Optimization of GF surface alternation and Fe3O4 depositionHerein, the multifunction one-step method is proposed to synthesize Fe3O4 in the crystalline nanostructure over the in-situ functionalized GF surface with high areal capacity. The GF functionalization and Fe3O4 electrodeposition are done by sweeping the potential from the oxidative potentials (positive potentials) followed by the reductive potentials (negative potentials) via the LSV technique, respectively, in the deposition bath containing FeCl3. While the role of the oxidative potentials is to functionalize the GF surface, the role of the reductive potentials is to deposit the Fe3O4. Functionalization of hydrophobic GF surface as seen from Fig. 1 occurred at the oxidative potentials with the aid of the formed oxygen, H2O, and ClO3− species that are in-situ formed at high positive potential32,37,38,39. The inserted oxygenated functional groups at the GF surface were probed by various surface characterization tools, particularly XPS studies (c.f. Figure 3). Additionally, the in-situ functionalized GF assisted in the homogenous uniform deposition of Fe3O4 redox active material around almost all GF fibers (c.f. Figure 5). The formation of the Fe3O4 is started in the reductive region by the reduction of Fe3+ to Fe2+ followed by the formation of Fe3O4 assisted by the hydrogen evolution reaction at the electrode surface according to the following Eqs. (3–5)39,40:Figure 1Graphic illustration of the synthesis of Fe3O4/GF electrode.$${Fe}^{3+}+{e}^{-}\to {Fe}^{2+}$$
(3)
$$2{H}_{2}O+2{e}^{-}\to {H}_{2}+2{OH}^{-}$$
(4)
$${Fe}^{2+}+2{Fe}^{3+}+8{OH}^{-}\to {Fe}_{3}{O}_{4}+4{H}_{2}O$$
(5)
Thus, according to the above-mentioned idea, the conditions were optimized by investigating the width of the potential window together with the electrolyte concentration. Firstly, three electrodes were prepared, using a concentration of the deposition bath equal to 100 mM FeCl3, applying various potential windows (2 to − 2 V, 2 to − 3 V, and 2 to − 4 V), see Fig. 2A. Importantly, the amounts of the loaded Fe3O4 are increased by opening the potential to the higher negative values. As a result, and from Fig. 2B, the electrode that is obtained at the extended potential window (+ 2 to − 4 V) is the best-prepared electrode due to the integrated surface area under the CVs resulting in a higher areal capacity (see Fig. 2C). Further, the effect of the FeCl3 concentration at the optimized deposition potential window is investigated using two other concentrations, 150 and 200 mM. Advantageously, as the deposition bath concentration is increased, the level of the functionalization is enhanced, and this is clear from the positive current’s region of all electrodes that is assisting in depositing the active materials in a uniform and homogeneous manner utilizing the maximum available GF surface area (see Fig. 2D). Also, the degree of the deposition of the Fe3O4 active materials is increased by extending the potentials to the higher reductive potentials, see Fig. 2D. Accordingly, the electrode that is prepared at 200 mM and wide potential window is the best and this is notable from the area under CVs and the calculated areal capacity as shown in Fig. 2E,F. Therefore, the electrode prepared using 200 mM FeCl3 solution and applying a potential window from + 2 to − 4V (Fe3O4/GF) is selected as the best electrode for further physical characterizations and electrochemical measurements.Figure 2(A) LSVs, (B) CVs, and (C) variation of Cs with varying deposition potential window curves of the electrodes prepared at potential windows of (2 to − 2 V), (2 to − 3 V), and (2 to − 4 V). (D) LSVs, (E) CVs, and (F) variations of Cs with varying concentrations of deposition bath curves of the electrodes prepared using various ferric chloride concentrations (100, 150, and 200 mM). Note that: all CV measurements were performed in 1 M KOH solution at a potential scan rate of 10 mV s−1.Physical characterizationXPS analysis is utilized to confirm the existence of Fe in the form of Fe3O4, and the incorporation of the oxygenated functional groups at the GF surface. From Fig. 3, the incorporation of the Fe element during the synthesis process is clear from the existence of the extra peak around 712 eV in comparison with the survey of pure GF surface. Also, the approval of the Fe3O4 preparation is verified by the existence of the two oxidation states of iron (Fe2+ and Fe3+). While the existence of Fe2+ is proven by the appearance of two peaks around 711 and 725 eV that are ascribed to the Fe 2p3/2 and Fe 2p1/2, respectively, the existence of the Fe3+ is proven by the appearance of the peaks around 713 and 728 eV that are assigned to the Fe 2p3/2 and Fe 2p1/2, respectively (see Fig. 3)41,42,43. Additionally, from O 1s spectra as displayed in Fig. 3, the formation of Fe3O4 is confirmed by the appearance of a peak assigned to the Fe–O bond44. Also, from O 1s spectra, one concludes that the successful in-situ GF functionalization is confirmed by displaying more oxygenated functional groups compared to the pristine GF surface. Additionally, the percentage of the O in the Fe3O4/GF electrode is 4 times that of the GF surface indicating the massive deposition and the in-situ functionalization. Further from C 1s, the appearance of the hump at high binding energy around 288 eV indicates the oxidation of the GF surface, and this is clear from the deconvoluted C 1s compared to that of the GF surface32,35,45.Figure 3XPS analysis of Fe3O4/GF and GF electrodes.Additionally, Raman analysis as a strong characterization tool for differentiation between the iron oxides and oxyhydroxide is done to further confirm the formation of Fe3O4. The theoretical studies of Raman analysis of the Fe3O4 materials display several active bands of A1g, Eg, and T2g modes between wavenumbers 200–1000 cm−1. From Fig. 4A, the appearance of three bands around 294, 538, and 661 cm−1 matches the vibration’s mode of Fe3O4, which concludes the deposition of Fe3O4 over the GF surface46. Remarkably, the Fe3O4/GF electrode’s G and D bands vanish, indicating that there is an excessive amount of Fe3O4 deposited that completely encases the GF fibers (see Fig. 4A).Figure 4(A) Raman spectra, (B) contact angle measurements, and (C) XRD patterns of GF (black line) and Fe3O4/GF (red line) electrodes.As a result of the above analysis that confirms the in-situ GF surface fluctuations and heavy deposition of the Fe3O4 materials, the Fe3O4/GF electrode shows exceptional hydrophilicity. As displayed in Fig. 4B, the contact angles of GF and Fe3O4/GF electrodes are 174.29° and 0°, respectively. This improves the capacity performance by smoothing the electrolyte ions’ diffusion paths for the reaction with redox-active materials.XRD analysis is done and boosted the above results. Firstly, from Fig. 4C, the formation of the Fe3O4 is established by obtaining peaks around 35°, 42°, 52°, 58°, 62°, and 82° assigned to the crystalline structure of Fe3O4 (JCPDS No. 19-0629)47,48. Secondly, as consistent with Raman analysis, the diffraction peaks assigned to the graphitic structure are not present in the Fe3O4/GF electrode, showing that the active materials have completely covered the fibers of GF with high thickness confirming the massive and homogeneous deposition.The surface morphology and elements distribution of GF and Fe3O4/GF electrodes are investigated by SEM and mapping EDX analysis, respectively. From Fig. 5, GF demonstrates a fiber’s flawless surface with little white spots that represent carbon dust after manufacture34,49. Whereas Fe3O4/GF, as shown in Fig. 5, exhibits uniform encasement of Fe3O4 on the in-situ altered GF surface in nanowire shape. Uniform encasement indicates effective surface modification via inserting oxygenated functional groups during the preparation of Fe3O4/GF which compiles with the findings obtained from XRD and Raman analysis. The complete encasement of GF fibers by active materials and the extraordinary hydrophilicity facilitates the electrolyte ions’ pathways toward the active sites predicting a high Cs (c.f. Figure 8). Moreover, mapping EDX of Fe3O4/GF demonstrates the massive deposition of the Fe3O4 by obtaining a massive amount of Fe element, high O percent, and low C percentage indicating the complete encasement that is consistent with the previous analyses (see Fig. 5).Figure 5SEM images with various magnifications and the corresponding mapping EDX of GF and Fe3O4/GF electrodes.TEM and HR-TEM analyses are conducted for Fe3O4/GF electrode for further confirmation of morphology and crystallinity nature of Fe3O4 active material. From Fig. 6, TEM images at different magnifications (Fig. 6) display a nanowire morphology with a small diameter (see Fig. 6B) that is consistent with SEM analysis. Furthermore, the Fe3O4 active particles’ selected area electron diffraction (SAED) is shown in Fig. 6D which indicates the polycrystalline nature of Fe3O4 due to the presence of bright spots surrounding the rings that are consistent with XRD analysis50,51.Figure 6HR-TEM images with various magnifications (A–C), and (D) the corresponding SAED pattern of the Fe3O4/GF electrode.Electrochemical storage performance of Fe3O4/GF electrodeThe peak intensities and the diffusion accessibility of the electrolyte ions to the underneath layers of the active materials vary throughout the first several cycles of operation. Thus, the electrochemical activation must be done to reach a steady state before the evaluation of the electrochemical storage performance of the Fe3O4 active materials52,53. From Fig. 7A, there are characteristic peaks of Fe3O4 active materials at the cathodic branch (I, and II) and three peaks at the anodic branch (III, IV, and V). During the cathodic direction, the Fe3+ is electrochemically reduced to Fe2+ (peak I) and then to Fe0 (peak II). Moving towards the anodic direction, i.e., toward high potential values, the three characteristics of anodic peaks appeared due to; (a) formation of Fe(OH) by the adsorption (peak III); (b) electrochemical oxidation of the Fe0 to the Fe2+; and (c) overcharging of the Fe2+ to Fe3+52,53. As shown in Fig. 7A, while the peak intensity of II, III, and IV is decreased by repetitive cycling, the intensity of peaks I and V is increased with cycling. Thus, the activation step is done to reach a steady state and nearly the same behavior, i.e., the same areal capacity (see Fig. 7B).Figure 7(A) CVs of Fe3O4/GF electrode before (black line) and after (red line) the electrochemical activation. (B) The variation of the specific capacity with cycle number during the electrochemical activation of the Fe3O4/GF electrode.After the electrochemical activation, the electrochemical storage performance of the Fe3O4/GF electrode is studied utilizing the CV, GCD, and EIS measurements. As seen from Fig. 8A, CVs of Fe3O4/GF and GF electrodes show that the value of capacity is not significantly influenced by the GF support. However, the electrochemical change of Fe between its oxidation states, as indicated by the peaks in the anodic and cathodic scan, results in a CV of Fe3O4/GF (red line) with a large surface area indicating a higher capacity, and this reflected the 3-D porous structure of GF support, complete encasement of GF fibers by redox-active Fe3O4, and the exceptional ionic conductivity of the prepared electrode. The Cs of the Fe3O4/GF is calculated using Eq. 1 and displays a value of 1418 mC cm−2. Advantageously, the Cs value of the Fe3O4/GF electrode is highly related to other based Fe3O4 electrodes (see Table 1). Additionally, by using Eq. 1 and normalizing capacity by the loaded mass (9.3 mg cm−2) instead of area, the Fe3O4/GF electrode displays a specific capacity of 153 C g−1 which is also higher than other reported values, e.g. Fe3O4/SDS54, CNT/Fe3O455, Fe3O4 nanoparticles56. Interestingly, one of the significantly effective parameters for the application and performance of SCs is the operational voltage window. Fe3O4/GF electrode exhibits an extended voltage window up to 1.45 V which is one of the widest potential windows obtained for the storage performance of Fe3O4 materials in the negative potential region (see Table 1). Additionally, from Fig. 8B, the Cs value of the Fe3O4/GF electrode is calculated at various potential scan rates. Figure 8C displays the variation of Cs and the obtained values are 1418.3, 1160.7, 1044.2, 968.4, 909.6, 825.6, and 769.6 mC cm−2 at various potential scan rates of 10, 15, 20, 25, 30, 40, and 50 mV s−1, respectively. The stability test is one of the most crucial factors in determining how well the SC material performs. The stability of Fe3O4 is shown in Fig. 8D for 200 cycles at a potential sweep rate of 20 mV s−1. Fe3O4/GF exhibits limited stability behavior, which is explained by the mechanical stress that results from the electrolyte ions’ intercalation and deintercalation within the active materials during the charge transfer process that occurs throughout the charging and discharging process.Figure 8(A) CVs of GF (black line) and Fe3O4/GF (red line) electrodes at a potential scan rate of 10 mV s−1. (B) CVs of Fe3O4/GF electrode at various potential scan rates. (C) The variation of Cs with the variation potential scan rate of the Fe3O4/GF electrode. (D) The capacity retention of Fe3O4/GF electrode at a potential scan rate of 20 mV s−1.Table 1 Comparison of the Cs and the operating potential window of Fe3O4/GF material with other reported Fe-based materials.Interestingly, the origin of the storage mechanism of the synthesized electrode can be investigated by the analysis of the CVs at various potential sweep rates. Therefore, the Trasatti method is introduced to calculate the percentage of the contribution of the different storage processes of the synthesized materials using Eqs. 6 and 75,6,65:$$q={q}_{\infty }+a{\nu }^{-0.5}$$
(6)
$$\frac{1}{q}=\frac{1}{{q}_{t}}+b{\nu }^{0.5}$$
(7)
where q represents the charge (C) at various potential scan speeds (ν), a and b are constants, q∞ denotes the charge owing to the surface process (fast surface faradaic and adsorption of electrolyte ions), and qt is the total charge (bulk and surface processes). The values of q∞ and qt are 0.362 and 1.36 C, respectively, based on the intercept values of Fig. 9A,B. The ratio of the surface process (surface faradaic and ions adsorption) to the overall charge is 26.6% suggesting the combination storage mechanism during the charging and discharging of Fe3O4/GF electrode materials. But the Trasatti method gives this ratio based on the extrapolation of q∞ and qt at the ν equal ∞ and 0, respectively. Thus, the storage mechanism of the active materials over the selected potentials sweep rates is mainly dependent on the intercalation and de-intercalation within the active materials and this is clear from the appearance of various redox peaks of CVs of Fe3O4/GF. Also, this behavior is confirmed by change separation using the Dunn method66,67,68. Figure 9C displays that the percentage of bulk faradic process is the main percentage of storage mechanism. Interestingly, at high-speed scan (50 mV s−1) the bulk faradic reaction contributes by 67.2%.Figure 9Trasatti method for the surface (A) and bulk reaction (B) of Fe3O4/GF electrode. Dunn method for determining bulk contribution at a potential scan rate of 10 mV s−1 (C). The variation of log(Ip) with log(v) of I (D) and V (E) peaks (from Fig. 7A) of the Fe3O4/GF electrode.Furthermore, the bulk faradaic process may be further analyzed by using the relation concerning redox peak current and the potential scan rate using Eq. 869:$${I}_{p}=a{\nu }^{b}$$
(8)
where Ip is the redox peak current, v is the potential sweep rate, and a and b are constants. In this instance, the b-value represents the rate at which the electrolyte ions inside the active materials intercalate and de-intercalate throughout the charging and discharging process. For the diffusion-controlled process (battery behavior), the b-value is equal to 0.5, while for the intercalation pseudocapacitive, it is equal to 1. Bot mechanism, i.e. Bulk faradaic and intercalation pseudocapacitive, is suggested when the b value is between 0.5 and 1. The b-value, which is ascribed to the relation between the redox current of peaks I and V (from Fig. 7A) with the potential scan rate, was obtained from the slopes of Fig. 9D,E and are found to be 0.67 and 0.59, respectively. One can conclude from b values that the bulk faradaic process is mixed between intercalation and diffusion-limited mechanism. However, the predominant mechanism is the diffusion-controlled process due to the values being close to 0.5.Secondly, the GCD test is obtained for Fe3O4/GF at various current densities. Figure 10A shows that the shape of the GCD curve is a battery-like electrode by displaying the plateau which is consistent with the data and results obtained from the investigation of CVs of the Fe3O4/GF electrode. Figure 10B displays the variation of Cs with various current densities and the obtained values are 1300, 944.7, 830.33, 718.4, and 653 mC cm−2 at current densities of 5, 10, 15, 20, and 25 mA cm−2, respectively.Figure 10(A) GCD curves of Fe3O4/GF electrode at various current densities. (B) The variation of Cs of Fe3O4/GF electrode with various current densities. (C) Nyquist plots of GF and Fe3O4/GF electrodes at OCP (D) Nyquist for the Fe3O4/GF electrode. (E) Admittance plots of GF and Fe3O4/GF electrodes at OCP.To further emphasize the improving function of the preparation process and to provide clarification for the previously described findings, EIS measurements of the pristine support and Fe3O4/GF electrodes are conducted. From Fig. 10C, the Fe3O4/GF displays the steeper and shorter length line in low frequency region in comparison with pure GF demonstrating the excellent capacitive performance of the Fe3O4/GF electrode. Additionally, from Fig. 10D and its inset, the equivalent circuit of Fe3O4/GF electrode (inset of Fig. 10D displays four components: equivalent series resistance (ESR) (is represented by the intercept in the real axis at the high-frequency region), constant phase element concerning double layer capacitance (CPE 1), charge transfer resistance (Rct), and constant phase element concerning the capacity of (CPE 2))70,71. Interestingly, the encasement of Fe3O4 active materials around GF fibers makes the ESR of GF increase resulting in a decrease in the total electronic mobility. However, compared to the GF electrode, the Fe3O4 electrode has a lower ESR value. This suggests that the preparation method improves the GF material’s electronic conductivity. The admittance plot (see Fig. 10E) provides additional confirmation of this.As a consequence of the Fe3O4/GF electrode’s electrochemical studies above, which are conducted as a negative electrode for SC applications. the Fe3O4/GF electrode compared to other related Fe3O4-based materials shows distinct features. The characteristics and features of the prepared electrode materials are concise as follows: (i) it is prepared with just one inexpensive, and simple electrochemical procedure, (ii) binder-free method, (iii) in contrast to the other work, where a pretreatment step is used, the GF is in-situ altered, improving both ionic and electronic conductivity, and (iv) Fe3O4/GF has an exceptionally broad potential window for operation (1.45 V). However, the prepared electrode suffers from limited stability due to deterioration of the swelling and shrinking of the electrode materials throughout the cyclic performance which may be improved in future work by addressing many parameters such as the incorporation of other metals to develop a binary system that can relieve the stress during the cyclic performance, and coating the active materials with the above layer from carbon material that is increasing the stability without affecting the other parameters.
Tailoring a facile electronic and ionic pathway to boost the storage performance of Fe3O4 nanowires as negative electrode for supercapacitor application
