Nanocellulose-Enabled, All-Nanofiber, High-Performance

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Energy, Environmental, and Catalysis Applications

Nanocellulose enabled all-nanofiber, high performance supercapacitor Qi Zhang, Chaoji Chen, Wenshuai Chen, Glenn Pastel, Xiaoyu Guo, ShouXin Liu, Qingwen Wang, Yixing Liu, Jian Li, Haipeng Yu, and Liangbing Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17414 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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ACS Applied Materials & Interfaces

Nanocellulose Enabled All-Nanofiber, High Performance Supercapacitor Qi Zhang†,§, Chaoji Chen‡,§, Wenshuai Chen†, Glenn Pastel‡, Xiaoyu Guo†, Shouxin Liu†, Qingwen Wang†, Yixing Liu†, Jian Li†, Haipeng Yu*†, Liangbing Hu*‡ †

Key laboratory of Bio-based Material Science and Technology of Ministry of Education, Northeast Forestry University, Harbin 150040, China.

‡

Department of Materials Science and Engineering, University of Maryland, College Park, Maryland, 20742

§

These authors contributed equally to this work.

* Corresponding to: [email protected]; [email protected]

KEYWORDS:

cellulose

nanofibers;

forest

nanomaterials;

nanocellulose cathode, asymmetric supercapacitors

ACS Paragon Plus Environment

nanocellulose

carbon,

ACS Applied Materials & Interfaces 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

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ABSTRACT Nanocellulose has been used as a sustainable nanomaterial for constructing advanced electrochemical energy storage systems with renewability, lightweight, flexibility, high performance, and satisfying safety. Here, we demonstrate a high-performance all-nanofiber asymmetric supercapacitor (ASC) assembled using a forest-based, nanocellulose-derived hierarchical porous carbon (nanocellulose-carbon, HPC) anode, a mesoporous nanocellulose membrane separator (nanocellulose-separator) and a NiCo2O4 cathode with nanocellulose carbon

as

support

matrix

(nanocellulose-cathode,

HPC/NiCo2O4).

HPC

has

a

three-dimensional (3D) porous structure comprised of interconnected nanofibers with an ultrahigh surface area of 2046 m2 g−1. When integrated with the mesoporous feature of the nanocellulose membrane separator, these properties facilitate both the quick delivery of ions and electrons even with thick (up to several hundreds of micrometres) and highly-loaded (5.8 mg cm2) ASC design. Consequently, the all-nanofiber ASC demonstrates a high electrochemical performance (64.83 F g−1 (10.84 F cm−3) at 0.25 A g−1 and 32.78 F g−1 or 5.48 F cm−3 at 4 A g−1) that surpasses most cellulose-based ASCs ever reported. Moreover, the nanocellulose components promise renewability, low cost, and biodegradability, thereby representing a promising direction toward high-power, environmentally friendly, and renewable energy storage devices.


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1. INTRODUCTION Sustainable carbon nanomaterials derived from renewable biomasses have caught intensive research interest because of a wealth of raw materials, green synthesis approaches, and good electrochemical properties.1–8 As a value-added refinery product of biomass, nanocellulose can be massively extracted via a combined method of chemical treatment and mechanical nanofibrillation process.9 Owing to the high Young’s modulus and rich hydroxyl group on the surfaces, nanocellulose and its derivates have been modified and assembled into membranes and aerogels with robust mechanical strength and high porosity for electrodes and separators. Hence, nanocellulose is regarded as one of the best candidates for fabricating novel composites and energy devices with low cost, good flexibility, and friendliness.10 Recently, porous carbon derived biomass has attracted much attention as the electrode material used in supercapacitors, lithium ion batteries and lithium-sulphur batteries. Nanocellulose as the predominant component of biomass can be directly converted into carbon nanofibers by removing the organic constituents through pyrolysis in an inert atmosphere without destroying the natural morphology of the precursor.11,12 The density and porosity of the resulting carbon products are regulated by adjusting the initial content of nanocelluloses, which is conducive to the preparation of porous carbon materials. Besides, the nanocelluloses contain abundant hydroxyl moieties on the surfaces, which can be utilized as active sites for the decoration of heteroatoms before pyrolysis, thus enabling various types of functionalization to design new cathodes. One successful example comes from Ji’s group13, where cellulose-derived carbon with abundant nanopores served as an anode material for sodium-ion

batteries.

Zhang

and

co-workers

developed


a

dopant-tunable


cellulose/polyaniline-derived

hard

carbon

anode

with

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enhanced

electrochemical

performance14. In addition to act as electrode materials after carbonized, nanocellulose can also be directly used as binders, matrix, separator, and electrolyte in the energy storage device. Recently, Lee and co-workers reported on an all inkjet-printed supercapacitor based on a cellulose-based paper with reliable electrochemical performance.15,16 The same group developed a terpyridine-nanocellulose nanoporous membrane as separator.17,18 This type of nanocellulose-based separator suppressed the capacity fading caused by Mn2+, consequently obtaining a better performance under high temperature cycling. Lars Wågberg et al. demonstrated that nanocellulose was an excellent binder in fabricating a flexible nanopaper/LiFePO4 cathode for lithium-ion batteries.19 Here, we demonstrate an integration strategy by using nanocellulose both as electrodes (anode and cathode) and separator. All-nanofiber asymmetric supercapacitor (ASC) is successfully fabricated by employing nanocellulose-derived hierarchical porous carbon (HPC) as an anode, mesoporous nanocellulose membrane as a separator, and spinel NiCo2O4 in-situ growth on HPC (HPC/NiCo2O4) as a cathode. ZnCl2 as a pore regulation agent is introduced during the aerogel foam formation, leading to high porosity and large specific surface area in the HPC and HPC/NiCo2O4. Meanwhile, the nanocellulose membrane is prepared featured a high porosity (~59%), a high electrolyte uptake (770%), a high ionic conductivity (0.265 S cm−1) and a robust mechanical property (Figure S1). The integration of the three nanocellulose-based components into a single ASC device contributes to all-nanofiber architecture with an interconnected conductive network and abundant porous spaces. As a result, both the transport of ions and electrons is accelerated, leading to high capacitance,


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high energy/power density, and long circulation performance.

2. EXPERIMENTAL SECTION 2.1. Preparation of HPC. The nanocelluloses were extracted from the poplar wood according to our previously reported method.9 The 0.8 wt% nanocellulose suspension was mixed with ZnCl2 at the mass ratio of 1:4 and freeze-dried into an aerogel foam. Subsequently, the aerogel foam was pyrolyzed in furnace under argon. The temperature was increased to 750 °C at the rate of 3 °C min−1 and kept at this value for 2 h. Later, the temperature was cooled to room temperature at a decreasing rate of 5 °C min–1. Finally, the resulting HPC was washed with 1 mol L−1 hydrochloric acid, ethanol and distilled water to eliminate the remnant ZnCl2. 2.2. Preparation of the Mesoporous Nanocellulose Membrane. The mesoporous nanocellulose membrane was prepared by pump filtration of 10 g 0.8 wt% nanocelluloses into a membrane-shaped hydrogel and dried at 60 °C. The porosity (P) of the membrane is figured out using the bulk density of nanocellulose membrane (ρa) and the density of the dense cellulose (ρ0):

P(%) = 1

ρa ρ0

(1)

where ρa denotes the density of the nanocellulose membrane (g cm‒3), and ρ0 is the dense density of cellulose (1.59 g cm‒3) which is obtained from the simple mixing rule with a negligible gas density. 2.3. Preparation of the HPC/NiCo2O4 Composite. 1 mmol Ni(NO3)2·6H2O and 2 mmol Co(NO3)2·6H2O were dissolved in the 40 mL mixed solution (ethanol: water is 1:1). 5 mmol



urea was added in the solution and stirred for 10 min. The HPC aerogel foam was then added in the solution and stirred for 30 min. The mixture was reacted in a reactor at 100 °C for 24 h. The resulting product was dried and calcined for 2 h at 250 °C to yield the HPC/NiCo2O4 composite foam. The loading mass ratio of NiCo2O4 to HPC is 3:2.

3. RESULTS AND DISCUSSION Owing to the nano-sized dimension, high specific surface area, reactive surfaces containing hydroxyl groups, entangled web-like structures, and advantageous mechanical and thermal properties, nanocellulose can be utilized as a promising one-dimensional (1D) nanobuilding block for (1) constructing self-standing electrodes or separators, (2) integration with other electrochemical active materials and developing carbon-based porous materials and carbon hybrid materials, and (3) constructing multiple kinds of electrodes for electrochemical energy storage devices.10 The design concept and fabrication of the all-nanofiber ASC based on a nanocellulose-derived HPC anode, HPC/NiCo2O4 composite cathode, and mesoporous nanocellulose membrane separator is illustrated in Figure 1a and 1b.


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Figure 1. Schematic illustrations of (a) the fabrication of nanocellulose-derived HPC and HPC/NiCo2O4 composite aerogels, and their assembly into an all-nanofiber ASC device (b). The nanocelluloses were first extracted from wood by a combination of chemical pretreatment and high-intensity ultrasonic nanofibrillation.9 The resulting nanocelluloses exhibited fibrous morphology with diameters of 2−20 nm and lengths beyond 1 μm (Figure 2a). Because of high aspect ratios and massive exposed hydroxyl groups, the nanocelluloses, particularly for those with mass content above 0.8 wt%, easily intertwine together and facilitate the gelation. When ZnCl2 was added in the nanocellulose suspension, the bonding of Zn2+ ions to the nanocelluloses promoted the agglomeration of adjacent slender nanocelluloses and caused their assembly into bundles, which also intensified the gelation of the suspension (Figure 2b). After freeze-drying, the nanocellulose hydrogel was transformed into a lightweight nanocellulose aerogel foam. The nanocellulose foam displayed a 3D network structure formed by the nanocellulose fibre bundles (Figure 2c).



Figure 2. (a) TEM image showing the nanocelluloses. (b) Optical photographs of the nanocellulose suspension before and after ZnCl2 addition. SEM images of (c) nanocellulose aerogel foam, (d) HPC foam and (e) the inner nanoporous structure in the HPC. (f) TEM image showing the micropores within the surfaces of HPC. Electrochemical measurements of the HPC electrode: (g) CV curves at ascending scan rates, (h) GCD profiles taken at various current densities, and (i) specific capacitance changes with different current densities. After being carbonized in argon gas, the nanocellulose foam was transformed into HPC that inherits the porous fibrous interconnected structure of the nanocellulose foam with small shrinkage (Figure 2d). The backbone surface of the HPC is coarse and full of nanopores and mesopores (Figure S2). Meanwhile, the HPC preserved the inner 3D nanoporous structure


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which is composed of nanofibers and bundles (Figure 2e). In the Raman spectrum of HPC, both the D-band at 1350 cm−1 which is related to structural defects and the sp3 carbon plane, and the G-band at 1580 cm−1 assigned to the hexagonal sp2 carbon plane were detected (Figure S3). The ratio of ID to IG was 0.7, implying a high extent of graphitization.20-22 The nitrogen adsorption–desorption isotherm of the HPC exhibit a Type-I profile (Figure S4), which arises from the typical adsorption of micropores (Figure 2f). The corresponding surface area is 2046 m2 g−1. Such a highly porous and conductive 3D structure provides abundant reactive sites for charge transfer reactions and short delivery routes for electrons and electrolyte ions. Moreover, the good conductivity of HPC contributes to a low ohmic drop.23−25 The electrochemical property of the HPC-based electrode was tested in a three-electrode configuration. Figure 2g presents the cyclic voltammetry (CV) curves under the scan rates between 1–200 mV s−1. When the scan rate is 200 mV s−1, the CV curve still exhibits quasi-rectangular shape, demonstrating efficient and rapid charge transfer. Figure 2h shows the GCD profiles in the range of 0.5−10 A g−1, in which the almost symmetrical triangular shapes suggest the formation of double electrochemical layers and fast ions transfer inside the electrode. The electrode exhibits a competitive specific capacitance, demonstrated by 235, 211, 196, 170 and 150 F g−1 at 0.5−10 A g−1, respectively (Figure 2i). The specific capacitance of 235 F g−1 for HPC is higher than those of most biomass-derived carbon materials. The favourable electrochemical performance of HPC can benefit from its hierarchical porous and highly conductive structure which facilitates the electrolyte infiltration and quick ions and electrons delivery.4,26−29



The HPC/NiCo2O4 composite was fabricated via a solvothermal method. The SEM image shows that the 3D interconnected network of the composite is mainly composed of interweaving microfibres (Figure 3a). Massive nanoparticles evenly grow on the surfaces of the microfibres (Figure 3b). SEM image with a higher-magnification shows that the nanoparticles have a needle-like morphology with approximate lengths of 1 μm (Figure 3c). These vertically aligned needles display an enlarged accessible surface area (Figure 3d).30 The lattice fringes of the nano-domains with an interplane spacing of 0.28 nm related to the (220) plane confirms the crystalline nature of NiCo2O4 (Figure 3e). This can be further confirmed by the selected-area electron diffraction (SAED) pattern with diffraction rings corresponding to the (220), (311), (400) and (442) planes of NiCo2O4, respectively (Figure 3f) and the XRD results (Figure 3g).31


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Figure 3. (a-c) SEM images of the HPC/NiCo2O4 revealing the NiCo2O4 needles attached on the microfibrous HPC surfaces. (d,e) TEM images showing the morphology and nano-domain lattice fringes of NiCo2O4. (f) SAED image showing the cyclic diffraction patterns of NiCo2O4. (g) XRD pattern of the HPC/NiCo2O4 composite. Nitrogen sorption analysis of the HPC/NiCo2O4 composite: (h) Nitrogen adsorption–desorption isotherm; (i) Cumulative pore volume and (inset) pore-size distribution. Oxidation state distributions of the metal cations in NiCo2O4/HPC composite are also vital for electrical performance in addition to the phase structure. We carried out XPS measurements to further investigate the oxidation state and chemical composition of the



NiCo2O4 needles (Figure S5). Considering the two spin-orbit doublets characteristic of Ni2+ (855.6 eV, 873.5 eV), Ni3+ (855.9 eV, 872.5 eV) and two shakeup satellites (Figure S5b), the Ni 2p spectrum is fitted using the Gaussian fitting method. The Co 2p spectrum is also fitted by considering the two spin-orbit doublets characteristic of Co2+ and Co3+, and two shakeup satellites (Figure S5c). The satellites may be associated when the electrons of carbon from the HPC and the neighboring Ni and Co atoms become delocalized.32,33 The O1s spectra contains four oxygen contributions including O1 at 529.3 eV from typical metal–oxygen bonds, O2 at 530.7 eV associated with oxygen in OH− groups, O3 at 531.3 eV associated with a higher amount of defect sites with low oxygen coordination, and O4 at 532.8eV attributed to the multiplicity of physi- and chemi-sorbed water (Figure S5d).31 The presence of the O2 contribution suggests that the surface of NiCo2O4 has been hydroxylated to a certain degree. These results show that Ni3+/Ni2+ and Co3+/Co2+ coexisted in the as-synthesized HPC/NiCo2O4. The metal ions with a high valence state and oxygen defective sites present in the HPC/NiCo2O4, favoring the surface-dependent electrochemical reactions.30,32 Nitrogen adsorption-desorption isotherm measurement was conducted to examine the porous nature of the HPC/NiCo2O4 composite. The profile of the N2 isotherms in Figure 3h is identified as type-IV with a very small hysteresis loop, confirming the existence of a mesoporous structure. The isotherms are characteristic of micropore filling in the low-pressure region and display a steep adsorption in the high-pressure region, indicating that both micropore and mesopore filling occurred. The BET specific surface area of the composite is 588.1 m2 g−1, which is higher than 148.5 m2 g−1 of pure nanostructured NiCo2O4.31 The total pore volume is 0.491 mL g−1 (Figure 3i). The cumulative pore volume


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calculated by the NLDFT model shows that micropores account for 51.1% of the total pore volume. The average pore size of the multilevel pore structure is 0.8 nm. The mesopores have a favourable effect on the transport of electrolyte ions and are also responsible for charge storage. Consequently, the charge-transfer resistance can be reduced and the time required for charged ion motion will be shortened. To evaluate the capacitive performance of the composite, electrochemical performance of the HPC/NiCo2O4 electrode was also measured in a three-electrode configuration.34−38 CV curves at scan rates between 1–200 mV s−1 display similar shapes with redox peaks (Figure 4a), implying a battery characteristic with fast redox reaction kinetics.33 As the scan rate increased, the cathodic and anodic peak potentials gradually shift toward the positive/negative potential and the redox current increases, indicating diffusion controlled reaction kinetics.32 Along with the double layer effect of HPC, a pair of wide redox peaks can be observed at 0.21 V and 0.45 V under a scan rate of 20 mV s−1, which mainly correspond to the successive multiple Faradic reactions related to the redox couple of Ni2+/Ni3+ and Co2+/Co3+ (Figure 4b). The redox reactions in an alkaline electrolyte are expressed as: NiCo2O4 + OH- + H2O ↔ NiOOH + 2CoOOH + e−; CoOOH + OH− ↔ CoO2 + H2O + e−.39−41 The GCD profiles under 0.5–20 A g−1 exhibit obvious potential plateaus (Figure 4c), which are ascribed to a surface-confined Faradic contribution of NiCo2O4 and high reversibility of the composite electrode. On basis of the discharge curves, the specific capacitance is approximately 706 F g−1 at 0.5 A g−1 (Figure 4d). At a high current density of 20 A g−1, the capacitance can remain as high as 623 F g−1. It presents favourable rate capability with 88% capacitance retention when the discharge current increased from 0.5 A



g−1 to 20 A g−1. This result may result from the excellent charge transfer of NiCo2O4 that is assisted by the porous HPC.42 The electrodes also exhibit good reversibility with an efficiency of 96.8% after 1000 cycles at 10 A g−1 (Figure 4e). The GCD profiles corresponding to the 1st and 1000th charge–discharge cycles are almost identical, demonstrating that the structure of HPC/NiCo2O4 was well-retained after the prolonged stability test (Figure 4f).43,44 Such superior performance may ascribe to the high specific capacitance of NiCo2O4 and the interconnected pores in the HPC/NiCo2O4 composite. Compared with the previously reported NiCo2O4-based electrodes (Table 1), the electrochemical properties of this material are modest, but the retention of capacitance is superior.

Figure 4. Electrochemical measurements of the HPC/NiCo2O4 electrode in 6 mol L-1 KOH solution. (a) CV curves with the scan rates of 1−200 mV s−1; (b) CV curves at 20 mV s−1; (c) GCD profiles at different current densities; (d) Specific capacitance at current densities of 0.5−20 A g−1; (e) Stability of long-term charge-discharge at 10 A g−1. The inset shows the GCD profiles for the initial three and the final three cycles; (f) Comparison of the GCD


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profiles between the 1st and 1000th charge–discharge cycles at 1 A g−1.

Table 1. Comparison of the electrochemical performance of the HPC/NiCo2O4 electrode in this work with other reported NiCo2O4-based electrodes in literatures. Electrode structure

Specific

Mass

capacitance

loading

(F

g-1)

(mg)

Capacitance retention

Cycling reduction

Reference

HPC/NiCo2O4 foam

623 at 20 A g-1

3.0

91% from 1 to 20 A g-1

3.2%, 1000 cycles

this work

NiCo2O4/graphite paper

530 at 10 A g-1

1.0

81% from 1 to 10 A g-1

2.0%, 1000 cycles

[45]

NiCo2O4 naoflakes

700 at 50 A g-1

1.29

87% from 1 to 20 A g-1

26.4%, 15000 cycles

[33]

NiCo2O4 nanosheet/ Ni foam

610 at 20 A g-1

0.8

67% from 1 to 20 A g-1

15.1%, 6000 cycles

[45]

Porous NiCo2O4 nanowires

650 at 20 A g-1

1.54

68% from 1 to 20 A g-1

6.2%, 3000 cycles

[47]

NiCo2O4 nanosheet/ Ni foam

532 at 20 A g-1

1.0

70% from 1 to 20 A g-1

19.0%, 3000 cycles

[48]

To further study the reaction kinetics, electrons and ions-transfer resistance of the HPC and HPC/NiCo2O4-based electrodes for ASCs, EIS was performed on a scale from 0.01−100 kHz with a 5 mV voltage amplitude. The Nyquist plot in Figure S6a features a relatively large phase angle, which corresponds to almost-ideal capacitive behaviour and low Warburg impedance. In the low frequency region, it is obvious that the HPC electrode presents an almost vertical line which is close to the imaginary coordinates, representing ideal electric double layer capacitive behavior.49−52 In the same low frequency region, the HPC/NiCo2O4-based electrode shows a more inclined line, indicating the Faradic redox reactions. In the high frequency region, the small-diameter semicircle implies a low Rct



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(charge-transfer resistance) originating from the reversible redox reactions. Based on the fitting of an equivalent circuit, the Rct of HPC is calculated as 0.54Ω, which is larger than that of the HPC/NiCo2O4 (0.26 Ω). In the middle frequency region, the slope of Nyquist plot (i.e., the length of the Warburg-type line) expresses the ion diffusion behavior. By contrast, the Warburg-type line of HPC/NiCo2O4 is shorter, suggesting fast ion diffusion in the conductive network. In addition, the Bode phase curve in Figure S6b shows that the phase angle of HPC is 86°, which approaches to that of an ideal capacitor of 90°. The phase angle of the HPC/NiCo2O4 is 75°, owing to the Faradic reaction.53,54 An all-nanofiber ASC device based on a HPC anode, a HPC/NiCo2O4 cathode, and a KOH-saturated nanocellulose membrane separator was assembled (Figure 5a). The relative mass on the two electrodes is calculated based on the Coulombic charge balance (Q = C △V m, where C, △V, m is specific capacitance, the operating potential window and mass of HPC/NiCo2O4 or HPC, respectively). The mass balancing follows the equation (2) 51: m C  V  m C  V

(2)

Based on the electrochemical performance of the HPC/NiCo2O4 and HPC, the optimal mass ratio of the electrodes should be m+(HPC/NiCo2O4)/m−(HPC) = 0.66 in the ASC. Specifically, the loading masses of positive and negative active materials are 2.3 and 3.5 mg, respectively. The typical CV plots of the HPC and HPC/NiCo2O4 electrodes at 50 mV s−1 are displayed in Figure 5b. The nearly rectangular-shaped CV plot of the HPC shows an double layer capacitance behavior, while the deformed CV curve of the HPC/NiCo2O4 displays a redox behavior along with the double layer effect.55 Figure 5c shows the quasi-rectangular CV plots of the ASC under the scan rate of 50 mV s−1 in various potential windows,


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suggesting that a potential range of 0−1.6 V can be achieved by combining the two electrode materials. Figure 5d illustrates the CV curves for the ASC in the potential window of 0−1.6 V, where only slight shape distortion is observed at various scan rates of 5-200 mV s−1. All CV curves have similar shapes and retain a pair of redox peaks, corresponding to the Faradic reaction of the HPC/NiCo2O4 in the KOH electrolyte and the double layer contribution from the HPC. Figure 5e shows the GCD profiles of the ASC at different current densities. The charge-discharge curves demonstrate nonlinear nature, supporting the rapid and reversible oxidation/reduction peaks. The specific capacitance as a function of current density is plotted in Figure 5f, which are calculated to be 64.83 (10.84 F cm−3), 53.42, 44.39, 39.70, 34.89, and 32.78 F g−1 (5.48 F cm−3) at the current densities of 0.25, 0.5, 1, 2, 3, and 4 A g−1, respectively, based on the total mass of the ASC device. The Ragone plot in Figure 5g compares the energy/power densities between the all-nanofiber ASC and other reported ASC devices.56–63 The ASC exhibits an energy density of 23.05 Wh kg−1 (3.85 mWh cm−3) at a power density of 213 W kg−1 (35.63 mW cm−3) and maintains 11.65 Wh kg−1 (1.9 mWh cm−3) at an extremely high power density of 3.38 kW kg−1 (0.56 W cm−3). As compared with different carbon-metal oxides/hydroxides (Table S1), both energy density and power density of the all-nanofiber ASC are one of the highest values among previously reported metal oxide-based ASC devices. Continuous GCD measurements over 1000 cycles were performed to evaluate the cyclability of the all-nanofiber ASC device. The capacitance retention is shown in Figure 5h, which remains over 93% of the initial value after 1000 cycles. The good cycling stability is ascribed to the hierarchically porous structure that facilitates ions diffusion during the charge/discharge processes.47,64−66 For potential



practical application, we also demonstrate that the all-nanofiber ASC device is able to power a LED or drive a fan rotating (Figure 5i and Movie S1).

Figure 5. (a) Graphical diagram of the all-nanofiber ASC. (b) CV curves of HPC and HPC/NiCo2O4 in a three-electrode configuration in different working potential windows. Electrochemical performance of the ASC device: (c) CV curves in different potential windows (v=50 mV s−1); (d) CV curves and (e) GCD profiles of the ASC at different scan rates; (f) Specific capacitance of the ASC at various current densities, and (g) Ragone plots of the ASC and some reported values from other ASCs. (h) Cyclic stability and Coulombic


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efficiency for 1000 cycles (i) Optical photographs of the green LEDs and a fan powered by a prototype ASC cell. 4. CONCLUSIONS In summary, an all-nanofiber ASC has been proposed based on a nanocellulose-derived HPC anode, a mesoporous nanocellulose membrane separator, and a HPC/NiCo2O4 cathode. The all-nanofiber ASC device with a unique all-fibril structure possesses two key advantages: 1) highly conductive 3D interconnected fibril networks provide continuous pathways for fast electron transport; 2) hierarchical pores (macro-, meso- and micro-pores) not only benefit the electrolyte penetration thus accelerating the ions transfer but also provide rich active sites (micropores) for charge storage. As a result, the all-nanofiber ASC shows a high capacitance of 64.83 F g−1 (10.84 F cm−3) at 0.25 A g−1 and 32.78 F g−1 (5.48 F cm−3) at 4 A g−1 on a device level, which is among the highest values for cellulose-based ASC devices. The presented strategy starts with all-nanofiber electrodes and separator design and is able to balance the pore structure, energy/power densities, renewability and biodegradability, representing a rational design for renewable electrochemical devices that is not confined to supercapacitors.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b0xxxx. Characterization and electrochemical test methods; Information of the nanocellulose



membrane; SEM images, Raman spectrum, and nitrogen adsorption–desorption isotherms of the HPC. XPS spectroscopy, Nyquist plots and Bode phase diagram of the HPC/NiCo2O4 electrode; table of electrochemical performance comparison (PDF). A fan powered by a prototype ASC cell (AVI). AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (H.Y.). *E-mail: [email protected] (L.H.). §

These authors contributed equally to this work.

ORCID Haipeng Yu: 0000-0003-0634-7913 Liangbing Hu: 0000-0002-9456-9315 Shouxin Liu: 0000-0002-0491-8885 Notes The authors declare no competing interests.

ACKNOWLEDGEMENTS Authors gratefully acknowledge the funding from National Natural Science Foundation of China (No. 31622016), and the Natural Science Foundation of Heilongjiang Province of China (JC2016002). We also thank the grant of Fundamental Research Funds for the Central Universities (2572017DG01).


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Nanocellulose-Enabled, All-Nanofiber, High-Performance

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