Building better chemistries for renewable energy
A world that runs on solar and wind energy needs better ways to convert and store that energy. Yang Shao-Horn, MIT's JR East Professor of Engineering, focuses on this global challenge, conducting research at the boundaries of chemistry and physics to improve the options for converting chemical energy to electrical energy.
Often collaborating with industry partners, Shao-Horn develops advanced battery technologies, hydrogen-based energy solutions, and other technologies to help decarbonize energy, transportation, and industrial processes.
One of her major efforts is to find improved ways to store electricity from solar or wind in chemical forms, such as reducing water to generate hydrogen or reducing nitrogen to make ammonia. For example, today when we make ammonia from nitrogen and hydrogen, the hydrogen is generated from fossil fuels. Potentially, we could instead turn to "green hydrogen" made with renewable energy. Optimizing production via this sustainable alternative is key for decarbonizing the manufacture of ammonia, a huge generator of greenhouse gases, she says.
In another line of research, Shao-Horn examines how catalysts interact with ions - charged particles - in electrolytes. For the past decade, she and her colleagues have investigated methods to change the electronic structure of catalysts such as metal oxides for greater catalyzing efficiency.
Shao-Horn examines how catalysts interact with ions (charged particles) in electrolytes.
"We want to minimize the barriers for all the different steps in order to transform one molecule to another form, let's say from water to oxygen," she says. "We have developed or discovered new oxide chemistries with record-setting catalytic activity."
In the past five years, her research group – the Electrochemical Energy Laboratory - has broadened its research beyond the electronic structure of the catalyst to experiment with the chemical physics of the electrolyte. The researchers have shown that by controlling the electrolyte's water structure, they can boost catalytic activity by two or three orders of magnitude.
In addition, the MIT team has demonstrated that coating the catalytic surfaces with ionic liquids that can boost the exchange of protons from the electrolyte can change the catalytic activity of oxygen reduction.
Such technologies may enable devices that can produce hydrogen at a much lower cost, easing the use of hydrogen fuels in transportation, Shao-Horn says.
Another major theme in her lab is improving batteries - such as the lithium ion batteries - in today's electric vehicles. In these batteries, lithium ions are carried by an electrolyte from a negative electrode to a positive electrode as energy is discharged, and back again as the battery is recharged. "We are looking at ways to develop better electrolyte-enhanced stability between electrolyte and electrode, so that we can store more energy per unit weight or unit volume," she says.
One strategy is to enrich the amount of nickel in the negative electrodes, replacing cobalt, which could potentially lower the cost and increase the energy density. The downside for this approach has been that such batteries can't be recharged as many times. But Shao-Horn has partnered with MIT colleagues Jeremiah Johnson, professor of chemistry, and Ju Li, professor of nuclear science and engineering and of materials science and engineering, on new electrolyte solutions that stabilize the electrolyte/electrode interface to overcome this problem.
Similarly designed electrolytes may also allow replacing graphite in positive electrodes with lithium. In a practical prototype, the technology might produce energy density approaching 500 watt-hours per kilogram, double what we see in car batteries now, Shao-Horn says. Such batteries might also prove powerful enough to fly future electric planes.
"These inventions are built upon a decade of work of understanding fundamentally how the conventional lithium ion battery electrolyte reacts with the nickel-rich positive electrode materials we use in those batteries," she says.
The advances also drew on a separate line of research done in collaboration with Johnson on lithium oxygen batteries, one of many battery architectures the Shao-Horn lab has investigated.
Lithium oxygen batteries offer the promise of much greater energy density for a given weight, but the oxygen chemistry is extremely reactive. Lithium oxygen batteries will not likely be practical for years, but the collaborators developed an electrolyte solvent that greatly enhanced stability in these batteries.
With that achievement in hand, "Ju Li, Jeremiah Johnson, and my group came together spontaneously to apply the new solvent from our lithium oxygen experiments to lithium ion batteries," she says.
The long-term collaborations that are paying off in lithium ion batteries are key to overall progress in energy conversion and storage, Shao-Horn emphasizes. "Support for open-ended fundamental research is really critical to seed scientific innovation in batteries or in fuel cells," she says, "because invention and discovery very often are unexpected."
Her lab's engagement with one corporate sponsor, BMW, offers a positive example of partnership. BMW for instance asked what happens over time with lithium ion batteries. "We need to predict life for lithium ion batteries for 10 years, but we have no idea what's happening on the electrode/electrolyte interface," Shao-Horn says. Her team has played a leading role in gathering research groups with relevant expertise in batteries, materials, computation, and surface science to understand the fundamental molecular processes underway over time.
"MIT students, postdocs, and faculty can look at a problem deeply and from unconventional ways of thinking," she emphasizes. "With an essentially unrestricted path to explore solutions, we tend to be more creative and we tend to be more innovative."
Some of the resulting discoveries will pay off in other applications and industries. Shao-Horn is now looking to engage traditional large energy companies and large chemical companies to develop science and technology roadmaps for a sustainable world. "What technology solutions do we need at the end of this decade, in 20 years, and in 30 years to truly achieve zero carbon emission by 2050?" she asks.