A New Porous Polymer for Highly Efficient Capacitive Energy Storage

Nov 26, 2017 - ... Debabrata Pradhan‡ , Taku Hayashi§, Yuh Hijikata§∥, Stephan Irle§∥ ... at 2 mV s–1 scan rate, together with a high speci...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 202−209

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A New Porous Polymer for Highly Efficient Capacitive Energy Storage Piyali Bhanja,† Sabuj K. Das,† Kousik Bhunia,‡ Debabrata Pradhan,‡ Taku Hayashi,§ Yuh Hijikata,§,∥ Stephan Irle,§,∥ and Asim Bhaumik*,† †

Department of Materials Science, Indian Association for the Cultivation of Science, 2A & B, Raja S. C. Mullick Road, Jadavpur 700 032, India ‡ Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur 721302, India § Department of Chemistry, Graduate School of Science, and ∥Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan S Supporting Information *

ABSTRACT: The new porous polymer TPDA-1 has been synthesized via solvothermal Schiff base condensation reaction between two organic monomers, i.e., 2,4,6-trihydroxyisophthalaldehyde and 1,3,5-tris(4-aminophenyl)triazine. The TPDA-1 material showed a very high specific capacitance of 469.4 F g−1, at 2 mV s−1 scan rate, together with a high specific surface area of 545 m2 g−1. It also exhibited excellent cyclic stability with 95% retention of its initial specific capacitance after 1000 cycles at 5 A g−1, suggesting its potential as a high performance supercapacitor. Extended π-conjugation and ion conduction inside the micropores throughout the whole polymeric matrix and high BET surface area could be responsible for this high supercapacitor performance in energy storage device. TPDA-1 has been characterized thoroughly by various electrochemical techniques such as cyclic voltammetry, galvanic charge−discharge, and electrochemical impedance spectroscopy. Our experimental results suggested a high potential of this porous polymer in energy storage devices for future generation. KEYWORDS: Porous organic polymer, Electrochemical study, Supercapacitor, Schiff base condensation

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INTRODUCTION Due to the rapid industrialization, urbanization, and population explosion over the past few decades, it is highly desirable to develop recyclable and safe sources of energy, as well as advanced and eco-friendly use of conventional energy through energy storage devices. The ease of gaining electricity and sustainability from these intermittent renewable energy storage sources has given them a significant role in energy applications in recent years.1−3 To obtain a device with high energy density and power, significant interest has been paid to the innovation of redox-active materials for electrochemical supercapacitors.4,5 In the advancement of related technologies, electrochemical capacitors (supercapacitors) are widely applied in electrical vehicles, modern electronic devices, and military devices, comprised of two closely spaced layers with opposing charges.6,7 Electrochemical supercapacitors have the ability to overcome the drawbacks of batteries which possess high energy densities but slow charging performance, and are superior over traditional capacitors, which charge faster but have lower energy density.8,9 Two different processes have been demonstrated to explain the mechanism of an electrochemical supercapacitor: one that occurs in the electrochemical double layer capacitor (EDLC) and is specified as a nonfaradaic process, and the another one, a faradaic process that emerges from electrodebound reversible redox processes is so-called pseudocapacitors.10−12 Recently, carbon-based materials ranging from © 2017 American Chemical Society

activated carbon to reduced graphene oxide materials are most widely used for energy storage application. In EDLCs, the charge was stored electrostatically using reversible adsorption of ions onto active material surfaces that are electrochemically stable and have high accessible surface areas. As a result, the EDLC offers very high stability during the charge−discharge cycles. However, limited energy density at the electrode surface has restricted its ultimate practical application. Thus, attempts have been made to exploit carbon-based electrode materials for supercapacitor applications by incorporating redox active materials such as transition metal oxides and conductive polymers. Subsequently, carbon-based transition metal oxide nanocomposites such as NiO/reduced graphene oxide (rGO),13 rGO/carbon,14 Ni(OH)2/multiwalled carbon nanotube (MWCNT),15 MnO2/carbon,16 MnO2/rGO,17 Co3O4/ rGO,18 and Co3O4/MWCNT19 have been synthesized for energy storage applications.20,21 In recent years, porous polymers such as covalent organic polymers (COPs) or related ordered covalent organic frameworks (COFs) have attracted immense attention due to their high surface areas, consistent nanopores, and possibility of designing the network structure using a wide range of functional building blocks.22−25 A quasiReceived: July 5, 2017 Revised: November 7, 2017 Published: November 26, 2017 202

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dioxane and mesitylene were used as received from Spectrochem, India, without further purification. Instrumentation. The X-ray diffraction pattern of the porous polymer was obtained from a Bruker AXS D-8 Advanced SWAX diffractometer using Cu Kα (0.154 06 nm) radiation. A Quantachrome Autosorb 1-C was used to obtain N2 sorption isotherms of TPDA-1 at 77 K, and the bulk sample was activated for 12 h at 120 °C under high vacuum conditions to remove any adsorbed solvent molecules. The nonlocal density functional theory (NLDFT) pore-size distribution of TPDA-1 was estimated by employing N2 at 77 K using a slit pore model in Autosorb-1 software from the resultant N2 adsorption isotherm. To record the FTIR spectra of the porous polymer and the monomers, a PerkinElmer Spectrum 100 spectrophotometer was used. UV−visible diffuse reflectance spectra were recorded with the use of a Shimadzu UV 2401PC spectrometer with an integrating sphere attachment where BaSO4 was used as background standard sample. The liquid state 1H and 13C NMR spectra of 1,3,5-tris(4aminophenyl)triazine (TAPT-1) and 2,4,6-trihydroxyisophthalaldehyde (PDA) were recorded on a Bruker Advance 500 MHz NMR spectrometer. The solid state 13C cross-polarization magic angle spinning NMR spectroscopic (13C CP MAS NMR) measurement was conducted on a Bruker Advance 500 MHz NMR spectrometer using a 4 nm MAS probe at a spinning rate of 5000 Hz, having sideband suppression. To observe the structural morphology as well as particle size, FE-SEM analysis was conducted with the use of a JEOL JEM 6700. The HR-TEM images of the material were recorded with the use of a JEOL 2010 TEM, operated at 200 kV, where the sample was prepared by dropping a sonicated absolute ethanolic suspension over a carbon-coated surface of a copper grid. The TG/DTA analysis was performed employing a temperature ramp of 10 °C/min with a thermal analyzer TA-SDT Q-600 under air flow. PerkinElmer 2400 Series II CHN analyzer was used for the elemental analysis. The electrochemical performances of the as-synthesized sample were conducted on a Bio-Logic SP-150 electrochemical workstation using a three-electrode configuration. The electrical conductivity of the sample was measured by the four-probe van der Pauw method using an ezHEMS (NanoMagnetic Instruments, U.K.). Synthesis of 2,4,6-Trihydroxyisophthalaldehyde. Following the literature,36 we prepared the Vilsmeier reagent, where 3.67 g (0.024 mol) of phosphorus oxychloride (POCl3) was added dropwise to anhydrous DMF (1.75 g, 0.024 mol) with continuous stirring at room temperature (298 K) under inert N2 atmosphere for about 30 min. Then the reagent was slowly added to a solution mixture of previously prepared anhydrous 1.0 g (0.008 mol) of phloroglucinol in dioxane under a N2 atmosphere (10 mL) maintaining the reaction temperature at ca. 315−320 K. This solution mixture was allowed to stir for 12 h to get a yellow amorphous solid and cooled to 0 °C. Then, this solid mixture was poured into an ice−water slurry (∼40 mL) and allowed to reach room temperature, and stirring was continued for a further 4 h. During this procedure, a cream colored precipitate appeared. Then, the precipitate was filtered off and washed with sufficiently large quantities of distilled water several times. Then, the solid product was dissolved in a very small amount of distilled water, and extracted with ethyl acetate two times. After washing with a brine solution, the obtained organic layer was dried using magnesium sulfate, and the ethyl acetate was removed in a rotary evaporator. The resultant solid was dried under vacuum at 360 K for 16 h to obtain constant weight (1.27 g, 7.0 mmol, 88%): 1H NMR (400 MHz, DMSO-d6) δ ppm 5.87 (s, 1 H, Ar−H), 9.98 (s, 2 H, CHO), 12.48 (br S, 2 H, OH), 13.48 (br S, 1 H, OH); 13C NMR (100 MHz, DMSO-d6) δ ppm 94.1 (C-5), 103.7 (C-1, C-3), 169.0 (C-2), 169.5 (C-4, C-6), 191.4 (2 × CHO); IR (KBr, cm−1) 3083 (br M), 2975 (br M), 2890 (br M), 1639 (s), 1620 (s), 1603 (s), 1440 (s), 1394 (s), 1259 (s), 1193 (s), 804 (s), 611 (s). Synthesis of 1,3,5-Tris(4-aminophenyl)triazine (TAPT-1). TAPT-1 was synthesized by trimerization of 4-aminobenzonitrile using superacid as a catalyst. Following a standard synthetic procedure,37 1.544 g (13.076 mmol) of 4-aminobenzonitrile was placed in a round-bottom flask on an ice bath at first. Then, 4 mL (44.4 mmol) of trifluoromethanesulfonic acid was added dropwise

ordered state in the polymer network could be responsible for high charge carrier mobilities.26 These porous polymers found wide-scale applications in various fields such as catalysis,27 gas storage and separation,28 drug delivery,29 sensing,30 optoelectronics,31 energy storage,32 and so on. COPs/COFs are conventionally prepared using different synthetic routes such as vapor-assisted conversion,33 microwave assisted solvothermal synthesis,34 and ultrahigh vacuum surface assisted synthesis.35 In this present work, we have synthesized a new porous polymer, TPDA-1, through solvothermal polycondensation reaction by incorporating 2,4,6-trihydroxyisophthalaldehyde moieties into a porous network bearing β-ketoenamines (Scheme 1), and it has been thoroughly characterized. High Scheme 1. Schematic Representation for the Synthesis of the Porous Polymer TPDA-1 and Its Polymeric Network

surface area, tunable porosity, and extended π-conjugation of the two-dimensional (2D) TPDA-1 polymer is responsible for this three-dimensional porous network material as supercapacitor electrode material. The structural and physiochemical properties of as-synthesized TPDA-1 material has been systematically investigated by powder X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, N2 sorption, UV− visible spectroscopy, 13C magic angle spinning (MAS) NMR spectroscopy, high resolution transmission electron microscopy (HR-TEM), field emission scanning electron microscopy (FESEM), thermogravimetry/differential thermal analysis (TG/ DTA), and CHN analysis. The electrochemical measurements showed that the energy storage ability of TPDA-1 is mainly contributed by the EDLC behavior, which offers excellent electrochemical stability as tested for 1000 charge−discharge cycles.

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EXPERIMENTAL SECTION

Chemicals. Phloroglucinol (M = 126.11 g/mol), phosphoryl chloride (M = 153.33 g/mol), and hexamine (M = 140.18 g/mol) were purchased from Sigma-Aldrich, India. Further, 4-aminobenzonitrile (M = 118.14) and trifluoromethanesulfonic acid were also obtained from Sigma-Aldrich, India. Organic solvents such as 1,4203

DOI: 10.1021/acssuschemeng.7b02234 ACS Sustainable Chem. Eng. 2018, 6, 202−209

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ACS Sustainable Chemistry & Engineering very slowly to maintain a temperature of 0 °C. After 1 h, the resultant slurry was stirred for 24 h at room temperature in an inert N2 atmosphere. After completion of the reaction, 20 mL of distilled water was poured into the mixture, followed by neutralization after adding 2 M NaOH solution until a final pH ∼7 was obtained. The pH level was monitored experimentally using litmus paper. The resultant yellow solid product was filtered off using simple filtration techniques and washed with distilled water five times to remove additional NaOH. The purified product has been characterized thoroughly by using FTIR, 1H NMR, and 13C NMR spectroscopies. The yield was 93.37 mol %. Synthesis of Porous Polymer TPDA-1. The TPDA-1 material was synthesized by using a Schiff base condensation polymerization method. The reaction was carried out using PDA (0.055 g; 0.3 mmol) and TAPT-1 (0.071 g; 0.2 mmol) in 2 mL of mesitylene and 2 mL of dioxane solvent mixture in the presence of 0.3 mL of 3 M acetic acid as catalyst. The reactants were first dispersed in a Pyrex tube by sonication for 10 min to form a completely homogeneous mixture. The reaction mixture was degassed in three subsequent freeze− pump−thaw cycles using liquid nitrogen. After that the tube was flame-sealed and kept at 120 °C in an oven for 72 h. Finally, the obtained deep red colored material was filtered and repeatedly washed with dioxane, water, and acetone, and dried under vacuum at 120 °C for 12 h to obtain TPDA-1. The presence of elements such as carbon, hydrogen, and nitrogen are revealed by CHN analysis, where C = 69.23%, H = 4.52%, and N = 13.84%. This compares favorably with theoretical values for the carbon, hydrogen, and nitrogen contents of the TPDA-1 polymer matrix, which are C = 63.12%, H = 4.30%, and N = 13.39%.

Surface Area and Porosity Analysis. The N2 sorption analysis was employed to measure the specific surface area with architectural rigidity and permanent porosity of TPDA-1. Figure 2 represents the N2 sorption isotherm of TPDA-1,

Figure 2. N2 sorption isotherms of TPDA-1, where filled and open circles indicate the adsorption and desorption isotherms, respectively. NLDFT pore size distribution plot is shown in the inset.

which was classified as a type I isotherm without any hysteresis loop according to IUPAC nomenclature.39 The sharp increase in the N2 uptake at low pressure (0.0−0.10 P/P0) indicates the presence of microporosity throughout the material. The Brunauer−Emmett−Teller (BET) surface area and pore volume of TPDA-1 were estimated to be 545 m2 g−1 and 0.2546 cm3 g−1, respectively. Also, the pore size distribution plot has been drawn by employing the NLDFT (nonlocal density functional theory) method as shown in the inset of Figure 2. Two sharp peaks appear at 1.4 and 1.6 nm, and one broad peak appears at 4.1 nm, which are attributed to the existence of micropores and interparticle mesopores throughout the entire polymer matrix. Moreover, to analyze the microporosity and mesoporosity contribution, De Boer statistical thickness (t-plot) analysis has been performed and reveals surface areas of 513 and 32 m2 g−1, respectively. Solid State 13C MAS NMR Analysis. To investigate the chemical environment of various types of carbon atoms in this porous polymer network, solid state 13C CP MAS NMR measurements have been carried out. The solid state 13C MAS NMR spectrum of TPDA-1 is displayed in Figure 3. The more down field resonance signals appeared at 185 and 174 ppm due to the presence of carbonyl carbon and the sp2 hybridized carbon atoms adjacent to the triazine ring.40 The sharp signals observed at 170 and 149 ppm are attributed to the hydroxyl groups bearing carbon atoms and sp2 hybridized benzene ring containing carbon atoms adjacent to the −NH groups, respectively. The strong resonance peak at 141 ppm appears due to the existence of enamine carbon atoms. The other sharp signals at 131, 121, and 114 ppm designate TAPT-1 containing different aromatic carbon atoms. The remaining two sp2 carbon atoms of the phloroglucinol moiety give signals at 108 and 101 ppm. As is apparent from Figure 3, there is no characteristic peak observed at ∼192 ppm which would indicate the existence of an aldehyde group, suggesting the full consumption of the PDA molecule during the Schiff base condensation reaction. Spectroscopic Analysis (FTIR and UV−Vis). The FTIR spectral bands of TPDA-1 and starting monomers have been

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RESULTS AND DISCUSSION Nanostructure Analysis. To investigate the crystalline nature of porous polymeric material, powder X-ray diffraction analysis has been carried out; this is shown in Figure 1. A broad

Figure 1. Wide angle powder X-ray diffraction pattern of TPDA-1.

peak is observed at a 2θ value of 5.5°, which indicates very low crystalline nature of the material. Another major broad peak was observed from 23 to 28° with a maximum at 26.7°, indicating the amorphous nature of TPDA-1. This low crystalline nature of the polymer could be attributed to the presence of angle strain between two monomers. TAPT-1 is a C3 symmetric triamine, and PDA is a dialdehyde which is not linear but bent. Therefore, extended hexagonal network formation is geometrically unfavorable. Thus, we observed broad diffraction peaks corresponding to the quasi-ordered state for this porous polymer material in contrast to the related crystalline COFs reported in the literature.38 204

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suggesting the polymeric framework is not composed from the enol form. The absorption peak observed at 1619 cm−1 could be attributed to the formation of α,β-unsaturated ketone in the polymer moiety. For newly formed −C−N groups, the characteristic stretching frequency is observed at 1280 cm−1. The weak signal at 1450 cm−1 appeared due to the presence of CC stretching vibration of the aromatic benzene rings. The two peaks at 1574 and 1372 cm−1 indicate the presence of the triazine ring in Figure 4a,b. The two separated peaks at 3455 and 3323 cm−1 originate from stretching vibrations of primary amine (NH2) groups in as-synthesized TAPT-1. As seen from Figure 4c, the strong adsorption peak at 1644 cm−1 confirms the presence of carbonyl (CO) stretching vibration of the 2,4,6-trihydroxyisophthalaldehyde compound containing aldehyde (CHO) groups. The remaining hydroxyl (−OH) groups of the 2,4,6-trihydroxyisophthalaldehyde moiety are noticeable by 3433 cm−1 stretching vibrations in Figure 4a,c. Moreover, the absence of a distinctive absorption signal at 2235 cm−1 for −CN group suggests the successful synthesis of the TAPT-1 molecule. The UV−visible reflectance spectrum of TPDA-1 in Figure S1 of the Supporting Information showed the broad peak at 550 nm due to the n → π* transition of polymer containing heteroatom.42 Further, another broad absorption peak from 300 to 400 nm could be attributed to the π → π* transition of the conjugated system adjacent to different heteroatoms.43 Furthermore, the band gap has been estimated as 1.6 eV by employing the UV−visible spectrum and this is shown in the inset of Figure S1. Microscopic Analysis. To investigate the structural morphology of TAPT-1, field emission scanning electron microscopy (FE-SEM) has been employed. Figure 5 represents the FE-SEM images of TPDA-1 at two different magnifications. As seen from Figure 5a, spherelike particles are self-assembled to form large agglomerated particles. Further, high resolution transmission electron microscopy (HR-TEM) was used to analyze the nanoscale porosity in the polymer matrix. HR-TEM images of the TPDA-1 polymer are shown in Figure 6, where the presence of micropores is seen throughout the material together with hollow spherelike particle morphology. Thermal Stability and Elemental Analysis. In order to investigate the thermal stability of TPDA-1, TG/DTA has been performed using a temperature ramp of 10 °C/min under air flow in the temperature range 25−800 °C. Figure S2 represents the TG/DTA profile diagram of TPDA-1, where the first weight loss at 98 °C could be attributed to the evaporation of

Figure 3. Solid state 13C CP MAS NMR spectrum of TPDA-1. Polymeric network of TPDA-1 is shown in the inset.

recorded from 4000 to 400 cm−1 to characterize the different organic functional groups. In Figure 4a, the broad peak at 3394

Figure 4. FTIR spectra of TPDA-1 (a) and of TAPT-1 (b) and PDA (c) monomers.

cm−1 and the sharp signal at 1510 cm−1 corroborate the −N−H stretching and −N−H bending vibrations in the polymer matrix. The strong adsorption band at 1578 cm−1 could be assigned to the existence of the CC stretching band for keto−enol tautomerization process.41 However, no characteristic stretching band for imine (CN) group is observed,

Figure 5. FE-SEM images of TPDA-1 at different magnifications (a and b). 205

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Figure 6. HR-TEM images of TPDA-1 at different magnifications (a and b).

mV s−1. From the CV curves, the specific capacitance of TPDA1 was estimated using eq 1, and this is shown in Figure 8.

surface adsorbed moisture, and the second weight loss up to 400 °C is attributed to the decomposition of organic functional groups. The third weight loss from 400 to 800 °C indicates combustion of the residual parts of the sample. Thus, TG/DTA analysis results indicate that TPDA-1 possesses good thermal stability. Electrochemical Study. The electrochemical energy storage performance of the as-synthesized TPDA-1 was measured on a Bio-Logic SP-150 electrochemical workstation using a three-electrode configuration, where a TPDA-1 coated graphite sheet was used as working electrode, Pt wire was used as counter electrode, and a saturated calomel electrode (SCE) was used as reference electrode. The graphite sheet (thickness 0.25 mm) was procured from Nickunj Eximp Entp P Ltd., India. The working electrode was prepared by drop-casting aliquot electrode materials over 1 cm2 graphite sheet. An aliquot was prepared by dispersing 4 mg of material in 1 mL of ethanol with 20 μL of 5% Nafion solution. TPDA-1 was tested as electrode material for the energy storage application by standard cyclic voltammetry (CV), galvanic charge−discharge (GCD), and electrochemical impedance study (EIS). The CV was conducted in 1 M H2SO4 as electrolyte with a potential window of 0−1 V. The corresponding CV curve is presented in Figure 7. The shape of the CV curve indicates EDLC type charge storage behavior of TPDA-1. In order to measure the performance of TPDA-1 in terms of charge storage capability, CV was performed at different scan rates between 2 and 100

Csp =

(∫ I dV )/νmV

(1)

Figure 8. Specific capacitances of TPDA-1 at different scan rates in 1 M H2SO4 electrolyte.

I is the response current, V is the potential window, ν is the potential scan rate, and m is the mass of the active material. The highest capacitance value obtained at 2 mV s−1 scan rate was 469.4 F g−1, which was found to be decreased with increasing the scan rate (Figure 8). This is because of fewer electrolyte ions undergoing diffusion and adsorption on the electrode surface at a higher scan rate.44,45 The high specific capacitance of TPDA-1 could be attributed to the presence of π-conjugated system in it, which facilitates electrical conductivity and ion adsorption.46,47 The electrical conductivity of the as-synthesized TPDA-1 was 0.05 ± 0.002 ohm−1 m−1 at room temperature. Details of the energy storage ability of TPDA-1 were further studied from GCD measurements. The GCD behavior over different load current densities is presented in Figure 9A. The charge−discharge curves exhibit equilateral triangle shapes, suggesting EDLC behavior and a good rate reversibility of the electrode material during the charge−discharge process. The specific capacitance (Csp) from the GCD plots was estimated from the discharge slope using eq 2.

Figure 7. Cyclic voltammograms of TPDA-1 at different scan rates in 1 M H2SO4 electrolyte.

Csp = (I Δt )/(mΔV ) 206

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Figure 9. Galvanostatic charge−discharge plots (A) and specific capacitance at different current densities (B) for TPDA-1 electrode measured in 1 M H2SO4 electrolyte.

I is the load current density, Δt is the discharge time, ΔV is the potential window, and m is the mass of the active materials. In Figure 9B, the estimated specific capacitance of TPDA-1 electrode was plotted as a function applied current densities. At a specific current density of 0.5 A g−1, the estimated specific capacitance was found to be 348 F g−1. TPDA-1 obviously was able to store charge via electrostatic attachment/adsorption at the electrode surface. The obtained specific capacitance was found to be comparable with nitrogen-doped graphitic carbon (255 F g−1 at 2 A g−1),48 porous carbon derived from metal− organic framework (233 F g−1 at 2 A g−1),49 amorphous terephthalonitrile derived nitrogen-rich network (298 F g−1 at 0.2 A g−1),50 benzimidazole grafted graphene (410 F g−1 at 0.4 A g−1),51 and nitrogen-enriched porous carbon sphere (388 F g−1 at 1 A g−1).52 The present result is also comparable with the state-of-the-art nanostructure material MnO2/rGO (242 F g−1 at 1 A g−1),53 TiO2/rGO (255 F g−1 at 0.125 A g−1),54 Fe3O4 nanosheet on one-dimensional carbon nanofibers (135 F g−1 at 0.42 A g−1),55 and Fe3O4 nanoparticles (207 F g−1 at 0.4 A g−1).56 Moreover, compared to benzimidazole-based COP material TpDAB (specific capacitance of 335 F g−1 at 2 mV s−1 scan rate),57 this TPDA-1 displayed a considerable enhancement in supercapacitor performance (specific capacitance to 469.4 F g−1 at 2 mV s−1 scan rate). It is known that the pore size, surface functional groups, and the surface electrolyte accessibility play important roles for the electrical double layer performance. The well-organized pore distribution in stacked 2D layers of COP creates huge interstitial space for free transport of electrolyte ions and provides more surface active sites on TPDA-1 to accommodate charges. Hence EDLC behavior is predominantly the charge storage mechanism for TPDA-1. The cycle stability of TPDA-1 was tested with up to 1000 charge−discharge cycle GCD at a fixed current density of 5 A g−1 as presented in Figure 10. TPDA-1 retains 95% of the capacitance after 1000 cycles, revealing good stability. The interfacial electrochemical properties of TPDA-1 were studied using EIS. The EIS was carried out at a frequency range of 100 kHz−100 mHz as presented in Figure 11. The typical Nyquist plot consists of two distinct regions: (1) a small semicircle in the high to medium frequency region, indicating the charge transport resistance, where the starting cross point indicates the contact resistance, and (2) the straight line in the low frequency region corresponding to the ion diffusion/transport at the electrode−electrolyte interface. The vertical lines with the small semicircle indicate the charge

Figure 10. Cyclic stability, i.e., specific capacitance vs cycle number (up to 1000 cycles) of TPDA-1 electrode as measured from galvanostatic charge−discharge measurement at an applied current density of 5 A g−1.

Figure 11. Nyquist plot for TPDA-1 material in the frequency range 100 kHz−100 mHz in 1 M H2SO4 electrolyte.

transport with good capacitive behavior without any diffusion limitation.52,58,59

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CONCLUSION Our experimental results suggested that a new highly stable porous polymer can be synthesized through the solvothermal 207

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Schiff base condensation reaction between two organic monomers, i.e., 2,4,6-trihydroxyisophthalaldehyde and 1,3,5tris(4-aminophenyl)triazine. This porous polymer material exhibited excellent energy storage capacity with a maximum specific capacitance of 469.4 F g −1 at 2 mV s−1 scan rate. It also provides very good cyclic stability with 95% retention of its initial specific capacitance after 1000 cycles, recommending the future potential of this porous polymer in designing a sustainable energy storage device.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02234. 1 H and 13C NMR of PDA and TAPT, UV−visible spectrum, TG/DTA profile, electrochemical data over TPDA-1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sabuj K. Das: 0000-0003-1284-385X Debabrata Pradhan: 0000-0003-3968-9610 Asim Bhaumik: 0000-0002-4907-7418 Author Contributions

P.B. and S.K.D. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.B. thanks CSIR, New Delhi, for a senior research fellowship. S.K.D. thanks UGC, New Delhi, for a senior research fellowship. A.B. wishes to thank DST, New Delhi, for instrumental facilities through the DST Unit on Nanoscience.

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REFERENCES

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