Three-dimensional nickel hydroxide/graphene composite hydrogels and their transformation to NiO/graphene composites for energy storage

Abstract

Graphene and its functionalized derivatives like graphene oxide (GO) have become handy and convenient building blocks for self-assembly to fabricate graphene-based functional materials with three-dimensional (3D) macroscopic structures. Herein, a convenient one-step hydrothermal method for preparing Ni(OH)₂/graphene composite hydrogels (NGHs) with interconnected networks is described. This procedure includes the reduction of GO sheets by hydrazine and the in situ deposition of Ni(OH)₂ nanoplates on graphene sheets. Notably, the obtained NGH_7.5 (the calculated mass ratio of formed Ni(OH)₂ with GO is 7.5 : 1) can offer a high specific capacitance of 1125.4 F g⁻¹ at a charge/discharge rate of 0.5 A g⁻¹, which is almost 2.2 times and 8.1 times higher than Ni(OH)₂ and graphene, respectively. Meanwhile, the NGHs also present stable cycling performances with 87.3% capacitance retention after 1000 cycles, which are significantly higher than that of its counterparts of Ni(OH)₂. More interestingly, the 3D structure of the NGH can be easily transmitted to NiO/graphene (NiO/G) composites utilizing a facile thermal treatment procedure. The electrode based on the NiO/G composite delivers a discharge capacity of 1349 mA h g⁻¹ and a charge capacity of 992 mA h g⁻¹ at the 1st cycle with a coulombic efficiency of about 73.5%. This work opens a considerable way to fabricate functional graphene-based 3D structures and their composite materials, and makes a significant contribution to energy storage/conversion from alternative energy sources.