Emulsion‐assisted interfacial polymerization strategy: Controllable architectural engineering of anisotropic and isotropic nanoparticles for high‐performance supercapacitors

Anisotropic nanoparticles have attracted extensive attention due to their potential applications in material transport, energy storage, and biopharmaceutical. However, due to the inadequate understanding of microscopic particle formation, controllable asymmetric growth is still a great challenge. Herein, we report a facile emulsion‐assisted interfacial polymerization strategy for the synthesis of nitrogen‐doped porous carbon particles (NPCPs) with tunable anisotropic/isotropic architectures. During the synthesis process, we can form emulsion droplets with different nanostructures directionally through dual routes, thereby assisting and mediating the polymerization and growth process of the monomer to obtain poly‐diaminopyridine nanoparticles with various architectures. The corresponding NPCPs with tunable specific surface area (125–362 m2 g−1), nitrogen content (10%–14%), and diverse morphologies can be acquired by calcination under N2 atmosphere at 700 °C. The synergetic effect of abundant microporous structures and active nitrogen species content contributes to improve the physicochemical properties, while the unique anisotropic architecture increases the charge diffusion efficiency and enhances the high‐rate stability. Therefore, the resultant NPCPs electrode exhibits a specific capacitance up to 275 F g−1 at 0.2 A g−1 and surface‐area‐normalized capacitance of 83.0 μF cm−2, indicating a promising material for high‐performance supercapacitors.


| INTRODUCTION
Porous carbon-based materials (PCMs), possessing stable physical properties, good electrical and thermal conductivity, and high specific surface area, have been a trending topic in a wide range of fields. [1][2][3][4][5][6] PCMs with various morphologies have been developed through refined design, of which integration of advantages of anisotropy and high-porosity materials attract wide attention due to their enhanced properties. 7-10 Anisotropic nanoparticles possess higher packing density and better biocompatibility than their corresponding isotropic ones, so that anisotropic nanoparticles exhibit wide application outlook and potential value in adsorption, catalysis, energy storage, and drug delivery. 11,12 However, due to the nature symmetry criteria and the need to minimize the surface energy, isotropic nanoparticles, for example, the nanospheres are generally produced, 13 so the development of the refined fabrication of anisotropic nanoparticles still exists big challenges.
A great diversity of strategies have been developed to synthesize PCMs among which the solvent-based synthesis system for polymerization followed by calcination has sparked a lot of interest because of its easy handling, high product uniformity, and environmental friendliness. [14][15][16][17] Although great efforts have been devoted, inevitable shortcomings and limitations still remain. Polymerization reactions suitable for the solution synthesis method are still relatively restricted, and the resulting products only involve single isotropic morphology, lacking control of architectural changes. In addition, most of PCMs possess hydrophobic surfaces and a scarce number of active sites, leading to limitations in practical application. 18 Doping carbon networks with heteroatoms (especially N) are regarded as a beneficial way to introduce abundant basic sites, which can enhance electron transport and electrical conductivity significantly. [19][20][21][22][23] Therefore, it is of great practical significance to develop anisotropic carbon nanoparticles with high nitrogen content and controllable architectures.
Herein, we demonstrate the controllable synthesis of N-doped porous carbon nanoparticles (donated as NPCPs) with tunable architectures through a facile and versatile emulsion-assisted interfacial polymerization strategy. NPCPs are synthesized by the utilization of dual surfactants of Pluronic F127 as well as hexadecyl trimethyl ammonium bromide (CTAB), the water-immiscible liquid 1, 3, 5-trimethyl benzene (TMB) as an emulsion phase, and 2, 6-diaminopyridine (DAP) as the monomer. NPCPs involving bowl-like, litchilike, and raspberry-like nanoparticles are fabricated in a distinctive continuous evolution process. The key point of the strategy is to precisely control the F127/CTAB/TMB nanoemulsion structures by varying the amount of TMB and the mass ratio of F127 to CTAB, thereby mediating the polymerization and growth process of DAP at the oil/water (O/W) interface and forming polydiaminopyridine nanoparticles with different structures and their derived high N-doped carbon nanoparticles. The resultant NPCPs possess tunable Brunauer-Emmett-Teller (BET) surface areas (125-362 m 2 g −1 ), nitrogen contents (10%-14%), and diverse anisotropic/ isotropic architectures. As a supercapacitor electrode, the electrochemical activity of NPCPs is influenced by the porous structures and nitrogen content, which can respectively contribute to electrical double-layer capacitance as well as pseudocapacitance and play a synergistic role in the improvement of the electrochemical performance. The raspberry-like NPCPs exhibit better capacitive and rate performance with an ultrahigh surface-areanormalized capacitance up to 83.0 μF cm −2 due to the acceleration of charge-transfer rate contribution from the anisotropic effect. This strategy elaborates the growth mechanism about how the emulsion system assists the generation of different kinds of particles, the as-obtained high nitrogen content porous carbon nanoparticles are expected to be a guidance for the synthesis of new supercapacitor materials.

| RESULTS AND DISCUSSION
The NPCPs with various architectures can be fabricated through a facile emulsion-assisted strategy at the O/W interface. In the mild water/ethanol system, F127 and CTAB are added as surfactants and the waterimmiscible liquid TMB is chosen to be an oil phase to form emulsion droplets. The polymerization of DAP was induced by adding the initiating agent APS to form polydiaminopyridine nanoparticles. Because there exists hydrogen and π-π bonding between the DAP monomers and the F127/CTAB/TMB emulsion, the tunable anisotropic and isotropic poly-diaminopyridine nanoparticles can be synthesized at the O/W interface by precisely controlling the construction of the emulsions. The corresponding NPCPs are obtained followed by calcination at 700°C under N 2 atmosphere. The controllable architectural engineering of NPCPs can be achieved in two routes. In Route I, by varying the amount of TMB, the structure can be adjusted from nonporous smooth nanospheres to litchi-like and then to raspberry-like nanoparticles. In Route II, the morphology of NPCPs can be tuned continuously from bowl-like to raspberry-like, next to litchi-like nanoparticles, and finally to nanospheres with smooth surface by varying the mass ratio of F127 to CTAB.
The morphology of the NPCPs samples fabricated through Route I was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). By increasing the amount of TMB with a gradient from 0.1 to 0.4 ml, the architectures of the carbon nanoparticles are changed from smooth-surfaced nanospheres to litchi-like nanospheres with grooved surface, then to nanospheres with deeper grooves, and finally to raspberry-like nanoparticles ( Figure 1A-D). The unique architectural variability clarifies that F127/CTAB/TMB emulsion droplets act as assisting agents for NPCPs, which plays a vital role in coordinating the whole reaction system. The nanoparticles are named NPCPs-x, where x is the amount of TMB. It can be seen from TEM images (Supporting Information: Figure S1e-h) that all the NPCPs samples exhibit a uniform morphology and abundant micropores due to the removal of surfactants and pyrolysis of organic frameworks, which contribute to rapid ion transport. SEM and TEM images show that the NPCP samples exhibit a particle size of~350 nm, and the as-synthesized NPCP-0.4 possesses a large single-cavity (~120 nm) and multigrooved surface, whose shape is similar to raspberry-like nanoparticles. According to the scanning TEM (STEM) image, the original hollow feature and grooved surface can be further confirmed ( Figure 1E). The element mapping images of NPCP-0.4 show that C, N, and O elements are distributed uniformly in the carbon frameworks.
The porous structure properties of NPCPs products were investigated by N 2 sorption measurements at 77 K. The N 2 adsorption-desorption isotherms ( Figure 1F) of NPCPs show typical type I curves with a sharp nitrogen uptake and rapid increases at the low relative pressure region (P/P 0 < 0.1), demonstrating the presence of abundant micropores in the frameworks. The BET specific surface areas exhibit in the range of 125-362 m 2 g −1 , all of which have a micropore specific surface area contribution of more than 89% (Supporting Information: Figure S3). The specific surface area of NPCPs-0.1 is 125 m 2 g −1 , which is much smaller than other samples (>300 m 2 g −1 ). The pore size distribution curves ( Figure 1G) are based on the Barret-Joyner-Halenda theory method. It can be confirmed from the curves that the curves of NPCPs-0.3 and NPCPs-0.4 exhibit dual peaks centred at 1.7 and 12 nm, where the former corresponds to the abundant microporous structures and the latter may be derived from the existence of the grooved surfaces. This result indicates that TMB not only conduces to form a stable nanoemulsion system to synthesize particles with different morphologies but also plays an important role in the formation of pores. 24 X-ray photoelectron spectroscopy (XPS) was conducted to analyze the elemental identifications of NPCPs samples. The XPS spectrum of NPCPs-0.4 shows three typical peaks of C 1 s, N 1 s and O 1 s ( Figure 2A). The corresponding relative element contents of C, N, and O are 79.4%, 14.2%, and 6.4%, which were supported by the carbon, hydrogen, and nitrogen elemental analysis (Supporting Information: Table S1). As shown in Figure 2B, the N 1 s XPS spectrum of NPCPs-0.4 was deconvoluted into four typical peaks at 402.1, 401.0, 400.2, and 398.6 eV, which respectively correspond to oxidic N (N-X, 1.1%), graphitic N (N-Q, 21.2%), pyrrolic N (N-5, 35.5%), and pyridinic N (N-6, 42.2%). Notably, the active nitrogen species (N-5 and N-6, sum to 77.7% of the total nitrogen content) are abundant in the carbon frameworks, which are helpful in tuning the electronic supply property. 19 The Raman spectrum ( Figure 2C) of the NPCPs-0.4 shows two broad peaks at around 1360 (D band) and 1580 cm −1 (G band), which attribute to the disordered sp 3 carbon and graphitic carbon, respectively. The intensity of D band is a bit higher than G band, and the intensity ratio of the D band to G band (I D /I G ) were calculated to be 1.02, indicating the existence rich defects in the carbon frameworks. The functional groups and evolution of the chemical composition of the poly-diaminopyridine nanoparticles were characterized by Fourier transform infrared (FTIR) spectroscopy ( Figure 2D). Compared with the FTIR spectra of the poly-diaminopyridine obtained without surfactants, the spectra of the emulsion/poly-diaminopyridine exhibits enhanced peaks originated from the stretching vibration of the C─H bond of saturated carbon of F127 and CTAB between 2827 and 3000 cm −1 , indicating the combination of the nanoemulsions and the poly-diaminopyridine nanoparticles. The FTIR spectrum of poly-diaminopyridine exhibit two characteristic peaks corresponding to the stretching vibration of imine groups (C═N) at about 1650 cm −1 and the adsorption peaks of amino groups (N─H) centred at 3435 cm −1 . The nitrogen-enriched frameworks can improve the surface wettability of the materials and contribute to enhancing the pseudocapacitance through redox reactions. Thermogravimetric analysis curve (Supporting Information: Figure S4) reveals that the residual carbon content under 800°C of the raspberry-like poly-diaminopyridine nanoparticles is 16%.
The growth mode and nanostructure of NPCPs are mainly assisted by the emulsions consisting of oil phase TMB and surfactants F127 and CTAB to control the polymerization process of monomers. By varying the mass ratio of F127 to CTAB, in Route II, another continuous trend of morphology change is shown in Figure 3. It can be found that monodisperse nanoparticles cannot be formed when only F127 exists (Supporting Information: Figure S5a). When the mass ratio (F127/ CTAB) changing from 1:0 to 2:1, the structure of NPCPs varies from aggregated particles to well-defined nanobowls (donated as NPCPs-5). Further increasing the F127/CTAB mass ratio to 1:1, then to 1:2, raspberry-like and litchi-like nanoparticles are formed. When the mass ratio is increased to 1:4, nanospheres with a smooth surface are obtained (Supporting Information: Figure S5b). According to the aforementioned observations, it exhibits that adding CTAB is contribute to form well-defined nanoparticles. But adding an excess amount of CTAB may weaken the interactions between the DAP molecule and F127/CTAB/TMB emulsion droplets, which attributes to the too much positively charged CTA + . 25 In the appropriate dosage range of CTAB, we observe a continuous changing process of the morphology of NPCPs, from smooth-surfaced nanobowls to raspberry-like nanoparticles with grooved surface, and finally to rough-surfaced litchi-like nanoparticles, which demonstrates the refined regulation of architectures.
Moreover, the effect of solvent (Supporting Information: Figure S6) and initiating agent (Supporting Information: Figure S7) on the nanostructure of NPCPs are also detailedly investigated. When decreasing the ethanol concentration to 30%, walnut-shaped nanoparticles are formed. By increasing the ethanol concentration to 50%, the raspberry-like nanoparticles mentioned above are obtained. Further increasing the ethanol concentration to 70%, aggregated solid nanospheres appear, without a well-define morphology. It is suggested that proper amount of ethanol helps to form dynamically stable nanoemulsions system, but too much ethanol will destroy the stability of the O/W interface, forming aggregated nanospheres. In addition, under the condition of adding 0.3 ml TMB, the surface roughness of NPCPs is tuned from shallow grooves to deep ones, with the increase amount of initiating agent APS. This is all because that APS can control the rate of polymerization. Larger amount of APS results in faster reaction rate and the formation of smaller nanospheres. 26 By varying the amount of APS, the diameter of the nanospheres can be continuously tuned from 250 to 350 nm.
On account of the above observations, the proposed formation mechanism for the architectural engineering of NPCPs can be summarised as follows (Scheme 1). During the process, because of the strong hydrophobic effect between TMB and the polyoxypropylene (PPO) segments of F127 as well as the alkyl chains tail of CTAB, the TMB molecules can embedded into the hydrophobic sections of the surfactants to form the F127/CTAB/TMB nanoemulsions. The DAP molecules aggregate at the interface between the oil droplets and water, integrating with the polyoxyethylene segments of F127 as well as the cationic CTAB head through hydrogen bonding and TMB through π─π bonding, respectively. 25,27 With the further process of nucleation and growth, the polydiaminopyridine nanoparticles are formed and the emulsion droplets are coated and embedded outside the surface of nanoparticles, preventing aggregation and assisting the polymerization procedure to form different architectures. Followed by calcination at 700°C under N 2 atmosphere, the corresponding NPCPs are obtained.
The key feature of the mechanism is that F127/CTAB/ TMB composite nanoemulsions with various nanostructures assist the polymerization and growth procedure of DAP molecules to form nanoparticles with different architectures at the O/W interface. The TMB oil droplets can be stabilized by surfactants and interact with DAP through π bonds, which play an important role in coordinating the liquid interfaces. It is well known that due to the swelling effect, the increase of TMB leads to an increase in the diameter of the F127/CTAB/TMB nanoemulsion. 28 The dynamic light scattering (DLS) results show that as the amount of TMB increase from 0.1 to 0.4 ml, the main diameter of the emulsion droplets increased from 10 to 68 nm (Supporting Information: Figure S9). The surface features of NPCPs can be finely regulated according to the change (Scheme 1, Route I). Small number and size of oil droplets are formed under a low amount of TMB (0.1 ml), which can mediate the stability of the O/W interface to a certain, leading to the formation of smooth poly-diaminopyridine nanospheres. With the increasing amount of TMB to 0.2 and 0.3 ml, medium-diameter F127/CTAB/TMB nanoemulsions appear, assisting the polymerization process of DAP at the O/W interface and thus coated and embedded on the surface of the poly-diaminopyridine nanoparticles as the reaction goes on. It can be observed visually that the surface roughness of NPCPs samples deepens due to the larger droplet size, changing from litchi-like nanospheres with a grooved surface to nanospheres with deeper grooves. As the amount of TMB further increases to 0.4 ml, the emulsion droplets become larger and the DLS curve exhibits a broad peak, demonstrating the coexistence of large and medium sizes of oil droplets in the system. The DAP molecules are absorbed through π bonds and deposit at the O/W interface of the largediameter droplets, grow and extend circumferentially along the emulsion interface, forming segregated island seeds. For the reason of reducing the number of interfaces between the poly-diaminopyridine island seeds and the nanodroplets and minimizing the interfacial energy, the DAP molecules tends to deposit on the already formed seeds rather than on the droplets, growing continuously in the radial direction of the droplets to form a single-cavity structure. While the remaining medium droplets are coated on the outer surface, forming a multigrooved structure and eventually leading to raspberry-like nanoparticles. The formation process is similar to the Volmer-Weber growth mode. When the bond between deposited materials is stronger than that between the materials and the substrate, the molecules of the deposited materials are more favorable to bond themselves rather than to the substrate, so that the molecules of the deposited materials combine on the substrate surface first to form numerous small isolated seeds, and finally to form three-dimensional islands. [29][30][31] The emulsion-assisted interfacial polymerization strategy is based on the H-bond interaction between the monomer DAP molecules and the structuredirecting agent F127 and CTAB. To investigate the respective roles of the two surfactants, we carry out a series of explorations around changing the mass ratio of F127 to CTAB. Without adding F127 and CTAB to the system, aggregated particles are obtained (Supporting Information: Figure S10). When only adding F127, aggregated particles turn to smaller ones, some of which have inwardly concave hollows. In the case of adding F127 and CTAB of a 2:1 mass ratio, it leads to uniform nanobowls with obvious boundaries. It is because that when CTAB is added, the as-generated positive CTA + ions can form strong electrostatic interaction between the positively charged micelles, leading to a more stable system and the avoidance of aggregation between particles. From the evident changes, it can be inferred that in the system, F127 mainly acts as a steric stabilizer, limiting particle growth, and CTAB mostly plays the role of preventing particle aggregation due to the electrostatic interaction. 14 The two surfactants cooperate with each other and are indispensable for the constitution of NPCPs with different nanostructures. Subsequently, we further adjust the molar ratio to 1:1 and 1:2. As the amount of CTAB increases while TMB remains the same, it generates more micelles to conduct hydrophobic interaction with TMB. Therefore, the resultant nanoemulsion droplets become smaller in diameter and larger in number, so raspberry-like nanoparticles are obtained when the molar ratio reaches 1:1. Due to the small number but large size of droplets at the mass ratio of 2:1, the smooth-surfaced nanobowls are obtained. Further changing to 1:2, due to the increase of surfactant, large-diameter droplets could not be formed, so isotropic nucleation and growth occur, thus forming a litchi-like structure. When it finally reaches 1:4, smooth-surfaced nanoparticles are gained because the emulsion droplets are too small.
Owing to the tunable anisotropic and isotropic architectures, well-developed porosity and the beneficial N-doped features, the electrochemical performance of NPCPs as a supercapacitor material is worth expecting. To evaluate the effect of NPCPs with different architectures on the supercapacitor performance systematically, we chose the litchi-like NPCPs-0.3, raspberry-like NPCPs-0.4, and bowl-like NPCPs-5 as supercapacitor electrodes in a three-electrode system using 6 M KOH solution as the electrolyte. The samples possess similar specific surface areas (315-362 m 2 g −1 ) and impressive anisotropic/isotropic structures, which can provide effective approaches for the structure-activity relationship. As shown in Figure 4A, all the cyclic voltammetry (CV) curves exhibit a nearly rectangular shape at the same scan rate of 10 mV s −1 , among which the CV curve of NPCPs-0.4 possesses a larger area than the other samples, demonstrating a better capacitance performance and a rapid electrochemical response. 32 It can be observed that all the CV curves of the as-made NPCPs-0.4 and NPCPs-5 electrodes (Supporting Information: Figures S11 and S12c) retain nearly rectangular shapes even at the scan rate up to 200 mV s −1 , which are better maintained than that of the NPCPs-0.3 electrode (Supporting Information: Figure S12a), indicating an excellent high-rate stability. This result can be attributed to the unique anisotropic single-cavity morphology of NPCPs-0.4 and NPCPs-5, offering short charge diffusion distances to facilitate transportation rate and frequency response. 33,34 The galvanostatic charge/discharge (GC) tests of NPCPs samples were conducted at different current densities from 0.1 to 5 A g −1 (Figures 4B and Supporting Information: Figure S12b,d). The smooth curves exhibit a typical triangular shape and are almost symmetric, which visually demonstrates the ideal double-layer capacitance behaviors as well as the reversibility. [35][36][37] The GC curves of different NPCPs samples at the constant current density of 0.2 A g −1 are shown in Figure 4C. NPCPs-0.4 possessing abundant microporous structure, plentiful nitrogen content and a hollow single-cavity morphology conducive to mass transfer, thus, exhibits the longest discharging time and the highest specific capacitance (275 F g −1 ) at 0.2 A g −1 , which is consistent well with the results of the CV tests. For porous carbon materials, as the charging frequency increases, the penetration of electrolyte ions becomes worse, the efficient contact surface area becomes smaller, and the barriers to ion diffusion increases. [38][39][40][41] Under intense charge-discharge condition (high current density of 5 A g −1 ), NPCPs-0.4 (61.1% retention), and NPCPs-5 (59.4% retention) maintain better than NPCPs-0.3 (54.9% retention), showing excellent high-speed handling capability ( Figure 4D).
The NPCPs samples exhibit outstanding capacitance performance and fast frequency response. This excellent property can attribute to the fact that NPCPs materials have concentrated distribution of micropores at 1.7 nm, which contribute to a high electrical double-layer capacitance (EDLC), while beneficial and effective nitrogen doping provides enough active functional groups, which contribute to a certain pseudocapacitance (PC). [42][43][44] According to Trasatti's method, we quantified the respective contributions from EDLC and PC of NPCPs samples. The total specific capacitance (C S,T,M ) can be partitioned into EDLC (C DL ) and PC (C P ) contributing capacitance. The N content and N species percentage of NPCPs samples can be observed in Figure 4E, the content of active nitrogen species in the carbon frameworks are all higher than 89%. Such high content of N-5 and N-6 is due to the unique molecular structure of the monomer DAP, which provides plentiful N-containing heterocycles in the backbone. The beneficial N-doped can improve the conductivity and electrochemical activity of NPCPs and can offer additional pseudocapacitance. [45][46][47] As expected, NPCPs-0.4 exhibits the highest C S,T,M of 241.5 F g −1 and C P of 118.3 F g −1 ( Figure 4F). Besides, the corresponding PC contribution is 49% of the total specific capacitance. Due to the similar single-cavity anisotropic structure and BET of NPCPs-5 and NPCPs-0.4, NPCPs-5 also possesses a high EDLC contribution (111.5 F g −1 ), but the total specific capacitance is lower than that of NPCPs-0.4 because of its low total nitrogen content and active nitrogen species content. The isotropic sphere morphology of NPCPs-0.3 also results in a lower C DL and C S,T,M . The surface-area-normalized capacitance (C A ) is an important value to evaluate the capacitance. 48,49 The capacitive properties of the carbon surface can be evaluated through dividing the specific capacitance by the corresponding specific surface area (C A = C s /S BET ), which can quantitatively evaluate the charge density on the carbon material surface. According to the calculation results, the C A values of NPCPs-0.3, NPCPs-0.4, and NPCPs-5 are respectively to be 63.6, 83.0, and 81.3 μF cm −2 . NPCPs-0.4 and NPCPs-5 exhibit similar C A and much higher than NPCPs-0.3, indicating the hollow large single-cavity structure provides faster charge diffusion and higher capacitance during the charge/discharge process. As for NPCPs-0.3, it possesses the highest BET specific surface area, but the lowest specific capacitance, leading to the lowest C A value among the NPCPs samples. It is clearly proved that the anisotropic raspberry-like NPCPs-0.4 has better electrochemical performance than its corresponding isotropic nanosphere NPCPs-0.3. The surface-area-normalized capacitances of NPCPs are roughly three to five times higher than those of traditional activated carbon materials (15-25 μF cm −2 ), 50 as well as most of the reported porous carbon materials ( Figure 5A and Supporting Information: Table S2). The abundant porous structure, finely designed anisotropic architecture, and high active nitrogen species content are helpful to improve EDLC and PC, endowing the NPCPs with the enhancement of electrochemical performance.
Electrochemical impedance spectroscopy is an effective measurement to analyze kinetic information of electron transport in carbon materials and interfacial property between electrolyte and electrode interface. Nyquist plots of NPCPs samples are given in Figure 5C. On the whole, the plots can be divided into three representative parts: (1) the linear, nearly vertical lines in the low-frequency region indicating an ideal capacitive behavior, (2) the semicircular curves representing the charge transfer resistance (R ct ) in the middle frequency region, and (3) in the high-frequency region, the Z′-axis intercept values representing the equivalent series resistance (R s ). 51 All the NPCPs samples endow low R s values (below 0.5 Ω), which indicates rapid charge transfer. 52,53 According to the high-frequency region of Nyquist plots (inset in Figure 5C inside the whole circuit is similar, resulting in close equivalent series resistance values. In the middle frequency region, NPCPs-0.4 exhibits the smallest semicircular curve, that is, the lowest charge transfer resistance, suggesting that a lower resistance is contribute to higher charge diffusion rate to some extent. 54,55 R ct is mostly effected by the electrode/electrolyte interface. The electrode provides a good electron and ion transfer pathway for the interface, which can be expected to a great application potential in practical energy storage. In addition, NPCPs-0.4 exhibits excellent high-rate and long-term cycling stability. [56][57][58][59] The GC curves and specific capacitance keep almost unchanged over 4000 cycles at the current density of 5 A g −1 ( Figure 5D). NPCPs are expected to be promising superior electrochemical materials, of which the microporous structure, beneficial nitrogen doping, and unique anisotropic large single-cavity features play a synergistic role in improving the properties of the materials. The abundant nitrogen sites can better optimize the ion diffusion process, thus taking full advantage of the specific surface area and providing more effective space for storing charges and ions. At the same time, larger surface area can also provide more channels for charge transport, expanding the contact interface between the active sites and the ions. The structures of NPCPs have an obvious impact on their physicochemical properties and further influence on the electrochemical performances, among which the hollow large single-cavity structure provides faster charge diffusion and exhibits impressive electrochemical behaviors including high surface-area-normalized capacitance and superior long-term cyclic stability.

| CONCLUSION
In summary, we have developed a facile and versatile emulsion-assisted interfacial polymerization strategy for the controllable synthesis of nitrogen-doped carbon nanoparticles with tunable architectures. The F127/ CTAB/TMB composite nanoemulsions acting as assisting agents during the nucleation and anisotropic/isotropic growth stages. The main core of this method is to control the structure of nanoemulsions in two routes, so as to assist and mediate the polymerization process of DAP molecules at the O/W interface, leading to carbon particles with different architectures. Due to the abundant micropores, high N content, and the anisotropic architectures, the NPCPs exhibit high specific capacitance and excellent cyclic stability. We estimated the respective contribution of double-layer capacitance and pseudocapacitance to the total capacitance and further demonstrated that the unique pore structure and morphology can supply more transport paths to improve the rate capacity, while the abundant nitrogen sites can improve the surface wettability, both of which have significant effects on the electrochemical performance. The study is of great significance for the precise regulation and synthesis of anisotropic nitrogen-doped carbon nanoparticles with different surface structures, and provides a new option for the efficient synthesis of high-performance supercapacitor materials.