Alkaline earth metal carboxylate hydrate-mediated controllable self-assembly of three-dimensional hierarchical nanoporous graphene for high-performance supercapacitors

Abstract

Three-dimensional (3-D) hierarchical nanoporous graphene (NPG) is highly promising as an electrode material for high-performance supercapacitors, but its fabrication presents formidable challenges. For the first time, 3-D hierarchical NPG has been synthesized via M(RCOO)₂·xH₂O-mediated controllable self-assembly, in which the noncovalently bonded M(RCOO)₂·xH₂O simultaneously create metal oxide NPs to serve as spacers and CO₂ and H₂O as pore generators. The pore size and pore density in the basal planes can be tailored by varying the amount of M(RCOO)₂·xH₂O. These unique nanostructured graphenes are subsequently investigated as electrode materials for supercapacitors, exhibiting a highly tunable specific capacitance. The most outstanding one achieves a specific capacitance of 186 F g⁻¹ at 0.5 A g⁻¹, surpassing all reported graphene-based supercapacitors in the same electrolyte. In addition, it maintains 79.6% of the capacitance at 20 A g⁻¹, revealing a superior rate performance compared to reported supercapacitors. Furthermore, no significant decrease in capacitance is observed even after 10 000 charge–discharge cycles, demonstrating excellent cycling stability. The ultrahigh capacitance and rate capability are ascribed to the 3-D hierarchical nanoporous structure of graphene with extremely rich edge sites remarkably increasing the surface area and promoting electron transfer and ion diffusion. This study not only presents an innovative strategy for synthesizing unique graphene-based materials with superior capacitive properties but also provides deep scientific insights into the self-assembly mechanism, which can be further extended to fabricate various other 3-D hierarchical nanoporous carbons for diverse important applications.