Design of 3D Graphene‐Oxide Spheres and Their Derived Hierarchical Porous Structures for High Performance Supercapacitors

Abstract

Graphene‐oxide (GO) based porous structures are highly desirable for supercapacitors, as the charge storage and transfer can be enhanced by advancement in the synthesis. An effective route is presented of, first, synthesis of three‐dimensional (3D) assembly of GO sheets in a spherical architecture (GOS) by flash‐freezing of GO dispersion, and then development of hierarchical porous graphene (HPG) networks by facile thermal‐shock reduction of GOS. This leads to a superior gravimetric specific capacitance of ≈306 F g−1 at 1.0 A g−1, with a capacitance retention of 93% after 10 000 cycles. The values represent a significant capacitance enhancement by 30–50% compared with the GO powder equivalent, and are among the highest reported for GO‐based structures from different chemical reduction routes. Furthermore, a solid‐state flexible supercapacitor is fabricated by constructing the HPG with polymer gel electrolyte, exhibiting an excellent areal specific capacitance of ≈220 mF cm−2 at 1.0 mA cm−2 with exceptional cyclic stability. The work reveals a facile but efficient synthesis approach of GO‐based materials to enhance the capacitive energy storage.

An efficient carbon-based ORR catalyst from low-temperature etching of ZIF-67 with ultra-small cobalt nanoparticles and high yield

“…[29][30][31][38][39][40][41][42][43] For instance, Zhong et al reported a nitrogen-doped carbon system by carbonization of ZIF-8/GO at 950 C. 29 It is worth noting that the precursors exhibit a signicant mass loss with a catalyst yield of less than 30 wt%. 29 Since the MOFs/ZIFs can be calcined at as low as 350 C in ambient air, the high concentration of thermally decomposed oxygen functional groups (CO/CO 2 /H 2 O) from GO can cause signicant gasication of the ZIF ligand, in the form of CO/CO 2 , [44][45][46] at the expense of the mass yield of the catalyst. Furthermore, owing to the low specic surface area of GO (less than 100 m 2 g À1 ), 44 with its packed layer-structure, it is difficult to achieve a homogeneous hybrid system from the surface grown MOF/ZIF crystals.…”

“…29 Since the MOFs/ZIFs can be calcined at as low as 350 C in ambient air, the high concentration of thermally decomposed oxygen functional groups (CO/CO 2 /H 2 O) from GO can cause signicant gasication of the ZIF ligand, in the form of CO/CO 2 , [44][45][46] at the expense of the mass yield of the catalyst. Furthermore, owing to the low specic surface area of GO (less than 100 m 2 g À1 ), 44 with its packed layer-structure, it is difficult to achieve a homogeneous hybrid system from the surface grown MOF/ZIF crystals. 30,38,40,47 However, the derivatives of the ZIF@rGO framework with increased surface area can boost the overall activity by facilitating the accessibility and distribution of reactants and products.…”

“…48 In principle, thermal-shock exfoliated GO (EGO) would be an ideal substrate, as it not only exhibits a high specic surface area (z700 m 2 g À1 ) but also has a exible pore structure with an extensive network of hierarchical meso/macropores (up to 6 cm 3 g À1 ; with sizes between 5 and 200 nm), for pore-conned growth of ZIF nanocrystals. 44,45 Moreover, the lower oxygen content (z13 atom%), with highly concentrated defects as anchoring sites can promote homogeneous crystal nucleation within its hyperporous nanospaces. The thermolysis of ZIFs/MOFs at mild temperatures before their full decomposition or carbonization at >600 C has yielded a defect-rich active structure by preserving their intrinsic pore structure with highly exposed (open) metal centres from the partly decomposed framework links at a minimal loss of the sample.…”

“…49,50 The active cobalt centres then readily react with thermally liberated oxygen from the EGO substrate. Here, it is worth noting that EGO exhibits limited residual oxygen content (as it is already exfoliated by thermal-shock at 400 C), 44,45 and the temperature plays a critical role in dissociating these remaining oxygen functional groups from EGO. Due to this, the hybrid, ZIF-67@EGO treated at 400 C shows little evidence for the oxidation of cobalt metal centres.…”

“…Control over dissociation of the framework, Co-N bonds and ligands, imidazolate ring in ZIF-67 is the key to produce a nanophase of Ndoped Co 3 O 4 /Co. This overview of the structural manipulation mechanism under mild oxidation conditions (EGO exhibits a very low amount of oxygenated groups compared to that of pristine GO, 13 atom% to 35 atom%) 44,45 is schematically represented in Scheme 1. Such structures in the hyperporous graphene networks should provide extensive surface accessibility and enhanced mass activity of N-doped Co 3 O 4 /Co (or CoO/Co) centres for oxygen reduction electrocatalysis.…”