The chemical composition and structure of MXene, as portrayed in this illustration, create channels that can trap gas molecules – making it a useful material for gas separation. Image: Drexel University.
The chemical composition and structure of MXene, as portrayed in this illustration, create channels that can trap gas molecules – making it a useful material for gas separation. Image: Drexel University.

Hydrogen is one of the most abundant elements on Earth and an exceptionally clean fuel source. While it is making its way into the fuel cells of electric cars, buses and heavy equipment, its widespread use is hampered by the expensive gas-separation process required to produce pure hydrogen. But that process could soon become more efficient and cost-effective thanks to a discovery by an international team of researchers, led in the US by Drexel University. This team has uncovered exceptionally efficient gas separation properties in a nanomaterial called MXene that could be incorporated into the membranes used to purify hydrogen.

While hydrogen is present in a wide variety of molecules and materials in nature – water foremost among them – it does not naturally exist in its pure elemental form on Earth. There are currently two main ways to produce pure hydrogen by separating it from the other elements to which it commonly bonds. One involves using an electric current to excite and split apart the atoms in water molecules; the other involves filtering a gaseous mixture containing hydrogen through a membrane to separate the hydrogen from any carbon dioxide or hydrocarbons.

The process of gas separation via a membrane is the more effective and affordable option, so in recent years researchers have been ramping up efforts to develop membranes that can thoroughly and quickly filter out hydrogen.

In a paper in Nature Communications, the international team reports that using MXene material in gas-separation membranes could be the most efficient way to purify hydrogen gas. The research, led by Haihui Wang, a professor from South China University of Technology, and Yury Gogotsi, a professor in the Department of Materials Science and Engineering at Drexel, shows that the nanomaterial's two-dimensional structure allows it to selectively reject large gas molecules while letting hydrogen slip between the layers.

"In this report, we show how exfoliated two-dimensional MXene nanosheets can be used as building blocks to construct laminated membranes for gas separation for the first time," said Gogotsi. "We demonstrated this using model systems of hydrogen and carbon dioxide."

Working in collaboration with researchers from South China University of Technology, Jilin University in China and Leibniz University in Germany, the Drexel team found that membranes created using MXene nanosheets outperform the top-of-the-line membrane materials currently in use – both in permeability and selectivity.

Many different kinds of membranes are currently in use throughout the energy industry, for applications such as purifying coolant water before it is released and for refining natural gas before it is distributed for use. Gas separation facilities also use membranes to retrieve nitrogen and oxygen from the atmosphere. This study opens the door to an expanded use of membrane technology, with the possibility of tailoring the filtration devices to sift out a large number of gaseous molecules.

The advantage MXene has over materials currently being used and developed for gas separation is that both its permeability and filtration selectivity are tied to its structure and chemical composition. By contrast, other membrane materials, such as graphene and zeolites, do their filtering only by physically trapping – or sieving – molecules in tiny grids and channels, like a net.

MXenes possess these special filtration properties because they are created by chemically etching out layers from a solid piece of material, called a MAX phase. This process forms a structure that is more like a sponge, with slit pores of various sizes. Gogotsi's Nanomaterials Research Group, which has been working with MXenes since 2011, can predetermine the size of the channels by using different types of MAX phases and etching them with different chemicals.

The channels themselves can be created in a way that makes them chemically active, so they are able to attract – or adsorb – certain molecules as they pass through. Thus, a MXene membrane functions more like a magnetic net and can be designed to trap a wide variety of chemical species as they pass through.

"This is one of the key advantages of MXenes," Gogotsi said. "We have dozens of MXenes available which can be tuned to provide selectivity to different gasses. We used titanium carbide MXene in this study, but there are at least two dozen other MXenes already available, and more are expected to be studied in the next couple of years, which means it could be developed for a number of different gas separation applications."

The versatile two-dimensional material, which was discovered at Drexel in 2011, has already shown an ability to improve the efficiency of electric storage devices, stave off electromagnetic interference and even purify water. Studying its gas separation properties was the next logical step, according to Gogotsi.

"Our work on water filtration, the sieving of ions and molecules, and supercapacitors, which also involves ion sieving, suggested that gas molecules may also be sieved using MXene membranes with atomically thin channels between the MXene sheets," he said. "However, we were lacking experience in the gas separation field. This research would not have been possible without our Chinese collaborators, who provided the experience needed to achieve the goal and demonstrated that MXene membranes can efficiently separate gas mixtures."

In order for MXene to make its way into industrial membranes, Gogotsi's group will continue to improve its durability, and chemical and temperature stability, and also reduce the cost of production.

This story is adapted from material from Drexel University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.