Title

Method for creating a passivating oxide layer on a stainless steel component of an electrochemical cell

State of the Art

The invention relates to the field of electrochemical cells, and, in particular, relates to metal-supported electrochemical cells. More specifically, the invention relates to the field of solid oxide cells, including metal-supported 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 and 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" 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, a 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 from 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 stainless steel in SOCs is that under SOC operating conditions (e.g. oxidizing atmospheres and temperatures in the range of 550 - 700 °C) steel components can be prone to corrosion, in particular to oxidation, which limits the life-time of the SOC. In addition, evaporation of chromium from stainless steel components may lead to chromium poisoning of the anode and, thus, to a degeneration of the fuel cell. 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. This class of steel is, however, expensive compared to standard steel grades. Alternatively, it is known to use standard steel grades and coat them with a corrosion protective layer. However, the known coating methods usually require expensive coating equipment, which often outweighs the price advantage of using standard steel grades. Furthermore, aluminium-containing stainless ferritic steels (e.g. 1.4742) are known, in which a protective layer of aluminum oxide forms on the surface. However, aluminum oxide has an electrically insulating effect, which can reduce the efficiency of the SOC. This becomes even more pronounced over time as aluminium atoms diffuse to the surface and lead to a growth of the alumina layer. Summary of the Invention

The invention relates to a method for creating a passivating oxide layer on a stainless steel component of an electrochemical cell. The method comprises a step (i) of enriching a surface zone of the stainless steel component with at least one oxide-forming donor element selected from the list consisting of Cr, Mn, Si, Mo, Nb, Ti, V, Al, Hf, and Ce. In other words, in step (i) the concentration of at least one donor element selected from the list consisting of Cr, Mn, Si, Mo, Nb, Ti, V, Al, Hf, and Ce is increased in a region near the surface of the stainless steel component.

This step (i) of enriching the surface zone comprises a step of heat treating the stainless steel component in an enclosure, e.g. in a vacuum oven. In the enclosure, at least one donor component is present together with the stainless steel component. The at least one donor component comprises, preferably consists of, one or more of the donor elements Cr, Mn, Si, Mo, Nb, Ti, V, Al, Hf, and Ce. The heat treatment is conducted at a temperature of at least 900°C and at most 1300°C. The heat treatment is conducted under a pressure of at least 5 mbar and at most 500 mbar.

During heat treatment of the stainless steel component in the enclosure under operating conditions (i.e. low pressure atmosphere, temperatures between 900 °C and 1300 °C), the at least one donor element contained in the at least one donor component partially evaporates. This results in a certain partial pressure of the at least one donor element being present in the enclosure and, consequently, to adsorption of the at least one donor element at the surface of the stainless steel component. The at least one donor element diffuses into the stainless steel component creating a surface zone with an increased (enriched) concentration of the at least one donor element compared to the bulk.

After step (i), the stainless steel component is oxidized in a step (ii) by a second heat treatment. By heat treating the stainless steel component in step (ii), the at least one donor element enriched in the surface zone of the stainless steel component is oxidized to form a passivating oxide layer on the stainless steel component. The second heat treatment of step (ii) may be performed in the same enclosure as the heat treatment of step (i). Alternatively, the second heat treatment may be performed in a different enclosure, e.g. in a vacuum oven.

The proposed method allows for cheap manufacturing of a protective layer that is highly oxidation resistant and prevents chromium evaporation from the stainless steel component. In particular, the method does not require expensive coating equipment, which reduces process costs. In contrast to known coating methods, the proposed method does not rely on coating a full layer of an oxide-forming donor element on the stainless steel component, but is based on increasing the concentration of the at least one donor elements in a surface zone of the stainless steel component. By varying process parameters, e.g. elemental composition of the at least one donor component, pressure in the enclosure and temperature in the enclosure, the amount of material diffusing into the stainless steel component can be precisely tuned, e.g. dependent on the elemental composition of the stainless steel component. This allows the concentration of donor element in the surface zone to be controlled such that the resulting passivating oxide layer formed after step (ii) is thick enough to provide sufficient corrosion protection but thin enough to not influence other properties of the stainless steel component, in particular electrical resistance. A further advantage of the proposed method is that the process of adsorption of the at least one donor element on the surface of the stainless steel component is not directed, i.e. not limited to surfaces exposed to a certain coating direction. This allows protective coatings to also be formed on complex shaped components, e.g. in undercuts or holes.

The stainless steel component may be a ferritic steel component. The stainless steel component may be a mechanical support, a gas permeable carrier or an interconnector component of an electrochemical cell. The stainless steel component may be any plate or sheet stainless steel component forming part of the electrochemical cell (for example, a spacer plate, or current collection plate). Preferably, the stainless steel component is a component of a solid oxide fuel cell or a solid oxide electrolyser cell. The stainless steel component may be a mechanical support or a gas permeable support layer of a metal-supported electrochemical cell, such as, for example, a SOC. The stainless steel component 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, however, is not limited to such components, but can be applied to other stainless steel components as well.

Preferably, in step (i) the heat treatment is conducted at a temperature of at least 1000°C and at most 1150°C.

Preferably, in step (i) the heat treatment is conducted for at least 1h and at most 24h.

In order to avoid oxidation of the stainless steel component during the heat treatment in step (i), the 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 or in addition, the heat treatment in step (i) 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 (i) is conducted in an atmosphere consisting of a mixture of two or more of Ar, He, N2 and H2.

For an efficient transfer of donor elements from the at least one donor component to the stainless steel component it may be advantageous if the concentration of the at least one donor element in the donor component is at least twice as high as the initial concentration of the at least one donor element in the stainless steel component before the step of enriching the surface zone.

The material composition of the surface zone of the stainless steel component and, thus, the material composition of the passivating oxide layer formed after heat treatment in step (ii) may be tuned depending on the configuration of the at least one donor component, e.g. depending on the number and elemental composition of donor components present in the enclosure. In some embodiments, a single donor component may be present in the enclosure, wherein the single donor component comprises or consists of one or more of the donor elements selected from the list consisting of Cr, Mn, Si, Mo, Nb, Ti, V, Al, Hf, Ce. In some embodiments, two or more donor components may be present in the enclosure. Then, a first donor component may be present consisting of a first donor element selected from the list consisting of Cr, Mn, Si, Mo, Nb, Ti, V, Al, Hf, Ce and a second donor component may be present consisting of a second donor element selected from from the list consisting of Cr, Mn, Si, Mo, Nb, Ti, V, Al, Hf, Ce, said first donor element and said second donor element being different.

Preferably, in step (ii) the second heat treatment is conducted at a temperature of at least 700°C and at most 1000°C. For promoting the formation of the passivating oxide layer, it may be advantageous if the second heat treatment step is conducted in an oxygen donating atmosphere. Preferably, the second heat treatment step is conducted in an H2O and/or O2 containing atmosphere.

In step (ii), the second heat treatment may be conducted for at least 30 min and at most 16h. Preferably, the second heat treatment is conducted for at least 30 min and at most 3h.

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. 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)MnOs, (La, Sr)CoOs, LaNiOs, or LaFeOs.

The method for manufacturing the electrochemical cell comprises a step of providing the at least one stainless steel component and creating a passivating oxide layer on said at least one stainless steel component by a method as described above. The features and advantages explained above in connection with the method of creating a passivating oxide layer are also applicable to the method for manufacturing the electrochemical cell. After creating the passivating oxide layer on the at least one stainless steel component, that is to say, after performing at least step (i) and step (ii) 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 (where the order of the electrode layers may be reversed depending on the type of cell), in particular on the at least one stainless steel component.

The electrochemical cell may be a solid oxide cell, preferably a metal-supported 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.

In some embodiments, the at least one stainless steel component may be a mechanical support of the electrochemical cell, more preferably a gas permeable carrier, for example, of a solid oxide cell. Then, the step of assembling the electrochemical cell may comprise steps of coating the mechanical support having the passivating oxide layer thereon with a fuel electrode layer, an electrolyte layer, and an air or oxidant electrode layer (where the order of the electrode layers may be reversed depending on the type of cell). The stainless steel component 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.