NiCo<sub>2</sub>O<sub>4</sub>/NiCo Layered Double Oxide Heterojunction Nanocomposite as a Cathode Material for Supercapacitors
Yi Gao, Ping Cao, Bin Cai, Ruixiang Xu, Hao Gu, Jingwei Zhou, Yuxin Sun, Junlai Jiang, Fei Yu
Abstract
NiCo oxides are used as one of the cathode materials for supercapacitors, but their low conductivity and low stability are limited in practical applications. This study successfully fabricated a nanoflower-structured heterojunction composite (NiCo 2 O 4 /NiCo-LDO) by integrating NiCo layered double oxide (NiCo-LDO) and NiCo 2 O 4 using a combined strategy of hydrothermal synthesis, electrodeposition, and calcination. The specific capacitances of the three electrodes, NiCo 2 O 4, NiCo 2 O 4 /NiCo-LDH, and NiCo 2 O 4 /NiCo-LDO, at current densities of 6 mA cm –2 were 2.15 F cm –2, 2.18 F cm –2, and 3.85 F cm –2, with NiCo 2 O 4 /NiCo-LDO performing the best. The asymmetric supercapacitor fabricated using this cathode exhibited a maximum energy density of 0.48 mWh cm –2 and a power density of 40 mW cm –2 while maintaining 81.03% capacitance retention after 5000 cycles. The morphological structures of NiCo 2 O 4, NiCo-LDH, and NiCo-LDO have been analyzed by finite element simulations under stress, and it has been demonstrated that the stability of the nanoflower structure of NiCo-LDO is due to the presence of uniformly distributed internal stresses. Based on first-principles calculations, the band structures and density of states of NiCo 2 O 4, NiCo-LDH, NiCo-LDO, NiCo 2 O 4 /NiCo-LDH, and NiCo 2 O 4 /NiCo-LDO were analyzed. The results indicate that after the hydrogen atoms within NiCo-LDH escape, an in situ transformation to NiCo-LDO occurs, resulting in a reduction of the bandgap from 1.46 to 0.52 eV, significantly enhancing the electrochemical performance. Furthermore, the bandgap of the heterojunction formed by NiCo-LDO and NiCo 2 O 4 narrows to 0.14 eV, demonstrating the superior electron transfer capability of the NiCo-LDO and NiCo 2 O 4 -based heterojunction.