GB2504959A

GB2504959A - Hydrogen generators and methods of generating dry hydrogen

Info

Publication number: GB2504959A
Application number: GB1214509.0A
Authority: GB
Inventors: William Francis Houlton Dean; Stephen Wood Powell
Original assignee: Ion Science Ltd
Current assignee: Ion Science Ltd
Priority date: 2012-08-14
Filing date: 2012-08-14
Publication date: 2014-02-19
Anticipated expiration: 2032-08-14
Also published as: GB201214509D0; GB2504959B

Abstract

A hydrogen generator includes a composite electrolytic cell comprising a pair of membrane electrode assemblies 1, 17 each incorporating a solid polymer electrolyte 2a polarised so as to generate hydrogen from water at a cathode and oxygen at an anode, the electrode assemblies dividing the interior of the generator into three successive chambers 11, 12, 23 including a common chamber 11 in which the respective cathode surfaces of the two electrode assemblies face each other, wherein the only external supply of liquid water 2 is to the chamber 12 bounded by the anode surface of the first electrode assembly 1. The polymer electrolyte is preferably a proton conducting polymer. The generator may comprise a comparator for measuring the ratio of respective currents passing through the first and second electrode assemblies. Also disclosed is a method of generating dry hydrogen using the generator of the invention, in which both cathodes generate hydrogen in the common chamber from water passing through the first electrode assembly.

Description

tM:;: INTELLECTUAL .*.. PROPERTY OFFICE Application No. 0B1214509.0 RTM Date:22 August 2012 The following terms are registered trademarks and should be read as such wherever they occur in this document: Nafion Du Pont Intellectual Properly Office is an operaling name of Ihe Patent Office www.ipo.gov.uk

HYDROGEN GENERATION

Field of the Invention

This invention concerns the electrolysis of water to form hydrogen.

Background of the Invention

Hydrogen is used in large quantities in oil refining, for example in the breaking down large hydrocarbon molecules into smaller ones in the process known as hydrocracking. It is also used in manufacture of ammonia by the Haber process.

Such hydrogen is usually sourced from the reaction of a fossil fuel, typically coal or methane, with water.

Hydrogen is also required as a chemical intermediate in various synthetic processes. In semiconductor manufacture it is occasionally required in an ultrapure state. It is used as a specialist fuel source, for example in the operation of flame ionisation detectors. Hydrogen is also widely recognised as a clean alternative to fossil fuels.

The sole product of its combustion in air is water, and water is a readily available source of hydrogen.

A few per cent of the hydrogen used worldwide is produced by water electrolysis. The process requires the application of a positive electric potential to one electrode, known as an anode, relative to another electrode, known as the cathode, in between which is either an acid or alkaline electrolyte. This applied potential is known hereinafter for convenience as the cell potential. In an acid electrolyte, the electrode reactions are: H20 => 2H' + -i 02 + 2e at the anode, and 2H + 2e => H2 at the cathode.

The minimum cell potential required to electrolyse the water is 1.229 V. cell operating at this potential is said to have a cell efficiency of 100%, and since the hydrogen is produced at the rate of one molecule per two electrons passed through the cell, this corresponds to producing hydrogen at a rate of 33 kW.hr per kilogram hydrogen. Any inefficiency in hydrogen production is manifest as waste heat.

Cell efficiencies of at least 801 have been reported for alkaline water electrolysis which is commonly used for large scale hydrogen production. However, according to the present art, alkaline electrolytes are typically liquids, which afford a particular risk of gas migration between the anode and cathode, and often carry a hazard associated with their being highly caustic.

Efficiencies of at least 601 can be achieved by acid electrolysis using proton conducting polymer electrolytes in sheet form, such as perfluorinated sulfonic acid polymer known under the trade name Nafion 114 of Dupont Chemicals.

The electrolyte contains an open network of channels and inverted micelles though which water is freely transported and along whose walls protons readily move under an electric field. There exists substantial art in supporting on or just within both polymer electrolyte faces a layer of particles of a metal or metal oxide catalyst often mixed with graphite to form what is referred to hereinafter as a membrane electrode assembly. In contact with suitable electric contacts and subjected to a sufficient electric potential, the surfaces of the membrane electrode assembly are caused to be electrically conductive and conducive to the production and evolution of oxygen and hydrogen at an anode and cathode respectively formed thereby. Typically, the anode contains as one surface ingredient iridium oxide, and the cathode contains as one surface ingredient platinum.

Water, removed from the electrolyte by electrolysis and evaporation into the produced hydrogen and oxygen, can be simply replaced by contact of liquid water with the electrolyte. Under electrolysis, liquid water is deposited at the cathode side, despite the consumption of water at the anode, in a process known as electro-osmosis.

A particular advantage of the use of a membrane electrolyte assembly in electrolysis of water is in the physical separation of produced hydrogen and oxygen. Any admixture of the two gases in the vicinity of the cathode or anode substantially reduces the cell efficiency, because hydrogen at an anode is more readily oxidised to protons than water to oxygen, and oxygen at a cathode is more readily reduced to water than protons to hydrogen.

Moreover, mixtures of pure hydrogen and oxygen, particularly in the ratio of 2:1 in which they are formed by electrolysis, can be explosive. This is a particular concern where it is intended to use the hydrogen in an explosive atmosphere, such as in its being a source for a flame to operate a flame ionisation detector in the detection of gases which may cause an explosion.

Water itself is also known as a oatalyst for hydrogen ignition and therefore it is desirable to dry hydrogen from the cathode before admixing it with oxygen. Moreover, downstream of a hydrogen generator it is often convenient for the means of conveyance of hydrogen to be restricted, such as to assure its pressure is controlled during mixing with other gases. The means of conveyance of hydrogen, by way of example, a capillary, may also be restrictive, but engaged downstream of the hydrogen generator with a view to minimising the volume of hydrogen contained within the means of conveyance, such as to ensure a rapid response of a flow of hydrogen delivered remote from the water electrolyser to a change in the electrolyser cell current which regulates hydrogen production. These restrictions are all liable to blockage by water condensate, particularly in view of hydrogen exiting the cell being warm and therefore prospectively being inclined to condensation. For similar reasons, it may be desirable to dry electrochemically produced oxygen, particularly if the oxygen is to be premixed with hydrogen before its combustion in a flame ionisation detector, as can be desirable in testing gases where the oxygen content of the test gas is low, such as in testing smoke stack gas.

Water-free hydrogen may be desirable in other applications too, for example in its use as a feedstock for ultrapure hydrogen purification through a palladium membrane that may be degraded by the presence of water.

Conventionally, the hydrogen stream is dried using a desiccant such as silica gel.

WO 01/19728, The Robert Gordon University, describes the use of a cellulose acetate membrane to separate hydrogen from entrained water.

In US patent 6,096,178, Amirov et al. describe the use of a Peltier in removal of water from electrolytically produced hydrogen for use in a flame ionisation detector.

Whilst very effective, a Peltier requires considerable electrical power and therefore is a distinct disadvantage in a portable and battery operated tool. Further, the condensed water trapped by any means needs to be removed and is liable to migrate where it is not wanted when the Peltier cooler is turned off.

Our co-pending patent application GB 2482744 A and equivalents elsewhere describe and claim a hydrogen generator including at least two electrolytic cells, each comprising a membrane electrode assembly incorporating a solid polymer electrolyte. The membrane electrode of a first cell is exposed to water and the hydrogen produced in this cell contacts the cathode surfaces of the successive electrolytic cells, wherein any water entrained in or carried over with the hydrogen produced in the first cell is the only significant feedstock for the successive electrolytic cells.

The present invention is a modification and improvement of GB 2482744 A. The present invention enables electrolytically-produced hydrogen to be dried not only more efficiently, but also so as to eliminate the need for condensate removal. The present invention confers an increased efficiency on a hydrogen electrolyser, and affords a diagnostic for monitoring the dryness of hydrogen produced by the water generator.

Summary of the Invention

According to the present invention we provide an apparatus for and a method of generating dry hydrogen by electrolysis of water in a composite electrolytic cell comprising a pair of membrane electrode assemblies each incorporating a solid polymer electrolyte polarized so as to generate hydrogen from water at a cathode and oxygen at an anode, which membrane electrode assemblies divide the interior of the hydrogen generator into three successive chambers, wherein (a) the only external supply of liquid water is to a chamber bounded by the anode surface of a first membrane electrode assembly and wherein (b) the respective cathode surfaces of the two membrane electrode assemblies face each other in a common chamber in such a manner that both cathodes generate hydrogen in the common chamber from water passing through the first membrane electrode assembly or otherwise entrained in the hydrogen.

In essence, the membrane electrode assemblies function as a pair of water electrolyzing cells arranged back-to-back, in which the first cell is at least partially exposed on the anodic side to a supply of liquid water and the hydrogen produced in this cell flows over the cathode surfaces of both water electrolyzing cells. The first cell acts both as a primary hydrogen generator and as a partially water-retentive membrane. As well as generating hydrogen, the second (downstream) cell acts as a drier for excess water that gets through the primary membrane. In so removing excess water, the second cell takes an increased burden of the hydrogen generation, thus lowering the water throughput from the primary generator.

Hereinafter for convenience the water-wetted membrane electrode assembly is referred to as the primary cell, with the evolved gas being introduced into the secondary cell.

Preferably, both membrane electrode assemblies are operated at substantially the same cell potential, thereby balancing water consumption: the more water entering the common cathodic chamber through the primary cell the more humidified is the secondary cell, such that more electrolysis is facilitated in this secondary cell, causing more water to be removed from the common cathodic chamber.

However, it is typically deleterious for cells to be driven at cell potentials exceeding 3 V. The hydrogen generator may incorporate a comparator for measuring the ratio of respective currents passing through the first and second membrane electrode assemblies. Such comparison can provide a measure of the guantity of water carried through to the common cathodic chamber or a diagnostic of operation of the hydrogen generator, such as cell efficiencies. The cell potentials and the ratio of the respective currents passing through the primary and secondary cells indicate the humidity of the hydrogen produced in the secondary cell and is also diagnostic of operation of the hydrogen generator.

Preferably, electrical contact is made with the membrane electrode assemblies by means of at least one electrically conducting mesh which overlies the electrode surfaces in electrical contact therewith. In particular, a relatively fine mesh directly overlies the electrode surfaces and a second, relatively coarse mesh overlies the fine mesh. Relatively fine or relatively coarse refers to the pitch of the respective meshes relative to each other.

Means are provided to urge the relatively stiff coarse mesh against the fine mesh, resulting in the fine mesh making intimate contact with the membrane electrode assembly, in order to disperse a relatively uniform force between the mesh and the membrane electrode assembly.

In every embodiment of the invention, it is necessary for there to be some region of confinement of water in contact with the anodic surface of the primary cell. It is frequently an advantage to confine such water with a material such as porous PTFE which allows for the facile passage of gas, but prevents passage of water droplets from one location to another.

It may be convenient for all cells provided according to the present invention to be electrically operated so as to deliver a controllable total operating current by which means the flow of hydrogen produced may also be controlled.

By way of example, a fairly precise flow of hydrogen may be required for the stable maintenance of a hydrogen flame.

That flow of hydrogen may need to be adjusted to ensure flame stability during different times of the flame's use, such as whenever it is ignited or when it is operating in particular ambient conditions of heat cold and such like.

It should be noted that the invention relies upon a particular quality of acid polymer electrolytes in becoming increasingly water adsorbing as they become increasingly dry. The invention encompasses the drying of hydrogen from a state of complete humidification to hydrogen that may exit the hydrogen generator either partially dried or extremely dry.

Brief Description of the Drawings

The invention will now be described by reference to the Figures wherein:-Figure 1 shows a schematic of an embodiment of the invention in which hydrogen is dried by a two cell configuration sharing a common gaseous chamber; and Figure la shows an enlarged view of a membrane electrode assembly.

Description of a Preferred Embodiment

Figure 1 shows a hydrogen generator comprising an overall housing divided into three chambers, 12, 11 and 23, by a pair of opposed membrane electrode assemblies 1 and 17.

Each assembly constitutes an electrolytic cell. 7\s described below, water is fed to the first chamber 12 and hydrogen is generated in the second, common chamber 11. In the first chamber 12, the anodic surface of a primary cell 1 is in contact with liquid water 2.

Primary cell 1 is depicted in detail in Figure la. The membrane electrode assembly comprises a solid polymer electrolyte 2a, integral anodic coating 2b conducive to oxygen generation, and integral cathodic coating 2c conducive to hydrogen generation. Overlaying 2b and 2c are fine electrically conducting meshes 3 and 4 respectively.

Overlaying meshes 3 and 4 are more robust and rigid electrically conducting screens 5 and 6, to which are welded non-corrosive contact wires annotated + and -in Figure la, corresponding, for example, to leads 7 and 8 respectively in Figure 1. Preferably, meshes 3 and 4 and screens 5 and 6 are substantially made of titanium.

Preferably, solid polymer electrolyte 2a is a proton conducting polymer such as a perfluorinated sulfonic acid polymer known under the trade name Nafion 114 of Dupont Chemicals.

Turning to Figure 1, the whole assembly is held in place by virtue of 0-rings 9, which provide gas tight seals between the membrane electrode assemblies and cell support members, and by 0-ring 10, which imparts a force upon rigid support screens 5 and 6, by which means electrical resistances between all members of the assembly shown in Figure la are kept low.

Electrical polarization of the assembly applied to leads 7 and 8 as shown by the plus and minus signs in Figures 1 and la causes hydrogen to be evolved in the common gaseous chamber 11 and oxygen in the watery chamber 12. The oxygen is vented from the first chamber 12 in conventional manner (not shown) . A presence of water 2 is maintained in chamber 12 by means of a water entry port 13.

Hydrogen passing though the membrane electrode assembly 1 enters the common gaseous chamber 11, where it is dried by contact with the cathodic sides of the primary cell 1 and secondary cell 17. The latter is of similar construction to primary cell 1, noting that the hydrogen generating, cathodic side of the cell 17 faces cathodic side of cell 1, the electrical lead 18 is positively polarized relative to the secondary cell electrical lead 19, and the components of cell 17 are as shown in Figure la but oppositely oriented.

Dried hydrogen exits common chamber 11 at gas exit port 20.

A further port 22 enables venting of oxygen from the third chamber 23. The whole assembly is held in place by nuts such as 24 and threaded sections such as 25.

The invention will now be described by way of how it may be operated. A supply of de-ionised water, which could by way of example be a collapsible bag under constraint, admits water into the first chamber 12 via port 13.

Electrical circuitry, not illustrated, causes a current of I amps to flow through the membrane electrode assembly 1 from lead 8 to lead 7. Typically, a flow through the primary cell is around 1 A per square centimetre, achieved by applying a typical maximum voltage to lead 7 of +3 V relative to lead 8. The primary cell provides a flow of hydrogen F1, which is typically the major fraction of hydrogen demanded by a device engaging the invention. The primary cell provided by the invention is sized accordingly.

Moist hydrogen is generated in common enclosure 11, where it is exposed to the cathodic surface of secondary cell 17, as is any water dragged through to the cathodic side of primary cell 1 by protons and dumped' there when they combine with electrons to form molecular hydrogen.

Electric circuitry, not shown, causes an electric potential, typically not exceeding +3 V and preferably identical with the cell potential across the primary cell 1, to be applied to lead 18 relative to lead 19. This causes a flow of current, 12, arising from the electrolysis of water which has been carried across from the primary cell 1 or adsorbed from the hydrogen gas stream by the electrolytic membrane in the secondary cell 17. The current 12 is typically less than half of the current I generated in the primary cell 1. The exact ratio of currents I2:I provides a -10 -useful diagnostic indication of the humidity of hydrogen and the quantity of water presented to the seoondary cell.

It should be noted that during typical operation all the current 12 moving through the secondary cell 17 will generate a flow of hydrogen F'2 in addition to the flow of hydrogen F'1 generated at the primary cell 1. Therefore, the electrical and electronic circuitry may be contrived to deliver a constant current supply to the several cells engaged according to the invention, as that total current will determine the total flow of hydrogen Ftc-ai generated by the invention.

= 0. 127 tota when flow is measured in mL/s at 1 bar and 20 degc and current in amps.

Exactly a flow of F-r)ta/2 of oxygen is concurrently generated by the several cells provided according to the present invention.

Thus the invention not only provides for the drying of an electrolytically generated hydrogen stream, but for the complete removal of the water, for its gainful use as feedstock to deliver more hydrogen, and for a diagnostic of correct or normal operation of the gas generator. In such respects the present invention provides for dry hydrogen generation which is reliable, requires no waste water disposal, which is electrically efficient, and which can be monitored. i:i

Claims

CLAIMS1. A hydrogen generator including a composite electrolytic cell comprising a pair of membrane electrode assemblies each incorporating a solid polymer electrolyte polarized so as to generate hydrogen from water at a cathode and oxygen at an anode, which membrane electrode assemblies divide the interior of the hydrogen generator into three successive chambers, wherein (a) the only external supply of liguid water is to a chamber bounded by the anode surface of a first membrane electrode assembly and wherein (b) the respective cathode surfaces of the two membrane electrode assemblies face each other in a common chamber in such a manner that both cathodes generate hydrogen in the common chamber from water passing through the first membrane electrode assembly or otherwise entrained in the hydrogen.
2. A hydrogen generator as claimed in Claim 1 in which both membrane electrode assemblies are operated at substantially the same cell potential.
3. A hydrogen generator as claimed in Claim 2 incorporating a comparator for measuring the ratio of respective currents passing through the first and second membrane electrode assemblies.
4. A hydrogen generator as claimed in any one of the preceding claims wherein the solid electrolyte is a proton conducting polymer.
5. A hydrogen generator as claimed in Claim 4 wherein the electrolyte is a perfluorinated sulfonic acid polymer.
6. A hydrogen generator as claimed in any one of the preceding claims in which at least one electrically conducting mesh overlies the electrode surfaces in electrical contact therewith.
7. A hydrogen generator as claimed in Claim 6 in which a relatively fine mesh directly overlies the electrode surfaces in electrical contact therewith, and a second, relatively coarse mesh overlies the fine mesh in electrical contact therewith.
8. A hydrogen generator as claimed in Claim 7 including means to urge the relatively stiff coarse mesh against the fine mesh.
9. A method of generating dry hydrogen by electrolysis of water in a composite electrolytic cell comprising a pair of membrane electrode assemblies each incorporating a solid polymer electrolyte polarized so as to generate hydrogen from water at a cathode and oxygen at an anode, which membrane electrode assemblies divide the interior of the hydrogen generator into three successive chambers, wherein (a) the only external supply of liquid water is to a chamber bounded by the anode surface of a first membrane electrode assembly and wherein (b) the respective cathode surfaces of the two membrane electrode assemblies face each other in a common chamber in such a manner that both cathodes generate hydrogen in the common chamber from water passing through the first membrane electrode assembly or otherwise entrained in the hydrogen.
10. A method of generating dry hydrogen as claimed in Claim 9 wherein both membrane electrode assemblies are operated at substantially the same cell potential.
11. A method of operating a hydrogen generator as claimed in Claim 10 wherein the generator incorporates a comparator for measuring the ratio of respective currents passing through the first and second membrane electrode assemblies.
12. A method of operating a hydrogen generator as claimed in Claim II wherein the ratio of the respective currents passing through the first and second cells is a diagnostic of operation of the hydrogen generator.
13. A method of operating a hydrogen generator as claimed in any one of Claims 9 to 12 wherein both membrane electrode assemblies are electrically operated so as to deliver a controllable total operating current, by which means the flow of hydrogen produced may also be controlled.
14. A hydrogen cell generator substantially as described and with reference to the accompanying drawings.
15. A method of operating a hydrogen cell generator substantially as described and with reference to the accompanying drawings.