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volumetric capacitances for supercapacitor
Carbon 100 (2016) 151e157
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Lignin-derived interconnected hierarchical porous carbon monolith with large areal/volumetric capacitances for supercapacitor Hui Li a, 1, Du Yuan a, 1, Chunhua Tang a, Suxi Wang b, Jiaotong Sun a, Zibiao Li b, Tao Tang a, Fuke Wang b, Hao Gong a, Chaobin He a, b, * a b
Department of Materials Science and Engineering, National University of Singapore, 117576, Singapore Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 117602, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 July 2015 Received in revised form 3 December 2015 Accepted 20 December 2015 Available online 6 January 2016
A large scale 3D interconnected hierarchical porous carbon monolith was developed by a novel synthetic approach from the 2nd most abundant natural polymer, lignin. This newly derived carbon monolith possess desirable microstructure and high conductivity that enables the fabrication of a binder-/ conductive additive-free thick electrode. The carbon monolith delivered simultaneously excellent areal/ volumetric capacitances of 3.0 F cm2/97.1 F cm3, at a high mass loading of 14.4 mg cm2. The areal capacitances obtained in our work are the highest among the reported carbon-based electrodes. The carbon monolith based symmetric supercapacitor achieved a high areal energy density of 0.16 mWh cm2 at 1.75 mW cm2, with excellent cycling performance (~95% retention after 10,000 cycles). Our work demonstrates the successful conversion of biomass waste into high-performance eletrode, and the promising areal/volumetric performances further suggest the developed approach may provide a positive route to improving the energy density of supercapacitor. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction With the depletion of fossil fuels and rapid-growing energy demand, great effort has been dedicated to high efficient, cost effective, and environmental friendly energy storage systems, like lithium ion battery and supercapacitor [1e5]. Supercapacitor receives wide applications due to its high power density and long lifetime. Recent research focuses primarily on improving areal/ volumetric energy densities of supercapacitor with mounting demand for compact and portable energy storage systems [6e8]. However, areal and volumetric performances are generally in conflict with each other [9]. For example, the volumetric performance of carbon-based electrodes do not scale up linearly with their thickness that hinders enhancing volumetric energy density, though significant progress achieved in advancing specific gravimetric capacitances (~100e400 F g1) [8,10e14]. Promising volumetric capacitance obtained with thin electrode (up to tens of micrometers) is commonly accompanied by a compromise of low
* Corresponding author. Department of Materials Science and Engineering, National University of Singapore, 117575, Singapore. E-mail address: [email protected] (C. He). 1 Hui Li and Du Yuan contribute equally to this work. http://dx.doi.org/10.1016/j.carbon.2015.12.075 0008-6223/© 2016 Elsevier Ltd. All rights reserved.
areal capacitance with poor mass loading (~1 mg cm2), comparing to commercially available products (10 mg cm2, 100e200 mm) [6,15e19]. When areal performance is under central pursuit with high mass loading/thickness, volumetric capacitance is largely sacrificed owing to high internal resistance and poor ion diffusion [16,19]. The great challenge therefore lies in simultaneous achievement of the true performance metrics of supercapacitor [6], where carbon-based electrodes typically offer low areal capacitance (tens to a few hundred of mF cm2) and volumetric capacitance (<60 F cm3) [16]. Since the true performance matrices are intimately correlated to both material and microstructure of the electrode, a thick carbonbased electrode should integrate the following desirable merits. A porous structure is essential with incorporation of macropores and mesopores to minimize ion diffusion distance for fast charge transport and increase surface area for promoting electrolyte/ electrode contact [20,21]. A binder-/conductive additive-free structure continuity is ideal to eliminate dead cell components and consequently retain the high conductivity of carbon, which benefits volumetric performance at high mass loading [2]. However, it is still challenging to synthesize continuously 3D networking hierarchical porous carbon that can be directly applied as electrode, especially in large macroscopic scale and from
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renewable biomass. Conversion of biomass feedstock (such as cellulous, sucrose, seaweed, etc.) into green carbon has recently become a promising route for energy application due to their unique merits of renewability, vast availability, and economical sustainability [21e24]. Lignin, the second most abundant natural polymer after cellulose, constitutes 20e30 wt% lignocellulose biomass, which is an underutilized byproduct of the pulp and paper industry and often treated as a waste that is burned for heat and power [25]. There are now growing interests of converting lignin into electrode for energy storage devices as a beneficial approach [26e29]. Though the derived carbon electrode show improved performance [30e32], current design is not aimed towards simultaneous high volumetric/areal performance. An effective way is to develop binder- and conductive additive-free electrode in order to circumvent the adverse effects by conventional electrode preparation route on active surface area, ion percolation, and conductivity [2,21]. Also, the lack of macro/mesopores with predominant micropores in the derived carbon likely hinders ion diffusion [33]. Controlling the porosity, especially mesopores (2e50 nm) and macropores (above 50 nm), in the highly branched lignin molecules becomes challenging. Hence, one desirable route to address above issues is to form an interconnected hierarchical porous carbon monolith from lignin as a thick electrode. Herein, we report a novel fabrication of large-scale hierarchical porous carbon monolith from lignin source via a dual-template approach with superior areal/volumetric capacitances. This newly derived interconnected porous 3D carbon monolith possess high pore volume of macropores and mesopores, and hence a large surface area, with high conductivity. The resultant desirable properties for capacitive performance enables the fabrication of a binder- and conductive additive-free 310 mm thick electrode, which delivered simultaneously excellent areal/volumetric capacitance of 3.0 F cm2/97.1 F cm3, at a high mass loading of 14.4 mg cm2. This is attributed to the hierarchical porous carbon network that promotes ion diffusion and electron transport. A remarkably high areal capacitance of 6.2 F cm2 could be further achieved with volumetric capacitance retained at 72.7 F cm3. To our knowledge, the areal capacitances obtained by our material are the highest for any other carbon-based electrodes reported in the literature. The carbon monolith based symmetric supercapacitor achieved a high areal energy density of 0.16 mWh cm2 at 1.75 mW cm2, with excellent cycling performance (~95% retention after 10,000 cycles at a high current density). Our work therefore demonstrates the feasibility of converting abundant, environmentally-benign, and sustainable lignin to high-performance product. The promising areal/volumetric performances suggest the large scale 3D interconnected hierarchical porous carbon network may provide a positive route to improving the energy density of supercapacitor. 2. Experimental details 2.1. Synthesis of hierarchical porous carbon monolith from lignin The lignin was used after acid-washing procedure. Mesopores silica KIT-6 was prepared following the reported procedure(Chem. Commun. 2003, 2136). 1 g lignin and 1.6 g Pluronic P123 were mixed in 20 mL THF with 150 mL of 6 M HCl solution, which was stirred at room temperature. Subsequently, 0.4 g KIT-6 dissolved in THF was added in the mixture which was allowed to be stirred for 24 h. The mixture was then poured into alumina porcelain boat to dry at ambient and then maintained at 70 C for 2 days. The dried monolithic lignin precursor was carbonized in a tube furnace under Ar environment. The temperature was ramped from 25 to 400 C at 1 C min1, 400 Ce900 C at 2 C min1, and maintained at 900 C for 15 min. After that, the sample was immersed in 2 M NaOH
solution to remove silica and then dried at 120 C. For comparison, lignin-derived carbon material without KIT-6 (LC) was synthesized with all other experimental conditions remaining identical. Moreover, the thickness of the films was measured with five individual batches and the average value (310 mm) was used to calculate the corresponding packing density, which was ~0.45 g cm3. 2.2. Characterizations Morphology of the hierarchical porous carbon monoliths were investigated under Zeiss Supra 40 field-emission scanning electron microscope (SEM) at an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) micrographs were obtained with a JEOL 2010F system. The conductivity of the carbon monoliths were 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 adsorptionedesorption at liquid-N2 temperature (77 K). The pore size distribution was obtained by the density function theory (DFT). Raman spectra were obtained by a U1000 Raman spectrometer equipped with a double monochromator using an Ar green laser as a 514.5 nm excitation source. 2.3. Electrochemical measurements Cyclic voltammetry measurements were performed on the electrochemical analyzer (Solartron S1 1287) under ambient condition. A platinum foil, a saturated Hg/HgCl electrode (SCE), and KOH solution (6 M) were used as the counter electrode, reference electrode and electrolyte, respectively. 2.4. Calculation of areal/volumetric capacitances and energy densities In the three-electrode configuration, the specific galvanostatic capacitance (Cg, F g1) was evaluated according to
Cg ¼ IDt=ðV mÞ
(1)
where I (A) is the discharge current, DtðsÞ is the discharge time, V (V) is the potential window and m (g) is the mass of active materials. Volumetric capacitances (Cv, F cm3) and areal capacitances (Ca, F cm2) at various current densities were calculated from following equations:
Cv ¼ Cg r
(2)
Ca ¼ Cg M
(3)
where r (g cm3) is the packing density of the carbon monolith, M (mg cm2) is the mass loading of the electrode. For the two-electrode full cell device, the gravimetric specific capacitance (Cg, F g1) and areal capacitance (Ca, F cm2) for a single electrode was calculated from each galvanostatic charge discharge curve according to
Cg ¼ 4IDt=ðV mÞ
(4)
Ca ¼ Cg M
(5)
Cv ¼ Cg r
(6)
where I (A) is the discharge current, DtðsÞ is the discharge time, V
H. Li et al. / Carbon 100 (2016) 151e157
(V) is the potential window, m (g) is the total mass of both electrodes, M (mg cm2)/r (g cm3) are the mass loading/packing density for one electrode. The areal energy density (E, Wh cm2) and areal power density (P, W cm2) of the full cell device were calculated using
E¼
1 1 CV 2 2 3600
(7)
P¼
E 3600 Dt
(8)
where V, Dt have the same notations as above, and C is the areal capacitance of the device calculated by
C¼
Ca 2 4
(9)
where the factor 2 arises from the double mass loading in the symmetric device. The thickness of the films was measured with five individual batches and the average value (310 mm) was used to calculate the corresponding packing density, which was ~0.45 g cm3.
3. Results and discussion The carbon monolith was synthesized through the combination of soft template (Pluronic P123, EO20PO70EO20) and hard template (mesoporous silica, KIT-6, Fig. S1, ESIy) (hereafter denoted as SLC, and the monolith by P123 is denoted as LC) as schematically illustrated in Fig. 1a (see also Experimental section). Monolithic precursors can be easily casted via evaporation of organic solvent in the mixture of lignin, P123, and silica, with flexible tuning of sizes and shapes (Fig. 1b and Fig. S2). The precursor was then carbonized into carbon monolith at 900 C under Ar with a volume contraction~20% (Fig. 1b). Centimeter-scale monolith can be easily achieved (Fig. 1b). Also, the dimension along the thickness direction can be adjusted from a few hundreds of micrometers to near millimeter. Both indicate clearly the potential of scaling up via this facile synthesis approach. In our work, P123 was applied to synthesize the continuous carbon monolith, comparing to the porous powder obtained by the hard template only [29]. This suggests the essence of structure continuity in the micelle assembly for the monolith preparation (see also the monolith formed by F127 in Fig. S3). The roles of soft template include stabilizing the micelle in the polar solvent via hydrogen bonding between abundant phenolic hydroxyl groups and miscible segments of the block copolymer, maintaining the monolithic feature during solvent evaporation, and being the scaffold to support the framework during pyrolysis [34e36]. A porous network throughout the entire sample is clearly shown in Fig. 1c. TEM image (Fig. 1d) reveals that the mesoporous entities with an average size of ~30 nm constituted the carbon network. The N2 adsorptionedesorption plots (Fig. 1g) showed type IV isotherms according to IUPAC, confirming the presence of mesopores. The differential pore size distribution by density function theory (DFT) analysis (Fig. 1h) further depicted a broad mesopore distribution of 10e50 nm in LC, which may be attributed to the branched molecular structure of lignin or the collapse of mesopores during pyrolysis [27]. In order to allay the pore collapse, we applied KIT-6 with pores of ~9 nm and wall thickness of ~3 nm (Fig. S1) as the hard template to support carbon framework with increasing surface area after template removal [37]. From DFT analysis, SLC-silica (SLC before KIT-6 etching) exhibited 6.3 nm emerging pores that took dominant pore volume. The 6.3 nm pores were maintained during
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pyrolysis with the support from silica [38]. After the removal of KIT6, DFT indicated that 3.4 nm pores appeared in SLC, which was attributed to the replication of silica wall [39]. Meanwhile, a sharp rise in the isotherm as p/p0 approaching 1 indicates the contribution from macropores [40], which might be due to incomplete infiltration of lignin precursor into KIT-6 [29,41]. As shown in Fig. 1e, SLC exhibited a well-interconnected 3D porous network even after the template removal. Together with TEM image (Fig. 1f), it indicates consistently the hierarchical porous structure in SLC that consisted of ~100 nm open macropores and mesopores of 3.4, 6.3 and ~50 nm. The high porosity in SLC further yields a much higher BrunauereEmmetteTeller (BET) surface area of 803 m2 g1 with a total pore volume of 0.86 cm3 g1, comparing to 350 m2 g1 and 0.35 cm3 g1 of LC. Furthermore, SLC possessed a high electrical conductivity of 1700 S m1 determined by the 4-probe method. This excellent electrical conductivity could be attributed to the continuous carbon network and graphitation confirmed by Raman spectrum [42]. The Raman bands (Fig. 1i) at ~1590 cm1 and ~1340 cm1 correspond to the G band (the stretching mode of the sp [2] carbon) and D band (the breathing mode of aromatic rings), respectively (see also similar Raman features of LC in Fig. S4). The D band may be attributed to the lattice defects induced by structure distortion to accommodate the formed mesopores [8,43]. The intense G band indicates the presence of graphitation structure in SLC that provides continuous pathways for fast electron transportation [28]. We then directly evaluate the electrochemical performance of these synthesized mechanically stable and conducting 3D carbon monoliths based on a three-electrode configuration, as an assembly of electrode and current collector without additional binder and conductive additive. The CV curves of SLC and LC were performed within the potential window of 1 to 0 V with a series of scan rates from 1 to 50 mV s1 (Fig. 2a and b). At slow scan rates, the plots exhibited nearly rectangular cycle voltammogram, indicating typical electric double-layer capacitor (EDLC) performance [12]. Though tendency of forming shuttle-shape CV observed at higher scan rate, more rectangular shape for SLC suggests better penetration of electrolyte into pores and elevated transient current comparing to LC [44]. SLC showed an excellent specific capacitance of 208.4 F g1 at a current density of 0.1 A g1, remarkably higher than that of LC (112.8 F g1). The specific capacitance of our SLC is comparable to the current state-of-art values of carbon electrode materials for EDLC (see Table S1 for literature data summary) [6,11]. At 50 mV s1, SLC was able to deliver a specific capacitance of 105.0 F g1 (Fig. 2c). The typical triangle-like charge/discharge plots in Fig. 2d and e exhibit the reversible capacitive behavior of EDLC. At a high current density of 10 A g1, SLC showed a better rate capacitance of 46.4% than LC (15.6%). To verify the roles of macropores and mesopores, 100 nm and 20 nm silica nanoparticles were selected as the hard templates to incorporate the corresponding macro- or mesopores where 25% and 77% enhancements at 0.1 A g1 were obtained respectively (Fig. S5). The Nyquist plot of SLC (Fig. S6) features a vertical line at the end of semicircle region, indicating a nearly ideal capacitive behavior of the cell, and the internal resistance was estimated to be ~1.6 U indicating desirable channel for electron and ion conduction. The excellent capacitive performance of SLC can then be attributed to the interconnected carbon network with large surface area and high volume hierarchical pores that ensure the high bi-continuous electronic and ionic conductivity, which facilitates fast ion transportation of the electrolyte into the interior surfaces of the electrode, increases electrolyte/electrode contact area, and provides continuous pathway for electron transportation [20,40,45]. Besides, the use of monoliths as electrodes eliminates extra inactive materials. In order to investigate the long-term cycling stability, the specific capacitance
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Fig. 1. (a) Scheme depicting the synthesis of hierarchical porous carbon monolith from lignin; (b) Photographic image of monolithic lignin precursors (upper image) and porous carbon monoliths after carbonization (lower image); SEM and TEM images of LC (c, d) and SLC (e, f) monoliths showing the existence of macropores (indicated by blue circles) and mesopores (indicated by red circles); (g) N2 adsorptionedesorption isotherms with (h) differential pore size distributions for LC, SLC-silica, and SLC; (i) Raman spectrum of SLC where the D (~1340 cm1) and G (~1590 cm1) bands are labelled. (A colour version of this figure can be viewed online.)
of SLC as a function of cycle number was conducted at a current density of 2.0 A g1 (Fig. 2f). ~96% capacitance retention was obtained after 1500 cycles, demonstrating good electrochemical stability of SLC. Importantly, SLC showed an areal capacitance of 3.0 F cm2 and a volumetric capacitance of 97.1 F cm3 at a current density of 1.4 mA cm2 (0.1 A g1) (Fig. 3a), where the areal capacitance is the highest in currently reported carbon materials and the volumetric capacitance is among the highest values in current literature (Fig. 3b) [8,11,13,17,19,20,46,47]. The high capacitances were obtained with a thickness of ~310 mm and mass loading of 14.4 mg cm2, which are orders of higher than most of the reported carbon materials (tens of mm, 0.1e1 mg cm2), and even higher than commercial capacitors (10 mg cm2, 100e200 mm). Realizing the diverse roles of material loading/packing on areal/volumetric capacitances, mass loading is considered when evaluating the
performance, especially for the thick electrode as in our case. When comparing to most advancing carbon materials developed with close mass loadings [8,20,47], SLC presents its outstanding advantage in areal performance (Fig. S7). The superior areal capacitance can be understood by the inphase increase of areal capacitance and mass loading as revealed in current study [15,19,48]. At a high current density of 144.5 mA cm2 (10 A g1), the areal capacitance of 1.4 F cm2 was still promising with the volumetric capacitance of 45.2 F cm3. Meanwhile, a current of 10 A g1 with the mass loading of SLC corresponds effectively to 144.5 A g1 on 1 mg cm2. This shows the advances of our materials under high current operation. On the other hand, the high volumetric capacitance of this thick electrode directly links to the gravimetric capacitance as discussed above. Note that SLC presented a typical packing density of porous carbon (~0.45 g cm3), where packing density is under intense focus to
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Fig. 2. Electrochemical performance of the carbon monoliths tested in a 3-electrode system: cyclic voltammograms of (a) SLC and (b) LC; (c) Specific capacitance as a function of (c) scan rate and (d) charge/discharge current density for SLC and LC; (e) Galvanostatic charge/discharge curves at 0.5 A g1; (f) Evaluation of specific capacitance versus the number of cycles at 2 A g1 for SLC, where the inset shows the galvanostatic charge/discharge curves at 2 A g1. (A colour version of this figure can be viewed online.)
increase volumetric capacitance [8,10]. Hence, the volumetric performance highlights the roles of interconnected hierarchical porous network with the bi-continuous conductivity, which draw positive influence on largely deteriorated areal/volumetric capacitances due to high internal resistance and poor ion diffusion during scaling up [16,19]. Moreover, SLC was able to deliver an areal energy density of 0.42 mWh cm2 with maximum power density of 70 mW cm2, and a volumetric energy density of 13.5 Wh L1 with maximum power density of 2255 W L1 (Fig. 3d), which demonstrates its great potential of enhancing energy density. To study the potential of increasing areal/volumetric capacitances of the carbon monolith, the relation between areal/volumetric capacitances and the dimension of monolith was investigated. A monotonic increasing trend was observed for the areal capacitance of SLC within the thickness range from 310 to 850 mm (Fig. 3c) that is approximately linearly proportional to the corresponding mass loading. An ultrahigh areal capacitance of 6.2 F cm2 was achieved at ~850 mm with
a very high mass loading of 40 mg cm2. Despite a decreasing trend was observed for the volumetric capacitance, a decent value of 72.7 F cm3 could be still retained at 850 mm. Also, at high current density of ~145 mA cm2, both 600 mm and 810 mm SLCs were able to present high areal capacitances ~3.5 F cm2 (Fig. S8). We further fabricated a symmetric full cell supercapacitor based on SLC to demonstrate its real application value. Fig. 4a shows nearrectangular CV curves at sweep rates from 1 to 100 mV s1 without any redox peaks, indicating ideal electrical double layer formation, small ion transfer resistance, and efficient charge propagation behavior in the device [6]. The SLC electrode in the full cell was able to deliver a remarkably high areal capacitance of 2.2 F cm2 (152.5 F g1) at 1 mV s1 in 6 M KOH, with a volumetric capacitance of 68.0 F cm3. From the typical triangle-like charge/discharge plots at different current densities (Fig. 4b), power densities and energy densities were calculated in the Ragone plot (Fig. 4c). The full cell delivered a maximum areal energy density of 0.16 mWh cm2 at
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Fig. 3. (a) Areal/volumetric capacitances as a function of charge/discharge current density for SLC; (b) Comparison on the areal capacitance (solid star) and volumetric capacitance (open star) of SLC with the reported literature values (solid symbols for areal capacitances and open symbols for volumetric capacitances) shows the superior performance in our work; (c) Areal/volumetric capacitances as a function of electrode mass loading/thickness, showing further enhancement towards ultrahigh areal capacitance; (d) Per-area/volume Ragone plot for SLC. (A colour version of this figure can be viewed online.)
Fig. 4. (a) CV curves of different scan rates for the symmetric supercapacitor by SLCs; (b) Galvanostatic charge/discharge curves collected at different current densities; (c) areal Ragone plots (energy density vs power density) for the full cell; (d) Cycling stability of the full cell collected at 289 mA cm1 up to 10,000 cycles. (A colour version of this figure can be viewed online.)
H. Li et al. / Carbon 100 (2016) 151e157
1.75 mW cm2, and a maximum volumetric energy density of 2.6 Wh L1 at 28.5 W L1. The areal energy density is among the highest of current state-of-art carbon based supercapacitors as summarized in Table S1. Note that the areal/volumetric energy densities can be further improved via enlarging potential window as in organic electrolyte. Though an increase in the power density compromises the energy density, it still provided 0.08 mWh cm2 at 309.5 mW cm2. The long-term cyclic stability is another critical requirement for supercapacitor performance, which was tested at a high current density of 289 mA cm2 (10 A g1) (Fig. 4d). After 10,000 cycles, the full cell continued to deliver 95.2% capacitance. It indicates directly the excellent cycle durability of the carbon monolith as electrode material for supercapacitor. 4. Conclusion A 3D interconnected hierarchical porous carbon monolith has been successfully synthesized from naturally abundant lignin as the high-performance electrode for electric double layer supercapacitor. The newly derived carbon monolith possessed desirable microstructure for efficient bi-continuous ion diffusion and electrical conduction, which can be directly applied as electrode and current collector that reduces the use of extra inactive materials. The thick (310 mm) carbon monolith presented simultaneously a superior areal capacitance of 3.0 F cm2 and a high volumetric capacitance of 97.1 F cm3 among currently reported carbon materials, at a large mass loading of 14.4 mg cm2. The areal capacitance could be further enhanced to an ultrahigh value of 6.2 F cm2 at 40 mg cm2, with a descent volumetric capacitance of 72.7 F cm3. The areal capacitances obtained in our study are the highest for any other carbon-based electrodes reported in the literature. The symmetric supercapacitor formed by the monoliths showed a high areal energy density of 0.16 mWh cm2 at 1.75 mW cm2 with good cycling stability (95.2% retention after 10,000 cycles at a high current density of 289 mA cm2). Our work demonstrates the successful utilization of biomass waste as highperformance electrode material for energy storage system. The large scale 3D hierarchical porous carbon network may offer promising route for supercapacitor with promising areal/volumetric energy densities. Acknowledgment x This work was supported by the SERC grant to NUS (R-284000-112-305). xx The authors would like to acknowledge Dr. Xiaolei Huang for the in-depth discussion on the performance of supercapacitor. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2015.12.075. References [1] X. Wang, X. Lu, B. Liu, D. Chen, Y. Tong, G. Shen, Adv. Mater 26 (2014) 4763. [2] A.D. Roberts, X. Li, H. Zhang, Chem. Soc. Rev. 43 (2014) 4341. [3] X. Lu, M. Yu, G. Wang, Y. Tong, Y. Li, Energ Environ. Sci. 7 (2014) 2160.
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Carbon journal homepage: www.elsevier.com/locate/carbon
Lignin-derived interconnected hierarchical porous carbon monolith with large areal/volumetric capacitances for supercapacitor Hui Li a, 1, Du Yuan a, 1, Chunhua Tang a, Suxi Wang b, Jiaotong Sun a, Zibiao Li b, Tao Tang a, Fuke Wang b, Hao Gong a, Chaobin He a, b, * a b
Department of Materials Science and Engineering, National University of Singapore, 117576, Singapore Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 117602, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 July 2015 Received in revised form 3 December 2015 Accepted 20 December 2015 Available online 6 January 2016
A large scale 3D interconnected hierarchical porous carbon monolith was developed by a novel synthetic approach from the 2nd most abundant natural polymer, lignin. This newly derived carbon monolith possess desirable microstructure and high conductivity that enables the fabrication of a binder-/ conductive additive-free thick electrode. The carbon monolith delivered simultaneously excellent areal/ volumetric capacitances of 3.0 F cm2/97.1 F cm3, at a high mass loading of 14.4 mg cm2. The areal capacitances obtained in our work are the highest among the reported carbon-based electrodes. The carbon monolith based symmetric supercapacitor achieved a high areal energy density of 0.16 mWh cm2 at 1.75 mW cm2, with excellent cycling performance (~95% retention after 10,000 cycles). Our work demonstrates the successful conversion of biomass waste into high-performance eletrode, and the promising areal/volumetric performances further suggest the developed approach may provide a positive route to improving the energy density of supercapacitor. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction With the depletion of fossil fuels and rapid-growing energy demand, great effort has been dedicated to high efficient, cost effective, and environmental friendly energy storage systems, like lithium ion battery and supercapacitor [1e5]. Supercapacitor receives wide applications due to its high power density and long lifetime. Recent research focuses primarily on improving areal/ volumetric energy densities of supercapacitor with mounting demand for compact and portable energy storage systems [6e8]. However, areal and volumetric performances are generally in conflict with each other [9]. For example, the volumetric performance of carbon-based electrodes do not scale up linearly with their thickness that hinders enhancing volumetric energy density, though significant progress achieved in advancing specific gravimetric capacitances (~100e400 F g1) [8,10e14]. Promising volumetric capacitance obtained with thin electrode (up to tens of micrometers) is commonly accompanied by a compromise of low
* Corresponding author. Department of Materials Science and Engineering, National University of Singapore, 117575, Singapore. E-mail address: [email protected] (C. He). 1 Hui Li and Du Yuan contribute equally to this work. http://dx.doi.org/10.1016/j.carbon.2015.12.075 0008-6223/© 2016 Elsevier Ltd. All rights reserved.
areal capacitance with poor mass loading (~1 mg cm2), comparing to commercially available products (10 mg cm2, 100e200 mm) [6,15e19]. When areal performance is under central pursuit with high mass loading/thickness, volumetric capacitance is largely sacrificed owing to high internal resistance and poor ion diffusion [16,19]. The great challenge therefore lies in simultaneous achievement of the true performance metrics of supercapacitor [6], where carbon-based electrodes typically offer low areal capacitance (tens to a few hundred of mF cm2) and volumetric capacitance (<60 F cm3) [16]. Since the true performance matrices are intimately correlated to both material and microstructure of the electrode, a thick carbonbased electrode should integrate the following desirable merits. A porous structure is essential with incorporation of macropores and mesopores to minimize ion diffusion distance for fast charge transport and increase surface area for promoting electrolyte/ electrode contact [20,21]. A binder-/conductive additive-free structure continuity is ideal to eliminate dead cell components and consequently retain the high conductivity of carbon, which benefits volumetric performance at high mass loading [2]. However, it is still challenging to synthesize continuously 3D networking hierarchical porous carbon that can be directly applied as electrode, especially in large macroscopic scale and from
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renewable biomass. Conversion of biomass feedstock (such as cellulous, sucrose, seaweed, etc.) into green carbon has recently become a promising route for energy application due to their unique merits of renewability, vast availability, and economical sustainability [21e24]. Lignin, the second most abundant natural polymer after cellulose, constitutes 20e30 wt% lignocellulose biomass, which is an underutilized byproduct of the pulp and paper industry and often treated as a waste that is burned for heat and power [25]. There are now growing interests of converting lignin into electrode for energy storage devices as a beneficial approach [26e29]. Though the derived carbon electrode show improved performance [30e32], current design is not aimed towards simultaneous high volumetric/areal performance. An effective way is to develop binder- and conductive additive-free electrode in order to circumvent the adverse effects by conventional electrode preparation route on active surface area, ion percolation, and conductivity [2,21]. Also, the lack of macro/mesopores with predominant micropores in the derived carbon likely hinders ion diffusion [33]. Controlling the porosity, especially mesopores (2e50 nm) and macropores (above 50 nm), in the highly branched lignin molecules becomes challenging. Hence, one desirable route to address above issues is to form an interconnected hierarchical porous carbon monolith from lignin as a thick electrode. Herein, we report a novel fabrication of large-scale hierarchical porous carbon monolith from lignin source via a dual-template approach with superior areal/volumetric capacitances. This newly derived interconnected porous 3D carbon monolith possess high pore volume of macropores and mesopores, and hence a large surface area, with high conductivity. The resultant desirable properties for capacitive performance enables the fabrication of a binder- and conductive additive-free 310 mm thick electrode, which delivered simultaneously excellent areal/volumetric capacitance of 3.0 F cm2/97.1 F cm3, at a high mass loading of 14.4 mg cm2. This is attributed to the hierarchical porous carbon network that promotes ion diffusion and electron transport. A remarkably high areal capacitance of 6.2 F cm2 could be further achieved with volumetric capacitance retained at 72.7 F cm3. To our knowledge, the areal capacitances obtained by our material are the highest for any other carbon-based electrodes reported in the literature. The carbon monolith based symmetric supercapacitor achieved a high areal energy density of 0.16 mWh cm2 at 1.75 mW cm2, with excellent cycling performance (~95% retention after 10,000 cycles at a high current density). Our work therefore demonstrates the feasibility of converting abundant, environmentally-benign, and sustainable lignin to high-performance product. The promising areal/volumetric performances suggest the large scale 3D interconnected hierarchical porous carbon network may provide a positive route to improving the energy density of supercapacitor. 2. Experimental details 2.1. Synthesis of hierarchical porous carbon monolith from lignin The lignin was used after acid-washing procedure. Mesopores silica KIT-6 was prepared following the reported procedure(Chem. Commun. 2003, 2136). 1 g lignin and 1.6 g Pluronic P123 were mixed in 20 mL THF with 150 mL of 6 M HCl solution, which was stirred at room temperature. Subsequently, 0.4 g KIT-6 dissolved in THF was added in the mixture which was allowed to be stirred for 24 h. The mixture was then poured into alumina porcelain boat to dry at ambient and then maintained at 70 C for 2 days. The dried monolithic lignin precursor was carbonized in a tube furnace under Ar environment. The temperature was ramped from 25 to 400 C at 1 C min1, 400 Ce900 C at 2 C min1, and maintained at 900 C for 15 min. After that, the sample was immersed in 2 M NaOH
solution to remove silica and then dried at 120 C. For comparison, lignin-derived carbon material without KIT-6 (LC) was synthesized with all other experimental conditions remaining identical. Moreover, the thickness of the films was measured with five individual batches and the average value (310 mm) was used to calculate the corresponding packing density, which was ~0.45 g cm3. 2.2. Characterizations Morphology of the hierarchical porous carbon monoliths were investigated under Zeiss Supra 40 field-emission scanning electron microscope (SEM) at an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) micrographs were obtained with a JEOL 2010F system. The conductivity of the carbon monoliths were 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 adsorptionedesorption at liquid-N2 temperature (77 K). The pore size distribution was obtained by the density function theory (DFT). Raman spectra were obtained by a U1000 Raman spectrometer equipped with a double monochromator using an Ar green laser as a 514.5 nm excitation source. 2.3. Electrochemical measurements Cyclic voltammetry measurements were performed on the electrochemical analyzer (Solartron S1 1287) under ambient condition. A platinum foil, a saturated Hg/HgCl electrode (SCE), and KOH solution (6 M) were used as the counter electrode, reference electrode and electrolyte, respectively. 2.4. Calculation of areal/volumetric capacitances and energy densities In the three-electrode configuration, the specific galvanostatic capacitance (Cg, F g1) was evaluated according to
Cg ¼ IDt=ðV mÞ
(1)
where I (A) is the discharge current, DtðsÞ is the discharge time, V (V) is the potential window and m (g) is the mass of active materials. Volumetric capacitances (Cv, F cm3) and areal capacitances (Ca, F cm2) at various current densities were calculated from following equations:
Cv ¼ Cg r
(2)
Ca ¼ Cg M
(3)
where r (g cm3) is the packing density of the carbon monolith, M (mg cm2) is the mass loading of the electrode. For the two-electrode full cell device, the gravimetric specific capacitance (Cg, F g1) and areal capacitance (Ca, F cm2) for a single electrode was calculated from each galvanostatic charge discharge curve according to
Cg ¼ 4IDt=ðV mÞ
(4)
Ca ¼ Cg M
(5)
Cv ¼ Cg r
(6)
where I (A) is the discharge current, DtðsÞ is the discharge time, V
H. Li et al. / Carbon 100 (2016) 151e157
(V) is the potential window, m (g) is the total mass of both electrodes, M (mg cm2)/r (g cm3) are the mass loading/packing density for one electrode. The areal energy density (E, Wh cm2) and areal power density (P, W cm2) of the full cell device were calculated using
E¼
1 1 CV 2 2 3600
(7)
P¼
E 3600 Dt
(8)
where V, Dt have the same notations as above, and C is the areal capacitance of the device calculated by
C¼
Ca 2 4
(9)
where the factor 2 arises from the double mass loading in the symmetric device. The thickness of the films was measured with five individual batches and the average value (310 mm) was used to calculate the corresponding packing density, which was ~0.45 g cm3.
3. Results and discussion The carbon monolith was synthesized through the combination of soft template (Pluronic P123, EO20PO70EO20) and hard template (mesoporous silica, KIT-6, Fig. S1, ESIy) (hereafter denoted as SLC, and the monolith by P123 is denoted as LC) as schematically illustrated in Fig. 1a (see also Experimental section). Monolithic precursors can be easily casted via evaporation of organic solvent in the mixture of lignin, P123, and silica, with flexible tuning of sizes and shapes (Fig. 1b and Fig. S2). The precursor was then carbonized into carbon monolith at 900 C under Ar with a volume contraction~20% (Fig. 1b). Centimeter-scale monolith can be easily achieved (Fig. 1b). Also, the dimension along the thickness direction can be adjusted from a few hundreds of micrometers to near millimeter. Both indicate clearly the potential of scaling up via this facile synthesis approach. In our work, P123 was applied to synthesize the continuous carbon monolith, comparing to the porous powder obtained by the hard template only [29]. This suggests the essence of structure continuity in the micelle assembly for the monolith preparation (see also the monolith formed by F127 in Fig. S3). The roles of soft template include stabilizing the micelle in the polar solvent via hydrogen bonding between abundant phenolic hydroxyl groups and miscible segments of the block copolymer, maintaining the monolithic feature during solvent evaporation, and being the scaffold to support the framework during pyrolysis [34e36]. A porous network throughout the entire sample is clearly shown in Fig. 1c. TEM image (Fig. 1d) reveals that the mesoporous entities with an average size of ~30 nm constituted the carbon network. The N2 adsorptionedesorption plots (Fig. 1g) showed type IV isotherms according to IUPAC, confirming the presence of mesopores. The differential pore size distribution by density function theory (DFT) analysis (Fig. 1h) further depicted a broad mesopore distribution of 10e50 nm in LC, which may be attributed to the branched molecular structure of lignin or the collapse of mesopores during pyrolysis [27]. In order to allay the pore collapse, we applied KIT-6 with pores of ~9 nm and wall thickness of ~3 nm (Fig. S1) as the hard template to support carbon framework with increasing surface area after template removal [37]. From DFT analysis, SLC-silica (SLC before KIT-6 etching) exhibited 6.3 nm emerging pores that took dominant pore volume. The 6.3 nm pores were maintained during
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pyrolysis with the support from silica [38]. After the removal of KIT6, DFT indicated that 3.4 nm pores appeared in SLC, which was attributed to the replication of silica wall [39]. Meanwhile, a sharp rise in the isotherm as p/p0 approaching 1 indicates the contribution from macropores [40], which might be due to incomplete infiltration of lignin precursor into KIT-6 [29,41]. As shown in Fig. 1e, SLC exhibited a well-interconnected 3D porous network even after the template removal. Together with TEM image (Fig. 1f), it indicates consistently the hierarchical porous structure in SLC that consisted of ~100 nm open macropores and mesopores of 3.4, 6.3 and ~50 nm. The high porosity in SLC further yields a much higher BrunauereEmmetteTeller (BET) surface area of 803 m2 g1 with a total pore volume of 0.86 cm3 g1, comparing to 350 m2 g1 and 0.35 cm3 g1 of LC. Furthermore, SLC possessed a high electrical conductivity of 1700 S m1 determined by the 4-probe method. This excellent electrical conductivity could be attributed to the continuous carbon network and graphitation confirmed by Raman spectrum [42]. The Raman bands (Fig. 1i) at ~1590 cm1 and ~1340 cm1 correspond to the G band (the stretching mode of the sp [2] carbon) and D band (the breathing mode of aromatic rings), respectively (see also similar Raman features of LC in Fig. S4). The D band may be attributed to the lattice defects induced by structure distortion to accommodate the formed mesopores [8,43]. The intense G band indicates the presence of graphitation structure in SLC that provides continuous pathways for fast electron transportation [28]. We then directly evaluate the electrochemical performance of these synthesized mechanically stable and conducting 3D carbon monoliths based on a three-electrode configuration, as an assembly of electrode and current collector without additional binder and conductive additive. The CV curves of SLC and LC were performed within the potential window of 1 to 0 V with a series of scan rates from 1 to 50 mV s1 (Fig. 2a and b). At slow scan rates, the plots exhibited nearly rectangular cycle voltammogram, indicating typical electric double-layer capacitor (EDLC) performance [12]. Though tendency of forming shuttle-shape CV observed at higher scan rate, more rectangular shape for SLC suggests better penetration of electrolyte into pores and elevated transient current comparing to LC [44]. SLC showed an excellent specific capacitance of 208.4 F g1 at a current density of 0.1 A g1, remarkably higher than that of LC (112.8 F g1). The specific capacitance of our SLC is comparable to the current state-of-art values of carbon electrode materials for EDLC (see Table S1 for literature data summary) [6,11]. At 50 mV s1, SLC was able to deliver a specific capacitance of 105.0 F g1 (Fig. 2c). The typical triangle-like charge/discharge plots in Fig. 2d and e exhibit the reversible capacitive behavior of EDLC. At a high current density of 10 A g1, SLC showed a better rate capacitance of 46.4% than LC (15.6%). To verify the roles of macropores and mesopores, 100 nm and 20 nm silica nanoparticles were selected as the hard templates to incorporate the corresponding macro- or mesopores where 25% and 77% enhancements at 0.1 A g1 were obtained respectively (Fig. S5). The Nyquist plot of SLC (Fig. S6) features a vertical line at the end of semicircle region, indicating a nearly ideal capacitive behavior of the cell, and the internal resistance was estimated to be ~1.6 U indicating desirable channel for electron and ion conduction. The excellent capacitive performance of SLC can then be attributed to the interconnected carbon network with large surface area and high volume hierarchical pores that ensure the high bi-continuous electronic and ionic conductivity, which facilitates fast ion transportation of the electrolyte into the interior surfaces of the electrode, increases electrolyte/electrode contact area, and provides continuous pathway for electron transportation [20,40,45]. Besides, the use of monoliths as electrodes eliminates extra inactive materials. In order to investigate the long-term cycling stability, the specific capacitance
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Fig. 1. (a) Scheme depicting the synthesis of hierarchical porous carbon monolith from lignin; (b) Photographic image of monolithic lignin precursors (upper image) and porous carbon monoliths after carbonization (lower image); SEM and TEM images of LC (c, d) and SLC (e, f) monoliths showing the existence of macropores (indicated by blue circles) and mesopores (indicated by red circles); (g) N2 adsorptionedesorption isotherms with (h) differential pore size distributions for LC, SLC-silica, and SLC; (i) Raman spectrum of SLC where the D (~1340 cm1) and G (~1590 cm1) bands are labelled. (A colour version of this figure can be viewed online.)
of SLC as a function of cycle number was conducted at a current density of 2.0 A g1 (Fig. 2f). ~96% capacitance retention was obtained after 1500 cycles, demonstrating good electrochemical stability of SLC. Importantly, SLC showed an areal capacitance of 3.0 F cm2 and a volumetric capacitance of 97.1 F cm3 at a current density of 1.4 mA cm2 (0.1 A g1) (Fig. 3a), where the areal capacitance is the highest in currently reported carbon materials and the volumetric capacitance is among the highest values in current literature (Fig. 3b) [8,11,13,17,19,20,46,47]. The high capacitances were obtained with a thickness of ~310 mm and mass loading of 14.4 mg cm2, which are orders of higher than most of the reported carbon materials (tens of mm, 0.1e1 mg cm2), and even higher than commercial capacitors (10 mg cm2, 100e200 mm). Realizing the diverse roles of material loading/packing on areal/volumetric capacitances, mass loading is considered when evaluating the
performance, especially for the thick electrode as in our case. When comparing to most advancing carbon materials developed with close mass loadings [8,20,47], SLC presents its outstanding advantage in areal performance (Fig. S7). The superior areal capacitance can be understood by the inphase increase of areal capacitance and mass loading as revealed in current study [15,19,48]. At a high current density of 144.5 mA cm2 (10 A g1), the areal capacitance of 1.4 F cm2 was still promising with the volumetric capacitance of 45.2 F cm3. Meanwhile, a current of 10 A g1 with the mass loading of SLC corresponds effectively to 144.5 A g1 on 1 mg cm2. This shows the advances of our materials under high current operation. On the other hand, the high volumetric capacitance of this thick electrode directly links to the gravimetric capacitance as discussed above. Note that SLC presented a typical packing density of porous carbon (~0.45 g cm3), where packing density is under intense focus to
H. Li et al. / Carbon 100 (2016) 151e157
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Fig. 2. Electrochemical performance of the carbon monoliths tested in a 3-electrode system: cyclic voltammograms of (a) SLC and (b) LC; (c) Specific capacitance as a function of (c) scan rate and (d) charge/discharge current density for SLC and LC; (e) Galvanostatic charge/discharge curves at 0.5 A g1; (f) Evaluation of specific capacitance versus the number of cycles at 2 A g1 for SLC, where the inset shows the galvanostatic charge/discharge curves at 2 A g1. (A colour version of this figure can be viewed online.)
increase volumetric capacitance [8,10]. Hence, the volumetric performance highlights the roles of interconnected hierarchical porous network with the bi-continuous conductivity, which draw positive influence on largely deteriorated areal/volumetric capacitances due to high internal resistance and poor ion diffusion during scaling up [16,19]. Moreover, SLC was able to deliver an areal energy density of 0.42 mWh cm2 with maximum power density of 70 mW cm2, and a volumetric energy density of 13.5 Wh L1 with maximum power density of 2255 W L1 (Fig. 3d), which demonstrates its great potential of enhancing energy density. To study the potential of increasing areal/volumetric capacitances of the carbon monolith, the relation between areal/volumetric capacitances and the dimension of monolith was investigated. A monotonic increasing trend was observed for the areal capacitance of SLC within the thickness range from 310 to 850 mm (Fig. 3c) that is approximately linearly proportional to the corresponding mass loading. An ultrahigh areal capacitance of 6.2 F cm2 was achieved at ~850 mm with
a very high mass loading of 40 mg cm2. Despite a decreasing trend was observed for the volumetric capacitance, a decent value of 72.7 F cm3 could be still retained at 850 mm. Also, at high current density of ~145 mA cm2, both 600 mm and 810 mm SLCs were able to present high areal capacitances ~3.5 F cm2 (Fig. S8). We further fabricated a symmetric full cell supercapacitor based on SLC to demonstrate its real application value. Fig. 4a shows nearrectangular CV curves at sweep rates from 1 to 100 mV s1 without any redox peaks, indicating ideal electrical double layer formation, small ion transfer resistance, and efficient charge propagation behavior in the device [6]. The SLC electrode in the full cell was able to deliver a remarkably high areal capacitance of 2.2 F cm2 (152.5 F g1) at 1 mV s1 in 6 M KOH, with a volumetric capacitance of 68.0 F cm3. From the typical triangle-like charge/discharge plots at different current densities (Fig. 4b), power densities and energy densities were calculated in the Ragone plot (Fig. 4c). The full cell delivered a maximum areal energy density of 0.16 mWh cm2 at
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Fig. 3. (a) Areal/volumetric capacitances as a function of charge/discharge current density for SLC; (b) Comparison on the areal capacitance (solid star) and volumetric capacitance (open star) of SLC with the reported literature values (solid symbols for areal capacitances and open symbols for volumetric capacitances) shows the superior performance in our work; (c) Areal/volumetric capacitances as a function of electrode mass loading/thickness, showing further enhancement towards ultrahigh areal capacitance; (d) Per-area/volume Ragone plot for SLC. (A colour version of this figure can be viewed online.)
Fig. 4. (a) CV curves of different scan rates for the symmetric supercapacitor by SLCs; (b) Galvanostatic charge/discharge curves collected at different current densities; (c) areal Ragone plots (energy density vs power density) for the full cell; (d) Cycling stability of the full cell collected at 289 mA cm1 up to 10,000 cycles. (A colour version of this figure can be viewed online.)
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1.75 mW cm2, and a maximum volumetric energy density of 2.6 Wh L1 at 28.5 W L1. The areal energy density is among the highest of current state-of-art carbon based supercapacitors as summarized in Table S1. Note that the areal/volumetric energy densities can be further improved via enlarging potential window as in organic electrolyte. Though an increase in the power density compromises the energy density, it still provided 0.08 mWh cm2 at 309.5 mW cm2. The long-term cyclic stability is another critical requirement for supercapacitor performance, which was tested at a high current density of 289 mA cm2 (10 A g1) (Fig. 4d). After 10,000 cycles, the full cell continued to deliver 95.2% capacitance. It indicates directly the excellent cycle durability of the carbon monolith as electrode material for supercapacitor. 4. Conclusion A 3D interconnected hierarchical porous carbon monolith has been successfully synthesized from naturally abundant lignin as the high-performance electrode for electric double layer supercapacitor. The newly derived carbon monolith possessed desirable microstructure for efficient bi-continuous ion diffusion and electrical conduction, which can be directly applied as electrode and current collector that reduces the use of extra inactive materials. The thick (310 mm) carbon monolith presented simultaneously a superior areal capacitance of 3.0 F cm2 and a high volumetric capacitance of 97.1 F cm3 among currently reported carbon materials, at a large mass loading of 14.4 mg cm2. The areal capacitance could be further enhanced to an ultrahigh value of 6.2 F cm2 at 40 mg cm2, with a descent volumetric capacitance of 72.7 F cm3. The areal capacitances obtained in our study are the highest for any other carbon-based electrodes reported in the literature. The symmetric supercapacitor formed by the monoliths showed a high areal energy density of 0.16 mWh cm2 at 1.75 mW cm2 with good cycling stability (95.2% retention after 10,000 cycles at a high current density of 289 mA cm2). Our work demonstrates the successful utilization of biomass waste as highperformance electrode material for energy storage system. The large scale 3D hierarchical porous carbon network may offer promising route for supercapacitor with promising areal/volumetric energy densities. Acknowledgment x This work was supported by the SERC grant to NUS (R-284000-112-305). xx The authors would like to acknowledge Dr. Xiaolei Huang for the in-depth discussion on the performance of supercapacitor. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2015.12.075. References [1] X. Wang, X. Lu, B. Liu, D. Chen, Y. Tong, G. Shen, Adv. Mater 26 (2014) 4763. [2] A.D. Roberts, X. Li, H. Zhang, Chem. Soc. Rev. 43 (2014) 4341. [3] X. Lu, M. Yu, G. Wang, Y. Tong, Y. Li, Energ Environ. Sci. 7 (2014) 2160.
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