Carbon dioxide reduction device and method for reducing carbon dioxide

Abstract

A device for reducing CO 2 by light, including: a cathode chamber holding a first electrolyte solution that contains CO 2 ; an anode chamber holding a second electrolyte solution; a proton conducting membrane disposed in a connecting portion between these chambers; a cathode electrode; and an anode electrode. The cathode electrode has a CO 2 reduction reaction region composed of a metal or a metal compound, and the anode electrode has a photochemical reaction region composed of nitride semiconductors. The photochemical reaction region of the anode electrode has a multilayer structure of a GaN layer and an Al x Ga 1-x N layer containing Mg (0<x≦0.25). The content of Mg in the Al x Ga 1-x N layer is 1×10 15 or more and 1×10 19 or less in terms of the number of Mg atom per cm 3 . The anode electrode is disposed in such a manner that the Al x Ga 1-x N layer can be exposed to light.

What is claimed is: 
     
         1 . A carbon dioxide reduction device for reducing carbon dioxide by light energy, comprising:
 a cathode chamber holding a first electrolyte solution that contains carbon dioxide;   an anode chamber holding a second electrolyte solution, connected to the cathode chamber;   a proton conducting membrane that is disposed in a connecting portion between the anode chamber and the cathode chamber so as to serve as a separator between the first electrolyte solution and the second electrolyte solution and to conduct hydrogen ions between the first and second electrolyte solutions;   a cathode electrode disposed in the cathode chamber so as to be in contact with the first electrolyte solution; and   an anode electrode disposed in the anode chamber so as to be in contact with the second electrolyte solution, wherein   the cathode electrode has a carbon dioxide reduction reaction region that is in contact with the first electrolyte solution and is composed of a metal or a metal compound,   the anode electrode has a photochemical reaction region that is in contact with the second electrolyte solution and is composed of nitride semiconductors,   the photochemical reaction region of the anode electrode has a multilayer structure of a GaN layer and an Al x Ga 1-x N layer containing Mg (0<x≦0.25),   a content of Mg in the Al x Ga 1-x N layer is 1×10 15  or more and 1×10 19  or less in terms of the number of Mg atoms that are contained in a unit volume (1 cm 3 ) of the Al x Ga 1-x N layer,   the anode electrode is disposed in the anode chamber in such a manner that the Al x Ga 1-x N layer in the photochemical reaction region can be exposed to light, and   the cathode electrode and the anode electrode are electrically connected to each other without an external power source interposed therebetween.   
     
     
         2 . The carbon dioxide reduction device according to  claim 1 , wherein the content of Mg in the Al x Ga 1-x N layer is 1×10 16  or more and 1×10 18  or less in terms of the number of Mg atoms that are contained in the unit volume (1 cm 3 ) of the Al x Ga 1-x N layer. 
     
     
         3 . The carbon dioxide reduction device according to  claim 1 , wherein the x has a value of 0.10 or more and 0.15 or less. 
     
     
         4 . The carbon dioxide reduction device according to  claim 1 , wherein the GaN layer is composed of an n-type GaN. 
     
     
         5 . The carbon dioxide reduction device according to  claim 1 , wherein a metal oxide containing Ni is disposed on the Al x Ga 1-x N layer in the photochemical reaction region. 
     
     
         6 . The carbon dioxide reduction device according to  claim 5 , wherein the metal oxide is in the form of fine particles. 
     
     
         7 . The carbon dioxide reduction device according to  claim 1 , wherein the metal constituting the reduction reaction region includes at least one selected from copper, gold, silver, tantalum, and indium. 
     
     
         8 . The carbon dioxide reduction device according to  claim 1 , wherein the first electrolyte solution is an aqueous solution containing at least one electrolyte selected from potassium bicarbonate, sodium bicarbonate, potassium chloride, and sodium chloride. 
     
     
         9 . A method for reducing carbon dioxide using a carbon dioxide reduction device, wherein
 the device is the carbon dioxide reduction device according to  claim 1 , and   the method comprises the step of irradiating the Al x Ga 1-x N layer in the photochemical reaction region of the anode electrode with light having a wavelength of 365 nm or less, with the first electrolyte solution and the second electrolyte solution being held in the cathode chamber and the anode chamber respectively, so as to allow generation of electrons and hydrogen ions to proceed in the photochemical reaction region and to allow a reaction of reducing carbon dioxide contained in the first electrolyte solution to proceed in the reduction reaction region of the cathode electrode.   
     
     
         10 . The method for reducing carbon dioxide according to  claim 9 , further comprising the step of introducing a gas containing carbon dioxide into the first electrolyte solution held in the cathode chamber. 
     
     
         11 . The method for reducing carbon dioxide according to  claim 9 , wherein the step is performed with the device being placed at room temperature and atmospheric pressure. 
     
     
         12 . The method for reducing carbon dioxide according to  claim 9 , wherein the reaction of reducing carbon dioxide produces at least one selected from methanol, ethanol, acetaldehyde, formic acid, methane, ethylene, and carbon monoxide.