GAS DIFFUSION LAYER COMPRISING COMPOSITE CARBON FIBERS, AS WELL AS METHOD FOR OBTAINING SAME AND ITS USES

Abstract

The present invention relates to a gas diffusion layer comprising composite carbon fibers, as well as a method for obtaining it and its uses. (no figure)

Description

Title of the invention: GAS DIFFUSION LAYER COMPRISING COMPOSITE CARBON FIBERS, AS WELL AS METHOD FOR OBTAINING SAME AND ITS USES

[0001] The present invention relates to a gas diffusion layer comprising composite carbon fibers, as well as a method for obtaining it and its uses.

[0002] Fuel cells provide a more environmentally friendly way to generate electricity for a variety of applications. They also offer greater energy efficiency, longer range, better durability, and greater reliability than other energy sources, such as combustion engines and batteries.

[0003] Fuel cells are electrochemical devices that directly convert chemical energy stored in a fuel, usually hydrogen, into electricity, producing water as a by-product.

[0004] Among the currently available fuel cells, proton exchange membrane fuel cells (PEMFCs) have become a relevant choice for power generation, particularly when used for transportation and handling applications. In the future, fuel cells may be the first choice for powering vehicles.

[0005] Proton exchange membrane fuel cells have several advantages over other energy production systems: - PEMFCs have a high efficiency in converting hydrogen into electricity, generally greater than 50%, contributing to a more efficient use of energy resources; - the chemical reaction taking place in PEMFCs produces only water as a by-product, eliminating direct emissions of greenhouse gases and air pollutants. This makes it a clean and environmentally friendly solution; - PEMFCs have the ability to start up quickly and respond quickly to changes in power demand. This makes them suitable for applications where rapid response is essential, such as in electric vehicles; - PEMFCs are relatively compact and lightweight compared to other power generation technologies, making them suitable for various mobile applications, such as electric vehicles and electrical appliances. Portable ironics; - PEMFCs operate at relatively low temperatures compared to other types of fuel cells, which simplifies their thermal management and makes them more suitable for certain applications; - PEMFCs have demonstrated longer lifetimes compared to other fuel cell technologies, which helps reduce maintenance costs and increase reliability.

[0006] PEMFCs operate on the principle of electrolysis, where hydrogen is separated into protons (H+ ions) and electrons at the anode. The protons pass through a proton exchange membrane (PEM) to the cathode, while the electrons follow an external circuit, creating an electric current. At the cathode, the protons, electrons, and oxygen react to form water.

[0007] The gas diffusion layer (GDL) is a crucial component in the structure of PEMFCs. It is a porous structure, generally placed between the bipolar plate and the fuel cell electrode. It allows: - uniform distribution of hydrogen on the anodic side and oxygen (or air) on the cathodic side on the surface of the electrodes. This ensures an efficient and homogeneous chemical reaction; - electrical conduction, allowing electrons generated during the reaction at the anode to move easily to the external electrical circuit; - the evacuation of water produced during the chemical reaction. This prevents the accumulation of water, thus ensuring constant performance of the PEMFC; - to provide structural support to the catalyst layers and the proton exchange membrane, contributing to the mechanical stability of the fuel cell.

[0008] The substrate (sometimes called Diffusion Medium) of the diffusion layers can be a porous material made of electronically conductive carbon fibers and is in the form of paper, fabrics or felts depending on the manufacturing process used. This substrate is then generally treated to be hydrophobic by soaking in a PTFE solution. The substrate sometimes has a microporous layer (MPL: Micro Porous Layer) placed between the fiber mat and the active layer. Thus, the hydrophobicity of these carbon fibers is obtained by fluorination. However, these fluorinated compounds are currently criticized for their toxicity and their extreme persistence in the environment and have been widely identified in wildlife and humans throughout the world.

[0009] The aim of the invention is to provide non-fluorinated materials and their methods of obtaining which avoid the aforementioned drawbacks.

[0010] Another objective of the invention is to enable the production of gas diffusion layers comprising these materials, and which, in particular when used in fuel cells proton exchange membrane fuel, allowing improved oxygen transport to the active sites, reducing the flooding of these GDLs by water produced by the cell in operation, while also maintaining electrical conductivity.

[0011] Thus, according to a first aspect, the invention relates to a composition consisting of or comprising carbon fibers with a diameter ranging from 350 to 1000 nm, and carbon black at a level of 3 to 8% by mass relative to the total mass of carbon fibers and carbon black.

[0012] The diameter of the carbon fibers can be measured by one of the techniques well known to those skilled in the art, for example by scanning electron microscopy (SEM).

[0013] By “diameter” is meant in particular the median diameter of the carbon fibers, which can in particular be determined by analysis of the 3D structures.

[0014] In particular, the 3D structures can be analyzed using image analysis software, for example GeoDict. These structures have for example been obtained by X-ray nanotomography (xCT for X-Ray Computed Tomography). The median diameters can in particular be determined on 2 or 3 acquisitions for each of the materials and averaged.

[0015] According to a particular embodiment, the diameter of the carbon fibers is from 500 to 1000 nm, in particular from 550, 600 or 650 to 1000 nm. According to a particular embodiment, the invention relates to a composition as described above, which has a porosity of from 87 to 97%.

[0016] According to a more particular embodiment, the invention relates to a composition as described above, which has a porosity of from 87 to 94, 95 or 96%.

[0017] By "porosity" is meant in particular the ratio between the volume of the voids and the total volume of the composition (i.e. in particular the total volume occupied by the composition), its value thus being between 0 and 1 (or, as a percentage, between 0 and 100%).

[0018] Porosity can be measured by one of the techniques well known to those skilled in the art, for example by analysis of 3D structures.

[0019] In particular, 3D structures can be analyzed using image analysis software, for example GeoDict. These structures have for example been obtained by X-ray nanotomography (xCT for X-Ray Computed Tomography). The porosity corresponds in particular to the number of voxels corresponding to the solid material out of the total number of voxels of the structure. Median values ​​can in particular be determined on 2 or 3 acquisitions for each of the materials and averaged.

[0020] According to a particular embodiment, the invention relates to a composition as described above, which comprises pores whose radius is from 0.8 to 1.5 μm.

[0021] According to a more particular embodiment, the invention relates to a composition as described above, which comprises pores whose radius is from 1.0 to 1.4 μm.

[0022] The radius of said pores can be measured by one of the techniques well known to those skilled in the art, for example by analysis of 3D structures.

[0023] In particular, the 3D structures can be analyzed using image analysis software, for example GeoDict. These structures have for example been obtained by X-ray nanotomography (xCT for X-Ray Computed Tomography). The radius can for example be determined by numerically simulating a MICP (Mercury Intrusion Capillary Pressure) experiment, i.e. an intrusion of mercury into the structure. In particular, the relationship between the volume of mercury introduced for a pressure P gives an associated pore radius R (Washburn relationship). Median values ​​can in particular be determined over 2 or 3 acquisitions for each of the materials and averaged.

[0024] By carbon black is meant in particular a powdery composition of carbon in amorphous form, which is in particular in the form of a powder consisting of or comprising spherical or spheroidal particles of 5 to 500 nm (largest dimension), in particular less than 15 nm.

[0025] According to a particular embodiment, the invention relates to a composition as defined above, which is free of polyvinylpyrrolidone (PVP).

[0026] According to a particular embodiment, the invention relates to a composition as defined above, which is free of polyacrylonitrile (PAN).

[0027] According to a particular embodiment, the invention relates to a composition as defined above, which is free of fluorinated compound.

[0028] According to a particular embodiment, the invention relates to a composition as defined above, which is free of PVP, PAN, and / or fluorinated compound.

[0029] According to another aspect, the present invention also relates to a process for preparing a composition A comprising carbon fibers, and carbon black, said process comprising the following steps: i. A step of mixing using a ball mill a composition B comprising polyacrylonitrile (PAN), and carbon black (CB) at a level of 0.5 to 50% by mass relative to the total mass of PAN and carbon black; ii. A step of electrospinning composition B as obtained at the end of the previous step to obtain a composition C; iii. A step of heat treatment of composition C as obtained at the end of the previous step, comprising the following sub-steps: a. A heating step, in particular at a temperature of between 180 to 300°C, in particular to about 240°C, and / or for 1 to 3 hours, in particular for about 2 hours; b. A pre-carbonization step, in particular at a temperature of 600 to 800°C, in particular at approximately 700°C, and / or for 45 minutes to 2 hours, in particular for approximately 1 hour; c. A carbonization step, in particular at a temperature of 1000 to 3000°C, in particular at approximately 1500°C, and / or for 45 minutes to 2 hours, in particular for approximately 1 hour;

[0030] to obtain said composition A.

[0031] According to a particular embodiment, composition A is a composition as defined previously, consisting of or comprising carbon fibers and carbon black at a level of 3 to 8% by mass relative to the total mass of carbon fibers and carbon black.

[0032] According to a particular embodiment, composition A is a composition as defined previously, consisting of or comprising carbon fibers with a diameter ranging from 350 to 1000 nm, and carbon black at a level of 3 to 8% by mass relative to the total mass of carbon fibers and carbon black.

[0033] All embodiments defined previously in relation to a composition also apply here, alone or in combinations.

[0034] According to a particular embodiment, composition A has a porosity of 87 to 97%.

[0035] According to a particular embodiment, composition A comprises pores whose radius is from 0.8 to 1.5 μm.

[0036] According to a particular embodiment, the carbon black is present in composition B at a rate of 1 / 200 to 1 / 5, in particular 1 / 200 to 1 / 10, in particular 1 / 200 to 1 / 20, by mass relative to the total mass of PAN and carbon black.

[0037] According to a particular embodiment, the carbon black is present in composition B at a rate of 1 / 100 to 1 / 5, in particular 1 / 100 to 1 / 10, in particular 1 / 100 to 1 / 20, by mass relative to the total mass of PAN and carbon black.

[0038] According to a particular embodiment, the carbon black is present in composition B at a rate of 1 / 50 to 1 / 5, in particular 1 / 50 to 1 / 10, in particular 1 / 50 to 1 / 20, by mass relative to the total mass of PAN and carbon black.

[0039] According to a particular embodiment, composition B is a suspension of black of carbon in a solution B' of PAN in a solvent, in particular chosen from N,N-dimethylformamide (DMF), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), ethanol, for example DMF.

[0040] According to a particular embodiment, the concentration of PAN in solution B' is from 5 to 15%, in particular approximately 10% by mass.

[0041] Ball mills are well known to those skilled in the art. It may, for example, be a cylindrical container comprising, in addition to composition B, balls, for example zirconium balls, which cylindrical container is rotated around its axis.

[0042] According to a particular embodiment, step (i) is carried out for a period of 10 to 30 hours, in particular for approximately 10 hours.

[0043] According to a particular embodiment, step (i) is followed, prior to step (ii), by a sonication step.

[0044] Electrospinning (or electrostatic spinning) is a fiber production method that uses electrical force to pull charged polymer strands in solution or melt to diameters in the hundreds of nanometers.

[0045] When a sufficiently high electrical voltage is applied to a liquid droplet, the body of the liquid becomes charged, and the electrostatic repulsion force then opposes the surface tension forcing the droplet to stretch; at a critical point, a stream of liquid erupts from the surface. This point of the eruption is called a "Taylor cone". If the molecular cohesion of the liquid is sufficiently high, there is no break in the stream and a charged jet of liquid is formed (if a break occurs, droplets are electrosprayed).

[0046] As the jet dries in flight, the current flow pattern changes from ohmic to convective as charges migrate to the fiber surface. The jet is then elongated by a process caused by electrostatic repulsion initiated at small folds on the fiber, until it is finally deposited on the collector ground. The elongation and thinning of the fiber resulting from this bending instability leads to the formation of uniform fibers with diameters in the nanometer range.

[0047] Electrospinning is well known to those skilled in the art. They will be able to implement it without difficulty according to their general knowledge, for example as described by Bin Ding et al. (Electrospinning: nanofabrication and applications William Andrew, San Diego, 2018).

[0048] Electrospinning can for example be carried out: - By applying a voltage to the nozzle of 15kV to 25kV, for example approximately 19 kV; - By applying a voltage to the collector from OkV to -12kV, for example approximately -10 kV; - With a flow rate of ImL / h to 2mL / h, for example approximately 1.5 mL / h; and / or - For 4 to 8 hours, for example for about 6 hours.

[0049] According to another aspect, the present invention also relates to a composition comprising carbon fibers and carbon black, capable of being obtained according to the process as defined above.

[0050] According to another aspect, the present invention also relates to the use of a composition as defined above, as a gas diffusion layer (GDL), in particular within a proton exchange membrane fuel cell (PEMFC).

[0051] According to a particular embodiment, the composition is a composition as defined previously, consisting of or comprising carbon fibers and carbon black at a level of 3 to 8% by mass relative to the total mass of carbon fibers and carbon black.

[0052] According to a particular embodiment, the composition is a composition as defined previously, consisting of or comprising carbon fibers with a diameter ranging from 350 to 1000 nm, and carbon black at a level of 3 to 8% by mass relative to the total mass of carbon fibers and carbon black.

[0053] All embodiments defined previously in relation to a composition also apply here, alone or in combinations.

[0054] According to a particular embodiment, the composition has a porosity of 87 to 97%.

[0055] According to a particular embodiment, the composition comprises pores whose radius is from 0.8 to 1.5 μm.

[0056] According to another aspect, the present invention also relates to the use of a composition as defined above, as a gas diffusion layer (GDL), in particular within a current collector for batteries.

[0057] According to a particular embodiment, the composition is a composition as defined previously, consisting of or comprising carbon fibers and carbon black at a level of 3 to 8% by mass relative to the total mass of carbon fibers and carbon black.

[0058] According to a particular embodiment, the composition is a composition as defined previously, consisting of or comprising carbon fibers with a diameter ranging from 350 to 1000 nm, and carbon black at a level of 3 to 8% by mass relative to the total mass of carbon fibers and carbon black.

[0059] All embodiments defined previously in relation to a composition also apply here, alone or in combinations.

[0060] According to a particular embodiment, the composition has a porosity of 87 to 97%.

[0061] According to a particular embodiment, the composition comprises pores whose radius is from 0.8 to 1.5 μm.

[0062] According to another aspect, the present invention also relates to the use of a composition as defined above, as a gas diffusion layer (GDL), in particular within a supercapacitor.

[0063] According to a particular embodiment, the composition is a composition as defined previously, consisting of or comprising carbon fibers and carbon black at a level of 3 to 8% by mass relative to the total mass of carbon fibers and carbon black.

[0064] According to a particular embodiment, the composition is a composition as defined previously, consisting of or comprising carbon fibers with a diameter ranging from 350 to 1000 nm, and carbon black at a level of 3 to 8% by mass relative to the total mass of carbon fibers and carbon black.

[0065] All embodiments defined previously in relation to a composition also apply here, alone or in combinations.

[0066] According to a particular embodiment, the composition has a porosity of 87 to 97%.

[0067] According to a particular embodiment, the composition comprises pores whose radius is from 0.8 to 1.5 μm.

[0068] According to another aspect, the present invention also relates to a gas diffusion layer (GDL) consisting of or comprising a composition as defined above.

[0069] According to a particular embodiment, the composition is a composition as defined previously, consisting of or comprising carbon fibers and carbon black at a level of 3 to 8% by mass relative to the total mass of carbon fibers and carbon black.

[0070] According to a particular embodiment, the composition is a composition as defined previously, consisting of or comprising carbon fibers with a diameter ranging from 350 to 1000 nm, and carbon black at a level of 3 to 8% by mass relative to the total mass of carbon fibers and carbon black.

[0071] All embodiments defined previously in relation to a composition also apply here, alone or in combinations.

[0072] According to a particular embodiment, the composition has a porosity of 87 to 97%.

[0073] According to a particular embodiment, the composition comprises pores whose radius is from 0.8 to 1.5 μm.

[0074] According to another aspect, the present invention also relates to a proton exchange membrane fuel cell (PEMFC) monocell comprising a gas diffusion layer (GDL) made of or comprising a composition as defined above.

[0075] According to a particular embodiment, the composition is a composition as defined previously, consisting of or comprising carbon fibers and carbon black at a level of 3 to 8% by mass relative to the total mass of carbon fibers and carbon black.

[0076] According to a particular embodiment, the composition is a composition as defined previously, consisting of or comprising carbon fibers with a diameter ranging from 350 to 1000 nm, and carbon black at a level of 3 to 8% by mass relative to the total mass of carbon fibers and carbon black.

[0077] All embodiments defined previously in relation to a composition also apply here, alone or in combinations.

[0078] According to a particular embodiment, the composition has a porosity of 87 to 97%.

[0079] According to a particular embodiment, the composition comprises pores whose radius is from 0.8 to 1.5 μm.

[0080] According to a particular embodiment, the monocell as defined previously further comprises electrode membrane, bipolar plates, and gas diffusion layer (GDL) at the anode.

[0081] These elements are well known to those skilled in the art and can, for example, be found commercially.

[0082] According to another aspect, the present invention also relates to a proton exchange membrane fuel cell (PEMFC) comprising at least one gas diffusion layer (GDL) consisting of or comprising a composition as defined above.

[0083] According to a particular embodiment, the composition is a composition as defined previously, consisting of or comprising carbon fibers and carbon black at a level of 3 to 8% by mass relative to the total mass of carbon fibers and carbon black.

[0084] According to a particular embodiment, the composition is a composition as defined previously, consisting of or comprising carbon fibers with a diameter ranging from 350 to 1000 nm, and carbon black at a level of 3 to 8% by mass relative to the total mass of carbon fibers and carbon black.

[0085] All embodiments defined previously in relation to a composition also apply here, alone or in combinations.

[0086] According to a particular embodiment, the composition has a porosity of between 87 to 97%.

[0087] According to a particular embodiment, the composition comprises pores whose radius is from 0.8 to 1.5 μm. Definitions

[0088] As used herein, the value ranges in the form of "xy" or "from x to y" or "between x and y" include the bounds x and y, the integers between these bounds, as well as all other real numbers between these bounds. For example, "1-5", or "from 1 to 5" or "between 1 and 5" designates the integers 1, 2, 3, 4 and 5, as well as all other real numbers between 1 and 5. Preferred embodiments include each individual integer in the value range, as well as any subcombination of these integers and any set of real numbers between these integers. For example, preferred values ​​for "1-5" might include the integers 1, 2, 3, 4, 5, 1-2, 1-3, 1-4, 1-5, 2-3, 2-4, 2-5, etc.

[0089] As used herein, the term "about" refers to a range of values ​​within ± 10% of a specific value. For example, the term "about 20" includes values ​​of 20 ± 10%, or values ​​from 18 to 22. FIGURES

[0090] [Fig.l] shows the scanning electron microscopy (SEM) images of: (A) electrospun GDL without carbon black (0) (x5000), (B) electrospun GDL with addition of carbon black at a ratio of 1 / 20, (1) (x5000), (C) electrospun GDL with addition of carbon black at a ratio of 1 / 20, (1) (x80000), (D) electrospun GDL with addition of carbon black at a ratio of 1 / 10, (2) (x5000), (E) electrospun GDL with addition of carbon black at a ratio of 1 / 5, (3) (x5000).

[0091] [Fig.2] shows the polarization curves of GDL 0-3 at (A) 50% RH, and (B) 100% RH.

[0092] [Fig.3] illustrates the overvoltages associated with the polarization curves of GDL without carbon black (0) and GDL with a 1 / 20 ratio of carbon black (1) at (A) 50% RH, and (B) 100% RH.

[0093] [Fig.4] shows the diffusion values ​​measured at the cathode, according to example 2, depending on the GDL used.

[0094] [Fig.5] relates to the curves of compositions outside the invention according to example 3, at 80% RH. EXAMPLES

[0095] Example 1: preparation of composite carbon fibers according to the invention

[0096] PAN (Polyacrilonitrile, molar mass 150000 g / mol, Sigma-Aldrich) was diluted in DMF (N,N-dimethylforamide, Sigma-Aldrich) to obtain a 10% by mass solution. For this, 3g of PAN were dissolved in 27g of DMF, and the mixture The resulting solution was stirred on a hot plate at 80°C with a magnetic stirrer for several hours until the PAN was completely dissolved. The desired mass of carbon black (CB, carbon black CL-08 from Continental Carbon, particle size < 15nm) was then added to the resulting solution to obtain the compositions at the different carbon black / PAN ratios as shown in Table 1 below (the different compositions, before electrospinning). Mixing on the hot plate and magnetic stirring were maintained for 3 hours. Compositions Mass CB (g) Mass PAN (g) Mass DMF (g) 0% CB (excluding invention^ 0) 0 3 ■77 CB 1 / 20 PAN (1) 0.15 3 77 CB 1 / 10 PAN (2) 0.30 3 77' CB 1 / 5 PAN (3) 0.60 3 77

[0097] Compositions 1-3 were then mixed using grinding balls. Zirconium balls were added to the compositions to do this, and left under ball milling type stirring for 10 min. To finish homogenizing the mixtures, compositions 1-3 thus obtained were stirred under ultrasound for 1 min.

[0098] Electrospinning with the obtained compositions 0-3 was then carried out. The parameters used were a voltage applied to the nozzle of 19 kV, and a voltage applied to the collector of -10 kV. The collector is rotatable and rotates at a speed of 500 rpm. The distance between the nozzle and the collector is 30 cm. The flow rate is 1.5 mL / h. The collector is surrounded by an aluminum foil in order to recover the deposit at the end of spinning. Spinning was allowed to proceed for 6 h.

[0099] For each composition 0-3, the fibers were recovered after deposition on the aluminum foil. A sample of size 60 x 80 mm was taken for each deposit of each composition 0-3.

[0100] Each sample then underwent a heat treatment to graphitize the polymer fibers. The first step of the heat treatment takes place in air. It is carried out at 240°C for 2 hours with a ramp of 1°C / min in a Nabertherm muffle furnace. During this step, the polymer chain is stabilized through several chemical reactions. The slow temperature rise is necessary due to the complexity of the reaction, which involves cyclization, aromatization, oxidation and the formation of intermolecular bonds.

[0101] A pre-carbonization step at 700°C, under argon, was then carried out for 1 hour with a 5-minute step at 350°C and 550°C, and a heating rate of 5°C / min. During this step, all non-carbon molecular groups are removed. It is therefore carried out under an argon atmosphere to avoid oxidation. The objective of this step is to begin to release the nitrogen groups (NH3, HCN, N2). It is carried out in a Nabertherm tube furnace.

[0102] Finally, carbonization (graphitization) took place at 1500°C for 1 hour. The heating rate is 3°C / min. A high-temperature Nabertherm tube furnace was used. At this stage, all fibers are completely carbonized.

[0103] At each step, the samples are placed between aluminum oxide plates to maintain a flat surface. The cooling of the furnaces is natural. The nanofibers outside the invention (0), as well as the graphitized materials of the invention (1-3) composed of composite carbon nanofibers with carbon black nanoparticles were thus obtained. These materials, obtained in the form of samples as mentioned above, constitute the gas diffusion layers (GDL) which will be used in Example 2.

[0104] The materials thus manufactured are observed under a scanning electron microscope ([Fig. 1]). Carbon black agglomerates are observed on the surface of the carbon fibers which are not present on the control material (0) without carbon black ([Fig.l]-A). The surface of the fibers is rougher with carbon black ([Fig.l]-B and 1-C). When the concentration of carbon black increases, the fibers are more closely linked together and carbon black agglomerates are formed along and at the intersection of the fibers ([Fig.l]-D and 1-E).

[0105] The structural properties of said materials were measured according to an analysis of the 3D structures, averaged over 3 structures for each material except for CB 1 / 10 (2 structures), and recorded in the following table.

[0106] The 3D structures were analyzed using image analysis software, for example GeoDict. These structures were for example obtained by X-ray nanotomography (xCT for X-Ray Computed Tomography). The porosity corresponds to the number of voxels corresponding to the solid matter out of the total number of voxels in the structure. The radius was determined by numerically simulating a MICP (Mercury Intrusion Capillary Pressure) experiment, i.e. an intrusion of mercury into the structure. The relationship between the volume of mercury introduced for a pressure P gives an associated pore radius R (Washburn relationship).

[0107] The uncertainties (±) correspond to the standard deviations associated with the differences in values ​​on different acquisitions of the same material. Compositions Porosity (¾) Median fiber diameter (nm) Median pore radius Qun) 0% CB (outside invention.. 0) 94 (±2) 506 (±94} 1.47 (±0.15) CB 1 / 20 PAN (I) 93 (±4) 574 (- AP 1.35 (±0.10) CB 1 / 10 PAN (2) 92(±2) 658(±308) 1.03 (±0.04) CB 1 / 5 PAN (3) 89 (±2} 647(±120) 1.02 (±0.15)

[0108] Example 2: use of composite carbon fibers according to the invention

[0109] The materials obtained according to Example 1 were tested as gas diffusion layers (GDL).

[0110] The performance of these materials was thus determined on a test bench with a single cell. The active surface is 1.8 cm2.

[0111] A commercial FuelCon evaluator-C 70351 bench was used to control the reactant gases and flow rates. Hydrogen and air were supplied at 500 NmL / min and 1500 NmL / min to the anode and cathode respectively. This corresponds to stoichiometries at the anode and cathode of 39 and 47 at lA / cm2 respectively, meaning that the gas supply is much higher than what the electrochemical reaction requires. Thus the partial pressure of the reactant gas at the outlet is approximately equal to the partial pressure at the inlet. There is therefore no partial pressure gradient along the active surface. The variations in behavior and performance tested therefore come solely from the materials used. The gas pressure is fixed at 1.5 bar for the anode and cathode. A thermostatically controlled bath regulates the cell temperature to 80°C via a water circuit. The reactive gases come from bipolar plates.They are designed with 30 parallel channels separated by teeth. The channels and teeth are each 250 pm wide. The channel depth is 400 pm. The relative humidity (RH) of the inlet gases at the anode and cathode is controlled by gas humidifiers. The current and voltage are controlled by a Biologie ®VMP2 potentiostat with a VMP3B-10 booster (10A / 20V). The positive electrode is the working electrode. The reference electrode and the counter electrode are both included in the negative electrode. The tests were carried out using commercial Gore A510.1 / M765.08 / C580.4 CCMs (Catalyst Coat Membranes). These CCMs are loaded with platinum at 0.1 mg / cm2 at the anode and 0.4 mg / cm2 at the cathode. The reaction at the cathode is kinetically decisive. This is why only the GDL at the cathode is tested. The GDL at the anode is an identical reference during the tests. It is the GDL 22BB of the Sigracet ® brand with an initial thickness of 220 pm. The initial thickness of the electrospun GDLs tested is 200 (+- 50) pm. A 25 pm PET film is interposed on each side of the membrane electrode assembly. A surface of 1.8 cm2 is left free in the middle of this film and determines the active surface. The GDLs are surrounded by a PTFE gasket which performs two functions: ensuring lateral sealing of the cell, and compressing the GDL to a desired thickness. Indeed, the initial thickness of the GDL is greater than that of the PTFE gasket. It is considered that the PTFE gasket does not deform and is not compressed when the cell is mounted, the same for the PET film surrounding the active surface. A 25 pm gasket is used on the cathode side, and 150 pm on the anode side.

[0112] [Fig.2] shows the polarization curves obtained for two relative humidities (RH): 50% and 100%. The polarization curves show a better performance of all the GDLs of the invention (GDL 1 to 3) over the control GDL outside the invention (GDL 0), and this, at these two relative humidities.

[0113] The polarization curves were also analyzed through the exploitation of three overvoltages ([Fig.3]): the activation overvoltage, the ohmic overvoltage, and the mass transfer overvoltage. The control GDL (0), without carbon black, and the composition (1) with carbon black at 1 / 20. Without wanting to be restricted to any theory, the improvement in performance, whether at 50% RH or 100% RH, is due to a lower mass transfer resistance, which means that oxygen reaches more of the active sites. Under these conditions, mass transfer is improved, as well as the electrochemical activation phenomenon even though the membrane electrode assembly is identical. This means that the GDLs of the invention induce better management of liquid water: the liquid water produced by the reaction in quantity at high humidity is less present in the structure of the GDL, and prevents less oxygen from passing through.In addition, liquid water is retained closer to the membrane, which improves proton transport and therefore better access to the active sites, which explains the better electrochemical behavior of the material.

[0114] Finally, the addition of carbon black does not alter the electrical conductivity and the ohmic overvoltage is identical under all conditions.

[0115] In addition, oxygen diffusion values ​​were obtained on the test bench ( [Fig.4]). These measurements consisted of varying the partial pressure of oxygen and measuring the associated limiting current. The proportionality relationship between the two quantities makes it possible to obtain the diffusion value by determining the direction coefficient of the line thus drawn. The results show an increase in oxygen diffusion with the GDLs of the invention. This partly explains the improvement in the performance of the GDLs with carbon black (1-3) compared to the GDL without carbon black (0).

[0116] In conclusion, the addition of carbon black according to the invention in carbon fibers created by electrospinning improves the performance of the GDL, whether at 50% or 100% humidity. Still without wanting to be restricted to any theory, the modification of the surface state changes the behavior of the passage of fluids. The diffusion and transfer of oxygen are improved, and the phenomenon of flooding of the structure by the liquid water produced is reduced, providing better management of the liquid water. Example 3: comparative example outside the invention

[0117] A composition outside the invention was prepared according to Example 1 by replacing the carbon black with nanotubes. The nanotubes / PAN ratio is 1 / 40. Beyond this, the mixture is not fluid enough for electrospinning.

[0118] The calculated porosity is approximately 96%, higher than the porosities measured with the compositions comprising carbon black. The performance is less good than with the compositions of the invention, in fact less good than the reference without carbon black, and without nanotubes, as for example illustrated in [Fig.5] (at 80% RH).

Claims

Claims

1. Composition consisting of or comprising carbon fibers with a diameter ranging from 350 to 1000 nm, and carbon black at a level of 3 to 8% by mass relative to the total mass of carbon fibers and carbon black.

2. A composition according to claim 1, which has a porosity of from 87 to 97%.

3. A composition according to claim 1 or 2, which comprises pores having a radius of 0.8 to 1.5 pm.

4. A process for preparing a composition according to any one of claims 1 to 3, said process comprising the following steps: i. A step of mixing, using a ball mill, a composition B comprising polyacrylonitrile (PAN) and carbon black (CB) at a rate of 0.5 to 50% by mass relative to the total mass of PAN and carbon black; ii. A step of electrospinning composition B as obtained at the end of the preceding step to obtain a composition C; iii. A step of heat treatment of composition C as obtained at the end of the preceding step, comprising the following sub-steps: a. A heating step, in particular at a temperature of from 180 to 300°C, in particular at approximately 240°C, and / or for 1 to 3 hours, in particular for approximately 2 hours; b.A pre-carbonization step, in particular at a temperature of 600 to 800°C, in particular at approximately 700°C, and / or for 45 minutes to 2 hours, in particular for approximately 1 hour; c. A carbonization step, in particular at a temperature of 1000 to 3000°C, in particular at approximately 1500°C, and / or for 45 minutes to 2 hours, in particular for approximately 1 hour; to obtain said composition A.

5. A method according to claim 4, wherein the carbon black is present in composition B at a rate of 1 / 200 to 1 / 5, in particular 1 / 200 to 1 / 10, in particular 1 / 200 to 1 / 20, by mass relative to the total mass of PAN and carbon black.

6. Use of a composition according to any one of claims 1 to 3, as a gas diffusion layer (GDL), in particular within a proton exchange membrane fuel cell (PEMFC), a current collector for batteries, or a supercapacitor.

7. A gas diffusion layer (GDL) made of or comprising a composition according to any one of claims 1 to 3.

8. A proton exchange membrane fuel cell (PEMFC) monocell comprising a gas diffusion layer (GDL) made of or comprising a composition according to any one of claims 1 to 3.

9. The monocell of claim 8, further comprising a membrane electrode, bipolar plates, and a gas diffusion layer (GDL) at the anode.

10. A proton exchange membrane fuel cell (PEMFC) comprising at least one gas diffusion layer (GDL) made of or comprising a composition according to any one of claims 1 to 3.

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