In this work, a series of NixCo1-xSe2 selenides are prepared with x ranging from 0 to 1 and investigated as electrode materials for supercapacitor. The sample Ni0.6Co0.4Se2 shows 1580 and 1205 F g(-1) at 1 and 20 A g(-1), respectively. At 10 A g(-1), 90% capacitance retention is sustained after 20,000 cycles. A Ni0.6Co0.4Se2-based asymmetric supercapacitor (ASC) is assembled and exhibits energy density of 44.1 and 36.8 Wh kg(-1) at power density of 691.3 and 15070.6 W kg(-1) and long stability. Systematic physicochemical/electrochemical analysis reveals that the high active MOOH (M = Ni, Co)/Ni0.6Co0.4Se2 phases generated by electrooxidation act as the actual active materials during the charge storage/release process. The collective effects of large ion-accessible active area, high conductivity and electrochemically active MOOH (M = Ni, Co)/Ni0.6Co0.4Se2 phases convincingly account for the remarkable performance, especially the excellent rate capability and stability. (C) 2019 Elsevier Inc. All rights reserved.