FLEXIBLE ELECTRICAL CONDUCTOR WITH ELEMENTS CONNECTED TO EACH OTHER BY TIG WELDING, MANUFACTURING METHOD FOR SUCH FLEXIBLE ELECTRICAL CONDUCTOR, USE OF AT LEAST ONE SUCH ELECTRICAL CONDUCTOR, AND ELECTROCHEMICAL SYSTEM COMPRISING SUCH FLEXIBLE ELECTRICAL CONDUCTOR

Description

TECHNICAL FIELD

[0001] The present invention relates to the general field of high temperature electrolysis (HTE), in particular high temperature steam electrolysis (HTE), respectively designated by the English terms "High Temperature Electrolysis" (HTE) and "High Temperature Steam Electrolysis" (HTSE), of carbon dioxide (CO2) electrolysis, or even of the co-electrolysis of water vapor and carbon dioxide (CO2) at high temperature.

[0002] More specifically, the invention relates to the field of high-temperature electrochemical devices, such as high-temperature solid oxide electrolyzers, usually designated by the acronym SOEC (for "Solid Oxide Electrolysis Cell" in English), and high-temperature solid oxide fuel cells, usually designated by the acronym SOFC (for "Solid Oxide Fuel Cells" in English), but also high-temperature steam co-electrolyzers with carbon dioxide, reversible fuel cell and high-temperature electrolyzer systems, or even so-called medium-temperature fuel cells or electrolyzers, on the order of 400°C, also called PCFCs for "Proton Ceramic Fuel Cell" in English.

[0003] More generally, the invention relates to the field of SOEC / SOFC type solid oxide cell stacks operating at high temperatures. These stacks can operate at atmospheric pressure or under negative pressure.

[0004] Beyond such stacks of SOEC / SOFC type solid oxide cells, the invention is relevant to any system where there is a need for electrical conduction in an oxidizing environment at high temperature or under conditions leading to the rapid degradation of electrically conductive materials.

[0005] More particularly, the invention relates to the supply of electric current to a stack of electrochemical cells in the hot zone, and more particularly, a flexible electrical conductor, a method of manufacturing such an electrical conductor, a use of at least one such electrical conductor, and an electrochemical system comprising such a flexible electrical conductor (see for example JP H01 283772 A, describing the preamble of claims 1 and 15). PREVIOUS STATE OF THE ART

[0006] In a high-temperature solid oxide electrolyzer (SOEC), the process involves converting water vapor (H₂O) into hydrogen (H₂), or other fuels such as methane (CH₄), natural gas, biogas, and oxygen (O₂) using an electric current within the same electrochemical device. It also involves converting carbon dioxide (CO₂) into carbon monoxide (CO) and oxygen (O₂). In a high-temperature solid oxide fuel cell (SOFC), the operation is reversed, producing both electricity and heat when supplied with hydrogen (H₂) and oxygen (O₂), typically from air and natural gas, specifically methane (CH₄). For the sake of simplicity, the following description focuses on the operation of a high-temperature solid oxide electrolyzer of the SOEC type performing the electrolysis of water vapor.However, this principle is applicable to the electrolysis of carbon dioxide (CO2), and even to the co-electrolysis of high-temperature steam (HTS) with carbon dioxide (CO2). Furthermore, this principle can be transposed to the case of a high-temperature solid oxide fuel cell (SOFC).

[0007] As is well known, a high-temperature water vapor (H₂O) electrolyzer, or EVHT electrolyzer, comprises a stack of several elementary solid oxide electrochemical cells. Referring to the figure 1, a solid oxide cell 10, or "SOC" (Anglo-Saxon acronym "Solid Oxide Cell") includes in particular: a) a first porous conductive electrode 12, or "cathode", intended to be supplied with steam for the production of dihydrogen; b) a second porous conductive electrode 14, or "anode", through which dioxygen (O2) produced by the electrolysis of water injected onto the cathode escapes; and c) a solid oxide membrane (dense electrolyte) 16 sandwiched between the cathode 12 and the anode 14, the membrane 16 being anionically conductive for high temperatures, usually temperatures above 600°C.

[0008] By heating cell 10 to at least this temperature and injecting an electric current I at anode 14, there is then a reduction of water on cathode 12, which generates dihydrogen (H 2 ) at cathode 12 and dioxygen (O 2 ) at anode 14.

[0009] A stack of 20 such cells, designed to produce a significant amount of hydrogen, is illustrated by the schematic view of the figure 2 In particular, the cells 10 are stacked one on top of the other, separated by interconnecting plates 18 or interconnectors. These plates serve both to ensure electrical continuity between the different electrodes of the cells 10, thus allowing them to be connected in series, and to distribute the various gases necessary for the operation of the cells, as well as, where applicable, a carrier gas to help with the removal of the electrolysis products and / or the thermal management of the stack.

[0010] To achieve this, the plates 18 are connected to a steam supply 22 for injecting this steam onto the cathodes of the cells 10 at a constant steam flow rate ΔH₂O controlled by a pilot-operated valve 24. The plates 18 are also connected to a gas collector 26 for collecting the gases produced by the electrolysis. An example of the stacking and interconnecting plate structure is described, for example, in International Patent WO 2011 / 110676 A1.

[0011] For the effective implementation of electrolysis by the stack 20, the stack is heated to a temperature above 600°C, usually between 650°C and 900°C, the gas supply is switched on at a constant flow rate, and an electrical power supply 28 is connected between two terminals 30, 32 of the stack 20 in order to pass a current through it. I .

[0012] The intensity IThe electric current is usually on the order of a few hundred amperes, which generates significant heat losses due to the Joule effect in the electrical conductors. To optimize the energy efficiency of solid oxide electrochemical systems, it is necessary to limit these heat losses by developing, in particular, specific electrical conductors, also known as "current supply rods" (or "bus bars").

[0013] A current-carrying rod in the stack is generally in the form of a metal rod. Taking the example of a cylindrical rod, the electrical resistance R is expressed by the following formula: R = ρ ⋅ l S where ρ is the resistivity of the rod (in Ω.m), l is the length of the rod (in m) and S is the cross-section of the rod (in m²).

[0014] Since Joule heating losses are proportional to the resistance R, to limit this effect, it is therefore necessary to reduce the electrical resistance of the current-supplying rod. Possible optimizations therefore consist of: limit the length of the rod, increase its cross-section, find a material with lower resistivity and stable at high temperature.

[0015] The first two options are geometric choices that generally depend on the shape of the electrochemical system. Therefore, there are constraints affecting them, and / or state-of-the-art rods are already optimized for the electrochemical system. The final point concerns the rod's constituent material, which must have minimal resistivity to reduce ohmic losses.

[0016] The optimization of this last point has not been sufficiently considered. Indeed, for all laboratory developments of the technology, energy efficiency is not paramount. However, as explained below, a current-carrying rod is immersed in a highly corrosive environment, so the standard solution implemented is to use solid stainless steel alloy rods, which therefore constitute the reference solution in all international publications. While the resistivity at room temperature (20°C) of these rods is already high, on the order of 75 x 10⁻⁸ Ω·m, it should be noted that this resistivity increases sharply with temperature.

[0017] Thus, at 900°C, which is a high operating temperature for a solid oxide electrolyzer, the electrical resistance of a stainless steel rod is 117 x 10⁻⁸ Ω·m, resulting in a very significant ohmic loss. These aspects were notably described in French patent application FR 3 036 840 A1.

[0018] However, if the goal is to optimize electrical resistivity, copper is generally the recommended material for electrical conductors subjected to high current. An experimental study conducted by the Applicant determined the resistivity curve of copper as a function of temperature and confirmed that choosing copper reduces ohmic losses by at least a factor of 10 compared to the reference material across the entire operating temperature range of solid oxide systems.

[0019] However, one of the major constraints that needs to be taken into consideration is the corrosion problem related to the stacking environment.

[0020] By referring to the figure 3 The stack of 20 is indeed enclosed in a so-called "thermal" chamber, the temperature of which is maintained between 650 and 900°C under air purging, a classic electrochemical system thus comprising: the EVHT 20 electrolyzer, for example the one described in relation to the figures 1 and 2and comprising a set of conduits 52, 54, 56, 58 for supplying and collecting gases from the anodes and cathodes of the electrochemical cells of the electrolyzer; an enclosure 60 in which the electrolyzer 20 is housed, the conduits 52, 54, 56, 58 passing through a wall of the enclosure 60 for their connection to gas supply and collection circuits (not shown). The enclosure 60 also includes an air inlet conduit 62 and an air outlet conduit 64, the enclosure 60 being, for example, everywhere else airtight to gases and liquids. The conduit 62 is suitable for being connected to an air supply circuit (not shown) so as to apply air sweeping to the hot zone surrounding the electrolyzer 20, the sweeping air being discharged through the outlet conduit 64; and two electrical conductors 66, 68 connected to terminals 30, 32 of stack 20 and passing through enclosure 60 for their connection to the current source 28.

[0021] Under these conditions, two copper rod conductors 66 and 68, at least part of which are contained within enclosure 60, will oxidize very rapidly. Furthermore, copper does not resist oxidation at high temperatures because the oxide formed on the surface is not sufficiently impermeable and adherent to protect the underlying metal. Materials known to resist oxidation at high temperatures are chromium- and alumina-forming alloys such as stainless steels and nickel-stainless alloys, as these form chromium and / or alumina, which are much more protective oxides. However, as mentioned above, these alloys have such high electrical resistivity that their use results in significant energy losses.

[0022] A high-temperature solid oxide fuel cell (SOFC) experiences similar problems. Indeed, an EVHT electrolyzer and an SOFC are identical structures, differing only in their operating mode. The electrolyzer operates in carbon dioxide (CO2) reduction mode or in co-electrolysis mode, meaning with a gas mixture at the cathodic inlet composed of water vapor (H2O) and carbon dioxide (CO2). The mixture at the cathodic outlet then consists of hydrogen (H2), water vapor (H2O), carbon monoxide (CO), and carbon dioxide (CO2). Referring to the figure 4An electrochemical cell constituting a SOFC fuel cell comprises the same elements (anode 12, cathode 14, electrolyte 16) as an electrolyzer cell. However, the fuel cell is supplied, at constant flow rates, with dihydrogen at its anode and dioxygen at its cathode, and connected to a load C to deliver the produced electric current. Given the electric current produced, which is several amperes, the fuel cell therefore experiences the same problems as the electrolyzer.

[0023] One solution would be to protect a copper rod (or any other metal deemed suitable in terms of electrical resistivity) with a coating to provide good oxidation resistance, for example, a chromium or alumina coating. This presents several problems. First, the coating's seal and adhesion to the copper substrate during heating must be guaranteed. It should be emphasized that copper has a high coefficient of thermal expansion, so significant differential thermal expansion stresses can occur and damage the coating and / or the coating / copper interface. Furthermore, at the hot end of the rod, an electrical connection to the stack must be made without exposing the copper. The connection must therefore be made on the coating without damaging it, which is technically challenging.

[0024] Another solution is to encase the copper rod in a sheath made of oxidation-resistant material. This solves the problem of resistance to differential thermal expansion stresses since the two materials are not bonded. Such an assembly (copper + stainless steel sheath) is already known from the prior art for other applications (e.g., a strong acid environment at low temperature, 50-80°C), notably from the Chinese document CN 202608143 U, which describes a copper bar simply inserted into a steel tube. This type of conductor performs satisfactorily at low temperatures, but it has been observed that it is unsuitable as is for solid oxide systems. Indeed, the limited contact between the conductor and the sheath results, at high temperatures, in the deterioration of the electrical contact between the two materials and an increase in ohmic losses.In other words, there is no optimized electrical conduction system in the state of the art suitable for a high electric current and able to withstand significant thermal cycling in an oxidizing environment.

[0025] French patent application FR 3 036 840 A1 discloses an electrical conductor suitable for currents of several hundred amperes, resistant to oxidation at high temperature and able to withstand thermal cycling up to 900°C. This electrical conductor comprises a rod made of a first metallic material and a sheath, completely covering the rod, made of a second metallic material, the two being welded together using hot isostatic compression (HIC).

[0026] More specifically, this application proposes to shape a rod composed of a copper core protected by an Inconel® 600 steel tube sheath, with a "whistle" made of Inconel® 600 steel which serves as the connection terminal, and a closing tip also made of Inconel® 600 steel through which a vacuum is drawn. These parts are assembled by TIG (Tungsten Inert Gas) arc welding. The resulting rod then undergoes a hot isostatic pressing (HIP) process, which allows for diffusion welding of the different materials without the addition of filler metal.

[0027] However, this solution has several drawbacks, including the use of hot isostatic compression (HIC), which is an expensive process and can only be carried out by specific companies, given a temperature and high pressure cycle of around 900°C and 1000 bar, with a cycle time of a few hours.

[0028] In addition, the current supply rod, or "busbar", consists of a single high-temperature connection range, which does not allow for internal connections within the high-temperature zone. DESCRIPTION OF THE INVENTION

[0029] The invention aims to remedy at least partially the needs mentioned above and the drawbacks related to prior art achievements.

[0030] The invention thus relates, according to one of its aspects, to a flexible electrical conductor as defined in claim 1, comprising: an assembly comprising: a flexible conductive core made of a first metallic material, a sheath covering the conductive core and made of a second metallic material, in particular stainless or refractory, with an electrical resistivity higher than the electrical resistivity of the first metallic material, a first connecting tab made at least in part of the second metallic material and connected to a first end of the assembly, where, at the first end of the assembly, the sheath and the first connecting tab are welded by means of TIG welding (acronym for "Tungsten Inert Gas" in English), in particular over the entire circumference, and the conductive core and the first connecting tab are welded by brazing or brazing, in particular high-temperature brazing or brazing.

[0031] The sheath and the first connecting tab can be welded using TIG welding, preferably with a filler material made of the second metallic material. However, the filler material could also be stainless or refractory metal, and / or metallic or refractory alloys, particularly stainless or refractory steel. The filler material is advantageously resistant to high-temperature oxidation and compatible with the materials used for the sheath and the connecting tab.

[0032] A "flexible" electrical conductor is defined as a conductor used for connection to a stack, designed to prevent the transmission of vibrations, expansion, and other unwanted movements between the stack and its environment (e.g., the oven floor, chassis, gas lines, etc.). This allows for a potential electrical connection between stacks during assembly without mechanical transitions, unlike a "rigid" electrical conductor, which acts mechanically within a main connection and is not directly connected to a stack. The flexibility of a flexible electrical conductor facilitates wiring, primarily at the stack level. It also allows it to adapt to different shapes. For a flexible electrical conductor, the forming torque is less than 2 Nm, whereas for a rigid electrical conductor, it is greater than 10 Nm.

[0033] The electrical conductor according to the invention may further comprise one or more of the following characteristics taken individually or in any possible technical combinations.

[0034] Advantageously, the electrical conductor may include a second connecting tab made at least partially of the second metallic material and connected to a second end of the assembly. At this second end, the sheath and the second connecting tab may be welded using TIG welding, particularly with filler material composed of the second metallic material, and the conductor core and the second connecting tab may be welded by brazing or soldering. The TIG welds at both ends of the assembly and the sheath may completely cover the conductor core along its entire length.

[0035] Furthermore, at least one gap may be present between the external surface of the conductive core and the internal surface of the sheath over at least part of the length of the conductive core.

[0036] The conductive core, the primary metallic material, can be made of copper, nickel, or silver and / or alloys of copper, nickel, or silver, or any other metal or alloy with good electrical conductivity. In particular, any other metal or alloy with good electrical conductivity that is susceptible to oxidation at high temperatures, on the order of 900°C, such as brass or bronze.

[0037] In addition, the sheath, the second metallic material, can be made of stainless or refractory metal and / or metallic or refractory alloys, in particular stainless or refractory steel, for example based on nickel, chromium or cobalt, in particular Inconel ®< , for example Inconel ®< 600 or 625, or any other metal or alloy resistant to oxidation at high temperature, for example 316L stainless steel.

[0038] The first connecting leg and / or the second connecting leg can be made entirely of the second metallic material.

[0039] Alternatively, in order to limit possible electrical losses, the first connecting leg and / or the second connecting leg may each comprise a connecting conductive core made of the first metallic material, and a connecting sheath covering the connecting core entirely along its entire length and made of the second metallic material.

[0040] The connecting sheath may have a thickness of approximately 0.5 mm.

[0041] According to a particular embodiment aimed in particular at obtaining a flexible and electrically insulated power cable, the assembly comprising the conductive core and the sheath can be flexible, in particular the conductive core and the sheath being made of a flexible material and the sheath being entirely covered by an electrical insulating sheath, or electrical insulating protection, in particular a braided ceramic sheath.

[0042] Furthermore, the invention also relates, according to another aspect, to a method for manufacturing an electrical conductor as defined above (see claim 8), the method comprising the following steps: cleaning of surfaces, in particular by means of a detergent and / or a solvent, in particular of surfaces intended to be welded, namely electrical conduction surfaces and surfaces required for sealing the electrical conductor, insertion of the conductive core into the sheath, welding by brazing or brazing between the conductive core and the first connecting tab, welding by TIG welding between the sheath and the first connecting tab, possibly, vacuuming of the sheath by pumping.

[0043] The electrical conductor may include a second connecting tab made at least partly of the second metallic material and connected to a second end of the assembly, and the process may include, after the TIG welding step between the sheath and the first connecting tab, the following steps: brazing or brazing between the conductive core and the second connecting lug, TIG welding between the sheath and the second connecting lug.

[0044] Manufacturing can be carried out under ambient atmosphere (air) or under a neutral atmosphere, for example of the argon type.

[0045] The first and / or second connecting leg can be formed by assembling a connecting conductor core and a connecting sheath that completely covers the core. The connecting core can be manufactured by die forging. However, other processes besides die forging could be used, such as machining or forging. The connecting sheath can be manufactured by stamping or by assembling several pieces made of the second metallic material.

[0046] Furthermore, the assembly of the first connecting leg and / or the second connecting leg may involve at least the following steps: cleaning of the component parts of the connecting leg, in particular with the aid of a detergent or solvent, insertion of the connecting core into the connecting sheath, vacuuming of the connecting leg, application of a Hot Isostatic Compression (HIC) diffusion welding cycle.

[0047] The Hot Isostatic Compression (HIC) diffusion welding cycle can be performed under the following operating conditions: bring the assembly consisting of the connecting core and the connecting sheath to a temperature between 600°C and 1060°C, preferably between 800°C and 1000°C, in particular a temperature of 920°C, apply to the connecting sheath a pressure between 500 bar and 1500 bar, preferably between 800 bar and 1200 bar, in particular a pressure of 1020 bar, apply a pressure and temperature plateau for a period of 30 minutes to several hours, preferably 1 hour to 3 hours, in particular 2 hours, allow the assembly to cool and depressurize.

[0048] In addition, to ensure good electrical conductivity, the conductive core and the connecting core of the first connecting leg and / or the second connecting leg can be connected together by a brazing or high-temperature brazing process.

[0049] Furthermore, according to another aspect of the invention, the use of at least one electrical conductor as defined above, as the electrical conductor of an electrochemical system (see claim 14) comprising: an enclosure for the circulation of air in the volume delimited by it, an electrochemical device housed in the enclosure, comprising: a stack, with solid oxides of the SOEC / SOFC type operating at high temperature, of elementary electrochemical cells each comprising an electrolyte intercalated between a cathode and an anode and connected in series between two electrical terminals, and said at least one electrical conductor connected to at least one of the two electrical terminals.

[0050] Furthermore, the invention also relates, according to another aspect, to an electrochemical system (see claim 15) comprising: an enclosure for air circulation within the volume delimited by it, an electrochemical device housed within the enclosure, comprising: a stack, with solid oxides of the SOEC / SOFC type operating at high temperature, of elementary electrochemical cells each comprising an electrolyte intercalated between a cathode and an anode and connected in series between two electrical terminals, and at least one electrical conductor as defined above, connected to at least one of the two electrical terminals. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The invention will be better understood upon reading the detailed description that follows, the non-limiting examples of its implementation, and upon examination of the schematic and partial figures in the attached drawing, on which: [ Fig. 1 ] is a schematic view of an elementary electrochemical cell of an EVHT electrolyzer, [ Fig. 2] is a schematic view of a cell stacking according to [ Fig. 1 ], [ Fig. 3 ] is a schematic view of a system incorporating a stacking according to [ Fig. 2 ], [ Fig. 4 ] is a schematic view of an electrochemical cell in a SOFC fuel cell, [ Fig. 5 ] is a schematic view of a flexible electrical conductor according to the invention, [ Fig. 6 ] is a schematic cross-sectional view along plane VI-VI of [ Fig. 5 ], [ Fig. 7 ] is a schematic view of another flexible electrical conductor according to the invention, [ Fig. 8 ] is a schematic cross-sectional view along plane VIII-VIII of [ Fig. 7 ], [ Fig. 9 ] is an enlarged view along A of [ Fig. 8 ], [ Fig. 10 ] is a perspective and exploded view of a connecting tab of the electrical conductor of [ Fig. 7 ], [ Fig. 11 ] is an assembled perspective view of a connecting tab of the electrical conductor of [ Fig. 7 ].

[0052] Throughout these figures, identical references may designate identical or analogous elements.

[0053] Furthermore, the different parts represented in the figures are not necessarily shown on a uniform scale, in order to make the figures more legible. DETAILED DESCRIPTION OF THE IMPLEMENTATION METHODS

[0054] THE figures 1 to 4 have already been described previously in the section relating to the prior art and the technical context of the invention.

[0055] With reference to figures 5 to 11 Examples of flexible electrical conductors, such as flexible power cables, are shown. Such a flexible connection facilitates wiring and absorbs expansion and vibration, among other things.

[0056] With reference to figures 5 and 6An example of a flexible electrical conductor 70 according to the invention is described. It comprises an assembly 72 consisting of a conductive core 74 made of a first metallic material, here copper, inserted in a sheath 79 made of a second metallic material, here stainless alloy, with an electrical resistivity greater than that of the first metallic material.

[0057] It should be noted that the conductive core 74 is here made of copper, but the invention applies to other metals that are good electrical conductors but sensitive to oxidation, for example nickel, silver, brass, bronze and / or copper alloys, such as those hardened by dispersoids.

[0058] In addition, the electrical conductor 70 has a first connecting tab 78 made of the second metallic material and connected to a first end 72a of the assembly 72, and a second connecting tab 78 made of the second metallic material and connected to a second end 72b of the assembly 72.

[0059] The connecting tabs 78, or "whistles," here made of Inconel®-type stainless alloy, hermetically seal the ends 72a and 72b of the assembly 72, thus preventing gas leakage. They provide the electrical connection terminals. They have a shape complementary to the electrolyzer plate to which the tabs 78 are fixed for the electrolyzer's electrical connection.

[0060] The shape or geometry of the connecting lugs 78 can be the usual shape of a lug, as shown here, or any other different shape, for example cylindrical and intended to fit into a bore or clamped between two half-shells attached to the device to be powered.

[0061] At the first end 72a of the assembly 72, the sheath 79 and the first connecting tab 78 are welded by means of a TIG weld over the entire circumference, represented by P on the figure 6 , for example, an orbital weld, with a filler material preferably composed of the second metallic material. The conductive core 74 and the first connecting tab 78 are welded by brazing or brazing, represented by B on the figure 6 There is no welding between the conductive core 74 and the sheath 79.

[0062] Similarly, at the second end 72b of the assembly 72, the sheath 79 and the second connecting tab 78 are welded by TIG welding (reference P) with a filler material preferably composed of the second metallic material. The conductive core 74 and the second connecting tab 78 are welded by brazing or brazing (reference B). The TIG welds at both ends 72a and 72b of the assembly 72 and the sheath 79 completely cover the conductive core 74 along its entire length L, as shown in the figure 6 There is no welding between the conductive core 74 and the sheath 79. TIG welding protects the conductive core 74 from oxidation. The weld is performed in such a way as to seal the connections between the whistles 78 and the sheath 79.

[0063] As schematically represented on the figure 6, one or more gaps J may be present between the external surface of the conductive core 74 and the internal surface of the cladding 79 over at least part of the length L of the conductive core 74. In particular, atmosphere may be trapped between the conductive core 74 and the cladding 79, for example air, or an inerting atmosphere, for example argon.

[0064] In the case of air trapped between the sheath 79 and the conductive core 74, during use, particularly at high temperatures, this air will be consumed by the oxidation of the copper and the Inconel®. However, since the volume is small and non-renewable (due to weld sealing), the oxidation layer will remain very thin. In the case of a neutral atmosphere, for example with argon gas purging, the formation of the oxidation layer can be prevented.

[0065] It is also possible to evacuate the assembly 72 using a tube added for this purpose. A degassing tube is then added to one end, and the sheath is evacuated by pumping through the tube. A swaging can then be performed to permanently maintain the vacuum, allowing the tube to be sealed airtight and permanently. This evacuation step can also be used to perform a leak test.

[0066] The stainless steel alloy of the sheath 79 and the connecting tabs 78 is chosen according to the thermal stresses to which the electrical conductor 70 is exposed. In particular, for a temperature range up to 900°C, the sheath 79 and the tabs 78 can be made of Inconel® < 600. The conductor 74 can have a diameter of approximately ten millimeters. However, the cross-section can be modified according to requirements, for example, in terms of current, voltage drop, etc. The conductor 74 can also be composed, in whole or in part, of one or more multi-strand cables, for example, composed of a multi-strand braid.

[0067] The invention thus aims to shape a core 74 composed of a core (or "heart" or "core") made of copper (or any other metal deemed satisfactory in terms of electrical resistivity) protected by a sheath 79 made of stainless or refractory metal, in particular stainless steel or stainless nickel alloy, all welded by TIG welding with the presence of two connecting tabs 78. The invention can therefore be implemented without the use of a Hot Isostatic Compression (HIC) process to allow the assembly between the core 74, the sheath 79 and the connecting tabs 78.

[0068] Advantageously, the invention can therefore allow for reduced manufacturing costs, as well as simplified manufacturing that even allows for shaping and cutting to length directly on the site of use (shaping, cutting to length, welding of the whistles). The electrical conductor 70 can be used entirely in high-temperature zones and also as a bulkhead penetration, enabling the connection between the high-temperature zone and the ambient temperature zone.

[0069] The manufacturing process of such an electrical conductor 70, intended for use as an electrical conductor for supplying current in an electrochemical system, for example that of figures 1 to 4 For example, it includes the following steps: manufacture the parts described above (core, sheath, tabs), cleaning the parts and in particular the surfaces intended to be welded, namely the electrical conduction surfaces and the surfaces necessary for sealing the electrical conductor, by means of a detergent and / or a solvent, or any other means, insertion of the conductive core 74 into the sheath 79, welding by brazing or brazing between the conductive core 74 and the first connecting tab 78, welding by TIG welding between the sheath 79 and the first connecting tab 78, welding by brazing or brazing between the conductive core 74 and the second connecting tab 78, welding by TIG welding between the sheath 79 and the second connecting tab 78, where appropriate, if necessary, vacuuming the sheath 79 by pumping.

[0070] In addition, a radiography step of the welds can be carried out in order to confirm the quality of the welds from a mechanical, electrical and sealing point of view.

[0071] The ends equipped with the 78 tabs are hot ends that can be pierced, as seen on the figures 5 and 6 , perpendicular to the axis of the sheath 79, to be screwed onto the stack.

[0072] TIG welding is best performed by a skilled professional, especially for welding between copper and Inconel® to ensure a good electrical connection, and for welding between Inconel® and Inconel® to ensure a watertight weld.

[0073] By comparing the resistance obtained for an electrical conductor 70 of 1 m, in Table 1 below, in the case of an electrical conductor 70 of diameter of 12 mm made entirely of Inconel ®< 600 (production according to the prior art) and in the case of an electrical conductor 70 of diameter of 12 mm made with a sheath 79 of Inconel ®< 600 and a core 74 of copper (production according to the invention), it can be seen that the invention makes it possible to reduce electrical losses by a factor of 10, at the operating temperature of 800°C. [Table 1]

[0074] Temperature (°C) Conductor resistance 70 Inconel ® < 600 (Ω) Conductor resistance 70 Inconel ® < 600 + copper (Ω) 20 9,1.10 -3< 0,21.10 -3< Table 1 800 10.10 -3< 0,87.10 -3<

[0075] For the results in Table 1, the resistivity of copper is 17.24 x 10⁻⁹ Ω·m at room temperature (20°C) and 70 x 10⁻⁹ Ω·m at 800°C. The resistivity of Inconel® 600 is 1.03 x 10⁻⁶ Ω·m at room temperature (20°C) and 1.13 x 10⁻⁶ Ω·m at 800°C.

[0076] The electrical conductor 70 obtained according to the principle of the invention is thus an electrical conductor suitable for the high temperature and high current of SOEC / SOFC type solid oxide cell stacks. However, electrical losses may occur in the connection tabs 78, and it is possible to modify the design of these connection tabs 78 in order to limit these losses.

[0077] THE figures 7 to 11 are related to another embodiment of an electrical conductor 70 according to the invention in which the connecting tabs 78 are of a different design, being then referred to as "high conductivity" connecting tabs 78 or whistles 78.

[0078] Specifically, the first connecting leg 78 and the second connecting leg 78 each comprise a connecting conductive core 80 made of the first metallic material, here copper, but any other metal previously described is possible, and a connecting sheath 81 completely covering the connecting core 80 along its entire length l, as visible in the Figure 10 , and made of the second metallic material, here Inconel® < 600, but any other metal previously described is possible. Advantageously, the connecting sheath 81 has a thickness eg, visible on the Figure 10 , which is on the order of 0.5 mm. Achieving a low thickness eg contributes significantly to reducing electrical losses.

[0079] Furthermore, each connecting tab 78 has a sleeve-forming tube 82 inserted into corresponding holes in the connecting core 80 and the connecting sleeve 81 to allow attachment to the stack, as seen in the Figures 10 and 11 .

[0080] The resulting connecting leg 78, as shown on the Figures 10 and 11This allows for the reduction of electrical losses in the whistle by replacing a portion of the second metallic material with the first metallic material, which has good conductivity. Indeed, by retaining an Inconel® connecting sleeve 81 to protect the copper connecting core 80 from oxidation, it is possible to reduce the electrical losses of the whistle 78. However, since such a whistle 78 is the connection point, electrical continuity across the entire connection surface is necessary between the connecting sleeve 81 and the connecting core 80. To achieve this, the manufacturing method for such a whistle 78, described below, uses the Hot Isostatic Compression (HIC) process, employed in the invention solely for the manufacture of such "high conductivity" whistles 78, in order to guarantee a weld across the entire connection surface between the connecting core 80 and the connecting sleeve 81.

[0081] The electrical conductor 70 of the embodiment of figures 7 And 8 therefore presents a better conductivity than that described with reference to figures 5 and 6 , thanks to the use of "high conductivity" 78 connection tabs. Specifically, a "high conductivity" 78 connection tab can have a resistivity of only about ten percent compared to a 78 connection tab made entirely of the second metallic material.

[0082] For manufacturing the "high conductivity" connecting tabs 78, the connecting core 80 can be obtained by die forging. Die forging consists of forming, by plastic deformation after heating, blanks made of alloys, such as aluminum, copper, titanium, nickel, etc. The die forging of steels is also called "stamping." Die forging is a forging operation performed using tools called "dies," specifically upper and lower half-dies. These have the shape of the part to be manufactured imprinted in the die.

[0083] Furthermore, the connecting sleeve 81 can be obtained by stamping or assembling several parts made of the second metallic material. This stamping manufacturing technique makes it possible to obtain, from a flat sheet of metal, an object whose shape is not developable. This technique is suitable for mass production.

[0084] The sheath 79 is here a flexible sheath, for example made of 321 stainless steel from the XS range in DN16 from the company Kenovel. In addition, the flexible conductive core 74 is for example multi-strand copper in 70 mm².

[0085] Furthermore, and advantageously, an electrically insulating sheath is added to the assembly 72 formed, as described previously, in particular a braided ceramic sheath of the Nefatex 1390 type (alumina-silica sheath with a standard dielectric strength of 700 V at 1000°C). Such an electrically insulating sheath is not shown in the examples described.

[0086] Note that on the figure 9 Brazing B is also shown, carried out between the connecting tab 78 and the conductive core 74. A TIG weld (reference P) is carried out between the whistle 78 and the flexible sheath 79 in order to ensure sealing.

[0087] The assembly process for a 78 "high conductivity" connecting tab or "high conductivity" whistle then comprises the following steps: cleaning of the constituent parts of the whistle 78, for example using detergents, solvents or any other suitable means, insertion of the connecting core 80 into the connecting sleeve 81, insertion of the sleeve tube 82, made of the first metallic material, TIG welding between the connecting sleeve 81 and the sleeve tube 82 to seal the joints on each side, optionally with a filler material, in particular composed of stainless steel, addition of parts 86 and 87 on the connecting core 80: part 86 is made of the first material and provides mechanical support between the connecting sleeve 81 and the flexible sleeve 79, and protection against oxidation (continuity of sealing between 81 and 79);Part 87 is made of the second material and allows the electrical connection between the connecting core 80 and the conductive core 74, addition of the closing cover 85 formed by a closing plate 83 and a queusoting tube 84, as shown on the; Figure 10 , TIG welding between the connecting sleeve 81 and the closing cover 85 to seal the joint, vacuuming of the whistle 78, a vacuum pump being connected to the tube 84 so as to create a vacuum inside the connecting sleeve 81, then a swaging of the tube 84 is carried out so as to seal it hermetically and permanently.

[0088] Subsequently, the application of a Hot Isostatic Compression (HIC) diffusion welding cycle is carried out with the following operating conditions: bring the assembly 78, consisting among other things of the connecting core 80 and the connecting sheath 81, to a temperature between 600°C and 1060°C, preferably between 800°C and 1000°C, in particular a temperature of 920°C, apply to the connecting sheath 81 a pressure between 500 bar and 1500 bar, preferably between 800 bar and 1200 bar, in particular a pressure of 1020 bar, apply a pressure and temperature plateau for a period of 30 minutes to several hours, preferably 1 hour to 3 hours, in particular 2 hours, allow the assembly to cool and depressurize.

[0089] Finally, each "high conductivity" connecting tab 78 can be machined to allow direct connection to the connecting core 80, resulting in a whistle 78 as shown in the figure 11 .

[0090] The resulting "high conductivity" whistles 78 are then connected to the assembly 72 by a low-resistivity connection. Specifically, the conductive core 74 and the connecting core 80 of each connecting tab 78 can be joined by brazing or high-temperature soldering. This provides a high-conductivity electrical connection. The choice of filler metal can ensure the connection up to maximum operating temperatures of approximately 900°C. Assembly can be carried out, for example, using the commercially available Castolin® solder 146 and the recommended flux 146 M. This solder consists of 60% copper, 39% zinc, and 1% tin-manganese.

[0091] Next, as previously described, the mechanical connection and sealing are achieved by TIG welding, as shown in the figure 8at point P around the entire circumference, with the addition of material preferably composed of the second metallic material. A vacuum step may be carried out, and a radiography step of the welds and brazes may also be implemented as described previously.

[0092] The invention can be applied to a high-temperature steam electrolyzer, a high-temperature co-electrolyzer fed with a mixture of steam (H2O) and carbon dioxide (CO2), a high-temperature solid oxide fuel cell, a reversible system, fuel cell and high-temperature electrolyzer, to "medium temperature" fuel cell or electrolyzer, i.e. 400°C, or PCFC for "Proton Ceramic Fuel Cell", as described above.

[0093] The invention applies to the systems described above operating at atmospheric pressure but also to systems under pressure.

[0094] Apart from the technical field of solid oxide electrochemical systems, the invention applies to all fields where there is a need for electrical conduction in an oxidizing environment at high temperature or under conditions leading to the rapid degradation of electrically conductive materials.

Claims

1. Flexible electrical conductor (70) comprising: - an assembly (72) comprising: - a flexible conductive core (74) made of a first metal material, - a sheath (79) covering the conductive core (74) and made of a second metal material having an electrical resistivity higher than the electrical resistivity of the first metal material, - a first connection strip (78) connected to a first end (72a) of the assembly (72), characterised in that the first connection strip (78) is made of the second metal material, and in that, at the first end (72a) of the assembly (72), the sheath (79) and the first connection strip (78) are bonded by TIG welding, and the conductive core (74) and the first connection strip (78) are bonded by braze-welding or brazing.

2. Conductor according to Claim 1, characterised in that it comprises: - a second connection strip (78) made at least in part of the second metal material and connected to a second end (72b) of the assembly (72), and in that, at the second end (72b) of the assembly (72), the sheath (79) and the second connection strip (78) are bonded by TIG welding, and the conductive core (74) and the second connection strip (78) are bonded by braze-welding or brazing, the TIG welds at the two ends (72a, 72b) of the assembly (72) and the sheath (79) completely covering the conductive core (74) over its entire length (L).

3. Conductor according to Claim 1 or 2, characterised in that at least one gap (J) is present between the outer surface of the conductive core (74) and the inner surface of the sheath (79) over at least part of the length (L) of the conductive core (74).

4. Conductor according to one of the preceding claims, characterised in that the conductive core (74) is made of copper, nickel or silver and / or copper, nickel or silver alloys.

5. Conductor according to one of the preceding claims, characterised in that the sheath (79) is made of stainless or refractory metal and / or metal or refractory alloys, in particular stainless or refractory steel.

6. Conductor according to one of the preceding claims, characterised in that the first connection strip (78) and / or the second connection strip (78) each has a conductive connection core (80) made of the first metal material, and a connection sheath (81) completely covering the connection core (80) over its entire length (l) and made of the second metal material.

7. Conductor according to one of the preceding claims, characterised in that the assembly (72) comprising the conductive core (74) and the sheath (79) is completely covered by an electrically insulating jacket.

8. Method for manufacturing an electrical conductor (70) according to one of the preceding claims, the method comprising the following steps: - cleaning the surfaces, in particular using a detergent and / or a solvent, - inserting the conductive core (74) into the sheath (79), - bonding the conductive core (74) to the first connection strip (78) by braze-welding or brazing, - bonding the sheath (79) to the first connection strip (78) by TIG welding, - optionally, evacuating the sheath (79) by pumping.

9. Method according to Claim 8, characterised in that the electrical conductor (70) comprises a second connection strip (78) made at least in part of the second metal material and connected to a second end (72b) of the assembly (72), and in that the method comprises, after the step of bonding the sheath (79) to the first connection strip (78) by TIG welding, the following steps: - bonding the conductive core (74) to the second connection strip (78) by braze-welding or brazing, - bonding the sheath (79) to the second connection strip (78) by TIG welding.

10. . Method according to Claim 8 or 9, characterised in that the first connection strip (78) and / or the second connection strip (78) are formed by assembling a conductive connection core (80) and a connection sheath (81) completely covering the connection core (80), the connection core (80) being manufactured by die-forging and the connection sheath (81) being manufactured by deep-drawing or by assembling a plurality of parts made of the second metal material.

11. Method according to Claim 10, characterised in that assembling the first connection strip (78) and / or the second connection strip (78) comprises at least the following steps: - cleaning the constituent elements of the connection strip (78), in particular using a detergent or a solvent, - inserting the connection core (80) into the connection sheath (81), - evacuating the connection strip (78), - applying a diffusion welding cycle by hot isostatic pressing (HIP).

12. Method according to Claim 11, characterised in that the diffusion welding cycle by hot isostatic pressing (HIP) is carried out with the following operating conditions: - heating the assembly formed by the connection core (80) and the connection sheath (81) to a temperature comprised between 600°C and 1060°C, preferably between 800°C and 1000°C, in particular a temperature of 920°C, - applying a pressure comprised between 500 bar and 1500 bar, preferably between 800 bar and 1200 bar, in particular a pressure of 1020 bar, to the connection sheath (81), - applying a pressure and temperature plateau for a period of 30 minutes to several hours, preferably 1 hour to 3 hours, in particular 2 hours, - allowing the assembly to cool and depressurising.

13. Method according to any one of Claims 10 to 12, characterised in that the conductive core (74) and the connection core (80) of the first connection strip (78) and / or of the second connection strip (78) are joined together by a high-temperature braze-welding or high-temperature brazing method.

14. Use of at least one electrical conductor (70) according to any one of Claims 1 to 7, as an electrical conductor of an electrochemical system comprising: - an enclosure (60) for air circulation in the volume delimited thereby, - an electrochemical device housed in the enclosure (60), comprising: - a high-temperature SOEC / SOFC-type solid oxide stack (20) of elementary electrochemical cells (10) each comprising an electrolyte (16) interposed between a cathode (12) and an anode (14) and connected in series between two electrical terminals (30, 32), and - said at least one electrical conductor (70) connected to at least one of the two electrical terminals (30, 32).

15. Electrochemical system comprising: - an enclosure (60) for air circulation in the volume delimited thereby, - an electrochemical device housed in the enclosure (60), comprising: - a high-temperature SOEC / SOFC-type solid oxide stack (20) of elementary electrochemical cells (10) each comprising an electrolyte (16) interposed between a cathode (12) and an anode (14) and connected in series between two electrical terminals (30, 32), the electrochemical system being characterised in that it comprises: - at least one electrical conductor (70) according to any one of Claims 1 to 7, connected to at least one of the two electrical terminals (30, 32).