Interconnected Hierarchical Porous Carbon from Lignin-Derived

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Interconnected Hierarchical Porous Carbon from Lignin-derived Byproducts of Bioethanol Production for Ultrahigh Performance Supercapacitors Liming Zhang, Tingting You, Tian Zhou, Xia Zhou, and Feng Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02774 • Publication Date (Web): 16 May 2016 Downloaded from http://pubs.acs.org on May 19, 2016

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ACS Applied Materials & Interfaces

Interconnected Hierarchical Porous Carbon from Lignin-derived Byproducts of Bioethanol Production for Ultrahigh Performance Supercapacitors Liming Zhang, Tingting You, Tian Zhou, Xia Zhou, Feng Xu*

Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing, 100083, China

KEYWORDS: bioethanol production, lignin-derived byproducts, nitrogen-doped, hierarchical porous carbon, supercapacitors

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ABSTRACT: The advent of bioethanol production has generated abundant lignin-derived byproducts, which contain proteins and polysaccharides. These byproducts are inapplicable for direct material applications. In this study, lignin-derived byproducts were used for the first time as carbon precursor to construct an

interconnected

hierarchical porous nitrogen-doped

carbon

(HPNC) via

hydrothermal treatment and activation. The obtained HPNC exhibited favorable features for supercapacitor applications, such as hierarchical bowl-like pore structures, high specific surface area of 2218 m2 g−1, high electronic conductivity of 4.8 S cm−1, and a nitrogen doping content of 3.4%. HPNC-based supercapacitors in a 6 M KOH aqueous electrolyte exhibited high-rate performance with high specific capacitance of 312 F g−1 at 1 A g−1 and 81% retention at 80 A g−1, as well as excellent cyclic life of 98% initial capacitance after 20 000 cycles at 10 A g−1. Moreover, HPNC-based supercapacitors in the ionic liquid electrolyte of EMI-BF4 displayed an enhanced energy density of 44.7 Wh kg−1 (remaining 74% of max value) at a ultrahigh power density of 73.1 kWkg−1. The proposed strategy may facilitate lignin utilization and build a green bioethanol production process.

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1 Introduction Sustainable biorefinery technology has attracted considerable attention as an alternative to chemical refinery because of global crises, such as pollution and depletion of fossil fuels.1-4 Cellulosic bioethanol is one of the most applicable products from the biorefinery process.5 The operation of the world’s largest industrial cellulosic bioethanol plant of DuPont in Iowa (October 30, 2015) will shape bioethanol research and development. Large amount of lignin-derived byproducts have been generated during biorefinery process, e.g., approximately 90,000 tons of these byproducts were produced by the DuPont plant with an annual ethanol output of 114 million litres.6 Traditionally, most large-scale industrial bioethanol processes burn these waste lignin to generate power,1 which is one of the primary reasons of the high cost of bioethanol production. Thus, many studies are being conducted to transform the lignin-derived byproducts into value-added products.3 The potential high-value products derived from pure lignin include carbon fiber, engineering plastics, thermoplastic elastomers, polymeric foams and membranes, and various fuels and chemicals. However, lignin-derived byproducts from bioethanol production after enzymatic deconstruction and microbial fermentation contain proteins (cellulase and Saccharomyces cerevisiae) and recalcitrant polysaccharides, a mixture that limits the suitability of these byproducts for direct material applications.1 Nevertheless, the basic composition of waste lignin/polysaccharides and adhering protein can be carbonized into heteroatom-doped carbons through a simple high-temperature process, which have broad prospect in energy storage.7

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Note that carbon-based nanomaterials, such as graphene and nanoporous carbons, are one of most advanced supercapacitor electrode materials, because of their large specific surface areas, excellent chemical stability, and high conductivity.8-10 Recently, graphene-based

electrodes,

microwave-exfoliated

such

as

liquid

graphene12,

electrolyte-mediated

graphene11,

graphene,13

laser-scribed

and

chemical/thermal-reduced graphene,14-15 were used as supercapacitor electrodes with high-level supercapacitive performance. Compared with graphene-based electrodes, nanoporous carbon electrodes from natural biomass have lower cost and are more renewable, eco-friendly and cost-effective. Lignocellulose-inspired nanoporous carbons are rapidly developed for advanced supercapacitor electrodes through hydrothermal carbonization of rye straw,16 hemps,17 bamboo-based industrial byproduct,10 eucalyptus wood sawdusts,18 spruces,2 pollens,19 paper pulp mill sludge,20 microalgae,21 coconut shell,22-23 and high-temperature pyrolysis of waste corn cob,24 waste coffee grounds,25 seaweeds,26-27 and dead leaves9. Meanwhile, bioresource

materials

containing

proteins

such

as

egg

whites

and

eggshell

membranes,28-30 shiitake mushroom,31 silk,32 and human hairs33 have also been constructed to heteroatom-doped (nitrogen, sulfur and oxygen) nanoporous carbons for high-performance supercapacitors. Lignin-derived byproducts from bioethanol production are advantageous over the aforementioned kinds of resources. More lignin-derived byproducts will be generated with the rapid development of cellulosic fuel refineries. Additionally, self-doped nitrogen in lignin-derived byproducts, which can be incorporated into carbon frameworks during carbonization, can effectively enhance the surface ion-accessibility and electronic conductivity to benefit the electrochemical process.32, 34


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If the abundant lignin-derived byproducts from bioethanol production can be converted into nitrogen-doped nanoporous carbon electrodes for supercapacitors, a promising door will be opened to explore high-value products from the lignin-derived byproducts and effectively reduce cost of bioethanol production. In this study, the lignin-derived byproducts are for the first time used as carbon resources to construct nitrogen-doped nanoporous carbon materials. An interconnected hierarchical porous nitrogen-doped carbon (HPNC) was created using combined hydrothermal carbonization and chemical activation approach. The resulting HPNC as supercapacitor electrode exhibits the high-level supercapacitive performance.

2 Experimental 2.1 Materials The lignin-derived byproducts were obtained from the solid residues during bioethanol fermentation of poplar.3, 5 The byroducts were washed with deionized water, and then dried at 55 oC overnight. The dried sample was used directly as the carbon precursor. The analysis of the lignin-derived byproducts composition was shown in supporting information.

The

potassium

hydroxide

(KOH),

hydrochloric

acid

(HCl)

and

N-methyl-2-pyrrolidone (NMP) were purchased from Beijing Chemical Plant (Beijing, China). 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4) was bought from Aladdin Industrial Corporation (Shanghai, China). All reagents used are of analytically pure.


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2.2 HPNC Preparation The HPNC materials were synthesized as follows: 1 g of dried lignin-derived byproducts was dispersed in 20 mL of deionized water including 0.3 mL of sulfuric acid by ultrasound treatment. The dispersion placed in a 50 mL sealed stainless-steel autoclave with Teflon-lined was performed up to 200 oC for 24 h and then was naturally cooled to room temperature. The resulting hydrochar was isolated by filtration, rinsed with deionized water and dried overnight at 70 oC. The hydrochar was chemically activated using KOH at a mass ratio of 1:2 (hydrochar/KOH), which were mixed in an agate mortar and heated at 800 oC at a heating rate of 3 oC min-1 for 1 h under Ar flow (100 mL min-1) in a horizontal tubular oven. After that, the activated samples were thoroughly rinsed with 10 vol% HCl solution to remove the inorganic impurities and then deionized water, and dried in an oven at 110 oC for 10 h. The final obtained product was named as HPNC and the yield of the carbon material was 28.3 wt%.

2.3 Material Characterization Structural features of HPNC were analyzed on a Hitachi S3400N scanning electron microscope (SEM). Field emission transmission electron microscopy (FE-TEM) was conducted using JEM2100F, JEOL. The Raman spectra were tested using a Horiba Jobin-Yvon LabRAM HR800. The X-ray powder diffraction (XRD) data were recorded on a Shimadzu diffractometer X-6000, maintaining the scanning angular range from 5° to 65°. The electrical conductivity was measured using four-point probe technique. X-ray photoelectron spectroscopy (XPS) was obtained from Escalab 250 Xi, Thermo. The nitrogen adsorption was tested with Quantachrome Instrument. The specific surface area


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was calculated by using multipoint Brumauer-Emmett-Teller (BET) method, and the pore size distributions were obtained by Non-Local Density Functional Theory (NLDFT). Elemental analysis was carried out using an elemental analyzer Vario EL III (Elementar, Hanau, Germany).

2.4 Electrochemical Measurement Two-electrode symmetric cell was assembled to examine the supercapacitive performances of the HPNC. The electrodes were manufactured by coating a slurry of 90 wt% HPNC and 10 wt% poly(vinylidenedifluoride) binder in NMP without using any conductive additive onto conducting carbon-coated substrate and then drying at 100 oC overnight in vacuum oven. The as-prepared electrodes pressed under 10 MPa had a 5 mg cm-2 mass loading . The electrode thickness is ~100 µm which is well comparable to those of the reported biomass-derived carbon and graphene electrodes.15, 17, 25, 33, 35 A glass-fiber membrane was used as the separator. The 6 M KOH and EMI-BF4 were used as electrolytes, respectively. The ionic liquid symmetrical supercapacitors were assembled in an argon-filled glove box. The cyclic voltammetry (CV) curves, galvanostatic charge/discharge voltage profiles, and electrochemical impedance spectroscopy were tested with a CHI660D electrochemical workstation. The gravimetric specific capacitance C (F g-1) of a single electrode was calculated from the upper part of discharge curves after voltage drop, according to C = 2I/[(dV/dt)m] where I is the current (A), dV/dt (V s-1) is the slope obtained by fitting a straight line to the upper part of discharge curve in the range of Vmax to ½ Vmax, and m is the mass (g) of active electrode material of one electrode. The energy density E (Wh kg-1) and power density P (W kg-1) based on electrode materials


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were calculated according to E = CV2/(8×3.6) and P = E/t, respectively, where V (V) is the discharge voltage and t (h) is the discharge time.

3 Results and discussion 3.1 Composition analysis of lignin-derived byproducts Lignin-derived byproducts were separated from the reaction mixture after ethanol fermentation of poplar (Fig. S1). The composition of lignin-derived byproducts was as follows: lignin, 64.6%; protein (from cellulase and S. cerevisiae), 18.9%; and carbohydrates connected with the lignin, 6.3% on a dry weight basis. The elemental composition of lignin-derived byproducts obtained by elemental analysis is carbon 50.5 wt%, oxygen 34.5 wt%, hydrogen 5.4 wt%, and nitrogen 9.6 wt%, respectively. The weight average (Mw) and number-average (Mn) molecular weight of the lignin were 709 g/mol and 1130 g/mol, respectively. These values are lower than those of the milled wood lignin of poplar (Mw 1773 g/mol and Mn 2555 g/mol, respectively). The low molecular weight was due to the degradation of lignin during bioethanol production. The lignin sample was analyzed by using the 2D HSQC NMR technique to investigate in detail its chemical structures (Fig. S2). The main structural characteristics of lignin, including basic composition (S and G units) and various substructures linked by ether and carbon-carbon bonds (β-O-4′, β–β′, β-5′, etc.), were labeled in detail in the electronic supplementary information. These unique structures can be transformed into the nitrogen/oxygen-doped graphitic aromatic carbon structures through the complex reactions, such as decarboxylation, dehydration and dehydrogenation during high-temperature carbonization process,24, 36-39 as illustrated in Fig. 1 and the following discussion.


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3.2 Characterisation of HPNC Figure 2 shows the typical morphology and microstructure of the HPNC. The interconnected porous carbon nanosheet frameworks were observed from the SEM image (Fig. 2a). The FE-TEM image (Fig. 2b) highlights a mass of non-open bowl-like pores existing on single carbon nanosheet, where plentiful nanopores were embedded on the walls of bowl-like pores (Fig. 2c). These unique hierarchical bowl-like pore systems can serve as electrolyte ion-buffering reservoirs during the electrochemical process, thus enhancing the ion transport kinetics by promoting ion transfer into the interior pore walls of the electrode and decreasing diffusion resistances.10, 30 This phenomenon also results in a high performance rate. The high-resolution FE-TEM image (Fig. 2d) exhibits that the nanosheets consist of several graphene-like layers and that the pore wall are graphitized partially. The unique nanostructures of interconnected hierarchical bowl-like porous nanosheets are attributed to the combination of the preparation process of hydrothermal treatment and chemical activation, as well as the intrinsic structures of lignin-derived byproducts. Fig. 1 illustrates a schematic of the synthesis process for the HPNC. The biomass fibers consist of cellulose intertwined by hemicellulose and lignin. During the microbial fermentation process, the celluloses were used to produce bioethanol, and vast amounts of the byproducts of the degraded lignin were generated where the remaining microbial protein adhered on the surface of degraded lignin. Then, the lignin-derived byproducts underwent the hydrothermal treatment in acid solution. This process carbonized the precursors into micro/nanoscale-fragmented biochars with high content of oxygen/nitrogen groups and closed macro/mesopore structures.18, 40 The following activation at a high temperature of


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800 °C induced the reaction between KOH and heteroatom-containing groups on the surface and in the bulk of biochars simultaneously to generate the hierarchical pore system. During this activation, the mainly intermediate phase of K2CO3 was formed first at below 700 °C,10,

41

which might contribute to the macropore formation.42 As the

temperature increased to around 800 °C, another important intermediate phase of K2O generated by the decomposition of K2CO3 further accelerated to etch carbon atoms and removed oxygen groups as CO2 and H2O,8, 42 which is responsible to the formation of hierarchical pores on the bowl-like macropore walls. Raman spectrum of HPNC (Fig. 3a) shows two characteristic peaks of G-band related to sp2-phase at 1596 cm−1 and D-band attributed to sp3-phase at 1353 cm−1, respectively.8, 32

The intensity ratio of IG/ID (1.08) related to the graphitization degree,29 is higher than

that of activated carbon Norit (0.52).17 The second-order 2D peak ascribed to the second-order zone boundary phonons,43 is also shown in HPNC. The results indicate that HPNC has high degree of graphitization with a large graphene domain (4.75 nm) and good electronic conductivity (4.8 S cm−1). The XRD pattern displays two peaks attributed to the (002) reflection of the stacking graphene layers at 20.8° and the ordered graphitic structure (100) reflection at 43.5°, respectively (Fig. 3b).25,

44

The average interlayer

distance d (0.43 nm) of HPNC was calculated from the central position of (002) peak, which is obviously superior to that of functionalized graphene (0.37 nm).15 This suggests a loosened structure in our carbon nanosheets in favor of the high-rate electrochemical process.45 Additionally, an obvious increase in the intensity at the low-angle region of XRD demonstrate a high porosity in the sample,12 which is consistent with the TEM observation.


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The nitrogen adsorption–desorption test of HPNC shows a hybrid I/IV-type isotherm (Fig. 3c). This result suggests the existence of a hierarchical pore size distribution in our sample, which is further verified by the NLDFT analysis (Fig. 3c). The hierarchical pore size distributions mainly consist of two narrow micropore systems around 0.72 and 1.4 nm (67% of total pore volume) and a small mesopore system at 2–7 nm originating from the nanopores on the wall of bowl-like pore walls, and a meso/macropore system at 25–75 nm stemming from the no-open bowl-like pores. The results are in good agreement with the morphology analysis. The specific surface area of HPNC based on MBET method is up to 2218 m2 g−1. The integration of high specific surface area and hierarchical pore size distribution is favorable for high-rate electrochemical process. XPS was used to measure the chemical composition and surface functionalities of HPNC. As shown in Fig. 3d, HPNC was composed of 91.4 at% carbon, 5.2 at% oxygen, and 3.4 at% nitrogen (Fig. 3d), which were in consistent with the values obtained by elemental analysis (carbon 89.1 wt%, oxygen 6.7 wt%, nitrogen 3.8 wt%, and hydrogen 0.4 wt%, respectively). The oxygen content of HPNC is well comparable to that of thermal-reduced commercial

graphene

activated

(4.96%),15

carbon

for

Na-NH3-reduced supercapacitor

graphene

(Norit,

(5.6%),46

4.5%).28

Note

and that

nitrogen-doping and high C/O ratio (18) can increase the conductivity of carbon materials, which are beneficial to electrochemical stability particularly at high current rates.47-48 High-resolution C 1s and N 1s spectra of HPNC are shown in Fig. 3e. The highresolution C1s spectrum of HPNC was deconvoluted by a primarily single peak of sp2C (284.8 eV) and several small peaks e.g., C-N (285.4 eV), C-O (286.7 eV), C=O (287.8 eV), and O=C-O (289.8 eV).32, 49-51 The high-content sp2 C phase of 72% manifests a


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mass of conjugated C=C domain constructed in our sample. The C-N peak indicates successful incorporation of nitrogen into the carbon framework, which is further confirmed by the energy dispersive X-ray images with the corresponding element mappings in Fig. 3f. The high-resolution N 1s spectrum of HPNC was deconvoluted by four peaks including quaternary N (400.6 eV), pyridinic N (398.2 eV), pyrrolic N (399.7 eV), and pyridinic N-oxidized (402.4 eV).28-30 The important N groups of pyridinic N (39%) and pyrrolic N (21%) sited at the edges and defects of graphite, can offer vast accessible active sites for charge storage.29 The other significant quaternary N groups (36%) located in the center of the defective graphite lattice can effectively improve electronic conductivity.52 In addition, this heteroatom (O, N) doping can improve the wettability of the electrodes with the electrolyte ion.26, 53 3.3 Supercapacitive performances Supercapacitive performances of HPNC were tested by two-electrode symmetric systems, denoted as HPNC-SC, in 6 M KOH aqueous electrolyte and

EMI-BF4 ionic

liquid electrolyte based on the most recommended reliable practical industrial method.54-56 The CV curves of HPNC-SC (Figs. 4a and c) display nearly symmetrical rectangular shapes in the range from 100 mV s−1 to 1000 mV s−1 in both aqueous and ionic liquid electrolytes. Moreover, the charge/discharge voltage profiles of HPNC-SC (Figs. 4b and d) also exhibit linearly symmetric triangular forms and small voltage drop even at a high current density of 80 A g−1 in both electrolyte systems. These results indicate that HPNC-SC exhibits significant electrical double-layer capacitive behavior and a small equivalent series resistance (ESR) leading to a good rate capability.8 The rate capability of HPNC-SC in both electrolytes is shown in Fig. 4e based on the


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charge/discharge curves. In aqueous electrolyte system, the devices showed high weight-normalized specific capacitances of 312 and 254 F g−1 at high current densities of 1 and 80 A g−1, respectively, resulting in high-rate capability of 81%. The area-normalized capacitance of HPNC-SC reached 14.1 µF cm−2, which is well comparable to the theoretical value of double-layer capacitance (15–25 µF cm−2).29-30 In ionic liquid electrolyte system, the specific capacitances of 141 and 105 F g−1 were obtained at 1 and 80 A g−1, respectively. This resulted in a 74% rate capability, which is higher than that of silk-derived nitrogen-doped carbon nanosheets (64% at 10 A g−1).32 This characteristic is ascribed to not only the electrode materials but also the testing condition at 60 °C, which evidently alleviated the ion “traffic jam” phenomenon in electrode pores.3, 22 For practical applications, the cycling stability of HPNC-SC in both electrolyte systems were tested. Fig. 4f shows the high cycling stability of 98% capacitance retention and nearly 100% Coulombic efficiency after 20 000 cycles at 10 A g−1 in 6 M KOH. While the 93% capacitance retention was obtained after 10 000 cycles at 10 A g−1 in EMI-BF4 electrolyte because of the partial electrolyte degradation on the heteroatom-doped surface of HPNC-based electrodes (Fig. S3). Compared with the reported state-of-the-art carbon electrodes (Table S1), HPNC presents high-level electrochemical properties in both electrolyte systems. Electrochemical impedance spectroscopy was performed to further understand the supercapacitive performance of HPNC electrodes, especially the ionic and electronic transport process. As shown in the expanded Fig. 5a, in the high-frequency region the equivalent series resistances (ESR) obtained from the corresponding value of the first intercept on the real axis57 present low values in aqueous and ionic liquid electrolyte


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systems, because of the excellent conductivity of HPNC. From the high- to mid-frequency region (inset of Fig. 5a), the low interfacial charge transfer resistances Rct calculated from the diameter of the semicircle were obtained for the aqueous (0.29 Ω) and ionic liquid (1.25 Ω) electrolyte systems. These characteristics are due to the unique bowl-like hierarchical pore structures and large heteroatom-doped surface areas of HPNC which lead to the efficient charge/ion transport between the electrode and electrolyte.33, 58-59

In the low-frequency region (Fig. 5a), almost vertical curves are shown in both

electrolyte systems, indicating the nearly ideal electrical double-layer capacitive behaviors and the small equivalent diffusion resistance.60 In addition, the total cell resistance obtained from extrapolating the vertical portion to the real axis is 0.78 and 2.45 Ω for aqueous and ionic liquid electrolyte systems, respectively. The Bode plots of the frequency responses are shown in Fig. 5b. The characteristic frequencies fo at phase angle of -45° are 2.03 and 0.62 Hz for aqueous and ionic liquid electrolyte systems, corresponding to the time constant τo (1/fo) of 0.49 and 1.61 s, respectively. These values are well superior to those of commercial active carbon electrode (10–100 s).13, 18 Note that the aqueous electrolyte system has lower ESR, Rct, and τo values than the ionic liquid system because of the higher conductivity of aqueous electrolyte. These results further demonstrate that the unique bowl-like hierarchical pore structures present efficient electronic/ionic transport pathways with excellent rate capability. The Ragone plot (Fig. 6a) was used to evaluate the overall performance of HPNC-SC in different electrolytes. HPNC-SC in aqueous electrolyte exhibits an energy density of 10.3/8.8 Wh kg−1 corresponding to the power density of 1.3/21.3 kW kg−1. Owing to the operating voltage window up to 3.5 V in ionic liquid electrolyte, the energy density is


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enhanced to 59.8 Wh kg−1 at power density of 875 W kg−1. Whereas a high energy density 44.7 Wh kg−1 (74% of max value) still remains at ultrahigh power density of 73.1 kWkg−1. These results indicate that our devices have the integrated high energy-power densities, which are superior to or comparable to those of state-of-the-art carbon electrodes from biomass or organic salts.17, 25, 32, 35, 61 For a practical packaged supercapacitor device, the normalized energy and power densities could be estimated to be 1/4 of those of electrode materials.12, 32, 59 The energy densities of assumed packaged devices are 15–11 Wh kg−1 in the ionic liquid electrolyte system, which is higher than that of commercial active carbon-based supercapacitors of ~5 Wh kg−1.8,

17

Fig. 6b shows that the normalized

energy densities are comparable to those of nickel-metal hydride (NiMH) and lead acid batteries even at high power densities of electrochemical capacitors.62-63

4 Conclusions The interconnected HPNC was successfully constructed using lignin-derived byproducts from bioethanol production through combined hydrothermal treatment and activation. The HPNC displayed bowl-like nanostructure with hierarchical pore distributions and high specific surface area of 2218 m2 g−1. The nitrogen-doping of 3.4% in HPNC was originated from the conversion of residual microbial protein adhering on lignin-derived byproducts after microbial fermentation. The nitrogen-doping resulted in a high conductivity of 4.8 S cm−1 and good surface wettability with electrolyte ions. In the aqueous electrolyte, the HPNC-SC exhibited high-rate performance with high specific capacitance of 312 F g−1 and 81% retention at 80 A g−1. The excellent cyclic life of 98% capacitance retention was achieved after 20 000 cycles at 10 A g−1. Moreover, in the ionic liquid electrolyte, the HPNC-SC exhibited enhanced energy density of 59.8 Wh kg−1 and


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remained 74% of maximum value at a power density of 73.1 kWkg−1. The high-rate capability and cycle life of HPNC-SC could be attributed to high conductivity and the efficient ion transport pathways from the unique hierarchical bowl-like pore structures. This study provides a potential strategy for high value-added utilization of lignin-derived byproducts and promotes large-scale application of bioethanol process. Acknowledgements This work was supported by the National Science Fund for Distinguished Young Scholars (31225005) and the Chinese Ministry of Education (113014A). ASSOCIATED CONTENT Supporting Information Figure S1-3 and Table S1 are shown in the supplementary information. Additional information is about the lignin byproduct and comparison of the supercapacitive performances of HPNC with that of the reported state-of-the-art carbon materials. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Corresponding author: Feng Xu. Email: [email protected] Notes The authors declare no competing financial interest. Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Graphene-Enhanced Electrode Materials with Ultrahigh Energy Density. Energy Environ. Sci. 2013, 6 (5), 1623-1632.


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Fig. 1. The schematic diagram of the process for fabricating HPNC.


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Fig. 2. Morphological characterizations of HPNC. (a) SEM, (b and c) FE-TEM, and (d) high-resolution FE-TEM images. The orange rings in (b, c) show the bowl-like pore structures; insert of (c) is the schematic of HPNC with the nanopores siting on the walls of the bowl-like pores; the orange arrows in (d) exhibit the graphene-like layer structures.


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Fig. 3. Structure and composition characterizations of HPNC. (a) Raman spectroscopy, (b) XRD pattern, (c) N2 adsorption-desorption isotherm and pore size distribution, (d) XPS spectrum and (e) its C1s and N1s spectrum, and (f) energy dispersive X-ray images with the corresponding mapping of C, N and O elements.


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Fig. 4. Supercapacitive performances of HPNC-SC. (a) CV curves and (b) charge/discharge voltage profiles in 6 M KOH electrolyte; (c) CV curves and (d) charge/discharge voltage profiles in EMI BF4 electrolyte; (e) gravimetric-normalized specific capacitances at different current densities; and (f) cycling stability and the Coulombic efficiency at 10 A g-1 in 6 M KOH electrolyte.


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Fig. 5. (a) The Nyquist plots and (b) the Bode plots of phase angle versus frequency of HPNC-SC in different electrolyte.


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Fig. 6. Energy and power densities of HPNC-SC in different electrolytes. (a) The Ragone plots based on electrode materials mass, and (b) performance comparison of the normalized packaged HPNC-based cells with the commercial electronic energy storage devices based on total device mass.


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TOC


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Interconnected Hierarchical Porous Carbon from Lignin-Derived

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