Making things that didn’t exist before. It was that possibility that initially led Paula T. Hammond, the David H. Koch Professor in Engineering, to study the sciences. Growing up in Detroit, she had a wide range of interests. She thought she might become a writer, but a chemistry course her junior year of high school gave her a glimpse into a possible future in science. “I was really intrigued by [the course] because it had a lab that allowed me to actually make new materials and look at color changes and transformations that were created just through chemistry,” she remembers. Nowadays, Hammond has her own lab and is using materials science to create novel solutions to today’s energy challenges. In exploring different material properties, she came to recognize that materials could have a major impact on a range of energy systems. Indeed, she believes that innovation in materials science could “allow us to get past some of the barriers that are known for batteries… for fuel cells…and for photovoltaic and solar applications. A lot of those limitations are materials science limitations.”
To that end, much of Hammond’s work involves creating and deploying polymers, a tool she first became interested in while an undergraduate at MIT. Broadly speaking, a polymer is a macromolecule made up of repeating structural units. Polymers can be either naturally occurring (natural rubber, cellulose) or synthetic (nylon, polyethylene) and can have a wide variety of properties and uses. Chemically modifying those polymers further expands the possibilities. “You can think of a polymer chain as something upon which you can hang different functional groups by attaching them to the side groups of polymers and still retain the property of the primary polymer, which has a flexible backbone,” she explains. Hammond is fascinated by the potential for using polymers to create dynamic, functional materials. “The fact that polymers can be extremely active—can be the active component in the system—always got me excited,” she says.
To achieve the benefits of combined polymers, Hammond and her team have pioneered new methods of “layer-by-layer assembly.” Normally, polymers do not mix readily. “The rules of thermodynamics are such that two large molecules have a very low driving force for mixing because there’s no real gain in entropy—unlike with a bunch of small molecules mixing together,” she explains. Her team’s novel approach uses electrical charge to assemble macromolecules that have unique properties. Applying this design concept, they have greatly improved the performance of methanol fuel cells and of carbon nanotube thin films for battery electrodes.
The fuel cell work serves as a striking example of Hammond’s research approach. Methanol fuel cells have many favorable characteristics: Methanol has high energy density, is easy to transport, and is much more stable than hydrogen, which is also used in fuel cell systems. But today’s devices have a major drawback. The most commonly used electrolyte material—Nafion—efficiently carries ions from electrode to electrode, but it is expensive, and it is permeable to methanol, which causes fuel seepage and efficiency losses. To preserve the high ion conductivity of the Nafion membrane while reducing its permeability, Hammond and her team tried coating it with a thin film. They found that just six layers yielded a 50% increase in power—a major gain requiring little material use. In collaborations with colleagues, her group is also developing a fuel cell with no Nafion at all. In its place they use a porous membrane coupled with a membrane interface of their own design and construction.
Clearly, Hammond and her group do not shoot only for incremental improvement. Indeed, they push themselves to the edge to develop novel systems— and then consider how to make them practical at large scale. Says Hammond, “With most of our systems, we’re thinking about how we can go big. Can we roll this out—literally? Can we spray-manufacture a system that does this? Can we develop a system that uses a small amount of a unique or unusual material but gets high value and high gain from that small amount?”
For Hammond, part of the appeal of working with polymers is their aesthetic beauty and the thrill of interacting so closely with potent natural forces. Her PhD research—also at MIT—involved polymers that change color when they are stretched. “There’s a visceral joy in watching that happen or in looking under the optical microscope and observing a liquid crystalline phase, which is just gorgeous,” she says. “And the topper is…knowing that your research can have this huge potential impact on humankind. That makes you push to find the system that works.”
Hammond relishes the fact that her neighbors on campus are similarly inspired. “The thing that gets me about MIT is that you can turn to your left or your right and you’ll find someone interesting and engaging who’s doing top-of-the-line work, who’s at the edge, pushing a frontier, and you can engage with them and find something to work on,” she says. Hammond has been an active collaborator, notably teaming up with Angela Belcher, W.M. Keck Professor of Energy, and Yet-Ming Chiang, Kyocera Professor of Ceramics, to develop and construct microbatteries using, for the first time, a microcontact printing and virus-based process to assemble the battery’s anodes. “It’s an extremely collaborative community and also a continually stimulating one,” she says with a laugh. “Some days you almost get a little overwhelmed by how stimulating it is.”
Recently, Hammond was part of a faculty team, led by colleague Clark Colton, professor of chemical engineering, that developed a new class for undergraduates. In 2009, the Energy Education Task Force launched MIT’s new Energy Studies Minor and was looking to expand the set of interdisciplinary, energy-related, project-based classes. Hammond worked with Colton and others to put together a curriculum for 10.27, Energy Engineering Projects Laboratory. Each faculty member devised plans that would both draw on ongoing research and engage students from across the Institute. In Hammond’s case, the students worked on the methanol fuel cell, a front-line project. “It ended up being a really synergistic group because we got to talk about fundamental engineering questions—transport through the membrane and why changing this parameter is going to give us this effect, for example—but we also tried entirely different things than we would normally try.” Based on their own ideas, the students made membranes and performed experiments to see how well their systems worked. Says Hammond, “The class allowed them to have their hands on everything.”
Energy is not the only field in which Hammond’s novel techniques are proving valuable. In other work, she is developing a new drug delivery system that uses a nanoparticle, built using layer-by-layer assembly, to target cancerous tumors based on their acidity. She also has helped develop bandages that utilize a nanoscale coating of thrombin, a clotting agent, to quickly stop bleeding. When beginning her career, Hammond did not expect to be active in fields as diverse as energy and biomaterials, but both share a common feature: ample opportunity to make something that didn’t exist before, and to make it count. “The thing that really gets me going is the ability to create a polymer material that is able to adjust, adapt, or perform because of its response to light, to a field, to human physiological pH, or to the endosome of a cancer cell in a way that gives you the desired effect,” she says. “It’s the ability to design a polymer to do what you want, essentially, that really excites me.”
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