WO2025228831A1

WO2025228831A1 - Electrolyte flow electrodeposition

Info

Publication number: WO2025228831A1
Application number: PCT/EP2025/061399
Authority: WO
Inventors: Bart Juul Wilhelmina Van Den Bossche
Original assignee: Elsyca NV
Current assignee: Elsyca NV
Priority date: 2024-04-29
Filing date: 2025-04-25
Publication date: 2025-11-06
Anticipated expiration: 2026-10-29
Also published as: EP4644582A1

Abstract

Example embodiments relate to a method and a system for electrodepositing a catalyst layer on a porous substrate. The method comprises, during an electrodeposition period, providing an electrolyte flow (320) through the porous substrate (312), wherein the electrolyte (302) comprises ions of at least one catalyst material; applying electrical pulses (341-344) between an anode (301) and the porous substrate (312) at inter-pulse intervals (352) to electrodeposit the ions of the at least one catalyst material on the porous substrate, thereby forming the catalyst layer and decreasing a local concentration of the ions at the porous substrate interface (311) during the respective electrical pulses (341-344); and wherein the electrical pulses are applied such that the electrolyte flow (320) at least partially restores the local concentration of the ions at the porous substrate interface (311) during the respective inter-pulse intervals (352).

Description

ELECTROLYTE FLOW ELECTRODEPOSITION

Field of the Invention

[01] The present invention generally relates to electrodepositing a catalyst layer on a porous substrate, in particular for manufacturing a porous electrode.

Background of the Invention

[02] Porous electrodes are electrodes that have a structure with interconnected pores, also referred to as openings or voids, throughout their material. These pores increase the surface area of the electrode, allowing for greater contact with the electrolyte in electrochemical systems, i.e. increasing the surface area of the electrode-electrolyte interface. This increased active surface area enhances the efficiency of electrochemical processes such as charge transfer and mass transport, which are vital for the performance of electrochemical devices and systems such as, for example, batteries, fuel cells, electrolysers, and capacitors.

[03] Porous electrodes typically comprise a porous substrate coated with a catalyst layer. The porous substrate, sometimes also referred to as the support, is a material or composite that does not participate in the electrochemical reactions but provides structural integrity and stability for the active catalytic species. The catalyst layer comprises at least one catalyst material for facilitating the desired electrochemical reactions at the electrode-electrolyte interface. Examples of catalyst materials include metals like platinum, palladium, or nickel; metal oxides; or other compounds with catalytic properties.

[04] A problem with manufacturing porous electrodes is that electrodepositing a catalyst layer on a porous substrate has a limited throwing power within the porous substrate. In other words, it is a problem to electrodeposit a catalyst layer with a substantially uniform thickness across the interior surfaces of a porous substrate. Summary of the Invention

[05] It is an object of the present invention, amongst others, to solve or alleviate the above identified problems and challenges by improving the throwing power of electrodepositing a catalyst layer on a porous substrate.

[06] According to a first aspect, this object is achieved by a method for electrodepositing a catalyst layer on a porous substrate. The method comprises, during an electrodeposition period providing an electrolyte flow through the porous substrate, wherein the electrolyte comprises ions of at least one catalyst material; and applying electrical pulses between an anode and the porous substrate at interpulse intervals to electrodeposit the ions of the at least one catalyst material on the porous substrate, thereby forming the catalyst layer and decreasing a local concentration of the ions at the porous substrate interface during the respective electrical pulses; wherein the electrical pulses are applied such that the electrolyte flow at least partially restores the local concentration of the ions at the porous substrate interface during the respective inter-pulse intervals.

[07] Thus, while an electrical pulse is applied between the anode and the porous substrate, ions of the at least one catalyst material are deposited onto the surface of the porous substrate. This deposition consumes catalyst ions within the electrolyte thereby decreasing the local concentration of the catalyst ions in the electrolyte at the interface between the porous substrate and the electrolyte, also referred to as the porous substrate interface. Thus, a portion of the catalyst layer is electrodeposited onto the porous substrate during the respective electrical pulses. By repeating the electrical pulses, a catalyst layer with a desired thickness can be obtained, thereby coating the porous substrate with the catalyst layer. The coated porous substrate may form a porous electrode for use in electrochemical devices and systems such as, for example, batteries, fuel cells, electrolysers, and capacitors. The electrical pulses may, for example, be voltage pulses applied between the anode and the porous substrate or current pulses between the anode and the porous substrate.

[08] Substantially no current is applied between the anode and the porous substrate during the period between two successive electrical pulses, i.e. during the inter-pulse interval. Alternatively, a smaller voltage difference is applied between the anode and the porous substrate during the inter-pulse interval, resulting in a very limited current relative to the electrical pulse. As such, substantially no electrodeposition occurs during the inter-pulse interval and, thus, the consumption of catalyst ions at the porous substrate interface is temporarily paused between successive electrical pulses. During the inter-pulse intervals, the provided electrolyte flow through the porous substrate supplies fresh electrolyte to the porous substrate interface throughout the porous substrate. As such, the provided electrolyte flow allows at least partially restoring the local concentration of catalyst ions at the porous substrate interface between successive pulsed electrodepositions.

[09] By providing the electrolyte flow, sufficient catalyst ions are made available at the porous substrate interface for substantially uniform electrodeposition during the next electrical pulse. This avoids a local deficit of catalyst ions within the pores of the porous substrate which typically results in an uneven thickness of the electrodeposited catalyst layer, i.e. which results in a low throwing power. The electrolyte flow may be provided continuously during the electrodeposition period, e.g. a steady electrolyte flow with a velocity of around 0.1 mm/s. Alternatively, the electrolyte flow may be provided intermittently as long as the flow is provided during the inter-pulse intervals, e.g. only during the inter-pulse intervals. The magnitude of this velocity will be practically limited by, for example, the required pressure difference for a given pump system, or by the risk for damaging the porous structure.

[10] The degree of restoration in local catalyst ion concentration during the interpulse intervals may depend on the duration of the inter-pulse intervals and the velocity of the electrolyte flow. The degree of depletion in local catalyst ion concentration during the electrical pulses may depend on a magnitude of the applied electrical pulses and the duration of the electrical pulses. The electrical pulses may, for example, be applied such that a balance is achieved between the depletion and restoration of the local concentration of catalyst ions at the porous substrate interface.

[11] Providing an electrolyte flow in addition to applying pulsed electrodeposition thus allows electrodepositing a porous substrate with a catalyst layer having a substantially uniform thickness across the entire surface of the substrate. In other words, it allows electrodepositing a porous substrate with a high throwing power. The method further allows consistently electrodepositing catalyst layers onto porous substrates having a thickness within specified tolerances, i.e. it allows electrodepositing porous substrates with an improved process capability. This has the advantage that the method can enable effective manufacturing of porous electrodes.

[12] According to an example embodiment, the electrolyte flow may be provided through a thickness of the porous substrate, and wherein a duration of the inter-pulse interval is at least equal to a ratio of the thickness of the porous substrate and a velocity of the electrolyte flow.

[13] The electrolyte flow may thus be provided such that it flows substantially perpendicular to an outer plane of the porous substrate which is defined by the length and width of the porous substrate. The velocity of the electrolyte flow may, for example, be around 0.1 mm/s. The thickness of the porous substrate may, for example, be around 200 pm. Applying the electrical pulses with an inter-pulse interval at least equal to a ratio of the thickness and a velocity of the electrolyte flow allows completely refreshing the electrolyte within the pores of the porous substrate during each inter-pulse interval. In other words, this allows fully restoring the local concentration of the ions at the porous substrate interface throughout the entire porous substrate during the inter-pulse interval. This has the advantage that it can further improve the throwing power of the electrodeposition.

[14] According to an example embodiment, a duration of the respective electrical pulses may be based on a bulk concentration of the ions of the at least one catalyst material within the electrolyte and a magnitude of the electrical pulse.

[15] The magnitude of the electrical pulse may for example be expressed in terms of applied voltage, charge, current, or power. An electrical pulse with a higher magnitude results in a higher consumption rate of the catalyst ions, i.e. a faster electrodeposition of the catalyst ions on the porous substrate. The duration of the respective electrical pulses may be smaller than one second, e.g. 0.1 s. [16] According to an example embodiment, a duration of the respective electrical pulses may be at most equal to a time interval for decreasing the local concentration of the ions at the porous substrate interface below a threshold concentration in absence of the electrolyte flow.

[17] The threshold concentration may be the lower limit of the local concentration of catalyst ions at the porous substrate interface that still allows obtaining electrodeposition according to process specifications. In other words, performing electrodeposition with a local ion concentration below the threshold concentration may result in a catalyst layer that does not meet specifications in terms of layer quality or composition.

[18] According to an example embodiment, the anode may be located upstream and/or downstream of the porous substrate relative to the direction of the electrolyte flow.

[19] The anode may thus be located upstream of the porous substrate relative to the direction of the electrolyte flow, or downstream of the porous substrate relative to the direction of the electrolyte flow. Alternatively, the anode may be located both upstream and downstream of the porous substrate. In other words, two separate electrodes may be provided at opposite sides of the porous substrate that jointly serve as the anode.

[20] According to an example embodiment, the method may further comprise, during a second electrodeposition period following the first electrodeposition period, reversing the direction of the electrolyte flow through the porous substrate.

[21] Thus, an electrolyte flow may be provided through the porous substrate in a first direction during a first electrodeposition period. During this first electrodeposition period, electrical pulses are applied between the anode and the porous substrate at inter-pulse intervals such that the electrolyte flow at least partially restores the local concentration of the ions at the porous substrate interface during the inter-pulse intervals. Upon completion of the first electrodeposition period, e.g. after 30 seconds, a second electrodeposition period may start during which an electrolyte flow is provided through the porous substrate in the reverse direction relative to the first electrolyte flow direction provided during the first electrodeposition period. During the second electrodeposition period, electrical pulses are applied between the anode and the porous substrate in a similar manner as during the first electrodeposition period, i.e. such that the reverse electrolyte flow at least partially restores the local concentration of the ions at the porous substrate interface during the inter-pulse intervals. The inter-pulse intervals and the electrical pulses may be substantially the same as during the first electrodeposition period, e.g. they may have the same duration and magnitude. Alternatively, the electrical pulses during the first and second electrodeposition period may be different as long as they allow partial restoration of the local ion concentration at the porous substrate interface during the inter-pulse intervals.

[22] Reversing the electrolyte flow allows electrodepositing a catalyst layer with a more uniform thickness onto a porous substrate. In other words, this further improves the throwing power of the electrodeposition compared to providing the electrolyte flow in a single direction. This has the further advantage that it greatly improves the process capability of electrodepositing porous substrates with a catalyst layer, as it allows consistently electrodepositing catalyst layers onto porous substrates within more narrow tolerances compared to providing the electrolyte flow in a single direction.

[23] According to an example embodiment, the method may further comprise, during the second electrodeposition period, applying the electrical pulses between a second anode and the porous substrate at an inter-pulse interval, wherein the second anode is located at an opposite side of the porous substrate relative to the first anode.

[24] Thus, the anode of the first electrodeposition period may be an electrode located upstream or downstream of the porous substrate relative to the direction of the electrolyte flow. This first anode may only be used during the first electrodeposition period. The second anode is a distinct electrode located at an opposite side of the porous substrate. Thus, if the first anode is located upstream of the porous substrate relative to the direction of the electrolyte flow during the first electrodeposition period, the second anode is located downstream of the porous substrate relative to the direction of the electrolyte flow during the first electrodeposition period, and vice-versa.

[25] According to an example embodiment, the method may further comprise repeating the first electrodeposition period and the second electrodeposition period.

[26] The first electrodeposition period and second electrodeposition period may thus be repeated until a catalyst layer of a desired thickness is deposited onto the porous substrate. The first and second electrodeposition periods may have the same duration or may have different durations.

[27] According to an example embodiment, the first electrodeposition period and the second electrodeposition period may have a duration at least equal to the combined duration of one electrical pulse and one inter-pulse interval.

[28] According to an example embodiment, the method may further comprise depositing a conductive seed layer between the porous substrate and the catalyst layer.

[29] Depositing the conductive seed layer can, for example, be achieved by an electroless deposition process, also referred to as autocatalytic deposition. This may be performed before electrodepositing the catalyst layer onto the porous substrate. In other words, the catalyst layer may be electrodeposited onto the conductive seed layer. The conductive seed layer may, for example, be made of Nickel, Silver, or Copper. The conductive seed layer may be substantially thin, i.e. having a thickness smaller than 1 pm. The conductive seed layer allows increasing the conductivity of the porous substrate. This has the advantage that it enables electrodepositing a catalyst layer onto initially non-conductive porous substrates. The conductive seed layer may further promote adhesion and improve uniform electrodeposition of the catalyst layer.

[30] According to an example embodiment, the method may further comprise providing the electrolyte flow in a plating tank comprising the electrolyte wherein the porous substrate is submerged. [31] According to an example embodiment, providing the electrolyte flow may further comprise pumping electrolyte through the porous substrate or mechanically oscillating the porous substrate in the plating tank.

[32] According to an example embodiment, the porous substrate may be a foam material, a packed bed material, or a fibre cloth material.

[33] A foam material may be made from, for example, polyurethane, polyethylene, or other polymers. A fibre cloth material may be made from woven or non-woven fibres. The fibres can be natural fibres, e.g. cotton, wool, or silk, or synthetic fibres, e.g. polyester, nylon, fibreglass, metal, or carbon. Packed bed materials may include a matrix or grid of, for example, carbon-based materials, metals, ceramic materials, polymeric materials, and composite materials.

[34] According to an example embodiment, the catalyst layer may be free of platinum group metals.

[35] This allows manufacturing Platinum Group Metal, PGM, free porous electrodes which are more cost-effective, more sustainable, and less reliant on scarce resources compared to flat plain or flat mesh type electrodes plated with PGM layers.

[36] According to an example embodiment, the at least one catalyst material may comprise Nickel, Cobalt, Copper, Tin, or Silver.

[37] Ions of one or more of these catalyst materials may thus be provided within the electrolyte for electrodeposition onto the porous substrate. As a result, the catalyst layer will comprise one or more of these catalyst materials.

[38] According to a second aspect, the invention relates to a system for electrodepositing a catalyst layer on a porous substrate, the system comprising:

- a plating tank comprising an electrolyte that comprises ions of at least one catalyst material;

- a receptacle configured to hold the porous substrate submerged in the electrolyte within the plating tank, wherein the receptacle is in electrical contact with the porous substrate; - at least one flow generating means configured to provide an electrolyte flow through the porous substrate;

- at least one anode located upstream and/or downstream of the porous substrate relative to the direction of the electrolyte flow; and

- at least one pulse generating means configured to apply electrical pulses between the at least one anode and the porous substrate at inter-pulse intervals to electrodeposit the ions of the at least one catalyst material on the porous substrate, thereby forming the catalyst layer and decreasing a local concentration of the ions at the porous substrate interface during the respective electrical pulses; and wherein the at least one pulse generating means is further configured to apply the electrical pulses such that the electrolyte flow at least partially restores the local concentration of the ions at the porous substrate interface during the respective inter-pulse intervals.

Brief Description of the Drawings

[39] Fig. 1 shows a cross section of a typical electrodeposition process for coating a porous substrate with a catalyst layer;

[40] Fig. 2 shows the method for electrodepositing a catalyst layer on a porous substrate according to example embodiments;

[41] Fig. 3 shows the at least partial restoration of the local concentration of catalyst ions at the porous substrate interface by providing an electrolyte flow during inter-pulse intervals and the resulting uniform catalyst layer thickness, according to example embodiments;

[42] Fig. 4A shows the evolution of the local concentration of catalyst ions at the porous substrate interface and the resulting catalyst layer thickness when performing a pulsed electrodeposition process without providing an electrolyte flow;

[43] Fig. 4B shows the evolution of the local concentration of catalyst ions at the porous substrate interface and the resulting catalyst layer thickness when performing an electrodeposition process with an electrolyte flow but without applying electrical pulses; and

[44] Fig. 5 shows the method for electrodepositing a catalyst layer on a porous substrate further comprising reversing the direction of the electrolyte flow though the porous substrate, according to example embodiments.

Detailed Description of Embodiment(s)

[45] Fig. 1 shows a cross section 100 of a typical electrodeposition process during which a porous substrate 110 is coated with a catalyst layer. A porous substrate 110 may refer to a material that comprises a network of interconnected pores or voids within its structure. These pores can range in size from nanometres to millimetres and may be distributed uniformly or non-uniform ly throughout the porous substrate 110. During electrodeposition, the porous substrate 110 is typically submerged in an electrolyte 102. As such, the electrolyte 102 fills the interconnected pores 111 of the porous substrate and contacts the porous substrate interface. It will be apparent that 100 shows an illustrative example of a porous substrate 110 with a structure 112 having uniformly distributed circular cross sections with equal surface areas, but that the cross sections of a porous substrate structure 112 may vary in size and/or shape. Porous substrates 110 can, for example, be made from metals, ceramics, polymers, and composites.

[46] Electrolyte 102 typically comprises ions of at least one catalyst material that is to be electrodeposited onto the porous substrate 110. To this end, a constant voltage or current is typically applied between an anode 101 and the porous substrate 112. Catalyst materials are active substances responsible for facilitating a desired chemical or electrochemical reaction when the coated porous substrate is in use, e.g. in an electrochemical system. Catalyst materials typically provide active sites where reactants can undergo chemical or electrochemical transformations. Examples of catalyst materials include metals like platinum, gold, nickel, and nickel alloys, as well as metal oxides as for example Iridium oxide, or other compounds with catalytic properties. [47] The layer comprising the at least one catalyst material that is deposited onto the porous substrate 110 is referred to as the catalyst layer. The catalyst layer may further comprise one or more modifying materials, e.g. nanoparticles, to further enhance its catalytic performance or stability. The catalyst layer may thus be a composite material or metal alloy, comprising one or more catalyst materials, which is coated onto a porous substrate.

[48] The structure obtained by electrodepositing a catalyst layer onto a porous substrate can be used as a porous electrode in electrochemical systems such as, for example, batteries, capacitors, fuel cells, and electrolysers. The porosity of the porous substrate 110 provides a large surface area for electrochemical reactions to occur relative to the geometrical volume occupied by the porous structure. Porous electrodes are promising for developing Platinum Group Metal, PGM, free electrodes as their enhanced surface area can compensate for the reduced catalytic activity typically associated with PGM free electrodes.

[49] A problem with manufacturing porous electrodes is that electrodepositing a catalyst layer on a porous substrate has a limited throwing power due to the fact that the main transport means for catalyst ions into the pores of the substrate is by diffusion. Diffusion is a relatively slow process, whereas the consumption rate of catalyst ions by electrodeposition reaction on the surface of the pores is relatively high. As a result, pores located deeper within the porous substrate structure will be deprived more from fresh supply of catalyst ions. In other words, it is a problem to electrodeposit a catalyst layer with a substantially uniform thickness across the entire surface of a porous substrate. Fig. 1 illustrates this problem by showing an example 120 of the local concentration of catalyst ions within the electrolyte 102 during a typical electrodeposition process 100. Fig. 1 further shows the resulting thickness 140 of the deposited catalyst layer throughout 121 the porous substrate, i.e. at positions P1 - Pn along the entire thickness of the porous substrate.

[50] The catalyst layer thickness 140 indicates that the electrodeposition of the catalyst layer in a typical electrodeposition process is limited to the outer surface areas of the porous substrate, i.e. positions P1 , P2, Pn-1 , and Pn. In this example, the obtained catalyst layer has a thickness of around 45 m at positions P1 and Pn; and a thickness of around 5 pm at positions P2 and Pn-1 . Deeper within the porous substrate, i.e. at positions P3 - Pn-2, the thickness of the deposited catalyst layer is around 0 pm. This is because the concentration of catalyst ions in the electrolyte within the pores of the porous substrate 110 is completed depleted, as illustrated by 120. This results in a very poor throwing power as the thickness 140 of the catalyst layer varies greatly along the surface of the porous substrate, i.e. the thickness is uneven or non-uniform.

[51] Therefore, it is an object of the present invention to provide a method for electrodepositing a catalyst layer onto a porous substrate that improves the throwing power, thereby enabling effective manufacturing of porous electrodes.

[52] Fig. 2 illustrates the method for electrodepositing a catalyst layer onto a porous substrate 212 according to an example embodiment of the invention. The method comprises, during an electrodeposition period, applying electrical pulses 241 - 244 between an anode 201 and the porous substrate 212 at inter-pulse intervals 252 - 254 to electrodeposit ions of at least one catalyst material on the porous substrate 212 during the electrical pulses 241 - 244. The catalyst ions are included within the electrolyte 202. The at least one catalyst material may, for example, include Nickel, Cobalt, Copper, Tin, or Silver. Ions of one or more of these catalyst materials may be provided within the electrolyte 202 for electrodeposition onto the porous substrate 212. The electrolyte 202 surrounds and contacts the porous substrate 212 at the porous substrate interface 211 , i.e. the contact surface between the porous substrate 212 and the electrolyte 202. By the electrodeposition of the catalyst ions, a catalyst layer is formed onto the porous substrate 212 and the local concentration of the ions at the porous substrate interface 211 decreases as the free catalyst ions within the electrolyte 202 are consumed. It will be apparent that, when a portion of the catalyst layer has been deposited onto the porous substrate 212, e.g. after applying a first electrical pulse 241 , the porous substrate interface 211 may refer to the contact surface between this portion of the catalyst layer and the electrolyte 202.

[53] The electrical pulses 241 - 244 may be characterized by a pulse duration or length 251 , and by a magnitude 255. The electrical pulses 241 - 244 may, for example, be voltage pulses applied between the anode 201 and the porous substrate 212 or current pulses applied between the anode 201 and the porous substrate 212. The magnitude 255 of the electrical pulse 241 - 244 may, for example, be expressed in terms of applied voltage, charge, current, or power. Substantially no current is applied between the anode 201 and the porous substrate 212 during the period 252 between two successive electrical pulses 241 , 242, i.e. during an inter-pulse interval 252 - 254. Alternatively, a smaller voltage difference is applied between the anode

201 and the porous substrate 212 during the inter-pulse intervals 252 - 254, resulting in a very limited current during the inter-pulse intervals 252 - 254 relative to during the electrical pulses 241 - 244. As such, substantially no electrodeposition occurs during the inter-pulse intervals 252 - 254 and, thus, the consumption of catalyst ions at the porous substrate interface 211 is temporarily paused between successive electrical pulses 241 - 244. It will thus be apparent that, during the inter-pulse intervals 252 - 254 a limited current or voltage may still be provided as long as it is limited relative to the voltage or current provided during the electrical pulses 241 - 244 and thus does not result in substantial electrodeposition during the inter-pulse intervals. For example, when applying a voltage of around 1 .0 V during the electrical pulses 241 - 244, a voltage of around 0.3 V may still be provided during the interpulse intervals 252 - 254.

[54] The method further comprises, during the electrodeposition period, providing an electrolyte flow 220 through the porous substrate 212. In other words, electrolyte

202 is moved through the interconnected pores or voids of the porous substrate 212. The electrolyte flow 220 may be provided continuously during the electrodeposition period, e.g. by providing a steady electrolyte flow 220 of around 0.1 mm/s. Alternatively, the electrolyte flow 220 may be provided intermittently as long as the flow 220 is provided during the inter-pulse intervals 252 - 254, e.g. only during the inter-pulse intervals 252 - 254. In doing so, the flow 220 through the interconnected pores within the porous substrate 212 supplies fresh electrolyte 202 to the porous substrate interface 211 throughout the porous substrate 210 during the inter-pulse intervals 252 - 254. As such, providing the electrolyte flow 220 allows at least partially restoring the local concentration of the catalyst ions at the porous substrate interface 211 between the pulsed electrodepositions 240, i.e. between the electrical pulses 241 - 244. [55] Fig. 3 illustrates the at least partial restoring of the local concentration of catalyst ions at the porous substrate interface by providing an electrolyte flow 220 during inter-pulse intervals 252 - 254. Fig. 3 shows an example of the evolution of the local concentration 301 - 304 of catalyst ions within the electrolyte when providing a constant electrolyte flow at a velocity of 0.1 mm/s through a porous substrate 210 with a thickness D of 0.2 mm. The applied electrical pulses 241 - 244 in this example have a duration of 0.1s and the inter-pulse intervals 252 - 254 have a duration of 2s.

[56] The evolution of the local catalyst ion concentration 301 - 304 is shown in the middle of the first electrical pulse 241 , i.e. at timestep 301 ; at the end of the first electrical pulse 241 , i.e. at timestep t₂ 302; in the middle of inter-pulse interval 252, i.e. at timestep t₃ 303; and at the end of the inter-pulse interval 252, i.e. at timestep t₄ 304. At the start of the first electrical pulse 241 , the concentration of catalyst ions at the porous substrate interface may be substantially equal to the bulk concentration of catalyst ions provided within the electrolyte, e.g. around 220 mol/m³ At timestep 301 , the electrodeposition during the first half of electrical pulse 241 has consumed a portion of the catalyst ions in the electrolyte within the pores of the porous substrate, thereby reducing the local concentration of catalyst ions at the porous substrate interface to around 160 mol/m³. After applying the entire electrical pulse 241 , i.e. at timestep t₂ 302, the concentration of catalyst ions at the porous substrate interface may further decrease to around 100 mol/m³. By providing the electrolyte flow 220 and pausing the electrodeposition during the inter-pulse interval 252, fresh electrolyte 305 is supplied through the porous substrate 210. This is illustrated in 303 at timestep t₃, which shows that the local concentration of catalyst ions in the electrolyte within the pores of the porous substrate is partially restored when half of the inter-pulse interval 252 has elapsed. By the end of inter-pulse interval 252, i.e. at timestep t₄ 304, the local concentration of catalyst ions is substantially restored within the porous substrate. It will be apparent that in the example of Fig. 3 the local concentration of catalyst ions is completely restored to the starting concentration, i.e. around 220 mol/m³, during the inter-pulse interval 252, but that the extent of restoration during a single inter-pules interval may be less as long as sufficient catalyst ions are provided by the electrolyte flow 220 for the next electrical pulse. [57] Thus by providing the electrolyte flow 220 during the inter-pulse interval 252, sufficient catalyst ions are made available at the porous substrate interface for substantially uniform electrodeposition during the next electrical pulse 242. This avoids a local deficit of catalyst ions within the pores of the porous substrate which typically results in an uneven thickness of the electrodeposited catalyst layer, i.e. which results in a low throwing power. Fig. 3 further shows the thickness 310 of the catalyst layer obtained during the example electrodeposition with forced electrolyte flow and pulsed electrodeposition according to embodiments of the invention. The thickness of the catalyst layer is plotted at positions P1 - Pn throughout the thickness 311 of the porous substrate 210. The catalyst layer thickness 310 gradually decreases from around 9 pm at position P1 to around 3 pm at position Pn along the direction of the electrolyte flow 220. This example electrodeposition according to embodiments of the invention yields an average catalyst layer thickness d_avg of around 5 pm with a standard deviation a_s of around 1 .24 pm.

[58] Providing an electrolyte flow 220 in addition to applying pulsed electrodeposition 240 thus allows electrodepositing a porous substrate with a catalyst layer having a substantially uniform thickness across the entire surface of the substrate 210. In other words, it allows electrodepositing a porous substrate with a high throwing power. It further allows consistently electrodepositing catalyst layers onto porous substrates having a thickness within specified tolerances, i.e. it allows electrodepositing porous substrates with an improved process capability. For example, the process capability index or ratio C_pk of the example electrodeposition process described in relation to Fig. 3 is +0.27, while a typical electrodeposition process as described in relation to Fig. 1 only has a process capability index of around -0.15. The process capability index or ratio may be determined as wherein N expresses the number of points P1 - Pn, d_max expresses the maximal allowable thickness, for example defined as d_avg - 2|im, d expresses the thickness of the catalyst layer at point n, and d_min expresses the minimal required thickness as d_avg + 2|im. A positive capability index indicates that the method can enable effective manufacturing of porous electrodes.

[59] For comparison, Fig. 4A and Fig. 4B show the evolution of catalyst ion concentrations 401 , 402, 410 and the resulting catalyst layer thickness 403, 412 when performing an electrodeposition process similar to the electrodeposition according to example embodiments described in relation to Fig. 3 but respectively without providing an electrolyte flow in Fig. 4A, and without applying electrical pulses in Fig. 4B.

[60] The electrodeposition process of Fig. 4A is performed on the same porous substrate with a thickness of 0.2 mm as the electrodeposition process of Fig. 3. Electrical pulses are applied that have a duration of 0.1s at inter-pulse intervals of 2s. No electrolyte flow is provided. Fig. 4A shows the local concentration of catalyst ions within the porous substrate at the end of an electrical pulse 401 . This illustrates that the local concentration of catalyst ions at the porous substrate interface is substantially depleted at the end of an electrical pulse, in particular at the middle portion of the porous substrate. Fig. 4A further shows the local concentration of catalyst ions at the end of an inter-pulse interval 402, i.e. momentarily before applying a next electrical pulse. This illustrates that, in absence of an electrolyte flow, an inter-pulse interval between electrical pulses only allows limited catalyst ion diffusion towards the depleted electrolyte regions within the porous substrate. Thus, without providing an electrolyte flow, the local concentration of catalyst ions is insufficiently restored. As a result, insufficient catalyst ions are made available at the porous substrate interface for substantially uniform electrodeposition during the next electrical pulse. As such, the obtained thickness 403 of a catalyst layer when performing pulsed electrodeposition without providing an electrolyte flow varies substantially from around 24 pm and 15 pm at the outer surfaces of the porous substrate to around 0 pm in the centre of the porous substrate. While the average catalyst layer thickness d_avg is similar to the process of Fig. 3, i.e. around 5 pm, the standard deviation a_s is significantly higher at around 4.92 pm. The process capability index C_pk of the electrodeposition process described in relation to Fig. 4A is also significantly worse at -0.12, which makes this electrodeposition process unsuitable to reliably manufacture porous electrodes.

[61] The electrodeposition process of Fig. 4B is also performed on the same porous substrate with a thickness D of 0.2 mm as the electrodeposition process of Fig. 3 and Fig. 4A. During this electrodeposition process, a constant electrolyte flow 411 is provided through the porous substrate at a velocity of 0.1 mm/s. No electrical pulses are applied. Instead, a constant current or voltage may be applied between an anode and the porous substrate. Fig. 4B shows in 412 that this electrodeposition process fails to form a catalyst layer on the entire surface of the porous substrate, as most catalyst ions are deposited at the outer surface areas of the porous substrate located upstream relative to the electrolyte flow 411. Again, the average catalyst layer thickness d_avg is similar to the process of Fig. 3 and Fig. 4A, i.e. around 5 pm. However, the standard deviation a_s is even higher at around 11.76 pm. The process capability index C_pk of the electrodeposition process described in relation to Fig. 4B is also significantly worse at -0.15, which also makes this electrodeposition process unsuitable to reliably manufacture porous electrodes.

[62] Returning to Fig. 2, the electrolyte flow 220 may preferably be provided through a thickness D of the porous substrate 210. The electrolyte flow 220 may thus be provided such that it flows substantially perpendicular to an outer plane of the porous substrate 210, which is defined by the length and width of the porous substrate. This may be achieved by providing the electrolyte flow 220 in a plating tank that holds the electrolyte 202 and by submerging the porous substrate 210 therein. To this end, the porous substrate may be provided into a receptacle configured to hold the porous substrate submerged in the electrolyte 202 within the plating tank. The receptacle may further be in electrical contact with the porous substrate 210 such that the electrical pulses may be applied via the receptacle. The receptacle may thus hold the porous substrate submerged within electrolyte 202 in a position that allows the provided electrolyte flow 220 to flow through the thickness D of the porous substrate. Alternatively, the electrolyte flow 220 may be provided through the porous substrate along any other direction. [63] The electrolyte flow 220 may be provided by pumping electrolyte through the porous substrate. This can, for example, be achieved by one or more pumps that move fresh electrolyte 202 through the plating tank. Alternatively, the electrolyte flow 220 may be provided by mechanically oscillating the porous substrate within the plating tank, e.g. by mechanically oscillating the receptacle holding the porous substrate.

[64] When providing an electrolyte flow 220 through the thickness D of the porous substrate 210, the duration t_pause of the inter-pulse intervals 252 - 254 may preferably be at least equal to a ratio of the thickness of the porous substrate D and a velocity v of the electrolyte flow 220, i.e. t_pause > ^D/_v. This allows completely refreshing the electrolyte within the pores of the porous substrate during each interpulse interval 252 - 254. In other words, this allows fully restoring the local concentration of catalyst ions at the porous substrate interface 211 throughout the entire porous substrate 210 during each inter-pulse interval 252 - 254. This has the advantage that it can further improve the throwing power of the electrodeposition.

[65] The duration 251 of the electrical pulses 241 - 244 may be at most equal to a time interval for decreasing the local concentration of the catalyst ions at the porous substrate interface 211 below a threshold concentration in absence of the electrolyte flow 220. In other words, the duration t_puise of the applied electrical pulses 241 - 244 is preferably wherein T_exhaustion represents the time to completely exhaust the catalyst ions at a certain catalyst ion consumption rate CR without providing an electrolyte flow, and wherein u represents the fraction between a threshold concentration C_T and the bulk concentration C_B of the catalyst ions in the electrolyte. The threshold concentration C_T u may be the lower limit of the local concentration of catalyst ions at the porous substrate interface 211 that still allows obtaining an electrodeposited catalyst layer according to process specifications in terms of layer quality or composition. In other words, performing electrodeposition with a local ion concentration below the threshold concentration may result in a catalyst layer that does not meet quality or composition specifications. The catalyst ion consumption rate CR is indicative for the rate at which the electrodeposition process during the electrical pulses 241 - 244 consumes the catalyst ions from the electrolyte. This rate may depend on the magnitude 255 of the applied electrical pulses 241 - 244, the used catalyst materials, and the chemistry of the electrolyte containing the catalyst ions. The catalyst ion consumption rate may be determined as wherein j represents the average electrode current density on the porous substrate; e represents the plating efficiency for the ions of the at least one catalyst material; z represents the charge of the ions of the at least one catalyst material; F is the Faraday constant; and s represents the active surface area of the porous substrate per volume unit.

[66] The time T_exhaustion to completely exhaust the catalyst ions may then be determined as wherein C_B represents the bulk concentration of the at least one catalyst material within the electrolyte 202, and p represents the volume fraction of the pores within the porous substrate 210.

[67] The duration t_puise 251 of the electrical pulses 241 - 244 may, for example, be 0.032s or 0.096s when the duration t_pause of the inter-pulse intervals 252 - 254 is 0.133s or 0.400s, respectively. The electrical pulses 241 - 244 may be applied in this manner when, for example, the thickness of the porous substrate D is 400 pm; the velocity of the provided electrolyte flow v is 1 mm/s; the fraction it is 0.7; the average electrode current density j is 200 A/m²; the plating efficiency for the ions of the at least one catalyst material e is 0.5; the charge of the ions of the at least one catalyst material z is 2; the active surface area of the porous substrate per volume unit s is 2.4E+5 m²/m³; the bulk concentration of the at least one catalyst material within the electrolyte C_B is 100 mol/m³; and the volume fraction of the pores within the porous substrate p is 0.4.

[68] It will further be apparent that the electrical pulses 241 - 244 in the example embodiment of Fig. 2 are substantially the same, i.e. that the duration and magnitude of the electrical pulses 241 - 244 and the duration of the inter-pulse intervals 252 - 254 remain substantially the same during the electrodeposition period. However, the electrical pulses 241 - 244 may also be applied dynamically in time, i.e. by varying the duration of the respective electrical pulses, the magnitude of the respective electrical pulses, and/or the duration of the respective inter-pulse intervals.

[69] The anode 201 may be located upstream of the porous substrate 212 relative to the direction of the electrolyte flow 220, as illustrated in Fig. 2. Alternatively, the anode may be located downstream of the porous substrate 212 relative to the direction of the electrolyte flow 220, i.e. at the opposite end of 200 (not shown in Fig. 2). Alternatively, the anode may be located both upstream and downstream of the porous substrate (not shown in Fig. 2). In other words, two separate electrodes may be provided at opposite sides of the porous substrate 212 that jointly serve as the anode.

[70] The method may further comprise depositing a conductive seed layer between the porous substrate 210 and the catalyst layer. The conductive seed layer may, for example, be made of Gold, Nickel, Silver, or Copper. The conductive seed layer may be substantially thin, i.e. having a thickness smaller than 1 pm. Such a conductive seed layer allows ensuring or increasing the conductivity of the porous substrate prior to performing the electrodeposition process. This can be achieved by, for example, performing an electroless deposition process, also referred to as autocatalytic deposition. Depositing the conductive seed layer may be performed before electrodepositing the catalyst layer onto the porous substrate. In other words, the catalyst layer may be electrodeposited onto the conductive seed layer. This has the advantage that it enables electrodepositing a catalyst layer onto non-conductive porous substrates. The conductive seed layer may further promote adhesion and improve uniform electrodeposition of the catalyst layer. [71] The porous substrate may be a foam material made from, for example, polyurethane, polyethylene, or other polymers. Alternatively, the porous substrate may be a fibre cloth material made from, for example, woven or non-woven fibres. These fibres can be natural fibres such as cotton, wool, or silk; or synthetic fibres such as polyester, nylon, fibreglass, steel, or carbon. Alternatively, the porous substrate may be a packed bed material made from a matrix or grid of carbon-based materials, metals, ceramic materials, polymeric materials, or composite materials.

[72] The at least one catalyst material may preferably be Nickel, Cobalt, Copper, Tin, or Silver. By using one or more of these catalyst materials, a catalyst layer may be obtained that is substantially free of Platinum Group Metals, PGM. This allows manufacturing PGM-free porous electrodes which are more cost-effective, more sustainable, and less reliant on scarce resources compared to plain or flat mesh type electrodes plated with PGM layers.

[73] Fig. 5 shows an example embodiment of the method for electrodepositing a catalyst layer on a porous substrate further comprising reversing 500b the direction of the electrolyte flow 520b through the porous substrate 510 during a second electrodeposition period 552 following a first electrodeposition period 551 . During the first electrodeposition period 551 , the method as described in relation to Fig. 2 may be performed, i.e. an electrolyte flow 520a may be provided through the porous substrate 510 in a first direction during the first electrodeposition period 551. During this first electrodeposition period 551 , electrical pulses 553, 554 are applied between the anode 501a and the porous substrate 312 at inter-pulse intervals 561 , 562 such that the electrolyte flow 520a at least partially restores the local concentration of the catalyst ions at the porous substrate interface 511 during the inter-pulse intervals 561 , 562. Upon completion of the first electrodeposition period 551 , e.g. after 30 seconds, the second electrodeposition period 552 may start during which an electrolyte flow 520b is provided through the porous substrate 510 in the reverse direction relative to the first electrolyte flow direction 520a.

[74] During the second electrodeposition period 552, electrical pulses 555, 556 may be applied in a similar manner as during the first electrodeposition period 551 , i.e. such that the reverse electrolyte flow 520b at least partially restores the local concentration of the catalyst ions at the porous substrate interface 511 during the inter-pulse intervals 557. The electrical pulses 555, 556 may be applied between a second anode 501 b and the porous substrate 312, wherein the second anode 501 b may be located at an opposite side of the porous substrate 510 relative to the first anode 501a. The second anode 501 b may thus be a distinct electrode located at an opposite side of the porous substrate 510. For example, if the first anode 501a is an electrode located upstream of the porous substrate 510 relative to the first electrolyte flow direction 520a during the first electrodeposition period 551 , the second anode 501 b may be located upstream of the porous substrate 510 relative to the reversed electrolyte flow direction 520b, and vice-versa. When the second electrodeposition period 552 starts at timestep t_rev 560, the direction of the electrolyte flow may thus be reversed and the anode used for applying the electrical pulses 555, 556 may be switched from the first anode 501 a to the second anode 501 b.

[75] Alternatively, the same anode 501a may be used during both electrodeposition periods 551 , 552. In this case, the anode may be located at any side of the porous substrate or at both sides simultaneously as discussed above in relation to Fig. 2. It will further be apparent that Fig. 5 only shows an illustrative example where the first anode 501 a and second anode 501 b are located upstream relative to the respective electrolyte flow directions 520a, 520b; and that the first and second anode may also be located downstream relative to the respective electrolyte flow directions 520a, 520b.

[76] The inter-pulse intervals 563 and the electrical pulses 555, 556 during the second electrodeposition period 552 may be substantially the same as the intervals 561 - 562 and pulses 553, 554 during the first electrodeposition period 551 , e.g. they may have the same duration 251 and magnitude 255. Alternatively, the electrical pulses during the first and second electrodeposition period may be different as long as they are applied such that they allow partial restoration of the local catalyst ion concentration at the porous substrate interface during the inter-pulse intervals.

[77] The first 551 and second electrodeposition period 552 may further be repeated one or more times. The periods 551 , 552 may, for example, be repeated until a catalyst layer of a desired average thickness is deposited onto the porous substrate. In other words, the direction of the electrolyte flow may be reversed a plurality of times. The first and second electrodeposition periods 551 , 552 may have the same duration or may have different durations. The duration of the first and second electrodeposition periods 551 , 552 may at least be equal to the combined duration of one electrical pulse 553 - 556 and one inter-pulse interval 352, 557 during said period.

[78] Reversing the electrolyte flow allows electrodepositing a catalyst layer with a more uniform thickness onto a porous substrate. In other words, this further improves the throwing power of the electrodeposition compared to providing the electrolyte flow in a single direction, e.g. as described in relation to Fig. 2. Fig. 5 shows the obtained thickness 570 of the catalyst layer along the thickness 503 of the porous substrate 510. When performing the electrodeposition at the same conditions as the electrodeposition in Fig. 3, Fig. 4A, and Fig. 4B, the formed catalyst layer has an average thickness d_avg of around 5 pm with a standard deviation a_s of only around 0.36 pm. Reversing the electrolyte flow has the further advantage that it greatly improves the process capability of electrodepositing porous substrates with a catalyst layer. The process capability index C_pk of the electrodeposition process described in relation to Fig. 5 is significantly improved to +1.62, which allows consistently electrodepositing catalyst layers onto porous substrates within more narrow tolerances compared to providing the electrolyte flow in a single direction. This has the advantage that this electrodeposition process is highly suitable to reliably manufacture porous electrodes.

[79] Although the present invention has been illustrated by reference to specific embodiments, it will be apparent to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied with various changes and modifications without departing from the scope thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. In other words, it is contemplated to cover any and all modifications, variations or equivalents that fall within the scope of the basic underlying principles and whose essential attributes are claimed in this patent application. It will furthermore be understood by the reader of this patent application that the words “comprising” or “comprise” do not exclude other elements or steps, that the words “a” or “an” do not exclude a plurality, and that a single element, such as a computer system, a processor, or another integrated unit may fulfil the functions of several means recited in the claims. Any reference signs in the claims shall not be construed as limiting the respective claims concerned. The terms “first”, “second”, third”, “a”, “b”, “c”, and the like, when used in the description or in the claims are introduced to distinguish between similar elements or steps and are not necessarily describing a sequential or chronological order. Similarly, the terms “top”, “bottom”, “over”, “under”, and the like are introduced for descriptive purposes and not necessarily to denote relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and embodiments of the invention are capable of operating according to the present invention in other sequences, or in orientations different from the one(s) described or illustrated above.

Claims

1. A method for electrodepositing a catalyst layer on a porous substrate (312); the method comprising, during an electrodeposition period:

- providing an electrolyte flow (320) through the porous substrate (312), wherein the electrolyte (302) comprises ions of at least one catalyst material;

- applying electrical pulses (341 - 344) between an anode (301 ) and the porous substrate (312) at inter-pulse intervals (352) to electrodeposit the ions of the at least one catalyst material on the porous substrate, thereby forming the catalyst layer and decreasing a local concentration of the ions at the porous substrate interface (311 ) during the respective electrical pulses (341 - 344); and wherein the electrical pulses are applied such that the electrolyte flow (320) at least partially restores the local concentration of the ions at the porous substrate interface (311 ) during the respective inter-pulse intervals (352).

2. The method according to claim 1 , wherein the electrolyte flow (320) is provided through a thickness of the porous substrate, and wherein a duration of the inter-pulse interval (352) is at least equal to a ratio of the thickness of the porous substrate and a velocity of the electrolyte flow.

3. The method according to any of the preceding claims, wherein a duration of the respective electrical pulses (351) is based on a bulk concentration of the ions of the at least one catalyst material within the electrolyte and a magnitude of the electrical pulse.

4. The method according to any of the preceding claims, wherein a duration of the respective electrical pulses (351 ) is at most equal to a time interval for decreasing the local concentration of the ions at the porous substrate interface below a threshold concentration in absence of the electrolyte flow.

5. The method according to any of the preceding claims, wherein the anode (301 ) is located upstream and/or downstream of the porous substrate (312) relative to the direction of the electrolyte flow (320).

6. The method according to any of the preceding claims, further comprising, during a second electrodeposition period (552) following the first electrodeposition period (551 ), reversing (500b) the direction of the electrolyte flow (520b) through the porous substrate (312).

7. The method according to claim 5 and 6, further comprising, during the second electrodeposition period (552), applying the electrical pulses (555, 556) between a second anode (501 b) and the porous substrate (312) at an interpulse interval (557), wherein the second anode is located at an opposite side of the porous substrate relative to the first anode.

8. The method according to claim 6 or 7, further comprising repeating the first electrodeposition period (551 ) and the second electrodeposition period (552).

9. The method according to any of the preceding claims, further comprising depositing a conductive seed layer between the porous substrate and the catalyst layer.

10. The method according to any of the preceding claims, further comprising providing the electrolyte flow (320, 520a, 520b) in a plating tank comprising the electrolyte (302, 502) wherein the porous substrate (312) is submerged.

11 . The method according to claim 10, wherein providing the electrolyte flow (320, 520a, 520b) comprises pumping electrolyte (302, 502) through the porous substrate (312) or mechanically oscillating the porous substrate in the plating tank.

12. The method according to any of the preceding claims, wherein the porous substrate (312) is a foam material, a packed bed material, or a fibre cloth material.

13. The method according to any of the preceding claims, wherein the catalyst layer is free of platinum group metals.

14. The method according to any of the preceding claims, wherein the at least one catalyst material comprises Nickel, Cobalt, Copper, Tin, or Silver.

15. A system for electrodepositing a catalyst layer on a porous substrate, the system comprising:

- a plating tank comprising an electrolyte (302, 502) that comprises ions of at least one catalyst material;

- a receptacle configured to hold the porous substrate submerged in the electrolyte within the plating tank, wherein the receptacle is in electrical contact with the porous substrate;

- at least one flow generating means configured to provide an electrolyte flow (320, 520a, 520b) through the porous substrate (302, 502);

- at least one anode (301 , 501a, 501b) located upstream and/or downstream of the porous substrate relative to the direction of the electrolyte flow; and

- at least one pulse generating means configured to apply electrical pulses (341 - 344, 553 - 556) between the at least one anode and the porous substrate at inter-pulse intervals (352, 557) to electrodeposit the ions of the at least one catalyst material on the porous substrate, thereby forming the catalyst layer and decreasing a local concentration of the ions at the porous substrate interface (311 , 511 ) during the respective electrical pulses (341 - 344, 553 - 556); and wherein the at least one pulse generating means is further configured to apply the electrical pulses such that the electrolyte flow (320, 520a, 520b) at least partially restores the local concentration of the ions at the porous substrate interface (311 , 511 ) during the respective inter-pulse intervals (352, 557).