Schematic showing the graphene-niobia nanocomposite for energy storage.
Illustration of the 3D hierarchical porous composite network.
Scanning electron microscopy image of the graphene-niobia nanocomposite.Energy is typically stored in batteries, which offer high energy density but low power density, or supercapacitors, which offer the reverse. Finding a material that combines both high energy and power density — in other words, one that stores a large amount of charge and delivers it swiftly — is challenging. Nanostructured materials are proving promising and now researchers believe that they may have come up with the winning formula in the form of a graphene-niobia (Nb2O5) nanocomposite [Sun et al., Science (2017) 356, 599].
Nanostructured energy storage materials have been hampered by the fact that they are often limited to ultrathin electrodes and very low mass loadings. But researchers from the University of California, Los Angeles, Hunan University, and King Saud University led by Xiangfeng Duan appear to have overcome these twin problems by fabricating a three-dimensional holey graphene network that acts as a conductive scaffold for electroactive Nb2O5 nanoparticles. The highly interconnected graphene structure provides a framework for electron transport, while the tunable pores allow for the rapid movement of ions.
“By systematically tailoring the porosity in the holey graphene backbone, charge transport in the composite architecture is optimized to deliver high areal capacity and current density at practical mass loadings,” says Duan. “Our work a critical step toward the use of high-performance electrode materials in practical cells.”
Holey graphene frameworks have been reported for supercapacitor applications — where the large surface area is an advantage — but diffusion limitations have proven challenging in thick electrodes until now. The combination of interpenetrating electron and ion transport pathways in the new material enables high capacity at high charge/discharge rates at a mass loading of 10—20 mg/cm2.
“In thicker electrodes, the mass transport limit for ions and the resistance to electron transport become increasingly critical,” explains Duan. “These effects lead to rapid degradation of capacity retention at high mass loadings in state-of-the-art commercial graphite, silicon and carbon/silicon anodes, as well as carbon/sulfur cathodes.”
The problem becomes even worse at high power densities, he adds. But because the graphene/Nb2O5 nanocomposite can deliver charge more efficiently, it enables much better charge transport and capacity retention at high mass loadings and current densities even in thick electrodes.
Yury Gogotsi, of Drexel University and director of the AJ Drexel Nanomaterials Institute, believes that this is what sets the work apart. Electrodes made from the new nanocomposite with the right architecture can have a practically useful weight (11 mg/cm2), yet still show high rates and areal capacity.
“We all want to charge our cell phones and, eventually, electric cars within minutes, not hours,” he points out. “Materials like this porous graphene-niobia composite possess high electronic and ionic conductivity, offering high-rate energy storage.”
But before the new composite can be commercialized for energy storage applications, the two-step fabrication process needs to be optimized for large-scale production, admits Duan.
“Continued efforts in designing novel electrode structures that could further improve the charge delivery rate will lead to even higher rate capabilities, accelerating the development of superior active materials in practical cells,” he says.
This article was originally published in Nano Today (2017), doi: 10.1016/j.nantod.2017.06.005