It is crucial to design advanced electrode materials to meet the rising and urgent demand for high-performance supercapacitors. An inexpensive carbon-based electrode would not only provide a cost advantage but would also maximize the device energy density. Peanut shell-derived few-layer graphene (PS-FLG) was so chosen as the active material for supercapacitor. First, peanut shell precursor was carbonized followed by activation with KOH. Potassium reacts violently to rip apart the layers between carbons. To get the highly dispersed PS-FLG, probe ultrasonication was preferred to accelerate the aqueous exfoliation of PS-AC in H 2 SO 4 because a definite power delivery into the aqueous system can be ensured in this process; entirely different from the variable bath ultrasonication47. Probe sonication was performed on a QSonica (Part no.- Q700, USA) ultrasonicator using a replaceable microtip of a diameter of 1/16″ (1.6 mm) at an amplitude of 112 µm for 1 h with a pulse on-and-off time of 5 seconds.

To check the dispersion of the as derived PS-FLG material in different solutions, several aqueous, as well as organic solvents have been selected. Figure S1 shows the digital photographs of PS-FLG dispersed in various solvents like water, isopropanol, dimethylformamide, dimethyl sulfoxide and carbon tetrachloride. The PS-FLG sample exhibits high dispersibility in polar (aqueous) as well as non-polar (organic) solvents. Fourier-Transform Infra-Red (FT-IR) analysis was conducted to establish the functional group present in PS-FLG material. As can be observed in Fig. S2, an array of both polar and non-polar functional groups is present in PS-FLG. The absorption peak at 3429 cm−1 can be assigned for the -OH stretching vibrations of the hydroxylic group and chemisorbed water48. The skeletal ring vibration of graphene-like sheets can be observed at 1631 cm−1 49,50. The peak at 3160 cm−1 stands for C=C-H stretching mode51. Medium broad vibration observed at 2786 cm−1 is ascribed to the overtones of O-H bending mode which arise from proton tunneling and Fermi resonance interactions52. The peaks at 1397 cm−1 and 1464 cm−1 appears because of the-CH 2 and -CH symmetric bending modes53. Presence of both polar and non-polar groups in its structure, makes PS-FLG disperse both in aqueous or organic solvents.

In order to investigate the structural transformations of the as-synthesized PS derived carbon materials, Raman spectral analysis was carried out. Figure 2a records all the Raman peaks corresponding to the PS-derived carbon structures at various stages of synthesis. Raman spectra of graphene-like materials are regularly characterized by two featured bands, D-band & G-band54. The D-band arise due to the breathing mode of k-point phonons of A 1g symmetry and G-band is assigned to the first order scattering of E 2g phonon of sp2 carbon atoms21,55. As can be observed in Fig. 2a there is a gradual red shift in the D-peak from PS-C (1351 cm−1) to PS-AC (1344 cm−1) to PS-FLG (1342 cm−1); it can be assigned to the gradual attainment of the more ordered structure of the PS-derived material at each stage. The G-band appears because of degenerate in-plane E 2g optical mode at the center of the Brillouin zone of graphitic carbon56. PS-derived carbons show a red-shift also in their G-band from PS-C (1593 cm−1) to PS-AC (1590 cm−1) to PS-FLG (1588 cm−1); gradually reaching for the more graphitic character. A characteristic overtone peak (2D band) at 2680 cm−1and (S3 band) at 2909 cm−1 appeared corresponding to PS-FLG, indicating graphitization of the materials57. The degree of crystallinity is directly proportional to the intensity ratio of D & G-peaks. As calculated from the obtained data, I G /I D for PS-FLG is 1.01, gradually increasing from 0.99 for PS-AC & 0.98 for PS-C; suggesting the increase in crystalline nature of the material12.

Figure 2 (a) Raman spectrum; (b) XRD patterns; (c) Comparison of N 2 sorption analysis isotherm (magnification in inset showing a type-IV isotherm for PS-FLG) and (d) Pore size distribution plot by NLDFT method of PS-derived materials at different stage of synthesis;(inset shows achievement of mesopores for PS-FLG). Full size image

Wide angle X-ray powder diffraction analysis of the PS-FLG sample was conducted under monochromatized Cu-K α to study the crystallinity and phase structure. As shown in Fig. 2b, the prominent diffraction peak at 2θ = 30.1° originates because of the characteristic reflection from the (002) graphitic plane. This peak is much sharper than the small peak at 2θ = 43.5° assigned to the (100) plane of distorted graphitic sheets, confirming the high degree of crystallinity of the exfoliated sample58. Figure S3 gives a comparative view of the XRD patterns of WS-C and AS-C. It shows the presence of some turbostratic disordered carbon phase along with the characteristic diffraction peaks for carbonaceous materials, at 2θ = 23.9° corresponding to the (002) plane and 2θ = 43.1° for the (100) plane in both AS-C and WS-C59.

Specially designed layered carbon nanomaterials with a good balance between mesoporosity and microporosity is highly desirable to achieve high energy/power density simultaneously60. Materials with high surface-to-volume ratio and abundance of mesopores promote sufficient charge storage (high energy density) and fast charge transfer kinetics (high power density) respectively which are very significant for applications in advanced energy storage systems. The N 2 adsorption-desorption isotherm of PS-FLG shows a mixed type I and type IV curve as observed in Fig. 2c; typical of a material with both microporosity and mesoporosity. A magnified image, of the curve corresponding to PS-FLG shown in the inset of Fig. 2c, shows a typical type IV curve with an H 2 type hysteresis loop, inherent of a mesoporous material, in the relative pressure (P/P 0 ) range of 0.4–0.712,60. However, this type of hysteresis loop is absent in the case of PS-C and PS-AC (Fig. 2c), suggesting the presence of only micropores in them. Only micropores are not suitable for either charge storage or propagation12. A steep increase in N 2 uptake at lower relative pressure also suggests the presence of micropores in PS-FLG61. Mechanical exfoliation not only separates the graphene sheets, but it is also involved in increasing the porosity, suitable for supercapacitors. Pore size distribution was computed using the adsorption branch of the isotherm by the NLDFT method, showing an average pore size of 1.3 nm in Fig. 2d. Magnified view of the NLDFT pore size distribution plot of PS-FLG (Fig. 2d, inset) reveals achievement of mesopores (2.1–5.4 nm) during exfoliation. Microporous substances with an enhanced surface area is ideal for elevated capacitance and power density in aqueous electrolyte because of high conductivity and smaller ionic size62. But to explore its potential in the organic electrolyte, it is necessary to have a reasonable mesopore volume and their good interconnectivity for the electrolyte ions to effectively realize all the available surface area of the carbon framework63,64. The specific surface area of the PS-FLG is measured to be 2070 m2 g−1, whereas, the surface area of PS-C and PS-AC is estimated to be 645 and 1554 m2 g−1, respectively. This high surface area acknowledged the superiority of the methods used to produce PS-FLG material compared to other methods reported recently for producing graphene-like carbon from biomass13,22,23,65,66 (Table 1). Such a high specific surface area (2070 m2 g−1) with high pore volume (1.33 cm3 g−1) can afford more active sites during the charge-discharge process and thus makes PS-FLG an attractive material for energy storage devices. WS-C and AS-C were also subjected to BET surface area analysis and the specific surface area of the WS-C and AS-C are measured to be 363 and 403 m2 g−1, respectively. Figure S4a gives the comparative N 2 adsorption-desorption isotherm plot of PS-FLG with the other two nutshells-derived carbon. Magnified view of WS-C and AS-C are shown in Figure S4b, which represents that both the materials possess type IV curve with an H2 type hysteresis loop, typically observed for mesoporous materials. Though the average pore size of both the material is around 2 nm, the presence of pores with larger diameters (Figure S4c) are also observed, confirming the presence of mesopores in them. Although the materials have mesoporous architecture, their not-so-high specific surface area along with low pore volume (0.047 cm3 g−1 for WS-C and 0.035 cm3 g−1 for AS-C) is responsible for their lower capacitive properties than PS-FLG. Table S1 summarizes all the surface morphological analysis data obtained from the peanut, walnut and almond shell-derived carbon material.

Table 1 Comparison of different graphene-based materials synthesized from waste biomass and their applications. Full size table

To investigate the microstructures, surface morphology and topology of the as-synthesized materials, SEM and AFM investigations were performed. The morphology of PS-FLG can be attributed to our synthesis strategy which is tailored accordingly to take maximum structural advantage of the peanut shell. SEM images in Fig. 3a–c reveals that the so formed PS-FLG contains a broad distribution of sheet-like structure. Figure S5a shows the distinctive SEM micrograph of the pre-carbonized PS, where the naturally abundant lignocellulosic microfibril networks quantify the rough surface morphology, a characteristic feature of any biomass. Figure S5b and c show the SEM images of the carbonized sample PS-C, and KOH-activated sample PS-AC, respectively. As observed, on activation with KOH at 800 °C the samples transformed to sheet-like morphology. Further, as seen in Fig. 3a–c, on exfoliation the otherwise stacked sheet-like structure was transformed to few layers graphene-like morphology with nearly transparent appearance. SEM images of walnut and almond waste shell based carbon were also captured. As can be observed in Figure S6, both WS-C and AS-C have a porous surface with profoundly broken sheet structures forming agglomerates unlike the continuous few-layer graphene-like nanosheet morphology, observed for PS-FLG in Fig. 3.

Figure 3 Microscopic surface analysis of exfoliated PSC (PS-FLG): (a), (b) and (c) SEM images at different resolutions. Scale bars: 10 µm and 5 µm. (d) Tapping-mode AFM images representing 2D mapping and height profile. Full size image

EDS was carried out to find out the elemental composition of the as developed PS-FLG, as observed in Fig. S7. The abundance of carbon is clearly seen with almost 94 wt.% of it with the minimal occurrence of oxygen (only about 6 wt.%) confirming the effective carbonization of the active materials. Atomic force microscopic (AFM) measurements were conducted to study the top-view images and cross-sectional thickness of the FLG material. The average cross-sectional thickness of the graphene sheets is found to be around 3.2 nm. Figure 3d reveals the existence of 6–8 layers of graphene sheets, considering the thickness of the monolayer graphene is 0.33 nm67. A detailed three-dimensional mapping of the PS-FLG nanosheets is presented in Fig. S8.

TEM measurement was performed to confirm the morphology of the few-layered graphene materials further. As can be seen in Fig. 4a–c, transparent few-layer graphene-like nanosheets are to be clearly observed at various magnifications on a holey carbon grid. High-resolution TEM (HRTEM) images of selected regions are presented in Fig. 4d and e revealing distorted nanosheets consisting about 5–6 layers of graphene nanosheets. Further, HRTEM confirms the interlayer spacing of ~0.34 nm (Fig. 4e), which corresponds to the (002) plane of few-layered graphene nanosheets. Selected area electron diffraction (SAED) analysis reveals the presence of a hexagonal lattice of crystalline graphene-like sheets with long-range order62. As can be observed from the diffraction pattern in Fig. 4f, there are six well-defined diffraction spots corresponding to a graphene-like lattice, indicating the crystalline nature of the sheets68,69. The active crystal planes of the nanosheets of PS-FLG are calculated to be (002) and (100) planes of the reciprocal lattice, which is in line with the data obtained from XRD measurements (Fig. 2b).

Figure 4 (a–c) TEM images of PS-FLG at different magnifications; (d) and (e) HRTEM images showing crystallite size of about 4.2 nm; (f) SAED pattern of PS-FLG clearly showing hexagonal graphene like lattice. Full size image

Electrochemical Analysis

Electrochemical performances were measured to evaluate the potential applications of the PS-derived few layer graphene-like nanosheets as an active electrode material in aqueous as well as organic electrolyte using three electrode setup. Cyclic voltammetric curves obtained in 1 M H 2 SO 4 at different stages of synthesis show the gradual increase in specific capacitance (C SP ) from the carbonized sample, PS-C; to the activated sample, PS-AC; and to the exfoliated sample, PS-FLG as exhibited in Fig. 5a. The increase in the C SP of PS-FLG with respect to PS-C and PS-AC is due to the increased specific surface area as well as the better distribution of pores of slightly larger diameter (as shown in Fig. 2d, inset); which favours the charge storage capacity of the exfoliated material. It explains the phenomenon of better ion transport of the PS-FLG materials; which further establishes the need for exfoliation to attain well-separated nanosheets. A distinctive capacitive behavior with quasi-rectangular CV curve was maintained throughout a good range of scan rates from 10 mV s−1 to 1000 mV s−1 for PS-FLG as shown in Fig. 5b, implying quick dynamics of high power behavior of the PS-FLG material. The presence of a bump in CV at slow scan rate indicates a little contribution of redox species to the electrochemical capacitance of the PS-FLG materials. The closely rectangular shape of the CV was maintained above the scan rate of 100 mV s−1, which may be attributed to the optimum amount of combined micropore and mesopore volume as well as good electrical conductivity63. The capacitive potential of the PS-FLG based electrode was further investigated by galvanostatic charge-discharge (GCD) experiments. An expected increase in discharge time was observed in the case of PS-FLG from PS-AC or PS-C at a current density of 1 A g−1 as shown in Fig. 5c.

Figure 5 Electrochemical capacitive behavior of different PS based electrodes in 1 M H 2 SO 4 : (a) CV at a scan-rate of 100 mV s−1 at different stages of synthesis; (b) typical CV response of PS-FLG at various scan rates (10–1000 A g−1); (c) GCD curves comparing at 1 A g−1; (d) GCD of PS-FLG at different current densities from 0.5–50 A g−1; (e) change in specific capacitance of PS-FLG as a function of current density; (f) comparative Ragone plot. Full size image

The highest C SP of 186 F g−1 was obtained at a current density of 0.5 A g−1 for PS-FLG as can be calculated from Fig. 5d. The superior performance of PS-FLG can be ascribed to its porous sheets like structure and the high BET surface area (2070 m2 g−1) which is crucial for better penetration of electrolyte ions and fast charge propagation and storage. Exfoliation promotes a few atomic layered thick graphene sheets, increasing its electrical conductivity dramatically and thus PS-FLG shows a huge increment in capacitive performance as an active electrode material for supercapacitor. The C SP (F g−1) of PS-FLG-based electrode material were calculated at various current densities from 0.5 A g−1 to 50 A g−1 (Fig. 5d). Rate capability is one of the determining parameters that are essential for practical applications of the active electrode material. Here as can be seen, even at a higher current density (50 A g−1), the typical quasi-rectangular shape of the GCD curve is maintained showing appreciable charge-discharge reversibility. As with enhanced current density, limited charge diffusion tends to reduce the specific capacity; Fig. 5e demonstrates how excellently PS-FLG can retain C SP , even with increasing current density.

For the fabrication of highly advanced supercapacitor for practical use, energy density must be improved without sacrificing the power supply. Enhanced C SP with a wide operating potential window is appropriate for increasing the energy density of the supercapacitor material. From the Ragone plot (Fig. 5f), it can be observed that PS-FLG exhibits highest energy density of 58.13 W h Kg−1 against a reasonably good power density of 375 W Kg−1 in a large potential window of 1.5 V (−0.4 V–1.1 V). This is maintained at 15.63 W h Kg−1 even at the highest power density of 37.5 KW Kg−1 in an aqueous electrolyte of 1 M H 2 SO 4 . The PS-AC and PS-C show highest energy densities at 6.8 W h Kg−1, and 1.2 W h Kg−1 with highest power densities attained at 15 W Kg−1 and 7.5 W Kg−1, respectively (Fig. 5f).

Electrochemical responses of the walnut and almond shells-derived carbon (WS-C or AS-C) were also studied as potential materials for different carbon sources for comparison. Cyclic voltammetry (Fig. 6a) at a scan rate of 100 mV s−1 and galvanostatic charge-discharge study at 0.5 A g−1 (Fig. 6b) was conducted for WS-C and AS-C to compare the capacitive behavior with PS-FLG material. As can be observed in Fig. 6b, PS-FLG shows a huge C SP of 186 F g−1, much more (about 4.5 times and 2.5 times higher) than that indicated by WS-C (42 F g−1) or AS-C (78 F g−1) at a current density of 0.5 A g−1. Detailed electrochemical performance of WS-C and AS-C are shown in Fig. S9. Fig. S9a and b gives the cyclic voltammetric responses at various scan rates while Fig. S9c and d represent the galvanostatic charge- discharge profile at different scan- rates of WS-C and AS-C based electrodes. As can be seen in Fig. S9c and d, both WS-C and AS-C are stable only up to a current density of 10 A g−1, much lower than PS-FLG which can retain their rate capability even at a much higher discharge current of 50 A g−1.

Figure 6 Comparison of electrochemical capacitive behavior of different nutshell-based electrodes in 1 M H 2 SO 4 : (a) CV at a scan-rate of 100 mV s−1; (b) GCD curves at a current density of 0.5 A g−1. Full size image

Electron impedance spectroscopy (EIS) was further done at the open circuit potential (OCP) to ensure that the as developed few-layer graphene-like nanosheets are well equipped to promote fast charge-discharge as well as efficient charge storage12. As demonstrated in Fig. 7a, at lower frequency region, the imaginary part increases more sharply for PS-FLG, than PS-C & PS-AC, indicating diffusion limited electron transfer characteristic of ideal capacitive behavior of the material. A closer view of the mid-frequency region reveals that the line due to Warburg resistance cuts the real axis at the smallest angle for PS-FLG as compared to other PS-based electrode materials. It indicates that the ion diffusion pathways for electrolyte ions to the pores of the electrode surface is shortest for PS-FLG12. A magnified image (inset, Fig. 7a) in the higher frequency region shows the real axis intercept provides equivalent series resistance (ESR, R s ) along with the semicircular impedance loop whose diameter gives the charge transfer resistance (R ct ). The R s value for the PS-FLG material is found to be 6.1 Ω, whereas, it was 6.8 Ω and 25.4 Ω, for PS-AC and PS-C, respectively. The R s value clearly tells the PS-FLG material has high conductivity and low internal resistance. The nanosheet-like structure allows for fast ion-transfer kinetics between the electrode surface and electrolyte making it most suitable for use as an active material. Figure 7b is the Bode phase plot, which demonstrates that in the low-frequency region when the phase shift approaches −70°, the material performs more like capacitors attends with the diffusion process70,71. The phase angle reaches −45° at the capacitor response frequency of 5.6 Hz, which is much higher than activated carbon (0.15 Hz)72.

Figure 7 Electrochemical Impedance Spectroscopy: (a) Comparative Nyquist plot at an AC amplitude of 5 mV. (b) Bode plot of PS-FLG at a range of 0.01 Hz to 10 kHz. (c) Charge-discharge cycling test, retention of specific capacitance and coloumbic efficiency with number of cycles. (d) Cycling stability of PS-FLG showing initial & final 5 cycles of GCD at a current density of 5 A g−1. Full size image

Galvanostatic charge-discharge cycling stability is one of the critical parameters to look out for in commercially available supercapacitors. The active material must endure fast charging-discharging at higher current densities and yet maintain its inherent property even after large number of GCD cycles. Figure 7c exhibits a promising 87% C SP retention even after 5000 GCD cycles at a sufficiently higher current density (10 A g−1). Fig. 7d shows the initial and final five GCD cycles having almost similar triangular pattern retained as is expected for good charge propagation of electrolyte ions into PS-FLG electrodes. CV plots before and after GCD cycling test can be observed in Figure S10, showing a slight decrease in the area under the curve but the characteristic CV shape is retained. This confirms the cycling stability of the active material. Figure 7c also shows retaining of coloumbic or faradaic efficiency around 100% indicating an absence of any side reactions or faradaic processes were taking place during GCD cycling test.

The capacitive behavior of PS-FLG material was tested in an organic electrolyte. Briefly, the CV and charge-discharge test were performed in 1 M Et 4 NBF 4 in 1:1 mixture of EC: DCE solvent with a three-electrode setup. Figure 8a exhibits the cyclic voltammetric response of the PS-FLG active electrode material in the organic electrolyte system. A high C SP of 78.3 F g−1 at a current density of 1 A g−1 during galvanostatic charge-discharge study was achieved (Fig. 8b). Introducing organic electrolyte allows an increment of the potential window to 2.5 V, thereby increasing the energy density to 68 W h Kg−1.

Figure 8 Electrochemical performance of PS-FLG in 1 M Et 4 NBF 4 in 1:1 EC: DCE; (a) CV response at a scan rate of 100 mV s−1; (b) GCD test at a current density of 1 A g−1. (c) Plot of discharge voltage of a SSD recorded with time. Inset is the optical image of a SSD device. (d) Lighting a red LED by two devices connected in series. Full size image

A comparative study of carbonaceous materials obtained from various waste biomass sources and their capacitive performances are presented in Table 2. A supercapacitor needs to have sufficiently high energy density without sacrificing on its instant power delivery, to be employed for commercialization. It is mostly observed that any material, which is stable even on a more extensive potential range, subsequently has higher energy storage capacity. Mostly reported supercapacitors based on carbon nanostructures are limited to function within a potential range of 1 V in an aqueous electrolyte12,21,73,74,75,76,77. However, our as developed PS-FLG material is shown to be functional in a much higher potential window of 1.5 V. Another key factor lies in the attainment of morphologies suitable for maximum charge storage. The predominant product morphology for the carbonaceous materials from the majority of the waste sources is found to be mostly activated or porous carbon. Only activated carbons from dried corn grains or sugarcane bagasse has got limited energy densities at a range of 10–30 W h Kg−1 75,77. On the other hand, nano or micro porous carbons from various sources like wood sawdust, dead neem leaves or waste coffee beans exhibit slightly better energy densities in the range of 8–55 W h Kg−1 (power densities ranging from 5–12 KW Kg−1) mostly because of the presence of well-distributed pores48,57,78. This establishes the need for a combination of both a layered structure with well-distributed pores and sufficiently high surface area for the achievement of higher energy density. PS-FLG possesses all these qualities along with an enhanced potential window. It shows almost 6 times higher energy density (58.25 W h Kg−1) and 3.75 times higher power density (37.5 KW Kg−1) than a similar morphology of few-layer graphene-like nanosheets achieved from waste coconut shell (9.58 W h Kg−1 at a maximum power delivery of 10 KW Kg−1)12.

Table 2 Comparison of capacitive performances for various carbon materials synthesized from waste biomass. Full size table

Performance evaluation of the fabricated symmetric solid-state supercapacitor