304 capillary tube Nanocomposites Based on Tungsten Oxide/Fullerene as Electrocatalysts and Inhibitors of Parasitic VO2+/VO2+ Reactions in Mixed Acids

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Stainless Steel 304 Coil Tube Chemical Composition

304 Stainless Steel Coil Tube is a kind of austenitic chromium-nickel alloy. According to the Stainless Steel 304 Coil Tube Manufacturer, the main component in it is Cr (17%-19%), and Ni (8%-10.5%). In order to improve its resistance to corrosion, there are small amounts of Mn (2%) and Si (0.75%).

Stainless Steel 304 Coil Tube Mechanical Properties

The mechanical properties of 304 stainless steel coil tube are as follows:

• Tensile strength: ≥515MPa
• Yield strength: ≥205MPa
• Elongation: ≥30%

Applications & Uses of Stainless Steel 304 Coil Tube

        The relatively high cost of vanadium redox flow batteries (VRFBs) limits their widespread use. The kinetics of electrochemical reactions must be improved in order to increase the power density and energy efficiency of the VRFB, thereby reducing the kWh cost of the VRFB. In this work, hydrothermally synthesized hydrated tungsten oxide (HWO) nanoparticles, C76 and C76/HWO, were deposited on carbon cloth electrodes and tested as electrocatalysts for the VO2+/VO2+ redox reaction. Field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX), high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), infrared Fourier transform Spectroscopy ( FTIR) and contact angle measurements. It has been found that the addition of C76 fullerene to HWO can enhance the kinetics of the electrode with respect to the VO2+/VO2+ redox reaction by increasing the conductivity and providing oxygen-containing functional groups on its surface. The HWO/C76 composite (50 wt% C76) proved to be the most suitable for the VO2+/VO2+ reaction with ΔEp of 176 mV compared to 365 mV for untreated carbon cloth (UCC). In addition, the HWO/C76 composite showed significant inhibition of the parasitic chlorine evolution reaction due to the W-OH functional groups.
        Intense human activity and the rapid industrial revolution have led to an unstoppably high demand for electricity, which is growing at about 3% per year1. For decades, the widespread use of fossil fuels as a source of energy has led to greenhouse gas emissions, leading to global warming, water and air pollution, threatening entire ecosystems. As a result, by 2050 the share of clean renewable energy and solar energy is projected to reach 75% of total electricity1. However, when renewable energy production exceeds 20% of total electricity production, the grid becomes unstable 1. The development of efficient energy storage systems is critical to this transition, as they must store excess electricity and balance supply and demand.
        Among all energy storage systems such as hybrid vanadium redox flow batteries2, all vanadium redox flow batteries (VRFBs) are the most advanced due to their many advantages3 and are considered the best solution for long term energy storage (~30 years). Use of renewable energy sources4. This is due to the separation of power and energy density, fast response, long life and relatively low annual costs of $65/kWh compared to $93-140/kWh for Li-ion and lead-acid batteries and 279-420 USD/kWh. /kWh batteries respectively 4.
        However, their widespread commercialization continues to be hampered by relatively high system capital costs, mainly due to battery packs4,5. Thus, improving battery performance by increasing the kinetics of two half-cell reactions can reduce battery size and thus reduce cost. Therefore, fast electron transfer to the electrode surface is required, depending on the design, composition and structure of the electrode, which must be carefully optimized. Although carbon-based electrodes have good chemical and electrochemical stability and good electrical conductivity, if left untreated, their kinetics will be slow due to the absence of oxygen functional groups and hydrophilicity7,8. Therefore, various electrocatalysts are combined with carbon electrodes, especially carbon nanostructures and metal oxides, to improve the kinetics of both electrodes, thereby increasing the kinetics of the VRFB electrodes.
        Many carbon materials have been used, such as carbon paper9, carbon nanotubes10,11,12,13, graphene-based nanostructures14,15,16,17, carbon nanofibers18 and others19,20,21,22,23, except for the fullerene family. In our previous study on C76, we reported for the first time the excellent electrocatalytic activity of this fullerene towards VO2+/VO2+, compared to heat-treated and untreated carbon cloth, the charge transfer resistance was reduced by 99.5% and 97%24. The catalytic performance of the carbon materials for the VO2+/VO2+ reaction compared to C76 is shown in Table S1. On the other hand, many metal oxides such as CeO225, ZrO226, MoO327, NiO28, SnO229, Cr2O330 and WO331, 32, 33, 34, 35, 36, 37, 38 are used due to their increased wettability and high oxygen content. groups. Table S2 shows the catalytic performance of these metal oxides in the VO2+/VO2+ reaction. WO3 has been used in a significant number of works due to its low cost, high stability in acidic media, and high catalytic activity31,32,33,34,35,36,37,38. However, WO3 showed little improvement in cathode kinetics. To improve the conductivity of WO3, the effect of using reduced tungsten oxide (W18O49) on positive electrode activity was tested38. Hydrated tungsten oxide (HWO) has never been tested in VRFB applications, although it has shown higher activity in supercapacitor applications due to faster cation diffusion compared to anhydrous WOx39,40. The third generation all-vanadium redox flow battery uses a mixed acid electrolyte composed of HCl and H2SO4 to improve battery performance and improve the solubility and stability of vanadium ions in the electrolyte. However, the parasitic chlorine evolution reaction has become one of the disadvantages of the third generation, so finding ways to suppress the chlorine evaluation reaction has become the task of several research groups.
        Here, VO2+/VO2+ reaction tests were carried out on HWO/C76 composites deposited on carbon cloth electrodes in order to find a balance between the electrical conductivity of the composites and the redox reaction kinetics on the electrode surface while suppressing parasitic chlorine deposition. reaction (KVR). Hydrated tungsten oxide (HWO) nanoparticles were synthesized by a simple hydrothermal method. Experiments were carried out in a mixed acid electrolyte (H2SO4/HCl) to simulate third generation VRFB (G3) for convenience and to investigate the effect of HWO on the parasitic chlorine evolution reaction42.
       Vanadium(IV) sulfate oxide hydrate (VOSO4, 99.9%, Alfa-Aeser), sulfuric acid (H2SO4), hydrochloric acid (HCl), dimethylformamide (DMF, Sigma-Aldrich), polyvinylidene fluoride (PVDF, Sigma-Aldrich), sodium Tungsten oxide dihydrate (Na2WO4, 99%, Sigma-Aldrich) and hydrophilic carbon cloth ELAT (Fuel Cell Store) were used in this study.
        Hydrated tungsten oxide (HWO) was prepared by a hydrothermal reaction in which 2 g of the Na2WO4 salt was dissolved in 12 ml of HO until a colorless solution was obtained, and then 12 ml of 2 M HCl was added dropwise until a light yellow suspension was obtained. suspension. The hydrothermal reaction was carried out in a Teflon coated stainless steel autoclave in an oven at 180 ºC for 3 hours. The residue was collected by filtration, washed 3 times with ethanol and water, dried in an oven at 70°C for ~3 h, and then ground to obtain a blue-gray HWO powder.
        The obtained (untreated) carbon cloth electrodes (CCTs) were used in the form in which they were obtained or subjected to heat treatment in a tube furnace at 450°C for 10 h at a heating rate of 15°C/min in air to obtain treated UCC (TCC), s Same as previous work 24. UCC and TCC were cut into electrodes approximately 1.5 cm wide and 7 cm long. Suspensions of C76, HWO, HWO-10% C76, HWO-30% C76 and HWO-50% C76 was prepared by adding 20 mg of active material powder and 10 wt% (~2.22 mg) of PVDF binder to ~1 ml of DMF prepared in and sonicated for 1 hour to improve uniformity. Then 2 mg of C76, HWO and HWO-C76 composites were applied to approximately 1.5 cm2 of the UCC active electrode area. All catalysts were loaded onto UCC electrodes and TCC was used for comparison purposes only, as our previous work has shown that heat treatment is not required 24 . Impression settling was achieved by brushing 100 µl of the suspension (load 2 mg) for greater uniformity. Then all the electrodes were dried in an oven overnight at 60°C. The electrodes are measured before and after to ensure accurate stock loading. In order to have a certain geometric area (~1.5 cm2) and prevent the rise of the vanadium electrolyte to the electrodes due to the capillary effect, a thin layer of paraffin was applied over the active material.
        A field emission scanning electron microscope (FESEM, Zeiss SEM Ultra 60.5 kV) was used to observe the HWO surface morphology. Energy dispersive X-ray spectroscopy equipped with Feii8SEM (EDX, Zeiss AG) was used to map the HWO-50%C76 elements on the UCC electrodes. A high resolution transmission electron microscope (HR-TEM, JOEL JEM-2100) operating at an accelerating voltage of 200 kV was used to obtain high resolution images and diffraction rings of HWO particles. Use the Crystallographic Tool Box (CrysTBox) software to analyze HWO diffraction rings using the ringGUI function and compare the results with XRD models. The structure and graphitization of UCC and TCC was determined by X-ray diffraction (XRD) at a scan rate of 2.4°/min from 5° to 70° with Cu Kα (λ = 1.54060 Å) using a Panalytical X-ray diffractometer. (Model 3600). XRD shows the crystal structure and phases of HWO. The PANalytical X’Pert HighScore software was used to match the HWO peaks to the tungsten oxide maps available in the database45. Compare the HWO results with the TEM results. The chemical composition and state of the HWO samples were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, ThermoScientific). The CASA-XPS software (v 2.3.15) was used for peak deconvolution and data analysis. Fourier transform infrared spectroscopy (FTIR, using a Perkin Elmer class KBr FTIR spectrometer) measurements were performed to determine the surface functional groups of HWO and HWO-50%C76. Compare the results with the XPS results. Contact angle measurements (KRUSS DSA25) were also used to characterize the wettability of the electrodes.
        For all electrochemical measurements, a Biologic SP 300 workstation was used. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to study the electrode kinetics of the VO2+/VO2+ redox reaction and the effect of reagent diffusion (VOSO4 (VO2+)) on the reaction rate. Both technologies use a three-electrode cell with an electrolyte concentration of 0.1 M VOSO4 (V4+) dissolved in 1 M H2SO4 + 1 M HCl (mixed acid). All electrochemical data presented are IR corrected. A saturated calomel electrode (SCE) and a platinum (Pt) coil were used as the reference and counter electrode, respectively. For CV, scan rates (ν) of 5, 20, and 50 mV/s were applied to a potential window (0–1) V compared to SCE for VO2+/VO2+, then corrected on the SHE scale to plot (VSCE = 0.242 V relative to HSE) . To investigate the retention of electrode activity, a CV recycle was performed on UCC, TCC, UCC-C76, UCC-HWO and UCC-HWO-50% C76 at ν equal to 5 mV/s. For EIS measurements for the VO2+/VO2+ redox reaction, a frequency range of 0.01-105 Hz and an open circuit voltage (OCV) disturbance of 10 mV were used. Each experiment was repeated 2-3 times to ensure the consistency of the results. The heterogeneous rate constants (k0) were obtained by the Nicholson method46,47.
        Hydrated tungsten oxide (HVO) has been successfully synthesized by the hydrothermal method. SEM image in fig. 1a shows that the deposited HWO consists of clusters of nanoparticles with particle sizes in the range of 25–50 nm.
        The X-ray diffraction pattern of HWO shows peaks (001) and (002) at ~23.5° and ~47.5°, respectively, which are characteristic of nonstoichiometric WO2.63 (W32O84) (PDF 077–0810, a = 21.4 Å, b = 17.8 Å, c = 3.8 Å, α = β = γ = 90°), which corresponds to its apparent blue color (Fig. 1b)48,49. Other peaks at approximately 20.5°, 27.1°, 28.1°, 30.8°, 35.7°, 36.7° and 52.7° are at (140), (620), (350 ), (720), (740), (560). and (970) diffraction planes, respectively, 49 orthorhombic WO2.63. Songara et al. 43 used the same synthetic method to obtain a white product, which was attributed to the presence of WO3(H2O)0.333. However, in this work, due to different conditions, a blue-gray product was obtained, indicating the coexistence of WO3(H2O)0.333 (PDF 087-1203, a = 7.3 Å, b = 12.5 Å, c = 7.7 ) in Å, α = β = γ = 90°) and the reduced form of tungsten oxide. Semiquantitative analysis with X’Pert HighScore software showed 26% WO3(H2O)0.333: 74% W32O84. Since W32O84 consists of W6+ and W4+ (1.67:1 W6+:W4+), the estimated content of W6+ and W4+ is about 72% W6+ and 28% W4+, respectively. SEM images, 1-second XPS spectra at the nucleus level, TEM images, FTIR spectra and Raman spectra of C76 particles were presented in our previous paper24. According to Kawada et al.50,51, X-ray diffraction pattern of C76 shows the monoclinic structure of FCC after removal of toluene.
        SEM images in fig. 2a and b show the successful deposition of HWO and HWO-50%C76 on and between the carbon fibers of the UCC electrodes. Elemental mapping of tungsten, carbon and oxygen in the SEM image in Fig. 2c is shown in fig. 2d–f showing that the tungsten and carbon are uniformly mixed (showing a similar distribution) over the electrode surface and the composite is not deposited evenly. due to the nature of the precipitation method.
        SEM images of deposited HWO particles (a) and HWO-C76 particles (b). EDX mapping uploaded to HWO-C76 at UCC using the area in image (c) shows the distribution of tungsten (d), carbon (e), and oxygen (f) in the sample.
        HR-TEM was used for high magnification imaging and crystallographic information (Figure 3). The HWO demonstrates the nanocube morphology as shown in Figure 3a and more clearly in Figure 3b. By magnifying the nanocube for diffraction of a selected area, the grating structure and diffraction planes satisfying Bragg’s law can be visualized as shown in Figure 3c, confirming the crystallinity of the material. In the inset to Fig. 3c shows the distance d 3.3 Å corresponding to the (022) and (620) diffraction planes in the WO3(H2O)0.333 and W32O84, 43, 44, 49 phases, respectively. This is consistent with the above XRD analysis (Fig. 1b) since the observed grating plane distance d (Fig. 3c) corresponds to the strongest XRD peak in the HWO sample. Sample rings are also shown in fig. 3d, where each ring corresponds to a separate plane. The WO3(H2O)0.333 and W32O84 planes are colored white and blue, respectively, and their corresponding XRD peaks are also shown in Fig. 1b. The first ring shown in the ring pattern corresponds to the first marked peak in the x-ray pattern of the (022) or (620) diffraction plane. From (022) to (402) rings, d-distances of 3.30, 3.17, 2.38, 1.93, and 1.69 Å were found, which are consistent with XRD values ​​of 3.30, 3.17, 2 .45, 1.93 and 1.66. Å, 44, 45, respectively.
        (a) HR-TEM image of HWO, (b) shows an enlarged image. Images of the grating planes are shown in (c), and inset (c) shows an enlarged image of the planes and the interval d 0.33 nm corresponding to the (002) and (620) planes. (d) HWO ring pattern showing the planes associated with the WO3(H2O)0.333 (white) and W32O84 (blue) phases.
        XPS analysis was performed to determine the surface chemistry and oxidation state of tungsten (Figures S1 and 4). The spectrum of the wide-range XPS scan of the synthesized HWO is shown in Fig. . S1, indicating the presence of tungsten. The XPS narrow-scan spectra of the main W 4f and O 1s levels are shown in Figs. 4a and b, respectively. The W 4f spectrum is split into two spin-orbit doublets corresponding to the binding energy of the oxidation state W. The peaks W 4f5/2 and W 4f7/2 at binding energies of 37.8 and 35.6 eV belong to W6+, and the peaks W 4f5/2 and W 4f7/2 at 36.6 and 34.9 eV are characteristic of the W4+ state, respectively. The presence of the oxidation state (W4+) further confirms the formation of non-stoichiometric WO2.63, while the presence of W6+ indicates stoichiometric WO3 due to WO3(H2O)0.333. The fitted data showed that the atomic percentages of W6+ and W4+ were 85% and 15%, respectively, which were relatively close to the values ​​estimated from the XRD data, given the difference between the two technologies. Both methods provide quantitative information with low accuracy, especially XRD. In addition, the two methods analyze different parts of the material because XRD is a bulk method while XPS is a surface method that only approaches a few nanometers. The O 1s spectrum splits into two peaks at 533 (22.2%) and 530.4 eV (77.8%). The first corresponds to OH, and the second to oxygen bonds in the lattice in WO. The presence of OH functional groups is consistent with the hydration properties of HWO.
        An FTIR analysis was also performed on these two samples to examine the presence of functional groups and coordinated water molecules in the hydrated HWO structure. The results show that the HWO-50% C76 sample and the FT-IR HWO results look the same due to the presence of HWO, but the intensity of the peaks differs due to different amounts of sample used during preparation for analysis (Fig. 5a). HWO-50% C76 All fullerene 24 peaks are shown except for the tungsten oxide peak. Detailed in fig. 5a shows that both samples exhibit a very strong broad band at ~710/cm, attributed to OWO stretching vibrations in the HWO lattice structure, and a strong shoulder at ~840/cm, attributed to WO. the sharp band at ~1610/cm is related to the bending vibration of OH, and the broad absorption band at ~3400/cm is related to the stretching vibration of OH in the hydroxyl group43. These results are consistent with the XPS spectrum in Fig. 4b, where the WO functional group can provide active sites for the VO2+/VO2+ reaction.
       FTIR analysis of HWO and HWO-50% C76 (a) showing functional groups and contact angle measurements (b, c).
        The OH group can also catalyze the VO2+/VO2+ reaction, thereby increasing the hydrophilicity of the electrode, thereby promoting diffusion and electron transfer rates. The HWO-50% C76 sample shows an additional C76 peak as shown in the figure. The peaks at ~2905, 2375, 1705, 1607, and 1445 cm3 can be assigned to the CH, O=C=O, C=O, C=C, and CO stretching vibrations, respectively. It is well known that the oxygen functional groups C=O and CO can serve as active centers for the redox reactions of vanadium. To test and compare the wettability of the two electrodes, contact angle measurements were used as shown in Fig. 5b, c. The HWO electrode immediately absorbs water droplets, indicating superhydrophilicity due to the available OH functional groups. HWO-50% C76 is more hydrophobic, with a contact angle of about 135° after 10 seconds. However, in electrochemical measurements, the HWO-50%C76 electrode was completely wetted in less than a minute. The wettability measurements are consistent with XPS and FTIR results, suggesting that more OH groups on the HWO surface makes it relatively more hydrophilic.
        The VO2+/VO2+ reactions of HWO and HWO-C76 nanocomposites were tested and it was expected that HWO would suppress the evolution of chlorine gas that occurs during VO2+/VO2+ reactions in mixed acids, while C76 would further catalyze the desired VO2+/ VO2+. HWO suspensions containing 10%, 30% and 50% C76 were applied to UCC electrodes with a total load of about 2 mg/cm2.
        As shown in fig. 6, the kinetics of the VO2+/VO2+ reaction on the electrode surface was examined using CV in mixed acidic electrolytes. Currents are shown as I/Ipa to facilitate comparison of ΔEp and Ipa/Ipc. Various catalysts are obtained directly from the figure. The current area unit data is shown in Figure 2S. On fig. Figure 6a shows that HWO slightly increases the electron transfer rate of the VO2+/VO2+ redox reaction on the electrode surface and suppresses the reaction of parasitic chlorine evolution. However, C76 significantly increases the electron transfer rate and catalyzes the chlorine evolution reaction. Therefore, a complex with the correct composition of HWO and C76 should have the best activity and the highest ability to inhibit the chlorine reaction. It was found that after increasing the C76 content, the electrochemical activity of the electrode improved, as evidenced by a decrease in ΔEp and an increase in the Ipa/Ipc ratio (Table S3). This was also confirmed by the RCT values ​​extracted from the Nyquist plot in Fig. 6d (table S3), where it was found that the RCT values ​​decreased with increasing content of C76. These results are also consistent with Lee’s study in which the addition of mesoporous carbon to mesoporous WO3 improved the charge transfer kinetics on VO2+/VO2+35. This suggests that a positive reaction may depend more on the conductivity of the electrode (C=C bond)18,24,35,36,37. Due to the change in the coordination geometry between [VO(H2O)5]2+ and [VO2(H2O)4]+, C76 can also reduce the response overstrain by reducing tissue energy. However, this may not be possible with HWO electrodes.
        (a) Cyclic voltammetric behavior of UCC and HWO-C76 composites with different HWO:C76 ratios in VO2+/VO2+ reactions in 0.1 M VOSO4/1 M H2SO4 + 1 M HCl electrolyte (at ν = 5 mV/s). (b) Randles-Sevchik and (c) Nicholson’s VO2+/VO2+ method for estimating diffusion efficiency and obtaining k0 values ​​(d).
        Not only was HWO-50% C76 exhibiting almost the same electrocatalytic activity as C76 for the VO2+/VO2+ reaction, but, more interestingly, it additionally suppressed the evolution of chlorine gas compared to C76, as shown in the figure. 6a, in addition to showing the smaller semicircle in fig. 6g (lower RCT). C76 showed a higher apparent Ipa/Ipc than HWO-50% C76 (Table S3), not due to improved reaction reversibility, but due to overlap with the chlorine reduction peak at 1.2 V compared to SHE. The best performance of HWO-50% C76 is attributed to the synergy between the negatively charged highly conductive C76 and the high wettability and catalytic functionalities of W-OH on HWO. While less chlorine emission will improve the charging efficiency of the full cell, improved kinetics will increase the efficiency of the full cell voltage.
        According to equation S1, for a quasi-reversible (relatively slow electron transfer) reaction controlled by diffusion, the peak current (IP) depends on the number of electrons (n), electrode area (A), diffusion coefficient (D), number of electrons transfer coefficient (α) and scanning speed (ν). In order to study the diffusion controlled behavior of the tested materials, the relationship between IP and ν1/2 was plotted and shown in Fig. 6b. Since all materials show a linear relationship, the reaction is controlled by diffusion. Since the VO2+/VO2+ reaction is quasi-reversible, the slope of the line depends on the diffusion coefficient and the value of α (equation S1). Due to the constant diffusion coefficient (≈ 4 × 10–6 cm2/s)52, the difference in line slope directly indicates different values ​​of α and hence different rates of electron transfer to the electrode surface, with C76 and HWO -50% C76, exhibiting the steepest slopes (highest electron transfer rate).
        The calculated low-frequency Warburg slopes (W) shown in Table S3 (Fig. 6d) have values ​​close to 1 for all materials, indicating perfect diffusion of redox particles and confirming the linear behavior of IP versus ν1/2 for CV . measurements . For HWO-50% C76, the Warburg slope deviates from unity to 1.32, suggesting a contribution not only from the semi-infinite diffusion of reactants (VO2+), but also possibly thin-layer behavior in the diffusion behavior due to electrode porosity.
        To further analyze the reversibility (electron transfer rate) of the VO2+/VO2+ redox reaction, the Nicholson quasi-reversible reaction method was also used to determine the standard rate constant k041.42. This is done by plotting the dimensionless kinetic parameter Ψ as a function of ΔEp as a function of ν−1/2 using the S2 equation. Table S4 shows the resulting Ψ values ​​for each electrode material. Plot the results (Figure 6c) to obtain k0 × 104 cm/s (written next to each row and presented in Table S4) using equation S3 for the slope of each plot. HWO-50% C76 was found to have the highest slope (Fig. 6c) and hence the highest k0 value of 2.47 × 10–4 cm/s. This means that this electrode provides the fastest kinetics consistent with the CV and EIS results in Figures 6a and d and Table S3. In addition, the k0 values ​​were also obtained from the Nyquist plots (Fig. 6d) of Equation S4 using the RCT values ​​(Table S3). These k0 results from EIS are summarized in Table S4 and also show that HWO-50% C76 exhibits the highest electron transfer rate due to the synergistic effect. Even though the value of k0 differs due to the different origin of each method, it still shows the same order of magnitude and shows consistency.
        To fully understand the excellent kinetics that can be achieved, it is important to compare the optimal electrode material with uninsulated UCC and TCC electrodes. For the VO2+/VO2+ reaction, HWO-C76 not only showed the lowest ΔEp and better reversibility, but also significantly suppressed the parasitic chlorine evolution reaction compared to TCC, as indicated by a significant current drop at 1.45 V compared to see OHA (Fig. 7a). In terms of stability, we assumed that HWO-50% C76 is physically stable because the catalyst was mixed with a PVDF binder and then applied to the carbon cloth electrodes. Compared to 50 mV for UCC, HWO-50% C76 showed a peak shift of 44 mV after 150 cycles (degradation rate 0.29 mV/cycle) (Figure 7b). It may not be a big difference, but the kinetics of UCC electrodes is very slow and degrades with cycling, especially for back reaction. Although the reversibility of TCC is much better than that of UCC, TCC was found to have a large peak shift of 73 mV after 150 cycles, which may be due to the large amount of chlorine released from its surface. To ensure that the catalyst adheres well to the electrode surface. As can be seen on all electrodes tested, even those without supported catalysts exhibit varying degrees of cycling instability, suggesting that changes in peak separation during cycling are due to material deactivation due to chemical changes rather than catalyst separation. Also, if a large amount of catalyst particles were to be separated from the electrode surface, this would lead to a significant increase in peak separation (not only by 44 mV), since the substrate (UCC) is relatively inactive for the VO2+/VO2+ redox reaction.
        Comparison of CV (a) and stability of the redox reaction VO2+/VO2+ (b) of the optimal electrode material with respect to CCC. In the electrolyte 0.1 M VOSO4/1 M H2SO4 + 1 M HCl, all CVs are equal to ν = 5 mV/s.
        To increase the economic attractiveness of VRFB technology, improving and understanding the kinetics of the vanadium redox reaction is essential to achieving high energy efficiency. Composites HWO-C76 were prepared and their electrocatalytic effect on the VO2+/VO2+ reaction was studied. HWO showed little kinetic enhancement but significantly suppressed chlorine evolution in mixed acidic electrolytes. Various ratios of HWO:C76 were used to further optimize the kinetics of HWO-based electrodes. Increasing the content of C76 to HWO can improve the electron transfer kinetics of the VO2+/VO2+ reaction on the modified electrode, among which HWO-50% C76 is the best material because it lowers the charge transfer resistance and further suppresses chlorine gas evolution compared to C76. and TCC are released. This was due to the synergistic effect between C=C sp2 hybridization, OH and W-OH functional groups. The degradation rate of HWO-50% C76 was found to be 0.29mV/cycle under multiple cycling while UCC and TCC are 0.33mV/cycle and 0.49mV/cycle respectively, making it very stable in mixed acid electrolytes. The presented results successfully identify high performance electrode materials for the VO2+/VO2+ reaction with fast kinetics and high stability. This will increase the output voltage, thereby improving the power efficiency of the VRFB, thereby reducing the cost of its future commercialization.
       The datasets used and/or analyzed in the current study are available from the respective authors upon reasonable request.
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