Method for the preparation of a gas diffusion layer using calendering and a gas diffusion layer obtainable by such method

Abstract

The present invention relates to a method for preparing gas-diffusion layers comprising the steps of: combining a layer of supporting material, a layer of the adhesive composition and a layer of the carrier-binder paste into a layered structure; heating the layered structure to a temperature in the range of between equal to or more than 70 °C and equal to or less than 90 °C with a maximum average heating rate of 2.0 °C / min; followed by at least two calendering steps comprising calendering the layered structure at least once at a temperature in the range 90-110 °C; and calendering the layered structure at least once at a temperature of at least 150 °C; wherein each calendering step is carried out using a gap width of 0-0.5 mm smaller than the gap width used in an immediately preceding calendering step.

Description

METHOD FOR THE PREPARATION OF A GAS DIFFUSION LAYER USING CALENDERING AND A GAS DIFFUSION LAYER OBTAINABLE BY SUCH METHODField of the invention

[0001] The present invention relates to a method for the preparation of a gas diffusion layer and a gas diffusion layer obtainable by such method. The invention further relates to methods for preparing a gas diffusion electrode, a gas diffusion electrode obtainable by such method, electrochemical cells comprising such a gas diffusion layer or electrode and to their use in electrochemical conversion processes.Description of the background art

[0002] Gas diffusion electrodes (GDE's) are a known type of electrodes. They can be used in electrochemical processes where one or more reactants are in the gas phase. Examples of such processes are the electrochemical reaction of hydrogen and oxygen in fuel cells and the electrochemical reduction of carbon dioxide.

[0003] As explained by Bidault in his thesis titled "Development of Alkaline Fuel Cell Gas Diffusion Cathodes using new Substrate Materials", Imperial College London, March 2010, chapter 2, in general, gas diffusion electrodes consist of several layers, which fulfil different functions. Bidault describes an electrode structure comprising a backing material (BM), a gas diffusion layer (GDL) and a catalytic or active layer (AL). The BM can serve as a current collector and can for example be a metal mesh. The GDL can supply the reactant gas to the AL and can prevent liquid electrolyte from passing through the electrode. For common, so-called bipolar designs, a conductive GDL comprising mixtures of polytetrafluoroethylene (PTFE) with conducting carbon support are favored. The active layer (AL) contains the catalyst which is usually supported on carbon black and bonded together with PTFE.

[0004] US2014 / 0227634 describes a gas diffusion electrode comprising a gas diffusion layer laminated on a reinforcement member, said gas-diffusion layer consisting of a sintered and cast conductive powder / fluorinated binder composition. The gas diffusion electrode is described to be manufactured by preparing a paste comprising a fluorinated binder and a conductive powder, such as carbon black, calendering the paste into layers, laminating said layers onto a reinforcement member under a heated press at a temperature of 100 °C to 150 °C and a pressure of 12 to 24 kilopascal, and subsequently bringing the temperature up to 300 °C - 400 °C and the pressure to 25-50 kilopascal. Hereafter the pressure was released to atmospheric and the laminated structure was exposed to air to induce sintering. Thereafter the sintered structure was casted under a heated press at a temperature of 300 °C to 400 °C and a pressure of 30 to 60 kilopascal.

[0005] EP4358196 Al describes a method for producing a gas diffusion layer for a fuel cell comprising providing a planar electrically conductive fiber material, coating the fiber material with a precursor to form a microporous layer, and subjecting the coated fiber material to a posttreatment at elevated pressure and optionally elevated temperature.

[0006] W02013 / 037902 describes a method of manufacturing a gas-diffusion electrode comprising the steps of incorporating a fluorinated binder and a conductive powder in a paste, calendering the paste into thin layers and laminating each layer onto a reinforcement layer under a heated press at 100-150 °C and under a pressure of 12-24 kPa until obtaining a laminated structure, which is then further heated to 300-400 °C with an increased pressure of 25-50 kPa toremove the aqueous phase. This is followed by releasing to pressure to induce full sintering and then casting the sintered structure under a heated press at 300-400 °C under a pressure of 30-60 kPa.

[0007] W02020 / 165074 describes a method for the preparation of a gas-diffusion layer comprising the steps of preparing a carrier-binder paste comprising a solvent, a fluorinated binder and conductive particles and preparing an adhesive composition comprising a solvent, a fluorinated binder and essentially no or equal to or less than 15 wt.% of conductive carrier particles. Then, a layer of supporting material, a layer of the adhesive composition and a layer of the carrier binder paste are combined and pressed at a pressure of at least 15 kPa and / or heated to a temperature of at least 300 °C.

[0008] A disadvantage of the methods as described above is that these methods are typically batch-procedures. There is a desire for a more continuous or semi-continuous process to produce multiple gas-diffusion layers in a continuous manner.

[0009] Another disadvantage of the methods of the prior art is that laminating or pressing the layers to remove the aqueous phase may result in crack formation in the achieved layers. Without being bound to theory, the inventors believe that laminating or treatment at high temperatures and / or under high pressures result in an uncontrolled rapid solvent evaporation. The gas formation or solvent evaporation causes cracks to form in the achieved layers. Cracked surfaces in gas-diffusion layers compromise the stability of the gas-diffusion layer or the gas-diffusion electrodes. Gas-diffusion electrodes, comprising a gas-diffusion layer, allow for gaseous reagents to diffuse through the electrode into an electrode compartment in which an electrolyte flows. The presence of cracks in the gas-diffusion layer may disrupt a uniform gas distribution within the electrode leading to areas with an insufficient supply of the gaseous reagent. Furthermore, cracked surfaces may allow the electrolyte to flow through the structure resulting in the flooding of the electrode.

[0010] In particular for the application in electrochemical carbon dioxide reduction, a gasdiffusion electrode needs to be able to withstand increased hydrostatic pressure in large electrochemical cells. In electrochemical cells with sizes larger than small lab-scale cells (for example, cells that are 1 meter tall), the combination of cracks and increased hydrostatic pressure in said cells can lead to the mechanical instability of the GDL. In addition, the additional pressure may cause existing cracks to widen and new cracks to form. Therefore, a stable gas-diffusion electrode that can withstand increased hydraulic pressures is important.Therefore, there is a desire for gas-diffusion layers and electrodes with good mechanical properties which remain mechanically stable during operation for a prolonged time.Brief summary of the invention

[0011] According to a first aspect of the present invention, an improved method for preparing gas-diffusion layers has been developed comprising the steps of:a) preparing a carrier-binder paste comprising a first solvent, a first fluorinated binder and conductive carrier particles; wherein the first solvent comprises water and an alkanol; b) preparing an adhesive composition comprising a second solvent and a second fluorinated binder; wherein the second solvent comprises water;wherein the first and second fluorinated binder comprise one or more fluorinated polymers chosen from the group consisting of polytetrafluoroethylene (PTFE) polymers, perfluoroalkoxy(PFA) polymers, fluorinated ethylene propylene (FEP) polymers and polyvinylidene difluoride (PVDF) polymers;c) providing a supporting material;d) combining a layer of supporting material, a layer of the adhesive composition and a layer of the carrier-binder paste into a layered structure, wherein the layer of the adhesive composition is applied between the layer of supporting material and the layer of the carrier-binder paste;e) heating the layered structure to a temperature in the range of between equal to or more than 70 °C and equal to or less than 90 °C with a maximum average heating rate of 2.0 °C / min; followed byf) at least two calendering steps comprisingfl) calendering the layered structure at least once at a temperature in the range of equal to or more than 90 °C to equal to or less than 110 °C; andf2) after step fl), calendering the layered structure at least once at a temperature of at least 150 °C;wherein each calendering step is carried out using a gap width in the range of equal to or greater than 0 to equal to or smaller than 0.5 mm smaller than the gap width used in an immediately preceding calendering step.

[0012] The method according to the invention allows for a controlled and slow evaporation of the solvent while forming the gas diffusion layer as a mechanically stable layered structure. The method advantageously results in a gas diffusion layer with reduced levels of cracking.

[0013] The method according to the invention allows for a more continuous process to produce gas-diffusion layers with suitable mechanical stability and a low level of cracks.

[0014] In a second aspect, the present invention relates to a gas-diffusion layer obtainable by or obtained by the method according to the invention. In particular, the present invention relates to a gas-diffusion layer comprising a microporous carrier-binder layer; a macroporous supporting material layer; an adhesive layer, which adhesive layer is present between the carrier-binder layer and the supporting material layer; wherein the gas diffusion layer has a thickness of at least 0.4 mm; wherein the gas diffusion layer has a Gurley number of at least 74 seconds; and wherein the gas diffusion layer has a bulk density of at least 0.45 g / cm3.

[0015] In a third aspect, the present invention relates to a gas-diffusion electrode comprising the gas-diffusion layer according to the invention.

[0016] In a fourth aspect, the present invention relates to an electrochemical cell comprising a gas-diffusion layer or gas-diffusion electrode according to the invention, in particular to an electrochemical cell which is suitable for the electrochemical reduction of carbon dioxide.Brief description of the drawings

[0017] The features and advantages of the invention will be appreciated upon reference to the following drawings, in which

[0018] Figure 1 shows photographs of the surface of a gas-diffusion layer prepared according to the method of the invention.

[0019] Figure 2 shows a photograph of the surface of a gas-diffusion layer prepared according to the method of the invention.

[0020] Figure 3 shows a photograph of the surface of a gas-diffusion layer not prepared according to the method of the invention.

[0021] Figure 4 shows a photograph of the surface of a gas-diffusion layer not prepared according to the method of the invention.

[0022] Figure 5 shows a photograph of the surface of a gas-diffusion layer prepared according to the method of the invention.

[0023] Figure 6 shows a photograph of the surface of a gas-diffusion layer prepared according to the method of the invention.

[0024] The drawings are intended for illustrative purposes only, and do not serve as a restriction of the scope or the protection as specified in the claims.Detailed description of the invention

[0025] The following is a description of certain embodiments of the invention, given by way of example only and with reference to the drawings.

[0026] By a gas diffusion electrode is herein suitably understood an electrode that can be used in electrochemical reactions where one or more of the reactants are in the gas phase. In addition, one or more of the other reactants and / or one or more products can be in the liquid phase or can be dissolved in a liquid.

[0027] The gas diffusion electrode described herein can contain a gas diffusion layer (GDL) and an active layer (AL). The active layer (AL) contains the catalyst and is herein also referred to as the catalytic layer. In addition a backing material (BM) can be present. The backing material can serve to reinforce the gas diffusion electrode. The backing material can be present as a part of an electrochemical cell that is used to attach the gas diffusion electrode to. It is also possible to attach a backing material directly to the gas diffusion electrode, i.e. where the backing material would be part of the gas diffusion electrode. When part of the gas diffusion electrode, the backing material is preferably attached to the layer of supporting material of the gas diffusion layer. Preferably the backing material consists of a conductive material, such as a metal. The backing material can for example be an expanded or woven metal, a metal foam or metal mesh or another rigid structure.

[0028] Gas diffusion layers (GDL's) can have complex structures and can comprise multiple porous layers and / or components. The gas diffusion layer described herein suitably contains a carrier-binder layer prepared from a carrier-binder paste combined with a layer of supporting material, attached to each other via an adhesive layer prepared from an adhesive composition. The layer of carrier-binder is preferably microporous, i.e. resulting in a microporous carrier-binder layer. The layer of supporting material is preferably macroporous, i.e. a macroporous supporting material layer. The combination of such microporous carrier-binder layer and such macroporous supporting material layer can advantageously be used for the transport of a gas, such as carbon dioxide, into the gas diffusion electrode, as explained in W02017112900A1.

[0029] Step a) of the method according to the invention comprises preparing a carrier-binder paste comprising a first solvent, a first fluorinated binder and conductive carrier particles.

[0030] Examples of suitable conductive carrier particles include carbon particles and / or conductive ceramic particles. Preferably the conductive carrier particles are carbon particles. More preferably, so-called carbon black particles are used. For example, Shawinigan acetylenecarbon black or Vulcan carbon can be used. The conductive carrier particles preferably have a particle size distribution having a mean diameter in the range from equal to or more than 5 nanometers to equal to or less than 350 nanometers, more preferably in the range from equal to or more than 10 nanometers to equal to or less than 200 nanometers and most preferably in the range from equal to or more than 15 nanometers to equal to or less than 100 nanometers. The particle size can be measured indirectly by measurement of the so-called nitrogen surface area (for example according to ASTM method D6556-17 titled "Standard Test Method for Carbon Black— Total and External Surface Area by Nitrogen Adsorption"). The conductive carrier particles preferably have a surface area in the range from equal to or more than 5 square meter per gram (m2 / g) to equal to or less than 150 m2 / g, more preferably in the range from equal to or more than 10 m2 / g to equal to or less than 150 m2 / g, and most preferably in the range from equal to or more than 20 m2 / g to equal to or less than 150 m2 / g. The use of conductive carrier particles, preferably carbon particles, having the above particle size and / or surface area can allow one to obtain a suitably conductive carrier-binder paste.

[0031] The first fluorinated binder comprises one or more fluorinated polymers chosen from the group consisting of polytetrafluoroethylene (PTFE) polymers, perfluoroalkoxy (PFA) polymers, fluorinated ethylene propylene (FEP) polymers and polyvinylidene difluoride (PVDF) polymers. Preferably, the fluorinated binder comprises or consists of PTFE polymers. Use of the above fluorinated binders as binder can allow one to obtain a suitably hydrophobic carrier-binder paste. When used in the methods according to the invention the use of the fluorinated binders as binder can further suitably allow one to obtain suitably stable gas diffusion layer and / or a conductive gas diffusion electrode.

[0032] Due to agglomeration of fluorinated binder particles, the mean diameter of the particle size distribution of the fluorinated binder particles may change when carrying out the methods according to the invention. At the start of the method, however, the fluorinated binder preferably has a particle size distribution with an average diameter in the range from equal to or more than 10 nanometers (corresponding to about 0.01 micrometer) to equal to or less than 1000 nanometers (corresponding to about 1 micrometer), more preferably in the range from equal to or more than 50 nanometers to equal to or less than 500 nanometers and most preferably in the range from equal to or more than 100 nanometers to equal to or less than 300 nanometers. For example, at the start of the method the fluorinated binder may have a particle size distribution with an average diameter of about 0.2 micrometer (corresponding to about 200 nanometer). The average diameter of the fluorinated binder particles can conveniently be determined according to ISO method ISO 13321, titled "Particle size analysis - Photon correlation spectroscopy".

[0033] Preferably, the carrier-binder paste contains, based on the total weight of the first fluorinated binder and conductive carrier particles together, in the range from equal to or more than 20 wt.% to equal to or less than 60 wt.% of fluorinated binder and in the range from equal to or more than 40 wt.% to equal to or less than 80 wt.% of conductive carrier particles. More preferably the carrier-binder paste contains, based on the total weight of fluorinated binder and conductive carrier particles together, in the range from equal to or more than 25 wt.% to equal to or less than 40 wt.% of fluorinated binder and in the range from equal to or more than 60 wt.% to equal to or less than 75 wt.% of conductive carrier particles.

[0034] In addition, the carrier-binder paste may contain additives, such as one or more pore forming agents and / or surfactants. Suitably the carrier-binder composition may further contain, based on the total weight of fluorinated binder and any additives, a total in the range from equalto or more than 0.05 wt.% to equal to or less than 10 wt.% of additives, more preferably in the range from equal to or more than 0.1 wt.% to equal to or less than 5 wt.% of additives. The carrierbinder paste can for example contain additives to decrease ohmic resistance, surfactants and / or pore forming agents. Examples of additives to decrease ohmic resistance include titanium and nickel and their oxides. Examples of surfactants include polyoxyethylene alkylethers. Examples of pore forming agents include sugars and ammonium carbonate.

[0035] The first solvent in the carrier-binder paste of step a) comprises water and an alkanol. Preferably the solvent is a mixture consisting of water and an alkanol. Preferably the alkanol is an alkanol comprising in the range from 1 to 8 carbon atoms, more preferably in the range from 1 to 6 carbon atoms and most preferably in the range from 1 to 4 carbon atoms. Examples of alkanols that can be used include methanol, ethanol, n-propanol, isopropanol, n-butanol, iso-butanol, tertbutanol, pentanol, hexanol and mixtures thereof. Preferred alkanols are ethanol, n-propanol and isopropanol. Without wishing to be bound by any kind of theory, the presence of the alkanol is believed to improve wettability of the conductive carrier particles. In addition, the presence of an alkanol may induce gelling upon mixing of the fluorinated binder in the solvent. In a preferred embodiment, the alkanol has a boiling temperature lower than 100 °C, i.e. the boiling temperature of water.

[0036] If a mixture consisting of water and an alkanol is used as a first solvent, such mixture preferably comprises in the range from equal to or more than 5 vol. % to equal to or less than 95 vol. % of water and in the range from equal to or more than 5 vol. % to equal to or less than 95 vol. % of alkanol, based on the total volume of the mixture. More preferably such a mixture of water and alkanol comprises in the range from equal to or more than 8 vol. % to equal to or less than 75 vol. % of water and in the range from equal to or more than 25 vol. % to equal to or less than 92 vol. % of alkanol, based on the total volume of the mixture. For example, a mixture of water and alkanol of about 50 vol. % water and about 50 vol. % alkanol is also possible. In another embodiment, a mixture of water and alkanol of about 75 vol.% water and 25 vol% of alkanol is used as a first solvent. Most preferably, the first solvent consists of 8-20 vol% of water and 80-92 vol% of alkanol. In one embodiment, the first solvent consists of 12 vol.% water and 88 vol.% of alkanol, such as isopropanol. Most preferably a solvent mixture comprising or consisting of ethanol and water or a solvent mixture comprising or consisting of isopropanol and water is used as a solvent.

[0037] Preferably the carrier-binder paste is prepared by making a suspension of the fluorinated binder in the solvent and subsequently mixing the conductive carrier particles with the suspension. Alternatively the carrier binder paste can be prepared by making a suspension of the conductive carrier particles in the solvent and subsequently mixing the fluorinated binder with the suspension; or the fluorinated binder and the conductive carrier particles can be mixed simultaneously with the solvent to prepare the suspension. The mixing can be carried out at a wide range of temperatures, but is preferably carried out at a temperature equal to or less than 40 °C and most preferably at ambient temperature (about 20 °C). Further, the mixing can be carried out at a wide range of pressures, but is preferably carried out at a pressure equal to or less than 0.2 megapascal (corresponding to about 2 bar) and most preferably at atmospheric pressure (about 0.1 megapascal, corresponding to about 1 bar). Preferably the mixing is carried out at ambient humidity. By humidity is herewith understood the amount of water in the air. Suitably, the mixing is carried out at a humidity in the range from 20% to 60%, more suitably in the range from 40% to 60%.

[0038] A suspension of the fluorinated binder in the solvent can be prepared in any manner known to be suitable for such by a person skilled in the art. The fluorinated binder is commercially available either as a dry powder or as a ready-made aqueous dispersion in water. The suspension can for example be prepared by mixing a fluorinated binder as a dry powder with the solvent. It is also possible to use a commercially available aqueous dispersion of fluorinated binder and diluting such to a desired concentration to prepare the desired suspension of fluorinated binder in solvent. Dispersions of fluorinated binder in water are commercially available. Preferences, such as for the type and particle size of the fluorinated binder are as mentioned above.

[0039] Preferably a suspension of fluorinated binder in solvent for use in the carrier-binder paste comprises a concentration of fluorinated binder in the range from equal to or more than 0.05 gram / milliliter (g / ml) to equal to or less than 0.5 g / ml, more preferably in the range from equal to or more than 0.1 g / ml to equal to or less than 0.3 g / ml.

[0040] The carrier binder paste of step a) preferably comprises a solid-to-liquid weight-based ratio in the range of 1:3.0 to 1:4.0, preferably in the range of 1:3.2 to 1:3.9. This ratio of solids to liquids allows for a suitable viscosity of the paste to be applied in the method according to the invention. A paste containing a too high liquid content makes it difficult to apply a layer of the paste. Such a composition is rather a suspension that requires the supporting material to be dipped into said suspension. Such suspensions are typically used in the manufacture of fuel cells wherein the support material is typically soaked into a solvent comprising conductive particles to impregnate the support material rather than apply a layer on the support material.

[0041] For step a), the first solvent in the carrier-binder paste comprises water and an alkanol and a suspension of the fluorinated binder in such solvent may be prepared by:-combining the fluorinated binder and the solvent; and-mixing the fluorinated binder and the solvent during a period in the range from equal to or more than 10 seconds to equal to or less than 15 minutes, more preferably to equal to or less than 10 minutes and most preferably to equal to or less than 3 minutes. More preferably the fluorinated binder and the solvent are mixed during a period in the range from equal to or more than 30 seconds to equal to or less than 3 minutes.

[0042] Preferably the obtained suspension of the fluorinated binder in the solvent is thereafter immediately combined with the conductive carrier particles. Preferences for the conductive carrier particles are as described above. The amount of conductive carrier particles is preferably targeted such that a carrier-binder paste is obtained having the preferred percentages of fluorinated binder and conductive carrier particles as explained above.

[0043] Without wishing to be bound by any kind of theory it is believed that the mixing of the fluorinated binder with a solvent containing an alkanol causes gelling and can be beneficial for the mechanical stability of the gas diffusion layer and the gas diffusion electrode. Mixingfor a too long period (for example longer than 15 minutes), however, is believed to may cause too much gelling which may make the suspension more difficult to mix with the conductive carrier particles.

[0044] The adding and mixing of the conductive carrier particles with / to the suspension of fluorinated binder in the solvent can be carried out in any manner known to be suitable for such by a person skilled in the art. The mixing may for example be carried out by manual mixing or kneading, or with the help of an industrial mixer or kneader.

[0045] In one preferred embodiment, the mixing of the conductive carrier particles with / to the suspension of the fluorinated binder in the solvent takes place for 1-3 minutes with a flat beater head followed by 30 seconds - 2 minutes of manual mixing. This resulted in a suitable carrier-binder paste structure for rolling the paste into layers. Too long mixing times of the conductive carrier particles with the suspension of the fluorinated binder in the solvent may result in dry and rubber-like structures which are unsuitable for rolling the paste into layers, whereas too short mixing times may result in that not all components are properly mixed into a suitable carrierbinder paste.

[0046] After adding and mixing the conductive carrier particles with / to the suspension of fluorinated binder in the solvent, a carrier-binder paste is obtained that can be used in subsequent steps. The carrier-binder paste so obtained may or may not already have a desired porous structure. Suitably the carrier-binder paste may obtain the desired porous structure during any of steps d)-f) of the method according to the invention.

[0047] A layer of the carrier-binder paste can be prepared in any manner known by a person skilled in the art. In the preparation of a layer of the carrier-binder paste, extruding, pressing, rolling, or a combination of such, can be used. Rolling techniques, such as calendering, are most preferred. Preferably a layer of carrier-binder paste is prepared that has a thickness in the range from equal to or more than 0.2 millimeter (mm) to equal to or less than 3.0 mm, more preferably in the range from equal to or more than 0.5 mm to equal to or less than 2.5 mm. However, the thickness of the carrier-binder layer may vary depending on the desired application of the resulting gas-diffusion layer.

[0048] Step b) comprises preparing an adhesive composition comprising a second solvent and a second fluorinated binder. The adhesive composition in step b) may comprise conductive particles in an amount of equal to or less than 15 wt.% of conductive carrier particles, based on the total weight of fluorinated binder and any conductive carrier particles. In a preferred embodiment, however, the adhesive composition in step b) comprises no or essentially no conductive particles. In one embodiment, the adhesive composition consists of a second solvent and a second fluorinated binder.

[0049] The second fluorinated binder in step b) can be chosen independently from the first fluorinated binder in step a). The second fluorinated binder in step b) can therefore be the same or different from the first fluorinated binder in step a). Preferably the second fluorinated binder in step b) is the same as the first fluorinated binder in step a). Preferences for the type of second fluorinated binder in step b) are as described above for the first fluorinated binder in step a).

[0050] If present, the conductive carrier particles in step b) can be chosen independently from the conductive carrier particles in step a). The conductive carrier particles in step b), if present, can therefore be the same or different from the conductive carrier particles in step a). Preferably the type and particle size of the conductive carrier particles in step b), if present, is the same or similar as the type and particle size of the conductive carrier particles in step a). Preferences for the type of conductive carrier particles in step b), if present, are as described above for step a). If present, the conductive carrier particles in step b) are preferably present in the adhesive composition in an amount of equal to or less than 10 wt.%, more preferably equal to or less than 5 wt.%, still more preferably equal to or less than 1 wt.% and most preferably equal to or less than 0.5 wt.%, based on the total weight of the second fluorinated binder and conductive carrier particles. Most preferably, however, no or essentially no conductive carrier particles, such as carbon particles, are present in step b). With essentially no conductive carrier particles is meant less than 0.1 wt.%, more preferably less than 0.05 wt.% based on the total weight of the second fluorinated binder and conductive carrier particles. That is, most preferably the adhesive composition is an adhesive composition in which such conductive carrier particles, such as carbonparticles, are absent or essentially absent. Without wishing to be bound by any kind of theory, it is believed that the presence of such conductive carrier particles, such as carbon particles, can reduce the adhesive strength of the adhesive composition. The absence of any carbon particles is believed to allow one to obtain a more stable gas diffusion layer and / or a more stable gas diffusion electrode.

[0051] The second solvent in the adhesive composition of step b) comprises water and may optionally comprise an alkanol and can be chosen independently from the first solvent in the carrier-binder paste of step a). Such second solvent in step b) can therefore be the same or different from the first solvent in step a). Preferably the second solvent in the adhesive composition of step b) consists of either water or consists of water and an alkanol. Further preferences for the type of any solvent in step b) are as described above for step a).

[0052] In addition, the adhesive composition may contain additives, such as one or more pore forming agents and / or surfactants. Suitably the adhesive composition may contain, based on the total weight of fluorinated binder and any additives, a total in the range from equal to or more than 0.05 wt.% to equal to or less than 10 wt.% of additives, more preferably in the range from equal to or more than 0.1 wt.% to equal to or less than 5 wt.% of additives. The adhesive composition can for example contain additives to decrease ohmic resistance, surfactants and / or pore forming agents. Examples of additives to decrease ohmic resistance include titanium and nickel and their oxides. Examples of surfactants include polyoxyethylene alkylethers. Examples of pore forming agents include sugars and ammonium carbonate.

[0053] Preferably, in step b), the adhesive composition is prepared by making a suspension of the second fluorinated binder, and optionally one or more additives, in the second solvent.

[0054] The suspension of the fluorinated binder in the solvent for use in the adhesive composition for step b) can be prepared in a similar manner as the suspension of the fluorinated binder in the solvent for use in the carrier-binder paste for step a).

[0055] Preferably the suspension of fluorinated binder in solvent for use in the adhesive composition in step b) comprises more fluorinated binder than the suspension of fluorinated binder in solvent for use in the carrier-binder paste. As a consequence the adhesive composition preferably comprises a higher concentration of fluorinated binder, based on grams of fluorinated binder per milliliter of solvent, than the carrier-binder paste. More preferably, the carrier-binder paste and / or the suspension of fluorinated binder in solvent for use in the carrier-binder paste comprises equal to or less than 0.3 grams fluorinated binder per milliliter of solvent, whereas the adhesive composition and / or the suspension of fluorinated binder in solvent for use in the adhesive composition preferably comprises equal to or more than 0.3 grams fluorinated binder per milliliter of solvent.

[0056] Preferably a suspension of fluorinated binder in solvent for use in the adhesive composition comprises a concentration of fluorinated binder in the range from equal to or more than 0.1 g / ml (grams fluorinated binder per milliliters of solvent) to equal to or less than 1.0 g / ml, more preferably in the range from equal to or more than 0.2 g / ml to equal to or less than 0.7 g / ml, and most preferably in the range from equal to or more than 0.3 g / ml to equal to or less than 0.6 g / ml.

[0057] In step b), an adhesive composition can be obtained that can be used in subsequent steps. The adhesive composition so obtained may or may not already have a desired porous structure. Suitably the adhesive composition may obtain the desired porous structure during any of steps d)-f) of the method according to the invention.

[0058] A layer of the adhesive composition can be prepared in any manner known by a person skilled in the art. Preferably an appropriate layer is prepared by painting, coating, spraying, airbrushing or dipping the surface of either the layer of supporting material or the layer of carrierbinder paste with the adhesive composition. Preferably a layer of adhesive composition is prepared that has a thickness in the range from equal to or more than 1 micrometer (pm) to equal to or less than 1000 micrometer (pm). More preferably, a layer of adhesive composition is prepared that has a thickness in the range from equal to or more than 1 micrometer (pm) to equal to or less than 100 micrometer (pm). More preferably a layer of adhesive composition is prepared that has a thickness in the range from equal to or more than 5 micrometer (pm) to equal to or less than 50 micrometer (pm), more preferably in the range from equal to or more than 10 micrometer (pm) to equal to or less than 30 micrometer (pm).

[0059] The layer of supporting material, as provided in step c), is preferably a layer of a porous supporting material. Preferred supporting materials include carbon cloth, graphitized carbon felt, carbon weave, carbon paper, metallic mesh, metallic felt and metallic foams and combinations of one or more of these. The supporting material can vary widely in thickness. Preferably the supporting material has a thickness in the range from equal to or more than 100 micrometer, preferably equal to or more than 200 micrometer to equal to or less than 1 centimeter, suitably equal to or less than 5 millimeter, more suitably equal to or less than 1 millimeter and even more suitably equal to or less than 600 micrometer.

[0060] Step d) in the method according to the invention comprises combining a layer of supporting material, a layer of the adhesive composition and a layer of the carrier-binder paste, wherein the layer of the adhesive composition is applied between the layer of supporting material and the layer of the carrier-binder paste.

[0061] In one embodiment, in step d), a layer of the adhesive composition is coated onto a side of the layer of the carrier-binder paste, whereafter the layer of supporting material is attached to the coated side of the layer of carrier-binder paste. In an alternative embodiment, in step d), a layer of the adhesive composition is coated onto a side of the layer of supporting material, whereafter the layer of carrier-binder paste is attached to the coated side of the layer of the supporting material.

[0062] The layer of the adhesive composition can be coated onto a side of the layer of the carrierbinder paste or, as applicable, onto a side of the layer of supporting material, in any manner known by the person skilled in the art to be suitable therefore. For example the coating can be applied by a hand coater, a paint roller, spraying, airbrushing, knife coating or dipping. Application by means of rolling or brushing is preferred.

[0063] The total thickness of the layered structure that is formed in step d) before it is subjected to a calendering step depends on the desired application of the gas-diffusion layer that is achieved. In case the GDL that is obtained with the method is applied in large electrochemical cells, the GDL needs to be able to withstand increased hydraulic pressures. This requires a relatively thick material and therefore more material than when it would be used in a lab-scale small electrochemical cell. Furthermore, the desired porosity of the obtained GDL is also influenced by the total amount of material that is used in the carrier-binder paste in combination. In addition, the thickness of the layered structure may also be tailored to minimize the electrical resistance when applied in an electrochemical process. In general, good results are achieved with the methodaccording to the invention when the layered structure has a total thickness of 0.3 to 20 mm, preferably 0.5 to 10 mm, more preferably 1.0 to 5.0 mm before the calendering.

[0064] In step e), the layered structure, as formed in step d) of the process according to the invention for preparing a gas-diffusion layer, is heated to a temperature in the range of between equal to or more than 70 °C and equal to or less than 90 °C with a maximum average heating rate of 2.0 °C / min.

[0065] The inventors found that a pre-heating step at a temperature below the boiling point of water, which is present in the solvents employed in steps a) and b), allows for a more controlled evaporation of the solvent, which results in a lower surface roughness and an increased homogeneity of the obtainable gas-diffusion layer. Immediate exposure to temperatures above 90 °C without a slow build-up of the temperature was found to lead to structural defects of the gas-diffusion layer. Therefore, both the pre-heating temperature and the temperature ramp during the pre-heating step e) are relevant for preventing structural defects to occur.

[0066] In one embodiment, the layered structure is heated to a temperature below the lowest boiling temperature of the one or more alkanols and water present in the layered structure in step e). For example, if the first solvent comprises isopropanol and water, the layered structure is heated to a temperature below 82 °C (for example to 75 °C or to 80 °C) in step e) as this is the boiling temperature for isopropanol under atmospheric conditions. This may allow for the controlled evaporation of the alkanol and / or the water.

[0067] The pre-heating step e) may take place in equipment known to the skilled worker, such as ovens set at a certain temperature and set to apply a certain heating rate. In one embodiment of a (semi-)continuous process, the layered structure may be placed on a conveyor belt, said belt set at a certain speed, passing through a number of ovens or dryers, each oven set at an increased temperature compared to the previous. As such, the layered structure can be heated to the desired temperature at a controlled temperature ramp.

[0068] In step e), the layered structure is heated to a temperature in the range of between equal to or more than 70 °C and equal to or less than 90 °C. Heating to temperatures below 70 °C were not found to be effective as they are too far below the boiling point of water. Heating to a temperature below 70 °C, immediately followed by calendering at a temperature in the range of 90 to 110 °C, at which a majority of the solvent, comprising water, evaporates, may result in a too rapid and uncontrolled evaporation of the solvent during calendering, leading to structural deformations. On the other hand, heating to temperatures above 90 °C before calendaring was also found to result in structural deformations such as cracks in the gas-diffusion layer.

[0069] The maximum average heating rate of 2.0 °C / min may be used as a constant heating rate of, for example, 0.5 °C / min, 1.0 °C / min, 1.5 °C / min, or 2 °C / min, at which the layered structure is heated from room temperature to a temperature of 70-90 °C. However, it may also be used as an averaged heating rate from room temperature to 70-90 °C. As such, the layered structure may be consecutively placed in ovens set at increasing temperatures. For example, the layered structure may first be treated in an oven set at 50 °C for 30 minutes to allow heating from room temperature (e.g. 20 °C), followed by 30 minutes in an oven set at 70 °C. In another example, the layered structure is placed on a conveyor belt which moves at a speed of 0.2 m / min through consecutive dryers with a length of 1 meter each and set at increasing temperatures with the first dryer set at 30 °C and the last dryer set at 90 °C.

[0070] In step f), following the pre-heating in step e), the layered structure is calendered at least twice of which one calendering step (step fl) comprises calendering the layered structure at least once at a temperature in the range of equal to or more than 90 °C to equal to or less than 110 °C. In a preferred embodiment, the layered structure is calendered at least once at a temperature in the range of equal to or more than 90 °C to equal to or less than 105 °C.

[0071] Calendering at said temperature ranges advantageously results in the controlled evaporation of the solvent components such that a majority of a solvent evaporates as a consequence of these one or more calendering steps in step fl). With "majority" herein is meant that at least 50 wt.% of the solvent has evaporated, preferably at least 60 wt.%, even more preferably at least 70 wt.%, such as at least 80 wt.,%; 90 wt.% or even at least 95 wt.%. The skilled person is aware of methods to verify that the majority of the solvent has evaporated. In some embodiments, a visual inspection can be enough to determine whether the majority of the solvent has evaporated. In other embodiments, the mass difference before and after the calendering is indicative of the amount of solvent that has evaporated.

[0072] It is advantageous to have the majority of the solvent evaporate before calendering at a temperature higher than 110 °C. Calendering at temperatures above 110 °C while the layered structure still comprises a large amount of water and / or alkanol may result in a too rapid and uncontrolled evaporation of the solvent which results in cracks to be formed in the layered structure. A slow and controlled evaporation allows for the carrier-binder paste to remain a more homogeneous layer.

[0073] Calendering is a mechanical process typically used to produce thin and uniform sheets of material by passing them through a series of, optionally heated, rollers. The rolls used in calendering are typically made from hard materials to withstand high pressure and temperature, such as steel or chromium-plated steel. Rolls can also be coated with rubber where a softer surface is needed.

[0074] For steps fl) and f2), known and commercially available calenders may be used. It is therefore possible to use calenders withs 2, 3, 4, or more than 4 calender rolls. In one embodiment, each calendering step is carried out using a 2-roll calender. The size of the calender rolls may be chosen based on the desired size of the resulting gas-diffusion layer. The calender rolls may be 5-100 ton calenders. The calenders may be equipped with oil-heating systems to allow the calendering steps to take place while heating the layered structure. The gap width between each calender roll can be tailored and influences the pressure that is applied on the layered structure with each calendering step.

[0075] Applying increased temperatures during the calendering steps may result in the (partial) evaporation of a solvent from the carrier-binder paste in step a) and / or from the adhesive layer in step b). A skilled worker is aware of the boiling points or boiling temperatures of the used solvents. The solvents according to the invention comprise water and an alkanol. Depending on the choice of alkanol, at a certain calendering temperature one solvent component will evaporate more easily than the other solvent component. For example, in case wherein the solvent consists of water (boiling temperature at 100 °C) and isopropanol (boiling temperature at 82 °C), calendering at a temperature of 85 °C will result in the evaporation of at least a part of the isopropanol, but a substantial smaller part of the water, whereas calendering at 110 °C may result in the evaporation of a substantially larger portion of both the water and the isopropanol.

[0076] The extent to which a portion of a solvent evaporates depends on the used temperature, but may also depend on the time it takes to calender the layered structure once and the gap widthat which the calender rolls are set. Therefore, if necessary, the layered structure is calendered at least once but may be calendered multiple times at various temperatures and / or various gap widths to achieve that the majority of the solvent has evaporated.

[0077] In one embodiment, the layered structure is calendered once at a temperature at which a majority of the solvent, comprising water and alkanol, evaporates. In another embodiment, the layered structure is calendered twice at different temperatures to achieve that the majority of the solvent is evaporated. For example, in case the solvent employed consists of water and an alkanol with a boiling point below the boiling point of water ( / .e. 100 °C), the layered structure may be calendered once at a temperature of 105 °C to achieve the evaporation of the majority of the solvent. In another embodiment, the layered structure is first calendered at a lower temperature and a second time at a higher temperature to achieve the same effect.

[0078] In a preferred embodiment, step fl) is carried out at a temperature of 0-10 °C above a boiling temperature of the water or the alkanol in the first and second solvent, whichever boiling temperature of water or the alkanol is the lowest. For example, in case the first and / or second solvent comprise isopropanol (boiling temperature is 82 °C), step fl) is preferably carried out at least once at a temperature between 82 and 92 °C to evaporate a portion of the isopropanol. Subsequently, step fl) may then be carried out a second time at a temperature between 100 and 110 °C to evaporate a portion of the water. The two calendering steps carried out together may then result in the majority of the solvent being evaporated.

[0079] The gap width used in each calendering step in the method according to the invention may be chosen by the skilled worker based on a number of parameters, such as the desired porosity in the obtained gas-diffusion layer, the thickness of the individual layers before calendering, and the rate of solvent evaporation at a given temperature and a given calendering speed.

[0080] In one embodiment, the gap width is set at a smaller width with each increasing temperature step. It is beneficial to lower the gap width gradually with each consecutive calendering step to prevent the layered structure from being pressed to hard in between the rolls, which may lead to structural deformations, such as cracks.

[0081] In the method according to the invention, each calendering step is carried out using a gap width in the range of equal to or greater than 0 to equal to or smaller than 0.5 mm smaller than the gap width used in an immediately preceding calendering step. This means that after the skilled person has decided on the gap width to be set for the very first calendering step, each immediately following calendering step may be carried out having the same gap width or maximally 0.5 mm smaller.

[0082] In a preferred embodiment, the gap width for the first calendering step is set 0.01 - 1.0 mm smaller, preferably 0.01 - 0.5 mm smaller than the thickness of the layered structure before the calendering, more preferably 0.05 to 0.2 mm smaller than the thickness of the layered structure before calendering.

[0083] In a preferred embodiment, the difference in gap width between each immediately consecutive calendering step is 0.5 mm or less, more preferably 0.3 mm or less and at least 0.01 mm. Large differences between the gap widths with each calendaring step (for example a calendering step at a gap width of 1.5 mm directly followed by a calendaring step at a width of 0.8 mm) may lead to structural deformation of the layered structure which may result in cracked surfaces of the gas-diffusion layer. Therefore, the number of calendering steps carried out in totalin the method according to the invention may depend on the initial thickness of the layered structure before calendering and the desired final thickness of the gas-diffusion layer.

[0084] In one embodiment, step fl) is carried out at least once with a calendering gap width set at between equal to or more than 0.5 mm to equal to or less than 8.0 mm, preferably between equal to or more than 1.2 mm and equal to or less than 2.0 mm. The calendering gap width is set to prevent large structural deformations to occur due to the evaporation of solvent. A too small gap width leaves too little space for the evaporated solvent to escape the layered structure, whereas a too large gap width may result in the layers not being able to form a structure together.

[0085] In a preferred embodiment, in the method according to the invention, the layered structure is subjected to another step of calendering at a temperature between 120 and 140 °C, preferably between 125 and 135 °C after step fl) and before step f2). This is particularly preferred in case the first solvent comprises or consists of water and an alkanol with a boiling temperature below 100 °C, more preferably ethanol or isopropanol, most preferably isopropanol. Without being bound to theory, it appears that an additional calendering step in this temperature range improves the structural integrity of the layered structure and leads to less structural deformation and less cracks.

[0086] In step f2) of the method according to the invention, the layered structure is further calendered at least once at a temperature of at least 150 °C, preferably at least 170 °C, even more preferably at least 190 °C. Calendering at this temperature results in the at least partial degradation of the first and second fluorinated binder and initiates a fluidization process of the binder. This process allows the layers to bind together and allows the layered structure to be stored or shelved before it may be used as a gas-diffusion layer.

[0087] In one embodiment, step f2) is carried out with a calendering gap width set at between equal to or more than 0.5 mm and less than 1.5 mm. Too small gap widths may lead to too dense gas-diffusion layers with a decreased porosity. It may also lead to structural deformation in the form of cracks or even breakage of the gas-diffusion layer. Too large gap widths may lead to a gasdiffusion layer with a low mechanical stability. Step f2) may be carried out, however, at a too large gap, followed by an additional calendering step with a smaller set gap width.

[0088] Good results were obtained in embodiments according to the invention in which the first solvent consists of water and isopropanol.

[0089] Good results were also obtained in embodiments according to the invention in which the first and second fluorinated binder consist of PTFE polymers.

[0090] The method according to the invention may further comprise a step after step f), wherein the layered structure is further subjected to a heat treatment at a temperature of between equal to or greater than 300 °C and equal to or less than 350 °C. In this temperature range, the fluorinated binder(s) may (partially) decompose, resulting in a fluidization of the binder. This may result in an optimization of the microporous layer distribution and porosity for enhanced and / or a more uniform dispersion for effective gas transfer when in use. Furthermore, it may contribute to an overall improved structural integrity of the gas-diffusion layer, as it can withstand increased hydrostatic pressure differences and prevent flooding

[0091] The temperature treatment at 300-350 °C may be carried out in an oven or during an additional calendering step. It may also take place while applying pressure using, for example, a coin press.

[0092] The method according to the invention advantageously allows one to prepare a gas diffusion layer that has improved mechanical stability and / or a low degree of cracks. The gas diffusion layer thus obtained is therefore also believed to be novel and inventive. This invention therefore also provides a gas diffusion layer obtained or obtainable by the method as described above.

[0093] The invention therefore further relates to a gas diffusion layer obtainable by the method according to the invention.

[0094] In a preferred embodiment, the invention relates to a gas-diffusion layer that is obtainable by the method according to the invention, which gas-diffusion layer comprises a microporous carrier-binder layer, a macroporous supporting material layer, and an adhesive layer, which adhesive layer is present between the carrier-binder layer and the supporting material layer.

[0095] The gas-diffusion layer according to the invention preferably has a thickness of between equal to or more than 0.4 mm to equal to or less than 1.2 mm, more preferably between equal to or more than 0.7 mm to equal to or less than 1.0 mm. These thicknesses result in a suitable porosity and stability to be applied in gas-diffusion electrodes used for carbon dioxide reduction processes. The gas-diffusion layer obtained with the method according to the invention may, however, have other thicknesses depending on the required stability and porosity for its desired applications.

[0096] The gas-diffusion layer according to the invention preferably has a Gurley number of at least 74 (as measured according to the ISO 5636-5 standard). This number is a representation of the gas permeability. This level of gas permeability makes the gas-diffusion layer applicable to be used in an gas-diffusion electrode for carbon dioxide reduction processes. The gas-diffusion layer obtained with the method according to the invent may, however, have a different gas permeability, expressed by a different Gurley number, depending on the required permeability for its desired application.

[0097] The gas-diffusion layer according to the invention preferably has a bulk density of between equal to or more than 0.45 g / cm3to equal to or less than 1.35 g / cm3. These bulk densities result in a suitable porosity and stability to be applied in gas-diffusion electrodes used for carbon dioxide reduction processes. The gas-diffusion layer obtained with the method according to the invention may, however, have other bulk densities, depending on the required stability and porosity for its desired applications. Bulk density may be measured according to methods known to the skilled worker. As an example, bulk density may be measured by determining the mass of the gasdiffusion layer and dividing it by its volume. The volume of the gas-diffusion layer can be determined by measuring the length and width of the gas-diffusion layer. The thickness of the gasdiffusion layer may be an average thickness, averaged over multiple points on the gas-diffusion layer.

[0098] Herein a distinction can be made between a gas-diffusion layer that is obtainable with the method according to the invention before the optional subjection to the heat-treatment at a temperature of between equal to or greater than 300 °C and equal to or less than 350 °C and after said heat-treatment. The gas-diffusion layers obtainable before and after heat-treatment have the same preferred properties as discussed above, i.e. they have similar preferences for thickness, bulk density, gas permeability and level of cracking on the surfaces. In case the further heat treatment is carried out under increased pressure in, for example, a coin press, the thickness ofthe GDL typically decreases slightly after the heat treatment and the bulk density may slightly increase a result. The gas-diffusion layer obtainable by the method according to the invention up to but not including the optional heat-treatment are herein referred to as the intermediate gasdiffusion layer, whereas after the heat-treatment it is herein referred to as the baked gas-diffusion layer.

[0099] The baked gas-diffusion layer obtainable according to the method of the invention has improved flooding properties. When applied in an electrochemical cell, the gas-diffusion layer in the gas-diffusion electrode allows gas (e.g. carbon dioxide) to diffuse through the electrode from a gas chamber into a chamber in which an electrolyte flows (liquid). The stability, composition, porosity, and hydrophobicity of the gas-diffusion electrode prevents the liquid to diffuse through the electrode towards the gas chamber. However, under the influence of the hydrostatic pressure differences between the gas chamber and the liquid chamber, this may happen anyway. This process is called flooding. When the size of an electrochemical cell is scaled up ( / .e. taller cells), the hydrostatic pressure difference between gas chamber and liquid chamber increases, requiring mechanically stronger gas-diffusion layers in the electrode.

[0100] Flooding is expressed as the hydrostatic pressure difference at which a GDL floods as herein measured with a Hydrostatic Head Tester TF163C. The baked gas-diffusion layer according to the invention does not flood at hydrostatic pressure differences of between 0 and 200 kPa, whereas the intermediate gas-diffusion layer according to the invention will flood at these hydrostatic pressure differences. The heat-treatment at a temperature of between equal to or greater than 300 °C and equal to or less than 350 °C results in the (partial) decomposition of the fluorinated binder, resulting in a fluidization of the binder. This may result in an optimization of the microporous layer distribution and porosity for enhanced and / or a more uniform dispersion for effective gas transfer when in use. Furthermore, it may contribute to an overall improved structural integrity of the gas-diffusion layer, as it can withstand increased hydrostatic pressure differences and prevent flooding.

[0101] An advantage of the method according to the invention is that it allows for a more controlled evaporation of the first and second solvent decreasing the risk of crack formation in the so-produced gas-diffusion layer. As a result, a mechanically more stable GDL is obtained.

[0102] Another advantage of the method according to the invention is that the method allows for a (semi-)continuous process to produce gas-diffusion layers. From the calendering steps, a continuous product of intermediate gas-diffusion layer is produced having a width as determined by the size of the calender rolls. The length can be then determined based on the desired application. For example, the product can be cut in pieces of 1 m long.

[0103] Another advantage of the method according to the invention is that the method produces an intermediate gas-diffusion layer which has a substantial shelf-life due to an improved mechanical stability with the appropriate properties such as bulk density, thickness, and low level of cracking. The calendering step at a temperature of at least 150 °C (step f2), wherein the fluorinated binder already partially decomposes contributes to this.

[0104] The intermediate gas-diffusion layer can therefore be stored for a substantial amount of time before it may be subjected to the heat-treatment to produce a baked GDL to achieve the desired flooding properties.

[0105] The baked gas diffusion layer preferably comprises at least:(i) a microporous carrier-binder layer having a pore size distribution with a mean pore diameter in the range from equal to or more than 1 micrometer to equal to or less than 100 micrometer, preferably equal to or less than 50 micrometer; and(ii) a macroporous supporting material layer having a pore size distribution with a mean pore diameter in the range from equal to or more than 100 micrometer to equal to or less than 5000 micrometer (corresponding to 5 millimeter), more preferably equal to or less than 2000 micrometer (corresponding to 2 millimeter), even more preferably equal to or less than 1000 micrometer (corresponding to 1 millimeter), still more preferably equal to or less than 800 micrometer and most preferably equal to or less than 500 micrometer;wherein an adhesive layer is present between the microporous carrier-binder layer and the macroporous supporting layer, which adhesive layer is attached to both the microporous carrierbinder layer and the macroporous supporting layer.

[0106] The adhesive layer can be microporous, having a pore size distribution with a mean pore diameter in the range from equal to or more than 1 micrometer to equal to or less than 100 micrometer, preferably equal to or less than 50 micrometer; or can be macroporous, having a pore size distribution with a mean pore diameter in the range from equal to or more than 100 micrometer to equal to or less than 5000 micrometer (corresponding to 5 millimeter), more preferably equal to or less than 2000 micrometer (corresponding to 2 millimeter), even more preferably equal to or less than 1000 micrometer (corresponding to 1 millimeter), still more preferably equal to or less than 800 micrometer and most preferably equal to or less than 500 micrometer.

[0107] Preferences for the carrier-binder layer are as mentioned above for the layer of carrierbinder paste, except that the solvent is removed from such, and preferences for the adhesive layer are as mentioned above for the layer of adhesive composition, except that the solvent is removed from such.

[0108] Preferences for the supporting material layer are as mentioned above for the layer of supporting material.

[0109] In addition the present invention provides a method for preparing a novel and inventive gas diffusion electrode comprising such a gas diffusion layer.

[0110] Suitably the present invention provides a method for the preparation of a gas diffusion electrode, containing the steps of:a) preparing a carrier-binder paste comprising a first solvent, a first fluorinated binder and conductive carrier particles; wherein the first solvent comprises water and an alkanol; b) preparing an adhesive composition comprising a second solvent and a second fluorinated binder; wherein the second solvent comprises water;wherein the first and second fluorinated binder comprise one or more fluorinated polymers chosen from the group consisting of polytetrafluoroethylene (PTFE) polymers, perfluoroalkoxy (PFA) polymers, fluorinated ethylene propylene (FEP) polymers and polyvinylidene difluoride (PVDF) polymers;c) providing a supporting material;d) combining a layer of supporting material, a layer of the adhesive composition and a layer of the carrier-binder paste into a layered structure, wherein the layer of the adhesive composition is applied between the layer of supporting material and the layer of the carrier-binder paste;e) heating the layered structure to a temperature in the range of between equal to or more than 70 °C and equal to or less than 90 °C with a maximum average heating rate of 2.0 °C / min; followed byf) at least two calendering steps comprisingfl) calendering the layered structure at least once at a temperature in the range of equal to or more than 90 °C to equal to or less than 110 °C; andf2) after step fl), calendering the layered structure at least once at a temperature of at least 150 °C;wherein each calendering step is carried out using a gap width in the range of equal to or greater than 0 to equal to or smaller than 0.5 mm smaller than the gap width used in an immediately preceding calendering step;g) subjecting the layered structure to a heat treatment at a temperature of between equal to or greater than 300 °C and equal to or less than 350 °C; andh) applying a catalytic layer, which catalytic layer comprises a catalyst, onto the layer of the carrier-binder paste on the side opposite of the side where the layer of the adhesive composition was applied.

[0111] Preferences for steps a)-g) are as described above for the method for preparation of gas diffusion layer.

[0112] Step h) can be carried out in any manner known by a person skilled in the art to be suitable therefore. The catalytic layer in step h) can be any catalytic layer known by a person skilled in the art to be suitable and can for example include catalysts as mentioned in WO2019141827. Preferably the catalytic layer includes a metallic catalyst supported on conductive carrier particles. More preferably the catalytic layer comprises a catalyst, a binder (preferably a fluorinated binder, nation, or polybenzimidazole) and conductive carrier particles. Examples of metallic catalysts include indium (In), tin (Sn), copper (Cu), nickel (Ni), cobalt (Co), bismuth (Bi), zinc (Zn), and combinations thereof. Most preferred is a catalyst comprising indium, bismuth, zinc or a combination of indium and / or bismuth and / or zinc. Preferences for the fluorinated binder and the conductive carrier particles are as described above for steps a) and b).

[0113] The method according to the invention advantageously allows one to prepare a gas diffusion electrode that has improved mechanical stability. The gas diffusion electrode thus obtained is therefore also believed to be novel and inventive. This invention therefore also provides a gas diffusion electrode obtained or obtainable by the method as described above.

[0114] The gas diffusion layer and gas diffusion electrode according to the invention can advantageously be applied in an electrochemical cell. The present invention therefore also provides an electrochemical cell comprising a gas diffusion layer and / or a gas diffusion electrode as described above. Further preferences for such an electrochemical cell are as described in WO2015184388A1 and W02017112900A1.

[0115] Such electrochemical cell can advantageously be used for an electrochemical process such as the electrochemical reduction of carbon dioxide. The present invention therefore also provides a process for electrochemically reducing carbon dioxide, comprising:- introducing an anolyte to a first cell compartment of an electrochemical cell, the first cell compartment comprising an anode;- introducing a catholyte and carbon dioxide to a second cell compartment of the electrochemical cell, the second cell compartment comprising a cathode; and- applying an electrical potential between the anode and the cathode sufficient to reduce the carbon dioxide to a reduced reaction product;wherein the cathode comprises a gas diffusion layer and / or a gas diffusion electrode as described herein.

[0116] Further preferences for such a process are as described in WO2015184388A1 and W02017112900A1. Most preferably the process is a process wherein carbon dioxide is reduced to a reduced reaction product selected from carboxylates and / or carboxylic acids, such as for example formate, formic acid, oxalate or oxalic acid.

[0117] The process can be carried out in an aqueous or non-aqueous medium.Examples

[0118] Porosimetry was determined by mercury intrusion porosimetry. Samples were degassed in vacuum at 100°C for 16 h. Subsequently, intrusion and extrusion curves were recorded on a Micromeritics Autopore IV 9505 analyzer from a pressure range of 0.002 - 220 MPa, all in accordance with ISO 15901-1:2016.LAB SCALE

[0119] In the lab, a carrier-binder paste was prepared with the following materials and amounts: 15 g of ACETYLENE BLACK 75 % (SOLTEX), 13.5 ml of Teflon™ PTFE DISP 30 Fluoropolymer Dispersion, 80 ml of 12 v% / 88 v% water / IPA solution. Herein, with water is meant distilled ultrapure water. The PTFE dispersion was mixed using a glass rod with 40 ml of the IPA / water solution for approximately 1.5 min. The obtained mixture was added to the carbon powder (acetylene black) and mixed using a glass rod to obtain a carrier-binder paste. The beaker was then rinsed with the remaining 40 ml of IPA / water, which was then added to the carrier-binder paste. The carrier-binder paste is further mixed by hand and by folding and passing it 1-2 times through rollers to ensure homogeneous consistency. The carrier-binder paste was rolled to a thickness of 1.8 mm and placed on an aluminium foil.

[0120] PTFE dispersion (Teflon™ PTFE DISP 30 Fluoropolymer, comprising 55-65% PTFE, 1-5% polyoxyethylene alkylether and water as solvent) was brushed on top of the layer of the carrierbinder paste to create a layer of the adhesive composition. On top of the layer of the adhesive composition layer, a carbon fiber layer having a thickness of 0.2286 mm was placed. The obtained layered structure was then sealed within aluminium foil to prevent solvent loss before heating and maintain integrity. The sealed "sandwich" structure was then pre-heated and rolled at desired temperatures and thickness through a 2-roll TMAXCN calendering machine for a certain number of passes as described in the following examples.

[0121] Preheating step in the lab was carried out in a hot press without applying any force onto the sealed layered structure and put on a steel plate at 50 °C for 30 min followed by 30 min at 70 °C. The average heating rate to achieve 70 °C is therefore calculated as follows: 70 °C - 20 °C (RT) = 50 °C increase which is allowed to take place over 30 min, which equals an average heat rate of approximately 1.67 °C / min.EXAMPLE 1:

[0122] The layered structure was prepared and pre-heated as described above. Next, the layered structure was (a) calendered once at 90 °C to a thickness of 1.75 mm, followed by (b) calenderingthe structure twice at 105 °C to a thickness of 1.55 mm, followed by (c) calendering the layered structure twice at 130 °C to a thickness of 1.15 mm. Finally, the layered structure was (d) calendered twice at 200 °C to a thickness of 0.85 mm. Figure 1 shows photographs taken of the surfaces of the GDL after each consecutive calendering step. The figure shows that with calendering at 90 and 105 °C, (a) and (b) respectively, the majority of the solvent has evaporated without deteriorating the surface of the GDL. After each consecutive calendering step, the surface becomes more homogeneous and smooth with a low level of cracking.

[0123] The obtained GDL is an intermediate GDL having a bulk density of 0.58 g / cm3.EXAMPLE 2

[0124] Example 2 was carried out exactly as in Example 1, except that in step (c), the layered structure was calendered at 142 °C instead of at 130 °

[0125] Figure 2 is a photograph of the GDL that was obtained after step (c). The figure shows the same pattern as was obtained after step (c) in Example 1, but the surface of the GDL is less homogeneous and smooth than in Example 1COMPARATIVE EXAMPLE 1

[0126] Comparative example 1 was carried out exactly as in Example 1 up to and including step (b), except that the layered structure was not pre-heated before calendering. Figure 3 shows a photograph of the surface of the resulting GDL structure after the calendering at 105 °C. The surface of the GDL shows significant cracking and is not smooth or homogenous.COMPARATIVE EXAMPLE 2

[0127] Comparative example 2 was carried out exactly as in Example 1, except for the set gap width in step c): 1.0 mm instead of 1.15 mm. This means that the decrease in gap width going from step b) to step c) was 0.55 mm, which is more than 0.5 mm difference.

[0128] Figure 4 shows a photograph of the GDL after step c) in comparative example 2. The figure shows that the GDL was broken with large cracks and unsuitable to become a proper gas-diffusion layer.COMPARATIVE EXAMPLE 3

[0129] Comparative example 3 was prepared and pre-heated as in Example 1 but the first calendering step (using a gap width of 1.45 mm) was carried out at 120 °C. The inventors found that immediately calendering at 120 °C yielded a heavily cracked GDL that was unsuitable to be further calendered or further processed to become a proper gas-diffusion layer.COMPARATIVE EXAMPLE 4

[0130] In comparative example 4, the sample was pre-heated at 50 °C for 30 min followed by 30 min at 90 °C. The average heating rate to achieve 90 °C is therefore calculated as follows: 90 °C -20 °C (RT) = 70 °C increase, which is allowed to take place over 30 min, which equals an average heat rate of approximately 2.33 °C / min. This too high average heat rate to achieve 90 °C yielded a heavily cracked GDL that could not be smoothed using a calender.LARGER SCALE

[0131] For large scale operations, the carrier-binder paste was prepared using ten times the amounts of the ingredients compared to the lab-scale experiments. The PTFE dispersion was mixed using a glass rod with the IPA / water solution. The obtained mixture was added to the carbon powder (acetylene black) and mixed using a glass rod to obtain a carrier-binder paste. The carrier-binder paste was mixed using a kitchen mixer with a flat beater attachment for 2 minutes, followed by 1 minute of hand mixing to ensure a homogeneous dough-like structure.

[0132] The carrier-binder paste was rolled to a thickness of approximately 0.93 mm and placed on an aluminium foil. The PTFE dispersion was brushed on top of the layer of the carrier-binder paste to create a layer of the adhesive composition. On top of the layer of the adhesive composition layer, a carbon fibre layer having a thickness of 0.2286 mm was placed. The obtained layered structure, having a total thickness of 1.25 mm, was then sealed within aluminium foil to prevent solvent loss and maintain integrity. The sealed "sandwich" structure was then pre-heated and rolled at desired temperatures and thickness through a 2-roll calendering machine for a certain number of passes as described in the following examples.

[0133] Preheating of the layered structure was carried out in 11 consecutive dryers (lm each in length) with a speed of 0.2 m / min gradually building up the temperature to 90 °C in the following order: two dryers at 30 °C, three dryers at 50 °C, three dryers at 70 °C and three dryers at 90 °C. The average heating rate to achieve 90 °C is therefore calculated as follows: 90 °C - 20 °C (RT) = 70 °C increase which is allowed to take place over 40 min (8 dryers, each taking 5 min), which equals an average heat rate of approximately 1.75 °C / min.EXAMPLE 3

[0134] The layered structure was prepared and pre-heated as described above for the larger scale operation. Next, the layered structure was (a) calendered twice at 105 °C to a thickness of 1.2 mm, followed by (b) calendering the layered structure twice at 130 °C to a thickness of 1.0 mm. Finally, the layered structure was (c) calendered twice at 200 °C to a thickness of 0.9 mm. Figure 5 shows photographs taken of the surfaces of the GDL after each consecutive calendering step. The figure shows that with calendering at 105 °C, the majority of the solvent has evaporated without deteriorating the surface of the GDL. After each consecutive calendering step, the surface becomes more homogeneous and smooth with a low level of cracking.

[0135] The obtained GDL is an intermediate GDL having a bulk density of 0.76 g / cm3. The gas permeability, as expressed in a Gurley number, was measured at 10 different locations on the obtained GDL and was in the range of 450-820 seconds.EXAMPLE 4

[0136] The intermediate GDL as obtained in Example 3 was placed in a coin press between steel plates and was heated starting from 200 °C to a temperature of 330 °C with a heating rate of 5 °C / min, while applying a force of 0.04 kN / cm2with a Fontijne Hot Press (equipped with a minimum available force of 7.5 kN)to obtain a baked GDL. Figure 6 shows a photograph of the surface of the intermediate GDL (a) and the surface of the obtained baked GDL. The figure shows that both the intermediate and the baked GDL have a smooth homogeneous surface showing no cracks.

[0137] The obtained baked GDL has a bulk density of 0.71 g / cm3and a thickness of approximately 0.8 mm The gas permeability, as expressed in a Gurley number, was measured at 10 different locations on the obtained GDL and was in the range of 162-740 seconds.

[0138] Table 1 shows results from the porosimetry measurements on Examples 3 and 4 For the baked GDL of Example 4, the Porosimetry measurements were performed on two pieces of the GDL and are hereinafter referred to 4(a) and 4(b).Table 1

Claims

-23- CLAIMS1. A method for the preparation of a gas diffusion layer, the method comprising the steps of:a) preparing a carrier-binder paste comprising a first solvent, a first fluorinated binder and conductive carrier particles;wherein the first solvent comprises water and an alkanolb) preparing an adhesive composition comprising a second solvent and a second fluorinated binder;wherein the second solvent comprises water;wherein the first and second fluorinated binder comprise one or more fluorinated polymers chosen from the group consisting of polytetrafluoroethylene (PTFE) polymers, perfluoroalkoxy (PFA) polymers, fluorinated ethylene propylene (FEP) polymers and polyvinylidene difluoride (PVDF) polymers;c) providing a supporting material;d) combining a layer of supporting material, a layer of the adhesive composition and a layer of the carrier-binder paste into a layered structure, wherein the layer of the adhesive composition is applied between the layer of supporting material and the layer of the carrierbinder paste;e) heating the layered structure to a temperature in the range of between equal to or more than 70 °C and equal to or less than 90 °C with a maximum average heating rate of 2.0 °C / min; followed byf) at least two calendering steps comprisingfl) calendering the layered structure at least once at a temperature in the range of equal to or more than 90 °C to equal to or less than 110 °C; ;f2) after step fl), calendering the layered structure at least once at a temperature of at least 150 °C;wherein each calendering step is carried out using a gap width in the range of equal to or greater than 0 to equal to or smaller than 0.5 mm smaller than the gap width used in an immediately preceding calendering step.

2. The method according to claim 1, wherein the alkanol is chosen from the group consisting of methanol, ethanol, n-propanol, iso-propanol, n-butanol, iso-butanol, tert-butanol, pentanol, hexanol, and mixtures thereof.

3. The method according to any of the preceding claims, wherein the alkanol has a boiling temperature below 100 °C.

4. The method according to any of the preceding claims, wherein the first solvent consists of water and isopropanol.

5. The method according to any of the preceding claims, wherein the first solvent contains between 5-95 vol% of water, preferably between 5-50 vol% of water, more preferably between 5-20 vol.% of water, the remainder being the alkanol.

6. The method according to any of the preceding claims, wherein the first and second fluorinated binders consist of PTFE polymers.

7. The method according to the any of the preceding claims, wherein the thickness of the layered structure is between equal to or more than 0.5 mm and equal to or less than 10 mm prior to calendering.

8. The method according to any of the preceding claims, wherein the method further comprises a step of calendering the layered structure at a temperature between 120-140 °C, after step fl) and before step f2).

9. The method according to any of the preceding claims, wherein step fl) is carried out at a temperature of 0-10 °C above the boiling temperature of water or the alkanol in the first and second solvent, whichever boiling temperature of water or the alkanol is the lowest.

10. The method according to any of the preceding claims, wherein step fl) is carried out at least once with a calendering gap width set at between equal to or more than 0.5 mm and equal to or less than 8.0 mm).

11. The method according to any of the preceding claims, wherein step f2) is carried out at least once with a calendering gap width set at between equal to or more than 0.5 mm and less than 1.5 mm.

12. The method according to any of the preceding claims, wherein, after step f), the layered structure is further subjected to a heat treatment at a temperature of between equal to or greater than 300 °C and equal to or less than 350 °C.

13. A gas diffusion layer obtainable by the method according to any of claims 1-12.

14. A gas diffusion layer according to claim 13, comprisinga) a microporous carrier-binder layer;b) a macroporous supporting material layer;c) an adhesive layer, which adhesive layer is present between the carrier-binder layer and the supporting material layer;wherein the gas diffusion layer has a thickness of at least 0.4 mm;wherein the gas diffusion layer has a Gurley number of at least 74 seconds; andwherein the gas diffusion layer has a bulk density of at least 0.45 g / cm3.

15. The gas diffusion layer according to claim 14, wherein the gas diffusion layer has a thickness in the range of between equal to or greater than 0.7 mm to equal to or smaller than 1.0 mm.

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