Polymer electrolyte membrane with MnO2 nanoparticles

Abstract

A polymer electrolyte membrane, wherein the polymer electrolyte membrane optionally has an anode catalyst layer on one main surface and / or a cathode catalyst layer on the other main surface, and wherein MnO2 nanoparticles (7) are applied to at least one main surface of the polymer electrolyte membrane or the anode catalyst layer and / or cathode catalyst layer, if present, wherein the MnO2 nanoparticles (7) are applied in a process step which comprises: (a) spraying a suspension containing MnO2 nanoparticles (7) and subsequent drying; (b) depositing MnO2 nanoparticles (7) by physical or chemical vapor deposition; or (c) Applying a paste containing MnO2 nanoparticles (7) by screen printing and subsequent drying.

Description

[0001] The present invention relates to a polymer electrolyte membrane. Furthermore, the present invention relates to a method for producing such a polymer electrolyte membrane, as well as to the use of MnO2 nanoparticles to extend the service life of a polymer electrolyte membrane in a fuel cell. Furthermore, the present invention relates to a fuel cell comprising such a polymer electrolyte membrane. Background of the invention

[0002] Fuel cells are electrochemical devices that directly convert chemical energy into electrical energy. Fuel cells have a cathode, an electrolyte, and an anode. An oxidant, such as oxygen or air, is supplied to the cathode, and a fuel, such as hydrogen or methanol, is supplied to the anode. Polymer electrolyte membranes are often used as the electrolyte. The polymer electrolyte membrane is electrically non-conductive and insulates the cathode from the anode. However, the polymer electrolyte membrane is permeable to protons. Therefore, polymer electrolyte membranes are also called proton-conducting electrolyte membranes or proton exchange membranes (PEMs). Protons formed during the oxidation of the fuel are transported from the anode through the polymer electrolyte membrane to the cathode. The polymer electrolyte membrane also serves as a barrier between the fuel and the oxidant.A typical material used as a polymer electrolyte membrane in fuel cells is sulfonated polytetrafluoroethylene, also known as Nafion.

[0003] Polymer electrolyte membranes in fuel cells are subject to a certain degree of mechanical or thermal stress. Furthermore, chemical degradation of polymer electrolyte membranes also occurs, which is due, among other things, to the formation of hydrogen peroxide, H2O2, at the cathode and / or anode. The formation of H2O2 in fuel cells was described, for example, by AB LaConti et al., Handbook of Fuel-Cells - Fundamentals, Technology and Applications, Vol. 3, p. 647 (2003). During the decomposition of the unstable H2O2, radicals and / or atomic oxygen are formed, which chemically react with the polymer electrolyte membrane and decompose it. This impairs the performance of the polymer electrolyte membrane (e.g., by reducing proton conductivity or barrier properties) over time, thus reducing the service life of the polymer electrolyte membrane.

[0004] The subsequently published DE 10 2012 212 420 A1 describes an ion-conducting membrane for fuel cell applications. It is described that the membrane has a first and second layer, wherein the first layer comprises a first ion-conducting polymer and nanofibers distributed therein, and the second layer comprises a second ion-conducting polymer without nanofibers. Multilayer membranes with fiber-free layers as well as layers in which nanofibers are dispersed in the ion-conducting polymer are described. Furthermore, materials for the nanofibers are mentioned in DE 10 2012 212 420 A1, wherein it is described, by way of example, that the nanofibers can comprise MnO2.

[0005] DE 10 2011 114 818 A1 describes a fuel cell with an ion-conductive membrane containing a stabilizer and platinum to inhibit fluoride loss from the membrane. For example, cerium ions and platinum metal are distributed within at least a portion of the ion-conductive membrane. It is also described that the stabilizer may comprise MnO2.

[0006] DE 10 2011 101 675 A1 also refers to fuel cell arrangements with reduced loss of fluorine.

[0007] DE 10 2007 048 872 A1 describes a fuel cell substrate, e.g. a membrane, which contains an ionomer and an additive.

[0008] Chemical decomposition of the polymer electrolyte membrane by H2O2 is therefore undesirable. Consequently, there is a need for polymer electrolyte membranes that are better protected against chemical decomposition, particularly by H2O2. The present invention seeks to provide such an improved polymer electrolyte membrane. Summary of the invention

[0009] According to the present invention, unwanted H2O2 is catalytically decomposed on the cathode and / or anode sides of the polymer electrolyte membrane. For this purpose, MnO2 nanoparticles are applied to the corresponding main surface of the polymer electrolyte membrane. The MnO2 nanoparticles decompose any H2O2 that forms immediately upon its formation, thereby neutralizing its damaging effect on the polymer electrolyte membrane.

[0010] Accordingly, the present invention provides a polymer electrolyte membrane having the features of claim 1.

[0011] Various materials are known that catalyze the decomposition of H2O2 into oxygen and water. Compared to other catalysts, however, manganese dioxide (MnO2) exhibits exceptionally favorable kinetics and high efficiency in the decomposition of H2O2. Furthermore, unlike other materials such as platinum, MnO2 is inexpensive and readily available. The use of MnO2 as a nanoparticle also provides a large surface area for the catalytic reaction without impairing the passage of the fuel or oxidant through the layer comprising MnO2 nanoparticles.

[0012] The present invention further provides a method for producing a polymer electrolyte membrane as described above having the features of claim 9.

[0013] The present invention also discloses the use of MnO2 nanoparticles to extend the lifetime of a polymer electrolyte membrane in a fuel cell.

[0014] Another aspect of the present invention is a fuel cell comprising a polymer electrolyte membrane as described above.

[0015] Further embodiments of the present invention are described in the appended claims. Description of the figure

[0016] The figure shows schematically a fuel cell arrangement according to an embodiment of the invention. Detailed description of the invention

[0017] The present invention relates to a polymer electrolyte membrane having the features of claim 1, which has a layer comprising MnO2 nanoparticles on at least one main surface.

[0018] According to the present invention, "nanoparticles" are understood to mean particles with particle sizes in the nanometer to micrometer range. In one embodiment, the MnO2 nanoparticles have a particle size of 1 nm to 10,000 nm. Preferably, the MnO2 nanoparticles have a particle size in the range of 50 nm to 5,000 nm. In a more preferred embodiment, the MnO2 nanoparticles have a particle size in the range of 50 nm to 2,500 nm. The use of nanoparticles leads, on the one hand, to a high surface area for the catalytic decomposition reaction. On the other hand, the use of nanoparticles ensures that the passage of the fuel or oxidant, depending on whether the anode or cathode side of the polymer electrolyte membrane is affected, is essentially unaffected by the layer comprising MnO2 nanoparticles.

[0019] Depending on the operating conditions, H2O2 can form in a fuel cell on the anode side and / or the cathode side. According to the invention, the layer comprising MnO2 nanoparticles is therefore applied to at least one main surface of the polymer electrolyte membrane, i.e., the anode side if it is affected by the formation of H2O2, or the cathode side if H2O2 is formed there under the prevailing operating conditions. In one embodiment, the layer comprising MnO2 nanoparticles is applied to both main surfaces of the polymer electrolyte membrane, i.e., the anode side and the cathode side.

[0020] As explained above, according to the invention, it is desirable to substantially not impair the passage of the fuel or oxidant through the layer comprising MnO2 nanoparticles. In this regard, it is preferable to design the layer comprising MnO2 nanoparticles on the polymer electrolyte membrane in a discontinuous manner. This can be achieved according to the invention, for example, by adjusting the amount of MnO2 nanoparticles applied to the respective main side of the polymer electrolyte membrane such that no continuous layer of significant thickness can form.

[0021] Various types of polymer electrolyte membranes can be used in fuel cells. In principle, these are all subject to chemical decomposition by reactive species resulting from H2O2. Therefore, the present invention can be advantageously used with all polymer electrolyte membranes known in the prior art. Typically, however, sulfonated polytetrafluoroethylene, i.e., a tetrafluoroethylene copolymer which, in addition to a polytetrafluoroethylene backbone, also has sulfonic acid groups (ionomer), is used as the polymer electrolyte membrane. This material is also known under the trade name Nafion. In a preferred embodiment of the present invention, the polymer electrolyte membrane, i.e., the uncoated polymer electrolyte membrane as such, therefore consists of sulfonated polytetrafluoroethylene.

[0022] In the production of fuel cells, catalyst layers can also be applied to the main sides of the polymer electrolyte membrane. The conversion of fuel or oxidant takes place on these catalyst layers. To convert the fuel, e.g., hydrogen or methanol, an anode catalyst is coated onto the anode side of the polymer electrolyte membrane. To convert the oxidant, e.g., oxygen or air, a cathode catalyst is applied to the cathode side of the polymer electrolyte membrane. In such embodiments, the layer comprising MnO2 nanoparticles according to the invention is applied to the respective catalyst layer (i.e., anode catalyst and / or cathode catalyst layer).

[0023] Accordingly, in one embodiment, the present invention relates to a polymer electrolyte membrane on whose anode side an anode catalyst layer is applied, wherein the anode catalyst layer is coated with or comprises a layer comprising MnO2 nanoparticles as defined above. Furthermore, in one embodiment, the present invention relates to a polymer electrolyte membrane on whose cathode side a cathode catalyst layer is applied, wherein the cathode catalyst layer is coated with or comprises a layer comprising MnO2 nanoparticles as defined above.In a further embodiment, the present invention relates to a polymer electrolyte membrane on whose anode side an anode catalyst layer is applied, wherein the anode catalyst layer is coated with a layer comprising MnO2 nanoparticles as defined above, and on whose cathode side a cathode catalyst layer is applied, wherein the cathode catalyst layer is coated with a layer comprising MnO2 nanoparticles as defined above.

[0024] Typically, the aforementioned catalyst layers for converting fuel or oxidizer comprise an electrically conductive carbon support material. Examples of this carbon-containing support material include Vulcan XC-72, carbon nanotubes, or graphene. The anode catalyst or cathode catalyst is applied to this support material. Suitable materials as catalysts include platinum or platinum alloys. A platinum / ruthenium alloy, e.g., Pt, has proven particularly effective as an anode catalyst. 0,7 Ru 0,3 , and platinum as the cathode catalyst. The amount of catalyst in the anode catalyst layer and / or cathode catalyst layer is preferably 0.01 mg / cm 2 up to 10mg / cm 2 , relative to the respective main surface of the polymer electrolyte membrane.

[0025] Such anode and cathode catalyst layers can be produced in a manner known per se in the prior art. For example, a particulate support material as described above, coated with the respective catalyst, can be introduced into a suspension that is sprayed onto the corresponding main side of the polymer electrolyte membrane until the desired surface weight of catalyst is reached. Volatile compounds such as methanol, ethanol, or isopropanol are particularly suitable as the liquid phase of this suspension. The suspension can also contain other additives, such as Nafion, to improve the conductivity of the catalyst layer. After spraying, the coated membrane is dried using conventional methods.

[0026] In one embodiment of the present invention, the amount of MnO2 nanoparticles on the respective main side of the polymer electrolyte membrane is 1 wt.% to 10 wt.%, based on the amount of catalyst in the anode catalyst layer and / or cathode catalyst layer. In another embodiment, the amount of MnO2 nanoparticles on the respective main side of the polymer electrolyte membrane is 0.1 µg / cm 2 up to 1 mg / cm 2 , preferably 0.04mg / cm 2 up to 0.4 mg / cm 2 .

[0027] The present invention also relates to a method for producing a polymer electrolyte membrane having the features of claim 9. The method according to the invention for producing a polymer electrolyte membrane as described above comprises the step of applying a layer comprising MnO2 nanoparticles to at least one main surface of the polymer electrolyte membrane.

[0028] In a variant of the method according to the invention, the application step comprises spraying a suspension containing MnO2 nanoparticles, followed by drying. Accordingly, MnO2 nanoparticles, for example MnO2 nanoparticles of the size defined above, are suspended in a suspension medium. Suitable suspension media are volatile compounds, for example alcohols such as methanol, ethanol, or isopropanol, or a mixture thereof with other common solvents, such as acetone, esters, and mixtures thereof, optionally in combination with water. The suspension is sprayed onto a main side of the polymer electrolyte membrane, which may already be coated with a catalyst layer as described above, using conventional methods. For example, a spray gun can be used for this purpose. The process can take place at room temperature and atmospheric pressure and should be completed within a few seconds, e.g.5 to 10 s. After deposition of the suspension on the membrane, it is dried until the volatile compounds are completely removed, for example by drying in air for a sufficient period of time, e.g., at least 15 minutes. Before deposition, the suspension can be treated in an ultrasonic bath to ensure uniform suspension. The suspension can advantageously be adjusted to a suitable viscosity, e.g., to a viscosity range of 500 to 5000 mPas, preferably 800 to 3000 mPas, more preferably 1000 to 2000 mPas.

[0029] In another variant of the method according to the invention, the application step comprises the deposition of MnO2 nanoparticles by means of physical or chemical vapor deposition. Physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods are known in the art. Using these methods, a preferably discontinuous layer of polycrystalline MnO2 can be deposited on the surface of the polymer electrolyte membrane, or the surface of the catalyst layers applied thereto. This is done using conventional PVD or CVD equipment at temperatures preferably not exceeding 100°C for preferably less than 10 minutes. The thickness of the optionally discontinuous layer of MnO2 nanoparticles obtainable in this way should preferably not exceed 100 monolayers of MnO2. More preferably, the layer thickness is 1 to 10 monolayers.The thickness of a single monolayer is about 2 to 5 nm.

[0030] In PVD processes, the evaporation of MnO2 is achieved, for example, by interaction with an Ar + -ion ​​beam or an electron beam, e.g., using a conventional ion source with MnO2 as the target. In CVD processes, the MnO2 nanoparticles can also be deposited, for example, by thermal evaporation of Mn under a low-pressure oxygen atmosphere (e.g., 0.001 to 0.01 bar).

[0031] In a further variant of the process according to the invention, the MnO2 nanoparticles are applied to the polymer electrolyte membrane or the catalyst layers deposited thereon by screen printing. A suitable paste containing the MnO2 nanoparticles is used for this purpose. After applying the paste using a screen-printing mesh, the coating is dried using conventional methods.

[0032] The present invention also discloses the use of MnO2 nanoparticles to extend the service life of a polymer electrolyte membrane in a fuel cell. As explained above, the MnO2 nanoparticles can directly decompose the H2O2 produced in the reactive zones of a fuel cell (particularly the anode catalyst layer and the cathode catalyst layer), thereby preventing harmful effects on the polymer electrolyte membrane. As a result, the proton conductivity and barrier properties of the membrane, or its mechanical integrity, are maintained for a longer period during fuel cell operation.

[0033] The present invention also relates to a fuel cell comprising a polymer electrolyte membrane as described above. The present invention is applicable to all types of fuel cells that use polymer electrolyte membranes. The basic design of such fuel cells is known in the art. For example, a fuel cell according to the invention contains a polymer electrolyte membrane, optionally coated with an anode catalyst layer and a cathode catalyst layer, and a layer comprising MnO2 nanoparticles applied thereto on at least one main side, preferably on both main sides. Furthermore, a typical fuel cell contains gas diffusion electrodes as the cathode and anode, as well as supply devices for fuel and oxidant.

[0034] A possible fuel cell arrangement is shown schematically in the attached figure. In the figure, 1 denotes the cathode side and 2 the anode side of the fuel cell. Reference numeral 3 denotes the uncoated polymer electrolyte membrane itself. On this polymer electrolyte membrane, corresponding catalyst layers are located on both the cathode and anode sides. MnO2 nanoparticles 7 are deposited on these. The coated membrane is held on both sides by a holder (graphite plate) 4. Fuel 5 and oxidant 6 are supplied via suitable supply lines.

Claims

[1] Polymer electrolyte membrane, wherein the polymer electrolyte membrane optionally has an anode catalyst layer on one main surface and / or a cathode catalyst layer on the other main surface, and wherein MnO2 nanoparticles (7) are applied to at least one main surface of the polymer electrolyte membrane or the anode catalyst layer and / or cathode catalyst layer, if present, wherein the MnO2 nanoparticles (7) are applied in a process step which comprises: (a) spraying a suspension containing MnO2 nanoparticles (7) and subsequent drying; (b) depositing MnO2 nanoparticles (7) by physical or chemical vapor deposition; or (c) Applying a paste containing MnO2 nanoparticles (7) by screen printing and subsequent drying. [2] Polymer electrolyte membrane according to claim 1, wherein the MnO2 nanoparticles (7) have a particle size of 1 nm to 10,000 nm, preferably 50 nm to 5,000 nm, more preferably 50 nm to 2,500 nm. [3] Polymer electrolyte membrane according to one of claims 1 or 2, wherein MnO2 nanoparticles (7) are applied to both main surfaces. [4] Polymer electrolyte membrane according to one of claims 1 to 3, wherein the MnO2 nanoparticles (7) are each applied in the form of a discontinuous layer. [5] A polymer electrolyte membrane according to any one of the preceding claims, wherein the polymer electrolyte membrane comprises sulfonated polytetrafluoroethylene (Nafion). [6] Polymer electrolyte membrane according to one of the preceding claims, further comprising an anode catalyst layer and / or a cathode catalyst layer, on which the MnO2 nanoparticles (7) are each applied. [7] Polymer electrolyte membrane according to claim 6, wherein the anode catalyst layer and / or cathode catalyst layer comprises electrically conductive support material made of carbon coated with a catalyst, preferably selected from platinum and platinum alloys; and wherein preferably the amount of catalyst in the anode catalyst layer and / or cathode catalyst layer is 0.01 mg / cm 2 up to 10mg / cm 2 and more preferably wherein the anode catalyst is a platinum / ruthenium alloy and the cathode catalyst is platinum. [8] The polymer electrolyte membrane according to claim 7, wherein the amount of MnO2 nanoparticles (7) is 1 to 10 wt% based on the amount of the catalyst in the anode catalyst layer and / or cathode catalyst layer. [9] A method for producing a polymer electrolyte membrane according to any one of claims 1 to 8, comprising the steps of: (i) providing a polymer electrolyte membrane, the polymer electrolyte membrane optionally having an anode catalyst layer on one main surface and / or a cathode catalyst layer on the other main surface; (ii) applying MnO2 nanoparticles (7) to at least one main surface of the polymer electrolyte membrane or the anode catalyst layer and / or cathode catalyst layer, if present, wherein the step of applying comprises: (a) spraying a suspension containing MnO2 nanoparticles (7) and subsequent drying; (b) depositing MnO2 nanoparticles (7) by physical or chemical vapor deposition; or (c) Applying a paste containing MnO2 nanoparticles (7) by screen printing and subsequent drying. [10] A fuel cell comprising a polymer electrolyte membrane according to any one of claims 1 to 8.

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