An ion exchange membrane fuel cell

Abstract

A polymer electrolyte membrane (PEM) 2 for use in an ion exchange membrane fuel cell, comprising a sensing layer 11 on a side of the PEM having two or more - optionally at least six - sensing elements

Description

Technical Field of the Invention The present invention relates to ion exchange membrane fuel cells and methods of manufacturing ion exchange membrane fuel cells. In particular, it relates to cathode-catalyst-mcmbranc assemblies and polymer electrolyte membranes comprising sensing elements for use in ion exchange membrane fuel cells and their manufacture, especially, but not exclusively, where they are configured to detect oxygen concentrations in proton exchange membrane fuel cells. Background to the Invention Ion exchange membrane fuel cells, such as Proton Exchange Membrane Fuel Cells (PEMFCs), hold promise for clean power generation in a range of applications including aerospace, marine, heavy duty automotive and stationary power. The electrochemical reactions of hydrogen and oxygen responsible for power generation in PEMFCs take place at the surface of electrodes which typically have a geometric surface areas of several hundred cm2. However, the distribution of these reactions across the electrodes is not uniform and there exists significant spatial variation across the cell in its efficiency. The efficiency of the electrochemical reactions is due to many different factors such as the availability of reactants, current density, and temperature. There is therefore a need to make spatially-resolved measurements in PEMFCs so that the variations in electrochemical reaction efficiency can be better understood and the cell performance optimised. A typical PEMFC comprises a layered structure including anode and cathode electrodes separated by a polymer electrolyte membrane (PEM). In operation, hydrogen is supplied to the anode where it is split into protons and electrons by a catalyst, and O2 is supplied to the cathode. The protons permeate the PEM, which is electrically insulating but proton conducting, and combine with an oxygen atom and electrons to form H2O and generate an electric potential between the anode and cathode that can be used to draw power. This process is facilitated by a catalyst provided on the cathode. Existing ways of measuring properties of a PEMFC include that disclosed in S. Takaichi et al., Electrochimica Acta 53 (2008) 4699-4705. In this example, a PEM is constructed by forming two separate membranes of equal thickness and then heat sealing them together with the platinum wires of inbetween. This results in a PEM with platinum sensing wires extending through the middle. Another existing PEMFC that has sensing properties is disclosed in Epting et al., ECS Transactions, 33 (1) 1241-1249 (2010). In this example, an electrode scaffold is constructed comprising alternating layers of insulating material and platinum. The scaffold is drilled to form a hole that is subsequently filled with the cathode catalyst material. The scaffold / catalyst is then bonded to the surface of the PEM such that measurements at different intervals along the cathode catalyst’s thickness are possible. It is an aim of an embodiment or embodiments of the invention to overcome or at least partially mitigate the one or more problems with the prior art and / or to provide an improved ion exchange membrane fuel cell. Summary of the Invention In a broad sense, the invention concerns a sensing layer comprising one or more sensing elements for use in an ion exchange membrane fuel cell. The one or more sensing elements may be provided on or in an electrically insulating medium. Preferably, the one or more sensing elements may be covered by an electrically insulating film. The invention also concerns a polymer electrolyte membrane (PEM) for use in an ion exchange membrane fuel cell. The PEM may therefore comprise the sensing layer. The film may cover the one or more sensing elements. Accordingly in a broad aspect there is provided a polymer electrolyte membrane (PEM) for use in an ion exchange membrane fuel cell, the PEM comprising a sensing layer provided on a side of the PEM, the sensing layer comprising one or more sensing elements covered in an electrically insulating film. The one or more sensing elements may comprise two or more sensing elements. The two or more sensing elements may be spaced across a plane of the PEM. Consequently, in a preferred embodiment there is provided a polymer electrolyte membrane (PEM) for use in an ion exchange membrane fuel cell, the PEM comprising a sensing layer provided on a side of the PEM, the sensing layer comprising two or more sensing elements covered in an electrically insulating film and spaced across a plane of the PEM. Advantageously, this allows the PEM to comprise sensing elements on or close to the surface of the PEM. The sensing elements thereby facilitate measurement of spatial variations of the properties of a fuel cell across the PEM / catalyst interface in use. This construction also allows the PEM to be easily produced and then used with different catalyst layers as required. The PEM of this embodiment may include any one or more of the features, optional or otherwise, of a PEM, the catalyst-coated-membrane assembly (CCM) of the first aspect, or ion exchange membrane fuel cell (IEMFC) as described herein. The invention also concerns a catalyst-coated-membrane assembly (CCM) for use in an ion exchange membrane fuel cell. The CCM may comprise a PEM. The CCM may comprise a catalyst layer. The CCM may comprise a sensing layer. The sensing layer may be provided between the PEM and the catalyst layer. The film may be configured to electrically insulate the one or more sensing elements from the catalyst layer. According to a first aspect of the invention there is provided a catalyst-coated-membrane assembly for use in an ion exchange membrane fuel cell (IEMFC) comprising a polymer electrolyte membrane (PEM), a catalyst layer and a sensing layer provided between the PEM and catalyst layer, wherein the sensing layer comprises one or more sensing elements covered in a film, the film configured to electrically insulate the one or more sensing elements from the catalyst layer. Thus, the sensing elements are beneficially arranged to detect properties of the IEMFC at or close to the PEM / catalyst layer interface in use. This is enabled as the sensing layer and sensing elements are between and thereby effectively sandwiched by the catalyst layer and PEM. Advantageously, the PEM facilitates easier construction of an ion exchange membrane fuel cell with sensing capability than the prior art as the sensing layer can be simply integrated into the design, for example by printing the sensing elements and film onto the PEM, then adding the catalyst layer over the top. This provides a cost-effective and efficient way to make measurements of properties at or close to the interface between the PEM and the catalyst layer. In addition as the sensing elements and film can be made very thin, they are suitable for measuring the properties of state of the art cells which can typically comprise a PEM of only 10-15 microns in thickness. In addition, as the PEM and catalyst layer can have an otherwise conventional bulk structure, the PEM of the present invention is less susceptible to inefficiency or performance losses due to the sensing elements and can also be incorporated into existing cells and stacks in place of regular non-sensing PEMs. This can reduce the cost of testing and developing new ion exchange membrane fuel cells and also provides the possibility of enhanced measurement accuracy as the sensing elements can be placed flexibly across the PEM. The sensing layer may be arranged across the plane of the PEM. The catalyst layer may extend over the sensing layer. The catalyst layer may extend over and / or around the one or more sensing elements. The catalyst layer may extend over and / or around the film. The catalyst layer may extend over the majority of the PEM. The sensing layer may extend over a minority of the PEM. Thus, the sensing layer is able to make measurements across the PEM / catalyst layer interface while minimising any reduction in cell performance due to the sensing layer. Preferably, the PEM / CCM is for use in a proton exchange membrane fuel cell (PEMFC). However, it may also be for use in other types of IEMFC such as an anion exchange membrane fuel cell. The catalyst layer may be a cathode catalyst layer. Advantageously, this allows the sensing elements to operate at or close to the PEM / cathode catalyst layer interface where the majority of the electrochemical reactions responsible for power generation in the cell occur, especially for a PEMFC. Alternatively, the catalyst layer may be an anode catalyst layer. This can allow the sensing elements to determine properties of the anode / PEM interface. In some embodiments, the PEM may comprise a sensing layer provided on both sides of the PEM. One sensing layer may be provided between the cathode catalyst layer and the PEM. Another sensing layer may be provided between the anode catalyst layer and the PEM. The PEM may comprise a sensing layer provided on each side of the PEM. Of course, the sensing layers of this PEM may each be coated with a catalyst layer to form a CCM and then be integrated into an IEMFC. Consequently, the PEM described herein may have any one or more of the optional features of the sensing layer, PEM, CCM or IEMFC as described herein. The film may be formed of an ion conducting material. The film may be formed of a proton conducting material. The film may comprise an ionomer. The film may comprise an ionomer of 700-900 equivalent weight (EW). The film may comprise Perfluorosulfonic Acid (PFSA). Preferably, the film comprises a 700-900 EW PFSA ionomer. The film may be formed of the same material as the PEM. The sensing elements may therefore be effectively embedded in the PEM. Advantageously, as the film is ion conducting, it helps to provide additional pathways for ions to travel around the sensing elements. This helps to reduce losses of efficiency in a cell due to the introduction of sensing elements for example due to the sensing elements physically blocking the passage of ions through the cell. The film may extend around and over each of the one or more sensing elements. The film may extend around each sensing element to provide an ion path around each sensing element. The film around each sensing element may extend (in the plane of the PEM) at least 1 mm from the outer edge of the sensing element. Thus, the film ensures the sensing elements are better electrically insulated from the catalyst layer and can provide additional pathways for ions around the sensing elements. The sensing elements may be provided in direct contact with the PEM. Thus, the sensing elements are closer to the PEM / catalyst layer interface. The film may not be provided between the sensing elements and the PEM. Thus, there is less unnecessary material between the PEM and catalyst layer, reducing any negative effects on cell performance. The film may have a thickness of no more than 10 microns. The film may have a thickness of 3 to 5 microns. This ensures that the sensing elements are adequately electrically shielded and insulated from the catalyst layer while minimising the impact of the film on the overall cell performance. The film may be porous. This can provide better mass transport to and from the sensing elements. The one or more sensing elements may comprise at least three sensing elements. Preferably, the sensing layer comprises at least six sensing elements, or at least ten sensing elements, or at least twenty sensing elements, or at least forty sensing elements. The sensing elements may be provided in an array across the plane of the PEM. This allows for the measurement of variations of a property across the plane of the PEM. Each sensing element may be spaced from every other sensing element. Thus, space is provided between the sensing elements to ensure the cell can operate efficiently. The two or more sensing elements may be evenly spaced across the plane of the PEM. This can advantageously provide a map of variations in a property across the PEM. Alternatively, the two or more sensing elements may be arranged in positions corresponding to a feature of interest. The two or more sensing elements may be arranged at or towards flow field edges of the PEM. This can allow detection of edge effects. The two or more sensing elements may be provided on adjacent interdigitated channels. This can facilitate measurement of inter-channel convection in the PEM. The film may be discontinuous. The film may not cover an entire side of the PEM. The film may cover no more than half of one side of the PEM. The film may have a surface area of less than the overlap area of the PEM and catalyst layer. Where there are two or more sensing elements, the film may comprise lateral gaps between sensing elements. The catalyst layer may extend into the lateral gaps. The catalyst layer may be in direct contact with the PEM in the lateral gaps. The catalyst layer may be in direct contact with the PEM over a majority of the surface area of the catalyst layer. Thus, the film area is minimised where possible to reduce the impact of the film on the performance of the cell. Alternatively, the film may be continuous. The film may extend between adjacent sensing elements. Where the film is continuous, it is preferably formed of the same material as the PEM. The film may therefore extend the PEM. The sensing elements may therefore be embedded at or close to the surface of the PEM. Each sensing element may have a thickness of no more than 5 microns. Each sensing element may have a thickness of at least 3 microns. Preferably, each sensing element has a thickness of 3 to 5 microns. Thus, the sensing elements are thin to ensure that the effect of them on the overall cell construction is minimised and that the PEM and CCM incorporating the sensing elements according to the invention can be more easily swapped with regular PEMs or CCMs in existing cells and stacks. Each sensing element may cover no more than 5%, or no more than 1%, of the area of the PEM. Preferably, each sensing element may cover no more than 0.1% of the area of the PEM. Each sensing element may cover no more than 5%, or no more than 1%, of the area of overlap between the PEM and catalyst layer. Preferably, each sensing element may cover no more than 0.1 % of the area of overlap between the PEM and catalyst layer. Each sensing element may cover an area of the PEM of no more than 20 mm2. Thus, the area of each sensing element is very small in comparison to the PEM and catalyst overlap area. The PEM and catalyst overlap area may be 200 cm2 or more. This helps to ensure that the sensing elements have a small or negligible effect on the overall cell performance. Each sensing element may have a diameter of no more than 5 mm. Each sensing element may have a diameter of at least 1 mm. Preferably, each sensing element has a diameter of 1 to 5 mm. Preferably, each sensing element is circular or disk-shaped. Of course, the sensing elements could have other shapes. In such cases, the diameter mentioned above may be a width of the sensing element as appropriate. An area of the film corresponding to one sensing element may therefore be circular with a diameter of 3 to 15 mm depending on the size of the sensing element. Such a film section may be centred with respect to the sensing element. As described above, this ensures adequate electrical insulation and additional ion pathways to compensate for those lost due to the sensing element. The sensing layer may comprise one or more conductive tracks. Each conductive track may be electrically conductive. Each conductive track may extend from one of the sensing elements to an edge of the PEM or CCM. The film may cover the conductive tracks. The film may electrically insulate each conductive track from the catalyst layer. Thus the conductive tracks provide a convenient electrical connection between each sensing element and the outside of the PEM or CCM allowing measurements to be taken. Each conductive track may be formed of a different material to the sensing element it is connected to. Each conductive track may be relatively, or preferably completely, insensitive to the property intended to be measured by the sensing element it is connected to. Each conductive track may not comprise an electrocatalyst material, or may be a non-electrocatalyst material. Thus, the measured signal is dominated by the signal generated by the sensing element. This ensures that the measurement is more indicative of the property measured at a specific location in the lEMFC / across the PEM surface. Each conductive track may be substantially straight. The conductive tracks may not cross one-another. Each conductive track may have a width of approximately 1 mm. Each conductive track may have a thickness of no more than 5 microns. Each conductive track may have a thickness of no less than 3 microns. Preferably, each conductive track has a thickness of 3 to 5 microns. Thus, the conductive tracks are sized to ensure they can effectively conduct electricity from the sensing elements and are also thin to ensure they do not significantly affect the cell performance. The film may extend around and over each of the one or more conductive tracks. The film may extend around each conductive track to provide an ion path around each conductive track. The film around each conductive track may extend at least 0.5 to 1 mm wider than the conductive track. Thus, the film ensures the conductive tracks are better electrically insulated from the catalyst layer and can provide additional pathways for ions around the conductive tracks. Each conductive track may comprise an electrically conducting material. Each conductive track may comprise a metal, such as for example, colloidal gold. Each conductive track preferably comprises carbon black, for example Vulcan XC-72 carbon black. Each conductive track may be printed onto the PEM. Each sensing element may be printed onto the PEM. The film may be printed onto the PEM. Preferably, the conductive tracks, sensing elements and film are all printed onto the PEM. This helps to ensure simple, accurate and effective construction of the PEM and CCM. Each of the conductive tracks, sensing elements and / or film may therefore comprise a printable material. The sensing elements may comprise an electrocatalyst material. Thus, the sensing elements are able to facilitate electrochemical reactions. This generates electrical signals that can be used to measure the properties of the cell. The electrocatalyst material may be active towards a property of interest in the cell. The electrocatalyst material may be active towards oxygen reduction reactions, for example the electrocatalyst material may be Platinum (Pt). The sensing elements may comprise Platinum supported on Carbon (C), for example 40% wt Pt / C or Platinum-Cobalt (Co)-Nickel (Nl) supported on Carbon, for example 40% wt Pt-Co-Ni / C. In such embodiments, the resulting Pt concentration may be about 0.1 mg Pt / cm2 across a sensing element. This can enable the sensing elements to detect variations in oxygen concentration across the PEM which is particularly important as local oxygen starvation reduces cell performance and leads to degradation in the cell over time. The electrocatalyst material may be active towards peroxide reduction reactions. This can enable the sensing of peroxide concentrations in the cell which are indicative of suboptimal cell performance and can lead to cell damage. The sensing elements may detect any one or more of temperature, electrode potential, oxygen concentration, peroxide concentration, pH, proton flux, or the like. This can enable the PEM and CCM of the present invention to enable measurement and optimisation of the cell design. The PEM is preferably planar. The PEM may be substantially two dimensional with a defined thickness. The PEM may have edges defining lateral edges of the PEM and its surface area of the PEM. The PEM may have two faces bounded by its edges. A side of the PEM may be parts of the PEM to one side of a midpoint through the thickness of the PEM, preferably at or close to the surface of the PEM. The PEM may comprise a central region across the thickness of the PEM. The central region may include a midpoint through the thickness of the PEM, and is preferably centred on the midpoint. The central region may extend substantially all the way between the lateral edges of the PEM. The central region may extend at least 50% of the thickness though the PEM. The central region may be at least 10 microns thick, but could be smaller depending on the thickness of the PEM. The central region may be free from sensing elements. Thus, the PEM is free of sensing elements through the middle which helps reduce any effect of the sensing elements on cell performance and also ensures the sensing elements are able to sense the properties of the PEM / catalyst layer interface. The PEM may have a surface area of at least 100 cm2. The PEM may have a surface area of no more than 400 cm2. Preferably, the PEM has a surface area of about 250 cm2. The PEM may have any suitable thickness for the given cell it is intended for. Preferably, the PEM has a thickness of at least 10 microns. The PEM may have a thickness of no more than 15 microns. The PEM may have a thickness of 10 to 15 microns. The PEM may be any suitable shape, preferably, the PEM is square or rectangular. Thus, the PEM is commercially relevant to modern thin cells and the sensing layer is suitable for accurate measurement of the properties of them without significantly altering their performance. The PEM may be formed from an ionomer. The PEM may be formed from PFSA. The PEM may be formed from the same material as the film. The PEM may be formed from an ionomer of 700-900 equivalent weight (EW). The PEM may comprise Perfluorosulfonic Acid (PFSA). Preferably, the PEM comprises a 700-900 EW PFSA ionomer. The catalyst layer is preferably planar. The catalyst layer may have a surface area that is the same as or smaller than the PEM. The catalyst layer may have a surface area that is 10% smaller, 20% smaller or 30% smaller than the PEM. The PEM may extend, for example 5 cm outside the edges of the catalyst layer. The sensing elements may only be provided in regions of overlap between the catalyst layer and PEM. The sensing elements are thereby only provided where the reactions take place. The catalyst layer may be provided centrally on the PEM. The catalyst layer may have a thickness of at least 1 micron. The catalyst layer may have a thickness of no more than 20 microns. The catalyst layer preferably has a thickness of 5 to 10 microns. Where the catalyst layer is an anode catalyst layer, it may have a thickness of 5 microns. Where the catalyst layer is a cathode catalyst layer, it may have a thickness of 10 microns. Thus, the catalyst layer is efficiently formed. The catalyst layer may be formed from an electrocatalyst material specific to the intended 1EMFC. Where the IEMFC is a proton exchange membrane fuel cell, the catalyst layer may comprise platinum. The catalyst layer may comprise Platinum supported on Carbon (C), for example 40% wt Pt / C or Platinum-Cobalt (Co)-Nickel (Ni) supported on Carbon, for example 40% wt Pt-Co-Ni / C. In such embodiments, the resulting Pt concentration in the catalyst layer may be about 0.1 to 0.5 mg Pt / cm2. The Pt concentration may be about 0.2 to 0.4 mg Pt / cm2 across a cathode catalyst layer or 0.1 mg Pt / cm2 across an anode catalyst layer. Any one or more of the sensing elements, conductive tracks and film may comprise an ionomer. Preferably, all of the sensing elements, conductive tracks and film comprise an ionomer. For example 700-900 EW PFSA ionomer. Preferably, the ionomerxarbon mass ratio in the sensing elements and / or conductive tracks is 0.7 to 0.9. Thus, the sensing elements, conductive tracks and film all provide additional pathways for ions through the sensing layer reducing any negative effect on cell performance. According to a second aspect of the present invention there is provided an ion exchange membrane fuel cell (IEMFC) comprising an anode, a cathode and the catalyst-coated-membrane assembly (CCM) of the first aspect. The IEMFC of the second aspect may of course include any one or more of the features of the sensing layer, PEM or CCM described generally herein or in relation to the first aspect above. The anode may comprise a gas diffusion layer. The anode may comprise an anode catalyst layer, for example as described above. The anode catalyst layer may be provided by the catalyst layer of the first aspect. The gas diffusion layer may be configured to facilitate gas transport to the anode catalyst layer. The anode catalyst layer may be provided between the gas diffusion layer and the PEM. The cathode may comprise a gas diffusion layer. The cathode may comprise a cathode catalyst layer, for example as described above. The cathode catalyst layer may be provided by the catalyst layer of the first aspect. The gas diffusion layer may be configured to facilitate gas transport to the cathode catalyst layer. The cathode catalyst layer may be provided between the gas diffusion layer and the PEM. The gas diffusion layers of the anode and cathode may each be constructed by any suitable means. For example, they may comprise 0.2-0.3 mm thick unwoven carbon paper with a 5% wt Polytetrafluoroethylene (PTFE) treatment as is known in the art. The edges of the PEM, cathode and / or anode may be sealed with an inert subgasket. The inert subgasket may be configured to insulate the PEM and contain the reaction inside the cell. The inert subgasket may be formed of an inert material. The inert subgaskets may be formed of an inert polymer, for example polyethlyene napthalate (PEN). The conductive tracks may extend around the inert subgasket. Thus, the conductive tracks are easily accessible. The IEMFC may comprise a pair of bipolar plates on the outside of the anode and cathode gas diffusion layers respectively. The bipolar plates may be configured to provide structural support to the IEMFC, dissipate heat, facilitate electrical connections to the anode / cathode, distribute reactants to the gas diffusion layers, and / or manage removal of reaction products from the cell. The IEMFC may comprise a sealing structure between the bipolar plates and the inert subgaskets respectively. The sealing structure may assist in sealing the IEMFC. According to a third aspect of the present invention there is provided a system for measuring the properties of an IEMFC, the system comprising: an IEMFC according to the second aspect; and a measurement apparatus connected to the anode and at least one of the one or more sensing elements, the measurement apparatus comprising an ammeter configured to measure a current flowing between the anode and a sensing element, and a processor configured to receive a signal indicative of the measured current from the ammeter and calculate a property of the IEMFC. The system of the third aspect may of course include any one or more of the features of the sensing layer, PEM, CCM or IEMFC described generally herein or in relation to the first and second aspects above. The measurement apparatus may comprise a potentiostat. The potentiostat may be connected to the anode. The potentiostat may be connected to at least one of the one or more sensing elements. The potentiostat may be configured to control a voltage of at least one of the one or more sensing elements. The potentiostat may be configured to control a voltage of at least one of the one or more sensing elements such that it is offset with respect to a voltage of the anode. The voltage offset may be any suitable offset such that the sensing element is operating in a diffusion limited or mass-transport limited regime that limits competing parasitic reactions. For example, in such a regime, the signal from the sensing element is limited primarily by the property of interest and so an accurate measurement can be made. Where the sensing element is configured to detect an oxygen concentration, the voltage offset may be 0.4 to 0.45 V. This provides a good balance between optimising the voltage for oxygen reactions while limiting parasitic reactions such as hydrogen evolution and peroxide formation. The measurement apparatus may comprise a multiplexer. The multiplexer may be configured to connect the measurement apparatus, e.g. the ammeter, to different sensing elements. The multiplexer may be configured to sequentially connect the measurement apparatus to each of the sensing elements. Thus, measurements can be made across the IEMFC to determine spatial variations in a property without requiring separate components to measure the current at each sensing element. Of course, in an alternative embodiment a measurement apparatus without multiplexer may be provided for each sensing element. Any one or more of the ammeter, potentiostat and multiplexer may be integrated with the processor and implemented in software. The processor may be configured to generate a calibration curve based on measurements taken at known values of the property of interest. A calibration curve may be generated for each of the sensing elements. The processor may be configured to perform chronoamperometry to determine the value of the property of interest. The processor may be configured to compare the measured current with the calibration curve to determine the value of the property of interest. The system may comprise a stack of two or more IEMFCs according to the second aspect. A measurement apparatus may be provided for each IEMFC. The measurement apparatus may alternatively be connected to all of the IEMFCs. Thus, the properties of multiple IEMFCs can be detected simultaneously. This is facilitated by the sensor layer design that enables it to be integrated into conventional fuel cell stacks. According to an aspect of the present invention there is provided a method of manufacturing a polymer electrolyte membrane (PEM) for use in an ion exchange membrane fuel cell, the method comprising providing a PEM and arranging two or more sensing elements spaced apart on a side of the PEM and covering the two or more sensing elements with an electrically insulating film to form a sensing layer. This method may of course include any feature of the fourth aspect below, or the CCM of the first aspect, or a PEM as described herein, or the IEMFC of the second aspect described above. The method may comprise forming a CCM by providing a catalyst layer over the side of the PEM containing the sensing layer. According to a fourth aspect of the present invention there is provide a method of manufacturing a catalyst-coated-membrane assembly for use in an ion exchange membrane fuel cell, the method comprising providing a polymer electrolyte membrane (PEM), providing a catalyst layer, and arranging one or more sensing elements between the PEM and catalyst layer, wherein the method comprises covering the one or more sensing elements in a film to electrically insulate them from the catalyst layer. The method of the fourth aspect may be a method of manufacturing the CCM of the first aspect, or a PEM as described herein, or the IEMFC of the second aspect described above, and may include any one or more of the optional features of the PEM, CCM or IEMFC described herein. The method may comprise providing or forming a PEM. The method may comprise printing the sensing layer onto the PEM. The method may comprise printing one or more sensing elements onto the PEM. The method may comprise printing one or more conductive tracks onto the PEM. The method may comprise printing the film over the sensing elements. The method may comprise printing the film over the conductive tracks. The steps of printing described above may comprise any suitable printing method, for example, inkjet printing, decal printing, screen printing, lithographic printing, slot-die printing, or the like. The method may comprise forming a catalyst ink slurry. The catalyst ink slurry may comprise a catalyst and an ionomer. The catalyst ink slurry may comprise 40% wt Pt / C catalyst and 700-900EW PFSA ionomer dispersion in a mixture of iso-Propyl Acrylate (IPA) and ultrapure water. The ionomer:carbon mass ratio may be in the range 0.7-0.9. The method may comprise printing the catalyst ink slurry to form the sensing elements. The method may comprise forming a conductive ink slurry, for example a carbon ink slurry. The conductive ink slurry may comprise a conductive compound mixed with ionomer. The conductive ink slurry may comprise Vulcan XC-72 and 700-900 EW PFSA ionomer dispersion in a mixture of IPA and ultrapure water. The ionomer:carbon mass ratio may in be the range 0.7-0.9. The method may comprise printing the conductive ink slurry to form the conductive tracks. The method may comprise forming an ionomer ink. The method may comprise diluting an ionomer dispersion in IPA and water. The method may comprise printing the ionomer ink to form the film. The method may comprise depositing the catalyst layer onto the PEM and sensing layer such that the sensing layer is provided between the PEM and catalyst layer. The catalyst layer may be deposited by any suitable means, such as ultrasonic spray coating, decal transfer, inkjet printing. The method may also comprise forming an IEMFC. In such embodiments, the method may comprise depositing a second catalyst layer onto the other side of the PEM. The method may comprise providing the anode and cathode gas diffusion layers on either side of the PEM. The method may comprise sealing the cell by providing an inert subgasket and bipolar plates around the gas diffusion layers. According to a fifth aspect of the present invention there is provided a method of measuring a property of an IEMFC, the IEMFC comprising a catalyst-coated-membrane assembly of the first aspect, the method comprising receiving a signal indicative of the property from the one or more sensing elements, and making a measurement based on the signal. The IEMFC may be an IEMFC according to the second aspect. The method of the fifth aspect may use the CCM of the first aspect, or a PEM as described herein, and may include any one or more of the optional features of the PEM, CCM or IEMFC described herein. It may also include any one or more features of the method of the fourth aspect, and vice versa. The method may comprise holding the one or more sensing elements at a constant voltage offset with respect to the anode. The method may comprise measuring a current between a sensing element and the anode. The method may comprise taking a chronoamperometric measurement. The method may comprise controlling the internal properties of the IEMFC. The method may comprise measuring the response of the sensing elements to known properties of the IEMFC. The method may comprise generating a calibration curve, preferably for each sensing element. A calibration curve may be generated based on the known properties of the IEMFC. For example, the level of oxygen inside the IEMFC may be controlled to generate an oxygen calibration curve for each sensing element. The method may comprise comparing measurements from each sensing element to a calibration curve to determine the value of the property of interest. The method may comprise taking measurements sequentially from two or more sensing elements. The method may comprise building a map of the value of a property across the plane of the PEM. Detailed Description of the Invention In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a schematic cross-section of an edge of a proton exchange membrane fuel cell (PEMFC); Figure 2 is a schematic cross-section of a catalyst-coated-membrane assembly of the PEMFC of Figure 1; Figure 3 is a schematic plan view of the sensing layer and polymer electrolyte membrane of the PEMFC of Figure 1; Figure 4 is a schematic diagram of a system for measuring oxygen concentrations in the PEMFC of Figure 1; Figure 5 is a diagram of the method of taking an oxygen concentration measurement in the PEMFC of Figure 1; and Figure 6 is a diagram for the method of manufacturing the PEMFC of Figure 1. Referring to Figures 1 to 4, a proton exchange fuel cell (PEMFC) 1 comprises a layered structure including a polymer electrolyte membrane (PEM) 2 provided between an anode and a cathode. While this embodiment is a PEMFC, other embodiments may use other ions to generate power, for example it may be an anion exchange membrane fuel cell. The invention thereby applies to ion exchange membrane fuel cells generally. The PEM 2 in this embodiment comprises a proton-conducting and electrically insulating non-porous material, such as an ionmer material as is standard in the art. For example 700-900 EW Perfluorosulfonic Acid (PFSA). The PEM 2 is thereby configured to allow protons to pass from the anode to the cathode in use and as described below. The PEM 2 is rectangular and planar with a surface area of several hundred square centimeters, for example about 250 cm2. In this embodiment, the PEM 2 has a thickness of no more than 15 microns, for example 10-15 microns. In this embodiment, the cathode comprises a cathode catalyst layer 4 and a cathode gas diffusion layer 5. The anode similarly comprises an anode catalyst layer 6 and an anode gas diffusion layer 7. The catalyst layers 4, 6 being on an inside of the cathode and anode respectively and adjacent to the PEM 2. In this embodiment, the catalyst layers 4, 6 comprise materials suitable to catalyse the required electrochemical reaction for the PEMFC 1 and which are electrically conducting to allow power to be drawn from the PEMFC 1. In this embodiment, the catalyst layers 4, 6 thereby comprise Platinum (Pt), for example they may be formed of Platinum supported on Carbon (C), for example 40% wt Pt / C or Platinum-Cobalt (Co)-Nickel (Nl) supported on Carbon, for example 40% wt Pt-Co-Ni / C. As the majority of the reaction occurs at the cathode catalyst layer 4, the cathode catalyst layer 4 is thicker and has more Pt than the anode catalyst layer 6. The cathode catalyst layer 4 comprises 0.2 to 0.4 mg Pt / cm2 and a thickness of about 10 microns. The anode catalyst layer 6 comprises about 0.1 mg Pt / cm2 and a thickness of about 5 microns. Both the catalyst layers 4, 6 extend across substantially all of the PEM 2, except for a small section at the edges as described below. The gas diffusion layers 5, 7 are configured to facilitate gas diffusion to and from the catalyst layers 4, 6, as well as providing mechanical support to the PEMFC 1 as well as their other typical functions. In this embodiment, the gas diffusion layers 5, 7 comprise 0.2-0.3 mm thick unwoven carbon paper with a 5% wt Polytetrafluoroethylene (PTFE) treatment. The outside layers of the PEMFC 1 comprise bipolar plates 8 which sandwich the other layers therebetween. In this embodiment, the PEM 2 extends out past the edges of the anode and the cathode. In this region, the PEM 2 and catalyst layers 4, 6 are sealed with an inert gasket 9 provided around the edge of these layers. The inert gasket 9 comprises an inert polymer, for example polyethlyene napthalate (PEN). The inert gasket 9 is further held in place and sealed via sealing structures 10 which extend between the inert gasket 9 and the bipolar plates 8. In this embodiment, the PEM 2 comprises a sensing layer 11 on a side facing the cathode catalyst layer 4. The sensing layer 11 comprises one or more sensing elements 12. The sensing elements are configured to facilitate detection of a property of the PEMFC 1 in use, as described below. As the sensing elements 12 are positioned effectively on the surface of the PEM 2, there is a large central region, spanning effectively the entire PEM 2 thickness, that is free of sensing elements 12 which helps mitigate any reduction in cell performance due to the sensing elements 12. In this embodiment, the sensing elements 12 are formed from the same electrocatalyst material as the cathode catalyst layer 4 and are thereby configured to detect oxygen concentrations at the PEM 2 / cathode interface. In this embodiment, each sensing element 12 comprises 0.1 Pt / cm2 across a sensing element to allow it to be active towards oxygen reduction reactions. In this embodiment, each sensing element 12 is disc shaped, with a thickness of 3 to 5 microns and a diameter of 1 to 5 mm. This ensures that each sensing element covers less than 0.1% the surface area of the PEM 2 to minimise their effect on the overall cell performance. In this embodiment, the sensing layer 11 comprises six sensing elements 12 evenly spaced across the PEM 2, as shown in Figure 3. This facilitates the generation of a map of the oxygen concentration across the PEM 2 in use and as described below. In other embodiments, the sensing elements 12 may be arranged around a specific location on the PEM 2 to probe certain effects, for example edge effects. In this embodiment, the sensing layer 11 further comprises one or more conductive tracks 13 which extend from each sensing element 12 to an edge of the PEMFC 1. The conductive tracks 13 comprise an electrically conducting material that is insensitive to the property probed by the sensing element 12 it is connected to. This allows an electrical signal to be taken from the conductive track 13 that is indicative of the property of interest at the location of the sensing element 12 in the PEMFC 1. In this embodiment, the conductive tracks 13 are therefore formed of a material that is not active towards oxygen reduction reactions, for example Vulcan XC-72 carbon black or colloidal gold. In this embodiment, the conductive tracks 13 are straight and extend directly from each sensing element 12 to an edge of the PEM 2 without crossing one another. The conductive tracks 13 have a width of about 1 mm and a thickness of 3 to 5 microns. The sensing layer 11 also comprises a film 14 provided over and around the sensing elements 12 and conductive tracks 13. The film 14 is configured to electrically insulate the sensing elements 12 and conductive tracks 13 from the cathode catalyst layer 4. This ensures that measurements are possible, as if they were not electrically insulated from the cathode catalyst layer 4, the sensing elements 12 would effectively become part of the cathode catalyst layer 4. In this embodiment, the film 14 comprises the same material as the PEM 2, that is a proton conducting and electrically insulating ionomer, such as 700-900 EW PFSA. However, in other embodiments, the film 14 may not be ion conducting and may be non-porous. Preferably, the film 14 is ion conducting as it then provides additional pathways for ions around the sensing elements 12, further reducing the effect of the sensing layer 11 on the overall PEMFC 1 performance. In this embodiment, the film 14 is only provided on and around the sensing elements 12 and conductive tracks 13. In these regions, it is 3-5 microns thick and extends out beyond the lateral edges of the sensing elements 12 and conductive tracks 13 by 1 to 5 mm. This helps ensure good electrical insulation and the provision of additional ion pathways as mentioned above. In this embodiment, there are therefore large lateral gaps 15 provided between the parts of the film covering adjacent sensing elements 12 and conductive tracks 13. The cathode catalyst layer 4 extends into these gaps 15 where it is in direct contact with the PEM 2. As the film 14 covers a minority of the surface of the PEM 2, then the cathode catalyst layer 4 is in direct contact with the PEM 2 over a majority of its surface. This helps reduce any reduction in PEMFC 1 performance due to the sensing layer 11. As described above, the PEM 2 comprises a sensing layer 11 on one side and the remaining layers of the PEMFC 1 can be flexibly selected depending on the fuel cell requirements. Thus, embodiments of the invention may simply comprise a PEM 2 comprising a sensing layer 11 as described above. The PEM 2 can then be integrated into existing fuel cells in place of membranes without sensing elements to allow the measurement of their properties. Furthermore, the PEM 2, sensing layer 11 and cathode catalyst layer 4 together form a cathode-coated-membrane assembly (CCM) 3. Thus, embodiments of the invention encompass such an assembly, whether the catalyst layer is a cathode catalyst layer or anode catalyst layer. Similarly, this CCM 3 with sensing elements could be used in place of existing non-sensing CCMs to allow the creation of fuel cells with sensing capabilities. In use, the PEMFC 1 operates in the usual way. The anode and cathode are electrically connected to a load such that electrical power can be drawn from the PEMFC 1. The reactants are supplied to the gas diffusion layers 5, 7 via the bipolar plates 8, for example Hydrogen is supplied to the anode and Oxygen to the cathode. The anode catalyst layer 4 splits the Hydrogen into protons and electrons. The protons diffuse through the PEM 2 to the cathode catalyst layer 4. At the cathode catalyst layer 5, protons combine with electrons and oxygen to form water while simultaneously generating an electric potential that is drawn as electrical power. Referring to Figure 4, to make measurements of oxygen concentration in the PEMFC 1 during operation, the PEMFC 1 is connected to a measurement apparatus 110 to form a system for measuring oxygen concentrations 100 comprising the PEMFC 1 and measurement apparatus 110. In this embodiment, the measurement apparatus 110 comprises a potentiostat 20 connected to each sensing element 12 and the anode. The potentiostat 20 is configured to maintain a constant voltage offset between each sensing element 12 and the anode. In this embodiment, the voltage offset is 0.4-0.45V. This ensures that the electrochemical reaction at each sensing element 12 is in a mass transport limited regime whereby the reaction speed and electricity generated is primarily dependent on the available oxygen. This limits the effect of competing reactions such as Hydrogen evolution on the measurement and differs from inside the cathode catalyst layer 4 where the reaction is typically limited by kinetic and ohmic effects rather than oxygen availability. The measurement apparatus 110 also comprises a multiplexer 21 and an ammeter 22. The multiplexer 21 is connected to the ammeter 22 as well as the anode and each sensing element 12 and is configured to sequentially connect the ammeter 22 to the anode and one of the sensing elements 12. The ammeter 22 is configured to measure a current between the anode and the sensing element 12 it is connected to via the multiplexer 21. The measurement apparatus 110 also comprises a processor 23, the processor is connected to the ammeter 22 and is operable to receive signals indicative of the current between the anode and a sensing element 12 and then perform chronoamperometric analysis to determine an oxygen concentration at the sensing element 12 based on the measured current as described below. Referring to Figure 5, in order to take measurements, a calibration curve is required to map the response of each sensing element 12 to the measured property, such as oxygen concentration. Starting at step SI, the potentiostat sets the voltage offset of the sensing elements 12 to the anode of 0.4-0.45V. At step S2, a calibration gas is introduced into the PEMFC 1 with a known oxygen concentration. At step S3, the multiplexer 21 sequentially connects the ammeter 22 to each of the sensing elements 12 and simultaneously the processor 23 records a response for each sensing element 12 via the ammeter 22. Steps S2 and S3 are then repeated for several other calibration gases with different known oxygen concentrations. Of course, in other embodiments, an alternative means of controlling the properties of the cell may be used, for example the temperature of the cell may be controlled to determine a response of each sensing element to temperature. Once enough calibration measurements have been taken, which is typically at least three, at step S4 the processor 23 generates a calibration curve for each sensing element 12 by fitting the data collected over steps S2 and S3. For example, the processor 23 may conduct linear regression or another suitable mathematical modelling technique to generate a calibration curve. Now that the calibration curves are generated, the system 100 is ready to take measurements during live operation of the PEMFC 1. At step S5, the PEMFC 1 is therefore turned on and put into use. The multiplexer 21 continuously cycles through the sensing elements 12, connecting and disconnecting them from the ammeter 22. The processor 23 continuously measures the oxygen concentration at each sensing element 12 based on the current measurements received from the ammeter 22. At step S6, the resulting data can be plotted or otherwise analysed, for example it may be plotted as a graph to show the temporal variation in oxygen concentration or as a map to show the spatial variation in oxygen concentration. This can help improve fuel cell design, for example by minimising local oxygen starvation across the PEM 2 / cathode interface. In some embodiments, it is also envisaged that two or more PEMFCs are stacked with respect to each other. A measurement apparatus may comprise a multiplexer that allows it to measure signals from sensing elements in all the PEMFCs and thereby take measurements across a PEMFC stack. This is facilitated by the efficient and non-invasive sensing layer design of the present invention. Referring to Figure 6, the PEMFC 1 may be manufactured as follows. At step Ml, a catalyst ink slurry containing 40% wt Pt / C catalyst and 700-900EW PFSA ionomer dispersion in a mixture of iso-Propyl Acrylate (IPA) and ultrapure water is prepared with an ionomerxarbon mass ratio in the range 0.7-0.9. The concentration of solids in this ink can be adjusted to yield the desired deposition layer thickness. At step M2, a conductive ink slurry in the form of a carbon ink slurry containing Vulcan XC-72 and 700-900 EW PFSA ionomer dispersion in a mixture of IPA and ultrapure water is prepared with an ionomerxarbon mass ratio in the range 0.7-0.9. The concentration of solids in the ink can be adjusted to yield the desired deposition layer thickness and also other substances, such as colloidal gold, can be used in place of carbon black. At step M3, an ionomer ink is prepared by diluting a 700-900EW PFSA ionomer dispersion in IPA and water. At step M4, the PEM 2 is provided or constructed according to known methods. The catalyst ink slurry is then printed on the PEM 2 to form the sensing elements 12, for example, by inkjet printing. At step M5, the carbon ink slurry is printed onto the PEM 2 to form the conductive tracks 13, also by inkjet printing. At step M6, the ionomer ink is printed onto the PEM 2 to form the film 14. Of course, in steps M4-M6 alternative printing methodologies may be employed such as printing onto a decal substrate such as Polytetrafluoroethylene (PTFE) or Polyethylene naphthalate (PEN) and then hot-pressing onto the PEM 2. Following step M6, the PEM 2 comprising sensing layer 11 is formed and is ready to be integrated into a PEMFC 1 in place of a regular non-sensing PEM. At step M7, the cathode catalyst layer 4 is deposited over the PEM 2 and film 14 using any standard method, for example ultrasonic spray coating, decal transfer, inkjet printing. Following step M7, the CCM comprising the sensing layer is formed, and is ready to be integrated into a PEMFC 1 in place of a regular non-sensing CCM. At step M8, the remainder of the PEMFC 1 layers are constructed around the CCM for example by spray coating the anode catalyst layer 5 and attaching the other layers. The one or more embodiments are described above by way of example only. Many variations are possible without departing from the scope of protection afforded by the appended claims.

Claims

1. A polymer electrolyte membrane (PEM) for use in an ion exchange membranefuel cell, the PEM comprising a sensing layer provided on a side of the PEM, the sensing layer comprising two or more sensing elements covered in an electrically insulating film and spaced across a plane of the PEM.

2. A PEM according to any preceding claim wherein lateral gaps are provided in the film between the sensing elements.

3. A PEM according to any preceding claim wherein the sensing elements are provided in direct contact with the PEM.

4. A PEM according to any preceding claim wherein the sensing elements comprise an ionomer.

5. A PEM according to any preceding claim wherein the sensing elements and / or film are deposited onto the PEM by printing.

6. A PEM according to any preceding claim comprising at least six sensing elements provided in an array across the plane of the PEM.

7. A PEM according to any preceding claim wherein the film has a surface area ofless than half the area of the PEM.

8. A PEM according to any preceding claim wherein the film is formed of an ion conducting material and extends around each sensing element to provide an ion path around each sensing element.

9. A PEM according to any preceding claim wherein the film has a thickness of no more than 10 microns, preferably 3 to 5 microns.

10. A PEM according to any preceding claim wherein each sensing element has a diameter of no more than 5 mm.

11. A PEM according to any preceding claim wherein each sensing element has a thickness of no more than 5 microns.

12. A PEM according to any preceding claim further comprising conductive tracks extending laterally from each sensing element to an edge of the PEM, wherein the film covers the conductive tracks.

13. A PEM according to claim 12 wherein the conductive tracks are printed onto the PEM.

14. A PEM according to any preceding claim wherein the sensing elements comprise an electrocatalyst material.

15. A PEM according to any preceding claim for use in a proton exchange membrane fuel cell.

16. A PEM according to claim 15 wherein the sensing elements comprise an electrocatalyst material active towards an oxygen reduction reaction.

17. A catalyst-coated-membrane assembly (CCM) for use in an ion exchange membrane fuel cell comprising a PEM according to any preceding claim and a catalyst layer provided over the sensing layer.

18. A CCM according to claim 17 wherein the catalyst layer is a cathode catalyst layer.

19. An ion exchange membrane fuel cell (IEMFC) comprising an anode, a cathode and a catalyst-coated-membrane according to claim 17 or 18 provided between the anode and the cathode.

20. A system for measuring the properties of an IEMFC, the system comprising: an IEMFC according to claim 19; and a measurement apparatus connected to the anode and at least one of the one or more sensing elements, and the measurement apparatus comprising an ammeter configured to determine a current flowing between the anode and a sensing element and a processor configured to receive a signal indicative of the current from the ammeter and calculate a property of the IEMFC.

21. A system according to claim 20 wherein IEMFC comprises two or more sensing elements and the measurement apparatus comprises a multiplexer configured tosequentially connect the measurement apparatus to each of the two or more sensing elements.

22. A system according to any of claims 20 or 21 comprising a stack of two or moreIEMFCs according to claim 19 wherein a measurement apparatus is provided 5 for each IEMFC, or the measurement apparatus is connected to all the IEMFCs,to allow for the measurement of properties in each IEMFC.

23. A method of manufacturing a polymer electrolyte membrane (PEM) for use in an ion exchange membrane fuel cell, the method comprising providing a PEM, arranging two or more sensing elements spaced apart on a side of the PEM, and 10 covering the two or more sensing elements with an electrically insulating filmto form a sensing layer.

24. The method of claim 24 further comprising printing the sensing elements onto the PEM and then printing the film over the sensing elements.

25. A method of manufacturing a catalyst-coated-membrane assembly for use in an 15 ion exchange membrane fuel cell, the method comprising forming a PEMaccording to the method of claims 23 or 24, or providing a PEM according to any of claims 1 to 16, and providing a catalyst layer over the side of the PEM containing the sensing layer.

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