NiC 2 O 4 ⋅ 2H 2 O Nanoflakes: A Novel Redox‐mediated Intercalative Pseudocapacitive Electrode for Supercapacitor Applications in Aqueous KOH and Neutral Na 2 SO 4 electrolytes

Abstract Pseudocapacitors with the accessibility of different oxidation states for redox‐mediated charge storage can achieve higher energy density compared to EDLC. NiC 2 O 4 ⋅ 2H 2 O is envisaged here as a potential pseudocapacitive electrode that works with the accessibility of the Ni 2+/3+ redox couple in the flexible structural network due to the presence of planar oxalate anions (C 2 O 4 2− ) supported by the 3‐dimensional hydrogen bonding network of crystal water. The NiC 2 O 4 ⋅ 2H 2 O electrode showed a superior specific capacitance equivalent to 990 F/g in the potential window of 0 to 0.45 V observed in aqueous KOH electrolyte and 440 F/g in 1 M neutral Na 2 SO 4 electrolyte in the potential window of 0 to 0.85 V. Predominant intercalative mechanism seems to play an important role behind the high charge storage capacity of NiC 2 O 4 ⋅ 2H 2 O electrode and the interactive contribution was found to be ∼84 % and surface contribution was found to be ∼16 % respectively. Further, in full cell asymmetric supercapacitor (AAS) mode in KOH electrolyte, in which NiC 2 O 4 ⋅ 2H 2 O is made as the positive electrode and Activated Carbon (AC) is made as the negative electrode, the highest specific energy of 141.5 Wh/kg and specific power of ∼559 W/kg at 0.2 A/g current density was obtained with superior cyclic stability. The detailed electrochemical studies confirm high cyclic stability and stable performance that makes NiC 2 O 4 ⋅ 2H 2 O a potential pseudocapacitive electrode for large‐scale energy storage applications.