NiC2O4 ⋅ 2H2O Nanoflakes: A Novel Redox-mediated Intercalative Pseudocapacitive Electrode for Supercapacitor Applications in Aqueous KOH and Neutral Na2SO4 electrolytes

Abstract

Pseudocapacitors with the accessibility of different oxidation states for redox-mediated charge storage can achieve higher energy density compared to EDLC. NiC2O4 ⋅ 2H2O is envisaged here as a potential pseudocapacitive electrode that works with the accessibility of the Ni2+/3+ redox couple in the flexible structural network due to the presence of planar oxalate anions (C2O42−) supported by the 3-dimensional hydrogen bonding network of crystal water. The NiC2O4 ⋅ 2H2O 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 Na2SO4 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 NiC2O4 ⋅ 2H2O 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 NiC2O4 ⋅ 2H2O 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 NiC2O4 ⋅ 2H2O a potential pseudocapacitive electrode for large-scale energy storage applications. © 2022 Wiley-VCH GmbH.