DE1993970U - FUEL CELL. - Google Patents

Description

The innovation relates to a fuel cell with an electrolyte chamber with anode and cathode, which is separated by a hydrogen-permeable membrane in the anode from a conversion zone in which a nickel conversion catalyst is arranged.

In fuel cells such as those described in U.S. Government Report No. AD 613031, referring to a development from liquid hydrocarbon fuel cells, a mixture of a carbon containing fuel and steam is introduced into a conversion zone located in the fuel cell to generate hydrogen which is then passed into the electrolyte through suitable means for an electrochemical reaction in the cell will. In general, the reforming catalyst located within the reforming zone into which the steam carbon-containing fuel is introduced contains a nickel catalyst which is used for its catalytic effectiveness and its relatively low cost. It has been found, however, that at fuel cell operating temperatures in the range of about 250 ° C to about 550 ° C, the pressure drop of the steam fuel supply across the nickel catalyst contained in the conversion zone increases over prolonged periods of cell operation. This increase in pressure drop is particularly pronounced when the feed ratio of steam to carbon-containing fuel is below 5 moles of steam per mole of carbon in the fuel. The use of carbon-containing fuels that contain at least two carbon atoms per molecule (e.g., hydrocarbons over methane) result in an even greater increase in pressure drop over extended periods of cell operation. If z. B. a steam-propane mixture in a conversion zone in the ratio of

5 moles of water per mole of carbon is introduced, the pressure drop over a conventional nickel catalyst as contained in the conversion zone gradually increases to a value of approximately one atmosphere after approximately 12 hours of conversion or operating at approximately 500 ° C. Since in the previous case the conversion takes place at approximately one atmosphere, the formation of the pressure drop of this magnitude prevents further generation of hydrogen in the conversion zone. When the steam / carbon ratio is decreased, it is found that the increase in pressure drop is accelerated. Thus, if a steam / carbon ratio of 3.3 moles of steam per mole of carbon is used, the pressure drop will reach one atmosphere after approximately 9 hours of operation. With a steam to carbon ratio of approximately 2, the pressure drop will reach one atmosphere in approximately 4 hours of operation. As the molecular weight of the hydrocarbon fuel is increased, the pressure drop reaches one atmosphere in considerably less time, and in the event that octane is used as fuel and a steam to carbon feed ratio of 2.5 is used, hydrogen production ceases after about 3 hours of steam conversion at approximately 500 ° C in one atmosphere.

However, since it is necessary for many fuel cell applications that the fuel cell operate continuously and reliably over times longer than 8 or 10 hours, the object of the present innovation is to create a fuel cell that is continuously operational for many thousands of hours, wherein the pressure drop leading to failure of the fuel cell after a short time is avoided.

This object is achieved according to the invention in that the particles of the catalyst in the conversion zone comprise a core made of refractory material and a nickel layer enveloping the core, the refractory material of the particle core being composed of approximately 0.5% to approximately 20% of an alkali or alkaline earth metal oxide, hydroxide or carbonate, based on the weight of the metal, or that the catalyst particles comprise an alkaline earth oxide coated with nickel.

A mixture of from 2 to about 5 moles of steam per mole of carbon in a carbon-containing fuel, which has at least two carbon atoms per molecule, is introduced into the conversion zone with the conversion catalyst which mixture reacts over this conversion catalyst at a temperature of about 250 ° C to about 550 ° C and under a pressure of less than about 10 atmospheres to form hydrogen.

The carbon containing fuels include hydrocarbons such as e.g. B. ethane, propane, butane, pentane, hexane, heptane, octane, etc .; and mixtures thereof; Petroleum fractions; aliphatic alcohols including ethanol, propanol, butanol, etc .; liquefied petroleum gases and JP fuels.

The steam used can be introduced into the conversion zone at any suitable temperature, preferably at a temperature which corresponds to the temperature reached in the conversion zone of the cell during cell operation. Preferably, the steam is admixed with the hydrocarbon-containing fuel in order to produce a suitable mixture at some point prior to the introduction of the steam and the fuel into the conversion zone of the cell.

The temperature at which the conversion reaction is carried out in the fuel cell can range from about 250 ° C to about 550 ° C. It has been found that cell operation is ineffective at temperatures below about 250 ° C. At temperatures above about 550 ° C, corrosion problems arise and the use of expensive construction materials becomes necessary.

The optimum transition temperature is in the range of about 400 ° C to about 500 ° C, and more particularly about 450 ° C.

The pressure for the conversion reaction can range up to about 10 atmospheres. Operating the cell at higher pressures, but below about 10 atmospheres, generally results in the production of hydrogen at higher partial pressures of the fuel anode. Of course, from the point of view of uncomplicated operation, it is advisable to carry out the conversion reaction at atmospheric pressure.

Examples of suitable fire-resistant materials are silica, kieselguhr, zirconia, alumina, natural clays, silica-alumina, metal carbides including carborundum, titanium oxide and molecular sieves.

The alkali or alkaline earth metal oxides, hydroxides and carbonates contained in the aforementioned catalysts include the oxides, hydroxides and carbonates of metals such as sodium, potassium, lithium, rubidium, cesium, calcium, strontium, magnesium and barium. Examples of specific compositions falling into this class are sodium oxide, potassium hydroxide, lithium carbonate, calcium oxide, strontium hydroxide and magnesium carbonate.

In addition, the catalyst used can be an alkaline earth oxide coated with nickel (e.g. calcium oxide, magnesium oxide, barium oxide) which has a nickel content of approximately 4 to approximately 70% by weight.

A group of catalysts can advantageously be prepared by treating a porous refractory catalyst containing from about 4 to about 70% nickel with one or more of the above-mentioned oxides, hydroxides or carbonates. For an alkali oxide, hydroxide or carbonate, this can be accomplished by immersing the catalyst in an aqueous solution of the composition and then drying the catalyst in a suitable oven.

If water-insoluble alkaline earth oxides, hydroxides or carbonates are used, a water-soluble alkaline metal composition can be used to impregnate the nickel-plated refractory material and the hydroxide or carbonate of the metal can then be precipitated in place.

In addition, a nickel-coated fire-resistant catalyst made with a heat-decomposable alkali or alkaline earth metal composition such as a nitrate,

Acetate or format, impregnated e.g. B. be filled directly into the conversion zone of the fuel cell, and the oxide and / or carbonate can be in place during the conversion in the zone due to the conditions that exist therein during fuel cell operation, i. H. high temperature and presence of CO [low] 2.

Part of a wall of the reforming zone, which contains the above-mentioned catalyst, forms a permeable membrane. Such a membrane can contain any hydrogen-permeable metal membrane that is compatible with the environment, e.g. H. is sufficiently resistant to the effects of the acidic or corrosive electrolyte and which retains its physical nature at the temperatures at which the conversion takes place. Examples of such membranes are palladium- or hydrogen-permeable palladium alloys, such as a silver-palladium alloy containing 25% silver.

The fuel cell according to the invention is further illustrated with the aid of an exemplary embodiment shown in the accompanying drawing, which shows a cross section through a fuel cell arrangement.

As can be seen from the drawing, the fuel cell assembly 10 comprises a fuel cell which has an outer housing 11 made of a non-conductive, impenetrable material

Material and two electrodes, an anode 12 and a cathode 14, which are held in the housing by spacers 16, 18 and 20 so that a conversion zone 22 containing a conversion catalyst 24 distributed over the zone 22, a Electrolyte space 26 and an oxidant space 28 is formed. The catalyst 24 contains particles of either a nickel coated refractory material containing from about 0.5% to about 20% of an alkali or alkaline earth metal oxide, hydroxide or carbonate (such as, for example, an aluminum catalyst which is about 33% Nickel and including sodium hydroxide in an amount of approximately 5.75% expressed as sodium metal) or a nickel-coated alkaline earth oxide such as nickel-coated magnesia which contains 25% nickel. The size of the catalyst particles will vary depending on the width 22a of the conversion zone 22. Preferably, the particles should have diameters ranging from one fifth to one tenth the width of the zone. The cell cover or lid 29 is provided with suitable lines 30 and 32 for introducing the fuel-vapor mixture and the oxidizing agent into the conversion zone 22 and the oxidizing agent space 28, respectively. The outlet lines 34 and 36 are provided for drawing off the gases. The cover 29 is provided with a suitable passage opening 37 in order to draw off the water which is formed during cell operation from the electrolyte which is contained in the space 26.

During cell operation, the electrolyte compartment 26 is filled with a suitable electrolyte, such as a molten alkali hydroxide, e.g. A mixture of sodium hydroxide and potassium hydroxide, and a fuel feed comprising a mixture of a carbon-containing fuel and steam is introduced into the conversion zone 22 through line 30. The ratio of steam to carbon-containing fuel and the flow rate with which the line 30 is fed can be controlled by suitable adjustment of valves 38 and 40, with which the control of the steam and the hydrocarbon can be made, respectively. In zone 22, the steam reacts with the hydrocarbon over catalyst 24, producing hydrogen gas. The anode 12 contains a microporous pad, such as. B. microporous carbon, which has a non-porous hydrogen-permeable metal membrane 42, for example made of palladium, on its end face that is exposed to the electrolyte. Although the thickness of the membrane 42 can vary widely, thin membranes that are on the order of 0.00025 cm (0.0001 inches) or less are preferred. It is of course possible to use thicker hydrogen permeable metal membranes which are self-supporting, and accordingly the porous carbon pad or support 12 can be omitted.

When the cell is to be started, the electrolyte compartment 26 is filled with a suitable electrolyte, such as a molten mixture of sodium hydroxide and potassium hydroxide, and the cell is heated to the operating temperature, which can be on the order of about 450 ° C.

A mixture of a carbon containing fuel (such as propane gas) and steam is then introduced into the reforming zone 22 via line 30. The ratio of fuel to steam and the rate at which the mixture is introduced is regulated by appropriate actuation of the control valves 38 and 40. Upon entering the conversion zone 22 and upon contact with the catalyst 24, the fuel and steam react to produce hydrogen gas. When the current is withdrawn from the cell through lines 44 and 44a, the result is that the hydrogen generated penetrates or is conducted through the membrane 42 and into the electrolyte in the electrolyte space 26 to an electrochemical reaction. For example, with a feed ratio of 6.9 moles of steam per mole of propane (steam to carbon ratio 2.3), 4.4 ml (STP) of propane per minute was introduced into the conversion zone which was maintained at 450 ° C and approximately one atmosphere. 4.8 amps of current was drawn from the cell at a voltage of 0.76 volts. An analysis of the leaking gases showed that it did not contain propane, indicating that the conversion of the fuel was 100%.

During cell operation, the large delta P pressure drop across catalyst bed 24 is measured by any suitable means, such as, for. B. a mercury manometer 46, measured, one leg of which is connected to the line 30, in which the supplied steam and the carbon-containing fuel are mixed and through which the mixture enters the conversion zone 22 and the other leg of which is connected to the outlet conduit 34 through which the exhaust gases are withdrawn from the conversion zone 22.

A mixture of steam and propane in a ratio of 2.5 moles of steam per mole of carbon was introduced into a fuel cell of the type shown in the drawing. The conversion zone of the cell contained a nickel-coated alumina conversion catalyst containing 33 1/3 percent nickel and 5.75 percent sodium hydroxide, expressed as sodium metal. Reshaping in the zone took place at a temperature of approximately 450 ° C and a pressure of approximately one atmosphere. Under these conditions, continuous cell operation was achieved without any noticeable change in the pressure drop across the catalyst bed 24 after a continuous period of operation in excess of 1000 hours.

Several experiments were carried out with untreated, nickel-coated aluminum oxide catalysts, a propane or octane steam mixture being introduced into the reforming zone (steam to carbon ratio in the range from 2 to 5). Conversion was carried out at 450 ° and approximately one atmosphere pressure. Even with that At higher steam to carbon ratios (i.e., on the order of 5), the increase in pressure drop across the catalyst in the zone took place rapidly and reached such a level that the run had to be stopped after 10 hours after it was started. With lower steam to carbon ratios (2 to 3), the pressure drop over the zone increased even more rapidly, so that the run had to be aborted within 6 or fewer hours.

The present innovation thus specifies devices with which a continuous and reliable operation of a fuel cell, in which hydrogen is generated on site, can be achieved over longer periods of time by increasing the pressure drop over time over the conversion zone of the cell in which a nickel conversion catalyst is included. As a result of the present innovation, the possible uses for fuel cells which contain effective and inexpensive nickel catalysts are considerably expanded.

Claims (1)

Fuel cell with an electrolyte chamber with anode and cathode, which is separated by a hydrogen-permeable membrane in the anode from a conversion zone in which a nickel conversion catalyst is arranged, characterized in that the particles (24) of the catalyst in the conversion zone (22) have a Core made of refractory material and a nickel layer enveloping the core, wherein the refractory material of the particle core consists of about 0.5% to about 20% of an alkali or alkaline earth metal oxide, hydroxide or carbonate, based on the weight of the metal, or that the catalyst particles comprise an alkaline earth oxide coated with nickel.