- Email: [email protected]
Charge-modulated CO2 capture
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Review Article Charge-modulated CO2 capture
Xin Tan, Hassan A. Tahini and Sean C. Smith∗ Recently, the concept of using charge-modulation to manipulate the interaction of adsorbate molecules such as CO2 on certain sorbent materials has been advanced through a series of first principle computational predictions. The interactions switched on through charge-modulation shifting the Fermi level are in part, but by no means exclusively, electrostatic. In addition to electrostatics, the Fermi level shifting can be viewed as a way to modulate not only the position but also the identity and character of the frontier orbitals on the sorbent materials that engage with the adsorbate molecules. Thus, a rich new space for electrochemical modulation of surface-molecular interactions—guided by first principle computational modeling—suggests itself. Here we summarize the growing computational literature on switchable CO2 capture strategies. We also provide some new insights into contrasting electrostatic versus chemical responses to charge-modulation as exemplified by water versus CO2 on N-doped graphene surfaces, which suggest that this CO2 capture strategy could be water tolerant.
nitride (h-BN) with boron vacancy [14], and silicon carbide (SiC) nanotubes and nanosheets [15,16], have been demonstrated as promising adsorbents for CO2 capture. However, the CO2 molecules bind strongly to these adsorbents, and the regeneration processes are difficult because of the large adsorption energy. Their practical applications are hampered by the high temperature requirement to release the captured CO2 .
Introduction
The urgent need to reduce CO2 forces scientists to develop improved methods and/or more efficient adsorbents for capturing and/or storing CO2 . A breakthrough came in early experiments by Lee et al. [17•• ] who demonstrated that CO2 exhibits an unusually high binding to anionic pyridine clusters in cold molecular beams. This enhanced CO2 binding to pyridinic-nitrogen was only realized when an excess electron was introduced, thus, negatively charging the cluster (Figure 1A). A similar theme was discovered by Yoon et al. [18• ] in which they theoretically demonstrated that charged fullerenes Cn (20 ≤ n ≤ 82) act as strong sorbents for H2 . More recently, density functional theory (DFT) computations were used to formulate a novel charge-modulated switchable CO2 capture/storage strategy [19• –25• ]. To control the capture/release process, Sun et al. suggested a simple on/off charge switching mechanism in h-BN nanomaterials as shown in Figure 1B [19• ]. However, this can be problematic as h-BN is an insulator with a band gap of about 5.8 eV which poses a challenge on how effectively the material can be charged up. To solve this, Jiao et al. [20•• ] proposed using carbon nanotubes with pyridinic-nitrogen which are conductive and could therefore act as an alternative absorbent to charge-modulated switchable CO2 capture. Following these early investigations, several experimentally feasible materials exhibiting charge-modulation for gas capture have been proposed [21•• –25• ]. Notably, conductive substitutional N-doped graphene (the majority defect, in contrast to the minority pyridinic N defect [20•• ]) has been predicted to be efficacious for chargemodulated CO2 capture [25• ] and conductive substitutional B-doped graphene has been predicted to exhibit charge-modulated H2 storage [26• ].
Carbon dioxide (CO2 ) makes up the vast majority of greenhouse gas emissions, which is one of the leading causes of global warming and climate change [1–4]. Not surprisingly, seeking for efficient, safe, reliable, and costeffective gas-adsorbent materials is a key challenge for efficiently separating, capturing, storing and/or converting CO2 . Several solid materials, such as metal-organic frameworks (MOFs) [5–9], carbon nanotubes [10–12], aluminum nitride (AlN) nanotubes [13], graphene-like boron
The main advantage of the charge-modulated strategy is controllable kinetics and reversibility. In detail, the gas CO2 molecules are weakly adsorbed on certain neutral sorbent materials; however, when excess electrons are introduced into the adsorbent, the adsorption of CO2 molecules can be dramatically enhanced. As the excess electrons are removed, the adsorbed CO2 can in principle be easily released. Different from other methods, the CO2
Address Integrated Materials Design Centre (IMDC), School of Chemical Engineering, UNSW Australia, Sydney, NSW 2052, Australia ∗
Corresponding author: Smith, Sean C. ([email protected])
Current Opinion in Electrochemistry 2017, 4:118–123 This review comes from a themed issue on Electrochemistry
Physical & Nano-
Edited by Feng, Jens For a complete overview see the Issue and the Editorial Available online 17 August 2017 http://dx.doi.org/10.1016/j.coelec.2017.08.006 2451-9103/© 2017 Elsevier Ltd. All rights reserved.
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Figure 1
(A) Neutral (left) and anionic (right) geometries and calculated energies of Pz1 (CO2 )n optimized at the B3LYP/6-31++G∗∗ level. The van der Waals bonds in the neutral complex turn into covalent bonds in the anion (modified with permission from Ref. [17•• ]). (B) Schematic illustrations of charge-modulated CO2 capture on h-BN nanomaterials (modified with permission from Ref. [19• ]).
capture/release process occurs spontaneously once excess electrons are introduced or removed, and the kinetics of uptake and release can be simply controlled by manipulating the charge state in the adsorbent. In principle, this strategy could overcome the “tyranny of thermochemistry”, which typically demands high regeneration costs due to the large CO2 adsorption energy in adsorbents. Water vapor (H2 O) is commonly found coexisting with CO2 in air pre-purification [27] and post-combustion CO2 capture [28]. The real industrial flue gas streams contain 8–10% water vapor, and previous studies have demonstrated that the adsorption process of CO2 capture is greatly affected by the presence of high water levels in flue gas streams [29–32]. Water molecules may compete for adsorption sites, which significantly reduce the CO2 capacity of sorbent materials. So far, competitive coadsorption of water is a major challenge for developing of adsorbent-based CO2 capture. This prompts an interesting question as to whether the charge-modulation switchable CO2 capture strategy is tolerant of water, which we shall see below, goes to the heart of electrostatics versus chemical response mechanisms in this approach.
Charge-modulation effect on adsorption of CO2 /H2 O on N-doped graphene As noted above, conductive N-doped graphene is a promising, fully conductive sorbent material for chargemodulated switchable CO2 capture [25• ] and we use it as an exemplar for the discussion below. Computational methods are as elaborated in detail previously [e.g., 25• ]. In contrast to its adsorption to neutral N-doped graphene (Figure 2A), the CO2 molecule is chemisorbed at an active site—a C atom adjacent to the N-dopant—of negatively charged N-doped graphene (Figure 2B). The adsorption of CO2 is remarkably enhanced as negative charge is added, as shown in Figure 2E. When more than 3 excess electrons are introduced in the supercell, the adsorption energy of a CO2 could increase to larger than −1.5 eV, which is much larger than the adsorption energies of CO2 www.sciencedirect.com
on other high-performance adsorbents (−0.4 ∼ −0.8 eV) [33]. Figure 2 also shows comparative data for the adsorption of water on N-doped graphene. On neutral Ndoped graphene (Figure 2C), geometry optimized H2 O molecule lies flat on the N-doped graphene above the center of a hexagonal ring close to the N-dopant. It is weakly adsorbed (i.e. physisorbed) with a small adsorption energy of −0.20 eV. As excess electrons are introduced, the interaction between H2 O and negatively charged N-doped graphene is significantly enhanced (Figure 2D). Although the H2 O molecule also locates above the center of a hexagonal ring close to N-dopant, it is now perpendicular to negatively charged N-doped graphene with two H atoms pointing downwards. Compared to the neutral case, the adsorption energy of H2 O on negatively charged N-doped graphene increases significantly, as shown in Figure 2E. This indicates that H2 O molecules may compete for active sites on negatively charged N-doped graphene, which may be detrimental to charge-modulated switchable CO2 capture.
Different adsorption characters of CO2 /H2 O on negatively charged N-doped graphene Although the adsorptions of both CO2 and H2 O on Ndoped graphene are remarkably enhanced as excess electrons are introduced, the interactions in each case are fundamentally different in nature. For CO2 on 4e− negatively charged N-doped graphene (Figures 2B and 3B), the CO2 molecule is strongly adsorbed at one C atom adjacent to the N-dopant by forming of a new chemical bond between the C atom of CO2 and surface C atom of N-doped graphene. The O–C–O angle is bent, the distance between the C atom of CO2 and the C atom adjacent to the N-dopant is shortened from 3.196 (neutral N-doped graphene, Figure 2A) to 1.652 A˚ (Figure 2B), and the charge transfer from N-doped graphene to CO2 increases to 0.72e− (Figure 3B). This suggests a chemisorption event, which is very sensitive to the local Current Opinion in Electrochemistry 2017, 4:118–123
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Figure 2
Top (upper) and side (lower) views of the lowest-energy configurations of a single CO2 molecule on (A) neutral and (B) 4e− negatively charged N-doped graphene. Top (upper) and side (lower) views of the lowest-energy configurations of a single H2 O molecule on (C) neutral and (D) 4e− negatively charged N-doped graphene. The blue, grey, red and white balls represent N, C, O and H atoms, respectively. (E) The adsorption energies of a CO2 /H2 O molecule on neutral and negatively charged N-doped graphene as functions of charge states (charge densities). The grey area indicates the chemisorption region of CO2 .
Figure 3
tion originates from the electrostatic interaction between the dipole of H2 O and the negatively charged N-doped graphene. The distance between H atoms of H2 O and negatively charged N-doped graphene is relatively large ˚ and the charge transfer from N-doped (about 2.388 A) graphene to H2 O molecule is only 0.01e− (Figure 3D). Since there is no substantive overlap of the electron densities of H2 O molecule and surface C atom of N-doped graphene, no new chemical bond is formed. The interaction is fundamentally electrostatic and hence is not expected to be strongly dependent on the local atomic environment. Thus, we expect that lateral movement of H2 O on 4e− negatively charged N-doped graphene surface should be relatively facile.
Diffusion barrier of CO2 /H2 O on negatively charged N-doped graphene
atomic environment. In this case, lateral movement of the chemisorbed CO2 molecule should be difficult.
Figure 4 shows the diffusion pathways and the corresponding diffusion barriers of CO2 diffusion on the 4e− negatively charged N-doped graphene surface. Consistent with our expectation, CO2 adsorption energies at different adsorption sites on 4e− negatively charged Ndoped graphene are quite different (Figure 4B): at the active site, i.e. the C atom adjacent to the N-dopant, the adsorption energy is about −1.37 eV, which is 0.46 ∼ 0.54 eV stronger than other adsorption sites. Moreover, we also found that the diffusion barrier of CO2 at the active site is large (0.86 eV). The strong adsorption and large diffusion barrier of CO2 at the active site compared to other adsorption sites indicate that CO2 molecule will bind specifically to the C atom adjacent to the N-dopant.
However, for H2 O on 4e− negatively charged N-doped graphene (Figures 2D and 3D), the enhanced adsorp-
In contrast, the adsorption energies of H2 O at different adsorption sites on 4e− negatively charged N-doped
The total charge density distribution of a single CO2 molecule on (A) neutral and (B) 4e− negatively charged N-doped graphene. The total charge density distribution of a single H2 O molecule on (C) neutral and (D) 4e− negatively charged N-doped graphene. The isosurface value is 0.8 e/A˚ 3 , and the red arrow shows the charge transfer between the N-doped graphene and the gas molecule. The overlap of the electron densities of the C atom of CO2 and surface C atom of N-doped graphene in (B) indicates the formation of a new chemical bond.
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Figure 4
(A) The diffusion pathways (1 → 2 → 3) of CO2 diffusion on the 4e− negatively charged N-doped graphene surface and (B) the corresponding diffusion barriers. The numbers and red arrows in (A) denote different adsorption sites and diffusion pathways considered here. Top and side views of the lowest-energy configurations of a single CO2 molecule adsorbed at (C) 1, (D) 2 and (E) 3 sites on the 4e− negatively charged N-doped graphene. The blue, grey and red balls represent N, C and O atoms.
Figure 5
(A) The diffusion pathways (1 → 2 and 1 → 3 →4) of H2 O diffusion on the 4e− negatively charged N-doped graphene surface and (B) the corresponding diffusion barriers. The numbers and arrows with different colors in (A) denote different adsorption sites and diffusion pathways considered here. Top and side views of the lowest-energy configurations of a single H2 O molecule absorbed at (C) 1, (D) 2, (E) 3 and (F) 4 sites on the 4e− negatively charged N-doped graphene. The blue, grey, red and white balls represent N, C, O and H atoms.
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graphene are very similar with the difference less than 0.03 eV (Figure 5B). Moreover, the diffusion barriers of H2 O on the 4e− negatively charged N-doped graphene surface are less than 0.05 eV (Figure 5B). All of this indicates that there is no specific adsorption site for the H2 O molecule and it can easily undergo lateral diffusion on the 4e− negatively charged N-doped graphene surface.
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In general, although CO2 and H2 O have comparable adsorption energy on negatively charged N-doped graphene, the CO2 molecule specifically binds to an active site, i.e. the C atom adjacent to the N-dopant, while the H2 O molecule can easily diffuse to any adsorption site on negatively charged N-doped graphene surface. Therefore, even when H2 O molecules bind to the surface, they should not block the active sites due to the ease of lateral diffusion to other sites during the CO2 capture process. These results indicate that the negatively charged N-doped graphene should be capable of efficient capture of CO2 molecules even in the presence of H2 O molecules and that the charge-modulated switchable CO2 capture strategy is indeed likely to be tolerant of water.
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Conclusions
10. Cinke M, Li J, Bauschlicher CW, Ricca A, Meyyappan M: CO2 adsorption in single-walled carbon nanotubes. Chem Phys Lett 2003, 376:761–766.
The growing computational literature on the possibilities of exploiting charge-modulation on certain materials to manipulate gas capture and release proffers a fascinating opportunity for experimental studies. Of course, it also offers a challenge in the form of determining precisely how to modulate charge at the heterogeneous solid–gas interface. Charging of electrode surfaces is well understood on one hand in the context of aqueous phase electrochemistry and also on the other hand in the context of device physics; hence one anticipates that cross fertilization of knowledge from these domains may lead to successful realization of charge-modulation gas capture in the near future. The importance of the applications such as CO2 capture and H2 storage are manifest and offer strong motivation to realize the remarkable potential of this approach for superior control and energy/cost savings.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: •
Paper of special interest Paper of outstanding interest.
••
11. Su F, Lu C, Chung A-J, Liao C-H: CO2 capture with amine-loaded carbon nanotubes via a dual-column temperature/vacuum swing adsorption. Appl Energy 2014, 113:706–712. 12. Zhang T, Xue Q, Zhang S, Dong M: Theoretical approaches to graphene and graphene-based materials. Nano Today 2012, 7:180–200. 13. Jiao Y, Du A, Zhu Z, Rudolph V, Smith SC: A density functional theory study of CO2 and N2 adsorption on aluminium nitride single walled nanotubes. J Mater Chem 2010, 20:10426. 14. Jiao Y, Du A, Zhu Z, Rudolph V, Lu GQ, Smith SC: A density functional theory study on CO2 capture and activation by graphene-like boron nitride with boron vacancy. Catal Today 2011, 175:271–275. 15. Zhao JX, Ding YH: Can silicon carbide nanotubes sense carbon dioxide? J ChemTheory Comput 2009, 5:1099–1105. 16. Zhang P, Hou XL, Mi JL, Jiang Q, Aslan H, Dong MD: Curvature effect of SiC nanotubes and sheets for CO2 capture and reduction. RSC Adv 2014, 4:48994–48999. 17••Lee . SH, Kim N, Ha DG, Kim SK: “Associative” electron attachment to azabenzene-(CO2 )n van der Waals complexes: stepwise formation of covalent bonds with additive electron affinities. J Am Chem Soc 2008, 130:16241–16244. Demonstrated that CO2 exhibits a usually high binding to anionic pyridine clusters in cold molecular beams. 18• . Yoon M, Yang S, Wang E, Zhang Z: Charged fullerenes as high-capacity hydrogen storage media. Nano Lett 2007, 7:2578–2583.
Acknowledgments
19• . Sun Q, Li Z, Searles DJ, Chen Y, Lu GM, Du A: Charge-controlled switchable CO2 capture on boron nitride nanomaterials.. J Am Chem Soc 2013, 135:8246–8253.
This research was undertaken with the assistance of resources provided by the National Computing Infrastructure (NCI) facility at the Australian National University; allocated through both the National Computational Merit Allocation Scheme supported by the Australian Government and the Australian Research Council grant LE160100051 (“Maintaining and enhancing merit based access to the NCI National Facility, 2016–2018”).
20••Jiao . Y, Zheng Y, Smith SC, Du A, Zhu Z: Electrocatalytically switchable CO2 capture: first principle computational exploration of carbon nanotubes with pyridinic nitrogen.. ChemSusChem 2014, 7:435–441. This paper reported a conductive absorbent to charge-modulated switchable CO2 capture.
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21••Tan . X, Tahini HA, Smith SC: Computational design of two-dimensional nanomaterials for charge modulated CO2 /H2 capture and/or storage. Energy Storage Mater 2016. http://dx.doi.org/10.1016/j.ensm.2016.12.002. Review article on computational design of two-dimensional nanomaterials for charge modulated CO2 /H2 capture and/or storage. 22• . Tan X, Kou L, Smith SC: Layered graphene-hexagonal BN nanocomposites: experimentally feasible approach to charge-induced switchable CO2 capture. ChemSusChem 2015, 8:2987–2993. 23• . Tan X, Tahini HA, Seal P, Smith SC: First-principle framework for total charging energies in electrocatalytic materials and charge-responsive molecular binding at gas−surface interfaces. ACS Appl Mater Interfaces 2016, 8:10897–10903. 24• . Tan X, Kou L, Tahini HA, Smith SC: Conductive graphitic carbon nitride as an ideal material for electrocatalytically switchable CO2 capture. Sci Rep 2015, 5:17636. 25• . Tan X, Tahini HA, Smith SC: Materials design for electrocatalytic carbon capture. APL Mater, vol 4 2016 053202. 26• . Tan X, Tahini HA, Smith SC: Conductive boron-doped graphene as an ideal material for electrocatalytically switchable and high-capacity hydrogen storage. ACS Appl Mater Interfaces 2016, 8:32815–32822.
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27. Rege SU, Yang RT, Qian KY, Buzanowski MA: Air-prepurification by pressure swing adsorption using single/layered beds. Chem Eng Sci 2001, 56:2745–2759. 28. Li G, Xiao P, Webley P, Zhang J, Singh R: Capture of CO2 from high humidity flue gas by vacuum swing adsorption with zeolite 13X. Adsorption 2008, 14:415–422. 29. Liu J, Benin AI, Furtado AMB, Jakubczak P, Willis RR, LeVan MD: Stability effects on CO2 adsorption for the DOBDC series of metal-organic frameworks. Langmuir 2011, 27:11451–11456. 30. Kizzie AC, Wong-Foy AG, Matzger AJ: Effect of humidity on the performance of microporous coordination polymers as adsorbents for CO2 capture. Langmuir 2011, 27:6368–6373. 31. Keskin S, van Heest TM, Sholl DS: Can metal–organic framework materials play a useful role in large-scale carbon dioxide separations? ChemSusChem 2010, 3:879–891. 32. Furukawa H, Gandara F, Zhang YB, Jiang J, Queen WL, Hudson MR, Yaghi OM: Water adsorption in porous metal-organic frameworks and related materials. J Am Chem Soc 2014, 136:4369–4381. 33. Chu S, Majumdar A: Opportunities and challenges for a sustainable energy future. Nature 2012, 488:294–303.
Current Opinion in Electrochemistry 2017, 4:118–123
Review Article Charge-modulated CO2 capture
Xin Tan, Hassan A. Tahini and Sean C. Smith∗ Recently, the concept of using charge-modulation to manipulate the interaction of adsorbate molecules such as CO2 on certain sorbent materials has been advanced through a series of first principle computational predictions. The interactions switched on through charge-modulation shifting the Fermi level are in part, but by no means exclusively, electrostatic. In addition to electrostatics, the Fermi level shifting can be viewed as a way to modulate not only the position but also the identity and character of the frontier orbitals on the sorbent materials that engage with the adsorbate molecules. Thus, a rich new space for electrochemical modulation of surface-molecular interactions—guided by first principle computational modeling—suggests itself. Here we summarize the growing computational literature on switchable CO2 capture strategies. We also provide some new insights into contrasting electrostatic versus chemical responses to charge-modulation as exemplified by water versus CO2 on N-doped graphene surfaces, which suggest that this CO2 capture strategy could be water tolerant.
nitride (h-BN) with boron vacancy [14], and silicon carbide (SiC) nanotubes and nanosheets [15,16], have been demonstrated as promising adsorbents for CO2 capture. However, the CO2 molecules bind strongly to these adsorbents, and the regeneration processes are difficult because of the large adsorption energy. Their practical applications are hampered by the high temperature requirement to release the captured CO2 .
Introduction
The urgent need to reduce CO2 forces scientists to develop improved methods and/or more efficient adsorbents for capturing and/or storing CO2 . A breakthrough came in early experiments by Lee et al. [17•• ] who demonstrated that CO2 exhibits an unusually high binding to anionic pyridine clusters in cold molecular beams. This enhanced CO2 binding to pyridinic-nitrogen was only realized when an excess electron was introduced, thus, negatively charging the cluster (Figure 1A). A similar theme was discovered by Yoon et al. [18• ] in which they theoretically demonstrated that charged fullerenes Cn (20 ≤ n ≤ 82) act as strong sorbents for H2 . More recently, density functional theory (DFT) computations were used to formulate a novel charge-modulated switchable CO2 capture/storage strategy [19• –25• ]. To control the capture/release process, Sun et al. suggested a simple on/off charge switching mechanism in h-BN nanomaterials as shown in Figure 1B [19• ]. However, this can be problematic as h-BN is an insulator with a band gap of about 5.8 eV which poses a challenge on how effectively the material can be charged up. To solve this, Jiao et al. [20•• ] proposed using carbon nanotubes with pyridinic-nitrogen which are conductive and could therefore act as an alternative absorbent to charge-modulated switchable CO2 capture. Following these early investigations, several experimentally feasible materials exhibiting charge-modulation for gas capture have been proposed [21•• –25• ]. Notably, conductive substitutional N-doped graphene (the majority defect, in contrast to the minority pyridinic N defect [20•• ]) has been predicted to be efficacious for chargemodulated CO2 capture [25• ] and conductive substitutional B-doped graphene has been predicted to exhibit charge-modulated H2 storage [26• ].
Carbon dioxide (CO2 ) makes up the vast majority of greenhouse gas emissions, which is one of the leading causes of global warming and climate change [1–4]. Not surprisingly, seeking for efficient, safe, reliable, and costeffective gas-adsorbent materials is a key challenge for efficiently separating, capturing, storing and/or converting CO2 . Several solid materials, such as metal-organic frameworks (MOFs) [5–9], carbon nanotubes [10–12], aluminum nitride (AlN) nanotubes [13], graphene-like boron
The main advantage of the charge-modulated strategy is controllable kinetics and reversibility. In detail, the gas CO2 molecules are weakly adsorbed on certain neutral sorbent materials; however, when excess electrons are introduced into the adsorbent, the adsorption of CO2 molecules can be dramatically enhanced. As the excess electrons are removed, the adsorbed CO2 can in principle be easily released. Different from other methods, the CO2
Address Integrated Materials Design Centre (IMDC), School of Chemical Engineering, UNSW Australia, Sydney, NSW 2052, Australia ∗
Corresponding author: Smith, Sean C. ([email protected])
Current Opinion in Electrochemistry 2017, 4:118–123 This review comes from a themed issue on Electrochemistry
Physical & Nano-
Edited by Feng, Jens For a complete overview see the Issue and the Editorial Available online 17 August 2017 http://dx.doi.org/10.1016/j.coelec.2017.08.006 2451-9103/© 2017 Elsevier Ltd. All rights reserved.
Current Opinion in Electrochemistry 2017, 4:118–123
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Physical & Nano-Electrochemistry Tan, Tahini and Smith
119
Figure 1
(A) Neutral (left) and anionic (right) geometries and calculated energies of Pz1 (CO2 )n optimized at the B3LYP/6-31++G∗∗ level. The van der Waals bonds in the neutral complex turn into covalent bonds in the anion (modified with permission from Ref. [17•• ]). (B) Schematic illustrations of charge-modulated CO2 capture on h-BN nanomaterials (modified with permission from Ref. [19• ]).
capture/release process occurs spontaneously once excess electrons are introduced or removed, and the kinetics of uptake and release can be simply controlled by manipulating the charge state in the adsorbent. In principle, this strategy could overcome the “tyranny of thermochemistry”, which typically demands high regeneration costs due to the large CO2 adsorption energy in adsorbents. Water vapor (H2 O) is commonly found coexisting with CO2 in air pre-purification [27] and post-combustion CO2 capture [28]. The real industrial flue gas streams contain 8–10% water vapor, and previous studies have demonstrated that the adsorption process of CO2 capture is greatly affected by the presence of high water levels in flue gas streams [29–32]. Water molecules may compete for adsorption sites, which significantly reduce the CO2 capacity of sorbent materials. So far, competitive coadsorption of water is a major challenge for developing of adsorbent-based CO2 capture. This prompts an interesting question as to whether the charge-modulation switchable CO2 capture strategy is tolerant of water, which we shall see below, goes to the heart of electrostatics versus chemical response mechanisms in this approach.
Charge-modulation effect on adsorption of CO2 /H2 O on N-doped graphene As noted above, conductive N-doped graphene is a promising, fully conductive sorbent material for chargemodulated switchable CO2 capture [25• ] and we use it as an exemplar for the discussion below. Computational methods are as elaborated in detail previously [e.g., 25• ]. In contrast to its adsorption to neutral N-doped graphene (Figure 2A), the CO2 molecule is chemisorbed at an active site—a C atom adjacent to the N-dopant—of negatively charged N-doped graphene (Figure 2B). The adsorption of CO2 is remarkably enhanced as negative charge is added, as shown in Figure 2E. When more than 3 excess electrons are introduced in the supercell, the adsorption energy of a CO2 could increase to larger than −1.5 eV, which is much larger than the adsorption energies of CO2 www.sciencedirect.com
on other high-performance adsorbents (−0.4 ∼ −0.8 eV) [33]. Figure 2 also shows comparative data for the adsorption of water on N-doped graphene. On neutral Ndoped graphene (Figure 2C), geometry optimized H2 O molecule lies flat on the N-doped graphene above the center of a hexagonal ring close to the N-dopant. It is weakly adsorbed (i.e. physisorbed) with a small adsorption energy of −0.20 eV. As excess electrons are introduced, the interaction between H2 O and negatively charged N-doped graphene is significantly enhanced (Figure 2D). Although the H2 O molecule also locates above the center of a hexagonal ring close to N-dopant, it is now perpendicular to negatively charged N-doped graphene with two H atoms pointing downwards. Compared to the neutral case, the adsorption energy of H2 O on negatively charged N-doped graphene increases significantly, as shown in Figure 2E. This indicates that H2 O molecules may compete for active sites on negatively charged N-doped graphene, which may be detrimental to charge-modulated switchable CO2 capture.
Different adsorption characters of CO2 /H2 O on negatively charged N-doped graphene Although the adsorptions of both CO2 and H2 O on Ndoped graphene are remarkably enhanced as excess electrons are introduced, the interactions in each case are fundamentally different in nature. For CO2 on 4e− negatively charged N-doped graphene (Figures 2B and 3B), the CO2 molecule is strongly adsorbed at one C atom adjacent to the N-dopant by forming of a new chemical bond between the C atom of CO2 and surface C atom of N-doped graphene. The O–C–O angle is bent, the distance between the C atom of CO2 and the C atom adjacent to the N-dopant is shortened from 3.196 (neutral N-doped graphene, Figure 2A) to 1.652 A˚ (Figure 2B), and the charge transfer from N-doped graphene to CO2 increases to 0.72e− (Figure 3B). This suggests a chemisorption event, which is very sensitive to the local Current Opinion in Electrochemistry 2017, 4:118–123
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Figure 2
Top (upper) and side (lower) views of the lowest-energy configurations of a single CO2 molecule on (A) neutral and (B) 4e− negatively charged N-doped graphene. Top (upper) and side (lower) views of the lowest-energy configurations of a single H2 O molecule on (C) neutral and (D) 4e− negatively charged N-doped graphene. The blue, grey, red and white balls represent N, C, O and H atoms, respectively. (E) The adsorption energies of a CO2 /H2 O molecule on neutral and negatively charged N-doped graphene as functions of charge states (charge densities). The grey area indicates the chemisorption region of CO2 .
Figure 3
tion originates from the electrostatic interaction between the dipole of H2 O and the negatively charged N-doped graphene. The distance between H atoms of H2 O and negatively charged N-doped graphene is relatively large ˚ and the charge transfer from N-doped (about 2.388 A) graphene to H2 O molecule is only 0.01e− (Figure 3D). Since there is no substantive overlap of the electron densities of H2 O molecule and surface C atom of N-doped graphene, no new chemical bond is formed. The interaction is fundamentally electrostatic and hence is not expected to be strongly dependent on the local atomic environment. Thus, we expect that lateral movement of H2 O on 4e− negatively charged N-doped graphene surface should be relatively facile.
Diffusion barrier of CO2 /H2 O on negatively charged N-doped graphene
atomic environment. In this case, lateral movement of the chemisorbed CO2 molecule should be difficult.
Figure 4 shows the diffusion pathways and the corresponding diffusion barriers of CO2 diffusion on the 4e− negatively charged N-doped graphene surface. Consistent with our expectation, CO2 adsorption energies at different adsorption sites on 4e− negatively charged Ndoped graphene are quite different (Figure 4B): at the active site, i.e. the C atom adjacent to the N-dopant, the adsorption energy is about −1.37 eV, which is 0.46 ∼ 0.54 eV stronger than other adsorption sites. Moreover, we also found that the diffusion barrier of CO2 at the active site is large (0.86 eV). The strong adsorption and large diffusion barrier of CO2 at the active site compared to other adsorption sites indicate that CO2 molecule will bind specifically to the C atom adjacent to the N-dopant.
However, for H2 O on 4e− negatively charged N-doped graphene (Figures 2D and 3D), the enhanced adsorp-
In contrast, the adsorption energies of H2 O at different adsorption sites on 4e− negatively charged N-doped
The total charge density distribution of a single CO2 molecule on (A) neutral and (B) 4e− negatively charged N-doped graphene. The total charge density distribution of a single H2 O molecule on (C) neutral and (D) 4e− negatively charged N-doped graphene. The isosurface value is 0.8 e/A˚ 3 , and the red arrow shows the charge transfer between the N-doped graphene and the gas molecule. The overlap of the electron densities of the C atom of CO2 and surface C atom of N-doped graphene in (B) indicates the formation of a new chemical bond.
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Figure 4
(A) The diffusion pathways (1 → 2 → 3) of CO2 diffusion on the 4e− negatively charged N-doped graphene surface and (B) the corresponding diffusion barriers. The numbers and red arrows in (A) denote different adsorption sites and diffusion pathways considered here. Top and side views of the lowest-energy configurations of a single CO2 molecule adsorbed at (C) 1, (D) 2 and (E) 3 sites on the 4e− negatively charged N-doped graphene. The blue, grey and red balls represent N, C and O atoms.
Figure 5
(A) The diffusion pathways (1 → 2 and 1 → 3 →4) of H2 O diffusion on the 4e− negatively charged N-doped graphene surface and (B) the corresponding diffusion barriers. The numbers and arrows with different colors in (A) denote different adsorption sites and diffusion pathways considered here. Top and side views of the lowest-energy configurations of a single H2 O molecule absorbed at (C) 1, (D) 2, (E) 3 and (F) 4 sites on the 4e− negatively charged N-doped graphene. The blue, grey, red and white balls represent N, C, O and H atoms.
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graphene are very similar with the difference less than 0.03 eV (Figure 5B). Moreover, the diffusion barriers of H2 O on the 4e− negatively charged N-doped graphene surface are less than 0.05 eV (Figure 5B). All of this indicates that there is no specific adsorption site for the H2 O molecule and it can easily undergo lateral diffusion on the 4e− negatively charged N-doped graphene surface.
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In general, although CO2 and H2 O have comparable adsorption energy on negatively charged N-doped graphene, the CO2 molecule specifically binds to an active site, i.e. the C atom adjacent to the N-dopant, while the H2 O molecule can easily diffuse to any adsorption site on negatively charged N-doped graphene surface. Therefore, even when H2 O molecules bind to the surface, they should not block the active sites due to the ease of lateral diffusion to other sites during the CO2 capture process. These results indicate that the negatively charged N-doped graphene should be capable of efficient capture of CO2 molecules even in the presence of H2 O molecules and that the charge-modulated switchable CO2 capture strategy is indeed likely to be tolerant of water.
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The growing computational literature on the possibilities of exploiting charge-modulation on certain materials to manipulate gas capture and release proffers a fascinating opportunity for experimental studies. Of course, it also offers a challenge in the form of determining precisely how to modulate charge at the heterogeneous solid–gas interface. Charging of electrode surfaces is well understood on one hand in the context of aqueous phase electrochemistry and also on the other hand in the context of device physics; hence one anticipates that cross fertilization of knowledge from these domains may lead to successful realization of charge-modulation gas capture in the near future. The importance of the applications such as CO2 capture and H2 storage are manifest and offer strong motivation to realize the remarkable potential of this approach for superior control and energy/cost savings.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: •
Paper of special interest Paper of outstanding interest.
••
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Acknowledgments
19• . Sun Q, Li Z, Searles DJ, Chen Y, Lu GM, Du A: Charge-controlled switchable CO2 capture on boron nitride nanomaterials.. J Am Chem Soc 2013, 135:8246–8253.
This research was undertaken with the assistance of resources provided by the National Computing Infrastructure (NCI) facility at the Australian National University; allocated through both the National Computational Merit Allocation Scheme supported by the Australian Government and the Australian Research Council grant LE160100051 (“Maintaining and enhancing merit based access to the NCI National Facility, 2016–2018”).
20••Jiao . Y, Zheng Y, Smith SC, Du A, Zhu Z: Electrocatalytically switchable CO2 capture: first principle computational exploration of carbon nanotubes with pyridinic nitrogen.. ChemSusChem 2014, 7:435–441. This paper reported a conductive absorbent to charge-modulated switchable CO2 capture.
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21••Tan . X, Tahini HA, Smith SC: Computational design of two-dimensional nanomaterials for charge modulated CO2 /H2 capture and/or storage. Energy Storage Mater 2016. http://dx.doi.org/10.1016/j.ensm.2016.12.002. Review article on computational design of two-dimensional nanomaterials for charge modulated CO2 /H2 capture and/or storage. 22• . Tan X, Kou L, Smith SC: Layered graphene-hexagonal BN nanocomposites: experimentally feasible approach to charge-induced switchable CO2 capture. ChemSusChem 2015, 8:2987–2993. 23• . Tan X, Tahini HA, Seal P, Smith SC: First-principle framework for total charging energies in electrocatalytic materials and charge-responsive molecular binding at gas−surface interfaces. ACS Appl Mater Interfaces 2016, 8:10897–10903. 24• . Tan X, Kou L, Tahini HA, Smith SC: Conductive graphitic carbon nitride as an ideal material for electrocatalytically switchable CO2 capture. Sci Rep 2015, 5:17636. 25• . Tan X, Tahini HA, Smith SC: Materials design for electrocatalytic carbon capture. APL Mater, vol 4 2016 053202. 26• . Tan X, Tahini HA, Smith SC: Conductive boron-doped graphene as an ideal material for electrocatalytically switchable and high-capacity hydrogen storage. ACS Appl Mater Interfaces 2016, 8:32815–32822.
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