The use of asymmetric supercapacitors (ASCs) with high energy density remains challenging for practical applications. In this study, three-dimensional (3D) hierarchical Ni1-xCoxMoO4 center dot xH(2)O microspheres assembled with nanosheets are successfully synthesized using a cation exchange method and used as an ASC electrode material. The optimal synergistic effect of Ni and Co metal ions is obtained by controlling their molar ratio in the aqueous solution. Compared with pure NiMoO4 center dot xH(2)O, the presence of Co ion promotes the redox reaction and induces rapid diffusion of electrons. As such, double-metal Ni1-xCoxMoO4 center dot xH(2)O exhibits improved electrochemical performance. The developed ASC device of Ni0.67Co0.33MoO4 center dot xH(2)O//AC shows excellent electrochemical properties and a specific capacitance of 114.6 F g(-1) at a current density of 1 A g(-1). The ASC also exhibits outstanding cycling life with 73.28% of the specific capacitance retained after 10000 cycles. Moreover, Ni0.67Co0.33MoO4 center dot xH(2)O//AC possesses energy density as high as 40.76 Wh kg(-1) at a power density of 261.82 W kg(-1). These results indicate that the synergistic effect of Ni-Co can improve the performance of ASCs. (C) 2017 Elsevier Ltd. All rights reserved.