Molecular Design of Mesoporous NiCo2O4 and NiCo2S4 with Sub-Micrometer-Polyhedron Architectures for Efficient Pseudocapacitive Energy Storage

Spinel-type NiCo2O4 (NCO) and NiCo2S4 (NCS) polyhedron architectures with sizes of 500-600 nm and rich mesopores with diameters of 1-2 nm are prepared facilely by the molecular design of Ni and Co into polyhedron-shaped zeolitic imidazolate frameworks as solid precursors. Both as-prepared NCO and NCS nanostructures exhibit excellent pseudocapacitance and stability as electrodes in supercapacitors. In particular, the exchange of O2- in the lattice of NCO with S2- obviously improves the electrochemical performance. NCS shows a highly attractive capacitance of 1296 F g(-1) at a current density of 1 A g(-1), ultrahigh rate capability with 93.2% capacitance retention at 10 A g(-1), and excellent cycling stability with a capacitance retention of 94.5% after cycling at 1 A g(-1) for 6000 times. The asymmetric supercapacitor with an NCS negative electrode and an active carbon positive electrode delivers a very attractive energy density of 44.8 Wh kg(-1) at power density 794.5 W kg(-1), and a favorable energy density of 37.7 Wh kg(-1) is still achieved at a high power density of 7981.1 W kg(-1). The specific mesoporous polyhedron architecture contributes significantly to the outstanding electrochemical performances of both NCO and NCS for capacitive energy storage.