FR2843635A1 - Evaluation of gas diffusion electrode, particularly for fuel cells and electrode support for use in electrochemical evaluation cell - Google Patents

Abstract

The electrically conducting part (11) of a gas supply pipe (10) is enclosed in a non-conducting sleeve (13) and is electrically connected to a measuring device via an orifice (12). The sleeve has a vent (14) for immersion in the electrolyte bath. The gas diffusion electrode (2) is placed to block the orifice and form a seal between the sleeve vent and the conducting part of the pipe The orifice in the conducting part and the vent in the sleeve are located opposite each other and have identical areas, with the orifice at most 10mm2. An expansion chamber (28) is placed upstream of the gas supply pipe with a gas inlet (29) and a calibrated passage (30) connecting to the gas supply pipe. The axis of the gas inlet into the expansion chamber is perpendicular to the axis of the calibrated passage. The gas supply pipe has an external tube one end of which is sealed to a disk of electrically conducting material in which the orifice of the gas supply pipe is formed. The gas supply pipe has an internal tube with one end opening near the orifice in the disk. The other end of the internal tube is connected to a gas inlet and the gas circulates from inside the internal tube to the space between the two tubes. The sleeve has a tubular part around the external tube and extending for at least the length of the support which is to be immersed in the electrolyte bath. The sleeve is fixed to the gas supply pipe. The gas supply pipe is metallic. An Independent claim is included for evaluating a gas diffusion electrode using an electrochemical cell with an electrode support as above. The support is immersed in the electrolyte bath and supplied with the reactive gas, while connected electrically to the measuring device to detect the voltage between the gas diffusion electrode and the reference electrode and the current intensity from the electrochemical reaction with the reactive gas circulating in the gas diffusion electrode.

Description

i

  GAS DIFFUSION ELECTRODE SUPPORT FOR ELECTROCHEMICAL EVALUATION CELL AND EVALUATION METHOD

OF A GAS DIFFUSION ELECTRODE

  The present invention relates to a gas diffusion electrode support 5 for use in an electrochemical evaluation cell further comprising a reference electrode, a counterelectrode, an electrolyte bath and a measuring device, said counterelectrode. and reference electrode being electrically connected to the meter and immersed in the

electrolyte bath.

  The development and optimization of fuel cells for the production of electrical energy necessitates an accurate evaluation of the characteristics of the gas diffusion electrodes. Indeed, a fuel cell comprises two gas diffusion electrodes, a first electrode, called an anode, one side of which is in contact with hydrogen gas and the other side of which is in contact with a generally liquid electrolyte, and a second electrode, called cathode, one side of which is in contact with the electrolyte and the other side of which is in contact with pure oxygen or air. These electrodes generally comprise two layers containing carbon particles, a catalyst, a binder and additives such as Teflon in variable proportion. A fabric, most often made of carbon fibers, is placed between the two layers and is used to collect the current produced. A hydrophilic layer generally containing the catalyst is arranged in contact with the electrolyte which can be a liquid or a solid polymer. The other hydrophobic layer is in contact with the reactive gas which diffuses through it to come into contact with the electrolyte in the

  hydrophilic layer where the electrochemical reaction takes place.

  To evaluate the characteristics of gas diffusion electrodes or of the materials intended for their manufacture, and in particular their capacity to transform the reactive gas or their resistance to pollutants contained in the reactive gas or the air, one usually uses a

  electrochemical cell provided with an electrode support with 5 rotating disc. The electrode or the material to be evaluated placed on the rotating disc is successively bathed in the electrolyte and brought into contact with the reactive gas so as to dissolve the reactive gas in the electrolyte and in order to obtain an electrical signal generated by the electrochemical reaction.

  However, the operating conditions of this type of evaluation cell are not representative of the actual operation of a fuel cell in which the electrode permanently has one face in contact with the gas and another face in contact with the electrolyte. On the other hand, the low solubility of the gas in the electrolyte limits the intensity of the electrical signals produced. In addition, this type of cell is relatively expensive due to the

  need to use a running engine.

  The object of the present invention is to overcome these drawbacks by proposing a gas diffusion electrode support for an electrochemical evaluation cell which makes it possible to reproduce, as faithfully as possible, the behavior of at least one of the electrodes 25 of a fuel cell, while making it possible to carry out numerous tests with different gas diffusion electrodes. To this end, the electrode support for an electrochemical evaluation cell is characterized in that it comprises, on the one hand, a gas supply duct having at least one electrically conductive part through which an orifice passes. and intended to be electrically connected to the measuring device, and on the other hand, a sleeve of non-electrically conductive material which surrounds at least said electrically conductive part of the gas supply pipe, in that said sleeve has an opening intended to be immersed in the electrolyte bath, and in that the gas diffusion electrode is disposed against said electrically conductive part and inside said sleeve so as to close off the orifice and sealing between the opening of the sleeve and the conductive part of

  electricity from the gas supply pipe.

  Thanks to this arrangement, the electrode has one face in contact with the gas and another face in contact with the electrolyte, which makes it possible to faithfully reproduce the operation of this electrode when it is used in a fuel cell. In addition, the electrically conductive part of the gas supply pipe 15 ensures very good electrical contact between the electrode and the measuring device, without requiring a bonding operation of a conductor on the electrode. . Thus, the mounting of the different electrodes to be tested or of the different

  materials to be assessed is facilitated.

  In preferred embodiments of the invention, it is optionally possible to have recourse to one and / or the other of the following arrangements: the orifice of the electrically conductive part and the opening of the sleeve are arranged opposite one another and have substantially identical areas; the orifice of the electrically conductive part and the opening of the sleeve are arranged opposite one another and have substantially identical surfaces; - An expansion chamber is arranged upstream of the gas supply duct, said chamber having a gas inlet and a calibrated passage communicating with said gas supply duct; - The axis of the gas inlet into the expansion chamber is substantially perpendicular to the axis of the calibrated passage; the gas supply pipe comprises an external tube which has a first end to which a disc of electrically conductive material in which the orifice of said gas supply pipe is formed is tightly attached; the gas supply duct comprises an inner tube disposed in the outer tube and a first end of which opens near the disc of conductive material and facing the orifice of said disc; the inner tube has a second end which communicates with a gas inlet, and a gas outlet is arranged near a second end of the outer tube, the gas flowing from the internal space of the internal tube towards the space between the outer tube and the inner tube; - The sleeve comprises a tubular part surrounding the outer tube and extending over at least the length of the support intended to be immersed in the electrolyte bath; - the sleeve is removably mounted on the gas supply pipe;

  - the gas supply pipe is metallic.

  The subject of the invention is also a method for evaluating a gas diffusion electrode using an electrochemical cell comprising a reference electrode, a counter electrode, an electrolyte bath and a measuring device, said counter-electrodes and the reference electrode being electrically connected to the measuring apparatus and immersed in the electrolyte bath, in which: - the gas diffusion electrode is mounted in a support as defined above; - The support is immersed in the electrolyte bath so as to immerse the opening of the sleeve; - Reactive gas is supplied to the gas supply pipe; - The gas diffusion electrode is electrically connected to the measuring device thanks to the electrically conductive part; - then, using the measuring device, the voltage V2 between the gas diffusion electrode and the reference electrode is noted on the one hand, and the intensity of the current resulting from the electrochemical reaction of the reactive gas and flowing in

the gas diffusion electrode.

  Other features and benefits of

  the invention will appear during the description which follows, given by way of nonlimiting examples, with reference to the appended drawings in which:

  - Figure 1 is a longitudinal sectional view of a gas diffusion electrode support according to the invention; FIG. 2 is a schematic view of an electrochemical evaluation cell 20 using the gas diffusion electrode support shown in FIG.

figure 1.

  In the different figures we have kept the same

  references to designate identical or similar elements.

  In Figure 1 is shown a support 1 for a gas diffusion electrode 2 and intended for use in an electrochemical evaluation cell 3 shown

in Figure 2.

  The electrochemical evaluation cell comprises, in known manner, a reference electrode 4, a counterelectrode 5, an electrolyte bath 6 contained in a tank

7 and a measuring device 8.

  The reference electrode 4, placed near the electrode support 2, can be formed by a tube

Luggin capillary.

  The counter electrode 5 may consist of a simple graphite or platinum bar, but it may also be formed by a second gas diffusion electrode placed in a support similar to support 1. For the measuring device 8, it is possible to use a

  potentiostat / galvanostat such as for example the model 263 distributed by the company Perkin Elmer. Of course, the counter electrode 5 and the reference electrode 4 are electrically connected to the measuring device 8.

  The tank 7 may include an agitator, for example a magnetic bar, and means for heating the electrolyte. According to the invention, the electrode support 2 15 comprises a gas supply pipe 10 having an electrically conductive part 11, formed in the example represented by a metal disc, and electrically connected to the apparatus for measure 8. The metal disc 11 is crossed by an orifice 12 formed in the center of the disc. A sleeve 13 of insulating material, such as plastic, surrounds the part of the duct

  gas supply 10 comprising the metal disc 11.

  The sleeve has an opening 14 opening out

  in the electrolyte bath when the support 1 is placed in the electrochemical cell 3.

  The gas diffusion electrode 2 is held against the metal disc 11 by the sleeve 13 so as to close the orifice 12 of the gas supply pipe 10 and to ensure the seal between the metal disc 11 and the opening 14 of the sleeve 13, so that the electrolyte does not come into contact with this metal part and so as not to have a leakage of reactive gas in the electrolyte bath. Thanks to these arrangements, the electrode 2 has a face in contact with the gas and a face in contact with the electrolyte, thus the measurements carried out with the support 1 are representative of the actual operation of one of the two electrodes. a fuel cell. Although the electrolyte of the evaluation cell 3 is a liquid electrolyte, this cell makes it possible to faithfully simulate a fuel cell containing an electrolyte in the form of solid polymer by adjusting the electrolytic bath composition so as to obtain an identical conductivity to that of a solid polymer type electrolyte. 10 Note that the disc 11 ensures by its simple support against the electrode 2 a good electrical contact therewith. An annular seal 15 of small thickness may be disposed between the inner periphery of the opening 14 of the sleeve 13 and the electrode 2, in order to improve the seal.

to the electrolyte.

  Preferably, the opening 14 of the sleeve 13 and the opening of the annular seal 15 are arranged opposite the orifice 12, and have an area identical to that of the orifice 11. This makes it possible to precisely delimit the active surface of electrode 2 which is in contact with

reactive gas and electrolyte.

  In a particularly advantageous manner, the orifice 12 has a reduced surface area, that is to say less than 25 to 10 mm2, and preferably equal to 3 mm2, so as to

  reduce the active surface of electrode 2.

  In fact, unlike fuel cells in which large electrode surfaces are sought in order to increase the production of electricity, it is preferable to have a reduced surface in order to evaluate the

  characteristics of the gas diffusion electrode.

  This reduced surface makes it possible to obtain a high current density in the electrode while maintaining a moderate output current, thus it is possible to highlight the phenomena of saturation of the electrode without reaching the maximum intensity supported by the device which

  is generally 100 milliamps.

  The small working surface also makes it possible to overcome the homogeneity defects of the electrode. Indeed, the composition of the electrode, and in particular the density of the catalyst, can vary from one region to another, especially if it is a sample of electrode produced in the laboratory. With a reduced working surface, the risk of carrying out an average measurement of the characteristics of an electrode exhibiting a lack of homogeneity is limited. Finally, the reduced working surface makes it possible to place the reference electrode 4 close to the entire active surface of the electrode to be evaluated. For example, 15 if the reference electrode is produced using a tube

  Luggin capillary, it is possible to place the end 4a of the capillary tube so that the entire working surface of the gas diffusion electrode 2 is located less than 5 times the outlet diameter of the tube 20 capillary.

  In the embodiment shown, the gas diffusion electrode 2 is in the form of a flexible membrane consisting of two layers (18, 19) between which is embedded a fabric of carbon fibers 20. The hydrophobic layer 18 is in contact with the gas and the layer

  hydrophilic 19 is in contact with the electrolyte 6.

  However, the structure of the support 1 allows the mounting of a single layer devoid of the carbon fiber fabric, in order to evaluate experimentally the characteristics of a material intended to be used for the realization of a

gas diffusion electrode.

  As shown in Figure 1, the gas supply conduit 10 comprises an outer metal tube 17 which has an end integral with the disc 11 to facilitate the electrical connection of the disc with the apparatus

of measurement.

  Preferably, the gas supply pipe 10 comprises an inner tube 21, a first end 21a of which opens near the metal disc 11 so as to be opposite the orifice 12. The gas arrives through a second end 2lb of the tube inside, flows from the internal space of the tube 21 to the space between the external tube 17 and the internal tube 21 to be evacuated by a gas outlet 22 10 arranged near a second end 17b of the tube

  outside. The short distance between the first end 21a of the inner tube and the metal disc 11 allows good contacting of the reactive gas with the hydrophobic layer 18 of the electrode. This distance is preferably less than 1 mm.

  The sleeve 13 of insulating material comprises a

  tubular part surrounding the outer tube 17 and extending over at least the length of the support 1 immersed in the electrolyte bath 6 to isolate the outer tube 17 from the electrolyte bath.

  The gas supply pipe 10 can then

  advantageously be metallic in order to ensure electrical contact between the disc 11 and a connector 23 arranged on a part of the support 1 which emerges from the electrolyte bath 6.

  The sleeve 13 is preferably removably mounted on the gas supply pipe 10 so that the gas diffusion electrode 2 can be easily changed. This removable mounting can be carried out using screws 24 30 passing through a shoulder 25 formed on the emerging end of the sleeve 13 and screwed into a single shoulder 26 of the outer tube 17 of the gas supply pipe. Of course, this assembly can be carried out differently and in particular to

  by means of fast-acting locking means.

  An expansion chamber 28, disposed upstream of the gas supply pipe 10 has a gas inlet 29 and a calibrated passage 30 which communicates with the inner tube 21 of the gas supply pipe. This expansion chamber 28 makes it possible to dampen a possible shock wave 5 caused by the opening of a reserve of reactive gas under pressure, or to avoid propagation inside the wave gas supply duct. stationary generated by

an air compressor.

  In the embodiment shown, the expansion chamber 28 is delimited by a cylindrical element 31 disposed in the extension of the outer tube 17 of the gas supply pipe, by a transverse wall 32 in which the calibrated passage 30 is made and by one

upper transverse wall 33.

  In order to have an electrical connector 23 on the upper end of the support, the walls of the expansion chamber 28 are metallic to ensure electrical continuity with the outer tube 17 of the conduit

gas supply 10.

  The axis of the gas inlet 29 in the expansion chamber 28 is substantially perpendicular to the axis of the calibrated passage 30 to improve the attenuation effect of

pressure waves from chamber 28.

  To evaluate the characteristics of the gas diffusion electrode 2 previously mounted in the support 1, the end of the sleeve 13 having the opening 14 is immersed in the electrolyte bath, the gas diffusion electrode 2 is electrically connected to the measuring device 8 using the connector 23, the counter electrode 5 and the reference electrode 4 also being connected to this device, and the gas supply pipe is supplied with gas

  reagent via the expansion chamber 28.

  The device 8 of the potentiostat / galvanostat type provides electrical continuity between the gas diffusion electrode 2 and the counterelectrode 5 necessary to obtain the electrochemical reaction. This type of measuring device makes it possible: - to send variable gas signals to the gas diffusion electrode to cause an electrochemical reaction; - record the responses of the gas diffusion electrode following the electrochemical reaction caused

by signals.

  The measuring device 8 is further adapted to take a reading of the characteristics of the electrode to

  gas diffusion 2 using the reference electrode 4.

  For example, the device records for different voltages V2 between the gas diffusion electrode and the reference electrode, the intensity of the current flowing in the electrochemical cell. So we can draw different curves

  voltage / current density characteristics corresponding to the behavior of the gas diffusion electrode 2 as a function of the gas used, the quantity of pollutant in the reactive gas, the composition of the electrode used, or the nature of the electrolytic bath .

  It will be noted that in order to evaluate the gas diffusion electrode in different baths, for example an alkaline bath then an acid bath, it suffices to remove the support 1 from the tank 7 and to place it in another tank containing the second bath. This can be done quickly without removing the electrode 2 from the support 1 and does not require the

  purging of the electrochemical cell.

  The electrode support according to the invention therefore makes it possible to carry out precise measurements of the characteristics of a gas diffusion electrode under the conditions of

  fuel cell operation. In addition, it is particularly suitable for use in the laboratory thanks to the possibility of easily changing the gas diffusion electrode or the operating conditions of the electrochemical cell.

Claims (12)

  1. Gas diffusion electrode support intended for use in an electrochemical evaluation cell (3) further comprising a reference electrode (4), a counter electrode (5), an electrolyte bath (6 ) and a measuring device (8), said counter electrode and reference electrode being electrically connected to the measuring device and immersed in the electrolyte bath, characterized in that it comprises on the one hand, a gas supply pipe (10) having at least one electrically conductive part (11) through which an orifice (12) passes and intended to be electrically connected to the measuring device (8), and the other part, a sleeve (13) of non-electrically conductive material which surrounds at least said electrically conductive part (11) of the gas supply pipe (10), in that said sleeve (13) has a opening (14) intended to be immersed in the electrolyte bath (6), and in that q ue the gas diffusion electrode (2) is disposed against said electrically conductive part (11) and inside said sleeve (13) so as to seal the orifice (12) and to ensure the seal between the opening (14) of the sleeve and the electrically conductive part (11) of the conduit gas supply.   2. An electrode holder according to claim 1, in which the orifice (12) of the electrically conductive part (11) and the opening (14) of the sleeve (13) are arranged facing the each other and present   substantially identical areas.   3. electrode holder according to claim 1 or   2, in which the orifice (12) of the electrically conductive part (11) has an area at most equal to 35 10 mm2.   4. electrode holder according to any one of   Claims 1 to 3, in which an expansion chamber (28) is arranged upstream of the gas supply pipe (10), said chamber having a gas inlet (29) and a calibrated passage (30) communicating with said gas supply pipe.   5. An electrode support according to claim 4,   wherein the axis of the gas inlet (29) in the expansion chamber (28) is substantially perpendicular to the axis of the calibrated passage (30).   6. An electrode holder according to any one of   Claims 1 to 5, in which the gas supply pipe comprises an outer tube (17) which has a first end to which a disc (11) made of conductive material is sealed.   the electricity in which is formed the orifice (12) of said gas supply pipe.   7. An electrode holder according to claim 6, in which the gas supply duct (10) comprises an inner tube (21) disposed in the outer tube (17) and a first end (21a) of which opens near the disc (11) of conductive material and facing the orifice (12) of said disc.   8. An electrode holder according to claim 7, in which the inner tube (21) has a second end (21b) which communicates with a gas inlet (29), and in which a gas outlet (22) is arranged. near a second end (17b) of the outer tube, the gas flowing from the inner space of the inner tube 30 (21) to the space between the outer tube (17) and the inner tube (21).   9. An electrode holder according to any one of   Claims 6 to 8, in which the sleeve (13) comprises a tubular part surrounding the outer tube (17) and 35 extending over at least the length of the support (1) intended   to be immersed in the electrolyte bath (6).   10. An electrode holder according to claim 9, in which the sleeve (13) is removably mounted on the gas supply pipe (10).   11. An electrode holder according to any one of   Claims 1 to 10, in which the supply duct of gas (10) is metallic.   12. Method for evaluating a gas diffusion electrode using an electrochemical cell 10 (3) comprising a reference electrode (4), a counterelectrode (5), an electrolyte bath (6) and a measuring device (8), said counter electrode and reference electrode being electrically connected to the measuring device and immersed in the electrolyte bath, in which: - the gas diffusion electrode (2) is mounted in a support (1) defined according to any one of claims 1 to 11;   - The support (1) is immersed in the electrolyte bath (6) 20 so as to immerse the opening (14) of the sleeve (13); - Reactive gas is supplied to the gas supply pipe (10); - The gas diffusion electrode 25 (2) is electrically connected to the measuring device (8) thanks to the electrically conductive part (11); - Then, using the measuring device (8), the voltage V2 is noted on the one hand between the gas diffusion electrode (2) and the reference electrode (4), and on the other hand 30 hand, the intensity of the current resulting from the electrochemical reaction with the reactive gas and flowing in   the gas diffusion electrode (2).