Abstract
Activated carbon fibers (ACFs) have been prepared from liquefied wood (Wliq) by chemical activation with KOH, with a particular focus on the effect of KOH/fiber ratio in term of porous texture and surface chemistry. ACFs based on steam activation served as a blank for comparison. The properties of the ACFs were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), nitrogen adsorption/desorption, Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The results show that the KOH-activated ACFs have rougher surfaces and more amorphous structure compared with the blank. The pore development was significant when the KOH/fiber ratio reached 3, and achieved a maximum Brunauer-Emmett-Teller (BET) surface area of 1371 m2 g-1 and total pore volume (Vtot) of 0.777 cm3 g-1, of which 45.3% belong to mesopores with diameters of 2–4 nm, while the blank activated at the same temperature had a BET surface of 1250 m2 g-1 and Vtot of 0.644 cm3 g-1, which are mainly micropores. The surface functional groups are closely associated with the KOH/fiber ratios. KOH-activated ACFs with KOH/fiber ratio of 3 have more oxygenated surface functional groups (C-O, C=O, -COOH) than the blank.
Introduction
Activated carbon fibers (ACFs) have unique characteristics compared with granular or powdered activated carbons (ACs) due to their well-developed pore structure and special surface reactivity. The application field comprises adsorption and separation, electronic materials, catalyst support, and the storage of natural gas (Suzuki 1994). ACFs are usually prepared from polyacrylonitrile, pitch or phenolic resin fibers. Wood is the most abundant terrestrial biomass which contains up to three-quarter polysaccharides and around one-quarter lignin (John and Anandjiwala 2008) and it has been utilized for the preparation of ACFs. Uraki et al. (2001) prepared ACFs from lignin, which was separated from wood by atmospheric acetic acid pulping. The same is true for bamboo (Li et al. 2014), jute (Phan et al. 2006) and cotton waste (Zheng et al. 2014), which are also good alternatives to ACF production based on polyacrylonitrile. Liquefied wood (Wliq) was also tested for ACF production (Liu and Zhao 2012; Zhang and Zhang 2013). This approach does not require the separation of chemical components of wood and leads to higher wood utilization ratios (Pu and Shiraishi 1993; Alma and Basturk 2006). ACFs prepared from Wliq by steam activation can reach specific surface areas (SSAs) around 2000 m2 g-1. Such materials have very high C contents and thus contain surface functionalities with low oxygen contents. Moreover, this approach is more economic than the other methods for ACF production. By contrast, the yields of ACFs from Wliq via steam activation are relatively low and the microporous material has a very narrow pore size distribution (PSD).
It was also demonstrated that chemical activation with KOH can widen the PSDs (Ruiz-Fernández et al. 2011) and introduce oxygen-containing surface groups (Babel and Jurewicz 2004). To date, one of the most extended utilizations of activated carbon materials is electric double-layer capacitors (EDLCs) (Castro-Muñiz et al. 2011). ACs from wood waste (Dobele et al. 2013) as well as ACFs from hardwood acetic acid lignin (You et al. 2015) were shown to have high potential for EDLCs. From the point of view of this application not only the wide PSDs and more mesopores with high SSA are required (Ma et al. 2014a), but the surface chemistry, especially the O functionalities, also has a significant effect on the capacity of EDLCs on account of their enhancement in pseudocapacitance (Oda et al. 2006). The expectation is that chemical activation with KOH of Wliq-based fibers could deliver ACFs with added mesoporosity and surface chemical properties that may be advantageous for application in EDLCs.
The aim of this work was to control the porosity and surface chemical properties of Wliq based ACFs. To this end, ACFs should be prepared by KOH activation and the KOH/fiber ratio should be investigated in terms of the ACFs’ porous structure and surface chemistry. The results will be compared with those obtained from ACFs based on steam activation (blank). The surface morphology of the prepared ACFs will be observed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The pore structure will be evaluated by N2 adsorption/desorption. Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) should contribute to the characterization of the materials.
Materials and methods
Preparation of ACFs
Chinese fir (Cunninghamia lanceolata) as a raw material sourced from Fujian, China, was firstly ground and screened to particle size of 60–80 meshes for use in the experiments. All the chemicals in the study were purchased from Beijing Chemical Works, Beijing, China, which were reagent grade and used without further purification. The mixture of wood flour, phenol and H3PO4 at a mass ratio of 1:5:0.4 was heated in an oil bath at 160°C for 2.5 h under continuous stirring to obtain Wliq. Then, 5% (based on the weight of liquefied solution) hexamethylenetetramine as synthetic agent was added to the Wliq to prepare the spinning solution by heating the mixture from room temperature (r.t.) up to 170°C in 40 min. The spun filaments were prepared by melt-spinning at 120°C with a laboratory spinning apparatus. Then, the resultant fibers were cured by soaking in a solution with HCHO and HCl (1:1 v/v) as the main components at 90°C for 2 h and then washed with distilled water and finally dried at 80°C for 4 h to obtain the precursor fibers. An amount of 20 g of the resultant precursors were then carbonized in a horizontal transparent tube furnace (Y02PB, Thermcraftinc, USA) at 500°C in N2 flow for 1 h carbonization. The carbonization yield was calculated by the weight ratio of the resultant carbon fibers (CFs) to precursor fibers.
About 6 g of CFs were put in the tube furnace after impregnation with KOH at KOH/fiber ratios of 1, 2 and 3 (w/w). About 2 g of each CF was carbonized by heating the mixtures under N2 flow (200 cm3 min-1) at a heating rate of 4°C min-1. The activation temperature was 850°C and the holding time was 1 h. The obtained ACFs were washed with distilled water until neutral pH and dried at 103±2°C. The KOH-activated ACFs are labeled as ACF-1, ACF-2, and ACF-3, where the numbers represent the impregnation ratios KOH/fiber as indicated above. The activation yield was calculated by the weight ratio of ACFs to CFs, thereby the total yield was obtained by multiplying activation yield by carbonization yield.
For comparison, steam activation was also conducted, where 2 g of CFs were similarly heated from r.t. to 850°C and were held at this temperature for 1 h by introducing a steam flow and then cooled down to r.t. These products are labeled as ACF-S or simply as the blank.
Characterization of ACFs
Surface morphology of the ACFs was examined with a scanning electron microscopy (S-3400N, Hitach Company, Japan). Before observation, the samples were metalized with a thin layer of Pt.
XRD analysis
XRD analysis (SHIMADZU, XRD-6000) was performed with CuKα radiation (0.154 nm) at 40 kV and 30 mA. The scanning rate was 2° min-1 with a scanning step of 0.02° from 5° to 60° (2θ).
Nitrogen adsorption measurement
The textural parameters of the ACFs were determined from N2 adsorption-desorption isotherm at -196°C (Autosorb-iQ, Quantachrome). Before analysis, the samples were degassed at 300°C for 3 h. The SSA (SBET) was calculated by the Brunauer-Emmett-Teller (BET) method based on N2 adsorption isotherm data (Brunauer et al. 1938). The total pore volume (Vtot) was evaluated by converting the amount of N2 adsorbed at a relative pressure of 0.995 to the volume of liquid adsorbate. The micropore area (Smicro) and micropore volume (Vmicro) were obtained the by t-plot method (de Boer et al. 1966). PSDs were calculated based on the density functional theory (Lastoskie et al. 1993), which relies on calculated adsorption isotherms for pores of different sizes.
FTIR analysis
The KBr pellet method containing 5% of samples was applied. The samples were pulverized (100 mesh) and mixed with KBr before pellet preparation. Instrument: FTIR, BRUKER Tensor 27, Germany.
XPS analysis
XPS analysis was performed by using a spectrophotometer (Escalab 250Xi, Thermo Scientific, USA) with a monochromated AlKα X-ray source (hν=1486.6 eV); 10 mA, 13 kV. The survey scans were collected from the binding energy ranging from 0 to 1350 eV. A nonlinear least squares curve-fitting program (XPSPEAK software, Version 4.1) was employed for XPS spectral deconvolution.
Results and discussion
Surface morphological observation
SEM micrographs of the ACFs are presented in Figure 1. A very smooth surface is clearly identifiable for the blank (activated by steam, Figure 1a). The morphological features of the KOH-activated ACFs are different (Figure 1b–d). Their surfaces are rugged due to the ablative effect of KOH. As the KOH/fiber ratio increases, highly cracked and collapsed surfaces are obtained, indicating the violent action between KOH and surface carbon with increasing KOH amounts. This action leads to the creation of pores and high surface areas. The even and clear distribution of the folds demonstrates that KOH was uniformly deposited in the material.
Scanning electron microscopy (SEM) photographs of surface of KOH-activated activated carbon fibers (ACFs) and steam-activated ACF-S: (a) ACFs-S; (b) ACFs-1; (c) ACFs-2; (d) ACFs-3. The numerals 1–3 refer to the KOH/fiber ratios.
The cross section of ACFs appears to be oval rather than circular (Figure 2). The ACF-1 has a very flat section (in the inset of Figure 2b), however, when the KOH/fiber ratio exceeds 1, holes appear at the core of the ACFs owing to the skin-core structure of the ACFs (Ma et al. 2014b). The edge structure is tight while the core structure is relatively loose and holes are formed at places of interaction between KOH and carbon. The holes become larger gradually with increasing mass ratios (in the inset of Figure 2c–f), which could be related to the more intense KOH penetration into the interior of the fibers, where the structures are then destroyed more severely.
Scanning electron microscopy (SEM) photographs of cross section of KOH-activated carbon fibers (ACFs); (a) ACF-1; (b) magnified ACF-1; (c) ACF-2; (d) magnified ACF-2; (e) ACF-3; (f) magnified ACF-3. The numerals 1–3 refer to the KOH/fiber ratios.
XRD analysis
Figure 3 shows the XRD patterns of the ACFs as a function of KOH/fiber ratio in comparison with the steam-activated blank. The latter displays a peak at approximately 24° and another at approximately 43°, which were assigned to the disordered graphitic 002 plane and 10 plane (overlapped 100 and 101), respectively (Ryu et al. 2002), suggesting that the ACFs prepared from Wliq were made of graphite-like microcrystallites (Wang et al. 2011). Compared with the blank, the peaks at 24° shift to a low angle in the case of KOH-activated ACFs, which means that their amorphous structures are strengthened. In other words, their graphitic microcrystallites are destroyed more severely, which may lead to widening of the pores because the pore walls are consisting of graphitic microcrystallites.
X-ray diffraction patterns of KOH-activated carbon fibers (ACFs) (1–3) and steam-activated ACF-S. The numerals 1–3 refer to the KOH/fiber ratios.
All of the KOH-activated ACFs show peaks at about 30.7° (corresponding to the standard JCPDS card of K2CO3, the number is 71-1466) due to the residual K2CO3 particles in the fibers, which are reaction products of KOH and carbon (Lillo-Ródenas et al. 2003). As the mass ratio increases, the peaks related to 002 plane move to lower angles and become broader, while the peaks attributed to K2CO3 are better resolved. This is because more intense reactions between KOH and carbon bring about more severely destructed crystal structures; this interpretation is consistent with the SEM observations. By contrast, high KOH/fiber ratio means that the presence of more KOH causes the production of more residual K2CO3, which is left after washing, and this K2CO3 is more perceptible on the spectrum.
N2 adsorption/desorption isotherm
Figure 4 shows the adsorption/desorption isotherms of N2 for the ACFs. In the blank, the volume adsorbed increases quickly with increasing pressure at low pressure ranges but levels off when the relative pressure is above 0.1. This means that the isotherms belong to type I according to the IUPAC classification (Qian et al. 2007), where microporous adsorption is dominating. As for the KOH-activated ACFs, however, the volume adsorbed maintains an upward trend when the relative pressure exceeds 0.1, especially for the ACF-1 and ACF-2. It is worth noting that adsorption/desorption hysteresis is visible in all KOH-activated ACFs, which is not the case for the blank. This observation is probably related to differences in mesopores between KOH-activated ACFs and the blank.
N2 adsorption/desorption isotherms of KOH-activated carbon fibers (ACFs) (1–3) and steam-activated ACF-S. The numerals 1–3 refer to the KOH/fiber ratios.
Pore properties of all of the ACFs including SBET, Smic, Smeso, Vtot, and yield are listed in Table 1. SBET and Vtot of the ACFs gradually increase with increasing KOH/fiber ratio while the yield decreases correspondingly. However, the Smeso of ACF-2 reaches the maximum of 525 m2 g-1 and then reduces with increasing KOH/fiber ratios, while the Smic keeps its increasing trend, most likely because of the limited availability of KOH when KOH is attached on the surface. Therefore, the activation took place only on the surface, and metallic K was generated in low amounts. As metallic K can intercalate into the carbon lattices and contributes to porosity development, the porosity is lower in the case of low KOH availability (Raymundo-Piñero et al. 2002). With the prolongation of the activation, micropores were gradually enlarged to mesopores (Dai et al. 2006). When the ratio KOH/fibers exceeded 2, there was enough KOH available for fiber penetration. The etching of the carbon framework was conducted from the outer to inner part. The intercalated metallic K could further contribute to the development of micropores. SBET, Smic and Smeso of steam-activated ACFs are 1240 m2 g-1, 969 m2 g-1 and 281 m2 g-1, respectively, which are slightly less than those of ACF-3. However, the ACF-3 has a higher yield and the ratio of mesopore volume to Vtot is 45.3%, while that of the blank is only 9.3%. This shows that mainly microporous structures arise during steam activation, while in the case of KOH activation the mesopores are dominant.
Pore properties of the KOH-activated activated carbon fibers (ACFs) (1–3) and steam activated ACF-S.
| Sample | SBET (m2 g-1) | Smicro (m2 g-1) | Smeso (m2 g-1) | Vtot (cm3 g-1) | Vmeso (cm3 g-1) | Vmeso/Vtot (%) | Yield (%) |
|---|---|---|---|---|---|---|---|
| ACF-1 | 536 | 35 | 501 | 0.504 | 0.471 | 93.5 | 66.8 |
| ACF-2 | 700 | 175 | 525 | 0.611 | 0.516 | 84.5 | 58.2 |
| ACF-3 | 1371 | 1024 | 347 | 0.777 | 0.352 | 45.3 | 35.6 |
| ACF-S | 1250 | 969 | 281 | 0.644 | 0.060 | 9.3 | 29.7 |
The numerals 1–3 refer to the KOH/fiber ratios.
The PSDs based on the density functional theory method are presented in Figure 5. The PSDs in the mesopore region are very similar in the range around 2–5 nm, while a clear difference can be observed in micropore region, where the micropores mainly accumulate in a narrow range below 1 nm for ACFs with low KOH/fiber ratios. In the case of AFCs-3, the distribution range is widened to 1–2 nm, where two distinct peaks at 1.25 nm and 1.45 nm are visible. Accordingly, increasing KOH/fiber ratios lead to more micropores and enlarge the pore width. Although micropores of the blank at 0.65 nm are similar to that of the KOH-activated ACFs, the PSDs in the mesopore region can hardly be recognized in the blank.
Pore size distributions (PSDs) of KOH-activated carbon fibers (ACFs) (1–3) and steam-activated ACF-S. The numerals 1–3 refer to the KOH/fiber ratios.
FTIR analysis
As seen from Figure 6, oxygen-containing functional groups such as C=O (1736 cm-1) (Ma and Ouyang 2013) and C-O (1161 cm-1, 1116 cm-1 and 1044 cm-1) (Benadjemia et al. 2011) can be identified, indicating the various oxygen containing functional groups in KOH-activated ACFs. In the blank, no pronounced bands are visible between 1300 cm-1 and 1000 cm-1, demonstrating the elimination of surface functional groups. Furthermore, there are few bands at 900–650 cm-1, indicating the formation of multi-benzene fused ring structures. In other words, the blank has more graphitic structure, which is consistent with the results of XRD analysis.
Fourier transform infrared (FTIR) spectra of KOH-activated carbon fibers (ACFs) (1–3) and steam-activated ACF-S. The numerals 1–3 refer to the KOH/fiber ratios.
XPS analysis
It is apparent from XPS spectra (Figure 7) that the C signal is larger than that of O. The element K is faintly visible in the spectra of KOH-activated samples. Atomic concentrations on the surface of the ACFs are summed up in Table 2. With increasing KOH/fiber ratios, carbon content increases from 85.2% to 88.6%, while the oxygen content decreases from 13.8% to 9.8%. The decreasing intensity of oxygen-containing functional groups was already seen on the FTIR spectra as a function of increasing KOH/fiber ratios. However, O/C atomic ratio of the steam-activated blank is only 9.4%, reflecting the fact that KOH-activated ACFs have probably more oxygen-containing functional groups. This result is similar to that obtained by Babel and Jurewicz (2004) who observed viscose fibers as precursors to prepare ACFs by KOH and steam activation. The authors interpreted the higher O contents of KOH activated ACFs by oxidation of C in an alkaline environment. Conversely, the present work revealed that O suffered a great loss in abundance of available KOH. The C1s region of the XPS spectra of all samples was evaluated (Table 3). As visible, the C1s spectra of all samples are very similar, therefore only the ACF-3 sample is presented in Figure 8 as an example. The C1s region exhibits an asymmetric tailing, which is partially due to the intrinsic asymmetry of the graphite peak or to the contribution of oxygen surface complexes. The C1s peak of ACF-3 can be subdivided into four components corresponding to the graphite (284.7 eV), C-O (285.6 eV), C=O (286.5 eV) and -COOH (288.3 eV) (Pavlidis et al. 2012), with the relative percentages of 58.6%, 16.7%, 12.2%, and 12.5%, respectively, compared to 68.1%, 15.0%, 7.7%, and 9.2% for the steam-activated ACFs, confirming the less oxygen-containing functional groups in the latter. The graphite content of the ACFs gradually decreases from 66.4% to 58.6% with increase of the KOH/fiber ratio, indicating the preferential reaction of KOH with carbon. Probably, the fiber matrix structure is disrupted due to the partial degradation of graphene layers by the interaction between KOH and carbon. Our interpretation is that KOH as an activating agent generated more unstable carbons in the AFC matrix leading to mesopore formation.
X-ray photoelectron spectroscopy (XPS) spectra of KOH-activated carbon fibers (ACFs) (1–3) and steam-activated ACF-S. The numerals 1–3 refer to the KOH/fiber ratios.
Surface composition of KOH activated carbon fibers (ACFs) (1–3) and that of steam activated ACF-S as determined by X-ray photoelectron spectroscopy (XPS).
| Sample | C (at.%) | O (at.%) | K (at.%) | O/C (%) |
|---|---|---|---|---|
| ACF-1 | 85.2 | 13.8 | 1.0 | 16.3 |
| ACF-2 | 86.1 | 12.4 | 1.5 | 14.4 |
| ACF-3 | 88.6 | 9.8 | 1.6 | 11.1 |
| ACF-S | 91.4 | 8.6 | – | 9.4 |
The numerals 1–3 refer to the KOH/fiber ratios.
Results of the fits of the C1s region. BE: binding energy; M: mole percentage.
| Sample | Graphite | C-OH | C=O | -COOH | ||||
|---|---|---|---|---|---|---|---|---|
| BE (eV) | M (%) | BE (eV) | M (%) | BE (eV) | M (%) | BE (eV) | M (%) | |
| ACF-1 | 284.7 | 66.4 | 285.6 | 15.1 | 286.5 | 10.4 | 288.3 | 8.1 |
| ACF-2 | 284.7 | 58.8 | 285.7 | 20.2 | 286.6 | 11.7 | 288.4 | 9.3 |
| ACF-3 | 284.7 | 58.6 | 285.6 | 16.7 | 286.5 | 12.2 | 288.3 | 12.5 |
| ACF-S | 284.7 | 68.1 | 285.5 | 15.0 | 286.6 | 7.7 | 288.4 | 9.2 |
The numerals 1–3 in the context of activated carbon fibers (ACFs) refer to the KOH/fiber ratios.
X-ray photoelectron spectroscopy (XPS) spectra of C1s region of the activated carbon fiber (ACF-3). The numeral 3 refers to the KOH/fiber ratios.
Conclusions
ACFs were prepared from Wliq by KOH activation. In the case of KOH/fiber ratio 3, the BET surface area of ACFs could reach values up to 1371 m2 g-1. Compared with the ACFs activated by steam at same temperature, the samples ACF-3 have more small micropores (<0.7 nm) in addition to more mesopores in the range of 2–4 nm. The KOH-activated ACFs had more oxygenated functional groups on their surfaces than the steam-activated ACF-S, which would play an important role when the ACFs should be used in EDLCs.
Acknowledgments
The research was financially supported by Specialized Research Fund for the Doctoral Program of Higher Education (No. 20130014130001).
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