electrolyser

Abstract

An electrolyser comprises a first porous electrode; a first outlet for a first electrolysis product; a second porous electrode; and a second outlet for a second electrolysis product. A spacer region having a longitudinal axis separates the first porous electrode and second porous electrode in a direction that is transverse to the longitudinal axis. The first porous electrode comprises: a first interface side that interfaces with the spacer region; and a first interface region extending from the first interface side. The second porous electrode comprises: a second interface side that interfaces with the spacer region; and a second interface region extending from the second interface side. The average pore size in at least the first interface region of the first porous electrode and in at least the second interface region of the second porous electrode is 25 μm or less.

Description

ELECTROLYSER

[0001] This invention relates to an electrolyser. In particular this invention relates to an electrolyser having a first porous electrode and a second porous electrode.BACKGROUND

[0002] Commercial electrolysis cells, or electrolysers, require a resistive, ion- permeable, largely gas-impermeable separator membrane between the electrodes to minimise the mixing of the electrolysis products. The electrolysis products may be dissolved species or gas bubbles, for example bubbles of hydrogen and oxygen in the case of water electrolysis. Membranes are expensive, can be intolerant of some cell chemistries and can be a limiting factor on the efficiency of a cell.

[0003] Attempts have been made to design electrolysis cells, which function without the use of a membrane. However, such systems have struggled to adequately prevent mixing of electrolysis products. In addition, such systems have issues of non-uniform flow through the cell or electrodes which limit the scalability.

[0004] It is an aim of the present invention to solve, mitigate or obviate, at least partly, at least one of the problems and / or disadvantages associated with known membraneless electrolysers.BRIEF SUMMARY

[0005] Embodiments of the present invention provide an electrolyser that operates without a physical barrier between the electrodes to prevent mixing of electrolysis products. Advantageously, the resultant system is cost effective as the membranes used for physical barriers can be expensive. In addition, this will result in replacing the solid membrane electrolyte with 10-100times higher conductivity liquid electrolyte in the gap between the electrodes. This in turn results in lower ohmic overpotentials and higher efficiency.

[0006] Embodiments of the present invention provide an electrolyser that is scalable in size, without a detrimental impact on efficiency or the amount of gas cross-over.

[0007] According to a first aspect of the invention there is provided an electrolyser comprising: a first electrode compartment comprising: a first porous electrode; and a first outlet for a first electrolysis product; a second electrode compartment comprising: a second porous electrode; and a second outlet for a second electrolysis product; a spacer region having a longitudinal axis, the spacer region separating the first porous electrode and second porous electrode in a direction that is transverse to the longitudinal axis, wherein the spacer region comprises an electrolyte inlet and permits the flow of an electrolyte from the electrolyte inlet along the longitudinal axis; wherein the first porous electrode comprises: a first interface side that interfaces with the spacer region; and a first interface region extending from the first interface side; wherein the second porous electrode comprises: a second interface side that interfaces with the spacer region; and a second interface region extending from the second interface side.

[0008] In certain embodiments, the average pore size in at least the first interface region of the first porous electrode is 25 pm or less. In certain embodiments, the average pore size in at least the second interface region of the second porous electrode is 25 pm or less.

[0009] In certain embodiments, the porosity of the first and second porous electrodes is from 20% to 90%.

[0010] In certain embodiments, the average pore size in at least the first interface region of the first porous electrode is from 1 pm to 10 pm, wherein the average pore size in at least the second interface region of the second porous electrode is from 1 pm to 10 pm.

[0011] In certain embodiments, the average pore size in at least the first interface region of the first porous electrode is from 3pm to 10 pm, wherein the average pore size in at least the second interface region of the second porous electrode is from 3pm to 10 pm.

[0012] In certain embodiments, the electrolyser further comprises a porous barrier layer positioned between the first interface side and the spacer region and / or between the second interface side and the spacer region.

[0013] In certain embodiments, the average pore size of the porous barrier layer is 25 pm or less.

[0014] In certain embodiments, the average pore size of the porous barrier layer is equal to or smaller than the average pore size of the first and second porous electrodes.

[0015] In certain embodiments, the thickness of the porous barrier layer is from 1-100 pm.

[0016] In certain embodiments, the porosity of the porous barrier layer is >20%.

[0017] In certain embodiments, the porous barrier layer is coated on, or attached to, at least one of the first interface side and the second interface side.

[0018] In certain embodiments, the porous barrier layer positioned between the first interface side and the spacer region is spaced from first interface side by a distance that is greater than the average pore size of the porous barrier layer, and / or wherein the porous barrier layer positioned between the second interface side and the spacer region is spaced from second interface side by a distance that is greater than the average pore size of the porous barrier layer.

[0019] In certain embodiments, the porous barrier layer is hydrophilic.

[0020] In certain embodiments, the porous barrier layer is inactive.

[0021] In certain embodiments, the electrolyser further comprises a pump configured to pump electrolyte through the electrolyte inlet.

[0022] In certain embodiments, the electrolyte inlet is at a first longitudinal end of the spacer region, wherein the second longitudinal end of the spacer region comprises a wall or boundary element extending between the first porous electrode and the second porous electrode.

[0023] In certain embodiments, the first porous electrode comprises an active material, wherein the second porous electrode comprises an active material.

[0024] In certain embodiments, the first porous electrode comprises a hydrophilic material, wherein the second porous electrode comprises a hydrophilic material.

[0025] In certain embodiments, the first electrode compartment comprises an anode chamber and the first porous electrode is a porous anode, wherein the second electrode compartment comprises a cathode chamber and the second porous electrode is a porous cathode.

[0026] In certain embodiments, wherein the first electrode compartment and the second electrode compartment comprise first and second electrically conductive elements in contact with the first porous electrode the second porous electrode, respectively,wherein the first and second electrically conductive elements are connected to a DC power source.

[0027] In certain embodiments, the maximum permeability, kmax, for at least one of the first and second porous electrodes is defined by:where xgapis the separation between the first porous electrode and the second porous electrode in a direction that is transverse to the longitudinal axis; xthickis the thickness of the porous electrode; and xlengthis the length of electrolyte within the spacer region.

[0028] In certain embodiments, the maximum average pore size, Dmax, in the first porous electrode and the second porous electrode is defined by:where e is the combined porosity of the first and second porous electrode, xgapis the separation between the first porous electrode and the second porous electrode in a direction that is transverse to the longitudinal axis; xthickis the thickness of the porous electrode; and xlengthis the length of electrolyte within the spacer region.

[0029] According to another aspect of the invention there is provided an electrolyser comprising: a first electrode compartment comprising: a first porous electrode; and a first outlet for a first electrolysis product; a second electrode compartment comprising: a second porous electrode; and a second outlet for a second electrolysis product; a spacer region having a longitudinal axis, the spacer region separating the first porous electrode and second porous electrode in a direction that is transverse to the longitudinal axis, wherein the spacer region comprises an electrolyte inlet and permits the flow of an electrolyte from the electrolyte inlet along the longitudinal axis; wherein the first porous electrode comprises: a first interface side that interfaces with the spacer region; and a first interface region extending from the first interface side; wherein the second porous electrode comprises: a second interface side that interfaces with the spacer region; and a second interface region extending from the second interface side, wherein the maximum average pore size, Dmax, in the first porous electrode and the second porous electrode is defined by:where e is the combined porosity of the first and second porous electrode, xgapis the separation between the first porous electrode and the second porous electrode in a direction that is transverse to the longitudinal axis; xthickis the thickness of the porous electrode; and xlengthis the height of electrolyte within the spacer region.

[0030] According to another aspect of the present invention there is provided an electrolyser stack comprising an assembly of two or more electrolyser cells of the first aspect of the invention.

[0031] In certain embodiments the two or more electrolyser cells of the electrolyser stack are electrically connected in series in bipolar configuration.

[0032] According to another aspect of the present invention there is provided a method of using the electrolyser of the first aspect of the invention, wherein the method comprises: applying a potential or current across the first porous electrode and the second porous electrode; and pumping an electrolyte through the spacer region.

[0033] In certain embodiments, the method comprises pumping the electrolyte through the spacer region at a Reynolds number of below 1000, aptly below 500.

[0034] In certain embodiments, the method comprises pumping the electrolyte through the spacer region at a Reynolds number of at least 25, aptly at least 35, aptly at least 50, aptly at least 60.

[0035] As used herein, the term ‘porous electrode’ refers to an electrode that is electrolyte permeable by convective permeation and capillary action of the electrolyte through the porous structure of the porous electrode. The porous structure of the porous electrode may be considered to be an interconnected network of pores and pore throats. The interconnected network of pores and pore throat may have a tortuosity of below 2 (that is, a passage through the interconnected network of pores and pore throats from a first point to a second point may have a length that is below 2 times larger than the absolute, or straight-line, distance between the first point and the second point), aptly below 1.5. It would be understood that the associated ‘pore size’ of the porous electrolyte may refer to both the pores and the pore throats. The porous structure of the porous electrode may be formed from a mesh, compressed or sintered powders or a weave ofwires. Alternatively, the layer can be fabricated through templating or additive manufacturing method. The porous electrode can be made of active materials or be made of electrically conductive materials coated with active materials.

[0036] As used herein, ‘pore size’ refers to a dimension taken across a cross-section of the pore. Put another way, the ‘pore size’ of a pore is a dimension extending from a first position at the perimeter of the pore cross-section to a second position at the perimeter of the pore cross-section, the first position being diametrically opposed to the second position. For example, for a pore having a circular cross-section the pore size is a diameter of the cross-section. For a pore having a cross-section of irregular shape, the pore size may refer to a minimum smallest dimension (for example the smallest distance between diametrically opposed points that can be found around the perimeter of the cross-section) or an effective dimension (for example the diameter of a circle that approximates or circumscribes the cross-section). It would be understood that the crosssection referred to herein is a cross-section that is transverse to a longitudinal axis through the pore. Put another way, the cross-section referred to herein is a cross-section that is transverse to the direction of flow through the pore. As used herein, the term ‘porous barrier layer’ refers to a barrier layer that is electrolyte permeable by convective permeation and capillary action of the electrolyte through the porous structure of the porous electrode. The porous structure of the porous barrier layer may be formed from a mesh or a weave of wires or as a coated or deposited porous layer on top of the porous electrode through various deposition techniques.

[0037] As used herein, the use of ‘average’ in the term ‘average pore size’ may refer to a mean average. It would be understood that the average pore size refers to the average size of those pores that allow convective permeation and capillary action of electrolyte through the porous structure of the porous electrode. That is, the porous electrodes are continuous porous media, with open interconnected pores providing a fluid flow path therethrough. The average pore size refers to the average size of those open pores that are interconnected with adjacent pores or pore throats to provide a potential fluid flow path.BRIEF DESCRIPTION OF THE DRAWINGS

[0038] FIG. 1 illustrates an example electrolyser according to an embodiment of the invention.

[0039] FIG. 2 is a graph showing the relationship between average pore radius in the porous electrodes and the ratio between the pressure drop across the electrodes and the pressure drop along the spacer region of an example electrolyser.

[0040] FIG. 3 is a graph showing the relationship between the flow of electroyte on the gas cross-over for an example electrolyser.

[0041] FIG. 4A and FIG. 4B are graphs showing the relationship between the flow of electrolyte, longitudal length and current density on the gas cross-over for an example electrolyser.

[0042] FIG. 5A to 5C are graphs showing the derived equations from simulation results to predict the maximum permeability for electrode and maximum average pore diameter along with reference to locations of experimental prototypes.DETAILED DESCRIPTION

[0043] FIG. 1 shows an example of a single electrolyser or electrolyser cell 100. The electrolyser 100 includes a first electrode compartment 102. The first electrode compartment 102 includes a first porous electrode 104 and a first outlet 106 for a first electrolysis product. The electrolyser 100 includes a second electrode compartment 114. The second electrode compartment 114 includes a second porous electrode 116 and a second outlet 118 for a second electrolysis product.

[0044] In this example, the first electrode compartment 102 is an anode chamber with the first porous electrode 104 being an anode and the second electrode compartment 114 is a cathode chamber with the second porous electrode 116 being a cathode. However, in another examples the first electrode compartment 102 may be the cathode chamber and the second electrode compartment 114 may be the anode chamber.

[0045] In this example, the first electrode compartment 102 includes an internal volume 136 positioned between the first porous electrode 104 and the first outlet 106. The second electrode compartment 114 includes an internal volume 136 positioned between the second porous electrode 116 and the second outlet 118.

[0046] In this example the first electrode compartment 102, the second electrode compartment 114 and the spacer region 112 are housed within an external housing 150.

[0047] The electrolyser 100 includes a spacer region 112 having a longitudinal axis 126. Using the direction convention shown in FIG. 1 , the longitudinal axis 126 extends in the Z-direction. The spacer region 112 separates the first porous electrode 104 and the second porous electrode 116 in a direction that is transverse to the longitudinal axis 126 (the X-direction). The separation between the first porous electrode 104 and the second porous electrode 116 is labelled as xgapin FIG. 1 .

[0048] The spacer region 112 includes an electrolyte inlet 128 and permits the flow of an electrolyte from the electrolyte inlet 128 along the longitudinal axis 126. In the illustrated example, the electrolyte inlet 128 is at a first longitudinal end of the spacer region 112. In this manner, flow of electrolyte passing through the electrolyte inlet 128 is generally towards a second longitudinal end of the spacer region 112.

[0049] The first porous electrode 104 includes a first interface side 108 that interfaces with the spacer region 112. The first porous electrode 104 includes a first interface region 110 extending from the first interface side 108. In this example, the first interface region 110 extends from the first interface side 108 in a traverse direction to the longitudinal axis 126 (the X-direction), away from the spacer region 112.

[0050] The second porous electrode 116 includes a second interface side 120 that interfaces with the spacer region 112. The second porous electrode 116 includes a second interface region 122 extending from the second interface side 120. In this example, the second interface region 122 extends from the second interface side 120 in a transverse direction to the longitudinal axis 126 (the X-direction), away from the spacer region 112.

[0051] Each of the first porous electrode 104 and the second porous electrode 116 may have any suitable thickness (in the X-direction). For example, the thickness of each of the first porous electrode 104 and the second porous electrode 116 may between 0.3mm and 10mm. The thickness of the first porous electrode 104 and the second porous electrode 104 are each labelled as xthick in FIG. 1.

[0052] The average pore size in at least the first interface region 110 of the first porous electrode 104 is 25 pm or less. That is the maximum average pore size is 25 pm or less. For example the average pore size in at least the first interface region 110 of the firstporous electrode 104 may be from 0.1 pm to 25 pm, aptly from 3 pm to 20 pm, aptly from 3 pm to 10 pm, aptly from 1 pm to 20 pm, aptly from 1 pm to 10 pm, aptly from 3 pm to 7 pm, aptly about 5 pm.

[0053] The average pore size in at least the second interface region 122 of the second porous electrode 116 is 25 pm or less. That is the maximum average pore size is 25 pm or less. For example the average pore size in at least the second interface region 122 of the second porous electrode 116 may be from 0.1 pm to 25 pm, aptly from 3 pm to 20 pm, aptly from 3 pm to 10 pm, aptly from 1 pm to 20 pm, aptly from 1 pm to 10 pm, aptly from 3 pm to 7 pm, aptly from 3 pm to 7 pm, aptly about 5 pm.

[0054] The variance of pore sizes within at least the first interface region 110 and the second interface region 122 may be 50% or less, for example.

[0055] In use of the electrolyser 100, a potential or current is applied across the first porous electrode 104 and the second porous electrode 116. An electrolyte is then pumped through the spacer region 112. In the illustrated example, the electrolyser 100 includes a pump 124 configured to pump electrolyte through the electrolyte inlet 128.

[0056] The electrolyte flows or permeates from the spacer region 112 through each of the first porous electrode 104 and the second porous electrode 116. For the first porous electrode 104, the electrolyte flows from the first interface side 108 of the first porous electrode 104 to the opposing side 146 of the first porous electrode 104. For the second porous electrode 116, the electrolyte flows from the second interface side 120 of the second porous electrode 116 to the opposing side 148 of the second porous electrode 116.

[0057] As the electrolyte flows through each of the first porous electrode 104 and the second porous electrode 116 it reacts with the corresponding active materials in the electrode producing an electrolysis product. For example where the electrolyte is water, hydrogen and oxygen are produced at the cathode (second porous electrode 116) and anode (first porous electrode 104) respectively. The electrolysis products may begin in a dissolved gas state before nucleating into gas bubbles.

[0058] Once the electrolysis products and remaining electrolyte have exited the opposing sides 146, 148 of each electrode, the electrolysis products and remaining electrolyte flow through the internal volume 136, 138 of the corresponding electrode compartment 102, 114 and out through the respective outlet 106, 118. Specifically, afirst electrolysis product and electrolyte flows from the first porous electrode 104, through the internal volume 136 of the first electrode compartment 102 and out through the first outlet 106. Similarly, a second electrolysis product and electrolyte flows from the second porous electrode 116, through the internal volume 138 of the second electrode compartment 114 and out through the second outlet 118. The direction of electrolyte flow (and electrolysis product flow where present) through the electrolyser 100 is shown by the arrows in FIG. 1. In this example, the electrolyser 100 is arranged such that the longitudinal axis 126 extends substantially vertically, with the electrolyte inlet 128 at a lower end thereof. In this manner, bubbles of electrolysis product benefit from the buoyancy effect and can be more easily carried to the respective outlet 106, 118.

[0059] The arrangement of the electrolyser 100 and the porosity of the first porous electrode 104 and second porous electrode 116 is such that the pressure drop of electrolyte in a transverse direction across each of the first porous electrode 104 and second porous electrode 116 is greater (> 5 times greater) than the longitudinal pressure drop of electrolyte along the spacer region 112. The pressure drop through each of the porous electrodes 104, 116 is largely driven by the pore size, shape, tortuosity, porosity, solid feature size and layer thickness in the corresponding interface region 110, 122. The relatively large pressure drop across each of the first porous electrode 104 and second porous electrode 116 (in comparison to the longitudinal pressure drop along the spacer region 112) helps maintain a uniform flow velocity through the electrode. A uniform flow velocity is maintained even when the electrolyser 100 is scaled-up in size or in height. For example, with the described arrangement a uniform laminar flow velocity can be maintained in electrodes up to 1 m2to 3 m2in size with a longitudinal height of 1 m to 2m since the main factor determining velocity through the porous active layer is pressure drop from flow through the porous electrodes 104, 116 and not from flow through the spacer region 112. The uniform flow is achieved while ensuring power loss is minimal. In addition, the laminar flow between the porous electrodes 104, 116 helps provide a velocity field barrier, with fluid velocity vectors pointing parallel to or through the respective electrodes 104, 116. This opposes the diffusion of dissolved gases across the spacer region 112 and encourages convection to transport the dissolved gases towards the outlets of the electrode compartments 106, 118, rather than to accumulateat the interface regions 110, 122. This effect is otherwise termed convective flow detachment.

[0060] In addition, operation at higher electrolyte pressure increases the solubility of the gaseous electrolysis products, such that a larger fraction of the generated gases remain dissolved in the electrolyte in regions of high pressure. This helps ensure any products generated in the interface regions 110, 122 remain dissolved or formed bubbles has smaller size. The velocity in the pores caused by the pressure gradient normal to the electrode surface causes convection of dissolved gas. This enables the accumulation of dissolved gas to be lower, leading to a retardation of bubble nucleation in the interface regions 110, 122 (even at high current density).

[0061] To exemplify the electrolyte pressure drops described above, the height of electrolyte within the spacer region 112 may result in a pressure difference between the electrolyte inlet 128 and the boundary element 140 (the height of electrolyte within the spacer region 112 is labelled as xiength in FIG. 1 ). This pressure difference is typically between 10 kPa and 15 kPa for every 1 m of the height of the spacer region 112 (in the Z-direction) depending on electrolyte density. In addition there is a hydrodynamic pressure difference from electrolyte flow along the spacer region 112. This will depend on a number of factors (for example, the Reynolds number of the flow, porosity within the spacer region 112, width of spacer region 112 and electrolyte dynamic viscosity and density). The total pressure difference along the spacer region 112 will typically be less than 500 kPa and usually below 300 kPa and can be below 150 kPa. The hydrodynamic pressure drop across the porous electrodes layer may be in the range of 300 kPa to 1500 kPa

[0062] FIG. 2 is a graph showing a physics-based analytical model for the relationship (the curve marked ‘ratio’) between the pore radius, displayed on the x-axis, and the ratio between the pressure drop of electrolyte in a transverse direction across each of the first and second porous electrodes and the longitudinal pressure drop of electrolyte along the spacer region 112, displayed on the y-axis. The pressure drops were calculated as followed:Where p is the viscosity of electrolyte, Qin is the flow rate of electrolyte in the studied region, xiength is the length of the electrode in the cell (in the z-direction, which may otherwise be termed the height of the electrode), xgapis the distance between the anode and cathode (separator gap distance in the x-direction), k is the absolute permeability of the porous media (m2) and xthick is the thickness of the porous electrode (in the x- direction).

[0063] For the model shown, the electrodes had a porosity of between 20 to 100%, a height of 0.014m and a thickness of 500 pm. The distance between the electrodes was 0.002m.

[0064] FIG. 2 also shows the parameters used for two lab scale experimental prototypes - Prototype 2 with an average pore radius of 95 pm (i.e. a pore size of 190 pm); and Prototype 3 with an average pore radius of 25 pm (i.e. a pore size of 50 pm). During operation bubbles were seen in the separator region of Prototype 2 due to the relatively small pressure drop of electrolyte in a transverse direction across each of the first and second porous electrodes. In contrast, no bubbles were seen in the separator region of Prototype 3 due to the relatively large pressure drop of electrolyte in a transverse direction across each of the first and second porous electrodes, which was over 5 times larger than the longitudinal pressure drop of electrolyte along the spacer region 112.

[0065] The trend shown in FIG. 2 demonstrates an increase in the ratio (between the pressure drop of electrolyte in a transverse direction across each of the first and second porous electrodes and the longitudinal pressure drop of electrolyte along the spacer region 112) as pore size decreases. It would be understood that this trend continues as the pore size is decreased beyond that shown in FIG. 2, for example to a pore size of 25 pm or less (i.e. a pore radius of 12.5 pm or less).

[0066] Referring back to FIG. 1 , in this example, any suitable electrode material may be used for the first porous electrode 104 and the second porous electrode 116. Each of the first porous electrode 104 and the second porous electrode 116 comprises an active material. The first porous electrode 104 and / or the second porous electrode 116 may be formed entirely of the active material. For example, the first porous electrode 104 and / or the second porous electrode 116 may be formed of nickel, stainless steel or a suitablemetal alloy. In other examples, the first porous electrode 104 and / or the second porous electrode 116 may be formed from an electrically conductive inert material coated with an active material. The active coating may comprise one or more of nickel, nickel double hydroxide, nickel oxyhydroxide, ruthenium oxide or platinum, for example.

[0067] In this example the first porous electrode 104 and the second porous electrode 116 comprise a hydrophilic material. This helps ensure the electrolyte can easily wet the surface / interface by reducing contact angle and therefore can move continuously through the structure of the porous electrodes 104, 116.

[0068] The porosity of the first porous electrode 104 and the second porous electrode 116 may be from 20% to 90%. Providing electrodes with a porosity above 20% helps ensure that pores are sufficiently interconnected to allow for efficient permeation of the electrolyte therethrough and that maximum pressure is not exceeded under desired flow rate. Providing electrodes with a porosity below 90% helps ensure that the electrodes can be easily manufactured and handled and allows pressure drop through the porous electrode to be increased.

[0069] In certain examples the maximum permeability (kmax) for each of the first and second porous electrodes 104, 116 may be determined by the following equation:where xgapis the separation between the first porous electrode 104 and the second porous electrode 116 in a direction that is transverse to the longitudinal axis 126; xthickis the thickness of the porous electrode 104, 116 (in the X-direction); and xlengthis the height of electrolyte within the spacer region 112.

[0070] Advantageously, the inventors have found that by limiting the permeability of the porous electrodes 104, 116 as defined by the equation in the preceding paragraph, the uniformity of electrolyte flow along the length of the interface side of the porous electrodes 104, 116 can be improved I maximized.

[0071] In certain examples the the maximum average pore size, Dmax, in the first porous electrode and the second porous electrode is defined by:where e is the combined porosity of the first porous electrode 104 and the second porous electrode 116, xgapis the separation between the first porous electrode 104 and the second porous electrode 116 in a direction that is transverse to the longitudinal axis 126;xthickisthe thickness of the porous electrode 104, 116; and xlengthis the height of electrolyte within the spacer region 112.

[0072] Advantageously, the inventors have found that limiting the maximum average pore size within the porous electrodes 104, 116 as defined by the equation in the preceding paragraph, the uniformity of electrolyte flow along the length of the interface side of the porous electrodes 104, 116 can be improved / maximized.

[0073] In this example, the electrolyser 100 includes a porous barrier layer positioned between the first interface side 108 and the spacer region 112 and / or the second interface side 120 and the spacer region 112. In the illustrated example there is a first porous barrier layer 132 positioned between first interface side 108 and the spacer region 112 and a second porous barrier layer 134 positioned between the second interface side 120 and the spacer region 112.

[0074] At high current density, the distribution of local current density in the active electrode reaction tends more towards the spacer region 112. The first porous barrier layer 132 and second porous barrier layer 134 reduce or prohibit activity at the interface between the spacer region 112 and the corresponding electrode. That is, reaction at the interface between the spacer region 112 and the electrodes is reduced or prevented by the presence of a barrier layer. As such, no dissolved gas or bubble will be produced at the interface. In this manner, providing a barrier layer between the interface and the spacer region 112 helps shift the reaction zone inside the porous electrodes, ensuring all bubbles or dissolved gases are formed inside the porous structure of the active electrode where convection will carry them away to the corresponding outlet. This, in turn, helps reduce any mixing of the separate dissolved gases.

[0075] The porous structure of the first porous barrier layer 132 and second porous barrier layer 134 also helps prevent electrolysis products from passing to the spacer region 112 from the corresponding electrode. Specifically, the entry capillary pressure of the pores of the first porous barrier layer 132 and second porous barrier layer 134 provides a barrier to the passage of electrolysis products from the electrodes to the spacer region 112. For example, if the gas phase produced from an electrode reachesthe pore throats of the corresponding barrier layer, it will not enter unless the gas phase pressure is greater than the entry capillary pressure of the throat.

[0076] Furthermore, in case of pump failure, the first porous barrier layer 132 and second porous barrier layer 134 help ensure that system remains safe, with limited gas mixing enabled by the capillary pressure phenomenon in porous barrier layers. Specifically, gas build up at the porous barrier layers interface will stop supply of electrolyte and consequently stop the reaction from proceeding and maintain the pressure below that required to overcome the capillary pressure.

[0077] The porous barrier layers 132, 134 may be coated on, or attached to, the corresponding interface side 108, 120. Coating or depositing the porous barrier layers 132, 134 directly onto the corresponding interface side 108, 120 helps ensure that the pores of the porous barrier layers 132, 134 are aligned with the pores in the corresponding interface side 108, 120.

[0078] In other examples, the porous barrier layers 132, 134 may be spaced from the corresponding interface side 108, 120 by a distance that is greater than the average pore size of the porous barrier layer 132, 134. Advantageously, this helps prevent the non-porous sections of the porous barrier layers 132, 134 from blocking the pore throats in the corresponding interface side 108, 120. Such blockage may otherwise lead to the production of electrolysis products on the corresponding interface side 108, 120. In this case the interface of porous barrier may be inactive by either applying additional blocking layer, or be made from non-active materials.

[0079] The porous barrier layers 132, 134 may comprise a film or mesh. For example the porous barrier layers 132, 134 may comprise a polymer material. Suitable polymer materials may include polyethylene or Polyethylene terephthalate. The polymer material may be a functionalized polymer, for example an ion conducting polymers with sulfonic acid or quaternary ammonium groups.

[0080] The porous barrier layers 132, 134 may be hydrophilic to the electrolyte used. The hydrophilic properties of the porous barrier layers 132, 134 help prevent bubbles from entering the spacer region 112 from within the porous electrodes 104, 116.

[0081] The porous barrier layers 132, 134 may be inactive. This helps prevent the formation of electrolysis products in the spacer region 112.

[0082] The average pore size of the porous barrier layers 132, 134 may be 25 pm or less. For example, the average pore size of the porous barrier layers 132, 134 may be from 0.1 pm to 25 pm, aptly from 3 pm to 20 pm, aptly from 3 pm to 10 pm, aptly from 1 pm to 20 pm, aptly from 1 pm to 10 pm, aptly from 3 pm to 7 pm, aptly about 5 pm.

[0083] The average pore size of the porous barrier layers 132, 134 may be equal to or smaller than the pore size of the corresponding porous electrode 104, 116. That is, the average pore size of the first porous barrier layer 132 may be equal to or smaller than the average pore size of the first porous electrode 104. Similarly, the average pore size of the second porous barrier layer 134 may be equal to or smaller than the average pore size of the second porous electrode 116.

[0084] Sizing the average pore size of the porous barrier layers 132, 134 to be equal to, or preferably smaller than, the average pore size of the corresponding porous electrode 104, 116 ensures there is an entry capillary pressure gradient inside the corresponding porous electrode 104, 116. As such, even if the gas phase produced within the porous electrode 104, 116 reaches the corresponding porous barrier layer 132, 134, it is unlikely to enter the pores of the porous barrier layer 132, 134 as the gas phase pressure is typically lower than the entry capillary pressure of the pores of the porous barrier layer 132, 134.

[0085] The average pore size of the porous barrier layers 132, 134 is larger than the typical bubble size generated by the electrolyser 100 in use. As the average pore size within the porous barrier layers 132, 134 is equal to or smaller than the typical bubble size generated by the electrolyser 100 in use, the bubbles are prevented from entering the spacer region 112 by the capillary pressure. Instead, there is pressure relief for the bubbles to exit through the opposing side of the porous electrodes 104, 116. This is particularly true if the average pore size within the porous barrier layers 132, 134 is smaller than the typical bubble size generated by the electrolyser 100 in use. It would be understood that the exact bubble sizes will depend on particular operating conditions, for example the operating current.

[0086] The porous barrier layers 132, 134 may have any suitable thickness. For example, the thickness of the porous barrier layers 132, 134 may be from 1 pm to 100 pm, aptly 1 pm to 50 pm. Advantageously the relatively thin thickness of the porous barrier layers 132, 134 results in lower ohmic losses in the electrolyte and thereforehelps the electrolyser 100 achieve a greater efficiency. In particular, a lower thickness for the porous barrier layers 132, 134 helps reduce the ohmic losses of the ions flowing through the porous barrier layers 132, 134. For example, a thickness of from 1 pm to 100 pm may help ensure an ASR (area-specific resistance) of from 20 mQ cm2to 150 mQ cm2.

[0087] The porosity of the porous barrier layers 132, 134 may be at least 20%. For example, the porosity of the porous barrier layers 132, 134 may be from 20% to 90%, aptly 20% to 60%. Advantageously, using porous barrier layers with a porosity of at least 20% helps ensure that ohmic losses in the electrolyte are minimised (with ohmic losses decreasing with increased porosity) and therefore system efficiency is maximised. In particular, a porosity of at least 20% provides an open structure for ion flow therethrough, which reduces the associated ohmic losses. In addition, a porosity of above 90% leads to handling difficulties with the porous barrier layers.

[0088] In the illustrated example, the second longitudinal end of the spacer region 112 includes a wall or boundary element 140 extending between the first porous electrode 104 and the second porous electrode 116. In this manner, at the second longitudinal end of the spacer region 112 the flow of electrolyte is substantially blocked creating a stagnation point. As such, the flow must separate, which creates a region of large flow rate from the spacer region 112 through the first porous electrode 104 and the second porous electrode 116. This helps divert any dissolved gas that has entered the spacer region 112, back into its respective electrode.

[0089] In this example, the wall or boundary element 140 includes an optional purge outlet 130. The purge outlet 130 may be a gas release valve or may allow a small fraction of the total liquid flow <20% therethrough, which is used to remove unwanted gases from the spacer region 112 and / or to balance the pressure in the system. This is particularly helpful during start-up of the electrolyser 100.

[0090] The current or potential difference across the first porous electrode 104 and the second porous electrode 116 may be applied in any suitable manner. For example, the electrolyser 100 may include first and second electrically conductive elements or terminals (not shown) in contact with the first porous electrode 104 and the second porous electrode 116, respectively. The first and second electrically conductive terminals may be connected to a DC power source. Where the electrolyser 100 is part of anelectrolyser stack (multiple electrolyser cells arranged in series I a bipolar configuration), the first electrically conductive terminal may be connected to the anode of the first electrolyser in the stack and the second electrically conductive terminal may be connected to the cathode of the last electrolyser in the stack.

[0091] The first electrode compartment 102 and second electrode compartment 114 may include a conductive layer that interfaces between a terminal and the corresponding electrode 104, 116 and / or that interfaces between electrodes of adjacent electrolysers in an electrolyser stack. In the illustrated example, the first electrode compartment 102 includes a first conductive layer 142 and the second electrode compartment 114 includes a second conductive layer 144.

[0092] The first conductive layer 142 and second conductive layer 144 may be porous. For example the porosity may be achieved with a series of conductive channels and / or a porous metal mesh. The porous metal mesh may include an average pore size of 0.5 mm to 20 mm, aptly 1 mm to 2 mm. The porous metal mesh may include a porosity of 60% to 90%. The conductive channels may be part of a metal plate, for example. The conductive channels may be metal, carbon or another electrically conductive materials.

[0093] The first conductive layer 142 and second conductive layer 144 may form a lining of the internal volumes 136, 138. Alternatively, the first conductive layer 142 and second conductive layer 144 may occupy the entirety of the internal volumes 136, 138. The first conductive layer 142 and second conductive layer 144 engage with the opposing side 146, 148 of the corresponding electrode. That is, the first conductive layer 142 engages with the opposing side 146 of the first porous electrode 104. The second conductive layer 144 engages with the opposing side 148 of the second porous electrode 116. The use of a first conductive layer 142 and second conductive layer 144 that interface with rear surfaces of the electrodes allows the applied current to be distributed uniformly along the longitudinal length of the electrodes 104, 116.

[0094] In some examples the first conductive layer 142 and second conductive layer 144 may include a non-porous plate layer. For example, the non-porous plate layer may form the lining or exterior of the first conductive layer 142 and the second conductive layer 144 and the volume defined by the non-porous plate may include a porous mesh (of the type described in the preceding paragraphs, for example) or a series of metal channels (of the type described in the preceding paragraphs, for example). The non-porous plate layer (otherwise termed a bipolar plate) functions to create a sealed system and connect the electrode electrically to an external terminal or to the adjacent electrode of the next cell if forming an electrolyser stack.

[0095] The first conductive layer 142 and second conductive layer 144 allow the passage of electrical current from the terminals to the electrodes (for example through the porous structure of the porous mesh or through the series of conductive channels). The porosity in the first conductive layer 142 and the second conductive layer 144 also allow the flow of electrolysis products to the outlets 106, 118 The outlets of each cell if assembled in a stack can be either joined externally through external manifold or internal manifold.

[0096] The relatively large pores of the first conductive layer 142 and second conductive layer 144 (compared to the electrodes) help ensure that the hydrodynamic pressure drop through internal volumes 136, 138 is negligible in comparison to the hydrodynamic pressure drop through the electrodes. By way of example, the hydraulic pressure drop through the electrodes may account for 60% to 90% of the total hydraulic pressure drop of the electrolyte flowing from the spacer region 112 to the outlets 106, 118. This helps improves uniformity of electrolyte flow across the entire electrode area and allows scale- up of the electrolyser 100.

[0097] The pump 124 may be configured to provide a flow of electrolyte along the spacer region 112 at a Reynolds number of below 500. The pump 124 may be configured to provide a flow of electrolyte along the spacer region 112 at a Reynolds number of below 500 for each cell in the stack. For example, this may equate to an inlet velocity below 1 m / s at the electrolyte inlet 128. Providing flow of electrolyte along the spacer region 112 at a Reynolds number of below 500 has been found to be particularly effective at avoiding the formation of vortices and the break-up of laminar boundary layers, which may help gas crossover within the spacer region 112. This helps ensure flux uniformity is maintained through the first porous electrode 104 and the second porous electrode 116 even when the electrolyser 100 is scaled up in the Z-direction.

[0098] The pump 124 may be configured to provide a flow of electrolyte along the spacer region 112 at a Reynolds number of at least 25, aptly at least 35, aptly at least 50, aptly at least 60. Providing flow of electrolyte along the spacer region 112 at a Reynolds number of at least 25 helps ensure that the gas cross-over in the spacer region 112remains relatively low. In addition, this helps ensure that a target velocity inside the electrode pores is achieved. The target velocity inside the electrode pores is the velocity which is sufficient for convective flux to remove produced dissolved gases or gas bubbles formed away from electrode and replenish any consumed water from the reaction. This in turn, helps achieve higher electrode throughput (for example >0.5 A / cm2). The target velocity within the electrode pores may be from 0.03 mm / s to 3 mm / s in the X-direction.

[0099] FIG. 3 shows computational fluid dynamics data for example electrolysers of the type described herein being used for electrolysis of water. In the illustrated example, a Reynolds number of at least 25 was required to maintain the H2 content in the O2 outlet below 0.5% when the electrolyser length in the Z-direction is 0.5 m. It can be seen that a Reynolds number of 250 reduces the H2 content in the O2 outlet to a negligible amount (below 0.01 %).

[0100] FIG. 4A and FIG. 4B show computational fluid dynamics data for example electrolysers of the type described herein being used for electrolysis of water. In the illustrated example, a Reynolds number of at least 35 was required to maintain the H2 content in the O2 outlet below 2% at 1 A cm’2and a Reynolds number of at least 75 for a current density of 0.1 A cm’2when the electrolyser length in the Z-direction is 0.5 m. It can be seen that a Reynolds number of 250 reduces the H2 content in the O2 outlet to a negligible amount (below 0.01 %).

[0101] FIG. 5A shows simulation results, predicting the maximum permeability, kmax, for which uniform flow was present across along the length of the interface side of the porous electrodes for different electrode lengths (where electrolyser length refers to the height of the electrolyte within the spacer region 112).

[0102] The results shown in FIG. 5A were found by solving a finite volume flow conductivity network model consisting of different permeability for channel and porous regions. The maximum permeability when the flow uniformity was greater than 99% was extracted.

[0103] From the extracted results a predictive equation for combined porous electrodes permeability was found to be:where xgapis the spacing between anode and cathode electrodes, xthickis the thickness of the composite porous electrode and xlengthis the length of the electrolyser in the longitudinal direction.

[0104] When combined with the Ergun equation, the equation in the preceding paragraph was used to calculate the maximum average pore diameter, Dmax, required for uniform flow defined as:where e is the porosity of the first and second porous electrode combined. FIG. 5B shows three plots following this equation for three porosity levels for the first and second porous electrodes (0.5, 0.2 and 0.9).

[0105] FIG. 5C shows the parameters used for three lab scale experimental prototypes - Prototype 2 with an average pore radius of 95 pm (i.e. a pore size of 190 pm), Prototype3 with an average pore radius of 26 pm (i.e. a pore size of 52 pm) and Prototype 4 with an average pore radius of 2.5 pm (i.e. a pore size of 5 pm). During operation bubbles were seen in the separator region of Prototype 2 due to the relatively small pressure drop of electrolyte in a transverse direction across each of the first and second porous electrodes. In contrast, no bubbles were seen in the separator region of Prototype 3 or4 due to the pore size being smaller than the maximum value defined by equation 2.

[0106] Various modifications to the above described examples or embodiments are possible. For example, the electrolyser 100 described herein may be used with any suitable electrolyte. For example, the electrolyte may be water or acid dissolved in water or the water may include 30% KOH, for example.

[0107] The average pore size in the first porous electrode 104 may differ from the average pore size in the second porous electrode 116. The average pore size in the first porous barrier layer 132 may differ from the average pore size in the second porous barrier layer 134.

[0108] In the illustrated example the first interface region 110 and the second interface region 122 are shown as occupying only a portion of the thickness of the corresponding electrode 104, 116. The thickness of the first interface region 110 and the second interface region 122 may be at least 15 pm thick, aptly at least 20 pm thick aptly, 0.3 mmto 5 mm thick. In other examples, the thickness of the first interface region 110 and the second interface side 120 may occupy substantially all of the thickness of the corresponding electrode 104, 116. However, it would be understood that the average pore size in the electrodes 104, 116 at positions towards the opposing sides 146, 148 is less critical in terms of controlling the pressure drop across the electrodes 104, 116. The thickness of the first interface region 110 and the second interface region 122 may differ.

[0109] In the illustrated example the spacer region 112 is shown as an empty region or channel. However, the spacer region 112 may include an open, non-conductive, mesh or porous layer to provide mechanical support to the adjacent porous barrier layers 132, 134. For example, the spacer region 112 may include a porous layer having a porosity of between 80% and 100% and an average pore size in the order of 0.1 mm to 1 mm.

[0110] In the illustrated embodiment each electrode 104, 116 includes a single porous barrier layer 132, 134. In other examples, each electrode 104, 116 may include multiple porous barrier layers, for example 2, 3 or more.

[0111] It will be clear to a person skilled in the art that features described in relation to any of the embodiments described above can be applicable interchangeably between the different embodiments. The embodiments described above are examples to illustrate various features of the invention. 1

Claims

CLAIMS1. An electrolyser comprising: a first electrode compartment comprising: a first porous electrode; and a first outlet for a first electrolysis product; a second electrode compartment comprising: a second porous electrode; and a second outlet for a second electrolysis product; a spacer region having a longitudinal axis, the spacer region separating the first porous electrode and second porous electrode in a direction that is transverse to the longitudinal axis, wherein the spacer region comprises an electrolyte inlet and permits the flow of an electrolyte from the electrolyte inlet along the longitudinal axis; wherein the first porous electrode comprises: a first interface side that interfaces with the spacer region; and a first interface region extending from the first interface side; wherein the second porous electrode comprises: a second interface side that interfaces with the spacer region; and a second interface region extending from the second interface side; wherein the average pore size in at least the first interface region of the first porous electrode is 25 pm or less, wherein the average pore size in at least the second interface region of the second porous electrode is 25 pm or less.

2. The electrolyser of claim 1 , wherein the porosity of the first porous electrode and the second porous electrode is from 20% to 90%.

3. The electrolyser of claim 1 or 2, wherein the average pore size in at least the first interface region of the first porous electrode is from 1 pm to 10 pm, wherein the average pore size in at least the second interface region of the second porous electrode is from 1 pm to 10 pm.

4. The electrolyser of any one of claims 1 to 3, further comprising a porous barrier layer positioned between the first interface side and the spacer region and / or between the second interface side and the spacer region.

5. The electrolyser of claim 4, wherein the average pore size of the porous barrier layer is 25 pm or less.

6. The electrolyser of claim 4 or 5, wherein the average pore size of the porous barrier layer is equal to or smaller than the average pore size of the first porous electrode and the second porous electrode.

7. The electrolyser of any one of claims 4 to 6, wherein the thickness of the porous barrier layer is from 1 pm to 100 pm.

8. The electrolyser of any one of claims 4 to 7, wherein the porosity of the porous barrier layer is >20%.

9. The electrolyser of any one of claims 4 to 8, wherein the porous barrier layer is coated on, or attached to, at least one of the first interface side and the second interface side.

10. The electrolyser of any one of claims 4 to 8, wherein the porous barrier layer positioned between the first interface side and the spacer region is spaced from the first interface side by a distance that is greater than the average pore size of the porous barrier layer, and / or wherein the porous barrier layer positioned between the second interface side and the spacer region is spaced from the second interface side by a distance that is greater than the average pore size of the porous barrier layer.

11. The electrolyser of any one of claims 4 to 10, wherein the porous barrier layer is hydrophilic.

12. The electrolyser of any one of claims 4 to 11 , wherein the porous barrier layer is inactive.

13. The electrolyser of any one of claims 1 to 12, further comprising a pump configured to pump electrolyte through the electrolyte inlet.

14. The electrolyser of any one of claims 1 to 13, wherein the electrolyte inlet is at a first longitudinal end of the spacer region, wherein the second longitudinal end of the spacerregion comprises a wall or boundary element extending between the first porous electrode and the second porous electrode.

15. The electrolyser of any one of claims 1 to 14, wherein the first porous electrode comprises an active material, wherein the second porous electrode comprises an active material.

16. The electrolyser of any one of claims 1 to 15, wherein the first porous electrode comprises a hydrophilic material, wherein the second porous electrode comprises a hydrophilic material.

17. The electrolyser of any one of claims 1 to 16, wherein the first electrode compartment comprises an anode chamber and the first porous electrode is a porous anode, wherein the second electrode compartment comprises a cathode chamber and the second porous electrode is a porous cathode.

18. The electrolyser of any one of claims 1 to 17, wherein the first electrode compartment and the second electrode compartment comprise first and second electrically conductive elements in contact with the first porous electrode the second porous electrode, respectively, wherein the first and second electrically conductive elements are connected to a DC power source.

19. The electrolyser of any one of claims 1 to 18, wherein the maximum permeability, kmax, for at least one of the first and second porous electrodes is defined by:where xgapis the separation between the first porous electrode and the second porous electrode in a direction that is transverse to the longitudinal axis; xthickis the thickness of the porous electrode; and xlengthis the height of electrolyte within the spacer region.

20. The electrolyser of any one of claims 1 to 19, wherein the maximum average pore size, Dmax, in the first porous electrode and the second porous electrode is defined by:where e is the combined porosity of the first and second porous electrode, xgapis the separation between the first porous electrode and the second porous electrode in a direction that is transverse to the longitudinal axis; xthickis the thickness of the porous electrode; and xlengthis the height of electrolyte within the spacer region.

21. An electrolyser comprising: a first electrode compartment comprising: a first porous electrode; and a first outlet for a first electrolysis product; a second electrode compartment comprising: a second porous electrode; and a second outlet for a second electrolysis product; a spacer region having a longitudinal axis, the spacer region separating the first porous electrode and second porous electrode in a direction that is transverse to the longitudinal axis, wherein the spacer region comprises an electrolyte inlet and permits the flow of an electrolyte from the electrolyte inlet along the longitudinal axis; wherein the first porous electrode comprises: a first interface side that interfaces with the spacer region; and a first interface region extending from the first interface side; wherein the second porous electrode comprises: a second interface side that interfaces with the spacer region; and a second interface region extending from the second interface side; wherein the maximum average pore size, Dmax, in the first porous electrode and the second porous electrode is defined by:where e is the combined porosity of the first and second porous electrode, xgapis the separation between the first porous electrode and the second porous electrode in adirection that is transverse to the longitudinal axis; xthickis the thickness of the porous electrode; and xlengthis the height of electrolyte within the spacer region.

22. The electrolyser of claim 21 , wherein the maximum permeability, kmax, for at least one of the first and second porous electrodes is defined by:where xgapis the separation between the first porous electrode and the second porous electrode in a direction that is transverse to the longitudinal axis; xthickis the thickness of the porous electrode; and xlengthis the height of electrolyte within the spacer region.

23. A method of using the electrolyser of any preceding claim, wherein the method comprises: applying a potential or current across the first porous electrode and the second porous electrode; and pumping an electrolyte through the spacer region.

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