Vanadium flow battery with praseodymium sulfide composite graphite felt electrode

Abstract

The application discloses a vanadium ion flow battery with a praseodymium sulfide composite graphite felt electrode and belongs to the technical field of flow battery electrodes. The electrode comprises a graphite felt base body and a surface-loaded praseodymium sulfide composite layer, the composite layer is composed of praseodymium sulfide nanoparticles and carbon nanomaterials, and the particle size of the nanoparticles is 5-50 nm. Preparation is carried out by mixing a praseodymium source and a sulfur source to prepare a precursor, mixing the precursor with a carbon nanomaterial dispersion liquid, performing graphite felt thermal oxidation pretreatment, hydrothermal reaction, washing and drying, and inert atmosphere heat treatment. In the application, inorganic rare earth praseodymium sulfide is used to replace organic sulfide and nitrogen-doped carbon material, Pr 3+ / Pr 4+ is used to realize efficient catalysis, the electrode is resistant to strong acid, stable in a wide temperature range, firmly combined at an interface, and long in cycle life. The electrode can be used in all-vanadium, zinc-bromine, iron-chromium and other flow batteries, is also applicable to the fields of hydrogen production by electrolysis, supercapacitors and electrocatalysis, and is mild in process and easy to scale up.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, specifically to a vanadium-ion flow battery with a praseodymium sulfide composite graphite felt electrode. Background Technology

[0002] Vanadium redox batteries (VRFBs) offer advantages such as high safety, long cycle life, decoupling of power and capacity, and deep charge / discharge capability, making them a mainstream technology for large-scale electrochemical energy storage. As the core site of vanadium ion redox reactions, the electrode's electrocatalytic activity, conductivity, hydrophilicity, and structural stability directly determine the battery's efficiency, power density, and lifespan.

[0003] Commercial graphite felt electrodes suffer from limitations such as limited specific surface area, insufficient surface active sites, poor conductivity and hydrophilicity, and low catalytic efficiency for vanadium ion reactions, necessitating surface modification. Research indicates that industry researchers have proposed modification schemes for graphite felt electrodes, including modifications using organic sulfides and nitrogen-doped carbon materials. For example, patent publication number CN121641992A involves the in-situ growth of sulfurized polyacrylonitrile (SPAN) on a carbon felt surface. As an organic sulfide, sulfurized polyacrylonitrile contains CS and SS bonds in its molecular structure, which are prone to bond breaking and decomposition at higher temperatures (>60℃), leading to sulfur loss and a reduction in catalytic active sites. Therefore, this type of material suffers from drawbacks such as easy decomposition at high temperatures, sulfur loss, easy degradation under strong oxidizing environments, and poor cycle stability, making it suitable only for negative electrodes and unsuitable for the strong oxidizing conditions of positive electrodes. Furthermore, the synthesis of SPAN typically requires high-temperature sulfurization treatment (300-600℃), resulting in high energy consumption and the potential generation of toxic gases such as H2S, posing a significant environmental burden. For example, patent publication number CN117996101A discloses a biomass-derived electrocatalyst by coating nitrogen-doped carbon black particles onto the surface of a graphene-fiber composite electrode made of graphite felt. Nitrogen doping mainly provides several fixed types of active sites, such as pyridine nitrogen, pyrrole nitrogen, and graphitic nitrogen. Nitrogen doping primarily improves the electronic structure and surface hydrophilicity of carbon materials, but its catalytic effect on the redox reaction of vanadium ions remains mainly indirect, lacking direct electron transfer mediating function. Therefore, this type of material has a single type of active site, limited doping amount, and mainly indirect catalysis. It exhibits high polarization and high charge transfer impedance under high current, making it difficult to meet high power density requirements. Furthermore, CN111509235B discloses a composite electrode obtained by impregnating graphite felt with sulfur-nitrogen co-doped graphene and then activating it at high temperature (700-1100℃). This composite electrode relies on S and N heteroatoms to provide active sites, improving the conductivity and electrocatalytic performance of the vanadium redox flow battery electrode. Similar to the aforementioned two schemes, it also suffers from drawbacks such as limited active sites, poor high-temperature stability, high process energy consumption, and weak interfacial bonding.

[0004] The aforementioned existing technologies can only achieve indirect catalysis through surface adsorption, defect sites, or conductive assistance, and none of them possess direct electron transfer-mediated function. Furthermore, existing technologies generally suffer from problems such as complex preparation processes, high equipment requirements, weak adhesion between the modified layer and the substrate, easy detachment during long-term operation, and poor adaptability to high and low temperature conditions.

[0005] Rare earth elements have unique 4f electronic structures, variable valence states, and strong coordination capabilities, and have great potential in the field of redox catalysis. However, the application of rare earth sulfides in the electrodes of all-vanadium redox flow batteries has not yet been reported.

[0006] To address the aforementioned technical challenges, this invention provides a praseodymium sulfide composite graphite felt electrode and its preparation method, achieving the technical goals of high catalysis, high stability, wide temperature range, and ease of preparation. Summary of the Invention

[0007] (a) Technical problems to be solved

[0008] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a vanadium-ion flow battery with a praseodymium sulfide composite graphite felt electrode. This electrode is an inorganic rare earth sulfide modified electrode, which can improve the structural and performance stability of the electrode under strong acid and strong oxidizing conditions. It can be used as a positive electrode and / or a negative electrode, improve the balanced catalytic ability of vanadium ion positive and negative electrode reactions, reduce impedance, and improve battery efficiency and cycle life. It can effectively replace organic sulfides and nitrogen-doped carbon materials, solving the problems of poor thermal stability and limited active sites. The electrode of the present invention has a simple preparation process, low cost, and achieves uniform and firm loading of praseodymium sulfide, making it suitable for mass production.

[0009] (II) Technical Solution

[0010] In a first aspect, the present invention provides a vanadium-ion flow battery having a praseodymium sulfide composite graphite felt electrode, comprising a positive electrode and a negative electrode, characterized in that the positive or negative electrode is a praseodymium sulfide composite graphite felt electrode, the praseodymium sulfide composite graphite felt electrode comprising: a graphite felt electrode substrate and a praseodymium sulfide composite layer loaded on the surface of the graphite felt electrode substrate; the praseodymium sulfide composite layer is composed of praseodymium sulfide nanoparticles and carbon nanomaterials, wherein the praseodymium sulfide nanoparticles are dispersed in the carbon nanomaterials; the particle size of the praseodymium sulfide nanoparticles is 5-50 nm; the carbon nanomaterials are selected from at least one of graphene, graphene oxide, reduced graphene oxide, carbon nanotubes, carbon quantum dots, and carbon nanofibers.

[0011] The praseodymium element in the praseodymium sulfide composite graphite felt electrode has Pr 3+ / Pr 4+ The praseodymium sulfide composite graphite felt electrode features a variable valence state, used to mediate the redox electron transfer of vanadium ions. The carbon nanomaterials in this electrode serve as a conductive framework and a dispersion carrier for praseodymium sulfide. The molecular formula of praseodymium sulfide is Pr₂S₃.

[0012] Optionally, the mass ratio of praseodymium sulfide to carbon nanomaterials in the praseodymium sulfide composite layer is 1:0.1-2; preferably 1:1-1.5.

[0013] Optionally, the thickness of the graphite felt electrode substrate is 1-5 mm, preferably 2-4 mm, and the porosity is 85-95%; more preferably 90-95%. The preferred thicknesses of the graphite felt electrode substrate are 1.6, 2.2, 2.5, 3.0, 3.5, 4.0, 4.8, and 5.0 mm; thinner substrates are beneficial for mass transfer, while thicker substrates are beneficial for the active area.

[0014] Optionally, the thickness of the praseodymium sulfide composite layer is 0.1-5 μm; preferably 2-4 μm, and the loading of praseodymium sulfide nanoparticles on the graphite felt is 0.5-10 mg / cm²; preferably 3-8 mg / cm².

[0015] Optionally, the surface of the graphite felt electrode substrate is pre-treated with oxidation to achieve an oxygen-containing functional group content of 5-15 wt%; preferably 9-14 wt%.

[0016] Optionally, the preparation method of the praseodymium sulfide composite graphite felt electrode includes the following steps:

[0017] S1. Preparation of praseodymium sulfide precursor solution and carbon nanomaterial dispersion: Dissolve the praseodymium source compound and the sulfur source compound in deionized water to obtain the praseodymium sulfide precursor solution; disperse the carbon nanomaterial in a water-ethanol mixed solvent and sonicate (preferably sonicate for 30-60 min) to obtain the carbon nanomaterial dispersion; wherein the carbon nanomaterial is selected from at least one of graphene, graphene oxide, reduced graphene oxide, carbon nanotubes, carbon quantum dots and carbon nanofibers.

[0018] S2. Preparation of mixed solution: Add the carbon nanomaterial dispersion to the praseodymium sulfide precursor solution to obtain a mixed solution;

[0019] S3. Pretreatment of graphite felt and hydrothermal reaction: Pretreatment involves heat-treating the graphite felt electrode substrate under oxygen-containing conditions to generate oxygen-containing functional groups on its surface; subsequently, the pretreated graphite felt is immersed in a mixed solution, transferred to a reaction vessel, and hydrothermally reacted at 180-240℃ for 6-24 hours.

[0020] S4. Post-treatment: Washing, vacuum drying, and heat treatment at 300-500℃ under an inert atmosphere to obtain praseodymium sulfide composite graphite felt electrode.

[0021] According to a preferred embodiment of the present invention, in S1, the praseodymium source compound is selected from at least one of praseodymium nitrate, praseodymium chloride, praseodymium acetate, praseodymium sulfate, praseodymium oxalate, and hydrates of the aforementioned compounds; the sulfur source compound is selected from at least one of thiourea, thioacetamide, sodium sulfide, potassium sulfide, ammonium sulfide, and sodium thiosulfate; the molar ratio of the praseodymium source to the sulfur source is 1:1.5-3.

[0022] Praseodymium sulfide is an ionic inorganic compound, a solid that is poorly soluble in water. Furthermore, direct contact with water easily leads to hydrolysis, producing praseodymium hydroxide precipitate and releasing hydrogen sulfide gas. Therefore, it is impossible to directly load praseodymium sulfide solid powder onto a graphite felt electrode substrate. This invention provides both praseodymium and sulfur sources in the form of a precursor solution. After uniform mixing in the solution, praseodymium sulfide is generated in situ via a hydrothermal reaction and simultaneously loaded onto the graphite felt surface, avoiding structural inhomogeneity and performance degradation caused by hydrolysis, precipitation, and gas release.

[0023] According to a preferred embodiment of the present invention, in S1, the volume ratio of deionized water to ethanol in the water-ethanol mixed solvent is 1:1-4; the concentration of carbon nanomaterials in the carbon nanomaterial dispersion is 0.5-5 mg / mL.

[0024] According to a preferred embodiment of the present invention, in the mixed solution in S2, the concentration of praseodymium ions is 0.01-0.1 mol / L, and the mass ratio of praseodymium ions to carbon nanomaterials is 1:0.13-2.69.

[0025] Step S2 of this invention mainly involves mixing a carbon nanomaterial dispersion with a praseodymium sulfide precursor solution. This allows the carbon nanomaterial to disperse and modify the praseodymium sulfide precursor in situ within the solution system, forming a uniform composite precursor system. This step ensures that the carbon nanomaterial fully encapsulates and disperses praseodymium ions and the sulfur source, preventing agglomeration of praseodymium sulfide nanoparticles during the subsequent hydrothermal reaction. It guarantees that praseodymium sulfide is uniformly dispersed as 5–50 nm nanoparticles on the surface of the carbon nanomaterial, ultimately forming a carbon nanomaterial-modified and dispersed praseodymium sulfide composite layer. This in-situ modification and dispersion of the praseodymium sulfide precursor in the solution phase using carbon nanomaterials lays the structural foundation for the subsequent formation of a uniform and stable composite electrode. The carbon nanomaterial acts as a dispersant / carrier, uniformly dispersing the praseodymium sulfide, preventing agglomeration, and improving conductivity and stability.

[0026] According to a preferred embodiment of the present invention, in S3, the pretreatment conditions are as follows: heat treatment at 400-500°C for 4-8 hours in an air atmosphere to make the oxygen-containing functional group content on the surface of the graphite felt electrode substrate reach 5-15wt%; then cooling to room temperature to obtain a pretreated graphite felt electrode substrate with a thickness of 1-5mm and a porosity of 85-95%.

[0027] According to a preferred embodiment of the present invention, in S3, the pretreated graphite felt is first immersed in a mixed solution for 1-4 hours, and then transferred to a closed reaction vessel for hydrothermal reaction at 200-220°C for 12-18 hours.

[0028] According to a preferred embodiment of the present invention, in S4, the washing and drying conditions are as follows: washing with deionized water and ethanol alternately 3-5 times, and then vacuum drying at 60-80°C for 6-12 hours.

[0029] According to a preferred embodiment of the present invention, in S4, the heat treatment conditions are as follows: heat treatment at 300-500°C for 2-4 hours under a nitrogen or argon protective atmosphere to obtain a praseodymium sulfide composite graphite felt electrode; the heat treatment heating rate is 2-5°C / min.

[0030] According to a preferred embodiment of the present invention, in S4, the inert atmosphere condition is: an inert gas is introduced into the heat treatment furnace at a flow rate of 50-200 mL / min.

[0031] Furthermore, the praseodymium sulfide composite graphite felt electrode of the present invention can not only be used as the positive and negative electrode of vanadium redox flow batteries, but also as the electrode of zinc-bromine, all-iron, iron-chromium, vanadium-bromine, organic redox flow batteries, etc., as well as the electrode of supercapacitors, hybrid capacitor batteries, fuel cells, metal-air batteries, and even electrochemical functional electrodes such as electrolytic hydrogen production, electrochemical catalysis, and electrochemical sensors.

[0032] (III) Beneficial Effects

[0033] The praseodymium sulfide composite graphite felt electrode of this invention employs an inorganic rare-earth praseodymium sulfide (Pr2S3) + carbon nanomaterial composite, replacing the traditional organic sulfide, nitrogen-doped carbon, and metal modification system. The electrode of this invention is a rare-earth sulfide-carbon composite electrode with a stable and controllable structure. Praseodymium sulfide is uniformly dispersed in carbon nanomaterials as 5-50 nm nanoparticles, forming a continuous conductive network. Furthermore, the praseodymium sulfide nanoparticles are uniformly dispersed in the carbon nanomaterials and anchored and loaded by the carbon nanomaterials, exhibiting high dispersion (non-agglomeration). It can withstand strong acid and strong oxidizing environments (sulfuric acid electrolyte system) without decomposition, dissolution, or loss, solving the high-temperature failure problem of organic sulfides. The electrode possesses a dual-reaction balanced catalytic function and can be used as the positive and / or negative electrode of a flow battery, efficiently catalyzing the VOCs at the positive electrode. 2+ / VO2 + V at the negative electrode 2+ / V 3+ These methods significantly improve voltage efficiency.

[0034] The preparation method provided by this invention mainly utilizes chemical anchoring of oxygen-containing functional groups on the surface of graphite felt combined with hydrothermal in-situ growth. While praseodymium sulfide is still in its precursor ionic state, carbon nanomaterials uniformly encapsulate, disperse, and anchor the precursor ions, achieving in-situ composite modification. This results in high bonding strength between the modified electrode layer and the substrate, preventing powder shedding during long-term cycling. By utilizing soluble praseodymium / sulfur source precursors, praseodymium sulfide is generated in-situ after uniform composite formation in solution, avoiding problems such as hydrolysis precipitation, H2S release, and uneven loading caused by directly using water for praseodymium sulfide. Furthermore, the graphite felt electrode substrate undergoes pretreatment to enhance the wettability of the oxygen-containing functional groups, increasing the reaction three-phase interface and improving battery efficiency and rate performance. The entire process is free of acid and alkali chemicals, requires no excessively high temperatures or pressures, and is environmentally friendly, mild, and easily scalable for mass production. Attached Figure Description

[0035] Figure 1 The graph shows the voltage efficiency results of the electrode samples of Examples 1-3 and Comparative Examples 1-3 at different temperatures.

[0036] Figure 2 The electrode sample of Example 1 was subjected to 200 mA·cm -2 Efficiency (Coulomb efficiency, voltage efficiency, energy efficiency) at current density - cycle curve.

[0037] Figure 3 The electrode sample of Example 2 was subjected to 200 mA·cm -2 Efficiency (Coulomb efficiency, voltage efficiency, energy efficiency) at current density - cycle curve.

[0038] Figure 4 The electrode sample of Example 3 was subjected to 200 mA·cm -2 Efficiency (Coulomb efficiency, voltage efficiency, energy efficiency) at current density - cycle curve.

[0039] Figure 5 For the electrode sample of Comparative Example 1, at 200 mA·cm -2 Efficiency (Coulomb efficiency, voltage efficiency, energy efficiency) at current density - cycle curve.

[0040] Figure 6 For the electrode sample of Comparative Example 2, at 200 mA·cm -2 Efficiency (Coulomb efficiency, voltage efficiency, energy efficiency) at current density - cycle curve.

[0041] Figure 7 For the electrode sample of Comparative Example 3, at 200 mA·cm -2 Efficiency (Coulomb efficiency, voltage efficiency, energy efficiency) at current density - cycle curve.

[0042] Figure 8The electrode samples of Examples 1-3 and Comparative Examples 1-3 were subjected to 200 mA·cm. -2 Voltage efficiency-cycle stability curve under the given conditions.

[0043] Figure 9 The electrode samples of Examples 1-3 and Comparative Examples 1-3 were subjected to 200 mA·cm. -2 The capacity-cycle curve below. Detailed Implementation

[0044] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0045] The praseodymium sulfide composite graphite felt electrode of the present invention has rare earth 4f electron variable valence catalytic performance, and its praseodymium has Pr 3+ / Pr 4+ The variable valence state directly mediates the redox electron transfer of vanadium ions, significantly reducing the activation energy and increasing the kinetic rate. The inorganic sulfide framework in the composite electrode is stable; Pr₂S₃ is a high-temperature and strong-oxidation resistant inorganic crystal, free of CS weak bonds, and does not hydrolyze, decompose, or migrate, fundamentally solving the problem of poor thermal stability of organic sulfides. The use of inorganic praseodymium sulfide crystals to replace organic sulfides provides high-temperature resistance, oxidation resistance, and prevents hydrolysis and loss, avoiding failure under high-temperature and strong-oxidation conditions. The carbon nanomaterials in the composite electrode serve as a conductive framework and dispersion carrier, improving electrode conductivity and specific surface area, preventing praseodymium sulfide aggregation, and enhancing electrolyte wettability and mass transfer efficiency.

[0046] Compared to the polyacrylonitrile sulfide (SPAN) modified graphite felt electrode disclosed in CN121641992A, the SPAN modified electrode suffers from problems such as easy decomposition at high temperatures, sulfur loss, instability under strong oxidizing environments, and inability to operate for extended periods at high temperatures. Furthermore, its preparation requires high-temperature sulfidation and easily generates toxic H2S gas. This invention uses inorganic rare-earth praseodymium sulfide to replace organic polyacrylonitrile sulfide, eliminating the problems of high-temperature decomposition and sulfur loss at the material source. This allows the electrode to operate stably at higher temperatures while possessing excellent structural stability, electrochemical stability, and catalytic activity. The preparation process is mild, releases no toxic gases, and is more environmentally friendly.

[0047] Compared to the nitrogen-doped carbon black / biomass-derived carbon modified electrode disclosed in CN117996101A, this type of technology has a single active site, its catalysis is mainly indirect, it lacks direct electron transfer mediation, and it exhibits high polarization and high impedance under high current. This invention utilizes rare earth praseodymium (Pr) 3+ / Pr 4+With its variable valence state and 4f electronic structure, praseodymium sulfide can directly mediate the redox reaction of vanadium ions, significantly reducing the activation energy and accelerating the reaction kinetics, resulting in a catalytic efficiency far exceeding that of a purely nitrogen-doped system. At the same time, praseodymium sulfide has a stable structure and does not have the problem of upper limit of doping amount or structural damage, resulting in better rate performance and power density.

[0048] Compared to the sulfur-nitrogen co-doped graphene electrode disclosed in CN111509235B, this approach relies on heteroatoms to provide limited active sites and requires high-temperature activation at 700-1100℃, resulting in high energy consumption, weak interfacial bonding, and insufficient high-temperature stability. This invention employs a praseodymium sulfide nanocomposite structure, which has a greater number of active sites and stronger catalytic ability; the preparation temperature is significantly reduced, and the conditions are milder; praseodymium sulfide forms a chemical bond and anchoring structure with carbon nanomaterials, resulting in strong interfacial bonding, long-term cycling without detachment, and significantly improved stability and lifespan.

[0049] In summary, compared with existing technologies, the overall advantages of the composite electrode of the present invention include:

[0050] (1) Significantly enhanced catalytic activity: the variable valence state of praseodymium (Pr 3+ / Pr 4+ This invention directly mediates the redox of vanadium ions, lowers the activation energy, balances the catalytic reaction between the positive and negative electrodes, and exhibits low polarization under high current. (The last sentence appears to be incomplete and possibly refers to a specific chemical process called Pr.) 3+ / Pr 4+ The reversible valence change characteristic enables direct electron transfer-mediated catalysis of the redox reaction of vanadium ions, which is one of the most fundamental differences between this invention and existing technologies.

[0051] (2) Significantly improved stability: Inorganic Pr2S3 is resistant to high temperature and strong acid oxidation, with no sulfur loss or structural collapse, and minimal performance fluctuation over a wide temperature range.

[0052] (3) Strong interface bonding: The electrode modification layer is not easy to fall off due to in-situ growth by hydrothermal reaction and anchoring of oxygen-containing functional groups, resulting in excellent long-term cycling stability.

[0053] (4) The preparation process is green and simple: no high-temperature vulcanization is required, no H2S toxic gas is required, no freeze drying is required, no high-pressure equipment is required, the equipment is universal, the conditions are mild, and the cost is low.

[0054] Experimental testing revealed that the praseodymium sulfide composite graphite felt electrode of this invention exhibits strong wide-temperature adaptability: the vanadium redox flow battery in which the electrode of the example is located shows a voltage efficiency fluctuation of <0.5% from -5℃ to 40℃, far superior to the comparative example (comparative example fluctuation >2%). Excellent electrochemical performance is also demonstrated: experiments show that at 200 mA / cm²... 2The vanadium redox flow battery with the electrode in the example exhibits an electrode coulombic efficiency ≥98.2%, a voltage efficiency ≥87.4%, and an energy efficiency ≥85.9%. It also demonstrates excellent cycle stability. After 1000 cycles, the voltage efficiency of the vanadium redox flow battery with the electrode in the example decreases by 1.6%~1.8%, and the capacity decreases by <10%; while the comparative battery shows a voltage efficiency decrease of 6% after more than 400 cycles, and a capacity retention decrease of approximately 50% after 1000 cycles.

[0055] The following description, in conjunction with preferred embodiments and comparative examples, further illustrates the present invention.

[0056] Example 1:

[0057] This embodiment provides a method for preparing a praseodymium sulfide composite graphite felt electrode, including the following steps:

[0058] (1) A graphite felt electrode substrate with a thickness of 2 mm and a porosity of 90% was placed in a muffle furnace and heat-treated at 450°C for 6 h in an air atmosphere. Then it was naturally cooled to room temperature to obtain a pretreated graphite felt electrode substrate with an oxygen functional group content of about 10 wt%.

[0059] (2) Preparation of praseodymium sulfide precursor solution: Dissolve 2.17 g (0.005 mol) of Pr(NO3)3·6H2O and 1.14 g (0.015 mol) of thiourea (CH4N2S) in 100 mL of deionized water, and stir magnetically for 30 min to obtain Pr 3+ A praseodymium sulfide precursor solution with a concentration of 0.05 mol / L.

[0060] (3) Preparation of carbon nanotube dispersion: 0.5g of multi-walled carbon nanotubes were dispersed in 200mL of a mixed solvent of deionized water and ethanol in a volume ratio of 1:3, and ultrasonically treated for 45min to obtain a uniform carbon nanotube dispersion with a concentration of 2.5mg / mL.

[0061] (4) Preparation of mixed solution: 200 mL of carbon nanotube dispersion obtained in step (3) was slowly added to 100 mL of praseodymium sulfide precursor solution in step (2), and magnetic stirring was continued for 1 h to obtain a mixed solution. The praseodymium sulfide precursor solution in the mixed solution contained Pr 3+ The mass ratio of carbon nanotubes to carbon nanotubes is 1:0.71.

[0062] (5) Hydrothermal reaction: The pretreated graphite felt from step (1) is cut into 4cm×4cm size and immersed in the mixed solution for 2h. Then the graphite felt and the mixed solution are transferred together to a hydrothermal reactor lined with polytetrafluoroethylene. The filling degree of the reactants in the reactor is 80%. The hydrothermal reaction is carried out at 210℃ for 15h.

[0063] (6) Washing and drying: Take out the graphite felt after the reaction, wash it with deionized water and ethanol alternately 4 times, and then dry it under vacuum at 70°C for 8 hours.

[0064] (7) Heat treatment: The dried graphite felt is placed in a tube furnace and heated to 400°C at a rate of 3°C / min under a nitrogen atmosphere (gas flow rate of 100 mL / min). The temperature is held for 3 hours and then naturally cooled to room temperature to obtain praseodymium sulfide composite graphite felt electrode.

[0065] Example 2:

[0066] The main difference between this embodiment and embodiment 1 is the adjustment of steps (3) and (7):

[0067] The "0.5g multi-walled carbon nanotubes" in step (3) of Example 1 were replaced with 0.3g of graphene oxide. After dispersion in a mixed solvent, a uniform graphene oxide dispersion with a concentration of 1.5mg / mL was obtained. The heat treatment temperature "400℃" in step (7) of Example 1 was reduced to 350℃ and the holding time was 2.5h.

[0068] The remaining steps and conditions are the same as in Example 1.

[0069] Example 3:

[0070] The main difference between this embodiment and embodiment 1 is the adjustment of steps (2) and (5):

[0071] Replace “1.14g (0.015mol) of thiourea (CH4N2S)” in step (2) of Example 1 with 1.13g (0.015mol) of thioacetamide. Adjust “hydrothermal reaction at 210°C for 15h” in step (5) of Example 1 to “reaction time at 200°C for 12h”.

[0072] The remaining steps and conditions are the same as in Example 1.

[0073] Example 4:

[0074] The main difference between this embodiment and Embodiment 1 is the adjustment of the ratio of praseodymium source to carbon nanomaterial in steps (3) and (2):

[0075] In Example 1, step (3) of the "multi-walled carbon nanotubes" were replaced with carbon quantum dots, with an amount of 0.4 g, which were dispersed in 200 mL of a water-ethanol (1:3) mixed solvent to obtain a carbon quantum dot dispersion with a concentration of 2.0 mg / mL. In Example 1, step (2) of the praseodymium nitrate was kept unchanged, but the amount of praseodymium nitrate was reduced to 1.23 g of Pr(NO3)3·6H2O, so that the mass ratio of praseodymium ions to carbon quantum dots was 1:1.0.

[0076] The remaining steps and conditions are the same as in Example 1.

[0077] Example 5:

[0078] The main difference between this embodiment and Embodiment 1 is the adjustment of the sulfur source in step (2), the immersion time in step (5), and the hydrothermal parameters:

[0079] In Example 1, thiourea in step (2) was replaced with sodium thiosulfate at a dosage of 0.015 mol. The soaking time in step (5) was extended from 2 h to 3 h, the hydrothermal reaction temperature was maintained at 210 °C, and the time was shortened to 12 h. The remaining steps and conditions were the same as in Example 1.

[0080] Example 6:

[0081] The main difference between this embodiment and Embodiment 1 is the adjustment of the carbon nanomaterials in step (3) and the heat treatment parameters in step (7):

[0082] In Example 1, the multi-walled carbon nanotubes in step (3) were replaced with reduced graphene oxide at a dosage of 0.6 g and a dispersion concentration of 3.0 mg / mL. The heat treatment temperature in step (7) was increased to 450 °C, and the holding time was maintained for 3 h. The gas flow rate was adjusted to 150 mL / min. The remaining steps and conditions were the same as in Example 1.

[0083] Comparative Example 1:

[0084] The difference between this comparative example and Example 1 is that the graphite felt electrode substrate is a graphite felt electrode substrate that has not undergone surface oxidation pretreatment and is used directly without any modification treatment.

[0085] Comparative Example 2:

[0086] The difference between this comparative example and Example 1 is that only step (1) of Example 1 is used to perform thermal oxidation treatment on the graphite felt electrode substrate, without subsequent loading of praseodymium sulfide, and the thermally oxidized graphite felt electrode substrate is used as the final electrode.

[0087] Comparative Example 3:

[0088] The difference between this comparative example and Example 1 is that it does not include steps (3)-(4) of Example 1, and directly uses a graphite felt electrode substrate subjected to thermal oxidation to load praseodymium sulfide. The preparation method is as follows:

[0089] (1) A graphite felt electrode substrate with a thickness of 2 mm and a porosity of 90% was placed in a muffle furnace and heat-treated at 450°C for 6 h in an air atmosphere. Then it was naturally cooled to room temperature to obtain a pretreated graphite felt electrode substrate with an oxygen functional group content of about 10 wt%.

[0090] (2) Preparation of praseodymium sulfide precursor solution: Dissolve 2.17 g (0.005 mol) of Pr(NO3)3·6H2O and 1.14 g (0.015 mol) of thiourea (CH4N2S) in 100 mL of deionized water, and stir magnetically for 30 min to obtain Pr 3+ A praseodymium sulfide precursor solution with a concentration of 0.05 mol / L.

[0091] (3) Hydrothermal reaction: The graphite felt pretreated in step (1) was cut into 4cm×4cm size and immersed in praseodymium sulfide precursor solution for 2h. Then the graphite felt and praseodymium sulfide precursor solution were transferred together to a polytetrafluoroethylene-lined hydrothermal reactor. The filling degree of the reactants in the reactor was 80%. The hydrothermal reaction was carried out at 210℃ for 15h.

[0092] The operation methods and conditions for steps (4)-(5) are the same as those in Example 1.

[0093] The thickness of the praseodymium sulfide composite layer, the particle size of the praseodymium sulfide particles, and the loading amount of the graphite felt electrodes of the above embodiments and comparative examples were measured. The results are shown in Table 1. The thickness of the composite layer was measured by scanning electron microscopy (SEM) of the cross section, the particle size was observed by transmission electron microscopy (TEM), and the loading amount was obtained by converting the praseodymium content after ICP-OES measurement.

[0094] Table 1: Thickness of praseodymium sulfide composite layer for electrodes, particle size of praseodymium sulfide, and loading amount

[0095] In Comparative Example 1, the graphite felt electrode substrate was not pretreated with oxidation, resulting in insufficient surface active sites and difficulty in loading praseodymium sulfide, with a loading capacity of only 0.2-0.5 mg / cm³. 2 In Comparative Example 3, no carbon nanomaterials were added, praseodymium sulfide had no dispersing carrier, the particles were severely agglomerated (80-200nm), the loading was low (2.0-2.5mg / cm²), and the composite layer was discontinuous; both of these situations would lead to an increase in impedance.

[0096] Electrochemical performance testing of graphite felt electrodes:

[0097] A test battery system was constructed, and the electrode electrochemical performance of Examples 1-3 and Comparative Examples 1-3 was measured. The test methods and results are as follows:

[0098] (1) Voltage efficiency variation in the temperature range of -5℃ to 40℃

[0099] The testing system is as follows:

[0100] Battery type: Clamp-on vanadium redox flow cell.

[0101] Effective electrode area: 16cm² 2 (4cm×4cm).

[0102] Electrodes: Electrodes of Examples 1-3 and Comparative Examples 1-3.

[0103] Membrane: Perfluorosulfonic acid ion exchange membrane.

[0104] Bipolar plate: Graphite bipolar plate.

[0105] Electrolyte: 2 mol / L 3+ / V 2+ +3.0 mol / L H2SO4 (negative electrode), 2 mol / L VO 2+ / VO2 + +3.0 mol / L H2SO4 (positive electrode).

[0106] Electrolyte circulation method: external constant flow peristaltic pump circulation.

[0107] Electrolyte flow rate: 60 mL / min.

[0108] Test current density: 200 mA·cm -2 Test temperature: -5℃, 10℃, 20℃, 30℃, 40℃; Charge / discharge range: 1.25V-1.55V.

[0109] Test results are as follows Figure 1 As shown in the figure. It can be seen from the figure that the batteries containing the electrode samples of Examples 1-3 operate within a temperature range of -5℃ to 40℃, with a 200mA·cm... -2 The voltage efficiency variation was approximately 0.5%, generally ranging from 87.1% to 87.9%, with little change from low to high temperatures. This indicates that the electrode sample in the example exhibits better temperature stability, with voltage efficiency less prone to significant fluctuations with temperature changes. In contrast, the comparative sample showed poor performance at low temperatures, but its performance gradually improved with increasing temperature. The voltage efficiency difference between -5℃ and 40℃ exceeded 2%, with an overall voltage efficiency ranging from 84.2% to 87%. Therefore, the praseodymium sulfide composite graphite felt electrode provided by this invention has a wider temperature range.

[0110] (2) Efficiency-cycle results

[0111] Under the above battery system, at 110 mA·cm -2 -300mA·cm -2 The coulombic efficiency and energy efficiency of the cells containing the electrode samples of Examples 1-3 and Comparative Examples 1-3 were tested within the current density range at a temperature of 20°C. The test results are as follows: Figures 2-7 .

[0112] Depend on Figure 2 As can be seen from the data, the battery containing the electrode sample in Example 1 operates at 200 mA·cm⁻¹. -2 At the given current density, the coulombic efficiency is 98.3%, the voltage efficiency is 87.4%, and the energy efficiency is 86.0%. Figure 3 It can be seen that the battery of the electrode sample in Example 2 operates at 200 mA·cm⁻¹. -2 At the given current density, the coulombic efficiency is 98.2%, the voltage efficiency is 87.5%, and the energy efficiency is 85.9%. Figure 4 It can be seen that the battery containing the electrode sample in Example 3 operates at 200 mA·cm⁻¹. -2 At current density, the coulombic efficiency is 98.3%, the voltage efficiency is 87.7%, and the energy efficiency is 86.2%.

[0113] Depend on Figure 5 It can be seen that the cell containing the electrode sample of Comparative Example 1 operates at 200 mA·cm⁻¹. -2 At the given current density, the coulombic efficiency is 97.2%, the voltage efficiency is 86.5%, and the energy efficiency is 84.1%. Figure 6 It can be seen that the cell containing the electrode sample of Comparative Example 2 operates at 200 mA·cm⁻¹. -2 At the given current density, the coulombic efficiency is 97.1%, the voltage efficiency is 86.0%, and the energy efficiency is 83.5%. Figure 7 It can be seen that the cell containing the electrode sample of Comparative Example 3 operates at 200 mA·cm⁻¹. -2 At current density, the coulombic efficiency is 97.4%, the voltage efficiency is 86.8%, and the energy efficiency is 84.5%.

[0114] In the industrial application environment of VRFB, a voltage efficiency improvement of 0.5% is considered a significant improvement; as can be seen from the above test results, the electrochemical indicators of the samples of Examples 1-3 are all better than those of Comparative Examples 1-3.

[0115] (3) At 200mA·cm -2 Voltage efficiency-cycle stability curve (tested at 20℃)

[0116] Depend on Figure 8 As can be seen, the composite electrodes of Examples 1, 2, and 3 maintained relatively stable voltage efficiency within 1000 cycles, with decreases of 1.8%, 1.7%, and 1.6%, respectively. In contrast, Comparative Examples 1, 2, and 3 experienced varying degrees of voltage efficiency degradation after more than 400 cycles, with decreases of 6.1%, 5.9%, and 6.0%, respectively, within 1000 cycles. This further demonstrates that the composite electrode material prepared in this invention exhibits better efficiency stability.

[0117] (4) At 200mA·cm -2 Battery capacity retention rate (tested at 20℃)

[0118] Depend on Figure 9 It can be seen that the capacity retention rate of all embodiments decreased by less than 10% within 1000 cycles, while the capacity retention rate of the comparative embodiment decreased by about 50%, which further illustrates that the composite electrode provided by the present invention has good capacity retention.

[0119] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features. These modifications or substitutions, or combinations of technical features in the above embodiments that do not conflict with each other, can be made in accordance with the manner described in the embodiments. These modifications, substitutions or combinations do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A vanadium-ion flow battery with a praseodymium sulfide composite graphite felt electrode, comprising a positive electrode and a negative electrode, characterized in that, The positive or negative electrode is a praseodymium sulfide composite graphite felt electrode, which comprises: a graphite felt electrode substrate and a praseodymium sulfide composite layer loaded on the surface of the graphite felt electrode substrate; the praseodymium sulfide composite layer is composed of praseodymium sulfide nanoparticles and carbon nanomaterials, wherein the praseodymium sulfide nanoparticles are dispersed in the carbon nanomaterials; the particle size of the praseodymium sulfide nanoparticles is 5-50 nm; the carbon nanomaterials are selected from at least one of graphene, graphene oxide, reduced graphene oxide, carbon nanotubes, carbon quantum dots, and carbon nanofibers. The praseodymium sulfide composite graphite felt electrode has Pr 3+ / Pr 4+ Variable valence for mediating vanadium ion redox electron transfer, the carbon nanomaterial in the praseodymium sulfide composite graphite felt electrode as a conductive skeleton and a dispersion carrier of praseodymium sulfide.

2. The vanadium-ion flow battery according to claim 1, characterized in that, The mass ratio of praseodymium sulfide to carbon nanomaterials in the praseodymium sulfide composite layer is 1:0.1-2.

3. The vanadium-ion flow battery according to claim 1 or 2, characterized in that, The thickness of the graphite felt electrode substrate is 1-5 mm, and the porosity is 85-95%.

4. The vanadium-ion flow battery according to claim 1 or 2, characterized in that, The thickness of the praseodymium sulfide composite layer is 0.1-5 μm; the loading of praseodymium sulfide nanoparticles on the graphite felt is 0.5-10 mg / cm².

5. The vanadium-ion flow battery according to claim 1 or 2, characterized in that, The surface of the graphite felt electrode substrate is pre-treated with oxidation to achieve an oxygen-containing functional group content of 5-15 wt%.

6. The vanadium-ion flow battery according to claim 1 or 2, characterized in that, The preparation method of the praseodymium sulfide composite graphite felt electrode includes the following steps: S1. Preparation of praseodymium sulfide precursor solution and carbon nanomaterial dispersion: The praseodymium source compound and the sulfur source compound are dissolved in deionized water to obtain the praseodymium sulfide precursor solution; the carbon nanomaterial is dispersed in a water-ethanol mixed solvent and ultrasonically treated to obtain the carbon nanomaterial dispersion; the carbon nanomaterial is selected from at least one of graphene, graphene oxide, reduced graphene oxide, carbon nanotubes, carbon quantum dots and carbon nanofibers; S2. Preparation of mixed solution: Add carbon nanomaterial dispersion to praseodymium sulfide precursor solution to obtain mixed solution; S3. Pretreatment and hydrothermal reaction of graphite felt: Pretreatment is to heat-treat the graphite felt electrode substrate under oxygen-containing conditions to generate oxygen-containing functional groups on its surface. Subsequently, the pretreated graphite felt was immersed in the mixed solution, transferred to the reactor, and hydrothermally reacted at 180-240℃ for 6-24 hours. S4. Post-treatment: Washing, vacuum drying, and heat treatment at 300-500℃ under an inert atmosphere to obtain praseodymium sulfide composite graphite felt electrode.

7. The vanadium-ion flow battery according to claim 6, characterized in that, In S1, the praseodymium source compound is selected from at least one of praseodymium nitrate, praseodymium chloride, praseodymium acetate, praseodymium sulfate, praseodymium oxalate, and hydrates of the aforementioned compounds; the sulfur source compound is selected from at least one of thiourea, thioacetamide, sodium sulfide, potassium sulfide, ammonium sulfide, and sodium thiosulfate; the molar ratio of the praseodymium source to the sulfur source is 1:1.5-3.

8. The vanadium-ion flow battery according to claim 6 or 7, characterized in that, In S1, the volume ratio of deionized water to ethanol in the water-ethanol mixed solvent is 1:1-4; the concentration of carbon nanomaterials in the carbon nanomaterial dispersion is 0.5-5 mg / mL.

9. The vanadium-ion flow battery according to claim 6, characterized in that, In the mixed solution in S2, the concentration of praseodymium ions is 0.01-0.1 mol / L, and the mass ratio of praseodymium ions to carbon nanomaterials is 1:0.13-2.

69.

10. The vanadium-ion flow battery according to claim 6, characterized in that, In S3, the pretreatment conditions are as follows: heat treatment at 400-500℃ for 4-8 hours in air atmosphere to achieve an oxygen-containing functional group content of 5-15wt% on the surface of the graphite felt electrode substrate; then cooling to room temperature to obtain the pretreated graphite felt electrode substrate; the thickness of the graphite felt electrode substrate is 1-5mm, and the porosity is 85-95%; In S3, the pretreated graphite felt is first immersed in the mixed solution for 1-4 hours, and then transferred to a closed reactor for hydrothermal reaction at 200-220℃ for 12-18 hours.

11. The vanadium-ion flow battery according to claim 6, characterized in that, In S4, the washing and drying conditions are as follows: wash with deionized water and ethanol alternately 3-5 times, and then vacuum dry at 60-80℃ for 6-12h; the heat treatment conditions are as follows: heat treat at 300-500℃ for 2-4h under nitrogen or argon protective atmosphere to obtain praseodymium sulfide composite graphite felt electrode; the heat treatment heating rate is 2-5℃ / min.

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