Patent Publication Number: US-2012024716-A1

Title: Device and method for reducing carbon dioxide

Description:
This application is a Continuation of PCT Application No. PCT/JP2011/001520 filed on Mar. 15, 2011, the entire content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD  
     The present disclosure relates to a device and a method for reducing carbon dioxide. 
     SUMMARY  
     The purpose of the present disclosure is to provide a novel device and method for reducing carbon dioxide. A device for reducing carbon dioxide includes a vessel for holding an electrolyte solution including carbon dioxide, a working electrode, and a counter electrode. The working electrode contains at least one boride selected from the group consisting of strontium hexaboride (SrB 6 ), calcium hexaboride (CaB 6 ), barium hexaboride (BaB 6 ), lanthanum hexaboride (LaB 6 ), and cerium hexaboride (CeB 6 ). The counter electrode may contain one of platinum, gold, silver, copper, nickel and titanium. 
     The working electrode may contain particles of the metal boride disposed on a substrate. The substrate may be a carbon paper, a noble metal substrate, a glassy carbon substrate or a conductive silicon substrate. 
     The device may further include a solid electrolyte membrane interposed between the working electrode and the counter electrode. The device may further include a reference electrode. 
     A method for reducing carbon dioxide according to the present disclosure includes a step (a) of preparing any one of the devices as set forth above. The vessel holds an electrolyte solution containing carbon dioxide. The metal boride of the working electrode is in contact with the electrolytic solution. The method further includes a step (b) of applying a negative voltage and a positive voltage to the working electrode and the counter electrode respectively to reduce the carbon dioxide contained in the electrolytic solution. By the reduction reaction, at least one of methane, ethylene, ethan and formic acid is generated. 
     In the step (b), a potential difference applied between the working electrode and the counter electrode is not less than 2.0 volts. 
     The present disclosure provides a novel device and method for reducing carbon dioxide. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS  
         FIG. 1  shows an exemplary device for reducing carbon dioxide according to the embodiment 1. 
         FIG. 2  shows a graph of the result of the reduction current—electric field potential measurement (C-V measurement) in the example 1, using an electrode comprising SrB 6 . 
         FIG. 3  shows a graph of the result of the gas chromatography in the example 1. 
         FIG. 4  shows a graph of the result of the liquid chromatography in the example 1. 
         FIG. 5A  shows a graph of the result of the reduction current—electric field potential measurement (C-V measurement) in the example 2, using an electrode comprising CaB 6 . 
         FIG. 5B  shows a graph of the result of the reduction current—electric field potential measurement (C-V measurement) in the example 2, using an electrode comprising BaB 6 . 
         FIG. 5C  shows a graph of the result of the reduction current—electric field potential measurement (C-V measurement) in the example 2, using an electrode comprising LaB 6 . 
         FIG. 5D  shows a graph of the result of the reduction current—electric field potential measurement (C-V measurement) in the example 2, using an electrode comprising CeB 6 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The exemplary embodiments are described below. 
     (Step (a)) 
     In the step (a), a device for reducing carbon dioxide is prepared. As shown in  FIG. 1 , the device comprises a vessel  21 , a working electrode  11 , and a counter electrode  13 . An electrolytic solution  15  is hold in the vessel  21 . An example of the electrolytic solution  15  is a potassium hydrogen carbonate aqueous solution. The electrolytic solution  15  contains carbon dioxide. It is preferable that the electrolytic solution  15  is mild acidic in the condition where carbon dioxide is dissolved in the electrolytic solution  15 . 
     The working electrode  11  contains metal hexaboride such as strontium hexaboride (SrB 6 ), calcium hexaboride (CaB 6 ), barium hexaboride (BaB 6 ), lanthanum hexaboride (LaB 6 ), or cerium hexaboride (CeB 6 ). The working electrode  11  can be fabricated as below. 
     First, boride particles are dispersed in an organic solvent to form slurry. Next, the slurry is applied to a porous conductive base material to obtain the working electrode  11 . This base material preferably has a shape of a film. An example of the base material is a carbon paper, a noble metal substrate, a glassy carbon substrate, or a conductive silicon substrate. The working electrode may be formed by a sputtering method. 
     The working electrode  11  is in contact with (immersed in) the electrolytic solution  15 . To be exact, the boride which the working electrode  11  comprises is in contact with the electrolytic solution  15 . In  FIG. 1 , the working electrode  11  is immersed in the electrolytic solution  15 . As long as the boride is in contact with the electrolytic solution  15 , only a part of the working electrode  11  may be immersed in the electrolytic solution  15 . 
     The counter electrode  13  contains metal. An example of the preferred metal is platinum, gold, silver, copper, nickel, and titanium. As long as the metal is not electrolyzed, the material of the metal is not limited. 
     The counter electrode  13  is in contact with the electrolytic solution  15 . To be exact, the metal which the counter electrode  13  comprises is in contact with the electrolytic solution  15 . In  FIG. 1 , the counter electrode  13  is immersed in the electrolytic solution  15 . As long as the metal is in contact with the electrolytic solution  15 , only a part of the counter electrode  13  may be immersed in the electrolytic solution  15 . 
     As shown in  FIG. 1 , it is preferable that the vessel  21  comprises a tube  17 . Carbon dioxide is supplied through the tube  17  to the electrolytic solution  15 . One end of the tube  17  is immersed in the electrolytic solution  15 . 
     It is preferred that a solid electrolyte membrane  16  is provided in the vessel  21 . This reason is described later in the step (b). The solid electrolyte membrane  16  is interposed between the working electrode  11  and the counter electrode  13  to divide the electrolytic solution  15  into a first liquid  15 L and a second liquid  15 R. The counter electrode  13  is in contact with the first liquid  15 L. The working electrode is in contact with the second liquid  15 R. 
     (Step (b)) 
     In the step (b), a negative voltage and a positive voltage are applied to the working electrode  11  and the counter electrode  13 , respectively. This causes the carbon dioxide contained in the electrolytic solution  15  (to be exact, the second liquid  15 R) to be reduced on the working electrode  11 . As a result, at lease one of carbon monoxide, formic acid, and methane is generated on the working electrode  11 . On the counter electrode  13 , water is oxidized to form oxygen. 
     It is preferred to use a potentiostat  14  to apply a potential difference between the working electrode  11  and the counter electrode  13 . 
     The potential difference applied between the working electrode  11  and the counter electrode  13  is preferably not less than  1 . 8  volts. This corresponds to the fact that carbon dioxide reduction current is measured at not more than −0.5 volts (and not less than −1.6 volts) in the example 1, which is described later. 
     In the preferable embodiment, the solid electrolyte membrane  16  is provided. Only a proton penetrates the solid electrolyte membrane  16 . An example of the solid electrolyte membrane  16  is a Nafion (Registered Trademark) film, which is available from Dupont Kabushiki Kaisha. 
     The solid electrolyte membrane  16  prevents a reverse reaction on the counter electrode  13 . Namely, when the carbon monoxide, formaldehyde, or methane, which is generated on the working electrode  11 , reaches the counter electrode  13 , it is oxidized on the counter electrode  13  to return to carbon dioxide. The solid electrolyte membrane  16  prevents this reverse reaction. 
     As shown in  FIG. 1 , it is preferred that a reference electrode  12  is further provided. The reference electrode  12  is in contact with the electrolytic solution  15 . When the solid electrolyte membrane  16  is used, the reference electrode  12  is in contact with the second liquid  15 R. The reference electrode  12  is electrically connected to the working electrode  11 . An example of the reference electrode  12  is a silver/silver chloride electrode. 
     The present device and method are described in more detail by the following example. 
     EXAMPLE 1 
     Particles of strontium hexaboride (SrB6, Furuuchi Chemical, purity of 99%) having an average particle size of several microns are disposed, with a distribution density of 1×10 7  particle/cm 2 , on a conductive carbon paper (CP) having a thickness of 0.5 mm, thereby making an electrode catalyst (working electrode) according to the present subject matter. Using this electrode catalyst, electrochemical reduction reaction of CO 2  was performed.  FIG. 1  shows a structural sectional view of the electrochemical cell used for this measurement. The electrochemical cell includes three electrodes, i.e., the boride particle supported electrode as set forth above as the working electrode  11 , a silver/silver chloride electrode (Ag/AgCl electrode) as the reference electrode  12 , and a platinum electrode (Pt-electrode) as the counter electrode  13 . The electric potential applied to the three electrodes was changed by using potensiostat  14 , and the reduction reaction of CO 2  was performed and evaluated. As the electrolyte  15 , 0.1M potassium bicarbonate aqueous solution (KHCO 3  aqueous solution) was used. The working electrode  11  and the counter electrode  13  were partitioned off with a solid electrolyte membrane  16  to prevent the gases produced by the catalytic reaction from being mixed. CO 2  gas was introduced into the electrolyte  15  though the gas instruction tube  17  arranged in the vessel  21  by being bubbled in the KHCO 3  electrolytic solution  15 . 
     First of all, (1) a nitrogen gas was introduced into an electrolyte for 30 minutes with a flow rate of 200 ml/min, keeping a bubbling state to exclude CO 2  from the electrolyte solution. Under this state, the electric potential was changed, and a curve of reduction current—electrolysis voltage (C-V curve) was measured. Next, (2) the gas was switched from nitrogen to CO 2  and the CO 2  gas was introduced into the electrolyte  15  for 30 minutes with the same flow rate of 200 ml/min so that the electrolyte  15  was saturated with CO 2 . Under this state, the electric potential was changed, and a C-V curve was measured. A reduction current by CO 2  reduction reaction was evaluated by taking a difference between the C-V curve in the state (2) (the state saturated with CO 2  ) and the C-V curve in the state (1) (the state that CO 2  was excluded).  FIG. 2  shows the result of the difference between the two C-V curves. 
     In this figure, the state that the current value (vertical axis) is negative shows that CO 2  reduction reaction occurred. As shown in  FIG. 2 , at the applied voltage is around −0.5 V, the reduction current changes from zero to negative in the experimental result of this example. In other words, when the electrode catalyst including the boride particles is used, the reduction current of CO 2  was observed at the voltage of approximately −0.7V with respect to the silver/silver chloride electrode (Ag/AgCl electrode) as the reference electrode. This result means that the reduction reaction has started at about −0.5V in a case using the standard hydrogen-electrode. On the other hand, when a CO 2  reducing experiment was conducted with an electrode catalyst of Cu in the same measurement system, the voltage smaller than −1.1V (i.e., larger in an absolute value) was necessary to cause the reduction reaction of CO 2 . This comparison shows that the electrode catalyst which includes boride is effective in reduction of voltage for reduction reaction of CO 2 . 
     Next, the product of the reduction reaction of CO 2  using the electrode on which the boride particles was supported was analyzed. For the analysis of gas components, a gas chromatograph of the hydrogen flame ion detector (FID) method was employed, and for the analysis of liquid components, a liquid chromatograph of the UV detection method was employed. 
       FIG. 3  shows the measurement result of detected methane (CH 4 ), ethylene (C 2 H 4 ) and ethan (C 2 H 6 ) with the gas chromatograph of FID. By using a separate column of PrapakQ and controlling a valve with a predetermined time-sequence, the FID gas chromatograph is programmed so that CH 4  is detected at around 1.5 minutes after the start of the measurement, C 2 H 4  is detected at around 4.5 minutes, and C 2 H 6  is detected at around 6.5 minutes, respectively. As a result, as shown in  FIG. 3 , the peaks of voltage were observed by time domains which correspond to the respective times, and it was confirmed that CH 4 , C 2 H 4  and C 2 H 6  were generated. 
     The measurement result of formic acid (HCOOH) by the liquid chromatograph is shown in  FIG. 4 . By using a column of TSKgel SCX—H+, the liquid chromatograph was set so that a peak of HCOOH is detected at around 11.5 minutes after the start of the measurement. As a result, as shown in  FIG. 4 , a peak of the voltage was observed in the range correspond to this time. As a result, it was confirmed that HCOOH was generated by the reduction reaction of CO 2  by using the electrode catalyst that includes the boride particles. As set forth above, the generation of methane (CH 4 ), ethylene (C 2 H 4 ), ethan (C 2 H 6 ) and formic acid (HCOOH) were confirmed finally by the result of analysis of the product produced by the catalytic reaction. 
     EXAMPLE 2 
     As another CO 2  reduction electrode catalyst material, similar experiments were conducted with the use of calcium hexaboride (CaB 6 ), barium hexaboride (BaB 6 ), lanthanum hexaboride (LaB 6 ), and cerium hexaboride (CeB 6 ). As a result, similarly to the result obtained in with use of the strontium hexaboride (SrB 6 ), the generation of methane (CH 4 ), ethylene (C 2 H 4 ), ethan (C 2 H 6 ) and formic acid (HCOOH) were confirmed. In addition, similarly to the case of SrB 6 , CO 2  reduction current was observed under lower voltage than that of copper. 
       FIGS. 5A to 5D  show CO 2  reduction currents obtained with the use of the electrode catalyst supporting CaB 6  particles, BaB 6  particles, LaB 6  particles, and CeB 6  particles, respectively. The reduction current was observed at a voltage of around −0.5 volts in the case of the CaB 6  electrode and at a voltage of around −0.8 volts in the case of the BaB 6  electrode, with respect to the Ag/AgCl electrode as the reference electrode. With respect to the LaB 6  electrode catalyst, the reduction current was observed at a voltage of around −0.8 volts. With respect to CeB 6  electrode catalyst, the reduction current was observed at a voltage of around −0.6 volts. 
     COMPARATIVE EXAMPLE 1 
     For a comparison, electrolytic reaction was measured with only the carbon paper (CP) which was used to support a boron particle. As a result, an electric current by the reduction of CO 2  was not observed, and it was confirmed that CP was inactive for reducing of CO 2 . The product by the electrolytic reaction was only hydrogen (H 2 ). 
     COMPARATIVE EXAMPLE 2 
     For another comparison, electrolytic reaction was measured with the borides of titanium (Ti) and zirconium (Zr). As a result, hydrogen (H 2 ) was a main product for the experiment, and hydrocarbon or formic acid (HCOOH) as a product by the electrolysis reaction was not generated. 
     INDUSTRIAL APPLICABILITY 
     The present device and method provide a novel method for reducing carbon dioxide.