Optimizing the electron spin state of hierarchical NiCo2O4@MXene@LDH 3D composite electrode by heterointerface engineering to enhance energy density and excellent cyclic stability of supercapacitor

The transition metal oxide of nickel cobaltite (NiCo 2 O 4 ) has gained more and more attention from battery industry for its excellent properties of high capacitance and cycling stability. However, its performance when used in supercapacitors is compromised due to its limited active sites and poor conductivity. Herein, a 3D (three-dimensional) hierarchical and high-performance material of NiCo 2 O 4 @MXene@LDH (LDH: layered double hydroxide) composite was designed and synthesized. In the material, NiCo 2 O 4 nanorods form a skeleton of nanosheet arrays on layered MXene; and on the nanosheet arrays, NiCo-LDH nanoparticles are attached to form a 3D hierarchical structure, extending the electrochemically active sites, suppressing the agglomeration of NiCo-LDH and producing hierarchical channels that promote electron and ion transport. Density functional theory (DFT) calculations and the results of Ultraviolet–visible (UV–Vis) spectrophotometry indicated that the introduction of MXene and NiCo-LDH induces a redistribution of the electronic states of NiCo 2 O 4 due to the formation of a heterointerface, thereby decreasing its bandgap by about 1.42 eV. The X-ray photoelectron spectroscopy (XPS) analysis further demonstrated the negative shift in the initial binding energies of nickel (Ni) and cobalt (Co) atoms, which is due to their transition of spin states; and this is in agreement with the high intensity of electron paramagnetic resonance (EPR) peak in the NiCo 2 O 4 @MXene@LDH composite. The in situ Raman analysis reveals that the enhanced energy storage performance originates from the accelerated formation and increased exposure of NiCoOOH active species during electrochemical cycling. The synergistic effect of MXene and NiCo-LDH introduced into NiCo 2 O 4 enhanced the supercapacitor in terms of power and energy densities. It was indicated that the supercapacitor fabricated from NiCo 2 O 4 @MXene@LDH supercapacitor exhibited a specific capacitance of 5066 F g −1 at 1 A g −1 , and a capacitance retention of 97.82 % after 10,000 charge cycles at 10 A g −1 . Moreover, an asymmetric supercapacitor (ASC) device with a NiCo 2 O 4 @MXene@LDH//AC configuration was developed. The supercapacitor presented an energy density and power density of 70.11 Wh kg −1 and 850.22 W kg −1 , respectively. The redistribution of electronic states and decreased bandgap could facilitate the creation of NiCo 2 O 4 -based materials with customized features for supercapacitors.