Mariko
Ago
*a,
Maryam
Borghei
a,
Johannes S.
Haataja
b and
Orlando J.
Rojas
*ab
aBio-Based Colloids and Materials and Centre of Excellence on “Molecular Engineering of Biosynthetic Hybrid Materials Research” (HYBER), Department of Forest Products Technology, Aalto, University, FIN-00076, Espoo, Finland. E-mail: mariko.ago@aalto.fi; orlando.rojas@aalto.fi
bMolecular Materials, Department of Applied Physics, Aalto University, FIN-00076, Espoo, Finland
First published on 30th August 2016
Flexible electrodes with supercapacitance were developed from highly mesoporous carbon fibers synthesized from lignin. Polyvinyl alcohol (PVA) facilitated the electrospinning of aqueous solutions of lignin and was used as a sacrificial polymer. Most importantly, PVA produced phase-separated domains for extreme surface area (>2000 m2 g−1) and mesoporous volume (0.7 cm3 g−1). An optimized sequential thermal treatment that initially included stabilization at 250 °C, allowed the formation of flexible, freestanding carbon networks upon PVA evolution to the gas phase and carbonization of the as-spun lignin-based fibers. Their main morphological and chemical characteristics were assessed by field emission scanning microscopy, transmission electron tomography reconstructions and Raman spectroscopy. The carbon fiber networks were used directly as electrodes with electrochemical double layer capacitance as determined by cyclic voltammetry and galvanostatic charge/discharge methods. Excellent electrochemical performance was demonstrated from the measured high rate capability and long-term cycling stability. The determined specific capacitance (∼205 F g−1 in 0.5 M Na2SO4 electrolyte) is one of the highest recorded for electrodes obtained from biopolymer precursors. Moreover, the electrical conductivity of the carbon fiber network (386 S m−1) was significantly higher, by two-orders of magnitude, than that obtained from the precursor (non-fibrous, powder) sample (2.47 S m−1). The remarkable performance of the synthesized electrodes is ascribed to the robust network morphology and mesoporosity obtained by soft-templating from the phase-separated sacrificial polymer. This is a demonstration of lignin valorization for novel application in advanced materials.
Carbons for double-layer type supercapacitors should ideally have three main properties, namely, (1) high specific surface areas, of the order of 1000 m2 g−1, (2) good intra- and inter-particle conductivity in porous matrices, and (3) good electrolyte accessibility to the intrapores.16 Several methods have been developed to obtain large specific surface areas as well as suitable porosity in carbon materials, including activation in alkaline or acidic media,17–19 steam20,21 or carbon dioxide treatments,22 hard-templating via filling of inorganic particles within an organic precursor, organic–organic assembly via soft-templating,23,24 among others. Particularly, soft-templating methods are suitable for developing mesoporosity in carbon materials towards conductive electrodes. This is mainly because of the small amounts of impurities left behind after carbonization.
Prior studies have reported successfully on the fabrication of ordered mesoporous carbons by using soft-templating synthesis with thermosetting network polymers. This has included phenolic-type resins as a carbon source and block copolymers; resorcinol–formaldehyde and diblock copolymers (PS-b-P4VP);25 resorcinol–formaldehyde and Pluronic surfactants26,27 and PAN-b-PMMA.28 In these studies, the pore size developed from the precursor structure was well controlled, at the molecular scale.
However, some drawbacks exist in these systems, such as the complex procedure used for polymer synthesis, time-consuming protocols and relatively high cost. Furthermore, additional limitations include the choice of both carbon and porogens precursor materials, to avoid the collapse of the porous structure during high-temperature treatment. In fact, only few materials meet associated requirements.23 In this context, lignin has attracted attention as a precursor of carbon materials owing to its high carbon content (more than 60% in phenyl propane groups), as well as its thermal stability, and favorable stiffness.29,30 Thus, we propose lignin as a widely available (exceeding 300 billion tons31 as a byproduct of the pulp and paper industries and biorefineries),32 renewable and inexpensive aromatic polymer for the generation of carbon electrodes and as an attractive option to replace synthetic thermosetting polymers as source of mesoporous carbon. Soft-templating, on the other hand, can be achieved by addition of a sacrificial polymer, polyvinyl alcohol (PVA).
The few, recent studies related to supercapacitors by using activated carbon from lignin are listed in Table 1. A capacitance as high as 268 F g−1 (ref. 33) was recorded in the presence of a conductive additive in the working electrodes.
Carbon source (reference) | Electrode and composition | Conductive additive | Electrolyte | Specific capacitance, F g−1 |
---|---|---|---|---|
a PVDF = polyvinylidene fluoride; PTFE = polytetrafluoroethylene; NEt4BF4 = tetramethyl ammonium tetrafluoroborate; CB = carbon black. “Carbon” in the “Electrode” column refers to the material obtained from lignin. | ||||
Kraft lignin13 | Carbon:comercial supercarbon:PVDF, 75:25:10 | Commercial high-surface-area supercarbon | 6 M KOH | 102.3 |
Black liquor33 | Carbon:carbon black:PTFE, 89:5:6 | Commercial functionalized CB | 6 M KOH/LiOH (50/1, w/w) | 267.8 |
Alkali lignin34 | Carbon fibrous mat | None | 6 M KOH | 64 |
Black liquor35 | Carbon:PTFE, 95:5 | None | 1.5 M NEt4BF4 | 87 |
Kraft lignin, alkali extracted lignin36 | Carbon:conductive carbon black:PVDF, 95:5:5 | Commercial conductive carbon black | 1 M H2SO4 | 114.4 |
Alkali lignin (present work) | Carbon fibrous mat | None | 0.5 M Na2SO4 | 205 |
In this study we developed a highly mesoporous nano- and micro-carbon fiber network as conductive electrode for supercapacitance by using lignin as a carbon precursor. The distinct advantage of the proposed approach is the synthesis of a mesoporous carbon network that can be utilized directly as an electrode with no need of additives and even in the absence of binders.
Recently, we demonstrated that the sub-micron fibers can be synthesized easily by electrospinning of aqueous solution of lignin. We also concluded that the physical properties (fiber diameter, morphology, etc.) can be tuned by the addition of polyvinyl alcohol (PVA) as a minor component.37,38 Moreover, our previous studies indicated microphase separation between lignin and PVA, which spontaneously segregated during the electrospinning process.38 For the system containing 75 wt% lignin (PVA, 25 wt%), discontinuous lignin/PVA phases formed within a lignin-rich, external phase. Based on these results, we hypothesize that highly mesoporous carbon fiber networks are feasible upon carbonization and by templating the two-phase morphology, from microphase separation of lignin and PVA. Here, the main lignin domain would act as carbon scaffold, while the mesopore domain would result from the PVA phases acting as a soft template. Fig. 1 illustrates the proposed component distribution in lignin/PVA electrospun fibers.
Free-standing fibrous carbon mats can be useful as performance conductive electrode in supercapacitors. A continuous fibrous morphology would also enable mechanical robustness, flexibility and stability. Furthermore, flexible electrodes, which may operate flawlessly even upon twisting, are desirable in the design of light and flexible devices, which are demanded in applications such as flexible circuits,39 displays,40 solar cells,41,42 and pressure sensors.43,44
The electrospinning technique has been widely adopted for fabricating fibrous mats from various polymer solutions.45–47 Such process balances electrostatic repulsive forces in the polymer solution and an externally-applied electric filed. Additionally, it can be scaled up to ensure low production costs. Here, we propose that high surface area electrospun mats obtained from lignin/PVA solutions, which are flexible and mechanically robust, are suitable, directly after carbonization, as freestanding electrodes. Therefore, we produced electrospun mats from lignin/PVA (75:25) as carbon precursor to fabricate highly mesoporous carbon networks using soft-templating. The morphology and porous structure of the resultant carbon network were investigated. In addition, electrochemical measurements for supercapacitance was performed to afford new storage devices from such mesoporous system. Overall, this contribution attempts to demonstrate the use of a renewable biopolymer to synthesize fibrous networks for free-standing and flexible carbon mats that can be used as supercapacitor electrodes.
Sample | Isothermal treatment 250 °C, h | Heating rate, °C min | Yield, % | Morphology |
---|---|---|---|---|
Mat 0 | 0 | 5 | 0 | N/A |
Mat 1 | 1 | 4 | 11 | Flexible mat |
Mat 2 | 2 | 4 | 10 | Flexible mat |
Mat 3 | 2 | 10 | 13 | Flexible mat |
Mat 4 | 2 | 20 | 0 | N/A |
Powder | 2 | 10 | 26 | Powder |
Solid film | 2 | 10 | 45 | Brittle film |
The morphology of the as-spun fibers, before and after carbonization, is shown in Fig. 2. Fibers with a radius of 148 ± 8 nm were observed as a randomly oriented network. Fig. 2c and d show the morphology for Mat 3 after carbonization followed by isothermal treatment and chemical activation. Some fibers adopted a higher curvature and fused into the network structure possibly due to the melting of PVA distributed in the fibers during the isothermal treatment (250 °C). The diameter of the fibers remained unchanged after carbonization, as concluded from repeated experiments under the same conditions. A micrograph obtained with high magnification revealed numerous small pores on the surface of the fibers, which could be accessible to the electrolyte. The carbon mats could be bent easily (Fig. 2e) and recovered the original flat shape after the stress was release, indicating that the mats were mechanically strong and the fibers formed an entangled and interconnected network. As a reference, lignin powder was subjected to carbonization by using the same conditions as the ones used for Mat 3 (herein indicated as “Powder” sample). A lignin/PVA solid film prepared by solvent casting was also prepared for comparison (“Solid film”). Note that the lignin/PVA solid film is expected to be closer to thermodynamic equilibrium because of a “slow” consolidation upon solvent evaporation.38 In this slow process, the interfacial tension between lignin and PVA drives phase segregation to yield PVA domain sizes as large as 500 nm.38 The effect of the characteristic time of treatment on the porous structure of the derived carbon material (from electrospun fiber mats or solid films) is discussed later. Both the powder sample (powder) and the solid film (Solid film) presented a higher yield (26% and 45%, respectively) compared to the fibrous mats (Table 2).
Nitrogen adsorption measurements (BET) were carried out in order to examine the specific surface area, pore volume, and pore size distribution of the carbon fibrous mats, powder and solid films (Table 3). The NLDFT model was used to analyze the BET isotherm.52 The pore size distributions for the powder and the mat samples are shown in Fig. 3. For the fibrous mats, highly specific surface areas were recorded (1689, 1387, and 2005 m2 g−1 for Mat 1, 2 and 3, respectively). Also, a high pore volume was determined (0.60, 0.71, and 0.70 cm3 g−1) with a dominant mesoporosity (58, 77, and 66%, respectively). The isotherms for the mat samples and the solid film presented a typical type-IV isotherm with a hysteresis loop in the P/P0 = 0.45–0.6 relative pressure region, which is attributed to mesoporous structure (Fig. S1a†). The isotherm displayed a rapid adsorption rise in the low-pressure region, indicating the presence of micropores. In contrast, the powder sample exhibited a BET specific surface area of 1236 m2 g−1 with total pore volume of 0.25 cm3 g−1 along with a half of mesoporous volume (51% mesopore). Besides, the isotherm of the powder sample showed type-I isotherm. These results suggest that the powder sample possessed a highly microspore structure (Fig. S1b†). The fibrous mats and the powder sample exhibited two prominent peaks in the pore size distribution, in the micropore (<2 nm) and mesopore (2–50 nm) regions. An interesting observation is that compared to the powder sample, the fibrous carbon mats presented a broader distribution in the mesopore region, centered around 2.5 nm (Mat 1 and Mat 3) or 2.9 nm (Mat 2). The BET surface area and pore size distribution confirmed the presence of highly mesoporous structures with high specific surface area. The pore size distribution can be explained in terms of microphase separation of the precursor electrospun fibers. In the 75:25 lignin/PVA system, the two-phase morphology arises from discontinuous PVA domains separated from the lignin, main phase.38 As the electrospun fibers were subjected to carbonization, the lignin scaffold was carbonized while the PVA phase evolved into gas, leaving behind a porous network. Further insights into the effect of the PVA domain size on the mesopore distribution can be gained from the solid films prepared by the evaporation-casting method. In this process, the characteristic time for densification is much longer than in electrospinning, which leads to conditions that are closer to equilibrium. In turn, this induces large phase-separated domains, with sizes of the order of 500 nm. BET surface and pore volume determinations for the solid films revealed significantly reduced values compared to the mat samples. Moreover, the isotherm exhibited type-IV profile characteristic of mesoporous structures (Fig. S1c†).
Sample | V total, cm3 g−1 | V micro, cm3 g−1 | V meso, cm3 g−1 | Mesopore ratio, % | BET SSA, m2 g−1 |
---|---|---|---|---|---|
Mat 1 | 0.602 | 0.256 | 0.346 | 58 | 1689 |
Mat 2 | 0.714 | 0.163 | 0.551 | 77 | 1387 |
Mat 3 | 0.703 | 0.242 | 0.461 | 66 | 2005 |
Powder | 0.252 | 0.121 | 0.131 | 51 | 1236 |
Solid film | 0.031 | 0.068 | 0.024 | 77 | 217 |
The results suggest a lower pore volume for the films compared to the electrospun fibers, owing to the larger PVA domains in the former structure. The PVA domains were effective in introducing the mesoporous structure in the solid film, depending on the domain size. The mesoporous structure of the carbon fibrous mat is evident in TEM micrographs and transmission electron tomography reconstructions (Fig. 4), which indicate the presence of numerous randomly distributed mesopores (the lighter areas in Fig. 4b) with diameter of ∼5 nm. They can be ascribed to the PVA domains that soft-templated as pores that, in turn, would allow electrolyte access from the outer surfaces (Fig. 4c). The results of SEM and TEM analyses are consistent with the results of nitrogen adsorption. It was later noticed that the capacitance strongly depended on the specific surface area as well as the pore size distribution of the electrode materials. Since higher accessibility of the electrolyte can contribute on higher charge accumulation at the electrode-electrolyte interface.53
Raman spectroscopy was applied to estimate the degree of graphitization of the carbonized samples (carbon fibrous mat as well as powder sample) (see Fig. S2†). Two Raman bands in the region between 1100 and 1800 cm−1 were distinctively observed for both the mat and powder samples. A strong band at 1590 cm−1, the G-band, and another band at 1350 cm−1, the D-band, were apparent. A similar peak position of the D and G peaks for the different samples was noted. The presence of graphitic bonding due to the periodic sp2 valance in crystalline carbonaceous material would strengthen the intensity of the G-band, while the D-band would reflect the presence of disorder and defective carbonaceous constituents. Therefore, the relative D-to-G band intensity ratio, ID/IG, can be related to the perfection of the graphitic layered structure, reflecting the graphitization degree of the material.54 The ID/IG ratios were similar for the samples tested, around 1.25–1.26, which indicates that the carbonization condition and the morphology of the samples (fibrous mat or powder) did not influence the graphitization.
A high electrical conductivity is of great significance in the development of electrodes for supercapacitors. The electrospun carbon mats derived from the lignin/PVA bi-component fibers were not completely graphitized; however, the electrical conductivity of the mat samples (386 S m−1) was significantly larger, by two orders of magnitude, than the one measured for the powder sample (2.47 S m−1). This can be explained by the morphology of the fiber network that provides a continuous pathway for effective electron transport and for a reduced electrochemical resistance in the electrode. In contrast, the powder samples contain micron/nano-sized carbon grains that may not have enough entanglement to yield a proper electron conductivity.
Fig. 5 compares the CV curves from Mat 3 (a) and powder (b) at scan rates of 5 to 50 mV s−1. It can be seen that all of the CV profiles exhibited a nearly symmetrical rectangular shape at low scan rates. This corresponds to an ideal reversible capacitive behavior with good performance characteristics, fast ion diffusion and electron transfer rates for EDLC. Nevertheless, a distortion of the CV curves was observed at high scan rates, likely due to limitations in electrolyte transport and diffusion. Although this phenomenon can be seen quite often in carbon-based electrode materials, further studies are needed to overcome this effect. It is noticeable that the current density for Mat 3 was higher than that of the powder sample.
The capacitance values at a scan rate of 5 mV s−1 was calculated; under this condition a prominent rectangular shape in the CV plots, with maximum plateau flatness for both mat and powder samples, was observed. As expected, Mat 3 produced a higher specific capacitance, 205 F g−1, than that of the powder, 93 F g−1. The other samples, Mat 1 and Mat 2, exhibited capacitance values of 204 F g−1 and 155 F g−1, respectively. The remarkably high supercapacitance of the mat samples can be attributed to the high surface area and well-developed mesoporous structure, with open pores accessible to ion/electrolyte transport. Thus, the freestanding carbon network structure in the fibrous mats provides effectively more active sites due to high specific surface area and mesoporosity, as well as highly electrical conductivity. The featured high capacitance and the flexibility of the free-standing fibrous carbon mats offer promising advantages for use in electrochemical supercapacitor devices. At the same time, they involve facile and fast synthesis. As comparison, we further studied the Solid film that was prepared by solvent casting and noted a quite low capacitance (6.4 F g−1). Fig. 5c and d show typical charge–discharge profiles of Mat 3 and powder samples in 0.5 M Na2SO4 solution at current densities from 0.3 A g−1 to 2 A g−1. The charge–discharge curves were similar in shape between 0 and 0.6 V, indicating that the double layer capacitance can form reversibility in a wide range of current densities, for both mat and powder samples. Mat 3 electrode, with a large BET specific surface area, exhibited a longer discharging interval, which suggests a higher electrical capacity. The results were in accordance with the CV profiles. However, voltage drops at the turning point of charge–discharge can be seen, especially in the measurement with higher current density, i.e., 2 A g−1, for both mat and powder samples. This indicates energy losses due to the internal resistance by diffusion-limited mobility of the electrolyte ions in the electrode pores.
The cycling stability of the Mat 3 and powder samples was also examined by CV measurements over 1500 cycles, at a scan rate of 10 mV s−1, and the supercapacitance retention was monitored as a function of cycling number (Fig. 6). Mat 3 kept ∼83% of the initial supercapacitance after 1500 cycles, indicating good electrochemical stability as a supercapacitor electrode material. The fibrous carbon network in mat sample can work as a support matrix, which provide structural robustness and stability. For the powder sample, a steep decline in supercapacitance was observed, which may be due to the disintegration of active material and mechanical detachment from the glassy carbon electrode.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17536h |
This journal is © The Royal Society of Chemistry 2016 |