Hydrogen is an environmentally friendly energy carrier owing to its high energy density per weight. This is further decorated by the fact that there is no greenhouse gas emissions associated with the use of hydrogen. Hydrogen is currently generated through steam reforming processes from natural gas. This is perhaps the most common and economical large-scale production process. Other by-products of the steam reforming are carbon monoxide and methane.
In order to get pure hydrogen for applications such as fuel cell, a purification process is needed in a bid to eliminate these by-products and other impurities. 2-Dimensional membranes such as polyphenylene and fused Pentagon have been demonstrated and analyzed to eliminate such impurities owing to their excellent performance and low energy consumption as compared to conventional separation methods.
A 2-dimensional membrane for hydrogen purification could exhibit a good balance between permeability and separation selectivity. High selectivity would led to high purity gas production. Unfortunately, the tradeoff between permeability and hydrogen selectivity limits the current 2-dimensional membranes. For example, a membrane with high permeability can be identified to have low selectivity.
Luckily, 2-dimensional stanene, a member of the honeycomb lattice, has been developed through molecular beam epitaxy. Theoretical computations have shown that the pore sizes of the 2-dimensional stanene are larger than for other graphene, silicene and germanene membranes. Fluorinating the defect-free 2-dimensional stanene can be used to enlarge the pore sizes further.
A team of researchers led by Professor Aijun Du from Queensland University of Technology, Australia, demonstrated in their study that the 2-dimensional stanene membrane could be implemented for hydrogen purification. The membrane could be strained further in order to get the desired hydrogen permeability. They then compared the purification performance of the 2-diemnsional stanene based membranes with other 2-dimensional membranes based on a theoretical upper bound relationship model. Their work is published in International Journal of Hydrogen Energy.
The authors fabricated the 2-D stanene and fluorostanene model counting on experimental results. As opposed to the typical planer geometry of graphene, the authors found that the low-bucked configuration was stable for the 2-D stanene and fluorinated stanene with pore uniformly distributed on the surface. The equilibrium lattice constant for the 2-D stanene was recorded as 4.66Å. This was larger than that of graphene and silicene. Fluorination could further enlarge these pores to 4.97Å.
The 2-D stanene membranes had the largest pore densities as compared to densities of previously investigated membranes thanks to the intrinsic honeycomb pores. High pore density reduces the area needed for gas mixture separation. This parameter is beyond the conventional tradeoff relationship that enables such membranes to attain high permeability without sacrificing selectivity.
Fluorination resulted in sp3-hybridized fluorostanene that had improved inertness towards syngas adsorption. For this reason, 2-dimensional stanene-based membranes are superior hydrogen purification membranes compared to conventional porous materials.
Combining strain and fluorination enabled the research team to realize excellent hydrogen purification performances. Their computations highlighted a new material for hydrogen separation for advanced validation.
Guoping Gao, Yan Jiao, Fengxian Ma, Yalong Jiao, Liangzhi Kou, Eric Waclawik, Aijun Du. Versatile two-dimensional stanene-based membrane for hydrogen purification. International Journal of Hydrogen energy, volume 42 (2017), pages 5577-5583.Go To International Journal of Hydrogen energy