Nanoconfined and Chemically Bonded MnO@Mn ₂ O ₃ Heterojunctions Within Carbon Nanotubes for Fibrous Supercapacitor with Ultra‐Long Cycle Stability

Abstract Carbon‐based fibrous supercapacitors (FSSCs) are promising power sources for wearable electronics, often compounding with transition metal oxides (TMOs) to improve energy density. However, conventional methods introducing TMOs onto exterior surfaces of carbon‐based fibers typically degrade electrical transport and cycle stability. Herein, nanoconfined MnO@Mn ₂ O ₃ heterojunctions within carbon nanotube (CNT) (MOIC) composite FSSCs stabilized by Mn─O─C bonds, exhibiting record cycle stability with 95.7% capacitance retention after 10 000 cycles and 89.4% after 50 000 cycles are reported. X‐ray absorption near edge structure (XANES), X‐ray diffraction, and X‐ray photoelectron spectroscopy (XPS) analyses confirm MnO@Mn ₂ O ₃ heterostructure, which arises through a partial phase transformation from MnO to Mn ₂ O ₃ , as further supported by density functional theory calculations. Mn─O─C chemical bonds, as verified through XPS, extended X‐ray absorption fine structure, and XANES analyses, facilitate 3D electron transport, enabling MOIC composite fiber superior electrical conductivity than CNT fiber. The nanoconfinement of Mn ²⁺ within CNTs, driven by capillary effects and electrostatic repulsion between protonated CNTs and Mn ²⁺ , preserves the clean exterior surfaces of CNTs. This configuration enables the successful wet‐spinning of MOIC composite fibers with three times the tensile strength of fibers without nanoconfinement. This work opens new pathways for designing carbon/metal oxide hybridized supercapacitors for wearable energy storage applications.