Hierarchical porous carbon monolith derived from lignin for high areal capacitance supercapacitors

Journal Pre-proof Hierarchical porous carbon monolith derived from lignin for high areal capacitance supercapacitors Hui Li, Yuhui Zhao, Siqi Liu, Pengcheng Li, Du Yuan, Chaobin He PII:

S1387-1811(19)30819-4

DOI:

https://doi.org/10.1016/j.micromeso.2019.109960

Reference:

MICMAT 109960

To appear in:

Microporous and Mesoporous Materials

Received Date: 5 September 2019 Revised Date:

10 December 2019

Accepted Date: 13 December 2019

Please cite this article as: H. Li, Y. Zhao, S. Liu, P. Li, D. Yuan, C. He, Hierarchical porous carbon monolith derived from lignin for high areal capacitance supercapacitors, Microporous and Mesoporous Materials (2020), doi: https://doi.org/10.1016/j.micromeso.2019.109960. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.

Author Contribution Statement Hui Li: Investigation, Conceptualization, Writing - Original Draft Yuhui Zhao: Investigation Siqi Liu: Investigation Pengcheng Li: Investigation, Data curation, Writing- Reviewing and Editing Du Yuan: Conceptualization, Writing- Reviewing and Editing Chaobin He: Supervision, Writing- Reviewing and Editing

Hierarchical Porous Carbon Monolith Derived from Lignin for High Areal Capacitance Supercapacitors Hui Li,a Yuhui Zhao,a Siqi Liu,b Pengcheng Li,a* Du Yuan,b* Chaobin Heb,c* a. Hubei Key Laboratory of Plasma Chemistry and Advanced Materials, School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan, 430205, China. b. Department of Materials Science and Engineering, National University of Singapore, 117576, Singapore. c. Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 117602, Singapore. Keywords: Hierarchical porous carbon; Lignin; Areal capacitance; Supercapacitors.

ABSTRACT: Hierarchical porous carbon with high areal energy density in supercapacitor has attracted extensive attention with increasing demand for compact and portable devices. Lignin based porous carbon monolith was fabricated via dual templates (P123 and silica nanoparticles) and carbonization procedure, where hierarchical porous structure with tuneable mesopore sizes and distributions could be modulated via incorporation of silica nanoparticles with size of 200 nm, 100 nm, and 7 nm, respectively. Due to higher surface area and optimized porous structure which facilitate fast ion diffusion, carbon monolith fabricated from silica nanoparticle of 7 nm

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exhibited superior electrochemical performance, which reached a high areal capacitance of 2.7 F cm-2 and excellent volumetric capacitance of 104.5 F cm-3 with mass loading of 13.6 mg cm-2 in 6 M KOH aqueous solution. Moreover, the symmetric cell assembled with this carbon electrode gave rise to a large amount of energy (131 µWh cm-2) at high power densities (1368 µW cm-2) and excellent cycle stability of 92.9% after 10 000 cycles, facilitating implementation of ligninderived materials in commercial energy storage products.

1. Introduction

With the merits of environmental friendliness, high power density, and excellent stability, electrical double layer supercapacitors (EDLCs) have been extensively explored for the application of energy storage devices.[1, 2] In the design of supercapacitor electrode materials, the favoured electrode materials should include the advancing properties as follows: (1) high specific surface area, meaning more electrolyte/electrode contact surfaces, (2) proper pore size distribution and pore network for facilitating ions diffusion, (3) high electrical conductivity for efficient charge transport, and (4) excellent electrochemical and mechanical stability for cycling performance.[3] The hierarchical porous carbons, especially those possessing macropores and meso/micropores, are currently considered as the ideal electrode materials to pursuit high performance supercapacitors.[4, 5] On one hand, macropores provide ion buffering reservoirs to minimize the ion diffusion distance to the inner surfaces. On the other hand, meso/micropores give rise to high surface area to enhance the contact between electrolyte and electrode.[6] With the superior hierarchical porous structure to facilitate ion diffusion, excellent capacitance performance could be achieved even at high mass loading, which results in high areal capacitance and high device capacities to fulfil the demand for compact, portable and mobile

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energy storage devices.[7, 8] There are numerous reports to construct hierarchical porous carbon materials.[2, 9, 10] However, the relatively high production cost of precursors impeded their widespread commercial application. Lignin, the second most abundant polymer of lignocellulosics after cellulose, has attracted intense interest.[11, 12] Due to the impressive advantages of being renewable, abundant, antioxidant, and low cost, lignin has been exploited for a number of applications, such as adhesive, toughener agent, and raw materials for thermal-setting materials.[13-15] Moreover, with its large quantities of aromatic groups, lignin has receiving increasing interest as sustainable carbon source for the application of energy storage devices, especially supercapacitors.[16, 17] However, many approaches are based on direct carbonization of lignin, which resulted in mainly micropores and undesirable ion diffusion.[18, 19] Therefore, modulating carbon porous structure is extremely crucial for the biomass materials on the application of commercial supercapacitors. Naskar et al. synthesized mesoporous carbon from lignin with surfactant of F127, which exhibited high capacitance of 102.3 F g-1 after activation by KOH.[20] Cazorla-Amoros et al. prepared hierarchical porous carbon materials by the direct carbonization of lignin/zeolite and subsequent basic etching of the inorganic template, which showed a specific capacitance of 250 F g-1.[6] Despite the enhanced mesoporosity via the strategies, most of the reported products are carbon powders. Polymeric binder and conductive additives are required to hold the particulate together for the preparation of electrode, leading to undesirably electrode resistance, increased electrode mass, and reduced surface area of the electrode.[21, 22] While carbon monolith without extra inactive materials, which usually exhibits remarkably high conductivity and superior mechanical properties, could directly be used as device electrode.[23] Therefore,

3

hierarchical porous carbon monolith derived from biomass precursors with high conductivity is expected as one of the most promising candidates for high performance supercapacitors. Herein we fabricated the lignin-derived carbon monolith with tunable hierarchical porous structure via the template of P123 and silica nanoparticles and carbonization procedure, which can be directly used as electrode for high areal capacitance supercapacitors. With modulating the size and content of incorporated silica nanoparticles, hierarchical porous carbon with tuneable well-defined mesopore sizes and distributions were fabricated. Ascribing to higher surface area and optimized porous structure which facilitates fast ion diffusion, carbon monolith fabricated from silica nanoparticle of 7 nm exhibited superior electrochemical performance, which reached high areal capacitance of 2.7 F cm-2 and excellent specific/volumetric capacitance of 200.2 F g-1 and 104.5 F cm-3 with mass loading of 13.6 mg cm-2. This work not only offers a facile strategy to fabricate tuneable porous structure but also improves the development of lignin based carbon monolith for the application of high performance energy storage systems.

2. Experimental section

2.1 Materials Kraft lignin, Pluronic P123 (EO20PO70EO20), and silica nanoparticles (7 nm) were purchased from Sigma Aldrich. Tetrahydrofuran (THF), tetraethyl orthosilicate (TEOS), sodium hydroxide (NaOH), potassium hydroxide (KOH), ammonia, and ethanol were bought from Sigma Aldrich and used as received. 2.2 Preparation of lignin-derived carbon monolith

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For the preparation of silica-modified lignin derived carbon monolith (SLC), three different sizes of silica (200 nm, 100 nm, and 7 nm, denoted as SLC-200, SLC-100 and SLC-7) were used. Silica nanoparticles of 200 nm and 100 nm were prepared as reported procedure.[24, 25] The silica nanoparticles of 200 nm and 100 nm were synthesized by the hydrolysis of tetraethylorthosilicate (TEOS) in base solution. 50 mL ethanol and 4 mL ammonia were mixed and stirred for 10 min. After that, 1 mL TEOS was added in the mixture and stirred at room temperature for 7 h to obtain the silica nanoparticles of 200 nm. The silica nanoparticles of 100 nm were synthesized in the mixture of 25 mL ethanol, 25 mL H2O and 4 mL ammonia. And then the mixture was stirred with 1 mL TEOS for 7 h at room temperature to obtain the products. The lignin was used after acid-washing procedure.[15, 26] 1.0 g lignin and 1.6 g P123 were dispersed in 20 mL THF and 150 µL of 6 M HCl solution. After 0.4 g (40 wt%, silica to lignin) silica nanoparticles were added in the mixture and stirred at room temperature for 24 h, the resulting composites suspensions were transferred to alumina porcelain boat to dry at ambient and then maintained at 70 oC for 2 days. Next, the dried sample was carbonized in tube furnace under Ar environment using heating rate of 1 °C min−1 up to 400 °C, resuming heating with 2 °C min−1 up to 900 oC, and finally maintained at 900 oC for 15 min (denoted as SLC-y-silica, y is particle size of silica, which is about 200, 100, and 7 nm). After that, the sample was immersed in 2 M NaOH solution for 6 h at 90 oC to remove silica nanoparticles and then dried at 120 oC. And then the carbon monolith was directly used as supercapacitor electrode (denoted as SLC-y). Carbon monoliths with different contents of silica were investigated and denoted as SLC-y-x wt% (x is weight ratios of silica to lignin). And the thickness/mass loading of the carbon monoliths were modulated by tuning the amount of the mixture suspension. For comparison, lignin-derived

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carbon material without silica nanoparticles (LC) was synthesized keeping all other conditions the same. 2.3 Characterization Morphology of the mesoporous carbon was observed under Zeiss Supra 40 field-emission scanning electron microscope (SEM) at an accelerating voltage of 5 kV. Energy dispersive X-ray spectroscopy (EDS) measurements were conducted on Oxford instruments with X-MaxN system. Transmission electron microscopy (TEM) micrographs were obtained with a JEOL 100CX system. The conductivity of the carbon materials was measured by four-probe method with a Keithley 2400 source/meter. The pore textural characteristics of the samples were measured with Surface Area and Porosity Analyzer (ASAP 2020) by N2 adsorption-desorption at liquid-N2 temperature (77 K). The pore size distribution was obtained by the density functional theory (DFT). Raman spectroscopy was measured by Jobin Yvon Horiba LabRam HR 800 Raman system. X-ray photoelectron spectroscopy (XPS) analysis was obtained by Kratos Axis Ultra Xray photoelectron spectroscope. Fourier-transform infrared (FTIR) spectroscopy was obtained by Thermo Fisher NICOLET6700 FTIR spectrometer. 2.4 Electrochemical Measurements

The electrochemical impedance spectroscopy (EIS) was conducted using electrochemical analyzer (Solartron 1287) in three-electrode system at open circuit potential, which were about 0.25 V, -0.27 V, and -0.39 V for SLC-7-13.6 mg cm-2, SLC-7-31.9 mg cm-2, and SLC-7-59.0 mg cm-2, respectively. The frequency limits were typically set between 0.1 Hz and 700 kHz. Cyclic voltammetry measurements were performed on Solartron 1287 under ambient condition. A platinum foil, a saturated Hg/Hg2Cl2 electrode (SCE), and KOH solution (6 M) were used as the

6

counter electrode, reference electrode and electrolyte, respectively. The carbon monolith was wrapped by nickel foam and directly used as working electrode for electrochemical measurements in the three-electrode configuration. The cyclic voltammetry test was performed within a potential window of -1 − 0 V and at different scan rates of 1 − 50 mV s-1. The galvanostatic charge-discharge measurements were performed with the current density of 0.1 − 10 A g-1. The specific capacitance (Cg) / areal capacitance (CS) / volumetric capacitance (Cv) of the electrode were calculated according to the equations:

Cg = I∆t / (V × m)

(1)

CS = I∆t / (V × S) = Cg × M (2)

Cv = I∆t / (V × v) = Cg × ρ (3)

where I (A) is the applied constant current, ∆t (s) is the discharge time, V (V) is the voltage window, m (g) is the mass of the electrode, S (cm2) is the surface area of the electrode, v (cm3) is the volume of the electrode, M (mg cm-2) is the mass loading of the electrode, and ρ (mg cm-3) is the mass density of the electrode materials.

The symmetric supercapacitor of SLC was performed using two-electrode configuration. Two nearly identical electrodes were firstly soaked in the electrolyte overnight. Next, they were separated by the electrolyte soaked filter paper and stacked together in a Swagelok-type cell. Electrochemical measurements were carried out in a voltage range from 0 to 1 V and the galvanostatic charge-discharge measurements were performed with the current density of 0.1 − 2 A g-1. The cell specific capacitance (Cg) and areal capacitance (Cs) of the SLC-based supercapacitors was calculated according to

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Cg = I∆t / (V × m)

(4)

CS = I∆t / (V × S)

(5)

where m (g) is the total mass of the two electrodes, and S (cm2) is the geometrical surface area of the electrodes which is about 0.25 cm2. The areal energy density (E, Wh cm-2) and areal power density (P, W cm-2) of the supercapacitors were calculated by using

= × .

=

× ∆

(6)

(7)

3. Results and discussions 3.1 Morphology and Structures of the Porous Carbon Monolith Lignin-derived hierarchical porous carbon monolith was fabricated by the combination of P123 and silica nanoparticles as templates. The schematic representation of the formation of porous carbon monolith from lignin is depicted in Fig.1a (the reaction process by the chemical equation was shown in Fig. S1). With abundance of aliphatic and phenolic hydroxyl groups, lignin/P123 micelle was easily formed through strong hydrogen bond between hydroxyl groups of lignin and the miscible segment of P123 macromolecule.[27] Therefore, the monolithic carbon precursor can be obtained from the mixture of lignin, P123, and silica. Furthermore, P123 worked as the scaffold to support the framework during pyrolysis, and the micelle domains were responsible for producing the mesopores in the resultant carbon. [20, 28] For more details see

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Supplementary Note 1 and Fig. S2. On the other hand, the silica nanoparticles provided a suitable support for the carbon matrix during carbonization and produced new macro/mesopores for carbon framework after base etching.[29] Successful removal of silica particles from carbon monoliths was confirmed by FTIR and EDS measurements (see Supplementary Notes 2 and 3, and Fig. S3 and S4). Consequently, hierarchical porous carbon was fabricated and the porous network with tuneable well-defined mesopore sizes and distributions could be modulated via incorporation of silica nanoparticles. As shown in Fig.1b and 1f, SEM and TEM images revealed that interconnected mesoporous structure with the pore size of around 30 nm entity throughout LC matrix. While for silica modified lignin-derived carbons (SLC), Fig.1(c,d,g,h) showed mesoporous carbons with additional macropores around 200 nm and 100 nm respectively, indicating that the silica nanoparticles were well replicated (SEM images of SLC-200-silica, SLC-100-silica and SLC-7-silica were shown in Fig. S5). When silica nanoparticles with particle size of 7 nm were used as templates, mesopores were generated in the carbon wall and distributed throughout the samples, leading to more highly porous structure (Fig.1e and 1i). These results confirmed the feasibility of this strategy to modify the nanostructure of porous carbon monolith. Furthermore, as shown in Fig.1a, the prepared carbon monolith was stiff and could be directly used as electrodes without any binders, facilitating the commercial application of this material for supercapacitors. The pore structures of the carbon were further analyzed using N2 adsorption-desorption measurement. The Brunauer-Emmett-Teller (BET) surface area and pore size distributions were obtained with non-local density functional theory (NLDFT). SLC-7 before silica nanoparticles removal (SLC-7-silica) was included to study the pores formation. As shown in Fig.2a, all the isotherm were characterized as type Ⅳ according to IUPAC classification, implying the existence

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of mesopores.[30, 31] The pore size distribution of LC indicated that the porosity was essentially made up of mesopores, which depicted a broad pore widths within the range of 10 - 60 nm, attributing to the branched structure of lignin and collapses during thermal pyrolysis. Moreover, the BET surface area was calculated to be 350 m2 g-1 with a total pore volume of 0.35 cm3 g-1. With the incorporation of silica nanoparticles (SLC-7-silica), pore volume increased within the pore distribution compared with LC, suggesting the support of silica nanoparticles to prevent collapses of the pores.[9, 32]. After removal of silica nanoparticles, SLC-7 exhibited a new peak of pore volume around 7 nm, attributing to the replicate of the silica nanoparticles. Moreover, pore size distribution extended to 80 nm, ascribing to pores interconnection of the neighboring channels after silica nanoparticles removal.[33] Such results indicated a hierarchical porous structure mainly containing mesopores and macropores, which is consistent with SEM and TEM images. Moreover, BET surface area was increased to 635 m2 g-1 with a total pore volume of 1.43 cm3 g-1 after removal of silica template. This hierarchical structure could provide proper porous structure for fast ion diffusion and high surface area to enhance electrolyte/electrode contact.[34, 35] SLC-7 with different amounts of silica nanoparticles, including 10 wt% and 60 wt%, were also fabricated to modulate the porous structure of the electrodes (Fig.2a, 2b and S6). BET surface area of SLC-7-10 wt% was 407 m2 g-1 with a total pore volume of 0.54 cm3 g-1. When more silica nanoparticles was added, SLC-7-60 wt% exhibited high surface area of 636 m2 g-1 with a total pore volume of 1.63 cm3 g-1. However, the carbon monolith became fragile, which was probably resulted from the cracks induced by the increasing connected pores. This would deteriorate electron transport within the carbon monolith and cause an undesired effect on the electrochemical performance. Furthermore, the X-ray photoelectron spectroscopy (XPS) survey was conducted to investigate C and O elements in the SLC-7 (Fig. S7). The C 1s spectrum of SLC can be fitted with

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five different peaks associated with sp2 C = C (284.5 eV), sp3 C - C (285.4 eV), C - O (286.5 eV), C = O (287.7 eV), and O = C - O (289.3 eV) (Fig.2c).[1, 36] The sp2 C = C peak is generally associated with the graphite structure, and the prominent intensity of the peak indicates the graphitization of the prepared carbon monolith. Consistent with XPS results, Raman spectra exhibited intense G band (1590 cm-1, graphite structure) in the carbon monolith (Fig.2d), which was favorable for fast charge transportation.[36-38] Consequently, high electrical conductivity about 1300 S m-1 could be reached for this carbon monolith, facilitating charge transport within the carbon monolith and contributing to the excellent capacitance performance. 3.2 Electrochemical Performances The electrochemical performance of SLC-7 and LC was evaluated in three-electrode configuration within the potential window of -1 – 0 V and the CV plots were shown in Fig.3a and 3b, respectively. All the CV curves were nearly rectangular without any obvious redox peaks, indicating good electrical double layer capacitance (EDLC) behavior.[29] SLC-7 exhibited larger voltammetry current response than LC at the same scan rates. Thus the capacity of SLC-7 was 178.8 F g-1, much higher than that of LC (107.3 F g-1) at a scan rate of 1 mV s-1. In addition, the GCD curves for SLC-7, SLC-100, SLC-200, and LC recorded at current densities of 0.1 A g-1 – 10 A g-1 were rather symmetric (Fig.3c and S8). The specific capacitance as a function of current density was calculated from the GCD curves (Fig.3d) and the values of 200.2, 141.2, and 136.6 F g-1 were obtained at a current density of 0.1 A g-1 for SLC-7, SLC-100, and SLC-200, respectively, all higher than 112.8 F g-1 for LC. Moreover, the effect of pure Ni foam on the capacity contribution to the carbon electrodes were elaborated to be negligible (as shown in Supplementary Notes 4 and Fig. S9). As the scan rate and current density increasing, the capacitance decreased because of the limited ion diffusion at the electrode/electrolyte

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interfaces,[34] especially for LC. At high current density of 10 A g-1, SLC-7 exhibited a superior rate performance of 120.9 F g-1 (60.4% capacitance retention), higher than 28.0 F g-1 for SLC100 (19.8%), 48.0 F g-1 for SLC-200 (35.1%), and 17.6 F g-1 for LC (15.6%). The superior specific capacitance and rate capability of SLC-7 is attributed to the unique hierarchical 3D porous structure. The meso/micropores structure provides large surface area to enhance the ion storage space, which is beneficial for ion adsorption and desorption, resulting in high capacitance. Furthermore, the macropores provide ion buffering reservoirs to minimize the ion diffusion distance to the inner surfaces, attributing to the efficient charge/discharge and excellent electrochemical performance especially at high current density. [23, 30] Compared with SLC-7, LC exhibits lower surface area and poor porosity which limits ion diffusion and charge transport, resulting in inferior capacitance performance. In addition, the capacitance performance of SLC-7 with different content of silica (SLC-7-10 wt% and SLC-7-60 wt%) were also investigated (Fig. S10) and shown in Fig.3e. The capacitance performance at a current density of 0.1 A g-1 was almost equal to each other. However, both of the samples decreased to a larger extent at higher current densities, probably ascribing to poor porosity and deteriorated charge transport. Therefore, the following studies will focus on SLC-7 with silica content of 40 wt%. And excellent cycling stability with 96% capacitance retention was achieved after 1000 charge/discharge cycles at a current density of 2 A g-1 (Fig.3f), indicating good stability of the electrode. With the demand for compact, portable, and mobile energy storage devices, high areal/volume energy density of devices becomes crucial when assessing device design and integration, since there is usually a limited surface area or volume to integrating capacitive materials in the devices.[7, 39-41] Areal capacitance (F cm-2) and volumetric capacitance (F cm-3) are further

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assessed and the results are calculated in Fig.4. Fig.4a showed that our materials exhibited a high areal capacitance of 2.7 F cm-2 with desirable volumetric capacitance of 104.5 F cm-3 at current density of 1.4 mA cm-2 (0.1 A g-1). Furthermore, the areal and volumetric capacitance could still retain at 1.6 F cm-2 and 63.1 F cm-3 at high current density of 135.7 mA cm-2 (10 A g-1), respectively. The areal capacitance is much larger than other reported carbon materials (tens to few hundreds of mF cm-2), [22, 42-44] and the superior performance was achieved at high mass loading of 13.6 mg cm-2 (thickness of 260 µm), higher than the most of reported carbons (<10 µm, <5 mg cm-2) and commercial carbons (100 - 200 µm, <10 mg cm-2). [45-47] The capacitive performance as a function of electrode mass loading/thickness was further investigated (Fig. S11) and shown in Fig.4b. With increasing mass loading from 13.6 mg cm-2 to 59.0 mg cm-2, specific capacitances decreased from 200.2 F g-1 to 147.9 F g-1 at current density of 0.1 A g-1. Moreover, at a high current density of 10 A g-1, the electrode remains 60%, 42%, and 14% of capacitance retention for the electrodes with mass loading of 13.6 mg cm-2, 31.9 mg cm-2, and 59.0 mg cm-2, respectively. The deteriorated rate performance could be ascribed to the increased charge transport distance/resistance and poor ion diffusion with increasing thickness of the electrodes, which was further evidenced by the electrochemical impedance spectroscopy (EIS) measurement (Fig.4c). The equivalent series resistance (ESR) of the carbon monolith was calculated to be 0.89, 1.25, 1.30 Ω for the electrodes with mass loading of 13.6 mg cm-2, 31.9 mg cm-2, and 59.0 mg cm-2, respectively. The low ESR is crucial for enhancing rate capability.[48] Moreover, the radius of the semicircle at high frequency was related to the charge transfer resistance and a smaller semicircle indicated smaller charge transfer resistance at electrode/electrolyte interface.[37, 49] With ~5 - fold increase of mass loading, the ion diffusion was usually limited due to long charge transport distance and the internal resistance was

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gradually increased, which resulted in deteriorated specific capacitance and rate capability, consistent

with

the

results

of

electrochemical

performance.

Although

gravimetric

specific/volume capacitance decreased with increasing mass loading of the electrodes, the areal capacitance gradually increased by increasing the mass loading/thickness of electrodes (from 260 µm to 1130 µm) (Fig.4d), and the maximum areal capacitance of 8.7 F cm-2 at 0.1 A g-1 was achieved with mass loading of 59.0 mg cm-2, which is one of the highest reported values for carbon materials to date (6.7 F cm−2 for graphene ribbon films,[1] 1.31 F cm-2 for porous carbon nanosheets,[10] 1.56 F cm-2 for activated carbon fiber paper, [45] 2.62 F cm-2 for holey graphene frameworks, [50]). Meanwhile, the electrode possessed gravimetric specific/volume capacitance of 147.9 F g-1 and 77.2 F cm-3, comparable to other reported carbon materials.[51-53] These results demonstrated the uniqueness of this material with high areal and specific/volumetric capacitance, which is promising for the practical application of energy storage products. Moreover, symmetric SLC-7 supercapacitor was fabricated to evaluate the device performance for real application. As shown in Fig.5a, the CV curves of our device were close to rectangular without redox peaks. Moreover, the charge/discharge curves exhibited the typical symmetric triangular shape (Fig.5b), which both represented a perfect electrical double layer formation.[54] The cell capacitance of the supercapacitor was calculated at different current densities ranging from 0.1 A g-1 to 2 A g-1 (Fig.5c). A high specific/areal capacitance of 34.7 F g-1 and 944 mF cm-2 were obtained at current density of 0.1 A g-1. The areal capacitance is higher than most of the reported carbon materials (Fig. 5d and Table S1), such as laser-scribed graphene (4.04 mF cm-2 at the current density of 1 A g-1),[55] electrolyte-mediated chemically converted graphene (102 mF cm-2 at 0.1 A g-1), [56] graphene (186 mF cm-2 at 1 A g-1), [7] activated carbon fiber paper (590 mF cm-2 at 10 mV s-1), [45] and SWNT (480 mF cm-2 at 200 µA cm-2). [39] Moreover, the

14

supercapacitor exhibited relative high retention capacitance of 74% (702 mF cm-2) at 2 A g-1. Ascribing to the excellent porous structure and areal capacity performance, the device gave rise to a high areal energy density of 131 µWh cm-2 at the power density of 1368 µW cm-2, which is higher or comparable to other carbon-based materials (Fig.5e), such as graphene fiber fabrics of 23.5 µWh cm-2,[57] graphene films of 8.4 µWh cm-2,[58] graphene/carbon particles of 27 µWh cm-2,[59] rGO-MWNT of 11 µWh cm-2,[60] N-CNFs/rGO/BC of 110 µWh cm-2,[47] CNTcarbonized cotton of 110 µWh cm-2,[36] FGH/FCC of 113.3 µWh cm-2,[61] and graphenecellulose paper of 6 µWh cm-2. [41] When power density increased to 63.8 mW cm-2, the energy density was retained at 69 µWh cm-2. To further characterize the cycle stability of the device, charge/discharge tests were carried out at a high current density of 2 A g-1 and shown in Fig.5f. The capacitance retention of 92.9% is achieved after 10 000 cycles, indicating the excellent longtime cycle performance. Considering the merits of renewable biomass, low cost, and high capacitance, this lignin derived carbon demonstrated potential application in widespread commercial applications as energy storage devices. 4. Conclusion In this work, hierarchical porous carbon monolith directly used as supercapacitor electrode was fabricated from abundant byproduct lignin via dual templates approach. The porous structure with tuneable porosity was modulated via the size and content of incorporated silica nanoparticles. With high surface area of 645 m2 g-1 and optimized porous structure for SLC-7, this material gave rise to superior areal capacitance of 2.7 F cm-2 and volumetric capacitance of 104.5 F cm-3 with mass loading of 13.6 mg cm-2 (thickness of 260 µm) in 6 M KOH aqueous solution. With increasing mass loading of 59.0 mg cm-2, highest areal capacitance of 8.7 F cm-2 was reached. Furthermore, the symmetric full cell supercapacitor exhibited a high areal energy density of 131

15

µWh cm-2 at the power density of 1368 µW cm-2 and excellent cycle stability of 92.9% after 10 000 cycles, indicating the potential application of the lignin based carbon materials for energy storage products.

Notes The authors declare no competing financial interest. Acknowledgements This work was supported by the National Natural Science Foundation of China [Grant No. 51803156, 51903194] and Hubei Provincial Natural Science Foundation of China [2018CFB102, 2019CFB190].

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Fig.1 (a) Schematic depiction of porous carbon monolith from lignin; SEM (b,c,d,e) and TEM (f,g,h,i) images of LC (b,f), SLC-200 (c,g), SLC-100 (d,h), and SLC-7 (e,i). Fig.2 (a) Nitrogen adsorption-desorption isotherms and (b) DFT pore size distribution for LC, SLC-7-silica, SLC-7, and the SLC-7 with different contents of silica nanoparticles (10 wt% and 60 wt%); (c) C 1s spectrum of SLC-7; (d) Raman spectrum of LC and SLC-7. Fig.3 Electrochemical characterization of lignin-derived carbons in a three-electrode system. Cyclic voltammetry curves at different scan rates for SLC-7 (a) and LC (b); (c) Galvanostatic charge/discharge curves of SLC-7 at current density of 0.1-10 A g-1; Calculated specific capacitances at various current densities (d); (e) specific capacitances for SLC-7 materials with different silica contents; (f) Cycling stability of SLC-7 at a current density of 2 A g-1, inset are charge/discharge curves for the 1st cycle and 1000th cycle. Fig.4 (a) Areal and volumetric capacitances under different current densities for SLC-7-13.6 mg cm-2; (b) specific/volumetric capacitances for SLC-7 with various mass loadings at current densities of 0.1-10 A g-1; (c) Nyquist plots of electrodes with various mass loadings; (d) Areal and volumetric capacitances as a function of mass loading/thickness at current density of 0.1 A g1

.

Fig.5 (a) Cyclic voltammetry curves at different scan rates for SLC-7-based symmetric supercapacitor; (b) Galvanostatic charge/discharge curves under different current densities; (c) Cell capacitance and areal capacitance as a function of current density; (d) Comparison of areal capacitance with reported carbon-based materials; (e) Ragone plot of SLC-7 (areal energy density vs areal power density), reported values of other carbon-based materials were included as

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blue square for comparison; (f) cycle stability of the device at current density of 2 A g-1 after 10 000 cycles, inset are charge/discharge curves for the 1st cycle and 10 000th cycle.

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Highlights:

Lignin derived hierarchical porous carbon monolith was fabricated by dual templates The porous structure can be tuned by the size and amount of silica nanoparticles High capacitance performance can be realized by optimizing porous structure 2.7 F cm-2, 104.5 F cm-3, and 200.2 F g-1 were achieved at 0.1 A g-1

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: