Innovative energy storage devices such as tiny, flexible batteries, solar-powered fabrics and fuel cell components that actually build themselves are the focus of a new MIT Energy Initiative (MITEI) center.
The Center for Self-Assembling Materials for Energy is directed by an MIT chemical engineer and a materials scientist who have come up with a way to mimic nature’s ability to self-assemble complex systems from disparate elements. Through their novel techniques, the researchers hope to revolutionize the design and manufacture of a variety of energy devices.
With seed funding of $250,000 from MITEI, the researchers are developing, among other things, easily commercialized techniques to manufacture more efficient thin films for photovoltaic devices and batteries that are low-cost, less toxic, lightweight and flexible.
Thin, lightweight solar cells that can be incorporated into plastic films or fabrics for tents, backpacks, soldiers’ gear and more are among the portable and easily deployable power sources that could result from the center’s work.
Nanoscale control
Through a technique that provides control down to the nanometer level, the researchers are manipulating the interface between organic and inorganic components to optimize desired characteristics.
While silicon is the most common material for photovoltaics, silicon solar cells are heavy, rigid and expensive to manufacture. The MIT researchers’ approach could incorporate a wider range of materials to create new kinds of electrical components and thin films.
“By generating nano-structured versions of cobalt and gold, we can greatly increase the efficiency of traditional photovoltaics,” said Paula T. Hammond, Mark Hyman Jr. Professor of Chemical Engineering and center co-director. “Exploiting nano-assemblies of inorganic and organic materials such as titania and ion-transport polymers may significantly boost the efficiency of converting solar light into electricity.
“If we could generate structures only a few nanometers long that maintained a densely packed structure, it would be possible to significantly increase the surface area of photovoltaic metal oxides such as titania or other semiconductors,” she said.
Self-assembling batteries
Center co-director Angela M. Belcher, the Germeshausen Professor of Materials Science and Engineering and Biological Engineering, genetically programs bacteria and viruses to build solar cells and batteries. Given a certain genetic code and the right ingredients, she said, the organisms self-assemble into tiny, nanoscale working devices and structures such as semiconductors. When the process is complete, there is no longer any living entity in the component, although it does contain organic parts.
The tube-shaped viruses are several nanometers wide and a micron—a hundredth the width of a human hair—long. They work as templates by attracting different metals or metal oxides from solution, resulting in densely ordered nanowires on solid-state components.
Layer-by-layer deposition of materials is a way to make a kind of sandwich with nanometer-scale layers or slices. It is an inexpensive and relatively new water-based process for manufacturing extremely thin films that can be used in electrochemical devices such as displays and optical coatings.
Hammond’s research group builds up the films with the help of ionic charges. Positively and negatively charged polymers or nanoparticles can be added by alternately dipping the substrate into solutions containing the plus- and minus-charged species. The negatively charged viruses are kinetically arranged on the top surface and used as a template to complete the assembly of the sandwich-like film.
The ionic bonds holding the components together are strong and stable. The process, which takes place at room temperature, allows each layer to be precisely patterned and fine-tuned, Hammond said. The technique, which allows ultrathin films to be produced in large quantities at low cost, could be used for water-based processes, which would be handy for fuel cells; or in the dry state, for batteries. MIT researcher Yet-Ming Chiang, Kyocera Professor of Materials Science and Engineering, an integral member of the Center for Self-Assembling Materials for Energy, also works on creating batteries as small as a grain of rice that cram as much electrical energy into as small or lightweight a package as possible.
Substrates can range from simple polymer materials based on acrylics or polyesters to more complex materials such as quantum dots or nanoparticles.
The high surface areas and dense packing of the systems being developed by the center could make ideal batteries and solar cells because energy can be stored over a broader surface area, significantly improving efficiency, Hammond said. The ultrathin nature of the films and the use of materials that interface at the nanometer scale can allow for greater efficiency as well.
Mimicking nature
Belcher, who applies natural processes to the creation of new materials, points to abalone shells as an example of a self-assembling system. “Around 500 million years ago, organisms challenged by changing ocean environments started making hard materials, because all of the sudden, they had the opportunity,” she said. “Male and female abalone make millions and millions of baby abalone and build beautiful materials. They don’t use any toxic materials and they don’t add toxic materials back to their environment.”
Human beings are themselves “examples of self-assembling, self-correcting systems,” she said, so it’s not so far-fetched to think of such systems being put to technology’s use.
“Why didn’t organisms make solar cells and batteries? They just haven’t had the opportunity yet,” Belcher said.