Acid-assisted strategy combined with KOH activation to efficiently

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Acid-assisted strategy combined with KOH activation to efficiently optimize carbon architectures from green copolymer adhesive for solid-state supercapacitors Yanyan Lu, Nannan Chen, Zhengyu Bai, Hongyu Mi, Chenchen Ji, and Luyi Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03377 • Publication Date (Web): 13 Sep 2018 Downloaded from http://pubs.acs.org on September 17, 2018

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ACS Sustainable Chemistry & Engineering

Acid-assisted strategy combined with KOH activation to efficiently optimize carbon architectures from green copolymer adhesive for solid-state supercapacitors Yanyan Lu,a Nannan Chen,a Zhengyu Bai,b Hongyu Mi,*a Chenchen Ji,a Luyi Sun*c a

Xinjiang Uygur Autonomous Region Key Laboratory of Coal Clean Conversion and Chemical

Engineering Process, School of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China b

School of Chemistry and Chemical Engineering, Key Laboratory of Green Chemical Media and

Reactions, Ministry of Education, Henan Normal University, Xinxiang 453007, China c

Department of Chemical & Biomolecular Engineering and Polymer Program, Institute of

Materials Science, University of Connecticut, Storrs, CT 06269-3136, USA

ACS Paragon Plus Environment

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Author for correspondence: Hongyu Mi Address: Xinjiang University, School of Chemistry and Chemical Engineering, Shengli Road 666, Urumqi, China E-mail: [email protected].

Luyi Sun Address: University of Connecticut, Department of Chemical & Biomolecular Engineering and Polymer Program, Storrs, CT 06269-3136, USA E-mail: [email protected].


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ABSTRACT: Portable carbon-based solid-state supercapacitor (SC), together with ideal performance and cost-effectiveness, is currently drawing extensive attention, of which the carbon electrode with tuned structure and properties is the key component. The present work opens up an innovated avenue to full utilization of novel acrylonitrile copolymer waterborne adhesive with dual functions as both carbon and nitrogen sources for fabricating a honeycomb-like nitrogen/oxygen (N/O) co-doped porous carbon with remarkably enhanced texture by a simple acid-assisted strategy combined with KOH activation. The final carbon (A-LAC2) achieves quite high capacitance of 317.5 F g−1 at 0.5 A g−1 and superb rate capability in a three-electrode cell. Specially, a promising solid-state symmetric SC is demonstrated by relatively high energy/power densities of 6.7 Wh kg−1/3486 W kg−1 as well as long lifespan over 10000 cycles, as a result of synergistic coupling of large surface area, hierarchical porous texture, rich surface species, and multidimensional ionic/electronic transfer channels presented by this material. This research provides a new yet efficient strategy to build high-performance carbon material from clean copolymer adhesive and holds great prospect in energy storage applications.

KEYWORDS: Waterborne adhesive, Acrylonitrile copolymer, Porous carbon, High surface area, Solid-state supercapacitor INTRODUCTION Significant demand of portable electronics and hybrid electric vehicles has stimulated enormous research activities on clean and sustainable energy technologies for decades.1−4 Among these energy storage devices, supercapacitors (SCs) typically exhibits excellent power capability, fast charge/discharge rates, environmental benignity and long lifetime, thereby giving rise to considerable attention.5−10 However, their real applications to date are still impeded by their low


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energy density, which is much lower than that of rechargeable batteries.7,11,12 In this regard, many efforts have been devoted to construct well performing architectures with high specific capacitance in hopes of increasing their energy density while retaining superior power and cycling stability.13 Hierarchical carbon nanostructures show prominent superiorities including intrinsic conductivity, high chemical/mechanical stability, and high porosity, which are thus broadly used as the electrode for diverse energy devices. In general, both carbon source and synthesis approach strongly affect physicochemical properties of final carbon products.14,15 In most cases, carbon materials obtained from traditional carbon sources and common synthesis methods may inevitably bring some shortcomings. For example, the carbons from coal might contain too many impurities, graphene and carbon nanotube are too expensive, and some carbons might involve a complicated synthetic process.16−18 Therefore, seeking for an inexpensive and efficient strategy to synthesize nanostructured carbons with hierarchical porosity for increased energy storage efficiency is one of the most critical challenges.19,20 Carbon-based SC stores the energy by the accumulation of electrostatic charges at the electrode/electrolyte interface.21 Thus, enlarging specific surface area (SSA) accessible for electrolyte ions is vital to improve the charge-storage efficiency of carbon materials toward SC applications.22−24 Typically, massive SSA can be realized in two ways, physical activation with different oxidizing atmosphere (CO2 or steam), and chemical activation with different activating agents (KOH, H3PO4, ZnCl2, etc).25−31 The latter shows distinct advantages, such as mature technique, low activation temperature, short activation time, low cost, and large surface area.32,33 By this way, nanoscale carbon architectures with tailored porosity are rationally designed, and a certain amount of the heteroatom (e.g. oxygen) is introduced into carbon matrix enhancing the specific capacitance contributed by faradaic reactions of the heteroatom. A suitable carbon


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source is another important factor determining the structure and performance of functional materials.7,12,15,34−36 Polyacrylonitrile (PAN) is a popular and powerful candidate because of welldeveloped carbonization chemistry,28 inherently high carbon and nitrogen densities, and spinability. It is often converted into porous carbons by a templating strategy,29,37,38 copolymerization reaction and carbonization,39 and electrospinning.40−44 However, these synthetic methods may be unsuitable for scalable production, which generally involve either complicated procedures or the use of poisonous and expensive organic solvent (such as dimethylformamide, DMF), thereby resulting in environmental pollution and prohibitive cost. Waterborne adhesive such as commercial LA133 would be a preferred carbon source. LA133 is an aqueous solution with 15% acrylonitrile copolymer, whose usage for producing functional carbons can address the above problems of PAN. For instance, when employing the electrospinning method, poisonous DMF can be avoided. It is also a low-cost nitrogenous polymer solution, readily yielding competitive N-doped carbons by carbonization. At present, LA133 basically serves as a kind of cheap adhesive, and hardly research claiming the potential of LA133-derived carbons is published. Based on environmental and economic considerations, LA133 adhesive is explored as carbon/nitrogen sources for developing honeycomb-like N/O co-doped carbons through a HClassisted flocculation strategy together with simple oxidation stabilization and two-in-one carbonization/activation procedures. Encouragingly, the combination of acid assistance and KOH activation strategies shows high efficiency to regulate the SSA and hierarchical porosity, which would lead to higher charge storage ability of resultant carbon active material (A-LAC2). Benefiting from intrinsic features of A-LAC2 (multidimensional structure, large SSA, high porosity, and N/O co-doping), A-LAC2 based aqueous and solid-state symmetric SCs display


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desirable device performance. High performance combined with efficient method verifies the prospect of carbonaceous materials from cheap and green waterborne adhesive in SC and other sustainable devices. EXPERIMENTAL SECTION Materials and reagents The reagents including potassium hydroxide (KOH) and polyvinyl alcohol (PVA, Mw = 95000, Sigma-Aldrich) were of analytical grade and used as received. LA133 waterborne adhesive (15% acrylonitrile copolymer in water) was acquired from Sichuan Yindile Technology Ltd. Deionized water was used in all experiments. Preparation of A-LAC2 Typically, commercial LA133 (50 g) was diluted into a low-concentration LA133 (2.5% acrylonitrile copolymer) with deionized water. About 350 mL HCl solution (3.0 M) was added dropwise to the diluted solution with vigorous stirring at room temperature. After further stirring for 30 min, the white floc was centrifuged and freeze-dried. The dried intermediate (LA2) was thermally pre-oxidized in air for 2 h at 230 oC, with a ramp of 1 oC min−1. Next, the pre-oxidized product (LAC2) was chemically activated with KOH in a mass ratio of 1:3 in a tube furnace. The activation condition was 750 oC for 3 h under a N2 flow with a heating rate of 5 oC min−1. To avoid the corrosion and pollution of quartz tube during thermal treatment, cheap graphite paper as a reusable lining was put into the tube. Finally, the treated product was repetitively purified in 1 M HCl solution and deionized water, and collected after drying for 12 h at 80 oC in an oven. The obtained powder was labelled as A-LAC2. For comparative studies, another carbon product (A-LAC1) was made by the above route without the acid-assisted step. Physicochemical characterization


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Scanning electron microscopy (SEM, SU-8010, Japan) and transmission electron microscopy (TEM, Hitachi-600; high-resolution TEM, JEM-2100, Japan) were used to characterize the morphology of the products. The structure was conducted by X-ray diffraction (XRD, BRÜKER D8, Germany) equipped with a Cu Kα radiation (λ = 0.154056 nm) and Raman spectrum (BRÜKER SENTERRA, Germany) operated with a 532 nm laser. Surface chemical property was examined using an X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific ESCALAB 250Xi, USA). Thermogravimetric analysis (TGA, STA 7300, Japan) was performed at a heating rate of 5 oC min−1 under an Ar atmosphere. Porous texture was analyzed by N2 adsorption-desorption test using a 3H-2000PM2 adsorption apparatus, in which SSA and pore size distribution (PSD) were acquired by Brunauer-Emmet-Teller (BET) method and BarrettJoyner-Halenda (BJH) method, respectively. Electrochemical measurement Electrochemical tests were carried out on a CHI660D electrochemical workstation by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance spectroscopy (EIS) techniques. Cyclic stability was measured using a CT2001A battery tester. To prepare the working electrode (WE), the active material obtained was mixed with acetylene black and polytetrafluoroethylene in 80:10:10 mass ratio, and rolled into a carbon film, which was further dried, cut into a round film, and pressed on a fresh Ni foam as the WE (the loading of active material in one electrode: ~2.7 mg cm−2). The three-electrode test was carried out in a 6.0 M KOH electrolyte with Hg/HgO and platinum as the reference and counter electrodes. The specific capacitance (Cm) was determined by galvanostatic discharge (GD) process using the following equation: Cm = I ∆t⁄m ∆V

(1)


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where I (A), ∆t (s), m (g), and ∆V (V) stand for the discharge current, the discharge time, the mass of active material, and the potential window after ohmic drop, respectively. The two-electrode cell was used to investigate the performance of symmetrical SC device. An aqueous SC was composed of the same electrodes as both anode and cathode, Whatman filter paper as the separator, and a 6 M KOH solution as the electrolyte. And the solid-state SC consisted of two identical electrodes and precuring PVA/KOH electrolyte without the use of additional separator, which was sealed with a polypropylene film. To obtain PVA/KOH electrolyte, 1 g of KOH and 1 g of PVA were added into 60 mL of deionized water, followed by heating up to 85 oC under stirring until the solution became clear. The PVA/KOH gel was precured under room temperature overnight for use. In these SC devices, the specific capacitance (Cs, F g−1) of single electrode, and energy density (E, Wh kg−1) and power density (P, W kg−1) of SC were given by the following equations: Cs = 2I∆t⁄m∆V

(2)

E = Cs (∆V)2 /(8×3.6)

(3)

P = 3600E⁄∆t

(4)

where m (g) is the mass of active material in one electrode, and ∆V (V) is the cell voltage after ohmic drop. RESULTS AND DISCUSSION Material structure


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Scheme 1. The procedure for the preparation of A-LAC1 and A-LAC2.

A facile protocol to synthesize N/O co-doped carbon (A-LAC2) derived from acrylonitrile copolymer adhesive is briefly presented in Scheme 1. As depicted, the whole process mainly involves several steps of H2O dilution, acid flocculation, freeze-drying, pre-oxidation, and onestep carbonization/activation. To illustrate the effect of the acid, A-LAC1 without the acid was also prepared. First, sticky LA133 was diluted with deionized water to obtain a stable lowconcentrated colloidal solution (2.5% LA133). After the addition of HCl solution, white floc appears, indicating the flocculant effect of HCl solution. The flocculation mechanism may be attributed to electrostatic interaction between acrylonitrile copolymer and flocculant agents, which changes the charge state on the surface of the colloid and thereby leads to the instability of the colloid. Next, 2.5% LA133 and the flocculated product were freeze-dried to form porous bulks. The latter freeze-dried sample displays a looser character compared with the former one, which may benefit for the formation of more ideal pore structure and higher SSA. Afterward, both dried samples were pre-oxidized under an air atmosphere and subsequent one-step carbonization/activation under a N2 atmosphere. Such two steps are generally necessary for achieving high surface area carbons from PAN and its derivatives,28,45 which may enhance the


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carbon retention in the precursor, greatly enlarge the SSA, and form heteroatom-doped carbon matrix enabling well control over electrochemical activity of materials. As a result, two different N/O co-doped porous carbons (A-LAC1 and A-LAC2) are produced. Their N/O heteroatomdoped structure is provided in Scheme 1, which shows their defect structure that will beneficial to increase the pseudocapacitance. A higher surface area of A-LAC2 (2446 m2 g−1) compared with A-LAC1 (1580 m2 g−1) obtained from N2 adsorption-desorption test demonstrates that the acid-assisted flocculation strategy combined with KOH activation is of particular efficiency in boosting texture properties of the materials.

Figure 1. SEM images of (a) A-LAC1 and (b) A-LAC2; TEM and high-resolution TEM images of (c, e) A-LAC1 and (d, f) A-LAC2. The morphologies of A-LAC1 and A-LAC2 are analyzed by SEM and TEM. SEM image in Figure 1a shows high bulk density A-LAC1. With the acid assistance, the as-synthesized ALAC2 clearly displays a honeycomb-like framework shape with massive interconnected and accessible macropores (Figure 1b), which ensures good structural stability and fast ion diffusion


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throughout the entire framework during electrochemical process. TEM results (Figure 1c and d) are nearly identical to their corresponding SEM results. High-resolution TEM images of both products (Figure 1e and f) indicate the presence of a large quantity of uniform nanopores responsible for high SSA.

Figure 2. (a) N2 adsorption-desorption isotherms (inset: the isotherm of A-LAC2 at a relative pressure ranging from 0.2−0.99) and (b) PSD curves of A-LAC1 and A-LAC2. Table 1. Textural parameters of A-LAC1 and A-LAC2 Sample

SBET (m2 g−1)

Smicro (m2 g−1)

A-LAC2

2446

2285

Vtotal (cm3 g−1) 1.29

Vmicro (cm3 g−1) 0.97

Dap (nm)

Smicro/SBET (%)

2.11

93.4

A-LAC1 1580 1306 1.10 0.61 2.77 82.7 SBET: specific surface area; Smicro: specific surface area of micropores; Vtotal: total pore volume; Vmicro: micropore volume; Dap: average pore diameter. Figure 2 represents N2 adsorption-desorption isotherms and PSD curves of A-LAC1 and ALAC2 for analyzing textural properties. In Figure 2a and its inset, both isotherms are found to contain high adsorption uptake and small hysteresis loop as well as small vertical tail at low, middle, and high relative pressures (P/P0: < 0.1, 0.4–0.95, and > 0.95), manifesting hierarchical porosity with high microporosity and low macroporosity for both samples.41,46 Besides, the multilevel nature of pores can be also verified by the peaks of micropores and mesopores


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respectively located at 0.47–1.2 nm and 2.0–4.2 nm in PSD (Figure 2b). Three types of pores play different roles, in which micropores and mesopores offer ion-storage sites and iontransporting channels while macropores function as ion-buffering reservoirs.13,47 Detailed results for BET areas and pore parameters are collected and presented in Table 1. Despite high BET area for A-LAC1 (1580 m2 g−1 with ~82.7% contribution from micropores), it still is significantly lower than that of A-LAC2 (2446 m2 g−1 with ~93.4% contribution from micropores). Moreover, pore volume of A-LAC2 is also quite high (1.29 cm3 g−1). For A-LAC2, such ideal textural properties are capable of full exposure of material surface beneficial for the mass/charge transport endowing high capacitive performance.

Figure 3. (a) XPS survey spectra, and high-resolution (b) O1s and (c) N1s XPS spectra of ALAC1 and A-LAC2. The electrochemical activity and wettability of the material are strongly associated with their surface chemistry. Thus, XPS spectra on A-LAC1 and A-LAC2 are presented in Figure 3. Survey spectra in Figure 3a show C, N and O signals, in which the peak intensity of C element is the highest, suggesting successful preparation of heretoatom-doped carbon products. Higher N/O contents (2.63/9.5 at%) of A-LAC2, compared with the corresponding values of A-LAC1 (1.71/7.71 at%), reveal high N/O co-doped level on A-LAC2. High-resolution O1s spectrum in Figure 3b is fit well with three component peaks around binding energies of around 531.2, 532.5, and 534.3 eV, arising from O-I (C=O), O-II (C-O), and O-III (chemisorbed oxygen and/or water), respectively.48 The appearance of O species can enhance the permeation of the electrolyte


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and provide more active sites allowing for efficient mass/electron transfer.48,49 High-resolution N1s spectrum (Figure 3c) mainly shows three N states assigned to pyridinic nitrogen (N-6, 398.2 eV), pyrrolic and/or pyridone nitrogen (N-5, 400.2 eV), and quaternary nitrogen (N-Q, 401.2 eV), respectively.41,50 of these N groups, N-5 and N-6 mainly contribute to the pseudocapacitance, whereas N-Q promotes electron transport and enhance the electronic conductivity.51,52 These results testify higher electroactivity and better electrolyte permeability of A-LAC2.

Figure 4. (a) XRD patterns and (b) Raman spectra of A-LAC1 and A-LAC2; (c) TG curves of LA2, LAC2 and A-LAC2 under an Ar flow atmosphere. XRD pattern and Raman spectroscopy are obtained for structural analysis of the products. XRD patterns in Figure 4a exhibit broad peaks around 26o and 43o assigned to typical (002) and (100) planes of amorphous carbon, which is often associated with the turbostratic structure of the carbon.53,54 Their disordered structure is further confirmed by Raman spectra. As shown in Figure 4b, both samples appear two peaks located at 1336 and 1581 cm−1 ascribed to the D-band and G-band, of which the former is due to the structural defects and disorder degree in graphitic layers and the latter to the in-phase vibration of the graphitic lattice.55,56 The intensity ratio (ID/IG) values of both bands are 1.04 and 1.05 for A-LAC1 and A-LAC2, respectively. Their low graphitization partly results from more defects induced by thermal decomposition of the materials and the destruction of graphitic carbon induced by KOH activation.57−59 Also, it infers


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high-level doping of the heteroatom. TGA curves in Figure 4c show thermal behavior of LA2, LAC2 and A-LAC2 under an Ar atmosphere. Approximately 71.9 and 58.1 wt% weight losses occur after 900 oC for LA2 and LAC2, respectively, indicating that the pre-oxidation process contributes to the improvement of structural stabilization and carbon yield. For the activated ALAC2, its weight loss is only 26.8 wt% at high temperature (900 oC), which is much less than those of LA2 and LAC2, meaning the highest thermal stability on A-LAC2. Electrochemical performance of carbon materials

Figure 5. Electrochemical properties of A-LAC1 and A-LAC2 on a three-electrode cell in a 6.0 M KOH electrolyte: (a) CV curves at a sweep rate of 10 mV s−1; (b) CV and (c) GCD curves of A-LAC2 at different sweep rates and current densities; (d) specific capacitances at different current densities; (e) Nyquist plots (inset: the enlarged EIS at high frequency); (f) cycling stability and Coulombic efficiency. A standard three-electrode cell was used to systematically investigate capacitive behaviors of ALAC1 and A-LAC2 in a 6.0 M KOH electrolyte (Figure 5). Figure 5a presents CV curves of both electrodes conducted at a sweep rate of 10 mV s−1, while Figure 5b shows CV curves of A-LAC2 at sweep rates of 3−100 mV s−1. All these quasi-rectangular curves indicate the characteristic of


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typical double-layer capacitor (EDLC) (Figure 5a and b), in which small deformation on curves arises from faradic reactions of N/O groups. Generally, under the same test conditions, the larger the circulated CV area is, the higher the specific capacitance is.51 It is thus speculated that ALAC2 may deliver a higher capacitance. Well-preserved CV shape with small distortion at a large sweep scan rate up to 100 mV s−1 infers fast electrochemical response and good reversibility on A-LAC2 (Figure 5b). The capacitance and rate capability of both materials were quantified by GCD technique at current densities of 0.5−30 A g−1. GCD curves of A-LAC2 are given in Figure 5c. Based on Equation (1), Cm values of A-LAC2 are 317.5, 298.8, 268.9, and 258.9 F g−1 at current densities of 0.5, 1, 10, and 30 A g−1, respectively, which are much higher than the values of A-LAC1 (209.3, 200, 179.3, and 170.6 F g−1), as plotted in Figure 5d. It is also observed that both electrodes show identical capacitance retention (81.5%) and A-LAC2 has higher capacitance value (258.9 F g−1) at large current of 30 A g−1, verifying high-capacitance and excellent rate characters of A-LAC2. Slow capacitance decline for A-LAC2 may arise from low internal resistance values (0, 0.02, 0.025, 0.048, and 0.07 V for 0.5−3, 5, 10, 20, and 30 A g−1, respectively). Moreover, these results obtained in our work show better charge storage ability as compared to previous studies, such as CESM-300 (297−196 F g−1 at 0.2−20 A g−1),60 CN-GLS (294−226 F g−1 at 0.5−20 A g−1),61 Ccel-LE (253−105 F g−1 at 0.5−10 A g−1),62 N,S codoping PCNs1-1 (298−233 F g−1 at 0.5−50 A g−1)63, HPCs-750 (314−237 F g−1 at 0.5−20 A g−1),64 and C/KOH-700 (348−290 F g−1 at 0.05−2 A g−1)65. Figure 5e presents Nyquist plots of both materials. All curves display a semicircle at high frequency followed by a straight line at low frequency. The enlarged EIS at high frequency is inserted in Figure 5e. The semicircle is related to an electron-transfer-limited process and the straight line to ion diffusion/transport in the electrolyte. According to the diameter of the


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semicircle, the charge transfer resistance (Rct) values of A-LAC1 and A-LAC2 are estimated to 0.15 and 0.12 Ω, respectively, inferring that A-LAC2 has faster charge transfer. From the intercept of semicircle with the Z' axis (the inset of Figure 5e), internal resistance (RS) of ALAC2 is estimated to be 0.3 Ω, smaller than that of A-LAC1 (0.6 Ω). Almost vertical lines in low frequency are noted, indicating nearly ideal capacitive behavior for these two electrode. The above comprehensive analysis reveals that A-LAC2 has a good charge storage profile. Cycling lifetime is a vital factor in evaluation of energy storage performance, which is examined by continuous GCD test at a current density of 4 A g−1. As shown in Figure 5f, their capacitance retentions approach 91% after 10000 cycles, indicative of long cycling life. Additionally, the Coulombic efficiency of around 100% manifests excellent electrochemical reversibility during cycling. Excellent energy storage on A-LAC2 electrode can be attributed to the following critical features: (1) the greatly improved SSA coupled with favorable honeycomb-like structure enables full utilization of active material significantly increasing active site accessibility; (2) hierarchical porous structure (micro-, meso- and macropores) enables rapid mass/electron transport by shortening the transfer distance and enhancing the ion accessibility of the electrode material; (3) 3D structure not only allows multidirectional ionic/electronic transport but also enhances the mechanical stability of the material; (4) the N/O co-doping on porous carbons well tunes electronic properties and/or conductivity as well as the wettability enhancing the pseudocapacitive contribution.

Electrochemical Performance of Aqueous Symmetric SC


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Figure 6. Electrochemical properties of the A-LAC2//A-LAC2 aqueous SC using a 6.0 M KOH electrolyte: (a) CV curves at different sweep rates; (b) GCD curves at different current densities; (c) Rogone plots of A-LAC1 and A-LAC2 based aqueous SCs (inset: specific capacitances of single electrode in the devices at various current densities); (d) cycling stability of A-LAC1 and A-LAC2 based SCs. The A-LAC2//A-LAC2 aqueous symmetric SC using Whatman filter paper separator and a 6.0 M KOH electrolyte are assembled in order to verdict the practical application of A-LAC2 in aqueous SCs. As seen in Figure 6a, with the sweep rate sharply rising from 10 to 300 mV s−1, nearly rectangular and symmetric CV curves are well maintained, proving an outstanding EDLC behavior of this device. GCD profiles at various current densities from 0.5 to 30 A g−1 are provided in Figure 6b. According to Equation (2), the specific capacitance (Cs) of single electrode in the A-LAC2 based device is in a range of 253.8−212.1 F g−1 at 0.5−30 A g−1, while 171.5−141.3 F g−1 for the A-LAC1 based device (the inset of Figure 6c). Thus, A-LAC2 based SC exhibits better rate response by 83.6% capacitance retention at 60-folded current. By calculation from GD curves using Equations (3) and (4), its maximum energy and power densities reach up to 8.8 Wh kg−1 at a power density of 125 W kg−1 and 6210 W kg−1 at an


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energy density of 5.0 Wh kg−1, respectively, which are higher than those of A-LAC1 based device (6.0 Wh kg−1/6039 W kg−1) (Figure 6c), and previous devices based on C/KOH-700 (8.3 Wh kg−1/1105 W kg−1)65, HC/KOH (8.1 Wh kg−1/1000 W kg−1)66, and HPCSLS-700-1 (8.6 Wh kg−1/ 5735.6 W kg−1)67. Moreover, the value of 8.8 Wh kg−1 is also comparable to those of those devices based on GNC1/4AP (9.60 Wh kg−1)68, CGS-700 (9.0 Wh kg−1)69, and HLPC (9.4 Wh kg−1)70. Long-term stability of both aqueous devices is tested at 4 A g−1 up to 20000 cycles (Figure 6d). Cs values slightly reduce for A-LAC1 and A-LAC2 based device, and still remain 91.1% and 94.6% of initial capacitances, respectively. By comparison, the latter device exhibits excellent stability. In terms of various aspects including specific capacitance, rate capability, and cyclability, the A-LAC2//A-LAC2 aqueous SC can achieve superior and stable electrochemical performance. Electrochemical performance of solid-state symmetric SC

Figure 7. Electrochemical properties of the A-LAC2//A-LAC2 solid-state SC using the PVAKOH electrolyte: (a) schematic diagram of the solid-state SC; (b) CV curves at different sweep rates; (c) GCD curves at different current densities; (d) Rogone plots (insert: cycling stability).


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To validate the application of A-LAC2 in solid-state SCs, the A-LAC2//A-LAC2 solid-state SC is further constructed with two identical A-LAC2 electrodes sandwiched by the PVA/KOH electrolyte (Figure 7a). Typical CV and GCD profiles of this device are presented in Figure 7b and c. As the sweep rate and current density increase, nearly symmetrically rectangular CV curves (Figure 7b) and triangular GCD curves can be well retained (Figure 7c). This is an indicator of typical DELC behavior and excellent reversibility, which is consistent with the results from aqueous device. From GCD curves in Figure 7c, Cs values of the solid-state device are evaluated to be 192.8, 188, 160.4, and 147.7 F g−1 at 0.5, 1, 10, and 20 A g−1, respectively. Further, the maximum energy and power densities of this device are determined as 6.7 Wh kg−1 at 0.5 A g−1 and 3486 W kg−1 at 20 A g−1 based on Equations (3) and (4) (Figure 7d). Cyclic life of the device is also examined at 2 A g−1 for continuous 10000 cycles. As shown in the insert of Figure 7d, the Cs of the device initially drops (ca. 11% capacitance loss over 1200 cycles), and then remains relatively stable up to 10000 cycles. High capacitance retention of 88.2% during cycling corresponds to 0.001% loss per cycle, verifying outstanding cycling performance on the A-LAC2 solid-state device. CONCLUSIONS To conclude, we report the finding of unique acid-assisted strategy combined with KOH activation to greatly promote textural properties of honeycomb-like N/O co-doped porous carbons (A-LAC2) with rich nanopores on sheets by employing inexpensive and eco-friendly waterborne adhesives. This novel carbon architecture delivers excellent electrode performance. Particularly, the solid-state symmetric SC using A-LAC2 as the cathode and anode and PVA/KOH as the electrolyte can achieve relatively high energy and power densities of 6.7 W h kg−1 and 3486 W kg−1, with good long-term durability up to 88.2% retention over 10000 cycles.


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The encouraging results demonstrate that LA133-based carbon has the significant potential in portable solid-state SCs, which opens up a unique way to guide further rational construction of advanced carbonaceous materials from green sources for use in next-generation sustainable energy storage devices. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The financial supports from the National Natural Science Foundation of China (21563029) and the 111 project (D17007) are greatly appreciated. The authors also wish to acknowledge Physical and Chemical Detecting Centre of Xinjiang University for material characterization. REFERENCES (1) Ghidiu, M.; Lukatskaya, M. R.; Zhao, M. Q.; Gogotsi, Y.; Barsoum, M. W. Conductive two-dimensional titanium carbide 'clay' with high volumetric capacitance. Nature 2014, 516 (7529), 78−81. (2) Béguin, F.; Presser, V.; Balducci, A.; Frackowiak, E. Carbons and electrolytes for advanced supercapacitors. Adv. Mater. 2014, 26 (14), 2219−2251.


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Scheme 1. The procedure for the preparation of A-LAC1 and A-LAC2.

Figure 1. SEM images of (a) A-LAC1 and (b) A-LAC2; TEM and high-resolution TEM images of (c, e) A-LAC1 and (d, f) A-LAC2.


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Figure 2. (a) N2 adsorption-desorption isotherms (inset: the isotherm of A-LAC2 at a relative pressure ranging from 0.2−0.99) and (b) PSD curves of A-LAC1 and A-LAC2.

Figure 3. (a) XPS survey spectra, and high-resolution (b) O1s and (c) N1s XPS spectra of A-LAC1 and A-LAC2.

Figure 4. (a) XRD patterns and (b) Raman spectra of A-LAC1 and A-LAC2; (c) TG curves of LA2, LAC2 and A-LAC2 under an Ar flow atmosphere.



Figure 5. Electrochemical properties of A-LAC1 and A-LAC2 on a three-electrode cell in 6.0 M KOH electrolyte: (a) CV curves at a sweep rate of 10 mV s−1; (b) CV and (c) GCD curves of A-LAC2 at different sweep rates and current densities; (d) specific capacitances at different current densities; (e) Nyquist plots (inset: the enlarged EIS at high frequency); (f) cycling stability and Coulombic efficiency.

Figure 6. Electrochemical properties of the A-LAC2//A-LAC2 aqueous SC using a 6.0 M KOH electrolyte: (a) CV curves at different sweep rates; (b) GCD curves at different current densities; (c) Rogone plots of A-LAC1 and A-LAC2 based aqueous SCs (inset: specific capacitances of single electrode in the devices at various current densities); (d) cycling stability of A-LAC1 and A-LAC2 based SCs.


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Figure 7. Electrochemical properties of the A-LAC2//A-LAC2 solid-state SC using the PVA-KOH electrolyte: (a) schematic diagram of the solid-state SC; (b) CV curves at different sweep rates; (c) GCD curves at different current densities; (d) Rogone plots (insert: cycling stability).



For Table of Contents Use Only Synopsis

Acid-assisted strategy combined with KOH activation efficiently optimizes architectures from novel adhesive, providing a new avenue to construct high-performance carbons for solid-state supercapacitors.


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Acid-assisted strategy combined with KOH activation to efficiently

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