Hierarchically Porous Graphitic Carbon with Simultaneously High

Oct 18, 2017 - Developing hierarchical porous carbon (HPC) materials with competing textural characteristics such as surface area and pore volume in o...
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Hierarchically Porous Graphitic Carbon with Simultaneously High Surface Area and Colossal Pore Volume Engineered via Ice Templating Luis Estevez,*,† Venkateshkumar Prabhakaran,† Adam L. Garcia,† Yongsoon Shin,† Jinhui Tao,† Ashleigh M. Schwarz,† Jens Darsell,† Priyanka Bhattacharya,†,‡ Vaithiyalingam Shutthanandan,† and Ji-Guang Zhang*,† †

Pacific Northwest National Laboratory, 902 Battelle Boulevard, P.O. Box 999, Richland, Washington 99352, United States Energy Technologies and Materials Division, University of Dayton Research Institute, 300 College Park Avenue, Dayton, Ohio 45469, United States

‡

S Supporting Information *

ABSTRACT: Developing hierarchical porous carbon (HPC) materials with competing textural characteristics such as surface area and pore volume in one material is difficult to accomplish, particularly for an atomically ordered graphitic carbon. Herein we describe a synthesis strategy to engineer tunable HPC materials across micro-, meso-, and macroporous length scales, allowing the fabrication of a graphitic HPC material (HPC-G) with both very high surface area (>2500 m2/g) and pore volume (>11 cm3/g), the combination of which has not been attained previously. The mesopore volume alone for these materials is up to 7.53 cm3/g, the highest ever reported, higher than even any porous carbon’s total pore volume, which for our HPC-G material was >11 cm3/g. This HPC-G material was explored for use both as a supercapacitor electrode and for oil adsorption, two applications that require either high surface area or large pore volume, textural properties that are typically exclusive to one another. We accomplished these high textural characteristics by employing ice templating not only as a route for macroporous formation but as a synergistic vehicle that enabled the significant loading of the mesoporous hard template. This design scheme for HPC-G materials can be utilized in broad applications, including electrochemical systems such as batteries and supercapacitors, sorbents, and catalyst supports, particularly supports where a high degree of thermal stability is required. KEYWORDS: hierarchically porous carbon, graphitic, ice templating, supercapacitors, water treatment, pore volume

H

ierarchical porous carbon (HPC) materials have garnered much interest recently.1−3 The specific ability to utilize a multimodal pore size distribution across multiple length scales has been shown to be useful in various applications. The inherent characteristics of carbon (relatively inert, good electrical conductivity, and low density) combines well with a tunable hierarchal porosity for various energy-based applications that utilize porous carbons for electrode materials such as supercapacitors,4,5,45,46 batteries,6,7 and fuel cells.8,9 HPCs have also been successfully utilized for environmental applications such as sorbents for CO 2 capture,10,11 materials for water treatment in the form of contaminant sorbents12 and capacitive deionization,13 and scaffolds for the impregnation of catalyst materials.14,15 © 2017 American Chemical Society

The advantages of multimodal pore sizes in HPC materials differ depending on the application(s) for which they are utilized. For energy storage applications, researchers have found that a hierarchical morphology can be of benefit in lowering the kinetic barriers for electrolyte penetration. This can be easily seen in electric double-layer capacitor (EDLC)-based supercapacitors,16,17 where the high surface area sites are typically made up of smaller micropores (defined as 1500 m2/g) are usually associated with smaller pores (3 cm3/g) require larger sized pores, typically at least tens of nanometers. Such HPCs have proven to be beneficial when utilized as catalyst supports in direct methanol fuel cells, where high surface area smaller pores are needed for catalyst loading, but at the same time larger pores are required for efficient fuel transport.20 Many researchers have recently utilized dual high surface area/pore volume HPC materials as solid supports for the impregnation of amines with excellent results for CO2 capture.11 Additionally, the ability to generate HPCs with macropores (>50 nm) has been utilized successfully in flow battery electrodes21 and water treatment via capacitive desalination with flow-through electrodes.22 However, it is extremely rare to find HPC materials with both exceptionally large surface area (>2500 m2/g) and large pore volume (>5 cm3/g), together in one HPC material. Atomic morphology, such as the extent of the ordering within the HPC material (or level of graphitization), is extremely important as well.23,24 The level of graphitization for porous carbon materials has been well documented to be, in large part, due to the carbon precursor utilized and the final pyrolysis temperature employed.25 Electrical conductivity and improved thermal and chemical stability26 of the porous carbons tend to scale with the level of graphitization;23,24 thus these carbons are typically preferred in electrochemical applications. A good example of this required stability is found in platinum supports utilized for catalysis, where the carbon materials require a more graphitic carbon to offset the Pt-induced carbon oxidation at lower temperatures.27,28 Herein, we report a rational approach for synthesizing a graphitic HPC system (labeled as HPC-G) and utilizing it as a tunable materials platform with exquisite morphological control of the HPC that enables the realization of extremely high surface area (>2900 m2/g) and colossally large pore volume (>11 cm3/g) values, never before seen together in one HPC material (graphitic or otherwise). We also show how we utilized ice templating to synergistically achieve these textural characteristics. For comparison of the graphitic structure in the HPC-G materials platform, a hard carbon (more atomically disordered) material was also synthesized with similar textural properties to the HPC-G materials and denoted as HPC-S. The HPC-S series of materials is similar to our previous work,29 and details of the synthesis and characterization can be found in the Supporting Information, whereas the differences in the synthesis technique for the HPC-G materials can be found in the Methods section. 11048

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Figure 1. Results of electron microscopy for the HPC-G-12-21 sample reveals (a) the macroporous structure as seen by SEM and the (b) mesopores present in the macroporous walls under TEM, as well as (c, d) the graphitic ordering under HRTEM. The heavily activated HPCG-12-21-5h sample shows (e) similar mesopores present under TEM and (f) graphitic ordering via HRTEM.

Table 1. Textural Characteristics, Level of Graphitization, and Oil Sorption Data for Various Porous Carbons porous carbon

BET SSA [m2/g]

micropore SSA [m2/g]a

total pore volume [cm3/g]b

micropore volume [cm3/g]a

mesopore volume [cm3/g]c

D band fwhm [cm−1]

G band fwhm [cm−1]

oil sorption [g/gcarbon]

HPC-G1000C HPC-G-1221 HPC-G-1221-3h HPC-G-1221-5h HPC-S-4-21 HPC-S-421-10h Norit AC KB-600JD

652

83

2.01

0.042

1.49

619

74

2.47

0.035

1.70

90

67

45

1831

259

7.03

0.131

4.85

2933

488

11.51

0.247

7.53

85

77

65

1292 2675

187 465

4.89 10.84

0.098 0.242

4.40 7.91

180 165

68 79

25

1948 1477

389 43

1.14 4.75

0.209 0.014

0.61 2.24

120 140

69 78

7 20

a Microporous surface area determined via the t-plot method. bTotal pore volume obtained via nitrogen uptake at the single-point P/Po value of ∼0.995. cMesopore volume was calculated by selecting the BJH cumulative pore volume value at a pore size of 50 nm and subtracting the t-plot micropore volume.

Scanning electron microscopy (SEM) (Figure 1a) reveals the typical macroporous structure present in the HPC-G materials platform. Transmission electron microscopy (TEM) images (Figure 1b) of the HPC-G-12-21 sample show a mesoporous PSD distribution centered at ∼15 nm, consistent with the colloidal silica particle size. More significantly, the highresolution TEM (HRTEM) images confirm the presence of a graphitic structure. Figure 1c shows a typical higher magnification HRTEM image of the individual mesopores for the HPC-G material. The image reveals the mesopore walls are made up of layers of graphitic sheets spaced ∼0.34 nm apart (Figure 1d), consistent with a more ordered, graphitic

increase the already high surface area and pore volume of our HPC-G materials. We employed a 12 nm colloidal silica template at a silica-to-PAN mass ratio of 2:1, resulting in the HPC-G-12-21 sample. An alternative HPC-G example with a larger colloidal silica template (∼115 nm) and resultant porous structure is presented in the Supporting Information (Figure S7) as a proof of concept toward this facile tunability. The HPC-G-12-21 samples underwent physical activativation,38 resulting in extremely high textural properties of both specific surface area (2933 m2/g) and pore volume (11.23 cm3/g) together in one material. 11049

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Figure 2. Nitrogen porosimetry data for the HPC-G materials at varying levels of activation showing (a) pore size distribution curves by volume and (b) surface area, as well as (c) cumulative pore volume as a function of pore size, all based on BJH theory. (d) Isotherms for the various levels of activation.

Figure 3. Micro-Raman spectroscopy data for the various HPC-S and HPC-G materials (both heavily activated and nonactivated) as well as commercially available porous carbons Norit AC and KB-600JD. A nonporous graphite power was also included for comparison, showing graphite’s telltale D, G, and G′ bands at ∼1360, ∼1575, and ∼2700 cm−1, respectively.

carbons, with most cases in the literature being below 2000 m2/ g.23−25 This high specific surface area (SSA) combined with the massively large measured pore volume values displayed by our HPC-G materials has not been seen in the literature until now. Nitrogen porosimetry was also used to better elucidate the morphology present in the HPC-G-12-21 samples during various stages of synthesis. Figure 2 shows the nitrogen porosimetry data for the HPC-G-12-21 samples before hightemperature pyrolysis (HPC-G-1000C) and after the ensuing high-temperature pyrolysis (HPC-G-12-21), as well as after later increasing levels of activation. As can be seen in Figure 2a,

structure. The TEM images for the heavily activated HPC-G12-21-5h carbon (Figure 1e,f) reveal the mesoporous PSD broadens out to ∼20 nm, in good agreement with the nitrogen porosimetry data (vide inf ra), while the HRTEM image (Figure 1f) also shows a more ordered and graphitic structure when compared to the HPC-S materials (Figure S1c,e), albeit with thinner mesoporous walls when compared to the unactivated HPC-G-12-21 carbon. This graphitic structure for the HPC-G materials is noteworthy particularly when combined with the textural properties for the activated HPC-G-12-21 materials (Table 1). Such high surface areas are rare for porous graphitic 11050

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at 180 and 165 cm−1 for the unactivated and heavily activated HPC-S samples, respectively. Also of importance is the G′ band found in the graphite sample (at ∼2700 cm−1), which is only present for the two (high temperature pyrolyzed) HPC-G samples, giving further credence to their graphitic/ordered structure. X-ray diffraction (XRD) analysis (Supporting Information, Figure S3) reveals somewhat broad peaks centered at 2θ = ∼24° for all of the samples due to the small crystalline domains in the pores. Of interest, is the highly activated HPC-G material, which has a sharp peak beginning to emerge at 2θ = ∼26°, just to the right of the broad peak at 24°, which is consistent with the earlier work of Ryoo et al.25 In Ryoo’s work, XRD spectra showed the same small sharp peak, to the right of the broader peak, as the carbons under investigation began to graphitize. As has been previously discussed, thermal stability for carbon materials is an important property in carbons utilized for catalyst supports, especially when utilizing platinum catalysts in electrochemical convertors. We compared the thermal stability of various samples (Figure S4) including our HPC samples, KB-600JD, Norit AC, and a nonporous graphite powder. As expected, the nonporous graphite powder was the most thermally stable, whereas the carbon with the least degree of ordering and extremely large SSA, the HPC-S-4-21-10h sample, was the least thermally stable. The onset of oxidation was determined via thermogravimetric analysis and shows a clear exothermic release upon heating that intensifies with mass loss, as would be expected with oxidation/burning of the carbon for all the samples. The onset value for the graphite powder is 664 °C, followed by nonactivated HPC-G (646 °C), KB (622 °C), Norit AC (600 °C), highly activated HPC-G (597 °C), and finally the highly activated HPC-S sample (490 °C). Decreasing oxidation onset values are inversely correlated with increasing surface area for all of the samples, but the HPCG-12-21-5h sample shows relatively good thermal stability despite its extremely high surface area (>2900 m2/g, via Brunauer−Emmett−Teller or BET theory). The HPC-G-1221-5h sample, despite having roughly twice the surface area and pore volume of the KB, has an oxidation onset value separated by only ∼25 °C from the KB sample and within 70 °C of the nonporous graphite powder. Such impressive thermal stability at such a high SSA is indicative of a more ordered graphitic structure, further corroborating the same conclusions from the Raman and HRTEM analysis previously discussed. HPC-G Materials with Very High Surface Area and Pore Volume as Supercapacitor Electrodes and Oil Adsorbents. To demonstrate the HPC-G materials’ high specific surface area, the application of a supercapacitor electrode was chosen. Supercapacitors (SCs) store energy via electrical double layer charge accumulation and thus require an electrically conductive material with high specific surface area to achieve higher performance, making nanoporous carbons with high surface area a natural choice.1,4 Additionally, to demonstrate the HPC-G materials’ colossally high pore volume values and graphitic/hydrophobic nature, we tested the materials as oil adsorbents. Due to the HPC-G’s structure, we were required to use two different commercial carbons as baselines for comparison: one with high SSA (Norit AC) and another with high pore volume (KB-600JD). For the supercapacitor electrode application, a solid membrane incorporated with an ionic liquid (IL)-based electrolyte, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4), was chosen, in a symmetrical two-electrode

the HPC-G-1000C sample’s PSD (by volume) reveals mesopores centered at ∼15 nm. After pyrolysis at 1600 °C, the PSD broadens slightly to larger mesopores, increasing the mesopore volume. Furthermore, there is a smaller 2−3 nm peak also present in the PSD (Figure 2a,b) in addition to the 15 nm peak. Upon activation of the HPC-G-12-21 sample, that smaller 2−3 nm peak becomes more prominent, while another peak centered at ∼8 nm emerges from the shoulder of the unactivated HPC-G-12-21 sample. There are also micropores that contribute to the higher SSA values that, while beyond the limit of the Barrett−Joyner−Halanda (BJH) analysis, can be measured by way of the calculated t-plot micropore surface area. Figure 2c reveals that roughly 82% (9.21 cm3/g) of the pore volume is from pores of ≤100 nm, and roughly 70% of the pore volume arises from pores sized at ≤50 nm (∼7.78 cm3/g). Removing the pore volume calculated via the t-plot analysis indicates that the mesopore volume is 7.53 cm3/g, extremely high, especially considering the graphitic nature of the material. Although the HRTEM images (vide supra) reveal good graphitic ordering, bulk characterization techniques such as Raman spectroscopy (Figure 3) were also employed to explore the nature of the chemical bonds for the various porous carbons. As can be seen from the Raman spectra data, all the carbons tested had the characteristic D (at ∼1350 cm−1) and G (at ∼1582 cm−1) bands that are present in many carbon materials.39,40 The G band (named after graphite) is common to all sp2 carbon systems, whereas the D band is indicative of a hybridized vibrational mode related to the graphene edges and thus reveals the presence of disorder in the graphene structure. Although the ratio of G band to D band can be a good indicator of the level of ordering/graphitization in carbon materials (as shown in the graphite powder sample), a nonporous microcrystalline natural graphite sample will have a much larger D band intensity when compared to its G band,39 due to the small crystal grains in the structure. Thus, despite being naturally occurring graphite, these small graphitic domains can lead to a misdiagnosis of the material being nongraphitic if one only analyzes the ratio of the two bands. Similarly, a highly porous graphitic carbon can be thought to be comprised of “small crystalline domains”, with the well-ordered/graphitic carbon located in between the small nanometer pores. Thus, the narrowness of the G and D peaks (indicating crystallinity) will then provide a good indication of the graphitic ordering for the porous carbons. In addition to the narrow crystalline D and G bands, the presence of a G′ band (∼2700 cm−1) will also indicate that the carbon material is graphitic.39,40 Compared to the well-defined and separated D and G bands present in the graphite powder, The HPC-G sample pyrolyzed to 1000 °C shows very broad bands with little separation of the two bands, consistent with a carbon having many functional groups, as confirmed via XPS (Supporting Information, Table S3). Table 1 shows the full width at half-maximum (fwhm) values (see Supporting Information, Figure S5 for peak fit details) for both G and D bands for the remaining porous carbons. For the G band, the values are all quite similar, ranging from 67 to 79 cm−1. The D band fwhm values are more telling and indicate a good graphitic ordering for the nonactivated and highly activated HPC-G samples (90 and 85 cm−1, respectively). Commercial porous carbons were also investigated and found to have a wider D band, indicative of a more disordered atomic structure (fwhm values of 120 and 140 cm−1 for the Norit AC and KB-600JD samples, respectively). The HPC-S samples, being essentially hard carbons,25,41 have the broadest D bands 11051

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Figure 4. Comparison of high surface area commercial carbon (Norit AC) and HPC-G-12-21-5h as electrochemical capacitor electrodes. Specific capacitance values were obtained from (a) galvanostatic charge−discharge curves and (b) were plotted as a function of rate (based on active material). (c) GCD curves for both materials at a rate of 0.6 A/g reveals the superior rate performance imbued by the morphology of the HPC-G; further corroborated by (d) cyclic voltammetry curves at a rate of 10 mV/s.

setup (each at ∼3.5 mg/cm2 loading of active material; see Supporting Information for further setup details). An IL-based electrolyte was primarily chosen to measure and compare the specific capacitance at a low rate where the porous carbons’ SSA would dictate most of the SC performance and, second, at higher rates to see the effects of bulky IL ions in an open hierarchical structure versus a more ramified microporous structure such as for traditional activated carbons (Norit AC). It should be noted, however, that such an exploration was limited in scope for this article, and the SC electrode was primarily utilized to comparatively demonstrate the very high SSA of our HPC materials alongside another high SSA porous carbon. This is particularly relevant considering the use of solid film membranes being utilized as an electrolyte, compared to the more commonly utilized liquid IL electrolytes employed in the literature. Figure 4 shows the results of the supercapacitor experiments. At a very low rate of 0.1 A/g, the galvanostatic charge− discharge (GCD) curves (Figure 4a) reveal stable supercapacitor performance. At such low rates, the bulky IL ions can more easily find the high surface area sites on both the Norit AC and HPC-G electrodes and the SSA plays a greater role in the measured specific capacitance values of the carbons. At 0.1 A/g, the specific capacitance values for the HPC-G-12-21-5h (henceforth referred to as HPC-G for brevity) and Norit AC samples are roughly 90 and 75 F/g, respectively, in good agreement with the SSA values for the respective carbons (Table 1). Interestingly, even at the low GCD rate of 0.1 A/g, the Norit AC has a noticeable increased IR drop when compared to that of the HPC-G material (0.23 and 0.07 V, respectively). We attribute this increased IR drop for the Norit AC material to the increased electrical conductivity from the ordered graphitic nature of the HPC-G material, since differences in morphology are minimized at such a low rate. Figure 4b shows the specific capacitance of the two carbons as a

function of rate. When the rate is doubled (0.2 A/g), the HPCG electrode maintains most of its capacitance at a value of ∼88 F/g, which, despite the limitations imposed by the solid electrolyte, performs surprisingly well when compared to other similar IL electrolyte SC systems (Table S4), even with ILs in liquid form (and sometimes with organic solvents added).42 Relative to the HPC-G material, the Norit AC electrode starts to lose capacitance relatively quickly at higher rates, culminating in a dramatic drop off in capacitance (∼39 F/g, or ∼52% of the original value) at 0.6 A/g. The HPC-G material retains more of its capacitance (∼81 F/g or ∼90%) at the same 0.6 A/g rate and beyond, as at 0.8 A/g, we were unable to calculate the capacitance for the Norit AC. At 0.6 A/g, the GCD curves (Figure 4c) for both the Norit AC and HPC-G electrodes show an increased IR drop of 1.32 and 0.37 V, respectively. Thus, it is clear that at 0.6 A/g the ions are difficult to move within the high surface area sorption sites of the Norit AC electrode, whereas they are easy to move within the HPC-G electrode. Additionally, the HPC-G electrodes still maintain good symmetry for its GCD curve at 0.6 A/g, indicative of its better performance at higher rates. This is further corroborated via cyclic voltammetry (Figure 4d), where the more distorted rectangular shape of the Norit is indicative of the poor ion transport in the purely microporous material. These promising results show the superior benefits of an HPC-G morphology as potential electrodes in IL-based SCs. Further testing for longterm cyclic stability performance shows good capacity retention (∼92%) at 10 000 cycles, typical for a supercapacitor (Figure S9). Furthermore, although beyond the scope of this work, the ability to control the ultimate carbonization temperature for the HPC-G precursor can be utilized in future work, as the lower pyrolysis temperature of 1000 °C leads to a porous carbon material with abundant oxygen (∼3.8 at. %) and nitrogen groups (∼6.6 at. %), as shown on Table S3. An intrinsic electrochemical activity of the electrode that contains an HPC 11052

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ACS Nano material (HPC-G-1000C) evaluation via cyclic voltammetry showed the presence of additional Faradaic activity, attributed to the presence of oxygen and nitrogen functional groups (see Figure S10). To demonstrate the colossally large void space for the HPCG materials, they were further investigated as oil sorbents. Recently, many efforts have been made to utilize porous carbon materials for oil adsorption.43,44 Two of the main prerequisites for good oil sorption performance in porous carbons are hydrophobicity and available void space (pore volume), and thus, oil adsorption was chosen as an application to demonstrate the extremely high pore volume of the HPC-G materials. KB-600JD was chosen as the commercial carbon for comparison, due its high pore volume (∼4.75 cm3/g) and good hydrophobicity, the latter corroborated by its low oxygen content (1.4 at. % via XPS, Table S3). The samples were examined for oil adsorption by utilizing 100−200 mg of the porous carbon material in a Petri dish, whereupon motor oil was pipetted, dropwise, onto the carbon until saturated (see Supporting Information for details and optical images in Figure S8). It was found that the KB-600JD sample could adsorb ∼20 times its own mass in oil, relatively high when compared to a low pore volume porous carbon such as Norit AC, which could only adsorb ∼6 times its weight. The unactivated HPC-S-4-21 sample (measured total BJH pore volume of 4.78 cm3/g) was found to adsorb ∼25 times its own weight, slightly higher than the KB-600. For the HPC-G system of materials the oil adsorption results were even higher. The unactivated HPC-1221 had a very high oil sorption capability of ∼45 times its own weight. Note that this high oil adsorption performance comes from a material with a relatively lower pore volume of 2.44 cm3/g. We believe this is due to the relatively high hydrophobicity of the unactivated HPC-G-12-21 material, as evidenced by its very low oxygen content (0.5 at. % via XPS). After extensive activation, the HPC-G-12-21-5h material is still quite hydrophobic, but the process does impart additional oxygen groups, which increases its elemental concentration of oxygen to 2.0 at. % (Supporting Information, Table S3). Despite the slight increase in hydrophilicity, the colossally high pore volume of >10 cm3/g combined with a still relatively high hydrophobic nature allows the HPC-G-12-21-5h sample to adsorb 65 times its own weight in oil, quite high considering recent results for similar materials.43,44

direction in the rational design of highly porous graphitic carbon materials.

METHODS Synthesis of the HPC-S materials follows an optimized route based, in part, on our previous work,29 which is addressed in detail in the Supporting Information. Synthesis of the HPC-G materials involved using a triple templating strategy whereby ice templating, a colloidal silica hard template, and CO2 activation were employed for the formation and control of macropores (>50 nm), mesopores (2−50 nm), and micropores (2500 m2/g via BET) and large pore volume (>10 cm3/g via BJH) in one material, a combination that has not been previously accomplished. The incorporation of ice templating was found to play a highly significant role in enhancing these advanced textural characteristics. This synthesis allows for the tunability of not only the porous morphology but also the adjustability down to the atomic structure, permitting the same high textural characteristics in a graphitic/ordered highly porous carbon material. Utilizing one single HPC-G material, we were able to outperform two separate commercial carbons, specifically selected for either high specific surface area or pore volume in applications that favor each specific textural property. The potential applications presented in this work demonstrated the advantageous textural properties of the HPC-G material, while the synthesis strategy presented herein leads to an excellent

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b05085. Additional figures and tables (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Luis Estevez: 0000-0002-8057-1078 Venkateshkumar Prabhakaran: 0000-0001-6692-6488 Ji-Guang Zhang: 0000-0001-7343-4609 Notes

The authors declare no competing financial interest. 11053

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ACKNOWLEDGMENTS L.E. would like to acknowledge the financial support of the Linus Pauling Distinguished Postdoctoral Fellowship program, a Laboratory Directed Research and Development program at Pacific Northwest National Laboratory, a multiprogram national laboratory operated by Battelle for the U.S. Department of Energy (DOE) under contract no. DE-AC05-76RL01830. J.G.Z. is financially supported by the Office of Vehicle Technologies of the U.S. DOE through the Advanced Battery Materials Research (BMR) program under contract no. DEAC02-05CH11231, subcontract no. 18769. A portion of the research was performed using Environmental Molecular Sciences Laboratory (EMSL), a National Scientific User Facility located at Pacific Northwest National Laboratory (PNNL) and supported financially by the U.S. DOE Office of Biological and Environmental Research, as well as the DOE, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences & Biosciences Division. The authors would also like to thank M. E. Gross for aid with experimental setup, Y. Cheng for help in supercapacitor electrode fabrication, and B. A. Legg and G. Zhu for TEM assistance. REFERENCES (1) Dutta, S.; Bhaumik, A.; Wu, K. C.-W. Hierarchically Porous Carbon Derived from Polymers and Biomass: Effects of Interconnected Pores in Energy Applications. Energy Environ. Sci. 2014, 7, 3574−3592. (2) Tan, K. W.; Jung, B.; Werner, J. G.; Rhoades, E. R.; Thompson, M. O.; Wiesner, U. Transient Laser Heating Induced Hierarchical Porous Structures From Block Copolymer−Directed Self-Assembly. Science 2015, 349, 54−58. (3) Hao, P.; Zhao, Z.; Leng, Y.; Tian, J.; Sang, Y.; Boughton, R. I.; Wong, C. P.; Liu, H.; Yang, B. Graphene-Based Nitrogen Self-Doped Hierarchical Porous Carbon Aerogels Derived from Chitosan for High Performance Supercapacitors. Nano Energy 2015, 15, 9−23. (4) Cheng, P.; Gao, S.; Zang, P.; Yang, X.; Bai, Y.; Xu, H.; Liu, Z.; Lei, Z. Hierarchically Porous Carbon by Activation of Shiitake Mushroom for Capacitive Energy Storage. Carbon 2015, 93, 315−324. (5) Ma, Y.; Ma, C.; Sheng, J.; Zhang, H.; Wang, R.; Xie, Z.; Shi, J. Nitrogen-Doped Hierarchical Porous Carbon with High Surface Area Derived from Graphene Oxide/Pitch Oxide Composite for Supercapacitors. J. Colloid Interface Sci. 2016, 461, 96−103. (6) Sahore, R.; Estevez, L. P.; Ramanujapuram, A.; DiSAlvo, F. J.; Giannelis, E. P. High-Rate Lithium-Sulfur Batteries Enabled by Hierarchical Porous Carbons Synthesized Via Ice Templation. J. Power Sources 2015, 297, 188−194. (7) Werner, J. G.; Johnson, S. S.; Vijay, V.; Wiesner, U. CarbonSulfur Composites from Cylindrical and Gyroidal Mesoporous Carbons with Tunable Properties in Lithium-Sulfur Batteries. Chem. Mater. 2015, 27, 3349−3357. (8) Liang, J.; Zhou, R. F.; Chen, X. M.; Tang, Y. H.; Qiao, S. Z. Fe-N Decorated Hybrids of CNTs Grown on Hierarchically Porous Carbon for High-Performance Oxygen Reduction. Adv. Mater. 2014, 26, 6074−6079. (9) Liang, H.-W.; Zhuang, X.; Bruller, S.; Feng, X.; Mullen, K. Hierarchically Porous Carbons with Optimized Nitrogen Doping as Highly Active Electrocatalysts for Oxygen Reduction. Nat. Commun. 2014, 4973, 1−7. (10) Srinivas, G.; Krungleviciute, V.; Guo, Z.-X.; Yildirim, T. Exceptional CO2 Capture in a Hierarchically Porous Carbon with Simultaneous High Surface Area and Pore Volume. Energy Environ. Sci. 2014, 7, 335−342. (11) Gadipelli, S.; Patel, H. A.; Guo, Z. An Ultrahigh Pore Volume Drives Up the Amine Stability and Cyclic CO2 Capacity of a SolidAmine@Carbon Sorbent. Adv. Mater. 2015, 27, 4903−4909. 11054

DOI: 10.1021/acsnano.7b05085 ACS Nano 2017, 11, 11047−11055

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DOI: 10.1021/acsnano.7b05085 ACS Nano 2017, 11, 11047−11055