Method for treating a porous transport layer and coated transport layer

Abstract

The present invention relates to a method for treating a porous transport layer, which comprises: - providing a porous transport layer having an oxide layer, - depositing a metal on the porous transport layer using a physical vapor deposition process, wherein the physical vapor deposition process is a High-Power Impulse Magnetron Sputtering Deposition (HiPIMS-D) process thus forming a coating layer on the porous transport layer to obtain a coated porous transport layer, or wherein the physical vapor deposition process is a High-Power Impulse Magnetron Sputtering-Metal Ion Etching (HiPIMS-MIE) process thus forming an etching metal layer on the porous transport layer, wherein the HiPIMS-MIE process further comprises: - removing at least partly native oxides from the native oxide layer thus forming an interfacial layer of the porous transport layer.

Description

[0001] Industrie De Nora S . p . A. , Nano4Energy S . L . N . E .

[0002] - 1 -

[0003] Method for Treating a Porous Transport Layer and Coated Transport Layer

[0004] Technical Field

[0005] The present invention relates to a method for treating a porous transport layer, preferably for coating the porous transport layer, and the coated porous transport layer obtained therefrom .

[0006] Prior Art

[0007] Proton Exchange Membrane ( PEM) water electrolysis is considered one of the most promising technologies for the production of hydrogen by splitting water . PEM water electrolyzers comprise a solid polymer electrolyte that conducts protons , separates produced gases , and insulates the electrodes .

[0008] The porous transport layer ( PTL ) is a crucial component in various electrochemical devices including PEM water electrolyzers as it plays multiple functions including facilitating the movement of water / gas to / from the catalyst layer and providing good electrical conductivity . Commercially available porous transport layers are typically made of porous titanium, stainless steel or carbon . Titanium has excellent corrosion resistance and good electrical conductivity, whereas carbon-based materials and stainless steel are less expensive alternatives , though they do not of fer the

[0009] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0010] -2- same level of corrosion resistance as titanium. The porous transport layer affects the overall performance of a PEM electrolyzer, such as minimizing ohmic losses.

[0011] The PTL can be coated with protective metals such as platinum group metals to improve its durability by preventing its oxidation and to enhance its conductivity. The coating can be achieved using various methods including physical vapor deposition (PVD) , electroplating, and thermal deposition.

[0012] The drawback with all those methods is the relatively high amount of platinum group metals needed to be deposited on the PTL to ensure its durability, i.e. in a range of at least 2-5 grams per square meter of platinum group metal. Additionally, the coatings obtained by those methods, especially electroplating and thermal deposition, do not provide adequate uniformity, adhesion, and conductivity.

[0013] The PTL is usually subjected to cleaning and native oxide layer removal before the deposition of the protective metals on its surface. This results in better adhesion of the protective layer and improved durability of the PTL. Native oxide layer removal can be done using plasma etching including argon (Ar) plasma etching.

[0014] The drawback of the argon plasma etching is that the native oxide layer is not entirely removed. Consequentially, the obtained coated PTL may not have

[0015] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0016] - 3 - sufficient uniformity, adhesion and durability.

[0017] Thus, it would be desirable to provide a method for treating and / or coating a porous transport layer with a protective metal that can solve the aforementioned drawbacks without affecting the performance and durability of the porous transport layer.

[0018] Summary of the Invention

[0019] The Applicant has now found a method for treating a porous transport layer using a High-Power Impulse Magnetron Sputtering Deposition (HiPIMS-D) process , which produces a coated porous transport layer with improved uniformity and conductivity, and a method for treating a porous transport layer using a High-Power Impulse Magnetron Sputtering-Metal Ion Etching (HiPIMS-MIE) process, which produces a porous transport layer with improved adhesion properties and reduced oxygen content, as described in the appended claims.

[0020] Furthermore, the methods of the present invention allow for reducing the platinum group metal loadings in the coating without sacrificing the performance and durability of the porous transport layer.

[0021] These and other objects and advantages of the present invention will become obvious from the following detailed description.

[0022] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S . p . A. , Nano4Energy S . L . N . E .

[0023] - 4 -

[0024] Brief Description of the Figures

[0025] Figure 1 shows a schematic representation of the method according to Example 2 of the present invention .

[0026] Figure 2 shows a schematic representation of the direct-current magnetron sputtering ( DCMS ) process according to Comparative Examples 1 and 2 .

[0027] Figure 3 shows a schematic representation of the High-Power Impulse Magnetron Sputtering (HiPIMS ) process according to Examples 1 and 2 of the present invention .

[0028] Figure 4 shows schematic cross section representations of coated porous transport layers , wherein Figure 4a shows a PTL obtained according to Comparative Example 1 , Figure 4b shows a PTL obtained according to Comparative Example 2 , Figure 4c shows a PTL obtained according to Example 1 of the invention and Figure 4d shows a PTL obtained according to Example 2 o f the invention .

[0029] Detailed Description of the Invention

[0030] For the purposes of the invention, definitions of some terms and / or expressions used in the present description and in the claims will be provided below .

[0031] For the purposes of the invention, the porous transport layer is a porous and conductive layer suitable to be used in electrochemical devices including PEM water electrolyzers . It may have a thickness comprised between 50 and 1000 micrometers , preferably comprised between

[0032] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0033] 100 to 500 micrometers, a porosity comprised between 20% and 90%, preferably comprised between 25% and 60% (Volume of Voids / Total Volume) x 100 measured by Mercury intrusion porosimetry) and an electrical conductivity comprised between 1 and 100 S / cm, preferably comprised between 10 and 60 S / cm.

[0034] For the purpose of the present invention, a High- Power Impulse Magnetron Sputtering Deposition (HiPIMS- D) process is a method for physical vapor deposition of thin films which is based on magnetron sputter deposition. The HiPIMS-D process is a pulsed mode of magnetron sputtering. The power of each pulse can be in the range of 0.1 kW to 1 MW and preferably within the range 10 kW to 1 MW for typical areas of the targets conventionally used in sputtering devices. The pulses can have a duration in the range of less than a hundred microseconds up to hundreds of microseconds and the intervals between pulses can range from hundreds of microseconds up to hundreds of milliseconds, in very special cases up to seconds. A magnetic field is arranged at the surface of a target, the magnetic field having the conventional structure such as a magnetron configuration. The target is a usually stationary object, from which material is to be sputtered in order to be deposited onto a substrate. A gas which is chosen so that it can be ionized is supplied to the chamber containing the target. A negative voltage is applied between an anode and a cathode in the chamber, the cathode being the target and the anode e.g. parts of the

[0035] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0036] -6- walls of the chamber. An electric discharge then occurs between the anode and the cathode, producing electrons trapped in the magnetic field by cooperation of the electric field produced by the applied voltage. Thus, an extremely high absolute level of pulsed electric power or equivalently an extremely high level of the power density in the generated electric pulses is provided to the sputtering device. This is accomplished using these electric pulses directed to the cathode, which have an extremely high-power density as referred to the area of the cathode. In the discharge very high currents and high current densities occur. This high-power level permits the production of a nearly fully ionized plasma in the vicinity of the cathode, for a sufficiently high current density up to 10 A / cm2and a sufficiently high applied voltage up to 3 kV. The high ionization can be achieved in the pressure range of 10-5-10-1mbar .

[0037] In the HiPIMS-D process, a negative bias voltage in the range of 10 to 400 V is applied to the substrate, depending on materials used and desired coating density. Preferably, a negative bias voltage in the range of 50 to 150 V is applied to the substrate in the HiPIMS-D process .

[0038] In contrast to HiPIMS-D, in the HiPIMS-MIE process larger negative bias voltages are applied to the substrates to achieve etching effects and ion implantation during pretreatment. Negative bias voltages in HiPIMS-MIE are generally between 100 and 1500 V,

[0039] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0040] -7- preferably in a range of 400 to 1400 V and more preferably in a range of 500 to 1200 V, again depending on the materials used and desired ion implantation depth. By substrate biasing at higher voltages than HiPIMS-D, the sputtering yield at the substrate side is increased and the net deposition rate is reduced. Film growth ceases as the average yield approaches unity, which is for most elements at energies in the range of 400 eV and 1400 eV, and the surface is etched as the energy is increased even further. Metal ion etching is technologically used in the pre-deposi tion step in order to remove contaminants and reduce the oxide scale of the substrate, generating a gradual interface to the subsequent metal deposition.

[0041] In the present patent application, all the operating conditions reported in the text must be understood as preferred conditions even if not expressly declared .

[0042] For the purposes of the present patent application the term "to comprise" or "to include" also comprises the term "to consist of" or "essentially consisting of".

[0043] For the purposes of the present patent application the definitions of the ranges always comprise the extreme values unless otherwise specified.

[0044] A first object of the invention therefore relates to a method for treating a porous transport layer, which

[0045] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S . p . A. , Nano4Energy S . L . N . E .

[0046] - 8 - comprises :

[0047] - providing a porous transport layer having a native oxide layer,

[0048] - depositing a metal on the porous transport layer using a physical vapor deposition process , wherein the physical vapor deposition process is a High- Power Impulse Magnetron Sputtering Deposition (HiPIMS- D) process , thus forming a coating layer on the porous transport layer to obtain a coated porous transport layer, or wherein the physical vapor deposition process is a high- power impulse magnetron sputtering-metal ion etching (HiPIMS-MIE ) process , thus forming an etching metal layer on the porous transport layer, wherein the HiPIMS-MIE process further comprises :

[0049] - removing at least partly native oxides from the native oxide layer thus forming an interfacial layer of the porous transport layer .

[0050] The Applicant has surprisingly found that using a HiPIMS-D process for the deposition of a coating layer or using a HiPIMS-MIE process for the deposition of an interlayer and the removal of native oxide layer from a porous transport layer leads to maintaining the interfacial contact resistance ( ICR) , performance and durability of the porous transport layer, even with reduced amount of the metal deposited on the PTL .

[0051] Advantageously, the porous transport layer or at least the native oxide layer of the porous transport

[0052] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0053] -9- layer comprises a material selected from porous titanium (Ti) , porous nickel (Ni) , porous stainless steel (SS) or a combination thereof, preferably titanium. The porous transport layer can be made of a planar or a mesh substrate of the same materials (Ti, Ni, SS) .

[0054] In one embodiment, the physical vapor deposition process is a High-power Impulse Magnetron Sputtering Deposition (HiPIMS-D) process, thus obtaining a coated porous transport layer.

[0055] The HiPIMS-D process utilizes extremely high-power densities in short pulses at low duty cycle for the deposition of thin films.

[0056] The metal plays a protective role for the PTL. For the HiPIMS-D process, the coating metal can preferably be selected from a group consisting of platinum, palladium, rhodium, ruthenium, iridium, gold, silver, tantalum and a combination thereof, more preferably platinum.

[0057] Platinum deposited from the HiPIMS-D process has crystal structures Pt

[0111] and Pt

[0200] , wherein the ratio Pt

[0200] : Pt

[0111] is less than 0.04, preferably less than 0.02. Other PVD methods, such as DCMS, yield higher Pt

[0200] : Pt

[0111] ratios, typically higher than 0.05.

[0058] Advantageously, the thickness of the coating layer is comprised between 1 and 100 nm, preferably comprised

[0059] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S . p . A. , Nano4Energy S . L . N . E .

[0060] - 10 - between 2 and 50 nm, more preferably comprised between 5 and 25 nm .

[0061] Alternatively, the physical vapor deposition process is a High-Power Impulse Magnetron Sputtering- Metal Ion Etching (HiPIMS-MIE ) process , thus forming an etching metal layer on the porous transport layer .

[0062] The HiPIMS-MIE process removes the native oxide layer and results in the implantation of the etching metal in the native oxide layer . The native oxide layer may have a thickness of 1 to 10 nm. For the purpose of the present invention, the native oxides layer is named interfacial layer after the removal of oxygen . The etching metal atoms replace the oxygen atoms in the interfacial layer for an implantation depth in a range of 1 to 10 nm during the removal of native oxides , then the etching metal atoms accumulate on the interfacial layer and form an etching metal layer . The etching metal layer may have a thickness comprised between 1 to 30 nm, preferably comprised between 1 to 20 nm, more preferably in a range of 1 to 10 nm .

[0063] The etching metal is advantageously selected from a group consisting of niobium, chromium, aluminum, tungsten, titanium or a combination thereof , preferably niobium or titanium, more preferably niobium .

[0064] For the purposes of the present invention, the expression "removing at least partly native oxides from

[0065] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E. the native oxide layer" covers the formation of an oxygen-depleted interfacial layer in which the oxygen atoms originally present in the native oxide are replaced or displaced by atoms of the etching metal, while maintaining the overall surface integrity of the porous transport layer. The expression refers to a process in which the oxide compounds naturally present on the surface of the porous transport layer are chemically and / or physically reduced, displaced, or thinned by energetic ion bombardment during the HiPIMS-MIE step. The extent of oxide removal is controlled by the bias voltage and ion energy. In the invention, the oxide layer originally having a thickness of about 2 - 10 nm under atmospheric exposure is partly or fully eliminated, depending on the applied parameters. When the expression "at least partly" is used, it encompasses both a partial removal, in which residual oxygen or oxide fragments remain within the interfacial region, provided that the oxygen concentration is reduced by at least 30 % compared with the untreated surface. Partial removal is typically achieved when the substrate bias voltage is at the lower range of the HiPIMS-MIE operating window (e.g. 400-600 V) , resulting in an oxygen-reduced interfacial layer still containing some oxide species. Complete removal occurs when the bias voltage and ion flux are higher (e.g. 800-1200 V) , generating a substantially metallic interface .

[0066] Advantageously, the method further comprises depositing a second metal, different from the coating

[0067] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0068] - 12- metal or the etching metal, on the coating layer or the etching metal layer, thus obtaining a coated porous transport layer further provided with a second metal layer .

[0069] The depositing of the second metal may advantageously be conducted using a High-Power Impulse Magnetron Sputtering Deposition (HiPIMS-D) process, thus obtaining a coated porous transport layer further provided with a second metal layer.

[0070] The second metal is preferably selected from a group consisting of platinum, palladium, rhodium, ruthenium, osmium, iridium, gold, silver, tantalum and a combination thereof, more preferably platinum.

[0071] Both alternative methods of the present invention, (i.e. HiPIMS-D or HiPIMS-MIE) can be preceded by subjecting the PTL substrate to a pre-treatment process to remove contaminants or debris. The pre-treatment process may involve washing the substrate in isopropyl alcohol (IPA) and deionized water (DI H20) to remove organic contaminants, particles, and ionic residues in a simple, low-cost, and effective manner. In addition or alternatively, the pre-treatment process may involve plasma assisted etching such as argon plasma assisted etching. In this preferable step, the native oxides and other contaminants including organic contaminants are removed at a thickness or penetration in a range of 1 to 2 nm.

[0072] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0073] A second object of the present invention relates to a coated porous transport layer comprising a porous transport layer and at least one coating layer, wherein the coating layer is obtained by depositing a coating metal via a High-Power Impulse Magnetron Sputtering Deposition (HiPIMS-D) process onto the porous transport layer. The coating metal is preferably selected from a group consisting of platinum, palladium, rhodium, ruthenium, iridium, gold, silver, tantalum and a combination thereof, preferably platinum.

[0074] In one embodiment, the coating metal comprises platinum. According to this embodiment, the present invention relates to a coated porous transport layer comprising a porous transport layer and a coating layer, wherein the coating layer comprises platinum having crystal structures Pt

[0111] and Pt

[0200] , wherein the ratio Pt

[0200] : Pt

[0111] is less than 0.04, preferably less than 0.02.

[0075] As mentioned, platinum deposited using HiPIMS-D process has crystal structures Pt

[0111] and Pt

[0200] , wherein the ratio of Pt

[0200] : Pt

[0111] is less than 0.04. Other deposition methods yield higher ratios of Pt

[0200] : Pt

[0111] , typically 0.05 or more.

[0076] In one embodiment, the porous transport layer comprises a substrate and an interlayer to the coating layer, i.e. arranged between the substrate and the coating layer. The interlayer can, for instance, be a

[0077] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S . p . A. , Nano4Energy S . L . N . E .

[0078] - 14 - native oxide layer or a reduced native oxide layer obtained by pre-treatment of a native oxide layer using plasma etching .

[0079] The interlayer can also be an interfacial layer obtained by treating the native oxide layer with a HiPIMS-MIE process as described above .

[0080] A third obj ect of the present invention relates to a coated porous transport layer comprising a porous transport layer, a first metal layer and a second metal layer, di f ferent from and adj acent to the first metal layer, wherein the porous transport layer comprises an interfacial layer adj acent to the first metal layer, and wherein the interfacial layer and the first metal layer are obtained via a High-Power Impulse Magnetron Sputtering-Metal Ion Etching (HiPIMS-MIE ) process . The etching metal used in the HiPIMS-MIE process is typically selected from a group consisting of niobium, chromium, aluminum, tungsten, titanium or a combination thereof , preferably niobium or titanium, more preferably niobium .

[0081] In the context of the method of the present invention, the first metal layer is also denoted "etching metal layer" .

[0082] Accordingly, in one embodiment , the coated porous transport layer comprises a porous transport layer, a first metal layer and a second metal layer, di f ferent from and adj acent to the first metal layer, wherein the

[0083] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E. porous transport layer comprises an interfacial layer adjacent to the first metal layer, wherein the first metal layer comprises a first metal selected from a group consisting of niobium, chromium, aluminum, tungsten, or a combination thereof, preferably niobium, or the first metal layer is titanium, and the interfacial layer comprises less than 70 atomic % titanium.

[0084] Preferably, the first metal layer has a thickness comprised between 1 and 30 nm, more preferably comprised between 1 and 20 nm, even more preferably in a range of 1 to 20 nm.

[0085] The interfacial layer of the porous transport layer comprises implanted atoms of the first metal layer at an implantation depth comprised between 1 to 10 nm, preferably in a range of 1 to 10 nm.

[0086] When the PTL is made of titanium and the etching metal is titanium, the first metal layer cannot be distinguished from the PTL.

[0087] Advantageously, the second metal is selected from a group consisting of platinum, palladium, rhodium, ruthenium, iridium, gold, silver, tantalum and a combination thereof, preferably platinum, more preferably platinum having crystal structures Pt

[0111] and Pt

[0200] , and wherein the ratio Pt

[0200] : Pt

[0111] is less than 0.04, even more preferably platinum having crystal structures Pt

[0111] and Pt

[0200] , and wherein the

[0088] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0089] - 16- ratio Pt

[0200] : Pt

[0111] is less than 0.04 and having a thickness comprised between 1 and 100 nm.

[0090] A fourth object of the present invention relates to an electrolyzer or fuel cell, preferably proton exchange membrane (PEM) water electrolyzer comprising the coated porous transport layer of the present invention.

[0091] A PEM water electrolysis cell may comprise an electrolyte membrane, an anode, and a cathode, a gas diffusion layer for the cathode, a porous transport layer (PTL) for the anode, a cathode bipolar plate, and an anode bipolar plate.

[0092] Advantageously, the coating layer has a thickness comprised between 1 and 100 nm, preferably comprised between 2 and 50 nm, more preferably comprised between 5 and 25 nm.

[0093] The following examples are provided for illustrative purposes only of the present invention and must not be understood as limiting the scope of protection defined by the appended claims.

[0094] Examples

[0095] Figure 2 shows a schematic representation of a direct-current magnetron sputtering (DCMS) system 20 as used in the Comparative Examples. The system 20 comprises substrate fixturing 21 holding various PTLs 22. In a

[0096] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0097] - 17- pre-treatment chamber 23, the substrate is subjected to an argon plasma etching process while the substrate is biased at - 450 V, DC-pulsed at 150 kHz. In a magnetron sputtering chamber 24, a coating material source 25 is provided. The DCMS process operates with a constant power density of about 1.25 W / cm2and the substrate bias voltage is -150 V.

[0098] Figure 3 shows a schematic representation of a high- power impulse magnetron sputtering (HiPIMS) system 30 according to the invention. The system 30 comprises substrate fixturing 31 holding various PTLs 32. In a pre-treatment chamber 33, the substrate is subjected to an argon plasma etching process while the substrate is biased at - 450 V, DC-pulsed at 150 kHz. In a magnetron sputtering chamber 34, a HiPIMS-MIE process followed by a HiPIMS-D process are carried out. A Nb-MIE source 35 and a Pt coating material source 36 are provided. In contrast to DCMS, HiPIMS applies short, high-power pulses to the target, generating a dense, highly ionized plasma containing metal ions. Magnetron power is at 7.5 W / cm2(avg) , 100 ps pulse length at a rate of 500 Hz for HiPIMS-MIE and at 1.25 W / cm2(avg) , 50 ps pulse length at a rate of 200 Hz for HiPIMS-D platinum deposition. During HiPIMS-MIE, the substrate is biased at about - 600 V, promoting energetic ion bombardment, at least partial sputter-etching of native oxides, and metal ion implantation. During HiPIMS-D, the bias voltage is reduced to about -150 V to favor net film growth. The pulsed operation produces extremely high instantaneous

[0099] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0100] - 18 - power densities— up to 7.5 W / cm2for metal-ion etching and 1.25 W / cm2for platinum deposition— resulting in dense and adherent coatings with finely controlled morphology and thickness.

[0101] Figure 4 summarizes the comparative examples and the examples according to the invention.

[0102] Fig. 4a: coated porous transport layers 40a obtained according to Comparative Example 1 comprising a substrate 41, a reduced oxide layer 42 obtained by Ar etching, and a DCMS coating layer 43 having a thickness of 100 nm ;

[0103] Fig. 4b: coated porous transport layers 40b obtained according to Comparative Example 2, comprising a substrate 41, a reduced oxide layer 42 obtained by Ar etching, and a DCMS coating layer 44 having a thickness of 10 nm ;

[0104] Fig. 4c: coated porous transport layers 40c obtained according to Example 1 of the invention, comprising a substrate 41, an interfacial layer 45 obtained by HiPIMS-MIE, an etching metal layer (first metal layer) 46 obtained by HiPIMS-MIE and a DCMS coating layer 47 having a thickness of 10 nm; and

[0105] Fig. 4d: coated porous transport layers 40d obtained according to Example 2 of the invention comprising a substrate 41, an interfacial layer 45 obtained by HiPIMS-MIE, an etching metal layer (first metal layer) 46 obtained by HiPIMS-MIE and a coating layer 48 obtained by HiPIMS-D having a thickness of 10 nm.

[0106] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0107] Example 1 according to the present invention (HiPIMS-MIE then DCMS) .

[0108] Example 1 of the method of the present invention starts with a porous transport layer or a substrate having a native oxide layer. The substrate is made of porous titanium. Native oxide layer has a thickness in a range of 2 and 10 nm at normal atmospheric conditions.

[0109] The substrate is subjected to argon plasma assisted etching. The native oxides (TiOx) and any surface contaminants are removed at a thickness or penetration in a range of 1 to 2 nm.

[0110] Afterwards, the porous titanium substrate is subjected to a HiPIMS-MIE process. Niobium is used as an etching metal. Initially, the native oxides (TiOx) are removed at a thickness or penetration in a range of 1 to 10 nm and at the same time niobium is implanted into the porous titanium substrate at an implantation depth in a range of 1 to 10 nm. Then, niobium is no longer implanted but rather starts to build up on top of the substrate forming a layer having a thickness in a range of 1 to 20 nm.

[0111] The substrate is lastly subjected to protective metal deposition. The direct-current magnetron sputtering (DCMS) process is used for the deposition of platinum. The thickness of the protective metal is around

[0112] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0113] -20-

[0114] 10 nm, and the Pt load is 0.2 g .m-2.

[0115] Example 2 according to the present invention (HiPIMS-MIE then HiPIMS) .

[0116] Example 2 of the method of the present invention can be represented by Figure 1 which starts in step a) with a porous transport layer (PTL) 10 or a substrate 11 having a native oxide layer 12. The substrate 11 is made of porous titanium. Native oxide layer 12 has a thickness in the range of 2 and 10 nm at normal atmospheric conditions .

[0117] In step b) the PTL or substrate is subjected to argon plasma assisted etching. The native oxides (TiOx) are partially removed and any surface contaminants are removed, yielding a reduced native oxide layer 13 having a thickness in a range of 1 to 2 nm.

[0118] Afterwards, in step c) , the porous titanium substrate 11 is subjected to a HiPIMS-MIE process. Niobium is used as an etching metal. Initially, the native oxides (TiOx) are removed at a thickness or penetration in a range of 1 to 10 nm and at the same time niobium is implanted into the porous titanium substrate at an implantation depth in a range of 1 to 10 nm, yielding an interfacial layer 14 on the substrate. Then, in step d) , niobium is no longer implanted but rather starts to build up on top of the substrate forming an etching metal layer (or first metal layer) 15 having a

[0119] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0120] -21 - thickness in a range of 1 to 20 nm.

[0121] In step e) , the substrate is lastly subjected to protective metal deposition. In this case, High-Power Impulse Magnetron Sputtering Deposition (HiPIMS-D) is used for the deposition coating layer 16 (or second metal layer) of platinum. The thickness of the protective coating layer is around 10 nm, and the Pt load is 0.2 g .m-2.

[0122] Comparative Example 1 (DCMS - high Pt loading)

[0123] Comparative example 1 starts with a porous transport layer or a substrate having a native oxide layer. The substrate is made of porous titanium. Native oxide layer has a thickness in a range of 2 and 10 nm at normal atmospheric conditions.

[0124] The substrate is subjected to argon plasma assisted etching. The native oxides (TiOx) are partially removed and any surface contaminants are removed yielding a reduced native oxide layer at a thickness in a range of 1 to 2 nm.

[0125] The substrate is lastly subjected to protective metal deposition. The direct-current magnetron sputtering (DCMS) process is used for the deposition of platinum. The thickness of the protective metal is around 100 nm, and the Pt load is 2 g.m-2.

[0126] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0127] Comparative Example 2 (DCMS - low Pt loading)

[0128] Comparative example 2 starts with a porous transport layer or a substrate having a native oxide layer. The substrate is made of porous titanium. Native oxide layer has a thickness in a range of 2 and 10 nm at normal atmospheric conditions.

[0129] The substrate is subjected to argon plasma assisted etching. The native oxides (TiOx) are partially removed and any surface contaminants are removed yielding a reduced native oxide layer at a thickness in a range of 1 to 2 nm.

[0130] The substrate is lastly subjected to protective metal deposition. The direct-current magnetron sputtering (DCMS) process is used for the deposition of platinum. The thickness of the protective metal is around 10 nm, and the Pt load is 0.2 g.m-2.

[0131] Operating conditions

[0132] Argon etching

[0133] Argon etching is conducted using the operating conditions as follows. Substrate is biased at 450V, DC- Pulsed 150 kHz. This results in a power comprised between 0.1 to 0.2W / cm2. Ar pressure in the low-pressure range of 4xl0“3mbar. The process last for around 10 minutes without heating.

[0134] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0135] Nb HiPIMS -MIE

[0136] This step provides enhanced adhesion. MIE reduces the oxide thickness at titanium interface and improves contact between surface and Pt coating deposition.

[0137] Magnetron generates energetic ions with HiPIMS working at high current densities compared to conventional DCMS . The operating conditions are as follows.

[0138] - The substrate is negatively biased and collects the positive metal niobium (Nb) ions,

[0139] - Ar pressure is set at 4xl0“3mbar.

[0140] - The average power density of 7.5 W / cm2is applied and distributed in pulses of 100 ps width and 500 Hz repetition frequency. This results in a peak current density of 0.75 A / cm2.

[0141] - Bias voltage to the substrate: DC mode, Arc control management is essential to withstand bias current higher than 10 A.

[0142] - Substrate is biased at 600V.

[0143] - No extra heating is applied.

[0144] - HiPIMS peak current around 0.75A / cm2guarantees the formation of Nb ions.

[0145] Negative voltage bias controls the energy of the arriving ions. Higher negative bias voltage increases metal ion etching rate and prevents the formation of

[0146] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0147] -24- oxides .

[0148] Pt HiPIMS:

[0149] The aim of this step is obtaining a denser and finegrained Pt coating with enhanced finishing at ultra-low loading. The emission of secondary electrons in intrinsic Pt is lower as compared to Nb. Therefore, lower current densities are obtained even when higher voltages are applied.

[0150] The operating conditions are as follows.

[0151] - Ar pressure 4xl0“3mbar.

[0152] - An average power density of 1.25W / cm2is applied and distributed in pulses of 50 ps width and 200 Hz repetition frequency. This results in a peak current density of 0.15 A / cm2.

[0153] - Substrate is biased at 150 V without heating.

[0154] Pt DCMS

[0155] Platinum deposition is conducted under the following operating conditions in Direct Current magnetron sputtering.

[0156] - Ar pressure 4x10-3mbar.

[0157] - A constant power density of 1.25W / cm2is applied which results in a current density of 2.27xl0-3A / cm2.

[0158] - Substrate is biased at 150 V without heating.

[0159] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0160] -25 -

[0161] Characterization studies

[0162] Ohmic decay over time for the PTL of the examples 1 and 2 according to the invention and comparative examples 1 and 2 were conducted.

[0163] Electrochemical cell

[0164] Cell Hardware

[0165] 25 cm2fuel cell technologies hardware is used. The cathode is flow f ield / bipolar plate (BPP) - graphite, triple serpentine (Fuel Cell Technologies) , while the anode is flow field / BPP - Ti, platinized, parallel channel (sputter coated, lOOnm thickness) . A gasketing made of polytetrafluoroethylene (PTFE) is used.

[0166] Operating Conditions

[0167] The 25cm2cell is operated at 60 °C using ultra-pure water (>2 megaohm (MQ) ) with the flow rate of 300 mL / min under atmospheric pressure. Water is fed to the anodic side only.

[0168] MEA Configuration:

[0169] Standard catalyst coated membranes (CCMs) are made using the methods known in the art. In the electrochemical tests reported herein, the membrane of the MEA was a proton exchange membrane of the

[0170] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0171] -26- perf luorosulfonic acid type, specifically a Nafion™ 117 membrane, although other PEM membranes known in the art can likewise be used.

[0172] Anode Catalyst - Ir Black, 2mg / cm2.

[0173] Anode (PTL) - Porous Ti (sintered powder) , PVD Pt coating .

[0174] Cathode Catalyst - 50wt% Pt / C, 0.5mg / cm2Pt.

[0175] Cathode (gas diffusion layer (GDL) ) - Carbon Paper, 5% wet-proofed.

[0176] Test Protocol

[0177] The test is run with constant current, at a current density of 4A / cm2(100A with 25cm2active area) . Intermittently, an electrochemical impedance spectroscopy (EIS ) / Polarization protocol is performed to gain an understanding of any voltage decay taking place. This is done with the intent to separate out contributions from ohmic, kinetic, and mass transport losses. For the PTL protective coating, of interest for this invention, the key performance indicator (KPI) is the ohmic contribution, which can be extracted using the high frequency resistance (HER) from the EIS data. Multiplying this value (ohms) by the operating current (100A) gives us the ohmic contribution in Volts. Furthermore, due to expected variability in cell builds, the main KPI for this testing is the decay in ohmic contribution (final - initial) .

[0178] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0179] -27 -

[0180] The ohmic decay values are reported in the Table 1 below .

[0181] Polarization - 0 - 4A / cm2, 500mA / s scan rate Galvanostatic Electrochemical Impedance

[0182] Spectroscopy (GEIS) - 2.5A, 20kHz to 0.1Hz, 100mA amplitude ( Potentiostatic Electrochemical Impedance Spectroscopy (PEIS) equivalent at ~1.45V) . Table 1: Ohmic Decay Over Time Results

[0183] M / 66050-PCT1 (414PCT-PTL) Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.

[0184] -28-

[0185] The results show that:

[0186] - with standard DCMS process, the benchmark sample (comparative example 1) , has an ohmic decay (in 500hrs of testing) of 38 pV / hr.

[0187] - Lowering the Pt loading to 10 nm, using the same DCMS process (comparative example 2) , increases the ohmic decay to 86 pV / hr.

[0188] - keeping the loading at 10 nm, but introducing the HiPIMS-MIE (Nb) step (Example 1) , there is a significant improvement in the ohmic degradation, surpassing that of the benchmark sample (comparative example 1) by 12 pV / hr, and by comparative example 2 by 62 pV / hr.

[0189] - Example 2 introduces the HiPIMS Pt Deposition step, as well as the HiPIMS-MIE step, and shows further improvement of the ohmic degradation to 0 pV / hr in the first 500 hrs tested.

[0190] - These results show clear, incremental improvement enabled by both the HiPIMS-MIE step and HiPIMS Pt deposition step to minimize Pt loading, while maintaining / improving stability of the ohmic contribution (main indicator for protective coating functionality) .

[0191] M / 66050-PCT1 (414PCT-PTL)

Claims

Industrie De Nora S . p . A. , Nano4Energy S . L . N . E .- 29 -Claims1 . Method for treating a porous transport layer, which comprises :- providing a porous transport layer having a native oxide layer,- depositing a metal on the porous transport layer using a physical vapor deposition process , wherein the physical vapor deposition process is a High-Power Impulse Magnetron Sputtering Deposition (HiPIMS-D) process , thus forming a coating layer on the porous transport layer to obtain a coated porous transport layer, or wherein the physical vapor deposition process is a High-Power Impulse Magnetron Sputtering-Metal Ion Etching (HiPIMS-MIE ) process , thus forming an etching metal layer on the porous transport layer, wherein the HiPIMS-MIE process further comprises :- removing at least partly native oxides from the native oxide layer thus forming an interfacial layer of the porous transport layer .2 . Method for treating a porous transport layer according to claim 1 , wherein the physical vapor deposition process is a High-Power Impul se Magnetron Sputtering Deposition (HiPIMS-D) process , thus obtaining a coated porous transport layer, and wherein the metal isM / 66050-PCT1 (414PCT-PTL)Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.-30- selected from a group consisting of platinum, palladium, rhodium, ruthenium, iridium, gold, silver, tantalum and a combination thereof, preferably platinum.

3. Method for treating a porous transport layer according to claim 1, wherein the physical vapor deposition process is a High-Power Impulse Magnetron Sputtering-Metal Ion Etching (HiPIMS- MIE) process, thus forming an etching metal layer on the porous transport layer, and wherein the etching metal is selected from a group consisting of niobium, chromium, aluminum, tungsten, titanium or a combination thereof, preferably niobium or titanium, more preferably niobium.

4. Method for treating a porous transport layer according to any one of the preceding claims, wherein the method further comprises depositing a second metal, different from the coating metal or the etching metal, on the coating layer or the etching metal layer, thus obtaining a coated porous transport layer further provided with a second metal layer.

5. Method for treating a porous transport layer according to claim 4, wherein the second metal is selected from a group consisting of platinum, palladium, rhodium, ruthenium, osmium, iridiumM / 66050-PCT1 (414PCT-PTL)Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.- 31 - and a combination thereof, preferably platinum.

6. Method for treating a porous transport layer according to any one of the preceding claims 4 or 5, wherein the depositing of the second metal is conducted using a High-Power Impulse Magnetron Sputtering Deposition (HiPIMS-D) process, thus obtaining a coated porous transport layer further provided with a second metal layer.

7. Coated porous transport layer comprising a porous transport layer and at least one coating layer, wherein the coating layer is obtained by depositing a coating metal via a High-Power Impulse Magnetron Sputtering Deposition (HiPIMS- D) process onto the porous transport layer.

8. Coated porous transport layer comprising a porous transport layer and a coating layer, wherein the coating layer comprises platinum having crystal structures Pt [111] and Pt [200] , wherein the ratio Pt [ 200 ] : Pt [ 111 ] is less than 0.04, preferably less than 0.02.

9. Coated porous transport layer according to claim 8, wherein the porous transport layer comprises a substrate and an interfacial layer adjacent to the coating layer.M / 66050-PCT1 (414PCT-PTL)Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.-32-10. Coated porous transport layer comprising a porous transport layer, a first metal layer and a second metal layer, different from and adjacent to the first metal layer, wherein the porous transport layer comprises an interfacial layer adjacent to the first metal layer, and wherein the interfacial layer and the first metal layer are obtained via a High-Power Impulse Magnetron Sputtering-Metal Ion Etching (HiPIMS-MIE) process.

11. Coated porous transport layer comprising a porous transport layer, a first metal layer and a second metal layer, different from and adjacent to the first metal layer, wherein the porous transport layer comprises an interfacial layer adjacent to the first metal layer, wherein the first metal layer comprises a first metal selected from a group consisting of niobium, chromium, aluminum, tungsten, or a combination thereof, preferably niobium, or the first metal layer is titanium, and the interfacial layer comprises less than 70 atomic % titanium.

12. Coated porous transport layer according to claim 11, wherein the second metal is selected from a group consisting of platinum, palladium, rhodium, ruthenium, iridium, gold, silver, tantalum and a combination thereof, preferablyM / 66050-PCT1 (414PCT-PTL)Industrie De Nora S.p.A. , Nano4Energy S.L.N.E.- 33 - platinum, more preferably platinum having crystal structures Pt [111] and Pt [200] , and wherein the ratio Pt [ 200 ] : Pt [ 111 ] is less than 0.04, even more preferably platinum having crystal structures Pt [111] and Pt [200] , and wherein the ratio Pt [ 200 ] : Pt [ 111 ] is less than 0.04 and having a thickness comprised between 1 and 100 nm.

13. Electrolyzer or fuel cell, preferably proton exchange membrane (PEM) water electrolyzer, comprising the coated porous transport layer according to any one of the preceding claims 7 to 12.M / 66050-PCT1 (414PCT-PTL)

Images