Hydrogen and CO2 storage in high surface area covalent triazine–based frameworks
Hydrogen storage and CO2 capture are of great importance for efficient fuel usage and environmentally clean methods. Here, we report a series of 7,7,8,8-tetracyanoquinodimethane-derived covalent triazine frameworks (TCNQ-CTFs) with different specific surface areas for hydrogen and CO2 storage. Such TCNQ-CTFs exhibit maximum H2 and CO2 adsorption capacities up to 2.79 wt% (77 K, 1 bar) and 5.99 mmol/g (273 K, 1 bar), respectively, which are the highest values among reported covalent triazine frameworks.Theory simulation by using the Grand Canonical Monte Carlo (GCMC) method revealed that abundant nitrogen and defects induced by annealing treatment are the reasons for the high adsorption capacity of the material. This work not only contributes a superior material for both hydrogen and CO2 storage under ambient conditions but also deepens the knowledge on its adsorption mechanism, thus guiding people to engineer more efficient storage materials.
Nitrogen doped carbon derived from 7,7,8,8-tetracyanoquinodimethane covalent triazine frameworks exhibits ultra-high H2 and CO2 adsorption capacities under ambient pressure, which are among the highest level of reported CTF to our knowledge.
Recently, due to increasingly prominent energy and environmental problems, hydrogen storage and carbon dioxide capture have attracted increasing attention [1,2]. To improve storage efficiency and practicability, various strategies have been adopted, such as high-pressure tanks (mainly for H2 storage) , chemical reaction by the formation of hydrides  or chemicals , and physical adsorption on solid materials [, , ]. Concerning safety, feasibility, and reversibility, physical adsorption using solid-state materials is currently attracting a great deal of attention. Storing H2 or CO2 with high volume or weight density and work under environmental thermodynamic conditions is a central task for the development of solid-state materials.
Porous materials, such as porous carbons [9,10], metal-organic frameworks (MOFs) [11,12], and covalent organic frameworks (COFs) , have recently been the subject of intense study as potential stores for H2 storage and CO2 capture. Various studies have suggested that the most important factors in determining adsorptive efficiency for H2 and CO2 in porous materials are specific surface area (SSA), pore volume, and pore size. For example, Furukawa et al.  and Farha et al.  reported ultra-igh surface area MOFs, MOF-210 and NU-100, which exhibited exceptional H2 and CO2 storage capacity under high pressure. Ben et al.  reported a kind of COF named PAF-1 (SSA = 6500 m2/g), which showed high uptake capacities for hydrogen (10.7 wt % at 77 K, 48 bar) and carbon dioxide (1300 mg/g at 298 K, 40 bar). In addition to physical structure, the chemical interaction between the adsorbed gas and functional groups also plays an important role in H2 or CO2 adsorption. Oxygen- or nitrogen-containing functional groups [9,, , ] are reported to be effective atoms for strengthening the interactions between adsorbed gas (H2 or CO2) and porous materials. Therefore, an efficient design strategy for hydrogen storage or carbon dioxide capture materials is becoming increasingly clear, that is, materials with high SSAs and abundant functional groups.
As a branch of porous organic materials, covalent triazine frameworks (CTFs) have abundant N atoms and adjustable pore structures [, , , , ] and are regarded as a potential material for H2 and CO2 storage. For example, Tuci et al.  reported a 2,6-dicyanopyridine–derived CTF, named CTF-pyHT, which showed H2 adsorption capacities up to 2.63 wt% at 77 K and 1 bar, much higher than those of most other microporous materials. Hug et al.  also reported the excellent carbon dioxide adsorption performance of bipyridine-CTF (5.58 mmol/g at 273 K and 1 bar), which was also a higher carbon dioxide absorption capacity than that of other reported microporous materials. However, the SSAs of reported CTFs are generally less than ~3000 m2/g, which limits their wide application in hydrogen storage and carbon dioxide capture. In our previous work, we reported a kind of 7,7,8,8-tetracyanoquinodimethane-derived covalent triazine framework (TCNQ-CTF) with a SSA up to 4000 m2/g . Combined with its rich nitrogen content, this type of TCNQ-CTF may be an ideal candidate for both H2 storage and CO2 capture. Herein, the adsorption properties of TCNQ-CTFs for H2 and CO2 were systematically investigated. We found that TCNQ-CTFs indeed exhibited ultrahigh H2 and CO2 adsorption capacities (2.79 wt%, 77 K; 5.99 mmol/g, 273 K) under ambient pressure, which were the highest among reported CTFs to our knowledge. In addition, we discussed the relationship between the adsorption capacity and SSA and nitrogen content referring to previous reports.
- The synthesis diagram of TCNQ-CTFs. TCNQ-CTFs, 7,7,8,8-tetracyanoquinodimethane-derived covalent triazine frameworks.
- Structural characterization of TCNQ-CTFs. (a) FTIR spectra. (b) Thermogravimetric (TG) analysis curve of the TCNQ-CTF. (c) SEM image. (d) Carbon, nitrogen, and oxide elemental mapping image of TCNQ-CT…
- H2 adsorption and desorption isotherms for TCNQ-CTFs at (a) 77Kand (b) 87K. TCNQ-CTFs, 7,7,8,8-tetracyanoquinodimethane-derived covalent triazine frameworks.
- CO2 adsorption and desorption isotherms for TCNQ-CTFs at (a) 273K and (b) 298K. TCNQ-CTFs, 7,7,8,8-tetracyanoquinodimethane-derived covalent triazine frameworks.
- Dependencies of (a) H2 and (b) CO2 uptake on the specific surface area with some reported CTFs as the reference. [25,26,28,29].
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Synthesis and characterization
As shown in Scheme 1, the specific synthesis of TCNQ-CTFs has been reported in detail in our previous article . Typically, TCNQ-CTFs were formed by the trimerization of 7,7,8,8-tetracyquinodimethane (TCNQ) in molten ZnCl2 at 400 C and further rearrangement at higher temperatures (500 C and 700°C). The corresponding samples were denoted as TCNQ-CTF-400, TCNQ-CTF-500, and TCNQ-CTF-700, respectively. TCNQ-CTF-900 was acquired by further calcination of the TCNQ-CTF-700 sample at 900°C for 1 h
In summary, we reported a series of TCNQ-CTFs with different synthesis temperatures for hydrogen and CO2 storage. With ultrahigh SSA (up to 4000 m2/g), TCNQ-CTF-900 exhibited the highest H2 (2.79 wt% at 77 K and 1 bar) and CO2 (5.99 mmol/g at 273 K and 1 bar) uptakes. These values are among the top levels for all reported CTFs measured under identical conditions. Combining the literature reports and the data herein, we illustrate that there is a good linear relationship between the H2 uptake
Qiwen Deng: Made the synthesis, performed the computational calculations, wrote the paper and all authors contributed to the data analysis. Guoqing Ren: wrote the paper. Yajuan Li: made the synthesis. Li Yang: performed the computational calculations. Shengliang Zhai: performed the materials characterization. Tie Yu: performed the materials characterization. Lei Sun: Methodology, Formal analysis. Weiqiao Deng: Resources, Supervision, Project administration. An Li: Supervision, Writing-
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
This work was financially supported by the National Key Research and Development Program of China (No. 2017YFA0204800), the Fundamental Research Funds of Shandong University (2019GN021 to G.-Q. Ren, 2019GN111 to Y. Tie, 2019HW016 to L. Sun), the National Natural Science Foundation of China (Grant No. 21525315), and the China Postdoctoral Science Foundation Grant 2019M651936.
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