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. 2020 Sep 1;5(36):23460–23467. doi: 10.1021/acsomega.0c03497

Molten Salt Template Synthesis of Hierarchical Porous Nitrogen-Containing Activated Carbon Derived from Chitosan for CO2 Capture

Peiyu Wang 1,*, Guoheng Zhang 1, Wanjun Chen 1, Qiong Chen 1, Haiyan Jiao 1, Liwei Liu 1, Xiangli Wang 1, Xiaoyan Deng 1
PMCID: PMC7496027  PMID: 32954199

Abstract

graphic file with name ao0c03497_0009.jpg

Here, hierarchical porous nitrogen-containing activated carbons (N-ACs) were prepared with LiCl-ZnCl2 molten salt as a template derived from cheap chitosan via simple one-step carbonization. The obtained N-ACs with the highest specific surface area of 2025 m2 g–1 and a high nitrogen content of 5.1 wt % were obtained using low molten salt/chitosan mass ratio (3/1) and moderate calcination temperature (1000 °C). Importantly, using these N-ACs as CO2 solid-state adsorbents, the maximum CO2 capture capacities could be up to 7.9/5.6 mmol g–1 at 0 °C/25 °C under 1 bar pressure, respectively. These CO2 capture capacities of N-ACs were the highest compared to reported biomass-derived carbon materials, and these values were also comparable to most of porous carbon materials. Moreover, as-made N-ACs also showed good selectivity for CO2/N2 separation and excellent recyclability. The unique hierarchical porous structure of N-ACs thus provided the right combination of adsorbent properties and could enable the design of high-performance CO2 solid adsorbents.

1. Introduction

Activated carbons (ACs) are versatile materials because of their series of advantages such as abundant porosity and thermal and chemical stability. They can be widely used in environmental protection (CO2 solid adsorbents and regulation SOx and NOx emissions from fuel combustion in automobiles), energy storage (supercapacitors and batteries), sensors, and so on.1 Taking into account business costs and energy/environment problems, a series of low-cost, easy-to-obtain, carbon rich precursors and sources have been explored for the production of ACs. Especially, preparation of carbon materials from biomass has become one of the research hotspots in recent years. Until now, many biomass-derived ACs, such as from eucalyptus sawdust,2 asparagus lettuce leaves,3 pollen,4 pine cone,5 and gelatin and starch,6 have been reported. These ACs have shown good potential applications as CO2 solid state adsorbents.1−6

It is well known that the CO2 capture capacity of ACs depends on the porous structure of ACs.7 In addition, in order to further improve the CO2 capture capacity of ACs, introduction of nitrogen atoms into ACs is also an effective strategy.8 However, it is difficult to have an excellent porous structure and rich nitrogen functional groups in ACs at the same time. Complex and limited methods to manufacture such superior ACs thus hinder their practical application in CO2 capture. Many methods have been developed to prepare ACs,1 such as soft template method,9 hard template method,10 chemical vapor deposition,11 chemical-activation (KOH,12 NaOH,13), and physical activation (CO214). Unfortunately, in these activation technologies, the pores are etched into the carbon material, and the formation of the pores is accompanied by significant mass loss, especially the significant reduction of nitrogen and other heteroatoms’ content. Therefore, it is urgent to develop a new method to prepare ACs, which can build a rich pore structure without significant mass loss.

A new “mixed molten salt” method may meet this demand, which often uses a non-carbonizable inorganic salt (such as LiCl-ZnCl2; NaCl-ZnCl2; or KCl-ZnCl2) mixed with ionic liquid. Then, carbon precursor was condensed and scaffolded in the presence of the molten salt at elevated temperatures.15 However, the use of the ionic liquid as a raw material in their report will make ACs too expensive, which is not conducive to the wide application of this method.15,16 For these reasons, it is more attractive to use this “mixed molten salt” method while choosing suitable cheaper carbon sources. For example, Pampel and Fellinger prepared nitrogen-containing activated carbons (N-ACs) with high porosity in a NaCl-ZnCl2 molten salt medium derived from adenine.17 Li et al. prepared N-AC submicrospheres in a KCl-ZnCl2 molten salt derived from diaminomaleonitrile, and N-AC submicrospheres exhibited great potential applications in energy storage, dye removal, and CO2 capture.18

In this study, we reported a simple one-step carbonization method to prepare hierarchical porous N-ACs for CO2 capture. LiCl-ZnCl2 (LiZ) molten salt was used as a salt template, and low-cost, eco-friendly chitosan was used as a raw material in this facile method. The molar ratio of LiZ molten salt is 23 mol % LiCl and 77 mol % ZnCl2, and the melting point of the LiZ is 294 °C.15 Our method of preparing N-ACs had the following advantages: (1) compared to other activation techniques, this method can avoid significant mass loss; (2) structural nitrogen can be preserved from raw materials; (3) compared to other expensive techniques such as zeolite templating,19 this method is a simple, cheap, and sustainable technique to address high surface area carbons; (4) the raw chitosan is cheap, sufficient, and eco-friendly; and (5) the molten salt can be recovered for further use. These advantages can lay the foundation for the practical application of N-ACs. The as-made N-ACs showed a high CO2 adsorption capacity of 7.9/5.6 mmol·g–1 at 0 °C/25 °C under 1 bar pressure, respectively. The as-made N-ACs also showed good selectivity for CO2/N2 separation and excellent recyclability.

2. Results and Discusion

2.1. Surface Chemical Properties of N-ACs

In our work, chitosan was converted to carbon using LiZ molten salt template carbonization at 800–1200 °C for 1 h. The resulting N-ACs were confirmed by XRD, Fourier-transformed infrared (FT-IR), and X-ray photoelectron spectroscopy (XPS) spectra. The XRD pattern of N-AC-3-1000 (Figure 1a) showed a high intensity in the low angle region, which suggested the presence of high density pores.21 The FT-IR spectra (Figure 1b) of the chitosan and N-AC-3-1000 showed the similar form. The strong bands in both spectra at 3477, 2924, and 2846 cm–1 can be ascribed to N–H stretching bonds, C–H asymmetric stretching, and symmetric stretching bonds, respectively.22 The prominent peak in both spectra at 1642 cm–1 can be associated to the deformation mode of a nearly free amine group.23 The peaks at 1385 and 1092 cm–1 can be associated to the symmetric carboxylate stretching bond and the C–O–C stretching band. The existence of nitrogen in samples was also proved by XPS.

Figure 1.

Figure 1

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(a) XRD pattern of N-AC-3-1000 and (b) FT-IR spectra of chitosan and N-AC-3-1000.

The N contents of all samples summered from XPS measurement were in the range of 4.8–5.6 wt % (shown in Table 1), which could confirm the successful doping of nitrogen into the frameworks of N-AC samples. The well-fitted N 1s region (Figure 2a–c) showed three species presented on the surfaces of N-AC samples, which can be attributed to quaternary nitrogen (401.5 eV),24 pyrrolic-N (400.3 eV),25 and pyridinic-N structure (398.7 eV),25 respectively. The detailed contents of the nitrogen species are listed in Table S1. It showed that the pyrrolic-N was the dominant N species on sample surfaces. Although all three forms of N are beneficial to CO2 adsorption, pyrrolic-N is more prominent.26 It can be seen from Figure 2a–c that the mass ratio of molten salt/chitosan had little effect on the dominant N species. It can be seen from Figure 2b,d,e that pyridinic-N will increase with the increase of temperature, as reported before.27

Table 1. Texural Properties of N-ACs.

sample SBET (m2 g–1)a Vt (cm3 g–1)b Vultra (cm3 g–1)c V (<1 nm) (cm3 g–1)c V (<2 nm) (cm3 g–1)c V (<5 nm) (cm3 g–1)c N (wt %)d CO2 uptake at 0 °C (mol g–1)e CO2 uptake at 25 °C (mol g–1)e
N-AC-L 168 0.08 - - 0.02 0.06 5.8 2.6 -
N-AC-Z 1330 0.66 0.14 0.23 0.45 0.55 5.0 4.0 -
N-AC-2-1000 1389 0.68 0.16 0.28 0.47 0.59 5.6 5.3 3.5
N-AC-3-1000 2025 1.15 0.21 0.32 0.56 1.02 5.1 7.9 5.6
N-AC-4-1000 1978 1.18 0.13 0.26 0.44 1.03 4.8 7.3 5.1
N-AC-3-800 1655 0.78 0.12 0.25 0.50 0.67 5.4 6.2 4.2
N-AC-3-1200 1902 1.10 0.18 0.29 0.45 0.93 4.9 7.6 5.2
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a

Surface area was calculated using the BET method at P/P0 = 0.01–0.2.

b

Total pore volume at P/P0 = 0.99.

c

Cumulative pore volume at pore sizes up to 0.7/1.0/2.0/5.0 nm deduced from N2 adsorption isotherms at 77 K using NLDFT method.

d

Data derived from XPS.

e

Data derived from CO2 adsorption isotherm at 1 bar.

Figure 2.

Figure 2

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N 1s XPS spectra of N-ACs: (a) N-AC-2-1000, (b) N-AC-3-1000, (c) N-AC-4-1000, (d) N-AC-3-800, and (e) N-AC-3-1200.

The yield of the obtained N-AC-3-1000 was 30 wt % in our method; at the same time, a contrast sample was made by directly carbonizing chitosan to 1000 °C under the same experimental conditions, and the yield was 32 wt %. This fully showed that the molten salt template was rather to a templating process than chemical activation. The yield was distinctly higher than KOH activated carbons (total yield ∼15–10 wt %)28 and also compared to other molten salt method (27 wt %, KCl/ZnCl2).29

The obtained N-AC-3-1000 showed a three-dimensional hierarchical porous structure with dense and interconnected pores in the scanning electron microscope (SEM) image (Figure 3a). The HR-SEM image (Figure 3b) showed the surface roughness of the actived carbon. The addition of molten salt could directly provide enough solid surfaces to form chitosan coating, which led to the formation of three-dimensional nanostructures via carbonization. Similar results were also found in the literatures, which used ionic liquid as the carbon source and molten salts as template.30 The structural characterization was further described using a transmission electron microscope (TEM) in Figure 3c in which a large number of pores were formed and evenly distributed in the particles. The hierarchical porous structure of N-ACs was further confirmed in N2 adsorption/desorption measurement.

Figure 3.

Figure 3

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(a) SEM images, (b) HR-SEM, and (c) TEM image of the N-AC-3-1000.

2.2. N2 Adsorption/Desorption

In order to further understand the microstructure and pore structure of N-ACs, N2 adsorption/desorption measurements were used to analyze all samples.

2.2.1. Effect of Salts on the Performance of N-ACs

The effect of salt on the carbonization of chitosan was investigated, which could exhibit the benefits using molten salts (LiZ) as the template. The N-ACs were obtained using pure LiCl, LiZ salts, and pure ZnCl2 as templates. The mass ratios of salt/chitosan were all adjusted to 3, and the carbonization temperatures were all 1000 °C. The obtained N-ACs were denoted as N-AC-L, N-AC-3-1000, and N-AC-Z using LiCl, LiZ, and ZnCl2 as templates, respectively. N2 adsorption/desorption curves and pore size distributions of N-AC-L, N-AC-3-1000, and N-AC-Z are shown in Figure S1. According to the IUPAC classification, the curves of N-AC-L and N-AC-Z all appeared to be type I, while the curve of N-AC-3-1000 was type IV, which had an obvious hysteretic loop.31,32 The Brunauer–Emmett–Teller (BET) surface area of N-AC-L, N-AC-3-1000, and N-AC-Z were 168, 2025, and 1330 m2 g–1, respectively. The pore volume of N-AC-L, N-AC-3-1000, and N-AC-Z were 0.08, 1.15, and 0.66 cm3 g–1. The pore size distributions calculated by non-local density functional theory (NLDFT) method are shown in Fig. S1b. It clearly demonstrated that the N-AC-3-1000 and N-AC-Z mainly had meso/microporous structures, while N-AC-L had a little porous structure. Table 1 listed the main parameters deriving from N2 adsorption/desorption measurement. It could be found that N-AC-3-1000 exhibits the highest BET surface area of 2025 m2 g–1, the biggest pore volume of 1.15 cm3 g–1, and a micropore volume of 0.56 cm3 g–1. These results showed that N-AC-3-1000 had a hierarchically porous structure with abundant micropores and mesopores. With respect to this point, LiZ showed the best performance among the three type salts for producing high-quality N-ACs.

2.2.2. Effect of Molten Salt/Chitosan Mass Ratio on the Performance of N-ACs

The mass ratio of LiZ/chitosan could affect the performance of N-ACs. The N-ACs obtained from three mass ratio of LiZ/chitosan (2, 3, 4) at 1000 °C were denoted as N-AC-2-1000, N-AC-3-1000, and N-AC-4-1000. Meanwhile, N2 adsorption/desorption curves and pore size distributions of the N-AC-x-1000 (x = 2, 3, 4) using NLDFT method were carried out (Figure 4a,b). The curve of N-AC-2-1000 appeared to be type I, suggesting the presence of micropores. N-AC-3-1000 and N-AC-4-1000 both had type IV curves with obvious hysteretic loops.31,32 The PSD curves of N-AC-2-1000, N-AC-3-1000, and N-AC-4-1000 are shown in Figure 4b. The curves showed that N-AC-2-1000 was composed of micropores (<2 nm) and mesopores centered at 2.11 nm, N-AC-3-1000 was composed of micropores and mesopores centered at 1.21 and 2.71 nm and N-AC-4-1000 was composed of micropores (<2 nm) and mesopores centered at 2.71 nm. These indicated that the hierarchical porous structure would be formed in N-ACs using LiZ salt templating method. As shown in Table 1, the specific surface areas were 1389, 2025, and 1948 m2 g–1 for N-AC-2-1000, N-AC-3-1000, and N-AC-4-1000, respectively. The total pore volumes were 0.68, 1.15, and 1.18 cm3 g–1 for N-AC-2-1000, N-AC-3-1000, and N-AC-4-1000, respectively. Cumulative pore volume at pore sizes up to 0.7/1.0/2.0/5.0 nm deduced from N2 adsorption isotherms at 77 K using NLDFT method are also listed in Table 1. The pore volumes at pore sizes up to 0.7/1.0/2.0 nm were all the highest in N-AC-3-1000 for all samples. With increasing the initial weight ratio of LiZ to chitosan from 2/1 to 4/1, the mesopore volume ratio of samples increased according to the N2 adsorption/desorption curves and pore size distribution curves. The results showed that excessive LiZ might form small clusters and act as templates to promote the formation of mesopores.15,16,30

Figure 4.

Figure 4

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(a) N2 adsorption/desorption curves of the N-AC-2-1000, N-AC-3-1000, and N-AC-4-1000; (b) corresponding pore size distribution curves calculated with NLDFT method of the N-AC-2-1000, N-AC-3-1000, and N-AC-4-1000.

2.2.3. Effect of Temperature on the Performance of N-ACs

Figure 5 showed the N2 adsorption/desorption curves and pore size distributions of the N-ACs calcined at different temperatures (800, 1000, and 1200 °C) with a LiZ/chitosan mass ratio of 3:1. According to the IUPAC classification, type IV curves with an obvious hysteretic loop associated with capillary condensation taking place in mesopores for the N-AC-3-1000 and N-AC-3-1200 and type I curve for N-AC-3-800 was also obtained.31,32 The PSD curves of N-AC-3-y are shown in Figure 5b. With increasing the calcination temperature from 800 to 1200 °C, the mesopores in the N-AC-3-y tend to increase, showing a hierarchical porous structure in N-ACs. As indicated in Table 1, these N-ACs had BET surface areas in the range of 1655–2025 m2 g–1 and the pore volumes were 0.78–1.15 cm3 g–1. In Table 1, it is shown that N-AC-3-1000 has highest BET surface areas and pore volume among N-AC-3-y (y = 800, 1000, 1200) materials. The micropore/mesopore volumes of samples are also listed in Table 1. Especially, the micropores and mesopores in N-AC-3-1000 were all rich, and the micropore volume could be as high as 0.56 cm3 g–1 (shown in Table 1). The mechanism and the effect of molten salt system on pore structure of materials are discussed in Supporting Information (Figure S2).

Figure 5.

Figure 5

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(a) N2 adsorption/desorption curves of N-AC-3-800, N-AC-3-1000, and N-AC-3-1200; (b) corresponding pore size distribution curves calculated with NLDFT method of the N-AC-3-800, N-AC-3-1000, and N-AC-3-1200.

2.3. CO2 Capture

The CO2 capture capacities of N-ACs at 0 °C for 1.0 bar are also listed in Table 1. CO2 adsorption curves for CO2 uptake of N-ACs samples at 0 °C are shown in Figures 6, S3, S4, and S5. Figure S3 shows the CO2 adsorption curves at 0 °C and 1 bar of N-AC-3-1000, N-AC-Z, and N-AC-L, which also showed the benefits using LiZ as template. Figure S4a shows that the CO2 adsorption curves at 0 °C and 1 bar of N-AC-x-1000 (x = 2, 3 and 4) are in the range of 5.3–7.9 mmol g–1. Figure S4b also shows the CO2 adsorption curves of N-AC-3-y (y = 800, 1000 and 1200) at 0 °C and 1 bar. N-AC-3-y materials have CO2 uptakes in the range of 6.2–7.9 mmol g–1 at 0 °C and 1 bar. These values were comparable to almost all biomass-derived carbon materials for CO2 adsorption (seen from Table 2).2−6,26,33−35

Figure 6.

Figure 6

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(a) CO2 adsorption curves of N-AC-3-1000 at 0 and 25 °C, the N2 adsorption at 0 and 25 °C; (b) CO2 multi-circle adsorption curves of N-AC-3-1000 at 0 and 25 °C.

Table 2. CO2 Uptakes of Various Porous ACs in Comparison with N-ACs Derived from Chitosan at 0 °C/25 °C, under 1 bar Pressure.

class carbon source CO2 adsorbed (0 °C/25 °C, mmol g–1) refs
biomass-derived ACs cellulose 5.8/3.9 (2)
  eucalyptus sawdust 6.1/4.8 (2)
  asparagus lettuce leavesa 6.0/4.4 (3)
  black locust 7.2/5.0 (4)
  pine cone 7.7/4.8 (5)
  gelatin and starch 7.5/3.8 (6)
  coconut shell 7.0/4.7 (26)
  Coca Cola 5.7/5.2 (33)
  water chestnut shell 6.0/4.5 (34)
  glucosamine hydrochloride 6.4/4.5 (35)
  sawdust -/5.8 (36)
  chitosan 7.9/5.6 this work
non-biomass-derived ACs carbide derived carbon 7.1/- (7)
  acetonitrile 6.9/4.4 (8)
  diaminomaleonitrile -/3.4 (18)
  polypyrrole 6.2/3.9 (37)
  petroleum coke 6.1/3.4 (38)
  benzimidazole-linked polymers 8.3/5.8 (39)
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a

CO2 adsorption capacity at 0/25 °C and 1 atm.

Figure S5a also showed the CO2 adsorption curves of N-AC-x-1000 (x = 2, 3 and 4) at 25 °C and 1 bar; the CO2 adsorption capacities were in the range of 3.5–5.6 mmol g–1. Figure S5b shows the CO2 adsorption curves of N-AC-3-y (y = 800, 1000, and 1200) at 25 °C and 1 bar, and N-AC-3-y materials had CO2 uptakes in the range of 4.2–5.6 mmol g–1 under this condition. It was worth mentioning that N-AC-3-1000 exhibited noticeably the best CO2 uptake capacity of 7.9 mmol g–1 at 0 °C and 1 bar and the highest CO2 uptake capacity of 5.6 mmol g–1 at 25 °C under 1 bar1,10(also shown in Figure 6). This value was comparable to biomass-derived carbon materials for CO2 adsorption2−6,26,33−36 or most of porous carbon materials (seen from Table 2).7,8,18,37−39 These results might be related to the unique hierarchical porous structure, the highest ultramicropore/micropore volume, and high N content of N-AC-3-1000.

Under the same experimental conditions (0 °C/25 °C, 1 bar), the N2 adsorption capacity of N-AC-3-1000 was also determined. As showen in Figure 6a, the N2 adsorption capacity was much lower, just reaching a maximum of 0.62/0.30 mmol g–1 at 0 °C (CO2: 7.9/5.6 mmol g–1, 0 °C/25 °C, 1 bar). This indicated that N-ACs could be used as a potential agent for selective adsorption separation of CO2 and N2. Figure 6b also showed the CO2 adsorption curves of N-AC-3-1000 at 0 °C/25 °C during five consecutive simple regeneration operations. It can be seen from Figure 6b that the adsorption performance does not decrease even after 5 cycles, indicating the stability of the N-ACs.

The Qst results calculated by adsorption isotherms of N-AC-3-1000 at 0 and 25 °C are shown in Figure 7. The initial Qst of N-AC-3-1000 under low CO2 absorption was in the range of 24–37 kJ/mol, which was equivalent to the reported values of other N-ACs.35,39 The higher Qst value was due to the adsorption in the pores and the interaction with the surface heterogeneity when the CO2 coverage was low.40Qst decreased with the increase of CO2 absorption, and the initial Qst of nitrogen-doped carbon was due to the interaction between CO2 and nitrogen-containing groups. With the increase of CO2 loading, these preferential adsorption sites were occupied and adsorbed on the weaker binding sites. It was clear that under the same conditions, the CO2 adsorption capacity of these N-ACs was very excellent. These N-ACs were of high stability and could be recycled to capture CO2.

Figure 7.

Figure 7

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Isosteric heat of CO2 adsorption on N-AC-3-1000 calculated from the experimental adsorption isotherms at 0 °C and 25 °C.

3. Experimental Section

3.1. Preparation of N-ACs Derived from Chitosan

First, 6 g of molten salt (23 mol % LiCl, 77 mol % ZnCl2) was dissolved in 50 mL of deionized water. Meanwhile, different mass chitosan (molten salt/chitosan mass ratio = 2, 3, 4) and 0.3 g of acetic acid were dissolved in 50 mL of deionized water. Then, these two solutions were mixed together, and the mixture was freeze-dried. The dried mixture was carbonized at different temperatures (800, 1000, and 1200 °C) for 1 h in Ar flow of 40 sccm. The obtained sample was washed using deionized water several times. The resulting samples were named N-AC-x-y; x is referred to the corresponding molten salt/carbon mass ratio, where y is presented the corresponding carbonization temperature. For comparison, 6 g of pure ZnCl2 or 6 g of LiCl were the replaced molten salt; the mass ratio of salt/chitosan was 3, and the carbonization temperature was 1000 °C; the result samples were denoted as N-AC-Z or N-AC-L.

3.2. Characterization

Crystallite structures were measured by a XRD (X’Pert Pro, Philips) using Cu Kα radiation. FT-IR spectra were recorded using a 60-SXB IR spectrometer. XPS measurements were performed on Physical Electronics, USA, Al Ka radiation. The microstructure of N-ACs was observed using a field emission scanning electron microscope on JSM-6701F and a transmission electron microscope on JEOL JEM-2100FEG at 200 kV. N2 adsorption/desorption measurements were performed on a Micrometitics ASAP 2020 volumetric adsorption analyzer at 77 K. The specific surface area was calculated from the BET method using adsorption data with relative pressure (P/P0) range from 0.01 to 0.2. The total pore volume (Vt) was assessed based on the amount of N2 adsorbed at P/P0 of 0.99. The pore size distribution (PSD) was obtained from N2 adsorption data using NLDFT method and assuming a slit pore model.

3.3. CO2 Capture Measurements

Low-pressure CO2 adsorption data were investigated by the Micromeritics ASAP 2020 static volumetric analyzer at 0 °C in the pressure range between 0.03 and 1 bar.20 In order to evaluate the adsorption selectivity, the N2 adsorption data of the N-ACs were measured at 0 °C. The cyclic adsorption test of CO2 was carried out through simple regeneration by evacuating at 200 °C for 4 h under a pressure of 10–3 mbar. The isosteric heats of adsorption were derived from Clausius–Clapeyron equation as following

3.3.

where, P, T, and R are the pressure, temperature, and ideal gas constant.

4. Conclusions

Hierarchical porous N-ACs were prepared using low price chitosan as carbon source with a LiCl–ZnCl2 molten salt as template via simple one-step carbonization. The prepared N-ACs showed the highest specific surface area of 2025 m2 g–1 and a high nitrogen content of 5.1 wt %. It was demonstrated that the N-ACs can provide high CO2 capture capacities, good selectivity, and excellent recyclability. Considering the low cost of chitosan, the recyclability of molten salts, and the excellent CO2 capture performance of N-AC materials, we believed that our method could promote the application of biomass in the synthesis of high performance N-ACs for CO2 capture.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (21663026, 51462031, 11764036 and 11564034) and Program for Young Talent of SEAC ((2018)98). This work was supported by the National Natural Science Foundation of Gansu Province (1606RJZA064 and 17JR5RA283) and project of science and technology of Gansu Province (17YF1GA025). This work was also supported by the Central Universities Basic Service Fee (31920150244, 31920150245, 31920170008, and 31920190006).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c03497.

  • Binding energies and relative contents of different nitrogen species evaluated by XPS; additional N2 adsorption/desorption curves and the corresponding pore size distribution; schematic representation of pore formation for N-ACs; additional CO2 adsorption curves; and relationship between adsorption capacity (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c03497_si_001.pdf (655.8KB, pdf)

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