Catalytic efficiency of synthesized, thermally treated, and oxidized carbon nanotube catalysts for water splitting supported by density functional theory

Abstract

The development of efficient, low-cost, and noble-metal-free electrocatalysts is crucial for sustainable hydrogen production via alkaline water splitting. Herein, pristine carbon nanotubes (CNT), high-temperature-treated CNT (CNT-HT), and nitric-acid-oxidized CNT (CNT-OX) were systematically engineered to elucidate the effects of surface chemistry and defect modulation on bifunctional electrocatalytic performance. Structural characterization by XRD, Raman spectroscopy, SEM, TEM, and EDS revealed that thermal treatment improved graphitic ordering and intertube connectivity, whereas oxidation introduced uniformly distributed oxygen-containing functional groups and defect sites while preserving the CNT framework. Electrochemical measurements in 1?M KOH demonstrated that CNT-OX exhibited the best catalytic activity, delivering Tafel slopes of 221.20 mV/dec for OER and 82.53 mV/dec for HER, together with reduced onset potentials and lower charge-transfer resistance. In addition, electrochemical double-layer capacitance (Cdl) analysis showed that CNT-OX possessed the highest Cdl value (0.825?mF/cm2), indicating a larger electrochemically active surface area and a greater number of accessible active sites. Chronoamperometric analysis confirmed its superior durability. DFT calculations further showed that oxidation decreases the HOMO-LUMO energy gap from 5.093 to 5.061?eV. This combined experimental and theoretical study identifies surface oxidation as an effective route to activate CNTs for high-performance, metal-free alkaline water splitting. © 2026 Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies.