WO2016096806A1

WO2016096806A1 - Method for hydrogen production and electrolytic cell thereof

Info

Publication number: WO2016096806A1
Application number: PCT/EP2015/079727
Authority: WO
Inventors: Atsushi URAKAWA; Jordi AMPURDANÉS VILANOVA; TachmajaL CORRALES SÁNCHEZ
Original assignee: Fundació Institut Català D'investigació Química (Iciq)
Priority date: 2014-12-16
Filing date: 2015-12-15
Publication date: 2016-06-23

Abstract

The present invention relates to a process for producing hydrogen in an electrolytic cell comprising a polymer electrolyte membrane, the process comprising contacting protons with a cathode comprising a hydrogen evolution catalyst comprising Co₃O₄ and a carbon material, wherein the carbon material is conductive or semi-conductive, and the process is carried out applying a potential equal to or higher than 1.8 V between the cathode and the anode.

Description

Method for hydrogen production and electrolytic cell thereof

The present invention relates to a process for the production of hydrogen by reduction of protons using a noble metal-free catalyst comprising cobalt oxide and a conductive or semi-conductive carbon material. The process of the invention allows for a high productivity of hydrogen over time when the potential applied to an electrolytic cell comprising a polymer electrolyte membrane is equal to or higher than 1 .8 V. BACKGROUND ART

The production of clean renewable energy is one of the most challenging research objectives of the present century. Hydrogen has been presented as one of the possible means to store energy into chemicals, with a high number of potential applications, ranging from ammonia synthesis and methanol synthesis to fuel cell applications. Hydrogen indeed finds applications as an energy source in the fields of mobility (fuel cell engines), power generation and heating. Most of the hydrogen produced worldwide is still obtained through steam reforming of fossil fuels, and the development of carbon neutral, competitive renewable and sustainable ways of producing hydrogen still remains a challenge in which water electrolysis has been proposed as a solid candidate solution.

Water electrolysis allows for the generation of hydrogen and oxygen according to the following half-reactions which have their own standard oxidation and reduction potentials:

(1 ) 2 H₂O→O₂ + 4 H⁺ + 4 e^" E° = -1 .23 V

(2) 2 H⁺ + 2 e^"→ H₂ E° = 0.00 V

These two reactions are the ones that take place in electrolytic cells at the anode (reaction (1 ), also known as Oxygen Evolution Reaction - OER) and cathode (reaction (2), also known as Hydrogen Evolution Reaction - HER) respectively. Catalysts are commonly used to speed up these reactions taking place at the cathode and at the anode. Most commonly, catalysts for HER are based on noble metals, such as platinum and ruthenium. Although very efficient as catalysts, these metals are rare and not abundant on earth and are usually expensive, leading to the need to minimize as much as possible the amount of these metals onto the electrode for further implementation of these technologies at larger scale, which somehow limits the hydrogen productivity of such systems. Other catalysts based on non-noble metals have been developed for the HER. In particular, Blanc and co-workers reported catalysts based on molybdenum sulfide doped with cobalt prepared by chemical vapor deposition, with platinum-like efficiency. The authors suggest that doping with cobalt allows for increasing active sites of the molybdenum based catalyst. Nevertheless, the authors are silent about the productivity of hydrogen over time of the catalyst and about the potential used in electrolysis experiments carried out with these materials.

In a different work, Yan and co-workers reported a photocatalyst based on a titanium oxide-cadmium selenide composite loaded with cobalt oxide. Upon irradiation with light, this catalyst efficiently catalyzes the HER. When the loading of the composite lied around 2.1 % in weight of the catalyst, the optimal production of hydrogen was found to be 660 μηηοΙ per gram of catalyst per hour (i.e. about 1 .32 mg of hydrogen per gram of catalyst per hour), which is far from matching the productivity achieved by electrolysis carried out in the presence of platinum. Moderate current densities (several mA cm²) were also observed. There is no further study of the behavior of the catalyst when light is not applied. The productivity of hydrogen at higher potentials was not further investigated and the authors are silent about the effects of applying high potentials on catalyst activity.

Chandrasekaran and co-workers reported a hybrid material made of reduced graphene oxide and cobalt oxide as a photocatalyst for the HER. The reduced graphene oxide is crumpled after a treatment at high temperature. Upon irradiation with light, this catalyst triggers the HER, especially when the thermal treatment took place around 500 °C. Nevertheless, the productivity of hydrogen is moderate and light irradiation is required to enhance catalyst activity. The potential and current density in the reported experiments are however low. Electrolysis experiments at high potentials were also not carried out and the influence of high potentials on the catalytic activity has not been studied. In a different approach, Fontecave and co-workers report a method for the preparation of cobalt-based catalysts for the HER based on the

electrodeposition of cobalt organic complexes onto an electrode, made of a (semi)-conductive support. According to the authors, the deposited catalyst consists in nanoparticles of cobalt oxo/hydroxo phosphate on the surface and metallic cobalt in the bulk. When used in electrolysis experiments, these catalysts exhibit moderate productivity of hydrogen. The behavior of such catalysts when higher potentials are applied is not known. Cobalt oxide is also a known catalyst for the Oxygen Evolution Reaction

(reaction 1 above). For instance, Nocera extensively described and reported the water oxidation capacity of cobalt oxide films formed onto electrodes in phosphate buffers. In a different approach, Jeon and co-workers studied the influence of the size of cobalt oxide particles on the catalytic activity for OER when the particles are placed onto polymer films. Electrolysis experiments were in this case carried out at 3V and platinum foil was used to catalyze the HER.

Also, Park and co-workers reported that carbon black particles tend to enhance the capacitative strength of composites formed of cobalt oxide nanoparticles (5-7 nm size) and graphene nanosheets. Authors attributed this phenomenon to the ability of carbon black particles to act as conductive links between the different sheets of graphene. The authors are however silent about the use of such materials as HER catalysts.

Further, in the Chinese patent CN103014750, a modified electrode for catalyzing water electrolysis and a process for its preparation were reported. Particularly, the modified electrode comprises a working electrode for water electrolysis and a catalyst material containing cobalt or nickel deposited on the working electrode. The modified electrode is obtained by electrolyzing a buffer solution which contains cobalt or nickel. Further, It is also disclosed the use of the modified electrode in standard electrolysis methods for obtaining current densities of 1 .5 mA cm² at 1 .29 V. This standard experiments comprises the use of an electrolyte that separates both electrodes. The authors are however silent about the use of such modified electrode in a proton exchange membrane device. Finally, in the Chinese patent CN104043453, a supported cobaltosic oxide nanometer composite catatyst and its use were reported. The supported cobaltosic oxide nanometer composite catalyst comprises solid powder, where the cobaltosic oxide particles have a particle diameter of about 10 nm being uniformly loaded on the surface of the acidified carbon black. This supported cobaltosic oxide nanometer composite catalyst can be used for the

preparation of a gas diffusion electrode useful in the chlor-alkali industrial oxygen cathode technology. The authors are however silent about the use of such supported cobaltosic oxide nanometer composite catatyst in a proton exchange membrane device.

Therefore, from what is known in the art it is derived that there is still the need of providing alternative catalysts and methods for hydrogen production, employing noble-metal free catalysts and having high hydrogen productivity.

SUMMARY OF THE INVENTION

Inventors have developed a new catalyst and process for the production of hydrogen using a mixture of cobalt oxide and a conductive or semi-conductive carbon material as HER catalyst in a proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane wherein high potentials, i.e. potentials equal to or higher than 1 .8 V, are applied, thereby allowing for high productivity of hydrogen over time.

The developed process is advantageous as the catalyst used in the process of the invention is based on non-precious metals, inexpensive and abundant on earth. Its activity is surprisingly enhanced when high potentials are applied to the electrolytic cell, thereby resulting in higher current densities within the proton exchange membrane cell which has an anode and a cathode

separated by a polymer electrolyte membrane.This therefore leads to a higher productivity of hydrogen over time than in the methods using cobalt oxide based catalysts of the state of the art.

Therefore, a first aspect of the invention relates to a process for producing hydrogen in an electrolytic cell comprising contacting protons with a cathode comprising a catalytically effective amount of a hydrogen evolution catalyst, wherein the hydrogen evolution catalyst comprises Co₃O₄ and a carbon material, which is conductive or semi-conductive, and the process is carried out applying a potential equal to or higher than 1 .8 V between the cathode and the anode, wherein the electrolytic cell is a proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane. In a second aspect, the invention relates to a proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane comprising a catalytically effective amount of a hydrogen evolution catalyst, wherein the hydrogen evolution catalyst comprises Co₃O₄ and a carbon material.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the current density (y axis), expressed in ampere per square centimeter, as a function of the potential (x axis, expressed in Volt) applied to a PEM electrolytic cell (see Figure 4) for:

Black line: platinum black as a catalyst (0.6 mg/cm²);

Black triangles: cobalt oxide and carbon black composite as catalyst (9 weight% cobalt oxide, 5 mg/cm² carbon black);

Black diamonds: cobalt oxide and carbon black composite as catalyst (33 weight% cobalt oxide, 5 mg/cm² carbon black);

Black crosses: cobalt oxide and carbon black composite as catalyst (47 weight% cobalt oxide, 5 mg/cm² carbon black);

Black squares: cobalt oxide and carbon black composite as catalyst (70 weight% cobalt oxide, 5 mg/cm² carbon black);

Black dashes: cobalt oxide as catalyst (100 weight% cobalt oxide, 5 mg/cm²).

Figure 2 shows the current density (y axis), expressed in ampere per square centimeter, over time (x axis), expressed in hours, of the electrolysis

experiment carried out in a PEM electrolytic cell (see Figure 4) with cobalt oxide and carbon black composite as catalyst described in Example 2 (47 weight% cobalt oxide, 5 mg/cm² carbon black) when:

Top curve: a voltage of 2.5 V is applied;

Bottom curve: a voltage of 2 V is applied. Figure 3 shows the productivity of hydrogen (y axis), expressed in kilograms of hydrogen per gram of catalyst (metal) per hour, as function of the potential (x axis, expressed in Volt) applied to a PEM electrolytic cell (see Figure 4) for: Black line: platinum black as a catalyst (0.6 mg/cm²); Black squares: cobalt oxide and carbon black composite as catalyst (9 weight% cobalt oxide, 5 mg/cm² carbon black);

Black circles: cobalt oxide and carbon black composite as catalyst (33 weight% cobalt oxide, 5 mg/cm² carbon black);

Black triangles: cobalt oxide and carbon black composite as catalyst (47 weight% cobalt oxide, 5 mg/cm² carbon black);

Black diamonds: cobalt oxide and carbon black composite as catalyst (70 weight% cobalt oxide, 5 mg/cm² carbon black);

Black dashes: cobalt oxide as catalyst (100 weight% cobalt oxide, 5 mg/cm²).

Figure 4 shows (a) a picture of the electrochemical PEM cell (Polymer electrolyte Membrane cell) fully assembled and (b) layer-by-layer reactor components, from membrane to back plate: (1 ) Membrane Electrode

Assembly (MEA), (2) gas diffusion layer (GDL), (3) fluoropolymer (PTFE) gasket, (4) current collector, (5) PTFE back gasket and (6) front or end plate.

Figure 5 shows a picture of the current collector (piece (4) on Figure 4) wherein (1 ) represents a serpentine flow-field, (2) is the water outlet, (3) is the water inlet and (4) is the electrical conector.

DETAILED DESCRIPTION OF THE INVENTION

All terms as used herein in this application, unless otherwise stated, shall be understood in their ordinary meaning as known in the art. Other more specific definitions for certain terms as used in the present application are as set forth below and are intended to apply uniformly throughout the specification and claims unless an otherwise expressly set out definition provides a broader definition. Unless otherwise expressly set out in this document, potentials are expressed in volts, taking as a reference the Hydrogen Evolution Reaction (HER, reaction (2) above) for which the thermodynamic potential is equal to zero volt.

In the context of the invention, the term "electrolytic cell" refers to a device able to trigger non-spontaneous chemical reactions upon introduction of electrical energy. An electrolytic cell comprises two half cells, each consisting of an electrode (cathode or anode) and an electrolyte, the half cells being in contact through a diaphragm (alkaline electrolysis), an electrolyte membrane or by mixing of electrolytes. Oxidation occurs at the anode and reduction occurs at the cathode. The anode of an electrolytic cell is positively charged (cathode is negatively charged), and the anode attracts anions from the solution. Oxidation takes place at the anode and electrons flow from the anode to the cathode. A half cell also contains an electrolyte, which is a substance containing free ions that make the substance electrically

conductive. In the electrolytic cell of the invention, the chemical reaction taking place at the cathode is the Hydrogen Evolution Reaction (HER) described above. A suitable chemical reaction taking place at the anode of the

electrolytic cell of the invention is an oxidation reaction releasing protons and electrons, preferably with a potential comprised from 0 to 1 .5 V (vs. HER), such as, in a non-limitative way, OER (reaction (1 ) described above), oxidation of methanol to formaldehyde or to carbon dioxide, metal oxidation mediated by water, and nitrogen to nitrate. The electrolytic cell of the invention can be an alkaline electrolytic cell, a "proton exchange membrane" cell, also referred to as "polymer electrolyte membrane" cell (PEM cell), solid oxide electrolyte membrane cell or a photo-electrochemical cell (PEC cell).

In the context of the invention, the term "potential", when applied to or referring to an electrolytic cell, refers to the electric potential difference or voltage existing between the anode and the cathode of said electrolytic cell.

The term "hydrogen evolution catalyst" refers to a composition of matter triggering the transformation of hydrogen ions or protons (H⁺), solvated or not, into hydrogen (H₂).

In the context of the invention, the term "alkaline cell" refers to an electrolytic cell where the half cells are physically separated, yet connected through a water permeable diaphragm selective to hydroxide ions (OH^").

In the context of the invention, the terms "proton exchange membrane cell", "polymer electrolyte membrane cell" or "PEM cell", used interchangeably, refer to an electrolytic cell where the half-cells (anode and cathode) are separated by a polymer electrolyte membrane.

In the context of the invention, the terms "proton exchange membrane", "polymer electrolyte membrane" or "PEM", used interchangeably, refer to a solid polymer membrane acting as an electrolyte, said membrane being responsible for the conduction of protons, the separation of product gases, electrical insulation and separation of the electrodes. Suitable membranes are made of copolymers of sulfonated tetrafluoroethylene based fluoropolymers or polybenzimidazole copolymers. The polymer electrolyte membrane may also serve as a support for the hydrogen evolution catalyst at the cathode and as a support for the oxygen evolution catalyst at the anode.

The terms "photo-electrochemical cell" or "PEC" refers to an electrochemical cell where at least one redox-reaction is conducted by light energy. PECs contain a cathode and an anode in contact with an electrolyte containing reactants that are oxidized or reduced at the respective electrodes of the separated and connected half-cells. In photo-electrochemical cells, the light is converted into chemical energy. The term "electrolyte" refers to any substance containing free ions that make the substance electrically conductive. Examples of suitable electrolytes for the present invention can be, among others, ionic solution electrolytes; molten and solid electrolytes; and gas electrolytes. The term "diaphragm"refers to a porous barrier which prevents the

spontaneous mixing of the electrolytes of the anode of the first chamber and the cathode of the second chamber, but allows the migration of ions from the anode to the cathode, and vice versa, to maintain electrical neutrality. In the context of the invention, the term "nanoparticle" refers to a particle with at least two dimensions at the nanoscale, particularly with all three dimensions at the nanoscale, where the nanoscale is the range about 1 nm to about 100 nm. Particularly, when the nanoparticle is substantially rod-shaped with a substantially circular cross-section, such as a nanowire or a nanotube, the "nanoparticle" refers to a particle with at least two dimensions at the

nanoscale, these two dimensions being the cross-section of the nanoparticle.

The term "size" refers to a characteristic physical dimension. For example, in the case of a nanoparticle that is substantially spherical, the size of the nanoparticle corresponds to the diameter of the nanoparticle. In the case of a nanoparticle that is substantially rod-shaped with a substantially circular cross- section, such as nanowire or a nanotube, the size of the nanoparticle corresponds to the diameter of the cross-section of the nanoparticle. In the case of a nanopartide that is substantially box-shaped, such as a nanocube, a nanobox, or a nanocage, the size of the nanopartide corresponds to the maximum edge length. When referring to a set of nanoparticles as being of a particular size, it is contemplated that the set of nanoparticles can have a distribution of sizes around the specified size. Thus, as used herein, a size of a set of nanoparticles can refer to a mode of a distribution of sizes, such as a peak size of the distribution of sizes. The size of a nanopartide can be determined following methods known in the state of the art, by using, as a non-limiting illustrative example, electron microscopy techniques or size exclusion chromatography.

In the context of the invention, the term "weight fraction", when applied to or referring to a substance, expressed as a percentage or as "wt%", or "weight %", refers to the amount in grams of a substance in one hundred grams of a mixture comprising said substance.

In the context of the invention, the term "weight ratio", when applied to a mixture of two substances A and B, expressed as a ratio, refers to the amount of substance A, in grams, with respect to the amount of substance B, in grams.

In the context of the invention, the terms "membrane electrode assembly" and "MEA" are used interchangeably, and refer to an assembly of a polymer electrolyte membrane with a catalyst, said catalyst being deposited or supported onto the membrane, thereby forming the electrode. Catalyst deposition over PEM can be accomplished by means of several techniques such as screen printing, decal transfer, spray coating, electrospray coating, inkjet printing and electroless deposition using formulations of catalyst as pastes or inks.

In the context of the invention, the term "cobalt oxide loading", expressed in milligrams per square centimeter (mg/cm²), refers to the amount of cobalt oxide comprised in one square centimeter of a support (active surface area). When the electrolytic cell is a PEM cell, the term "cobalt oxide loading" refers to the amount of cobalt oxide comprised in one square centimeter of the polymer electrolyte membrane. The term "catalytically effective amount" refers to the fact that the amount of catalyst is sufficient for the reaction to take place.

In the context of the invention, the term "hydrogen productivity", expressed in grams or kilograms (of hydrogen) per hour and per gram (of active metal or metal oxide in the catalyst), refers to the maximal amount of hydrogen that can be produced during one hour of operation of an electrolytic cell comprising one gram of the active metal or metal oxide in the HER catalyst. When the electrolytic cell is a PEM cell, hydrogen productivity can be derived from current density, expressed in Ampere per square centimeter of the membrane, the cobalt oxide loading (mg/cm²) and its composition (gram of active metal or metal oxide per gram of dry catalyst) using Faraday's law of electrolysis and ideal gas law. The term "active metal or metal oxide in the catalyst" refers to, when referring to the cathode, either platinum or cobalt oxide and, when referring to the anode, iridium, ruthenium, iridium oxide, ruthenium oxide or a mixture thereof.

The terms "cobalt oxide" or "C03O are used interchangeably and refer to a cobalt(ll)cobalt(lll)oxide. Co3O₄ adopts the normal spinel structure, with Co²⁺ ions in tetrahedral interstices and Co³⁺ ions in the octahedral interstices of the cubic close-packed lattice of oxide anions. Co3O₄ is also referred as cobalt (11,111) oxide since it has mixed valences; hence, its formula is sometimes written as Co^llCo^lll ₂O₄ or CoO Co₂O₃.

As mentioned above, the inventors have developed a process for producing hydrogen in an electrolytic cell comprising contacting protons with a cathode comprising a catalytically effective amount of a hydrogen evolution catalyst, wherein the hydrogen evolution catalyst comprises Co₃O₄ and a carbon material, which is conductive or semi-conductive, wherein the electrolytic cell is a proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane, and the process is carried out applying a potential equal to or higher than 1 .8 V between the cathode and the anode.

In a preferred embodiment of the first aspect of the invention, optionally in combination with one or more features of the various embodiments described above or below, the conductive or semi-conductive carbon material is selected from the group consisting of carbon black, graphite, graphene, reduced graphene oxide, carbon nanotubes, fullerene and mixture thereof. More preferably, optionally in combination with one or more features of the various embodiments described above or below, the conductive or semi-conductive carbon material is carbon black.

In a preferred embodiment of the first aspect of the invention, optionally in combination with one or more features of the various embodiments described above or below, the process comprises contacting protons with a cathode comprising a catalytically effective amount of a hydrogen evolution catalyst, wherein Co3O₄ is in the form of nanoparticles. Preferably, optionally in combination with one or more features of the various embodiments described above or below, the size of the nanoparticle is comprised from 10 to 50 nanometers. More preferably, the size of the nanoparticle is comprised from 20 to 40 nanometers.

In another embodiment of the first aspect of the invention, optionally in combination with one or more features of the various embodiments described above or below, the weight ratio of Co3O₄ to carbon material is comprised from 1 :99 to 99:1 . More preferably, the weight ratio of Co3O₄ to carbon material is comprised from 1 :99 to 3:1 . Even more preferably, optionally in combination with one or more features of the various embodiments described above or below, the weight ratio of Co3O₄ to carbon material is comprised from 1 :99 to 1 :1 . Even more preferably, optionally in combination with one or more features of the various embodiments described above or below, the weight ratio of Co3O₄ to carbon material is comprised from 1 :20 to 3:1 . Even more preferably, the weight ratio of Co3O₄ to carbon material is comprised from 1 :20 to 1 :1 . In even more preferred embodiments, optionally in combination with one or more features of the various embodiments described above or below, the weight ratio of Co3O₄ to carbon material is 9:91 . In another more preferred embodiment, optionally in combination with one or more features of the various embodiments described above or below, the weight ratio of Co3O₄ to carbon material is 33:66. In another more preferred embodiment, optionally in combination with one or more features of the various embodiments described above or below, the weight ratio of Co3O₄ to carbon material is 47:53. In another more preferred embodiment, optionally in combination with one or more features of the various embodiments described above or below, the weight ratio of Co3O₄ to carbon material is 70:30.

In another embodiment of the first aspect, optionally in combination with one or more features of the various embodiments described above or below, the process is carried out applying a potential equal to or higher than 2 V. In another embodiment of the first aspect, optionally in combination with one or more features of the various embodiments described above or below, the process is carried out applying a potential equal to or higher than 2.1 V; more preferably the process is carried out applying a potential equal to or higher than 2.2 V; more preferably the process is carried out applying a potential equal to or higher than 2.3 V; more preferably the process is carried out applying a potential equal to or higher than 2.4 V and even more preferably the process is carried out applying a potential equal to or higher than 2.5 V.

In another preferred embodiment of the first aspect, optionally in combination with one or more features of the various embodiments described above or below, the process is carried out applying a potential comprised from 1 .8 to 3 V. In another preferred embodiment of the first aspect, optionally in

combination with one or more features of the various embodiments described above or below, the process is carried out applying a potential comprised from 2 to 3 V. In another preferred embodiment of the first aspect, optionally in combination with one or more features of the various embodiments described above or below, the process is carried out applying a potential comprised from 2.2 to 3 V. In a more preferred embodiment, optionally in combination with one or more features of the various embodiments described above or below, the process is carried out applying a potential comprised from 2.5 to 3 V.

In another embodiment, optionally in combination with one or more features of the various embodiments described above or below, the hydrogen evolution catalyst further comprises a compound facilitating the adhesion of the carbon material and Co3O₄. It is advantageous as it allows for supporting the catalyst onto a solid support. When the electrolytic cell is a PEM cell, this agent may be a polymer electrolyte. In a preferred embodiment of the first aspect, optionally in combination with one or more features of the various

embodiments described above or below, the hydrogen evolution catalyst further comprises a compound facilitating the adhesion of the carbon material and Co₃O₄ and the weight fraction of the compound facilitating the adhesion of the carbon material and Co3O₄ is comprised from 10 to 20%. Preferably, the weight fraction of the compound facilitating the adhesion of the carbon material and Co3O₄ is comprised from 15 to 16%. More preferably, the compound facilitating the adhesion of the carbon material and Co3O₄ is a polymer electrolyte.

In a preferred embodiment, optionally in combination with one or more features of the various embodiments described above or below, the hydrogen evolution catalyst further comprises a polymer electrolyte. When the hydrogen evolution catalyst further comprises a polymer electrolyte, this facilitates the formation of the membrane electrode assembly by adhesion of the catalyst to the membrane thanks to the presence of the polymer electrolyte in the hydrogen evolution catalyst. In a preferred embodiment of the first aspect, optionally in combination with one or more features of the various embodiments described above or below, the hydrogen evolution catalyst further comprises a polymer electrolyte and the weight fraction of the polymer electrolyte is comprised from 10 to 20%. Preferably, the weight fraction of the polymer electrolyte is comprised from 15 to 16%.

In a preferred embodiment, optionally in combination with one or more features of the various embodiments described above or below, the hydrogen evolution catalyst is a supported catalyst. More preferably, the hydrogen evolution catalyst is supported onto a polymer electrolyte membrane. Even more preferably, the hydrogen evolution catalyst further comprises a polymer electrolyte and is supported onto a polymer electrolyte membrane. It is advantageous as it allows for straightforward adhesion of the cathode to the proton exchange membrane.

In a preferred embodiment of the first aspect, optionally in combination with one or more features of the various embodiments described above or below, the cobalt oxide loading on the support is comprised from 0.1 mg per cm² to 20 mg per cm². In a more preferred embodiment, optionally in combination with one or more features of the various embodiments described above or below, the cobalt oxide loading on the support is comprised from 0.4 mg per cm² to 15 mg per cm². In an even more preferred embodiment, optionally in combination with one or more features of the various embodiments described above or below, the cobalt oxide loading on the support is comprised from 0.4 mg per cm² to 5 mg per cm². In another preferred embodiment, optionally in combination with one or more features of the various embodiments described above or below, the cobalt oxide loading on the support is comprised from 0.1 mg per cm² to 1 mg per cm².

In another embodiment of the first aspect of the invention, optionally in combination with one or more features of the various embodiments described above or below, the protons are produced by water oxidation at the anode. Suitable catalysts to carry out anodic water oxidation are known in the art.

Therefore, in a more preferred embodiment, optionally in combination with one or more features of the various embodiments described above or below, the water oxidation reaction comprises contacting water with the anode of the electrolytic cell, wherein the anode comprises a catalytically effective amount of a catalyst selected from RuO₂, Ru, Ir, lrO₂ and mixtures thereof. More preferably, the water oxidation catalyst is lrO₂.

In a more preferred embodiment, optionally in combination with one or more features of the various embodiments described above or below, the loading of the catalyst in the anode is comprised from 0.4 to 5 mg per cm². More preferably, the loading of the catalyst in the anode is about 2 mg per cm².

In another embodiment of the first aspect of the invention, optionally in combination with one or more features of the various embodiments described above or below, the catalysts comprised at the anode and the cathode are supported onto a polymer electrolyte membrane.

In a more preferred embodiment, optionally in combination with one or more features of the various embodiments described above or below, the polymer electrolyte membrane is a sulfonated tetrafluoroethylene based fluoropolymer- copolymer. Even more preferably, the polymer electrolyte membrane is a copolymer of tetrafluoroethylene and 1 ,1 ,2,2-tetrafluoro-2-((1 ,1 ,1 ,2,3,3- hexafluoro-3-((1 ,2,2-trifluorovinyl)oxy)propan-2-yl)oxy)ethanesulfonic acid (commercially available with the tradename Nafion 1 17). Other suitable polymer electrolyte membranes are known in the art and will become apparent to the skilled in the art upon reduction to practice of the invention. When the process of the invention is carried out at potentials comprised from 1 .8 to 3 V and with low cobalt oxide loading (from 0.4 mg per cm² to 15 mg per cm²), the productivity of hydrogen is high, since high current densities are obtained.

In a more preferred embodiment, optionally in combination with one or more features of the various embodiments described above or below, the

membrane is at a temperature comprised from 70 °C to 90 °C. More preferably, the temperature of the membrane is about 80 °C.

In another embodiment of the first aspect of the invention, water flows continuously through the anodic half-cell. This is advantageous as this allows for continuous electrolytic operation. In another embodiment, optionally in combination with one or more features of the various embodiments described above or below, the invention relates to a process for producing hydrogen in an electrolytic cell, wherein the electrolytic cell is a proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane comprising contacting protons with a cathode comprising a catalytically effective amount of a hydrogen evolution catalyst comprising Co₃O₄ and a carbon material wherein:

the carbon material is carbon black; Co3O₄ is in the form of nanoparticles and the size of the nanoparticles is comprised from 10 to 50 nanometers; and the weight ratio of catalyst to carbon material is comprised from 1 :20 to 3:1 .

In another embodiment, optionally in combination with one or more features of the various embodiments described above or below, the invention relates to a process for producing hydrogen in an electrolytic cell, wherein the electrolytic cell is a proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane, comprising contacting protons with a cathode comprising a catalytically effective amount of a hydrogen evolution catalyst comprising Co₃O₄ and a carbon material wherein:

the carbon material is carbon black; Co3O₄ is in the form of nanoparticles and the size of the nanoparticles is comprised from 10 to 50 nanometers, the hydrogen evolution catalyst is supported and the cobalt oxide loading on the support is comprised from 0.4 mg per cm² to 15 mg per cm². In another embodiment, optionally in combination with one or more features of the various embodiments described above or below, the invention relates to a process for producing hydrogen in an electrolytic cell, wherein the electrolytic cell is a proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane, comprising contacting protons with a cathode comprising a catalytically effective amount of a hydrogen evolution catalyst comprising Co₃O₄ and a carbon material wherein:

the carbon material is carbon black; Co3O₄ is in the form of nanoparticles and the size of the nanoparticles is comprised from 10 to 50 nanometers;

the weight ratio of catalyst to carbon material is comprised from 1 :20 to 3:1 ; the hydrogen evolution catalyst is supported and the cobalt oxide loading on the support is comprised from 0.4 mg per cm² to 15 mg per cm².

the carbon material is carbon black; Co3O₄ is in the form of nanoparticles and the size of the nanoparticles is comprised from 10 to 50 nanometers; and the process is carried out applying a potential comprised from 1 .8 to 3 V. In another embodiment, optionally in combination with one or more features of the various embodiments described above or below, the invention relates to a process for producing hydrogen in an electrolytic cell, wherein the electrolytic cell is a proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane, comprising contacting protons with a cathode comprising a catalytically effective amount of a hydrogen evolution catalyst comprising Co₃O₄ and a carbon material wherein:

the carbon material is carbon black; Co3O₄ is in the form of nanoparticles and the size of the nanoparticles is comprised from 10 to 50 nanometers and the process is carried out applying a potential comprised from 2.2 to 3 V.

In another embodiment, optionally in combination with one or more features of the various embodiments described above or below, the invention relates to a process for producing hydrogen in an electrolytic cell wherein the electrolytic cell is a proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane, comprising contacting protons with a cathode comprising a catalytically effective amount of a hydrogen evolution catalyst comprising Co₃O₄ and a carbon material wherein:

the weight ratio of catalyst to carbon material is comprised from 1 :20 to 3:1 ; the hydrogen evolution catalyst is supported, the cobalt oxide loading on the support is comprised from 0.4 mg per cm² to 15 mg per cm²; and

the process is carried out applying a potential comprised from 1 .8 to 3 V.

the process is carried out applying a potential comprised from 2.2 to 3 V.

the weight ratio of catalyst to carbon material is comprised from 1 :20 to 3:1 ; the hydrogen evolution catalyst is supported; the cobalt oxide loading on the support is comprised from 0.4 mg per cm² to 15 mg per cm²;

the process is carried out applying a potential comprised from 1 .8 to 3 V; and the protons are produced by water oxidation at the anode.

Co3O₄ is in the form of nanoparticles and the size of the nanoparticles is comprised from 10 to 50 nanometers;

the carbon material is carbon black;

the process is carried out applying a potential comprised from 1 .8 to 3 V; the protons are produced by water oxidation at the anode; and

the anode comprises a catalytically effective amount of a catalyst selected from Ru, RuO2, lrO2, lr and mixtures thereof.

the carbon material is carbon black;

the catalysts comprised at the anode and the cathode are supported onto a polymer electrolyte membrane. In another embodiment, optionally in combination with one or more features of the various embodiments described above or below, the invention relates to a process for producing hydrogen in an electrolytic cell wherein the electrolytic cell is a proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane, comprising contacting protons with a cathode comprising a catalytically effective amount of a hydrogen evolution catalyst comprising Co₃O₄ and a carbon material wherein:

the carbon material is carbon black;

the weight ratio of catalyst to carbon material is comprised from 1 :20 to 3:1 ; the hydrogen evolution catalyst is supported;

the cobalt oxide loading on the support is comprised from 0.4 mg per cm² to 15 mg per cm²;

the process is carried out applying a potential comprised from 1 .8 to 3 V; the protons are produced by water oxidation at the anode;

the anode comprises a catalytically effective amount of a catalyst selected from Ru, RuO2, lrO2, lr and mixtures thereof; and

the catalysts comprised at the anode and the cathode are supported onto a polymer electrolyte membrane.

the carbon material is carbon black;

the weight ratio of catalyst to carbon material is comprised from 1 :20 to 3:1 ; the hydrogen evolution catalyst further comprises a compound facilitating the adhesion of the carbon material and Co3O₄;

the hydrogen evolution catalyst is supported;

the carbon material is carbon black;

the hydrogen evolution catalyst is supported;

the anode comprises a catalytically effective amount of a catalyst selected from Ru, RUO2, lrO2, lr and mixtures thereof;

the catalyst loading of the catalyst in the anode is about 2 mg per cm²; and the catalysts comprised at the anode and the cathode are supported onto a polymer electrolyte membrane.

In another embodiment, optionally in combination with one or more features of the various embodiments described above or below, the invention relates to a process for producing hydrogen in an electrolytic cell, wherein the electrolytic cell is a proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane, comprising contacting protons with a cathode comprising a catalytically effective amount of a hydrogen evolution catalyst comprising Co₃O₄ and a carbon material wherein: Co3O₄ is in the form of nanoparticles and the size of the nanoparticles is comprised from 10 to 50 nanometers;

the carbon material is carbon black;

the hydrogen evolution catalyst is supported;

the process is carried out applying a potential comprised from 1 .8 to 3 V;

the protons are produced by water oxidation at the anode;

the catalyst loading of the catalyst in the anode is about 2 mg per cm²;

the catalysts comprised at the anode and the cathode are supported onto a polymer electrolyte membrane; and

the temperature of the membrane is about 80 °C.

The process of the invention is advantageous as it allows for reaching high productivity of hydrogen using noble-metal free catalyst for the hydrogen evolution reaction. Despite the application of relatively high potentials to the electrolytic cell, which implies high energetic costs for the process, the exceptionally high current densities observed with the process of the invention at low hydrogen evolution catalyst loadings allows balancing the energetic cost of the process with a high amount of hydrogen produced with respect to the amount of time, energy and catalyst engaged in the reaction.

In another preferred embodiment, optionally in combination with one or more features of the various embodiments described above or below, the process of the invention further comprises the previous step of preparing the membrane electrode assembly comprising the steps of:

(i) preparing the hydrogen evolution catalyst by physical mixing of cobalt oxide and the conductive or semi-conductive carbon material;

(ii) suspending the mixture of step (i) and an amount of the polymer electrolyte in a solvent; and

(iii) depositing the suspension obtained in (ii) onto the membrane. Preferably, the solvent used in step (ii) is selected from the group consisting of water, methanol, ethanol, propanol, butanol and isopropanol. More preferably, the solvent is isopropanol. Suitable ways of supporting the catalyst onto the membrane are known in the art and include screen printing, decal transfer, spray coating, electrospray coating, inkjet printing and electroless deposition. The preferred method for carrying out step (iii) is spray coating. The invention discloses a hydrogen evolution catalyst comprising cobalt oxide nanoparticles and carbon black wherein the weight ratio of cobalt oxide to carbon black is comprised from 1 :20 to 3:1 and the size of the cobalt oxide nanoparticles is comprised from 10 to 50 nm. All the embodiments disclosed above for the weight ratio of Co3O₄ to carbon material, the optional presence in certain amounts of a compound facilitating the adhesion of the carbon material and Co3O₄, size and morphology of the cobalt oxide nanoparticles comprised in the hydrogen evolution catalyst of the first aspect also apply for the hydrogen evolution catalyst.

More preferably, optionally in combination with one or more features of the various embodiments described above or below, the size of the cobalt oxide nanoparticles is comprised from 20 to 40 nm. In another embodiment, optionally in combination with one or more features of the various embodiments described above or below, the hydrogen evolution catalyst comprises cobalt oxide nanoparticles and carbon black wherein the weight ratio of cobalt oxide to carbon black is comprised from 1 :20 to 1 :1 . In another embodiment, the hydrogen evolution catalyst comprises cobalt oxide nanoparticles and carbon black wherein the weight ratio of cobalt oxide to carbon black is 9:91 . In another embodiment, the hydrogen evolution catalyst comprises cobalt oxide nanoparticles and carbon black wherein the weight ratio of cobalt oxide to carbon black is 33:67. In another embodiment, the hydrogen evolution catalyst comprises cobalt oxide nanoparticles and carbon black wherein the weight ratio of cobalt oxide to carbon black is 47:53. In another embodiment, the hydrogen evolution catalyst comprises cobalt oxide nanoparticles and carbon black wherein the weight ratio of cobalt oxide to carbon black is 70:30.

In another embodiment, optionally in combination with one or more features of the various embodiments described above or below, the hydrogen evolution catalyst further comprises a compound facilitating the adhesion of the carbon material and Co3O₄. Preferably, the compound facilitating the adhesion of the carbon material and Co3O₄ is a polymer electrolyte. In a more preferred embodiment, optionally in combination with one or more features of the various embodiments described above or below, the polymer electrolyte is a sulfonated tetrafluoroethylene based fluoropolymer-copolymer.

In a preferred embodiment, optionally in combination with one or more features of the various embodiments described above or below, the hydrogen evolution catalyst further comprises a compound facilitating the adhesion of the carbon material and Co3O₄ and the weight fraction of the compound facilitating the adhesion of the carbon material and Co3O₄ is comprised from 10 to 20%. Preferably, the weight fraction of the compound facilitating the adhesion of the carbon material and Co3O₄ is comprised from 15 to 16%. In another embodiment, the invention relates to a hydrogen evolution catalyst comprising cobalt oxide nanoparticles and carbon black wherein:

the weight ratio of cobalt oxide to carbon black is comprised from 1 :20 to 3:1 ; the size of the cobalt oxide nanoparticles is comprised from 20 to 40 nm; and the hydrogen evolution catalyst further comprises a polymer electrolyte.

In another embodiment, the hydrogen evolution catalyst comprises cobalt oxide nanoparticles and carbon black wherein the weight ratio of cobalt oxide to carbon black is comprised from 1 :20 to 1 :1 ; and the hydrogen evolution catalyst further comprises a polymer electrolyte.

In another embodiment, the hydrogen evolution catalyst comprises cobalt oxide nanoparticles and carbon black wherein the weight ratio of cobalt oxide to carbon black is comprised from 1 :20 to 1 :1 ; the size of the cobalt oxide nanoparticles is comprised from 20 to 40 nm; and the hydrogen evolution catalyst further comprises a polymer electrolyte.

In another embodiment, the hydrogen evolution catalyst comprises cobalt oxide nanoparticles and carbon black wherein the weight ratio of cobalt oxide to carbon black is comprised from 1 :20 to 3:1 ; and the hydrogen evolution catalyst further comprises a polymer electrolyte.

In another embodiment, the hydrogen evolution catalyst comprises cobalt oxide nanoparticles and carbon black wherein the weight ratio of cobalt oxide to carbon black is comprised from 1 :20 to 3:1 ; the size of the cobalt oxide nanoparticles is comprised from 20 to 40 nm; and the hydrogen evolution catalyst further comprises a polymer electrolyte. The invention discloses a hydrogen evolution catalyst comprising cobalt oxide nanoparticles and carbon black wherein:

the weight ratio of cobalt oxide to carbon black is comprised from 1 :20 to 3:1 ; the hydrogen evolution catalyst further comprises a polymer electrolyte; and the weight fraction of the polymer electrolyte is comprised from 10 to 20%.

The invention discloses a hydrogen evolution catalyst comprising cobalt oxide nanoparticles and carbon black wherein:

the weight ratio of cobalt oxide to carbon black is comprised from 1 :20 to 3:1 ; the size of the cobalt oxide nanoparticles is comprised from 20 to 40 nm; the hydrogen evolution catalyst further comprises a polymer electrolyte; and the weight fraction of the polymer electrolyte is comprised from 10 to 20%.

In another embodiment, the hydrogen evolution catalyst comprises cobalt oxide nanoparticles and carbon black wherein the weight ratio of cobalt oxide to carbon black is comprised from 1 :20 to 1 :1 ; the size of the cobalt oxide nanoparticles is comprised from 20 to 40 nm; the hydrogen evolution catalyst further comprises a polymer electrolyte; and the weight fraction of the polymer electrolyte is comprised from 10 to 20%. In another embodiment, the hydrogen evolution catalyst comprises cobalt oxide nanoparticles and carbon black wherein the weight ratio of cobalt oxide to carbon black is comprised from 1 :20 to 1 :1 ; the hydrogen evolution catalyst further comprises a polymer electrolyte; and the weight fraction of the polymer electrolyte is comprised from 10 to 20%.

The hydrogen evolution catalyst is obtainable by physically mixing Co3O₄, the carbon material and, optionally, a compound facilitating the adhesion of the carbon material and Co3O₄. As described above, a second aspect of the invention relates to a proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane comprising a catalytically effective amount of a hydrogen evolution catalyst, wherein the hydrogen evolution catalyst comprises Co3O₄ and a carbon material.

All the embodiments disclosed above for the hydrogen evolution catalyst also apply for the proton exchange membrane cell of the second aspect of the invention.

In an embodiment of the second aspect, optionally in combination with one or more features of the various embodiments described above or below, the hydrogen evolution catalyst comprised in the cathode of the proton exchange membrane cell is that wherein the Co3O₄ is in form of nanoparticles; the carbon material is carbon black; the weight ratio of Co₃O₄ to carbon black is comprised from 1 :20 to 3:1 ; and the size of the Co3O₄ nanoparticles is comprised from 10 to 50 nm.

In an embodiment of the second aspect, optionally in combination with one or more features of the various embodiments described above or below, the hydrogen evolution catalyst comprised in the cathode of the proton exchange membrane cell is that wherein the weight ratio of Co3O₄ to carbon black is comprised from 1 :20 to 1 :1

In an embodiment of the second aspect, optionally in combination with one or more features of the various embodiments described above or below, the hydrogen evolution catalyst comprised in the cathode of the proton exchange membrane cell is that wherein the size of the Co3O₄ nanoparticles is comprised from 20 to 40nm.

In an embodiment of the second aspect, optionally in combination with one or more features of the various embodiments described above or below, the hydrogen evolution catalyst comprised in the cathode of the proton exchange membrane cell comprises nanoparticles of Co3O₄ and carbon black wherein the weight ratio of Co₃O₄ to carbon black is 9:91 . In an embodiment of the second aspect, optionally in combination with one or more features of the various embodiments described above or below, the hydrogen evolution catalyst comprised in the cathode of the proton exchange membrane cell comprises nanoparticles of Co3O₄ and carbon black wherein the weight ratio of Co₃O₄ to carbon black is 33:67. In an embodiment of the second aspect, optionally in combination with one or more features of the various

embodiments described above or below, the hydrogen evolution catalyst comprised in the cathode of the proton exchange membrane cell comprises nanoparticles of Co3O₄ and carbon black wherein the weight ratio of Co3O₄ to carbon black is 47:53. In an embodiment of the second aspect, optionally in combination with one or more features of the various embodiments described above or below, the hydrogen evolution catalyst comprised in the cathode of the proton exchange membrane cell comprises nanoparticles of Co3O₄ and carbon black wherein the weight ratio of Co₃O₄ to carbon black is 70:30.

In an embodiment of the second aspect, optionally in combination with one or more features of the various embodiments described above or below, the hydrogen evolution catalyst comprised in the cathode of the proton exchange membrane cell further comprises a compound facilitating the adhesion of the carbon material and Co3O₄. Preferably, the compound facilitating the adhesion of the carbon material and Co3O₄ comprised in the hydrogen evolution catalyst of the proton exchange membrane cell of the second aspect is a sulfonated tetrafluoroethylene based fluoropolymer-copolymer; more preferably, the compound facilitating the adhesion of the carbon material and Co3O₄ comprised in the hydrogen evolution catalyst of the proton exchange membrane cell of the second aspect is a co-polymer of tetrafluoroethylene and 1 ,1 ,2,2-tetrafluoro-2-((1 ,1 ,1 ,2,3,3-hexafluoro-3-((1 ,2,2- trifluorovinyl)oxy)propan-2-yl)oxy)ethanesulfonic acid. In an embodiment of the second aspect, optionally in combination with one or more features of the various embodiments described above or below, the hydrogen evolution catalyst comprised in the cathode of the proton exchange membrane cell is that wherein the weight fraction of the compound facilitating the adhesion of the carbon material and Co3O₄ is comprised from 10 to 20%.

In an embodiment of the second aspect, optionally in combination with one or more features of the various embodiments described above or below, the proton exchange membrane cell which has an anode and a cathode

separated by a polymer electrolyte membrane is such that the polymer electrolyte membrane and the compound facilitating the adhesion of the carbon material and Co3O₄ are a sulfonated tetrafluoroethylene based fluoropolymer-copolymer. Preferably, the sulfonated tetrafluoroethylene based fluoropolymer-copolymer is a co-polymer of tetrafluoroethylene and 1 ,1 ,2,2- tetrafluoro-2-((1 ,1 ,1 ,2,3,3-hexafluoro-3-((1 ,2,2-trifluorovinyl)oxy)propan-2- yl)oxy)ethanesulfonic acid.

In an embodiment of the second aspect, optionally in combination with one or more features of the various embodiments described above or below, the proton exchange membrane cell comprises a sulfonated tetrafluoroethylene based fluoropolymer-copolymer; preferably a co-polymer of

tetrafluoroethylene and 1 ,1 ,2,2-tetrafluoro-2-((1 ,1 ,1 ,2,3,3-hexafluoro-3-((1 ,2,2- trifluorovinyl)oxy)propan-2-yl)oxy)ethanesulfonic acid, and an anode comprising lrO₂ as a catalyst.

In a preferred embodiment of the second aspect, optionally in combination with one or more features of the various embodiments described above or below, the proton exchange membrane cell comprises the hydrogen evolution catalyst wherein the weight ratio of Co₃O₄ to carbon black is comprised from 1 :20 to 3:1 , said hydrogen evolution catalyst further comprising a compound facilitating the adhesion of the carbon material and Co₃O₄ consisting of a copolymer of tetrafluoroethylene and 1 ,1 ,2,2-tetrafluoro-2-((1 ,1 ,1 ,2,3,3- hexafluoro-3-((1 ,2,2-trifluorovinyl)oxy)propan-2-yl)oxy)ethanesulfonic acid, wherein the weight fraction of the compound facilitating the adhesion of the carbon material and Co3O₄ is comprised from 10 to 20%; a proton exchange membrane consisting of a co-polymer of tetrafluoroethylene and 1 ,1 ,2,2- tetrafluoro-2-((1 ,1 ,1 ,2,3,3-hexafluoro-3-((1 ,2,2-trifluorovinyl)oxy)propan-2- yl)oxy)ethanesulfonic acid; and an anode comprising lrO₂ as a catalyst.

In a preferred embodiment of the second aspect, optionally in combination with one or more features of the various embodiments described above or below, the proton exchange membrane cell comprisesa hydrogen evolution catalyst selected from the group consisting of the hydrogen evolution catalyst wherein the weight ratio of Co₃O₄ to carbon black is 9:91 , the hydrogen evolution catalyst wherein the weight ratio of Co₃O₄ to carbon black is 33:67, the hydrogen evolution catalyst wherein the weight ratio of Co₃O₄ to carbon black is 47:53, the hydrogen evolution catalyst wherein the weight ratio of Co₃O₄ to carbon black is 70:30, said hydrogen evolution catalyst further comprising a compound facilitating the adhesion of the carbon material and Co3O₄ consisting of a co-polymer of tetrafluoroethylene and 1 ,1 ,2,2- tetrafluoro-2-((1 ,1 ,1 ,2,3,3-hexafluoro-3-((1 ,2,2-trifluorovinyl)oxy)propan-2- yl)oxy)ethanesulfonic acid, wherein the weight fraction of the compound facilitating the adhesion of the carbon material and Co3O₄ is comprised from 10 to 20%; a proton exchange membrane consisting of a co-polymer of tetrafluoroethylene and 1 ,1 ,2,2-tetrafluoro-2-((1 ,1 ,1 ,2,3,3-hexafluoro-3-((1 ,2,2- trifluorovinyl)oxy)propan-2-yl)oxy)ethanesulfonic acid; and an anode comprising lrO₂ as a catalyst.

In an embodiment of the second aspect, optionally in combination with one or more features of the various embodiments described above or below, the proton exchange membrane cell further comprises: (i) at least a diffusion layer, and (ii) current collectors with internal flow field. In a preferred embodiment of the second aspect, optionally in combination with one or more features of the various embodiments described above or below, the proton exchange membrane cell further comprises: (i) at least a diffusion layer consisting of porous titanium gas diffusion layer, and (ii) current collectors with internal flow field consisting of titanium collectors.

In an embodiment of the second aspect, optionally in combination with one or more features of the various embodiments described above or below, the proton exchange membrane cell comprises a hydrogen evolution catalyst selected from the group consisting of the hydrogen evolution catalyst wherein the weight ratio of Co₃O₄ to carbon black is 9:91 , the hydrogen evolution catalyst wherein the weight ratio of Co₃O₄ to carbon black is 33:67, the hydrogen evolution catalyst wherein the weight ratio of Co₃O₄ to carbon black is 47:53, the hydrogen evolution catalyst wherein the weight ratio of Co₃O₄ to carbon black is 70:30, said hydrogen evolution catalyst further comprising a compound facilitating the adhesion of the carbon material and Co3O₄ consisting of a co-polymer of tetrafluoroethylene and 1 ,1 ,2,2-tetrafluoro-2- ((1 ,1 ,1 ,2,3,3-hexafluoro-3-((1 ,2,2-trifluorovinyl)oxy)propan-2- yl)oxy)ethanesulfonic acid, wherein the weight fraction of the compound facilitating the adhesion of the carbon material and Co3O₄ is comprised from

10 to 20%; a proton exchange membrane consisting of a co-polymer of tetrafluoroethylene and 1 ,1 ,2,2-tetrafluoro-2-((1 ,1 ,1 ,2,3,3-hexafluoro-3-((1 ,2,2- trifluorovinyl)oxy)propan-2-yl)oxy)ethanesulfonic acid; and an anode comprising lrO₂ as a catalyst; (i) at least a diffusion layer consisting of porous titanium gas diffusion layer; and (ii) current collectors with internal flow field consisting of titanium collectors. In the proton exchange membrane cell of the invention, the diffusion layer and current collector are preferably separated with a PTFE gasket. In an embodiment of the second aspect, optionally in combination with one or more features of the various embodiments described above or below, the proton exchange membrane cell comprises a hydrogen evolution catalyst wherein the weight ratio of Co3O₄ to carbon black is comprised from 1 :20 to 3:1 , said hydrogen evolution catalyst further comprising a compound facilitating the adhesion of the carbon material and Co₃O₄ consisting of a copolymer of tetrafluoroethylene and 1 ,1 ,2,2-tetrafluoro-2-((1 ,1 ,1 ,2,3,3- hexafluoro-3-((1 ,2,2-trifluorovinyl)oxy)propan-2-yl)oxy)ethanesulfonic acid, wherein the weight fraction of the compound facilitating the adhesion of the carbon material and Co3O₄ is comprised from 10 to 20%; a proton exchange membrane consisting of a co-polymer of tetrafluoroethylene and 1 ,1 ,2,2- tetrafluoro-2-((1 ,1 ,1 ,2,3,3-hexafluoro-3-((1 ,2,2-trifluorovinyl)oxy)propan-2- yl)oxy)ethanesulfonic acid; and an anode comprising lrO₂ as a catalyst; (i) at least a diffusion layer consisting of porous titanium gas diffusion layer; and (ii) current collectors with internal flow field consisting of titanium collectors. In the proton exchange membrane cell of the invention, the diffusion layer and current collector are preferably separated with a PTFE gasket(s). As detailed in the Examples below, Co3O₄ was found to be an efficient, inexpensive cathode catalyst for HER in PEM electrolysis. Addition of carbon materials to the HER active material (Co3O₄) positively influenced the performance of the physically mixed composites due to enhanced electrical conductivity and efficient use of active HER sites. The material enhanced its performance particularly at high potentials and notably it showed linear response of current density against cell voltage variations. Strikingly excellent long-term stability at 2.5 V up to 24 h of continuous PEM electrolysis was shown, elucidating robustness of Co3O₄ based materials under harsh conditions. Low cost and high abundance of Co3O₄, excellent stability and high current density when used as cathode material in PEM electrolysis, and facile preparation of MEA are encouraging features to use Co3O₄ for practical application, especially as a composite with carbon material to boost HER activity. It is also important to mention about practical implications of the required high cell voltage to achieve high current density which can exceed Pt-black cathode.

Throughout the description and claims the word "comprises" and variations of the word, are not intended to exclude other technical features, additives, components or steps. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention. The following examples and drawings are provided by way of illustration, and they are not intended to be limiting of the present invention. Furthermore, the present invention covers all possible combinations of particular and preferred embodiments described herein.

EXAMPLES

Preparative example 1 : Preparation of the catalysts

Carbon black (CAS number: 1333-86-4, Vulcan, Fuel Cell Store) was used as received. Its surface area is around ca. 250 m²/g and the electrical conductivity is about 70 S/m. Carbon black has amorphous crystalline structure and commonly acts as a conductive support or substrate.

Co3O₄ (>99.5%, nanopowder,particle size <50 nm) was used as purchased from Sigma Aldrich (CAS number: 1308-06-1 ) without further purification.

The catalysts used in the process of the invention were prepared by mixing physically the amounts of carbon black and cobalt oxide shown in Table 1 using a mortar and pestle.

Table 1 Preparative example 2: Preparation of the electrodes a) Pre-treatment of the electrolyte membrane

Nafion 1 17 membrane (CAS number: 31 175-20-9, 175 μηη thick, Ion-Power) was used as the proton transport medium. Prior to use, the Nafion membrane was treated by washing in 5 vol% H2O2 aqueous solution for 1 h at 80 °C to eliminate residual organic impurities, then in 0.5 M sulphuric acid for 1 h at 80 °C to incorporate water molecules and to activate the membrane by hydration. The membrane was further treated with boiling water for 1 h in order to remove possible impurities remaining from previous treatments. The pre- treated membranes were stored in water at ambient temperature prior to deposition of the catalyst materials. Furthermore, treated membranes were kept in flat shape so that further catalyst deposition would be easier, avoiding possible problems related to membrane bending and fixation over the holder used for catalyst deposition . b) Preparation of the catalyst inks and deposition

As solid particles, catalysts cannot be coated over the membrane or the electrode since there would not be any physical binding area between catalyst and membrane surface and as a result catalysts could get easily removed from the Nafion surface. In order to avoid this, the catalyst should be prepared in the form of either paste (for screen printing) or ink (for decal transfer, spray coating, inkjet printing or electroless deposition). Typical ink formulation contains dry catalyst, Nafion solution and solvent (e.g. methanol, ethanol, isopropanol or water). Catalyst ink composition varies depending on the purpose and the amount of the material to be deposited over the membrane, referred as a dry cobalt oxide loading per unit of surface area or most commonly known as loading (mgcat cm²). In order to provide ionic transport to catalytic active sites, addition of Nafion solution (resulting in dry Nafion after deposition) to the catalyst ink is required. Nafion helps to form a thin layer of catalyst after the deposition over the membrane surface ensuring a proper proton transport from the membrane to the catalyst surface. Catalyst inks were made of a mixture of catalyst powder (amount depending on cobalt oxide loading over membrane surface, mgcat cm²), isopropanol (4 ml_) and Nafion solution (Nafion D-521 dispersion, 5 wt% in water and 1 - propanol, > 0.92 meq/g exchange capacity, Alfa Aesar); in a weight fraction of 15.4% dry Nafion in total dry solid. The ink was sonicated for 1 h using an ultrasound bath (120W/ 60Hz, BANDELIN SONOREX).

Catalysts were coated over a Nafion membrane (active area of 2x2 cm²) by using a common airbrush (dnozzie: 0.5 mm, Ventus Titan) and fixing Nafion membrane in a heated holder with open window. Deposition temperature was set at 85 °C to evaporate the solvent during the deposition avoiding

membrane swelling and allowing the formation of a homogeneous and uniform catalyst layer on both sides of Nafion membrane. The total membrane surface covered by catalyst is termed as active area; this is the portion of area which is used to determine the cobalt oxide loading. The final catalyst layer will act as an electrode (i.e., anode or cathode depending on catalyst).

Table 2 shows the different cathode compositions prepared:

Metal loading is referred to as the amount of metal (i.e. Co₃0₄ or Pt black) per square centimeter of the electrode.

Table 2

In electrolysis experiments described below, the anode, prepared according to a similar procedure as for cathode preparation, contains a catalyst loading of 2 mg/cm² of lrO₂, where the dry ink contains 84.6 wt% lrO₂ and 15.4 wt%

Nafion.

Preparative example 3: PEM cell assembly

A PEM cell is based on a multilayer arrangement where the most important parts are (i) membrane electrode assembly (MEA, Figura 4 b (1 )), (ii) diffusion layers (Figure 4 b (2)) and (iii) current collectors with internal flow field (Figure 4 b (4)). In our design, we made use of: (1 ) Nafion 1 17 MEA, coated with electrode catalysts (anode and cathode), as described above, (2) porous titanium (Ti) gas diffusion layers (GDL, 2x2 or 4x4 cm², 1 mm thick, dpores: 50 pm, Sintered-Filter, XINXIANG FILTER TECHNOLOGY CO., LTD) and (4) Ti current collectors (2 mm thick) with a CNC machined flow field (1 cm³ of total channel volume, with face filling single serpentine groove).

The MEA (Figure 4 b (1 )) and GDL (Figure 4 b (2)) were sandwiched between the current collectors (Figure 4 b (4)), which were held together by 8 screws aluminium cell housing plates (1 cm thick) (Figure 4 b (6)). Sealing and internal electrode insulation was provided by PTFE gaskets (Figure 4 b (3) and (5)). Screw torque was set at 5 N m, providing a gas and liquid leak-free system.

Power supply electrical connectors were directly plugged to the current collector (Figure 4 b (4)) plates through the connection pins. Figure 4 shows the assembly of the PEM cell and Figure 5 shows the designed current collector (Figure 4 b (4)).

Example 1 : Electrolysis experiments The system for electrolysis experiments consists in four main components, namely (i) PEM electrolyser, (ii) peristaltic pump system and water reservoir, (iii) temperature controller and (iv) computer controlled potentiostat. A peristaltic pump (Ismatec Reglo Digital, 2 channels), connected to the current plate (Figure 4, (4)) was employed to continuously supply water to the system. The temperature controller was used to set the desired working temperature

(80 °C, optimum value for Nafion membrane operation) inside the

electrochemical reactor. Voltage (EMF, V), current (I, A) and current density (j, A/CIT12) values, were controlled, measured and recorded using a power supply/potentiostat system (CPX400DP, 420 DC Power Supply, 60 V max, 20 A max), controlled by a custom LabVIEW based interface. Evolved gases (O2 from anode and htefrom cathode) were released to the atmosphere. MEA layer-by-layer composition for all water electrolysis tests was lrO2 1 Nafion I CC, where CC refers to the cathode catalyst described in Table 2 (Pt black, C03O4, and Co3O4/Vulcan).

Polarization curves were recorded from 1 .2 V up to 2.7 V by means of potential sweep measurements with AV= 0.05 V/step. Liquid water flow rate of 0.5 mL/min was found to be optimum for our studies. Table 3 shows the results obtained for electrolysis experiments carried out with the electrodes described in Table 2 and using the catalysts of Table 1 . Figure 1 shows a polarization plot of the data compiled in Table 3. The results of Table 3 and Figure 1 show that the process of the invention, using cobalt-based hydrogen evolution catalysts at high potential, i.e. equal to or higher than 1 .8 V, allow for obtaining higher current density values than with cobalt based catalysts of the state of the art. In particular, when high potentials are applied, current density values equal to or higher than those obtained using platinum hydrogen evolution catalyst can be obtained, even at comparable cobalt oxide loadings. Such high values have not been reported for the cobalt-based catalyst systems of the state of the art.

Catalyst U (V) j (A/cm²) Catalyst U (V) j (A/cm²) Catalyst U (V) j (A/cm²)

1.20 0.000 1.20 0.001 1.20 0.000

1.25 0.000 1.25 0.001 1.25 0.000

1.30 0.000 1.30 0.001 1.30 0.000

1.35 0.000 1.35 0.001 1.35 0.000

1.40 0.000 1.40 0.001 1.40 0.000

1.45 0.001 1.45 0.002 1.45 0.000

1.50 0.005 1.50 0.004 1.50 0.001

1.55 0.001 1.55 0.007 1.55 0.001

1.60 0.002 1.60 0.012 1.60 0.002

1.65 0.004 1.65 0.019 1.65 0.004

1.70 0.008 1.70 0.028 1.70 0.005

1.75 0.013 1.75 0.040 1.75 0.008

1.80 0.032 1.80 0.054 1.80 0.014

1.85 0.067 1.85 0.090 1.85 0.034

1.90 0.101 1.90 0.169 1.90 0.086

Co⁹ 1.95 0.175 Co⁴⁷ 1.95 0.290 Co¹⁰⁰ 1.95 0.166

2.00 0.287 2.00 0.420 2.00 0.276

2.05 0.407 2.05 0.550 2.05 0.359

2.10 0.521 2.10 0.675 2.10 0.466

2.15 0.637 2.15 0.788 2.15 0.573

2.20 0.751 2.20 0.900 2.20 0.678

2.25 0.864 2.25 1.012 2.25 0.777

2.30 0.975 2.30 1.121 2.30 0.866

2.35 1.076 2.35 1.232 2.35 0.948

2.40 1.172 2.40 1.339 2.40 1.018

2.45 1.262 2.45 1.449 2.45 1.078

2.50 1.346 2.50 1.554 2.50 1.124

2.55 1.426 2.55 1.661 2.55 1.169

2.60 1.503 2.60 1.761 2.60 1.204

2.65 1.583 2.65 1.862 2.65 1.229

2.70 1.668 2.70 1.958 2.70 1.255

Catalyst U (V) j (A/cm²) Catalyst U (V) j (A/cm²) Catalyst U (V) j (A/cm²)

1.20 0.000 1.20 0.000 1.20 0.000

1.25 0.000 1.25 0.000 1.25 0.000

1.30 0.001 1.30 0.000 1.30 0.000

1.35 0.001 1.35 0.000 1.35 0.000

1.40 0.001 1.40 0.000 1.40 0.001

1.45 0.001 1.45 0.000 1.45 0.007

1.50 0.002 1.50 0.000 1.50 0.027

1.55 0.004 1.55 0.000 1.55 0.075

1.60 0.01 1 1.60 0.000 1.60 0.148

1.65 0.027 1.65 0.001 1.65 0.229

1.70 0.040 1.70 0.003 1.70 0.303

1.75 0.069 1.75 0.010 1.75 0.365

1.80 0.108 1.80 0.027 1.80 0.433

1.85 0.156 1.85 0.069 1.85 0.501

1.90 0.21 1 1.90 0.144 1.90 0.569

Co³³ 1.95 0.259 Co⁷⁰ 1.95 0.250 Pt 1.95 0.636

2.00 0.352 2.00 0.363 2.00 0.704

2.05 0.471 2.05 0.475 2.05 0.771

2.10 0.586 2.10 0.583 2.10 0.839

2.15 0.698 2.15 0.687 2.15 0.907

2.20 0.807 2.20 0.782 2.20 0.974

2.25 0.914 2.25 0.871 2.25 1.042

2.30 1.019 2.30 0.949 2.30 1.109

2.35 1.1 18 2.35 1.020 2.35 1.177

2.40 1.217 2.40 1.083 2.40 1.245

2.45 1.313 2.45 1.139 2.45 1.312

2.50 1.406 2.50 1.191 2.50 1.380

2.55 1.492 2.55 1.240 2.55 1.447

2.60 1.606 2.60 1.283 2.60 1.515

2.65 1.707 2.65 1.323 2.65 1.583

2.70 1.809 2.70 1.357 2.70 1.650

Table 3 According to Faraday's law and ideal gas law, the measured current density is theoretically directly proportional to the amount of hydrogen evolved at the electrode. Table 4 reports the theoretical maximal productivity of hydrogen, expressed in grams of hydrogen produced in one hour per gram of metal or metal oxide in the catalyst, as a function of the potential applied to the electrolytic cell. Figure 3 graphically shows the evolution of the productivity as a function of potential for each catalyst.

Table 4

The results of Table 4 demonstrate that the process of the invention allows for obtaining higher productivity of hydrogen than with the cobalt-based catalysts of the state of the art. In some cases, the productivity is even equal to or higher than when platinum is used as a catalyst. Example 2: Stability test A water electrolysis experiment was carried out during 24 hours using Co as a catalyst on the cathode in the same conditions as in Example 1 but for the applied potential. Figure 2 shows the evolution over time of the current density in a water electrolysis experiment carried out at 2.0 V or at 2.5 V. Operating at 2.0 V, a stable and relatively high current density (ca. 0.42 A/cm²) was reached after 4 h. At 12 h indication of slight performance deterioration was observed. Such signal drift was quantified and determined to be around 5 mA cm²-h from 12h to 24h. This small signal drift may arise from the fact that at 2.0 V the electrolysis performance lies within low-high current density transition region. When a potential of 2.5 V was applied, the performance slightly increased over time at the beginning (1 h) because of the hydration effects. After this stabilization period, the system was completely stable without noticeable performance deterioration in PEM electrolysis along the tested period. The example shows that Co3O₄/carbon black is a very stable HER electrocatalyst in PEM electrolysis, especially operated at high potential regime (from 2.0 to 2.7 V) with outstanding long-term stability.

REFERENCES CITED IN THE APPLICATION - Chinese patent CN103014750.

- Chinese patent CN104043453.

- Blanc and co-workers, Abstracts, 66th Southeast Regional Meeting of the American Chemical Society, Nashville, TN, United States, October 16-19 (2014), SERMACS-1 139

- Yan and co-workers, International Journal of Hydrogen Energy, 39, 2014,

13353-13360.

- Chandrasekaran and co-workers, Materials Letters, 136, 2014, 1 18-121 .

- Fontecave and co-workers, patent application WO20131501 16

- Nocera and co-workers, Chem. Soc. Rev., 38, 2009, 109-1 14.

- Jeon and co-workers, International Journal of Hydrogen Energy, 36, 201 1 ,

10587-10592.

- Park and co-workers, Electrochimica Acta, 89, 2013, 516- 522.

Claims

1 . A process for producing hydrogen in an electrolytic cell comprising contacting protons with a cathode comprising a catalytically effective amount of a hydrogen evolution catalyst, wherein the hydrogen evolution catalyst comprises Co3O₄ and a carbon material, which is conductive or semi- conductive, and the process is carried out applying a potential equal to or higher than 1 .8 V between the cathode and the anode;

wherein the electrolytic cell is a proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane.

2. The process according to claim 1 wherein the carbon material is selected from the group consisting of carbon black, graphite, graphene, reduced graphene oxide, carbon nanotubes, fullerene and mixture thereof.

3. The process according to any of the claims 1 and 2 wherein the carbon material is carbon black.

4. The process according to any of the claims 1 to 3 wherein Co3O₄ is in the form of nanopartides and the size of the nanopartides is comprised from 10 to

50 nanometers.

5. The process according to any of the claims 1 to 4 wherein the weight ratio of Co3O₄ to carbon material is comprised from 1 :20 to 3:1 .

6. The process according to any of the claims 1 to 5 wherein the process is carried out applying a potential comprised from 1 .8 to 3 V.

7. The process according to claim 6 wherein the process is carried out applying a potential comprised from 2 to 3 V.

8. The process according to any of the claims 6-7 wherein the process is carried out applying a potential comprised from 2.5 to 3 V.

9. The process according to any of the claims 1 to 8 wherein the hydrogen evolution catalyst further comprises a compound facilitating the adhesion of the carbon material and Co3O₄.

10. The process according to any of the claims 1 -9 wherein the polymer electrolyte membrane and the compound facilitating the adhesion of the carbon material and Co3O₄ are a sulfonated tetrafluoroethylene based fluoropolymer-copolymer.

1 1 . The process according to claim 10 wherein the membrane is at a temperature comprised from 70 °C to 90 °C.

12. The process according to any of the claims 1 -1 1 wherein the cobalt oxide loading on the support is comprised from 0.4 mg per cm² to 15 mg per cm².

13. The process according to any of the claims 1 to 12 wherein the protons are produced by water oxidation at the anode.

14. The process according to claim 13 wherein the water oxidation reaction comprises contacting water with the anode of the electrolytic cell, wherein the anode comprises a catalytically effective amount of a catalyst selected from the group consisting of RuO2, Ru, Ir, lrO2 and mixtures thereof.

15. A proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane comprising a catalytically effective amount of a hydrogen evolution catalyst, wherein the hydrogen evolution catalyst comprises Co3O₄ and a carbon material .

16. The proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane according to claim 15, wherein the Co3O₄ is in form of nanoparticles; the carbon material is carbon black; the weight ratio of Co3O₄ to carbon black is comprised from 1 :20 to 3:1 ; and the size of the Co3O₄ nanoparticles is comprised from 10 to 50 nm.

17. The proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane according to claim 16, wherein the weight ratio of Co3O₄ to carbon black is comprised from 1 :20 to 1 :1 .

18. The proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane according to any of the claims 16-17, wherein the size of the Co3O₄ nanoparticles is comprised from 20 to 40nm.

19. The proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane according to any of the claims 15-18, further comprising a compound facilitating the adhesion of the carbon material and Co3O₄.

20. The proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane according to claim 19, wherein the weight fraction of the compound facilitating the adhesion of the carbon material and Co3O₄ is comprised from 10 to 20%.

21 . The proton exchange exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane according to any of the claims 19-20, wherein the polymer electrolyte membrane and the compound facilitating the adhesion of the carbon material and Co3O₄ are a sulfonated tetrafluoroethylene based fluoropolymer-copolymer.

22. The proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane according to any of the claims 15-21 , further comprising:

(i) at least a diffusion layer, and

(ii) current collectors with internal flow field.

23. The proton exchange membrane cell which has an anode and a cathode separated by a polymer electrolyte membrane according to claim 22, wherein: (i) the diffusion layer is a porous titanium gas diffusion layer; and

(ii) the current collectors with internal flow field are Ti current collectors.