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ELECTROCHEMICAL PROCESS FOR THE PRODUCTION OF PRESSURIZED GASEOUS HYDROGEN BY ELECTROLYSIS THEN BY ELECTROCHEMICAL CONVERSION

20210079536 ยท 2021-03-18

    Inventors

    Cpc classification

    International classification

    Abstract

    An electrochemical process implements, in a decoupled manner, a first step of electrolysis of an electrolyte to produce gaseous oxygen in a chamber and a second step of electrochemical conversion of H+ ions into gaseous hydrogen in a chamber which contains a liquid phase and a gas phase not dissolved in the liquid phase. Gaseous hydrogen produced in the conversion step is partly present in the gaseous headspace of chamber and as bubbles in the electrolyte, and partly dissolved in the electrolyte which is saturated with hydrogen. The electrolyte has at least one redox pair (A/B) forming at least one intermediate vector enabling the decoupling of the first and second steps. The interface between the gas and liquid phases is increased during the second step to accelerate the diffusion, from liquid phase to gas phase, of the dissolved hydrogen able to supersaturate the electrolyte. Pressurized gaseous hydrogen is then collected.

    Claims

    1. Electrochemical process for the production of pressurized gaseous hydrogen, characterized in that it consists essentially of implementing, in a decoupled manner, at least one step of custom-character electrolysis of a preferably aqueous electrolyte, this electrolysis step custom-character producing gaseous oxygen in a chamber custom-character, and at least one step C of electrochemical conversion of H.sup.+ ions into gaseous hydrogen in a chamber C which is identical to or different from chamber custom-character and which contains a liquid phase L and a gas phase G not dissolved in this liquid phase; wherein: the electrolysis step custom-character involves at least one cathode on which at least one ionic species is electrochemically reduced; the gaseous hydrogen produced in the conversion step C is partly present in the gaseous headspace of chamber C and as bubbles in the electrolyte, and partly dissolved in the electrolyte which is thus saturated, or even supersaturated, with hydrogen; the electrolyte comprises at least one redox pair (A/B) forming at least one intermediate vector enabling the decoupling of steps custom-character & C, with: acidic electrolyte:
    E(A/B)<E(O.sub.2/H.sub.2O);
    |[E(A/B)E(O.sub.2/H.sub.2O)]|100 mV basic electrolyte:
    E(A/B)<E(O.sub.2/OH.sup.);
    |[E(A/B)E(O.sub.2/OH.sup.)]|100 mV the interface between the undissolved gas phase G and the liquid phase Lhereinafter referred to as the G/L interfaceis increased at least during step C, so as to accelerate the diffusion, from the liquid phase to the gas phase, of the dissolved hydrogen able to supersaturate the electrolyte; and the pressurized gaseous hydrogen is collected in a storage tank.

    2. Process according to claim 1, wherein the increase of the G/L interface is carried out by implementing at least one of the following operations: (i) forced circulation which preferably consists of generating an electrolyte flow in chamber custom-characterC or C, more preferably using at least one pump so as to evacuate and replenish the gas bubbles present on the electrode or electrodes or the catalyst(s) and on any roughnesses of chamber custom-characterC or C; (ii) at least partial substitution of the dissolved hydrogen by at least one neutral gas, by injection of the latter into chamber custom-characterC or C to generate bubbles of neutral gas intended to evacuate and replenish the gas bubbles present on the electrode or electrodes, on any catalyst(s) not dissolved in the electrolyte, and/or on any roughnesses of chamber custom-characterC or C; (iii) partial decompression, which consists of isolating the storage tank from chamber custom-characterC or C, in order to increase the pressure of gaseous hydrogen in the chamber; then when this pressure is greater than that in the storage tank, a decompression of chamber custom-characterC or C is performed, which creates or increases the supersaturation of H.sub.2 in the electrolyte and therefore promotes the formation of bubbles; (iv) spraying the electrolyte as droplets into the gaseous headspace of chamber custom-characterC or C, (v) at least one heating, preferably at least one localized heating, of the electrolyte, which advantageously consists of locally reducing the solubility of the dissolved gaseous hydrogen, thus promoting the nucleation of bubbles, (vi) subjecting the electrolyte to ultrasound to generate bubbles, (vii) at least one depolarization, preferably at least one localized depolarization of the electrolyte, which preferably consists of accelerating the kinetics of the hydrogen formation reaction in order to locally increase supersaturation and promote the formation of bubbles, (viii) making use of nanoparticles and/or at least one porous nucleation material in the electrolyte, to promote the nucleation of bubbles and increase the number of bubbles nucleation sites, (ix) mechanical stirring of the electrolyte, which promotes the nucleation of bubbles by providing the energy necessary to counteract the surface tension.

    3. Process according to claim 1, wherein: With an acidic electrolyte:
    E.sub.th(A/B)<E.sub.th(H.sup.+/H.sub.2);
    Advantageously, |[E.sub.th(A/B)E.sub.th(H.sup.+/H.sub.2)]|100 mV With a basic electrolyte:
    E.sub.th(A/B)<E.sub.th(H.sub.2O/H.sub.2);
    Advantageously, |[E.sub.th(A/B)E.sub.th(H.sub.2O/H.sub.2)]|100 mV

    4. Process according to claim 3 characterized by a redox pair (A/B) wherein: A is composed of at least one metal ion of metal M; B is composed of at least metal M; with M preferably chosen from the metals, and more preferably from the group comprisingideally composed of: Zn; Cd; Sn, Ni, Mn, Fe, Pb, Co; Zn being particularly preferred.

    5. Process according to claim 3 characterized by a redox pair (A/B) wherein: A is composed of at least one ion I.sup.A whose number of valence electrons is V1; B is composed of at least one ion I.sup.B whose number of valence electrons V2<V1; with I preferably chosen from the ions resulting from atoms chosen from the group comprisingideally composed of: Fe; U; Cr; S; V; iron and vanadium being particularly preferred.

    6. Process according to claim 4, wherein: M(i). the electrolysis step custom-character and the conversion step C are carried out in a same chamber custom-characterC containing electrolyte in which are immersed at least three electrodes, namely at least one cathode on which the reduced metal M is deposited during the electrolysis step custom-character, at least one anode in the vicinity of which is produced gaseous oxygen resulting from the oxidation of water during the electrolysis step custom-character, and at least one hydrogen electrode inactive during the electrolysis step custom-character in the vicinity of which is produced the gaseous hydrogen resulting from reduction of the H.sup.+ ions of the electrolyte during the conversion step C; M(ii). during the electrolysis step custom-character, a power supply connected to the cathode and to the anode delivers an electric current, such that the metal M is deposited on the cathode and gaseous oxygen is released at the anode; M(iii). for the implementation of the conversion step C: the chamber custom-characterC is hermetically sealed; the cathode of step custom-character, which becomes the anode of step C, is connected to the hydrogen electrode by an electrical conductor so as to function as a battery being discharged, such that the metal M is dissolved in the electrolyte at the anode of C and gaseous hydrogen is released and compressed in the gaseous headspace of the sealed chamber custom-characterC; and means for increasing the G/L interface are put into operation to promote the transformation of the dissolved gas in the electrolyte, in particular hydrogen, into undissolved gas.

    7. Process according to claim 1, wherein: With an acidic electrolyte:
    E.sub.th(A/B)>E.sub.th(H.sup.+/H.sub.2);
    Advantageously, |[E.sub.th(A/B)E.sub.th(H.sup.+/H.sub.2)]|100 mV With a basic electrolyte:
    E.sub.th(A/B)>E.sub.th(H.sub.2O/H.sub.2);
    Advantageously, |[E.sub.th(A/B)E.sub.th(H.sub.2O/H.sub.2)]|100 mV

    8. Process according to claim 6 characterized by a redox pair (A/B) wherein: A is composed of at least one metal ion of metal M; B is composed of at least metal M; with M preferably chosen from the metals, and more preferably still from the group comprisingideally composed of: Cu, Mn, Ag; Cu being particularly preferred.

    9. Process according to claim 6 characterized by a redox pair (A/B) wherein: A is composed of at least one ion I.sup.A whose number of valence electrons is V1; B is composed of at least one ion I.sup.B whose number of valence electrons V2<V1; with I preferably chosen from the ions resulting from the atoms chosen from the group comprisingideally composed of: Fe, V, Mn; iron and vanadium; iron and vanadium being particularly preferred.

    10. Process according to claim 8, wherein the electrolysis step custom-character and the conversion step C are carried out in one and the same chamber custom-characterC; the chamber custom-characterC comprises: at least one electrochemical compartment (J) containing an electrolyte including the redox pair (A/B), and at least one electrochemical compartment (K) containing an electrolyte in which at least one hydrogen electrode is immersed, these two electrochemical compartments being separated by at least one ion-exchange membrane; at least one anode; at least one cathode; during the electrolysis step custom-character, a power supply connected to the cathode and to the anode delivers an electric current, such that the A ions are reduced to B at the cathode and such that the oxidation of the water leads to a release of gaseous oxygen at the anode; for the implementation of the conversion step C: the chamber is custom-characterC is hermetically sealed, the cathode of step custom-character, which becomes the anode of step C, is connected to the hydrogen electrode by a power supply which delivers an electric current so that the reducing agents B are oxidized to ions A in compartment (J) concurrently with a reduction of the H.sup.+ ions contained in compartment (K) to gaseous hydrogen, which is released and compressed in the gaseous headspace of the sealed chamber custom-characterC, and means for increasing the G/L interface are put into operation to promote the transformation of dissolved gas in the electrolyte, in particular hydrogen, into undissolved gas.

    11. Device for implementing the process according to claim 6, wherein it comprises: a) at least one sealed chamber custom-character C intended to contain at least one electrolyte; b) at least one cathode intended to be immersed in the electrolyte; c) at least one anode intended to be immersed in the electrolyte; d) at least one hydrogen electrode intended to be immersed in the electrolyte; e) a power supply intended to connect the cathode to the anode; f) an electrical conductor intended to connect the cathode to the hydrogen electrode; g) and means for increasing the G/L interface; h) possibly means for circulating the electrolyte or electrolytes in the chamber custom-characterC; i) possibly means for regulating the temperature of the electrolyte or electrolytes in the chamber custom-character C.

    12. Device for implementing the process according to claim 3, wherein it comprises: aa. at least one sealed chamber custom-character C comprising: at least one electrochemical compartment (J) intended to contain a catholyte including the redox pair (A/B), and at least one electrochemical compartment (K) intended to contain an anolyte, and at least one ion-exchange membrane, preferably cationic, separating these two compartments; ab. at least one cathode of step custom-character intended to be immersed in the catholyte; ac. at least one anode of step custom-character intended to be immersed in the anolyte; ad. at least one hydrogen electrode intended to be immersed in the anolyte; ae. at least one power supply intended to connect the cathode of step custom-character to the anode (d) of step custom-character; af. at least one electrical conductor intended to connect the cathode of step custom-character, which has become the anode of step C, to the hydrogen electrode; ag. means for increasing the G/L interface; ah. possibly means for circulating the electrolyte or electrolytes in the chamber custom-characterC; ai. and possibly means for heating and/or cooling the electrolyte or electrolytes in the chamber custom-characterC.

    13. Device for implementing the process according to claim 3, wherein it comprises: a.2. at least one chamber custom-character comprising: at least one electrochemical compartment (J) intended to contain a catholyte including the redox pair (A/B), and at least one electrochemical compartment (K) intended to contain an anolyte, at least one membrane, preferably cationic, separating these two compartments; b.2. at least one chamber C, Identical to or different from chamber custom-character, comprising: at least one catalyst; c.2. at least one cathode of step custom-character intended to be immersed in the catholyte; d.2. at least one anode of step custom-character intended to be immersed in the anolyte; e.2. a power supply intended to connect the cathode of step custom-character to the anode of step custom-character; f.2. means for increasing the G/L interface; g.2. means for circulating the electrolyte or electrolytes between chambers custom-character and C; h.2. and possibly means for heating and/or cooling the electrolyte or electrolytes in chambers custom-character and C.

    14. Device for implementing the process according to claim 7, wherein it comprises: aa. at least one sealed chamber custom-character C comprising: at least one electrochemical compartment (J) intended to contain an electrolyte including the redox pair (A/B), and at least one electrochemical compartment (K) intended to contain an electrolyte, at least one membrane, preferably cationic, separating these two compartments; ab. at least one cathode of step custom-character intended to be immersed in the electrolyte of compartment (J); ac. at least one anode of step custom-character intended to be immersed in the electrolyte of compartment (J) or of compartment (K); ad. at least one hydrogen electrode intended to be immersed in the electrolyte of compartment (K); ae. at least one power supply intended to connect the cathode of step custom-character to the anode (a) of step custom-character; af. at least one power supply intended to connect the cathode of step custom-character, which has become the anode of step C, to the hydrogen electrode; ag. means for increasing the G/L interface; ah. possibly means for circulating the electrolyte or electrolytes in chambers custom-character and C; ai. and possibly means for regulating the temperature of the electrolyte or electrolytes in chambers custom-character and C.

    15. A kit for implementing the process according to claim 1, wherein the kit includes: a device comprising: a) at least one sealed chamber custom-character C intended to contain at least one electrolyte; b) at least one cathode intended to be immersed in the electrolyte: c) at least one anode intended to be immersed in the electrolyte; d) at least one hydrogen electrode intended to be immersed in the electrolyte: e) a power supply intended to connect the cathode to the anode; f) an electrical conductor intended to connect the cathode to the hydrogen electrode: g) and means for increasing the G/L interface; h) possibly means for circulating the electrolyte or electrolytes in the chamber custom-characterC i) possibly means for regulating the temperature of the electrolyte or electrolytes in the chamber custom-character C. and at least some of the components for the preparation of the electrolyte intended to be contained in the chamber of the device.

    16. A kit for implementing the process according to claim 3, wherein the kit includes: a second device comprising: i) at least one sealed chamber custom-character C comprising: at least one electrochemical compartment (J) intended to contain a catholyte including the redox pair (A/B), and at least one electrochemical compartment (K) intended to contain an anolyte, and at least one ion-exchange membrane, preferably cationic, separating these two compartments; ii) at least one cathode of step custom-character intended to be immersed in the catholyte; iii) at least one anode of step custom-character intended to be immersed in the anolyte; iv) at least one hydrogen electrode intended to be immersed in the anolyte; v) at least one power supply intended to connect the cathode of step custom-character to the anode (d) of step custom-character; vi) at least one electrical conductor intended to connect the cathode of step custom-character, which has become the anode of step C, to the hydrogen electrode; vii) means for increasing the G/L interface; viii) possibly means for circulating the electrolyte or electrolytes in the chamber custom-characterC; and ix) possibly means for heating and/or cooling the electrolyte or electrolytes in the chamber custom-characterC; and at least some of the components for the preparation of the catholyte and anolyte intended to be contained in the two electrochemical cells of the chamber of the second device; or a third device comprising: aa) at least one chamber custom-character comprising: at least one electrochemical compartment (J) intended to contain a catholyte including the redox pair (A/B), and at least one electrochemical compartment (K) intended to contain an anolyte, at least one membrane, preferably cationic, separating these two compartments; bb) at least one chamber C, identical to or different from chamber custom-character, comprising: at least one catalyst; cc) at least one cathode of step custom-character intended to be immersed in the catholyte; dd) at least one anode of step custom-character intended to be immersed in the anolyte; ee) a power supply intended to connect the cathode of step custom-character to the anode of step custom-character; ff) means for increasing the G/L interface; gg) means for circulating the electrolyte or electrolytes between chambers custom-character and C; and hh) possibly means for heating and/or cooling the electrolyte or electrolytes in chambers custom-character and C; and at least some of the components for the preparation of the catholyte and anolyte intended to be contained in the two electrochemical cells of the chamber of the third device.

    17. A kit for implementing the process according to claim 7, wherein the kit includes: a fourth device comprising: ai) at least one sealed chamber custom-character C comprising: at least one electrochemical compartment (J) intended to contain an electrolyte including the redox pair (A/B), and at least one electrochemical compartment (K) intended to contain an electrolyte, at least one membrane, preferably cationic, separating these two compartments; bi) at least one cathode of step custom-character intended to be immersed in the electrolyte of compartment (J); ci) at least one anode of step custom-character intended to be immersed in the electrolyte of compartment (J) or of compartment (K); di) at least one hydrogen electrode intended to be immersed in the electrolyte of compartment (K); ei) at least one power supply intended to connect the cathode of step custom-character to the anode (a) of step custom-character; fi) at least one power supply intended to connect the cathode of step custom-character, which has become the anode of step C, to the hydrogen electrode; gi) means for increasing the G/L interface; hi) possibly means for circulating the electrolyte or electrolytes in chambers custom-character and C; ii) and possibly means for regulating the temperature of the electrolyte or electrolytes in chambers custom-character and C; and at least some of the components for the preparation of the catholyte and anolyte intended to be contained in the two electrochemical cells of the chamber of the fourth device.

    Description

    EXAMPLES

    [0344] The following examples illustrate variants No. 1 & No. 2 of the first (F1) and second (F2) embodiments of the process according to the invention.

    [0345] The description of these examples is made with reference to the appended figures, in which:

    [0346] FIGS. F.1; F1.1; F1.2; F1.2.1; F1.2.2; F1.2.2.1; F1.2.2.2; F2; F2.1; F2.2; are schematic representations of some of the possible variations of the embodiments of the process according to the invention and of the embodiments of the device according to the invention;

    [0347] FIG. 1A is a schematic representation of the device used in the electrolysis step custom-character of Example 1;

    [0348] FIG. 1B is a schematic representation of the device used in the electrochemical conversion step C of Example 1; custom-character

    [0349] FIG. 2A is a schematic representation of the device used in the electrolysis step custom-character of Example 2;

    [0350] FIG. 2B is a schematic representation of the device used in the electrochemical conversion step C of Example 2.

    [0351] FIG. 3A is a schematic representation of the device used in the electrolysis step custom-character of Example 3;

    [0352] FIG. 3B is a schematic representation of the device used in the electrochemical conversion step C of Example 3.

    [0353] Summary table corresponding to figures F.1; F1.1; F1.2; F1.2.1; F1.2.2; F1.2.2.1; F1.2.2.2; F2; F2.1: F2.2

    TABLE-US-00001 F1: an electrolysis step and a step of operating as a battery F2: Two electrolysis steps F1.1: A is an ion/B is a metal F1.1: A is an ion/B is a metal (preferably a single chamber) (preferably a single chamber) F1.2: A & B are ions: F1.2: A & B are ions F1.2.1: H2 electrode (preferably a single chamber) (preferably a single chamber) F1.2.2: catalyst F1.2.2.1: a single chamber F1.2.2.2: two chambers

    [0354] Legend for figures F.1; F1.1; F1.2; F1.2.1; F1.2.2; F1.2.2.1; F1.2.2.2; F2; F2.1; F2.2: [0355] (c): cathode [0356] (H.sub.2): hydrogen electrode [0357] (a): anode [0358] Mb: membrane [0359] Ct: catalyst [0360] Milieu acide: Acid medium [0361] Milieu basique: Basic medium [0362] tape: Step

    Example 1: 1st Embodiment/Variant No. 1: F1.1

    [0363] In the following example, hydrogen was produced at 200 bar in the device shown in FIG. 1A. This device is an electrochemical reactor composed of a sealed chamber 1 in which are three electrodes 3,4,5 bathed in an acidic aqueous solution (electrolyte) 2. The three electrodes, which have a surface area of 1 m.sup.2, are as follows: [0364] An electrode on which the metal is deposited (cathode), of aluminum 3; [0365] An electrode on which oxygen is released (anode), of lead-silver-calcium alloy 4; [0366] An electrode on which hydrogen is released (hydrogen electrode), of platinum 5.

    [0367] The electrolyte 2 is composed of zinc ions (concentration 1.5 mol/L) and sulfuric acid (2.55 mol/L). It is prepared by mixing 67 kg of sulfuric acid (37.5%, Brenntag) in 28.6 L of deionized water, then adding 45 kg of zinc sulfate (ZnSO.sub.4,7H.sub.2O) (97.5%, Platret) to this mixture. The reactor 1 is equipped with heating means 6 consisting of exchangers and which enable maintaining the temperature of the electrolyte 2 at 30 C. The reactor 1 is provided with a gas outlet pipe 8, this pipe being subdivided into a pipe 9 for discharging the gaseous oxygen and a pipe 10 for discharging the gaseous hydrogen. Each pipe 9,10 is equipped with a valve 12, 11, respectively the O.sub.2 valve and H.sub.2 valve, enabling independent extraction of these two gases from the high pressure chamber 1.

    [0368] During the first step custom-character (FIG. 1A), a power supply 7 connected to the cathode 3 and to the anode 4 provides a current density of 595 A/m.sup.2 for 5 h, which makes it possible to deposit 3267 g of zinc on the cathode (with a Faraday efficiency of 90%).

    [0369] The second step C is a step of converting the electrochemical energy, stored in the form of zinc deposited on the cathode, into electricity (FIG. 1B). To do this, the cathode 3 is electrically connected to the hydrogen electrode 5 via an electronic load 13. The rate of hydrogen evolution is 350 mol/h/m.sup.2 and it takes 4 hr40 to produce 50 mol of hydrogen. In the absence of a device to suppress supersaturation, the supersaturation reaches a value of approximately 6. This results in a shift in the potential of the hydrogen electrode, measured using a reference electrode, of 24 mV relative to its equilibrium value, and an achieved pressure of about 85 bar, with about 70% of the produced hydrogen trapped in dissolved form. Ultrasound is then applied by a piezoelectric generator 14 in a frequency range corresponding to the appearance of the acoustic cavitation phenomenon. The supersaturation value is returned to 1, the shift in potential of the hydrogen electrode at equilibrium to 0, and the pressure then reaches about 200 bar, with the amount of trapped hydrogen in dissolved form now only about 26%.

    Example 2: With an Ion/Ion Redox Pair 1st Embodiment/Variant No. 2: F1.2

    [0370] In the example which follows, the device represented in FIG. 2A comprises a chamber Et which is an electrochemical cell 1 composed of two compartments (J, K) respectively containing a cathode 4 and an anode 5, each having a surface area of 150 cm.sup.2, and an electrolyte (catholyte 2 and anolyte 3) of 250 mL each. The two electrolytes 2 and 3 are separated by a cationic membrane 6 made of Nafion 125. The electrodes are made of carbon felt.

    [0371] A power supply 7 is connected to the cathode 4 and anode 5.

    [0372] The catholyte 2 is prepared from sodium polysulfide Na.sub.2S.9H.sub.2O (1.6 mol.Math.L.sup.1), and the anolyte 3 is a 1 mol.Math.L.sup.1 sulfuric acid solution. The catholyte is prepared by mixing 96.1 g of Na2S, 9H2O (99.99%, Sigma Aldrich) and 185 g of deionized water. The anolyte is prepared by mixing 66 g of sulfuric acid (37.5%, Brenntag) in 200 mL of deionized water.

    [0373] During the electrolysis step custom-character with oxygen production (FIG. 2A), the S.sub.4.sup.2 ions are reduced to S.sub.2.sup.2 at the cathode:


    S.sub.4.sup.2+2e.sup.=2S.sub.2.sup.2E=0.26V

    [0374] At the anode, the reduction of the water leads to the release of oxygen:


    H.sub.2O=O.sub.2+2H.sup.++2e.sup.E=1.23V

    [0375] This oxygen is discharged via a pipe 8 equipped with a valve 9.

    [0376] The power supply 7 makes it possible to apply a current of 50 mA/cm.sup.2 for 2 hr20. The Faraday efficiency is 80%, and the concentration of S.sub.4.sup.2 ions is 0.6 mol/L at the end of this step custom-character.

    [0377] At the end of this step, the catholyte is transferred via a pipe 10 equipped with a valve 11 to the chamber where the second step takes place.

    [0378] Step C (FIG. 2B) allows the production of hydrogen; the catholyte 2 is sent via a pump 13 to a chemical reactor 12 containing beads 14 of tungsten carbide. Two simultaneous reactions take place:


    2S.sub.2.sup.2=S.sub.4.sup.2+2e.sup.E=0.26V


    2H.sup.++2e.sup.=H.sub.2E=0V

    [0379] After 4 hours of electrochemical conversion catalyzed by tungsten carbide, the concentration of S.sub.4.sup.2 sulfide ions is again 1.6 mol/L, and 0.25 g of H.sub.2 is generated. With a supersaturation value of 6, this hydrogen can inflate a small 15 mL cartridge to about 110 bar, with a shift in potential of the hydrogen electrode of 24 mV, measured using a reference electrode. Alumina nanoparticles present in the reactor 12 enable accelerating the nucleation of gaseous hydrogen and reducing the supersaturation to 1. Desorption of the hydrogen then makes it possible to reach 250 bar in the cartridge.

    Example 3: 2.SUP.nd .Embodiment/Variant 2 (Ion/Ion Redox Pair) F2.2

    [0380] In the following example, hydrogen was produced at 16 bar in the device shown in FIG. 3A. This device is a sealed electrochemical reactor/chamber 1 composed of two compartments (J, K) of 1 L each. The 1st compartment J contains a carbon electrode 2 (which acts as a cathode during the 1st electrolysis step custom-character). The 2nd compartment K contains an electrode where oxygen is released during the 1st electrolysis step custom-character (anode), made of lead-silver-calcium alloy 3, and an electrode where hydrogen is released during the 2nd step C (hydrogen electrode), made of platinum 4. The surface area of each electrode is 155 cm.sup.2. A Nafion 125) 5 membrane separates the electrodes of each compartment.

    [0381] The electrolyte (catholyte) 6 contained in 1, blue in color, is composed of vanadium ions VO.sub.2.sup.+ (concentration 2 mol/L) and sulfuric acid ions (2 mol/L). It is prepared by mixing 520 g of sulfuric acid (37.5%, Brenntag) in 590 mL of deionized water, then adding 325 g of hydrated vanadium sulfate oxide (OSO.sub.4.xH.sub.2O; 99.9% Alfa Aesar) to this mixture.

    [0382] The electrolyte (anolyte) 7 contained in the second compartment K is a 2 mol/L solution of sulfuric acid. It is prepared by mixing 520 g of sulfuric acid (37.5%, Brenntag) in 590 mL of deionized water.

    [0383] During the first electrolysis step custom-character (FIG. 3A), a power supply 8 connected to the cathode 2 and anode 3 provides a current density of 300 A/m.sup.2 for 1 h.

    [0384] The following reactions occur:


    at the cathode: VO.sup.2+2H.sup.++2e.sup.=V.sup.3+H.sub.2O E=0.34V


    at the anode: H.sub.2O=O.sub.2+2H.sup.+2e.sup.E=1.23V

    [0385] The oxygen is discharged through pipe 9, controlled by valve 10.

    [0386] During the 2nd electrochemical conversion step C (FIG. 3B), the cathode 2 (which then acts as an anode) and the H.sub.2 electrode 4 are connected to the power supply 8 which supplies 300 A/m.sup.2 for 1 hour. 1.2 g of hydrogen at 16 bar are produced.

    [0387] The following reactions take place:


    At the H.sub.2 electrode 4: 2H.sup.++2e.sup.=H.sub.2E=0V


    At electrode 2: V.sup.3++H.sub.2O=VO.sup.2++2H.sup.++2e.sup.E=0.34V

    [0388] The hydrogen released alone then rises under pressure as the electrolysis proceeds. Localized heating 11 allows the temperature of the electrolyte to be increased in a localized manner, which results in desupersaturation of the electrolyte. The gaseous hydrogen formed is evacuated through pipe 12 controlled by valve 13 when the target pressure (16 bar) is reached.