Polymers Take Charge
Most people think of polymers in terms of plastic boxes, moldings, grocery bags, and a host of other common structural uses where the mechanical properties of plastics are tuned for the purpose of support. Think of your lunch box, for example. Polymers are also everywhere in the form of thin films. The paper in this newsletter is coated with polymers designed to enhance the optical quality of what is probably poor quality paper. Another very common use of polymers is in insulation of electrical wire, to prevent the passage of electrical current. Paradoxically, one of the most exciting new uses of polymers is now to conduct electricity.
You might think this is a new development, but the means by which you sense and even understand the words on this page involve an electrical charge moving along or through polymer membranes. All life depends on membranes that interact with charged species, generally called ions, to produce concentration gradients that give rise to the generation of voltages and thus to the action of nerve cells. Many similar membrane processes occur throughout living things that provide for separation of chemicals and generation or supply of energy, in addition to generating electrical signals. In a case of technology imitating life, emerging energy technologies are making use of membranes that conduct current through the passage of either ions or electrons. The most familiar technology today would be fuel cells for electric vehicles, which use remarkable polymer membrane materials that conduct high currents but also stand up to rigorous chemical conditions. Lest you think these are laboratory curiosities, ion exchange membranes are used in the production of caustic soda and chlorine, an activity that pushes several percent of the nation's electricity supply through the membranes. Electrically conducting polymer membranes are therefore a major factor in energy technologies.
Two other technologies that depend on organic membranes are lithium batteries for electric vehicles and electrochromic windows for buildings. As a result of the Division's Advanced Energy Technologies Department (AETD) battery group's participation in the U.S. Advanced Battery Consortium program to develop batteries for electric vehicles, a polymer synthesis and characterization program has been developed. This activity, sponsored in large part by support from 3M and DOE, has helped the Laboratory provide membranes for batteries and fuel cells that use geometries familiar to anyone who visits the produce section. Figure 1 shows a typical jelly-roll configuration for a lithium battery. If this looks a bit like a roll of scotch tape, it's not surprising. The intent is to use technologies developed for other uses to package the batteries. 3M and polymer companies that make rolls of "poly" bags are adept at this. Although electrochromic windows have not yet made a big impact on the market, electrochromic mirrors are now quite common. These devices automatically darken the mirror to avoid glare and are available in vehicles made by Lexus, BMW, and Daimler-Chrysler. The operation of these devices again depends on driving ions through a polymer separator by means of an electric field. Figure 2 shows the principle of these devices. Ions, electrons, and electron holes, the charges, are driven through the membrane by applying an electric field. This results in the production or storage of energy, separation of chemicals, or production of light.
This last device is a very exciting development that has occurred only in the last decade and has the potential to revolutionize computer screens, for example. At EETD, Steve Johnson, Head of the Lighting Group, has identified the potential of these devices for area lighting. Soon, polymer membranes will be lighting up rooms in Berkeley Lab's Building 62 as well as helping to store or convert the energy needed to run them.
Other polymer developments in EETD involve research in which polymer membranes are used for biological processes. Working with Dick Fish of the AETD, we are developing membranes that support selective enzymatic chemical reactions that mimic in many ways the biological processes. However, by applying the principles developed in batteries and fuel cells and the chlor-alkali industry, we are moving toward accelerating bioprocesses to rates that make the use of biomass feasible as feedstock for chemicals and fuels. The polymer story has only just begun in EETD, so watch for further articles on these exciting fields.
For more information, contact:
- John Kerr
- (510) 486-6279; fax (510) 486-7397
This work is supported by the U.S. Department of Energy, the Advanced Battery Consortium and 3M.