Feb 20 2019
According to an update on a newly reported study on porous carbon fibers, these materials can possibly be employed in an industrial environment, representing a significant step from the theoretical to the application phase.
This figure shows the synthesis of porous carbon fibers and loading of MnO2. (a) A diblock copolymer of polyacrylonitrile-block-polymethyl methacrylate (PAN-b-PMMA) is spun into a polymer fiber mat. In the magnified view, the block copolymer microphase separates into a bicontinuous network structure. (b) After pyrolysis, the block copolymer fibers are converted to porous carbon fibers (black) with continuous and uniform mesopores (white channels), which afford high loadings of transition metal oxides. (c) The porous carbon fibers are loaded with manganese oxide (magenta). In the magnified view, the continuous carbon fiber matrix and partially filled mesopores provide effective expressways for electron conduction and ion diffusion, respectively. (Image credit: Virginia Tech)
Guoliang “Greg” Liu is an assistant professor of chemistry in the College of Science and is also a member of the Macromolecules Innovation Institute. He has been exploring various methods to create carbon fibers that have homogenous porous structures. Liu has described in a journal article, recently published in Science Advances, how block copolymers were used by his lab to develop carbon fibers with mesopores evenly distributed throughout, just like a sponge.
Merely a week later, Liu had published one more article, but this time in Nature Communications. The latest article demonstrates how the porous carbon fibers developed by Liu can facilitate high energy density as well as high ion/ electron charging rates, which are normally mutually exclusive in the case of electrochemical energy storage devices.
This is the next step that will be relevant to industry. We want to make an industrial-friendly process. Now industry should seriously look at carbon fiber not only as a structural material but also an energy storage platform for cars, aircrafts, and others.
Guoliang “Greg” Liu, Assistant Professor, Department of Chemistry, College of Science, Virginia Tech.
Introducing pseudocapacitive materials
Carbon fibers exhibit high performance in a wide range of areas, including weight and mechanical strength, and as a result, they are extensively employed in the automotive and aerospace sectors. The long-term objective of Liu is to develop exterior car shells from porous carbon fibers that can potentially store energy inside the pores.
However, carbon by itself is not enough. Despite being a structurally sound material, carbon does not have sufficient energy density to produce supercapacitors for applications that are highly challenging.
In today’s industry standard, carbon is combined with the so-called pseudocapacitive materials and while such materials can unlock the potential to store huge amounts of energy, they cause another issue of slow charge-discharge rate.
Manganese oxide (MnO2) is an oft-used pseudocapacitive material, thanks to its reasonable and low-cost performance. Before loading MnO2 onto carbon fiber or another similar material, Liu saturates the fibers in a KMnO4 precursor solution. This precursor subsequently reacts with carbon, etching away a thin carbon layer, and then adheres onto the remaining carbon, producing a thin coat of approximately 2 nm in thickness.
However, the industry encounters some difficulties with regards to MnO2. While a less amount of MnO2 results in reduced storage capacity, an excess amount of MnO2 produces an extremely thick coat that is electrically insulating. Added to this, it also slows down ion transport. Both play an active role in slow charge-discharge rates.
We want to couple carbon with pseudocapacitive materials because they together have a much higher energy density than pure carbon. Now the question is how to solve the problem of electron and ion conductivity.
Guoliang “Greg” Liu, Assistant Professor, Department of Chemistry, College of Science, Virginia Tech
Yet, Liu has found that his porous carbon fibers will be able to resolve these problems. Tests conducted in his laboratory demonstrated the best of both worlds: continuous high charging and discharging rates, and high loading of MnO2.
Liu’s laboratory showed that they can load around 7 mg/cm2 of MnO2 before there is a drop in performance; that is double or almost triple the amount of MnO2 that can be presently utilized by industry.
“We have achieved 84 percent of the theoretical limit of this material at a mass loading of 7 mg/cm2,” stated Liu. “If you load 7 mg/cm2 of other materials, you will not reach this.”
Short-term applications
At the speed Liu’s laboratory is reporting results, cars driven by exterior shells could become a reality sooner than ever; however, Liu puts the brakes on that concept.
“In a long-term vision, we could replace gasoline with just electric supercapacitor cars,” stated Liu. “At this moment, the minimum of what we could do is to utilize this as an energy storage part in cars.”
According to him, a shorter-term application would be to use the carbon fiber parts to supply plenty of energy in a short period of time to speed up cars faster. However, besides the automotive sector, Liu is also looking into other transportation applications.
If you want a drone to deliver products for Amazon, you want the drone to carry as much weight as possible, and you want the drone to be as lightweight as possible. Carbon fiber-based drones can do both jobs. The carbon fibers are strong structural materials for carrying the goods, and they are energy storage materials to provide power for transportation.
Guoliang “Greg” Liu, Assistant Professor, Department of Chemistry, College of Science, Virginia Tech.
The study on this material is ongoing in Liu's laboratory, and according to him, he still has several more concepts that need to be tested.
“What I believe is that porous carbon fibers are a platform material,” said Liu. “The first two papers, we focused on energy storage for vehicles. But we believe that this material can do more than that. Hopefully we'll be able to tell more stories soon.”
Tianyu Liu, a postdoctoral associate in the Liu lab, is the first author of the paper. Yichen Guo and Zhengping Zhou—two former postdoctoral associates—and Dong Guo, a third-year doctoral student in the Department of Chemistry, were also involved in the study.