Surface oxygen-functional groups in zeolite-templated carbons: Effect on electrical energy storage characteristics

Abstract

Zeolite-templated carbons (ZTCs) can now be synthesized on a large scale (e.g., 70 g of carbon) with the aid of zeolite-embedded $Ca^{2+}$ ions as a carbonization catalyst, thereby giving much attention to the carbon materials for electrical energy storage applications. In this thesis, the anode characteristics of the ZTC electrode in Li-ion battery application will be addressed. Due to sub-nanometric pore sizes of the ZTC, $Li^+$ ions could diffuse into the carbon pores possibly without associated with solvent molecules or counter anions, and thus the Li storage capacity was increased to 8 times the capacity of the graphite electrode. However, there still remained a critical safety issue in the synthesis of ZTCs arising from the use of HF solutions for the dissolution of zeolites. To avoid the toxic HF, the detailed procedures using aqueous ethanolic NaOH and HCl solutions for the template removal have been developed while maintaining ordered microporous structures of ZTCs. In this thesis, the template-dissolution methods and the supercapacitor characteristics of the ZTC products from the NaOH-washing will be discussed. During the washing processes using the alkaline solutions, oxygen-containing functional groups were introduced on the ZTC structures. The increased oxygen content of the ZTCs resulted in a remarkable enhancement of supercapacitance in an aqueous electrolyte, whereas the opposite phenomenon was observed in an organic electrolyte. This result may be due to a decrease in the electrical conductance of the ZTCs, and an improvement in the hydrophilicity. The effect of the surface oxygen functionalities in ZTCs will provide valuable insights for research on the electrochemical applications of the nanoporous carbons as an electrode material.