Hydrothermal Synthesis of Boron-Doped Graphene for High-Performance Zinc-Ion Hybrid Capacitor Using Aloe Vera Gel Electrolyte

Abstract

The great interest in developing emerging zinc-ion capacitors (ZIC) for energy storage applications is due to their inexpensiveness and the future necessity for hybrid electrical energy storage devices. The Zn-ion hybrid capacitor device was assembled using boron (B)-doped reduced graphene oxide (B-RGO) material, which acts as the cathode, and pure zinc metal as an anode. This research work aims to study the influence of B-doped reduced graphene oxide (B-RGO) with Aloe vera gel as an electrolyte. The reduced graphene oxide (RGO) and B-RGO electrode active materials were confirmed through X-ray diffraction (XRD), RAMAN, Fourier transformation infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM) and field emission-transmission electron microscopy (FE-TEM) analysis. The surface morphological images reveal that a few-layered nanostructure B-RGO was used in the Zn-ion hybrid capacitor device. The electrochemical performance of the Zn-ion hybrid capacitor was evaluated through cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurements, with a wide active potential range of 0–2 V versus Zn/Zn⁺. The mixture composition of Aloe vera extract and 1M ZnSO₄ electrolyte generated a stable voltage and exhibited good capacitive behavior. The fabricated ZIC coin cell device with the Aloe vera gel semi-gel electrolyte containing ZnSO₄ demonstrated improved Zn⁺ ionic exchange and storage efficiency. Moreover, the B-RGO electrode active material exhibited excellent cycle stability. The simple one-step electrochemical technique is the most suitable process for boron doping into graphene nanosheets for future energy storage applications.

1. Introduction

The past decades have seen incredible growth in supercapacitors, like electric double-layer capacitors (EDLC) [1], and pseudo-capacitors, with a high power density, lightweight construction, low-cost device assembly, and long cycle life [2,3]. Currently, there is growing interest in the development of aqueous electrolyte-based battery types, specifically zinc-ion hybrid capacitors (ZICs), which combine the advantages of metallic foil anodes found in batteries with carbon-based electrodes found in capacitors for energy storage devices [4], since the ZIC devices exhibit good energy and power density. The use of MXene materials, such as Ti₃C₂T_x and V₂CT_x electrodes, shows promise for the development of hybrid flexible energy storage ZIC devices [5]. In comparison, other hybrid capacitor devices based on different battery types require time-consuming and multi-step pre-lithiation and pre-sodiation of battery-type anodes [6,7]. Among these, zinc (Zn) metal batteries (ZMBs) have a high volumetric capacity of 5849 mAh/cm and a gravimetric capacity of 819 mAh/g. Therefore, ZICs hybrid devices with Zn metal anodes have been predicted as an outstanding alternative source for future electrochemical energy storage [8,9].

In general, there are three major types of electrolytes used in symmetric or asymmetric hybrid supercapacitors (SCs) which are included in batteries: liquid-state, solid-state, and gel-state [10]. Organic or non-aqueous electrolytes have higher dissociation voltage [11]. However, the higher resistivity of organic-based electrolytes limits cell power. The most common aqueous electrolytes are sulphuric acid (H₂SO₄) and potassium hydroxide (KOH), and also include zinc sulfate (ZnSO₄) for zinc-metal based supercapacitor applications [12]. The aqueous zinc-ion batteries (ZIBs) with liquid-state electrolytes are usually considered as ultra-intrinsic batteries [13,14]. Nevertheless, aqueous electrolytes present various side reactions of dendrite growth and metal zinc foil corrosion and Zn metal foil surface passivation [15]. The hydrogel-based gel electrolytes possess higher ionic conductivity, excellent flexibility, and good mechanical stability for future flexible-based batteries and hybrid supercapacitors [16,17]. Recently, a few reports have explored the introduction of Aloe vera gel in natural polymeric gel devices for electrochromic applications and cross-linked PVA/graphene-based materials combined with Aloe vera hydrogel for bio-compatible applications [18]. Liu et al. report self-healable ZIB based on guar gum/ZnSO₄/glycerol electrolyte gel for ZIB applications [19]. These aqueous hydrogel electrolytes, when combined with ZnSO₄ and mixed electrolytes, exhibit good ionic conductivity and lower internal resistance, resulting in superior power delivery [20]. The chemical composition of Aloe vera gel, a natural bio-polymer containing polysaccharides, has attracted significant attention for gel-state electrolytes, due to its ability to extend the electrochemical stability window and provide flexibility [21,22,23]. Moreover, compared with all-solid-state electrolytes, the gel-state electrolyte can offer safe and fast ionic insertion/extraction during the charge/discharge process for high capacity and good cyclic stability [24,25].

Commonly, graphene-based materials are widely used for energy storage applications in supercapacitors [26,27]. Recently, heteroatoms like Boron (B), Nitrogen (N), Phosphor (P), Fluorine (F) and Sulfur (S) were used to dope 2D carbonaceous materials, such as graphene nanosheets. Interestingly, boron (B)-doped graphene has been a promising active material in supercapacitor research for energy storage [28,29,30]. Boron-doped graphene materials with improved conducting/semiconducting properties have been proposed for high-energy storage applications. Boron-doped reduced graphene oxide sheets (B-RGO) represent a suitable candidate for the dopant source, as they strongly reveal an electrical energy storage sector with high power density and a long cycle life [31]. Boron atomic doping, stably combined with carbon atoms, increases electronic conductivity, which is favorable for electrochemical high-energy storage like supercapacitor applications [32,33,34,35]. Furthermore, B-doped graphene has been utilized in the development of lithium-ion batteries [36] and solar cells [37]. In the context of B-doping of carbon nanomaterials such as graphene, this can increase the specific capacitance of electric double-layer capacitors (EDLCs). The incorporation of boron atoms into the hexagonal sp²-bonded carbon framework creates stable 2D layered structures through boron–carbon (B–C) bonds, facilitated by electron-donating neighboring carbon atoms [38,39]. However, several methods have been employed to produce pure graphene, including chemical oxidation using Hummer’s method [40], microwave-assisted boron-doped few-layer graphene synthesis [41], and chemical vapor deposition (CVD) [42]. However, these methods have limitations, such as low yields and time-consuming purification processes. A single-step hydrothermal preparation has been utilized to produce boron-doped reduced graphene with high yield, specifically for supercapacitor electrode nanosheets in Zn-ion hybrid supercapacitor applications [43,44,45,46,47].

In this paper, the facile one-step synthesis of boron-doped graphene was achieved through hydrothermal doping with boric acid (H₃BO₃) as a boron source. A zinc-ion capacitor (ZIC) with pure reduced graphene oxide (RGO) and as-prepared B-RGO cathode was reported for the first time using an Aloe vera gel semi-gel film electrolyte. In addition to excellent energy storage performances, the as-prepared boron-doped graphene nanosheets are strong enough to be used for supercapacitors with 5000 charge–discharge cycles. Additionally, ionic conductivity and electrochemical studies for zinc-ion hybrid battery-type capacitors are carried out. We show that Zn-ion storage properties are improved through B-RGO.

2. Experimental

2.1. Materials and Methods

All reagents were of analytical grade and used without further purification. Graphene oxide (GO) was synthesized using a modified Hummer method. Pure graphite powder and boric acid (H₃BO₃) (the dopant source of the boron atom) was purchased from Alfa Aesar (Kandel, Germany). GO reduction into RGO was obtained using reagents with Hydrazine monohydrate (N₂H₄) reagent grade, 98%. The boron-doped graphene nanosheets (B-RGO) were prepared by facile one-step hydrothermal technique preparation, as shown in the scheme (Figure 1).

2.2. Experimental Procedure of B-RGO

The synthesis of boron-doped reduced graphene oxide (B-RGO) was performed in the following way. As-prepared fine GO powder of 1 g was dispersed in 180 mL of distilled water (DI) by sonication (1 h). After that, 0.3 g of H₃BO₃ was added to the suspension and stirred for 1 h at room temperature. The above-attained well-mixed solution was transferred into a 300 mL Teflon line with stainless steel autoclave and placed in the oven at 150 °C for 10 h. Finally, we allowed it to cool in the oven to room temperature. After that, the sample was filtered and washed with centrifugation (7000 rpm for 10 min), and dried at 80 °C. Afterward, the final product was collected and the graphene sample doped with boron was named as B-RGO. In order to prepare clean RGO without boron doping, 0.5 g of GO was first treated with 5 mL of hydrazine hydrate and then added to a beaker containing 100 mL of hydrazine hydrate. The above mixture was then placed in an 80 °C water bath pot for one hour; the resulting boron-doped reduced graphene oxide (B-RGO) and the active materials applied to the Zn-ion hybrid capacitor device are shown in Figure 1.

2.3. Material Characterization

The crystalline phases of the boron-doped graphene (B-RGO) and RGO samples were characterized by an X-ray diffraction (XRD) system using (PANalytical X’PERT-PRO model: Bruker D8 Focus, Almelo, The Netherlands) with Cu-Kα radiation (λ = 0.154056 nm), using a voltage of 40 kV, a current of 30 mA, and diffraction scanning ranges of a 2θ angle between 5° and 80°. RAMAN spectroscopy, an efficient technique for identifying the quality of the atomic doped graphene sheets, was performed using an XPLORA Plus (HORIBA Scientific, Horiba Jobin Yvon, Palaiseau, France) with a TE air-cooled charge-coupled device (CCD) detector with spectrum gaining time 100 sec and operating power at 10 mW laser excitation (room temperature excited with a YAG (Nd) laser source). The functional group identifications from the Fourier-transform infrared spectroscopy (FT–IR) spectra were recorded with Perkin Elmer (Model no. Spectrum-100, Thermo Scientific; Waltham, MA, USA), and the spectra were observed in transmittance (T%) mode in the wavenumber ranges from 400 to 4000 cm⁻¹ using KBr pellet field-emission scanning electron microscopy (FE-SEM S-4800 coupled with energy dispersive X-ray spectroscopy (EDS), Tokyo, Japan); it was used to analyze the surface morphology of RGO and B-RGO nanosheets; EDS was performed with the field emission SEM (FESEM; S-4800, Hitachi, Tokyo, Japan) at a 15 Kv accelerated voltage. The elemental composition analysis was quantified by HR-TEM energy dispersive X-ray (EDX) spectroscopy analysis to confirm the presence of RGO and B-RGO nanosheets. The field-emission transmission electron microscopy (FE-TEM) nanoscale images were recorded with an (FEI-Tecnai) TF-20 (FEI Company, Hillsborough, OR, USA) through an operating accelerating voltage of 200 kV; this instrument was utilized to study the nanoscale few-layered sheet morphology of the sample after being exfoliated and the B-doped sample. The surface element compositions, chemical compositions and electronic structures were resolved by X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo Scientific, Waltham, MA, USA) with the source of mono-chromatic Al Kα radiation (hν = 1486.6 eV; X-ray energy source, 15 kV and 150 W; spot size, 400–500 μm).

2.4. Electrochemical Characterization and Electrode Preparations

Electrochemical characterizations performed with an electrochemical workstation were examined using the BIOLOGIC-VSP multichannel potentiostat/galvanostat using a two-electrode device system (from Biologic, 4. rue de Vaucanson, Seyssinet-Pariset, FRANCE. A zinc metal foil (99%, MTI, Seoul, South Korea) was used as the negative electrode and as-prepared pure RGO and B-RGO active material electrodes acted as a positive electrode. We assembled a CR-2032 model coin-cell, zinc metal foil as anode and the working electrode composition of 80:10:10 weight ratio consisted of (i) active material (RGO and B-RGO); (ii) super-P carbon block, and (iii) polyvinylidene fluoride (PVDF) as binder with a solvent N-methyl-2-pyrrolidone (NMP). The above well-mixed (i, ii, and iii) compositions were homogeneously mixed in an agate mortar up to a gel-like slurry solution with high viscosity. Then the slurry was cast onto conductive carbon paper, HCP020 graphite paper (99.9%, battery grade, MTI) for making current collectors for the cathode material, and finally small circular discs were cut to make current collectors for the cathode material (cathode die-cutter of size 16 mm). Then the RGO and B-RGO slurry-coated electrodes were dried at 80 °C for 8 h in a vacuum oven. The active materials mass loading was around ~1–3 mg/cm².

2.5. Semi-Hydrogel Film Preparations

One of the novel aspects of this research work is the development of activated semi-organic aqueous gel electrolytes for zinc-ion hybrid capacitors. A semi-organic or organic Aloe-vera-based hydrogel electrolyte was derived from aloe leaves. Initially, pure Aloe vera (purchased from a local agricultural mart in Gyeongsan, South Korea) was first washed, then its outer leaf skin was carefully peeled off. Then the inner flexible part of the Aloe vera gel was very carefully sliced into thin layers. Furthermore, a thin portion of Aloe vera gel was kept at room temperature for 2 to 3 h, and a small lab syringe was used to take it out, since there was too much water in it. Afterward, the Aloe vera gel film was immersed in 1M ZnSO₄ electrolyte using a Petrik dish for 5 h. As a final step, the slime Aloe vera semi-hydro gel was cut using a 20 mm separator hand cutter for the zinc-ion hybrid supercapacitor device, using a CR-2032 model in semi-wet condition. Furthermore, Aloe vera semi-aqueous hydrogel film and ionic measurements were conducted to compare RGO and B-RGO cathodes. The cyclic voltammetry (CV) was carried out at varied scan rates from 10 mV/s to 110 mV/s. The impedance was measured in the frequency range of 1 MHz–100 mHz with a 10 mV of voltage bias. The galvanostatic charge/discharge (GCD) curves were studied from 0.0 to 0.2 V vs. Zn/ZnSO₄.

3. Results and Discussion

3.1. X-ray Diffraction Analysis

The XRD analysis was carried out on the structural properties and inter-planar expansion of raw graphite and boron-doped graphene nanosheets (Figure 2). The XRD pattern of reduced graphene oxide (RGO) (Figure 2) shows a major characteristic of the highly intensive peak at 2θ-26.34°, corresponding to the (002) crystal plane; the resultant d-spacing value is 3.31 Å. The additional peaks were observed at 42.38°, 54.39° and 77.40°, corresponding to the planes (100) (111) and (200), respectively. After doping/reduction of B-RGO (inset Figure 1), a large broadening diffracted peak was observed at 24.36°, corresponding to the (002) plane with the resultant d-spacing value being 3.42 Å. For the sake of comparison, in pure hydrazine-reduced GO, the broadening B-RGO material peak was shifted towards slightly lower angles of 2θ–24.36°, and the magnified view of the XRD pattern is shown in Figure S1 of the Supplementary Materials. From the XRD results, graphene layers (B-RGO) have an increased interatomic layer distance (an interlamellar d-spacing value of 3.42 Å), as boron atoms are incorporated in interlayer spacing within graphitic nanostructures, resulting in an increase in interlayer distances. Hence, XRD results confirm hydrothermally produced boron-doped graphene nanosheets as good crystalline materials.

3.2. Micro-Raman Analysis

Figure 3 shows the Raman analysis, a unique informative technique used to study the structural characterization of graphitic materials, for B-RGO and RGO. The main features in the Raman spectra of pure RGO were represented by the sp² hybridized carbon G-band and the sp³ hybridized carbon as the D-band. The D and G bands in pure RGO were seen at 1349.97 cm⁻¹ and 1573.92 cm⁻¹, respectively. (The RGO magnified view of the Raman data is shown in the Supplementary Materials (Figure S2)). The boron-doped few-layer graphene nanosheets (B-RGO) show a very highly disordered (D) band at 1345.98 cm⁻¹ and the graphitic (G) band at 1591.86 cm⁻¹ intensity also increases gradually [35]. The small disorder in the bulk graphite may be due to the sp³ hybridized carbon atoms being polyaromatic hydrocarbons [36,37]. The intensity disorder ratio (I_D/I_G) for B-RGO and RGO corresponds to the values of 0.93 and 0.32, respectively. The (I_D/I_G) higher disorder (D-band-sp³) ratio confirms that the boron atom is doped in crystalline graphene. As a result, the Raman spectrum confirms that the as-prepared graphene nanosheets are B-doped.

3.3. FT-IR Analysis

The vibrational characteristics of functional groups were analyzed using Fourier-transform infrared spectroscopy (FTIR). From Figure 4, the B-RGO sample shows that an intense and high broad peak was observed at 3437.39 cm⁻¹, corresponding to the O–H stretching vibrations of the carboxylic functional group. A major transmittance, present at 1631.62 cm⁻¹, corresponds to the C=C contest bond matching the aromatic stretching vibrations for the sp² carbon network. The short broadband at 1380.01 cm⁻¹ corresponds to the C=O carboxylic group. In the FT-IR spectrum, the B-RGO major functional groups were commonly attributed to the bands like the boron–carbon band, which is observed in the range of 1050–1200 cm⁻¹. The very sharp peak at 1052 cm⁻¹ can be attributed to the B–C stretching bond, and corresponds to the carbon network interacting with boron atoms with sp³ (the trigonal boron atom leads to a B–C intense peak) [38]. These boron-containing functional groups resulting in all cases can be attributed to the presence of B-RGO. The major characteristic peaks of the pure RGO spectrum were assigned to functional groups: 3437.39 (O–H), 1575.16 (C=O), 1632.44 (C=C), and 1188.63 cm⁻¹ (C–O–C), respectively. In conclusion, the comparison of RGO and B-RGO characteristics in infrared (IR) spectrums confirms that boron was successfully incorporated into carbon lattices [39].

3.4. FE-SEM Morphological Analysis

Figure 5a,b shows the FE-SEM microscale morphology images of the few-layered structure of expanded graphene oxide prepared by a facile one-step hydrothermal process. The reduced graphene (RGO) shows a highly graphitic stacked layer morphology in the interlayer structure. The micrographs of the as-synthesized B-RGO at low magnification view are shown in Figure 5c,d. The B-doped few-layered graphene sheets display reduced domain size, improved surface exfoliation morphologies, and twisted edges. Figure 5e,f shows hydrothermally doped graphene nanosheets that range in thickness from 20 to 30 nanometers. When graphene is exfoliated into thin platelets with B-atom doping, it forms wrinkles and folded layers. As a result, the SEM images of the graphene nanosheets that are doped/reduced with boron reveal a few-layered structure.

3.5. Energy-Dispersive X-ray (EDX) and Element Mapping Spectroscopy Analysis

Energy-dispersive X-ray (EDX) spectroscopy elemental mapping analysis was performed on the B-RGO major sample, obtained by hydrothermal doping/reductions using GO with boric acid. The EDX mapping analysis (Figure 6) confirms the presence of boron (B), carbon (C), and oxygen (O), and is normally associated with materials that are coated on carbon tape substrates.

From the EDX elemental atomic composition, percentages were qualitatively found to be (at. %) level at B-5.37 at. %, C-72.80 at. %, and O-21.82 at. %. The EDX clearly indicates that the boron atom doped or boron functional groups were still present in the graphene (carbon) structure. This results in an effective atomic doped graphene with a total percentage of boron (B) of 5.37 at.% in the carbon matrix. As can be seen from the boron atomic-doped RGO, the hydrothermal method was used to prepare atomic-doping graphene with a high yield.

3.6. FE-TEM Analysis

Figure 7a–c presents field-emission transmission electron microscope (FE-TEM) images of the synthesized B-RGO material. The typical FE-TEM image (Figure 7a), shows clearly that the reduced graphene prepared is twisted and crumpled in nature. Figure 7b shows the edge-folded few-layer graphene sheets; each graphene nanolayer was chemically exfoliated and the boron atom functionalized into RGO. The field-emission TEM nanoscale image in Figure 7c clearly depicts a few layers of graphene (around 3–4 layers), with warped edges, and a highly transparent section of heteroatom-doped graphene is observable. Notably, Figure 7d–g illustrates that the B-RGO images demonstrate minimal restacking of the few layers: they do not restack. Furthermore, in Figure 7d,h, the selected area electron diffraction (SAED) pattern exhibits a ring-like pattern, comprising multiple diffraction spots corresponding to the hexagonal lattice of boron-doped graphene. Therefore, for successful energy storage applications, one-step hydrothermal doping for the doped graphene from graphene oxide (B-RGO) is crucial for large-scale production.

3.7. XPS Analysis

X-ray photoelectron spectroscopy (XPS) was performed to find out the chemical composition, and is a powerful tool for characterizing the doping levels of heteroatoms. As shown in Figure 8a,b, the presence of B1s, C1s and O1s for the B-RGO sample was confirmed. The wide scan spectra details of the XPS peak de-convolution are presented in pure RGO and B-doped RGO samples. The pure RGO (Figure 8a) material was de-convoluted for the core-level high-resolution C1s spectrum which was de-convoluted, fitting into three different components accordingly: the sp² carbon networks were 284.32 (C=C), 286.35 (C=O) eV and 288.12 eV (O–C=O), respectively. The graphene edge sites are composed of unsaturated carbon atoms that are bonded to pure RGO, which is chemically reduced with a considerable number of carboxyl functional groups. Figure 8c,d shows that the de-convoluted B-RGO spectrum indicates three distinct components: C–O/C–N (284.24 eV), C=O (285.46 eV) and C–B/O–C=O (287.83 eV). It is evident from Figure 8e that boron atoms achieve the B1s peak at 193.2 eV, which indicates that boron atoms exist in the graphene sample obtained from hydrothermal doping [40,41]. In (d) B-RGO (O1s), the deconvoluted fitting forms of the C–O (531.87 eV) and B–O/O–C=O (535.37 and 533.11 eV) groups, it appears that, due to the difference between boron and oxygen, O₂ can be readily absorbed by the boron dopant. Figure 8e indicates that boron has been successfully integrated into the graphene and sp³ carbon frameworks. In addition, boron is capable of substituting carbon at the trigonal sites (BC₃) [40,41,42,43]. The oxygen content confirms the presence of structural defects contained by the sample. Therefore, the high-resolution B1s spectrum component that corresponds to the boron atoms can be discerned from the XPS spectrum [44,45].

3.8. Electrochemical Measurements ZIC

3.8.1. Cyclic Voltammetry and Electrochemical Impedance Spectroscopy (EIS)

Figure 9 shows the cyclic voltammetry (CV) curves of B-RGO martial at different scan rates of 10–50 mV/s obtained by a three-electrode cell system using 1 M ZnSO₄ with Aloe vera semi-gel electrolyte. The CV curves of pure RGO and B-RGO electrodes’ active material at various scan rates of 10, 30, 50, 70, 90 and 110 mV/s in the potential range of 0.0–2.0 V are shown in Figure 9a. From the CV curves, the pure RGO materials clearly showed a near-rectangular nature indicating typical double-layer capacitor (EDLC) behavior. In addition, the B-RGO materials show the CV curves revealing EDLC with a partially pseudo-capacitive (Faradaic reaction) nature. The cyclic voltammetry (CV) curves with a higher active current density reveal that boron atoms are effectively doped into reduced graphene oxide (B-RGO). In Figure 9c, as expected, the increase in scan rate resulted in increased current density and an integrated area with a fairly small change in the shape of the CV curves. The CV curves exhibit a pair of redox peaks that can be attributed to diffusion kinetics for a fixed potential window of 0–2 V, which indicates quasi-reversible redox reactions. In Figure 9c the CV curves obtained at 110 mV/s for the bare electrode conductive substrate graphite paper, pure RGO and B-RGO, show a gradual increase in the electrochemical active window.

Electrochemical impedance spectroscopy (EIS) was performed to investigate the impedance spectra of the B-RGO material, as depicted in Figure 9d. The EIS Nyquist plot exhibited a semicircle at the high-frequency region, indicating the presence of internal solution resistance (Rs) and charge transfer resistance (Rct) within the system. The Nyquist plot also revealed a straight line with a slope of approximately 45° in the low-frequency region, indicating diffusive resistance characterized by the Warburg (W) impedance. The EIS spectrum was collected over a frequency range from 100 kHz to 0.01 Hz, and the inset in Figure 9d provides an expanded view of the high-frequency region. These observations confirm the presence of Rs, Rct, and W impedance components in the electrochemical behavior of both the RGO and B-RGO materials. In comparison, Nyquist EIS measurements of charged transfer resistances result in Rct = 40.22 Ω and 57.51 Ω for B-doped reduced graphene (B-RGO) and pure RGO, respectively (inset shown in Figure 9d); the parameters of the equivalent circuit are comparatively discussed in Table S1 of the Supplementary Materials. According to these EIS measurements, heteroatom-doped graphene electrodes have much lower resistances (Rct) than pure RGO electrodes. EIS Nyquist plots showed concurrent increases in the B-doped graphene’s electrical conductivity and electrochemical activity. Because of the simultaneous reduction in GO and doping, the hydrothermal technique for doping is simple and cost-effective. The EIS results can be attributed to the enhanced surface wettability of the electrolyte after boron doping, which also plays an important role in improving the electrochemical properties.

3.8.2. Galvanostatic Charging/Discharging (GCD) Measurements

There has been great interest in aqueous zinc-based electrochemical storage devices that use zinc-ion battery-type galvanostatic charging and discharging, especially when they use Aloe vera gel film-based aqueous electrolytes. It was therefore decided to construct hybrid zinc-ion based capacitors (ZICs) by coupling the capacitive cathode active materials (RGO and B-RGO), with pure zinc foil for the anode and Aloe vera semi-gel film as the separator and Zn-ion transport in an aqueous electrolyte. As shown in Figure 9, the galvanostatic charging/discharging study was performed on boron-doped graphene nanosheets (B-RGO) at different current densities of 1, 2, 5, 7 and 10 mA/g. The specific capacitance (Csp) was observed from charge–discharge with different current densities, using the following CGD battery-type curves.

The electrochemical performance of the RGO electrodes was evaluated using a galvanostatic charging/discharging plot. The charge–discharge results for the pure RGO electrodes exhibited specific capacity values of 184.89 mAh/g, 127.41 mAh/g, 74.82 mAh/g, 49.71 mAh/g, and 32.09 mAh/g, corresponding to current densities of 1, 3, 5, 7, and 10 mA/g, respectively. These values indicate the capacity of the electrodes during the charge and discharge processes. This figure (Figure 10a) presents the electrochemical analysis of galvanostatic charge–discharge; a maximum specific capacitance of 184.89 mAh/g was obtained at an estimated current density of 1 mA/g.

Figure 10b shows the electrochemical galvanostatic charging/discharging (GCD) performance of Aloe vera gel-based ZICs for different current densities for the B-RGO electrodes. Figure 10b B-RGO electrodes shows that the discharge-specific capacities are 305.95, 197.55, 145.28, 97.93, and 61.88 mAh/g at current densities of 1, 3, 5, 7 and 10 mA/g, respectively. Compared with a maximum capacity of 305.95 mAh/g at a current density of 1 mA/g, the electrochemical performance suggests that the boron atomic functional groups with inner RGO lattice defects (B–C) enhance the charge transfer kinetics through Aloe vera semi-gel electrolyte thin films. A heteroatom-doped reduced GO (B-RGO) is an effective candidate for battery-type zinc-ion storage by taking advantage of the surface redox reactions. Moreover, the comparative galvanostatic charging/discharging curve of Zn//RGO and Zn//B-RGO @ 1 mA/g data is displayed (Figure S3) and in the Supplementary Materials.

In Figure 11, Ragone shows that in the observed Zn//RGO and B-RGO devices when the different current densities (1–10 mA/g) were applied, specific capacity values in the range of (184.89–32.09 mAh/g) were obtained. In comparison with the Zn//B-RGO device, the Ragone plot shows two instances of good capacitive behavior at specific capacity ranges of 305.95–61.88 mAh/g at the current density range of 1–10 mA/g. Therefore, graphene nanosheets doped with boron atoms have excellent properties as electrode-active material in Zn-ion-based battery-type hybrid supercapacitors. It is determined that the good specific capacitance value of graphene is due to its enhanced electrical conductivity and its higher capacitance, due to boron atoms doped into the graphene networks. As a result, bulk synthesis of graphene nanoplatelets can be achieved via the exploration of direct doping techniques. Furthermore, uniform doping with high efficiency is essential for improving the electrical properties of doped graphene nanoplatelets and developing their energy-storage capabilities.

3.8.3. Cyclic stability of Zn//RGO and Zn//B-RGO

GCD cyclic stability tests were conducted on RGO and B-RGO electrodes in 1M ZnSO₄ electrolyte at 10 mA/g during a galvanostatic charge–discharge at 5000 cycles (Figure 12). The Coulombic efficiency (CE) of the hydrothermally B-doped RGO nanosheets was 99.5% over 5000 cycles. In addition, both Zn//RGO and Zn//B-RGO active electrodes possess a specific capacity stability of 60–53 mAh/g and 30–22 mAh/g, respectively. In this case, the high surface morphological changes in the graphene nanosheets may be caused by the activation of boron doping RGO during long cycles, resulting in good charge–discharge stability for graphene nanosheets. In comparison with the previous reports [28,44,48,49], these results of cycling stability are relatively excellent (see Table S2 of the Supplementary Materials). In addition, a comparison study of pure graphene electrodes and B-doped graphene nanosheets was conducted. It demonstrates that the easily prepared B-RGO electrode materials are suitable for semi-gel-based electrolytes for future hybrid energy storage devices, with excellent electrochemical performance and excellent cyclic stability.

4. Conclusions

We demonstrated the Zn-ion battery-type hybrid capacitors (ZIC) using heteroatom-doped and reduced few-layered graphene (B-RGO). Morphological imaging using FE-SEM and FE-TEM confirmed the presence of boron-doped crumbled reduced graphene nanosheets. EDX analysis revealed that the boron content was 8.69 at.%. The cyclic voltammetry (CV) curves exhibited a capacitive nature characteristic of electric double-layer capacitors (EDLCs). Galvanostatic charge–discharge analysis showed that the B-RGO sample exhibited a specific capacity of 96.33 mAh/g. The Zn//B-RGO configuration demonstrated high Coulombic efficiency (over 99.5%) and stable specific capacity (60 mAh/g at 10 mA/g) with excellent cycle stability over 5000 cycles. In comparison, the maximum specific capacities of RGO and B-doped RGO devices were found to be 184.89 mAh/g and 305.95 mAh/g at 1 mA/g, respectively. In conclusion, the Zn vs. B-RGO device displayed superior Zn⁺ storage capacity compared with undoped reduced graphene, thereby increasing the storage capacity and cycle life. Heteroatom doping is an innovative approach to reducing the cost of graphene production. We demonstrated a high-performance electrochemical Zn-ion capacitors, and we can enhance the hydrothermal performance of single-step GO reduction while simultaneously achieving effective B-doping for future energy storage materials.