WO2016177951A1

WO2016177951A1 - Catalyst supported by carbon nanotubes and by graphene, and method for preparing same

Info

Publication number: WO2016177951A1
Application number: PCT/FR2016/050909
Authority: WO
Inventors: Emeline REMY; Frédéric FOUDA-ONANA; Marie Heitzmann; Pierre-André Jacques; Yohann THOMAS
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2015-05-06
Filing date: 2016-04-19
Publication date: 2016-11-10
Also published as: FR3035800B1; FR3035800A1

Abstract

The present invention relates to a catalyst having the formula M¹(CNT/Gr) supported by carbon nanotubes and by graphene. It also relates to the method for preparing this catalyst, comprising the following steps: a) preparation of M¹/CNT particles of a transition metal M¹ and supported by carbon nanotubes CNT, via thermal reduction of a salt of a transition metal M¹in the presence of carbon nanotubes, b) preparation of M¹/Gr particles of a transition metal M¹and supported by graphene Gr, via chemical reduction of a salt of a transition metal M¹in the presence of graphene, c) mixing of the M¹/CNT particles obtained in step a) with the MVGr particles obtained in step b), d) obtaining of a catalyst supported by carbon nanotubes and by graphene having the formula M¹(CNT/Gr). The invention further relates to using the catalyst having the formula M¹(CNT/Gr) in a proton exchange membrane fuel cell or in a proton exchange membrane electrolyzer.

Description

CATALYST SUPPORTED BY CARBON NANOTUBES AND GRAPHENE, AND PROCESS FOR PREPARING THE SAME

FIELD OF THE INVENTION

The present invention relates to a metal catalyst supported by both carbon nanotubes and graphene.

The field of use of the present invention relates in particular to fuel cells and electrolysers PEM (the acronym "proton exchange membrane" means proton exchange membrane).

PRIOR STATE OF THE ART A fuel cell makes it possible to convert chemical energy from a fuel such as hydrogen, methanol, ethanol or formic acid into electrical energy.

In general, a fuel cell comprises an anode, an electrolyte and a cathode. A PEMFC battery (proton exchange membrane fuel cell) is a battery having a proton exchange membrane. A PEMFC comprises an electrode membrane assembly (AME) consisting of an anode, a proton exchange membrane and a cathode. The operation of a fuel cell involves an oxidation reaction of the fuel at the anode, and a reduction reaction of the oxidant at the cathode.

In the case of a PEMFC, oxidation of the hydrogen fuel at the anode generates H ⁺ protons and electrons. The electrons are directed to the cathode via a circuit external to the fuel cell, while the protons pass through the membrane. At the cathode, the reduction of the oxidant, oxygen for example, makes it possible to form water in the presence of protons and electrons.

Because of the multitude of species involved in these reactions and their mobility necessary for the proper functioning of the battery, the electrodes generally comprise a diffusion layer, a microporous layer and an active layer. The diffusion layer provides not only the distribution of the oxidant and fuel to the electrodes, but also the evacuation of formed or unreacted compounds. In addition, it allows the conduction of electrons.

The active layer ensures electrochemical reactions. It is therefore exposed to drastic conditions related in particular to the oxidizing or reducing environment, to the possible strong local acidity, to the appearance of hot spots, or cycles of drying and flooding.

The active layer generally comprises a catalyst in the form of nanoparticles supported by a carbon substrate, in particular carbon black.

Numerous studies have made it possible to improve the properties of the active layer, whether in terms of the activity of the catalyst, the dispersion of the catalyst in the active layer or electrode materials.

Thus, the optimization of the catalyst is important in order to limit the amount needed for the proper functioning of the fuel cell, and for the purpose of improving the corrosion resistance of the carbon substrate on which the catalyst is deposited.

It is for this purpose that graphene or carbon nanotubes have been used to substitute carbon black as a carbon support for the catalyst. It has been shown that these graphitic type supports offer better resistance to corrosion

However, the use of graphene can prevent reagents (oxidant or fuel) from being accessed by the catalyst because of its 2D geometrical configuration in sheets. This phenomenon thus reduces the performance of the fuel cell. However, it can be attenuated by separators of the carbon black or carbon nanotube (CNT) type between the graphene sheets (Gr). These separators increase the porosity of graphene, which facilitates access to the catalyst and reduces the mass transport resistance.

Even though, these NTC / Gr carbon substrates improve the electrochemical performances of the catalyst relative to graphene alone or to CNTs alone, the catalyst particles are not homogeneously distributed over the two types of carbonaceous materials and form agglomerates of the order of 50 to 100 nanometers. The presence of agglomerates does not make it possible to optimize the use of all the catalyst particles, some sites remaining inactive.

The Applicant has developed a hybrid catalyst to solve this technical problem. The catalyst according to the present invention comprises metal particles distributed homogeneously on a carbon substrate in NTC and in graphene. Thus, the use of the deposited metal particles is optimized by avoiding the formation of agglomerates. SUMMARY OF THE INVENTION

The present invention relates to a catalyst in the form of metal particles supported by carbon nanotubes (CNTs) and graphene (Gr). This hybrid catalyst, of formula M NTC / Gr), is obtained by means of a method making it possible to optimize the distribution of the particles by depositing them on the CNTs and on graphene separately.

Unlike the catalysts of the prior art, the hybrid catalyst according to the invention has a homogeneous distribution of the metal particles on the two types of supports (NTC and graphene), and improves the diffusion of the gases and therefore the accessibility to the particles of metal. In addition, the catalyst according to the invention has a better stability during its use. It is the process for preparing the catalyst that makes it possible to obtain this homogeneous distribution of the metal particles.

More specifically, the present invention relates to a process for the preparation of a catalyst of the formula M NTC / Gr) supported by carbon nanotubes and by graphene, comprising the following steps:

a) preparation of IV / NTC particles of a transition metal M ¹ and supported by carbon nanotubes NTC, by thermal reduction of a salt of a transition metal M ¹ in the presence of carbon nanotubes;

b) preparing M Gr particles of a transition metal M ¹ and supported by graphene Gr, by chemical reduction of a salt of a transition metal M ¹ in the presence of graphene;

c) mixing the M '/ NTC particles from step a) with the MVGr particles from step b), advantageously in solution; d) obtaining a catalyst supported by carbon nanotubes and by graphene of the formula M NTC / Gr).

At the end of step d), if necessary, the catalyst M NTC / Gr) is separated from the solvent possibly used in step c). This solvent is advantageously chosen from the group comprising water and alcohol. The catalyst / solvent separation is carried out according to conventional techniques including in particular evaporation.

Advantageously, the transition metal M ¹ is a group 10 metal, more preferably platinum. It may also be an alloy of a Group 10 metal, such as a platinum alloy, for example an alloy selected from the group comprising: platinum / cobalt (Pt / Co); platinum / palladium (Pt / Pd); and platinum / ruthenium (Pt / Ru). The degree of oxidation of the metal M ¹ of the catalyst M NTC / Gr) is equal to 0.

The thermal reduction of step a) making it possible to obtain the IV / NTC particles advantageously comprises the following steps:

preparing a suspension comprising a solvent, carbon nanotubes and a salt of a transition metal M ¹ , advantageously by mixing between a suspension of CNT and a solution of a salt of a transition metal M ¹ ;

homogenization of this suspension by stirring, advantageously under ultrasound;

drying this suspension by removing the solvent so as to obtain a powder;

optionally, grinding the powder;

heat treatment of this powder under a reducing atmosphere; preferably at a temperature between 150 ° C and 800 ° C, more preferably between 200 ° C and 400 ° C;

obtaining M ¹ / NTC particles.

The thermal reduction of the salt of the metal M ¹ in the presence of CNT is advantageously carried out under an atmosphere of hydrogen, for example under H ₂ / Ar. The solvent used to prepare the M '/ NTC particles is advantageously chosen from the group comprising water; ethanol; and their mixtures. Advantageously, in the IV / NTC particles, the transition metal M ¹ represents 20% and 50% by weight relative to the total weight of the M '/ CNT particles, more advantageously 30%> and 40%>. The CNTs are advantageously multi-walled nanotubes, that is to say carbon nanotubes wound on themselves or carbon nanotubes arranged in concentric cylinders.

According to a particular embodiment, the CNTs can be doped with nitrogen atoms. In this case, the nitrogen atoms represent 0.2 to 10% by weight relative to the weight of the CNTs (before doping), more advantageously 0.4 to 3%.

The chemical reduction of step b) making it possible to obtain the particles M Gr advantageously comprises the following steps:

- Preparation of a suspension comprising a solvent, graphene oxide and a salt of a transition metal M ¹ , preferably by mixing a suspension of graphene oxide and a solution of a salt of a metal M ¹ transition;

homogenization of this suspension by stirring, advantageously under ultrasound;

addition of a reducing agent in the suspension;

stirring the suspension, advantageously under ultrasound;

reflux heating of the suspension;

separating the liquid phase from the solid phase to obtain MVGr particles, advantageously by filtration;

optionally, rinsing MVGr particles, preferably with water and / or acetone, and then drying the particles MVGr.

In the preparation of the suspension, the use of graphene oxide makes it possible to offer points of attachment to the metal particles, unlike graphene.

The solvent used to prepare the MVGr particles is advantageously chosen from the group comprising water; ethanol and their mixtures. The chemical reduction of the salt of the metal M ¹ is advantageously carried out in the presence of a reducing agent chosen from the group comprising ethylene glycol (EG); NaBH ₄ ; acetic acid; and formic acid. Advantageously, in the MVGr particles, the transition metal M ¹ represents 20 to 50% by weight relative to the total weight of the MVGr particles, more advantageously 30 to 40%. Graphene can also be graphene doped with nitrogen atoms. In this case, the graphene advantageously comprises between 0.2 and 10% by weight of nitrogen atoms, more advantageously between 0.4 and 3%.

Step c) relating to the mixing of the particles IV ^ / NTC and M Gr is advantageously carried out in solution. Even more advantageously, it is carried out in an electrode ink that can in particular comprise an ionomer and a solvent such as water and / or an alcohol.

As already indicated, the present invention also relates to the catalyst MnCNTC / Gr) obtained by the process above.

In the catalyst M NTC / Gr) according to the invention, the ratio by weight between the particles MVGr and the particles M '/ NTC is advantageously between 0.2 and 3, more preferably between 0.2 and 0.5.

By way of example, the weight ratio MVGr: M ^! / NTC can be between 20:80 and 75:25, advantageously between 20:80 and 30:70.

It can thus be of the order of 20:80, 25:75, 30:70, 50:50, or 75:25.

Advantageously, the particles of M ¹ are nanoparticles having a size advantageously between 1 and 10 nanometers, more advantageously between 1 and 6 nanometers. By "size" is meant the largest dimension of the particles, namely the diameter for spherical particles or the length for parallelepipedic particles for example.

The homogeneous distribution of the metal particles M ¹ improves the electrocatalytic performance as well as the electroactive activity and surface (ECSA). In addition, the nature of the carbon substrate (NTC + Gr) improves the stability of the catalyst during aging cycles through improved corrosion resistance. The improvement of the homogeneous distribution of the particles on the carbon substrates into NTC and Gr is due to the separation of the synthesis steps of the particles M '/ NTC and MVGr. The reduction of the metal salt M ¹ is in fact adapted according to the carbon substrate, and more particularly to the surface chemistry of CNTs and graphene.

The present invention also relates to an electrode ink comprising the catalyst of formula M NTC / Gr).

This electrode ink generally comprises a solvent and an ionomer.

The ionomer is advantageously a proton-conducting polymer, for example a perfluorosulphonated polymer such as Nafion®, for example.

The solvent of this electrode ink is advantageously water, an alcohol such as isopropanol and mixtures thereof.

According to a particular embodiment, the process for preparing this ink may especially comprise the following steps:

a ') preparation of M' / NTC particles of a transition metal M ¹ supported by carbon nanotubes NTC, by thermal reduction of a salt of a transition metal M ¹ in the presence of carbon nanotubes;

b ') preparing M Gr particles of a transition metal M ¹ on graphene Gr, by chemical reduction of a salt of a transition metal M ¹ in the presence of graphene;

c ') mixing the M' / NTC particles from step a ') with the MVGr particles from step b');

(d) obtaining a catalyst supported by carbon nanotubes and graphene of the formula (NTC / Gr);

e ') obtaining an electrode ink by mixing the MnCNTC / Gr catalyst with an ionomer and a solvent.

According to another particular embodiment, the process for preparing this ink may notably comprise the following steps: a ") preparation of M '/ NTC particles of a transition metal M ¹ supported by carbon nanotubes NTC, by thermal reduction of a salt of a transition metal M ¹ in the presence of carbon nanotubes;

b ") preparing M Gr particles of a transition metal M ¹ on graphene Gr, by chemical reduction of a salt of a transition metal M ¹ in the presence of graphene;

c ") mixing in a solvent the M '/ NTC particles from step a") with the M Gr particles from step b ") and with an ionomer;

d ") obtaining an electrode ink containing a catalyst supported by carbon nanotubes and by graphene of the formula M NTC / Gr), an ionomer and a solvent.

Whatever the embodiment, during the preparation of the electrode ink (steps e 'and c "), the solvent is advantageously identical to the solvent used in the preparation of IV ^ / NTC and MVGr particles.

In addition, the conditions of preparation of the particles M '/ NTC and MVGr are in accordance with those specified in the description. The present invention also relates to a cathode comprising the catalyst M ¹ (NTC / Gr).

It also relates to the use of this cathode in a proton exchange membrane fuel cell or in a PEM electrolyser (proton exchange membrane electrolyser).

On the other hand, the invention relates to the fuel cell, preferably proton exchange membrane, or PEM electrolyser comprising this cathode. The invention further relates to a method for preparing an electrode comprising the following steps:

preparing an electrode ink comprising the catalyst M NTC / Gr) according to the invention, a solvent, and an ionomer;

depositing this electrode ink on a support, for example on a current collector including a metal sheet;

drying of this ink. Advantageously, the deposition of the electrode ink is carried out by sputtering on a GDL (gaseous diffusion layer).

The catalyst M n TCN / Gr) according to the invention can also be used in all applications subject to the corrosion phenomenon of the substrate supporting the catalyst. It can thus be used in heterogeneous catalysis for example.

The invention and the advantages thereof will appear more clearly from the following figures and examples given to illustrate the invention and not in a limiting manner.

DESCRIPTION OF THE FIGURES

FIG. 1 represents the image by transmission electron microscopy of Pt / NTC particles obtained by thermal reduction.

Figure 2 shows the size distribution of Pt / NTC particles obtained by thermal reduction.

FIG. 3 represents the image by transmission electron microscopy of Pt / Gr particles obtained by chemical reduction.

Figure 4 shows the size distribution of Pt / Gr particles obtained by chemical reduction.

FIG. 5 represents the voltammograms obtained for the Pt catalyst (NTC / Gr) according to the invention and catalysts according to counterexamples.

FIG. 6 represents the comparison of the electroactive surface and activity values for Pt (CNT / Gr) catalysts according to the invention and catalysts according to counterexamples.

FIG. 7 represents the evolution of the active surface as a function of the mass ratio CNT: Gr for Pt (CNT / Gr) catalysts according to the invention and catalysts according to counterexamples.

FIG. 8 represents the evolution of the active surface as a function of the number of rings for Pt (NTC / Gr) catalysts according to the invention and catalysts according to counterexamples.

FIG. 9 represents the image by transmission electron microscopy of particles Pt (NTC / Gr) according to a counterexample of the invention.

FIG. 10 represents the electro-active surface for a Pt (NTC / Gr) catalyst according to the invention and a Pt (NTC / Gr) catalyst according to a counterexample. EXAMPLES OF CARRYING OUT THE INVENTION

1 / Preparation of supported catalysts Pt / NTC and Pt / Gr 1.1 Thermal reduction in a hydrogen / argon atmosphere

Platinum is deposited on CNTs according to the following steps:

suspending 600 mg of NTC in 500 mL of water using an ultrasonic probe for one hour.

solution of 1.066 g of platinum salt H ₂ PtCl 6 in 100 ml of water,

addition of the aqueous solution of H ₂ PtCl 6 in the suspension of CNT.

homogenization of the resulting suspension using an ultrasound probe for 1 hour. The dispersion obtained is kept under magnetic stirring (magnetic stirrer + magnetic bar) at room temperature for 12 hours.

evaporation of the solvent (water) by heating the solution at 100 ° C with magnetic stirring for 6 hours, then in an oven at 95 ° C until complete evaporation of the solvent and obtaining a dry powder,

grinding of the powder thus obtained.

- reduction of platinum salt under a reducing atmosphere (hydrogen). The ground powder is placed in a silica nacelle inside a tubular furnace. Inert gas purge (argon) is carried out for 30 minutes. The thermal reduction is then carried out at 300 ° C. under a mixture of hydrogen and argon (90:10) for 3 hours.

The ultrasonic probe used in the examples is a high intensity sonifactor of Bioblock Scientific reference 75041 (maximum power output of 750 Watt). It has generally been used for 15 minutes with 01: 01 ON: OFF.

The CNTs used in this synthesis are marketed by Nanocyl under the reference NANOCYL ™ NC3152. It is carbon nanotubes multi-wall nitrogen. FIG. 1 represents an image obtained by transmission electron microscopy on the Pt / NTC catalyst thus obtained. The platinum nanoparticles are homogeneously distributed on the CNTs. The particle size is centered at 2.9 nm (Figure 2). 1.2 Chemical reduction in the presence of ethylene glycol

Platinum is deposited on graphene according to the following steps:

suspending 200 mg of graphene oxide in 180 mL of distilled water using an ultrasound probe for 15 minutes.

dissolving 354 mg of H ₂ PtCl 6, 6H ₂ O (platinum salt) in 20 ml of water, dropwise addition of this platinum salt solution to the suspension of graphene.

homogenization of the resulting suspension by use of an ultrasound probe for 15 minutes.

addition of this suspension to 160 mL of ethylene glycol.

heating the suspension at reflux at 110 ° C for 12 hours.

recovery of the catalyst supported by graphene by filtration under vacuum.

rinsing several times with deionized water and acetone of the supported catalyst, drying the supported catalyst in an oven at 95 ° C for 24 hours.

The graphene used in this synthesis is marketed by the company Angstron Materials under the reference N006-010-P.

FIG. 3 represents an image obtained by transmission electron microscopy on the Pt / Gr catalyst thus obtained. Platinum nanoparticles are homogeneously distributed on graphene. The particle size is centered at 3.9 nm (FIG. 4).

21 Preparation of an electrode from supported Pt / NTC and Pt / Gr catalysts

Several electrode inks, the compositions of which are summarized in Table 1, have been prepared. Table 1 Composition of Electrode Inks According to the Invention (INV) and Counterexamples (CE)

platinum on carbon black, comprising 47.3% by weight of Pt under the reference Tanaka TEC10V50E

platinum particles on carbon nanotubes obtained by thermal reduction (point 1.1 / above)

graphene platinum particles obtained by chemical reduction (point 1.2 / above)

perfluorosulfonated polymer Nafion®

isopropanol

The catalyst Pt (NTC / Gr): 75/25 corresponds to a mixture of platinum particles on NTC Pt / NTC obtained by thermal reduction and Pt / Gr graphene platinum particles obtained by chemical reduction. The ratio by weight between the particles M '/ NTC and the particles MVGr is equal to 75/25. 3 / Electrochemical tests

The cyclic voltammograms obtained on the samples, by cycling the potential of 0.06 V _ERH to 1.2 V _ERH , are represented by FIG. 5 (loading of 0.1 mgPt / cm ² , 0.5M H ₂ SO ₄ , under nitrogen ). Values of active surfaces (ESCA _Pt in m ² / gpt) and activity (j ^_'02 A / g _Pt) are reported in Figure 6.

The electrochemical properties of the catalysts INV-1 to 5 and CE-1 to 3 are tested by rotating disk electrode (EDT) in a standard three-electrode arrangement, under a nitrogen flow to determine the electro-active surface and under oxygen for to quantify the activity of the catalysts with respect to the reduction of oxygen.

Figure 5 shows that graphene alone as the catalyst support (CE-2) has low electroactive surface area (ESCA) and activity (J0 ₂ ) values compared to conventional catalyst CE-1. The curves of FIG. 5 correspond, from top to bottom at 0.2V, to Examples CE-1, INV-5, INV-4, INV-3, INV-2, CE-3, INV-1 and CE-2. .

The graph of FIG. 6 clearly shows the influence of the mass ratio NTC / Gr on the electrochemical performances of the catalysts studied. The presence of NTC makes it possible to increase the electrochemical performances significantly with a maximum for the mass ratio NTC / Gr (75:25, INV-4).

The carbon nanotubes act as separators by increasing the porosity of the graphene matrix, thus facilitating the access of the reactive gases (0 ₂ ) to the platinum particles.

Thus, for the optimal mass ratio (75:25, INV-4), the electrochemical performances of the catalyst based on graphene and carbon nanotubes exceed those of the reference catalyst (CE-1) conventionally used for the same platinum loading. .

FIG. 7 (CNT = NTC, G = Gr) shows an identical evolution of the electro-active surface as a function of the percentage of nanotubes in the graphene matrix, the maximum performances being obtained for the mass ratio NTC / Gr (75:25 , INV-4). In order to study the stability of the catalysts, accelerated aging tests were carried out on the INV-4 catalyst (FIG. 8). These tests are carried out by cycling the potential between 0.06 V _ERH and 1.2 V _ERH at 20 mV / s over 2700 cycles. Characterization cycles at 5 mV / s between 0.06 V _ERH and 1.2 V _ERH are performed at regular intervals to visualize the electro-active surface value.

Figure 8 shows a loss of active area after 2700 cycles less important for the catalyst INV-4 compared to the conventional catalyst CE-1. These results show increased stability during cycling for catalyst INV-4 relative to catalyst CE-1.

4 / Simultaneous deposition of the catalyst on both types of substrate (CE-4)

The counter-example CE-4 makes it possible to evaluate the influence of the operating mode on the homogeneity of the distribution of metallic nanoparticles.

In this counterexample, the precursor of metal particles (platinum salt) was deposited simultaneously on the two types of carbon supports: NTCs and graphene. The formation of the particles was carried out by chemical reduction of the platinum salt.

The microscopy results (FIG. 9) as well as the rotary electrode characterization for evaluating the electroactive surface of the catalysts (FIG. 10) were compared with the results obtained for the catalyst according to the invention INV-4.

The microscopy results highlight the presence of zones without metal particles which results from a non-optimal distribution of the particles on all the NTC and Gr supports. This is particularly visible on the NTCs (see the black rectangles of FIG. 9). ). This heterogeneous distribution results in a low value of electro-active surface obtained for CE-4 45 m ² .gPt ^_1, compared with that obtained for INV 4 (52 m ² ^.gPf!).

The comparative test CE-4 makes it possible to highlight the importance of depositing the metal particles on each of the supports prior to mixing them.

Claims

A process for the preparation of a catalyst of the formula M NTC / Gr) supported by carbon nanotubes and graphene, comprising the following steps:

c) mixing the M '/ NTC particles from step a) with the MVGr particles from step b);

d) obtaining a catalyst supported by carbon nanotubes and by graphene of the formula M NTC / Gr).

Process for the preparation of a catalyst according to Claim 1, characterized in that the transition metal M ¹ is a Group 10 metal or a Group 10 metal alloy.

Process for the preparation of a catalyst according to claim 1 or 2, characterized in that the thermal reduction of step a) comprises the following steps: preparation of a suspension comprising a solvent, carbon nanotubes and a salt of a transition metal M ¹ ;

homogenization of this suspension by stirring;

drying this suspension by removing the solvent so as to obtain a powder;

heat treatment of this powder under a reducing atmosphere;

obtaining M '/ NTC particles.

Process for the preparation of a catalyst according to one of claims 1 to 3, characterized in that the chemical reduction of step b) comprises the following steps:

preparing a suspension comprising a solvent, graphene oxide and a salt of a transition metal M ¹ ;

homogenization of this suspension by stirring;

addition of a reducing agent in the suspension;

agitation of the suspension; reflux heating of the suspension;

separation of the liquid phase from the solid phase to obtain MVGr particles.

Process for the preparation of a catalyst according to one of Claims 1 to 4, characterized in that:

in the IV / NTC particles, the transition metal M ¹ represents 20 to 50% by weight relative to the total weight of the M '/ CNT particles, advantageously 30 to 40%;

in the MVGr particles, the transition metal M ¹ represents 20 to 50% by weight relative to the total weight of the MVGr particles, advantageously 30 to 40%.

Process for the preparation of a catalyst according to one of Claims 1 to 5, characterized in that the weight ratio between the MVGr particles and the M '/ NTC particles is between 20:80 and 30:70.

Process for the preparation of a catalyst according to one of Claims 1 to 6, characterized:

in that graphene is graphene doped with nitrogen atoms; and in that the CNTs are multi-wall nanotubes advantageously doped with nitrogen atoms.

Catalyst of formula M n TCN / Gr) obtained by the method according to one of claims 1 to 7.

An electrode ink comprising the catalyst of formula M NTC / Gr) according to claim 8. The process for preparing the ink according to claim 9, comprising the following steps:

a ') preparation of particles M ^! / NTC of a transition metal M ¹ supported by carbon nanotubes NTC, by thermal reduction of a salt of a transition metal M ¹ in the presence of carbon nanotubes;

b ') preparing MVGr particles of a transition metal M ¹ on Gr graphene, by chemical reduction of a salt of a transition metal M ¹ in the presence of graphene; c ') mixing the M' / NTC particles from step a ') with the particles

MVGr from step b ');

The method of preparing the ink of claim 9, comprising the steps of:

a ") preparation of IV / NTC particles of a transition metal M ¹ supported by carbon nanotubes NTC, by thermal reduction of a salt of a transition metal M ¹ in the presence of carbon nanotubes;

c ") mixing in a solvent particles M '/ NTC from step a") with the particles MVGr from step b ") and with an ionomer; d") obtaining an electrode ink containing a catalyst supported by carbon nanotubes and by graphene of the formula M NTC / Gr), an ionomer and a solvent.

Cathode comprising the catalyst of claim 8.

13. Use of the cathode according to claim 12 in a proton exchange membrane fuel cell or in a proton exchange membrane electrolyser.