Method for producing membrane electrode assembly for electrochemical cell

Abstract

The invention relates to a method for producing an electrochemical cell, preferably a fuel cell, in particular a PEM fuel cell, in which a catalyst material is deposited on a carbon-based support material having a porous structure in order to form an electrode of a membrane-electrode assembly and the carbon-based support material is subsequently subjected to a thermal post-treatment, the carbon-based support material is selectively oxidized by means of a reaction gas containing at least one reactant, preferably carbon dioxide (CO2), ammonia (NH3), nitrogen dioxide (NO2), nitrogen monoxide (NO) and / or water vapor (H2O). The invention also relates to an electrochemical cell, preferably a fuel cell, in particular a PEM fuel cell.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for manufacturing a membrane electrode assembly of an electrochemical cell, preferably a fuel cell, in particular a PEM fuel cell. BACKGROUND

[0002] Fuel cells are electrochemical energy converters which convert chemical energy stored in a fuel, for example hydrogen or methane, into electrical energy in an electrochemical reaction. If hydrogen is used as energy source, only water is produced as a by-product upon conversion, so that fuel cells are particularly suitable for zero-emission drives in motor vehicles. Another field of application is stationary plants for generating energy.

[0003] Polymer electrolyte membrane (PEM) fuel cells are a widespread type. A polymer electrolyte membrane fuel cell essentially consists of a membrane electrode assembly (MEA) in which a negative electrode, i.e. an anode, and a positive electrode, i.e. a cathode, are separated by a polymer electrolyte membrane. In combination with bipolar plates for gas supply and cooling, a plurality of membrane electrode assemblies can be connected in series to achieve the (high) voltage required for the respective end application.

[0004] The electrodes of the MEA are usually composed of a porous carbon-based carrier material on which nanoscale platinum particles or platinum alloy particles are deposited as catalyst material. Since the catalyst material is decisive for the performance of the fuel cell, an increase in performance can be achieved by applying a greater amount of platinum and / or by developing a more active catalyst material. The latter allows a reduction of the total amount of platinum required, which has a significant cost proportion to the manufacturing costs of the fuel cell.

[0005] Likewise, the porous structure of the carbon-based carrier material of the electrodes, in particular of the cathode, is decisive for the performance. The respective reaction gas (anode side: hydrogen, cathode side: oxygen) reaches the catalyst material through the porous structure. In principle, it applies that particles of the catalyst material which are in the depth of the pores of the carrier material are better protected from adverse interactions with other electrode components and aging. However, pores which are too deep and / or too narrow limit the transport of gas molecules to the catalyst material and thus limit the performance of the fuel cell, in particular at high current densities. Carbon-based carrier materials with an adverse porous structure are also referred to as "badly accessible carbon". Carbon-based carrier materials which enable a protection of the catalyst material in sufficiently large, non-limiting pores are referred to as "accessible carbon".

[0006] The "accessible carbon" can be synthetically produced by means of a laborious multi-stage template method. In a subsequent step, a platinum-containing catalyst material is deposited on the carbon. Alternatively, the "poorly accessible carbon", which already has catalyst material in its pores, can be converted into "accessible carbon" by means of a thermal aftertreatment. The thermal aftertreatment requires a reaction gas which reacts with the carbon-based carrier material and which etches the carrier material purposefully, preferably in the immediate vicinity of the particles of catalyst material. The previously limited transport pores can be enlarged in this way without losing the protective effect described above. The thermal aftertreatment has the following advantage over the template method: significantly lower-cost carbon-based carrier materials can be used. The disadvantage, however, is a high heat generation when using oxygen as the reaction gas. If the reaction heat is not purposefully removed, the reaction itself is accelerated, so that uncontrolled burning of the carbon-based catalyst material occurs. A controlled reaction process requires low temperatures, long reaction times and small batch sizes, which limit the scale-up of the thermal aftertreatment (Hochkalierung). SUMMARY

[0007] The present application is directed to this task and gives a method for producing an electrochemical cell, in which method a carbon-based carrier material is subjected to an aftertreatment in order to optimize the pore structure of the electrodes of the membrane-electrode assembly. The method should enable a controlled reaction process and should be implemented as simply and cost-effectively as possible.

[0008] In order to solve the stated task, a method having the features of claim 1 is proposed. Advantageous further extensions of the application can be gathered from the dependent claims. Furthermore, an electrochemical cell, preferably a fuel cell, in particular a PEM fuel cell, is given.

[0009] In the method proposed for producing an electrochemical cell, preferably a fuel cell, in particular a PEM fuel cell, in order to construct the electrodes of the membrane-electrode assembly, a catalyst material is deposited on a carbon-based carrier material having a porous structure. Subsequently, the carbon-based carrier material is subjected to a thermal aftertreatment, in which the carbon-based carrier material is selectively oxidized by means of a reaction gas which contains at least one reactant, preferably carbon dioxide (CO2), ammonia (NH3), nitrogen dioxide (NO2), nitrogen monoxide (NO) and / or water vapor (H2O).

[0010] By means of the selective oxidation, the carbon in the immediate vicinity of the catalyst material is purposefully removed. This means that the pores are enlarged, so that the gas transport to the catalyst material is improved. The resulting optimization of the porous structure of the carbon-based carrier material leads to an improvement in the properties of the embedded catalyst material and thus to an improvement in the properties of the electrochemical cell produced according to the method. The selective oxidation leads to the pores in the immediate vicinity of the catalyst material being enlarged; this can also be said to be a selective pore enlargement.

[0011] The widening of the pores in the carbon-based carrier material can be shown by means of nitrogen adsorption measurements. In this case, a defined amount of nitrogen condenses in the pores of the carbon-based carrier material. From the measurement, a pore size distribution can be derived, which shows the differential volume of the pores having a certain diameter.

[0012] In order to measure the performance of the catalyst material in the fuel cell, a current-voltage characteristic curve can be recorded. The higher the cell potential at a defined current density, the better the performance of the cell.

[0013] By means of the proposed method, "badly accessible carbon" can be converted into "accessible carbon". This means that cost-advantageous carbon-based carrier materials can be used. In contrast to the thermal aftertreatment using oxygen explained at the outset, however, no or only a significantly reduced degree of heat generation occurs in the proposed method, so that the reaction can always remain controllable. In particular, a self-accelerating reaction is reliably prevented until the carbon-based carrier material is completely combusted.

[0014] In the thermal aftertreatment using oxygen (O2), the reaction proceeds strongly exothermically, since the carbon is combusted. The reaction can be represented by the following equation:

[0015] In the selective oxidation, for example by means of water vapor (H2O) as a reactant, the reaction proceeds endothermically according to the following equation, in contrast:

[0016] This applies analogously in the case of the use of carbon dioxide (CO2) or ammonia (NH3) as a reactant, in which the endothermic reaction represented by the following equation is run:

[0017] If the reaction proceeds endothermically, i.e. without releasing heat, the course of the thermal aftertreatment can be accelerated and / or shortened by increasing the temperature, without the reaction running out of control. In the case of an endothermic reaction, the selective oxidation is thus easier to control. In the case of a strongly exothermic reaction, there would even be a danger of complete combustion of the carbon-based carrier material.

[0018] According to a preferred embodiment of the application, therefore, the selective oxidation is an endothermic reaction. Preferably, water vapor (H2O), carbon dioxide (CO2) and / or ammonia (NH3) are used as reactants, since they lead to an endothermic reaction.

[0019] According to another preferred embodiment of the present application, the selective oxidation is a slightly exothermic reaction, wherein "slightly exothermic" is to be understood in contrast to "strongly exothermic" reactions in the case of using oxygen (O2) as reactant (see equation 1 above). In the case of a slightly exothermic reaction, heat is also released, however, in a smaller amount. This is the case by performing the selective oxidation with nitrogen dioxide (NO2) and / or nitric oxide (NO) as reactants.

[0020] For performing the proposed method, it is thus also possible to use nitrogen dioxide (NO2) and / or nitric oxide (NO) as reactants. Since the reaction is slightly exothermic, i.e. only a small amount of heat is released, the reaction remains controllable. In addition, a further advantage can be achieved by additionally embedding nitrogen or nitrogen groups on the carbon surface. The reaction which is associated therewith can be derived from the following equation:

[0021] It is furthermore proposed to use a gas mixture containing carbon dioxide (CO2), ammonia (NH3), nitrogen dioxide (NO2), nitric oxide (NO) and / or water vapor (H2O) as reactant. The above-mentioned gases can thus also be mixed, wherein the mixture or the mixed gas contains at least two of the above-mentioned gases as reactants. The mixing ratio or the respective concentration of the gases contained in the mixture then determines whether the selective oxidation is an endothermic or a (slightly) exothermic reaction.

[0022] Preferably, in the case of an exothermic reaction, the amount of heat released during the selective oxidation is controlled via the concentration of at least one of the reactants in the reaction gas. In this way, it can be ensured that the reaction remains unchanged in a slightly exothermic and thus controllable manner. This applies not only in the case of using a gas mixture as reactant, but also in the case of using a gas as reactant. However, it is also preferred that the gas used as reactant is not present as pure gas, but as a gas mixture comprising the reactant and an inert carrier gas (Trägergas).

[0023] It is furthermore proposed that the mass loss of the carbon-based carrier material achieved via the selective oxidation is controlled via the reaction temperature and / or the reaction duration during the thermal aftertreatment. A higher reaction temperature in the case of a constant reaction duration leads to a greater relative mass loss, which means that the pores are enlarged more strongly. In addition, a defined mass loss can be achieved at a higher reaction temperature in a shorter time. This alternative of the reaction parameters is only observed to a limited extent in the known thermal aftertreatment using oxygen, since the self-acceleration of the reaction at too high a reaction temperature leads to an uncontrolled complete combustion of the carbon-based carrier material.

[0024] The reaction temperature is preferably selected in the range from 200 to 1000°C, preferably from 300 to 700°C, and / or the reaction duration is between one minute and 24 hours, preferably between 1 hour and 24 hours. A shorter reaction duration is advantageous, in particular for the mass production of electrochemical cells. In order to shorten the reaction duration, the reaction temperature should be selected as high as possible, which is possible in principle in the proposed method.

[0025] Furthermore preferably, a carbon-based carrier material having a mass-specific surface area in the range from 200 and 2000 m 2 / g, preferably between 4000 and 1000 m 2 / g is used. The mass-specific surface area of the carrier material can be inferred from the pore volume of the carrier material. For example, at the same pore volume, micropores having a pore size of up to 2 nm have a greater mass-specific surface area than mesopores having a pore size between 2 and 14 nm. Experiments with different carbon-based carrier materials have shown that the selective oxidation leads to a slightly increased pore volume in the case of micropores and to a significantly increased pore volume in the case of mesopores. The desired pore enlargement effect is thus significantly more pronounced in the case of mesopores.

[0026] Preferably, platinum (Pt), ruthenium (Ru), rhodium (Rh), iridium (Ir), palladium (Pd) and / or alloys thereof are used as catalyst material. Platinum belongs to the particularly efficient catalyst materials, so that platinum is particularly preferably used.

[0027] Furthermore preferably, the carbon-based carrier material contains 5 to 60% by weight of catalyst material, relative to the total weight of the carbon-based carrier material, after deposition of the catalyst material and before the thermal aftertreatment. By the proposed method, an advantageous pore enlargement is produced by the selective oxidation in the vicinity of the catalyst material.

[0028] According to a further preferred embodiment of the application, a gas mixture composed of water vapor (H2O) and an inert carrier gas, for example argon (Ar) and / or molecular nitrogen (N2), is used as reaction gas. The method can be carried out in an advantageous manner particularly simply and cost-effectively.

[0029] Furthermore preferably, in order to produce the gas mixture, the inert carrier gas is guided through a humidifier which is filled with water. In the humidifier, the inert carrier gas absorbs water or water vapor. The gas mixture which comes out of the humidifier has a water vapor fraction which corresponds to the saturation vapor pressure of water at the temperature of the humidifier.

[0030] The temperature in the humidifier is preferably 10 to 100°C, further preferably 20 to 50°C. The partial pressure of the water vapor in the gas stream at a temperature of 10 to 100°C is 1 to 100 kPa, and at a temperature of 20 to 50°C is 2 to 12 kPa. The partial pressure of the water vapor in the gas stream again depends on the temperature in the humidifier, by which the reaction for the selective oxidation of the carbon-based support material can likewise be controlled.

[0031] In principle, the temperature in the humidifier can also exceed 100°C. In this case, the partial pressure of the water vapor can be increased to more than 100 kPa.

[0032] The thermal post-treatment of the carbon-based support material is preferably carried out in an oven, through which the reaction gas is guided. The oven makes it possible to set a defined reaction temperature and thus to achieve a controlled reaction process. Further reaction control is possible via the volume flow of the reaction gas, as long as an under-stoichiometric volume flow is introduced into the oven. Preferably, however, an over-stoichiometric volume flow is used in order to safeguard a uniform post-treatment of the carbon-based support material.

[0033] Furthermore, an electrochemical cell, preferably a fuel cell, in particular a PEM fuel cell, is proposed, which is manufactured according to the method according to the application. The electrochemical cell is characterized by an improved performance. BRIEF DESCRIPTION OF DRAWINGS

[0034] The application and its advantages are explained further below with reference to the drawings. The drawings show: Figure 1 : schematic diagram of a test construction for the thermal post-treatment of a carbon-based support material, Figure 2 : schematic diagram for describing the relative mass loss (%) in relation to the reaction temperature (°C) during the thermal post-treatment, Figure 3 : schematic diagram for describing the differential pore volume in relation to the corresponding pore size of the carbon-based support material, (a) before and (b) after the thermal post-treatment, and Figure 4 : schematic diagram for describing the current-voltage characteristic of a PEM fuel cell operated with hydrogen and air (21% oxygen in nitrogen) at 80°C, 95% relative humidity and 170 kPa abs inlet pressure with a 5 cm 2 large active surface, the carbon-based support material of the cathode (a) without thermal post-treatment and (b) with thermal post-treatment. DETAILED DESCRIPTION

[0035] From Figure 1An exemplary test setup for the thermal post-treatment of the carbon-based support material 7 doped with catalyst material is shown. The test setup comprises a humidifier 2 and an oven 5. The carbon-based support material 7 doped with catalyst material is arranged in a quartz tube 6 of the oven 5. For the thermal post-treatment of the carbon-based support material 7 doped with catalyst material, a reaction gas is introduced into the oven 5.

[0036] The reaction gas is currently a gas mixture 4 generated in the humidifier 2. For this purpose, an inert gas 1 is introduced into the humidifier 2, which is filled with water 3. The inert gas 1 absorbs water or water vapor in the humidifier 2, so that the gas mixture 4 is generated, which is used as reaction gas.

[0037] By means of the gas mixture 4 introduced into the oven 5, the carbon-based support material 7 is selectively oxidized, wherein the selective oxidation can be controlled via the partial pressure of the water vapor in the gas mixture 4, the reaction temperature in the oven 5 and the reaction duration. The partial pressure of the water vapor is in turn related to the temperature in the humidifier 2.

[0038] If another reaction gas, for example carbon dioxide (CO2), is used for the selective oxidation, the reaction gas can be introduced directly into the oven 5. The humidifier 2 is not necessary.

[0039] The selective oxidation of the carbon-based support material 7 doped with catalyst material takes place with a mass loss, which is desired. Because the mass loss means a greater pore volume and thus a better gas transport capability. Accordingly, the performance of the catalyst material and thus of the electrochemical cell is improved. The mass loss can additionally be controlled via the reaction temperature. The reaction temperature should be set so that excessive mass loss is avoided, because excessive mass loss can cause negative effects.

[0040] As shown exemplary in Figure 2 the relative mass loss (%) is related to the reaction temperature (°C) and the reaction duration, wherein these measurements are made after 12 hours and after 1 hour, respectively. The higher the reaction temperature, the greater the mass loss or rather a specific mass loss can be reached faster. The relative mass loss in the non-reacting gas (here: argon) is indicated by the hatching in Figure 2 .

[0041] The observed mass loss is accompanied by a widening of the pores in the carbon-based support material. This can be proven by means of nitrogen adsorption measurements. Here, a defined amount of nitrogen condenses in the pores. From these measurements, the pore size distribution can be determined. As shown exemplary in Figure 3As can be seen by way of example, the thermally post-treated carbon-based support material (curve (b)) exhibits a slightly increased pore volume in the microporous region (up to 2 nm) and a significantly increased pore volume in the mesoporous region (between 2 and 14 nm) compared to the untreated original material (curve (a)). These measurements thus suggest that pores, especially mesopores, can be successfully enlarged.

[0042] In addition, by Figure 4 The current-voltage characteristic curves can be observed, which demonstrate the performance improvement of the electrochemical cell manufactured according to the proposed method. Under a given current density, the higher the cell potential, the better the cell performance.

[0043] At low current density (<500 mA / cm) 2 In the region of [missing information], the achieved cell voltage (V) is virtually the same for both the untreated raw material (curve (a)) and the thermally treated carbon-based support material (curve (b)). This indicates that the active catalyst material particles are located within the pores of the support material in both cases and are protected from adverse interactions with other electrode components. At high current densities (>1000 mA / cm²), [missing information]. 2 In the region of [the cell], the thermally post-treated carbon-based support material achieves significantly higher battery voltage and thus higher conversion efficiency of the hydrogen used. This is explained by the improved gas transport capacity of the pores in the carbon-based support material, as these pores are enlarged through thermal post-treatment.

Claims

1. A method for manufacturing an electrochemical cell, preferably a fuel cell, and especially a PEM fuel cell, wherein, in order to construct the electrode of a membrane electrode assembly, a catalyst material is deposited on a carbon-based support material having a porous structure, and the carbon-based support material is subsequently subjected to a thermal post-treatment, wherein, The carbon-based support material is selectively oxidized by means of a reaction gas containing at least one reactant, preferably carbon dioxide (CO2), ammonia (NH3), nitrogen dioxide (NO2), nitric oxide (NO) and / or water vapor (H2O).

2. The method according to claim 1, Its features are, Selective oxidation is an endothermic reaction.

3. The method according to claim 1 or 2, Its features are, Water vapor (H2O), carbon dioxide (CO2), and / or ammonia (NH3) are used as reactants.

4. The method according to claim 1, Its features are, Nitrogen dioxide (NO2) and / or nitric oxide (NO) are used as reactants.

5. The method according to claim 1, Its features are, A gaseous mixture containing carbon dioxide (CO2), ammonia (NH3), nitrogen dioxide (NO2), nitric oxide (NO) and / or water vapor (H2O) is used as a reactant.

6. The method according to claim 4 or 5, Its features are, In the case of an exothermic reaction, the heat released during the selective oxidation is controlled by the concentration of at least one reactant in the reaction gas.

7. The method according to any one of the preceding claims, Its features are, The mass loss of the carbon-based support material achieved by the selective oxidation is controlled by the reaction temperature and / or reaction time during the thermal post-treatment.

8. The method according to any one of the preceding claims, Its features are, Choose a reaction temperature in the range of 200 to 1000°C, preferably in the range of 300 to 700°C, and / or a reaction duration between one minute and 24 hours, preferably between one hour and 24 hours.

9. The method according to any one of the preceding claims, Its features are, Using those with 200 and 2000m 2 / g, preferably at 400 and 1000 m 2 Carbon-based carrier materials with a mass specific surface area between / g.

10. The method according to any one of the preceding claims, Its features are, Platinum (Pt), ruthenium (Ru), rhodium (Rh), iridium (Ir), palladium (Pd), and / or their alloys are used as catalyst materials.

11. The method according to any one of the preceding claims, Its features are, The carbon-based support material contains 5 to 60% by mass of catalyst material relative to the total weight of the carbon-based support material after the deposition of the catalyst material and before the thermal post-treatment.

12. The method according to any one of the preceding claims, Its features are, A gas mixture consisting of water vapor (H2O) and an inert carrier gas, such as argon (Ar) and / or nitrogen molecules (N2), is used as the reaction gas.

13. The method according to claim 12, Its features are, In order to generate the gas mixture, the inert carrier gas is guided through a water-filled humidifier, wherein the temperature in the humidifier is preferably 10 to 100°C, more preferably 20 to 50°C.

14. The method according to any one of the preceding claims, Its features are, The thermal post-treatment of the carbon-based carrier material is carried out in an oven, through which the reaction gases are guided.

15. An electrochemical cell, preferably a fuel cell, especially a PEM fuel cell, said cell being manufactured according to the method according to any one of the preceding claims.

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