Ordered
Assembly of NiCo<sub>2</sub>O<sub>4</sub> Multiple
Hierarchical Structures for High-Performance Pseudocapacitors
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Abstract
The
design and development of nanomaterials has become central
to the advancement of pseudocapacitive performance. Many one-dimensional
nanostructures (1D NSs), two-dimensional nanostructures (2D NSs),
and three-dimensional hierarchical structures (3D HSs) composed of
these building blocks have been synthesized as pseudocapacitive materials
via different methods. However, due to the unclear assembly mechanism
of these NSs, reports of HSs simultaneously assembled from two or
more types of NSs are rare. In this article, NiCo<sub>2</sub>O<sub>4</sub> multiple hierarchical structures (MHSs) composed of 1D nanowires
and 2D nanosheets are simply grown on Ni foam using an ordered two-step
hydrothermal synthesis followed by annealing processing. The low-dimensional
nanowire is found to hold priority in the growth order, rather than
the high-dimensional nanosheet, thus effectively promoting the integration
of these different NSs in the assembly of the NiCo<sub>2</sub>O<sub>4</sub> MHSs. With vast electroactive surface area and favorable
mesoporous architecture, the NiCo<sub>2</sub>O<sub>4</sub> MHSs exhibit
a high specific capacitance of up to 2623.3 F g<sup>–1</sup>, scaled to the active mass of the NiCo<sub>2</sub>O<sub>4</sub> sample
at a current density of 1 A g<sup>–1</sup>. A nearly constant
rate performance of 68% is achieved at a current density ranging from
1 to 40 A g<sup>–1</sup>, and the sample retains approximately
94% of its maximum capacitance even after 3000 continuous charge–discharge
cycles at a consistently high current density of 10 A g<sup>–1</sup>