Metal/metal chalcogenide electrode with high specific surface area

Abstract

The present invention relates to an electrode comprising an electrically conductive substrate of which at least one portion of the surface is covered with a metal deposit of copper, the surface of said deposit being in an oxidised, sulphurised, selenised and/or tellurised form and the deposit having a specific surface area of more than 1 m.sup.2/g; a method for preparing such an electrode; and a method for oxygenising water with dioxygen involving such an electrode.

Claims

1. An electrode comprising an electrically conductive substrate having a surface, wherein at least one part of the surface of the electrically conductive substrate is covered with a porous copper metal deposit, wherein the surface of the porous copper metal deposit is oxidized, wherein the porous copper metal deposit has a specific surface area greater than or equal to 1 m.sup.2/g and greater than or equal to 15 cm.sup.2/cm.sup.2.sub.geometric, and an average pore size of between 10 μm and 500 μm, wherein the oxidized surface of the porous copper metal deposit is obtained by oxidizing the surface of the porous copper metal deposit at a temperature comprised between 100° C. and 400° C., and wherein copper oxide is optionally deposited on the oxidized surface of the porous copper metal deposit.

2. The electrode according to claim 1, wherein the metal deposit has a thickness comprised between 10 μm and 2 mm.

3. The electrode according to claim 1, wherein the average pore size is between 30 μm and 70 μm.

4. The electrode according to claim 1, wherein the electrically conductive substrate consists, at least in part, of an electrically conductive material selected from a metal; a metal oxide; a metal sulphide; carbon; a semiconductor; and a mixture thereof.

5. The electrode according to claim 4, wherein the metal is copper, steel, aluminium, or zinc; the metal oxide is fluorine-doped tin oxide (FTO) or indium tin oxide (ITO); the metal sulphide is cadmium sulphide or zinc sulphide; the carbon is in the form of carbon felt, graphite, vitreous carbon, or boron-doped diamond; and the semiconductor is silicon.

6. The electrode according to claim 1, wherein the metal deposit has a specific surface area comprised between 1 m.sup.2/g and 500 m.sup.2/g.

7. The electrode according to claim 6, wherein the metal deposit has a specific surface area comprised between 3 m.sup.2/g and 50 m.sup.2/g.

8. The electrode according to claim 1, wherein the metal deposit has a specific surface area of between 15/cm.sup.2, cm.sup.2.sub.geometric and 50 cm.sup.2/cm.sup.2.sub.geometric.

9. An electrochemical device comprising an electrode according to claim 1.

10. The electrochemical device according to claim 9, being an electrolysis device or a fuel cell.

11. An electrode obtainable by a process comprising the following successive steps: (i) electrodepositing copper on at least one part of the surface of an electrically conductive substrate so as to form a copper metal deposit on the at least one part of the surface of the electrically conductive substrate, the copper metal deposit having a surface, (ii) oxidizing the surface of the copper metal deposit at a temperature comprised between 100° C. and 400° C. to form an oxidized surface, and (iii) optionally depositing copper oxide on the oxidized surface of the copper metal deposit.

12. An electrochemical device comprising an electrode according to claim 11.

13. A process for preparing an electrode according to claim 1 comprising the following successive steps: (i) electrodepositing copper on at least one part of the surface of the electrically conductive substrate so as to form a copper metal deposit the at least one part of the surface of the electrically conductive substrate, (ii) oxidizing the surface of the copper metal deposit at a temperature between 100° C. and 400° C., and (iii) optionally depositing copper oxide on the oxidized surface of the copper metal deposit.

14. The process according to claim 13, wherein the step (i) comprises the following successive steps: (a) immersing at least partially the electrically conductive substrate in an acidic aqueous solution containing ions of the copper to be deposited, and (b) applying a current between the electrically conductive substrate and a second electrode.

15. The process according to claim 13, wherein the step (iii) comprises the following successive steps: (1) immersing at least the part of the electrically conductive substrate covered with the copper metal deposit, the surface of which is oxidized, obtained in the step (ii) in a solution containing copper ions, and (2) applying a potential between the electrically conductive substrate and a second electrode, the electric potential applied to the electrically conductive substrate being negative and then positive, wherein the step (iii) may be repeated once or several times.

16. The process according to claim 14, wherein the current of step (b) has a current density comprised between 0.1 mA/cm.sup.2 and 5 A/cm.sup.2.

17. The process according to claim 14, wherein the acidic aqueous solution containing ions of the copper to be deposited is an acidic aqueous solution containing a water-soluble salt of the copper to be deposited.

18. The process according to claim 17, wherein the water-soluble salt of the copper to be deposited is selected from CuSO.sub.4, CuCl.sub.2, Cu(ClO.sub.4).sub.2, and a mixture thereof.

Description

FIGURES

(1) FIGS. 1A to 1D show scanning electron microscopy (SEM) images of the Cu/Cu.sub.xO.sub.y electrodes obtained in example 1 (FIGS. 1A and 1C) and the Cu/Cu.sub.xS.sub.y electrodes obtained in example 2 (FIGS. 1B and 1D).

(2) FIG. 2 shows the X-ray powder diffractogram of the Cu/Cu.sub.xO.sub.y electrode obtained in example 1.

(3) FIGS. 3A and 3B show, respectively, the linear scan voltammetry (A) and the Tafel plot (B) of the Cu/Cu.sub.xO.sub.y electrode obtained in example 1 in a 1.0 M NaOH aqueous solution.

(4) FIGS. 4A and 4B show the wide spectrum (A) and the high-resolution spectrum centred on sulphur (B) obtained by X-ray photoelectron spectroscopy of the Cu/Cu.sub.xS.sub.y electrode obtained in example 2.

(5) FIGS. 5A and 5B show, respectively, the linear scan voltammetry (A) and the Tafel plot (B) of the Cu/Cu.sub.xS.sub.y electrode obtained in example 2 in a 1.0 M NaOH aqueous solution.

(6) FIG. 6 illustrates a water electrolysis process/device in which water is oxidized to dioxygen at the anode which is an electrode according to the present invention.

(7) FIGS. 7A and 7B show scanning electron microscopy (SEM) images of the Cu/Cu.sub.xO.sub.y/CuO NP electrode obtained in example 6.

(8) FIG. 8 shows the X-ray powder diffractogram of the Cu/Cu.sub.xO.sub.y/CuO NP electrode obtained in example 6.

(9) FIGS. 9A and 9B show, respectively, the linear scan voltammetry (A) and the Tafel plot (B) of the Cu/Cu.sub.xO.sub.y/CuO NP electrode obtained in example 6 in a 1.0 M KOH aqueous solution.

(10) FIGS. 10A and 10B show, respectively, the linear scan voltammetry (A) and the Tafel plot (B) of the Cu/Cu.sub.xO.sub.y (FTO) electrode obtained in example 7 in a 1.0 M KOH aqueous solution.

(11) FIGS. 11A and 11B show, respectively, the linear scan voltammetry (A) and the Tafel plot (B) of the Cu/Cu.sub.xO.sub.y (FC) electrode obtained in example 8 in a 1.0 M KOH aqueous solution.

(12) FIGS. 12A and 12B show, respectively, the linear scan voltammetry (A) and the Tafel plot (B) of the Cu/Cu.sub.xO.sub.y (Ti) electrode obtained in example 9 in a 1.0 M KOH aqueous solution.

(13) FIG. 13 shows TEM images of an electrode section made by focused ion beam: (a) STEM−HAADF image indicating by circles the areas where the SAED images were measured (b)(c) STEM-XEDS analyses (Cu in dark grey, O in light grey) (Cu in green and O in red on the original colour image) (d)(e)(f) SAED images of the areas indicated in (a).

EXAMPLES

General Considerations

(14) The electrocatalytic measurements and the electrolysis experiments are performed in a three-electrode, two-compartment cell, allowing separation of gas phase products in the anodic and cathodic compartments using a Bio-Logic SP300 potentiostat. An Ag/AgCl reference electrode is placed in the same compartment as the working electrode. A platinum counter electrode is placed in a separate compartment connected by a porosity 5 sintered glass filled with the electrolyte solution. The potentials are referenced to the reversible hydrogen electrode (RHE) using the equation below:
E.sub.RNE=E.sub.Ag/AgCl+0.197+0.059*pH

(15) The result of linear scanning voltammetry is not compensated for the ohmic drop. The faradaic yield was obtained by comparing the theoretical amount of oxygen produced on the basis of the charge consumed with the amount of oxygen determined by gas chromatography. The scanning electron microscopy (SEM) images were acquired using a Hitachi S-4800 scanning electron microscope. The images of X-ray powder diffraction of patterns were recorded using an analytical X'Pert Pro P diffractometer provided with a Cu-Ka radiation source (λKα1=1.540598 Â, λKα2=1.544426 nm) or a Co-Ka radiation source (λKα1=1.78897 Å, λKα2=1.79285 Å) with an X'Celerator detector. Gas chromatography was performed on a Shimadzu GC-2014 chromatograph equipped with a Quadrex Molsieve 5A plot column, a thermal conductivity detector and using He as carrier gas (30 ml/min). To prepare the electrode, the surface of the Cu plate (3 cm×1 cm) was cleaned using sandpaper (p 1200) followed by immersion in a 5.0M HCl solution for 30 s. The plate is then rinsed with ethanol before being air-dried. The Randles-Sevcik equation (1) is used to calculate λ.sub.diff, the electroactive surface area of the electrode:
i.sub.p=2.69×10.sup.5n.sup.3/2D.sup.1/2A.sub.diffCv.sup.1/2  (1)

(16) The current i.sub.p is the peak current corresponding to the reduction of the redox pair (Fe.sup.3+/Fe.sup.2+), determined by cyclic voltammetry of K.sub.3[Fe(CN).sub.6], n is the number of electrons exchanged, D is the diffusion coefficient of the analyte (7.5×10.sup.−6 cm.sup.2.Math.s.sup.−1 for K.sub.3[Fe(CN).sub.6]), C (mol.Math.cm.sup.−3) is the molar concentration in the analyte and v is the scanning rate (V.Math.s.sup.−1). The electroactive surface area of the electrodes is measured using an electrode of 1 cm.sup.2 geometric surface area plunged into a solution containing 5 mM K.sub.3[Fe(CN).sub.6] and 0.1 M pH 7.0 phosphate buffer. Application of equation (1) then allows the determination of the electroactive surface area value A.sub.diff, and consequently of the specific surface area determined by electrochemistry by dividing this value by the geometric surface area of the electrode according to the relationship: Specific surface area determined by electrochemistry=A.sub.diff/A.sub.geometric (in cm.sup.2/cm.sup.2.sub.geometric). The samples used for the BET measurements were obtained by mechanical abrasion, using a 1 mm thick PVC (poly(vinyl chloride)) blade, of the metal deposit present on the substrate.

Example 1: Preparation of a Copper/Copper Oxide Electrode According to the Invention on Copper Substrate

(17) 1 cm.sup.2 of a freshly cleaned copper plate is immersed in 20 ml of a 0.2 M CuSO.sub.4, 1.5 M H.sub.2SO.sub.4 solution and a current of 0.5 A is applied using a galvanostatic method for a period of 80 s. The electrode is then removed from the solution and cleaned with large amounts of distilled water and dried under vacuum (10 mbar). The electrode is then transferred to an oven under static air atmosphere (1 bar). The temperature is raised to 310° C. at a rate of 10° C. per minute, and the temperature is kept constant for 1 hour. After this step, the electrode is cooled to room temperature and stored in air. This electrode is subsequently named Cu/Cu.sub.xO.sub.y electrode. FIGS. 1A and 1C show SEM images of this electrode and illustrate the porous nanostructures and the high specific surface area of the material. The X-ray powder diffractogram of this electrode is presented in FIG. 2 and shows the presence of Cu, Cu.sub.2O and CuO. Linear scanning voltammetry between 1.2V and 2.0V vs RHE (Reversible Hydrogen Electrode) in 1.0M NaOH aqueous solution with a scanning rate of 10 mV/s is presented in FIG. 3A. A current density of 10 mA.Math.cm.sup.−2 was obtained at 340 mV overvoltage for the production of dioxygen. The Tafel plot of the electrode is presented in FIG. 3B. Catalytic activity increases linearly from 1.56V to 1.66V vs RHE. The Tafel slope was determined in this region to be equal to 38 mV.Math.dec.sup.−1. The specific surface area determined electrochemically is 19.6 cm.sup.2/cm.sup.2.sub.geometric. The specific surface area determined by BET is 3.4 m.sup.2/g.

Example 2: Preparation of a Copper/Copper Sulphide Electrode According to the Invention on Copper Substrate

(18) 1 cm.sup.2 of a freshly cleaned copper plate is immersed in 20 ml of a 0.2 M CuSO.sub.4, 1.5 M H.sub.2SO.sub.4 solution and a current of 0.5 A is applied using a galvanostatic method for a period of 80 s. The electrode is then removed from the solution and cleaned with large amounts of distilled water and dried under vacuum (10 mbar). The electrode is then transferred to a glass reactor containing 15 mg of elemental sulphur powder in a compartment separated from but connected to the rest of the reactor where the electrode is located. Vacuum (0.01 mbar) is created in the reactor (including in the compartment containing elemental sulphur), which is kept under static vacuum and placed in an oven. The temperature of the whole is raised to 150° C. at a rate of 15° C. per minute, and the temperature is maintained at 150° C. for 2 minutes. After this step, the reactor is removed from the oven and allowed to cool to room temperature, all while creating vacuum in the reactor (dynamic vacuum). After cooling to room temperature, the compartment containing elemental sulphur is disconnected from the reactor containing the electrode and the reactor containing the electrode is placed in the oven again, all while creating vacuum (dynamic vacuum—0.01 mbar). The oven temperature is raised to 150° C. at a rate of 15° C. per minute, and the temperature is maintained at 150° C. for 1 h. After this annealing step, the electrode is cooled to room temperature under dynamic vacuum and used quickly after its preparation. This electrode is subsequently named Cu/Cu.sub.xS.sub.y electrode. FIGS. 1B and 1D show SEM images of this electrode and illustrate the porous nanostructures and the high specific surface area of the material. The spectrum obtained by X-ray photoelectron spectroscopy of this electrode is presented in FIG. 4 and reveals the presence of copper and copper sulphide. Linear scanning voltammetry between 1.2V and 2.0V vs RHE in 1.0M NaOH aqueous solution with a scanning rate of 10 mV/s is presented in FIG. 5A. A current density of 10 mA.Math.cm.sup.−2 was obtained at 340 mV overvoltage for the production of dioxygen. The Tafel plot of the electrode is presented in FIG. 5B. Catalytic activity increases linearly from 1.56V to 1.66V vs RHE. The Tafel slope was determined in this region to be equal to 40 mV.Math.dec.sup.−1.

(19) The specific surface area determined electrochemically is 23.0 cm.sup.2/cm.sup.2.sub.geometric. The specific surface area determined by BET is 3.8 m.sup.2/g.

Example 3: Measurement of Catalytic Activity for Water Oxidation of the Cu/Cu.SUB.x.O.SUB.y .Electrode

(20) Water electrolysis is carried out with a fixed applied overvoltage of 400 mV using the Cu/Cu.sub.xO.sub.y electrode as working electrode in a 1.0 M NaOH aqueous solution. The generation of a large amount of oxygen gas is observed at the electrode. The oxygen produced is quantified by gas chromatography. After 10 minutes of electrolysis, the faradaic yield for O.sub.2 evolution is estimated at 89% (2.2 C consumed, 5.14 μmol O.sub.2 generated, 0.5 cm.sup.2 electrode plunging into the solution).

Example 4: Measurement of Catalytic Activity for Water Oxidation of the Cu/Cu.SUB.x.S.SUB.y .Electrode

(21) Water electrolysis is carried out with a fixed applied overvoltage of 400 mV using the Cu/Cu.sub.xS.sub.y electrode as working electrode in a 1.0 M NaOH aqueous solution. The generation of a large amount of oxygen gas is observed at the electrode. The oxygen produced is quantified by gas chromatography. After 10 minutes of electrolysis, the faradaic yield for O.sub.2 evolution is estimated at 92% (2.0 C consumed, 4.83 μmol O.sub.2 generated, 0.5 cm.sup.2 electrode plunging into the solution).

Example 5: Durability of the Cu/Cu.SUB.x.O.SUB.y .Electrode Under Catalytic Conditions

(22) Water electrolysis is carried out with a fixed applied overvoltage of 600 mV using the Cu/Cu.sub.xO.sub.y electrode as working electrode in a 0.1 M NaOH aqueous solution. Throughout the duration of the experiment (4 h), a large amount of oxygen gas is constantly generated. No sign of deactivation is observed, a stable current density of 20 mA/cm.sup.2 being observed throughout the experiment.

Example 6: Preparation of a Copper/Copper Oxide Electrode According to the Invention on Copper Substrate

(23) 1 cm.sup.2 of a freshly cleaned copper plate is immersed in 20 ml of a 0.2 M CuSO.sub.4, 1.5 M H.sub.2SO.sub.4 solution and a current of 0.5 A is applied using a galvanostatic method for a period of 80 s. The electrode is then removed from the solution and cleaned with large amounts of distilled water and dried under vacuum (10 mbar). The electrode is then transferred to an oven under static air atmosphere (1 bar). The temperature is raised to 310° C. at a rate of 10° C. per minute, and the temperature is kept constant for 1 hour. After this step, the electrode is cooled to room temperature. The electrode thus prepared is then plunged into a solution of Cu(imidazole).sub.2Cl.sub.2 (0.2 mM) in an acetonitrile/3% water (v/v) mixture also containing 0.1 M supporting salt TBAPF.sub.6 (tetrabutylammonium hexafluorophosphate). The electrode is then subjected to two cycles of cyclic voltammetry between −0.5 V and 1 V (vs Ag/AgCl) and a cycle rate of 50 mV/s. The electrode thus obtained is removed from the solution and cleaned with large amounts of distilled water and air-dried at room temperature.

(24) This electrode is subsequently named Cu/Cu.sub.xO.sub.y/CuO NP electrode.

(25) FIG. 7 shows SEM images of this electrode and illustrates the porous nanostructures and the high specific surface area of the material. The X-ray powder diffractogram of this electrode is presented in FIG. 8 and shows the presence of Cu, Cu.sub.2O and CuO. Linear scanning voltammetry between 1.2V and 2.0V vs RHE (Reversible Hydrogen Electrode) in a 1.0M KOH aqueous solution with a scanning rate of 10 mV/s is presented in FIG. 9A. A current density of 10 mA.Math.cm.sup.−2 was obtained at 290 mV overvoltage for the production of dioxygen. The Tafel plot of the electrode is presented in FIG. 9B. Catalytic activity increases linearly from 1.56V to 1.66V vs RHE. The Tafel slope was determined in this region to be equal to 64 mV.Math.dec.sup.−1. The specific surface area determined electrochemically is 20.6 cm.sup.2/cm.sup.2.sub.geometric.

(26) FIG. 13 shows TEM images of an electrode section made by focused ion beam: (a) STEM−HAADF (Scanning Transmission Electron Microscope/High-Angle Annular Dark-Field) image indicating the areas where the SAED (Selected Area Electron Diffraction) images were measured by circles (b)(c) STEM-XEDS (Scanning Transmission Electron Microscope/X-Ray Energy-Dispersive Spectroscopy) analyses (Cu in dark grey, O in light grey) (Cu in green and O in red on the original colour image) (d)(e)(f) SAED images of the areas indicated in (a) showing the presence of copper metal, Cu.sub.2O (area (e)) or CuO (area(f)).

(27) The same example was carried out while replacing the supporting salt TBAPF.sub.6 (tetrabutylammonium hexafluorophosphate) with TBABF.sub.4 (tetrabutylammonium tetrafluoroborate).

Example 7: Preparation of a Copper/Copper Oxide Electrode According to the Invention on an FTO Substrate

(28) 1 cm.sup.2 of a layer of FTO deposited on a freshly cleaned glass slide is immersed in 20 ml of a 0.2 M CuSO.sub.4, 1.5 M H.sub.2SO.sub.4 solution and a current of 0.5 A is applied using a galvanostatic method for a period of 80 s. The electrode is then removed from the solution and cleaned with large amounts of distilled water and dried under vacuum (10 mbar). The electrode is then transferred to an oven under static air atmosphere (1 bar). The temperature is raised to 310° C. at a rate of 10° C. per minute, and the temperature is kept constant for 1 hour. After this step, the electrode is cooled to room temperature. This electrode is subsequently named Cu/Cu.sub.xO.sub.y (FTO) electrode.

(29) Linear scanning voltammetry between 1.2V and 2.0V vs RHE (Reversible Hydrogen Electrode) in a 1.0M KOH aqueous solution with a scanning rate of 10 mV/s is presented in FIG. 10A. A current density of 10 mA.Math.cm.sup.−2 was obtained at 540 mV overvoltage for the production of dioxygen. The Tafel plot of the electrode is presented in FIG. 10B. Catalytic activity increases linearly from 1.45V to 1.6V vs RHE. The Tafel slope was determined in this region to be equal to 121 mV.Math.dec.sup.−1.

Example 8: Preparation of a Copper/Copper Oxide Electrode According to the Invention on a Carbon Felt Substrate

(30) 1 cm.sup.2 of freshly cleaned carbon felt is immersed in 20 ml of a 0.2 M CuSO.sub.4, 1.5 M H.sub.2SO.sub.4 solution and a current of 0.5 A is applied using a galvanostatic method for a period of 80 s. The electrode is then removed from the solution and cleaned with large amounts of distilled water and dried under vacuum (10 mbar). The electrode is then transferred to an oven under static air atmosphere (1 bar). The temperature is raised to 310° C. at a rate of 10° C. per minute, and the temperature is kept constant for 1 hour. After this step, the electrode is cooled to room temperature. This electrode is subsequently named Cu/Cu.sub.xO.sub.y (FC) electrode. Linear scanning voltammetry between 1.2V and 2.0V vs RHE (Reversible Hydrogen Electrode) in a 1.0M KOH aqueous solution with a scanning rate of 10 mV/s is presented in FIG. 11A. A current density of 10 mA.Math.cm.sup.−2 was obtained at 350 mV overvoltage for the production of dioxygen. The Tafel plot of the electrode is presented in FIG. 11B. Catalytic activity increases linearly from 1.48V to 1.61V vs RHE. The Tafel slope was determined in this region to be equal to 84 mV.Math.dec.sup.−1.

Example 9: Preparation of a Copper/Copper Oxide Electrode According to the Invention on a Titanium Substrate

(31) 1 cm.sup.2 of freshly cleaned titanium plate is immersed in 20 ml of a 0.2 M CuSO.sub.4, 1.5 M H.sub.2SO.sub.4 solution and a current of 0.5 A is applied using a galvanostatic method for a period of 80 s. The electrode is then removed from the solution and cleaned with large amounts of distilled water and dried under vacuum (10 mbar). The electrode is then transferred to an oven under static air atmosphere (1 bar). The temperature is raised to 310° C. at a rate of 10° C. per minute, and the temperature is kept constant for 1 hour. After this step, the electrode is cooled to room temperature. This electrode is subsequently named Cu/Cu.sub.xO.sub.y (Ti) electrode. Linear scanning voltammetry between 1.2V and 2.0V vs RHE (Reversible Hydrogen Electrode) in a 1.0M KOH aqueous solution with a scanning rate of 10 mV/s is presented in FIG. 12A. A current density of 10 mA.Math.cm.sup.−2 was obtained at 410 mV overvoltage for the production of dioxygen. The Tafel plot of the electrode is presented in FIG. 12B. Catalytic activity increases linearly from 1.45V to 1.55V vs RHE. The Tafel slope was determined in this region to be equal to 89 mV.Math.dec.sup.−1.

REFERENCES

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