Article Cite This: Energy Fuels 2019, 33, 6568−6576
pubs.acs.org/EF
Nitrogen-Doped Porous Carbons from Lotus Leaf for CO2 Capture and Supercapacitor Electrodes Shenfang Liu,† Pupu Yang,† Linlin Wang,‡ Yuliang Li,§ Zhenzhen Wu,† Rui Ma,*,† Jiayi Wu,† and Xin Hu*,† †
Downloaded via BUFFALO STATE on July 19, 2019 at 07:15:54 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Zhejiang Normal University, Jinhua 321004, P. R. China ‡ College of Engineering, Zhejiang Normal University, 688 Yingbin Ave., Jinhua 321004, P. R. China § Canwell Medical Co. Ltd., Jinhua, Zhejiang 321016, China S Supporting Information *
ABSTRACT: In this work, N-doped porous carbons were synthesized by a one-step sodium amide activation of carbonized lotus leaf at 450−500 °C. The CO2 adsorption properties of the as-synthesized carbonaceous materials were carefully investigated. In addition, the supercapacitor performance of the optimized sample was also preliminarily explored to examine its potential as the electrode material. These lotus leaf-derived carbons possess good CO2 adsorption capacity up to 3.50 and 5.18 mmol/g at 25 and 0 °C under atmospheric pressure, respectively. It was found that the synthetic effects of narrow microporosity, N content, pore size, and pore size distribution of the sorbents decide their CO2 adsorption abilities under the ambient conditions. These lotus leaf-based carbons also demonstrate many excellent CO2 adsorption properties, such as good selectivity of CO2 over N2, quick adsorption kinetics, moderate heat of adsorption, excellent recyclability, and high dynamic adsorption capacity. In addition, preliminary electrochemical studies show that the optimized sample has high capacitance (266 F/g) and excellent stability in cycling tests. These results indicate these lotus leaf-derived N-doped porous carbons have good potential in the application of CO2 capture and supercapacitor.
1. INTRODUCTION Excessive CO2 emission has caused serious climate changes, which is an urgent problem to be solved.1,2 To mitigate CO2 emission, various CO2 capture techniques have been widely explored, such as amine scrubbing,3 membrane separation,4 ionic liquid absorption,5 adsorption by solid sorbents,6−9 etc. Of these methods, adsorption by solid sorbents is considered as the most promising and sustainable technology for CO2 capture. Solid sorbents with good CO2 capture properties are required for the successful implementation of this technique. Currently, widespread studies have been conducted on the CO2 adsorption of various porous materials, such as zeolites,10 porous carbons,11−17 metal−organic frameworks (MOFs),18 porous polymers,19,20 etc. Among these adsorbents, porous carbons stand out due to their well-known multiple merits like low cost, easy processability, stable chemical properties, adjustable porosity, and hydrophobicity.21−24 It has been reported that narrow micropores ( 0.1, the isotherms demonstrate an almost flat adsorption feature. When the P/P0 is close to 1.0, an obvious increase in adsorption was found for the samples synthesized at higher activation temperature (500 and 550 °C) and NaNH2/LC ratio (2 and 3), indicating the presence of some macropores in these adsorbents. Furthermore, a small hysteresis between adsorption and desorption branches at P/P0 of 0.4−0.9 was also found for some samples, illustrating the presence of mesopores. The coexistence of micropore and mesopore is affirmed by the pore size distribution (PSD) curves of these porous carbons (Figure 2). It can also be found that the N2 adsorption−desorption isotherms of adsorbents prepared at a NaNH2/LC ratio of 1 6569
DOI: 10.1021/acs.energyfuels.9b00886 Energy Fuels 2019, 33, 6568−6576
Article
Energy & Fuels
preferential generation of narrow microporous structures at a small NaNH2 amount. Although LC-550-2 possesses the most developed porous texture among all adsorbents, it possesses the largest difference between Vn and Vt, suggesting that a high activation temperature is unfavorable for creation of narrow micropores. 3.2. Morphology, Phase Structure, and Chemical Information of Lotus Leaf-Derived Carbonaceous Adsorbents. The morphology of LC and the selected Ndoped porous carbon (LC-550-1) was observed by scanning electron microscopy (SEM). As illustrated in Figure 3a, LC exhibits a bulky morphology with some winkles but no holes found on the surface. After treatment by NaNH2, the LC was broken into smaller pieces but still no obvious pores can be found on the surface of LC-550-1 (Figure 3b,c). Transmission electron microscopy (TEM) was further explored to explore the detailed morphology of LC-550-1. As demonstrated in Figure 3d, abundant disordered slitlike microspores can be clearly observed for LC-550-1, which agrees well with the results of nitrogen sorption. The phase structural information of the lotus leaf-derived porous carbon was obtained by X-ray diffraction (XRD) measurement. As illustrated in Figure S1 (Supporting Information), two weak and broad peaks at 2θ equal to ca. 23 and 43° were found, which can be indexed as (002) and (100) diffractions of amorphous carbon.38 The amorphous nature judged from the XRD results is in line with the finding through the TEM observation. From elemental analysis, LC was found to contain 64.37 wt % C, 2.57 wt % H, and 1.55 wt % N. Upon NaNH2 activation, the N content of the activated adsorbents increased compared with LC, suggesting the successful integration of N into the carbon skeleton. N content in the activated carbons increases with the increase of NaNH2 amount but decreases with the increase of activation temperature. The more NaNH2 was used, more N-containing functionalities were available to dope into the carbon skeleton. Although a higher activation temperature was explored, more unstable N species will be decomposed, leading to the decreased N content in the resultant sample. X-ray photoelectron spectroscopy (XPS) was applied to identify the essence of N moieties of samples under different treatment conditions. All selected samples exhibit two peaks centered at 398.4 and 400.0 eV (Figure 4), indexed to pyridinic-N (N-6) and pyrrolic-/pyridonic-N (N-5), respec-
Figure 2. Pore size distribution of the samples prepared at different conditions.
in-depth research should be conducted to clarify the mechanism of NaNH2 activation. The narrow microporosity of the as-synthesized adsorbents was calculated by the CO2 adsorption data measured at 0 °C using the Dubinin−Radushkevich (D−R) equation. As shown in Table 1, the Vn for these adsorbents is from 0.42 to 0.70 cm3/g. A higher Vn than Vt was found in the samples synthesized at a NaNH2/LC ratio of 1, suggesting the
Table 1. Porous Properties, Elemental Compositions, and CO2 Uptakes of Sorbents Derived from Lotus Leaf under Different Conditions CO2 uptake (mmol/g) sample
SBETa (m2/g)a
V0b (cm3/g)
Vtc (cm3/g)
Vnd (cm3/g)
N (wt %)
C (wt %)
H (wt %)
25 °C
0 °C
LC LC-450-1 LC-450-2 LC-450-3 LC-500-1 LC-500-2 LC-500-3 LC-550-1 LC-550-2 LC-550-3
4 792 1651 1557 833 1924 1667 1087 1883 1311
0.02 0.42 0.83 0.78 0.43 1.00 0.92 0.54 1.24 0.85
0 0.29 0.67 0.54 0.31 0.79 0.72 0.45 0.90 0.56
0.13 0.42 0.64 0.47 0.44 0.70 0.59 0.54 0.58 0.47
1.55 3.53 3.85 4.27 3.24 3.64 4.04 2.55 3.04 3.65.
64.37 52.46 51.21 51.79 52.98 53.24 53.35 53.30 58.75 59.62
2.57 3.62 3.64 3.68 5.06 5.75 5.81 5.19 5.04 5.52
1.00 3.24 3.33 2.67 3.38 3.22 3.04 3.50 2.21 2.44
1.16 4.60 5.18 3.81 4.87 4.92 4.44 5.04 3.58 3.68
a
Surface area was calculated using the BET method at P/P0 = 0.01−0.1. bTotal pore volume at P/P0 = 0.99. cEvaluated by the t-plot method. dPore volume of narrow micropores (
