JP7287234B2 - CO2 reduction reactor - Google Patents

Description

The present invention relates to a _CO2 reduction reactor for generating a _CO2 reduction reaction at the cathode.

Conventionally, various devices have been proposed as CO ₂ reduction reactors for causing a CO ₂ reduction reaction. For example, in many cases, an anode and a cathode to which a predetermined voltage is applied are immersed in an electrolytic solution in which _CO2 is dissolved, and a _CO2 reduction reaction occurs at the cathode.

In Non-Patent Document 1, _CO2 is supplied as a gas, and a gas diffusion membrane is used to separate the gas from the electrolyte, and the _CO2 is diffused into the electrolyte through the gas diffusion membrane to reduce the _CO2. shown to do. Such reduction of _CO2 can produce CO, CH4, _C2H4 , _{H2 , C2H5OH} .

Patent Literature 1 shows that CO ₂ is brought into contact with a catalyst under pressure to produce CO and the like. By controlling the pressure of _CO2 , the concentration of _CO2 can be controlled, and the amount of CO and hydrogen produced can be controlled.

Non-Patent Document 2 shows the production of CO using a low-concentration CO ₂ -containing gas.

JP 2014-62038 A

Qingxin Jia et al, “Direct Gas-phase CO2 Reduction for Solar Methane Generation Using a Gas Diffusion Electrode with a BiVO4:Mo and a Cu-In-Se Photoanode”, Chem. Lett. 2018, 47, 436-439 "Electrocatalytic reduction of low concentration CO2", Chem. Sci., 2019, 10, 1597-1606

Here, in the electrolyte with dissolved _CO2 , due to the low diffusivity of _CO2 , at low concentrations of _CO2 , the _CO2 supply to the cathode electrode becomes insufficient and the faradaic efficiency of _CO2 reduction decreases. do.

Non-Patent Document 1 uses a Cu-based catalyst and an Ag-based catalyst. In this case, reduction product selectivity is low, various substances are produced at the same time as described above, and the Faraday efficiency of a specific substance cannot be increased.

Patent Document 1 is based on the assumption that high-concentration CO ₂ gas is used, and the reduction reaction cannot be well controlled for low-concentration CO ₂ gas such as factory exhaust gas.

In Non-Patent Document 2, low to high concentrations of CO ₂ can also be reduced, but the product is CO gas. Therefore, it is not as easy to store as formic acid, which is liquid at room temperature.

The CO ₂ concentration of factory exhaust gas varies widely from 10% to 100%, and the application requires a CO ₂ reduction device that operates over a wide range of concentrations. Most previous studies on _CO2 reducers have limited the _CO2 concentration to 100%.

The CO ₂ reduction reaction apparatus according to the present invention is arranged such that an anode electrode arranged in an electrolytic solution, one side is in contact with the electrolytic solution, and the other side is in contact with the CO ₂ -containing gas, and functions as a gas-liquid separation membrane, a porous and conductive cathode electrode; and a power source for applying a voltage between the anode electrode and the cathode electrode. Alternatively, a Ru complex polymer is supported as a reduction catalyst, and a _CO2 reduction reaction occurs between the protons in the electrolytic solution, the electrons from the cathode electrode, and the _CO2 from the other side, resulting in an organic substance is synthesized.

The porous conductor may be made of one or more materials selected from carbon paper, porous metal, and porous glass coated with conductive carbon or metal.

It is preferable that the cathode electrode is configured by supporting a Ru complex or Ru complex polymer and carbon nanotubes on the base material.

The cathode electrode may be arranged horizontally, with a lower side contacting the electrolyte and an upper side contacting the CO2 _- containing gas.

According to the invention, _CO2 is supplied from a CO2 _- containing gas. Since the diffusion rate of gas molecules in the gas phase is about 10 ⁴ higher than that in the liquid, the Faraday efficiency of formic acid production can be maintained high even at low CO ₂ concentrations. In addition, the high formic acid production selectivity of the Ru complex or Ru complex polymer can increase the Faradaic efficiency of formic acid.

1 is a diagram showing an outline of the configuration of a CO ₂ reduction device according to an embodiment (configuration of a device used in experiments); FIG. It is the result of the current (Current density (mA/cm ² )) vs. potential measurement of the cathode electrode in a small cell using a gas phase/liquid phase reduction reaction. CO ₂ concentration and reaction time (Time (sec)) dependence of gas-liquid CO ₂ reduction reaction current (current density (mA/cm ² )) (constant potential: -0.37 V (vs. RHE) applied ). Figure 2 is a graph plotting the Faradaic efficiency (%) of formic acid production in a small _cell using gas/liquid phase reduction reactions versus _CO2 concentration (%). FIG. 3 is a diagram showing another configuration example of a CO ₂ reduction reactor.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described based on the drawings. It should be noted that the invention is not limited to the embodiments described herein.

In the present embodiment, a Ru complex or Ru complex polymer, which has a high selectivity for converting CO ₂ to liquid formic acid, is supported on carbon paper that also functions as a gas-liquid separation to form a cathode electrode. One side of the cathode electrode is opposed to the anode electrode through the electrolyte, and CO ₂ gas is supplied to the other side to enable CO ₂ reduction from low concentration to high concentration at the cathode electrode.

"Configuration example"
FIG. 1 is a diagram showing an outline of the configuration of a CO ₂ reduction device according to an embodiment (the configuration of the device used in experiments).

The reaction tank 10 contains an electrolytic solution. A bulging portion 12 that bulges upward is provided on a portion of the upper wall of the reaction vessel 10 . The upper surface of the bulge 12 is flat and horizontally oriented, and an opening 16 is provided in the center thereof leaving a peripheral area.

A cathode electrode 18 is arranged to cover the upper side of the opening 16 . The bottom surface of the cathode electrode 18 is in contact with the electrolyte in the reaction vessel 10 through the opening 16 . The cathode electrode 18 is also oriented horizontally. The cathode electrode 18 is larger than the opening 16 and covers the entire opening 16. A sealing material 20 is arranged between the upper surface of the bulging portion 12 around the opening 16 and the cathode electrode 18, so that the electrolytic solution can flow around. Sealed to prevent leakage. The cathode electrode 18 functions as a gas-liquid separation membrane.

A CO ₂ gas container 22 containing a CO ₂ -containing gas is provided above the cathode electrode 18 . The content of CO ₂ in the CO ₂ -containing gas may be 100%, or may be about 10%. In this example, a CO ₂ -containing gas is circulated in the CO ₂ gas container 22 .

At the bottom of the CO ₂ gas container 22 , a bulging portion 24 that bulges downward in a shape corresponding to the bulging portion 12 of the reaction tank 10 is provided. An opening 26 is provided in the bulging portion 24 at a position corresponding to the opening 16 of the reaction vessel 10 , so that the upper surface of the cathode electrode 18 comes into contact with the CO ₂ -containing gas in the CO ₂ gas container 22 . That is, a sealing material 28 is arranged between the cathode electrode 18 and the lower surface of the bulging portion 24 around the opening 26 to seal the cathode electrode 18 so that the CO ₂ gas does not leak to the surroundings.

An anode electrode 30 and a reference electrode 32 are placed in the reaction bath 10 and immersed in the electrolytic solution.

The cathode electrode 18, the anode electrode 30, and the reference electrode 32 are connected to a separately provided potentiostat 34 as a power source, and the potential of each electrode is controlled by the potentiostat 34.

A porous conductor is used as a base material for the cathode electrode 18 , and the CO ₂ -containing gas in the CO ₂ gas container 22 dissolves and diffuses into the electrolyte through the cathode electrode 18 . The base material of the cathode electrode 18 supports a CO ₂ reduction catalyst, and Ru complex or Ru complex polymer is used as the CO ₂ reduction catalyst. The base material of the cathode electrode 18 is preferably composed of one or more materials selected from carbon paper, porous metal, and porous glass coated with conductive carbon or metal.

As for the electrolyte, anode electrode 30, and reference electrode 32, various substances employed in known CO ₂ reduction devices can be used. For example, a phosphoric acid buffer is used as the electrolyte, platinum (Pt) is used as the anode electrode 30, and silver/silver chloride (Ag/AgCl) is used as the reference electrode 32. Note that the reference electrode 32 is unnecessary in the actual device.

By applying a predetermined voltage between the anode electrode 30 and the cathode electrode 18 in such an apparatus, a CO ₂ reduction reaction occurs at the cathode electrode 18 . That is, a _CO2 reduction reaction occurs between the protons in the electrolyte, the electrons from the cathode electrode, and the _CO2 from the other side to synthesize organic matter, especially formic acid.

Here, according to the present embodiment, the cathode electrode 18 is a porous conductor, the electrolytic solution exists on the lower side, the _CO2 gas exists on the upper side, and the _CO2 on the upper side dissolves in the electrolytic solution. In this way, formic acid is obtained as a reduction product of _CO2 gas in the electrolyte.

Since the diffusion rate of gas molecules in gas phase is about 10 ⁴ higher than that in liquid, effective CO ₂ reduction reaction can occur even at low CO ₂ concentration. Therefore, it becomes possible to effectively reduce CO ₂ for factory exhaust gas and the like.

In addition, by using carbon paper as the base material of the cathode electrode 18 and supporting Ru complex or Ru complex polymer on it, high formic acid production selectivity can be obtained in a wide range of CO ₂ concentration, and formic acid can be produced with high Faradaic efficiency. can be generated.

In order to maintain the liquid level of the electrolyte in the reaction vessel 10 at the position of the cathode electrode 18, a liquid level gauge is provided to control the supply of the electrolyte, or an overflow weir is provided to control the liquid level. You can It is also preferable to determine the liquid level in consideration of the surface tension in the cathode electrode 18 .

[Description of Experimental Example]
Electrochemical measurements were performed with the experimental setup shown in Fig. 1 to determine the _CO2 concentration dependence of the Faradaic efficiency of formic acid production.

The faradaic formic acid efficiency of the cathode electrode 18 is determined by electrochemical measurement using an apparatus in which the reaction vessel 10 and the CO ₂ gas container 22 as shown in FIG. Calculated.

A method for manufacturing the cathode electrode 18 will be described below. A carbon paper was coated with the multi-walled carbon nanotube dispersion solution and dried to obtain a carbon substrate. By attaching carbon nanotubes, the surface area can be increased and the performance of the catalyst can be improved.

Next, an acetonitrile solution of an Ru complex monomer, a Ru complex polymer polymerized using pyrrole and iron chloride was applied to the carbon substrate. Here, the Ru complex monomer is [Ru(C3pyrrol-dcbpy)(CO) ₂ Cl ₂ ]. After that, the solvent (acetonitrile) was removed to obtain the cathode electrode 18 supporting the Ru complex polymer.

A cathode electrode 18 was sandwiched between the reactor 10 and the CO ₂ gas container 22 to separate the two chambers. Sealing materials 20 and 28 were sandwiched between the upper and lower chambers of the cathode electrode 18 in order to keep the connecting portion airtight. The seal members 20 and 28 are made by sandwiching a plastic plate between two sheets of silicone rubber, each having a through hole of 10 mm×10 mm at the center. A Cu tape sandwiched between sealing materials 20 and 28 and the cathode electrode 18 was used for current collection of the cathode electrode 18 .

A Pt wire was used as the anode electrode 30 as a counter electrode, and an Ag/AgCl electrode was used as the reference electrode 32 . The electrolytic solution was a 0.1 M phosphate buffer (pH 6.8) prepared with K ₂ HPO ₄ and KH ₂ PO ₄ and injected into the reaction vessel 10 until it touched the cathode electrode 18 .

A CO ₂ -containing gas adjusted to a predetermined concentration was flowed into the CO ₂ gas container 22 . The concentrations used in the experiments were 100, 50, 20, 10, 5% and were made by diluting with _N2 .

The measurement procedure is shown below. First, CO ₂ gas with a concentration of 100% was flowed into the CO ₂ gas container 22 for 30 minutes, and then current-potential measurement was performed in the applied potential range of 0.6 V to -1.0 V (vs RHE). Subsequently, constant potential measurement was performed at an applied potential of −0.37 V (vs. RHE) for 30 minutes.

After the measurement, the electrolyte was sampled for quantitative determination of formic acid. Duplicate samples were taken to account for variability. After that, the constant potential measurement was performed again for 30 minutes, and the electrolytic solution was similarly sampled twice. Subsequently, the concentration of CO ₂ gas was changed to 50%, and the gas was allowed to flow into the CO ₂ gas container 22 for 30 minutes. As in the case of 100%, constant potential measurement was performed at −0.37 V (vs. RHE), and the electrolytic solution was sampled twice every 30 minutes. Constant potential measurements in 20, 10, and 5% gases and sampling of the electrolytic solution were performed in the same manner. The amount of formic acid in the sampled electrolyte was quantified by ion chromatography, and the Faradaic efficiency of formic acid production was calculated.

[Experimental result]
FIG. 2 shows the results of current (current density (mA/cm ² )) versus potential measurement of a cathode electrode in a small cell using a gas phase/liquid phase reduction reaction. Figure 3 shows the CO ₂ concentration and reaction time (Time (sec)) dependence of the gas-liquid CO ₂ reduction reaction current (current density (mA/cm ² )) (constant potential: -0.37 V (vs. RHE )).

The spikes observed every 30 minutes are transient currents at the start and stop of the measurement. The current decreased as the _CO2 concentration decreased. It is considered that this is because the CO ₂ reduction reaction probability decreases when the concentration of CO ₂ , which is a reaction substrate, decreases. No significant current drop is seen during the 1 hour measurement at each concentration.

FIG. 4 is a graph plotting the Faradaic efficiency (%) of formic acid production in _{a small cell using gas phase/liquid phase reduction reaction against CO 2 concentration} (%). This value is the average of all the values of two samples at 30 minutes of potentiostatic measurement and two samples at 1 hour. The faradaic efficiency was almost constant above 80% in the CO ₂ concentration range of 100% to 20%. At 10% and 5%, it showed a decreasing trend, but still maintained a Faraday efficiency of 60% or more, indicating that high formic acid conversion is possible over a wide range of CO ₂ concentrations.

"Other configuration examples"
FIG. 5 is a diagram showing another configuration example of the CO ₂ reduction reactor. Thus, the anode electrode 30 is immersed in the reaction bath 10 . On the other hand, the cathode electrode 18 is supported on the water surface by the float 40 . The float 40 may be cylindrical, spherical, or non-circular in cross section. Moreover, the place where the float 40 is provided is arbitrary.

By setting the buoyancy of the float 40 and the weight of the cathode electrode, the liquid surface of the electrolytic solution is positioned substantially above the porous cathode electrode 18 . In addition, by installing the reaction tank 10 in a place where the atmosphere is a CO ₂ -containing gas, the upper surface of the cathode electrode 18 and the liquid surface of the electrolyte come into contact with the CO ₂ -containing gas. In this state, by applying a predetermined voltage between the anode electrode 30 and the cathode electrode 18 by the power supply 42, _CO2 is dissolved in the electrolyte near the cathode electrode 18, and _CO2 is reduced in the same manner as described above. A reaction can occur to produce formic acid.

In the case of this example, even if the liquid level of the electrolyte changes, the effect that the cathode electrode 18 is held at the liquid level is obtained. Also, the Faradaic efficiency of formic acid production can be kept high over a broad band of _CO2 concentrations.

"Actions and effects in the embodiment"
The diffusion rate of gas molecules in the gas phase is about 10 ⁴ higher than the diffusion rate in liquid. Therefore, the faradaic efficiency of formic acid production can be maintained high even at low _CO2 concentrations. In addition, the high formic acid generation selectivity of CO ₂ over a wide concentration range by the Ru complex or Ru complex polymer can keep the Faradaic efficiency of formic acid high.

10 reaction vessel, 12,24 bulging part, 16,26 opening, 18 cathode electrode, 20,28 sealing material, 22 CO ₂ gas container, 30 anode electrode, 32 reference electrode, 34 potentiostat (power supply), 40 float, 42 power supply.

Claims (4)

an anode electrode disposed in an electrolyte;
a cathode electrode which is arranged so that one side is in contact with the electrolyte and the other side is in contact with the CO ₂ -containing gas, functions as a gas-liquid separation membrane, and is porous and conductive;
a power source that applies a voltage between the anode electrode and the cathode electrode;
including
The cathode electrode is configured by supporting a Ru complex or Ru complex polymer as a reduction catalyst on a substrate made of a porous conductor,
A _CO2 reduction reaction occurs between the protons in the electrolyte, the electrons from the cathode electrode, and the _CO2 from the other side to synthesize an organic substance.
_CO2 reduction reactor. A _CO2 reduction reactor according to claim 1, comprising:
The porous conductor is composed of one or more selected from carbon paper, porous metal, and porous glass coated with conductive carbon or metal.
_CO2 reduction reactor. A _CO2 reduction reactor according to claim 1 or 2, comprising:
The cathode electrode is configured by supporting a Ru complex or Ru complex polymer and carbon nanotubes on the base material,
_CO2 reduction reactor. A CO ₂ reduction reactor according to any one of claims 1 to 3,
The cathode electrode is arranged in a horizontal direction and contacts the electrolytic solution on the lower side and the CO2 _- containing gas on the upper side.
_CO2 reduction reactor.

Images