Electrochemical stack

Abstract

An electrochemical stack (200) is described, the stack comprising: a first electrochemical cell (200a) having a first cell frame (202a) and a second electrochemical cell (200b) having a second cell fr

Description

Field of Invention This invention relates to an electrochemical stack comprising electrochemical cells, and more specifically to the arrangement of a bipolar plate and seal between the electrochemical cells in the stack. Background An electrolysis cell typically comprises: a frame; an anodic half-cell; a cathodic half-cell; a membrane or separator between the two half cells; and, means to isolate each cell within a stack. During the electrolysis of water, the cathodic half-cell produces hydrogen which is often done at pressure to limit downstream compression requirements. The anodic half-cell normally contains a liquid solution and a gas mixture at substantially atmospheric pressure. During operation the anodic compartment is fed with an electrolyte mixture, which is circulated through the system along with the generated gas, typically oxygen. The anodic and cathodic half-cells are typically separated by an ionic membrane such as an anion exchange membrane (AEM) or proton exchange membrane (PEM) or a separator as seen in alkaline systems. The unintentional mixture of gases, notably hydrogen and oxygen, created in each compartment is called crossover. In the anodic compartment the gas mixture is oxygen and hydrogen, which could create a potential explosive gas mixture. Therefore, the hydrogen content should be kept below the lower explosive limit which is approximately 4% hydrogen in oxygen. Crossover from the anodic to cathodic compartment is usually much lower, especially in the presence of a pressure differential, with a higher pressure in the cathodic half-cell, and is not normally a concern. Normally, electrochemical cells in a stack are separated completely by a bipolar plate arranged between each cell in a stack. This means any potential leak from the cathodic halfcell interface with the bipolar plate could result in an external leak of hydrogen to the surrounding atmosphere which may result in an accumulation of hydrogen in the vicinity of the stack, possibly creating a hazardous mixture. Ventilation is generally used to overcome this issue. The present disclosure focuses on the example of electrolysers for water electrolysis, but the disclosure can apply to other types of electrolysis or electrochemical devices. An object of the present disclosure is to provide improved sealing of an electrochemical cell in a stack of electrochemical cells. Summary of the Invention Aspects and embodiments of the present invention are set out in the appended claims. These and other aspects and embodiments of the invention are also described herein. According to at least one aspect described herein, there is provided an electrochemical stack comprising: a first electrochemical cell having a first cell frame and a second electrochemical cell having a second cell frame, wherein the first and second electrochemical cells are adjacent in the stack; a bipolar plate between the first and second cell frames to separate a cathodic half-cell of the first electrochemical cell and an anodic half-cell of the second electrochemical cell; and a frame interface seal arranged to seal an interface between the first and second cell frames beyond the outer edge of the bipolar plate. In this way, gas leaks from the cathodic half-cell are prevented from passing between the first and second cell frames and out into the ambient environment because the frame interface seal is located beyond the outer edge of the bipolar plate. As the leaked gases are less likely to bypass the frame interface seal due to the pressure drop experienced by the gas having left the cathodic half-cell, they are instead redirected into the anodic half-cell of the adjacent cell in the stack, where the gases can be vented from the device properly. Preferably, the bipolar plate is narrower and / or shorter than the first and / or second cell frames, more preferably narrower and / or shorter than both the first and second cell frames. In this way, there will be a portion of the cell frames which is not separated by the bipolar plate, because the bipolar plate does not extend all the way to the edge of the cell frame. In this portion, the frame interface seal forces leaked gases back between the bipolar plate and the anodic half-cell of the adjacent cell. Preferably, the anodic half-cell is narrower and / or shorter than the bipolar plate. Therefore, the bipolar plate can cover the entire area of the anodic half-cell of the second cell to separate it from the cathodic half-cell of the first cell. Preferably, the cathodic half-cell is narrower and / or shorter than the anodic half-cell. The cathodic half-cell may then be deeper / thicker to maintain the same volume as the anodic half-cell. As used herein, the terms “narrower” and “shorter” refers to the two longest dimensions (i.e., width and height) of the cell frame, bipolar plate, anodic or cathodic half-cells, or membrane / MEA, bearing in mind that these components are substantially planar (typically, they are formed as square or rectangular plates). These terms do not refer to the thickness or depth of these components. Preferably, the frame interface seal is arranged to surround the bipolar plate and / or at least part of the anodic half-cell. In other words, the bipolar plate between the first and second cells is entirely within the perimeter of the frame interface seal which seals the interface between the first and second frames. Thus, the frame interface seal prevents leaked gases escaping between the cells to the ambient environment. Preferably, the frame interface seal is coupled to the second cell frame and arranged to contact the first cell frame, more preferably wherein the frame interface seal is coupled to the second cell frame by being located in a groove of the second cell frame. Alternatively, the frame interface seal is coupled to the first cell frame and arranged to contact the second cell frame, preferably wherein the frame interface seal is coupled to the first cell frame by being located in a groove of the first cell frame. Ideally, each electrochemical cell in the stack forms a repeating element, such that each stack comprises only one frame interface seal to seal against the adjacent cell in a stack, for example. Preferably, the electrochemical stack comprises a cathodic seal arranged between the bipolar plate and the first cell frame to seal against leaks from the cathodic half-cell of the first electrochemical cell. Preferably, the cathodic seal is arranged to surround at least part of the cathodic half-cell. In the path of gases leaking from the cathodic half-cell, the cathodic seal is upstream of the frame interface seal. Thus, the frame interface seal acts as a backup seal to redirect leaks internally in the event of failure of the cathodic seal. Preferably, the cathodic seal is coupled to the bipolar plate and arranged to contact the first cell frame. For example, the cathodic seal is coupled to the bipolar plate by being located in a groove of the bipolar plate. This arrangement may make the stack more compact, and improve the seal between the cathodic seal and bipolar plate. Alternatively, the cathodic seal may be coupled to the first cell frame and arranged to contact the bipolar plate. For example, the cathodic seal is coupled to the first cell frame by being located in a groove of the first cell frame. This arrangement may make the stack more compact and improve the sealing. Preferably, the first cell frame and / or second cell frame comprises a bipolar plate step, and the bipolar plate is seated on the step. The step may have a depth equivalent to the thickness of the bipolar plate, such that when the bipolar plate is seated on the step it is flush with the cell frame, forming a substantially flat surface to abut the next cell in the stack. Preferably, the cathodic seal is coupled to the bipolar plate step of the first cell frame, preferably by being located in a groove of the step and arranged to contact the bipolar plate seated on the step. This arrangement may make the stack more compact and improve the sealing. Preferably, the first and / or second cell frame comprises an opening to accommodate its respective cathodic and anodic half-cells. The opening is located roughly centrally in the frame. In this example, the opening comprises plate-shaped spaces to accommodate the plate-shaped components (e.g., the half-cells, membrane / MEA, electrodes, etc.). Preferably, the opening is stepped to create a first space to accommodate one of the halfcells and a second space to accommodate the other of the half-cells, the second space being narrower than the first space. The spaces may be shaped to accommodate plate-like components. The spaces act as slots to receive the plate-like components. Preferably, the first (wider) space accommodates the anodic half-cell and the second (narrower) space accommodates the cathodic half-cell. Preferably, a membrane or membrane electrode assembly (MEA) is arranged to span the opening to separate the cathodic and anodic half-cells, wherein the membrane is arranged to be seated on a step of the stepped opening. Preferably, the electrochemical stack comprises a membrane seal arranged to seal between the membrane or MEA and the step of the stepped opening. The membrane seal seals the membrane / MEA with the cell frame. Preferably, the membrane seal is arranged to surround at least part of the cathodic half-cell. Preferably, the membrane is an anion exchange membrane. More preferably, the electrochemical cells operate with a dry cathode. Preferably, the frame interface seal, cathodic seal, and / or membrane seal is a gasket made from one or more of: nitrile butadiene rubber; ethylene propylene diene monomer; fluoroelastomer; fluoroelastomer rubber material; polytetrafluoroethylene; and thermoplastic polyurethane, ora metal gasket, porous gasket or liquid applied gasket. Preferably, the bipolar plate is made from one or more of: nickel; stainless steel; or surface-treated steel. Preferably, the first and / or second cell frame is made from any one of: Polyphenylenesulphide; Polypropylene; Polyethylene; Polyoxymethylene; Polyphenylsulfone; Polyetheretherketone; and an equivalent material with additional glass fibre content. Preferably, the pressure in the cathodic half-cell of the first electrochemical cell is higher than the pressure in the anodic half-cell of the second electrochemical cell. Preferably, the first and / or second cell frames comprise: one or more channels for the supply and / or removal of water and / or electrolyte to or from the electrochemical cell; and / or one or more gas removal channels for the removal of generated gases from the electrochemical cell. According to another aspect described herein, there is provided an electrochemical cell assembly for an electrochemical stack as aforementioned. The electrochemical cell assembly comprises: a cathodic half-cell and an anodic half-cell in a cell frame; a bipolar plate to be secured to the cell frame; and a frame interface seal, wherein the width and / or height of the bipolar plate is less than the width and / or heigh of the frame interface seal. According to another aspect described herein, there is provided a stack of electrochemical cells, the stack comprising at least two electrochemical cells, wherein each cell comprises: • A cell frame, the cell frame having a recess; • A separator in the recess spanning the substantial width of the recess, with one side of the separator being an anodic half-cell and the other side being a cathodic halfcell; • A bipolar plate between the cathodic half-cell of a first electrochemical cell and anodic half-cell side of a second electrochemical cell, wherein o The is wider than the cathodic half-cell, but narrower than the cell frame, • A first sealing means for sealing the and the first cell frame, and • A second sealing means between the two cell frames, with the second sealing means being between the anodic half-cell of the second electrochemical cell and the external side of the cell frame, and arranged such that any gas from the cathodic half-cell of the first cell which bypasses the first sealing means is directed by the second sealing means to enter the anodic half-cell of the second cell instead of venting to atmosphere. As used herein, the term “first sealing means” may refer to a cathode sealing means or a cathodic seal, and the term “second sealing means” may refer to an anode sealing means or a frame interface seal. Preferably, the cell frame material is Polyphenylenesulphide alternative materials may be used such as but not limited to Polypropylene (PP), Polyethylene (PE), Polyoxymethylene (POM), Polyphenylsulfone (PPSU), Polyetheretherketone (PEEK) or an equivalent material with additional glass fibre content. Whilst it is envisaged the separator may be any separator, preferably the separator is a membrane, more preferably still a polymeric membrane. Said membrane may be an anion exchange membrane (AEM) or and proton exchange membrane (PEM). Other separators include diaphragms. Yet more preferably still, the separator, when a membrane, forms a membrane electrode assembly (MEA) with any one or more of: anode gas diffusion layer (GDL), anode, cathode, carbon cloth, cathode GDL. Preferably, the recess of the cell frame comprises a support for the separator / membrane and optionally the frame recess has accommodation for a third sealing means, an MEA sealing means (i.e., a membrane seal). Normally the support is such that the cathodic half-cell is narrower than the anodic half-cell (e.g., stepped). It is envisaged that the volume of the half-cells may be different, or substantially similar. The area at the interface between the two half-cells may be substantially similar. Alternatively the cathodic half-cell may be larger than the anodic half cell of a given cell, or the anodic halfcell may be larger than the cathodic half-cell of a given cell. Given the differing cross section likely resulting from the aforementioned support, the narrower half-cell may be deeper than the wider half-cell to maintain similar volumes. The surface area of the half-cell interface, at the membrane / MEA, is envisaged as being at least 1cm2, more preferably over 5cm2, more preferably still over 10cm2. It is envisaged that the cell interface may be between 10cm2 and 60cm2, or 60cm2 and 180cm2, or 180cm2 and 300cm2, or 300cm2 and 600cm2, or 600cm2 and 1200cm2, or over 1200cm2. In a preferred embodiment of the present invention the depth (i.e., thickness) of each halfcell is between 0.01mm and 10mm, more preferably between 0.1mm and 10mm, more preferable still between 0.1mm and 5mm, and even more preferably between 0.5mm and 5mm or 1 mm and 2mm or 1 mm and 1,5mm. The overall cell volume is preferably in the range of 0.01cm3 and 1cm3 or 0.05cm3 and 5cm3, or 0.1cm3 and 10cm3 or 0.6cm3 and 60cm3 or 1.8cm3 and 180cm3 or 3cm3 and 300cm3 or 6cm3 and 600cm3 or 12cm3 and 1200cm3 or over 1200cm3 or even above 2000cm3 Whilst not intended to be a limiting feature of the present disclosure, each electrochemical half-cell will comprise electrodes and catalytic material, which are not described in detail herein. Preferably, the bipolar plate is wider than the anodic and cathodic half-cell of the first cell. Alternatively, the bipolar is wider than only the cathodic half-cell such that the anodic half-cell is bordered by the bipolar plate and the cell frame. Nevertheless, the anode sealing means (i.e., frame interface seal) shall not be in part of the anodic half-cell, the anode sealing means (i.e., frame interface seal) being in contact with the cell frames of two adjacent electrochemical cells. It is preferred that there will be a pressure differential between the half-cells in each cell, with each cell in the stack having similar pressure differential. Alternatively, each cell in the stack may have different pressure differentials. The pressure in the cathodic half-cell should be higher than the pressure in the anodic half-cell of the adjacent cell within the stack. The pressure difference may be as low as 1pa. It could theoretically be even lower, but preferably is at least 1 bar. More preferably still the pressure differential shall be at least 5 bar, 10 bar, 25 bar, 35 bar or 50 bar or 100 bar or 500+ bar. It is envisaged that the pressure differential may be in the range of 5 bar to 100 bar. The pressure differential ensures the flow of gases in the correct direction. In a preferred embodiment the bipolar plate is made from Nickel, Stainless Steel or Steel with dedicated surface treatment. The purpose of the bipolar plate is to isolate the cells, conduct current between the cells and facilitate the flow of electrolyte. Further the bipolar plate can be used as an access point to perform cell voltage measurements. Whilst each of the three sealing means (the membrane seal, cathodic seal, and frame interface seal) may be any suitable type of sealing, preferably gaskets are used. More preferably still the gaskets are made from resistant materials such as nitrile butadiene rubber, ethylene propylene diene monomer, fluoroelastomer, fluoroelastomer rubber material, polytetrafluoroethylene, or thermoplastic polyurethane. Other sealing options include metal gaskets, porous gaskets or liquid applied gaskets. In the present invention, the bipolar plate is wider than either of the anodic or cathodic halfcells, but narrower than the cell frame. More preferably still the bipolar plate is narrower than the span across the second (anodic) sealing means (i.e., the frame interface seal), where the second sealing means sits between the outer edge of the bipolar plate and within the boundary of the cell frame. Each cell frame is preferably provided with an electrolyte channel for the introduction and removal of electrolyte (and generated gases), and a hydrogen channel for the removal of generated hydrogen. The hydrogen channel may have its own sealing means. A stack of electrochemical cells in accordance with the present invention may be 2 or more cells, in fact it could be in the range of 2 to 100 cells, 5 and 50 cells, 10 and 30 cells, or indeed over 100 cells. Preferably, the electrochemical stack is forms an electrolyser, more preferably still an AEM electrolyser and even more preferably still, an AEM electrolyser operating with a substantially dry cathode, that being a cathodic half-cell which sees no direct introduction of electrolyte during operation. A benefit of the present invention is that any leaked gas is re-directed internally for circulation and proper venting downstream and is prevented from leaking externally into the vicinity of the electrochemical device. This improves the safety of the present invention and mitigates the need for excessive ventilation. Other advantages include the mitigation of shunt currents in the electrolyte channel by ensuring the flow of each cell is isolated, thereby reducing corrosion and increasing efficiency. Other benefits include cost saving by having a reduced (in size) bipolar plate with less material cost. Any apparatus feature as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure. Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. Furthermore, any, some and / or all features in one aspect can be applied to any, some and / or all features in any other aspect, in any appropriate combination. It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and / or supplied and / or used independently. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure. The invention extends to methods, system and apparatus substantially as herein described and / or as illustrated with reference to the accompanying figures. One or more aspects will now be described, by way of example only and with reference to the accompanying drawings having like-reference numerals, in which: Figure 1 is perspective view of an electrochemical stack of the prior art; Figure 2 is an exploded view of the electrochemical stack of Figure 1; Figure 3 is an exploded view of an electrochemical cell for an electrochemical stack in accordance with the present invention; Figure 4 is a cross-section view along the line A-A in Figure 3 of two of the electrochemical cells shown in Figure 3, arranged in a stack in accordance with the present invention; Figure 5 is a cross section view along the line A-A in Figure 3 of a single electrochemical cell for the electrochemical stack of the present invention; and Figure 6 is a cross section view of an alternative exemplary single cell for the electrochemical stack of the present invention. Detailed description Figure 1 is a perspective view of an electrochemical stack 10 of the prior art and Figure 2 is an exploded view of the electrochemical stack 10 of Figure 1. The stack 10 comprises a plurality of individual electrochemical cells 100 stacked between end plates 102a, 102b. The end plates are secured together via pull rods 104 and nuts 106 which couple to the pull rods. The end plates securely fasten the components of the stack 10 and maintain pressure. There are hydrogen ports 108a, 108b for the supply or removal of hydrogen to or from the electrochemical cells 100 of the stack via a fluid connector 110 which is separated from the electrochemical cells 100 by an insulator 112. In the prior art electrochemical stack 10, the electrochemical cells 100 are separated from one another by a bipolar plate 114 which spans the entire area of the electrochemical cell. In other words, the bipolar plate extends to match the dimensions (length and width) of the electrochemical cell. The bipolar plate therefore entirely separates one electrochemical cell from the next electrochemical cell in the stack, such that adjacent electrochemical cells are not in contact with one another. Problematically, this arrangement means that in the event of a failure of the seal between the bipolar plate and the frame of the electrochemical cell, any hydrogen escaping from the electrochemical cell will flow between the electrochemical cell and the bipolar plate towards the edge of the stack and onwards into the ambient environment. This may result in an accumulation of hydrogen in the vicinity of the stack, possibly creating a hazardous mixture. Figure 3 shows an exploded view of an electrochemical cell 200 for an electrochemical stack in accordance with the present invention. The electrochemical stack comprises at least two of the electrochemical cells 200 shown in Figure 3, such as a first and second electrochemical cell arranged adjacent one another in the stack. Each electrochemical cell 200 comprises a cell frame 202, wherein the cell frames are arranged adjacent one another in the stack. The cell frames 202 may be made from any one of: Polyphenylenesulphide; Polypropylene; Polyethylene; Polyoxymethylene; Poly-phenylsulfone; Polyetheretherketone; and an equivalent material with additional glass fibre content. The cell frame has an opening to accommodate a cathodic half-cell and an anodic half-cell of the electrochemical cell. A membrane 204 spans this opening such that the cathodic halfcell and anodic half-cell are separated by the membrane. The membrane in this example is an anion exchange membrane. In the cathodic half-cell is a cathodic electrode 206 and, in this example, two sheets of a carbon cloth 208 and a cathodic gas diffusion layer 210. In the anodic half-cell on the other side of the membrane 204 is an anodic electrode 212 and an anodic gas diffusion layer 214 which abuts the anode 212. Hydrogen seals 216 fit within corresponding apertures in the electrochemical cell frame 202, and may be kept in place by the pressure of the generated hydrogen. A bipolar plate 218 is located as an end plate of the electrochemical cell 200 such that the bipolar plate separates the cell 200 from the next cell in the stack. In this example the bipolar plate 218 is located on the cathodic side of the cell 200, and would separate the cathodic half-cell of the electrochemical cell 200 shown in Figure 3 from the anodic half-cell of the next electrochemical cell in the stack (not shown). The bipolar plate is made from one or more of: nickel; stainless steel; or surface-treated steel. In the electrochemical cell 200 of Figure 3, three separate seals are shown. First, a membrane seal 220 is arranged to seal between the membrane 204 and the cell frame 202 as described more specifically with reference to Figure 4. The membrane seal 220 is formed as a gasket arranged to surround or encircle at least part of the cathodic half-cell as shown in Figure 4. Second, a cathodic seal 222 is arranged between the bipolar plate 218 and the cell frame 202 to seal against leaks from the cathodic half-cell of the electrochemical cell 200. The cathodic seal is formed as a gasket arranged to surround or encircle at least part of the cathodic half-cell. In this example, the cathodic seal is coupled to the cell frame 202 and arranged to contact the bipolar plate. In particular, in this example, the cathodic seal is coupled to the cell frame 202 by being located in a groove (not shown) of the cell frame 202. In other examples, the cathodic seal may be coupled to the bipolar plate 218 and arranged to contact the cell frame 202, for example the seal may be located in a groove of the bipolar plate 218. Third, a frame interface seal 224 is arranged on the opposite side of the electrochemical cell 200 to the bipolar plate 218. When the electrochemical cell 200 is arranged with other cells into a stack, the frame interface seal is arranged to seal the interface between two electrochemical cell frames beyond the outer edge of the bipolar plate. Thus, the frame interface seal of a first electrochemical cell is arranged to surround the bipolar place of an adjacent electrochemical cell in the stack. In this example, the cell frame 202 has a groove and the frame interface seal is arranged to fit within the groove to couple to the frame. Any of the frame interface seal, the cathodic seal, and / or the membrane seal may be a gasket made from one or more of: nitrile butadiene rubber; ethylene propylene diene monomer; fluoroelastomer; fluoroelastomer rubber material; polytetrafluoroethylene; and thermoplastic polyurethane, or a metal gasket, porous gasket or liquid applied gasket. Figure 4 is a cross-section view along line A-A in Figure 3 of two of the electrochemical cells shown in Figure 3, arranged in a stack 20 in accordance with the present invention. In particular, a first electrochemical call 200a is stacked adjacent a second electrochemical cell 200b. The first electrochemical cell 200a comprises a first cell frame 202a, and the second electrochemical cell 200b comprises a second cell frame 202b. The respective cell frames have respective openings to accommodate the anodic and cathodic half-cells of each electrochemical cell. Specifically, the first cell frame 202a accommodates a cathodic halfcell 205a and an anodic half-cell 211a and the second cell frame 202b also accommodates a cathodic half-cell 205b and an anodic half-cell 211b. The cathodic half-cells 205a, 205b may include the cathodic electrode (not shown), the carbon cloth (not shown) and the cathodic gas diffusion layer (not shown) and the anodic half-cells 211a, 211b may include the anodic electrode (not shown) and the anodic gas diffusion layer (not shown). The cathodic and anodic half-cells are separated in each electrochemical cell by a membrane (204a in the first electrochemical cell 200a, and 204b in the second electrochemical cell 200b). The opening within each cell frame 202a, 202b comprises a stepped structure so that the opening creates two spaces with different dimensions (specifically, different widths). The narrower space in each cell frame is arranged to accommodate the cathodic half-cells 205a, 205b and the wider space is arranged to accommodate the anodic half-cells 211a, 211b. The membrane 204a, 204b is arranged to be seated on the stepped structure in the opening of each cell frame such that it spans the wider space in each cell frame, and separates the anodic and cathodic half-cells. It should be noted that, in this example, or in other examples, the membrane may be a membrane electrode assembly (MEA), which is an assembled stack of one or more membranes (e.g., anion exchange membranes (AEMs)) together with other components such as: electrodes (anode and / or cathode), anodic and / or cathodic gas diffusion layers, or carbon cloths; therefore, aspects described herein as being part of anodic and cathodic half-cells may in fact be formed as part of the MEA. The membrane enables selective transfer of ions from the cathodic half-cell to the anodic half-cell during operation, as indicated by the arrows. When the electrochemical stack 20 is used for electrolysis of water to produce hydrogen, and the membrane is an AEM, the membrane allows transport of hydroxide anions (OH-) from the cathodic half-cell to the anodic half-cell, while rejecting cations. The membrane seal 220a, 220b seals between the membrane (or MEA) 204a, 204b and the cell frame 202a, 202b. In this example, the membrane seal seals between the membrane (or MEA) 204a, 204b and a step of the stepped structure of the opening in the cell frame. For example, the membrane seal may be located in a groove of the step and arrange to contact the membrane / MEA when the membrane / MEA is seated on the step. Alternatively, the membrane seal may be located in a groove of the membrane / MEA and arranged to contact the seat of the opening when the membrane / MEA is seated. In any case, the membrane seal keeps the membrane / MEA 204a, 204b secured between the cathodic and anodic halfcells of its respective electrochemical call 200a, 200b. The first electrochemical cell 200a comprises a bipolar plate 218a which is arranged to separate the cathodic half-cell 205a of the first electrochemical cell 200a from the anodic half-cell 211 b of the second electrochemical cell 200b. The second electrochemical cell 200b also comprises a bipolar plate 218b which is arranged to separate the cathodic half-cell 205b of the second electrochemical cell 200b from the anodic half-cell of the next electrochemical cell in the stack (not shown). Thus, in each electrochemical cell 200a, 200b, the bipolar plate 218a, 218b is located on the other side of the cathodic half-cell to the membrane / MEA 204a, 204b. The cell frames 202a. 202b may each comprise a bipolar plate step upon which the bipolar plate can be seated when it is attached to the cell frame. In this way, when the bipolar plate 218a, 218b is attached to the cell frame the bipolar plate is flush with the cell frame. Therefore, the bipolar plate and cell frame together form a flat face of the electrochemical cell to be stacked against other electrochemical cells in the electrochemical stack. The cathodic seals 222a, 222b are arranged between their respective cell frames 202a, 202b and the bipolar plates 218a, 218b. In particular, in this example the cathodic seals 222a, 222b are arranged between the bipolar plates and the bipolar plate step. The cathodic seal therefore seals between the bipolar plate and the cell frames to seal against leaks (e.g., of hydrogen) from the cathodic half-cell. The bipolar plates are narrower and / or shorter than the cell frames. In other words, the bipolar plate does not span the entire area of the cell frame, and instead stops short of extending all the way to the edges of the cell frame. In this way, a part of the cell frame is left uncovered by the bipolar plate, beyond the outer edge of the bipolar plate, such that when the first electrochemical cell 200a is stacked against the second electrochemical cell 200b, there is an interface 203 between the first and second cell frames at which the cell frames are not separated by the bipolar plate. As can be seen from Figure 4, the anodic halfcell is also narrower and / or shorter than the bipolar plate, and the cathodic half-cell is narrower and / or shorter than the anodic half-cell. In this example, a frame interface seal 224b is located at the interface 203 where the first cell frame 202a and the second cell frame 202b meet without the bipolar plate therebetween. As the bipolar plate is narrower / shorterthan the cell frame, the bipolar plate does not extend all the way to the edge of the cell frames. Thus, beyond the outer edge of the bipolar plate, the first and second cell frames meet frame-to-frame at the interface 203. In this way, the frame interface seal 224b surrounds or encircles the bipolar plate and / or at least part of the anodic half-cell. In this example the frame interface seal 224b is coupled to the second cell frame 202b and arranged to contact the first cell frame 202a. Specifically, the frame interface seal 224b is coupled to the second cell frame by being located in a groove of the second cell frame. In other examples, the frame interface seal may be coupled instead to the first cell frame, such as located in a groove of the first cell frame, and arranged to contact the second cell frame. The cathodic seal 222a (which seals the bipolar plate 218a between the first and second cell frames) can occasionally fail, for example, as a result of degradation of the seal overtime. Whilst some hydrogen escaping from the cathodic half-cell 205a of the first cell 200a could crossover to the anodic half-cell 211a of the same cell (due to the pressure differential between the higher pressure cathodic half-cell and lower pressure anodic half-cell), the membrane / MEA limits such crossover. Thus, in the event of a failure of the cathodic seal 222a, gas escaping from the cathodic half-cell 205a of the first cell 200a will tend to flow out between the bipolar plate 218a and the first cell frame 200a as this gap is not properly sealed by the failed cathodic seal 222a. Common practice in the prior art is for the bipolar plate to span the entire area of two adjacent cells in a stack, or beyond, meaning that if the leaked hydrogen passes the cathodic seal it would be able to flow between the bipolar plate and the cell frame until it reaches the edge of the frame and leaks into the ambient atmosphere in the vicinity of the stack. The arrangement disclosed herein differs from the prior art in that the bipolar plate does not extend all the way to the edge of the cell frame, and an additional seal is provided along the potential hydrogen leak path in the form of the frame interface seal 224b. The frame interface seal 224b prevents the escaped hydrogen from flowing out between the first and second cell frames 202a, 202b and instead directs the escaped hydrogen to flow back between the bipolar plate 218a and the second cell frame 202b, as indicated by the dashed line in Figure 4. In this way, the escaped hydrogen from the cathodic half-cell 205a of the first electrochemical cell 200a is directed into the anodic half-cell 211b of the second electrochemical cell 200b (adjacent the first cell in the stack) where the escaped hydrogen can be circulated with the electrolyte through the anodic half-cell and vented from the stack accordingly. Figure 5 is a cross section view along the line A-A in Figure 3 of a single electrochemical cell 200 for the electrochemical stack of the present invention. Figure 5 shows the membrane 204 which separates the cathodic and anodic sides of the cell. On the cathodic side, the cathode electrode 206, the carbon cloths 208, and the cathodic gas diffusion layer 210 are shown. On the anodic side, the anode electrode 212 and the anodic gas diffusion layer 214 are shown. The membrane / MEA seal 220 seals between the membrane and the cell frame 202 as described above. The cathodic seal 222 seals between the bipolar plate 218 and the cell frame 202 as described above. Finally, the frame interface seal 224 seals between the cell frame 202 and the frame of another cell to be stacked on top of the cell 200 in an electrochemical stack as described above. Figure 6 is a cross section view of an alternative exemplary single cell for the electrochemical stack of the present invention. The components of Figure 5 are also shown in Figure 6, with the same reference numerals. Figure 6 differs from Figure 5 in that the example shown in Figure 6 includes the hydrogen channel seal 216 described above with respect to Figure 3. It will be understood that the invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention. Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination. Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims.

Claims

1. An electrochemical stack comprising:a first electrochemical cell having a first cell frame and a second electrochemical cell having a second cell frame, wherein the first and second electrochemical cells are adjacent in the stack;a bipolar plate between the first and second cell frames to separate a cathodic half-cell of the first electrochemical cell and an anodic half-cell of the second electrochemical cell; anda frame interface seal arranged to seal an interface between the first and second cell frames beyond an outer edge of the bipolar plate.

2. An electrochemical stack according to Claim 1, wherein the bipolar plate is narrower and / or shorter than the first and / or second cell frames.

3. An electrochemical stack according to Claim 1 or 2, wherein the anodic half-cell is narrower and / or shorter than the bipolar plate.

4. An electrochemical stack according to any preceding claim, wherein the cathodic half-cell is narrower and / or shorter than the anodic half-cell.

5. An electrochemical stack according to any preceding claim, wherein the frame interface seal is arranged to surround the bipolar plate and / or at least part of the anodic half-cell.

6. An electrochemical stack according to any preceding claim, wherein the frame interface seal is coupled to the second cell frame and arranged to contact the first cell frame, preferably wherein the frame interface seal is coupled to the second cell frame by being located in a groove of the second cell frame.

7. An electrochemical stack according to any of Claims 1 to 5, wherein the frame interface seal is coupled to the first cell frame and arranged to contact the second cell frame, preferably wherein the frame interface seal is coupled to the first cell frame by being located in a groove of the first cell frame.

8. An electrochemical stack according to any preceding claim, comprising a cathodic seal arranged between the bipolar plate and the first cell frame to seal against leaks from the cathodic half-cell of the first electrochemical cell.

9. An electrochemical stack according to Claim 8, wherein the cathodic seal is arranged to surround at least part of the cathodic half-cell.

10. An electrochemical stack according to Claim 8 or 9, wherein the cathodic seal is coupled to the bipolar plate and arranged to contact the first cell frame, preferably wherein the cathodic seal is coupled to the bipolar plate by being located in a groove of the bipolar plate.

11. An electrochemical stack according to Claim 8 or 9, wherein the cathodic seal is coupled to the first cell frame and arranged to contact the bipolar plate, preferably wherein the cathodic seal is coupled to the first cell frame by being located in a groove of the first cell frame.

12. An electrochemical stack according to any preceding claim, wherein the first cell frame and / or second cell frame comprises a bipolar plate step, and the bipolar plate is seated on the step.

13. An electrochemical stack according to Claims 11 and 12, wherein the cathodic seal is coupled to the bipolar plate step of the first cell frame, preferably by being located in a groove of the step and arranged to contact the bipolar plate seated on the step.

14. An electrochemical stack according to any preceding claim, wherein the first and / or second cell frame comprises an opening to accommodate its respective cathodic and anodic half-cells.

15. An electrochemical stack according to Claim 14, wherein the opening is stepped to create a first space to accommodate one of the half-cells and a second space to accommodate the other of the half-cells, the second space being narrower than the first space.

16. An electrochemical stack according to Claim 15, wherein the first space accommodates the anodic half-cell and the second, narrower space accommodates the cathodic half-cell.

17. An electrochemical stack according to Claim 15 or 16, wherein a membrane or membrane electrode assembly (MEA) is arranged to span the opening to separatethe cathodic and anodic half-cells, wherein the membrane is arranged to be seated on a step of the stepped opening.

18. An electrochemical stack according to Claim 17, comprising a membrane seal arranged to seal between the membrane or MEA and the step of the stepped opening.

19. An electrochemical stack according to Claim 18, wherein the membrane seal is arranged to surround at least part of the cathodic half-cell.

20. An electrochemical stack according to any of Claims 17 to 19, wherein the membrane is an anion exchange membrane.

21. An electrochemical stack according to any preceding claim, wherein the frame interface seal, cathodic seal, and / or membrane seal is a gasket made from one or more of: nitrile butadiene rubber; ethylene propylene diene monomer; fluoroelastomer; fluoroelastomer rubber material; polytetrafluoroethylene; and thermoplastic polyurethane, or a metal gasket, porous gasket or liquid applied gasket.

22. An electrochemical stack according to any preceding claim, wherein the bipolar plate is made from one or more of: nickel; stainless steel; or surface-treated steel.

23. An electrochemical stack according to any preceding claim, wherein the first and / or second cell frame is made from any one of: Polyphenylenesulphide; Polypropylene; Polyethylene; Polyoxymethylene; Polyphenylsulfone; Polyetheretherketone; and an equivalent material with additional glass fibre content.

24. An electrochemical stack according to any preceding claim wherein the pressure in the cathodic half-cell of the first electrochemical cell is higher than the pressure in the anodic half-cell of the second electrochemical cell.

25. An electrochemical stack according to any preceding claim, wherein the first and / or second cell frames comprise: one or more channels for the supply and / or removal of water and / or electrolyte to or from the electrochemical cell; and / or one or more gas removal channels for the removal of generated gases from the electrochemical cell.

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