WO2013190158A1

WO2013190158A1 - Method for producing electrodes for polymer fuel cells

Info

Publication number: WO2013190158A1
Application number: PCT/ES2013/070393
Authority: WO
Inventors: Antonio José MARTÍN FERNÁNDEZ; Antonio Alfonso MARTÍNEZ CHAPARRO
Original assignee: Centro De Investigaciones Energéticas, Medioambientales Y Tecnológicas (Ciemat)
Priority date: 2012-06-19
Filing date: 2013-06-18
Publication date: 2013-12-27
Also published as: ES2396664B1; ES2396664A1

Abstract

The invention relates to an electrochemical cell suitable for electrodepositing a catalyst on a substrate, and to a method for producing an electrode by means of the electrodeposition technique, using said electrochemical cell.

Description

METHOD FOR THE PREPARATION OF ELECTRODES FOR BATTERIES

POLYMER FUEL

FIELD OF THE INVENTION The present invention falls within the field of fuel cells, and in particular refers to procedures that allow the manufacture of electrodes intended for application in polymeric fuel cells, as well as electrochemical deposit cells.

BACKGROUND OF THE INVENTION

Fuel cells are the most efficient devices that exist for the production of electricity from hydrogen. Its widespread use in different applications, portable, mobile and stationary, will allow the transition to a sustainable economy based on hydrogen as fuel. Among the different fuel cell technologies, PEMFC ("Proton Exchange Membrane Fuel Cells") batteries are closer to commercialization and widespread use, having demonstrated their viability in many different applications, such as electric vehicles (urban buses, cargo transporters, cars), stationary and portable applications. For example, the results of demonstration projects carried out in different places, inside and outside Spain, show that electric vehicles powered by a hydrogen-powered fuel cell, combined with some auxiliary power element such as batteries or supercapacitors, work with similar benefits (power, speed, autonomy, reliability) to traditional vehicles based on internal combustion engine, with the additional advantage of not emitting pollutants.

There are two main obstacles to the development and generalization of fuel cells, such as the cost of manufacturing the battery and its durability. This is due to the fact that PEMFC batteries, because they operate at a low temperature (<80 ° C), require a catalyst for electrochemical reactions that is platinum. This precious metal is expensive and scarce; it is estimated that its annual production would not be sufficient to meet the requirements of fuel cells according to the current state of technology, which

REPLACEMENT SHEET (Rule 26) they need about 0.5 to 1 gPt-kW ^"1 on their electrodes. On the other hand, the durability of PEMFCs is limited by the reactivity of platinum with its carbon support, which limits it to about 4000 hours of operation in applications automotive, even below what is required to be competitive with current vehicles.In the electrochemical reactions that take place in the electrodes of a PEMFC battery, species in the gas phase participate, usually oxygen in the cathode and hydrogen in the anode, and in the liquid phase, usually protons, on the solid catalyst particle, which is why they are known as three-phase reactions (gas-liquid-solid), and require that the electrode have a particular structure that allows its coexistence on a microscopic scale. achieved by means of what are known as gas diffusion electrodes, integrated by a series of microporous layers, usually three, that allow electric conduction and sim transport of reagents and liquid and gaseous products: the gas diffuser layer (GDL), the microporous layer and the catalyst layer. The first consists of a 0.5-1 mm thick carbon cloth or paper, the second is usually a layer of microporous carbon such as carbon black of about 0.1-0.5 mm, and the third is a thinner layer, about 5-30 μιη, of platinum particles supported on carbon black.

For the manufacture of these electrodes, the substrate is normally used (carbon cloth or paper) on which the other two layers of carbon black and platinum deposited on carbon black are successively deposited. Different methods are used for this, such as impregnation or airbrush. These methods, although simple, distribute the catalyst randomly, which means that part of the deposited platinum is not active because it is not exposed to the reagents in the fuel cell. In light of the problems existing in the state of the art, it becomes necessary for the development of fuel cells to have electrode manufacturing techniques that allow reducing the amount of platinum necessary, and increasing its durability, as well as distributing the catalyst homogeneously over the entire surface of the substrate so that all deposited catalyst is active. It is also of interest that these methods are affordable on a large scale, and can be used to prepare electrodes with different types of catalyst, of activity similar to that of platinum, but with lower cost.

The method of electrochemical deposition for the preparation of sheet metal is well known, and is used industrially in different applications: synthesis of materials, metallurgical refinement, coatings, preparation of electronic circuits, among others. It is a method that requires little investment in equipment and is usually carried out at low temperature (below 80 ° C) and ambient pressure. Its use for the preparation of electrodes for fuel cells is described by Martín, E.J. et al. (J. Power Sources, 2009, 192, 14-20). The procedure described there uses a rectangular electrochemical cell provided with three electrodes: the substrate on which the catalyst will be deposited, a microporous platinum counter electrode as a counter electrode and a reference electrode based on mercury / mercury sulfate, electrodeposition being carried out under potentiostatic conditions by the application of a program consisting of scans of potential or pulses of potential using a solution of hexachloroplatmate as an electrolyte. It is shown that with this technique electrodes with a lower catalyst charge and high activity are obtained.

However, the development of techniques and / or devices that allow the electrodeposition process to be carried out more efficiently is necessary, further improving the distribution of the catalyst on the surface of the substrate in which they are deposited.

BRIEF DESCRIPTION OF THE INVENTION

The authors of the present invention have developed an electrochemical cell that allows the electrodeposition or electrochemical deposition process to be carried out more efficiently and distributing the catalyst homogeneously over the surface of the substrate. As an additional advantage, said electrochemical cell allows the deposition of thin layers of various catalysts.

The electrodeposition process using said electrochemical cell makes it possible to significantly reduce the amount of platinum necessary, further increasing its durability. One of the main characteristics of the developed cell is the incorporation of an ion conductive membrane with low electrical resistivity that prevents the gases generated in the electrolyte in contact with the counter electrode from reaching the deposit substrate, thus avoiding reactions parallel to those of electrodeposition which decrease the faradaic efficiency of said process and cause interference in the properties of the deposited catalyst.

Another additional characteristic of the cell is that it allows the control of the atmosphere in the outer part of the substrate, since since it is a paper or cloth substrate, it has high permeability to ambient oxygen, which can by this way reach the interface of the substrate with the electrolyte and cause parallel reactions.

This electrochemical cell has the additional advantage of being able to be used not only to carry out the electrodeposition reaction, but also as a cell for the study or testing of the electrodes in a controlled atmosphere, since in the same cell the electrode can be prepared, carry out the electrochemical and / or chemical post-treatments that may be necessary, and the first characterization tests to be carried out: measurement of the area of electrodeposited platinum and the activity for oxygen reduction.

Thus, a first aspect of the present invention is an electrochemical cell suitable for the electrodeposition of a catalyst on a substrate, wherein said electrochemical cell comprises:

- a first chamber adapted to house an electrolyte, where said chamber is limited by at least two parallel walls, where: a first wall comprises:

- a substrate comprising an inner face and an outer face, where the outer face comprises a gas diffuser layer and the inner face comprises a microporous layer;

- a first electrical contact located in contact with the outer face of the substrate;

where the outer face of the substrate and the first electrical contact are located inside a second chamber comprising at least two conduits for the entry and exit of gases, suitable to allow control of the atmosphere on the outside of the substrate,

A second wall comprises:

- a counter electrode comprising an inner face and an outer face,

- a second electrical contact located in contact with the outer face of the counter electrode;

- an ion conductive membrane and impermeable to the passage of gases located inside the chamber by separating it into two sub-chambers, a first sub-chamber in contact with the inner face of the substrate and a second sub-chamber in contact with the inner face of the counter electrode; where the substrate, the membrane and the counter electrode are located in parallel arrangement.

In addition, said cell may have flexible devices or means that allow the first electrical contact to be pressed on the opposite side of the substrate, thus favoring the homogeneous contact between them and isolating said rear face of the electrode from the electrolyte liquid, essential aspects for achieving homogeneous deposits in large areas.

In a particular embodiment, the electrochemical cell comprises: a housing in the form of a parallelepiped open at least on one of its faces, where the face opposite the open face comprises a first window;

- a closing block adapted to cover the open face of the housing, where said closing block comprises a recess arranged in the outer face and a second through window arranged at the bottom of the recess, such that the face of the block of Closing with the second through window is the face adapted to cover the open face of the housing, where:

- the housing and the closing block, are connected defining inside the first subchamber in contact with the inner face of the substrate, - the first window of the housing is closed externally by an assembly that consecutively comprises the membrane, the counter electrode and the second electrical contact, so that the membrane and the counter electrode are distanced from each other giving rise to the second subchamber which is in contact with the inside face of the counter electrode,

- the recess of the closing block houses the substrate and the first electrical contact, the inner face of the substrate being in contact with the second through window, where said recess is closed by the outer face of the closing block by means of a cover hermetic, so that inside said recess the second chamber is formed in which the outer face of the substrate and the first electrical contact are housed.

A further aspect of the present invention is a process for the preparation of an electrode, wherein said process comprises the deposition of a catalyst on a substrate by means of the electrodeposition technique, where the substrate is a hydrated hydrated substrate comprising a layer of carbon microporous, characterized in that the electrodeposition is carried out in an electrochemical cell as previously defined, and where said procedure further comprises: prior to electrodeposition, coating the microporous carbon layer with a microporous layer comprising a mixture of carbon black and a ionomer, and subjecting the substrate to a suitable activation process to provide the coated face with ionic and hydrophilic conductivity, where said activation process is carried out by chemical, electrochemical or a combination of both; and / or subsequently to electrodeposition, subjecting the electrode obtained to a treatment selected from a heat treatment, an electrochemical treatment and a combination thereof, where the electrochemical treatment comprises subjecting the electrode to an electrochemical cycling in sulfuric acid at scanning rates. and controlled sweep limits until a stable response of the intensity between two consecutive sweeps is obtained and the heat treatment comprises subjecting the electrode to a heating stage at a temperature between 100 ° C and 150 ° C in a humid atmosphere.

Another aspect of the invention is directed to a test bench for the characterization and measurement of the catalytic activity of an electrode, wherein said test bench comprises:

- an electrochemical cell as previously defined, wherein the substrate is replaced by the electrode to be tested,

- an inert electrolyte, and

- a source of electricity or potentiostat / galvanostat capable of controlling and recording the electrochemical potential of the electrode and the current intensity in the cell.

A final aspect of the present invention relates to a method for the selection of an electrode suitable for use in a fuel cell, wherein said method comprises: - depositing the electrode in a test bench as defined previously, in the place intended to house the substrate;

- fill the chamber adapted to house the electrolyte with an inert electrolyte and ion conductor;

- supplying oxygen or air to the outside of the electrode through the gas inlet duct of the second chamber;

- apply a suitable electrochemical potential between the electrode and a reference electrode; Y

- Check if there is an electric current passing due to oxygen reduction, where the production of said electric current is indicative that the electrode is suitable for use in a fuel cell.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 corresponds to a side view of the electrodeposition cell assembly. Figure 2 corresponds to a PEMFC monocell polarization curve with Pt-WC catalyst> 3 electrodeposited at the cathode.

DETAILED DESCRIPTION OF THE INVENTION In a first aspect, the present invention relates to an electrochemical cell that allows the deposition of a catalyst on a substrate by means of the electrodeposition technique.

Said electrochemical cell comprises a chamber adapted to house an electrolyte and an ion conducting membrane and impermeable to the passage of gases located inside said chamber, so that the membrane separates the chamber into two subchambers.

The chamber is limited by at least two parallel walls. The first wall comprises a substrate, which in turn comprises an inner face and an outer face, and a first electrical contact located in contact with the outer face of the substrate. For its part, the second wall comprises a counter electrode, which in turn comprises an inner face and an outer face, and a second electrical contact located in contact with the outer face of the counter electrode.

The arrangement of the different elements causes the membrane to separate the chamber into two subchambers: a first subchamber that is in contact with the inner face of the substrate and a second subchamber that is in contact with the inner face of the counter electrode.

The outer face of the substrate and the first electrical contact are located inside a second chamber that comprises at least two conduits for gas inlet and outlet, respectively. This allows controlling the composition of the atmosphere on the outer face of the substrate, preventing the ambient oxygen from reaching said substrate.

The substrate used in said cell is a conventional substrate for gas diffusion electrodes, which comprises on its outer face a gas diffuser layer and on its inner face a microporous layer, which allow the electrical conduction and simultaneous transport of reagents and liquid products And soda. Said gas diffuser layer preferably comprises a carbon cloth or paper with a thickness of between 0.5 and 1 mm. In a preferred embodiment, the gas diffuser layer is impregnated with a hydrophobic agent.

On the other hand, the microporous layer is a layer of a material comprising pores of micro and nanometric size. Preferably, it is a layer of microporous carbon, such as carbon black, and has a thickness between 0.1 and 0.5 mm. In a preferred embodiment, this microporous layer is also impregnated with a hydrophobic agent.

The microporous layer is deposited on the gas diffuser layer, so that on said microporous layer a catalyst is deposited during the electrodeposition process. The deposition of the microporous layer can be done by techniques such as airbrushing or electrospray, widely known in the state of the art.

In a particular embodiment, the substrate is square in shape with varying between lxl cm ² and 10x10 cm ² surface, more preferably the surface is between 3x3 and 6x6 cm ^2, even more preferably between 4x4 and 5x5 cm ^2. Higher dimensions may make it difficult to obtain a homogeneous catalyst deposit, while lower dimensions may cause the so-called "edge effect" which also entails the difficulty of obtaining a homogeneous deposit.

Said substrate is connected by its outer face, that is, by the gas diffuser layer, to a first electrical contact that allows electrical conduction to an external circuit.

In a preferred embodiment, said first electrical contact has the same dimensions as the substrate. Also preferably, said first electrical contact is a copper foil.

In a preferred embodiment, the second chamber in which the outer face of the substrate is housed and the first electrical contact comprises a cover that allows said chamber to be closed. The aforementioned cover comprises the at least two conduits for the entry and exit of gases. These ducts can be two holes between 1 and 3 mm in diameter with standard fittings. On the other hand, the counter electrode of the electrochemical cell of the invention is an electrode on whose surface a catalyst with an electrochemical area per unit of geometric area greater than that of the substrate is deposited. The counter electrode must be tight and stable. Preferably, said counter electrode comprises a graphite plate in contact with a cloth or carbon paper electrode on whose inner face platinum is deposited as a catalyst.

In a preferred embodiment, the counter electrode has geometric dimensions somewhat larger than those of the substrate to be deposited, more preferably it has dimensions of between 5-15% larger than those of said substrate, and is arranged centrally with respect to said substratum. That is, in their location all their faces protrude the same distance with respect to the faces of the substrate.

Said counter electrode is connected on its outer face to a second electrical contact that allows electrical conduction to an external circuit. In a preferred embodiment, said second electrical contact has the same dimensions as the counter electrode. Also preferably, said second electrical contact is a copper foil.

The electrochemical cell of the invention is characterized in that it comprises an ion conducting membrane with low electrical and hydrophilic resistivity, but impermeable to the passage of gases, located between the substrate and the counter electrode, where said membrane separates the first subchamber in contact with the substrate of the second subchamber in contact with the counter electrode. The presence of this membrane prevents the gases generated in the electrolyte in contact with the counter electrode from reaching the deposit substrate, thus increasing the performance of the electrodeposition reaction and avoiding its interference in the deposition reactions.

The substrate, the membrane and the counter electrode are arranged in parallel arrangement in the electrochemical cell of the invention, so that the membrane is housed in parallel between the front face of the counter electrode and the front face of the substrate.

In a preferred embodiment, the ion conducting membrane is housed in parallel to the front face of the counter electrode at a distance between 2 and 5 mm. In a particular embodiment, the electrochemical cell comprises an electrolyte disposed in the first subchamber and in the second subchamber, so that the ion conducting membrane and impermeable to the passage of gases separates the electrolyte disposed in the first subchamber from the electrolyte disposed in the second subchamber In another particular embodiment, the electrochemical cell of the invention further comprises flexible means located on the outer face of the first electrical contact, which allow said first electrical contact to be pressed on the substrate to be deposited, obtaining a homogeneous and low-resistive contact of importance between them. to get homogeneous deposits in large areas. Said flexible means are preferably at least one spring or a foam padding, and can be found attached to a metal sheet of somewhat smaller dimensions than the electrical contact that contacts the outer face of the substrate. In a particular embodiment, the flexible means are compressed on the first electrical contact by means of the cover that allows the second chamber to close. Particular embodiment

A particular embodiment of the electrochemical cell of the present invention is represented in Figure 1. Said figure corresponds to a side view of the electrodeposition cell assembly.

In this particular embodiment, the electrochemical cell comprises: a housing (1) in the form of an open parallelepiped at least on one of its faces, where the face opposite the open face comprises a first window (2);

- a closing block (3) adapted to cover the open face of the housing, wherein said closing block comprises a recess (4) disposed on the outer face and a second through window (5) arranged at the bottom of the recess, such that the face of the closing block with the second through window is the face adapted to cover the open face of the housing, where:

- the housing (1) and the closing block (2), are joined defining inside the first sub chamber (6) in contact with the inner face of the substrate, - the first window (2) of the housing is closed externally by an assembly that consecutively comprises the membrane (7), the counter electrode (8) and the second electrical contact (9), so that the membrane (7) and the counter electrode (8) are spaced apart from each other giving rise to the second subchamber that is in contact with the inside face of the counter electrode (8),

- the recess (5) of the closing block (3) houses the substrate (10) and the first electrical contact (11), the substrate (10) being in contact with the second through window (5), where said recessed is closed by the outer face of the closing block by means of an airtight cover (20), so that the second chamber is formed inside said recess in which the outer face of the substrate and the first are housed inside electrical contact.

In a preferred embodiment, the first and second windows (2 and 4) have a square shape.

This electrochemical cell comprises a parallelepiped-shaped housing (1), where said parallelepiped is open at least on one of its faces, and the opposite side of it comprises a window (2) that allows the passage of ions and current during the reaction of electrodeposition between the active zone of the substrate (10) and the counter electrode (8).

Said housing (1) houses in the external part comprising the window (2), an assembly that consecutively comprises the ion conducting membrane (7), the counter electrode (8) and the second electrical contact (9).

In particular, the membrane (7) is attached to the window of the housing (1), so that said membrane (7) covers the entire window (2). The membrane can be joined to the window (2) by incorporating a first gasket (12) that allows the membrane edges to be sealed.

The counter electrode (8) comprises a substrate (8a), on whose surface a catalyst is deposited, and a graphite plate (8b). Said counter electrode (8) is placed next to the membrane (7) so that said membrane (7) and the counter electrode (8) are spaced apart from each other, giving rise to the second subchamber in contact with the counter electrode. In a preferred embodiment, the distance between the counter electrode (8) and the membrane (7) is 2-5 mm. Preferably, the substrate (8a) is a carbon cloth or paper and the catalyst deposited thereon is platinum.

Between the membrane and the counter electrode a second joint (13) can be located to seal the edges of both elements. The second electrical contact (9) that provides the electric current to the external circuit is located on the outer face of the counter electrode.

In a particular embodiment, the constituent material of the cell housing (1) is a material sufficiently resistant to chemical agents, in particular to acids, and especially to sulfuric acid comprised in the electrolyte. Preferably, said material is a polymethacrylate, although others such as Teflon, PVC, or some other sufficiently rigid, resistant and impermeable polymer can also be used.

The electrochemical cell shown in Figure 1 further comprises a closing block (3) adapted to cover the open face of the housing (1). Said closing block comprises a recess (4) disposed on the outer face connected to a second window (5) arranged at the bottom of the recess, such that the face of the closing block having the second through window is the adapted face to cover the open face of the housing (1).

The closing block houses inside the substrate (10) and the first electrical contact (11), the substrate (10) being in contact with the second through window (5).

The chamber closure block (3) is connected to the housing (1) so that at its junction they define the first sub-chamber (6) in contact with the substrate (11).

Preferably, the connection between both pieces is carried out by means of screws and a silicone gasket (14), in order to obtain a sufficient seal.

Preferably, the interior dimensions of the first subchamber formed between the housing and the closing block should be such that once the desired amount of catalyst has been deposited, the concentration of its precursors in the electrolyte has not decreased by more than 10% . The through window (5) has the same dimensions as the geometric active area of the substrate.

The term "geometric active area of the substrate" means the projected area of the substrate on which the catalyst is deposited. In a particular embodiment, the closing block (3) can incorporate, at least, another window (15) of dimensions larger than the window (5), which allows the preparation of electrodes of those dimensions in the same cell without having to perform Modifications on it. As many stepped windows can be incorporated as desired for the preparation of electrodes with different active areas, as long as the closing elements and electrical contacts according to the different sizes are used.

Said closing block (3) houses inside the substrate (10) and the first electrical contact (11) located on the outer face thereof, so that the inner face of the substrate is in contact with the first sub-chamber (6) formed by the union of the housing (1) with the closing block (3) through the window (5). In a preferred embodiment, the substrate (10) is square in shape with varying between lxl cm ² and 10x10 cm ² surface, more preferably the surface is between 3x3 and 6x6 cm ^2, even more preferably between 4x4 and 5x5 cm ²

The substrate (10) can be attached to the hole (5) of the closing block (3) by means of a gasket (16), preferably, a gasket with sufficient hydrophobicity, such as a teflon gasket, for sealing the edges of the electrode that prevents the electrolyte from leaving.

All the elements that constitute this electrochemical cell must be arranged in parallel.

The parallelepiped shape and the described dimensions of the electrochemical cell are best suited to obtain:

1) a better process economy, since larger volumes require more electrolyte and, consequently, more platinum, and

2) a homogeneous deposit that requires that the substrate and counter electrode be close and opposite. The recess (4) included in the closing block (3) is closed by the external face of said closing block by means of a hermetic cover (20), so that the recessed forms the second chamber in which the face is housed exterior of the substrate and the first electrical contact. This hermetic cover (20) also includes holes intended for the passage of some gas to the block (3) to carry out the deposition of the catalyst in a controlled atmosphere, which allows to achieve anoxic conditions during the deposition, since dissolved oxygen is a reagent that gives place to parallel reactions (oxygen reduction) and influences the characteristics of the deposit, especially when the catalyst is platinum and tungsten oxide.

Said cover (20) can also comprise the flexible means (19) that allow the first electrical contact to be pressed on the substrate to be deposited, obtaining a homogeneous and low resistive contact between them, perfectly isolating the substrate from the electrolyte liquid. Said flexible means are preferably at least one spring or a foam padding.

In a particular embodiment, said cover (20) comprises a hole, preferably in its geometric center, which allows the passage of the electric cable connected to the first electrical contact (11) and having its same diameter to favor the sealing. In a particular embodiment, the recess (4) also houses a frame (18) whose internal dimensions coincide with the active area of the substrate or electrode to be prepared, while the external dimensions must match the measurements of the substrate. Said frame is constituted by one or more pieces of electrically inert and insulating material, and is located on the outside of the first electrical contact (11) and firmly presses the edges of said first electrical contact and of the substrate against the edges of the recess ( 4) intern of the closing block (3), thus improving the tightness of the cell.

Said frame (18) allows the flexible means (19) that press the first electrical contact on the substrate to pass through it. In a preferred embodiment, the electrochemical cell further comprises an access opening to the first sub chamber that is in contact with the substrate, and has a cover (17) adapted to close said opening to isolate the electrolyte from ambient air.

Said cover (17) also allows an orderly electrode to be placed in order, if necessary, and the conduits for the passage of gases.

The electrochemical cell can also comprise a series of joints (21,22) that allow the union of the frame (18) and the hermetic cover (20) and of the frame (18) with the first electrical contact (11).

The electrochemical cell of the invention has the additional advantage of being able to be used not only to carry out the electrodeposition reaction, but also as a cell for the study or testing of electrodes in a controlled atmosphere, since the electrode can be prepared in the same cell , carry out the electrochemical and / or chemical post-treatments that may be necessary, and carry out the first characterization tests: measurement of the area of electrodeposited platinum and the activity for oxygen reduction.

Therefore, in a particular embodiment, when the cell is used to effect an electrodeposition reaction of a catalyst on a substrate, the electrolyte is located in the first and second subchambers and comprises an aqueous solution comprising a precursor of the catalyst material to be deposited. on the substrate. Preferably, the catalyst to be deposited is platinum, whereby in that case the electrolyte can be an aqueous solution comprising a platinum precursor, such as for example a platinum salt such as hexachloroplatinate.

Additionally, the electrolyte may comprise additional agents to facilitate the electrodeposition process, such as sulfuric acid, and additional compounds that may be part of the catalyst together with platinum, such as cobalt or tungsten salts.

In another particular embodiment, when the cell is used for electrode testing and verifying oxygen reduction, the electrolyte is an inert electrolyte and ion conductor. Preferably, the electrolyte is a solution of sulfuric acid or perchloric acid. In this particular embodiment, the holes intended for the passage of gases from the cover (20) are used for the passage of oxygen or air during the test phase of the electrode.

The connection of the cover (20) to the closing block (3) can be carried out by means of screws, although it is also possible to carry it out by elastic elements, such as clamps or jaws for quick assembly and disassembly operations.

In another aspect, the present invention relates to a method for preparing an electrode comprising the deposition of a catalyst on a substrate by means of the electrodeposition technique, characterized in that the electrodeposition is carried out in an electrochemical cell as previously defined. Said procedure allows the deposit to be carried out under potentiostatic conditions or under galvanostatic conditions depending on the parameter to be controlled, the electrochemical potential of the substrate or the intensity of the deposit current, respectively.

Under potentiostatic conditions, suitable equipment is required, such as a potentiostat and a reference electrode, which allows the electrochemical potential to be set with respect to a reference electrode on the substrate surface, and varies according to a suitable potential program. Said program may consist of potential scans or potential pulses, applying as many scans as necessary to ensure the deposit of the desired amount of catalyst. Typical values include between 25 and 100 sweeps. In a particular embodiment, the electrodeposition is performed by applying a potential sweep between 0.05 V and 0.9 V using as an electrolyte an aqueous solution of hexachloroplatinate in sulfuric acid and boric acid.

Under galvanostatic conditions, an electric current, for example between 0.1 and 5 mA / cm ² , is passed between the substrate and the counter electrode by means of a current source or electroplating. The electrodeposition process can have different stages depending on the catalyst profile that one wishes to obtain. In addition, it may require the bubbling of nitrogen gas into the solution and the passage of nitrogen through the outer face of the substrate to ensure anoxic conditions. The amount of deposited catalyst can be estimated from the electric current, according to Faraday's law. In a particular embodiment, the substrate on which the electrodeposition is performed is a hydrophobicized substrate that prevents the electrolyte from percolating therethrough. There are commercially available hydrophobic substrates (PEMEAS®, SIGRACET®) comprising a layer of carbon paper or cloth and a layer of hydrophobic microporous carbon. Alternatively, a hydrophobic substrate can be prepared starting from a layer of carbon paper or cloth, one of its faces being coated with a layer of hydrophobic microporous carbon with an appropriate agent such as Teflon.

In another particular embodiment, the microporous carbon layer of the hydrophobic substrate is coated with another hydrophilic microporous layer by a mixture comprising carbon black, an ionomer and a dispersing agent. The ionomer is added as an ionic conductor in order to provide the active layer of the proton conduction substrate, but in concentrations sufficiently small to avoid an excessive diffusion barrier of the catalyst precursor anions to the grains of the carbon. Preferably, said ionomer is commercially available under the name of Nafion.

On the other hand, the dispersing agent is a liquid that is added to the mixture to form a suspension and thus make the deposit possible by suitable techniques. Preferably, the dispersing agent is of an alcoholic nature, the use of isopropanol being even more preferred. The ionomer remains in the microporous layer after deposition, while the dispersing agent is removed by evaporation and is not part of the deposited final layer.

The deposition of the different layers that constitute the substrate can be carried out by airbrushing or electrospray techniques.

In another particular embodiment, and as a prior step to the electrodeposition process, the process of the invention further comprises subjecting the substrate to an activation process in order to provide it with ionic conductivity and hydrophilic character exclusively on the deposit surface. Said activation process can be carried out by chemical or electrochemical means.

In a particular embodiment, the activation by chemical means comprises contacting the face of the substrate on which the electrodeposition is to be carried out with a concentrated nitric acid solution for a time ranging from 1 to 10 seconds, and subsequent washing with water to remove acid residues.

In another particular embodiment, electrochemical activation comprises subjecting the substrate on which the electrodeposition is to be cycled to electrochemical potential. Preferably, the potential cycling is performed at a scan rate of between 10 and 100 mV / s and controlled scan limits (for example anodic limit of 0.0-0.1 V vs NHE, cathodic limit of 1.0 to 1.5 V vs NHE, where NHE comes from Normal Hydrogen Electrodé), using sulfuric acid, in a concentration for example of 0.5 M, as electrolyte. In this case, the substrate must be subjected to a sufficient number of sweeps, until a desired value of the double layer charge current is achieved, which indicates the hydrophilic character achieved, without penetration of the electrolyte into the face. back of the substrate. Typical values range between 25 and 100 sweeps. This method has the advantage of being able to monitor the activation of the substrate in real time from the measurement of the current, which allows a better process control.

Both types of activation can be combined. A gentle chemical activation prior to electrochemistry shortens the time needed for the latter.

Electrochemical activation is performed with the substrate in the electrochemical cell of the invention, while chemical activation can be performed outside said cell. However, both could be carried out in the cell without disassembling the substrate.

The substrate has a way in which the active area is surrounded by additional substrate to ensure correct mounting and sealing. As an example, for an active area of 4x4 cm ^2, we start from substrates of 5x5 cm ² . The additional area is pressed by a hydrophobic joint that prevents electrolyte from flowing between the substrate and the cell. In another particular embodiment, the electrolyte used in the electrochemical cell that allows the electrodeposition process to be carried out is an aqueous solution comprising the precursor of the catalyst material to be deposited on the substrate. Preferably, the catalyst to be deposited is platinum, whereby in that case the electrolyte can be an aqueous solution comprising a platinum precursor, such as for example a platinum salt such as hexachloroplatinate. Additionally, the electrolyte may comprise additional agents to facilitate the electrodeposition process, such as sulfuric acid, and additional compounds that may be part of the catalyst together with platinum, such as cobalt or tungsten salts. This electrolyte is poured into the appropriate amount in the electrodeposition cell.

Once the substrate and electrolyte are located in the electrodeposition cell, the electrodeposition process is carried out. This process can be carried out in potentiostatic conditions or in galvanostatic conditions depending on the parameter to be controlled, as previously mentioned. The electrodeposition process can be carried out in successive stages. For example, after completing the first electrodeposition, the formed electrode can be removed from the cell, to deposit another hydrophilic microporous layer on it and again complete another electrodeposition process. Electrodes are thus obtained with layers of catalysts composed of thin sheets with structure: substrate-hydrophobic microporous layer-hydrofoil-catalyst microporous layer -hydro-catalyst-microporous layer ...

Once the electrodeposition process has been carried out, the electrode is disassembled from the cell and can be used, for example, for application in a fuel cell.

However, in a preferred embodiment, the process of the invention further comprises subjecting the electrode obtained to a subsequent treatment selected from an electrochemical treatment, a heat treatment and a combination thereof.

In a particular embodiment, the subsequent treatment is an electrochemical treatment comprising subjecting the electrode to electrochemical cycling in order to remove labile elements from the reservoir, that is, that part of the electrodeposit that has not been properly fixed to the surface of the substrate. This treatment therefore seeks to leave only the well adhered part of the deposited material. In a preferred embodiment, this electrochemical treatment comprises subjecting the electrode to an electrochemical cycling in sulfuric acid, more preferably at a concentration of 0.5 M, at controlled scanning rates and scanning limits, until a stable response of the current intensity between Two consecutive sweeps. This subsequent treatment can be carried out in the same electrochemical cell of the invention. In another particular embodiment, the subsequent treatment is a heat treatment comprising subjecting the electrode to a heating stage at a temperature between 100 and 150 ° C in a humid atmosphere. Preferably, said heating is carried out for a time between 10 and 60 minutes. This heat treatment is carried out in order to cure components such as the ionomer used in the initial coating of the substrate. Thus, when a solution of the ionomer, for example Naphion solution in alcohols, is used as starting material for the preparation of the substrate, this treatment can facilitate the polymerization and formation of the ionomer layer that gives the electrode a proton conductivity. In addition, this process allows to improve the integrity of the electrode, as well as to eliminate possible additives from the electrodeposition process.

The process of the invention allows to obtain an electrodeposited electrode in which the catalyst is distributed in a surface layer of about 1-10 microns, compared to 15-30 microns of the electrodes prepared by other procedures. The catalyst, preferably Pt, Pt-Co or Pt-WC "3 alloys, is coating part of the carbon grains of the microporous layer only in those areas where it has been exposed to the electrolyte. Therefore, electrodes with a very thin layer of catalyst, which results in a better performance mainly at high current demands. The images obtained by SEM show a different catalyst distribution.In addition, the mass activity (by weight of Pt) is higher in the electrodes obtained with the process of the invention.

Accordingly, an additional aspect described is an electrode obtainable according to a procedure as defined above. In a particular embodiment, said electrode is characterized in that the deposited catalyst is distributed in a surface layer of between 1 and 10 microns.

A further aspect described is a fuel cell comprising an electrode as previously described, that is, an electrode obtainable according to the process of the invention. Also preferably, said fuel cell comprises an electrode obtainable according to the method of the invention characterized because the deposited catalyst is distributed in a surface layer of between 1 and 10 microns.

The electrochemical cell of the present invention can be used in a test bench to carry out the first characterization tests of an electrode intended for incorporation into a fuel cell, which can be obtained by the electrodeposition technique mentioned above. Such tests include the measurement of the electrodeposited catalyst area and the activity for oxygen reduction.

Therefore, another aspect of the invention is a test bench for the characterization and measurement of the catalytic activity of an electrode, wherein said test bench comprises:

- an electrochemical cell as previously defined, wherein the substrate is replaced by the electrode to be tested, and

- an inert electrolyte, and - a source of electricity or potentiostat / galvanostat capable of controlling and recording the electrochemical potential of the electrode and the current intensity in the cell.

In a particular embodiment, the inert electrolyte is an aqueous solution of sulfuric acid or perchloric acid. A final aspect of the invention relates to a method for selecting an electrode suitable for use in a fuel cell, wherein said method comprises: a) depositing the electrode in an electrochemical cell as defined previously, in the place intended for host the substrate; b) fill the chamber adapted to house the electrolyte with an inert electrolyte; c) supplying oxygen or air to the outside of the electrode through the gas inlet of the second chamber; d) apply a suitable electrochemical potential between the electrode and a reference electrode, for example between 0.1 and 0.5 V against the reference electrode; and e) verify if an electric current occurs due to oxygen reduction, where the production of said electric current is indicative that the electrode is suitable for use in a fuel cell.

Step a) of this procedure consists in locating the electrode to be tested in the electrochemical cell described above in the same place where the substrate is located. If the electrode has been formed in the same electrochemical cell, it is deposited in the same place where it was formed.

The electrolyte used must be inert, that is, it must not react with the electrode or any other species of the solution. In a particular embodiment, said electrolyte is a solution of sulfuric acid or perchloric acid. The oxygen can be supplied by bubbling the solution with gas 0 ₂ and passing 0 ₂ through the outer face of the electrode so that it reaches all the catalyst deposited in the electrode where, in contact with the electrolyte, the desired electrochemical reaction undergoes, that is, its reduction with water formation. That is, the recording of an increase in the intensity of electric current by putting oxygen in the cell, which is proportional to the rate of reduction, is indicative that the electrode is suitable for use in a fuel cell.

In a particular embodiment, the oxygen is supplied through the holes intended for this purpose located in the sealed cap (20) of the cell described in the particular embodiment of the invention. The test to verify the reduction of oxygen can be carried out in potentiostatic or galvanostatic form, as previously mentioned, and allows the initial selection of the electrodes suitable for final assembly in a fuel cell. Example 1. Electrodeposition procedure using the electrochemical cell of the invention

The process has been carried out in a conventional laboratory, on a work table, with electricity and gas connection (N ₂ , 0 ₂ / air). It has been required for this: Equipment: A potentiostat / electroplating (Autolab, Eco Chemie) has been used as a current source and / or voltage source, capable of supplying at least 10V and 1A in direct current for an active area of 4x4 cm ² .

Materials and chemicals:

- Salts of the elements that constitute the catalysts: H ₂ PtCl6, Na ₂ W0 ₄ , Co (N0 ₃ ).

- Electrolyte: H ₂ S0 ₄ 0.5M.

- Carbon black for substrate preparation

- Teflon (solution in water) as hydrophobic agent

- Nafion (solution in aliphatic alcohols) as an ionomer - Carbon fabrics or carbon paper hydrophobicized with Teflon as a substrate.

- Electrochemical cell. Designed in Ciemat (Fig. L). This cell has been used for the preparation of electrodes of 4x4 cm ² .

A microporous layer between 20 and 50 microns thick was added to the hydrophobic carbon paper or cloth substrate by aerosol deposition of an ink consisting of carbon black, dissolved ionomer and dispersing agent (isopropanol). After removal of the dispersing agent by drying in an air stream, the substrate was pretreated by electrochemical activation. For this, the substrate was located in the electrochemical cell described, filled with 0.5M H ₂ S04 electrolyte, dissolved oxygen was removed by upper and back bubbling with N ₂ for 30 min, and which continued during the electrodeposition process, since The substrate was then subjected to an electrochemical cycling of 50 or 100 mV / s between 0.07 (at 0.1) and 1 (at 1.3) V vs. NHE After 10-50 cycles, a stable intensity response was reached and electrochemical activation was terminated. The tank was then emptied and filled with the solution containing the catalyst and electrolyte precursors inert (H ₂ SO ₄ + H ₂ PtCl6 + Na ₂ W04). The substrate was subjected to potential cycling of 50 or 100 mV / s between -0.5 (at 0.1) and 1 (at 1.3) V vs. NHE or subjected to galvanostatic conditions (1-5 mA-cm ^"2 for 20-60 minutes).

Once the deposit was made, the deposit electrolyte was emptied (H ₂ SO ₄ + H ₂ PtCl6 + Na ₂ W04), and filled with 0.5M H ₂ S04. Dissolved oxygen was removed by upper and posterior bubbling with N ₂ for 30 min, and that continued during the process. Next, the electrode was subjected to an electrochemical cycling of 50 or 100 mV / s between 0.07 (at 0.1) and 1.3 (at 1.5) V vs. NHE During this process the adsorption / desorption current of H on Pt was monitored, which is proportional to the area of deposited Pt. Once it reached a constant value, after about 5-20 cycles, the cycling was completed and with it the process.

Subsequently, the electrochemical area was measured, that is, the catalytically active area, using the process described in the International Journal of Hydrogen Energy, Vol 34, íssue 11, 2009, Pages 4838-4846, and the electrocatalytic activity measurement to reduce oxygen, by bubbling oxygen inside the cell and behind the substrate.

The electrode obtained with Pt-WC> 3 electrodeposited was tested as a cathode in a PEMFC monocell). The other elements of the monocell were: commercial Pt / C electrode with 0.25 mg / cm ² as anode, Nafion 212R as electrolyte. After 300 hours of continuous operation of the monocell under a constant demand of 200 mA-cm ^"2 , the polarization curve shown in Figure 2 was obtained using the following conditions: feeding with H ₂ at anode and 0 ₂ at cathode, at 80 ° C, 100% humidification, and constant stoichiometric ratio H _2/0 _2:. 1.5 / 3.0 relative pressure: 1 atm.

Claims

An electrochemical cell suitable for electrodeposition of a catalyst on a substrate, wherein said cell comprises:

where the outer face of the substrate and the first electrical contact are located inside a second chamber comprising at least two conduits for the entry and exit of gases, suitable to allow control of the atmosphere in the exterior part of the substratum,

A second wall comprises:

- a counter electrode comprising an inner face and an outer face,

- an ion conductive membrane and impermeable to the passage of gases located inside the first chamber separating it into two sub-chambers, a first sub-chamber in contact with the inner face of the substrate and a second sub-chamber in contact with the inner face of the counter electrode;

where the substrate, the membrane and the counter electrode are located in parallel arrangement.

Electrochemical cell according to claim 1, further comprising an electrolyte disposed in the first subchamber and in the second subchamber, such that the

REPLACEMENT SHEET (Rule 26) ion conductive membrane and impermeable to the passage of gases separates the electrolyte disposed in the first subchamber from the electrolyte disposed in the second subchamber. 3. Electrochemical cell according to any of claims 1 to 2, further comprising flexible means located behind the first electrical contact adapted to press said first electrical contact on the outer face of the substrate. 4. Electrochemical cell according to claim 3, wherein the flexible means is a spring or a foam padding.

5. An electrochemical cell according to any one of claims 1 to 4, comprising: a housing in the form of a parallelepiped open at least on one of its faces, where the face opposite the open face comprises a first window;

- the housing and the closing block, are connected defining inside the first subchamber in contact with the inner face of the substrate,

- the first window of the housing is closed externally by an assembly that consecutively comprises the membrane, the counter electrode and the second electrical contact, so that the membrane and the counter electrode are distanced from each other giving rise to

REPLACEMENT SHEET (Rule 26) the second subchamber that is in contact with the inside face of the counter electrode,

- the recess of the closing block houses the substrate and the first electrical contact, the inner face of the substrate being in contact with the second through window, where said recess is closed by the outer face of the closing block by means of a cover airtight comprising at least two conduits for the entry and exit of gases, so that inside said recess the second chamber is formed in which the outer face of the substrate and the first electrical contact are housed.

6. Electrochemical cell according to claim 5, wherein the first and second window have a square shape. 7. Electrochemical cell according to any of claims 5 to 6, wherein the housing constituent material is a polymethacrylate.

8. Electrochemical cell according to any one of claims 5 to 7, wherein the substrate has a surface comprised between lxl cm ² and 10x10 cm ² .

9. Electrochemical cell according to any of claims 5 to 8, wherein the substrate is attached to the second window of the closure block by means of a hydrophobic joint.

10. Electrochemical cell according to any of claims 5 to 9, wherein the hermetic cover comprises flexible means that press the first electrical contact on the substrate. 11. Electrochemical cell according to any of claims 5 to 10, wherein the locking of the closing block further houses a frame whose internal dimensions coincide with the active area of the substrate and the external dimensions coincide with

REPLACEMENT SHEET (Rule 26) the substrate measures, where said frame is located on the outside of the first contact.

12. Electrochemical cell according to claim 1, wherein the seal and the frame are constituted by an electrically inert and insulating material.

13. Electrochemical cell according to any of claims 5 to 12, wherein the housing comprises an access opening to the first sub chamber that is in contact with the substrate, and has a cover adapted to close said opening to isolate the electrolyte from ambient air .

14. Electrochemical cell according to any of claims 1 to 13, wherein the electrolyte disposed in the first and second subchamber is an aqueous solution comprising a precursor of the catalyst material.

15. A process for the preparation of an electrode, wherein said procedure comprises the deposition of a catalyst on a substrate by means of the electrodeposition technique, where the substrate is a hydrophobic substrate comprising a layer of microporous carbon, characterized in that the electrodeposition is performed in an electrochemical cell as defined in any one of claims 1 to 14, and wherein said method further comprises: prior to electrodeposition, coating the microporous carbon layer with a microporous layer comprising a mixture of carbon black and an ionomer, and subjecting the substrate to a suitable activation process to provide the coated face with ionic and hydrophilic conductivity, where said activation process is carried out by chemical, electrochemical or a combination of both;

I

after electrodeposition, subject the electrode obtained to a treatment selected from a heat treatment, an electrochemical treatment and a combination thereof, where the electrochemical treatment comprises subjecting the electrode to an electrochemical cycling in sulfuric acid at scanning rates and limits of controlled sweeping until a stable response of the intensity between two consecutive sweeps and the

REPLACEMENT SHEET (Rule 26) Heat treatment comprises subjecting the electrode to a heating stage at a temperature between 100 ° C and 150 ° C in a humid atmosphere.

16. The method according to claim 15, wherein the electrodeposition is carried out under potentiostatic or galvanostatic conditions.

17. Method according to any of claims 15 to 16, wherein the chemical activation comprises contacting the face of the substrate on which the electrodeposition is carried out with a concentrated nitric acid solution for a time ranging from 1 to 10 seconds, and subsequent washing with water.

18. A method according to any of claims 15 to 16, wherein electrochemical activation comprises subjecting the substrate to a cycle of electrochemical potential.

19. A test bench comprising:

- an electrochemical cell as defined in any one of claims 1 to 13 wherein the substrate is replaced by an electrode, an inert electrolyte, and

20. Test bench according to claim 19, wherein the inert electrolyte is a solution of sulfuric acid or perchloric acid.

21. A procedure for the selection of an electrode suitable for use in a fuel cell, wherein said procedure comprises:

- depositing the electrode in a test bench as defined in any one of claims 19 to 20, in the place intended to house the substrate;

- fill the chamber adapted to house the electrolyte with an inert electrolyte;

REPLACEMENT SHEET (Rule 26) - apply a suitable electrochemical potential between the electrode and a reference electrode; Y

- verify if electric current is produced due to oxygen reduction, where the production of said current is indicative that the electrode is suitable for use in a fuel cell.

REPLACEMENT SHEET (Rule 26)