Enhancement of vanadium redox flow battery performance with nitrogen-functionalized graphite felt electrodes etched by K2FeO4

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 functionalization by means of K2FeO4 and NH3 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 VO2+/VO2+ redox couple. The combined approach of Fe etching and nitrogen functionalization significantly enhances the electrochemical activity of the GF electrode. This improves the performance of the VRFB.


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][2][3][4][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 consisting solely of varying oxidation states of vanadium ions in both the positive and negative electrolyte solutions, which assists in minimizing crosscontamination during operation.However, the chargingdischarging efficiency of VRFB primarily relies on the performance of its electrodes on which 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.
Many materials have been investigated, including metal electrodes, graphite electrodes, carbon electrodes, organic composite electrodes, among others [8,9].However, graphite felt (GF) 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].However, GF electrodes do not have a sufficiently high specific surface area or dynamic reversibility.They are of low hydrophilicity, thus limiting 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 (such as Pt [14], Sn [15]) or metal oxides (MoO 2 [16], NiO [17], ZrO 2 [18], etc.) on the surface of the GF electrode to enhance its catalytic activity.However, this approach is costly, and using precious metals may lead to impurities in the electrolyte solution over extended periods of operation.In recent years, carbon-based electrocatalysts have been widely used to functionalize the electrodes, such as carbon nanotubes [19], graphene oxides [20], carbon dots [21], and carbon nanoparticles [22].These carbon-based electrocatalysts generally have high electronic conductivity, stability in acidic conditions and cost-effectiveness.However, maintaining the stability of these electrocatalysts on the GF surface during the extended period of electrolyte flow in the VRFB poses a challenge.
Activated GF has demonstrated superior stability in VRFB applications and etching treatments are influential in introducing defects and oxygen-containing functional groups [23][24][25][26].Additionally, incorporating nonmetal elements, especially N [27], S [28], P [29], and B [30], 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 implemented hydrothermal and heat treatment processes to synthesize oxygen-nitrogen-functionalized graphite felt combined with Fe etching and nitrogen functionalization, 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 functionalization under comparatively milder conditions.Consequently, the graphite felt acquires a large number of active sites, leading to an improvement in its electrochemical properties.This results in better conductivity, chemical stability, and electrocatalytic activity, making oxygen-nitrogen-functionalized GF an excellent electrode material for VRFB.

Preparation of modified GF electrode
The preparation of the GF electrolyte was conducted via a hydrothermal treatment followed by heat treatment.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 0.1 mol/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.Finally, the sample was washed thoroughly with deionized water and dried at 60 ℃.The obtained sample is labelled as EGF(N).For comparison, another sample was obtained under similar conditions, but the second treatment was performed in flowing Ar.This sample is labelled as EGF(A).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 surface compositions of GFs were determined by X-ray photoelectron spectroscopy (ESCALAB 250Xiic).The binding energy calibration was based on the C1 peak (284.8 eV).

Electrochemical characterization
The cyclic voltammetry (CV) and electrochemical impedance spectroscopic (EIS) analyses were conducted on an electrochemical workstation with a three-electrode system (CS310).The working electrode was the 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 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 was compressed by 25% in thickness.The initial electrolyte concentration for 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 with a cut-off voltages of 0.8 V and 1.65 V, respectively.

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 as per the following equations [5,25,31]: 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).The surface of the fiber may become 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 (I D ) in Raman spectroscopy is commonly associated with defects and disorder of carbon atoms in a given sample.Conversely, the intensity of the G peak (I G ) typically represents the in-plane vibration of sp 2 carbon atoms of graphitized carbon [32].The I D /I G is an indicator of their structural defects.Notably, 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 [33].
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 [34].The broad XPS spectrum (Fig. 4) clearly shows that K 2 FeO 4 treatment significantly reduces the intensity of the C1(284.8eV) 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 [35].In addition, the O1s (532 eV) signal in EGF(A) and EGF(N) is quite clear in comparison to GF.The spectra of O1s (Fig. 5a, 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 1 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 [36].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 [33], we deduce that it is because the atoms in the surface layer become confused after hydrothermal treatment, which is more favorable to nitrogen functionalization.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 corresponding to pyridine nitrogen, quaternary nitrogen, pyridine nitrogen and oxide  nitrogen, respectively.Figure 5d 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 More oxygen-nitrogen-functionalized groups will lead to a better wettability that is good for sufficient contact between the electrode and the electrolyte.To validate this hypothesis, 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 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. 7a), EGF(N) demonstrated a much higher peak current (308.7 mA•cm −2 ) compared to GF(A) (246.3 mA•cm −2 ) and GF (231.4 mA•cm −2 ).Additionally, EGF(N) (-329.1 mA•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 nitrogencontaining functional groups on GF surface.The peak potential separation(ΔEp) values were also reduced, EGF(N) shows the lowest ΔEp of 0.39 V, while for the EGF(A), it is 0.46V, 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. 7b, c.All the peaks are visible in the EGF(N), but not in the GF.As shown in the Fig. 7d, 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 [37].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 [38].
The resistances of GF, EGF(A), and EGF(N) were determined by electrochemical impedance spectroscopy (EIS).The Nyquist plot can be divided into two parts: semicircular and linear.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 derived from 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 between electrode and electrolyte, and Warburg impedance W is related to diffusion of vanadium ions [39][40][41].
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 charge transfer resistance decreases after treatment with K 2 FeO 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. 9a.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 [42].Figure 9b summarizes the energy efficiency (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 [43].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.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. 10a.The diffusion of vanadium ions leads to the decrease of discharge capacity with increased cycle number [44].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), The EGF(N) cell 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 0.989 A•h, higher than that of cells using GF electrode (0.385 A•h) and cells using EGF(A) electrode (0.662 A•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. 10b.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. 10b.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 [45].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% to 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 [46].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 hydrothermal and heat treatment, K 2 FeO 4 etching and nitrogen functionalization were achieved.A significant quantity of oxygen-nitrogen functionalized surface structures were successfully synthesized on 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.

Fig. 1
Fig. 1 Schematic illustration of a graphite felt electrode modified with oxygen-nitrogen-functionalized groups

Fig. 10
Fig. 10 (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.−2

Table 1
XPS analyses of the primary elements present on the surface of GF with oxygen-nitrogen-functionalized groups