NiO-decorated rGO functional layer on the graphite felt as the negative electrode of vanadium redox �ow batteries

Vanadium redox �ow batteries (VRFBs) are prospective energy storage medium owing to their �exible design and long lifetime. However, the problem of sluggish negative electrode dynamics of VRFBs has become a great resistance to their large-scale commercial applications. To solve this problem, we employed a facile and cost-effective approach to synthesize NiO/rGO composites using hydrothermal and calcination processes. The NiO/rGO nanocatalysts were evenly applied onto the heat-treated graphite felt (HGF) to prepare a high-performance negative electrode for VRFBs. This coating process was achieved using an ultrasonic spraying system, resulting in NiO/rGO-HGF. The NiO/rGO electrocatalysts provided enhanced adsorption characteristics of vanadium ions and su�cient redox-reactive sites, which improved electrochemical performance (9.41% higher energy e�ciency of NiO/rGO-HGF compared with HGF at 160 mA cm − 2 ) and high cycle stability (84.7% electrolyte capacity after 100 cycles) of the VRFB cells. In conclusion, our work with the NiO/rGO-HGF anode represents a promising direction for the development of highly e�cient and stable VRFB anodes for broadening commercial applications.


Introduction
With decreasing fossil fuel reserves and the growing environmental crisis, the global challenge lies in advancing the renewable energy sources, including wind and solar power.[1][2][3].Accordingly, separated energy storage systems have been adopted to compensate the disadvantages of renewable power generators such as instability and intermittency during the operation.After the pioneering work of Skyllas-Kazacos research group, the vanadium redox ow batteries (VRFBs) have known for their various bene ts, including long-term cyclability and low levelized cost of energy.VRFBs utilize same electrochemical reaction species (vanadium) in both electrodes at the different valence states, which strategy effectively mitigates the risks associated with cross-contamination [4].
The electrode takes crucial part for the VRFB and serves as a locus for redox reactions.Its e ciency directly governs the power density and the energy-conversion e ciency of the VRFB.Graphite felt (GF) is the prevalent choice for VRFB electrodes, owing to its corrosion-resistant three-dimensional porous structure and cost-effectiveness.However, the utilization of pristine GF as the negative electrode in VRFBs introduces challenges, including suboptimal hydrophilicity, insu cient active sites, and pronounced side reactions leading to hydrogen precipitation.To improve the energy storage capability of VRFBs and improve commercial viability, researchers have redirected their efforts toward augmenting the surface characteristics of electrodes.For example, to increase the effective active sites during the charge and discharge, thermal activation [5], acid treatment [6], water activation [7], plasma treatment [8], electrochemical oxidation treatment [9] and microwave treatment [10] are used to modify the surface functional groups of electrodes.Modi cation with carbonaceous materials, such as carbon nanoparticles [11], carbon spheres [12], carbon akes [13], carbon nanotubes [14], graphene [15], and graphene oxide [16], increases the electrochemical active sites and provides accelerated electrical conductivity.Furthermore, some inexpensive metal oxides (including TiO 2 [17], Mn 3 O 4 [18], WO 3 [19], CeO 2 [20], and PbO 2 [21]) have been used as catalysts to provide more reactive sites and increase the hydrophilicity of the GF.Yun et al. [22] synthesized NiO nanocatalysts on GF surfaces via distributed thermal decomposition.In this study, nickel ions modi ed the oxygen-related surface functional groups by substituting H + to the hydroxyl groups.Moreover, the negatively charged NiO nanoparticles also showed good vanadium ions adsorption, which effectively improved the energy storage capability.However, the agglomeration and uneven distribution of nanoparticles on the electrode remain a challenge.Furthermore, these nanoparticles are often detached from the substrate during VRFB cycling, degrading the cycle stability.Thus, the uniform anchoring of metal oxide particles is a crucial technique for improving the e ciency and stability of VRFB.
Reduced graphene oxide (rGO), on the other hand, boasts a two-dimensional layered structure with an expansive active speci c surface area [23].When employed as a carrier for NiO, it effectively mitigates the intrinsic low electrical conductivity associated with metal oxides [24].Additionally, rGO provides an abundance of anchoring sites for NiO, ensuring a uniform distribution and preventing the agglomeration of NiO nanoparticles.Furthermore, during battery cycling, the nanoparticles adhered securely to the electrode surface.The synergistic effect of the NiO-decorated rGO functional layer on the GF electrode has not been investigated in detail for VRFB applications.
In this study, an NiO/rGO composite structure was successfully fabricated using a hydrothermal method.
The prepared NiO/rGO nanocatalysts were then uniformly coated on the surface of HGF by ultrasonic spraying system.Because rGO with a high electrical conductivity provided anchor sites for NiO, the NiO nanoparticles were well distributed on the rGO surface and did not easily agglomerate.Introducing NiO nanoparticles into rGO provided abundant catalytically active sites and improved its charge transfer capability.This strategy was applied to a VRFB anode that exhibited high catalytic activity and excellent cycling stability.

Experimental
To remove impurities at the surface, graphite felt (GF, 4.2 mm thickness, Sinopro) was repeatedly cleaned with deionized water (DI water) and ethanol.After drying, the felt was heated in a 500°C oven using air atmosphere for 10 h to activate the surface of GF (thereafter referred to as HGF).NiO-decorated rGO was successfully fabricated via a hydrothermal method and following calcination process.The 15 min ultrasonic process was used to disperse the 0.05 g) into 10 ml of DI water.Then, 30 ml of 2mM nickel chloride hexahydrate (NiCl 2 •6H 2 O, SAMCHUN) and 2 mM urea (CH 4 N 2 O, Aldrich) solution were homogeneously mixed with rGO solution after stirring for 3 h.As shown in Fig. 1, the resulting precursor was then poured to a 50 mL reaction chamber, to progress the hydrothermal reaction for 6 h under 180°C.The precipitates were then centrifuged several times with ethyl alcohol and DI water to eliminate the residual reactants.The resulting product was dried at 80 ℃.Then, the resultant sample was heat treated at 400°C for 3 h using N 2 gas to synthesize NiO/rGO powder.The dispersion solution of the sample, containing 0.5 wt% NiO/rGO and 0.1 wt% CMC dissolved in DI water was uniformly sprayed onto the HGF surface using an ultrasonic spray-coating system.For ultrasonic spray coating, the suspension feed rate, frequency, spray time, hot plate temperature and distance between nozzle and HGF were set to 4 mL/h, 130 kHz, 5 min, 100°C and 25 cm, respectively.Furthermore, rGO-HGF was prepared using the same procedure to con rm the role of NiO layer.
The surface morphology was characterized using eld-emission scanning electron microscopy (FESEM).
X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were conducted to investigate the crystalline phases and verify the chemical bonding states.Electrochemical impedance spectroscopy (EIS) measurements with cyclic voltammetry (CV) were executed utilizing an AUTOLAB instrumentation platform, under a xed voltage range from − 0.8 V to 0 V.During the electrochemical characteristic evaluation, 0.15 M V 3+ electrolyte with three-electrode measurement system at a scan rate of 5 mV s − 1 .The prepared felts, Ag/AgCl, and Pt electrodes were served as working, reference, and counter electrodes, respectively.
A VRFB full-cell was introduced to evaluate the practical energy storage performance at the voltage range from 0.7 to 1.6 V. NiO/rGO-HGF and HGF were utilized as the negative and positive electrode to construct full-cell measurement system.Rectangular samples with dimensions of 20 × 30 mm were cut from pristine HGF and used as the cathode side in the VRFB tests.For comparison, the anode side was tested successively with HGF, rGO-HGF and NiO/rGO-HGF samples.For the electrolyte, the 1.5 M V 3+ (20 mL) with 3.0M H 2 SO 4 solvent and 1.5 M V 4+ (20 mL) with 3.0M H 2 SO 4 solvent was adopted for the negative and positive part, respectively.Energy storage performance was investigated at current densities of 40, 80, 120, and 160 mA cm − 2 and a rapid cycling test was carried out at a current density of 160 mA cm − 2 for 100 cycles.

Results and Discussion
Figure 2 appears the morphologies of the HGF, rGO-HGF, and NiO/rGO-HGF felt.As demonstrated in previous research and shown in Fig. 2(a, d), HGF showed a smooth surface.In contrast, the surfaces of rGO-HGF and NiO/rGO-HGF treated by ultrasonic spraying exhibited rough surface morphologies.Speci cally, Fig. 2(b, e) show the 2D lamellar structure of rGO, which uniformly envelops the HGF surface.This phenomenon is contributed by the van der Waals gravitational force, which causes rGO to curl and fold [25].The increased speci c surface area of rGO facilitated interfacial interactions between the electrode and the vanadium electrolyte solution.In Fig. 2(c) and (f), uniform decoration of small-sized NiO nanoparticles was observed on the rGO layer.This was a consequence of the defect sites in rGO, which provided favorable locations for the generation of NiO nanoparticles [26].Additionally, in Fig. 2(g), we present EDS elemental mapping of the NiO/rGO-HGF sample.These results clearly demonstrated the uniform elemental distribution on the electrode surface.
XPS was performed on the NiO/rGO sample to investigate its chemical bonding structure.In Fig. 4(a), the binding energy exhibits a primary peak along with satellite peaks at ~ 854.8 eV and ~ 861.2 eV, corresponding to the Ni 2p 3/2 spin-orbit energy levels.At the same time, the Ni 2p 1/2 spin-orbit energy level of nickel oxide is corresponding to the main peak at ~ 872.6 eV and the satellite peak at ~ 879.4 eV, which supports the predominance of Ni in the + 2 valence state [30].Figure 5(a) shows the high contact angle of 131.7° obtained for the HGF electrode, indicating poor hydrophilicity and low electrolyte wettability.In contrast, when V 3+ electrolyte was dropped onto the surfaces of rGO-HGF and NiO/rGO-HGF (Fig. 5b, c), the droplets were rapidly absorbed.This observation demonstrated the high hydrophilicity of the rGO-HGF and NiO/rGO-HGF electrodes.The increased hydrophilicity was owing to the substantial incorporation of oxygen related surface functional groups into the NiO/rGO catalyst.These incorporated functional groups promote more favorable electrolyte wetting process, thereby accelerating the diffusion of the V 3+ electrolyte.
Cyclic voltammetry (CV) was performed to examine the effects of rGO and NiO/rGO on the V 3+ /V 2+ reaction kinetics.Figure 6(a) and Table (1) show CV plots of HGF, rGO-HGF, and NiO/rGO-HGF for the V 3+ /V 2+ redox pair containing 0.15 M V 3+ +3 M H 2 SO 4 at a scan rate of 5 mV s − 1 .Notably, compared with pristine HGFs, HGFs treated with rGO and NiO/rGO layers showed reduced peak separation and increased peak current values.Thus, the rGO and NiO/rGO exhibit favorable reversibility and electrochemical activity as catalysts for V 3+ /V 2+ redox reactions.EIS was performed systematically on all electrodes to validate this conclusion.EIS is a valuable and indispensable technique for probing the kinetic properties at the surface of electrode during the electrochemical reactions.In Fig. 6(b), the negative Nyquist plots for different graphite felts are presented, where the diameters of the semicircles are inversely correlated with the charge transfer resistance (R ct ) [35,36].Notably, HGFs with the rGO and the NiO/rGO layer was showed signi cantly lower R ct than that of the pristine HGF, which also supports the CV results.Therefore, based on the EIS and CV data, the effect of NiO/rGO functional layer on the graphite felt was con rmed, which improved the catalytic activity of the HGFs during the vanadium redox reactions.
For evaluating the electrochemical activity and stability of the electrodes, we performed negative cyclic voltammetry (CV) tests with varying scan rates from 1 to 10 mV s − 1 .The results are displayed in Figs.7(a) and (b).It is noteworthy that the NiO/rGO-HGFs consistently exhibited improved peak current values at all scan rates.This suggests that the stability and reversibility of HGF are inferior to those of NiO/rGO-HGF [37].
Figure 8(a) shows the galvanostatic charge/discharge data with HGF, rGO-HGF, and NiO/rGO-HGF electrodes at a current density of 120 mA cm − 2 .Interestingly, the NiO/rGO-HGF negative electrode VRFB had an increased discharge and a reduced charge plateau.This indicates that NiO/rGO-HGF effectively mitigates electrochemical polarization, thereby improving the overall battery performance.For demonstrating the performance of the VRFB with the NiO/rGO-HGF anode, single-cell experiments were conducted at various applied current densities, as exhibited in Fig. 8(b-d).These gures show energy e ciency (EE), coulombic e ciency (CE) and voltage e ciency (VE) data for current densities from 40 to 160 mA cm − 2 .The VE improved owing to the increased vanadium ion penetration rate according to the current increment [38].Nonetheless, the simultaneous rise in current density results in an accelerated charge and discharge rate, which in turn leads to an increased overpotential and reduces both EE and VE [39].Because VE is closely correlated with the overpotential, the NiO/rGO-HGF negative electrode consistently exhibited a higher VE compared to pure HGF at various current densities.This suggests that the NiO/rGO layer as an anode exhibits superior electrocatalytic activity and excellent charge-transfer capability.In addition, because EE is calculated using CE and VE, the trend of EE re ects that of VE.The EE is a crucial marker for assessing battery performance, as it signi es the battery's ability to convert and store energy e ciently.Figure 8(b) appears the NiO/rGO-HGF negative electrode achieves an EE of up to 93.51% at a current density of 40 mA cm − 2 .Interestingly, the difference in the EE between the HGFnegative electrode and the NiO/rGO-HGF negative electrode gradually increased with increasing current density during battery operation.Speci cally, the NiO/rGO-HGF negative electrode exhibited higher EE compared with the HGF negative electrode by 7.226%, 6.912%, 8.236%, and 9.410% at current densities range of 40, 80, 120, and 160 mA cm − 2 , respectively.This improvement is attributed to the NiO/rGO layer, which acts as an electrode catalyst and provides a more effective active surface area, signi cantly reducing the polarization characteristic.The assembled full cells with HGF cathode and NiO/rGO-HGF anode were subjected to 100 cycles at a current density of 160 mA cm − 2 to evaluate their cycle stability.In Fig. 9(a), the initial capacity of the NiO/rGO-HGF negative electrode was 607.4 mAh.After 100 cycles, the capacity was 514.6 mAh, indicating a superior retention rate of 84.7%.Conversely, the discharge capacity of the HGF negative electrode decreased from 550.5 mAh to 278.1 mAh, corresponding to a poor cycle stability with the retention rate of 50.5%.Furthermore, Fig. 9(b) shows that the NiO/rGO HGFnegative electrode maintained a higher EE even after 100 cycles (76.35%).This highlights the excellent energy-storage capacity and superior electrochemical stability of NiO/rGO-HGF.This enhancement can be attributed to the following three factors.First, an NiO-decorated rGO functional layer was uniformly applied to the HGF surface, resulting in enhanced electrocatalytic activity.Secondly, rGO modi ed the surface functional groups, thereby increasing its cycle stability.Third, the negatively charged O 2− ions on NiO enhance the adhesion of vanadium ions to HGF, which contributes to the high EE and improved electrical conductivity.

Conclusion
We synthesized highly active and low-cost NiO/rGO functional layer-decorated graphite felt electrodes for VRFB using a simple hydrothermal method and ultrasonic spraying.Analysis of the data obtained from CV and EIS measurements showed that the NiO/rGO catalysts exhibited remarkable kinetic reversibility and substantial electrochemical activity because of their ability to facilitate the V 3+ /V 2+ redox reaction.Thus, the electrochemical performance of VRFB using NiO/rGO-HGF was enhanced, including the high EE as high as 93.51% at a current density of 40 mA cm − 2 .In addition, the NiO/rGO-HGF negative electrode exhibits a high capacity retention rate (84.7%) during a 100 cycle durability test at a current density of 160 mA cm − 2 .These results are attributed to the following: (1) The NiO-decorated rGO functional layer effectively enhanced the speci c surface area and provided high electrocatalytic activity.( 2) rGO provides oxygen-containing functional groups together with anchoring sites for NiO, allowing superior cyclability.
(3) The negatively charged O 2− ions of NiO facilitated vanadium ion adhesion to the electrode, which contributed to the EE.These results suggest that NiO/rGO-HGF has signi cant potential as an electrode material for enhancing the electrochemical activity and stability of VRFB anodes.

Figure 4 (
b) presents the tted XPS data for the C 1s region, including three distinguishable peaks.These peaks are located at ~ 284.5 eV, ~ 286.2 eV, and ~ 288.0 eV respectively, corresponding to the binding energies of C-C, C-O and O-C = O at the 1s electron level.Notably, the successful reduction of GO to rGO during heat treatment is indicated by the weaker peaks of the C-O and O-C = O peaks than those of C-C [31].Figure 4(c) shows the XPS spectrum of the O 1s region, that indicates the existence of O-containing functional groups within the NiO-rGO composite.Among these peaks, the characteristic peak at ~ 529.3 eV and ~ 529.5 eV corresponds to general metal oxide bonding, including the C-O-Ni and Ni-O bonds [32].Generally, nickel forms bonds with the oxygen-containing functional groups or carbon atoms in rGO.However, NiO-rGO sample showed no characteristic peak at ~ 283.5 eV (indicative of Ni-C) in the C1s spectrum, suggesting that NiO interacts with rGO primarily through C-O-Ni bonds [33].In addition, the characteristic peak at ~ 531.1 eV corresponds to C = O bonds, and the peak at ~ 532.9 eV corresponds to C-OH bonds [34].

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