A conceptually new approach has been developed for the fabrication of magnetite (Fe 3 O 4 )-decorated carbon nanotubes (M-CNTs) for negative electrodes of electrochemical supercapacitors. M-CNTs were prepared by an ultrasonic-assisted chemical synthesis method, which involved dispersion of functionalized CNTs in water, Fe 3 O 4 formation on the CNTs surface, and particle extraction through liquid-liquid interface (PELLI). Palmitic acid was found to be an efficient new extractor for PELLI. The slurries produced after drying and redispersing M-CNTs and slurries obtained using PELLI were used for electrode fabrication. The electrodes prepared using PELLI showed superior performance due to reduced particle agglomeration. Testing results provided an insight into the influence of Fe 3 O 4 /CNTs mass ratio on the capacitance and capacitance retention at high charge-discharge rates. A capacitance of 5.82 F cm −2 (145.4 F g −1 ) was achieved in Na 2 SO 4 electrolyte using electrodes with high active mass of 40 mg cm −2 and ratio of active mass to current collector mass of 0.6. Good electrochemical performance was achieved at low impedance.The capacitance of the negative M-CNTs electrodes was comparable with capacitance of advanced positive MnO 2 -CNTs electrodes, which was beneficial for the fabrication of asymmetric devices. The asymmetric device has been fabricated, which showed promising performance in a voltage window of 1.6 V. K E Y W O R D Sasymmetric device, capacitance, carbon nanotube, composite, magnetite, supercapacitor

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This investigation is motivated by the need in the development of manganese oxide cathodes for supercapacitors with high capacitance at high charge–discharge rates and enhanced capacitance retention in a wide range of charge–discharge rates. It also addresses the challenge of eliminating the time-consuming activation procedure, which limits the applications of Mn3O4 cathodes. The new approach is based on the use of environmentally friendly and biocompatible pH modifiers–dispersants, such as polyethylenimine (PEI) and meglumine (MG) for hydrothermal synthesis. In this approach, the use of inorganic alkalis is avoided. We demonstrate the benefits of this approach for the fabrication of manganese oxide nanoparticles, such as Mn-PEI and Mn-MG. Electrodes with a high active mass of 40 mg cm−2 are fabricated and electrochemically tested in 0.5 M Na2SO4 electrolyte. The method of electrode material fabrication offers benefits for the accelerated electrode activation procedure, which is practically eliminated for Mn-MG electrodes. The Mn-MG electrodes showed a remarkably high capacitance of 3.68 F cm−2 (93.19 F g−1) at a sweep rate of 100 mV s−1 and a high capacitance retention of 90.6% in the CV sweep range of 1–100 mV s−1.

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