Ultralight Flexible Electrodes of Nitrogen-Doped Carbon Macrotube Sponges for High-Performance Supercapacitors

Light-weight and flexible supercapacitors with outstanding electrochemical performances are strongly desired in portable and wearable electronics. Here, ultralight nitrogen-doped carbon macrotube (N-CMT) sponges with 3D interconnected macroporous structures are fabricated and used as substrate to grow nickel ferrite (NiFe₂O₄) nanoparticles by vapor diffusion–precipitation and in situ growth. This process effectively suppresses the agglomeration of NiFe₂O₄, enabling good interfacial contact between N-CMT sponges and NiFe₂O₄. More remarkably, the as-synthesized NiFe₂O₄/N-CMT composite sponges can be directly used as electrodes without additional processing that could cause agglomeration and reduction of active sites. Benefiting from the tubular structure and the synergetic effect of NiFe₂O₄ and N-CMT, the NiFe₂O₄/N-CMT-2 exhibits a high specific capacitance of 715.4 F g⁻¹ at a current density of 1 A g⁻¹, and 508.3 F g⁻¹ at 10 A g⁻¹, with 90.9% of capacitance retention after 50 000 cycles at 1 A g⁻¹ in an alkaline electrolyte. Furthermore, flexible supercapacitors are fabricated, yielding areal specific capacitances of 1397.4 and 1041.2 mF cm⁻² at 0.5 and 8 mA cm⁻², respectively. They also exhibit exceptional cycling performance with capacitance retention of 92.9% at 1 mA cm⁻² after 10 000 cycles under bending. This work paves a new way to develop flexible, light-weight, and high-performance energy storage devices.