Improving the pore-ion size compatibility between poly(ionic liquid)-derived carbons and high-voltage electrolytes for high energy-power supercapacitors

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Improving the pore-ion size compatibility between poly(ionic liquid)derived carbons and high-voltage electrolytes for high energy-power supercapacitors Ling Miaoa, Hui Duana, Zhiwei Wangb, Yaokang Lvc, Wei Xiongd, Dazhang Zhua, Lihua Gana, ⁎ ⁎ Liangchun Lia, , Mingxian Liua,b, a

Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, PR China State Key Laboratory of Pollution Control and Resources Reuse, Shanghai Institute of Pollution Control and Ecological Security, School of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China c College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, PR China d Key Laboratory for Green Chemical Process of Ministry of Education, School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430073, PR China b

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

carbon nanosheets are • N/O-doped derived from a high-carbon-yield poly (ionic liquid).

3D porous architecture is tuned • The via alkali ion exchange and in-situ activation.

pores and ion-transport • Ion-matching paths work along for a maximized capacitance.

size compatibility achieves • Pore-ion high energy density and high-voltage tolerance.

A R T I C LE I N FO

A B S T R A C T

Keywords: Hierarchical porous carbon Poly(ionic liquid) High-voltage electrolyte Pore-ion size compatibility Supercapacitor High energy-power density

Maximizing carbon capacitance in high-voltage electrolytes has gained increasing interests to resolve the low energy storage concern in supercapacitors. Yet the large ion sizes and high viscosity of such electrolytes greatly thwart their compatibility with the pore diameters of carbon electrodes, leading to sluggish charge transport and unsatisfied energy-power outputs. Herein, heteroatom-doped, hierarchical porous carbons are derived from a high-carbon-yield main-chain poly(ionic liquid) bearing NH2+: HSO4− ion pairs and rigid aromatic backbones, followed by tailoring the 3D porous architecture through alkali ion exchange and in-situ activation. The typical sample (PIL-RbC) has sheet-like geometry, electron-rich N/O heterogeneous dopants, and a vast adsorbing surface (3021 m2 g−1). More importantly, PIL-RbC with ion-matching pores (dominated at 0.80 nm) and iontransport paths (> 1 nm pores) shows a superb compatibility with 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid electrolyte, giving a maximized electrode capacitance of 228 F g−1 in a symmetric supercapacitor. The PIL-RbC-based device delivers a high energy density up to 119.4 Wh kg−1 at 397 W kg−1, and maintains 41.7 Wh kg−1 at a high power-output of 19.7 kW kg−1, along with a satisfactory tolerability (91% retention after 10,000 consecutive cycles at 4 V). This strategy sheds light on both synthesizing poly(ionic

⁎ Corresponding authors at: Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, PR China (L. Li and M. Liu). E-mail addresses: [email protected] (L. Li), [email protected] (M. Liu).

https://doi.org/10.1016/j.cej.2019.122945 Received 1 August 2019; Received in revised form 8 September 2019; Accepted 22 September 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Ling Miao, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.122945

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liquid)-derived heteroatom-doped porous carbons and matching well-designed carbon electrodes with highpotential electrolytes for integrated enhancements in supercapacitor performances.

1. Introduction

polymeric chain via multiple choice of cations and anions, thus favorably yielding active sites in the final carbon skeleton [29–31]. Besides, the polymeric state enables a flexible tailoring of chemical structures and morphologies, and meanwhile can crosslink template species to regulate the pore architectures [32,33]. The thermostability of poly (ionic liquid)s strongly depends on the ionic nature and cross-linking moieties, such as cyano groups that can withstand harsh carbonization owing to the polytriazine intermediate network formed by the trimerization reaction under pyrolysis conditions [34–36]. The negligible vapor pressures owing to the charged ion pairs in such poly(ionic liquid)s suppress the mass loss of the precursor at the initial stage of the carbonization, and appending cross-linkable moieties into the cyanobased cationic structures can avoid the deformation of polymeric chains at higher temperatures [37–39]. However, complex and high-cost synthetic steps are generally involved in the preparation of cyanocontaining poly(ionic liquid)s [36,39]. Herein, we develop a facile and efficient fabrication of advanced carbon electrode materials derived from a main-chain poly(ionic liquid), poly(5-carboxybenzene-1,3-diamine disulfate) (p[DABA] [2HSO4]), bearing both NH2+: HSO4− ion pairs to improve thermal stability and rigid aromatic backbones to form a high-crosslinking carbonaceous framework. Taking advantage of the high carbonization yield of p[DABA][2HSO4] and fine regulation of alkali ion exchange/ activation, a model sample (PIL-RbC) with electron-rich N/O heterogeneous dopants and 3D hierarchical porous sheets is obtained. Ionmatching micropores dominated at 0.80 nm and ion-transport paths give a high-compatible PIL-RbC electrode with a high capacitance of 228 F g−1 in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4) electrolyte. Moreover, the PIL-RbC-based device achieves an ultrahigh energy density of 119.4 Wh kg−1 at 397 W kg−1 and still retains 41.7 Wh kg−1 with the increasing power output of 19.7 kW kg−1. The slight capacitance loss of 9% after 10,000 consecutive cycles further demonstrates long cycling lifespan at a high operating voltage of 4 V.

Growing concerns on energy shortage and climate change promote worldwide demands for sustainable, green energy-storage and generation technologies [1-4]. Carbon-based supercapacitors, which store energy through reversible adsorption of ions at the electrode-electrolyte interface, are high-power devices that can be charged/discharged on short time scales, but suffer from delivering a limited amount of energy compared with lithium-ion batteries and fuel cells [5–7]. The energy density (E) of a supercapacitor depends on both the device capacitance (C) and voltage window (V) based on the equation E = 0.5CV2 [8]. As such, maximizing carbon capacitance in a high-voltage electrolyte can be a win-win strategy for high-energy storage. High-voltage electrolytes, such as water-in-salt, organic electrolytes and ionic liquids, are used to resolve water decomposition concern (1.23 V) of conventional aqueous electrolytes [9–11]. However, these electrolytes with high viscosity and large ion size cannot match well with porous carbon electrodes in which high-level micropores are required for achieving high capacitance, leading to insufficient energy-power performances because of limited charge-transport dynamics [12,13]. Therefore, improving the pore-ion size compatibility between carbon electrodes and high-voltage electrolytes for a maximized capacitance can balance on the teeterboard between high-energy storage and high-power output. Fine design of pore structure and surface property of carbons is an effective way to fabricate a high-capacitance electrode [14–17]. Carbon capacitance can be intuitively interpreted as the amount of electrolyteion species adsorbed on the electrode surface, and the high-rate capacitance is also dependent on transport of the ions to/from the surface [13]. The milestone discovery in the capacitance increase was realized at micropore sizes less than 1 nm owing to the denser packing of the desolvation shell within the pores, along with a further finding of a maximized capacitance when matching the micropore diameter with the electrolyte ion size [18,19]. Larger pores can decrease diffusion distance and interface resistance of electrolyte ions as easy ion-transport channels to the fine micropores [20,21]. Besides, the introduction of electron-donor heteroatoms into carbons endows a dense positive charge contribution of adjacent carbon atoms to form a polarized electrode surface for highly efficient electron and ion transport, and also provides additional pseudocapacitance in acid/basic electrolytes [6,15,22]. Given all of the above, a high-compatible carbon electrode with a high-voltage electrolyte for an advanced supercapacitor should integrate the following requirements of: (1) ion-matching spaces to accommodate plentiful of large electrolyte ions for high-energy storage, (2) adequate ion-transport paths to ensure fast diffusion of viscous electrolytes to/from the electrode surface for outstanding high-rate capacitance and power property, (3) ion-accessible wetting surface to enhance the interaction between electrolyte ions and active sites on the electrode surface for improved ion transport, and (4) continuous conductive skeleton to provide an efficient electron-transport path for long high-voltage lifespan [23–27]. Consequently, designing advanced carbon electrode with heteroatom modification and hierarchical porous architecture is highly desired to overcome the drawbacks of high-voltage electrolyte including high viscosity and large ion size, but it still remains challenging to develop a facile and efficient synthesis approach. Poly(ionic liquid)s, generally formed by the polymerization of monomeric ionic liquids, have emerged as attractive carbon precursors that combine the polymeric chain and ionic liquid moieties as repeating units [28]. Poly(ionic liquid)s inherit the molecular designability of ionic liquids to introduce diverse heteroatoms such as N, S, B into the

2. Experimental section 2.1. Synthesis In a typical synthesis, 3.043 g 5-carboxybenzene-1,3-diamine (DABA) in 30 mL N, N-dimethylformamide is dropwise into 20 mL diluted H2SO4 (16 wt%), and stirred for 1 h to form the ionic liquid [DABA][2HSO4]. Then, 13.692 g ammonium persulfate (APS) in 20 mL water is added and the mixture is stirred at room temperature for 24 h. After filtration and washing, the vacuum dried poly(ionic liquid) p [DABA][2HSO4] is mixed with alkali hydroxide (ROH, R = Li, Na, K, Rb, Cs) in water at a mass ratio of 1:1.5, and then freeze-dried to constant weight. The mixture (R@p[DABA][2HSO4]) is carbonized at 800 °C (10 °C min−1) for 2 h under N2 atmosphere. After washing with diluted HCl and water, the dried product is denoted as PIL-RC. For a comparison, p[DABA][2HSO4] is direct carbonized (without alkali activation) and the resultant sample is denoted as PIL-C. 2.2. Characterization The AVANCE III HD (600 MHz) nuclear magnetic resonance spectrometer is utilized to record the 1HNMR and CNMR spectra with DMSO‑d6 as solvent. Fourier-transform infrared spectroscopy (FT-IR) is recorded on a Thermo Scientific Nicolet iS10 FT-IR spectrometer. Element compositions and binding states of p[DABA][2HSO4] and carbon samples are recorded by an X-ray photoelectron spectrometer 2

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(XPS, AXIS Ultra DLD) using Mg Kα radiation. A Hitachi S-4800 scanning electron microscope (SEM) is employed to obtain SEM images and X-ray energy dispersive spectroscopy (EDS) mapping spectra. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements are conducted with the heating program from 25 to 900 °C using a STA409 thermogravimetric analyzer. Nitrogen sorption experiments are conducted with a Micromeritics ASAP 2460 instrument at −196 °C. The Brunauer–Emmett–Teller (BET) method is employed to calculate the specific surface area, and a Dubinin–Radushkevich plot is used to obtain the total pore volume. The morphology of the products is further characterized by a transmission electron microscope (TEM, JEM-2100). The wetting ability is tested on a contact angle analysis instrument (KRÜSS GmbH FM40Mk2). The Raman microscope (Renishaw Invia) is employed to collect the graphitization degree of carbons with a 514 nm laser. X-ray diffraction (XRD, D8 advance, Germany) patterns are obtained in the range of 10–80° with a monochromatic Cu Kα radiation source.

C=

4 I× Δt V×m

(1)

E=

1 Ccell V 2 2 × 3.6

(2)

P=

3600 × E Δt

(3)

where Δt, I, m and V are the discharge time (s), the discharge current (A), the total weight of electroactive materials in two electrodes (g), and the voltage window during the discharge process (V), respectively. 3. Results and discussion Fig. 1 depicts the schematic synthesis of PIL-RCs that takes advantage of the high carbonization yield of the main-chain p[DABA] [2HSO4] and tunable alkali ion exchange/activation. Typically, the protic ionic liquid [DABA][2HSO4] (Fig. S1a) is selected as the monomer because of its easy preparation via the simple neutralization of the protic acid and N-containing base (Fig. 1, step A). With the addition of the initiator APS, the main-chain poly(ionic liquid) p[DABA] [2HSO4] (Fig. S1b) is subsequently obtained by the oxidative polymerization reaction of the monomeric [DABA][2HSO4] at room temperature (Fig. 1, step B). The oxidation state of p[DABA][2HSO4] further investigated by FT-IR and XPS measurements indicates that the polymeric chain mainly consists of benzenoid amine-like units: the two peaks at 1445 and 1574 cm−1 in FT-IR spectrum (Fig. 2a) are assigned to the stretching mode of benzenoid amine-like units (with the CeN stretching vibration at 1196 cm−1) and quinoid imine-like units, and the XPS result in Fig. 2b supports the higher molar content of the eNHe in benzenoid amine-like units (400.2 eV, 59 mol%) than that of the neutral eN] in quinoid imine-like units (399.2 eV, 41 mol%) [40,41]. After that, the alkali ion exchange occurs between the eCOOH on the p [DABA][2HSO4] chain and ROH solution to form the dispersed eCOOR functional groups in the R@p[DABA][2HSO4] skeleton (Fig. 1, step C). Ultimately, the freeze-dried R@p[DABA][2HSO4] is subjected to direct activation under inert atmosphere, followed by the removal of metal residues (Fig. 1, step D). As confirmed by EDS-mapping, the uniform alkali distribution in R@p[DABA][2HSO4] (Fig. 2c) promotes

2.3. Electrochemical measurements The carbon samples are mixed with carbon black and polytetrafluoroethylene binder at a mass ratio of 8:1:1 in ethanol. The mixture film is cut into an electrode disk with the diameter of 8 mm and the mass of ~3.5 mg, dried at 100 °C overnight and then pressed on the stainless-steel current collector. The supercapacitor device is assembled by facing two as-obtained electrodes separated with a Celgard 3501 polypropylene membrane in a glove box with Ar atmosphere (H2O and O2 < 1 ppm) using 80 μL of 7 m Li-TFSI (7 mol kg−1 lithium bis(trifluoromethane sulfonyl)imide in deionized water), 1 M TEA-BF4/ACN (1 mol L−1 tetraethylammonium tetrafluoroborate in deuterated acetonitrile) and neat EMI-BF4 (1-ethyl-3-methylimidazolium tetrafluoroborate) electrolyte. Electrochemical performances are evaluated on a CHI660E electrochemical analyzer, involving cyclic voltammetry (CV), gravimetric charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements. The electrode capacitance (C, F g−1), energy density (E, Wh kg−1) and power density (P, W kg−1) are calculated from the discharge branch of GCD curves based on the following Eqs:

Fig. 1. Schematic synthesis of PIL-RCs. 3

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Fig. 2. FT-IR spectrum (a) and high-resolution N 1 s XPS spectrum (b) of p[DABA][2HSO4]. EDS mapping of the R element in R@p[DABA][2HSO4] (c). TGA curves of DABA, p[DABA][2HSO4] and R@p[DABA][2HSO4] (the inset is the DSC curve of R@p[DABA][2HSO4]) (d).

Fig. 3. SEM images of PIL-C (a), PIL-LiC (b), PIL-NaC (c), PIL-KC (d), PIL-RbC (e), PIL-CsC (f). 4

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homogeneous and efficient in-situ activation effect. The carbonization process of p[DABA][2HSO4] and R@p[DABA] [2HSO4] is monitored by TGA in Fig. 2d. For the main-chain p[DABA] [2HSO4], the sharply reduced weight in the range of 232–291 °C and 443–560 °C result from the release of volatile fragments in [HSO4] anions and the decomposition of benzenoid amine-like units in the p [DABA][2HSO4] chain respectively. In contrast, the parent amine DABA was completely evaporated at 411 °C. The strong protonation from H2SO4 to DABA significantly increases the thermostability of its volatile parent amine to withstand harsh carbonization. The high carbonization yield of 59.5 wt% (800 °C) for p[DABA][2HSO4] is attributed to its unique structure bearing both heteroatom-containing ion pairs (NH2+: HSO4−) to improve thermal stability and rigid aromatic backbones to serve as cross-linking moieties for carbon skeleton [39]. More importantly, such high-yield p[DABA][2HSO4] delivers a high flexibility in porous architecture optimization as R@p[DABA][2HSO4] still owns a high carbonization yield of 56.5 wt% at 800 °C. The DSC curve (Fig. 2d inset) and XRD patterns (Fig. S2a) reveal different stages in the alkali activation process. The eCOOR functional groups in R@p [DABA][2HSO4] primarily decompose into R2CO3 with the release of volatile H2O and CO2 below 400 °C [42]. R2CO3 is further decomposed to R2O in the temperature range of 492–705 °C along with subsequent reactions between R2O/R2CO3/CO2 and C [43]. The produced alkali (R2O + C → 2R + CO) begins to severely etch framework carbon atoms for widening interlayer spacings, and R species intercalate between carbon layers for thinning carbon sheets [44–46]. The interlayer spacing and the intercalated R thickness depend on alkali ion size and activation conditions (amount/temperature), giving rise to hierarchical porous architectures in PIL-RCs via nanospace occupation and swelling effect. To solve the inaccessibility of small micropores to large electrolyte ions, the varying ROH activators are employed to obtain enlarged micropores (Pmatching) matching with the electrolyte-ion size (generally < 1 nm) in PIL-RCs. As observed by SEM images, the non-activated PIL-C consists of microscale blocks with no obvious pores on the smooth surface (Fig. 3a), while alkali activation results in a transformed morphology (Fig. 3b–f) and a porous network. The pore structure and parameters of PIL-RCs are shown in Fig. 4 and Table 1. All the N2 sorption isotherms of PIL-RCs (Fig. 4a) present the characteristic of typical I curves with a narrow knee at P/P0 < 0.05, indicating abundant micropores in PIL-RCs [47-49]. The sharp rise at P/P0 = 0.9–1 for PIL-NaC can be interpreted as large quantity of macropores owing to the swelling effect of Na species [50]. Alkali activation involves the primary decomposition of eCOOR to R2O and subsequent continuous reactions between R2O/R2CO3/CO2 and C [51,52]. The removal of carbon atoms generates developed micropores which contribute to the obvious gain of overall surface area and total pore volume (from 468 to 3021 m2 g−1, 0.21 to 1.59 cm3 g−1, respectively) in Table 1. As shown in Fig. 4b and Table 1, the non-activated PIL-C owns a single diameter of Pmatching at 0.50 nm, along with insufficient pores related with ion-

Table 1 Pore structure parameters of PIL-RCs and PIL-C.a Samples

SBET (m2 g−1)

Pmatching (nm)

Ptransport (nm)

Vtotal (cm3 g−1)

Vtransport (cm3 g−1)

PIL-LiC PIL-NaC PIL-KC PIL-RbC PIL-CsC PIL-C

1603 1969 2885 3021 1887 468

0.50, 0.80 0.50, 0.65 0.50, 0.69 0.50, 0.80 0.54, 0.80 0.50

1.18, 1.18, 1.18, 1.18, 1.18 1.01

0.95 1.31 1.44 1.59 0.65 0.21

0.73 1.03 1.19 1.33 0.25 0.07

2.73 2.00 2.16 2.16

(77%) (79%) (83%) (84%) (38%) (23%)

a

SBET, specific surface area; Pmatching, pores smaller than 1 nm, the dominant one in bold type; Ptransport, pores larger than 1 nm; Vtotal, total pore volume; Vtransport, volume of pores larger than 1 nm.

transport dynamics (Ptransport, > 1 nm). Owing to the pore generation and enlarging effect by alkali activation from LiOH to RbOH [43,53], PIL-RCs exhibit the dual-level Pmatching of 0.50 and 0.65–0.80 nm, coupled with considerable Ptransport of 1.18 and 2.00–2.73 nm; the two sizes of Pmatching are generated by the decomposition and removal of dispersed eCOOR functional groups in R@p[DABA][2HSO4], and the larger Ptransport are further formed by the aggregation of the remaining R species over higher temperature. PIL-RbC with the largest surface area of 3021 m2 g−1 owns two size of Pmatching at 0.50 and 0.80 nm, and a high Vransport of 84%. However, CsOH activation leads to the significantly increased primary Pmatching peaked at 0.54 nm, coupled with the minor presence of secondary Pmatching (0.80 nm) and Ptransport (1.18 nm). Owing to the strong basicity of CsOH, the complete ion-exchange between eCOOH and CsOH enables the molecular-level dispersion of Cs+ and thus a uniform distribution of primary Pmatching (i.e., 0.54 nm), and unreacted CsOH is not reserved for further formation of enough Ptransport, as shown in Fig. 3f. Consequently, the continuous transformation of dispersed eCOOR functional groups in R@p[DABA] [2HSO4] promotes the in-situ occupation effect of R fillers into the carbon framework, yielding the corresponding emptied nanospace after acid washing [42,54]. With the increasing size of alkali ions (i.e., R fillers), the dominant diameter of Pmatching (i.e., ion-matching spaces) can be tuned in the range of 0.50–0.80 nm to accommodate large electrolyte ions (Fig. 4c). The optimization of ion-transport porous architecture is further conducted by regulating the activation amounts (RbOH/p[DABA] [2HSO4] ratio of 0.5–2) and temperatures (700–900 °C). The PIL-RbCs activated under different conditions are denoted as PIL-RbCx, y, where x and y represent the activation amount and temperature, respectively. As shown in Fig. S3, all the PIL-RbCs exhibit a sheet-like morphology, which is assigned to the inward/outward carbon etching by alkali species both in the bulk and on the surface of R@p[DABA][2HSO4]. With the RbOH/p[DABA][2HSO4] ratio increasing from 0.5 to 2, more unreacted RbOH are gradually involved in the aggregation and intercalation into the PIL-RbCs over higher temperature, evidenced by the larger macropores and thinner sheets in the final carbons (Figs. S3a–c,

Fig. 4. Nitrogen sorption isotherms (a) and pore size distribution curves (b) of PIL-RCs and PIL-C. The trendies of alkali ion sizes and dominant Pmatching diameters for PIL-RCs (c). 5

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of carbon electrode to electrolyte with similar polarity, thus favoring the wetting ability of carbon materials and the efficiency of ion transportation [57,58]. The electron-rich species in heteroatoms account for high electrode conductivity owing to more delocalized electrons within the conjugated carbon skeleton [15,59]. In the wettability measurements (Fig. 5e), the electrode interface of PIL-RCs (except PIL-CsC) can be easily wetted by EMI-BF4 ionic liquid electrolyte with low contact angles of 53–74°. In the XRD patterns (Fig. S2b), two broad peaks at 23 and 44° are related with the (0 0 2) and (0 0 1) diffraction modes, characteristic of the disordered carbon materials [60]. That can be further demonstrated by the Raman spectra (Fig. S6a), which show the typical D band and G band at 1351 and 1583 cm−1 attributed to the defects of disordered carbon atoms in the hexagonal ring and the vibrations of sp2 carbon atoms in the graphitic lattice [61]. Fig. S6a are deconvoluted into Gaussian shapes using four bands at 1220 (I), 1351 (D), 1487 (D′) and 1583 (G) cm−1, respectively (Figs. S6b–f) [62]. The peak area ratio of two bands (ID/IG) in PIL-RCs is an evaluation criterion for the structural order level, which is 1.20–1.33, indicating a similar crystallinity degree [62,63]. The same temperature-programmed activation of PIL-RCs triggers a series of decompostion reactions from eCOOR into R species in the analogous temperature range, contributing to the similar structural disorder to the hexagonal rings within graphene layers (i.e., ID/IG). To evaluate electrochemical properties of PIL-RCs electrodes, symmetric devices are assembled and tested using 7 m Li-TFSI water-in-salt electrolyte at 2.2 V and 1 M TEA-BF4/ACN organic electrolyte at 2.8 V. Rectangular-like CV curves tested in different voltage windows at 10 mV s−1 (Fig. S7a) are representative of double layer capacitance because an ingenious electrolyte–electrode interphase is formed between carbon surface and high-concentration Li-TFSI layer to block water electrolysis of conventional aqueous supercapacitors beyond 1.23 V [12,64]. Based on the integrated CV areas in Fig. 6a, PIL-RbC electrode exhibits the highest capacitance/energy storage. Capacitance performances of all the carbon electrodes are further studied from GCD measurements in Fig. S7b. At 0.2 A g−1 and in 7 m Li-TFSI, the calculated capacitances based on the discharging branches are listed in descending order: PIL-RbC (189 F g−1) > PIL-KC (149 F g−1) > PIL-

e). As shown in Fig. S4a and Table S1, increasing RbOH loading contributes to the enhanced surface area of 1734–3021 m2 g−1 and total pore volume of 0.58–1.59 cm3 g−1 for micropore-dominated PILRbC800 materials. The highest content of Ptransport reaches 84% under the optimized RbOH/p[DABA][2HSO4] ratio of 1.5 (Fig. S4b and Table S1), and PIL-RbC800 samples prepared by insufficient or excess activation amount exhibit lower content of Ptransport (36% for PIL-RbC0.5, 800, 77% for PIL-RbC1.0, 800 and 70% for PIL-RbC2.0, 800). The role of R intercalation between carbon layers is also demonstrated by increasing activation temperature from 700 to 900 °C. Alkali activation at higher temperature can promote the redox reaction between R2O and C [46], giving thicker Rb intercalants and thus thinner sheets (Figs. S3d–f). As shown in Fig. S4c and Table S1, the optimized activation temperature of 800 °C can both dredge the island tunnels within the thick sheets of PILRbC1.5, 700, and remedy the poor interconnectivity between ionmatching spaces and larger pores that is missing within the thin sheets of PIL-RbC1.5, 900. Therefore, increasing activation amounts and temperatures are responsible for enlarging the pore diameter larger than 1 nm and thinning carbon sheets owing to the swelling effect of the aggregated R species. The optimized ion-transport porous architecture is constructed with abundant Ptransport (the 84% ratio in volume) nested on macroporous sheets (with the pore diameter of 150–300 nm, Fig. S5). The surface states of PIL-RCs that can affect the wetting ability and electrical conductivity are investigated via XPS. All the samples are mainly consisted of C, N, and O elements (Fig. 5a), and the slight presence of S (0.12–0.53 at.%) inherited from [HSO4] anions (Table S2). The high-resolution XPS spectrum of C 1 s is curve-fitted to three types of C species including sp2-bonded C (CeC, 284.6 eV), C single bonded with heteroatoms (CeN/CeO, 285.6 eV) and C double bonded with O in functional groups (C]O, 287.3 eV) (Fig. 5b) [55]. In the case of N species, three peaks at the binding energies of 398.4, 400.5 and 401.0 eV are assigned to pyridinic N (N-6), pyrrolic N (N-5) and graphitic N (N-Q), respectively (Fig. 5c) [26]. For O species, the curvefitted spectrum contains C]O, OeCeO and O]CeO bonds at the binding energies of 531.2, 532.3, and 533.5 eV (Fig. 5d) [56]. N/O species bonded with adjacent carbon atoms improve the surface affinity

Fig. 5. XPS spectra of PIL-RCs (a). High-resolution XPS spectra of PIL-RbC: C 1 s (b), N 1 s (c) and O 1 s (d). EMI-BF4 contact angles of PIL-RCs and the activated carbon (AC) (e). 6

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Fig. 6. Electrochemical properties using 7 m Li-TFSI water-in-salt electrolyte at 2.2 V: CV curves of PIL-RCs and PIL-C electrodes at 10 mV s−1 (a), GCD curves of PILRbC electrode (b) and Nyquist plots of PIL-RCs electrodes (c). Electrochemical properties using 1 M TEA-BF4/ACN organic electrolyte at 2.8 V: CV curves of PIL-RbC electrode tested in different voltage windows at 10 mV s−1 (d) and the relationship between capacitance and current density of PIL-RCs and PIL-C electrodes (e). Ragone plots of PIL-RbC-based devices (f).

NaC (127 F g−1) > PIL-LiC (122 F g−1) > PIL-CsC (80 F g−1) > PILC (41 F g−1), respectively. Additionally, PIL-RbC electrode finish a charging-discharging process within 14 s at 10 A g−1, and preserve 68% of its capacitance (i.e., 129 F g−1), demonstrating ideal ion-absorption behaviors at high rates (Fig. 6b) [65]. EIS measurements are further conducted to detect electrochemical behaviors in frequency range of 0.01–106 Hz. As presented in Fig. 6c and Fig. S8, the intrinsic ohmic resistances (Rs) are only 1.19–1.94 Ω for all the PIL-RCs electrodes, and the stepped slopes at the intermediate-frequency region (expect for PILCsC) indicate rapid ionic migration within accessible porous architecture, effectively contributing to high-rate capacitance and then power properties [66]. The high-frequency semicircle indicates the smallest charge transfer resistance of 2.76 Ω for PIL-RbC electrode, demonstrating satisfactory electronic conductivity improved by electron-rich heterogeneous species [67]. TEA-BF4 is a common organic electrolyte used to extend voltage windows in pursuit of high-energy device (Fig. 6d). At 0.2 A g−1 and in 1 M TEA-BF4/ACN, the calculated capacitances are listed in descending order: PIL-RbC (168 F g−1) > PIL-KC (154 F g−1) > PIL-NaC (101 F g−1) > PIL-LiC (97 F g−1) > PIL-CsC (68 F g−1) > PIL-C (37 F g−1), in agreement with CV results (Figs. S7c, d). The relationship between specific capacitance and current density of PIL-RCs and PIL-C electrodes is plotted in Fig. 6e. Ragone plots reveals the remarkable energy density of 45.7 Wh kg−1 with the corresponding power density of 276 W kg−1 (0.2 A g−1) for TEA-BF4/ACN-based device, outperforming that of Li-TFSI-based device (31.8 Wh kg−1, 219 W kg−1) (Fig. 6f). The comparison between capacitive performances of various carbon electrodes in aqueous and nonaqueous electrolytes is summarized in Table S3. The electrochemical properties of PIL-RCs electrodes are further tested in symmetric devices using EMI-BF4 ionic liquid electrolyte at 3.5 V. PIL-RCs-based device possesses a higher operating voltage of 3.5 V in EMI-BF4 than those in Li-TFSI and TEA-BF4 because EMI+/ BF4− at a larger size can withstand higher cathodic/anodic potential respectively to extend the device voltage (Fig. 7a) [9,68]. At 0.2 A g−1 and in neat EMI-BF4, specific capacitances are calculated to be 165, 170, 194, 228, 101 and 56 F g−1 for PIL-LiC, PIL-NaC, PIL-KC, PIL-RbC,

PIL-CsC and PIL-C, respectively (Fig. 7b). Additionally, PIL-RbC electrode delivers excellent rate capability with a fast discharge time of 13 s at 10 A g−1 and the capacitance loss of 33% (Figs. S9a, b). Based on the above results tested in three high-voltage electrolytes, the electrode capacitance trend (i.e., PIL-RbC > PIL-KC > PIL-NaC > PIL-LiC > PIL-CsC > PIL-C) can be explained by the relationship between the dominant Pmatching diameter and electrolyte ion size (Fig. 7c). Among PIL-RCs electrodes activated by different alkali ions, the highest content of Pmatching dominated at 0.80 nm in PIL-RbC not only serve as ionmatching spaces to accommodate large electrolyte ions (TFSI−: 0.79 nm, EMI+: 0.76 nm) to make full use of electrode surface area, but also minimize the distance (the double-layer thickness) between electrode surface and adsorbed ions, thus delivering the highest capacitance [23,69]. According to C = εA/d, the smaller dominant Pmatching in other PIL-RCs limit plenty of electrolyte ions for the formation of electrical double layers, while the wider dominant Pmatching decrease the utilization of a vast electrode surface and thus lead to comparable capacitances of PIL-RbC and PIL-KC (Pmatching dominated at 0.69 nm) in TEABF4/ACN (TEA+: 0.67 nm) [19,70]. Among three high-voltage electrolytes, the higher capacitance of all the PIL-RCs electrodes in neat EMI-BF4 can be explained by the presence of primary Pmatching centered at 0.50–0.54 nm. These primary Pmatching whose diameter is close to the BF4− size of 0.48 nm, can lower the strong cation-anion interaction by dissociating part of anions from ion pairs, and then facilitate EMI+ diffusion into the secondary Pmatching (0.80 nm in PIL-RbC) [14,71]. The pore-ion size compatibility of PIL-RbC electrodes with the EMIBF4 electrolyte is optimized by adjusting activation amounts and temperatures to ensure efficient ion transport, and demonstrated by electrochemical measurements tested in EMI-BF4 at 4 V. Typical CV curves of the double-layer capacitor with rectangular shape are obtained at the operating voltage up to 4 V (Fig. 7d). At 0.2 A g−1, specific capacitances are calculated to be 103, 203, 215, 191, 123 and 174 F g−1 for PILRbC0.5, 800, PIL-RbC1.0, 800, PIL-RbC1.5, 800, PIL-RbC2.0, 800, PIL-RbC1.5, 700 and PIL-RbC1.5, 900, respectively (Fig. 7e). The calculated capacitances of PIL-RbC1.5, 800 are 215, 195, 182, 169, 151 and 135 F g−1 at the current densities of 0.2, 0.5, 1, 2, 5 and 10 A g−1, respectively 7

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Fig. 7. Electrochemical properties using EMI-BF4 ionic liquid electrolyte at 3.5 V: CV curves of PIL-RCs-based devices at 10 mV s−1 (a) and GCD curves of PIL-RCs and PIL-C electrodes at 0.2 A g−1 (b). The trendies of the dominant ion-matching micropore diameter and capacitance in high-voltage electrolytes for PIL-RCs electrodes (c). Electrochemical properties using EMIBF4 ionic liquid electrolyte at 4 V: CV curves of the PIL-RbC1.5, 800-based device tested in different voltage windows at 10 mV s−1 (d), GCD curves of PIL-RbCs electrodes at 0.2 A g−1 (e), the relationship between specific capacitance and current density of PIL-RbCs electrodes (f), GCD curves of PIL-RbC1.5, 800 electrode (g), Ragone plots of PIL-RbCs-based devices (h) and cycling stability and Coulombic efficiencies of the PIL-RbC1.5, 800-based device at 1 A g−1 over 10,000 cycles (i).

(Fig. 7f). A fast charging-discharging process finished within 27 s at 10 A g−1 and the corresponding capacitance retention of 63% demonstrate excellent rate capability of PIL-RbC1.5, 800, compared with PIL-RbC0.5, −1 , 47% retention), PIL-RbC1.0, 800 (124 F g−1, 61% reten800 (48 F g tion), PIL-RbC2.0, 800 (111 F g−1, 58% retention), PIL-RbC1.5, 700 (64 F g−1, 52% retention) and PIL-RbC1.5, 900 (92 F g−1, 53% retention) (Fig. 7g and Fig. S9c). The relationship between energy density and power density of PIL-RbCs-based device is plotted in Ragone plots (Fig. 7h). The PIL-RbC1.5, 800-based device delivers a high energy density of 119.4 Wh kg−1 with the corresponding power density of 397 W kg−1 (0.2 A g−1) and maintain 41.7 Wh kg−1 at 19.7 kW kg−1 (10 A g−1). After 10,000 consecutive cycles at 1 A g−1, the PIL-RbC1.5, 800-based device displays long cycling life with 91.1% capacitance retention and 90.7% Coulombic efficiency (Fig. 7i). To sum up, the high pore-ion size compatibility between poly(ionic liquid)-derived PIL-RbC electrode and the high-voltage EMI-BF4 electrolyte contributes to the large energy storage, high power output and long high-voltage tolerability. First, multi-level ion-matching spaces centered at 0.50 and 0.80 nm maximize the utilization of the large surface area (3021 m2 g−1) by increasing the amount of large BF4− and EMI+ closely adsorbed on the electrode for the establishment of electric double layers [19,45]. Second, hierarchical ion-transport paths provide efficient highways to the fine ion-matching spaces for reversible and fast ion adsorption [72–74]. Third, 3D macroporous sheet-like

morphology can alleviate stacking and curling of ultrathin sheets, reduce transportation route and function as ion buffer reservoirs, which increases diffusion coefficient for immediate access of electrolyte to the electrode surface [75,76]. Fourth, the ion-accessible electrode surface with a small contact angle of 53° modified by heteroatom doping improves interfacial affinity with EMI-BF4, thus enhancing effective surface area and the efficiency of ion transfer [77]. Finally, a robust and continuous conductive skeleton derived from the main-chain p[DABA] [2HSO4] as well as electron-rich species in N/O heteroatoms are responsible for high electrical conductivity and satisfactory high-voltage tolerability [59,78].

4. Conclusion In conclusion, a novel main-chain poly(ionic liquid) p[DABA] [2HSO4] bearing heteroatom-containing ion pairs on the benzenoid amine-like polymeric chain is employed as carbon precursor with a high carbon residual of 59.5 wt%. Through tailored alkali ion exchange/ activation, the optimized sample integrates the characteristics of ionmatching spaces (dominated at 0.80 nm), ion-transport paths (the 84% volume ratio), macroporous sheet skeleton (with the pore diameter of 150–300 nm) and electron-rich N/O heterogeneous dopants. These overwhelming structural properties guarantee a maximized adsorbing surface and highly efficient paths for charge (ion and electron) 8

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transport, enabling a superb pore-ion size compatibility of the resultant electrode with a maximized capacitance of 228 F g−1 in an EMI-BF4based symmetric supercapacitor. Furthermore, the assembled device reaches a high energy density of 119.4 Wh kg−1 at 397 W kg−1, and maintains 41.7 Wh kg−1 with the corresponding power density of 19.7 kW kg−1, along with satisfactory high-voltage tolerability (91% retention after 10,000 consecutive cycles at 4 V). These results not only advance the facile and efficient concept to develop heteroatom-doped porous nanocarbons derived from high-carbonization-yield poly(ionic liquid)s, but also resolve the disharmonious energy-power density concerns by matching well-designed carbon electrodes with electrolytes with wide operating voltage windows.

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