DE29823321U1 - Combination of electrolysis and fuel cells - Google Patents

Description

Combination of electrolysis and fuel cell

The invention relates to an electrochemical cell with polymer electrolytes that can be operated as a fuel cell or as an electrolysis cell. The redox system H2,O2/H2O can thus be reversibly shifted.

The cell contains two different electrodes, each provided with a catalyst, which are arranged opposite each other on the surfaces of a gas-tight polymer electrolyte membrane that conducts H+ ions. This unit consisting of membrane and electrodes (membrane-electrode assembly, MEA) is sealed between the two halves of a suitable cell body so that gases can be supplied to the electrodes or gases can be removed from the electrodes without loss or mixing. It is also possible to connect the cells in series to form a stack using bipolar plates.

Two operating modes are possible for the cell according to the invention:

a) Fuel cell operation: generation of electrical power by consuming hydrogen and oxygen; the waste product is water.

b) Electrolysis operation: consumption of electrical power and water to produce hydrogen and oxygen.

The hydrogen is always in the same half of the cell, regardless of the operating mode. The electrode in this half of the cell serves as the anode in fuel cell operation and as the cathode in electrolysis operation. The opposite is true for the electrode in which oxygen is converted: in fuel cell operation it acts as the cathode and in electrolysis operation as the anode.
In both operating modes, the oxygen electrode has a positive potential compared to the hydrogen electrode.

The change between the two operating modes is determined only by the choice of the cell's terminal voltage: At voltages of less than 1.01 V, the cell acts as a fuel cell, i.e. hydrogen is consumed at the negative pole and oxygen at the positive pole. The cell then emits electrical power. If the terminal voltage generated by an external energy source is more than 1.48 V, hydrogen and oxygen are generated at the corresponding poles. The cell draws the necessary power from the external electrical energy source.

The voltage range between 1.01 V and 1.48 V is undefined, but very little current flows here compared to the operating ranges.

The reaction gases hydrogen and oxygen can be processed by the electrochemical cell according to the invention under either overpressure or underpressure in both operating modes. However, the operating voltage ranges shift slightly.

If operating modes a) and b) alternate with the same, but not too high, material conversion, the water used or produced can be stored in a specially designed electrode structure - primarily in the oxygen electrode. An external water supply is then not necessary.

The cell according to the invention can be used together with two gas containers as an energy storage device for electrical energy. The advantage over accumulators is the enormously large number of cycles and the fact that the amount of energy that can be stored depends only on the capacity of the gas container and not on the size of the cell. Large amounts of energy can therefore be stored inexpensively. The self-discharge rate is negligible; it only depends on the tightness of the gas storage system. In order to eliminate the gas tank for oxygen, it is possible and in certain applications sensible to operate the cell with oxygen from the air.

The high charge-discharge efficiency desirable for energy storage devices has been optimized in the present invention while maintaining inexpensive materials and a simple structure. It is a maximum of 68% and, at current densities typical for this cell, about 50%.

The applications concern in detail:

1) Electric vehicles: Using the present invention, energy from the generator-operating electric motors of a vehicle can be stored during braking (operating mode b). This energy is then available again in the form of electrical energy for acceleration phases (operating mode a).

2) Due to the long service life of the combined electrolysis and fuel cell, it can be used advantageously as an emergency power supply.

3) In the area of grid control^, long-term and short-term voltage fluctuations in the power grid and peak loads can be compensated. The present invention^ can be used particularly advantageously in short-term charging and discharging cycles due to the simple water management.

4) Energy storage in space travel.

5) As a teaching aid, the invention, including two small gas containers, serves as the simplest possible representation of the conceivable future hydrogen economy.

In the literature one can already find descriptions of electrochemical cells of a similar design, which, however, have serious disadvantages compared to the present invention.
The cell described in "Hydrogen/Oxygen SPER electrochemical devices for Zero-G applications, Proceedings of the European Space Power Conference; Madrid, Spain, October 2-6, 1989" does contain an electrode that consumes oxygen in fuel cell operation and generates it in electrolysis operation. However, the electrolysis efficiency is relatively poor due to the choice of platinum as the catalyst. The overpotential for water oxidation to oxygen on platinum is typically about 0.25V. Platinum was used in fuel cell operation due to its superior properties in oxygen reduction.

A further disadvantage of the excessive anodic potential when using pure platinum catalysts in the oxygen electrode is the fact that commercially available and inexpensive electrode materials made of carbon are damaged by oxidation within a short time (see DE 40 27 655 Cl). Sponges or sintered materials made of titanium can be used for relatively short operating times, but here too a thickening of the superficial oxide skin is observed and thus a reduction in electrical conductivity.

Although effective electrocatalysts for water oxidation are known (e.g. iridium, rhodium and their oxides), which, however, have only very low activity with regard to oxygen reduction (but good with regard to hydrogen oxidation), the above-mentioned problems of low efficiency and (partial) material decomposition in DE 40 27 655 Cl could be solved by exchanging the gases when changing the operating mode. If, for example, you want to change from operating mode a) to operating mode b), both electrode chambers must first be flushed with an inert gas. Hydrogen is then introduced into the previous oxygen electrode chamber.

and vice versa. This procedure causes gas losses and requires a complicated valve system and the storage of purge gas. Another disadvantage is the amount of time that elapses when changing operating modes. Use as an energy storage device in an electric vehicle is therefore impossible or at least severely limited.

The invention is therefore based on the object of developing a combined electrolysis and fuel cell that operates with low overvoltages and can be constructed from inexpensive carbon materials. Furthermore, the cell according to the invention can switch between the two operating modes within a few milliseconds. Cycles with a balanced charge-discharge balance can be carried out without water circulation.

This object is achieved according to the invention by using a mixed catalyst on the oxygen electrode, in which one component optimally catalyzes the oxygen reduction, while the other component is responsible for water oxidation. By gradually reducing the hydrophobicity of the catalyst carrier, a water supply can be maintained in the electrode without flooding the catalyst.

The hydrogen electrode including catalyst is not critical for the present invention. Standard anodes from fuel cell technology can be used. For cost reasons, the gas diffusion electrodes according to DE 195 44 323 Al are optimal, as they use small amounts of a supported catalyst, namely platinum on carbon. The hydrogen oxidation in operating mode a) and the reduction of the H+ ions in operating mode b) are both catalyzed with almost no overvoltage. The gas supply and removal is made possible by the porous electrode material.

The oxygen electrode is best catalyzed with a mixture of platinum-black and iridium-black. The mass ratio of platinum and iridium is preferably in the range of 1:10 to 10:1. The total precious metal coverage is between 0.2 mg/cm ² and 10 mg/cm ² . In fuel cell operation, platinum catalyzes oxygen reduction with the smallest possible overvoltage, while iridium enables water oxidation with very good efficiency. Surprisingly, the two catalysts do not interfere with each other. Alloy catalysts made from these elements can even be used. Furthermore, iridium oxide, rhodium, osmium and their oxides can also be used instead of or in addition to iridium.

If long service lives are required, catalysts without soot as a carrier material should be used. In order to optimally bind the catalyst to the H+ ion-conducting membrane material, the catalyst particles are mixed with small amounts of H+ ion-conducting polymer and applied in a thin layer close to the membrane. In addition to the catalytic effect, the oxygen electrode must also enable gas exchange and the addition or removal of water. It is a significant disadvantage for fuel cell operation if the catalyst layer is completely or partially flooded with product water. This prevents oxygen from entering. In electrolysis operation, however, water is required as close to the catalyst layer as possible.

The electrode therefore consists of a highly hydrophobic, fine-pored catalyst layer and one or more gas diffusion layers that change from hydrophobic to hydrophilic as the distance between the catalyst and the membrane increases. In operating mode a), the water that forms is forced out of the pores of the catalyst and collects in the more hydrophilic part of the electrode.

During electrolysis, this water can evaporate again and is even available in vapor form on the catalyst. Increased operating temperatures are therefore advantageous. The part of the electrode facing away from the catalyst acts as a product or reaction water reservoir. With large-area electrodes, it is advantageous to interrupt the hydrophilic outer layer of the electrode at certain intervals to allow gas access to the hydrophobic layer. The total thickness of the oxygen electrode is typically 0.1 to 2.5 mm, preferably in the range of 0.4 to 1.4 mm.

The gas diffusion layer is preferably made from one or more layers of differently hydrophobized carbon fiber paper (Toray, Jap.) or from gas diffusion layers with different PTFE contents according to DE 195 44 323 Al. The carbon fiber paper can be hydrophobized by impregnating it with a PTFE suspension (or a suspension of another suitable material) and then drying and possibly sintering. The degree of hydrophobicity depends on the concentration of the suspension. If PTFE is used for hydrophobicization, a content of between 5 and 60% in the carbon fiber paper is preferably aimed for. Pore size also influences the degree of hydrophobicity: small pores appear more hydrophobic than large ones.

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In order to protect the carbon material additionally against the oxidative effect of the high positive potential of the oxygen electrode, small amounts of iridium, rhodium and/or osmium and/or their oxides with a high surface area can be added to the suspension in order to eliminate locally occurring increases in the potential. This impregnation can also be carried out in a separate process step. Another possibility for protecting the carbon-containing gas diffusion layer is to apply a thin, hydrophobic layer of noble metal Mohr without the addition of ion-degrading polymer. A noble metal loading of 1 to 10 mg/cm ² and a binder content of 5 to 50% are preferred. PTFE, for example, is suitable as a binder and is processed as a suspension.

A bond between the electrodes and the membrane is preferably created by hot pressing. The pressing pressures are between 10 and 500 bar, preferably between 50 and 120 bar. The temperatures are in the range from 20 ⁰ C to 200 ⁰ C, preferably between 80 ⁰ C and 140 ⁰ C.

Example:

The Nafion 115 membrane serves as the electrolyte. Before the membrane electrode assembly is manufactured, it is first conditioned in 1 molar sulfuric acid and then in deionized water. The membrane is dried at room temperature.

The hydrogen electrode is manufactured according to the process of DE 195 44 323 Al with two gas diffusion layers and with 0.2 mg/cm ² platinum as catalyst.

The catalyst carrier of the oxygen electrode is a carbon fiber paper with more than 70% open pores and a thickness of 1.4 mm (eg Toray TGPH-1.4 t). It is first impregnated with the PTFE suspension (TF 5235 from Dyneon), which has been diluted with water in a ratio of 1:6, and then dried. This material is then post-treated at a temperature of 275 ⁰ C.

The catalyst suspension for the oxygen side is prepared from 0.6 g platinum Mohr, 0.4 g iridium Mohr, 2 g water, 1.25 g Nafion solution (Aldrich) and 0.2 g PTFE powder (TF 9202 from Dyneon) by intensive mixing in a ball mill.

This suspension is applied to the carbon fiber paper and dried. It does not penetrate due to the hydrophobic treatment. The precious metal coverage is about 5 mg/cm ² .

The membrane and electrodes are assembled by hot pressing for 30 seconds at 80 bar pressure at 130 ⁰ C. This unit is then backed with a hydrophilic carbon fiber felt (eg GFD2 from SGL Carbon AG) on the oxygen side and installed in a suitable cell body. After a running-in phase of a few hours at about 80 ⁰ C with sufficient water supply, the following values can be achieved:

Operating temperature 80° C:

At a current density of 150 mA/cm ^2, the electrolysis voltage is 1.67 V and the voltage in fuel cell operation is 0.81 V

Operating temperature 20° C:

At a current density of 150 mA/cm ^2, the electrolysis voltage is 1.74 V and the voltage in fuel cell operation is 0.74 V.

The maximum short-term current carrying capacity is well over 1 A/cm ² . The time required to switch between the operating modes is determined only by the time constant of the RC element, which is formed from the electrode resistance and the double-layer capacitance of the cell. It is in the range of about 50 ms.

For example, in periodic charging and discharging operations with a balanced shock balance, approximately 240 As per cm ² electrode area can be converted in one operating mode without water supply or removal.

Claims (8)

1. Combination of electrolysis and fuel cell consisting of an oxygen electrode acting as an anode or cathode, a hydrogen electrode also acting as a cathode or anode, and a solid electrolyte membrane, characterized in that the catalyst of the oxygen electrode consists of several components, of which at least one essentially carries out the oxygen reduction and another essentially carries out the water oxidation. 2. Combination of electrolysis and fuel cell according to claim 1, characterized in that the catalyst for oxygen reduction contains platinum and/or that the catalyst for water oxidation contains one of the elements iridium, rhodium, osmium. 3. Combination of electrolysis and fuel cell according to claim 1 or 2, characterized in that the catalyst of the oxygen electrode contains an alloy of at least two of its components. 4. Combination of electrolysis and fuel cell according to claims 1 to 3, characterized in that H+ ion-conductive material is added to the catalyst on the oxygen side. 5. Combination of electrolysis and fuel cell according to claims 1 to 4, characterized in that the catalyst of the oxygen side is located in a layer as close as possible to the membrane. 6. Combination of electrolysis and fuel cell according to claims 1 to 5, characterized in that the oxygen electrode is more hydrophobic on the side facing the membrane than on the side facing away from the membrane. 7. Combination of electrolysis and fuel cell according to claims 1 to 6, characterized in that the gas diffusion layer of the oxygen electrode consists essentially of carbon. 8. Combination of electrolysis and fuel cell according to claims 1 to 7, characterized in that a thin, hydrophobized noble metal layer is used to protect the gas diffusion layer against anodic oxidation.