Porous Carbon Materials with Ultrahigh Surface Area for High Energy Density Supercapacitors and High Power Density Electrochemical Thermocell.
Authors: Zhang Long , Baughman Ray H. , Chen Yongsheng 
Published: February 2020

Low energy density of energy storage devices such as supercapacitors1-5 and thermocells6 has always been a problem to be solved urgently, which is due to the low specific surface area and low density of the electrode materials. The graphene based materials with high surface area have successfully increased the energy density of energy storage devices, but the density of graphene materials is still low, which leads to low energy density based on the weight of the whole devices. Currently most efforts have been focused on increasing the specific surface area of graphene materials, instead of increasing the density of graphene.
Herein, we have successfully synthesized graphene based materials with both high specific surface area and density. Phenolic precursors were in-situ polymerized on the surface of graphene 3D network through the hydrothermal reaction. The mass ratio between the phenolic precursors and graphene oxide was varied from 1:4 to 1:100. The results indicated that when the mass ratio between the phenolic resin precursor and graphene oxide is 1:16, graphene based 3D cross-linked nano-composites with high density can be realized. The corresponding specific surface area is 700 m2/g and the density is 0.55 g/cm3; Chemical activation by using activation agents to the as-prepared products was performed for the further increasing of the surface area. Different activation agents such as potassium hydroxide, sodium hydroxide and potassium carbonate were used, in which the chemical activation effect is the best for potassium hydroxide is the best. The optimal mass ratio between potassium hydroxide and Intermediate products ratio is 4:1, while the optimized activation temperature is 900 oC. The optimized graphene based materials have the specific surface area up to 3000 m2/g and the density about 0.5 g/cm3, which was used as the electrode materials for supercapacitors and thermocells3,6. Supercapacitors based on our materials exhibited a high energy density of 90 wh/kg in organic electrolyte, which is much higher than 54 wh/kg for supercapacitors based on the commercial activated carbon. Thermocells based on our materials achieved a output power of 12 W/m2, which is also much higher than 2 W/m2 for thermocells based on the traditional carbon materials. This project provides a key material for energy storage devices to achieve high energy density and output power. Furthermore, it will strongly promote the research and development of supercapacitors and thermocells for their application in the area of small ignition device and harvesting the low-grade and wasted thermal energy.
Reference:
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(6) Long Zhang; Taewoo Kim; Na Li; Tae June Kang; Jun Chen; Jennifer M. Pringle; Mei Zhang; Ali H. Kazim; Shaoli Fang; Carter Haines; Danah Al-Masri; Baratunde A. Cola; Joselito M. Razal; Jiangtao Di; Stephen Beirne; Douglas R. MacFarlane; Anuncia Gonzalez-Martin; Sibi Mathew; Yong Hyup Kim; Gordon Wallace; Ray H. Baughman*. High Power Density Electrochemical Thermocells for Inexpensively Harvesting Low-Grade Thermal Energy. Adv Mater
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