POROUS ELECTRODE FOR PROTON-EXCHANGE MEMBRANE

Abstract

A process for manufacturing a catalytic electrode includes depositing an electrocatalytic ink on a carrier, wherein the electrocatalytic ink includes an electrocatalytic material and a product polymerizable into a protonically conductive polymer. The process also includes solidifying the electrocatalytic ink so as to form an electrode wherein the composition of the product polymerizable into a protonically conductive polymer and its proportion in the ink is defined so that the electrode formed has a breaking strength greater than 1 MPa. The process further includes separating the electrode formed from the carrier.

Claims

1. A manufacture comprising an electrochemical cell, wherein said electrochemical cell comprises a proton-exchange membrane, a conductive current collector, and an electrode, wherein said electrode is inserted between said proton-exchange membrane and said current collector, wherein said electrode comprises a protonically conductive polymer and an electrocatalytic material, wherein said electrode has a breaking strength that is greater than 1 MPa, and wherein said electrode has an area of which at least 95% is not mechanically bonded to said conductive current collector and not mechanically bonded to said proton-exchange membrane.

2. The manufacture of claim 1, wherein said electrode comprises methylcellulose, wherein said methylcellulose is present in a proportion by weight of between 2 and 10.5%.

3. The manufacture of claim 1, wherein said electrode comprises a carrier, wherein said electrocatalytic material is deposited on said carrier, and wherein said carrier has a roughness lower than 5 five micrometers.

4. The manufacture of claim 1, further comprising an ink that comprises a polymerizable product, wherein said polymerizable product is polymerizable into a protonically conductive polymer, and wherein as a result of a ratio in which said polymerizable product is present, said electrode has a porosity of between 20% and 40%.

5. The manufacture of claim 1, further comprising electrocatalytic ink, wherein said electrocatalytic ink comprises solid content, wherein said solid content comprises said electrocatalytic material, and wherein said electrocatalytic material comprises between 60% and 75% of said solid content by weight.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG. 1 is a schematic sectional view of an example electrolysis device according to the invention; and

[0017] FIGS. 2 to 5 illustrate various steps of an example process for manufacturing an electrode for such an electrolysis device.

DETAILED DESCRIPTION

[0018] FIG. 1 is a schematic sectional view of an example electrolysis device 1 comprising two electrodes. The electrolysis device 1 is equipped with a proton-exchange membrane 30. An anode 32 and a cathode 31 are fixed on either side of the proton-exchange membrane 30. The electrolysis device 1 furthermore comprises first and second porous current-collectors 35, 36 making contact with the cathode 31 and the anode 32, respectively. The electrolysis device 1 also comprises first and second electrical-supply plates 33, 34 making contact with the first and second porous current-collectors 35, 36, respectively. The first porous current-collector 35 is thus interposed between the first supply-plate 33 and the cathode 31. The second porous current-collector 36 is interposed between the second supply-plate 34 and the anode 32. The electrolysis device 1 is in this case sealed by insulating clamping plates associated with a suitable seal 37.

[0019] The first electrical supply plate 33 contains a water supply duct in communication with the cathode 31 via the first porous current-collector 35. The first electrical supply plate 33 also contains a duct, for removing dihydrogen, in communication with the cathode 31 via the first porous current-collector 35.

[0020] The second electrical supply plate 34 contains a water supply duct in communication with the anode 32 via the porous current-collector 36. The second electrical supply plate 34 also contains a duct, for removing dioxygen, in communication with the anode 32 via the porous current-collector 36.

[0021] The proton-exchange membrane 30 allows protons to be transferred between the anode 32 and the cathode 31 and permits separating the gases generated. The proton-exchange membrane 30 also acts as an electronic insulator between the anode 32 and the cathode 31. The proton-exchange membrane 30 may for example be made of a perfluorosulfonic acid polymer membrane. Such materials are especially distributed under the trade name Nafion by DuPont. Such a membrane frequently is about 100 m in thickness.

[0022] In operation, the anode 32 and the cathode 31 are brought into contact with water. A voltage source 2 applies a potential difference is applied between the anode 32 and the cathode 31. The voltage source 2 is configured to apply a DC voltage of between 1.3 volts and 3.0 volts to cause a current density in the first and second electrical supply plates 33, 34 of between 10 and 40,000 amperes per square meter.

[0023] When such a voltage is applied, the following reaction occurs at the anode 32:

2H.sub.2O.fwdarw.4H.sup.++4e.sup.+O.sub.2.

[0024] An oxidation reaction of the water at the anode 32 produces dioxygen. The oxidation at the anode 32 also generates H.sup.+ ions that pass through the proton-exchange membrane 30 to the cathode 31 and electrons are sent back to the cathode 31 by the source 2. At the cathode 31, the H.sup.+ ions are reduced to generate dihydrogen.

[0025] The reaction at the cathode 31 is thus the following:

2H.sup.++2e.sup..fwdarw.H.sub.2.

[0026] At least one of the cathode 31 and the anode 32, hereafter referred to generically as an electrode, is formed of a layer that forms neither a coating secured to the membrane 30, nor a coating secured to the first or second porous current-collectors 35, 36. One or more of the electrodes is thus a mechanically self-supporting layer.

[0027] The porosity of the first and second porous current-collector collectors 35, 36 to water is thus preserved. It also optimizes use of the catalyst material of the cathode 31 and the anode 32, thereby improving the performance of the electrolysis device 1. Recycling of the electrolysis device 1 is made easier due to the absence of catalyst in the first and second porous current-collectors 35, 36.

[0028] As a result, it possible to avoid any interaction between the membrane 30 and the anode or cathode 31, 32 during the process used to manufacture the anode or cathode 31, 32. Thus, the deformation inherent to wetting and drying of the membrane 30 when an ink is applied thereto is avoided. Cracks of the cathode and anode 31, 32 are thus prevented, and likewise the decreased performance that results therefrom. Degradation of the membrane 30 due to solvents possibly used when forming the cathode and anode 31, 32 is thus also avoided. The impermeability of the membrane 30 to gases is thus improved.

[0029] FIGS. 2 to 4 illustrate various steps of an example process for manufacturing a cathode and an anode 31, 32.

[0030] A first step, illustrated in FIG. 2, includes depositing liquid electrocatalytic ink 6 on a carrier 4. The ink 6 may be deposited by any appropriate means, for example using a coating technique, or by spraying with a nozzle 5 as illustrated. To produce an electrode with a uniform thickness, it is preferable to deposit the ink 6 on a carrier 4 that is substantially flat and held horizontal.

[0031] As detailed below, the electrocatalytic ink 6 and the carrier 4 adhere weakly to each other. This is contrary to the conventional aim of making the ink adhere to a membrane or a current collector.

[0032] On the one hand, the electrocatalytic ink 6 comprises a product polymerizable into a protonically-conductive polymer, and on the other hand an electrocatalytic material. The polymerizable product is intended to be solidified, thus giving the electrode mechanical strength while also making it possible for water and ions to diffuse to the electrocatalytic material when the electrode is assembled in the catalysis device 1. The polymerizable product may take the form of a dissolved polymer or of an ionomer.

[0033] The electrocatalytic material has catalytic properties matched to the catalytic reaction to be carried out. The electrocatalytic material may take the form of particles or nanoparticles containing metal atoms. The catalyst material may comprise metal oxides. In the formulations mentioned below, the electrocatalytic material is iridium oxide. Metals such as platinum, gold, silver, cobalt, and ruthenium may also be used. The electrocatalytic ink 6 advantageously contains a thickening product, an example of which is methylcellulose. The various components of the electrocatalytic ink 6 are advantageously dissolved in a solvent such as water.

[0034] To limit the adhesion between the ink 6 and the carrier 4, it is preferable for the carrier 4 to have a roughness lower than 5 m, and preferably lower than 3 m. In an effort to limit adhesion, it is particularly preferable for the carrier 4 to have an interface energy that is lower than 60 millinewtons per meter, and in some cases, even lower than 45 millinewtons per meter.

[0035] In some embodiments, the carrier 4 has an interface energy higher than 20 millinewtons per meter and preferably at least equal to 25 millinewtons per meter in order to limit the formation of scattered drops before the ink 6 has solidified.

[0036] To limit the adhesion between the ink 6 and the carrier 4, it is particularly desirable for the deposited ink to have a surface tension that is higher than the carrier's interface energy.

[0037] A second step, illustrated in FIG. 3, includes solidifying the electrocatalytic ink 6 that has been deposited on the carrier 4. In this step, the polymerizable product reacts to form a polymer. This results in the formation of a film on the electrode is thus formed on the carrier 4. A polymerizable product is an ionomer.

[0038] The step of solidifying the electrocatalytic ink may involve a drying step to evaporate the solvent. Any drying process may be used. Examples include drying with an oven or a flow of hot air. In some practices, drying the electrocatalytic ink includes placing it in an environment at a temperature of between 50 and 150 C.

[0039] A third step includes separating the electrode formed by the solidified ink 6 from the carrier 4. In some examples, this involves using a force to peel or lift the solidified ink 6 from the carrier 4. Because the solidified ink 6 is not strongly bonded to the carrier 4, this force can be relatively small. This weak adhesion promotes separability of the electrode formed with a minimal risk of deterioration.

[0040] To allow the electrode to be handled during its separation from the carrier 4, and to give it mechanical strength during its subsequent use, the composition of the polymerizable product and its proportion in the ink are defined so that the electrode formed has a breaking strength greater than 1 MPa, preferably greater than 5 MPa, and typically comprised between 1 and 15 MPa.

[0041] An electrode thus formed will advantageously have a thickness comprised between 2 and 20 m and preferably between 5 and 10 m.

[0042] A fourth step, illustrated in FIG. 5, includes integrating an electrode thus formed into an electrochemical cell, here an electrolysis device. Here, the electrode formed is a cathode 31 that is inserted between a proton-exchange membrane 30 and a conductive current collector 35.

[0043] Advantageously, any fastening of the middle part of the electrode is avoided in order not to degrade the performance or lifetime thereof. Advantageously, at least 95% of the area of the electrode is not mechanically bonded to the current collector and not mechanically bonded to the membrane. Advantageously, the electrode is held in position by compressing it between the proton-exchange membrane 30 and a current collector. A pressure of between 0.05 and 1.5 MPa will for example be applied to the electrode in order to ensure it is held in position.

[0044] Advantageously, the electrocatalytic material includes a thickening product. This thickening product advantageously comprises methylcellulose.

[0045] Trials have demonstrated that a proportion of thickening product, and in particular methylcellulose, of between 2 and 10.5% of the solid content of the ink proves to be particularly advantageous to facilitate separation of the solidified ink and the carrier 4. Optimally, this proportion of thickening product is between 3 and 6% and preferably between 4 and 5%.

[0046] To ensure the electrode preserves optimal water diffusion properties, the proportion of the polymerizable product in the ink is advantageously defined so that the electrode formed has a porosity of between 20 and 40%. The ink 6 will possibly and advantageously have a proportion of polymerizable product of between 20 and 30% of its solid content.

[0047] To facilitate the catalytic effect of the electrode formed, the proportion of electrocatalytic material is advantageously between 60 and 75% of the solid content of the ink.

[0048] The following is a first example of a possible formulation for the ink 6:

TABLE-US-00001 % by % by weight (solid Products weight content) IrO.sub.2 12.3 69.9 Nafion ionomer 4.9 27.84 DE2020 Methylcellulose 0.4 2.26 Water 82.3

[0049] The following is a second example of a possible formulation for the ink 6:

TABLE-US-00002 % by % by weight (solid Products weight content) IrO.sub.2 12.3 68.33 Nafion ionomer 4.9 27.22 DE2020 Methylcellulose 0.8 4.44 Water 82.0

[0050] The following is a third example of a possible formulation for the ink 6:

TABLE-US-00003 % by % by weight (solid Products weight content) IrO.sub.2 12.2 64.21 Nafion ionomer 4.8 25.26 DE2020 Methylcellulose 2.0 10.53 Water 81.0

[0051] In the examples given, the various components of the ink are dissolved in water. However, it may also be envisioned to dissolve these components in alcohol or in a mixture of alcohol and water.

[0052] In the process for manufacturing the electrode described above, the electrode does not contain reinforcing fibers. Such an absence of reinforcing fibers allows the protonic permeability of the electrode to be improved and the density of incorporated electrocatalytic material to be maximized. However, it may of course be envisioned to include reinforcing fibers in the ink in order to increase its breaking strength properties.

[0053] Although the example described illustrates the integration of an electrode into an electrolysis device, the invention of course also applies to the integration of such an electrode into a fuel cell. Such an electrode may then be bonded to a membrane adapted to a fuel cell, for example having a thickness of about 25 m.