A catalyst-free nanoporous graphite felt negative electrode, a colloidal etching preparation method thereof, and application thereof in a full vanadium redox flow battery

Abstract

This invention discloses a catalyst-free nanoporous graphite felt anode, its colloidal etching preparation method, and its application in a vanadium redox flow battery, belonging to the fields of electrode preparation and flow battery technology. The preparation method includes: adding a saturated FeCl3 solution dropwise to boiling deionized water to prepare a Fe(OH)3 colloidal system; immersing the graphite felt in the colloidal solution for impregnation and drying; heat-treating at 300℃ to convert Fe(OH)3 into iron oxide; annealing at 1050℃ under an argon atmosphere to induce iron-assisted carbothermic etching; and removing residual iron-containing impurities by acid washing to obtain the nanoporous graphite felt anode. The FeGF-2 electrode prepared by this invention has a large specific surface area, improved wettability, increased defect density, and a charge transfer resistance reduced from 219.1 Ω·cm. 2 Reduced to 9.6 Ω·cm 2 A vanadium redox flow battery with vanadium as the negative electrode operates at 300 mA cm⁻¹. ‑2 The energy efficiency reaches 79.67% at 500mA cm⁻¹. ‑2 After 2500 stable cycles, the energy efficiency decreased by only 2.12%.

Description

Technical Field

[0001] This invention belongs to the field of electrode preparation and flow battery technology, and specifically relates to a catalyst-free nanoporous graphite felt anode, its colloidal etching preparation method, and its application in an all-vanadium redox flow battery. Background Technology

[0002] Vanadium redox flow batteries (VRFBs) are considered ideal for large-scale stationary energy storage due to their advantages such as independent design of power and energy, long cycle life, high safety, and the use of vanadium ions in both positive and negative electrodes to avoid irreversible cross-contamination. As a key component of VRFBs, the electrodes provide active sites for the vanadium redox reaction, directly affecting the reaction rate, mass transfer efficiency, and overall energy efficiency of the battery. Graphite felt is widely used as an electrode material for VRFBs due to its good conductivity, chemical stability, and low cost. However, virgin graphite felt typically has a small specific surface area and insufficient active sites, resulting in slow electrochemical reaction kinetics. This problem is particularly evident in V... 2+ / V 3+ This is particularly prominent in the negative electrode reaction.

[0003] To enhance the electrochemical activity of graphite felt electrodes, researchers have conducted extensive work. Surface oxidation methods, such as heat treatment, acid treatment, and electrochemical oxidation, can introduce oxygen-containing functional groups onto the carbon fiber surface, thereby improving the wettability of the electrode. However, these methods are limited by the low specific surface area of ​​the carbon fiber itself, resulting in limited effectiveness. Another commonly used strategy is to load electrocatalysts, such as metal or metal oxide nanoparticles and carbon-based nanostructures, onto the graphite felt. Although these added catalysts can improve the initial activity to some extent, their interfacial stability is problematic under continuous electrolyte flow conditions, easily leading to catalyst detachment or partial blockage of mass transfer channels. Therefore, developing modification strategies that do not require added catalysts and directly construct stable active structures on the carbon fiber surface has significant research and application value.

[0004] Subtractive etching provides an effective way to increase the usable surface area of ​​carbon electrodes by constructing porous structures on the surface and exposing edge-planar carbon sites. Currently, researchers have explored various etchants for activating carbon felt electrodes, including KOH, water vapor, HF-based systems, and transition metal-assisted etching. However, these methods often suffer from harsh etching conditions, uneven etchant distribution, and uncontrollable pore formation, easily leading to non-uniform electrode surface structures and even excessive damage to the carbon framework. For VRFB electrodes, the ideal etching strategy should create a large number of usable active sites while maintaining sufficient mass transfer channels and structural integrity. Achieving uniform nanoscale etching on bent, interwoven graphite fibers remains a pressing problem. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a catalyst-free nanoporous graphite felt anode, its colloidal etching preparation method, and its application in an all-vanadium redox flow battery.

[0006] This invention utilizes a colloidal ferric hydroxide etching strategy to construct nanoporous graphite felt as a VRFB anode. Fe(OH)3 colloid, prepared from inexpensive FeCl3, serves as a dispersible iron-based precursor, capable of relatively uniform deposition on the graphite fiber surface before high-temperature treatment. During subsequent heat treatment and acid washing, a porous structure forms on the carbon fiber surface, while residual iron-containing species are removed. The optimized FeGF-2 electrode exhibits an increased specific surface area of ​​8.58 m². 2 g -1 Improved wettability and increased defect density. These structural features contribute to V 2+ / V 3+ The negative electrode reaction is manifested by a charge transfer resistance decreasing from 219.1 Ω·cm at the PGF. 2 The value of FeGF-2 decreased to 9.6 Ω·cm. 2 In single-cell tests using commercial graphite felt as the positive electrode and FeGF-2 as the negative electrode, the battery achieved a voltage of 300 mA cm⁻¹. -2 It achieved an energy efficiency of 79.67% and can operate at 500 mA cm⁻¹ -2 It is running stably.

[0007] This invention is achieved through the following technical solution: A method for preparing a catalyst-free nanoporous graphite felt anode using colloidal etching includes the following steps: S1. Dissolve excess FeCl3·6H2O in deionized water to obtain a saturated FeCl3 solution; add the saturated FeCl3 solution dropwise to boiling deionized water to prepare a Fe(OH)3 colloidal system. S2. Immerse the graphite felt sample in Fe(OH)3 colloidal solution, dry it after impregnation, and obtain the colloidal impregnation precursor; S3. Heat-treat the colloid-impregnated precursor to convert Fe(OH)3 deposited on the carbon fiber surface into iron oxide. S4. Anneal the heat-treated sample in an argon atmosphere to induce iron-assisted carbon thermal etching. S5. Immerse the annealed electrode in 4M HCl to remove residual iron impurities, wash until neutral and dry to obtain a nanoporous graphite felt negative electrode.

[0008] Furthermore, in S1, the amount of saturated FeCl3 solution added is 1-3 mL of saturated FeCl3 solution per 60 mL of boiling deionized water.

[0009] Furthermore, the S3 heat treatment conditions are a preliminary heat treatment at 300°C for 1 hour.

[0010] Furthermore, the S4 annealing conditions are annealing at 1050℃ for 5 hours.

[0011] The present invention also provides the preparation method described above for obtaining a catalyst-free nanoporous graphite felt anode.

[0012] Furthermore, the specific surface area of ​​the catalyst-free nanoporous graphite felt anode is 7.98–8.58 m². 2 g -1 The contact angle is 88.4°~89.3°, and the Raman spectrum ID / IG ratio is 1.27~1.34.

[0013] Furthermore, the surface of the catalyst-free nanoporous graphite felt negative electrode has a mesoporous structure, with the pore size distribution concentrated in the mesoporous range.

[0014] The present invention also provides an all-vanadium redox flow battery, comprising a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode is the catalyst-free nanoporous graphite felt negative electrode described above.

[0015] Furthermore, at 300mA cm -2 Energy efficiency ≥79.67% at current density, at 500mA cm -2 Energy efficiency at current density ≥ 67.66%, peak power density ≥ 992.75 mW / cm² -2 .

[0016] Furthermore, at 500mA cm -2 After 2500 cycles at the current density, the energy efficiency decreases by ≤2.12%.

[0017] Beneficial technical effects of the present invention: (1) The present invention adopts a colloidal iron hydroxide subtractive etching strategy to directly construct a porous and defect-rich structure on the carbon fiber surface without the need for external metal or carbon-based catalysts. The active structure formed by etching is integrated with the graphite felt substrate and is not easily detached under continuous electrolyte cycling, overcoming the problems of easy detachment and easy blockage of transport channels of traditional supported catalysts, and significantly improving the structural stability and long-term operational reliability of the electrode.

[0018] (2) The optimized FeGF-2 electrode has a specific surface area of ​​8.58 m². 2 g -1 It has 3.13 times the surface area of ​​untreated graphite felt, providing more electrochemical active sites. At the same time, the contact angle decreased from 134.4° to 88.4°, and the surface changed from hydrophobic to hydrophilic, effectively improving the penetration and wetting of electrolyte inside the electrode, which is beneficial to the efficient transport and utilization of active materials.

[0019] (3) The numerous structural defects and edge carbon sites introduced by etching exhibit higher electrocatalytic activity for the vanadium redox reaction. V 2+ / V 3+ The charge transfer resistance of the negative electrode reaction is 219.1 Ω·cm. 2 Reduced to 9.6 Ω·cm 2 The oxidation-reduction ratio was reduced by nearly 23 times. CV tests showed that FeGF-2 exhibited more pronounced redox peaks and a stronger current response, with significantly better electrochemical reversibility and reaction kinetics than the untreated electrode.

[0020] (4) At 300 mA cm -2 At this level, the FeGF-2 cell achieved an energy efficiency of 79.67%, significantly higher than the 60.88% of the untreated electrode; at 500 mA cm⁻¹ -2 It still maintains an energy efficiency of 67.66%. Peak power density is 561.23 mW / cm². -2 Increased to 992.75 mW cm -2 This represents an improvement of approximately 77%. The untreated electrode at 300 mA cm⁻¹... -2 The above exhibits severe polarization, while FeGF-2 cells can achieve polarization at 500 mA cm⁻¹. -2 It operates stably under low current conditions, and its ability to operate at high current density is significantly enhanced.

[0021] (5) FeGF-2 cells at 500 mA cm -2 The electrode exhibits excellent long-term operational stability, maintaining stable operation for 2500 cycles at high current densities with an energy efficiency decrease of only 2.12%. This result is attributed to the integrated nature of the etched porous structure and the graphite felt substrate, which enables the electrode to maintain stable electrochemical performance under continuous flow conditions. Attached Figure Description

[0022] Figure 1 Morphological evolution of raw and modified graphite felt electrodes. (a) SEM images of graphite felt impregnated with PGF and (b) Fe(OH)3 colloidal precursors. SEM images of etching electrodes prepared with different FeCl3 precursor volumes: (c) FeGF-1, (d) FeGF-2, (e) FeGF-3, and (f) FeGF-4. FeGF-4 was used as an over-concentration control group to evaluate the effect of colloidal precipitation on etching morphology. EDS elemental mapping of FeGF-2, showing the distribution of (g) carbon (C), (h) oxygen (O), and (i) iron (Fe).

[0023] Figure 2Structural characteristics, surface chemical properties, and porous structures of PGF and FeGF electrodes. (a) XRD patterns of PGF and FeGF-1 / 2 / 3; (b) Raman spectra; (c) High-resolution O 1s XPS spectrum of FeGF-1 / 2 / 3; (d) N2 adsorption-desorption isotherm at 77 K; (e) BJH pore size distribution curves of PGF and FeGF-1 / 2 / 3.

[0024] Figure 3 PGF and FeGF electrode pairs V 2+ / V 3+ Electrochemical behavior of redox reactions. (a) PGF and (b) FeGF-2 at scan rates of 2, 5, 10, and 15 mV s. -1 CV curves at 10 mV s. (c) FeGF-1, FeGF-2, and FeGF-3 at 10 mV s. -1 CV curves under [variable name]. (d) Relationship between peak current density of PGF and FeGF-2 and the square root of scan rate. (e) CV curves of PGF and FeGF-1 / 2 / 3 at V [variable name]. 2+ / V 3+ Nyquist plot measured near the redox couple's formal potential. (f) Charge transfer resistance (Rct) value obtained from EIS data fitting. Electrolyte: 0.1 MV. 3+ +3.0M H2SO4.

[0025] Figure 4 In-situ EIS and DRT analyses of batteries using PGF and FeGF-2 as negative electrodes. DRT spectra of batteries using (a) PGF and (b) FeGF-2 as negative electrodes at different SOCs from 5% to 80%. DRT contour plots of batteries using (c) PGF and (d) FeGF-2 as negative electrodes. Both batteries used commercially available graphite felt as the positive electrode. Tests were conducted in a commercially available 1.5 M V3.5+ mixed acid electrolyte with an AC amplitude of 10 mV.

[0026] Figure 5 VRFB single-cell performance with PGF or FeGF electrodes as negative electrodes. (a) Cells with PGF, FeGF-1, FeGF-2, and FeGF-3 as negative electrodes at 300 mA cm⁻¹ -2 (b) Constant current charge-discharge voltage curves of the corresponding battery at 100 to 500 mA cm⁻¹ -2 Rate performance at current density. (c) Polarization and power density curves of the battery with PGF and FeGF-2 as negative electrodes. (d) Rate performance of the battery with PGF as negative electrode at 300 mA cm⁻¹. -2Long-term cycling performance, (e) FeGF-2 as negative electrode at 300 mA cm⁻¹ -2 The long-cycle performance at 500 mA cm⁻¹, and (f) the battery with FeGF-2 as the negative electrode at 500 mA cm⁻¹. -2 Long-term cycle performance. All batteries use commercially available graphite felt as the positive electrode. Detailed Implementation

[0027] Example 1: Preparation of FeGF-1 Electrode S1. Dissolve excess FeCl3·6H2O in deionized water to obtain a saturated FeCl3 solution. Then, add 1 mL of the saturated FeCl3 solution dropwise to 60 mL of boiling deionized water to prepare a Fe(OH)3 colloidal system.

[0028] S2. The graphite felt sample was immersed in the corresponding Fe(OH)3 colloidal solution for at least 1 hour, and then dried overnight to obtain the colloidal impregnation precursor. During the initial heat treatment at 300°C for 1 hour, the Fe(OH)3 deposited on the carbon fiber surface was converted into iron oxide. Subsequently, the sample was annealed in a tube furnace at 1050°C for 5 hours under an Ar atmosphere to induce iron-assisted carbothermic etching. After cooling, the electrode was immersed in 4M HCl for at least 1 hour to remove residual iron-containing impurities, then washed with deionized water until neutral, and dried at 120°C for 6 hours.

[0029] Example 2: Preparation of FeGF-2 Electrode S1. Dissolve excess FeCl3·6H2O in deionized water to obtain a saturated FeCl3 solution. Then, add 2 mL of the saturated FeCl3 solution dropwise to 60 mL of boiling deionized water to prepare a Fe(OH)3 colloidal system.

[0030] S2. The graphite felt sample was immersed in the corresponding Fe(OH)3 colloidal solution for at least 1 hour, and then dried overnight to obtain the colloidal impregnation precursor. During the initial heat treatment at 300°C for 1 hour, the Fe(OH)3 deposited on the carbon fiber surface was converted into iron oxide. Subsequently, the sample was annealed in a tube furnace at 1050°C for 5 hours under an Ar atmosphere to induce iron-assisted carbothermic etching. After cooling, the electrode was immersed in 4M HCl for at least 1 hour to remove residual iron-containing impurities, then washed with deionized water until neutral, and dried at 120°C for 6 hours.

[0031] Example 3: Preparation of FeGF-3 Electrode S1. Dissolve excess FeCl3·6H2O in deionized water to obtain a saturated FeCl3 solution. Then, add 3 mL of the saturated FeCl3 solution dropwise to 60 mL of boiling deionized water to prepare a Fe(OH)3 colloidal system.

[0032] S2. The graphite felt sample was immersed in the corresponding Fe(OH)3 colloidal solution for at least 1 hour, and then dried overnight to obtain the colloidal impregnation precursor. During the initial heat treatment at 300°C for 1 hour, the Fe(OH)3 deposited on the carbon fiber surface was converted into iron oxide. Subsequently, the sample was annealed in a tube furnace at 1050°C for 5 hours under an Ar atmosphere to induce iron-assisted carbothermic etching. After cooling, the electrode was immersed in 4M HCl for at least 1 hour to remove residual iron-containing impurities, then washed with deionized water until neutral, and dried at 120°C for 6 hours.

[0033] Comparative Example 1: Preparation of FeGF-4 Electrode S1. Dissolve excess FeCl3·6H2O in deionized water to obtain a saturated FeCl3 solution. Then, add 4 mL of the saturated FeCl3 solution dropwise to 60 mL of boiling deionized water to prepare a Fe(OH)3 colloidal system.

[0034] S2. The graphite felt sample was immersed in the corresponding Fe(OH)3 colloidal solution for at least 1 hour, and then dried overnight to obtain the colloidal impregnation precursor. During the initial heat treatment at 300°C for 1 hour, the Fe(OH)3 deposited on the carbon fiber surface was converted into iron oxide. Subsequently, the sample was annealed in a tube furnace at 1050°C for 5 hours under an Ar atmosphere to induce iron-assisted carbothermic etching. After cooling, the electrode was immersed in 4M HCl for at least 1 hour to remove residual iron-containing impurities, then washed with deionized water until neutral, and dried at 120°C for 6 hours.

[0035] Comparative Example 2: Preparation of PGF Electrode As a control, the original graphite felt was subjected to the same heat treatment and impurity removal process as in Example 1 without being impregnated with Fe(OH)3 colloid, and the resulting electrode was denoted as PGF.

[0036] In Examples 1-3, the systems prepared using 1-3 mL of FeCl3 solution maintained a colloidal dispersion, while the 4 mL system showed significant precipitation and was used as an over-concentrated control group. This was used to evaluate the effect of colloidal precipitation on the etching morphology.

[0037] 1. Physical characterization First, the morphological evolution of the graphite felt electrode was studied using SEM. For example... Figure 1 As shown in Figure a, PGF exhibits a relatively smooth carbon fiber surface after heat treatment. After impregnation with Fe(OH)3 colloidal solution, the precursor is distributed on the surface of the graphite fibers, forming an intermediate of the loaded colloid (…). Figure 1(b) During the subsequent high-temperature treatment in an Ar atmosphere, the deposited iron-containing species can induce localized carbothermal etching of the carbon fibers. After acid washing, a nanoporous structure is formed on the fiber surface.

[0038] The etching morphology showed a significant dependence on the amount of FeCl3 precursor used. For FeGF-1 to FeGF-3 ( Figure 1 (ce) As the amount of FeCl3 solution increased, the surface pores became more developed, indicating that the etching degree could be controlled by the concentration of the colloidal precursor. However, when the amount of FeCl3 added increased to 4 mL, obvious precipitation appeared in the colloidal system. The obtained FeGF-4 sample had poor uniformity of porous structure and was not fully developed. Figure 1 f) indicates that excessive precursor disrupts the colloidal dispersion, weakening etching uniformity due to colloidal aggregation and precipitation. This observation is consistent with the trend of Fe content variation in the colloidal solution; due to precipitation, the measured Fe concentration in the 4 mL system is actually lower than that in the 3 mL system. Therefore, FeGF-4 should only be used as an over-concentration control group, not as an optimized electrode.

[0039] EDS elemental mapping analysis was performed on FeGF-2 to examine the elemental distribution after etching and acid washing. Figure 1 As shown in Figure gi, carbon and oxygen are uniformly distributed on the etched fiber surface, and no obvious Fe signal was detected. This result indicates that HCl acid washing effectively removed residual iron species, reducing them below the EDS detection limit. Removal of residual iron species is crucial for minimizing potential side reactions in acidic vanadium electrolytes. Based on both morphology and elemental mapping results, FeGF-2 exhibits a relatively uniform porous carbon surface and was therefore selected as the optimized sample for subsequent structural and electrochemical evaluation.

[0040] To evaluate the impact of colloidal etching on the crystal structure and defect density of graphite felt, XRD and Raman spectroscopy analyses were performed. Figure 2 As shown in figure a, both PGF and FeGF-1 / 2 / 3 exhibit characteristic diffraction peaks of the graphitic carbon (002) crystal plane. The preservation of graphite diffraction characteristics indicates that the high-temperature etching process did not cause substantial damage to the carbon framework. No obvious iron impurity-related crystal phases were observed after acid washing, indicating that residual iron impurities were effectively removed or below the XRD detection limit.

[0041] Raman spectroscopy was further used to evaluate the defect structure of the electrode. For example... Figure 2 As shown in b, the intensity ratio of the D band to the G band (I D / I G The concentrations of PGF increased from 1.08 to 1.27, 1.32, and 1.34 for FeGF-1, FeGF-2, and FeGF-3, respectively. D / I GThe increase in the ratio indicates that the etching process introduces more structural defects and edge sites onto the carbon fiber surface. Since edge planar carbon sites are generally more reactive to the vanadium redox reaction than basal site sites, the increase in defect density is expected to promote improved electrochemical kinetics.

[0042] The surface chemical composition of the electrode was analyzed using X-ray photoelectron spectroscopy. For example... Figure 2 As shown in Figure c, the oxygen contents of PGF, FeGF-1, FeGF-2, and FeGF-3 are 3.35, 4.42, 3.97, and 4.00 at.%, respectively. The relatively small variation in oxygen content indicates that the colloidal etching process did not introduce excessive surface oxidation. Therefore, the improvement in electrochemical performance should not be attributed solely to changes in oxygen-containing functional groups, but rather to the synergistic effect of the porous structure, increased specific surface area, and defect-rich carbon sites.

[0043] The wettability of the electrodes was evaluated by contact angle measurements. The apparent contact angle decreased from 134.4° for PGF to 89.3°, 88.4°, and 88.7° for FeGF-1, FeGF-2, and FeGF-3, respectively. These results indicate that colloidal etching improves the surface wettability of the graphite felt. Considering the relatively similar oxygen content among the samples, the improvement in wettability is more reasonably attributed to the roughened porous surface and enhanced capillary penetration, rather than a significant increase in surface oxygen-containing groups.

[0044] The porous structure was further characterized by N2 adsorption-desorption tests. Figure 2 As shown in d, the nitrogen adsorption capacity of the FeGF series samples is higher than that of PGF, indicating the formation of additional porous structures after etching. (BJH pore size distribution) Figure 2 e) This shows that the generated pores are mainly concentrated in the mesoporous range, with a low proportion of micropores. As shown in Table 1, the specific surface area of ​​FeGF-2 reaches 8.58 m². 2 g -1 It is PGF (2.738m) 2 g -1 FeGF-2 exhibited the highest pore volume among all tested samples, being 3.13 times that of the graphite felt. These results demonstrate that the colloidal etching strategy effectively increases the available surface area of ​​the graphite felt and constructs mesoporous channels, thereby exposing more active sites and promoting electrolyte wetting during VRFB operation.

[0045] Table 1. BET specific surface area, pore volume, and pore size of PGF, FeGF-1, FeGF-2, and FeGF-3

[0046] 2. CV, EIS and in-situ EIS testing To evaluate the effect of the electrode on V 2+ / V3+ Electrocatalytic activity of redox reactions at scan rates of 2, 5, 10, and 15 mVs -1 CV testing was conducted below. Figure 3 As shown in a and b, compared with PGF, the FeGF-2 electrode exhibits more pronounced redox peaks and a stronger current response, indicating improved electrochemical reversibility and reaction kinetics. At 10 mV s -1 Further comparison of FeGF-1, FeGF-2, and FeGF-3 shows that FeGF-2 exhibits the best redox response in the modified electrode. Figure 3 c). Furthermore, the peak current density exhibits an approximately linear relationship with the square root of the scan rate ( Figure 3 d), indicating that V under the tested conditions 2+ / V 3+ The redox process is primarily diffusion-controlled. The improved CV response of FeGF-2 can be attributed to its increased available surface area, improved wettability, and defect-rich porous structure.

[0047] The interfacial charge transfer behavior of each electrode was further compared using EIS. For example... Figure 3 As shown in e, the PGF exhibits a large semicircle in the Nyquist plot, indicating that V 2+ / V 3+ Charge transfer in redox reactions is relatively slow. In contrast, the semicircle of the FeGF series electrodes is significantly reduced, indicating a decrease in charge transfer resistance after colloidal etching. The fitted Rct value decreases from 219.1 Ω cm⁻¹ for PGF. 2 The Ω cm⁻¹ of FeGF-1 decreased to 17.59 Ω·cm. 2 9.6Ω cm of FeGF-2 2 and 15.32Ω cm of FeGF-3 2 ( Figure 3 f). Among these electrodes, FeGF-2 exhibits the lowest R. ct This confirms that it possesses optimal interfacial charge transfer kinetics. These EIS results are consistent with CV observations, further demonstrating that porous, defect-rich carbon surfaces are beneficial for the negative electrode V. 2+ / V 3+ reaction.

[0048] To further evaluate the polarization evolution under operating conditions of the flow battery, in-situ EIS tests were performed at different SOCs ranging from 5% to 80%. During the tests, the electrolyte was dispensed at a rate of 20 mL / min. -1The flow rate was circulated. In these tests, commercial graphite felt was used as the positive electrode, and PGF or FeGF-2 was used as the negative electrode. The obtained in-situ EIS spectra were further analyzed by DRT using the DRTtools software package based on Tikhonov regularization, converting the frequency domain impedance response to the relaxation time domain.

[0049] like Figure 4 As shown in the diagram, the battery with PGF as the negative electrode exhibits a stronger DRT signal in the main relaxation time region, indicating more significant polarization during battery operation. In contrast, the battery with FeGF-2 as the negative electrode shows a significantly weaker DRT response throughout the entire tested SOC range. The shorter to medium relaxation time region is generally related to interfacial charge transfer processes, while the longer relaxation time region is related to mass transfer polarization. Therefore, the lower DRT intensity of the FeGF-2 battery indicates that both charge transfer and mass transfer-related polarization are reduced under operating conditions.

[0050] These results are consistent with the lower R obtained in the three-electrode EIS test. ct The values ​​were consistent, further confirming that FeGF-2 is beneficial in promoting V 2+ / V 3+ Negative electrode reaction. The reduction in operating polarization is attributed to the increased available surface area of ​​FeGF-2, improved wettability, and mesoporous defect-rich structure, which together promote interfacial reaction kinetics and electrolyte wetting.

[0051] 3. Single-cell test To evaluate the practical application performance of the modified anode, VRFB single cells were assembled using commercial graphite felt as the positive electrode and PGF or FeGF-1 / FeGF-2 / FeGF-3 as the anode. For example... Figure 5 As shown in a, at 300mA cm -2 Under these conditions, the battery using FeGF as the negative electrode exhibits a smaller charge-discharge voltage difference compared to the battery using PGF, indicating reduced polarization during operation. Among the modified electrodes, FeGF-2 exhibits the lowest voltage polarization and the highest charging capacity. The charging capacity of the FeGF-2 battery reaches 418.9 mAh, significantly higher than that of the PGF battery. This improvement can be attributed to the porous and defect-rich FeGF-2 electrode reducing negative electrode polarization and improving electrolyte utilization.

[0052] Further at 100-500mA cm -2 Rate performance was evaluated at current densities. Figure 5 As shown in b, the battery using FeGF anode maintains a CE comparable to that of the PGF battery, while its EE value is significantly improved, especially at high current densities. At 300 mA / cm²... -2At this level, the battery using FeGF-2 as the negative electrode achieved an EE of 79.67%, higher than the 60.88% of the PGF battery. Furthermore, the PGF battery at 300 mA cm⁻¹… -2 At current densities above these levels, severe polarization is observed, while FeGF cells can exhibit polarization at 500 mA cm⁻¹. -2 The FeGF-2 battery operates at 500 mA cm⁻¹. -2 It achieved an EE of 67.66%, demonstrating its improved high current density capability.

[0053] The power output capabilities of the batteries were further compared through polarization measurements. Figure 5 As shown in Figure c, the battery with FeGF-2 as the negative electrode exhibits lower voltage polarization and higher power density than the PGF battery. The peak power density is significantly higher than that of the PGF battery, which has a peak power density of 561.23 mW / cm². -2 Increased to 992.75 mW cm⁻² for FeGF-2. -2 This enhancement is consistent with the decrease in charge transfer resistance and the improvement in redox kinetics observed in the three-electrode test.

[0054] The operational stability of the modified anode was evaluated through long-cycle testing. Figure 5 As shown in d and e, at 300mA cm -2 Under these conditions, the battery using FeGF-2 as the negative electrode maintained a more stable electrical conductivity (EE) than the PGF battery. More importantly, the FeGF-2 battery exhibited better EE at 500 mA / cm². -2 It ran stably for 2500 cycles. Figure 5 (f) EE decreased by only 2.12%. These results indicate that the etched porous structure of FeGF-2 can maintain stable electrochemical performance under continuous flow operation conditions.

[0055] The stable cycling performance is primarily attributed to the porous structure and defect sites generated directly on the carbon fiber surface during colloidal etching. Unlike externally loaded nanoparticle catalysts, these active structures are integrated with the graphite felt substrate, thus preventing them from detaching during continuous electrolyte cycling. This structural stability, combined with improved interfacial reaction kinetics, enables the FeGF-2 anode to maintain stable electrochemical performance during high current density VRFB operation.

[0056] In summary, this invention develops a colloidal ferric hydroxide etching strategy for constructing nanoporous graphite felt as a VRFB anode. By using FeCl3-derived Fe(OH)3 colloid as a dispersible etching precursor, a porous structure is generated on the carbon fiber surface after high-temperature treatment and acid washing. The optimized FeGF-2 electrode exhibits an increased specific surface area of ​​8.58 m². 2 g -1Improved wettability and increased defect density, these properties together provide more available active sites and promote V 2+ / V 3+ Electrolyte wetting in redox reactions. Electrochemical tests show that FeGF-2 exhibits a superior redox response and lower charge transfer resistance compared to PGF, with a fitted R... ct The value decreased from 219.1 Ω·cm² for PGF to 9.6 Ω·cm² for FeGF-2. 2 In single-cell tests using commercial graphite felt as the positive electrode and FeGF-2 as the negative electrode, the battery achieved a voltage of 300 mA cm⁻¹. -2 It achieved an EE of 79.67% and at 500mA cm -2 Under these conditions, the EE was maintained at 67.66%. The peak power density also increased from 561.23 mW / cm² at the PGF. -2 Increased to 992.75 mW cm⁻² for FeGF-2. -2 Long-cycle testing further demonstrates that this battery performs well at 500 mA cm⁻¹. -2 After 2500 cycles of stable operation, the EE decreased by only 2.12%. These results demonstrate that colloidal ferric hydroxide etching is an effective subtractive strategy that can provide a porous, defect-rich carbon surface for graphite felt anodes without relying on external catalysts, thereby improving their performance.

Claims

1. A method for preparing a catalyst-free nanoporous graphite felt anode using colloidal etching, characterized in that: Includes the following steps: S1. Dissolve excess FeCl3·6H2O in deionized water to obtain a saturated FeCl3 solution; add the saturated FeCl3 solution dropwise to boiling deionized water to prepare a Fe(OH)3 colloidal system. S2. Immerse the graphite felt sample in Fe(OH)3 colloidal solution, dry it after impregnation, and obtain the colloidal impregnation precursor; S3. Heat-treat the colloid-impregnated precursor to convert Fe(OH)3 deposited on the carbon fiber surface into iron oxide. S4. Anneal the heat-treated sample in an argon atmosphere to induce iron-assisted carbon thermal etching. S5. Immerse the annealed electrode in 4M HCl to remove residual iron impurities, wash until neutral and dry to obtain a nanoporous graphite felt negative electrode.

2. The method for preparing a catalyst-free nanoporous graphite felt negative electrode by colloidal etching according to claim 1, characterized in that: In S1, the amount of saturated FeCl3 solution added is 1-3 mL of saturated FeCl3 solution per 60 mL of boiling deionized water.

3. The method for preparing a catalyst-free nanoporous graphite felt negative electrode by colloidal etching according to claim 1, characterized in that: The conditions for S3 heat treatment are: initial heat treatment at 300℃ for 1 hour.

4. The method for preparing a catalyst-free nanoporous graphite felt negative electrode by colloidal etching according to claim 1, characterized in that: The S4 annealing process is performed at 1050℃ for 5 hours.

5. A catalyst-free nanoporous graphite felt anode prepared by any one of claims 1-4.

6. The catalyst-free nanoporous graphite felt anode according to claim 5, characterized in that: The specific surface area of the non-catalyst nanoporous graphite felt negative electrode is 7.98-8.58 m 2 g -1 The contact angle is 88.4-89.3°, and the Raman spectrum ID / IG ratio is 1.27-1.

34.

7. The catalyst-free nanoporous graphite felt anode according to claim 5, characterized in that: The surface of the catalyst-free nanoporous graphite felt negative electrode has a mesoporous structure, with the pore size distribution concentrated in the mesoporous range.

8. A vanadium redox flow battery, characterized in that: It includes a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is the catalyst-free nanoporous graphite felt negative electrode according to any one of claims 5-7.

9. The all-vanadium redox flow battery according to claim 8, characterized in that: Energy efficiency ≥ 79.67% at 300 mA cm -2 Energy efficiency ≥ 67.66% at 500 mA cm -2 Energy efficiency ≥ 67.66% at 500 mA cm -2 .

10. The all-vanadium redox flow battery according to claim 8, characterized in that: After 2500 cycles at 500 mA cm -2 The energy efficiency decay was < 2.12% after 2500 cycles at a current density of 500 mA cm-2.

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