Patent Publication Number: US-2016248112-A1

Title: Fuel cell with optimised operation along the air flow channel

Description:
The invention relates to fuel cell stacks, and in particular proton exchange membrane (PEM) fuel cell stacks. 
     Fuel cell stacks are envisioned as systems for supplying electricity to mass-produced automotive vehicles in the future, and for many other applications. A fuel cell stack is an electrochemical device that converts chemical energy directly into electrical power. Dihydrogen is used as fuel of the fuel cell stack. Dihydrogen is oxidized and ionized at an electrode of the stack and dioxygen from the air is reduced at another electrode of the stack. The chemical reaction produces water at the cathode, oxygen being reduced and reacting with the protons. The great advantage of the fuel cell stack is preventing releases of atmospheric pollutants at the electricity generation site. 
     Proton exchange membrane (PEM) fuel cell stacks have particularly advantageous properties of compactness. Each cell comprises an electrolytic membrane that allows only the passage of protons and not the passage of electrons. The membrane comprises an anode on a first face and a cathode on a second face in order to form a membrane electrode assembly (MEA). 
     At the anode, the dihydrogen is ionized in order to produce protons that pass through the membrane. The electrons produced by this reaction migrate toward a flow plate, then pass through an electrical circuit external to the cell in order to form an electric current. At the cathode, oxygen is reduced and reacts with the protons to form water. 
     The fuel cell stack may comprise several flow plates, for example made of metal, stacked on top of one another. The membrane is positioned between two flow plates. The flow plates may comprise flow channels and orifices in order to guide the reactants and the products to/from the membrane. The plates are also electrically conductive in order to form collectors for the electrons generated at the anode. Gas diffusion layers are inserted between the electrodes and the flow plates and are in contact with the flow plates. 
     The MEAs have a heterogeneous or non uniform operation over the length of the air and hydrogen flow channels. On the cathode side for example, the change in the relative humidity of the gases between the inlet (drying conditions) and the outlet (flooding conditions) of the flow channel has an effect on the heterogeneity of the current density. The current density is lower at the inlet of the flow channel due to an insufficient humidity. The current density is also lower at the outlet of the flow channel due to an excessive humidity that may flood the MEA. This heterogeneity of current density promotes degradation phenomena such as the localized corrosion of the carbon or the maturation of the catalyst. 
     Document US 2004/038808 describes a membrane electrode assembly structure. In this structure, the catalyst concentration of the cathode varies with a gradient along an axis. This document describes a homogeneous gas diffusion layer. 
     Document EP 1 176 654 describes a fuel cell stack structure in which a same electrode combines a catalytic layer and a gas diffusion layer, the properties of which vary in various zones. 
     Document U.S. Pat. No. 6,933,067 proposes producing a cathode having an increasing platinum loading from the air outlet up to the air inlet. Thus, a large amount of water is generated at the inlet of this flow channel in order to increase the current density thereof. Such a cathode is however relatively difficult to produce correctly on an industrial scale. 
     The invention aims to solve this drawback and to propose an alternative solution to this technical problem, while facilitating the precise positioning of a gas diffusion layer. The invention thus relates to a fuel cell stack as defined in the appended claims. The invention also relates to a process for manufacturing a fuel cell stack, as defined in the appended claims. 
    
    
     
       Other features and advantages of the invention will become clearly apparent from the description that is given thereof below, by way of nonlimiting illustration, and with reference to the appended figures, in which: 
         FIG. 1  is an exploded perspective view of an example of a fuel cell stack; 
         FIG. 2  is a top view of a flow plate comprising an example of a flow channel route; 
         FIG. 3  is a bottom view of a cathode gas diffusion layer according to one example of embodiment of the invention; 
         FIG. 4  is a bottom view of a membrane electrode assembly provided with a reinforcement, intended to be combined with the gas diffusion layer from  FIG. 3 ; 
         FIG. 5  is a cross-sectional view of one cell of a fuel cell stack according to one example of embodiment of the invention; 
         FIG. 6  is a graph illustrating the respective performances of a fuel cell stack according to the prior art and according to one embodiment of the invention; 
         FIG. 7  illustrates a sequence of steps of an example of a manufacturing process according to the invention. 
     
    
    
       FIG. 1  is a schematic exploded perspective view of a stack of cells  1  of a  10  fuel cell stack  2 . The fuel cell stack  2  comprises several superposed cells  1 . The cells  1  are of proton exchange membrane or polymer electrolyte membrane type. 
     The fuel cell stack  2  comprises a fuel source  120  supplying an inlet of each cell  1  with dihydrogen. The fuel cell stack  2  also comprises an air source  122  supplying an inlet of each cell with air, containing oxygen used as oxidant. Each cell  1   15  also comprises exhaust channels. Each cell  1  may also have a cooling circuit (illustrated in  FIG. 2 ). 
     Each cell  1  comprises a membrane electrode assembly  110 . The fuel cell stack  2  illustrated especially comprises a number of membrane electrode assemblies or MEAs  110 . A membrane electrode assembly  110  comprises an electrolyte  113 ,  20  cathode  112  (not illustrated in  FIG. 1 ) and an anode  111  which are placed on either side of the electrolyte and fastened to this electrolyte  113 . 
     Between each pair of adjacent MEAs, a pair of flow guides is positioned. The flow guides of each pair are firmly attached in order to form a bipolar plate  103 . Each flow guide is for example formed from a metal sheet, usually made of stainless steel.  25  A bipolar plate  103  thus comprises a metal sheet  102  oriented toward a cathode of an MEA  110  and a metal sheet  101  (not illustrated in  FIG. 1 ) oriented toward an anode of another MEA  110 . The metal sheets  101  and  102  have surfaces in relief defining flow channels  106  (not illustrated in  FIG. 1 ). The metal sheets  101  and  102  are firmly attached by welds  104 . 
     In a manner known per se, during the operation of the cell  1 , air flows between the MEA and the metal sheet  102 , and dihydrogen flows between the MEA and the metal sheet  101 . At the anode  111 , the dihydrogen is ionized in order to produce protons that pass through the MEA. The electrons produced by this reaction are collected by the metal sheet  102 . The electrons produced are then applied to an electrical load connected to the fuel cell stack  2  in order to form an electric current. At the cathode  112 , oxygen is reduced and reacts with the protons in order to form water. The reactions at the anode and the cathode are governed as follows: 
       H 2 →2H + +2 e   −  at the anode;
 
       4H + +4 e +O 2 →2H 2 O at the cathode.
 
     During its operation, one cell of the fuel cell stack usually generates a DC voltage between the anode and the cathode of the order of 1 V. The catalyst material used at the anode  111  or at the cathode  112  is advantageously platinum, for its excellent catalytic performance. 
       FIG. 2  is a top view of an example of a metal sheet  102  of a fuel cell stack  2 . The metal sheet  102  delimits flow channels  106 . The flow channels  106  extend between an air inlet duct  125  and a water outlet duct  126 . The metal sheet  102  is furthermore passed through by a coolant flow duct  124 . 
       FIG. 3  is a bottom view of an example of a gas diffusion layer  22  placed in contact with the metal sheet  102  and covering the flow channels  106 . The gas diffusion layer  22  comprises a first portion  24  and a second portion  25 . The first portion  24  covers a portion of the flow channels  106  from the air inlet  125 . The second portion covers a portion of the flow channels  106  from the water outlet  126 . 
     The portions  24  and  25  of the gas diffusion layer  22  are adjoining. The portions  24  and  25  are here two separate components, that adjoin at an interface  26 . The portions  24  and  25  are advantageously adjoining without overlapping, in order to avoid forming an overthickness at the interface  26 . 
     The portions  24  and  25  have different compositions. Thus, the composition of the portion  24  has a current density under dry conditions greater than that of the composition of the portion  25 . The portion  24  thus makes it possible to obtain a greater current density in the vicinity of the air inlet, at the start of the flow channel  106 , under drying conditions when only a little water has been generated in the flow channel  106 . The portion  24  extends for example between 15 and 50% of the length of the flow channel  106  from the air inlet. The composition of the portion  25  has a current density under wet conditions greater than that of the composition of the portion  24 . The portion  25  extends for example between 50 and 85% of the length of the flow channel  106  From the water outlet. The median portion of the flow channel  106 , in which the humidity level is intermediate, thus benefits from the composition of the portion  25 . 
     A person skilled in the art will be able to determine more precisely the distribution of the portions  24  and  25  over the length of the flow channel  106  with an acquisition card for acquiring the localized currents that is positioned in the stack of the cells  1 , with a prior test on a uniform gas diffusion layer  21 . Such a card makes it possible in particular to determine the zones in which the current density is lower, in order to determine up to where the portion  25  should extend. 
     Tests have in particular been carried out with a current acquisition card having a 20×24 matrix, each element of the matrix having a surface area of 0.45 cm 2 . 
     The portion  24  may be formed from a gas diffusion layer sold by Freudenberg FCCT under the commercial reference H2 415-I2-C3. The portion  25  may be formed from a gas diffusion layer sold by SGL Group under the commercial reference 24BC.  FIG. 6  is a graph that compares the respective polarization curves of fuel cell stacks R and I. The fuel cell stack R includes a gas diffusion layer consisting solely, and as  20  one piece, of the layer sold by SGL Group under the commercial reference 24BC. The fuel cell stack I includes a gas diffusion layer having a portion  24  of Freudenberg H2 415-I2-C3 type and a portion  25  of SGL Group 24BC type. A substantial increase in the average current density and a homogenization of this density are noted irrespective of the operating conditions. 
     The dry conditions are for example determined for a relative humidity of 20%. The wet conditions are for example determined for a relative humidity of 100%. 
     In order to facilitate a satisfactory positioning between the first and second portions  24  and  25 , these advantageously have adjoining edges of complementary and non-rectilinear shapes, as illustrated in the example from  FIG. 3 . 
       FIG. 4  is a bottom view of an example of a reinforcement  132  that proves particularly advantageous within the context of the invention. The reinforcement  132  is fastened to a membrane electrode assembly  103 . The reinforcement  132  comprises a first median opening  134  and a second median opening  135 . These median openings  134  and  135  are separated by a strip  133 . The median openings  134  and  135  reveal a portion of the cathode  112 . The reinforcement  132  is additionally passed through by the air inlet duct  125 , by the water outlet duct  126  and by the coolant flow duct. 
       FIG. 5  is a cross-sectional view of one cell  1  of the assembled fuel cell stack  2 . 
     A reinforcement  131  is fastened to the membrane electrode assembly. The reinforcement  131  comprises an inner edge which covers the periphery of the anode  111 . The inner edge is firmly attached to the anode  111 . The reinforcement  131  extends beyond the periphery of the anode  111  and forms an overlap on the membrane  113 . The reinforcement  131  is firmly attached to the membrane  113 . The firm attachment of the reinforcement  131  to the anode  111  and to the membrane  113  may be achieved by any suitable means, for example by hot pressing or by printing of the anode  111  on the reinforcement  131 . The reinforcement  131  comprises a median opening. This median opening reveals the median portion of the anode  111 . 
     The gas diffusion layer  21  is compressed between the anode  111  and the metal sheet  101 . The gas diffusion layer  21  thus crosses the median opening of the reinforcement  131  and is in contact with the anode  111 . 
     The reinforcement  132  is fastened to the membrane electrode assembly and to the reinforcement  131 . The reinforcement  132  comprises inner edges which cover the periphery of the cathode  112 . The inner edges are firmly attached to the cathode  112 . The reinforcement  132  extends beyond the periphery of the cathode  112  and forms an overlap on the membrane  113 . The reinforcement  132  is firmly attached to the membrane  113 . The reinforcements  131  and  132  are fastened to one another at their periphery. 
     The portion  24  of the gas diffusion layer  22  is compressed between the cathode  112  and the metal sheet  102 . The portion  24  thus crosses the median opening  134  of the reinforcement  132  and is in contact with the cathode  112 . The portion  25  of the gas diffusion layer  22  is compressed between the cathode  112  and the metal sheet  102 . The portion  25  thus crosses the median opening  135  of the reinforcement  132  and is in contact with the cathode  112 . The interface  26  between the portions  24  and  25  is superposed on the strip  133  separating the openings  134  and  135 . The risk of asperities potentially present at the edges of the portions  24  and  25  impairing or even piercing the cathode  112  or the membrane  113  is thus avoided. It is possible to avoid an additional component in the cell  1 , by using a strip  133  formed as one piece with the reinforcement  132  already used. 
     Seals  23  may be positioned around the gas diffusion layers  21  and  22 , in order to guarantee the sealing between the reinforcement  131  and the metal sheet  101  or the sealing between the reinforcement  132  and the metal sheet  102 . 
     The gas diffusion layer  22  is compressed between the cathode  112  and the metal sheet  102 . Under this compression, the first and second portions  24  and  25  of the gas diffusion layer  22  have the same thickness, in order to limit the deformations and heterogeneities of the stack of cells  1  and to prevent possible sealing problems at the periphery of the stack. The first and second portions  24  and  25  may have different thicknesses in the absence of compression and be sized as a function of their modulus of elasticity in order to have the same thickness when they are subjected to the compression of the cell  1 . 
     For a compression of 1 MPa after assembly, the portions  24  and  25  advantageously have a thickness of around 190 μm±40 μm. 
       FIG. 7  illustrates a sequence of several steps of an example of a process for manufacturing one cell  1  of a fuel cell stack  2  according to one embodiment of the invention. 
     A reinforcement  132  is provided in step  301 . The reinforcement  132  is advantageously flat. The reinforcement  132  has for example precut contours corresponding to the openings  134  and  135  to be formed, these contours being separated by the strip  133 . 
     In step  302 , an electrocatalytic ink is deposited in the liquid phase, which is intended to form the cathode  112  after drying. The cathode  112  may be solidified by any suitable means. The cathode  112  formed extends beyond the precut contours. Thus, a superposition is created between inner edges of the reinforcement  132  and the periphery of the cathode  112 . The anode  111  may be formed in a similar manner on a reinforcement  131  having a precut contour that corresponds to its median opening. 
     The electrocatalytic material has catalytic properties suitable for the catalytic reaction to be carried out. The electrocatalytic material may be in the form of particles or nanoparticles containing metal atoms. The catalyst material may in particular comprise metal oxides. The electrocatalytic material may be a metal such as platinum, gold, silver, cobalt or ruthenium. 
     In step  303 , a membrane electrode assembly is produced by firmly attaching on one hand the reinforcement  132  and the cathode  112  to one face of a membrane  113 , and by firmly attaching on the other hand the reinforcement  131  and the anode  111  to another face of the membrane  113 . A reinforcement and an electrode may thus be firmly attached to the membrane  113  during a same hot-pressing step. 
     In order to promote the adhesion of the electrodes to the membrane  113  during a hot-pressing step, the membrane  113  and the electrodes advantageously comprise the same polymer material. This polymer material advantageously has a glass transition temperature below the hot-pressing temperature. The polymerizable material used to form this polymer material could be the ionomer sold under the commercial reference Nafion DE2020. 
     In step  304 , the portions inside the precut contours of the reinforcements  131  and  132  are removed. The median openings of the reinforcements  131  and  132  are thus made, so as to reveal the respective median portions of the anode  111  and of the cathode  112 . Reinforcements were thus formed from supports for the deposition of an electrocatalytic ink. 
     In step  305 , it is possible to form the ducts  124 ,  125  and  126  by cuts through the periphery of the stacks of layers produced. 
     In step  306 , the gas diffusion layers  21  and  22  are provided. The gas diffusion layer  21  is thus placed in contact with the revealed portion of the anode  111 , through the opening of the reinforcement  131 . The periphery of the gas diffusion layer  21  covers the inner edge of the reinforcement  131 . The portions  24  and  25  of the gas diffusion layer  22  are placed in contact with the revealed portions of the cathode  112 , through the openings  134  and  135 . The periphery of the gas diffusion layer  22  covers the inner edge of the reinforcement  132 . 
     In step  307 , in order to obtain the fuel cell stack cell  1  illustrated in  FIG. 5 , the membrane electrode assembly provided with the gas diffusion layers  21  and  22  may then be included between two metal flow guide sheets  101  and  102 .