A graphical overview of numerous papers published on the nickel-based supercapacitors is shown in Fig. 4. The data are retrieved from the Google scholar database. Most of these papers demonstrate that the achievable capacitance is around 500–2500 F g −1 (250–1250 C g −1).
The fundamental difference between nickel-based batteries and electrochemical capacitors is that the redox reactions in batteries occur in the bulk phase; while the energy stored in supercapacitors is mainly due to the surface-involved processes. This disparity leads to the different theoretical limits of the stored energy.
To the best of our knowledge, the highest value reported so far is 4172.5 F g −1 (1669 C g −1) at a current density of 1 A g −1 ; however, this value was achieved for a very small loading mass (only ∼1 mg) on a nickel foam. Figure 4. Graphical overview of papers published on the nickel-based supercapacitors within the last decade.
This system showed a specific capacitance of 1337 F g −1 (602 C g −1) at 5 A g −1 . Nickel cobalt sulfide and its composite with carbon-based materials, similar to its counterpart nickel cobalt oxide and derivatives, have become popular materials nowadays , , .
Nickel–copper carbonate hydroxide has also been investigated as a supercapacitor electrode . Figure 13. Typical FESEM images of surface morphology of (a) original copper foam, (b) after the first processing step, oxidization, and (c) after the second processing step, i.e., hydrothermal reaction.
Monolayer nickel cobalt hydroxyl carbonate with an average thickness of 1.07 nm was synthesized and a distinguished capacitance of 2266 F g −1 (1133 C g −1) was obtained at 0.5 A g −1. When the current density was enlarged to 20 A g −1, 83.6% of the original capacitance was maintained .