EP-4741532-A1 - VOLTAGE CONTROL METHOD

Abstract

A voltage control method of an embodiment is for applying a voltage to an electrochemical device that has an electrode with a catalyst containing a noble metal and periodically reversing a polarity of the voltage to perform an electrolytic reaction and recover a valuable resource from the electrochemical device. The voltage control method includes continuously increasing an absolute value of the voltage and then holding the value while periodically reversing the polarity of the voltage.

Inventors

• KANAMURA, SHOHEI
• FUJIMAKI, TAKURO

Assignees

• Kabushiki Kaisha Toshiba
• Toshiba Energy Systems & Solutions Corporation

Dates

Publication Date

20260513

Application Date

20250721

Claims (5)

1. A voltage control method for applying a voltage to an electrochemical device that has an electrode with a catalyst containing a noble metal and periodically reversing a polarity of the voltage to perform an electrolytic reaction and recover a valuable resource from the electrochemical device, the voltage control method comprising continuously increasing an absolute value of the voltage and then holding the value while periodically reversing the polarity of the voltage.
2. The method according to claim 1, comprising: a first period in which the value of the voltage is decreased linearly or curvilinearly from a first value to equal or lower than a second value that is lower than the first value and then held; and a second period in which the value of the voltage is increased linearly or curvilinearly from a third value the same as or different from the first value to equal to or higher than a fourth value that is higher than the third value and then held.
3. The method according to claim 2, comprising: a third period in which the voltage is not applied between the first period and the second period.
4. The method according to claim 2, wherein a time necessary for the value of the voltage to reach the second value from the first value is 0.6 seconds or longer, in the first period, and a time necessary for the value of the voltage to reach the fourth value from the third value is 0.6 seconds or longer, in the second period.
5. The method according to claim 1, wherein the catalyst contains at least one noble metal selected from the group consisting of platinum, iridium, ruthenium, rhodium, palladium, gold, and rhenium.

Description

FIELD Embodiments relate to a voltage control method. BACKGROUND As global transition to a carbon neutral society progresses, hydrogen is being focused on as an alternative to fossil energy. In a society where hydrogen is used for various purposes, electrochemical devices such as a water electrolysis device that produces hydrogen by electrolysis of water and a fuel cell that generates power by using obtained hydrogen are important. There are various types of water electrolysis devices and fuel cells, depending on differences in operating temperature and device construction. Among those devices, a PEM-type water electrolysis device and a polymer electrolyte fuel cell (PEFC) using a solid polymer membrane as an electrolyte and a noble metal as an electrode catalyst have merits such as possibilities of size reduction and low-temperature operation, and are expected to become further widespread. Structures of the PEM-type water electrolysis device and the PEFC are quite similar, and both use a membrane electrode assembly (MEA) in which electrocatalysts are coated on both sides of a solid polymer membrane. As transition to the hydrogen society progresses in the future, the number of PEM-type water electrolysis devices and PEFCs on the market increases, and a total amount of noble metals used for electrode catalysts also increases. Thus, in order for those devices to become widely used, it is necessary to secure a sufficient amount of noble metals. As a method for securing the amount of noble metals, there is cited recycling of a used PEM-type water electrolysis device and PEFC to recover noble metals. Procurement costs are expected to be decreased by recovering noble metals that are present in high concentration in waste products than by recovering noble metals contained in quite low concentration in ores. Further, recovering unevenly existing resources from waste articles enables stable row material procurement to thereby allow sustainable product manufacturing. As a method of recovering a noble metal from a fuel cell, generally, after wastes are burned in an incinerator or the like, a noble metal component contained in ash is dissolved with a strongly acidic solution such as aqua regia, separated, and recovered (Patent Document 1). Further, as a method simpler and low in environmental load, a technology for dissolving and recovering a noble metal from a fuel cell by using electrolysis (Patent Documents 2, 3, and 4) is also developed, and development of a fuel cell recycling technology is expected to progress further in the future. The technology for dissolving a noble metal from a MEA by electrolysis, the technology described in Patent Documents 2, 3, and 4, enables easy dissolution of a normally indissoluble noble metal by using dilute hydrochloric acid at a room temperature and is expected to be implemented in society as a method with a low environmental load. In the electrolysis method used here, the noble metal is dissolved by periodically reversing a polarity of an applied voltage, unlike in ordinary electrolysis that is performed at a constant voltage or constant current. When a voltage is applied to an electrochemical device from the outside, there are observed a nonfaradaic current associated with charging of an electric double layer at an interface between an electrolytic solution and an electrode and a faradaic current associated with progress of an electrochemical reaction. The nonfaradaic current flows at a moment when the voltage is applied and does not flow when and after charging at the interface is completed, and only the faradaic current by the electrochemical reaction is observed (Non-patent Document 1). When electrolysis is performed such that the polarity of the applied voltage is periodically reversed, a nonfaradaic current flows at every moment of polarity switching. Since an electricity quantity saved at the interface increases as a polarity area becomes large, a large nonfaradaic current flows intermittently in a device such as a PEM-type water electrolysis device and a PEFC for which a porous body with a large electrode area is used as an electrode material. Since the nonfaradaic current flowing immediately after reversing of the polarity is quite large compared with the faradaic current, it is necessary to prepare a power supply with a large output size that can withstand a current larger than the current actually used for noble metal dissolution when noble metal dissolution by the electrolytic method is performed. Patent Document 4 shows an example of performing electrolysis by applying a rectangular wave voltage. Though noble metal dissolution is possible also by this method, when considering a size increase of a device, a large current flows at a time of polarity reversal, and there is required an increase in a device cost and a safety mechanism against a the large current. REFERENCE Patent Document 1: Japanese Patent No. 1626036Patent Document 2: Japanese Patent No. 6652518Patent D