Cell unit with improved chemical layer arrangement, stack of cell units and method for manufacturing

Abstract

The invention provides an electrochemical cell unit (10) with the following general features according to which the cell unit comprises: a support structure (14) carrying cell chemistry layers (18, 20, 22). The cell chemistry layers comprise a fuel electrode layer (18), an oxidant electrode layer (20) and an electrolyte layer (22). The electrolyte layer (22) is located between the fuel electrode layer (18) and oxidant electrode layer (20), and the fuel electrode layer (18) is located between the support structure (14) and the electrolyte layer (22). The support structure (14) comprises a porous region (24) surrounded by a non-porous region (26). At least the electrolyte layer (22) of the cell chemistry layers extends past a perimeter of the porous region (24). An active area of the cell chemistry layers comprises at least the fuel electrode layer (18) and the oxidant electrode layer (20). Typically, the electrolyte layer is present in the active area of the cell chemistry layers and is located between the electrode layers.

Description

[0001] Description

[0002] Title

[0003] Cell unit with improved chemical layer arrangement, stack of cell units and method for manufacturing

[0004] The present invention relates to an electrochemical cell unit. Such a cell unit can be used in a stack comprising a plurality of electrochemical cell units. Such a stack also is a part of the invention at hand. The invention also provides a method for manufacturing. A stack can be part of an electrochemical cell assembly, for example a fuel cell or electrolyser cell.

[0005] Preferably the cell unit is a solid oxide cell unit, more preferably a metal-supported solid oxide cell unit for a fuel cell or electrolyser cell.

[0006] The cell unit can be used in a stack as a fuel cell that can produce electricity by using an electrochemical conversion process that oxidizes fuel to produce electricity. Electrolyser cell units may be considered fuel cells running in reverse mode, i.e. using electricity to decompose a compound into its constituent parts, for example, water into hydrogen and oxygen. Reversible cells are capable of operating in both modes. Fuel cell units and electrolyser cell units typically comprise electrochemically active layers that may be configured to allow for conversion of electrochemical fuel to electricity (fuel cells) or for decomposing a compound into its constituent parts using electricity (electrolyser cells).

[0007] A solid oxide fuel cell that produces electricity is based upon a solid oxide electrolyte that conducts negative oxygen ions from a cathode to an anode located on opposite sides of the electrolyte. For this, a fuel, or reformed fuel, contacts the anode (fuel electrode) and an oxidant, such as air or an oxygen rich fluid, contacts the cathode (air electrode).

[0008] Conventional ceramic-supported (e.g., anode-supported) SOFCs have low mechanical strength and are vulnerable to fracture. Hence, metal-supported SOFCs have recently been developed which have the active fuel cell component layers supported on a metal substrate. In these cells, the ceramic layers can be very thin since they only perform an electrochemical function: that is to say, the ceramic layers are not self-supporting but rather are thin coatings / films laid down on and supported by the metal substrate. Such metal supported SOFC stacks are more robust, lower cost, have better thermal properties than ceramic- supported SOFCs and can be manufactured using conventional metal welding techniques. The present invention seeks to improve the robustness and reliability of the cell unit.

[0009] The invention provides an electrochemical cell unit with the following general features according to which the cell unit comprises: a support structure carrying cell chemistry layers. The cell chemistry layers comprise a fuel electrode layer, an oxidant electrode layer and an electrolyte layer. The electrolyte layer is located between the fuel electrode layer and oxidant electrode layer, and the fuel electrode layer is located between the support structure and the electrolyte layer. The support structure comprises a porous region surrounded by a non- porous region. At least the electrolyte layer of the cell chemistry layers extends past a perimeter of the porous region. An active area of the cell chemistry layers comprises at least the fuel electrode layer and the oxidant electrode layer. Typically, the electrolyte layer is present in the active area of the cell chemistry layers and is located between the electrode layers.

[0010] In this context, the support structure comprising a porous region surrounded by a non-porous region maybe understood as the perimeter of the porous region being within the perimeter of the non-porous region. Areas of the cell chemistry layers in which at least the fuel electrode layer and the oxidant electrode layer are present, may be understood as constituting the active area of the cell chemistry layers. The oxidant electrode is sometimes also referred to as air electrode.

[0011] The invention provides an arrangement of the chemistry layers in the cell unit such that the active area of the cell chemistry layers is arranged such that fuel can be provided in a sufficient manner to the active area of the cell chemistry layers to ensure that there is a large enough voltage drop between the electrodes (at the edges thereof) to ensure the prevention of negative effects on the electrochemical cell unit, like fuel electrode oxidation.

[0012] In areas of the active area of the cell chemistry layers that are provided atop the non-porous region undesirable effects may occur. These effects are avoided by the aspects of the invention described above and hereafter.

[0013] For example, in fuel cell operation fuel is supplied via the porous region of the support structure to the chemistry layers. In areas, in which both electrode layers (active area) (i.e. , all three of the chemistry layers) are present but they are located over a section of the support structure that is non-porous, the fuel that is available for reaction is limited and may be insufficient as it needs to be supplied from the porous region via porous parts of the chemistry layers.

[0014] Lateral diffusion of fuel in the fuel electrode is typically limited. Hence the available fuel, e.g., hydrogen, to undergo reaction in the chemistry layers atop the non-porous region may be insufficient to uphold a certain voltage drop and oxidation of the fuel electrode (anode in this case) may occur and lead to excessive wear and degradation of the cell unit. E.g., the voltage drop from anode (fuel electrode layer) to cathode (oxidant electrode layer) may be below the local oxidation potential of components, e.g., metal in cermet (for example nickel (Ni)), of the fuel electrode (anode in fuel cell operation). This may lead to oxidation which may later be followed by reduction, e.g., over thermal or stop-start cycles in lifetime of the cell unit, which may degrade the mechanical properties of the cell unit and lead to cracking, leaking and cell failure. The configurations according to the invention ensure advantageous cell chemistry layer placement and e.g., in fuel cell operation ensure that there is sufficient fuel available for a large enough voltage drop between the electrode layers to ensure reliable and safe operation. The mechanism of deterioration avoided by the invention at hand is described on the example of a cell unit in fuel cell operation comprising a cermet fuel electrode. Cermet is a common material for fuel electrodes and is metal-containing. If excess oxygen is present in the fuel electrode layer, then metal in the cermet could be oxidized. Oxidization of metal leads to change in volume (typically larger), which may lead to cracking of cell chemistry layers, especially if the layers are subject to redox cycles, and eventually this may lead to failure of the cell chemistry layers - and therefore of the cell unit as a whole.

[0015] In areas in which only one electrode layer (not part of the active area) is provided on the non- porous region, there is no risk of damaging reactions due to insufficient fuel, as both electrode layers (active area) are required for reactions. Related mechanisms exist when the cell unit is operated in electrolysis or electrolyser operation (in which the fuel electrode layer functions as the cathode and the oxidant electrode layer functions as the anode), and the overlaps of the claimed invention provide a balance in electrolysis between cost and efficiency. The invention provides sufficient coverage of the porous region with the electrolyte layer and minimizes the extent of both electrode layers over the non-porous region of the support in which fuel supply is not ensured.

[0016] According to a first aspect of the invention an overlap of the active area of the cell chemistry layers past the perimeter of the porous region and over the non-porous region is less than or equal to a threshold z. The threshold z is an extent of the overlap in which it is ensured that sufficient fuel is supplied to the parts of the active area that are within the threshold z. The provision of fuel is influenced by the permeability of the fuel electrode layer for fuel. The fuel electrode layer is typically a porous structure and fuel is transported predominantly along an interconnected network of pores in the material of the electrode layer. Thus, thickness of the fuel electrode layer, its porosity, the size of the pores, the degree of pore interconnectedness and other transport related parameters influence how far fuel is transported in a lateral direction in the fuel electrode layer into the area atop the non-porous region covered by the fuel electrode layer.

[0017] Preferably the threshold z is such that fuel supply to the active area of the chemistry layers atop the non-porous region is sufficient to maintain a voltage drop from the fuel electrode layer (anode in fuel cell operation) to the oxidant electrode layer (cathode in fuel cell operation) that is above the local oxidation potential of components, e.g., metal in cermet (for example nickel (Ni)), of the fuel electrode (anode in fuel cell operation). Oxidation and degradation of the cell unit is efficiently prevented. The threshold z maybe such that fuel supply to the active area of the chemistry layers atop the non-porous region is sufficient to maintain a voltage drop that is at least 5%, preferably 10%, preferably 15%, preferably 20%, preferably 25% above the local oxidation potential of components of the fuel electrode (anode in fuel cell operation).

[0018] The threshold z may be 7mm, preferably 5mm, preferably 3mm, preferably 2,5mm, preferably 2mm, preferably 1,75mm, preferably 1 ,5mm, preferably 1 ,25mm, preferably 1mm, preferably 0,75mm, preferably 0,7mm. These values provide a threshold that for most of the commonly used materials of fuel electrode layers, their usual dimensions and fuel transport relevant material properties, like porosity, ensure a sufficient supply of fuel to active areas that is within this threshold.

[0019] The exact value of the maximum "allowed" overlap, i.e. the threshold z, is influenced by geometrical parameters, material properties of the chemistry layers and their scattering between different cell units in mass production. Further, the conditions at which the cell unit is operated play a role.

[0020] The geometrical parameters and material properties most relevant for the maximum "allowed" overlap, i.e. the threshold z, include all parameters influencing the fuel transport through the electrolyte layer and the fuel electrode layer. Such parameters include but are not limited to: individual layer thicknesses, porosities, pore sizes, pore connectivity and tortuosity. Operating conditions can play a role in influencing the maximum "allowed" overlap, i.e. the threshold z. The threshold z is influenced fuel utlilisation and hence by parameters affecting fuel utlilisation, including the fuel flow rate, current density, temperature boundary conditions and the gas mixture the cell unit is operated at.

[0021] To ensure a robust design of the maximum "allowed" overlap, both tolerances in the manufacturing processes as well as changes in material properties and operating conditions due to degradation over lifetime should to be considered.

[0022] The following qualitative cause-effect relationships can be applied when designing the maximum "allowed" overlap for a given cell unit design and operating conditions:

[0023] Increasing the porosity of the fuel electrode layer decreases the transport resistance in the fuel electrode layer, which in turn increases the maximum "allowed" overlap, i.e. the threshold z. Decreasing porosity respectively decreases the maximum "allowed" overlap.

[0024] Increasing the pore size of the fuel electrode layer decreases the transport resistance in the fuel electrode layer, which in turn increases the maximum "allowed" overlap, i.e. the threshold z. Decreasing pore size respectively decreases the maximum "allowed" overlap.

[0025] Increasing the thickness of the fuel electrode layer decreases the transport resistance in the fuel electrode layer, which in turn increases the maximum "allowed" overlap, i.e. the threshold z. Decreasing the thickness of the fuel electrode layer respectively decreases the maximum "allowed" overlap.

[0026] Increasing the thickness of the support structure (in the porous region) increases the transport resistance in the porous region of the support structure, which in turn decreases the maximum "allowed" overlap, i.e. the threshold z, and vice versa.

[0027] Increasing the hole pitch of the support structure (in the porous region) or in general decreasing its transmissiveness decreases fuel availability in the electrode above and near the porous region of the support structure, which in turn decreases the maximum "allowed" overlap, i.e. the threshold z, and vice versa. Increasing the hole diameter of the support structure (in the porous region) increases fuel availability in the electrode above and near the porous region of the support structure, which in turn increases the maximum "allowed" overlap, i.e. the threshold z, and vice versa.

[0028] Increasing the Fuel utilization reduces the local partial pressure of the fuel which in turn decreases the maximum "allowed" overlap, i.e. the treshhold z, and vica versa.

[0029] Increasing the current density reduces the local partial pressure of the fuel which in turn decreases the maximum "allowed" overlap, i.e. the treshhold z, and vica versa.

[0030] Preferably the entirety of the porous region is covered by the active area of the cell chemistry layers. This maximises active area size, and therefore power density.

[0031] Preferably the active area of the cell chemistry layers extends past the perimeter of the porous region and over the non-porous region less than or equal to a threshold z along at least 50% of the circumferential extent of the perimeter of the porous region. Preferably this is the case along 75%, more preferably along 80%, more preferably along 85%, more preferably along 90%, more preferably along 95%, more preferably along the entirety of the circumferential extent of the perimeter of the porous region.. Mechanical stability may be provided by neighboring regions even if undesired oxidation occurs locally such that the overall performance and lifetime of the cell unit is not impaired.

[0032] For this reason, cell units in which the active area of the cell chemistry layers extends past the perimeter of the porous region and over the non-porous region less than or equal to a threshold z along at least 50% of the circumferential extent of the perimeter of the porous region have an improved lifetime over those that do not satisfy the threshold z around the perimeter. Lifetime of cell units having larger proportions of the perimeter having an extension less than the threshold z is correspondingly improved.

[0033] Preferably the active area of the cell chemistry layers extends past the perimeter of the porous region and over the non-porous region less than or equal to a threshold z along the entirety of the circumferential extent of the perimeter of the porous region. This maximises stability and lifetime of the cell unit.

[0034] The invention also provides further aspects. The features according to these further aspects constitute inventive concepts by itself and in combination with each other and / or the afore described first aspect. Hence the aspects relate to a cell unit with the general features and optionally the features of the aforementioned first aspect and / or the subsequently described other aspects.

[0035] According to another aspect of the invention, there is provided a cell unit, wherein an areal extent of the fuel electrode layer is greater than an areal extent of the porous region. Areal extent refers to the area covered by the respective layer. The entirety of the porous region may e.g. be covered by the fuel electrode layer. Covering as much of the porous region as possible by the fuel electrode layer enables design of an efficient cell unit. Parts of the porous region not covered by the fuel electrode layer are not covered by active area of the cell chemistry layers and hence are not providing area in which reaction occurs.

[0036] According to another aspect of the invention, there is provided a cell unit, wherein an areal extent of the oxidant electrode layer is greater than an areal extent of the porous region. Covering the porous region with oxidant electrode layer provides efficient use of the cell geometry and e.g. the amount of converted fuel can be maximized.

[0037] According to another aspect of the invention, there is provided a cell unit, wherein an areal extent of the fuel electrode layer is greater than an areal extent of the oxidant electrode layer. This enables control of the size and shape of the active area of the cell chemistry layers by control of the arrangement of the oxidant electrode layer. The sealing of the porous region is independent of the extent of the oxidant electrode layer. The oxidant electrode layer can be e.g. chosen to lie within the porous region without any issues regarding fuel leakage. Choosing the oxidant electrode layer to be "smaller" than the fuel electrode layer allows the oxidant electrode layer to be used efficiently to control the size and arrangement of the active area during manufacture.

[0038] According to another aspect of the invention, there is provided a cell unit, wherein a perimeter of the oxidant electrode layer lies within the perimeter of the fuel electrode layer. The perimeter of the oxidant electrode layer may, according to another aspect, also be within the perimeter of the fuel electrode layer along at least 50% of the circumferential length of the perimeter of the fuel electrode layer, more preferable along at least 60%, preferably along at least 70%, preferably along at least 80%, preferably along at least 90%, preferably along at least 95%, of the circumferential length of the perimeter of the fuel electrode layer. This enables that the extent of the active area of the cell chemistry layers is (mainly) prescribed by the oxidant electrode layer. This facilitates precise manufacturing and e.g. enables optical inspection of the extent of the active area after application of all the cell chemistry layers. The size and arrangement of the active area may even be modified after application of the oxidant electrode layer by partial removal, e.g. in the case of manufacturing errors. This ensures reliable high-quality production and the option of immediately identifying cell units not up to specification.

[0039] According to another aspect of the invention, there is provided a cell unit, wherein a perimeter of the oxidant electrode layer lies within the perimeter of the porous region. The perimeter of the oxidant electrode layer may, according to another aspect, also lie within the perimeter of the porous region along at least 50% of the circumferential length of the perimeter of the porous region, more preferable along at least 60%, preferably along at least 70%, preferably along at least 80%, preferably along at least 90%, preferably along at least 95%, of the circumferential length of the perimeter of the porous region. By limiting the extent of the oxidant electrode layer to be within the perimeter of the porous region the extent of the active area can be ensured to be within the porous region. This aspect provides a specifically safe arrangements that eliminates the possibility of the active area extending into the non- porous region.

[0040] According to another aspect of the invention, there is provided a cell unit, wherein an areal extent of the oxidant electrode layer is greater than an areal extent of the fuel electrode layer. Manufacture and application of the oxidant electrode layer maybe simplified. The layer controlling the extent and arrangement of the active area may be chosen to be the fuel electrode layer in this aspect.

[0041] According to another aspect of the invention, there is provided a cell unit, wherein a perimeter of the fuel electrode layer lies within the perimeter of the oxidant electrode layer. The perimeter of the fuel electrode layer may, according to another aspect, also be within the perimeter of the oxidant electrode layer along at least 50% of the circumferential length of the perimeter of the oxidant electrode layer, more preferable along at least 60%, preferably along at least 70%, preferably along at least 80%, preferably along at least 90%, preferably along at least 95%, of the circumferential length of the perimeter of the oxidant electrode layer. The extent and arrangement of the active area can be (mainly) determined by the fuel electrode layer in this aspect. The extent of the later active area can be controlled directly after application of the fuel electrode layer. A faulty application during application may be corrected, which may be easier as only one layer is applied yet, or the workpiece maybe discarded, which is cost efficient, as only one layer is applied yet. Requirements for the positioning of the other layers are low as the active area is determined by the fuel electrode layer in this aspect.

[0042] According to another aspect of the invention, there is provided a cell unit, wherein a perimeter of the fuel electrode layer lies within the perimeter of the porous region. The perimeter of the fuel electrode layer may, according to another aspect, also lie within the perimeter of the porous region along at least 50% of the circumferential length of the perimeter of the porous region, more preferable along at least 60%, preferably along at least 70%, preferably along at least 80%, preferably along at least 90%, preferably along at least 95%, of the circumferential length of the perimeter of the porous region. This aspect provides a specifically safe arrangements that eliminates the possibility of the active area extending into the non-porous region. The later extent of the active area can also be directly examined upon application of the fuel electrode layer, which may reduce costs in the case of work pieces not produced according to specification.

[0043] There are preferred embodiments of the invention that describe additional preferred features that improve the invention according to the described aspect (one or several of them) further. These preferred features are to be understood as preferential implementations of the aspects in isolation and in conjunction with the other preferred features.

[0044] Preferably the electrolyte layer covers the entire extent of the fuel electrode layer. This provides an efficient seal and prevents leakage. The porous pathways of the porous region and the porous pathways in the fuel electrode are covered by the electrolyte layer to provide an efficient gas seal. Preferably the porous region is covered by the fuel electrode layer. Via the porosity of the porous region a flow path for fluid, in particular fuel (in fuel cell or electrolyser cell operation) is provided from the cell volume to the cell chemistry layers (specifically an innermost layer, the fuel electrode layer, of the cell chemistry layers) and the electrolyte layer forms a seal that prevents undesired gas leakage. This preferred embodiment simplifies manufacturing as the areal arrangement of the electrolyte layer is not subject to strict specifications but merely has to exceed the specified minimum.

[0045] Preferably the electrolyte layer is in contact with the non-porous region of the support structure around a perimeter of the fuel electrode layer. It is also preferred if the electrolyte layer is in contact with the non-porous region of the support structure along at least 50%, preferably 60%, preferably 70%, preferably 80%, preferably 90%, preferably 95% of the perimeter of the fuel electrode layer. This provides a particularly save encapsulation / sealing which is easily manufactured.

[0046] Preferably the fuel electrode layer comprises a cermet. This makes the fuel electrode layer particularly robust.

[0047] Preferably the support structure is a metal substrate. The support structure is flexible yet durable and provides structural integrity even when dimensions are chosen small.

[0048] Preferably the porous region of the support structure comprises laser-drilled through-holes. Preferably the fuel electrode is in fluidic communication with the cell volume via the porous region. This allows for a defined permeability of fluid I the fuel. Also, the extent of the porous region can be controlled precisely.

[0049] Preferably the cell unit further comprises an interconnector structure, wherein the support structure and the interconnector structure overlay one another to enclose a cell volume therebetween (e.g., the support structure and the interconnector structure at least partially delimit the cell volume), also referred to as first fluid volume. Preferably the fuel electrode is in fluidic communication with the cell volume via the porous region. In fuel cell operation, fuel, e.g., hydrogen may be provided to the chemistry layers via the cell volume. The support structure and the interconnector structure can be plate like in configuration, i.e. , have an essentially overall planar extent. A support structure may be provided as a support plate and the interconnector structure may be provided as an interconnector plate.

[0050] Preferably the support structure is attached to the interconnector structure along an outer periphery of the support structure. This allows easy manufacture and potentially optical inspection of the connection. Further, the connection can be provided spatially separate from the cell chemistry layers and interference can be avoided.

[0051] The invention also provides a stack of cell units, comprising a plurality of cell units according to any one or multiple of the afore mentioned aspects and preferable implementations of the invention, said plurality of cell units being stacked upon one another along a stacking direction. The stack provides long lifetime and reliable operation.

[0052] The invention also provides a method of manufacturing an electrochemical cell unit. The method comprises: providing a support structure comprising a porous region surrounded by a non-porous region; coating or depositing cell chemistry layers on the support structure, the cell chemistry layers comprising a fuel electrode layer, an oxidant electrode layer and an electrolyte layer. The electrolyte layer is located between the fuel electrode layer and the oxidant electrode layer, and the fuel electrode layer is located between the support structure and the electrolyte layer. At least the electrolyte layer of the cell chemistry layers extends past a perimeter of the porous region. An active area of the cell chemistry layers comprises at least the fuel electrode layer and the oxidant electrode layer. Wherein the chemistry layers are applied such that an overlap of the active area of the cell chemistry layers past the perimeter of the porous region and over the non-porous region is less than or equal to a threshold z. The method allows manufacturing of a durable, reliable cell unit that has a long lifetime.

[0053] Further embodiments are derivable from the following description and the drawings.

[0054] In the drawings:

[0055] Figure 1 shows a cross section through a cell unit;

[0056] Figure 2 shows a detail of the cell unit in Figure 1 ;

[0057] Figure 3 shows a simplified schematic of the arrangement of different areas and layers of the cell unit of Figure 1 ;

[0058] Figure 4 shows a simplified schematic cross-sectional of two cell units stacked upon one another;

[0059] Figure 5 shows an electrochemical cell assembly comprising a number cell units stacked into a cell stack;

[0060] Figure 6 shows a simplified schematic of the arrangement of different areas and layers of the cell unit of an alternative embodiment of the invention;

[0061] Figure 7 shows a simplified schematic of the arrangement of different areas and layers of the cell unit of an alternative embodiment of the invention; and Figure 8 shows a simplified schematic of the arrangement of different areas and layers of the cell unit of an alternative embodiment of the invention.

[0062] Figure 1 shows an electrochemical cell unit 10 comprising an interconnector structure 12 and a support structure 14 in a cross-sectional view. Figure 2 shows an enlarged view of a section on the right-hand side of Figure 1 and Figure 3 shows a schematic top view in which the shapes of the shown components are strongly simplified.

[0063] In the described examples the interconnector structure 12 and the support structure 14 are plate like. Other configurations are possible within the scope of the invention. The interconnector structure 12 and the support structure 14 may also be referred to as interconnector plate 12 and the support plate 14. The support structure 14 and the interconnector structure 12 overlay one another. A cell volume 15 is delimited and enclosed by the support structure 14 and the interconnector structure 12. Preferably the support structure 14 is attached to the interconnector structure 12. Preferably the support structure 14 and the interconnector structure 12 are metallic and are connected by welding. Other materials and other methods of connection are possible in the sense of the invention at hand.

[0064] Cell chemistry layers 16 are applied (e.g., coated or deposited) on the support structure 14. The support structure 14 is located between the cell chemistry layers 16 and the interconnector structure 12.

[0065] The cell chemistry layers 16 comprise a fuel electrode layer 18, an oxidant electrode layer 20 and an electrolyte layer 22.

[0066] The fuel electrode layer 18 is applied on the support structure 14 (i.e., is an innermost layer of the cell chemistry layers, in other words a layer of the cell chemistry layers that is nearest to the support structure), the oxidant electrode layer 20 is an outermost layer of the cell chemistry layers, in other words a layer of the cell chemistry layers that is furthest from the support structure. The electrolyte layer 22 is located between the fuel electrode layer 18 and oxidant electrode layer 20 and separates the fuel electrode layer 18 and oxidant electrode layer 20.

[0067] The support structure 14 comprises a porous region 24 surrounded by a non-porous region 26. The porous region 24 is covered by at least the electrolyte layer 22 and in some cases all of the cell chemistry layers 16, as depicted in Figs. 1-3. The porous region may comprise a number of (e.g., laser-drilled) through holes 36. Porosity in the porous region 24 may additionally or alternatively also be provided by other means.

[0068] The cell volume 15 which is delimited and enclosed by the support structure 14 and the interconnector structure 12 provides an enclosed inner fluid volume (cell space). The fluid volume serves to supply operating fluid (e.g., fuel or oxidant in case of fuel cells) to the cell chemistry layers 16 (electrochemically active layers). For this, the support plate 14 (support structure 14) comprises the porous region 24 below the cell chemistry layers 16 (electrochemically active layers) such that fluid (fuel) may pass through the porous region 24 (via its pores) from the fluid volume (cell volume 15) to the electrochemical layers (cell chemistry layers 16, specifically the fuel electrode layer 18). In order to supply fluid to the cell volume 15, the cell unit 10 may comprise one or more fluid ports 106, e.g., in the form of through-holes (see Figure 5).

[0069] The porous region 24 of the support structure 14 is delimited by its perimeter 30 as schematically shown in Figure 3. The fuel electrode layer 18 is delimited by its perimeter 32. The non-porous region 26 of the support structure 14 extends from the perimeter 30 of the porous region 24 of the support structure 14 to the edge 34 of the outer periphery 36 of the support structure 14. Preferably the connection between the support structure 14 and the interconnector structure 12 is provided in this outer periphery 36 of the support structure 14.

[0070] In the shown example the areal extent of the oxidant electrode layer 20 is smaller than the areal extent of the fuel electrode layer 18. In other words, the area covered by oxidant electrode layer 20 is smaller than the area covered by the fuel electrode layer 18. The areal extent of the electrolyte layer 22 is bigger than the areal extent of the fuel electrode layer 18. In other words, the area covered by electrolyte layer 22 is bigger than the area covered by the fuel electrode layer 18.

[0071] The areas covered by the cell chemistry layers 16 mostly overlap, but as mentioned above, the different layers have different sizes and extend over each other. In the illustrated example the fuel electrode layer 18 extends past the oxidant electrode layer 20 along the entire perimeter 30 of the porous region 24 of the support structure 14, when viewed in a stacking direction 28, which extends perpendicular to the areal extent of the cell chemistry layers 16. More specifically, as illustrated in Figure 3, the perimeter 30 of the porous region 24 is within (i.e., smaller areal extent than) a perimeter 38 of the oxidant electrode layer 20. The perimeter 38 of the oxidant electrode layer 20 is within the perimeter 32 of the fuel electrode layer 18. In other words, and as best shown in Figure 2, the fuel electrode layer 18 extends past (by outward overlap 40) the perimeter 38 of the oxidant electrode layer 20. In a similar way, there is an outward overlap 42 of the oxidant electrode layer 20 over the porous region 24 of the support structure 14 along the perimeter 30 of the porous region 24 or in other words the oxidant electrode layer 20 extends past (by outward overlap 42) the porous region 24. Further, the perimeter 32 of the fuel electrode layer 18 is within the perimeter 46 of the electrolyte layer 22 and hence there is an outward overlap 44 of the electrolyte layer 22 over the fuel electrode layer 18 along the perimeter 32 of the fuel electrode layer 18. In this outward overlap 44 the electrolyte layer 22 is in contact with the non-porous region 26 of the support structure 14.

[0072] An active area 48 of the cell chemistry layers 16 comprises at least the fuel electrode layer 18 and the oxidant electrode layer 20. Hence in the example of Figure 3 the perimeter 50 of the active area 48 coincides with the perimeter 38 of the oxidant electrode layer 20, as the perimeter 38 of the oxidant electrode layer 20 is within the perimeter 32 of the fuel electrode layer 18.

[0073] An overlap of the active area 48 of the cell chemistry layers 16 past the perimeter 30 of the porous region 24 and over the non-porous region 26 is less than or equal to a threshold z, see Fig. 3.

[0074] In the example of Figure 3 an areal extent of the fuel electrode layer 18 is greater than an areal extent of the porous region 24. An areal extent of the oxidant electrode layer 20 is also greater than the areal extent of the porous region 24.

[0075] The areal extent of the fuel electrode layer 18 is greater than an areal extent of the oxidant electrode layer 20. The layers are arranged such that the perimeter 38 of the oxidant electrode layer 20 is within the perimeter 32 of the fuel electrode layer 18.

[0076] In fuel cell operation a drop of the cell voltage below the Nernst voltage of oxidation of metal in, e.g. cermet-containing, fuel electrode is reliably avoided by the arrangements according to the invention and in particular as explained in the context of the examples. The local voltage drop between anode (fuel electrode layer 18) and cathode (oxidant electrode layer 20) remains higher than the local oxidation potential due to sufficient supply of fuel to the edge regions of the electrode layers.

[0077] Preferably, the electrolyte layer 22 covers the entire extent of the fuel electrode layer 18. Preferably, the electrolyte layer 22 is in contact with the non-porous region 26 of the support structure 14 around the perimeter 32 of the fuel electrode layer 18.

[0078] Preferably, the area of contact between the electrolyte layer 22 and the non-porous region 26 of the support structure 14 is smaller than the area of contact between the fuel electrode layer 18 and the non-porous region 26 of the support structure 14. That is the area in which the electrolyte layer 22 extends past the fuel electrode layer 18 and is in direct contact with the support structure 14, is smaller than the area in which the fuel electrode layer 18 extends past the porous region 24 and is in contact with the non-porous region 26 of the support structure 14

[0079] Preferably the entire extent of the porous region 24 of the support structure 14 is covered by the sandwich-structure (laminate forming the cell chemistry layers) formed by the fuel electrode layer 18, the oxidant electrode layer 20 and the electrolyte layer 22 in between them.

[0080] Several cell units 10 are usually stacked on top of each other to form a stack 100 of cell units. The arrangement of two cell units 10 on top of each other is shown in Figure 4. Figure 5 shows a stack 100 of cell units arranged within a housing and held in a compressed state between a first end plate 102 and a second end plate 104.

[0081] In the stack 100, the fluid ports 106, of adjacent cell units 10 are aligned along the stacking direction 28.

[0082] In the illustrated example, the stack 100 further comprises gaskets 108 that are interposed between the cell units 10 and surround the fluid ports 60 of the cell units 10. Specifically, each fluid port 106 is associated with a gasket 108 on both sides of the cell unit 10. Preferably, the gaskets 108 are annular sealing rings having a central opening 110. The gaskets 108 may be formed from an exfoliated vermiculite material. Other materials are also possible within the scope of the invention. The stack 100 of cell units 10 may be arranged between two end plates 102, 104 provided on opposite sides of the stack 100, thus forming an electrochemical cell assembly 200.

[0083] The electrochemical cell assembly 200 comprises a fluid access port 112 for supplying fluid from an exterior of the electrochemical cell assembly 200 and the individual cell units 10 and a fluid exhaust port 114 for discharging exhaust fluid from the cell units 10 to the exterior. In the example, the fluid access port 112 and the fluid exhaust port 114 are each formed by a respective through-hole 106 formed in the first end plate 102 arranged on the bottom of the electrochemical cell assembly 200.

[0084] The electrochemical cell assembly 200 may further comprises a housing 118 encasing the stack 100 of cell units 10. In preferred examples, the housing 118 is formed from metal, preferably steel. The housing 118 may be welded to the end plates 102, 104.

[0085] The housing 118 and the end plates 102,104 together form an enclosure for a second fluid volume 120, preferably for air or oxidant, around the stack 100 of cell units 10.

[0086] Referring to Figure 4, it can be seen that the interconnector structure 12 comprises optional inward projections 122 extending into the fluid volume 15. The inward projections 122 form a supporting structure helping to maintain the fluid volume 15 (cell space) open. The inward projections 122 define first fluid passageways 124 inside the cell unit 10. In the stacked configuration as shown in Figure 4, outward projections 126 formed in the interconnector plate 12 engage at their ends against an outer surface of the cell chemistry layers 16 of an adjacent cell unit 10, thus defining second fluid passageways 128 between adjacent cell units 10. During operation, said second fluid passageways 128 may transport fluid (in fuel cell operation mode air or oxidant) to the cell chemistry layers 16 (in fuel cell operation mode to the cathode layer (oxidant electrode layer 20) forming the top of the cell chemistry layers 16).

[0087] In operation of the cell stack 100 in a fuel cell mode, fuel (e.g. hydrogen) is supplied to the fuel electrode layer 18 operating as anode layer of the cell chemistry layers 16 (e.g., via the fluid access port 112, the fluid volume 15 and the porous region 24), whereas air or oxidant is supplied to the oxidant electrode layer 20 operating as cathode layer of the cell chemistry layers 16 (e.g., via the second fluid volume 120 enclosed by the housing 118 and the end plates 102,104 and via the second fluid passageways 128 formed between the outwards projections 126). Operation in electrolysis mode may be similar, with fuel such as water (steam) or carbon dioxide being supplied to the fuel electrode layer 18 operating as a cathode and, in the case of an oxygen ion conducting electrolyte, oxygen being produced at the oxidant electrode layer 20 operating as an anode.

[0088] In Figure 6 an alternative embodiment of the cell unit of the invention is shown in a depiction corresponding to the one in Figure 3.

[0089] In the embodiment of Figure 6 the areal extent of the oxidant electrode layer 20 is greater than an areal extent of the fuel electrode layer 18. Further, in the embodiment of Figure 6 the arrangement of the layers is such that the perimeter 32 of the fuel electrode layer 18 is within the perimeter 38 of the oxidant electrode layer 20.

[0090] The electrolyte layer 22 in Figure 6, just as in Figure 3, covers the entire extent of the fuel electrode layer 18 and further, the electrolyte layer 22 is in contact with the non-porous region 26 of the support structure 14 around the perimeter 32 of the fuel electrode layer 18.

[0091] In the example of Figure 6 the perimeter 50 of the active area 48 (the active area 48 of the cell chemistry layers 16 comprising at least the fuel electrode layer 18 and the oxidant electrode layer 20) coincides with the perimeter 32 of the fuel electrode layer 18 as the perimeter 32 of the fuel electrode layer 18 is within the perimeter 38 of the oxidant electrode layer 20.

[0092] An overlap of the active area 48 of the cell chemistry layers 16 past the perimeter 30 of the porous region 24 and over the non-porous region 26 is less than or equal to a threshold z in the example of Figure 6.

[0093] In the embodiment of Figure 7 the areal extent of the oxidant electrode layer 20 is greater than an areal extent of the fuel electrode layer 18. Further, in the embodiment of Figure 7 the arrangement of the layers is such that the perimeter 32 of the fuel electrode layer 18 is within the perimeter 38 of the oxidant electrode layer 20.

[0094] The electrolyte layer 22 in Figure 7, just as in Figure 3 and Figure 6, covers the entire extent of the fuel electrode layer 18 and further, the electrolyte layer 22 is in contact with the non- porous region 26 of the support structure 14 around the perimeter 32 of the fuel electrode layer 18. In the example of Figure 7 the perimeter 50 of the active area 48 (the active area 48 of the cell chemistry layers 16 comprising at least the fuel electrode layer 18 and the oxidant electrode layer 20) coincides with the perimeter 32 of the fuel electrode layer 18, as the perimeter 32 of the fuel electrode layer 18 is within the perimeter 38 of the oxidant electrode layer 20.

[0095] In the example of Figure 7 an overlap of the active area 48 of the cell chemistry layers 16 past the perimeter 30 of the porous region 24 and over the non-porous region 26 is less than or equal to a threshold z as the active area 48 actually does not extend over the non-porous region 26. Instead in the example of Figure 7 the perimeter 50 of the active area 48 is entirely within the perimeter 30 of the porous region 24.

[0096] In the example of Figure 8 the arrangement is similar to Figure 7, except that in Figure 8 the perimeter 50 of the active area 48 coincides with the perimeter 38 of the oxidant electrode layer 20, as the perimeter 38 of the oxidant electrode layer 20 is within the perimeter 32 of the fuel electrode layer 18. As in Figure 7 the overlap of the active area 48 of the cell chemistry layers 16 past the perimeter 30 of the porous region 24 and over the non-porous region 26 is less than or equal to a threshold z, as the active area 48 actually does not extend over the non-porous region 26 but is entirely within the perimeter 30 of the porous region 24.

Claims

1. Claims1. An electrochemical cell unit (10) comprising: a support structure (14) carrying cell chemistry layers (16); the cell chemistry layers (16) comprising a fuel electrode layer (18), an oxidant electrode layer (20) and an electrolyte layer (22); wherein: the electrolyte layer (22) is located between the fuel electrode layer (18) and oxidant electrode layer (20), and the fuel electrode layer (18) is located between the support structure (14) and the electrolyte layer (22); the support structure (14) comprises a porous region (24) surrounded by a non-porous region (26), and at least the electrolyte layer (22) of the cell chemistry layers (16) extends past a perimeter (30) of the porous region (24); an active area (48) of the cell chemistry layers (16) comprises at least the fuel electrode layer (18) and the oxidant electrode layer (20); wherein an overlap (42) of the active area (48) of the cell chemistry layers (16) past the perimeter (30) of the porous region (24) and over the non-porous region (26) is less than or equal to a threshold z.

2. The electrochemical cell unit (10) according to the preceding claim, wherein an areal extent of the fuel electrode layer (18) is greater than an areal extent of the porous region (24).

3. The electrochemical cell unit (10) according to one of the preceding claims, wherein an areal extent of the oxidant electrode layer (20) is greater than the areal extent of the porous region (24).

4. The electrochemical cell unit (10) according to one of the preceding claims, wherein the perimeter (38) of the oxidant electrode layer (20) is within the perimeter (32) of the fuelelectrode layer (18) along at least 50% of the circumferential length of the perimeter (32) of the fuel electrode layer (20).

5. The electrochemical cell unit (10) according to one of the preceding claims, wherein the perimeter (38) of the oxidant electrode layer (20) is within a perimeter (30) of the porous region (24).

6. The electrochemical cell unit (10) according to one of the preceding claims, wherein the perimeter of the oxidant electrode layer (20) is within the perimeter (30) of the porous region (24) along at least 50% of the circumferential length of the perimeter (30) of the porous region (24).

7. The electrochemical cell unit (10) according to one of the preceding claims, wherein the areal extent of the oxidant electrode layer (20) is greater than the areal extent of the fuel electrode layer (18).

8. The electrochemical cell unit (10) according to one of the preceding claims, wherein the perimeter of the fuel electrode layer (18) is within the perimeter of the oxidant electrode layer (20).

9. The electrochemical cell unit (10) according to one of the preceding claims, wherein the perimeter of the fuel electrode layer (18) is within the perimeter of the oxidant electrode layer (20) along at least 50% of the circumferential length of the perimeter of the oxidant electrode layer.

10. The electrochemical cell unit (10) according to one of claims 1 to 6, wherein the areal extent of the fuel electrode layer (18) is greater than the areal extent of the oxidant electrode layer (20).

11. The electrochemical cell unit (10) according to one of the preceding claims 1 to 6 and 10, wherein a perimeter (38) of the oxidant electrode layer (20) is within a perimeter (32) of the fuel electrode layer (18).

12. The electrochemical cell unit (10) according to one of the preceding claims, wherein the perimeter of the fuel electrode layer (18) is within the perimeter of the porous region (26).

13. The electrochemical cell unit (10) according to one of the preceding claims, wherein the perimeter of the fuel electrode layer (18) is within the perimeter of the porous region (26) along at least 50% of the circumferential length of the perimeter of the porous region.

14. The electrochemical cell unit (10) according to any one of the preceding claims, wherein the electrolyte layer (22) covers the entire extent of the fuel electrode layer (18).

15. The electrochemical cell unit (10) according to the preceding claim, wherein the electrolyte layer (22) is in contact with the non-porous region (26) of the support structure (14) around the perimeter (32) of the fuel electrode layer (18).

16. The electrochemical cell unit (10) according to the preceding claim, wherein the electrolyte layer (22) is in contact with the non-porous region (26) of the support structure (14) around the perimeter (32) of the fuel electrode layer (18) along at least 50% of the perimeter of the fuel electrode layer.

17. The electrochemical cell unit (10) according to any one of the preceding claims, wherein the fuel electrode layer (18) comprises a cermet.

18. The electrochemical cell unit (10) according to any one of the preceding claims, wherein the support structure (14) is a metal substrate.

19. The electrochemical cell unit (10) according to any one of the preceding claims, wherein the porous region (24) of the support structure (14) comprises laser-drilled through- holes.

20. The electrochemical cell unit (10) according to any one of the preceding claims, further comprising an interconnector structure (12), wherein the support structure (14) and the interconnector structure (12) overlay one another to enclose a cell volume (15) therebetween.

21. The electrochemical cell unit (10) according to the preceding claim, wherein the fuel electrode (18) is in fluidic communication with the cell volume (15) via the porous region (24).

22. The electrochemical cell unit (10) according to claim 20 or 21 , wherein the support structure (14) is attached to the interconnector structure (12) along an outer periphery23. A stack (100) of cell units, comprising a plurality of cell units (10) according to any one of claims 1 to 22, said plurality of cell units (10) being stacked upon one another along a stacking direction (28).

24. A method of manufacturing an electrochemical cell unit, comprising: providing a support structure (14) comprising a porous region (24) surrounded by a non- porous region (26) coating or depositing cell chemistry layers (16) on the support structure, the cell chemistry layers (16) comprising a fuel electrode layer (18), an oxidant electrode layer (20) and an electrolyte layer (22); wherein: the electrolyte layer (22) is located between the fuel electrode layer (18) and oxidant electrode layer (20), and the fuel electrode layer (18) is located between the support structure (14) and the electrolyte layer (22); at least the electrolyte layer (22) of the cell chemistry layers (16) extends past a perimeter (30) of the porous region (24); and an overlap of the cell chemistry layers (16) past the perimeter (30) of the porous region (24) and over the non-porous region is less than or equal to a threshold z.

25. An electrochemical cell unit (10) comprising: a support structure (14) carrying cell chemistry layers (16); the cell chemistry layers (16) comprising a fuel electrode layer (18), an oxidant electrode layer (20) and an electrolyte layer (22); wherein: the electrolyte layer (22) is located between the fuel electrode layer (18) and oxidant electrode layer (20), and the fuel electrode layer (18) is located between the support structure (14) and the electrolyte layer (22); the support structure (14) comprising a porous region (24) surrounded by a non-porous region (26), and at least the electrolyte layer (22) of the cell chemistry layers (16) extends past a perimeter (30) of the porous region (24) along the entire perimeter of the porous region (24); wherein an areal extent of the fuel electrode layer (18) is greater than an areal extent of the oxidant electrode layer (20) and / or wherein a perimeter (38) of the oxidant electrode layer (20) lies within a perimeter (32) of the fuel electrode layer (18).

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