(Left) Schematic of the LaB6 nanowires deposited onto carbon fiber cloth as electrode material for supercapacitors. (Top right) Transmission electron microscopy image of the as-prepared LaB6 nanowires. (Bottom right) Current-voltage curves of the LaB6-CFC electrode at different scan rates.Researchers report, for first time, that boride nanowires deposited on carbon fiber cloth could form the basis of high capacity, stable supercapacitors for energy storage devices [Xue et al., Materials Today Energy 10 (2018) 28-33].
Supercapacitors could support clean, sustainable energy development by providing effective, high capacity storage for renewable supplies such as solar or wind, which although abundant cannot be readily turned off or on in response to demand. Supercapacitors are now used as efficient power sources in consumer electronics and electric vehicles, but new, more effective electrode materials could open up more applications.
There are two mechanisms by which supercapacitors store energy: either through the absorption of ions or by means of redox reactions. In either case, the electrode material plays a significant role in determining the supercapacitor’s performance.
The team from City University of Hong Kong and Sun Yat-sen University in China have created a novel active electrochemical material based on LaB6 nanowires deposited on carbon fiber cloth, which demonstrates high capacitance and good stability over 10 000 cycles of charging and discharging.
The 400-nm-diameter nanowires were synthesized in a dense and even layer on the surface of the carbon fiber cloth using a low-cost, moderate chemical vapor deposition (CVD) technique. The nanowires appear highly crystalline with very smooth surfaces.
“The high aspect ratio of the nanowire structure greatly enhances the surface area of the material,” explains Chunyi Zhi of City University of Hong Kong, who led the research effort with Fei Liu of Sun Yat-sen University. “The LaB6 nanowires work as the electrochemical active material, providing capacitance in the supercapacitor.”
The researchers report that the LaB6 nanowire electrode material works in a three-electrode test cell with both Na2SO4 and H2SO4 electrolytes, although it demonstrates a higher capacity in Na2SO4 (of 17.34 mF cm−2) at the same current density (0.1 mA cm−2).
“This is the first demonstration of using this material for supercapacitors,” says Zhi. “The LaB6 nanowires on carbon fiber cloth demonstrate a binder-free and free-standing configuration, providing good contact between the interface of electrode material and the electrolyte, which facilitates low charge transfer resistance and good performance,” he adds.
From their analysis of the electrical charge/discharge processes, the researchers believe that the energy storage mechanism in LaB6 nanowires is dominated by chemical or Faradaic reactions near the surface of the electrode material.
The results are impressive compared with graphene powders and SnSe nanocrystals, say the researchers, and could extend the range of electrode materials for supercapacitors beyond traditional carbon, metal oxides, and conducting materials to borides.
“We plan to further develop LaB6 nanowires as the electrochemical active material for wearable energy storage devices,” Zhi told Materials Today.