Patent Application
20160248112
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:
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 115 also comprises exhaust channels. Each cell 1 may also have a cooling circuit (illustrated in
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
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
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:
H2→2H++2e− at the anode;
4H++4e+O2→2H2O 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.
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 cm2.
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.
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
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.
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
| Number | Date | Country | Kind |
|---|---|---|---|
| 1360644 | Oct 2013 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2014/052683 | 10/22/2014 | WO | 00 |
