Introduction

The energy costs related to the separation and sanitization of industrial commodities, such as gases, currently presents approximately 15% of global energy production, and the requirement of such commodities is anticipated to be triple by 20501. Today the need of innovating effective separation and purification methods with lesser energy footprints is more for carbon dioxide (CO2) than for any other gas. This is probably due to its contribution to climate change as well as the contamination of other gas streams including natural gas, biogas and syngas2. Apart from CO2 adsorption and separation, the storage of H2, a next-generation energy carrier with a high chemical energy (39 kW h/kg) and ability to provide pollution-free ignition is also need of the present time. Recently, porous carbons are considered as an emerging candidates as H2 storage materials by virtue of their facile preparation, low cost, light weight, fast kinetics, and high surface area3,4.

Different methods have been reported to fabricate microporous carbon materials with a high surface area5,6,7. Microporosity and high nitrogen content render Schiff-base polymers a highly interesting class of materials for the synthesis of carbons via pyrolysis. For the polymer synthesis, an organic media is used as the solvent for the reaction between primary amines as nucleophilic center and electrophilic carbons of the carbonyl groups to synthesize the polymer framework8,9. The resulting carbons obtained by the pyrolysis of the polymers possess a moderate surface area of 500–1500 m2/g which can be improved further using KOH as the chemical activating agent. Although chemical activation by KOH is a widely used technique to produce a porous network10, the number of variables in the experimental parameters and the differences in the reactivity of precursors limits a comprehensive understanding of the mechanism. In general, the reaction between carbon and KOH begins with the solid–solid interaction followed by the solid–liquid reaction which may reduce the potassium compound to metallic potassium (K), oxidize carbon to carbon oxides and carbonates accompanied by the formation of various active intermediates11. However, experimental and theoretical data present a general reaction between carbon and KOH, as presented in Eq. (1).

$$6{\rm{KOH}}+2{\rm{C}}\to 2{\rm{K}}+3{{\rm{H}}}_{2}+2{{\rm{K}}}_{2}{{\rm{CO}}}_{3}$$
(1)

KOH is entirely consumed when carbon is heated at ~600 °C. At temperatures higher than 700 °C, the as-formed K2CO3 starts to decompose into CO2 and K2O and finally vanishes at 800 °C. At temperatures above 700 °C, potassium compounds including K2O and K2CO3 are reduced to metallic potassium, which remains embedded in the carbon matrix.

$${{\rm{K}}}_{2}{{\rm{CO}}}_{3}\to {{\rm{K}}}_{2}{\rm{O}}+{{\rm{CO}}}_{2}$$
(2)
$${{\rm{CO}}}_{2}+{\rm{C}}\to 2\mathrm{CO}\,$$
(3)
$${{\rm{K}}}_{2}{{\rm{CO}}}_{3}+2{\rm{C}}\to 2{\rm{K}}+3\mathrm{CO}\,$$
(4)
$${\rm{C}}+{K}_{2}{\rm{O}}\to 2{\rm{K}}+\mathrm{CO}\,$$
(5)

According to the above discussions, the KOH activation mechanisms proposed are (a) Chemical activation by various potassium compounds involves the etching of carbon frameworks due to the redox reactions resulting in the generation of a porous network and (b) the intercalation of the metallic form of potassium into the carbon framework during the activation resulting in the expansion of this framework12. Activation followed by washing removes the intercalated metallic as well as other forms of potassium. As a result, the carbon framework remains expanded without reverting to its nonporous structure, and thus, high microporosity is generated. Sodium hydroxide is also widely used as activating agent. According to previous studies, anthracite NaOH activation significantly increases the surface area from 1017 to 2208 m2/g and the micropore volume from 0.05 to 0.4 cm3/g, with the NaOH to char ratio varying from 1 to 313. However, at high ratios, the micropores enlarged into mesopores, owing to excessive NaOH etching. Moreover, these conventional methods of activation exhibit limitations in controlling the pore size and formation of bottlenecks and/or micropores, typically with a size below 0.7 nm. Contrary to this, one of the interesting synthetic protocol proposed recently involves the condensation and activation of polymer precursors in molten salt medium with these salts playing the role of an alternative solvent as well as porogen for designing porous carbons.

In this context, a direct synthetic strategy is reported for the one step polymerization, carbonization, and activation of the carbon precursors to produce microporous carbons. Two types of carbon materials were prepared at two different temperatures using KOH and a eutectic mixture of KOH-NaOH as the two molten salt media. The resulting carbon materials were evaluated in terms of their ability to serve as gas storage materials.

Results and Discussion

Porous materials are an emerging platform of scientific interest in the context of their applicability in gas storage, energy storage, and catalytic activities. The successful transformation of the monomers to microporous carbons (Fig. 1) is evident from FTIR spectra. Attenuation of the peak corresponding to the aldehyde at 1690 cm−1 as well as the appearance of weak signals at ~3410 cm−1 corresponding to the stretching vibration of secondary amine (N−H), and methylene (CH) at 2917 cm−1 as the remnants of aminal linkages are confirmation of polymerization of organic precursors (Fig. 2). In addition, the peak of the triazine ring is evident at 1556 cm−1. The presence of a broad band at 978–1210 cm−1 is ascribed to the vinyl group (=C–H) and (C–N) bonds. The decrease in the intensity of the bands between 1000–1600 cm−1 suggest the removal of the O/C–H/C atomic species during carbonization. However, the absence of sharp peaks of any characteristic functionality indicates the effective carbonization of the polymer structure at the targeted temperatures. Moreover, the results indicate the leftovers of nitrogen functionalities which plays a vital role for enhancing CO2 adsorption13.

Figure 1
figure 1

Synthetic procedure for activated microporous carbons.

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Figure 2
figure 2

FTIR spectra of fabricated carbon materials.

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For a better understanding the effects of the activating agents on the polymerization–carbonization–activation processes, we analyzed the thermal stability of the precursors. Figure 3 presents the thermogravimetric (TG) analysis curves for the three samples (melamine + isophthalaldehyde (MI), melamine + isophthalaldehyde + KOH (MIK), and melamine + isophthalaldehyde + KOH + NaOH (MIKN)). An overlap is observed in the TG curves of the samples with KOH and eutectic KOH–NaOH mixture, indicating that a similar mechanism is involved in the weight loss in the course of one-pot processes. Moreover, it reveals that any dissimilarity in the microstructure of the activated carbonaceous materials are rather due to the physical state of the activating agent during polymerization. Furthermore, eutectic mixture is the physical mixture of NaOH and KOH. The mixture is used for lowering the melting point of individual salt and thus provides a molten medium for monomers to condense at desired polymerization temperature. As anticipated, the sample with this eutectic mixture shows a rapid mass loss at temperatures below 200 °C, which is attributed to its low melting point. However, it should be noted that a strong mass loss that appeared at 346 °C in the TG curve of MI has disappeared in the curves of the sample mixtures with the activating agents. This is justified by the notable evolution of unreacted melamine in MI. Thus, the activating agents act as an efficient reaction media for the in situ polymerization in comparison to the hydroxide-free process. Subsequently, a progressive weight loss observed at temperatures above 600 °C is attributed to the carbonization of the polymer. The major component left behind is the carbon with traces of heteroatoms, as proved by elemental analysis. Henceforth, TG curves are used to explore the polymerization–carbonization processes occurring in the eutectic mixtures, resulting in the excellent yield of carbons.

Figure 3
figure 3

TGA thermograms of (a) MI (b) MIK and (c) MIKN before carbonization.

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SEM images (Fig. 4) clearly show that the carbons fabricated in the presence of hydroxide activating agents exhibit a different morphology than those prepared without them. MI-700 and MI-800 show rough surfaces formed by irregularly interconnected networks with voids, in accordance with their low specific surface areas. However, MIK-700 and MIK-800 exhibit smooth surfaces with some spherical voids. Apparently, polymerization occurred in the liquid media, and the polymer framework has the ability to entrap the generated bubbles released as reaction products or drops of the eutectic mixture. Hence, spherical voids formed in the carbonized material, as observed in the SEM images. Furthermore, MIKN-700 and MIKN-800 exhibit hierarchical porosity with a large number of mesopores. This is attributed to excessive NaOH etching in the eutectic mixture. The diffraction patterns (XRD) of all the carbon materials are presented in Figure S1. From the figure, it is revealed that washing remove all the impurities of sodium and potassium, as no crystalline peak was observed. Consequently, the only peaks observed were attributed to carbon-based materials with the graphitic nature. The diffraction peak of the (002) plane attributed to the periodicity in the ABAB hexagonal close packing of graphitic structures along the z-axis is absent in almost all the diffraction patterns. Apart from this, a high intense scattering peak is observed at low 2θ values, indicating the disorderness with a low stacking order in graphene sheets, possibly owing to the extensive micropore generation during the activation process.

Figure 4
figure 4

HR-SEM images of fabricated carbon materials.

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X-ray photoelectron spectroscopy reveals the surface characteristics of prepared carbons. The survey scans (Fig. 5) show the presence of a high percentage of carbon along with a moderately low oxygen content. The high-carbon content reveals that the prepared sample mainly composed of carbon. The moderate to low nitrogen contents are in accordance with the elemental analysis data. Elemental analysis data (Table 1) indicates that the material contains mainly carbon with a small amount of nitrogen. A careful analysis of the consequences of the carbonization process reveals a large loss of mass with the release of large volumes of volatile gases and some tarry substances14,15 and as a result, H and N contents decrease with an increase in the carbon content16. All the samples are rich in carbon because the process combined carbonization and activation. The carbon content of MIKN increased gradually from 75.9 to 90.6% as the temperature was increased to 700 to 800 °C, whereas the content of nitrogen decreases from 1.65 to 0.77%. The increase in the carbon content during the carbonization is attributed to two factors: (i) Increased formation of carbon basal planes and (ii) aromatization17.

Figure 5
figure 5

XPS survey scan of adsorbents prepared.

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Table 1 Elemental composition of the adsorbents prepared.
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Physical properties including the textural features (specific surface area, pore-size distributions, and pore volumes) of the prepared materials were evaluated to assess their role in gas (CO2 and H2) adsorption. For this purpose, adsorption-desorption isotherms were obtained at 77 K. The isotherms and pore size distributions are presented in Fig. 6(a,b). The textural properties are summarized in Table 2. The adsorption profiles (Fig. 6a) of all the prepared samples indicate rapid uptake of nitrogen gas at low pressures with Type I adsorption characteristic, according to the IUPAC classification18. This kind of isotherm indicates a high affinity of the adsorbate and adsorbent for each other, and also suggests that the adsorbent in question is comprised mainly of micropores. However, MIKN-700 and MIKN-800 exhibit both Type I and Type IV isotherms. The isotherms exhibit nitrogen uptake at high pressures, indicating the presence of medium-sized or large mesopores along with the micropores. Furthermore, upon comparison of specific surface areas it is revealed that the initial blank samples, MI-700 and MI-800, prepared without any hydroxide activating agent exhibit a moderately low surface area of 330 and 360 m2/g, respectively, which is still better than those of the samples prepared by the old conventional synthesis methods8,9,19. Samples synthesized in the presence of KOH only, MIK-700 and MIK-800, possess a high surface area of 2101 and 2090 m2/g, respectively. This could be because the KOH activation leads to the generation of micropores primarily, resulting in a higher micropore volume and hence a porous carbon with a higher surface area20. On the other hand, samples prepared with the KOH-NaOH mixture, MIKN-700 and MIKN-800, leads to the generation of a hierarchical porous structure with both meso- and micropores with the surface areas of 3246 and 2984 m2/g, respectively. It is worth noting that with an increase in the carbonization temperature, hierarchical meso- and microporosity is induced with higher mesopore volume and wider pore size diameter. This can be explained by the decomposition mechanism of salts which upon high temperature reduces to metallic species (sodium and potassium). With the passage of time, a portion of metallic species tends to evaporate from the lattice. However, some proportion of it remains intercalated. Consequently, all these phenomena generate a hierarchical meso- and microporous structure. However, all the superactivated microporous structures were prepared with the mass ratio of 2/1 between the activating agents and monomers. The specific surface area and adsorption profiles of MIK and MIKN are better than those of the materials obtained by chemical activation at high activating agent/precursor ratios21. On comparison with the single salt activation method, MIKN-700 exhibit excellent textural features than UAK2–80020. Similarly, UANa-960p presents a method with use of 3:1 activating agent to carbon precursor ratio13. However, in our work we used lesser amount of activating agents (2:1) with excellent results. Present method generates microporous carbon structure with high specific surface area and narrow pore size diameters. This is accredited to the use of KOH-NaOH eutectic mixture rather than single salt activation. Hence, it is claimed that the present strategy provides a facile way to design highly microporous carbons.

Figure 6
figure 6

(a) Nitrogen adsorption-desorption isotherms and (b) pore size distributions of the adsorbents prepared.

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Table 2 Textural properties of the adsorbents prepared.
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The micropore volumes of the prepared samples were estimated using the Dubinin–Radushkevich (D–R) equation shown below:

$$W={W}_{o}{\exp }[-B(\frac{T}{\beta })Lo{g}^{2}(\frac{P}{{P}_{o}})]$$

where, W indicates the amount of the liquid adsorbed at P/P0, W0 is the total amount of the adsorbate in the micropores, B is the adsorbent constant, and β represents the affinity coefficient (0.34 for N2)22. The ratio of the micropore volume to total volume turns out to be 80, 1.5, 3.5, 80.7, 95.6, and 88.2% for MI-800, MIK-800, MIKN-800, MI-700, MIK-700, and MIKN-700, respectively. The samples prepared at 700 °C possesses a higher micropore volume compared to that prepared at 800 °C, suggesting that the activation at higher temperature is detrimental for the generation of micropores; instead, it stimulates the formation of mesopores. Similar outcomes were observed by Prahas et al. who reported that the elevation of the activation temperature drive the widening of the micropores into mesopores23.

To investigate the textural parameters deeply, pore size distribution curves were obtained by the nonlocal density functional theory (NLDFT). Figure 6(b) illustrates the pore size distributions of all the prepared samples. The pore size of the samples prepared in the presence of molten salts lies in the range of 0.5 and 3 nm. In contrast, MI possesses mesopores in combination with micropores, and therefore, the average pore size of these samples lies at ~6 nm. It is well known phenomenon that the pore size less than 1 nm is highly beneficial for gas adsorption and storage. Consequently, samples with micropores of an optimum pore size can be employed as efficient candidates for gas adsorption24.

At pressures up to 1 bar, the CO2 adsorption isotherms obtained at 273, 283, and 298 K, respectively, are presented in Fig. 7 and the complete adsorption data is listed in Table 3. At 1 bar and 273 K, microporous carbons exhibit an excellent CO2 uptake of 9.7 mmol/g (429.6 mg/g), 8.9 mmol/g (392.1 mg/g), 8.3 mmol/g (368.4 mg/g), 7.5 mmol/g (333.1 mg/g), 3.1 mmol/g (137.6 mg/g), and 2.9 mmol/g (130.1 mg/g), respectively, for MIK-700, MIK-800, MIKN-700, MIKN-800, MI-700, and MI-800, which are comparable to the CO2 uptake of recently reported CO2 adsorbents (Table 4). The CO2 adsorption isotherms of all the sorbents are nonlinear, indicating their microporous nature. All the prepared samples suffered a clear loss in the CO2 uptake with an increase in temperature from 273 to 298 K. This trend is in accordance with the exothermic nature of the adsorption process, implying that the amount of adsorbed CO2 decreases with an increase in the temperature25. One of the representative samples exhibits a CO2 uptake of 9.7 mmol/g (429.6 mg/g) at 273 K which decreases to 7.0 mmol/g (312.2 mg/g) and 5.4 mmol/g (238.7 mg/g) at 283 and 298 K, respectively.

Figure 7
figure 7

CO2 adsorption-desorption isotherms.

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Table 3 CO2 and H2 adsorption capacities along with the isosteric heats of adsorption (Qst) for CO2 gas determined at 273 and 283 K, shown by the prepared microporous carbons.
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Table 4 Comparison of CO2 uptake by various adsorbents at 273 K and 1 bar.
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Furthermore, the relationship between the CO2 adsorption ability and the textural properties of the activated carbons materials was evaluated. From the textural studies it is revealed that the samples prepared in the presence of KOH exhibit well-defined micropores with a higher micropore volume and narrower pore size distribution. More specifically, MIK-700 and MIK-800 exhibit narrow pore sizes, suggesting that the contribution of narrow micropores to the adsorption capacity is greater than those of the wider micropores and mesopores. This is because, in low pressure studies, these narrow micropores are easily filled by the CO2 molecules with a greater adsorption potential compared to wider pores. Furthermore, the higher activation temperature (MIKN-800) and presence of NaOH (MIKN-800 and MIKN-700) lead to wider micropores: 1.95 nm for MIKN-800 and 1.79 nm for MIKN-700. Consequently, there is an increase in the specific surface area and pore volume (2984 m2/g and 1.98 cm3/g for MIKN-800 and 3246 m2/g and 1.79 cm3/g for MIKN-700, respectively) but they contributed less to CO2 adsorption at the atmospheric pressure because no pore filling occurred in these cases. It has been reported that narrow micropores play a crucial role in CO2 adsorption26,27; however, the correlation between the pore size and CO2 adsorption has been rarely investigated. Presser et al. reported a perfectly linear relationship between the pore structure (with pore diameter of <0.8 nm) and CO2 uptake at 1 bar for carbide-derived carbons28. Apart from a suitable texture, MIK-700 also possesses higher nitrogen content (2.66%) compared to other samples prepared in molten salt media. This can improve the Lewis acid-base interactions between CO2 molecules and nitrogen moieties, leading to efficient adsorption. Consequently, the results confirm that the fabrication of efficient CO2 carbon sorbents requires a precise control of the porosity, with a pore size of approximately 1 nm along with the hetero-atom doping.

Easy regeneration is another critical feature that must be considered while designing CO2 sorbents. In this respect, porous carbons offer certain advantages over other materials, such as zeolites. The isosteric heat of adsorption (\({\rm{\Delta }}{H}_{ads}\,\)or Qst) was calculated on the basis of the Clausius−Clapeyron equation shown below.

$$\mathrm{ln}(\frac{{P}_{2}}{{{\rm{P}}}_{1}})=-\frac{{{\rm{\Delta }}{\rm{H}}}_{{\rm{ads}}}}{{\rm{R}}}(\frac{1}{{T}_{2}}-\frac{1}{{T}_{1}})$$

where, P2 and P1 are the values of p(CO 2 ), R denotes the universal gas constant, and T 2 and T 1 are 273 and 283 K, respectively. Figure S2 shows the Q st values as a function of adsorbed CO2; the corresponding data is listed in the Table 3. The initial values of Q st are in the range of 31.6–40.5 kJ/mol at a low surface coverage. However, the values show a decreasing trend with an increase in the amount of adsorbed gas, indicating that the interactions between CO2 and the pore walls are stronger than those between the CO2 molecules. The Qst values for the prepared samples are comparable to those of the nitrogen-doped templated carbons from zeolite and mesoporous silica (31−36 kJ/mol)29,30, nitrogen-incorporated hierarchical porous carbons (33−37 kJ/mol)31,32, and triptycene-derived benzimidazole-linked polymers (29 kJ/mol)33. This implies that the adsorption is physical in nature, which strongly depends on the pore structure of porous carbons. If Qst is too large, the regeneration of the sorbents becomes too costly, and if it is too small, the CO2 uptake and CO2 selectivity over N2 selectivity will be too small34. However, while choosing a suitable adsorbent material, the absolute uptake of CO2 is not the only factor to be considered; selectivity over other competing gases and particulates is also an important feature. For a material showing excellent adsorption profile for pure CO2 gas but simultaneously adsorbing a large amount of another gas in a more realistic gas mixture, the working capacity would be dramatically reduced. Ideally, CO2 adsorbents must exhibit a selective adsorption behavior over all other sorbates. For this reason, CO2/N2 selectivity studies were performed at 273 K. Selective adsorption behaviors of the adsorbents can be well-explained from the view point of molecular sieving, which demonstrates that kinetic diameters plays a dominant role during the gas adsorption. The kinetic diameters of CO2 and N2 are 3.30 and 3.64 Å, respectively, as reported earlier35. Hence, owing to the small diameter, CO2 is adsorbed more than N2. Apart from this, another factor to be considered is the thermodynamics, emphasizing the role of the critical temperature of the two gases in the selective adsorption behavior. The critical temperatures (T c ) of gaseous CO2 and N2 are reported to be 304.2 and 126.2 K, respectively. As CO2 has a higher critical temperature, CO2 molecules easily condense and adsorb on the porous framework. The selectivities for CO2 over N2 (Fig. 8) determined by initial slope method comes out to be 238, 141, 53, 230, 111, and 71 at 273 K for MI-800, MIK-800, MIKN-800, MI-700, MIK-700, and MIKN-700, respectively, which are comparable to those of highly selective CO2 adsorbents36,37,38.

Figure 8
figure 8

CO2 and N2 adsorption isotherms obtained at 273 K for selectivity measurements.

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The hydrogen adsorption isotherms obtained at 77 K and 1 bar are presented in Fig. 9 and the details are listed in Table 3. The adsorption capacity can be enhanced either by increasing the storage pressure or decreasing the adsorption temperature to 77 K. Here, we chose a low temperature adsorption phenomenon. All the isotherms are Type I (Langmuir-type) isotherms, demonstrating that the formation of a hydrogen monolayer results in a saturation condition which is a usual characteristic for microporous surfaces. However, with an increase in the pressure up to 1 bar, the gas uptake increases continuously but a clear plateau does not appear because of the low pressure measurements. Sample MIKN-800 shows the highest hydrogen storage capacity, reaching up to 4.0 wt% at 1 bar, which is attributed to its highest surface area as well as the maximum pore volume. The present work demonstrates a high performance than the previously reported data, 2.4 wt% for AX2139, 2.8 wt% for activated carbons40, 2.0 wt% for corncob41, 2.6 wt% for wood42, and 2.7 wt% for Quercus agrifolia-based ACs43. Additionally, MIKN-800 outperforms many MOFs and POPs in terms of its ability to store hydrogen gas at 77 K. This is due to high surface area with optimum pore structure, narrow pore-diameter with hierarchical meso-micro and ultra-micropores as well as nitrogen-doping. Heteroatom doping shows beneficial impact on gas storage property, as already reported in literature44,45. Moreover, samples MI-700 and MI-800 have the lowest surface areas and pore volumes and exhibit the lowest H2 uptake of 0.95 wt%. Based on these results, we conclude that good correlation exists between the textural features (surface area and pore volume) and gas adsorption capacity of the carbons46,47.

Figure 9
figure 9

Hydrogen adsorption-desorption isotherms obtained at 77 K up to 1 bar.

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Conclusions

Sustainable microporous carbons were synthesized via a one-step condensation-activation strategy in the presence of molten salts. The hydroxide-containing molten salts are responsible for generating well-developed microporous structures with a high surface area. CO2 and H2 adsorption properties of the fabricated materials were investigated and the results reveal that the microporous carbons prepared using KOH at 700 °C exhibit an exceptionally high CO2 adsorption (uptake = 9.7 mmol/g at 273 K and 1 bar). This outstanding CO2 adsorption performance of the prepared carbons is principally attributed to the narrow micropores (1.05 nm) and ultimately the optimum micropore volume (1.33 cm3/g). Similarly, the high specific area and pore volume contribute significantly to the H2 storage performance at 77 K, with 4.0 wt% uptake of H2 by MIKN-800 (specific surface area and pore volume of 2984 m2/g and 1.98 cm3/g, respectively). Hence, present work contributes an effort to design solvent-free single step strategy for high surface area containing microporous carbons as novel materials for efficient CO2 and H2 adsorption with an excellent CO2/N2 selective adsorption behavior.

Methods

Materials and synthesis

All the reagents comprising melamine (99%), isophthalaldehyde (99%), sodium hydroxide (98%), potassium hydroxide (95%), and hydrochloric acid were obtained from Sigma–Aldrich. In this study, melamine and isophthalaldehyde were carbonized to design two different types of porous carbons. The first, labeled as MIK-X, was synthesized using 1.24 g of melamine (M) with 1.36 g of isophthalaldehyde (I), and 5.0 g of KOH (K). All the reagents were mechanically ground and heated directly at 250 °C for 180 min followed by carbonization at the targeted temperature for 60 min with the heating rate of 1 °C min−1. The resulting product was neutralized to pH 7 by washing sequentially with 3 M HCl solution and distilled water. The obtained material was allowed to dry overnight at 120 °C. The other sample is denoted as MIKN-X. The preparation was carried out following the same procedure described for MIK-X, except that the composition of the activating agents was 2.5 g KOH and 2.5 g NaOH (KN). For evaluating the role of molten salts, blank samples containing a mixture of melamine and isophthalaldehyde (MI-X) were prepared in the absence of the hydroxides. In the names of the prepared samples, X denotes the targeted temperature of carbonization (700 or 800 °C).

Physicochemical characterization

The synthesized carbon materials were characterized by different analytical techniques. FTIR spectra of the fabricated samples were collected between 4000−400 cm−1 using a Fourier transform-infrared vacuum VERTEX 80 V spectrometer. A thermal gravimetric analyzer (TGA; TG209F3) determined the thermal stability of the prepared materials under nitrogen atmosphere. X-ray diffraction patterns were recorded on a D2 PHASER, BRUKER, X-ray diffractometer. All the recordings were made in the range of 2° to 80° (2θ). For exploring the morphological structures, field emission scanning electron microscopy (FESEM; Model SU8010, Hitachi Co., Ltd.) was used. Elemental analysis was performed using an EA1112 element analyzer. Further surface characterization was performed by X-ray photoelectron spectroscopy using XPS, VG Scientific Co., ESCA LAB MK-II. The textural features of the prepared materials were determined by obtaining N2 adsorption−desorption isotherms from a Model Belsorp Max instrument (BEL Japan, Inc.). Carbon dioxide adsorption capacity at 273, 283, and 298 K and hydrogen storage at 77 K were determined by a Model Belsorp Max instrument (BEL Japan, Inc.). For the CO2/N2 selectivity measurements, adsorption measurements for both the gases were carried out at 273 K using a Model Belsorp Max instrument (BEL Japan, Inc.). Prior to all the adsorption measurements, the fabricated materials were stored under vacuum conditions by heating at 120 °C for 8 h.