Porous transport layer

Abstract

A method for manufacturing a porous transport layer for use in a PEM electrode, comprises the steps of: providing a first powder having a first grade, and a second powder having a second grade, wherein the second grade of the second powder is finer than the first grade of the first powder; blending together the first and second powders; and constructing a porous construct by compacting together and sintering the blended powder; wherein the first grade of the first powder is selected such that the porous construct has a constant fluid transfer rate in use, and the second grade of the second powder is selected so as to produce enhanced fluid transfer properties in a surface-adjacent region of the porous construct in use.

Description

[0001] POROUS TRANSPORT LAYER

[0002] BACKGROUND TO THE INVENTION

[0003] Hydrogen is emerging as a pivotal player in the pursuit of decarbonizing industries and achieving the goal of net-zero emissions. Its significance lies in its potential to act as a pristine and adaptable energy carrier, allowing a diverse range of industries and modes of transport to become more sustainable. Hydrogen offers a tangible solution for industries heavily reliant on fossil fuels, such as transportation, manufacturing, and power generation. Through the substitution of carbon-intensive fuels with hydrogen, these sectors can substantially diminish their carbon footprint. Hydrogen fuel cells emerge as a promising alternative to conventional internal combustion engines, as they solely emit water vapor and efficiently generate electricity.

[0004] Hydrogen enables the integration of renewable energy sources into existing energy systems. Surplus electricity stemming from renewable sources, such as wind and solar power, can be harnessed to produce hydrogen via electrolysis. This process facilitates energy storage and mitigates the intermittency challenges associated with renewable power, ensuring a dependable and steadfast energy supply. Hydrogen produced via electrolysis from renewable energy sources is known as green hydrogen because no greenhouse gas is emitted (in contrast to other methods of producing hydrogen, such as from natural gas).

[0005] Hydrogen can be produced using PEM (proton exchange membrane) electrolysis. This method is in essence the reverse of a fuel cell. Water is passed through an electrically charged PEM electrode, which splits the water into hydrogen and oxygen. Unlike other methods of hydrogen production, PEM electrolysis does not produce carbon-dioxide or greenhouse gas as a by-product. The membrane is impermeable to gases ensuring safety and is able to self- pressurise gases produced at each electrode. This, however, requires a finely tuned electrode / membrane interface. Additionally, PEM electrolysis can be powered by renewable energy sources such as wind turbines. As a result, PEM electrolysis can be used to produce hydrogen with very low carbon emissions.

[0006] PEM electrolysis is typically performed by passing water through an electrochemical cell and applying a direct-current (DC) voltage at two electrodes: a negatively charged cathode and positively charged anode, separated by a gas impermeable proton exchange membrane. The membrane is coated by a catalyst layer on one or both sides. Water is oxidised giving protons and oxygen at the anode, the protons are transferred through the proton exchange membrane and are then reduced at the cathode with electrons, producing hydrogen. The protons (positively charged hydrogen ions) pass through the ion transport membrane (a solid polymer electrolyte -PEM) to the cathode, where they combine with electrons to form molecular hydrogen. The molecular hydrogen, in gaseous form, can then be collected at pressure. A porous transport layer (PTL) is positioned between the membrane and the anode and / or cathode, in order to manage the flow of water to the membrane.

[0007] However, existing systems and methods for PEM electrolysis have drawbacks. Electrolysis is expensive, as electrical power must be supplied to the electrolyser to perform the reaction. Electricity may be wasted by efficiency losses inside the electrolyser. In addition, the rate of hydrogen production from electrolysis may be slow compared to other forms of hydrogen production, such as from natural gas. Accordingly, there exists a need for PEM electrolysis systems which provide increased efficiency and reduce overall costs.

[0008] SUMMARY OF THE INVENTION

[0009] According to a first aspect, there comprises a method for manufacturing a porous transport layer for use in a PEM electrode, comprising the steps of: providing a first powder having a first grade, and a second powder having a second grade, wherein the second grade of the second powder is finer than the first grade of the first powder; blending together the first and second powders; and constructing a porous construct by compacting together and sintering the blended powder; wherein the first grade of the first powder is selected such that the porous construct has a constant fluid transfer rate in use, and the second grade of the second powder is selected so as to produce enhanced fluid transfer properties in a surface-adjacent region of the porous construct in use.

[0010] PTLs can be the source of significant inefficiencies. In PEM electrolysis, for long life, good water saturation of the catalyst layer is required, especially at maximum operating regime. PTLs can limit performance and reduce the lifetime of PEM water electrolysers. Inefficiencies can arise from impaired transport of reactant and products to and from active reaction sites, which is difficult to detect. These transports take place in opposite direction at the anode and in the same direction in the case of the cathode within the PTL. Over time, initial voltaic inefficiency is often followed by degradation. This may require replacement of the PTL or refurbishment of the system. In the worst design cases, degradation is exponential. The grade of a powder may be defined as the nominal size of particles in the powder, as shown in column 403. Since sieves come in standard sizes of apertures, nominal powders can be graded in relation to the standard sieve size which the powder will pass through.

[0011] The second powder fills in the interstices between the particles of the second powder, resulting a finer porosity network in the porous construct. In addition, the smoothness of the contact surface between the PTL and the membrane-catalyst layer is improved. These features enhance the capillary effect in regions adjacent to the surface of the porous construct, resulting in fluids being transferred more uniformly through the PTL and ensuring that the membrane-catalyst layer remains saturated with water. This results in a more efficient electrolysis reaction and less wear on the membrane components of the electrolyser.

[0012] Rather than combining two separate powders, the first and second powders may initially be separated from a bulk powder containing a wide range of particle grades. The bulk powder may be sieved or otherwise separated by particle grades, with the first powder being taken from one grade and the second powder being taken from another, finer grade. The first and second powders are then recombined in the correct proportions to form the blended powder.

[0013] The first and second powders may comprise titanium, due to titanium's good electrical conductivity, mechanical robustness and good corrosion resistance. The PTL must have good conductivity in order to conduct electrons from the electrolysis reaction away from the reaction site with minimal losses.

[0014] Optionally, the method further comprises the step of locally compacting the surface- adjacent region of the porous construct. This results in increased smoothness of the contact surface. Compacting the surface-adjacent region also reduces the diameter of the pores within this region, further enhancing the capillary effect.

[0015] Optionally, the surface-adjacent region is compacted by rolling. As will be discussed below, rolling advantageously concentrates compression forces into the surface- adjacent region, ensuring that the surface-adjacent region is compacted while the core of the porous construct is unaffected. Optionally, the porous construct is re-sintered following compaction. This ensures that the surface-adjacent region retains its compacted structure in use and preceding cold work is fused permanently.

[0016] Optionally, the fluid transfer rate is evaluated by measuring, over a period of time, a volume of a volume of water exiting the porous construct. The water flow rate is a proxy for the proton transfer rate, as will be discussed below. The water flow rate is measured over a prolonged period of time to ensure that the water transfer rate is constant when the PTL is operating in the steady state, as opposed to simply being constant during a transient response.

[0017] Optionally, the first powder has particle sizes of 80 pm or more, and the second powder has particle sizes of less than 80 pm.

[0018] Optionally, the first powder comprises powder sizes between 85 pm and 125 pm, and the second powder comprises powder sizes between 35 pm and 75 pm.

[0019] Optionally, the blended powder comprises at least 90% the first powder by mass. This ensures that the bulk fluid transfer rate of the porous construct is substantially set by the first powder, with the second powder serving primarily to enhance the fluid transfer properties in the surface-adjacent region.

[0020] According to a second aspect, there comprises a porous transport layer for use in a PEM electrode, comprising a porous construct; wherein the porous construct is constructed by compacting together and sintering a blend of a first powder and a second powder, the first powder having a first grade and the second powder having a second grade, wherein the second grade of the second powder is finer than the first grade of the first powder; and wherein the first grade of the first powder is selected such that the porous construct has a constant fluid transfer rate in use, and the second grade of the second powder is selected so as to produce enhanced fluid transfer properties in a surface-adjacent region of the porous construct in use.

[0021] According to a third aspect, there comprises a method for selecting a first grade of a first powder for use in manufacturing of a porous transport layer for use in a PEM electrode, the method comprising steps of: a) providing a plurality of sample powders each having a distinct sample powder grade; b) selecting a sample powder having an initial sample powder grade; c) manufacturing a sample porous construct by compacting together and sintering the sample powder; d) evaluating a fluid transfer rate of the sample porous construct; e) if the fluid transfer rate of the sample porous construct is constant, identifying the initial sample powder grade as the first grade of the first powder; f) if the water transfer rate of the sample porous construct is not constant, selecting a coarser sample powder grade from the plurality of sample powders; and g) repeating steps c-f iteratively until a sample powder grade is found from which a sample porous construct with a constant water transfer rate can be manufactured.

[0022] The mass transport properties of a PTL can be qualitatively observed, in theory, by the polarisation curve shape, or estimated mathematically using standard equations, such as the Darcy or 'Darcy modified' equations. However, the inventors have found that polarisation curves do not give accurate measurements of the steady-state water transfer rate, and instead record the transient response of the PTL. Moreover, the standard equations accurately predict unidirectional single-phase flow of gas or air through a PTL, but do not accurately model the interactions between water flowing in one direction through the porous network of the porous construct while gas and reactants flow in the other direction. This iterative method discussed above is slower and more labour intensive, but results in a porous construct with an experimentally- verified constant fluid transfer rate.

[0023] Optionally, the fluid transfer rate is evaluated by measuring a water transfer rate of the sample porous construct.

[0024] Optionally, measuring the water transfer rate comprises measuring, over a period of time, a volume of a volume of water exiting the porous construct.

[0025] Optionally, the period of time is at least 1000 hours, so as to ensure the PTL operates for a prolonged period in the steady state. The period of time may even be at least 2000 hours.

[0026] According to a fourth aspect, there comprises a method for selecting a second grade of a second powder for use in providing enhanced water transfer properties in the surface-adjacent region of a porous transport layer for use in a PEM electrode, the porous transport layer being formed from a first powder having a first grade selected such that the porous construct has a constant water transfer rate in use, the method comprising steps of: a) providing a plurality of sample powders each having a distinct sample powder grade; b) selecting a sample powder having an initial sample powder grade, wherein the initial sample powder grade is finer than the first grade of the first powder; c) forming a sample blended powder by blending the sample powder together with the first powder; d) manufacturing a sample porous construct by compacting together and sintering the blended powder; e) evaluating fluid transfer properties in a surface-adjacent region of the sample porous construct; f) if the fluid transfer properties of the surface-adjacent region of the sample porous construct are enhanced, identifying the initial sample powder grade as the second grade of the second powder; g) if the water transfer properties of the surface-adjacent region of the sample porous construct are not enhanced, selecting smaller sample powder grade from the plurality of sample powders; and h) repeating steps c-g iteratively until a sample powder grade is found from which a sample porous construct with a constant water transfer rate can be manufactured.

[0027] As with the method of the third aspect, the method of the fourth aspect is iterative and involves physical construction and testing of sample porous constructs. The second powder is selected only if it provides an experimentally-proven enhancement of the surface-adjacent region.

[0028] The first grade of the first powder may be selected using the method of the third aspect.

[0029] Optionally, selecting the second powder further comprises characterising the porosity of the sample porous construct. As the second powder fills the interstices between particles of the first powder, the fineness of the porosity network increases. However, if the grade of the second powder is too fine, then the particles of the second powder may fully fill the pores, blocking off the porosity network and diminishing the fluid transfer properties of the porous construct. It is advantageous to experimentally measure the porosity of the sample porous construct, to ensure that the pores remain open.

[0030] BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figures la-c graphically illustrate the powder particles of a prior art porous construct. Fig. la is an isometric view of a section of a single layer of powder in the porous construct. Fig. lb is a side view of a section of the porous construct. Fig. 1c is an isometric view of multiple stacked layers within a section of the porous construct.

[0032] Figures 2a-b graphically illustrate the powder particles of an exemplary porous construct according to the invention. Fig. 2a is an isometric view of a section of a single layer of powder in the porous construct. Fig. 2b is an isometric view of multiple stacked layers within a section of the porous construct. Fig. 2c is a side view of a section of the porous construct.

[0033] Figures 3a-b are side views of the porous construct of Figure 2b, following compaction of a surface-adjacent region. Fig. 3a shows the porous construct following a first compaction of the surface-adjacent region, while Fig. 3b shows the porous construct of Figure 3a following a second compaction of the surface adjacent region and resintering of the porous construct.

[0034] Figure 4 is a table showing the powder grades of different powders by percentage of the total weight of the powder. Two exemplary powders according to the invention are compared against two prior art powders.

[0035] Figure 5 is a flowchart showing an exemplary method for manufacturing a PTL from a first and second powder.

[0036] Figure 6 is a flowchart showing an exemplary method for selecting a first grade of a first powder for use in manufacturing a PTL.

[0037] Figure 7 is a flowchart showing an exemplary method for selecting a second grade of a second powder for use in manufacturing a PTL.

[0038] DETAILED DESCRIPTION

[0039] A prior art porous construct will be described with reference to Fig. 1. The prior art porous construct 100 is formed from a single powder population 101, which comprises only one calibre of powder. That is, each powder particle 101 in the porous construct

[0040] 100 is approximately the same diameter. For the purposes of clarity, the powder particles 101 are shown arranged in flat layers 102, with wide spacings between the particles 101. In reality, the powder particles 101 may be more tightly packed, such that each particle 101 makes contact with at least one other particle 101. The particles

[0041] 101 may also not be arranged in distinct layers - for example, the particles 101 may be distributed randomly throughout the porous construct 100.

[0042] The porous construct 100 comprises a contact surface illustrated by the line 103, which is the external face of the porous construct 100 configured to make contact with the membrane-catalyst layer. It is beneficial to prevent membrane catalyst excessive interpenetration into the PTL to prevent cracks from appearing. The contact surface 103 is defined by the outermost powder particles 101, or in the example shown, by the outermost layer 102. As a result, the contact surface 103 is not entirely flat. Due to the gaps between the powder 101 of the outermost layer 102, there are pores 103A. Even if the particles 101 were to be positioned in direct contact with each other, there would still be pores 103A, because the powder particles 101 do not perfectly tesselate and pack together to form a flat surface. This is particularly the case for approximately spherical particles or shape factor controlled particles, as spheres have a low packing efficiency.

[0043] The flatness of a surface may be characterised by its Developed Interfacial Area Ratio (referred to by the notation Sdr). This is the ratio between the real, interfacial area of a surface, and the projected, definition area of that surface. The prior art porous construct 100 has a comparatively low Sdr, due to the pores 103A caused by the interstices between powder particles 101. This means that only a small fraction of contact surface 103 actually makes contact with the membrane (not shown).

[0044] The single powder population used in the prior art porous construct 100 optimises for short term efficiency. This has directed development of PTLs towards using finer and finer powder stock. However, this approach ignores the importance of multi fluid mass transport for PTLs and high current density to reduce total capital cost of equipment.

[0045] A exemplary porous construct according to the invention is described with reference to Figure 2. The porous construct 200 comprises two distinct powders 201A and 201B blended together. The first powder 201A has a first grade, and the second powder 201B has a second grade. The grade of the second powder 201B is finer than the grade of the first powder 201A, that is, the diameters of the particles of the second powder 201B are smaller than the diameters of the particles of the first powder 201A. As per Fig. 1, for the purposes of clarity the powder particles 101 are shown arranged in flat layers 202, with wide spacings between the powder articles 201. In some examples, the porous construct 200 comprises at least 80% the first powder 201A by weight, with the remainder being the second powder 201B. In some examples, the porous construct 200 comprises at least 90% the first powder 201A by weight. The second powder 201B at least partially fills the interstices between particles of the first powder 201A randomly.

[0046] The first 201A and second 201B powder each comprise irregularly-shaped particles, That is, the particles of the powders are shaped so that they do not fit together to form a regularly-spaced lattice. This encourages a porosity network to form within the porous construct 200. The irregularly shaped particles also enhance the capillary effect compared to smooth, regularly shaped particles.

[0047] The first grade of the first powder 201A is selected such that the porous construct has a constant fluid transfer rate in use. That is, the first powder 201A is selected such that the porous construct 200 provides a constant rate of water transfer towards the catalyst-coated membrane, and a constant rate of gas transfer away from the catalyst- coated membrane. The water transfer rate may be dependent on external parameters of the porous construct, such as size, shape, aspect ratio, contact angle and many manufacturing parameters. Therefore the first powder grade which produces a porous construct with a constant fluid transfer rate may vary depending on e.g. the specific dimensions of the porous construct. The second grade of the second powder 201B is selected so as to produce enhanced fluid transfer properties in a surface-adjacent region of the porous construct 200 in use, as will be discussed below. The second powder grade which produces enhanced fluid transport in a surface-adjacent region may also vary depending on external parameters of the porous construct, and additionally depending on the selected first powder grade.

[0048] By filling in the interstices between particles of the first powder 201A, the second powder 201B reduces the empty space between adjacent particles. This results in a smoother and flatter contact surface 203, with shallower and narrower pores 203A. The Sdr of the contact surface 103 is increased compared to prior art porous transport layers. In some examples, the Sdr > 35%. Reducing the depth and diameter of the pores 203A means that a greater proportion of the contact surface 203 makes contact with the catalyst-coated membrane (not shown), increasing the rate of the electrolysis reaction. Since the interstices in the porous construct are reduced in size, the capillary effect of the interstices in the surface-adjacent region 204 of the porous construct 200, close to the contact surface 203, is enhanced. The enhanced capillary effect increases the rate and uniformity of fluid transfer through the surface-adjacent region 204. Water is drawn more quickly through the surface-adjacent region towards the catalyst- coated membrane, and gases produced from the electrolysis reaction are drawn away from the membrane, through the surface-adjacent region 204 into the porous construct 200.

[0049] In some examples, the surface-adjacent region 204 is of order of a few tens of micron deep (that is, of the order of one or two particle diameters of the first powder 201A). In other words, the thickness 204A of the surface-adjacent region 204 is significantly smaller than the thickness 200A of the porous construct 200. In some examples, the electrical resistance at the interface between the membrane and contact surface 203 is also decreased, allowing higher electrical currents to be safely transferred. This in turn allows higher rates of hydrogen production. In addition, the porous transport layer is able to provide more uniform support to the membrane, mitigating mechanical deformation of the membrane.

[0050] In some examples and as illustrated in Figure 3a, the efficiency of the PTL may be further improved by compacting a surface-adjacent region 204 of the porous construct 200. For the purpose of clarity, the porous construct 200 is illustrated as being only a few layers of powder 201 thick. In reality, the powder particles 201 are smaller compared to the thickness of the porous construct 200, such that the porous construct

[0051] 200 is 10 to 100 layers thick. A compression force 305 is applied to the porous construct 200, perpendicular to the contact surface 203. This has the effect of densification of the surface-adjacent region 204. The compression force 305 compacts the surface-adjacent region 204 of the porous construct 200, such that the interstices between adjacent particles or layers of particles are reduced in size and diameter, resulting in a finer network of pores. This may further enhance the capillary effect in the surface-adjacent region 204.

[0052] The compression force 305 may be applied by rolling or pressing, that is, by pressing a roller or a die across the contact surface 203 while holding the porous construct 200 in place. Applying the compression force 305 may be particularly effective for spreading out protruding material on the contact surface 203 over a larger area. The protruding material may be pushed into the pores 203A, further narrowing the diameter of the pores 203A and enhancing the capillary effect in the surface-adjacent region 204. Rolling also concentrates the compression force 305A into a thin load line moving across the contact surface 203. Within the surface-adjacent region 204, the compression pressure is sufficiently high to compact the powder particles 201 together, but further inside the porous construct 200 the compression load is spread out and the compression pressure is no longer high enough to compact the particles

[0053] 201 together. As a result, the permeability in the core 306 of the porous construct 200 (which makes up the majority of the thickness of the porous construct 200) is unaffected.

[0054] The compaction of the surface-adjacent region 204 may be performed in several stages. For example, the roller may be applied to the porous construct 200 first in one direction, and then in another direction. To account for the compacting of the surface- adjacent region 204, the porous construct 200 may initially be constructed (that is, initially compacted and sintered) with a thickness greater than its design thickness. Compacting the surface-adjacent region 204 reduces the thickness of the porous construct 200 down to the desired design thickness.

[0055] Figure 3b shows a porous construct 200 after two successive stages of compacting the surface-adjacent region 204, followed by re-sintering of the porous construct. Successive compaction of the surface-adjacent region 204 causes the particles of the first powder 201A to be pushed down further than the particles of the second powder 201B, as the larger particles of the first powder 201A are the primary load-bearers in the porous construct 200. This results in relative net movement or reorganisation of the smaller particles of the second powder 201B outwards, towards the contact surface 203. Moving the smaller particles of the second powder 201B towards the contact surface 203 smooths out undulations on the contact surface 203 between particles 201A of the first powder, further increasing the Sdr of the contact surface 203. Resintering the porous construct 200 ensures that the powder particles 201 in the surface-adjacent region 204 remain compacted while the porous construct 200 is in use.

[0056] The compaction of the surface-adjacent region 204 and re-sintering result in a smoother contact surface 203 with smaller and shallower pores 203A. In other words, the Sdr of the contact surface 203 is further increased. As a result, the fluid transfer properties of the surface-adjacent region are further enhanced and the efficiency of the electrolysis reaction is further improved. Specifically, the catalyst turnover ratio is improved, measured in Amperes / grams, facilitating a reduction in catalyst loading.

[0057] Two exemplary blended powders 401A, 401B, each consisting of a blend of the first powder 201A and second powder 201B, is described in relation to the table 400 in Figure 4. The table also shows two prior art powders 402A, 402B, for comparison.

[0058] The standard sieve sizes are given in US Sieve sizes in column 404, and in Tyler Equivalent sieve sizes in column 405. The size of the sieve openings is given in column 406. The percentage of particles in each powder 401-402 graded at each standard sieve size are given in the columns 401 - 402.

[0059] The prior art powders 402 each comprise a narrow set of powder sizes. The first prior art powder 402A has particle sizes between 0.074 mm and 0.125 mm. The minimum grade of the first prior art powder 402A is a 200 mesh Tyler Equivalent. The second prior art powder 402B has particle sizes between 0.0788 mm and 0.149 mm. The minimum grade of the second prior art powder 402B is a 170 mesh Tyler Equivalent.

[0060] The exemplary powders 401A, 401B according to the invention each comprise a blend of a first powder and a second powder. The first powder's minimum grade is a 170 mesh Taylor Equivalent, meaning that the minimum particle size of the first powder is 0.088mm. The second powder comprises an even blend of different particle sizes, all of which are smaller than a 170 mesh Taylor Equivalent. In other words, the first powder has particle sizes of 80 pm or more, and the second powder has particle sizes of less than 80 pm. Both blended powders 401 comprise at least 90% the first powder by mass. For example, the first blended powder 401A comprises 91% (22.75% 115 mesh + 45.5% 150 mesh + 22.75% 170 mesh, all Taylor equivalent) the first powder. Both blended powders 401 comprises a first powder with powder sizes between 85 pm and 125 pm, and a second powder with powder sizes between 35 pm and 75 pm. The second blended powder 401B further comprises a small amount third powder with powder sizes above 125 pm.

[0061] A method 500 for manufacturing a PTL is described with reference to Figure 5. The first step of the method is to provide 501A, 501B a first powder and a second powder. The second powder has a finer grade than the first powder, that is, the diameters of particles of the second powder are finer than diameters of particles of the first powder. The first and second powders are blended together 502 to form a blended powder. A V shaped powder blender may be used for blending the powders. The powders may be blended 502 such that particles of the second powder are distributed uniformly through the interstices between particles of the first powder.

[0062] The blended powder is then compacted 503 to form a porous construct. The compaction 203 may be performed by introducing the blended powder into a mould, and then pressing the powder. The as-yet unsintered porous construct may be referred to as a 'green'.

[0063] The green is sintered 504 by heating it to a high temperature under vacuum. The particles of the first powder 501A diffuse into one another in the green body in a equidistant manner. The green body may be compressed while being heated, to ensure that the powder particles are pressed together - usually a pile of sinters is arranged between ceramic plates. The compacting mould may also be used to apply heat to the green body, such that the compacting 503 and sintering 504 steps occur simultaneously or successively. In some examples, the green body is subject to several alternating stages of sintering and compaction.

[0064] A surface-adjacent region of the porous construct is then compacted 505. As previously described in relation to Figure 3, the compaction of the surface-adjacent region 505 may be performed by pressing a roller across a surface of the porous construct. Multiple stages of compaction of the surface-adjacent region 505 may be performed. Next, the porous construct is re-sintered 506. This fixes the new compacted structure of the surface-adjacent region in place.

[0065] A method 600 for selecting a first grade of the first powder for a porous construct is described with reference to Figure 6. The method comprises providing a plurality of sample powders each having a distinct sample powder grade. The plurality of powders may be provided according to the previously described standard sieve sizes.

[0066] A sample powder having an initial sample powder grade is selected 601. For example, the initial sample powder grade may be 100 mesh Tyler Equivalent, meaning that the sample powder cannot pass through a 100 mesh Tyler Equivalent sieve. The initial sample powder grade may be selected 601 may be selected based on the current level to be used in the electrolyser. For example, a given particle diameter may be expected to produce a porous construct with a certain electrical resistance.

[0067] A sample porous construct is manufactured by compacting 602 together and sintering 603 the sample powder. For a given thickness of porous construct, a certain range of permeability will be expected to be obtained.

[0068] The bulk fluid transfer rate of the sample porous construct is then evaluated 604. The fluid transfer rate can be evaluated 604 by measuring the water transfer rate through the sample porous construct. The water transfer rate may be the measured water flow rate in millilitres per proton transported through sample porous construct into the membrane, when the sample porous construct is connected to the membrane and electrolysed.

[0069] The water transfer rate has been found to be proportional to the proton transport rate (approximately a ratio of 2.5 molecule of water for every proton), and thus also proportional to the gas transfer rate. The fluid transfer rate, as measured by the water transfer rate, should remain constant at all regime, especially at the maximum targeted fluid transfer rate. The water transfer rate, which proceeds from anode to cathode, if not steady, indicates a deficit of water transport at the anode (the water consuming electrode). Operating with an unsteady water transfer rate means operating the electrolyser with a partially de-saturated catalyst.

[0070] The water transfer rate may be measured over a long period, to ensure that the water transfer rate is constant and the flow is established as a steady state. For example, the water transfer rate may only be considered constant if it remains steady for a minimum of 1000 hours (41 days), or even 2000 hours (83 days). The water transfer rate may be measured directly, by measuring the volume and content of the water existing one side of the porous construct (for example, the cathode side). Long- duration iterative testing of the water transfer rate is more time consuming than methods known in the art, but provides more accurate evaluation of the PTL.

[0071] If the water transfer rate is constant, then the initial sample powder grade is identified 606 as the as the first grade of the first powder. That is, powder with the initial sample powder grade will be used as the first powder grade for manufacturing porous constructs. If the water transfer rate is not constant, then a coarser sample powder grade is selected 606. That is, the next sieving grade up should be selected as the initial sample grade. Steps 602-605 are repeated until a sample powder is found from which a sample porous construct with a constant water transfer rate can be manufactured.

[0072] A method 700 for selecting a second grade of a second powder for use in manufacturing a porous construct is described with reference to Figure 7. The second powder is for use in providing enhanced water transfer properties in the surface-adjacent region of a porous transport layer for use in a PEM electrode, the porous transport layer being formed from a first powder having a first grade selected such that the porous construct has a constant water transfer rate in use. The first powder may be selected according to the method 600 described in relation to Figure 6.

[0073] The method 700 comprises providing a plurality of sample powders each having a distinct sample powder grade. The plurality of powders may be provided according to the previously described standard sieve sizes. A sample powder having an initial sample powder grade is selected 701, such that the initial sample powder grade is finer than the first grade of the first powder. For example, if the first powder grade is 100 mesh Tyler equivalent, the initial sample powder grade may be 115 mesh Tyler Equivalent. A sample blended powder is formed 702 by blending the sample powder together with the first powder. This may be done using a V shaped powder blender, as previously described.

[0074] A sample porous construct is manufactured by compacting 703 together and sintering 704 the sample blended powder. Optionally, a surface-adjacent region of the porous construct may be compacted and the porous construct may be re-sintered, as previously described in relation to Figure 3.

[0075] The fluid transfer properties in a surface-adjacent region of the sample porous construct are then evaluated 705. This can be done by measuring the water transfer rate over a period of time, as previously described. The fluid transfer properties can also be assessed by measuring the Sdr of the contact surface. In addition, the fluid transfer properties can be assessed by characterising the porosity of the porous construct and in particular the surface-adjacent region. Well-known porosity characterisation methods include porosimetry and mercury intrusion . It is advantageous to characterise the porosity, to ensure that the densification of the porous construct caused by the presence of the second powder and / or the compaction of the surface adjacent region does not lead to blockage or non-uniform features on the contact surface. Evaluating 705 the fluid transfer properties of the porous construct may involve a comparative analysis. For example, the fluid transfer properties porous construct constructed from the sample blended powder may be compared against the fluid transfer properties of porous constructs made from only the first powder, or from a different sample blended powder comprising the first powder and a different initial sample powder, or from a different ratio of the first powder to the sample powder.

[0076] If the fluid transfer properties of the surface-adjacent region of the sample porous construct are enhanced, the initial sample powder grade is identified 707 as the second grade of the second powder. That is, powder with the initial sample powder grade will be used as the second powder grade and blended with the first powder to manufacture porous constructs. As previously mentioned, the water transfer rate may be dependent on external parameters of the porous construct, such as size, shape, aspect ratio, contact angle and many manufacturing parameters. Therefore the initial powder grade which produces enhanced fluid transfer rate in the surface-adjacent region may vary depending on e.g. the specific dimensions of the porous construct. Additionally, the initial powder grade which produces enhanced fluid transfer rate in the surface- adjacent region may depend on the grade of the first powder. If the sample second powder does not provide enhanced fluid transfer properties (or the fluid transfer are insufficiently enhanced), then a finer sample powder grade from the plurality of sample powders is selected 706. Steps 702-706 are repeated until a sample powder is found from which a sample porous construct with enhanced fluid transfer properties in the surface-adjacent region can be manufactured.

Claims

CLAIMS1. A method for manufacturing a porous transport layer for use in a PEM electrode, comprising the steps of: providing a first powder having a first grade, and a second powder having a second grade, wherein the second grade of the second powder is finer than the first grade of the first powder; blending together the first and second powders; and constructing a porous construct by compacting together and sintering the blended powder; wherein the first grade of the first powder is selected such that the porous construct has a constant fluid transfer rate in use, and the second grade of the second powder is selected so as to produce enhanced fluid transfer properties in a surface- adjacent region of the porous construct in use.

2. The method of claim 1, further comprising the step of locally compacting the surface-adjacent region of the porous construct.

3. The method of claim 2, wherein the surface-adjacent region is compacted by rolling.

4. The method of claim 2 or 3, wherein the porous construct is re-sintered following compaction.

5. The method of any previous claim, wherein the fluid transfer rate is evaluated by measuring, over a period of time, a volume of a volume of water exiting the porous construct.

6. The method of any preceding claim, wherein the first powder has particle sizes of 80 pm or more, and the second powder has particle sizes of less than 80 pm.

7. The method of claim 6, wherein the first powder comprises powder sizes between 85 pm and 125 pm, and the second powder comprises powder sizes between 35 pm and 75 pm.

8. The method of any preceding claim, wherein the blended powder comprises at least 90% the first powder by mass.

9. A porous transport layer for use in a PEM electrode, comprising a porous construct; wherein the porous construct is constructed by compacting together and sintering a blend of a first powder and a second powder, the first powder having a first grade and the second powder having a second grade, wherein the second grade of the second powder is finer than the first grade of the first powder; and wherein the first grade of the first powder is selected such that the porous construct has a constant fluid transfer rate in use, and the second grade of the second powder is selected so as to produce enhanced fluid transfer properties in a surface- adjacent region of the porous construct in use.

10. A method for selecting a first grade of a first powder for use in manufacturing of a porous transport layer for use in a PEM electrode, the method comprising steps of: a) providing a plurality of sample powders each having a distinct sample powder grade; b) selecting a sample powder having an initial sample powder grade; c) manufacturing a sample porous construct by compacting together and sintering the sample powder; d) evaluating a fluid transfer rate of the sample porous construct; e) if the fluid transfer rate of the sample porous construct is constant, identifying the initial sample powder grade as the first grade of the first powder; f) if the water transfer rate of the sample porous construct is not constant, selecting a coarser sample powder grade from the plurality of sample powders; and g) repeating steps c-f iteratively until a sample powder grade is found from which a sample porous construct with a constant water transfer rate can be manufactured.

11. The method of claim 10, wherein the fluid transfer rate is evaluated by measuring a water transfer rate of the sample porous construct.

12. The method of claim 11, wherein measuring the water transfer rate comprises measuring, over a period of time, a volume of a volume of water exiting the porous construct.

13. The method of claim 12, wherein the period of time is at least 1000 hours.

14. A method for selecting a second grade of a second powder for use in providing enhanced water transfer properties in the surface-adjacent region of a porous transport layer for use in a PEM electrode, the porous transport layer being formed from a first powder having a first grade selected such that the porous construct has a constant water transfer rate in use, the method comprising steps of: a) providing a plurality of sample powders each having a distinct sample powder grade; b) selecting a sample powder having an initial sample powder grade, wherein the initial sample powder grade is finer than the first grade of the first powder; c) forming a sample blended powder by blending the sample powder together with the first powder; d) manufacturing a sample porous construct by compacting together and sintering the blended powder; e) evaluating fluid transfer properties in a surface-adjacent region of the sample porous construct; f) if the fluid transfer properties of the surface-adjacent region of the sample porous construct are enhanced, identifying the initial sample powder grade as the second grade of the second powder; g) if the water transfer properties of the surface-adjacent region of the sample porous construct are not enhanced, selecting smaller sample powder grade from the plurality of sample powders; and h) repeating steps c-g iteratively until a sample powder grade is found from which a sample porous construct with a constant water transfer rate can be manufactured.

15. The method of any of claims 10 to 15, wherein selecting the second powder further comprises characterising the porosity of the sample porous construct.

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