KR20070030837A - Membrane electrode assembly with controlled water management/ion flow - Google Patents

Abstract

Membrane-electrode combinations include a grid for controlling ion flow, a separate layer of membrane material and / or an ionically inert material for the transfer of liquid or gaseous reaction components from and / or to one or more electrodes. .

Membrane Electrode Assembly, Water Management, Ion Flow

Description

Membrane electrode assembly with controlled water management / ion flow {MEMBRANE ELECTRODE ASSEMBLY WITH CONTROLLED WATER MANAGEMENT / ION FLOW}

The present invention relates to electrochemical cells and, in particular, membrane electrode combinations with controlled water management / ion flow.

Many ionic polymer membranes used as electrolytes in electrochemical cells contain only one active material. Inert materials are often used as scaffolds and mechanical structures for active polymers. Such films are stacked to provide different properties to each electrode. In all cases, the properties and materials are the same at each point of the two membrane electrode surfaces.

Effective water management in electrochemical cells is essential for good performance. For example, in fuel cells, performance strongly depends on hydration control in the solid polymer electrode (SPE). During operation of the fuel cell, ions travel through the SPE. In so doing, they accompany water molecules with them. This is known as electroosmotic drag. The end result of this phenomenon is that, for example in proton-conducting, cationic exchange (CE) SPEs, the positive electrode tends to dry and the negative electrode tends to flood. This has a negative impact on fuel cell performance by reducing the ease of ion conduction in the SPE and preventing access of oxygen to the cathode. Water is produced as a result of the electrochemical reaction at the cathode, thus creating a problem of cathode overflow. In a hydroxyl ion-conducting anionic exchange (AE) SPE, the situation is reversed, but the problem remains the same: water is produced at the anode, and used at the cathode, thus the anode tends to overflow and the cathode Tends to dry.

Attempts to overcome these problems have been made through two main routes. First, the fuel feed has been wetted to help the water be transported to the anode side of the SPE membrane and to reduce drying. While this presents a risk of anode overflow, it requires a substantial balance of equipment, which increases device costs and reduces power. Secondly, thinner SPE membranes have been sought to virtually reduce the distance that water has to flow back from the cathode to the anode. However, electro-osmotic drag still limits the water balance through this mechanism, while at the same time mechanically less stable SPE membranes have higher fuel cross over.

The voltage produced by the fuel cell generally depends on the electrochemical potential of the reactants and the efficiency of any catalyst used. The current generated by the cell depends on its effective area, the ionic resistance of the membrane and how effective the delivery of fuel is at the contact surface of the catalyst-membrane. The output is the product of voltage and current, and is usually provided by slowing or limiting the rate of fuel flow to the cell, or providing unrestricted fuel to the cell and adjusting the energy surplus to the requirements (e.g., By emptying). These methods of adjusting the output are, first of all, undesirable for reasons of inefficiency.

WO03 / 023980 discloses their use in hydrophilic polymers and membrane electrode combinations (MES's) that can be used in or as a fuel cell.

Summary of the Invention

The present invention provides a means by which water production and / or flow of ions through the membrane of the MEA can be controlled.

In a first aspect of the invention, the membrane comprises a material that is ionically inert to the transfer of liquid or gaseous reaction components to and / or from one or more electrodes. Alternatively, the membrane comprises an anionic exchange material and a cationic exchange material, and these materials are separated by an ionically inert material. The distribution of materials that are hydrophilic but not ionically active can maintain the effective operation of the ionomer under hydraulic conditions, especially the redistribution of product water in the cell, and the Nafion (or other low water content, low water permeable membrane) based solid polymer fuel Adjustments can be made to prevent or reduce flooding of the catalytic electrode structure, which is a common problem for cells.

In a second aspect of the invention, the membrane is in the form of a sheet or layer of essentially ionically active material, for example one or more anionic exchange materials in the form of one or more thin planar parallel layers and also one or more planar parallel layers One or more sheets or layers of cationic material, the layers being in ionic contact and arranged so that flow of ions from one electrode to the other passes through the contact surface between the layers of material. One system (called CE) contacts the anode and the other material (called AE) contacts the anode. In this case, the potent charge carriers are protons in CE and OH ions in AE. This arrangement means that the catalyst on each electrode can be independently selected so that one is operated in an acid environment while the other catalyst is operated in an alkaline environment. In addition, in the case of oxygen-hydrogen fuel cells, the product water will be produced at the interface between the AE and CE components of the composite membrane, thus avoiding the problem of flooding of the catalyst-electrode structure.

In another aspect of the invention, the MEA comprises an electrically conductive grid or grids. This approach, including regulating ion flow by providing an appropriate potential difference through at least a portion of the combination, is completely separate from obstructing and discarding the aforementioned techniques. It is particularly suitable for using essentially hydrophilic materials that do not suffer from hydration problems associated with other SPE technologies.

The MEA will be in the form of a plurality of such combinations, for example a stack. The MEA or stack will be coupled to the cell. The cell will be used as a fuel cell or an electrochemical cell, for example an electrolytic cell or a photovoltaic cell.

The present invention provides membranes of composites that may include a plurality of active materials that are cationic and anionic, and optionally also non-active materials that are hydrophilic and / or ionically inert and non-hydrophilic. These may be formed from suitable precursors by polymerisation in in -situ to form a solid polymer composite electrolyte (SPCE), which will also be polymerized individually to be included in the final film.

The use of SPCE in electrochemical cells will provide a path for ion conduction through ionically active phase (s). In the case of fuel cells, electro-osmotic drag will occur here, as in the case of conventional SPE materials. However, there is no transport of ions through the ionically inert phase (s) of the material. Therefore, the hydrophilic, ionically inert phase, water freely moves from high to low concentrations through these relatively high thermal diffusivity pathways to help maintain hydraulic balance during operation. Water movement through this phase will not be delayed by electro-osmotic drag. Using both AE and CE phases in the same SPE provides an additional route for hydration control, while rate change allows for a variety of water generation / use combinations.

The present invention allows for effective water management in the SPE of an electrochemical cell via a route that does not require additional balance of the plant. For example, hydraulic stability in SPE helps to maintain fuel cell efficacy. Hydraulic stability also reduces dimensional changes in the SPE due to drying and re-hydration, which is another common failure modality observed in fuel cells using conventional SPE materials. It is believed to contribute to de-stratum.

Another advantage associated with the present invention is that the present invention allows direct control of the electrochemical activity of the cell. In particular, the present invention provides a method of directly regulating the electrical output of a fuel cell.

desirable Implementation  Explanation

The present invention describes a novel composite material that provides improved retention of hydration. There are three main embodiments.

The first major embodiment, if hydrophilic, comprises an ionically inert material that provides a path for water migration within the membrane and is not delayed by electro-osmotic drag. In the case of CE materials in fuel cells, this allows the back-diffusion of water from cathode to anode. Drying of the anode is reduced as part of the excess water at the cathode is back-diffused through the ionically inactive active ingredient. This helps to maintain high efficiency by maintaining high ionic conduction through the SPE. For AE materials in fuel cells, this allows back-diffusion from anode to cathode; The same benefits apply as when applied to CE materials. No additional fuel humidification is required. Since the ionically inert material is not electrochemically active, any such region on the SPCE surface does not require catalysis. Non-hydrophilic, ionically inert materials can be added to increase physical strength and dimensional stability, if advantageous and / or desired. The membrane will comprise a microporous, non-hydrophilic, ionically active material for permeation of water through capillary action.

The second major embodiment uses a combination of AE and CE phases in SPCE to control the production and use of water at the anode and cathode positions. In fuel cells, because CE materials generate water at the cathode and AE materials use water at the cathode and water at the anode, the use of AE and CE materials in different proportions results in water generation at both the anode and cathode. Allow adjustment. An ionically inert phase should be used to separate the ionic phase; It can be hydrophilic to increase water diffusion or can be non-hydrophilic to provide separation and additional strength or dimensional stability.

The third major implementation involves the use of a grid. This can be used to regulate ion flow.

These three embodiments can be used individually or together to provide extensive access to hydration control and ion flow through the membrane.

The product of the present invention will be produced by a process comprising two steps: the production of ionically inactive phase (s), and the implantation of ionically active hydrophilic polymer (s).

Ionically  Inert Active phase

The ionically inert material may be hydrophilic or non-hydrophilic or a combination, depending on the application.

The ionically inert phase should produce enough dimensions to allow effective water transfer and to provide a suitable area of the ionically active material. These dimensions will vary depending on the application of the final product.

An example of an ionically inert phase is a mesh having a thickness between 10 and 2000 μm and a pore size between 100 and 100,000 μm. The hole may be any three-dimensional shape.

A further example is an ionically inert powder comprising particles of between 10 and 2000 μm in diameter filled between ionically active materials prior to the polymerization reaction.

Most polymers will absorb some water, even when described as hydrophobic; For example, most polymers, including those that are considered hydrophobic, will absorb water up to 5%. The polymer should be chosen to meet the water transport requirements in the electrochemical cell.

Particular examples of ionically inert and non-hydrophilic materials are polyesters or polyethylenes.

Examples of monomers having hydrophilic properties are amino-alkyl acrylates and methacrylates, in particular alkyl groups having 1 to 4 carbons, for example methyl or ethyl. Amino group mono- or di-substituted with any substituent, and is preferably to the C _{1 - 4} alkyl groups such as methyl or ethyl. Specific examples are aminoethyl acrylate, dimethylaminoethyl acrylate, methylaminoethyl methacrylate and diethylaminoethyl methacrylate. Hydroalkyl acrylates and methacrylates will also be used as hydrophilic monomers in which the alkyl groups are preferably 1 to 4 carbon atoms, for example methyl or ethyl. Specific example is hydroxyethyl methacrylate (HEMA).

Preferred hydrophilic monomers include N-vinylpyrrolidone (VP) and other vinyl lactams, and acrylamides and methacrylamides and their N-substituted derivatives. Substituted acrylamide and methacrylamide derivatives are mono- or di-substituted, and preferred substituents are alkyl, hydroxyalkyl and aminoalkyl (including mono- and di-substituted aminoalkyl), for example Di-alkyl aminoalkyl groups. Preferably any alkyl group appears to contain 1 to 4 carbon atoms, with methyl and ethyl being particularly preferred. Examples of such derivatives are N-methylacrylamide, N-isopropylacrylamide, N, N-dimethylacrylamide, N, N-dimethylaminomethylacrylamide, N, N-dimethylaminoethylacrylamide and N-methylamino Isopropylacrylamide.

The polymer is preferably, for example, a copolymer of two or more hydrophilic monomers as described above, or one or more hydrophilic monomers and one or more other monomers. Preferably, the hydrophilic monomer is an alkyl acrylate or methacrylate, in particular where the alkyl group has 1 to 4 carbon atoms, for example methyl or ethyl, or copolymerized with acrylonitrile. Specific examples of suitable copolymers are copolymers of VP and methyl methacrylate (MMA), of VP and HEMA, and of VP, styrene and acrylonitrile. Co-polymers of terephthalic acid and VP will also be suitable.

The polymer should preferably be crosslinked, and this will be achieved by including di- or poly-functional crosslinkers in the monomer system. Examples of suitable crosslinkers include two or more ethylenically unsaturated groups such as allyl methacrylate, divinylbenzene, ethylene glycol dimethacrylate, trimethylpropane trimethacrylate and trimethylpropane trimethacrylate. It is a compound containing. Usually the polymer should be relatively lightly crosslinked and the crosslinker preferably used in an amount of, for example, about 1% by weight of the monomer system.

High-strength polymer systems are also preferred in some cases, such as those obtained from acrylonitrile and VP.

Composites containing methacrylic acid can be produced including inherent integrated pH adjusting materials. As such, the material is a composite comprising both acid and alkali components as part of the same structure. Such materials can be used in SPCE as ionically active ingredients or in their own range as novel composite SPE. Methacrylic acid is passed through a number of routes including gamma radiation and thermal initiation, for example one or more acrylonitrile, MMA, HEMA, N, N, N-trimethylammonium chloride, vinylbenzene trimethylammonium chloride, water and allyl Will polymerize with methacrylate. Examples of such materials are obtained from methacrylic acid, acrylonitrile, vinylbenzene trimethylammonium chloride, water and allyl methacrylate.

Ionically  Inert Active phase Polymerization

Polymerization of the hydrophilic ionically inert phase will be carried out by the use of chemical initiators and by heating or by the use of irradiation, in which case no heating or initiator is required. Electro-polymerization is also possible.

Particularly sufficient chemical initiators are organic peroxides such as azobispropylpercarbonate or benzoyl peroxide. It is generally appropriate to heat at a temperature of 30 to 80 ° C., preferably 30 to 80 ° C., and it is generally preferred to carry out the heating in a series of steps of increasing the temperature.

After the basic polymerization has been carried out, the post-cure treatment will be achieved, for example, by heating the polymer under a temperature of 85 to 95 ° C., preferably under vacuum.

If the polymerization is carried out by irradiation, various forms of radiation will be used, for example ionizing radiation such as UV light, X-rays or gamma rays, or particulate radiation such as electron or photon beams. Preferably the radiation is ionized gamma radiation from a beam of cobalt 60 origin or from an electron. After the basic polymerisation is complete, the post-curing treatment will be carried out by further investigation.

Although most hydrophilic polymers are polymerized preferably at doses between 1 and 4 Mr, polymerisation irradiation can be used between 0.1 and 20Mr. This preferred dose should be applied over a period between 1 hour and 10 days.

Typically, the mixture undergoing polymerization will consist of a monomer or monomers, a crosslinking agent and any initiator needed, but if desired, will include a solvent of one or more monomers, eg water.

Once the ionically inactive active phase is produced, an ionically conductive material can be used to fill in the voids provided by the ionically inactive phase. This will then be in-situ polymerized. Alternatively, the phases can be polymerized independently and then combined to make a film.

Ionically In active phase  For use Polymer

There are two main categories of ionically active materials, AE SPE and CE SPE. Such materials are described in WO03 / 023890, the contents of which are incorporated by reference. According to the invention, these materials are used separately or in combination with the ionically inert phase. In all cases, the AE and CE phases will contain many different polymeric compounds with different ionic groups and different ionic strengths.

Specific examples are the use of AE and CE phases separated by ionically inert hydrophilic phases and used with hydrogen and oxygen fuel cells. This is shown in FIG. 1. In the examples of CE and AE in a 1: 1 ratio, the following reaction will occur. At anode, 4 H _{2 in} , 4 H ₂ O exit; At the cathode, 2O ₂ enters.

The CE phase uses hydrogen from the anode, oxygen from the cathode and produces water at the cathode. Water is also dragged through the membrane and into the anode as a result of the osmotic drag.

The AE phase uses hydrogen from the cathode, oxygen and water from the cathode, and produces water at the anode. Any osmotic drag also carries water from the cathode to the anode. Hydrophilic passageways allow water to diffuse through the membrane.

Further examples use a CE: AE ionic phase in a 3: 1 ratio separated by a non-hydrophilic ionically inert phase. In hydrogen: oxygen fuel cells this will result in the same amount of water being produced at the anode and at the cathode. Since the correct ratio of AE: CE depends on the electro-osmotic drag level, a system will be created that eliminates any need for any gas hydration and provides a self-hydrating SPE.

Polymerization of the ionically active material for this second step follows the same principle as was required for the hydrophilic phase.

The use of AE-CE laminates, whether physically separated or not, allows the catalyst on each electrode to be individually selected for optimal chemical reactions, and higher than is available when using single membrane tissue from the cell. Open circuit potential. Moreover, when hydrogen and oxygen are used as fuels and oxidants, the product water is produced in the membrane structure at the contact surface between the AE and CE layers. These layers will be forcibly separated by the hydraulic pressure of the product water, and thus a preferred embodiment of the present invention is mutually compatible to form connections between the various layers and to maintain the (hydraulic and electrical) internal integrity of the composite structure. Involves the use of infiltrated network structures.

The joints between the laminates must be rigid enough to prevent water buildup and subsequent water pressure separation. This will be achieved by the use of a suitable adhesive, but the preferred system is the interpenetrating network polymer system described in GB-A-1463301, the contents of which are incorporated by reference.

Use of contrast grid

In a preferred embodiment, the cell of the invention comprises an electrically conductive grid arranged in the film of the cell. The voltage will be applied across the electrodes and grid, allowing the flow of ions through the membrane to be regulated. In fact, the grid can be seen to act as an electrostatic screen. The use of such a grid is desirable because it allows for precise regulation of the cell's output, thereby improving the cell's efficacy.

The grid will be made of metal (eg gold) or carbon fiber and will include a catalyst. It will be coated with an electrically and / or ionically resistant material and will allow to maintain the potential of the film at other potential, without providing electrons or removing electrons from the system. Such a grid is particularly desirable when the membrane only conducts a single ionic species, for example a proton exchange membrane such as Nafion®.

A particularly preferred embodiment of the present invention is an AE and CE material laminate layer in which ions flow into the interlayer. Thus, quantum-conductive and OH-conductive layers provide a laminate in which water can be produced at the contact surface of the layer rather than at the surface of the film. In this way, any catalyst overflow present can be prevented. The two opposing layers are in ionic contact, for example, through the open space of the grid mentioned above. This arrangement provides an adjustable fuel cell. It will also allow various catalyst materials to be used at the cathode and anode.

Embodiments of the present invention will be described by way of example only with reference to the accompanying drawings, FIG. This figure shows an electrochemical cell comprising electrodes 1a and 1b separated by a membrane. The membrane is a laminate of layers of anionic exchange material 2a and cationic exchange material 2b, which layers are in ionic contact through the open space of grid 3. The rate of ion flow through the membrane can be controlled by applying a potential difference across the electrode and the grid. This arrangement provides an adjustable electrochemical cell.

The use of a control grid has been shown to have a significant effect even when embedded in a film of the same nature, ie an AE-AE structure.

More than one grid may be used, for example a pair of grids, with the potential applied between the grids or between the grids and one of the electrodes forming the MEA.

The time-varying applied potential (a / c potential) will be applied, so the accompanying change in cell output or cell characteristics is also time-varying (in the effect of producing a / c output from a fuel cell; in general Unique d / c device).

The following examples illustrate the invention.

Example 1 shows the effect when a single insulated control grid was used in a fuel cell made of AE-CE bond. In this case, the output of the battery can vary widely as shown in the figure.

Example 2 shows the effect in the film of the AE-AE structure of the non-insulated gold grid in the film of the AE-AE structure. In this case the effective conductivity of the membrane can be controlled to vary by 30%.

1 and 2 are schematic views of each of the embodiments of the present invention.

3 and 5 are schematic views of each of the circuits implementing the present invention.

4A-4C are graphical representations of the results obtained in the operation of the present invention.

Example  One

Test cell

The test cell included both CE and AE polymers separated but not sequestered by a metallic mesh (gold or platinum) coated to insulate it electrically and ionically from the two polymers. The CE polymer was adjacent to the oxidant feed and the AE was adjacent to the fuel feed. The catalyst was platinum black coated on a carbon fabric.

Electrical circuit

A high impedance, adjustable voltage supply was connected to the control grid. The electrical circuit is shown in FIG.

Test system

A) The voltage between the fuel cell anode and cathode was kept constant at 0.3V using a Prodigit electronic load. During this time the current carried in the fuel cell was recorded over a period of 5 minutes. This procedure was repeated while various voltages were applied to the control grid.

B) The circuit between the fuel cell anode and the cathode was opened, preventing any current flow between the two. The cell voltage was recorded 1 and 2 minutes after opening the circuit.

C) The fuel cell was polarized using a decade resistor box. The cell current and voltage were recorded and the output calculated.

The results obtained from Test Schemes A to C are shown in FIGS. 4A to 4C, respectively. Each is a graph plotting cell current (mA) against potential difference between oxidant electrode and control grid (V).

Example  2

The gold grid was pressed between two AE materials. The combination and the electrical circuit are shown in FIG. The Solartron Complex Impendence test apparatus provided 10mV modulation with a 1V DC bias. The current through the grid was monitored to ensure that it was always zero.

Initially, the PSU (power supply) was turned on, but the grid was not connected, and Solartron applied a frequency scan to the membrane to establish a frequency where the conductivity was completely real. This was 130 kHz.

The membrane was then scanned for 5 minutes at a constant frequency of 130 kHz and sampled every 5 minutes. After 1 minute, the conductivity was stabilized to 3.40 mScm ⁻¹ .

The grid was then connected to the PSU and applied 1V through it. Constant frequency scans were repeated. The conductivity was increased to 4.66 mScm ⁻¹ .

The grid was separated and the conductivity was reduced to 3.36 mScm ⁻¹ . The grid was re-connected and the conductivity increased to 4.77 mScm ⁻¹ .

Claims (19)

A membrane-electrode combination comprising electrodes separated by an ion-exchange membrane, and also means for regulating the flow of ions through the membrane. The combination of claim 1, wherein said film comprises, as said means, an electrically conductive grid. The combination of claim 2, wherein the grid is electrically and / or ionically coated with a resistive material. 4. The combination according to claim 2 or 3, wherein a means for providing a potential difference through at least a portion of the combination is involved. A membrane-electrode combination, comprising electrodes divided into at least two layers of ionic material, whereby ions can flow between the electrodes and through the contact surface of adjacent layers. The combination of claim 5, comprising a different catalyst at each electrode. The membrane-electrode combination, wherein the membrane comprises an ionically inert material for permeating liquid or gaseous reaction components to and / or from one or more electrodes. 8. The combination of claim 7, wherein said reaction component is water. The combination of claim 7 or 8, wherein the ionically inert material is hydrophilic. 10. The combination according to claim 1, wherein said membrane is a cationic exchange (CE) membrane. 10. The combination according to claim 1, wherein said membrane is an anionic exchange (CE) membrane. 12. The combination according to any one of the preceding claims, wherein the membrane comprises an AE material and a CE material. 7. Combination according to claim 5 or 6, wherein the layers are CE and AE materials, respectively. 10. The membrane of any of claims 7-9, wherein the membrane is a CE membrane, and in use, the ionically inert material allows the product, water, to diffuse from the cathode to the anode. Combination. 10. The combination according to any one of claims 7 to 9, wherein the membrane is an AE membrane, and in use, the ionically inert material allows the product water to diffuse from the anode to the cathode. 16. The combination according to any one of claims 1 to 15, wherein the membrane is hydrophilic. 17. The combination according to any one of the preceding claims, wherein the membrane is at least 0.5 mm thick. Stack of combinations according to claim 1. Use of the combination or stack according to any one of claims 1 to 18 as a fuel cell or an electrolytic cell.

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