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

Abstract

An electrochemical process comprises a step E.sup.l of electrolysis of an electrolyte in order to produce gaseous oxygen and a step of converting oxidation-reduction chemical energy into electrical energy with production of H.sub.2. The electrolyte comprises M.sup.m+ ions of a metal M corresponding to the redox pair (M.sup.m+/M), and A.sup.a+ ions of a depolarization additive A corresponding to a redox pair (A.sup.a+/A). Current is supplied between the anode and the cathode, A.sup.a+ and M.sup.m+ are deposited on the cathode respectively in the form of A and M during the electrolysis and gaseous oxygen is released at the anode. The supply of current between the anode and the cathode is then cut off. Depolarization occurs corresponding to the conversion step C°, with production of H.sub.2 and dissolution of M and A into M.sup.m+ and A.sup.a+ at the electrode acting as the cathode during step E.sup.l and the produced H.sub.2 is collected.

Claims

1. Electrochemical process for the production of pressurized gaseous hydrogen, characterized in that it consists essentially of implementing, in at least one chamber, at least one step E.sup.l of electrolysis of an electrolyte comprising at least one solvent, preferably aqueous, this electrolysis step E.sup.l converting electrical energy into chemical energy, with production of gaseous oxygen in a chamber E.sup.l, and at least one step C° of converting this chemical energy into oxidation-reduction energy with production of gaseous hydrogen in a closed chamber C° that is identical to or different from, preferably identical to, chamber E.sup.l; wherein: the electrolyte comprises M.sup.m+ ions, M corresponding to the redox pair (M.sup.m+/M), and A.sup.a+ ions of at least one depolarization additive A corresponding to a redox pair (A.sup.a+/A) where: the absolute value of the overvoltage of the hydrogen evolution reaction on the metal M is greater than the difference E.sub.th(H.sup.+/H.sub.2)−E.sub.th(M.sup.m+/M) in acidic medium and than the difference E.sub.th(H.sub.2O/H.sub.2)−E.sub.th(M.sup.m+/M) in basic medium; E.sub.th(A.sup.a+/A)<E.sub.th(H.sup.+/H.sub.2) in acidic medium; E.sub.th(A.sup.a+/A)<E.sub.th(H.sub.2O/H.sub.2) in basic medium; m is an integer; preferably between −5 and 5, and more preferably between −4 and 4 a is an integer; preferably between −5 and 5, and more preferably between −4 and 4 the absolute value of the overvoltage of the hydrogen evolution reaction on the metal A is less than the difference E.sub.th(H.sup.+/H.sub.2)−E.sub.th(M.sup.m+/M) in acidic medium and than the difference E.sub.th(H.sub.2O/H.sub.2)−E.sub.th(M.sup.m+/M) in basic medium; the electrolysis step E.sup.l is initiated by supplying current between the anode and the cathode; A.sup.a+ and M.sup.m+ are respectively deposited in the form of A and M on the cathode during the electrolysis step E.sup.l and gaseous oxygen is released at the anode; the electrolysis step E.sup.l is stopped by cutting off the supply of current between the anode and the cathode; local depolarization effects then appear between A, M, and the H.sup.+ ions, said effects leading to the production of gaseous hydrogen and dissolution of M and A into M.sup.m+ and A.sup.a+ at the electrode which serves as the cathode during step E.sup.l; which corresponds to the conversion step C′; The gaseous hydrogen thus produced is collected, preferably under a pressure P.sup.Hyd; The gaseous hydrogen thus collected is possibly stored outside the chamber.

2. Process according to claim 1, wherein: M is a metal, preferably chosen from the group comprising—ideally composed of: Zn, Cd, Sn, Ni, Mn, Fe, Pb, Co, Hg, their alloys, and mixtures thereof; Zn being particularly preferred; A is a metal, preferably chosen from the group comprising—ideally composed of: Fe, Co, Sn, Ni, Ta, Mo, W, Pd, Rh, In, Ge, their alloys, and mixtures thereof; Fe and Ni being particularly preferred.

3. Process according to claim 1, characterized by at least one of the following: The ions of the metal M are supplied to the electrolyte by at least one precursor, preferably chosen from the group comprising—ideally composed of: salts, in particular sulfates, oxides, nitrates, chlorides, citrates, phosphates, carbonates, fluorides, bromides, oxides, aqueous hydroxide solutions of alkali metals or alkaline earth metals, and mixtures thereof. The ions of the depolarization additive A are supplied to the electrolyte by at least one precursor, preferably chosen from the group comprising—ideally composed of: salts, in particular sulfates, oxides, cyanates, phosphates, ammonias, nitrates, chlorides, hydrated ions, complex ions, and mixtures thereof; and more preferably from the complex ions in oxygenated, cyanated, ammoniated, or fluorosilicic form, and mixtures thereof. The electrolyte is an aqueous saline solution further comprising at least one Bronsted-Lowry acid or base for which the counterion is preferably identical to the ion of the salt M and/or of A.

4. Process according to claim 1, wherein each chamber E.sup.l comprises at least one cathode and at least one anode.

5. Process according to claim 1, wherein the cathode is made from a material enabling deposition of the metal M with a Faraday efficiency of at least 30%, preferably of at least 50%, this material preferably being selected from the group of metals and/or metal alloys comprising—and ideally composed of: Al, Pb and Pb alloys, materials based on carbon, on nickel, and/or on iron, stainless steels, and combinations of these materials.

6. Process according to claim 1, wherein the anode is either made from a material chosen from the group of metals and/or metal alloys comprising and ideally composed of: Pb and Pb alloys, in particular Pb—Ag—Ca or Pb—Ag alloys, steels, nickel, or iron, and combinations of these materials; or is composed of a dimensionally stable anode (DSA), or at least one oxide.

7. Process according to claim 1, wherein, during the electrolysis step E.sup.l, the supply of direct current delivers a current density i (A/m.sup.2) of between 100 and 5000, preferably 200 and 3000, and more preferably 400 and 2000.

8. Process according to claim 1, wherein the interface between the undissolved gas phase G and the liquid phase L—hereinafter referred to as the G/L interface—is increased at least during step C°, so as to accelerate the diffusion, from liquid phase to gas phase, of the dissolved hydrogen saturating or even supersaturating the electrolyte.

9. Device for implementing the process according to claim 1, characterized in that it comprises: a) at least one closed chamber E.sup.l 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) a power supply connected to the at least one cathode (b) and to the at least one anode (c); e) at least one gas discharge pipe equipped with at least one valve, this discharge pipe preferably subdividing into at least one pipe intended for discharging gaseous oxygen, possibly mixed with gaseous hydrogen, and into at least one pipe intended for discharging gaseous hydrogen; each of these pipes being equipped with at least one valve; f) possibly means for increasing the G/L interface; g) possibly means for circulating the electrolyte in the chamber; h) possibly means for heating the electrolyte in the chamber.

10. A kit for implementing the process according to claim 1, wherein the kit includes: a device comprising: a) at least one closed chamber E.sup.l 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) a power supply connected to the at least one cathode (b) and to the at least one anode (c); e) at least one gas discharge pipe equipped with at least one valve, this discharge pipe preferably subdividing into at least one pipe intended for discharging gaseous oxygen, possibly mixed with gaseous hydrogen, and into at least one pipe intended for discharging gaseous hydrogen; each of these pipes being equipped with at least one valve; f) possibly means for increasing the G/L interface; g) possibly means for circulating the electrolyte in the chamber; h) possibly means for heating the electrolyte in the chamber; and components for preparing the electrolyte intended to be contained in the chamber of the device,

Description

EXAMPLE 1

[0143] The description of this example is made with reference to the appended figures, in which:

[0144] FIG. 1 is a schematic representation of the device for implementing the process according to the invention;

[0145] FIG. 2 is a straight cross-section along line II-II of FIG. 1.

[0146] The device shown in these figures comprises a high-pressure chamber 1, inside of which are arranged an anode 2 and a cathode 3, which are bathed in an electrolyte 4. This chamber 1 is a closed chamber provided with a gas outlet pipe 5, said pipe being subdivided into a pipe 6 for discharging gaseous hydrogen and a pipe 7 for discharging gaseous oxygen. Each pipe 6,7 is equipped with a valve 8,9, respectively the H.sub.2 valve and O.sub.2 valve, enabling the independent extraction of these two gases from the high-pressure chamber 1.

[0147] The anode 2 and the cathode 3 are connected to a DC generator 10, capable of supplying them with power in order to induce electrolysis.

[0148] Means 11 for heating the chamber 1 are schematically represented in FIG. 1.

[0149] The chamber 1, the anode 2, the cathode 3, and the electrolyte 4 form an electrolytic cell. For industrial production of pressurized gaseous hydrogen, this cell can be multiplied to increase production capacity.

[0150] In the following example, hydrogen was produced at 80 bar in an electrochemical reactor composed of a chamber 1 and two electrodes (anode 2, cathode 3) bathed in an acidic aqueous solution (electrolyte 4). The two electrodes, each with a surface area of 0.5 m.sup.2, are as follows: [0151] An electrode on which the deposition of metal takes place (cathode), made of aluminum (ref. EN AW 1050A H14 (Al≥99.5%)); [0152] An electrode on which oxygen is released (anode), made of lead-silver-calcium alloy (JL Goslar);

[0153] The electrolyte 4 is composed of zinc ions—metal M—(concentration 1.5 mol.L.sup.−1), of sulfuric acid (1.5 mol.L.sup.−1), and of iron sulfate salt—ion A.sup.++—(8.4×10.sup.−4 mol.L.sup.−1). The temperature is set at 30° C.

[0154] The electrolyte 4 is prepared by mixing 15.84 kg of sulfuric acid (37.5%, Brenntag) in 7.085 L of deionized water, then adding to this mixture 2.44 kg of ZnO (99.9%, Brenntag). 4.7 g of iron sulfate heptahydrate (99%, Sigma Aldrich) are then added to this solution.

[0155] Initially, the two electrodes 2 and 3 are immersed in the electrolyte 4. The oxygen valve 9 is open, and the hydrogen valve 8 is closed.

[0156] During the first electrolysis step C, the generator 10 delivers a current density of 595 A/m.sup.2 for 2 hr, which makes it possible to deposit 653 g of metal M: zinc on the cathode (with an efficiency of 90%). Simultaneously, iron (depolarization additive A) is co-deposited at the cathode 3. Oxygen exits the chamber 1 through the outlet pipe 5 and the pipe 7 for which the valve 9 is in the open position.

[0157] In the second step C°, the power supply 10 is turned off and hydrogen begins to form. The H.sub.2 gas leaving the chamber 1 initially contains oxygen and hydrogen; this mixture is sent via outlet pipe 5 and pipe 7 for which the valve 9 is in the open position, while the valve 8 of the H.sub.2 pipe 6 is closed, in a capacity which allows diluting this mixture with another gas (argon for example). An O.sub.2 sensor located in pipe 7 makes it possible to measure the O.sub.2 content in the gas in real time. When it no longer contains oxygen, valve 9 is closed. The pressure P.sup.Hyd of the hydrogen produced in chamber 1 increases as the gas is generated. When the desired pressure P.sup.Hyd is reached, valve 8 is opened and the hydrogen is sent, via outlet pipe 5 and pipe 6, to a tank not shown in FIG. 1. The pressure P.sup.Hyd inside the chamber 1 is measured using a pressure sensor.

[0158] The rate of the hydrogen evolution is 29 g/h/m.sup.2, and it takes 1 hr24 to produce 20 g of hydrogen at 80 bar.

EXAMPLE 2

[0159] An electrochemical cell was used to produce hydrogen at atmospheric pressure. The cell contains two compartments, separated by a polyester diaphragm. The first compartment contains the electrode which acts as the cathode in the first step, and a solution called the catholyte (1 L). The second compartment contains the electrode which acts as the anode in the first step, and a solution called the anolyte (1 L). Two circulation systems, each driven by a pump, replenish the electrolytes at a flow rate of 20 mL/min.

TABLE-US-00001 Description of operating conditions: Compartment 1 Catholyte 12 g/L of Mn, 150 g/L of (NH.sub.4).sub.2SO.sub.4, 20 mg/L of Co Cathode Stainless steel 316, 1 m.sup.2 Cathode current density 485 A/m.sup.2 Compartment 2 Anolyte 12 g/L of Mn, 130 g/L of (NH.sub.4).sub.2SO.sub.4, 40 g/L of H.sub.2SO.sub.4 Anode Alloy of PbAg (1 mass % Ag), 0.485 m.sup.2 Operating conditions Anode current density 1000 A/m.sup.2 Temperature 40° C. Faraday efficiency 65% Cell voltage 5.5 V Deposition duration 2 hours Amount of manganese 645.75 g deposited Amount of hydrogen 23.52 g produced

[0160] In the first step, the two electrodes are connected to a power supply which delivers a current of 10.9 A. The manganese is deposited at the cathode, and oxygen is released at the anode. Cobalt acts as a depolarizing additive; it is co-deposited with the manganese at the cathode. At the end of the first step, the power is cut off and hydrogen begins to be released at the stainless steel electrode. At the same time, the manganese oxidizes to Mn.sup.2+ ions and the cobalt to Co.sup.2+ ions.