Energy storage is near the top of the priority list at MIT and research institutions around the world, and for good reason. Cost-effective energy storage will be essential for making intermittently-generated renewable energy more competitive with fossil fuels.
MIT Chemistry Department
Next generation energy storage and conversion devices use a wide variety of active materials. Yet, they all share a common set of underlying processes. “In all these technologies, you’re taking electrons and storing them in a chemical bond,” says Yogesh Surendranath, an assistant professor of chemistry at MIT. “It could be the ions that flow through a battery or the chemical bonds that you make or break in a fuel cell or electrolyzer, or the charges you add to a redox flow battery. It all involves the conversion of electrons into chemical energy.”
The chief mission of the Surendranath Lab is to control the properties of the interface, or boundary, between a conductor and liquid electrolyte solution where energy is converted. By doing so with molecular level precision, Surendranath aims to make energy storage more “efficient, selective, robust, and long-lived,” he says.
The movement of electrons across an interface can be classified as outer or inner sphere electron transfers. Outer sphere transfers are found in standard batteries, in which the electrode serves as an “inert current collector,” says Surendranath “The electrode simply delivers the electrons, which basically hop into solution or the charge storage active material. The electrodes themselves don’t form or break chemical bonds.”
The interfaces that primarily interest Surendranath use inner sphere electron transfers, in which bonds are made and broken synchronously with the electron transfer. For example, in a fuel cell, the electrodes use inner sphere transfers to release the energy stored in the fuel.
“Most emerging energy technologies use inner sphere transfers to store charge in the most energy-packed way possible,” says Surendranath. “This usually involves pairing an electron with another material in a chemical bond. Making that happen, especially with cheap, abundant, scalable materials, is really difficult.”
The Surendranath Lab is not only studying such interfaces, but is developing new catalysts by precisely tuning interfacial structure. “Sometimes it involves combining organic with inorganic materials or nanostructuring materials to expose certain facets that will make or break bonds,” says Surendranath. “Sometimes it means coupling a mediator in solution with an electrode. All these strategies are used to engineer interfacial structure at the atomistic and molecular level.”
Spicing up Carbon
One of the lab’s most promising projects is the study of the interfacial chemistry of carbon, a material that often occupies more volume in electrochemical conversion devices than the active material itself. “Carbon is in your phone battery, in fuel cells, and in redox flow batteries, so the strategies we develop to improve carbon’s reactivity could have broad applicability,” says Surendranath.
So far, the chemistry of carbon surfaces has remained ill defined and hard to modify. Yet, the Surendranath Lab has begun using scalable organic chemistry tools to engineer carbon surfaces at the molecular level. In one project, the lab is designing embedded functional groups on the carbon surface that are fluorinated.
“Fluorine-carbon bonds, which are used in making Teflon, are really strong,” says Surendranath. “We want to make carbon surfaces that are as robust as Teflon so you could greatly extend the lifetime of your battery.”
In the case of fuel cells, you want to make the carbon electrode much more catalytically active for inner sphere charge transfer. In the case of a redox flow battery, however, the carbon interface should be much less reactive, thereby enabling more selectivity for the outer sphere chemistry. “With redox flow you don’t want your electrode to be a bad actor and undergo parasitic reactions or degrade over time,” he says.
In either case, the Surendranath Lab has found ways to tune surface properties by changing small functional groups or handles. “Once we have a protocol in hand, we can change that in a systematic way to get the desired property, whether it’s longevity, chemical inertness, or robust catalytic activity,” says Surendranath.
Bridging the Phase Boundary in CO2 Reduction
One of the Surendranath Lab’s most ambitious projects is the study of the interfacial reactivity at phase boundaries that occur when you combine two different materials. Examples include industrial catalysts used for the Fischer–Tropsch reaction, where hydrogen and carbon monoxide gas are converted into long-chain hydrocarbons, or the catalytic reforming process used to change the octane rating of gasoline.
“Catalysts are almost never made of a single material with a single chemical or crystal structure,” says Surendranath. “They almost always involve a catalyst on a particular support or bound to a different type of material, like a metal bound to a metal oxide or metal sulphide. The reactivity exposed at a metal/metal oxide boundary is very different than what happens with the metal alone.”
The research could lead to more affordable ways to accomplish the “costly, high-end, high temperature processes used by industry,” says Surendranath. “We want to turn it into a low temperature process that could be used, for example, to convert electricity from carbon dioxide into a fuel like ethanol.”
Directly converting CO2 from the air into a liquid fuel is a challenging reaction because many fuel products have similar energy content, necessitating highly selective catalysts.
“The selectivity gets really hard,” says Surendranath. It is particularly difficult to convert CO2 into a single fuel product “because of all the other reactions that could happen just as easily.”
His lab is exploring a number of techniques to make the appropriate phase boundaries, including adding molecules to the surface or even nanostructuring a new surface. “It’s all about the catalyst, and how the interfacial structure looks and what active sites are on display,” he says.
The technology has a long way to go, but the payoff could be immense, says Surendranath. “If we can accomplish that energy [conversion] reaction by exploiting the phase boundary, it would be a game changing way to make synthetic fuels. All the challenges involved with doing energy storage become a lot easier because you now have an energy dense way to store electricity. You can combust it directly with current infrastructure, or use a fuel cell to recover the energy later on.”
Ultimately, the research could lead to a “room temperature, perhaps even portable device that could directly make fuel from a renewable energy source,” says Surendranath. “If you have a solar farm and have a CO2 sequestering unit, you could do onsite conversion to fuel.”
Nanoscale Tools to Tame Tiny Catalysts
Almost all the catalysts the Surendranath Lab works with range from two to 20 nanometers. “To maximize the surface area to volume ratio, you have to shrink the catalyst particles as much as possible while still displaying the same or better catalytic properties compared to macro-sized catalysts,” says Surendranath.
To see what’s going on with these nanoscale particles, let alone try to engineer them, the Surendranath Lab depends greatly on the latest imaging technologies. These include vibrational or non-linear optical spectroscopy, as well as new nanoscale microscopy techniques.
“We can now do advanced transmission electron microscopy where we can image individual atoms on the surface of a nanocrystal,” says Surendranath. “This gives us unprecedented power to understand how the surface structure relates to the chemical reactivity in ways we could only dream about 10 years ago. We try to correlate how the structural features directly transfer into catalytic properties. Nanoscience tools allow us to select the faceting, the surface composition, and the bound ligands, all of which help tune the reactivity of the interface.”
In the CO2-to-fuels project, for example, such techniques are used to engineer a nanocrystal shape so that it displays a faceting that is ideal for the selective generation of a particular type of fuel. “You can play a lot of interesting games at the nanoscale, like generating metastable phases that give rise to outsized reactivity, or exchanging ions to turn one material into another,” says Surendranath.
One area in which the lab has seen considerable progress is tuning alloys for CO2 reduction. “At the nanoscale, you can tune the alloy composition in ways you could never do in the bulk because the lattice structure is more fluid,” he says. “You’re not constraining the material by the forces of an extended solid.”
The new tools have also enabled the lab to develop entirely new fabrication techniques. “We can now grow metal sulfides in an atomically precise layer-by-layer fashion within an electrochemical environment,” says Surendranath. “Because we can change the metal composition, it allows us an unprecedented level of control in terms of tuning catalyst activity and selectivity.”
In the process of exploring the interfacial chemistry of energy conversion, Surendranath hopes to “transform our energy landscape,” he says. “Ultimately, we want to show that chemistry and electricity can be viewed as one and the same.”