Thin membranes could help people go to Mars and clean the air here on Earth.

by Karen Miller

The ideal technology for space travel would be simple, robust, reliable, lightweight, and volumetrically efficient. It would have no moving parts, which would make it less likely to break. It would be a passive technology, not requiring any energy from the outside. It would be small. It would be light. An ideal technology for space, says chemical engineer Doug Way, is the membrane.

Well, OK, membranes can't do everything. Membranes won't boost us into space. And they won't carry us to Mars. But membranes could solve some of the problems of travelling there. And once we arrive, they could help us get back.

Basically, membranes are a semi-permeable barrier. They're like a wall, except that gases, and even liquids, can seep through them. But - here's the key point - different molecules move through membranes at different rates. Membranes can therefore be used to sort things out, separating one type of molecule from another.

Doug Way of the Colorado School of Mines and Lockheed engineer Larry Mason are working on a project that uses membranes to help produce rocket fuel from the Martian atmosphere. The principle is simple: The Martian atmosphere is 95% carbon dioxide (CO₂). Using membranes, explorers could extract some of that CO₂, which when mixed with hydrogen and then heated yields methane - a useful propellant for rockets or rovers.

Water is a byproduct of this type of methane production, called the Sabatier process (discovered by the French chemist Paul Sabatier in the nineteenth century). What's more, water can be electrolyzed into oxygen, for breathing, and hydrogen, which can be used to produce another round of methane.

Image credit: Doug Way (Colorado School of Mines) and Larry Mason (Lockheed Martin). A membrane inside a test cell

Although the Martian atmosphere is almost pure CO₂, it's not pure enough for the Sabatier process. Carbon dioxide must be separated from the other atmospheric gases before it's processed. Otherwise unused gases - mostly nitrogen and argon - build up, and will eventually keep the procedure from working. Way and Mason are developing a membrane that will separate out CO₂.

The specialized polymers that make up these membranes, some of which were developed at the Idaho National Environmental and Engineering Laboratory, are engineered to increase carbon dioxide solubility. "We add in groups of molecules that are polar - they carry an electric charge," says Way. Because carbon dioxide molecules are also polar, they're attracted to charged groups in the membrane.

The membranes are tested in a special chamber that simulates the Martian environment, explains Larry Mason. The device, which is about a metre high, is divided into two compartments. One contains a Mars-like atmosphere, and the other side contains a vacuum. They’re separated by a membrane that's about one square inch in surface area. A mass spectrometre measures how easily each gas moves into the vacuum side.

"In the best [membrane] material we've found," says Way, "at Martian conditions, CO₂ was transferred across the membrane about 50 times faster than nitrogen."

A schematic diagram of the Membrane Test Facility at Larry Mason's Lockheed Martin laboratory.

"Right now," adds Mason, “we’re screening different candidate materials to find the ones that permeate CO₂ the best. Once we find that, we can concentrate on getting enough through in an appropriate amount of time, by changing the amount of area, packaging it, and so on."

The researchers want to design a device that produces a gas that's 99.8 percent CO₂ at a rate of 2.5 liters per minute. To do that, Way says, will require quite a bit of membrane. Although the membrane is very thin - about 25 microns, one-quarter of the diametre of a strand of hair - it will probably need to be about 300 square feet in area, the size of a small room. All that will have to fit into a package of about 1 square foot.

But a membrane that separates CO₂ from other gases can do more than provide the raw material for rocket fuel. "This is fundamental technology," says Mason. "It's got all kinds of uses."

Membranes might help reduce carbon dioxide emissions from factory smoke stacks.

It could, for example, be used to filter air on the space station or on a spaceship bound for Mars. Carbon dioxide, which is a waste product of our metabolism, must continually be removed from the atmosphere of self-contained spacecraft. Membranes that are permeable only to carbon dioxide would be perfect, says Mason. "The CO₂ would just passively go through the membrane into a holding chamber - or out into space. Oxygen and other gases would stay intact inside the habitat."

These membranes could potentially help slow global greenhouse warming, too. "There's some thought," says Mason, "that a membrane could be used in extracting CO₂ from factory smoke stacks - reducing the amount of carbon dioxide that's dumped into the atmosphere." Such an application still lies in the future, he says.

"The biggest potential Earth-application," adds Way, "is the removal of CO₂ from natural gas.  CO₂ is the most common contaminant in natural gas besides water vapour.  Membrane separations are one of the primary processes used to filter natural gas so that it meets pipeline specifications of less than 2% CO₂." This is a big deal because "the natural gas industry is huge - more than 100 billion dollars per year in retail value," according to Way.

To Mason, "the most exciting part of this technology is the fact that it may leverage us to actually go to Mars and live and work there someday." And, in the meantime, there are plenty of uses for it right here on Earth.