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High-yield Harvest of Nanofibers/Mesoporous Carbon Composite by Pyrolysis of Waste Biomass and Its Application for High Durability Electrochemical Energy Storage Wu-Jun Liu, Ke Tian, Yan-Rong He, Hong Jiang, and Han-Qing Yu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es504184c • Publication Date (Web): 05 Nov 2014 Downloaded from http://pubs.acs.org on November 10, 2014

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Environmental Science & Technology

High-yield Harvest of Nanofibers/Mesoporous Carbon Composite by Pyrolysis of Waste Biomass and Its Application for High Durability Electrochemical Energy Storage

Wu-Jun Liu, Ke Tian, Yan-Rong He, Hong Jiang*, Han-Qing Yu Department of Chemistry, University of Science and Technology of China, Hefei 230026, China

∗

Corresponding author: Dr. Hong Jiang

E-mail: [email protected]

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ABSTRACTS

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Disposal and recycling of the large scale biomass waste is of great concern.

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Themochemically converting the waste biomass to functional carbon nanomaterials and bio-

4

oil is an environmentally-friendly apporach by reducing greenhouse gas emissions and air

5

pollution caused by open burning. In this work, we reported a scalable, “green” method for

6

the synthesis of the nanofibers/mesoporous carbon composites through pyrolysis of the

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Fe(III)-preloaded biomass, which is controllable by adjustment of temperature and additive of

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catalyst. It is found that the coupled catalytic action of both Fe and Cl species is able to

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effectively catalyze the growth of the carbon nanofibers on the mesoporous carbon and form

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magnetic nanofibers/mesoporous carbon composites (M-NMCCs). The mechanism for the

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growth of the nanofibers is proposed as an in-situ vapor deposition process, and confirmed by

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the XRD and SEM results. M-NMCCs can be directly used as electrode materials for

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electrochemical energy storage without further separation, and exhibit favorable energy

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storage performance with high EDLC capacitance, good retention capability, and excellent

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stability and durability (more than 98% capacitance retention after 10,000 cycles).

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Considering that biomass is a naturally abundant and renewable resource (over billions tons

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biomass produced every year globally) and pyrolysis is a proven technique, M-NMCCs can

18

be easily produced at large scale and become a sustainable and reliable resource for clean

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energy storage.


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TOC Art

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INTRODUCTION

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With the increasing of agricultural/industrial production or consumption, the generation

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of waste biomass has been greatly accelerated.1 For example, in China, it is estimated to

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generate more than 2.4 billion tons of waste biomass in the year 2010, an increase of 18.1%

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over the previous year.2 The waste biomass will be an abundantly available resources,

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whereas if mishandled, it will become a major source of environmental pollution. For instance,

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the energy contained in the biomass waste of China is amount to that of one billion of

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standard coal, while the fertilizer value of the nitrogen (N) and phosphorus (P) is more than

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that of the total chemical fertilizer produced in China. However, due to the lack of facilities,

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high labor costs, low economic benefit, and tough environmental regulations, it is not

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economically friendly to recycle the solid waste.3 Instead, most of the biomass waste are

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currently open burnt directly in many developing countries, causing serious environmental

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pollution. For example, the incineration of biomass waste will result in serious air pollution

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due to the emission of heavy metals, particulate matters (e.g., PM 2.5), and dioxin-like

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compounds during the combustion process.4-7 Therefore, it urgently needs to develop an

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environmentally friendly and economically feasible technology for recycling the large scale

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biomass waste.

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Fast pyrolysis, an thermochemical decomposition process of lignocellulosic biomass

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under anoxic conditions at an ultrahigh heating-up rate, has been heavily studied and

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commercially used in recent years8-10 (see Table S1 of Supporting Information (SI)). In a

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pyrolysis process, not only the renewable liquid fuel, but also the tons of biochar has been

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produced and has not found its value-added utilization. On the other hand, different

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carbonaceous materials, such as nanotubes and graphene, have been massively produced and

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found their application in separation, energy sorage, drug deliver, and bioimaging.11,12 For

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instance, carbonaceous materials are the most widely used electrical double layer capacitors

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(EDLC).13-15 Due to the more favorable cycle durability and electrical conductivity of the 4 ACS Paragon Plus Environment

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different carbonaceous materials have led to their use in almost 100% of commercial

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supercapacitors since 1960th.16 Conventional synthetic routes of cabon nanomaterials, such as

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chemical vapor deposition, and arc discharge synthesis, etc., usually need tedious and

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expensive synthesis paths as well as organic solvent and electrochemical treatment, which

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limit their large-scale production and commercialization.17-19 Although some researchers

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developed facile methods to prepare carbon nanomaterials by pyrolyzing silk and feathers,12,20

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using the complicated lignocellulosic biomass which contains ligin, cellulose, semicellulose,

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and ash to prepare carbon nanomaterials have not been found to date.

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Recently, we found that the functionalized carbonaceous materials could be formed in

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the pyrolysis of lignocellulosic biomass by adjusting the pyrolysis temperature.21,22 It is

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envisioned that we can develop a conceptually new method for in-situ growth of carbon fibers

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by catalytically reducing and depositing certain volatile low-molecular-weight organic

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compounds on the carbonaceous skeletons. In addition, the morphology and properties of as-

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prepared carbon composites could be tunable via deliberated adjustment of temperature and

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selection of catalysts. Thus, the main objective of this study is to (1) demostrate a new

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approach to synthesize magnetic nanofiber/mesoporous carbon composites (M-NMCCs) in

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one-pot by catalytic pyrolysis of lignocellulosic biomass with FeCl3, (2) evaluate the energy

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storage performance of M-NMCCs, and (3) reveal the formation mechanism of M-NMCCs.

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EXPERIMENTAL SECTION

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Materials. The biomass (sawdust, a naturally abundant lignocellulosic biomass waste) used in

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this work was collected from a local timber treatment plant in Hefei, China. Before used, the

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biomass was first washed several times with distilled water to remove the impurity particles

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and then dried at 378 K overnight. The dried biomass was then crushed by a high-speed rotary

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cutting mill, and the produced particles with the particle size smaller than 0.12 mm (120 5 ACS Paragon Plus Environment


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mesh) were screened for further use. The proximate analysis and elemental composition of the

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biomass was analyzed and shown in our previous work.23

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The precursor for the magnetic carbon nanofibers (M-NMCCs) was the Fe(III)-preloaded

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biomass, which was prepared in a biosorption process using sawdust as adsorbent at ambient

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temperature. Briefly, 10.0 g of sawdust and 1,000 mL of FeCl3 solution with a concentration

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of 10 mmol L–1 were mixed in a flask and shaken in a constant temperature oscillator at 200

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rpm for 300 min. Afterwards, the water in the mixture was evaporated under reduced pressure,

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and the solid residue was dried at 378 K overnight and sieved again to collect the particles

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with size smaller than 120 mesh, and the Fe-preloaded biomass with Fe content of

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approximately 1.0 mmol g–1 was obtained. For comparison, the Fe(III) preloaded biomass

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with other Fe source (e.g., Fe2(SO4)3 and Fe(NO3)3) and other metal preloaded biomass (e.g.,

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CuCl2 and NiCl2) were prepared in the same way.

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Synthesis of the M-NMCCs. The M-NMCCs were directly synthesized by fast

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pyrolysis of the Fe-loaded biomass under N2 flow. The fast pyrolysis experiments were

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performed in a pyrolyzed reactor described in our previous work.23 Briefly, A 4.0 g portion of

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the Fe-preloaded biomass sample was first placed in the feed pipe under a 400 mL min–1 of N2

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flow, which was maintained for 20 min to remove air from the pyrolysis system. Once the

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temperature rose to the setting value (873-1073 K), the Fe-loaded biomass sample was

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inserted into the quartz tubular reactor with a piston for pyrolysis. The volatiles produced in

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the pyrolysis process were swept out by the N2 flow (200 mL min–1), and condensed by cold

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ethanol to obtain bio-oil. When the fast pyrolysis process was accomplished, the solid residue

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in the heating zone was maintained for another one hour for further carbonization. Finally, the

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reactor was moved out of the heating zone and cooled in the N2 flow (200 mL min–1) to room

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temperature, and the M-NMCCs were obtained.

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Characterizations. The structural features of the M-NMCCs were investigated by

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nitrogen adsorption-desorption isotherms, which were carried out at 77 K on a Micromeritics 6 ACS Paragon Plus Environment

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Gemini apparatus (ASAP 2020 M+C, Micromeritics, USA). The surface morphology of the

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M-NMCCs was analyzed by SEM (Sirion 200, FEI electron optics company, USA) coupled

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with energy dispersive X-ray spectroscopy (EDX, INCA energy, UK) and TEM (JEOL-2100F,

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Japan). The surface composition and chemical state of the M-NMCCs were studied by X-ray

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photoelectron spectroscopy (XPS). XRD analysis of the M-NMCCs was carried out in an 18

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kW rotating anode X-ray diffractometer (MXPAHF, Rigaku, Japan) using nickel-filtered Cu

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Kα radiation source (30 kV/160 mA, λ=0.154056 nm). The Raman analysis of the M-NMCCs

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was performed in a Laser Raman spectrometer (LabRamHR, HORIBA Jobin Yvon, France).

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The radiation source was a laser operating at a wavelength of 514 nm and a power of 25 mW.

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Energy Storage Performance of the M-NMCCs. The electrochemical tests were

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conducted on a CHI 760D electrochemical workstation at room temperature in a three-

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electrode system, in which the Ag/AgCl electrode was used as a reference electrode, platinum

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wire was used as a counter electrode, and 0.5 mol L–1 of K2SO4 solution served as electrolyte.

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The work electrode was prepared by loading a slurry which consisted of 80 wt% active

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material (e.g., M-NMCCs), 10 wt% of conducting agent (carbon black), and 10 wt% of binder

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(polytetrafluoroethylene (PTFE) in methanol) on a nickel foam and dried at 353 K for 30 min.

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The determination and calculation of specific capacitance were described in Supporting

126

Information (SI). The electrochemical impedance spectroscopy (EIS) of the material was

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analyzed in a frequency range of 0.01 to 100,000 Hz, at the open circuit voltage with an

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alternate current amplitude of 5 mV.

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RESULTS AND DISCUSSION

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Effects of Temperature on the Structure and Composition of M-NMCCs. Pyrolysis

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temperature is a primary factor influencing the microstructure and morphology of M-NMCCs.

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Representative images of the M-NMCCs prepared at different temperatures are shown in 7 ACS Paragon Plus Environment


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Fig.S1 (623–773 K) and Fig. 1a-c (873–1073 K). As shown in Figures S1a, b, the nanofiber

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structure could not be formed at low pyrolysis temperatures (e.g., 623 and 673 K), while as

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the temperature was increased to 773 K, some small nanofiber structure was found in Fig. S1c.

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When the temperature was further increased to 873–1073 K, the nanofiber structure

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continuously grew to several micrometers long, suggesting that a high temperature was

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favorable to the growth of the nanofiber structure. The high coverage SEM images of the M-

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NMCCs are presented in Fig.S2, which shows that the carbon nanofibers tightly covered on

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the mesoporous carbon surface.

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Fig. 2a shows the Raman spectra of the M-NMCCs. The peaks around wavenumber of

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1355 and 1592 cm–1 are the characteristic D (defect) and G (graphitic) bands of carbon,

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respectively. The D band (~1355 cm–1) is attributed to the breathing mode of κ-point phonons

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of A1g symmetry, which reflects the disorder degree of the carbon materials, while the G band

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is ascribed to the E2g phonon of sp2 carbon atoms and reflects the graphitization degree of the

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carbon materials.24 For the M-NMCCs in this work, the intensity of D bands (ID) decreased

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with the increasing temperature, while the intensity of G bands (IG) increased with the

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increasing temperature. The high IG/ID value of the M-NMCCs-1073 indicates that the carbon

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nanofibers growing at a higher temperature had a greater graphitization degree, which is

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confirmed by the X-Ray Diffraction (XRD) results.

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The texture features of the M-NMCCs, e.g., surface area and pore structure, were

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analyzed by the nitrogen adsorption-desorption method. As shown in Fig. 2b, the nitrogen

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sorption isotherms of the M-NMCCs-873 and M-NMCCs-973 exhibited a typical IV type

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pattern according to the IUPAC classification,25 confirming the presence of mesopores in

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these materials. Strangely, differing from most of the porous materials reported

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previously,16,26 the adsorption and desorption branches of the M-NMCCs-873 and M-

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NMCCs-973 did not close completely in the relatively low pressures region. These

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observations were mainly resulted from the presence of a large amount of micropores in the 8 ACS Paragon Plus Environment

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materials, which trended to be collapsed under high-vacuum conditions in the nitrogen

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adsorption-desorption process. Similar results were also reported previously.27,28 Differing

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from the M-NMCCs-873 and M-NMCCs-973, the isotherm of the M-NMCCs-1073 exhibited

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a classic IV type pattern with an H2 type hysteresis loops, implying the presence of some

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macropores with non-uniform size and shape.29 With the nitrogen quantity adsorbed at

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different relative pressures, the surface properties (e.g., surface area, pore volume and size) of

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the M-NMCCs were calculated and are shown in Table S2. The Brunauer, Em-mett and Teller

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(BET) surface areas of the M-NMCCs decreased from 421.4 to 360.0 m2 g–1 with an increase

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in the pyrolysis temperature. The micropore area and volume were dominant in the M-

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NMCCs-873 and M-NMCCs-973, but significantly decreased in the M-NMCCs-1073,

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indicating that a high pyrolysis temperature is favorable to the growth of the pore size and

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volume.

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The transformation of the Fe species in the pyrolysis process was monitored by XRD

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(Fig. 2c). The pattern of the M-NMCCs-873 shows 7 peaks at 2θ of 30.1°, 35.7°, 37.1°, 43.2°,

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53.6°, 56.9°, and 62.7°, which are attributed to the Fe3O4 lattice planes (JCPDS, 19-0629).

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For the M-NMCCs-973, the new peak at 2θ of 40.6° is assigned to the Fe3C (201) lattice

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plane (JCPDS, 35-0772). With a further increase in the pyrolysis temperature to 1073 K, the

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Fe3O4 crystalline phase disappeared, but metallic Fe and Fe3C crystalline were formed.

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Furthermore, a strong peak due to graphitic carbon (2θ=26.1°) was observed for the M-

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NMCCs-1073, indicating the formation of the graphitic carbon at high temperatures with the

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catalysis of the Fe species.30

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With the XRD results, the transformation of the Fe species in the pyrolysis process can

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be explained by Eqs. 1–4: Fe3+ was initially hydrolyzed to some amorphous Fe species (e.g.,

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Fe(OH)3 and FeO(OH)) at 623 K (XRD pattern a in Fig. S3); The amorphous Fe species were

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then transformed to Fe2O3 at 673 K (XRD pattern b in Fig. S3), which was further reduced to

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Fe3O4 under mesothermal conditions (e.g., 773–973 K) by the reducing components (e.g., H2, 9 ACS Paragon Plus Environment


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CO, and amorphous carbon) formed in the biomass pyrolysis process.

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metallic Fe was attributed to the further reduction of the Fe3O4 by the amorphous carbon

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formed during biomass pyrolysis process, while the formation of Fe3C was mainly due to the

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partial dissolution of carbon atoms into metallic Fe.33 The energy-dispersive X-ray (EDX)

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spectroscopy reveals that there was 3.79 atom% of Fe in the material, but there were no

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nanoparticles of Fe species (e.g., Fe, Fe3C, and Fe3O4) in the SEM images (Fig.S4), the

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possible reason may be that the Fe species were coated by the in situ grown carbon nanofibers.

The formation of

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Fe3++3H2O →Fe(OH)3+3H+

(1)

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Fe(OH)3→FeO(OH) →Fe2O3

(2)

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3Fe2O3+4H2(CO, C)→2Fe3O4+4H2O(CO2, CO)↑

(3)

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Fe3O4+4C →3Fe+4CO ↑

(4)

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Effects of Catalysis on the Growing of the Nanofibers. As shown in Fig. 3, the Fe(III)-

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preloaded biomass was pyrolyzed to produce bio-oil, a renewable liquid as fuel or source of

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chemicals,8, 34 meanwhile to obtain M-NMCCs. It is a rapid, cost-effective, organic solution

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free, and readily-scalable route to prepare M-NMCCs. Another important factor affcting the

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formation of M-NMCCs was the catalyst, and numerous studies were undertaken to prepare

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nanofibers by catalysis and activation of metalic compounds. It is reported that iron

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compounds can catalyze the formation of carbon nanofibers and metal chloride can activate

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the porous carbon.35,36 However, we found that not all ferric salts and chloride can catalyze

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the formation of M-NMCCs. The nanofibers have not formed by pyrolysis of Fe(NO3)3 and

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Fe2(SO4)3) or CuCl2 and NiCl2 preloaded biomass (Fig.S5). Hence, it is supposed that the

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formation of the carbon nanofiber structure was the combined catalytic effects of both Fe(III)

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and Cl–. To verify this assumption, we pyrolyzed the Fe(NO3)3 and CuCl2 co-loaded biomass,

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in which Fe(III) and Cl– coexisted, the produced materials exhibited carbon nanofiber

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structure (Fig.S6). In the thermochemical reaction process without oxygen, FeCl3 can improve

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the decomposition of celluloses and hemicelluloses into small-molecular hydrocarbons, e.g., 10 ACS Paragon Plus Environment

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CH4, C2H4 and C2H2.37 These hydrocarbons, can deposit on the in-situ formed Fe species, e.g.,

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Fe3O4 and Fe3C, to grow the carbon nanofibers in a way similar to the chemical vapor

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deposition process (Fig.3). As evidenced by the transmission electron microscope (TEM)

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images (insert in Fig. 1a-c), some Fe species nanoparticles were observed on the bottom of the

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carbon nanofibers, suggesting the deposition and growth of the carbon nanofibers on the Fe

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species. For Fe(NO3)3 and Fe2(SO4)3, despite of their catalysis on the carbon nanofiber growth,

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they could not catalyze the formation of small-molecular-weight hydrocarbons, which are the

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carbon source for the growth of carbon nanofibers. While for CuCl2 and NiCl2, they have the

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similar effects on the catalytic formation of the small-molecular-weight hydrocarbons in

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biomass pyrolysis process, but could not catalyze the deposition and growth of the carbon

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nanofibers. To further confirm the vapor deposition and growth mechanism, we carried out

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another pyrolysis experiment, in which the feedstock was the FeCl3-preloaded biochar

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(without further release of small-molecular-weight hydrocarbons). As shown in Fig.S7,

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though the Fe3O4, Fe, and Fe3C species could be found in the XRD patterns of the products

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after pyrolysis, the carbon nanofiber structure could not be observed in the SEM images

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(Fig.S8).

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Application of the M-NMCCs for Electrochemical Energy Storage. To explore the

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potential applications of the as-synthesized M-NMCCs, the materials were fabricated into

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supercapacitor electrodes and characterized with cyclic voltammograms (CV) measurements.

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Fig. 4a shows the CV curves of the M-NMCCs prepared at different temperatures. The M-

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NMCCs-1073 exhibited almost rectangular and symmetric CVs, indicating that the dominant

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contribution of the capacitance was originated from EDLC38,39 The specific capacitance of the

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M-NMCCs-1073 calculated from its CV curve with a potential scan rate of 50 mV s–1 was 89

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F g–1, much higher than those of other two M-NMCCs (17 and 24 F g–1 for the M-NMCCs-

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873 and M-NMCCs-973, respectively). The high capacitance of the M-NMCCs-1073 was

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mainly attributed to its abundant mesopore and macropore structure (Table S2). It is known 11 ACS Paragon Plus Environment


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that the mesopore and macropore structure can improve the infinitely fast ion transport and

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decrease the ion “traffic jam” within the pores.40,41 As stated by Dahn et al.,42 When the pores

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size is less than 1.5 nm in diameter, it can be fully filled by the adsorbed electrolyte ions,

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causing ion “traffic jam”. While for the large mesopores, due to its big size, it can

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accommodate more ions within its pore structure.43,44 As the GCD curves shown in Fig. 4d,

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there is a plateau close to 0 V in the charged curve, suggesting that more electrolyte ions can

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accommodate, agreeing with the above discussion well.

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Moreover, the carbon nanofibers were found to enhance the ion diffusion in the electrode

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and act as electronic bridges between the activated carbon particles, thus greatly improving

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the capacitance performance of the M-NMCCs-1073.45 Furthermore, as revealed by the

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Raman spectra (Fig. 3a), the M-NMCCs-1073 showed a higher graphitization degree,

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endowing it with a good electronic conductivity, thus improving its capacitance

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performance.46 We also compared the capacitance performance of the carbon materials

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synthesized by pyrolysis of different Fe(III) (including FeCl3, Fe(NO3)3 and Fe2(SO4)3)

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preloaded biomass (Fig. S9), the specific capacitances of the carbon materials derived from

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Fe(NO3)3 and Fe2(SO4)3 preloaded biomass are only 19 and 21 F g–1 at scan rate of 50 mV s–1,

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much lower than that of the carbon materials derived from FeCl3 (i.e., M-NMCCs-1073, 89 F

256

g–1 at 50 mV s–1). The main reason for this phenomenon is that there is no carbon nanofiber

257

structure in the carbon materials derived from the Fe(NO3)3 and Fe2(SO4)3 preloaded biomass

258

(Fig. S5) and cannot effectively diffuse the ions and transport the electrons between the

259

carbon particles.

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Fig. 4b shows the CV curves of the M-NMCCs-1073 at different potential scan rates.

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The CV curves kept the “rectangular shape” even at a potential scan rate of 100 mV s–1. With

262

a further increase in the potential scan rate to 150 and 200 mV s–1, the CV curves were

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slightly distorted (Fig. S10), which was mainly due to the limited ion incorporation into the

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active electrode material at high scan rates.47 The specific capacitance of the M-NMCCs-1073 12 ACS Paragon Plus Environment

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was up to 128 F g–1 at a potential scan rate of 2 mV s–1 (Fig. 4c), and the capacitance per

266

surface area reached 0.36 F m–2, much higher than those of many other electric double-layer

267

capacitances reported previously (Table S3).

268

When the potential scan rate was continuously increased to 10 and then to 200 mV s–1,

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the specific capacitance decreased to 101 and 54 F g–1, respectively. Such a decrease in the

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specific capacitance was universal, which was possibly caused by the insufficient time

271

available for ion diffusion and transport inside the pores in the materials.19,48 Indeed, at a

272

potential scan rate of 200 mV s–1, the distortion of the rectangular shape of the CV curve was

273

clearly observed (Fig. S11, SI), which is a characteristic of ion diffusion and transport being

274

restricted on the EDLC electrode surface.48 To further understand the ion diffusion and

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transport characteristic of the M-NMCCs-1073 at different scan rates, the Trasatti’s method

276

was used to analyze the contribution of the total charge stored, which was described by the

277

following equation (Eq.5):

278

q = q0 + k ( v -0.5 )

(5)

279

where q (F/g) is the total stored charge involved in the slow access of electrolyte ions into the

280

electrode materials, v (mV/s) is the scan rate, q0 is the stored charge related to the accessible

281

outer surface, k is a constant, and the kv-0.5 is the stored charge controlled by diffusion. The

282

values of k and q0 can be obtained by plotting the v-0.5 dependence of the total stored charge.

283

Based on the above analysis, the stored charge controlled by the diffusion (kv-0.5) as a

284

function of the scan rates was presented in the insert of Fig. 4c, which further confirmed that

285

with the increasing of the scan rate, the diffusion of the ions into the electrode materials is

286

greatly restrained, thus causing the decrease of the specific capacitance. A small hump was

287

also observed at 0.4–0.6 V (vs Ag/AgCl) of the CV curves (Fig.4b), which can be attributed

288

to pseudocapacitive contribution of the redox process of remained Fe compounds.49,50 As

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shown in the XRD patterns (Fig. 2c), the main Fe species remained in the carbon nanofibers 13 ACS Paragon Plus Environment


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are Fe3O4 and Fe3C, which are very stable and hard to be released from composites after acid

291

treatment.51

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To further evaluate the capacitance performance of the M-NMCCs-1073, galvanostatic

293

charge/discharge (GCD) experiments were carried out at different current densities in a three-

294

electrode system. As shown in the GCD curves (Fig. 4d), the discharging curves show a small

295

Ohmic drop of at its start, which is attributed to the present of internal resistance in the

296

electrode materials.52 This internal resistance, as analyzed by the electrochemical impedance

297

spectroscopy, was 1.26 Ω. The specific capacitance was 99.1 F g–1 at a current density of 0.5

298

A g–1, and decreased to 72.0 F g–1 with an increase in the current density to 6.0 A g–1. About

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72.6% of the specific capacitance was retained when the current density was increased from

300

0.5 to 6.0 A g–1. The reason for a decrease in capacitance is that the effective interactions

301

between the ions and the electrode materials become deficient with the increase in current

302

density. 53

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The energy and power densities of the M-NMCCs-1073 were calculated by the following

304

equations (Eqs. 6 and 7)[52]

305

1 E= × Cm × (∆V ) 2 2

306

P=

(6)

E ∆t

(7)

307

where E (J g–1) is the energy density, Cm (F) is the specific capacitance deduced from the

308

GCD curves, P is the power density (W g–1), ∆V (V) is the potential change within the

309

discharge time ∆t (s). A Ragone plot is presented in Fig. 4e for the M-NMCCs-1073, which

310

shows that the energy density for the electrode material is about 11.3 Wh/kg at a current

311

density of 0.5 A g–1, and still keeps at 8.0 Wh/kg at a high current density of 6 A g–1. Note

312

that for the state-of-the-art of current electrochemical energy storage devices, the

313

specifications of the energy density are 70-100 and 5-8 wh/kg for the lithium ion batteries and


Page 15 of 29


314

commercial carbon supercapacitors, respectively,54,55 electrodes materials from this work

315

could deliver energy densities slightly better than those of state-of-the-art commercial carbon

316

supercapacitors.

317

The capacitive behavior of the M-NMCCs-1073 was further investigated by the

318

electrochemical impedance spectroscopy (EIS), in which the transport characteristics of the

319

charge carriers within the electrode material were illuminated. The EIS spectrum of the M-

320

NMCCs-1073 electrode was presented in Fig. 4f, and an equivalent circuit (lower inset of

321

Fig.3f) was proposed to fit the spectrum.56 This equivalent circuit was composed of Rs, a

322

resistor accounting for the solution resistance between the reference electrode (Ag/AgCl) and

323

the working electrode (M-NMCCs-1073); Rict, an electrode-electrolyte interfacial charge

324

transfer resistor; Zw, a finite length Warburg diffusion element including the encountered

325

impedances when the electrolyte ions diffuse from the bulk to the electrode-electrolyte

326

interfaces; Rect, a reaction electron transfer resistor accounting for the electron transfer

327

resistance involved in the redox reactions; Cdlc, a capacitor accounting for the electrical

328

double layers formed on the surfaces of the composite electrode in contact with the

329

electrolyte; and Cp, a capacitor accounting for the psudocapacitance generated from the

330

Faradaic redox reactions of the electrode material.

331

As shown in Fig. 4f, a semicircle was found at decreasing frequency, which was more

332

clearly observed from the upper inset of Fig. 4f, indicating that the composite interfacial

333

impedance and psudocapacitance impedance were non-negligible.57 An ‘onset’ frequency can

334

be defined as the highest frequency where the impedance begins to be dominated by the

335

capacitive behavior (i.e., the Nyquist plot starts to go vertical), which reflects the highest

336

frequency to achieve most of the capacitance.38 The ‘onset’ frequency of the M-NMCCs-1073

337

is 89 Hz, higher than many other carbon supercapacitors (e.g., 50 Hz),38 indicating the fast

338

electrolyte ion transfer in the electrode material.



Page 16 of 29

339

One of the most important advantages for the EDLC electrode material is its durability

340

and stability. The long term cycle performance of the M-NMCCs-1073 was evaluated by the

341

CV measurements at a potential scan rate of 50 mV s–1 (Fig. 4g). The specific capacitance still

342

remained at about 87.4 F g–1 after 10,000 cycles, more than 98% retention of the initial

343

capacitance (89.1 F g–1), indicating an excellent durability of the M-NMCCs-1073 used as a

344

supercapacitive electrode material. Furthermore, the rectangle shape of CV curves remained

345

unchanged even after 10,000 times of CV scan, showing the favorable stability of the

346

materials.

347

Environmental Implications. Energy storage in supercapacitors is mainly based on the

348

diffusion and transport of electrolyte ions on the surface of electrode materials.58 The

349

complicated porous structures and disordered texture of porous carbon often limits the

350

electron transfer efficiency and results in a poor electrical conductivity, thus greatly limiting

351

the performance of the supercapacitors.59 An effective method for this problem is to

352

composite the highly conductive carbon frame, e.g., carbon nanotubes and nanofibers, on

353

porous activated carbon, in which the carbon nanotubes/nanofibers can act as a structural

354

frame and an electron collector to improve the electron transfer in the electrochemical energy

355

storage

356

nanofibers/mesoporous carbon composites have much better performance in EDLC energy

357

stroage due to the synergistic effects of nanofibers and mesoporous carbon in which the

358

mesoporous carbon can offer more accessible surface area and pore structure for the migration

359

and diffusion of the electrolyte ions, thus improving the electrochemical energy storage

360

performance.61 Therefore, in this study, about 45%-50% of the initial biomass feedstock can

361

be converted to nanofibers/mesoporous carbon composites, all of which could be used as

362

supercapacitors.

process.60

Whereas,

compared

to

the

pure

carbon

nanofibers,

the

363

Though the specific capacitance of the as-synthesis is releative lower than those of

364

conventional porous carbon based materials, its excellent stability and durability (more than 16 ACS Paragon Plus Environment

Page 17 of 29


365

98% capacitance retention after 10,000 cycles) can make them find its applications in many

366

consumer pocket electronics. Considering that biomass is a naturally abundant and renewable

367

resource (over billions tons biomass produced every year globally) and pyrolysis is a proven

368

technique, the M-NMCCs can be easily produced at large scale and become a sustainable and

369

reliable resource for clean energy storage. In addition, a notable observation is that the

370

aggregation of the carbon nanofibers was avoided in pyrolysis, which might be one of the

371

factors responsible for the high performance of M-NMCCs. Besides, we also found that both

372

the yield and quality of bio-oil derived from the FeCl3 proloaded biomass have been improved

373

(Table S4 and Fig.S11), which is beneficial to the large-scale application of this technology.

374 375

Supporting Information

376

Tables S1-S4 and Figures S1-S11 is provided as supporting information for this manuscript.

377

This material is available via the Internet at http://pubs.acs.org

378 379 380

ACKNOWLEDGEMENTS

381

This work is sponsored by the Key Special Program on the S&T for the Pollution Control and

382

Treatment of Water Bodies (No.2012ZX07103-001), National 863 Program (2012AA063608-

383

01), National Key Technology R&D Program of the Ministry of Science and Technology

384

(2012BAJ08B00).

385

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Figure Captions

547 548

Figure 1. SEM and TEM (insert) images of the M-NMCCs: (a) M-NMCCs-873; (b) M-

549

NMCCs-973; and (c) M-NMCCs-1073.

550 551

Figure 2. (a) Raman spectra of the M-NMCCs; (b) nitrogen adsorption-desorption isotherms

552

of the M-NMCCs; and (c) XRD patterns of the M-NMCCs.

553 554

Figure 3. Schematic and mechanism illustration of the synthesis of the magnetic carbon

555

nanofiber from the lignocellulosic biomass waste.

556 557

Figure 4. Electrochemical performance of the M-NMCCs in a three-electrode system. (a)

558

Cyclic voltammograms (CV) curves of M-NMCCs synthesized at different temperature

559

(potential scan rate: 50 mV s-1); (b) CV curves of the M-NMCCs-1073 at different potential

560

scan rate; (c) Specific capacitances of the M-NMCCs-1073 at different potential scan rate; (d)

561

Galvanostatic charge-discharge curves of M-NMCCs-1073 at different current densities; (e)

562

Ragone plots of the M-NMCCs-1073; (f) Electrochemical impedance spectra (Nyquist plot)

563

of the M-NMCCs-1073, the upper inset is an enlargement of the high-frequency region of the

564

Nyquist plot, and the lower inset is the proposed equivalent circuit. (g) Long term cycling

565

performance of M-NMCCs-1073 (potential scan rate: 50 mV s-1).

566



Figure 1


Page 26 of 29

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Figure 2



mechanism

Figure 3


Page 28 of 29

Page 29 of 29


Figure 4 29 ACS Paragon Plus Environment

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