Polymer-based membranes for gas separation currently favor high gas permeability at the expense of high gas selectivity or vice versa because of theoretical performance limits.
Research taking place under the European Union’s €1-billion ($1.2-billion) Graphene Flagship project is tackling this trade-off by combining individual graphene oxide (GO) sheets with polymer spacers made of a 2-nm-thick polyethyleneimine (PEI) layer in a sandwich-style structure.
Figure 1. This equipment produces the new graphene-based gas separation material. Source: Graphene Flagship.
This highly reproducible, periodic multi-layered structure with a constant spacing of 3.7-nm between combined GO/PEI layers gives a record combination of gas permeability and selectivity, say the researchers.
The tunable permeability, high selectivity and possibility to produce similar coatings on a wide range of polymers also represent a new approach to making gas separation membranes for large-scale applications, they note.
“By switching from the standard 3D membrane to a layered polymer structure, we achieved gas separation over the theoretical limit in a membrane only 100-nm thick,” explains Vincenzo Palermo, a professor at Chalmers University of Technology, Gothenburg, Sweden, which is coordinating the project. He also is vice director of Graphene Flagship.
“The practical challenges in scaling-up the manufacturing process are mainly related to the need to apply a multilayer coating, which will require a significant process optimization,” he adds.
However, Palermo points out that continuous roll-to-roll surface treatments requiring multiple solution coating steps already are common and efficient at the industrial level — for example, the four-color CMYK (cyan, magenta, yellow and black) process used to print magazines. “Our process requires only two different types of deposition, GO and PEI, and so the 25–50 layers needed could ideally be obtained using just two liquid reservoirs.”
The membranes’ tunability should make them ideal for industrial-scale use for separating and purifying many gaseous streams produced by the process industries. “However, the physical processes that lie behind this material mean it could be suitable for many other applications including as a selective gas barrier material and in water purification,” he believes.
A pilot project between the chemical and material engineering department at the University of Bologna, Italy, and Graphene XT, also in Bologna, now is kicking off and should spur increasing industrial interest, says Palermo.
“This research is focused on developing a coating line at the pilot plant level. It will involve different projects that involve the fabrication of thin coatings of graphene on top of polymer films or membranes,” he concludes.