Multifunctional nanoporous biocarbon derived from ginger: a promising material for CO₂ capture and supercapacitor

INTRODUCTION

The ever-increasing demand for energy to meet the needs of the growing population has led to a continuous depletion of fossil fuels and a significant increase in greenhouse gas emissions. To address this dual challenge effectively, a combination of strategies is required: reducing CO₂ emissions and a revolutionary shift towards renewable energy technologies^[1-3]. Consequently, tackling CO₂ emissions along with safe and efficient storage of energy using devices such as supercapacitors is of paramount importance. Therefore, it is crucial to rationally fabricate high-performance materials with specific characteristics for effective gas adsorption and high-performance energy storage^[4-8].

Nanoporous carbons, particularly those derived from various biomass precursors, are one of the most extensively studied materials that have demonstrated great potential as CO₂ adsorbents and electrode materials for supercapacitors^[9-13]. These materials are cost-effective, environmentally friendly and easily scalable to industry levels^[14] and are characterized by high surface area, flexibility towards surface modification, strong adsorption capacity, and good thermal and chemical stability^[8,15-22]. These characteristic features assert the utility of nanoporous carbons as adsorbents for CO₂ capture or electrodes for energy storage. However, the key to enhancing their practical utility lies not only in developing simple synthesis approaches that are both environmentally safe and economically viable but also in using the appropriate bio sources that can offer better surface properties and unique surface functionalities. For example, nanoporous biocarbon derived from high oxygen or nitrogen-containing biosources can offer heteroatom functionality on the surface of the materials, which is highly critical not only for adsorption but also for energy storage^[7,23-31].

For this purpose, various biomass materials, including green wastes, serve as efficient starting materials for porous carbon synthesis. Pine cone flowers as a biomass source were converted to honeycomb-like activated porous carbon conducting scaffolds and coated on Ni(OH)₂ nanosheets to form composites, which exhibited excellent electrochemical properties^[32]. The 3D N/O self-doped heteroatom honeycomb-like porous carbon was prepared from bio-waste oyster shells which exhibited exceptional electrochemical properties, including a high specific capacitance, large surface area, and good cycling stability. It was reported that the hierarchical structure and heteroatom doping are mainly responsible for their enhanced electrochemical performance^[33]. Likewise, Nallapureddy et al. prepared graphitic carbon nano-onions (CNO) and hetero-atom-doped CNO from chicken fat oil biowaste through an oil-wick flame pyrolysis approach. This approach seems quite interesting as it offers a simple and economical synthesis process, resulting in CNO materials with good capacitance and h-CNO materials with even higher specific capacitance^[34]. Utilizing these materials not only helps the efficient synthesis of porous carbon but also provides a concurrent safe disposal method for large amounts of such waste materials^[35-38]. Among the various biomasses, ginger is produced in large quantities for food and fragrance purposes, one of the key seasoning elements for food cooking for its sweetness and pungent smell due to the existence of chemicals such as zingerone and gingerol. These chemicals may induce surface extra functionalities if ginger acts as the precursor for synthesizing porous carbon. Around five million tons of ginger are produced worldwide which ultimately generates a huge amount of waste. By using waste ginger as the carbon source, it is possible that new nanocarbons with surface functional groups could be prepared. In addition, this strategy offers a safe and economical approach to the disposal of ginger waste biomass. Keeping this in mind, we report here on the synthesis of heteroatom functionalized nanoporous carbons using a solid activation approach with KOH as the activating agent. The prepared materials have been utilized for both CO₂ adsorption and energy storage applications. The optimized material showed a CO₂ adsorption capacity of 25.8 mmol/g at 0 °C (30 bar) and exhibited the highest specific capacitance of 244 F g^-1 at the current density of 0.5 A g^-1.

EXPERIMENTAL

RESULTS AND DISCUSSION

The process of synthesizing ginger-activated nanoporous biocarbon is schematically shown in Figure 1. Ginger was initially pyrolyzed by varying the temperature (400, 500, and 600 °C) to prepare nonporous carbon (GPC) and further activated with KOH in the mass ratio of 1:3 (GPC:KOH) to prepare GNBC. The details of the synthesis conditions for the nonporous carbon and GNBC are summarized in Supplementary Table 1. The powder XRD patterns of the as-synthesized GNBC samples are shown in Figure 2A and Supplementary Figure 1. GNBC2-5 samples exhibit diffraction peaks at 2θ = 24.6° and 43.3°, attributed to the (002) and (100) reflections of the graphitic stackings in the carbon structure)^[39]. Moreover, a shift towards a high angle in the (002) peak position with improved intensity was observed with the increase in the pyrolysis temperature to 500 °C [Supplementary Table 2], suggesting an increase in the ordering of the carbon structure. Similarly, a decrease in the interlayer spacing was also observed [Supplementary Table 2], reflecting improved carbon atom rearranged into a more graphitic structure. In contrast, the XRD pattern of GNBC6 showed a (002) peak shift towards a low angle (2θ = 23.4°) with an increase in the d-spacing to 0.381 nm, demonstrating the expansion of the graphene layers due to an increase in the pyrolysis temperature. Moreover, this could be attributed to the structural deformation of the material due to the formation of harsh activation conditions (volatilized KOH) at high temperatures as it is expected that this would create more surface functional groups due to the oxidation of carbon layers, resulting in the increase of the d-spacing^[40-42].

Figure 1. Schematic representation of the synthesis of porous activated carbon from ginger waste.

Figure 2. (A) X-ray diffraction patterns, (B) N₂ adsorption/desorption isotherms of porous activated carbon from ginger, (C) SEM, (D) TEM and EDS mapping of (E) carbon and (F) oxygen element of GNBC6 sample.

The textural properties of the GNBC materials were further investigated using the N₂ adsorption/desorption analysis and the results are shown in Figure 2B and Supplementary Figure 2. The GNBC samples exhibit a typical type I-like isotherm corresponding to the classification of the International Union of Pure and Applied Chemistry (IUPAC), reflecting the presence of micropore size distribution, and the H4-type hysteresis confirmed the presence of slit-like pores. The surface area and the pore volume of the GNBC materials increased from 1,328.1 to 2,330.6 m²/g and 0.66 to 1.10 cm³/g, respectively, with the pyrolysis temperature rising from 200 to 600 °C [Supplementary Table 2]. GNBC6 displayed the highest BET-specific surface area of 2,330.6 m²/g and pore volume of 1.10 cm³/g among all the samples. These results highlight the importance of regulating the pyrolysis temperature for enhancing the surface area and pore volume when activated with KOH. The pore diameter was analyzed using the Horvath Kawazoe method. As noticed from Supplementary Figure 3 and Supplementary Table 2, the pore diameter increased while the pyrolysis temperature grew from 400 to 500 °C (1.33 to 1.73 nm) and gradually decreased to 1.55 nm when heated at 600 °C. The presence of micropores with a size < 2 nm is highly favorable for CO₂ uptake at ambient conditions^[43-45]. These results validated the importance of regulating the pyrolysis temperature during KOH activation for enhancing the surface area and pore volume. The high surface area and predominantly microporous nature of the materials are expected to be beneficial for gas adsorption and electrochemical activity.

Further structural and morphological details of GNBC were investigated using SEM and TEM analyses. The highly disordered nature of the synthesized materials, with few small pores and voids, was observed by SEM analysis [Figure 2C, Supplementary Figure 4]. A porous sheet-like structure was observed by TEM analysis of the GNBC materials, which is due to accelerated activation by KOH [Figure 2D, Supplementary Figure 5]. Furthermore, as shown in Figure 2E and F, and Supplementary Figure 6, the energy-dispersive X-ray spectroscopy (EDS) elemental mapping of GNBC samples displayed the presence of carbon (C) and oxygen (O). The elemental composition analysis for the GNBC samples showed a high amount of carbon (> 96 At.%) and less oxygen content [Supplementary Table 3].

The surface functionalities of the GNBC samples were analyzed using the FTIR spectroscopy, and the results are displayed in Figure 3A. The appearance of an extensive band from 3,200 to 3,600 cm^-1 centered at approximately 3,421 cm^-1, confirmed the presence of O-H stretching vibrations which are attributed to the moisture and adsorbed hydroxyl groups. As the synthesis temperature increased, a decrease in the intensity of this band is observed due to the loss of OH functional groups at high temperatures. The bands at 1,719 and 1,612 cm^-1 correspond to C=C and C-C stretching vibrations of -COO-, while those centered at 2,318, 1,417, and 1,137 cm^-1 can be attributed to the C=O/C-O functional groups of the carboxylate, respectively^[28]. The peaks noticed between 550 and 1,000 cm^-1 are ascribed to the presence of aromatic C-H vibrations^[46]. The difference in the intensity of the band at 1,137 cm^-1 noticed in the samples GNBC4, GNBC5 & GNBC6 indicates the influence of pyrolysis temperature and the formation of oxygen functional groups in the alkene structure due to KOH activation^[46]. Moreover, the alkene structures (C=C) functionalized with oxygen may behave more electrically conducting than samples with less oxygen functional groups^[28].

Figure 3. (A) FTIR spectra (inset shows enlarged view of peak 2,318 and 776 cm^-1) and (B) survey scan spectra of GNBC samples, (C) deconvoluted C 1s spectra of GNBC6 and (D) NEXAFS C K-edge spectra of GNBC samples.

The binding environment and composition of GNBC samples were further studied utilizing the XPS. Survey scans of prepared samples show peaks related to C 1s and O 1s without any noticeable impurity [Figure 3B]. The high-resolution C 1s profiles of the GNBC6 sample are fitted into three peak positions which are centered at 284.3, 285.5, and 288.6 eV binding energies, as shown in Figure 3C. Similarly, GNBC4 and GNBC5 demonstrated similar types of C 1s spectra [Supplementary Figure 7A and B]. The strong peak at 284.3 eV is assigned to the sp² hybridized C-C bond, revealing the existence of graphitic carbon. The fitted peaks at 285.5 and 288.6 eV correspond to the C-O-C and O-C=O groups, and the details of the compositions are outlined in Supplementary Table 3, respectively. As shown in Supplementary Figure 8A-C, the O 1s spectra can be deconvoluted to two peaks at 532.1 and 533.9 eV, which are ascribed to the functionalities of C-O and O-C=O, respectively. The XPS analysis further reveals that the proportions of oxygen-containing functional groups (C-O) increased for the sample pyrolyzed at high temperature (GNBC6 sample), and the chemical activation could also promote high oxygen content on the carbon material [Supplementary Table 3]. The results obtained from the FTIR analysis are in good agreement with these XPS results demonstrating that the activation of the ginger biomass creates a huge amount of the surface oxygen functional groups, which may either come from the ginger biomass itself or through KOH activation due to the easy cleavage of C-O-C linkages. The C K-edge spectra obtained from the near-edge X-ray absorption fine structure (NEXAFS) analysis of GNBC4, GNBC5 & GNBC6 show four peaks represented as C1 (284.7 eV), C2 (285.2 eV), C3 (288.2 eV) and C4 (293 eV), as shown in Figure 3D. The broad bump at 284.7 eV and the sharp peak at 285.2 eV correspond to the C 1s-π* transition of C=C, and the high intensity and broadening of π* transition are observed for the samples treated at high temperatures that reflected the formation of defects in the carbon structure. The peak at 288.2 eV is ascribed to the C 1s-π* transition of C=O excitation, and the C4 feature in the range of 292 to 300 eV represents the C 1s-σ* transition of C=C resonance region, respectively^[47,48].The smaller peak intensity in C1 and the broader C4 peak indicate that the graphitic plane is imperfect^[49].The O K-edge spectra, as shown in Supplementary Figure 8D, display distinct peaks at 530.3 eV assigned to the O 1s-π* transition of COOH, and 531.8 and 534.5 eV, corresponding to the O 1s-π* transition of C=O and C-O-C, respectively^[50]. The broad peak from 535 to 545 eV corresponds to the O 1s-σ* transition of C=O, which is also consistent with the FTIR and XPS results.

The electrochemical performance of GNBC samples was investigated using the three-electrode system employing 3M KOH as the electrolyte. The cyclic voltammetry (CV) curves of GNBC4, GNBC5, and GNBC6 were measured at the scanning rate of 100 mV/s [Figure 4A]. The CV curve of GNBC6 exhibited a quasi-rectangular shape, a typical behavior of an electrical double-layer capacitor (EDLC). GNBC4 and GNBC5 samples show marginally inaccurate rectangular shapes due to the restriction in the diffusion of the ions into the layers, and this behavior could be seen at different scan rates, as shown in Supplementary Figure 9. Likewise, in the GNBC6 sample, while increasing the scan rate of the CV from 5 to 100 mV s^-1, the shape of the CV remained as rectangular, indicating that the ions may possibly diffuse into the pore channels despite the decreasing diffusion time. Also, as evident from the XRD, TEM, and XPS results, a disordered structure and oxygen functionalities could attract the hydrophilic electrolyte, thereby increasing the EDLC behavior.

Figure 4. (A) Cyclic voltammetry curves of GNBC samples at 100 mV/s, (B) Charge-discharge profile of GNBC samples at 0.5 A/g, (C) comparison of specific capacitance at different current densities for GNBC samples, (D) CV curves measured using GNBC6 electrode at different scan rates in a two-electrode cell, (E) specific capacitance of GNBC6 at different current densities and the inset displays the corresponding GCDs, and (F) two electrode EIS spectra of GNBC6 sample.

The GCD (galvanostatic charge-discharge) tests were conducted to study the rate capabilities of the electrodes. The GCD curves measured in the current densities of 0.5 to 10 A/g from 0 to -0.8 V are shown in Supplementary Figure 10. The GNBC6 sample showed a prolonged discharge time compared to GNBC4 and GNBC5 samples at 0.5 A/g [Figure 4B], and these results are consistent with the CV. The triangular shape of the GCD curves at different current densities [Supplementary Figure 10] demonstrates ideal supercapacitive behavior of the GNBC electrodes, characterized by a minor internal resistance (IR) drop, which signifies good coulombic efficiency and excellent reversibility. The graph shown in Figure 4C illustrates the specific capacitance values of GNBC samples calculated at various current densities. Among the samples studied, GNBC6 demonstrated the highest specific capacitance of 244 F/g, followed by GNBC5 of 215 F/g and GNBC4 of 189 F/g at a current density of 0.5 A/g, respectively. Compared with other biomass activated carbons and waste derived activated carbons [Supplementary Table 4], the supercapacitance performance of the GNBC6 is much better, with an improved specific capacitance value and high stability. It could be noticed that even though the current density was varied to 10 A/g, the samples GNBC6, GNBC5, and GNBC4 displayed a specific capacitance of 125, 75 and 100 F/g. This unique performance could be due to material properties such as thin carbon sheet morphology that features a high BET-specific surface area and large pore volume, as well as the presence of micropores that offer high conductivity and better electronic transport, leading to higher capacitance at high current density. As evident from the textural properties [Supplementary Table 2], the gradual loss in the capacitance observed at high current densities in GNBC samples could be due to a hindrance in the ion diffusion caused by the pore dynamics. The electrochemical impedance spectroscopy (EIS) measurements were carried out on the GNBC electrodes to understand the mechanism of charge transfer, and the results are shown in Supplementary Figure 11. The Nyquist plots displayed a typical capacitance-like behavior in all the samples. When performing the simulation analysis using an equivalent circuit, the lowest resistance for charge transfer is observed in GNBC6 (0.69 Ω), followed by GNBC5 (0.84 Ω) and GNBC4 (0.98 Ω).

As three-electrode supercapacitance performance is exceptional in the GNBC6 sample, the sample was further evaluated in a two-electrode setup. The CV curves exhibited a nearly rectangular shape at a scan rate of 5 mV/s, which is characteristic of EDLC behavior in the carbon structure [Figure 4D]. As seen from Figure 4D, there were no significant changes observed in the CV plots as increasing the rate of scan from 5 to 100 mV s^-1, implying its excellent charge transfer capability, low resistance and excellent capacitance retention. Similarly, when the potential window was increased to 1.4 V, a sharp rise in electrode current was observed, accompanied by a dramatic change in the curve’s shape. This behavior suggests that the high potential caused polarization of the electrode material, leading to electrolyte decomposition^[28].The relationship between voltage and time and the impact of current densities on specific capacitance values were examined through the GCD method (inset of Figure 4E). The characteristic feature of EDLC was exhibited through the triangular shape during charging and discharging, and the GNBC6 sample demonstrated a high specific capacitance of 119 F/g at 0.5 A/g and 63 F/g at 10 A/g, suggesting its promising potential for supercapacitor applications [Figure 4E]. The exceptional capacitive performance observed by the GNBC is primarily due to the minimal resistance encountered by electrolyte ions during diffusion and the confinement of ions within the pore structure, which limits the ion penetration time at higher current densities. To further investigate the ion transport and charge transfer characteristics, EIS measurements were conducted, as illustrated in Figure 4F. The Nyquist plot revealed a distinct semicircle in the high-frequency region, indicative of charge transfer resistance, and the low-frequency range displays a distinct linear line, which reflects the Warburg impedance associated with ion diffusion within the electrode material^[51]. From these measurements, the equivalent series resistance (R_s) was determined to be 0.39 Ω, while the charge transfer resistance (R_ct) was found to be 0.75 Ω. The sample also achieved a high energy density of 23.8 Wh kg^-1 and a power density of 8,168 W Kg^-1 [Figure 5A]. Furthermore, the GNBC6 electrode demonstrated exceptional cyclic stability, retaining 93% of its capacitance after 10,000 continuous charge-discharge cycles at 20 A/g, with outstanding coulombic efficiency [Figure 5B]. In addition, the SEM images in Supplementary Figure 12 reveal that the GNBC6 sample exhibited no visible damage after undergoing multiple charge-discharge cycles, reinforcing its structural stability. This retention in capacity could be due to the electro-activation of the carbon material during longer charge-discharging cycling processes^[16]. Ginger-derived activated carbon (GNBC6) has a hierarchical porous structure with interconnected micro, meso and macro pores. This unique structure provides a large surface area for charge storage, efficient electrolyte ion transport and short diffusion paths for ions. The interconnected pore network allows for rapid charge/discharge cycles without degradation of the structure over many cycles.

Figure 5. (A) The Ragone plot of GNBC6 sample, (B) long cycling stability and coulombic efficiency of GNBC6 sample, (C) CO₂ adsorption isotherms of GNBC4, GNBC5 and GNBC6 at 0 °C, (D and E) CO₂ adsorption isotherms of GNBC5 and GNBC6 at 10 and 25 °C, (F) Isosteric heat of adsorption of GNBC5 and GNBC6 samples.

The porous carbon materials derived from ginger biochar synthesized at the pyrolysis temperatures of 400, 500, and 600 °C demonstrated a high BET-specific surface area with enormous micropores and significantly large pore volume, which should be beneficial for enhanced gas adsorption. Therefore, the CO₂ adsorption capabilities of GNBC samples were measured at 0 °C by gradually varying the pressure from 0-30 bar. Figure 5C shows the sorption isotherms of GNBC4, GNBC5, and GNBC6 samples, demonstrating a massive rise in the adsorption of CO₂ at ambient pressure and gradually increasing as the pressure rises without any saturation. The summary of CO₂ adsorption at different pressures for the samples is shown in Supplementary Table 5. Among the samples, GNBC5 adsorbed 25.8 mmol/g of CO₂, demonstrating the highest uptake at 30 bar pressure compared to GNBC4 - 20.1 mmol/g and GNBC6 - 21.7 mmol/g. The superior CO₂ adsorption performance of GNBC5 could be attributed to the high BET-specific surface area (2,140.4 m²/g) and pore volume (1.043 cm³/g), where the surface area provides more sites for CO₂ molecules to interact with and the pore volume offers space for CO₂ molecules to be physically trapped^[28]. Interestingly, the sample GNBC6 displayed the highest BET-specific surface area of 2,330.6 m²/g and showed a relatively lower CO₂ uptake (21.7 mmol/g) compared to GNBC5. Thoroughly analyzing the textural parameters, we noticed that the percentage of mesopore area in sample GNBC5 (8.5%) is two-fold higher than GNBC6 (4.4%), which may lead to the higher adsorption of GNBC5 at high pressure. The results indicate that the mesopore area is crucial for increasing the CO₂ adsorption capacity under high pressures^[16].However, at the low-pressure region (0 to 5 bar pressure), the sample GNBC6 displayed the highest CO₂ adsorption of 4.87 mmol/g at 1 bar, followed by GNBC5 at 3.82 mmol/g and GNBC4 at 2.65 mmol/g. The high CO₂ uptake at the low-pressure region in GNBC6 is correlated with its large proportion of micropores (95.6%) and huge micropore volume (0.99 cm³/g). The CO₂ adsorption at low pressure is very high in micropore materials due to their small pore size, creating a large surface area for adsorption when comparing the surface area to volume ratio. Additionally, the confinement effect in micropores enhances the interactions between CO₂ molecules and the pore walls, further promoting adsorption^[28].

The CO₂ adsorption was also carried out at 10 and 25 °C for GNBC5 and GNBC6 samples [Figure 5D and E] to further evaluate the influence of temperature. At elevated temperatures, the adsorption efficiency of GNBC5 and GNBC6 decreases to 24.1 and 20.2 mmol/g at 10 °C and further reduces to 20.2 and 17.7 mmol/g at 25 °C. Increasing the temperature of the adsorption process can reduce the CO₂ uptake due to several factors, such as weaker interaction between CO₂ molecules and the carbon surface and the increase in the mobility of the molecules which makes them harder to be trapped within the pores^[16,18]. The significant change in the CO₂ uptake noticed in GNBC6 and GNBC5 samples arises from the adsorption kinetics that should have slowed down due to adsorbate-adsorbent interactions at relatively higher temperatures. Therefore, the strength of interactions between the CO₂ molecules and the GNBC5 and GNBC6 adsorbent materials was evaluated using the isosteric heat of adsorption (Q_st) calculated using the Clausius-Clapeyron equation.

As shown in Figure 5F, Q_st values remain between 27.9-22.3 and 23-20.9 kJ mol^-1 for GNBC6 and GNBC5, respectively, indicating the adsorption process is physical in nature. In addition, when adsorption increases, the Q_st values of GNBC6 and GNBC5 decrease, indicating that during the adsorption process, more CO₂ molecules are adsorbed onto the surface, forming multiple layers of adsorption, which weakens the interaction with the surface, leading to a decrease in the Q_st value. Moreover, the lower strength of these interactions facilitates the easy regeneration of adsorbed gas molecules and enhances its potential for practical applications.

CONCLUSION

In this study, we sucessfully prepared nanoporous biocarbons with surface functional oxygen groups and high specific surface area from ginger biomass and applied them for high-energy supercapacitors and CO₂ capture applications. We demonstrated that the micro and mesoporous pore structures in the nanoporous biocarbon can be tuned by adjusting the pyrolysis temperatures. The developed best material showed a high BET-specific surface area of 2,330 m²/g and a high specific capacitance of 247 F/g at a current density of 0.5 A/g in a three-electrode system which is much better than that of the activated carbon and multiwalled carbon nanotubes. It was found that the presence of rich micropores and oxygen-based functional groups, generated through the activation and from the ginger biomass, really helped to enhance the active sites, promoting the diffusion and confinement of the ions in the layers, which resulted in a high capacitance with a capacitance retention of 100% even after 2,500 cycles. In addition, the developed nanoporous biocarbon achieved an excellent CO₂ adsorption capacity of 25.8 mmol/g at 30 bar pressure. The high microporosity in the developed material facilitated a high amount of CO₂ adsorption of 4.87 mmol/g at ambient pressure and 0 °C. The enhanced performance of the synthesized materials in their intended applications could be attributed to the combined effect of their distinct porosity features. The advantages of transforming food waste to nanostructure carbon open an excellent platform for developing low-cost energy storage, CO₂ capture, and conversion materials.