In this paper, we synthesize a new type of Ni-Co selenides (Ni0.34Co0.66Se2 nanorod) on carbon fiber paper (CFP) as flexible electrode for supercapacitor. Comparing with NiCo2O4 and NiCo2S4, the Ni0.34Co0.66Se2 exhibits superior capacitive performance including high areal capacitance (2.61 F cm (2) at 4 mA cm (2)), good rate capability (75% of retention from 4 to 20 mA cm (2)) and excellent long-term cycling stability (only 7.4% of loss after 6000 cycles). When assembled into a symmetric supercapacitor, the as-obtained, Ni0.34Co0.66Se2 based device possesses a volumetric capacitance of 14.55 F cm (3) at 1 mA cm (2), and a volumetric energy density of 0.47 mWh cm (3) at 10 mA cm (2), which are superior to most of the symmetric supercapacitors reported previously. Then we explore the energy storage mechanism of Ni0.34Co0.66Se2 by monitoring the changes in component, morphology, electroactive surface area (ESA) and electron transport characteristics. It is found that the Ni0.34Co0.66Se2 species suffer from serious Se loss and transform into NixCo1-x(O) gradully during charge/ discharge cycles, but the ESA increases significantly in this process. In addition, the Ni0.34Co0.66Se2 electrode possesses lower internal resistance, indicating its good electron transfer properties. Both these factors will compensate the performance degradation from Se loss, resulted in better rate capability and cycling stability for Ni0.34Co0.66Se2 electrode. Therefore, it is believed that the proposed 3D Ni0.34Co0.66Se2 nanorod array modified CFP with optimizational component and structual design opens a new horizen in the development of highperformance flexible supercapacitor electrode with new charge storage mechanism. (C) 2017 Elsevier Ltd. All rights reserved.