Chunqi Xianga,
Ao Lia,
Shulin Yang*a,
Zhigao Lana,
Wei Xiea,
Yiming Tanga,
Huoxi Xu*a,
Zhao Wangb and
Haoshuang Guab
aSchool of Physics and Electronic Information, Hubei Key Laboratory for Processing and Application of Catalytic Materials, Huanggang Normal University, Huanggang 438000, P. R. China. E-mail: yangsl@hgnu.edu.cn; xuhuoxi@hgnu.edu.cn
bFaculty of Physics and Electronic Sciences, Hubei University, Wuhan 430062, P. R. China
First published on 15th August 2019
The hydrogen storage performances of novel graphene nanoflakes doped with Cr atoms were systematically investigated using first-principles density functional theory. The calculated results showed that one Cr atom could be successfully doped into the graphene nanoflake with a binding energy of −4.402 eV. Different from the H2 molecule moving away from the pristine graphene nanoflake surface, the built Cr-doped graphene nanoflake exhibited a high affinity to the H2 molecule with a chemical adsorption energy of −0.574 eV. Moreover, the adsorptions of two to five H2 molecules on the Cr-doped graphene nanoflake were studied as well. It was found that there were a maximum of three H2 molecules stored on the graphene nanoflake doped with one Cr atom. Also, the further calculations showed that the numbers of the stored H2 molecules were effectively improved to be six (or nine) when the graphene nanoflakes were doped with two (or three) Cr atoms. This research reveals that the graphene nanoflake doped with Cr atom could be a promising material to store H2 molecules and its H2 storage performance could be effectively enhanced through modifying the number of doped Cr atoms.
Hydrogen (H2), a renewable and clean energy resource, has been recognized as an ideal substitution for the excessively consumed fossil fuels.16 This outstanding energy carrier is highly friendly to the environment with a wide range of sources and an extremely high energy density (143 kJ g−1).17 Some countries have devoted their efforts to studying the effective application of H2 in the typical industries of aerospace and clean-energy vehicles.18 The efficient storage of hydrogen gas is of significant importance to the safe and full use of this promising energy.19–21 According to the recent reports, one of those effective ways to store hydrogen energy is to design the solid-state material to store the gas in molecules, which has been widely reported in the modified graphene-based sheets.9,22,23 The theoretical and experimental studies of Yang et al. showed that the hydrogen storage capacity of the microporous carbon-based material could be significantly enhanced by modified with Ru due to the spillover effect.24 Fan and his workmates have also done a first-principles study on the hydrogen storage properties of the Sc-decorated graphene. They found that there were six H2 molecules in maximum adsorbed on the modified Sc atom.25 Moreover, the Ti-doped and Os-doped graphene nanoflakes were also calculated to be the potential material to achieve the high hydrogen storage due to the partially occupied 3d orbitals.26,27 Reasonably, it could be inferred that the hydrogen storage performance of the graphene-based materials could be successfully and effectively enhanced through doped with transition metal atoms.
The Cr atom, one of the typical transition metal atoms, is also reported to be selected to modify the graphene-based materials to improve their interactions with the adsorbed gas molecules.28–30 For instance, Zhang et al. have researched the strongly chemical adsorption of the formaldehyde molecule on Cr-doped graphene surface.29 The study of Villagracia and his group also showed that the Cr-doped penta-graphene could interact with the hydrogen gas molecule with the adsorption energy of −0.25 eV, exhibiting higher affinity to hydrogen gas than the pristine penta-graphene.31 However, few literatures were reported to study the interaction between the hydrogen gas and the Cr-doped graphene nanoflake, let alone systematically investigating the hydrogen storage capacity on this promising nanosheet. Furthermore, a majority of the published researches mainly focused on the hydrogen gas molecules stored on the graphene-based sheet modified with only one metal atom, the hydrogen storage performance of the sheets modified with two or more metal atoms was little reported.
In this paper, the graphene nanoflake with H atoms at the end of the C atoms located at the edge and the modified nanoflakes doped with Cr atoms were constructed to study the effects of Cr atom on the hydrogen storage performance of the graphene-based materials. The interaction between the built graphene-based sheets and the stored hydrogen molecules were studied through analysis their optimized morphologies, the density of states and electron densities. Also, the hydrogen gas molecules stored on the graphene nanoflakes doped with two or three Cr atoms were also investigated in details.
The average binding energy (Ēb) of Cr atoms in the PGNF is calculated with the equation of
Ēb = [EnCrGNF − Esub − nECr]/n, | (1) |
The adsorption energies (Ead) and the average adsorption energies (Ēad) of the H2 molecules adsorbed on graphene nanoflakes doped with Cr atoms are defined as
Ead = EnH2+CrGNF − E(n−1)H2+CrPGNF − EH2 | (2) |
Ēad = [EnH2+CrGNF − ECrGNF – nEH2]/n, | (3) |
Fig. 1 The calculated optimized geometries (top view and side view) of the built PGNF (a) and CrGNF (b). |
In the following research, the hydrogen molecules with different orientations were placed on the built nanoflakes to investigate the interactions between the hydrogen gas and the PGNF (or CrGNF). Two typical adsorption modes were constructed for one hydrogen molecule adsorbed on PGNF (or CrGNF): (i) the hydrogen molecule was parallel to the nanoflake with H atom above the active site (C atom for PGNF or Cr atom for CrGNF); (ii) the hydrogen molecule was perpendicular to the nanoflake with the H atom above the active site.24 Interestingly, we found that the final optimized geometries of the built adsorption modes for PGNF or CrGNF were little affected by the orientations of the placed hydrogen molecule. The optimized structures (top view and side view) of the H2 adsorbed on PGNF (mode P1H) or CrGNF (mode Cr1H) systems are shown in Fig. 2. The results presented that the hydrogen molecule placed on PGNF ran far away from the nanoflake with the long-distance (d) of 3.124 Å, indicating the weak interaction between the H2 and the PGNF, which agreed well with the reported results.46,47 While in the case of the Cr doped system, the placed H2 molecule was found to be adsorbed stably above the active Cr atom in the CrGNF. The d between the adsorbed H2 and the CrGNF was found to be 1.756 Å, much shorter than that of the pure system. Meanwhile, the bond length (l) of the H2 adsorbed on the CrGNF expanded to be 0.864 Å, which is longer than that of the H2 adsorbed on the PGNF (0.753 Å). Furthermore, the Ead of the H2 adsorbed on the CrGNF was calculated to be −0.574 eV, higher than that in the pure graphene system (−0.098 eV). The shorter d, longer l and higher Ead implied the stronger interaction between the adsorbed H2 and the graphene nanoflake doped with Cr atom, indicating that the CrGNF could be applied as a promising candidate for the storage of H2 molecules.
Fig. 2 The optimized structures (top view and side view) of built modes of the H2 adsorbed on PGNF (a) and CrGNF (b). |
The electron densities and the partial density of states (PDOS) of the systems of one H2 molecule adsorbed on PGNF and CrGNF are further studied to better understand their gas storage performance, as displayed in Fig. 3. The results showed that the electron density of the H2 molecule in the mode P1H was mainly distributed within the molecule (seen in Fig. 3a), indicating the weak interaction between the adsorbed H2 and the substrate of graphene nanoflake. In the case of mode Gr1H, the significant overlap in the electron densities of the adsorbed H2 and that of the CrGNF (seen in Fig. 3b) meant that there were certain electrons transferred between the gas molecule and the substrate, implying their strong interaction with each other.26 All the calculated results provided clear clues that the H2 molecule only exhibited a weak physical adsorption on the PGNF but a strong chemical adsorption on the CrGNF. This different adsorption of H2 molecule on PGNF or CrGNF could also be confirmed through the analysis of their PDOS shown in Fig. 3c and d. The PDOS of the Cr-doped system showed that the peaks of the H2 molecule undertook a stronger overlap with those of the Cr atom in CrGNF than those of the PGNF system due to the hybridization of σ and σ* orbitals of H2 molecule with d orbitals of the Cr atom, further indicating the more effective interaction between the adsorbed H2 molecule and the graphene nanoflake doped with Cr atom.48
Fig. 3 The electron density and PDOS of the systems of one H2 adsorbed on PGNF (a and c) and CrGNF (b and d). |
Also, more H2 molecules were also introduced on the CrGNF to fully investigate its hydrogen storage property. Fig. 4 displays the calculated optimized geometries of the adsorption systems in which two, three, four and five H2 molecules were adsorbed on CrGNF, and they were defined as the modes Cr2H, Cr3H, Cr4H and Cr5H, respectively. The corresponding H2 molecules were labeled with different numbers in the above modes. The d between the H2 molecules and the active Cr atom in the doped nanoflake were studied in detail, which was listed in Table 1. As can be seen in Fig. 4a, the two molecules adsorbed on CrGNF in mode Cr2H exhibited a bilaterally symmetric structure with the d of 1.922 Å and 1.942 Å for H2 molecule numbered #1 and #2, respectively, which were comparable to the d of the one H2 molecule adsorbed on CrGNF. In mode Cr3H, the H2 molecules numbered #1, #2 and #3 presented a symmetric structure of a triangle with slightly larger d being 2.177 Å, 1.971 Å and 2.081 Å, respectively.49 However, when the fourth H2 molecule was introduced on the surface of the CrGNF, the obtained results showed that this H2 molecule numbered #4 in mode Cr4H stayed farther away from the substrate than the other three molecules (as seen in Fig. 4c and listed in Table 1), exhibiting a d of ∼3.682 Å. Moreover, the H2 molecules number #4 and #5 in mode Cr5H also showed similar tendencies to exhibit weak interaction with the CrGNF, staying away from the substrate with the d of 3.610 Å and 3.418 Å, respectively. The bond lengths (l) of the H2 molecules in all the built modes were also studied and listed in Table 1. The results showed that the l of the H2 molecule numbered #4 in mode Cr4H and the H2 molecules numbered #4 or #5 in mode Cr5H almost retained the original value of free H2 molecule (approximate 0.754 Å), while the l of the other stored H2 molecules in the studied modes expanded to larger values of over 0.766 Å. Our calculated research is well consistent with the studies of the hydrogen molecules stored on the 8B transition metal-doped silicon carbide nanotubes and graphene quantum dots.27,49
Fig. 4 The optimized geometries (top view and side view) of two (a), three (b), four (c) and five (d) H2 molecules adsorbed on CrGNF. |
Sensing material | Mode | Numbers of H2 | l (Å) | d (Å) | Eab (eV) | Ēab (eV) |
---|---|---|---|---|---|---|
CrGNF | Cr1H | H2#1 | 0.846 | 1.756 | −0.574 | −0.574 |
Cr2H | H2#1 | 0.802 | 1.922 | — | −0.363 | |
H2#2 | 0.792 | 1.942 | −0.160 | |||
Cr3H | H2#1 | 0.770 | 2.177 | — | −0.310 | |
H2#2 | 0.815 | 1.971 | — | |||
H2#3 | 0.779 | 2.081 | −0.182 | |||
Cr4H | H2#1 | 0.766 | 2.092 | — | −0.261 | |
H2#2 | 0.799 | 1.942 | — | |||
H2#3 | 0.779 | 2.087 | — | |||
H2#4 | 0.754 | 3.682 | −0.115 | |||
Cr5H | H2#1 | 0.766 | 2.096 | — | −0.227 | |
H2#2 | 0.784 | 2.024 | — | |||
H2#3 | 0.780 | 2.045 | — | |||
H2#4 | 0.755 | 3.610 | — | |||
H2#5 | 0.755 | 3.418 | −0.093 |
The Ead and the Ēad of the H2 molecules adsorbed on CrGNF were also calculated according to the eqn (2) and (3), as listed in Table 1. It was found that there was a negative relationship between the calculated Ead (or Ēad) and the number of the H2 molecules adsorbed on the CrGNF, which agreed with the results of the hydrogen stored on the Y or Ti decorated graphene-based materials.50,51 The Ead of the second (or third) H2 molecule in the mode Cr2H (or Cr3H) from eqn (2) were calculated to be −0.160 eV (or −0.182 eV). However, the Ead of the fourth H2 molecule was calculated to be only −0.114 eV, which was lower than the Ēad of four H2 molecules (−0.261 eV) in the mode Cr4H obtained with the eqn (3). A similar difference between the Ead of the fifth H2 molecule (−0.093 eV) and the Ēad of the five H2 molecules (−0.227 eV) was also found in mode Cr5H. Generally speaking, the low Ead of gas molecules usually means the weak interaction between the adsorbed gas molecules with the substrates, whereas the high Ēad of gas molecule adsorbed on substrate indicates a strong interaction between them.48 Then, according to Ead and Ēad, it was difficult to confirm whether the physical or chemical interaction took place when the fourth (or fifth) H2 molecule was adsorbed on the CrGNF. To solve this problem leading to the misunderstanding in the interaction between the stored H2 molecules and the CrGNF, the electron densities of the modes Cr2H, Cr3H, Cr4H and Cr5H were further studied, which were displayed in the Fig. 5. As shown, there was a distinct overlap among the electron densities of the two (or three) H2 molecules and that of the Cr atom in the CrGNF in the mode Cr2H (or Cr3H), confirming the strong chemical adsorption of the H2 molecules on CrGNF. In the case of the Cr4H, we found that the electron densities of three H2 molecules numbered #1, #2 and #3 presented a similarly strong overlap with that of the Cr atom, but the electron density of the H2 molecule numbered #4 presented extremely weaker interactions with those of other H2 molecules or Cr atom in the adsorption system. For the H2 molecules numbered #4 and #5 in mode Cr5H, their electron densities were also discovered to be distributed mainly just within them, interacting weakly with other molecules or Cr atom. Based on all the calculated results above, it could be inferred that the H2 molecule numbered #4 in mode Cr4H (numbered #4 or #5 in Cr5H) exhibited only weak physical interaction with the CrGNF, which consisted well with the reported results in the previous literatures.27,49 Therefore, it was reasonable to speculate that there were three H2 molecules in maximum being chemisorbed and stored stably on the monolayer graphene nanoflake doped with one Cr atom, indicating the promising hydrogen storage property of the Cr doped graphene-based nanoflake.
Fig. 5 The electrons distributions of two (a), three (b), four (c) and five (d) H2 molecules adsorbed on CrGNF. |
Based on the research discussed above, another C atom in the CrGNF was further replaced by the Cr atom to establish 2CrGNF to improve the hydrogen gas storage performance of the doped graphene nanoflakes. The optimized geometry of the built 2CrGNF was studied and we numbered the two Cr atoms as #1 and #2, respectively, as shown in Fig. 6a. Similar with the CrGNF, the second doped Cr atom (numbered #2) also moved up from the plan of the graphene nanoflake with the highness of 1.736 Å. The highness of the first doped Cr atom (numbered #1) in the 2CrGNF was calculated to be 1.783 Å, little affected by the doped Cr atom numbered #2. The average binding energy (Ēb) of two doped Cr atoms was calculated to be −3.86 eV, indicating the stability of the 2CrGNF. Based on the studies of mode Gr3H, six H2 molecules were introduced on the 2CrGNF with three ones above each Cr atom (mode 2Cr6H). The calculated results showed that all the H2 molecules could be adsorbed and stored on the 2CrGNF, as presented in Fig. 6c. The three H2 molecules numbered #1, #2 and #3 interacted with the Cr atom numbered #1 with the d of 2.083 Å, 1.938 Å and 2.028 Å, respectively. For the three H2 molecules stored on the Cr atom numbered #2, the corresponding d were studied to be 2.103 Å, 1.911 Å and 2.033 Å for H2 molecules numbered #4, #5 and #6, respectively. The calculated distances between the three stored H2 molecules and each Cr atom in the 2CrGNF were comparable with those obtained in the mode Cr3H. The bond lengths of the H2 molecules numbered #1, #2, #3, #4, #5 and #6 were 0.777 Å, 0.798 Å, 0.783 Å, 775 Å, 0.804 Å and 0.781 Å, respectively, which were also similar with those in mode Cr3H, as shown in Fig. 6c and listed in Table 2. Meanwhile, we have simulated the electronic density of the mode 2Cr6H to systematically study the H2 storage performance of graphene nanoflake doped with two Cr atoms, as shown in Fig. 6e. The overlaps between the electronic densities of the three H2 molecules and each Cr atom indicated the extremely strong interactions within them, clearly implying that the six H2 molecules could be chemically stored on the 2CrGNF. Moreover, the Ēab of the H2 molecules in modes 2Cr6H was calculated to be −0.305 eV, further confirming the possibility of the storage of the six H2 molecules on the 2CrGNF. As expected, it was found that there were nine H2 molecules stored on the graphene quantum doped with 3 Cr atoms (3CrGNF), which could be proved by the stable optimized geometries (seen in Fig. 6b and d) and the strong overlap in the electron densities (seen in Fig. 6f) of this storage system as well as the detailed parameters of the d or l (listed in b) in this storage system. From our research, it is reasonable to imply that the H2 storage performance of the graphene-based material could be significantly enhanced through modification with more than one metal atom.
Sensing material | Active site | Numbers of H2 | l (Å) | d (Å) | Ēab (eV) |
---|---|---|---|---|---|
2CrGNF | Cr#1 | H2#1 | 0.777 | 2.083 | −0.305 |
H2#2 | 0.798 | 1.938 | |||
H2#3 | 0.783 | 2.028 | |||
Cr#2 | H2#4 | 0.775 | 2.103 | ||
H2#5 | 0.804 | 1.911 | |||
H2#6 | 0.781 | 2.033 | |||
3CrGNF | Cr#1 | H2#1 | 0.788 | 2.006 | −0.303 |
H2#2 | 0.785 | 2.023 | |||
H2#3 | 0.782 | 2.053 | |||
Cr#2 | H2#4 | 0.797 | 1.946 | ||
H2#5 | 0.791 | 1.977 | |||
H2#6 | 0.778 | 2.071 | |||
Cr#3 | H2#7 | 0.785 | 2.033 | ||
H2#8 | 0.796 | 1.962 | |||
H2#9 | 0.771 | 2.174 |
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