Nickel hydroxide-carbon nanotube nanocomposites as supercapacitor electrodes: Crystallinity dependent performances

Nickel hydroxide (Ni(OH)₂) is a promising pseudocapacitive material to increase the energy storage capacity of supercapacitors. Ni(OH)₂ has three common crystalline structures: amorphous (amor-), α-, and β-Ni(OH)₂. There is a lack of good understanding on their pros and cons as supercapacitor electrodes. In this work, we synthesized three nanocomposites with thin layers (10-15 nm) of amor-, α-, and β-Ni(OH)₂ deposited on conductive multi-walled carbon nanotubes (MWCNTs). The mass loading of Ni(OH)₂ is analogous in these nanocomposites, ranging from 49.1-52.2 wt% with a comparable narrow-pore size distribution centered around 4-5 nm. They were fabricated into supercapacitor electrodes at a mass loading of 6 mg cm^-2 with a thickness of ∼250 μm, similar to the electrodes used in commercial supercapacitors. Our results show that MWCNT/amor-Ni(OH)₂ has the highest specific capacitance (1495 or 2984 F g^-1, based on the mass of total active materials or Ni(OH)₂ only at the scan rate of 5 mV s^-1 in 1 M KOH electrolyte). It also has the best rate capability among the three nanocomposites. Better performances can be attributed to its disordered structure, which increases its effective surface area and reduces diffusion resistance for redox reactions. However, superior performances gradually deteriorate to the same level as that of MWCNT/β-Ni(OH)₂ over 3000 charge/discharge cycles, because amor- and α-Ni(OH)₂ transform slowly to more ordered β-Ni(OH)₂. Our results highlight that the electrochemical performances of MWCNT/Ni(OH)₂ nanocomposites depend on the crystallinity of Ni(OH)₂, and the performances of electrodes change upon the crystalline structure transformation of Ni(OH)₂ under repeated redox reactions. Future research should focus on improving the structure stability of amor-Ni(OH)₂.