Targeted Synthesis of Polymer and Microporous Carbon

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Targeted Synthesis of Polymer and Microporous Carbon Nanofibers via Temperature-Dependent and Molecularly-Triggered Interfacial Assembly

Lei Shi, Wen-Cui Li, Xiang-Qian Zhang, and An-Hui Lu*

DOI: 10.1002/admi.202000381

deposition, have been applied in the fab-rication of porous CNFs.[13–18] However, the diameters of CNFs via electrospin-ning and vapor deposition are generally greater than 200 nm.[16,19–21] Notably, the capacity of porous CNFs to store charge is positively related to their surface-to-volume ratio and the surface-to-volume ratio scales inversely with the diameter of porous CNFs.[22] Therefore, the synthesis of porous CNFs with tunable diameters, especially diameters of less than 100 nm, is of great desire nowadays. Yu's group has already obtained CNFs by using Te nanowires as the template and glucose as the carbon precursor.[23,24] After hydro-thermal treatment, doping nitrogen, car-bonization and removing the Te nanowire templates, CNFs exhibit a large specific surface area of over 1000 m2 g−1 and a mean diameter of ≈100 nm. Previously we have reported that by using a soft template (triblock copoly mer F127), CNFs with a diameter about 100 nm can be prepared, which exhibited superior rate capacity as electrode material in Li-ion batteries.[25]

However, the growth process is still unclear, giving rise to the difficulties in precisely regulating the morphology and size of PNFs. Besides, Fujikawa and workers used cetyltrimethylam-monium bromide as a template, 1,3,5-trimethylbenzene and tert-butanol as cosurfactants, to prepare polymer nanofibers.[26] Carbonization of the obtained polymer nanofibers led to the microporous CNFs, which have a diameter of ≈55 nm. Although scientists have succeeded in preparing CNFs with small dia-meters, the literature documents are rather rare, especially those about the insight of molecular interfacial assembly and CNFs’ growth mechanism; thus the precise control of CNFs diameter is waiting to be addressed. In order to obtain CNFs with desired properties, the key issue is to control the growth of their poly-meric counterpart under proper conditions. Therefore, we need to explore and understand the growth mechanism of the PNFs, to figure out the key influence factors, and to establish a control-lable alternative synthesis method for porous CNFs.

In this study, taking one step further, we investigated the for-mation mechanism of the PNFs via the temperature-dependent and molecularly-triggered interfacial assembly process. As a result, PNFs with tunable diameters in the range of 30–80 nm can be prepared. And we figured out two crucial factors that

Microporous carbon nanofibers (CNFs) derived from polymer nanofibers (PNFs), featured with short radial diffusion length of ions, have been proposed as ideal electrode materials for energy storage devices. However, lacking precise control of selective formation of fibrous micelle templates, the targeted synthesis of microporous CNFs is still a challenge. To address this issue, PNFs are first fabricated by using triblock copolymer F127 as the structure directing agent via the temperature-dependent and molecularly-triggered interfacial assembly process. Two crucial factors are figured that determine the fibrous morphology of the assembled polymer. At suitable temperatures, F127 starts to form fibrous micelles, and molecular trigger hexamethylenetetramine generates formaldehyde and ammonia in situ. Then, phenolic polymer assemble around the fibrous micelles, following with the formation of PNFs. This feasible synthesis method can grow PNFs and precisely control the PNFs’ diameters ranging from 30 to 80 nm. Microporous CNFs with high specific surface area of 1175 m2 g−1, and an average diameter of 40 nm can be prepared through carbonization of PNFs and an activation step. Taking such microporous CNFs as electrode materials for a supercapacitor, they exhibit an excellent specific capacitance 295 F g−1 at a high current density 50 A g−1.

L. Shi, Prof. W.-C. Li, Dr. X.-Q. Zhang, Prof. A.-H. LuState Key Laboratory of Fine ChemicalsSchool of Chemical EngineeringDalian University of TechnologyDalian 116024, P. R. ChinaE-mail: [email protected]

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.202000381.

1. Introduction

As important electrode materials, porous carbon nanofibers (CNFs) derived from polymer nanofibers have a great poten-tial in energy storage devices and other applications due to their remarkable performance.[1–12] In comparison with other types of electrode materials, porous CNFs not only provide a short radial diffusion length for electrolyte, but also give long axial electron transport paths to promote rapid redox reac-tion between electrode interface and electrolyte ions. There-fore, great efforts have been made to prepare porous CNFs. So far, the most common methods, electrospinning and vapor

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determine the fibrous morphology of the assembled polymer products, ensuring easy manipulation of the current synthesis method. Through in-depth exploration of the formation process of PNFs, the precise control of the morphology and the diam-eter of PNFs has been realized from the origin. This work has significance for the future hydrothermal synthesis of polymer fibers. We also demonstrated that when used as electrode mate-rials in supercapacitors, microporous CNFs derived from PNFs can be obtained with a superior specific capacitance of 295 F g−1 at a high current density of 50 A g−1.

2. Result and Discussion

After hydrothermal treatment of mixture of F127, resorcinol and hexamethylenetetramine (HMT) dissolved in water at 110 °C, the collected sample (PNFs) display uniform fibrous morphology with a small diameter from 45 to 55 nm and the length basically above 500 nm (Figure 1a). After carboniza-tion of PNFs under argon atmosphere, the obtained CNFs-1 retain well the original fibrous shape with a diameter of around 40 nm (Figure 1b). The shrinkage of the fiber diameter before and after carbonization is about 20%. The nitrogen adsorption

isotherm of CNFs-1 exhibits essentially type-I curve (Figure S1, Supporting Information). After slight activation of CNFs-1 by CO2, the obtained sample CNFs-2 still shows a type-I isotherm (Figure 1c). In general, the decomposition of F127 spherical micelles will produce mesoporous channels,[27] resulting in the type IV nitrogen adsorption isotherm. The conformation of transitional curve might be due to the disappearance of spher-ical micelles. Based on the adsorption branch of the nitrogen adsorption isotherm, and using the density functional theory model to calculate pore size distribution of CNFs-2, its pos-sesses the micropore sizes in the range of 0.4–2 nm (the inset of Figure 1c), and the total pore volume is 0.85 cm3 g−1. Specific surface area is accordingly of 1175 m2 g−1 based on Brunner–Emmet–Teller model and the increase in the specific surface area is about 75% as compared to that of CNFs-1, indicating that an additional activation step can dramatically increase the porosity meanwhile retain the fiber shape very well. The trans-mission electron microscopy (TEM) image in Figure 1d clearly shows that the sample CNFs-1 has a diameter of ≈40 nm, which is slightly smaller than that of PNFs because of the contraction effect caused by the carbonization process. In addition, dimly visible linear pores along the axial direction of the CNFs-1 are seen due to the decomposition of F127.

Figure 1. SEM images of a) PNFs and b) CNFs-1, c) N2 sorption isotherm and pore size distribution of CNFs-2 after activation by CO2, d) TEM image of CNFs-1.





Aiming to figure out the growth mechanism of the PNFs, we first conducted a series of experiments by varying the hydro-thermal temperature of 90, 100, 120, or 140 °C. The morphology of the obtained products is shown in Figure S2, Supporting Information. The products appear as large sized polymer aggregates consisting of short polymer rods with an average diameter of ≈50 nm at a hydrothermal temperature of 90 °C (Figure S2a, Supporting Information). Different from the PNFs obtained at 110 °C, few products were separated by high speed centrifugation (Figure S3, Supporting Information) because that no enough polymer was formed due to tiny amount of HMT decomposed at a low temperature. When the hydro-thermal temperature increases to 100 °C, fibrous F127 micelles are formed due to the decrease of critical micelle concentra-tion (CMC),[28] and PNFs can be observed but less uniform (Figure S2b, Supporting Information). The diameter of PNFs increases to around 70 nm when the hydrothermal temperature rises to 120 °C (Figure S2c, Supporting Information). Further increase the hydrothermal temperature to 140 °C, the collected polymer shows a large-sized spherical morphology with diame-ters ranging 3–5 um (Figure S2d, Supporting Information) and is composed of stacked-layer structure due to the disappearance of F127 micelles at this temperature. It can therefore be con-cluded that the PNFs can be formed at the hydrothermal tem-peratures between 100 and 120 °C, and have a homogeneous fibrous morphology. The yields at 100, 110, and 120 °C were as high as 51.3%, 57.8%, and 63.3%, respectively.

To reveal the function of F127 in the synthesis, the fol-lowing experiment was designed and carried out. First, the F127 aqueous solution was heated in a flask under reflux in a 110 °C oil bath for 1 h. And then, a previously prepared solu-tion of HMT and resorcinol was added into the F127 solution, and subsequently heated for additional 4 h at 110 °C. It should be noted that the reaction temperature in the flask was about 96 °C. After the ambient pressure synthesis, the collected polymer product also showed fibrous morphology (Figure S4, Supporting Information), confirming that F127 was first formed fibrous micelles and then subject to interfacial assembly by phenolic polymer. Interestingly, the polymer product was mostly attached to the bottom of the flask in contact with the oil bath, due to the temperature of the flask wall was higher than the solution in the flask, promoting the growth of polymer.

As a control experiment, when HMT was substituted by formaldehyde (37 wt%) and ammonia (25 wt%) (equivalent

to the formaldehyde and ammonia theoretically obtained from HMT) during the hydrothermal process, the polymeric products showed micron-sized irregular particles (Figure S5, Supporting Information). This result indicates that the unsub-stitutability of HMT during the growth process of PNFs, again revealing that the in situ formed formaldehyde and ammonia from HMT is necessary. One can envisage that F127 first forms fibrous micelles at the temperatures ranging 100–120 °C, and meanwhile the in situ formed formaldehyde and ammonia from HMT participate in the generation of nanofibers. We thus call this synthesis as: temperature-dependent and molecularly- triggered interfacial assembly process.

To further test the above conjecture, the effect of the inter-facial assembly polymerization process on the morphology of PNFs was investigated by varying the hydrothermal period. As a result, the product obtained in 1 h hydrothermal synthesis cannot be separated by centrifugation (15 000 rpm, 15 min), and the yield after natural drying is extremely low, hard to measure. The obtained products show no specific morphology (Figure S6a, Supporting Information). After 3 h hydrothermal synthesis, fibrous product can be observed (Figure S6b, Sup-porting Information). One possibility is that short polymeriza-tion reaction time cannot generate enough phenolic polymers to instantly assemble around the F127 fibrous micelles.

On the basis of above results, we propose the formation pro-cess of PNFs as presented in Figure 2. As the hydrothermal temperature increases, fibrous micelles constituted by triblock copolymer F127 gradually appear with the sharp decrease of CMC. Short rod-shaped micelles can be formed at 90 °C (inset a in Figure 2). As the insufficient reaction time and low temper-ature, HMT did not decompose to generate enough formalde-hyde and ammonia at this period, leading to the fibrous micelles were not stabilized by phenolic polymer through polymeriza-tion reaction. After that, as the temperature increased and the reaction proceeded, HMT more efficiently decomposed into formaldehyde and ammonia. This can be confirmed by increased product yield. The self-assembly of resorcinol was guided by enriched-ether and -hydroxyl groups F127 fibrous micelles and a controlled polymerization of resorcinol with for-maldehyde around fibrous micelles was catalyzed by ammonia. Fibrous micelles were instantly stabilized by pheno lic resin and exhibited fiber morphology (inset b in Figure 2). Finally, PNFs with improved dispersion, high uniformity and high yield were obtained (inset c in Figure 2).

Figure 2. The proposed formation process for the polymer nanofibers.





We set a careful control on the HMT/resorcinol molar ratio in order to prepare PNFs with tailorable diameters. Scan-ning electron microscopy (SEM) images of PNFs show that the diameter can be tuned from 74.2, 51.3, 40.9, and 36.4 nm by varying the molar ratio of HMT/resorcinol (Figures 1a and 3a–c). That is to say, the diameters of the PNFs decrease as the molar ratio of HMT/resorcinol increase. It can be interpreted that a low concentration of HMT would slow the reaction rate, probably initiating fewer nuclei in the reaction solution. Hence, PNFs with large diameter are formed as the time progresses.[29] In contrast, nuclei form rapidly in the reac-tion systems with a high concentration of HMT. Therefore, as the reaction time progresses, PNFs with a small diameter are formed. Further decreasing the molar ratio of HMT/res-orcinol leads to a mixture of PNFs and polymer nanospheres (Figure S7, Supporting Information), strongly reflecting the necessity and importance of the amount of HMT. By taking the molar ratio of HMT/resorcinol and the diameters of PNFs with the equation fitting, a quadratic function of diameters versus molar ratio is obtained (Figure 3d). Hence, we believe that the current synthesis method can be used to precise control of the growth of PNFs with continuously tunable dia meters in cer-tain range.

One would expect that CNFs can provide a short radial dif-fusion length for electrolyte, and microporous, high surface area CNFs would be ideal electrode materials for supercapaci-tors. We therefore prepared sample CNFs-2 which has high specific surface area of 1175 m2 g−1 and an average diameter of 40 nm, and tested its electronic and ion transmission efficiency in a supercapacitor. As shown in Figure 4a,b, a three-electrode system was used to conduct a cyclic voltammetry (CV) test at different scan rates and a galvanostatic charge-discharge (GC) test at different current densities. As the scan rate increased from 5 to 200 mV s−1, the CV curves are all kept in a rectan-gular-like shape (Figure 4a), which indicates that CNFs-2 have a good capacitive performance at a high scan rate. Normally, the element composition of phenolic polymer-based carbon materials mainly consists of carbon with tiny amount of oxygen and hydrogen. Even though the nitrogen-containing chemical has been used in the poly merization process, the nitrogen content basically can be ignored due to a high carbonization temperature above 900 °C.[30] In addition, at current densities of 0.5 to 50 A g−1, all GC curves appear as isosceles triangles (Figure 4b), which indicate that CNFs-2 can still maintain relatively standard capacitor charging and discharging behav-iors at high current densities. Further, its capacitance decays

Figure 3. SEM images a–c) of PNFs with different diameters synthesized by different ratios of HMT and resorcinol (R). d) Relationship between HMT/R molar ratios and diameters of PNFs.





slowly with the increase of discharge rates, showing a high specific capacitance of 295 F g−1 at a high current density of 50 A g−1 (Figure 4c). The CV and GC curves of CNFs-1 without CO2 activation treatment also show nearly rectangular and tri-angular traces, respectively, indicating good capacitive behavior (Figure S8a,b, Supporting Information). The unique fibrous structure is favorable for improving the efficiency of ion and electron transport by providing more entrances and shorter distances for electrolyte accessibility. CNFs-1 provided a capaci-tance of 223 F g−1 at a current density of 0.5 A g−1 and retained 140 F g−1 at a current density of 50 A g−1 (Figure S8c, Sup-porting Information). Compared with CNFs-1, the capacitance of CNFs-2 increased significantly, which is attributed to the interconnectivity of micropores and fibrous morphology with high length-diameter ratio.

Therefore, we constructed a symmetrical supercapacitor using two pieces of CNFs-2 electrodes with the same size and weight and tested it in 1.0 m H2SO4 electrolyte. When the scan rate of the symmetrical cell is gradually increased from 5 to 200 mV s−1, the CV curves still retain a rectan-gular shape (Figure S9a, Supporting Information), indicating that the symmetrical cell still has good capacitance perfor-mance at high scan rates. And all GC curves show the isos-celes triangle shapes at current densities from 0.5 to 50 A g−1 (Figure S9b, Supporting Information). In addition, the capaci-tance of the symmetrical cell still decays slowly with increasing discharge rate (Figure S9c, Supporting Information). And the ragone plots showing the relationship between energy den-sity and power density was obtained under similar test con-ditions (Figure S9c, Supporting Information). Moreover, the

electrochemical stability of the supercapacitor was tested by GC charge–discharge cycling at a current density of 2.0 A g−1. As seen in Figure 4d, the capacitance value is slightly reduced after 50 cycles compared to the initial capacitance. It may be caused by electrolyte filling and wetting the micropores before achieving a stable charge and discharge state. After that, the specific capacitance remains a value of ≈51 F g−1 after another 4950 cycles.

Table S1, Supporting Information summarizes the latest electrochemical performance results of various carbon mate-rials used in supercapacitors. It should be noted that the current CNFs-2 electrode exhibits better electrochemical per-formance compared with most reported ordered mesopore carbon,[32,33] carbon nanobowl,[34] carbon nanobelts,[35] carbon nanofibers,[31,37–39] and so on.[36] Combined with the high spe-cific capacity at high current density, it is believed that the CNFs-2 electrode can be used as a very promising candidate as a high-performance supercapacitor. The excellent elec-trochemical performance can be attributed to the following problems: 1) the abundant micropores provide a fast trans-port channel for the electrolyte, and the high surface area also provides a large number of reaction interfaces for electrolyte ions. 2) The fibrous morphology of CNFs-2 enables fast axial transmission of electrons and greatly reduces the radial trans-mission distance of the electrolyte. And because the diam-eter of CNFs-2 is less than 100nm, the utilization ratio of the material surface has been further improved. 3) The fibrous carbon material has good structural stability and chemical stability, which is beneficial to the application in long-cycle supercapacitors.

Figure 4. a) CV curves of CNFs-2 at different scan rates, b) GC curves of CNFs-2 at different current density, c) specific capacitances of CNFs-2 at various current densities, d) cycling performance of CNFs-2 at current density of 2 A g−1 (voltages ranging 0–1 V).





3. Conclusion

We have demonstrated a controllable synthesis of PNFs and CNFs via the temperature-dependent and molecularly-triggered interfacial assembly process. Two crucial factors that determine the fibrous morphology of the assembled polymer were figured out. Mechanism insight has shown that as the temperature increases, soft template F127 starts to form fibrous micelles. Simultaneously, HMT begins to decompose into formaldehyde and ammonia in situ. By using F127 fibrous micelles as the core and ammonia as the catalyst, phenolic polymer would assemble around the fibrous templates, thus forming PNFs through con-tinuous polymerization reaction. The current synthesis method can precisely grow PNFs with continuously tunable diameters in the range of 30–80 nm. By using PNFs as the carbon pre-cursors, CNFs can be obtained with abundant micropores and high surface area. When CNFs were used as electrode materials in supercapacitors, they can display a high specific capacitance of 295 F g−1 at 50 A g−1. We believe that this controllable syn-thesis method ensures easy preparation of 1D nanosized fiber materials.

4. Experimental SectionChemical: Resorcinol (R, 99.5%) and HMT were purchased from

Sinopharm Chemical Reagent Co., Ltd. Triblock copolymer F127 was purchased from Sigma.

Synthesis of PNFs and CNFs: In the typical fabrication process of PNFs, a mixed aqueous solution of resorcinol and HMT was added to the aqueous solution of F127 at 40 °C with gently stirring. The total molar ratio of R, HMT, F127 and deionized water was 1.0:x:0.191: 5556 (x = 0.2, 0.4, 0.6, 0.8, 1.0). After being stirred for 30 min at 40 °C, the reaction mixture was transferred into an autoclave and placed in oven at 110 °C for 4 h. The resulting products were separated by centrifugation at 15 000 rpm for 15 min and then adequately freeze-dried.

The CNFs were obtained by carbonization of PNFs at 900 °C for 2 h under Ar atmosphere, which was named as CNFs-1. In order to improve the surface area, CNFs was also activated with CO2 at 900 °C for 2 h with a flow rate of 60 mL min−1, which was named as CNFs-2.

Materials Characterization: SEM images were taken with a Hitachi S-4800 instrument at 10 kV. TEM images were carried out with a FEI Tecnai G2 F20 microscope operating at 300 kV. Nitrogen sorption isotherm of CNFs was measured at 77 K with an ASAP 2020 adsorption analyzer (Micromeritics).

Electrochemical Measurements: Electrochemical experiments were measured by a three-electrode system. The working electrode was fabricated using a mixture of 80 wt% active materials (CNFs), 10 wt% conductive carbon (Ketchen Black), and 10 wt% binder (poly(vinylidene fluoride)) in NMP to obtain a slurry, which was painted onto a carbon paper disk. The mass loading is about 1 mg cm−2. The reference electrode was Hg/Hg2SO4 electrode and the counter electrode was platinum sheet. 1 m H2SO4 was used as the electrolyte. GC charge and discharge experiments were performed in the range of −0.6 to −0.35 V at room temperature on an electrochemical workstation (CH Instruments Inc., Shanghai, China, CHI660D), which CV measurements were carried out on.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis work was supported by the National Natural Science Foundation of China (No. 21776041 and No. 21875028) and Cheung Kong Scholars Programme of China (T2015036).

Conflict of InterestThe authors declare no conflict of interest.

Keywordscarbon nanofibers, fibrous micelles, high rate capacity supercapacitors, hydrothermal treatment, polymer fibers

Received: February 28, 2020Revised: April 4, 2020

Published online:

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Targeted Synthesis of Polymer and Microporous Carbon