For the Next Generation of Batteries, a Plentiful Compound Shows Promise

Jennifer Hampton, Ph.D. | Professor of Physics

Dr. Jennifer Hampton is captivated by interesting materials — especially those in the electrifying world of electrochemistry. She’s currently studying iron hexacyanoferrate, better known as Prussian blue. This name, unsurprisingly, comes from the compound’s striking hue. Prussian blue has been used as a colorant in numerous applications, from the paint of van Gogh’s “Starry Night” to a stain applied to medical samples to make iron visible under a microscope (such as when diagnosing a certain type of anemia). Beneath the blue, though, are electrochemical properties — invisible to the naked eye, but fascinating to Hampton for the battery power possibilities they hint at.

“I’m interested in straightforward ways to make materials that have interesting, real-world applications,” Hampton says. Her research area, however, is hardly straightforward. “I ended up in this interdisciplinary field, which is sort of physical chemistry/materials engineering/nanoscale science/surface science/electrochemistry,” she says. “The boundaries between the traditional disciplines get pretty fuzzy. I’ve been in that boundary for a while — the physics-y end of electrochemistry — using electrochemistry to make materials.”

Her Hope College lab group has been concocting “analogues,” or variants, of Prussian blue — tweaking the compound and testing each variant’s charge and discharge capabilities. A $169,279 National Science Foundation grant awarded in 2016 supports the project. In a 2020 paper published in the Journal of Electroanalytical Chemistry, Hampton and co-authors reported on related work she did at her grad school alma mater Cornell University during a 2014-15 sabbatical from Hope — research that was the impetus to initiate her current research. In late 2019, she and Hope physics and chemistry double major Amanda Rensmo ’19 published an article in the journal Metals reporting on the project’s findings to that point.

Prussian blue consists of iron, carbon, and nitrogen. It was first compounded in the 1700s, which makes it one of the earliest modern synthetic compounds. It gets its blue color from the electronic transitions in the iron it contains.

In addition to being useful as a pigment, Prussian blue can store electricity, and it’s that feature that has captured Hampton’s professional imagination. The charge-storing properties of the compound and its analogues are similar to the properties of the better-known lithium ion battery, one of the current industry standards. Like the cobalt-oxide and carbon structure that forms a lithium ion battery, Prussian blue and its analogues have a porous structure that ions — electrically-charged elemental particles — can pass into, changing the material’s charge state in the process. As that happens, the structure stores a large quantity of electrons — holds a charge — which can be released later on, the defining feature of a battery.

In the materials Hampton works with, it’s iron that switches between charge states (denoting how many electrons it’s holding onto) as ions move through and electricity is stored and released.

“It’s this open cube, Tinkertoy® kind of structure,” Hampton explains. “There are spaces where ions can flow in and out.”

In lithium ion batteries, it’s lithium ions that flow back and forth as a battery charges and discharges, but with other materials — Prussian blue and its analogues among them — different ions could be used.

“For this material, because of the larger pores in the structure, you can actually use sodium, or even calcium or potassium or magnesium,” Hampton says, “which makes them interesting, because those are more earth-abundant than lithium.”

“There are some applications where you really want to have a good amount of charge storage and not a lot of weight,” Hampton says. (Think laptop.) For those applications, lithium ion batteries are still the top choice, because lithium is so light. “But for other applications where you’re less worried about weight but more interested in it being more affordable, or being more earth-abundant, then these could potentially be more useful.”

To create the analogues that these different ions could pass through, Hampton and her team run raw materials through two processes. First, they deposit a thin film of nickel (or cobalt, or iron, or copper or an alloy) onto a gold or indium tin oxide substrate. This produces a strip of metal several hundred nanometers to one micron thick, which is just one-fiftieth as thick as an average human hair’s thicker end.

Then things get reactive. They apply hexacyanoferrate to the thin film of metal — let’s say they’re using nickel — to fabricate a Prussian blue analogue. “It sounds scarier than it is — because cyanide’s a poison, right?” Hampton says. “But free cyanide is a poison because it binds to the iron in your blood, and the iron in your blood then can’t carry oxygen around. This cyanide is already strongly, strongly bound to the irons, so it’s about as deadly as salt — other kinds of salts. I wouldn’t eat it, but it’s not going to kill you like free cyanide would.”

The reaction isn’t perilous, either. The researchers simply place the thin film and hexacyanoferrate in solution and the two can’t keep apart. A thin film of nickel hexacyanoferrate builds over the thin film of nickel, and the Prussian blue analogue is ready for testing.

“Once we have the film, we can study it electrochemically,” Hampton says. “We can put it back in the electrochemical system without any reactive species in the solution, and look at the reactions that are occurring in the material by forcing it to charge and looking at the current, and then forcing it to discharge and looking at the current again.”

They’re determining how much electrical charge a tiny piece of the material can store and release, and how quickly, and how much it changes based on the smoothness and thickness of the film, and which metal film holds the most promise.

“I don’t think that any one new battery technology is going to be the be-all-end-all of batteries,” Hampton says. “I think it’s going to be more of a variety, for different kinds of applications.”

Lithium ion batteries won’t be disappearing anytime soon, in any case. Personal electronic devices will always need to be light; home and car batteries will always need to be relatively compact. “You get a lot of bang for your buck in terms of that charge for the weight,” Hampton says.

But the downside is it’s not as easy to come by. “Calcium, magnesium, even sodium, are literally in the rocks of the earth,” she says. “Lithium is pretty rare in comparison.” Large-scale, industrial operations could use a cheaper, more abundant battery — even if it’s heavier and bulkier than the industry standard.

Weaving electrochemistry into physics takes an interdisciplinary mindset. It’s one Hampton shares with her students, many of whom are working on majors other than physics.

“The fact that I’ve had students from lots of different fields, Hampton says, “is evidence of the interdisciplinary nature of the work”— call it physical chemistry/materials engineering/nanoscale science/surface science/electrochemistry.

“There’s a lot of exciting science that happens in those boundary regions,” Hampton says.