Fabrication And Electrochemical Capacitance Of Graphene Electrode Materials With Three - Dimensional Network Structure

Electrochemical capacitors, also known as supercapacitors, have received tremendous attention as energy storage devices because of their unique properties including fast charge-discharge rate, excellent cycling performance and environmental friendly property. Electrode materials play an important role in determining the performance of a supercapacitor. Graphene-based materials have emerged as one of promising electrode for electrochemical energy storage (EES) because of itshigh specific surface area, favorable electronic properties, extraordinarily thermal and excellent mechanical properties. However, the two-dimensional (2D) graphene suffers from overlapping and restacking because of π-π stack and hydrophobic interaction between graphene layer, as well as the physical cross-linking and Van der Waals’ interaction during processing of graphene materials. Building 2D graphene into three-dimensional (3D) structure not only favors the transport of electrons, but also promotes the ion diffusion in the interior of electrode, leading to an unexpected capacitive performance.Herein, a 3D highly conductive graphite network was prepared by a chemical vapor deposition (CVD) method with commercially available Ni foam as the sacricial template and styrene as the carbon precursor. The seamlessly interconnected 3D network with high graphitization makes the 3DG an ideal scaffold to accommodate various pseudocapacitive components. The hydrothermal reaction between 3DG and aqueous KMnO4 allows the growth of uniform MnO2 nanoflakes on the surface of 3DG backbone to form a hybrid nanostructure (3DG-MnO2). The 3DG-MnO2 composite retained a similar interconnected morphology to that of pristine 3DG. The intimate junction between the 3DG backbone and MnO2 minimizes the interfacial contact resistance and greatly promotes electron migration from the 3DG network to MnO2, while the MnO2 nanoflakes emanating from the 3DG backbone offer a large electrochemically active surface area, promote the capacitance of composite materials, largely shortening the diffusion pathway for effective electrolyte penetration. In a three electrode system, the 3DG-MnO2 electrode delivers a specific capacitance of 210 F/g at 2.0 A/g, reasonable cycling stability (75%) and an outstanding coulombic efficiency (97.8%) after 4000 consecutive charge-discharge cycles, making the 3DG-MnO2 composite one of the promising electrodes for EES.Graphene aerogel (GA) have drawing wide attentions in view of its abundant porosity and 3D nature. However, during the hydrothermal gelation process, the partial sheets overlapped or coalesced through physical cross-linking of individual reduced GO, resulting in dramatic decrease of specific surface area and a low energy storage capacity. To make the GA meet the requirements for high energy storage device, we adopted a facile yet efficient route to introduce 2.0-8.0 nm nanopores on graphene sheets to form a hierarchical macro-and mesoporous structure. The interconnected macropores of GA and the presence of 2D in-plane nanopores potentially improve the utilization efficiency of electrode for charge storage and greatly promoted the ion kinetics of electrolyte. According to the N2 absorption anlysis, the GA activated with 0.5 M H3PO4 (aGA-0.5) exhibited a maximum specific surfeca area of 1145 m2/g. Electrochemical measurement showed that this electrode exhibited a maximum capacitance of 204 F/g with an excellent cycling stability (92% capacitance retention after 10000 cycles). This method opens up a new venues to prepare various graphene-based functional materials for emerging applications in biosensors, solar cell, nanoelectronics and electrochemical energy storage.