Cookie Policy Research Square My Dashboard MY ARTICLE ADMIN Workflows Draft AUTHOR'S VIEW Peer Review Timeline Preview AUTHOR SERVICES SUPPORT In ReviewNature Communications 18-279619 ARTICLE Chemical Engineering Energy Engineering +1 General and Predictable Synthesis of Ultrahigh-Surface-Area Porous Carbons with Superior Yield via Preferential Removal of sp2-Hybridized Atoms Peifeng Yu, Weicai Zhang, Yingliang Liu, Fei Xu, Yeru Liang Abstract A grand challenge in the state-of-the-art porous carbons is the lack of reliable synthesis strategy for achieving ultrahigh surface areas while maintaining a high carbonization yield. Ultrahigh surface area generally depends on trial and error activation with poor understanding of structural information in the starting carbonaceous matter to predict the ultrahigh porosity. Meanwhile, excessive development of porosity (> 3500 m2 g− 1) will undoubtedly give rise to low carbonization yield (< 10%), thus far restricting cost-effective applications. Here, we report a general and predictable protocol via constructing nitrogen-doped sp2-hybridized carbon atoms in the carbonaceous matter, which guides the pore-creating agents (e.g., KOH) to preferentially etch over sp2- rather than sp3-hybridized atoms, thus greatly increasing the activation reaction efficiency to simultaneously accomplish ultrahigh porosity without sacrificing carbonization yield, a critical paradox in producing carbons. A highest surface area (4482 m2 g− 1) with 10 wt.% carbonization yield and 3500 m2 g− 1 with an unparalleled yield of 35% are achieved so far, which enables great potential in adsorptive-related applications as exemplified by their record-high gas adsorption and supercapacitve performances. Our findings reveal important insights on directed synthesis of ultrahigh-surface-area carbons and provide an impetus for their on-demand applications. KEYWORDS ultrahigh-surface-area porous carbons, synthesis strategy, superior yield, sp2-hybridized atoms Edit Fulltext Introduction
Carbon materials, such as graphenes1, carbon nanotubes2, fullerenes3, and porous carbons4, constitute a class of versatile materials that find indispensable utility in a wide range of potential and practical applications. Porous carbons, integrating the advantages of carbon materials with porous solids, have specially gathered intense interests due to their outstanding physicochemical properties and their extensive application fields5, 6. Benefiting from the fast-growing synthesis strategy and nanotechnology during the two past decades, porous carbons have reached a new level of precision control over pore structure with rapid advancement of surface areas. Despite tremendous previous efforts, porous carbons produced by activation still stand out to be competitive taking into account of its feasibility to attain much higher surface area and the low cost, thus dominating the market of porous carbons.
Essentially, pursuing ultrahigh specific surface areas has been a long-pursued goal for the field of porous materials due to the high porosity for providing large surface area for dispersion of active sites could lead to unconventional properties, and thus opens unprecedented opportunities for breakthroughs in the emerging applications or further expands in their application scopes. Continuous progress in specific surface area has been achieved in the state-of-the-art porous materials, such as, zeolites(i.e., 4100 m2 g-1)7, metal-organic framework (i.e., MOF, 9140 m2 g-1)8 and porous organic polymers (i.e., POP, 6461 m2 g-1)9. For porous carbons, chemical activation of biomass and polymer has been well established to prepare porous carbons with large surface area generally up to 3000 m2 g-1, and only very few examples have been reported to prepare higher values over 3500 m2 g-110, 11 by selecting specific precursors. Nevertheless, the realization of these high surface areas is on account of trial and error activation, and there are no material parameters or characteristics of the starting carbonaceous matter that can be used to predict the ultrahigh porosity. Moreover, the harsh chemical activation condition usually gives rise to a low yield below 10 wt.% when the surface area up to 3500 m2 g-1, which is mainly caused by the burn-off effect during the forming of pores. This yields a high preparation cost of super activated carbons, such as the price of classical activated carbon YP-50 up to about 257 $ per kilogram. Therefore, obtaining the ultrahigh BET surface area along with the satisficed yield can vitalize the high performance activated carbon market for industrialized producing at a low cost. However, the research in this field lacks attention.
In this contribution, we show a concept for combination of ultrahigh porosity and large yield in carbon materials through preferential removal of sp2-hybridized carbon atoms. Key to this strategy is construction of nitrogen-doped (N-doped) sp2-hybridized carbon atoms in the precursors, which act as “magnet” with a strong attraction to pore-creating agents, e.g., KOH, to guide and preferentially etch over the sp2-hybridized carbon atoms. Such a priority of removal of sp2-hybridized carbon atoms but not sp3-hybridized carbon atoms greatly increases the activation reaction efficiency when compared with routine approaches, in which the sp3-hybridized carbon atoms are more reactive and preferred to be etched. As a consequence, the specific surface area and the yield are enhanced simultaneously. In this way, we prepared a carbon material with the highest Brunauer-Emmett-Teller (BET) specific surface area (i.e., 4482 m2 g− 1) reported so far, accompanying with an unconventionally large yield of 10.0 wt.%. These values are not only 1.3 times (for specific surface area) or 3.2 times (for yield) enhancement compared to those of prepared by conventional means, but also hold a significant superiority over all those reported in the literatures (Supplementary Table 1). The present synthetic strategy is rather general, which can be utilized to synthesize a series of porous carbons with simultaneously ultrahigh specific surface area over 3500 m2 g− 1 and large yield up to 35.1 wt.% from various carbon precursors including biomass (e.g., mushroom), natural macromolecule (e.g., cellulose) and synthetic polymer (e.g., phenolic resin). Benefiting from the well-developed porous structure, the resulting porous carbons offer a promising opportunity to boost the performances to a new stage. For instance, high H2 adsorption capacity of 8.4 wt.% at 40 bar, large CO2 capture capacity of 39.6 mmol g− 1 at 40 bar, huge supercapacitive capacitance of 634 F g− 1 can be achieved, which exceed those of nearly all the benchmark porous carbons.
Preview Carbon materials, such as graphenes1, carbon nanotubes2, fullerenes3, and porous carbons4, constitute a class of versatile materials that find indispensable utility in a wide range of potential and practical applications. Porous carbons, integrating the advantages of carbon materials with porous solids, have specially gathered intense interests due to their outstanding physicochemical properties and their extensive application fields5, 6. Benefiting from the fast-growing synthesis strategy and nanotechnology during the two past decades, porous carbons have reached a new level of precision control over pore structure with rapid advancement of surface areas. Despite tremendous previous efforts, porous carbons produced by activation still stand out to be competitive taking into account of its feasibility to attain much higher surface area and the low cost, thus dominating the market of porous carbons. Essentially, pursuing ultrahigh specific surface areas has been a long-pursued goal for the field of porous materials due to the high porosity for providing large surface area for dispersion of active sites could lead to unconventional properties, and thus opens unprecedented opportunities for breakthroughs in the emerging applications or further expands in their application scopes. Continuous progress in specific surface area has been achieved in the state-of-the-art porous materials, such as, zeolites(i.e., 4100 m2 g-1)7, metal-organic framework (i.e., MOF, 9140 m2 g-1)8 and porous organic polymers (i.e., POP, 6461 m2 g-1)9. For porous carbons, chemical activation of biomass and polymer has been well established to prepare porous carbons with large surface area generally up to 3000 m2 g-1, and only very few examples have been reported to prepare higher values over 3500 m2 g-110, 11 by selecting specific precursors. Nevertheless, the realization of these high surface areas is on account of trial and error activation, and there are no material parameters or characteristics of the starting carbonaceous matter that can be used to predict the ultrahigh porosity. Moreover, the harsh chemical activation condition usually gives rise to a low yield below 10 wt.% when the surface area up to 3500 m2 g-1, which is mainly caused by the burn-off effect during the forming of pores. This yields a high preparation cost of super activated carbons, such as the price of classical activated carbon YP-50 up to about 257 $ per kilogram. Therefore, obtaining the ultrahigh BET surface area along with the satisficed yield can vitalize the high performance activated carbon market for industrialized producing at a low cost. However, the research in this field lacks attention. In this contribution, we show a concept for combination of ultrahigh porosity and large yield in carbon materials through preferential removal of sp2-hybridized carbon atoms. Key to this strategy is construction of nitrogen-doped (N-doped) sp2-hybridized carbon atoms in the precursors, which act as “magnet” with a strong attraction to pore-creating agents, e.g., KOH, to guide and preferentially etch over the sp2-hybridized carbon atoms. Such a priority of removal of sp2-hybridized carbon atoms but not sp3-hybridized carbon atoms greatly increases the activation reaction efficiency when compared with routine approaches, in which the sp3-hybridized carbon atoms are more reactive and preferred to be etched. As a consequence, the specific surface area and the yield are enhanced simultaneously. In this way, we prepared a carbon material with the highest Brunauer-Emmett-Teller (BET) specific surface area (i.e., 4482 m2 g− 1) reported so far, accompanying with an unconventionally large yield of 10.0 wt.%. These values are not only 1.3 times (for specific surface area) or 3.2 times (for yield) enhancement compared to those of prepared by conventional means, but also hold a significant superiority over all those reported in the literatures (Supplementary Table 1). The present synthetic strategy is rather general, which can be utilized to synthesize a series of porous carbons with simultaneously ultrahigh specific surface area over 3500 m2 g− 1 and large yield up to 35.1 wt.% from various carbon precursors including biomass (e.g., mushroom), natural macromolecule (e.g., cellulose) and synthetic polymer (e.g., phenolic resin). Benefiting from the well-developed porous structure, the resulting porous carbons offer a promising opportunity to boost the performances to a new stage. For instance, high H2 adsorption capacity of 8.4 wt.% at 40 bar, large CO2 capture capacity of 39.6 mmol g− 1 at 40 bar, huge supercapacitive capacitance of 634 F g− 1 can be achieved, which exceed those of nearly all the benchmark porous carbons. Results And Discussion
The chemical etching processes for generating porous structure are schematically illustrated in Fig. 1, in which KOH is used as the pore-creating agents for a model demonstration. As the starting materials, semi-carbonized precursors obtained at low temperatures usually consist of sp2-hybridized and sp3-hybridized carbon atoms12. With increasing the activation temperature, the KOH will start to react with carbon atoms. In a normal KOH activation procedure, the sp3-hybridized carbon atoms are preferred to be etched because of the more reactive of sp3-hybridized carbon atoms than sp2-hybridized carbon atoms (Fig. 1a)13. It is known that the disordered packing of the carbon sheets that is crosslinked by sp3-hybridized carbon atoms synergistically significantly contribute to the generation of the pore structure12, 14. Therefore, the preferential removal of sp3-hybridized carbon atoms not only easily collapse the porous architectures, but also leads to a poor activation efficiency, resulting in relatively low porosity and low yield (Fig. 1a).
To circumvent this intrinsic limitation, we proposed to construct N-doped sp2-hybridized carbon atoms in the semi-carbonized precursors to guide KOH to preferentially etch over the sp2-hybridized carbon atoms. We first performed density functional theory (DFT) to evaluate the effect of the N species on KOH etching process. In the DFT calculations, KOH is simulated as an individual hydroxyl group for convenience. As shown in Fig. 2a, site 1, site 2 and site 3 indicate three different distances between the hydroxyl groups adsorbed on carbon atoms and N atoms. The corresponding adsorption energy (∆E) between KOH and N-doped carbon was calculated and listed in Fig. 2b. It can be seen that the hydroxyl group at site 1 nearest to N atom has the most negative ∆E of -2.64 eV. Moreover, the ∆E gradually increased from − 2.07 to -2.00 when increasing the distance between the hydroxyl group and N atom. However, all the ∆E of N-doped carbon is smaller than that of pristine carbon without N atom (i.e., -1.35 eV), which indicates that the hydroxyl group has a stronger interaction with N-doped carbon than pristine carbon. Meanwhile, the reactant and product were predicted by intrinsic reaction coordinate by calculating the transition state (Fig. 2c). As shown in Fig. 2d, the energy barrier between the hydroxyl group and N-doped carbon (i.e., 0.12 eV) is obviously smaller than that of pristine carbon (i.e., 0.66 eV). All above results indicate that KOH is preferred to react with N-doped carbon; especially, the region of N atom is preferentially etched by KOH.
Guided by the theoretical calculations, we then set out to introduce N heteroatoms by pyrolysis of carbon precursor and N-enrich resource, which utilize mushroom and melamine as typical demonstrations, respectively. As shown in Supplementary Fig. 1, the resultant semi-carbonized mushroom possesses a high N content of 10.9 at.% as determined by X-ray photoelectron spectroscopy (XPS). High-resolution N1s spectrum shows that the main peak is centered at 399.18 eV assigned to pyridinic N, illustrating that the hybridization between N and carbon atoms is mainly sp2 in the semi-carbonized mushroom scaffolds (Supplementary Fig. 2)15.
We further heated the mixture composed of semi-carbonized mushroom and KOH at high temperature in N2 flow. After washing and drying, we obtained the mushroom-based porous carbon (M-PC). Its pore structure characteristic was probed by N2 adsorption-desorption measurements at 77 K (Supplementary Fig. 3). As shown in Supplementary Fig. 3a, the M-PC exhibits type I isotherm with significant adsorption at low relative pressure (P/P0 < 0.01), which indicates the presence of tremendous micropores. After that, the adsorption amount increases gradually until the P/P0 near 0.4, demonstrating the formation of abundant small-sized mesopores. According to the pore size distribution curves in Supplementary Fig. 3b, the size of these micropores and mesopores are mainly centered at about 1.2 and 2.5 nm, respectively. High-resolution transmission electron microscopy (TEM) image confirmed the existence of these nanopores distributed in the carbon framework (Fig. 3a). A BET calculation reveals that the BET specific surface area of M-PC is as high as 4482 m2 g-1 (Fig. 3b). To the best of our knowledge, this is the highest specific surface area for all carbon materials reported so far (Fig. 3c and Supplementary Table 2). In sharp contrast, a mushroom-based control carbon sample (M-control sample) prepared by a normal strategy without N atoms has an obviously smaller BET specific surface area of 3428 m2 g-1. More importantly, although possessing an ultrahigh specific surface area, M-PC displays an unusual enhanced yield as high as 10.0 wt.%, which is almost 3.2 times larger than that of the control sample, i.e., 3.1 wt.% (Fig. 3b).
In addition to natural mushroom, this efficient strategy can be well extended to other kinds of carbon precursors such as natural macromolecule (e.g., cellulose) and synthetic polymer (e.g., phenolic resin). Similarly, both BET specific surface area and yield of the as-prepared carbon materials are enhanced at the same time. For example, as shown in Fig. 3d, the BET specific surface areas of the cellulose-based porous carbon (C-PC) and phenolic resin-based porous carbon (PR-PC) are calculated to be 3504 and 3954 m2 g-1, respectively, which are significantly higher than those of the corresponding control samples (i.e., 2929 and 3167 m2 g-1). Meanwhile, the yields of C-PC and PR-PC are remarkably enhanced from 6.0 wt.% and 20.8 wt.% to 10.0 wt.% and 35.1 wt.%, respectively (Fig. 3e). As shown in Fig. 3f and Supplementary Table 1, these resulting porous carbons are super to these reported previous considering the comprehensive properties of specific surface area and yield. These results indicate that ultrahigh porosity and large yield, which is an intrinsic conflict under normal circumstances, can be simultaneously combined into carbon material by constructing N heteroatoms in the semi-carbonized precursors for preferential removal of sp2-hybridized carbon atoms. Considering its thermally decomposable characteristics (Supplementary Fig. 4), the N-enrich resource, i.e., melamine makes very little contribution to carbon yield and is believed to just provide N heteroatoms to affect the pore-marking process.
To understand the detailed pore-making mechanism with the introduction of N heteroatoms, we evaluated the phase and structural change of the intermediate products. As well known, the pore formation in the KOH activation underling the process complexity usually originates from the combustion of carbon species treated with KOH36. It has been demonstrated the traditional KOH activation generally proceeds as 6KOH + C = 2K + 3H2 + 2K2CO3 followed by reaction of K/K2CO3/CO2 with carbon species and/or decomposition of K2CO3 (Supplementary Eqs. 1 ~ 5)37. Typically, KOH starts to react with carbon atoms to yield K2CO3 at around 400 oC according to Supplementary Eq. 138, which was confirmed by the XRD measurement (Fig. 4a). At this stage, the sp3-hybridized carbon atoms but not the sp2-hybridized carbon atoms are preferred to be etched due to its stronger reactivity (Fig. 1a)13. This reaction continues far till 700 oC (Fig. 4a). When the reaction temperature achieves to 700 oC, the K2CO3 reacts further as the Supplementary Eqs. 2 ~ 5 with the release of CO and CO2 gas to create the pores38. Since those sp3-hybridized carbon atoms plays important roles in crosslinking the graphite-like microcrystals within the semi-carbonized framework (Supplementary Fig. 5), the graphite-like microcrystals are no longer restricted upon the removal of the sp3-hybridized carbon atoms39. Correspondingly, the carbonaceous porous framework is destroyed with constantly increasing the temperatures, collapsing into nanosheet-like structure with a low porosity (Fig. 4b and Supplementary Fig. 6) rather than the original irregular bulk (Supplementary Fig. 5). Furthermore, in order to construct a high porous structure, more carbon species have to be consumed, leading to a low yield (Supplementary Fig. 7).
On the contrary, with the presence of N heteroatoms, KOH is inclined to react with carbon atoms close to N atoms (i.e., sp2-hybridized carbon atoms) due to the strong interaction between KOH and N atoms. Then the KOH reacts as the Eq. 1 with the N atoms and their neighboring sp2-hybridized carbon atoms to form KCN and KOCN with gradually increasing the activation temperatures (Fig. 4a). Such a priority of removal of the sp2-hybridized carbon atoms not only retains the original carbonaceous porous bulk framework (Fig. 4b and Supplementary Fig. 8), but also makes fewer carbon atoms etched to realize a high porosity when the activation process is performed (Eqs. 2 ~ 4), thus increasing the activation reaction efficiency accompanied by ultrahigh porosity and large yield. As a consequence, the emerged carbon materials possessed a higher proportion of sp3-hybridized carbon atoms (i.e., a lower proportion of sp2-hybridized carbon atoms) when compared with those materials obtained in a regular activation procedure. For example, the M-PC exhibited a decreased (002) diffraction intensity assigned to the sp2-hybridized carbon in the XRD patterns (Supplementary Fig. 9), a lower characteristic peak intensity around 1580 cm− 1 (G-band) ascribed to the sp2-hybridized carbon in the Raman spectra (Supplementary Fig. 10) and a calculated higher value of ID/IG as compared to the control carbon sample (Supplementary Fig. 11). Moreover, the calculated values of stack height (Lc) and stack width (La) based on the results of XRD and Raman measurements for M-PC is much lower than the control sample, intuitively highlighting that the lower ratio sp2-hybridized carbon atoms in the sample M-PC (Fig. 4c and 4d). Quantitative calculation from X-ray photoelectron spectroscopy (XPS) analysis showed that the ratio between the sp3-hybridized carbon and sp2-hybridized carbon atoms in the M-PC is as high as 0.9, which is 3 times higher than that in the control sample (i.e., 0.3) as illustrated in Supplementary Fig. 12 and Fig. 4e. This result is well consistent with those obtained from Raman and XRD measurements, further confirming that the introduction of N heteroatoms indeed affects the removal process of carbon atoms and improve the activation efficiency and yield concurrently.
The exceptionally well-developed porosity together with the remarkable yield makes the as-fabricated porous carbons very attractive in their practical application fields. Particularly, the ultrahigh specific surface area of 4482 m2 g-1 in M-PC provides an unprecedented platform for boosting their performance to a new stage. Herein, we focused on the adsorption (i.e., H2 uptake and CO2 capture) and electrochemical (i.e., supercapacitive) performances. Given that H2 storage in porous carbons is favored by high specific surface area, we assessed the H2 uptake properties of M-PC at 77 K in the pressure range from 20 to 40 bar (Supplementary Fig. 13). As shown in Fig. 5a, the excess hydrogen uptake of M-PC is 5.8 wt.% at 20 bar and increases in line with the rise in adsorption pressure. An impressive excess hydrogen uptake achieves to 8.4 wt.% at 40 bar, which is at the top end of those values previously reported for carbon materials, and indeed is comparable to or even superior to those ones obtained in metal organic framework with BET specific surface area exceeding 6000 m2 g-1 (Fig. 5c and Supplementary Table 3). Such superiority in adsorption also enables great potential for their application in CO2 capture (Supplementary Fig. 14). As summarized in Fig. 5b, the M-PC delivers CO2 adsorption capacities of 24.4, 30.2 and 39.6 mmol g-1 at 20, 30 and 40 bar, respectively. So far as known, these values are the highest adsorption capacities ever reported for porous carbons (Fig. 5c and Supplementary Table 4).
The preponderance of the presented carbons in application is further demonstrated in energy storage. For example, when used as supercapacitor electrode, the M-PC can supply sufficient active sites for ion storage and then possesses outstanding electrochemical performance. Galvanostatic charge-discharge test was utilized to evaluate the capacity of the as-constructed supercapacitors (Supplementary Fig. 15). A calculation based on these galvanostatic discharge curves demonstrates that M-PC showed an intriguing specific capacitance value as high as 634 F g-1 at 0.1 A g-1, which is higher than that of commercial activated carbon benchmark and is one of the highest values obtained from all porous carbons (Fig. 5d and Supplementary Table 5). Even increasing the current density 500 times to an ultrahigh value, i.e., 50 A g-1, a remarkable value of 266 F g-1 can be achieved (Fig. 5d), indicative of its fast ion storage ability and rate performance. These very attractive electrochemical performances highlight the great potential of suitability of M-PC for advanced supercapacitors, which presents a positive potential for bridging the performance gap between supercapacitors and lithium-ion battery.
In summary, we have developed an efficient strategy to prepare porous carbons with ultrahigh porosity and large yield by preferential removal of sp2-hybridized carbon atoms. The introduction of N-doped sp2-hybridized carbon atoms was demonstrated to guide the KOH to preferentially etch over the sp2-hybridized carbon atoms, which greatly improved the activation reaction efficiency to simultaneously intensify the porosity and yield. The resulting carbon material from natural mushroom showed an ultrahigh specific surface area of 4482 m2 g− 1 and an impressively large yield of about 10 wt.%, which gave rise to excellent adsorption and electrochemical performance in practical application. Also, this strategy had proved its generalization to other kinds of carbon precursors such as natural macromolecule (e.g., cellulose) and synthetic polymer (e.g., phenolic resin), which could provide a powerful conceptual approach to generate advanced porous carbon materials. Overall, this study provides deep understating and opens up the possibilities for overcoming the major issues that how to cost-effectively prepare porous carbon with high specific surface area and desirable yield through an ingenious design for the precursors.