful of electrical energy in their
production of hydrogen. This inefficiency has severely
disadvantaged hydrogen as, for example, a potential transportation
fuel for a future economy.
[0007] For example, in the era of the George W. Bush presidency,
the U.S.A. considered hydrogen to be strategically important as an
alternative transportation fuel. However, since that time, in the
Obama presidency, it has been recognised that electric batteries
can provide a better overall efficiency for the conversion of grid
electrical energy into automotive power than is achieved by the
current commercial water electrolysers combined with the use of
high-efficiency fuel cells (powered by hydrogen). The U.S.A. has,
consequently, revised its strategic focus away from
hydrogen-powered automobiles to electric-powered automobiles in the
period 2009-2012. The Department of Energy in the U.S.A.,
nevertheless, has, as one of its critical targets, the development
of water electrolysers which achieve 90% overall energy efficiency,
HHV.
[0008] A key problem with current commercial water electrolysers is
that they suffer from electrical losses caused by their operation
at extremely high electrical current densities (of typically
1000-8000 mA/cm.sup.2). This is commercially unavoidable because
the only way to achieve a low cost of production of hydrogen is to
minimize the quantity of materials required in the electrolyser per
kilogram of hydrogen that is generated. Many of the materials used
in commercial electrolysers are exceedingly expensive--for example,
the precious metal catalysts used at the anode/cathode and the
proton exchange membrane diaphragm used to separate the gases. The
only way to achieve a low overall price for the hydrogen produced,
is therefore to generate the largest reasonable amount of hydrogen
per unit area for the cost of manufacturing the electrolyser. In
other words, a high current density is needed to lower the capital
cost of the electrolyser per kilogram of hydrogen produced. The
Department of Energy in the U.S.A. has, as another of its critical
targets, the development of water electrolysers that minimise the
quantity of precious metal catalysts and other expensive components
required and thereby reduce the capital costs.
[0009] At such high current densities the energy losses which occur
in the water-splitting process are large. These energy losses
include Ohmic losses at the electrodes and within the electrolyte,
as well as so-called overpotential losses, which occur when a
larger voltage than is theoretically necessary must be applied to
drive the water-splitting process. These losses combine to create
the energy inefficiencies displayed by commercially-available water
electrolysers.
[0010] In the Applicant's earlier International Patent Application
No. PCT/AU2011/001603, the Applicant described a water splitting
cell which employed spacers that allows the cell to be manufactured
from inexpensive and thin materials. The key advantage of employing
inexpensive manufacturing techniques to produce water splitting
cells, is that it makes it commercially viable to build cells with
large surface areas and operate them at low current densities. Much
higher overall energy efficiencies can be realised in this way than
is possible in present-day commercial water electrolysers.
Traditional approaches to the manufacture of water electrolysers
involve high capital expenditure which precludes the additional
capital cost involved in manufacturing the large electrode areas
required at low current densities.
[0011] Operating at low current densities leverages the ability to
produce hydrogen at very high efficiencies. In such devices, it is
important to minimise energy losses so that the operational
efficiencies and reduced manufacturing costs compensate for the
increase in electrode area.
[0012] An important energy loss is the so-called "bubble
overpotential", which occurs at both electrodes during the
formation of gas bubbles of hydrogen (cathode) and oxygen (anode).
For example, the concentrations of O.sub.2 bubbles required not
only produce overpotential at the anode, but also represent a very
reactive environment that challenges the long term stability of
many catalysts.
[0013] Low current densities are generally consistent with high
energy efficiencies because they minimise the losses that occur,
including Ohmic losses and the like, during the water-splitting
reaction. However, it is presently not commercially feasible to use
low current densities in current commercial water electrolysers
because of the high cost of materials used in such devices.
[0014] In summary, there presently exists a pressing need to
improve water electrolyser technology to achieve higher energy
efficiency HHV and lower the overall cost of hydrogen manufactured
by electrolytic water splitting. In one example problem, reducing
or eliminating a key energy loss--the bubble overpotential--could
diminish the energy losses and improve the overall energy
efficiency of water splitting.
[0015] Numerous other electrochemical liquid-to-gas transformations
have similar problems as those described above for water
electrolysis, namely high costs of materials, which force the use
of high current densities in the device or cell, with associated
low overall energy efficiencies. For example, the electrochemical
production of chlorine from brine (aqueous sodium chloride) is
extremely wasteful of energy. The same is true for numerous
electrochemical gas-to-liquid transformations. For example,
hydrogen-oxygen fuel cells are generally only 40-70% energy
efficient for similar reasons to those described above.
[0016] There is a need for electrochemical devices or cells,
electrodes, methods of manufacture thereof, and/or methods for
electrochemical or electrolytic reactions or processes, which
address or at least ameliorate one or more problems inherent in the
prior art, for example allowing higher energy efficiencies to be
achieved.
[0017] The reference in this specification to any prior publication
(or information derived from it), or to any matter which is known,
is not, and should not be taken as an acknowledgment or admission
or any form of suggestion that the prior publication (or
information derived from it) or known matter forms part of the
common general knowledge in the field of endeavour to which this
specification relates.
SUMMARY
[0018] This Summary is provided to introduce a selection of
concepts in a simplified form that are further described below in
the Examples. This Summary is not intended to identify key features
or essential features of the claimed subject matter, nor is it
intended to be used to limit the scope of the claimed subject
matter.
[0019] It will be convenient to describe embodiments of the
invention in relation to electrochemical devices or cells,
electrodes or methods for water splitting, however it should be
appreciated that the present invention can be applied to other
types of liquid-to-gas or gas-to-liquid electrochemical
reactions.
[0020] In one form there is provided an electrode for a water
splitting device, comprising a gas permeable material. Also
included in the electrode, or as part of an associated electrode or
anode/cathode, for example positioned adjacent the electrode, is a
second material. A spacer layer is positioned between the gas
permeable material and the second material, the spacer layer
providing a gas collection layer, for example within the electrode,
between an anode-cathode pair, an anode-anode pair or a
cathode-cathode pair. A conducting layer is also provided as part
of the electrode. The second material may be part of the electrode,
or an associated or adjacent electrode, cathode or anode, and in
one form may also be a gas permeable material.
[0021] Reference to a gas permeable material should be read as a
general reference also including any form or type of gas permeable
medium, article, layer, membrane, barrier, matrix, element or
structure, or combination thereof.
[0022] Reference to a gas permeable material should also be read as
including a meaning that at least part of the material is
sufficiently porous or penetrable to allow movement, transfer,
penetration or transport of one or more gases through or across at
least part of the gas permeable material. The gas permeable
material can also be referred to as a "breathable" material.
[0023] In various examples: the conducting layer is provided
adjacent to or at least partially within the gas permeable
material; the conducting layer is associated with the gas permeable
material; the conducting layer is deposited on the gas permeable
material; the gas permeable material is deposited on the conducting
layer; and/or the gas collection layer is able to transport gas
internally in the electrode. In another example, the gas permeable
material is a gas permeable membrane. In another example, the
second material is a further or additional gas permeable
membrane.
[0024] Preferably, the gas collection layer is able to transport
gas internally in the electrode to at least one gas exit area
positioned at or near an edge or an end of the electrode.
[0025] In various other example aspects: the gas permeable material
and the second material are separate layers of the electrode; the
second material is part of an adjacent anode or cathode; the second
material is a gas permeable material; and/or the second material is
a gas permeable material and a second conducting layer is provided
adjacent to or at least partially within the second material. Thus,
in one example the spacer layer providing a gas collection layer is
provided between a gas permeable layer and a second layer being a
further gas permeable layer of the electrode. In another example,
the second material is a gas permeable material and a second
conducting layer is associated with, positioned adjacent to, or
deposited on the second material.
[0026] In yet other example aspects: the electrode is formed of
flexible layers; the electrode is at least partially wound in a
spiral; and/or the conducting layer includes one or more
catalysts.
[0027] In an example aspect, the spacer layer is positioned
adjacent to an inner side of gas permeable material, and the
conducting layer is positioned adjacent to, on or partially within
an outer side of the gas permeable material.
[0028] Optionally, the gas permeable material is made at least
partially or wholly from a polymer material, for example PTFE,
polyethylene or polypropylene.
[0029] In other example aspects: at least a portion of the
conducting layer is between the one or more catalysts and the gas
permeable material; the spacer layer is in the form of a gas
channel spacer; and/or the spacer layer includes embossed
structures on an inner surface of the gas permeable material and/or
the second material.
[0030] In another form there is provided an electrode for a water
splitting device, comprising: a first gas permeable material; a
second gas permeable material; a spacer layer positioned between
the first gas permeable material and the second gas permeable
material, the spacer layer providing a gas collection layer; a
first conducting layer associated with the first gas permeable
material; and, a second conducting layer associated with the second
gas permeable material.
[0031] In various examples: the first conducting layer is provided
adjacent to or at least partially within the first gas permeable
material; the second conducting layer is provided adjacent to or at
least partially within the second gas permeable material; the
electrode is formed of flexible layers wound in a spiral; the
electrode is formed of planar layers; the first conducting layer
includes a catalyst; and/or the second conducting layer includes
another catalyst.
[0032] In another form there is provided a water splitting device,
comprising: an electrolyte; at least one electrode including: a gas
permeable material; a second material; a spacer layer positioned
between the gas permeable material and the second material, the
spacer layer providing a gas collection layer; and, a conducting
layer.
[0033] In another form there is provided a water splitting device,
comprising: at least one cathode including: a first gas permeable
material and a first conducting layer associated with the first gas
permeable material; a second gas permeable material and a second
conducting layer associated with the second gas permeable material;
a spacer layer positioned between the first gas permeable material
and the second gas permeable material, the spacer layer providing a
gas collection layer; and, at least one anode including: a third
gas permeable material and a third conducting layer associated with
the third gas permeable material; a fourth gas permeable material
and a fourth conducting layer associated with the fourth gas
permeable material; a further spacer layer positioned between the
third gas permeable material and the fourth gas permeable material,
the further spacer layer providing a gas collection layer; wherein
the at least one cathode and the at least one anode are at least
partially within an electrolyte in operation.
[0034] In one example, the at least one electrode is a gas
permeable electrode comprising two gas permeable materials having
the spacer layer positioned between the materials and against an
inner side of each material, and wherein each material includes a
conducting layer on the outer side of each material. In another
example, there is provided a plurality of cathodes and anodes
interleaved with water permeable spacers defining electrolyte
layers. In an example aspect the electrolyte is in fluid
communication and connected to an electrolyte inlet and an
electrolyte outlet, and the gas collection layer is in gaseous
communication to a gas outlet.
[0035] In various other examples, there are provided methods for
treating water comprising applying a low current density to the
water splitting device, including: producing hydrogen gas and
collecting the hydrogen gas via the gas collection layer; and/or
pressurizing the electrolyte. In other examples, the low current
density is less than 1000 mA/cm.sup.2; the low current density is
less than 100 mA/cm.sup.2; the low current density is less than 20
mA/cm.sup.2; producing hydrogen gas is at 75% energy efficiency HHV
or greater; and/or producing hydrogen gas is at 85% energy
efficiency HHV or greater.
[0036] In one form, there is provided a gas permeable electrode for
a water splitting device comprising at least one gas permeable
material and a spacer layer positioned against, adjacent or forming
part of, an inner side of the material and between the material and
another layer, said spacer layer defining a gas collection layer,
and wherein the material includes a conducting layer. Optionally,
the conducting layer includes or is associated with one or more
catalysts, and wherein the conducting layer is on the outer side of
the material.
[0037] In another form, there is provided a gas permeable electrode
assembly for a water splitting device comprising two gas permeable
materials having a spacer layer positioned between the materials
and against, adjacent or forming part of, an inner side of each
material, said spacer layer defining a gas collection layer and
wherein each material includes a conducting layer. Optionally, one
or both conducting layers include one or more catalysts, and
wherein the conducting layer is on the outer side of each
material.
[0038] In one example embodiment, the gas permeable material
includes PTFE, polyethylene or polypropylene, or a combination
thereof. In another example embodiment, at least a portion of the
conducting layer is disposed between the catalyst and the material.
Preferably, the gas permeable material is gas permeable and
electrolyte impermeable. In another example embodiment, there is
provided a gas permeable electrode wherein the spacer layer is in
the form of a gas channel spacer or embossed structures positioned,
attached, incorporated or placed on, near or at least partially
within, an inner side of at least one of the gas permeable
materials.
[0039] In another example form, the gas permeable electrodes can be
interleaved with water permeable spacers to produce a multi-layered
water splitting cell. An advantage of these electrodes is that they
sandwich a gas collection layer between two gas permeable
electrodes and may provide a cheap way of manufacturing a
multi-layered water splitting cell.
[0040] In another example embodiment, there is provided a water
splitting device comprising at least one cathode and at least one
anode, wherein at least one of the least one cathode and at least
one anode is a gas permeable electrode assembly comprising two gas
permeable materials having a spacer layer positioned between or
intermediate the materials and against, adjacent, or at least
partially within, an inner side of each material, said spacer layer
defining a gas collection layer, and wherein each material includes
or is associated with a conducting layer. Optionally, the
conducting layer includes one or more catalysts, and wherein the
conducting layer is on the outer side of each material.
[0041] In another example embodiment, there is provided a water
splitting device comprising a plurality of cathodes and anodes
interleaved with water permeable spacers defining electrolyte
layers, wherein the cathodes and the anodes are in the form of a
gas permeable electrodes assembly comprising two gas permeable
materials having a spacer layer positioned between or intermediate
the materials and against, or at least partially within, an inner
side of each material, said spacer layer defining a gas collection
layer, and wherein each material includes a conducting layer.
Optionally, the conducting layer includes one or more catalysts,
and wherein the conducting layer is on the outer side of each
material.
[0042] In further example forms, the water splitting devices may be
configured into modular devices in which the footprint and gas
handling infrastructure may be reduced. In one example embodiment,
there is provided a water splitting device comprising a spiral
wound multi-layered water splitting cell. In a further example, the
water splitting cell includes a plurality of cathodes and anodes
interleaved with water permeable spacers defining electrolyte
layers, and wherein the cathodes and the anodes are in the form of
gas permeable electrode assemblies comprising two gas permeable
materials having a spacer layer positioned between or intermediate
the gas permeable materials and against, or at least partially
within, an inner side of each material, said spacer layer defining
a gas collection layer, and wherein each material includes a
conducting layer that includes at least one catalyst, and wherein
the conducting layer is on the outer side of each material, said
electrolyte in fluid communication and connected to an electrolyte
inlet and an electrolyte outlet, said gas collection layer between
the anodes in fluid communication to an oxygen outlet and said gas
collection layer between the cathodes in fluid communication to a
hydrogen outlet. The spiral wound water splitting device is a
practical example way to reduce the footprint and gas handling
infrastructure. Spiral wound devices permit the electrolyte to
permeate through electrolyte layers along the water splitting
device. The gases can be extracted laterally, for example oxygen in
one direction to a collection channel and hydrogen in the other
direction to another collection channel.
[0043] The example spiral wound water splitting device allows the
cell to be manufactured from inexpensive and thin materials. A key
advantage of employing inexpensive manufacturing techniques to
produce water splitting cells, is that it makes it commercially
viable to build cells with large surface areas and operate them at
low current densities. These example water splitting cells are
flexible and can be configured into a spiral wound water splitting
device.
[0044] According to further example forms, in order to form spiral
wound water splitting devices a multi-layered arrangement of
flat-sheet materials may be rolled up into a spiral-wound
arrangement. The spiral wound arrangement may then be encased in a
casing, which holds the spiral-wound element in place within a
module whilst allowing for water to transit through the module.
Collection tubes may be positioned to plumb the respective gases,
hydrogen and oxygen from the water splitting device. Conveniently,
the collection tubes may be attached to the water splitting device
with the desired collection channels being open to the collection
tube for the respective gas. For example, all of the hydrogen gas
channels may be open at a matching location and communicate with
the collection tube for the hydrogen gas. At that location, the
oxygen gas channels can be closed or sealed. At a different
location on the water splitting cell, the oxygen gas channels may
be open and communicate with the collection tube for the oxygen
gas. At that location the hydrogen gas channels can be closed or
sealed.
[0045] In another example embodiment, there is provided a water
splitting device comprising a plurality of hollow fibre cathodes
and a plurality of hollow fibre anodes, wherein said plurality of
hollow fibre cathodes comprise a hollow fibre gas permeable
material having a conducting layer that may include a catalyst, and
wherein said plurality of hollow fibre anodes comprise a hollow
fibre gas permeable material having a conducting layer that may
include a catalyst.
[0046] One of the advantages addressed by example embodiments is
the elimination of the need for a proton exchange membrane between
the electrodes, as used in known water splitting cells. Proton
exchange membranes are generally not required where gas permeable
or breathable (preferably "bubble-free" or "substantially
bubble-free") electrodes are employed. Moreover, proton exchange
membranes swell in aqueous media and, as a result, make it
difficult to provide the packing efficiencies and modular designs
desirable to produce water splitting cells having low capital
expenditure requirements and low operating costs.
[0047] The inventors have found that the water splitting cells
allow the efficient use of space between the anode and cathode. In
one example, the water splitting cells permit at least 70% of the
volume between the anode and the cathode to the occupied by
electrolyte whilst maintaining the anode and cathode in a spaced
apart relationship. In addition, the water splitting cells may
allow a non-electrolyte component (e.g. the spacer layer) in the
electrolyte chamber to be less than 20% of the total resistance of
the electrolyte chamber. The water splitting cells may also permit
the diffusion of both cations and anions across the electrolyte
chamber without impedance, which would otherwise occur with use of
a proton exchange membrane diaphragm.
[0048] In one example embodiment, the spacer layer or component
within the electrolyte chamber may be gas permeable. In addition to
use in water splitting cells, various example embodiments may be
useful in performing other gas-to-liquid or liquid-to-gas
transformations, such as fuel cells or water treatment devices.
Various example forms address the pressing need for electrochemical
cells capable of performing gas-to-liquid or liquid-to-gas
transformations with high energy efficiencies. Specifically,
various example forms address the need for an electrolyser capable
of manufacturing hydrogen from water at high energy efficiency and
low cost.
[0049] The inventors have realised or implemented one or more of
the following example aspects, features or advantages, thus
providing various example embodiments: [0050] (1) when optimally
fabricated and implemented, gas permeable or breathable electrode
structures diminish the overall energy losses arising in a water
electrolyser from the bubble overpotential. The effect of reducing
or eliminating the bubble overpotential is to increase the overall
energy efficiency of the water electrolysis process. The gas
permeable or breathable electrode structures may be formed from a
variety of gas permeable materials. In one form, the gas permeable
materials may be porous, allowing the gases to migrate across the
material through its porous structure. In another form, the gas
permeable material may allow the gas to diffuse through a
non-porous structure. [0051] (2) low-cost catalysts containing
earth-abundant elements can be used to catalyse the water-splitting
reactions at the anode and cathode in gas permeable or breathable
electrode structures. While such catalysts are often not amenable
to energy efficient operation at high current densities, they are
capable of achieving exceedingly high energy efficiencies at lower
current densities than are currently used in commercial water
electrolysers. Some catalysts are electrically conductive and in
some embodiments, the catalyst may be used to form the conducting
layer. An example of a electrically conducting material that is
suitable for use as a catalyst is nickel. [0052] (3)
commercially-available and low-cost materials and material
structures can be economically applied to the fabrication of gas
permeable or breathable electrode structures which split water with
high energy efficiency. [0053] (4) reactor structures can be used
to fabricate modular, multi-layer water electrolysis cells having
exceedingly large internal surface areas, but relatively small
external footprints and low overall costs. The effect of this
realisation is to make it possible to build inexpensive, modular
water electrolysis cells having high internal surface area but low
external footprint. [0054] (5) the availability of low-cost
catalysts and materials, as well as low-cost reactor configurations
with high internal surface areas, makes it possible to fabricate an
entirely new type of electrolyser that generates hydrogen at
low-cost and high energy efficiency by operating at lower current
densities than has hitherto been commercially viable.
[0055] In various example forms, the high energy efficiency is
achieved by one or more of: (a) low current density, which
minimises the electrical losses, (b) low-cost catalysts, for
example Earth-abundant elements which operate highly efficiently at
lower current densities, and (c) the use of gas permeable or
breathable electrode or material structures, which reduce or
eliminate the bubble overpotential at each electrode.
[0056] In various example forms, the low cost is achieved by one or
more of the following features within the electrolyser: (i)
low-cost materials as the substrate for the gas permeable or
breathable anodes and/or cathodes, (ii) low-cost catalysts, for
example Earth-abundant elements, as the catalysts at the anode and
cathode (instead of high-cost precious metals), and (iii) low-cost
reactor structures that have relatively high internal surface areas
but relatively small external footprints. Preferably, the
combination of these factors allows for relatively high overall
rates of gas generation even when relatively small current
densities per unit surface area are employed.
[0057] In further example forms, the anodes and cathodes may
comprise hollow flat-sheets or tubes whose external surfaces are
porous and either hydrophobic (in the case where the liquid used is
hydrophilic--e.g. water) or hydrophilic (in the case where the
liquid used is hydrophobic--e.g. petroleum ether), to thereby allow
the gases but not the liquids, or other electrolyte fluids, to pass
through them into the associated gas channels.
BRIEF DESCRIPTION OF THE DRAWINGS
[0058] Illustrative embodiments will now be described solely by way
of non-limiting examples and with reference to the accompanying
figures. Various example embodiments will be apparent from the
following description, given by way of example only, of at least
one preferred but non-limiting embodiment, described in connection
with the accompanying figures.
[0059] FIG. 1 graphically depicts the performance of example water
electrolysers containing at each of the anode and the cathode: (a)
Ni-coated flat-sheet breathable electrode in 1 M NaOH (without
formation of bubbles or substantial formation of bubbles at the
electrode), or (b) Pt-coated flat-sheet breathable electrodes in 1
M strong acid (without formation of bubbles or substantial
formation of bubbles at the electrode), relative to (c) an
electrolyser comprising of known solid Pt flat-sheet electrodes in
1 M strong acid at the anode and cathode (with formation of
bubbles).
[0060] FIG. 2 graphically depicts the performance of example water
electrolysers containing at each of the anode and the cathode: (a)
Pt-coated hollow-fibre breathable electrodes (sealed at the bottom
and open at the top) in 1 M strong acid (without formation of
bubbles or substantial formation of bubbles at the electrode),
relative to (b) an electrolyser comprising of known solid Pt wire
electrodes in 1 M strong acid at the anode and cathode (with
formation of bubbles).
[0061] FIG. 3 depicts: (a) a perspective view of the example cell
used to perform the measurements in FIG. 1; (b) a cross-sectional
schematic of the structure of the example cell.
[0062] FIG. 4 depicts: (a) a photograph of a water electrolysis
experiment containing a known standard Pt wire at one electrode
(with bubbles clearly visible) and an, example Pt-coated
hollow-fibre (i.e. an example gas permeable electrode) (sealed at
the bottom, open at the top) at the other electrode, with no
bubbles visible; (b) a schematic explaining the fabrication of
example gas permeable electrodes with Pt-coated hollow-fibres for
use in an example water splitting cell.
[0063] FIG. 5 depicts electron microscope pictures of the surface
of the example Pt-coated hollow-fibre electrode of FIG. 4.
[0064] FIG. 6 depicts: (a) a schematic explaining the fabrication
of example hollow-sheet gas permeable or breathable electrodes for
an anode and cathode in an example electrolyser; (b) an electron
micrograph of a dense, and robust example spacer (also referred to
as a "permeate" or "gas-transport" spacer or spacer layer) that can
be incorporated within a hollow space inside or between a rolled
gas permeable material or gas permeable sheet materials.
[0065] FIG. 7 depicts an electron micrograph of the "flow-channel"
example.
[0066] FIG. 8 depicts schematically an example process or method by
which example electrodes can be formed for use as spiral-wound or
flat electrodes in an electrolyser.
[0067] FIG. 9 depicts schematically: (a) an example electrolyser or
cell having flat-sheet electrodes; (b) and (c) example
electrolysers or cells having a spiral-wound electrode; (d) and (e)
example electrical connections for a unipolar design and a bipolar
design.
[0068] FIG. 10 depicts schematically an example process or method
by which further example electrodes can be formed for use as
spiral-wound or flat electrodes in an electrolyser.
[0069] FIG. 11 depicts schematically (a) a further example
electrolyser or cell having flat-sheet electrodes; (b) and (c)
further example electrolysers or cells having spiral-wound
electrodes; using the example electrodes of FIG. 10.
[0070] FIG. 12 depicts a cut-away view of an example electrolyser
module containing hollow-fibre gas permeable or breathable
materials.
[0071] FIG. 13 is a schematic illustration of the operation of one
type of example electrolyser module involving hollow-fibre gas
permeable or breathable materials.
[0072] FIG. 14 is a schematic illustration the operation of a
second type of example electrolyser module involving hollow-fibre
gas permeable or breathable materials.
[0073] FIG. 15 is a schematic illustration showing how separate
modules of an example spiral wound electrolyser may be combined
within a second, tube housing to generate a larger quantity of
hydrogen from water.
[0074] FIG. 16 illustrates how separate tube housings containing
multiple modules may be combined within a plant.
[0075] FIG. 17 illustrates an example circuit for converting
three-phase AC electricity into DC electricity with near-100%
energy efficiency, for use with example electrolysers.
[0076] FIG. 18 illustrates (a) in an exploded view, and (b) in an
assembled view, how single, flat-sheet gas permeable or breathable
material electrodes may be combined into an example
`Plate-and-Frame` style electrolyser. FIG. 18(c)-(d) illustrates
how two such example anode-cathode cells may be combined into an
example multi-layer electrolyser.
[0077] FIG. 19(a)-(c) depicts typical rates of gas generation by
the example `plate-and-frame` style electrolyser from FIG. 18, over
three days of operation under conditions of constant switching "on"
and "off". (a) depicts data for part of day 1; (b) depicts data for
part of day 2; and (c) depicts data for part of day 3.
EXAMPLES
[0078] The following modes, features or aspects, given by way of
example only, are described in order to provide a more precise
understanding of the subject matter of a preferred embodiment or
embodiments. In the figures, incorporated to illustrate features of
example embodiments, like reference numerals are used to identify
like parts throughout the figures.
[0079] Example gas permeable or breathable electrodes may be formed
by any convenient means. For example, gas permeable electrodes can
be formed by depositing a conducting layer on a gas permeable
material and subsequently depositing a catalyst on the conducting
layer. In one example, one could start with a gas permeable
non-conducting material and then form the conducting layer on the
material, and thereafter, deposit the catalyst. Alternatively, one
could start with a gas permeable conducting material and then
deposit the catalyst.
[0080] In another example, a gas permeable or breathable electrode
may be formed by holding or positioning a conductive layer,
incorporating a catalyst or not, in close association with a gas
permeable or breathable material. In this example, one would form
the conducting layer with catalyst separately and then position,
place or attach the conducting layer against a gas permeable
material. The inventors have found that by simply pressing the
conducting layer against a gas permeable material one is able to
have a significant proportion of gas reaction products to migrate
across the material and not form bubbles, or not substantially form
bubbles or at least visible bubbles, in the electrolyte. The
conducting layer with catalyst may be chemically or physically
bound to the gas permeable material.
[0081] The anode and cathode layers may be separated by suitable
liquid-permeable, electrically-insulating spacers, which allow
liquid ingress to the anodes and cathodes whilst simultaneously
preventing short circuits from forming between the anodes and
cathodes. One example of such a spacer is the "feed-channel"
spacers used in commercially-available reverse osmosis membrane
modules. The spacer is suitably robust to allow the transit of
liquids but prevent the anodes and cathodes from collapsing on
themselves, even under high applied pressures.
[0082] In one example there is provided an electrode for a water
splitting device. The electrode comprises a gas permeable material
and a second material, being part of the electrode, and/or an anode
or a cathode adjacent to the electrode. A spacer layer is
positioned between the gas permeable material and the second
material, the spacer layer providing a gas collection layer, i.e.
within the electrode or between the electrode and an adjacent anode
or cathode. A conducting layer is also provided as part of the
electrode and is associated with the gas permeable material. The
second material may be part of the electrode or an adjacent
electrode (e.g. anode-anode, cathode-cathode or anode-cathode
pairs), and in a preferred example is also a gas permeable or
breathable material. The conducting layer can be provided adjacent
to or at least partially within the gas permeable material,
preferably on an outer side of the gas permeable material.
Preferably, the conducting layer is associated with, positioned
next to or is deposited on the gas permeable material. The gas
collection layer is able to transport gas internally in the
electrode, preferably to an exit area or region of the electrode.
In another example, the gas permeable material is a gas permeable
membrane and the second material is a further or additional gas
permeable membrane.
[0083] Preferably, the gas collection layer is able to transport
gas internally in the electrode to at least one gas exit area
positioned at or near an edge or an end of the electrode. In
another example the gas permeable material and the second material
are separate layers of the electrode. The second material is
preferably a gas permeable material or membrane. The second
material can be a gas permeable material and a second conducting
layer can be provided adjacent to or at least partially within the
second material. Thus, in one example the spacer layer providing a
gas collection layer is provided or positioned between a gas
permeable layer and a second layer (i.e. the second material) being
a further gas permeable layer of the electrode. In another example,
the second material is a gas permeable material and a second
conducting layer is associated with, positioned adjacent to, or
deposited on the second material.
[0084] Spacer layers are provided to maintain the respective gas
collection channels as well as the electrolyte channels. Suitable
spacer layers can be selected for each channel. The gas collection
layer in the respective electrodes is maintained by a spacer layer
which may be in the form of embossed structures on the inner
surfaces of the materials or as a separate spacer device such as a
gas diffusion spacer or the like. The electrolyte layer between the
anodes and cathodes may be maintained by the use of a spacer layer
in the form of a "flow channel" spacer. Other suitable spaces may
be used that allow the electrolyte to permeate the electrolyte
layer and contact the respective anode and cathodes.
[0085] The internal vacancies, voids or spaces within the hollow
sheets or fibres comprising the anodes and cathodes, may be filled,
or at least partially filled, with a spacer or spacer layer,
preferably a robust spacer or spacer layer, that allows gases to
pass through the spacer or spacer layer, but which prevents the
walls of the hollow structure from collapsing on themselves, even
under high applied pressures. An example of such a spacer is the
"permeate" spacer used in commercially-available reverse osmosis
membrane modules.
[0086] The gas permeable or breathable anodes and cathodes may be
constructed by depositing electrically conductive metallic layers
on an outer surface or surfaces of the gas permeable or breathable
materials and then, if necessary, depositing suitable
(electro)catalysts on the electrically conductive layers.
Alternatively, the electrically conductive metallic layers may
serve as (electro)catalysts in their own right. The catalysts may
be so chosen as to facilitate and accelerate the liquid-to-gas or
gas-to-liquid transformation.
[0087] The gas permeable or breathable electrodes may be
conveniently constructed whereby the gas flux across the gas
permeable material is tuned to the production rate of the reaction
product that may form a gas at the electrode. In an alternative
example, the gas permeable or breathable anodes and cathodes are
constructed by co-assembling in close and tight-fit proximity to
each other: (1) a gas permeable or breathable material with (2) a
free-standing, planar, porous metallic or conductive structure
coated, where necessary, with suitable catalysts. The
free-standing, planar, porous conductive structures may be fine
metal meshes, grids, felts, or similar planar, porous conductors.
Conductive structures of this type are commercially available from
a wide variety of vendors.
[0088] The gas permeable or breathable materials maintain a
well-defined liquid-gas interface at all of the anodes and cathodes
in the cell during the reaction. This may be achieved by ensuring
that the differential pressure across the gas permeable or
breathable materials of the anodes and cathodes (from the liquid
side to the gas side) is less than the capillary pressure to wet
their pores. In this way, liquid is not driven into the gas
channels nor gas driven into the liquid chambers, as a result of
the applied pressure.
[0089] In liquid-to-gas or gas-to-liquid transformations in which a
pressure larger than atmospheric is applied to either the liquid or
the gases, the reactor may be designed so that the applied pressure
does not exceed the capillary pressure at which liquid is driven
into the gas channels or gas driven into the liquid channels. That
is, the pores of the materials are so chosen as to ensure the
maintenance of a distinct liquid-gas interface at the anodes and
cathodes during operation under the applied pressure.
[0090] Washburn's equation is used to calculate the maximum pore
size required to maintain a clear liquid-gas interface at the gas
permeable or breathable electrodes when a pressure is applied to
either the gases or the liquids in the reactor, as described in the
non-limiting case in example 5. In the non-limiting case of PTFE
materials with water as an electrolyte in a water electrolyser,
where the contact angle is 115.degree. and a 1 bar pressure
differential is applied across the material, the pores should
preferably have a radius or other characteristic dimension of less
than 0.5 microns, more preferably less than 0.25 microns, and still
more preferably about 0.1 microns or lower. In the case where the
contact angle is 100.degree., the pores should preferably have a
radius or other characteristic dimension of less than 0.1 microns,
more preferably less than 0.05 microns, and still more preferably
about 0.025 microns or lower.
[0091] The materials used to fabricate the gas permeable or
breathable anodes and cathodes in one example swell by less than 1%
in water, or in the liquid employed in the device. The gases
associated with the anodes and cathodes are kept separate from each
other by engineering the gas channels within the reactor such that
the anode gases are separated at all points from the cathode gases.
In another example, the multi-layered structure of anodes and
cathodes comprising the electrochemical cell is housed within a
tight-fitting and robust casing which holds within it, all of the
anodes and cathodes, as well as the gas and liquid channels. That
is, the multi-layered structure of anodes and cathodes and their
associated gas and liquid channels are fabricated in a modular
form, which maybe readily linked to other modules to form larger
overall reactor structures. Moreover, in the case of failure, they
may be readily removed from and replaced in such structures by
other identically constructed modules.
[0092] In another example, the multi-layered structures of anodes
and cathodes within a single module have a relatively high internal
surfaces area, but a relatively low external area or footprint. For
example, a single module may have an internal structure of more
than 2 square meters, but external dimensions of 1 square meter. In
another example, a single module could have an internal area of
more than 10 square meters, but external area of less than 1 square
meter. A single module may have an internal area of more than 20
square meters, but an external area of less than 1 square meter. In
another example, the multi-layered structure of anodes within a
single module, may have the gas channels associated with the anode
connected into a single inlet/outlet pipe.
[0093] In another example, the multi-layered structure of cathodes
within a single module, may have the gas channels associated with
the cathode connected into a single inlet/outlet pipe, which is
separate from the anode inlet/outlet pipe. In a further example,
the multi-layered structure of anodes and cathodes within a single
module may be configured as a multi-layered material arrangement.
The multi-layer spiral wound structure may comprise one or more
than one cathode/anode electrode assembly pairs, and may comprise
one or more leaf assemblies.
[0094] The modular units described above may be so engineered as to
be readily attached to other, identical modular units, to thereby
seamlessly enlarge the overall reactor to the extent required. The
combined modular units as described above may themselves be housed
within a second, robust housing that contains within it all of the
liquid that is passed through the modular units and which serves as
a second containment chamber for the gases that are present within
the interconnected modules. The individual modular units within the
second, outer robust housing may be readily and easily removed and
exchanged for other, identical modules, allowing easy replacement
of defective or poorly operational modules.
[0095] An example water splitting cell may be operated at
relatively low current densities in order to achieve high energy
efficiencies in the production of gases-from liquids, or
liquids-from-gases. The water splitting cells may be operated at a
current density that accords with the highest reasonable energy
efficiency under the circumstances. For example, in the case of a
reactor which converts water into hydrogen and oxygen gas (a water
electrolyser), the reactor may be operated at a current density
that accords with more than 75% energy efficiency in terms of the
higher heating value (HHV) of hydrogen. As 1 kg of hydrogen
contains within it a total of 39 kWh of energy, 75% energy
efficiency may be achieved if the electrolyser generates 1 kg of
hydrogen upon the application of 52 kWh of electrical energy.
[0096] The water electrolyser may be operated at a current density
that accords with more than 85% energy efficiency according to the
higher heating value (HHV) of hydrogen; 85% energy efficiency may
be achieved if the electrolyser generates 1 kg of hydrogen upon the
application of 45.9 kWh of electrical energy. The water splitting
cell may be operated at a current density that accords with more
than 90% energy efficiency according to the higher heating value
(HHV) of hydrogen; 90% energy efficiency may be achieved if the
electrolyser generates 1 kg of hydrogen upon the application of
43.3 kWh of electrical energy. The removal of produced gas across
the gas permeable material results in a device capable of,
separating the gas from the reaction at the electrode. Greater than
90% of the gas produced at the at least one electrode can be
removed from the cell across the gas permeable material. Desirably,
greater than 95% and greater than 99% of the gas produced can be
removed across the gas permeable material. The water splitting cell
may operate to produce hydrogen gas at greater than 75% energy
efficiency HHV. Desirably, the water splitting cell may produce
hydrogen gas at greater than 90% energy efficiency HHV.
[0097] The inventors have found that the water splitting cells may
be operated efficiently by managing the pressure differential
across the gas permeable materials. The management of the pressure
differential may prevent wetting of the materials and drives the
gas reaction products across the material. The selection of
pressure differential will be typically dependent upon the nature
of the water splitting materials and may be determined with
reference to Washburn's equation as described below. Pressurizing
the electrolyte may also be useful in providing a pressurised gas
product in the gas collection layers.
[0098] In another example there is provided a process for
generating hydrogen comprising applying low current density to a
water splitting cell pressurizing an electrolyte, splitting water
and producing hydrogen gas and oxygen gas; and collecting the
respective pressurised gases with the respective gas collection
layers. The water splitting cell may be operated at temperatures
that are desirably less than 100.degree. C., less than 75.degree.
C. and less than 50.degree. C.
[0099] The individual electrochemical cells within the reactor may
be so configured in series or parallel, as to maximize the voltage
(Volts) and minimise the current (Amps) required. This is because,
in general, the cost of electrical conductors increases as the
current load increases, whereas the cost of AC-DC rectification
equipment per unit output decreases as the output voltage
increases. The overall configuration of the individual cells in
series or parallel within the reactor may be configured as to best
match the available three-phase industrial or residential power.
This is because a close match of the overall power requirements of
the electrolyser and the available three-phase power generally
allows for low-cost AC to DC conversion with near 100% energy
efficiency, thereby minimising losses.
[0100] A preferred embodiment typically includes an electrochemical
reactor for direct electrical transformation of water into hydrogen
and oxygen, the water electrolyser preferably but not exclusively,
comprising hollow gas permeable or breathable electrode structures
(e.g. flat-sheets or fibres) as anodes and cathodes in
multi-layered arrangements: [0101] i. where the anodes have
associated with them discrete oxygen gas channels, [0102] ii. where
the cathodes have associated with them discrete hydrogen gas
channels, [0103] iii. each of which hydrogen or oxygen channels are
linked to their respective electrodes by the pores in the gas
permeable or breathable materials, [0104] iv. where the gas
permeable or breathable materials maintain a distinct liquid-gas
interface during the reaction, [0105] v. where the pore sizes and
qualities of the gas permeable or breathable materials are such
that they maintain distinct liquid-gas interfaces under conditions
where the liquids and/or gases are subjected to an applied pressure
greater than atmospheric during operation, [0106] vi. where the
spaces between the anodes and cathodes are occupied by robust
electrically insulating spacers ("feed-channel spacers") that allow
the ingress of electrolyte water to the anodes and cathodes, whilst
preventing the anodes and cathodes from contacting each other and
thereby forming short circuits, [0107] vii. where the gas channels
are preferably, but not exclusively, occupied by robust spacers
("gas-channel spacers") that allow for the transport of gases
through them but prevent the walls of the gas channels from falling
in upon themselves even in circumstances where a pressure larger
than atmospheric is applied to the water electrolyte, [0108] viii.
where the hydrogen gas channels are linked to a single hydrogen gas
outlet, [0109] ix. where the oxygen gas channels are linked to a
single oxygen gas outlet, [0110] x. where the water is allowed to
permeate between the anodes and cathodes, [0111] xi. where the
entire multi-layered arrangement of anodes, cathodes, spacers and
gas channels, is incorporated within a single module having
relatively high internal surface area but low external footprint,
[0112] xii. where the modular units can be readily attached to
other, identical modular units, to thereby seamlessly enlarge the
electrolyser to the extent required, [0113] xiii. where the
combined modular units are themselves housed within a second,
robust housing that contains within it all of the water that is
passed through the modular units and which serves as a second
containment shield for the flammable hydrogen gas that is generated
within the modules, [0114] xiv. where individual modular units
within the second housing can be readily and easily exchanged for
other, identical modules, [0115] xv. where the electrolyser is
operated at low overall current density in order to achieve high
energy efficiencies in the production of hydrogen gas from water;
preferably at a current density according with 75% energy
efficiency, or, more preferably, at 85% energy efficiency, or still
more preferably at more than 90% energy efficiency, [0116] xvi.
where the individual cells within the overall electrolyser assembly
are so configured in series or parallel as to generally maximize
the voltage (Volts) and minimise the current (Amps) required,
and/or [0117] xvii. where the individual cells within the overall
electrolyser assembly are so configured in series or parallel as to
best match the available three-phase industrial or residential
power.
Example 1
Demonstration of the Potential of Gas Permeable or Breathable
Electrodes to Achieve High Energy Efficiencies in Water
Electrolysis
[0118] To assess whether the use of gas permeable or breathable
electrodes could improve the energy efficiency of the liquid-to-gas
transformation that occurs in water electrolysers, we examined the
optimal fabrication of gas permeable or breathable electrodes. The
gas permeable or breathable electrodes were then tested by
incorporation in bubble-free, laboratory-scale water electrolysers
where their performance was compared under optimum conditions of
acidity/basicity with standard, industry-best catalysts which
generated bubbles. For this comparison we selected solid platinum
(Pt) in 1 M strong acid as the "industry-best" catalyst. The reason
for this choice was that the other alternative--namely, nickel (Ni)
catalyst in strongly basic alkaline electrolysers--is generally
considered less energy efficient overall than Pt in strong
acid.
[0119] All of the comparisons involved the use of very simply
deposited, smooth metals with low surface area. The idea was to see
how they compare in their efficiency and overall output, and
whether the use of gas permeable or breathable electrodes could
improve the overall energy efficiency of water electrolysis
compared to the best available industry catalysts. The data in
FIGS. 1-2 compares typical performances of the various bubble-free
electrolysers with the industry-best Pt catalyst in 1 M strong
acid, where bubbles are generated.
Example 1A
Water Electrolysers Employing Flat-Sheet Gas Permeable or
Breathable Electrodes
[0120] The first set of data displayed in FIG. 1 examines two
"bubble-free" electrolysers incorporating flat-sheet breathable
electrodes at both the cathode and anode: a Ni-catalyzed alkaline
electrolyser in 1 M strong base (FIG. 1(a)), and a Pt-catalyzed
acid electrolyser in 1 M strong acid (FIG. 1(b)). The acid used was
sulphuric acid. The base used was sodium hydroxide. The same
catalysts were used at both of the anode and cathode
simultaneously.
[0121] The data in FIG. 1(a)-(b) was collected using the cell
depicted in FIG. 3. The cell in FIG. 3(a) is depicted schematically
in FIG. 3(b). The cell comprises the following parts: a central
water reservoir 100 has a water-free hydrogen collection chamber
110 on the left side and a water-free oxygen collection chamber 120
on the right side. Between the water reservoir 100 and the hydrogen
collection chamber 110 is a gas permeable or breathable electrode
130. Between the water reservoir 100 and the oxygen collection
chamber 120 is a gas permeable or breathable electrode 140. On, or
close to, or partially within, the surface of the gas permeable or
breathable electrodes 130 and 140 is a conductive layer containing
a suitable catalyst 150, or more than one catalysts. When an
electrical current is applied to the conductive layers 150 by an
electrical power source 160, such as a battery, then electrons flow
along the outer circuit as shown in circuit pathways 170. That
current causes water to be split into hydrogen on the surface of
the breathable electrode 130 (called the cathode) and oxygen on the
surface of the breathable electrode 140 (called the anode). Instead
of forming bubbles at these surfaces, the oxygen and hydrogen
passes through the hydrophobic pores 180 into the oxygen and
hydrogen collection chambers 120 and 110, respectively. Liquid
water cannot pass through these pores since it repels the
hydrophobic surfaces of the pores and the surface tension of the
water prevents droplets of water from disengaging from the bulk of
the water to thereby pass through the pores. Thus, the electrodes
130 and 140 act as gas-permeable, water-impermeable barriers.
[0122] For the data in FIG. 1(a), the Ni catalyst was a
commercially available, thin. Ni-coated flexible textile, which is
used for electromagnetic shielding. The textile was pushed and held
tight against a gas permeable or breathable hydrophobic material.
This worked just as well as depositing the metal directly onto the
material surface as was done for the data in FIG. 1(b), where the
Pt catalyst was deposited directly on the material by vacuum
metallization, a standard commercial process. In both cases, the
catalysts were subjected to extended conditioning before the
representative data shown in FIG. 1(a)-(b) was collected. By this
is meant that the electrolysers were left in operation in the 1 M
strong acid/base conditions shown with an applied voltage
(typically 2-3 V) for several hours before measuring the data. The
conditioning allows the systems to get to a clear steady state and
ensures that the measurements are reliable.
[0123] The current density at a fixed cell voltage of 1.6 V (=93%
energy efficiency HHV) was then measured for the two bubble-free
electrolysers. As can be seen, both of the breathable Ni and Pt
systems gave current densities of 1 mA/cm.sup.2 or more. The Pt one
gave a stable current within 1 min of being switched on. The Ni one
took about 5 min to reach a stable current. But both of the
currents are over 1 mA/cm.sup.2 and both are maintained unchanged
for extended periods of time (data not shown in FIG. 1 for
clarity).
[0124] By comparison, and referring to FIG. 1(c), the inventors
have previously studied the "industry-best" Pt catalyst in 1 M
strong acid with bubble formation. Those studies showed that, after
conditioning for 1 h and under the most optimum possible conditions
(more optimum than for the results in FIGS. 1(a) and (b)), solid
bare Pt generates a steady-state current of, on average, 0.83
mA/cm.sup.2. This is the absolute maximum steady-state current
density one can get at a Pt cathode when using a very large Pt mesh
electrode at the anode. If two equally-sized electrodes were used
at the anode and cathode (as was the case in the data in FIGS.
1(a)-(b)), the current density would be lower.
[0125] By this measure both of the bubble-free electrolysers
incorporating alkaline Ni-catalyzed and acid Pt-catalysed
breathable cells at each of the anode and cathode convincingly beat
simple electrolysers employing the industry-best catalyst, Pt, at
both anode and cathode in a configuration where bubbles were
generated. Moreover, the material-based electrolysers do not
exhibit the usual jagged chronoamperogram profiles associated with
bubble-formation, nor a slowly declining output until a
steady-state is generated, as is found with bare Pt.
Example 1B
Water Electrolysers Employing Hollow-Fibre Gas Permeable or
Breathable Electrodes
[0126] The second set of data in FIG. 2 compares, under optimum
conditions of acidity (1 M strong acid): [0127] (1) a bubble-free
electrolyser incorporating gas permeable or breathable hollow-fibre
electrodes coated with Pt at both the anode and cathode (the Pt was
deposited directly onto the materials using vacuum metallization, a
standard commercial process), and [0128] (2) The same electrolyser
cell, but with known bare Pt wire electrodes at both anode and
cathode.
[0129] FIG. 4(a) depicts a photograph of an example electrolyser
contrasting a known bare Pt wire for the cathode and a Pt-coated
hydrophobic hollow-fibre gas permeable electrode for the anode. As
can be seen, the known bare Pt wire becomes covered in bubbles
during the water electrolysis, whereas the hollow-fibre gas
permeable electrode is bubble free, i.e. without bubble formation
or without substantial bubble formation, at least visible bubble
formation.
[0130] FIG. 4(b) depicts a schematic of a method or process by
which the bubble-free electrolyser in point (1) above was
fabricated and how it operates. Hydrophobic hollow-fibre materials
200 were obtained. These were then coated by vacuum metallization
of Pt--a standard commercial process--to yield the Pt-coated
hollow-fibre material 210. (FIG. 5 depicts a scanning electron
micrograph of the surface of 210, showing the thickness of the
coating to be 20-50 nm). Two Pt-coated hollow-fibre materials are
then sealed at the bottom using araldite glue and dipped into an
aqueous solution of 1 M strong acid. The open tops of the
hollow-fibre materials are left to protrude above the surface of
the liquid water. Electrical connections at their surfaces (on the
conducting Pt) are connected to a power source, such as battery
220, which is used to drive an electrical current between the two,
with the electron movement shown at conducting pathway 230. As a
result of the applied voltage, water is split into hydrogen gas at
the surface of the cathode and oxygen gas at the surface of the
anode. The gases do not form bubbles however, as they instead
transit through the hydrophobic pores of the hollow fibre gas
permeable materials 240. Liquid water does not pass through these
pores because under the atmospheric test conditions, liquid water
is not capable of wetting the hydrophobic porous surface, in this
example based on Goretex.RTM. material, a porous form of
polytetrafluoroethylene (PTFE) with a micro-structure characterized
by nodes interconnected by fibrils. Thus, hydrogen gas is collected
in the hydrogen gas channel 260 within the center of the cathode
hollow-fibre material. Oxygen gas is collected within the oxygen
gas channel at the center of the anodic hollow-fibre.
[0131] The operation of the above example electrolyser yields the
data shown in FIG. 2(a). To obtain this data, we applied a fixed
current density of 2 mA/cm.sup.2 to the electrolyser and then
examined how the voltage (energy efficiency) varied over time. The
data is illustrated in this way to demonstrate how a commercial
electrolyser of this type may be operated. The use of a fixed
current density may be the most suitable mode of operation since it
guarantees the generation of a particular quantity of hydrogen per
day. (The rate of hydrogen generation is dependent on the current
employed). The data in FIG. 2(b) shows comparable results with a
known bare Pt wire at both the anode and cathode under otherwise
identical conditions. In both cases, the catalysts were not
pre-conditioned in order to demonstrate what happens during the
first hour of operation of an electrolyser and to show why
conditioning is necessary to obtain accurate data.
[0132] For the known bare Pt wire, one observes a clear decline in
energy efficiency over an hour of conditioning; this is very
typical of bare Pt electrodes and occurs before a steady-state is
established (after 1-2 hours of operation). During the conditioning
process, the energy efficiency can be seen to decline to around 88%
(1 hour). One hour later it is typically around 85%, which is at or
near to the steady state current density. The solid Pt electrodes
have been previously studied by the inventors and yielded an energy
efficiency of around the 83-85% mark at 2 mA/cm.sup.2 after a
steady-state was established. By contrast, the hollow-fibre gas
permeable electrodes do not display a similar decline. Their
chronovoltammetric profile is virtually flat, at around 96% energy
efficiency, and with only relatively small declines to the
steady-state. Moreover, they maintain higher energy efficiencies
than the comparable known bare Pt wire "industry-best" catalysts
over extended periods (e.g. 12 h of continuous testing). They are
noticeably more energy efficient than the industry-best Pt catalyst
in a configuration where bubbles are generated.
Conclusions for Example 1
Electrolysers Comprising of Gas Permeable or Breathable Electrodes
at Both Anode and Cathode May Achieve High Energy Efficiencies in
Water Electrolysis
[0133] Thus, it can be concluded that bubble-free water
electrolysers, i.e. that operate without substantial bubble
formation, comprising of gas permeable or breathable electrodes at
both the cathode and anode may achieve higher energy efficiencies
than systems which generate bubbles in liquid-to-gas
transformations. This is due to the reduction or elimination of the
bubble overpotential, which comprises a major source of energy loss
in such systems.
[0134] Furthermore, if this is true for water electrolysis, which
is one of the most challenging electrochemical liquid-to-gas
transformations, then it may also be true for other electrochemical
liquid-to-gas transformations. Moreover, the stability of the
gas-liquid interface in such systems will, likely, also greatly
facilitate and improve the energy efficiency of comparable
gas-to-liquid electrochemical transformations in such reactors.
Example 2
An Electrochemical Reactor Comprising a Multi-Layer, Hollow
Flat-Sheet Configuration (`Spiral-Wound Module`)
[0135] FIG. 6(a) schematically depicts a double-sided, flat-sheet
hydrophobic material 710. The material comprises of an upper and a
lower hydrophobic surface with a spacer, known generically as a
"permeate" spacer 740 between them. The upper and lower surfaces
contain hydrophobic pores which allow gases, but not liquid water
to pass through unless sufficient pressure is applied and/or the
water surface tension is sufficiently lowered. The "permeate"
spacer is typically dense but porous. FIG. 6(b) illustrates a
typical microscopic structure of this spacer. The microscopic
structure of a "flow channel" spacer of this type is depicted in
FIG. 7. As can be seen in FIG. 7, whereas this spacer has an open
structure that is suitable for transport of water through it, the
spacer in FIG. 6(b) has a more dense structure, making it suitable
for gas, but not liquid transport. To construct a flat-sheet water
electrolyser reactor, one can start with the hydrophobic
double-sided material with built-in gas spacer 710. Upon the
surface of this material a conductive layer is deposited, typically
using vacuum metallization. In the case of an alkaline
electrolyser, the conductive layer is typically nickel (Ni). Using
this technique, Ni layers of 20-50 nm may be deposited. The
Ni-coated materials may then be subjected to dip-coating using, for
example, electroless nickel plating, to thicken the conducting Ni
layer on their surface. After this, a catalyst, or more than one
catalyst, may be deposited upon or otherwise attached to the
conducting Ni surface. A range of possible catalysts exist and are
known in the art.
[0136] For water oxidation (namely the reaction that occurs at the
anode in water splitting), catalysts such as Co.sub.3O.sub.4,
LiCo.sub.2O.sub.4, NiCo.sub.2O.sub.4, MnO.sub.2, Mn.sub.2O.sub.3,
and other catalysts are available. The catalyst may be deposited by
various means known in the art. A representative example of
depositing such a catalyst upon a nickel surface is given in the
publication entitled: "Size-Dependent Activity of Co.sub.3O.sub.4
Nanoparticle Anodes for Alkaline Water Electrolysis" by Arthur J.
Esswein, Meredith J. McMurdo, Phillip N. Ross, Alexis T. Bell, and
T. Don Tilley, in the Journal of Physical Chemistry C 2009, Volume
113, pages 15068-15072. By means such as these, the anode 720 in
FIG. 6(a) may be prepared.
[0137] For the cathode, various catalysts exist that may be
deposited on the nickel surface, such as nanoparticulate Ni or
nanoparticulate nickel and other metal alloys. The publication
entitled: "Pre-Investigation of Water Electrolysis", document
PSO-F&U 2006-1-6287, issued jointly by the Department of
Chemistry, Technical University of Denmark, The Riso National
Laboratory of Denmark and DONG Energy, in 2008, describes means to
deposit such materials on the anode (starting from page 50). The
cathode 730 in FIG. 6(a) may thus be prepared. The document goes on
to describe anode catalysts and means of depositing them on the
anode.
[0138] FIG. 8 illustrates one approach to making an example water
electrolyser using the hollow, flat sheet cathode 730 and anode
720, thus prepared. The cathode 730 is sealed 731 at three of the
four edges, with the fourth edge half sealed 731 and half left
unsealed 732 as shown. The sealing may be carried out by snap
heating and melting the edges of the hollow flat-sheets to thereby
block the movements of gases and liquids out of the edges. Laser
heating may also be used to seal the edges of the cathode. The
anode 720 is sealed 721 at three of the four edges, with the fourth
edge half sealed 721 and half left unsealed 722 as shown. The
sealing may be carried out by snap heating and melting the edges of
the hollow flat-sheets to thereby block the movements of gases and
liquids out of the edges. Laser heating may also be used to seal
the edges of the anode. The sealing depicted in FIGS. 8(a) and (b)
may be carried out before the deposition of the conducting Ni layer
and deposition of the catalysts, if this is more suitable. As shown
in FIG. 8(c), the anodes and cathodes are then stacked with
intervening flow-channel spacers of the type depicted in FIG. 7.
Note that the unsealed edges of the anodes all line up with each
other along the back left edge, whereas the unsealed edges of the
cathodes line up with each other along the front left edge. Note
that the unsealed edges of the anodes and cathodes do not overlap
each other.
[0139] FIG. 9(a) depicts how the assembly in FIG. 8(c) may be
turned into an example water electrolyser. A hollow tube (typically
comprising of an electrically insulating polymer) is attached to
the assembly in FIG. 8(c) as shown in FIG. 9(a). The tube is
segregated into a forward chamber 910 and a rear chamber 920 which
are not connected to each other. The anodes and cathode are
attached to the tube in such a way that their unsealed edges open
into the internal chambers of the tube. The unsealed edges of the
cathode open exclusively into the rear chamber of the tube 920,
while the unsealed edges of the anode open exclusively into the
forward chamber of the tube 910. The anodes and cathodes may be
electrically connected in series (bipolar design) or parallel
(unipolar design), with a single external electrical connection for
the positive pole and another single external electrical connection
for the negative pole (as shown in FIG. 9(a)). FIG. 9(d)-(e)
depicts possible, non-limiting connection pathways for a unipolar
design (FIG. 9(d)) and a bipolar design (FIG. 9(e)). Other
connection pathways are possible.
[0140] During operation of the electrolyser, water is allowed to
permeate through the flow-channel spacers in the direction (out of
the page) shown in FIG. 9(a)). Thus, during operation, water is
present at and fills the intervening space between the anodes and
cathodes.
When a voltage is now applied across the anodes and cathodes,
hydrogen is generated at the surface of the cathodes and passes
through the pores of the cathode materials as depicted in FIG.
6(a). Oxygen is simultaneously generated at the surface of the
anodes and passes through the pores of the anode materials as
depicted in FIG. 6(a). The oxygen and hydrogen then fill the vacant
space about the spacer within the hollow sheet anodes and cathodes.
The only escape for the hydrogen is to exit the hollow sheet
cathode by the unsealed edges into the rear chamber 920 of the
attached tube. The only escape for the oxygen is to exit the
hollow-sheet anodes by the unsealed edges into the forward chamber
910 of the attached tube. In this way, the gases are channeled and
collected separately in the forward 910 and rear 920 chambers of
the attached tube.
[0141] To minimise the overall footprint of the reactor, the
multi-layered arrangement of flat-sheet materials may be rolled up
into a spiral-wound arrangement as shown in 940 (FIG. 9(b). The
spiral wound arrangement may then be encased in a polymer casing
950, which holds the spiral-wound element in place within a module
(950) whilst nevertheless allowing for water to transit through the
module as shown in FIG. 9(b). When a suitable voltage is applied to
such a module, hydrogen gas is generated and exits the module at
the rear tube as shown. Oxygen gas is generated at the forward tube
as shown.
[0142] An alternative arrangement is depicted in FIG. 9(c). In this
arrangement, the collection tube is not segmented into a forward
and a rear collection chamber. Rather the tube is segmented down
its length into two separate chambers. The flat-sheet anodes and
cathodes are attached to the tube in such a way that the unsealed
edges of the anodes empty into one of these chamber and the
unsealed edges of the cathodes empty into the other of these
chambers. Thus, when spiral wound as shown in 940 in FIG. 9(c), and
modularized by encasing in a polymer case 950, the module allows
for water to transit through as shown in FIG. 9(c). When a suitable
voltage is applied to such a module, hydrogen gas is generated and
exits the module from one of the segmented gas channels within the
collection tube, while oxygen is generated and exits the module
from the other of the segmented chambers as shown. Water
electrolysis modules of the type depicted in 950 typically display
high internal surface area but a relatively small overall
footprint. A range of other options exist to fabricate a spiral
wound water electrolysis module. In order to demonstrate some of
the other, non-limiting options for fabricating spiral wound
electrolysers, reference is made to FIGS. 10 and 11.
[0143] FIG. 10 illustrates another approach to the manufacture of a
spiral wound electrolyser module. The cathode 730 is sealed 731 at
three of the four edges, with the fourth edge left unsealed 732 as
shown (FIG. 10(a)). The anode 720 is sealed 721 at three of the
four edges, with the fourth edge left unsealed 722 as shown (FIG.
10(b)). The anodes and cathodes are then stacked as shown in FIG.
10(c) with intervening flow-channel spacers of the type depicted in
FIG. 7. Note that the unsealed edges of the anodes all line up with
each other along the left edge, whereas the unsealed edges of the
cathodes line up with each other along the right edge.
[0144] FIG. 11(a) depicts how the assembly in FIG. 10(c) may be
turned into a water electrolyser of the present invention. A hollow
tube 1110 is attached to the left side of the assembly in FIG.
10(c) as shown in FIG. 11(a). The anodes are attached to the tube
1110 in such a way that their unsealed edges open into the internal
vacancy of the tube 1110. Another tube 1120 is attached to the
right side of the assembly. The cathode is attached to the tube
1120 in such a way that their unsealed edges open into the internal
vacancies of the tube 1120. Thus, when water permeates through the
assembly and a suitable voltage is applied, the hydrogen gas that
is generated is collected by the right-hand tube 1120, while the
oxygen gas generated is separately collected by the left hand tube
1110.
[0145] When this arrangement is spiral wound 1130 (FIG. 11(b)-(c),
two possible modular arrangement may be fabricated. The modular
arrangement shown in 1140 in FIG. 11(b) comprises of two, roughly
equally thick, spiral wound elements encased by a polymer casing
1140. The casing allows water to pass through the module as shown.
The two inner tubes separately collect and yield the hydrogen and
oxygen that is generated. The modular arrangement shown in 1150 in
FIG. 11(c) comprises of one spiral wound element incorporating the
left hand collection tube (oxygen generation) and encased by a
polymer casing 1140, with the other collection tube (hydrogen
generation) located on the outer surface of the module. The casing
allows water to pass through the module as shown. The inner tube
collects and supplies the oxygen that is generated. The outer tube
collects and supplies the hydrogen that is generated.
[0146] Because such water electrolysis modules have a high internal
surface area but a relatively small overall footprint or external
area, they can be operated at relatively low overall current
densities. A typical current density would be 10 mA/cm.sup.2, which
is two orders of magnitude smaller than the current densities
currently employed in most commercial water electrolysers. At so
low a current density, it is possible to generate hydrogen with
near to or greater than 90% energy efficiency HHV. The electrical
power requirements and options for series and parallel electrical
arrangement of the individual cells in such modules are discussed
in detail in Example 6.
Example 3
An Electrochemical Reactor Comprising a Multi-Layer, Hollow-Fibre
Configuration (`Hollow-Fibre Module`)
[0147] FIG. 12 depicts schematically and in principle how a set of
hollow-fibre anode and cathode electrodes may be configured for an
example water electrolyser. A set of conductive catalytic
hollow-fibre materials 1200 may be aligned and housed within a
casing 1200 that allows for water to be transported around the
array of hollow-fibre materials. To construct a hollow-fibre water
electrolyser reactor, one can start with the hydrophobic
hollow-fibre material with built-in gas spacer 200 depicted in FIG.
4(b). Upon the surface of this material a conductive layer is
deposited, typically using vacuum metallization. In the case of an
alkaline electrolyser, the conductive layer is typically nickel
(Ni). Using this technique, Ni layers of 20-50 nm may be deposited.
The Ni-coated materials may then be subjected to dip-coating using
electroless nickel plating, to thicken the conducting Ni layer on
their surface. After this, a catalyst may be deposited upon the
conducting Ni surface. A range of possible catalysts exist and are
known in the art. Methods of depositing them are described in
Example 3.
[0148] To ensure that the hollow fibre anode or cathode thus
prepared is electrically isolated from other electrodes when in
operation, it would typically be further coated with a layer of
porous Teflon or sulfonated fluorinated polymer using a standard
dip-coating procedure well-known in the art. By means such as
these, the hollow-fibre anode 1320 and hollow-fibre cathode 1310 in
FIG. 13 may be prepared. The cathodes and anodes thus prepared are
then sealed at their both ends using simple heat sealing or a laser
sealing process. If necessary, the hollow-fibre gas permeable
materials may be sealed prior to the deposition of the conductive
and catalytic layers upon their surface.
[0149] The cathode and anode hollow fibres are then interdigitated
as shown schematically in FIG. 13, with their ends lying in a
non-interdigitated fashion on opposite sides. In FIG. 13, the anode
hollow fibres 1320 have their non-interdigitated ends on the right
and the cathode hollow fibres 1310 have their non-interdigitated
ends on the left. A conductive adhesive is then cast about the
non-interdigitated ends of the anode hollow-fibres 1320. The
adhesive is allowed to set, whereafter a conductive adhesive is
cast about the non-interdigitated ends of the cathode hollow-fibres
1310. After the two adhesives are set, they are sawn through with a
fine bandsaw, opening up the one end of the sealed hollow fibres.
The anode hollow fibres 1320 are now open on the right-hand side of
the interdigitated assembly (as shown in FIG. 13), while the
cathode hollow fibres 1310 are open at the left hand side of the
interdigitated assembly (as shown in FIG. 13). The interdigitated
assembly is then encased in a polymer case 1330 which allows water
to pass between the interdigitated hollow-fibres but not into their
internal gas collection channels.
[0150] The anodes and cathodes are then preferably, though not
necessarily, connected in parallel with each other (unipolar
design), with the negative external pole connected to the left-hand
(cathode) conducting adhesive plug and the positive external pole
connected to the right-hand (anode) conducting adhesive plug.
Bipolar designs are also possible in which individual fibres, or
bundles of fibres are connected in series with each other so that
hydrogen is generated in the hollow-fibres open at the left-hand
side of the electrolyser and oxygen in the hollow-fibres open at
the right-hand side of the electrolyser.
[0151] Upon applying an electrical voltage to the two conducting
adhesive plugs at either end of the interdigitated arrangement, in
the presence of water, hydrogen gas is formed at the surface of the
cathode hollow-fibres. As shown in FIG. 4(b), the hydrogen passes
through the hydrophobic pores 240 of the hollow fibre into the
internal gas collection channel 260, without forming bubbles at the
surface of the cathode. The hydrogen is channeled as shown in FIG.
13 into the hydrogen outlet at the left of the reactor in FIG.
13.
[0152] At the same time, oxygen is generated at the surface anode
hollow-fibres. As shown in FIG. 4(b), the hydrogen passes through
the hydrophobic pores 240 of the hollow fibre into the internal gas
collection channel 270 of the anodes, without forming bubbles at
the surface of the anode. The oxygen is channeled as shown in FIG.
13 into the oxygen outlet at the right of the reactor in FIG.
13.
[0153] Thus, the module depicted in FIG. 13 generates hydrogen and
oxygen upon application of a suitable voltage and when water is
passed through the module. A range of other options exist to
fabricate a hollow-fibre water electrolysis module of the present
invention. In order to demonstrate another, non-limiting option,
reference is made to FIG. 14.
[0154] In FIG. 14, the anode and cathode hollow fibres have not
been interdigitated, but have instead been incorporated in two
separate multi-layer arrangements that face each other. On the left
hand side, a set of parallel hollow-fibre cathodes 1410 have been
located together within the module housing 1430, while on the right
hand side, a set of parallel hollow-fibre anodes 1420 have been
located together in the module housing 1430. A proton exchange
membrane or material may optionally be present between the cathode
and the anode hollow-fibres.
[0155] Upon applying an electrical voltage to the two conducting
adhesive plugs at either end of the module, in the presence of a
suitable aqueous electrolyte filling the module, hydrogen gas is
formed at the surface of the cathode hollow-fibres 1410 and is
transported to the hydrogen exit via the pores of the materials and
their hollow interiors. Oxygen gas is similarly formed at the
surface of the anode hollow-fibres 1420 and is transported to the
oxygen exit via the pores of the materials and their hollow
interiors. Thus, the module depicted in FIG. 14 generates hydrogen
and oxygen upon application of a suitable voltage and when the
module is filled with a suitable aqueous electrolyte.
[0156] Because such hollow-fibre based water electrolysis modules
have a high internal surface area but a relatively small overall
footprint, they can be operated at relatively low overall current
densities. A typical current density would be 10 mA/cm.sup.2. which
is two orders of magnitude smaller than the current densities
currently employed in most commercial water electrolysers. At so
low a current density, it is possible to generate hydrogen with
near to or greater than 90% energy efficiency HHV. The electrical
power requirements and options for series and parallel electrical
arrangement of the individual cells in such modules are discussed
in detail in Example 6.
Example 4
Assembling Water Electrolyser Modules into Electrolyser Plants
[0157] FIG. 15 depicts schematically how water electrolyser modules
may be assembled into larger units that constitute an electrolyser
plant. Three modules 1510 (of the same type described as 950 in
FIG. 9(c)) are attached to each other via robust "quick-fit"
fittings 1520, that correctly connect the separate hydrogen and
oxygen gas collections channels together in a secure way. The
combined modules are then pushed into a thick metal tube 1530 which
is sealed with a thick metal cover plate 1540 at each end. The
cover plates 1540 allow for the transportation of water through the
tube and permit the gas collection tubes to protrude outside of the
tube. Water is then passed through the sealed tube 1550 as shown,
while a voltage is applied to the combined anodes and cathodes in
the modules within the tube. The resulting hydrogen and oxygen that
is generated is collected as shown at the bottom right of FIG.
15.
[0158] The tube 1530 acts as a second containment vessel for the
hydrogen that is generated and thereby carries out a safety
function for the electrolyser. The configuration depicted in FIG.
15 is for a water electrolyser plant. In such plants, multiple
tubes containing modules may be combined as shown in the photograph
in FIG. 16. Tubular arrangements of water electrolyser modules may
be combined in a similar way.
Example 5
Fabricating an Electrolyser to Generate Pressurised Hydrogen
[0159] In many applications, it is desirable to produce hydrogen at
a pressure greater than atmospheric. For this reason, most
commercial electrolysers generate pressurised hydrogen. For
example, commercial alkaline electrolysers generally produce
hydrogen at pressures of 1-20 bar. In order to generate pressurised
hydrogen in an example electrolyser, it is necessary to pressurise
the water, whilst simultaneously ensuring that that a stable
gas-liquid interface is maintained at the breathable electrodes
under the applied pressure. That is, the breathable electrode must
typically be so designed that water will not be pushed through the
pores into the associated gas channels under the applied
pressure.
[0160] The equation relating the wetting of the pores of a porous
material to the liquid used and the pressure differential is
Washburn's equation:
P c = 2 .gamma. r cos .phi. ##EQU00001##
where P.sub.C=the capillary pressure, r=the pore radius,
.gamma.=the surface tension of the liquid, and .phi.=the contact
angle of the liquid with the material. Using this equation, one may
calculate the optimum pore size (for round pores) to achieve the
desired, distinct liquid-gas interface at a particular differential
pressure.
[0161] For example, for a polytetrafluoroethylene (PTFE) material
in contact with liquid water, the contact angles are typically
100-115.degree.. The surface tension of water is typically 0.07197
N/m at 25.degree. C. If the water contains an electrolyte such as 1
M KOH, then the surface tension of the water typically increases to
0.07480 N/m. Applying these parameters to the Washburn equation
yields the following data:
TABLE-US-00001 Contact Angle of the liquid Pressure to Pressure to
Pressure to Pore size of with the wet/dewet wet/dewet wet/dewet
material, material, pore, Pa pore, Pa pore, Pa micrometers degrees
(N/m2) (bar) (psi) 10 115 6322 0.06 0.9 5 115 12645 0.13 1.8 1 115
63224 0.63 9.2 0.5 115 126447 1.26 18.3 0.3 115 210746 2.11 30.6
0.1 115 632237 6.32 91.7 0.05 115 1264474 12.64 183.3 0.025 115
2528948 25.29 366.7 0.013 115 4863361 48.63 705.2 0.01 115 6322369
63.22 916.7 10 100 2598 0.03 0.4 5 100 5196 0.05 0.8 1 100 25978
0.26 3.8 0.5 100 51956 0.52 7.5 0.3 100 86593 0.87 12.6 0.1 100
259778 2.60 37.7 0.05 100 519555 5.20 75.3 0.025 100 1039111 10.39
150.7 0.013 100 1998290 19.98 289.8 0.01 100 2597777 25.98
376.7
[0162] While many PTFE materials have oblong, not round pores, this
data indicates that for a 1 bar pressure differential across the
breathable materials in a liquid-to-gas transformation involving 1
M KOH (aq) and PTFE materials where the contact angle was
115.degree., the pores should preferably have a radius of less than
0.5 microns, more preferably less than 0.25 microns, and still more
preferably around 0.1 microns or lower. In this way there would be
a diminishing possibility of an applied pressure causing water to
be driven into the gas channels.
[0163] If the contact angle were 100.degree., then for a 1 bar
pressure differential across the material in a liquid-to-gas
transformation involving 1 M KOH (aq) and PTFE materials, the PTFE
material pores should preferably have a radius or other
characteristic dimension of less than 0.1 microns, more preferably
less than 0.05 microns, and still more preferably about 0.025
microns or lower.
Example 6
The Power Requirements of Electrolysers. Tailoring the Electrolyser
to the Available Three-Phase Power for Maximum AC to DC Conversion
Efficiency
[0164] As noted earlier, individual anode-cathode cells within
modules of the types depicted in 950, 1140, 1150, 1210, 1330, and
1430 may be connected in series or parallel, or combinations
thereof. Modules containing cells in parallel electrical
arrangements are termed unipolar modules. Modules containing cells
in series electrical arrangements are termed bipolar modules (see,
for example, FIG. 9(d)-(e)). Moreover, the modules (e.g. 1510 in
FIG. 15) may themselves be electrically connected in series or
parallel.
[0165] The overall electrical arrangement--whether cells are
connected in series or parallel, or combinations
thereof--significantly affects the electrical power requirements
for the electrolyser. In general it is desirable for reasons of
cost, energy efficiency, and complexity of design, to construct the
overall electrolyser to utilize a higher overall voltage and a
lower overall current. This is because the cost of electrical
conductors increases as the current load increases, whereas the
cost of AC-DC rectification equipment per unit output decreases as
the output voltage increases. Still more preferably, because DC
power is required, the overall electrolyser should be constructed
in such a way that the electrical losses involved in converting
residential or industrial AC power to DC are minimized to, ideally,
well less than 10%. Ideally, the power requirements of the overall
electrolyser configuration will be matched to the three-phase
residential or industrial power supply that is available. This
ensures virtually 100% efficiency in AC to DC conversion.
[0166] To illustrate the various permutations discussed above,
reference is made to an example of a module of the types depicted
in 950, 1140, 1150, 1210, 1330, and 1430. For the purposes of the
example it will be assumed that each module is so constructed as to
contain 20 individual cells containing one breathable anode and one
breathable cathode of 1 m.sup.2 each, where each cell operates at
1.6 V DC(=93% energy efficiency HHV) and a current density of 10
mA/cm.sup.2. Under these conditions, each cell will generate 90
grams of hydrogen per day (24 hours), and each module will generate
1.8 kg of hydrogen per day.
[0167] The permutations for the electrical power requirements of a
module of this type would be as follows: [0168] (1) If the module
was unipolar with the cells arranged exclusively in parallel, then
it would require a power supply capable of providing 1.6 Volts DC
and 2000 Amps of current (3.2 kW overall). [0169] (2) If the module
was bipolar with the cells arranged exclusively in series, then it
would require a power supply capable of providing 32 Volts DC and
100 Amps of current (3.2 kW overall). In general, the bipolar
module would be cheaper, more efficient, and less complex to power
as it would employ a lower current and higher voltage.
[0170] If 60 modules of the above types were electrically combined,
then this could, again, be in parallel or series. The permutations
for the power requirements are as follows: [0171] (1) In a parallel
arrangement of unipolar modules, the overall power requirement
would be 1.6 Volts DC and 120,000 Amps (192 kW overall) [0172] (2)
In a series arrangement of unipolar modules, the overall power
requirement would be 96 Volts DC and 2000 Amps (192 kW overall)
[0173] (3) In a parallel arrangement of bipolar modules, the
overall power requirement would be 32 Volts DC and 6,000 Amps (192
kW overall) [0174] (4) In a series arrangement of bipolar modules,
the overall power requirement would be 1920 Volts DC and 100 Amps
(192 kW overall). Under all of these conditions, the electrolyser
will generate 108 kg of hydrogen per day.
[0175] The optimum overall electrical configuration for an example
electrolyser can be determined by aiming to match its power
requirement to the industrial or residential three-phase power that
is available. If this can be achieved, then the power loss in going
from AC to DC can be limited to essentially zero, since only diodes
and capacitors are required for the rectifier, and not a
transformer.
[0176] For example, in Australia three-phase mains power provides
600 Volts DC, with a maximum current load of 120 Amps. If the
individual cells in the electrolyser operate optimally at 1.6 V DC
and a current density of 10 mA/cm.sup.2, and contain one breathable
anode and one breathable cathode of 1 m.sup.2 each, then the
electrolyser would need 375 cells in series in order to draw 600
Volts DC. Each individual cell will then experience a voltage of
1.6 Volt DC. The overall current drawn by such an electrolyser
would be 100 Amps, giving an overall power of 60 kW.
[0177] To build such an electrolyser one would combine 19 of the
bipolar version of the above modules in series. This would yield
380 cells in total, each of which would experience 600/380=1.58
Volts DC. The overall current drawn by the electrolyser would be
101 Amps, which is well within the maximum current load of the
Australian three-phase power supply. Such an electrolyser would
generate 34.2 kg of hydrogen per 24 hour day, with near to 100%
efficiency in its conversion of AC to DC electricity. It could be
plugged into a standard three-phase wall socket.
[0178] The AC to DC conversion unit in the power supply required
for such an electrolyser would be a very simple arrangement of six
diodes and beverage-can sized capacitors wired in a delta
arrangement of the type shown in FIG. 17. Units of this type are
currently commercially available (for example, the "SEMIKRON--SKD
160/16--BRIDGE RECTIFIER, 3 PH, 160A, 1600V". Thus, the cost of the
power supply would also be minimized and, effectively, trivial or
non limitation overall.
[0179] Several alternative approaches exist in which the available
three-phase power may be efficiently harnessed. For example,
another approach is to subject the three-phase power to half-wave
rectification using a very simple circuit that again utilizes only
diodes and capacitors and thereby avoids electrical energy losses.
An electrolyser tailored to half-wave rectified 300 Volt DC would
ideally contain 187 individual cells of the above type in series.
Such an electrolyser could be constructed of 9 bipolar modules
connected in series, which comprise of 180 individual cells. Each
cell would experience 1.67 Volts DC. The overall current drawn
would be 96 Amps. Such an electrolyser would generate 16.2 kg of
hydrogen per 24 hour day. It could be plugged into a standard
three-phase wall socket.
Example 7
An Electrochemical Reactor Comprising a Multi-Layer, Flat-Sheet
Configuration (`Plate-and-Frame Type Module`)
[0180] FIG. 18(a) provides an exploded view that illustrates how
multiple, single-ply or sheet material electrodes may be combined
within a `plate-and-frame` type electrolyser. The following items
are sandwiched or adjoined into an example electrolyser structure:
[0181] (1) Two end plates 1600, each of which contain a recessed
gas collection chamber 1610 into which a porous plastic support
1620 is incorporated; [0182] (2) A gas permeable material electrode
1630 (the anode), which can involve a Gortex.RTM. material, or like
material, coated with a conductive catalytic layer on the side
facing the middle of the device held within a polymer laminate
1640. The laminate also affixes a fine conductive mesh 1650 over
the conductive, catalytic side of the material electrode. The mesh
connects up to the copper connector 1660; [0183] (3) A spacer 1670,
within which the electrolyte (1 M KOH solution) resides; [0184] (4)
A second gas permeable material electrode 1680 (the cathode), which
involves a Gortex.RTM. material, or like material, coated with a
conductive catalytic layer on the side facing the middle of the
device held within a polymer laminate 1690. The laminate also
affixes a fine conductive mesh 1700 over the conductive, catalytic
side of the material electrode. The mesh connects up to the copper
connector 1710.
[0185] When screwed together, or otherwise attached together or
joined, for example by glues, adhesives or melt processes, as shown
in FIG. 18(b), then assembly 1720 may act as a highly efficient
electrolyser. Aqueous solution (1 M KOH) is introduced into the
space between the electrodes via ports 1730 and 1740. The water
fills the volume within spacer 1670. When an electrical voltage is
then applied over the copper connectors 1660 and 1710, then the
water is split into hydrogen and oxygen. The gases move through
their respective material electrodes. Oxygen gas exits the device
at ports 1750 and 1760. Hydrogen exits the device at the
corresponding ports on the back side of assembly 1720.
[0186] Multiple such assemblies may be combined into a multi-layer
assembly. FIG. 18(c)-(d) illustrates how this may be done. In FIGS.
18(c)-(d), two assemblies 1720 are combined by incorporating a gas
collecting spacer unit 1770 between them. The spacer unit contains
a hydrogen outlet 1780, that collects hydrogen from each of the
adjacent assemblies 1720. To facilitate this arrangement, both of
the cathodes 1690 of assemblies 1720 are attached to the spacer
1770, which has a porous internal structure 1790, through which the
generated hydrogen may pass prior to exiting at outlet 1780. The
anodes 1640 of assemblies 1720 are located on the outside of the
stack, causing oxygen to be transmitted via outlet 1750 and 1760,
on the outer sides of the resulting `plate-and-frame`
electrolyser.
[0187] FIG. 19 depicts data for the operation of the device shown
in FIG. 18 (a)-(b) at an applied cell voltage of 1.6 V (94%
electrical efficiency, HHV), over three days of operation, with
repeated, intermittent `on` and `off` switching. As can be seen,
the device generates gases at a relatively constant rate, consuming
around 10-12 mA/cm.sup.2 of current in doing so. During the third
day of operation (FIG. 19(c)), the device was tested at both 1.5 V
(99% electrical efficiency, HHV) and 1.6 V (94% electrical
efficiency, HHV), as shown.
[0188] Multiple assemblies of this type may be combined into a
single, multi-layer `plate-and-frame` type electrolyser, as shown
in FIG. 18(c)-(d).
[0189] Throughout this specification and the claims which follow,
unless the context requires otherwise, the word "comprise", and
variations such as "comprises" or "comprising", will be understood
to imply the inclusion of a stated integer or step or group of
integers or steps but not the exclusion of any other integer or
step or group of integers or steps.
[0190] Optional embodiments may also be said to broadly consist in
the parts, elements and features referred to or indicated herein,
individually or collectively, in any or all combinations of two or
more of the parts, elements or features, and wherein specific
integers are mentioned herein which have known equivalents in the
art to which the invention relates, such known equivalents are
deemed to be incorporated herein as if individually set forth.
[0191] Although a preferred embodiment has been described in
detail, it should be understood that many modifications, changes,
substitutions or alterations will be apparent to those skilled in
the art without departing from the scope of the present
invention.
* * * * *