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. Specifically, 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 specific 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.
The rGO and NiO/rGO powder XRD spectra are shown in Fig. 3. For the rGO samples, the peak at ~ 25.74° was detected, regarding the graphite (002) structures [27, 28]. The NiO/rGO sample shows slightly decreased graphite peaks at approximately 29.1°, together with additional peaks at 37.1°, 43.1°, 62.9°, and 75.04°. These observations correspond to the diffraction peaks of NiO, implying that the crystallographic planes are indexed as (111), (200), (220), (311), and (222) [29]. This confirms that the hydrothermal synthesis and subsequent thermal treatment successfully produced the NiO/rGO composite structure.
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 2p3/2 spin-orbit energy levels. At the same time, the Ni 2p1/2 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 4(b) presents the fitted 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]. 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 V3+ 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 V3+ electrolyte.
Cyclic voltammetry (CV) was performed to examine the effects of rGO and NiO/rGO on the V3+/V2+ reaction kinetics. Figure 6(a) and Table (1) show CV plots of HGF, rGO-HGF, and NiO/rGO-HGF for the V3+/V2+ redox pair containing 0.15 M V3++3 M H2SO4 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 V3+/V2+ 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 (Rct) [35, 36]. Notably, HGFs with the rGO and the NiO/rGO layer was showed significantly lower Rct 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 confirmed, 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 figures show energy efficiency (EE), coulombic efficiency (CE) and voltage efficiency (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 reflects that of VE. The EE is a crucial marker for assessing battery performance, as it signifies the battery's ability to convert and store energy efficiently. 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 HGF-negative electrode and the NiO/rGO-HGF negative electrode gradually increased with increasing current density during battery operation. Specifically, 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, significantly 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 HGF-negative 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 modified the surface functional groups, thereby increasing its cycle stability. Third, the negatively charged O2− ions on NiO enhance the adhesion of vanadium ions to HGF, which contributes to the high EE and improved electrical conductivity.