WO2025157402A1

WO2025157402A1 - Electrochemical cell assembly

Info

Publication number: WO2025157402A1
Application number: PCT/EP2024/051652
Authority: WO
Inventors: Nils Giesselmann; Konrad MEISTER; Rajan THANDI; Ashok ANDERMANN; Andrew Proctor
Original assignee: Robert Bosch GmbH; Ceres Power Ltd
Current assignee: Robert Bosch GmbH; Ceres Power Ltd
Priority date: 2024-01-24
Filing date: 2024-01-24
Publication date: 2025-07-31
Anticipated expiration: 2026-07-24

Abstract

The invention relates to an electrochemical cell assembly (10) comprising a stack (16) of cell units (18), wherein in said stack there is provided a fluid inlet manifold (28) and a fluid outlet manifold (30), each cell unit has a fluid inlet port (38) being in communication with the fluid inlet manifold and a fluid outlet port (40) being in communication with the fluid outlet manifold, each cell unit defines an inner fluid flow path (36) for fluid to flow from the fluid inlet port to the fluid outlet port, each cell unit has a fluid permeability along its fluid flow path, an average fluid permeability of the cell units in an upper half (52) of the stack is higher than an average fluid permeability of the cell units in a lower half (54) of the stack.

Description

Title

Electrochemical cell assembly

State of the Art

The invention relates to the field of electrochemical cell stacks, in particular, fuel cell stacks and electrolyser cell stacks. More specifically, the invention relates to an electrochemical cell assembly.

Fuel cells and electrolyser cells are examples of electrochemical cells. Fuel cells are energy conversion devices that allow for conversion of electrochemical fuel (e.g. H₂) to electricity. Electrolyser cells are fuels cells running in reverse mode, i.e. using electricity to decompose a fuel (e.g. H₂O) into its constituent parts (e.g. H₂ and O₂). Reversible cells are capable of operating in both modes. Such electrochemical cells typically comprise electrochemically active layers that may be configured to allow for conversion of electrochemical fuel to electricity (fuel cells) or for decomposing a fuel into its constituent parts using electricity (electrolyser cells).

The present invention specifically relates to solid oxide cells (SOCs). Such solid oxide cells (SOCs) typically comprise an electrolyte layer formed from a solid oxide, e.g. from Yttria-stabilized zirconia (YSZ), Gadolinia-doped Ceria, or Cerium Gadolinium Oxide (CGO). SOCs can be run as solid oxide fuel cell (SOFC) or as solid oxide electrolyser cell (SOEC).

Typically, multiple of such cell units are stacked upon one another along a stacking direction to form a “stack” of cell units (also referred to as ‘cell repeat units’). Said stack is commonly arranged between two end plates provided on opposite sides of the stack, thus forming an electrochemical cell assembly.

Such cell assemblies typically comprise a fuel inlet manifold for supplying fluid, preferably fuel, to the cell units and a fluid outlet manifold for removing exhaust fluid from the cell units. Typically, the fluid inlet manifold and the fluid outlet manifold extend through the stack of cell units and are in fluid communication with each other via the cell units. Specifically, each cell unit may define a fluid flow path for fluid to flow from the fluid inlet manifold to the fluid outlet manifold. For this, each cell unit may have a fluid inlet port being in communication with the fluid inlet manifold and a fluid outlet port being in communication with the fluid outlet manifold. Such an electrochemical cell assembly is known, for example, from WO 2020/126486 or WO 2022/175679.

It is an object of the present invention to improve performance of an electrochemical cell assembly.

Description of the Invention

According to the invention, there is provided an electrochemical cell assembly according to claim 1. Preferably, the electrochemical cell assembly is a fuel cell assembly or electrolyser cell assembly. The electrochemical cell assembly comprises a stack of cell units. The stack of cell units comprises a plurality of cell units that are stacked upon one another along a stacking direction. Preferably, each cell unit extends in a cell plane perpendicular to the stacking direction in a first direction, preferably longitudinal direction, and in a second direction, preferably width direction, perpendicular to the first direction. The electrochemical cell assembly further comprises a fluid inlet manifold and a fluid outlet manifold. The fluid inlet manifold is configured to supply fluid, preferably fuel, to the cell units. The fluid outlet manifold is configured to discharge fluid, preferably exhaust fuel, from the cell units. The fluid inlet manifold and the fluid outlet manifold each extend through the stack of cell units, preferably along the stacking direction. The fluid inlet manifold is configured to supply fluid in an inflow direction, preferably parallel to the stacking direction. The fluid outlet manifold is configured to remove fluid in an outflow direction, preferably opposite the inflow direction. Each cell unit has a fluid inlet port, preferably in the form of a through-hole, and a fluid outlet port, preferably in the form of a through-hole. Preferably, the fluid inlet port and the fluid outlet port are spatially separated from each other, preferably along the first direction of the cell units. The fluid inlet port is in fluid communication with the fluid inlet manifold. The fluid outlet port is in fluid communication with the fluid outlet manifold. Each cell unit defines an inner fluid flow path from the fluid inlet port to the fluid outlet port. Thus, each cell unit defines an inner fluid flow path for fluid to flow from the fluid inlet manifold to the fluid outlet manifold, said fluid flow path extending from the fluid inlet port of the cell unit to the fluid outlet port of the cell unit. During use of the electrochemical cell assembly, fluid may flow along the fluid inlet manifold (preferably upwards along the stacking direction), circulate through the cell units, and then flow along the fluid outlet manifold (preferably down against the stacking direction). Each cell unit has a fluid permeability along its fluid flow path between the fluid inlet port and the fluid outlet port. According to the invention, the cell units are configured and stacked along the stacking direction such that an average fluid permeability of the cell units in the upper half of the stack of cell units is larger than an average fluid permeability of the cell units in the lower half of the stack of cell units.

The proposed configuration improves performance of the electrochemical cell assembly. Specifically, the proposed configuration aids homogeneous fluid distribution, preferably fuel distribution, inside the stack of cell units. In particular, increasing fluid permeability in inflow direction of the fluid helps to compensate for changes in pressure difference between the fluid inlet manifold and the fluid outlet manifold occurring along the inflow direction. Specifically, it has been recognized that a pressure difference between the fluid inlet port and the fluid outlet port of a cell unit is smaller for the cell units (upper cell units) of the stack which are later in the inflow direction (e.g., further from the supply to the fluid inlet manifold, for example, the cell units closer to the second end plate) than for the cell units (lower cell units) of the stack which are earlier in the inflow direction (e.g., closer to the supply to the fluid inlet manifold, for example, the cell units closer to the first end plate). Increasing fluid permeability of the cell units in the upper half of the stack allows compensating for this pressure difference, thus aiding homogeneous fluid supply.

As used herein, the term "fluid permeability" refers to the ability of a cell unit to let fluid pass through the inner fluid flow path from the fluid inlet port to the fluid outlet port. Specifically, a fluid flow through the inner fluid flow path of a cell unit may be approximated by the following formula: m~K x (p_{fluid iniet p}ort ~ Pfiuta outlet port), wherein m denotes the mass flow, K the fluid permeability, and p; the pressure at the respective fluid port. Thus, the fluid permeability of a cell unit may be defined as the quotient of flow rate through the inner fluid flow path from the fluid inlet port to the fluid outlet port and pressure difference being between the fluid inlet port and the fluid outlet port.

As generally known in the art, the fluid permeability of a cell unit may be determined by measuring a pressure at the fluid inlet port and a pressure the fluid outlet port of the cell unit, e.g. with pressure sensors known in the art, and by simultaneously measuring a mass flow through the fluid inlet port, e.g. with a flow meter known in the art. The mass flow may be measured a position upstream of the fluid inlet port, e.g. in a supply line connected to the fluid inlet port. The fluid permeability may then be calculated according to the above formula.

As used herein, the term "average fluid permeability" refers to the average value of the fluid permeabilities of the cell units of a group of cell units, e.g. the cell units of the upper half of the stack of cell units or the cell units of the lower half of the stack of cell units.

As used herein, the term "upper half' of the stack of cell units refers to the half of the stack of cell units that is later in the inflow direction, and the term "lower half' of the stack of cell units refers to the half of the stack of cell units that is earlier in the inflow direction.

In preferred embodiments, the inflow direction is parallel to the stacking direction. Thus, the "upper half" of the stack of cell units may refer to the half of the stack of cell units that is later in the stacking direction, and the "lower half" of the stack of cell units may refer to the half of the stack of cell units that is earlier in the stacking direction.

In some embodiments, the cell units in the upper half of the stack of cell units are relatively further from a supply to the fluid inlet manifold than the cell units in the lower half of the stack of cell units. In such embodiments, the term "upper half' of the stack of cell units refers to the half of the stack of cell units that is further from the supply to the fluid inlet manifold, i.e. later in the inflow direction, and the term "lower half" of the stack of cell units refers to the half of the stack of cell units that is closer to the supply to the fluid inlet manifold, i.e. earlier in the inflow direction.

Alternatively or additionally, the cell units in the upper half of the stack of cell units may be relatively further from an exhaust from the fluid outlet manifold than the cell units in the lower half of the stack of cell units. Thus, in such embodiments, the "upper half" of the stack of cell units may refer to the half of the stack of cell units that is further from the exhaust from the fluid inlet manifold, and the term "lower half' of the stack of cell units may refer to the half of the stack of cell units that is closer to the exhaust from the fluid inlet manifold.

In preferred embodiments, the electrochemical cell assembly comprises a fluid access port for supplying fluid from an exterior of the cell assembly to the fluid inlet manifold, and a fluid exhaust port for discharging exhaust fluid from the fluid outlet manifold to the exterior. In such embodiments, cell units with higher fluid permeability may be positioned further from the fluid access port than cell units with lower fluid permeability. Alternatively or additionally, cell units with higher fluid permeability may be positioned further from the fluid exhaust port than cell units with lower fluid permeability. Thus, the "upper half" of the stack of cell units may be relatively further from the fluid access port than the "lower half" of the stack of cell units and/or the "upper half" of the stack of cell units may be relatively further from the fluid exhaust port than the "lower half" of the stack of cell units.

In preferred embodiments, the electrochemical cell assembly further comprises an end plate, on which the stack of cell units is arranged. Preferably, the electrochemical cell assembly further comprises a first end plate and a second end plate, wherein the stack of cell units is arranged, preferably held in compression, between said first end plate and said second end plate. Thus, the stacking direction preferably points from the first end plate to the second end plate, preferably perpendicular to the first end plate.

In such embodiments, the term "upper half' of the stack of cell units may refer to the half of the stack of cell units that is closest to the second end plate (or which are later in the inflow direction - e.g., further from the supply to the fluid inlet manifold), and the term "lower half" of the stack of cell units may refer to the half of the stack of cell units that is closest to the first end plate (or which are earlier in the inflow direction - e.g., closer to the supply to the fluid inlet manifold).

Preferably, the first end plate is a base plate of the electrochemical cell assembly. Thus, during a use of the electrochemical cell assembly as intended, the first end plate is positioned under the stack of cell units. Alternatively, the first end plate is a top plate of the electrochemical cell assembly. That is to say, during a use of the electrochemical cell assembly as intended, the first end plate is positioned on top of the stack of cell units. Thus, the terms "upper" and "lower" half of the stack as used herein are not necessarily to be understood as upper and lower halves in terms of gravity, and may instead or additionally relate to being relatively further from (“upper”) and closer to (“lower”) supply of fluid to the fluid inlet manifold.

In embodiments comprising an end plate, the end plate, preferably first end plate, may comprise a fluid access port for supplying fluid from an exterior of the cell assembly to the fluid inlet manifold, and a fluid exhaust port for discharging exhaust fluid from the fluid outlet manifold to the exterior. Thus, the electrochemical cell assembly may be configured such that fluid, preferably fuel, may enter the cell assembly via the fluid access port, flow up along the fluid inlet manifold, pass through the respective inner fluid flow path of each cell unit, and flow down along the fluid outlet manifold to the fluid exhaust port. The "upper half' of the stack of cell units may be relatively further from the fluid access port and the fluid exhaust port than the "lower half" of the stack of cell units. Cell units with higher fluid permeability may be positioned further from the fluid access port than cell units with lower fluid permeability. Cell units with higher fluid permeability may be positioned further from the fluid exhaust port than cell units with lower fluid permeability.

In preferred embodiments, the fluid access port and the fluid exhaust port are each formed by a respective through-hole formed in the first end plate. Thus, the fluid inlet manifold may be in fluid communication with a first through-hole formed in the first end plate, and the fluid outlet manifold may be in fluid communication with a second through-hole formed in the first end plate.

The fluid permeability of a cell unit is inversely proportional to a flow resistance for fluid to flow from the fluid inlet port to the fluid outlet port along the inner fluid flow path. Hence, the ability to let fluid pass through the inner fluid flow path may also be characterised by the flow resistance. Thus, each cell unit may have a flow resistance along its fluid flow path from the fluid inlet port to the fluid outlet port, preferably from the fluid inlet manifold to the fluid outlet manifold, wherein an average flow resistance of the cell units in the upper half of the stack is lower than an average flow resistance of the cell units in the lower half of the stack.

In some embodiments, the electrochemical cell assembly comprises electrochemically active cell units and electrochemically inactive "dummy" cell units. Typically, such dummy cell units are provided at one or both ends of the stack of cell units in stacking direction. As used herein, the term "cell unit" refers solely to electrochemically active cell units. Thus, dummy cell units that are optionally disposed at one or both sides of the stack of cell units are not considered when referring to an "average fluid permeability" of the cell units in the upper or lower half of the stack of cell units.

In some embodiments, an average fluid permeability of the cell units in the upper half of the stack of cell units is at least 2.5 %, preferably at least 5%, more preferably at least 7%, more preferably at least 10% larger, than the average fluid permeability of the cell units in the lower half of the stack of cell units. This range has proven advantageous with regards to a homogeneous fluid distribution inside the stack of cell units.

In some embodiments, the upper half of the stack of cell units comprises at least one high permeability cell unit and the lower half of the stack of cell units comprises at least one low permeability cell unit, wherein a high permeability cell unit is distinguished from a low permeability cell unit in that is has a higher specific fluid permeability for fluid to flow along the inner fluid flow path. Preferably, a respective high permeability cell unit has a higher fluid permeability than an average fluid permeability of the cell units of the stack of cell units. Preferably, a respective low permeability cell unit has a lower fluid permeability than an average fluid permeability of the cell units of the stack of cell units.

Preferably, a fluid permeability of a high permeability cell unit is at least 0.5 %, preferably at least 1.0 %, more preferably at least 2.5 %, more preferably at least 5.0 % higher than the fluid permeability of a low permeability cell unit.

In some embodiments, the cell units are stacked upon one another such that for a majority of the cell units, preferably for at least 70% of the cell units, more preferably for at least 80% of the cell units, more preferably for at least 90% of the cell units, more preferably for at least 95 % of the cell units, the fluid permeability of the cell units is the same or increases from cell unit to cell unit in stacking direction. Thus, when looking at pairs of neighbouring cell units, for at least a majority of said pairs of cell units, the fluid permeability of the cell units in a pair is the same or the cell unit being located later in the stacking direction (e.g., later in the inflow direction) has a higher fluid permeability than the cell unit located earlier in the stacking direction (e.g., earlier in the inflow direction).

In some embodiments, at least 95%, preferably at least 98%, of the cell units are stacked upon one another such that a fluid permeability of the cell units increases from cell unit to cell unit in stacking direction. Thus, the fluid permeability of the cell units may gradually increase in the stacking direction. This allows for a particularly homogeneous fuel distribution inside the stack as it allows compensating for a gradually decreasing pressure difference in the stacking direction between the fluid inlet manifold and the fluid outlet manifold. Preferably, the fluid inlet ports of adjacent cell units are aligned such that they overly along the stacking direction and form part of the fluid inlet manifold. Preferably, the fluid outlet ports of adjacent cell units are aligned such that they overly along the stacking direction and form part of the fluid outlet manifold.

In some embodiments, each fluid port is formed by a respective through-hole formed in the cell unit, said through-hole extending through the cell unit along the stacking direction. Thus, the fluid inlet port may be formed by a first through-hole formed in the cell unit and the fluid outlet port may be formed by a second through- hole formed in the cell unit, the first and second through-holes each extending through the respective cell unit along the stacking direction. Preferably, the through- hole forming the fluid inlet port forms a section of the fluid inlet manifold and the through-hole forming the fluid outlet port forms a section of the fluid outlet manifold.

In some embodiments, the stack of cell units further comprises gaskets interposed between the cell units. Preferably, each fluid port (each through-hole) is surrounded by a respective gasket. Preferably, said gasket has a gasket opening. In preferred embodiments, a column of the aligned fluid inlet/outlet ports (through-holes) of the cell units and the gasket openings of the gaskets surrounding the fluid inlet/outlet ports form the fluid inlet/outlet manifold or at least a section of the fluid inlet/outlet manifold. Thus, a plurality of gaskets may be provided around each of the fluid inlet manifold and the fluid outlet manifold to prevent loss of fluid. Preferably, between adjacent cell units at least one of said gaskets or a group of said gasket is provided. Preferably, the gaskets are formed from a, particularly exfoliated, vermiculite material.

In preferred embodiments, each cell unit encloses an inner fluid volume (cell volume), said inner fluid volume defining the inner fluid flow path or at least a section of the inner fluid flow path. Preferably, said fluid volume is in fluid communication with the fluid inlet manifold via the fluid inlet port of the cell unit and in fluid communication with the fluid outlet manifold via the fluid outlet port of the cell unit. Preferably, the fluid volume is in communication with electrochemically active layers of the cell unit (see below).

In some embodiments, the fluid inlet port and/or the fluid outlet port of at least some of the cell units is surrounded by a fluid guiding structure. Thus, at least some of the cell units may have a fluid guiding structure positioned around the fluid inlet port and/or a fluid guiding structure positioned around the fluid outlet port. Said fluid guiding structure defines a fluid pathway for conveying fluid between the assigned fluid port and the fluid volume defined by the cell unit. Thus, the fluid guiding structure positioned around the fluid inlet port may be configured for conveying fluid that is supplied to the fluid inlet port via the fluid inlet manifold to the inner fluid volume. The fluid guiding structure positioned around the fluid outlet port may be configured for conveying fluid from the fluid volume to the fluid outlet port, and thus to the fluid outlet manifold. Preferably, the fluid guiding structure associated with the fluid outlet port is configured to collect fluid from the fluid volume and discharge it to the fluid outlet port.

The fluid pathway defined by the fluid guiding structure has a flow cross-section for fluid to flow from the associated fluid port to the fluid volume (cell volume). Preferably, a flow cross-section of said fluid pathway is higher for high permeability cell units than for low permeability cell units.

The fluid pathway may be provided by one or more fluid channels defined by the fluid guiding structure or formed in the fluid guiding structure. Thus, the fluid guiding structure may define one or more fluid channels. In some embodiments, the fluid guiding structure has radially extending channels defining said fluid pathway.

In some embodiments, for high permeability cell units a number of said channels and/or a flow cross-section of said channels is larger than for low permeability cell units.

In some embodiments, for high permeability cell units a total flow cross-section of said channels is larger than for low permeability cell units.

If the fluid pathway is provided by a single fluid channel, the flow cross-section of the fluid pathway may be defined by the flow cross-section, preferably a minimum cross-sectional area, of said channel. In embodiments, wherein the fluid pathway is provided by multiple fluid channels, the flow cross-section of the fluid pathway may be defined by the total flow cross-section of said channels, in particular by the total minimum cross-sectional area of said channels.

In some embodiments, a low permeability cell unit may be distinguished from a high permeability cell unit in that some of said channels are closed or narrowed. The fluid guiding structure may be formed integral with the cell unit, e.g. integral with a support plate or an interconnector plate of the cell unit (see below). Alternatively, the fluid guiding structure may be formed by a separate piece that is connected to or positioned within the cell units. For example, the fluid guiding structure may be bonded to the cell units, e.g. by welding. The fluid guiding structure may be formed by an inset that is inserted in the fluid volume defined by the cell unit. The fluid guiding structure may be formed by a metal disc having radially extending channels.

In preferred embodiments, each cell unit comprises a support plate and an interconnector plate, said support plate and said interconnector plate overlying one another along the stacking direction. Preferably, the support plate and the interconnector plate are formed from metal, more preferably stainless steel. The support plate and the interconnector plate enclose a fluid volume therebetween (i.e. the above-described fluid volume or cell volume). Preferably, the support plate and the interconnector plate are, in particular sealingly, attached to each other.

In embodiments having a support plate and an interconnector plate, the fluid inlet port and the fluid outlet port may each be formed by a respective through-hole extending in stacking direction through both the support plate and interconnector plate.

In embodiments having a support plate and an interconnector plate, the abovedescribed fluid guiding structure may be formed by a fluid guiding insert that is interposed between the support plate and the interconnector plate, preferably in the fluid volume enclosed between the support plate and the interconnector plate. Thus, the fluid inlet port and/or the fluid outlet port may be associated with a fluid guiding insert, said fluid guiding insert being disposed between the support plate and the interconnector plate.

The fluid guiding insert preferably defines a fluid pathway for conveying fluid between the assigned fluid port and the fluid volume in a lateral direction, i.e. in a direction perpendicular to the stacking direction. Thus, the fluid guiding insert associated with the fluid inlet port may be configured to spread fluid being supplied to the fluid inlet port via the fluid inlet manifold in a plane that is perpendicular to the stacking direction. The fluid guiding insert associated with the fluid outlet port may configured to collect fluid from the fluid volume and discharge it to the fluid outlet port. In embodiments having a support plate, an interconnector plate and a fluid guiding insert, the fluid inlet port and/or the fluid outlet port may be provided by a through- hole extending through the interconnector plate, the fluid guiding insert and the support plate. In some embodiments, the fluid inlet port and/or the fluid outlet port may be provided by a through-hole formed in the interconnector plate, a through- hole formed in the fluid guiding insert and a through-hole formed in the support plate, said through-holes being aligned with each other.

In some embodiments, the fluid guiding insert is positioned adjacent to the associated fluid port. In some embodiments, the fluid guiding insert surrounds the associated fluid port in a plane perpendicular to the stacking direction (sometimes referred to as a lateral direction).

In some embodiments, the fluid inlet port and/or the fluid outlet port, preferably the at least one through-hole forming a respective fluid port, is circular. In such embodiments, the fluid guiding insert may be circular. The fluid guiding insert may be arranged coaxially with the associated fluid port.

In some embodiments, the fluid guiding insert may be a metal disc having radially extending channels, said channels defining said fluid pathway. The metal disc may be fixed to either or both of the support plate and the interconnector plate, preferably by welding.

The fluid volume may be defined by a gap formed between the support plate and the interconnector plate. In such embodiments, preferably for high permeability cell units a gap height of said gap is larger than for low permeability cell units (i.e. , the dimension of the gap, or spacing between the plates which thereby forms the fluid volume, is the height of the fluid volume, and varying that height between cell units varies the permeability of said cell units).

In some embodiments, the interconnector plate and/or the support plate, preferably the interconnector plate, may have a concavity, said concavity defining the fluid volume (e.g., the height thereof). In such embodiments, preferably for high permeability cell units a depth of said concavity along the stacking direction is larger than for low permeability cell units. The concavity may be bordered by flanged perimeter features formed around the perimeter of the respective plate. The interconnector plate and/or the support plate, preferably the interconnector plate, may be shaped to a concave configuration. In some embodiments, the support plate and/or the interconnector plate, preferably the interconnector plate, has a structured area with a plurality of, preferably spatially separated, protrusions formed therein. The protrusions extend into the fluid volume enclosed between the support plate and the interconnector plate (which may affect the fluid permeability of the cell unit) and define fluid passageways (fluid channels) therebetween for distributing a fluid within the fluid volume during operation of the electrochemical cell unit. In such embodiments, preferably for high permeability cell units an average height of said protrusions - and thus of the fluid passageways - is larger than for low permeability cell units.

Preferably, the protrusions contact the respective other plate. Thus, a height of the protrusions defines a gap height between the support plate and the interconnector plate.

In preferred embodiments, the interconnector plate and/or the support plate, preferably the interconnector plate, has a concavity and the protrusions are arranged within said concavity.

The protrusions may extend along the stacking direction. Thus, the height of a protrusion may mean the extension of the protrusion along the stacking direction. The protrusions may be provided as dimples. The protrusions may be formed by pressing the interconnector plate.

In preferred embodiments, the support plate has a periphery and a central portion surrounded by the periphery, wherein the interconnector plate has a periphery and a central portion surrounded by the periphery, wherein the support plate and the interconnector plate are stacked upon one another along the stacking direction, wherein the periphery of the support plate is (preferably directly and) sealingly attached to the periphery of the interconnector plate, wherein the central portion of the support plate and the central portion of the interconnector plate enclose a fluid volume (cell volume) therebetween, wherein the central portion of the interconnector plate has a structured area with a plurality of protrusions formed therein, said protrusions extending into the fluid volume enclosed between the support plate and the interconnector plate and defining fluid passageways therebetween. In such embodiments, preferably for high permeability cell units an average height of said protrusions is larger than for low permeability cell units. Preferably, the central portion of the support plate has a porous region and the cell chemistry layers are disposed on a surface of said porous region that is facing away from the interconnector plate. Preferably, the central portion of the interconnector plate has a concavity, wherein the structured area is within said concavity.

In some embodiments, the cell units are fuel cell units. In some embodiments, the cell units are electrolyser cell units. Preferably, the cell units are solid oxide cell units (i.e. , solid oxide fuel cell units or solid oxide electrolyser cell units). Preferably, the cell units are metal-supported cell units.

The cell units may be flat or planar. The cell units may each comprise multiple layers or plates overlying each other. Preferably, each cell unit comprises electrochemically active layers (also referred to as cell chemistry layers). The electrochemically active layers may comprise a fuel electrode layer, an electrolyte layer and an air or oxidant electrode layer. The electrochemically active layers may be deposited (e.g. as thin coatings or films) on and supported by the support plate, e.g. by a metal support plate, such as a metal foil. Preferably, the fluid inlet port and the fluid outlet port of a respective cell unit are in fluid communication with said electrochemically active layers of the cell unit, preferably via the fluid volume.

In some embodiments, the electrochemical cell assembly comprises a housing surrounding the stack of cell units around the stacking direction to define or enclose a fluid volume. The housing may be a stack enclosure defining a fluid volume containing the stack of cell units. The housing may be welded to the first and second end plates. The housing, the first end plate and the second endplate together may form a stack enclosure defining a fluid volume containing the stack of cell units.

In embodiments comprising a housing, the electrochemical cell assembly preferably comprises a fluid inlet port for supplying fluid, preferably air or oxidant, from the exterior of the electrochemical cell assembly to the fluid volume enclosed by the housing, and a fluid outlet port for removing fluid, preferably exhaust air or oxidant, from the fluid volume. Preferably, the electrochemical cell assembly comprises an air or oxidant inlet port and an air or oxidant outlet port. Preferably, between adjacent cell units, there is provided a fluid flow path, preferably air or oxidant flow path, for fluid, preferably air or oxidant, to flow from the fluid inlet port to the fluid outlet port. The fluid inlet port and the fluid outlet port may be formed by a respective through-hole formed in the first or second end plate. In preferred embodiments, the cell units extend between the fluid inlet port and the fluid outlet port in the first direction perpendicular to the stacking direction. The invention also relates to a method of preparing a stack of cell units for use in an electrochemical cell assembly, preferably for use in an electrochemical cell assembly as described above. The invention also relates to a method of preparing an electrochemical cell assembly.

According to a first aspect, there is provided a method of preparing (i.e. , manufacturing) a stack of cell units comprising the steps of providing a plurality cell units having different permeabilities and stacking said plurality of cell units on top of each other along a stacking direction to form a stack of cell units, wherein the cell units are stacked such that an average fluid permeability of the cell units in the upper half of the stack is higher than an average fluid permeability of the cell units in the lower half of the stack. Thus, the cell units are stacked such that at least one cell unit having a higher fluid permeability is located on top of a cell unit having a lower fluid permeability.

Preferably, the plurality of cell units are stacked on top of each other such that for a majority of the cell units, preferably for at least 70% of the cell units, more preferably for at least 80% of the cell units, more preferably for at least 90% of the cell units, more preferably for at least 95 % of the cell units, a fluid permeability of the cell units is the same or increases from cell unit to cell unit in the stacking direction. In some embodiments, the plurality of cell units are stacked on top of each other such that for at least 95%, preferably at least 98%, of the cell units a fluid permeability of the cell units increases from cell unit to cell unit in stacking direction.

In some embodiments, the step of providing the plurality cell units having different permeabilities comprises the substeps of providing a plurality of cell units and modifying the fluid permeability of at least some of said cell units to provide cell units having lower and/or higher permeabilities.

In some embodiments, each cell unit has a fluid inlet port and a fluid outlet port, wherein each cell unit defines an inner fluid flow path for fluid to flow from its fluid inlet port to its fluid outlet port, wherein each cell units encloses an inner fluid volume, said fluid volume being in fluid communication with the fluid inlet port and the fluid outlet port, wherein a fluid guiding structure, preferably fluid guiding insert, is provided around the fluid inlet port and/or around the fluid outlet port, said fluid guiding structure defining channels for conveying fluid between the assigned fluid port and the fluid volume in a lateral direction, wherein at least some of said channels are closed, preferably by a respective web, preferably at an end proximal to the fluid port. In such embodiments, preparing cell units having different permeabilities may comprise selectively opening one or more or all of said initially closed channels. Advantageously, this may comprise removing one or more or all of said webs, for example by punching or laser cutting.

According to a second aspect, there is provided a method of preparing a stack of cell units comprising providing a plurality of cell units, wherein each cell unit has a fluid inlet port and a fluid outlet port, and defines an inner fluid flow path for fluid to flow from its fluid inlet port to its fluid outlet port; characterising each of said plurality of cell units with respect to a fluid permeability along its fluid flow path; stacking said plurality of cell units on top of each other in a stacking direction, such that an average fluid permeability of the cell units in the upper half of the stack is higher than an average fluid permeability of the cell units in the lower half of the stack.

In some embodiments, characterising the plurality of cell units with respect to the fluid permeability comprises determining the fluid permeability of each cell unit.

In some embodiments, characterising the plurality of cell units with respect to the fluid permeability comprises measuring a pressure difference between the fluid inlet port and the fluid outlet port and measuring a mass flow through the fluid inlet port. Measuring the pressure difference between the fluid inlet port and the fluid outlet port may comprise measuring a pressure at the fluid inlet port and measuring a pressure at the fluid outlet port, for example by pressure sensors known in the art. Measuring the mass flow through the fluid inlet port may comprise measuring, for example by a flow meter known in the art, a mass flow at a position upstream of the fluid inlet port, e.g. in a supply line connected to the fluid inlet port.

Alternatively or in addition, characterising the plurality of cell units with respect to the fluid permeability may comprise characterising a geometric shape of the cell units.

In some embodiments, the method further comprises, after characterising the plurality of cell units with respect to a fluid permeability and before stacking the cell units, classifying said plurality of cell units into two or more permeability-classes, each permeability class comprising cell units having a fluid permeability within a specific permeability range of said permeability class. Thus, cell units having a fluid permeability in a specific permeability range are grouped into the same permeability class. The permeability ranges of the permeability classes may overlap or may not overlap.

In such embodiments, the plurality of cell units may be stacked in stacking direction such that cell units of higher permeability classes (i.e. permeability classes comprising cell units having a relatively high fluid permeability) are stacked on top of cell units of lower permeability classes (i.e. permeability classes comprising cell units having a relatively low fluid permeability).

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

In the drawings:

Fig. 1 shows a simplified perspective view of an exemplary embodiment of an electrochemical cell assembly;

Fig. 2 shows a cross-sectional view of the electrochemical cell assembly according to Fig. 1;

Fig. 3 shows a perspective exploded view of an exemplary cell unit;

Fig. 4 shows a simplified perspective section of an electrochemical cell assembly comprising cell units according to Fig. 3;

Fig. 5 shows a cross-sectional view of an example of a high permeability cell unit;

Fig. 6 shows a cross-sectional view of an example of a low permeability cell unit;

Fig. 7 shows a perspective view of a detail of a cell unit comprising a fluid guiding structure; and

Fig. 8 shows a perspective view of a detail of a cell unit comprising a fluid guiding insert; and

Fig. 9 shows a top view of an exemplary fluid guiding insert. Repeat use of reference symbols in the present specification and drawings is intended to represent the same or analogous features or elements.

Referring to Figures 1 and 2, there is shown an exemplary configuration of an electrochemical cell assembly 10. Figures 1 and 2 are intended to primarily provide an overview of the electrochemical cell assembly 10 and its components in general by way of example. The invention, however, is not limited to this specific design.

The electrochemical cell assembly 10 comprises a first end plate 12, a second end plate 14, and a stack 16 of cell units 18 (also referred to as ‘cell repeat units’) arranged between the first end plate 12 and the second end plate 14.

Specifically, the first end plate 12 and the second end plate 14 are arranged on opposite sides of the stack 16 and the stack 16 is arranged in a receiving volume defined between the first end plate 12 and the second end plate 14. In this example, the first end plate 12 forms a lower end plate or base plate of the cell assembly 10. The second end plate 14 forms a top plate of the cell assembly 10.

Referring to Figure 2, it can be seen that the stack 16 comprises a plurality of cell units 18 that are stacked upon each other along a stacking direction 20. The cell units 18 extend in a respective cell plane that is perpendicular to the stacking direction 20 in a first direction 22 and in a second direction 24 perpendicular to the first direction 22. The cell units 18 are electrically connected in series.

In the example, the electrochemical cell assembly 10 further comprises optional insulation plates 26 located between the end plates 12, 14 and the stack 16 of cell units 18. The electrochemical cell assembly 10 preferably also comprises a current collection or delivery system, known in the art (not shown). For example, the electrochemical cell assembly 10 may comprise one or more current collector plates, e.g. provided between the insulation plates 26 and the stack 16 of cell units 18. In addition, the current collection or delivery system may comprise one or more electrical connection members, such as busbars, for electrically connecting said current collector or delivery plates.

Preferably, the electrochemical cell assembly 10 further comprises a housing (not shown) circumscribing the stack 16 of cell units 18 around the stacking direction 20. In preferred examples, the housing is formed from metal, preferably steel. The housing may be welded to the end plates 12, 14. As schematically illustrated in Figure 2, the stack 16 of cell units 18 comprises a fluid inlet manifold 28 and a fluid outlet manifold 30. The fluid inlet manifold 28 is configured to supply fluid, preferably fuel, to the cell units 18 in an inflow direction 32, in the example parallel to the stacking direction 22. The fluid outlet manifold 30 is configured to remove exhaust fluid, preferably exhaust fuel, from the cell units 18 in an outflow direction 34, in the example opposite the inflow direction 32.

The fluid inlet manifold 28 and the fluid outlet manifold 30 are fluidically connected via the cell units 18. Specifically, each cell unit 18 defines an inner fluid flow path 36 (schematically illustrated in Figure 2 for some of the cell units 18) for fluid to flow from the fluid inlet manifold 28 to the fluid outlet manifold 30. Thus, during use of the electrochemical cell assembly 10, fluid, in particular fuel, may flow along the fluid inlet manifold 28 in stacking direction 20 up, circulate through the cell units 18, and then flow down against the stacking direction 20 along the fluid outlet manifold 30.

Specifically, each cell unit 18 has a fluid inlet port 38, preferably in the form of a through-hole 74, and a fluid outlet port 40, preferably in the form of a through-hole 74 (further details see below). The fluid inlet port 38 is in fluid communication with the fluid inlet manifold 28. The fluid outlet port 40 is in fluid communication with the fluid outlet manifold 30. Preferably, the fluid inlet port 38 forms a section of the fluid inlet manifold 28 and the fluid outlet port 40 forms a section of the fluid outlet manifold 30.

Preferably, each cell unit 18 defines an inner fluid volume 42 defining the inner fluid flow path 36 or at least a section of the inner fluid flow path 36. Said fluid volume 42 is in fluid communication with the fluid inlet manifold 28 via the fluid inlet port 28 of the cell unit 18 and in fluid communication with the fluid outlet manifold 30 via the fluid outlet port 40 of the cell unit 18. Preferably, the fluid volume 42 is in communication with electrochemically active layers of the cell unit 18 (details see below).

As shown in Figure 2, the first end plate 12 comprises a fluid access port 44 for supplying fluid from an exterior of the cell assembly 10 to the fluid inlet manifold 28, and a fluid exhaust port 46 for discharging exhaust fluid from the fluid outlet manifold 30 to the exterior. In the example, the fluid access port 44 and the fluid exhaust port 46 are each formed by a respective through-hole 48, 50 formed in the first end plate 12. As set out above, each cell unit 18 has a fluid permeability along the fluid flow path 36 from the fluid inlet port 38 to the fluid outlet port 40. According to the invention, the cell units 18 are configured and stacked along the stacking direction 20 such that an average fluid permeability of the cell units 18 in the upper half 52 of the stack 16 of cell units 18 is larger than an average fluid permeability of the cell units 18 in the lower half 54 of the stack 16 of cell units 18 (further details below).

In the following, an exemplary configuration of the cell units 18 will be described with reference to Figures 3 to 5.

As set out above, the cell units 18 may be fuel cell units, electrolyser cell units or reversible cell units. In the example, the cell units 18 are metal-supported solid oxide fuel cells.

Each cell unit 18 exemplarily comprises an interconnector plate 56 (also referred to as interconnect or separator plate) and a support plate 58 (also referred to as substrate), which are stacked upon each other along the stacking direction 20. The interconnector plate 56 and the support plate 58 are formed from metal, preferably stainless steel.

Referring to Figure 3, it can be seen that the interconnector plate 56 and the support plate 58 each have a periphery 60, 62 and a central portion 64, 66 surrounded by the periphery 60, 62. The interconnector plate 56 and the support plate 58 are attached to each other at their peripheries 60, 62, preferably by welding, to enclose a fluid volume 68 therebetween (see Figure 5).

The support plate 58, in its central portion 64, carries the electrochemically active layers 70 over a porous region 72 (see Fig. 5). In the example, the support plate 58 is a flat component.

In the example, the interconnector plate 56 is tub-shaped having flanged perimeter features 69 around its periphery 60, preferably formed by pressing the interconnector plate 56 to a concave configuration. As can be seen from Figure 5, the flanged perimeter features 69 extend out of the predominant plane of the interconnector plate 56 to create a concavity 71 (and a convexity to the outside surface) in the interconnector plate 56. In the assembled state of the cell unit 18, said concavity 71 forms the fluid volume 68 enclosed between the interconnector plate 56 and the support plate 58. The fluid volume 68 is in fluid communication with the electrochemically active layers 70 via said porous region 72.

As shown in Figures 3 and 5, the interconnector plate 56 has a structured area 80 in its central portion 64, in particular within said concavity 71 , having shaped inward protrusions 82 extending into the fluid volume 68. The inward protrusions 82 form a supporting structure helping to maintain the fluid volume 68 (cell space) open. The inward protrusions 82 define fluid passageways 83 therebetween. Preferably, the inward protrusions 82 abut the support plate 58.

As set out in detail below, by tuning a depth 85 of said concavity 71 , and thus a height 84 those protrusions 82, the fluid permeability of the cell unit 18 may be tuned.

In the example, the interconnector plate 56, in its structured area 80, further comprises shaped outward protrusions 86. Said outward protrusions 86 form contact portions of the cell unit 18 for contacting an adjacent cell unit 18, specifically the electrochemically active layers 70 of an adjacent cell unit 18 (see Figure 4). More specifically, the outward protrusions 86 engage at their ends against an outer surface of the electrochemically active layer 70 of an adjacent cell unit 18. The outward protrusions 86 define fluid passageways 88 for air or oxidant to flow between adjacent cell units 18.

The outward protrusions 86 and the inward protrusions 82 are - for simplicity - shown as cubes, but may have other cross-sectional shapes such as pyramids, flattopped pyramids, cones, domes or bumps. The outward protrusions 86 and the inward protrusions 82 may be formed by dimples.

The outward protrusions 86 and the inward protrusions 82 may be formed in the interconnector plate 12 by pressing or forming the interconnector plate 12.

In order to supply fluid, in particular fuel, to the fluid volume 68 between the interconnector plate 56 and the support plate 58 (and thus to the electrochemically active layers 70) or to discharge fluid from the fluid volume 68, the interconnector plate 56 and the support plate 58 each have respective through-holes 74 formed therein. In the assembled state, the through-holes 74 formed in the interconnector plate 56 and the through-holes 74 formed in the support plate 58 are aligned, wherein a pair consisting of one through-hole 74 in the interconnector plate 56 and the associated through-hole 74 in the support plate 58 forms an above-mentioned fluid port 38, 40 of the cell unit 18. In the specific example, each cell unit 18 comprises two fuel inlet ports 38 and two fuel outlet ports 40 (in Figures 2 and 4 only one fuel inlet port 38 and one fuel outlet port 40 are visible).

Thus, within each cell unit 18, fuel flows from the two fuel inlet ports 38 to the fuel outlet ports 48 via the fluid volume 68, thus forming the above-mentioned fluid flow path 36. In the example, a net fuel flow direction extends along the first direction 22. In practice, there will be multiple flow paths through the cell volume 68, and they may be straight or convoluted, dependent upon the design of the cell unit 18.

As can be seen in Figure 4, the fluid ports 38, 40 of adjacent cell units 18 are aligned such that they overlie along the stacking direction 20.

In the example, the stack 16 of cell units 18 further comprises gaskets 76 that are interposed between the cell units 18 and surround the fluid ports 38, 40 (through- holes 74) of the cell units 18. Specifically, each fluid port 38, 40 is associated with a gasket 76 on both sides of the cell unit 18. Preferably, the gaskets 76 are annular sealing rings having a central opening 78. The gaskets 36 may be formed from a vermiculite material.

As shown in Figures 2 and 4, an aligned column of the gasket openings 78 of the gaskets 76 and the through-holes 74 (fluid ports 38, 40) of the cell units 18 forms a respective fluid manifold 28, 30. As set out above, each fluid manifold 28, 30 is in fluid communication with, preferably coaxially to, an assigned through-hole 48, 50 in the first end plate 12 (see Fig. 4).

As set out above, by tuning the depth 85 of the concavity and thus the height 84 of the protrusions 82, the permeability of a cell unit 18 may be changed. Exemplarily, Figure 5 shows a high permeability cell unit 90 and Figure 6 shows a low permeability cell unit 92. When comparing Figures 5 and 6, it can be seen that a depth 85 of the concavity and thus a height 84 of the inward protrusions 82 is larger for the high permeability cell unit 90 (see Fig. 5) than for the low permeability cell unit 92 (see Fig. 6). Thus, for the high permeability cell unit 90, a flow cross-section through the fluid passageways 83 is increased, leading to a lower flow resistance (and thus to a higher fluid permeability) for fluid to flow through the fluid volume 68. According to a general aspect illustrated in Figure 7, at least some of the fluid ports 38, 40 of a cell unit 18, preferably each fluid port 38, 40 (through-hole 74), may be surrounded by a fluid guiding structure 94.

Figure 7 shows a detail of a cell unit 18 in the area of the fluid inlet port 38 (the fluid outlet port 40 may be configured identically) in a partial cut-away view, wherein a fluid guiding structure 94 is provided between the support plate 58 and the interconnector plate 56 of the cell unit 18. Exemplarily, the fluid guiding structure 94 is provided in the form of a fluid guiding insert 96 that is interposed between the support plate 58 and the interconnector plate 56. In other embodiments, the fluid guiding structure 94 may be integrally formed with the support plate 58 or the interconnector plate 56.

As shown in Figure 7, the fluid guiding structure 94 (fluid guiding insert 96) defines radially extending channels 98. The channels 98 are configured to convey fluid between the fluid port 38,40 and the fluid volume 68 in a lateral direction.

Specifically, in case of a fluid inlet port 38, the channels 98 are configured to convey fluid that is supplied to the fluid inlet port 38 via the fluid inlet manifold 28 to the fluid volume 68. In case of a fluid outlet port 40, the channels 98 are configured to collect fluid from the fluid volume 68 and guide it to the fluid outlet port 40 (and thus to the fluid outlet manifold 30).

As set out above, alternatively or in addition to varying the height 84 of the inward protrusions 82 or the depth 85 of the concavity 71, the fluid permeability of a cell unit 18 may be tuned by tuning a geometry of said fluid guiding structure 94. For example, by varying a geometry of the channels 98, a fluid permeability of the cell unit 18 may be varied.

According to a first aspect, during manufacture of the electrochemical cell assembly 10, cell units 18 having fluid guiding structures 94 of different geometries may be provided (i.e. cell units 18 having different permeabilities may be provided) and said cell units 18 may be stacked on top of each other such that a permeability increases from cell unit 18 to cell unit 18 in stacking direction 20.

According to a second aspect illustrated in Figure 8, in an initial state, at least some of the channels 98, in the example all channels 98, may be closed at an end 100 proximal to the fluid port 38 by a respective web 102. Thus, in an initial state, the fluid port 38, 40 may be fluidically decoupled from the fluid volume 68. In a subsequent process step, at least some of the webs 102 may be cut off, e.g. by means of a laser, so that fluid can flow between the fluid port 38, 40 and the fluid volume 68 (a possible cut line 108 is schematically shown in Figure 9). Thus, depending on the number of channels 98 opened, cell units 18 having higher or lower fluid permeability may be provided. Accordingly, a high permeability cell unit 90 may have more channels 98 opened than a low permeability cell unit 92.

Exemplarily, the fluid guiding insert 96 may be provided by a disc 104 having a central opening 106 (see Figure 9). The disc 104 may be a metal disc. The disc 104 may be welded to either or both the support plate 58 and the interconnector plate 56.

Claims

1. An electrochemical cell assembly (10), preferably fuel cell assembly or electrolyser cell assembly, comprising a stack (16) of cell units (18) having a plurality of cell units (18) stacked upon one another in a stacking direction (20), wherein: in said stack (16) of cell units (18), there is provided a fluid inlet manifold (28) for supplying fluid to the cell units (18) in an inflow direction (32) and a fluid outlet manifold (30) for removing fluid from the cell units (18) in an outflow direction (34) , said fluid inlet manifold (28) and said fluid outlet manifold (20) each extend through the stack (16) of cell units (18), each cell unit (18) has a fluid inlet port (38) being in communication with the fluid inlet manifold (28) and a fluid outlet port (40) being in communication with the fluid outlet manifold (30), each cell unit (18) defines an inner fluid flow path (36) for fluid to flow from the fluid inlet port (38) to the fluid outlet port (40), each cell unit (18) has a fluid permeability along its fluid flow path (36) from the fluid inlet port (38) to the fluid outlet port (40), an average fluid permeability of the cell units (18) in an upper half (52) of the stack (16) is higher than an average fluid permeability of the cell units (18) in a lower half (54) of the stack (16).

2. The electrochemical cell assembly (10) according to claim 1 , wherein the average fluid permeability of the cell units (18) in the upper half (52) of the stack (16) of cell units (18) is at least 2.5 %, preferably at least 5%, more preferably at least 7.5%, more preferably at least 10%, higher than the average fluid permeability of the cell units (18) in the lower half (54) of the stack (16) of cell units (18).

3. The electrochemical cell assembly (10) according to claim 1 or 2, wherein the upper half (52) of the stack (16) of cell units (18) comprises at least one high permeability cell unit (90) and the lower half (54) of the stack (16) of cell units (18) comprises at least one low permeability cell unit (92), wherein a high permeability cell unit (90) has a higher fluid permeability than a low permeability cell unit (92).

4. The electrochemical cell assembly (10) according to the preceding claim, wherein the fluid permeability of a high permeability cell unit (90) is at least 0.5%, preferably at least 1 %, preferably at least 2.5%, preferably at least 5%, higher than the fluid permeability of a low permeability cell unit (92).

5. The electrochemical cell assembly (10) according to any one of the preceding claims, wherein the cell units (18) are stacked upon one another such that at least for a majority of cell units (18) the fluid permeability of the cell units (18) is the same or increases from cell unit (18) to cell unit (18) in the stacking direction (20).

6. The electrochemical cell assembly (10) according to any one of the preceding claims, wherein the cell units (18) in the upper half (52) of the stack (16) of cell units (18) are relatively further from a supply to the fluid inlet manifold (28) than the cell units (18) in the lower half (54) of the stack (16) of cell units (18), and/or wherein the cell units (18) in the upper half (52) of the stack (16) of cell units (18) are relatively further from an exhaust from the fluid outlet manifold (30) than the cell units (18) in the lower half (54) of the stack (16) of cell units (18).

7. The electrochemical cell assembly (10) according to any one of the preceding claims, further comprising an end plate, preferably first and second end plates (12, 14), wherein the stack (16) of cell units (18) is arranged on said end plate, preferably between said first end plate (12) and said second end plate (14), wherein the end plate, preferably at least the first end plate (12), comprises a fluid access port (44) for supplying fluid from an exterior of the cell assembly (10) to the fluid inlet manifold (28), and a fluid exhaust port (46) for discharging exhaust fluid from the fluid outlet manifold (30) to the exterior of the cell assembly (10).

8. The electrochemical cell assembly (10) according to any one of the preceding claims, wherein each fluid port (38, 40) is formed by a respective through-hole (74) formed in the cell unit (18), said through-hole (74) extending through the cell unit (18) along the stacking direction (20), wherein the through-hole (74) forming the fluid inlet port (38) of a cell unit (18) forms a section of the fluid inlet manifold (28) and the through-hole (74) forming the fluid outlet port (40) of a cell unit (18) forms a section of the fluid outlet manifold (30).

9. The electrochemical cell assembly (10) according to any one of the preceding claims, wherein each cell unit (18) encloses an inner fluid volume (68) defining at least a section of the inner fluid flow path (36), wherein said inner fluid volume (68) is in fluid communication with the fluid inlet manifold (28) via the fluid inlet port (38) of the cell unit (18) and in fluid communication with the fluid outlet manifold (30) via the fluid outlet port (40) of the cell unit (18).

10. The electrochemical cell assembly (10) according to the preceding claim, wherein the fluid inlet port (38) and/or the fluid outlet port (40) of at least some of the cell units (18) is surrounded by a fluid guiding structure (94), said fluid guiding structure (94) defining a fluid pathway for conveying fluid between the assigned fluid port (38, 40) and the fluid volume (68).

11. The electrochemical cell assembly (10) according to the preceding claim, wherein a flow cross-section of said fluid pathway is higher for high permeability cell units (90) than for low permeability cell units (92).

12. The electrochemical cell assembly (10) according to claim 10 or 11 , wherein the fluid guiding structure (94) has radially extending channels (98) defining said fluid pathway.

13. The electrochemical cell assembly (10) according to the preceding claim, wherein for high permeability cell units (90) a number of said channels (98) and/or a total flow cross-section of said channels (98) is larger than for low permeability cell units (92).

14. The electrochemical cell assembly (10) according to any of claims 9 to 13; wherein each cell unit (18) comprises a support plate (58) and an interconnector plate (56), said support plate (58) and said interconnector plate (56) overlying one another in stacking direction (20), wherein the fluid volume (68) is enclosed between the support plate (58) and the interconnector plate (56).

15. The electrochemical cell assembly (10) according to the preceding claim when referring to claim 10, wherein the fluid guiding structure (94) is formed by a fluid guiding insert (96), said fluid guiding insert (96) being disposed between the support plate (58) and the interconnector plate (56), preferably in the fluid volume (68).

16. The electrochemical cell assembly according to any one of claims 14 to 15, wherein the interconnector plate (56) and/or the support plate (58), preferably the interconnector plate (56), has a concavity (71), said concavity defining the fluid volume (68), wherein for high permeability cell units (90) a depth (85) of said concavity (71) is larger than for low permeability cell units (92).

17. The electrochemical cell assembly according to any one of claims 14 to 16, wherein either or both the support plate (58) and the interconnector plate (56) have a structured area (80), preferably within said concavity (71), with a plurality of protrusions (82) formed therein, said protrusions (82) extending into the fluid volume (68) enclosed between the support plate (58) and the interconnector plate (56) and defining fluid passageways (68) therebetween, wherein for high permeability cell units (90) an average height (84) of said protrusions (82) is larger than for low permeability cell units (92).

18. Method of preparing a stack (16) of cell units (18) for use in an electrochemical cell assembly (10), comprising providing a plurality of cell units (18), wherein at least some of said plurality of cell units (18) have different fluid permeabilities, stacking said plurality of cell units (18) on top of each other along a stacking direction (20) such that an average fluid permeability of the cell units (18) in an upper half (52) of the stack (16) is higher than an average fluid permeability of the cell units (18) in a lower half (54) of the stack (16).

19. The method according to the preceding claim, wherein providing the cell units (18) having different fluid permeabilities comprises providing a plurality of cell units (18) and modifying the fluid permeability of at least some of said cell units (18).

20. The method according to the preceding claim, wherein each cell unit (18): has a fluid inlet port (38) and a fluid outlet port (40), defines an inner fluid flow path (36) for fluid to flow from its fluid inlet port (38) to its fluid outlet port (40), and encloses an inner fluid volume (68), said fluid volume (68) being in fluid communication with the fluid inlet port (38) and the fluid outlet port (40), wherein around the fluid inlet port (38) and/or around the fluid outlet port (40), there is provided a fluid guiding structure (94), preferably fluid guiding insert (96), said fluid guiding structure (94) defining channels (98) for conveying fluid between the assigned fluid port (38, 40) and the fluid volume (68) in a lateral direction, wherein at least some of said fluid channels (98) are closed, preferably by a respective web (102), wherein modifying the cell units (18) comprises selectively opening one or more or all of said initially closed channels (98), preferably by removing one or more or all of said webs (102).

21. The method according to any of claims 18 to 20, wherein the plurality of cell units (18) are stacked such that a fluid permeability of the cell units (18) increases from cell unit (18) to cell unit (18) in stacking direction (20).

22. Method of preparing a stack (16) of cell units (18), comprising: providing a plurality of cell units (18), wherein each cell unit (18) has a fluid inlet port (38) and a fluid outlet port (40), and defines an inner fluid flow path (36) for fluid to flow from its fluid inlet port (38) to its fluid outlet port (40); characterising each of said plurality of cell units (18) with respect to a fluid permeability along its fluid flow path (36); stacking said plurality of cell units (18) on top of each other in a stacking direction (20), such that an average fluid permeability of the cell units (18) in the upper half (52) of the stack (16) is higher than an average fluid permeability of the cell units (18) in the lower half (54) of the stack (16).