Byeong Ho Min and
Kyeong Youl Jung*
Department of Chemical Engineering, Kongju National University, 1223-4 Cheonan-Daero, Seobuk-gu, Cheonan, Chungnam 331-717, Republic of Korea. E-mail: kyjung@kongju.ac.kr; Fax: +82-41-554-2640; Tel: +82-41-521-9365
First published on 18th April 2017
Porous carbon spheres were synthesized by spray pyrolysis from an aqueous solution containing sucrose and sodium bicarbonate. Tetraethyl orthosilicate (TEOS) was tested as a porogen agent, and the microstructure and the porosity of carbon particles were investigated with changing TEOS content. According to the TEM and element mapping analysis, Si elements are uniformly distributed throughout the carbon matrix, and most of them are easily removed by ultrasonic washing with purified water. As a result, the surface area was increased by introducing TEOS into the spray solution. In particular, the external surface area of carbon can be largely increased by using TEOS, about 2.4 times larger than that of carbon produced without TEOS. The microstructure of carbon spheres was also influenced by the use of TEOS, which was discussed based on TEM, element mapping and XRD analysis. According to the evaluation of the electrochemical properties, the carbon prepared using TEOS exhibited enhanced specific capacitance, mainly due to an increase of the external surface area. The ion-sorption capacitance or the ion diffusion coefficient of carbon prepared by changing the TEOS content was found to have a linear relationship with the external surface area. These results demonstrate experimentally that adding TEOS to the precursor solution in the spray pyrolysis is a simple and effective way of producing highly mesoporous carbon spheres with improved electrochemical properties.
Mesoporous carbons have been prepared by various methods such as sol–gel,23 hydrothermal synthesis,24 template-based carbonization,25–27 chemical/physical activation,28–30 and spray pyrolysis.31–37 Among those, the spray pyrolysis is considered as a potential technique for the preparation of porous carbon because it has several advantages including continuous operation, one-step synthesis in a short time less than several seconds, spherical morphology and relatively narrow particle size distribution. When carbons are prepared by the spray pyrolysis, the microstructure and porosity can be easily controlled by the selection of the carbon source or by changing the additive and its concentration. The carbon sources frequently used in the spray pyrolysis are sucrose,36–38 phenolic resin,34,39 alkali benzoates32 and alkali propiolates (HCCCO2M, M = Li, Na and K).31,33 Also, surfactants or colloidal particles can be used as an additive to control the pore size.40,41 A general overview for the synthesis of carbon materials via spray pyrolysis was highlighted in the ref. 42.
PCMs with high porosity and controlled-microstructure can be produced by the carbonization of RF resin and block copolymers or the decomposition of alkali carboxylates, but these precursors are too expensive, considering the practical application of PCMs prepared by the spray pyrolysis. Carbonization using hard templates including colloidal silica, mesoporous silica and zeolite makes it possible to yield PCMs with well-defined pores and high surface area. Hampsey et al. introduced a spray-pyrolysis approach for the synthesis of hierarchical porous carbons particles using sucrose as a carbon source and silicate templates.41 When the similar hard-templating method is carried out in the spray pyrolysis to prepare PCMs, the additional etching process using excess HF, KOH or NaOH is needed to remove the template. Then, the synthetic process becomes expensive and inefficient. Thus, the development of a novel and relatively inexpensive synthetic method is a challenging work when PCMs are prepared by the spray pyrolysis.
Fortunato et al. reported a facile synthesis of PCM using ultrasonic spray pyrolysis without a sacrificial template.38 They used sucrose as the carbon source and sodium bicarbonate (or sodium carbonate) as a base catalyst for the decomposition of sucrose. The sodium bicarbonate also plays a role as porogens since the gaseous products from the decomposition can create high porosity. In addition, the salts nanoparticles produced in carbon matrix can be easily removed by a simple washing using water. The porosity and microstructure of spherical carbon particles were reported to be easily controlled by changing the concentration or the kinds of the base catalyst.38 Given this, the use of sucrose and sodium bicarbonate in spray pyrolysis for the preparation of PCMs is a good approach, but the resulting carbon particles have mostly micropores and hollow structure. In this work, the spray pyrolysis using sucrose and sodium bicarbonate was applied to prepare porous carbon particles. To control the surface area, mesoporosity and microstructure of carbon particles, a small amount of TEOS was added to the spray solution. Then, TEOS-derived intermediates can be easily removed by a simple ultrasonic washing without using any high-concentration basic chemicals. Resultantly, the porosity and microstructure of carbon spheres was controlled by changing the TEOS content. The electrochemical properties of the porous carbons prepared were evaluated by using cyclic voltammetry and electrochemical impedance spectroscopy. Finally, the use of TEOS with sodium bicarbonate was found to an effective way to produce porous carbon with large mesopores and enhanced ion sorption capacitance.
The prepared precursor solution was atomized by the ultrasonic aerosol generator, and the formed droplets were carried into the quartz reactor of 800 °C by N2 (10 L min−1). The generated powders were withdrawn by the Teflon bag filter installed at the end of the quartz tube. Finally, the as-prepared carbon powders were washed several times using purified water in order to remove alkali salts. The carbon powder prepared without the use of TEOS was denoted as SPC00. The carbons prepared at 0.05, 0.10 and 0.15 in the mole ratio of TEOS to sucrose were named as SPC05, SPC10 and SPC15, respectively.
The electrochemical properties of carbon spheres were investigated by the cyclic voltammetry (CV) and electrical impedance spectroscopy (EIS) measurements in a typical three-electrode system. A carbon paste to make working electrodes was prepared by mixing the prepared carbon powder (1.5 g) with 0.2 g of polyvinylidene fluoride (PVdF, Mw = 530000) and 5.8 g of di-methylacetamide (DMAc) using a planetary centrifugal mixer (Thinky mixer, ARE-310). A carbon film was formed on a graphite sheet (Dong-bang Carbon Co., Korea) via a doctor-blade method and tested as the working electrode (2.54 cm2 in area) in 0.5 M KCl solution through the CV and EIS measurements. In the used three-electrode system, a carbon-coated platinum rode and an Ag/AgCl electrode were used as a counter and a reference electrode, respectively. Cyclic voltammetry was performed using a potentiostat (AutoLab PGST30) in the potential range from −0.5 V to 0.5 V (vs. Ag/AgCl) at a scan rate of 10 mV s−1. The EIS analysis was measured by the potentiostat connected with a frequency response analyser (FRA) at a frequencies range from 20 mHz to 100 Hz.
The carbons (SPC05, SPC10, and SPC15) prepared by using TEOS have the BET surface area of 1067–1180 m2 g−1, and the carbon (SPC00) prepared without using TEOS has the BET surface area of 921 m2 g−1. Clearly, the TEOS addition to the spray solution makes it possible to enlarge the specific surface area. The largest BET surface area was achieved when the mole ratio of TEOS to sucrose was 0.1, and it is about 1.3 times larger than the carbon powder (SPC00) prepared without TEOS. The increase of the surface area is attributed to the increment in both micropores (Smicro) area and external surface area (Sext). Fig. 2 shows the total surface area and pore volume for the carbon samples. Micropores are generated by small gas molecules evolved by pyrolysis of sucrose or TEOS. The external surface area is mostly due to mesopores which is formed by the removal of salt nanoparticles remained in the as-prepared carbon powder. All the changes observed in the surface area or the pore volume are attributed to the added TEOS because the NaHCO3 and sucrose contents are fixed. The increase of the TEOS content generates more gaseous elements by the pyrolysis so that the micropore surface area increases slightly with increasing the TEOS content. The external surface area is also increased by the use of TEOS, but it does not monotonically increase with increasing the TEOS content and shows a maximum value when the ratio of TEOS to sucrose is 0.1. The external surface area of the SPC00 carbon is about 8.0% of the total BET area. When the carbon particles were prepared by adding TEOS, the percentage of the external surface was 9.0% (SPC05), 15.2% (SPC10) and 11.2% (SPC15) higher than that of the SPC00 carbon. That is, the use of TEOS produces more mesopores. The SPC10 carbon has the largest external surface of 179 m2 g−1, approximately 2.4 times larger than the SPC00 carbon (74 m2 g−1) prepared without TEOS. The change in pore volume with varying the TEOS content is similar to the behavior of the surface area. The micropore volume of SPC05 carbon (0.462 cm3 g−1) is larger than that of the SPC00 carbon (0.360 cm3 g−1), but the mesopore volume (Vtotal − Vmicro) of the two carbons is approximately the same: 0.207 cm3 g−1 (SPC00) and 0.191 cm3 g−1 (SPC05). In the SPC10 sample, however, the mesopore volume (0.318 cm3 g−1) is about 1.5 times larger than that of the SPC00. BJH desorption pore sizes are 3.9, 3.2, 3.4, 3.3 nm for the SPC00, SPC05, SPC10 and SPC15 carbons, respectively. That is, the added TEOS did not significantly change the average pore size calculated from the desorption isotherm.
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Fig. 2 Total surface area (a) and pore volume (b) of carbon particles synthesized by changing the TEOS/sucrose mole ratio via spray pyrolysis. |
In interconnected pore systems with a hysteresis loop, the pore size distribution (PSD) is affected by the pore network effects. The sudden desorption in the P/P0 range of 0.4–0.5 is caused by the evaporation of metastable pore liquid, which is referred to as the tensile strength effect (TSE).43 Given this, the PSD based on the desorption branch reveals an artificial peak and do not reflect the real pore size distribution. Instead, the PSD derived from the adsorption branch does not exhibit artificial peaks and makes it possible to explain more realistic pore size. The inset of Fig. 1(b) is the PSD derived from the adsorption isotherm. The SPC00 carbon does not have any peak in the region of 10–50 nm, but there is a broad distribution in macropore regions (>50 nm). The SPC05 carbon shows a weak peak at about 40 nm. The SPC10 carbon has a sharp distribution centered about 15 nm. For SPC15 carbon, there are two peaks, one observed around 15 nm and the other observed around 40 nm. From the pore size distribution derived from the adsorption branch, it is clear that many mesopores larger than 10 nm are generated by preparing carbon powders with using TEOS. From the results achieved so far, it has been found that the use of TEOS is an effective way for increasing the specific surface area. In particular, adding TEOS to the spray solution makes it possible to increase the mesopores surface. In terms of the surface area and the pore volume, the optimal TEOS content was determined as 0.1 in mole ratio with respect to sucrose.
The washing of as-prepared carbon particles was carried out only using purified water. Thus, NaHCO3 and TEOS should exist as a water-soluble form in carbon matrix. Fig. 4 shows XRD patterns before and after the washing for the SPC00 and SPC10 carbons. For the SPC00 carbon obtained before the washing, two hydrated crystal peaks were observed. One is Na2CO3–H2O (JCPDS no. 008-0448) and the other is Na3H(CO3)2–2H2O (JCPDS no. 029-1447). For the SPC10 carbon before the washing, the observed peaks are well indexed to Na3H(CO3)2–2H2O crystals. There is no crystal form related to silicon oxide. The formed Na2CO3–H2O or Na3H(CO3)2–2H2O salts are highly soluble in water. Thus, they can be easily removed from the carbon matrix via a simple water-based washing process, which was confirmed by the XRD results for the carbon powder obtained after the washing.
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Fig. 4 XRD patterns of carbon particles before and after the ultrasonic washing using purified water. |
For the SPC10 carbon particles, it is surmised that amorphous silicon oxide is formed from the pyrolysis of TEOS. To identify that silicon oxides can be extracted by the ultrasonic washing, the element mapping analysis was carried out. Fig. 5 shows TEM and element mapping results for the SPC10 carbon before and after the washing. Before the washing, the Si elements exist in the whole region of carbon matrix without any phase separation. After the washing, the quantity of Si elements was largely reduced. According to EDX analysis (Fig. S2†), the percentage of Si in as-prepared carbon particles was about 1.15 at%, and it was reduced to 0.01% after the washing. That is, about 99% of Si elements were removed by the ultrasonic washing. It is well known that SiO2 can be dissolved out in NaOH solution. Na3H(CO3)2–2H2O salts are dissociated to Na+, OH−, and HCO3− ions in the washing water. As a result, the washing solution becomes highly basic condition enough to dissolve SiO2 in carbon matrix because the mole concentration of Na ions is ten times higher than that of Si in the precursor solution. For the SPC05 and SPC15 carbons obtained after the ultrasonic washing, the element mapping was carried out, and the results were shown in Fig. S3.† It was confirmed that the Si elements are mostly removed in the SPC05 carbon, but a considerable amount of Si elements remains in the SPC15 carbon. Thus, the remained silicon-based oxides plugged the pores, resulting in the decrease in the surface area of the SPC15 carbon, especially in the external surface area, compared with the SPC10 carbon.
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Fig. 5 TEM and element mapping for the SPC10 carbons before (a) and after (b) the ultrasonic washing using purified water. |
On the base of the results achieved from SEM, TEM, element mapping and XRD analysis, the mechanism of the porous carbon formation in spray pyrolysis was expressed in Fig. 6. In spray pyrolysis, the microstructure of particles is changeable depending on the precipitation behavior of salts during the drying step. As droplets are heated, water evaporation proceeds, which leads to increase the surface concentration of salts. Then, the surface precipitation of salts begins because the surface concentration first reaches a supercritical saturation due to the fast drying of water. On the contrary, the volumetric precipitation occurs when the drying of water is low enough to induce the concentration increment of salts in the whole droplets. Also, the volumetric precipitation of slats can occur in the case that ultrafine colloidal particles exist initially in droplets, because the precipitation tends to first begin on the colloid surface.
When TEOS is not used, the fast water evaporation induces the surface precipitation which generates the surface shell layer of the sucrose/NaHCO3 composite. As droplet temperature increases further, the dehydration of NaHCO3 proceeds in the inside region of droplets, and Na2CO3 nanocrystals are generated progressively. After the drying is complete, the pyrolysis and carbonization of sucrose begins. In this step, NaHCO3 is turned into Na3H(CO3)2 nanocrystals via partial pyrolysis under a N2 environment. The formed Na2CO3 or Na3H(CO3)2 nanocrystals are removed easily by the ultrasonic washing and will contribute to form mesopores or macropores. In the case of using TEOS, SiO2 embryos exist initially in droplets. As the water evaporation proceeds, the sucrose/NaHCO3 composite particles are formed via the volumetric precipitation. In this step, SiO2 embryos are trapped within the sucrose/NaHCO3 matrix. Also, some SiO2 embryos take part in forming NaHCO3/SiO2 composited nanoparticles. Thereafter, the resulting sucrose/NaHCO3 composite particles are changed into carbon and Na3H(CO3)2 or Na3H(CO3)2/SiO2 composited nanoparticles via pyrolysis and carbonization processes. The existence of Na3H(CO3)2/SiO2 composited nanoparticles formed in carbon matrix are identified from the HR-TEM images of particles before the washing (Fig. S4†). Finally, Na3H(CO3)2/SiO2 nanoparticles are extracted out of carbon matrix in the ultrasonic washing step, forming a lot of mesopores as shown in Fig. 3(e).
In precipitation and carbonization processes, the size or distribution of Na3H(CO3)2/SiO2 composited nanoparticles within the carbon matrix can be affected by the content of TEOS. When no TEOS is used, the precipitated Na2CO3 is decomposed and grown as Na3H(CO3)2 nanocrystals of about 100 nm in the core of the hollow carbon matrix in the pyrolysis and carbonization step. This is the cause of the observed macropores of about 100 nm in the TEM image (Fig. 3(c)) and the pore size distribution (Fig. 1(b)). However, according to the pore size distribution derived from the adsorption isotherm (Fig. 1(b), inset), all carbons prepared using TEOS do not have macropores of about 100 nm. This result indicates that the SiO2 embryos do change the size and distribution of water-soluble salt precipitates. That is, the SiO2 embryos are involved in forming Na3H(CO3)2/SiO2 nanoparticles during the carbonization and prevent Na2CO3 from growing into large crystals.
To confirm the effect of TEOS contents, TEM images and pore size distributions derived from the adsorption branch are shown in Fig. 7. The SPC05 carbon has mesopores of about 40–50 nm, which indicates that the Na3H(CO3)2/SiO2 nanoparticles of about 40–50 nm are formed and distributed in the carbon matrix. At low TEOS content, the SiO2 embryos do not seem to be enough to avoid the growth of sodium carbonates in the carbonization step. With increasing the TEOS content, more SiO2 embryos are present in the initial precursor solution and act as seeds for precipitation. Resultantly, most sodium carbonates are converted to Na3H(CO3)2/SiO2 nanoparticles of approximately 15–20 nm rather than growing into large individual crystals. Also, the Na3H(CO3)2/SiO2 nanoparticles are distributed throughout the carbon matrix and contribute to generate a lot of mesopores when removing through a washing process. This hypothesis is evidenced from the TEM image (Fig. 7(b)) or the pore size distribution (Fig. 7(b), inset) for the SPC10 carbon. When the TEOS content is high (in the case of SPC15 carbon), the number of Na3H(CO3)2/SiO2 nanoparticles seems to be too large to be independently distributed in the carbon matrix. That is, in the carbonization step, particle growth occurs by agglomeration between Na3H(CO3)2/SiO2 nanoparticles. As a result, the SPC15 carbon has mesopores of about 40–50 nm with the pores of 15–20 nm in the pore size distribution (inset of Fig. 7(c)).
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Fig. 8(a) is the CV curves which have nearly a rectangular shape, indicating that the adsorption and desorption of ions are well achieved reversely. The specific capacitance values are displayed in Fig. 8(b) as a function of the applied voltages. The carbons (SPC05, SPC10, SPC15) prepared by using TEOS have higher specific capacitance than the SPC00 carbon. The specific capacitance at 0.0 V was summarized in Table 2. The SPC10 carbon has the largest capacitance (148 F g−1) which is about 1.5 times larger than that (96 F g−1) of the SPC00 carbon. If ions can be adsorbed on all surfaces, the capacitance should increase linearly with the total surface area. To confirm the surface area effect, the specific capacitance at 0.0 V was expressed as a function of the BET surface area (Fig. 8(c)). The specific capacitance does not have a linear relation with the BET surface area, indicating that all surfaces do not contribute to the adsorption of ions. The external surface area (Sext) and the specific capacitance were displayed as a function of the TEOS/sucrose mole ratio in Fig. 8(d). Both the external surface and the specific capacitance exhibit a similar behavior for the change of the TEOS/sucrose mole ratio. As shown in the inset of Fig. 8(d), the specific capacitance increases linearly with the external surface area. These results indicate that the increase in the specific capacitance of carbons prepared by using TEOS is mainly attributed to the increase of the external surface due to the formation of many mesopores.
Sample name | Specific capacitance [F g−1] | Rd [Ω] | σ [Ω s−1/2] | D [cm2 s−1] |
---|---|---|---|---|
SPC00 | 96 | 4.9 | 1.878 | 0.60 × 10−8 |
SPC05 | 110 | 7.0 | 1.087 | 1.79 × 10−8 |
SPC10 | 148 | 5.6 | 0.646 | 5.07 × 10−8 |
SPC15 | 127 | 7.8 | 0.847 | 2.95 × 10−8 |
EIS experiments were conducted to further examine the basic electrochemical behavior of synthesized carbon materials. Fig. 9(a) is the Nyquist plot for the impedances obtained in the frequency range from 0.01 Hz to 500 Hz. The intercept of the real axis (Z′) in the Nyquist plot is corresponding to the equivalent series resistance (ESR) associated with the bulk electrolyte/electrode material/current collector interfaces. As the frequency decreases, the impedance is close to a vertical curve that can be observed in a typical supercapacitor. As shown in the inset of Fig. 9(a), there is a liner relation with a slope of 45° between Z′(Re) and Z′′(Im) in the high frequency region. This is corresponding to the Warburg impedance, which represents the frequency dependent diffusion of ions within the porous electrode. That is, the real axis value (Z′) in this linear region is corresponding to the diffusion resistance (Rd) of ions. The estimated diffusion resistance values were summarized in Table 2 and showed in Fig. 9(b). The carbons prepared by using TEOS have the diffusion resistance slightly greater than the SPC00 carbon. In terms of ion diffusion, mesopores are more helpful than micropores. If carbon has only micropores, the ion adsorption is limited near the surface region of microporous carbon matrix. That is, the ion diffusion length is short. If carbon has mesopores along with micropores, the mesopores allows ions to penetrate deeper into the porous carbon matrix, thereby increasing the effective surface area that is substantially involved in ion sorption. This situation is good for improving the specific capacitance because of the substantially increment in the effective surface area for ion sorption. On the other hand, the deep penetration of ions lengthens the diffusion path, resulting in the increase of the ion diffusion resistance. Thus, the increase in the diffusion resistance (Rd) of carbons prepared using TEOS is due to the elongation of the diffusion path by the formation of more mesopores with increasing micropores. The SPC10 carbon has a smaller Rd value than the SPC05 or SPC15 carbons, but has the largest number of mesopores (Sext). According to Fig. 1(b), large mesopores of about 15 nm were generated only at the SPC10 carbon, which is thought to be a cause of relatively small diffusion resistance.
The carbon porosity directly affects the diffusion rate of ions. Warburg impedance (ZW) is basically created by the diffusion of ions in porous carbon electrodes and expressed by the following equation:44
ZW = σω−1/2(1 − j) | (2) |
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Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra01999h |
This journal is © The Royal Society of Chemistry 2017 |