Graphene aerogel, assembled from graphene sheets, with an ntrinsically high specifc surface area, and continuous thermal and electrical conductivity networks, has demonstrated unparalleled advantages in energy storage, sensing and adsorption fields. However, due to the instinctive random-stacking of graphene sheets during the sol–gel process of graphene oxide (GO) solution, the traditional graphene aerogel usually consists of disordered graphene porous network, which undermines the ions/charge transmission in electrode. The three-dimensional porous network of conventional graphene aerogels is irregular, caused high interfacial resistance and zigzagging ion channels, which has negative effect on charge-ion transport and effective electrochemical area in electrochemical process. Those problems restricts the use of graphene materials in electrochemical energy devices. Therefore, how to design a new strategy to assemble graphene to fabricate high-performance graphene aerogels is still an important chanllenge.
Aim at the existing problems of graphene aerogel, Holey graphene oxide (HGO) was synthesized via a facile mild defect-etching reaction. HGO suspensions exhibited a bright birefringence texture under a cross polarized light microscope, reﬂecting the formation of the lyotropic liquid crystalline phase with high-speed centrifugation. The uniform alignment in the anisotropic holey graphene hydrogels was further enhanced due to the capillary effect within the syringe piston via in situ reduction of oriented holey graphene oxide, as shown in figure 1. As obtained HGO aerogel has a regular three-dimensional porous network structure, low density of 42–55 mg cm-3, good conductivity of 165 S m-1, and high specific surface area of 537~825 m2 g-1. In this work, an effcient cryo-thermocell system has been developed by assembling the above HGO aerogel and electrolyte by the mixture of water and formamide. This cryo-thermocell system can steady operate under -40oC, which demonstrated the outstanding power density and low ion transport resistance, 3.6 W m-2 and 15.7 Ω. Moreover, assembled thermocells in series packaging substantially enhance the voltage of the open-circuit, i.e., from 140 mV (1-cylinder thermocell) to 2.1 V (15-cylinder thermocells), showing an important application prospect in the application of low temperature energy devices.
Figure 1 a,b) TEM images of graphene oxide (a) and holey graphene oxide (b). c,d) The optical texture of graphene oxide liquid crystal (c) and holey graphene oxide liquid crystal (d), under a cross-polarized-light optical microscope (POM). e) Raman spectra of graphene oxide and holey graphene oxide. f) A digital photo of cylinders of graphene aerogel (GA), anisotropic graphene aerogel (AGA), holey graphene aerogel (HGA), and anisotropic holey graphene aerogel (AHGA) supported by the petals of a lily. g,h) SEM images of the facture surfaces of perpendicular (g, top view) and parallel (h, side view) to the long axis of AHGA cylinder. The inset in (g) is the illustration direction of AHGA cylinder. i) Compressive stress–strain curves measured from the cubic samples. The inset in (i) is a 15 mg anisotropic holey graphene aerogel cylinder supporting a 200 g counterpoise, more than 13 000 times of its own weight. j) Conductivity and mechanical strength of the anisotropic holey graphene aerogel in different directions. k) Specifc surface area of the different aerogels.