The scientists from the Federal Polytechnic School of Lausanne, Switzerland and the University of Bath, the United Kingdom have developed an exceptionally efficient carbon membrane able to separate hydrogen from other gases. It is just 10 nanometers thick, which is ten thousand times thinner than a human hair. A key factor in its production was the adding of a small amount of oxygen, which radically altered its internal structure, at the same time increasing both the rate of hydrogen flow and the accuracy of its separation from nitrogen.
In industry, hydrogen is seldom obtained in its pure form. It is usually mixed with nitrogen, methane, carbon dioxide, and other substances. Complex and energy-intensive technologies are used for purification, such as cryogenic separation or high-pressure adsorption. However, a more economical option is membranes acting as a molecular sieve allowing some gases to pass through easily while blocking others. The problem is that such systems either allow gas to pass through quickly but do not separate it well, or they separate it well but operate too slowly.
The researchers made an attempt to solve it with an unconventional approach. They used a polymer (4-vinylpyridine) heated on a nickel substrate at a temperature of about 500 °C, transforming it into an ultra-thin carbon film. When heating the polymer, the scientists added a small amount of oxygen to the atmosphere – no more than 0.8%. Typically, the membrane technologies consider oxygen undesirable because it can oxidize and degrade the material. However, in this case, oxygen turned out to act as a kind of ‘molecular scissors’: it breaks the weakest carbon bonds, forming a more complex system of ultra-small pores within the membrane.
As a result, pores of several sizes formed simultaneously within the material. Narrower regions effectively trapped large nitrogen molecules, while the wider channels allowed hydrogen to pass through quickly. The combination of these two types of structures allowed unusually high gas separation efficiency.
The result was unusual for membrane technologies: a membrane just 10 nanometers thick allowed hydrogen to pass through very quickly, while barely letting nitrogen through. Usually, in membrane technologies you have to choose between high flow rate and good gas separation.
In addition, many carbon-based materials over time experience what is called aging: their structure gradually becomes denser, and their performance declines. In the new study, the membrane maintained stable performance for over 190 hours of a continuous operation at a temperature of 150 °C.
The researchers believe the technology might be in demand for hydrogen purification in the ammonia industry and natural gas processing. At the same time, the production method itself appears relatively simple and fast: a complete membrane manufacturing cycle takes less than two and a half hours which increases the chances of industrial scale development.
