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DOI: 10.1039/D4CC02532F (Communication) Chem. Commun., 2024, Advance Article

Waste biomass-based 3D graphene aerogels for high performance zinc-ion hybrid supercapacitors†

Gouri Sankar Dasa, Lokesh Kumar Jangirb, K Sandeep Rajua, Yogesh Chandra Sharma*b and Kumud Malika Tripathi*c
aDepartment of Chemistry, Indian Institute of Petroleum and Energy, Visakhapatnam, Andhra Pradesh-530003, India
bDepartment of Chemistry, Indian Institute of Technology BHU, Varanasi-221005, India
cCenter for Emerging Technologies for Sustainable Development (CETSD), Indian Institute of Technology Jodhpur, Jodhpur-342037, India. E-mail: ysharma.apc@iitbhu.ac.in; kumud20010@gmail.com

Received 24th May 2024 , Accepted 27th August 2024

First published on 28th August 2024

Abstract

Biomass-based 3D graphene aerogels were explored as cathode materials for the fabrication of high-performance zinc-ion hybrid supercapacitors. These hybrid supercapacitors delivered a high specific capacitance of ∼353.1 F g−1 at a current density of 0.1 A g−1 and maximum specific energy and power of 158.9 W h kg−1 (at 84 W kg−1 of specific power) and 14.8 kW kg−1 (at 77.2 W h kg−1 of specific energy), respectively. These devices exhibited a remarkable rate capability at high current densities and excellent capacity retention, retaining ∼84.2% of their capacity with ∼100% coulombic efficiency up to 10[thin space (1/6-em)]000 cycles at 10 A g−1.

The newly emerging Zn-ion hybrid supercapacitors (ZHSCs) are some of the prominent devices for electrochemical energy storage because of their intrinsic multivalent feature of both batteries and supercapacitors.1 ZHSCs possess the distinctive features of elevated capacitance, high power density, environmental friendliness, low-cost, safer workability, elevated energy storage capability, a high theoretical capacity (823 mA h g−1) and a low redox potential of −0.76 V. ZHSCs employing green nano-carbons as cathode materials offer considerable potential to overcome the main drawback of sluggish reaction kinetics between electrodes.1 However, the development of nano-carbon based cathodes having both zincophilicity and hydrophilicity with high-performance remains a great challenge.2

The use of graphene-based materials as cathodes in ZHSCs is intensively investigated owing to their fascinating structure, excellent electrical conductivity and intriguing features.3 But, the aggregation of graphene sheets in electrolyte solutions limits the performance due to the loss of ion-transport channels, conductivity and specific surface area.4 Graphene aerogels (GAs) as 3D porous structures of graphene have precise structural advantages as cathode materials such as the existence of diverse micropores and mesopores, which increase specific capacity and reduce the diffusion resistance, respectively.5 A 3D nanoarchitecture of GAs with interconnected micro-/mesopores and open micropores offers high accessibility to electrophilic sites for enhanced charge storage.6 The use of biomass as a precursor for the synthesis of 3D porous graphene-based structures offers several advantages, including low cost, an easy synthesis process, abundance, easy availability and environmental friendliness.7

The prospects of using bio-waste derived 3D GAs as efficient cathodes in ZHSCs have been investigated in the present work. Waste pear fruits are used as a natural and economically viable carbon source for the synthesis of highly porous 3D interconnected hierarchical graphene. The synthesis approach involves a simple hydrothermal carbonization of pear fruit followed by a freeze-drying and pyrolysis process as demonstrated in Fig. 1. Initially, pear fruit undergoes hydrothermal treatment, which involves several reactions such as aromatization, dehydration, and carbonization, resulting in the formation of a carbonaceous aerogel.8 However, the precise mechanism behind the formation of a carbonaceous aerogel from biomass remains unknown. Subsequently, freeze-drying is employed to remove adsorbed water, followed by high-temperature annealing in an inert environment to achieve graphitization. The resulting ZHSCs exhibited impressive specific capacitance and excellent rate capability due to their structural advantages.


image file: d4cc02532f-f1.tif
Fig. 1 Schematic of the synthesis of GAs from biomass.

The scanning electron microscopy (SEM) image of GAs in Fig. 2a shows honeycomb-like cells in a 3D interconnected sponge-like architecture. The SEM image in Fig. 2b indicates the formation of randomly oriented adjacent nanosheets assembled in 3D as GAs. Fig. 2c illustrates a low-resolution TEM image of GAs, which displays the uneven alignment of graphene sheets. The internal microstructure of GAs as observed in high-resolution TEM (HR-TEM) images is shown in Fig. 2d and Fig. S1a (ESI†). The HR-TEM image indicates the formation of defective graphene nanosheets, as pointed out by white arrows (Fig. 2d). The resulting GAs mainly composed of non-uniformly distributed graphitic carbon layers having an average interlayer spacing of ∼0.36 nm (Fig. S1a, ESI†).7 These structural characteristics of GAs result in compact electrical resistances and shortened ion-diffusion pathways, making them advantageous for potential utilization as electrode materials in ZHSCs. Raman spectroscopy was utilized to examine the presence of defects, such as vacancies or disorder in the graphene lattice, and graphitic nature of carbon in GAs. Two characteristic peaks were observed in the Raman spectrum (Fig. 2e) of GAs corresponding to the D band and G band at ∼1361.4 cm−1 and ∼1588.7 cm−1, respectively. The D band is attributed to the vibrations of defective carbons (sp3), whereas the G band corresponds to the in-plane vibration of sp2 carbon atoms (in E2g mode) in the graphene matrix.7 The intensity of the D band relative to the G band (ID/IG) = 0.856 suggests a lower degree of disorder or a higher degree of graphitization in the GA structure.7 The high graphitic composition of GA facilitates fast charge transport, thereby improving its rate capability.


image file: d4cc02532f-f2.tif
Fig. 2 Characterization of GAs. (a) and (b) SEM images. (c) TEM image of GAs at low resolution. (d) HRTEM image showing defective graphitic arrangement of nanosheets. (e) Raman spectrum. High-resolution XPS spectra showing the different bonding environments of C 1s (f) and O 1s (g). (h) The N2 adsorption/desorption isotherm and (i) pore size distribution curve.

The phase purity and nature of crystallinity of GAs were investigated using the obtained X-ray diffraction (XRD) patterns. Fig. S1b (ESI†) shows two distinct peaks at 2θ = 24.7° and 43.1° attributed to the (002) and (101) planes of graphitic carbon, respectively, in the XRD pattern of GAs.8 The elemental composition and chemical binding sites of the as synthesized GAs were analysed using X-ray photoelectron spectroscopy (XPS). The XPS survey scan of GA in Fig. S1c (ESI†) shows two characteristic peaks at binding energies of 286.08 and 532.64 eV corresponding to C 1s and O 1s with elemental compositions of C (92.56%) and O (7.44%), (Fig. S1d, ESI†). The high resolution XPS spectra of C 1s can be deconvoluted into five peaks as displayed in Fig. 2f. The peak centred at 285.2 eV is attributed to C[double bond, length as m-dash]C (sp2 hybridized carbon), whereas the peak positioned at 285.7 eV corresponds to C–C (sp3 hybridized carbon). The weak peaks at binding energies of 286.4, 287.2 and 289.3 eV belong to C–O, C[double bond, length as m-dash]O and COO−, respectively. The detailed examination of the high-resolution O 1s spectra (Fig. 2g) reveals three distinct peaks located at binding energies 532.6, 533.2 and 534.4 eV, which are assigned to C–O, C[double bond, length as m-dash]O and COO− species, respectively.8 A peak at 3421 cm−1 in the FTIR spectrum of GA in Fig. S2 (ESI†) is attributed to the presence of O–H stretching. Doublet peaks at 2937 and 2851 cm−1 are attributed to –C–H bending vibrations. Distinctive peaks at 1701, 1578, and 1153 cm−1 are attributed to the vibrations of C[double bond, length as m-dash]O, C[double bond, length as m-dash]C, and C–O/C–C, respectively. A peak at 656 cm−1 corresponds to [double bond, length as m-dash]C–H bending vibrations.8 The presence of oxygenated surface functional groups in GAs enhances their surface wettability and accessibility facilitating improved pseudocapacitive activity.6 BET surface analysis was conducted to examine the surface area, pore diameter, and pore size distribution of GAs. The as-synthesized GAs exhibited an Ib type isotherm as illustrated in Fig. 2h. This indicates a diverse pore distribution.9 N2 adsorption/desorption measurement of GAs showed a high BET surface area of ∼210.32 m2 g−1 and a total pore volume (P/P0 = 0.990) of 0.1817 cm3 g−1. Nonlocal density functional theory (NLDFT) was employed to compute the distribution of pore sizes, depicted in Fig. 2i. The average pore diameter of GAs was determined to be 3.46 nm.

Benefitting from the mesoporous structure, large pore volume and high surface area properties, GAs exhibit considerable potential as promising cathode materials in ZHSCs. The electrochemical energy storage capabilities of GAs were examined in an assembled cointype cell composed of stainless steel supported GAs as a cathode and zinc coated carbon fiber as an anode with 1.5 M aqueous ZnSO4 solution as an electrolyte. These ZHSCs consist of a battery type anode with a redox reaction of Zn2+ ions and a supercapacitor type cathode with SO42− ion adsorption/desorption on the surface of GAs, which together provided the excellent current density and power density of the devices.10 Fig. 3a displays the cyclic voltammetry (CV) response of GA-based ZHSCs in the potential range of 0–1.8 V at various scan rates varying from 5 to 100 mV s−1. The CV curves of the GA-based ZHSCs exhibited a fusiform shape and the size of the curve increases with increasing scan rate without showing any severe deformation. These results strongly suggest that the as-synthesized GA-based ZHSCs could possess reversible and fast electrochemical reactions. The non-ideal rectangular shape with reversible weak redox humps of the CV response indicates the pseudo-capacitance behaviour of the fabricated ZHSCs. The primary mechanism for energy storage in GA-based ZHSCs involves the deposition/dissolution of zinc ions (reversible battery-type reactions) occurring at the zinc anode, coupled with the electrical double-layer capacitance resulting from the adsorption/desorption of SO42− ions on the surface of the GA cathode. The low redox potential of the Zn anode is beneficial to achieve a greater operational potential and higher energy density in ZHSCs as compared to aqueous symmetric supercapacitors.


image file: d4cc02532f-f3.tif
Fig. 3 The electrochemical measurements of GA-based ZHSCs. (a) CV response at various scan rates in a potential range of 0–1.8 V. (b) and (c) GCD curves at different current densities from 0.1 A g−1 to 20 A g−1. (d) Specific capacitance calculated at various current densities. (e) The EIS spectrum fitted with an electrical equivalent circuit before and after 5000 cycles. (f) Ragone plot of the GA-based ZHSCs. The cycling performance of ZHSCs in terms of coulombic efficiency measured at current densities of (g) 10 A g−1 and (h) 20 A g−1.

Furthermore, the electrochemical properties of GA-based ZHSCs were evaluated through galvanostatic charge–discharge (GCD) curves. Fig. 3b and c represent the GCD curves of ZHSCs obtained at various current densities ranging from 0.1 A g−1 to 20 A g−1. The quasi-triangular shape of GCD curves is an indication of excellent reversibility. The specific capacitance values at different specific current densities such as 0.1, 0.5, 1, 2, 4, 6, 8, 10 and 20 A g−1 were calculated to be 353.1, 317, 285.8, 257.84, 220.4, 204.1, 194.64, 187.73, and 171.7 F g−1 and the maximum specific capacitance of ∼353.1 F g−1 was obtained at a current density of 0.1 A g−1 (Fig. 3d). The specific capacitance value still remained 171.7 F g−1 upon increasing the current density to 20 A g−1, which strongly suggested the high-rate capability. The excellent energy storage performance of the ZHSCs corresponded to the highly porous structure of the GA-based cathode, which were highly beneficial for the electrolyte ion adsorption/desorption and diffusion during the fast charging/discharging process.

The Nyquist plot of GA-based ZHSCs before and after 5000 cycles in the frequency range of 0.1 Hz–100 kHz is demonstrated in Fig. 3e. The charge transfer resistance is indicated by the semi-circular region in the higher frequency range, while the linear region in the lower frequency range reveals the Zn2+ diffusion process in the electrolyte. The smaller diameter observed in a semicircle of the Nyquist plot measures 17.6 Ω, indicating minimal electrode/electrolyte interfacial charge transfer resistance (Rct) and rapid electrochemical reaction kinetics,11 which does not change much after 5000 cycles (20.5 Ω). The Rs value (equivalent series resistance) of 3.54 Ω indicates the low internal resistance of the fabricated device. The inset of Fig. 3e shows the equivalent electronic circuit of ZHSCs. The maximum specific energy of 158.9 W h kg−1 was evaluated at a specific power of 84 W kg−1 and the maximum specific power of 14.8 kW kg−1 from the ZHSCs device was obtained at a specific energy of 77.2 W h kg−1. The excellent energy storage performance of the GA-based ZHSCs was compared with that of different types of reported carbon-based materials and shown in Fig. 3f and Table S1 (ESI†). Fig. 3g demonstrates the long-term (charge/discharge) cycling stability of GA-based ZHSCs at current densities of 10 A g−1 and 20 A g−1, respectively. ZHSCs exhibited ∼84.2% specific capacitance retention of its initial capacitance with ∼100% coulombic efficiency after 10[thin space (1/6-em)]000 cycles at a current density of 10 A g−1, whereas a specific capacitance retention of ∼96.3% with ∼100% of coulombic efficiency after 1000 cycles was observed at a current density of 20 A g−1. This performance can be attributed to the outstanding stability at high current density and long-term durability, which make GA-based ZHSCs reliable for practical applications.

The working and energy storage mechanisms of ZHSCs entail highly reversible battery-like processes (such as the movement of Zn ions) occurring on the Zn anode, along with electrical double-layer capacitance on the GA cathode. Fig. 4a shows a schematic illustration of the charging and discharging mechanism of ZHSCs. The as-synthesized GA-based cathode followed the adsorption/desorption of SO42− ions on the surface, which may be described using the following electrochemical reactions eqn (1) and (2):12

 
Anode Zn2+ + 2e− ↔ Zn (1)
 
Cathode GA + SO42− ↔ GA//SO42− (2)


image file: d4cc02532f-f4.tif
Fig. 4 (a) A schematic representation of the charge–discharge mechanism of GA-based ZHSCs. (b) Plot of log (current) as a function of log (scan rate) for cathodic peaks in GAs and anodic peaks in Zn. (c) Area under CV curves showing the capacitive and diffusion contributions at a scan rate of 5 mV s−1. (d) The percentage contribution of both surface capacitive and diffusion involvement at various scan rates from 5 to 100 mV s−1.

For better understanding exceptional electrochemical performance of ZHSCs, the electrochemical kinetics of the GCD process was investigated. The quantification calculations were carried out and the electrochemical reaction mechanism was evaluated according to Dunn’ reports. According to the power-law equation (eqn (3) and (4)), the relationship between the peak current density (i) and the scan rate (v) is given as follows:3

 
i = avb (3)
 
log(i) = log(a) + b[thin space (1/6-em)]log(v) (4)
where the variable parameters a and b were derived from the intercept and slope of the linear plot between log(i) and log(v) given by eqn (4). The energy storage mechanism depends on the value of parameter b. The diffusion-control behaviour is observed at b = 0.5, while the surface capacitive process is observed at b = 1. The values of b were calculated from the slope of the linear plot in Fig. 4b and were found to be 0.76 and 0.78 for anodic and cathodic currents, respectively. These calculated values conform to the surface capacitive control for the energy storage process in GA-based ZHSCs. In addition, the contributions to the total charge storage in GA-based Zn-HSCs can be split into capacitive effects and diffusion-controlled charges using the following eqn (5) and (6).13,14
 
i = k1v + k2v1/2 (5)
 
i/v1/2 = k1v1/2 + k2 (6)
where k1 and k2 are the capacity and diffusion control constant. The values of these constants can be calculated from the linear fitting of the plot of i/v1/2 vs. v1/2. The capacitive control contribution was calculated to be ∼34.7% at a sweep rate of 5 mV s−1. In Fig. 4c, the percentage of surface capacitive control contribution indicated by the red colour and diffusion control contribution indicated by the black colour are changes with changing scan rate. The surface capacitive control contribution of GA-based ZHSCs increases from 34.7% and reaches its maximum value of 96.2% with increase in the swap rate as depicted in Fig. 4d. This notable capacitive control component enables quick transfer of charges, and reversible adsorption/desorption of SO42− ions occurs on the surface of the GAs and deposition/stripping of Zn2+ occurs on the anode. The efficient ion diffusion and fast electron transfer provided the outstanding electrochemical performance of ZHSCs.

In summary, waste pear fruit was utilized as a natural carbon source for the synthesis of highly porous GAs in an economical approach. The as-synthesized 3D GAs were explored as electrode materials for the fabrication of high-performance ZHSCs. Experimental findings indicate that ZHSCs based on GAs exhibit remarkable electrochemical characteristics. These include a high capacitance of ∼353.1 F g−1 at a current density of 0.1 A g−1, a maximum specific power of 14.8 kW kg−1 and specific energy of 158.9 W h kg−1. In addition, GA-based ZHSCs showed impressive capacity retention of ∼84.2% even after 10[thin space (1/6-em)]000 charge–discharge cycles at 10 A g−1 and 96% after 1000 cycles at 20 A g−1. This study highlights the potential use of pear fruit biomass derived GAs as cathode materials with high performance and long-term stability for next-generation energy storage devices.

Gouri Sankar Das: editing, data curation, and writing first draft. Lokesh Jangir: editing and data curation. K Sandeep Raju: editing and data curation. Yogesh Chandra Sharma: supervision, funding acquisition, and editing. Kumud Malika Tripathi: supervision, funding acquisition, and editing.

This work was supported by Center Mine Planning & Design Institute Limited (CMPDI LTD) India 34012/01/2022-CCT (FTS-354022).

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cc02532f
This journal is © The Royal Society of Chemistry 2024