Manufacturing process for catalytic layers for proton exchange membrane fuel cells

Abstract

Method for manufacturing a catalytic layer (6) for a membrane-electrode assembly (200) of proton exchange membrane fuel cells (100), the membrane-electrode assembly (200) comprising an electrolytic membrane (3), two gas diffusion layers (7) on either side of the electrolytic membrane (3) and two catalytic layers (6) disposed at the interfaces between the gas diffusion layers (7) and the electrolytic membrane (3), the method for manufacturing a catalytic layer (6) comprising the following steps: a) preparation of a catalytic ink (8) comprising: the preparation of a suspension comprising ionized water, an alcohol, and a catalyst based on Pt nanoparticles on a graphitic carbon support, the nanoparticles having an average size greater than 30 Å, the addition of an ionomer material,b) deposition of the catalytic ink (8) on at least one surface of the electrolytic membrane (3) and / or on at least one gas diffusion layer (7), so as to obtain a catalytic layer (6). Figure for the abbreviation: Figure 1,

Description

Title of the invention: Method for manufacturing catalytic layers for proton exchange membrane fuel cells

[0001] The present invention relates to the field of proton exchange membrane fuel cells, better known by the English expression "Proton Exchange Membrane Fuel Cell" or by the acronym PEMFC. In particular, the invention relates to the manufacture of a catalytic layer for these cells, making it possible to significantly increase their durability. According to another aspect, the invention also relates to a membrane-electrode assembly comprising at least one catalytic layer thus prepared.

[0002] The operating principle of PEMFC batteries is based on the conversion of chemical energy into electrical energy by catalytic reaction between a fuel, for example hydrogen, and an oxidizer, for example oxygen. Membrane-electrode assemblies (MEAs), commonly called "battery cores", constitute the basic elements of PEMFCs. As illustrated in [Fig.l], the MEA is generally formed of an electrolyte made by a polymer membrane, which constitutes an electronic insulating medium, but which is proton conductive. This electrolyte is in contact with a catalytic layer (electrode) on either side of its surface. Current collectors ensure the transfer of electrons to the external surface of the electrodes. In addition, gas diffusion layers are arranged on either side of the MEA to ensure electrical conduction, homogeneous distribution of reactive gases and evacuation of the water produced.

[0003] The electrochemical reactions which occur in PEMFC batteries are kinetically favored by the presence of a catalyst constituting the electrodes.

[0004] Several materials can be used depending on the type of reaction and fuel, but platinum proves to be the most effective catalyst for most reactions and fuels. As already indicated, the catalyst can be in the form of catalytic layers, which are generally made up of platinum nanoparticles supported by carbon particles. The catalyst can be uniformly deposited using a catalytic ink on the surface of the membrane or on the diffusion layer. This catalytic ink is composed in particular of the catalyst supported on carbon (platinum-platinized carbon), a carrier liquid and a proton-conducting polymer. The latter is generally of the same nature as that of the electrolyte.

[0005] The catalyst plays a main role in the performance of the cell, since it is the one that activates the electrochemical reactions taking place in the active layers. It also acts on durability, in particular thanks to its carbon support which can be more or less graphitized in order to withstand as long as possible the drastic conditions generated during the operation of the PEMFC (low pH, high temperature, high potential, oxidizing atmosphere at the cathode). However, it has been found that deposits of inks based on graphitized carbon are complex to achieve. Indeed, catalysts with graphitic carbon supports are difficult to implement, the active layer is inhomogeneous and cracks easily, the performances achieved fluctuate enormously and the quality of the deposits is not reproducible even from the same mother suspension.

[0006] Thus, one of the aims of the present invention is to overcome the aforementioned drawbacks. To this end, the present invention provides a method for manufacturing a catalytic layer for a membrane-electrode assembly (MEA) of proton exchange membrane fuel cells (PEMFC), the membrane-electrode assembly comprising an electrolytic membrane, two gas diffusion layers on either side of the electrolytic membrane and two catalytic layers arranged at the interfaces between the gas diffusion layers and the electrolytic membrane, the method for manufacturing a catalytic layer comprising the following steps:

[0007] a) preparation of a catalytic ink comprising: i. the preparation of a suspension comprising ionized water, an alcohol and a catalyst based on Pt nanoparticles on a graphitic carbon support, the nanoparticles having an average size greater than 30 Å, in particular an average size of between 40 Å and 100 Å, for example between approximately 45 and 80 Å, on a graphitic carbon support, and

[0008] ii) the addition of an ionomeric material,

[0009] b) depositing the catalytic ink on at least one surface of the electrolytic membrane and / or on at least one gas diffusion layer, so as to obtain a catalytic layer.

[0010] Thanks to the method of the invention, it is possible to obtain a catalytic layer based on Pt / Cg (Pt nanoparticles on graphitic carbon) so as to manufacture PEMFC batteries having a longer lifespan than conventional PEMFCs, for example using amorphous carbon, while retaining the performance of conventional batteries. It is known that graphitic carbon has a more resistant structure under the conditions of high temperatures and the highly oxidizing operating environment of the batteries, which helps to increase its longevity. In this context, the inventors have noticed that the average size of the platinum nanoparticles combined with the use of a graphitic carbon support has a great impact on the performance of these durable batteries.

[0011] It is understood herein that the Pt nanoparticles have an overall shape of a coarse sphere or spheroid. The average size is in particular measured by TEM (transmission electron microscopy) on the largest dimension of the spheroidal Pt nanoparticle.

[0012] According to one possibility, the percentage of dry matter in the catalytic ink is between 9% by weight and 12% by weight. These mass proportions of dry matter are lower compared to the usual proportions of around 17%, the deposits of which have proven to be complex to produce. As previously explained, catalysts with graphitic carbon supports and those in particular loaded with platinum at less than 40% by weight are difficult to implement, the active layer is inhomogeneous and cracks easily. To overcome this drawback, reducing the mass proportions of dry matter compared to the usual proportions has made it possible to obtain continuous and homogeneous catalytic layers, without cracking and reproducible performances. Thanks to this percentage of dry matter in the catalytic ink, it has been possible to vary the Pt content in the catalytic layer in a range from 0.05 mg^ / cm2 to 0.4 mg^ / cm2.

[0013] It is understood in this document that the dry materials consist of the catalyst and the dried ionomer material. This expression corresponds to an expression also known to those skilled in the art under the formulation 'dry extract' or 'solid content' in English.

[0014] According to one arrangement, the mass ratio of ionomer to the I / C catalyst is between approximately 0.90 and 1.30, and in particular between approximately 1.00 and 1.25. This makes it possible to achieve more satisfactory current densities than with a ratio of around 0.70 which can be found with amorphous carbon-based catalysts. Without being bound by any theory, this I / C ratio range seems to allow appropriate ionomer coverage around the catalyst, which would promote proton conduction in the cell core. On the other hand, it seems unwise to increase the VC ratio beyond 1.35 because this entails a risk of densification of the electrode, leading to phenomena of slower diffusion of the reactants in the cell core (also called AME).

[0015] According to one possibility, the preparation of the catalytic ink further comprises the following steps:

[0016] k) addition of inert beads to the suspension obtained in step i) and dispersion on a roller agitator,

[0017] 1) addition of the ionomeric material according to step ii) in the suspension obtained at step k) so as to obtain a suspension comprising the inert beads, and

[0018] m) dispersion of the suspension obtained in step 1) on the roller agitator, so as to obtain the catalytic ink.

[0019] These dispersion steps, which can also be called 'agitation step', are carried out in two stages, one without ionomer and the second with ionomer, allowing to achieve optimal dispersion which contributes to the good quality of the coating deposit.

[0020] Of course, the preparation of the catalytic ink comprises, after step m), the removal of the inert beads from the suspension before its use for the deposition of a catalytic layer.

[0021] The roller agitator used is an IKA Roller 10 basic model. Its speed is set at 30 revolutions per minute and the lifting angle is 3°.

[0022] According to one arrangement, steps k) to m) are carried out at room temperature.

[0023] According to one possibility, the duration of the dispersion of step k) is between 12 and 36 hours, for example between 18 and 30 hours.

[0024] According to one arrangement, the duration of the dispersion of step m) is between 24 and 84 hours, for example between 36 and 72 hours.

[0025] According to one possibility, the cumulative duration of the dispersions of steps k) and m) is less than or equal to 120 hours so as not to reach the moment when the composition of the suspension begins to degrade.

[0026] Advantageously, the inert beads added in step k) have a mass corresponding to twice the mass of dry matter of the catalytic ink.

[0027] According to one possibility, the inert balls have an average millimetric size, for example an average size around 3 mm.

[0028] The Pt / Cg catalyst is chosen from TEC10EA30E-HT® and TEC10EA50E-HT®. These Pt catalysts on graphitic carbon supports have high resistance to oxidation in the drastic conditions generated during operation of the PEMFC cell. They are available from the company Tanaka®. The number 30 (or 50) corresponds to the percentage of platinum in the catalyst and the letters HT (for Heat Treatment) reflect the presence of graphitized carbon, obtained by treatment of an amorphous carbon at high temperature which makes it more durable.

[0029] According to one arrangement, the ionomer material is Nafion®, available from the supplier Dupont®.

[0030] According to one possibility, the step b) of depositing the catalytic ink is carried out by coating, in particular at a temperature of approximately 60°C.

[0031] According to other characteristics, the manufacturing method of the invention comprises one or more of the following optional characteristics considered alone or in combination: - The graphitic carbon support of the catalyst comprises or consists of an aggregate of graphitic carbon particles with an average size of between 30 and 100 nm. - The inert balls are made of zirconium oxide. - The deposition of the catalytic ink is carried out by coating with a speed of approximately 10 mm / s. - The electrolytic membrane comprises an ionomer material. - The ionomer material is of the perfluorosulfonic acid polymer (PFSA) type or Nafion®.

[0032] According to another aspect, the invention proposes a membrane-electrode assembly comprising at least one catalytic layer deposited on at least one face of the electrolytic membrane and / or on at least one gas diffusion layer of said membrane-electrode assembly, the at least one catalytic layer comprising an ionomer dispersed around a catalyst comprising Pt nanoparticles on a graphitic carbon support, Pt nanoparticles having an average size greater than 30 Å.

[0033] According to one possibility, the at least one catalytic layer comprises a Pt loading of between 0.05 mg^ / cm2 and 0.4 mg^ / cm2.

[0034] Example of an AME, the anode of the membrane-electrode assembly is a conventional anode formed from a Pt catalyst on an amorphous carbon support, such as the TEC10V50E® catalyst available from the supplier Tanaka®. The AME is produced by hot transfer of the electrodes (loaded at 0.1 mgpt / cm2) onto a commercial membrane. Pressing is carried out at 145 °C, 1 MPa for 180 seconds.

[0035] According to one variant, the membrane-electrode assembly comprises a catalytic layer at the anode and / or at the cathode.

[0036] Other characteristics and advantages will appear on reading the detailed description below, of a non-limiting example of implementation, made with reference to the appended figures in which:

[0037] [Fig.l] represents a cross-sectional view of a proton exchange membrane fuel cell (PEMFC) comprising an MEA, according to one embodiment of the invention.

[0038] [Fig.2] represents two catalytic layers deposited by coating with a ca ink talytic comprising A a dry mass percentage of 17% and B a dry mass percentage of 10% according to one embodiment of the invention.

[0039] [Fig.3] illustrates polarization curves of different AMEs obtained from the same catalytic ink having a dry mass percentage of 17%.

[0040] [Fig.4] illustrates high frequency resistance bias curves for different I / C ratios for a relative humidity level of 50% according to one embodiment of the invention.

[0041] [Fig.5] illustrates high frequency resistance polarization curves for different I / C ratios for a relative humidity level of 80% according to a rea- lization of the invention.

[0042] [Fig.6] illustrates high frequency resistance bias curves for different I / C ratios for a relative humidity level of 100% according to one embodiment of the invention.

[0043] [Fig.7] represents a diagram of the catalytic ink dispersion protocol according to an embodiment of the invention.

[0044] [Fig.8] represents two optical microscope images of a catalytic layer deposited A by coating a catalytic ink prepared according to a conventional dispersion and B according to a dispersion according to an embodiment of the invention.

[0045] [Fig.9A] represents a top view SEM image of a catalytic layer deposited after dispersion by magnetic stirring.

[0046] [Fig.9B] represents a top view SEM image of a catalytic layer deposited after dispersion according to an embodiment of the invention.

[0047] [Fig. 10] shows polarization curves for catalysts comprising amorphous carbon and graphitic carbon.

[0048] As illustrated in [Fig.l], the proton exchange membrane fuel cell (PEMFC) conventionally comprises a membrane-electrode assembly 200 (MEA) surrounded on either side by a current collector 1, itself surrounded by a fluidic cooling system 2. The membrane-electrode assembly 200 as such comprises an electrolytic membrane 3 formed mainly of Nafion® near which the cathode 4 and the anode 5 are arranged. An ionomer of the perfluorosulfonic acid polymer (PFSA) type is also conceivable as a variant. The anode 5 like the cathode 4 comprises a catalytic layer 6 juxtaposed or deposited on a gas diffusion layer 7.

[0049] In the present invention, the catalytic layer 6 is formed by coating a catalytic ink 8 comprising a catalyst based on Pt nanoparticles on a graphitic carbon support, the nanoparticles having an average size greater than 30 Å. To this catalyst is added ionized water, an alcohol, and ionomer material, the whole forming a suspension which it is necessary to disperse before deposition. Effect of dry matter percentage:

[0050] The effect of the percentage of dry matter in the catalytic ink 8 is studied in particular during the deposition of two catalytic layers 6 having a percentage of dry matter of approximately 17% by weight ([Fig.2] A) and 10% by weight ([Fig.2] B). The reduction of the dry extract in the inks 8 makes it possible to obtain continuous, homogeneous and crack-free deposits, as illustrated in [Fig.2] B, unlike the deposition with the ink comprising a higher percentage of dry matter.

[0051] This lack of homogeneity is also reflected in the performance of the stack cores. 200. These are very heterogeneous, even if the electrodes come from the same deposit. As illustrated in [Fig.3] representing the polarization curves at the conditions of UC = 0.5 H2 / air 80°C, PTot = 2 bara, P02 = 0.4 bara, the performances of eight MEAs intended to be identical fluctuate by more than 50% in current density for a voltage at 0.65 V. This variation is found whatever the relative humidity content ([Fig.3] A 50% RH, [Fig.3] B 80% RH and [Fig.3] C 100% RH). Effect of ionomer / carbon ratio:

[0052] An improvement in performance and reproducibility is observed with increasing ionomer / carbon ratio. Figures 4 to 6 illustrate polarization curves (left A, C and E) and high-frequency resistances measured at four relevant cell voltages (right B, D and F) for AME 200 prepared with a Pt / Gc cathode 4 with different FC ratios, varying between 0.5 and 1.2. The tests were carried out at Tcell. = 80°C, 2 bar absolute pressure and H2 / air flows at (A, B) 50% RH, (C, D) 80% RH and (E, F) 100% RH.

[0053] The results in Table 1 (below) and Figures 4A, 5C, 6E unambiguously demonstrate that the I / C ratio is of major importance for the performance of the AME 200. Whatever the relative humidity tested (from 50 to 100%), the lowest ionomer content (I / C = 0.5) leads to a significant decrease in the performance of the AME 200 (see polarization curves A, C, E). The trend is more accentuated in dry conditions (50% RH and to a lesser extent 80% RH), in which I / C = 0.7 is still insufficient to allow a correct performance of the cell core 200. On the contrary, the higher values ​​of the I / C ratio (1 and 1.2) lead to almost similar polarization curves (probably reaching the optimum). The diagrams of 4B, 5C and 5D also illustrate a high frequency resistance which decreases with increasing FC ratio until reaching a settling towards FC = 1.2.Thus, an improvement in the reproducibility of the tests is observed from an ionomer / carbon ratio of 1 (this is even better for an ionomer / carbon ratio of 1.2) but there is no notable increase in performance between an ionomer / carbon ratio of 1 and 1.2.

[0054] [Tables 1] I / C ratio 50% RH 80% RH 100% RH 0.5 0.11 Acm2 ± 60% 0.30 Acm2 ± 53% 1.2 Acm2 ± 9% 0.7 0.18 Acm2 ±70% 0.61 Acm2 ±21% 1.6 Acm2 ±10% 1 0.61 Acm2 ± 15 % 0.96 Acm2 ± 9% 2.0 Acm2 ± 4% 1.2 0.56 Acm2 ±11% 0.92 Acm2 ± 5% 1.9 Acm2 ± 4%

[0055] Table 1: Comparison of electrochemical performance and test reproducibility, at 0.65 V Effect of dispersion

[0056] The effect of the two-step dispersion (without ionomer then with ionomer) of the catalytic ink 8 is now described in relation to Table 2, [Fig.7] illustrating the diagram of the dispersion protocol and Figures 8A, 8B which are optical microscope images of catalytic layers 6 obtained from different dispersions.

[0057] Table 2 indicates the proportions and quantities of the different constituents of the catalytic ink 8 to form the catalytic layer 6 of the cathode 4. As described above, the anode 5 is obtained from an amorphous carbon catalyst.

[0058] [Tables2] Components Dry extract (%) Mass (g) Ink (%) Dry extract (%) Catalyst TEC10EA30E-H T® (30.3% Pt) 99.00% 1 5% 54.5% Material Ionomer Nafion D2020® 22.00% 3.75 20% 45.5% Ethanol 0.00% 2.2 12% 0.0% Deionized Water 0.00% 11.6 63% 0.0%

[0059] Table 2: Components of catalytic ink 8 (for cathode 4)

[0060] According to a first step, a suspension is prepared by mixing 1 g of ca TEC10EA30E-HT® catalyst available from Tanaka® comprising Pt nanoparticles with an average size of 48 Å on a graphitic carbon support with 11.6 g of ionized water and 2.2 g of ethanol. Inert millimeter-sized zirconium oxide beads are introduced into the suspension before dispersing for approximately 24 hours on a roller mixer 9 at room temperature (20-25°C). This dispersion may be referred to as 'Light Bail Milling' in the remainder of the document.

[0061] The ionomer material Nafion D2020® available from the supplier Dupont® is then introduced with a quantity (3.75 g) making it possible to obtain a mass ratio 1 / C of approximately 1.2, and a percentage of dry matter (or a dry extract) of 9.8% by weight (mass concentration of approximately 0.10 g / ml solvent). The mixture is again dispersed on a roller stirrer 9 for 72 hours. The inert beads are filtered and the catalytic ink 8 thus obtained is deposited by coating at 60°C with a speed of 10 mm / s.

[0062] The deposition of this catalytic ink 8 obtained according to the method of the invention is compared with that of a second reference sample prepared in the same way with the only difference being that the dispersion was produced from a suspension comprising all the components, including the ionomer material, in a single operation and for only 24 hours.

[0063] The quality of the catalytic layers 6 obtained by coating is observed by optical microscope (figures 7 A and 7 B). These images illustrate that the dispersion of the 2nd sample is not adequate since it does not lead to a satisfactory deposition and that many cracks are present. The catalytic layer 6 obtained from the catalytic ink 8 according to the dispersion developed by the invention shows a homogeneous deposition without cracks.

[0064] The effect of the dispersion according to the invention is compared to that of a dispersion by magnetic stirring. SEM images in top view (Figures 8 A and 8 B) illustrate the quality of the two layers 6 deposited by coating from respectively an ink 8 dispersed according to the method of the present invention with an I / C ratio = 0.5 (Figures 8B) and a different catalytic ink because dispersed by magnetic stirring with the same FC ratio of 0.5 ([Fig.8] A).

[0065] We can see that the deposit is not homogeneous in terms of platinum loading when the ink is dispersed only by magnetic stirring: we note the presence of agglomerates during the top view by SEM ([Fig.8] A), When the ink 8 is dispersed by LBM (Light Bail Milling), the SEM images show a great improvement in the homogeneity of the deposits 6 (figures 8 B).

[0066] Platinum loading analysis of X-ray florescence deposits shows a large loading disparity: Lpt = 0.13 mgPt cm2 ± 0.03 when the ink was dispersed by magnetic stirring. Platinum loading analysis of X-ray florescence deposits shows a large loading disparity: Lpt = 0.13 mgPt cm2 ± 0.03 when the ink was dispersed by magnetic stirring. X-ray fluorescence of the dispersed catalytic ink according to the invention has a standard deviation much lower than for a dispersion by magnetic stirring: LPt = 0.10 mg^ cm 2 ± 0.01.

[0067] All these results tend to prove the effectiveness of the solution implemented by the invention, both in the choice of the percentage of dry masses, the choice of the catalyst, the choice of the FC ratio and in the dispersion process leading to the catalytic ink 8. Effect of a graphitic carbon catalyst

[0068] [Fig. 10] now illustrates the performance results obtained for an AME comprising an electrode obtained according to the invention and conventional electrodes. [Fig. 10] allows the comparison of the polarization curves for different catalysts at the following conditions: H2 / air 80°C, PTot = 2 bara, P02 = 0.4 bara, 80% RH (TEC36V52® conventional activated carbon - TEC10V50E® conventional activated carbon and TEC10EA3E-HT® graphitic carbon according to the invention). Thus, the curve illustrating the results with the graphitized catalyst prepared according to the invention shows the formation of an AME 200 which has performances comparable to those obtained with less durable conventional catalysts.

[0069] Thus, the present invention makes it possible to improve the durability of the battery cores 200 (Membrane-Electrode Assembly AME), to produce much more homogeneous coating deposits, and to achieve performances similar to those of conventional commercial catalysts with more resistant and more durable materials.

Claims

Claims

1. A method of manufacturing a catalytic layer (6) for a membrane-electrode assembly (200) of proton exchange membrane fuel cells (100), the membrane-electrode assembly (200) comprising an electrolytic membrane (3), two gas diffusion layers (7) on either side of the electrolytic membrane (3) and two catalytic layers (6) arranged at the interfaces between the gas diffusion layers (7) and the electrolytic membrane (3), the method of manufacturing a catalytic layer (6) comprising the following steps: a) preparing a catalytic ink (8) comprising: i. preparing a suspension comprising ionized water, an alcohol, and a catalyst based on Pt nanoparticles on a graphitic carbon support, the nanoparticles having an average size greater than 30 Å, ii.the addition of an ionomeric material, b) deposition of the catalytic ink (8) on at least one surface of the electrolytic membrane (3) and / or on at least one gas diffusion layer (7), so as to obtain a catalytic layer (6).

2. Manufacturing method according to claim 1, wherein the percentage of dry matter in the catalytic ink (8) is between 9% by weight and 12% by weight.

3. Manufacturing process according to one of claims 1 to 2, in which the mass ratio of ionomer to the I / C catalyst is between approximately 0.90 and 1.

30.

4. Manufacturing method according to one of claims 1 to 3, wherein the preparation of the catalytic ink (8) further comprises the following steps: k) adding inert beads to the suspension obtained in step i) and dispersing on a roller agitator (9), 1) adding the ionomer material according to step ii) to the suspension obtained in step k) so as to obtain a suspension comprising the inert beads, and m) dispersing the suspension obtained in step 1) on the roller agitator (9), so as to obtain the catalytic ink (8).

5. A manufacturing method according to claim 4, wherein the duration of

6.

7.

8.

9.

10. the dispersion of step k) is between 12 and 36 hours. Manufacturing process according to one of claims 4 or 5, in which the duration of the dispersion of step m) is between 36 and 84 hours. Manufacturing method according to one of claims 4 to 6, in which the inert beads added in step k) have a mass corresponding to twice the mass of dry matter of the catalytic ink (8). Manufacturing method according to one of claims 4 to 7, in which the inert balls have an average millimetric size. Membrane-electrode assembly (200) comprising at least one catalytic layer (6) formed on an electrolytic membrane (3) and / or on at least one gas diffusion layer, the at least one catalytic layer (6) comprising an ionomer dispersed around a catalyst comprising Pt nanoparticles on a graphitic carbon support, Pt nanoparticles having an average size greater than 30 Å. Membrane-electrode assembly (200) according to claim 9, wherein the at least one catalytic layer (6) comprises a Pt loading of between 0.05 mg^ / cm2 and 0.4 mg^ / cm2.

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