Method and apparatus for preparing reduction product of carbon dioxide by electrochemically reducing carbon dioxide

Abstract

The present invention relates to a method and an apparatus of preparing a reduction product of carbon dioxide by electrochemically reducing carbon dioxide. The present invention can prepare, in an energy-efficient manner, a reduction product of high-concentration carbon dioxide with high Faraday efficiency as in a liquid reduction reaction by producing the reduction product of carbon dioxide by supplying water or an electrolytic solution to an anode region; supplying humidified carbon dioxide gas having a second temperature higher than a first temperature to a cathode region within an electrochemical cell having the first temperature so as to supply the carbon dioxide gas which has been humidified to be in a condition where the relative humidity is greater than 100%, while applying a voltage between the anode region and the cathode region so as to generate hydrogen ions (H.sup.+) in the anode region; and transporting the hydrogen ions to the cathode region through the electrolyte membrane, thereby electrochemically reducing the carbon dioxide gas.

Claims

1. An apparatus of preparing a reduction product of carbon dioxide (CO.sub.2) by electrochemically reducing carbon dioxide, comprising: an electrochemical cell, which comprises: an anode; a cathode; an electrolyte membrane which is disposed between the anode and the cathode to separate the anode region and the cathode region, and is configured to transport hydrogen ions (H.sup.+) from the anode to the cathode region therethrough; an inlet configured to supply water or an electrolytic solution to the anode region; and an inlet configured to supply humidified carbon dioxide to the cathode region; an energy supply source, which is operably linked to the anode and the cathode, and is configured to apply a voltage between the anode and the cathode to generate hydrogen ions in the anode region and reduce the carbon dioxide in the cathode to produce a reduction product of the carbon dioxide; and a humidifier or atomizer, which is linked to the inlet for supplying humidified carbon dioxide to the cathode region, and is configured to humidify carbon dioxide so that humidified carbon dioxide having the relative humidity in a range of 150% to 2,000% is supplied to the cathode region within the electrochemical cell.

2. The apparatus of claim 1, wherein, upon applying a voltage, the moisture in which carbon dioxide is dissolved is condensed as the relative humidity within the cathode region is in a range of 150% to 2,000% and forms a water film on the cathode surface, and the reduction product is produced from the carbon dioxide dissolved in the water film as a reactant by electrochemical reduction.

3. The apparatus of claim 1, wherein the electrochemical cell comprises a membrane-electrode assembly having an anode catalyst layer and a cathode catalyst layer formed on each surface of the electrolyte membrane, respectively, wherein a gas diffusion layer of supplying the humidified carbon dioxide to the catalyst layer, which is placed in the cathode side of the membrane-electrode assembly.

4. The apparatus of claim 1, wherein the electrochemical cell has a first temperature, and the supply of the carbon dioxide gas which has been humidified in a condition where the relative humidity is in a range of 150% to 2,000% is achieved by providing a humidified carbon dioxide gas having a second temperature higher than the first temperature to the cathode region within the electrochemical cell.

5. The apparatus of claim 1, wherein the relative humidity is in a range of 300% to 1,500%.

6. A method of preparing a reduction product of carbon dioxide (CO.sub.2) by electrochemically reducing carbon dioxide within an electrochemical cell, which comprises an anode, a cathode, and an electrolyte membrane which is disposed between the anode and the cathode to separate the anode region and the cathode region: (1) supplying water or an electrolytic solution to the anode region; (2) supplying a carbon dioxide gas, which has been humidified by using a humidifier or atomizer in a condition where the relative humidity is in a range of 150% to 2,000%, to the cathode region within the electrochemical cell; and (3) applying a voltage between the anode region and the cathode region to generate hydrogen ions (H.sup.+) in the anode region, wherein the hydrogen ions move through the electrolyte membrane to the cathode region and then electrochemically reduce the carbon dioxide gas so as to produce a reduction product of carbon dioxide.

7. The method of claim 6, wherein, upon applying a voltage in step (3), the moisture in which carbon dioxide is dissolved is condensed as the relative humidity within the cathode region is in a range of 150% to 2,000% and forms a water film on the cathode surface, and the reduction product is produced from the carbon dioxide dissolved in the water film as a reactant by electrochemical reduction.

8. The method of claim 6, wherein the electrochemical cell comprises a membrane-electrode assembly having an anode catalyst layer and a cathode catalyst layer formed on each surface of the electrolyte membrane, respectively, and a gas diffusion layer of supplying the humidified carbon dioxide to the catalyst layer, which is placed in the cathode side of the membrane-electrode assembly.

9. The method of claim 6, wherein the reduction product of the carbon dioxide is formic acid, formaldehyde, formate, acetaldehyde, acetate, acetic acid, acetone, 1-butanol, 2-butanol, 2-butanone, ethanol, isopropanol, lactate, lactic acid, methanol, 1-propanal, 1-propanol, propionic acid, or a mixture thereof.

10. The method of claim 6, wherein a first temperature of the electrochemical cell is in the range of 10 C. to 25 C.

11. The method of claim 6, wherein a second temperature of the humidified carbon dioxide gas is in the range of 30 C. to 100 C.

12. The method of claim 6, wherein the reduction product of the carbon dioxide in step (3) is dissolved in the water derived from the humidified carbon dioxide gas and water which has undergone a crossover from the anode region, and produced in the form of an aqueous solution.

13. The method of claim 6, wherein the reduction product of the carbon dioxide produced in step (3) has a concentration of 1% (w/v) to 20% (w/v).

14. The method of claim 6, wherein the reduction product of the carbon dioxide in step (3) is produced with a Faradaic efficiency of 80% or higher.

15. The method of claim 6, wherein the cathode is selected from the group consisting of Sn, a Sn alloy, Al, Au, Ag, C, Cd, Co, Cr, Cu, a Cu alloy, Ga, Hg, In, Mo, Nb, Ni, NiCo.sub.2O.sub.4, a Ni alloy, a NiFe alloy, Pb, Rh, Ti, V, W, Zn, an alloy in accordance with ASTM F 1058, nichrome, austenitic steel, duplex steel, ferrite steel, martensitic steel, stainless steel, degenerately-doped p-Si, degenerately doped p-Si:As, degenerately-doped p-Si:B, degenerately-doped n-Si, degenerately-doped n-Si:As, degenerately-doped n-Si:B, and mixtures thereof.

16. The method of claim 15, wherein the Sn alloy as the cathode is a SnPb alloy.

17. The method of claim 6, wherein the electrolytic solution is an aqueous solution comprising an electrolyte of KHCO.sub.3, K.sub.2CO.sub.3, KOH, KCl, KClO.sub.4, K.sub.2SiO.sub.3, Na.sub.2SO.sub.4, NaNO.sub.3, NaCl, NaF, NaClO.sub.4, CaCl.sub.2), guanidinium cations, H.sup.+ ions, alkali metal cations, ammonium cations, alkylammonium cations, halide ions, alkylamines, borates, carbonates, guanidinium derivatives, nitrites, nitrates, phosphates, polyphosphates, perchlorates, silicates, sulfates, tetraalkylammonium salts, or a mixture thereof.

18. The method of claim 6, wherein the electrochemical cell has a first temperature, and the supply of the carbon dioxide gas which has been humidified in a condition where the relative humidity is in a range of 150% to 2,000% in the step (2) is achieved by providing a humidified carbon dioxide gas having a second temperature higher than the first temperature to the cathode region within the electrochemical cell.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIGS. 1A and 1B are schematic diagrams that thermodynamically explain the limit of the existing gas-phase reduction reaction of carbon dioxide.

(2) FIG. 2 is a conceptual diagram schematically illustrating the reduction reaction of carbon dioxide occurring in an apparatus and each part of the apparatus according to an embodiment of the present invention.

(3) FIG. 3 is a schematic conceptual diagram of an apparatus of preparing formic acid (HCOOH) by electrochemically reducing carbon dioxide (CO.sub.2) according to an embodiment of the present invention.

(4) FIG. 4 shows the measurement results of Faradaic efficiency according to relative humidity in preparing formic acid (HCOOH) by electrochemical reduction of carbon dioxide (CO.sub.2) according to the method of the present invention.

(5) FIG. 5 shows the measurement results of recovery rate of liquid-phase products according to relative humidity in preparing formic acid by electrochemical reduction of carbon dioxide according to the method of the present invention.

(6) FIG. 6 shows the measurement results of current density according to relative humidity in preparing formic acid by electrochemical reduction of carbon dioxide according to the method of the present invention.

(7) FIG. 7 shows the measurement results of concentration of formic acid production according to relative humidity in preparing formic acid by electrochemical reduction of carbon dioxide according to the method of the present invention.

BEST MODE

(8) Preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily carry out the present invention

(9) Hereinafter, a configuration of an apparatus of preparing a reduction product of carbon dioxide (CO.sub.2) (e.g., formic acid (HCOOH)) by electrochemically reducing carbon dioxide according to an embodiment of the present invention will be described.

(10) FIG. 3 is a schematic conceptual diagram of an apparatus of preparing a reduction product of carbon dioxide (e.g., formic acid (HCOOH)) by electrochemically reducing carbon dioxide according to an embodiment of the present invention.

(11) Referring to FIG. 3, the apparatus of preparing a reduction product of carbon dioxide by electrochemically reducing carbon dioxide according to an embodiment of the present invention may include:

(12) an electrochemical cell, which includes an anode; a cathode; and an electrolyte membrane which is disposed between the anode and the cathode to separate the anode region and the cathode region; an inlet for supplying water or an electrolytic solution to the anode region; and an inlet for supplying humidified carbon dioxide to the cathode region;

(13) an energy supply source, which is operably linked to the anode and the cathode through current collector, and applies a voltage between the anode and the cathode to reduce the carbon dioxide in the cathode to a reduction product of the carbon dioxide; and

(14) a humidifier or atomizer, which is linked to the inlet for supplying humidified carbon dioxide to the cathode region and thereby supplies humidified carbon dioxide to the cathode region.

(15) The energy supply source may be configured in such a manner that the energy supply source is operably linked to the anode and the cathode, and applies a voltage between the anode and the cathode to reduce the carbon dioxide to the reduction product of carbon dioxide in the cathode.

(16) The electrical energy for the electrochemical reduction of carbon dioxide may come from conventional energy supply sources, including conventional nuclear energy sources and alternative energy supply sources from solar cells or other non-fossil fuel electricity sources (e.g., hydro power, wind power, solar power generation, geothermal, etc.). Preferably, the electricity source can supply a voltage higher than 1.6 V across the cell. Adjustment to different voltage values may be made depending on the internal resistance of the cell used.

(17) In the above apparatus, hydrogen ions (H.sup.+) can be generated as shown in Reaction Scheme 1 below when water or an electrolytic solution is supplied to the anode region and current is applied.
2H.sub.2O.fwdarw.O.sub.2+4H.sup.++4e.sup.[Reaction Scheme 1]

(18) That is, the reactant in the anode is in a liquid phase in the present invention. As described above, in the anode region, hydrogen ions (H.sup.+) can be generated by supplying only water, and hydrogen ions (H.sup.+) can also be generated even by supplying an electrolytic solution, specifically an aqueous solution containing an electrolyte. The electrolytic solution may be an aqueous electrolytic solution at a concentration of 0.1 M to 1 M.

(19) On the other hand, carbon dioxide gas can be electrochemically reduced to formic acid by hydrogen cations (H.sup.+) that have moved to the cathode region through the electrolyte membrane as shown in Reaction Scheme 2 below, under conditions where relative humidity exceeds 100%, by supplying humidified carbon dioxide gas having a second temperature higher than a first temperature to the cathode region within the electrochemical cell having the first temperature.
2CO.sub.2+4H.sup.+++4e.sup..fwdarw.2HCOOH[Reaction Scheme 2]

(20) That is, the reactant in the cathode region is CO.sub.2 gas which is humidified to be in a condition where the relative humidity exceeds 100%, that is, in a gaseous phase.

(21) Relative humidity is defined as follows, and thus relative humidity can be adjusted via the temperature of the electrochemical cell and the temperature of the humidifier or atomizer.
Relative humidity=(Saturated water vapor pressure at a temperature within a humidifier or atomizer/Saturated water vapor pressure at a temperature inside of a cell)100(%)

(22) That is, excess water can be supplied to the electrochemical cell, specifically to the cathode region, with the reactant (i.e., CO.sub.2) by passing the CO.sub.2 gas by setting the temperature of the humidifier or atomizer at a temperature higher than that of the electrochemical cell. In particular, the water supplied in excess is condensed and forms a water film on the surface of the electrode (i.e., the cathode) and the reaction proceeds easily as in a liquid-phase reaction because the CO.sub.2 supplied together with the water is dissolved in the condensed water film and used as a reactant. Additionally, CO.sub.2 (i.e., the reactant) is delivered to the surface of the electrode (i.e., the cathode) in a gaseous phase, and the CO.sub.2 consumed by the reaction in the water film is constantly replenished by the CO.sub.2 gas being supplied, and thus it is possible to minimize mass transfer resistance without being limited by solubility.

(23) Additionally, since the activation energy of the reaction is lowered using a minimum amount of water, the formic acid being produced can be recovered at a high concentration. Accordingly, the cost required for separation, purification, and concentration can be reduced.

(24) In Example of the present invention, it was confirmed that highly-concentrated formic acid in the range of 1.4% (w/v) to 8.3% (w/v) (i.e., 14,000 ppm to 83,000 ppm in terms of ppm) can be recovered with a Faradaic efficiency of 80% or higher when humidified CO.sub.2 gas having a relative humidity of 100% or higher is supplied to the cathode region. More specifically, it was confirmed that highly-concentrated formic acid in the range of 1.4% (w/v) to 7% (w/v) (i.e., 14,000 ppm to 70,000 ppm in terms of ppm) can be recovered with a Faradaic efficiency of 80% or higher when the humidified CO.sub.2 gas having a relative humidity of 300% to 1,500% is supplied to the cathode region.

(25) Additionally, since an aqueous solution is supplied to the anode region, the water that undergoes a crossover in the anode region is supplied to the cathode region and thereby more spontaneously promotes the electrochemical reduction reaction of carbon dioxide, and thus it is possible to recover highly-concentrated formic acid with much greater efficiency. Furthermore, the highly-concentrated formic acid can be purified using a gas/liquid separation apparatus as shown in FIG. 3.

(26) Hereinafter, the method of preparing formic acid using an apparatus of preparing formic acid (HCOOH) by electrochemically reducing the carbon dioxide (CO.sub.2) according to the present invention will be described in more detail with reference to the following Examples. However, these Examples are for illustrative purposes only and the invention is not limited by these Examples.

Example 1: Preparation of Apparatus of Preparing Formic Acid by Electrochemically Reducing Carbon Dioxide

(27) As shown in FIGS. 2 and 3, an apparatus of preparing formic acid (HCOOH) by electrochemically reducing carbon dioxide (CO.sub.2) according to an embodiment of the present invention was prepared.

(28) The catalysts used for the anode and the cathode were platinum (Pt) and tin (Sn), respectively, and particulate metal catalyst powders were mixed and dispersed in an alcohol together with a Nafion ionomer, which served as a binder, and the resultant was coated on each side of an electrolyte membrane to form an electrode catalyst layer, and thereby a membrane-electrode assembly was prepared. In order to smoothly supply humidified CO.sub.2 to the catalyst layer, a carbon paper layer having a thickness of 0.1 mm to 0.5 mm was used as a gas diffusion layer in the cathode side of the prepared membrane electrode assembly. A current collector was inserted into both sides of a membrane-electrode assembly including the gas diffusion layer so as to allow a current to flow between the two electrodes when a voltage is applied thereto, and a flow path was formed to supply reactants and release products.

Experimental Example 1: Examination of Efficiency of Method for Preparing Formic Acid According to the Present Invention

(29) The performance of formic acid preparation was evaluated at a relative humidity of 100% to 1,500% by varying the humidifier temperature where the internal temperature of the cell was set at 25 C., using the apparatus of preparing formic acid (HCOOH) by electrochemically reducing carbon dioxide (CO.sub.2) prepared in Example 1.

(30) In particular, the experimental conditions were as follows and the humidification conditions were as shown in Table 1:

(31) Reaction voltage: 3 V (constant voltage operation)

(32) Reaction temperature: 298 K

(33) Reaction pressure: 1 atm (atmospheric pressure)

(34) Electrolyte membrane: Nafion 115

(35) Anode catalyst: Pt black

(36) Cathode catalyst: Sn powder

(37) Electrode area: 25 cm.sup.2

(38) Anode reactant: aqueous solution of 0.5 M KHCO.sub.3 40 mL/min

(39) Cathode reactant: humidified CO.sub.2 gas 300 mL/min (relative humidity 100% to 1,500%)

(40) TABLE-US-00001 TABLE 1 Relative Internal Temperature Humidifier Humidity (%) of Cell ( C.) Temperature ( C.) 100 25 25 300 25 45 700 25 63 1000 25 71 1500 25 80

(41) The results are shown in FIGS. 4 to 7.

(42) FIG. 4 shows the measurement results of Faradaic efficiency according to relative humidity. In FIG. 4, it was confirmed that formic acid can be produced at a Faradaic efficiency of 80% or higher under the condition where relative humidity exceeds 100% using the method according to the present invention.

(43) FIG. 5 shows the measurement results of recovery rate of liquid-phase products according to relative humidity. In FIG. 5, it was confirmed that formic acid can be produced at a recovery rate of 1 mL/h to 10 mL/h under the condition where relative humidity exceeds 100% using the method according to the present invention.

(44) FIG. 6 shows the measurement results of current density according to relative humidity. In FIG. 6, it was confirmed that the current density slightly increases as the relative humidity increases.

(45) FIG. 7 shows the measurement results of concentration of formic acid produced according to relative humidity. In FIG. 7, it was confirmed that highly-concentrated formic acid can be produced at a concentration of 1.4% (w/v) to 8.3% (w/v) under the condition where relative humidity exceeds 100% using the method according to the present invention.