Voltage control method

Abstract

This invention provides a voltage control method for recovering valuable materials from electrochemical devices, which can suppress the generation of large currents. [Solution] The voltage control method is a voltage control method for recovering valuable substances from an electrochemical device equipped with an electrode having a catalyst containing a precious metal, by applying a voltage to the device and periodically reversing the polarity of the voltage to perform an electrolytic reaction, wherein the polarity of the voltage is periodically reversed by increasing the absolute value of the voltage over time and then holding it.

Description

[Technical Field]

[0001] Embodiments of the present invention relate to a voltage control method. [Background technology]

[0002] As the global transition to a carbon-neutral society progresses, hydrogen is attracting attention as an alternative to fossil fuels. In a society where hydrogen is used for various purposes, electrochemical devices such as water electrolyzers that produce hydrogen by electrolyzing water, and fuel cells that generate electricity using the obtained hydrogen, will become important. Various types of water electrolyzers and fuel cells exist, differing in operating temperature and device configuration. Among these, PEM-type water electrolyzers and polymer electrolyte membranes (PEFCs), which use a solid polymer membrane as the electrolyte and a precious metal as the electrode catalyst, are expected to become even more widespread due to their compact size and low-temperature operation capabilities. The structures of these PEM-type water electrolyzers and PEFCs are extremely similar, and both use a membrane electrode assembly (MEA) in which electrode catalysts are coated on both sides of a solid polymer membrane.

[0003] As the world moves towards a hydrogen-based society, the number of PEM-type water electrolyzers and PEFCs on the market will increase, but so will the total amount of precious metals used in electrode catalysts. Therefore, securing a sufficient supply of precious metals is necessary for the widespread adoption of these devices. One way to secure a sufficient supply of precious metals is to recycle used PEM-type water electrolyzers and PEFCs to recover them. Recovering precious metals present in high concentrations in waste products is expected to reduce procurement costs compared to recovering precious metals present in extremely low concentrations in ore. Furthermore, recovering resources that are unevenly distributed from waste products will enable a stable supply of raw materials and sustainable product manufacturing.

[0004] Generally, methods for recovering precious metals from fuel cells involve burning the waste in an incinerator, then dissolving the precious metal components in the ash with a strong oxidizing solution such as aqua regia and separating and recovering them (Patent Document 1). Furthermore, a simpler and more environmentally friendly method has been developed to dissolve and recover precious metals from fuel cells using electrolysis (Patent Documents 2, 3, and 4), and it is expected that the development of fuel cell recycling technologies will advance further in the future.

[0005] The electrolytic technology for dissolving precious metals from MEA described in Patent Documents 2, 3, and 4 is expected to be implemented in society as an environmentally friendly method because it can easily dissolve precious metals that are normally insoluble using dilute hydrochloric acid at room temperature. Unlike conventional electrolysis, which is performed at a constant voltage or current, the electrolytic method used here dissolves precious metals by periodically reversing the polarity of the applied voltage.

[0006] When an external voltage is applied to an electrochemical device, a non-Faraday current is observed due to the charging of the electric double layer at the interface between the electrolyte and the electrode, and a Faraday current is observed due to the progress of the electrochemical reaction. The non-Faraday current flows at the moment the voltage is applied, and once charging at the interface is complete, it stops flowing, and only the Faraday current due to the electrochemical reaction is observed (Non-Patent Literature 1). When electrolysis is performed in a way that periodically reverses the polarity of the applied voltage, a non-Faraday current flows at each moment of polarity switching. As the electrode area increases, the amount of electricity stored at the interface also increases, so in devices that use porous materials with a large electrode area, such as PEM-type water electrolyzers and PEFCs, a large non-Faraday current flows intermittently. The non-Faraday current that flows immediately after polarity reversal is much larger than the Faraday current, so when dissolving precious metals by electrolysis, 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 dissolving the precious metals.

[0007] Patent Document 4 shows an example of electrolysis performed by applying a square wave voltage. While this method can also dissolve precious metals, when considering scaling up the apparatus, a large current flows when the polarity is reversed, which increases the cost of the apparatus and necessitates safety mechanisms against high currents. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Patent No. 1626036 [Patent Document 2] Patent No. 6652518 [Patent Document 3] Patent No. 6652454 [Patent Document 4] Patent No. 6109769 [Non-patent literature]

[0009] [Non-Patent Document 1] Kitamura, Fusao, “Chronoamperometry”, Electrochemistry, 68, No.1, pp.63-68 (2000) [Overview of the project] [Problems that the invention aims to solve]

[0010] The problem that this invention aims to solve is to provide a voltage control method for recovering valuable materials from electrochemical devices that can suppress the generation of large currents. [Means for solving the problem]

[0011] The voltage control method of the embodiment is a voltage control method for recovering valuable substances from an electrochemical device by applying a voltage to an electrochemical device equipped with an electrode having a catalyst containing a precious metal, and periodically reversing the polarity of the voltage to perform an electrolytic reaction, wherein the polarity of the voltage is periodically reversed by increasing the absolute value of the voltage over time and then holding it.

Brief Description of the Drawings

[0012] [Figure 1] It is a block diagram showing a configuration example of an electrochemical system. [Figure 2] It is a block diagram showing a configuration example of an electrochemical system. [Figure 3] It is a block diagram showing a configuration example of an electrochemical system. [Figure 4] It is a graph showing the time change of the voltage applied to an electrochemical device for recovering valuable substances by conventional electrolysis. [Figure 5] It is a graph representing the time change of the voltage in an embodiment. We [Figure 6] It is a schematic diagram showing an electric circuit representing the inside of an electrochemical device [Figure 7] It is a diagram showing the time change of voltage E. [Figure 8] It is a diagram showing the time change of current i. [Figure 9] It is a diagram showing an example of the time change of voltage E. [Figure 10] It is a diagram showing an example of the time change of current i. [Figure 11] It is a graph representing another example of the time change of the voltage wave in an embodiment. [Figure 12] It is a graph showing the change of current over time in an example.

Mode for Carrying Out the Invention

[0013] Hereinafter, embodiments will be described with reference to the drawings. In each of the embodiments shown below, substantially the same constituent parts are denoted by the same reference numerals, and the description thereof may be partially omitted. The drawings are schematic, and the relationship between the thickness and the planar dimensions, the ratio of the thicknesses of each part, etc. may be different from the actual ones.

[0014] In this specification, "connect" includes not only directly connecting but also indirectly connecting in some cases unless otherwise specified.

[0015] This embodiment is a voltage control method for recovering valuable substances from an electrochemical device by applying a voltage to an electrochemical device equipped with an electrode having a catalyst containing a precious metal, and periodically reversing the polarity of the voltage to perform an electrolytic reaction. The catalyst has the function of promoting electrolytic reactions such as oxidation and reduction reactions by the electrochemical device, as well as electrolytic reactions of electrolytes or solutes. Examples of precious metals include platinum (Pt), iridium (Ir), ruthenium (Ru), rhodium (Rh), palladium (Pd), platinum (Pt), gold (Au), rhenium (Re), etc. The catalyst may also contain an oxide containing at least one of the above precious metals. The electrolytic reaction is carried out by contacting an object (e.g., an electrode having an oxide catalyst) with a solution containing at least one ionic species and chemical species that can form a complex with the precious metal, applying a voltage to the object in contact with the solution, and periodically reversing the polarity of the voltage. In other words, the electrolytic reaction involves alternating between oxidation and reduction reactions. The solution contains, for example, hydrochloric acid (HCl).

[0016] Figures 1, 2, and 3 are block diagrams showing an example configuration of an electrochemical system capable of performing the above-described voltage control method. The electrochemical system comprises an electrochemical device 100, a power supply 200, and a voltage control device 300.

[0017] The electrochemical device 100 includes an electrode having a catalyst containing a precious metal. The precious metal is a valuable material to be recovered. The electrochemical device 100 can be, for example, a fuel cell or a PEM-type water electrolysis apparatus as described in Patent Documents 2 and 3, or an electrolytic cell equipped with electrodes containing valuable materials such as soda electrolysis electrodes or insoluble electrodes for plating may be used.

[0018] The power supply 200 can generate a voltage for electrolysis by the electrochemical device 100. The power supply 200 is connected, for example, directly or indirectly to the electrochemical device 100.

[0019] The voltage control device 300 can, for example, control the value of the voltage supplied from the power supply 200 to the electrochemical device 100. The voltage control device 300 is connected to the power supply 200, for example.

[0020] The voltage control device 300 may operate in a manner that controls the voltage output from the power supply 200 by supplying a control signal to the power supply 200, as shown in Figure 1, or it may operate in a manner that directly changes the voltage output from the power supply 200, as shown in Figure 2. Furthermore, the power supply 200 and the voltage control device 300 may be provided independently, or they may be configured to have the functions of both the power supply 200 and the voltage control device 300 within a single power supply unit 400, as shown in Figure 3.

[0021] In the dissolution of precious metal catalysts such as Pt and Ru reported in Patent Documents 2, 3, and 4, a solution containing components capable of forming complexes with the target precious metal is injected into an electrochemical device. After converting the extremely thin precious metal oxide film present on the surface of the metal catalyst into metal through a reduction treatment, the exposed metal is dissolved through an oxidation treatment. During the oxidation treatment, not only dissolution but also oxide film formation proceeds simultaneously, so the dissolution reaction stops when the metal is covered with an oxide film. The oxide film is removed by performing a reduction treatment again, so by alternating between reduction and oxidation, it becomes possible to dissolve precious metals that would normally not dissolve. In other words, the conventional method is a technique for dissolving precious metals that have a thin oxide on their surface.

[0022] Figure 4 is a graph showing the time variation of the voltage applied to an electrochemical device for the conventional electrolysis-based recovery of valuable materials. The horizontal axis of the graph represents time [seconds]. The vertical axis of the graph represents the voltage [V] applied to the electrochemical device. In the conventional electrolysis-based recovery of valuable materials described in Patent Documents 2, 3, and 4, as shown in Figure 4, voltages X and Y are periodically applied to the electrochemical device in a rectangular wave to dissolve platinum group metals such as platinum and ruthenium. Voltage X is a lower value than voltage Y. Voltage Y is a higher value than voltage X, for example, a positive value. However, with this voltage control method, a spike-like non-Faraday current flows at the moment of switching from voltage X to voltage Y and from voltage Y to voltage X, so there is a problem that electrolysis cannot be performed if the output of the electrochemical device is insufficient. Thus, although precious metal dissolution is possible with the above method, when considering the scaling up of the equipment, a large current flows when the polarity is reversed, which increases the cost of the equipment and necessitates safety mechanisms for large currents.

[0023] Therefore, a voltage control method that enables the melting of precious metals while suppressing the large non-Faraday current that flows when the polarity is reversed is described below. In this embodiment, instead of a square wave, the polarity of the voltage is periodically reversed by increasing the absolute value of the voltage over time and then holding it, as shown in Figure 5. This suppresses the instantaneous flow of a large non-Faraday current. In other words, in this embodiment, when reversing the polarity of the voltage, the slope of the voltage wave from a reference value such as 0V to the target value of the applied voltage is made larger than that of a square wave. Examples of such voltage waves include trapezoidal waves.

[0024] Figure 5 is a graph showing the time variation of the voltage in the embodiment. The horizontal axis of the graph represents time [seconds]. The vertical axis of the graph represents the voltage [V] applied to the electrochemical device 100. The embodiment has a period P1 with a first polarity and a period P2 with a second polarity opposite to the first polarity, in order to perform an electrolytic reaction by applying a voltage to the electrochemical device 100 and periodically reversing the polarity of the voltage. When recovering valuable materials, periods P1 and P2 are alternately switched and repeated. The voltage is applied, for example, to an electrode having the catalyst.

[0025] During period P1, the voltage applied to the electrochemical device 100 is gradually decreased from time T1 to time T2 until it is less than or equal to the value of voltage X. Voltage X is lower than voltage Y. Note that voltage X is not limited to a negative value lower than 0V, but may also be a positive value. Figure 5 shows an example where the voltage is gradually decreased from 0V to voltage X, but it may also be gradually decreased from a value different from 0V to voltage X.

[0026] Next, the voltage applied to the electrochemical device 100 is kept below the value of voltage X from time T2 to time T3. Figure 5 shows an example where the voltage is kept at the value of voltage X, but it does not necessarily have to be a constant value. The voltage that is held until time T3 changes to 0V when the application is stopped. At this time, the voltage value may be increased over time from the value of voltage X until it reaches 0V.

[0027] Next, during period P2, the voltage applied to the electrochemical device 100 is gradually increased from time T3 to time T4 until it is greater than or equal to the value of voltage Y. Voltage Y is, for example, higher than 0V. Figure 5 shows an example where the voltage is gradually increased from 0V to voltage Y, but it may also be increased gradually from a value different from 0V to voltage Y.

[0028] Next, the voltage applied to the electrochemical device 100 is maintained at or above the value of voltage Y from time T4 to time T5. Figure 5 shows an example where the voltage is maintained at the value of voltage Y, but it does not necessarily have to be a constant value. The voltage, whose value is maintained until time T5, then changes to 0V when the application is stopped. At this time, the voltage value may be gradually decreased from the value of voltage Y to 0V.

[0029] Here, we explain that spike-like non-Faraday currents can be suppressed by changing the voltage wave from a square wave to a trapezoidal wave. Figure 6 is a schematic diagram showing the electrical circuit (equivalent circuit) representing the inside of an electrochemical device. When the inside of an electrochemical device is represented by an electrical circuit, the electrolyte resistance R is as shown in Figure 6. s And, electric double layer (capacitance) C dThis forms an RC circuit. Figure 6 further shows a switch SW that controls the supply of voltage E to the electrochemical device, and the current i flowing inside the electrochemical device.

[0030] Voltage E across the electric double layer c , amount of electricity charged q, capacity C d Equation (1) holds true.

[0031]

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[0032] When the switch SW is in the ON position, if E is the voltage applied to the electrochemical device, then the voltage drop iR across the electrolyte is... s and the voltage E across the electric double layer c Since the sum of must be equal to E, equation (2) holds true.

[0033]

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[0034] Furthermore, since current i is the amount of electric current flowing per unit time, it can be expressed by equation (3).

[0035]

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[0036] Substituting equation (3) into equation (2) yields equation (4).

[0037]

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[0038] Solving the differential equation (4) under the condition that q=0 at t=0 and then differentiating it, we can find the change in current i with respect to time t, as shown in equation (5). This current i is a non-Faraday current.

[0039]

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[0040] A qualitative illustration of equation (5) is shown in Figures 7 and 8. Figure 7 shows an example of the time variation of voltage E. Figure 8 shows an example of the time variation of current i. From Figures 7 and 8, it can be seen that a large current flows when a voltage is applied.

[0041] Next, we will explain the case where a voltage is applied to an electrochemical device using a trapezoidal wave. In this case, the voltage E gradually increases with respect to time t, so it can be expressed by equation (6).

[0042]

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[0043] Substituting equation (6) into equation (4) yields equation (7).

[0044]

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[0045] Solving this under the condition that q=0 when t=0, and then differentiating, we obtain equation (8).

[0046]

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[0047] Equation (8) can be qualitatively illustrated as shown in Figures 9 and 10. Figure 9 shows an example of the time variation of voltage E. Figure 10 shows an example of the time variation of current i. As is clear from Figures 9 and 10, unlike the case of a square wave, the non-Faraday current gradually increases and eventually becomes a constant value determined by the voltage control conditions and the electrochemical device. Therefore, compared to when the voltage is applied with a square wave, a large instantaneous non-Faraday current can be suppressed by gradually increasing the absolute value of the voltage over time when reversing the polarity. Although Figure 5 described a trapezoidal wave in which the voltage increases linearly with respect to time, it is also possible to suppress the flow of a large instantaneous non-Faraday current when the voltage is changed curvilinearly. Furthermore, if at least a part of it increases over time, for example, when the absolute value of the voltage is changed in a stepwise manner, it is also possible to suppress the flow of a large instantaneous non-Faraday current.

[0048] In this embodiment, if the absolute value of the voltage can be increased over time when reversing the polarity of the voltage, a period of no voltage application may be provided, for example, before decreasing or increasing the voltage over time. Figure 11 is a graph showing another example of the time variation of the voltage in the embodiment. The horizontal axis of the graph represents time [seconds]. The vertical axis of the graph represents the voltage [V] applied to the electrochemical device 100. Figure 11 shows, as an example, an example in which period P3 is between period P1 and period P2 in Figure 5, but is not limited to this, and period P3 may be provided before period P1 or before period P2. These periods are repeated in the order of period P1, period P3, period P2 or period P2, period P3, period P1. In period P3, for example, the application of voltage is stopped from time T3 to time TX, and then the value of the voltage is changed over time from time TX to time T4. By providing period P3, it is possible to suppress the backflow of charge charged to the electrodes to the power supply 200.

[0049] The time it takes for the voltage to drop from 0V to below the value of voltage X during period P1, and the time it takes for the voltage to rise from 0V to above the value of voltage Y during period P2, are preferably 0.6 seconds or more, and more preferably 1 second or more, respectively. By setting the time to 0.6 seconds or more, it is possible to suppress the instantaneous flow of a large non-Faraday current. The upper limit of the above time is not particularly limited, but for example, it is 60 seconds or less. The above time can be controlled, for example, by adjusting the rate of increase of the absolute value of the voltage (slope of the voltage wave) by the voltage control device 300.

[0050] As described above, in this embodiment, when recovering valuable materials by electrolysis, the polarity of the voltage is reversed by increasing the absolute value of the voltage over time and then holding it, thereby suppressing the instantaneous flow of a large non-Faraday current. Therefore, valuable materials can be recovered without preparing a power supply with a large output size. [Examples]

[0051] (Comparative Example 1, Examples 1 and 2) Figure 12 shows the change in current over time during a voltage application test (applying square wave and trapezoidal wave voltages) to a fuel cell cell stack. The horizontal axis of the graph in Figure 12 represents time [seconds]. The vertical axis of the graph in Figure 12 represents the current [A] in the cell stack. The voltage application test was performed without removing the electrodes containing precious metals from the cell stack.

[0052] In Comparative Example 1, when a voltage of 18.2 V was applied to the cell stack at a slope of less than 30.3 V / s with a rectangular wave for less than 0.6 seconds, a current of 91 A flowed immediately after the voltage application, and then the current gradually decreased and a steady current of about 20 A flowed. In this case, even though the electrolysis operation was performed with a steady current of 20 A, a power supply 200 capable of an output current of about 100 A had to be prepared. On the other hand, in Example 1, when a voltage was applied to the cell stack while increasing it at a slope of 18.2 V / s with a trapezoidal wave for 1 second and then holding it at 18.2 V, the current gradually increased from 0 A, a maximum current of 35 A flowed, and finally a steady current of about 20 A, which was the same as in the case of the rectangular wave, flowed. Also, in Example 2, when a voltage was applied to the cell stack while increasing it at a slope of 30.3 / s with a trapezoidal wave for 0.6 seconds and then holding it at 18.2 V, the current gradually increased from 0 A, a maximum current of 37.5 A flowed, and finally a steady current of about 20 A, which was the same as in the case of the rectangular wave, flowed. From these results, it was confirmed that by increasing the absolute value of the voltage over time and then holding it to reverse the polarity of the voltage, it was possible to suppress the flow of a large instantaneous non-Faraday current. For example, when applying a voltage with a trapezoidal wave, it was sufficient to prepare a power supply 200 capable of an output current of about 40 to 50 A.

[0053] As described above, several embodiments of the present invention have been explained, but these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are included in the invention described in the claims and its equivalent scope.

Explanation of Reference Numerals

[0054] 100... Electrochemical device, 200... Power supply, 300... Voltage control device, 400... Power supply device, C d ... Capacitance, E... Voltage, E c ... Voltage, P1... Period, P2... Period, P3... Period, R s...resistance, SW...switch, T1...time, T2...time, T3...time, T4...time, T5...time, TX...time, X...voltage, Y...voltage, i...current, iR s ...voltage drop, q...electric charge, t...time, v...slope.

Claims

1. A voltage control method for recovering valuable substances from an electrochemical device by applying a voltage to an electrochemical device equipped with an electrode having a catalyst containing a precious metal, and periodically reversing the polarity of the voltage to carry out an electrolytic reaction, The polarity of the voltage is periodically reversed by increasing the absolute value of the voltage over time and then holding it. Voltage control method.

2. A first period during which the voltage value is linearly or curvilinearly decreased from a first value to a second value lower than the first value and then held thereafter, A second period during which the voltage value is increased linearly or curvilinearly from a third value that is the same as or different from the first value to a fourth value or higher that is higher than the third value, and then held thereafter. Equipped with, The voltage control method according to claim 1.

3. The system comprises a third period during which the voltage is not applied between the first period and the second period. The voltage control method according to claim 2.

4. During the first period, the time it takes for the voltage value to change from the first value to the second value is 0.6 seconds or more. During the second period, the time it takes for the voltage value to change from the third value to the fourth value is 0.6 seconds or more. The voltage control method according to claim 2.

5. The catalyst includes at least one precious metal selected from the group consisting of platinum, iridium, ruthenium, rhodium, palladium, platinum, gold, and rhenium. The voltage control method according to claim 1.

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