Alkaline aqueous all-iron flow battery negative electrode electrolyte

Abstract

The invention belongs to the technical field of redox flow batteries, and provides an alkaline aqueous all-iron flow battery negative electrode electrolyte. The negative electrode electrolyte adopts a full water-soluble design, a triple synergistic mechanism is constructed through molecular engineering, and the negative electrode electrolyte is composed of an iron-based active substance, a stabilizer, a difunctional organic ligand, an alkaline supporting electrolyte and a conductive enhancer; the total concentration of the difunctional organic ligand is regulated and controlled to be 1.0-2.0 mol / L. According to the invention, a multifunctional organic ligand is introduced to optimize a negative electrode electrolyte system, so that stable coordination of iron ions and double improvement of redox capacity of the electrolyte under an alkaline condition are realized. The system comprises an iron active material, a redox active organic ligand, a supporting electrolyte and an auxiliary electrolyte. Experiments show that the cycle life and the capacity density of an all-iron flow battery constructed based on the electrolyte are improved by about 40% compared with those of a traditional system.

Description

Technical Field

[0001] The present invention belongs to the technical field of redox flow batteries and relates to a negative electrode electrolyte for an alkaline aqueous all-iron flow battery. Background Art

[0002] Flow batteries achieve electrical energy storage and conversion through electrochemical reactions of redox couples in liquid electrolytes, and have technical advantages such as flexible system design, high energy efficiency, deep charge-discharge capabilities, and intrinsic safety, showing important application value in the field of large-scale energy storage. The most commercially developed all-vanadium system is restricted by the scarcity of vanadium resources, and the electrolyte cost accounts for more than 40% of the total, seriously pushing up the total system cost. Developing a new low-cost aqueous system has become an urgent need in the industry. Although the traditional all-iron system uses an iron-based redox couple of the same element to significantly reduce the system cost, it is still urgently needed to improve its energy efficiency and cycle life due to technical bottlenecks such as the solubility of active substances, the tendency of side reactions, and the selective migration of ions.

[0003] The existing negative electrode electrolyte systems for iron-based flow batteries still face the dual challenges of coordination chemistry and interfacial stability: Although the acidic system (such as CN113328124A) inhibits Fe 2+ hydrolysis through multiple additives, the two-phase reaction mechanism of Fe 2+ deposition / dissolution is difficult to avoid dendrite growth and hydrogen evolution side reactions, resulting in a capacity decay rate of more than 0.5% / cycle; the alkaline system (such as CN118336062A, CN118738483A) uses TEA-based ligands to construct a six-coordination structure, but small molecule auxiliary ligands are prone to desorption and migration during long-term cycling, leading to a reduction in coordination number (six → five coordination) and the formation of Fe(OH)3 precipitation and elemental iron; the chelating iron ability of organic phosphine complexing agents (such as CN119481191A) is stronger than that of EDTA-based and phosphate-based ones, and the complexing capacity is high. Although the strong coordination effect of phosphonic acid groups enhances the thermodynamic stability, it may overly bind the d-orbital electrons of iron ions, resulting in sluggish redox reaction kinetics and affecting high-rate performance.

[0004] In summary, the current negative electrode electrolyte system for all-iron flow batteries still faces three core challenges: First, although the existing modification schemes can improve the cycle stability, their applicability is restricted by a narrow pH window and a limited working temperature range, making it difficult to adapt to the requirements of complex working conditions; second, the solid-liquid two-phase reaction mechanism leads to kinetic retardation in the Fe deposition / dissolution process; third, commercial applications urgently need to balance performance and cost, and the existing high-cost organic ligands increase the electrolyte cost.

[0005] Therefore, the present invention provides a negative electrolyte for an alkaline all-iron flow battery. By adding organic phosphates and phosphomolybdic acid, a phosphonic acid group-molybdenum oxide molecular cluster is formed. Through the synergistic coordination of the phosphonic acid group-molybdenum oxide molecular cluster, a six / eight-coordinate supramolecular structure with high symmetry is formed, and its ligand field stabilization energy is significantly better than that of the traditional five-coordinate system. Summary of the Invention

[0006] Aiming at the key problems of fast capacity decay and insufficient electrochemical stability of alkaline all-iron flow batteries, the present invention proposes an electrolyte structure optimization strategy based on bifunctional ligands. By designing an organic molecule with both coordination and redox active centers, a "ligand-iron ion" synergistic mechanism is constructed in an alkaline medium: the coordination end forms a thermodynamically stable Fe(II) / Fe(III) coordination structure through polydentate chelation, effectively inhibiting the hydrolysis precipitation and disproportionation side reactions of iron species; at the same time, the redox active groups embedded in the molecular structure can establish an electron transfer channel independent of the metal center, realizing a doubling effect of the intrinsic capacity of the electrolyte. This synergistic mechanism breaks through the performance limitations of traditional monofunctional ligands, enabling the electrolyte system to simultaneously possess excellent cycle stability and high charge storage capacity.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A negative electrolyte for an alkaline aqueous all-iron flow battery, specifically a high-performance composite negative electrolyte system suitable for alkaline aqueous all-iron flow batteries. Its core components are: iron-based active substances, stabilizers, bifunctional organic ligands, alkaline supporting electrolytes, and conductive enhancers. This system adopts a fully water-soluble design and constructs a triple synergistic mechanism through molecular engineering. In particular, the total concentration of the bifunctional organic ligand is regulated to 1.0 - 2.0 mol / L, maximizing the charge storage density while ensuring the kinetic performance of the electrolyte, effectively solving the technical contradiction that it is difficult to simultaneously improve the capacity and stability of the traditional system.

[0009] The iron-based active substances are selected from ferric salts, preferably at least one of ferric sulfate, ferric phosphate, ferric nitrate, and ferric acetate. The concentration of the iron-based active substances in the electrolyte is 0.5 - 1.5 mol / L, and further preferably 0.6 - 1.2 mol / L.

[0010] The stabilizer is at least one of polyols and polycarboxylates, preferably at least one of glycerol and sodium citrate. The molar ratio of the stabilizer to the Fe element in the iron-based active substances is 0.5:1 - 1:1.

[0011] The alkaline supporting electrolyte is an alkaline inorganic compound, preferably at least one of sodium hydroxide, lithium hydroxide, and potassium hydroxide. The concentration of the alkaline supporting electrolyte in the electrolyte solution is 1.0 - 4.0 mol / L, and further preferably 2.0 - 3.0 mol / L.

[0012] The conductive enhancer, as an auxiliary electrolyte, is selected from inorganic salts that do not participate in electrochemical reactions, preferably at least one of potassium sulfate, potassium nitrate, potassium acetate, sodium sulfate, sodium nitrate, and sodium acetate. The concentration of this auxiliary electrolyte in the electrolyte solution is 0.5 - 1.5 mol / L.

[0013] The bifunctional organic ligand is a mixture of an active organic ligand and phosphomolybdic acid, serving as a functional component of the electrolyte solution. The active organic ligand includes at least one of sodium ethylenediaminetetramethylenephosphonate, potassium ethylenediaminetetramethylenephosphonate, potassium hexamethylenediaminetetramethylenephosphonate, and sodium hexamethylenediaminetetramethylenephosphonate. The molar ratio of the organic phosphate group in the bifunctional organic ligand to phosphomolybdic acid is 1:1. The concentration of the bifunctional organic ligand in the electrolyte solution is 1.0 - 2.0 mol / L, and further preferably 1.2 - 1.8 mol / L; the molar ratio of the bifunctional organic ligand to the iron-based active material is 1:1 - 2:1.

[0014] A method for preparing the negative electrode electrolyte of an aqueous alkaline all-iron redox flow battery is as follows:

[0015] Step (1): Add the iron-based active material and deionized water to a reactor with stirring, and heat to 90 °C while stirring.

[0016] Step (2): Slowly add the stabilizer to the solution in step (1) and stir until completely dissolved, then continue stirring and reacting for 1 hour.

[0017] Step (3): Add the active organic ligand substance to the mixed solution in step (2) and stir until completely dissolved.

[0018] Step (4): Add the alkaline inorganic compound to the mixed solution in step (3) and stir until completely dissolved.

[0019] Step (5): Slowly add the conductive enhancer electrolyte to the mixed solution in step (4) and stir until completely dissolved.

[0020] Step (6): Slowly add the alkaline inorganic compound to the mixed solution in step (5) to obtain the negative electrode electrolyte.

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

[0022] (1) The present invention innovatively introduces active organic ligands (such as ethylenediaminetetramethylenephosphonate) and phosphomolybdic acid as functional components of the negative electrolyte. In their molecular structures, multifunctional functional groups such as hydroxyl groups, phosphonic acid groups, amino groups, and molybdenum-oxygen clusters are coordinately distributed. Based on the coordination advantage of the hybridization of high electronegativity atoms such as N / P / O with the Fe(II) d orbital, the coordination vacancies of ferrous ions are effectively occupied through a multi-dentate coordination mechanism to construct a six / eight-coordination chelate structure with high symmetry. This coordination mode is significantly superior to the geometric stability of the traditional five-coordination system, and can effectively inhibit the formation of Fe(OH)2 precipitation and the 2+ / Fe 3+ disproportionation reaction of Fe

[0023] at the same time, enhancing the kinetic reversibility of the redox process, thereby comprehensively improving the cycle stability of the electrolyte. Brief Description of the Drawings

[0024] Figure 1 It is the charge-discharge curve diagram of Example 1.

[0025] Figure 2 It is the charge-discharge curve diagrams of Comparative Example 1 and Comparative Example 4.

[0026] Figure 3 Capacity retention rate of all-iron flow batteries in Examples 1-5.

[0027] Figure 4 Capacity retention rate of all-iron flow batteries in Comparative Examples 1-5. Detailed Embodiments

[0028] In order to make the technical problems, technical solutions, and beneficial effects solved by the present invention clearer and more understandable, the following further describes the implementation solutions of the present invention in combination with examples. 1. Preparation process of all-iron flow battery:

[0029] 1.1 Preparation process of negative electrolyte:

[0030] Examples 1-6 and Comparative Examples 1-3 are prepared in the following manner:

[0031] (1) Add iron-based active substances and deionized water into a reactor with stirring, and heat up to 90 °C while stirring.

[0032] (2) Slowly add a stabilizer to the solution in step (1) and stir until completely dissolved, then continue stirring and reacting for 1 hour.

[0033] (3) Add active organic ligand substances to the mixed solution in step (2) and stir until completely dissolved.

[0034] (4) Add alkaline inorganic compounds to the mixed solution in step (3) and stir until completely dissolved.

[0035] (5) Slowly add a conductive enhancer electrolyte to the mixed solution in step (4) and stir until completely dissolved.

[0036] (6) Slowly add alkaline inorganic compounds to the mixed solution in step (5) and stir until completely dissolved.

[0037] 1.2 Preparation process of the positive electrode electrolyte:

[0038] Dissolve 1 mol of potassium hydroxide, 1 mol of potassium hydroxide, 0.4 mol of potassium ferrocyanide, and 0.4 mol of sodium ferrocyanide in deionized water at 80 °C, and make up the volume to 1 L to obtain an alkaline positive electrode electrolyte.

[0039] 1.3 Assembly of the full cell:

[0040] The battery assembly sequence is: left steel splint, positive copper plate electrode, rubber gasket, positive flow channel plate, 30*30*5 mm graphite felt, ion exchange membrane (polytetrafluoroethylene sulfonic acid ion exchange membrane), negative 30*30*5 mm graphite felt, negative flow channel plate, rubber gasket, negative copper plate electrode, and right steel splint.

[0041] Example 1

[0042] A negative electrode electrolyte for an aqueous alkaline all-iron flow battery: 0.5 mol of iron phosphate, 0.5 mol of sodium ethylene diamine tetra(methylene phosphonate), 0.5 mol of phosphomolybdic acid, 0.25 mol of ethylene glycol, 0.5 mol of sodium sulfate, and 4 mol of potassium hydroxide, and make up the volume to 1 L to prepare the negative electrode electrolyte.

[0043] Example 2

[0044] A negative electrode electrolyte for an aqueous alkaline all-iron flow battery: 0.75 mol of iron sulfate, 1 mol of potassium ethylene diamine tetra(methylene phosphonate), 1 mol of phosphomolybdic acid, 1.5 mol of ethylene glycol, 1.5 mol of potassium sulfate, and 4 mol of sodium hydroxide, and make up the volume to 1 L to prepare the negative electrode electrolyte.

[0045] Example 3

[0046] The negative electrolyte of an aqueous alkaline all-iron redox flow battery: 0.8 mol of iron nitrate, 0.6 mol of sodium hexamethylenediaminetetramethylenephosphonate, 0.6 mol of phosphomolybdic acid, 0.8 mol of sodium citrate, 1 mol of potassium nitrate and 2 mol of potassium hydroxide, made up to 1 L to prepare the negative electrolyte.

[0047] Example 4

[0048] The negative electrolyte of an aqueous alkaline all-iron redox flow battery: 1.2 mol of iron acetate, 0.9 mol of potassium hexamethylenediaminetetramethylenephosphonate, 0.9 mol of phosphomolybdic acid, 1.2 mol of sodium citrate, 12 mol of sodium acetate and 3 mol of potassium hydroxide, made up to 1 L to prepare the negative electrolyte.

[0049] Example 5

[0050] The negative electrolyte of an aqueous alkaline all-iron redox flow battery: 1 mol of iron sulfate, 0.8 mol of potassium hexamethylenediaminetetramethylenephosphonate, 8 mol of phosphomolybdic acid, 0.8 mol of sodium citrate, 1.5 mol of sodium nitrate and 3 mol of potassium hydroxide, made up to 1 L to prepare the negative electrolyte.

[0051] Comparative Example 1

[0052] The negative electrolyte of an aqueous alkaline all-iron redox flow battery: 0.5 mol of iron phosphate, 0.5 mol of sodium ethylenediaminetetramethylenephosphonate, 0.25 mol of triethylene glycol, 0.5 mol of sodium sulfate, 1 mol of potassium sulfate and 1 mol of potassium hydroxide, made up to 1 L to prepare the negative electrolyte.

[0053] Comparative Example 2

[0054] The negative electrolyte of an aqueous alkaline all-iron redox flow battery: 0.75 mol of iron sulfate, 1 mol of potassium ethylenediaminetetramethylenephosphonate, 1.5 mol of triethylene glycol, 1 mol of sodium sulfate, 1.5 mol of potassium sulfate and 4 mol of sodium hydroxide, made up to 1 L to prepare the negative electrolyte.

[0055] Comparative Example 3

[0056] The negative electrolyte of an aqueous alkaline all-iron redox flow battery: 0.8 mol of iron nitrate, 0.6 mol of sodium hexamethylenediaminetetramethylenephosphonate, 0.8 mol of sodium citrate, 1 mol of potassium nitrate and 2 mol of potassium hydroxide, made up to 1 L to prepare the negative electrolyte.

[0057] Comparative Example 4

[0058] The negative electrolyte of an aqueous alkaline all-iron redox flow battery: 1.2 mol of iron acetate, 1.2 mol of sodium citrate, 12 mol of sodium acetate and 3 mol of potassium hydroxide, made up to 1 L to prepare the negative electrolyte.

[0059] Comparative Example 5

[0060] 2. Anode electrolyte of an aqueous alkaline all-iron flow battery: 1 mol of ferric sulfate, 0.8 mol of sodium citrate, 1.5 mol of sodium nitrate, and 3 mol of potassium hydroxide, made up to 1 L to prepare the anode electrolyte. Electrochemical performance test of the all-iron flow battery

[0061] Battery parameters: The volume of the positive electrode electrolyte is 100 ml, the volume of the negative electrode electrolyte is 50 ml, and the flow rates of the positive and negative electrode electrolytes are both 250 ml / min.

[0062] Charge and discharge tester: BT-4008T of Neware Testing Technology Co., Ltd.

[0063] Charge and discharge mode: Constant current and constant voltage charging mode.

[0064] The charging cut-off voltage for Examples 1-5 and Comparative Examples 1-5 is 1.75 V, the charging time is 1 hour, the discharging cut-off voltage is 0.5 V, and the number of charge and discharge cycles is 200 times. Table 1 shows the Coulombic efficiency, voltage efficiency, and energy efficiency of Examples 1-5 and Comparative Examples 1-5. Figure 3 Indicates the capacity retention rate of the neutral flow battery composed of Examples 1-5, Figure 4 Indicates the capacity retention rate of the all-iron flow battery composed of Comparative Examples 1-5.

[0065] Table 1 Battery performance of examples and comparative examples

[0066]

[0067]

[0068] As can be seen from Table 1, the battery performance of Examples 1-5 is significantly higher than that of Comparative Examples 1-5.

[0069] From Figure 1 Compared with Figure 2 , it can be seen that the discharge platform, discharge capacity, and efficiency of Example 1 are significantly better than those of Comparative Example 1 and Comparative Example 4. Based on Figures 3 - 4 The cycle performance analysis shows that the all-iron flow battery system constructed by Examples 1-5 exhibits excellent capacity retention characteristics. After 400 deep cycles, it still maintains >95% of the initial capacity, and its capacity decay rate (<0.012% / cycle) is reduced by one order of magnitude compared with the comparative example system. This significant difference stems from the following synergistic mechanism:

[0070] (1) Redox activity regulation: The introduction of phosphomolybdic acid broadens the charge compensation path of the iron-based reaction through multi-electron transfer characteristics and increases the effective capacity of the negative electrode by more than 38%;

[0071] (2) Inhibition of interfacial side reactions: Phosphomolybdic acid anions form a dynamic adsorption layer on the electrode surface, selectively passivating the hydrogen evolution active sites and reducing the side reaction current density;

[0072] (3) Control of transmembrane permeation: The steric hindrance effect of the phosphomolybdic acid-iron supramolecular complex inhibits the iron ion permeability.

[0073] In summary, the present invention innovatively constructs an organic phosphine-phosphomolybdic acid composite ligand system, combines a polyol / carboxylic acid composite stabilizer, and achieves a breakthrough regulation of iron coordination chemistry in an alkaline environment. Based on the concept of multinuclear coordination design, through the synergistic coordination of phosphonic acid groups and molybdenum oxide molecular clusters, a six / eight-coordinate supramolecular structure with high symmetry is formed, and its ligand field stabilization energy is significantly better than that of the traditional five-coordinate system. At the same time, the multi-stabilizer regulates the interfacial microenvironment through the synergistic action of hydroxyl and carboxyl groups, and the alkaline electrolyte environment effectively inhibits the side reactions of hydrogen and oxygen evolution and the formation of an iron-based passivation layer, significantly enhancing the reversibility of the dissolution-deposition process of iron species. This strategy breakthroughly solves the problems of disproportionation decomposition and dendrite growth caused by the structural relaxation of ferrous complexes, and improves the cycle stability of the all-iron flow battery.

[0074] The above embodiments only represent the implementation modes of the present invention, but should not be construed as limiting the scope of the present invention. It should be noted that for those skilled in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention.

Claims

1. An alkaline aqueous all-iron redox flow battery negative electrolyte, characterized in that, The negative electrode electrolyte adopts a fully water-soluble design, and a triple synergistic mechanism is constructed through molecular engineering. Its composition is: iron-based active substance, stabilizer, bifunctional organic ligand, alkaline supporting electrolyte and conductivity enhancer; the total concentration of the bifunctional organic ligand is regulated to 1.0 - 2.0 mol / L.

2. The negative electrode electrolyte of an alkaline aqueous all-iron redox flow battery according to claim 1, characterized in that The bifunctional organic ligand is a mixture of an active organic ligand and phosphomolybdic acid, and serves as a functional component of the electrolyte.

3. The negative electrode electrolyte of an alkaline aqueous all-iron redox flow battery according to claim 2, wherein The molar ratio of the organic phosphate group to phosphomolybdic acid in the bifunctional organic ligand is 1:

1.

4. An alkaline aqueous all-iron redox flow battery negative electrolyte according to claim 2, characterized in that, The active organic ligand includes at least one of sodium ethylenediaminetetramethylenephosphonate, potassium ethylenediaminetetramethylenephosphonate, potassium hexamethylenediaminetetramethylenephosphonate, and sodium hexamethylenediaminetetramethylenephosphonate.

5. An alkaline aqueous all-iron redox flow battery negative electrolyte according to claim 1, characterized in that, The iron-based active substance is selected from ferric salts, preferably at least one of ferric sulfate, ferric phosphate, ferric nitrate, and ferric acetate.

6. The negative electrode electrolyte of an alkaline aqueous all-iron redox flow battery according to claim 1, characterized in that The stabilizer is at least one of polyols and polycarboxylates; the alkaline supporting electrolyte is an alkaline inorganic compound; the conductivity enhancer serves as an auxiliary electrolyte and is selected from inorganic salts that do not participate in electrochemical reactions.

7. An alkaline aqueous all-iron redox flow battery negative electrolyte according to claim 1, characterized in that, The stabilizer is preferably at least one of glycerol and sodium citrate; the alkaline supporting electrolyte is preferably at least one of sodium hydroxide, lithium hydroxide, and potassium hydroxide; the conductivity enhancer is preferably at least one of potassium sulfate, potassium nitrate, potassium acetate, sodium sulfate, sodium nitrate, and sodium acetate.

8. The negative electrode electrolyte of an alkaline aqueous all-iron redox flow battery according to claim 1, wherein: The molar ratio of the bifunctional organic ligand to the iron-based active substance is 1:1 - 2:1; The concentration of the iron-based active substance in the electrolyte is 0.5 - 1.5 mol / L; The molar ratio of the stabilizer to the Fe element in the iron-based active substance is 0.5:1 - 1:1; The concentration of the alkaline supporting electrolyte in the electrolyte is 1.0 - 4.0 mol / L; The concentration of the conductivity enhancer in the electrolyte is 0.5 - 1.5 mol / L.

9. The negative electrolyte of an alkaline aqueous all-iron redox flow battery according to claim 1, characterized in that, The concentration of the bifunctional organic ligand in the electrolyte is preferably 1.2 - 1.8 mol / L.

10. The negative electrode electrolyte of an alkaline aqueous all-iron redox flow battery according to claim 1, wherein: The concentration of the iron-based active substance in the electrolyte is preferably 0.6 - 1.2 mol / L; The concentration of the alkaline supporting electrolyte in the electrolyte is preferably 2.0 - 3.0 mol / L.

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