Highly Compressible Nitrogen-Doped Carbon Foam Electrode with

Subscriber access provided by HACETTEPE UNIVERSITESI KUTUPHANESI

Article

Highly Compressible Nitrogen-Doped Carbon Foam Electrode with Excellent Rate Capability via a Smart Etching and Catalytic Process Kang Xiao, Yanhua Zeng, Jin Long, Hongbin Chen, Liangxin Ding, Suqing Wang, and Haihui Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b02381 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 18, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Highly Compressible Nitrogen-doped Carbon Foam Electrode with Excellent Rate Capability via a Smart Etching and Catalytic Process Kang Xiao, Yanhua Zeng, Jin Long, Hongbin Chen, Liang-Xin Ding,* Suqing Wang and Haihui Wang* School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China Keywords: KMnO4, nitrogen-doped carbon, compressible electrode, symmetric supercapacitor, excellent rate capability

ABSTRACT

Freestanding three-dimensional (3D) nitrogen-doped carbon foam (NCF) with large pore architectures is proposed as a promising electrode configuration for elastic electronics. However, although it exhibits excellent mechanical performance, the available capacitive performances (especially its rate capability) are still unsatisfactory. By using KMnO4, we demonstrate a smart etching and catalytic process to form highly graphitized and etched nitrogen-doped carbon foam (denoted as ENCF) with an exfoliated carbon shell architecture. These compositional and structural features endow the ENCF electrodes with excellent electron conductivity as well as

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 23

more ion-accessible electrochemical active sites. Significantly, all-solid-state symmetric supercapacitor (SSC) devices based on the ENCF electrodes exhibit enhanced specific capacitance and marked high-rate capability. Furthermore, the integrated device has no significant capacity loss under 60% compressive strain.

INTRODUCTION Wearable, flexible and elastic electronic devices have attracted increasing attention in recent years. To power these elastic electronic devices, flexible and compressible/stretchable power sources with robust mechanical properties are needed.1-4 Among the various power sources, lithium-ion, sodium ion, lithium-air and lithium-sulfur batteries offer higher energy densities than supercapacitors (SC), however, concerns regarding their poisonous organic electrolyte and rising cost have prompted extensive research into using SCs in flexible power sources due to their unique features of high safety, long cycling life and ultrahigh power density.1,5 Numerous previous efforts have shown that 3D porous structures are suitable configurations to prepare the compressible electrodes, and great progress has been achieved using materials such as graphene and carbon nanotubes (CNTs) foams.6-12 However, those graphene and CNTs based compressible electrodes suffered from relatively small pore structures and low porosity, which hinders the infiltration of the gel electrolytes and leads to many of the inner pores or active sites in the electrode not being fully utilized. Indeed, many excellent studies show superior electrochemical performances and mechanical properties in aqueous electrolyte, but similar studies of all-solid-state devices have not been reported. The attractive approach of coating active materials on commercial sponges with large pore structures was developed to overcome this limitation. For example, Niu et al. first reported compressible all-solid-state SC devices based on PPy-SWCNTs-sponge electrodes, where the commercial sponges endowed the


2

Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


materials with superior compressibility and polyaniline provided pseudocapacitance.13 However, the active material (PPy-SWCNTs) was directly deposited on the framework of the sponge through weak binding forces; as a result, they are not capable of withstanding large deformations over long time periods. Moreover, the inactive mechanical support may deteriorate the rate performance due to its low electron transport ability. Within this context, electrode materials with large pores and freestanding structures are desired. Recently, our group developed a type of freestanding, hydrophilic and highly compressible N-doped carbon foam (NCF) that can be used as the electrode material in supercapacitors.14 An integrated all-solid-state symmetric supercapacitor (SSC) device based on NCF electrodes can be arbitrarily compressed under 60% strain without significant changes in the electrochemical performance. Additionally, a specific capacitance of 343 mF cm-2 could be achieved without any other active capacitive materials. However, significantly boosting the highrate capability while maintaining the mechanical performance still remains a big challenge, which is perhaps due to the low degree of graphitization of NCF. If its capacitance and rate performance could be further enhanced while retaining its mechanical properties, it would provide a significantly useful alternative electrode for commercial all-solid-state SC applications. It has been recognized that carbon materials with large specific surface areas and higher degrees of graphitization offer more active sites and superior conductivity, which results in higher capacitances and rate capabilities.15-18 Consequently, numerous studies have mainly focused on the two aforementioned aspects to improve the capacitive performance of carbon material electrodes. For instance, Wang et al. demonstrated that the direct use of chemically activated carbon cloth as the electrode material greatly enhanced the areal capacitance.19 A more efficient electrochemical exfoliation strategy was proposed by Lu and co-workers, which shows that


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 23

surface exfoliation of the carbon fibers in carbon cloth enhances the surface area, which could significantly boost the capability while preserving the original mechanical properties.20 Inspired by this, we hypothesize that if the surface of the carbon fibers in NCF could be exfoliated and transferred to graphitized carbon, remarkable compressible NCF electrode materials with high surface roughness and conductivity could be achieved. Herein, we reported a smart etching and catalytic process to distinctly enhance the capacitive performances of NCF electrodes using commercial KMnO4 as the etchant and catalyst precursor, which leads to faultless implementation in highly compressible allsolid-state SSC devices. Our strategy to improve the electrochemical performance by etching the NCF (final product denoted ENCF) electrode can be grouped as follows: i) a melamine sponge (MS) precursor was etched with KMnO4 solution, yielding an ENCF consisting of exfoliated carbon fibers with more ion-accessible surface area sites; ii) MnO2 (derived from the pyrolysis of KMnO4) as a graphitization catalyst can improve the degree of graphitization of ENCF, and thus contribute to high electric conductivity;21-22 iii) a small quantity of MnO2 nanoparticles embedded in ENCF can produce extra pseudocapacitance.23-26 and iv) hierarchical micro-, meso-, and macropores channels formed a rapid ion and electron transport network for further improvement in the rate performance.27-31


4

Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. a-c) Schematic illustration of the formation of ENCF. The structure evolution images: a-2) Melamine sponge, b-2) Melamine sponge after etching, c-2) ENCF. RESULTS AND DISCUSSION The experimental preparation of ENCF electrodes involves two steps, as schematically illustrated in Figure 1 (upper), and the digital photos of the respective samples are also presented in Figure 1 (lower). The etching of MS was achieved by applying a preferential etching process to the preformed ENCF precursor. In the etching process, the unsaturated hydrocarbons in the melamine sponge can be oxidized to hydroxy or carboxyl. Next, after selective removal of redundant KMnO4 and subsequent calcination treatment in Ar atmosphere, an ENCF with shaggy carbon fibers was obtained. The scanning electron microscopy (SEM) images in Figure 2a-b shows that MS has a rather smooth surface over the whole fiber and after etching, the surface morphology of carbon fiber is completely different, indicating that the surface of the fiber was etched, leading to the formation of rough surface morphologies. A subsequent calcining process was carried out to convert the MS to ENCF. As seen in the SEM observations (Figure 2c),


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 23

the final ENCF underwent an obvious volume shrinkage compared with the pre-etched MS precursor but still inherited the rough surface morphology.

Figure 2. SEM images of a) melamine sponge, b) etched melamine sponge and c) ENCF. d) Low-magnification TEM image of single ENCF fiber. e-f) High-resolution TEM image collected at the edge of the single ENCF fiber as indicated by the cusp in (d). STEM (g) and the elemental mapping of ENCF (h). Figure 2d depicts the transmission electron microscopy (TEM) image of ENCF, from which representative carbon fibers with rough surfaces can be observed. The composition of ENCF was investigated by X-ray photoelectron spectroscopy (XPS). The survey XPS spectrum (Figure S1) clearly revealed that the product contains C, N, O, and


6

Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mn elements. Based on a previous report, the peak binding energy separation between Mn 2p3/2 and Mn 2p1/2 of approximately 11.8 eV is characteristic of MnO2.23 Careful examination by thermogravimetric (TG) analysis in air revealed that the weight percentage of MnO2 was estimated to be approximately less than 10% (Figure S2). The essence of this strategy lies in the residual KMnO4 transformed into MnO2 at high temperatures, which triggers the enhanced graphitization of the carbon fiber. As verified by high-resolution TEM images (Figure 2e and Figure S3 in the Supporting Information), MnO2 nanoparticles were embedded in the graphitized carbon matrix. Interestingly, TEM observation reveals the homogeneous distribution of mesopores with diameters in the range of 10-20 nm (Figure 2f), while Brunauer-Emmett-Teller (BET) measurements (Figure S4a) also verify the mesoporous nature. Significantly, the specific surface area of ENCF increased by more than one order of magnitude from 5.3 m2 g-1 to 61.2 m2 g-1 in comparison to untreated NCF (Figure S4b). The elemental mappings of a single carbon fiber reveal a uniform distribution of carbon, nitrogen, oxygen and manganese in the ENCF fiber (Figure 2g-h). An important advantage of this smart etching process is the dramatically increased number of effective active sites (surface area accessible to the electrolyte ions) in ENCF. To prove the enhancement in the accessible surface area, the dye-absorption method was used to investigate ENCF and untreated NCF samples (Figure 3a). The ENCF and untreated NCF samples were immersed in a 2 mg L-1 aqueous methylene blue (MB) solution for 2 h in the dark, which is the most commonly used solid absorber for determining the ion-accessible surface area.19-20 Clearly, the absorption peaks of MB with ENCF almost disappear, while the absorption peaks for the untreated NCF sample are


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 23

greatly changed but still exist. Digital photographs of the adsorption tests vividly demonstrate the enhanced adsorption properties. From observation of the digital photos, the final color of the MB solution with ENCF became transparent, and that with the untreated NCF sample was light blue. This indicates that MB molecules were absorbed more easily by the ENCF sample than the untreated NCF sample, indicating a distinct enhancement in the ion-accessible surface area upon chemical etching. Moreover, Raman spectroscopy analysis (Figure S5) confirms that more graphitized carbon was produced during high-temperature processes with the MnO2 catalyst. The G peak in the spectrum of ENCF associated with crystalline carbon (located at approximately 1600 cm-1) is significantly enhanced compared with that of untreated NCF, producing an obvious increase in the ratio of the G mode to the D mode (associated with carbon defects) (IG/ID) from 0.801 to 0.967.32 The increased IG/ID ratio indicates that the calcining process with MnO2 nanoparticles can promote an increase in the carbonization degree, with partial structural transformation from defective carbon to crystallized carbon. As a consequence, superior electrical conductivity of the ENCF samples can be obtained by this catalytic thermal treatment process.22 The electrical conductivity was calculated from the I-V curves presented in the Supporting Information (Figure S6). Taking various NCF samples as examples, the electrical conductivity of the untreated NCF samples was enhanced by increased temperature and ranged from 0.46 S cm-2 to 3.8 S cm-2 (Figure 3b). However, high graphitization of carbon tends to collapse or distortion under compression due to the relatively poor compressibility and springiness limit. Therefore, the samples of NCF-800 and NCF-900 could not maintain their original morphology after several continuous compressions.14 Significantly, the highly graphitized ENCF achieved the high


8

Page 9 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrical conductivity of 6.4 S cm-2, higher than that of the untreated NCF-900 sample, while still maintaining its outstanding mechanical properties. The volume of ENCF after compression can expand to recover to its original volume without discernible variation, owing to the excellent elasticity and robust carbon fiber network of ENCF (Figure 3c). Figure S7(a-b) shows the stress-strain measurements of the NCF samples under different compressive degrees with 20%, 40%, 60% and 80% strain. During the unloading process, the compressive stress always remains above zero and almost returns to the original point, indicating that the reduction of stress is accompanied by the immediate recovery of ENCF and without plastic deformation.14 The cyclic compression curves of ENCF for 1000 loading-unloading cycles at a strain rate of 50% for 15 s with a maximum strain of 50% are presented in Figure S7c. Although the first compression curve is different from the subsequent cycles due to the higher Young’s modulus, maximum stress and large energy loss coefficient,33 the representational stressstrain curves of the 500th and 1000th cycle perfectly overlap, which highlights the a high recovery rate and that the mechanical properties can be maintained by ENCF upon longterm deformation. Moreover, the relative height changes of multi-cycle compression testing for ENCF are shown in Figure 3d. It was clear that the relative height of NCF after 1000 cyclic compressions remained almost the same without observable height deformation.


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 23

Figure 3. a) UV-vis absorption spectra and digital image (inset) collected for the original MB solution, untreated NCF and ENCF after reacting for 2 h. b) Comparison of the conductivity of the untreated NCF at various temperature and ENCF. c) The digital images showing compressibility of ENCF. d) The variation of height as a function of time at maximum strain of 50% for 1000 cycles. Bottom: The first five cycles. Upper: The last five cycles. The advantage of this unique structure, which allows for large and long-term deformations combined with increased ion-accessible active sites and superior electrical conductivity, makes ENCF suitable to be used directly as a high-performance and compressible electrode for supercapacitors. We first tested the electrochemical performance in a three-electrode electrochemical cell. CV curves of freestanding ENCF as the working electrode directly in 5 M LiCl electrolyte shows typical electrochemical double-layer capacitive behavior.34 The ENCF electrodes (Figure S8) display an enhanced specific capacitance of 473 mF cm-3 under the scan rate of 1 mV s-1 and superior rate capability (196 mF cm-3 for ENCF at 1 V s-1) compared with the NCF electrode.14 The rate capability during ultrafast charging/discharging is critical for high-


10

Page 11 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


performance electrode materials. In general, large distortions in the CV shape under ultrafast scan rates result in an unpromising rate capability.35 The rectangular CV curves of the ENCF electrode are retained even at ultrahigh scan rates (e.g., up to 20 V s-1), and the linear dependence of the discharge current on the scan rate was observed at scan rates of up to 10 V s-1, showing that our ENCF electrode has excellent rate capability. The enhanced electrochemical performance of the ENCF electrode compared with the untreated NCF electrode suggests that a large specific surface area and good electrical conductivity of the electrode is crucial to boost specific capacitance and rate capabilities. Having determined that the chemically etched and highly graphitized ENCF electrode exhibits enhanced capacitive behavior as well as a fascinating ability to withstand various compression deformations, we sought to test its practical application as a two-electrode symmetric supercapacitor (SSC). In this work, a simple all-solid-state SSC device with two ENCF electrodes and 5 M LiCl/PVA as the gel electrolyte was assembled (Figure S9). The electrochemical properties of the as-fabricated SSC device were evaluated by CV and GCD testing. The CV and GCD curves of the SSC device present nearly rectangular (Figure 4a-b) and symmetric triangular traces (Figure S10a-b), respectively, which are indicative of ideal capacitive behavior and fast charge-discharge properties. In particular, the SSC device is robust and capable of charging/discharging at scan rates as high as 100 V s-1 and can complete one charge/discharge cycle within 3 s at a current density of 20 mA cm-2. Note that previously reported data in the literature on the excellent rate capability of supercapacitors are often provided over a different scan rate range. In this case, the certain specific capacitances at scan rates of 10 mV s-1 and 1 V s-1 were chosen for better comparison. Specifically, a remarkable capacitance retention rate of 56% was achieved, which is noticeably higher than the rate


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 23

capability of the untreated NCF electrode and even comparable to the best rate capability reported previously in the literature obtained under a similar scan rate range (10 mV s-1 to 1 V s1 19, 36-37

).

The volumetric capacitance of the SSC device was calculated from the CV curves at

various scan rates (Figure 4d). Since the mass of all-solid-state (active material or entire device) supercapacitors vary in the literature, volumetric capacitance is a fair parameter for better comparison. At the slow scan rate of 10 mV s-1, the volumetric capacitance is 56 mF cm-3, which is among the superior of all-solid-state compressible supercapacitors (Table S1). These findings indicate that chemical etching promotes the formation of more efficient EDLC active sites and that high graphitization is beneficial to rapid ion transport within the electrodes.

Figure 4. a) CV curves of the ENCF SSC device at low scan rates from 10 mV s-1 to 800 mV s-1, and b) at high scan rates from 1 V s-1 to 100 V s-1. c) A linear dependence of the discharge


12

Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


current versus scan rates. d) Volumetric capacitance calculated from CV curves at various scan rates. e) Capacitance retention of ENCF SSC device at different degrees of compression, and (inset) CV curves under different degrees of compression. f) Long-term cycling performance of the ENCF SSC device at different compress states for 4 000 cycles. g) Schematic diagram of the PET substrates with Au film for assembling three ENCF devices into one unit in series. h) CV curves of the three ENCF SSC devices and a single s ENCF SSC device at the scan rate of 400 mV s-1. i) GCD curves of the three ENCF SSC devices and a single s ENCF SSC device at the current density of 6 mA cm-2. Another fascinating feature of this ENCF electrode is its natural ability to act as a compressible supercapacitor. To study the compression stability of the ENCF-SSC device, the as-prepared ENCF-SSC device was examined by CV and GCD measurements under different compression conditions, as shown in Figure 4e. When the as-prepared ENCFSSC device was compressed from its normal state by 20%, 40% and 60% (Based on the whole height of device include substrates and separator), it almost maintains its curvilinear shape. Moreover, the long-term durable stability of the ENCF-SSC device under various compression states was further tested and is given in Figure 4f and Figure S11. Our ENCF-SSC device shows excellent long-term durability (less than 4% decrease in capacitance at each compression state). This suggests that the ENCF electrodes are highly stable and durable under compression. In general, the voltage window of a single supercapacitor device is too low (less than 1 V for SSC and less than 2 V for ASC) to power electronic equipment in practical applications. Therefore, several supercapacitor units were arranged in series to enhance the output voltage to necessary levels. In this case, a smart design of a PET/Au current collector was developed to assemble the ENCFSSC device in series on one chip to produce high working voltages. As shown in Figure 4g, three ENCF-SSC units could be assembled on two PET collectors with patterned Au


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 23

films to form “one” integrated device. The working voltage output of the integrated device can be readily increased to 3.0 V (Figure 4h-i). Our integrated device design was not only effective at removing redundant interconnects between device units but also readily increased the voltage by increasing the unit numbers. CONCLUSIONS In summary, N-doping carbon foam electrodes were developed via a smart etching and catalytic process to fabricate highly compressible all-solid-state supercapacitors. The as-prepared supercapacitor device based on treated carbon foam electrodes reveals enhanced specific capacitance and marked high-rate capability that is much higher than those presented in previous reports. Moreover, the excellent electrochemical performances are well maintained under a large compressive strain of 60% and after compression over 4000 cycles. Considering the enhanced mechanical and electrochemical properties, the compressible ENCF electrodes are particularly promising for next-generation flexible and elastic electronics. METHODS Preparation of ENCF. Melamine sponge (MS) were purchased from from Outlook Company (Chengdu). Through surface etching of MS fibers, ENCF precursor was obtained. Specifically, MS (5 × 3 × 2.5 cm3) was immersed in 50 mL of 10 mM KMnO4 solution for 48 h at room temperature. The resulting etched MS was commutative washed by deionized water and ethanol until the colour of final cleansing solution was clear. The obtained etched MS were dried in vacuum drying oven at 60 °C for 48 h. After that, the ENCF precursor was annealed in a tubular furnace at 700 °C under an argon atmosphere for 2 h (heating rate: 5 °C min-1). For comparison, the NCF was annealed under the same steps while without KMnO4 to processed.


14

Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fabrication of solid-state SSCs.

The LiCl/PVA gel electrolyte was prepared as previous

repored.[14] 2 g PVA powder and 4.24 g LiCl were mixed in 20 mL deionized water and heating at 85 °C under vigorous stirring for 2 h. The NCF-SSCs were assembled by two pieces of NCF and PET/Au film substrates with with one piece of separator sandwiched between, the LiCl/PVA gel electrolyte was spread onto the electrode and separator until the NCF was saturated. Then, all parts were pressed together and kept at 45 °C for 24 h to remove excess water. Materials characterization and electrochemical measurements. The X-ray diﬀraction (XRD) patterns were performed using a Bruker D8 Advance with filtered Cu-Kα radiation. The structure and morphology were characterized by scanning electron microscopy (SEM, FEI Nano430), transmission electron microscope (TEM, JEOL 2100F), X-ray photoelectron spectroscopy (XPS, ESCALAB 250) and Raman spectra (LabRAM Aramis). The BET specific surface area, total pore volume and pore size distribution were obtained from nitrogen sorption measurements (Micromeritics analyzer ASAP 2010 (USA)).The ultraviolet-visible (UV-vis) absorbance spectra were collected from Shimadzu UV-2450 spectroscopy. Mechanical tests were carried out by using an Instron Universal Testing Machine (model 5567). The electrochemical performance of individual electrodes were investigated in a conventional three-electrode with a saturated calomel reference electrode and Pt counter electrode in 5 M LiCl aqueous solution. Calculations. Single electrode: Areal capacitances of the ENCF electrodes were calculated from the CV curve using the following formula:38 ࢂࢉ

૚ ࡯= න ࡵ(ࢂ)ࢊࢂ ࡿ࢜(ࢂࢉ − ࢂࢇ) ࢂࢇ


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 23

Where S is the area of NCF electrode (cm-2), v is the potential scan rate (V s-1), Vc-Va is the potential range, and I(V) is the response current (A). NCF-SSC Devices: The volumetric capacitance (CV) of the NCF-SSC devices were calculated from the CV curve using the following formula:38 ࢂࢉ

૚ ࡯= න ࡵ(ࢂ)ࢊࢂ ࡿ࢜(ࢂࢉ − ࢂࢇ) ࢂࢇ

Where S is the volume of ENCF SSC device (cm-3), v is the potential scan rate (V s-1), Vc-Va is the potential range, and I(V) is the response current (A). The slope of I-V curve is the conductance of electrode material. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Additional XPS patterns, TG curve, HRTEM image, Pore-size distribution curve, Raman spectrum, stress-strain curves, and electrochemical characterization of ENCF electrode, I-V curves of ENCF and NCF (PDF) AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; [email protected]


16

Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Notes The authors declare no competing financial interest ACKNOWLEDGMENT This study was supported by the Natural Science Foundation of China (no. 21406078 and 21536005), the Pearl River and S&T Nova Program of Guangzhou (201610010076), and Fundamental Research Funds for the Central Universities. REFERENCES (1) Wang, X.; Lu, X.; Liu, B.; Chen, D.; Tong, Y.; Shen, G. Flexible Energy-storage Devices: Design Consideration and Recent Progress. Adv. Mater. 2014, 26, 4763-4782. (2) El-Kady, M. F.; Kaner, R. B. Scalable Fabrication of High-power Graphene Microsupercapacitors for Flexible and On-chip Energy Storage. Nature Commun. 2013, 4, 1475. (3) Wang, X.; Liu, B.; Liu, R.; Wang, Q.; Hou, X.; Chen, D.; Wang, R.; Shen, G. Fiber-based Flexible All-solid-state Asymmetric Supercapacitors for Integrated Photodetecting System. Angew. Chem. Int. Ed. 2014, 53, 1849-1853. (4) Li, L.; Wu, Z.; Yuan, S.; Zhang, X.-B. Advances and Challenges for Flexible Energy Storage and Conversion Devices and Systems. Energy Environ. Sci. 2014, 7, 2101-2122. (5) Li, S.; Qiu, J.; Lai, C.; Ling, M.; Zhao, H.; Zhang, S. Surface Capacitive Contributions: Towards High Rate Anode Materials for Sodium Ion Batteries. Nano Energy 2015, 12, 224230 . (6) Zhao, Y.; Liu, J.; Hu, Y.; Cheng, H.; Hu, C.; Jiang, C.; Jiang, L.; Cao, A.; Qu, L. Highly Compression-tolerant Supercapacitor Based on Polypyrrole-mediated Graphene Foam Electrodes. Adv. Mater. 2013, 25, 591-595.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 23

(7) Qin, Y.; Peng, Q.; Ding, Y.; Lin, Z.; Wang, C.; Li, Y.; Xu, F.; Li, J.; Yuan, Y.; He, X. Lightweight, Superelastic, and Mechanically Flexible Graphene/Polyimide Nanocomposite Foam for Strain Sensor Application. ACS Nano 2015, 9, 8933-8941. (8) Li, P.; Kong, C.; Shang, Y.; Shi, E.; Yu, Y.; Qian, W.; Wei, F.; Wei, J.; Wang, K.; Zhu, H. Highly Deformation-tolerant Carbon Nanotube Sponges as Supercapacitor Electrodes. Nanoscale 2013, 5, 8472-8479. (9) Zhu, C.; Han, T. Y.-J.; Duoss, E. B.; Golobic, A. M.; Kuntz, J. D.; Spadaccini, C. M.; Worsley, M. A. Highly Compressible 3D Periodic Graphene Aerogel Microlattices. Nature Commun. 2015, 6, 6962. (10) Qin, Y.; Yuan, J.; Li, J.; Chen, D.; Kong, Y.; Chu, F.; Tao, Y.; Liu, M. Crosslinking Graphene Oxide into Robust 3D Porous N-Doped Graphene. Adv. Mater. 2015, 27, 51715175. (11) Wu, C.; Huang, X.; Wu, X.; Qian, R.; Jiang, P. Mechanically Flexible and Multifunctional Polymer-based Graphene Foams for Elastic Conductors and Oil-water Separators. Adv. Mater. 2013, 25, 5658-5662. (12) Zhao, J.; Lai, H.; Lyu, Z.; Jiang, Y.; Xie, K.; Wang, X.; Wu, Q.; Yang, L.; Jin, Z.; Ma, Y. Hydrophilic Hierarchical Nitrogen-doped Carbon Nanocages for Ultrahigh Supercapacitive Performance. Adv. Mater. 2015, 27, 3541-3545. (13) Niu, Z.; Zhou, W.; Chen, X.; Chen, J.; Xie, S. Highly Compressible and All-solid-state Supercapacitors Based on Nanostructured Composite Sponge. Adv. Mater. 2015, 27, 60026008.


18

Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Xiao, K.; Ding, L. X.; Liu, G.; Chen, H.; Wang, S.; Wang, H. Freestanding, Hydrophilic Nitrogen-doped Carbon Foams for Highly Compressible All Solid-state Supercapacitors. Adv. Mater. 2016. 28, 5997-6002. (15) Candelaria, S. L.; Shao, Y.; Zhou, W.; Li, X.; Xiao, J.; Zhang, J.-G.; Wang, Y.; Liu, J.; Li, J.; Cao, G. Nanostructured Carbon for Energy Storage and Conversion. Nano Energy 2012, 1, 195-220. (16) Yan, J.; Wang, Q.; Wei, T.; Fan, Z. Interconnected Frameworks with a Sandwiched Porous Carbon Layer/Graphene Hybrids for Supercapacitors with High Gravimetric and Volumetric Performances. Adv. Energy Mater. 2014, 4, 1300816. (17) Zhu, D.; Cheng, K.; Wang, Y.; Sun, D.; Gan, L.; Chen, T.; Jiang, J.; Liu, M. Nitrogendoped Porous Carbons with Nanofiber-like Structure Derived from Poly (aniline-co-pphenylenediamine) for Supercapacitors. Electrochim. Acta 2017, 224, 17-24. (18) Miao, L.; Duan, H.; Liu, M.; Lu, W.; Zhu, D.; Chen, T.; Li, L.; Gan, L. Poly(ionic liquid)derived, N, S-codoped Ultramicroporous Carbon Nanoparticles for Supercapacitors. Chem. Eng. J. 2017, 317, 651-659. (19) Wang, G.; Wang, H.; Lu, X.; Ling, Y.; Yu, M.; Zhai, T.; Tong, Y.; Li, Y. Solid-state Supercapacitor Based on Activated Carbon Cloths Exhibits Excellent Rate Capability. Adv. Mater. 2014, 26, 2676-2682. (20) Wang, W.; Liu, W.; Zeng, Y.; Han, Y.; Yu, M.; Lu, X.; Tong, Y. A Novel Exfoliation Strategy to Significantly Boost the Energy Storage Capability of Commercial Carbon Cloth. Adv. Mater. 2015, 27, 3572-3578. (21) Dupuis, A.-C. The Catalyst in the CCVD of Carbon Nanotubes-A Review. Prog. Mater Sci. 2005, 50, 929-961.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

(22) Chen, Y.; Fu, K.; Zhu, S.; Luo, W.; Wang, Y.; Li, Y.; Hitz, E. M.; Yao, Y.; Dai, J.; Wan, J. Reduced Graphene Oxide Films with Ultrahigh Conductivity as Li-ion Battery Current Collectors. Nano Lett. 2016, 16, 3616-3623. (23) Xiao, K.; Li, J.-W.; Chen, G.-F.; Liu, Z.-Q.; Li, N.; Su, Y.-Z. Amorphous MnO2 Supported on 3D-Ni Nanodendrites for Large Areal Capacitance Supercapacitors. Electrochim. Acta 2014, 149, 341-348. (24) Lu, Q.; Chen, J. G.; Xiao, J. Q. Nanostructured Electrodes for High-performance Pseudocapacitors. Angew. Chem. Int. Ed. 2013, 52, 1882-1889. (25) Cheng, S.; Yang, L.; Chen, D.; Ji, X.; Jiang, Z.-J.; Ding, D.; Liu, M. Phase Evolution of an Alpha MnO2-based Electrode for Pseudo-capacitors Probed by in Operando Raman Spectroscopy. Nano Energy 2014, 9, 161-167. (26) Liu, M.; Shi, M.; Lu, W.; Zhu, D.; Li, L.; Gan. L. Core-shell Reduced Graphene oxide/MnOx@carbon Hollow Nanospheres for High Performance Supercapacitor Electrodes. Chem. Eng. J. 2017, 313, 518-526. (27) Xu, C.; Xu, Y.; Tang, C.; Wei, Q.; Meng, J.; Huang, L.; Zhou, L.; Zhang, G.; He, L.; Mai, L. Carbon-coated Hierarchical NaTi2(PO4)3 Mesoporous Microflowers with Superior Sodium Storage Performance. Nano Energy 2016, 28, 224-231. (28) Pan, A. Q.; Wu, H. B.; Zhang, L.; Lou, X. W. Uniform V2O5 Nanosheet-assembled Hollow Microflowers with Excellent Lithium Storage Properties. Energy Environ. Sci. 2013, 6, 14761479. (29) Yang, T.; Zhou, R.; Wang, D; Jiang, S.; Yamauchi, Y.; Qiao, S.; Monteiro, M.; Liu, J. Hierarchical Mesoporous Yolk-shell Structured Carbonaceous Nanospheres for High


20

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Performance Electrochemical Capacitive Energy Storage. Chem. Commun. 2015, 51, 25182521. (30) Chen, S.; Duan, J.; Tang, Y.; Qiao, S. Hybrid Hydrogels of Porous Graphene and Nickel Hydroxide as Advanced Supercapacitor Materials. Chem. Eur. J. 2013, 19, 7118-7124. (31) Chen, S.; Xing, W.; Duan, J.; Hu, X.; Qiao, S. Nanostructured Morphology Control for Efficient Supercapacitor Electrodes. J. Mater. Chem. A 2013, 1, 2941-2954 (32) Wang, S.; Xia, L.; Yu, L.; Zhang, L.; Wang, H.; Lou, X. W. Free-standing Nitrogen-doped Carbon Nanofiber Films: Integrated Electrodes for Sodium-ion Batteries with Ultralong Cycle Life and Superior Rate Capability. Adv. Energy Mater. 2016, 6, 1502217. (33) Hu, H.; Zhao, Z.; Wan, W.; Gogotsi, Y.; Qiu, J. Ultralight and Highly Compressible Graphene Aerogels. Adv. Mater. 2013, 25, 2219-2223. (34) Xiao, K.; Ding, L.-X.; Chen, H.; Wang, S.; Lu, X.; Wang, H. Nitrogen-doped Porous Carbon Derived from Residuary Shaddock Peel: A Promising and Sustainable Anode for High Energy Density Asymmetric Supercapacitors. J. Mater. Chem. A 2016, 4, 372-378. (35) Xu, Y.; Lin, Z.; Huang, X.; Liu, Y.; Huang, Y.; Duan, X. Flexible Solid-state Supercapacitors Based on Three-dimensional Graphene Hydrogel Films. ACS Nano 2013, 7, 4042-4049. (36) Wang, S.; Zhang, L.; Sun, C.; Shao, Y.; Wu, Y. Lv, J.; Hao, X. Gallium Nitride Crystals: Novel Supercapacitor Electrode Materials. Adv. Mater. 2016, 28, 3768-3776. (37) Yang, P.; Chao, D.; Zhu, C.; Xia, X.; Zhang, Y.; Wang, X.; Sun, P.; Tay, B. K.; Shen, Z. X.; Mai, W. Ultrafast-charging Supercapacitors Based on Corn-like Titanium Nitride Nanostructures. Adv. Sci. 2015. 3, 1500299.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

(38) Chen, G.; Su, Y.; Kuang, P.; Liu, Z.; Chen, D.; Wu, X.; Li, N.; Qiao, S. Polypyrrole Shell@3D-Ni Metal Core Structured Electrodes for High-performance Supercapacitors. Chem. Eur. J. 2015, 21, 4614-4621.


22

Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents


23

Highly Compressible Nitrogen-Doped Carbon Foam Electrode with

Recommend Documents