Abstract:

NiMn-LDH (NiMn-layered double hydroxide) is a promising cathode material for hybrid supercapacitors due to its inherent environmental sustainability, exceptionally high theoretical specific capacitance, and robust cycling stability. However, its widespread practical application faces significant limitations due to poor electronic conductivity, which results in low specific capacitance and rate capability. Particularly at mg·cm^-2 magnitude loading, the specific capacitance of NiMn-LDH at high current densities of 50 A·g^-1 and above is much lower than 1500 F·g^-1, a performance threshold critically insufficient for the energy-power balance required in commercial hybrid supercapacitor devices. To address this limitation, this work innovatively developed a novel NiMn-LDH@Ni₉₅Cu₅ electrode via a simple two-step electrodeposition strategy. Ni₉₅Cu₅ dendrites foam with hierarchical porous structure were prepared by hydrogen bubble template method, and NiMn-LDH was anchored to the Ni₉₅Cu₅ substrate by electrochemical deposition. By systematically adjusting the Mn/Ni stoichiometric ratio in NiMn-LDH electrodeposited on the surface of Ni₉₅Cu₅ dendritic foam through variations of the metal ion ratios in electrodeposition solution, its influence on the NiMn-LDH’s composition, elemental valence state, crystal structure, morphology, energy band configuration, and electrochemical behavior were investigated. With the Mn content in NiMn_x-LDH increases, the size of NiMn-LDH nanosheets decreases. The optimized NiMn_0.6-LDH@Ni₉₅Cu₅ electrode exhibits superior crystallinity, minimized charge-transfer resistance, the narrowest band gap, synergistically yielding exceptional electrochemical performance, delivering outstanding specific capacitances of 2365 F·g^-1 at a current density of 1 A·g^-1 and 1803 F·g^-1 at an ultrahigh current density of 50 A·g^-1, even under high mass loadings (>2 mg·cm^-2). Furthermore, it demonstrates remarkable cycling stability, retaining 88.8% of its initial capacity after 3000 cycles at 20 A·g^-1. Collectively, this study confirms that the composition, crystallinity and energy band structure of LDH can be synergistically optimized by precisely tuning the bimetallic ratio, thus solving the problem of specific capacitance and multiplicity performance degradation of high-loading electrodes, and provides a new idea for the design of next-generation high-performance hybrid supercapacitor electrodes.

Key words: hybrid supercapacitors, NiMn-LDH, Mn/Ni ratio, specific capacitance, rate capability