Three-dimensional Interconnected Microporous Carbon Network

Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Three-Dimensional Interconnected Microporous Carbon Network Derived from Aniline Formaldehyde Resin/Sodium Polyacrylate Interpenetrating Polymer Networks (AF/PAAS IPNs) with Controllable Porosity for Supercapacitors Yan-Dong Ma,† Xi-Wen Chen,† and Ling-Bin Kong*,†,‡

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†

State Key Laboratory of Advanced Processing and Recycling of Non-Ferrous Metals, Lanzhou University of Technology, Lanzhou 730050, P. R. China ‡ School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, P. R. China S Supporting Information *

ABSTRACT: A facile strategy for the synthesis of threedimensional nitrogen-doped microporous carbon (MC-IPNx) is reported here. We adopt the advantages of cross-linking individually of interpenetrating polymer networks to synthesize porous carbon with well-controlled micropores (∼0.6 nm) and interconnected pores by one-step carbonization of aniline formaldehyde resin/sodium polyacrylate interpenetrating polymer networks (AF/PAAS IPNs), a radically different approach that allows AF/PAAS IPNs to be directly converted to nitrogendoped microporous carbon (MC-IPNx) without any pretreatment. Due to the interpenetrating pores and abundant porosity, MC-IPNx acquired a high specific surface area (1275 m2 g−1), which reveals remarkable specific capacity (223 F g−1 at a current density of 1 A g−1) and superb capacity retention (80% retention in a current density within the range 0.5−10 A g−1). The unique pore structure of MC-IPNx provides an innovative idea for the control of pore size, and advanced electrochemical performance indicates its application prospects. KEYWORDS: interpenetrating polymer networks, microporous carbon, controllable pore size, supercapacitors, electrochemical performance

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INTRODUCTION Supercapacitors, also known as electrochemical capacitors, have emerged as the times require with the increasingly global challenges of an energy shortage and environmental pollution caused by the overconsumption of fossil fuels; these materials are characterized by preeminent power densities, rapid charge/ discharge kinetics, and prodigious cycle lifetimes.1−5 Several important indicators for evaluating supercapacitors, the specific capacity, ratio, and cycle life, are closely related to the electrode materials. In the past few decades, scholars have conducted in-depth research on transition metal compounds,6−8 conductive polymers,9,10 and carbon materials.11−13 Among them, porous nanostructured carbon has attracted extensive attention owing to its good conductivity, vast specific surface area (SSA), adjustable pore size, good thermal stability/chemical stability, and easy availability of raw materials.14−16 The energy storage mechanism of carbon-based supercapacitors is contingent on electrostatic adsorption of ionic charges to constitute an electric double layer presenting at the interface between the electrolyte and the electrode.17 As a result, the pore structure, including accessible surface area (ASA), pore size distribution (PSD), and porosity of carbon © XXXX American Chemical Society

framework, is the most considerable factor in determining the properties of carbon-based supercapacitors.18 A variety of strategies for synthesizing carbon materials have been reported to overcome the bottleneck constraints of pore structure, such as electrospinning technology for carbon nanofibers,19 chemical vapor deposition (CVD) for carbon nanotubes20 and carbon nanosheets,21 Hummers method for graphene,22 high-temperature chlorination of carbides for carbide-derived carbon,23 and polymer pyrolysis method for carbon nanocage24 and porous carbon.25 Among these, porous carbon is one of the promising materials used for electrochemical energy storage. Mainly, common strategies for synthesizing porous carbon materials are hard templates,26−28 soft templates,29,30 and template-free methods.31,32 Just as the name suggests, hard template methods fabricate carbon material by carbonizing the amalgam of carbon precursors and templates with given structural characteristics followed by dislodging the template introduced before by either chemical Received: May 26, 2019 Accepted: August 5, 2019 Published: August 5, 2019 A

DOI: 10.1021/acsaem.9b01044 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials etching or dissolution in strong acid and/or alkaline, so that the carbon skeleton is retained, imitating the morphology and structure reversely. There is no doubt that preparation of nanostructured porous carbon with a hard template method can optimize pore structure and pore size up to the hilt; nevertheless, dishearteningly, we are aware it is not the easiest path to synthesize large batches of different categories of templates. Worse still, for casting out the hard template completely, several hazardous chemicals are recommended such as strong acids and alkaline chemicals, which produce many problems in industrial production and environmental protection. For soft template methods, precursors bond with templates which allow the conformation of supramolecular self-assemblies via hydrogen bonding or electrostatic attraction followed by pyrolysis in the noble atmosphere at high temperatures to dehydrogenation and deoxygenation of organic compounds that lead to templates decomposing and volatilizing, leaving behind the carbon framework. For example, Chen and co-workers chose polyacrylonitrile (PAN) as a carbon source and poly(methyl methacrylate) (PMMA) as the soft template to synthesize porous carbon nanofibers (CNFs) by coaxial and emulsion electrospinning which exhibit good electrochemical and mechanical properties, simultaneously.33 Du et al. adopted ionic liquids as soft template and nitrogen source, and resin as the carbon source to prepare N-doped core−shell mesoporous spheres and carbon sheet composites (N-CMCS/CS) with excellent electrochemical properties.34 Other soft templates such as Pluronic F12735 and (PEO−PPO−PEO)-type triblock copolymer36,37 are also employed as common pore formers. However, problems of the thermal stability and the interaction between soft templates and precursors, and the cross-linking ability of the precursors, need to be considered comprehensively, which limits the practical application of it unquestionably. With regard to this, efforts have been targeted at polymer blends, which are composed of carbon precursor polymers and thermoplastic polymers.38,39 Among the diverse polymers, phenolic resin is one of the most significant examples, and the intentional design of porous morphology has been extensively studied because of the manageable procedure of the dehydration condensation reaction. However, as pore formers, these decomposing polymers such as poly(methyl methacrylate),40 polystyrene,41 and poly(ethylene oxide)39 are combined with phenolic resins solely by noncovalent interaction including hydrogen bonding, electrostatic interaction, and van der Waals forces.42,43 Consequently, the macro-phase-separation between PF and decomposable polymer that occurs during the cross-linking curing process becomes an insurmountable problem which may cause uneven pore size, small pore volume, noninterconnected pore structure, and low surface area in the complex nanostructured carbon after pyrolysis of precursors.15,44 In addition, the electrochemical performance of these carbon materials is not very satisfactory. The combination of carbon and heteroatoms is one of the promising methods to improve it.25 Also, the most typical one is nitrogen doping, which can not only form active sites45 but also increase the wettability of the porous carbon surface,31 thereby improving its electrochemical performance. The selection of nitrogen-rich precursors followed by carbonization to synthesize nitrogen-doped porous carbon makes the nitrogen-doping process effective and uniform and thus is widely spread.46

Carbonizing interpenetrating polymer networks (IPNs) directly is considered a novel strategy to fabricate porous carbon with controlled pore size. Interpenetrating polymer networks is a polymer blend of two or more polymer networks that are respectively cross-linked and interpenetrated. The molecular structure of one of the polymers is controlled to be a “retained polymer”, which is carbonized at a high temperature to form a carbon framework; the other polymer is controlled to become a “sacrificial polymer”, and the pores are formed at a high temperature. By controlling the relative content of the “retained polymer” and the “sacrificial polymer”, the pore structure characteristics and pore size distribution can be controlled. On this basis, several works have been reported to make a thorough inquiry in the method. Zhang et al. synthesized and carbonized PF/PMMA IPNs to acquire porous carbon with a high SSA of 865 m2 g−1.15 Chen and co-workers fabricated hierarchical porous carbon by pyrolysis PF/PAAS IPNs and acquired a specific capacitance of 201 F g−1 at 0.5 A g−1 for supercapacitors.47 In fact, IPNs facilitate two polymer networks “forcing compatibility”, which is an effective method to avoid macro-phase-separation resulting from feeble noncovalent bonds. Furthermore, the method solves the problem of difficulty in template synthesis and removal, controls the pore diameter of the carbon material, and ensures that it is adjusted within a certain range just so that the relative content of the two polymers can range from 0 to 100% theoretically. However, to date, no studies have focused on controlling the pore size to the micropore level by carbonizing IPNs, or have further discussed the matching between the pore size and the ionic dimensions. In addition, a well-defined quantitative relationship between the micropore size and the specific capacitance is also needed in order to better understand their relationship and to design effective carbonbased supercapacitors. Inspired by this, here, we propose a simple strategy for synthesizing porous carbon material with a controllable pore size prepared from aniline formaldehyde resin (AF)/sodium polyacrylate (PAAS) IPN. The aniline formaldehyde prepolymer is introduced into the internal network space of the PAAS to form an IPN system in which only hydrogen bonding interactions exist between the two polymers. During pyrolysis, the AF network tends to form a carbon matrix, and the PAAS evaporates into a gaseous product while acting as a porous constructor. The sponge-like microporous carbon (MC-IPNx) with an amazing specific surface area of 1275 m2 g−1 reveals excellent rate capability, retaining 80% from 0.5 to 10 A g−1 and an exceptional specific capacitance of 240.5 F g−1 at 0.5 A g−1. At the same time, the electrode has good cycle stability, and there is hardly any capacitance decay after 10 000 charge and discharge cycles at 6 A g−1 in a 6 mol L−1 KOH aqueous electrolyte. In addition, in a two-electrode system, the material exhibits a wonderful energy density of 43 Wh kg−1 at 900 W kg−1. This work provides a new IPN option for the development of porous carbon with controllable pore size for advanced supercapacitor components.

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RESULTS AND DISCUSSION Superiority of Nanostructured Carbon Derived from AF/PAAS IPNs. The procedure for the preparation of nitrogen-doped microporous carbon with controllable pore sizes via the pyrolysis of AF/PAAS IPNs route is schematically illustrated in Scheme 1. Formaldehyde and aniline are dissolved in an aqueous solution of sodium hydroxide B


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ACS Applied Energy Materials

spectrum of PAAS does not contain unsaturated CC, which means that the monomer has been polymerized and contains characteristic absorption peaks of two carboxylates at 1622 and 1029 cm−1. Withal, for the spectrum of AF/PAAS IPNs, it can be described as a superposition of the spectra of AF and PAAS. Characteristic absorption peaks such as 3412 cm−1 (N−H bond) were also shown in the FTIR spectrum of AF/PAAS IPNs, and there is no new chemical bond generation, which means that there is only physical entanglement between the two polymer networks. The XRD patterns and Raman spectroscopy characterizations are presented in Figure 2 to explore the phase structure of C-IPN, C-AFB, and C-AF. All samples exhibit two conspicuous diffraction peaks, which are 2θ = 23.3° and 43.7° (Figure 2a), corresponding to the (002) and (100) planes, respectively, and no sharp peaks are observed, indicative of the amorphous structure of as-obtained carbon material which can be ascribed as the turbostratic structure with randomly accumulated graphene layers.51,52 In other words, the formation of carbonous materials that originated from the polymer precursor has been demonstrated owing to the characteristic diffraction peaks of carbon materials displayed in the XRD patterns. This disordered state is also described by Raman spectroscopy which is exhibited in Figure 2b and evidently reveals the typical D-band peak (∼1340 cm−1) and G-band peak (∼1593 cm−1). Between them, the D-band represents a structural disordered peak which occurs with graphite lattice defects, a disordered arrangement of edges, and an increase in the low-symmetry carbon structure, while the Gband originates from the stretching vibration of the (C−C) bond in the plane of the graphite lattice. As the grade of order of the carbon structure and the degree of graphitization lower, the intensity of the D-band gradually increases and that of the G-band gradually decreases. The structural order of carbon materials is usually characterized by the integrated intensity ratio (ID/IG) of the D-band to the G-band.51 Also, the calculated ID/IG value of C-IPN is ascertained to be approximately 1.00, which is close to that of C-AFB (0.98) and C-AF (0.99). The very close result indicates a similar crystal structure which is in line with the XRD results. To further analyze its chemical nature, the C-IPN sample was specifically selected for XPS measurement. The existence of C, N, and O is clearly revealed (Figure 3a), which means that N-doping carbon material is fabricated after the PAAS network is thermally decomposed; namely, the nitrogen atoms present in the precursor are successfully incorporated into the carbon skeleton. The corresponding high-resolution spectrum of C 1s in the C-IPN sample is shown in Figure 3b, which showcases four individual constituent peaks after a deconvolution procedure. The predominant peak at 284.4 eV is mainly owing to the contribution of the C−C group, and others at 285.0, 285.8, and 288.2 eV result from C−N, C−O, and CO groups, respectively.53 The N 1s spectrum shown in Figure 3c is deconvoluted into three individual peaks centered at 400.8, 399.9, and 398.3 eV corresponding to quaternary (N-Q), pyrrolic (N-5), and pyridinic nitrogen (N-6), separately.54 For O 1s, it can be deconvoluted into O-I at 531.4 eV (CO/O− CO), O-II at 532.2 eV (C−OH/C−O−C), and O-III at 536 eV (COOH).55 Abundant heteroatomic-containing functional groups are attributed to the reasonable choice of precursors. Furthermore, the nitrogen atom bound to the carbon material generates additional functional groups by changing the donor− acceptor characteristics of the electron to cause a surface

Scheme 1. Synthesis Illustration of Microporous Carbons Derived from AF, Blend of AF/PAAS, and AF/PAAS IPNs

sequentially, and then the functional group of the compound transforms from hydroxymethyl (−CH2OH) to stabilized methine (=CH2) via prepolymerization under alkaline conditions at an opportune temperature. After that, the prepolymer is adequately assimilated into the networks of PAAS followed by hydrothermal synthesis to facilitate further dehydration condensation of the prepolymer. Then, the intertwined polymer network structure composed of AF resin and PAAS takes shape. During the process of pyrolysis, PAAS thermally decomposes to form a porous structure on the carbon substrate transformed from AF resin; the porous carbon as-obtained is marked as C-IPN. In order to explore the difference in structure and electrochemical properties, we also synthesized blending of two polymers and pure phase AF resin and carbonized material, in which the process of swelling is countermanded with other conditions remaining unchanged, and as-prepared carbon material is marked as C-AFB and CAF, respectively. FTIR spectra for AF, PAAS, and AF/PAAS IPNs are given in Figure 1 to demonstrate the interpenetrating structure. In

Figure 1. FTIR spectra of the cured AF, PAAS, and AF/PAAS IPNs.

the spectrum of AF, the characteristic absorptions of the flexural vibration of (N−H) bond in methylene secondary amine appeared at 3412 cm−1. Peaks at 1600−1500 cm−1 were generated from the aromatic (−CC−) bonds.48 There are peaks of (−CH2−N) at 1400−1300 cm−1.49 Peaks between 3091 and 2846 cm−1 result from the aromatic and aliphatic C− H stretches, respectively. The bands between 1387 and 1228 cm−1 are caused by (C−N) stretching (band between the aromatic ring and amino group).50 Peaks at 650−750 cm−1 arise from the vibration of the stretched aromatic proton. The C


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Figure 2. (a) XRD patterns of the samples synthesized under different conditions and (b) Raman spectra of C-IPN, C-AFB, and C-AF.

Figure 3. (a) XPS spectra of C-IPN and (b−d) high-resolution XPS spectra of C-IPN: (b) C 1s, (c) N 1s, and (d) O 1s.

remarkable difference in electrochemical surface activity can be easily recognized. To be more specific, the sweep area of CIPN presents a rectangular-like shape with the largest scan area corresponding to good electric double layer behavior and high capacitance and C-AFB secondarily. Incomprehensibly, the curve of C-AF deviates from the rectangular shape to a large extent with minimum coverage area. In order to explore this problem, we conducted electrochemical tests on foamed nickel containing no active material for comparison (Figure S1). It was found that its CV curve is similar to that of C-AF with a value of area almost the same as the latter. Therefore, we predict that C-AF has basically no capacity. To further investigate the electrochemical performance of the samples, the GCD measurements at current density of 0.5 A g−1 were carried out, as shown in Figure 4b. The discharge time of CIPN is significantly longer than that for other samples, indicating that it provides a relatively large specific capacitance,

Faraday reaction, thereby providing pseudocapacitance to ameliorate the capacitance value and energy density of the carbon supercapacitor.56,57 The porous carbon obtained by carbonizing AF/PAAS IPNs is demonstrated to have a highly disordered structure with partial graphitization and abundant nitrogen atom doping in the carbon framework. However, in order to prove the superiority of the scheme, it is also necessary to consider a series of important parameters of electrochemical performance. With this in mind, the cyclic voltammetry (CV) curves, galvanostatic charge−discharge (GCD) curves, and specific capacitances of samples at different current densities are performed in the three-electrode system under a potential window of 1 V, in which the electrolyte is 6 M KOH and the saturated calomel electrode (SCE) is used as reference electrode. From the CV curves of C-IPN, C-AFB, and C-AF at a scan speed of 5 mV s−1 demonstrated in Figure 4a, the D


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Figure 4. (a) CV curves at 5 mV s−1, (b) GCD curves at 0.5 A g−1, (c) the specific capacitances at different current densities, and (d) Nyquist plots.

in the EIS (Figure 4d) is approximately perpendicular to the real axis to represent the ideal capacitive behavior of C-IPN caused by a rapid ion diffusion/transfer process, indicating the perfect channels for ions of C-IPN rather than C-AFB or CAF.60,61 Various electrochemical performance tests have shown that C-IPN has great advantages as an electrode material. Significantly, the reasonable pore structure characteristic is an important condition to ensure its electrochemical performance. Therefore, it is necessary to explore the microstructure of different samples and make a reasonable explanation for its electrochemical performance. Under these circumstances, a hypothesis about the structural characteristics of the three types of samples was proposed. For C-IPN, it has crosscoupled and interconnected pore structure characteristics with uniform pore size; for C-AFB, its pores should be relatively independent with uncertain size. For C-AF, there should be only a few pores, but a stack of multiple monoliths. We have proposed three sets of models to illustrate and confirm these assumptions. As shown in Figure 5a, the two polymers of AF and PAAS are each polymerized and interpenetrated on a microscopic scale to form an interpenetrating structure. In addition, the internal components of the interpenetrating polymer are equably distributed. Thus, after carbonizing, the PAAS network is pyrolyzed and volatilized, and the space it occupies will undoubtedly form an interconnected pore. Since the two elements are wellproportioned in the polymer system, the size of the formed pores should be within a narrow range, and the specific area of nanostructured carbon will be very impressive. The large specific surface area accelerates the provision of a sufficient electrode electrolyte interface for accumulating charge or ions, which is desirable for carbon-based supercapacitor energy storage processes. Additionally, the ample and interconnected

which is consistent with the results of the CV tests. In addition, the galvanostatic charge/discharge curve is almost symmetrical, with a gradual slope change, showing the shape of an isosceles triangle, which shows that the electrode based on C-IPN has good electric double layer capacitance performance.58 On the contrary, the GCD behavior of C-AF is not so perfect with poor symmetry and a short discharge time, which is also true for unloaded foamed nickel. As a result, the calculated specific capacitance values of C-IPN, C-AFB, C-AF, and foamed nickel are 213, 168, 5, and 4 F g−1, respectively. The distinct difference in electrochemical performance on one hand confirms that the nanostructured carbon as-synthesized from the precursor of AF/PAAS IPNs for the supercapacitor is a successful and effective project; on the other hand, it also shows that C-AF has almost no capacitance contribution, which is further evidence of the prediction we put forward in the CV test. Figure S2a shows typical CV curves of the C-IPN electrode at different scan rates. The “rectangular shape” cyclic voltammetry was maintained for the CV curves even at a very high-potential scan rate of 100 mV s−1, implying its excellent electrochemical behavior. Moreover, the galvanostatic charge/discharge curve of C-IPN (Figure S2b) is nearly symmetric while the current load increases from 0.5 to 10 A g−1, exhibiting only a very small IR drop at a high current density. Figure 4c shows the relationship between specific capacitance and charge−discharge current density. When the current density increases to 10 A g−1, there is a slight decrease in the capacitance value of C-IPN, which is 78.4% of the value at the current density of 0.5 A g−1, suggesting a good capacitance retention rate.59 As a comparison, the specific capacitance drop of C-AFB is more obvious; the specific capacitance is as low as 120 F g−1 when the current density increases to 10 A g−1. The tail line of the low-frequency region E


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ions lose the carrier and cannot form an electric double layer structure, so the specific capacitance is as low as that of the unloaded foamed nickel. The FE-SEM image of C-AF in Figure 5c displays a relatively smooth monolith with few pores. To further prove our speculation and explore the pore structure of the sample, the N2 adsorption−desorption test is carried out. As shown in Figure 6a, the sharp increase in nitrogen uptake at low relative pressures in the adsorption− desorption isotherm confirms the presence of large numbers of micropores within the C-IPN, whereas when the relative pressure is gradually increased, the adsorption amount increases slowly, indicating the absence of mesopores or macropores. As for C-AFB, the adsorption amount shown in the figure is significantly smaller, and the hysteresis loop in the relative pressure of 0.5−0.99 p/p0 indicates the existence of mesopores.63 Unlike the other samples, the low nitrogen absorption of C-AF indicates that the number of pores is very scarce. This result can be further confirmed by the pore size distribution (PSD) curve in Figure 6b and Figure S3. We found that the specific surface areas of C-IPN, C-AFB, and CAF samples were 976, 637, and 16 m2 g−1, respectively, and the pore volume decreased from 0.591 to 0.430 to 0.062 cm3 g−1, respectively. It is apparent that C-IPN has the highest specific surface area and pore volume, which indicates that carbonization of AF/PAAS IPNs is an effective method for porous carbon synthesis. The pore structure characteristics were also further explored by TEM technology. TEM images of C-IPN, C-AFB, and CAF at the same magnification (Figure 7a−c) clearly suggest

Figure 5. Interpretation of the microscopic mechanism of synthesis of (a) C-IPN, (b) C-AFB, and (c) C-AF and the corresponding SEM images.

channel characteristics are beneficial to reduce the diffusion resistance of electrolyte ions, optimize the diffusion kinetics of ions, and enable electrolyte ions to favorably enter the pores and adhere to the electrode interface.62 The FE-SEM image in Figure 5a displays that the microscopic morphology of asobtained C-IPN possesses a micrometer-sized lump with a sponge-like shape, which is strong proof of this hypothesis. In contrast, the blend product of AF and PAAS (as shown in Figure 5b) appears to be less regular, with no well-defined morphology, and it is simply a random combination of the two components. As a result, the pores as-formed are isolated and noninterconnected just like the pattern of one component of the decomposed mixture. Moreover, since the PAAS network is not fully embedded inside the AF block, the specific surface area of the C-AFB is significantly smaller than that of the CIPN, which is the non-negligible reason for why the energy storage capacity of the C-AFB is inferior. The FE-SEM image of C-AFB is consistent with the expected result, so it can be seen that there is only a rough surface with no obvious pores existing in it. For C-AF, it lost the pore-forming agent PAAS. During the pyrolysis process, the AF resins only experienced dehydrogenation and deoxidation and could not form pores, resulting in a very small specific surface area value (Figure 5c). Consequently, when it is used as an electrode material, charged

Figure 7. TEM images of (a) C-IPN, (b) C-AFB, and (c) C-AF.

that C-IPN has a higher porosity than others. Simultaneously, there are only a few pores of C-AF, which is consistent with the calculated pore size distribution (Figure 6b and Figure S3). Obviously, C-IPN with an interconnected pore feature makes

Figure 6. (a) N2 adsorption/desorption isotherms and (b) the pore size distributions (0−2.3 nm) of the samples synthesized under different conditions. F


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Figure 8. (a) N2 adsorption/desorption isotherms, (b, c) pore size distributions, and (d) cumulative pore volume of the as-synthesized MC-IPNx.

pore size, which is in line with our expectation. The strongest peak of MC-IPN3 indicates that it has the highest porosity, meaning the largest specific surface area (Figure 8a) and the largest pore volume accumulation (Figure 8d). In addition, a sharp increase of 0.2−1.0 nm and a near-horizontal pore volume accumulation curve after about 2 nm (Figure 8d) further confirmed this result. Adequate microporosity is further confirmed by the porosity parameters of the as-obtained MCIPNx in Table 1. SBET and Vtotal exhibited the same pattern of

it more promising than C-AFB and C-AF as an electrode material for carbon-based supercapacitors. Porosity Adjustment for MC-IPNx. On the basis of the unique structural characteristics and excellent electrochemical performance of C-IPN, we synthesized a series of interpenetrating polymers by changing the relative content of aniline formaldehyde resin and PAAS to further optimize its potential application value as an efficacious electrode material for the electric double layer capacitor. Specifically, we increase the content of aniline from 2.5 to 4 g while ensuring a molar ratio of A:F = 1:1, so that the content of PAAS and other conditions remain unchanged. The sample obtained after carbonization was labeled as MC-IPNx (x = 1−5, corresponding aniline content was 2, 2.5, 3, 3.5, 4 g). Measurement of the N2 adsorption−desorption isotherm is a good choice for analysis of the specific surface area (SSA), total pore volume, and pore size (PSD) distribution of the resulting sample. As shown in Figure 8a, the N2 adsorption−desorption isotherms of all samples showed a typical type I classification in light of International Union of Pure and Applied Chemistry (IUPAC), indicating the presence of a large number of micropores.64,65 In detail, the amount of gas adsorption increases rapidly at low relative pressures,66 due to the filling of the microporous pores; the subsequent horizontal or near horizontal platform indicates that the micropores have been filled with adsorbate, with almost no further adsorption occurring. MC-IPN3 demonstrates the largest SSA of 1275 m2 g−1 as compared to others with 795, 976, 1082, and 887 m2 g−1 for MC-IPN1, MC-IPN2, MC-IPN4, and MC-IPN5 based on the Brunauer−Emmett−Teller theory, respectively. In Figure 8b,c, the pore size distribution curve shows a distinct peak at 0.6 nm in a range 0.2−1.0 nm, indicating that the carbonized AF/PAAS IPNs can achieve precise control of the

Table 1. Porosity Parameters Including Specific Surface Areas and Pore Volume of MC-IPNx sample

SBETa (m2 g−1)

Smicb (m2 g−1)

Vtotalc (cm3g −1)

Vmicd (cm3 g−1)

MC-IPN1 MC-IPN2 MC-IPN3 MC-IPN4 MC-IPN5

795 976 1275 1082 887

743 829 792 993 781

0.418 0.591 0.770 0.591 0.505

0.353 0.453 0.516 0.499 0.400

a

Specific surface area, estimated using a model of Brunauer− Emmett−Teller (BET). bMicropore specific surface area, calculated by t-plotting. cTotal pore volume, obtained by using nitrogen as the adsorbent at a maximum relative pressure of P/P0 = 0.99. dMicropore volume, estimated according to the t-plot method.

variation, reaching a maximum at the aniline content of 3 g (corresponding to sample MC-IPN3). Obviously, these samples are porous carbon materials dominated by micropores. The rich porous structure is also demonstrated by TEM technology, as shown in Figure S5, and this indicates its excellent electrochemical performance. In addition, the water contact angle was also tested to demonstrate the difference in surface wettability of the as-prepared porous carbon. As shown G


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Figure 9. Electrochemical performance of MC-IPNx: (a) CV curves at 5 mV s−1, (b) GCD curves at 0.5 A g−1, (c) the specific capacitances at different current densities, and (d) Nyquist plots of MC-IPNx.

Figure 10. Electrochemical performance of MC-IPN3: (a) CV curves at the scan rate up from 5 to 200 mV s−1, (b) GCD curves at 0.5−10 A g−1, (c) electrochemical impedance spectroscopy (the inset reveal the high resolution of the spectrum), and (d) the cycling stability at 6 A g−1 after 10 000 cycles.

in Figure S4, all samples exhibited very close angular values, which means that the surface wettability of the material in this system will not be a major factor affecting its electrochemical performance in this system.

To further investigate the effect of microporous structure on electrochemical performance, the sample was assessed in a single electrode system with 6 M KOH as the electrolyte. The result is shown in Figure 9. Among them, Figure 9a shows the cyclic voltammetry (CV) of the sample at a sweep rate of 5 mV H


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ACS Applied Energy Materials Table 2. Comparison of Specific Surface Area and Specific Capacitance (Cs) of Different Porous Carbon Materials sample

SBETa (m2 g−1)

RM-6-4 HPC4.2‑16.8‑6C HCNS N/C-900 APC-bib NHPC HHCSs-700 MC-IPN3

477.6 1330 515 684 1290 1091 253 1275

Cs (F g−1)

electrolyte 2 6 6 6 6 6 6 6

M M M M M M M M

KOH KOH KOH KOH KOH KOH KOH KOH

139 194 227 217 164 234 241 240.5

capacitance measurement at 0.5 0.1 0.5 0.5 0.5 0.5 0.5 0.5

A A A A A A A A

g−1 g−1 g−1 g−1 g−1 g−1 g−1 g−1

ref 68 69 70 71 72 73 74 this work

a

Specific surface area, estimated using a model of Brunauer−Emmett−Teller (BET).

Figure 11. Electrochemical performance in a two-electrode system (MC-IPN3//MC-IPN3 symmetric supercapacitor) with different electrolyte: (a) CV curves at 20 mV s−1, (b) GCD curves at 0.5 A g−1, (c) electrochemical impedance spectroscopy, and (d) Ragone plot of MC-IPN3 symmetric supercapacitor compared to others (CS-v,45 3D IHPNC,75 dr-Bi2S3/S-NCNF,76 O−N−S codoped HPC,31 B-g-CN800,77 PHC78).

s−1, and all samples show an approximately rectangular closed loop. In an electric double layer capacitor, this near-perfect shape indicates the ability of ions to rapidly adsorb and desorb on the surface of the carbon substrate. As can be seen, the MCIPN3 curve integral area which is proportional to the specific capacity is significantly larger than that for the other samples, indicating its high energy storage capacity. The outcome can be further acknowledged by the galvanostatic charge/discharge (GCD) curve at a current density of 0.5 A g−1 in Figure 9b. The curves of all samples exhibited an approximately symmetric isosceles triangle shape, reflecting its good capacitive behavior and near-ideal electrochemical reversibility. The galvanostatic charge/discharge curve is considered to be an advanced technique for calculating the specific capacitance of the electrode material. As a result, the specific capacitance of MC-IPN3 is 240.5 F g−1 when the discharge current density is 0.5 A g−1, which is significantly superior to that of others with the specific capacitance of 160, 213, 220, and 200.5 F g−1 for MC-IPN1, MC-IPN2, MC-IPN4, and MC-IPN5. The

prominant electrochemical performance of the sample may be due to the interconnected microporous structure and higher specific surface area (due to higher porosity). The specific capacitance at different current densities is summarized in Figure 9c. Obviously, while the current density is gradually increased to 10 A g−1, the specific capacity of all samples has a slight decrease, and the capacitance retention rate is as high as about 80% of that of MC-IPN3, which may be because the interpenetrating porous channels reduce the diffusion resistance of ions; thus, the diffusion kinetics of ions is greatly enhanced. The measurement of electrochemical impedance spectroscopy (EIS) is an important means to study the ion/ electron transport kinetics of supercapacitors. Figure 9d shows the Nyquist plot for different MC-IPNx samples at a frequency of 105−10−2 Hz. We can find out that all spectra include a semicircular arc in the high-frequency region (relating to interface charge transfer).57 The 45° sloped line corresponds to the diffusion resistance in the medium-frequency region, and the vertical line in the low-frequency region represents I


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0.5 A g−1. The 100% capacity retention rate after 10 000 cycles at the current load of 6 A g−1 indicates long-term chemical stability. At the same time, the assembled symmetrical capacitor has an exceptional energy density of 43 Wh kg−1 at 900 W kg−1 in the neutral electrolyte. Therefore, this work enriches the strategy of synthesizing microporous carbon with controlled pore size and exerts great potential as an electrode material in supercapacitors.

higher-capacitance behavior.67 The half arc of MC-IPN3 is smaller than that for other samples in the high-frequency region of the EIS spectrum, indicating that its interface chargetransfer resistance is lower than that for other samples obtained under the same conditions. The steeper line of the lowerfrequency region indicates its higher capacitance behavior. The electrochemical performance of MC-IPN3 is illustrated in Figure 10. The rectangular-like shape exhibited by MCIPN3 is preferably maintained even at the scan rate up to 200 mV s−1 in 6 M KOH aqueous solution (Figure 10a). The GCD curve obtained at a current load of 0.5−10 A g−1 (Figure 10b) exhibits a symmetrical profile and an insignificant IR drop, which also represent a much low internal resistance of the system. From the EIS results in Figure 10c, MC-IPN3 not only has a lower internal resistance (0.52 Ω) composed of the ionic resistance of the electrolyte and the contact resistance of the collector interface/active material but also shows a shorter Warburg region. In addition, compared with the initial state, MC-IPN3 also exhibited high stability of 100% after 10 000 cycles with a current density of 6 A g−1 (Figure 10d), indicating its good stability and high reversibility as a prominent supercapacitor electrode material. Table 2 shows a comparison of the specific surface area and specific capacitances of different porous carbon materials. The asprepared MC-IPN3 exhibits a high specific surface area and an excellent capacitance value. Electrochemical Performance for the SSC Device Assembled with MC-IPN3. Symmetric supercapacitors (SSC) were assembled with 6 M KOH and 1 M Na2SO4 aqueous solution to evaluate the practical application potential of MC-IPN3. Figure 11a shows the CV curve of the capacitor with a sweep speed of 20 mV s−1. It can be seen that a good rectangular loop can be maintained even in the wide potential window of 0−1.8 V in 1 M Na2SO4, which is because the high overpotential of the dihydrogen evolution in the electrolyte allows the voltage window to be extended.79 The GCD curves for different electrolytes at current densities of 0.5 A g−1 are shown in Figure 11b. The specific capacitance based on the GCD curve is 155 and 95.5 F g−1, respectively. Figure 11c shows the Nyquist plot of the MC-IPN3 symmetric supercapacitor. In comparison, the impedance of the supercapacitor is much smaller in the 6 M KOH electrolyte, which is probably due to the fact that the radius of K+ is smaller than the radius of Na+ in aqueous solutions,80 so that it can better match with the pore size of 0.6 nm. However, thanks to the wide potential window, the power density and energy density of the supercapacitor in 1 M Na2SO4 are much higher than other materials reported before (Figure 11d). As a result, the cell exhibits a high energy density of 43 Wh kg−1 at 900 W kg−1.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b01044. Experimental details and characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ling-Bin Kong: 0000-0002-2271-4202 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (51762031) and the Foundation for Innovation Groups of Basic Research in Gansu Province (1606RJIA322).

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REFERENCES

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CONCLUSIONS In summary, we demonstrate a breakthrough approach to synthesis of the nitrogen-doped microporous carbon (MCIPNx) with well-defined unimodal pore size distribution and outstanding electrochemical performance. AF/PAAS IPNs produce porous carbons via “forcing compatibility” and cross-linking, separately, effectively avoiding various disadvantages caused by the template method, and eliminating the uncontrollable structural features caused by macroscopic phase separation. In addition, AF/PAAS IPNs provide a wellcontrolled pore size allowing it to be distributed over a narrow range. More importantly, MC-IPN3 revealed a higher specific capacitance of 240.5 F g−1 when the current density is J


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