Boosting Electrochemical Energy Storage of Carbon Nanotubes by Creating a Hybrid Material From Polyoxometalate Molecules

Carbon nanotubes (CNTs) are considered good electrochemical energy storage materials because they exhibit sizeable theoretical surface area, high electrical conductivity, and ideal chemical stability. However, they tend to form clusters because of the strong van der Waals forces that lessen the electrochemically active areas.

This problem is also encountered in single-walled carbon nanotubes (SWNTs) because they possess a high length-to-diameter ratio. A potential solution to this problem is the enhancement of electrochemical energy storage of CNTs, which was made possible by a joint research team from different institutions in China.

Creating Hybrids for Supercapacitors

Dr. Wang Xiao from the Shenzhen Institute of Advanced Technology (SIAT) of the Chinese Academy of Sciences collaborated with Dr. Zhu Sheng from Shanxi University and Prof. Li Yan from Peking University. Their research involves encapsulating polyoxometalate guest molecules within SWNTs with a diameter of about 1.4 nm.

Electrons could transfer from carbon nanotubes to polyoxometalate clusters to loosen pi stacking. Due to the confinement ability of CNTs, one-dimensional chain-like patterns were created by the polyoxometalate molecules in the CNT cavity.

The researchers observed enhanced electrochemical energy storage properties in the resulting one-dimensional hybrid material. It has higher specific capacitance than pure SWNTs at ten millivolts per second. It also provided a capacity retention rate of 91.3% to the assembled supercapacitor after 10,000 cycles.

The confinement effects protect the encapsulated polyoxometalate molecules from chemical degradation and improve their cycling ability. By delivering high specific gravity and cyclic stability, well-defined hybrids can be potential electrode materials for supercapacitors.

Dr. Xiao believes that the result of their study can provide an important contribution to the investigation of the confinement effect of CNTs and holds potential in utilizing high-performance energy storage and conversion materials.

Role of Capacitors as Electrochemical Energy Storage

Electrochemical energy storage (EES) systems have traditionally been used as energy storage devices for generating power. The system is based on chemical energy storage, converted to electrical energy when needed. There are three types of EES systems: electrochemical capacitors, batteries, and fuel cells.

As an EES system, capacitors use two-terminal passive electronic components to store electric charge between their metal plates. They store less energy per unit mass or unit volume but have high power density because they discharge the energy rapidly to create a large amount of power.

The supercapacitors, also known as electric double-layer capacitor (EDLC), acts as an EES device by exploiting the accumulation of charge in an electric double-layer found between a carbon electrode with a high surface area and a liquid electrolyte.

Aside from using EDLC, supercapacitors also store charge through an electrochemical pseudo-capacitance known as hybrid capacitance. Hybrid capacitors store charge electrostatically and electrochemically.

Hybrid capacitors employ both the Faradaic and non-Faradaic processes in doing their job as storage of charge. Compared to supercapacitors, they have higher operating voltage, capacitance, and energy density. Hybrid capacitors are also known for having lower self-discharge and standby currents.