What’s in a name? More than you'd care to think about when it comes to energy storage, a team of researchers from France and the United States argued last week in the journal Science. As the energy storage field has taken off in the past five to seven years, the line between batteries and supercapacitors (also called ultracapacitors) has started to blur and scientists and engineers have become less and less consistent when naming these devices, the researchers say.
Much too often, battery materials are called supercapacitors in the scientific literature, unknowingly or perhaps deliberately, says Yury Gogotsi, a materials science and engineering professor at Drexel University and one of the authors of an essay in last week's Science. “Confusion doesn’t help progress,” he says. “Attempts to sell a poor material as a good one by using wrong terminology really holds back research and leads to a waste of money and time.”
To understand, let’s cover the basics. Batteries store charge through a redox reaction, which involves a material giving up electrons and the transport of ions through some other material. So batteries can store a lot of energy but they typically take hours to recharge.
Supercapacitors, also called ultracapacitors, store charge electrostatically on high surface-area electrodes. They store less energy but can charge or discharge in seconds. They’re commonly used to provide short bursts of power in buses and cranes and hold much promise for electric cars and the green grid.
What everyone wants is a device that can store a lot of energy and charge or discharge quickly.
Between 2006 and 2013, the number of materials research articles with the word supercapacitor or ultracapacitor in the title more than quadrupled, Gogotsi says. Technology and terminology mixups tend to happen in such rapidly growing fields. The field of graphene faces similar naming confusions.
In the case of energy storage, one reason for the confusion is that new materials—especially those on the nanoscale—combine the characteristics of the batteries and supercapacitors. This throws off scientists who are new to the field. “There are many people from a chemistry, materials science, nanoscience background who don’t know engineering rules [or] understand electrical engineering or electrochemistry well enough.”
But sometimes, scientists will deliberately try to sell a battery material as a supercapacitor, Gogotsi points out. “Numerous papers in the past year take materials like cobalt or nickel-based compounds which are typically battery materials, and represent them as pseudocapacitors,” he says. “They show up to 10 times higher capacitance than common supercapacitor materials. The problem is they are not, because they don’t have other characteristics of supercapacitors, such as thousands to a million of cycles lifetime. It’s really a battery material simply packaged and sold in literature as a supercapacitor. So an investor may invest money not understanding that they’re simply funding a type of battery development.”
So how do you define a battery versus a supercapacitor?
“The defining character is really the electrical response,” Gogotsi says. A linear change of the electric potential during discharge typically results in a constant current for a supercapacitor, whereas in a battery the current peaks at two specific potentials because of the oxidation (electron loss) and reduction (electron gain) reactions that happen at the electrodes.
Researchers need to look at the material’s electrochemical characteristics and then accurately name the device. This responsibility falls on scientists but also on journal editors and reviewers.
It’s really a matter for the entire research community, Gogotsi says. The community needs to agree on definitions and terminology, he adds. Standards would help, but they’re neither easy to create or enforce. “We know from experience that engineers like and use standards,” he says. “Scientists? Not so much.”