Thermal conversion of polypyrrole nanotubes to nitrogen-doped carbon nanotubes for efficient water desalination using membrane capacitive deionization

Thermal conversion of polypyrrole nanotubes to nitrogen-doped carbon nanotubes for efficient water desalination using membrane capacitive deionization

Separation and Purification Technology 235 (2020) 116196 Contents lists available at ScienceDirect Separation and Purification Technology journal hom...

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Separation and Purification Technology 235 (2020) 116196

Contents lists available at ScienceDirect

Separation and Purification Technology journal homepage:

Thermal conversion of polypyrrole nanotubes to nitrogen-doped carbon nanotubes for efficient water desalination using membrane capacitive deionization


Pengfei Shi, Chen Wang, Jiayue Sun, Peng Lin, Xingtao Xu , Tao Yang State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, 1 N. Xikang Rd., Nanjing 210-098, China



Keywords: Membrane capacitive deionization Nitrogen-doped carbon nanotubes Polypyrrole nanotubes In situ polymerization Thermal conversion

The exploration of new family of nitrogen-doped carbon materials with superior performance is of significant interests in capacitive deionization (CDI) or membrane CDI (MCDI) field. In this work, nitrogen-doped carbon nanotubes (nit-CNTs) were synthesized by thermal conversion of polypyrrole nanotubes in nitrogen atmosphere. Thanks to their interconnected nanotube structure providing more accessible space for ion accommodation, shortened ion diffusion pathway for fast ion adsorption/desorption, and optimized nitrogen doping species for improved electrical conductivity and increased sodium ion adsorption, the nit-CNTs exhibit a maximum desalination capacity of 17.18 mg g−1 and a good cycling stability with little capacity fading after 20 cycles, highlighting their practicality for water desalination. This work not only showcases a new case of nitrogen-doped carbon materials for MCDI application, but also highlights the significance of one-dimensional hollow nanotubes and nitrogen doping chemistry.

1. Introduction For decades, water shortage has always been amongst the serious problems on the planet, along with population growth, industrialization, environmental pollution and climate change [1–8]. The desalination of saline water could provide additional affordable freshwater beyond traditional hydrologic cycle, but the practical desalination technologies, such as ion exchange, membrane filtration, reverse osmosis, and electroosmosis usually exhibit serious drawbacks of high cost, energy consuming and secondary pollution [9–12]. Capacitive deionization (CDI) or membrane CDI (MCDI), known as an electrochemical desalination technique based on electrical double layer (EDL) principle, is recognized as an promising alternative, owing to the low energy consumption combined with high water utilization efficiency and environmentally friendliness [13–17]. Generally, the desalination performance of CDI (or MCDI) electrode is highly related to the structural properties of the active materials [18–20]. Porous carbons are undoubtedly the most widely used materials due to their abundance and high porosity. However, the currently used carbon materials usually have a low desalination capacity, limiting the large-scale implementation of CDI and MCDI. In recent years, nitrogenization of carbon materials is of particular significance to improve the electrochemical performance of carbon materials, since

nitrogen dopants could improve electrical conductivity and wettability of carbon matrix [21], and enhance ion adsorption ability [22]. Since the pioneering work regarding nitrogen-doped graphene for CDI application [23], many nitrogen-doped carbon materials have been synthesized with different morphologies such as nanofibers [24,25], nanorods [26,27], nanosheets [28], nanospheres [29–31], and nanopolyhedra [32–34]. One-dimensional hollow tubes, as reported in many studies, have demonstrated excellent performances in many electrochemical applications due to their higher surface area with less utilization of mass, improved contact between electrode and electrolyte, and shortened diffusion pathways for both ions and electrons [35–37]. However, the preparation of nitrogen-doped carbon nanotubes (abbreviated as nit-CNTs) for CDI and MCDI applications still remains as a great challenge. To address this issue, herein nit-CNTs were prepared by thermal conversion of polypyrrole (PPy) nanotubes (PNTs) in nitrogen atmosphere. Different from commercially available CNTs prepared by chemical vapor deposition method, the prepared nit-CNTs in this work show several advantageous features: (i) three-dimensional (3D) interconnected nanotube structure providing more accessible space for ion accommodation and continuous pathway for electron transfer, (ii) plentiful nitrogen dopants within the carbon matrix for improved electrical conductivity and ion adsorption, and (iii) porous tube walls

Corresponding author. E-mail address: [email protected] (X. Xu). Received 15 July 2019; Received in revised form 11 September 2019; Accepted 8 October 2019 Available online 08 October 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Schemes for the synthetic procedure of nit-CNTs.

2.4. Electrochemical analysis

for enhanced ion adsorption. Consequently, nit-CNTs exhibit a maximum desalination capacity of 17.18 mg g−1, and good cycling stability with little capacity fading after 20 cycles.

The electrochemical measurements were performed on an electrochemical workstation (CS2350H) in a three-electrode system comprising 1 M NaCl aqueous electrolyte, a platinum (Pt) counter electrode and an Ag/AgCl reference electrode. Cyclic voltammetry (CV) measurements was carried out in the potential range of 0–1 V. The Nyquist plots were obtained from electrochemical impedance spectroscopy (EIS) measurements with a frequency range of 10 mHz to 100 kHz. The specific capacitances were calculated from the CV curves by using the following equation:

2. Experimental section 2.1. Chemicals Pyrrole, methyl orange (MO), ferric chloride (FeCl3), sodium chloride (NaCl), polyvinylidene fluoride (PVDF) and ethanol were purchased from Sinopharm Chemical Reagents Co., Ltd. Vulcan XC 72 was purchased from Cabot Corporation.


2.2. Synthesis of PNTs and nit-CNTs

where i is the current (A), m is the mass of the electrode (g), v is the scan rate (mV s−1) and ΔV is the voltage window (V).

The synthetic procedure of nit-CNTs is schematically illustrated in Fig. 1, and the detailed experimental process is described as follows. Firstly, 0.1 g of MO was dispersed into 120 mL of deionized water under sonication of 15 min to make a MO aqueous solution. Then, 0.486 g of FeCl3 was mixed with MO aqueous solution under stirring and an ice bath to form a reactive self-degraded template. After that, 0.210 mL of pyrrole was added into the mixed solution and kept for 24 h under stirring at dark condition, which thus leads to the formation of PPy shell by in situ polymerization of pyrrole on the template surface. Subsequently, the collected PNTs were washed with ethanol and water for several times to remove the template, and then dried at 60 °C for 12 h. Finally, the nit-CNTs were obtained through thermal conversion of the as-made PNTs at 900 °C for 4 h in N2 atmosphere.

∫ idV /2mvΔV


2.5. MCDI tests Each individual electrode with a geometric area of around 16 cm2 was fabricated by coating a mixture of the sample, Vulcan XC 72 and PVDF binder on graphite paper (thickness: 1 mm). The weight ratio of sample, Vulcan XC 72 and PVDF was 8:1:1. The mixture was pressed onto a graphite paper and dried under vacuum at 60 °C for 12 h. For each desalination test, a pair of symmetry electrodes were assembled into a MCDI unit system combined with ion exchange membranes. The variance in concentration of the de-aerated NaCl solution was continuously recorded and measured by the ion conductivity meter. The volume was set at 50 mL, the flow rate was fixed at 20 mL min−1, and the operating voltage was varied in range of 0–1.2 V. The desalination capacity (Γ, mg g−1) and mean desalination rate (v, mg g−1 min−1) at t min were calculated by as follows:

2.3. Characterization The morphological characterizations were performed with a fieldemission scanning electron microscope (FE-SEM, HITACHI SU-8230), and transmission electron microscope (TEM, JEOL JEM-2100F). Powder X-ray diffraction (XRD) patterns were obtained on an Ultima Rint 2000 X-ray diffractometer (RIGAKU, Japan) using Cu Kα radiation (40 kV, 40 mA, 2° min−1 scan rate). Raman spectra were obtained by Renishaw inVia microscope. A He-Ne laser (633 nm) was used as the light source for excitation. Nitrogen adsorption/desorption isotherms were obtained by using a BELSORP-mini (Bel Japan, Inc.). The specific surface area (SSA) was analyzed by Multipoint Brunauer-Emmett-Teller (BET) technique. The pore size distribution was studied by the nonlocal density functional theory (NLDFT) method. The chemical state of elements was investigated using X-ray photoelectron spectroscopy (XPS, PHI Quantera SXM) with Al Kα radiation (20 kV, 5 mA). The shift of binding energy was calibrated using the C1s level at 284.5 eV.

Γ = (C0 − Ct ) × V / m


v = Γ/ t


in which C0 and Ct represent the concentration of NaCl at initial stage and t min, respectively (mg L−1), V represents the volume of the NaCl solution (L) and m represents the overall mass of the electrode material (g). 3. Results and discussion 3.1. Structural evolution from PNTs to nit-CNTs The structural evolution from PNTs to nit-CNTs was first investigated by FESEM and TEM observations. Compared with commercial CNTs (Fig. S1, Supporting Information), both PNTs and nit-CNTs exhibit a 3D interconnected fibrous structure (Fig. 2a, d). In addition, 2

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Fig. 2. (a, d) FESEM and (b–c, e–f) TEM images of (a–c) PNTs and (d–e) nit-CNTs.

The structural evolution from PNTs to nit-CNTs was then studied by XRD and Raman observations (Fig. 3). As shown in Fig. 3a, PNTs exhibit an amorphous nature with a broad hump corresponding to their repeating unit (i.e. pyrrole ring), indicating a highly oriented polymer chain [13]. After carbonization, the obtained nit-CNTs show two board peaks centered at ~26° and ~42°, indicating the formation of disordered carbon microstructure [38]. Further Raman spectra (Fig. 3b) show that both PNTs and nit-CNTs exhibit two obvious peaks at ~1344

the diameters of nit-CNTs are a little lower than those of PNTs, possibly due to the structural shrink during carbonization process. Further EDS mapping image demonstrates that little iron-containing species remain in nit-CNTs (Fig. S2). The TEM images showcase the hollow nanotube structures for PNTs and nit-CNTs (Fig. 2b–c, e–f). It is obvious that the nit-CNTs show a thinner and more porous tube wall, further suggesting that thermal conversion process would cause structural shrink and create more pores in nit-CNTs.

Fig. 3. (a) XRD and (b) Raman spectra of PNTs and nit-CNTs. 3

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Fig. 4. (a) N2 sorption/desorption isotherms and (b) corresponding pore size distribution curves of PNTs and nit-CNTs.

and ~1590 cm−1, corresponding to D-band and G-band, respectively. The intensity ratio (ID/IG) of D band to G band is often used as a measure of defect or disorder degree in the materials [12]. The ID/IG value of nit-CNTs is 0.93, higher than that of PNTs (0.86), indicating more defects generated after carbonization [39]. The porosity of PNTs and nit-CNTs was investigated by N2 sorption/ desorption isotherms. The isotherms shown in Fig. 4a display the typeIV curves, indicating mesoporous structures [40]. The higher N2 sorption capacity usually corresponds to the larger SSA [21]. Compared to PNTs (19.5 m2 g−1), nit-CNTs show a higher SSA of 200.9 m2 g−1 possibly because of the formation of porous carbon texture after thermal conversion [39]. Furthermore, the corresponding pore size distribution curves of PNTs and nit-CNTs suggest a mesoporous structure, which is favorable to the fast ion diffusion and enhanced the ion adsorption [41]. To get more insights about elements and oxidation states of the PNTs and nit-CNTs, XPS analysis was performed and the corresponding results are presented in Fig. 5. As shown in Fig. 5a, both the PNTs and nit-CNTs show the obvious presence of characteristics C1s, N1s and O1s peaks without obvious Fe-based peaks, indicating little Fe-containing species remains in the product. As shown in Fig. 5b, the high resolution N1s XPS spectra of PNTs exhibit a single peak at 399.8 eV, corresponding to the typical peak of nitrogen in PPy ring [39] (denoted as pyrrolic N). After carbonization, the obtained nit-CNTs show two distinct peaks at 398.4 and 400.9 eV, corresponding to the pyridinic N and graphitic N, respectively [28], which is consistent with Yu’s work [42]. The corresponding percentage of nitrogen dopants decreases from 16.08 at.% for PNTs to 14.52 at.% for nit-CNTs, possibly due to the loss of nitrogen dopants during carbonization process.

80.2 F g−1 for nit-CNTs and PNTs, respectively, suggesting that nitCNTs possibly have a higher desalination capacity. EIS analysis further provided insights into the electrical resistance and ion diffusion of a carbon electrode [40]. The Nyquist impedance spectra for nit-CNTs and PNTs electrodes in 1 M NaCl aqueous solution are presented in Fig. 6b. Obviously, the plots display similar shapes, consisting of a linear trait at the low frequency region and a small quasisemicircle at the high frequency region. The diameter of the small quasi-semicircle at the high frequency region corresponds to the charge transfer resistance (Rct) [44,45]. The fitted Rct value for PNTs electrode is 0.87 Ω and after carbonization, the obtained nit-CNTs show a lower Rct value of 0.58 Ω. Furthermore, a steeper line gradient corresponds to faster ion diffusion and is therefore more representative of an ideal capacitor [45]. Clearly, the slope for nit-CNTs is larger than that for PNTs, suggesting a faster ion diffusion rate.

3.3. MCDI properties analysis The desalination properties of nit-CNTs were investigated in 10 mM (corresponding to 584.4 mg L−1) NaCl solution at 0–1.2 V. Fig. 7a shows the variations of desalination capacity with time for nit-CNTs. It can be seen that without an applied voltage, no obvious variation could be found. Once the voltage is imposed, the saline ions will be driven onto the oppositely charged electrodes and stored within the internal pores, resulting in the increase of desalination capacity until the system achieves the equilibrium. Moreover, a higher applied voltage corresponds to a better desalination performance, indicating that higher applied electric field is effective to improve the desalination performance of nit-CNTs. The desalination capacity of nit-CNTs at 1.2 V is calculated to be 13.26 mg g−1, which is much higher than those of CNTs (4.88 mg g−1; Fig. S3) and PNTs (6.92 mg g−1; Fig. S4) under same condition. The Ragone plots displayed in Fig. 7b show that with the increase of the applied voltage, the corresponding Ragone plots shift towards the upper and more right region relative to the plots at 0 V, indicating higher desalination capacity and rate. To further evaluate the practicability of nit-CNTs for water desalination, nit-CNTs electrode was then investigated in NaCl solution ranging from 5 to 40 mM (Fig. 8a). The desalination capacity of the nit-

3.2. Electrochemical properties analysis The electrochemical properties of nit-CNTs were studied by CV and EIS, respectively, in a three-electrode system with 1 M NaCl solution as the electrolyte. For comparison, PNTs were investigated under same conditions. The CV curves of all samples shown in Fig. 6a exhibit rectangular shapes without any obvious redox peaks, indicating capacitive behavior [43]. The specific capacitances calculated are 158.6 and 4

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Fig. 5. (a) Full XPS survey and (b) high resolution N1s XPS spectra of PNTs and nit-CNTs.

Fig. 6. (a) CV curves at 10 mV s−1 and (b) Nyquist impedance spectra of the nit-CNTs and PNTs.

CNTs increases obviously from 7.52 mg g−1 for 5 mM to 17.18 mg g−1 for 40 mM. In addition, nit-CNTs also showcases a good cycling stability with little capacity fading after 20 cycles (Fig. 8b). Through further cycling CV tests, the nit-CNTs based cell also show a stable cycling ability with little capacitance loss after 50 cycles (Fig. S5), further suggesting a good stability of nit-CNTs.

structural property, the nitrogen dopants within the carbon matrix are also believed to have positive effects on the improvement of desalination capacity of carbon materials. In detail, the graphitic-N species could promote the electron transfer within the carbon matrix, thus improving the electrical conductivity of carbon materials, and the pyridinic-N and pyrrolic-N species are favorable to the adsorption of metal ions through specific interactions between the nitrogen dopants and metal ions. For the adsorption of NaCl, pyridinic-N species, recognized as the harder Lewis base (electron donors) [53] compared to pyrrolic-N species, are much more sutiable for the adsorption of Na+ (which is recognized as a hard acid) on the basic of the hard-soft interaction principle. Taken the above-mentioned points into consideration, although the SSA of nit-CNTs is not very high, their desalination capacity is still comparable to most of the reported carbon materials (Table 1), possibly due to the following reasons: (i) The structural advantages of nit-CNTs endow them with accelerated mass transport/electron transfer properties, thus guaranteeing faster ion adsorption/desorption. As revealed by

3.4. Performance evaluation and discussion Generally, the desalination capacity of the carbon materials is highly related to their SSA value. A higher SSA usually leads to a higher desalination capacity [55]. However, in addition to the SSA, the pore structure and nitrogen dopants within the carbon matrix are also instructive to the improvement of the desalination capacity of the carbon materials [22]. Compared with micropore-dominated carbon materials, mesopore-structured carbon materials usually show the faster ion diffusion rate and enhanced ion adsorption capability, which thereby leads to a higher desalination capacity [32]. In addition to the 5

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Fig. 7. (a) Desalination capacity variations and (b) corresponding Ragone plots of nit-CNTs at various applied voltages.

Fig. 8. (a) Desalination capacity in various concentration NaCl solution and (b) cycling desalination performance (10 mM NaCl solution) of nit-CNTs.

Table 1 Comparison between nit-CNTs and other carbon electrodes. Sample

SSA (m2 g−1)

Voltage (V)

SAC (mg g−1)


Activated carbon-1-2.0 Porous carbon spheres-1000 N-doped hierarchical porous carbon Cotton-derived carbon sponge Porous Carbon Nanosheets Mesoporous activated carbon Nitrogen-doped graphene sponge Mesoporous graphene Nitrogen-doped carbon-800 Porous carbon-900 Porous carbon polyhedral-1200 nit-CNTs

2105 1321 609 2680 2853 1968 526.7 474.0 798 1911 1187.8 200.9

1.0 1.6 1.2 1.2 1.1 1.6 1.2 1.2 1.2 1.2 1.2 1.2

9.72 5.81 10.27 16.1 15.6 11.7 21.0 14.2 8.52 10.90 13.86 17.18

[46] [47] [48] [49] [50] [51] [21] [52] [34] [53] [54] This work

4. Conclusion

the morphological observations, the nit-CNTs show a three-dimensional conductive network composed of interconnected 1D mesoporous nanotubes. Such novel architecture is believed to supply a continuous pathway for electron transfer, provide more accommodation space for electrolyte, and shorten ion diffusion pathway for faster ion adsorption/ desorption. (ii) nit-CNTs show the optimized nitrogen dopants comprising graphitic-N and pyridinic-N (Fig. 9). Benefited from such optimized nitrogen dopants, nit-CNTs exhibit an improved electrical conductivity and enhanced ability for the adsorption of Na+. Taken the aforementioned points into consideration, we believe that nit-CNTs should be a promising candidate as CDI electrode materials.

In this work, the nit-CNTs were successfully prepared via thermal conversion of PNTs in nitrogen atmosphere. Benefited from three-dimensional interconnected nanotube structures, and optimized nitrogen dopants comprising graphitic-N and pyridinic-N, nit-CNTs show a maximum desalination capacity of 17.18 mg g−1, and good cycling stability with little capacity fading after 20 cycles. It is believed that nitCNTs should be promisingly used as MCDI electrode materials for largescale practical application.


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Fig. 9. Illustration for the specific role of nitrogen species in nit-CNTs.


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