Synthesis of N-doped zeolite-templated carbons via depolymerized oligomer filling: applications in EDLC electrodes

Abstract

Zeolite-templated carbons (ZTCs) are widely studied from basic research to applied research owing to their characteristic pore structures. To synthesize ZTCs, molecules with a size smaller than the pore sizes of template zeolites have been used as carbon sources for their carbonization in the zeolite pores. Therefore, the type of carbon sources has been limited to molecules with a size smaller than the pore sizes of zeolites. In this study, highly structurally regular N-doped zeolite-templated carbons are synthesized using propylene as a carbon source and chitin as both carbon and nitrogen sources via a depolymerized oligomer filling (DOF) mechanism. Chitin, the second most abundant biopolymer on the Earth, consists of N-acetylglucosamine (GlcNAc) as its unit structure and has a much larger size than the zeolite pores. NaY zeolite is used as a template without drying and mixed with chitin. The mixture is subjected to chemical vapor deposition (CVD) using propylene and subsequent heat treatment for graphitization, followed by HF etching for zeolite removal. Upon heating the mixture of the zeolite and chitin, chitin is catalytically depolymerized into chitin oligosaccharide radicals by the zeolite, and the radicals are absorbed into the zeolite pores below 450 °C, which is supported by electron spin resonance and N₂ adsorption/desorption analyses. The ZTC structure is completed by propylene CVD for adequately filling carbon into the zeolite pores. A validation experiment is conducted using GlcNAc instead of chitin to confirm that the N-doped ZTC is synthesized via the DOF mechanism. The resulting N-doped ZTCs have high structural regularity and high surface areas ranging from 3420 to 3740 m² g⁻¹, and show a higher area-normalized capacitance than undoped ZTC as electric double-layer capacitor electrodes. Utilizing chitin from crustacean shells as one of the raw materials highlights an innovative approach to waste reduction and advances sustainable materials science, contributing to the circular economy and sustainable development goals.