Two-dimensional CoNi nanoparticles@S,N-doped carbon composites derived from S, N-containing Co/Ni MOFs for high performance supercapacitors
Owing to controllable morphologies, tunable porous structures, diverse compositions and easy to fabricate, metal organic frameworks (MOFs) have been a class of ideal precursor materials 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 in room temperature using thiophene-2,5-dicarboxylate (Tdc) and 4,4ˊ-bipyridine (4,4’-Bpy) as organic linkers. As precursor material, the as-synthesized 2D Co/Ni MOFs nanosheets were further pyrolized at 550 °C in N2 atmosphere to obtain 2D CoNi alloy nanoparticles incorporated into S, N-doped carbon nanosheets (CoNi@SNC) with a surface area of 224 m2 g-1, porous structure, and good conductivity. Interestingly, it was found that 2D Co/Ni MOFs nanosheets can be directly used as electrode material for supercapacitor, delivering a specific capacitance of 312 F g-1 at 1 A g-1, whereas CoNi@SNC derived from its MOFs precursor as electrode material for supercapacitor exhibits 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 excellence rate capability at a high charge/discharge current. Further, an asymmetric supercapacitor device was also constructed with CoNi@SNC as the positive electrode and active carbon as the negative electrode, exhibiting an energy density of 55.7 Wh kg-1 at the power denstity of 0.8 kW kg-1 with lifetime stability up to 4000 charge-discharge cycles (capacitance retention of ~90.6%). The results demonstrate that electrochemical activation generated CoNi oxides/oxyhydroxides on the surface of CoNi alloy nanoparticles in alkaline electrolyte during electrochemical measurements should be the electrochemical active species of CoNi@SNC constructued supercapacitor. Besides of that, the high performance of CoNi@SNC constructed supercapacitor could be collectively due to its relatively high surface area favourbale for the exposure of electrochemical active sites, porous structure to promote redox-related mass transport, and CoNi alloy nanoparticles combined with graphitic carbon as electron collector to improve electron transfer.