Two-dimensional CoNi nanoparticles@S,N-doped carbon composites derived from S,N-containing Co/Ni MOFs for high performance supercapacitors

Due to their controllable morphologies, tunable porous structures, diverse compositions and easy fabrication, metal-organic frameworks (MOFs) are an ideal class of precursor material to develop high performance carbon-based materials for energy applications. In this work, two-dimensional (2D) Co/Ni MOFs nanosheets with a molar ratio of Co2+ to Ni2+ of 1 : 1 were first synthesized at room temperature using thiophene-2,5-dicarboxylate (Tdc) and 4,4'-bipyridine (4,4'-Bpy) as organic linkers. As a precursor material, the as-synthesized 2D Co/Ni MOFs nanosheets were further pyrolized at 550 degrees C in N-2 atmosphere to incorporate 2D CoNi alloy nanoparticles into S, N-doped carbon nanosheets (CoNi@SNC) with a surface area of 224 m(2) g(-1), a porous structure, and good conductivity. Interestingly, it was found that the 2D Co/Ni MOFs nanosheets can be directly used as electrode materials for supercapacitors, delivering a specific capacitance of 312 F g(-1) at 1 A g(-1), whereas CoNi@SNC derived from its MOFs precursor as an electrode material for supercapacitors exhibits a much higher specific capacitance (1970, 1897 and 1730 F g(-1) at 1, 2, 5 A g(-1), respectively) with long cycling life (retaining 95.1% of the value at 10 A g(-1) after 3000 cycles) and excellent rate capability at a high charge/discharge current. Further, an asymmetric supercapacitor device was constructed with CoNi@SNC as the positive electrode and active carbon as the negative electrode, exhibiting an energy density of 55.7 W h kg(-1) at a power density of 0.8 kW kg(-1) with lifetime stability up to 4000 charge-discharge cycles (capacitance retention of similar to 90.6%). The results demonstrate that electrochemical activation-generated CoNi oxides/oxyhydroxides on the surface of the CoNi alloy nanoparticles in alkaline electrolyte during electrochemical measurements are the electrochemical active species of the CoNi@SNC-constructed supercapacitor. Additionally, the high performance of the CoNi@SNC-constructed supercapacitor can be collectively attributed to its relatively high surface area, which is favourable for the exposure of electrochemical active sites; its porous structure, which promotes redox-related mass transport; and the combination of CoNi alloy nanoparticles with graphitic carbon, which functions as an electron collector to improve electron transfer.