A negative electrode electrolyte for an all-iron flow battery, a preparation method thereof, and an application thereof

Abstract

The present invention discloses a negative electrode electrolyte for an all-iron flow battery, a preparation method thereof, and an application thereof. The negative electrode electrolyte is composed of the following components: ferrous chloride at a concentration of 1.8 to 3.6 mol / L, magnesium chloride at a concentration of 0.25 to 1.5 mol / L, and sodium chloride at a concentration of 0.05 to 0.15 mol / L, with the balance being water and the pH being -1.0 to 0. The high-efficiency negative electrode electrolyte for the all-iron flow battery of the present invention adopts an extremely low pH of -1.0 to 0, and in a strong acidic environment, the problems of hydrogen evolution and poor reversibility of the deposition / dissolution reaction existing in the negative electrode of the all-iron flow battery are improved by magnesium chloride in the inorganic interface regulation, so as to obtain an all-iron flow battery with high coulombic efficiency and high cycle life.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrochemical energy storage, and in particular relates to an all-iron liquid flow battery negative electrode electrolyte and a preparation method and application thereof. Background Art

[0002] With the rapid development of renewable energy such as wind power and solar power, energy storage technology has become the core to solve their inherent intermittent and volatility problems.

[0003] As a new energy storage technology, all-iron flow battery uses iron ions as the active material of electrolyte in both positive and negative electrodes. 2+ / Fe 3+ As a redox couple, the negative electrode converts Fe 2+ / Fe 0 As a redox couple, it has shown significant commercial potential due to its cost advantage brought by its abundant iron reserves, the high safety of aqueous electrolytes, and its environmentally friendly characteristics without heavy metal pollution. However, its practical application still faces key challenges: during the operation of all-iron liquid flow batteries, there will be some problems such as hydrogen evolution side reactions and poor reversibility, which will cause irreversible loss of active materials and reduce the efficiency of the battery. At the same time, the iron deposition reaction can easily lead to an imbalance in the pH value of the electrolyte and induce the precipitation of Fe(OH)3 colloids. The continuous accumulation of such precipitates at the diaphragm will significantly increase the internal resistance of the battery and cause rapid decay of the cycle performance. These problems ultimately restrict the large-scale engineering application of this technology.

[0004] Currently, the number of patents that have been publicly authorized or are under review for all-iron flow batteries is limited, and patents related to alkaline systems occupy the mainstream position, while the technical development of acidic systems mainly revolves around iron-based organic complexes. After reading the literature on related research, it is not difficult to find that the current academic research focus in acidic flow batteries is mainly on the exploration of the basic electrochemical properties of ferrous chloride (FeCl2) as an active substance, as well as the application optimization of various organic complexing agents such as citrate, cyanide, DMSO and other coordination additives.

[0005] In the prior art, (CN114709459B) discloses a negative electrode electrolyte for an aqueous all-iron flow battery, which uses a double complexing agent (such as triethanolamine and oxalate or imidazole compound) to synergistically complex Fe 3+ / Fe 2+ , by stabilizing the iron ion coordination structure to inhibit the deposition of metallic iron, it solves the problem of metallic iron precipitation and the resulting battery capacity decay and low efficiency. However, its technology has the following limitations: it needs to add high concentrations of organic complexing agents, which increases the cost of the electrolyte; at the same time, imidazole compounds have potential biological toxicity and do not meet the requirements of green energy storage; the pH of the electrolyte needs to be maintained in the neutral range, which cannot adapt to high concentrations of Fe 2+system, and the upper limit of the cycle life is only in the thousands.

[0006] (CN114388859B) discloses a negative electrolyte for an all-iron flow battery. By adding dimethyl sulfoxide (DMSO) or its derivatives to form a coordination structure with ferrous ions, the morphology of iron deposition is regulated, and the hydrogen evolution reaction is inhibited. This method also relies on organic coordination, the pH of the electrolyte needs to be maintained weakly acidic, the upper limit of the iron ion concentration is only 1.5 mol / L, and the cycle life verification is only 100 times. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to overcome the above-mentioned deficiencies and defects in the background technology, and provide a negative electrolyte for an all-iron flow battery, its preparation method and application, so as to improve the coulomb efficiency and cycle life.

[0008] To solve the above technical problem, the technical solution proposed by the present invention is:

[0009] A negative electrolyte for an all-iron flow battery, the negative electrolyte is composed of the following components: ferrous chloride at 1.8 to 3.6 mol / L, magnesium chloride at 0.25 to 1.5 mol / L, and sodium chloride at 0.05 to 0.15 mol / L, with the balance being water and the pH being -1.0 to 0.

[0010] As a further improvement, the concentration of ferrous chloride is 3.0 to 3.6 mol / L.

[0011] As a further improvement, the concentration of magnesium chloride is 0.8 to 1.2 mol / L.

[0012] As a further improvement, the concentration of sodium chloride is 0.08 to 0.12 mol / L.

[0013] As a further improvement, the pH is -0.5 to 0.

[0014] The present invention also provides a preparation method of the negative electrolyte for an all-iron flow battery, including: dissolving ferrous chloride, magnesium chloride and sodium chloride in water according to the concentration requirements, and adjusting the pH to the required range, then the negative electrolyte is obtained.

[0015] The present invention also provides an application of the negative electrolyte in the preparation of an all-iron flow battery.

[0016] The present invention also provides an all-iron flow battery, which includes the negative electrolyte. Its positive electrolyte is an aqueous solution of ferrous chloride and sodium chloride.

[0017] Compared with the prior art, the beneficial effects of the present invention are:

[0018] The negative electrolyte of the high-efficiency all-iron flow battery of the present invention uses a very low pH of -1.0 to 0. In a strong acidic environment, magnesium chloride is used to regulate the inorganic interface to improve the problems of hydrogen evolution and poor reversibility of deposition / dissolution reactions existing in the negative electrode of the all-iron flow battery, and a high coulombic efficiency and high cycle life all-iron flow battery are obtained. Specifically, it has:

[0019] (1) High coulombic efficiency and high cycle life: It can perform a long cycle test of 10,000 cycles, and the highest coulombic efficiency can reach 99.5%.

[0020] (2) Optimize the electrode reaction performance: Cyclic voltammetry tests show that the addition of magnesium chloride inhibits the hydrogen evolution reaction, reduces the degree of battery potential separation, and promotes the reversibility and kinetic performance of the Fe / Fe² + reaction, thereby improving the coulombic efficiency.

[0021] (3) Improve the iron deposition morphology: SEM characterization shows that the structure after adding magnesium chloride is denser and more uniform than that without addition; XRD shows a single structural orientation of iron deposition, thereby improving the cycle life. Description of the Drawings

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0023] Figure 1 Cyclic voltammogram of the embodiment: (a) Cyclic voltammograms obtained in baths containing 3.6 mol / L ferrous chloride and different concentrations of magnesium chloride from 0 to 1.5 mol / L and different pH values of 0, -0.5, and -1.0, (b) Comparison of the hydrogen evolution reaction current peaks.

[0024] Figure 2 Scanning electron microscope images of electro-deposited iron in baths with different concentrations of magnesium chloride at a magnification of 10,000: (a) 0 mol / L magnesium chloride (b) 1.0 mol / L magnesium chloride.

[0025] Figure 3 X-ray diffraction patterns of electro-deposited iron in 0 mol / L magnesium chloride and 1.0 mol / L magnesium chloride.

[0026] Figure 4 Comparison and determination of the coulombic efficiency of electrolyte systems with 1.0 mol / L and without magnesium chloride at different pH values.

[0027] Figure 5It is the coulombic efficiency after 10,000 cycles of the battery. Detailed implementation manners

[0028] To facilitate the understanding of the present invention, the present invention will be described more comprehensively and in detail below in conjunction with the accompanying drawings of the specification and preferred embodiments, but the protection scope of the present invention is not limited to the following specific embodiments.

[0029] Unless otherwise defined, all professional terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention.

[0030] Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment, etc. used in the present invention can be obtained through the market or can be prepared by existing methods.

[0031] In some specific implementation manners, the negative electrode electrolyte of the all-iron flow battery of the present invention contains ferrous chloride, sodium chloride, and magnesium chloride, and its composition is as follows:

[0032] Ferrous chloride: 1.8 - 3.6 mol / L, preferably 3.0 - 3.6 mol / L;

[0033] Magnesium chloride: 0.25 - 1.5 mol / L, preferably 0.8 - 1.2 mol / L;

[0034] Sodium chloride: 0.05 - 0.15 mol / L, preferably 0.08 - 0.12 mol / L;

[0035] Deionized water: the balance.

[0036] The pH of the negative electrode electrolyte is: -1.0 - 0, preferably -0.5 - 0.

[0037] As a redox active substance, the increase in the concentration of ferrous ions can increase the charge storage amount per unit volume and directly improve the mass specific capacity and volume energy density of the system. Therefore, the concentration of ferrous chloride is preferably 3.0 - 3.6 mol / L.

[0038] When the concentration of magnesium chloride is 1.5 mol / L, the effect of enhancing reversibility begins to be inhibited (such as Figure 1 the cyclic voltammogram), so the preferred concentration of magnesium chloride is 0.8 - 1.2 mol / L.

[0039] In the range of pH from -1.0 to 0, as the pH increases, the hydrogen evolution side reaction weakens and the Coulombic efficiency increases. However, when the pH is greater than 2, it will cause hydrolysis precipitation of iron ions, affecting the proton membrane, increasing side reactions and performance attenuation, etc., all of which will lead to a decrease in the Coulombic efficiency of the battery. Moreover, when pH = 1, during long-term cycling, the pH stability is worse due to the consumption of hydrogen ions, so the long-term cycling performance is worse than that at pH = 0. Therefore, the pH is controlled within -1.0 to 0.

[0040] Example 1 Preparation of Ferrous Chloride-based Basic Electrolyte

[0041] Taking 3.6 mol / L ferrous chloride as an example, 45.63 grams of anhydrous ferrous chloride (purity ≥ 99.9%) and 0.58 grams of sodium chloride were successively added to an appropriate amount of deionized water, and magnetically stirred at 500 rpm for 30 minutes at 25 ± 1 °C until the solid was completely dissolved to obtain a homogeneous and transparent mixed solution; then it was transferred to a 100 mL volumetric flask, the inner wall of the beaker was rinsed 3 times with a small amount of deionized water, the rinsing solutions were combined and made up to the mark, shaken well and left to stand to defoam, and finally 100 ml of a basic electrolyte containing 3.6 mol / L ferrous chloride and 0.1 mol / L sodium chloride was prepared.

[0042] Example 2 Preparation of Gradient Magnesium Chloride Composite Electrolyte

[0043] Taking the 3.6 mol / L ferrous chloride-based solution prepared in Example 1 as an example, 4 groups of 100 mL basic electrolytes (containing 3.6 mol / L ferrous chloride and 0.1 mol / L NaCl) were taken. In each group, 2.38 grams (corresponding to 0.25 mol / L), 4.76 grams (0.5 mol / L), 9.52 grams (1.0 mol / L), and 14.28 grams (1.5 mol / L) of anhydrous magnesium chloride (purity ≥ 99.9%) were accurately weighed in sequence and added to the corresponding group of solutions, and magnetically stirred (500 rpm, 25 ± 1 °C) for 30 minutes until completely dissolved, and after volume fixing, four groups of electrolytes with different magnesium chloride concentrations were obtained, and the solutions were all transparent and without precipitation.

[0044] Example 3 Construction of pH Gradient Electrolyte

[0045] Taking the pH adjustment of the negative electrode electrolyte as an example, a precision pH meter calibrated at three points (standard buffer solution pH 1.68, 4.01, 7.00) was used to measure the initial pH value of the basic electrolyte; 12 mol / L concentrated hydrochloric acid and deionized water were mixed at a volume ratio of 1:3 to prepare 3 mol / L dilute hydrochloric acid for standby.

[0046] Take 100 mL of the electrolyte containing magnesium chloride (at any concentration) in Example 2, transfer it to a 25 mL volumetric flask for volume fixation, and divide it equally into 4 clean beakers, with each portion being 25 mL; one of them is sealed and stored as a blank control solution, and the remaining three are respectively added dropwise with 3 mol / L dilute hydrochloric acid, and the pH is monitored and adjusted to the target values (0, -0.5, -1.0, with an error of ±0.05) by magnetic stirring (300 rpm).

[0047] Repeat the above operations to successively process the other three groups of electrolytes containing magnesium chloride at different concentrations (0.25, 0.5, 1.0, 1.5 mol / L), and finally obtain 12 negative electrode electrolytes with a pH gradient and 4 control solutions.

[0048] Example 4 Performance Test

[0049] Simulation device: An H-type double-chamber electrolytic cell (material: borosilicate glass, chamber volume 30 mL, Nafion212 proton exchange membrane) is used, and 4×4 cm 2 graphite felt and 1×1 cm 2 platinum-titanium electrodes are respectively used at the positive and negative electrodes.

[0050] Electrochemical characterization: The cyclic voltammetry (CV) test is used to study the electrochemical behavior of iron deposition / dissolution. Figure 1 Among them: (a) Cyclic voltammograms obtained in baths containing 3.6 mol / L ferrous chloride and different concentrations of magnesium chloride from 0 to 1.5 mol / L as well as different pH values of 0, -0.5, and -1.0, (b) Comparison of the current peaks of the hydrogen evolution reaction. Among them, the ferrous chloride-based electrolyte in Example 1 is used for 0 mol / L magnesium chloride. The chronoamperometry method is used for quantitative evaluation of the coulombic efficiency, and the average value is taken through three repeated charge-discharge cycles to ensure the reliability of the data. Figure 4 Shows the comparison results of the short-term cyclic coulombic efficiency measured under different pH conditions (0, -0.5, -1.0) in an electrolyte system based on 3.6 mol / L ferrous chloride (with or without 1 mol / L magnesium chloride).

[0051] Structure and morphology characterization: The scanning electron microscope (SEM) and X-ray diffraction (XRD) are used to study the influence of magnesium chloride on the iron deposition process. Figure 2 Are SEM images of iron electrodeposited in baths with different concentrations of magnesium chloride at a magnification of 10,000: (a) 0 mol / L magnesium chloride (pH = -1.0) (b) 1.0 mol / L magnesium chloride (pH = -1.0). Figure 3 - X-ray diffraction patterns of iron electrodeposited in 0 mol / L magnesium chloride (pH = -1.0) and 1.0 mol / L (pH = -1.0) magnesium chloride.

[0052] As can be seen from the above results, the electrolyte of the present invention improves iron deposition:

[0053] Figure 1 -a, as the concentration of magnesium chloride increases (0 - 1.5 mol / L), the potential difference (ΔEp) of the redox peak gradually decreases, and the peak shape tends to be symmetric. It is proved that the addition of magnesium chloride reduces the polarization effect, thereby enhancing the reversibility of iron deposition and dissolution in the electrode reaction. As the pH increases, the polarization effect decreases.

[0054] While Figure 3 in the XRD pattern of, the iron deposition without the addition of magnesium chloride has diffraction peaks of two planes of iron (110) and (211). With the addition of magnesium chloride, the iron deposition tends more towards the diffraction peak of the iron (211) plane. The addition of magnesium chloride changes the preferred orientation of the growth of the iron plating layer, and thus changes the deposition morphology. Figure 2 The SEM image of shows that after the addition of magnesium chloride, the iron deposition layer is more uniform and dense, proving that the addition of magnesium chloride has a good effect on grain refinement and leveling of the iron plating layer.

[0055] That is, the addition of magnesium chloride optimizes the morphology of iron deposition by enhancing the reversibility of the negative electrode and changing the preferred orientation of the growth of the iron plating layer.

[0056] As can be seen from the above results, the electrolyte of the present invention can inhibit hydrogen evolution:

[0057] Figure 1 -b shows that at the same pH, as the concentration of magnesium chloride increases (especially ≥1.0 mol / L), the current peak value of HER decreases significantly. At the same concentration of magnesium chloride, as the pH increases (weakening of acidity), the HER peak current also naturally decreases, but the presence of magnesium chloride further amplifies this inhibitory effect.

[0058] The mechanism of hydrogen evolution inhibition is that Mg 2+ and form an electrolyte with high ionic strength, reducing the activity of , while Mg 2+ is preferentially adsorbed on the electrode surface due to its high charge density, forming a "shielding layer", reducing the adsorption and reduction of on the electrode surface. At the same time, in a strong acidic environment (pH = -1.0 - 0), the buffering effect of Mg 2+ can effectively stabilize the pH of the electrolyte, avoid the precipitation of Fe(OH)3 caused by local pH increase, indirectly reduce the consumption of , inhibit the generation of H2, and enhance the long cycle life.

[0059] As can be seen from the above results, the electrolyte of the present invention can improve the Coulomb efficiency of the battery:

[0060] Under the condition of no magnesium chloride and at different pH values (0, -0.5, -1.0), the coulombic efficiencies are 94.9%, 91.3%, and 88.9% respectively; while the coulombic efficiencies corresponding to the pH conditions with the addition of 1.0 mol / L magnesium chloride are increased to 97.9%, 97.1%, and 95.1%. The results show that the introduction of magnesium chloride can effectively improve the coulombic efficiency of the battery. Especially in a strongly acidic environment (pH = -1.0), the improvement of the coulombic efficiency is more obvious, which further verifies its optimization effect on the redox reversibility.

[0061] Example 5 Long-cycle performance test

[0062] Taking the 3.6 mol / L ferrous chloride base solution as an example, a symmetric electrolysis system was configured: the positive electrolyte was 25 mL of the base electrolyte prepared in Example 1 (pH controlled at about -1.0), and the negative electrolyte was 25 mL of a composite electrolyte containing 1.0 mol / L magnesium chloride (pH adjustable range of -1.0 to 0). A pre-treated 4×4 cm² graphite felt positive electrode and a 1×1 cm² platinum sheet counter electrode were respectively placed in a H-type double-chamber electrolytic cell (material: borosilicate glass, chamber volume 30 mL, Nafion 212 proton exchange membrane). The device was sealed to isolate oxygen, and after assembly, it was connected to a CT3001A battery test system (Landt Instruments, USA). The constant current charge-discharge mode (20 mA / cm², cut-off voltage 0.1 - 5 V) was set, and the coulombic efficiency of the 1st - 10000th cycles was recorded.

[0063] Figure 5 The long-cycle test results (1.0 mol / L magnesium chloride, pH = 0) show that in the 10000-cycle long-cycle test, the coulombic efficiency was maintained at an average of 95% in the first 3000 cycles. After reaching the pH dynamic balance, the coulombic efficiency was maintained at an average of 99.2 ± 0.3% in the last 7000 cycles.

[0064] The above are only the preferred embodiments of the present invention and do not impose any form of limitation on the present invention. Therefore, any simple modification, equivalent change, and modification made to the above embodiments based on the technical essence of the present invention without departing from the technical solution of the present invention shall fall within the scope of protection of the technical solution of the present invention.

Claims

1. A negative electrode electrolyte for an all-iron flow battery, characterized in that, The negative electrode electrolyte consists of the following components: ferrous chloride at a concentration of 3.0 - 3.6 mol / L, magnesium chloride at a concentration of 0.8 - 1.2 mol / L, and sodium chloride at a concentration of 0.05 - 0.15 mol / L, with the balance being water and the pH being -1.0 - 0.

2. The negative electrode electrolyte of the all-iron flow battery according to claim 1, characterized in that The concentration of the sodium chloride is 0.08 - 0.12 mol / L.

3. The negative electrode electrolyte of the all-iron flow battery according to claim 1, characterized in that, The pH is -0.5 - 0.

4. A method for preparing the negative electrode electrolyte of an all-iron flow battery according to any one of claims 1 to 3, characterized in that, It includes: Dissolve ferrous chloride, magnesium chloride, and sodium chloride in water according to the concentration requirements and adjust the pH to the required range to obtain the negative electrode electrolyte.

5. Use of the negative electrode electrolyte according to any one of claims 1 - 3 in the preparation of an all - iron redox flow battery.

6. A all-iron flow battery, characterized in that, It contains the negative electrode electrolyte according to any one of claims 1 - 3.

7. The all-iron flow battery according to claim 6, wherein Its positive electrode electrolyte is an aqueous solution of ferrous chloride and sodium chloride.

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