Strained graphitic carbon nitride for hydrogen purification
Introduction
The demand for cleaner energy has incited greater interest in hydrogen energy as it offers a superior alternative to conventional fossil fuel combustion, thanks to its high energy density [1], higher energy conversion efficiency and its environmental friendly nature [2]. Current H2 production methods include, steam-methane reforming, coal gasification, water-electrolysis etc. [3]. Irrespective of the production method, a critical step common to every method is hydrogen purification step from a mixture of gases.
Among numerous gas separation techniques, membrane separation is regarded superior due to its intrinsic low energy requirements, simple operation [4] and higher tunability. Membrane technology for hydrogen separation exploits the difference in molecular size between hydrogen and other impurity gases. Among various membrane types, such as polymeric [5], inorganic [6], [7], [8], and hybrid types [9], graphene and graphene like two dimensional membranes attract greater interest due to their atomic thickness. A membrane's performance is measured by its selectivity and permeability, which normally shows inversely proportional behaviour. However, atomically thin single layer 2D material which has the ultimate thinness, is known to exceed this limitation. Therefore, porous graphene [10], [11], [12], [13], [14], [15], [16], [17], [18] and graphene like materials such as graphynes [19], [20], graphitic carbon nitrides [21], [22] have been explored for gas separation applications.
Two dimensional C3N4 is a graphene like carbon nitride material, which has attracted the interest of the scientific community due to its interesting chemical and photocatalytic properties, and excellent thermal properties [23], [24]. It is the most stable of the carbon nitride family, which consists of tri-s-triazine building blocks [25]. Another advantage of this material is its facile synthesis. The common top-down synthesis methods are liquid exfoliation under sonication and thermal exfoliation with the aid of intercalation compounds [26]. As a candidate for gas purification, it intrinsically owns uniformly distributed regular sized pores, unlike in porous graphene where achieving such a feat is challenging. Further, Cao et al. have successfully synthesised and shown that a g-C3N4 incorporated membranes can effectively purify water from ethanol mixtures where the sieving effects were rendered by the g-C3N4 layers [27].
Here we study the use of g-C3N4 for gas separation applications using first principles and molecular dynamic simulation approaches. Our study evaluates the permeability of small gas molecules: H2, CO2 and CH4, across a g-C3N4 membrane. The choice of gas molecules is based on the membrane pore area, and the kinetic diameters of the gas molecules. In this study we primarily focus on the possibility of improving the gas permeability of the g-C3N4 membrane, by subjecting it to small biaxial strains. The findings of this study can find applications in H2 separation from CO2 and CH4, in refinery waste recovery, H2 purification and pre combustion CO2 separation.
Section snippets
Computational details
In this study, molecular dynamic simulations were used to analyse the H2, CO2 and CH4 permeation across the membrane, and first principle DFT calculations were used for geometry optimization of g-C3N4 and potential energy scan (PES) for H2 passage under various strained conditions.
All DFT calculations were carried out using DMol3 module in Materials Studio [28], [29]. First, the g-C3N4 supercell was fully relaxed, allowing the relaxation of both atomic positions and the lattice parameters, and
Results and discussion
The structural changes induced by applied 2.5%, 5% and 7.5% strain values are summarized and compared against the unstrained cell, in Table 1. The results signify that the change of molecular structure is not directly proportional to the increase of cell dimensions. The stronger aromatic bonds B2 and B3 (Fig. 2) within the hexagonal rings demonstrate higher resistance to stretching compared to the B1 bond outside the ring. Similarly, the percentage increase to porous area of the monolayer (D1
Conclusion
In conclusion, gas selectivity and high permeability are the two dominant factors defining the effective performance of a gas separation membrane. Our combined classical MD simulation and first principles DFT simulation results provide an accurate trend of gas selectivity and a reasonable approximate for the gas flux across a membrane. Our findings show that with minor mechanical tuning, the g-C3N4 membrane displays exceptional gas permeability, without compromising on its superior H2 filtering
Acknowledgement
Support from the ARC Discovery Project (DP130102120) and the High Performance Computer resources provided by the Queensland University of Technology are gratefully acknowledged. The first author also gratefully acknowledge Ph.D. scholarship support from Queensland University of Technology.
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