Improving capacity of nickel phosphate Versailles Santa Barbara-5 with calcination for high-performance asymmetric supercapacitors

Versailles Santa Barbara-5 (VSB-5) is a prominent example of nanoporous nickel-based materials for energy storage applications due to their high theoretical specific capacitance, thermal and chemical stability, and ease of synthesis. Yet, improving the electrochemical properties of VSB-5 for high-performance supercapacitors remains in high interest to exceed the current state-of-the-art. This work demonstrates the effect of different calcination temperatures of nickel phosphate (NiP) VSB-5 on its structure, morphology, and electrochemical performances. NiP VSB-5 was synthesized via hydrothermal technique followed by calcination at 300 °C (NiP-300), 400 °C (NiP-400), 500 °C (NiP-500), 600 °C (NiP-600), 700 °C (NiP-700), and 800 °C (NiP-800). Interestingly, the calcination temperatures induce the crystal-amorphous phase change and affect the diameter size of the NiP VSB-5 rod structure. The NiP-600 delivers the highest specific capacity of 516 C g−1 at 0.62 A g−1 compared to as-synthesized VSB-5 (31.4 C g−1), NiP-300 (103 C g−1), NiP-400 (189 C g−1), NiP-500 (279 C g−1), NiP-700 (396 C g−1), and NiP-800 (0.97 C g−1). The high specific capacity of NiP-600 is triggered by the amorphous phase and smaller particle diameter size, which provide abundant active sites for redox reactions. Furthermore, we assembled the best performing NiP-600 as the cathode and activated carbon (AC) as the anode into an asymmetric supercapacitor (SC). The NiP-600//AC-based supercapacitors achieved a remarkable energy density of 52 Wh kg−1 at a power density of 434 W kg−1, which is superior to several reported nickel phosphate-based SC. The SC can maintain 71 % of original capacitance after 2000 cycles at a scan rate of 100 mV s−1. This study provides insight into the effect of calcination temperature to generate the best-performing NiP VSB-5 electrode for high-performance supercapacitor.