Thomas Edison went through thousands of materials before he finally found the right tungsten filament to create a working lightbulb. This type of trial-and-error research continues today and is responsible for countless inventions that improve our world. Battery systems that help power our lives in many seen (and unseen) ways are one example.
However, improving these materials and devices requires more than experimentation. Modern engineers must also form a deeper understanding of the general principles that govern material performance, from which they can design better materials to achieve challenging product requirements.
In a paper published Aug. 13 in the Proceedings of the National Academy of Sciences (PNAS), University of Delaware, Northwestern University and industry researchers report expanded understanding on how electrons move through the conductive parts of complex fluids called slurries that are found in electrochemical devices such as batteries and other energy storage devices.
It's important work that can help overcome existing knowledge gaps about how electrons hop between conductive particles found in these materials, as engineers seek new ways to improve that activity.
The paper is the result of collaborative research between UD's Norman Wagner, Unidel Robert L. Pigford Chair in Chemical and Biomolecular Engineering, and researchers led by Jeffrey Richards, assistant professor of chemical and biological engineering at Northwestern University, and a former UD postdoctoral researcher. Lead authors on the paper include UD alumna Julie Hipp, who earned her doctoral degree in chemical and biomolecular engineering in 2020 and now is a senior scientist at Procter and Gamble, and Paolo Ramos, a former NU graduate student now at L'Oreal. NU doctoral candidate Qingsong Liu also contributed to this work.
According to Wagner, by combining carefully designed and conducted experiments with state-of-the-art theory and simulation, the research team found that enhancing performance requires more than formulation chemistry. It also requires understanding how the electrical conductivity behaves as the slurry materials are processed and manufactured.
"To control the device performance, it's not enough just to control the chemistry, we have to control the microstructure, too," said Wagner. This is because the material's final microstructure — meaning how all the components come together — regulates how the electrons can move, impacting the device's power and efficiency.
Performance depends on the details
Though many electrochemical devices exist, let's stay with the battery example for a moment to break things down.
Batteries supply electricity when electrons move through a solution or "slurry" made of conductive materials and solvents via a chemical reaction. How well the battery system works depends on the materials, which includes both the chemistry and the manufacturing processes used in its creation.
Think of it like multiple racecars going around a racetrack. All the racecars have steering wheels, tires and engines, but the structure of each vehicle and how it's assembled may differ from car to car. So, just because a car with an engine and a steering wheel is on the track doesn't mean it gets the same performance as the other vehicles. The same is true for the critical components in batteries. The details matter in how you put them together.
Conductive versions of carbon black (or soot) are commonly used in batteries as well as a vast number of electrochemical devices. They are nano-sized crystals of carbon made in such a way that they stick together and form aggregates, or clusters, that can be mixed with various liquids to form a slurry. This slurry is then used to cast, or make, parts of a battery or other devices.
"In that mixture, electrons can move very fast within the carbon black, which is highly conductive like an electrical wire. But the electrons have to hop from one cluster of carbon-black particles to another because the carbon black is suspended in the slurry — the aggregate particles are not connected as a solid structure," explained Wagner.
The researchers had previously shown that the way the carbon black material flows — its rheology—plays a key role in the material's performance, using neutron-scattering techniques at the National Institute of Standards and Technology's Center for Neutron Research in Gaithersburg, Maryland, through UD's Center for Neutron Science. In this new study, the research team extended that work to create a universal roadmap for understanding how the conductivity of the flowing slurry depends upon the chemistry of the components from which it is comprised and — importantly — how the slurry is processed.
Together, these pieces form a blueprint for how to process energy storage devices during manufacturing. The promise in this kind of roadmap is an enhanced ability to systematically design materials and predict the behavior for electrochemical devices on the front end.
"What we've studied allows us to begin to understand how the structure of this carbon-black slurry, this aggregated suspension, impacts the efficiency and performance of these devices," said Wagner. "We're not solving anyone's specific battery problem. The hope is that others in practice can apply our foundational work to their own electrochemical systems and problems."
The researchers expect this work will have an impact on the formulation and processing windows for emerging electrochemical energy storage methods and water deionization technologies.
Wagner gave the example of electrolyzer devices that use electricity to split water into its component parts of hydrogen and oxygen. One of the most challenging parts of this process is mixing and controlling the properties of the material solutions that enable the electrolyzer to do its work and free up hydrogen molecules so they can be used for other purposes, say, as an energy resource. According to Wagner, future improvements in such devices will depend on processing.
"You can get the chemistry right, but if you don't process it right, you don't end up with the performance that you want," Wagner said.
