3D Ni-Co selenide nanorod array grown on carbon fiber paper: towards high-performance flexible supercapacitor electrode with new energy storage mechanism

In this paper, we synthesize a new type of Ni-Co selenides (Ni_0.34Co_0.66Se₂ nanorod) on carbon fiber paper (CFP) as flexible electrode for supercapacitor. Comparing with NiCo₂O₄ and NiCo₂S₄, the Ni_0.34Co_0.66Se₂ 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, Ni_0.34Co_0.66Se₂ 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 Ni_0.34Co_0.66Se₂ by monitoring the changes in component, morphology, electroactive surface area (ESA) and electron transport characteristics. It is found that the Ni_0.34Co_0.66Se₂ species suffer from serious Se loss and transform into Ni_xCo_1-xO gradully during charge/discharge cycles, but the ESA increases significantly in this process. In addition, the Ni_0.34Co_0.66Se₂ 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 Ni_0.34Co_0.66Se₂ electrode. Therefore, it is believed that the proposed 3D Ni_0.34Co_0.66Se₂ nanorod array modified CFP with optimizational component and structual design opens a new horizen in the development of high-performance flexible supercapacitor electrode with new charge storage mechanism.