Dense N-doped carbon nanotubes with encapsulated Fe nanoparticles directly grown within red brick as a sustainable monolithic electrode for high-performance supercapacitors

Abstract

Heteroatom-doped carbon nanomaterials are commonly employed as electrode materials for supercapacitors (SCs) due to their high accessible surface area, tunable surface chemistry, and unique electronic structures. However, they are generally prepared in fine powdery forms from expensive high-purity metal catalyst and carbon precursors via a tedious synthetic process, limiting their practical application. Herein, we develop a monolithic electrode, denoted as Fe@NCNTs/RB, by directly growing high-density N-doped carbon nanotubes (NCNTs) with encapsulated Fe nanoparticles within a red brick (RB) substrate via the chemical vapor deposition (CVD) method using melamine as the sole C and N sources. During the CVD process, the endogenous Fe species within the RB substrate act as efficient self-generated catalysts for catalyzing the in situ growth of high-density NCNTs from melamine pyrolysis, avoiding the use of external high-purity metal catalysts. The as-fabricated Fe@NCNTs/RB electrode is electrically conductive and mechanically strong and can be directly used as a binder-free electrode for SCs, exhibiting a high areal capacitance (C_a) of 918.75 mF cm⁻² at 1.0 mA cm⁻² and an excellent rate capability with 34% capacitance retention at 20 mA cm⁻². Notably, a symmetric SC assembled with an Fe@NCNTs/RB electrode delivers a high C_a of 277.48 mF cm⁻² at 1.0 mA cm⁻², an energy density of 11.13 μWh cm⁻² at a power density of 269.25 μW cm⁻² within a potential window of 0–1.1 V, and excellent cycling stability after 50 000 cycles with 92% capacitance retention and a unit coulombic efficiency at 10 mA cm⁻². This work paves a new way for the development of cost-effective and practically applicable monolithic electrodes for high-performance SCs.