Bipolar plate with a titanium nitride coating

Abstract

In accordance with a first aspect of the invention there is provided a method for coating a bipolar plate, comprising the steps of: - providing a bipolar plate wherein at least the outside layer of the bipolar plate comprises metallic titanium; - subjecting the bipolar plate to laser nitridation, thereby forming a coating of titanium nitride on the outer surface of the bipolar plate. The inventors have found that laser nitridation of titanium bipolar plates results in bipolar plates comprising a highly stable TiN coating with good conformality, chemical stability, and / or high electrical conductivity. Furthermore, applying a TiN coating by laser nitridation can be integrated well in the production of the bipolar plate, for instance in a roll-to-plate process that involves laser welding, and is efficient and simple compared to deposition methods known in the art. The TiN coating on the bipolar plate can for instance be used directly in an electrochemical reactor, or can serve as a substrate for further coatings.

Description

[0001] Title: Bipolar plate with a titanium nitride coating

[0002] BACKGROUND OF THE INVENTION

[0003] The invention is in the field of electrochemistry. In particular the present invention is directed to a method for producing a bipolar plate and to a bipolar plate obtainable by that method.

[0004] Electrochemical reactors are widely used in different fields of technology. Well-known examples of electrochemical reactors that are used in generation and storage of energy are fuel cells and electrolyzers. Other applications for electrochemical reactors include reactors for reduction of CO2, which can be used for reducing CO2 emissions, while at the same time producing valuable fuels or chemicals.

[0005] Different types of electrochemical reactors exist, which are often categorized based on their electrolyte material. Well known types of electrochemical reactors Proton Exchange Membrane (PEM) reactors, Anion Exchange Membrane (AEM) reactors, alkaline reactors and solid oxide reactors. In many cases, in order to scale up the reactions, electrochemical reactors are applied in the form of a stack of reactors. An important part in electrochemical reactors, and in particular of stacks of reactors, is the bipolar plate.

[0006] Bipolar plates server multiple functions. For instance, they isolate individual cells in a stack, conduct current between cells, facilitate water and thermal management through the cell, and provide conduits for providing reactant gases to the reactor as well as removing reaction products from the reactor.

[0007] Currently, bipolar plates are typically produced via stamping or hydroforming metal sheets. Typically, an anodic plate and a cathodic plate are welded together, to form a robust unified bipolar plate. This welding can for instance be done using laser welding. In some cases, especially in fuel cells, carbon bipolar plates are also used instead of metal bipolar plates.

[0008] The produced plates typically also undergo a step in which a coating of a chemically stable and electrically conductive material is applied on both the anode and cathode sides of the bipolar plate. Coating methods include electroplating, physical vapor deposition (PVD), and chemical vapor deposition (CVD). Typically, noble metals such as gold or platinum group metals (PGMs) are used to coat the bipolar plate.

[0009] However, there are downsides to using noble metals for providing bipolar plates with a chemically stable and electrically conductive coating. For instance, due to scarceness and price of the material used and thickness of the deposited layers, bipolar plates with noble metal coatings are relatively expensive, thereby having a negative impact on CapEx related to devices in which bipolar plates are applied.

[0010] In addition, applying a coating to bipolar plates can be cumbersome, because thorough cleaning of the surface of the bipolar plates is necessary before applying the coating, in order to make sure that the coating properly adheres to the surface. Applying the coating layer in such a way has to be done carefully in order to obtain a coating surface that is free of pores, pinholes and cracks. In order to prevent defects in the coating layer that leave underlying layers exposed, relatively thick coating layers are applied (e.g. 5 gm), making bipolar plates even more expensive.

[0011] Also, the sharp interface between the metal to be coated and the applied coating layer can be a weak point where defects can form.

[0012] Recently, instead of noble metal coatings, titanium nitride (TiN) has been used as a less expensive coating material for bipolar plates. Titanium nitride has good chemical resistance, hardness and electrical conductivity. Compared to metallic titanium, TiN has low H2 uptake, and a TiN coating can therefore act as a hydrogen barrier and protect titanium against hydrogen embrittlement. In addition, TiN coatings provide low interfacial resistance in electrochemical reactors between the bipolar plate and the porous transport layer, e.g. down to 10 mil

[0013] Typical methods for the application of TiN coatings involve electroplating, physical or chemical vapor deposition, or other additive processes that build up a layer of material onto a substrate. Often, these techniques require high temperature and / or vacuum conditions, meaning that they are expensive and inefficient and it can be difficult to apply these techniques on a commercial scale.

[0014] However, bipolar plates with TiN coatings also have downsides. For instance, it is very important that the entire bipolar plate is covered with protective material and that the coating is strong enough in terms of chemical and mechanical stability. Due to variations in thickness and chemical stability, it is typically needed to have an average thickness of the TiN of up to 5 gm or even higher in order make sure that there are no defects in the coating.

[0015] Another downside is that the chemical stability of TiN nitride is lower than that of noble metals. For instance, under the influence of oxygen, titanium oxynitrides with the general formula TiOxNymay be formed, which can negatively affect the stability and / or electrical conductivity of the bipolar plate. The stability of TiN against oxynitride formation is highly affected by the stoichiometry and phases present in the compound.

[0016] In addition, the chemical stability of TiN can be affected by the crystallite size and grain border, and the interface between metal substrate and TiN coating can be difficult to control.

[0017] It is generally very difficult to produce stoichiometric TiN coatings using deposition techniques such as PVD or CVD without a post-treatment under high temperatures. Annealing post-treatments are frequently needed to improve the crystallinity and adhesion of the coatings prepared by PVD. Best coatings in terms of electrical conductivity and interface adhesion require heat treatment > 1050 °C. In W02005101555, plasma nitriding of a fuel separator at temperatures of 500 - 890 °C is described.

[0018] An object of the invention is to produce bipolar plates with good properties, for instance high chemical resistance and / or good electrical conductivity.

[0019] Another object of the invention is to provide an efficient method for producing such bipolar plates, while addressing at least one of the downsides of known methods.

[0020] Another object of the invention is to produce bipolar plates in a practical and / or cost-effective way.

[0021] BRIEF SUMMARY OF THE INVENTION

[0022] In accordance with a first aspect of the invention there is provided a method for coating a bipolar plate, comprising the steps of: - providing a bipolar plate wherein at least the outside layer of the bipolar plate comprises metallic titanium; - subjecting the bipolar plate to laser nitridation, thereby forming a coating of titanium nitride on the outer surface of the bipolar plate.

[0023] The inventors have found that laser nitridation of titanium bipolar plates results in bipolar plates comprising a highly stable TiN coating with good conformality, chemical stability, and / or high electrical conductivity. Furthermore, applying a TiN coating by laser nitridation can be integrated well in the production of the bipolar plate, for instance in a roll-to-plate process that involves laser welding, and is efficient and simple compared to deposition methods known in the art. The TiN coating on the bipolar plate can for instance be used directly in an electrochemical reactor, or can serve as a substrate for further coatings.

[0024] According to a second aspect of the invention, there is provided a bipolar plate obtainable by the method according to the first aspect of the invention. There is also provided the use of a bipolar plate obtainable by the method as described herein in an electrochemical reactor, wherein the electrochemical reactor is preferably selected from the group consisting of an electrolyzer and a fuel cell.

[0025] BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is a schematic representation of a cross section of a cell in an electrochemical membrane reactor.

[0027] Figure 2 is a schematic representation of the production of a bipolar plate with a coating.

[0028] Figure 3 is a schematic representation of the production of a bipolar plate with a coating in a roll-to-plate process.

[0029] Figures 4-8 show SEM images of titanium bipolar plates coated with a titanium nitride coating using laser nitridation at different conditions.

[0030] Figure 9 shows optical microscopy images of three TiN coatings prepared by laser nitridation under different surface temperatures.

[0031] DETAILED DESCRIPTION OF THE INVENTION

[0032] In accordance with a first aspect of the invention there is provided a method for producing a bipolar plate with a coating. Preferably, the bipolar plate is suitable for being used in an electrochemical reactor. The method as disclosed herein comprises the steps of: - providing a bipolar plate wherein at least the outside layer of the bipolar plate comprises metallic titanium; - subjecting the bipolar plate to laser nitridation, thereby forming a coating of titanium nitride on the outer surface of the bipolar plate.

[0033] Laser nitridation, as used herein, refers to a sub-class of thermal nitridation, in which a laser source is used to heat the titanium surface in the presence of a nitrogen source. This results in the formation of TiN on the outer surface of the bipolar plate. In comparison with classical thermal nitridation, in which an entire part has to be heated in an oven to temperatures of ca. 1200 °C, laser nitridation involves heating the material to be coated very locally using a laser, which makes laser nitridation faster, more energy-efficient, and more easily integrated with continuous production processes than classical thermal nitridation.

[0034] Preferably, laser nitridation is performed at an ambient temperature of 80 °C or less, more preferably 50 °C or less, such as 40 °C or less. Locally, particularly at the surface of the bipolar plate that is irradiated by the laser, the temperature can be much higher due to the laser heating up the part of the bipolar plate being coated.

[0035] Laser nitridation of the bipolar plate results in formation of a TiN coating at the outer surface of the bipolar plate. Because the coating involves chemical reaction of the outer layer of the bipolar plate itself instead of deposition of a layer on top of the bipolar plate, the process does not significantly modify the component dimensions. Instead, laser nitridation process only affects the chemical structure of outside surface of the bipolar plate.

[0036] Another advantage compared to deposited coatings is that nitrogen is incorporated into the titanium framework, thereby forming titanium nitride. Therefore, compared to deposition of titanium nitride on a surface, laser nitridation does not lead to a sharp interface between the substrate and deposited material. Instead, laser nitridation may result in a gradient of nitrogen concentration along the sample depth, wherein in the outer layers a Ti : N ratio close to 1 : 1 is obtained, and wherein the Ti : N ratio increases with increasing depth.

[0037] Using laser nitridation, a coating with good chemical resistance and / or high electrical conductivity can be produced. Without wishing to be bound by theory, the inventors believe that this is caused by the fact that laser nitridation leads to TiN with a Ti : N atomic ratio that is close to the ideal atomic ratio of 1 : 1 at the outer surface of the bipolar plate, especially in comparison to TiN coatings that are obtained by deposition.

[0038] Depending degree of nitrogen incorporation in titanium, titanium nitrides can exist in different Ti : N ratio and correspondingly in different phases or crystal structures. Based the formula TiNx, wherein 0 < x < 1, different equilibrium phases exist. At low values of x up to around 0.3, ct-Ti with hexagonal crystal structure resembling metallic titanium is the predominant phase. At somewhat higher values of x, around 0.3 < x < 0.5, titanium nitride may exist in the form of 8-Ti2N with tetragonal structure. At even higher values of x, 0.3 < x < 1.0, and even for x > 1.0 titanium nitride increasingly occurs as 8-TiN with a cubic crystal structure, also sometimes referred to as osbornite. In addition, depending on the conditions, the formation of a martensitic (a-)titanium phase and needle formation may occur. The cubic 8-TiN osbornite phase is typically considered to be the most stable phase mechanically and chemically. Therefore, the relative amount of cubic 8-TiN, which can for instance be measured using X-ray diffraction, can be a good indication for the Ti : N ratio in the laser nitrided bipolar plate.

[0039] The stoichiometric Ti : N ratio can be measured using different techniques. In addition to X-ray diffraction which is mostly a bulk technique, X-ray Photoelectron Spectroscopy (XPS) or Energy-Dispersive X- ray spectroscopy (typically combined with Scanning Electron Microscopy (SEM-EDX) or Transmission Electron Microscopy (TEM-EDX) on a sample prepared using Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) can be used to determine a profile of the stoichiometric ratio of Ti : N along the depth of bipolar plate, for instance to verify whether the Ti : N ratio increases with increasing depth.

[0040] Preferably, the atomic ratio of titanium : nitrogen at the outer surface of the titanium nitride layer, for instance in the outermost 50 nm of the titanium nitride coating, is in the range of 0.8 : 1.0 - 1.2 : 1.0, more preferably 0.9 : 1.0 - 1.1 : 1.0. It will be appreciated that a higher Ti : N ratio will be seen in deeper layers of the bipolar plate after laser nitridation, as discussed above, but because these deeper layers are not exposed at the outer surface, this ratio does not negatively affect chemical stability of the coated bipolar plate.

[0041] In titanium and titanium nitride, X-rays typically have a penetration depth of 10-20 pm at 20 angles that are typically applied for X- ray diffraction, meaning that an X-ray diffractogram will reveal crystalline phases that are present in the outer 10-20 pm of the coated bipolar plate. A high abundance of 5-TiN as measured using XRD is indicative that the Ti : N ratio at the surface of the bipolar plate is in the range of 0.8 : 1.0 - 1.2 : 1.0, or in other words, that the outer layer comprises the stable osbornite phase. Preferably, the outer surface of the coated bipolar plate has an amount of 5-TiN of 20 % by weight or more, relative to the weight of all titanium-containing phases, preferably 30 wt.% or more, even more preferably 40 wt.% or more, such as 50 wt.% or more or even 60 wt.% or more or 70 wt.% or more. In embodiments, the amount of 5-TiN in the the outer surface of the coated bipolar plate is measured using X-ray diffraction, using the Rietveld method and / or the Reference Intensity Ratio (RIR). Preferably, XRD diffraction measurements and analysis are performed using the conditions as described in example 1.

[0042] Alternatively or additionally, the presence of titanium phases on the outer surface of the bipolar plate may also be assessed using grazing incidence X-ray diffraction, in which X-ray diffractograms are recorded at 20 angles of 0.5-5.0 degrees and using which information can be obtained on the outer 1.0-2.5 pm of the coated bipolar plate.

[0043] Because of the low electrical resistivity and the high chemical stability of TiN, the TiN coating can reduce the interfacial contact resistance between components in an electrochemical reactor, for instance between the bipolar plate and the porous transport layer.

[0044] The TiN coating layer can be applied to the anode side or the cathode side of the bipolar plate, or to both the anode and the cathode side. The inventors have found that especially good TiN coatings can be obtained when during laser nitridation the surface temperature bipolar plate does not get too high. Generally, it is believed that surface temperatures above the melting point of titanium and / or titanium nitride phases are needed, for instance above the melting point of metallic titanium (ca. 1670 °C) or even above the melting point of titanium nitride (ca. 2930 °C), in order to get sufficient incorporation of nitrogen into the titanium surface. However, surprisingly, when the surface temperature of the bipolar plate during laser nitridation is kept below 2000 °C, preferably even below the melting point of metallic titanium, coatings with less defects and a smoother surface can be obtained, which are particularly suitable for use in electrochemical reactors. In embodiments, the surface temperature of the bipolar plate during laser nitridation is in a range of 1000 °C - 1650 °C, more preferably 1200 °C - 1650 °C, such as 1300 °C - 1600 °C.

[0045] The surface temperature of the bipolar plate during laser nitridation can be influenced by various factors, such as thickness and thermal capacity of the bipolar plate, ambient temperature, laser power, laser wavelength, power density, size and geometry of the laser spot, and scan speed. To be able to maintain a desired surface temperature with precision, the surface temperature is preferably measured in situ during laser nitridation at the part of the surface where laser nitridation takes place, i.e., the spot that is being irradiated by the laser. Preferably, surface temperature measurement is performed using a remote sensing thermometer, such as a pyrometer or an infrared thermometer. Remote temperature sensing can be performed at an angle to the laser beam irradiating the sample during laser nitridation, or inline, i.e., from the same direction as the laser beam. Preferably, temperature measurements are performed inline, because this may allow for more accurate measurement.

[0046] The inventors have further found that better quality coating can be obtained when the surface temperature of the bipolar plate remains constant during laser nitridation. By measuring surface temperature during the laser nitridation treatment, and by adjusting the laser power, scan speed and / or spot size in accordingly in order to keep the temperature constant, the laser nitride coating is smoother, compared to processes in which laser power, scan speed and spot size are all kept constant during the process without temperature control. Therefore, in embodiments, the surface temperature during laser nitridation is preferably kept within a range of 100 °C below and 100 °C above a predefined setpoint temperature, preferably 50 °C below and above the setpoint, more preferably 25 °C below and above the setpoint, such as 10 °C below and above the setpoint.

[0047] The degree of absorption of the laser energy by the bipolar plate, and the resulting penetration depth of the laser used is important for e.g. the coating thickness, and may depend on the wavelength of the laser source. Preferably, laser nitridation is performed using a laser having a wavelength in the range of 900-2000 nm.

[0048] Laser nitridation may be performed using a pulsed and / or a continuous laser source. A pulsed laser might be preferred in some cases, because it leads to less unwanted heating of the bipolar plate (i.e., the entire bipolar plate including parts that are outside the region where laser nitridation is taking place) during laser nitridation.

[0049] In order to be able to apply the coating in a short time, the laser source used for laser nitridation preferably has an output power of 500 W or more, preferably 1000 W or more. Because laser sources are usually capable of delivering less than their maximum output power, the maximum output power of the laser source can be much higher than the power used for laser nitridation, and is not particularly limited. For instance, the laser source may have a maximum laser power of 1000-50000 W, such as 5000-25000 W. However, when aiming for a low enough surface temperature, laser sources with a output power of more than 10000 W or even more than 5000 W may not be necessary. Therefore, in embodiments, the laser source has an output power of 500-10000 W, such as 1000-5000 W. Particularly good results and efficiencies can be achieved when laser nitridation is performed using a diode laser, because this type of laser has a high efficiency, and advantageously, large spot sizes can be achieved, meaning that it is possible to apply coatings to a large surface area in a short time.

[0050] In embodiments, the spot size of the laser used for laser nitridation is in the range of 10-10000 mm2, preferably 50-1000 mm2. If the laser has a small spot size relative to the size of the bipolar plate, laser nitridation can be performed by scanning the surface of the bipolar plate along two dimensions with the laser spot. However, a larger laser spot that is elongated along one direction (i.e., a laser spot in the shape of a line) or elongated along two directions may also be used. In that case, scanning with the laser spot along two directions of the bipolar plate may not be necessary. Instead, it may be enough to move the laser spot and the bipolar plate relative to each other only in one direction. This makes it convenient to integrate the step of laser nitridation with continuous production of bipolar plates, for instance using a roll-to-plate process, as described below.

[0051] The power density during laser nitridation may be in the range of 0.1-5.0 kW / cm2. Preferably, in order to keep the surface temperature of the bipolar plate low, the power density is 3.0 kW / cm2or less, preferably 2.5 kW / cm2or less, more preferably 2.0 kW / cm2or less, even more preferably 1.8 kW / cm2or less or 1.5 kW / cm2or less. If the power density is too low, laser nitridation may take too long. Therefore, the power density is preferably 0.3 kW / cm2or higher, for instance 0.3-3.0 kW / cm2, or 0.5-2.0 kW / cm2.

[0052] Laser nitridation is preferably carried out using nitrogen gas as nitrogen source. Alternatively or additionally, ammonia gas can for instance be used as nitrogen source. Compared to using nitrogen, ammonia gas can be used to decrease the temperature that it required for laser nitridation. It is not necessary to add a titanium source in the coating process, because the metallic titanium outside surface of the bipolar plate itself is titanium source in the laser nitridation process.

[0053] In order to get good quality titanium nitride coatings, laser nitridation is preferably carried out in an atmosphere wherein the oxygen content is 100 ppm or less by volume relative of the total volume of the gas atmosphere, preferably 50 ppm or less, more preferably 30 ppm or less. Such low oxygen levels can be achieved for instance by providing a flow of the nitrogen source that displaces any oxygen that might be present and / or mixing the nitrogen source with a heavier gas such as argon.

[0054] The laser nitridation coating process can be synergistically combined with other steps in the production of bipolar plates. For instance, laser nitridation can be performed using the same equipment as for laser welding of cathode and anode plates, for instance in the same chamber, without needing to transport the bipolar plates to a different machine and / or cleaning the surface. Such steps would typically be needed for deposition-based coating processes. Therefore, efficiency of bipolar plate production can be increased, while keeping productions costs low. Laser nitridation can be performed on the bipolar plate after laser welding, and / or on the cathode and / or anode plate before laser welding. Preferably, laser nitridation is performed after laser welding, in order to prevent that laser welding might disturb the titanium nitride coating.

[0055] Typically, different types of laser sources will be used for laser welding and laser nitridation, because laser nitridation is directed at the outer surface of the bipolar plate, whereas laser welding is directed to deeper layers of the bipolar plate. However, other equipment such as (fiber) optics may be used for both laser processes.

[0056] A known method to produce bipolar plates from metal sheet material is to provide channels are formed in a plate, for instance by stamping, molding or hydroforming. After creating the channels in the plate, two plates are formed into a bipolar plate, for instance using laser welding. Laser welding is preferably performed under an inert atmosphere. The process of joining an anode plate and a cathode plate with channels into a bipolar plate is illustrated in figure 2. Bipolar plates have two opposites sides, one of which will act as a cathode, and one of which will act as an anode in an electrochemical reactor.

[0057] Making the plates from metal sheet material can advantageously be performed a so-called roll-to-plate process, which involves providing a flat metal sheet, rolled it out into a plate. Subsequently, the bipolar plates can be coated using laser nitridation as described herein.

[0058] In a roll-to-plate process, the production of bipolar plates can be performed continuously, and the bipolar plates can be cut to the desired size. Subsequent laser nitridation of the bipolar plates can also be performed continuously, before and / or after cutting bipolar plates to the desired size.

[0059] In embodiments, the metal sheet material has a thickness of 50- 5000 gm, preferably, 100-1000 gm, such as 200-500 gm.

[0060] Preferably, the metal sheet material comprises titanium. In order to facilitate laser nitridation, it is preferred that at least the outer surface of the metal sheet material is titanium.

[0061] By combining the laser nitridation step and the laser welding step as described above, it is not necessary to take out the bipolar plates from the laser welding equipment, and to transport them to different equipment for to apply a coating or vice versa. In addition, because the coating is produced by chemical reaction of the outer layer of the bipolar plate, there is also no need for complex surface cleaning and surface preparation prior to the coating.

[0062] Preferably, the laser nitridation step is performed in 15 minutes or less, preferably 10 minutes or less, more preferably 5 minutes or less, meaning that the surface of the bipolar plate is subjected to laser radiation for the specified duration. This is beneficial for large-scale production of these bipolar plates. During this time, a thick enough coating for protecting the titanium, such as a coating with an average thickness of 50 nm or more, is preferably produced. Depending e.g. on laser power and spot size, laser nitridation can even be performed in 3 minutes or less, or 2 minutes or less, or 1 minute or less. In some cases, exposure of the surface to laser radiation for less than a second can already be enough to coat the bipolar plate. Therefore, in embodiments, laser nitridation comprises exposing the surface of the bipolar plate to laser radiation for a duration of 0.1-300 seconds, preferably 0.2-100 seconds, more preferably 0.3-60 seconds. This duration may be achieved in a single pass, or in multiple passes, for instance 1-5 passes.

[0063] According to a second aspect of the invention, there is provided a bipolar plate comprising metallic titanium covered by a titanium nitride coating layer, obtainable using the method described herein.

[0064] Preferably, the titanium nitride coating of the bipolar plate has an average thickness of 50 nm or more, such as 100 nm or more. Using laser nitridation as described herein, a bipolar plate with a titanium nitride coating layer having a thickness of 500 nm or more, or 1.0 gm or more can be produced, for instance 500 nm — 50 gm, or 1.0-20 gm. Because of the good chemical stability, it is not necessary for the titanium nitride coating to be very thick. Therefore, the titanium nitride coating may also have an average thickness of 5000 nm or less. For instance, the titanium nitride coating may have an average thickness of 100-3000 nm while still providing enough chemical stability, but due to the good conductivity of titanium nitride, thicker coating layers may also be suitable. Thickness of the titanium nitride coating layer, as defined herein, refers to the shortest distance from the outer surface of the coated bipolar plate to a point inside the bipolar plate where nitrogen is present in an amount of 5.0 atom% or less, preferably measured using TEM-EDX.

[0065] The size of the bipolar plate may be chosen to match the electrochemical reactor in which it should be applied, typically, the bipolar plate has a length and / or width of 1 cm or more, such as 5 cm or more, or 10 cm or more. As long as the bipolar plate is structurally solid, there is no upper size limit for the bipolar plate. Typically, however, the bipolar plate has a length and / or width of 200 cm or less, or 100 cm or less.

[0066] In embodiments, the geometric surface area of the bipolar plate (i.e., the surface area based on the dimensions of the without taking channels into account) per side of the bipolar plate is 1.0-10000 cm2, preferably 100-2000 cm2.

[0067] As explained above, because the coating is applied using laser nitridation instead of deposition, the interface between metallic titanium and the titanium nitride coating comprises a gradient of decreasing Ti : N ratio towards the outer surface of the bipolar plate. Preferably, the thickness of the region where such a gradient is observed is 5 nm or more, preferably 10 nm or more, such as 10-500 nm or 20-200 nm. Depending on the thickness of the coating, the thickness of the region where such a gradient is observed may even be in the order of magnitude of micrometers, such as 1.0 - 100 gm, or 2.0 - 50 gm.

[0068] An electrochemical reactor with one or more bipolar plates as described herein is also provided.

[0069] Different kinds of electrochemical reactors can be equipped with bipolar plates according to the invention. Preferably, the electrochemical reactors in which the bipolar plates can be used are selected from the group consisting of a proton-exchange membrane water electrolyzer and a fuel cell.

[0070] There is also provided the use of a bipolar plate obtainable using the method as described herein and / or a bipolar plate as described herein in an electrochemical reactor, wherein the electrochemical reactor is preferably selected from the group consisting of an electrolyzer and a fuel cell.

[0071] The resulting titanium nitride coating can be used to provide high quality protection of the bipolar plate in both electrolyzers and fuel cells, at the cathode side, at the anode side, or both. By using laser nitridation to create high quality titanium nitride coatings, expensive noble metal coatings on bipolar plates can be replaced, leading to significant cost reductions.

[0072] The coating may also serve as interface between Ti substrate and noble metal coating. TiN terminated surfaces according to the invention can subsequently be coated with Ir or Pt. Thanks to the TiN coating, thinner layers of Ir or Pt can be sufficient to protect the bipolar plates from oxidizing and / or suffering from hydrogen embrittlement.

[0073] Alternatively or additionally, the titanium nitride coating can serve as a substrate for further coating layers.

[0074] Depending on the type of electrochemical reactor, and depending on the side (i.e., the cathode or anode side), the coating may serve different purposes. In a fuel cell, hydrogen is typically applied to the anode, and the TiN coating can serve to protect the anode side of the bipolar plate against hydrogen embrittlement. On the cathode side of a fuel cell, where oxygen is typically applied, the TiN coating protects the cathode side of the bipolar plate against oxidation. Conversely, in an electrolyzer, hydrogen is typically formed at the cathode, meaning that the TiN coating can be used to protect the cathode against hydrogen embrittlement. On the anode side of an electrolyzer, oxygen is formed, and the TiN coating can be used to protect the anode side of the bipolar plate against oxidation. Compared to fuel cells, which typically operate in a voltage range of ca. 0.6- 1.0 V, the conditions in electrolyzers are typically harsher. Electrolyzers typically operate at 1.5 V or more. Under these conditions, especially at the anode side at which oxygen is formed, the chemical stability of the TiN coating according to the invention may not always be enough to prevent oxidation and titanium oxynitrides may be formed. Therefore, the bipolar plate may be provided with a noble metal coating on top of the TiN coating on one or both sides of the bipolar plate. Alternatively, or additionally, the bipolar plate may also comprise a noble metal coating instead of the TiN coating on the anode or cathode side of the bipolar plate. In addition, the TiN coating may enhance the wettability of the bipolar plate, which leads to better distribution and / or removal of gases. It is believed that the wettability is enhanced because the nanostructured surface of TiN decreases the water contact angle.

[0075] Apart from being used in electrolyzers and fuel cells, the bipolar plates can be used in other electrochemical devices including but not limited to devices for CO2 reduction, NH3 production, H2O2 generation, etc.

[0076] Figure 1 is a schematic representation of a cross section of a single cell of an electrochemical membrane reactor. Membrane Electrode Assembly (1) comprises ion-exchange membrane (5), for instance a perfluorosulfonic acid membrane, and two catalyst layers (4, 6). Adjacent to the membrane on one side, typically referred to as the anode side, is anode catalyst layer (4). Similarly, on the cathode side, cathode catalyst layer (6) is placed adjacent to the membrane. Adjacent to each catalyst layer a porous transport layer (3, 7) is located. On the cathode side, the porous transport layer (7) is typically made of carbon, and on the anode side, the porous transport layer (3) is typically made of titanium. Adjacent to each porous transport layer, a bipolar plate (2, 8) is located. On the anode side of the cell, anode side (9) of bipolar plate (2) faces the side of the membrane where the anode catalyst layer (4) is located, whereas cathode side (10) of bipolar plate (2) faces the cathode catalyst layer of an adjacent cell (not shown) in case the cell is placed in a stack. Similarly, cathode side (11) of bipolar plate (8) faces the side of the membrane where the cathode catalyst layer (6) is located, and anode side (12) of bipolar plate (8) faces the anode catalyst layer of an adjacent cell (not shown) in case the cell is placed in a stack.

[0077] Figure 2 is a schematic representation of the production of a bipolar plate with a coating. Figure 2a shows cathode plate (20) and anode plate (21), which are both provided with channels (22) and lands (23). In Figure 2b, the plates are combined, for instance using laser welding, into bipolar plate (24) having cathode side (25) and cathode side (26). Cathode side (25) and anode side (26) of the bipolar plate may be interchangeable, for instance in case cathode plate (20) and anode plate (21) are identical. Figure 2c shows titanium nitride coating (27) applied onto bipolar plate (24). In figure 2c, both the cathode side and the anode side are coated. It will be appreciated that it is also possible to apply the coating only to one of the sides.

[0078] Figure 3 is a schematic representation of production of a bipolar plate with a coating in a roll-to-plate process starting from a roll (200) of sheet material. Channels are added to the plate of metal sheet material in channel forming unit (201). Subsequently, the plates are combined into a bipolar plate using laser welding and coated using laser nitridation in laser unit (202).

[0079] The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising”, “having”, “including” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to”) unless otherwise noted. Recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. For the purpose of the description and of the appended claims, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term “about”. Also, all ranges include any combination of the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.

[0080] EXAMPLES

[0081] Example 1

[0082] Five titanium bipolar plates (Ti grade 1, parallel channels 3,2 x 3,2 cm2active area with channel depth 1 mm and channel width 1.1 mm) were subjected to laser nitridation at different conditions as shown in Table 1. For samples 1 and 2, power density was kept constant, whereas for samples 3-5, the surface temperature was kept constant by varying power density.

[0083] Table 1

[0084] The coated bipolar plates were characterized using X-ray diffraction using a Bruker D8-A25 diffractometer equipped with a Lynxeye detector, using a Cu radiation (wavelength X=1.5405 A) at a voltage of 30 kV and a current of 40 mA. Diffraction patterns were recorded at 20 angles of 4- 70 ° with steps of 0.024 ° and equivalent measurement times of 192 s per step. Identification of the crystalline phases was carried out using the PDF- 2 database of the International Centre for Diffraction Data (ICDD) and the Crystallography Open Database (COD). Quantitative analysis of the crystalline phases in order to determine amounts of different crystalline phases in weight percent was performed using the Rietveld method, with Bruker's TOPAS v.4.2 software. An additional semi- quantitative analysis was performed based on the Reference Intensity Ratio (RIR) method. For this, the RIR values from the PDF-2 database were used for each of the identified phases. The results and corresponding uncertainties are obtained by combining the Rietveld and RIR results and are shown in Table 2.

[0085] Table 2

[0086] The surface of the coated bipolar plates was also characterized with SEM. Using FIB-SEM, a channel of 12 pm depth was cut in order to give access to a cross-section of the coated bipolar plates, allowing to qualitatively follow the Ti : N ratio increases with increasing depth using EDX. SEM images at different magnifications for samples 1-5 are shown in figures 4-8 respectively, and the locations at which the Ti : N ratio was determined are indicated. In all samples, it was observed that the Ti : N ratio gradually increases at increasing depth.

[0087] Example 2

[0088] Three additional bipolar plates were subjected to laser nitridation at conditions as shown in Table 3.

[0089] Table 3 In figure 9a-c, optical microscopy images of the coated samples 6-8 are shown. In figure 9a, the smooth coating of sample 6 is seen. Figure 9b shows that the surface of the coating of sample 7 is also generally smooth, with some slight ripples in the coating. In figure 9c, it can be seen that laser nitriding at surface temperatures exceeding 2000 °C leads to more drastic morphology changes such as ripples and / or roughening, which is not desired for electrochemical cells.

Claims

Claims1. Method for coating a bipolar plate, comprising the steps of:- providing a bipolar plate wherein at least the outside layer of the bipolar plate comprises metallic titanium;- subjecting the bipolar plate to laser nitridation, thereby forming a coating of titanium nitride on the outer surface of the bipolar plate; wherein laser nitridation is performed at a surface temperature of the bipolar plate of 2000 °C or lower.

2. Method according to claim 1, wherein laser nitridation is performed at a surface temperature of the bipolar plate of 1670 °C or lower, preferably 1200 - 1600 °C.

3. Method according to claim 1 or 2, wherein the atomic ratio of titanium : nitrogen at the outer surface of the titanium nitride layer is 0.8 : 1.0 - 1.2 : 1.0.

4. Method according to any one of claims 1-3, wherein laser nitridation is performed using a pulsed or continuous diode laser.

5. Method according to any one of claims 1-4, wherein laser nitridation is performed using a laser having a wavelength in the range of 900-2000 nm.

6. Method according to any one of claims 1-5, wherein laser nitridation is performed using a laser source having an output power of 500- 50000 W, preferably 1000-5000 W.

7. Method according to any one of claims 1-6, wherein laser nitridation is performed at a power density of 5.0 kW / cm2or less, preferably 3.0 kW / cm2or less, such as 0.5-2.0 kW / cm2.

8. Method according to any one of claims 1-7, wherein laser nitridation is performed using a laser spot size of 10-10000 mm2, preferably 50-1000 mm2.

9. Method according to any one of claims 1-8, wherein laser nitridation is carried out in a gas atmosphere with an oxygen concentration of 100 ppm by volume or less, preferably 30 ppm or less.

10. Method according to any one of claims 1-9, further comprising the steps of:- providing at least two plates of metal sheet material;- introducing channels in the plates, preferably by stamping and / or hydroforming, thereby creating one or more channels and one or more lands in each plate; and- joining the plates to yield a bipolar plate, preferably by laser welding.

11. Method according claims 10, wherein joining the plates to yield a bipolar plate is performed by laser welding and wherein laser welding and laser nitridation are performed in the same chamber.

12. Method according to claim 10 or 11, wherein the metal sheet material is provided in the form of a roll.

13. Method according to any one of claims 1-12, wherein the laser nitridation step is performed in 15 minutes or less, preferably 10 minutes or less, more preferably 30 seconds or less.

14. Bipolar plate comprising metallic titanium and a titanium nitride coating, obtainable using the method according to any one of claims 1-13.

15. Bipolar plate according to claim 14, wherein the outer surface comprises 50 % by weight or more of 5-TiN relative to the weight of all titanium-containing phases, preferably 60 wt.% or more, more preferably 70 wt.% or more.

16. Bipolar plate according to claim 14 or 15, wherein the titanium nitride coating has an average thickness of 100 nm to 50 gm, preferably 500 nm to 30 gm.

17. Bipolar plate according to any one of claims 14-16, wherein the interface between metallic titanium and the titanium nitride coating comprises a gradient of decreasing Ti : N ratio towards the outer surface of the bipolar plate.

18. Bipolar plate according to any one of claims 14-17, wherein one side or both sides of the bipolar plate comprise a titanium nitride coating.

19. Electrochemical reactor comprising one or more bipolar plates according to any one of claims 14-18.

20. Electrochemical reactor according to claim 19, wherein on at least one side of the bipolar plate, the titanium nitride coating is the outer surface of the bipolar plate.

21. Electrochemical reactor according to claim 19 or 20, wherein on at least one side of the bipolar plate, at least one further coating, such as a noble metal coating is provided on top of the titanium nitride.

22. Electrochemical reactor according to any one of claims 19-21, wherein the electrochemical reactor is a proton-exchange membrane water electrolyzer.

23. Electrochemical reactor according to any one of claims 19-22, wherein the electrochemical reactor is a fuel cell.

24. Use of a bipolar plate obtainable using the method according to any one of claims 1-13 and / or a bipolar plate according to any one of claims 14-18 in an electrochemical reactor, wherein the electrochemical reactor is preferably selected from the group consisting of an electrolyzer and a fuel cell.

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