Ideal metal-decorated three dimensional covalent organic frameworks for reversiblehydrogen storageYoon Jeong Choi, Jung Woo Lee, Jung Hoon Choi, and Jeung Ku Kang Citation: Applied Physics Letters 92, 173102 (2008); doi: 10.1063/1.2912525 View online: http://dx.doi.org/10.1063/1.2912525 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/92/17?ver=pdfcov Published by the AIP Publishing

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Ideal metal-decorated three dimensional covalent organic frameworksfor reversible hydrogen storage

Yoon Jeong Choi, Jung Woo Lee, Jung Hoon Choi, and Jeung Ku Kanga�

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology,Daejon 305-701, Republic of Korea

�Received 10 March 2008; accepted 31 March 2008; published online 28 April 2008�

The most stable H2 adsorption on pure covalent organic frameworks �COFs� occurs near O atoms,but its binding energy ��0.05 eV� is not sufficient to satisfy the minimum value �0.24 eV� requiredfor practical applications. Meanwhile, Li and Mg ion-decorated COFs have hydrogen adsorptionenergies of 0.28 and 0.30 eV, respectively, and their saturated hydrogen storage capacities exceedthe DOE target �6.5 wt % �. Also the effect of some counterions on hydrogen storage has beenexplored. Moreover, doped ions prefer to exist as isolated elements on COFs, which is anotheradvantage to realize hydrogen storage media. © 2008 American Institute of Physics.�DOI: 10.1063/1.2912525�

Hydrogen is widely considered as an economical, non-polluting, and renewable energy for automobile applications.Meanwhile, hydrogen storage is a key issue to realize thebenefits of hydrogen fueled vehicles. The U.S. Departmentof Energy �DOE� has targeted the maximum weight percentof reversibly available hydrogen at 6.5% for variousapplications.1

Thus far, several hydrogen storage methods have beensuggested such as carbon fiber reinforced high-strengthcontainers,2 liquid hydrogen,3 chemical metal hydrides,4 andcarbon-based materials such as carbon nanotubes �CNTs��Ref. 5� and metal organic frameworks �MOFs� �Refs. 6–8�.Among these, MOFs �Ref. 6� have attracted great attention inrecent years due to the possibility for the fast and reversibleH2 sorption on their large surface areas. However, these stud-ies on MOFs with even high surface areas demonstrate thatthe adsorption energies of hydrogen molecules are 0.07 and0.04–0.05 eV on zinc oxide corners and organic linkers ofMOFs, respectively.8�a� Meanwhile, these weak adsorptionenergies are too small to compensate the value for 0.24 eVrequired for practical applications at room temperature. Also,it was verified that modifications of metal center and organiclinkersin MOFs have failed to overcome this obstacle.9

Here, we report that the metal-doped covalent organicframeworks �COFs� could modify the hydrogen adsorptionenergies as well as the hydrogen storage capacities at roomtemperature. To investigate the adsorption sites and the ad-sorption energies of hydrogen molecules, we have taken allof the three dimensional �3D� COFs �termed as COF-102,COF-103, COF-105, and COF-108� reported so far.10

Figure 1 shows the fully optimized structures of pureand hydrogen molecules-adsorbed metal-decorated 3D COFsand model boron oxide rings, a B3O3 ring and a C2O2B ring,at the B3LYP level of theory.11,12 Then, single point calcula-tions have been performed using the second-order Møller–Plesset perturbation theory �MP2�8�a� level with these opti-mized geometries. We used 6-31G* basis set for B, C, H, andO atoms, and 6-311+ +G** basis set for adsorbed H atoms.We calculated the binding energies of Li, Mg, Li+, Mg2+, and

Ti atoms bound to C2O2B rings. For metal atoms and ions,we have used the basis sets of 6-311+ +G**.

The stable H2 adsorption sites are displayed in Fig. 2.We have determined the B3LYP H2 binding energies �Eb�and the bond distances �R� between the H2 and the boronoxide ring. In both cases of the two rings, we find that thebinding H2 to the center of phenyl group is not preferred.The most stable adsorption site for H2 is near the oxygenatom of a boron oxide ring. The bond distances from theoxygen atom to the H2 are 2.946 and 2.672 Å for a B3O3ring and a C2O2B ring, respectively. The Eb �0.05 eV� of themost stable H2 on a C2O2B ring is somewhat larger than the0.03 eV on a B3O3 ring. Each oxygen atom can be bound totwo H2 molecules with the average Eb of 0.03 and 0.04 eVfor a B3O3 ring and a C2O2B ring, respectively.

Table I lists the MP2 binding energies of molecular hy-drogen bound to a C2O2B ring with these geometries forcomparison with MOFs. The MP2 binding energy of 0.06 eVon a C2O2B ring is slightly smaller in magnitude than the0.07 eV on a zinc oxide cluster in MOFs. Despite of the factthat the excess amount of hydrogen �0.8 wt % � adsorbed onCOF-105 and COF-108 at ambient conditions is slightlylarger than 0.6 wt % of isoreticular MOF-14, it is still far

a�Author to whom correspondence should be addressed. TEL.: �82-42-869-3338. FAX: �82-42-869-3310. Electronic mail: jeungku@kaist.ac.kr.

TABLE I. Calculated binding energies �Eb in eV= �Ering-H2*�n−1�+EH2

�−Ering-H2

*n� and the maximum number of multiple bound H2 molecules on a

C2O2B ring and a decorated metal on a C2O2B ring, and Eb of a metal atomto a C2O2B ring at the MP2 level of theory.

Units C2O2B Li Mg Li+ Mg2+ Ti

Average Eba 0.06 0.14 0.28 0.30 0.45

Max H2b 2 2 0 3 6 4

1st H2Eb 0.06 0.14 0.50 0.61 0.612.22c

2nd H2Eb 0.05 0.13 0.20 0.47 0.103rd H2Eb 0.14 0.36 0.574th H2Eb 0.22 0.545th H2Eb 0.096th H2Eb 0.03Metald 0.42 0.14 1.62 6.09 4.59

aThe average Eb is the binding energy per one H2.bThe maximum number of adsorbed H2 per one oxygen or one metal.cThis is a dissociative adsorption.dThis is the binding energy for a metal atom bound to a C2O2B ring.

APPLIED PHYSICS LETTERS 92, 173102 �2008�

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from the DOE target 6.5 wt %.13 For viable practical hydro-gen storage at targeted conditions �−30–80 °C and1–100 bars�, the hydrogen adsorption enthalpy energyshould be in the range of about 0.1 to 0.62 eV.9�d�,9�e�,14

Metal-decorated C2O2B rings are found to have large H2adsorption energies and improved hydrogen uptake capacity.The employed metal atoms and ions are Li, Mg, Li+, Mg2+,and Ti. Table I summarizes their metal Eb on a C2O2B ringand the Eb of H2, and the maximum number of H2 adsorbedper a metal atom. All metal atoms can bind to the side phenylunit of a C2O2B ring. Each decorated Li can store only twoH2 and each Mg cannot give adsorption to any H2. Eachdecorated Ti atom can adsorb up to four H2 molecules and itsH2 binding energies �except first adsorption� are in the rangeof suitable hydrogen energies for practical use. However, asthe adsorption of the first H2 on Ti involves a dissociativeadsorption and clustering of decorated Ti atoms is more pre-ferred, consequently, it is considered that Ti might not be agood modifier being capable of fully accommodating mo-lecular hydrogen at room temperature.

Meanwhile, Li+ and Mg2+ ions have sufficient Eb of 1.62and 6.09 eV, respectively, to a C2O2B ring. Figures 3�a�–3�c�show that three metal ions can be doped in a C2O2B ring.Each decorated Li+ and Mg2+ can adsorb up to three and sixH2 molecules with the average Eb of 0.28 and 0.30 eV, re-spectively. In the case of multiple H2 adsorptions, the longestbond distance from the metal ion to the adsorbed H2 is about

2.245 Å for Li+ and about 4.323 Å for Mg2+, as shown inFigs. 2�d� and 2�e�. Considering the above-described idealhydrogen adsorption energy range, the Eb �0.03 eV� of thesixth H2for Mg2+ is too small to be stored up to room tem-perature, thus, five H2 molecules can be effectively bound toeach Mg2+ ion.

We also estimate the saturation values of gravimetrichydrogen storage capacities at room temperature and reason-able pressures using a method proposed by Sagara et al.8�b�

We consider two COF-108s composed of each C2O2B ringfully doped with three Li+ or Mg2+ ions and only counted thenumber of hydrogen molecules effectively adsorbed on eachmetal ion in the unit cell of COF-108. The calculated maxi-mum saturation values of hydrogen sorption capacity are 6.5and 6.8 wt%, for Li+- and Mg2+-decorated COF-108s, re-spectively. Meanwhile, we find that there is no experimentaldata reported for metal-doped COFs at present. Also, wehave explored the counterion effects on hydrogen storageproperties. For example, some possible approaches to dopeLi ions on COFs are to use LiF and LiCl. LiF on a C2B2Oring leads to the binding energy of 1.2 eV, where Li ion ofLiF binds to three H2 molecules. Although the binding en-ergy is less than Li ion one, we find that F ion leads to theadditional adsorption of one hydrogen molecule. Also, forLiCl, the H2 binding energies are less than those of LiF andCl ion is not able to bind with hydrogen molecule. Thus,

FIG. 1. �Color online� Model com-pounds; �a� a B3O3 ring for COF-102and COF-103, and �b� a C2O2B ringfor COF-105 and COF-108.

FIG. 2. �Color online� Hydrogen ad-sorption sites on a B3O3 ring for COF-102 and COF-103: two sites of �a� and�b� are located above the phenyl groupand one site �c� is located above theoxygen atom of the boron oxide ring.Similarly, two sites of �d� and �e� on aC2O2B ring for COF-105 and COF-108 are located above the phenylgroup, while one site �f� is locatedabove the oxygen atom of the boronoxide ring.

173102-2 Choi et al. Appl. Phys. Lett. 92, 173102 �2008�

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these results imply that LiF is a more preferable doping ele-ment to LiCl. Moreover, Li+ or Mg2+ ions prefer to exist asisolated elements. These behaviors are great benefits to real-ize the practically usable hydrogen storage media. It shouldbe noted that if the clustered or aggregated structures arepreferred instead of isolated elements, they could signifi-cantly reduce their maximum hydrogen storage capacitiesfeasible on isolated elements. Moreover, Mg2+-decoratedCOFs, which additionally include heavy elements of Mg ionsin pure COF matrices, provide the better moieties to improvevolumetric densities compared to those for pure COFs.

In conclusion, we have explored hydrogen storagemechanisms on pure and metal ion-decorated COFs via first-principles calculations. The H2 binding characteristics ofpure COFs are very similar to those of MOFs. The moststable H2 adsorption energy is still lower than 0.10 eV,which is too small to satisfy about 0.24–0.27 eV ideal forvarious applications at 1 bar condition. Meanwhile, we findthat the Li+- and Mg2+-decorated COFs having 0.28 and0.30 eV for their hydrogen adsorption energies, respectively,are promising candidates for reversible hydrogen storage me-dia, and that two metal-decorated COF structures are capableof satisfying the DOE target �6.5 wt % hydrogen�. Also, wehave explored the effects of counterions on hydrogen storagewith LiF. Moreover, Li+ and Mg2+ ions are found to prefer toexist as isolated elements on COFs.

This work was supported by Grant No.M103KW010017-06K2301-01720 from the Hydrogen En-ergy R & D program of the Ministry of Science & Technol-ogy �MOST�, by the Korea Science and Engineering Foun-dation �KOSEF� grant funded by the MOST �No. R0A-2007-000-20029-0�, and by the Korea Research Foundation Grant

�KRF-2005-005-J09703�.

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FIG. 3. �Color online� Li+- andMg2+-doped C2O2B rings, andH2-adsorbed structures; �a� one, �b�two, and �c� three Li+-doped C2O2Brings. �d� Three H2 bound to Li+- and�e� six H2 bound to Mg2+-dopedC2O2B rings.

173102-3 Choi et al. Appl. Phys. Lett. 92, 173102 �2008�

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