Chemical storage for renewable energy
Working with collaborators from the MIT campus to research groups in California and Singapore, MIT Professor Yang Shao-Horn’s group has produced research that could lead not only to practical improvements but to better scientific understanding of underlying reaction mechanisms in catalysis. The research is applicable to fuel cells, lithium-air or metal-air batteries and electrochemical photo-induced reactions.
|MIT Professor Yang Shao-Horn. Image courtesy of MIT Energy Intiative: Stuart Darsh, Copyright 2013|
"To build a device using, let’s say using either lithium as an energy carrier, or hydrogen as an energy carrier, you will have to combine either lithium or hydrogen with oxygen to form lithium oxide or water – so that basically is the energy generation process, where you can actually extract electricity from chemical energy. If you want to store that energy, you need to convert the electrical energy from the renewable source, say a solar cell, into a chemical form – that is where you need to do water splitting to generate the hydrogen or to do oxidation of metal oxide to generate lithium. So essentially if you want to utilize oxygen for storage and also for generation of electricity, we need to figure out how to promote (the) kinetics of oxygen reduction and oxygen evolution, because the kinetics of oxygen electrocatalysis really limits the efficiency, how we convert energy," Shao-Horn said.
Recent Electrochemical Energy Lab work conducted by materials science graduate student Kelsey Stoerzinger focuses on promoting the oxygen reduction reaction by tuning the chemical nature of the earth-abundant element manganese, while that of materials science graduate student Wesley T. Hong uses X-ray photoelectron spectroscopy and other techniques to characterize dynamic chemical changes in transition metal oxides such as lanthanum strontium cobaltite. Precious metals such as platinum and gold can be used as catalysts for these reactions, but they are expensive and operate at limited efficiency. "Each time we convert from chemical to electrical (energy), ideally we want the conversion efficiency to be 100 percent, and currently if you are going to use oxygen, the efficiency is fairly limited to 70 to 80 percent," Shao-Horn said. Her lab is working to develop cheaper, more efficient catalysts made up of earth abundant elements. "We look for design principles, for example, what are the key governing parameters that can control activity of oxides by several orders of magnitude, and then we (utilize and tune that design, or) apply the design principle, tune the governing parameter, to enhance the catalytic activity, and we seek more molecular level, or atomic level, understanding in terms of reaction mechanisms. What are the active sites, what are the limiting steps?" Shao-Horn said. The lab measures chemistry and structure under realistic operating conditions on model electrode surfaces. “We look at how we can store electricity efficiently in chemical form, through electrochemical technologies such as batteries and fuel cells, capacitors and flow batteries. We are interested in utilizing oxygen as a key component in the electrochemical storage devices, so we have to learn how to promote the reaction kinetics of oxygen reduction, oxygen evolution, to really understand the mechanism of oxygen electrocatalysis," Shao-Horn said.
Connecting oxide physics/chemistry with functions
A research effort led by Dr. Jin Suntivich (Ph.D. 2012), a former group member and now Assistant Professor in Materials Science/Engineering at Cornell University, established that the electronic configuration of the transition metal in oxide catalysts can serve as a design principle to screen for compounds that more efficiently promote oxygen reduction and evolution. The work was followed by a demonstration that a compound of barium, strontium, cobalt and iron, predicted by the design parameters, had oxygen evolution reactivity at least an order of magnitude higher than iridium oxide catalyst in alkaline media.
Stoerzinger studied thin films of manganese oxide containing lanthanum, strontium, or calcium, to show that Mn3+ (a manganese ion which has “given up” three electrons) is the active Mn electronic state for the oxygen reduction reaction. This work, conducted with the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory and the Canadian Light Source, opens a path to controlling the amount of Mn3+ and increasing the reactivity of oxides for oxygen reduction.
Hong’s work observed chemical decomposition at the surface of lanthanum strontium cobalt oxide, La0.8Sr0.2CoO3−δ (LSC113 or lanthanum strontium cobaltite), thin films. But LSC113 with the addition of another oxide chemistry, (La0.5Sr0.5)2CoO4+δ (LSC214), decorated on the surface showed no change upon heating. The research team’s report on the material, with potential use as a solid oxide fuel cell cathode, identifies methods for stabilizing surface chemistry for highly active and stable catalysts for oxygen electrocatalysis.
Stoerzinger’s studies are done at room temperature in water-based solution, while Hong’s studies are done by flowing oxygen gas over the sample at 500 degrees Celsius (932 degrees Fahrenheit). “The end application for these two is similar in that they can both be used to catalyze oxygen for hydrogen fuel cells, but they would be used in different types of devices,” Hong said.
Stoerzinger’s work applies to an alkaline anion exchange membrane fuel cell and Hong’s to a solid oxide fuel cell.
“They are really making, I think, great strides towards understanding oxides and oxide physics; how they may control the activity of the oxides. So you’re looking at the future leaders of oxides for catalysis,” Shao-Horn said.