Electrochemical reaction device and valuable material manufacturing system

An electrochemical reaction method, comprising:

• supplying gas containing carbon dioxide to a first flow path of an electrochemical reaction cell so that the gas is in contact with a reduction electrode, and supplying an electrolytic solution containing water or water vapor to a second flow path of the electrochemical reaction cell so that the electrolytic solution or water vapor is in contact with a oxidation electrode, wherein the electrochemical reaction cell comprises the first flow path, the second flow path, the reduction electrode, the oxidation electrode, and a diaphragm provided between the reduction electrode and the oxidation electrode, the reduction electrode having a first surface and a second surface opposite to the first surface, the first flow path facing on the first surface, the diaphragm being in contact with the second surface;
• applying a voltage to the electrochemical reaction cell from a power supply connected to the reduction electrode and the oxidation electrode, to reduce carbon dioxide and thus produce a carbon compound, and to oxidize water and thus produce oxygen;
• discharging a mixing gas containing a produced gas with the reduction electrode and an unreacted carbon dioxide from the first flow path and a mixture containing the electrolytic solution or water vapor and a produced gas with the oxidation electrode from the second flow path;
• measuring at least one of a first flow rate of the mixing gas discharged from the first flow path and a second flow rate of the mixture discharged from the second flow path to obtain a flow rate signal of at least one of the mixing gas and the mixture as a detection signal;
• and
• controlling an amount of the gas containing the carbon dioxide to be supplied to the first flow path based on a control signal which is obtained by converting the flow rate signal in consideration of temperature information inside the electrochemical reaction cell so that a concentration of the unreacted carbon dioxide in the mixing gas discharged from the first flow path takes a desired value, wherein
• the concentration of the unreacted carbon dioxide in the mixing gas discharged from the first flow path is controlled in a desired value by measuring a flow rate of a second gas discharged from the second flow path and adjusting a flow rate of the gas containing the carbon dioxide to be supplied to the first flow path when an allowable carbon dioxide concentration of the first gas discharged from the first flow path is denoted as Δy, where Δy is a numerical value, and a flow rate of the second gas per a unit electrode area varies within a range of −Δy/0.5665 ml/min/cm2 or more and −Δy/0.0335 ml/min/cm2 or less.