Enhancement of Vanadium Redox Flow Battery Performance with Nitrogen-Functionalized Graphite Felt Electrodes Etched by K2 FeO4

Enhancement of Vanadium Redox Flow Battery Performance with Nitrogen-Functionalized Graphite Felt Electrodes Etched by K2 FeO4 | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Enhancement of Vanadium Redox Flow Battery Performance with Nitrogen-Functionalized Graphite Felt Electrodes Etched by K2 FeO4 Hongwei Li, Huina Wang, Yueyang Xie, Yukun Wang, Guanghong Yan, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-2674166/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 7 You are reading this latest preprint version Abstract Doping with oxygen and nitrogen in graphite felt (GF) is critical for enhancing the activity of the electrode material in vanadium redox flow batteries (VRFB). In this paper, we present a combined approach that utilizes Fe etching and nitrogen doping by means of K 2 FeO 4 and NH 3 to modify the surface structure of graphite fibers. The results show that the innovative approach enhances the disordered structure of the surface carbon of GF and substantially improves the oxygen and nitrogen functionalized groups. This modified GF is completely hydrophilic, and its assembled electrode energy efficiency is 80.08% at a current density of 80 mA∙cm − 2 , compared with 69.87% for the pristine GF. The energy efficiency of the modified GF was maintained at 81.8% after 50 charge-discharge cycles. This can be attributed to the reduced internal resistance of these modified GF electrode as well as to the improved mass transport and charge redox exchange towards VO 2+ /VO 2 + redox couple. The approach of combined Fe etching and nitrogen doping is a simple and effective technique that significantly boosts the performance of VRFB. Graphite Felt Vanadium redox flow battery Etching Nitrogen doping Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Introduction With regards to large-scale energy storage solutions, Vanadium redox flow battery (VRFB) are considered an attractive alternative due to their versatile design, long operational life, and exceptional safety, as well as their high level of reliability and thepotential for high decouple power output and energy capacity [ 1 – 5 ] . The concept and technology behind VRFB were first introduced by Skyllas-Kazacos et al. in the 1980s [ 6 ] . Its uniqueness lies in a VRFB consists solely of varying oxidation states of vanadium ions in both the positive and negative electrolyte solutions, which assists in minimizing cross-contamination during operation. However, the charging-discharging efficiency of VRFB primarily relies on the performance of its electrode. Because it is through these materials that the redox reaction occurs during the flow of vanadium ions [ 7 ] . As a result, developing high-performing electrodes has become one of the most significant subjects in this field. Throughout the history of VRFB electrode material development, numerous materials have been investigated, including metal electrodes, graphite electrodes, carbon electrodes, organic composite electrodes, among others [ 8 , 9 ] . However, graphite felt has emerged as a highly valued material due to its distinct properties and characteristics, such as its affordability, high level of electron conductivity, remarkable cycle stability, and strong acid corrosion resistance [ 10 , 11 ] . Despite advantages, GF electrodes have a low specific surface area, poor dynamic reversibility, and low hydrophilicity, which limit VRFB's energy efficiency and power density [ 12 , 13 ] . In order to address these challenges, various approaches have been employed to optimize the GF electrode's performance. One such method involves depositing metals (Pt [ 14 ] , Sn [ 15 ] , etc.) or metal oxides (MoO 2 [ 16 ] , NiO [ 17 ] , ZrO 2 [ 18 ] , etc.) on the surface of the GF electrode, with the aim of enhancing its catalytic activity. However, this approach can be costly, and the use of precious metals may lead to impurities in the electrolyte solution over extended periods of operation. In recent years, the utilization of carbon-based electrocatalysts for electrode material functionalization has become increasingly popular, such as carbon nanotubes [ 19 ] , graphene oxides [ 20 ] , carbon dots [ 21 ] , carbon nanoparticles [ 22 ] and so on. This is mainly attributed to their affordability, high electronic conductivity, and stability in acidic conditions. However, maintaining the stability of these electrocatalysts on the GF surface during the extended period of electrolyte flow in the VRFB poses a challenge. The application of the "less is more" principle to the design of GF surface structures may offer a more advantageous approach to enhancing their electrode performance in practical applications. Activated GF has demonstrated superior stability in VRFB applications and etching treatments are influential in introducing defects and oxygen-containing functional groups [ 23 – 25 ] . Additionally, incorporating nonmetal elements, especially the doping of c-adjacent elements, for example, N [ 26 ] , S [ 27 ] , P [ 28 ] , and B [ 29 ] , contributes to optimizing the surface structure of GF. As a result, the charge transfer between vanadium ions and electrodes could be facilitated. In this work, we present a simple process for synthesizing oxygen-nitrogen-functionalized graphite felt combined Fe etching and nitrogen doping, which significantly enhances the performance of VRFB. The process of Fe 2 KO 4 etching not only provides a considerable number of defects and introduces oxygen species to the graphite fibers' surface, but also stimulates structural metamorphosis in the carbon framework, thereby expediting the formation of copious nitrogen doping under comparatively milder conditions at the same time. Consequently, the graphite felt acquires a large number of active sites, which leads to an improvement in its electrochemical properties. This results in better conductivity, chemical stability, and electrocatalytic activity, making oxygen-nitrogen-functionalized graphite felt an excellent electrode material for VRFB. Experiments Preparation of modified GF electrode The preparation of EGF(N) electrolyte was conducted via hydrothermal treatment and heat treatment process. The thermal-activated graphite felt was first subjected to pre-oxidation at 450°C for 6 h. Subsequently, the treated GF was immersed in a 0.1mol/L Fe 2 KO 4 (Aladdin, 98.0%) solution sealed in an autoclave and hydrothermal treated at 180°C for 6 h. The resulting sample was designated as EGF. The sample was then subjected to a secondary treatment at 700°C for 2 h in a NH 3 flow. A HCl solution of 1 mol/L was then applied to the sample to remove residual iron and iron-based compounds, and the sample was then washed completely neutrally with deionized water. Following drying, a sample of EGF(N) was obtained. EGF(A), a sample obtained under similar conditions, was compared only by changing the airflow to Ar. The synthesis process can be schematically shown in Fig. 1. Physical characterization All GFs were examined by field-emission scanning electron microscopy (S-4800). Raman spectra of GFs (after treated) were measured using a laser confocal Raman spectrometer with the range from 500 to 3500 cm -1 (DXR2). Goniometry was employed to determine the contact angle of water on the surface of the felts (JC2000D1). The elemental composition of GFs was determined by X-ray photoelectron spectroscopy (ESCALAB 250Xiic). The binding energy calibration was based on the C1 peak (284.8 eV). Electrochemical characterization For both CV and EIS analysis, the electrochemical measurements were conducted through an electrochemical workstation with a three-electrode system (CS310). The working electrode was an EGF(N) sample (10×20 mm 2 ), while a Hg/HgO electrode and Pt foil were used as reference and counter electrodes, respectively. The electrochemical performance of the positive electrode was investigated using a solution of 1.0 M VO 2+ and 2 M H 2 SO 4 . The voltage range for cyclic voltammetry was set at 0.8 ~ 1.2 V. The electrochemical impedance spectroscopy (EIS) measurements were carried out in a frequency ranging from 10 -2 to 10 5 Hz. In addition, a charge and discharge test is performed on a battery testing system (CT2001). The electrode, electrode frame, graphite bipolar plate, copper collector plate, and end plate are distributed symmetrically on both sides of the ion exchange membrane (Nafion 117). In this study, the GF electrodes had an active area of 30×30 mm 2 and a compression ratio of 25% in thickness. The initial electrolyte concentration of both positive and negative electrodes was 1.7 M V 3.5+ and 4.2 M H 2 SO 4 aqueous solution, and the electrolyte flowed through the battery through the double-channel peristaltic pump at a constant flow rate of 50 mL∙min -1 . The battery was charged and discharged at a current density of 80 mA∙cm ­­-2 . Cut-off voltages were 0.8 V and 1.65 V, respectively. Results And Discussion Physicochemical Characterization Figure 2 demonstrates SEM images of the pristine GF, EGF(A), and EGF(N) before and after treatment, illustrating their distinct morphological characteristics. Pristine GF fibers exhibit a smooth and almost defect-free surface prior to treatment, as shown in Fig. 2 (a) and (b). After hydrothermal treatment, the K 2 FeO 4 is decomposed, and the equation is as follows [ 5 , 25 , 30 ] : \({\text{K}}_{\text{2}}\text{Fe}{\text{O}}_{\text{4}}\text{+}{\text{H}}_{\text{2}}\text{O→}{\text{Fe}\left(\text{OH}\right)}_{\text{3}}\text{+KOH}\) \(\) (1) $${\text{Fe}\left(\text{OH}\right)}_{\text{3}}\text{→}{\text{Fe}}_{\text{2}}{\text{O}}_{\text{3}}\text{+}{\text{H}}_{\text{2}}\text{O}$$ 2 On one hand, the activation of the graphite felt surface occurs through exposure to a hydrothermal alkaline environment, while on the other hand, surface defects arise due to the iron-carbon reaction at elevated temperatures. Upon heating in Ar, the GF surface became rough and uneven inner surface with deeper grooves and crevices, as shown in Fig. 2 (c) and (d), which attribute to the iron-carbon reaction. When the heat treatment is carried out in NH 3 atmosphere, the fiber's surface displays even deeper grooves and crevices, along with etched fibers attached to the surface, resulting in a unique morphology, as illustrated in Fig. 2 (e) and (f). It is possible that the surface of the fiber becomes looser and more conducive to atom doping after hydrothermal treatment. To assess the structural evolution during carbonization, we conducted an analysis of Raman spectroscopy. Figure 3 shows that GF, EGF(N), and EGF(A) presented two peaks at roughly 1350 cm − 1 (D band) and 1590 cm − 1 (G band). The intensity of the D band in Raman spectroscopy is commonly associated with defects and disorder of carbon atoms in a given sample. Conversely, the intensity of the G peak typically represents the in-plane vibration of sp 2 carbon atoms of graphitized carbon [ 31 ] . The I D /I G (intensity of D band and G band) is an indicator of their structural defects It is noteworthy that while the I D /I G of pristine GF is 1.09, EGF(N) and EGF(A) after treatment showed higher I D /I G of 1.35 and 1.26. Consequently, the etching of Fe 2 KO 4 results in an increase in structural disorder. Moreover, EGF(N) exhibited a higher I D /I G than EGF(A). It can be explained NH 3 treatment led to an increase in I D /I G , suggesting an increase in surface roughness and defects [ 32 ] . Additionally, the surface functional groups of carbon fibers also affect the performance of VRFB. The XPS analysis was conducted in order to examine the differences in the chemical bonds between the samples and their composition of elements [ 33 ] . The broad XPS spectrum (Fig. 4 ) clearly shows that K 2 FeO 4 treatment significantly reduces the intensity of the C1(284.8 eV) signal in graphite fibers as compared to pristine graphite fibers. This is attributed to the loss of carbon quality caused by the etching effect, which breaks the surface integrity of the graphite fibers [ 34 ] . As well, the O1s (532eV) signal in EGF(A) and EGF(N) is quite clear in comparison to GF. The spectra of O1s (Fig. 5 a, b, c) reveal several oxygen functional groups: 531.5(O1), 532.5(O2), 533.6(O3), and 534.9(O4) eV, respectively, which correspond to the C = O bond, the C-OH bond, the OH bond, and the H-OH bond. Table I presents the results of analyses of the major surface elements. It is evident that the oxygen content of the treated EGF(N) (24.84%) and EGF(A) (25.37%) are both significantly higher than that of the pristine GF (5.29%). As a result, the oxygen atomic contents of the C-OH and C = O groups increased by 23.7% and 24.3%, respectively. However, the abundant oxygen functional groups have a direct impact on the formation of vanadium redox reactive sites, even though they are conducive to electrolyte transport [ 35 ] . The presence of N1s (401.3 eV) was also detected in sample EGF(N), confirming the successful doping of nitrogenous groups by heat treatment in an NH 3 atmosphere. This is close to the nitrogen content of the pristine GF treated at 900°C [ 32 ] , we deduce that it is because the atoms in the surface layer become confused after hydrothermal treatment, which is more favorable to nitrogen doping. According to Table 1 , EGF(N) (4.83%) had a higher N content than EGF(A) (1.71%) and GF (1.32%), which was attributed to the NH 3 atmosphere at elevated temperatures. N1s high-resolution spectra reveal those functional groups, which correspond to pyridine nitrogen, quaternary nitrogen, pyridine nitrogen and oxide nitrogen, respectively. Figure 5 .d illustrates the peak fitting of N1s for EGF(N). During heat treatment, nitrogen is introduced to the surface of the graphite felt and nitrogen-containing groups are formed. EGF(N) contains a lower percentage of N1 (41.19%) than N2 (48.12%). In this case, the hydrothermal reaction and the loose surface resulting at high temperature in more substitution of N to C may be the cause. N contributes to the stability of electronegativity sites by providing them with electrons, and it is an essential component of redox reactions. Table 1 XPS analyses of the primary elements present on the surface of GF with oxygen-nitrogen-functionalized groups Content(at%) Elements Peak BE(eV) GF EGF(A) EGF(N) C1s 284.8 93.39 72.92 70.33 O1s 532.5 5.29 25.37 24.84 N1s 401.2 1.32 1.71 4.83 More oxygen-nitrogen-functionalized groups will lead to a better wettability that is good for sufficient contact between the electrode and the electrolyte. To prove the speculation, GF electrode wettability was investigated using a contact angle instrument. Figure.6 shows that GF, EGF(A), and EGF(N) have contact angles of 131.5°, 0°, and 0°, respectively. The electrode surface changes from hydrophobic to hydrophilic as a result of the increase in surface defects and the increased oxygen concentration after heat treatment. In order to investigate the accessibility of vanadium ions in the electrolyte, electrodes were immersed in a vanadium electrolyte (0.1 M VOSO 4 + 2 M H 2 SO 4 ). GF was floating on the electrolyte due to the hydrophobic properties of the conductive electrolyte. EGF(A) and EGF(N) are deposited at the bottom of the electrolyte, indicating that they are more electrolyte accessible than pristine GF. Electrochemical Characterizations The electrochemical properties of the EGF(N) electrodes were determined by cyclic voltammetry, where the electrode's electrochemical activity is reflected by its redox initiation potential and current density (IP). On the positive side (Fig. 7 a), EGF(N) demonstrated a much higher peak current (308.7mA∙cm − 2 ) compared to GF(A) (246.3mA∙cm − 2 ) and GF (231.4mA∙cm − 2 ). Additionally, EGF(N) (-329.1mA∙cm − 2 ) and EGF(A) (-259.8mA∙cm − 2 ) showed a substantial reduction peak, whereas no peak was observed in the pristine GF. As compared to other samples, EGF(N) exhibits the highest peak redox current, which is consistent with the results of the SEM and XPS tests. A further improvement in electrochemical properties could be attributed to the addition of N atoms. The increased peak currents indicate the improvement in the reaction kinetics of VO 2+ /VO 2 + in the presence of oxygen-containing and nitrogen-containing functional groups on GF surface. The peak potential separation(ΔEp) values were also reduced, EGF(N) shows the lowest ΔEp of 0.39V, while the pristine is 0.61V, further implying the best reversibility toward vanadium redox reaction and a reduction in the polarization of the redox. The CV curves of the GF and EGF(N) for VO 2+ /VO 2 + redox reactions are also shown in Fig. 7 b, c. All the peaks are visible in the EGF(N), but not in the GF. As shown in the Fig. 7 d, a linear relationship exists between peak current density and the square root of scan rate, which suggests that reactions are primarily governed by transport processes [ 36 ] . The improvement of the properties is due to the introduction of oxygen and nitrogen groups, which can effectively change the electronegativity of the pristine graphite felt and enhance the attraction of vanadium ions to the electrode surface [ 37 ] . The resistances of GF, EGF(A), and EGF(N) were determined through electrochemical impedance spectroscopy (EIS). The Nyquist plot can be divided into two parts, semicircular part and a linear part. In the high-frequency region, indicating that the reaction at the electrode is mainly controlled by charge transfer and mass transfer [ 12 ] . Figure 8 illustrates an equivalent circuit for fitting the Nyquist plot. The Rs is solution resistance related to electrolyte, Rct is charge transfer resistance at the interface between electrode and electrolyte, Q is double-layer capacitance at the interface between electrode and electrolyte, and Warburg impedance W is related to diffusion of vanadium ions [ 38 – 40 ] . The pristine GF has a large Rct value (6.18Ω) for its hydrophobic surface and poor electrochemical activity. The results of EGF(A) and EGF(N) imply that the electron transfer resistance decreases after treatment with Fe 2 KO 4 . However, EGF(N) had the lowest Rct (0.64Ω), which indicated a significant enhancement in catalytic activity toward VO 2+ /VO 2 + reaction by N doping. The introduction of oxygen-containing and nitrogen-containing functional groups significantly reduced the mass and charge transfer resistance. Vrfb Single Cell Performance The electrochemical activity feature of the EGF(N) electrode was investigated by examining the charging/discharging performance of an assembled VRFB single cell. In contrast to the GF and EGF(A) electrodes, EGF(N) electrode shows a lower charging plateau and a higher discharging plateau, as shown in Fig. 9 a. The reason is ascribed to the reduction in electrochemical polarization in the presence of ECF(N) electrodes with improved electrochemical activity and enhanced reaction reversibility [ 41 ] . Figure.9b summarizes the EE values of the samples at different current densities. Obviously, EGF(N) batteries have always had the highest EE. As the current density increases, the potential decreases with the increase of polarization, resulting in high overpotential and side reaction [ 42 ] . Compared with other samples, the initial EE of EGF(N) was 81.48%, and when the current density was restored to 80 again, the EE of the battery still remained above 80%, which proved its high operational stability. On one hand, EGF(N) improves the catalytic activity of the electrode, and enhances the mass transfer performance of the electrode, thus the performance of the battery at high current density is also improved Cycling stability test. The VRFB cells with pristine GF, EGF(A), and EGF(N) were charged and discharged at 80 mA∙cm − 2 for 50 cycles to investigate the discharge capacities of different electrodes in Fig. 10 a. The diffusion of vanadium ions leads to the decrease of discharge capacity with the increase of cycle number [ 43 ] . As the cycle numbers increased, the discharge capacities of cells with pristine GF, EGF(A) and EGF(N) decreased gradually. A cell with an EGF(N) electrode has superior initial discharge capacity not only than a cell with GF or EGF(A), but it also experiences a slower reduction in capacity with increasing numbers of charge/discharge cycles as compared with a cell with pristine GF and EGF. After undergoing 50 cycles of charge and discharge, the discharge capacity of cells using EGF(N) was still 98.9 mA∙h, higher than that of cells using GF electrode (38.5 mA∙h) and cells using EGF(A) electrode (66.2 mA∙h). The EGF(N) electrode leads to the slowest decay in the discharge capacity of the battery. The cell with EGF(N) exhibited a significantly higher capacity retention rate of 81.1% compared to the cell with pristine GF of 48.4% and the cell with EGF(A) of 66.9%, indicating a relatively high cycling stability. Energy efficiency (EE), coulomb efficiency (CE), and voltage efficiency (VE) are shown in Fig. 10 b. CE is determined by its own discharge and side effects. In the pristine cell, the CE was higher than in the EGF(N) cell, due to the shorter charge and discharge times, as well as the lower penetration of active material. the current efficiency of all samples was higher than 95%, which indicated that the assembled cell had good tightness [ 4 ] . The voltage efficiency (VE) of the cell is also a key factor for evaluating the VRFB single cell as shown in Fig. 10 b. The VE of the cell built with EGF(N) electrode (89.3%) was higher than that of the cell built with CF electrode (79.6%), indicating enhanced charge-discharge capability. Owing to the increase in voltage efficiency, the cell with EGF(N) shows a higher energy efficiency of 80.08% than that of the cell with pristine GF (69.87%), which may be owing to the significantly reduced mass transfer and charge transfer resistances resulting from the introduced oxygen and nitrogen containing functional groups [ 44 ] . The cell assembled with EGF(N) shows a higher energy efficiency and almost no decay in energy efficiency after 50 cycles, demonstrating an excellent VRFB performance. However, the energy efficiency for cell with pristine GF decays from 69.87–65.01% after 50 cycles. The improvement of energy efficiency stability of single cell is attributed to the decrease of electrochemical polarization during charging and discharging [ 45 ] . Hence, this result highlights the superior electrochemical performance of the cell with EGF(N), indicating its potential as a promising candidate for high-performance energy storage applications. But the utilization of electrolyte is still limited, further efforts should be made to improve the battery capacity and the utilization rate of electrolyte in the future. Conclusions In summary, through the combination of a facile process involving K 2 FeO 4 etching and nitrogen doping at 700°C, a significant quantity of oxygen-nitrogen functionalized surface structures was successfully synthesized on the surface of commercially available GF. CV and EIS assessments reveal that the EGF(N) exhibits a high level of electrochemical activity towards the VO 2+ /VO 2 + redox reaction. The cell employing EGF(N) had an average energy efficiency of 80.08% at a current density of 80 mA∙cm − 2 , compared to pristine GF's 69.87%. The charge-discharge tests performed on EGF(N) demonstrated a remarkable retention of energy efficiency, indicating the suitability of EGF(N) for prolonged operation in energy storage applications. The exceptional electrochemical characteristics exhibited by EGF(N) can be attributed to enhanced electrical conductivity, increased availability of active sites, and improved wettability, facilitated by the activated surface and the introduction of oxygen-nitrogen-functionalized groups onto the surface of GF. Declarations Ethics approval This article does not contain any studies involving humans and animals performed by any of the authors. Competing interests The authors declared that they have no conflicts of interest to this work. Authors' contributions All the authors contributed to the concept and design of the study. HL contributed to reviewing and editing manuscripts. HW contributed to processing data and writing manuscript. YX contributed to methodology and writing original draft. YW contributed to data curation and writing manuscript. GY, FX and BW contributed to data analysis and revising manuscript. Funding: This work was supported by the Fundamental Research Funds for the Central Universities from Chang'an University (No.300102310110 and No.300102311403), College Students' Innovative Entrepreneurial Training Plan Program（No.X202210710376） Availability of data and material All data generated or analyzed during this study are included in this published article. References M. Ulaganathan, V. Aravindan, Q. Yan, et al., (2016) Recent Advancements in All‐Vanadium Redox Flow Batteries. J Advanced Materials Interfaces. 31, 1-22 https://doi.org/10.1002/admi.201500309 M. Park, J. Ryu, W. Wang, et al., (2016) Material design and engineering of next-generation flow-battery technologies. J Nature Reviews Materials. 21, 1-18 https://doi.org/10.1038/natrevmats.2016.80 L. Zhao, Q. Ma, Q. Xu, et al., (2021) Performance improvement of non-aqueous iron-vanadium flow battery using chromium oxide–modified graphite felt electrode. 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Shen, et al., (2015) Effects of nitrogen doping on the electrochemical performance of graphite felts for vanadium redox flow batteries. J International Journal of Energy Research. 395, 709-716 https://doi.org/10.1002/er.3291 B. Li, J. Xue, C. Han, et al., (2021) A hafnium oxide-coated dendrite-free zinc anode for rechargeable aqueous zinc-ion batteries. J J. Colloid Interface Sci. 599, 467-475 https://doi.org/10.1016/j.jcis.2021.04.113 L. Zhang, J. Yue, Q. Deng, et al., (2020) Preparation of a porous graphite felt electrode for advance vanadium redox flow batteries. J RSC Advances. 1023, 13374-13378 https://doi.org/10.1039/d0ra00666a L. M. Mohsen, Z.-J. Mohammad, N. Zeinab, et al., (2022) Antimony-decorated graphite felt electrode of vanadium redox flow battery in mixed-acid electrolyte: Promoting electrocatalytic and gas-evolution inhibitory properties. J J. Electroanal. Chem. 908, 116090-116102 https://doi.org/10.1016/J.JELECHEM.2022.116090 H. R. Jiang, W. Shyy, L. Zeng, et al., (2018) Highly efficient and ultra-stable boron-doped graphite felt electrodes for vanadium redox flow batteries. J Journal of Materials Chemistry A. 627, 13244-13253 https://doi.org/10.1039/c8ta03388a Q. Ma, C. Mao, W. Fu, et al., (2022) Numerical study of deep eutectic solvent electrolyte‐based vanadium‐iron redox flow battery with three‐dimensional multi‐layer porous electrode. J International journal of energy research. 469, 12820-12836 https://doi.org/10.1002/er.8055 M. Sheeraz, M. Asad, L. J. Young, et al., (2017) Excellent electrocatalytic effects of tin through in situ electrodeposition on the performance of all-vanadium redox flow batteries. J Journal of Materials Chemistry A. 533, 17388-17400 https://doi.org/10.1039/c7ta05657e G. P. C., B. Arjun, L. T. M., et al., (2021) In-Situ Tools Used in Vanadium Redox Flow Battery Research—Review. J Batteries. 73, 53-89 https://doi.org/10.3390/batteries7030053 S. A. Razali, Rusi and S. R. Majid, (2019) Fabrication of polyaniline nanorods on electro-etched carbon cloth and its electrochemical activities as electrode materials. J Ionics. 256, 2575-2584 https://doi.org/10.1007/s11581-018-2809-7 J. Dai, X. Teng, Y. Song, et al., (2017) Effect of casting solvent and annealing temperature on recast Nafion membranes for vanadium redox flow battery. J J. Membr. Sci. 522, 56-67 https://doi.org/10.1016/j.memsci.2016.09.014 Y. Yang, Y. Zhang, T. Liu, et al., (2019) Improved broad temperature adaptability and energy density of vanadium redox flow battery based on sulfate-chloride mixed acid by optimizing the concentration of electrolyte. J J. Power Sources. 415, 62-68 https://doi.org/10.1016/j.jpowsour.2019.01.049 Y. Huang, L. Peng, Y. Liu, et al., (2016) Biobased Nano Porous Active Carbon Fibers for High-Performance Supercapacitors. J ACS Appl Mater Interfaces. 824, 15205-15215 https://doi.org/10.1021/acsami.6b02214 J. Jutao, F. Xiaogang, L. Qiao, et al., (2013) Identifying the active site in nitrogen-doped graphene for the VO 2+ /VO 2 + redox reaction. J ACS nano. 76, 4764-4773 https://doi.org/10.1021/nn3046709 M. M. G., (2017) Energy Storage and Power Electronics Technologies: A Strong Combination to Empower the Transformation to the Smart Grid. J Proceedings of the IEEE. 10511, 2191-2219 https://doi.org/10.1109/JPROC.2017.2702627 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Major revision 20 Apr, 2023 Reviews received at journal 18 Mar, 2023 Reviewers agreed at journal 13 Mar, 2023 Reviewers invited by journal 13 Mar, 2023 Submission checks completed at journal 13 Mar, 2023 Editor assigned by journal 13 Mar, 2023 First submitted to journal 09 Mar, 2023 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-2674166","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":182948262,"identity":"38a4e573-2184-4605-9e33-1dcb097c9b77","order_by":0,"name":"Hongwei Li","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAp0lEQVRIiWNgGAWjYBACPmYeBoYPDAcYGxgYDIjTwgbUwjiDNC0MPAxAXSRpYec9Jm1Tc0e2gb15mwRDzR1iHMaXJp1z7JlxA8+xMgmGY8+I0cJjdju34XBig0SOmQRjw2EitViCtMi/IUULI9gWHuK1mP/sOXbYuI0nrdgi4RgRWvj5zxgb/Kg5LNvPfnjjjQ81RGhBWAciEkjQMApGwSgYBaMADwAAwxAzmr9xrDgAAAAASUVORK5CYII=","orcid":"","institution":"Chang'an University","correspondingAuthor":true,"prefix":"","firstName":"Hongwei","middleName":"","lastName":"Li","suffix":""},{"id":182948265,"identity":"8b7a5f7c-3697-4761-89b0-668933f0c4dd","order_by":1,"name":"Huina Wang","email":"","orcid":"","institution":"Chang'an University","correspondingAuthor":false,"prefix":"","firstName":"Huina","middleName":"","lastName":"Wang","suffix":""},{"id":182948267,"identity":"24607084-14a2-4a58-88dd-12807b82f64f","order_by":2,"name":"Yueyang Xie","email":"","orcid":"","institution":"Chang'an University","correspondingAuthor":false,"prefix":"","firstName":"Yueyang","middleName":"","lastName":"Xie","suffix":""},{"id":182948269,"identity":"6bafd763-d96f-4446-a3a4-3b7e0a19938d","order_by":3,"name":"Yukun Wang","email":"","orcid":"","institution":"Chang'an University","correspondingAuthor":false,"prefix":"","firstName":"Yukun","middleName":"","lastName":"Wang","suffix":""},{"id":182948270,"identity":"4930f00f-e4c1-4576-8f3c-f7ce6d970d58","order_by":4,"name":"Guanghong Yan","email":"","orcid":"","institution":"Chang'an University","correspondingAuthor":false,"prefix":"","firstName":"Guanghong","middleName":"","lastName":"Yan","suffix":""},{"id":182948272,"identity":"54bd7598-9217-4d5e-a25e-e693fe1dc903","order_by":5,"name":"Bin Wang","email":"","orcid":"","institution":"Chang'an University","correspondingAuthor":false,"prefix":"","firstName":"Bin","middleName":"","lastName":"Wang","suffix":""},{"id":182948274,"identity":"b2d7fb3f-25b6-4d03-961d-41c09c5851a5","order_by":6,"name":"Fei Xue","email":"","orcid":"","institution":"Chang'an University","correspondingAuthor":false,"prefix":"","firstName":"Fei","middleName":"","lastName":"Xue","suffix":""}],"badges":[],"createdAt":"2023-03-09 14:14:29","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-2674166/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-2674166/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":34380912,"identity":"b675e87d-9f06-40df-bdbc-9843d56d2c5c","added_by":"auto","created_at":"2023-03-16 20:57:54","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":189298,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of a graphite felt electrode modified with oxygen- nitrogen-functionalized groups\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-2674166/v1/f479ad386d76e99f0db4f84d.png"},{"id":34379008,"identity":"e3dd72a9-d7f9-461e-9b46-b7034ba483eb","added_by":"auto","created_at":"2023-03-16 20:41:55","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1498311,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of (a, b) the pristine GF, (c, d) EGF(A), (e, f) EGF(N)\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-2674166/v1/041b86420b804f6469d70937.png"},{"id":34377954,"identity":"ed88d975-11d6-4050-89c1-2840d9f41f40","added_by":"auto","created_at":"2023-03-16 20:33:54","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":31831,"visible":true,"origin":"","legend":"\u003cp\u003eRaman spectra of pristine GF, EGF(A) and EGF(N)\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-2674166/v1/67cdcb841f3c8481802d95d7.png"},{"id":34377957,"identity":"6f71aec5-f918-4221-8b17-a1606107a160","added_by":"auto","created_at":"2023-03-16 20:33:54","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":24227,"visible":true,"origin":"","legend":"\u003cp\u003eXPS spectra of pristine GF, EGF(A) and EGF(N)\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-2674166/v1/1561e5925dacd89d129a064b.png"},{"id":34379710,"identity":"ab9ad800-ebcd-4260-9cc8-1df3be3c6e24","added_by":"auto","created_at":"2023-03-16 20:49:54","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":273081,"visible":true,"origin":"","legend":"\u003cp\u003eXPS Element content (a) O1s of pristine GF, (b) O1s of EGF(A), (c) O1s of EGF(N), (d) N1s and fitting results of EGF(N)\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-2674166/v1/e88f6bcd3b77089d9c565e98.png"},{"id":34379714,"identity":"be38a646-ddfb-479c-9295-cdab3515c362","added_by":"auto","created_at":"2023-03-16 20:49:55","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":122598,"visible":true,"origin":"","legend":"\u003cp\u003eThe contact angle of (a) the pristine GF, (b) EGF(A), (c) EGF(N)\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-2674166/v1/d70c97806ca507737d1e43fb.png"},{"id":34379713,"identity":"ff59ca9f-b218-41c1-9fcb-450ed6d67233","added_by":"auto","created_at":"2023-03-16 20:49:54","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":265939,"visible":true,"origin":"","legend":"\u003cp\u003eCyclic voltammetry curves in 1.0 M VO\u003csup\u003e2+\u003c/sup\u003e + 2.0 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e (a) CV for GF, EGF(A) and EGF(N) at 10 mV/s, (b) CV for GF at various scan rates, (c) CV for EGF(N) at various scan rates, (d) comparison of peak current densities for GF and EGF(N)\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-2674166/v1/8db9839c0751ff92aca1a050.png"},{"id":34379006,"identity":"90d6ff58-7da7-437e-a269-50aaab9a24e8","added_by":"auto","created_at":"2023-03-16 20:41:54","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":28560,"visible":true,"origin":"","legend":"\u003cp\u003eNyquist plots of GF and EGF(N) samples in 1.0 M VO\u003csup\u003e2+\u003c/sup\u003e + 2 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrolyte towards VO\u003csup\u003e2+\u003c/sup\u003e/VO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e redox reaction.\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-2674166/v1/cbc8f9d1dbe1b2fe9019ae84.png"},{"id":34377961,"identity":"38e94669-5dfd-4d09-ad9f-0066f0923a48","added_by":"auto","created_at":"2023-03-16 20:33:54","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":106537,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Charge-discharge curves at the current densities of 80 mA∙cm\u003csup\u003e-2\u003c/sup\u003e for VRFBs with GF, EGF(A) and EGF(N) electrodes, (b) efficiency of the VRFBs with GF, EGF(A) and EGF(N) electrodes at various current densities\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-2674166/v1/9a3a9806a929ea8f9e1ec88d.png"},{"id":34379004,"identity":"59f9305b-2c0b-4ad6-ae61-8311c67e39f1","added_by":"auto","created_at":"2023-03-16 20:41:54","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":107464,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Discharge capacity of the VRFBs with GF, EGF(A) and EGF(N), (b) cycling performance of VRFB with GF and EGF(N) at a current density of 80 mA∙cm\u003csup\u003e-2\u003c/sup\u003e\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-2674166/v1/ad26ec4778471d80e001e2d2.png"},{"id":34381535,"identity":"55b05569-29ee-4736-ae18-2bd6eb607277","added_by":"auto","created_at":"2023-03-16 21:06:01","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2604163,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2674166/v1/c8cde781-5d1b-4b46-9460-da3513ed0914.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Enhancement of Vanadium Redox Flow Battery Performance with Nitrogen-Functionalized Graphite Felt Electrodes Etched by K2 FeO4","fulltext":[{"header":"Introduction","content":"\u003cp\u003eWith regards to large-scale energy storage solutions, Vanadium redox flow battery (VRFB) are considered an attractive alternative due to their versatile design, long operational life, and exceptional safety, as well as their high level of reliability and thepotential for high decouple power output and energy capacity\u003csup\u003e[\u003cspan additionalcitationids=\"CR2 CR3 CR4\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/sup\u003e. The concept and technology behind VRFB were first introduced by Skyllas-Kazacos et al. in the 1980s\u003csup\u003e[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e. Its uniqueness lies in a VRFB consists solely of varying oxidation states of vanadium ions in both the positive and negative electrolyte solutions, which assists in minimizing cross-contamination during operation. However, the charging-discharging efficiency of VRFB primarily relies on the performance of its electrode. Because it is through these materials that the redox reaction occurs during the flow of vanadium ions\u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e. As a result, developing high-performing electrodes has become one of the most significant subjects in this field.\u003c/p\u003e \u003cp\u003eThroughout the history of VRFB electrode material development, numerous materials have been investigated, including metal electrodes, graphite electrodes, carbon electrodes, organic composite electrodes, among others\u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e. However, graphite felt has emerged as a highly valued material due to its distinct properties and characteristics, such as its affordability, high level of electron conductivity, remarkable cycle stability, and strong acid corrosion resistance\u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e. Despite advantages, GF electrodes have a low specific surface area, poor dynamic reversibility, and low hydrophilicity, which limit VRFB's energy efficiency and power density\u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e. In order to address these challenges, various approaches have been employed to optimize the GF electrode's performance. One such method involves depositing metals (Pt\u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e, Sn\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e, etc.) or metal oxides (MoO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e, NiO\u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e, ZrO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e, etc.) on the surface of the GF electrode, with the aim of enhancing its catalytic activity. However, this approach can be costly, and the use of precious metals may lead to impurities in the electrolyte solution over extended periods of operation. In recent years, the utilization of carbon-based electrocatalysts for electrode material functionalization has become increasingly popular, such as carbon nanotubes\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e, graphene oxides\u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e, carbon dots\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e, carbon nanoparticles\u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e and so on. This is mainly attributed to their affordability, high electronic conductivity, and stability in acidic conditions. However, maintaining the stability of these electrocatalysts on the GF surface during the extended period of electrolyte flow in the VRFB poses a challenge.\u003c/p\u003e \u003cp\u003eThe application of the \"less is more\" principle to the design of GF surface structures may offer a more advantageous approach to enhancing their electrode performance in practical applications. Activated GF has demonstrated superior stability in VRFB applications and etching treatments are influential in introducing defects and oxygen-containing functional groups\u003csup\u003e[\u003cspan additionalcitationids=\"CR24\" citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e. Additionally, incorporating nonmetal elements, especially the doping of c-adjacent elements, for example, N\u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e, S\u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e, P\u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e, and B\u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e, contributes to optimizing the surface structure of GF. As a result, the charge transfer between vanadium ions and electrodes could be facilitated.\u003c/p\u003e \u003cp\u003eIn this work, we present a simple process for synthesizing oxygen-nitrogen-functionalized graphite felt combined Fe etching and nitrogen doping, which significantly enhances the performance of VRFB. The process of Fe\u003csub\u003e2\u003c/sub\u003eKO\u003csub\u003e4\u003c/sub\u003e etching not only provides a considerable number of defects and introduces oxygen species to the graphite fibers' surface, but also stimulates structural metamorphosis in the carbon framework, thereby expediting the formation of copious nitrogen doping under comparatively milder conditions at the same time. Consequently, the graphite felt acquires a large number of active sites, which leads to an improvement in its electrochemical properties. This results in better conductivity, chemical stability, and electrocatalytic activity, making oxygen-nitrogen-functionalized graphite felt an excellent electrode material for VRFB.\u003c/p\u003e "},{"header":"Experiments","content":"\u003cp\u003e\u003cstrong\u003ePreparation of modified GF electrode\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe preparation of EGF(N) electrolyte was conducted via hydrothermal treatment and\u0026nbsp;heat\u0026nbsp;treatment process. The thermal-activated graphite felt was first subjected to pre-oxidation at 450\u0026deg;C for 6 h. Subsequently, the treated GF was immersed in a 0.1mol/L Fe\u003csub\u003e2\u003c/sub\u003eKO\u003csub\u003e4\u003c/sub\u003e (Aladdin, 98.0%) solution sealed in an autoclave and hydrothermal treated at 180\u0026deg;C for 6 h. The resulting sample was designated as EGF. The sample was then subjected to a secondary treatment at 700\u0026deg;C for 2 h in a NH\u003csub\u003e3\u0026nbsp;\u003c/sub\u003eflow. A HCl solution of 1 mol/L was then applied to the sample to remove residual iron and iron-based compounds, and the sample was then washed completely neutrally with deionized water. Following drying, a sample of EGF(N) was obtained. EGF(A), a sample obtained under similar conditions, was compared only by changing the airflow to Ar. The synthesis process can be schematically shown in Fig. 1.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePhysical characterization\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll GFs were examined by field-emission scanning electron microscopy (S-4800). Raman spectra of GFs (after treated)\u0026nbsp;were measured using a laser confocal Raman spectrometer with the range from 500 to 3500 cm\u003csup\u003e-1\u0026nbsp;\u003c/sup\u003e(DXR2).\u0026nbsp;Goniometry was employed to determine the contact angle of water on the surface of the felts (JC2000D1). The elemental composition of GFs was determined by X-ray photoelectron spectroscopy (ESCALAB 250Xiic). The binding energy calibration was based on the C1 peak (284.8 eV).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElectrochemical characterization\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFor both CV and EIS analysis, the electrochemical measurements were conducted through an electrochemical workstation with a three-electrode system (CS310). The working electrode was an EGF(N) sample (10\u0026times;20 mm\u003csup\u003e2\u003c/sup\u003e), while a Hg/HgO electrode and Pt foil were used as reference and counter electrodes, respectively. The electrochemical performance of the positive electrode was investigated using a solution of 1.0 M VO\u003csup\u003e2+\u003c/sup\u003e and 2 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e. The voltage range for cyclic voltammetry was set at 0.8 ~ 1.2 V. The electrochemical impedance spectroscopy (EIS) measurements were carried out in a frequency ranging from 10\u003csup\u003e-2\u003c/sup\u003e to 10\u003csup\u003e5\u003c/sup\u003e Hz.\u003c/p\u003e\n\u003cp\u003eIn addition, a\u0026nbsp;charge and discharge test is performed on a battery testing system (CT2001). The electrode, electrode frame, graphite bipolar plate, copper collector plate, and end plate are distributed symmetrically on both sides of the ion exchange membrane (Nafion 117).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn this study, the GF electrodes had an active area of 30\u0026times;30 mm\u003csup\u003e2\u003c/sup\u003e and a compression ratio of 25% in thickness. The initial electrolyte concentration of both positive and negative electrodes was 1.7 M V\u003csup\u003e3.5+\u003c/sup\u003e and 4.2 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u0026nbsp;\u003c/sub\u003eaqueous solution, and the electrolyte flowed through the battery through the double-channel peristaltic pump at a constant flow rate of 50 mL∙min\u003csup\u003e-1\u003c/sup\u003e. The battery was charged and discharged at a current density of 80 mA∙cm\u003csup\u003e\u0026shy;\u0026shy;-2\u003c/sup\u003e. Cut-off voltages were 0.8 V and 1.65 V, respectively.\u003c/p\u003e"},{"header":"Results And Discussion","content":"\n\u003ch3\u003ePhysicochemical Characterization\u003c/h3\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e demonstrates SEM images of the pristine GF, EGF(A), and EGF(N) before and after treatment, illustrating their distinct morphological characteristics. Pristine GF fibers exhibit a smooth and almost defect-free surface prior to treatment, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a) and (b). After hydrothermal treatment, the K\u003csub\u003e2\u003c/sub\u003eFeO\u003csub\u003e4\u003c/sub\u003e is decomposed, and the equation is as follows\u003csup\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/sup\u003e:\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\({\\text{K}}_{\\text{2}}\\text{Fe}{\\text{O}}_{\\text{4}}\\text{+}{\\text{H}}_{\\text{2}}\\text{O\u0026rarr;}{\\text{Fe}\\left(\\text{OH}\\right)}_{\\text{3}}\\text{+KOH}\\)\u003c/span\u003e \u003c/span\u003e \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\)\u003c/span\u003e\u003c/span\u003e(1)\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$${\\text{Fe}\\left(\\text{OH}\\right)}_{\\text{3}}\\text{\u0026rarr;}{\\text{Fe}}_{\\text{2}}{\\text{O}}_{\\text{3}}\\text{+}{\\text{H}}_{\\text{2}}\\text{O}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eOn one hand, the activation of the graphite felt surface occurs through exposure to a hydrothermal alkaline environment, while on the other hand, surface defects arise due to the iron-carbon reaction at elevated temperatures. Upon heating in Ar, the GF surface became rough and uneven inner surface with deeper grooves and crevices, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(c) and (d), which attribute to the iron-carbon reaction. When the heat treatment is carried out in NH\u003csub\u003e3\u003c/sub\u003e atmosphere, the fiber's surface displays even deeper grooves and crevices, along with etched fibers attached to the surface, resulting in a unique morphology, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(e) and (f). It is possible that the surface of the fiber becomes looser and more conducive to atom doping after hydrothermal treatment.\u003c/p\u003e \u003cp\u003eTo assess the structural evolution during carbonization, we conducted an analysis of Raman spectroscopy. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows that GF, EGF(N), and EGF(A) presented two peaks at roughly 1350 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (D band) and 1590 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (G band). The intensity of the D band in Raman spectroscopy is commonly associated with defects and disorder of carbon atoms in a given sample. Conversely, the intensity of the G peak typically represents the in-plane vibration of sp\u003csup\u003e2\u003c/sup\u003e carbon atoms of graphitized carbon\u003csup\u003e[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/sup\u003e. The I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e (intensity of D band and G band) is an indicator of their structural defects It is noteworthy that while the I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e of pristine GF is 1.09, EGF(N) and EGF(A) after treatment showed higher I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e of 1.35 and 1.26. Consequently, the etching of Fe\u003csub\u003e2\u003c/sub\u003eKO\u003csub\u003e4\u003c/sub\u003e results in an increase in structural disorder. Moreover, EGF(N) exhibited a higher I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e than EGF(A). It can be explained NH\u003csub\u003e3\u003c/sub\u003e treatment led to an increase in I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e, suggesting an increase in surface roughness and defects\u003csup\u003e[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAdditionally, the surface functional groups of carbon fibers also affect the performance of VRFB. The XPS analysis was conducted in order to examine the differences in the chemical bonds between the samples and their composition of elements\u003csup\u003e[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]\u003c/sup\u003e. The broad XPS spectrum (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) clearly shows that K\u003csub\u003e2\u003c/sub\u003eFeO\u003csub\u003e4\u003c/sub\u003e treatment significantly reduces the intensity of the C1(284.8 eV) signal in graphite fibers as compared to pristine graphite fibers. This is attributed to the loss of carbon quality caused by the etching effect, which breaks the surface integrity of the graphite fibers\u003csup\u003e[\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]\u003c/sup\u003e. As well, the O1s (532eV) signal in EGF(A) and EGF(N) is quite clear in comparison to GF. The spectra of O1s (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, b, c) reveal several oxygen functional groups: 531.5(O1), 532.5(O2), 533.6(O3), and 534.9(O4) eV, respectively, which correspond to the C\u0026thinsp;=\u0026thinsp;O bond, the C-OH bond, the OH bond, and the H-OH bond. Table I presents the results of analyses of the major surface elements. It is evident that the oxygen content of the treated EGF(N) (24.84%) and EGF(A) (25.37%) are both significantly higher than that of the pristine GF (5.29%). As a result, the oxygen atomic contents of the C-OH and C\u0026thinsp;=\u0026thinsp;O groups increased by 23.7% and 24.3%, respectively. However, the abundant oxygen functional groups have a direct impact on the formation of vanadium redox reactive sites, even though they are conducive to electrolyte transport\u003csup\u003e[\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]\u003c/sup\u003e. The presence of N1s (401.3 eV) was also detected in sample EGF(N), confirming the successful doping of nitrogenous groups by heat treatment in an NH\u003csub\u003e3\u003c/sub\u003e atmosphere. This is close to the nitrogen content of the pristine GF treated at 900\u0026deg;C\u003csup\u003e[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e, we deduce that it is because the atoms in the surface layer become confused after hydrothermal treatment, which is more favorable to nitrogen doping. According to Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, EGF(N) (4.83%) had a higher N content than EGF(A) (1.71%) and GF (1.32%), which was attributed to the NH\u003csub\u003e3\u003c/sub\u003e atmosphere at elevated temperatures. N1s high-resolution spectra reveal those functional groups, which correspond to pyridine nitrogen, quaternary nitrogen, pyridine nitrogen and oxide nitrogen, respectively. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e.d illustrates the peak fitting of N1s for EGF(N). During heat treatment, nitrogen is introduced to the surface of the graphite felt and nitrogen-containing groups are formed. EGF(N) contains a lower percentage of N1 (41.19%) than N2 (48.12%). In this case, the hydrothermal reaction and the loose surface resulting at high temperature in more substitution of N to C may be the cause. N contributes to the stability of electronegativity sites by providing them with electrons, and it is an essential component of redox reactions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eXPS analyses of the primary elements present on the surface of GF with oxygen-nitrogen-functionalized groups\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c5\" namest=\"c3\"\u003e \u003cp\u003eContent(at%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eElements\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003ePeak BE(eV)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eGF\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eEGF(A)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eEGF(N)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC1s\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e284.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e93.39\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e72.92\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e70.33\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eO1s\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e532.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5.29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e25.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e24.84\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eN1s\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e401.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.71\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e4.83\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eMore oxygen-nitrogen-functionalized groups will lead to a better wettability that is good for sufficient contact between the electrode and the electrolyte. To prove the speculation, GF electrode wettability was investigated using a contact angle instrument. Figure.6 shows that GF, EGF(A), and EGF(N) have contact angles of 131.5\u0026deg;, 0\u0026deg;, and 0\u0026deg;, respectively. The electrode surface changes from hydrophobic to hydrophilic as a result of the increase in surface defects and the increased oxygen concentration after heat treatment. In order to investigate the accessibility of vanadium ions in the electrolyte, electrodes were immersed in a vanadium electrolyte (0.1 M VOSO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e). GF was floating on the electrolyte due to the hydrophobic properties of the conductive electrolyte. EGF(A) and EGF(N) are deposited at the bottom of the electrolyte, indicating that they are more electrolyte accessible than pristine GF.\u003c/p\u003e\n\u003ch3\u003eElectrochemical Characterizations\u003c/h3\u003e\n\u003cp\u003eThe electrochemical properties of the EGF(N) electrodes were determined by cyclic voltammetry, where the electrode's electrochemical activity is reflected by its redox initiation potential and current density (IP). On the positive side (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea), EGF(N) demonstrated a much higher peak current (308.7mA∙cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) compared to GF(A) (246.3mA∙cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) and GF (231.4mA∙cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e). Additionally, EGF(N) (-329.1mA∙cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) and EGF(A) (-259.8mA∙cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) showed a substantial reduction peak, whereas no peak was observed in the pristine GF. As compared to other samples, EGF(N) exhibits the highest peak redox current, which is consistent with the results of the SEM and XPS tests. A further improvement in electrochemical properties could be attributed to the addition of N atoms. The increased peak currents indicate the improvement in the reaction kinetics of VO\u003csup\u003e2+\u003c/sup\u003e/VO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e in the presence of oxygen-containing and nitrogen-containing functional groups on GF surface. The peak potential separation(ΔEp) values were also reduced, EGF(N) shows the lowest ΔEp of 0.39V, while the pristine is 0.61V, further implying the best reversibility toward vanadium redox reaction and a reduction in the polarization of the redox. The CV curves of the GF and EGF(N) for VO\u003csup\u003e2+\u003c/sup\u003e/VO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e redox reactions are also shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb, c. All the peaks are visible in the EGF(N), but not in the GF. As shown in the Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed, a linear relationship exists between peak current density and the square root of scan rate, which suggests that reactions are primarily governed by transport processes\u003csup\u003e[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]\u003c/sup\u003e. The improvement of the properties is due to the introduction of oxygen and nitrogen groups, which can effectively change the electronegativity of the pristine graphite felt and enhance the attraction of vanadium ions to the electrode surface \u003csup\u003e[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe resistances of GF, EGF(A), and EGF(N) were determined through electrochemical impedance spectroscopy (EIS). The Nyquist plot can be divided into two parts, semicircular part and a linear part. In the high-frequency region, indicating that the reaction at the electrode is mainly controlled by charge transfer and mass transfer \u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e illustrates an equivalent circuit for fitting the Nyquist plot. The Rs is solution resistance related to electrolyte, Rct is charge transfer resistance at the interface between electrode and electrolyte, Q is double-layer capacitance at the interface between electrode and electrolyte, and Warburg impedance W is related to diffusion of vanadium ions \u003csup\u003e[\u003cspan additionalcitationids=\"CR39\" citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/sup\u003e. The pristine GF has a large Rct value (6.18Ω) for its hydrophobic surface and poor electrochemical activity. The results of EGF(A) and EGF(N) imply that the electron transfer resistance decreases after treatment with Fe\u003csub\u003e2\u003c/sub\u003eKO\u003csub\u003e4\u003c/sub\u003e. However, EGF(N) had the lowest Rct (0.64Ω), which indicated a significant enhancement in catalytic activity toward VO\u003csup\u003e2+\u003c/sup\u003e/VO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e reaction by N doping. The introduction of oxygen-containing and nitrogen-containing functional groups significantly reduced the mass and charge transfer resistance.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eVrfb Single Cell Performance\u003c/h3\u003e\n\u003cp\u003eThe electrochemical activity feature of the EGF(N) electrode was investigated by examining the charging/discharging performance of an assembled VRFB single cell. In contrast to the GF and EGF(A) electrodes, EGF(N) electrode shows a lower charging plateau and a higher discharging plateau, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea. The reason is ascribed to the reduction in electrochemical polarization in the presence of ECF(N) electrodes with improved electrochemical activity and enhanced reaction reversibility \u003csup\u003e[\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]\u003c/sup\u003e. Figure.9b summarizes the EE values of the samples at different current densities. Obviously, EGF(N) batteries have always had the highest EE. As the current density increases, the potential decreases with the increase of polarization, resulting in high overpotential and side reaction \u003csup\u003e[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]\u003c/sup\u003e. Compared with other samples, the initial EE of EGF(N) was 81.48%, and when the current density was restored to 80 again, the EE of the battery still remained above 80%, which proved its high operational stability. On one hand, EGF(N) improves the catalytic activity of the electrode, and enhances the mass transfer performance of the electrode, thus the performance of the battery at high current density is also improved Cycling stability test.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe VRFB cells with pristine GF, EGF(A), and EGF(N) were charged and discharged at 80 mA∙cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e for 50 cycles to investigate the discharge capacities of different electrodes in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ea. The diffusion of vanadium ions leads to the decrease of discharge capacity with the increase of cycle number\u003csup\u003e[\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]\u003c/sup\u003e. As the cycle numbers increased, the discharge capacities of cells with pristine GF, EGF(A) and EGF(N) decreased gradually. A cell with an EGF(N) electrode has superior initial discharge capacity not only than a cell with GF or EGF(A), but it also experiences a slower reduction in capacity with increasing numbers of charge/discharge cycles as compared with a cell with pristine GF and EGF. After undergoing 50 cycles of charge and discharge, the discharge capacity of cells using EGF(N) was still 98.9 mA∙h, higher than that of cells using GF electrode (38.5 mA∙h) and cells using EGF(A) electrode (66.2 mA∙h). The EGF(N) electrode leads to the slowest decay in the discharge capacity of the battery. The cell with EGF(N) exhibited a significantly higher capacity retention rate of 81.1% compared to the cell with pristine GF of 48.4% and the cell with EGF(A) of 66.9%, indicating a relatively high cycling stability.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eEnergy efficiency (EE), coulomb efficiency (CE), and voltage efficiency (VE) are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003eb. CE is determined by its own discharge and side effects. In the pristine cell, the CE was higher than in the EGF(N) cell, due to the shorter charge and discharge times, as well as the lower penetration of active material. the current efficiency of all samples was higher than 95%, which indicated that the assembled cell had good tightness\u003csup\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e. The voltage efficiency (VE) of the cell is also a key factor for evaluating the VRFB single cell as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003eb. The VE of the cell built with EGF(N) electrode (89.3%) was higher than that of the cell built with CF electrode (79.6%), indicating enhanced charge-discharge capability. Owing to the increase in voltage efficiency, the cell with EGF(N) shows a higher energy efficiency of 80.08% than that of the cell with pristine GF (69.87%), which may be owing to the significantly reduced mass transfer and charge transfer resistances resulting from the introduced oxygen and nitrogen containing functional groups\u003csup\u003e[\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]\u003c/sup\u003e. The cell assembled with EGF(N) shows a higher energy efficiency and almost no decay in energy efficiency after 50 cycles, demonstrating an excellent VRFB performance. However, the energy efficiency for cell with pristine GF decays from 69.87\u0026ndash;65.01% after 50 cycles.\u003c/p\u003e \u003cp\u003eThe improvement of energy efficiency stability of single cell is attributed to the decrease of electrochemical polarization during charging and discharging \u003csup\u003e[\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]\u003c/sup\u003e. Hence, this result highlights the superior electrochemical performance of the cell with EGF(N), indicating its potential as a promising candidate for high-performance energy storage applications. But the utilization of electrolyte is still limited, further efforts should be made to improve the battery capacity and the utilization rate of electrolyte in the future.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn summary, through the combination of a facile process involving K\u003csub\u003e2\u003c/sub\u003eFeO\u003csub\u003e4\u003c/sub\u003e etching and nitrogen doping at 700\u0026deg;C, a significant quantity of oxygen-nitrogen functionalized surface structures was successfully synthesized on the surface of commercially available GF. CV and EIS assessments reveal that the EGF(N) exhibits a high level of electrochemical activity towards the VO\u003csup\u003e2+\u003c/sup\u003e/VO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e redox reaction. The cell employing EGF(N) had an average energy efficiency of 80.08% at a current density of 80 mA∙cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, compared to pristine GF's 69.87%. The charge-discharge tests performed on EGF(N) demonstrated a remarkable retention of energy efficiency, indicating the suitability of EGF(N) for prolonged operation in energy storage applications. The exceptional electrochemical characteristics exhibited by EGF(N) can be attributed to enhanced electrical conductivity, increased availability of active sites, and improved wettability, facilitated by the activated surface and the introduction of oxygen-nitrogen-functionalized groups onto the surface of GF.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis article does not contain any studies involving humans and animals performed by any of the authors.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe authors declared that they have no conflicts of interest to this work.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026apos; contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll the authors contributed to the concept and design of the study. HL contributed to reviewing and editing manuscripts. HW contributed to processing data and writing manuscript. YX contributed to methodology and writing original draft. YW contributed to data curation and writing manuscript. GY, FX and BW contributed to data analysis and revising manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding:\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThis work was supported by the Fundamental Research Funds for the Central Universities from Chang\u0026apos;an University (No.300102310110 and No.300102311403), College Students\u0026apos; Innovative Entrepreneurial Training Plan Program（No.X202210710376）\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and material\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data generated or analyzed during this study are included in this published article.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eM. Ulaganathan, V. Aravindan, Q. 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J Proceedings of the IEEE. 10511, 2191-2219 https://doi.org/10.1109/JPROC.2017.2702627\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"ionics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Ionics](https://www.springer.com/journal/11581) ","snPcode":"11581","submissionUrl":"https://mc.manuscriptcentral.com/ionics","title":"Ionics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Graphite Felt, Vanadium redox flow battery, Etching, Nitrogen doping","lastPublishedDoi":"10.21203/rs.3.rs-2674166/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-2674166/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eDoping with oxygen and nitrogen in graphite felt (GF) is critical for enhancing the activity of the electrode material in vanadium redox flow batteries (VRFB). In this paper, we present a combined approach that utilizes Fe etching and nitrogen doping by means of K\u003csub\u003e2\u003c/sub\u003eFeO\u003csub\u003e4\u003c/sub\u003e and NH\u003csub\u003e3\u003c/sub\u003e to modify the surface structure of graphite fibers. The results show that the innovative approach enhances the disordered structure of the surface carbon of GF and substantially improves the oxygen and nitrogen functionalized groups. This modified GF is completely hydrophilic, and its assembled electrode energy efficiency is 80.08% at a current density of 80 mA∙cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, compared with 69.87% for the pristine GF. The energy efficiency of the modified GF was maintained at 81.8% after 50 charge-discharge cycles. This can be attributed to the reduced internal resistance of these modified GF electrode as well as to the improved mass transport and charge redox exchange towards VO\u003csup\u003e2+\u003c/sup\u003e/VO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e redox couple. The approach of combined Fe etching and nitrogen doping is a simple and effective technique that significantly boosts the performance of VRFB.\u003c/p\u003e","manuscriptTitle":"Enhancement of Vanadium Redox Flow Battery Performance with Nitrogen-Functionalized Graphite Felt Electrodes Etched by K2 FeO4","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2023-03-16 20:33:49","doi":"10.21203/rs.3.rs-2674166/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major revision","date":"2023-04-21T01:45:49+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2023-03-18T06:57:54+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"da4c8c3f-211a-4043-bcc1-b71ec8a67db2","date":"2023-03-14T03:04:18+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2023-03-14T02:09:22+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2023-03-13T07:23:01+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2023-03-13T07:23:01+00:00","index":"","fulltext":""},{"type":"submitted","content":"Ionics","date":"2023-03-09T14:12:27+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"ionics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Ionics](https://www.springer.com/journal/11581) ","snPcode":"11581","submissionUrl":"https://mc.manuscriptcentral.com/ionics","title":"Ionics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"cddedc82-ae60-4aeb-b403-955a184c2cbe","owner":[],"postedDate":"March 16th, 2023","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2023-05-18T09:14:23+00:00","versionOfRecord":[],"versionCreatedAt":"2023-03-16 20:33:49","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-2674166","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-2674166","identity":"rs-2674166","version":["v1"]},"buildId":"_2-kVJe1T_tPrBINL-cwx","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}