Microwave-Assisted In Situ Synthesis of NiMn2O4 Nanoparticles Embedded in NiCo2O4 Nanosheets on Nickel Foam as Binder-Free Electrode Material for High-Performance Supercapacitors

Binder-free NiMn2O4@NiCo2O4 nanocomposites with NiMn2O4 nanoparticle (NP) surface coverage on NiCo2O4 nanosheets (NSs) are fabricated on nickel foam (NF) via a two-step microwave-assisted hydrothermal (MAH) method combined with annealing treatment, which can be used as a high-performance electrode material for supercapacitors. Specifically, a tulle-like NiCo2O4 nanosheet framework is first in situ grown on NF, followed by the growth of NiMn2O4 NPs on the surface of NiCo2O4 NSs via a secondary MAH process. To investigate the effect of the second-step holding time (HT) of MAH on material performance, a series of experiments were carried out with an HT of 15, 30, 45, and 60 min, and the microstructures and electrochemical properties of the products were analyzed. Structural characterization results confirm the successful synthesis of well-defined NiMn2O4-NPs@NiCo2O4-NSs composites. Electrochemical tests demonstrate that the product at an HT of 30 min has the best electrochemical performance with a higher specific capacitance of 441.56 F·cm−2 at 1 A·cm−2 and cycling stability (75% capacitance retention after 5000 cycles at 15 A·cm−2). The superior electrochemical properties are mainly attributed to the unique porous tulle-like NS structure with the largest specific surface area of the 30 min product. This distinctive structure affords abundant electrochemical active sites, effectively prevents structural collapse during long-term cycling, and shortens the transmission and diffusion pathways of electrons and electrolyte ions. The optimized NiMn2O4@NiCo2O4 electrode material presents extensive application prospects for high-performance supercapacitors.