AU2012213033A1

AU2012213033A1 - Electrolyser and assembly comprising same, in particular for the production of H2 and O2

Info

Publication number: AU2012213033A1
Application number: AU2012213033A
Authority: AU
Inventors: Arash Mofakhami
Original assignee: Ceram Hyd SA
Current assignee: Ceram Hyd SA
Priority date: 2011-02-03
Filing date: 2012-02-03
Publication date: 2013-08-22
Also published as: JP2014504680A; CA2826002A1; FR2971262A1; EP2670889A1; FR2971262B1; WO2012104819A1; US20140202875A1

Abstract

The present invention relates to an electrolyser for the production of at least one chemical substance, such as hydrogen, oxygen, chlorine or hypochlorous acid, or sodium hydroxide, by electrolysis of pure water or of water containing at least one salt, base and/or acid such as NaCl, H

Description

WO 2012/104819 PCT/IB2012/050508 Electrolyser and assembly comprising same, in particular for the production of

H

2 and 02 The present invention relates to the production of chemicals such as 5 dihydrogen, dioxygen, chlorine, hypochlorous acid or soda, by electrolysis of pure water or of water containing salts, bases and/or acids such as NaCl, H 2

SO

4 , KOH, NaOH, by means of an electrolyzer comprising several electrolytic cells each equipped with at least one ion-exchange membrane arranged between an anode and a cathode. The invention aims to improve the electrolyzers, so as to facilitate production 10 of the substance or substances in question and to lower their cost. The invention thus relates to an electrolyzer for the production of dihydrogen and dioxygen or other chemicals, comprising a stack of at least a first and a second consecutive electrolytic cell, each electrolytic cell having: - an anode, 15 - a cathode, - at least one ion-exchange membrane arranged between the anode and the cathode, the exchange membranes of the first and of the second electrolytic cells being separated by a bipolar electrode constituting on the one hand the anode of the first electrolytic cell and 20 on the other hand the cathode of the second electrolytic cell. The electrolyzer according to the invention is of simpler assembly compared to the known electrolyzers in which the anodes and cathodes of the different cells are separated from one another. Moreover, the circulation of electric current in the electrolyzer, notably in a 25 cell and/or between the different electrolytic cells, can be improved. The electrolyzer can be configured for the production of dihydrogen, dioxygen, chlorine, hypochlorous acid, soda, by electrolysis of pure water or of water containing at least one salt, an acid and/or a base such as NaCl, H 2

SO

4 , KOH or NaOH. The bipolar electrode can comprise a bipolar plate all in one piece, the bipolar 30 plate being associated if necessary with at least one grid and with at least one porous plate, notably with one or two grids and with one or two porous plates, it being possible for a grid and a porous plate to be arranged on either side of the bipolar plate. Throughout the WO 2012/104819 PCT/IB2012/050508 2 application, the term "plate" is to be understood broadly as a synonym of wall and not limited to a flat component, even if a flat shape is preferred. One grid can define at least partially the anodic chamber of the first cell, and another grid - the cathodic chamber of the second cell. Each anodic or cathodic chamber can be delimited on the one hand by the 5 bipolar plate and on the other hand by a porous plate. The porous plate can provide a suitable support for the adjacent ion-exchange membrane. The porous plate can also perform the role of diffuser for circulation of the electrolyte and gas, so as to promote the electrochemical reaction. Hermeticity of the chambers to the gases and to the electrolyte as well as the circulations in each of the anodic and cathodic chambers, being independent 10 and impervious between them, are also obtained by means of one and the same gasket assembled with an axial rotation of 1800. The bipolar electrode can be made entirely in one piece, the bipolar plate, the grids and the porous plates being in this case integral with one another prior to installation in the electrolyzer. 15 The bipolar electrode, or at the very least the bipolar plate, can comprise at least one of the following materials: nickel, iridium, ruthenium, palladium, cadmium, molybdenum, platinum, stainless steel, titanium, tantalum, iron alloy, nickel alloy, lead alloy and/or a thin layer of tantalum oxide, iridium oxide, ruthenium oxide, lead oxide, ferric oxide, platinum, platinum carbon, palladium, nickel, cadmium, and/or molybdenum. 20 The porous plate or plates can comprise at least one of the following materials: nickel, iridium, ruthenium, palladium, cadmium, molybdenum, platinum, stainless steel, titanium, tantalum, iron alloy, nickel alloy, lead alloy and/or a thin layer of tantalum oxide, iridium oxide, ruthenium oxide, lead oxide, ferric oxide, platinum, platinum carbon, palladium, nickel, cadmium, and/or molybdenum. 25 The porous plate or plates can for example comprise titanium covered with a layer of one of the above materials on the face adjacent to the ion-exchange membrane. The anode can comprise at least one of the following materials: titanium, tantalum, iridium, iron alloy, lead alloy, and/or a thin layer of tantalum oxide, iridium oxide, ruthenium oxide, lead oxide, and/or ferric oxide. The thin layer can notably be 30 arranged on the face of the anode adjacent to the ion-exchange membrane. The cathode can comprise at least one of the following materials: nickel, iridium, palladium, cadmium, molybdenum, platinum, titanium, tantalum, iron alloy, lead WO 2012/104819 PCT/IB2012/050508 3 alloy, nickel alloy and/or a thin layer of platinum, platinum carbon, palladium, nickel, cadmium, and/or molybdenum. The thin layer can notably be arranged on the face of the cathode adjacent to the ion-exchange membrane. The bipolar electrode can notably be made entirely of one and the same 5 material, for example titanium. More precisely, the bipolar plate can be made entirely of one and the same material, for example titanium. The grid or grids can be made entirely of one and the same material, for example titanium. The porous plate or plates can be made entirely of one and the same material, for example titanium. Frames and gaskets can be interposed between the electrodes. The electrolytes 10 can circulate between the cells owing to holes made in the frames and gaskets and to circulating ducts provided in the gaskets. The bipolar electrode, notably the bipolar plate, can as a variant comprise a coating of a material, for example tantalum. The coating can have a thickness between 10 and 100 pm, for example of the order of 50 pm. 15 The ion-exchange membrane preferably comprises boron nitride and more preferably activated boron nitride. The aim of "activation" of boron nitride is to promote ionic conduction in the boron nitride. In activated boron nitride, the activated [BN] crystallite generates -OH, -H,

-SO

3 H or -SO 4 H bonds on its surface, which will create N-H 2 *, B-OH 2 *, B-SOxH 2 * or N 20 SOxH 2 * groups. Conduction of ions can also take place owing to pairs available on oxygen atoms inserted in nitrogen holes in the boron nitride. These nitrogen holes containing oxygen atoms can notably be present when the boron nitride was obtained from B 2 0 3 or from H 3 BO3. The boron nitride used can comprise at least one, for example one or more 25 substituent element(s), from the following list: boron oxide, calcium borate, boric acid, sulfuric acid. The presence of such elements may promote activation, notably when they are present in a proportion by weight between 1 and 20%. The presence of boric acid, for example present in the pores of the boron nitride or in amorphous form, may promote the creation of B-OH and NH bonds. 30 For carrying out activation, boron nitride, or the membrane comprising it, can be exposed to a fluid for supplying H 3 0 or S0 4 2 - ions and for creating B-OH and/or B

SO

4 H, B-SO 3 H, N-SO 4 H, N-SO 3 H bonds and/or N-H bonds in the boron nitride. The fluid WO 2012/104819 PCT/IB2012/050508 4 can for example be an acid solution containing H 3 0' ions, for example strong acids such as HCl, H 2

SO

4 , H 3

PO

4 , H 2

S

2 0 7 , or weak acids, or need not be an acid solution, but for example a basic solution containing OH- ions, for example a solution of soda or of potash. The concentration of the solution can have an influence on the rate and level of activation 5 obtained, i.e. on the level of ionic conductivity obtained, but not on the appearance of the activation itself. The concentration of acid is for example between 1 and 18 mol/L and the concentration of soda can be between 0.5 and 1 mol/L. In order to promote the creation of bonds of the [BN] crystallite with -OH, SO 4 H, -SO 3 H, -SO 4 H, -SO 3 H and/or -H, boron nitride or the membrane containing boron 10 nitride can be exposed to an electric field, for example an electric field between 15 and 40 000 V/m in the presence of a 1 M solution of H 2

SO

4 acid for example. The electric field can be supplied by an external generator. The voltage applied is for example between 1.5 V and 50 V, for example of the order of 30 V. The source of voltage can be constant, or, as a variant, not constant. It can be configured to detect the 15 end of activation automatically, for example when the current density in the material increases sharply. The intensity of the current circulating during activation in the boron nitride can be of the order of 10 mA/cm 2 to 1000 mA/cm 2 . The activation with a fluid can be carried out at a temperature between 0 and 90'C, for example of the order of 60'C, or even at room temperature. 20 After being exposed to the solution, the boron nitride can be rinsed and optionally dried before being used for manufacturing the electrolyzer. The fluid can be removed so that its residual content is below 2%. The step of exposure to the fluid can have a duration of less than 50 hours. In a practical example of the invention, the boron nitride is activated by 25 mixing boron nitride, for example in powder form, with acid, for example concentrated sulfuric acid, for example 3M for a predetermined time, and then rinsing, before using the activated boron nitride for making the ion-exchange membrane, for example by mixing the activated boron nitride powder with a polymer matrix. The ion-exchange membrane can comprise a polymer matrix. The polymer 30 matrix can comprise at least one of the polymers from the following list: polyvinyl alcohol (PVA), vinyl caprolactam, PTFE (Teflon*), sulfonated polyether sulfone, this list not being exhaustive. The polymer matrix can for example comprise PTFE from the WO 2012/104819 PCT/IB2012/050508 5 DUPONT company, known by the trade name Teflon , or a PTFE from another company. The ionic conduction with PTFE can be as good as with other polymers, and can reach 0.2 S/cm. The proportion by weight of boron nitride in the membrane can be above 50%, 5 better still greater than or equal to 95%, notably in the case of combination with PTFE. In some embodiments it is for example of the order of 70%, and of the order of 90% in other embodiments. The mechanical strength of the ion-exchange membrane (mechanical strength at 300 pm) can be satisfactory for a small amount of PTFE, for example of the order of 4 10 MPa (Young's modulus) at 5 wt% of PTFE at 25'C, and increases significantly with a larger amount of PTFE, for example of the order of 6 MPa at 15%. The temperature range for use of the ion-exchange membrane can be fairly wide, and can be up to 180'C. The boron nitride present in the ion-exchange membrane can be in the form of 15 a powder composed of grains having a larger transverse dimension between 0.5 and 15 pm, and is for example centered on 5 pm. According to one hypothesis of the mechanism of action, the ionic conduction in the boron nitride takes place on the surface of the activated boron nitride crystallites making up the grains. 20 In one embodiment, the boron nitride is composed of a powder of nanoparticles, i.e. of grains having a single crystal of nanometric size, for example between 10 and 500 nanometers. The ion-exchange membrane can have a thickness between 50 and 500 pm, for example of the order of about 200 pm to 300 pm. A relatively small thickness can 25 improve the ionic conduction. Nevertheless, the thickness of the ion-exchange membrane is sufficient to allow the membrane to withstand high pressures in the electrolyzer, if necessary. This pressure can for example reach 30 bar in one practical example of the invention. "Permeability of a material" denotes an intrinsic characteristic of the latter, 30 which measures its capacity for allowing passage of a fluid or of a gas-laden liquid and which is independent of the porosity of the material. Dihydrogen or dioxygen dissolved in WO 2012/104819 PCT/IB2012/050508 6 water can pass through the ion-exchange membrane. This phenomenon gives rise to the presence of dihydrogen in the dioxygen and of dioxygen in the dihydrogen collected. The presence of dihydrogen in dioxygen can constitute a danger. The lower explosive limit (LEL) corresponds to presence of about 4% of dihydrogen in air or in 5 dioxygen. A scale of LEL is thus defined, with 100% LEL corresponding to presence of 4% of dihydrogen in dioxygen. The permeability of dihydrogen and dioxygen through the ion-exchange membrane is preferably low enough to allow the proportion of dihydrogen in dioxygen to be below 70% LEL at 30 bar and 90'C. The electrolyzer can comprise an alarm that is 10 triggered if this limit is exceeded. The assembly can comprise one or more sensors arranged at electrolyzer outlet for monitoring the levels of dihydrogen in dioxygen and of dioxygen in dihydrogen, which can make it possible to guarantee a sufficient degree of purity of the dihydrogen. In case of insufficient purity, operation can be stopped. "Porosity" denotes all the interstices, joined together or not, of a material that 15 can contain fluids, liquids or gases. The porosity is a numerical value that characterizes these interstices, corresponding to the ratio of the volume of the voids of the material divided by the total volume. The ion-exchange membrane is preferably nonporous in operation, so that it is impervious to gases in the operating conditions. Conversely, the dry ion-exchange membrane may not be nonporous. It may not be impervious to gases. 20 Each electrolytic cell can consume water, the reaction taking place in an 1 electrolytic cell being for example as follows: H 2 0 -> H 2 + -02. This reaction can take 2 place in an acid medium, which can facilitate circulation of the H 3 0 ions or of the protons H+ from the anode to the cathode through the ion-exchange membrane. The electrolyte can thus comprise water and acid. The acid can be selected 25 from the following list, which is not exhaustive: sulfuric acid, phosphoric acid, carboxylic acid. The acid can have a concentration of the order of 5 to 20 wt%, for example. This reaction can also take place in a basic medium, which can facilitate circulation of the OH- ions from the cathode to the anode through the ion-exchange membrane.

WO 2012/104819 PCT/IB2012/050508 7 The electrolyte can thus comprise water and a base. The base can be selected from the following list, which is not exhaustive: potash KOH and soda NaOH. The base can have a concentration of the order of 5 to 30 wt%, for example. Depending on the substance or substances that we wish to produce, it may not 5 be necessary to add acid or base during use, but only water for example. In operation, a voltage at the terminals of each of the electrolytic cells is for example between 1.24 and 5 V, being for example of the order of 1.48 V or more. In operation, the current circulating in the electrolytic cells can be between 200 and 1000 A, being for example of the order of 500 A for an active surface of 500 cm 2 . 10 In one embodiment, at least one cell of the stack can comprise a single ion exchange membrane between the anode and the cathode. The cell can comprise two chambers, an anodic chamber defined between the anode and the membrane and a cathodic chamber between the cathode and the membrane. In this configuration, the electrolyzer can be used for the production of H 2 and 02, of Cl 2 and NaOH or of Cl 2 and 15

H

2 . In one embodiment, at least one cell of the stack can comprise two ion exchange membranes, preferably two membranes providing an intermediate chamber between them. The cell can comprise three chambers. In this configuration, the electrolyzer can be used for the production of HClO and of NaOH or for desalination of 20 salt water and therefore for production of pure water. At least one cell can comprise a nonselective ion-exchange membrane such as a membrane comprising boron nitride, and a selective exchange membrane such as a membrane based on Nafion. "Nonselective exchange membrane" means a membrane having the capacity to 25 conduct both anions and cations. There may be means for establishing a circulation of electrolyte in the intermediate chamber, notably with a view to withdrawing a substance produced in the latter during operation of the electrolyzer. The invention further relates to an electrolytic assembly comprising: 30 - an electrolyzer as defined above, - a reservoir associated with cathodic production, for example of dihydrogen for supplying the dihydrogen obtained at a given pressure, and WO 2012/104819 PCT/IB2012/050508 8 - a reservoir associated with anodic production, for example of dioxygen for supplying the dioxygen obtained at a given pressure. The electrolyte can be stored in each of the two reservoirs. The assembly can comprise fluid communication between the two reservoirs, 5 notably at their base. The assembly can comprise a device monitoring the electrolyte level in each of the reservoirs. The fluid communication can be controlled by means of a transfer valve, as will be described later, in relation to the electrolyte levels in each of the reservoirs. As a variant, and depending on what is possible, this fluid communication can 10 be free and can ensure equilibrium of the electrolyte level in the two reservoirs. In that way, when electrolysis produces dihydrogen and dioxygen, the ratio of the relative volumes available for the gases obtained in each of the reservoirs is always constant, thus respecting the stoichiometry of the reaction and therefore equilibrium of the pressures in the reservoirs. The advantage of having a connection between the reservoirs is that it can 15 ensure equilibrium of the liquid levels in each of the reservoirs. The assembly can comprise a water supply. In one embodiment, this can be provided by the dioxygen reservoir, if this gas is produced. As a variant, it can be provided by the dihydrogen reservoir. The electrolyte can contain the following ions in addition to water: hydroxyl 20 and sulfate. On the anode side, in the presence of the voltage, the sulfate ions convert water to gaseous oxygen and hydroxyl ions. The oxygen is recovered and the hydroxyl ions pass through the membrane to the cathode. This therefore results in consumption of water. On the cathode side, in the presence of the voltage, the hydroxyl ions are converted 25 to hydrogen and water. The presence of sulfate ions makes it possible to maintain the level of the concentration of hydroxyl ions. In a practical example of the invention, the electrolyzer comprises seven successive electrolytic cells, with an active surface of 500 cm 2 per cell. In operation, such an electrolyzer may consume a power of 7 kW with an efficiency of 70%. 30 In another practical example of the invention, the electrolyzer comprises 70 successive electrolytic cells. In operation, such an electrolyzer can consume a power of 70 kW with an efficiency of 70%.

WO 2012/104819 PCT/IB2012/050508 9 Each ion-exchange membrane can have a total surface area of the order of 1050 cm 2 , its dimensions being for example 30 cm x 35 cm, or an active surface of 500 cm 2 . The electrolyzer can comprise a front end shield and a rear end shield, which 5 border the consecutive electrolytic cells. The front and rear end shields can comprise stainless steel, for example stainless steel 316L. In the presence of gaseous discharges that are not released to atmosphere, the assembly comprises a stabilizer for stabilizing the pressure in the reservoirs, for example to a value between 10 and 30 bar. It is also possible to work at atmospheric pressure. The 10 stabilizer can comprise a discharger for each reservoir for regulating the pressure in the corresponding reservoir, and for obtaining an identical pressure in each of the reservoirs, which can prevent damage to the electrolytic cells and in particular to the ion-exchange membranes. Each of the reservoirs can also be equipped with a control pressure sensor, as 15 well as a degassing outlet equipped with a safety valve that is operated in emergency. Each of the reservoirs can further comprise an outlet valve allowing the user to recover the gas produced. The gases produced can be recovered for direct use or to be compressed, for example to a value of 300 bar, for example for transport. In the case of production of dihydrogen and dioxygen, the outlet of the 20 dihydrogen reservoir can be equipped with a device containing a catalyst that makes it possible to burn the residual dioxygen that may be present in the dihydrogen reservoir, so as to obtain pure dihydrogen. It is also possible to use a dryer for removing the residual water, which may have been obtained for example by combustion of dihydrogen with the residual dioxygen. It is then possible to measure the flow rate of dihydrogen obtained, as 25 well as fit a sensor for verifying the purity of the gas obtained. The assembly can further comprise a temperature sensor of the electrolyte in each of the reservoirs on the one hand, and in the electrolytic cells themselves on the other hand, so as to control the temperature of the electrolyte and maintain a roughly constant operating temperature, for example at a value between 0 0 C and 120'C, or even between 30 70'C and 120'C. It can for example be of the order of about 70'C. Maintaining a high enough operating temperature can promote the electrochemical reaction, independently of the choice of pressure. Conversely, a temperature limit must not be exceeded, beyond WO 2012/104819 PCT/IB2012/050508 10 which there is a risk of degradation of the assembly. If necessary, the assembly can also comprise at least one, or even two devices for cooling the electrolyte before it enters the electrolyzer, optionally equipped with a temperature sensor for monitoring the effectiveness of cooling. 5 The assembly can further comprise a heating device, for example for use in cold environments, depending on the temperature difference between the operating temperature and the external temperature. The heating device can for example comprise resistances arranged in the electrolyte, for example in the electrolyte reservoirs or near the stack of cells. As a variant, the voltage can be increased at the start of operation to obtain 10 ohmic losses for heating the assembly, then returning to the operating voltage. The assembly can also comprise a thermal insulation from the exterior. Stabilizing the temperature to an operating temperature makes it possible to improve the efficiency and the working life of the electrolyzer. The solenoid valves can be made at least partially of PVDF. 15 The power supply of the electrolyzer is preferably housed in an electric cabinet comprising a process control computer for controlling the current and/or voltage of the power supplied to the stack of electrolytic cells starting from the mains current and voltage. The control cabinet can also be equipped with remote connection allowing 20 remote maintenance of the assembly. The operating time of the assembly can be of the order of 10 000 hours at least. Moreover, the assembly can be equipped with an electrolyte retention tank. The assembly can comprise an acidity sensor or conversely may be without it. 25 The invention further relates to a method of production of hypochlorous acid, by means of an intermediate-chamber electrolyzer. The anodic chamber of a cell can contain water, the cathodic chamber can contain water and the intermediate chamber can contain brine. The invention further relates to an electrolyzer cell, comprising: 30 - an anode, - a cathode, - two exchange membranes, notably: WO 2012/104819 PCT/IB2012/050508 11 a selective exchange membrane and a nonselective exchange membrane, arranged between the anode and the cathode, preferably a nonselective membrane based on activated boron nitride and preferably a selective membrane based on Nafion, the nonselective membrane preferably 5 protecting the selective membrane from a basic or acidic environment. The invention will be better understood on reading the detailed description, given hereunder, of practical examples and on examining the appended drawings, in which: - Fig. 1 is a perspective view of a stack of electrolytic cells according to the 10 invention, - Fig. 2 is an exploded view of the stack of electrolytic cells of Fig. 1, - Fig. 3 is an exploded view of an electrolytic cell, - Figs. 4a to 41 are top views of each of the components making up the stack in Figs. 1 and 2, 15 - Figs. 5 and 6 are schematic, partial cross-sectional views of the electrolytic cell in Fig. 3, - Figs. 7a to 7c are perspective views of assemblies according to the invention, - Fig. 8 is a schematic illustration of the operation of the assembly according 20 to the invention, - Figs. 9a and 9b illustrate control of the electrolyte temperature, - Figs. 10, 1 la and 1 1b, and 12a to 12c illustrate, schematically, management of the flows of electrolyte in the assembly according to the invention, - Fig. 13 shows schematically a variant of electrolyzer according to the 25 invention, - Fig. 14 illustrates the circulation of electrolyte in a stack of cells according to one embodiment of the invention, and - Fig. 15 illustrates the use of holes in the various elements in order to define the circulation in the various chambers. 30 Figs. 1 and 2 show a stack 1 of electrolytic cells according to the invention. In the example described, this stack comprises seven electrolytic cells 10 separated by six bipolar electrodes 4 and, at the ends, two end electrodes 4a.

WO 2012/104819 PCT/IB2012/050508 12 Each of the cells comprises, as shown in Figs. 3, 4a to 41, at least one ion exchange membrane 11, on either side of which porous plates 12 are arranged, each surrounded by a frame 13. The two porous plates 12 can be of different sizes, as illustrated in Figs. 4e and 4g, and one can be larger than the other so that it will be supported on the 5 frame around the other porous plate during installation of the stack, so as to guarantee good mechanical protection of the ion-exchange membrane, avoiding any shearing effect, as illustrated in Fig. 6. As the frames 13 have a shape corresponding to the associated porous plate, they consequently each have a different shape, as illustrated in Figs. 4d and 4h. The largest porous plate can either be on the cathode side or on the anode side. Each 10 frame 13 allows positioning of the corresponding porous plate. It provides mechanical protection. The frames 13 can be made of titanium, of plastic, for example Nylon, Teflon, PFA, PEHD, or of epoxy. On either side of the porous plates, grids 14 are arranged, which can be of identical size and shape, as in the example described. Each grid can define, at least 15 partially, on the one hand the anionic chamber and on the other hand the cathodic chamber. The grids 14 can be made of titanium. Each grid 14 is surrounded by a gasket 15. The same gasket is used for each cathodic and anodic chamber but arranged in an opposite direction, to avoid mixing of the electrolytes circulating in the anodic chamber and circulating in the cathodic chamber. In 20 the example described, the gasket 15 is serrated so that it crushes easily, for absorbing manufacturing deviations on the thickness of the stack, said deviations being due to the manufacturing tolerance for each component of the stack. The grids 14 can each comprise lugs 14a configured to project into circulating ducts 15a provided in the gasket 15. These lugs 14a provide support on the ion-exchange 25 membrane during clamping of the stack and can improve the hermeticity at this level, which means that mixing of the gases produced can be avoided. Finally, a bipolar plate 4 will close the anodic chamber of a first electrolytic cell and the cathodic chamber of a second, adjacent electrolytic cell. It can be made of titanium. The bipolar plate defines, together with the grids 14 and the porous plates, a 30 bipolar electrode 15, constituting on the one hand the anode of the first electrolytic cell and on the other hand the cathode of the second electrolytic cell, and it separates the ion exchange membrane of the first electrolytic cell from that of the second electrolytic cell.

WO 2012/104819 PCT/IB2012/050508 13 The bipolar electrode 15, in other words the two grids, the two porous plates as well as the bipolar component, constitutes an assembly of five components as illustrated in Fig. 5, and can be made all in one piece, for example of titanium, by diffusion bonding. For this purpose, the five components are placed in a mold, they are pressed to hold them 5 in position, and are then heated to a high temperature, for example of the order of 1500'C. Then bonding spots appear on the titanium, so that a bipolar electrode can be obtained all in one piece. Examples of values for the different thicknesses of an electrolytic cell are given below: 10 - porous plate and frame: 0.5 to 0.6 mm, - grid: 1.25 mm, which makes it possible to promote circulation of the electrolyte and avoid hydraulic head losses, - gasket: 1.5 mm, which is to reach 1.25 mm on crushing, - bipolar plate: 0.5 to 0.6 mm, 15 - ion-exchange membrane: 0.2 to 0.5 mm. We thus obtain a total cell thickness for example between 4.2 and 4.8 mm. The surface area of the ion-exchange membrane can be of the order of 1000 cm 2 in total. The active part, i.e. the part providing the electrochemical reaction, may only be of the order of half, for example 500 cm 2 . A portion of the surface of the 20 membrane can be used as gasket, being the same size as the frames associated with the porous plates. The electrolyzer comprises catalysts of the electrochemical reaction. These catalysts are preferably arranged between the ion-exchange membrane and the porous plates. The catalysts are preferably deposited on the ion-exchange membrane rather than 25 on the porous plates. In one embodiment, the catalysts comprise on the one hand a catalyst deposited on the ion-exchange membrane, and on the other hand a thin layer deposited on the porous plates forming the anode and/or the cathode, as described above. In this case, the porous plates comprise a thin layer of a catalytic material on their face adjacent to the 30 ion-exchange membrane. The ion-exchange membrane can comprise two layers of catalysts, one on each side, in the case when the cell comprises a single ion-exchange membrane.

WO 2012/104819 PCT/IB2012/050508 14 In one embodiment, each ion-exchange membrane comprises a single layer of catalyst, in the case when the cell comprises two ion-exchange membranes. In a practical example of the invention, the catalysts comprise on the one hand platinum on the (or one of the) ion-exchange membrane(s) on the hydrogen production 5 side and on the other hand IrO 2 on the (or the other of the) ion-exchange membrane(s) on the dioxygen production side. In a practical example of the invention, the catalysts comprise on the one hand platinum on the (or one of the) ion-exchange membrane(s) on the cathode side and on the other hand IrO 2 on the (or the other of the) ion-exchange membrane(s) on the anode side. 10 An example of a method of depositing the catalysts on the ion-exchange membrane will now be described. Masks the same size as the porous plates are used, and are arranged on the ion exchange membrane so that catalyst is only deposited on the portion of the ion-exchange membrane that is intended to be covered by the porous plate. 15 Deposition of IrO 2 is carried out by mixing the latter in the form of powder with ethanol and a liquid proton conductor, used as adhesive, such as Nafion@ or activated boron nitride mixed with PTFE. The liquid obtained can be placed in a sonotrode for breaking up the granules and is then sprayed on one face of the membrane. The membrane can be heated immediately after spraying or during spraying to a temperature of the order 20 of 50'C to facilitate evaporation of the ethanol present in the mixture. The same procedure can be used with platinum by adding activated carbon to the mixture. In one embodiment, catalysts deposited on the porous plates are used. In one practical example, 1 mg/cm 2 of platinum and 2 mg/cm 2 of iridium oxide are deposited. 25 In the example considered, the cells are moreover assembled together and are held clamped between end shields 2a and 2b by flexible washers 5. In one practical example, these washers are not flat, forming a spring, allowing adjustment of the pressure to which the stack of electrolytic cells is subjected, so as to provide a roughly constant resultant pressure. This pressure can be for example of the order of 100 bar. A stack of 30 washers can be used so as to increase the stiffness constant. Clamping of the stack of electrolytic cells can be done in a controlled manner, by calculating the appropriate tightening torque.

WO 2012/104819 PCT/IB2012/050508 15 The stack can comprise bipolar plates 4 that are all identical. The bipolar plates can notably all be flat. As a variant, the stack can comprise flat bipolar plates 4 arranged between the ion-exchange membranes and two bipolar plates 4a of a different shape at each end, 5 otherwise called anode collector and cathode collector. These can be configured so that they each make contact with a copper component 7 having a sleeve 8 intended to fit into a central hole in the corresponding end shield 2a, 2b, so as to allow power supply to the stack. The copper component 7 can be isolated from the end shield by a seal (not shown) and can be surrounded by a gasket 9 to ensure hermeticity and stress distribution. The 10 sleeve 8 can be surrounded by a Teflon@ insert to protect the electrical feed. The electrolyzer further comprises hydraulic connectors defining two inlets and two outlets, more precisely an inlet 3a and an outlet 3b for a cathodic end chamber and an inlet 3c and an outlet 3d for an anodic end chamber, the cathodic and anodic chambers between two successive cells communicating with one another. Each hydraulic 15 connector can comprise an intermediate insert, for example made of titanium. The front end shield 2a, also shown in Fig. 4k, houses the electrolyte inlets and outlets, i.e. more precisely the electrolyte inlet 3a on the dioxygen production side, the outlet for electrolyte laden with dioxygen 3b, as well as the electrolyte inlet on the dihydrogen production side 3c, and finally the outlet for electrolyte laden with dihydrogen 20 3d. The electrolyte circulates in the electrolyzer between the electrolytic cells depending on the shape of the gaskets 15 arranged around the grids. We shall now describe, referring to Figs. 7a, 7b and 8, an electrolytic assembly 20 comprising the electrolyzer described above, as well as a reservoir 21 of dihydrogen for 25 supplying the dihydrogen obtained, and a reservoir 22 of dioxygen for supplying the dioxygen obtained. In the example considered, the cross-sectional area of the dihydrogen reservoir is twice the cross-sectional area of the dioxygen reservoir, but it could be otherwise. The circulation of the electrolyte is controlled by solenoid valves V 1 , V 2 , V 3 , 30 and V 4 and circulation is provided by pumps Pi and P 2 , for example in "all or nothing". The electrolyte used in a practical example can be demineralized water with 10 wt% of H 2 SO4.

WO 2012/104819 PCT/IB2012/050508 16 The electrolyte is stored in each of the dihydrogen reservoir and the dioxygen reservoir, the assembly comprising fluid communication between the two reservoirs, at their base, controlled by a transfer valve EV 1 , for maintaining equilibrium of the electrolyte level and of the level of acidity in the two reservoirs. 5 Taking into account the reactions for production of dihydrogen and dioxygen, there is a flux of H 3 0' ions through the membrane. Moreover, taking into account that the membrane is able to conduct both anions and cations, there is an opposite flux of sulfate ions through the membrane. In that way, the concentration of sulfate ions decreases on the dihydrogen production side and increases on the dioxygen production side. In fact, the 10 ion-exchange membrane containing boron nitride is a nonselective ionic membrane, in contrast to a membrane comprising Nafion@. It thus permits circulation of the sulfate ions through the membrane, so that their concentration can vary in the chambers on either side of the membrane. It is therefore desirable to regulate this concentration. For this purpose, the assembly comprises a transfer valve EV 1 , a water feed pump P 3 as well as dischargers 15 DEVI and DEV2 on the dihydrogen outlet side and dioxygen outlet side respectively. The level of electrolyte in the dioxygen and dihydrogen reservoirs is, in normal operation, maintained between a high level and a low level. Thus, while the electrolyte level is maintained between these high and low levels, the feed pump P 3 and the transfer valve EV 1 remain inactive, as illustrated in Fig. 10. 20 When the level of electrolyte in the dihydrogen reservoir exceeds the high level as a result of the natural transfer of water through the membranes in normal operation of the electrolyzer, opening of the electrotransfer valve EV 1 and pressure control by the dihydrogen gas outlet discharger DEVI makes it possible to rebalance the levels to reach either the high level of the oxygen reservoir, as illustrated in Fig. 11 a, or the low 25 level of the dihydrogen reservoir, as illustrated in Fig. 1 1b, depending on the amount of electrolyte remaining in the assembly. When the levels of electrolyte in both reservoirs are low because all the available water has been consumed, the feed pump P 3 switches on and fills the dioxygen reservoir with water, as illustrated in Fig. 12a, from a demineralized water reservoir 23. 30 It should be noted that during normal operation of the electrolyzer, an imbalance of concentration of acid between the dihydrogen reservoir and the dioxygen reservoir is induced by the actual operation of the membranes. The dioxygen reservoir, WO 2012/104819 PCT/IB2012/050508 17 anionic side, recovers almost the majority of the sulfate ions SO 4 2- and the hydrogen 2 reservoir is depleted of sulfate ions SO 4 -. After filling the dioxygen reservoir, a two-way transfer between the two reservoirs makes it possible to rebalance the concentrations of sulfate ions SO42- between the two reservoirs, as shown in Figs. 12b and 12c. The cycle 5 therefore depends on the amount of water consumed between the high level and the low level. The same adjustment can be applied when the electrolyte used is low, except that the transfers are reversed. The assembly can also comprise two condensers 24 for recovering the water vapor essentially and electrolyte optionally, which may be released from the reservoirs. 10 The outlet pressure of the reservoirs 21, 22 can be controlled by means of the dischargers DEV 1 and DEV 2 . The pressures can be adjusted so as to have a maximum pressure on the dihydrogen production side, wherein the pressure difference can be positive and can be up to 10 bar on the dihydrogen production side. For this purpose, it is possible to use a porous plate of larger size on the dioxygen production side and of smaller 15 size on the other side, taking into account the pressure difference. This can make it possible advantageously to obtain hydrogen of greater purity for a longer time. Of course, reversing the size of the porous plates is still within the scope of the present invention. The user can select the operating pressure. The pressure in the reservoirs can 20 be controlled by a loop composed of two pressure sensors PH 2 and P0 2 and the two dischargers DEV 1 and DEV 2 . The control loop makes it possible to regulate the gas flow to adjust the pressure in the reservoirs. Each of the reservoirs 21, 22 can further comprise a safety valve 25 and an opening 26 for initial filling of the reservoirs. 25 The outlet of the reservoirs 21, 22 is also equipped with a dryer 27 for removing residual water, which could for example have been produced by combustion of dihydrogen with the residual dioxygen. It is then possible to measure, at 28, the flow rates of dihydrogen and of dioxygen obtained as well as provide sensors at 29 for verifying the purity of the gases 30 obtained. The assembly can further comprise sensors 30 of the temperatures of the electrolyte in each of the reservoirs on the one hand, and in the electrolytic cells WO 2012/104819 PCT/IB2012/050508 18 themselves on the other hand, so as to control the temperature of the electrolyte and maintain a roughly constant operating temperature, for example at a value between 70 and 120'C. Maintaining a high enough operating temperature can promote the electrochemical reaction, regardless of the pressure selected. However, a temperature limit, above which 5 there is a risk of deterioration of the assembly, must not be exceeded. If necessary, the assembly can also comprise at least one, or even two devices for cooling the electrolyte before it enters the electrolyzer, optionally equipped with a temperature sensor for monitoring the effectiveness of cooling. The assembly comprises, in the example described, two cooling devices 50, 10 each for cooling the electrolyte received from the dihydrogen and dioxygen reservoirs. In the example described, each cooling device 50 comprises three elements: cooling pump 51, liquid-liquid heat exchanger 52 receiving the hot electrolyte from the reservoirs and air-liquid heat exchanger 53, as illustrated in Fig. 9a. The cooling device can thus comprise two operating levels, as illustrated in 15 Fig. 9b. In a first level 55, cooling is effected by operating the cooling pump only, to circulate the electrolyte in the air-liquid exchanger before returning it cooled to the stack of cells. In a second level 56, the cooling pump and the fan of the air-liquid exchanger can be operated simultaneously. Finally, if this is not sufficient, the system is configured to reduce the current automatically at 57. The temperature thresholds determining the levels 20 used can be determined appropriately as a function of the operating temperature desired for the assembly. The thresholds indicated in Fig. 9b are in particular only a guide. Finally, the electric power supply of the electrolyzer is housed in an electric cabinet 40 comprising a process control computer for controlling the current intensity and voltage supplied to the stack of electrolytic cells starting from the mains current and 25 voltage. The control cabinet can also be equipped with a remote connection 41 allowing remote maintenance of the assembly. In the example that has just been described, the power used for producing the dihydrogen and dioxygen is 5 kW. Of course, when the power consumed is different, and 30 the assembly is of larger or smaller size, this is still within the scope of the present invention. As an example, Fig. 7c shows an assembly configured for a power consumption of 1 kW.

WO 2012/104819 PCT/IB2012/050508 19 The invention is not limited to the production of dihydrogen and dioxygen. The invention applies to the production of other substances and in particular hypochlorous acid. The invention is not limited to the presence of one exchange membrane per 5 cell. Thus, according to one aspect of the invention, the cell comprises at least two exchange membranes 11 between the anode and the cathode, defining an intermediate chamber I. An example of such a cell is shown in Fig. 13. 10 In such a cell, two membranes 11 are arranged between the anode and the cathode of the cell, which can further comprise all the elements described above. Thus, the cell can comprise the stack illustrated in Fig. 3, except that two membranes 11 are used instead of the single membrane 11 and they are separated by a frame so as to define the intermediate chamber I. Moreover, additional fluid 15 communications may be provided. For example, an inlet 3g and an outlet 3f are added to allow circulating the electrolyte in the intermediate chamber I as illustrated in Figs. 14 and 15. The electrolyzer can thus have three inlets and three outlets in the embodiment with replacement of the single membrane per cell with two membranes defining an 20 additional circulating chamber I. In this case, a third reservoir (not shown) can be provided for receiving the electrolyte. Circulation of the electrolyte in the anodic A, cathodic C and intermediate I chambers can take place as illustrated in Fig. 14. In the case of production of hypochlorous acid, for example water is circulated 25 in the anodic chamber or chambers, for example water in the cathodic chamber or chambers, for example brine (for example water/NaCl) in the intermediate chamber or chambers I, and hypochlorous acid is recovered in the intermediate chamber or chambers and soda in the cathodic chamber or chambers.

Claims

1. An electrolyzer for the production of at least one chemical substance, such as dihydrogen, dioxygen, chlorine or hypochlorous acid, or soda, by electrolysis of pure 5 water or of water containing at least one salt, base and/or acid such as NaCl, H

2 SO 4 , KOH or NaOH, comprising a stack of at least a first and a second consecutive electrolytic cell, each electrolytic cell (10) comprising: - an anode, - a cathode, 10 - at least one ion-exchange membrane (11) arranged between the anode and the cathode, the ion-exchange membranes (11) of the first and second electrolytic cell being separated by a bipolar electrode (15) constituting on the one hand the anode of the first electrolytic cell and on the other hand the cathode of the second electrolytic cell. 15 2. The electrolyzer as claimed in the preceding claim, in which the bipolar electrode (15) comprises a bipolar plate (4) all in one piece, the bipolar plate being associated if necessary with at least one grid (14) and with at least one porous plate (12).

3. The electrolyzer as claimed in one of the preceding claims, in which the bipolar electrode (15) is entirely in one piece, the bipolar plate (4), the grids (14) and the 20 porous plates (12) being integral with one another prior to installation in the electrolyzer.

4. The electrolyzer as claimed in any one of the preceding claims, in which the bipolar electrode (15) comprises at least one of the following materials: nickel, iridium, ruthenium, palladium, cadmium, molybdenum, platinum, stainless steel, titanium, tantalum, iron alloy, nickel alloy, lead alloy, and/or a thin layer of tantalum oxide, iridium 25 oxide, ruthenium oxide, lead oxide, ferric oxide, platinum, platinum carbon, palladium, nickel, cadmium, and/or molybdenum.

5. The electrolyzer as claimed in any one of the preceding claims, in which the anode comprises at least one of the following materials: titanium, tantalum, iridium, iron alloy, lead alloy, and/or a thin layer of tantalum oxide, iridium oxide, ruthenium 30 oxide, lead oxide, and/or ferric oxide.

6. The electrolyzer as claimed in any one of the preceding claims, in which the cathode comprises at least one of the following materials: nickel, iridium, palladium, WO 2012/104819 PCT/IB2012/050508 21 cadmium, molybdenum, platinum, titanium, tantalum, iron alloy, lead alloy, nickel alloy and/or a thin layer of platinum, platinum carbon, palladium, nickel, cadmium, and/or molybdenum.

7. The electrolyzer as claimed in any one of the preceding claims, in which 5 the ion-exchange membrane (11) comprises boron nitride, notably activated boron nitride.

8. The electrolyzer as claimed in any one of the preceding claims, in which the ion-exchange membrane (11) comprises a polymer matrix, notably based on PTFE.

9. The electrolyzer as claimed in any one of the preceding claims, in which the ion-exchange membrane (11) has a thickness between 100 and 500 P m.

10 10. The electrolyzer as claimed in the preceding claim, the electrolyte comprising water and acid, water and a base or water and a salt.

11. The electrolyzer as claimed in any one of the preceding claims, one cell comprising a single ion-exchange membrane between the anode and the cathode.

12. The electrolyzer as claimed in any one of claims 1 to 8, at least one cell of 15 the stack comprising two ion-exchange membranes (11), preferably two membranes providing an intermediate chamber between them (I).

13. The electrolyzer as claimed in any one of claims 1 to 8, at least one cell comprising a nonselective ion-exchange membrane such as a membrane comprising boron nitride, and a selective exchange membrane such as a membrane based on Nafion. 20

14. The electrolyzer as claimed in claim 12, comprising means for establishing a circulation of electrolyte in the intermediate chamber (I), notably with a view to withdrawing a substance produced in the electrolyzer during operation thereof.

15. An electrolytic assembly comprising: - an electrolyzer (1) as claimed in any one of the preceding claims, 25 - a reservoir, notably a dihydrogen reservoir (21) for storing the dihydrogen obtained, and - a reservoir, notably a dioxygen reservoir (22) for storing the dioxygen obtained.

16. The assembly as claimed in the preceding claim, in which the electrolyte is 30 stored in each of the two reservoirs.

17. The assembly as claimed in the preceding claim, having fluid communication between the two reservoirs, preferably at their base, more preferably WO 2012/104819 PCT/IB2012/050508 22 controlled by a transfer valve (EV 1 ), for maintaining equilibrium of the electrolyte level in the two reservoirs.

18. A method of production of hypochlorous acid, by means of the electrolyzer as claimed in claim 12, in which the anodic chamber of a cell preferably 5 contains water, the cathodic chamber contains water and the intermediate chamber contains brine.

19. An electrolyzer cell, comprising: - an anode, - a cathode, 10 - two exchange membranes (11), notably: - a selective exchange membrane and a nonselective exchange membrane (11), arranged between the anode and the cathode, preferably a nonselective membrane (11) based on activated boron nitride and preferably a selective membrane based on Nafion, the nonselective 15 membrane preferably protecting the selective membrane from a basic or acidic environment.