Patent Application
20150292094
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.
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+) are typically reduced according to the half reaction shown in equation (2). The overall reaction at the two electrodes is given in equation (3):
2H2O→O2+4H++4e− (anode) (1)
4e−+4H+→2H2 (cathode) (2)
2H2O→O2+2H2 (overall reaction) (3)
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.
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.
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).
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.
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.
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/cm2). 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.
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.
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.
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.
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 O2 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 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.
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.
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.
Optionally, the gas permeable material is made at least partially or wholly from a polymer material, for example PTFE, polyethylene or polypropylene.
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.
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.
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.
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.
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.
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.
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/cm2; the low current density is less than 100 mA/cm2; the low current density is less than 20 mA/cm2; producing hydrogen gas is at 75% energy efficiency HHV or greater; and/or producing hydrogen gas is at 85% energy efficiency HHV or greater.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The inventors have realised or implemented one or more of the following example aspects, features or advantages, thus providing various example embodiments:
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.
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.
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.
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.
a)-(c) depicts typical rates of gas generation by the example ‘plate-and-frame’ style electrolyser from
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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° 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°, 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.
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.
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.
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.
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.
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.
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.
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.
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° C., less than 75° C. and less than 50° C.
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.
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:
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.
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
The first set of data displayed in
The data in
For the data in
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/cm2 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/cm2 and both are maintained unchanged for extended periods of time (data not shown in
By comparison, and referring to
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.
The second set of data in
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.
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. (
The operation of the above example electrolyser yields the data shown in
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/cm2 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.
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.
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.
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.
For water oxidation (namely the reaction that occurs at the anode in water splitting), catalysts such as Co3O4, LiCo2O4, NiCo2O4, MnO2, Mn2O3, 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 Co3O4 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
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
a) depicts how the assembly in
During operation of the electrolyser, water is allowed to permeate through the flow-channel spacers in the direction (out of the page) shown in
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
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 (
An alternative arrangement is depicted in
a) depicts how the assembly in
When this arrangement is spiral wound 1130 (
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/cm2, 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.
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
The cathode and anode hollow fibres are then interdigitated as shown schematically in
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.
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
At the same time, oxygen is generated at the surface anode hollow-fibres. As shown in
Thus, the module depicted in
In
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
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/cm2. 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.
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
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.
The equation relating the wetting of the pores of a porous material to the liquid used and the pressure differential is Washburn's equation:
where PC=the capillary pressure, r=the pore radius, γ=the surface tension of the liquid, and φ=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.
For example, for a polytetrafluoroethylene (PTFE) material in contact with liquid water, the contact angles are typically 100-115°. The surface tension of water is typically 0.07197 N/m at 25° 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:
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°, 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.
If the contact angle were 100°, 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.
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,
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.
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 m2 each, where each cell operates at 1.6 V DC(=93% energy efficiency HHV) and a current density of 10 mA/cm2. 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.
The permutations for the electrical power requirements of a module of this type would be as follows:
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:
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.
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/cm2, and contain one breathable anode and one breathable cathode of 1 m2 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.
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.
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
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.
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:
When screwed together, or otherwise attached together or joined, for example by glues, adhesives or melt processes, as shown in
Multiple such assemblies may be combined into a multi-layer assembly.
Multiple assemblies of this type may be combined into a single, multi-layer ‘plate-and-frame’ type electrolyser, as shown in
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.
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012902448 | Jun 2012 | AU | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/AU2013/000617 | 6/11/2013 | WO | 00 |
