Enhanced electrochemical energy storage through confinement of carbon nanotubes to polyoxometalates

June 09, 2023

(Nanowerk News) As an outstanding electrochemical energy storage material, carbon nanotubes (CNTs) have a reputation for their superior electrical conductivity, large theoretical surface area and excellent chemical stability.

However, this prestige was overshadowed by significant drawbacks. Due to the influence of strong van der Waals forces, CNTs show a tendency to aggregate, which in turn reduces their electrochemical active area. This problem is especially amplified for single-walled carbon nanotubes (SWNTs), which is due to their high length-to-diameter ratio.

Seeking solutions, coordinated research efforts, led by Dr. WANG Xiao of the Shenzhen Institute of Advanced Technology (SIAT) of the Chinese Academy of Sciences, Dr. ZHU Sheng from Shanxi University, and Prof. LI Yan of Peking University, encapsulated polyoxometalate guest molecules in SWNTs (mean diameter of about 1.4 nm) to support the electrochemical energy storage capabilities of CNTs. Schematic illustration and electron microscopy characterization of the one-dimensional heterostructures of SWNT-delimited polyoxometalate clusters. (Image: WANG Xiao)

The results of this research are published in Cells Reporting Physics (“One-dimensional heterostructure of polyoxometalate-encapsulated carbon nanotubes to enhance capacitive energy storage”).

As a result of the confinement effect exhibited by the CNT, the polyoxometalate molecules construct a chain-like one-dimensional structure within the cavity of the CNT. This carefully structured (enamel protected) hybrid presents promising potential as an electrode material candidate for supercapacitors.

Electron transfer from the CNT to the polyoxometalate reduces the surface charge density of the nanotubes, consequently weakening the van der Waals forces and inhibiting aggregation. Thus, SWNTs infused with polyoxometalate exhibit a wider electrochemical active area and increased double-layer capacitance.

The polyoxometalate molecule contributes the apparent capacitance via a reversible redox reaction, thereby enhancing the capacitive performance of the hybrid (enamel protected). Notably, the CNT confinement effect significantly augments the cycle stability of the encapsulated polyoxometalate.

Ultimately, this one-dimensional hybrid material displays enhanced electrochemical energy storage properties, achieving a specific capacitance of 328.6 farads per gram (@ 10 millivolts per second), a feat that surpasses pure SWNT (172.2 farads per gram). In addition, the assembled supercapacitor maintains a capacity retention rate of 91.3% after 10,000 cycles.

Dr. WANG Xiao, corresponding author of the study, commented, “Our study provides valuable insights into research into the confinement effect of CNTs, which holds great potential for the development of high-performance energy conversion and storage materials.”

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