Apparatus and method for production of hydrogen gas

The present invention is concerned with an apparatus and a method for the production of hydrogen gas. More specifically the present invention is concerned with an apparatus and method employing a cycle of regenerative electro-dialysis.

Hydrogen gas is commonly seen as a potential solution to the world’s dependence on fossil fuels. When burned in the presence of oxygen, the subsequent oxidation reaction produces energy (in the form of heat) and water. Therefore, providing hydrogen can be generated in a low / zero carbon manner, it offers a potential alternative to fossil fuels when implemented in e.g. vehicles containing hydrogen fuel cells.

Hydrogen is also used in a number of other chemical processes, e.g. for the production of ammonia and methanol.

Hydrogen gas can be generated in a number of ways. Many such industrialised methods rely on fossil fuels- for example the steam reforming of natural gas, or partial oxidation of methane. This is clearly not addressing the problem discussed above- such methods inevitably produce carbon-based by products (and ultimately C02).

Electrolysis of water also produces hydrogen but requires large amounts of electricity. Electrolysis is somewhat inefficient, and depending on the technique and equipment used offers between 70 and 80% efficiency. Beneficially, such a process (consuming only electricity and water) can be powered from e.g. renewable sources, making it ‘green’. That said, there is room for improvement based on the efficiencies commonly found.

Systems and methods of incorporating reverse electrodialysis (RED) technology are known for generating electricity and hydrogen by decomposing water. The technology relies on a difference in salt concentration between two electrolytes. Typically, seawater and fresh water are fed through a stack of alternating cation and anion exchange membranes to create a chemical potential difference over each membrane so as to generate a voltage over the stack. While this technology can be a useful add-on to a large seawater desalination plant, the stacked arrangement of a plurality of membranes and the need for a continuous supply of electrolytes for a relatively low productive output makes it unsuitable as a standalone energy or hydrogen supply.

It is the aim of the present invention to mitigate the aforementioned problems.

According to a first aspect of the present invention there is provided an apparatus for the production of hydrogen gas having: a first electrodialysis cell, and a second electrodialysis cell, each electrodialysis cell comprising: a first reservoir portion having an inlet, a first (liquid) outlet and a second (gas) outlet; a second reservoir portion having an inlet, a first (liquid) outlet and a second (gas) outlet; an ion exchange membrane separating the first and second reservoir portions; an anode positioned within the first reservoir portion; a cathode positioned within the second reservoir portion; wherein: the first (liquid) outlet from the first reservoir portion of the first cell is in fluid communication with the inlet of the second reservoir portion of the second cell; the first (liquid) outlet from the second reservoir portion of the first cell is in fluid communication with the inlet of the first reservoir portion of the second cell; the first (liquid) outlet from the first reservoir portion of the second cell is in fluid communication with the inlet of the second reservoir portion of the first cell; the first (liquid) outlet from the second reservoir portion of the second cell is in fluid communication with the inlet of the first reservoir portion of the first cell; and wherein; an aqueous alkaline electrolyte is provided in the respective first reservoirs; water is provided in the respective second reservoirs; upon the supply or power to each anode / cathode pair: the aqueous alkaline electrolyte is separated into positively charged ions and hydroxyl radicals in the first reservoir by the anode; the water is separated into hydrogen and hydroxyl radicals by the cathode; the positively charged ions pass across the ion exchange membrane to combine with the hydroxyl radicals in the second reservoir to release hydrogen gas via the second (gas) outlet. Advantageously, the above arrangement provides a regenerative electrodialysis hydrogen production apparatus that consumes less electricity than most electrolysis-based systems. The arrangement only consumes deionised water. No primary electrolyte charge is consumed. The present invention has been tested and consumes up to 25% less energy than conventional electrolysis modes of hydrogen production per unit volume of hydrogen gas produced.

Preferably a catalyst is provided in the first and/or second reservoir portion.

Preferably the anode is a catalytic anode and/or the cathode is a catalytic cathode.

Alternatively a catalyst may be coupled to the anode and/or the cathode.

The apparatus may comprise a third electrodialysis cell, wherein the first second and third cells are arranged in series.

The apparatus may comprise a third electrodialysis cell, wherein the first second and third cells are arranged in parallel.

Preferably the electrolyte is an aqueous alkaline hydroxide, more preferably a metal hydroxide, and comprises one or more of the following: calcium hydroxide Ca(OH) ₂ ; potassium hydroxide KOH; sodium hydroxide NaOH; rubidium hydroxide RbOH; lithium hydroxide LiOH barium hydroxide Ba(OH)2; and, caesium hydroxide CsOH.

According to a second aspect there is provided a method for the production of hydrogen gas comprising the steps of: providing a first electrodialysis cell having first and second reservoir portions; providing a second electrodialysis cell having first and second reservoir portions; each electrodialysis cell comprising an anode positioned within the first reservoir portion, a cathode positioned within the second reservoir portion and, an ion exchange membrane separating the respective first and second reservoir portions; providing an aqueous alkaline hydroxide electrolyte in the respective first reservoirs; providing water in the respective second reservoirs; applying a potential difference between each respective anode and cathode to thereby: separate the aqueous alkaline electrolyte into positively charged ions and hydroxyl radicals in the first reservoir at the anode; and, separate the water into hydrogen and hydroxyl radials at the cathode; allowing the positively charged ions to pass across the ion exchange membrane to combine with the hydroxyl radicals in the second reservoir to release hydrogen gas via the second (gas) outlet; transporting water from the respective first reservoirs of one of the first and second cells to the respective second reservoirs of the other of the first and second cells; transporting aqueous alkaline electrolyte to the respective second reservoirs of one of the first and second cells to the respective first reservoirs of the other of the first and second cells.

Preferably the method comprises the step of: providing a catalyst in the first and/or second reservoir portion.

Preferably the anode is a catalytic anode and/or the cathode is a catalytic cathode.

Preferably the catalyst is coupled to the anode and/or the cathode.

Preferably the method comprises the step of: providing a third electrodialysis cell; and, arranging the first second and third cells in series.

Preferably the method comprises the step of: providing a third electrodialysis cell; and, arranging the first second and third cells in parallel.

An example apparatus and method in accordance with the present invention will now be described with reference to the accompanying Figures in which:

Figure 1 is a schematic view of an apparatus according to the present invention;

Figure 2 is a flow diagram showing the steps of a process according to the present invention; Figure 3 is a schematic view of a four-cell apparatus in a series configuration; and,

Figure 4 is a schematic view of a four-cell apparatus in a parallel configuration.

Configuration of the first embodiment

Referring to Figure 1 , an apparatus 100 according to the present invention is shown schematically. The apparatus 100 comprises a first cell 200 and a second cell 300. The cells 200, 300 are almost identical, and as such only the cell 200 will be described in detail here. Features of cell 200 that are also present on cell 300 will be identified by reference numerals 100 greater.

The cell 200 comprises a tank 202 enclosing a fluid reservoir 204. The tank 202 contains a semi-permeable membrane 206 dividing the reservoir into a first reservoir portion 208 and a second reservoir portion 210. The membrane 206 is an ion exchange membrane as known in the art. It is configured to be impermeable to gases and liquids, but permits passage of certain ions. For the purposes of the present invention, the membrane needs to permit at least passage of specific positively charged ions as will be discussed below.

The tank 202 defines a first reservoir portion inlet 212 and a first reservoir portion outlet 214 in fluid communication with the first reservoir portion 208. The tank 202 defines a second reservoir portion inlet 216 and a second reservoir portion outlet 218 in fluid communication with the second reservoir portion 210. A supply inlet 220 is also provided in fluid communication with the second reservoir portion 210. The first reservoir portion 208 and the second reservoir portion 210 each comprise a respective first reservoir 222 and second reservoir gas outlet 224.

A catalytically active anode 226 is positioned within the first reservoir portion 208. A catalytically active cathode 228 is positioned within the second reservoir portion 210.

In terms of the materials used to the catalytically active cathode, this may be at least partially constructed from one or more of the following non-exhaustive list of materials:

• Platinum on a carbon or graphene substrate;

• platinum black catalyst layer on Nafion;

• a ferro-nickel alloy;

• rhodium-iridium alloy;

• oxide based coatings;

• nickel-iron oxide composites;

• Lanthanum Nickel Cobaltite;

• Lanthanum Strontium Cobalt Ferrite; • Lanthanum Strontium Manganite coatings, composites and laminates.

In terms of the materials used at the catalytically active anode, this may be at least partially constructed from one or more of the following non-exhaustive list of materials:

• ferro-nickel alloy;

• ferro-nickel oxide on a titanium substrate;

• a combination of iron/nickel/cobalt powder on titanium;

• lead zinc dioxide on titanium mesh;

• Lead dioxide on a carbon substrate;

• Lead dioxide on zeolite;

• Mixed metal oxides on a suitable substrate.

Cathode or Anode construction may be based on a host material, for example including but not limited to:

• Ceramic;

• titanium plate, mesh or film;

• graphite;

• graphene;

• polyflouro mesh / catalyst compresses;

• one or more printed conductive catalyst elements on card or other substrate conducive to receiving printed materials.

The cell 300 comprises a tank 302 enclosing a fluid reservoir 304. The tank 302 contains a semi-permeable membrane 306 dividing the reservoir into a first reservoir portion 308 and a second reservoir portion 310. The tank 302 defines a first reservoir portion inlet 312 and a first reservoir portion outlet 314 in fluid communication with the first reservoir portion 308. The tank 302 defines a second reservoir portion inlet 316 and a second reservoir portion outlet 318 in fluid communication with the second reservoir portion 310. A supply inlet 320 is also provided in fluid communication with the second reservoir portion 310. The first reservoir portion 308 and the second reservoir portion 310 each comprise a respective first reservoir 322 and second reservoir gas outlet 324.

A catalytically active anode 326 is positioned within the first reservoir portion 308. catalytically active cathode 328 is positioned within the second reservoir portion 310. The anode 326 / cathode 328 may be constructed from the materials mentioned above with respect to the first cell 200.

The cells 200, 300 are connected as follows: The second reservoir portion outlet 318 of the cell 300 is connected to the first reservoir portion inlet 212 of the cell 200 via a first conduit 400.

The first reservoir portion outlet 314 of the cell 300 is connected to the second reservoir portion inlet 216 of the cell 200 via a second conduit 402.

The second reservoir portion outlet 218 of the cell 200 is connected to the first reservoir portion inlet 312 of the cell 300 via a third conduit 404.

The first reservoir portion outlet 214 of the cell 200 is connected to the second reservoir portion inlet 316 of the cell 300 via a fourth conduit 406.

Each conduit 400, 402, 404, 406 comprises a respective fluid pump 401, 403, 405, 407.

Before use, the first reservoir 208 of the first cell 200 is filled with an aqueous alkaline hydroxide electrolyte. It will be understood that the electrolyte may be selected from one or more of the following:

• Calcium hydroxide Ca(OH)2;

• Potassium hydroxide KOH;

• Sodium hydroxide NaOH;

• Rubidium hydroxide RbOH;

• Lithium hydroxide LiOH

• Barium hydroxide Ba(OH)2;

• Caesium hydroxide CsOH.

This is a non-exhaustive list, and the skilled addressee will understand that other electrolytes may be used. The second reservoir 210 of the first cell 200 is filled with water (H2O).

The first reservoir 308 of the second cell 300 is also filled with the same aqueous alkaline electrolyte as the first cell 200. The second reservoir 310 of the second cell 300 is filled with water (H2O).

The anode and cathode of each cell 200, 300 are connected to respective DC power sources (not shown in Figure 1).

Method of operation of the first embodiment

Turning to Figure 2, the following steps constitute operation of the apparatus 100.

Step 500 - DC power to the two cells 200, 300 is switched on; Step 502 - At the first cell anode 226, positively charged metal ions (for example Ca ²⁺ , K ⁺ etc.), under the influence of the induced electric charge, are driven through the membrane 206 from the first reservoir 208 to the second reservoir 210.

Step 504 - At the same time, at the anode 226 there is a self-reaction of remaining hydroxyl radicals (OH ) producing water and oxygen:

OH ^~ + OH ^~ = H ₂ 0 + O leading to oxygen being released via the first reservoir gas outlet 222. The aqueous alkaline electrolyte is reduced to H2O and O. As this occurs, the pH is rebalanced towards 7 (neutral). Electro-dialysis is occurring here.

Step 506 - The depleted electrolyte (i.e. H2O) is passed to the second reservoir 310 of the second cell 300 via the fourth conduit 406.

Step 508 - At the second reservoir 210 of the cell 200, the catalytically active cathode 228 splits the water into hydrogen (H) and hydroxyls (-OH radicals).

Step 510 - The surplus of positively charged ions now at the cathode side (after passing through the membrane 206) combines with the released hydroxyl radicals to form an aqueous hydroxide solution.

Step 512 - At the same time, hydrogen gas (H2) is formed and released via the second reservoir gas outlet 224.

Step 514 - The aqueous hydroxide concentrate passes from the second reservoir 210 to the first reservoir 308 of the second cell 300 via the third conduit 404.

Step 516 - At the second cell anode 326, positively charged ions, under the influence of the induced electric charge, are driven through the membrane 306 from the first reservoir 308 to the second reservoir 310.

Step 518 - At the same time, at the anode 326 there is a self-reaction of remaining hydroxyl radicals (OH ) producing water and oxygen:

OH ^~ + OH ^~ = H ₂ 0 + O leading to oxygen being released via the first reservoir gas outlet 322.

Step 520 - The depleted electrolyte (i.e. H2O) is passed to the second reservoir 210 of the first cell 200 via the second conduit 402.

Step 522 - At the second reservoir 310 of the cell 300, the catalytically active cathode 328 splits the water into hydrogen and hydroxyl radicals (H, OH ). Step 524 - The surplus of positively charged ions at the cathode side of the cell (after passing through the membrane 306) combines with the released OH to form an aqueous hydroxide concentrate.

Step 526 - At the same time hydrogen gas (H2) is formed and released via the second reservoir gas outlet 324.

Step 528 - The aqueous hydroxide concentrate passes from the second reservoir 310 to the first reservoir 208 of the first cell 300 via the first conduit 400.

The system therefore forms a cycle of regenerative electrodialysis.

During the process the apparatus 100 consumes electricity (powering the anode / cathode pairs of each cell, and the pumps) and deionised water, which is continuously fed to each second reservoir 210, 310 via inlets 220, 320. Each cell produces both oxygen gas and hydrogen gas.

This embodiment relies on a flow of fluid through the cells and across each anode and cathode. For example, the aqueous hydroxide electrolyte enters cell 200 at inlet 212, passed downwardly across the anode 226 where it is depleted. Water is collected at the other side of the anode (outlet 214).

In terms of how the apparatus 100 manages the flow of fluid, there are several options:

In a first sub-embodiment, a pH sensor is provided in the first reservoir(s) 208, 308. Once the pH drops to a predetermined level (towards neutral - i.e. pH 7) a controller (not shown) activates the pumps 401, 403, 405, 407 to cycle the fluid between the cells. This would act to “recharge” reservoirs 208, 308 with the cation-rich electrolyte from the cathode sides and increase the pH.

In a second, alternative sub-embodiment, the pumps are continuously active. The flow rate through the pumps is selected to keep the pH in each first reservoir 208, 308 above a predetermined level. This can either be sensed and controlled in “real time”, or alternatively the system’s characteristic depletion curve can be measured and used to determine this optimum flow rate.

Configuration of the second embodiment

Turning to Figure 3, an apparatus 1000 is shown in which four cells 1100, 1200, 1300, 1400 are shown, each having an anode side (A) and cathode side (C). The cells 1100, 1200, 1300, 1400 are the same configuration as the first embodiment. The cells are connected in series. Conduits passing fluid from each anode A to the cathode of the next cell in the system are shown in dashed lines. Conduits passing fluid from each cathode to the anode of the next cell are shown in solid lines.

Configuration of the third embodiment

Turning to Figure 4, an apparatus 2000 is shown in which four cells 2100, 2200, 2300, 2400 are shown, each having an anode side (A) and cathode side (C). The cells 2100, 2200, 2300, 2400 are the same configuration as the first embodiment. The cells are connected in parallel. Conduits passing fluid from the anodes A to the cathodes are shown in dashed lines. Conduits passing fluid from the cathodes to the anodes are shown in solid lines.

It will be understood that any of the features of the first embodiment may be provided in the second and/or third embodiments.

Variations of the embodiments

Instead of a catalyst being attached to the respective cathodes, the cathode may be a catalytic cathode - i.e. it may be partially or entirely constructed from catalytic material.

Electrode materials may comprise any, or a combination of, the following: plated, solid plate, mesh or sintered platinum, palladium, gold, titanium, carbon, graphite, graphene, carbon fibre, zinc, silver, lead, zirconium, tungsten, platinum black, cobalt and iridium. Further electrode materials may be formed from of alloys comprising of two or more of platinum, palladium, gold, titanium, carbon, graphite, graphene, carbon fibre, zinc, silver, lead, cobalt, zirconium, tungsten, platinum black or iridium. Electrode structure may include but not limited to, plated sintered, pressed or enhanced substrates of carbon, graphene, zeolite, carbon fibre, carbon granules, carbon composites or ceramic substrates treated with but not limited to one or more of platinum, palladium, gold, titanium, zink, silver, cobalt, lead, zirconium, tungsten, platinum black or iridium.