Negative electrode electrolyte of total-iron flow battery as well as preparation method and application of negative electrode electrolyte

Abstract

The invention discloses an all-iron flow battery negative electrode electrolyte as well as a preparation method and application thereof, the negative electrode electrolyte is prepared from the following components: 1.8 to 3.6 mol / L of ferrous chloride, 0.25 to 1.5 mol / L of magnesium chloride, 0.05 to 0.15 mol / L of sodium chloride and the balance of water, and the pH (Potential of Hydrogen) is-1.0 to 0. According to the negative electrode electrolyte of the efficient all-iron flow battery, the extremely low pH of-1.0-0 is adopted, the problems of hydrogen evolution and poor reversibility of a deposition / dissolution reaction existing in a negative electrode of the all-iron flow battery are solved by regulating and controlling magnesium chloride on an inorganic interface in a strong acid environment, and the all-iron flow battery with high coulombic efficiency and long cycle life is obtained.

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 reaction 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, causing Fe(OH) 3 The continuous accumulation of colloid precipitation at the separator will significantly increase the internal resistance of the battery and cause rapid decay of cycle performance. These problems ultimately restrict the large-scale engineering application of this technology.

[0004] Currently, there are only a limited number of patents that have been publicly authorized or are under review for all-iron flow batteries. Patents related to alkaline systems dominate, while the technical development of acidic systems mainly revolves around iron-based organic complexes. After reviewing the literature on related research, it is not difficult to find that the current academic research focus in acidic flow batteries is mainly on ferrous chloride (FeCl 2 ) as the basic electrochemical properties of active materials, 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 Fe2+ system, and the upper limit of cycle life is only thousands of times.

[0006] (CN114388859B) discloses an all-iron liquid flow battery negative electrode electrolyte, which forms a coordination structure with ferrous ions by adding dimethyl sulfoxide (DMSO) or its derivatives to regulate the iron deposition morphology and inhibit the hydrogen evolution reaction. This method also relies on organic coordination, the electrolyte pH needs to maintain weak acidity, the upper limit of 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 deficiencies and defects mentioned in the above background technology and provide an all-iron liquid flow battery negative electrode electrolyte and a preparation method and application thereof to improve the coulombic efficiency and cycle life.

[0008] In order to solve the above technical problems, the technical solution proposed by the present invention is: A negative electrode electrolyte for an all-iron liquid flow battery, the negative electrode electrolyte consisting of the following components: 1.8-3.6 mol / L of ferrous chloride, 0.25-1.5 mol / L of magnesium chloride and 0.05-0.15 mol / L of sodium chloride, the remainder being water, and the pH being -1.0-0.

[0009] As a further improvement, the concentration of ferrous chloride is 3.0~3.6 mol / L.

[0010] As a further improvement, the magnesium chloride concentration is 0.8~1.2 mol / L.

[0011] As a further improvement, the sodium chloride concentration is 0.08~0.12 mol / L.

[0012] As a further improvement, the pH is -0.5~0.

[0013] The present invention also provides a method for preparing the negative electrode electrolyte of the all-iron liquid flow battery, comprising: dissolving ferrous chloride, magnesium chloride and sodium chloride in water according to concentration requirements, and adjusting the pH to a required range to obtain the negative electrode electrolyte.

[0014] The present invention also provides an application of the negative electrode electrolyte in preparing an all-iron liquid flow battery.

[0015] The present invention also provides an all-iron liquid flow battery, which comprises the negative electrode electrolyte. The positive electrode electrolyte is an aqueous solution of ferrous chloride and sodium chloride.

[0016] Compared with the prior art, the present invention has the following beneficial effects: The high-efficiency all-iron liquid flow battery negative electrode electrolyte of the present invention adopts an extremely low pH of -1.0 to 0, and improves the problems of hydrogen evolution and poor reversibility of deposition / dissolution reactions existing in the negative electrode of the all-iron liquid flow battery by regulating magnesium chloride at the inorganic interface in a strong acidic environment, thereby obtaining an all-iron liquid flow battery with high coulomb efficiency and high cycle life. Specifically, it has: (1) High Coulombic efficiency and long cycle life: It can perform long cycle tests of 10,000 cycles, with a Coulombic efficiency of up to 99.5%.

[0017] (2) Optimizing electrode reaction performance: Cyclic voltammetry tests showed that the addition of magnesium chloride inhibited the hydrogen evolution reaction, reduced the degree of battery potential separation, and promoted the Fe / Fe² + The reversibility and kinetics of the reaction are improved, thereby increasing the Coulombic efficiency.

[0018] (3) Improvement of iron deposition morphology: SEM characterization shows that the structure after adding magnesium chloride is denser and more uniform than when it is not added; XRD shows the single structural orientation of iron deposition, thereby improving the cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0020] Figure 1 The cyclic voltammograms of the examples are as follows: (a) cyclic voltammograms obtained in a bath 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 current peaks of the hydrogen evolution reaction.

[0021] Figure 2 Scanning electron microscope images of electrodeposited iron in magnesium chloride baths of different concentrations at a magnification of 10,000: (a) 0 mol / L magnesium chloride (b) 1.0 mol / L magnesium chloride.

[0022] Figure 3 Shown are the X-ray diffraction patterns of iron electrodeposited in 0 mol / L magnesium chloride and 1.0 mol / L magnesium chloride.

[0023] Figure 4 This is a comparative measurement of the coulombic efficiency of electrolyte systems containing 1.0 mol / L magnesium chloride and those without magnesium chloride at different pH values.

[0024] Figure 5 It is the Coulombic efficiency of the battery after 10,000 cycles. DETAILED DESCRIPTION

[0025] In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively and meticulously below in conjunction with the accompanying drawings and preferred embodiments of the present invention, but the protection scope of the present invention is not limited to the following specific embodiments.

[0026] Unless otherwise defined, all professional terms used below have the same meanings as those generally 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 scope of protection of the present invention.

[0027] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0028] In some specific embodiments, the negative electrode electrolyte of the all-iron liquid flow battery of the present invention comprises ferrous chloride, sodium chloride and magnesium chloride, and its composition is: Ferrous chloride: 1.8 ~3.6 mol / L, preferably 3.0~3.6 mol / L; Magnesium chloride: 0.25 ~1.5 mol / L, preferably 0.8 ~ 1.2 mol / L; Sodium chloride: 0.05~0.15 mol / L, preferably 0.08~0.12 mol / L; Deionized water: balance.

[0029] The pH of the negative electrode electrolyte is: -1.0~0, preferably -0.5~0.

[0030] Ferrous ions are redox active substances. The increase in their concentration can increase the charge storage capacity per unit volume, directly improving the mass specific capacity and volume energy density of the system. Therefore, the preferred concentration of ferrous chloride is 3.0~3.6 mol / L.

[0031] The effect of enhancing reversibility began to be inhibited when the concentration of magnesium chloride was 1.5 mol / L (e.g. Figure 1 cyclic voltammogram), so the preferred concentration of magnesium chloride is 0.8~1.2 mol / L.

[0032] In the pH range of -1.0~0, as the pH increases, the hydrogen evolution side reaction weakens and the Coulomb efficiency increases. However, when the pH is greater than 2, it will cause the hydrolysis and precipitation of iron ions, affecting the proton membrane, increasing side reactions and performance degradation, all of which will lead to a decrease in the Coulomb efficiency of the battery. Moreover, at pH=1, the pH stability is worse during the long cycle due to the consumption of hydrogen ions, so the long cycle performance is worse than when pH=0. Therefore, the pH is controlled at -1.0~0.

[0033] Example 1 Preparation of ferrous chloride based electrolyte Taking 3.6 mol / L ferrous chloride as an example, 45.63 g of anhydrous ferrous chloride (purity ≥ 99.9%) and 0.58 g of sodium chloride were added to an appropriate amount of deionized water in sequence, and magnetic stirring was carried out at 500 rpm for 30 minutes at 25±1°C until the solid was completely dissolved to obtain a uniform and transparent mixed solution; then the solution was transferred to a 100 mL volumetric flask, and the inner wall of the beaker was rinsed with a small amount of deionized water for 3 times. The rinse solution was combined and the volume was adjusted to the mark, and the solution was shaken and allowed to stand for defoaming. Finally, 100 ml of a basic electrolyte containing 3.6 mol / L ferrous chloride and 0.1 mol / L sodium chloride was prepared.

[0034] Example 2 Preparation of gradient magnesium chloride composite electrolyte Taking the 3.6 mol / L ferrous chloride base solution prepared in Example 1 as an example, 4 groups of 100 mL basic electrolyte (containing 3.6 mol / L ferrous chloride and 0.1 mol / L NaCl) were taken respectively, and 2.38 g (corresponding to 0.25 mol / L), 4.76 g (0.5 mol / L), 9.52 g (1.0 mol / L), and 14.28 g (1.5 mol / L) of anhydrous magnesium chloride (purity ≥99.9%) were accurately weighed in each group, and added to the corresponding group solution, and magnetic stirring (500 rpm, 25±1°C) was performed for 30 minutes until completely dissolved. After constant volume, four groups of electrolytes with different magnesium chloride concentrations were obtained, and the solutions were all transparent and precipitated.

[0035] Example 3 Construction of pH Gradient Electrolyte Taking the pH adjustment of the negative electrode electrolyte as an example, a precision pH meter calibrated at three points (standard buffer 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 in a volume ratio of 1:3 to prepare 3 mol / L dilute hydrochloric acid for use.

[0036] Take 100 mL of the electrolyte containing magnesium chloride (any concentration) in Example 2, transfer it to a 25 mL volumetric flask to make up the volume, and divide it equally into 4 clean beakers, 25 mL each; one of them is sealed and stored as a blank control solution, and 3 mol / L dilute hydrochloric acid is added dropwise to the remaining three portions, and the pH is monitored in real time by magnetic stirring (300 rpm) and adjusted to the target value (0, -0.5, -1.0, error ±0.05).

[0037] The above operation was repeated to sequentially treat the remaining three groups of electrolytes containing magnesium chloride of different concentrations (0.25, 0.5, 1.0, and 1.5 mol / L), and finally 12 portions of pH gradient negative electrode electrolytes and 4 portions of control solutions were prepared.

[0038] Example 4 Performance Test Simulation device: An H-type double-chamber electrolyzer (material: borosilicate glass, chamber volume 30 mL, Nafion212 proton exchange membrane) was used, with 4×4 cm 2 Graphite felt and 1×1cm 2 of platinum-titanium electrodes.

[0039] Electrochemical characterization: Cyclic voltammetry (CV) tests were used to investigate the electrochemical behavior of iron deposition / stripping. Figure 1 In the figure: (a) Cyclic voltammetry curves obtained in a bath 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 current peaks of the hydrogen evolution reaction. The 0 mol / L magnesium chloride is the ferrous chloride-based electrolyte of Example 1. The coulombic efficiency was quantitatively evaluated by chronoamperometry, and the data reliability was ensured by repeating the charge and discharge cycles three times and taking the average value. Figure 4 The comparative results of short-term cyclic coulombic efficiency measured under different pH conditions (0, -0.5, -1.0) in an electrolyte system containing 3.6 mol / L ferrous chloride (with or without 1 mol / L magnesium chloride) are shown.

[0040] Structural and morphological characterization: The effect of magnesium chloride on the iron deposition process was studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Figure 2 These are SEM images of iron electrodeposited in magnesium chloride baths of different concentrations 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 MgCl2 (pH=-1.0) and 1.0 mol / L MgCl2 (pH=-1.0).

[0041] It can be seen from the above results that the electrolyte of the present invention improves iron deposition: Figure 1 -a, with the increase of magnesium chloride concentration (0~1.5 mol / L), the potential difference (ΔEp) of the redox peak gradually decreases, and the peak shape tends to be symmetrical. It proves 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.

[0042] and Figure 3In the XRD diagram, the iron deposition without magnesium chloride has two diffraction peaks of iron (110) and (211). With the addition of magnesium chloride, the iron deposition tends to be more inclined to the iron (211) diffraction peak. The addition of magnesium chloride changes the preferred orientation of the iron coating growth, and thus changes the morphology of the deposition. Figure 2 The SEM image shows that after adding magnesium chloride, the iron deposition layer is more uniform and dense, proving that the addition of magnesium chloride has a better grain refinement and leveling effect on the iron plating layer.

[0043] That is, the addition of MgCl2 optimizes the morphology of Fe deposition by enhancing the reversibility of the negative electrode and changing the preferred orientation of Fe plating growth.

[0044] It can be seen from the above results that the electrolyte of the present invention can inhibit hydrogen evolution: Figure 1 -b shows that at the same pH, as the magnesium chloride concentration increases (especially ≥1.0 mol / L), the HER current peak decreases significantly. At the same magnesium chloride concentration, as the pH increases (weakening acidity), the HER peak current also naturally decreases, but the presence of magnesium chloride further amplifies this inhibitory effect.

[0045] The mechanism of inhibiting hydrogen evolution is that Mg 2+ and Forming a high ionic strength electrolyte, reducing The activity of Mg 2+ Because the high charge density is preferentially adsorbed on the electrode surface, forming a "shielding layer", reducing Adsorption and reduction on the electrode surface. At the same time, in a strong acidic environment (pH = -1.0 ~ 0), Mg 2+ The buffering effect can effectively stabilize the pH of the electrolyte and avoid the Fe(OH) 3 Sedimentation, indirect reduction Consumption, inhibition of H 2 Generates, enhancing long cycle life.

[0046] From the above results, it can be seen that the electrolyte of the present invention can improve the coulombic efficiency of the battery: Without magnesium chloride, the coulombic efficiency at different pH values ​​(0, -0.5, -1.0) was 94.9%, 91.3%, and 88.9%, respectively; while the coulombic efficiency under the corresponding pH conditions of adding 1.0 mol / L magnesium chloride 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 strong acidic environment (pH = -1.0), the coulombic efficiency is improved more significantly, which further verifies its optimization effect on redox reversibility.

[0047] Example 5 Long cycle performance test Taking 3.6 mol / L ferrous chloride basic solution as an example, a symmetrical electrolysis system was configured: the positive electrode electrolyte was 25 mL of the basic electrolyte prepared in Example 1 (pH controlled at about -1.0), and the negative electrode electrolyte was 25 mL of a composite electrolyte containing 1.0 mol / L magnesium chloride (pH = -1.0~0 adjustable range). Take out the H-type double-chamber electrolytic cell (material: borosilicate glass, chamber volume 30 mL, Nafion 212 proton exchange membrane) and place the pretreated 4×4 cm² graphite felt positive electrode and 1×1 cm² platinum sheet counter electrode respectively. The device was sealed to isolate oxygen, and after assembly, it was connected to the CT3001A battery test system (LandtInstruments, USA). Set the constant current charge and discharge mode (20 mA / cm², cut-off voltage 0.1-5 V), and record the coulombic efficiency of the 1st to 10000th cycles.

[0048] Figure 5 The long cycle test results (1.0 mol / L magnesium chloride, pH=0) show that in the 10,000-cycle long cycle test, the Coulomb efficiency was maintained at an average of 95% in the first 3,000 cycles. After reaching pH dynamic equilibrium, the Coulomb efficiency was maintained at an average of 99.2±0.3% in the next 7,000 cycles.

[0049] The above is only a preferred embodiment of the present invention and does not limit the present invention in any form. Therefore, any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the scope of protection of the technical solution of the present invention.

Claims

1. An all-iron liquid flow battery negative electrode electrolyte, characterized in that: The negative electrode electrolyte consists of the following components: 1.8-3.6 mol / L of ferrous chloride, 0.25-1.5 mol / L of magnesium chloride and 0.05-0.15 mol / L of sodium chloride, the remainder being water, and the pH is -1.0-0.

2. The negative electrode electrolyte of the all-iron liquid flow battery according to claim 1, characterized in that: The concentration of ferrous chloride is 3.0-3.6 mol / L.

3. The negative electrode electrolyte of the all-iron liquid flow battery according to claim 1, characterized in that: The magnesium chloride concentration is 0.8-1.2 mol / L.

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

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

6. A method for preparing the negative electrode electrolyte of the all-iron liquid flow battery according to any one of claims 1 to 5, characterized in that: include: The negative electrode electrolyte is obtained by dissolving ferrous chloride, magnesium chloride and sodium chloride in water according to the concentration requirements and adjusting the pH to the required range.

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

8. An all-iron liquid flow battery, characterized in that: It comprises the negative electrode electrolyte according to any one of claims 1 to 5.

9. The all-iron liquid flow battery according to claim 8, characterized in that: The positive electrode electrolyte is an aqueous solution of ferrous chloride and sodium chloride.

Images