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Hierarchically Porous Carbon Materials for CO Capture: The Role of Pore Structure 2

Luis Estevez, Dushyant Barpaga, Jian Zheng, Sandip R Sabale, Rajankumar L Patel, Ji-Guang Zhang, B. Peter McGrail, and Radha Kishan Motkuri Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03879 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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Industrial & Engineering Chemistry Research

Hierarchically Porous Carbon Materials for CO2 Capture: The Role of Pore Structure Luis Estevez*†‡, Dushyant Barpaga†, Jian Zheng, Sandip Sabale¶, Rajankumar L Patel, Ji-Guang Zhang, B. Peter McGrail and Radha Kishan Motkuri* Pacific Northwest National Laboratory, 902 Battelle Boulevard, P.O. Box 999, Richland, WA 99352 (USA) ‡

Current address: Energy Technologies and Materials Division, University of Dayton Research

Institute, 300 College Park Avenue, Dayton, Ohio 45469, United States ¶

Current address: P.G. Department of Chemistry, Jaysingpur College, Jaysingpur 416101,

Maharashtra, India. KEYWORDS: hierarchically porous carbon, pore structure, surface area, pore volume, CO2 capture

ABSTRACT: With advances in porous carbon synthesis techniques, hierarchically porous carbon (HPC) materials are being utilized as relatively new sorbents for CO2 capture applications. These HPC materials were used as a platform to prepare samples with differing textural properties and morphologies to elucidate structure-property relationships. It was found

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that high microporous content, rather than overall surface area was of primary importance for predicting good CO2 capture performance. Two HPC materials were analyzed, each with near identical high surface area (~2700 m2/g) and colossally high pore volume (~10 cm3/g), but with different microporous content and pore size distributions, which led to dramatically different CO2 capture performance. Overall, large pore volumes obtained from distinct mesopores were found to significantly impact adsorption performance. From these results, an optimized HPC material was synthesized that achieved a high CO2 capacity of ~3.7 mmol/g at 25°C and 1 bar.

INTRODUCTION The burning of fossil fuels for energy accounts for just over two-thirds of the globally produced anthropogenic greenhouse gas (GHG) emissions. Of these GHG emissions, 90% are attributed to carbon dioxide (CO2).1 Though progress has been made in moving away from CO2 producing fossil fuels towards more renewable-based sources of energy,2,

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the capture and

sequestration of CO2 from fossil fuel-based power plants (typically coal) can be seen as a short term solution to mitigate current emission levels. Carbon capture and storage (CCS) technologies can also play a role in the utilization of natural gas as a “clean” energy source,4 given its low ratio of CO2 emitted per unit of energy generated.5 Unfortunately, these natural gas wells typically contain 10-20 mol% CO2 (up to 70 mol% for some wells),6 thus requiring CCS technology on-site. To address these CCS requirements, current state of the art efforts have focused on liquid amine scrubbing. In this process, the CO2 gas is passed through an aqueous amine solution where it is captured via a well understood reversible chemical reaction mechanism.7 To release the CO2 gas (regeneration), energy is supplied in the form of heat. Although these solution-based amine scrubbers are effective at


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capturing CO2, they suffer in application due to several issues such as limited surface utilization, corrosion, volatilization/degradation, and a significant energy penalty associated with material regeneration.8 The energy cost raises the price of energy production and thus, alternative capture and storage solutions are desired.9 This need has spurred a great deal of research directed at finding alternative approaches to CCS involving various novel adsorbent materials and technologies.9,

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Nanoporous materials

relying on physical adsorption have been heavily explored for CO2 capture applications in recent years.11-13 Specifically, commonly utilized solid sorbents for various adsorption/separation studies include zeolites,14, 15 metal-organic frameworks (MOFs),16-20 and porous carbons.21, 22 Of the various solid sorbents, carbon materials are one of the most utilized as they are easily obtainable, chemically and hydrothermally robust, and can have high surface area—usually in the form of activated carbons (ACs). There are many examples in the literature of various ACs employed as sorbents for CO2 capture; and though results are presented at various pressures and temperatures, most researchers show results including a temperature of 25°C and a pressure of 1 bar, which allows for ease of comparison amongst the proposed materials—as lower temperatures and higher pressures both lead to improved CO2 capture capacities. Typical results for commercially available ACs at 25°C and 1 bar (such as Maxsorb AC)23 vary from ~1.5 mmol/g to roughly 2.5 mmol/g, with most ACs typically in the middle at ~2 mmol/g.9,

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Recent examples in the literature of activated

almond shells or olive stones show optimized surface areas (via BET method) of 862 m2/g and 1215 m2/g, leading to carbon capture capacities of 2.7 mmol/g and 3.1 mmol/g, respectively.25 Current advances in porous carbon synthesis techniques have allowed researchers to achieve even higher specific surface area (SSA) carbons, even beyond 4000 m2/g. Jalilov et al.21


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demonstrated a porous carbon with ~4200 m2/g that was explored for high pressure CO2 adsorption and still achieved ~2.6 mmol/g capacity at 1 bar and 23°C. Others have utilized high surface area ACs combined with nitrogen groups to improve upon the CO2 capacity. Zhang et al.26 utilized various N doping methods on a high SSA AC (2350 – 2829 m2/g) to obtain high CO2 capture performances of 2.92 – 3.75 mmol/g; while He et al.6 synthesized an ultra-high SSA (4196 m2/g) polyaniline-based porous carbon with nitrogen groups to achieve an extremely high CO2 capture performance of 4.3 mmol/g. An interesting recent push to synthesize hierarchical porous carbon (HPC) materials using covalent organic frameworks by Shengqian Ma et al27, 28 creates extra pores that has allowed them to obtain porous carbons with good CO2 adsorption performance. Srinivas et al.24 have demonstrated record high adsorption characteristics for HPC materials at high pressures (>27 mmol/g at 30 bar) that were attributed to the unique combination of high surface area (2734 m2/g) and high total pore volume (5.23 cm3/g) that are typically unique to HPC materials. The same HPC material also showed good performance relative to the current literature at lower pressures (3.10 mmol/g at 25°C, 1bar) indicating that the larger pore volume, combined with the large surface area could be of benefit at even low pressures. Recently, our group has synthesized an HPC material with a tunable morphology,5,

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containing both high SSA (~2700 m2/g) and colossally large pore volume values of over 10 cm3/g. Herein we reveal how we were able to exquisitely control the textural characteristics of our HPC materials, enabling their use as solid sorbents for CO2 capture. We also show how despite having similar textural characteristics (SSA and PV) for two of the HPC materials, the performance for each varies dramatically due to differences in the pore size distribution (PSD). The elucidation of these and other structure-property relationships, allowed us to achieve a high CO2 capture performance for a morphologically optimized HPC material. We believe this work


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gives insight into material design for achieving good solid-based CCS performance and lays a foundation for future work for the utilization of emerging HPC materials in this field.

MATERIALS AND METHODS Materials The silica utilized for the hard template was a 4 nm colloidal silica, 15 wt.% suspension (Alfa Aesar). Sucrose (>99%) was purchased from Sigma Aldrich. The CO2 and N2 gases utilized for activation and adsorption measurements were purchased at a purity of >99.9% from OXARC Inc. Synthesis of HPCs HPC materials were synthesized based on procedures described in recent work that outlines the synthesis procedure and provides characterization data in greater detail beyond the scope of this work.5,

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Briefly, three different templating strategies were utilized to obtain three distinct

porosity regimes. Ice templating,30 hard colloidal silica templating5,

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and physical activation

(via CO2);32 were used to imbue macropores (>50 nm), mesopores (2-50 nm) and micropores (20 h. Isotherms were measured at 77 K and the specific surface area was calculated using the Brumauer-Emmett-Teller (BET) method, (7 points). The pore volume was estimated from single point adsorption at a targeted relative pressure of 0.995. The pore size distribution was determined from the adsorption branch, according to the Barrett-Joyner-Halanda (BJH) method (Figure 1c and 1d) as well as the nonlocal density functional theory (NLDFT) equilibrium model (Figure S3). The NLDFT model was evaluated using nitrogen as the adsorbate at 77K on a characteristic carbon assuming slit and cylindrical pore geometries. Scanning electron microscopy (SEM) images were acquired with a JEOL-5900 microscope utilizing a tungsten filament at ~5 keV, at a working distance of ~12 mm. Transmission electron microscopy (TEM) images were obtained using a Tecnai F20 TEM/STEM from Fisher. The F20 is a 200 kV field emission TEM. Digital imaging was accomplished via an eagle camera and TIA software. Sample preparation consisted of adding the sample (dried) into a vial of ethanol. The mixture was sonicated for 2-5 min, whereupon the sample was then mixed by shaking the vial. The larger particles were allowed to settle and the dilute solution at the top of the vial was pipetted out and placed on a carbon-film-containing copper TEM grid (EMS).

RESULTS AND DISCUSSION


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Three HPC materials were synthesized with varying pore characteristics including HPC-1-Act, HPC-1-OA and HPC-2-Act. In order to evaluate the textural properties for the synthesized materials, two commercial carbons were also considered for comparison. These included NORIT AC and KB600JD. Textural characterization results and adsorption analysis was performed on each of these five samples to elucidate the effect of differing porosity on CO2 adsorption. Textural Characterization The N2 adsorption isotherms and PSD of the various carbons utilized for this study are shown in Figure 1. The synthesized HPC-1-Act and HPC-2-Act carbons display a relatively uniform, well defined mesoporous structure (Figure 1c) unlike the commercial materials (Figure 1d) and the HPC-1-OA sample. Smaller pores were observed in HPC-2-Act (~7 nm), as compared to the HPC-1-OA (~40 - 50 nm) and HPC-1-Act (~40 nm) materials, as confirmed via transmission electron microscopy (Figure 2). Although the PSDs of the two HPC-1 samples are similar, noticeable shifts between the two suggest that as a result of over activation, small mesopores have merged to form more macropores, resulting in a broadened PSD curve in general for HPC1-OA (Figure 1c). These macropores are in addition to the larger macropores (1 – 10 µm) present for the HPC-1-Act (Figure 2a1), HPC-1-OA (Figure 2b1), and HPC-2-Act (Figure 2c1) samples that were outside of the limits of the nitrogen porosimetry characterization. Alternatively, the SEM images for the KB 600JD (Figure 2d1) and Norit AC (Figure 2e1) samples reveal a nondescript powder for each of the commercial carbons used.


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Figure 1. Nitrogen porosimetry results for the various carbons utilized in this study. N2 sorption isotherms (77K) for the (a) synthesized HPC materials and (b) commercial carbons and the derived adsorption-based BJH pore size distributions for (c) HPCs and (d) commercial carbons are shown.


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Figure 2. (1) SEM, (2) TEM and (3) higher magnification TEM for the (A) HPC-1-Act, (B) HPC-1-OA, (C) HPC-2-Act, (D) KB 600JD and (E) Norit AC carbon samples.


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Table 1 shows the textural characteristics for each carbon studied. As can be seen, for the three synthesized HPC materials, the surface areas are all quite similar ranging from 2560 m2/g to 2772 m2/g. Interestingly, even with the clear differences between the PSDs of the two HPC-1Act and HPC-1-OA samples (Figure 1, vide supra), the surface area and pore volume values were determined to both be extremely close in their measured values. Differences in total pore volume exist for the HPC materials, with the HPC-1-Act and HPC-1-OA having very similar PV values of ~10 cm3/g, whereas the HPC-2-Act sample had roughly half of the previous sample’s PV value at 4.31 cm3/g. The microporous SSA (as calculated via t-plot method) for the HPC-1Act and HPC-2-Act samples are similarly high, as would be expected for samples with optimized activation, resulting in microporous SSA values of 456 m2/g and 499 m2/g, respectively. For the over activated HPC-1-OA sample, the microporous content plummets to roughly half (255 m2/g), due to the collapse of smaller pores, as explained previously. Table 1. Textural characteristics and CO2 sorption data for the various porous carbons. BET SSA [m2/g]

Micropore SSA [m2/g]a

Total pore volume [cm3/g]b

Micropore volume [cm3/g]a

Carbon uptake [mmol/g]c

HPC-1-Act

2698

456

10.3

0.227

3.7

HPC-1-OA

2772

255

10.0

0.120

1.9

HPC-2-Act

2560

499

4.31

0.251

2.5

Norit AC

1948

389

1.14

0.209

2.6

KB 600JD

1477

43

4.75

0.014

1.6

Porous carbon

a

Microporous content determined via the t-plot method, b total pore volume obtained via nitrogen uptake at the single point P/Po value of ~0.995, c at 25°C, 1 bar Two commercially available porous carbon materials were also included in our study for comparison with the synthesized HPC materials—Ketjenblack-600JD (KB 600JD) and Norit


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DLC Supra 30 (Norit AC). As can be seen from their respective isotherms (Figure 1b), both carbons are distinctly different. The Norit AC is a microporous carbon as evidenced by its type I isotherm, whereas the KB 600JD sample has a type IV isotherm indicative of a mesoporous carbon. Interestingly, the isotherm for KB 600JD seems to have two different hysteresis regimes; one from roughly P/Po 0.4 – 0.9 and another at P/Po of 0.9 – 1.0. These regimes can be distinguished by the resulting PSD (Figure 1d), where two distinct peak profiles are observed: a broad one from 2 - 40 nm and another, much larger macroporous one that starts at ~50 nm and goes on beyond the limits of nitrogen porosimetry (>100 nm). The Norit AC is a classic microporous material with a t-plot SSA value of 389 m2/g, while the KB 600JD has very little microporosity, with a t-plot estimate of only 43 m2/g. This results in two very different materials for comparison with our HPCs: the Norit AC, a high surface area (1948 m2/g) low pore volume (1.14 cm3/g) microporous carbon; and the KB 600JD, a mesoporous/macroporous carbon with good SSA (1477 m2/g) and very high pore volume (4.75 cm3/g), with almost no microporosity. These commercial carbons, when combined with the HPCs, provided us the means to compare various carbons with divergent morphologies as CCS materials, from which to better elucidate structure-property relationships and ultimately provide insight into material design criteria for these potential applications.

CO2 Adsorption When utilized as solid sorbents, certain trends emerge based on the carbons’ various morphologies. As it has been noted in the literature,9 the CO2 capture performance tends to scale with surface area (Table S1). However, the CO2 adsorption performance (measured at 25°C and an ultimate pressure value of 1 bar) for the various carbons studied (Figure 3a) reveals that the


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microporous/small mesoporous SSA (obtained via t-plot method on Table 1) is a better predictor for performance. The nitrogen sorption (Figure 3b) for the same samples at the same conditions shows negligible uptake compared to CO2, indicative of the good separation potential of carbon based adsorbents. Of the carbons studied, the KB 600JD has the poorest adsorption performance at 1.6 mmol/g. Despite having a good SSA of 1477 m2/g, it has hardly any microporosity (microporous SSA of 43 m2/g) leading to relatively low performance when compared to other carbons of similar SSA.10 For the HPC-2-Act and Norit AC samples, both have high surface areas of 2560 m2/g and 1948 m2/g (microporous SSA of 499 m2/g and 389 m2/g), and observed to have similar CO2 capture performances (2.5 mmol/g and 2.6 mmol/g). The NLDFT model was utilized (Figure S3) to verify the microporous content found via t-plot method and to see the PSD at the microporous (and very small mesoporous) regime where the BJH method has limitations. The NLDFT PSD data supports the t-plot revealing a drop off in microporosity from the over activation of the HPC-1-Act to the HPC-1-OA sample as evidenced by the sharp decrease in both the sub 1.5 nm microporous peak, as well as the ~1.7-3 nm peak that is right on the micro/mesoporous boundary (Figure S3a). Also, the NLDFT PSD curves for the commercial carbons are in line with the t-plot data. The KB 600JD sample has no real microporosity content unlike the Norit AC sample that is heavily microporous with a sharp peak at ~1.2 nm and another peak at 2-3 nm (Figure S3b). Although the t-plot estimation shows that the microporosity is higher for the HPC-2-Act sample versus the Norit AC, the NLDFT PSD reveals that the mesopores present in the Norit AC are just over the micro/mesoporous boundary at a peak centered at 2-3 nm. This analysis reveals that the overwhelming majority of the pores found in the Norit are ≤ 3 nm as opposed to the HPC-2-Act, which has a predominant peak at ~7 nm. This is evidence of the importance of sorbent-sorbate interactions as a major influence for CO2


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capture. Even though the HPC-2-Act has a higher t-plot based microporous SSA, total surface area and pore volume than the Norit AC, the fact that the Norit is made up exclusively of sub 3 nm pores leads to a slightly better CO2 capture performance (2.6 versus 2.5 mmol/g).

Figure 3. The (a) CO2 and (b) N2 adsorption/desorption isotherms (25°C, Po = 1 bar) for the various porous carbon materials investigated in this work. Interesting total pore volume values were observed, with the HPC-2-Act having a similar pore volume to KB 600JD (4.31 and 4.75 cm3/g, respectively) and both having nearly four times the void space relative to the Norit AC (1.14 cm3/g). This difference is due to the PSD for each (Figures 1c-d) with the Norit being made up of mostly micropores with some mesopores present at just over the 2 nm micro/mesoporous threshold (Figure S3b). Alternatively, although the HPC2-Act sample also has small mesopores, they are distinct from the micropores and are present as a ~7 nm peak (Figure 1c) and contribute quite a bit more towards the pore volume. The large pore volume value for the KB 600JD sample is due to a very broad mesoporous peak as characterized, vide supra. It is interesting to note that in addition to the similarities in microporous content for the HPC-2-Act and Norit AC samples, the two samples also have a narrower PSD when compared to that of the KB 600JD.


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CO2 adsorption performance comparing the HPC-1-Act and HPC-1-OA samples also provided unique insight (Figure 3). Compositionally, the HPC-1-Act and HPC-1-OA samples are very similar, in that the HPC-1-OA sample is merely HPC-1-Act, which has been activated until the micropores (Figure S3a) and smaller mesopores (Figure 1c) start collapsing. Additionally, both materials show overall surface areas of ~2700 m2/g and pore volumes of ~10 cm3/g, (within 3% of each other). The only difference observed is the loss of microporous content in HPC-1-OA (255 m2/g) over HPC-1-Act (456 m2/g) due to over activation observed directly in Figure S3a. It is speculated that this microporous/small mesoporous content explains the dramatic difference in CO2 capture performance between the two carbons. The HPC-1-OA material had an ultimate CO2 capture performance of 1.9 mmol/g, somewhat lower than the values seen for ACs— typically ~2 mmol/g.23,

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Alternatively, the HPC-1-Act material had nearly double the CO2

capture ability at 3.7 mmol/g, a very high amount. The carbon capture performance for the HPC1-Act sample is among the highest seen in the literature for porous sorbents (Table S1) and even rivals that of the highest performing nitrogen doped carbons,6 while surpassing other notable high surface area N-doped porous carbons;26 despite no nitrogen being present in the HPC samples due to their synthesis scheme and utilization of sucrose as a carbon precursor.5, 29 The effect of the porous carbon’s pore volume also seems to play a role in the CO2 capture performance in addition to the microporous content. As it has been rare to encounter porous carbon materials with both simultaneously high SSA and PV, this aspect of the structureproperty relationship has not yet been thoroughly studied in the literature. HPC-1-Act and HPC2-Act, both with similar microporous content (456 m2/g and 499 m2/g, respectively), similar sub 3 nm pore size distributions (Figure S3a), similarly narrow mesopore size distributions, and similar total surface areas (~2600 m2/g) were observed to have CO2 uptakes of 3.7 mmol/g and


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2.5 mmol/g, respectively. From our study, the only morphological difference between these two samples is the PSD in the mesoporous regime, with the HPC-1-Act sample having a mesopore size centered at ~40 nm and the HPC-2-Act material having one centered at ~7 nm. This difference in pore size leads to a larger total pore volume for the HPC-1-Act material, roughly double that of the HPC-2-Act. There have been some recent studies using simultaneously high SSA and PV HPC materials as CO2 adsorbents, where the authors have found better CO2 capture due to the larger mesopore volume,24 but the experimental conditions involved high pressure CO2. We have found that for lower pressure CO2 adsorption at 1 bar, a larger mesopore size distribution (and its subsequently higher pore volume) with a distinct and narrow PSD peak, seems to also have the effect of increasing the CO2 sorption performance for HPC materials—all else being equal. This should aid researchers in the future, as more HPCs continue to be utilized as sorbents for CO2 capture.

CONCLUSIONS In summary, we have synthesized HPC materials and taken advantage of the tunability inherent in the synthesis strategy to explore the role of pore structure on HPCs utilized for CO2 capture. Commercially available porous carbons were also utilized for comparison of CO2 performance and to further aid in the elucidation of the structure-property relationships. We found that microporous and small mesoporous content is a much better predictor of CO2 adsorption performance than surface area—a relationship that can be difficult to deconvolute. We accomplished this by utilizing CO2 activation past the optimized point to collapse the small meso- and micropores present in our HPC-1-Act sample which dropped the carbon capture performance nearly in half—despite the surface area and pore volume values for the over


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activated sample being within 3% of the optimally activated sample. The optimally activated HPC-1-Act was found to achieve a very large CO2 capture performance of 3.7 mmol/g, a value amongst some of the higher performing porous carbon sorbents to date. Although microporous/small mesoporous content was found to be the best indicator of carbon capture performance, a large pore volume values originating from a distinct large mesoporous peak was found to improve CO2 performance as well. This work lays the foundation for CO2 capture, utilizing HPC materials—a relatively recent addition to porous carbon solid sorbents. Additionally, we anticipate our HPC martial with colossally large pore volume (~10 cm3/g) combined with a high SSA due to good microporous content, should be good candidates for future work in exploring their use as high pressure CO2 capture materials.

ASSOCIATED CONTENT Supporting Information. Figure S1: Nitrogen porosimetry PSD data for the HPC materials before and after activation. Figure S2: Cumulative pore volume as a function of pore size, via BJH adsorption. Figure S3: Micropore and small mesopore size distribution based on NLDFT equilibrium model. Table S1: Comparison of various porous materials for CO2 adsorption showing the characterized surface area and adsorption conditions. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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*E-mail: [email protected] Notes †

The following authors have contributed equally to the work. All authors declare no competing

financial interest. ACKNOWLEDGMENT L. Estevez would like to acknowledge the support of the Linus Pauling Distinguished Postdoctoral Fellowship program, a Laboratory Directed Research and Development program at Pacific Northwest National Laboratory, a multi-program national laboratory operated by Battelle for the U.S. Department of Energy. A portion of the research was performed using Environmental Molecular Sciences Laboratory. The authors acknowledge U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy’s Geothermal Technologies Office (GTO) for financial support. PNNL is operated by Battelle for the U.S. Department of Energy (DOE) under Contract DE-AC05-76RL01830. REFERENCES 1. Dutta, S.; Bhaumik, A.; Wu, K. C. W., Hierarchically porous carbon derived from polymers and biomass: effect of interconnected pores on energy applications. Energy & Environmental Science 2014, 7, (11), 3574-3592. 2. Wang, L.; Yang, Y.; Shen, W. L.; Kong, X. M.; Li, P.; Yu, J. G.; Rodrigues, A. E., CO2 Capture from Flue Gas in an Existing Coal-Fired Power Plant by Two Successive Pilot-Scale VPSA Units. Industrial & Engineering Chemistry Research 2013, 52, (23), 7947-7955. 3. Huck, J. M.; Lin, L. C.; Berger, A. H.; Shahrak, M. N.; Martin, R. L.; Bhown, A. S.; Haranczyk, M.; Reuter, K.; Smit, B., Evaluating different classes of porous materials for carbon capture. Energy & Environmental Science 2014, 7, (12), 4132-4146. 4. Dalton, S.; Heatley, F.; Budd, P. M., Thermal stabilization of polyacrylonitrile fibres. Polymer 1999, 40, (20), 5531-5543. 5. Estevez, L.; Dua, R.; Bhandari, N.; Ramanujapuram, A.; Wang, P.; Giannelis, E. P., A facile approach for the synthesis of monolithic hierarchical porous carbons - high performance materials for amine based CO2 capture and supercapacitor electrode. Energy & Environmental Science 2013, 6, (6), 1785-1790.


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Hierarchically Porous Carbon Materials for CO2 Capture - American

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