Fast, cheap method to make supercapacitor electrodes for cars and lasers
The researchers, led by University of Washington assistant professor of materials science and engineering Peter Pauzauskie, published a paper on July 17 in the journal Nature Microsystems and Nanoengineering describing their supercapacitor electrode and the fast, inexpensive way they made it. Their novel method starts with carbon-rich materials that have been dried into a low-density matrix called an aerogel. This aerogel on its own can act as a crude electrode, but Pauzauskie's team more than doubled its capacitance, which is its ability to store electric charge.
These inexpensive starting materials, coupled with a streamlined synthesis process, minimize two common barriers to industrial application: cost and speed.
"In industrial applications, time is money," said Pauzauskie. "We can make the starting materials for these electrodes in hours, rather than weeks. And that can significantly drive down the synthesis cost for making high-performance supercapacitor electrodes."
Effective supercapacitor electrodes are synthesized from carbon-rich materials that also have a high surface area. The latter requirement is critical because of the unique way supercapacitors store electric charge. While a conventional battery stores electric charges via the chemical reactions occurring within it, a supercapacitor instead stores and separates positive and negative charges directly on its surface.
"Supercapacitors can act much faster than batteries because they are not limited by the speed of the reaction or byproducts that can form," said co-lead author Matthew Lim, a UW doctoral student in the Department of Materials Science & Engineering. "Supercapacitors can charge and discharge very quickly, which is why they're great at delivering these 'pulses' of power."
"They have great applications in settings where a battery on its own is too slow," said fellow lead author Matthew Crane, a doctoral student in the UW Department of Chemical Engineering. "In moments where a battery is too slow to meet energy demands, a supercapacitor with a high surface area electrode could 'kick' in quickly and make up for the energy deficit."
To get the high surface area for an efficient electrode, the team used aerogels. These are wet, gel-like substances that have gone through a special treatment of drying and heating to replace their liquid components with air or another gas. These methods preserve the gel's 3-D structure, giving it a high surface area and extremely low density. It's like removing all the water out of Jell-O with no shrinking.
"One gram of aerogel contains about as much surface area as one football field," said Pauzauskie.
Crane made aerogels from a gel-like polymer, a material with repeating structural units, created from formaldehyde and other carbon-based molecules. This ensured that their device, like today's supercapacitor electrodes, would consist of carbon-rich materials.
Previously, Lim demonstrated that adding graphene - which is a sheet of carbon just one atom thick - to the gel imbued the resulting aerogel with supercapacitor properties. But, Lim and Crane needed to improve the aerogel's performance, and make the synthesis process cheaper and easier.
In Lim's previous experiments, adding graphene hadn't improved the aerogel's capacitance. So they instead loaded aerogels with thin sheets of either molybdenum disulfide or tungsten disulfide. Both chemicals are used widely today in industrial lubricants.
The researchers treated both materials with high-frequency sound waves to break them up into thin sheets and incorporated them into the carbon-rich gel matrix. They could synthesize a fully-loaded wet gel in less than two hours, while other methods would take many days. After obtaining the dried, low-density aerogel, they combined it with adhesives and another carbon-rich material to create an industrial "dough," which Lim could simply roll out to sheets just a few thousandths of an inch thick. They cut half-inch discs from the dough and assembled them into simple coin cell battery casings to test the material's effectiveness as a supercapacitor electrode.
Not only were their electrodes fast, simple and easy to synthesize, but they also sported a capacitance at least 127 percent greater than the carbon-rich aerogel alone.
Lim and Crane expect that aerogels loaded with even thinner sheets of molybdenum disulfide or tungsten disulfide - theirs were about 10 to 100 atoms thick - would show an even better performance. But first, they wanted to show that loaded aerogels would be faster and cheaper to synthesize, a necessary step for industrial production. The fine-tuning comes next.
The team believes that these efforts can help advance science even outside the realm of supercapacitor electrodes. Their aerogel-suspended molybdenum disulfide might remain sufficiently stable to catalyze hydrogen production. And their method to trap materials quickly in aerogels could be applied to high capacitance batteries or catalysis.
The research was conducted with the help of Energ2 Technologies, a UW start-up company based in Seattle that was recently acquired by BASF.
Abstract
Transition metal dichalcogenide (TMD) materials have recently demonstrated exceptional supercapacitor properties after conversion to a metallic phase, which increases the conductivity of the network. However, freestanding, exfoliated transition metal dichalcogenide films exhibit surface areas far below their theoretical maximum (1.2 %), can fail during electrochemical operation due to poor mechanical properties, and often require pyrophoric chemicals to process. On the other hand, pyrolyzed carbon aerogels exhibit extraordinary specific surface areas for double layer capacitance, high conductivity, and a strong mechanical network of covalent chemical bonds. In this paper, we demonstrate the scalable, rapid nanomanufacturing of TMD (MoS2 and WS2) and carbon aerogel composites, favoring liquid-phase exfoliation to avoid pyrophoric chemicals. The aerogel matrix support enhances conductivity of the composite and the synthesis can complete in 30"min. We find that the addition of transition metal dichalcogenides does not impact the structure of the aerogel, which maintains a high specific surface area up to 620"m2"ga'1 with peak pore radii of 10"nm. While supercapacitor tests of the aerogels yield capacitances around 80"F"ga'1 at the lowest applied currents, the aerogels loaded with TMD's exhibit volumetric capacitances up to 127% greater than the unloaded aerogels. In addition, the WS2 aerogels show excellent cycling stability with no capacitance loss over 2000 cycles, as well as markedly better rate capability and lower charge transfer resistance compared to their MoS2-loaded counterparts. We hypothesize that these differences in performance stem from differences in contact resistance and in the favorability of ion adsorption on the chalcogenides.
They have demonstrated a rapid, scalable nanomanufacturing process for the production of TMD-doped carbon aerogel composites via polycondensation of resorcinol and formaldehyde catalyzed with hydrochloric acid in acetonitrile. Compared to typical aerogel processing (24"h), the reaction presented here occurs in 2% of the time without sacrificing the narrow pore sizes or high surface areas of a standard RF aerogel. This synthesis outlines a general method to support TMD's with high electrical conductivity and porosity which is applicable to other stable TMD's. Any advances in TMD synthesis or exfoliation can be directly incorporated via this process. Given the wide potential range of TMD applications, including electrochemical, photovoltaic, and catalytic, this rapid synthesis will accelerate combinatorial optimization of design parameters to engineer new devices. As a proof of concept, we explored the performance of MoS2 and WS2-doped carbon aerogels as electrodes for supercapacitors. An initial screening of device performances indicates that the addition of TMD's yields electrodes that are cyclically stable and offer volumetric capacitances up to 127% higher than pyrolyzed RF alone. Further, the ability to rapidly process new materials into composites is magnified by the range of applications for high surface area, conductive supports, and we believe this scalable manufacturing methodology will find widespread use.
