AU2024305070A1 - Electrochemical cell - Google Patents

Abstract

An electrochemical cell comprising a non electrically-conductive structural frame for supporting components of the electrochemical cell and a tensioning element, wherein the structural frame comprises engagement means adapted to engage the tensioning element, wherein the engagement means comprises at least two bosses on the structural frame, each boss adapted to engage with a corresponding aperture on the tensioning element.

Description

Electrochemical Cell

TECHNICAL FIELD

The present invention relates to electrochemical cells and more particularly, electrochemical flow cells, electrochemical fuel cells and electrolyser cells for electrolysis of water or other electrolytes.

BACKGROUND ART

[0001] Due to its potentially lower carbon footprint, the use of hydrogen as a clean and sustainable energy source is increasingly attracting attention. Accelerating hydrogen energy industrialisation is a strategic initiative for many countries and participants.

[0002] The electrolysis of water is emerging as an alternative source of hydrogen compared to the currently employed extraction of hydrogen from petrochemical sources.

[0003] Classified by the electrolyte, there are three established electrolytic hydrogen production technologies using proton exchange membrane electrolysers, alkaline water electrolysers and anion exchange membrane electrolysers. The electrolysers generally comprise a membrane separating the anode and the cathode along with gas diffusion and catalyst layers. Each membrane type offers particular advantages over the others.

[0004] A proton-exchange membrane (PEM), is a semipermeable membrane adapted to conduct protons and insulate hydrogen and oxygen. They produce high-purity hydrogen gas without the need for additional purification steps and can produce hydrogen gas at higher pressures, reducing the need for additional compression steps. Electrolysers using proton exchange membranes are generally more compact and suitable for applications with space constraints and have faster response times to changes in electrical load.

[0005] Proton exchange membranes have higher catalyst costs than other membranes, requiring expensive platinum-based catalysts for the hydrogen evolution reaction. The membranes are more sensitive to impurities in the water feedstock, requiring extensive water purification systems. [0006] An anion exchange membrane (AEM) is a semipermeable membrane adapted to conduct anions by reject gases hydrogen and oxygen. AEM membranes use less expensive catalysts such as nickel than proton exchange membranes.

[0007] Alkaline electrolysis membranes do require the use of alkaline electrolytes, which can be corrosive to some materials and usually produce hydrogen gas at relatively lower pressures than proton exchange electrolysers. They have a larger physical footprint compared to other types of electrolysers and have slower response times to changes in electrical load. Alkaline electrolysers can utilise a wider range of feedstocks, including purified water, brackish water, and wastewater, caustic solutions such as sodium hydroxide and potassium hydroxide.

[0008] An alkaline water electrolyser is characterised by having two electrodes operating in a liquid alkaline electrolyte solution of potassium hydroxide or sodium hydroxide. These electrodes are separated by a separator, separating the product gases and transporting the hydroxide ions (OH") from one electrode to the other.

[0009] A fuel cell is an electrochemical device that converts the chemical energy of a fuel, typically hydrogen, into electrical energy. They comprise an electrolyte that allows the movement of ions between the anode and the cathode. It can be a solid, liquid, or polymer membrane, depending on the type of fuel cell. The anode is the electrode where the fuel (usually hydrogen) is oxidised, releasing electrons and generating positively charged ions and the cathode is the electrode where the oxidant (often oxygen from the air) is reduced, accepting electrons and reacting with the ions from the anode.

[0010] Different types of fuel cells, such as proton exchange membrane fuel cells (PEMFC), solid oxide fuel cells (SOFC), alkaline fuel cells (AFC)and molten carbonate fuel cells (MCFC) are known. The specific components and configurations may vary depending on the type of fuel cell technology.

[0011 ] A flow battery is an electrochemical energy storage device that uses two electrolyte solutions stored in separate tanks. When the battery is charging or discharging, the electrolytes flow through the battery cell stack.

[0012] Flow batteries consist of two separate electrolyte storage tanks, typically containing different redox-active species dissolved in a supporting electrolyte. The tanks may be made of various materials, such as plastic or metal, and their structural problems can include leakage, corrosion, or degradation over time.

[0013] Flow batteries use a membrane to separate the positive and negative electrolyte solutions while allowing the flow of ions.

[0014] Electrolysers, fuel cells and flow batteries all can experience mechanical stress due to factors such as fluid flow, pressure variations, thermal expansion, or vibrations. Excessive mechanical stress can result in structural damage, degradation, or even catastrophic failure.

[0015] Inadequate sealing in stacks of electrolysers, fuel cells and flow batteries can result in gas or electrolyte crossover, reduced efficiency and decreased performance of the overall device.

[0016] All three of these technologies have aspects in common and can all act as a pressure vessel albeit with differing internal pressures. They all utilise electrically non- conductive structural frames, made of for example, plastics, to retain various electrolysis cell components. The operating conditions of the cell as well as the inherent strength of the plastic impacts on the sizes of the frames. For example, higher operating pressures conditions generally require thicker frames to reduce frame distortion.

[0017] To address this problem, it is known to reinforce plastic frames with materials such as fibres of glass, aramid and basalt into the plastic to increase their strength and allow for a thinner frame wall thicknesses. However, the plastic frame materials can degrade and leach into the electrolyte releasing contaminants from the fibres which are not compatible for some forms of electrolysis which can deleteriously impact the electrolyser.

[0018] Electrolysers are known to operate at elevated temperatures under which conditions, the various components thermally expand at various rates. Additionally, plastic materials and gaskets have a tendency to thermally expand and then not elastically return to the same state. Internal loads radiate outwards and plastics can have a tendency to deform outwards over time.

[0019] The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge as at the priority date of the application.

[0020] Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

[0021 ] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referenced to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

[0022] The present invention is not to be limited in scope by the specific embodiments described herein, which are intended for the purpose of exemplification only. Functionally equivalent products, compositions and methods are clearly within the scope of the invention as described herein. The entire disclosures of all publications (including patents, patent applications, journal articles, laboratory manuals, books, or other documents) cited herein are hereby incorporated by reference.

SUMMARY OF INVENTION

[0023] In accordance with the present invention, there is provided an electrochemical cell comprising a non electrically conductive structural frame for supporting components of the electrochemical cell and a tensioning element, wherein the structural frame comprises engagement means adapted to engage the tensioning element, and wherein the engagement means comprises at least two bosses on the structural frame, each boss adapted to engage with a corresponding aperture on the tensioning element.

[0024] Preferably, the tensioning element is electrically conductive.

[0025] Advantageously, engagement of the structural frame and the tensioning element strengthens the structural frame. [0026] Engagement of the structural frame and the tensioning element facilitates the transfer of force from the structural frame to the tensioning element, reducing mechanical stresses on the structural frame. This enables the structural frame to be operated at higher pressures and temperatures than structural frames without an engaged tensioning element. It further enables structural frames to be made smaller and with less materials than structural frames of the prior art. It further enables structural frames to be prepared with less stress resistant materials than structural frames of the prior art.

[0027] Preferably, the tensioning element has a higher ultimate tensile strength than the structural frame.

[0028] Preferably, the tensioning element has a higher tensile modulus than the structural frame.

[0029] Preferably, the structural frame has as high a compressive, tensile strength as possible or at least adequate margin of safety for applied forces in whatever configuration, to reduce the likelihood of plastic deformation or rupture. Preferably, the thermal coefficients of expansion of adjacent materials such as those comprising the structural frame and the tensioning element are as close as possible.

[0030] Preferably, the structural frame is plastic with high resistance to hydrolysis and leaching. Suitable plastics may include those selected from the groups including but not limited to polysulfones, polyphenylsulfones, polyether ether ketones, polyphenylene sulfides, polyphenylene sulfones, polyamideimides, polyamides, polybutylterephthalates and polyethyl imides.

[0031] In one form of the invention, the structural frame may be provided in the form of an electrically conductive element overmoulded with a non-electrically conductive layer where the overmoulded component as a whole exhibits electrically insulating properties with respect to the next electrically conducting element.

[0032] During operation of an electrochemical cell, the cell is impacted by two different loads caused by thermal expansion and internal operating pressure. As these loads are applied to the structural frame, the frame tries to expand laterally. The engagement means transfers that pressure load onto the tensioning element and the load is not only required to push out the structural frame but also the tensioning element, thus reducing the deflection of the structural frame.

[0033] It is known to operate electrochemical cells at elevated temperatures. The present invention enables the structural frame to be operated at higher pressures and temperatures reducing creep stress.

[0034] In one form of the invention, the tensioning element is metallic. Metallic tensioning elements may be prepared from titanium, stainless steel or nickel.

[0035] Ine form of the invention, the tensioning element is non-metallic. Non-metallic tensioning elements may be prepared graphite.

[0036] Preferably, the tensile strength of the tensioning element is 50 to 1500 MPa. In one form of the invention, the tensile strength of the tensioning element is about 50 MPa. In one form of the invention, the tensile strength of the tensioning element is about 100 MPa. In one form of the invention, the tensile strength of the tensioning element is about 200 MPa. In one form of the invention, the tensile strength of the tensioning element is about 300 MPa. In one form of the invention, the tensile strength of the tensioning element is about 400 MPa. In one form of the invention, the tensile strength of the tensioning element is about 500 MPa. In one form of the invention, the tensile strength of the tensioning element is about 600 MPa. In one form of the invention, the tensile strength of the tensioning element is about 700 MPa. In one form of the invention, the tensile strength of the tensioning element is about 800 MPa. In one form of the invention, the tensile strength of the tensioning element is about 900 MPa. In one form of the invention, the tensile strength of the tensioning element is about 1000 MPa. In one form of the invention, the tensile strength of the tensioning element is about 1100 MPa. In one form of the invention, the tensile strength of the tensioning element is about 1200 MPa. In one form of the invention, the tensile strength of the tensioning element is about 1300 MPa. In one form of the invention, the tensile strength of the tensioning element is about 1400 MPa. In one form of the invention, the tensile strength of the tensioning element is about 1500 MPa.

[0037] In one form of the invention, the structural frame comprises plastic. Many types of plastics exist with varying tensile properties. Preferably, tensile strength of the structural frame is 10 to 200 MPa. In one form of the invention, the tensile strength of the structural frame is about 10 MPa. In one form of the invention, the tensile strength of the structural frame is about 20 MPa. In one form of the invention, the tensile strength of the structural frame is about 30 MPa. In one form of the invention, the tensile strength of the structural frame is about 40 MPa. In one form of the invention, the tensile strength of the structural frame is about 50 MPa. In one form of the invention, the tensile strength of the structural frame is about 60 MPa. In one form of the invention, the tensile strength of the structural frame is about 70 MPa. In one form of the invention, the tensile strength of the structural frame is about 80 MPa. In one form of the invention, the tensile strength of the structural frame is about 90 MPa. In one form of the invention, the tensile strength of the structural frame is about 100 MPa. In one form of the invention, the tensile strength of the structural frame is about 110 MPa. In one form of the invention, the tensile strength of the structural frame is about 120 MPa. In one form of the invention, the tensile strength of the structural frame is about 130 MPa. In one form of the invention, the tensile strength of the structural frame is about 140 MPa. In one form of the invention, the tensile strength of the structural frame is about 150 MPa. In one form of the invention, the tensile strength of the structural frame is about 160 MPa. In one form of the invention, the tensile strength of the structural frame is about 170 MPa. In one form of the invention, the tensile strength of the structural frame is about 180 MPa. In one form of the invention, the tensile strength of the structural frame is about 190 MPa. In one form of the invention, the tensile strength of the structural frame is about 200 MPa.

[0038] In one form of the invention, the tensile modulus of the tensioning element is 100 to 300 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 100 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 110 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 120 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 130 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 140 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 150 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 160 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 170 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 180 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 190 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 200 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 210 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 220 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 230 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 240 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 250 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 260 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 270 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 280 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 290 GPa. In one form of the invention, the tensile modulus of the tensioning element is about 300 GPa.

[0039] In one form of the invention, the tensile modulus of the structural frame is 0.1 to 10 GPa. In one form of the invention, the tensile modulus of the structural frame is about 0.1 GPa. In one form of the invention, the tensile modulus of the structural frame is about 0.2 GPa. In one form of the invention, the tensile modulus of the structural frame is about 0.3 GPa. In one form of the invention, the tensile modulus of the structural frame is about 0.4 GPa. In one form of the invention, the tensile modulus of the structural frame is about 0.5 GPa. In one form of the invention, the tensile modulus of the structural frame is about 0.6 GPa. In one form of the invention, the tensile modulus of the structural frame is about 0.7 GPa. In one form of the invention, the tensile modulus of the structural frame is about 0.8 GPa. In one form of the invention, the tensile modulus of the structural frame is about 0.9 GPa. In one form of the invention, the tensile modulus of the structural frame is about 1 GPa. In one form of the invention, the tensile modulus of the structural frame is about 2 GPa. In one form of the invention, the tensile modulus of the structural frame is about 3 GPa. In one form of the invention, the tensile modulus of the structural frame is about 4 GPa. In one form of the invention, the tensile modulus of the structural frame is about 5 GPa. In one form of the invention, the tensile modulus of the structural frame is about 6 GPa. In one form of the invention, the tensile modulus of the structural frame is about 7 GPa. In one form of the invention, the tensile modulus of the structural frame is about 8 GPa. In one form of the invention, the tensile modulus of the structural frame is about 9 GPa. In one form of the invention, the tensile modulus of the structural frame is about 10 GPa. [0040] Preferably there are provided a plurality of apertures on the tensioning element.

[0041] Preferably, there are provided a plurality of bosses on the structural frame.

[0042] In the context of the present specification, an aperture on the tensioning element is referred to as a tensioning element aperture.

[0043] In the context of the present specification, the term corresponding aperture shall be understood to mean a tensioning element aperture corresponding to a particular boss. For completeness, it shall be understood that where the structural frame comprises two bosses, the tensioning element will comprise at least two apertures, each boss corresponding to a different tensioning element aperture. Where the structural frame comprises three bosses, the tensioning element comprises at least three apertures, each boss corresponding to a different tensioning element aperture and so on as the number of bosses increases.

[0044] In one form of the invention, the structural frame comprises a plurality of apertures. In the context of the present specification, the plurality of apertures on the structural frame are referred to as structural frame apertures.

[0045] In one form of the invention, each boss is provided adjacent to a structural frame aperture. It will be understood that each boss is provided adjacent to a different structural frame aperture.

[0046] In one form of the invention, each boss is adapted to engage with an inner surface of each corresponding tensioning element aperture. Advantageously, engagement of each boss with each inner surface tensioning element aperture requires no lateral rotation of either the structural frame or the tensioning element.

[0047] In one form of the invention, the height of each boss is approximately the same as the thickness of the tensioning element. It will be appreciated that a gasket may be provided between the structural frame and the tensioning element. Where a gasket is provided between the structural frame and the tensioning element, the height of each boss is preferably, approximately the same as the thickness of the tensioning element and the gasket. [0048] In one form of the invention, the boss is between 0.1 mm and 10 mm high. In an alternate form of the invention, the boss is between 0.1 mm and 5 mm high. In an alternate form of the invention, the boss is between 0.1 mm and 3 mm high. In an alternate form of the invention, the boss is between 0.1 mm and 2 mm high. In an alternate form of the invention, the boss is between 0.1 mm and 1 mm high.

[0049] In one form of the invention, the boss is at least 0.1 mm high. In one form of the invention, the boss is at least 0.5 mm high. In one form of the invention, the boss is at least 1 mm high. In one form of the invention, the boss is at least 2 mm high. In one form of the invention, the boss is at least 3 mm high. In one form of the invention, the boss is at least 4 mm high. In one form of the invention, the boss is at least 5 mm high. In one form of the invention, the boss is at least 10 mm high.

[0050] In one form of the invention, the boss is less than 0.1 mm high. In one form of the invention, the boss is less than 0.5 mm high. In one form of the invention, the boss is less than 1 .0 mm high. In one form of the invention, the boss is less than 2 mm high. In one form of the invention, the boss is less than 3 mm high. In one form of the invention, the boss is less than 4 mm thick. In one form of the invention, the boss is less than 5 mm high. In one form of the invention, the boss is less than 10 mm high.

[0051] In one form of the invention, the boss is about 0.1 mm high. In one form of the invention, the boss is about 0.5 mm high. In one form of the invention, the boss is about 1 mm high. In one form of the invention, the boss is about 2 mm high. In one form of the invention, the boss is about 3 mm high. In one form of the invention, the boss is about 4 mm high. In one form of the invention, the boss is about 5 mm high. In one form of the invention, the boss is about 10 mm high.

[0052] It will be appreciated that the thickness of the boss will be impacted by the material of the boss as well as the forces the boss is intended to withstand. In one form of the invention, the boss is between 0.1 mm and 10 mm thick. In an alternate form of the invention, the boss is between 0.1 mm and 5 mm thick. In an alternate form of the invention, the boss is between 0.1 mm and 3 mm thick. In an alternate form of the invention, the boss is between 0.1 mm and 2 mm thick. In an alternate form of the invention, the boss is between 0.1 mm and 1 mm thick. [0053] In one form of the invention, the boss is at least 0.1 mm thick. In one form of the invention, the boss is at least 0.5 mm thick. In one form of the invention, the boss is at least 1 mm thick. In one form of the invention, the boss is at least 2 mm thick. In one form of the invention, the boss is at least 3 mm thick. In one form of the invention, the boss is at least 4 mm thick. In one form of the invention, the boss is at least 5 mm thick. In one form of the invention, the boss is at least 10 mm thick.

[0054] In one form of the invention, the boss is less than 0.1 mm thick. In one form of the invention, the boss is less than 0.5 mm thick. In one form of the invention, the boss is less than 1 mm thick. In one form of the invention, the boss is less than 2 mm thick. In one form of the invention, the boss is less than 3 mm thick. In one form of the invention, the boss is less than 4 mm thick. In one form of the invention, the boss is less than 5 mm thick. In one form of the invention, the boss is less than 10 mm thick.

[0055] In one form of the invention, the boss is about 0.1 mm thick. In one form of the invention, the boss is about 0.5 mm thick. In one form of the invention, the boss is about 1 mm thick. In one form of the invention, the boss is about 2 mm thick. In one form of the invention, the boss is about 3 mm thick. In one form of the invention, the boss is about 4 mm thick. In one form of the invention, the boss is about 5 mm thick. In one form of the invention, the boss is about 10 mm thick.

[0056] Where a boss engages with the inner surface of a tensioning element aperture, it will be appreciated that the transverse cross-sectional shape of the boss should be complimentary with the shape of said aperture. Where the tensioning element aperture is circular, the boss will be an arc with a similar internal radius. The radius of the boss should be such that it can reside inside the tensioning element aperture. Preferably, engagement of the tensioning element aperture with the boss provides a firm fit.

[0057] It will be appreciated that the apertures in the structural frame and apertures in the tensioning element will be aligned in the electrochemical cell such that both can receive a tie rod as is known in the art. This is particularly the case where the tensioning element is provided in the form of a bipolar pate or a current collector. [0058] As is known in the art, aperture dimensions are influenced by, amongst others, internal load, tie rod quantity, tie rod dimensions, active area size, structural frame strengths, thermal expansions of all materials used, cyclic fatigue loads.

[0059] Preferably, each tensioning element aperture and each structural frame aperture are concentric.

[0060] Preferably, the diameter of the tensioning element aperture is larger than the diameter of the structural frame aperture. In one form of the invention, the tensioning element aperture is about 30 mm. In an alternate form of the invention, the structural frame aperture is about 30 mm.

[0061] Where the structural frame and the tensioning element have different coefficients of thermal expansion, the diameters of the tensioning element aperture and the structural frame aperture will be such that an interference fit is between the boss and the tensioning element aperture on attainment of operating temperature.

[0062] In one form of the invention, the structural frame comprises a border which defines the active area of the electrochemical cell.

[0063] Where the structural frame comprises a border, the structural frame apertures reside in the structural frame border. It will be appreciated that the width of the border must be larger than the aperture diameters.

[0064] The present invention allows structural frame borders to be narrower than the prior art. Depending on the application, a structural frame border may be up to 75 % narrower than previously contemplated. For a PEM electrolyser, this may result in a reduction of a 300 mm frame to 75 mm.

[0065] The structural frame apertures are preferably the same size as each other.

[0066] It will be appreciated that the structural frame may contain a number of apertures for receiving tie rods as is known in the art. The present invention includes within its scope the utilisation of any number of bosses on the structural frame, ranging from two bosses up to a number of bosses that is equal to the number of structural frame apertures. [0067] It will be appreciated that where the apertures are for tie rods, the number of structural frame apertures and the number of tensioning element apertures may be affected by the preferred number of tie rods.

[0068] Additionally, if a small degree of tolerance is provided between the tie rod and the structural frame aperture, the structural frame can expand into said space reducing local stresses.

[0069] In one form of the invention, each structural frame aperture contains an adjacent boss. In an alternate form of the invention, alternating structural frame apertures contain an adjacent boss.

[0070] From an engineering perspective, an electrochemical cell is essentially a pressure vessel. Circular pressure vessels have more uniform stress distribution that non-circular pressure vessels which can result in higher structural integrity and better resistance to pressure-induced deformation. Other advantages include increased rigidity, improved pressure resistance and enhanced material usage than non-circular pressure vessels.

[0071] Quadrilateral pressure vessels offer their own particular advantages, including better space utilisation, versatility in orientation and better material optimisation.

[0072] The choice between circular and rectangular pressure vessel designs depends on the specific requirements, constraints, and priorities of the application at hand. Factors such as pressure levels, space limitations, installation considerations, and manufacturing costs play a crucial role in determining the most suitable design.

[0073] In one form of the invention, the structural frame and the tensioning element are quadrilateral.

[0074] It will be appreciated that the structural frame and the tensioning element may be similarly dimensioned as is known in the art.

[0075] In one form of the invention, the quadrilateral structural frame has external dimensions of approximately 450 mm x 750 mm. In such an embodiment, the longer sides may comprise four structural frame apertures and the shorter sides may comprise two or three structural frame apertures. It will be appreciated that variations are inherently possible. It will be appreciated that the number of apertures will be influenced by the number of ties rods which in turn is affected by the material structural capacities and the internal operating pressures. It is within the scope of the skilled addressee to determine the appropriate number of tie rods for a given electrochemical cell stack.

[0076] The structural frame preferably comprises apertures in the corners of structural frame border. More preferably, the structural frame comprises structural frame apertures in the corners of the border and along all four the sides of the border.

[0077] Preferably, each boss is provided as a pair of complimentary bosses. In the context of the present specification, a pair of complementary bosses shall be understood to refer to a pair of bosses on opposite edges of a structural frame.

[0078] For example, for a circular structural frame comprising two bosses, the pair of complimentary bosses will preferably be located approximately 180° apart from each other on the structural frame. In this way, they are able to act against forces radiating from the centre of the frame in opposite directions, thereby providing increased stability. For a circular structural frame comprising four bosses, they will preferably be located approximately 90° apart from each other and so on.

[0079] For a quadrilateral structural frame, a pair of complimentary bosses will preferably be located on opposing edges of the frame.

[0080] During operation of an electrochemical cell, the bosses will be impacted by two different loads caused by thermal expansion and internal operating pressure respectively. Both sets of loads act approximately in the same direction, radiating outwards from the centre of the structural frame towards its edges. Where the structural frame is circular, each boss will experience a similar load, irrespective of its location of the structural frame. However, where the structural frame is a quadrilateral, a boss on a side edge of the structural frame will experience a different load to a boss on a corner of the structural frame.

[0081] In one form of the invention, at least one boss is an elongate projection and the tensioning element aperture is a corresponding elongate shape adapted to receive the boss. [0082] In one form of the invention, at least one boss is a circular projection and the tensioning element aperture is a corresponding circular shape adapted to receive the boss.

[0083] In one form of the invention, at least one boss is arcuate.

[0084] In one form of the invention, at least one boss forms a 10° arc. In one form of the invention, at least one boss forms a 20° arc. In one form of the invention, at least one boss forms a 30° arc. In one form of the invention, at least one boss forms a 40° arc. In one form of the invention, at least one boss forms a 50° arc. In one form of the invention, at least one boss forms a 60° arc. In one form of the invention, at least one boss forms a 70° arc. In one form of the invention, at least one boss forms an 80° arc. In one form of the invention, at least one boss forms a 90° arc. In one form of the invention, at least one boss forms a 100° arc. In one form of the invention, at least one boss forms a 110° arc. In one form of the invention, at least one boss forms a 120° arc. In one form of the invention, at least one boss forms a 130° arc. In one form of the invention, at least one boss forms a 140° arc. In one form of the invention, at least one boss forms a 150° arc. In one form of the invention, at least one boss forms a 160° arc. In one form of the invention, at least one boss forms a 170° arc. In one form of the invention, at least one boss forms a 180° arc. In one form of the invention, at least one boss forms a 190° arc. In one form of the invention, at least one boss forms a 200° arc. In one form of the invention, at least one boss forms a 210° arc. In one form of the invention, at least one boss forms a 220° arc. In one form of the invention, at least one boss forms a 230° arc. In one form of the invention, at least one boss forms a 240° arc. In one form of the invention, at least one boss forms a 250° arc. In one form of the invention, at least one boss forms a 260° arc. In one form of the invention, at least one boss forms a 270° arc. In one form of the invention, at least one boss forms a 280° arc. In one form of the invention, at least one boss forms a 290° arc. In one form of the invention, at least one boss forms a 300° arc. In one form of the invention, at least one boss forms a 310° arc. In one form of the invention, at least one boss forms a 320° arc. In one form of the invention, at least one boss forms a 330° arc. In one form of the invention, at least one boss forms a 340° arc. In one form of the invention, at least one boss forms a 350° arc. It will be appreciated that the present invention may comprises combinations of bosses of different degrees of curvature. [0085] In one form of the invention, the structural frame comprises a plurality of 180° arcuate bosses.

[0086] In one form of the invention, the structural frame comprises a plurality of 270° arcuate bosses.

[0087] In one form of the invention, the structural frame comprises a plurality of 180° arcuate bosses and a plurality of 270° arcuate bosses.

[0088] Where a boss is located on a side edge of a quadrilateral structural frame, the boss is preferably an arc up to 180°. A boss in this location experiences load in one direction only. Such load forces the boss against the inside edge of the tensioning element aperture. An arc greater than 180° may contain redundant material.

Additionally, an arc of up to 180° allows the boss to thermally expand inwards towards the tie rod reducing some internal stresses.

[0089] Where a boss is located on adjacent a corner of a quadrilateral structural frame, the boss is preferably a semicircle up to 270°. A boss in this location experiences load in two directions. Such loads force the boss against the inside edge of the tensioning element aperture. A semicircle greater than 270° may contain redundant material.

[0090] It will be appreciated that an arc comprises a concave side and a convex side. Where a boss is located on a side edge of a quadrilateral structural frame, the convex side of the arc preferably faces the outer edge of the structural frame and the concave side preferably faces the inner side of the structural frame.

[0091] In one form of the invention, a recess is provided on the surface of the structural frame adjacent a boss. Preferably, the shape of the recess mirrors the shape of the boss. Where the boss is provided as an arc, the recess is preferably a similarly dimensioned arc. For example, where the boss is a 180° arc, the recess is preferably a 180° arc and where the boss is a 270° arc, the recess is preferably a 270° arc.

[0092] In one form of the invention, a recess is provided adjacent each boss.

[0093] The recess is adapted to reduce stress concentrations where the boss meets the structural frame surface. [0094] It will be appreciated that the depth of the recess will be limited by the thickness of the structural frame.

[0095] In one form of the invention, the recess is between 0.1 mm and 5 mm deep. In an alternate form of the invention, the recess is between 0.1 mm and 3 mm deep. In an alternate form of the invention, the recess is between 0.1 mm and 2 mm deep. In an alternate form of the invention, the recess is between 0.1 mm and 1 mm deep.

[0096] In one form of the invention, the recess is at least 0.1 mm deep. In one form of the invention, the recess is at least 0.2 mm deep. In one form of the invention, the recess is at least 3 mm deep. In one form of the invention, the recess is at least 0.4 mm deep. In one form of the invention, the recess is at least 0.5 mm deep. In one form of the invention, the recess is at least 1 mm deep. In one form of the invention, the recess is at least 2 mm deep. In one form of the invention, the recess is at least 3 mm deep. In one form of the invention, the recess is at least 5 mm deep.

[0097] In one form of the invention, the recess is less than 0.1 mm deep. In one form of the invention, the recess is less than 0.2 mm deep. In one form of the invention, the recess is less than 3 mm deep. In one form of the invention, the recess is at least 0.4 mm deep. In one form of the invention, the recess is less than 0.5 mm deep. In one form of the invention, the recess is less than 1 mm deep. In one form of the invention, the recess is less than 2 mm deep. In one form of the invention, the recess is less than 3 mm deep. In one form of the invention, the recess is less than 5 mm deep.

[0098] In one form of the invention, the recess is about 0.1 mm deep. In one form of the invention, the recess is about 0.2 mm deep. In one form of the invention, the recess is about 3 mm deep. In one form of the invention, the recess is about 0.4 mm deep. In one form of the invention, the recess is about 0.5 mm deep. In one form of the invention, the recess is about 1 mm deep. In one form of the invention, the recess is about 2 mm deep. In one form of the invention, the recess is about 3 mm deep. In one form of the invention, the recess is about 5 mm deep.

[0099] In one form of the invention, the recess is between 0.1 mm and 5 mm wide. In an alternate form of the invention, the recess is between 0.1 mm and 3 mm wide. In an alternate form of the invention, the recess is between 0.1 mm and 2 mm wide. In an alternate form of the invention, the recess is between 0.1 mm and 1 mm wide. [00100] In one form of the invention, the recess is at least 0.1 mm wide. In one form of the invention, the recess is at least 0.2 mm wide. In one form of the invention, the boss is at least 0.3 mm wide. In one form of the invention, the recess is at least 0.4 mm wide. In one form of the invention, the recess is at least 0.5 mm wide. In one form of the invention, the recess is at least 1 mm wide. In one form of the invention, the recess is at least 2 mm wide. In one form of the invention, the recess is at least 3 mm wide. In one form of the invention, the recess is at least 5 mm wide.

[00101 ] In one form of the invention, the recess is less than 0.1 mm wide. In one form of the invention, the recess is less than 0.2 mm wide. In one form of the invention, the boss is less than 0.3 mm wide. In one form of the invention, the recess is less than 0.4 mm wide. In one form of the invention, the recess is less than 0.5 mm wide. In one form of the invention, the recess is less than 1 mm wide. In one form of the invention, the recess is less than 2 mm wide. In one form of the invention, the recess is less than 3 mm wide. In one form of the invention, the recess is less than 5 mm wide.

[00102] In one form of the invention, the recess is about 0.1 mm wide. In one form of the invention, the recess is about 0.2 mm wide. In one form of the invention, the boss is about 0.3 mm wide. In one form of the invention, the recess is about 0.4 mm wide. In one form of the invention, the recess is about 0.5 mm wide. In one form of the invention, the recess is about 1 mm wide. In one form of the invention, the recess is about 2 mm wide. In one form of the invention, the recess is about 3 mm wide. In one form of the invention, the recess is about 5 mm wide.

[00103] Preferably, the boss comprises the same non-conductive material as the structural frame.

[00104] In one form of the invention, the boss is integral with the structural frame.

[00105] The structural frame with an integral boss may be prepared by any method known in the art, including but not limited to injection moulding, blow moulding, thermoforming, compression moulding, rotation moulding or 3D printing. Alternatively, the structural frame with an integral boss may be prepared by subtractive manufacturing where a portion of the frame is removed to expose a boss.

[00106] Advantageously, the boss can insulate the tie rod from the current collector and may remove the need to provide the tie rod with an insulating covering. [00107] The electrochemical cell may be provided in the form of a flow cell, a fuel cell or an electrolysis cell.

[00108] In one form of the invention, the tensioning element is provided in the form of a bipolar plate. In an alternate form of the invention, the tensioning element is provided in the form of a current collector. It will be appreciated that an electrolyser stack may comprise both current collectors and bipolar plates acting as conductive layers. In an alternate form of the invention, the tensioning element is provided in the form of an electrode.

[00109] It is known to use bipolar plates in electrochemical cell assemblies for a variety of purposes. They allow even gas distribution over the electrode surface are and provide electrical conductivity and assist in heat management by facilitating water outtake. They provide support to the membrane electrochemical assemblies and provide mechanical stability to a stack of cells.

[00110] Bipolar plates are generally alternately stacked with membrane electrochemical assemblies so that each plate provides electrical connection on one side for the anode of one membrane electrolyser assembly and on the second side for the cathode of the neighbouring membrane electrochemical assembly.

[00111] It will be appreciated that electrochemical cell may further comprise a separator selected from the group comprising proton exchange membranes, anion exchange membranes and alkaline separators.

[00112] The separator may form part of a membrane electrode assembly comprising a catalyst-coated membrane and an anode and a cathode on opposing sides of the membrane.

[00113] Alkaline electrolysis separators are known to include porous nickel materials such as nickel foam or mesh coated in a thin layer of nickel hydroxide, which would act as both the electrode and separator or Zirfon, which is a zirconium oxide based ceramic material, or porous ceramics like aluminium oxide or porous ceramic composites.

[00114] It will be appreciated that the membrane should have good mechanical stability, high porosity for efficient ion transport and chemical resistance to alkaline electrolytes. [00115] Proton exchange membranes are usually perfluorosulfonic acid polymers such as Nation. Other materials include PBI and SPEEK (sulfonated polyether ether ketone), PES (poly ether sulfone), PI (Polyimide)

[00116] It will be appreciated that the membrane should have excellent chemical and thermal stability, high proton conductivity and resistance to chemical degradation operable at elevated temperatures which allow for improved water management and higher system efficiency.

[00117] Anion exchange membranes are typically an anion conducting polymer membrane. Such as quaternary ammonium functionalised polymers like PVam, PPO, PS

[00118] Proton exchange membranes are typically coated with platinum on the cathode side and iridium oxide or ruthenium oxide at the anode side. Anion exchange membranes are typically coated with platinum, palladium, nickel, nickel oxide or nickel based alloys such as nickel-cobalt or nickel-iron.

[00119] Fuel cells use catalysts to facilitate the electrochemical reactions that occur at the electrodes. In a fuel cell, the anode catalyst facilitates the oxidation reaction of the fuel, such as hydrogen gas (H2) or a hydrocarbon, to release electrons and generate positively charged ions (e.g., H⁺). The cathode catalyst facilitates the reduction reaction of the oxidant, typically oxygen gas (O2), where the ions and electrons combine to form water (H2O) or other reaction by-products.

[00120] The most commonly used catalyst material in fuel cells is platinum or platinumbased alloys due to their high catalytic activity and stability. These catalysts are typically applied as a thin layer or coating onto the electrode surfaces, increasing the surface area available for the electrochemical reactions and enhancing the efficiency of the fuel cell.

[00121 ] Flow batteries typically do not use catalysts in the same way as fuel cells. In flow batteries, the redox reactions occur directly between the electrolyte solutions rather than at specific catalyst-coated electrodes. The redox-active species in the electrolytes undergo reversible oxidation and reduction reactions during the charging and discharging processes. [00122] Flow batteries employ different redox couples as the active species in the electrolyte solutions. These redox couples can consist of metal ions, organic compounds, or other electroactive species. The reactions between these species can occur without the need for catalysts.

[00123] However, some flow battery chemistries may utilise catalysts, albeit in a different context. For example, certain types of flow batteries, such as the vanadium redox flow battery (VRFB), may use catalysts in the electrode materials to enhance the kinetics of reactions occurring at the electrode surfaces. These catalysts are typically used to improve the efficiency and performance of the electrode reactions, rather than directly facilitating the redox reactions in the electrolyte.

[00124] The electrochemical cell may further be provided with at least one flow field.

[00125] It is known to provide flow fields as a metallic mesh or indented bipolar plate that allows passage of the oxygen and hydrogen gases whilst also being electrically conductive.

[00126] The electrochemical cell assembly may further comprise at least one gas diffusion layer. It is known to use a gas diffusion layer for each electrode. They are porous, conductive materials that facilitate the reactant gas distribution, provide a pathway for electron flow and allow efficient water management within the electrochemical cell. The anode gas diffusion layer distributes the hydrogen gas evenly across the surface of the electrode, while the cathode gas diffusion layer facilities the distribution of the oxygen gas in the same way. The gas diffusion layers also assist in the removal of water produced during the reactions and provide some mechanical support to the catalyst layers.

[00127] In one form of the invention, the electrochemical cell assembly comprises a gas diffusion layer and a porous transport layer.

[00128] In proton exchange membrane technology, both a porous transport layer and a gas diffusion layer may be provided. They have different functions and are positioned differently within the assembly. The porous transport layer is a thin, porous layer that is typically located between the catalyst and the gas diffusion layer. It acts as a transport medium for reactant gases and facilitates the movement of protons (H⁺) and water molecules within the electrode assembly. The primary functions of the PTL are proton transport and water management.

[00129] The gas diffusion layer is generally a thicker and more porous layer typically positioned between the catalyst layer and the flow field or bipolar plate. The main functions of the GDL are gas distribution and electron conduction.

[00130] It is known to coat gas diffusion layers and porous transport layers to enhance their performance and functionality. The coatings applied to these layers serve various purposes depending on the specific requirements of the electrochemical cell.

[00131 ] Gas diffusion layers and porous transport layers may be coated with hydrophobic materials to improve water management within the electrochemical cell. These coatings repel water and help prevent flooding or blockage of the porous layers, ensuring efficient gas and proton transport. Hydrophobic coatings can also enhance the overall stability and performance of the electrochemical cell by preventing water buildup and enabling better reactant gas access to the catalyst layers.

[00132] Gas diffusion layers and porous transport layers may be coated with catalyst materials to enhance the electrochemical reactions at the electrode interfaces. The catalyst coatings facilitate faster and more efficient reactions, such as the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode. Common catalyst materials used for coating include without being limited to precious metals like platinum (Pt), palladium (Pd), or non-precious metals like nickel (Ni) or cobalt (Co).

[00133] To enhance proton conductivity within the electrode assembly, gas diffusion layers and porous transport layers may be coated with proton-conductive materials. These coatings assist in the efficient transport of protons from the catalyst layers to the proton exchange membrane. Materials such as perfluorosulfonic acid (PFSA) or other ionomer coatings can enhance proton conduction, enabling better performance and higher efficiency of the electrochemical cell.

[00134] It will be appreciated that the specific choice of coating depends on the electrolyser design, the desired performance characteristics, and the compatibility with the operating conditions. Coatings are applied to gas diffusion layers and porous transport layers to optimise their functionality, improve water management, enhance catalytic activity, or facilitate proton transport, all of which contribute to the overall efficiency and performance of the electrolyser.

[00135] The electrochemical cell of the present invention may further be provided with at least one overmoulded gasket.

[00136] In one form of the invention, the gasket is overmoulded onto the structural frame to mechanically interlock the structural frame and the tensioning element. In an alternate form of the invention, the gasket is overmoulded onto the tensioning element to mechanically interlock the structural frame and the conductive layer. Suitable materials in include ethylene propylene diene monomer (ePDM), or a fluoroelastomer such as FKM dependant on electrolyte and temperature its subject to.

[00137] Advantageously, an overmoulded gasket can reduce the surface area requiring compression.

[00138] Advantageously, an overmoulded gasket can reduce the individual number of parts to assemble.

[00139] Proton exchange membrane electrolysers typically operate at a high differential pressure to improve their performance and efficiency. The high differential pressure refers to the pressure difference between the hydrogen and oxygen sides of the electrolyser. Advantages include improved mass transport or reactant gases to the catalyst layers allowing for more efficient electrochemical reactions. The pressure gradient helps ensure a steady supply of reactant gases to the catalyst sites, reducing concentration polarization and improving the overall reaction kinetics.

[00140] The high differential pressure aid in lowering compression costs downstream of the electrolyser. This high pressure on the cathode allows for energy saving downstream depending on the intended use. Operating at high differential pressure aids in effective water management within the electrolyser. It helps remove water produced at the cathode during the oxygen evolution reaction (OER) and prevents flooding or accumulation of water in the electrode assembly. Efficient water management is crucial for maintaining the proton conductivity of the membrane and avoiding performance degradation. [00141] High differential pressure reduces ohmic losses associated with proton transport through the membrane. By operating at higher pressures, the proton exchange membrane experiences better contact with the catalyst layers, reducing the resistance to proton conduction. This leads to lower voltage losses and improved overall electrolyser efficiency.

[00142] Hydrogen is typically used at high pressures, so outputting a higher hydrogen pressure reduces the cost/need for additional compressors to get to the needed hydrogen pressure for transportation.

[00143] The structural frame further comprises water inlet and water outlet apertures. It will be appreciated that said apertures are different to the tie rod apertures.

BRIEF DESCRIPTION OF THE DRAWINGS

[00144] Further features of the present invention are more fully described in the following description of several non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

Figure 1 is an exploded isometric view of an electrochemical cell assembly in accordance with an embodiment of the invention;

Figure 2 is an exploded isometric view of an electrolyser stack in accordance with an embodiment of the invention;

Figure 3 is an isometric view of a bipolar plate in accordance with an embodiment of the invention;

Figure 4 is a plan view of a bipolar plate in accordance with an embodiment of the invention;

Figure 5 is an isometric view of a structural frame in accordance with an embodiment of the invention;

Figure 6 is a plan view of a structural frame in accordance with an embodiment of the invention; Figure 7 is a side view of a structural frame and tensioning element in accordance with an embodiment of the invention; and

Figure 8 is an isometric view of a structural frame in accordance with an embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

[00145] Those skilled in the art will appreciate that the invention described herein is amenable to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

[00146] Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

[00147] Those skilled in the art will appreciate that the invention described herein is amenable to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more steps or features.

[00148] In Figure 1 there is provided an exploded isometric view of an electrochemical cell assembly in accordance with an embodiment of the present invention. The assembly 10 comprises a tensioning element in the form of a nickel bipolar plate 12, a 0.25 mm PTFE seal 14, a first polysulfone structural frame 16, a 0.25 mm PTFE seal 18, a hydrogen permeable nickel cathodic elastic element flow field 20, a nickel cathode 22, a Zirfon 200 separator 24, a nickel anode 26, a nickel anodic elastic element flow field 28, a second polysulfone structural frame 30 and a 0.25 mm PTFE seal 32.

[00149] In Figure 2 there is provided an exploded isometric view of an electrolyser stack 40 in accordance with an embodiment of the present invention. The stack 40 assembly comprises a first stainless steel 316 end plate 42, polysulfone seal plate 44, a stainless steel 316 current collector 46, a structural frame 48, and a bipolar plate 50.

[00150] The stack 40 further comprises a plurality of tie rods 60 running the length of the stack as is known in the art attached at one end to insulators 62 and retained against the end plate 42 with washers 64 and nuts 66.

[00151 ] Similar to an electrolysis cell, a fuel cell typically comprises a platinum-based material that facilitates the electrochemical reactions at the anode and cathode, promoting the movement of electrons and ions. Bipolar plates positioned between individual cells in a fuel cell stack, helping to distribute fuel and oxidant and providing electrical connections between cells. Gas diffusion layers are typically carbon-based materials that enable the distribution of fuel and oxidant gases to the electrodes while also facilitating the removal of reaction by-products. Current collectors are conductive plates that collect the electrons generated at the electrodes and deliver them to an external circuit for use.

[00152] Gaskets may be used to seal the interfaces between the different components of a fuel cell, such as the anode, cathode, and electrolyte. These gaskets help prevent gas leakage and maintain the integrity of the fuel cell. They are typically made of materials that can withstand the operating conditions of the fuel cell, such as high temperatures and corrosive environments.

[00153] Some fuel cell designs use bonding or adhesive materials to secure various components together. These materials can provide structural stability and prevent movement or separation of the components during operation.

[00154] Fuel cells are typically held together in a few different ways, depending on the specific design and type of fuel cell. In a fuel cell stack, which consists of multiple individual cells connected together, the cells are often held together by stack assembly hardware. This hardware includes clamping plates, bolts, and compression systems that apply pressure to maintain proper contact between the components and ensure good electrical conductivity.

[00155] Fuel cells may also be held together by a frame or housing structure that provides support and protection. This can be particularly relevant for larger fuel cell systems used in stationary or automotive applications. The frame or housing may be made of metal, plastic, or other suitable materials.

[00156] The cell stack in a flow battery consists of multiple individual cells connected in series. The stack is responsible for facilitating the electrochemical reactions and generating electrical power.

[00157] Flow batteries require pumps and valves to control the flow of electrolyte solutions. The structural problems associated with these components include wear, clogging, leakage, and reduced efficiency due to energy losses during pumping.

[00158] Flow batteries have current collectors and electrodes, typically made of conductive materials, to facilitate the electrochemical reactions. Structural problems can include corrosion, degradation, and loss of active material, which can reduce the overall performance and lifespan of the battery.

[00159] A flow battery cell assembly appears very similar to the electrolyser cell assembly of Figure 1 .

[00160] In Figure 3 there is provided an isometric view of a tensioning element in the form of a bipolar plate 12 in accordance with an embodiment of the present invention. The bipolar plate 12 comprises a plurality of tie rod apertures 70 for receiving tie rods 60. The bipolar plate 12 further comprises a plurality of water inlet apertures 72 and a plurality of water and oxygen outlet apertures 74. Hydrogen 76 outlet apertures are provided as is known in the art.

[00161 ] A plan view of the bipolar plate 12 is provided at Figure 4 wherein like numbering represent like parts.

[00162] In Figure 5 there is provided an isometric view of a structural frame 16 in accordance with an embodiment of the present invention. The structural frame 16 comprises a plurality of tie rod apertures 80 for receiving tie rods 60. The structural frame 16 further comprises a plurality of water inlet apertures 82 and a plurality of water and oxygen outlet apertures 84. Hydrogen 86 outlet apertures are provided as is known in the art.

[00163] A plan view of the structural frame 16 is provided at Figure 6 wherein like numbering represent like parts. [00164] The bipolar plate 12 and the structural frame 16 are the same size, with equal lengths and widths. Overlaying of the bipolar plate 12 and the structural frame 16 aligns the apertures such that tie rod apertures 70 and 80, water inlet apertures 72 and 82, water and oxygen outlet apertures 74 and 84 and hydrogen outlet apertures 76 and 86 are coincident with each other.

[00165] Where the structural frame is PSU plastic and the bipolar plate is stainless steel, the tensile strength of the former is with safety factors about 25 MPa and the tensile strength of the latter with safety factors is about 100 MPa. Advantageously, the stainless steel tensioning element has a higher tensile strength than the structural frame.

[00166] Where the structural frame is PSU plastic and the bipolar plate is stainless steel, the tensile modulus of the former is about 2-3 GPa and the tensile modulus of the latter is about 200 GPa. Advantageously, the stainless steel tensioning element has a higher tensile modulus than the structural frame.

[00167] The structural frame 16 is provided with a series of flow channels 90 for fluid distribution as is known in the art.

[00168] In Figure 7 there is provided a side view of a portion of an electrolyser cell assembly in accordance with an embodiment of the present invention. The Figure depicts bipolar plates 12 overlaying either side of a structural frame 16. The boss 100 extends from the structural frame 16 in one direction only i.e. towards the lower bipolar plate shown in the Figure. The height of the boss 100 is approximately the same as the thickness of the bipolar plate 12 and gasket 108. The boss 100 buts the internal face 102 of the bipolar plate aperture 70. The internal face 104 of the bipolar plate that does not engage the boss 100 does not extend the into the tie rod aperture 92 as far as the internal face 106 of the underlying structural frame 16. Consequently, a tie rod (not shown) entering the aperture 92 does not engage any of the metallic surfaces of the bipolar plate 12. Tie rod diameters may be in the order of 30 mm. Other components 109 of an electrolyser cell assembly such as membranes, separators, flow fields, electrodes, gas diffusion layers, hydrophobic layers and hydrophilic layers may be provided as is known in the art. [00169] In Figure 8, there is provided an isometric view of a portion of a structural frame 16. Two tie rod apertures are depicted, a corner tie rod aperture 120 and a side tie rod aperture 122. Adjacent the corner aperture 120 is a 270° boss 124. Adjacent the side aperture 122 is a 180° boss 126. Adjacent the 270° boss 124 is a 270° recess 127. Adjacent the 180° boss 126 is a 180° recess 128.

[00170] In use, electrolyte (either water or alkaline solution depending on the separator type) passes through the electrolyte inlet 72 into the one or more of the anodic and cathodic chambers either side of the separator. The electrolyte reacts with the electrode/s passing ions through the separator and excess electrolyte flows out electrolyte outlet 82 and then through all of the adjacent cells to the end plate and out of the system. Oxygen and hydrogen exit the respective outlets as is known in the art.

[00171] To assemble a cell assembly 10, the structural frame 16 is placed on a hard clean surface, and the parts are placed in the order as shown in exploded view of cell assembly in Figure 1 .

[00172] To assemble a stack, the parts are laid vertically as shown in Figure 2. The tie rods act as guiding rods to accurately place parts on top of each other. Finally, a tensioner system is used to pull the stack together and place the nuts on the tie rods.

[00173] The end plate should be made of a material that is suitably strong to maintain internal pressure of the stack and compressive forces applied to the tie rods. It will be appreciated that if the end plate doubles as the current collector, then the material has to be electrically conductive. End plates are commonly made of Stainless Steel 316.

[00174] A brief summary of potential operating conditions and potenital compnents of three electrolyser cells is shown below.

[00175] Simulation results demonstrate that a standard structural ring without reinforced PSU and a frame width of 150 mm at 67.5 bar (test pressure) would undergo internal deflection of 21 .14 mm and a stress of 164 MPa. To keep deflection under 1 mm and stress under 30 MPa, a frame width of over 600 mm would be required.

Claims

1 . An electrochemical cell comprising a non electrically-conductive structural frame for supporting components of the electrochemical cell and a tensioning element, wherein the structural frame comprises engagement means adapted to engage the tensioning element, wherein the engagement means comprises at least two bosses on the structural frame, each boss adapted to engage with a corresponding aperture on the tensioning element.

2. An electrochemical cell in accordance with claim 1 wherein the tensioning element is electrically conductive.

3. An electrochemical cell in accordance with claim 1 or claim 2, wherein the tensioning element has a higher tensile strength than the structural frame.

4. An electrochemical cell in accordance with any one of the preceding claims, wherein the tensioning element has a higher tensile modulus than the structural frame.

5. An electrochemical cell in accordance with any one of the preceding claims, wherein the structural frame is plastic, selected from the groups including but not limited to polysulfones, polyphenylsulfones, polyether ether ketones, polyphenylene sulfides, polyphenylene sulfones, polyamideimides, polyamides, polybutylterephthalates and polyethyl imides.

6. An electrochemical cell in accordance with any one of the preceding claims, wherein the tensioning element comprises stainless steel.

7. An electrochemical cell in accordance with any one of the preceding claims, wherein, the structural frame comprises a plurality of structural frame apertures and the tensioning element comprises a plurality of tensioning element apertures. An electrochemical cell in accordance with claim 6, wherein the diameter of the tensioning element aperture is larger than the diameter of the structural frame aperture.

8. An electrochemical cell in accordance with claim 6 or claim 7, wherein each tensioning element aperture and each structural frame aperture are concentric.

9. An electrochemical cell in accordance with any one of the preceding claims, wherein each boss is provided adjacent to a structural frame aperture.

10. An electrochemical cell in accordance with any one of the preceding claims, wherein each boss is adapted to engage with an inner surface of each tensioning element aperture. An electrochemical cell in accordance with any one of the preceding claims, wherein the height of each boss is approximately the same as the thickness of the tensioning element or the thickness of the tensioning element and a gasket.

11. An electrochemical cell in accordance with any one of the preceding claims, wherein each boss is provided as a pair of complimentary bosses.

12. An electrochemical cell in accordance with any one of the preceding claims, wherein at least one boss is arcuate.

13. An electrochemical cell in accordance with any one of the preceding claims, wherein at least one boss is a 180° arc.

14. An electrochemical cell in accordance with any one of the preceding claims, wherein at least one boss is a 270° arc.

15. An electrochemical cell in accordance with any one of the preceding claims, wherein the structural frame comprises a plurality of 180° arcuate bosses and a plurality of 270° arcuate bosses.

16. An electrochemical cell in accordance with any one of the preceding claims, wherein a recess is provided on the surface of the structural frame adjacent a boss.

17. An electrochemical cell in accordance with any one of the preceding claims, wherein the boss comprises the same non-conductive material as the structural frame.

18. An electrochemical cell in accordance with any one of the preceding claims, wherein the boss is integral with the structural frame.

19. An electrochemical cell in accordance with any one of the preceding claims, wherein the electrochemical cell further comprises a separator selected from the group comprising proton exchange membranes, anion exchange membranes and alkaline separators.

20. An electrochemical cell in accordance with any one of the preceding claims, wherein the conductive layer is provided in the form of a bipolar plate, a current collector and/or an electrode.

21 . An electrochemical cell in accordance with any one of the preceding claims, wherein the electrochemical cell is provided in the form of a flow cell, a fuel cell or an electrolyser cell.