Conductive Graphitic Carbon Nitride as an Ideal Material for Electrocatalytically Switchable CO2 Capture.

Good electrical conductivity and high electron mobility of the sorbent materials are prerequisite for electrocatalytically switchable CO2 capture. However, no conductive and easily synthetic sorbent materials are available until now. Here, we examined the possibility of conductive graphitic carbon nitride (g-C4N3) nanosheets as sorbent materials for electrocatalytically switchable CO2 capture. Using first-principle calculations, we found that the adsorption energy of CO2 molecules on g-C4N3 nanosheets can be dramatically enhanced by injecting extra electrons into the adsorbent. At saturation CO2 capture coverage, the negatively charged g-C4N3 nanosheets achieve CO2 capture capacities up to 73.9 × 10(13) cm(-2) or 42.3 wt%. In contrast to other CO2 capture approaches, the process of CO2 capture/release occurs spontaneously without any energy barriers once extra electrons are introduced or removed, and these processes can be simply controlled and reversed by switching on/off the charging voltage. In addition, these negatively charged g-C4N3 nanosheets are highly selective for separating CO2 from mixtures with CH4, H2 and/or N2. These predictions may prove to be instrumental in searching for a new class of experimentally feasible high-capacity CO2 capture materials with ideal thermodynamics and reversibility.

At the current rate of emissions of greenhouse gases, for which carbon dioxide (n class="Chemical">CO2) is the main component, global warming and climate change will continue to rise123. One crucial issue facing efficiently separating, capturing, storing and/or converting CO2 is the development of a practical sorbent material456. Liquid-amine, which is the most common adsorbent for current industrial process for CO2 capture, suffers from relatively low efficiency, equipment corrosion, solvent loss, and toxicity78910. Alternatively, various solid materials have been proposed as attractive adsorbents for CO2 capture, including metal-organic frameworks (MOFs)1112131415, aluminum nitride (AlN)16, carbon171819, hexagonal boron nitride (h-BN)20, and silicon carbide (SiC)2122 nanostructures. However, the difficult regeneration processes due to the large adsorption energy, which generally demands high temperatures to release captured CO2, significantly hinders their practical applications.

Recently, electrocatalytically switchable CO2 capture scheme has been proposed as a controllable, high selective, and reversible n class="Chemical">CO2 capture strategy for bare h-BN nanomaterials23. Specifically, CO2 molecules are weakly adsorbed (i.e. physisorbed) on neutral h-BN. By injecting extra electrons into h-BN adsorbent, density functional theory (DFT) calculations reveal that CO2 adsorption can be dramatically enhanced via a charge-induced chemisorption interaction. The chemically adsorbed CO2 can in principle be released when the extra electrons are removed. In contrast to previous methods, the CO2 capture/release occurs spontaneously once extra electrons are introduced or removed, and the process of CO2 capture/release can be simply controlled and reversed by switching on/off the charges carried by h-BN nanomaterials. However, h-BN is wide-gap semiconductor with band gap around 5.8 eV2425 and it is not clear how to charge up bare h-BN due to its insulating character.

To overcome the above disadvantage, Jiao et al.26 have investigated carbon nanotubes with n class="Chemical">pyridinic-nitrogen as an alternative absorbent to electrocatalytically switchable CO2 capture because of their good electron conductivity. On the other hand, we have proposed layered h-BN and graphene (hybrid BN/G) nanosheets, consisting of a single or double-layer h-BN and a substrate graphene layer, as an experimentally feasible approach to induce the requisite charge on h-BN for electrocatalytically switchable CO2 capture27. However, the synthesis of carbon nanomaterials with pyridinic nitrogen doping and hybrid BN/G are difficult to control in experiment. One natural question arise: can we find a conductive sorbent material for electrocatalytically switchable CO2 capture, which avoids complicated synthesis route?

Very recently, intense attention has been attracted by a new class of two-dimensional conjugated polymer, n class="Chemical">graphitic carbon nitride, due to the anisotropic two-dimensional geometric morphology and the aromatic π-conjugated framework. This endows carbon nitride nanosheets with attractive bandgap- and surface-engineered applications in both energy- and environment-related topics, such as photocatalysis for water splitting2829, hydrogen evolution30, CO2 reduction31, organosynthesis32, amongst others33.g-C3N4 and g-C4N3 are two kinds of two-dimensional conjugated nanosheets, which have been recently synthesized by using cross-linking nitride-containing anions in ionic liquid3435. Different from each other, g-C3N4 is semiconductor34, while g-C4N3 shows half-metallic property36.

Here we show that electrocatalytically switchable CO2 capture is indeed possible by considering conductive n class="Chemical">g-C4N3 nanosheets, of which the charge states can be easily modified experimentally because of the good electrical conductivity and high electron mobility. Using first-principle calculations, we found that the adsorption energy of CO2 molecules on g-C4N3 nanosheets can be dramatically enhanced from 0.24 to 2.52 eV by injecting extra electrons into the adsorbent. At saturation CO2 capture coverage, the negatively charged g-C4N3 nanosheets achieve CO2 capture capacities up to 73.9 × 1013 cm−2 or 42.3 wt%. Once the extra electrons are removed, the captured CO2 molecules can easily desorb from the adsorbent. In contrast to other CO2 capture approaches, the process of CO2 capture/release occurs spontaneously without any energy barriers once extra electrons are introduced or removed, and these processes can be simply controlled and reversed by switching on/off the charging voltage. In addition, these negatively charged g-C4N3 nanosheets are highly selective for separating CO2 from mixtures with CH4, H2 and/or N2. These predictions might pave the way in searching for a new class of experimentally feasible high-capacity CO2 capture materials with ideal thermodynamics and reversibility.

Results

Since good electrical conductivity and high electron mobility are prerequisite for injecting extra electrons into electrocatalytically switchable CO2 capture materials, we first studied the electronic structures of isolated n class="Chemical">g-C4N3. The lowest-energy configurations and the calculated band structures of g-C4N3 are shown in Fig. 1. Consistent with previous studies36, g-C4N3 is a (2 × 2) reconstructed structure with half-metallic state. This indicates that g-C4N3 has good electrical conductivity and high electron mobility, which should readily facilitate electron injection/release for electrocatalytically switchable CO2 capture.

Figure 1

Top (upper) and side (lower) views of (a) a (2 × 2) reconstructed g-C4N3 supercell. The blue and grey balls represent N and C atoms, respectively, and the unit cell of g-C4N3 is indicated by red dot lines. C1 and C2 denote different C atoms in g-C4N3 unit cell. The calculated band structures of (b) a (2 × 2) reconstructed g-C4N3. The blue dashed line denotes the Fermi level. The red and black lines in (b) denote the spin-up and spin-down states, respectively.

Single CO2 Adsorption on Neutral and 2 e− Negatively Charged g-C4N3 Nanosheets

We next shift our attention to a single CO2 adsorption on neutral and negatively charged n class="Chemical">g-C4N3. Since g-C4N3 is a (2 × 2) reconstructed structure, there are many different adsorption sites for a CO2 molecule. Here, we considered all the adsorption sites: directly on top of a C or N atom, above the midpoint of a bond linking the C and N atoms, and above the center of a honeycomb-like hexagon. Figure 2 shows the lowest-energy configurations of a CO2 absorbed on neutral and 2 e− negatively charged g-C4N3. On neutral g-C4N3 (Fig. 2(a)), the linear CO2 molecule is parallel to g-C4N3 and locates on top of three nitrogen atoms. The distance between the C atom of CO2 and closest N atom is 2.966 Å, and the linear CO2 molecule shows little structural change compared to a free CO2 molecule with the O-C-O angle and two double C=O bonds being 178.2° and 1.176 Å, respectively. Mulliken population analysis suggests that the amount of transferred electron from the absorbed CO2 molecule to g-C4N3 is negligible (about 0.004 e−). For the neutral case, the CO2 molecule is weakly adsorbed (i.e. physisorbed) onto neutral g-C4N3 with small adsorption energy of 0.24 eV.

Figure 2

Top and side views of the lowest-energy configurations of a single CO2 molecule absorbed on the (a) neutral and (b) 2 e− negatively charged g-C4N3. The blue, grey and red balls represent N, C and O atoms, respectively, and the adsorption energies of the CO2 molecule on neutral and 2 e− negatively charged g-C4N3 are listed.

After injecting two extra electrons into the g-C4N3 supercell (Fig. 2(b)), the n class="Chemical">CO2 is strongly adsorbed at surface N atom, and changes from physisorption into chemisorption on 2 e− negatively charged g-C4N3. The distance between the C atom of CO2 and surface N atom of g-C4N3 is shortened to 1.569 Å, the O-C-O angle is bent from 178.2° to 131.8°, the two double C=O bonds are elongated from 1.176 to 1.246 Å, and the charge transfer from g-C4N3 to CO2 increase to 0.56e−. In this case, the adsorption energy of a CO2 remarkably increases to 1.20 eV, which is much larger than the adsorption energies of CO2 on other high-performance adsorbents (0.4–0.8 eV)6, indicating that the negatively charged g-C4N3 is an excellent adsorbent for CO2 capture.

To understand the underlying mechanism of CO2 capture on negatively charged n class="Chemical">g-C4N3, we plotted the deformation electronic density of neutral and 2 e− negatively charged g-C4N3 by subtracting the electronic density of isolated N and C atoms from the sheet in Fig. 3. Obviously, for the neutral case (Fig. 3(a)), some electrons are extracted from the C atoms and delocalized over the N atoms, as implied by the green regions. Mulliken population analysis indicated that the electrons distribute at N, C1 and C2 are −0.302, 0.294 and −0.036 |e|, respectively. When two extra electrons are introduced (Fig. 3(b)), the extra electrons are almost evenly distributed on N and C atoms. Mulliken population analysis suggest that each atom gains −0.07 ~ −0.09 |e|, and the electrons distribute at N, C1 and C2 are −0.383, 0.222 and −0.122 |e|, respectively. Compared with the neutral case, more electrons are distributed and delocalized at N atoms, as implied by the green regions in Fig. 3(b). As CO2 is a Lewis acid and it prefers to accept, rather than donate, electrons during reaction, the N atom of negatively charged g-C4N3 can donate electrons to CO2, and form a new bond between the C atom of CO2 and surface N atom of g-C4N3 (Fig. 3(d)), which is significantly different from the case that CO2 on neutral g-C4N3 (Fig. 3(c). This is the reason why the CO2 molecule has a strong interaction with negatively charged g-C4N3.

Figure 3

The deformation electronic density of (a) neutral and (b) 2 e− negatively charged g-C4N3. Green and yellow refer to electron-rich and -deficient area, respectively. The isosurface value is 0.02 e/au. (c) The total charge density distribution of a single CO2 molecule on (c) neutral and (d) 2 e− negatively charged g-C4N3. The isosurface value is 0.8 e/au. The overlap of the electron densities of the C atom of CO2 and surface N atom of g-C4N3 in (d) indicates the formation of a new bond.

In order to investigate the kinetic process of CO2 capture/release on 2 e− negatively charged n class="Chemical">g-C4N3, we next studied the energy change of a CO2 molecule adsorbed on g-C4N3 after the introduction or removal of the two extra electrons. In Fig. 4(a), we started with the lowest-energy configuration of neutral g-C4N3 with a physisorbed CO2 molecule. Two electrons are then added to the neutral g-C4N3, and we examined the energy changes as the system relaxes to the 2 e− negatively charged optimized state. In Fig. 4(b), we started with the lowest-energy configuration of the 2 e− negatively charged g-C4N3 with a chemisorbed CO2 molecule. Two electrons are removed, and then the system is allowed to relax, forming a physisorbed CO2 molecule. When two extra electrons are introduced into g-C4N3, the interactions between the CO2 molecule and the 2 e− negatively charged g-C4N3 are significantly larger than that with neutral g-C4N3, and the CO2 molecule spontaneously relaxes to chemisorption configuration. This process is exothermic by 1.08 eV without any energy barrier. On the other hand, when two extra electrons are removed from the 2 e− negatively charged g-C4N3, the CO2 molecule spontaneously returns to the weakly bound state and desorbs from g-C4N3. This process is also exothermic by 1.39 eV without any energy barrier. Therefore, the CO2 storage/release processes on negatively charged g-C4N3 are reversible with fast kinetics, and can be easily controlled via adding/removing the extra electrons.

Figure 4

The energy change of (a) the relaxation (capture) of a CO2 molecule on g-C4N3 after two extra electrons are introduced, and (b) the reverse relaxation (release) process of a captured CO2 molecule from g-C4N3 after two extra electrons are removed from the adsorbent.

The Effects of Charge Density on Single CO2 Capture on Negatively Charged g-C4N3 Nanosheets

To investigate the effects of charge density on CO2 capture on negatively charged n class="Chemical">g-C4N3, we investigated a CO2 adsorption on negatively charged g-C4N3 with different charge densities. Here, we defined the charge densities of g-C4N3 as followswhere ρ, Q and S are the charge densities of g-C4N3, the total charge and the surface area in 2 × 2 supercell, respectively. For g-C4N3, the surface area in 2 × 2 supercell can be calculated as , where a is the lattice constant of 2 × 2 supercell.

Figure 5 shows the adsorption energies of a CO2 on negatively charged n class="Chemical">g-C4N3 and the charge transfer between CO2 and g-C4N3 as functions of charge densities. For small charge density case (<13.9 × 1013 cm−2), the adsorption energy of CO2 is small (0.24 ~ 0.35 eV), and charge transfer between CO2 and g-C4N3 is less than 0.06 e−. When charge density is larger than 13.9 × 1013 cm−2, the adsorption energy of CO2 and the charge transfer from g-C4N3 to CO2 increase dramatically with increasing charge density, indicating the CO2 molecule can only adsorb on negatively charged g-C4N3 with large charge density. Considering the adsorption energies of CO2 on other high-performance adsorbents is 0.4−0.8 eV 6, we define the minimum charging density for CO2 capture on negatively charged g-C4N3 is about 17.0 × 1013 cm−2.

Figure 5

The adsorption energies of a CO2 on negatively charged g-C4N3 and the charge transfer between CO2 and g-C4N3 as functions of charge densities.

The gray area indicates the adsorption region.

CO2 Capture Capacity on Negatively Charged g-C4N3 Nanosheets

To estimate CO2 capture capacity on negatively charged n class="Chemical">g-C4N3, we studied the maximum number and the average adsorption energy of captured CO2 molecules on negatively charged g-C4N3 with different charge densities (Fig. 6(a)). Here, we determinate the maximum number of captured CO2 for each negatively charged g-C4N3 with different charge density by gradually increasing the number of CO2 molecules on negatively charged g-C4N3 until no more CO2 can be absorbed. The average adsorption energy of captured CO2 is calculated as the total adsorption energy divided by the maximum number of captured CO2. The results show that no CO2 molecules can be captured by negatively charged g-C4N3 with small charge density (≤12.3 × 1013 cm−2). As the charge density increase from 18.5 × 1013 to 61.6 × 1013 cm−2, the negatively charged g-C4N3 can capture two, four and six CO2 molecules with the average adsorption energy of captured CO2 molecules ranging from 0.72 to 3.58 eV. We note that a further increase in the number of CO2 molecules leads to some CO2 molecules moving far away from the adsorbent during the geometry optimization even if we further increase the charge density of g-C4N3. Therefore, we define six CO2 molecules in each 2 × 2 supercell (i.e. CO2 capture capacity 73.9 × 1013 cm−2 or 42.3 wt%) as the likely saturation CO2 capture coverage (Fig. 6(b,c)). It should be noted that surface defective sites such as N vacancies or un-condensed amino group could lower CO2 capture capacity. However, considering the high CO2 capture capacity of negatively charged g-C4N3, we believe this may nevertheless represent a feasible high-capacity CO2 capture material.

Figure 6

(a) The maximum number and the average adsorption energies of captured CO2 molecules on negatively charged g-C4N3 with different charge densities. (b) Top and (c) side views of the lowest-energy configuration of six CO2 molecules adsorbed on negatively charged g-C4N3 with charge density 61.7 × 1013 cm−2.

Interestingly, we note that the CO2 molecules do not all bind equally but the capture process occurs discretely two at a time. To further confirm this phenomenon, we put four n class="Chemical">CO2 molecules on neutral g-C4N3 and gradually increase the charge density of negatively charged g-C4N3 until four CO2 are all captured (corresponding lowest-energy configurations are shown in Figure S1 of the Supporting Information). Clearly, four CO2 are physically adsorbed on neural and 1 e− negatively charged g-C4N3 (Figure S1(a,b), Supporting Information). On 2 e− negatively charged g-C4N3, two CO2 are chemisorbed while other two CO2 are physisorbed on adsorbent (Figure S1(c), Supporting Information). When three electrons are introduced, all the CO2 are captured on 3 e− negatively charged g-C4N3 (Figure S1(d), Supporting Information).

CH4, H2 and N2 Adsorption on g-C4N3 Nanosheets

CH4, n class="Chemical">H2, N2 are three types of gas mixtures that are currently most interesting for CO2 capture technologies, namely, postcombustion (predominantly CO2/N2 separation), natural gas sweetening (CO2/CH4), and precombustion (CO2/H2) capture37. In order to demonstrate the high selectivity of negatively charged g-C4N3 nanosheets for CO2 capture, we calculated the adsorption energies of CH4, H2 and N2 on neutral and negatively charged g-C4N3 and compared with those of CO2. In Fig. 7 we list the comparative adsorption energies of CO2, CH4, H2, and N2 on neutral, 1 e− and 2 e− negatively charged g-C4N3 (corresponding lowest-energy configurations are shown in Figure S2 of the Supporting Information). Clearly, the adsorptions of CH4, H2 and N2 on neutral, 1 e− and 2 e− g-C4N3 are all physical rather than chemical. The distance between the carbon atom of CH4 (the hydrogen atom of H2, the nitrogen atom of N2) and g-C4N3 is 3.157–3.159 (2.111–2.539, 2.865–3.236) Å, respectively. The adsorption energies of CH4, H2 and N2 on neutral, 1 e− and 2 e− g-C4N3 range from 0.06 to 0.39 eV. In contrast, although CO2 is physically adsorbed at neutral and 1 e− g-C4N3 with small adsorption energy in the range from 0.24 to 0.32 eV, CO2 is tightly chemisorbed on 2 e− g-C4N3 with large adsorption energy of 1.20 eV. The above comparisons demonstrate that negatively charged g-C4N3 has very high selectivity for capturing CO2 from CH4, H2 and/or N2 mixtures.

Figure 7

The adsorption energies of CO2, CH4, H2, N2 and H2O on neutral, 1 e– and 2 e– negatively charged g-C4N3.

Water Adsorption on g-C4N3 Nanosheets

Since water saturates most industrial gas streams, including flue gas, we also studied the adsorption energies of n class="Chemical">H2O on neutral and negatively charged g-C4N3 and compared with those of CO2, as shown in Fig. 7 (corresponding lowest-energy configurations are shown in Figure S2 of the Supporting Information). On neutral g-C4N3, both CO2 and H2O are physically adsorbed on adsorbents with small adsorption energies of 0.24 and 0.38 eV, respectively. On 1 e− g-C4N3, the adsorption energy of CO2 slightly increases to 0.32 eV, while the adsorption energy of H2O significantly increases to 0.60 eV, which is twice as much as that of CO2. On 2 e− g-C4N3, both CO2 and H2O are chemically adsorbed on adsorbents with large adsorption energies of 1.20 and 1.18 eV, respectively. These results indicate that the negatively charged g-C4N3 cannot selectively capture CO2 from a gas mixture with H2O present, and we should utilize some absorbent to eliminate water prior to CO2 adsorption. In fact, since the adsorption energy of H2O is twice as much as that of CO2 on 1 e− g-C4N3, utilization of 1 e− g-C4N3 to eliminate water prior to CO2 adsorption is one potentially viable approach. In this scanario, we could utilize g-C4N3 at lower voltage to eliminate water prior to a second stage of CO2 adsorption at slightly higher voltage.

Discussion

In summary, we have shown that modification of the charge state of conductive g-C4N3 nanosheets provides an experimentally feasible approach for electrocatalytically switchable n class="Chemical">CO2 capture. Compared with other CO2 capture approaches, the process of CO2 capture/release occurs spontaneously without any energy barriers once extra electrons are introduced or removed, and these processes can be simply controlled and reversed by switching on/off the charging voltage. In addition, these negatively charged g-C4N3 nanosheets are highly selective for separating CO2 from mixtures with CH4, H2 and/or N2.

Good electrical conductivity and high electron mobility of the sorbent materials are prerequisite for electrocatalytically switchable CO2 capture. The aim of the present paper is to explore conductive and easily synthetic sorbent material as an experimentally feasible adsorbent for electrocatalytically switchable n class="Chemical">CO2 capture. These predictions may prove to be instrumental in searching for a new class of high-capacity CO2 capture materials with ideal thermodynamics and reversibility, and we hope that this work will stimulate further theoretical and experimental research in this direction.

Methods

Our DFT calculations employed the linear combination of atomic orbital and spin-unrestricted method implemented in Dmol3 package38. The generalized gradient approximation (GGA) in Perdew-Burke-Ernzerhof (PBE) functional form39 together with an all-electron double numerical basis set with polarization function (Dn class="Chemical">NP) were adopted. Since the standard PBE functional is incapable of giving an accurate description of weak interactions, we adopted a DFT+D (D stands for dispersion) approach with the Grimme’s vdW correction in our computations40. The real-space global cutoff radius was set to be 4.1 Å.

To study CO2 capture/release on n class="Chemical">g-C4N3 nanosheets, we employed 2 × 2 supercell with periodic boundary conditions in the x-y plane (Fig. 1(a)). The vacuum space was set to larger than 20 Å in the z direction to avoid interactions between periodic images. In geometry optimizations, all the atomic coordinates were fully relaxed up to the residual atomic forces smaller than 0.001 Ha/Å, and the total energy was converged to 10−5 Ha. The Brillouin zone integration was performed on a (6 × 6 × 1) Monkhorst-Pack k-point mesh41.

In order to investigate the gas adsorption on adsorbent, we defined the adsorption energy E of CO2, n class="Chemical">CH4, H2 and N2 molecules on g-C4N3 as followswhere , , , and are the total energy of isolated g-C4N3 nanosheets, isolated gas molecule, g-C4N3 with the adsorbed gas, and number of gas molecules adsorbed on g-C4N3. According to this definition, a more positive adsorption energy indicates a stronger binding of the gas molecule to g-C4N3. The electron distribution and transfer mechanism are determined using the Mulliken method42.

Additional Information

How to cite this article: Tan, X. et al. Conductive Graphitic Carbon Nitride as an Ideal Material for Electrocatalytically Switchable n class="Chemical">CO2 Capture. Sci. Rep. 5, 17636; doi: 10.1038/srep17636 (2015).