Hierarchical design of Cu-Ni(OH)₂/Cu-Mn_xO_y core/shell nanosheet arrays for ultra-high performance of asymmetric supercapacitor

Supercapacitors lack high energy density and long term stability, so a new class of hybrid material with hierarchical structure is desirable. Herein, for the first time we are reporting a rational design of hierarchical Cu-Ni(OH)₂/Cu-Mn_xO_y core/shell nanosheet arrays. The Cu-Ni(OH)₂/Cu-Mn_xO_y nanosheet arrays were synthesized by a two-step hydrothermal reaction. The as-synthesized Cu-Ni(OH)₂/Cu-Mn_xO_y hybrid structure exhibits ultrahigh areal and specific capacitances of 10.14 F cm⁻² and 2535 F g⁻¹, respectively at 3 mA cm⁻². The electrode exhibits high rate-capability (59.37%) and excellent capacitance retention of 93.09% after 10 000 cycles. In Cu-Ni(OH)₂/Cu-Mn_xO_y core/shell hybrid structure, highly conductive and electroactive Cu-Ni(OH)₂ core and Cu-Mn_xO_y shell components played a vital role to develop a high-performance and stable electrode material. Furthermore, the unique porous structure of Cu-Ni(OH)₂/Cu-Mn_xO_y also provides enough space and short-diffusion lengths for Faradaic reaction. The asymmetric supercapacitors assembled of Cu-Ni(OH)₂/Cu-Mn_xO_y as positive electrode and GR/CNTs as negative electrode delivers a high specific capacitance of 267.15 F g⁻¹ at a current density of 1 A g⁻¹ with high-rate capability of 60.30%. Moreover, the as-assembled ASC device also shows an ultra-high energy density of 94.98 W h kg⁻¹ at a power density of 759.89 W kg⁻¹ and a maximum power density of 15 173.5 W kg⁻¹ at an energy density of 57.28 W h kg⁻¹ with capacitance retention of 89.79% after 10 000 cycles. These outstanding results suggest that Cu-Ni(OH)₂/Cu-Mn_xO_y hybrid structure can be considered as a cathode material for asymmetric supercapacitor in future.