“Pottery” of Porous Graphene Materials

DOI: 10.1002/aelm.201500004 Here, we report a prototype method of processing GOM into “pottery” of porous graphene materials (PPGM) with ultrahigh weight densities by imitating the process of preparing traditional pottery: mud-making, formation, and sintering ( Figure 1 a–c). In brief, GOM was prepared from the wet cake of GO without the purifi cation by dialysis; shaping GOM into desired structures (named as GOM workblanks) by either hands or using molds; “sintering,” i.e., the reduction of GOM workblanks via a solvothermal process to set their shapes, and removing impurities by hydrothermal reaction (Figure 1 d–f). In this case, GOM with a GO content around 40% (by weight, wt%) was employed as the precursor. As a result, the fi nal PPGM possesses a much higher weight density than those of other graphene-based porous materials, making it an ideal material for the use as the electrodes of supercapacitors. Typically, 60 μm thick PPGM plates were directly used as the electrodes of supercapacitors without blending any binders or conductive additives, showing high gravimetric and volumetric specifi c capacitances of about 200 F g −1 and 200 F cm −3 , as well as good rate capability and electrochemical stability. Moreover, these PPGMs exhibited interesting anisotropic microstructure and properties, which were corresponding to the direction of stress applied during the formation machining. GO was synthesized from graphite by a modifi ed Hummers’ method developed by our group, [ 32,33 ] and it was washed with diluted hydrochloric acid and deionic water after the fi ltration to form a wet cake. This cake was collected and dried under ambient condition for a certain time to form GOM (Supporting Information). In this case, the sonication, centrifugation, and dialysis processes usually involved in Hummers method for exfoliation and purifi cation of GO were not applied. Thus, our method is more simple, cheaper, and eco-friendly than those reported previously. The atomic force microscopic (AFM) images indicate that our GO sample purifi ed by dialysis (pGO for characterization only) composes of mainly monolayer GO sheets with lateral sizes from submicrometer up to several micrometres and a small amount of multilayer GO sheets (Figure S1, Supporting Information). The GOM with water content around 60% was tested to have the proper machinability for shaping. The GOM is a plastic mixture with a dark brown color like ceramic mud (Figure 1 d). The GOM can be easily processed into desired shapes such as fl ower by hands or using molds (Figure 1 e). In order to fi x the shapes of the soft and easily deformable GOM workblanks, “sintering,” a solvothermal treatment using hexane as the solvent was performed at 180 °C for 10 h (Experimental Section). Hexane is a hydrophobic poor solvent of GO or rGO. Thus, the water in GOMs was kept inside the workblanks to form pores and the GO component was reduced into rGO to fi x the shapes. After drying in air, rGO monoliths were obtained and they are nominated as crude PPGM (cPPGM). The cPPGM maintained the shape of its original GOM precursor with a small volume shrinkage Graphene has a unique atom-thick 2D structure with a large specifi c surface area, good physical and chemical stability, and excellent mechanical, optical, electrical, and thermal properties. [ 1–3 ] Therefore, it is an attractive building block to construct 3D macroscopic porous materials for the applications in sensors, [ 4,5 ] catalysis, [ 6 ] environmental protection, [ 7 ] and energy related systems, [ 8–13 ] etc. Actually, a variety of techniques have been successfully developed to fabricate graphene-based 3D porous materials, and most of them involve using graphene oxide (GO) as the precursor. [ 14–30 ] This is mainly due to that GO can be easily produced in a large scale by oxidizing and exfoliating graphite, and it is dispersible and processable. [ 31–33 ]