An iron-based negative electrolyte for a flow battery and a preparation method thereof

Abstract

An iron-based negative electrolyte for a flow battery and a preparation method thereof belong to the technical field of flow battery energy storage. The iron-based negative electrolyte for the flow battery comprises iron ions, a chelating agent, a supporting electrolyte, an auxiliary electrolyte and a solvent. The chelating agent comprises any one or a combination of at least two of tris(hydroxymethyl)methylglycine, N,N-bis(2-hydroxyethyl)glycine, and N-(2-hydroxyethyl)iminodiacetic acid, which improves the formula of the iron-based negative electrolyte for the flow battery and provides multiple new options for iron-based flow batteries. The present invention has the advantages of simple operation, easy scale-up, long cycle life, low cost, etc., and can be applied as a large-scale electric energy storage and efficient conversion device in solar and wind power generation systems, and also has broad application prospects in small energy storage devices such as automotive batteries and household energy storage.

Description

Technical Field

[0001] The present invention relates to the technical field of flow battery energy storage, and in particular to an iron-based flow battery negative electrolyte and a preparation method thereof, which can be applied in solar and wind power generation systems as large-scale electric energy storage and efficient conversion equipment, and also have broad application prospects in small energy storage devices such as automotive batteries and household energy storage. Background Art

[0002] Nowadays, clean and renewable energy sources (such as wind energy and solar energy) are widely used. However, these energy sources usually have problems of discontinuity and instability, so energy storage facilities need to be equipped during use. Iron-based flow batteries are expected to be used as large-scale or distributed energy storage systems in power facilities due to their advantages of high safety, low cost, and high power density, promoting the efficient utilization of clean and renewable energy.

[0003] The structure of a flow battery mainly consists of a liquid storage tank, a battery stack, and a pump. During battery operation, the pump injects the positive and negative electrolytes from the liquid storage tank into the battery interior, where they flow through the surfaces of the positive and negative electrodes and undergo electrochemical reactions. The positive and negative electrolytes are separated by an ion-selective permeable membrane. The positive and negative electrodes are connected to the power load, and electrons are transferred through the circuit, while positive and negative ions are transferred through the diaphragm inside the battery to form a complete current loop.

[0004] Currently, commercial all-vanadium flow batteries have shown their advantages in the energy storage field. However, in the long term, they still have some deficiencies in resource utilization, environmental protection, and cost control. Iron-based flow batteries have several advantages compared to all-vanadium flow batteries. First, the iron materials used in iron-based batteries have a lower cost, while the vanadium materials in all-vanadium batteries are more expensive, so they have an advantage in terms of initial investment and operating costs. In addition, iron is one of the most abundant elements on Earth, and its resources are relatively easy to obtain, while vanadium resources are relatively scarce and may face supply chain risks. The materials of iron-based flow batteries have less impact on the environment and meet the requirements of sustainable development, while the mining and processing of vanadium may have a certain impact on the environment. In terms of safety, iron-based batteries are safer during use and are not prone to fire or explosion, making them suitable for large-scale applications. At the same time, iron-based flow batteries show good cycle stability during multiple charge and discharge processes and are suitable for long-term energy storage needs. In addition, their design and manufacturing are relatively simple, which may reduce the complexity and maintenance cost of the system. These advantages make iron-based flow batteries a more attractive choice than all-vanadium flow batteries in certain application scenarios, especially in terms of cost and resource sustainability. Summary of the Invention

[0005] The object of the present invention is to improve the disadvantages of existing iron-based flow batteries, overcome the problems of poor energy density, reaction rate, safety and stability of the electrolyte, enrich the formula of the negative electrolyte of the iron-based flow battery, and provide various new iron-based flow battery selection schemes. The chelating agent in the present invention forms a stable chelate with iron ions, has a relatively high solubility, good thermal stability and chemical stability, and is suitable for use under various conditions.

[0006] The technical solution for achieving the object of the present invention is as follows:

[0007] A negative electrolyte for an iron-based flow battery and a preparation method thereof, characterized in that the negative electrolyte comprises an active substance, a ligand, a supporting electrolyte, and an auxiliary electrolyte; the active substance is one or more of ferric sulfate, ferric nitrate, ferric chloride, and ferric acetate, the ligand is one or more of tris(hydroxymethyl)methylglycine, N,N-bis(2-hydroxyethyl)glycine, and N-(2-hydroxyethyl)iminodiacetic acid, the supporting electrolyte is one or more of sodium hydroxide, potassium hydroxide, lithium hydroxide, potassium carbonate, sodium carbonate, and lithium carbonate, the auxiliary electrolyte is one or more of potassium chloride, potassium nitrate, sodium nitrate, sodium chloride, ammonium chloride, potassium sulfate, sodium sulfate, and ammonium sulfate, and the solvent is anaerobic deionized water.

[0008] Further, when the ligand is tris(hydroxymethyl)methylglycine, the corresponding supporting electrolyte is one or more of sodium hydroxide, potassium hydroxide, and lithium hydroxide; when the ligand is N,N-bis(2-hydroxyethyl)glycine or / and N-(2-hydroxyethyl)iminodiacetic acid, the corresponding supporting electrolyte is one or more of potassium carbonate, sodium carbonate, and lithium carbonate.

[0009] The molar ratio of the ligand to the iron ions in the active substance is 5:1 to 2:1, and the preferred ratio is 3:1.

[0010] Further, the molar concentration of the iron ions in the active substance is 0 to 1.2 mol / L and not 0, preferably 0.1 to 0.6 mol / L; the molar concentrations of tris(hydroxymethyl)methylglycine, N,N-bis(2-hydroxyethyl)glycine, and N-(2-hydroxyethyl)iminodiacetic acid as the ligand are 0 to 5.0 mol / L and not 0, preferably 0.5 to 2.5 mol / L; the molar concentration of the supporting electrolyte is 0 to 8.0 mol / L, preferably 1.0 to 4.0 mol / L; the molar concentration of the auxiliary electrolyte is 0 to 3.0 mol / L, preferably 0.2 to 1.5 mol / L.

[0011] A negative electrolyte for an iron-based flow battery and a preparation method thereof, characterized in that the preparation process of the negative electrolyte is as follows:

[0012] (1) Introduce an inert gas into the reactor to exhaust the air.

[0013] (2) Add the active substance, ligand, and auxiliary electrolyte. After mixing, add deionized water and stir for 1 to 3 hours;

[0014] (3) Then add the supporting electrolyte and stir well for 1 to 3 hours to make a homogeneous solution.

[0015] Let it stand for 6 hours before use, and detect and record the pH and viscosity.

[0016] An iron-based flow battery includes a positive electrode, a negative electrode, positive and negative electrode electrolytes, a separator, and a battery housing. The positive electrode, negative electrode, positive and negative electrolytes, and separator are disposed in the battery housing. The separator is located between the adjacent positive and negative electrodes, and both sides of the separator are in contact with the positive and negative electrode electrolytes. The negative electrode electrolyte is the negative electrode electrolyte of the present invention;

[0017] Further, the positive electrode electrolyte is potassium ferrocyanide, an auxiliary electrolyte, or / and a supporting electrolyte having the same supporting electrolyte anion as that of the corresponding negative electrode electrolyte.

[0018] Further, the positive electrode or the negative electrode is a porous carbon material electrode.

[0019] Further, the separator is a Nafion membrane with a thickness of 50 to 200 μm.

[0020] Further, the operating temperature of the negative electrode electrolyte is 10 to 70 °C.

[0021] Compared with the prior art, the present invention has the following remarkable advantages and beneficial effects:

[0022] 1. By adding tris(hydroxymethyl)methylglycine, N,N-bis(2-hydroxyethyl)glycine, N-(2-hydroxyethyl)iminodiacetic acid, or their chemical derivatives to complex with iron salts, the present invention improves the formula of the negative electrode electrolyte of the iron-based flow battery and provides multiple new options for the iron-based flow battery. Tris(hydroxymethyl)methylglycine can be paired with a strong base electrolyte, and N,N-bis(2-hydroxyethyl)glycine and N-(2-hydroxyethyl)iminodiacetic acid can be paired with a weak base electrolyte, which can be paired with the positive electrode electrolyte in neutral, weak base, and strong base environments to assemble different batteries. Moreover, compared with other carboxylic acid ligands, tris(hydroxymethyl)methylglycine has a further negative shift in the voltage window of the formed complex, increasing the single cell potential difference and thus improving the energy density.

[0023] 2. The chelating agent used in the present invention has a central nitrogen atom connected to multiple hydroxyl or carboxyl functional groups, and the formed complex has a relatively high solubility. The concentration of the active substance undergoing oxidation-reduction in the electrolyte increases, and the energy density is improved. The electrolyte can also provide fast charge and discharge capabilities at a relatively high current density and quickly respond to changes in power demand.

[0024] 3. The chelating agent selected in the present invention is inexpensive, the cost of the electrolyte is low, it is safe and stable, and the preparation method of the electrolyte is simple and easy to scale up. It can be applied not only to large-scale electric energy storage and high-efficiency conversion devices, but also to small energy storage devices such as automotive batteries and household energy storage. Description of the Drawings

[0025] Figure 1 Cyclic voltammogram of Example 1 within 10 - 100 mV·s -1

[0026] Figure 2 Cyclic voltammogram of Example 2 within 10 - 50 mV·s -1

[0027] Figure 3 Cyclic voltammogram of Example 3 within 10 - 50 mV·s -1

[0028] Figure 4 Charge and discharge efficiency, capacity - cycle curve within 280 cycles of Example 1

[0029] Figure 5 Charge and discharge efficiency, capacity - cycle curve within 70 cycles of Example 2

[0030] Figure 6 Charge and discharge efficiency, capacity - cycle curve within 90 cycles of Example 3 Detailed Description of the Invention

[0031] The present invention will be described in detail below through specific examples. However, the uses and purposes of these exemplified embodiments are only used to illustrate the present invention, and do not constitute any form of limitation to the actual protection scope of the present invention, nor will the protection scope of the present invention be limited thereto. Example 1

[0032] In this example, the main components of the negative electrode electrolyte include: iron chloride, tris(hydroxymethyl)methylglycine as a complexing agent, the supporting electrolyte is sodium hydroxide, the auxiliary electrolyte is potassium chloride, and the solvent is anaerobic deionized water. Among them, the concentration of iron chloride is 0.2 mol / L, the concentration of tris(hydroxymethyl)methylglycine is 0.8 mol / L, the concentration of sodium hydroxide is 1.6 mol / L, and the concentration of potassium chloride is 0.2 mol / L.

[0033] The specific steps for preparing the negative electrode electrolyte are as follows:

[0034] ​​​Purge the air in the reactor with nitrogen, and sequentially add 5.4 g of ferric chloride, 14.3 g of tris(hydroxymethyl)methylglycine, 6.4 g of sodium hydroxide, and 1.5 g of potassium chloride. After mixing, add deoxygenated ionized water, stir for 8 hours to make a homogeneous solution, volume to 100 mL, and let stand for 12 hours before use.

[0035] Perform electrochemical tests on the above-mentioned negative electrode electrolyte, and the results are as Figure 1 shown. Scan the cyclic voltammogram within a scan rate range of 2 to 100 mV·s -1 . There is a regular linear relationship between the scan rate and the current density, and the oxidation-reduction current intensities are similar, showing good reversibility. Among carboxylic acid ligands, the voltage window can shift to a lower voltage (more negative potential), and the reduction peak potential is after -1.0 V, increasing the single cell potential difference.

[0036] In the battery test, the electrolyte of the positive electrode uses a 0.5 mol / L potassium ferrocyanide and 0.5 mol / L potassium hydroxide electrolyte.

[0037] Add the positive and negative electrode electrolytes to the corresponding liquid storage tanks of the flow battery respectively. The flow battery system mainly consists of a battery, positive and negative liquid storage tanks, positive and negative peristaltic pumps, corrosion-resistant circulation pipelines, and a battery test system. The battery components include: aluminum end plates, polytetrafluoroethylene gaskets, current collector plates, graphite bipolar plates, fluororubber gaskets, proton exchange membranes, and graphite felt electrodes. The graphite felt electrodes are 3 cm × 3 cm, and the proton exchange membrane uses a Nafion 117 membrane.

[0038] After assembling the above components, first circulate an inert gas in the battery system to avoid inactivation of active substances, and then connect to the battery test system to start the test. The volume of the positive and negative electrode electrolytes is 10 ml each, and the positive and negative electrodes are both 3 × 3 cm 2 porous carbon felt electrodes, and the proton membrane is a Nafion 117 membrane.

[0039] Charge to 1.8 V at a relatively high current density of 80 mA / cm 2 , and discharge to 0.01 V at a relatively high current density of 80 mA / cm 2 . Perform 280 charge and discharge cycle tests on the assembled iron-based flow battery. The charge and discharge efficiency and capacity-cycle curve within 280 cycles are as Figure 4 shown. Within 280 cycles under the above high current density conditions, the Coulomb efficiency (CE) is about 90%, the energy efficiency (EE) and voltage efficiency (VE) are about 60% and 70% respectively, and its stability is good, and the charge and discharge capacity decays slowly. Example 2

[0040] In this embodiment, the main components of the negative electrode electrolyte include: iron nitrate, N,N-bis(2-hydroxyethyl)glycine as a complexing agent, potassium carbonate as a supporting electrolyte, sodium chloride as an auxiliary electrolyte, and anaerobic deionized water as a solvent. Among them, the concentration of iron nitrate is 0.2 mol / L, the concentration of N,N-bis(2-hydroxyethyl)glycine is 0.8 mol / L, the concentration of potassium carbonate is 1.2 mol / L, and the concentration of sodium chloride is 0.2 mol / L.

[0041] The specific steps for preparing the negative electrode electrolyte are as follows:

[0042] Nitrogen is introduced into the reactor to exhaust the air, and 8.1 g of iron nitrate, 13.1 g of N,N-bis(2-hydroxyethyl)glycine, 16.6 g of potassium carbonate, and 1.2 g of sodium chloride are successively added. After mixing, anaerobic deionized water is added, and the mixture is stirred for 24 hours to form a homogeneous solution, which is fixed to 100 mL and allowed to stand for 12 hours before use.

[0043] The electrolyte of the positive electrode is mainly a 0.4 mol / L sodium ferrocyanide and 0.4 mol / L sodium chloride electrolyte.

[0044] After the battery system is assembled, other reference conditions are the same as in Example 1.

[0045] Connect to the battery test system and start the test. The volume of the positive and negative electrode electrolytes is 20 ml each, and the positive and negative electrodes are both 3 × 3 cm 2 porous carbon felt electrodes, and the proton membrane is a Nafion 212 membrane. Charge to 1.0 V at a relatively high current density of 80 mA / cm 2 and discharge to 0.01 V at a relatively high current density of 80 mA / cm 2 .

[0046] The assembled iron-based flow battery is subjected to 70 charge and discharge cycle tests, and the charge and discharge efficiency and capacity-cycle curve within 70 cycles are as Figure 5 shown.

[0047] The Coulomb efficiency (CE) within 70 cycles is close to 100%, and the voltage efficiency (VE) and energy efficiency (EE) are about 70%. The charge and discharge capacity is relatively stable. Example 3

[0048] In this embodiment, the main components of the negative electrode electrolyte include: ferric sulfate, N-(2-hydroxyethyl)iminodiacetic acid as a complexing agent, potassium carbonate as a supporting electrolyte, sodium chloride as an auxiliary electrolyte, and anaerobic deionized water as a solvent. Among them, the concentration of ferric sulfate is 0.2 mol / L, the concentration of N-(2-hydroxyethyl)iminodiacetic acid is 0.8 mol / L, the concentration of potassium carbonate is 1.2 mol / L, and the concentration of sodium chloride is 0.2 mol / L.

[0049] The specific steps for preparing the negative electrode electrolyte are as follows:

[0050] Take 8.0 g of ferric sulfate, 14.2 g of N-(2-hydroxyethyl)iminodiacetic acid, and 1.2 g of sodium chloride, put them into a reactor, add 16.6 g of potassium carbonate, add anaerobic deionized water, and then stir for 6 h. After complete dissolution, transfer it to a 100 mL volumetric flask and make up the volume with anaerobic deionized water at room temperature. Let it stand for 24 h before use.

[0051] The electrolyte of the positive electrode is mainly a 0.5 mol / L lithium ferrocyanide and 0.5 mol / L lithium carbonate electrolyte.

[0052] After the battery system is assembled, other reference conditions are the same as in Example 1. To prevent the inactivation of the active material and the deterioration of the alkaline electrolyte, after the inert gas fills the entire battery system. Connect it to the battery test system and start the test.

[0053] Connect it to the battery test system and start the test. The volume of the positive and negative electrode electrolytes is 25 ml each, and the positive and negative electrodes are both 3 × 3 cm 2 porous carbon felt electrodes, and the proton membrane is a Nafion 212 membrane. At a relatively high current density of 80 mA / cm 2 charge to 0.9V, and at a relatively high current density of 80 mA / cm 2 discharge to 0.01 V.

[0054] Perform 90 charge and discharge cycle tests on the assembled iron-based flow battery. The charge and discharge efficiency and capacity-cycle curve within 90 cycles are as Figure 6 shown.

[0055] The Coulomb efficiency (CE) within 90 cycles is close to 90%, the voltage efficiency (VE) decays from 70% in the first cycle to about 50%, and the energy efficiency (EE) decays from 60% in the first cycle to about 50%. The charge and discharge capacity tends to be stable after 30 cycles.

Claims

1. An iron-based negative electrolyte for a flow battery applicable to a high current density of 80 mA / cm 2 , characterized in that The negative electrolyte includes active substances, ligands, supporting electrolytes, and auxiliary electrolytes; the active substances are one or more of iron sulfate, iron nitrate, iron chloride, and iron acetate, the ligand is tris(hydroxymethyl)methylglycine, or the ligand is one or more of N,N-bis(2-hydroxyethyl)glycine and N-(2-hydroxyethyl)iminodiacetic acid; the described iron-based flow battery is applicable to a high current density of 80 mA / cm 2 ; The supporting electrolyte is one or more of sodium hydroxide, potassium hydroxide, lithium hydroxide, potassium carbonate, sodium carbonate, and lithium carbonate. The auxiliary electrolyte is one or more of potassium chloride, potassium nitrate, sodium nitrate, sodium chloride, ammonium chloride, potassium sulfate, sodium sulfate, and ammonium sulfate. The solvent is anaerobic deionized water; When the ligand is tris(hydroxymethyl)methylglycine, the corresponding supporting electrolyte is one or more of sodium hydroxide, potassium hydroxide, and lithium hydroxide. When the ligand is N,N-bis(2-hydroxyethyl)glycine or / and N-(2-hydroxyethyl)iminodiacetic acid, the corresponding supporting electrolyte is one or more of potassium carbonate, sodium carbonate, and lithium carbonate.

2. The negative electrolyte of an iron-based flow battery applicable to a high current density of 80 mA / cm 2 , characterized in that The molar ratio of the ligand to the iron ions in the active substance is 5:1 to 2:

1.

3. The negative electrode electrolyte of an iron-based flow battery applicable to a high current density of 80 mA / cm 2 is characterized in that The molar ratio of the ligand to the iron ions in the active substance is 3:

1.

4. The negative electrolyte of an iron-based flow battery applicable to a high current density of 80 mA / cm 2 , characterized in that The molar concentration of the iron ions in the active substance is 0 to 1.2 mol / L and not 0; the molar concentrations of the ligands tris(hydroxymethyl)methylglycine, N,N-bis(2-hydroxyethyl)glycine, and N-(2-hydroxyethyl)iminodiacetic acid are 0 to 5.0 mol / L and not 0; the molar concentration of the supporting electrolyte is 0 to 8.0 mol / L and not 0; the molar concentration of the auxiliary electrolyte is 0 to 3.0 mol / L and not 0.

5. The negative electrolyte for an iron-based flow battery applicable to a high current density of 80 mA / cm 2 , characterized in that The molar concentration of the iron ions in the active substance is 0.1 to 0.6 mol / L; the molar concentrations of the ligands tris(hydroxymethyl)methylglycine, N,N-bis(2-hydroxyethyl)glycine, and N-(2-hydroxyethyl)iminodiacetic acid are 0.5 to 2.5 mol / L; the molar concentration of the supporting electrolyte is 1.0 to 4.0 mol / L; the molar concentration of the auxiliary electrolyte is 0.2 to 1.5 mol / L.

6. An iron-based flow battery applicable to a high current density of 80 mA / cm 2 comprises a positive electrode, a negative electrode, positive and negative electrode electrolytes, a separator and a battery housing. The positive electrode, negative electrode, positive and negative electrode electrolytes and separator are disposed within the battery housing. The separator is located between the adjacent positive and negative electrodes and contacts the positive and negative electrode electrolytes on both sides thereof. It is characterized in that The negative electrode electrolyte is a negative electrode electrolyte for an iron-based flow battery applicable to a high current density of 80 mA / cm 2 as described in any one of claims 1 to 5.

7. A kind of iron-based flow battery applicable to a high current density of 80 mA / cm 2 , characterized in that The positive electrode electrolyte uses potassium ferrocyanide, and an auxiliary electrolyte or / and a supporting electrolyte with the same anions as the supporting electrolyte of the corresponding negative electrode electrolyte.

8. An iron-based flow battery applicable to a high current density of 80 mA / cm 2 , characterized in that The positive electrode or the negative electrode is a porous carbon material electrode; the separator is a Nafion membrane with a thickness of 50 to 200 μm.

9. A kind of iron-based flow battery applicable to a high current density of 80 mA / cm 2 , characterized in that The operating temperature of the negative electrode electrolyte is 10 to 70 °C.

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