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The relatively high cost of all-vanadium flow-through redox batteries (VRFBs) limits their widespread use. Improving the kinetics of electrochemical reactions is required to increase the specific power and energy efficiency of the VRFB, thereby reducing the cost of kWh 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 fullerenes to HWO can improve electrode kinetics by increasing electrical conductivity and providing oxidized functional groups on its surface, thereby promoting the VO2+/VO2+ redox reaction. The HWO/C76 composite (50 wt% C76) proved to be the best choice for the VO2+/VO2+ reaction with ΔEp of 176 mV, while untreated carbon cloth (UCC) was 365 mV. In addition, the HWO/C76 composite showed a significant inhibitory effect on the parasitic chlorine evolution reaction due to the W-OH functional group.
Intense human activity and the rapid industrial revolution have led to an unstoppably high demand for electricity, which is increasing by about 3% per year1. For decades, the widespread use of fossil fuels as a source of energy has led to greenhouse gas emissions that contribute to global warming, water and air pollution, threatening entire ecosystems. As a result, the penetration of clean and renewable wind and solar energy is expected to reach 75% of total electricity by 20501. However, when the share of electricity from renewable sources exceeds 20% of the total electricity generation, the grid becomes unstable.
Among all energy storage systems such as the hybrid vanadium redox flow battery2, the all-vanadium redox flow battery (VRFB) has developed the most rapidly due to its many advantages and is considered the best solution for long-term energy storage (about 30 years). ) Options in combination with renewable energy4. This is due to the separation of power and energy density, fast response, long service life, and a relatively low annual cost of $65/kWh compared to $93-140/kWh for Li-ion and lead-acid batteries and 279-420 US dollars per kWh. battery respectively 4.
However, their large-scale commercialization is still constrained by their relatively high system capital costs, mainly due to cell stacks4,5. Thus, improving stack performance by increasing the kinetics of the two half-element reactions can reduce stack size and thus reduce cost. Therefore, fast electron transfer to the electrode surface is necessary, which depends on the design, composition and structure of the electrode and requires careful optimization6. Despite the good chemical and electrochemical stability and good electrical conductivity of carbon electrodes, their untreated kinetics are sluggish due to the absence of oxygen functional groups and hydrophilicity7,8. Therefore, various electrocatalysts are combined with carbon-based electrodes, especially carbon nanostructures and metal oxides, to improve the kinetics of both electrodes, thereby increasing the kinetics of the VRFB electrode.
In addition to our previous work on C76, we first reported the excellent electrocatalytic activity of this fullerene for VO2+/VO2+, charge transfer, compared to heat-treated and untreated carbon cloth. Resistance is reduced by 99.5% and 97%. 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 have been used because of their increased wettability and abundant oxygen functionality. , 38. group. The catalytic activity of these metal oxides in the VO2+/VO2+ reaction is presented in Table S2. 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, the improvement in cathodic kinetics due to WO3 is insignificant. To improve the conductivity of WO3, the effect of using reduced tungsten oxide (W18O49) on cathodic activity was tested38. Hydrated tungsten oxide (HWO) has never been tested in VRFB applications, although it exhibits increased activity in supercapacitor applications due to faster cation diffusion compared to anhydrous WOx39,40. The third generation 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 the search for ways to inhibit the chlorine evaluation reaction has become the focus 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 kinetics of the electrode surface while suppressing parasitic chlorine evolution. response (CER). 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 the third generation VRFB (G3) for practicality and to investigate the effect of HWO on the parasitic chlorine evolution reaction.
Vanadium(IV) sulfate 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 hydrothermal reaction 43 in which 2 g of the Na2WO4 salt was dissolved in 12 ml of H2O to give a colorless solution, then 12 ml of 2 M HCl was added dropwise to give a pale yellow suspension. The slurry was placed in a Teflon coated stainless steel autoclave and kept in an oven at 180° C. for 3 hours for hydrothermal reaction. The residue was collected by filtration, washed 3 times with ethanol and water, dried in an oven at 70°C for ~3 hours, and then triturated to give a blue-grey HWO powder.
The obtained (untreated) carbon cloth electrodes (CCT) were used as is or heat treated in a tube furnace at 450°C in air with a heating rate of 15 ºC/min for 10 hours to obtain treated CCs (TCC). as described in the previous article24. 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 were prepared by adding 20 mg .% (~2.22 mg) of PVDF binder to ~1 ml DMF and sonicated for 1 hour to improve uniformity. 2 mg of C76, HWO and HWO-C76 composites were sequentially applied to a UCC active electrode area of approximately 1.5 cm2. All catalysts were loaded onto UCC electrodes and TCC was used for comparison purposes only, as our previous work showed that heat treatment was not required24. Impression settling was achieved by brushing 100 µl of the suspension (load 2 mg) for a more even effect. Then all the electrodes were dried in an oven at 60° C. overnight. The electrodes are measured forward and backward 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 electrode due to the capillary effect, a thin layer of paraffin was applied over the active material.
Field emission scanning electron microscopy (FESEM, Zeiss SEM Ultra 60, 5 kV) was used to observe the HWO surface morphology. An energy dispersive X-ray spectrometer equipped with Feii8SEM (EDX, Zeiss Inc.) was used to map 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 image higher resolution HWO particles and diffraction rings. The Crystallography Toolbox (CrysTBox) software uses the ringGUI function to analyze the HWO ring diffraction pattern and compare the results with the XRD pattern. The structure and graphitization of UCC and TCC was analyzed 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 showed the crystal structure and phase of HWO. The PANalytical X’Pert HighScore software was used to match the HWO peaks to the tungsten oxide maps available in the database45. HWO results were compared with 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. To determine the surface functional groups of HWO and HWO-50%C76, measurements were made using Fourier transform infrared spectroscopy (FTIR, Perkin Elmer spectrometer, using KBr FTIR). The results were compared with 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 methods used a three-electrode cell with an electrolyte concentration of 0.1 M VOSO4 (V4+) in 1 M H2SO4 + 1 M HCl (mixture of acids). 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 the VO2+/VO2+ potential window for (0–1) V vs. SCE, then adjusted for SHE to plot (VSCE = 0.242 V vs. HSE) . To study the retention of electrode activity, repeated cyclic CVs were performed at ν 5 mV/s for UCC, TCC, UCC-C76, UCC-HWO, and UCC-HWO-50% C76. For EIS measurements, the frequency range of the VO2+/VO2+ redox reaction was 0.01-105 Hz, and the voltage perturbation at open-circuit voltage (OCV) was 10 mV. 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 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 their clear 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° were assigned to (140), (620), ( 350), (720), (740), (560°). ) ) and (970) diffraction planes orthogonal to WO2.63, respectively. The same synthetic method was used by Songara et al. 43 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 that WO3(H2O)0.333 (PDF 087-1203, a = 7.3 Å, b = 12.5 Å, c = 7 .7 Å, α = β = γ = 90°) and the reduced form of tungsten oxide. Semiquantitative analysis using 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 article. According to Kawada et al.,50,51 X-ray diffraction of C76 after removal of toluene showed the monoclinic structure of FCC.
SEM images in fig. 2a and b show that HWO and HWO-50%C76 were successfully deposited on and between the carbon fibers of the UCC electrode. EDX element maps of tungsten, carbon, and oxygen on SEM images in fig. 2c are shown in fig. 2d-f indicating that the tungsten and carbon are evenly mixed (showing a similar distribution) over the entire electrode surface and the composite is not uniformly deposited due to the nature of the deposition method.
SEM images of deposited HWO particles (a) and HWO-C76 particles (b). EDX mapping on HWO-C76 loaded on 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). HWO shows the nanocube morphology as shown in Fig. 3a and more clearly in Fig. 3b. By magnifying the nanocube for diffraction of selected areas, one can visualize the grating structure and diffraction planes that satisfy the Bragg law, as shown in Fig. 3c, which confirms 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 found in the WO3(H2O)0.333 and W32O84 phases, respectively43,44,49. This is consistent with the XRD analysis described above (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 diagram corresponds to the first marked peak in the x-ray pattern of the (022) or (620) diffraction plane. From the (022) to (402) rings, the d-spacing values are 3.30, 3.17, 2.38, 1.93, and 1.69 Å, consistent with XRD values of 3.30, 3.17, 2, 45, 1.93. and 1.66 Å, which is equal to 44, 45, respectively.
(a) HR-TEM image of HWO, (b) shows an enlarged image. Images of the grating planes are shown in (c), inset (c) shows an enlarged image of the planes and a pitch d of 0.33 nm corresponding to the (002) and (620) planes. (d) HWO ring pattern showing planes associated with WO3(H2O)0.333 (white) and W32O84 (blue).
XPS analysis was performed to determine the surface chemistry and oxidation state of tungsten (Figures S1 and 4). The wide range XPS scan spectrum of the synthesized HWO is shown in Figure S1, indicating the presence of tungsten. The XPS narrow-scan spectra of the W 4f and O 1s core levels are shown in Figs. 4a and b, respectively. The W 4f spectrum splits into two spin-orbit doublets corresponding to the binding energies of the W oxidation state. and W 4f7/2 at 36.6 and 34.9 eV are characteristic of the W4+ state of 40, respectively. )0.333. The fitted data show that the atomic percentages of W6+ and W4+ are 85% and 15%, respectively, which are close to the values estimated from the XRD data considering the differences between the two methods. Both methods provide quantitative information with low accuracy, especially XRD. Also, these 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 is divided 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 coordinating water molecules in the hydrated HWO structure. The results show that the HWO-50% C76 sample and FT-IR HWO results appear similar due to the presence of HWO, but the intensity of the peaks differs due to the different amount of sample used in preparation for analysis (Fig. 5a). ) HWO-50% C76 shows that all peaks, except for the peak of tungsten oxide, are related to fullerene 24. Detailed in fig. 5a shows that both samples exhibit a very strong broad band at ~710/cm attributed to OWO stretching oscillations in the HWO lattice structure, with a strong shoulder at ~840/cm attributed to WO. For stretching vibrations, a sharp band at about 1610/cm is attributed to bending vibrations of OH, while a broad absorption band at about 3400/cm is attributed to stretching vibrations of OH in hydroxyl groups43. These results are consistent with the XPS spectra in Figs. 4b, where WO functional groups can provide active sites for the VO2+/VO2+ reaction.
FTIR analysis of HWO and HWO-50% C76 (a), indicated functional groups and contact angle measurements (b, c).
The OH group can also catalyze the VO2+/VO2+ reaction, while increasing the hydrophilicity of the electrode, thereby promoting the rate of diffusion and electron transfer. As shown, the HWO-50% C76 sample shows an additional peak for C76. 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 taken as shown in Fig. 5b,c. The HWO electrode immediately absorbed 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 became completely wet in less than a minute. The wettability measurements are consistent with XPS and FTIR results, indicating 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 chlorine evolution in the VO2+/VO2+ reaction in mixed acid, and C76 would further catalyze the desired VO2+/VO2+ redox reaction. %, 30%, and 50% C76 in HWO suspensions and CCC deposited on electrodes with a total loading of about 2 mg/cm2.
As shown in fig. 6, the kinetics of the VO2+/VO2+ reaction on the electrode surface was examined by CV in a mixed acidic electrolyte. The currents are shown as I/Ipa for easy comparison of ΔEp and Ipa/Ipc for different catalysts directly on the graph. 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 correctly formulated composite of HWO and C76 is expected to have the best activity and the greatest ability to inhibit the chlorine evolution reaction. It was found that after increasing the content of C76, the electrochemical activity of the electrodes 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), which were found to decrease with increasing C76 content. These results are also consistent with Li’s study, in which the addition of mesoporous carbon to mesoporous WO3 showed improved charge transfer kinetics on VO2+/VO2+35. This indicates that the direct reaction may depend more on the electrode conductivity (C=C bond) 18, 24, 35, 36, 37. This may also be due to a change in the coordination geometry between [VO(H2O)5]2+ and [VO2(H2O)4]+, C76 reduces reaction overvoltage by reducing tissue energy. However, this may not be possible with HWO electrodes.
(a) Cyclic voltammetric behavior (ν = 5 mV/s) of the VO2+/VO2+ reaction of UCC and HWO-C76 composites with different HWO:C76 ratios in 0.1 M VOSO4/1 M H2SO4 + 1 M HCl electrolyte. (b) Randles-Sevchik and (c) Nicholson VO2+/VO2+ method to evaluate diffusion efficiency and obtain k0(d) values.
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 chlorine evolution compared to C76, as shown in Fig. 6a, and also exhibits the Smaller Semicircle in fig. 6d (lower RCT). C76 showed a higher apparent Ipa/Ipc than HWO-50% C76 (Table S3), not because of improved reaction reversibility, but because of the peak overlap of the chlorine reduction reaction with SHE at 1.2 V. The best performance of HWO- The 50% C76 is attributed to the synergistic effect between the negatively charged highly conductive C76 and the high wettability and W-OH catalytic functionality on HWO. Less chlorine emission will improve the charging efficiency of the full cell, while improved kinetics will improve 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 presented 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). Since the diffusion coefficient is constant (≈ 4 × 10–6 cm2/s)52, the difference in the slope of the line directly indicates different values of α, and hence the electron transfer rate on the electrode surface, which is shown for C76 and HWO -50% C76 Steepest slope (highest electron transfer rate).
The Warburg slopes (W) calculated for the low frequencies shown in Table S3 (Fig. 6d) have values close to 1 for all materials, indicating perfect diffusion of redox species and confirming the linear behavior of IP compared to ν1/ 2. CV is measured. For HWO-50% C76, the Warburg slope deviates from 1 to 1.32, indicating not only semi-infinite diffusion of the reagent (VO2+), but also a possible contribution of thin-layer behavior to 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 using the S2 equation to construct the dimensionless kinetic parameter Ψ, which is a function of ΔEp, as a function of ν-1/2. Table S4 shows the Ψ values obtained for each electrode material. The results (Fig. 6c) were plotted to obtain k0 × 104 cm/s from the slope of each plot using Equation S3 (written next to each row and presented in Table S4). HWO-50% C76 was found to have the highest slope (Fig. 6c), thus the maximum value of k0 is 2.47 × 10–4 cm/s. This means that this electrode achieves the fastest kinetics, which is consistent with the CV and EIS results in Fig. 6a and d and in Table S3. In addition, the value of k0 was also obtained from the Nyquist plot (Fig. 6d) of Equation S4 using the RCT value (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 k0 values differ due to the different origins of each method, they still show the same order of magnitude and show consistency.
To fully understand the excellent kinetics obtained, it is important to compare the optimal electrode materials with uncoated 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 measured by the current at 1.45 V relative to SHE (Fig. 7a). In terms of stability, we assumed that HWO-50% C76 was physically stable because the catalyst was mixed with a PVDF binder and then applied to the carbon cloth electrodes. HWO-50% C76 showed a peak shift of 44 mV (degradation rate 0.29 mV/cycle) after 150 cycles compared to 50 mV for UCC (Figure 7b). This may not be a big difference, but the kinetics of UCC electrodes is very slow and degrades with cycling, especially for reverse reactions. 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 formed on its surface. so that the catalyst adheres well to the electrode surface. As can be seen from all electrodes tested, even electrodes without supported catalysts showed varying degrees of cycling instability, suggesting that the change in peak separation during cycling is due to deactivation of the material caused by chemical changes rather than catalyst separation. In addition, if a large amount of catalyst particles were to be separated from the electrode surface, this would result in a significant increase in peak separation (not only 44 mV), since the substrate (UCC) is relatively inactive for the VO2+/VO2+ redox reaction.
Comparison of the CV of the best electrode material compared to UCC (a) and the stability of the VO2+/VO2+ redox reaction (b). ν = 5 mV/s for all CVs in 0.1 M VOSO4/1 M H2SO4 + 1 M HCl electrolyte.
To increase the economic attractiveness of VRFB technology, expanding and understanding the kinetics of vanadium redox reactions is essential to achieve high energy efficiency. Composites HWO-C76 were prepared and their electrocatalytic effect on the VO2+/VO2+ reaction was studied. HWO showed little kinetic enhancement in mixed acidic electrolytes but significantly suppressed chlorine evolution. Various ratios of HWO:C76 were used to further optimize the kinetics of HWO-based electrodes. Increasing C76 to HWO improves the electron transfer kinetics of the VO2+/VO2+ reaction on the modified electrode, of which HWO-50% C76 is the best material because it reduces charge transfer resistance and further suppresses chlorine compared to C76 and TCC deposit. . This is due to the synergistic effect between C=C sp2 hybridization, OH and W-OH functional groups. The degradation rate after repeated cycling of HWO-50% C76 was found to be 0.29 mV/cycle, while the degradation rate of UCC and TCC is 0.33 mV/cycle and 0.49 mV/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 increasing the energy efficiency of the VRFB, thus 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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