One-step strategy to graphene/Ni(OH)₂ composite hydrogels as advanced three-dimensional supercapacitor electrode materials

Graphene-based three-dimensional (3D) macroscopic materials have recently attracted increasing interest by virtue of their exciting potential in electrochemical energy conversion and storage. Here we report a facile one-step strategy to prepare mechanically strong and electrically conductive graphene/Ni(OH)₂ composite hydrogels with an interconnected porous network. The composite hydrogels were directly used as 3D supercapacitor electrode materials without adding any other binder or conductive additives. An optimized composite hydrogel containing ~82 wt.% Ni(OH)₂ exhibited a specific capacitance of ~1, 247 F/g at a scan rate of 5 mV/s and ~785 F/g at 40 mV/s (~63% capacitance retention) with excellent cycling stability. The capacity of the 3D hydrogels greatly surpasses that of a physical mixture of graphene sheets and Ni(OH)₂ nanoplates (~309 F/g at 40 mV/s). The same strategy was also applied to fabricate graphene–carbon nanotube/Ni(OH)₂ ternary composite hydrogels with further improved specific capacitances (~1, 352 F/g at 5 mV/s) and rate capability (~66% capacitance retention at 40 mV/s). Both composite hydrogels obtained here can deliver high energy densities (~43 and ~47 Wh/kg, respectively) and power densities (~8 and ~9 kW/kg, respectively), making them attractive electrode materials for supercapacitor applications. This study opens a new pathway to the design and fabrication of functional 3D graphene composite materials, and can significantly impact broad areas including energy storage and beyond.