Development of an Amorphous Nickel Boride/Manganese Molybdate Heterostructure as an Efficient Electrode Material for a High-Performance Asymmetric Supercapacitor

The exploration of heterostructure materials with unique electronic properties is considered a desirable platform for fabricating electrode/surface interface relationships for constructing asymmetric supercapacitors (ASCs) with high energy density. In this work, a heterostructure based on amorphous nickel boride (NiXB) and crystalline square bar-like manganese molybdate (MnMoO4) was prepared by a simple synthesis strategy. The formation of the NiXB/MnMoO4 hybrid was confirmed by powder X-ray diffraction (p-XRD), field emission scanning electron microscopy (FE-SEM), field-emission transmission electron microscopy (FE-TEM), Brunauer-Emmett-Teller (BET), Raman, and X-ray photoelectron spectroscopy (XPS). In this hybrid system (NiXB/MnMoO4), the intact combination of NiXB and MnMoO4 leads to a large surface area with open porous channels and abundant crystalline/amorphous interfaces with a tunable electronic structure. This NiXB/MnMoO4 hybrid shows high specific capacitance (587.4 F g-1) at 1 A g-1, and it even retains a capacitance of 442.2 F g-1 at 10 A g-1, indicating superior electrochemical performance. The fabricated NiXB/MnMoO4 hybrid electrode also exhibited an excellent capacity retention of 124.4% (10000 cycles) and a Coulombic efficiency of 99.8% at a current density of 10 A g-1. In addition, the ASC device (NiXB/MnMoO4//activated carbon) achieved a specific capacitance of 104 F g-1 at 1 A g-1 and delivered a high energy density of 32.5 Wh.kg-1 with a power density of 750 W·kg-1. This exceptional electrochemical behavior is due to the ordered porous architecture and the strong synergistic effect of NiXB and MnMoO4, which enhances the accessibility and adsorption of OH- ions that improve electron transport. Moreover, the NiXB/MnMoO4//AC device exhibits excellent cyclic stability with a retention of 83.4% of the original capacitance after 10000 cycles, which is due to the heterojunction layer between NiXB and MnMoO4 that can improve the surface wettability without causing structural changes. Our results show that the metal boride/molybdate-based heterostructure is a new category of high-performance and promising material for the growth of advanced energy storage devices.