|The diffusion of unwanted carbon dioxide through the new polymer membrane. Credit: Trent Schindler, National Science Foundation
A team of researchers from the University of Texas and the Research Triangle Institute (RTI) have developed a “reverse-selective” rubbery plastic film that can purify hydrogen efficiently and that could contribute to lowering the cost of purifying hydrogen for use in fuel cells.
The research team designed a material that is more permeable to impurities than to hydrogen, reversing the conventional approach.
In other words, rather than allowing hydrogen pass through, keeping the impurities behind, the new membrane allows larger gas molecules such as CO2 and polar molecules to pass through, keeping the hydrogen behind.
The membrane—a molecularly engineered, highly branched, cross-linked polyethylene oxide—works because the molecules in its structure have relatively “positive” parts that attract electrons and relatively “negative” parts that repel electrons. CO2 has some of these polar characteristics, so it is attracted to the membrane, dissolving into it as salt dissolves into a glass of water.
The molecules diffuse through the membrane at a rate that increases as more polar molecules become entrenched in the rubbery polymer, the researchers found. Even when the membrane is saturated with impurities, the polar properties continue to funnel the undesirable molecules along at a faster rate than for hydrogen, retaining most hydrogen molecules on the upstream side.
Unlike other methods, the new reverse-selective process can capture hydrogen at a pressure close to that of the incoming gas. This is a primary advantage for the membrane because high pressure is important for transport of the gas, and many applications, yet adds significant costs.
The researchers, who describe their work in the 3 Feb. issue of Science, tested the material at three different temperatures common for industrial hydrogen purification: 95º, 50º and -4º F.
The new membrane not only separated these two gases better than previous membranes, but did so when additional components such as hydrogen sulfide and water vapor were present as occurs in industrial settings. The membrane proved 40 times more permeable to carbon dioxide than hydrogen.
While other hydrogen extraction methods still have advantages, the researchers believe there is great potential for future approaches to be hybrid processes that incorporate the new membrane within established systems.
The research is part of the 2003 Hydrogen Fuel Initiative to develop alternative fuel options to gasoline by 2010. It is funded by the US Department of Energy, with additional support from the National Science Foundation.
“Plasticization-Enhanced Hydrogen Purification Using Polymeric Membranes”; Haiqing Lin, Elizabeth Van Wagner, Benny D. Freeman, Lora G. Toy, Raghubir P. Gupta; Science 3 February 2006: Vol. 311. no. 5761, pp. 639–642; DOI:10.1126/science.1118079
NSF animation of the diffusion process