Synthesis and characterization of hierarchical porous carbons for high-performance supercapacitors

Abstract

Climate change and energy crisis have become a serious concern because of the rapid industry development associated with large consumption of fossil resource. As an important energy storage device in renewable energy technologies, supercapacitors exhibit several competitive advantages over the lithium-ion batteries in high power density, rapid charge-discharge rate, and long cycle life. As a common electrode material of supercapacitor, activated carbon is generally produced from biomass carbonization followed by physical or chemical activation that involves either high-temperature treatments or toxic and corrosive activating agents, resulting in higher energy-consumption and environmental pollution. In this thesis, several new green and cost-effective methodologies are developed to synthesize hierarchical porous carbons for the high-performance supercapacitors.

We experimentally demonstrate a new one-step strategy of mild Na2CO3-NaCl molten salt assisted activation using glucose as the carbon precursor. During the pyrolysis, Na2CO3 acts as the activating agent and NaCl served as the reaction media, where the porous texture of the activated carbon can be manipulated by optimizing the synthesis parameters including pyrolysis temperature, composition of the molten salt and their mass ratio with the carbon precursor materials. In contrast, the activated carbon pyrolyzed from the mixture of sodium carboxymethylcellulose (CMC-Na) and NaCl exhibits superior capacitive performance in the aqueous supercapacitor. The experimental results show that the self-templated Na2CO3 facilitates the formation of hierarchical porous structure. Without NaCl additive, the porous carbons directly pyrolyzed from CMC-Na exhibit inferior capacitive performance, demonstrating the importance of NaCl addition in producing capacitive carbons.

We also report the high-performance ionic liquid pouch-type supercapacitors fabricated with the green aqueous binder and defect engineered graphene nanoparticles through a controllable ball-milling of pristine graphite. The parameters of the milling process, such as filling ratio, ball to graphite mass ratio, rotational speed, milling time, are systematically optimized to achieve the excellent gravimetric, volumetric and specific areal capacitances. Additionally, the high packing density of carbon electrodes and the post heat-treatment on ball-milled graphite can improve the cycle stability significantly. Our work provides a new insight to produce eco-friendly porous carbons for high throughout manufacturing production of large-scaled supercapacitors.