Patent Description:
Water electrolysers are used for the generation of hydrogen and oxygen by passing an electric current through an electrolyte to convert water electrochemically into hydrogen (gas) and oxygen (gas). Although solid polymer electrolytes can be used in such water electrolysers, aqueous electrolyte solutions are most often employed.

Existing systems are for example high-pressure water electrolysers, wherein a feed stream comprising water is fed into the electrochemical cell, i.e. an electrolysis cell, under operating pressures between <NUM> bar and <NUM> bar. High-pressure systems allow to generate a compressed hydrogen output, and thus eliminate the need for an external compressor for the hydrogen gas.

Since water is consumed during generation of hydrogen and oxygen, water electrolysers require a feed of water to be added to the electrolyte, in particular to an aqueous electrolyte solution. With a growing world population, global expansion of business activity, rapid urbanization and climate change, there is an increasing demand on fresh water, with increasing water scarcity. Hence, it would accordingly be beneficial if the water comes no longer from fresh water feeds, but from other liquid feed streams comprising water, for example a saline stream, such as sea water and brackish water.

<CIT> describes a system for preparing hydrogen and oxygen by using tidal current energy and seawater. The seawater is desalinated by reverse osmosis (RO) to obtain purified water that is used in an electrolysis unit, where water is converted to hydrogen and oxygen. Reverse osmosis extensively removes dissolved salts by forcing water having a high content of dissolved salts at high pressures through dedicated semi-permeable membranes, which can be made of various polymer materials. The membranes are susceptible to fouling and other processes need to precede the RO process to prevent fouling and increase RO membrane lifetime. In <CIT>, the purified water obtained by a RO process is directly used in the electrolysis cell, and no special electrolyte solutions are used, nor recirculated.

<CIT> discloses a high-pressure electrolyser for use on-board a marine vessel. Sea water is first purified (desalinated) using the purification installations available on-board, followed by electrolysis of the obtained fresh water into hydrogen gas.

<CIT> disclose an onshore installation wherein sea water is first purified into fresh water and the fresh water is then converted into hydrogen and oxygen by means of electrolysis.

<CIT> discloses a similar installation as <CIT>, further using the mechanical energy of the sea waves as energy source for the processes requiring electrical power.

One disadvantage of the above-mentioned installations is that the saline water has to be desalinated into a stream of fresh water before it can be fed to the electrolyser for conversion into hydrogen and oxygen.

<CIT> discloses a method and a device for producing hydrogen gas and oxygen gas by means of electrolysis of water in an aqueous electrolyte solution. The aqueous electrolyte solution is replenished with fresh water obtained from a liquid feed stream, in particular saline water, by means of a vacuum mediated forward osmosis process. In the vacuum mediated forward osmosis process, water is first evaporated from the liquid feed stream, wherein the water vapour passes through a vapour transfer tunnel and condensates into the aqueous electrolyte solution. One disadvantage of this system is that energy is required for the evaporation of the water from the saline feed stream to the aqueous electrolyte solution.

On the other hand, <CIT> describes a system for production of pure water by desalination of seawater that includes a forward osmosis (FO) reactor and an electrodeionization (EDI) reactor. An aqueous electrolyte solution flows between the FO reactor and the EDI reactor. In the FO reactor, the aqueous electrolyte solution draws water through the FO membrane. In the EDI reactor, the water is split into hydrogen and hydroxyl ions by the use of cation and anion exchange membranes along with strong ion-exchange resins enabling the EDI reactor to function electrochemically to transfer protons and hydroxyl ions across the membranes for recombination into pure water, leaving only a concentrated polymeric solution in the reactor for recycling to the FO reactor. An impressed voltage of <NUM> VDC is used, preventing electrolysis of water which would produce oxygen and hydrogen gases. In the system of <CIT>, water is hence not electrolyzed, but only ionic splitting and transport of hydrogen cations and hydroxyl anions take place across the relevant membranes, after which they recombine into pure water.

Osmotic driving forces in FO can be significantly greater than hydraulic driving forces in RO, leading to higher water flux rates and recoveries. One further advantage of FO systems is that they are non-pressurized systems, allowing design with lighter, compact, and less expensive materials. Another advantage is that operation of FO systems requires less energy. These factors translate to savings both in capital and operational costs. In addition, the lower amount of more highly concentrated reject brine produced by FO processes can also more easily managed. <CIT> discloses a device and method for electrolytic reduction of water with dissolved hydrogen molecules.

The present invention aims to solve one or more of the problems of the devices and methods of the state of the art. It is an aim of the invention to provide an improved method and related device for the generation of hydrogen and oxygen from a liquid feed stream comprising water, which, amongst other advantages, involves a reduced number of processing steps, is more efficient and reduces the energy consumption.

The invention further aims to provide an improved device for the generation of hydrogen and oxygen from a liquid feed stream comprising water, which, amongst other advantages, is of simpler construction.

According to a first aspect of the invention, there is therefore provided a method for the generation of hydrogen and oxygen from a liquid feed stream comprising water. The method comprises passing an electric current through an aqueous electrolyte solution. Hydrogen (gas) and oxygen (gas) are thereby generated. Advantageously, the hydrogen and/or the oxygen are separated or extracted from the electrolyte solution. Water is fed to the aqueous electrolyte solution by means of forward osmosis. The aqueous electrolyte solution is brought into contact with a first side of a forward osmosis membrane and the liquid feed stream comprising water is brought into contact with a second side of the forward osmosis membrane. Water permeates through the forward osmosis membrane from the second side to the first side by means of a difference in osmotic pressure between the liquid feed stream and the aqueous electrolyte solution.

Advantageously, the aqueous electrolyte solution has an osmolarity of at least <NUM> mOsm/L. Advantageously, the liquid feed stream has an osmolarity of <NUM> mOsm/L or less. Advantageously, the difference in osmotic pressure between the liquid feed stream and the aqueous electrolyte solution is at least <NUM> bar.

Advantageously, the aqueous electrolyte solution comprises an alkaline electrolyte. Advantageously, the aqueous electrolyte solution is an aqueous alkaline electrolyte solution. Advantageously, the alkaline electrolyte comprises KOH and/or NaOH.

Advantageously, the liquid feed stream is a saline water stream.

Advantageously, the electric current passes through the aqueous electrolyte solution in an electrochemical cell comprising an anode and a cathode. Water in the aqueous solution is advantageously consumed in the electrochemical cell.

According to a second aspect of the invention, there is provided a device for the generation of hydrogen and oxygen from a liquid feed stream comprising water. Devices according to the present aspect are advantageously configured to carry out methods for generation of hydrogen and oxygen as described herein. The device comprises an electrochemical cell and an osmosis unit. The electrochemical cell comprising an anode, a cathode and an electrolyte compartment for containing an aqueous electrolyte solution. The electrochemical cell is configured for passing an electric current through the electrolyte compartment. By so doing, hydrogen (gas) and oxygen (gas) are generated. The osmosis unit comprises a forward osmosis membrane, a first compartment fluidly connected to the electrolyte compartment, and a second compartment comprising a feed inlet and a feed outlet. The first compartment is in contact with a first side of the forward osmosis membrane for passing the aqueous electrolyte solution along the first side of the forward osmosis membrane. The second compartment is in contact with a second side of the forward osmosis membrane for passing a liquid feed stream along the second side of the forward osmosis membrane. Preferably, the electrolyte compartment is in fluid contact with the anode and/or the cathode.

The device according to the invention may comprise a first electrolyte compartment for feeding a first electrolyte solution to the anode and a second electrolyte compartment for feeding a second electrolyte solution to the cathode. The first compartment of the osmosis unit is advantageously fluidly connected to one or both of the first and second electrolyte compartments.

The device can comprise means for separating hydrogen and/or oxygen. The means for separating hydrogen and/or oxygen can be part of the electrolyte compartment and be fluidly connected with the osmosis unit, or can comprise a unit fluidly connected between the electrolyte compartment and the first compartment of the osmosis unit. The means for separating hydrogen and/or oxygen can be an extraction system comprising one or more extraction outlets for evacuating respectively hydrogen gas and/or oxygen gas from the electrolyte compartment.

Advantageously, the forward osmosis membrane of the device of the invention is a semi-permeable membrane. Preferably, the forward osmosis membrane is a non-porous, water selectively permeable membrane. Preferably, the forward osmosis membrane material is selected from the group consisting of asymmetric aromatic polyamide, cellulose acetate optionally reinforced with mineral fillers, and cellulose triacetate.

Advantageously, the first side of the forward osmosis membrane of the device of the invention forms a wall of the first compartment of the osmosis unit. Advantageously, the second side of the forward osmosis membrane of the device of the invention forms a wall of the second compartment of the osmosis unit.

Methods for the generation of hydrogen and oxygen from an aqueous liquid feed stream comprise an electrolysis step in which an electric current is passed through an aqueous electrolyte solution. The aqueous electrolyte solution is depleted from water contained therein as the water converts to hydrogen and oxygen, and a concentrated electrolyte solution is obtained. The concentrated electrolyte solution is replenished (diluted) with water in a forward osmosis step. According to an aspect of the invention, the concentrated electrolyte solution acts as the draw solution in the forward osmosis step, allowing water to be drawn from a feed stream. The concentrated electrolyte solution is hence brought into contact with a first side of a forward osmosis membrane. A liquid feed stream comprising water is brought into contact with a second side of the forward osmosis membrane, opposite to the first side. Water permeates through the forward osmosis membrane from the second side to the first side due to a difference in osmotic pressure between the liquid feed stream and the concentrated electrolyte solution.

According to the present invention, in the electrolysis step, hydrogen (gas) and oxygen (gas) are generated by passing an electric current through an aqueous electrolyte solution. By so doing, electrolysis of water contained in the aqueous electrolyte solution is obtained. As the electrolysis process depletes water in the aqueous electrolyte solution, fresh water must be fed into the electrolyte solution stream. The inventors now have surprisingly found that water can advantageously be fed to the aqueous electrolyte solution from a liquid feed stream comprising water by means of forward osmosis. Hence, the electrolyte salt can be used in an endless loop between the forward osmosis process to act as a so-called draw solution having a higher concentration of the electrolyte salt, and the electrolysis process where it acts as the electrolyte in an aqueous electrolyte solution having a lower concentration of the electrolyte salt, for converting the water into hydrogen and oxygen.

Water is fed from the liquid feed stream through the forward osmosis membrane to the aqueous electrolyte solution by means of a difference in osmotic pressure between the liquid feed stream and the aqueous electrolyte solution. In order for the water to be fed from the liquid feed stream to the aqueous electrolyte solution, the osmotic pressure of the aqueous electrolyte solution is larger than the osmotic pressure of the liquid feed stream. The osmotic pressure difference depends on the osmotic concentration or osmolarity of both liquid streams. In other words, the osmolarity of the aqueous electrolyte solution is larger than the osmolarity of the liquid feed stream in order to allow water to transfer from the liquid feed stream to the aqueous electrolyte solution through forward osmosis.

The aqueous electrolyte solution advantageously has an osmotic concentration or osmolarity of at least <NUM> mOsm/L, advantageously at least <NUM> mOsm/L, advantageously at least <NUM> mOsm/L, advantageously at least <NUM> mOsm/L, advantageously at least <NUM> mOsm/L, advantageously at least <NUM> mOsm/L. Advantageously, the osmolarity of the aqueous electrolyte solution is smaller than or equal to <NUM> Osm/L, advantageously smaller than or equal to <NUM> Osm/L, advantageously smaller than or equal to <NUM> Osm/L, advantageously smaller than or equal to <NUM> Osm/L, advantageously smaller than or equal to <NUM> Osm/L. The osmolarity of the aqueous electrolyte solution is advantageously determined for the (concentrated) solution that is fed to the osmosis unit for replenishment with water.

The aqueous electrolyte solution advantageously has an osmotic pressure of at least <NUM> bar, such as at least <NUM> bar, <NUM> bar, <NUM> bar, <NUM> bar, <NUM> bar, <NUM> bar, <NUM> bar, <NUM> bar, <NUM> bar, <NUM> bar, <NUM> bar, <NUM> bar, up to at least <NUM> bar. Preferably, the osmotic pressure is between <NUM> and <NUM> bar, even more preferably between <NUM> bar and <NUM> bar.

The liquid feed stream comprising water advantageously has an osmotic concentration or osmolarity of <NUM> mOsm/L or less, such as <NUM> mOsm/L or less, <NUM> mOsm/L or less, or <NUM> mOsm/L or less. Preferably the osmolarity of the liquid feed stream comprising water is about <NUM> mOsm/L. The osmolarity of the liquid feed stream is advantageously at least <NUM> mOsm/L, advantageously at least <NUM> mOsm/L, advantageously at least <NUM> mOsm/L. The osmolarity of the liquid feed stream is advantageously determined at entrance into the osmosis unit.

The liquid feed stream comprising water advantageously has an osmotic pressure between <NUM> and <NUM> bar, such as between <NUM> and <NUM> bar, between <NUM> and <NUM> bar, and preferably between <NUM> and <NUM> bar. For example, a saline water feed stream at <NUM> has an osmotic pressure of about <NUM> bar.

The difference in osmotic pressure between the liquid feed stream and the aqueous electrolyte solution is advantageously at least <NUM> bar, advantageously between <NUM> and <NUM> bar, such as between <NUM> and <NUM> bar, between <NUM> and <NUM> bar, between <NUM> and <NUM> bar, or between <NUM> and <NUM> bar. Preferably, the difference in osmotic pressure is between <NUM> and <NUM> bar.

Above values of osmolarity and osmotic pressure are determined at <NUM> and atmospheric pressure.

The aqueous electrolyte solution advantageously comprises an alkaline electrolyte, advantageously is an alkaline electrolyte solution. Preferably, the alkaline electrolyte comprises KOH, NaOH, or a mixture of both.

The aqueous electrolyte solution advantageously has a molar concentration between <NUM> and <NUM>, preferably between <NUM> and <NUM>, such as between <NUM> an <NUM>, or between <NUM> and <NUM>, for example <NUM>.

Preferably, the aqueous electrolyte solution is a KOH solution in a molar concentration range between <NUM> and <NUM>.

The liquid feed stream may be any type of waste stream comprising water. Preferably, the liquid feed stream is a saline water stream, such as seawater or brackish water.

The ionic concentration of the liquid feed stream is advantageously between <NUM> and <NUM>, such as between <NUM> and <NUM>, between <NUM> and <NUM>, or between <NUM> and <NUM>.

Preferably, the liquid feed stream is a saline water stream with an ionic concentration between <NUM> and <NUM>, such as seawater with an ionic concentration between <NUM> and <NUM>.

Advantageously, the electrolysis step as described above is performed in an electrochemical cell, advantageously an electrolysis cell. The electrochemical cell advantageously comprises an anode, a cathode and one or more electrolyte compartments arranged in an electric current path between the anode and the cathode. At least one of the one or more electrolyte compartments are configured for containing the aqueous electrolyte solution. The anode and cathode allow for passing the electric current through the aqueous electrolyte solution.

When the electric current passes through the aqueous electrolyte solution, water is advantageously consumed in the electrochemical cell. Preferably, water is consumed at the anode side, where it is converted into hydrogen (gas) and hydroxide ions according to formula (I). At the cathode side, the hydroxide ions are converted into oxygen (gas) and water according to formula (II). The net reaction clearly shows the consumption of <NUM> water molecules for the production of <NUM> oxygen molecule and <NUM> hydrogen molecules (formula (III)).

<NUM>H<NUM>O + <NUM>e- → <NUM> H<NUM> (g) + <NUM>OH-     (I).

<NUM> OH- → O<NUM>(g) + <NUM> H<NUM>O + <NUM>e-     (II).

<NUM>H<NUM>O → <NUM> H<NUM> (g) + O<NUM>(g)     (III).

The present invention further relates to a device for the generation of hydrogen and oxygen from a liquid feed stream comprising water. Referring to <FIG>, the device <NUM> comprises an electrochemical cell <NUM> comprising an anode <NUM>, a cathode <NUM> and an electrolyte compartment <NUM> for containing an aqueous electrolyte solution. Preferably, the electrochemical cell is an electrolysis cell. The electrochemical cell <NUM> is configured for passing an electric current <NUM> through the electrolyte compartment <NUM>. The electrolyte compartment <NUM> is advantageously in fluid contact with the anode <NUM> and/or the cathode <NUM>.

Preferably, the electrochemical cell comprises a separator membrane <NUM>. The separator membrane <NUM> divides the electrolyte compartment <NUM> in an anode electrolyte compartment <NUM> and a cathode electrolyte compartment <NUM>. The separator membrane may be an open mesh polyphenylene sulphide fabric symmetrically coated with a mixture of a polymer and zirconium oxide (Zirfon), or a generally known ion exchange membrane, such as an alkaline ion exchange membrane (alkaline AEM), an asbestos membrane, a NiO conductor membrane, or a ceramic OH- conductor membrane.

The device <NUM> further comprises an osmosis unit <NUM> comprising a forward osmosis membrane <NUM>, a first compartment (permeate side) <NUM> fluidly connected to the electrolyte compartment <NUM>, and a second compartment <NUM> (feed side) comprising a feed inlet <NUM> and a feed outlet <NUM>. The forward osmosis membrane <NUM> forms the separating wall between the first compartment <NUM> and the second compartment <NUM>. A first side <NUM> of the forward osmosis membrane <NUM> is in contact with the first compartment <NUM> containing the concentrated electrolyte solution drawn from the electrochemical cell <NUM>. A second side <NUM> of the forward osmosis membrane <NUM> is in contact with the second compartment <NUM> containing the liquid feed stream.

The concentrated electrolyte solution and the liquid feed stream can be made to flow along the membrane <NUM> as appropriate. Osmosis units with counter current flow, concurrent flow, cross flow or any suitable combination thereof can be contemplated.

Device <NUM> further comprises a unit <NUM> for separating hydrogen and/or oxygen from the electrolyte solution. The unit <NUM> is advantageously fluidly connected between the electrolyte compartment <NUM> and the first compartment <NUM> of the osmosis unit <NUM>.

Referring to <FIG>, a second embodiment of the device of the present invention is presented. The device <NUM> differs from the device <NUM> of <FIG> in that the anode electrolyte compartment <NUM> and the cathode electrolyte compartment <NUM> are fed with separate electrolyte solution streams. A first electrolyte compartment <NUM> is fluidly connected to the anode electrolyte compartment <NUM> of electrochemical cell <NUM> for feeding a first electrolyte solution to the anode <NUM>. A second electrolyte compartment <NUM> is fluidly connected to the cathode electrolyte compartment <NUM> for feeding a second electrolyte solution to the cathode <NUM>. The separator membrane <NUM> can divide the anode electrolyte compartment <NUM> from the cathode electrolyte compartment <NUM>. The first electrolyte solution flows in a loop <NUM> as a diluted solution from the first electrolyte compartment <NUM> to the anode electrolyte compartment <NUM> and as a concentrated solution from the anode electrolyte compartment back to the first electrolyte compartment <NUM>. Likewise, the second electrolyte solution flows in a loop <NUM> as a diluted solution from the second electrolyte compartment <NUM> to the cathode electrolyte compartment and as a concentrated solution back to the second electrolyte compartment.

In the first and second electrolyte compartments <NUM> and <NUM>, the concentrated electrolyte solutions are diluted with fresh water produced by the osmosis unit <NUM>. Advantageously, part of the concentrated electrolyte solution from the first and/or second electrolyte compartments can be drawn to the permeate compartment <NUM> of the osmosis unit and provide the driving force to extract water from the liquid feed solution of feed compartment <NUM>, i.e. through a difference in osmotic pressure. It will be convenient to note that the first and second electrolyte solutions can have substantial identical composition. By way of example, concentrated first and second electrolyte solutions can be drawn from the first and second electrolyte compartments <NUM> and <NUM> respectively, and fed via a loop <NUM> to the permeate compartment <NUM>. The concentrated first and second electrolyte solutions can be mixed in loop <NUM> and/or in the permeate compartment <NUM>.

The first electrolyte compartment <NUM> can act as a separator for extracting hydrogen gas produced at the anode <NUM> from the first electrolyte solution. The hydrogen gas can be evacuated from the first electrolyte compartment <NUM> at extraction outlet <NUM>. The second electrolyte compartment <NUM> can act as a separator for extracting oxygen gas produced at the cathode <NUM> from the second electrolyte solution. The oxygen gas can be evacuated from the second electrolyte compartment <NUM> at extraction outlet <NUM>.

The osmosis unit <NUM> of device <NUM> can be made substantially identical to the osmosis unit of device <NUM> of <FIG>. Preferably, the first compartment <NUM> of the osmosis unit <NUM> is delimited by the first side <NUM> of the forward osmosis membrane <NUM> and the second compartment <NUM> of the osmosis unit <NUM> is delimited by the second side <NUM> of the forward osmosis membrane <NUM>.

According to an alternative embodiment (not shown), the device <NUM> of <FIG> is altered to comprise two osmosis units instead of one. Each osmosis unit can be fluidly connected to a respective one of the first and second electrolyte compartments <NUM> and <NUM>. The second compartments <NUM> of the osmosis units can be fluidly connected in parallel or in series.

It is also possible to form separate loops between, on the one hand the anode electrolyte compartment <NUM>, the first compartment <NUM> and the first (permeate) compartment of the osmosis unit <NUM>, and on the other hand the cathode electrolyte compartment <NUM>, the second compartment <NUM> and the first (permeate) compartment of the osmosis unit <NUM>. Alternatively, each of the separate loops may be connected to a separate osmosis unit as outlined above.

The device of the present invention may comprise additional electrochemical cells, i.e. a stack of electrochemical cells. Preferably, each electrochemical cell comprises a separator membrane dividing the electrolyte compartment into an anode electrolyte compartment and a cathode electrolyte compartment.

All electrochemical cells of the stack are preferably fluidly connected to the osmosis unit as indicated above. The stack can comprise appropriate flow distribution manifolds for the anode electrolyte compartments and for the cathode electrolyte compartments which are fluidly connected to unit <NUM> or to the first and second electrolyte compartments <NUM> and <NUM> respectively. The anode electrolyte compartments of the stack, the cathode electrolyte compartments, or both may be fluidly connected in series. Alternatively, they may be fluidly connected in parallel.

The forward osmosis membrane <NUM> is advantageously a semi-permeable, porous membrane which is selective towards water molecules. When the liquid feed stream is a saline water stream, such as seawater or brackish water, the forward osmosis membrane is advantageously impermeable to salt, e.g. with a rejection higher than <NUM>%.

Preferably, the forward osmosis membrane <NUM> has a high density of an active layer for a high rejection of the solute, such as salt when the liquid feed stream is a saline water stream. The osmosis membrane may further comprise a thin support layer with minimum porosity to allow a high flux of water.

Preferably, the forward osmosis membrane is hydrophilic. A hydrophilic osmosis membrane contributes to the flux of water, i.e. the quantity of water being fed from the liquid feed stream to the aqueous electrolyte solution per unit of time. A hydrophilic membrane also contributes to a reduction of fouling of the membrane at the second side.

The forward osmosis membrane comprises advantageously an active layer substantially made of asymmetric aromatic polyamide, cellulose acetate optionally reinforced with mineral fillers, and cellulose triacetate. These membranes can be synthesized through phase inversion. An extensive review on the characteristics of forward osmosis membranes and their current application is provided in <NPL>.

Claim 1:
Method for the generation of hydrogen and oxygen from a liquid feed stream comprising water, the method comprising passing an electric current through an aqueous electrolyte solution to generate hydrogen and oxygen and a concentrated electrolyte solution is obtained, characterised in that water is fed to the concentrated electrolyte solution to obtain the aqueous electrolyte solution by means of forward osmosis, wherein the concentrated electrolyte solution is brought into contact with a first side (<NUM>) of a forward osmosis membrane (<NUM>) and the liquid feed stream comprising water is brought into contact with a second side (<NUM>) of the forward osmosis membrane, wherein water permeates through the forward osmosis membrane (<NUM>) from the second side (<NUM>) to the first side (<NUM>) by means of a difference in osmotic pressure between the liquid feed stream and the concentrated electrolyte solution.