Asymmetric Energy Storage Devices Based on Surface-Driven

Research Article pubs.acs.org/journal/ascecg

Asymmetric Energy Storage Devices Based on Surface-Driven Sodium-Ion Storage Min Yeong Song,†,§ Na Rae Kim,†,§ Se Youn Cho,† Hyoung-Joon Jin,*,† and Young Soo Yun*,‡ †

Department of Polymer Science and Engineering, Inha University, 100, Inha-ro, Nam-gu, Incheon 22201, Korea Department of Chemical Engineering, Kangwon National University, 346, Jungang-ro, Samcheok-si, Gangwon-do 25913, Korea

‡

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S Supporting Information *

ABSTRACT: Energy storage devices (ESDs) based on Na ions are potential sustainable power sources for large-scale applications. However, they suffer from an unsatisfactory electrochemical performance originating from the unfavorable intercalation of large and heavy Na ions. In this study, two different types of nanostructured carbons were fabricated from renewable bioresources by simple pyrolysis and used as an anode/cathode pair for surface-driven Na-ion storage. Hierarchically porous carbon nanowebs (HPCNWs) composed of highly defective pseudographitic layers were prepared from bacterial cellulose and used as the anode for Na-ion storage. In contrast, the corresponding cathode consisted of functionalized microporous carbon nanosheets (FM-CNSs) fabricated from waste coffee grounds. The HPCNWs and FM-CNSs exhibited pseudocapacitive Na-ion storage, achieving remarkably fast and stable energy storage for the anodic and cathodic potential ranges, respectively. Moreover, asymmetric ESDs based on HP-CNWs and FM-CNSs showed a high specific energy of ∼130.6 W h kg−1 at ∼210 W kg−1 and a high specific power of ∼15,260 W kg−1 at 43.6 W h kg−1 with a stable behavior over 3,000 cycles. KEYWORDS: Carbon nanofiber, Carbon nanoweb, Carbon nanosheet, Pseudocapacitor, Electrode, Hybrid capacitor

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ions.15−17 Therefore, the research in this field has been focused on developing surface-induced charge storage materials. Various nanostructured carbon-based materials (NCMs) have been reported as both anodes and cathodes for surfacedriven Na-ion storage.13,14,18−28 For the anodic potential region, NCMs such as 0-D carbon nanospheres,18,19 1-D carbon nanofibers,20,21 2-D carbon nanosheets,22−24 and 3-D carbon foams25 have shown high capacities and good rate capabilities with stable cycling. These NCMs have an amorphous carbon structure comprising nanometer-scale pseudographitic layers. It has been reported that the carbon microstructure plays an important role in the Na-ion storage anode.13 Na ions can be inserted in pseudographitic layers, while topological defect sites on hexagonal carbon layers can also store charges in a pseudocapacitive manner.13 On the other hand, for the cathodic potential range, heteroatoms such as O, N, S, and their hybrids located at the edge sites of the NCMs act as redox centers for the surface-driven charge storage.26−30 Additionally, the high surface area of NCMs can contribute to an increase in charge storage by forming electrochemical double layers (EDLs) on the electrode surface.26−30 These results suggest that highly porous and functionalized carbon structures are ideal cathode material platforms, while highly

INTRODUCTION

The demand for and consequent research interest in advanced high-energy and high-power power sources with long cycle lifetimes, applicable in both compact mobile electronic devices and electrical grid systems, have been increasing.1−3 Conventional Li-ion batteries (LIBs) show insufficient power densities and cycle lifetimes, despite possessing high-energy densities.3 Moreover, the limited availability of Li resources prevents their use for large-scale energy storage (e.g., electric vehicles and grid systems).4 Therefore, various energy storage systems, such as metal−air batteries,5−7 Li−S batteries,7,8 redox-flow batteries,9,10 among others,11−13 have been investigated as largescale power sources. Among them, Na-ion batteries (NIBs) are a feasible alternative for substituting LIBs, since Na is a sustainable and ubiquitous resource. Moreover, NIBs have a chemistry similar to that of LIBs, facilitating rapid advancements in several technological issues.4,13 The electrochemical performance of NIBs is highly dependent on the active electrode materials used for both the anode and cathode. Unfortunately, NIBs suffer from unfavorable Na-ion storage on the intercalation-based electrode typically used in LIBs, since Na ions are ∼55% larger and 330% heavier than Li ions, and show a 0.33 V higher electropotential (vs elemental Li).14 In contrast, Na-ion conductivity in conventional carbonate-based electrolytes is higher than that of Li-ion, indicating that surfacedriven charge storage could be more advantageous with Na © 2016 American Chemical Society

Received: August 18, 2016 Revised: November 2, 2016 Published: November 21, 2016 616

DOI: 10.1021/acssuschemeng.6b01991 ACS Sustainable Chem. Eng. 2017, 5, 616−624

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ACS Sustainable Chemistry & Engineering

Figure 1. Morphological characteristics of the FM-CNSs and HP-CNWs. (a) FE-SEM image of the FM-CNSs; (b and c) FE-TEM images of the FM-CNSs under different magnifications. (d) FE-SEM image of the HP-CNWs; (e and f) FE-TEM images of the HP-CNWs under different magnifications. frozen at −196 °C and subsequently freeze-dried at −45 °C and 4.5 Pa for 72 h. The dried WCGs (5 g) were mixed with KOH (2.5 g, 95%, Samchun Pure Chemical Co., Ltd., Korea) in a mortar, and the mixture was heated to 800 °C for 2 h at a rate of 10 °C min−1. The resulting product (FM-CNSs) was washed using distilled water and ethanol (99.9%, OCI Company, USA) and dried in a vacuum oven at 30 °C. Preparation of the HP-CNWs. BC pellicles were synthesized using Acetobacter xylinum BRC 5 in a Hestrin and Schramm (HS) medium according to a previously reported method.31 All cells utilized in this study were precultured in a test tube for 3 days until maximal activity was achieved. Then, 50 μL of active bacteria was injected in a culture dish with 10 mL of HS medium and incubated at 30 °C for 7 days. The fabricated BC hydrogels were immersed in a 0.25 M aqueous sodium hydroxide solution (NaOH, 97.0%, Daejung, Korea) for 48 h at room temperature to remove the bacteria and residual HS medium. The purified BC hydrogels were neutralized by washing with a large amount of deionized water, and immersed in tert-butanol. After freezing at −30 °C for 6 h, the BCs were freeze-dried at −45 °C and 4.5 Pa for 72 h. The as-prepared BC cryogels were heated to 800 °C for 2 h at a rate of 2 °C min−1. The resulting HP-CNWs were stored in a vacuum oven at 30 °C. Characterization. The sample morphology was examined using field-emission scanning electron microscopy (FE-SEM, S-4300, Hitachi, Japan) and field-emission transmission electron microscopy (FE-TEM, JEM2100F, JEOL, Japan). Topographical images of the samples were obtained using an atomic force microscope (AFM, Cypher, Oxford Instruments AFM Inc.) with a tapping mode cantilever. The Raman spectra were recorded using a linearly polarized continuous-wave laser (514.5 nm, 2.41 eV, 16 mW). The laser beam was focused using a ×100 objective lens, resulting in a spot with a diameter of ∼1 μm. The acquisition time and number of cycles collected for each spectrum were 10 s and 3, respectively. X-ray diffraction (XRD, Rigaku D/MAX 2500) analysis was performed using Cu Kα radiation (λ = 0.154 nm) at 40 kV and 100 mA. The chemical composition of the samples was examined by X-ray photoelectron spectroscopy (XPS, PHI 5700 ESCA, USA) using monochromatic Al Kα radiation (hν = 1,486.6 eV). The pore structure of the samples was analyzed using N2 adsorption and desorption isotherms obtained with a surface area and porosimetry analyzer (Tristar, Micromeritics, USA) at −196 °C.

defective pseudographitic nanostructures are required for the Na-ion storage anode. Hence, to maximize the surface-induced Na-ion storage performance, it is crucial to produce electrode materials with a sophisticated design containing a well-defined microstructure, high surface area, and numerous redox-active sites, as well as a high-aspect-ratio nanostructure. Furthermore, high-performance energy storage devices using Na ions as charge carriers can be fabricated using a combination of NCMbased anode and cathode pairs. Nevertheless, reports on the specific design of NCM-based electrodes and related devices are scarce. In this study, functionalized microporous carbon nanosheets (FM-CNSs) and hierarchically porous carbon nanowebs (HPCNWs) were fabricated from sustainable bioresources such as waste coffee grounds (WCGs) and bacterial cellulose (BC), respectively, by simple pyrolysis. The FM-CNSs and HPCNWs exhibited similar amorphous carbon microstructures composed of nanometer-scale hexagonal carbon layers. The FM-CNSs also possessed a high specific surface area with numerous micropores and a number of O- and N-containing species; the HP-CNWs exhibited a hierarchical pore structure (composed of high-aspect-ratio nanofibers), and a moderate surface area and oxygen group quantity. The unique characteristics of the FM-CNS and HP-CNW materials led to a high electrochemical performance of the corresponding Na-ion storage cathode and anode, respectively. Furthermore, the asymmetric energy storage devices (AESDs) based on FMCNSs//HP-CNWs electrode pairs showed a high specific energy of ∼130.6 W h kg−1 and high specific power of ∼15,260 W kg−1 with long-term cycling stability.

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EXPERIMENTAL SECTION

Preparation of the FM-CNSs. FM-CNSs were prepared using a previously reported method.28 The WCGs obtained from commercial beverage manufacturers were ultrasonicated in N,N-dimethylformamide (99.8%, Sigma-Aldrich, USA) using a horn-type sonicator. The supernatant was vacuum-filtered, and the solvent was exchanged for tert-butanol (99.0%, Sigma-Aldrich, USA). The exfoliated WCGs were 617


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Figure 2. Microstructure of the FM-CNSs and HP-CNWs. (a) XRD patterns, and (b) Raman spectra of the FM-CNSs and HP-CNWs.

Figure 3. Surface properties of the FM-CNSs and HP-CNWs. XPS (a) C 1s spectra and (b) O 1s spectra of the FM-CNSs and HP-CNWs. (c) XPS N 1s spectra of the FM-CNSs. Electrochemical Characterization. The electrochemical properties of the FM-CNSs, HP-CNWs, and their asymmetric full cells were characterized using a Wonatec automatic battery cycler and CR2032type coin cells. For the half-cells, coin cells were assembled in an Arfilled glovebox using the FM-CNSs or HP-CNWs as working electrodes and a metallic Na foil as both reference and counter electrodes. NaPF6 (Aldrich, 99.99%) was dissolved in a solution of ethylene carbonate (EC) and propylene carbonate (PC) (1:1 v/v) with a concentration of 1 M and used as an electrolyte for Na-ion storage. A glass microfiber filter (GF/F, Whatman) was used as separator. The working electrodes were prepared by mixing the active material (80 wt %) with the conductive carbon (10 wt %) and polyvinylidene difluoride (10 wt %) in N-methyl-2-pyrrolidone. The resulting slurries were uniformly cast on an Al foil, and the resulting electrodes were dried at 120 °C for 2 h and roll-pressed. The active material mass loading was ∼1 mg cm−2, and the total electrode weight was 2−3 mg. For the asymmetric full cells, coin cells were assembled in an Ar-filled glovebox using the FM-CNSs and HP-CNWs as cathode and anode, respectively. The same electrolyte and separator were used, and the total electrode weight (both anode and cathode) was 4−5 mg.

The mass ratio between the FM-CNS cathode and HP-CNW anode was ∼1.3:1. For the half-cells, the anode and cathode were galvanostatically cycled between 0.01 and 3.0 V vs Na+/Na and between 1.5 and 4.5 V vs Na+/Na, respectively, at various currents. In addition, the AESDs were galvanostatically cycled between 0.5 and 4.2 V. To assemble the AESDs, the FM-CNSs and HP-CNWs were precycled with Na for 10 cycles, and the onset potential of both electrodes was 1.5 V vs Na+/Na.

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RESULTS AND DISCUSSION The morphologies of the FM-CNSs and HP-CNWs are shown in Figure 1. The FM-CNSs consist of large, flat, and randomly shaped particles with a lateral size of several micrometers and a thickness of ∼8 nm [Figures 1(a), 1(b), and S1 in the Supporting Information (SI)]. In contrast, the HP-CNWs have a 3-D porous structure composed of entangled nanofibers with a diameter of 10−20 nm [Figures 1(d) and 1(e)]. Additional FE-TEM images are shown in Figures S2 and S3 in the SI. The microstructures of the FM-CNSs and HP-CNWs were characterized by high-resolution FE-TEM imaging [Figures 618


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Figure 4. Pore characteristics of the FM-CNSs and HP-CNWs. (a) N2 adsorption and desorption isotherms, and (b) pore size distribution of the FM-CNSs and HP-CNWs.

Figure 5. Electrochemical performance of the HP-CNWs in 1 M NaPF6 dissolved in EC:PC (1:1 v/v) as an electrolyte over a voltage window of 0.01−3.0 V vs Na+/Na. (a) Galvanostatic discharge/charge profiles at a current density of 0.1 A g−1. (b) CV curves at a sweep rate of 0.1 mV s−1. (c) Rate capabilities at current densities of 0.1−15 A g−1, and subsequently 0.1 A g−1 again. (d) Cycling performance over 1,000 cycles at a current density of 0.5 A g−1.

bands (ID/IG) for the FM-CNSs and HP-CNWs were ∼0.94 and ∼0.93, respectively, indicating nanometer-sized hexagonal carbon layers. These results confirmed that the carbon microstructures of the FM-CNSs and HP-CNWs are composed of poorly stacked nanometer-scale hexagonal carbon layers. The surface properties of the FM-CNSs and HP-CNWs were investigated by XPS [Figures 3 and S4 in the SI]. During pyrolysis, cellulose and lignin molecules are transformed in double bonded carbonaceous intermediates at 150−300 °C by thermal cleavage of the glycosidic and ether bonds. By heating further, the depolymerized structures interconnect with each other and form aromatic structures (termed carbonization).33,34 During carbonization, almost all heteroatoms are emitted as gases and some participate in the formation of thermostable

1(c) and 1(f)]. Both samples exhibited an amorphous carbon structure without a long-range carbon ordering. The specific microstructures of the FM-CNSs and HP-CNWs were further characterized by XRD and Raman spectroscopy [Figure 2]. The XRD patterns of both samples show a broad graphitic (002) peak at 23.0°, indicating poorly stacked carbon layers [Figure 2(a)]. On the other hand, the Raman spectra of the FM-CNSs and HP-CNWs showed distinct D and G bands centered at 1,348/1,584 cm−1 and 1,348/1,591 cm−1, respectively, as depicted in Figure 2(b); these correspond to the disordered A1g breathing mode of the six-member aromatic ring close to the basal edge, and the hexagonal carbon structure related to the E2g vibration mode of the sp2-hybridized C atoms, respectively.32 The intensity ratios between the D and G 619


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Figure 6. Electrochemical performance of the FM-CNSs in 1 M NaPF6 dissolved in EC:PC (1:1 v/v) as an electrolyte over a voltage window of 1.5− 4.5 V vs Na+/Na. (a) Galvanostatic charge/discharge profiles at current densities of 0.1−10 A g−1. (b) CV curves at sweep rates of 1, 2, and 5 mV s−1. (c) Rate capabilities at current densities of 0.1−10 A g−1, and subsequently 0.1 A g−1 again. (d) Cycling performance over 1,000 cycles at a current density of 0.5 A g−1.

4(a)]. While the FM-CNSs contained mainly ∼2 nm pores, the HP-CNWs exhibited a broader pore size distribution, as shown in Figure 4(b). The specific surface area of the FM-CNSs is ∼1,764.8 m2 g−1, which is ∼14 times higher than that of the HP-CNWs (123.2 m2 g−1). The large number of micropores of FM-CNSs is advantageous for capacitive EDL charge storage. Hence, the FM-CNSs exhibited suitable characteristics to act as cathodes for Na-ion storage. In contrast, the HP-CNWs were mainly composed of defective pseudographitic layers, being suitable anodes for Na-ion storage. The Na-ion storage performance of the HP-CNWs was tested in the voltage window of 0.01−3.0 V vs Na+/Na at various current densities [Figure 5]. The first galvanostatic discharge profile showed a linear voltage variation in the potential range of 1.5−3.0 V, indicating a capacitive charge storage [Figure 5(a)]. Cyclic voltammogram (CV) curves of the HP-CNWs swept at 0.1 mV s−1 also showed EDLs in a similar voltage range, coinciding with the galvanostatic discharge profile [Figure 5(b)]. The profile showed a large plateau section starting at ∼0.5 V [inset of Figure 5(a)], which is present only in the first discharge profile. This plateau was possibly due to the formation of a solid-electrolyte-interface (SEI) layer.38 The CV curve showed a large peak at ∼0.4 V, which also supports the SEI layer formation [Figure 5(b)]. The charge profiles showed a reversible capacity of ∼210 mA h g−1 and were composed of three sections with different slopes, i.e., ∼0.1, 0.1−1.5, and 1.5−3.0 V [Figure 5(a)]. The plateau-like profile in the low voltage region (∼0.1 V) corresponded to a capacity of ∼30 mA h g−1, which was attributed to Na-ion storage induced by Na nanoclustering.13 The linear voltage

conjugated structures below 300 °C, remaining as functional groups even after carbonization at 1200 °C.34,35 The XPS spectra of the HP-CNWs and FM-CNSs show the presence of oxygen and oxygen-/nitrogen-containing functional groups, respectively [Figure S4 in the SI]. As shown in Figure 3(a), several peaks assigned to sp2 CC, sp3 C−C, C−O, and CO bonds centered at 284.3, 284.7, 285.7, and 288.9 eV, respectively, are observed in the XPS C 1s spectra of the FM-CNSs.36,37 Similar peaks from sp2 CC, sp3 C−C, C−O, and CO bonds centered at 284.3, 284.9, 286.2, and 289.7 eV, respectively, were also observed for the C 1s spectra of the HPCNWs [Figure 3(a)]. The oxygen configuration of the FMCNSs features a main CO bond centered at 531.8 eV and a minor C−O bond centered at 533.5 eV, while the HP-CNWs exhibit two main oxygen peaks: CO centered at 531.3 eV and C−O centered at 532.7 eV [Figure 3(b)].36,37 Additionally, the FM-CNSs contain four different types of N atoms, i.e., pyridinic-N, pyrrole/pyridonic-N, quaternary-N, and N−O bonds centered at 398.3, 400.4, 401.5, and 402.3 eV, respectively [Figure 3(c)].37 The C/O ratios for the FMCNSs and HP-CNWs are 5.0 and 18.6, respectively, while the C/N ratio of the former is 28.9. These data indicate that FMCNSs have a large number of surface heteroatoms, with most of the oxygen species existing as CO bonds, favoring pseudocapacitive charge storage. The porosity of the FM-CNSs and HP-CNWs was characterized by N2 adsorption and desorption isotherm measurements [Figure 4]. The FM-CNSs and HP-CNWs showed IUPAC type-I and type-II isotherms, indicating a microporous and macroporous structure, respectively [Figure 620


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Figure 7. Electrochemical performance of the AESDs based on HP-CNWs and FM-CNSs as anodes and cathodes, respectively, in 1 M NaPF6 dissolved in EC:PC (1:1 v/v) as an electrolyte over a voltage window of 0.5−4.2 V. (a) Galvanostatic charge/discharge profiles at current densities of 0.2, 0.4, and 0.8 A g−1. (b) CV curves at sweep rates of 5, 10, and 20 mV s−1. (c) Ragone plots of several AESDs based on HP-CNWs//FM-CNSs, Na-TNT//graphite,44 TiO2-RGO//AC,45 V2O5-CNT//AC,46 Na-TNT//AC,47 and NiCo2O4//AC.48 (d) Cycling performance over 3,000 cycles at a current density of 0.5 A g−1.

and surface-controlled charge storage were involved. Similar Na-ion nanoclustering was observed for the carbon nanoplates, with a b value of ∼0.6.13 The rate capability of the HP-CNWs was investigated by applying different current densities (0.1−15 A g−1) [Figure 5(c)]. Highly stable capacities of ∼105 mA h g−1 could still be obtained even at a density of 15 A g−1. In addition, when the current density returned to 0.1 A g−1 after 80 cycles, the HP-CNWs recovered their initial capacity, demonstrating good reversibility. The cycling stability of the HP-CNWs was tested over 1,000 cycles at a current density of 0.5 A g−1 [Figure 5(d)]. A specific capacity decay was observed during the first ∼20 cycles; however, stable capacities were subsequently maintained, and a specific capacity of ∼165 mA h g−1 was achieved after 1,000 cycles, indicating a good cycling performance of the HP-CNWs. The Na-ion storage performance of the FM-CNSs was tested in a voltage window of 1.5−4.5 V vs Na+/Na at various current densities [Figure 6]. The galvanostatic charge/discharge profiles of the FM-CNSs were linear along the entire voltage range at all current rates, indicating capacitive charge storage [Figure 6(a)]. CVs of the FM-CNSs also showed close-torectangular shapes, supporting the capacitive charge storage mechanism [Figure 6(b)]. The b value of the FM-CNSs was ∼1 for the overall voltage window [Figure S6 in the SI]. The FMCNS sample has a high specific surface area and numerous heteroatoms, as shown by the N2 adsorption and desorption data and XPS results, respectively. The open surface area can store charges by a physical adsorption/desorption mechanism.

increase between 0.1 and 1.5 V could result from the pseudocapacitive Na-ion storage on the topological defect sites of hexagonal carbon layers.13,39 Most of the reversible capacities are induced from the pseudocapacitive Na-ion storage by ∼1.5 V. The steeper profile above ∼1.5 V is due to capacitive charge storage. These three different charge storage modes were also observed in the CV curves, as shown in Figure 5(b). The charge storage kinetics in the potential ranges of ∼0.1 V and 0.1−1.5 V were characterized using CV curves at different sweep rates [Figure S5 in the SI]. The peak currents from these curves increased with increasing sweep rate, which can be mathematically expressed as i = avb, where a and b are adjustable values.40 For diffusion-controlled charge storage, b is ∼0.5; in contrast, b is ∼1 for surface-controlled charge storage. In the potential range of 0.1−1.5 V, b is ∼1, indicating that charge storage is surface-controlled. Topological defect sites originating from Stone−Wales defects, vacancy defects, adatoms, edge defects, and highly distorted hexagonal carbons41 can act as redox centers for surface-induced charge storage.13,42 Although the HP-CNWs have a relatively low specific surface area (∼123.2 m2 g−1), the active surface area for Na-ion storage is not limited to the open surface area, because Na-ions can be inserted in the pseudographitic layers for the anodic potential region.43 Therefore, almost all topological defects of the HPCNWs, possessing poorly stacked carbon layers, could react with Na ions, resulting in a large specific capacity with a fast kinetic character. In contrast, b was ∼0.72 for the low-voltage plateau, indicating that both diffusion-controlled charge storage 621


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0.1 to 4.0 A g−1 were similar. The specific energy or specific power was calculated by multiplying the capacity or current density, respectively, by the average voltage. A high specific energy of 130.6 W h kg−1 was achieved at a specific power of 210 W kg−1, and a high specific power of ∼15,260 W kg−1 was obtained at a specific energy of 43.6 W h kg−1. More details can be observed in Table 1. Ragone plots of several AESDs based

In addition, oxygen- and nitrogen-containing functional groups can act as redox-active sites for the cathodic potential region.26−30 Hence, the unique microstructure and surface properties of the FM-CNSs facilitate their application as cathodes for Na-ion storage. The FM-CNSs exhibited a discharge capacity of ∼130 mA h g−1 at a current density of 0.1 A g−1, which gradually decreased with increasing current densities [Figure 6(c)]. At current densities of 0.2, 0.5, 1, 2, 5, and 10 A g−1, the FM-CNSs exhibited specific capacities of ∼123, ∼117, ∼111, ∼104, ∼95, and ∼85 mA h g−1, respectively. These data indicate that a high capacity retention (∼65%) was achieved over a 100-fold current density increase (0.1 to 10 A g−1). In addition, the FM-CNSs showed stable cycling performances over 1,000 cycles, as shown in Figure 6(d). The electrochemical performance of the HP-CNWs and FMCNSs was also tested in the cathodic and anodic voltage regions, respectively, as shown in Figure S7 in the SI. Both HPCNWs and FM-CNSs showed poor reversible capacities in the cathodic and anodic voltage regions, respectively. This result could originate from the different textural and surface properties between the HP-CNWs and FM-CNSs. Therefore, we used the HP-CNWs and FM-CNSs as anode and cathode, respectively, with an asymmetric configuration, instead of a symmetric one. The AESDs based on HP-CNWs and FM-CNSs as anode and cathode, respectively, were prepared after precycling the electrodes in half-cells with Na (the specific cell design is depicted in Figure S8 in the SI). During the first few cycles, both HP-CNWs and FM-CNSs do not have enough Coulombic efficiency, as shown in Figures 5 and 6, leading to an energy density loss of the AESDs. This issue can be solved by a precycling process, where stable SEI layers can be formed on the surface of the HP-CNWs/FM-CNSs. Accordingly, the electrochemical performances of both HP-CNWs and FMCNSs can be optimized. Furthermore, an additional charge injection is carried out during the precycling process. The HPCNW anode and FM-CNS cathode show most reversible capacities by ∼1.5 V vs Na+/Na, while their open-circuit voltages are ∼2.8 V vs Na+/Na. If both electrodes are assembled without the precycling process, the working voltage of the FM-CNSs is limited to 2.8 and 4.5 V vs Na+/Na. This originates a large energy imbalance between the anode and cathode, leading to a significant specific energy loss of the AESDs. Thus, the onset potential was tuned to 1.5 V vs Na+/ Na by charge injection during the precycling process, and both HP-CNW anode and FM-CNS cathode have a good energy balance in this controlled system, as shown in Figure S8 in the SI. The AESDs utilized Na ions as charge carriers, and they were operated over a wide voltage range of 0.5−4.2 V at different current densities. The corresponding galvanostatic charge/discharge curves showed a triangular shape, indicating typical supercapacitor-like charge storage [Figure 7(a)]. The continuous voltage increase/decrease originated from the surface-driven charge storage behavior of both electrodes, which was also confirmed by the rectangular-shaped CVs [Figure 7(b)]. The steep current variation slopes at switching potentials indicated a small mass transfer resistance, which was maintained even after increasing the scan rate by 20 mV s−1. The specific capacitance of the AESDs at 0.1 A g−1 was 59.6 F g−1, corresponding to 61.3 mA h g−1, and the calculated average voltage was ∼2.13 V. While the specific capacitance decreased with current density, the average voltages for current rates of

Table 1. AESD Electrochemical Performance Summary Current density (A g−1)

Capacity (mA h g−1)

Average voltage (V)

Specific energy (W h kg−1)

Specific power (W kg−1)

0.1 0.2 0.4 0.8 1.5 2.5 4.0 5.0 6.0 7.0 8.0

61.3 57.6 52.7 48.2 43.2 38.2 32.9 29.9 27.3 24.8 22.9

2.13 2.15 2.16 2.17 2.17 2.16 2.12 2.08 2.02 1.96 1.91

130.6 124.0 114.1 104.8 93.6 82.5 69.7 62.2 55.2 48.7 43.6

210 430 870 1,740 3,250 5,390 8,480 10,400 12,150 13,730 15,260

on HP-CNWs//FM-CNSs, Na-TNT//graphite, 44 TiO 2 RGO//AC,45 V2 O5 −CNT//AC,46 Na-TNT//AC, 47 and NiCo2O4//AC48 are shown in Figure 7(c). The HP-CNW// FM-CNS device shows the highest specific energy and power values, as well as an excellent cycling performance over 3,000 cycles with ∼100% Coulombic efficiency. About 85.4% of the initial capacitance was maintained after 3,000 cycles.

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CONCLUSIONS In summary, FM-CNSs and HP-CNWs were fabricated by simple pyrolysis of renewable bioresources, such as WCGs and BCs, respectively. Both FM-CNSs and HP-CNWs were composed of nanometer-scale hexagonal carbon layers with poor graphitic stacking. The FM-CNSs possessed a high specific surface area of ∼1,764.8 m2 g−1, numerous micropores, and abundant oxygen and nitrogen species, with C/O and C/N ratios of 5.0 and 28.9, respectively. In contrast, the HP-CNWs had a specific surface area of 123.2 m2 g−1, hierarchical pore structure, and some oxygen heteroatoms (C/O ratio of 18.6). The HP-CNWs and FM-CNSs showed high electrochemical performance for the anodic and cathodic potential ranges, respectively. Both electrodes stored Na-ions by a surface-driven pseudocapacitive mechanism, attaining fast and highly stable cycling. In addition, the HP-CNWs and FM-CNSs exhibited high specific capacities of ∼210 and 130 mA h g−1, respectively. Furthermore, the AESDs based on HP-CNW anodes and FMCNS cathodes exhibited a high specific energy of 130.6 W h kg−1 and high specific power of ∼15,260 W h kg−1, together with an excellent cycling performance over 3,000 cycles.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01991. AFM image of the FM-CNSs; FE-TEM images of the HP-CNWs and FM-CNSs under different magnifications; XPS spectra of the HP-CNWs and FM-CNSs; 622


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specific peak currents of the HP-CNWs characterized over different voltage ranges at different sweep rates, and of the FM-CNSs characterized at different sweep rates; galvanostatic discharge/charge profiles and rate capabilities of the HP-CNWs and FM-CNSs; and schematic image of the AESD anode and cathode potential curves during a galvanostatic charge/discharge cycle (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Young Soo Yun: 0000-0002-2937-9638 Author Contributions §

M.Y.S. and N.R.K. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by a grant from the Technology Development Program for Industrial Core funded by the Ministry of Trade, Industry & Energy, Republic of Korea (Project No. 10067368) and supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2016R1A2B4009601). This study was also supported by 2016 Research Grant from Kangwon National University.

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REFERENCES

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