Title

Method for creating a protective coating on a component of an electrochemical cell

State of the Art

The invention relates to the field of electrochemical cells. More specifically, the invention relates to the field of solid oxide cells.

Solid oxide cells (SOCs) comprise three basic parts: a fuel electrode, a solid electrolyte and an air or oxidant electrode, which are commonly arranged in layers. They may be tubular or planar in configuration. Multiple planar solid oxide cell units may be arranged overlying one another to form a "stack", with the individual cell units arranged electrically in series. SOCs typically operate at temperatures ranging from 600 °C to 1000 °C.

SOCs can be run as solid oxide fuel cell (SOFC) or as solid oxide electrolyser cell (SOEC). SOFCs are energy conversion devices that allow for conversion of electrochemical fuel to electricity. More specifically, SOFCs use an electrochemical conversion process that oxidises fuel to produce electricity. For this, a fuel, or reformed fuel, contacts the fuel electrode and an oxidant, such as air or an oxygen rich fluid, contacts the oxidant electrode. The solid oxide electrolyte then conducts negative oxygen ions from the oxidant electrode to the fuel electrode. Hence, for SOFCs, the fuel electrode constitutes the anode and the air or oxidant electrode constitutes the cathode. SOECs are SOCs run in reverse mode compared to SOFCs. SOECs are commonly used for the electrolysis of water, in particular for generating hydrogen and oxygen gas. In this case, the fuel electrode constitutes the cathode and the air or oxidant electrode constitutes the anode.

Fuel electrode, solid electrolyte and air or oxidant electrode may be selfsupported ("electrolyte-supported", "cathode-supported", or "anode-supported" EM2019/6090

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SOC) or arranged in layers on a mechanical support. In modern SOCs, the mechanical support and also other components forming part of the cell repeat unit (for example, a separator plate, an interconnector component, or spacer plate, or current collection plate) are preferably made of stainless steel which offers several advantages over conventionally used materials such as ceramics. In particular, mechanical supports made of stainless steel allow for a more compact design of the SOC and, thus, for a higher power density. Such SOCs are commonly referred to as metal-supported SOCs ("MS-SOC"). In a MS-SOC, the mechanical support may be an intrinsically porous metal substrate formed from a powder metal precursor (for example, by tape casting), or, more preferably, is formed from a metal support plate provided with a porous region in the form of through holes or small apertures surrounded by a non-porous (solid) region. The porous region is provided through the metal support plate, and a fuel electrode layer is coated over that region, and then successive layers coated on top, which layers are thus supported by the metal support plate.

One disadvantage of using steel in SOCs is that under SOC operating conditions (e.g. oxidizing atmospheres and temperatures in the range of 550 - 1000 °C) components made from steel are usually prone to high-temperature corrosion, in particular to oxidation, which limits the life-time of the SOC. For that reason, a special class of corrosion-resistant stainless steels is commonly used, e.g. Hitachi ZMG 232 G10, Sandvik Sanergy HT, Crofer 22 APU, Plansee ITM.

However, this class of steel is expensive compared to standard steel grades. Alternatively, it is known to use standard stainless steel grades and coat them with a corrosion protective layer. However, the known coating methods are usually expensive, too, and often result in coatings that are protective but electrically insulating, which impairs fuel cell performance.

Summary of the Invention

The invention relates to a method for creating a protective coating on a component of an electrochemical cell. Preferably, the electrochemical cell is a solid oxide cell, more preferably a metal-supported solid oxide cell.

The component is made of stainless steel with a content of Cr of more than 10%wt. Said component made of stainless steel may be a mechanical support, a EM2019/6090

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- 3 - gas permeable support layer of a metal-supported electrochemical cell, such as for example, a SOC, or an interconnector component of the electrochemical cell. Said component made of stainless steel may be any plate or sheet stainless steel component forming part of the electrochemical cell (for example, a spacer plate or current collection plate). Said component made of stainless steel may be an intrinsically porous metal substrate formed from a powder metal precursor or may be formed from a metal support plate provided with a porous region in the form of through holes or small apertures surrounded by a non-porous (solid) region.

The method comprises a step (i), a step (ii), and an optional step (iii) of treating the at least one component made of stainless steel.

In step (i) the at least one component made of stainless steel is coated with a coating. The at least one component made of stainless steel may be coated on its entire outer surface. Alternatively, the at least one component may be coated on part of its outer surface only, e.g. on one (or more) face(s) of its entire outer surface. Preferably, at least the entirety of one face of its surface is coated.

The coating may consist of a single layer composed of Nb, Ti, Zr, and/or W. Alternatively, the coating may be a multilayer coating that comprises at least one layer composed of Nb, Ti, Zr, and/or W (i.e. an intermetallic phase-forming layer).

In step (ii) the at least one coated component made of stainless steel is heat treated. The heat treatment is performed at a temperature of at least 1000 °C and at most 1200 °C. Preferably, the heat treatment is performed at a temperature of at least 1050 °C and at most 1100 °C. The heat treatment is performed for a duration of at least 30 min and of at most 5h, preferably for a duration of at most 2h. During the heat treatment step, the at least one coated component made of stainless steel is kept under a low pressure atmosphere of less than 50 mbar or in a vacuum. For this reason, the heat treatment step (ii) may be conducted in an enclosure, e.g. in a vacuum oven.

In the optional step (iii), the at least one coated component made of stainless steel may be oxidized by heat treating it at a temperature of at least 800 °C and at most 1000 °C. Preferably, in step (iii) the heat treatment is performed for a duration of at least 30 min and of at most 5h in an oxygen containing EM2019/6090

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- 4 - atmosphere. The heat treatment in step (iii) may be performed under ambient air, preferably at atmospheric pressure.

The proposed method allows for cheap manufacturing of a highly corrosion resistant protective coating on the component made of stainless steel, which increases cell life-time under operating conditions. The protective coating allows for the use of relatively cheap standard steel grades (e.g. 1.4016,1.4509), which leads to significant cost reductions. The corrosion protection is based on the formation of at least one intermetallic phase (also called intermetallic) in a surface zone of the component made of stainless steel. During the heat treatment in step (ii) the elements Nb, Ti, Zr, and/or W diffuse into the surface of the stainless steel component. As a consequence, iron-rich intermetallic phases are formed in a near-surface region of the stainless steel component. These intermetallic phases exhibit high corrosion resistance as well as good electrical conductivity.

The coating may be applied as a multilayer. This may further improve the corrosion-protection performance. For example, diffusion processes and, thus, the resulting phases and/or the microstructure of the protective coating may be tuned depending on the order and the material composition of the individual sublayers. In addition, multilayer coatings may exhibit enhanced adhesion on the component made of stainless steel.

In an example of the described process, in step (i) the coating is applied as a multilayer and the multilayer is then heat treated in step (ii). The multilayer may comprise at least one layer composed of Hf or a rare earth element, in particular Ce, La, or Y, in addition to the at least one (intermetallic phase-forming) layer composed of Nb, Ti, Zr, and/or W. The rare earth elements may contribute to a better corrosion resistance and may enhance adhesion of oxides forming during step (iii) or during SOFC operating conditions. Alternatively or in addition, the mutltilayer may comprise at least one layer composed of one or more of Cr, Mo, V, Mn, Co, Si, or Al, in addition to the at least one layer composed of Nb, Ti, Zr, and/or W. These elements may promote the formation of the at least one intermetallic phase. In particular, the elements Cr, Mo, V, Mn, Co, Si, or Al may be incorporated in the at least one intermetallic phase, which may further EM2019/6090

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- 5 - enhance corrosion resistance. Moreover, Si and Al may additionally form protective oxides (e.g. SiC>2, AI2O3), in particular after performing step (iii).

In an example of the described process, at least one additional layer may be applied after step (ii). It is possible that in step (i) the coating is applied as a multilayer and the at least one additional layer is applied onto this multilayer after performing step (ii). It is also possible that in step (i) the coating is applied as a single layer and the at least one additional layer is applied onto the single layer after performing step (ii). The at least one additional layer applied after step (ii) may be a layer composed of one or more of Cr, Mo, V, Mn, Co, Si, Ti, Cu, or Al. Preferably, the additional layer is composed of Al or Si. Preferably, the at least one additional layer may be applied before conducting step (iii).

In a further example of the described process, a first layer may be coated directly onto the component made of stainless steel and a second layer may be coated onto the first layer. Preferably, said first layer is composed of Nb, Ti, Zr, and/or W and said second layer is composed of one or more of Cr, Mo, V, Mn, Co, Si, Ti, Cu, or Al. The second layer may be coated before or after heat treating the first layer composed of Nb, Ti, Zr, and/or W in step (ii). The second layer may be oxidized in a step (iii) before further processing of the component made of stainless steel to form a protective oxide layer (e.g. AI2O3, SiC>2).

In a further example of the described process, in step (i) a first layer may be coated directly onto the component made of stainless steel and a second layer may be coated onto the first layer. The first layer may be composed of Hf or of a rare earth element, in particular Ce, La, or Y, and the second layer may be composed of Nb, Ti, Zr, and/or W. Such a multilayer structure may improve adhesion of the protective coating on the component made of stainless steel. It may be further advantageous, if a third layer is coated onto the second layer, wherein the third layer is composed of Cr, Mo, V, Mn, Co, Si, Ti, Cu, or Al. Preferably, the third layer is composed of Al or Si. The third layer may be coated before or after heat treating the first and second layers in step (ii). In particular, the third layer is applied before performing step (iii). Preferably, the third layer is oxidized in step (iii) before further processing of the component made of stainless steel to form a protective oxide layer (e.g. AI2O3, SiC>2) . EM2019/6090

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In order to avoid oxidation of the component made of stainless steel during the heat treatment in step (ii), this heat treatment may be conducted in an inert gas atmosphere. Preferably, the heat treatment is conducted in an atmosphere containing, in particular consisting of, one or more of Ar, He, or N2.

Alternatively, the heat treatment in step (ii) may be conducted in a reducing atmosphere, e.g. in an atmosphere containing, in particular consisting of, H2. It is also possible that the heat treatment in step (ii) is conducted in an atmosphere consisting of a mixture of two or more of Ar, He, N2, and H2.

In some embodiments, one or more of the layers of the coating may be applied using a physical vapor deposition method, in particular using a magnetron sputtering method. Alternatively or supplementary, one or more of the layers may be applied using a, in particular plasma-assisted, thermal or electron beam evaporation method. Alternatively or supplementary, one or more of the layers may be applied using a chemical or an electrochemical deposition method.

In some embodiments, the coating may have a coating thickness of at least 10 nm, in particular at least 25 nm, in particular at least 50 nm and/or a coating thickness of less than 2000 nm, in particular less than 1000 nm, in particular less than 500 nm.

The invention also relates to a method for manufacturing an electrochemical cell. The electrochemical cell comprises at least a fuel electrode, an electrolyte, an air or oxidant electrode, and at least one component made of stainless steel with a content of Cr of more than 10%wt. Exemplarily, the fuel electrode may be composed of nickel oxide or Ni-YSZ (Yttria-stabilized zirconia). The solid electrolyte layer may be composed of Yttria-stabilized zirconia (YSZ), Gadolinia- doped Ceria or Cerium Gadolinium Oxide (CGO). The air or oxidant electrode may be composed of (La,Sr)MnO3, (La, Sr)CoOs, LaNiOs, or LaFeOs.

The method for manufacturing the electrochemical cell comprises a step of providing the at least one component made of stainless steel and coating said at least one component made of stainless steel with a coating by a method as described above. The features and advantages explained above in connection with the method of creating a protective coating are also applicable to the method EM2019/6090

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- 7 - for manufacturing the electrochemical cell. After coating the stainless steel component with the protective coating, that is to say, after at least performing step (i), step (ii) and, optionally, step (iii), as discussed above, the electrochemical cell is assembled in a subsequent step. Preferably, the step of assembling the electrochemical cell comprises steps of applying a fuel electrode, an electrolyte, and an air or oxidant electrode.

The electrochemical cell may be a solid oxide cell. The electrochemical cell may be a solid oxide fuel cell (SOFC). Then, the fuel electrode constitutes an anode and the air or oxidant electrode constitutes a cathode of the cell. Alternatively, the electrochemical cell may be a solid oxide electrolyser cell (SOEC). Then, the fuel electrode constitutes a cathode and the air or oxidant electrode constitutes an anode of the cell.

The electrochemical cell may be a metal-supported electrochemical cell, such as, for example, a metal-supported solid oxide cell. In some embodiments, the at least one component made of stainless steel may be a mechanical support, a gas permeable carrier, or an interconnector component of an electrochemical cell (e.g. a cell repeat unit), and, in particular, a mechanical support of a solid oxide cell, more preferably a gas permeable carrier of a solid oxide cell. Then, the step of assembling the electrochemical cell may comprise steps of coating the mechanical support having the protective coating thereon with a fuel electrode layer, an electrolyte layer, and an air or oxidant electrode layer (the order of the respective electrode layers may be reversed depending on the type of cell).

Further embodiments are derivable from the following description and the drawings.

In the drawings, the figures show:

Figure 1 : an outline of a coated component made of stainless steel according to a first embodiment;

Figure 2: an outline of a coated component made of stainless steel according to a second embodiment; EM2019/6090

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Figure 3: an outline of a coated component made of stainless steel according to a third embodiment; and

Figure 4: an outline of a coated component made of stainless steel according to a fourth embodiment.

In the drawing, Figures 1 to 4 schematically depict different illustrating examples of a component 10 made of stainless steel with a protective coating 12 applied on a top face 14 of its outer surface. As described in detail below, the coating 12 may consist of a single layer (cf. Fig. 1) or comprise multiple layers (cf. Fig. 2 - 4). The component 10 made of stainless may be a mechanical support, an interconnector or a gas permeable support layer of an electrochemical cell (e.g. cell repeat unit), preferably of a solid oxide fuel cell or a solid oxide electrolyser cell (not shown). The electrochemical cell may be assembled after applying the protective coating 12 onto the component 10 made of stainless steel.

According to a first embodiment illustrated in Fig. 1, the component 10 made of stainless steel is coated with a single layer 16 composed of Nb, Ti, Zr, and/or W in a step (i). After that, the coated component 10 made of stainless steel is heat treated in a step (ii), wherein the heat treatment is performed at a temperature of at least 1000 °C and at most 1200 °C, in particular at a temperature of at least 1050 °C and at most 1100°C. The heat treatment is performed for a duration of at least 30 min and of at most 5h, in particular for a duration of at most 2h. During the heat treatment, the coated component 10 made of stainless steel is kept under a low pressure atmosphere of less than 50 mbar or in a vacuum. During the heat treatment, Nb, Ti, Zr, or W diffuses from the layer 16 into the surface of the component 10 such that iron-rich intermetallic phases are formed in a near- surface region of the component 10 made of stainless steel.

According to a second embodiment illustrated in Fig. 2, after step (ii) an additional layer 18 of aluminium or silicon is applied onto the first layer 16 of Nb, Ti, Zr, or W. After applying the additional layer 18, the coated component 10 made of stainless steel is oxidized in a step (iii) by an additional heat treatment to form a protective oxide layer, i.e. an AI2O3 or SiC>2 layer. The additional heat treatment in step (iii) is performed at a temperature of at least 800 °C and at most 1000 °C, for a duration of at least 30 min and of at most 5h in an oxygen EM2019/6090

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- 9 - containing atmosphere, in particular ambient air, in particular at atmospheric pressure.

According to a third embodiment illustrated in Fig. 3, in a step (ia) a first layer 20 is coated onto the component 10 made of stainless steel, wherein the first layer 20 is composed of Hf or of a rare earth element, in particular Ce, La, or Y. After that, in a step (ib), a second layer 22 is coated onto the first layer 20, wherein the second layer 22 is composed of Nb, Ti, Zr, or W. After that, the component 10 made of stainless steel coated with the first layer 20 and the second layer 22 is heat treated in a step (ii) to form intermetallic phases as explained above. Exemplarily, the heat treatment in step (ii) is performed at a temperature of at least 1000 °C and at most 1200 °C, in particular at a temperature of at least 1050 °C and at most 1100 °C. The heat treatment is exemplarily performed for a duration of at least 30 min and of at most 5h, in particular for a duration of at most 2h. During heat treatment, the coated component 10 made of stainless steel is kept under a low pressure atmosphere of less than 50 mbar or in a vacuum.

According to a fourth embodiment illustrated in Fig. 4, an additional layer 24 of aluminium or silicon is applied onto the second layer 22 of Nb, Ti, Zr, or W. After applying the additional layer 24, the coated component 10 made of stainless steel is oxidized in a step (iii) by an additional heat treatment to form a protective oxide layer, i.e. an AI2O3 or SiC>2 layer. The additional heat treatment in step (iii) is exemplarily performed at a temperature of at least 800 °C and at most 1000 °C, for a duration of at least 30 min and of at most 5h in an oxygen containing atmosphere, in particular ambient air, in particular at atmospheric pressure.

After applying the protective coating 12 onto the component 10 made of stainless steel, the electrochemical cell may be assembled. For instance, the component 10 made of stainless steel may be a mechanical support of a solid oxide fuel cell or a solid oxide electrolyser cell. Then, assembling the SOFC or the SOEC may include steps of coating the component made of stainless steel 10 having the protective coating 12 thereon with a fuel electrode layer, a solid electrolyte layer and an air or oxidant electrode layer (the order of the respective electrode layers may be reversed depending on the type of cell).