Abstract
The electrode material plays a key role in the performance of a supercapacitor. This study reports a method to prepare high conductivity hierarchical porous carbon with pre-crosslinking based on sodium lignosulfonate, which usually is burned as a waste effluent in pulp and paper mills. Results indicate pre-crosslinking with glutaraldehyde and ethyleneimine polymer, activation with less potassium hydroxide KOH, and finally carbonization to form carbon materials of hyperbranched macromolecules with ultra-rich specific surface area of 2780 m2/g and reasonable pore size distribution. The prepared carbon material showed high specific capacitance and good electrochemical performance when used as electrode materials for supercapacitors. At 0.5 A/g, the specific capacitance was 305 F/g, and the specific capacitance decay was only 6.2%, even after 2500 consecutive charge/discharge cycles, which revealed the huge potential for the supercapacitor characteristics with long service life. The results provided a high-valued application of industrial waste, which is important to further the sustainable development of the national economy.
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Preparation of High Conductivity Hierarchical Porous Carbon Based on Sodium Lignosulfonate with Pre-crosslinking
Yanjie Yi, Yi Hou,* and Youming Li
The electrode material plays a key role in the performance of a supercapacitor. This study reports a method to prepare high conductivity hierarchical porous carbon with pre-crosslinking based on sodium lignosulfonate, which usually is burned as a waste effluent in pulp and paper mills. Results indicate pre-crosslinking with glutaraldehyde and ethyleneimine polymer, activation with less potassium hydroxide KOH, and finally carbonization to form carbon materials of hyperbranched macromolecules with ultra-rich specific surface area of 2780 m2/g and reasonable pore size distribution. The prepared carbon material showed high specific capacitance and good electrochemical performance when used as electrode materials for supercapacitors. At 0.5 A/g, the specific capacitance was 305 F/g, and the specific capacitance decay was only 6.2%, even after 2500 consecutive charge/discharge cycles, which revealed the huge potential for the supercapacitor characteristics with long service life. The results provided a high-valued application of industrial waste, which is important to further the sustainable development of the national economy.
Keywords: Sodium lignosulfonate; Porous carbon; Supercapacitor; Pre-crosslinking
Contact information: State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, 510640, Guangzhou, China; *Corresponding author: ceyhou@scut.edu.cn
INTRODUCTION
With the development of society and technology and the increased demand for energy, the high-performance capacitor has attracted a lot of attention. The supercapacitor has the characteristics of high charging and discharging speed, high efficiency, environmental friendliness, long cycle life, wide application temperature range and high safety performance, and ease of use as a fast charging and discharging standby power (Mao et al. 2017; Gao et al. 2018; Men et al. 2019; Sun et al. 2019). The electrode of the supercapacitor is composed of carbon materials with high specific surface area (Beguin et al. 2014). Recently, biomass-based porous carbon materials have been attracting extensive attention as a way to achieve high utilization of resources. Various biomass resources, such as bamboo (Zhang et al. 2018), rice husk (Kumagai et al. 2013), corn stover (Kumagai et al. 2013), and tea (Song et al. 2019), have been used in the preparation of porous carbon to be then applied in supercapacitors.
At present, the preparation method of biomass porous carbon is mainly concentrated as a chemical activation method; the activator is mostly potassium hydroxide (KOH), and the carbon-alkali ratio is 1:3. High alkali consumption not only increases the production cost but also corrodes the carbonized instrument and reduces its service life. The prepared porous carbons are greatly influenced by the structure of the precursors. Different precursor structures lead to different pore structures of porous carbon. The random staggered arrangement between the pore channels results in lower electrolyte ion transport efficiency, high resistance, and lower capacitance utilization. Doping heteroatoms to the carbon framework can improve the wetting performance of the surface (Wang et al. 2020), storing a large number of ions. The presence of heteroatoms can form the center of redox reaction and thus introduce Faraday pseudocapacitors. For example, Wang et al. (2019) and Liu et al. (2019) prepared O-N-S co-doped hierarchical porous carbons to improve the specific capacitance.
As a by-product of pulping and papermaking, lignin and its derivatives are mostly excreted in the form of black liquid (Lora and Glasser 2002). Black liquor is usually burned, such that lignin and other organic compounds are not effectively used. This is a great waste of resources. As natural aromatic compounds, the carbon content of the lignin is as high as 60 to 70%; in addition, lignin is one of the most abundant organic matters in the natural world (Aro and Fatehi 2017). In recent years, Chang et al. (2016) attempted to prepare lignin directly into porous carbon using chemical activations. However, the prepared porous carbons have the disadvantages of relatively small specific surface areas, unreasonable pore size distribution on the carbon skeleton, difficulties to control the pore formation process during the carbonization process, and the poor electrochemical performance after assembling into capacitors. Later, Zhang et al. (2019) proposed a novel green bacterial activation method for the synthesis of lignin-based porous carbon materials. Under the treatment of bacterial decomposition, the specific surface area of the carbon precursor is improved, which has a positive effect on the subsequent carbonization process, but colony growth requires high environmental and material requirements, and the internal treatment of carbon precursor is not uniform, resulting in a large difference of carbon structure. Therefore, a simple and easy pretreatment method of precursors is necessary for the preparation of porous carbon with high specific area and high electrochemical performance.
In this work, porous carbon materials were prepared by the innovative introduction of pre-crosslinking reaction using lignin sulfonate produced in the pulping and papermaking process as the raw material. To explore the influence of pre-crosslinking and activation processes on the pore size, surface area, and electrochemical properties of carbon materials, it can provide a new technical route and data base for the high-value application of biomass resources.
EXPERIMENTAL
Materials
Sodium lignosulfonate (93 wt%) was obtained from Damas-Beta Corporation (Guangzhou, China). It was used without any further purification. Glutaraldehyde solution (25%) was purchased from Kermel Reagent Co., Ltd. (Tianjing, China). Ethleneimine polymer (M.W. 1800.99%) was purchased from Innochem Company (Beijng, China). Potassium hydroxide (KOH), hydrochloric acid (HCl), and ethanol absolute were all of analytical grade and purchased from Guangzhou Congyuan Fine Chemical Co., Ltd. (Guangzhou, China). Deionized water was used in all experiments.
Synthesis
Preparation of carbon materials
A total of 5 g sodium lignosulfonate was dissolved in 60 mL ethanol absolute, 2.5 g glutaraldehyde aqueous solution was added, 0.01 mol ethyleneiminepolymer was added as a crosslinking agent, the mixed solution was stirred, and ultrasonic treatment was applied for 1 h. The mixed solution was transferred into the polytetrafluoroethylene and reacted for 8 h at 160 C. After the reaction was finished, the mixed solution was cooled to room temperature, washed with ethanol absolute, and dried at 60 C. Precursor after pre-crosslinking reaction (labeled as SLP) was heated to 400 C at the rate of 2 C/min and pre-carbonized for 1 h at 400 C. The pre-carbonation sample was mixed with KOH (1:1) and carbonized at 800 C for 2 h at the rate of 4 C/min. The porous carbon material hereafter referred to as SLPA (sodium lignosulfonate that had undergone pre-crosslinking and activating) was obtained after being washed with 1 mol/L HCl and deionized water to pH = 7.
For comparison, the sample called SLPC (sodium lignosulfonate that had undergone pre-crosslinking but without activating) was synthesized by following the same process without KOH as activator. Sodium lignosulfonate directly carbonized without pre-crosslinking or activator was named SLC. Sodium lignosulfonate using KOH as activators but without pre-crosslinking reaction were marked as SLAC.
Methods
Characterization
The 3D morphology and microstructures of carbon powders were observed by a LEO1530VP scanning electron microscope (SEM) (Carl Zeiss Co., Ltd., ShangHai, China). The specific surface and density functional theory were studied with the Brunauer-Emmett-Teller (BET) method using N2 adsorption-desorption at 77 K after treating the samples at 300 C for 8 h. X-ray photoelectron spectroscopy (XPS) (Thermol/ESCALAB 250XI; (Thermo Fisher Scientific Co., Ltd., Waltham, America) was conducted to examine the surface chemical compositions of the samples. The crystallographic information of the samples was investigated using a Bruker D8 Advanced X-ray diffractometer (XRD) with Cu K1 radiation source and Raman spectroscopy (LabRAM Aramis, Paris, France).
Electrochemical characterization
The performances of the samples were first evaluated using a three-electrode system in an electrochemical workstation (CHI660E; CH Instruments Co., Ltd., Shanghai, China). In a three-electrode system, the SLPA electrode, platinum electrode, and Ag/AgCl electrode were used as the working electrode, counter electrode, and reference electrode, respectively. To prepare working electrodes, the samples, acetylene black, and polytetrafluoroethylene (PTFE) were mixed in a weight ratio of 8:1:1, followed by addition of absolute ethanol to make the mixture into a uniform slurry. After drying at 120 C for 8 h, the mixture was pressed onto nickel foam as the current collector.
The electrochemical characterization, such as cyclic voltammetry (CV), galvanostatic charging-discharging (GCD), and electrochemical impedance spectroscopy (EIS), measurements were conducted using 6 M KOH in the voltage range of -1 to 0 V.
The specific capacitance of electrode was calculated using Eq. 1,
Cm = I × Δt / (m × ΔV) (1)
where Cm (F g-1) is specific capacitance, Δt (s) is discharge time, m (g) is active material mass, and ΔV (V) is the potential window.
RESULTS AND DISCUSSIONS
Fabrication of SLPA Carbon Composites
The synthesis mechanism of SLPA is depicted in Schemes 1 and 2.
Scheme 1. Preparation process of the porous carbon
Scheme 2. Schematic diagram of the precursor preparation process
Due to the self-pressurization due to volatilization of the solvent in the hydrothermal reactor, the macromolecules undergo the processes of dissolution and crystallization (Barbier et al. 2012), resulting in the branched chain connected to the benzene ring breaking off, molecular complexity, and volume reduction. Hydroxyl groups on the benzene ring of sodium lignosulfonate and glutaraldehyde can react with the ethyleneimine polymer as a crosslinking agent. Due to the fracture of the branched chain, the linked molecules between the tightly connected sodium lignosulfonate macromolecules became smaller, and the whole surface of macromolecules became loose, so it is easy to form many and regular pores in the carbonation process. In the process of carbonization, due to the micropores present in the molecule, a more developed pore structure can be formed using less KOH as activator. The KOH etches the carbon skeleton to generate micro-pores, and the gas generated in the high-temperature carbonization process escapes to further amplify the pores to obtain carbon materials with ultra-high specific surface areas.
Materials Characterization
The SEM analysis can directly explore the morphology of the as-prepared samples. Figure 1 shows the 3D interconnected porous architecture.
Fig. 1. SEM images of (a) SLC, (b) SLAC, (c and d) SLPC, and (e and f) SLPA
As shown in Fig. 1a, the SLC samples exhibited a loose and irregular porous structure. Most of the pores were mesopores, and the depth of the pores was shallow. The results showed that the samples without pre-crosslinking had the characteristics of small and underdeveloped pore size in the carbonization process. Figure 1b demonstrates that the pores of SLAC samples became expanded, deeper, and the carbon bone was hard, indicating that the activators increased the number of pores and expanded pore size during the carbonization process. From Figs. 1c and 1d, the holes were interlaced and further deepened, and a large number of micropores appeared on the inner wall of the macrospores. At the same time, the surface of carbon material has the tendency of gradually becoming smooth. This phenomenon indicated that the pre-crosslinking reaction can increase the number of pore-forming active sites in the carbonization process, and it was easy to form a large number of micropores. With the carbonization process, the pores sizes were enlarged, the pore walls were thickened, and the internal stability of carbon structures were improved. Figures 1e and 1f show the morphology of the SLPA samples. The graphitization of SLPA increased and the pores became deeper and the pore sizes further increased compared to the SLPC. The pores contained a large number of micropores, and the wall was thickened. It was easy to find that the surface of the prepared carbon materials contained a large number of well-developed pores and the surface of carbon materials also became more smooth and the size of the pores were uniformed under the synergistic action of the pre-crosslinking reaction and activators.
The structural characteristics of the samples were investigated by XRD. Two broad diffraction peaks were observed at 2θ = 23° and 43°, corresponding to the (002) and (101) graphitic carbon, respectively. The peaks that are evident in Fig. 2 implied that the samples had amorphous structure. Peaks of SLC were weaker and broader than others, showing a lower degree of graphitization.
Fig. 2. The XRD patterns of SLC, SLAC, SLPC, and SLPA
Compared to the SLC and SLAC samples, the SLPC and SLPA samples (002) peaks were narrow and strong, indicating the extent of graphitization was obviously higher than that of others. This result suggested that the pre-crosslinking reaction had a positive effect on improving the graphitization of carbon materials in the subsequent carbonization process. A narrow and strong peak of SLPA at around 2θ = 26° showed the sample exhibited a high graphitization degree. These results were consistent with SEM images.
As shown in Fig. 3, the Raman spectroscopy of the samples were applied to verify the structural defects of them. Two peaks at around 1345 cm-1 (D-band for the structural defects), and 1589 cm-1 (G-band for the structural defects) can be observed. The graphitization extent of the samples can be calculated from the intensity ratio (ID/IG). The ID/IG values for SLC, SLAC, SLPC, and SLPA were 1.050, 1.045, 0.999, and 0.949, respectively. Clearly, the SLPA had the lowest ratio, revealing its lowest degree of structural disorder and higher graphitization. The SLPC sample just had more of a pre-crosslinking process than the SLC, and the other conditions remained the same. However, from the ID/IG values, the graphitization degree of the SLPC was greatly improved, which indicated that the pre-crosslinking treatment can improve the order degree of precursors and had a positive effect on the carbonization process. The authors can also draw the same conclusion from the ID/IG values of the SLPA and SLAC. The increase of graphitization degree can make up the carbon defect and improve the conductivity (Barranco et al. 2010).
Fig. 3. Raman spectra of SLC, SLAC, SLPC, and SLPA
The XPS method was employed to investigate the elemental composition and chemical structure of the carbon samples. Table 1 lists the C and O contents of the four samples, respectively. Figure 4a shows that the XPS survey spectra contained two peaks of carbon, i.e. the (C1s peak) and (O1s peak). As shown in Figs. 4b through e, the peaks of C1s were found at 284.4, 285.6, 286.5, and 289.2 eV, and they were assigned to C=C/C-C, C-O, C=O, and O=C-O, respectively (Ai et al. 2015; Gharehkhani et al. 2015). Compared to the other three samples, the enhanced C-O peaks in the spectrum of the SLPA samples revealed that the sample SLPA had the highest oxygen content.
Fig. 4. Surface composition analysis of the samples: (a) XPS survey spectra of the samples; the high-resolution of C1s feature of (b) SLC; (c) SLAC; (d) SLPC; and (e) SLPA
Table 1. The Specific Surface Area and Pore Structure of the SLC, SLAC, SLPC, and SLPA
Oxygen atoms doped on the carbon network could enhance the wettability and electrochemical activity and provide extra pseudocapacitance contribution (Wei et al. 2016). The incorporation of oxygen atoms on the surface of electrode materials will lead to redox reaction, thus introducing pseudocapacitance and increasing the specific capacitance of electrode materials. This inference coincides with the results of the electrochemical characterization.
The pore textures of the carbon samples were tested by nitrogen adsorption-desorption isotherms measurements. As shown in Fig. 5, four curves showed the typical IV-type isotherms according to the IUPAC classification (Thommes et al. 2015), in which an obvious intense increase in adsorption amount under low relative pressure can be found because of the existence of micropores. Under a medium relative pressure (0.45 < p/p0 < 0.9), every sample showed a type H4 hysteresis loop that revealed carbon samples containing mesopores. At p/p0 > 0.9, the curve slowly increased and demonstrated the presence of macropores.
Fig. 5. (a) N2 adsorption/desorption isotherms of the SLC, SLAC, SLPC, and SLPA; (b) Pore size distributions of the SLC, SLAC, SLPC, and SLPA
The porosity parameters of the as-prepared carbon samples are listed in Table 2. The SLPA samples had the highest specific surface area (2780 m2/g) compared to SLC (716.5 m2/g), SLAC (987.3 m2/g), and SLPC (1190 m2/g).
Table 3 compares the specific surface area of lignin-based porous carbon under different pretreatment methods. The authors found that the porous carbon prepared by the pre-linking reaction had the largest specific surface area. In the pre-crosslinking process, the outer layer of the macromolecule was far away from the nucleus. Consequently, it was less bound and was prone to fracture in the hydrothermal process. While many nucleations can be produced in the process, the broken chains around the nucleus formed new molecules with larger surface, smaller volume, and micropore framework, thus increasing the specific surface area of the precursors. Furthermore, from the pore size distribution, SLPA contained developed micropores and mesoporous pores. Those pores can provide a large number of storage space and transmission channels for electrons (Niu et al. 2017), which was in favor of the electrochemical performance of the samples.
Table 2. Specific Surface Area and Pore Structure of the SLC, SLAC, SLPC, and SLPA
a SBET is the total specific surface area;
b Smic is the micropore specific surface area;
c Sext is the external surface area (Sext = SBET – Smic);
d Vtotal is the total pore volume;
e Dm is the average pore diameter.
Table 3. Comparison of the Specific Surface Area of the SLPA and Other Reported Lignin Based Porous Carbon
a* Zhang et al. (2019); b* Ho et al. (2018); c* Fu et al. (2019); d* Wang et al. (2019)
Electrochemical Evaluation
To evaluate the performance of the prepared carbon materials as electrode-active materials for supercapacitors, the capacitive performance of the samples were tested in a three-electrode system in 6 mol/L KOH electrolyte. As shown in Fig. 6a, at the scanning rate of 50 mV/s, all the CV curves had a rectangular shape, indicating a typical double-layer capacitive behavior. From Fig. 6a, two peaks are apparent at ca. -0.54 V and -0.32 V, which may be due to the redox reaction of oxygen atoms on the surface of the materials.The slight deviation from the ideal rectangular shape in the high potential can be observed in the CV curves of the SLPC and SLPA samples, which may be attributed to the presence of oxygen atoms that initiate the faradaic redox reactions and provide additional pseudocapacitance (Nian and Teng 2002; Chen et al. 2008; Ren et al. 2013). Furthermore, the surrounded area by the CV curve of SLPA was larger than that of SLC, SLAC, and SLPC, indicating the improvement of specific capacitance due to the dramatically increased surface area. The CV curves of the SLPA retained their quasi-rectangular shape as the scan rate increased from 10 mV/s to 200 mV/s (Fig. 6b), revealing good rate capability. In Figs. 6c and d, galvanostatic charge/discharge tests further evaluated the electrochemical performance of the samples. As shown in Fig. 6c, at the current density of 0.5 A/g, the calculated specific capacitance of the SLA, SLAC, SLPC, and SLPA samples was 75.7, 107, 245, and 305 F/g, respectively. The GCD curves of SLAC, SLPC, and SLPA samples had obvious platforms that appeared during the charging process, indicating the existence of pseudocapacitor behavior (Milczarek and Nowicki 2013; Suktha et al. 2015). Even at 0.5A/g low current density, the SLPA IR drop was not obvious, revealing that its conductivity was higher than other samples. The GCD curves of the sample SLPA at different current density presented in Fig. 6d displayed typical capacitor triangle shapes. Table 4 compares the electrochemical performance of lignin-based porous carbon under different preparation methods. From the comparison results, it can be found that the porous carbon had a high specific capacitance after pre-crosslinking treatment, indicating that the pre-crosslinking treatment had a positive effect on the electrochemical performance of porous carbon.