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Hierarchically Porous Carbon Derived from PolyHIPE for Supercapacitor and Deionization Applications Wei Hu, Feifei Xie, Yuquan Li, Zhengchen Wu, Ke Tian, Miao Wang, Likun Pan, and Lei Li Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03175 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on October 27, 2017

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Hierarchically Porous Carbon Derived from PolyHIPE for Supercapacitor and Deionization Applications Wei Hu a, Feifei Xie a, Yuquan Li b, Zhengchen Wu a, Ke Tian a, Miao Wang b, Likun Pan b and Lei Li* a a

b

College of Materials, Xiamen University, Xiamen 361005, China

Shanghai Key Laboratory of Magnetic Resonance, School of Physics and Materials Science, East China Normal University, Shanghai 200062, China

KEYWORDS: Hierarchically porous carbon, High internal phase emulsions, Activation, Supercapacitor, Capacitive deionization.

ABSTRACT: Hierarchically porous carbon (HPC) materials with interconnected pore texture are produced from porous poly(divinylbenzene) precursor, synthesized by polymerizing high internal

phase

emulsion.

After

carbonation,

the

macroporous

structures

of

the

poly(divinylbenzene) precursor are preserved and enormous micro/mesopores via carbonation with KOH are produced, resulting in interconnected hierarchical pore network. The prepared HPC has a maximum specific surface area of 2189 m2 g-1. The electrode materials for supercapacitor and capacitive deionization (CDI) device employing the formed HPC exhibit high

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specific capacity of 88 mAh g-1 through a voltage range of 1V (319 F g-1 at 1 A g-1) and superior electrosorption capacity of 21.3 mg g-1 in 500 mg L-1 NaCl solution, respectively. The excellent capacitive performance could be ascribed to the combination of high specific surface area and favorable hierarchical porous structure.

INTRODUCTION Electrochemical capacitors (ECs) possessing superior power density and broad capacitive application prospects have attracted considerable attention as one of the most promising energy conversion and storage devices.1-4 Generally, both the kind and the morphology of electrode materials are the key point to determine the performance of ECs.5 According to energy storage mechanism, ECs can be classified into electrochemical double layer capacitors and pseudocapacitors, in which the energies are stored by electrostatic charges accumulated at the electrode/electrolyte interface and reversible redox reactions occurring on the surface of the electrodes, respectively.6-7 During the development of ECs, porous electrode materials with high effective surface area providing abundant interface for charge accumulation and porous structure offering sufficient active sites for electrochemical reactions have been widely investigated.8-9 Among them, porous carbon materials, including activated carbon, carbon aerogel, carbon nanotube and graphene,10-16 are believed to be promising candidates due to their excellent physicochemical stability, good conductivity, high specific surface area and controlled porosity.17 However, most porous-carbon-based ECs are subject to the limitations arising from the high ion-transport resistance and insufficient ionic diffusion within the electrode materials because of their microporous characteristic with a narrow micropore distribution.18-19 To circumvent the intractable disadvantages, hierarchical porous carbon (HPC)-based electrode materials possessing hierarchical structure combining macropores, mesopores and


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micropores are highly desirable.20-21 The macropores can play the role of ion-buffering reservoirs to reduce diffusion distance, the mesopores serve as channels with minimized ion-transport resistance and micropores can provide more sites for charge accumulation. 20, 22-23 Thanks to the high specific surface and optimized pore size distribution, HPC demonstrates a synergistic effect of each level pore during the charge/discharge process to improve its performance.24-25 Therefore, considerable efforts have been devoted to design and develop such carbon materials with hierarchically porous structure. However, the fabrication of HPC usually is time-consuming and high cost, since the preparation of templates is complicated and the removal of templates involves some dangerous chemical reagent.26-27 Therefore, seeking for a simple and efficient technology for HPC is attracting both industrial and academic interests. Porous carbon derived from polymeric precursors has good reproducibility and well-defined pore structure owing to their designable precursors with optional monomers and synthetic methods.20, 28 One potential route to HPC is via the carbonization of high internal phase emulsion (HIPE) -templated porous polymers with tailored porosity.29-30 HIPE refers to emulsion containing an internal phase (or dispersed phase) that occupies over 74% of the total volume of the system.31 Polymerization of monomers in the continuous phase of HIPE followed by the removal of internal phase yields highly porous polymeric network with micron-sized pores, which is known as polyHIPE.32-33 Owing to the inherent macroporous characteristic, polyHIPEs usually exhibit relative low specific surface area (~50 m2 g-1).34 In order to improve the porosity and surface area, more micro-/mesoporous structures should be introduced into the polyHIPE skeleton. Recently, microporous organic polymers (MOPs) with high specific surface area and porosity prepared by radical hyper-crosslinking reaction have been developed.35-36 For example, hyper-cross-linked MOPs based on alternating copolymerization of divinylbenzene (DVB) and


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bismaleimides has been successfully prepared. Similarly, DVB can act as sole monomer, leading to inherently microporous networks with a specific surface area of 989 m2 g-1.37 The cross-linked networks endow the polymer with excellent thermal stability, which is important for high carbonaceous residues after pyrolysis, resulting in the well-preserved porosity of precursor.30, 38 Moreover, interconnected macroporous structures and plentiful microporous structures facilitate the uniform dispersion of carbonization activator into the porous skeleton, which is helpful for the further enhancement of surface and porosity during the sequential carbonation process.39 Herein, we present a novel strategy for the preparation of interconnected HPC templating from a porous polyDVB monolith prepared by polyHIPE. The polymeric precursor was produced via water-in-oil high internal phase emulsion polymerization, without the need of costly template and subsequent etching process. Thanks to the radical hyper-crosslinking reaction and the introduction of porogens, the specific surface area of the polyDVB precursor could achieve up to 711 m2 g-1. Eventually, abundant mesopores and micropores were introduced into the skeleton to form an interconnected and hierarchical network structure during carbonation process with the aid of KOH activation. The obtained HPC has a maximum specific surface area of 2189 m2 g-1 and exhibits excellent electrochemical performances, including high specific capacity of 88 mAh g-1 through a voltage range of 1V (319 F g-1 at 1 A g-1) in the 1 M H2SO4 electrolyte and superior electrosorption capacity of 21.3 mg g−1 in 500 mg L−1 NaCl solution, respectively. EXPERIMENTAL SECTION Materials. DVB was purchased from J&K Scientific Co., Ltd. Toluene, sodium hydroxide (NaOH), sorbitan monooleate (Span 80, CP), calcium chloride (CaCl2) were all purchased from Xilong Scientfic Co., Ltd. KOH, ethanol and acetylene black were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). 2, 2'-azobisisobutyronitrile (AIBN) was


4

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purchased from Tianjin Guangfu Fine Chemical Research Institute. Tetraethylammonium tetrafluoroborate

in

acetonitrile

(TEABF4/AN),

polytetrafluoroethylene

(PTFE)

and

polyvinylidene fluoride (PVDF) was purchased from Sigma-Aldrich Co., Ltd. DVB was washed to remove inhibitor with 5 wt % NaOH aqueous solution and deionized water for three times, respectively. Unless specifically noted, all the chemical reagents were analytical grade and used as received without further purification. Preparation of polyHIPE. During the preparation process, all emulsions were made up of water immiscible DVB/toluene phase with 80% of internal phase volume fraction, stabilized by 6% of Span 80 versus the total volume of the oil phase. AIBN and Span 80 were dispersed in the mixture of organic phase consisting of toluene and DVB with different volume ratio. Under continuous mechanical stirring (300 rpm), an aqueous solution containing CaCl2 (10 g L-1) was added dropwise at a rate of 2 mL min-1. After complete addition of aqueous phase, the emulsion was stirred more vigorously (500 rpm) further 10 minutes. The obtained HIPE was transferred into a 50 mL free standing polypropylene centrifuge (Falcon) tube and then heated at 70 °C for 24 h in a convection oven to initiate polymerization. The resulting PolyHIPE was extensively washed with ethanol in a Soxhlet for 24 h and was dried in a vacuum oven at 80 °C overnight. The obtained polyHIPEs are denoted as polyHIPE-x, where x stands for the volume ratio of toluene/DVB in the range of 0.5/1 to 5/1. Preparation of HPC. Initially, KOH was dissolved in a small amount mixed solution of ethanol and deionized water. The polyHIPE precursor was mixed evenly with the above mixture and then was dried in a vacuum oven at 110 °C overnight. The carbonation process was carried out in a tubular furnace at 700 °C for 2 h with a heating rate of 2 °C min-1 under argon protection. After cooling to room temperature, the samples were thoroughly rinsed with 3 M HCl


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aqueous solution, followed by washing with excess deionized water and then were dried at 80 °C under vacuum. The weight ratio of KOH to precursor was controlled in a range from 0.5 to 3 and the obtained samples were denoted as HPC-x, where x is the mass ratio of KOH to precursor. For comparison, a non-activated carbon material (denoted as DC) was prepared by direct carbonization of the precursor at 700 °C for 2 h under argon atmosphere. Characterization. The Infrared spectra were recorded on a Fourier transform infrared (FT-IR) spectrometer (NICOLET iS10). The morphology of polyHIPE was observed by scanning electron microscopy (SEM) (TM3000, Hitachi) with an accelerating voltage of 15 kV and a working distance of 5 mm. The high magnification SEM was examined by SEM (SU-70, Hitachi) with an accelerating voltage of 10 kV and a working distance of 15 mm. Thermal Gravity Analysis (TGA) was performed on a thermal analyzer (SDT Q600 V20.9 Build 20) under N2 atmosphere with a heating rate of 10 °C min-1. Nitrogen adsorption/desorption isotherms were measured at 77 K using a Micromeritics TriStar II 3020 static volumetric analyzer. The Brunauer–Emmett–Teller surface area was calculated within the relative pressure range of 0.05 to 0.2. The pore size distributions were calculated from the adsorption isotherms by nonlocal density functional theory (NLDFT). Powder X-ray diffraction (XRD) was performed on a Bruker D8 Advance X-ray diffractometer. The Raman spectra were obtained on a Renishaw inVia Raman spectrometer using laser excitation at 532 nm. Elemental analysis was performed by CHNOS Elementar Vario EL III elemental analyzer. X-ray photoelectron spectroscopy (XPS) measurements were performed with an ESCALab 220I-XL system. Electrochemical measurement. For fabrication of working electrode, 70 wt % of HPC sample, 20 wt % of acetylene black and 10 wt % of PTFE were blended with ethanol. The slurry of the above mixture was pressed onto a platinum foil and then the resulting electrode was dried


6

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in oven at 80 °C for 24 h. The mass loading area of the electrode was ~4 mg cm-2 (efficient electrode area: ~0.5 cm2). For a three-electrode system, platinum foil and Hg/Hg2Cl2 electrodes were used as the counter and reference electrodes, respectively. For a two-electrode system in organic electrolyte, the slurry was pasting onto Al foil. The two symmetrical electrodes and a porous film separator were sandwiched in the coin cell (CR2025) under argon atmosphere. Cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) behavior were tested in an electrolyte of 1 M H2SO4, and the potential window was chosen in the range of 0 to 1 V. The electrochemical impedance spectroscopy was measured in a frequency ranging from 100 kHz to 10 mHz with an alternating-current amplitude of 10 mV. All of the electrochemical tests were performed using a CHI 660E instrument (Shanghai Chenhua Apparatus Co. Ltd.) at room temperature. The specific capacitance of the electrode was calculated from the GCD curves according to the following equation: ூ×∆௧

‫ = ܥ‬௠×∆௏ ‫=ܥ‬

ସ×ூ×∆௧ ௠×∆௏

(in three-electrode system) (in two-electrode system)

where ‫ ܥ‬is the specific capacitance (F g-1), ‫ ܫ‬is the discharge current (A), ∆‫ ݐ‬is the discharge time (s), ∆V is the potential window (V), and ݉ is single electrode mass in three-electrode system or total mass of both electrodes in two-electrode system (g). The energy density ‫( ܧ‬Wh kg-1) and power density ܲ (W kg-1) based on electrode materials were calculated according to the following formulas: ‫ ܸ∆ × ܥ‬ଶ ‫=ܧ‬ 8 × 3.6 ܲ=

3600‫ܧ‬ ∆‫ݐ‬


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where ‫ ܥ‬is the specific capacitance measured using two-electrode system, ∆V is the potential window (V) and ∆‫ ݐ‬is the discharge time (s). Electrosorption measurement. Each capacitive deionization (CDI) electrode was prepared by pasting a mixture of HPC sample (80 wt %), acetylene black (10 wt %) and PVDF (10 wt %) onto graphite paper. The mass of each electrode was ~75 mg. Each experiment was carried out in a continuous recycling system of NaCl solution including a unit cell with a flow rate of 100 mL min-1. The volume and temperature of the solution were kept at 100 mL and 298 K, respectively. A direct voltage of 1.2 V was applied on the opposite electrodes. The concentration change of the solution was monitored and measured at the outlet of the unit cell by using a conductivity meter. In our experiment, the electrosorption capacity (߁, mg g-1) was defined as follows: ߁=

(ܿ଴ − ܿ௘ ) × V ݉

where ܿ଴ and ܿ௘ are initial and final NaCl concentrations (mg L-1), V is the volume of NaCl solution (L) and ݉ is the total mass of the electrodes (g). Charge efficiency (߉) was calculated according to the following equation: ߉=

߁×‫ܨ‬ Σ

where ‫ ܨ‬is the Faraday constant (96485 C mol-1), ߁ is the electrosorption capacity (mol g-1) and Σ (charge, C g-1) was obtained by integrating the corresponding current. RESULTS AND DISCUSSION


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Scheme 1. Schematic illustration for the preparation of HPC derived from polyHIPE.

Figure 1. SEM images of (a) polyHIPE-0.5, (b) polyHIPE-1, (c) polyHIPE-2, (d) polyHIPE-3 and (e) polyHIPE-5. As illustrated in scheme 1, the fabrication of HPC mainly involved a strategy using DVBbased polyHIPE as precursor followed by a carbonation process. PolyDVB monolith was produced from the HIPE containing a mixed continuous phase of DVB and toluene. The


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interconnected macroporous structure was obtained by polymerizing the monomer and removing the water. With the aid of KOH activation, abundant micropores and mesopores were created in macroporous skeleton after carbonation. To identify the optimum precursor, various polyHIPEs were prepared by controlling the volume ratio between toluene and DVB. The chemical structure of these polyHIPEs was investigated by FTIR measurement. After polymerization, the intensities of the characteristic bands corresponding to the vinyl, such as the C−H stretching vibrations of =C−H at 3088 cm-1 and the stretching vibrations of C=C at 1630 cm-1 decrease dramatically. In addition, the band at 2925 cm-1 is attributed to the C−H stretching in −CH2−, indicating the formation of methylene in the polyHIPEs (Figure S1).37, 41 These results reveal the successful synthesis of polyHIPEs. The internal morphology of the polyHIPEs was investigated by SEM. As shown in Figure 1, all the samples exhibit typical macroporous structure with spherical cavities in the range of 9−11 µm and many small pores in the polymer walls. A highly interconnected, open-cell porous structure is clearly visible, as expected for polyHIPEs templated from standard surfactant-stabilized HIPEs.42 The similar morphology is attributed to the consistent proportion of oil and water phase in all emulsions.34

Figure 2. (a) N2 adsorption/desorption isotherms and (b) pore size distribution calculated using NLDFT method of the polyHIPEs.


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Table 1. Surface Areas and Pore Properties of PolyHIPEs. Sample

SBETa)

Vtotalb)

Daveragec)

(m2 g-1)

(cm3 g- (nm) 1 )

polyHIPE-0.5 444

0.45

4.0

polyHIPE-1

645

0.90

5.6

polyHIPE-2

668

1.59

9.5

polyHIPE-3

707

2.11

11.9

polyHIPE-5

711

2.34

13.2

a)

Surface area calculated from nitrogen adsorption isotherms at 77.3 K using the BET equation. b) Pore volume calculated from the nitrogen isotherm at P/P0 = 0.99, 77.3 K. c) Average pore size calculated from 4 × (pore volume)/(BET surface area). To further study the influence of toluene/DVB ratio on the specific surface area of polyHIPEs, N2 adsorption/desorption isotherms of polyHIPE samples were measured. As shown in Figure 2a, all polyHIPEs exhibit type II isotherms with a hysteresis loop. A steep N2 uptake at low relative pressure reflects the presence of microporosity. The sharp rise at relatively high pressure (P/P0>0.9) indicates the existence of macropores, which is attributed to the voids and interparticle porosity.43 As indicated in Figure 2b, the pore size distribution analysis based on NLDFT method reveals the presence of primarily macropores with a small microporous contribution. All the pore size distribution curves show a broad distribution spanning tens of nanometers. Determined by SEM observation, N2 adsorption/desorption isotherms and pore size distribution curves, it can be concluded that the obtained polyHIPEs possess hierarchical porosity ranging from micro- to nano-scale with interconnected structures. The BET surface areas and pore structure parameters of the polyHIPEs are summarized in Table 1. The specific surface area of the polyHIPEs ranges from 444 to 711 m2 g-1 and the pore volume increases from 0.45 to 2.34 cm3 g-1 with the increase of volume ratio of toluene/DVB from 0.5 to 5. It should be


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noted that the formed polyHIPEs from sole DVB have much higher specific surface area and pore volume in comparison with the conventional polyHIPE polymers, owing to the combination of highly crosslinked structure and an inert porogen.31,

34

The self-polymerization of rigid

moieties provided by the aromatic group within DVB into crosslinked network structures results in the formation of permanent microporosity.44-45 On the other hand, the addition of toluene as a solvating porogen creates a secondary structure within the developing polymer matrix and the higher level of porogen is more effective in solvating the growing polymeric network during polymerization.46-48 The prepared polyHIPEs turn to be more brittle and even collapsed during the drying process with high toluene volume fraction (Figure S2). Considering the balance between mechanical stability and specific surface area, polyHIPE-1 was selected as the precursor for the following carbonation process. Besides, the polyHIPE-1 also exhibits good thermal stability as revealed by the thermogravimetric analysis under N2 atmosphere (Figure S3). The corresponding temperature of dramatic weight losses is around 450 °C, which is beyond the melting point of KOH (380 °C). The good thermal stability and interconnected hierarchically porous structure is beneficial to uniform molten KOH penetration for sufficient activation.


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Figure 3. SEM images of (a) HPC-0.5, (b) HPC-1, (c) HPC-2 and (d) HPC-3 (inset: high resolution SEM images). The detailed morphology of the prepared HPC samples was investigated by SEM. As shown in Figure 3, all the HPCs exhibit rough surface and preserved cellular macroporous structure with interconnected voids. The average diameter of macropores is in the range of 4−6 µm, with an exception of HPC-3, in which the macropores exhibit partial deformation owing to the excessive KOH etching. A lot of cracks appear on the surface of the HPCs in comparison to the DC sample carbonized without KOH (Figure S4). The formation of the cracks is mainly attribute to the addition of KOH which can etch the carbon framework and even lead to collapse of the pores.4950

The interconnected hierarchically porous structure integrating the micro/mesopores into the


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macropore walls possesses good permeability, delivers more accessible surface for interfacial accumulation of ions, and reduces the diffusion distances of electrolyte ions.20, 25

Figure 4. (a) N2 adsorption/desorption isotherms and (b) pore size distribution calculated using NLDFT method of the HPC samples. Table 2. Surface Areas and Pore Properties of Prepared Samples. SBETa)

Smicrob)

Vtotalc)

Vmicrod)

(m2 g-1)

(m2 g-1)

(cm3 g-1)

(cm3 g-1)

DC

34

-

0.05

0.02

HPC-0.5

1009

961

0.55

0.46

HPC-1

1109

1063

0.61

0.50

HPC-2

2189

1952

1.11

0.96

HPC-3

1575

1471

0.84

0.69

Sample

a)

Surface area calculated from nitrogen adsorption isotherms at 77.3 K using the BET equation. b) Micropore surface area calculated from nitrogen adsorption isotherms at 77.3 K using the t-plot equation. c) Pore volume calculated from the nitrogen isotherm at P/P0 = 0.99, 77.3 K. d) Micropore volume calculated from the nitrogen isotherm at P/P0 = 0.15, 77.3 K using the t-plot equation. The

pore

properties

of

the

prepared

HPC

samples

were

investigated

by

N2

adsorption/desorption measurements. As shown in Figure 4a, all HPC samples show a


14

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combination of type I and type IV isotherms. Rapid saturation at low relative pressure indicates the existence of abundant micropores. The hysteresis loop in the relative pressure region of 0.4

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