GB2610372A

GB2610372A - Bipolar plate

Info

Publication number: GB2610372A
Application number: GB2110732.1A
Authority: GB
Inventors: Harding Whitehead Adam
Original assignee: Invinity Energy Systems Ireland Ltd
Current assignee: Invinity Energy Systems Ireland Ltd
Priority date: 2021-07-26
Filing date: 2021-07-26
Publication date: 2023-03-08
Also published as: GB202110732D0

Abstract

A bipolar plate for use in a flow battery cell or cell stack, which bipolar plate is less susceptible to corrosion and mechanical damage and decreases resistance. The bipolar plate comprises a first face 3 for disposing adjacent a porous electrode or membrane, an electrolyte flow conduit 5, and one or more openings 207, 307 for providing fluid communication between the first face and the electrolyte flow conduit, the one or more openings having a width less than a maximum width of the electrolyte flow conduit, wherein the electrolyte flow conduit is defined by an electrically insulating material or surface. The openings may comprise a slot 207 or perforations 307. A method of making the bipolar plate is also described, in which a pre-formed electrically insulating duct or tube, optionally with one or more pre-formed openings, is disposed in an electrically conductive material to form the bipolar plate. The bipolar plate is preferably carbon-based and the electrically insulating material or surface is suitably chemically stable in a highly acidic and/or oxidative environment, preferably a polymer such as a polyolefin. The electrolyte flow passages may have a curved (e.g. circular) or polygonal (e.g. quadrilateral or triangular) cross-section.

Description

Bipolar Plate

FIELD OF THE INVENTION

The present invention relates to bipolar plates for use in flow batteries and to flow batteries comprising bipolar plates, methods of storing and discharging power using such flow batteries comprising, and to methods of manufacturing the same

BACKGROUND OF THE INVENTION

Bipolar plates are used in flow battery cell stacks (i.e. a plurality of flow battery cells arranged in electrical series) to physically separate and electrically connect adjacent flow battery cells. A typical flow battery cell and flow battery cell stack are illustrated, respectively, in Figures IA and 1B (described below).

A bipolar plate typically has a first face for disposing adjacent a positive first flow battery cell and a second face, opposite the first face, for disposing adjacent a negative second cell. Some bipolar plate designs have smooth, continuous planar first and second faces, in which case electrolyte flows through a cell only via a porous electrode (e.g. through carbon felt) -this is known as an electrolyte 'flow through' arrangement. Some bipolar plate designs have generally planar first and second faces that comprise inlaid channels or grooves, which channels or grooves define a flow field geometry. In this latter case, electrolyte may flow through the channels or grooves (as opposed to only through a porous electrode) as it travels from an inlet side to an outlet side of a cell. In. this arrangement, an advantage is that resistivity through a cell (and thus through a flow battery cell stack) is reduced. However, also in this arrangement, a cell typically has poor electrolyte flow homogeneity while the bipolar plate is prone to corrosion and mechanical damage, leading to poor flow battery performance (e.g. degradation of performance over time).

US-A-2016/0308224 describes coating rectangular cross-section, open-topped channels of a bipolar plate with a dielectric material (e.g. by spraying), and in another option describes total occlusion of channels of a bipolar plate by a dielectric material.

EP-A-3136490 describes a bipolar plate that can decrease the internal resistance of a flow battery by facilitating smoother electrolyte flow.

Disclosed is a bipolar plate comprising channels that have an internal width larger than the width of the channel opening The present inventor has found a solution to the aforementioned shortcomings in bipolar plates for use in flow batteries.

PROBLEM TO BE SOLVED BY THE INVENTION

It is an object of the invention to provide an improved bipolar plate comprising electrolyte flow conduits. It is a further object of the invention to provide an improved flow battery cell and/or cell stack. It is a still further object of the invention to provide a method of manufacturing said improved bipolar plate.

SUMMARY OF THE INVENTION

In a first aspect of the invention, there is provided a bipolar plate for use in a flow battery cell or cell stack, the bipolar plate comprising: a first face for disposing adjacent a porous electrode or membrane, an electrolyte flow conduit, and one or more openings for providing fluid communication between the first face and the electrolyte flow conduit, wherein the electrolyte flow conduit is defined by an electrically insulating material or surface. Preferably, the one or more openings have a width less than a maximum width of the electrolyte flow conduit. Preferably, the bipolar plate is a carbon-based bipolar plate.

In a second aspect of the invention, there is provided a bipolar plate for use in a flow battery cell or cell stack, the bipolar plate comprising: a first face for disposing adjacent a first porous electrode or membrane, a second face opposite the first face for disposing adjacent a second porous electrode or membrane, a first electrolyte flow conduit (or a first set of electrolyte flow conduits) in fluid communication with the first face, and a second electrolyte flow conduit (or a second set of electrolyte flow conduits) in fluid communication with -2 -the second face, wherein the electrolyte flow conduits are disposed individually at (preferably, equal) intervals along a width of the bipolar plate. Preferably, the first and second electrolyte flow conduits are disposed alternately at (preferably, equal) intervals along the width of the bipolar plate. Preferably, a bottom portion of each flow conduit extends through a central plane located medially with respect to the first face and second face. Preferably, the first face, the second face, and the central plane are parallel. Preferably, the bipolar plate including the first electrolyte flow conduit (or the first set of electrolyte flow conduits) and/or the second electrolyte flow conduit (or the second set of electrolyte flow conduits) may be as defined in the first aspect.

In a third aspect of the invention, there is provided a flow battery cell comprising at least one bipolar plate as defined in the first or second aspect, the flow battery cell further comprising a membrane and preferably two porous electrodes.

In a fourth aspect of the invention, there is provided a flow battery cell stack of at least two (e.g. a plurality of) flow battery cells arranged in electrical series, the flow battery cell stack comprising at least one bipolar plate as defined in the first or second aspect.

In a fifth aspect of the invention, there is provided a method of manufacturing a bipolar plate for use in a flow battery cell or cell stack, the method comprising: providing a pre-formed electrically insulating duct or tube optionally comprising one or more pre-formed openings, disposing the pre-formed electrically insulating duct or tube in an electrically conductive material to form a bipolar plate comprising an electrolyte flow conduit defined by the electrically insulating duct or tube, and where the pre-formed duct or tube is not provided with one or more pre-formed openings, forming one or more openings in the preformed duct or tube. Preferably, the bipolar plate is as defined in the first or second aspect In a sixth aspect of the invention, there is provided a method of storing or discharging electrochemical energy using a flow battery cell as defined in the third aspect or a flow battery cell stack as defined in the fourth aspect

ADVANTAGES OF THE INVENTION

The products, apparatus and methods of the invention facilitate improved flow battery performance. In particular, the bipolar plate of the invention is less susceptible to shunt-current induced corrosion/damage, is less susceptible to mechanical damage, promotes a more homogeneous electrolyte flow through a cell, and/or facilitates thinner cells, thus reducing the resistivity (and in turn improving the efficiency and/or power density) of a flow battery.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 A and 1B are schematic illustrations of a typical flow battery cell and a typical flow battery cell stack, respectively; Figure 2A is a cross-sectional view of a flow conduit of one embodiment of a bipolar plate; Figure 2B is a perspective view of a bipolar plate of one embodiment; Figures 3A to 3K are cross-sectional views of flow conduits of further embodiments, Figure 4 is a perspective view of a bipolar plate of one embodiment; Figures 5A to SC are cross-sectional views of bipolar plates of further embodiments

DETAILED DESCRIPTION OF THE INVENTION

The invention concerns bipolar plates for use in flow batteries and flow batteries comprising bipolar plates. It also concerns methods of storing and discharging power using flow batteries comprising such bipolar plates and methods of manufacturing said bipolar plates.

There is provided a bipolar plate for use in a flow battery cell or cell stack, the bipolar plate comprising: a first face for disposing adjacent a porous electrode or membrane, an electrolyte flow conduit, and one or more openings for providing fluid communication between the first face and the electrolyte flow -4 -conduit, wherein the electrolyte flow conduit is defined by an electrically insulating material or surface.

A particular advantage of the invention is reduced shunt current induced damage to (e.g. reduced corrosion of) a bipolar plate. Cells arranged in electrical series (i.e. forming a flow battery cell stack) typically have common electrolyte distribution manifolds. Thus, in conventional flow battery cell stacks, an ionic short circuit can occur between individual cells of a flow battery cell stack via the electrolyte in a common manifold, causing shunt currents. Shunt currents may cause a charging reaction (and corresponding discharging reaction) of the bipolar plate, particularly at areas in or proximal to bipolar plate electrolyte flow conduits that are defined by an electrically conducting material. Said charging and discharging reactions of the bipolar plate may lead to overcharge of, and thus damage to, the bipolar plate (particularly when a bipolar plate is made from a carbon-based material). For example, a bipolar plate can be corroded by overpotential-induced oxidation of carbon-based material. Because the electrolyte flow conduits of the invention are defined by an electrically insulating material or surface (and preferably by an electrically insulating duct or tube), shunt current induced charging and discharging of a bipolar plate is reduced.

A further advantage of the invention, for example when the electrolyte flow conduits are defined by an electrically insulting duct or tube (as described below), is a better maintained flow field geometry throughout the life of the bipolar plate. Bipolar plates that comprise a plurality of electrolyte flow conduits typically also comprise a plurality of narrow, elongate ribs defining and separating said flow conduits. Electrically conducting materials (e.g. carbon-based materials), which are used to manufacture bipolar plates for use in flow batteries, are generally frangible. Thus, ribs of a conventional bipolar plate, that are made from an electrically conducting material, such as from a carbon-based material, are prone to break/shear as a flow battery is assembled (e.g, under compressive force) and/or over time during use of a flow battery (e.g. due to mechanical erosion or chemical corrosion). If said ribs break, this disrupts the designed flow field geometry of a bipolar plate leading to reduced flow homogeneity within a cell and consequently the resistance of a flow battery is increased. In the present invention, the electrically insulating material or surface (particularly an electrically insulting duct or tube) serves to mechanically reinforce and/or shield (from electrolyte flow erosion) the electrically conducting material (and particularly ribs) of the bipolar plate, reducing the incidence of rib breakage/shearing.

A still further advantage of the invention is reduced through plane resistivity of a flow battery cell stack, in turn reducing ohmic losses (at a given current density) and therefore improving energy efficiency. Or, at a given energy efficiency, power density of a flow battery cell stack may be increased -where power density is increased, for a given flow battery cell stack power, flow battery cell stack components can be decreased in size because less active area is required, reducing manufacturing cost. In some embodiments, through plane resistivity is reduced because flow homogeneity (e.g. electrolyte flow distribution and/or consistency of electrolyte pressure) through a cell is improved. Improved consistency of electrolyte pressure (i.e. reduced pressure drop) through a cell also improves energy efficiency because a lower pumping load is required. In some embodiments, through plane resistivity is reduced because the depth of a bipolar plate (and thus of a cell) can be reduced. Furthermore, bipolar plates of the invention can allow for thinner porous electrodes to be incorporated into a cell, reducing cell through plane resistivity.

Herein, features of the bipolar plate are discussed relative a bipolar plate orientation whereby the first face is generally level and facing upward, and thus, for example, the electrolyte flow conduit is disposed below the first face. However, in use in a flow battery, the bipolar plate may be orientated on its side such that the first face is generally vertical and facing sideways. Furthermore, where the context allows, the orientation of features discussed relative the first face may be inverted if and when the same features (e.g. of the electrolyte flow conduit) are discussed relative the second face. Length is the dimension that extends in the direction of electrolyte flow along the bipolar plate. Depth is the dimension between electrodes. Width is the dimension orthogonal to length and depth. -6 -

Preferably, the bipolar plate is for use in a redox flow battery or cell stack, more preferably a vanadium redox flow battery cell or cell stack. A vanadium redox flow battery is a rechargeable flow battery that employs vanadium ions in different oxidation states to store chemical potential energy.

Preferably, the bipolar plate is a carbon-based bipolar plate Preferably, the carbon-based bipolar plate comprises a composite material comprising a carbon material (i.e. a 'carbon composite') and/or the carbon-based bipolar plate comprises a generally pure carbon material. A generally pure carbon material contains at least 99 wt % carbon material. More preferably, the carbon-based bipolar plate consists of, in addition to the electrically insulating material or surface, a carbon composite and/or a generally pure carbon material. Preferably, the generally pure carbon material and/or the carbon material of the carbon composite is any one or two or more of graphite, expanded graphite, carbon black, amorphous carbon, glassy carbon, heteroatom-doped carbon, and carbon nanotubes. More preferably, the generally pure carbon material and/or the carbon material of the carbon composite is graphite and/or expanded graphite. A heteroatom-doped carbon may be carbon doped with any one or more of phosphorus, boron or sulfur by pyrolysis such as is disclosed in CN-A108539210.

Preferably, the carbon composite comprises a binder or binders.

Preferably, the binder is a polymer, more preferably a thermoset or thermoplastic polymer, e.g. a polyolefin or a fluorocarbon. For example, the polymer may be any one or two or more of epoxy resin, phenolic resin, polyethylene, polypropylene, polyvinyl chloride, ethylene vinyl acetate, ethylene tetrafluoroethylene, polyvinylidene fluoride, and polytetrafluoroethylene.

Preferably, the carbon composite comprises (or contains) at least 40 wt % carbon, more preferably at least 50 wt % carbon, still more preferably at least 55 wt % carbon, most preferably at least 60 wt % carbon, for example around 65 wt % carbon. Preferably, the carbon composite comprises (or contains) at most 95 wt % carbon, more preferably at most 90 wt 11O carbon, still more preferably at most 85 wt %, e.g. at most 80 wt %. For example, the carbon -7 -composite comprises (or contains) from 60 wt.'1O to 85 wt % carbon, more preferably from 65 wt % to 80 wt % carbon.

Preferably, the carbon composite comprises (or contains) at most 60 wt % binder, more preferably at most 50 wt % binder, still more preferably at most 45 wt % binder, most preferably at most 40 wt % binder, for example around 35 wt % binder. Preferably, the carbon composite comprises (or contains) at least wt °/-1) binder, more preferably at least 10 wt % binder, still more preferably at least 15 wt 9'1) binder, e.g. at least 20 wt % binder. For example, the carbon composite comprises (or contains) from 15 wt % to 40 wt % binder, more preferably from 20 wt % to 35 wt % binder.

Preferably, the bipolar plate is generally cuboid (e.g. rectangular cuboid) shaped. In one preferred option, the bipolar plate is generally square cuboid shaped (e.g. the first and a second face are square). Preferably, the bipolar plate has a length, a width and a depth.

Preferably, the bipolar plate comprises a second face. Preferably, the second face is opposite and/or parallel to the first face. Here, the term opposite refers to a face on the opposite side of the bipolar plate.

Preferably, the first face and/or the second face are generally planar.

Optionally, the second face comprises a smooth, continuous planar surface. Thus, in this option, the second face is not in fluid communication with any electrolyte flow conduit. As such, the bipolar plate may function as a terminal plate for use in a flow battery cell or cell stack whereby the second face is disposed adjacent a current collector.

Preferably, the depth of the bipolar plate is at least 0.5 mm, more preferably at least 1 mm, still more preferably at least 1.5 mm, for example at least 2 mm. Preferably, the depth of the bipolar plate is at most 2 cm, more preferably at most 1.5 cm, still more preferably at most 1 cm, most preferably at most 8 mm, for example at most 6 mm. For example, the depth of the bipolar plate is from 0.5 mm to 2 cm, more preferably from 1 mm to 1.5 cm, still more preferably from 1.5 mm to 1 cm, e.g. from 2 mm to 6 mm. -8 -

Preferably, the length of the bipolar plate is from 5 cm to 50 cm, more preferably from 8 cm to 40 cm, still more preferably from 10 cm to 30 cm, e.g. around 15 cm.

Preferably, the width of the bipolar plate is from 20 cm to 145 cm, more preferably from 25 cm to 130 cm, still more preferably from 30 cm to 115 cm, most preferably from 35 cm to 100 cm, e.g. around 70 cm.

Typically, the length of a bipolar plate is less than the width of a bipolar plate for minimising pressure drop across a cell.

Preferably, the first face (and thus a second face opposite the first face) has an area of from 300 cm' to 3 m2, more preferably the first face has an area of from 300 cm" to 3,500 cm2, still more preferably 400 cm" to 3,000 cm2, most preferably from 500 cm2 to 2,500 cm', e.g. around 1000 cm'.

Preferably, the bipolar plate comprises and inlet side face and/or an outlet side face. Preferably, the inlet side face and the outlet side face are opposite one another (e.g. located at opposite lengthwise ends of the bipolar plate).

Preferably, the electrolyte flow conduit is elongate and/or straight.

However, the electrolyte flow conduit, which defines a bipolar plate flow field geometry (or a part thereof), may be any suitable shape, for example it may be serpentine or wave shaped along its length, such as sine wave shaped.

Preferably, the electrolyte flow conduit has a first end and a second end. Preferably, at least one of the first end and the second end is an electrolyte inlet or an electrolyte outlet. Preferably, the electrolyte inlet or electrolyte outlet is formed in a side face of the bipolar plate. Preferably, an electrolyte inlet is formed in the inlet side face and/or is configured to communicate with a cell electrolyte inlet manifold. Preferably, an electrolyte outlet is formed in the outlet side face and/or is configured to communicate with a cell electrolyte outlet manifold.

Preferably, the electrolyte flow conduit extends along the length of the bipolar plate from a first inlet side face and/or to a second outlet side face.

More preferably, the electrolyte flow conduit extends along the length of the bipolar plate from the inlet side face to the outlet side face In one option, at least one of the first end or the second end of the electrolyte flow conduit comprises an end wall for blocking electrolyte flow. For example, an end of the electrolyte flow conduit may comprise an end wall defined by the electrically insulating material or surface, or by an electrically conducting (e.g. a carbon-based) material of the bipolar plate.

Preferably, the electrolyte flow conduit, which is defined by an electrically insulating material or surface, has a cross-sectional shape that is consistent along its length.

Preferably, the electrolyte flow conduit has a generally curved (e.g. generally continuous curved) cross-sectional shape. More preferably, the cross-sectional shape of the electrolyte flow conduit is continuously curved below the level located at the interface between the electrolyte flow conduit and the one or more openings. For example, the electrolyte flow conduit may have a generally semi-circular cross-sectional shape (i.e. wherein the semi-circle is orientated such that its flat side faces upwardly). More preferably, the electrolyte flow conduit has a generally circular (or ovular or egg-shaped) cross-sectional shape. A generally circular cross-sectional shape preferably includes the shape of a circle comprising a flattened (e.g. level) top -(i.e. a circle shape absent a top segment defined by a level secant). Preferably, the level secant is taken at at least 60 % the height of the circle, more preferably at least 70 %, still more preferably at least 80 °A, most preferably at least 90 %, e.g. at least 95%.

Preferably, the electrolyte flow conduit cross-sectional area is at least 0.2 mm2, more preferably at least 0.4 mm2, still more preferably at least 0.6 mm2, most preferably at least 0.8 mm2, e.g. at least 1 mm2. Preferably, the electrolyte flow conduit cross-sectional area is at most 40 mm2, more preferably at most 30 mm2, still more preferably at most 20 mm2, most preferably at most 15 mm2, e.g. at most 10 mm2. For example, the electrolyte flow conduit cross-sectional area is from 0.2 mm2 to 40 mm2, more preferably from 0.4 mm2 to 30 mm2, still more preferably from 1 mm2 to 10 mm2, e.g. around 5 mm2.

Preferably, in embodiments where the cross-sectional shape of the electrolyte flow conduit is generally circular, the radius of the electrolyte flow conduit is preferably at least 0.4 mm, more preferably at least 0.6 mm, still more -10-preferably at least 0.8 mm, most preferably at least 1 mm. Preferably, the radius of the electrolyte flow conduit is at most 10 mm, more preferably at most 8 mm, still more preferably at most 6 mm, most preferably at most 5 mm, e.g. at most 4 mm. For example, the radius of the electrolyte flow conduit is from 1 mm to 3 mm, e.g. around 2 mm.

The one or more openings provide fluid communication between the first face and the electrolyte flow conduit.

Preferably, the one or more openings have a width less than a maximum width of the electrolyte flow conduit. More preferably, the width of the one or more openings is at most 90 % the maximum width of the electrolyte flow conduit, still more preferably at most 85 'I/O, most preferably at most 80%, for example at most 75 °,/,'). Preferably, the width of the one or more openings is at least 5 % the maximum width of the electrolyte flow conduit, more preferably at least 10 %, still more preferably at least 15 %, most preferably at least 20 %, e.g. at least 25 %. An advantage of this feature is that, without significantly impacting flow of electrolyte from/to the electrolyte flow conduit to/from the first face (and thus in situ to/from a porous electrode), electrolyte flow characteristics within the flow conduit are improved by contrast to an 'open topped' flow conduit. By open topped flow conduit, it is meant a flow conduit having an opening having a width that is about or at least the maximum width of the flow conduit. This improves electrolyte flow homogeneity through a cell and reduces electrolyte pumping load. A further advantage of this feature is reduced porous electrode (e.g. carbon felt) ingress into the flow conduit, further improving electrolyte flow homogeneity through a cell. In a still further advantage, because electrolyte flow characteristics within the flow conduit are improved, the length of a bipolar plate (where electrolyte flows along the length of the bipolar plate) may be increased for a given pressure drop through the electrolyte flow conduit. This facilitates a squarer bipolar plate geometry.

Preferably, the width of the one or more openings is constant along the length of the bipolar plate. Optionally, the width of the one or more openings gradually reduces along the length of the bipolar plate (e.g. toward the outlet side of the bipolar plate).

Preferably, in one option, a depth of the one or more openings (i.e. the distance between the first face and a top of the electrolyte flow conduit) is at most a minimum thickness of the electrically insulating material or surface, and preferably is zero or approaching zero.

Optionally, a corner or corners between the one or more openings arid the electrolyte flow conduit and/or the first face is rounded (i.e. absent an angle). For example, where the width of the one or more openings is less than a maximum width of the electrolyte flow conduit, the one or more openings is defined by a bullnose surface(s). A particular benefit of this feature is improved electrolyte flow characteristics as electrolyte travels through the one or more openings between the electrolyte flow conduit and the first face.

Preferably, the one or more openings are a slot or perforations. Perforations are particularly suited to reducing porous electrode (e.g. carbon felt) ingress.

Where the one or more openings is a slot, the slot preferably has a width of from 20 to 60% of the maximum width of the electrolyte flow conduit. Where the one or more opening are perforations, the perforations preferably have a width of from 40 to 80 % of the maximum width of the electrolyte flow conduit.

Preferably, the perforations have a generally circular or elliptical cross-sectional shape and/or the perforations have a generally cylindrical or frustum (e.g. frustoconical) shape.

Preferably, the perforations are disposed at equal intervals along the length of the electrolyte flow conduit. Alternatively, in one embodiment, the perforations are disposed at decreasing intervals and/or the size of the perforations increases along the length of the electrolyte flow conduit toward the outlet side of the bipolar plate. Thus, as electrolyte pressure drops toward the outlet side of the bipolar plate this is counteracted by greater flow area available between the electrolyte flow conduit and the porous electrode, improving flow homogeneity through a cell.

The electrolyte flow conduit is (and preferably the one or more openings are) defined by an electrically insulating material or surface. -12-

Preferably, the electrically insulating material or surface is chemically stable in a highly acidic (e.g. around pH 0) and/or oxidative environment. Preferably, the electrically insulating material or surface is a polymer, more preferably a polyolefin, still more preferably polypropylene and/or polyethylene. In one option, the electrically insulating material or surface is a glass (e.g. a glass capillary).

Preferably, the thickness (and/or the minimum thickness) of the electrically insulating material is in the range of from 10 nm to 1 mm, more preferably from 50 p.m to 0.6 mm, still more preferably from 0.1 mm to 0.3 mm, most preferably around 0.2 mm. In one option, the thickness (and/or the minimum thickness) of the electrically insulating material is at least 1 nm, for example around 5 tim. The thickness of the electrically insulating material must be no less than the minimum thickness, for example where the electrically insulating material (e.g. a tube) has inner and outer cross-sectional shapes that are different (which causes the thickness at different points of a cross-section of the electrically insulating material to vary).

Preferably, the electrolyte flow conduit (and preferably the one or more openings) is defined by an electrically insulating duct (e.g. trough) or tube disposed (e.g. inset) in the bipolar plate. i.e. the electrically insulating material is an electrically insulating duct or tube. Preferably, the duct or tube is pre-formed.

A particular benefit of using a pre-formed duct or tube to define the electrolyte flow conduit is that the inner surface of said duct or tube may comprise a smooth internal surface without requiring a post-manufacturing surface treatment (e.g. abrasion). Thus, a consistently smooth electrolyte flow conduit surface may be achieved along the length of the flow conduit. This is particularly advantageous when the electrolyte flow conduit is difficult to access (e.g. when the one or more openings are perforations or are particularly narrow). A further advantage of using a duct or tube, where the one or more openings are perforations, is that said perforations are easier to fabricate (e.g. by hole-punching or drilling) because the material (e.g. polymer material) of the duct or tube is unlikely to crack or break.

Preferably, the electrically insulating duct or tube is chemically stable in a highly acidic (e.g. around pH 0) and/or oxidative environment.

-13 -Preferably, the electrically insulating duct or tube is a polymer, more preferably a polyolefin, still more preferably polypropylene and/or polyethylene. In one option, the electrically insulating material or surface is a glass (e.g. a glass capillary).

Preferably, an inner cross-sectional shape of the electrically insulating duct or tube defines the cross-sectional shape of the electrolyte flow conduit. Preferably, the inner cross-sectional shape and an external cross-sectional shape is consistent along the full length of the tube or duct.

Optionally, the external cross-sectional shape of the duct or tube is generally circular.

Optionally, the external cross-sectional shape of the duct or tube comprises a stabilising means (e.g. for prohibiting rotation and/or incorrect initial orientation of the duct or tube during manufacture of the bipolar plate). For example, where the external cross-sectional shape of the duct or tube is generally circular, the duct or tube may rotate during manufacture of the bipolar plate such that one or more openings of the duct or tube is incorrectly orientated relative the first face. The stabilising means may prohibit such rotation. In this option, the stabilizing means is preferably located at a top portion (e.g. for when a pre-formed duct or tube is lowered into a receiving recess of a pre-formed bipolar plate) or a bottom portion (e.g, for when a pre-formed duct or tube is placed onto a core sheet during manufacture of a bipolar plate) of the tube or duct. Preferably, the stabilising means comprises two arm portions, each arm portion extending laterally from alternate sides of the tube or duct. Preferably, the stabilising means (e.g, the two arm portions) of the duct or tube define a portion of the first face.

Preferably, the duct or tube has a non-circular external cross-sectional shape. More preferably, the duct or tube has a generally polygonal (e.g. quadrilateral or triangular) external cross-sectional shape. Still more preferably, the duct or tube has any one of a generally rectangular, square, or triangular external cross-sectional shape. There are several advantages to a duct or tube having a generally polygonal external cross-sectional shape (as discussed below in the description of Figure 3H). A still further advantage of this feature, where the one or more openings are perforations and the inner cross-sectional shape of the -14 -duct or tube is generally circular, is that along the length of the duct or tube and between the perforations a top surface of the duct or tube bridges the electrolyte flow conduit. The bridging structures serves to provide improved mechanical reinforcement of frangible ribs of the bipolar plate. Said advantage may be generalised to further embodiments employing perforations.

Preferably, the bipolar plate comprises a receiving recess having a cross-sectional shape corresponding to an external cross-sectional shape of the electrically insulating duct or tube and/or having a length corresponding to a length of the electrically insulating duct or tube.

Preferably, the duct or tube is fixedly attached to or retained by electrically conductive (e.g. carbon-based) material of the bipolar plate. Preferably, the duct or tube is fixedly attached in or retained by the receiving recess. Preferably, the duct or tube is fixedly attached by thermal bonding and/or by adhesive.

Preferably, a portion (i.e. an electrically insulating portion) of the first face adjacent the one or more openings (e.g. adjacent the slot or perforations) is defined by the electrically insulating material or surface (e.g. by the duct or tube). In one option, the first face preferably comprises a top surface of the duct or tube (such that a portion of the first face adjacent the one or more openings is defined by an electrically insulating material). A particular advantage of this feature is further reduced shunt current charging of the bipolar plate. Preferably, a top surface of the electrically insulating material or surface (e.g, of the duct or tube) and a remaining electrically conductive portion of the first face (e.g. defined by a carbon-based material) are planar and/or flush, such that the first face is generally planar.

Therefore, preferably, where the one or openings is a slot, the first face comprises two electrically insulating strips that each run along the length of the electrolyte flow conduit adjacent to the slot, one on each side of the slot. And preferably, where the one or more openings are perforations, the first face comprises a single electrically insulating strip that runs along the length of the electrolyte flow conduit and comprises a row of perforations aligned with the perforations of the electrolyte flow conduit.

-15 -Preferably the width of the strip, or cumulative width of the strips, is at least 5 % of the maximum width of the electrolyte flow conduit, more preferably at least 10 %, still more preferably at least 15 %, most preferably at least 20 %, for example at least 25 °,'O.

Preferably, where a portion of the first face is defined by the electrically insulating material or surface, at least 70% of the surface area of the first face is electrically conducting (e.g. is defined by a carbon-based material), more preferably at least 75 %, still more preferably at least 80 %, most preferably at least 85 %, for example at least 90 %, e.g. around 95 %, in one option around 99 %.

In one option, the bipolar plate further comprises a porous electrode (e.g. a carbon felt or carbon paper) permanently fixed (e.g. bonded) to the first face. A particular advantage of this option is reduced interfacial contact resistance between the bipolar plate and the porous electrode. Preferably, in this option, the porous electrode comprises one or more porous electrode openings corresponding to (e.g. disposed directly above) the one or more openings of the bipolar plate. Therefore, preferably, in situ, the electrolyte flow conduit is in direct (e.g. unobstructed) fluid communication with a membrane of a cell. A particular benefit of this feature is further reduced carbon felt ingress into the one or more openings of the bipolar plate (or still further into the electrolyte flow conduit).

Preferably, the bipolar plate comprises a plurality of (e.g. a first set of) electrolyte flow conduits (i.e. in fluid communication with the first face). Each of the plurality of the electrolyte flow conduits preferably are as defined in any embodiment of the electrolyte flow conduit disclosed above. Preferably, the plurality of electrolyte flow conduits are parallel and/or are disposed at intervals (e.g, equal intervals) along the width of the bipolar plate.

Preferably, each electrolyte flow conduit of the plurality of flow conduits comprise an end wall alternately (i.e. alternately across the width of the bipolar plate) at either an inlet side or an outlet side. This feature provides for an interdigitated flow field arrangement, whereby electrolyte cannot enter and exit a -16 -cell via the same electrolyte flow conduit. This arrangement promotes electrolyte flow through the porous electrode of a cell.

Preferably, the bipolar plate comprises a second face opposite (i.e. located on the opposite side of the bipolar plate to) the first face and for disposing adjacent a porous electrode or membrane, the second face in fluid communication with a second electrolyte flow conduit (or a second set of electrolyte flow conduits) via one or more openings as defined in any of the embodiments defined above.

Preferably, herein, an electrically insulating material refers to a material having a conductivity of a < 10 S/cm and/or an electrically conductive material refers to a material having a conductivity of a < 10' S/cm.

In a preferred embodiment, the bipolar plate for use in a flow battery cell or cell stack comprises a (preferably, generally planar) first face for disposing adjacent a porous electrode or membrane, an electrolyte flow conduit having a generally curved cross-sectional shape, and a slot or perforations for providing fluid communication between the first face and the electrolyte flow conduit, the one or more openings having a width less than a maximum width of the electrolyte flow conduit, wherein the electrolyte flow conduit is defined by an electrically insulating duct or tube. Preferably, the electrolyte flow conduit has a continuous curved, more preferably a generally circular, cross-sectional shape.

Preferably, the electrically insulating duct or tube comprises a non-circular (e.g a polygonal) external cross-sectional shape.

In a second aspect of the invention, there is provided a bipolar plate for use in a flow battery cell or cell stack, the bipolar plate comprising: a first face for disposing adjacent a first porous electrode or membrane, a second face opposite the first face for disposing adjacent a second porous electrode or membrane, a first electrolyte flow conduit (or a first set of electrolyte flow conduits) in fluid communication with the first face, and a second electrolyte flow conduit (or a second set of electrolyte flow conduits) in fluid communication with the second face, wherein the electrolyte flow conduits are disposed individually at (preferably, equal) intervals along a width of the bipolar plate. Preferably, the -17-first and second electrolyte flow conduits are disposed alternately at (preferably, equal) intervals along a width of the bipolar plate.

Preferably, a bottom portion of each flow conduit extends through a central plane located medially with respect to the first face and second face. A bottom portion of a flow conduit is distal the bipolar plate face (either first or second) with which the flow conduit is in fluid communication with Preferably, the first face, the second face, and the central plane are parallel.

Preferably, where the context allows, the bipolar plate including the first electrolyte flow conduit (or the first set of electrolyte flow conduits) and/or the second electrolyte flow conduit (or the second set of electrolyte flow conduits) may be as defined in any embodiment of the first aspect above.

A particular advantage of the bipolar plate of the second aspect is that a depth of the bipolar plate does not need to accommodate two electrolyte flow conduits and as such the bipolar plate may be thinner.

In a third aspect of the invention, there is provided a flow battery cell comprising at least one (preferably, two) bipolar plate(s) as defined in the first or second aspect.

Preferably, the flow battery cell further comprises one or any two or more of: porous electrodes (e.g. carbon felt or carbon paper electrodes), a membrane (e.g. an ion exchange membrane), a cell frame, an electrolyte inlet manifold, an electrolyte outlet manifold, and current collectors. The arrangement of components of the cell is known to the skilled person (e.g. as illustrated in Figure 1A). Optionally, at least one (e.g. two) bipolar plate (as defined in the first or second aspect) functions as a terminal plate.

Preferably, each of the porous electrodes have a depth of from 0.1 mm to 2 mm. A particular advantage of the bipolar plate of the present invention is that the depth of an adjacent porous electrode may be reduced (e.g, relative a conventional flow through-type porous electrode that may have a depth of around 3 mm) and as such the total through plane resistance of a cell may be reduced.

Preferably, the porous electrodes are any one or combination of two or more of carbon felt, carbon paper, carbon clothe and carbon veil.

-18 -In a fourth aspect of the invention, there is provided a flow battery cell stack of at least two (e.g. a plurality of) flow battery cells arranged in electrical series, the flow battery cell stack comprising at least one bipolar plate as defined in the first or second aspect. Preferably, the flow battery cell stack comprises a bipolar plate (as defined in the first or second aspect) between each pair of adjacent cells and optionally two bipolar plates (as defined in the first or second aspect) function as terminal plates of the flow battery cell stack.

Preferably, each cell of the flow battery cell stack is defined as per the third aspect.

In a fifth aspect of the invention, there is provided a method of manufacturing a bipolar plate for use in a flow battery cell or cell stack, the method comprising: providing a pre-formed electrically insulating duct or tube optionally comprising one or more pre-formed openings, disposing the pre-formed electrically insulating duct or tube in an electrically conductive (e.g. a carbon- 1 5 based) material to form a bipolar plate comprising an electrolyte flow conduit defined by the electrically insulating duct or tube, and where the pre-formed duct or tube is not provided with one or more pre-formed openings, forming one or more openings in the pre-formed duct or tube (e.g. by drilling, punching or laser cutting). Thus, the electrolyte flow conduit is in fluid communication with a face of the bipolar plate via the one or more openings. Preferably, the bipolar plate (and/or the electrolyte flow conduit) manufactured by the present method is as defined in any of the embodiments of the first or second aspect. Preferably, where the context allows, any of the features of the bipolar plate and/or of the electrolyte flow conduit(s) disclosed in the first or second aspect may be incorporated into the present method of manufacturing a bipolar plate. Preferably, the method (or portions of it) may be duplicated, consecutively or concurrently, in respect to additional pre-formed ducts or tubes defining electrolyte flow conduits in fluid communication with the same and/or an opposite face of the bipolar plate.

In one option, the method comprises extruding or casting the electrically conductive (e.g. carbon-based) material to form a bipolar plate comprising a receiving recess for receiving the pre-formed duct or tube, and disposing (e.g. inserting) the pre-formed duct or tube into the receiving recess -19-such that the pre-formed duct or tube is fixedly attached to the electrically conductive material defining the receiving recess (e.g. by providing adhesive or by thermal bonding). Optionally, the pre-formed duct or tube is disposed into the receiving recess, for example from a reel, concurrently with an extrusion process.

Optionally, the method further comprises cutting an extrudate to a pre-defined bipolar plate length before or after the pre-formed duct or tube is disposed in the receiving recess. Optionally, the method further comprises providing a bipolar plate mold or die. Optionally, the method further comprises providing an electrically conductive (e.g. carbon containing) reagent mixture.

In a further option, the method comprises providing an electrically conductive (e.g. carbon-based) sheet, arranging the pre-formed duct or tube in a pre-defined position on or relative to a first face of the sheet, applying an electrically conductive (e.g. carbon containing) reagent mixture to the first face of the sheet such that the pre-formed duct or tube is at least partially (preferably, fully) covered by the reagent mixture, and allowing the reagent mixture to solidify. Concurrently or consecutively, the method further comprises arranging a second pre-formed duct or tube in a pre-defined position on or relative to a second face of the sheet, applying an electrically conductive (e.g. carbon containing) reagent mixture to the second face of the sheet such that the pre-formed duct or tube is at least partially (preferably, fully) covered by the reagent mixture, and allowing the reagent mixture to solidify. Optionally, the method further comprises providing a bipolar plate mold. Optional, the method further comprises providing a sheet mold or die. Optionally, the method further comprises molding or extruding the electrically conductive sheet.

In one option, to arrange a pre-formed duct or tube having pre-formed openings in a pre-defined position, a frame for communicating with the pre-formed openings is provided. The frame is configured to temporarily hold the pre-formed duct or tube in the pre-defined position via the pre-formed openings. Thus, optionally, the pre-formed duct or tube is loaded onto the frame. For example, the frame comprises frame studs that correspond with and may be inserted into the openings of the duct or tube. Once the reagent mixture is solidified, the frame may be separated from the pre-formed duct or tube, which -20 -duct or tube is disposed in the solidified reagent mixture, by pulling. An advantage of the frame is that pre-formed openings cannot be obstructed by the reagent mixture during the manufacturing process and many (e.g. serpentine) electrolyte flow channel geometries can easily be achieved.

In a further option, sharing features with the option directly above where the context allows, a pre-formed duct or tube absent pre-formed openings is provided, wherein one or more openings are provided in the pre-formed duct or tube (e.g. by hole punching) by loading the pre-formed duct or tube onto the frame Preferably, the electrically conductive sheet of the option directly above and/or the electrically conductive reagent mixture of any of the above options are a carbon composite (i.e. a composite material comprising a carbon material). Preferably, the carbon composite comprises a binder or binders. Preferably, the binder is a polymer, more preferably a thermoset or thermoplastic polymer. For example, the polymer may be any one or two or more of epoxy resin, phenolic resin, polyethylene, polypropylene, polyvinyl chloride, ethylene vinyl acetate, ethylene tetrafluoroethylene, polyvinylidene fluoride, and polytetrafluoroethylene. Optionally, the electrically conductive sheet of the above option is made from a generally pure carbon material. Preferably, the carbon material of the generally pure carbon material and/or of the carbon composite is any one or two or more of graphite, expanded graphite, carbon black, amorphous carbon, glassy carbon, heteroatom-doped carbon, and carbon nanotubes.

In any of the above options, the method preferably further comprises any one or combination of two or more of: (a) subjecting the first face (and/or a second face) of the bipolar plate with the tube fixedly attached or retained therein to one more abrading treatments. For example, the abrading treatment may serve to smooth the first face and/or to remove polymer-rich layers and/or to expose the one or more openings. The abrading treatment preferably comprises any one or two or more of sand blasting, milling, planing, plasma etching, chemical etching and grinding.

(b) providing a porous electrode(s) and bonding it to the first face (and/or to a second face) of the bipolar plate. For example, the porous -2 I_ -electrode may be bonded to a face of the bipolar plate by thermal bonding or adhesive. In a further example, a portion of the porous electrode is disposed in an electrically conductive reagent mixture before said reagent mixture solidifies and the porous electrode is held in position until the reagent mixture solidifies.

(9) where the tube is not provided with a pre-formed slot or perforations, forming a slot or perforations in the tube (and optionally consecutively or concurrently in a porous electrode bonded to a face of the bipolar plate).

The invention will now be described in more detail, without limitation, with reference to the accompanying Figures (which are not drawn to scale unless indicated otherwise). Features common throughout the figures share reference numbers.

In Figure 1A, there is illustrated a schematic drawing of a typical known design of a flow battery 110 with a single flow battery cell 1H having a negative electrode 112 and a positive electrode 114 separated by a separator 116.

To drive electrochemically reversible redox reactions, a negative liquid electrolyte 117 is delivered from a storage tank 118 to the negative electrode 112 and a positive liquid electrolyte 119 is delivered from a storage tank 120 to the positive electrode 114. The separator 116 may be a micro-porous separator or an ion exchange membrane, functioning to separate the electrodes and electrolyte while allowing selected ions to pass through to complete the redox reactions. The flow battery 110 further comprises a first collector (or 'terminal') plate 122 and a second collector (or 'terminal) plate 124 which are arranged adjacent the porous electrodes 112 and 114, respectively. The porous electrodes 112 and 114 are made from a material (e.g, carbon felt or carbon sheet) that is electrically conductive and catalytically active with regard to the liquid electrolyte 117 and 119, and preferably corrosion resistant. The collector plates are coupled to conductors 126 and 128 which complete a circuit through either an electrical power source 130 (for charging) or an electrical power load 132 (for discharging) via an electrical switch 134.

In Figure 1B, there is illustrated a schematic drawing of a typical known design of flow battery 110 comprising a stack of two flow battery cells.

-22 -The flow battery 110 of Figure 1B has a design that is generally common with that of Figure IA and as such the two figures share reference numbers for common features. By contrast to Figure IA, Figure 1B additionally comprises a bipolar plate 136 which functions to physically separate, but electrically connect, the two cells of flow battery 110. Furthermore, flow battery 110 comprises a distribution (or 'inlet') manifold 139 (for distributing negative electrolyte 117, from a storage tank 118, to the two negative half cells of flow battery 110) and a corresponding collection (or 'outlet') manifold 141 (for collecting negative electrolyte 117 as it exits the two negative half cells of flow battery 110 and returning it to storage tank 118). The two positive half cells of the flow battery 110 likewise communicate with a distribution manifold 138 and a collection manifold 140. Although the flow battery cell stack 110 of Figure 1B is illustrated as consisting of two cells, a flow battery cell stack may comprise several (and often from 20 to 50) cells arranged in electrical series.

In Figure 2A, there is illustrated a cross-sectional view of an electrolyte flow conduit (flow conduit') 5 of an embodiment of a bipolar plate of the invention. Flow conduit 5 is defined by a trough 9 inset in a carbon-based bipolar plate ('bipolar plate') 1. The bipolar plate 1 has a first face 3 and a second face (not shown) opposite and parallel to the first face 3. In use, the bipolar plate 1 is arranged in a flow battery cell stack to physically separate and electrically connect two adjacent flow battery cells (for example, as illustrated in Figure 1B). In situ, the first face 3 is compressed against a porous electrode (not shown) such as carbon felt. Electrolyte, either catholyte or anolyte, may flow through the flow conduit 5 to and/or from the porous electrode via a slot 7 (although a row of perforations may alternatively provide fluid communication between the flow conduit 5 and a porous electrode).

The flow conduit 5 is defined by a pre-formed electrically insulating polymer (e.g. polyolefin) trough (i.e. an open-topped duct) 9, which trough 9 has been inserted into and thermally bonded to (carbon-based material of) a receiving recess 11 of the bipolar plate 1. The trough 9 may alternatively be fixed in the recess 11 using adhesive or other means. The flow conduit 5 has a generally rectangular cross-sectional shape having a width of around 2.5 mm and -23 -a depth of around 3 mm. The trough 9 has a generally uniform thickness A of around 200 um. The first face 3, which is planar, comprises a top surface 4 of the trough 9 such that a portion of the first face 3 adjacent the slot 7 is defined by an electrically insulating material that is resistant to acid and/or oxidative corrosion and/or electrolyte flow induced mechanical abrasion. The trough 9 comprises a generally right-angled corner 14 where a side wall 13 and the top surface 4 of the trough 9 meet.

Generally, where a bipolar plate comprises several flow conduits 5, there is typically disposed between recesses 11 of each flow conduit 5 fine carbon-based material ribs 15 which are fragile and thus prone to mechanical damage. An advantage of the trough 9 (and indeed of all pre-formed tubes and ducts described hereunder) is that it mechanically reinforces ribs 15 (adjacent side walls 13 of trough 9) of the bipolar plate 1. In a particular example, the trough 9 shieldsa cusp 17 of the bipolar plate 1 and thus minimises electrolyte flow induced mechanical abrasion at the cusp 17. A further advantage of the trough 9, which is electrically insulating, is that it reduces shunt current induced damage of the bipolar plate 1 (as described more fully above).

In Figure 2B, there is illustrated a perspective view of a bipolar plate 1 of an embodiment of the invention having four flow conduits 5 having rectangular cross-sectional shapes as illustrated in Figure 2A. The bipolar plate 1 has a first face 3, a second face 25, a length L of 20 cm, a width W of 70 cm, a depth D of 4 mm and is generally cuboidal in shape.

Each of the flow conduits 5 are generally straight, are the same length as the bipolar plate 1, and extend between a first (inlet) side face 27 and a second (outlet) side face 29 of the bipolar plate 1. A particular benefit of the straight flow conduits 5 (and hence straight receiving recesses 11) is that the bipolar plate 1 may be made by a continuous manufacturing process such as extrusion. However, the shape of the flow conduits, which define a bipolar plate flow field geometry, may be any suitable shape, e.g. serpentine or wave (e.g. sine wave) shaped.

A first pair of flow conduits 19 (19L and 19R) each have an electrolyte inlet 31 formed in side face 27 and an electrolyte outlet 33 formed in -24 -side face 29. The inlets 31 of the bipolar plate 1 are configured to communicate with a cell level electrolyte inlet manifold ('inlet manifold') (not shown) and the outlets 33 of the bipolar plate 1 are configured to communicate with a cell level electrolyte outlet manifold ('outlet manifold') (not shown). In one example, the inlet manifold will communicate with the inlets 31 of both flow conduits 19L and 19R and the outlet manifold will communicate with the outlets 33 of both flow conduits 19L and 19R. In this example, electrolyte may, within flow conduits 19L and 19R, 'flow by' a porous electrode. In a second example, the inlet manifold may communicate with the inlet 31 of flow conduit 19L and block the inlet 31 of flow conduit 19R, while the outlet manifold may block the outlet 33 of flow conduit 19L and communicate with outlet 33 of flow conduit 19R (or vice versa). Thus, in this example, electrolyte may flow into a cell via flow conduit 19L, flow out of flow conduit 19L into a porous electrode, and subsequently flow into flow conduit 19R from the porous electrode where it can exit the cell. The arrangement of this second example is known as an Interdigitated' flow field geometry.

A second pair of flow conduits 21 (21L and 21R) illustrate an alternative interdigitated flow field geometry arrangement. Flow conduit 21L is defined by a trough 9 comprising a first (inlet) end wall 23 such that flow conduit 21L does not comprise an inlet. Flow conduit 21R is defined by a trough 9 comprising a second (outlet) end wall 35 such that flow conduit 21R does not comprise an outlet.

Although in Figure 2B, for simplicity, flow conduits 5 are illustrated as only in fluid communication with the first face 3, a bipolar plate will typically comprise a second set of flow conduits 5 that are in fluid communication with the second face 25. The second set of flow conduits 5 may mirror (e.g. in a mirror plane parallel and medial relative the first face 3 and second face 25) the flow conduits 5 that are in fluid communication with the first face 3, or they may be offset relative the flow conduits 5 that are in fluid communication with the first face 3. However, it is noted that a terminal plate of a stack of fuel cells may comprise flow conduits 5 only in fluid communication with a single face. -25 -

Figures 3A to 3K, described hereunder, illustrate further embodiments of the invention as cross-sectional views of flow conduits 5 defined by a tube or duct 9 disposed (e.g. inset) in a carbon-based bipolar plate 1.

In Figure 3A, the flow conduit 5, defined by a trough 9, has a generally curved (in this example semi-circular) cross-sectional shape. An advantage of a generally curved flow conduit cross-sectional shape is improved in-conduit electrolyte flow characteristics and thus a reduction in pressure loss through a cell. Therefore, for a given electrolyte flow rate, pumping load may be reduced (or electrolyte flow rate can be increased for a given pumping load) - increasing the efficiency of a flow battery. Use of a tube or duct that is pre-formed, which has a generally curved (in this case semi-circular) and preferably generally circular cross-sectional shape is particularly advantageous because a uniform cross-sectional shape and smooth conduit flow surface 8 can be achieved consistently along the length of said tube or duct.

In Figure 3B, by contrast to Figure 3A, the trough 9 additionally comprises arm portions 18 extending laterally from a top 10 of a semi-circle portion of the trough 9. The arm portions 18 increase the area of the top surface 4 of the trough 9, such that a larger portion (by contrast to the bipolar plate of Figure 3A) of the first face 3 adjacent the slot 7 is defined by an electrically insulating material that is resistant to acid and/or oxidative corrosion and/or electrolyte flow induced mechanical abrasion. That is to say, the two strips 4 (of trough 9 material) that form a portion of the first face 3 and run adjacent to and parallel with the slot 7 have an increased width. By increasing the area of the top surface 4 of the trough 9, shunt current induced charging (and thus discharging) of the bipolar plate is further reduced. An additional advantage of the arms 18 is that they aid the initial location and maintain the orientation of the trough 9 during manufacture of the bipolar plate 1.

In Figure 3C, the trough 9, by contrast to the angled corner 14 of Figure 3A, comprises a rounded corner 14. A particular advantage of the rounded corner 14 is that electrolyte flow characteristics are improved proximal the corner 14 (i.e. as electrolyte enters and/or exits the flow conduit 5 proximal the corner -26 - 14). A non-illustrated embodiment of the invention may comprise the trough 9 illustrated in Figure 3B adapted to have rounded corners 14.

In Figure 3D, a tube 9 has been inserted into and fixed to (carbon-based material of) a receiving recess 11 of the bipolar plate 1 using adhesive (although the tube 9 may alternatively be thermally bonded to the receiving recess 11). The flow conduit 5 has a generally square cross-sectional shape having a width and depth of around 2 mm. The tube 9 has a generally uniform thickness A of around 300 itm.

There is illustrated a slot 7 for providing fluid communication between the first face 3 (and thus in situ between a porous electrode) and the electrolyte flow conduit 5, the slot 7 having a width approximately 35 that of the width of the electrolyte flow conduit 5. The slot 7 may alternatively be a perforation (of a row of perforations). A first advantage of the slot 7 (or perforations) having a width less than the maximum width of the flow conduits (i.e. by contrast to an 'open-topped' flow conduit) is improved electrolyte flow characteristics within the flow conduit 5. A second advantage is reduced ingress of porous electrode (e.g. carbon felt) into the flow conduit 5 -porous electrode ingress negatively impacts electrolyte flow homogeneity and pressure drop within a cell. Both advantages provide for a more efficient cell.

The first face 3, which is planar, comprises a top surface 4 of a top portion 16 of the tube 9 such that a portion of the first face 3 adjacent the slot 7 is defined by an electrically insulating material that is resistant to acid and/or oxidative corrosion and/or electrolyte flow induced mechanical abrasion.

In Figure 3E, the tube 9 is similar to that of Figure 3D, however, the cross-sectional shape of tube 9 is semi-circular as opposed to square in shape.

In Figure 3F, the outer and inner shape of the tube 9 and the flow conduit 5 have a generally circular cross-sectional shape. The bipolar plate I is made by applying/pouring a carbon containing reagent mixture around the tube 9 (e.g. within a mold) and allowing it to solidify. Thus, the tube 9 is retained by carbon-based material lips 37 of the bipolar plate 1. A slot or perforations 7 may either be pre-formed or fabricated after the carbon containing reagent mixture has solidified.

-27 -In Figure 30, the bipolar plate 1 is similar to that illustrated in Figure 3F, however, the first face 3 defines a plane that secants the inner circle of the tube 9. Thus, the generally circular tube 9 is pre-formed absent a top segment or a top segment of the generally circular tube 9 is removed (e.g. by abrading or planing) during the manufacture of the bipolar plate 1. Advantages of the tube 9 (by contrast to that of Figure 3F) is that the extent of the retaining lips 37 (which are made of a fragile carbon-based material that is prone to breakage) is reduced and that the first face 3 comprises a top surface 4 of the tube 9 (the benefits of which are discussed above).

In Figure 3H, the generally circular inner cross-sectional shape of the tube 9 (and thus of the flow conduit 5) is similar to that of Figure 3F.

However, the outer cross-sectional shape of the tube 9 is generally square, which feature provides several advantages. First, an essentially 'whole-circle' flow conduit cross-sectional shape (which it is envisaged provides optimum in-conduit electrolyte flow characteristics) can be provided while eliminating fragile carbon-based material lips (see 37 in Figures 3F and 30). Second, the first face 3 of the bipolar plate 1 comprises a top surface 4 of the electrically insulating tube 9, reducing shunt current charging of the bipolar plate 1. Third, the average thickness of the tube 9 is increased, increasing mechanical reinforcement of the fragile carbon-based ribs 15. Fourth, particularly where the slot or perforations are pre-formed in the tube 9, the non-circular outer shape of the tube 9 aids the initial location and maintains the correct orientation of the tube 9 during manufacture of the bipolar plate 1. Fifth, an essentially 'whole-circle' flow conduit cross-sectional shape can be provided without the need to manufacture the bipolar plate using a carbon containing reagent mixture that solidifies (i.e. economical manufacturing methods, such as extrusion, for forming the carbon-based portion of the bipolar plate may be used).

In Figure 31, by contrast to Figure 3H, the slot or perforations 7 has been adapted to have rounded (e.g. 'bull-nose) corners 14 for improved electrolyte flow characteristics as electrolyte enters and exits the flow conduit 5.

In Figure 3J, by contrast to Figure 3H, the outer cross-sectional shape of the tube 9 is triangular. In addition to the advantages described above for -28 -Figure 3H, a particular benefit of the triangular outer cross-sectional shape is that a bottom corner 39 aids the initial location of the tube 9 within snugly fitting receiving recess 11 during manufacture of the bipolar plate I. In Figure 3K, the inner shape of the tube 9 (and thus the flow conduit 5) is the same as in Figure 3G while the rectangular outer cross-sectional shape of the tube 9 is similar to that of Figure 3H. A particular benefit of tube 9 of this embodiment is that a channel disposed between the flow conduit 5 and the first face 3 is eliminated or nearly eliminated (that is to say, the depth of the slot or perforations 7 is zero or essentially zero), eliminating/reducing any negative effects on electrolyte flow characteristics that occur because of such a channel and which increase pressure drop through a cell.

Figure 4 is a perspective view of a bipolar plate 1 having inset two tubes 9L and 9R, each defining a flow conduit 5. Tube 9L provides fluid communication between its flow conduit 5 and the first face 3 of the bipolar plate 1 via a slot 207. Tube 9R provides fluid communication between its flow conduit and the first face 3 of the bipolar plate 1 via a row of generally circular perforations 307. It is noted that, when a cross-section is taken through plane AA, the resulting cross-sectional view (such as that illustrated in Figure 3H) may not differentiate between a slot and a row of perforations.

Figures 5A and 5B are cross-sectional views of bipolar plates 1, each having a first face 3 and a second face 25 parallel to the first face 3. Each of the bipolar plates 1 (in Figures 5A and 5B) comprise a first set of four flow conduits 205 in fluid communication the first face 3 and a second set of four flow conduits 305 in fluid communication with the second face 25. The flow conduits (and any associated ducts or tubes) of the sets 205 and 305 may be any described in the above aspects of invention and/or illustrated in the foregoing figures.

In Figure 5A, the positions of the two sets of flow conduits 205 and 305 min-or each other in a mirror plane B-B parallel and medial relative the first face 3 and second face 25.

In Figure 5B, the positions of the flow conduits, of the two sets of flow conduits 205 and 305, alternate (between sets) such that they are disposed at equal intervals along the width of the bipolar plate. Thus, preferably, a portion of -29 -each flow conduit extend through a mirror plane B-B parallel and medial relative the first face 3 and second face 25. An advantage of this arrangement is that the depth of the bipolar plate 1 may be reduced without decreasing the minimum distance (defined by carbon-based material of the bipolar plate) between individual flow conduits In Figure SC, the bipolar plate 1 comprises a core sheet 103 (e.g a pre-formed sheet of carbon-based material). The flow conduits of sets 205 and 305 abut the core sheet 103 (although in an alternative embodiment they do not directly abut, but are located proximal to, the core sheet 103). Carbon-based material 203 (for example, a hardened carbon containing reagent mixture) is disposed around the flow conduits of sets 205 and 305 in an amount such that the bipolar plate comprises a planar first face 3 and a planar second face 25.

The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention. -3 -

Claims

CLAIMS: 1. A bipolar plate for use in a flow battery cell or cell stack, the bipolar plate comprising: a first face for disposing adjacent a porous electrode or membrane, an electrolyte flow conduit, and one or more openings for providing fluid communication between the first face and the electrolyte flow conduit, the one or more openings having a width less than a maximum width of the electrolyte flow conduit, wherein the electrolyte flow conduit is defined by an electrically insulating material or surface.
2. The bipolar plate as claimed in claim 1, wherein the width of the one or more openings is at most 90 % the maximum width of the electrolyte flow conduit.
3. The bipolar plate as claimed in claim 1 or claim 2, wherein the one or more openings are defined by the electrically insulating material or surface.
4. The bipolar plate as claimed in any one of the preceding claims, wherein the bipolar plate is carbon-based.
5. The bipolar plate as claimed in any one of the preceding claims, wherein the one or more openings are a slot or perforations.
6. The bipolar plate as claimed in any one of the preceding claims, wherein the electrolyte flow conduit has a generally curved (e.g. continuous curved) cross-sectional shape.
7. The bipolar plate as claimed in any one of the preceding claims, wherein the electrolyte flow conduit has a generally circular cross-sectional shape.
-31 - 8. The bipolar plate as claimed in any one of the preceding claims, wherein a portion of the first face adjacent the slot or perforations is defined by the electrically insulating material or surface.
9. The bipolar plate as claimed in any one of the preceding claims, wherein at least 50% of the first face is defined by carbon-based material.
10. The bipolar plate as claimed in any one of the preceding claims, wherein the minimum thickness of the electrically insulating material is in the range of 10 from 10 gm to 1 mm.
11. The bipolar plate as claimed in any one of the preceding claims, wherein the electrolyte flow conduit (and preferably the one or more openings) is defined by an electrically insulating duct or tube disposed in the bipolar plate.
12. The bipolar plate as claimed in claim 11, wherein the bipolar plate comprises a receiving recess having a cross-sectional shape corresponding to an external cross-sectional shape of the electrically insulating duct or tube and/or having a length corresponding to a length of the electrically insulating duct or tube.
13 The bipolar plate as claimed in claim 11 or claim 12, wherein the duct or tube is fixedly attached to or retained by electrically conductive material of the bipolar plate 14. The bipolar plate as claimed in any one of claims 11 to 13, wherein the duct or tube has a generally polygonal (e.g. quadrilateral or triangular) external cross-sectional shape.15. The bipolar plate as claimed in any one of the preceding claims, wherein the electrically insulating material or surface (e.g. the tube) is chemically stable in -32 -a highly acidic and/or oxidative environment, preferably is a polymer, more preferably is a polyolefin.16. The bipolar plate as claimed in any one of the preceding claims, wherein the electrolyte flow conduit has a first end and a second end, and wherein at least one end is an electrolyte inlet or an electrolyte outlet, preferably wherein the electrolyte inlet or electrolyte outlet is formed in a side face of the bipolar plate 17. The bipolar plate as claimed in any one of the preceding claims, wherein the bipolar plate comprises a plurality of electrolyte flow conduits.18. The bipolar plate as claimed in any one of the preceding claims, wherein the perforations have a generally circular or elliptical cross-sectional shape and/or wherein the perforations have a generally cylindrical or frustum (e.g. frustoconi cal) shape.19. The bipolar plate as claimed in any one of the preceding claims, further comprising a second face opposite the first face and for disposing adjacent a porous electrode or membrane, the second face in fluid communication with a second electrolyte flow conduit via one or more openings as defined in any one of the preceding claims.20. A flow battery cell comprising at least one bipolar plate as defined in any one of claims I to 19.2k A flow battery cell stack of at least two (e.g. a plurality of) flow battery cells arranged in electrical series, the flow battery cell stack comprising at least one bipolar plate as defined in any one of claims 1 to 19.22. A method of manufacturing a bipolar plate for use in a flow battery cell or cell stack, the method comprising: fin providing a pre-formed electrically insulating duct or tube optionally comprising one or more pre-formed openings, disposing the pre-formed electrically insulating duct or tube in an electrically conductive material to form a bipolar plate comprising an electrolyte flow conduit defined by the electrically insulating duct or tube, and where the pre-formed duct or tube is not provided with one or more preformed openings, forming one or more openings in the pre-formed duct or tube, preferably wherein the bipolar is a bipolar plate as defined in any one of claims 1 to 19 23. A method of manufacturing as claimed in claim 22, the method further comprising: extruding or casting the electrically conductive (e.g. carbon-based) material to form a bipolar plate comprising a receiving recess for receiving the pre-formed duct or tube, inserting the pre-formed duct or tube into the receiving recess such that the pre-formed duct or tube is fixedly attached to the electrically conductive material defining the receiving recess (e.g. by providing adhesive or by thermal bonding).24. A method of manufacturing as claimed in claim 22, the method further comprising: providing an electrically conductive (e.g. carbon-based) sheet, arranging the pre-formed duct or tube in a pre-defined position on or relative to a first face of the sheet, applying an electrically conductive (e.g. carbon containing) reagent mixture to the first face of the sheet such that the pre-formed duct or tube is at least partially covered by the reagent mixture, and allowing the reagent mixture to solidify.25. A method of storing or discharging electrochemical energy using a flow battery cell as defined in claim 20 or a flow battery cell stack as defined in claim 21 -3 -