“Metallic-phase MoS2 (M-MoS2) is metastable and does not exist in nature,” explains team member Yelong Zhang of Northeastern University in Boston. “No one had investigated water-coupled and multi-layer M-MoS2 for use in supercapacitors until now, and we have found that this material can be used as an electrode in an ultrafast-rate supercapacitor with no need for extra additives or binders. We prepared M-MoS2 with a monolayer of water molecules covering both sides of the material nanosheet using a hydrothermal technique and found that the system reaches a high capacitance of 380 F/g at a scan rate of 5 mV/s.”

The device maintains a capacitance of 105 F/g at a scan rate of 10V/s, he adds, and the specific capacitance of a symmetric supercapacitor based on M-MoS2/H2O electrodes retains a value as high as 249 F/g under 50 mV.

Multilayer structures with lots of nanochannels

The researchers, led by Hongli Zhu, say that the excellent electrochemical properties of the M-MoS2 electrodes come thanks to their hydrophilicity and high electrical conductivity, as well as the fact that they have multilayer structures with lots of nanochannels that act as ion reservoirs and liquid-transfer channels.

The nanochannels between the layers of M-MoS2-H2O, which are around 1.18 nm wide, help increase the amount of space through which ions can diffuse. They also enlarge the surface area over which ions can be adsorbed. It is because of these channels – and the high electrical conductivity of the system – that it boasts such a high capacitance, says Zhang.

“As for applications, such ultrafast supercapacitor electrodes could find use in electric and hybrid vehicles, as well as portable devices,” he tells nanotechweb.org.

The electrodes are detailed in Nano Letters DOI: 10.1021/acs.nanolett.6b05134.

Further reading

• MoS2 nanosheets make good supercapacitor electrodes (May 2015)
• Field effect improves quantum capacitors and vertical transistors (Aug 2016)