EP4034695A1

EP4034695A1 - Textured anode for fluorine production and process for structuring a carbon substrate intended to be used in such an anode

Info

Publication number: EP4034695A1
Application number: EP20790371.7A
Authority: EP
Inventors: Tomy FALCON; Nicolas BATISSE; Marc Dubois; Katia GUÉRIN ARAUJO DA SILVA
Original assignee: Centre National de la Recherche Scientifique CNRS; Sorbonne Universite; Universite Clermont Auvergne; Sigma Clermont
Current assignee: Centre National de la Recherche Scientifique CNRS; Sorbonne Universite; Universite Clermont Auvergne; Sigma Clermont
Priority date: 2019-09-24
Filing date: 2020-09-24
Publication date: 2022-08-03
Also published as: WO2021058912A1; FR3101083B1; FR3101083A1

Abstract

The invention relates to the production of a microtextured anode and to the use of this anode in an electrochemical cell for the electrolytic production of gaseous fluorine in a bath of molten fluorinated salts.

Description

Textured anode for the production of fluorine and method of structuring a carbon substrate for use in such an anode

Technical area

[1] The present invention relates generally to the production of a microtextured anode and the use of this anode in an electrochemical cell for the electrolytic production of fluorine gas in a bath of molten fluorinated salts.

State of the art

[2] At present, the production of molecular fluorine F2 by electrolysis is typically carried out in an electrochemical cell in a molten salt medium of hydrofluorinated potassium fluoride, KF-2HF at a temperature close to 100 ° C because it is known those skilled in the art that the KF-2FIF mixture is the most suitable for the electrolysis of fluorine in terms of temperature and ionic conductivity ⁽¹⁾ . Few materials can be envisaged in the anode position in an electrochemical cell due to the high reactivity of nascent fluorine and of the HF medium. At present, the industrial production of fluorine is carried out using carbonaceous anodes on the surface of which a fluorocarbon film covering the anode is formed during the production of gaseous fluorine ^{(2) (4)} . As for the cathodes, they are most often made of steel and are protected against corrosion as long as they remain under cathodic polarization ^{(4) (5)} . The fluorocarbon film formed on the surface of the anode causes an anode overvoltage, which should be reduced in order to avoid excessively high current intensities and therefore excessively large amounts of energy to be supplied to the device.

[3] Various works ^{(1) (3)} show that the anode overvoltage observed during the generation of F2 is not only caused by the fluorination of the anode surface due to the aggressive environment of the electrolysis and to the conditions applied ( high applied voltage in the presence of fluorine), but also by the dynamics of bubble formation appearing when the voltage increases. There are various publications which deal with the nucleation on the surface of an anode and according to the work of H. Groult ⁽⁶⁾ , the bubbles are born at the level of the imperfections and the pores of the surface. to form a fluidized layer (as illustrated by FIG. 1 and in particular FIG. 1c). The latter is the result of the coalescence of several small bubbles created on the imperfections. The detachment of a bubble (figure 1e) reform the fluidized layer (figure 1c). [4] Thus, even today, the electrolysis of fluorine remains a poorly mastered technology. Indeed, because of the dangerousness of the process, it is difficult to make observations of the physico-chemical processes taking place inside the electrolysers, and therefore to quantify them precisely. However, the laboratory experiments which have been carried out show that the gas emissions at the electrodes are particularly dependent not only on the composition of the bath, but also on the chemical and morphological surface state of the electrodes ^{(6) (8)} . An improvement in the wettability of the electrodes by the bath would make it possible to control the release of bubbles and therefore the efficiency of the electrochemical reactions leading to the production of fluorine and dihydrogen, as has already been done for other industrial electrolyses: in particular that water ⁽⁹⁾ .

[5] Many recent advances show that surface structuring makes it possible to achieve wettability domains never seen before by simple modification of surface chemical groups ^{(10) (11)} . Thus, the wettability of different electrode materials by the KF-2HF molten salt in the presence of fluorine as well as the optimization of the generation of bubbles by a microstructuring of the surface of the electrodes have been particularly studied ⁽⁶⁾ ' ⁽¹²⁾ .

[6] The objective of the present invention is to provide an anode which makes it possible to better control the characteristics of the gas emissions at the anode in an electrochemical cell for the electrolytic production of gaseous fluorine in a bath of molten fluorinated salts. This should make it possible in particular to reduce the anode overvoltage appearing during the production of fluorine, to increase the faradic yields linked to the production of fluorine, to optimize the recovery of the gases in the dedicated gas compartments, and to limit the phenomena of entrainment of particles at the electrolyte / gas interface which are particularly detrimental to the process for the electrolytic production of fluorine gas in a bath of molten fluorinated salts. [7] To fulfill such an objective, various surface structures and nanostructurations have already been considered. Thus, patent applications US 20104/0126875 and EP2145984 from MITSUI CHEMICALS teach the creation of through channels (of the order of a hundred mm in diameter) in the thickness of the anode so that the fluorine F2 gas produced at the anode (or hydrogen H2 gas at the cathode) can escape from the inter-electrode space where the electrolyte is present to a space behind the electrode where there is no electrolyte but only a gas phase. This channel microstructuring changes the configuration of the electrode and forces an electrode thickness of 3 mm and the implementation of additional compartments to evacuate the gases produced.

[8] In addition, the generation of bubbles remains anarchic if the microstructuring is not controlled. It is necessary that the fluorine bubbles are generated within a well-defined geometric structure in relation to the size of the bubbles and that the bubble is accompanied by precise geometric patterns throughout its routing to the outside of the cell. electrolysis.

Description of the invention

[9] To this end, the applicant has developed an anode where only the surface is modified for use in an electrochemical cell for the electrolytic production of fluorine gas in a bath of molten fluorinated salts, this anode comprising a carbon substrate , said substrate having a surface having at least one textured zone, this anode being characterized in that said textured zone is a micro-structured surface comprising a plurality of contiguous and identical micro-patterns, which are repeated periodically in at least one direction D1 of said textured zone, said direction being coplanar with said surface, said micropatterns comprising:

- either a hollow element of length U and a protruding element of length L _j , which are adjacent and substantially parallel to each other,

- or a plurality of alternations of hollow elements of length U and protruding element of length L _j adjacent and substantially parallel to each other, the width l _j of each pattern in the direction perpendicular to the direction (D1) being between 5 and 200 pm. [10] By textured zone is meant, within the meaning of the present invention, a part of the surface comprising an organized arrangement of a plurality of elements (ridges, grooves, ribs, grooves, blades or bumps, bosses), consisting by repeating the same basic element or pattern.

[11] In the context of the present invention, the micro-patterns are contiguous.

[12] By contiguous micro-patterns is meant, within the meaning of the present invention, micro-patterns adjacent to one another without a separation zone so as to define a continuous textured zone.

[13] By micropatterns substantially parallel to each other is meant, within the meaning of the present invention, micromotives repeated periodically in directions D1 and D'1 of the textured zone respectively according to translation vectors respectively. and d (as illustrated in FIG. 7) forming between them an angle of at most 5 °.

[14] The micro-structured surface of the thus textured zone of the anode according to the invention makes it possible to increase the dynamics of formation and detachment of fluorine gas bubbles at the surface of the anode. This results in better convection of the electrolyte at the surface of the anode, which increases the release of fluorine. In fact, the electrodes conventionally used for the production of fluorine are often found saturated at the surface, with a coating of gaseous fluorine which separates the electrode from the electrolyte. In addition, by using the anode according to the invention, the shape of the bubbles which are detached is modified by virtue of the textured zone: they are spherical and of small size, and therefore detach quickly. The applied current density can be increased without the electrode being saturated with gas. We thus manage to produce more than 20% more fluorine at constant current density, and we can achieve current densities 3 to 4 times higher than those of a comparable anode, around 90 A / dm ² with this electrode. .

[15] This particular regime in which the fluorine bubbles are detached more easily, results in a change in slope of the curve conventionally obtained by monitoring the voltage as a function of the current density. It therefore occurs with the microstructured anode, by comparison with the unmodified electrode with which the initial slope is maintained. [16] Advantageously, the width l _j of each pattern may be between 5 and 100 µm, and preferably of the order of 50 µm.

[17] According to an advantageous embodiment of the anode according to the invention, the micro-patterns can be repeated periodically in two non-parallel directions D1, D2 coplanar with said textured surface, so that these micro-patterns are shaped essentially parallelepipedic (cf. [Fig 6]).

[18] By non-parallel coplanar directions D1, D2 is understood to mean, within the meaning of the present invention, two directions forming between them an angle greater than 5 ° and less than 175 °.

[19] If the directions D1 and D2 are perpendicular (cf. figures 4 and 5), the micro patterns are essentially rectangular in shape. In this case, Lj and Li are respectively the lengths of a protruding element and of a hollow element of a pattern along the pattern repetition direction D1, and l _j being both the width of an element in protrusion of the pattern along D1 and the length of this same element along D2. We define the length of the hollow pattern in direction D2 by In this case, if the sum (I, + l _j ) is equal to the sum of the lengths (U + L _j ), the micro pattern will have the overall shape of a square in both directions D1 and D2 (as shown by [Fig 6]).

[20] The present invention also relates to the use of the anode according to the invention in an electrochemical cell for the electrolytic production of gaseous fluorine in a bath of molten fluorinated salts ionic conductor by fluoride ions (for example a bath of CsF-xFIF or of molten KF-2FIF Preferably, in the context of the present invention, a bath of molten KF-2FIF will be used.

[21] A further subject of the present invention is a method for structuring a carbon substrate intended to be used as an electrochemically active substrate of an electrochemical cell anode according to the invention, said method comprising the following steps:

- providing a non-graphitized carbon substrate having at least one surface (20) to be textured, this substrate advantageously being able to be cleaned and then dried before the subsequent etching step;

- Etching of said surface to be textured by irradiation using a light beam substantially perpendicular to said surface emitted by a pulsed laser, preferably capable of generating ultrashort pulses, said light beam having a relative displacement movement with respect to said surface (20) to be textured in at least one direction (D1) of said textured surface (21) coplanar with said textured surface (21) .

[22] The term ultrashort pulse is understood to mean, within the meaning of the present invention, pulses the duration of which is between 2 femtoseconds and 5 picoseconds. The choice of laser ablation was motivated by its various advantages when ultra-short pulses, of the order of a femtosecond, are used. When the laser pulse arrives on the surface of the material and the interaction time is long enough (nano or microsecond), the energy of the beam will heat the surface and liquefy the material around the impact. Resolidification and / or elevation of the surface temperature can adversely affect the future properties of the material ⁽¹³⁾ . The ablation will also be diffuse with a “melted” appearance.

[23] For an application in fluorine electrolysis, it is necessary to obtain surfaces with exceptional cleanliness and finish in order to be able to control and model the geometries. The femtosecond laser, with its very short pulses, allows during the laser / material interaction, to limit the propagation and heating of the substrate. The ablation is carried out by sublimation of the material irradiated by the laser beam; the term athermal ablation can be used.

[24] Regarding the actual etching step, two configurations are possible: either the surface to be textured of the anode is fixed and the laser beam moves, or the reverse is the case. that is, the laser beam is stationary and the surface to be textured moves relative to the laser beam. Preferably, the preferred configuration will be that in which the laser beam is spatially fixed and the sample is moved in the different dimensions of space (typically X-YZ orthogonal). In this case, the procedure is as follows: a laser emits continuously and during replacement movements, the laser beam is blocked. Typically to make a "dotted" line, the sample is moved at constant speed and the laser beam is intermittently blocked. The power (combination of the energy and the repetition rate of the pulsed laser) of the laser beam depends on the material to be ablated, on the focus of the beam, on the speed of movement: speed and power are therefore interdependent. For the same depth of ablation, we can move 2X faster with 2 times more power: the energy deposited by surface element remains the same.

[25] Advantageously, the etching step can be carried out in a moderating medium such as an aqueous medium, vacuum or a gas such as argon, to drive away from the substrate during texturing the residues formed during of ablation.

Brief description of the figures

[26] The following examples illustrate the invention, in conjunction with the figures commented on above, without however limiting its scope:

[Fig 1] shows the evolution of the current density as a function of the size of the bubble spread on the surface ^{[6l [7]} :

[Fig 2] schematically represents an electrochemical cell for the electrolytic production of fluorine gas in a bath of molten fluorinated salts, operating in galvanostatic mode by imposing a defined current density varying from 5 to 90 A / dm ² between fixed voltage terminals at 3 and 7 V vs EN H;

[Fig 3] shows the evolution of the anode voltages as a function of the current density applied for an industrial raw anode of the prior art;

[Fig 4] shows four SEM images of the surface of carbon pitch anodes textured by the laser ablation process according to the invention when it is carried out in aqueous medium, in air, in argon and in vacuum;

[Fig 5] is a schematic representation of the textured anodes of Figure 4, in which the micropatterns are square in shape and repeated in two directions D1 and D2 perpendicular to each other;

[Fig 6] is a schematic representation of an anode textured area, in which the micropatterns are rectangular in shape and repeated in two directions D1 and D2 not parallel, but not perpendicular to each other;

[Fig 7] is a schematic representation of an anode textured area, in which the micropatterns are rectangular in shape and periodically repeated in substantially parallel directions D1 and D'1 of the textured area forming an angle between them. at most 5 °;

[Fig 8]: Figure 8A shows photographs of anode in operation at a current density of 12 A / dm ² in the case of an untextured and unpolished raw anode (a), of an unpolished raw anode. textured but polished (b), with a textured anode according to the invention by laser ablation in air (c), and FIG. 8B shows photographs of anode in operation at a current density of 12 A / dm ² in the case of a textured anode according to the invention by laser ablation under argon (d), a textured anode according to the invention by vacuum laser ablation (e), a textured anode according to the invention by laser ablation under water (f);

[Fig 9]: Figure 9A shows the evolution of the voltage as a function of the current density with a textured anode according to the invention; and FIG. 9B also shows the change in voltage as a function of current density with a textured anode according to the invention.

[27] Figures 1, 2, 5 to 7 are described in more detail in the prior art above, while Figures 3, 4, 8 and 9 are described in more detail in the following examples, which illustrate the invention without limiting its scope.

[28] Figure 2 schematically shows an example of an electrochemical cell for the electrolytic production of fluorine gas in a bath of molten fluorinated salts, which comprises:

- an anode according to the invention in carbon pitch, partially covered with Teflon to leave a textured active surface of about 1 cm ² , where the reaction takes place

, the anode being connected to a first brass current lead,

- a copper cathode made from a sheet 1 mm thick and a active surface of 2 cm ² , where the reaction takes place, the cathode being connected to a second brass current lead;

- a copper reference electrode,

- an F2 difluor collector,

.a collector of dihydrogen H2 to the outside atmosphere _,

.an electrolytic bath of hydrofluorinated potassium fluoride KF-2HF, in which the electrodes are immersed,

- a heating coil to maintain the temperature of a bath at 95 ° C, the coil being placed at the bottom of the cell.

[29] During electrolysis, a potential difference is applied between the anode and the cathode, leading to the production of gas. The difluorium is released at the anode and the hydrogen is produced at the cathode according to the following equations:

- at the anode:

- at the cathode:

- either the global reaction

[30] In this medium, the production of fluorine takes place at a minimum potential of 2.9 V according to the balance reaction presented above, corresponding to the decomposition voltage of the HF molecule. In reality, in a stabilized state, fluorine is released more towards 5.5 V, mainly because, on the one hand, of the overvoltage linked to the surface of the anode which becomes fluorinated and, on the other hand, to the ohmic drop which is very important in the electrolyte. The latter is generated by the low conductivity of the bath (electrolyte) while the anode overvoltage is caused by the presence of insulating fluorocarbon compounds formed at the anode / electrolyte interface. The energy efficiency is then lowered to only 30%.

[31] The very exothermic recombination of these two gases can cause localized or larger explosions, especially if the quantity of gas is large. In addition, the gas F2 consumed for this reaction is lost for the rest of the process, leading to a gross loss of yield (the cell is designed to resist recombinations). To overcome these problems, skirts are installed above the electrodes (Figure 2). The low point of the skirt (submerged part) is placed so as to ensure a good rate of gas recovery without hindering the current lines. The second important point lies in respecting the inter-electrode distance.

EXAMPLES

[32] Equipment for carrying out the engraving:

A femtosecond pulsed laser is used for the laser ablation, coupled with an optical device comprising the use of a focusing objective making it possible to obtain beam sizes of the order of a few micrometers. The laser and the optical device give the possibility of modifying the repetition frequency of the laser (time between two laser shots) and power, parameters which will greatly modify the ablation dynamics, bringing a significant degree of freedom to manipulations.

[33] Since the oxygen present at the surface constitutes a limiting point for an efficient production of fluorine, a cell was carried out to control the atmosphere of ablation under gas and under water.

[34] Samples:

- raw anodes in carbon pitch,

- raw anode in carbon pitch, untextured and polished,

- carbon pitch anode textured by the process according to the invention, in which the laser ablation is carried out in an aqueous medium, using a pulsed femtosecond laser according to the etching conditions of Example 2,

- carbon pitch anode textured by the process according to the invention, in which the laser ablation is carried out in a controlled atmosphere in air, using a pulsed femtosecond laser according to the etching conditions of Example 2,

- carbon pitch anode textured by the method according to the invention, in which the laser ablation is carried out under vacuum, using a pulsed femtosecond laser according to the etching conditions of Example 2,

- carbon pitch anode textured by the process according to the invention, in which the laser ablation is carried out under argon, using a pulsed femtosecond laser according to the etching conditions of Example 2.

[35] Prior to the laser ablation etching step, the raw pitch anodes are polished according to the following polishing protocol:

- polishing with a SiC 600 disc for a few minutes so as to eliminate any irregularities on the surface. The purpose of this first polishing being to obtain a flatness of the sample allowing to have a relatively smooth surface,

- polishing with a SiC 1200 disc for 3 minutes,

- polishing with a SiC 2400 disc for 9 minutes.

[36] The three polishing phases are carried out under water with new discs.

[37] Different ablation atmospheres (stated above) were implemented according to a pulsed laser ablation process allowing the interaction of ultrashort pulses of the order of a hundred femtoseconds with the surface of the sample (ablation of material by formation of a plasma) and moving the spot laser (focused to a diameter of the order of 2 μm) along rectilinear and parallel paths spaced 50 micrometers apart. Depending on the level of laser energy applied, different passes on the same paths are required at depth in order to generate a more or less deep microstructuring.

[38] EXAMPLE 1 (COMPARATIVE): non-textured anode

[39] For the electrolytic production of fluorine gas, a crude carbon pitch anode is used in the electrochemical cell in a molten salt medium of hydrofluorinated potassium fluoride schematically illustrated in FIG. 2. The generation of fluorine takes place at the carbon pitch anode according to the electrochemical equation:

[40] The more the current density applied to the electrochemical device is increased, the more the voltage of the device deviates from the theoretical potential of 3.05 V, as shown in Figure 3: this deviation is called anode overvoltage. It’s wasted energy for the electrochemical system. Note the quasi-linearity of the voltage / current density curve.

[41] In an industrial environment, a current density greater than 12A / dm ² is generally not used because every other anode no longer produces F2 above this value.

[42] Figure 3 also shows that in the case of an anode still producing F2, the anode voltage is very high, of the order of 6.5 V when the current density is becomes equal to 12A / dm ^2. .

[43] EXAMPLE 2 (ACCORDING TO THE INVENTION): nanostructuring of the anode and influence of the ablation atmosphere

[44] The following are used for the electrolytic production of fluorine gas:

- the four carbon pitch textured anodes according to the invention,

- in the electrochemical cell in a molten salt medium of hydrofluorinated potassium fluoride illustrated in FIG. 2,

- under the same conditions as in Example 1. [45] FIG. 4 shows four SEM images of the surface of carbon pitch anodes textured by the laser ablation process according to the invention when it is carried out in an aqueous medium, in air, under argon and under vacuum. The most interesting and regular structure is the one proceeding under water because, on the one hand, of the absence of carbon particles resulting from the ablation which can obstruct the microstructure and, on the other hand, of the presence of nano- corrugations formed instead of the active surface.

[46] In operation, when the laser ablation has been performed in air, it is observed that many disorganized carbon particles are produced and will be privileged sites of fluorination, requiring treatment to remove them before fluorination (cf. figure 8c).

[47] When the laser ablation is carried out under Argon (cf. FIG. 7d), in the absence of oxygen which could be grafted on the surface, many particles are produced along the path of the laser. Even if this carbon is not oxidized, the significant presence of disorganized particles leads to a significant fluorination of these particles, and therefore to a coating of the surface by fluorine (cf. FIG. 8d) or even to the production of CF ₄ gas. which will pollute the F ₂ produced.

[48] When the laser ablation is performed under vacuum (see figure 8e in operation), the samples have a satisfactory surface appearance with few or no particles and very well-defined shapes of the microstructure (see figure 4). . The electrochemical performances are similar to the polished raw anode sample serving as a non-rough reference (unlike the raw anode with uncontrolled irregular roughness: see figure 8b) although, visually (see figure 8e), the dynamics of fluorine release at the surface is very different.

[49] The bubbles generated on an anode etched under water (cf. figure 8f) are small, spherical and of homogeneous size, whereas on a non-textured and simply polished anode (cf. figure 8b), they appear larger and not- spherical.

[50] EXAMPLE 3 (ACCORDING TO THE INVENTION) [51] We compare the evolution of the voltage of the device as a function of the current density applied, for three types of anodes: a polished raw anode, an unpolished raw anode and a textured anode under water according to the invention.

[52] Figures 9A and 9B show that at low current density, the quantity of gas produced is already greater for the microstructured electrodes. It is only from 15 A / dm ² that the production of fluorine will increase significantly for the electrodes which have undergone a laser treatment. This change in regime is specific to microstructured electrodes as illustrated by the comparison with polished (Figure 8b) and raw (Figure 3) electrodes.

[53] At 32 A / dm ² , a clear classification is established: the sample which produces the least is the one not textured but previously polished, this is explained by the low roughness due to the polishing of the surface, thus reducing the number of germination points. The raw pitch sample is in an intermediate position. Its roughness, uncontrolled, makes it possible to have more germination points than the polished sample, but the spreading of bubbles on the surface reduces the active surface and therefore the production of fluorine. In the case of an anode according to the invention, the production of fluorine is the best. Overall, the structures made on the surface have two effects. The first is the increase and distribution of the number of germination points. The second effect is the modification of the dynamics of the bubbles at the surface leading to the formation of smaller bubbles of fluorine. The latter, by their ability to quickly unhook from the surface, will create convection loops close to the surface. The stirring of the bath in place will allow the other bubbles to better unhook. A virtuous circle is then formed which is all the more effective when the electrode is in a vertical position. For microstructured anodes, this results in a more moderate increase in the anode voltage with the current density. Thus, the microstructuring significantly impacts the production of fluorine gas. LIST OF REFERENCES

[54] 1. G. Cady, “Freezing Points and Vapor Pressures of the System Potassium

Fluoride-Hydrogen Fluoridel ”, J. Am. Chem. Soc., Vol. 56, no 7, p. 1431-1434, 1934. 2. H. Groult, “Electrochemistry of fluorine production”, J. Fluor. Chem., Vol. 119, no

2, p. 173-189, 2003.

3. H. Groult, D. Devilliers, M. Vogler, C. Flinnen, P. Marcus, and F. Nicolas,

“Electrochemical behavior and surface analysis of crude and modified carbon electrodes for fluorine production. », Electrochimica Acta, vol. 38, p. 2413-21, 1993. 4. H. Groult et al., “Role of elemental fluorine in nuclearfield”, J. Fluor. Chem., Vol. 128, no.4, p. 285-295, 2007.

5. B. E. Conway and S. Y. Qian, “Unusual kinetic behavior in cathodic H2 evolution from KF.2HF melts at mild-steel and alloy electrodes. », Hem. Ind., Vol. 58, p. 259-270, 2004. 6. H. Vogt, “Gas-evolving electrodes. », In Compr. Treatise Electrochem., 1983, vol.

6. p. 445-89.

7. J. Eigeldinger and H. Vogt, “The bubble coverage of gas-evolving electrodes in a flowing electrolyte. », Electrochimica Acta, vol. 45, p. 4449-4456, 2000.

8. A. T. Khun, Industrial electrochemical processes. Elsevier Publishing Company, 1971.

9. T. Rauscher et al., “Femtosecond-laser structuring of Ni electrodes for highly active hydrogen evolution”, Electrochimica Acta, vol. 247, p. 1130-1139, 2017.

10. R. N. Wenzel, “Surface roughness and contact angle. », J. Phys. Chem., Vol.

53, no 9, p. 1466-1467, 1949. 11. A. Cassie and S. Baxter, “Wettability of porous surfaces”, Trans. Faraday Soc., Vol. 40, p. 546-551, 1944.

12. S. G. Bankoff, “Entrapment of gas in the spreading of a liquid over a rough surface. », AIChE J., vol. 4, p. 24-6, 1958.

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Claims

[Claim 1] Anode (1) for use in an electrochemical cell for the electrolytic production of fluorine gas in a bath of molten fluorinated salts, said anode (1) comprising a carbon substrate (2), said substrate (2) having a surface (21) having at least one textured zone (210), characterized in that said textured zone (210) is a microstructured surface comprising a plurality of contiguous and identical micro-patterns (22), which are repeated periodically in at least one direction (D1) of said textured zone (210), said direction (D1) being coplanar with said surface (21), said micropatterns comprising: either a hollow element (220) of length U and a protruding element (221) of length L _j , which are adjacent and substantially parallel to each other, i.e. a plurality of alternations of hollow elements (220) of length U and protruding element of length L _j adjacent and substantially parallel to each other , the width r I, of each unit in said direction (D1) being between 5 and 200 µm.

[Claim 2] An anode according to claim 1, wherein the width, I, of each pattern in said direction (D1) is between 5 and 100 µm.

[Claim 3] The anode of claim 2, wherein the width of each pattern in said direction (D1) is of the order of 50 µm.

[Claim 4] An anode according to any one of claims 1 to 3, wherein the micro-patterns (22) are periodically repeated in two non-parallel directions (D1, D2) coplanar of said textured surface (21), so that said micro-patterns (22) are of parallelepipedal shape.

[Claim 5] An anode according to any one of claims 1 to 3, wherein the directions (D1) and (D2) are perpendicular, so that said micro patterns (22) are rectangular in shape.

[Claim 6] Use of the anode (1) as defined according to any one of claims 1 to 5 in an electrochemical cell for the electrolytic production of gaseous fluorine in a bath of molten fluorinated salts ionic conductor by fluoride ions, preferably in a bath of molten KF-2HF.

[Claim 7] A method of structuring a carbon substrate (2) intended to be used as an electrochemically active substrate (2) of an anode (1) of an electrochemical cell as defined according to any one of claims 1 to 5, said method comprising the steps of: providing a non-graphitized carbon substrate having at least one surface (20) to be textured; etching of said surface (20) to be textured by irradiation using a beam of light perpendicular to said surface (20) emitted by a pulsed laser, preferably capable of generating ultrashort pulses, said beam of light having movement of relative displacement with respect to said surface (20) to be textured in at least one direction (D1) of said textured surface (21) coplanar with said textured surface (21). [Claim 8] The method of claim 7, wherein the etching step is carried out in a moderating medium selected from aqueous medium, vacuum or a gas such as argon.