Highly porous carbons with superior performance for CO₂ capture through hydrogen-bonding interactions

Abstract

Highly porous carbons were prepared by using polyaniline (PANI) as a carbon precursor and KOH as an activating agent via a one-step chemical activation process. The effects of the activation parameters such as activation temperature, KOH–PANI weight ratio and pre-heating temperature were fully investigated, through which the pore structure and the materials chemistry of the activated porous carbons were optimized. When studied as an adsorbent for CO₂ capture, the optimized porous carbon exhibited a high CO₂ capture capacity of 4.50 mmol g⁻¹, high multi-cycle sorption/desorption stability and highly selective adsorption of CO₂ over N₂ (0.27 mmol g⁻¹) at 25 °C. This superior performance for CO₂ capture was found to be closely related to C–H groups on the carbon surface through hydrogen bonding interactions.

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1. Introduction

Reduction of CO₂ concentration in the atmosphere has attracted considerable attention because CO₂ is considered to be the main anthropogenic contributor to climate change.^1–3 In addition, a high concentration of CO₂ is harmful to humans, especially in space-limited chambers like submarines and space ships.⁴ The selective capture and storage of CO₂ at low cost in an energy-effective way are crucial for achieving substantial reduction of the emission level.^3,5–7

Most of the current commercial technologies for CO₂ capture are based on absorption–regeneration technology involving liquid amine-based process.^8,9 However, this technology has the problems of high energy consumption, solvent degradation, and equipment corrosion.^10,11 In order to overcome the disadvantages of the liquid amine-based process, researchers focused on the adsorption by solid adsorbents because of its low energy consumption and low equipment cost. Up to now, a large number of solid adsorbents have been employed for CO₂ capture, such as zeolites,^12–15 mesoporous silica,^5,16–18 metal–organic frameworks (MOFs),^19,20 metal oxides,²¹ ionic liquid²² and porous carbons.^23–28 Carbon-based adsorbents are most promising for CO₂ capture due to high chemical stability, low cost, high surface area and large adsorption capacity.^7,29,30

High surface area and/or high pore volume are the prerequisites for CO₂ capture by CO₂ gas physisorption^27,31 In addition, CO₂ is slightly acidic, and thus the basicity of adsorbents in surface chemistry plays an important role in achieving high CO₂ capture performance through acid–base interactions.³² Adsorbents functionalized with amines show high CO₂ adsorption capacity.³³ When nitrogen is incorporated or doped to porous carbons, their CO₂ adsorption capacity will be further improved.^25,34,35 However, the influence of N-containing groups on CO₂ capture is still controversial and not well-defined.³¹ One-to-one base–acid interaction cannot explain the fact that, in some cases, a small amount of doped nitrogen results in greatly increased CO₂ adsorption capacity.

Conducting polymer, polyaniline (PANI), has been intensively studied due to its facile synthesis, low cost, environmental stability and unique optical, electrical, and electrochemical properties.³⁶ This polymer has been previously employed as a precursor to fabricate N-doped porous carbons by means of nanocasting techniques. Two-step chemical activation process, carburization and then activation, was normally adopted to produce porous carbons using conducting polymers as carbon precursors.^{6,23,25,37–39} However, to our best knowledge, no work has been reported to produce N-doped, highly porous carbons from PANI through a one-step chemical activation process.

In this paper, PANI was used as the carbon precursor and KOH as the activating agent to produce N-doped, highly porous carbons through a one-step chemical activation process. The effects of the activation parameters such as activation temperature, KOH–PANI weight ratio and pre-heating temperature were fully investigated and optimized. The resulted activated carbon exhibited a high CO₂ capture capacity of 4.50 mmol g⁻¹, high multi-cycle sorption/desorption stability and highly selective adsorption of CO₂ over N₂ (0.27 mmol g⁻¹) at 25 °C. The contributions to the CO₂ capture performance from the specific surface area and N content were discussed. It was found that, in addition to these two factors, the superior performance for CO₂ capture is closely related to C–H groups on the carbon surface through hydrogen bonding interactions.

2. Experimental

2.1 Synthesis of highly porous carbons

PANI was prepared by aniline oxidation using ammonium persulfate as an oxidant. Briefly, aniline (0.23 M, 350 mL) acid solution was slowly dropped into an ammonium persulfate (0.54 M, 150 mL) acid solution at ice water temperature and the mixture was magnetically stirred for 6 h. Dark green PANI was then separated by filtration and washed thoroughly with de-ionized water until neutral pH was reached and then dried. The final PANI yield was around 100%. A physical mixture of KOH and PANI (weight ratio of KOH–PANI = 1–6) was preheated at (150–400 °C, 1 h) in a tube furnace under Ar flow (100 mL min⁻¹), followed by a high-temperature (500–700 °C, 1 h) activation process. The adsorbents were obtained by consequent washing the residue with de-ionized water and ethanol until neutral pH was reached and then dried in an oven at 120 °C. The as-synthesized samples were denoted as AC-x-y-z, where x is the KOH–PANI weight ratio, y is the preheating temperature (in °C) and z is the activation temperature (in °C). For example, sample AC-2-200-600 reflects the activation conditions KOH–PANI ratio of 2, preheating at 200 °C for 1 h and activation temperature at 600 °C 1 h. Sample AC-2-600D was prepared by the same method but omitted the preheating process. In order to determine the effect of nitrogen content, we use the sucrose as precursor instead of PANI polymer but the same activation denoted as sucrose-2-200-600. The conventional two-step activation procedure, PANI carbonization at 600 °C, then followed the same preparation process of AC-2-200-600 was identified as AC600-2-200-600.

2.2 Material characterization

The morphologies of the samples were examined by scanning electron microscopy (SEM) using a JEOL 6301F. X-ray photoelectron spectroscopy (XPS) was carried out with Kratos Axis 165 using a monochromated Al Kα excitation source. Elemental analyses were done on an Elementar Vario EL III microanalyzer. N₂ adsorption–desorption isotherms were measured at 77 K with Autosorb I (Quantachrome, USA). The samples were degassed in a high vacuum for 2 h at 250 °C before nitrogen adsorption measurement. The total pore volume was calculated as the adsorbed volume of liquid nitrogen at a relative pressure of 0.997 and surface area was measured based on the multipoint Brunauer–Emmett–Teller (BET) method at a relative pressure of 0.05–0.20. The pore size distributions (PSDs) and the volume of micropores smaller than a certain pore size were obtained from the N₂ adsorption isotherms using the non-local density functional theory (NLDFT) model, assuming a slit pore shape.

2.3 CO₂ capture measurements

CO₂ adsorption capacity was measured using a thermo-gravimetric apparatus (SDT Q600, TA Instruments, USA). The adsorbent particles were ground and sieved to 150–250 μm size range before use. The CO₂ capture was tested at TGA (SDT Q600, TA Instruments, USA), the samples were degassed at 105 °C 30 min, then 95% CO₂ + 5% N₂ (flow rate 100 mL min⁻¹) at 25 °C about 1 h. In the case of adsorption and desorption, during the adsorption, the carbon sample was exposed to a stream of CO₂ (100 mL min⁻¹). Once the sample was saturated, the CO₂ was switched to He (100 mL min⁻¹) and CO₂ was desorbed. The CO₂ adsorption experiments were also performed with ASAP 2020C (Micrometrics Corp.) at different temperatures (25, 50, 70, 95 °C). In the case of the adsorption–desorption cycles, the sample (≈30 mg) was degassed under the He gas at 200 °C before the cyclic experiments. During the adsorption, the carbon sample was exposed to a stream of pure CO₂ (100 mL min⁻¹). Once the sample was saturated, the gas was switched back from CO₂ to He (100 mL min⁻¹) and the carbon dioxide was desorbed. This adsorption–desorption cycle was repeated several times.

3. Results and discussion

Scanning electron microscopy (SEM) was used to image the microscopic morphology of the carbon precursor and the activated carbons. PANI, when used as the carbon precursor, exhibited a typical sponge-like architecture formed by interconnected microparticles of about 100 nm in diameter (Fig. 1a). This morphology offers a large interfacial area for the polymer to react with KOH, which is essential to achieve uniform activation reactions and a good pore development. The activated carbons demonstrate a variety of shapes depending on the degree of activation. The less-activated sample AC-1-200-600 (Fig. 1b) contains small pores on the surface, while the fully activated carbon AC-2-200-600 is made up of particles with large cavities and smooth surfaces (Fig. 1c). The over-activated carbon AC-4-200-600 is shown to have extra-large cavities (Fig. 1d).

Fig. 1 SEM images showing the structural characteristics of the carbon materials: (a) polyaniline (PANI), (b) sample AC-1-200-600, (c) sample AC-2-200-600 and (d) sample AC-4-200-600.

The N₂ sorption and pore size distribution (PSD) of the activated carbons, prepared at different activation temperatures, KOH–PANI weight ratios and preheating temperatures, are displayed in Fig. 2. All the activated carbons show an adsorption isotherm of type I in Fig. 2a, c and e, in which, the curves rapidly achieves a high adsorption plateau at very low relative pressure. It indicates that a large content of micropores exists in these samples.²³ The PSD graphs in Fig. 2b, d and f also suggest that these activated carbons are made up of mixed micro- and meso-pores,⁴⁰ which may be beneficial to the fast dynamics of CO₂ sorption.²⁸

Fig. 2 Effect of activation temperature (a and b), KOH ratio (c and d) and preheating temperature (e and f) on N₂ adsorption isotherms and pore size distributions.

The effect of the activation temperature on the pore structures of the resulted activated carbons are displayed in Fig. 2a and b. The textural parameters of the activated carbons are also concluded in Table 1. The surface area and the total pore volume for the sample AC-2-200-500 are only 184 m² g⁻¹ and 0.20 cm³ g⁻¹, respectively (Table 1), suggesting that the polymer was not fully carbonized at 500 °C. When increasing the activation temperature to 600 °C, the surface area and the pore volume increased at a staggering rate to 1668 m² g⁻¹ and 0.35 cm³ g⁻¹, respectively. Further increasing the temperature, the surface area and the pore volume kept increasing up to 2720 m² g⁻¹ and 0.42 cm³ g⁻¹, respectively. Fig. 2c and d demonstrates the effect of KOH–PANI weight ratio. From the pore structure data in Table 1, we can clearly see that the KOH–PANI ratio of 2 is the optimum ratio. Any deviation from this ratio will decrease both the surface area and the pore volume. Fig. 2e–f illustrates the importance of the preheating temperature on the final pore textures. Although the preheating temperature has been considered as a critical parameter for the activation process, few literatures studied its effect.⁴¹ Fig. 2e and f and Table 1 in this report reveal that, increasing the preheating temperature from 150 to 200 °C leads to higher surface area and volume; however, further increasing the preheating temperature to 300 or 400 °C results in lower surface area and pore volume. An optimum preheating temperature of 200 °C was determined. The effect of the preheating temperature will be discussed in detail later in this report.

Table 1 Properties of nitrogen-doped and highly porous carbons

Materials	SBET m^2 g^−1	Smicro^b m^2 g^−1	Vp^a cm^3 g^−1	Vmicro^c cm^3 g^−1	N%	N/C weight ratio	H/C weight ratio	CO2 uptake^d mmol g^−1	Normalize CO2 uptake mmol CO2 per mmol N	Normalized CO2 uptake mmol CO2 per m^2
a Total pore volume at p/p^o ∼ 0.99.b The micropore surface areas.c Volume of micropores with pore diameters less than 1 nm as determined by the NLDFT method.d CO2 capture capacities of the porous carbons at 25 °C and 0.95 atm. N/C and H/C weight ratio were determined by elemental analysis.
AC-1-200-600	775	724	0.34	0.19	8.30	0.1523	0.03771	2.45	0.44	3.16
AC-1.5-200-600	1473	1274	0.67	0.34	7.85	0.1463	0.04163	2.95	0.50	2.00
AC-2-200-600	1668	1328	0.69	0.35	5.16	0.1208	0.05140	4.30	1.17	2.58
AC-2.5-200-600	898	680	0.49	0.19	5.03	0.1176	0.03282	1.64	0.90	2.90
AC-3-200-600	859	631	0.40	0.17	4.09	0.08114	0.02687	1.59	1.18	1.93
AC-4-200-600	802	503	0.50	0.14	1.84	0.03420	0.02186	1.55	0.42	1.85
AC-2-150-600	1309	1094	0.56	0.29	7.18	0.1322	0.04594	3.85	0.75	2.93
AC-2-300-600	586	473	0.26	0.13	5.10	0.1180	0.04005	2.69	0.54	4.59
AC-2-400-600	783	579	0.41	0.16	3.93	0.1177	0.03504	2.01	0.72	2.57
AC-2-200-500	184	107	0.20	0.028	6.63	0.1624	0.02493	2.23	0.47	12.12
AC-2-200-700	2720	1441	1.68	0.42	3.20	0.04957	0.02588	2.55	1.11	0.94
AC600-2-200-600	910	790	0.47	0.20	5.25	—	—	1.95	0.13	2.14
Sucrose-2-200-600	39	10	0.29	0.0028	—	—	—	1.60	—	—

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High surface area and high pore volume are required for CO₂ gas physisorption.^27,42 Although some CO₂ adsorption data reported in Table 1 appear to support this physisorption mechanism, CO₂ adsorption data contradictory to the physisorption mechanism also exist. For example, sample AC-2-200-700 and sample AC-1-200-600 are shown to have almost the same CO₂ adsorption capacity (2.55 mmol g⁻¹), but they had a very different specific surface areas (2720 m² g⁻¹ for AC-2-200-700 and 775 m² g⁻¹ for AC-1-200-600). This indicates that the CO₂ uptake cannot be solely rationalized by the physio-characteristics of the activated carbons, such as specific surface area and/or micropore volume.

The chemisorption of CO₂ gas has also been reported by many researchers.^{3,33,39,42,43} It involves in acid–base interactions between N-containing basic functional groups and acidic CO₂ gas. However, the capacity of CO₂ capture observed in the investigation does not agree well with the chemisorption mechanism. As seen in Table 1, samples AC-2-200-600 and AC-2.5-200-600 contain almost the same nitrogen content, but the former yields much larger CO₂ uptake (4.30 mmol g⁻¹) than the latter (1.64 mmol g⁻¹).

More interestingly, as shown in Table 1, sample AC-1-200-600 has larger specific surface area (775 m² g⁻¹) and higher nitrogen content (8.3%) than those of AC-3-200-600 (a specific surface area of 586 m² g⁻¹ and a nitrogen content of 5.1%), but yields smaller CO₂ uptake (2.45 mmol g⁻¹) than AC-2-200-600 (2.69 mmol g⁻¹). It implies that a combined consideration of chemisorption and physisorption is still inconsistent with the experimental findings and therefore alternative factors must be explored in order to rationalize the high CO₂ capture capacity of 4.30 mmol g⁻¹ achieved.

To explore possible alternative factors affecting CO₂ uptake, CO₂ capture capacities were normalized by the N content that accommodates the effect of acid–base interactions. As shown in Table 1, sample AC-2-200-600 has a larger specific area (1668 m² g⁻¹) than that of AC-3-200-600 (859 m² g⁻¹), but they have almost the same CO₂ uptake after normalization. Similarly, CO₂ capture capacities were normalized by the specific surface area of the samples (Table 1). Although the N content of sample AC-3-200-600 (4.09%) is much higher than that of AC-4-200-600 (1.84%), both have almost the same CO₂ uptake when normalized by the specific surface area of the samples. It is obvious that the physical adsorption by surface area and the acid–base interactions by N content together are not able to explain the CO₂ adsorption behaviour found in this study. It further indicates that additional factors may also play an important role in CO₂ uptake.

The CO₂ adsorption capacity and the H/C weight ratio were plotted as a function of the activation temperature at the fixed KOH–PANI ratio of 2 and the preheating temperature of 200 °C in Fig. 3a. As the activation temperature increases, CO₂ adsorption capacity exhibits a volcanic change, with a maximum value of 4.30 mmol g⁻¹ obtained at the activation temperature of 600 °C. Interestingly, the changing trend of H/C weight ratio with the activation temperature coincides with that of CO₂ adsorption capacity (Fig. 3a). The H/C weight ratio also exhibits a volcanic change with the highest H/C weight ratio observed at 600 °C.

Fig. 3 The variation of CO₂ adsorption capacities, H/C weight ratio and N/C weight with activation temperature for samples processed with KOH–PANI = 2 and a preheating temperature of 200 °C (a and b), KOH–PANI weight ratio for samples processed at a preheating temperature of 200 °C and an activation temperature of 600 °C (c and d), and preheating temperature for samples processed at KOH–PANI = 2 and an activation temperature of 600 °C (e and f).

In contrast, the N/C weight ratio was found to decrease monotonically with increasing activation temperature (Fig. 3b). Similar trend was also found when the CO₂ adsorption capacity, the H/C weight ratio and the N/C weight ratio were plotted as a function of the KOH–PANI ratio in Fig. 3c and d and the preheating temperature in Fig. 3e and f, respectively. In Fig. 3c, the amount of CO₂ uptake shows a peak value at a KOH–PANI ratio of 2, which is similar to the variation of the H/C weight ratio with KOH–PANI ratio. However, the N/C weight ratio decreases monotonically from 0.15 to 0.03 with increasing KOH–PANI ratio (Fig. 3d). In Fig. 3e, both the CO₂ uptake amount and the H/C weight ratio demonstrate a maximum point at a preheating temperature of 200 °C, while the N/C weight ratio almost keeps the same at a level of around 0.12.

From the above discussion, it is clear that that the trend of CO₂ uptake amount is different from that of N/C weight ratio, but correlates well with that of H/C weight ratio. It is, therefore, suggested that the C–H groups on the carbon surface may play a vital role in CO₂ capture. The capture of CO₂ molecules by C–H group is suggested to be through hydrogen bonding interactions as C–H⋯O. Although the interaction of C–H⋯O is weaker than that of typical hydrogen bonds, the existence of this type of weak hydrogen bond has been validated in the experimental and theoretical studies reported previously.^31,44,45 Fig. 4a shows a direct correlation of the CO₂ uptake with the H/C weight ratio and a linear relation between the CO₂ uptake and the H/C weight ratio is seen. The slope of the linearly fitted line in Fig. 4a was determined to be 143 mmol CO₂ g⁻¹ (H/C weight ratio)⁻¹. In contrast, the relation between the CO₂ uptake and the N/C weight ratio is statistically insignificant (Fig. 4b). Fig. 4 further confirms the importance C–H group in CO₂ capture through hydrogen-bonding interactions. The observation of this type of hydrogen-bonding interaction, C–H⋯O, in CO₂ capture may be attributed to the adopted one-step chemical activation process, in which the carburization and the activation occurred concurrently and the formation of C–H groups in the activated carbons were obviously facilitated. The latter was found to contribute to the CO₂ capture through hydrogen-bonding interactions.³¹

Fig. 4 Variation of CO₂ adsorption capacities with H/C weight ratio (a) and N/C weight ratio (b) for samples processed at different KOH weight ratios and different preheating temperatures under the activation temperature of 600 °C.

From the above experiment results, it has been determined that the highest CO₂ adsorption can be achieved when the KOH–PANI mixtures were activated at the following optimized conditions: the activation temperature of 600 °C for 1 h, the KOH–PANI ratio of 2 and the preheating temperature of 200 °C for 1 h. It is worth to note that, although the pure dry KOH melts at 380 °C, KOH can be melted at a lower temperature when mixed with carbon precursors.⁴³ In this study, we observed that KOH starts to be melted at 200 °C when it was mixed with PANI. It is suggested that, for the preheating temperature lower than 200 °C, the unmelted KOH will not wet the KOH–PANI mixtures thoroughly; for the preheating temperature over 200 °C, the melted KOH may alter the morphology of PANI polymers. In both cases, the pore structures of the final activated carbons will be affected and the CO₂ capture capacities will be deteriorated. Therefore, the optimized preheating temperature was obtained at 200 °C.

The CO₂ capture behaviour of the porous carbons activated at the optimized conditions was further characterized. As shown in Fig. 5a, the prepared activated carbon AC-2-200-600 has shown a very high CO₂ capture capacity of 4.50 mmol g⁻¹ (under 1 atm 25 °C), which is amongst the best of the known porous carbons.^35,42,46 The commercial adsorbent SA9T, which is an amine immobilized on the polymer support, indicates only 0.44 mmol g⁻¹ capacity for CO₂ uptake.⁴⁷ Zeolite 13X, which is widely considered to be a promising CO₂ adsorbent, presents 3.64 mmol g⁻¹ capacity for CO₂ uptake.³⁵ Nitrogen-enriched porous carbons produced from polyacrylonitrile containing block copolymer yields a maximum capacity of 3.00 mmol g⁻¹.⁴⁶ N-enriched porous carbons by the carbonization of a polymeric material in the presence of an amino acid L-lysine show a maximum CO₂ adsorption uptake of 3.1 mmol g⁻¹.³⁴ All these comparison results further reveal the significance of the C–H group on CO₂ capture through hydrogen-bonding interactions.

Fig. 5 CO₂ adsorption of sample AC-2-200-600 at 25, 50, 70 and 95 °C (a) and isosteric heat of adsorption (Q_st) calculated from CO₂ adsorption curves (b).

To investigate the CO₂ capture performance at elevated temperatures, the adsorption isotherms of AC-2-200-600 at 25, 50, 70 and 95 °C were measured (Fig. 5a). It can be seen that the CO₂ uptake decreases significantly with increasing adsorption temperature, which is reasonable since the molecular kinetic energy of CO₂ rises with increasing temperature. The higher temperature leads to lower CO₂ adsorption capacity, indicating that the CO₂ adsorption process on the AC-2-200-600 is an exothermic process. To determine the strength of the interaction between CO₂ molecules and the activated AC-2-200-600, the isosteric heat of adsorption (Q_st) was calculated by applying Clausius–Clapeyron equation to the CO₂ adsorption isotherms at 25, 50, 70 and 95 °C (Fig. 5b).^25,34,48 The isosteric heat of adsorption decreases from 36 to 31 kJ mol⁻¹ as the CO₂ adsorption amount increases from 0.2 to 1.2 mmol g⁻¹. The high isosteric heat of adsorption is mainly due to hydrogen-bonding interactions (C–H⋯O) and acid–base interactions between N-containing basic functional groups and acidic CO₂ gas.

For the scale up application in CO₂ sequestration, the adsorbent should also show high stability during cyclic adsorption–desorption processes and high selectivity, in addition to high capacity. The stability during cyclic adsorption–desorption processes for the sample AC-2-200-600 was measured at 25 °C in Fig. 6, which shows only 3% drop in CO₂ adsorption capacity after 30 cycles demonstrating its high stability. For selectivity, N₂ adsorption on AC-2-200-600 shows a very small adsorption capacity of 0.27 mmol g⁻¹ for N₂ gas at 25 °C, indicating that AC-2-200-600 can be used for selective adsorption of CO₂ in the atmosphere (Fig. 7a). The selectivity of CO₂ over N₂ gas, defined as the ratio of the initial slopes of N₂ and CO₂ adsorption isotherms (Fig. 7b), was calculated using Henry's law, where the initial slopes were calculated from CO₂ and N₂ adsorption isotherms as reported earlier.^47,49 It is interesting that the selectivity of CO₂ over N₂ was determined to be 36 at 25 °C, which supports the result in Fig. 7a and demonstrates its potential for industrial applications.⁴⁷ The comparison data in Fig. 5–7 clearly show that the optimized N-doped activated carbons, prepared through a simple and scalable one-step chemical activation process, demonstrate a high adsorption capacity, high stability and high selectivity for CO₂ capture.

Fig. 6 Stability of CO₂ adsorption–desorption cycles obtained for sample AC-2-200-600 at 25 °C.

Fig. 7 Adsorption curves (a) and determination of initial slopes from CO₂ and N₂ adsorption isotherms at 25 °C for sample AC-2-200-600 (b).

4. Conclusion

In conclusion, nitrogen-doped, highly porous carbon was synthesized via a one-step chemical activation process by using polyaniline as the carbon precursor and KOH as the activating agent. The effects of the activation parameters such as activation temperature, KOH–PANI weight ratio and pre-heating temperature were fully investigated. The optimum conditions are: the activation temperature of 600 °C for 1 h, the KOH–PANI ratio of 2 and the preheating temperature of 200 °C for 1 h. The activated porous carbon with the optimized pore structure and material's chemistry exhibited a high CO₂ capture capacity of 4.50 mmol g⁻¹, high multi-cycle sorption/desorption stability and highly selective adsorption of CO₂ over N₂ (0.27 mmol g⁻¹) at 25 °C. This superior performance for CO₂ capture was found to be closely related to C–H groups on the carbon surface through hydrogen bonding interactions, in addition to specific surface area and N content of the activated carbons. The relatively large content of C–H groups in the activated carbons is believed to be achieved when the carburization process and the activation process occur simultaneously within the one-step chemical activation process.

Acknowledgements

The authors would like to acknowledge Carbon Management Canada for the financial support.

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