Porous transport element for membrane electrode assemblies, electrolyzer comprising said porous transport element and method for producing said porous element

Abstract

The present invention relates to a porous transport element (1) for membrane electrode assemblies comprising: -a porous main body (10) whereupon a first face (11) is identified that is intended in use to face a catalytic layer of a membrane electrode assembly, the porosity of said main body (10) being defined by a medium porosity Φ1; -a porous conductive material layer (20) which is arranged to cover said first face (11) of said main body (10) in order to be interposed in use between said main body (10) and said catalytic layer and that has a porosity defined by a medium porosity Φ2. Said layer of conductive material (20) comprises at least one electronic conductive organic polymer.

Description

"POROUS TRANSPORT ELEMENT FOR MEMBRANE ELECTRODEASSEMBLIES, ELECTROLYZER COMPRISING SAID POROUS TRANSPORT ELEMENT AND METHOD FOR PRODUCING SAID POROUS ELEMENT" DESCRIPTIONField of application

[0001] The present invention relates to a porous transport element for membrane electrode assemblies, an electrolyzer comprising said porous transport element, and a method for producing said porous element.

[0002] The porous transport element for membrane electrode assemblies according to the invention finds particular application in electrolyzers (Water Electrolyzers), but may also find application in other electrochemical devices that use membrane electrode assemblies (MEA), in particular fuel cells.Prior art

[0003] As shown schematically in Fig. 1, a membrane electrode assembly, better known in the field as an MEA, comprises:

[0004] - a membrane M, typically polymeric, for example of the polymer electrolyte type with ionic conductivity,

[0005] - an anode A arranged upon a first face of the membrane M (for example in the form of a layer with a catalyst and ionomer)

[0006] - a cathode C arranged upon a second face of themembrane M (for example in the form of a layer with a catalyst and ionomer).

[0007] Such assemblies (MEAs) are generally used in fuel cells and in electrolyzers, as components of an assembled stack.

[0008] The membrane is inserted between two electrodes that contain the catalyst. The electrodes are electrically insulated from each other by the membrane and constitute the anode and the cathode, respectively.

[0009] The polymer membrane is typically an electrical insulation barrier that is selectively permeable to ions, for example made of perfluorosulfonic acid (PFSA) ionomers. In the case of a membrane made using an anionic conductivity polymer, the membrane is called an AEM membrane - Anion Exchange Membrane. In the case of a membrane made of a polymer with protonic conductivity, the membrane is called a PEM - Proton Exchange Membrane.

[0010] In the assembled stack that makes up a fuel cell or an electrolyzer, each electrode consists of a catalytic layer and may be associated with a porous transport layer, known in the field as a PTL (porous transport layer) or GDL (gas diffusion layer).

[0011] In electrolysis, by means of electrical energy, hydrogen and oxygen are produced from water. Polymer membrane electrolyzers comprise one or more membraneelectrode assemblies (MEA). Chemical reactions take place within the catalytic layers, and gases are produced therein. The supply of water, discharging of gases, and the electrical connection all take place by means of the transport layers PTL which may also be defined as gas diffusion layers.

[0012] An MEA makes it possible to establish a triphasic, anodic, or cathodic boundary, which is essential for the establishment of the transport phenomena necessary for the operation of electrolyzers and fuel cells.

[0013] In the case of polymer electrolytic electrolyzers with protonic conductivity (PEMWE), the anodic three- phase boundary is the interface where the anode electrode, the reagent (H2O), and the polymer electrolyte (PEM) meet. Water is oxidized at this boundary thereby producing protons, electrons, and oxygen. The protons are transported through the membrane to the cathode, while the electrons flow through paths that are available in the external circuit. The efficiency of this process depends upon the quality of the anodic three-phase boundary which is influenced by factors such as the distribution of the catalyst, the diffusion of water through the electrode, and the transport of gaseous oxygen out of the electrode.

[0014] In polymer electrolyte electrolyzers with protonicconductivity (PEMWE), however, the three-phase cathodic boundary is the interface where the cathode electrode, the protons, and the electrons meet. Protons and electrons are combined at this boundary to produce gaseous hydrogen. The efficiency of this process also depends upon the quality and distribution of the three- phase cathodic boundary.

[0015] The three-phase boundaries are therefore fundamental to the transport phenomena that occur inside an electrolyzer because their distribution and accessibility determine the transfer of reagents, products, and charges between the electrodes and the membrane. The efficiency of these processes depends upon the quality of the interfaces and the transport properties of the materials involved. Optimization of said three-phase boundaries is therefore essential to improving the performance of electrolyzers .

[0016] The same problems that are associated with the three-phase boundary are also present in polymer electrolyte fuel cells with the difference that hydrogen oxidizes at the anode producing electrons and protons, while at the cathode the oxygen is reduced resulting in the production of water.

[0017] In particular, in acidic electrolysis, protons migrate through the membrane in the form of chargecarriers. The polymer membrane consists of a protonconducting polymer, for example, perfluorosulfonic acid (PFSA). Acid electrolysis is also called PEM electrolysis (using a proton exchange membrane). The catalytic layers are usually composed of nanoparticles, for example, IrOx or IrRuOx on the anodic side and platinum on carbon on the cathodic side, each with a proton-conducting polymer (for example, PFSA-based) as a binder. The transport layers usually consist of a non-woven fabric of titanium fibers, sintered titanium, or a stretched titanium lattice (anodic side) and a carbon fiber cloth (on the cathodic side).

[0018] A key objective for the next generation of PEMWE protonic conductivity polymer electrolyzers is to reduce the catalyst load in order to both reduce costs and mitigate the possibility of future catalyst supply shortages, catalysts consisting for example of iridium, an extremely scarce and critical material.

[0019] The prior art technology in the field of electrolyzers is currently based upon a membrane platform that is coated with a catalyst. In this configuration, an iridium-based catalytic layer is deposited upon a proton exchange membrane (PEM) to form the anodic electrode. Several techniques for direct and indirect ink deposition have been developed in order to obtain a uniform anduniformly distributed layer of iridium (JP6225890B2).However, all of the techniques developed thus far are unable to achieve desirable properties both in terms of performance and durability when attempting to reduce the catalyst load below 0.5 mg?cm-2. The lower currently reached limit is about 2 mg?cm-2.

[0020] Recent studies have identified the formation of islands and / or disconnected areas within the coated electrode as one of the main limiting mechanisms. These islands and / or disconnected areas are generated by local deformations of the membrane and by the dissolution of the iridium during operation, causing the iridium layer to be subdivided into multiple fragments. When one of these fragments loses the electrical connection with the interface of the porous transport layer and with the remaining part of the catalytic layer, it becomes electrically isolated and ceases to function, thereby effectively deactivating the corresponding section of the anodic electrode.

[0021] This problem is addressed in particular in patent application US2022 / 0307141 Al. It is proposed to interpose an intermediate layer consisting of conductive ceramic or metal nanofibers between at least one catalytic layer formed by nanoparticles and the relative porous transport layer. Alternatively, it is proposed tointroduce the conductive ceramic or metal nanofibers directly into the catalytic layer. The increase in conductivity at the interface between the catalytic layer and the membrane results in an increase in the electrical connectivity of the catalytically active nanoparticles, leading to an increase in conductivity at the interface. Furthermore, the nanofibers may reduce the contact resistance between the transport layer and the catalytic layer and further reduce the mass transport resistance by increasing porosity.Disclosure of the invention

[0022] Therefore, the main object of the present invention is to provide an alternative solution to that of the aforementioned prior art, by providing a porous transport element for membrane electrode assemblies that makes it possible to ensure electrical connectivity with a catalytic layer once said element is inserted into a membrane electrode assembly.

[0023] A further object of the present invention is to provide a porous transport element for membrane electrode assemblies that is easy to manufacture.Brief description of the drawings

[0024] The technical features of the invention, according to the aforesaid objects, may be clearly seen in the contents of the claims below, and its advantages willbecome more readily apparent in the detailed description that follows, made with reference to the accompanying drawings, which represent one or more purely exemplifying and non-limiting embodiments thereof, wherein:

[0025] - Fig. 1 is a not-to-scale schematic representation of a porous transport element for membrane electrode assemblies according to the invention, illustrated in association with a membrane electrode assembly MEA. Detailed description

[0026] A porous transport element for membrane electrode assemblies according to the invention has been indicated as a whole with reference numeral 1 in the accompanying Fig. 1.

[0027] A membrane electrode assembly MEA, intended to be integrated into an assembled stack that forms an electrolyzer or fuel cell, comprises: - a membrane M, in particular a polymer (for example of the polymer electrolyte type with ionic conductivity); - an anode A arranged at a first face of the membrane; - a cathode C arranged at a second face of the membrane.

[0028] In other words, in a membrane electrode assembly MEA, the membrane is inserted between two electrodes containing the catalyst. The electrodes are electrically insulated from each other by the membrane and constitute the anode and the cathode, respectively. In turn, eachelectrode consists of a catalytic layer.

[0029] Within an assembled stack forming an electrolyzer or fuel cell, in addition to one or more MEAs, there may be other components (such as gaskets and bipolar plates) which for simplicity of disclosure are not listed here, also taking into account the fact that the application of the porous transport element 1 according to the present invention is not limited to the type of assembled stack and to its specific components.

[0030] In particular, in an assembled stack, each catalytic layer within an MEA is associated with a porous transport layer, known in the industry as a PTL (Porous Transport Layer) or GDL (Gas Diffusion Layer).

[0031] The porous transport element 1 for membrane electrode assemblies according to the invention constitutes at least one of the two porous transport layers associated with an MEA, preferably the one associated with the anode.

[0032] According to one general embodiment of the invention, the porous transport element 1 for membrane electrode assemblies comprises a porous main body 10 whereupon a first face 11 is identified that is intended in use to face a catalytic layer A of a membrane electrode assembly.

[0033] The porosity of said main body 10 is defined by anaverage porosity ?1, defined as the ratio between the void volume and the total volume of the body.

[0034] Preferably, the average porosity ?1 of said main porous body 10 is equal to or greater than 30%, preferably equal to or greater than 50%, more preferably equal to 75%.

[0035] Preferably, the main porous body 10 has a thickness of between 100 micrometers and 2 millimeters, more preferably of between 300 micrometers and 1 millimeter.

[0036] In particular, the porous main body 10 may be made of a sintered metal, preferably titanium. In such a case, preferably, the average distance between sintered particles is between 5 micrometers and 60 micrometers, preferably equal to about 10 micrometers.

[0037] The porous main body 10 is preferably made of a sintered metal, preferably titanium, made of particles with an average diameter of between 30 and 70 micrometers, preferably equal to about 50 micrometers. Preferably, the porous main body 10 is made of sintered titanium and has a thickness of between 125 micrometers and 1 millimeter.

[0038] Alternatively, the porous main body 10 may be a nonwoven fabric of metal fibers, preferably titanium. In such a case, preferably, the metal fibers have an average diameter of between 10 micrometers and 50 micrometers.

[0039] The porous main body 10 is preferably a non-woven fabric of titanium fibers with an average diameter of about 20 micrometers and has a thickness of between 125 micrometers and 2 millimeters.

[0040] According to the aforesaid general embodiment of the invention, the porous transport element 1 for membrane electrode assemblies also comprises a porous conductive material layer 20 which is arranged to cover the first face 11 of said main body 10 to be interposed in use between the main body 10 and the catalytic layer A towards which, in use, the first face 11 of said main body 10 is facing.

[0041] Said porous conductive material layer 20 has a porosity defined by an average porosity ?2, understood as the ratio between the void volume and the total volume of the layer.

[0042] According to the invention, said porous conductive material layer 20 comprises at least one electronic conductive organic polymer.

[0043] According to a preferred embodiment of the invention, the porous conductive material layer 20 comprises at least one organic polymer with ionic conductivity in addition to said at least one electronic conductive organic polymer. In other words, preferably, the porous conductive material layer 20 comprises amixture of ionic conductive polymer material and electronic conductive polymer material.

[0044] Advantageously, said porous conductive material layer 20 is integral to the main porous body 10. In particular, the porous conductive material layer 20 may be obtained by depositing the polymer materials by electrodeposition using a dip coating technique or else using a spray coating technique.

[0045] Preferably, said at least one ion-conductive organic-type polymer is a protonic conductivity polymer, preferably selected from amongst polyanionic polymers.

[0046] According to a particularly preferred embodiment, said protonic conductive polymer is selected from the ionomers of perfluorosulfonic acid (PFSA), preferably with an equivalent molecular weight between 1,200 and 700 g / mol. In particular, the PFSA may be Nafion or Aquivion.

[0047] Alternatively, said protonic conductivity polymer may be poly (styrene sulfonate) (PSS), preferably with an equivalent molecular weight between 70,000 and 150,000 g / mol.

[0048] Advantageously, the electronic conductive organic polymer is selected from amongst conjugated polymers.

[0049] Preferably, the electronic conductive polymer is selected from the group consisting of PEDOT (poly (3,4- ethylene dioxythiophene) , PPY (polypyrrole), PANI(polyaniline) .

[0050] The electronic conductive polymer may be present within the porous conductive material layer 20 in fiber form .

[0051] In this case, preferably, said fibers also comprise a polymer with ionic conductivity, preferably with protonic conductivity.

[0052] In particular, said fibers have an average diameter of less than 1 micrometer, and preferably between 100 nanometers and 700 nanometers.

[0053] Alternatively, the electron-conductive polymer may be present within the porous conductive material layer 20 in the form of particles and / or aggregates.

[0054] In such case, preferably, said particles and / or aggregates also comprise a polymer with ionic conductivity, preferably with protonic conductivity.

[0055] Preferably, said porous conductive material layer 20 has a thickness of between 5 micrometers and 25 micrometers .

[0056] Advantageously, the average porosity ?2 of said porous conductive material layer 20 is equal to or greater than 30%.

[0057] Advantageously, the average porosity ?2 of said porous conductive material layer 20 is equal to or greater than 50%.

[0058] Advantageously, the average porosity ?2 of said porous conductive material layer 20 is equal to or greater than the average porosity ?1 of said main porous body 10.

[0059] Preferably, the average porosity ?2 of said porous conductive material layer 20 is greater than the average porosity ?1 of said main porous body 10. In the case wherein the porous conductive material layer consists of a set of fibers, preferably, the average value of the distance d2 between the fibers of the porous conductive layer 20 is less than:

[0060] - the average value of the distance between the metal fibers of the main body 10 (if said body consists of a non-woven metal fiber), or

[0061] - the average value of the distance between the sintered particles of the main body 10 (if said body consists of a sintered metal material).

[0062] In the event that the porous conductive material layer consists of a set of particles and / or aggregates, preferably, the average value of the distance d2 between the particles and / or aggregates of said porous conductive layer 20 is less than:

[0063] - the average value of the distance between the metal fibers of the main body 10 (if said body consists of a non-woven metal fiber), or

[0064] - the average value of the distance between the sintered particles of the main body 10 (if said body consists of a sintered metal material).

[0065] In the context of the present invention, the distance between the fibers or the distance between the particles / aggregates within the main porous body and / or within the porous conductive layer may be measured using any method suitable for the purpose. For the purposes of the present invention, what is relevant is not so much the measurement method used as the fact of using the same measurement method such as to allow for a homogeneous comparison between the various distances. In particular, such parameters may be measured in the main porous body and in the porous conductive layer by SEM observation, followed by a mutual comparison of the distances.

[0066] Advantageously, the porous conductive material layer 20 has an overall porosity that is sufficiently high so as to avoid the introduction of mass transport-related losses, while at the same time demonstrating, however, an average dimension for the gaps between the fibers or particles / aggregates that, in order to maximize contact with the catalytic layer, is less than the gaps between the fibers or particles of the main porous body.

[0067] According to a preferred embodiment, the porous conductive material layer 20 has a maximum surfaceroughness Rz that is lower than the maximum surface roughness Rz of the main body 10.

[0068] According to one embodiment, the main porous body 10 has a maximum surface roughness Rz of between 10 micrometers and 25 micrometers. ★ ★ ★

[0069] The object of the present invention is an electrolyzer .

[0070] The electrolyzer according to the invention comprises at least one membrane electrode assembly MEA.

[0071] In turn, such at least one membrane electrode assembly comprises:

[0072] - a polymer electrolyte membrane M, preferably a polymer electrolyte membrane with protonic conductivity,

[0073] - an anode A arranged at a first face of the membrane; and

[0074] - a cathode C arranged at a second face of the membrane opposite to the first.

[0075] Both the anode and the cathode are composed of a catalytic layer.

[0076] The electrolyzer comprises a porous transport layer (PTL or GDL) for each of the two catalytic layers of said at least one MEA.

[0077] Each porous transport layer (PTL or GDL) is associated with the respective catalytic layer on theopposite side with respect to the polymer electrolyte membrane.

[0078] According to the present invention, the porous transport layer PTL (or GDL) associated with the anode consists of a porous transport element 1 according to the invention, and in particular as described above.★ ★ ★

[0079] The present invention also pertains to a method for producing the porous transport element 1 for membrane electrode assemblies.

[0080] Such production method comprises the following operating steps:

[0081] - a) providing a porous main body 10, preferably made of a sintered metal or non-woven fabric of metal fibers;

[0082] - b) depositing upon a first face 11 of said main porous body 10 one or more organic-type electronic conductive polymers such as to form a porous conductive material layer 20.

[0083] Preferably, the electronic conductive polymer is deposited mixed with at least one organic polymer with ionic conductivity, preferably protonic conductivity.

[0084] Advantageously, step b) may be performed by electrodeposition .

[0085] In such case, step b) involves the deposition of aprecursor of an electronic conductive organic polymer mixed with at least one organic polymer with ionic conductivity, preferably protonic conductivity.

[0086] For example, in the case where the electronic conductive organic polymer is PEDOT, the precursor is EDOT.

[0087] Preferably, the deposition step b) by electrodeposition is preceded by a step of removing any oxides and impurities from the deposition surface (first face 11) on the main porous body 10.

[0088] Alternatively, step b) may be accomplished by means of:

[0089] - a dip coating technique; or

[0090] - a spray coating technique.~k ~k ~k

[0091] The invention provides a porous transport element for membrane electrode assemblies that allows for electrical connectivity to be ensured using a catalytic layer of a membrane electrode assembly, thus providing an alternative solution to that of the prior art mentioned above.

[0092] In fact, it has been experimentally verified that the presence of the porous layer, made of a polymer-type- conductive material, interposed between the main porous layer and the catalytic layer, reduces the loss of activeportions within the catalyst layer as a result of the formation of any fragments. Thus, the risk of the formation of islands and / or disconnected areas in the coated electrode and thus the risk of locally lost electrical connection with the interface of the porous transport layer and with the remaining part of the catalytic layer, is significantly reduced.

[0093] The porous layer made of polymer-type-conductive material thus leads to an increase in conductivity at the interface between the catalytic layer and the membrane, which translates into an increase in the electrical connectivity of the catalytically active particles, thereby leading to an increase in conductivity at the interface.

[0094] The porous transport element for membrane electrode assemblies according to the invention is also easy to manufacture .

[0095] The invention thus conceived therefore achieves the intended objectives.

[0096] Obviously, in practice it may also assume different forms and configurations from the one illustrated above, without thereby departing from the present scope of protection .

[0097] Furthermore, all details may be replaced with technically equivalent elements, and the dimensions,shapes, and materials used may be any according to the needs.

Claims

CLAIMS1. A porous transport element (1) for membrane electrode assemblies comprising:- a porous main body (10) on which a first face (11) is identified, intended in use to face a catalytic layer of a membrane electrode assembly, the porosity of said main body (10) being defined by an average porosity <|J1;- a porous layer (20) of conductive material which is placed to cover said first face (11) of said main body (10) to be interposed in use between said main body (10) and said catalytic layer and has a porosity defined by an average porosity <£>2, characterized in that said conductive material layer (20) comprises at least one electronic conductive organic polymer.

2. A porous transport element (1) according to claim 1, wherein said conductive material layer (20) comprises at least one ionic conductive organic polymer.

3. A porous transport element (1) according to claim 2, wherein said at least one ionic conductive organic polymer is a protonic conductive polymer, preferably selected from polyanionic polymers.

4. A porous transport element (1) according to claim 3, wherein said protonic conductive polymer is selected from perfluorosulfonic acid (PFSA) ionomers, preferably withequivalent molecular weight between 1,200 and 700 g / mol.

5. A porous transport element (1) according to any one of the preceding claims, wherein the electronic conductive polymer is selected from conjugated polymers.

6. A porous transport element (1) according to any one of the preceding claims, wherein the electronic conductive polymer is selected from the group consisting of PEDOT, PPY (polypyrrole), PANI (polyaniline).

7. A porous transport element (1) according to any one of the preceding claims, wherein the electronic conductive polymer is present in the form of fibers.

8. A porous transport element (1) according to claim 7, wherein said fibers also comprise an ionic conductive, preferably protonic conductive polymer.

9. A porous transport element (1) according to claim 7 or 8, wherein said fibers have an average diameter less than 1 micrometer, preferably between 100 nanometers and 700 nanometers.

10. A porous transport element (1) according to any one of claims 1 to 6, wherein the electronic conductive polymer is present in the form of particles and / or aggregates.

11. A porous transport element (1) according to claim 10, wherein said particles and / or aggregates also comprise an ionic conductive polymer, preferably protonicconductive polymer.

12. A porous transport element (1) according to any one of the preceding claims, wherein said conductive material layer (20) has a thickness between 5 micrometers and 25 micrometers .

13. A porous transport element (1) according to any one of the preceding claims, wherein the average porosity <£>2 of said conductive material layer (20) is equal to or greater than 30%, preferably equal to or greater than 50%.

14. A porous transport element (1) according to any one of the preceding claims, wherein the average porosity <£>2 of said conductive material layer (20) is equal to or greater than the average porosity <|J1 of said main body (10).

15. A porous transport element (1) according to any one of the preceding claims, wherein said porous main body (10) is made of sintered metal, preferably titanium, preferably said sintered metal being made with particles of average diameter between 30 and 70 micrometers, more preferably of about 50 micrometers, wherein preferably the average distance between sintered particles is between 5 micrometers and 60 micrometers, more preferably of about 10 micrometers.

16. A porous transport element (1) according to any oneof claims 1 to 15, wherein said porous main body (10) is a non-woven fabric of metal fibers, preferably titanium, in particular said metal fibers have an average diameter between 10 micrometers and 50 micrometers.

17. A porous transport element (1) according to claim 7, 8 or 9 and claim 16 or 17, wherein the average value of the distance d2 between the fibers of the porous conductive layer (20) is lower than the average value of the distance of the metal fibers of the main body (10) or the average value of the distance between the sintered particles of the main body (10).

18. A porous transport element (1) according to claim 10 or 11 and claim 16 or 17, wherein the average value of the distance d2 between the particles and / or aggregates of said porous conductive layer (20) is lower than the average value of the distance of the metal fibers of the main body (10) or the average value of the distance between the sintered particles of the main body (10).

19. A porous transport element (1) according to any one of the preceding claims, wherein the average porosity <|J1 of said porous main body (10) is equal to or greater than 30%, preferably equal to or greater than 50%, more preferably equal to 75%.

20. A porous transport element (1) according to any one of the preceding claims, wherein said porous main body(10) has a thickness between 100 micrometers and 2 millimeters, preferably between 300 micrometers and 1 millimeter .

21. A porous transport element (1) according to any one of the preceding claims, wherein said conductive porous layer (20) has a maximum surface roughness Rz less than the maximum surface roughness Rz of the main body (10), preferably said porous main body (10) having a maximum surface roughness Rz between 10 micrometers and 25 micrometers .

22. An electrolyzer comprising at least one membrane electrode assembly, which in turn comprises:- a polymer electrolyte membrane, preferably a protonic conductive polymer electrolyte membrane,- an anode positioned on a first face of the membrane;- a cathode positioned on a second face of the membrane opposite to the first; wherein said anode and said cathode both consist of a catalytic layer, and wherein said electrolyzer comprises a porous transport layer for each of the two catalytic layers of said at least one membrane electrode assembly, each porous transport layer being associated with the respective catalytic layer on the opposite side with respect to the polymer electrolyte membranecharacterized in that the porous transport layer associated with the anode of said at least one membrane electrode assembly consists of a porous transport element (1) according to any one of the preceding claims.

23. A method for producing a porous transport element (1) according to any one of claims 1 to 21, comprising the following operating steps:- a) providing a porous main body (10), preferably made of sintered metal or non-woven fabric of metal fibers;- b) depositing one or more electronic conductive organic polymers on a first face (11) of said porous main body (10) so as to form a porous layer (20) of conductive material.

24. A method according to claim 23, wherein said step b) is carried out by electrodeposition.

25. A method according to claim 23, wherein said step b) is carried out by dip-coating technique.

26. A method according to claim 23, wherein said step b) is carried out by spray coating technique.

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