Fuel cell with bipolar plate provided with a grid and associated manufacturing process

Abstract

The invention relates to a fuel cell comprising a first bipolar plate (BP1) provided with channels, a first electrode (AND) facing the channels of the first bipolar plate, a second bipolar plate (BP2) provided with channels, a second electrode (CTH) facing the channels of the second bipolar plate, and a central proton exchange membrane (MBN) arranged between the first electrode (AND) and the second electrode (CTH), characterized in that the fuel cell comprises at least one grid (GR1, GR2) which has a diamond pattern and is arranged on one of the first bipolar plate (BP1) and the second bipolar plate (BP2). The invention also relates to an associated manufacturing process.

Description

[0001] DESCRIPTION

[0002] TITLE: BIPOLAR PLATE FUEL CELL EQUIPPED WITH A GRID AND ASSOCIATED MANUFACTURING METHOD

[0003] Technical field of the invention

[0004] The invention relates to the field of energy production by electrolysis and more specifically to energy conversion by fuel cell.

[0005] Technical background

[0006] The PEMFC (for “Proton Exchange Membrane Fuel Cells”) is a fuel cell that contains several components assembled symmetrically on either side of a central MBN membrane, as shown in Figure 1.

[0007] On each side of the MBN membrane, the PEMFC comprises a bipolar plate PB1, PB2 equipped with CNX channels which aim to circulate and distribute a reactive gas (H2, O2) towards the center of the cell. The bipolar plate BP1, BP2 also serves to collect the electrons produced by the reaction occurring in the cell. The PEMFC also comprises a gas diffusion layer GDL, arranged opposite the channels of the bipolar plate BP, made of an (extremely) porous material to allow this diffusion towards the interior of the cell. The PEMFC also comprises a catalyst layer CL which is generally composed of electrochemically active platinum nanoparticles to promote the chemical reaction. The combination of a GDL gas diffusion layer and a CL catalyst layer forms a PEMFC electrode where, depending on the case, either the oxidation reaction (ROH) of hydrogen (anode, AND) or the reduction reaction of oxygen (ORR) (cathode, CTH) takes place.Finally, at the center of the PEMFC, the MBN membrane is a proton exchange membrane, which transports only protons - while the recovered electrons pass through an external electrical circuit to generate electricity. The membrane faces each CL catalyst layer of an electrode.

[0008] Below are the main reactions taking place within the PEMFC.

[0009] 1) Oxidation (anode):

[0010] H2^ 2H + + 2nd- (ROH)

[0011] 2) Reduction (cathode):

[0012] 1 / 2O2+ 2H + + 2e- H2O (ORR)

[0013] 3) Overall reaction:

[0014] H2 + 1 X O2 — > H2O + electricity + heat We are constantly seeking to improve the performance of a fuel cell, in particular its efficiency.

[0015] Also, an objective of the invention is to propose a fuel cell exhibiting improved performance, in particular in terms of efficiency.

[0016] Summary of the invention

[0017] To solve the aforementioned objective, the invention proposes a fuel cell comprising a first bipolar plate provided with channels, a first electrode arranged opposite the channels of the first bipolar plate, a second bipolar plate provided with channels, a second electrode arranged opposite the channels of the second bipolar plate and a central proton exchange membrane arranged between the first electrode and the second electrode characterized in that the fuel cell comprises at least one grid, having a diamond structure, covering the channels of the first bipolar plate or the channels of the second bipolar plate.

[0018] This improves the distribution of reactive gases towards the center of the fuel cell.

[0019] The fuel cell according to the invention may comprise at least one of the following additional characteristics, taken alone or in combination: the fuel cell comprises a first grid, having a diamond structure, covering the channels of the first bipolar plate and a second grid, having a diamond structure, covering the channels of the second bipolar plate; said at least one grid is made of a gold-coated stainless steel, or of graphite.

[0020] The invention also proposes a method for manufacturing a fuel cell according to the invention, said method comprising an additive manufacturing step for manufacturing said at least one grid.

[0021] In the process according to the invention, additive manufacturing may be carried out by laser fusion on a powder bed, for example selective laser fusion or selective laser sintering.

[0022] Brief description of the figures

[0023] Other objects and characteristics of the invention will appear more clearly in the following description, made with reference to the appended figures, in which:

[0024] [Fig. 2] represents a PEMFC according to the invention, according to an exploded view; [Fig. 3a] and [Fig. 3b] represent, respectively, a bipolar plate according to the prior art and a bipolar plate confirming to the invention;

[0025] [Fig. 4a], [Fig. 4b] and [Fig. 4c] represent, respectively, a sectional view in a first plane of a bipolar plate equipped with its grid, a front view of this same bipolar plate thus allowing the grid to be viewed from the front, and a sectional view in a second of this same plate at the level of the grid;

[0026] [Fig. 5] represents, in three dimensions, an elementary cell of a diamond-structured grid;

[0027] [Fig. 6] represents, in a front view, a grid with a honeycomb structure;

[0028] [Fig. 7] represents polarization curves for a fuel cell according to the prior art on the one hand and for a fuel cell according to an exemplary embodiment in accordance with the invention on the other hand.

[0029] Detailed description of the invention

[0030] The invention relates to a fuel cell (PEMFC) comprising a first bipolar plate BP1 provided with channels intended to circulate a reactive gas, a first electrode AND (anode) arranged opposite the channels of the first bipolar plate, a second bipolar plate BP2 also provided with channels intended to circulate a reactive gas, a second electrode CTH (cathode) arranged opposite the channels of the second bipolar plate BP2 and a central proton exchange membrane MBN arranged between the first electrode AND and the second electrode CTH.

[0031] Furthermore, the fuel cell according to the invention further comprises at least one grid GR1, GR2 covering the channels of the first bipolar plate BP1 or the channels of the second bipolar plate BP2 (see figure 3b).

[0032] Reference may be made to Figure 2 for a representation of a fuel cell according to the invention.

[0033] Advantageously, the fuel cell according to the invention comprises a first grid GR1 covering both the channels of the first bipolar plate BP1 and a second grid GR2 covering the channels of the second bipolar plate BP2.

[0034] Advantageously also, said at least one grid GR1, GR2 is a grid with a diamond-type architecture. When a grid GR1, GR2 is provided for each bipolar plate PB1, PB2 of the fuel cell, they can both be made with a diamond structure. By "diamond architecture" or "diamond structure", we refer to the structure in which the different carbon atoms of a diamond are arranged and connected in space. We can refer to: https: / / fr.wikipedia.org / wiki / Structure_diamant.

[0035] A diamond structure is shown in Figures 4a, 4b and 4c.

[0036] It will be described in more detail with the support of these figures in the context of the example of realization which will be presented.

[0037] However, it should be noted that the presence of a grid will generally induce a better distribution of gases and a better collection of electrons (collection surface increased with the grid), and ultimately result in a better efficiency. In the particular case of a diamond structure grid, the specific surface is particularly high because the diamond structure has orifices in different directions of space.

[0038] Furthermore, an elementary cell for the diamond structure can be provided with dimensions between 1 and 3 mm and strands with dimensions between 200 and 600 microns. In Figure 5, which represents in three dimensions an elementary cell of a diamond structure, we provide the case (example) of an elementary cell with dimensions of the order of 1 mm and a strand of 200 microns (transverse dimensions).

[0039] Of course, in order to improve the performance of a fuel cell, a diamond structure grid is not the only option. Other structures can be considered for the grid. Thus, as another example, it is possible to consider a GR1, GR2 grid with a honeycomb type structure. This is what has been shown in Figure 6, according to a front view (YZ plane) of a bipolar plate. In Figure 6, a honeycomb elementary cell is typically 1mm and a strand typically 200 microns. More generally, we can provide an elementary cell of the honeycomb with dimensions between 1 and 3mm and strands with dimensions between 200 and 600 microns. We are therefore here for the honeycomb structure on a dimensioning similar to that of the diamond structure.

[0040] Furthermore, the or each grid GR1, GR2 can be made of stainless steel, for example type 316L, covered with gold (with a bonding layer for example made of chrome), or of another material such as graphite. The objective is to have a material constituting the grid which is a good electrical conductor. In particular, in the case of stainless steel, as an oxide layer generally forms on the surface, polishing is carried out to remove the oxide layer and gold is deposited in a thin layer (which does not oxidize while also providing a completely negligible contact resistance) around the stainless steel. In the case of graphite, the material can remain in the state without deposition of an additional thin layer. The invention also relates to a method for manufacturing a fuel cell (PEMFC) according to the invention comprising an additive manufacturing step for manufacturing said at least one grid.Additive manufacturing can be carried out using a laser powder bed fusion technique (sometimes referred to in the literature as L-PBF), for example selective laser melting (SLM) or selective laser sintering (SLS).

[0041] We will now present a concrete example of implementation.

[0042] In this example, a single-piece bipolar plate is manufactured with a grid covering the channels intended to circulate the reactive fluids by laser fusion on a powder bed.

[0043] A stainless steel powder (316L) that meets the characteristics of the powders used for the L-PBF process (spherical powders, particle size 15-45 pm) was incorporated into a TruPrint 1000 machine with a Ytterbium-doped fiber laser (wavelength 1064 nm, laser spot 30 pm).

[0044] The part was manufactured vertically on a plate.

[0045] The possible parameters for manufacturing are the following: laser power: 50 W and 200 W, for example 120 W,

[0046] - scanning speed 300 mm / s to 3000 mm / s, for example 700 mm / s, layer thickness: 10 pm to 100 pm, for example 30 pm,

[0047] - inter-cord distance: 10 pm to 150 pm, for example 70 pm,

[0048] The pattern chosen for the grid of the PB bipolar plate is based on a diamond structure.

[0049] Once the diamond structure is obtained by L-PBF, other operations are then necessary due to the choice of a stainless steel to make the grid, this steel in fact forming a layer of oxides on the surface. However, the presence of such a layer is detrimental to the performance of the fuel cell, in particular for the collection of electrons by the bipolar plate.

[0050] Also, polishing of the bipolar plate was carried out with silicon carbide paper (SiC, 15 pm grain size) for 30s, followed by rinsing with ethanol. The oxide present is then removed.

[0051] Then, a thermal evaporation deposition of chromium was carried out, in a thin layer (20 to 50 nm) on the surface of the bipolar plate. This was followed by a thermal evaporation deposition of gold, also in a thin layer (70 to 200 nm) to cover the chromium. The thin layer of chromium serves as a bonding layer for the gold on the stainless steel (316L).

[0052] These steps (polishing, deposits) are not necessary when choosing a powder made of a material that does not form an oxide layer on the surface, after manufacturing by L-PBF.

[0053] Figure 3a shows a conventional bipolar plate (prior art) with the circulation channels for the reactive gases (in this case, dimensions of 7cm*7cm with a thickness of 0.8cm). Figure 3b shows a bipolar plate obtained with the implementation of the method according to the invention, with the grid (in this case, dimensions of 4.2cm*4.2cm with a thickness of 0.5mm) in diamond-type structure covering the circulation channels for the reactive gases.

[0054] A diagram of the grid obtained in figure 3b is made in support of figures 4a, 4b and 4c, according to three views defined with the three directions X, Y, Z of space.

[0055] Figure 4a is a representation of the bipolar plate with its diamond-structured grid in a cross-section (XY plane where the X axis defines the direction of the thickness of the bipolar plate, and therefore of the grid). Figure 4b is a front view of the grid (YZ plane) in diamond structure and corresponds to the view in Figure 2a. Finally, Figure 4c is another cross-section of the diamond-structured grid (XZ plane).

[0056] Figure 4b shows that the diamond structure grid has through holes in the X direction. Figure 4c shows that the diamond structure grid also has holes in the Y direction. This is even more visible in Figure 5, which is a three-dimensional representation of the diamond structure grid at the level of an elementary cell of the grid. More precisely, Figure 5 represents an elementary cell of the diamond structure such that (X,Y,Z) = (1mm, 1mm, 1mm), in this case and in the context of the example presented a strand size of 200 pm and a maximum ferret of approximately 600 pm.

[0057] To be able to accurately compare the results obtained with a PEMFC fuel cell comprising bipolar plates each equipped with a diamond structure grid thus manufactured on the one hand (invention, INV) with the results obtained with a fuel cell not comprising such a grid (figure 2a, conventional bipolar plate, prior art, AA) on the other hand, the treatment mentioned above (polishing, deposition of chromium then gold) was also carried out for the conventional bipolar plates obtained of course with the same L-PBF process under the same conditions and the same powder as in the present example. The only difference therefore lies in the presence of the diamond structure grid. Figure 7 represents the polarization curves obtained for the invention INV and the prior art AA. The representation of a polarization curve is the most common approach for testing the performance of a fuel cell.A polarization curve represents the evolution of the potential (ordinates) as a function of the current density (abscissa). A higher current density at the same potential characterizes a more efficient fuel cell, and therefore one with a higher efficiency. As can be seen in Figure 7, for example, for a potential of 0.6V, there is an improvement in current density of approximately 27%.

Claims

CLAIMS 1. Fuel cell comprising a first bipolar plate (BP1) provided with channels, a first electrode (AND) arranged opposite the channels of the first bipolar plate, a second bipolar plate (BP2) provided with channels, a second electrode (CTH) arranged opposite the channels of the second bipolar plate and a central proton exchange membrane (MBN), arranged between the first electrode (AND) and the second electrode (CTH) characterized in that the fuel cell comprises at least one grid (GR1, GR2), having a diamond structure, covering the channels of the first bipolar plate (BP1) or the channels of the second bipolar plate (BP2).

2. Fuel cell according to claim 1, characterized in that it comprises a first grid (GR1), having a diamond structure, covering the channels of the first bipolar plate (BP1) and a second grid (GR2), having a diamond structure, covering the channels of the second bipolar plate (BP2).

3. Fuel cell according to one of the preceding claims, characterized in that said at least one grid (GR1, GR2) is made of gold-coated stainless steel, or of graphite.

4. Method for manufacturing a fuel cell according to one of the preceding claims, characterized in that it comprises an additive manufacturing step for manufacturing said at least one grid.

5. Method according to the preceding claim, in which the additive manufacturing is done by laser powder bed fusion, for example selective laser melting (SLM) or selective laser sintering (SLS).

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