2D materials sandwich promises high energy density flexible supercaps
In their paper “One-Step Device Fabrication of Phosphorene and Graphene Interdigital MicroSupercapacitors with High Energy Density” published in the ACS Nano journal, the researchers explain their manufacturing process, starting with stable inks of electromechanically exfoliated graphene and phosphorene successively filtered/deposited through a customized interdigitated mask onto a PTFE membrane and then transferred onto a PET foil. A ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate or BMIMPF6) is then drop cast onto the pattern.
Analysing their devices under Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM), the scientists observed the compact layered structure of the Phosphorene-Graphene (PG) film, in which relatively small phosphorene nanosheets are uniformly incorporated into the interlayer of large graphene nanosheets, showing the excellent structural integration of the two materials.
They also report for this phosphorene-incorporated graphene (PG) film a high electrical conductivity of 319 S cm−1, an order of magnitude higher than what literature reports for graphene/polymer hybrid films (21−40 S cm−1), guaranteeing both high electron transport and fast ionic transport.
Working in ionic liquid, the finalized Micro SuperCapacitors were characterized with an areal capacitance of 9.8mF cm−2, a volumetric capacitance of 37.0 F cm−3, and an energy density of 11.6mWh cm−3, over twice higher than that of most nanocarbon-based MSCs according to the paper. Thanks to the ionic liquid used, the devices supported high operating voltage (3V) while avoiding the oxidation of phosphorene and exhibiting a high thermal stability (up to 200°C).
Multiple PG-MSCs were then connected in series and tested under flexure, exhibiting excellent flexibility and a very stable performance without capacitance fluctuation even under highly folded states. In their paper, the researchers attribute the excellent performance of their PG-MSCs to the complementary properties and synergistic combination of the 2D nanosheets together with the ionic liquid electrolyte.
“First, the 2D phosphorene and graphene nanosheets in PG films have a strong coupling effect for energy storage. The puckered phosphorene lamellae offer more ionic accommodation and fast transport pathway, and significantly prevent the restacking of graphene sheets, while high conductive capacitive graphene works as main mechanical skeleton and high-speed electron transport network, guaranteeing efficient utilization of phosphorene nanosheets for energy storage”.
“Second, the simplified device fabrication of PG-MSCs can efficiently avoid the oxidation of phosphorene and solvent contamination originating from multiple-step lithographic processing”, they write.
The researchers concluded their paper by emphasizing the simplicity and scalability of the device’s production process to create parallelly and serially interconnected modular power sources without the need for metal-based interconnects and contacts.