April 29, 2014 | 2 Comments
The flexible material is a made from nanoporous nickel-fluoride electrodes layered around a solid electrolyte. The design delivers battery-like supercapacitor performance that combines the best qualities of a high-energy battery and a high-powered supercapacitor without using the lithium found in commercial batteries.
The new work by Tour’s Rice lab team is detailed in the Journal of the American Chemical Society. The team has flexible, portable and wearable electronics squarely in its sights with the creation of a thin film for energy storage.
The team’s electrochemical capacitor is about a hundredth of an inch thick but can be scaled up for devices either by increasing the size or adding layers, said Rice postdoctoral researcher Yang Yang, co-lead author of the paper with graduate student Gedeng Ruan. They expect that standard manufacturing techniques may allow the battery to be even thinner.
The student’s tests found their square-inch device held 76% of its capacity over 10,000 charge-discharge cycles and 1,000 bending cycles. Your humble writer has double checked – they did say ten thousand cycles. One wonders how many cycles per percentage of capacity loss there would be without the bending.
The numbers that matter are an energy density of 384 Wh kg–1, and a power density of 112 kW kg–1.
Tour explained the team set out to find a material that has the flexible qualities of graphene, carbon nanotubes and conducting polymers while possessing much higher electrical storage capacity typically found in inorganmetal compounds. Inorganic compounds have, until recently, lacked flexibility.
Tour said, “This is not easy to do, because materials with such high capacity are usually brittle. And we’ve had really good, flexible carbon storage systems in the past, but carbon as a material has never hit the theoretical value that can be found in inorganic systems, and nickel fluoride in particular.”
Yang explained the material’s versatility, “Compared with a lithium-ion device, the structure is quite simple and safe. It behaves like a battery but the structure is that of a supercapacitor. If we use it as a supercapacitor, we can charge quickly at a high current rate and discharge it in a very short time. But for other applications, we find we can set it up to charge more slowly and to discharge slowly like a battery.”
To create the battery/supercapacitor, the team deposited a nickel layer on a backing. They etched it to create 5-nanometer pores within the 900-nanometer-thick nickel fluoride layer, giving it high surface area for storage. Once they removed the backing, they sandwiched the electrodes around an electrolyte of potassium hydroxide in polyvinyl alcohol. Testing found no degradation of the pore structure even after 10,000 charge/recharge cycles. The researchers also found no significant degradation to the electrode-electrolyte interface.
“The numbers are exceedingly high in the power that it can deliver, and it’s a very simple method to make high-powered systems,” Tour said, adding that the technique shows promise for the manufacture of other 3-D nanoporous materials. “We’re already talking with companies interested in commercializing this.”
Rice graduate student Changsheng Xiang and postdoctoral researcher Gunuk Wang are co-authors of the paper.
These results and the versatility opportunities have to be extremely enticing to manufacturers. The potential, even at the basic lab sample is extraordinary. With commercial interest already investigating this is one technology that may well come to market very quickly.
And we’re just at the very beginning of the development.