Improving flow battery electrodes Featured

    Summer Scholar Alexandra Oliveira contributes to work on redox flow batteries in Brushett Lab.

    Renewable energy technologies such as wind and solar are unpredictable and intermittent, creating a need for batteries to store electricity until it is needed, notes MIT Postdoctoral Associate Antoni Forner-Cuenca. Yet cost-effective technologies have been limited to date.

    2017 MPC-CMSE Alexandra Oliveira is working under Forner-Cuenca in the research group of Fikile R. Brushett, the Raymond A. (1921) and Helen E. St. Laurent Career Development Professor of Chemical Engineering at MIT, to improve the chemistry of porous carbon electrodes in one particular type of battery known as a redox flow battery.

    “Redox flow batteries are a very promising technology for large-scale energy storage, and, in particular, the work that Alex is doing with us over this summer internship is focusing on modifying the surface chemistry of these porous electrodes,” Forner-Cuenca explains.

    During a visit to the lab, Oliveira, who is a junior chemical engineering major at the University of Connecticut, described how liquid electrolytes that are separated by a selective nafion membrane flow through the water-based battery cell and back out. Because the nafion membrane is a selective membrane, ions are exchanged across it, but the two electrolytes circulate in separate halves of the battery. Leads connected to the battery allow the researchers to send through a voltage and then measure the current. “In this case, what we're using it for is to take a capacitance reading, which is an electrochemical property that we can then use to calculate the real surface area. So we want to know the entire area over which we are going to be reacting our electrolyte,” Oliveira says. These electrochemical methods provide valuable information about the material properties like the surface area, Forner-Cuenca says.

    Oliveira finds that carbon cloth electrodes, which transmit fluid through open pores, are among the more interesting and successful electrodes. “If you look up to the light, you can see how porous it is,” she says. “So the electrolyte can flow through this much more effectively than some of the other electrodes we have. For her summer project, Oliveira will be growing polymers onto the surface of the carbon electrodes to see if she can change the chemistry of the electrode and then measure the overall performance of the redox flow batteries.

    “In a flow battery, the electrolyte flows through the cell,” Oliveira explains. “A lot of research has been done on the electrolytes and active materials but not much on the electrodes themselves. So we’re actually looking at the microstructures of the electrodes and comparing to see what makes the perfect flow cell electrode.”

    Summer Scholar Alex Oliveira DP Web
    2017 MPC-CMSE Summer Scholar Alexandra Oliveira holds a metal connector that goes into a redox flow battery. Oliveira is interning in the Brushett lab this summer, focusing on improving the chemistry of porous carbon electrodes. Photo, Denis Paiste, Materials Processing Center.

    Oliveira’s work focuses on the porosity and permeability of the electrodes. “We’ve been running all sorts of pressure tests to measure permeability, and we are trying to figure out what the surface area of the electrodes is exactly, not just the geometric area on the top, but the area of the electrode that will react electrochemically when we run our flow cell,” she says. Oliveira also plans to use an electrochemical technique called electrographing, in molecules are added to the surface of the electrode in a thin layer. “What we can do then is we can keep the same electrode but we can change the chemical properties of it,” she explains.

    Forner-Cuenca suggests that the electrode is the heart of the flow battery in the sense that it has to fulfill different functions. “It needs to be a platform where the liquid electrolyte distributes well, at the same time, electrochemical reactions will happen on the surface of the fibers,” he explains. “So you can imagine that electrode as a very porous, carbonaceous material, which has carbon fibers that are kind of cylinders and the liquid electrolyte will wet those surfaces. The electrons will go from the electrolyte to the carbon fiber, or vice versa, and electrochemical reactions will happen. So you need to have a very selective material. You need to have activity for the products you want, and not secondary reactions, and at the same time it needs to be a material with mechanical edge stability. That means this is going to be inside the flow battery for many years and is going to be compressed under pressure so the electrode also needs to be also mechanically stable. So there are all these different properties that we need to optimize in order to have an advanced flow battery and we are focusing on the electrode in this particular work.”

    “What makes redox flow batteries so unique is that they can completely decouple power and energy,” Oliveira explains. “So if you think of your car, in this case, the engine and the fuel tank would be completely separate. So for our purposes, our reactor and our tank control power and energy separately.”

    Oliveira’s internship is supported in part by NSF’s Materials Research Science and Engineering Centers program [grant DMR-14-19807]. Participants in the Research Experience for Undergraduates, co-sponsored by the Materials Processing Center and the Center for Materials Science and Engineering, will present their results at a poster session during the last week of the program. The program runs from June 15, 2017, to August 5, 2017, on the MIT campus in Cambridge, Mass.

    – Denis Paiste, Materials Processing Center
    July 31, 2017