Water electrolysis system and control method of water electrolysis system

A control method executed by electronic equipment of a water electrolysis system comprising:

• a water electrolysis cell having an electrolyte membrane, and an anode and a cathode provided on both sides of the electrolyte membrane in a thickness direction, and configured to generate oxygen of a higher pressure on the anode than a pressure of an aqueous solution on the cathode while electrolyzing water of the aqueous solution supplied to the cathode by applying a voltage between the anode and the cathode;
• a power supply configured to apply the voltage between the anode and the cathode;
• an aqueous solution supply source configured to supply the aqueous solution containing hydroxide ions of a predetermined concentration to the cathode;
• a flow rate restricting unit configured to restrict a flow rate of oxygen in a flow channel for the oxygen discharged from the anode;
• and
• the electronic equipment,
• the control method of the water electrolysis system operated by the electronic equipment, comprising:
• an acquisition step of acquiring a concentration of the hydroxide ions of the aqueous solution corresponding to an acquisition value of each of the voltage and the current on the basis of information on predetermined correspondence among the voltage and the current between the anode and the cathode and the concentration of the hydroxide ions of the aqueous solution of the cathode;
• and
• a change step of changing an operation state of the water electrolysis system to increase the voltage while restricting a supply amount of the aqueous solution to the cathode so as to increase a film thickness of the electrolyte membrane through thermal expansion when the concentration of the hydroxide ions of the aqueous solution acquired by the acquisition step is higher than a predetermined reference concentration range corresponding to a predetermined combination of the voltage and the current.