Reinforced porous silicon membrane

Abstract

The invention provides a separator (1) comprising a first separator side (2), a second separator side (3), a porous membrane (100) and a reinforcement support element (200), wherein (i) the membrane (100) comprises a silicon comprising layer (110) having a first layer side (101) and a second layer side (102), wherein a distance between the first layer side (101) and the second layer side (102) defines a membrane thickness (d mem), (ii) the membrane comprises pores (140), the pores (140) defining open fluid channels from the first layer side (101) to the second layer side (102), (iii) a porosity of the membrane (100) is in the range 1- 10%, (iv) a mean pore cross-sectional dimension (dp) is equal to or less than 10 µm, (v) the membrane thickness (dmem) is equal to or less than 200 µm, and (vi) the membrane comprises a first layer (121) arranged at the first layer side (101) and a second layer (122) arranged at the second layer side (102), wherein the first layer (121) is electrically conductive, and the second layer (122) is electrically conductive, wherein the first layer (121) directly contacts the silicon comprising layer (110), and the second layer (122) directly contacts the silicon comprising layer (110), (vii) the reinforcement support element (200) is configured from the first separator side (2) to the second separator side (3), wherein the reinforcement support element (200) is connected to the silicon comprising layer (110), wherein the reinforcement support element (200) extends from the separator (1) at at least one side selected from the first separator side (2) and the second separator side (3), and (viii) a maximal distance between the first separator side (2) and the second separator side (3), and defined perpendicular to a virtual plane (P) of the silicon comprising layer (110), defines a separator thickness (ds), wherein the separator thickness (ds) is selected from the range of 50 µm – 3 cm.

Description

[0001] REINFORCED POROUS SILICON MEMBRANE

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a separator comprising a reinforced zero-gap porous membrane with defined pore size and porosity. The invention further relates to the alkaline water electrolysis using the separator.

[0004] BACKGROUND OF THE INVENTION

[0005] Alkaline water electrolysis methods and systems are known in the art. US20230143558 Al, e.g., describes a membrane-electrode-gasket assembly for alkaline water electrolysis, the assembly including: a separating membrane having first and second membrane faces; a first electrode arranged in contact with the first membrane face; and an insulating gasket holding the membrane and the electrode as one body, the gasket including: first and second faces for contacting with anode- and cathode-side frames respectively; a slit part opening toward an inner side of the gasket and receiving the entire peripheries of the membrane and the electrode; first and second parts facing with each other across the slit part; and a continuous part arranged on an outer periphery side of the slit part, uniting the first and second parts into one body, and sealing an outer periphery end of the slit part, wherein the first and second parts sandwich therebetween to hold the entire peripheries of the membrane and the electrode received in the slit part into one body. Further, US9011651B2 describes an apparatus for the electrolytic splitting of water into hydrogen and / or oxygen, the apparatus comprising: (i) at least one lithographically-patternable substrate having a surface; (ii) a plurality of microscaled catalytic electrodes embedded in said surface; (iii) at least one counter electrode in proximity to but not on said surface; (iv) means for collecting evolved hydrogen and / or oxygen gas; (v) electrical powering means for applying a voltage across said plurality of microscaled catalytic electrodes and said at least one counter electrode; and (vi) a container for holding an aqueous electrolyte and housing said plurality of microscaled catalytic electrodes and said at least one counter electrode. Electrolytic processes using the above electrolytic apparatus or functional mimics thereof are also described.

[0006] SUMMARY OF THE INVENTION

[0007] Water electrolysis offers a clean method to produce hydrogen when connected to renewable energy sources such as wind and solar power but only accounted for ~ 0.1 % of dedicated hydrogen production in 2020 (not including by-product hydrogen from chlor-alkali plants). Heavily polluting processes such as coal gasification and steam methane reforming (SMR) continue to be the dominant sources of hydrogen supply. The carbon footprint of renewable energy-powered water electrolysis is a fraction of that associated with SMR. Therefore, water electrolysis may be expected to play a critical role in reaching global climate change mitigation goals. However, hydrogen production from water electrolysis must reach cost parity with hydrogen from fossil fuels for large-scale adoption.

[0008] Alkaline water electrolysis and proton exchange membrane electrolysis are the two main low-temperature electrolysis technologies with wide commercial applications. Proton exchange membrane electrolyzers (PEMEs) may typically offer higher efficiencies and operate at a higher current density. However, the efficiency of PEMEs degrades twice as fast as alkaline water electrolyzers (AWEs) - nullifying their higher beginning-of-life efficiency. Furthermore, PEMEs use expensive titanium-based components, iridium-based anodes, and platinum-based cathodes. AWEs, on the other hand, may utilize cheaper non-platinum group metal (non-PGM) electrodes such as iron, cobalt, and nickel. AWEs may typically be cheaper to set-up and operate and offer larger stack sizes than PEMEs. This may make AWEs more suitable for the large-scale water electrolysis plants needed to achieve gigawatt-scale production. Nevertheless, improvements in AWE efficiency are crucial to reducing stack dimensions and, consequently, material costs.

[0009] A key development in AWEs may be the adoption of a zero-gap electrode architecture aimed at lowering the Ohmic overpotential by reducing the interelectrode distance, allowing the electrolyzer to operate at a higher current density. Zero-gap electrodes (ZGEs) are typically made by compressing two electrodes on either side of a porous separator. The interelectrode distance in a ZGE configuration is substantially equal to the thickness of the membrane or the separator. The Ohmic resistance of the cell may therefore be approximately equal to the Ohmic resistance of the separator alone. However, this is not the case for AWEs separators, such as Zirfon, where the Ohmic resistance of the cell has been found to be almost twice the resistance of the separator alone. Contrary to the expectation that the adoption of a zero-gap configuration would reduce losses due to bubbles, it has been shown that the additional Ohmic penalty in ZGEs arises from the formation of gas bubbles in the gap between the electrodes and the membrane. It has been proposed that a 0.2 mm gap may be used to mitigate a large part of the bubble losses in a ZGEs configuration.

[0010] The need to further reduce the resistance of porous separators may be a key step in making AWEs competitive with PEMs in terms of efficiency and operating current density. The resistivity of separators may be minimized by increasing the separator porosity, which essentially represents the percentage of the membrane volume filled with electrolyte. At higher porosities in the separator, ions may have to traverse (migrate) a shorter distance in the bulk before reaching a pore, which may lower the Ohmic loss. However, this may come at the cost of increased gas crossover, giving rise to another critical factor limiting the electrolyzer's maximum current density for safe operation.

[0011] Notably, resistance to gas crossover is expected to depend strongly on the (average) pore size, wherein smaller pores may be better at excluding gas bubbles. Conversely, anions are normally much smaller than gas bubbles - hence, separator resistance may be less sensitive to pore size. Further, other pore effects, such as tortuosity, should not be disregarded. High tortuosity can increase the path length for ionic migration within the separator pores, which may increase Ohmic resistance. On the other hand, tortuous pores might be beneficial for gas crossover inhibition. Tortuous pores may come at the cost of increased fouling, which remains challenge for small and tortuous pores like found in prior art separators.

[0012] A way to minimize gas crossover is the deposition of ionomers inside the pores. However, a key drawback of using ionomers is that they are typically incompatible with silicon- based monolithic microelectromechanical systems (MEMS). They may introduce additional stresses into the devices due to their volumetric expansion in electrolyte solutions. Unlike ionomers, porous silicon (PSi) may have no charged surface moieties which enable conduction, i.e., ionic conductivity may solely be provided by the electrolyte filling the pores. Therefore, prior art may describe strategies to attempt to impart ion conductivity to porous silicon separators, e.g., by silanization to introduce charged surface groups on the pore walls and filling the pores with concentrated liquid electrolytes. Yet, despite the inherent advantage of lower Ohmic losses in silicon separators, due to their small thickness, PSi-based devices may exhibit unsafe gas-crossover levels, severely limiting their applicability.

[0013] Furthermore, prior art membranes may be rather fragile. Pressure differences over the membrane and / or flowing fluids along the membrane may result in fractures in the membrane and eventually failure of the separator.

[0014] Hence, it is an aspect of the invention to provide an alternative separator, which preferably further at least partly obviates one or more of above-described drawbacks. It is a further aspect of the invention to provide an alkaline water electrolyzer, which preferably further at least partly obviates one or more of above-described drawbacks. The present invention may have as object to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. In a first aspect, the invention provides a separator. The separator may in embodiments be configured for an electrochemical cell. The separator may comprise a porous membrane. The porous membrane may especially be a porous silicon membrane. The separator may in further embodiments comprise a plurality of porous membranes. The membrane(s) may in embodiments be reinforced. The separator may further especially comprise a reinforcement support element. The separator may especially comprise a first separator side and a second separator side. Especially, the first separator side is configured opposite to the second separator side. The separator may especially comprise a planar separator. In embodiments, the reinforcement support element may extend from the separator at the first separator side. In further embodiments, the reinforcement support element may (also) extend from the separator at the second side. The membrane (or the plurality of membranes) may be configured recessed in the reinforcement support element, especially at at least one of the two separator sides. The membrane especially comprises a silicon comprising layer. Further, the silicon comprising layer may have, two (especially planar) sides. Herein, these (planar) sides may be referred to as a first layer side and a second layer side. The second layer side and the first layer side are especially configured at opposite sides (of the silicon comprising layer). Moreover, in embodiments, a (minimal) distance between the first layer side and the second layer side defines a membrane thickness (dmem). Further, especially, the membrane comprises pores. The pores especially define channels, especially channels with no interconnects (especially straight channels) from the first (layer) side to the second (layer) side (of the silicon comprising layer and / or of the separator). Further, especially a porosity of the membrane is in the range of 0.01- 50%. In specific embodiments, the porosity of the membrane is selected from the range of 0.1- 25%. The porosity may for instance be in the range of 1-10%. The porosity may especially be defined as a total volume of the pores relative to a total volume of the membrane (including the volume of the pores) comprising the pores. In further embodiments, a mean pore cross-sectional dimension (dp) (especially parallel to the first layer side (and / or second layer side)) is equal to or less than 200 pm, such as equal to or less than 100 pm. In specific embodiments, the mean pore cross-sectional dimension (or “pore size”) is equal to or less than 50 pm, such as equal to or less than 20 pm, even more especially equal to or less than 10 pm. The pore size may especially be at least 1 nm, such as at least 200 nm. In further embodiments, the pore size is equal to or less than 200 pm, such as equal to or less than 100 pm, especially equal to or less than 50 pm. In specific embodiments, the pore size is equal to or less than 10 pm, such as between 1 nm to 10 pm. Further, the membrane thickness (dmem) may in embodiments be equal to or less than 200 pm. In further embodiments, the membrane thickness (or thickness of the silicon comprising layer) is no more than 100 pm, such as no more than 50 pm. The membrane thickness may especially be at least 1 pm, such as at least 5 pm, even more especially at least 10 pm. In specific embodiments, the membrane may (optionally) comprise one or more of a first (planar) layer arranged at (or over) the first layer side and a second (planar) layer arranged at (or over) the second layer side. The (first and / or second) layer may especially be electrically conductive. In embodiments, the first layer may comprise an electrically conductive material. In further embodiments (also) the second layer comprises an electrically conductive material. The first layer may in embodiments directly contact the silicon comprising layer. The second layer may in further embodiments (also) directly contact the silicon comprising layer. In further specific embodiments, the silicon comprising layer is arranged between the first layer and the second layer. The first layer may in specific embodiments define a first electrode. Furthermore, the second layer may in further embodiments defines a second electrode. Hence, in embodiments, the membrane comprises a first and / or a second (conductive) layer. In alternative embodiments, the membrane may not comprise such conductive layer. In further embodiments, the reinforcement support element is connected to the silicon comprising layer. The reinforcement support element is in embodiments configured from the first separator side to the second separator side. The reinforcement support element may especially extend from the separator (relative to the membrane(s)) at at least one side selected from the first separator side and the second separator side. In embodiments a maximal distance between the first separator side and the second separator side may define a separator thickness. The maximal distance (between the two sides) may especially be defined perpendicular to a virtual plane of the silicon comprising layer. In embodiments, the separator thickness may be at least 10 pm, such as at least 50 pm, for instance at least 100 pm, and especially at least 200 pm. The separator thickness may especially be larger than the membrane thickness. Essentially, the separator thickness may have any arbitrarily large value. Yet, the separator thickness may be selected to minimize a total volume of the separator (while having a robust separator). The separator thickness may especially be selected to be no more than 3 cm, such as no more than 2000 pm, and especially no more than 1000 pm. In embodiments. In specific embodiments, the separator thickness is selected from the range of 50 pm - 3 cm. The separator thickness may in specific embodiments be determined by the used method of fabrication, especially by a thickness of a silicon wafer used. In further specific embodiments, the method may (further) comprise removing (such as etching) a part of the thickness of the wafer to reduce the separator thickness.

[0015] In a further aspect, the invention provides a method for fabrication of a separator, especially the separator described herein. The method especially comprises a silicon layer fabrication stage and (subsequently) a covering stage. In embodiments, the silicon layer fabrication stage comprises providing a silicon wafer. The silicon wafer may in embodiments be a silicon-on -insulator wafer. A silicon-on -insulator (“SOI”) wafer may be known to the skilled person. The silicon wafer especially comprises a silicon handle layer (configured) at (especially defining) a first side of the silicon wafer, a silicon device layer (configured) at (especially defining) a second side of the silicon wafer, and an insulating (buried) oxide layer configured between the device layer and the handle layer. In specific embodiments, the silicon wafer may consist of the silicon device layer, the silicon handle layer, and the insulating (buried) oxide layer configured between the device layer and the handle layer. The silicon layer fabrication stage may further comprise providing pits in the device layer (from the second side of the silicon wafer) to provide a patterned silicon wafer. The pits may in embodiments be provided using a photolithographic technique, such as ultraviolet lithography, deep ultraviolet lithography, extreme ultraviolet lithography (EUVL), X-ray lithography, reactive ion etching, deep reactive ion etching, or plasma etching. The pits may in specific embodiments especially be provided in a predetermined membrane zone of the silicon layer. In specific embodiments, the pits are provided by deep reactive ion etching. The silicon layer fabrication stage may further comprise removing the handle layer and the insulating layer, especially at the membrane zone, from the patterned silicon wafer to provide a silicon layer at the membrane zone (especially such that the device layer is isolated from the other layers, especially (substantially only) at the membrane zone). The handle layer and the insulating layer are in embodiments removed (at the membrane zone) such that the pits define channels (or (initial) pores) from a first side of the silicon layer to a second side of the silicon layer (at the membrane zone). In further specific embodiments, the handle layer and the insulating layer are maintained at other locations of the silicon wafer (i.e., locations not comprising the membrane zone). Especially each pit may define a respective channel from the first to the second layer side of the silicon comprising layer (at the membrane zone). Further, especially the other locations may define the reinforcement support element. Further, in embodiments, the covering stage comprises providing, especially depositing, one or more materials selected from the group of silicon nitride, silicon oxide, poly-silicon (polycrystalline silicon), and parylene on the silicon layer, especially wherein the (entire) silicon layer is covered with the one or more materials to provide a silicon comprising layer at the membrane zone comprising pores from a first layer side to a second layer side of the silicon comprising layer. In further embodiments, the depositing stage may comprise providing, especially depositing, one or more materials selected from the group of silicon nitride, silicon oxide, poly-silicon, and parylene on the (entire) silicon wafer (after having removed the handle layer and the insulating layer at the membrane zone), wherein the (entire, especially the remainders of) silicon wafer is covered with the one or more materials to provide a silicon comprising layer at the membrane zone comprising pores from a first layer side to a second layer side of the silicon comprising layer. In specific embodiments, the depositing stage may comprise providing, especially depositing, one or more materials selected from the group of silicon nitride, silicon oxide, poly-silicon, and parylene on the silicon wafer (after having removed the handle layer and the insulating layer at the membrane zone), a total surface area of the first side of the silicon wafer, (of) (ii) the second side of the silicon wafer, and (of) the channels is covered with silicon nitride, to provide a silicon comprising layer at the membrane zone comprising pores from a first layer side to a second layer side of the silicon comprising layer. During depositing the one or more materials, especially a (surface of a) wall of the channel s / initi al pores is also (at least partly) covered with the deposited material. In specific embodiments, the (fabrication) method (further) comprises an electrode provision stage (configured subsequent to the depositing stage). The electrode provision stage may especially comprise providing a first (especially electrically conductive) layer at (a first side of the (treated) silicon wafer comprising) the first layer side of the silicon comprising layer. In further embodiments, the electrode provision stage comprises providing a second (especially electrically conductive) layer at (a second side of the (treated) silicon wafer comprising) the second layer side of the silicon comprising layer.

[0016] The separator may in embodiments be applied as a separator in an electrochemical cell.

[0017] Hence, the invention provides in specific embodiments, a separator comprising

[0018] (i) a first separator side, (ii) a second separator side, (iii) a porous (silicon) membrane, and (iv) a reinforcement support element, wherein (i) the membrane comprises a silicon comprising layer having a first (planar) side and a second (planar) side, wherein a (minimal) distance between the first layer side and the second layer side defines a membrane thickness, wherein

[0019] (ii) the membrane comprises pores, the pores defining open fluid channels from the first layer side to the second layer side, (iii) a porosity of the membrane is in the range of 0.01-50%, such as in the range of 0.1-25%, especially in the range of 1-10%, (iv) a mean pore cross-sectional dimension (parallel to the first layer side) is equal to or less than 200 pm, such as equal to or less than 100 pm, especially equal to or less than 50 pm, even more especially equal to or less than 10 pm, and the membrane thickness is equal to or less than 200 pm, (v) the reinforcement support element is connected to the silicon comprising layer, wherein the reinforcement support element extends from the separator, and (vi) a maximal distance between the first separator side and the second separator side, and defined perpendicular to a virtual plane of the silicon comprising layer, defines a separator thickness, wherein the separator thickness is selected from the range of 50 pm - 3 cm. In further specific embodiments, the membrane optionally comprises (especially, comprises) a first layer arranged at the first layer side and / or (especially, and) a second layer arranged at the second layer side, wherein the first layer is electrically conductive, and the second layer is electrically conductive. Furthermore, in further embodiments also the support element may comprise the first layer and / or the second layer. The first layer and / or the second layer may extend over substantially the entire first separator side and / or the second separator side, in embodiments. Especially, the first layer (when present) directly contacts the silicon comprising layer, and the second layer (when present) directly contacts the silicon comprising layer. Moreover, in embodiments, the first layer (when present) may define a total external surface at the first separator side of the separation element, and the second layer (when present) may define a total external surface at the second separator side of. In further embodiments, the first layer (if present) defines a first electrode, and the second layer (if present) defines a second electrode.

[0020] Additionally or alternatively, the invention provides in embodiments a method for fabrication of a separator, the method (“fabrication method”) comprising a silicon layer fabrication stage, a subsequent covering stage, and optionally an electrode provision stage; wherein (i) the silicon layer fabrication stage comprises: providing a silicon wafer consisting of a silicon handle layer at a first side of the silicon wafer, a silicon device layer at a second side of the silicon wafer, and an insulating (buried) oxide layer configured between the device layer and the handle layer; providing pits in the device layer (especially by deep reactive ion etching) at a predetermined membrane zone of the silicon wafer to provide a patterned silicon wafer; and removing the handle layer and the insulating layer at the membrane zone from the patterned silicon wafer to provide a silicon layer at the membrane zone, wherein the pits (provided by etching) define channels from a first side of the silicon layer to a second side of the silicon layer; wherein (ii) the covering stage comprises depositing silicon nitride on the silicon wafer (10), wherein a total surface area of (i) the first side of the silicon wafer (10), (ii) the second side of the silicon wafer (10), and (iii) the channels (19) is covered with silicon nitride, to provide a silicon comprising layer comprising pores from a first layer side to a second layer side of the silicon comprising layer at the membrane zone; and wherein (iii) the optional electrode provision stage comprises providing a first (electrically conductive) layer at the first layer side of the silicon comprising layer, and providing a second (electrically conductive) layer at the second layer side of the silicon comprising layer. The present invention especially relates to (a fabrication of) robust and stable porous separators / membranes with, in embodiments, relatively small pore sizes, low ionic resistance and gas crossover. Separators with ordered and well-defined straight (or slightly tapering) pores may be fabricated using the invention, e.g., using photolithographic techniques. A deposition of a silicon nitride or of an alternative layer on the porous separator may impart chemical resistance, which may contribute to the stability of the separator in alkaline conditions. The separator is further also especially a reinforced separator which may contribute to the robustness of the separator during use. Moreover, the reinforcement support element (or: “support element”) may be provided in a non-complex way when producing the separator from a (commercially available) wafer. The separator may especially be formed from a (single) wafer, thereby providing a solid connection between the membrane and the support element. The separator, especially the membrane in combination with the support element, may especially comprise a monolithic structure.

[0021] The separator may especially be configured for installing in an electrochemical cell. The membrane may in specific embodiments be referred to a “zero-gap membrane electrode assembly” (“zero-gap MEA” or “zero-gap membrane electrode”). Moreover, the term “membrane” may refer to a plurality of membranes. The separator may in embodiments comprise a plurality of membranes, especially separated from each other by the support element (see further below).

[0022] A zero-gap configuration may be obtained by depositing an electrocatalyst - such as platinum or nickel - directly on either side of the silicon comprising layer. It has experimentally been found that the porosity, which is related to the pore size and spacing between the pores, may have a significant effect on the ionic resistance of the membrane and the purity of the gas produced during alkaline water electrolysis (using the membrane). It appears that a high purity of the produced gases may be obtained at lower porosities, while a lowest ionic resistance may be bee found at higher porosities. Embodiments of the membrane may generate an ionic resistance as low as 0.3 Q cm2or perhaps even lower, albeit compromising Faradaic efficiency. The current invention may especially result in ionomeric- free systems that may rely purely on the separator geometry applied for alkali-water electrolysis.

[0023] The separator may in further embodiments be used in a cell / reactor for separation applications. In yet further embodiments, the separator may be applied for organic electrochemical reactions. The separator may in embodiments be used for alkaline water electrolysis. In further specific embodiments, the electrochemical cell comprises a redox flow battery cell. The separator may be used in a redox flow battery in embodiments. The separator may in specific embodiments be used in an alkaline water electrolyzer, e.g., as described herein.

[0024] The separator may in embodiments be provided using etching techniques (see below). Moreover, the membrane may in embodiments be provided using etching techniques to provide the membrane comprising the pores and to remove parts of the handle layer and the insulating layer. The membrane may especially be configured from the first separator side to the second separator side. In embodiments, the membrane may comprise a continuous (single) membrane. In further embodiments, the membrane may comprise a plurality of (sub) membranes, or membrane areas, (side-by-side) distributed over the separator. The term “membrane” may for instance in embodiments refer to at least two (sub) membranes, such as at least 4 (sub) membranes, especially at least nine (sub) membranes, and in embodiments at least 100 (sub) membranes, especially at least 1000 (sub) membranes, such as at least 10000 (sub) membranes.

[0025] Essentially, the membrane may comprise any arbitrary number of (sub) membranes. A total number of (sub) membranes (or membrane areas) defining the (total) membrane may especially be based on a total area of the (sub) membranes. In embodiments, for instance a characteristic size, such as a diameter or a maximum width or length of a total area of the (sub) membrane may be in the micrometer range, whereas in further embodiments the characteristic size may be in the millimeter range. The characteristic size of the (sub) membrane may especially be selected from the range of 1 pm - 30 cm, such as from the range of 1 pm - 1 cm. The characteristic size of the (sub) membrane may in embodiments be substantially equal to a diameter of a wafer.

[0026] The term “membrane” may refer in embodiments to a plurality of “membranes” and / or to a plurality of “membrane areas”. In specific embodiments the separator comprises at least four membranes, especially at least nine membranes, such as at least 100 membranes, especially at least 1000 membranes, such as at least 10000 membranes, and especially the reinforcement support element may be configured enclosing any (single) one of the membranes. In further embodiments, the (especially any one of the) membranes are configured recessed in the reinforcement support element.

[0027] The support element may in embodiments be provided by removing parts of the handle layer and the insulating layer (especially at locations other than the membrane zone). The support element may in embodiments comprise a continuous support element. The support element may in further embodiments be configured from the first separator side to the second separator side. The support element may further especially enclose the membrane(s). Each single one) of the membrane areas may in embodiments be enclosed (surrounded) by the support element. The support element may especially be configured enclosing any (single) one of the membranes. The support element may in further embodiments be configured extending from one of the two separator sides. The support element may especially extend beyond the membrane (areas) at at least one of the two separator sides. The support element may in further embodiments be aligned with one of the first (membrane) side and the (second) membrane side at the other one of the separator sides. As such, the support element may reinforce the separator, especially the membrane(s).

[0028] The term “membrane zone” may in embodiments refer to a plurality of (sub) membrane zones. The number of (sub) membrane zones may in embodiments be equal to the number of membrane areas.

[0029] Especially, the separator thickness may in embodiments substantially be defined by the support element. A maximal thickness of the support element defined perpendicular to (a plane abutting the first separator side (or the second separator side) of) the separator may in embodiments define the separator thickness. The separator thickness may further especially be equal to the membrane thickness and a total distance that the support element extends beyond the membrane. The separator thickness may in embodiments be selected based on the fragility of the membrane. For instance, a first separator comprising a membrane with a first membrane thickness may especially have a larger separator thickness than a second separator comprising a membrane with a second membrane if the second membrane thickness is larger than the first thickness (especially if a total membrane area of both separators are the same). Furthermore, the separator thickness of a first separator having a smaller total membrane area than a total membrane area of a second separator may in embodiments be selected smaller than the separator thickness of the second separator. Furthermore, a single large membrane may in embodiment also require a larger separator thickness, then embodiment wherein the membrane area of the single large membrane is divided over a plurality of smaller membrane areas.

[0030] In specific embodiments, the separator thickness may be as large as a thickness of the wafer it is made of. Yet, in further embodiments, also part of the handle layer may have been removed and the separator thickness may e.g., be in the range of 10 pm to 500 pm. The separator thickness may in embodiments be at least 50 pm, such as at least 100 pm, and especially equal to or less than 3 cm. Moreover, the separator thickness may further be selected based on a distance between neighboring membrane areas. It is noted that in embodiments not all membrane areas of the membrane may have the same characteristics sizes. In embodiments, for instance, the membrane may comprise outer membrane areas enclosing center membrane areas. In embodiments, the outer membrane areas may have a larger characteristic size than the central membrane areas.

[0031] Furthermore, (also) the pores (or especially the pits provided in the fabrication method) (especially provided in the membrane zone) may be provided using etching. The pores may have a linear configuration in embodiments. The pores may have a (especially, right) cylindrical or tubular configuration. In embodiments, the pores have a straight tubular configuration. In further embodiments, the pores may have a (slightly) tapering configuration. The pores may in embodiments have a taper angle of 10° at maximum, such as a taper angle of 5° at maximum. The tapered configuration may be a result of the etching process. The cross- sectional configuration of the pores (based on the pits) may especially be controlled in an etching process (e.g., by a geometry of a mask used for etching, see further below). The cross- sectional geometry of the pits (and successively of the pores) may have any arbitrary shape. Further, the cross-sectional geometry may have a symmetrical shape, such as a squared shape or a triangular shape, an ellipsoidal shape, or a circular shape. In specific embodiments, the pores have a cylindrical configuration.

[0032] Further, based on a (straight) tubular shape of the pores, the porosity of the membrane may (also) be defined as a total cross-sectional area (Ap) of the pores (parallel to the first side of the membrane) relative to a total (surface) area (Amem) of the first side (of the (sub) membrane) (including the total cross-sectional area of the pores) comprising the pores. It will be understood that the porosity described herein is the (local) relevant porosity at locations comprising the pores. The skilled person will understand which part of the membrane refers to “the area comprising the pores” and the (relevant) porosity in this part of the membrane. If only a center region of the membrane comprises pores, then the surface area of the center region determines the total relevant area of the membrane. If the pores are configured over the entire area of the membrane, then the total surface area (at one side) of the membrane determines the relevant membrane area.

[0033] The pores may especially be evenly distributed over (part of the) the membrane. The pores may be configured in a symmetric pitch arrangement (e.g., a triangular pitch arrangement or a square pore pitch arrangement). The pores may e.g., be arranged in a triangular configuration. In alternative embodiments, the pore arrangement may change over the membrane. The membranes may comprise a gradient in porosity (over the membrane). A gradient porosity may, e.g., be advantageous for embodiments of the electrochemical cell (such as water electrolyzer) wherein a gaseous reaction product is formed. Gas crossover may sometimes not be uniformly distributed at all locations in a membrane. MEAs oriented vertically could have pressure differences between the top and the bottom. It might be beneficial to sacrifice porosity at “problem areas” where gas crossover may be greater. For instance, bubbles tend to coalesce as they move toward the outlet. Bubble size may determine crossover.

[0034] In further embodiments, dimensions of at least one of the pores and another one of the pores may be alike. Moreover, in embodiments, dimensions (all of) the pores may be alike. Herein the term “alike” when referring to the dimensions especially indicates that the dimensions may be the same or identical. Yet, it will be understood that the terms “alike”, ’’the same” and “identical” includes that based on the fabrication there may be a minimal difference, such as determined within a standard, e.g., industrial, tolerance.

[0035] In embodiments, (also) the first layer comprises (part of) the pores. In further embodiments, (also) the second layer comprises (part of) the pores. The pore may especially provide a (fluid) channel through the first layer, the silicon comprising layer and the second layer. In alternative embodiments, one or more of the first layer and the second layer may be porous, wherein the pores in said one or more of the first layer and the second layer may be distinct and / or different from the pores comprised by the silicon comprising layer.

[0036] The silicon comprising layer may especially be configured for being resistant to an alkaline environment. To this end (i.e., facilitating (or increasing) the resistance to an alkaline environment), the silicon layer may be covered with, e.g., one or more of silicon nitride, silicon oxide, poly-silicon, and parylene. Also the support element may comprise a layer comprising one or more of silicon nitride, silicon oxide, poly-silicon, and parylene. Said layer may in embodiments be configured contacting the first layer and / or the second layer. Such or layer covering may be supplied (to the silicon layer), especially as a coating, e.g., using CVD. Additionally, or alternatively, the covering may be a product of a reaction of the silicon (in the silicon layer and / or in the support element) with a further reactant, e.g., an oxide and / or high pressure steam, provided to the silicon layer (to provide a coating). Hence, in embodiments, the silicon comprising layer, and in embodiments also the support element, may comprise a silicon layer (entirely) covered with silicon nitride (and / or with one or more of silicon oxide, poly-silicon, and parylene). The silicon layer is especially substantially completely covered with the one or more of silicon nitride, silicon oxide, a poly-silicon and parylene. Moreover, in embodiments, the interior of the silicon comprising layer consists of a (perforated) silicon layer and the exterior (including the wall of the (respective) pores) of the silicon comprising layer consists of one or more of silicon nitride, silicon oxide, poly-silicon, and parylene. The silicon (comprising) layer may especially comprise a coating of one or more of silicon nitride, silicon oxide, poly-silicon, and parylene. Especially, the exterior of the silicon comprising layer (or “the coating”) consists of silicon nitride. Furthermore, in embodiments, the support element may comprise a layer consisting of silicon nitride. Silicon nitride may have the advantage that it does not cause additional stress or buckling. Stress and buckling may, e.g., arise from a (volumetric) expansion of the coating when contacted with an electrolyte or other fluid in the cell / reactor comprising the membrane. Silicon nitride may further provide a smooth layer and is especially electrically insulating. Paralyne may have the same advantages.

[0037] Hence, in embodiments, the pores may have a straight tubular (and optionally tapered) configuration. In further embodiments, the first layer may (substantially completely) extend over the reinforcement element at the first separator side (or at the second separator side), and especially the second layer may extend (substantially completely) over the reinforcement element at the second separator side (or first separator side).

[0038] The first layer and / or the second layer may define a respective electrode. The first layer and / or the second layer may comprise an electrically conductive material. The electrically conductive material may for instance be carbon, a metal, a metal oxide, a metal nitride, a metal halide, a metal alloy, a conducting polymer, or a combination thereof. Hence, in specific embodiments, the first layer (may comprise an electrically conductive material, especially wherein the electrically conductive material) may comprise one or more electrically conductive materials selected from the group consisting of carbon, metals, metal oxides, metal nitrides, metal halides, metal alloys, and conducting polymers. In further specific embodiments, the second layer (may comprise an electrically conductive material, especially wherein the electrically conductive material) may comprise one or more electrically conductive materials selected from the group consisting of carbon, metals, metal oxides, metal nitrides, metal halides, metal alloys, and conducting polymer. In embodiments, the first and second layer may have substantially the same configuration (composition). In alternative embodiments, the composition, such as the conductive material of the first layer differs from the conductive material of the second layer. Herein, the term “conductive” may especially refer to being “electrically conductive”.

[0039] In further embodiments, the first layer may comprise a first electrode material. Additionally or alternatively, the second layer may comprise a second electrode material. Especially, the first electrode material and the second electrode material may be one or more materials independently selected from each other (for the first electrode material and the second electrode material) from the group consisting of platinum, cobalt, nickel, iron, iridium, copper, aluminum, titanium. Herein, the separator is especially explained based on the use in an alkaline water electrolyzer, though the invention is not limited to such applications. In embodiments, the separator may comprise a membrane (especially having a zero-gap electrode configuration) for use in an alkaline water electrolysis cell.

[0040] The membrane may in embodiments have an ionic resistance of < 10 cm2. The ionic resistance may be measured by Galvano dynamic sweeps over a current of 0-10 mA / cm2, with an electrolyte concentration of IM NaOH, see also further in the experimental section. Further, experiments show that in embodiments, the membrane of the invention may have a gas permeability towards H2 of < 4% (of the total concentration of H2).

[0041] In further embodiments, the separator may comprise a membrane (especially having a zero-gap electrode configuration) for use in a redox flow battery and / or organic electrochemical reactions, especially for C-N coupling.

[0042] An example of such organic electrochemical reaction is for instance the dimerization of acrylonitrile (AN) to adiponitrile: 2 CH2=CHCN + 2 e“ + 2 H+— NCCH2-CH2-CH2-CH2CN. Another example may be the production of amino acids from (a) keto acids, such as given in the next scheme: s

[0043] Further reactions may, for example, comprise one or more of the following reactions: CO2+ 2 H++ 2 e“ HCOOH; CO2+ 2 H++ 2 e“ CO + H2O;

[0044] CO2+ 8 H++ 8 e“ CH4+ 2 H2O; 2 CO2+ 12 H++ 12 e“ C2H4+ 4 H2O;

[0045] CO2+ 12 H++ 12 e’^ C2H5OH + 3 H2O; N2+6 H++ 6 e 2 NH3; and

[0046] N2+ 6 H2O + 6 e ’ 2 NH3+ 6 OH'.

[0047] The reactions may especially be based on one or more reactants selected from the group consisting of aromatic and aliphatic, branched and straight-chain organic molecules, especially comprising 1 to 20 carbon atoms connected by any combination of single, double and triple bonds, and especially comprising one or more of halogen groups, carboxylate groups, carboxylic groups, carbonyl groups, ester linkages, amine groups, imine groups, nitrile groups, oxime groups, and thiol groups.

[0048] Hence, the separator may, in embodiments, be used in an electrochemical cell.

[0049] In a further aspect, the invention provides an electrochemical cell comprising the separator (described herein). The invention provides in embodiments an alkaline water electrolyzer comprising the separator (as described herein). The electrical cell, especially the alkaline water electrolyzer, may especially comprise an anode chamber, a cathode chamber, and the separator. The separator especially separates the anode chamber from the cathode chamber.

[0050] The first layer may especially be configured for coupling to a first (positive) electrode of a power supply. Further (also) the second layer may be configured for electrical coupling to a second (negative) electrode of the power supply.

[0051] In further embodiments the first layer may comprise a first coupling element for electrically coupling to a first (positive) electrode of a power supply and the second layer may comprise a second coupling element for electrically coupling to a second (negative) electrode of the power supply. In further specific embodiments, the first (and / or second) layer may be electrically coupled to the power supply via further electrically conductive layers, such as via a gas diffusion layer and / or a current collector.

[0052] In embodiments, the anode chamber may comprise a first gas diffusion layer contacting the membrane, a first endplate and a first current collector arranged between the first gas diffusion layer and the first endplate. Especially, the cathode chamber may comprise a second gas diffusion layer contacting the membrane, a second endplate and a second current collector arranged between the second gas diffusion layer and the second endplate. The gas diffusion layers may be electrically conductive, the current collectors may be electrically conductive. Further, especially, the first current collector and the first gas diffusion layer may be electrical conductively coupled. In embodiments (also) the second current collector and the second gas diffusion layer may be electrical conductively coupled.

[0053] The term “electrical conductively coupled” in phrases like “the first and the second element are electrical conductively coupled” especially indicates that an electrons may travel from the first element to the second element optionally via a third element. An electrical current may flow from the first element to the second element, and the first element and the second element especially define a part of an electric circuit.

[0054] In specific embodiments, a gas diffusion layer defines the first layer and / or the second layer. The (one or more) gas diffusion layer(s) may, e.g., be configured (directly) contacting one or more of the (first and second) side of the membrane. Alternatively, the (one or more) gas diffusion layer(s) may, e.g., be configured (directly) contacting one or more of the first and second layers (of the membrane).

[0055] Gas diffusion layers are known in the art and may, e.g., comprise carbon paper, carbon plates, carbon cloth and / or carbon felt. The gas diffusion layer may further comprise a metal mesh. The gas diffusion layer is especially configured electrically conductive, allowing an electric current to pass from the current collector to the membrane. The gas diffusion layer further comprises an open (continuous) space allowing a gas originating at the membrane or in the gas diffusion layer to travel through the gas diffusion layer (especially in a direction opposite to gravity). The gas diffusion layer may prevent a hold-up of bubbles of the generated gas at an edge of the membrane. The gas diffusion layers may facilitate efficient extraction of gases produced during electrolysis. The gas diffusion layer may further protect the electrodes from corrosion. A hold-up of (large) bubbles may negatively affect the electrochemical reaction at the membrane. The gas diffusion layer may in embodiments comprise an electrically conductive material or electrode material described herein in relation to the first layer and the second layer.

[0056] The anode chamber may further comprise a (first) fluid inlet for providing a fluid, such as an alkaline water or another fluid, especially a liquid, in the anode chamber. Further, the anode chamber may in embodiments comprise a first gas outlet for discharging a gas that is produced in the anode chamber (during operating the cell). The first gas outlet may further allow filling the anode chamber with the fluid (via the first fluid inlet), i.e., by allowing a gaseous material in the anode chamber to be replaced by a liquid fluid. Additionally, or alternatively, the cathode chamber may comprise a (second) fluid inlet for providing a fluid, such as an alkaline water or another fluid, especially a liquid, in the cathode chamber. The cathode chamber may also comprise a (second) gas outlet for discharging a gas that is produced in the cathode chamber (during operating the cell)

[0057] The current collector is especially made of electrically conductive material. The current collector may in embodiments, e.g., comprise (such as be made of) graphite or stainless steel. Yet in further embodiments, the current collector may be made from another electrically conductive material described herein, for instance in relation with the first and second layer. In embodiments the first current collector may comprise a first coupling element for electrically coupling to a first (positive) electrode of a power supply. In further embodiments, the second current collector may comprise a second coupling element for electrically coupling to a second (negative) electrode of a power supply). Coupling element may facilitate connecting a power source to the current collector (or one of the electrodes).

[0058] The endplates are in embodiments configured to electrically insulate the conductive material (the current collectors) from the outside. The endplates may especially be made of an electrically non-conductive material. The endplate may, e.g., comprise or be made of plastic (e.g., Poly ether ketone (PEEK), Polyoxymethylene (POM), or polytetrafluorethylene (Teflon). The endplates are especially electrically insulating. In further embodiments, the endplate may comprise steel. An endplate made of metal may be useful in high temperature electrolysis. When using a conductive endplate, gaskets may be used to isolate the endplates from the outside of the cell In further specific embodiments, the end plates may be left out. In yet further embodiments, the current collector may be configured to include the endplate. That is, in embodiments the current collectors may comprise the endplates.

[0059] The anode and / or the cathode chamber may in embodiments be flow-through chambers, especially wherein the fluid may flow through the chamber (during operating the cell). Hence, one or more of the anode chamber and the cathode chamber may in further embodiments comprise a fluid outlet.

[0060] In specific in embodiments, the anode chamber may comprise a (first) fluid inlet and a (first) fluid outlet and the cathode chamber may comprise a (second) fluid inlet and a (second) fluid outlet, especially for one or more of (i) providing alkaline water in the respective chambers and (ii) flowing alkaline water through the respective chambers. In embodiments, the anode chamber may be configured to provide oxygen gas upon application of a potential difference over the electrolyzer. The anode chamber may especially comprise a (first) gas outlet for discharging oxygen gas from the anode chamber. Moreover, in further embodiments, the cathode chamber may be configured to provide hydrogen gas upon application of a potential difference over the electrolyzer. In further embodiments, the cathode chamber may comprise a (second) gas outlet for discharging hydrogen gas from the cathode chamber.

[0061] A flow-through chamber may advantageously be configured by configuring a channel or recess in the current collector such that the channel / recess is in open fluid connection with the gas diffusion layer arranged against the current collector. A flow channel may be defined by the channel / recess in combination with the gas diffusion layer. In embodiments, the flow channel in combination with a free volume of the gas diffusion layer may define a total (flow-through) space of the chamber. The flow channel (or channel / recess) may especially be configured in fluid connection with the fluid inlet and the fluid outlet at the respective ends of the flow through channel. The term “flow channel” may refer to a network of (flow) channels.

[0062] In further embodiments, one or more of (i) the first current collector comprises a first channel (or recess) configured in open fluid connection with the first gas diffusion layer (wherein the first gas diffusion layer closes the first channel at a side of the first gas diffusion layer), wherein the first channel and the first gas diffusion layer together define a first flow channel. The first fluid inlet and the first fluid outlet may be (fluidly) connected to the first flow channel. Additionally, or alternatively, the second current collector may comprise a second channel (or recess) arranged in open fluid connection with the second gas diffusion layer, wherein the second channel and the second gas diffusion layer together define a second flow channel. The second fluid inlet and the second fluid outlet are especially (fluidly) connected to the second flow channel.

[0063] The electrochemical cell, especially the alkaline water electrolyzer, is especially configured to connect to a power supply. Moreover, in embodiments, the electrochemical cell comprises a power supply comprising a positive electrode and a negative electrode. Especially, the positive electrode (of the power supply) may be configured in electrical connection with the first layer (optionally via the first current collector) and the negative electrode (of the power supply) may be configured in electrical connection with the second layer (optionally via the second current collector).

[0064] The power supply may be configured for supplying a direct current. The power supply may be configured for providing a potential difference depending on the application. The potential difference may essentially have any arbitrary value. In embodiments the power supply may, e.g., be configured for providing a potential difference (dV) between the first layer and the second layer, with dV < 10V. It is noted that for embodiments of the electrochemical cell comprising a redox flow battery an electrical current may flow in an opposite direction through a power circuit compared to a circuit formed in an electrolyzer. In a redox flow cell, during a discharge cycle, power may be provided (by electrochemical reactions taking place in the cell), whereas in an electrolyzer power (energy) may be consumed (to provide the electrochemical (electrolysis) reactions in the cell. (During charging the redox flow cell, the operation is comparable to the operation of the electrolyzer) Therefore, in embodiments, the term “power supply” may be replaced by “power consumer”, such as a power grid, an equipment to be powered, etc. Also a (rechargeable) battery may be an embodiment of a power consumer.

[0065] Hence, the invention relates to many different configurations of the electrochemical cell. In embodiments, the electrodes are directly formed at the separator (especially the membrane) and may be part of the separator (especially the membrane). The separator comprising the electrodes may be a single (integral) unit. In further embodiments, electrodes may be configured against the membrane, e.g., by pressing or forcing the electrodes against (the sides of) the membrane. For instance in embodiments, a gas diffusion electrode may be arranged contacting the porous silicon comprising layer without having a further conductive layer between the silicon comprising layer and the gas diffusion layer. Both above- described configurations may also be known as a “zero-gap membrane electrode” configuration. In yet further embodiments, one or more of the electrodes may be arranged in the anode chamber and / or cathode chamber at a distance from the membrane.

[0066] Hence, in specific embodiments, the first layer comprises the first gas diffusion layer and / or the second layer comprises the second gas diffusion layer. The first gas diffusion layer may define the first electrode and / or the second gas diffusion layer may define the second electrode. In alternative embodiments, a (first) conductive layer is provided, especially deposited, at the first layer side of the silicon comprising layer and / or a (second) conductive layer is provided, especially deposited, at the second layer side of the silicon comprising layer. In further embodiments, the (first) conductive layer may further define the exterior of the support element at the first separator side, and / or the (second) conductive layer may further define the exterior of the support element at the second separator side.

[0067] In a further aspect, the invention provides a method for fabrication of a separator. The method (“fabrication method”) may provide the separator described herein. The method at least comprises a silicon layer fabrication stage and a covering stage, as described above. The silicon comprising layer (of the membrane) may be fabricated by the silicon layer fabrication stage together with the covering stage. The method may further comprise an electrode provision stage. In the electrode provision stage, electrodes (including electrically conductive layers that may function as an electrode) may be provided at least at the first layer side and the second layer side. The electrodes are in embodiments electrically conductive coupled to all of the membrane areas (in the electrode provision stage).

[0068] In the silicon layer fabrication stage, a porous silicon layer having channels from a first side to a second side of the silicon layer may be fabricated from a silicon wafer, as is explained above. The silicon layer may be fabricated using standard silicon micromachining techniques. The silicon layer fabrication stage may especially comprise micro-resolution (additive) manufacturing techniques. Pits may be provided in (the predetermined membrane zone of) the silicon device layer using standard etching techniques, especially deep reactive ion etching techniques. In embodiments, the silicon wafer may be selected for having a thickness in the range of 50 pm - 3 cm, especially in the range of 100 pm - 3 cm. such as in the range of 100 pm - 1000 pm, especially in the range of 100 pm - 750 pm. In further embodiments, the silicon wafer may be selected for having a thickness of the device layer in the range of 5- 200 pm, such as in the range of 5-100 pm. Moreover, the pits provided in the device layer in the silicon layer fabrication stage may have a depth being equal to or larger than the thickness of the device layer. Further, in specific embodiments, the pits (in the device layer) may be provided having a cross sectional dimension selected from the range of 1-400 pm, especially 1-200 pm, such as 1-100 pm, even more especially 1- 20 pm. The pit size (cross-sectional dimension) may in embodiments be at least 0.5 pm, such as at least 1 pm, especially at least 2 pm, such as at least 5 pm. The pit size may especially be 500 pm at maximum, such as 400 pmat maximum, especially 300 pm at maximum, and in embodiments no more than 200 pm, even more especially no more than 100 pm, such as no more than 50 pm, especially no more than 20 pm. Especially, a total cross-sectional area of the pits over a total area of the silicon layer comprising the pits may be configured in the range of 0.1-60%, such as in the range of 0.1-25%, especially in the range of 1-15%.

[0069] In embodiments the handle layer may be removed (at the membrane zone) by silicon plasma etching. Further, the insulating (buried) oxide layer may in embodiments be removed (at the membrane zone) using hydrogen fluoride.

[0070] In the covering stage, the silicon layer, and especially also remaining parts of the wafer not comprising the membrane zone, may be coated with an insulating material. To prevent any short-circuiting through the channels in the silicon layer, also walls of these channels are especially coated, resulting in the (final) pores in the silicon comprising layer (and eventually in the membrane). The insulating layer may protect the silicon layer (and the support element) against fluid that could weaken or dissolve the silicon. The insulating material may especially be selected to be resistant to fluids that may contact the membrane during operation of the electrochemical cell. To apply the separator in an alkaline water electrolyzer, the insulating material may be selected to be resistant to the alkalinity of the water. The insulating material may in embodiments be resistant to a fluid having a pH of 10 or more, or e.g., having a pH of about 14. Moreover, the separator may in embodiments be configured to be resistant to a fluid having a pH of 10 or more, or e.g., having a pH of about 14. In specific embodiments, the insulating material comprises silicon nitride.

[0071] The silicon layer fabrication stage may especially comprise: after providing the silicon wafer, providing a photoresist on the device layer. Further, the silicon layer fabrication stage may comprise patterning the photoresist using a mask (and especially developing the photoresist), to provide the device layer comprising a patterned photoresist. Subsequently, the device layer comprising the patterned photoresist may be subjected to an etching plasma to provide the patterned silicon wafer. The silicon layer fabrication stage may in further embodiments comprise providing a (second) photoresist on the handle layer, especially on remaining locations of the handle layer not being part of the membrane zone. Further, the silicon layer fabrication stage may comprise patterning the (second) photoresist using a mask (and especially developing the (second) photoresist), to provide the handle layer comprising a patterned photoresist. Subsequently, the handle layer comprising the patterned photoresist may be subjected to an etching plasma to provide the (further) patterned silicon wafer. In further embodiments, the insulating layer (at the membrane zone) may be etched from the further patterned silicon wafer, especially using hydrofluoric acid, to provide a completely patterned silicon wafer.

[0072] Providing a photo resist on the device and / or handle layer may in embodiments comprise spin coating the device and / or handle layer with the photoresist and solidifying (or “developing”) the photoresist.

[0073] In further embodiments, the covering stage may comprise depositing silicon nitride on the (entire) completely patterned silicon wafer. Hence, in embodiments, the covering stage comprises depositing silicon nitride on the silicon layer. The covering stage may in further embodiments also comprises depositing silicon nitride on remainders of the handle layer and the insulating layer. Moreover, in embodiments, the (entire) silicon layer (including walls of the channels) may be covered with silicon nitride, to provide a silicon comprising layer comprising pores from a first layer side to a second layer side of the silicon comprising layer. Different known technologies may be applied for coating the silicon layer and the remainders of the handle layer and the insulating layer. The silicon nitride layer may, e.g., be deposited by chemical vapor deposition. The coating may have a thickness in the range of ten to a few hundred of nanometers (although thicker coatings may also be provided). In embodiments the coating (especially the silicon nitride layer) has a thickness of 2 pm at maximum, such as 1 pm at maximum, especially 500 nm at maximum, such as 200 nm at maximum, even more especially 100 nm at maximum, such as especially 50 nm at maximum. The coating may further have a thickness of at least 5 nm, especially at least 10 nm, such as at least 20 nm, and in embodiments at least 50 nm.

[0074] The term “remainders of the handle layer and the insulating layer” and comparable terms may especially refer to parts of the handle layer and the insulating layer that are not etched away in the silicon layer fabrication stage.

[0075] In embodiments, the porous silicon comprising layer (at the membrane zone) defines the membrane. In alternative embodiments, a first and / or second (conductive) layer is provided at the first and / or second layer side of the porous silicon comprising layer resulting in the membrane. The first and / or second layer may define a first and / or second electrode, respectively. Providing one or more of these layers may be done in the electrode provision stage. The electrode provision stage is especially configured subsequent to the covering stage. In specific embodiments, the electrode provision stage comprises providing a first (electrically conductive) layer at the first side of the wafer, especially of the silicon comprising layer (and optionally especially also the remainders of the handle layer and the insulating layer- if present). In further specific embodiments, the electrode provision stage (also) comprises providing a second (electrically conductive) layer at the second side of wafer, especially of the silicon comprising layer (and optionally especially also on the remainders of the handle layer and the insulating layer - if present).

[0076] Depending on the material used to provide the electrode, it may be advantageous to first provide an adhesion layer on the wafer, especially on the silicon comprising layer before providing the electrode material. Hence, in embodiments the electrode provision stage may comprise: (i) depositing an adhesion material on the first side of the wafer and on the second side of the wafer for providing an adhesion layer covering the first side of the wafer and the second side of the wafer, and (ii) depositing a first metal comprising material at the adhesion layer configured at the first (layer) side (of the wafer and / or the silicon comprising layer) and a second metal comprising material at the adhesion layer configured at the second (layer) side (of the wafer and / or the silicon comprising layer), to provide the first layer and the second layer at the first side and the second side of the separator (respectively). The first metal comprising material and the second metal comprising material may especially comprise a material described above in relation to the configuration of the electrically conductive materials for the first layer and the second layer or a material described in relation with the first electrode material and the second electrode material. Typical adhesion materials that may be used are titanium, tantalum, and chromium, though other options are herein not excluded. The adhesion material may in embodiments be selected for comprising tantalum. In further embodiments, the adhesion layer comprises one or more of are titanium, tantalum, and chromium. The adhesion layer may especially comprise, even more especially consist of, tantalum.

[0077] In specific embodiments, one or more of the adhesion material, the first metal comprising material, and the second metal comprising material may be deposited via chemical vapor deposition. In further embodiments, DC sputtering or physical vapor deposition may be used to deposit one or more of the adhesion material, the first metal comprising material, and the second metal comprising material. In further embodiments depositing one or more of the adhesion material, the first metal comprising material, and the second metal comprising material may comprise electron beam evaporation. In further embodiments, depositing may be (performed) with the wafer placed at a non-straight angle, such as an angle between 5 and 30 degrees, especially an angle between

[0078] 5 and 10 degrees, relative to the direction of the chemical vapor flux. Especially, during depositing, the wafer may be placed at the non-straight angle relative to the direction of the chemical vapor flux. Depositing under a non-straight angle may prevent that walls of the pores are also completely covered, which may result in short circuiting via the pores.

[0079] In embodiments the first layer and the second layer may be provided having a thickness in the range of 50-500 nm. The first and / or the second layer may especially have a thickness of at least 10 nm, such as at least 25 nm, especially at least 50 nm. The thickness of the first and / or the second layer may especially be no larger than 1 pm, such as no larger than 500 nm, even more especially 200 nm at maximum. The first layer and the second layer (as well as the optional adhesion layer) may in embodiments be configured completely covering the first side and the second side of the separator. The adhesion layer may especially be provided having a thickness in the range of 1-50 nm. The adhesion layer may in embodiments have a thickness of at least 1 nm, such as at least 5 nm, especially at least 10 nm, even more especially at least 25 nm. A maximum thickness of the adhesion layer may especially be 100 nm, even more especially 50 nm. In embodiments the thickness of the adhesion layer may be no more than 390 nm.

[0080] BRIEF DESCRIPTION OF THE DRAWINGS

[0081] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: Fig. l schematically depicts an embodiment of a separator of the invention; Fig. 2 schematically depicts an embodiment of the electrochemical cell of the invention; Figs 3A and 3B schematically depicts some aspects of the separator; Figs 4A and 4B depict some further embodiments of the electrochemical cell; Fig. 5 depicts experimental results of operating embodiments of the electrochemical cell; Fig.

[0082] 6 schematically depicts an embodiment of the method of the invention; and Figs 7A. 7B, and 8 depict some further aspects of the separator. The schematic drawings are not necessarily to scale.

[0083] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0084] Fig. 1 schematically depicts a section an embodiment of the separator 1. The separator comprises a membrane 100 and a reinforcement support element 200. The membrane 100 is especially a porous (silicon) membrane 100. The separator may, e.g., be applied in an electrochemical cell 1000 and / or for separation applications. The electrochemical cell 1000 may, e.g., be used for carrying out electro-chemical reactions, alkaline water hydrolysis, alkaline water electrolysis, or energy generation in a redox flow battery. Herein, the electrochemical cell 1000 is especially explained based on an alkaline water electrolyzer 1000 (as an embodiment of the electrochemical cell 1000), though the invention is not limited to such applications. The figures also especially explain the invention based on the alkaline water electrolyzer 1000, herein also simply indicated as “electrolyzer” 1000.

[0085] The depicted separator 1 has a first separator side 2 and a second separator side. The separator 1 comprises a membrane 100 comprising a silicon comprising layer 110 having a first (planar) side 101 and a second (planar) side 102. The (minimal) distance between the first layer side 101 and the second layer side 102 is indicated as membrane thickness dmem. The membrane thickness dmem is in specific embodiments equal to or less than 200 pm. It is noted that in the depicted embodiment of Fig. 1, a total thickness or width of the membrane 100 is larger than the membrane thickness dmem. In the given embodiment the width of the membrane is made up of the membrane thickness dmem, the thickness of the first layer 121 and the thickness of the second layer 122. The term “membrane thickness” especially refers to a total thickness of the silicon comprising layer 110.

[0086] In the membrane 100 very schematically and exaggerated pores 140 are depicted. The pores 140 defining open fluid channels from the first layer side 101 to the second layer side 102. The mean pore cross-sectional dimension or size is indicated as dp. This is the dimension parallel to the first layer side 101(and second layer side 102). The pores size may in embodiments be equal to or less than 200 pm and in embodiments, e.g., between 1 nm to 10 pm. The pore 140 may basically have any cross-sectional configuration and is especially symmetrically, e.g., circular, squared, triangled, ellipsoidal, etc. Hence, in embodiments, the cross-sectional dimension / size dpcorresponds to a width or diameter of the pore 140. In other configurations dpmay refer to largest distance between a first location of the wall of the pore 140 to a second location of the wall of the pore 140. The term “mean” in mean pores size especially refers to the number average pores size, i.e., the total of all pore sizes divided by the number of all pores.

[0087] In the depicted embodiment, the pores 140 have a straight tubular configuration, especially a cylindrical configuration. This straight configuration is especially the result of embodiments of the fabrication method of the invention, wherein the pores 140 are the result of a photolithographic such as an etching technology. In embodiments, the pores 140 may be configured slightly tapering. The tapered configuration may in embodiments comprise a taper angle of no more than 5°. A tapering configuration of the (circular) pores may especially be found for membranes 100 having a relatively larger membrane thickness dmem. The figure further schematically depicts that the pores 140 may be evenly distributed in embodiments (as indicated in the lower portion of the membrane 100). Yet, the pores 140 may also be more randomly distributed (as is schematically indicated at the top of the figure).

[0088] The separator 1 further comprises the support element 200. Only a part of the separator element 200 is depicted and encircled with a dotted line. The reinforcement support element 200 is configured from the first separator side 2 to the second separator side 3. The support element 200 is connected to the silicon comprising layer 110 and extends from the separator 1. In the depicted embodiment, the support element 200 extends at first separator side 2. In alternative embodiments, the support element 200 may extend at the first separator side 2 and at the second separator side 3. Further also the separator thickness is indicated with reference ds, The separator thickness dsis defined perpendicular to a plane virtual plane of (or through) the silicon comprising layer 110. The virtual plane is schematically indicated wit reference P. The separator thickness dsmay in embodiments be in the range of 50 pm - 3 cm.

[0089] In the depicted embodiment, the membrane 100 comprises the first layer 121 arranged at the first layer side 101 and the second layer 122 arranged at the second side 102. The first layer 121 extends over the support element 200 at the first separation side 2. The second layer 122 extend over the support element 200 at the second separation side 3. These layers 121, 122 comprise electrically conductive material 1210 and electrically conductive material 1220, respectively. The first layer 121 of the embodiment may therefore also be indicated as first electrode 131 and the second layer 122 may be indicated as second electrode 132. The figure further depicts that the first layer 121 directly contacts the silicon comprising layer 110, and that the second layer 122 directly contacts the silicon comprising layer 110. This may be different in other embodiments. The first electrode 131 and / or second electrode 132 may, e.g., also be defined by other layers that may be remote from the membrane 100 (especially not being part of the membrane 100 and / or the support element 200).

[0090] The figure further depicts that the silicon comprising layer 110 comprises a silicon layer 115 that is covered or coated with a coating material, especially an insulating material such as silicon nitride 116. This coating in the pore 140 may prevent short-circuiting between the first electrode 131 and the second electrode 132 (via the pores 140).

[0091] The first layer 121 especially comprises an electrically conductive material 1210. Likewise, the second layer 122 comprises an electrically conductive material 1220, as is depicted in the figure. Examples of possible electrically conductive material that may be used alone, or in combination are carbon, metals, metal oxides, metal nitrides, metal halides, metal alloys, and a conducting polymer. The electrically conductive materials 1210, 1220, of respectively the first layer 121 and the second layer 122, may herein also be indicated as first electrode material 1210 and second electrode material 1220, respectively. These electrode materials 1210, 1220 may in embodiments independently from each comprise platinum, cobalt, nickel, iron, iridium, copper, aluminum, titanium, and combinations thereof.

[0092] The electrodes 131 and 132 are configured in direct contact with the membrane 100, especially with the silicon comprising layer 110. Therefore it may also be said that the membrane 100 depicted in the figure has a zero-gap electrode configuration. The separator 10 may be fabricated using the method of the invention, comprising at least a silicon layer fabrication stage and a covering stage; In embodiments, e.g., for fabricating the membrane 100 of Fig. 1 (including the electrodes 131, 132), the method further comprises electrode provision stage. In Fig. 1 a part of a single membrane 100 is depicted. It is noted that the membrane 100 may be defined by a plurality of different (sub) membranes 100 or membrane area (see e.g., Figs 7A-8).

[0093] In the silicon layer fabrication stage, a silicon wafer consisting of a silicon device layer, a silicon handle layer, and an insulating (buried) oxide layer configured between the device layer and the handle layer may be provided. Next, pits may be provided (etched) in the device layer by deep reactive ion etching (using a mask to configure the etching pattern) resulting in a patterned silicon wafer. The pits are especially provided through the entire device layer and perpendicular to the surface of the wafer. Subsequently, the handle layer and the insulating layer may be removed from the patterned silicon wafer to provide the silicon layer 115 of the membrane 100. The handle layer may for instance be (partly) removed (at locations of the membrane zone 15 (see Fig. 6) by silicon plasma etching, and the insulating layer may (partly) be removed using HF. After removing these layers, the pits provided by etching define (especially straight) channels 119, or ’’primary pores”, from a first side 117 of the silicon layer 115 to a second side 118 of the silicon layer 115, see Fig. 1. In the figure also one of the channels 119 is verry schematically indicated. The reference 119 especially refers to a pore 140 including part of the silicon nitride layer 116 forming the wall of the pore 140.

[0094] In the covering stage, for instance silicon nitride (or another material described herein) 116 may be deposited on the silicon layer 115 (including walls of the channels 119 and the support element 200) to provide the silicon comprising layer 110 comprising pores 140 from a first layer side 101 to a second layer side 102 of the silicon comprising layer 110. Further, in the optional electrode provision stage, the first (conductive) layer 121 may be provided at the first layer side 101 of the silicon comprising layer 110 and / or the second (conductive) layer 122 may be provided at the second layer sidel02 of the silicon comprising layer 110. The silicon nitride 116 or other material is in embodiments deposited on the silicon layer 115 by (low pressure) chemical vapor deposition or sputter deposition.

[0095] In embodiments, initially an adhesion layer, e.g. comprising tantalum, may be deposited on the first layer side 101 and on the second layer side 102, and next a first metal comprising material 1210 may be deposited at the adhesion layer configured at the first layer side 101 and a second metal comprising material 1220 may be deposited at the adhesion layer configured at the second layer side 102, to provide the first layer 121 and the second layer 122. Deposition of the first and the second layer 121, 122 may comprise chemical vapor deposition or physical vapor deposition, e.g., DC sputtering. During depositing, the wafer is especially placed at a non-straight angle, e.g., between 5 and 10 degrees, relative to the direction of the chemical vapor flux. This way only a minor fraction, or essentially none, of the material is deposited in the pores 140 to prevent electrical contact between the first layer 121 and the second layer 122.

[0096] The separator 1 may define a separator for use in for instance an alkaline water electrolysis cell, a redox flow battery, or for use in organic electrochemical reactions, such as for C-N coupling. In Fig. 2 schematically an embodiment of an electrochemical cell 1000 is given, more especially of an alkaline water electrolyzer 1000. In the figure, the support element 200 is not depicted for clarity reasons.

[0097] The depicted alkaline water electrolyzer 1000 comprises an anode chamber 1100, a cathode chamber 1200, and the separator 1 separating the anode chamber 1100 from the cathode chamber 1200. In the depicted embodiment, the first layer 121 comprises a first coupling element 1311 coupled to a first (positive) electrode 1301 of a power supply 1300, and the second layer 122 comprises a second coupling element 1321 electrically coupled to a second (negative) electrode 1302 of the power supply 1300. The power supply 1300 may provide a potential difference dV between the electrodes 1301 and 1302. During operation of the water electrolyzer 1000, wherein the chambers 1100 and 1200 are filled with an alkaline aqueous solution, electrons may flow from the first electrode 1301 to the second electrode 1302 and may react with water in the cathode chamber 1200, resulting in the formation of hydrogen gas and hydroxide ions, according to the formula 4 H2O - 4 e' - 2H2 + 4 OH'. Concurrently, at the first electrode oxygen gas may be formed based on the next reaction: 4 OH' -> 2 H2O + O2 + 4e'. These reactions may continue (as long as the power supply 1300 is connected) since the hydroxide ions may travel from the cathode chamber 1200 to the anode chamber 1100 through the porous membrane 100. The power supply especially provides a direct current to the electrodes 131, 132. Furthermore, the water is preferably alkaline water having an alkaline pH, higher than 7, especially higher than 10. The alkaline water may for instance comprise a 1 M NaOH solution or any other alkaline water solution known to the skilled person. It is noted that this first reaction is especially catalyzed by the first electrode material 1210 or first electrode conductive material 1210 and the second reaction may be catalyzed by the second electrode material 1220. The electrode materials 1210, 1210 or electric conductive material 1210, 1220 may therefore also be referred to as “catalyst”.

[0098] Further, also a fluid inlet 1101, a fluid outlet 1109, and a gas outlet 1105 of the anode chamber 1100 and a fluid inlet 1201, a fluid outlet 1209 and a gas outlet 1205 of the cathode chamber 1200 are depicted.

[0099] The membrane 100 of the invention may especially have an ionic resistance of < 10 cm2, facilitating a quick travel of the hydroxide ions through the membrane. The membrane 100 may further have low gas permeability towards H2, such as at maximum 4% (of the total amount of H2). An impurity of less than 4% H2 may be present in the oxygen gas produced in the anode chamber 1100.

[0100] It is noted that in Fig. 2, as well as in Figs 4A and 4B, for clarity reasons only the most relevant elements of the cells 1000 are depicted. Based on the described information and general knowledge, an operatable cell 1000 may be configured. For instance, there may be additional frames and / or gaskets in the electrolyzer which are not indicated in the figures, e.g., to prevent unwanted leakage and to maintain the different layers at their position (such as pressing them together). Moreover, a plurality of cells 1000 may be couple to each other, which is also not depicted in the given figures. Gaskets may e.g., comprise or consist of rubber (e.g., EPDM, Buna-S, Viton, Kalrez, Silicone, or natural rubber). Frames may comprise or consist of Teflon.

[0101] In Figs 3 A and 3B, some further aspects of the invention are given. The figures depict portions of the membrane 100, wherein the first layer side 101 (and second layer side 102) is configured parallel to the paper. As is described above, the pores 140 may in embodiments be evenly distributed over the membrane 100. Some configurations are depicted in these figures. In Fig. 3A a configuration with a square pore pitch arrangement is depicted. In Fig. 3B the pores 140 are configured in a triangular pitch arrangement. In the figures also the pore surface area Apand the membrane surface area Amem are very schematically indicated. These values may be used to determine the porosity of embodiments of the membrane. The porosity may be defined as a total cross-sectional area Apof the pores 140 parallel to the first layer side 101 (of the membrane 100) relative to a total area Amem of the first layer side 101. In the given examples the total (relevant) cross-sectional area Apof the pores 140 is the indicated area Apof one pore 140 (actually four times a quarter of the area Apfor each pore 140 in Fig. 3 A and three times a third of the area Apfor each pore 140 in Fig. 3B). The total (relevant) area Amem of the membrane 100 comprising the pores 140 is defined by the area in the dotted square or triangle (including the areas of the pores 140 in the square or triangle. In Fig. 3B also the cross-sectional dimension or size is indicated with the reference dp. The size dpmay in embodiments be less than 200 pm, such as equal to or less than 10 pm, for instance between 1 nm to 10 pm.

[0102] The porosity of membrane 100 may especially be at least 0.01%, such as at least 0.1%, even more especially at least 1%. Further, the porosity may especially be 50% at maximum, such as 25% at maximum, especially 10% at maximum.

[0103] These figures further depict that the dimensions of the pores 140 are alike (i.e., the same within a standard, e.g., industrial, tolerance).

[0104] In Figs 4A and 4B some further embodiments of the electrochemical cell 1000, especially the alkaline water electrolyzer 1000, are schematically depicted. In both embodiments, the anode chamber 1100 comprises a first gas diffusion layer 1130 contacting the membrane 100, a first endplate 1190 and a first current collector 1180 (or first “bipolar plate” 1180) configured between the first gas diffusion layer 1130 and the first endplate 1190 The cathode chamber 1200 is configured alike. The cathode chamber 1200 comprises a second gas diffusion layer 1230 contacting the membrane 100, a second endplate 1290 and a second current collector (bipolar plate) 1280 arranged between the second gas diffusion layer 1230 and the second endplate 1290. It is noted that especially the gas diffusion layers 1130, 1230 have a space for receiving a fluid. Yet, the combination of respective endplate 1190, 1290, current collector 1180, 1280, gas diffusion layer 1130, 1230 (and part of the membrane 100 / separator 100) may define the chambers 1100, 1200. The gas diffusion layers 1130, 1230 and the current collectors 1180, 1280 are electrically conductive and are part of an electrical circuit together with the power supply 1300. Being part of the electrical circuit may be the results of the fact that the first current collector 1180 and the first gas diffusion layer 1130 are electrical conductively coupled, and the second current collector 1280 and the second gas diffusion layer 1230 are electrical conductively coupled.

[0105] In figure 4A and 4B it is also very schematically indicated that the first current collector 1180 comprises a first coupling element 1181 for electrically coupling to the first (positive) electrode 1301 of the power supply 1300, and the second current collector 1280 comprises a second coupling element 1281 for electrically coupling to a second (negative) electrode 1302 of a power supply 1300. The coupling elements 1181, 1281, essentially may have any configuration to connect the power supply 1300 to the current collectors 1180, 1280 (the same holds for the first coupling element 1311 and the second coupling element 1321 depicted in Fig. 1).

[0106] Further, also a fluid inlet 1101 and fluid outlet 1109 of the anode chamber 1100 and a fluid inlet 1201 and a fluid outlet 1209 of the cathode chamberl200 are depicted. In the depicted embodiments, respectively given at the bottom end and the top end of the cell 1000. It will be understood that the inlet and outlet may also be changed around, or be configured at other locations. The inlets 1101, 1201 may be used for providing an alkaline aqueous solution (such as alkaline water) to the respective chambers 1100, 1200 and / or for flowing alkaline water through the respective chambers 1100, 1200 (see also below the explanation of Fig. 4B).

[0107] Furthermore, in Figs 4 (as well as in Fig. 2), it is also very schematically depicted that the chambers 1100, 1200 comprise a gas outlet, indicated with numbers 1105 and 1205, respectively. These outlets 1105, 1205 may be used for discharging oxygen gas from the anode chamber 1100 and for discharging hydrogen gas from the cathode chamber 1200 during electrolyzing alkaline water.

[0108] The embodiment depicted in Fig. 4B relates to an electrolyzer 1000 having flow- through (anode and cathode) chambers 1100, 1200. In the figure this is very schematically indicated with a shaded part of the first and second current collectors 1180, 1280; references 1185 and 1285 point to these parts. The first current collector 1180 comprises a first channel or first recess (not visible in the figure but being configured in the shaded part of the first current collector 1180) configured in open fluid connection with the first gas diffusion layer 1130 The first gas diffusion layer 1130 closes the first channel (at a side of the gas diffusion layer 1130. Further, especially the first channel (recess) and the first gas diffusion layer 1130 together define a first flow channel 1185. The first fluid inlet 1101 and the first fluid outlet 1109 are connected to the first flow channel 1185. Hence, when providing a liquid to the first inlet 1101, the liquid may flow via a space formed by the first flow channel 1185 in combination with an open space in the first gas diffusion layerl 130. In the depicted figure also the cathode chamber 1200 is a flow-through chamber. The second current collector 1280 also comprises a recess or a second channel (also not visible in the figure) arranged in open fluid connection with the second gas diffusion layer 1230 and together with the second gas diffusion layer 1230 defining the second flow channel 1285 which is connected to the second fluid inlet 1201 and the second fluid outlet 1209.

[0109] Also, in these figures further standard elements are not depicted, see also above. The electrochemical cells 1000 of Figs 4A and 4B may e.g., comprise frames to hold various elements in place between the two current collectors 1180, 1280.

[0110] As explained above, also alternative configurations for the electrochemical cell 1000 are part of the invention. Figs 4 A and 4B may be used to explain such configuration. In embodiments, the membrane 100 may not include one or more of the first layer 121 and the second layer 122. As such the total thickness of the membrane may correspond to the thickness of the silicon comprising layer 110 and to the membrane thickness dmem, see also the explanation with respect to Fig. l.The membrane 100 may in embodiments consist of (only) the silicon comprising layer 110 depicted in Fig. 1 (without the two layers indicated with reference numbers 121 and 122 in Fig. 1. If such membrane 100 is assumed to be in the electrochemical cells 1000 in Figs 4, then the gas diffusion layers 1130 and 1230 would directly contact the silicon comprising layer 110. Hence, in such embodiment, the first layer 121 comprises the first gas diffusion layer 1130 and the second layer 122 comprises the second gas diffusion layer 1230. The first gas diffusion layer 1130 may define the first electrode 131 and the second gas diffusion layer 1230 may define the second electrode 132. It will be understood that in other embodiments, (only) one of the first and second layer 121, 122, may already be part of the membrane 100. Hence, in embodiments, the width of the first and / or second layer 121, 122 may be defined by the width of the respective gas diffusion layer 1130, 1230. Typical gas diffusion layer may have a width in the range of 0.01 to 0.5 mm. Further, a typical porosity of the gas diffusion layer (providing the open space) is in the range of 50-90%.

[0111] For these embodiments as well as the embodiments depicted in Figs 2, 4A and 4B, the electrochemical cell 1000, especially the alkaline water electrolyzer 1000, may be connectable to, or comprise, the power supply 1300. The power supply 1300 may comprise the positive electrode 1301 and negative electrode 1302. When connected, the positive electrode 1301 is especially configured electrically connected with the first layer 121 (optionally via the first current collector 1180) and the negative electrode 1302 may be configured electrically connected with the second layer 122 (optionally via the second current collector 1280). It is noted that alternatively, the negative electrode 1302 and the positive electrode 1301 may be changed. Herein, the terms anode and cathode are used to refer to the different chambers 1100, 1200, and for a clear explanation, it is described that the negative electrode 1302 may be connected to the cathode chamber 1200 and the positive electrode 1301 may be connected to the anode chamber 1100. Yet, also when the electrodes are changed, the invention may still function.

[0112] Fig. 5 depicts the average cell potential V for four different membranes 100 in an alkaline water electrolyzer 1000 schematically depicted in Fig. 4B having membranes 100 with different porosities (a = 0.15 %, a = 0.6 %, a = 1.69 %, and a = 7.7 %, given at the x-axis) when applying different currents (I = 50 mA, I = 100 mA, I = 250 mA, I = 500 mA, and I = 800 mA; see y-axis). The depicted results of the alkaline water electrolyzers 1000 were operated at an electrolyte flow rate of 100 mL / min. The size of the circles indicates the magnitude of the average potential V (averaged over a thirty second period). The value of the average potential is also indicated in the circle. The figure is further explained in the Experimental section, further below. The figure depicts that the cell potential V increases with increasing current and decreases with increasing separator 100 porosity.

[0113] Fig. 6 depicts an embodiment of the method of the invention. Subsequent stages or steps are indicated with the references A to I. At stage A, a silicon wafer 10 consisting of a silicon device layer 11, a silicon handle layer 13, and an insulating oxide layer 12 configured between the device layer 11 and the handle layer 13. The silicon handle layer 13 is configured at, especially defines, a first side of the silicon wafer 10 and the silicon device layer 11 is configured at, especially defines, a second side of the silicon wafer 10. In stage B (the top of) the handle layer 13 is oxidized, e.g., thermally, resulting in an additional insulating oxide layer 12at the handle layer 13.. Stage B may be absent in other embodiments. In stage C, photoresist 21 is provided on the device layer 11, especially on the membrane zone 15, and the photoresist 21 is patterned using a mask.

[0114] In stage D, the device layer 11 comprising the patterned photoresist is subjected to an etching plasma to provide the pits in the device layer. In stage E, a photoresist 21 is provided on the handle layer 13 at locations not comprising the membrane zone 15, and the photoresist 21 is patterned using a mask, thereby providing the handle layer 13 comprising a patterned photoresist. Successively, in stage F the handle layer 13 comprising the patterned photoresist is subjected to an etching plasma, resulting in the silicon wafer 10 depicted for stage F. In stage G the insulating layer 12 is removed using hydrogen fluoride. At this stage there is an open fluid connection from one side to the other side of the device layer 11. Further, also the support element 200 is formed (yet without any further layer, such as a nitride layer and the electrode).

[0115] Next, in stage H, a nitride layer 116 is deposited over the silicon layer 115 as well as over the remaining parts of the insulating layer 12 and the handle layer 13. Herein this may also be indicated as “the nitride layer 116 is deposited on the silicon wafer 10, wherein a total surface area of (i) the first side of the silicon wafer 10, (ii) the second side of the silicon wafer 10, and (iii) the channels 19 is covered with silicon nitride 116”. Based on this, the silicon comprising layer 110 comprising pores 140 from the first layer side 101 to the second layer side 102 of the silicon comprising layer 110 at the membrane zone 15 is provided.

[0116] In stage I the final electrodes are provided on both sides of the separator 1. Note, that the nitride layer is provided in the pores as is depicted in stage H, whereas the electrodes are not configured in the pores 140.

[0117] In Figs 7A-7B, two embodiments of the separator 1 are depicted to further illustrate some aspects. The depicted embodiments have a (circular) shape of a wafer 10. It will be understood that by further tooling such shape and configuring a specific pattern of the membranes 100, other shapes of the separator 1 may be provided, (e.g., a rectangular shape). The embodiment in Fig. 7A comprises nine membranes 100, herein also indicated as “sub membranes”, or “membrane areas”. These may have been configured during etching the membrane zone 15. The figure depicts that also the membrane zone 15 may comprise a plurality of membrane zones 15. The membranes 100 have a characteristic size, such as width, diameter, or length, indicated with the reference Smem. Here, Smem corresponds to the diameter of the membrane 100. All membranes 100 are enclosed by the support element 200. Further, a minimal distance between neighboring membranes 100 is indicate with the reference WSUp. The minimal distance WSUpmay especially be at least 10 pm. The minimal distance WSUp may further depend on a diameter of the wafer 10 and may eventually be at maximum equal to the diameter of the wafer. In embodiments, WSUp 225 mm at maximum, especially 10 mm at maximum. Hence, in embodiments WSUp may be selected from the range of 10 pm-10 mm. Furthermore, the value of WSUp may vary over the separator 1. For instance WSUp may in embodiments be larger near the edge of the separator 1, and smaller in a center region of the separator 1.

[0118] The embodiment depicted in Fig. 7B comprises substantially more membranes 100 then the one in Fig. 7A. Yet, the membranes 100 are much smaller as can be noticed by the comparing the respective sizes Smem. The minimum distance between the membranes 100 in Fig. 7B is also smaller than the minimum distance in Fig. 7A (compare the respective WSUp sizes). It will be understood that configuration differences like the ones shown in these figures may result in more or less robust separators 1. Therefore, the separator thickness ds(not visible in the figures) may especially be selected at a different value for the different embodiments of Figs 7A and 7B. lin Fig. 8, some further examples are depicted of partitions of the separator, further indicating that the membrane(s) 100 may be configured in many different ways and with many different shapes.

[0119] The terms “substantially” or “essentially” herein, and similar terms, will be understood by the person skilled in the art. The terms “substantially” or “essentially” may also include embodiments with “entirely”, “completely”, “all”, etc. Hence, in embodiments the adjective substantially or essentially may also be removed. Where applicable, the term “substantially” or the term “essentially” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. Moreover, the terms ’’about” and “approximately” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. For numerical values it is to be understood that the terms “substantially”, “essentially”, “about”, and “approximately” may also relate to the range of 90% - 110%, such as 95%-105%, especially 99%-101% of the values(s) it refers to.

[0120] The term “comprise” also includes embodiments wherein the term “comprises” means “consists of’. The term “and / or” especially relates to one or more of the items mentioned before and after “and / or”. For instance, a phrase “item 1 and / or item 2” and similar phrases may relate to one or more of item 1 and item 2. The term “comprising” may in an embodiment refer to “consisting of’ but may in another embodiment also refer to “containing at least the defined species and optionally one or more other species”.

[0121] Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

[0122] The devices, apparatus, or systems may herein amongst others be described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation, or devices, apparatus, or systems in operation.

[0123] The term “further embodiment” and similar terms may refer to an embodiment comprising the features of the previously discussed embodiment but may also refer to an alternative embodiment.

[0124] It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.

[0125] Use of the verb “to comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, “include”, “including”, “contain”, “containing” and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The term “plurality” refers to two or more. Furthermore, the terms “a plurality of’ and “a number of’ may be used interchangeably.

[0126] The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim, or an apparatus claim, or a system claim, enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0127] The invention also provides a control system that may control the device, apparatus, or system, or that may execute the herein described method or process. Yet further, the invention also provides a computer program product, when running on a computer which is functionally coupled to or comprised by the device, apparatus, or system, controls one or more controllable elements of such device, apparatus, or system.

[0128] The invention further applies to a device, apparatus, or system comprising one or more of the characterizing features described in the description and / or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterizing features described in the description and / or shown in the attached drawings. Moreover, if a method or an embodiment of the method is described being executed in a device, apparatus, or system, it will be understood that the device, apparatus, or system is suitable for or configured for (executing) the method or the embodiment of the method, respectively.

[0129] The various aspects discussed in this patent can be combined in order to provide additional advantages. Further, the person skilled in the art will understand that embodiments can be combined, and that also more than two embodiments can be combined. Furthermore, some of the features can form the basis for one or more divisional applications. EXPERIMENTAL

[0130] Fabrication of a zero-gap electrode

[0131] Zero-gap electrodes with a porous silicon separator were fabricated from a silicon-on-insulator (SOI) wafer. First, a 40 pm thick, porous membrane is etched using reactive ion etching, which results in well-defined, straight pores arranged in a hexagonal pattern. The porosity of the separator was varied by changing the pore diameter, dpand the pore spacing, sp. A silicon nitride layer was deposited on all silicon surfaces to ensure electrical insulation as well as the chemical stability of the separator in alkaline conditions.

[0132] Materials and methods

[0133] The porous silicon membranes were fabricated from silicon on an insulator (SOI) wafer from Si-Mat of 100 mm in diameter. The SOI wafer consists of a 500 pm thick Si handle layer, a 1pm insulating buried oxide (BOX) layer, and a 40 pm thick Si device layer. Nine porous silicon (PSi) chips per wafer, each 24 x 24 mm2were fabricated. The pore diameter was varied between 4 and 10 pm. The spacing was varied between 14 and 300 pm. Resulting in a porosity ranging from 0.06% to about 14%.

[0134] The fabrication commences over the device layer. The device layer is spin- coated with photoresist (Olin OiR 907-17, 4000 rpm, 30s) and baked at 120 °C for 60 s. The photoresist is patterned with a mask and developed in OPD 4262 for 45 s. In this case, three separate mask designs with different pore diameters (4 pm, 6 pm, and 10 pm) and pore spacing were used in the study. After development, the pores are etched in a plasma etcher (5 pm / min, Plasma Pro 100 Estrelas), resulting in pores of depths similar to the device layer (i.e., 40 pm). Later the handle layer is etched at 20 pm / min in the silicon plasma etcher (Plasma Pro 100 Estrelas). The BOX layer is used as a protective layer during the etching steps of the device layer and handle layer. After the BOX layer is removed with HF both sides of the silicon membrane are open and ready for subsequent processing.

[0135] A SixNy layer was deposited using LPCVD over the perforated silicon spacer to protect the silicon separator from the alkaline electrolyte.

[0136] The zero-gap configuration was completed by vapor-depositing metal layers (platinum or nickel) on either side of the silicon membrane. Platinum was used as a proof-of- concept catalyst, while nickel demonstrated the possibility of using earth-abundant catalysts, which optionally, could substitute platinum. The electrocatalyst (Pt or Ni) was evaporated at an angle to limit metal deposition inside the pore. The evaporation was done at a 10° angle with a Balzers BAK 600 with a crucible E Gun. A Tantalum adhesion layer of 30 nm (0.15 nm / s, 220 mA) was deposited and followed by a 200 nm Pt (0.4 nm / s, 250 mA) or Ni (0.3 nm / s, 120 mA). The base pressure was in the range of 5 x 10-7mbar and the process pressure at 2 x 10-6mbar. The final, active zero-gap electrode is a circular region of 2 cm diameter at the center of each chip resulting in an effective surface area of 3.14 cm2per substrate.

[0137] SEM images showed that the diameter of the pores decreases from 4 pm, 6 pm, and 10 pm initially to 2.4 pm, 4.1 pm, and 7.9 pm. A representative SEM image of the deposited metal Pt deposited showed some side pore coverage. The inside of the pore however remains as SixNy.

[0138] Ionic resistance of the silicon zero-gap separator

[0139] The ionic resistance of the silicon zero-gap separator (hereinafter PSi-ZGE) is measured using Galvano dynamic sweeps using a potentiostat connected to a four-electrode system. The current is swept from 0 to ± 10 mA / cm2with a 30 s rest (0 mA) between positive and negative sweeps. The resistance is calculated from the slope of potential and current according to Ohm's law. The separator resistance (Rs) is calculated as the additional cell resistance introduced by the PSi-ZGE over the pure solution resistance measured without a separator between the two half-cells. It is essential to mention that the vapor-deposited metal electrodes (Pt or Ni) on the porous silicon separator are not connected to the potentiostat during ionic resistance measurements to avoid bubble generation on the PSi-ZGE. The ionic resistance of bare PSi separators with no deposited metal is nearly identical to that of a metal-coated PSi- ZGE. This, in addition to the SEM image showed that the pores remained unobstructed during the deposition of the metal layers. The variation in Rsas a function of the porosity of the separator (a) show a decrease in resistance with increasing porosity, ranging from above 100 Qcm2at the lowest porosity, via about 2 to 3 Qcm2at a porosity of 0.5- 1% to values of 1.5 Qcm2or lower for porosity values higher than 1%. The resistance showed to be independent from the pore diameter. This can be explained by the fact that the size of the charge-carrying ions is orders of magnitude smaller than the pore diameters.

[0140] The ionic resistance of Zirfon at room temperature measured identically to the PSi-ZGE is 1.4 Qcm2. The non-conductive nature of the Zirfon increases the resistivity; thus, their structure needs to be optimized for lessened iR losses. The results show that PSi-ZGE can achieve lower ionic resistance than Zirfon at much lower porosities (about 1 to 2 % versus 50% porosity). We must note that the Zirfon separator is about 12 times thicker than the PSi-ZGEs device layer (ca. 40 pm). This contributes to the lower ionic resistance of PSi-ZGEs. However, an important result is that the PSi-ZGEs of a < 1 exhibit similar ionic resistance to Zirfon with s of -50%, -10-50 times more porous than our PSi-ZGEs. This is advantageous for the PSi- ZGEs ionic resistance. Similitude in the ionic resistance between PSi-ZGEs and Zirfon might be due to the silicon pore dimension and geometry that allows ionic shortcuts within 40 pm thick of the Psi separator. As ion transport could be affected between randomly or uniformly oriented geometries, it is necessary to look at the tortuosity. For the PSi-ZGEs, the resulting tortuosity is 1, as the thickness of the separator is the same as the length of the pores. In the case of Zirfon, ionic transport is influenced by the high tortuosity which has been reported to range between 1.5 and 2.8. Such large values are due to the separator constituents. Zirfon is made from packed spheres that give ionic paths many turns, while PSi-ZGEs can be defined as pores with straight channels that create shortcuts to lower ionic paths.

[0141] The Faradaic efficiency (FE)

[0142] The Faradaic efficiency (FE) of PSi-ZGE is evaluated in for multiple porosity series with a defined pore diameter of 4.1 pm. Most pore diameters have similar ionic-resistant; hence the pore diameter of 4.1 pm is used. Calibration curves were made to estimate the EE and O2 concentration and FE. The PSi-ZGEs experiments use a 200 nm layer thickness of Pt over the anode / cathode. Pt is selected as the main electrocatalyst in the PSi-ZGEs. An alternative to Pt is an earth-abundant electrocatalyst, such as Ni. Ni has the same layer thickness as Pt, and may be a more stable electrocatalyst for the Oxygen forming reaction. This low-cost catalyst may be ideal for larger-scale zero-gap membranes for alkali electrolysis.

[0143] The experiments show that FE for hydrogen (EE) and oxygen (O2) are nearly 100%. However, this is not the case for porosities higher than 0.61%. In this case, porosity values between 1.69 and 7.78 reveal a decrease in the total FE (H2 + O2) of 20% to 40% compared to 0.61% porosity or lower. Similar behavior has been observed for Ni. For porosities of 0.61%, Ni and Pt are close to 100%, but this is not the case for porosities of 3.81%, for which total FE (FET) remains below 85%. Such a low FET can be related to pore blocking (e.g., bubbles) during electrolysis. PSi-ZGE pore blocking by other materials, such as evaporated Pt or Ni, is discarded as the ionic resistance is low. Alternatively, large pores can lead to promoting water recombination during electrolysis. Furthermore, electrocatalyst composition does not have an apparent effect. Rather than carbon, no impurities over Pt or Ni have been observed.

[0144] The gas-separation capability

[0145] The gas-separation capability of the PSi-ZGEs is analyzed to evaluate its FET contribution. Fundamentally, the permeability of the gas-separator is related to the bubble point pressure (BPP) - which is the pressure required to blow air through a liquid-filled separator. The BPP can be correlated to the Laplace equation (AP = 2ycos0 / r), where AP is the pressure difference across the PSi-ZGEs, y is the surface tension at the liquid-air interface, 0 is the contact angle of the liquid with the PSi-ZGEs, and r is the pore radius in PSi-ZGEs43. For higher threshold pressure, smaller pore sizes and smaller contact angles are beneficial in decreasing permeability. Therefore, it is assumed that pore size or porosity is a decisive resistance parameter.

[0146] The experiments show that the crossover at the cathode reveals that O2 remains below the explosion limit (<4%) values for porosities between 0.067 and 7.78. The crossover of H2 to the anode unveils is at larger quantities than 6%. 6% of O2 in H2 is flammable. It appears that porosities of 0.067% and 0.15% reduce the risk of explosion with values below (<4%). For higher porosity than 0.15%, the EE impurity can be flammable because it is present at 7.5% and increases to 15% as the porosity increases. This is the case for the 7.78% porosity.

[0147] Hydrogen permeability to the anode is a latent issue in current industrial separators and is related to the pore dimensions. This is the case of Zirfon PERL and may also be the case for other separators with average pore diameters between 70-130 nm and separator thickness of 500 pm. With an average pore diameter of 130 nm and porosity close to 50%, Zirfon PERL H2 permeability is relatively high and close to 20* 10-12mol.m-1bar-1sec-1. Hence, it is not surprising that for 40 pm thin PSi-ZGEs with a porosity of 0.27% or higher, H2 permeates. The H2 crossover in can be mitigated by placing the electrodes far apart and collecting the gas at their respective electrode. However, this decreases the efficiency of the process because the ions have to travel a large distance between electrodes. For practical applications, the cathode cell could remain open during the operation. Under low temperatures, a separate Pt wire or mesh may be placed at the anode to promote water recombination. A commercially available separator commonly used is a flexible limiting the gas crossover to a maximum of 2 %. However, such separators use ionomers and impose large ion conductivity penalties. Hence, the most exciting alternative is ionomeric-free separators for alkali electrolysis to reduce costs further. This is the case for porosities of 0.067 and 0.15%, where the gas crossover is below the explosion limit. Nevertheless, a trade-off must be found between ionic conductivity and gas permeability.

[0148] The stability test

[0149] The stability of a platinum-coated PSi-ZGE (porosity 0.61%) was tested by operating it at a relatively high, constant nominal current density of 100 mA / cm2for a continuous 24 hours following a ramp of 30 mA / cm2for 14 h and 100 mA / cm2for 24 h. A relatively high potential of ~ 4.5 V is observed between the anode and the cathode. This is much larger than the values commonly reported in the literature for ZGEs. However, it is worth noting that experiments in this study are carried out at room temperature. It is likely that further improvements in the performance of the cell can be achieved by optimizing the electrochemical surface area. Perhaps more important is the lack of forced convection of the electrolyte in the experimental configuration. Firstly, it is reasonable to expect large pH gradients to develop between the anode and the cathode, contributing to a high overpotential. Secondly, the lack of forced convection likely leads to the holdup of gas inside the cell, and the adhesion of gas bubbles to the electrode surface. The presence of gas bubbles in the vicinity of the electrode is known to exacerbate Ohmic losses in electrolytic systems4. It is known that bubbles nucleating on electrodes cause fluctuations in potential, which is also observed in the experiments. For a constant rate of gas generation at the electrode, i.e., a constant current, it is possible to estimate the average departure size of bubbles from the electrode based on the frequency of fluctuations in the cell potential. Peak analysis yields a mean peak frequency of - 0.7 Hz. For a constant current density of 100 mA / cm2, this frequency corresponds to a bubble diameter of - 30 mm. This is larger than the diameter of the electrode and thus confirms the buildup of large gas slugs inside the electrochemical cell which periodically bubble up towards the bubbler.

[0150] Flow cell for experiments

[0151] To study the effect of forced electrolyte convection, a simple flow cell, as described in relation to Fig. 4B was used to further characterize the performance of the PSi- ZGEs. The flow cell is comprised of a pair of endplates machined out of polyoxymethylene (POM), two rubber gaskets, two stainless steel (SS316L) current collectors and a Teflon spacer which holds the studied PSi-ZGE in place. The edges of a central cutout in the current collectors holds the PSi-ZGE in place and provides electrical conductivity. Flow channels are machined into the gaskets and the current collectors. Standard flat-bottom fluidic connectors were used to fasten tubing onto the end plates.

[0152] Currents from 50 mA to 800 mA were applied and the potential was measured over 30 second periods at electrolyte flow rates of 5, 25, and 100 ml / min, provided to the tubing at both of the endplates. A syringe pump (Harvard Apparatus PHD 2000) was used for all flow experiments. Potentiograms of PSI-ZGEs of different porosities (a = 0.15 %, 8 = 0.6 %, a = 1.69 %, and a = 7.7 %) at different applied currents (I = 50 mA, I = 100 mA, I = 250 mA, I = 500 mA, and I = 800 mA) and different flow rates (5 ml / min, 25 ml / min, and 100 ml / min) were determined.

[0153] The measured potentiograms (i.e., showing the potential V as a function of time) showed that flow rate does not significantly affect the cell potential for the range of flow rates considered in the study. On the other hand, it was observed that an increasing applied current resulted in an increasing potential. For instance, for a = 7.7 % at a flow rate of 100 ml / min, the average cell potential (averaged over the porosity and the time) increased from 1.73 to 3.22 V when the current was increased from 50 to 800 mA, and for a = 0.15 % at a flow rate of 100 ml / min, the average cell potential increased from 2.01 V to 3.47 V when the current was increased from 50 mA to 250 mA. As a further example, for a = 7.7 % at a flow rate of 5 ml / min, the average cell potential increased from 1.76 to 3.22 V when the current was increased from 50 to 800 mA, and for a = 0.15 % at a flow rate of 5 ml / min, the average cell potential increased from 2.02 V to 3.50 V when the current was increased from 50 mA to 250 mA. It is noted that a potential limit of 5 V was applied to all experiments and experiments for separators with e < 0. 15 was not possible to perform the measurement for an applied current above 250 mA because the cell potential exceeded the 5 V limit imposed on the potentiostat.

[0154] In Fig. 5, the average cell potential (averaged over the measurement time of 30 seconds) at different applied currents and separator porosities at an electrolyte flowrate of 100 mL / min is schematically represented. In the figure, the separator porosity (in %) is given at the horizontal axis, the applied current is given at the vertical axis in mA; The depicted circles represent the different applied current (the center of the circles representing the respective current values). For clarity reasons, different line types (^— ; ; — - and — . . — ) are used representing 50 mA, 100 mA, 250 mA, 500 mA, and 800 mA). The size of the circles depict the magnitude of the average cell potential. The value of the average cell potential in Volt is also indicated in the respective circles.

[0155] As expected, the cell potential increases with increasing current and decreases with increasing separator porosity. Due to the design of the flow cell, the exposed electrode area was 450 mm2. Therefore, the highest currents 500 mA and 800 mA correspond to current densities of 111.11 mA / cm2and 177.77 mA / cm2, respectively. As expected, the introduction of forced convection alleviates the high potential described above.

[0156] Conclusion

[0157] In conclusion, a stable, silicon-based porous separator for alkaline water electrolysis with vapor-deposited metal electrodes in a zero-gap configuration is demonstrated. The approach is sufficiently versatile as pore size and porosity are modulated. The approach is not limited to one metal electrode type, as for instance platinum or nickel can be used. The separator coated with silicon nitride is stable under alkaline water electrolysis. The PSi-ZGEs have comparable, if not even better, ionic resistance to Zirfon UTP300 at a much lower porosity. Even in the absence of forced electrolyte convection, O2 cross-over into the cathodic gas outlet stream was well below the safety limit for 4.1 pm pores for all porosities evaluated. In the absence of electrolyte convection, the H2 contamination at the anode remains lower than the safety limit for separator porosities, c < 0.15. For e > 0.15, significant hydrogen contamination has been observed at the anodic gas outlet. The stability of the separators was evaluated by operating them at 100 mA / cm2over a 24 hour period. SEM images confirm that the electrode coating remains largely unaffected after the experiment but for a minor increase in surface roughness. Finally, it is shown that current densities as high as 177 mA / cm2can be reliably achieved in a flow configuration. Such PSi-ZGE can become useful platforms for compact alkaline water electrolyzers compatible with MEMS technology.

Claims

CLAIMS:

1. A separator (1) comprising a first separator side (2), a second separator side (3), a porous membrane (100), and a reinforcement support element (200), wherein the membrane (100) comprises a silicon comprising layer (110) having a first layer side (101) and a second layer side (102), wherein a distance between the first layer side (101) and the second layer side (102) defines a membrane thickness (dmem), the membrane comprises pores (140), the pores (140) defining open fluid channels from the first layer side (101) to the second layer side (102), a porosity of the membrane (100) is in the range 1-10%, a mean pore cross-sectional dimension (dp) is equal to or less than 10 pm, the membrane thickness (dmem) is equal to or less than 200 pm, the membrane comprises a first layer (121) arranged at the first layer side (101) and a second layer (122) arranged at the second layer side (102), wherein the first layer (121) is electrically conductive, and the second layer (122) is electrically conductive, wherein the first layer (121) directly contacts the silicon comprising layer (110), and the second layer (122) directly contacts the silicon comprising layer (110), the reinforcement support element (200) is configured from the first separator side (2) to the second separator side (3), wherein the reinforcement support element (200) is connected to the silicon comprising layer (110), wherein the reinforcement support element (200) extends from the separator (1) at at least one side selected from the first separator side (2) and the second separator side (3), and a maximal distance between the first separator side (2) and the second separator side (3), and defined perpendicular to a virtual plane (P) of the silicon comprising layer (110), defines a separator thickness (ds), wherein the separator thickness (ds) is selected from the range of 50 pm - 3 cm.

2. The separator (1) according to claim 1, wherein the pores (140) have a straight tubular configuration, wherein the first layer (121) extends over the reinforcement element (200) at the first separator side (2), and wherein the second layer (122) extends over the reinforcement element (200) at the second separator side (3).

3. The separator (1) according to any one of the preceding claims, wherein the separator comprises at least 100 membranes (100), wherein the reinforcement support element (200) is configured enclosing any one of the membranes (100), and wherein the at least 100 membranes are configured recessed in the reinforcement support element (200).

4. The separator (1) according to any one of the preceding claims, wherein the silicon comprising layer (110) comprises a silicon layer (115) covered with silicon nitride (H6).

5. The separator (1) according to any one of the preceding claims, wherein the first layer (121) and the second layer (122) independently from each other comprise one or more electrically conductive materials selected from the group consisting of carbon, metals, metal oxides, metal nitrides, metal halides, metal alloys, and conducting polymers.

6. The separator (1) according to any one of the preceding claims, wherein the first layer (121) comprises a first electrode material (1210), wherein the second layer (122) comprises a second electrode material (1220), wherein the first electrode material (1210) and the second electrode material (1220) independently from each are one or more materials selected from the group consisting of platinum, cobalt, nickel, iron, iridium, copper, aluminum, titanium.

7. The separator (1) according to any one of the preceding claims, wherein the membrane (100) has an ionic resistance of < 10 cm2, wherein the ionic resistance is measured by Galvano dynamic sweeps over a current of 0-10 mA / cm2, with an electrolyte concentration of IM NaOH.

8. An alkaline water electrolyzer (1000) comprising an anode chamber (1100), a cathode chamber (1200), and a separator (1) according to any one of the preceding claims, wherein the separator (1) separates the anode chamber (1100) from the cathode chamber (1200).

9. The alkaline water electrolyzer (1000) according to claim 8, whereinthe anode chamber (1100) comprises a first gas diffusion layer (1130) contacting the membrane (100), a first endplate (1190), and a first current collector (1180) arranged between the first gas diffusion layer (1130) and the first endplate (1190), the cathode chamber (1200) comprises a second gas diffusion layer (1230) contacting the membrane (100), a second endplate (1290), and a second current collector (1280) arranged between the second gas diffusion layer (1230) and the second endplate (1290), the gas diffusion layers (1130, 1230) are electrically conductive, the current collectors (1180, 1280) are electrically conductive, and, the first current collector (1180) and the first gas diffusion layer (1130) are electrical conductively coupled, and the second current collector (1280) and the second gas diffusion layer (1230) are electrical conductively coupled.

10. The alkaline water electrolyzer (1000) according to any one of the claims 8-9, wherein: the anode chamber (1100) comprises a first fluid inlet (1101) and a first fluid outlet (1109) and the cathode chamber (1200) comprises a second fluid inlet (1201) and a second fluid outlet (1209), for one or more of (i) providing alkaline water in the respective chambers (1100, 1200) and (ii) flowing alkaline water through the respective chambers (1100, 1200), the anode chamber (1100) comprises a first gas outlet (1105) for discharging oxygen gas from the anode chamber (1100), and the cathode chamber (1200) comprises a second gas outlet (1205) for discharging hydrogen gas from the cathode chamber (1200).

11. The alkaline water electrolyzer (1000) according to claim 9 and claim 10, wherein one or more of the first current collector (1180) comprises a first channel configured in open fluid connection with the first gas diffusion layer (1130), wherein the first channel and the first gas diffusion layer (1130) together define a first flow channel (1185), wherein the first fluid inlet (1101) and the and the first fluid outlet (1109) are connected to the first flow channel (1185), and the second current collector (1280) comprises a second channel arranged in open fluid connection with the second gas diffusion layer (1230), wherein the second channel and the second gas diffusion layer (1230) together define a second flow channel (1285), whereinthe second fluid inlet (1201) and the second fluid outlet (1209) are connected to the second flow channel (1285).

12. The alkaline water electrolyzer (1000) according to any one of the claims 8-11, comprising a power supply (1300) comprising a positive electrode (1301) and a negative electrode (1302), wherein the positive electrode (1301) is configured in electrical connection with the first layer (121) and the negative electrode (1302) is configured in electrical connection with the second layer (122).

13. A method for fabrication of a separator (1), the method comprising a silicon layer fabrication stage, a subsequent covering stage, and an electrode provision stage; wherein the silicon layer fabrication stage comprises: providing a silicon wafer (10) consisting of a silicon handle layer (13) at a first side of the silicon wafer (10), a silicon device layer (11) at a second side of the silicon wafer (10), and an insulating oxide layer (12) configured between the device layer (11) and the handle layer (13), providing pits in the device layer (11) at a predetermined membrane zone (15) of the silicon wafer (10) by deep reactive ion etching to provide a patterned silicon wafer (10), and removing the handle layer (13) and the insulating layer (12) at the membrane zone (15) from the patterned silicon wafer (10) to provide a silicon layer (115) at the membrane zone (15), wherein the pits define channels (119) from a first side (117) of the silicon layer (115) to a second side (118) of the silicon layer (115); wherein the covering stage comprises: depositing silicon nitride (116) on the silicon wafer (10), wherein a total surface area of (i) the first side of the silicon wafer (10), (ii) the second side of the silicon wafer (10), and (iii) the channels (19) is covered with silicon nitride (116), to provide a silicon comprising layer (110) comprising pores (140) from a first layer side (101) to a second layer side (102) of the silicon comprising layer (110) at the membrane zone; wherein the electrode provision stage comprises: providing a first electrically conductive layer (121) at the first layer side (101) of the silicon comprising layer (110), and providing a second electrically conductive layer (122) at the second layer side(102) of the silicon comprising layer (110).

14. The method according to claim 13, wherein the silicon layer fabrication stage comprises after providing the silicon wafer: providing a photoresist (21) on the device layer (11), patterning the photoresist (21) using a mask, to provide the device layer (11) comprising a patterned photoresist, subjecting the device layer (11) comprising the patterned photoresist to an etching plasma, providing the photoresist (21) on the handle layer (13) at locations not comprising the membrane zone (15), patterning the photoresist (21) using a mask, to provide the handle layer (13) comprising a patterned photoresist, and subjecting the device handle layer (13) comprising the patterned photoresist to an etching plasma to provide the patterned silicon wafer (10).

15. The method according to any one of claims 13-14, wherein the electrode provision stage comprises: (i) depositing an adhesion material on the first layer side (101) and on the second layer side (102) for providing an adhesion layer covering the first layer side (101) and the second layer side (102), and (ii) depositing a first metal comprising material (1210) at the adhesion layer configured at the first layer side (101) and a second metal comprising material (1220) at the adhesion layer configured at the second layer side (102), to provide the first layer (121) and the second layer (122) at the first layer side (101) and the second layer side (102), wherein the adhesion material, the first metal comprising material (1210), and the second metal comprising material (1220) are deposited via chemical vapor deposition, wherein depositing is with the wafer placed at a non-straight angle relative to the direction of the chemical vapor flux.

16. The method according to any one of the claims 13-15, wherein the silicon wafer is selected for having a thickness in the range of 50 pm - 3 cm, and wherein the silicon wafer is selected for having a thickness of the device layer in the range of 5-200 pm, wherein the pits are provided having a cross sectional dimension selected from the range of 1- 20 pm, and wherein a total cross-sectional area of the pits over a total area of the silicon layer (115) comprising the pits is selected from the range of 0.1-50%.

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