GB2511615A - Fuel cell system - Google Patents

Abstract

A liquid electrolyte fuel cell system (10, fig 1) comprises at least one fuel cell 80, each fuel cell comprising a liquid electrolyte chamber 82 between opposed electrodes, the electrodes being an anode 83 and a cathode 84, and a duct (36, fig 1) for supplying a gas stream to a cathode gas chamber 86 adjacent to the cathode 84. The system also comprises a supply 92 of hydrogen peroxide, which is introduced into the cathode gas chamber 86. The hydrogen peroxide may be introduced within an aqueous liquid supplied to a humidification chamber (52, fig 1). The hydrogen peroxide may be sprayed through a nozzle (74, fig 4) into the gas stream. Alternatively it may be introduced into a hydrophilic layer 90 on a surface of the cathode 84 facing the cathode gas chamber 86.

Description

Fuel Cell System The present invention relates to liquid electrolyte fuel cell systems, preferably but not exclusively incorporating alkaline liquid-electrolyte fuel cells, and to methods of operating such fuel systems.

Background to the invention

Fuel cells have been identified as a relatively clean and efficient source of electrical power. Alkaline fuel cells with a liquid electrolyte are of particular interest because they operate at relatively low temperatures, are efficient and mechanically and electrochemically durable. Acid fuel cells and fuel cells employing other liquid electrolytes are also of interest.

Such fuel cells typically comprise an electrolyte chamber separated from a fuel gas chamber (containing a fuel gas, typically hydrogen) and a further gas chamber (containing an oxidant gas, usually air). The electrolyte chamber is separated from the gas chambers using electrodes. Typical electrodes for alkaline fuel cells comprise a conductive metal, typically nickel, that provides mechanical strength to the electrode, and the electrode also incorporates a catalyst coating which may comprise activated carbon and a catalyst metal, typically platinum.

In operation, chemical reactions occur at each electrode, generating electricity. For example, if a fuel cell is provided with hydrogen gas and with air, supplied respectively to an anode chamber and to a cathode chamber, the reactions are as follows, at the anode: H2+2OH -, 2H2O+2e; and at the cathode: 1⁄2O2+H2O+2e 20ft so that the overall reaction is hydrogen plus oxygen giving water, but with simultaneous generation of electricity, and with diffusion of hydroxyl ions from the cathode to the anode through the electrolyte. Water also evaporates at both electrodes. Problems can arise due to loss of water, particularly at the cathode, as water not only evaporates but is also used up in the electrochemical reaction. Over a prolonged period of operation, crystals may be formed at the surface of the electrode. Such crystals may for example consist of potassium carbonate or potassium hydroxide, at the air electrode, if the electrolyte contains potassium hydroxide. Any such crystals will have a detrimental effect on gas flow into the electrode.

Discussion of the invention The fuel cell system of the present invention addresses or mitigates one or more

problems of the prior art.

According to the present invention there is provided a liquid electrolyte fuel cell system comprising at least one fuel cell, each fuel cell comprising a liquid electrolyte chamber between opposed electrodes, the electrodes being an anode and a cathode, and an opposite face of the anode being in contact with an anode gas chamber and an opposite face of the cathode being in contact with a cathode gas chamber, and a duct for supplying a gas stream to the cathode gas chamber adjacent to the cathode; wherein the system also comprises means to introduce hydrogen peroxide into the cathode gas chamber.

In a second aspect, the invention provides a method of operating a liquid electrolyte fuel cell system comprising at least one fuel cell, each fuel cell comprising a liquid electrolyte chamber between opposed electrodes, the electrodes being an anode and a cathode, and an opposite face of the anode being in contact with an anode gas chamber and an opposite face of the cathode being in contact with a cathode gas chamber, and a duct for supplying a gas stream to the cathode gas chamber adjacent to the cathode; wherein the method comprises introducing hydrogen peroxide into the cathode gas chamber.

In one embodiment the hydrogen peroxide is introduced into the gas stream supplied to the cathode. For example the duct may include a humidification chamber in which the gas stream supplied to the cathode is subjected to humidification before being supplied to the cathode, and the hydrogen peroxide may be introduced into the humidification chamber. In another embodiment the hydrogen peroxide is introduced directly into the gas chamber adjacent to the cathode.

In the first embodiment, the duct may include both a gas heater and a humidification chamber, and means to supply an aqueous liquid containing hydrogen peroxide to the humidification chamber so the gas is humidified by contact with the aqueous liquid. The aqueous liquid supplied to the humidification chamber may be distilled water.

In use the gas heater preferably raises the temperature of the gas to within 5°C of the operating temperature of the fuel cell or cells, more preferably within 2°C. This may be an electrical heater, or alternatively may involve heat exchange with a heated liquid. This may involve direct or indirect heat transfer.

The humidification chamber may be separate from the gas heater, or integral with it.

Preferably the humidification chamber is designed not to impose a large pressure drop on the gas flowing through it. For example, although bubbling is an effective way of bringing a gas into contact with a liquid, it inevitably introduces a pressure drop, if only because of the depth below the surface of the liquid at which the bubbles are formed. If bubbles are formed at a depth of 50 mm below the surface this requires a pressure of at least 500 Pa.

One design of humidification chamber incorporates a plurality of baffles that are aligned with the gas flow direction to define gas flow channels, means to cause aqueous liquid to flow over surfaces of the baffles, and means to collect a pool of aqueous liquid at the bottom of each gas flow channel. The depth of aqueous liquid in such a pool may be maintained by a weir or overflow.

The aqueous liquid supplied to the humidification chamber may be water, and may also contain hydrogen peroxide.

As an altemative or in addition to supplying hydrogen peroxide to the gas stream by using the humidification chamber, hydrogen peroxide may be introduced into the vicinity of the cathode in another way. For example it may be introduced through a spray nozzle into the gas stream outside the fuel cell. Alternatively, hydrogen peroxide may be introduced directly into the cathode chamber. For example, it may be introduced on to a hydrophilic element in contact with the cathode. Such a hydrophilic element may for example be a hydrophilic polyurethane foam, which may be perforated to ensure the hydrophilic element does not inhibit gas transport.

It has been found that in operation of a fuel cell system without use of the present invention, there is a net evaporation of water from the electrodes, in particular from the cathode, which may lead to the formation of crystalline potassium hydroxide or potassium carbonate in pores in the electrode if the electrolyte is an aqueous solution of potassium hydroxide. This may cause delamination of the electrode, and may hinder mass transport for the gaseous reactants. Although this can be suppressed by humidifying and warming the air supplied to the cathode, which significantly reduces loss of water by evaporation, it has been found that small quantities of added hydrogen peroxide are very effective at removing any crystals that form. Furthermore, the breakdown of hydrogen peroxide produces oxygen, which enhances the concentration of oxygen in the gas chamber adjacent to the cathode, and can therefore increase the power available from the fuel cell system.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a schematic diagram of the fluid flows of a fuel cell system of the invention; Figure 2 shows a perspective view of a humidification chamber of the fuel cell system of figure 1, partly broken away; Figure 3 shows a longitudinal sectional view of the humidification chamber of figure 2, on the line 3-3; Figure 4 shows a longitudinal sectional view, partly as a schematic diagram, of a spray injection system suitable for use in the fuel cell system of figure 1; and Figure 5 shows a sectional view of a modification to a fuel cell which may be incorporated within the fuel cell system of figure 1.

Referring to figure 1, a fuel cell system 10 includes a fuel cell stack 20 (represented schematically), which uses an aqueous solution of potassium hydroxide as electrolyte 12, for example at a concentration of 6 moles/litre. The fuel cell stack 20 is supplied with hydrogen gas as fuel, air as oxidant, and electrolyte 12, and operates at an electrolyte temperature of about 65° or 70°C. Hydrogen gas is supplied to the fuel cell stack 20 from a hydrogen storage cylinder 22 through a regulator 24 and a control valve 26, and an exhaust gas stream emerges through a first gas outlet duct 28. Air is supplied by a blower 30, and any CO2 is removed by passing the air through a scrubber 32 and a filter 34 before the air flows through a duct 36 to the fuel cell stack 20, and spent air emerges through a second gas outlet duct 38.

The fuel cell stack 20 is represented schematically, but in this example it consists of a stack of fuel cells, each fuel cell comprising a liquid electrolyte chamber between opposed electrodes, the electrodes being an anode and a cathode. Each anode has a face in contact with the liquid electrolyte, and an opposite face in contact with hydrogen in a gas chamber; similarly each cathode has a face in contact with the liquid electrolyte, and an opposite face in contact with air in a gas chamber. In each cell, air flows through the gas chamber adjacent to the cathode, to emerge as the spent air in the outlet duct 38.

Similarly, in each cell, hydrogen flows through the gas chamber adjacent to the anode, and emerges as the exhaust gas stream in the outlet duct 28.

Operation of the fuel cell stack 20 generates electricity, and also generates water by virtue of the chemical reactions described above. In addition water evaporates in both the anode and cathode gas chambers so both the exhaust gas stream and the spent air contain water vapour. The rate of evaporation depends on the electrode surface area exposed to reactant gases, the flow rate of the reactant gases, and the operating temperature. It also depends on the partial pressure of water vapour in the anode and cathode gas chambers.

The overall result would be a steady loss of water from the electrolyte 12; the loss of water can be prevented by condensing water vapour from the spent air in the outlet duct 38 (or from the exhaust gas), for example by providing a condenser 39. In addition, the chemical reaction occurring at the cathode generates hydroxyl ions and consumes water, so concentrating the electrolyte in the vicinity of the cathode.

The electrolyte 12 is stored in an electrolyte storage tank 40 provided with a vent 41.

A pump 42 circulates electrolyte from the storage tank 40 into a header tank 44 provided with a vent 45, the header tank 44 having an overflow pipe 46 so that electrolyte returns to the storage tank 40. This ensures that the level of electrolyte in the header tank 44 is constant. The electrolyte is supplied at constant pressure through a duct 47 to the fuel cell stack 20; and spent electrolyte returns to the storage tank 40 through a return duct 48. The storage tank 40 includes a heat exchanger 49 to remove excess heat, or to provide heat to the electrolyte 12.

In the duct 35 the air stream passes through a heat exchanger 50, and then a humidification chamber 52. Distilled water is supplied through a duct 53 to the humidification chamber 52; in addition hydrogen peroxide is introduced into the flow of distilled water through a duct 54. Consequently the liquid that enters the humidification chamber 52 is a dilute aqueous solution of hydrogen peroxide. The hydrogen peroxide may be introduced continuously through the duct 54, or alternatively may be introduced at intervals. The excess water emerging from the humidification chamber 52 is discharged through a duct 55 to waste.

The water supplied through the duct 53 is preferably at an elevated temperature, for example it may be heated by heat exchange with the electrolyte in the return duct 48. This will ensure that it is at a temperature only a few degrees lower than the operating temperature of the fuel cell stack 20. For example the water may be passed through a heat exchanger (not shown) to exchange heat with the spent electrolyte in the return duct 48, before being supplied through the duct 53. In this case the humidification chamber 52 may be sufficiently warm that no separate heat exchanger 50 is required: the humidification chamber 52 both heats and humidifies the air stream at the same time, by direct contact with water.

It will be appreciated that the humidification chamber 52 may take a variety of different forms. One such design is shown in figures 2 and 3, to which reference is now made.

Referring now to figures 2 and 3, the humidification chamber 52 consists of a generally rectangular housing 60 subdivided into several flow channels 62 (five are shown in figure 2) by parallel baffles 63 which extend from the top wall to just above the bottom wall of the housing 60. The baffles 63 do not extend to the ends of the housing 60, so there is a gas distribution space 64 at each end. The duct 36 supplying air to the humidification chamber 52 communicates with the gas distribution space 64 at one end (the left-hand end as shown) through the top wall of the housing 60, while the duct that takes humidified air to the fuel cell stack 20 communicates with the gas distribution space 64 at the opposite end, through the end wall of the housing 60.

A dilute solution of hydrogen peroxide in water is supplied to the humidification chamber 52 through a duct 66 which is connected to the duct 53. The duct 66 extends across the top of the housing 60, and communicates with the flow channels 62 through small apertures 68 (shown schematically in figure 3) through the top wall of the housing 60 above the baffles 63, near the left-hand end of the baffles 63 as shown. The apertures 68 are typically of diameter between 0.5 and 3 mm, for example 1.5 mm. Hence the dilute aqueous solution of hydrogen peroxide forms a curtain of droplets or liquid jets, falling from the apertures 68 into the flow channels 62, through which the air must flow, and the solution also trickles down the baffles 63. The solution collects as a pool at the bottom of the housing 60. The baffles 63 do not contact the bottom wall, so the pool of solution is continuous, and is not divided by the baffles 63. The outflow duct 55 communicates with the end wall of the housing 60 at the right-hand end (as shown) at such a position as to ensure there is a consistent depth of solution at the bottom of the housing 60, which may for example be 10 mm. The solution then flows out of the duct 55 to be discharged as waste.

It will be appreciated that the effect of the humidification chamber 52 is that the air stream supplied to the cathode contains hydrogen peroxide either as droplets or as vapour.

The operating temperature of the fuel cell stack 20 is typically above 50°C, and may for example be between 60° and 70°C. At such a temperature the hydrogen peroxide will tend to break down chemically, forming water and oxygen. It has been found that it interacts with any crystals that have formed in or on the cathodes, and assists the dissolution of the crystals.

It will be appreciated that the fuel cell system 10 described above may be modified in various ways while remaining within the scope of the present invention. For example the number of flow channels 62 and the dimensions of the flow channels 62 may be different from that described. The air stream may be heated to the operating temperature of the fuel cell or cells, and the air stream may be saturated with water vapour at that operating temperature. Alternatively the air stream may be heated to a temperature slightly above the operating temperature, thereby enhancing its capacity to carry water vapour, and reducing the degree of condensation that may otherwise occur in the air duct 36 between the humidification chamber 52 and the fuel cell stack 20.

The gas stream may not be saturated with water vapour after passage through the humidification chamber 52, but should be humidified to achieve a relative humidity at least 65%, or above 75%, or above 80%, as it emerges from the chamber 52. It will be appreciated that humidification of the air stream decreases the partial pressure of oxygen in the air stream, so reducing the power of the fuel cell stack 20; whereas breakdown of the hydrogen peroxide increases the partial pressure of oxygen, and tends to increase the power of the fuel cell stack 20.

In a further modification, the hydrogen peroxide is injected into the duct 36 carrying humidified air, between the humidification chamber 52 and the fuel cell stack 20, via a duct 56; this is indicated as a broken line in figure 1. The hydrogen peroxide may be introduced through a spray nozzle so that droplets of hydrogen peroxide in the form of a fine mist are distributed throughout the humidified air flowing through the duct 36 downstream of the humidification chamber 52. Injection of hydrogen peroxide into the duct 36 through the duct 56 may be carried out instead of the injection of hydrogen peroxide through the duct 54 into the humidification chamber 52; or alternatively hydrogen peroxide may be introduced both into the humidification chamber 52 and also directly sprayed into the air stream via the duct 56.

Referhng now to figure 4, this shows a spray injection system 70 which may correspond to the duct 56 which is used to introduce droplets of hydrogen peroxide. A reservoir 71 contains an aqueous solution of hydrogen peroxide. This is connected via a pump 72 and a flow control valve 73 to an injection nozzle 74. The injection nozzle 74 is shown in longitudinal cross-section, and consists of a tube 75 extending along the axis of the duct 36 carrying air from the blower 32 the fuel cell stack 20, the tube 75 tapering to a narrow orifice 76. The tube 75 is installed such that the orifice 76 is at the centre of a venturi-shaped constriction 77 within the duct 36. The arrangement is such that droplets of aqueous hydrogen peroxide solution, in the form of a fine mist, are distributed throughout the air flowing through the duct 36, the constriction 77 helping to ensure thorough mixing of the mist of droplets into the air stream.

It is desirable for there to be sufficient distance along the duct 36 between the spray injection system 70 and the fuel cell stack 20 to ensure that all the droplets have evaporated by the time the air reaches the cathode compartments within the fuel cell stack 20. This may be assisted by preheating the air stream. Alternatively, as described below in relation to figure 5, the cathode may be provided with a hydrophilic gas-permeable coating 90.

Referring now to figure 5 there is shown a sectional view through a single cell 80 which would be suitable for incorporation within the fuel cell stack 20. As in the fuel cell described previously, the fuel cell 80 comprises a liquid electrolyte chamber 82 between opposed electrodes, an anode 83 and a cathode 84. Each anode 83 has one surface facing the liquid electrolyte chamber 82, and an opposite surface facing hydrogen in a gas chamber 85. Similarly each cathode 84 has one surface facing the liquid electrolyte chamber 82, and its opposite surface facing air in a gas chamber 86. In each cell 80 within the fuel cell stack 20, air flows through the gas chamber 86 adjacent to the cathode 84, to emerge as the spent air in the outlet duct 38. Similarly, in each cell 80, hydrogen flows through the gas chamber 85, and emerges as the exhaust gas stream in the outlet duct 28.

The anode 83 and the cathode 84 each consists of a metal current collector 87 and a catalyst-containing coating 88. The metal current collector 87 may be a metal mesh or a perforated metal sheet. The catalyst-containing coating 88 faces the electrolyte chamber 82, so there is catalyst adjacent to the electrolyte, and the coating 88 is hydrophobic at least in the portions further from the electrolyte to suppress migration of aqueous electrolyte through the catalyst-containing coating 88. The catalyst-containing coating 88 must also be electrically conducting, and may comprise a particulate conductor such as carbon and a hydrophobic binder such as polytetrafluoroethylene (PTFE). Indeed the coating 88 may consist of a plurality of different layers, for example a layer without catalyst which allows for gas diffusion, covered by a catalyst-containing layer. The catalyst material in the anode 83 may be different from that in the cathode 84.

Such an electrode structure is known. In use there is a gas/electrolyte interface within the catalyst-containing coating 88, and electrochemical reactions take place at the gas/electrolyte/catalyst interface. It will be appreciated that if droplets of an aqueous solution of hydrogen peroxide were to be carried by the air stream onto the surface of the cathode 84 facing the air in the gas chamber 86, the droplets may obstruct the pores through the hydrophobic coating 88.

In the cell 80 the cathode 84 is additionally provided with a porous hydrophilic coating 90 facing the air in the gas chamber 86. This may also be perforated, to ensure that oxygen diffusion is not significantly inhibited. It might for example be of perfolated filter paper, or hydrophilic carbon paper, or hydrophilic polyurethane foam. Such a material will tend to absorb moisture, because it is hydrophilic. The cell 80 also includes a micro-pump 92 arranged to inject small quantities of aqueous hydrogen peroxide solution onto the porous hydrophilic coating 90.

For example the micro-pump 92 may introduce a solution which is between 1% and 10% (by weight) hydrogen peroxide in water, such as 3%. It has been found that a rate of injection of 1 mI/day of such a hydrogen peroxide solution over a surface area of 25 cm2 is sufficient to prevent or remove any crystals that otherwise may form within the hydrophobic portion of the coating 88. In contrast, injection of water alone does not have this beneficial effect. The micro-pump 92 may be arranged to inject the solution on a slow and continuous basis, or alternatively it may inject the solution at intervals, for example once every hour, or every 12 hours, or once a day. The porous hydrophilic coating 90 absorbs the aqueous hydrogen peroxide, so that it does not obstruct the pores within the hydrophobic portion of the coating 88.

If hydrogen peroxide is not provided in the vicinity of the cathode, it has been observed that crystals tend to gradually form within the hydrophobic portion of the coating 88, forming initially at the gas-liquid-catalyst interface, and tend to creep towards the metal current collector 87. The rate at which this occurs depends upon the temperatures of the air stream and of the electrolyte, and on the humidity of the air stream. If hydrogen peroxide is provided in one or more of the ways described above, the hydrogen peroxide reacts with any such crystals, producing water (and so dilute potassium hydroxide solution) and oxygen. If this occurs adjacent to the gas-liquid-catalyst interface, the dilute potassium hydroxide solution can diffuse into the electrolyte, while the oxygen can participate in the electrochemical reaction at the catalyst. If this occurs at the rear surface of the cathode, the dilute potassium hydroxide solution is absorbed by the hydrophilic material, whereas the oxygen diffuses along with the air through the cathode 84 to the catalytically active interface with the electrolyte. This is beneficial to operation of the cell.

It will be appreciated that a fuel cell system may utilise one or more of the above described techniques for introducing hydrogen peroxide into the vicinity of the cathode. For example hydrogen peroxide may be introduced into the air stream supplied to the cathode gas chamber 86 by using a humidification chamber 52, and/or using a spray nozzle 74, and/or hydrogen peroxide may be introduced into a gas-permeable hydrophilic coating 90 on a surface of the cathode.

Claims (14)

1. Claims 1. A liquid electrolyte fuel cell system comprising at least one fuel cell, each fuel cell comprising a liquid electrolyte chamber between opposed electrodes, the electrodes being an anode and a cathode, and an opposite face of the anode being in contact with an anode gas chamber and an opposite face of the cathode being in contact with a cathode gas chamber, and a duct for supplying a gas stream to the cathode gas chamber adjacent to the cathode; wherein the system also comprises means to introduce hydrogen peroxide into the cathode gas chamber.
2. 2. A fuel cell system as claimed in claim 1 wherein hydrogen peroxide is introduced into the gas stream supplied to the cathode.
3. 3. A fuel cell system as claimed in claim 2 wherein the duct includes a humidification chamber in which the gas stream supplied to the cathode is subjected to humidification before being supplied to the cathode, and the system also comprises means to introduce the hydrogen peroxide into the humidification chamber.
4. 4. A fuel cell system as claimed in claim 2 or claim 3 comprising means to spray hydrogen peroxide into the gas stream supplied to the cathode.
5. 5. A fuel cell system as claimed in any one of the preceding claims wherein the cathode comprises a gas-permeable hydrophilic layer at a surface facing the gas stream, and means for introducing hydrogen peroxide into the gas-permeable hydrophilic layer.
6. 6. A fuel cell system as claimed in any one of the preceding claims wherein hydrogen peroxide is introduced in the form of an aqueous solution of concentration between 1% and 10% by weight.
7. 7. A method of operating a liquid electrolyte fuel cell system comprising at least one fuel cell, each fuel cell comprising a liquid electrolyte chamber between opposed electrodes, the electrodes being an anode and a cathode, and an opposite face of the anode being in contact with an anode gas chamber and an opposite face of the cathode being in contact with a cathode gas chamber, and a duct for supplying a gas stream to the cathode gas chamber adjacent to the cathode; wherein the method comprises introducing hydrogen peroxide into the cathode gas chamber.
8. 8. A method as claimed in claim 7 comprising introducing hydrogen peroxide into the gas stream supplied to the cathode.
9. 9. A method as claimed in claim 8 wherein the duct includes a humidification chamber, and the method comprises supplying an aqueous liquid containing hydrogen peroxide to the humidification chamber so the gas is humidified by contact with the aqueous liquid.
10. 10. A method as claimed in claim 8 comprising spraying hydrogen peroxide into the gas stream supplied to the cathode.
11. 11. A method as claimed in claim 7 wherein the cathode comprises a gas-permeable hydrophilic layer at a surface facing the gas stream, and the method comprises introducing hydrogen peroxide into the gas-permeable hydrophilic layer.
12. 12. A method as claimed in any one of claims 7 to 11 wherein hydrogen peroxide is introduced in the form of an aqueous solution of concentration between 1% and 10% by weight.
13. 13. A fuel cell system substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.
14. 14. A method of operating a fuel cell system substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.